Hsikwangshan Twinkling Star Co., Ltd

Hsikwangshan Twinkling Star Co., Ltd.(HTS)is a large 
comprehensive enterprise integrated with mining, dressing, smelting, 
chemical industry production and scientific research, the biggest  factory for producing antimony products, the cradle of the country antimony industry, and the major exporting base area of antimony products. The company owns two mines, four smelters, one technology center and one inspection and measure center. Its annual production capacity is 55,000 metric tons of ores, 40,000 metric tons of antimony products and 40,000 metric tons of zinc ingots. It is famous all over the world for its rich anti-mony reserves and antimony products, and has been titled beautiful name "World Antimony Capital" for a long time. 
    Since the commencement of the mining in 1897, Hiskwangshan has had 
antimony industry for more than one hundred years of history, and 
accumulated abundant antimony production experience. The antimony 
products have good quality and various specifications, of which over 90% 
are exported far away to more than 50 countries and regions such as Japan,America, Europe, and are famous domestic and overseas. The products have won the National Silver Medal for many times. The product trademark "Twinkling Star" brand has been appraised as famous trademark for many times. The quality management system has obtained ISO9002 series quality Certification.
    At present, of its over 7000 employees, more than 800 are the 
different kinds of specialized technical personnel. So HTS can keep  ahead in the production and scientific research field of antimony industry in the country. The related standards of antimony quality and chemical analysis methods were drawn up by the company under the government s accreditation.With antimony uses incercreasingly extending, other than keeping on producing antimony ingots, antimony trioxide, antimony sulfide, etc., traditional products with high grade, the company has been researched and developed wide antimony series such as sodium antimonate, dust-free antimony trioxide, antimony pentoxide,
golden yellow antimony, antimony series flame-retardant masterbatch. 
Especially the antimony oxide line with high technology that was put into production in 2002 and listed as one of the national "Double High & One Top"items introduced computer control, and practiced lots of scientific payoffs developed in donkey's years, therefore, all kinds of chemical indexes and physical characters of the products are more stable and the outside package improved visibly. Now they could completely meet the requirements of all kinds of customers.
    In order to keep up with the market changes and enhance economic benefit, while focusing on producing antimony products, the company has been actively developing other products. Presently, it has established several product lines with the annual production capacity of 10,000 metric tons of chlor-alkali, 5000 metric tons of hydrogen peroxide, 40,000 metric tons of zinc ingots, and 70,000 metric tons of sulfuric acid, zinc sulfate, indium metal, cadmium metal, etc.. A new pattern, of which antimony production is the major but coexisting with other production, has taken shape and sufficiently shows the obvious  advantages and large potential of "World Antimony Capital."
    There is only one "Antimony Capital"around the world, and "Antimony Capital" will be worthy of its name. "Twinkling Star" is a symbol of high quality, and satisfying all reasonable requirements of the customers is Hsikwangshan' s aim.We will provide favorable service and cooperate sincerely with the domestic and overseas customers with reciprocity.

Manganese flakes offers still rising but buyers reluctant to purchase

Manganese offers go up to almost RMB14,000/t (USD2,050/t) ex works but few deals can be concluded at prices above RMB13,500/t (USD1,977/t) ex works. Sources reported that the buyers feel reluctant to enter the market now for fear that the price would go higher once they accept such a high price. Actually, the downstream market is not so promising for the moment.
A Hunan-based smelter offering RMB14,000/t (USD2,050/t) ex works told Asian Metal that they received many inquiries these days. "While we only have about one to two cargos of stock at hand, we are in no hurry to sell for the moment as the price is still rising," said the source, thinking that most smelters still have not fully resumed production and manganese supply is not very sufficient for the moment.
The source reported that many foreign buyers also start to arrange some deals, expecting that manganese export price would also move up soon. "Many suppliers already have raised their offers to USD2,450/t FOB though the buyers are reluctant to purchase at above USD2,350/t."
The source believes manganese demand would keep strong in the coming weeks while most consumers only have very limited stock at hand. But the source is also cautious that more smelters would resume production, seeing the price going up. The source claimed that as the downstream stainless steel market needs time to recover, it is better not to resume wholly too fast.
A Jiangsu-based buyer confirmed that the suppliers are raising their price significantly on expectation that the downstream industry would enter the market soon to place orders for the coming months while the stainless steel market already achieves some improvement. The source confirmed that many consumers might not have much material at hand but as it takes time for the demand to increase, it is impossible for the price to hike so quickly to RMB13,000-14,000/t (USD1,903-2,050/t) ex works.
According to the source, many major smelters already have placed some orders at much lower prices of about RMB12,000-12,400/t (USD1,757-1,816/t) ex works in last two months. The source believes manganese price would soon return to a normal level in the range of RMB12,000-12,500/t (USD1,757-1,830/t) ex works again.

Manganese price rises a bit with slightly increasing inquiries

With more buyers back to the market, more inquiries have been received these days and the participants claim manganese price rises a bit again back to RMB12,300/t (USD1,801/t) ex works. The offer is about USD2,350/t FOB for export and RMB12,500/t (USD1,830/t) ex works for domestic deals.
A Hunan-based smelter with a capacity of 12,000tpy told Asian Metal that most suppliers raise their offers to RMB12,400-12,600/t (USD1,816-1,845/t) ex works. As most smelters are running low, manganese supply is a bit tight now while many downstream consumers are about to replenish some deals for the coming months.
"Manganese price is stable at above RMB12,000/t (USD1,757/t) ex works though many buyers claim the price is not the bottom line yet," said the source, claiming that as manganese smelters are mostly in low production, the supply seems tightening. And many suppliers are reluctant to sell at a price below RMB12,300/t (USD1,801/t) ex works. The source claimed that some exporters raise their offer to USD2,350-2,400/t FOB.
Another Jiangsu-based trader believed that it depends on the downstream industry that whether manganese price would move up in the coming weeks. Currently, the stainless steel market improves a bit in China with the price rising and slightly increasing demand. As a result, manganese demand also increases gradually. The source claimed that they can get some materials at RMB12,700-12,800/t (USD1,859-1,874/t) delivered to the consumers and are considering replenishing some stocks for the coming May.

Ammonium polyphosphate with high polymerization degree flame retardant

This product equivalent to Clariant GmbH's Exolit AP 422, 423 and Budenheim FR Cros 484.

Key properties:
GD-APP101 is a fine-particle ammonium polyphosphate produced by a special method.The pruduct is largely insoluble in water and completely insoluble in organic solvent. It is white, non-hygroscopic and non-flammable,whose crystalline form is II,n>1000.

1,Low water solubility GD-APP101 has low water solubility which makes it useful in application where the product is exposed to high humidity conditions or water.

2,High phosphorus content GD-APP101 contains 72.5% phosphorus (as P2O5) which makes it a very effective fire retardant.

3,Nearly neutral PH-GD-APP101 can be neutralized or brought to the alkaline side by the addition of small amounts od ammonium hydroxide.

4,Non-halogen GD-APP101 is a highly effective non-halogen fire retardant.

Applications:

1, Solvent based intumescent coatings.

2, Water based intumescent coatings.

3, Flame retardant for flexible and rigid urethane foams.

4, Flame retardant for unsaturated polyesters.

5, Flame retardant for epoxies.

6, Flame retardant for acrylics.  

7, Flame retardant for polyurethanes.

Package:
Kraft bag or woven bag 25 kg per bag with PE liner, or 500 kg, 600kg big bag or on customer's request.

Storage and Handling:
Store in a dry and cool area.
Avoid inhalation, ingestion, and contact with eyes and skin.

Production of ammonium polyphosphates

There is disclosed a method and apparatus for the production of ammonium polyphosphate solutions of high concentrations from phosphoric acid and ammonia. The process is ideally suited for the production of such solutions from wet-process phosphoric acid which has a P₂ O₅ content of from 56 to about 70 weight percent, impurity-free basis, and which, preferably, contains from 5 to about 50 percent of its phosphorus present as polyphosphoric acid. The method comprises introducing the phosphoric acid into a reactor bearing a Teflon liner and reacting it therein with vapors of ammonia while controlling the amount of ammonia from 0.2 to about 0.3 pound ammonia per pound of P₂ O₅ in the feed acid, sufficient to achieve a peak reaction temperature in the reactor between about 525° and 775°F., the absolute value of which is dependent on the bulk water content of the feed acid. The products from the reactor are quenched by direct contact with an aqueous solution of ammonium polyphosphate, additional ammonia is added to neutralize the phosphoric acid to a pH value between about 5.5 and about 8.5 and sufficient water is added to achieve a concentration of from 24 to about 55 weight percent P₂ O₅ in the aqueous solution. Preferably, the aqueous solution is continuously circulated, cooled and passed to a storage vessel which serves as a product accumulation and supply of circulating liquid. The preferred apparatus of the invention comprises a tubular reactor having inlet sparging means for the ammonia vapors and phosphoric acid to introduce these reactants into intimate contact at the inlet portion of the reactor. The reactor and the inlet spargers are formed of Teflon and it has been found that the Teflon provides a scale-resistant lining for the reactor surfaces such that the reactor can be operated for extended periods of time without requiring shutdown for cleaning and maintenance.

We claim:

1. The method of producing ammonium polyphosphates by the adiabatic reaction of ammonia and phosphoric acid including the steps of continuously introducing phosphoric acid having a P₂ O₅ content of about 56 to about 70 weight percent into an adiabatic reactor and into admixture with ammonia continuously injected into said reactor at a rate corresponding to about 0.12 to about 0.56 weight parts ammonia per weight part P₂ O₅, reacting said acid with said ammonia and partially neutralizing said acid, said acid concentration and ammonia injection rate being sufficient to adiabatically maintain a reaction temperature of about 525° to about 750°F., said reaction temperature being above the gel transition temperature of the polytetrafluoroethylene resin liner hereinafter detailed and sufficient to render said resin ablative in the presence of said acid and ammonia and to evaporate water from said acid, concentrate said acid and form ammonium polyphosphates therein, and carrying out said reaction in a reaction zone comprising a supporting reactor shell internally lined with a polytetrafluoroethylene liner having a weight average molecular weight of about 390,000 to about 9,000,000 and a gel transition temperature below said reaction temperature.

2. The method of claim 1 wherein said shell is lined with said solid polytetrafluoroethylene to a thickness of from 0.1 to about 1 inch.

3. The method of claim 2 wherein said reactor is tubular having a length to internal diameter ratio from 10 to about 35.

4. The method of claim 1 wherein said temperature is from 620° to about 775°F.

5. The method of claim 1 wherein said temperature is maintained at a maximum value by controlling the relative rates of ammonia and phosphoric acid to the reactor.

6. The method of claim 1 wherein the product effluent from the reaction zone is discharged into contact with a sufficient quantity of an aqueous solution having a concentration of from 15 to 40 weight percent P₂ O₅ as ammonium phosphates, a pH value from 5.5 to about 8.5 and containing from 0 to about 75 weight percent of said P₂ O₅ as acyclic polyphosphates, to reduce the temperature of said product effluent to a temperature no greater than about 190°F.

7. The method of claim 6 wherein the entire effluent from the reaction zone is contacted with said aqueous solution.

8. The method of claim 7 wherein said aqueous solution comprises cooled reaction product.

9. The method of claim 8 wherein said aqueous solution is continuously circulated in a recycle loop into contact with said product effluent at a recycle rate from 20:1 to about 60:1 parts per weight part of said product effluent and through a cooling step to maintain its temperature from about 125° to 175°F.

10. The method of claim 9 wherein ammonia is added to said aqueous solution in said recycle loop in a sufficient amount to maintain said pH value of said aqueous solution.

11. The method of claim 9 wherein water is added to said aqueous solution in said recycle loop in a sufficient amount to maintain said concentration of ammonium phosphates.

Description:

BACKGROUND OF THE INVENTION

Ammonium phosphate solutions which contain a high percentage of the phosphorus present as polyphosphates are highly desirable because the polyphosphates are subsequently more soluble than orthophosphate. When the phosphate is derived from wet-process phosphoric acid, which constitutes the majority of commercial phosphoric acid, the polyphosphates also function to chelate the congeneric, metallic impurities of the wet-process phosphoric acid and avoid their precipitation in the ammonium phosphate solution. A high concentration of polyphosphates for this purpose is also desired to avoid any instability upon storage of such solutions that could be caused by the hydrolysis of the polyphosphate to levels below its effective concentration for preventing precipitation of the metallic impurities.

The formation of polyphosphoric acid or polyphosphates requires the expenditure of a considerable amount of energy for the evaporative dehydration of the phosphoric acid. It has been suggested that the exothermic heat of neutralization of phosphoric acid with ammonia be utilized as a source of energy for the molecular dehydration of the acid; see German Patent 6321. While this technique is operative for laboratory demonstrations, heretofor it has not been utilized in an entirely successful commercial installation. A large number of prior attempts have been made and many of these have concentrated on the reaction of ammonia and phosphoric acid in a tubular reactor using a continuous flow system. Typical of these are U.S. Pat. Nos. 2,902,342; 3,419,379; 3,420,624; 3,464,808; and 3,649,175.

The tubular reactor is ideally suited for this reaction since it provides a minimal residence time of the reactants and products and provides sufficient turbulence for adequate mixing of the reactants, completion of the reaction and dehydration of the orthophosphate. Phosphoric acid, however, and, in particular, wet-process phosphoric acid, is not readily amenable to such processing. The high temperatures involved in the evaporative neutralization of the acid can cause formation of insoluble precipitates with the metallic impurities. These precipitates occur because the temperatures are often sufficient to form minute amounts of cyclic or metaphosphates. The latter react with the metallic impurities to form refractory precipitates which coat the inside surfaces of the reactor, often plugging it and requiring discontinuance of the reaction after a few hours of operation. Attempts to lessen the degree of this precipitation generally involve limiting the reaction to temperatures less than the maximum which could be attainable.

It is, therefore, an object of this invention to provide a process for the production of ammonium polyphosphates which will achieve the maximum conversion to polyphosphates.

It is, furthermore, an object of this invention to provide such a process which utilizes substantially all the exothermic heat of reaction between ammonia and phosphoric acid to concentrate the acid and effect molecular dehydration of the phosphates.

It is a further object of the invention to operate such a process at the maximum temperature that can be obtained and to operate the process under substantially abiatic conditions.

It is also an object of the invention to provide a reactor which can be used in the aforestated process for prolonged periods of time without requiring repair, maintenance and cleaning.

Other and related objects will be apparent from the following discussion of the invention.

BRIEF STATEMENT OF THE INVENTION

This invention comprises a method for the continuous ammoniation of phosphoric acid in a reactor under substantially abiatic conditions to molecularly dehydrate the acid and form polyphosphates therein. The invention also comprises a reactor that can be used in the process which is substantially free from the formation of scale deposits.

It has been found that when the reaction between ammonium and phosphoric acid is performed in a reaction zone defined by a polytetrafluoroethylene resin such as Teflon substantially no scale deposits accumulate in the reaction zone. It is believed that the polytetrafluoroethylene is ablative under the high temperatures and conditions of the reaction and that scale deposits, if formed in the reactor, are dislodged therefrom. Regardless of the mechanism, no accumulation of scale deposits occurs and the reactor remains clean and unobstructed even after prolonged periods of use. The use of a reaction zone defined by the polytetrafluoroethylene resin permits the neutralization of phosphoric acid with ammonia vapors under substantially abiatic conditions and the maximum amount of the exothermic heat of neutralization is available for the molecular dehydration of the orthophosphate. This permits operation of the process at the maximum obtainable temperatures, e.g., temperatures from about 525° to about 775°F., usually about 620° to about 775°F., depending on the feed acid concentration, and permits maximum conversion of the orthophosphate to the desirable polyphosphate species. The method of the invention, therefore, comprises the neutralization of phosphoric acid with ammonia vapors under substantially abiatic conditions at a temperature from 525° to about 775°F., the latter being controlled by the control of the amount of ammonia introduced into the neutralization reactor in proportion to the amount of phosphoric acid. The continuous process can be controlled by supplying phosphoric acid thereto at a substantially constant and controlled flow rate and introducing ammonia vapor into reaction therewith at a rate controlled to achieve the maximum reactor temperature.

The crude reaction product can be discharged directly into a solution of ammonium polyphosphate product and additional ammonia can be added to the stream to complete the neutralization of the phosphoric acid to a pH value from about 5.5 to about 8.5. Water can also be added to the solution to dilute the crude reaction product to the desired concentration, typically to a value from about 24 to about 55 weight percent P ₂ O ₅ . The solution can then be passed through heat exchange means in indirect contact with liquid ammonia to vaporize the ammonia for use in the process and, thereafter, through heat exchange means in indirect contact with cooling water.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by reference to the illustrated, preferred embodiment thereof shown in the FIGURES of which:

FIG. 1 is a flow diagram of the preferred process;

FIG. 2 is a diagram of the tubular reactor used in the preferred process;

FIGS. 3 and 4 illustrate the polyfluorohydrocarbonammonia distributor used in the tubular reactor;

FIGS. 5 and 6 illustrate the phosphoric acid inlet distributor of the tubular reactor;

FIGS. 7 and 8 illustrate the polyfluorohydrocarbon inlet flange of the tubular reactor;

FIGS. 9 and 10 are flow diagrams of modified processes; and

FIG. 11 illustrates another reactor for the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the flow diagram of the preferred process is illustrated. The reactor 10 is shown as a generally tubular reactor which discharges into a mixing tee 11 in line 12. Line 12 receives a stream of recycled aqueous ammonium phosphate solution at a recycle rate of approximately 15 to about 50, preferably from 20 to about 45, weight parts recycle per part of reactor product. The recycle is supplied to the mixing tee 11 by pump 14 which draws liquid from line 48 of a recycle loop containing aqueous ammonium phosphate solution. The inlet of the mixing tee portion of line 12 can bear mixing means in the form of orifice plate 18 to insure thorough turbulence and mixing of the crude product from reactor 10 with the recycle liquid. The admixture of crude reactor product and recycle liquid can be passed through line 20 to pump 22 for circulation return to the shell side of heat exchangers 24 and 26. A bleed stream of product is withdrawn from the recycle loop through valve 23 and this is passed through cooler 24 where its temperature is reduced to about ambient and then passed to storage tank 16 via line 47. Ammonium phosphate product is withdrawn from tank 16 by line 17. Water is added through line 28 in an amount sufficient to control the dilution of the reactor products and maintain the liquid in the recirculating stream at the desired concentration.

The preferred embodiment shows quenching of the entire effluent from the reactor in a recycle stream. Alternatively, the reactor effluent could be discharged into the liquid or vapor space of a vessel containing an ammonium phosphate solution, e.g., tank 16. When discharged into the vapor space of such a vessel, the vapor of the effluent, chiefly steam, will separate from the liquid, thereby increasing the amount of water which must be added to the liquid to maintain the desired concentration of ammonium phosphate in the solution within the tank 16 over that necessary when the entire effluent is quenched by injection into the liquid within the tank 16.

Wet-process phosphoric acid from supply 30 is passed through line 32 with its flow controlled by valve 34 at a relatively constant rate. The liquid is introduced into the reactor and is intimately contacted therein with ammonium vapors introduced through line 36 at a rate which is controlled by valve 38. The flow of ammonia vapors into admixture with the phosphoric acid is controlled in accordance with the invention at a rate which is sufficient to maximize the temperature of the reaction in reactor 10 to a value from about 525° to about 775°F., the temperature attained being dependent on the amount of water in the feed acid. The ammonia vapor is supplied to line 36 from the storage of liquid ammonia at 40 through line 42 which passes to the tube side of heat exchanger 24 where the ammonia is preheated sufficiently to vaporize the ammonia and supply the vapors for the reaction in reactor 10.

The amount of ammonia that is sufficient to maximize the temperature within reactor 10 is insufficient to achieve the neutralization of the phosphoric acid to the desired vaue, e.g., a pH value from about 5.5 to about 8.5. This will provide a nitrogen to P ₂ O ₅ weight ratio in the product from 0.25 to 0.08 throughout the operational temperature range. Since a substantial amount of ammonia is not fixed in the product at the elevated reaction temperature, the amount of ammonia supplied to the reactor is from 1.5 to 2.25 times the amount of ammonia fixed as ammonium phosphate in the reactor effluent. The excess ammonia is reacted, however, when the reactor effluent is quenched with the cool recycle solution. The weight ratio of gaseous ammonia to the P ₂ O ₅ of the feed acid supplied as reactants to the reactor can, therefore, be from about 0.12 to about 0.56, preferably from about 0.15 to about 0.5. Generally, the amount of ammonia introduced into the reactor will be less than that required for the neutralization. Accordingly, additional ammonia is passed through line 44 at a rate controlled by valve 46 to admixture with the recycle liquid being discharged from pump 22 and this amount is controlled to neutralize the acid to the aforementioned desired pH value.

The reaction is preferably controlled at the maximum or peak temperature that can be achieved. This will vary somewhat, depending on the acid concentration and impurity content. Since the maximum temperature achieves maximum conversion to polyphosphate, the reaction is performed at the maximum or peak temperature. This temperature can be readily determined for any feed acid by variation of the ammonia flow into the reactor until the peak temperature is achieved, i.e., until the temperature is observed to increase and then decrease as the amount of ammonia flow into the reactor is increased.

In some instances, particularly with acids of high concentrations of P ₂ O ₅ and/or impurities, the reaction with ammonia may not be readily initiated in a short reactor. This occurs since these acids are very viscous and the reaction becomes mass transfer limited. Under such conditions, the ammonia fails to become sufficiently mixed with the acid for initiation of the reaction within the short residence time in the reactor. Longer reactors with greater residence time to provide a preheating or initiation section can be used; however, this approach is not optimum for processing and, therefore, is not preferred.

More desirably, when the aforementioned difficulties are observed, reduction of the feed acid's viscosity will generally permit initiation of the reaction and attainment of the desired peak reaction temperature in a reactor of optimum length. To permit the use of reactors of minimal length, it has been found that the viscosity of the feed acid at the inlet to the reactor should be less than a maximum value which, typically, is about 2500 centipoises at the reactor inlet to achieve initiation of the reaction and, preferably, less than 2000 centipoises to insure sufficient mixing and reaction time to attain a peak reaction temperature. Various methods to reduce the acid's viscosity can be used, e.g, the acid can be diluted or can be preheated so that its inlet viscosity is lowered.

It has also been found that preheating of the ammonia to the reactor can eliminate or reduce the difficulties in initiating the reaction or in achieving the peak reaction temperature, apparently since the heat transfer from the ammonia to the acid in the inlet portion of the reactor is sufficiently efficient to lower the acid's viscosity rapidly and permit adequate mixing for the reaction. Accordingly, it is preferred to provide an ammonia preheater as heat exchanger 25. Steam can be supplied from line 27 to the tubes of this exchanger to heat the ammonia passed through the shell of the exchanger. The degree of preheating can then be varied, as needed, for any particular feed acid.

An advantage of the tubular reactor is that it can be employed at very high specfic mass flow rates, thereby minimizing the capital costs of a plant. Typically, mass flow rates from 75 to about 300, preferably from 100 to about 200 pounds per second per square foot of flow area can be used.

The reactor effluent is quenched from the aforementioned reaction temperature to a temperature of about 120° to about 190°F. immediately upon admixture with the recycle liquid in mixing tee 11. This liquid is cooled approximately 10° to about 20°F. during its passage through heat exchanger 24 and is further cooled to a temperature of about 150°F. by passage through the exchanger 26. Cooling water is introduced into the exchanger 26 through line 50 at a rate controlled by valve 52 to maintain the outlet temperature of the ammonium phosphate liquid in line 48 at a temperature of about 125° to about 175°F., preferably about 140° to about 160°F. The recycle solution of ammonium phosphate can have a concentration from 15 to 40 weight percent P ₂ O ₅ with from 0 to about 75 percent of the phosphate present as acyclic polyphosphate. The solution pH value should be from about 5.5 to about 8.5.

The amount of liquid ammonium phosphate solution circulated through the recycle facilities should be sufficient to provide a recycle ratio in tee 11 of from 20:1 to about 60:1, preferably from 30:1 to about 50:1, weight parts recycled liquid per weight part fresh product from the reactor.

FIG. 2 illustrates the tubular reactor used in the preferred process. The preferred reactor is generally tubular with a length to internal diameter ratio from 10 to about 35, preferably from 15 to about 25. The reactor comprises a supporting metal shell 60 which is internally lined to a substantial thickness, e.g., from 0.1 to about 1 inch, preferably from about 0.15 to about 1 inch, preferably from about 0.25 to about 0.7 inch, with solid polytetrafluoroethylene resin. A commercially available resin useful for this service is Teflon. The resin has a weight average molecular weight from about 390,000 to 9,000,000. The resin has a crystalline structure at its normal service temperature up to 500°F. At temperatures above 500°F., the physical properties of the resin degrade and, at about 600°F., the resin becomes an amorphous transparent gel. Accordingly, the resin is not recommended for use at temperatures above 500°F. and is considered entirely unsuited for use in services at temperatures of 600° F. or greater. This invention, however, utilizes the resin at temperatures where its physical properties are degraded and, preferably, at temperatures above its gel transition temperature. It is believed that under such conditions the resin ablates and thereby avoids the accumulation of scale deposits on the resin liner walls.

The supporting metal shell is employed to provide the necessary structural strength to the reactor. The metal shell can be of stainless steel or of mild steel. The resin lining is preferably a molded or extruded tubular member 86 which has an outside diameter permitting its insertion into the reactor shell 60. The inlet end of the reaction zone is defined by an ammonia sparger 72 and spacer 85, within a phosphoric inlet header, which are formed from plates or plugs of the solid polytetrafluoroethylene resin and these can be sealed to the tubular member 86 to isolate the metal shell 60 from all contact with the reaction zone contents, if desired. In practice, it has not been found necessary to seal the metal shell 60 from contact with the reaction zone contents since, even with a shell formed of mild steel, no significant corrosion has been experienced. Because the polytetrafluoroethylene liner 86 has a low heat conductivity, the metal shell 60 remains relatively cool and at a temperature where any minor amounts of the reaction product that may seep between the tubular member and shell are substantially non-corrosive and non-scaling, thereby eliminating the need for sealing of the metal shell from all contact with the reactants.

The metal shell 60 bears, on opposite ends, conventional pressure vessel flanges 62 and 64. The inlet assembly of the reactor is formed between end flange 62 and flange 63 and is secured by a plurality of tie bolts 65. The ammonium sparger 72 of the inlet assembly is positioned on the inboard side of flange 63. The construction details of this sparger are shown by FIGS. 3 and 4. Referring now to FIGS. 3 and 4, the ammonia sparger is formed of the polytetrafluoroethylene resin and comprises a plug 71 bearing an annular flange 73. The solid plug 71 is provided with a plurality of bores 74 spaced about its central portion. The sparger 72 also has a central bore 75 which serves to receive the free end of thermowell 66. Plug 71 of sparger 72 fits into the through opening of the phosphoric acid inlet header 76. This inlet header is secured to conduit 32 that supplies the phosphoric acid to the reactor. The details of construction of this inlet header are shown in FIGS. 5 and 6.

Referring now to FIGS. 5 and 6, the inlet header for the phosphoric acid can be seen to comprise a ring 77 having a through opening 78 and a hollow interior or chamber 79. The ring is secured to a nipple 81 that bears conventional male threads 80 for attachment to the phosphoric acid conduit 32. Ring 77 is formed of corrosion-resistant metal, preferably stainless steel and nipple 81 of similar construction is welded thereto.

Referring now to FIG. 2, the downstream side of inlet header 76 receives, in its through opening 78, spacer 85. The construction details of the spacer 85 is shown in FIGS. 7 and 8 where it is shown to comprise a plug 82 that bears an annular flange 83 and a central through opening 84. The through opening 84 is of the same diameter as the outside diameter of the reactor tubular member 86 while the external diameter of plug 82 is of the same diameter as through opening 78. The upstream end of plug 82 extends into chamber 79 within the inlet header 76 and opposes in a spaced opposition the inboard end of plug 71 of the ammonia sparger 72. These plug faces are separated by a distance of 0.1 to about 1, preferably from 0.25 to about 0.5 inch.

The tubular member 86 extends into the through opening 84 of spacer 85 and terminates flush with the upstream face of spacer 85 at the aforementioned spacing from the discharged face of the ammonia sparger 72. The tubular member 86 is coextensive with the metal shell 60 and, at its discharge end, terminates generally at the downstream face of flange 64. As previously mentioned, member 86 can fit loosely in the shell 60 and a plurality of radial spacing means in the form of machine screws 68 can be provided in bores through the reactor wall to restrain the member 86. The screws are retained by nuts 69 which are welded to the reactor 60.

The polytetrafluoroethylene tubular member 86 will exhibit some permanent deformation upon being heated to the reaction temperature. This deformation comprises from 1 to about 3 percent expansion. Accordingly, the dimensions of the member should be undersized to permit such deformation or it will be necessary to cool and disassemble the reactor to permit resizing of the member 86. The machine screws 68 will restrain the upstream end of tubular member 86 and prevent its axial expansion from closing the gap between the opposed faces of sparger 72 and spacer 85. If desired, abutments of fixed dimensions can be carried on the face of one or both of sparger 72 and the end of tubular member 86 to insure that the face-to-face spacing of these members is not decreased to less than the desired dimension.

The phosphoric acid which is introduced into the inlet header 76 through nipple 81 passes into the annulus within chamber 79 about the plug 82 and flows between the faces of plug 82 and plug 71 and into contact with the ammonia introduced through bores 74 of ammonium sparger 72. The combined phosphoric acid and ammonia streams then flow into the open end of tubular member 86.

Flange 64 is secured to mixing tee 11 by a flange plate 90 that is welded to one side thereof. Tie bolts 92 are used to secure this assembly. Tee 11 bears, at its opposite ends, conventional flange plates 94 and 96 which are secured to the recycle lines 12 and 20, shown in FIG. 1. The tee 11 also bears another flange plate 93 to which is secured line flange 95 by bolt fasteners. A thermowell 66 extends through bore 97 in line flange 95 and is welded thereto. This thermowell can be a metal tube which should be coated with a film of polytetrafluoroethylene or encased in a sheath of the polymer. A thermowell of solid polytetrafluoroethylene could also be used, however, a polymer-covered metal tube is preferred because of its greater strength and rigidity. To insure accurate temperature measurements, it is preferred to minimize the thickness of the polytetrafluoroethylene coating on the metal thermowell to not exceed about 0.1 inch. The inboard end of the thermowell extends through and is supported by the central bore 75 of sparger 72. The outboard end of this thermowell is threaded and bears a conventional nut 99. The nut 99 has a packing gland through which is passed a pair of electrical conductors to a thermocouple (not shown) which extends into the elongated thermowell 66. The thermocouple can, preferably, be positioned at any point along the distance of thermowell 66.

The capacity of a tubular reactor can be increased by pre-ammoniation of the wet-process phosphoric acid. FIGS. 9 and 10 illustrate typical processes employing pre-ammoniation reactors. As shown in FIG. 9, the wet-process acid can be introduced into a vessel 31 where it is contacted with ammonia that is introduced through line 35 at a rate controlled by valve 37 to achieve a temperature in vessel 31 from 150° to about 350°F., preferably from 200° to 300°F. The amount of ammonia required to achieve these temperatures comprises from 15 to about 30 percent of the ammonia which is passed through line 42 and reactor 10. The intermediate product from vessel 31 can then be pumped through line 32 to reactor 10. The vapors which separate from the intermediate product in vessel 31 can be passed through line 33 to blend with the ammonia from line 36. Preferably, vessel 31 is maintained at a superatmospheric pressure, e.g., up to about 100 psig, to permit the direct pressurization of the liquid and vapor effluents therefrom into reactor 10.

FIG. 9 illustrates an alternative but less preferred cooling of the recycle and product stream. If desired, the entire stream of quenched product and recycle can be passed through heat exchangers 24 and 26 and then to storage tank 16. This is less preferred than the system illustrated in FIG. 1 since it results in a higher temperature of the ammonium phosphate solution in tank 16, thereby creating a greater tendency for hydrolysis of the polyphosphate in the solution.

FIG. 10 illustrates an embodiment in which the preammoniation is performed in a tubular reactor 21. The entire effluent, which is a vapor and liquid mixture, is passed through the reactor into line 32. The temperature and pressure conditions in reactor 41 can be the same as the aforedescribed reactor 31.

Because reactors 31 and 41 are operated at mild conditions of temperature and pressure, solid deposits of precipitates and scale are not encountered. Accordingly, these reactors can be entirely metallic, without any polytetrafluoroethylene liner. Stainless steel is preferred for construction of these vessels to minimize corrosion problems. When a tubular reactor such as 41 is used, it can be integral with reactor 10 and can be an extension of the metal shell 60 of reactor 10.

Although tubular reactors are preferred to minimize the capital costs of a plant, other reactor designs can also be used. FIG. 11 illustrates a suitable reactor which has an outer supporting metal shell 61 and an inner liner 87 formed of the aforementioned polytetrafluoroethylene. The thickness and physical properties of this liner are as described with regard to liner 86 as shown in FIG. 2. The reactor has a lower section 89 and an upper section 91 of greater diameter connected together by a conical portion 81. The lower section 89 has an inlet nozzle for connection to the wet-process phosphoric acid supply line 32 and a distribution header 79 for connection to the ammonia line 36. The reactor could be of uniform diameter to simplify construction, however, the illustrated shape is preferred to minimize the liquid residence time while still providing adequate vapor-liquid disengagement surface. The liquid effluent is withdrawn through nozzle 77 and a level control means 69 can be provided to maintain the desired liquid level in the reactor. The vapor effluent is removed through nozzle 67.

The conditions maintained in the reactor are a peak reaction temperature and sufficient pressure to maintain a liquid phase but insufficient to reduce evaporative concentration of the liquid. The peak reaction temperature is maintained from 525° to 775°F., preferably from 550° to 710°F., and the pressure can be from 1 to about 5 atmospheres. Preferably, the pressure maintained is only that sufficient to overcome the flow pressure drop of the vapor and liquid effluents.

The liquid effluent from nozzle 77 is preferably quenched by direct contact with an aqueous solution of ammonium phosphate. Typically, the effluent can be introduced into mixing tee 11 shown in FIG. 1. The vapor effluent from nozzle 67 can be blended with the quenched product and recycle liquid to absorb therein condensible ammonia vapors and steam. The remainder of the process is as described with regard to FIG. 1.

The process is described with reference to the presently preferred apparatus in the following paragraphs.

In a typical embodiment, the reactor shell 60 is 5 feet in length and is formed of a mild or carbon steel tube having an outside diameter of 4 inches. The tubular member 86 is formed of Teflon with a length of 64 inches, a 3 inch outside diameter and a wall thickness of 1/2 inch. The mixing tee is formed of 4 inch outside diameter mild or carbon steel piping. A total of thirteen bores 74 are provided through the Teflon plug 72 of the ammonium distributor, each with a diameter of 0.3125 inch. The face-to-face spacing between sparger 72 and spacer 76 is 0.375 inch.

The aforedescribed reactor is used for the production of ammonium polyphosphates by supplying wet-process phosphoric acid having a concentration of 70 weight percent P ₂ O ₅ and containing approximately 50 percent of its phosphorus as polyphosphoric acids to the inlet header 76. The acid is supplied at a feed rate of about 10 gallons per minute and at ambient temperature. The inlet pressure of the reactor is about 20 psig. Ammonia is vaporized in heat exchanger 24 and the vapors are supplied to the ammonia sparger at a rate of 5 gallons per minute of liquid, the amount of ammonia being controlled to peak the reaction temperature at about 725°F. The maximum temperature is found to be attained at approximately 12 inches from the inlet end of the reactor and to remain relatively constant throughout the remainder of the reactor. An aqueous ammonium phosphate solution having a composition of 10 weight percent nitrogen and 34 weight percent P ₂ O ₅ is maintained in vessel 16 at 150°F. and is pumped to the mixing tee by pump 14 at a rate of about 300 gallons per minute, sufficient to supply a weight ratio of recycle liquid to fresh reactants of about 40/1. Approximately 7.5 gallons per minute of ammonia are supplied through line 44 to control the pH of the recycle liquid at a value of about 6.3. Water is also introduced through line 28 to maintain the concentration of the ammonium phosphate solution constant.

The reactor is used for a prolonged period of about 100 hours under the aforedescribed conditions with a variety of concentrated wet-process phosphoric acids. Throughout the use of the reactor, no difficulty is experienced with the deposition of precipitates. Upon completion of about 100 hours of use, the reactor is disassembled and it is observed that the Teflon tubular member 86 is intact and free of deposits. It is observed that the member 86 is changed in physical appearance from its original opaqueness to a clear, translucent state. Careful inspection of the tubular member 86 reveals that a slight amount of the member has been ablated by the reaction and pitting to depths of from 10 to about 20 mils is apparent on the inside surfaces of the member 86.

The reactor was employed for a series of experiments in which phosphoric acids of varied concentration are reacted. The varied concentration of the acids was achieved by blending together different proportions of a merchant grade, nominally 52 percent P ₂ O ₅ acid and a commercial super acid of nominally 72 percent P ₂ O ₅ content. The acids were blended together at the inlet of the acid feed pump which discharged into conduit 32. Samples of the acids were taken periodically during each experiment and the following tabulates the average acid composition for each experiment:

Ammonium polyphosphate production

BACKGROUND OF THE INVENTION

The utility and advantages of aqueous ammonium phosphate solutions are well known, particularly in the agricultural industry. Several advantages associated with the presence of acyclic polyphosphates are also recognized. For instance, products obtained from wet-process acids generally contain metallic impurities including iron, magnesium, aluminum and the like, which form unmanageable precipitates upon neutralization of acids containing insufficient polymeric phosphate. Secondly, ammonium phosphate solubility increases in proportion to polymeric phosphate content. Thus, an acid having an H ₂ O/P ₂ O ₅ molar ratio of 3 corresponding to a polyphosphate content of 10 percent based on total P ₂ O ₅ , can be converted directly to the ammonium phosphate solution commercially designated as 8-24-0 containing 8 weight percent nitrogen and 24 weight percent phosphate expressed as P ₂ O ₅ . The more concentrated solution 10-34-0 containing 10 and 34 weight percent nitrogen and P ₂ O ₅ , respectively, requires the use of ammonium phosphate of which at least 50 percent of the P ₂ O ₅ is polymeric. Similarly, 12-44-0 can be obtained only with ammonium phosphates in which 75 percent or more of the P ₂ O ₅ is polymerized. Maximum solubilities varies somewhat with pH and temperature, and are obtained under slightly acidic conditions, ie., pH levels below about 7, generally on the order of about 6.5. However, polyphosphate stability increases with pH with the result that pH should be at least about 5, preferably above about 6.

Starting materials include essentially any source of phosphoric acid including wet-process acids, so-called white acids, and the like. The wet-process acids are obtained by acidifying phosphate-containing rock with strong mineral acids such as sulfuric, which convert the calcium or other metal phosphates to phosphoric acid, calcium sulfate, etc. Insoluble sulfates are removed by filtration although the wet-process acids generally contain at least about 1 and often between about 1 and about 20 weight percent cogeneric metallic impurities expressed as the corresponding oxides.

The so-called "white acids" are obtained by the "electric furnace" process in which phosphate-containing rock is reduced by reaction with coke at extremely high temperature generated by electrical current. The phosphate rock is reduced to elemental phosphorus, which is then burned to P ₂ O ₅ and absorbed in water. While these acids are generally more expensive than wet-process acids, they often become available at prices low enough to justify their use in the manufacture of ammonium phosphates.

The possibility of increasing polymeric content by driving off free and chemically combined waters, i.e., polymerizing the acid, at very high temperatures, has been recognized for some time. Temperatures required to obtain any significant conversion to polymeric P ₂ O ₅ are at least about 400° F., usually between about 500° and about 750° F.

Earlier attempts at polymerization involved heating the crude acid, i.e., an acid having a relatively high H ₂ O/P ₂ O ₅ ratio, with an external heating source, to obtain the required polymerization, then neutralizing the polymerized product. It was then discovered that the required temperatures could be generated by the autogenous heat of neutralization with ammonia. This procedure accomplished two objectives in one step -- phosphate polymerization and ammonium phosphate or polyphosphate formation. This process could be carried out in either a batch or continuous basis, the latter often involving the use of so-called tubular reactors in which ammonia and the phosphoric acid feed were continuously passed through and contacted in the reactor tube. Numerous variations of both batch and continuous operations have been suggested.

All of these processes require relatively concentrated phosphoric acid feeds in order that the heat of neutralization will be sufficient to reach the required temperatures. For several reasons, the maximum temperatures obtainable by adiabatic neutralization decrease markedly as the H ₂ O/P ₂ O ₅ feed ratio increases. Reaction efficiency, which also depends on several variables, also effects maximum reaction temperature and, consequently, influences conversion, i.e., polymerization level. Even the best designed and controlled reactors can obtain at most 90 percent, and generally less than 90 percent of the theoretical polymer content.

These and other factors make it highly impractical and often impossible to obtain a required polymer content from a given acid feed without preconcentration of the acid to reduce H ₂ O/P ₂ O ₅ ratio prior to neutralization.

It is therefore one object of this invention to provide an improved method for converting phosphoric acids to ammonium phosphates. It is another object to provide a method for producing stable aqueous solutions of ammonium phosphates containing substantial amounts of ammonium polyphosphate. These and other objectives, variations and modifications of the concepts of this invention will be apparent to one skilled in the art in view of the following description, drawings and claims.

Therefore, in accordance with one embodiment, aqueous solutions of ammonium polyphosphates in which at least 25 percent of the P ₂ O ₅ is present as polyphosphate, are produced by mixing with the feed acid an anhydrous mineral acid selected from sulfuric, nitric and hydrochloric acids, in amounts of at least about 0.05 mole of mineral acid per mole of P ₂ O ₅ sufficient to produce a liquid phase temperature in the reaction melt of about 400° to about 750° F. upon adiabatic neutralization of the acid mixture with anhydrous ammonia. In another embodiment, similar conversions are obtained with feed acids having equivalent H ₂ O/P ₂ O ₅ mole ratios about about 3.3 and as high as 15.

The amount of ammonia required to obtain this objective is generally on the order of at least about 0.12 weight part of anhydrous ammonia per weight part of the acid mixture, i.e., P ₂ O ₅ plus mineral acid equivalent. In this disclosure the term "mineral acid" refers only to the additional sulfuric, nitric and/or hydrochloric acids. The term "mineral acid equivalent" refers to the molar equivalent acidity of the mineral acid at the conditions involved, i.e., the extent to which it will react with ammonia.

Reaction conditions are controlled to produce a product melt, using essentially only the autogenous heat of neutralization, at a temperature sufficient to increase the polyphosphate content by at least about 10 percent based on total P ₂ O ₅ as compared to the feed acid, and form a product of which at least about 25 percent of the P ₂ O ₅ is present as polyphosphate. The product melt is then quenched and neutralized, preferably to a temperature of about 200° F. or less, generally by immersion in an aqueous medium.

The several concepts involved in the use of this invention can be better understood by reference to the drawings of which:

FIG. 1 is a graphical representation of the interrelationship of H ₂ O/P ₂ O ₅ feed ratio, H ₂ SO ₄ /P ₂ O ₅ molar ratio, and percent conversion, i.e., percent polymeric P ₂ O ₅ in the product assuming 100 percent efficiency. Of course, 100 percent efficiency is impossible to obtain, the best possible efficiencies being on the order of about 80 to about 90 percent. Poor control of process conditions and/or high heat losses can lower efficiency to 60 percent or less.

Efficiency is here expressed as the percentage of polymeric P ₂ O ₅ actually obtained as compared to the percentage which would be obtained at the theoretical 100 percent efficiency level. Thus, it is possible to interpolate between the constant efficiency lines in FIG. 1 to determine, for instance, what H ₂ SO ₄ /P ₂ O ₅ molar ratio would be required to obtain a certain polymer content from a feed having a given H ₂ O/P ₂ O ₅ molar ratio in a system having an efficiency of, say, 80 percent.

FIG. 2 is a graphical representation of the manner in which conversion level is influenced by maximum reaction temperature for typical wet-process and white acids. The wet-process acid was typical of a material having an H ₂ O/P ₂ O ₅ ratio of about 7 and containing about 10 weight percent cogeneric metallic impurities determined as the corresponding oxides. The white acid contained less than 1 weight percent impurities and is representative of a material having an H ₂ O/P ₂ O ₅ ratio of 8.

FIG. 3 is a graphical representation of percent conversion to polyphosphates as a function of feed H ₂ O/P ₂ O ₅ mole ratio for both pure and typical impure (wet-process) systems in an ideal adiabatic reactor operating at 100 percent efficiency without benefit of this invention.

FIG. 4 is a graphical representation of the P ₂ O ₅ /nitrogen ratio in the liquid reactant phase prior to quench at equilibrium as a function of liquid phase temperature including only that nitrogen combined as ammonium phosphate, i.e., excluding nitrogen present as ammonium sulfate, nitrate or chloride.

FIG. 5 is a schematic flow diagram of one conversion system envisioned within the concept of this invention.

While this invention contemplates the use of either sulfuric, nitric or hydrochloric acids, or mixtures of these for purposes of simplicity, it is described primarily with reference only to sulfuric acid. The effects of each of these acids on the reaction are approximately the same on a molar basis. For instance, hydrochloric acid increases reaction temperature by about 5 percent more than an equal molar amount of sulfuric acid. Thus, theoretically, hydrochloric acid would induce a 105° F. temperature increase when used in a molar amount equal to the quantity of sulfuric acid required to produce an increase of 100° F. This difference is generally so slight that it is completely over-shadowed by other process parameters. Consequently, these three acids, or combinations thereof, can be considered essentially identical so far as they influence this process.

The process can be carried out in a variety of apparatus including batch or continuous flow systems such as the so-called "pipe reactors" of which numerous varieties are known. A particularly preferred system is described in copending application Ser. No. 591,056 of Harbolt and Young wherein the reaction is carried out in a highly dispersed, discontinuous, unconfined liquid phase surrounded by an ammonia atmosphere. That system is similar to the one illustrated in FIG. 5 detailed hereinafter.

Phosphoric acid feed can be obtained from essentially any source so long as the H ₂ O/P ₂ O ₅ ratio, determined on the basis of the total liquid water introduced to the system with any component in either free or chemically combined form is within the prescribed ranges. The most common feed acid sources are the white acids and wet-process acids. The phosphoric acid feed can also contain substantial amounts of polymeric phosphates, although it will usually contain less than 30, often less than 25 percent polymer. However, these methods are of greatest advantage with feeds containing less than 10 percent polyphosphate, and preferably containing no polymer at all.

H ₂ O/P ₂ O ₅ molar ratio is determined on the basis of total unvaporized water. Little, and preferably no unvaporized water is introduced with the ammonia. Consequently the majority, if not all, of the unvaporized water is introduced as free of chemically combined water with the phosphoric or mineral acids. This invention is of greatest advantage when that ratio exceeds 3.3 and is generally between about 3.3 and about 15. Even greater advantage is realized with acids having H ₂ O/P ₂ O ₅ ratios of about 4.5 or more or even about 5 or more due to the fact that this method will yield acceptable polyphosphate levels that can not be obtained by available alternative methods from similar feeds. Accordingly, this ratio will generally be between about 4.5 and about 15, preferably between about 5 and about 10. Nevertheless, these methods can also serve to increase conversion, even with concentrated feeds, i.e., acids having H ₂ O/P ₂ O ₅ ratios below 3.3. Temperature is limited by both water content and efficiency neither of which are ideal, even in the best systems. Thus, these methods increase conversion even in those instances.

While the wetter feeds, i.e., those having H ₂ O/P ₂ O ₅ ratios between about 10 and about 15 can be employed, higher ratios should be avoided, and levels within this range, i.e., 10 to 15, are preferably avoided due to the difficulty in controlling the extremely rapid water evolution and the difficulties involved in maintaining an even temperature distribution throughout the liquid phase required to avoid the formation of ammonium phosphate- ammonium sulfate, nitrate and/or ammonium chloride crusts. These crusts, if formed, inhibit ammonia transfer across the vapor-liquid interface. It is essential that the reaction be sufficiently rapid to obtain liquid phase temperature above the salt melting point in order to avoid crust formation and assure the maintenance of a single liquid phase. This is particularly true in the case of the unconfined liquid phase reaction system -- such as that illustrated in FIG. 5.

While the reasons for the success of such unconfined liquid phase reactors are not completely understood, it appears that reaction rate is increased sufficiently to generate heat at a rate greater than it is transferred to the immediate environment, even at the high reaction temperatures required. The rate of liquid phase temperature increase is necessarily a function of the rate of neutralization of the phosphoric and added mineral acid. This in turn involves the rate at which ammonia permeates the liquid phase. This would be true either in a discontinuous liquid phase or in the continuous liquid phases prevailing in both batch or tubular reactors.

In either situation, heat transfer rate, and consequently temperature elevation rate, are reduced by one or more types of heat transfer barriers or inhibiting films at the liquid-vapor interface due to one or more characteristics of the phosphoric acid feed, the combination of phosphoric acid and added mineral acid, the reaction products of one or both of these with ammonia, or some intermediate forms, or combinations thereof. Accordingly, when operating with the high water content acids employed herein, it is essential to assure that the combination of phosphoric and mineral acids is sufficient to obtain the reaction rate required to produce temperatures in excess of the ammonium phosphate melting point and avoid the formation of mass transfer limiting crusts or films.

It was not initially apparent that parameters could be found or, if found, could be controlled to maintain these essential balances. For instance, Y. A. K. Abdul-Rahman and E. J. Crosby observed the formation of an impervious crust upon treatment of phosphoric acid droplets with ammonia. Their results are reported in "Direct Formation of Particles from Drops by Chemical Reaction With Gases", Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin, appearing in Chemical Engineering Science (1973), Vol. 28, pages 1273-1284. The efforts of these authors, as indicated by the title of their work, was directed to the formation of solid droplets of ammonium phosphate. While their investigations were carried out at temperatures far below those required to promote polymerization, and at contact times far exceeding commercially practicable holding times, they did observe several factors which appeared to negate the utility of this process. These included the rapid formation of a crust surrounding the droplets, the attainment of only very low temperatures, even with anhydrous ammonia, and the actual explosion of the droplets in some cases due to containment of vaporized water by the ammonium phosphate crust.

These methods involve a number of variables, the most significant being H ₂ O/P ₂ O ₅ and mineral acid/P ₂ O ₅ molar ratios. If the required conditions are not maintained the elevated temperatures required for polymerization can not be reached, and the rate of the water release and the temperature increase will be insufficient to reach the product melting point prior to the occurrence of solid or semisolid crusts in the liquid phase.

The mineral acids should be anhydrous, i.e., should contain less than 5, preferably less than 2 weight percent free water. The accompanying drawings illustrate that the mineral acid/P ₂ O ₅ ratio must be correlated with H ₂ O/P ₂ O ₅ ratio and reaction temperature required to obtain a specified conversion level. As a general rule, however, this ratio will be at least about 0.05, preferably at least about 0.1. In the preferred embodiment, however, using feed acids having H ₂ O/P ₂ O ₅ molar ratios of about 5 or greater, this ratio should correspond to at least about 0.17, and will generally fall within the range of about 0.17 to about 4.

In most instances it is preferable to obtain a product of which at least 25 percent of the P ₂ O ₅ is present as a polyphosphate. Thus the mineral acid/P ₂ O ₅ ratio should be at least about 10 percent greater than that represented by the correlation for 25 percent polymer shown in FIG. 1 at an H ₂ O/P ₂ O ₅ ratio corresponding to that of the system. As mentioned above, FIG. 1 represents a correlation for an assumed ideal situation, i.e., a batch reactor achieving 100 percent efficiency. Thus the 10 percent differential above that called for by this correlation, in part, accounts for the inefficiency of actual flow systems and allows some leeway for process control sensitivity. Taking all of these factors into account, the interrelationship of these variables can be illustrated by the expression: [mineral acid]/P ₂ O ₅ = 0.267 [H ₂ O/P ₂ O ₅ ]- 1.16.

this expression holds true for conversion of phosphoric acid containing less than 25 percent polyphosphates to products containing at least about 50 percent polyphosphates determined as P ₂ O ₅ .

It is also preferable to employ anhydrous ammonia containing less than 5 volume percent water and to introduce the ammonia to the reactor at a temperature above the dewpoint. Ammonia injection rates into the reaction zone per se (as opposed to downstream of the reactor) should correspond to at least about 40, and generally about 40 to about 70 percent of the amount required to neutralize the acid mixture.

FIG. 4 illustrates that the amount of nitrogen associated with the molten phosphate reaction phase decreases substantially as temperature is increased. This is true of both phosphoric and sulfuric acid, the latter of which is neutralized only to ammonium bisulfate at reaction temperatures. Thus, as a practical matter, the nitrogen required to produce the neutralized end product can not be consumed in the reaction zone per se, but must be absorbed at some point downstream of the reactor, e.g., after quenching.

Therefore, as a general rule ammonia addition rate to the reactor should correspond to at least about 0.12, generally about 0.12 to about 0.4 weight parts ammonia per weight part P ₂ O ₅ . In addition to this amount, sufficient ammonia should be added to neutralize the mineral acid to the maximum extent possible at reaction temperature.

Reaction conditions, including the water and acid ratio, ammonia addition rate, and reactor holding time, should be correlated to obtain a reaction temperature within the range of about 400° to about 750° F., preferably 500° to 750° F. and sufficient to produce a product containing at least 10 percent more of the P ₂ O ₅ as polyphosphate than was present in the phosphoric acid feed. These conditions should generally be sufficient to produce a product containing at least about 25 percent, preferably at least about 50 percent of the P ₂ O ₅ as polymeric phosphates. In the preferred, discontinuous liquid phase system, these conditions should be sufficient to increase the polymeric phosphate content by at least about 10 percent determined as total P ₂ O ₅ within about five feet of the acid spray means described therein.

Sulfuric acid is neutralized to ammonium bisulfate, most of which remains in the product solution and contributes to the value of this material for agricultural applications as a source of both sulfur and nitrogen. Ammonium chloride or nitrate can also be retained in the product by operating with a closed reactor and quenching the melt in or immediately after the reaction zone.

However, if desired, substantial amounts, e.g., an excess of about 50 percent, of the ammonium chloride or nitrate can be removed from the melt prior to quenching. I have observed that these materials can be emitted as a vaporous fog from the partially neutralized melt if the melt is maintained at a temperature in excess of about 300° and preferably over 400° F. This objective can be readily accomplished by venting the reactor to a scrubber or passing the product melt from the reaction zone to any one of the numerous types of accumulating devices such as a separation vessel over which a vapor space is maintained. The chloride or nitrate can be continuously withdrawn from the vapor space overlying the hot product melt. The product should be rapidly quenched to a temperature below about 200° F. and neutralized to a pH above about 5.5 by direct immersion in water or an aqueous product solution.

The products are characterized as stable aqueous solutions containing at least about 15 weight percent P ₂ O ₅ of which at least 10, and preferably at least about 20 percent more of the total P ₂ O ₅ is polymeric phosphate than was present in the phosphoric acid feed. Polyphosphate levels will generally correspond to at least about 25, often at least about 40, and preferably at least about 50 percent of total P ₂ O ₅ . It is also preferable to minimize the chloride content of products intended for agricultural use, as described above, to less than about 50 and preferably less than 20 percent of the amount produced by neutralization of hydrochloric acid, when used.

The operation of this invention is described with reference to the embodiment illustrated schematically in FIG. 5. The acid feed -- a combination of the phosphoric and mineral acids -- is dispersed as a discontinuous liquid phase in an unconfined reaction zone and, in that condition, is reacted with ammonia injected into the acid spray within the reaction zone. The degree of dispersion achieved by any given spray means is at least, in part, a function of feed viscosity. Thus, as the viscosity of the feed increases, the degree of dispersion is reduced. Nevertheless, adequate dispersion can be maintained even with highly viscous acid feeds by heating the acid to a temperature between about 250° and about 500° F. Temperatures substantially above this level should be avoided with wet-process acids to prevent cyclic metaphosphate production and apparatus fouling upstream of the reactor or within the acid spray means itself. While there is, of course, no precise feed acid viscosity above which feed preheating should be employed, this embodiment is particularly useful with feeds having viscosities above about 4000 centipoise at 80° F.

Under any circumstances the feed acid temperature should be at least about 50° F., preferably at least about 100° F. With the more viscous feeds, temperatures on the order of 200° to about 500° F. are presently preferred. In fact, substantial advantage is realized by heating the feed acid, regardless of initial viscosity. Preheating reduces the heat load required to elevate liquid phase temperature. However, this aspect is of only minor significance when compared to the latent heat of vaporization involved in expelling water from the liquid reaction phase.

The mineral acid, in addition to serving the objectives described above, also reduces acid viscosity. Thus it reduces viscosity, increases acid dispersion and distribution and reduces droplet size.

Referring now to FIG. 5, phosphoric acid is passed from reservoir, pipeline or other container 1 to mixer 52 where it is combined with mineral acid entering by line 50. These materials are mixed by any suitable means such as agitator 51. The combination is then passed to the reaction zone through lines 2, 4, 6 and 7 to acid-spraying means 9. The feed may be heated, as described, by passage through heat exchanger 3 and lines 5, 6 and 7 as before.

In an alternative embodiment, preheating can be achieved by passing a portion or all of the acid through preneutralizer 8 where it is contacted with ammonia entering via line 19. The ammonia rate to neutralizer 8 should be sufficient to only partially neutralize the acid mixture and increase its temperature to a level in the range of about 200° to about 500° F.

More importantly, however, the extent of preneutralization should not be so great as to prevent the possibility of obtaining the high temperature required in reaction zone 11 to produce the desired conversion. Thus it is presently preferred that the amount of ammonia introduced into neutralizer 8 be below about 0.1, preferably below about 0.05 weight parts ammonia per weight part P ₂ O ₅ plus mineral acid. Of course feed temperatures above about 200° F. will require pressure control systems, pumps and valves not illustrated. Preferably, sufficient pressure is maintained at that point to prevent steam flashing within the process lines prior to the ejection of the feed acid from nozzle 9.

Ammonia can be obtained either as a gas or liquid from a tank or pipeline 12 and passed to the reaction zone through nozzle 9 directly through lines 13, 15 and 16 and intermediate heat exchanger 23 and heater 14. Depending on the ammonia storage conditions, heat exchangers 23 and heater 14 may or may not be required. In the event that ammonia is obtained as a liquid, sufficient heat should be added to completely vaporize the ammonia prior to ejection into the reaction zone.

Ammonia feed rate is controlled in proportion to the acid feed rate to obtain the highest liquid phase temperature in zone 11. While the total ammonia required for neutralization can be added to the reactor, maximum reaction temperatures are obtained generally by adding about 40 to about 70 percent of this amount to the reactor and injecting the remainder elsewhere in the system such as to quench zone 20, product and recycle lines, accumulator 27, or the like. Due to equilibrium limits, this range corresponds generally to the maximum amount taken up by the acid at reaction temperature. For instance, when producing 8-22-0, the highest liquid phase temperatures, and consequently the highest conversion levels, are obtained by adding approximately 60 percent of the stoichiometric ammonia to the reactor. This corresponds to approximately 0.21 weight parts ammonia per weight part total acid. A total of 0.35 weight parts ammonia per weight part acid is required to produce 10-34-0.

However, for convenience, it may be desirable to add all the ammonia to the reactor. Excess ammonia will reduce reaction temperature, albeit to a minor extent, and is readily absorbed in the product quench. However, ammonia rates substantially above those required for complete neutralization are of little or no benefit and serve only to increase product pH. Accordingly, the total ammonia injection rate, including the minor amount added to pre-neutralizer 8, if any, should be at least about 0.12 and is generally within the range of 0.12 to about 0.40 weight parts ammonia per weight part P ₂ O ₅ plus mineral acid.

Ammonia mass rate should also be taken into account in the design and operation of ammonia jets 17 or similar injection means. Concentrated phosphoric acids are generally very viscous. Therefore, the amount of energy required to adequately disperse the liquid feed can be very high in the absence of any supplemental dispersing effect. While it is not essential, this supplemental effect can be obtained as illustrated in this embodiment by injecting ammonia into the acid spray at high velocities. It is therefore particularly preferred that at least a substantial proportion, e.g., at least about 30 percent, preferably at least 50 percent, of the ammonia added to the reactor, be introduced as a high velocity stream radially inwardly into the acid spray from around the periphery of the acid spray and about one foot or less below the acid spray means at a velocity having an inward radial vector of at least about 200, usually at least about 500, and preferably at least about 1000 feet per second. This manner of ammonia injection accomplishes several functions, the most significant of which are increased liquid phase dispersion and subdivision, and directional control and shaping or containment of the liquid spray within the unconfined reaction zone 11. The remainder of the ammonia, or the total amount if jets are not employed, can be introduced at any point to zone 11, although preferably in a manner that achieves direct contact with the acid spray.

Acid is ejected from means 9 through one or more acid spray means 10 which can be any one of the numerous known apparatus elements for producing dispersed liquid sprays such as nozzles, orifices, jets, or the like. The acid is thus sprayed downwardly into reaction zone 11 where it is intimately contacted with and dispersed by ammonia, preferably introduced by nozzles or jets 17. One suitable spraying means is illustrated schematically in FIGS. 4, 5 and 6 of copending application Ser. No. 591,056, filed June 27, 1974, by Bruce A. Harbolt and Donald C. Young, incorporated herein by reference.

Although the entire reaction system should be contained in a housing 18, the great majority of the liquid spray will pass downwardly into aqueous quench 20 without contacting the interior walls of vapor housing 18. This housing is preferably a substantially gas-tight enclosure having a relatively wide internal diameter of at least about 2 feet, preferably at least about 4 feet, containing the ammonia-acid jet 9 and aqueous quench 20 as illustrated in FIG. 5. The height of enclosure 18 should be sufficient to provide a vertical distance between acid spray means 10 and the upper surface of quench 20 sufficient to allow adequate reaction temperature elevation and polymerization.

The reaction is extremely rapid in these systems and will go to completion in less than 5 seconds, often less than 2 seconds, and generally less than 1 second. Thus the distance between the ammonia-acid jet or jets and product quench 20 should be at least about 1, preferably at least about 2, and is generally at least about 4 feet. Furthermore, to minimize or completely eliminate the contact of dispersed acid droplets with the interior surfaces of housing 18, it is presently preferred that the ratio of the vertical distance between the acid spray and the aqueous quench to the internal diameter of housing 18 be less than about 5.

The short reaction times made possible by this process are far less than those required to obtain comparable conversions by alternative methods, e.g., confined tubular or batch reactors. While the nature and influence of all the factors contributing to these high reaction rates are not known with certainty, much shorter contact times are required. Undoubtedly, the prolonged contact required in the alternative confined liquid phase systems accentuates reactor fouling and corrosion.

The reactor can contain a baffle or shroud 33 surrounding the acid and ammonia injection means 10 and 17, respectively. This shroud does not contain or confine the acid spray. On the contrary, its function is to momentarily contain the high velocity ammonia gas and increases turbulence in the initial stages of the ammonia-acid interaction when ammonia jets are employed. Acid dispersion and surface area are increased while droplet size is reduced, thereby increasing reaction rate. Furthermore, by directing the flow of ammonia downwardly into quench 20, baffle 33 also serves to guide the acid spray droplets in the same direction thereby avoiding substantial contact of those materials with the inside surfaces of housing 18.

For the reasons mentioned above, I prefer that shroud 33 should not be of such length or internal diameter as to significantly confine the acid spray. Obviously some of the acid droplets will contact the shroud interior walls, yet the spray will not be confined to an extent that causes significant droplet coalescence or formation of a continuous liquid phase. Thus the shroud should have a substantially vertical longitudinal axis aligned with the axis of acid spray means 10. It should be completely open at its lower extremity and have a length of about 4 feet or less, preferably about 2 feet or less with a ratio of length to minimum internal diameter of about 10 or less, generally about 5 or less, and preferably less than 3. The artisan will appreciate that any shroud or baffle can be used which will accomplish the above objectives, e.g., cones or square enclosures having the described lengths and radices.

Even though the acid is ejected at relatively high velocity, the reaction between the ammonia and acid is substantially complete within 5 feet, generally within 3 feet, and often within 2 feet of the spray nozzle. Reaction rates are sufficient to create temperatures of at least 400° F., generally between 500° and 750° F., and preferably between about 550° and 700° F. in the acid droplets within these distances.

Nevertheless, depending upon the acid's downward velocity, some additional conversion may be obtained with greater vertical distances, e.g., about 5 feet or 5 to about 10 feet. Of course a similar effect can be achieved by reducing acid velocity. In other words, the same conversion that can be accomplished in a vertical drop of 5 feet might be obtained at lower velocity within 4 feet, all other things being equal. However, this latter qualification is a source of difficulty since, for the reasons mentioned above, the mass velocity through acid ejectors 10 contributes, in large part, to acid dispersion. Obviously the independent variable is contact time and these questions of acid velocity and vertical distance are the dependent variables determining that quantity. However, the system is best described in these respects due to the difficulty in determining contact time per se. It is presently believed that the required conversions can be obtained in less than 3 seconds, generally in less than 2 seconds, and often even within 1 second or less.

Reaction rate is determined by a number of factors including droplet size, ammonia concentration (water content), reactor pressure, feed acid and ammonia injection temperature and velocity, feed acid P ₂ O ₅ concentration, e.g., H ₂ O/P ₂ O ₅ and mineral acid/P ₂ O ₅ ratios, impurity level, and ambient gas temperature in zone 11. The influence of most of these variables is completely overshadowed by the effects of droplet size, feed acid P ₂ O ₅ content, mineral acid/P ₂ O ₅ ratio and ammonia purity.

The ammonia should be substantially anhydrous. While superatmospheric pressures can be used, they are not required to obtain the desired conversions. Both operating and capital cost can be minimized by operating at ambient conditions. In fact, a slight vacuum is created within vapor confinement housing 18 presumably due to the rapid absorption of ammonia in the acid spray and quench 20. The effects of H ₂ O/P ₂ O ₅ and mineral acid/P ₂ O ₅ feed ratios are discussed above.

The remaining variable -- acid spray efficiency or spray droplet size -- is determined by several variables well known in the art. A detailed description of the variables involved in producing liquid sprays or mists is found in Kirk-Othmer, Encyclopaedia of Chemical Technology, 2nd Edition, Vol. 18, Interscience Publishers (1969), pages 634-653. A Bibliography giving reference to the work of numerous investigators is also included. The article refers to numerous forms of liquid spraying and atomizing equipment known to the art which are suitable for use in this embodiment.

It is presently preferred, however, that the spray forming means comprise a system in which the ammonia and acid enter the reaction zone through separate lines as illustrated in FIG. 5 although these separate lines can be contained in the same housing, nozzle or spray means. This qualification is very desirable since reaction between ammonia and acid in a confined space such as in a reactor line, pipe reactor or closed mixing nozzle, sufficient to produce a temperature above 500° F. will cause fouling with wet-process acids and corrosion with white acids. Thus the major amounts of ammonia and acid should be ejected from separate orifices. In addition, the cooperation of the ammonia and acid spray means should be such that neutralized or partially neutralized hot acid spray, e.g., 500° F. or higher, is not directed onto the ammonia nozzles.

An apparatus of this type is illustrated in FIGS. 4-6 of Harbolt and Young, supra, wherein annular and peripheral ammonia injection means are positioned relative to each other such that a significant amount of the hot acid does not contact the ammonia injectors or other solid surface of the spray apparatus. In this manner the nozzles or jets are not heated by a hot reaction mixture, and they are cooled internally by incoming feed. Thus they do not present a hot surface for fouling accumulation.

The principal factors controlling acid distribution and droplet size include acid feed velocity, viscosity and nozzle size and design, i.e., Reynolds number, ammonia mass flow rate relative to the acid flow rate, ammonia velocity upon contact with the acid spray, and the position of the ammonia jets, orifices, etc. relative to the acid jets and spray. Droplet size can be reduced by reducing feed acid viscosity at the acid orifice, increasing the velocity of the acid and/or ammonia streams, increasing the relative ammonia mass flow rate, increasing the acid orifice Reynolds number, and positioning the ammonia jets in close proximity to the acid nozzle aligned in a manner to provide a substantial radial component of the ammonia stream relative to the acid stream. One or more of these variables can be adjusted to promote the desired reaction conditions as indicated by liquid phase temperature and/or conversion to polymeric phosphates.

Conversion can be determined by simply catching a sample of the melt ahead of the quench. The polymeric species in the melt are stable, even at elevated temperatures, if isolated from substantial moisture. Thus the dispersed reaction product can be sampled, retained in a sealed container and analyzed to determine conversion level.

Similarly, liquid phase temperature can be approximated by positioning a temperature-sensing device such as a thermister, thermocouple or the like within the acid spray at one or more elevations to determine the maximum temperature obtained within the reaction zone.

Returning to FIG. 5, the melt is passed directly into quench 20 at a temperature and pH sufficient to minimize hydration and depolymerization. This temperature should be less than about 200° F., preferably below about 150° F. The pH should be above about 5.5, preferably between 5.5 and about 8.5. However, product solubility is highest under slightly acidic conditions. Thus pH levels between about 5.5 and about 6.8 are presently preferred. Recycle is controlled at a rate sufficient to rapidly quench the product melt and corresponds to a volumetric recycle ratio of at least 10, preferably between about 20 to about 60.

Quench medium 20, containing dissolved ammonium phosphate product, is passed by lines 21, 22, 24 and 26 through heat exchangers 3 and 23 and cooler 25 to product accumulator 27. Additional ammonia required to produce the desired product, e.g., 10-34-0, can be added either in the reaction zone or at any point in the recycle system. Makeup water is added as required to accumulator 27 via line 29 or other means as required to obtain the desired concentration and product is withdrawn via line 28.

While the aforegoing description has centered primarily on the adaption of these concepts to the preferred embodiment involving an unconfined liquid phase reaction system, those concepts are obviously applicable to any systems involving the substantially adiabatic neutralization of phosphoric acid with ammonia to produce ammonium phosphate products containing substantial amounts of polyphosphates. Illustrative of alternative systems are either batch neutralizers or continuous tank or pipe reactors such as those illustrated in U.S. Pat. Nos. 3,752,990; 3,734,708; 3,382,059; 2,902,342 and 3,730,700.

The most common alternative systems are the pipe reactors illustrated in several of these patents which generally operate by the introduction of the acid feed and ammonia into one end of an elongate tube. The tube can be either straight or curvilinear. Reaction proceeds throughout the length of the tube and neutralized product is recovered from the opposite extremity and is generally passed directly to a quench zone.

EXAMPLE

This example demonstrates the conversion of a wet-process acid having an H ₂ O/P ₂ O ₅ molar ratio of 7.5 containing about 10 weight percent incident metallic impurities comprising aluminum, iron and magnesium expressed as the corresponding oxides. Under normal circumstances, using only the autogenous heat of neutralization, an acid having an H ₂ O/P ₂ O ₅ ratio of 7.5, could, at best, be converted to a product containing less than 3 weight percent polyphosphate even in a system which is 100 percent efficient.

Prior to introduction into the reaction zone the feed acid was mixed with sufficient 99 percent sulfuric acid to obtain an H ₂ SO ₄ /P ₂ O ₅ molar ratio of 0.96. This mixture was then injected from the acid spray means in a system such as that illustrated in FIG. 5. The acid and ammonia spray means were substantially as illustrated schematically in that Figure. The nozzle included a 4.5 inch O. D. cylindrical housing having a lower base plate perforated by 37 acid orifices three-eighths inch in diameter on 1/2-inch centers. Thirty-seven ammonia orifices 0.1175 inch I.D. and 0.25 inch O.D. were positioned concentrically within each of the acid orifices leaving an annular spacing (annular acid orifice) between the external surface of the ammonia orifices. Each of the ammonia orifices extended approximately one-fourth inch downwardly below the base plate. The lower portion of the assembly was surrounded by twelve 0.18 inch I.D. stainless steel ammonia jets spaced evenly about the circumference of the housing, the open ends of which tubes pointed radially inwardly into the spray created by the acid and ammonia jets mentioned above, at a level approximately 11/4 inch below the ammonia orifices positioned within the acid jets in the base plate.

The radial ammonia jets, in turn, were surrounded by a cylindrical baffle shroud 4 inches in length and 4 inches I.D. positioned approximately as illustrated at 33 in FIG. 5, and being axially aligned with the cylindrical housing to provide a turbulent gas mixing zone immediately below the housing. This assemblage was contained in a substantially vapor-tight rectangular vapor housing 3 feet square, containing a quench zone in the lower portion thereof and having a vertical dimension between the acid nozzle and the quench surface of approximately 5 feet.

The mixture of sulfuric and wet-process acid was passed to the acid nozzles at a rate of about 10 gallons per minute at a temperature of 150° F. Ammonia was ejected from the 49 ammonia nozzles at a rate of 2.5 gallons per minute corresponding to 0.6 weight parts ammonia per weight part P ₂ O ₅ or 0.38 weight parts ammonia per weight part P ₂ O ₅ plus H ₂ SO ₄ . The gas velocity through the axial and radial ammonia orifices was in excess of 1000 feet per second.

The acid and ammonia spray was directed downwardly into the aqueous quench which passed through the unconfined reaction zone at a rate corresponding to a recycle ratio of 20/1 at a temperature of 150° F. Recycle pH was maintained at 6.5 by the addition of 0.11 weight parts ammonia per weight part P ₂ O ₅ to the recycle system.

The product solution contained 13 weight percent nitrogen, 26 weight percent P ₂ O ₅ , and 6 weight percent sulfur. Seventy-four percent of the P ₂ O ₅ was present as polyphosphate.