WO1989003805A2

WO1989003805A2 - Production of low-fluorine calcium phosphate and phosphoric acid and of silica

Info

Publication number: WO1989003805A2
Application number: PCT/US1988/003746
Authority: WO
Inventors: Timothy W. Tobiason
Original assignee: Designer Premixes Inc.
Priority date: 1987-10-30
Filing date: 1988-10-28
Publication date: 1989-05-05
Also published as: WO1989003805A3; AU2809589A

Abstract

A process for producing low-fluorine calcium phosphate granules comprises a special roasting process in which the granules are heated to a temperature sufficient to expand the granules and evolve volatile fluorine compounds. The product resulting from the process according to the invention has a higher proportion of phosphorus than the granules from which it is produced, mixes well with conventional feeds, and exhibits good biological availability. The product is also useful as insulation. Processes are also disclosed for reducing the fluorine content of the phosphoric acid used to make the granules. The fluorine content of phosphoric acid can be reduced by precipitating fluorine salts from already-produced phsophoric acid in the presence of hydrochloric acid or by digesting phosphate rock with recycled phosphoric acid and hydrochloric acid to produce low-fluorine phosphoric acid directly. In either embodiment, reactive silica is used. Low-cost reactive silica can be made from vitreous crystalline silica by using the roasting process, eliminating the need for costly diatomaceous silica.

Description

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PRODUCTIONOF LOW-FLUORINE CALCIUMPHOSPHATE AND PHOSPHORICACID AND OF SILICA

5 Background of the Invention

The present invention relates to a process for producing low-fluorine calcium phosphate granules that are useful as an animal feed supplement and as an insulating material and to a process for producing

10 low-fluorine phosphoric acid that can be used to make the calcium phosphate granules.

Phosphates are essential to all forms of life, both animal and plant. For example, the genetic material of all living organisms, the nucleic acids,

15 contains phosphorus. Because of the essential role phosphorus plays, it is a major constituent of fertilizers and is also used in animal feeds. Among the animals, phosphorus is also important for bone development and is especially important as a

20 supplement for young animals when bone development is critical.

The principle source of phosphate is mined phosphate rock. There are several methods for treating phosphate rock to produce phosphates. In

25 the furnace process, the phosphatic rock is combined Z with silica and coke and reduced at high temperatures in an electric furnace. Elemental phosphorus is produced, along with a calcium silicate slag and iron phosphide. Phosphorus-containing fertilizers do not require elemental phosphorus, but are manufactured by the so-called "wet process" in which phosphate rock is treated with an acid, usually sulfuric acid. Calcium phosphate in the ore dissolves in the acid, and crystals of calcium sulfate (gypsum) are formed. After separation of the calcium sulfate by filtration, the acid is concentrated to the level required to make various phosphates.

Single superphosphate is produced in large quantities and is made by reacting phosphate rock with sulfuric acid. The reaction results in a solid mass of monocalcium phosphate and gypsum.

When sulfuric acid is added to phosphate rocks in a proportion greater than needed to make single superphosphate, orthophosphoric acid, H3PO4, is produced. This acid is used as an intermediate in preparing other phosphates.

Triple superphosphate is made by acidulating phosphate rock with phosphoric acid. Triple superphosphate has over twice the concentration of phosphorus as that found in superphosphate. When reacted with hydrated lime, a chemical mixture of approximately 80% monocalcium phosphate and 20% dicalcium phosphate is produced. However, as in the other products, the monocalcium phosphate is bound to two waters of hydration, reducing the concentration of the calcium and phosphorus in the compound from that possible if the waters of hydration could be removed. The calcium to phosphorus ratio of feed grade calcium phosphates ranges from about 21% phosphorus to 15-18% calcium for monocalcium phosphate to 18% phosphorus to 34% calcium for tricalciu phosphate. Generally, as the calcium to phosphate ratio increases, the biological availability (a measure of digestibility) decreases. Therefore, phosphate feed supplements having a lesser quantity of calcium (e.g., made from triple superphosphate) are more desirable. It would be especially desirable if waters of hydration could be easily removed, reducing the costs of transporting water associated with each unit of phosphorus.

Another problem in the production of phosphate feed supplements is that phosphate rock typically contains impurities, especially fluorine, but also magnesium, aluminum, iron and heavy metals, including cadmium and vanadium, the absolute amount of which varies depending on the origin of the rock. Many of the impurities present in the phosphate rock are also solubilized during digestion of the rock and are retained in the acid produced, resulting in acid that is green or black in color. The contaminants frequently tie up phosphorus, resulting in reduced solubility and biological availability, or settle out, causing plugging of pumps and lines. These contaminants pose an additional problem when the phosphates are to be used in animal feeds, since government regulations limit the amount of fluorine and other impurities allowed in animal feeds. For example, the Association of American Feed Control- has established a maximum fluorine content for phosphate substances used as animal feed supplements of one part fluorine per 100 parts phosphorus. The phosphoric acid produced from phosphate rock requires additional processing to reduce fluorine to an acceptable level when the end use is for animals. The processes used are carefully controlled thermochemical processes, typically some sort of steam stripping process. In the process, steam, or a mixture of silica and steam, is added to the wet process phosphoric acid and water is evaporated to carry off the fluorine. The treatment is repeated until the level of fluorine is acceptable. However, aluminum fluorides cannot be effectively steam stripped.

The stripping equipment used - in these processes requires a large capital outlay, and the process is energy intensive, further increasing the cost of the product. Because of the huge capital outlay, phosphoric acid and the phosphates produced therefrom have been the exclusive province of large companies. Furthermore, because of the large cost of the facilities, these companies have centralized their processing. Large amounts of by-products of the process, particularly gypsum, are produced.

Gypsum can be used as a fertilizer for alkaline soils and has enormous water retention capabilities for dry soils. If filter-wet gypsum is mixed with lime, it can used to produce plaster bricks. Gypsum can also be used as cement retarder or in other building products such as plasterboard. However, the costs of distribution ^"from the central processing facilities contribute greatly to mark-up price of gypsum. Summary of the Invention

It is therefore an object of the present invention to provide a process for .the production of calcium phosphate compounds for use in animal feeds. It is another object of the invention to provide a process for the digestion of phosphate rock that produces phosphoric acid with low levels of fluorine without additional processing.

It is yet another object of the invention to provide a process for removing fluorine from bulk phosphoric acid.

It is a further object of the invention to provide a process for the production of calcium phosphate compounds for use in animal feeds that reduces fluorine to acceptable levels.

It is a another object of the invention to provide a low-cost process for the production of calcium phosphate compounds for use in animal feeds.

It is a further object of the invention to provide a low-cost process for the production of clean phosphoric acid from phosphate rock.

It is a yet another object of the invention to provide a calcium phosphate product for use in animal feeds that has a higher proportion of phosphorus than that available with other processes.

It is a further object of the invention to provide a calcium phosphate product for use in animal feeds that has a high biological availability of phosphorus. It is another object of the invention to provide a product for use as an insulating material.

It is yet another object of the invention to provide a process for increasing the surface area and reactivity of vitreous crystalline silica. έ

These_, and other objects of the invention are achieved by a process for producing low-fluorine calcium phosphate granules comprising the steps of placing calcium phosphate granules in a roasting oven and roasting the granules at a temperature sufficient to expand the granules and drive off residual fluorine compounds.

The objects of the invention are also achieved by a process for digesting phosphate rock, comprising the steps of mixing a first solution of water and at least one metal salt of sodium or potassium and adding the first solution to a strong mineral acid to produce a second solution comprising hydrochloric acid, mixing a third solution of reactive silica and water, adding the second and third solutions to a phosphoric acid solution and phosphate rock, digesting the phosphate rock to produce phosphoric acid, filtering the digested rock, adding sulfuric acid to the filtrate to precipitate out calcium sulfate and produce phosphoric acid, vacuum filtering and washing the phosphoric acid to produce concentrated phosphoric acid and wash acid, and recycling the wash acid for use in phosphate rock digestion. The process produces phosphoric acid having low levels of fluorine and other impurities.

The objects of the invention are also achieved by a process for reducing the fluorine content of phosphoric acid by precipitating fluorine salts, comprising the steps of mixing a first solution of water and at least one metal, salt of sodium or potassium and adding the first solution to a strong mineral acid to produce a second solution comprising hydrochloric acid, mixing a third solution of reactive silica and water adding the second and third solutions to a phosphoric acid solution produced from phosphate rock to produce a fourth solution, forming reaction products consisting essentially of sodium silica fluorides, potassium silica fluoride, sodium potassium silica fluoride or a mixture thereof, and precipitating the reaction products from the fourth solution until equilibrium is achieved.

The objects of the invention are also achieved by a process for producing reactive silica, comprising the steps of placing vitreous crystalline silica in a roasting device and roasting the silica at a temperature sufficient to expand the silica. The expanded silica is a low-cost alternative to diatomaceous silica.

In a preferred embodiment of producing low- fluorine calcium phosphate granules, the process additionally comprises the step of reducing the fluorine content of the granules either by precipitating fluorine salts from phosphoric acid used to make the granules or by digesting the phosphate rock by a process that produces phosphoric acid having low levels of fluorine and other impurities without the necessity of additional processing.

The product resulting from the process according to the invention has a higher proportion of phosphorus than the granules from which it is produced and fluorine levels below those required by government regulations. Surprisingly, the product exhibits biological availability when used as an animal feed that is comparable to that of existing products, but at a much lower cost. The good biological availability may result from changes to both the chemical and physical form of the granules during the process (i.e., the increased phosphorus and calcium amount and the expansion of the granules.) An additional unexpected advantage of the roasted product is that it has a density very close to that of corn meal and soybean meal. Thus, when added to these feeds it does not exhibit a tendency to settle to the bottom.

The process of the invention requires a low capital outlay for equipment, allowing processing to be done on a local level. Furthermore, the process of the invention is much less energy intensive than prior art processes for producing phosphoric acid and phosphates.

Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Brief Description of the Drawings

Fig. 1 is a diagram of a rotary-type kiln for carrying out the process according to the invention. Fig. 2 is a schematic diagram illustrating the process for digesting phosphate rock to produce clean phosphoric acid without additional processing.

Description of the Preferred Embodiments

Referring now to Fig. 1, rotary kiln 10 may be of conventional design, having stationary end portions 12 and 14, rotary center section or cylinder 13 lined with a suitable refractory 15, and a mechanism for rotating the cylinder (not shown) .

Preferably a plurality of radially aligned and spaced fins 19 project from the inside of cylinder 13. Material enters kiln 10 via inlet 16 and exits via outlet 17. Volatile gases exit kiln 10 via conduit

18. Kiln 10 can be tilted to various degrees with respect to the horizontal so that material in the kiln flows from inlet to outlet. In an alternative arrangement, a non-rotating gravity flow kiln may be used. The gravity-flow kiln is stationary and generally of rectangular construction with a fixed ramp inside the kiln having its input end at a higher level than its output end so that material flows down the ramp due to gravity. The interior of this type of kiln and the upper surface of ramp are covered with a suitable refractory. Preferably a plurality of raised portions are included on the ramp so that the granules bounce back and forth as they move down the ramp. This type of kiln is less preferred since process control is more difficult in this arrangement.

In kiln 10 fuel and air are fed to burner 30 to produce flame 31 within the kiln. ϊt is preferred that burner 30 be placed below inlet 16. The term "flame" within the meaning of the present specification includes both the luminous portions of the oxidizing reaction and the hot gasses associated therewith.

Calcium phosphate granules 11a are introduced into kiln 10 at inlet 12. Before entering the kiln it is desirable to screen and clean the granules to a uniform size, e.g., 65-70 mesh. This provides greater uniformity in the resulting product. /D Oversize particles are recycled by grinding, before being rescreened.

As the granules proceed through the kiln they expand in size. The granules expand like popcorn and release volatile gases contained in the granules, especially fluorine compounds. Partially expanded granules are shown as lib, and fully expanded exiting granules are shown at lie. The fully expanded granules may be from two to twenty times the size of original granules 11a. However, it is preferred that the exiting granules be expanded from two to four times their original size. Large granules are less desirable as feed supplements and would generally require an extra pulverization step. The exiting granules are relatively fine, giving the appearance of sand.

Kiln 10 creates a tumbling action with respect to the granules as a result of its rotation. The use of vanes 19 in kiln 10 greatly assists proper tumbling action. (Gravity flow kilns are less preferred because they generally create less tumbling action, even when raised portions are incorporated on the ramp.) The assisted tumbling action improves the process and product according to the invention by causing more even exposure of the granules and by preventing the granules from sticking to each other as a result of insufficient motion.

Granules in the kiln that do not pass through the flame eventually tend to clump together (so- called "mudballing") and/or plasticize into hardened masses. In general, this happens either when there is a delay between the time the granules enter the kiln and their passage through the flame or when the granules are not thrown through the flame at all, situations that are more common in simple gravity- flow kilns. If one or both of these occur and there is also excessive contact between the granules, mudballing or plasticizing result. In this case, the clumps or plasticized masses can still be used to produce the product according to the invention by

°passing the them through the kiln a second time.

However, it is preferable to avoid a second pass through the kiln because of the concomitant decrease in throughput and increase in energy requirements. A second pass can be eliminated by providing sufficient tumbling action to insure that the granules pass through the flame.

Both the tilt and rotation of the kiln and the use and arrangement of vanes in the rotary kiln contribute to tumbling action, as noted above. The proper tilt and arrangement of vanes necessary for sufficient tumbling action depends on the particular calcium phosphate granules. While insufficient tumbling action is the primary cause of mudballing and/or plasticizing, processing time, temperature, phosphate intake rate and air flow all interact with the "tumbling" parameters and affect the resulting product. In particular, phosphate intake rate must not be too high or the granules will experience a _< elay before entering the flame, or be heated too slowly to expand.

Calcium phosphate granules vary considerably from batch to batch and the process can be optimized for a given batch. In order to optimize process parameters for a particular batch, the first step is to observe whether mudballing or plasticizing occur. If these do occur, the parameters are adjusted until mudballing and/or plasticizing are eliminated. The parameters can be further varied in order to achieve maximum particle size for the exiting granules and increased uniformity in particle size.

In general, for rotary kilns a drum rotation of 20 to 40 rpm and a tilt of approximately 2 to 10 degrees, preferably about 5 degrees, is used. The preferred angle between the drum radius and each vane is 45° and the free end of the vane is angled away from the direction of rotation. A planar vane without the lip at its free edge (common in conventional rotary kilns) will assist in throwing the granules towards the flame, although vanes concave in cross-section have also been found to work well. The preferred angle of the kiln with respect to the horizontal is about 5-15", with about 5" being preferred.

It has been found that by adjusting the ramp to minimal tilt, the process tends to "self- regulate," i.e., the air flow through the kiln determines the residence time of a granule in the kiln. The granules pass slowly through the kiln. When a given granule has expanded to the desired size, it is light enough to be picked up by the air flow and carried to the kiln outlet. Complete conversion of the granules into the expanded product is insured.

Higher temperatures during processing mean that smaller process times may be used. However, higher temperatures make it more difficult to control the other parameters to prevent mudballing and plasticizing, and are also undesirable because they tend to favor production of polyphosphates. The best way to control the process time is by adjusting the angle of slope of the kiln to make the process self- regulating, as described above. By adjusting the kiln to minimal slope and using lower temperatures, somewhat longer process times result, but the process is very reproducible and requires little monitoring and little batch-to-batch adjustment.

In a preferred embodiment according the invention, the fluorine level of the phosphate material used in the process is reduced before roasting. Frazier et al, "Chemical Behavior of

Fluorine in the Production of Wet-Process Phosphoric

Acid," TVA Bulletin Y-113. May 1977, the contents of which are incorporated herein by reference, have identified twelve fluoride salts which can be precipitated from filter-grade (30-32%) wet-process phosphoric acid. These twelve salts are Na₂SiFg, K₂SiF₆, NaKSiF₆, Ca₃(AlF₆)₂-4H₂0, Mg₃AlF₆, NaK₂AlF₆, Ca₄SO₄SiAlF₁₃-10H₂O, MgNaAlFg*2H₂0, MgSiF₆*6H₂0, CaSiF₆'2H 0, CaF₂ and MgF₂. The study attempted to identify, isolate and characterize these salts and factors affecting the precipitation of certain of these salts, chukhrovite and ralstonite, from acid simulating that produced from Florida phosphate rock.

Surprisingly, it has been found that addition of hydrochloric acid to merchant-grade phosphoric acid (52-54% P 0₅) results in removal of increased amounts of fluorine from the phosphoric acid. Hydrochloric acid appears to alter the balance in the solution in a way which favors precipitation of the fluorine salt. Although hydrochloric acid per se can be used, because of cost considerations, it is preferred to use its equivalent by combining a chloride salt, preferably NaCl or KCl, with another strong mineral acid, preferably sulfuric acid. When hydrochloric acid or its equivalent is used, process equipment must be able to withstand its effects. Therefore, polyvinylchloride (PVC) is used for reactor tanks and piping, and pumps are PVC or cast /≠ iron. Polypropylene filter cloth is used on the vacuum filters.

The acid/salt solution is added to the phosphoric acid and mixed. Then reactive silica dissolved in water is added. Reactive silica includes diatoms and other amorphous species which can provide a large surface area for reaction. Reactive silica can be purchased in the form of diatomaceous silica, or can be produced according to the invention by expanding vitreous crystalline silica, as will be described hereinafter. Further mixing produces clarified acid in which the level of fluorine has been reduced to about 2,000 to 3,000 ppm. This level of fluorine appears to be an equilibrium level that will be achieved regardless of the initial level of fluorine in the acid; as long as an excess of reactants are present the fluorine will precipitate out until it is in equilibrium with approximately 2,000 to 3,000 ppm in the acid solution.

Although any of the twelve fluoride salts can be precipitated, it is preferred to precipitate silica fluoride salts, especially Na₂SiF₆, K₂SiF₆ or NaKSiFg. These silica fluoride salts precipitate readily from acid solutions. Moreover, the portion that remains in the phosphoric acid, and is incorporated in the form of silica fluorides into the calcium phosphate granules made from the acid, is more easily evolved in the roasting process than other of the fluorine salts. Generally SiF₄ from silica fluorides evolves at 300°F or at even lower temperatures.

As with the other fluorine salts, while it is possible to determine the initial concentrations of fluorine and other ions in the bulk phosphoric acid as received, and then calculate the necessary stoichiometric amounts of salt(s) containing the elements found in the respective fluorine salt precipitate, it is simpler in practice to add fixed amounts of the reactants. For example, it has been found that merchant-grade phosphoric acid generally contains about 3,000-10,000 ppm fluorine. In order to precipitate Na₂SiFg, K₂SiF₆ or NaKSiFg, preferred reactants are sodium chloride (or potassium chloride) , sulfuric acid, and reactive silica. Based on an amount of 50,000 pounds of merchant-grade phosphoric acid, five hundred pounds of salt, five hundred pounds of sulfuric acid, and five hundred pounds of reactive silica is a sufficient excess to insure precipitation. (Because the salt and reactive silica are each mixed with water, the resulting solution will also contain approximately 6,000 pounds water.)

The fluorine reduction is achieved by first mixing a solution of acid and metal salt. This is done by first dissolving the salt in water and then adding the salt solution obtained to the acid. It is important to add the salt solution to the acid, since the high heat produced prevents the salt solution from reacting with the acid to form insoluble precipitates. A solution of reactive silica and water is also prepared. Both the acid/salt solution and dissolved reactive silica are then mixed with a phosphoric acid solution produced from phosphate rock. The reaction forms insoluble fluorine salts that precipitate from the acid solution.

The precipitate containing the fluorine salts also contains some precipitated phosphate compounds, amounting to 6-7% of the original phosphorus. Instead of wasting these phosphates, ammonia and / water is added to the precipitate, producing a product useful as a fertilizer.

It is also possible to add salt(s) without adding hydrochloric acid or its equivalent. However, the fluorine will not precipitate as readily, and much more will remain in solution. Furthermore, some of the fluorine in solution will be incorporated into the calcium phosphate granules produced from the phosphoric acid in a form which does not evolve easily in the roaster. Evolution of hydrogen fluoride from non-silica fluoride compounds requires temperatures of over 2,000"F. This greatly decreases phosphorus biological value by causing the formation of meta- and pyrophosphates. As in the case when hydrochloric acid forms part of the solution, Na SiF₆, K SiF₆ and NaKSiFg are the preferred salts because they precipitate more readily and because the silica fluorides are the most easily evolved from the calcium phosphate granules. However, their precipitation is not favored as it is when the solution contains hydrochloric acid. Corresponding salts that may be used to form the acid/salt solution include, in particular, sodium (or potassium) sulfate and sodium (or potassium) hydrogen sulfate. Quite surprisingly, it has been found that if fluorine removal is implemented at an earlier stage, i.e., by adding the salt and hydrochloric acid to the sulfuric acid used to process phosphate rock, aluminum, iron, and heavy metals, e.g., vanadium and cadmium, are also precipitated along with the fluorine salts. Thus, in a particularly preferred embodiment, removal of fluorine occurs at this earlier stage. Levels of fluorine of 1,000 ppm and even lower can be achieved. After the fluorine salt(s) have been precipitated from the acid, the clarified acid may additionally be contacted with a hot surface to further defluorinate if desired. Alternatively, the clarified acid may be subjected to steam stripping. Fewer steam stripping cycles will be necessary than is typical in the industry, because of the already reduced level of fluorine in the acid.

It has also been found that phosphoric acid with low levels of fluorine and other impurities can be produced directly from phosphate rock by using hydrochloric acid during phosphate rock digestion with recycled phosphoric acid. The phosphate rock is attacked by dilute phosphoric acid and water, forming a weaker acid mix and monocalcium phosphate in solution. Approximately 94-96% of the rock is digested. Less heat is generated than with sulfuric acid digestions. By keeping the reaction temperature below 35°C, preferably at approximately 15-20°C, low levels of fluorine are evolved to the atmosphere so that no scrubbers are needed. The preferred method of cooling is by directly pumping cold compressed air into the bottom of the reactor. The lower heat levels mean less attack on the gangue containing the metal impurities, and hence fewer metal impurities are found in the phosphoric acid. Also, heavy metal sulfates are not formed, and the heavy metals precipitate out with the other impurities instead of being held in solution. Filtering a cooler solution tends to allow more solids to be removed.

The phosphoric acid digestion is accomplished as shown in Fig. 2. To initiate the process, one ton of phosphate rock is added to from 2-5 tons of merchant-grade phosphoric acid (0-54-0) , 2-5 tons of water, and an acid/salt solution. The acid/salt solution is produced by dissolving 20-80 pounds of salt (NaCl or KCl) in approximately 160-640 pounds water and then pouring the salt solution into 20-80 pounds of sulfuric acid to form NaHS0₄ and/or KHS0₄ and hydrochloric acid. The amount of salt used is the same as the amount of sulfuric acid used and is equal to one-eighth the amount of water used; e.g., if forty pounds of salt are used, then it will be dissolved in 320 pounds of water and poured into forty pounds of sulfuric acid.

Reactive silica dissolved in water is also added to the reactor. When diatomaceous silica is used, 20-40 pounds of silica diatoms are generally dissolved in 60-120 pounds of water (i.e., at least 3 pounds of water to one pound of silica) . Since reactive silica produced by expanding vitreous crystalline silica is cheaper than diatomaceous silica, more silica can be used. More silica allows other impurities such as magnesium, aluminum and iron to be removed in the form of their silicates, producing even cleaner acid. Some of these silicates are less dense than the acid and float to the top, where they are removed. Other of the silicates will precipitate with the fluorosilicates. The digestion time ranges from approximately one hour when 5 tons of acid are used to two hours when two tons of acid are used. When only two tons of acid are used, 85-90% of the rock is digested in the first hour. The second hour is required if 94- 96% of the rock is to be digested. Alternatively, some sulfuric acid can be added at this stage to speed the digestion, or to drive the digestion to completion (i.e., 99% digested). However, the use of sulfuric acid in the final stage will lead to some increase in impurities in the acid, especially if heavy metals are present.

After the digestion, the phosphoric acid is pumped to a separate reactor for addition of sulfuric acid and precipitation of gypsum. One ton of rock normally contains 30% calcium and requires 1,440 pounds of sulfuric acid to precipitate all of the calcium as CaS0₄. Following reaction with the sulfuric acid to precipitate CaS0 , the acid is vacuum filtered in a countercurrent filtration unit. The first vacuum filter stage produces product acid. The gypsum produced is collected on the filter and can be sold for use on alkaline and/or dry soils and for use in building products. Acid from subsequent filter stages is recycled to the phosphate rock digestion reactor. The concentration of this acid is approximately 28-32% P₂θ5« Four to ten tons of this recycled acid are used to process one ton of rock. When recycled acid is used in the digestion, the amounts of other reactants remain the same except that the 2 to 5 tons of water added in the start-up digestion is not added.

The concentration of product acid (i.e., that removed at the first stage of filtration) varies from 40-55%, depending on the concentration and amount of recycled acid used in the process. If desired, the product acid can be concentrated.

Reactive silica for use in both the rock digestion and fluorine precipitation from already- produced phosphoric acid can be made inexpensively with a roasting process similar to that used for the calcium phosphate granules. By roasting mined, vitreous crystalline silica in a kiln as shown in Fig. 1, the surface area of the silica is expanded from 10 to 50 times, resulting in a form of silica which is as reactive with the fluorine and other impurities as the more-costly diatomaceous silica.

In the process, the density of the silica changes from approximately 50 lbs/ft³ to approximately 10 lbs/ft³. Because the silica is so light, some of it enters the exhaust. Therefore, when roasting silica, a cyclone collector is added to the exhaust to recover exiting product. After low-fluorine phosphoric acid has been produced, it is mixed with powdered calcium to form monocalcium and dicalciu phosphate. Approximately

1,770 pounds of clarified acid is pumped into a mixer. Powdered calcium carbonate (800 pounds) is added and mixed to form a reaction product of approximately 80-90% monocalcium phosphate and 10-20% dicalcium phosphate. The reaction is complete in 30 seconds to four minutes.

It has been found that addition of water to the clarified acid after precipitation of the fluoride compounds reduces the incidence of fines during formation of the calcium phosphate granules.

Approximately 15% of water by volume is added to the clarified acid before the acid is combined with the calcium carbonate. As noted above, it is preferred that the granules produced also be screened to increase uniformity in exiting granule size from the kiln.

The primary function of the roasting process is to evolve the fluorine compounds in the calcium phosphate granules. For example, when a fluorine precipitation step is used on merchant-grade phosphoric acid, approximately 2,000 ppm fluorine remain in the clarified acid and are incorporated in the calcium phosphate granules. However, by roasting the granules, fluorine content is reduced to approximately 500 ppm. The roasting process has other benefits. For example, it dries the product and drives off the waters of hydration, resulting in a product having a higher content of phosphorus.

Furthermore, the expanded product has a density very close to that of corn meal or soybean meal so that it does not settle to the bottom when added to these feeds. The temperature used in the roasting process will depend on whether an initial fluorine precipitation step is used. When the process includes the preferred embodiment of an initial fluorine precipitation step, lower temperatures may be used, e.g., temperatures in the range from 250°F to 900°F, more preferably in the range of 400°F to 800^βF. Temperatures at the lower end of the range, i.e., 400°F to 600"F, will require somewhat longer processing times than temperatures in the upper end of the range, i.e., 600°F to 800°F. Processing times ranging from about 20-40 seconds, preferably about 30 seconds, at higher temperatures (i.e., 600°F to 800°F) and from about 40-60 seconds at lower temperatures (i.e., 400"F to 600°F) are preferred. Lower temperatures are desirable so that undesirable polyphosphates are not formed.

When the roasting process is used alone, higher temperatures (e.g., 800^βF to 1,000°F, or even in excess of 2,000°F when HF must be evolved) are generally necessary to drive off fluorine in the granules.

The following examples are presented to more fully illustrate the present invention, but are not limitative. Example 1. Expansion of phosphate material - rotary kiln.

A kiln essentially as shown in Fig. 1 was used. The drum size was 28 inches in diameter by ten feet long. The tilt of the ramp was 5°. Phosphate intake rate was 6-8 tons/hr. The phosphate granules were screened on 70 mesh screen before entering the kiln. The air flow in the kiln was 2000 cfm provided by a built-in blower set at 1,760 rpm. A 4.2 million BTϋ Webster burner provided an exhaust temperature of approximately 700^βF. The roasting process reduced fluorine in the monocalcium phosphate from approximately 2000 ppm to 500 ppm.

Example 2. Removal of fluorine from bulk phosphoric acid.

Approximately 50,000 pounds of western (Idaho) merchant-grade phosphoric acid (0-54-0, testing 23.7% phosphorus and 3,000 ppm fluorine) was pumped into a polyvinylchloride tank. Five hundred pounds of NaCl was mixed into 4,000 pounds H₂0 to create a salt water solution. This solution was then poured into 500 pounds of sulfuric acid. The result was a solution of hydrochloric acid, Na₂S0₄ and NaHS0₄.

Five hundred pounds of silica diatoms was mixed into approximately 2000 pounds of water to dissolve the silica.

Both the acid/salt solution and the dissolved silica were then added to the acid in the tank, mixed for approximately eight hours, and then allowed to settle for 24-48 hours. At this point the level of fluorine in the clarified acid has been reduced to about 2,000 ppm. Example 3. Removal of fluorine during processing of phosphate rock.

One pound of salt (NaCl or KCl or a mixture of the two) was dissolved in approximately six pounds of water. The solution was poured into one pound of sulfuric acid. This was then added to a mixer containing ten pounds of phosphate rock, 0.12 pounds of diatomaceous silica and an additional six pounds of sulfuric acid. Additional water (approximately 16.5 pounds) was added to form a slurry which was mixed for one hour. The amount of phosphoric acid produced was 10.8 pounds. The acid contained 920 ppm of fluorine and 4.37% phosphorus by weight.

The solution was filtered with a polypropylene multifilament cloth, #522-113 made by Industrial

Filter of Minneapolis, Minn.

Example 4. Production of calcium phosphate granules from defluorinated phosphoric acid.

Clarified merchant-grade acid (1,772 pounds) was measured into a mixer. Calcium carbonate (800 pounds) was slowly added, mixed and reacted. The time for completion of the reaction was approximately

3-4 minutes. The resulting product was a mixture of approximately 80-90% monocalcium phosphate and 10-20% dicalcium phosphate.

Example 5. Reduction of fines in calcium phosphate granules.

Example 4 was repeated, except that the 15% by volume of water was added to the clarified merchant- grade acid before the 1,772 pounds of acid was measured and added to 800 pounds of calcium carbonate in the mixer. The resulting product had a decreased number of fines. Example 6. Digestion of phosphate rock with hydrochloric acid and recycled phosphoric acid.

Approximately one ton of finely-ground western (Idaho) phosphate rock was placed in a reactor.

Sixty pounds of sodium chloride is dissolved in approximately 480 pounds of water and then poured into sixty pounds of sulfuric acid to form an acid/salt solution of NaHS0₄ and hydrochloric acid. Eighty pounds of reactive silica produced according to Example 7 is added to approximately 400 pounds of water. The acid/salt solution and dissolved silica was added to the phosphate rock along with 5 tons of phosphoric acid (0-54-0) having a concentration of approximately 50-54% and 5 tons of water. The temperature was controlled to 15-20^βC, and the digestion lasted approximately one hour. After digestion, the digested rock was filtered on a vacuum filter. The filtrate was pumped to a second reactor where 1,440 pounds of sulfuric acid was added to precipitate out CaS0₄. After mixing for 1 1/2 to 2 hours to form good crystals, the reactor contents were vacuum filtered and washed in a countercurrent process. The first vacuum filter stage was product acid, and was placed in a settling tank where any solids that were small enough to pass through the filter could settle. The third product, gypsum, was removed from the filter and stored. Subsequent filtration stages were recycled to provide phosphoric acid for digestion of further rock. From 4 to 10 tons of the recycled wash acid were added to a ton of rock and the same amounts of acid/salt solution and dissolved silica as used initially. Filtration and precipitation of CaS0₄ was performed in the same manner. Example 7. Expansion of silica - rotary kiln.

A kiln essentially as shown in Fig. 1 was used. The drum size was 28 inches in diameter by ten feet long. A cyclone collector was added to the exhaust to catch the expanded silica which entered the exhaust. The tilt of the ramp was 5^β. Mined vitreous crystalline silica was fed into the kiln at an intake rate of 6-8 tons/hr. The air flow in the kiln was 2000 cfm provided by a built-in blower set at 1,760 rpm. A 4.2 million BTU Webster burner provided an exhaust temperature of approximately 700°F. The roasting process expanded the surface area of the silica from about 50 lbs/ft³ to about 10 lbs/ft³.

Claims

zά, What is claimed is;

1. A process for producing low-fluorine calcium phosphate granules, comprising the steps of: placing calcium phosphate granules in a roasting device; and roasting the granules at a temperature sufficient to expand the granules and evolve fluorine compounds.

2. A process as claimed in claim 1, wherein the roasting temperature is in the range of about 250°F to 800°F.

3. A process as claimed in claim 1, wherein the roasting temperature is in the range of about 600^βF to 800°F and the roasting time is about 20-40 seconds.

4. A process as claimed in claim 1 wherein the roasting temperature is in the range of about 400°F to 600"F and the roasting time is about 40-60 seconds.

5. A process as claimed in claim 1, wherein the roasting device comprises a rotary kiln having a burner that produces a flame, and the roasting step comprises rotating the kiln to cause the granules to pass through the burner flame.

6. A process as claimed in claim 5, wherein the rotary kiln further comprises vanes that aid in the passage of the granules through the flame.

1. A process as claimed in claim 1, wherein the granules are expanded to about two to four times in size.

8. A process as claimed in claim 1, additionally comprising the step of reducing the fluorine content of the granules by precipitating fluorine salts from phosphoric acid used to make the granules, comprising the steps of: mixing a first solution of water and at least one metal salt of sodium or potassium and adding the first solution to a strong mineral acid to produce a second solution comprising hydrochloric acid; mixing a third solution of reactive silica and water; adding the second and third solutions to a phosphoric acid solution produced from phosphate rock to produce a fourth solution; forming reaction products consisting essentially of sodium silica fluorides, potassium silica fluoride, sodium potassium silica fluoride or a mixture thereof; and precipitating the reaction products from the fourth solution until equilibrium is achieved.

9. A process according to claim 8, wherein the metal salt is selected from the group consisting of sodium or potassium sulfate and sodium or potassium hydrogen sulfate.

10. A process according to claim 8, wherein the metal salt is selected from the group consisting of sodium and potassium chloride. z2

11. A process according to claim 8, wherein the precipitation of the fluorine salts occurs during production of the phosphoric acid from phosphate rock.

12. A process as claimed in claim 1, additionally comprising the step of reducing the fluorine content of the granules by using low- fluorine phosphoric acid produced by a process comprising the steps of: mixing a first solution of water and at least one metal salt of sodium or potassium and adding the first solution to a strong mineral acid to produce a second solution comprising hydrochloric acid; mixing a third solution of reactive silica and water; adding the second and third solutions to a phosphoric acid solution and phosphate rock; digesting the phosphate rock to produce phosphoric acid; filtering the digested rock; adding sulfuric acid to the filtrate to precipitate out calcium sulfate and produce phosphoric acid; filtering and washing the phosphoric acid to produce concentrated phosphoric acid and wash acid; and recycling the wash acid for use in phosphate rock digestion.

13. A process according to claim 12, wherein the metal salt is selected from the group consisting of sodium and potassium chloride.

14. A process for reducing the fluorine content of phosphoric acid by precipitating fluorine salts, comprising the steps of: mixing a first solution of water and at least one metal salt of sodium or potassium and adding the first solution to a strong mineral acid to produce a second solution comprising hydrochloric acid; mixing a third solution of reactive silica and water; adding the second and third solutions to a phosphoric acid solution produced from phosphate rock to produce a fourth solution; forming reaction products consisting essentially of sodium silica fluorides, potassium silica fluoride, sodium potassium silica fluoride or a mixture thereof; and precipitating the reaction products from the fourth solution until equilibrium is achieved.

15. A process for digesting phosphate rock, comprising the steps of: mixing a first solution of water and at least one metal salt of sodium or potassium and adding the first solution to a strong mineral acid to produce a second solution comprising hydrochloric acid; mixing a third solution of reactive silica and water; adding the second and third solutions to a phosphoric acid solution and phosphate rock; digesting the phosphate rock to produce phosphoric acid; filtering the digested rock; adding sulfuric acid to the filtrate to precipitate out calcium sulfate and produce phosphoric acid; filtering and washing the phosphoric acid to produce concentrated phosphoric acid and wash acid; and recycling the wash acid for use in phosphate rock digestion.

16. A calcium phosphate product produced by the process according to claim 1.

17. A calcium phosphate product produced by the process according to claim 8.

18. A calcium phosphate product produced by the process according to claim 12.

19. A method of supplementing an animal feed with phosphorus, comprising the step of adding the product according to claim 16 to an animal feed.

20. A method of supplementing an animal feed with phosphorus, comprising the step of adding the product according to claim 17 to an animal feed.

21. A method of supplementing an animal feed with phosphorus, comprising the step of adding the product according to claim 18 to an animal feed.

22. A method of producing an insulating surface, comprising the steps of: mixing the product according to claim 16 with a binder; and applying the mixture to a surface.

23. A method of insulating an enclosed space, comprising the step of blowing the product according to claim 16 into the enclosed space.

24. A process for producing reactive silica, comprising the steps of: placing vitreous crystalline silica in a roasting device; and roasting the granules at a temperature sufficient to expand the silica.