CROSS-REFERENCE TO RELATED APPLICATIONS This application is based on and hereby claims priority to U.S. Provisional Application Nos. 60/574,930 filed May 28, 2004, 60/575,415 filed June 1 , 2004, 60/615,954 filed October 6, 2004, 60/574,927 filed May 28, 2004, 60/575,416 filed June 1 , 2004, and 60/615,955 filed October 6, 2004, the contents of which are hereby incorporated by reference.

BACKGROUND ART Phosphoric acid, which is a source material for producing phosphates such as calcium phosphate, is produced by a conventional wet process in which sulfuric acid is used to treat a fine phosphate rock as shown in the following reaction. (Reaction 1). The phosphate rock contains several impurities including uranium.

Ca5F(PO4)S + 5H2SO4 + 10H2O -» 5CaSO4-2H2Oj + 3H3PO4 + HF (1) In the reaction described in Reaction 1 , dihydrated calcium sulfate (also known as gypsum or phosphogypsum) is produced. The phosphate rock (Ca5F(PO4)3) contains among other impurities about 50 to 100 ppm of uranium. During the wet process (shown in Reaction 1), at least some of the uranium from the phosphate rock stays with the gypsum produced.

Therefore, the impure gypsum cannot be used commercially unless the uranium in the gypsum is reduced to an acceptable level. Currently, the gypsum produced in the fertilizer and phosphoric acid plants from the phosphate rock, such as shown in Reaction 1 , is piled on the ground according to such conventional process. A very large pile of the gypsum on the ground is referred to as a "gypsum stack."

Since, the impure gypsum produced from Reaction 1 cannot be used as a calcium resource, an extra calcium resource such as lime (calcium hydroxide) has to be used in order to produce feed grade phosphate (calcium phosphate).

The phosphoric acid obtained from the wet process described in Reaction 1 is reacted with lime to produce calcium phosphate according to the conventional process. In the conventional process, there is no method of using calcium in the phosphogypsum to produce among other useful products. DISCLOSURE OF THE INVENTION To address the above and/or different concerns, the inventors propose a method of treating phosphogypsum, which produces calcium carbonate and ammonium sulfate from the phosphogypsum. The calcium carbonate contains impurities from the phosphogypsum. The calcium carbonate is reacted with an organic acid to produce a liquid calcium salt of the organic acid and un-reacted solid impurities. The calcium salt of the organic acid is consumed through at least one of two processes. First, the calcium salt of the organic acid may be reacted with phosphoric acid to produce calcium phosphate. Second, the calcium salt of the organic acid may be reacted with the ammonium sulfate to produce the ammonium salt of the organic acid and gypsum, the ammonium salt of the organic acid being reacted with the phosphoric acid to produce ammonium phosphate. The gypsum produced in the second process may have a reduced impurity level relative to phosphogypsum and so be useful in traditional gypsum applications.

In addition, the calcium salt of the organic acid may also be consumed in a reaction with sulfuric acid to produce gypsum.

The process of reacting the ammonium salt of the organic acid with the phosphoric acid will reproduce the organic acid, which can be recycled to react with the calcium carbonate.

In the first process, when the calcium salt of the organic acid is reacted with phosphoric acid to produce calcium phosphate, there is excess ammonium sulfate. The excess ammonium sulfate may be reacted with calcium hydroxide to produce gypsum, which has a reduced impurity level relative to the phosphogypsum. In reacting the excess ammonium sulfate with calcium hydroxide, ammonia may be produced, in which case, the ammonia may be used to produce the calcium carbonate and ammonium sulfate from the phosphogypsum.

The inventors also propose to treat phosphogypsum, by (i) producing calcium carbonate and ammonium sulfate from the phosphogypsum through a reaction with ammonium carbonate, the calcium carbonate containing impurities from phosphogypsum, (ii) reacting calcium carbonate with an ammonium salt of an organic acid to produce a liquid calcium salt of the organic acid and un-reacted solid impurities, and (iii) reacting the calcium salt of the organic acid with the ammonium sulfate to produce the ammonium salt of the organic acid and gypsum. The ammonium salt of the organic acid may be recycled for reaction with the calcium carbonate. The calcium salt of the organic acid may be produced by reacting calcium carbonate with the ammonium salt of the organic acid, at a temperature of 60 degree C or higher.

The inventors further propose purifying impure gypsum by (i) reacting the impure gypsum with an ammonia-carbon dioxide source, producing ammonium sulfate and calcium carbonate, (ii) reacting the ammonium sulfate with a calcium salt of an organic acid and water, producing substantially purified gypsum and the ammonium salt of the organic acid, and (iii) reacting the ammonium salt of the organic acid with the calcium carbonate, producing ammonium carbonate, which is recycled back to react with the impure gypsum and the calcium salt the organic acid, which is recycled back to react with the ammonium sulfate.

Uranium may be removed from an impure, uranium-containing gypsum, by (i) reacting the uranium-containing gypsum with an ammonia-carbon dioxide source, producing calcium carbonate, and (ii) reacting an ammonium salt of an organic acid with the calcium carbonate to produce a liquid, a gas, and a uranium-concentrated solid.

Still further, the inventors propose producing calcium phosphate by (i) reacting calcium carbonate with an organic acid to produce the calcium salt of the organic acid, (ii) reacting phosphoric acid and the calcium salt of the organic acid, producing calcium phosphate and the organic acid, which is recycled back to react with calcium carbonate, and (iii) reacting the ammonium sulfate with lime, producing substantially purified gypsum and ammonia. The calcium carbonate may be produced by reacting impure gypsum with an ammonia-carbon dioxide source, producing ammonium sulfate and calcium carbonate.

It is also an aspect of the present invention to provide an integrated process for producing feed grade phosphates while substantially reducing the amount of gypsum waste generated.

It is another aspect of the present invention to react phosphogypsum with an ammonia-carbon dioxide source to produce ammonium sulfate and calcium carbonate.

In a further aspect of the present invention a calcium salt of an organic acid is produced when calcium carbonate produced from phosphogypsum is reacted with the organic acid.

In yet another aspect of the present invention substantially pure phosphoric acid is produced that is substantially free of impurities found in phosphate rock. It is another aspect of the present invention to produce calcium phosphate by reacting the phosphoric acid produced from phosphate rock with the calcium salt of the organic acid.

In a further aspect of the present invention substantially purified gypsum being substantially free of uranium, is produced by reacting the ammonium sulfate and the calcium salt of the organic acid.

In yet another aspect of the present invention ammonium phosphate is produced by reacting the phosphoric acid from phosphate rock and an ammonium salt of the organic acid.

It is another aspect of the present invention to produce calcium phosphate by reacting the ammonium phosphate with the calcium salt of the organic acid.

In a further aspect of the present invention the organic acid is produced and replenished in the process by reacting the phosphoric acid produced from phosphate rock with the ammonium or calcium salt of the organic acid.

In yet another aspect of the present invention radioactive materials such as uranium may be efficiently removed from the impure gypsum produced in Reaction 1 , without consuming substantially any extra chemicals in the overall phosphate production process.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar elements throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

Fig. 1 is a flow diagram showing a process for producing phosphoric acid. Fig. 2 is a flow diagram of a process for treating phosphate rock to produce useful chemicals, according to a first aspect of the invention. Fig. 3 is a schematic drawing of a reactor and associated components, which reactor is shown in Fig. 2. Figs. 4A and 4B are schematic views of alternate embodiments for the settling velocity filter shown in Fig. 1. Fig. 5 shows a flow diagram for a process for treating phosphate rock, according to a second aspect of the invention. Fig. 6 is a flow diagram of a first alternative embodiment for the process shown in Fig. 2. Fig. 7 is a flow diagram of a third alternative embodiment for the process shown in Fig. 2.

BEST MODE(S) FOR CARRYING OUTTHE INVENTION Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

Fig. 1 is a flow diagram showing a process for producing phosphoric acid. Reaction 1 , which is repeated below, occurs in Fig. 1.

Ca5F(PO4)3 + 5H2SO4 + 10H2O -» 5CaSO4-2H2O j + 3H3PO4 + HF (1) Reaction 1 shows phosphate rock being reacted with sulfuric acid. However, even if phosphate rock is very well ground when it is reacted with sulfuric acid, calcium sulfate will form only on the outside of the particles. The inside of the particles remains unreacted, which can increase the gypsum byproduct as well as lower phosphate recovery. To address this, Reaction 1 is performed as two separate reactions, Reactions 1 a and 1 b below.

Ca10F2(PO4)6 + 14 H3PO4 ■» 10 Ca(H2PO4)2 + 2 HF (1a) 10 Ca(H2PO4)2 + 10 H2SO4 + 20 H2O -» 20 H3PO4 + 10 CaSO4-2H2O (1b) In Reaction 1a, the phosphate rock is first reacted with phosphoric acid to produce calcium phosphate. Then, in Reaction 1b, the calcium phosphate is reacted with sulfuric acid to produce calcium sulfate. The process for producing phosphoric acid is described in The Fertilizer Manual published by the International Fertilizer Development Center. Herein, the terms "calcium sulfate" and "gypsum" are used somewhat synonymously.

Referring to Fig. 1 , after grinding the phosphate rock, it is introduced to a phosphoric acid attack tank, where Reaction 1a occurs. Phosphoric acid reacts the phosphate rock to form mono calcium phosphate, which is more soluble in water than the starting material. The mono calcium phosphate is then reacted with sulfuric acid. Fig. 1 shows the reaction of sulfuric acid as having six stages. After the sixth stage, a portion of the phosphoric acid/phosphogypsum slurry is recycled to the first reactor while the balance is filtered to separate the product phosphoric acid from the calcium sulfate in a filter feed tank. Calcium sulfate is subjected to three filtration stages with counter-current washing before repulping. In the second, intermediate stage a dilute phosphoric acid is separated from phosphogypsum and recycled back to the first reactor for Reaction 1a. The phosphogypsum is repulped with water.

Fig. 2 is a flow diagram of a process for treating phosphate rock to produce useful chemicals, which expands upon the phosphoric acid process shown in Fig. 1. In Fig. 2, the phosphate rock, after being finely ground, is sent to a pretreatment area. In the pretreatment area, the phosphate rock is soaked in phosphoric acid. Reaction 1a occurs in the pretreatment area, as discussed above. From pretreatment, the produced calcium phosphate is sent to Reactor 1. In Reactor 1 , sulfuric acid is added. Reaction 1 b occurs in Reactor 1 in a manner similar to that shown in Fig. 1.

In Fig. 2, the pretreatment area corresponds with the H3PO4 acid attack tank shown in Fig. 1. In Fig. 2, Reactor 1 corresponds with the H2SO4 acid attack tank shown in Fig. 1. Fig. 1 shows 6 stages for the phosphoric acid and sulfuric acid reactions. A staged reaction is not shown in Fig. 2 for simplicity. A staged reaction is certainly possible.

The process that occurs in Fig. 2 produces a slurry of dihydrated calcium sulfate (also called gypsum) at stream 10. This slurry is sent to a filter 15 to separate the gypsum from the phosphoric acid. The gypsum exits the filter 15 via stream 16. The impure phosphoric acid is recycled from the filter 15 to the pretreatment area via stream 17.

Stream 16 containing impure gypsum is then processed in Reactors 2 and 3. The purpose of Reactors 2 and 3 is to treat the impure gypsum with an ammonia-water solution in the presence of carbon dioxide (Reaction 2). In Reactors 2 and 3, at least one of the following Reactions 2-4 occurs.

CaSO4-2H2O + 2NH3 + CO2 ■» (NH4)2SO4 + H2O + CaCO3J (2) CaSO4-2H2O + (NH4)2CO3 •» (NH4)2SO4 + 2H2O + CaCO3 I (3) CaSO4-2H2O + 2NH4HCO3 -> (NH4)2SO4 + CaCO3 J + CO2 T + 3H2O (4) All of the Reactions 2-4 produce calcium carbonate, which is precipitated, and ammonium sulfate. Uranium and other impurities in the calcium sulfate may stay with the calcium carbonate precipitates. Process conditions determine which reaction(s) of Reactions 2-4 occur. At sufficiently high temperatures and sufficiently low pressures, ammonia is a gas, thus favoring Reaction 2. However, at ambient temperature and pressure, ammonia reacts with water and converts to ammonium hydroxide according to Reaction 2a below.

NH3 (g) + H2O (I) <=> NH4OH (aq) (2a) In addition, if both water and carbon dioxide are present with ammonia, then ammonium carbonate or ammonium bicarbonate are formed respectively according to Reaction 2b or 2c below.

2NH3 (g) + 2 H2O (I) + CO2 (g) <=> (NH4)2CO3 (aq) (2b) NH3 (g) + H2O (I) + CO2 (g) <=> (NH4)HCO3 (aq) (2c) In view of the foregoing, if the temperature is sufficiently high and the pressure is sufficiently low, Reaction 2 primarily occurs in Reactors 2 and 3. If the temperature is lower and/or the pressure is higher, then Reactions 3 and/or 4 primarily occur in Reactors 2 and 3. In practice, stream 52 leaving Reactor 5 may contain a gaseous stream (CO2 and NH3), which is bubbled through liquid contained in Reactor 3. Stream 32 may be a liquid, which is sent to Reactor 2.

Reactors 2 and 3 serve as a counter current reaction mode to react the gypsum. Calcium sulfate is gradually converted to calcium carbonate. According to a particularly preferred embodiment, stream 22 exits Reactor 2 having maximum purity ammonium sulfate, and stream 34 exits Reactor 3 having maximum purity calcium carbonate.

A quantitative conversion from CaSO4 to (NH4J2SO4 can be reached in about 50 min at room temperature. To do this, in Reactor 2, the remaining excess ammonia and CO2 (ammonium carbonate or ammonium bicarbonate) in stream 32 is reacted with fresh gypsum from filter 15 to produce ammonium sulfate. Because there is excess gypsum, substantially all of the remaining ammonia and CO2 is reacted. The product stream 22 contains ammonium sulfate. Reactor 3 has excess ammonia and CO2, all remaining gypsum in the stream 24 is converted to ammonium sulfate. With Reactors 2 and 3, the quantitative conversion from CaSO4 to (NH4)2SO4 is reached. Reactor 2 can be a stirred reactor. On a batch scale, all remaining excess ammonia and CO2 in stream 32 may be reacted after approximately 30 min of mixing. According to an alternative embodiment, these conditions can also be utilized in a continuous process.

The ammonium sulfate exits Reactor 2 via stream 22. The ammonium sulfate in stream 22 is sufficiently pure to have commercial use outside the process shown in Fig. 2. That is, stream 22 contains substantially no free ammonia or ammonium carbonate or ammonium bicarbonate.

The solids produced in Reactor 2 are a mixture of calcium sulfate and calcium carbonate. The solids exit Reactor 2 in stream 24. The calcium sulfate and calcium carbonate are treated with a fresh ammonia solution in presence of carbon dioxide gas, as discussed above, in Reactor 3. The quantitative conversion changes all of the calcium sulfate to ammonium sulfate.

The solids from Reactor 3 (calcium carbonate and impurities) exit via stream 34. The solids in stream 34 may be treated with 3-hydroxypropionic acid (3-HP), HOCH2CH2CO2H according to a preferred embodiment. 3-HP is an organic acid, which reacts with calcium carbonate to form calcium 3-hydroxypropionate and releases carbon dioxide gas and water. This reaction is shown below as Reaction 5, which occurs in Reactor 5.

CaCO3 + 2HOCH2CH2CO2H -> Ca(HOCH2CH2CO2)2 + CO2 | + H2O (5) Reaction 5 is an acid-base reaction, which is likely to proceed to the right hand side of the equilibrium rapidly and CO2 is removed. Carbon dioxide gas from Reactor 5 is sent to Reactor 3 via stream 52. One embodiment for Reactor 5 is shown in more detail in Fig. 3. Reaction 5 occurs as in Reactor 5A. Solids and liquids exit Reactor 5A via stream 58. A gas stream 55 is sent to an adsorption column 5B. In column 5B, carbon dioxide is collected or trapped by an ammonia water solution, which enters via stream 56. Stream 52 from adsorption column 5B contains carbon dioxide, ammonia and water (or ammonium carbonate or ammonium bicarbonate), and is sent to Reactor 3. Although stream 56 is necessary, it is not shown in Rg. 2. Stream 52 is used as the ammonia source for Reactor 3.

Although Fig. 3 shows adsorption column 5B, it is possible to run the process without carbon dioxide adsorption. Gas stream 55 contains primarily carbon dioxide. Instead of supplying Reactor 3 via stream 52, carbon dioxide can be supplied as a gas, from gas stream 55 to Reactor 3. In this case, the ammonia source from stream 56 is lost. Therefore, a separate ammonia stream must be introduced into Reactor 3 together with the carbon dioxide stream 55. Stream 55 is a gas, and the ammonia source for Reactor 3 may also be a gas. These two gases could be supplied separately or combined before introduction into Reactor 3. If gaseous, the carbon dioxide and ammonia are bubbled through Reactor 3. With adsorption column 5B, Reactor 3 is supplied with stream 52, which is at least partially a liquid. Without adsorption column 5B, Reactor 3 may be supplied with a gas. In either case, Reactor 3 is supplied with carbon dioxide and ammonia.

In addition to adsorbing carbon dioxide, adsorption column 5B may also function as a reservoir.

It is known that in the traditional process a significant proportion of the Uranium in the rock is in a chemical form that dissolves when subjected to concentrated sulphuric acid. The residual uranium compounds, together other impurities, are not leached significantly in that harsh environment, so it is unlikely that they will be significantly solublized under the conditions in reactors 2, 3 and 5.

The solids and liquids in stream 58 (see Fig. 3) are sent to separator 5C, which produces a solid stream 54 and a liquid stream 57. The solids in stream 54 contain uranium, which may be used to produce uranium compounds. The uranium level may be about 0.1-0.2 weight %. This concentration assumes that original uranium concentration in gypsum is about 50 to 100 ppm (0.005 weight % to 0.01 weight %). The solids in stream 16 (see Fig. 2) may include gypsum and 5 wt.% to 10 wt.% impurities. If stream 16 (phosphogypsum) contains 95 % of gypsum and all the gypsum is reacted, then the only solids that remain are the impurities, originally 5 wt.%, now approaching 100 wt.%. The removal of gypsum represents a concentration of about 20 times. Thus, an original concentration of 0.005 weight % to 0.01 weight % becomes 0.1 weight % to 0.2 weight %.

With regard to the liquid stream 57, calcium 3-hydroxypropionate freely dissolves in water. Hence, stream 57 contains both water and calcium 3-hydroxypropionate. The calcium 3-HP water solution in stream 57 has a plurality of uses in the Fig. 2 processes. One use is adding the calcium 3-HP into a phosphoric acid solution to produce calcium phosphate precipitates and release 3-HP according to Reaction 6 below.

H3PO4 + Ca(HOCH2CH2CO2)2 •» CaHPO4 | + 2HOCH2CH2CO2H (6) Reaction 6 occurs in Reactor 6. The released 3-HP exits Reactor 6 on stream 61. Stream 61 is sent to Reactor 5 along with stream 34 for Reaction 5. One product of Reactor 6 is CaHPO4. This chemical is also referred to as "dical." A related chemical, Ca(H2PO4)2, is referred to as "monocal." If phosphoric acid is used in Reactor 6, it is possible to select between dical and monocal by varying the amount of calcium 3HP.

As can be seen from Fig. 2, Reactor 6 is supplied with phosphoric acid on stream 62. This stream is labeled "Pure H3PO4". However, the phosphoric acid produced in the wet process (Reaction 1) is not pure. For example, the phosphoric acid contains HF, H2SiF6 and H3AIF6. In order to purify the phosphoric acid, stream 63 from Reaction 1 is sent to Reactor 6/7/8 in which Reactions 6, 7, 8 and 8A occur. Reaction 6 is shown above, and Reactions 7, 8 and 8A are shown below.

2HF + Ca(HOCH2CH2CO2)2 -» CaF2 | + 2HOCH2CH2CO2H (7) H3AIF6 + H3PO4 + 3Ca(HOCH2CH2CO2)2 -» 3CaF2 J, + AIPO4 J, + 6HOCH2CH2CO2H..(8) H2SiF6 + 2H2O + 3Ca(HOCH2CH2CO2)2 ■» 3CaF21 + SiO2 j + 6HOCH2CH2CO2H...(8A) Stream 65 contains the products of Reaction 6/7/8. In addition, because Reaction 6 competes with Reactions 7, 8 and 8A for the calcium 3-HP, Reaction 6 is not substantially completed in Reactor 6/7/8. Thus, stream 65 also contains phosphoric acid. Filter 68 can be used to separate the liquids from the solids. The solids exit filter 68 via stream 67 and include CaHPO4, CaF2, SiO2 and AIPO4. Assuming that Reactions 7/8 proceed further than reaction 6, the liquid (stream 62) that remains will have an increased phosphoric acid purity, which is supplied to Reactor 6.

Reactor 6/7/8 may be a key component in the separation. Together with filter 68, separation may be performed based on the differences in settling velocities of the precipitates. In other systems, CaF2 forms fine solid particles, which stay in suspension and do not readily precipitate. In other systems, AIPO4 forms the flake-like precipitates that also exist in suspension. If this behavior occurs in this system, then AIPO4 may have a higher settling velocity than CaF2. CaHPO4 forms large crystalline precipitates that easily settle. CaHPO4 therefore has the highest settling velocity. Although Fig. 2 shows a stream 67 as the only solid stream exiting filter 68, there may be two or three solid streams, one for CaF2, one for CaHPO4 and perhaps one for AIPO4.

Settling velocity concepts are described, for example, in U.S. Patent No. 4,540,484, U.S. Patent No. 5,549,734 and U.S. Patent No. 6,432,298. Fig. 4A is a schematic view showing how Reactor 6/7/8 and filter 68 work together to perform Reactions 6, 7 and 8 and separate the products thereof. In addition to Reactions 7 and 8, Reaction 8A removes H2SiF6 by converting H2SiF6 to SiO2. SiO2 may be produced and separated using Reactor 6/7/8 and filter 68. However, the production and separation is not shown in Fig. 4A. Together, Reactor 6/7/8 and filter 68 form a gravity filter. Fig. 4A shows an embodiment where filter 68 is embodied as two filters, filter 68a and filter 68b. A belt filter or a centrifuge, for example, can be used as the filters 68a, 68b for streams 65a, 65b. Although two filters are shown in Fig. 4A, it may not be necessary to separate the calcium fluoride from the aluminum phosphate. Both can be treated as impurities. Silicon dioxide, which is not shown in Fig. 4A, may be removed together with CaF2 and AIPO4 in this single filter 68 according to an alternative embodiment. In this case, streams 65a and 65b would be combined, and a single filter 68 would be used to separate phosphoric acid from impurities.

According to one potential embodiment, in a gravity reactor, liquid from streams 105/63 and 57 can be added to a liquid capacity of about 90 % of the total volume of the reactor 6/7/8. Fig. 4A shows streams 57 and 105/63 supplying Reactor 6/7/8 at an upper portion thereof. The place of liquid introduction is not restricted to an upper portion. In Reactor 6/7/8, he solids with a higher settling velocity proceed to the bottom of the Reactor. Aluminum phosphate, a flake-like precipitate, has a lower settling velocity than calcium phosphate. Aluminum phosphate exits Reactor 6/7/8 from a middle portion, at stream 65b. Calcium fluoride, a fine solid, has the lowest settling velocity. Calcium fluoride exits Reactor 6/7/8 from an upper portion at stream 65a. Fig. 4A shows liquid phosphoric acid being removed from Reactor 6/7/8 via filters 68a and 68b. In a gravity reactor/filter, perhaps the bottom 10 % of the volume contains mainly the heavier solids, CaHPO4.

Other configurations are of course possible. Fig. 4B is an alternate embodiment to the embodiment shown in Fig. 4A.

CaHPO4 is a commercially valuable chemical. Calcium phosphate which may be the heaviest solid may exit as a slurry. The calcium phosphate could be separated from stream 62c with a filter, in the same manner as the separation for CaF2 and AIPO4.

Reactor 6 is designed for producing calcium phosphate. Accordingly, the CaHPO4 slurry in stream 62c of Fig. 4A could be combined with the phosphoric acid solution (stream 62) obtained from the filters 68a and 68b and sent to Reactor 6 to produce calcium phosphate (Reaction 6).

Reactors 6/7/8 and 6 use calcium 3-HP as a calcium source. Chinese Patent Publication CN 1022556C, published October 27, 1993 proposed to use calcium hydroxide, calcium oxide and calcium carbonate as calcium sources. However, using these reactants entails a solid-liquid reaction, which takes longer and requires excess calcium. In addition, both the reactants and the products are solids. On the other hand, reactions 6, 7 and 8 use liquid reactants and produce solid products. It is relatively easy to determine when reactions 6, 7 and 8 are complete. When both the reactants and the products contain solids it is more difficult to judge completion. Furthermore, with calcium hydroxide, calcium oxide and calcium carbonate, a batch process may be required. When the reactants are liquid and the products are solid, the reaction can be run continuously.

As mentioned above, Reactor 2 produces ammonium sulfate in stream 22. This ammonium sulfate may be sold as a fertilizer or can be reacted with calcium 3-HP to produce high purity calcium sulfate and ammonium 3-HP according to Reaction 9.

(NhU)2SO4 + Ca(HOCH2CH2COa)2 2H2O^ CaSO4-2H2O 1 + 2NH4HOCH2CH2CO2 (9) Reaction 9 occurs in Reactor 9. Reactor 9 represents a third use for the calcium 3-HP produced in Reactor 5, exiting via stream 57. In Reactor 9, Reaction 9 proceeds quickly. On a batch scale, calcium sulfate is produced almost immediately after calcium 3-HP is added to an ammonium sulfate solution. After separation, solids are produced in stream 91. These solids comprise substantially pure calcium sulfate. The calcium sulfate is substantially free of radioactive materials and can be used commercially. Stream 93 contains the liquids from Reactor 9, including ammonium 3-HP. The ammonium 3-HP can be reacted with phosphoric acid to produce ammonium phosphate and release 3-HP according to Reaction 10 below.

H3PO4 + NH4HOCH2CH2CO2 -» NH4H2PO4 + HOCH2CH2CO2H (10) Reaction 10 occurs in Reactor 10. Reactor 10 is supplied with the impure H3PO4 from the wet process (Reaction 1). The products of Reaction 10, and the impurities contained in phosphoric acid, exit Reactor 10 via stream 101. Pure ammonium phosphate can be obtained from stream 101 using crystallizer 103. After crystallization, some ammonium phosphate remains in the mother liquor solution exiting crystallizer 103 via stream 105. Stream 105 contains 3-HP, the phosphoric acid impurities and remaining ammonium phosphate. Stream 105 can be combined with the phosphoric acid solution in stream 63 to produce calcium phosphate. As discussed above, calcium phosphate is produced with calcium 3-HP in Reactor 6. The calcium phosphate exits Reactor 6 via stream 69.

As an alternative, after crystallization, the ammonium phosphate remaining in the mother liquor can be converted into calcium phosphate by adding calcium 3-HP according to Reaction 11 and 1 1 A below.

2NH4H2PO4 + Ca(HOCH2CH2COa)2 ■» Ca(H2PO4)2j + 2NH4HOCH2CH2CO2 (11) NH4H2PO4 + Ca(HOCH2CH2CO2)2^ CaHPO4J + NH4HOCH2CH2CO2+ HOCH2CH2CO2H (11A) Referring to Fig. 5, this reaction occurs in Reactor 11. Ca 3-HP is provided via stream 57. Thus, with the alternative shown in Fig. 5, there are four uses for the Ca 3-HP produced in Reactor 5. The solids from Reactor 11 are calcium phosphate in stream 111. The liquids exit in stream 113, and comprise ammonium 3-HP and 3-HP. Stream 113 can be combined with stream 61 and sent to Reactor 5 to produce calcium 3-HP.

As an alternative to the process described above, 3-HP may be replaced with a different organic acid. One qualification for the organic acid is that the calcium salt thereof has a relatively high solubility in water. Calcium carbonate which enters Reactor 5 on stream 34 contains uranium. The products of Reaction 5 are calcium 3-HP (the calcium salt of the acid), carbon dioxide gas, and water. The calcium salt of 3-HP is highly soluble in water and will be present as a liquid. On the other hand, the uranium is not reacted in Reaction 5. The uranium stays in a solid state with the residue from Reactor 5. Thus, the solid stream 54 contains uranium, but substantially no Ca-3HP according to one embodiment. On the other hand, if the calcium salt of the organic acid had a lower solubility in water, then some of the calcium salt would be present as a solid. In this case, the uranium-containing residue would be mixed with the calcium salt of the organic acid according to one embodiment. For all of the calcium salt lost, the organic acid must be replenished, representing additional cost. To avoid this cost, it is desirable for the calcium salt of the organic acid to have a high water solubility.

To be more specific regarding the solubility guideline, the acid should have a calcium salt with a water solubility greater than 5%, more particularly with a water solubility greater than 10%, and still more particularly with a water solubility greater than 20%. Propionic acid (HPA) is one alternative. Calcium propionate, the calcium salt of HPA1 is produced in a reaction corresponding to Reaction 5 above. With propionic acid, the solubility of calcium propionate in water is 33% at 2O0C. To accommodate this, a dilute solution can be used in stream 57, for example. With more water, there is less calcium salt. The amount of propionic acid required to complete the above processes may be somewhat higher because of the lower solubility of its calcium salt. However, functionality is preserved.

Acetic acid (HAc) may also be used to replace 3-HP. The solubility of calcium acetate, the calcium salt of HAc, in water is about 25% at 2O0C. This solubility is also lower than that of calcium 3-hydroxypropionate. The amount of acetic acid required to complete the above processes may be higher because of the lower solubility of its calcium salt. Thus, although it is entirely possible to use any organic acid to proceed with the processes described above, 3-HP has an advantage of higher efficiency.

The method described above may enable the production of feed grade ammonium and calcium phosphates and purified byproducts of calcium and ammonium sulfates while generating substantially reduced quantities of wastes.

DISCUSSION - INTRODUCTION TO PROCESS OPTIONS Referring to reaction 1 , the primary component of phosphate rock used for the above process is calcium phosphate. When phosphoric acid is produced from phosphate rock according to reaction 1 , the phosphoric acid protons are derived from sulfuric acid. Each phosphoric acid requires three protons. Each sulfuric acid only provides two protons. Accordingly, one and one half moles of sulfuric acid are necessary to produce one mole of phosphoric acid.

For every mole of sulfuric acid consumed, a mole of calcium sulfate (gypsum) is formed. Thus, 1.5 moles of gypsum are produced for every one mole of phosphoric acid. In addition, the phosphate rock is not pure calcium phosphate. Phosphate rock also contains fluorine. Because of reactions with fluorine, two moles of sulfuric acid are necessary to produce one mole of phosphoric acid. These two moles of sulfuric acid produced two moles of gypsum. Thus, based on the usual impurity content, for every mole of phosphoric acid produced from phosphate rock, two moles of gypsum are produced.

The gypsum produced from the wet process (Reaction 1) contains impurities including uranium. It is a goal to treat the gypsum to produce useful chemicals. As a first step, reactions 2, 3 and 4 produce calcium carbonate from the gypsum. Each mole of calcium sulfate (gypsum) produces one mole of calcium carbonate. With reaction 5, calcium carbonate is converted to the calcium salt of an organic acid. Calcium 3-HP is one example of such a salt. In reaction 5, the molar ratio of calcium carbonate to calcium 3-HP is one to one. Therefore, the two to one ratio with phosphoric acid is not changed through reactions 2-5. Two moles of calcium 3-HP are produced for every single mole of phosphoric acid.

1 mol H3PO4 = 2 mol CaSO4 2 mol CaSO4= 2 mol CaCO3 2 mol CaCO3= 2 mol calcium 3-HP 1 mol H3PO4 = 2 mol calcium 3-HP In Fig. 2, three uses for stream 57 are shown. The first and second uses relate to producing calcium phosphate with phosphoric acid, according to reaction 6. In reaction 6, one mole of calcium phosphate is produced from one mole of phosphoric acid and one mole of calcium 3-HP. Therefore, according to a molar balance, 50% of the gypsum can be consumed. That is, phosphate rock produces two moles of calcium 3-HP for every one mole of phosphoric acid. For each mole of phosphoric acid used in reaction 6, one mole of gypsum (or calcium 3-HP) remains.

The third use for stream 57 is the production of ammonium phosphate, through reactors 9 and 10 and crystallizer 103. The molar ratio of phosphoric acid to calcium 3- HP is 1 :2. Referring to reaction 9, every mole of calcium 3-HP produces two moles of ammonium 3-HP. Therefore, for every mole of phosphoric acid produced, four moles of ammonium 3-HP are produced. Referring to reaction 10, one mole of ammonium 3-HP is consumed for every one mole of phosphoric acid. Three moles of ammonium 3-HP remain unreacted. Based on a molar balance, 25% of the gypsum can be consumed to produce ammonium phosphate.

The above description assumes that all of the phosphoric acid is used to produce either ammonium phosphate or calcium phosphate. On the other hand, phosphoric acid itself is a valuable chemical. If all of the phosphoric acid is sold, then none of the gypsum is consumed.

PROCESS OPTION 1

Referring to Fig. 2, calcium 3-hydroxypropionate can be added directly into Reactor 6 via stream 57 together with the phosphoric acid solution (stream 62) obtained from the wet process to precipitate calcium phosphate (stream 69) and release 3-HP (stream 61). Calcium phosphate is formed rapidly when calcium 3-hydroxypropionate is added into the phosphoric acid solution. The released 3-HP is recycled back to Reactor 5 via stream 61 to react with calcium carbonate to produce calcium 3-hydroxypropionate.

If all calcium 3-hydroxypropionate produced from the reaction 5 is used to react with all the phosphoric acid produced in reaction 1 to produce calcium phosphate, about 50 mol % of the calcium 3-HP would be consumed. This translates to eliminating 50% of the raw gypsum. Referring to Fig. 6, one possible way to consume the excess calcium 3- hydroxypropionate is by treating it with sulfuric acid to form pure calcium sulfate and regenerate 3-HP according to reaction 12 below.

H2SO4 + Ca(HOCH2CH2COa)2 ■» CaSO42H2O J, + 2HOCH2CH2CO2H (12) In Fig. 6, reaction 12 occurs in Reactor 12. The 3-HP can be recycled via stream 121 to treat CaCO3 in Reactor 5.

A further advantage of the formation of calcium phosphate in this manner is that fluorine impurities in the raw phosphoric acid stream (in the forms of H3AIF6, HF and H2SiF6), can be removed in the form of CaF2, a solid which can be precipitated by mixing an excess of raw phosphoric acid with calcium 3-hydroxypropionate. The calcium phosphate produced in this mode may have a low enough fluorine content to meet the feed grade or technical grade specifications.

PROCESS OPTION 2 The ammonium sulfate in stream 22 produced from the reaction of gypsum with ammonia solution and CO2 (Reactions 2-4) can be sold as a fertilizer. Excess ammonium sulfate will have to be treated, otherwise it will generate a new waste and consume ammonia.

In addition to or instead of selling the ammonium sulfate, it can be reacted with calcium 3-hydroxypropionate to produce high purity calcium sulfate and ammonium 3- hydroxypropionate (Reactor 9). The calcium sulfate precipitates immediately after calcium 3-hydroxypropionate is added to the ammonium sulfate solution.

After separation of the solids from the solution, purer CaSO42H2O is obtained at stream 91. This pure CaSO42H2O may be free of radioactive material and may be commercially usable. The ammonium 3-hydroxypropionate solution (stream 93) can be used to react with phosphoric acid in Reactor 10 to produce mono-ammonium phosphate (MAP) and release 3-HP (stream 101). The 3-HP released from Reaction 10 can be sent to react with calcium carbonate to produce calcium 3-hydroxypropionate (Reaction 5) as shown in Fig. 2, via streams 105, 65, 62 and 61.

If all the phosphoric acid obtained from the wet process is used to produce mono- ammonium phosphate (MAP) in Reactor 10, about 25% of the gypsum waste will be consumed (the production of 1 phosphoric acid produces 2.0 gypsum, one gypsum generates one ammonium sulfate which produces two ammonium 3-hydroxypropionate and one MAP requires only one ammonium 3-hydroxypropionate). Therefore, a maximum of 25% of the gypsum waste can be treated according to this process option.

If calcium phosphate has a larger market than ammonium phosphates (MAP can act as a precursor for diammonium phosphate production), using all calcium 3- hydroxypropionate to produce calcium phosphate will consume more gypsum waste (one CaHPO4 requires one calcium 3-hydroxypropionate).

However, if calcium 3-hydroxypropionate and all phosphoric acid produced in the wet process are used to produce CaHPO4, not all ammonium sulfate produced from the treatment of the gypsum waste (Reactions 2-4) can be treated by calcium 3- hydroxypropionate (50% remains). In order to avoid generating the new waste of ammonium sulfate, alternative treatment can be used. One option is to use lime or calcium hydroxide to treat remained ammonium sulfate to form pure gypsum and release ammonia which can be recycled back to Reactors 2 and 3 according to reaction 13 below.

(NH4)2SO4 + Ca(OH)2 ■» CaSO4-2H2O j + 2NH3T (13)

PROCESS OPTION 3 In treating phosphogypsum, ammonium 3-HP is one compound that should be eliminated. The process can produce more ammonium 3-HP than can be consumed with phosphoric acid. According to option 3, ammonium 3-HP is reacted with the CaCO3 according to reaction 14 below.

2NH4HOCH2CH2CO2 + CaCO3 •* (NH4J2CO3 + Ca(HOCH2CH2CO2)2 (14)

Fig. 7 is a flow diagram of a further alternative to the process shown in Fig. 2, relating to reaction 14. In Fig. 7, CaCO3 is reacted ammonium 3-HP instead of 3-HP. Reaction 14 occurs in Reactor 14. Ammonium 3-HP is supplied to Reactor 14 from Reactor 9 via stream 141. The process shown in Fig. 7 converts all of the impure gypsum to pure gypsum. Stream 16 is impure gypsum. Stream 91 is pure gypsum. In Reactor 5 of Fig. 2, an acid (3-HP) is reacted with a salt, CaCO3. This reaction can be done at room temperature.

Ammonium 3-HP is a salt. Most ammonia salts of an organic acid are unstable at high temperatures. Ammonium carbonate has a boiling point of 60 Deg C. If the reaction is at 6O0C or higher the ammonium carbonate produced can be evaporated from the reaction system. At the 60 degree C or higher reaction temperature ammonium 3- hydroxypropionate is unstable and decomposes to ammonia and 3-HP. The released 3- HP reacts with calcium carbonate to form calcium 3-HP and carbonic acid.

On a batch scale, a quantitative conversion from ammonium 3-hydroxypropionate to calcium 3-hydroxypropionate (or from CaCO3 to (NH4)2CO3) can be reached in about 1 hour at about 10O0C by reacting CaCO3 with ammonium 3-hydroxypropionate. This demonstrates that ammonium 3-hydroxypropionate from Reaction 9 can be recycled back to treat CaCO3 produced from the gypsum waste (Reactions 2-4). Reaction 14 forms calcium 3-hydroxypropionate and releases ammonium carbonate (or ammonia and CO2), which is recycled back to Reactors 2 and 3 via stream 52. This process demonstrates that all gypsum waste produced in the fertilizer plant can be treated without consuming substantial additional chemicals. Further, the existing gypsum waste from the gypsum stacks can also be treated with this technology.

In this technology the only discharged material is uranium containing material, which may be used directly to produce uranium by a conventional technology. The amount of discharged material will be about 5% of the amount of the gypsum waste produced in the fertilizer plant. 3-HP, NH3 and CO2 used in this invention are all recycled and are not consumed.

The third option is based upon uranium compounds remaining insoluble when exposed to a CO2 / NH3 / 3-HP environment. The uranium compounds in the gypsum waste have been exposed to phosphoric acid (reaction 1a) and sulfuric acid (reaction 1b), but did not dissolve. The chemicals of the CO2 / NH3 / 3-HP are much weaker chemicals (or acids and base) than sulfuric acid or phosphoric acid. If phosphoric acid and sulfuric acid did not dissolve the uranium compounds, the ammonium sulfate and the CO2 / NH3 / 3-HP environment are unlikely dissolve the uranium compounds.

The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected. For example, there have been substantial discussions regarding the use of 3-HP. However, other organic acids, such as propionic acid, formic acid, lactic acid and acetic acid can be used. These variations and modifications are within the spirit and scope of the invention.