or

---

Ind. Eng. Chem. Process Des. Dev. 1981,20,416-424

416

Goodsthe, S. L.; Comparato. J. R.; Matthews, F. T., paper presented at Second International Coal Utilization Conference, Houston, Texas, Nov 6-8, 1979. Highley, J.; Merrick, D. AICM Symp. Ser. 1071, 67(116), 219. Holcomb, R. S. "Proceedlngs of the Fourth International Conference on Fluidized-Bed Combustion"; MITRE Corporation: McLean, VA, 1975; p

Vlrr, M. J. "The Industrial Fluidized Bed Boiler"; StonePlatt Fluidfire Limited, West Mldlands, Engiand, 1978.

Received for review January 23, 1979 Revised mhnuscript received August 4, 1980 Accepted April 20, 1981

171

hy,'H.' R. "Proceedings Of the Fourth htm"Onai Conference On FluklizedBed Combustbn"; MITRE Corporatlon: McLean, VA. 1975; p 93. Kunii, D.; Levenspiel, 0. J. Chem. fng. Jpn. 1869, 2 , 122. Slncovec. R. F.; Madsen, N. K. ACM Trans. Math. Software 1975, 1 , 232.

This work was conducted under the sponsorship of the Engineering Experiment Station (Energy Study Project) of Kansas State University.

A New Process for Production of Purified Phosphoric Acid and/or Fertilizer Grade Dicalcium Phosphate from Various Grades of Phosphatic MateriaIs Grover L. Brldger,' Carl B. Drees,* and Amltava H. Roy*3 School of Chemical Engineering, Georgia Instnute of Technokgy, Atlanta, Georgia

30332

new process for production of purified phosphoric acid and/or fertilizergrade dicalcium phosphate has been developed and tested with various grades of phosphatic materials as the source of P2O5. This new process is based on dissociation of monocalcium phosphate with an organic solvent. Phosphate concentrates, colloidal phosphates, phosphate slimes, and phosphate matrix containing 7.5-34.3% P2O5 have all been found amenable to treatment by the above process to produce phosphoric acid of much greater purity than that produced by current commercial wet-acid processes. Altematively or simultaneously, the process will produce fertilizergrade dcalcium phosphate using much less sulfuric or phosphoric acid than previous processes. A highly concentratgd phosphoric acid can be made directly, without vacuum evaporation. A

Introduction For production of high-purity phosphoric acid by the wet process, high-grade phosphate rocks or concentrates are desirable. However, the supply of high-grade phosphates is rapidly decreasing, and it would be highly desirable to be able to produce high-purity phosphoric acid from low-grade phosphatic materials. In 1974 the domestic production of marketable phosphate rock was 45 686 OW tons. On the average P205content of this production was 30.8% (67.4% Bone Phosphate of Lime [BPL]). The average grade of phosphate ore mined in the United States was 13.3% Pz05,and the average PZO5 recovery from the ore was 67.7 % . The grade distribution of the marketable phosphate rock consumed in the United States in 1974 was as follows: less than 60 BPL, 5.6%; 60-66 BPL, 20.8%; 66-70 BPL, 42.0%; 70-72 BPL, 12.2%; 72-74 BPL, 11.6%; and over 74 BPL, 7.8%. More than two-thirds of the phosphate rock marketed in 1974 was less than 70 BPL. When the lower grades of presently marketable phosphate rock are used for manufacture of phosphoric acid by the wet process, the product acid contains substantially more impurities than when high-grade rock is used. A process which could produce high-purity phosphoric acid from low-grade phosphatic materials would be very de'G. L. Bridger was Director and Professor of Chemical Engineering at Georgia Institute of Technology,Atlanta, Ga., before his death on Nov 3, 1978. 2Chevronh e a r c h CO.,P.0.BOX1627, Richmond, CA 94802. To whom correspondence should be addresses International Fertilizer Development Center, P.O. Box 2040, Muscle Shoals, AL 35660. 0196-4305/81/1120-0416$01.25/0

sirable. In particular, a process which could produce high-purity acid from raw phosphate ore (matrix) would have the added advantages of eliminating the ore beneficiation cost and increasing the Pz05recovery, since PzO5 losses in washing and flotation would be eliminated. Also, many phosphatic materials not now economical for production of phosphoric acid could be used, thus extending the life of phosphate reserves. It is well known that monocalcium phosphate undergoes dissociation under suitable conditions to form phosphoric acid and dicalcium phosphate Ca(H2P0J2 = H3P04+ CaHP04 (1) The conditions under which this reaction proceeds in an aqueous system were investigated by Elmore et al. (19401, who found that high concentration and high temperature favored the reaction. The dissociation in the presence of a number of organic solvents, including ethanol, acetone, dioxane, tetrahydrofuran, and pyridine, was investigated by B o d e et aL (1959). However, the dissociation has never been successfully used as a basis of a commerical process. A number of studies have been made in the past concerned with the extraction of phosphoric acid by organic solvents and the solubility of phosphoric acid in various organic liquids. A variety of patents exist for chemical processes that produce a purified orthophosphoric acid . based on the extraction of'crude wet-process phosphoric acid by organic solvents with the subsequent recovery of the purified acid product. Very extensive literature reviews of the Processes, papers, and patents are given by Drees (1972), ROY (19761, and McCulloUgh (1976). The production of fertilizer-grade dicalcium phosphate by direct acidulation of phosphate rock with mineral acids 0 1981 American Chemical

Society

Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 417 r---PHOSPHATE ROCK

SULrUR/C ACID

__L

A DIGESTION __c

1

HI PO4

.... WATER

*

B FILTRATION

%PO4

I

-------____ RECYCLE II

1

I

I

IMP WATER

1

C REACTION

I

A 0 7DRYING

1 IMP

i

I

GYPSUM IMP

I

I

i

I

OlSSDCl ATlON RECYCLE I

PHOSPHORIC ACID PRODUCT OR RECYCLE

I I

SOLVENT

I I

OlCALClUM L -PHOSPHATE PRODUCTOR RECYCLE

-

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-0cp IMP

F FILTRATIoN

G

-

H] Po4 SOLVENT

SOLVENT SEPARATION

.

I MAKE-UP SOLVENT

MCP = MOkOCALClUM PHOSPHATE OCP = OlCALClUM PHOSPHATE IMP = iMPURlTlES

Figure 1. Process for production of low-impurity phosphoric acid and/or dicalcium phosphate.

has long been a goal of the fertilizer industry, since it would enable the production of a unit of available P205with much less mineral acid consumption than by conventional superphosphate and other processes. For example, if reaction 2 could be made to proceed easily, the consumption Calo(P04)6F2+ 4H3Po4 = 10CaHP04 + 2HF

(2)

of phosphoric acid per unit of available P205would be only 57% as much as in the production of triple superphosphate by reaction 3. Calo(P04)6F2+ 14H3P04= 10Ca(H2P04)2+ 2HF

(3)

Proposed Process A process was formulated in which the dissociation reaction 1is carried out in an organic solvent. This process has the following features: (1) production of a high-purity phosphoric acid, since the organic solvent would be expected to reject most of the common impurities found in wet-process phosphoric acid; (2) production of a highly concentrated phosphoric acid directly without the usual vacuum evaporation concentration step; (3) production of a fertilizer-grade dicalcium phosphate with substantially low mineral-acid consumption indicated in reaction 2; (4) flexibility of producing only pure phosphoric acid or only fertilizer-grade dicalcium phosphate, or any proportion of the two; (5) adaptability for processing low-grade as well as high-grade phosphate rocks, and also for processing phosphatic byproducts such as slimes, washings, and other rejected phosphatic materials. A generalized schematic flow diagram showing the major steps involved in the proposed process (Figure 1)consists of the following steps: (A) reaction of phosphate rock (and sometimes recycled dicalcium phosphate) with sulfuric acid to form crude phosphoric acid and calcium sulfate. This step and the following step are essentially the same as production of wet-process phosphoric acid. The principal reactions are Calo(P04)6F2+ lOH2So4= 6H3P04+ 10CaSo4 + 2HF (4)

CaHP04

+ H2SO4 = H,P04 + CaS04

(5)

The calcium sulfate and the dicalcium phosphate may be hydrated, depending on reaction conditions; (B) filtration of the reaction products to remove insoluble calcium

sulfate and some of the impurities, with washing of the filter cake with water and recycling the washings to step A; (C) reaction of the crude phosphoric acid with phosphate rock to produce monocalcium phosphate. Product phosphoric acid may be recycled to this step. The reaction product is a slurry unless the phosphoric acid is concentrated or the slurry is dried (D), in which case it is a solid product practically identical to triple superphosphate. T h e principal reaction in this step is Calo(P04)6F2+ 12H3P04 = 9Ca(H2PO4I2+caF2

(6) The monocalcium phosphate may be hydrated, depending on reaction conditions; (E) dissociation of the monocalcium phosphate with an organic solvent which dissolves the resulting phosphoric acid but not the dicalcium phosphate or impurities. The principal reaction in this step is Ca(H2PO4I2= CaHP04 + ( 1)

(F) filtration of the dissociation products to separate a solution of purified phosphoric acid in the organic solvent and water from the solid dicalcium phosphate which contains impurities. Washing of the filter cake with make-up solvent may be carried out; washings would be recycled to step E; (G)separation of purified phosphoric acid and water from the solvent by fractional distillation; (H) stripping of the occluded organic solvent and water from the dicalcium phosphate and recovering it for recycle to step E. The process as described above produces two products, namely a relatively pure if acid, which would be suitable for fertilizer production and many industrial applications, and an impure dicalcium phosphate, which would be suitable as a fertilizer. Alternatively, however, the process can be used to produce only one product if desired, namely, either pure phosphoric acid or impure dicalcium phosphate. If only pure phosphoric acid is desired, all of the dicalcium phosphate is recycled to step A, as indicated by the dotted line labeled Recycle I on the flowsheet. In this case the dicalcium phosphate is converted back to crude phosphoric acid and calcium sulfate, and the impurities are removed in step B with the calcium sulfate. If only dicalcium phosphate is desired, all of the phosphoric acid is recycled to step C, as indicated by the dotted line labeled Recycle 11, where it is converted to mono-

418

Ind. Eng. Chem. Process Des. Dev., Vol. 20,No. 3, 1981 SULFURIC AS10

PHOSPHATE

IMP

B FILTRATION 5

WATER

WATER

GYPSUM IMP

e l

7

'

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-

.

C SOLVENT SEPARATION A

SOLVENl MCP =MONOCALCIUM PHOSPHATE OCP= OICALCIUMPHOSPHATE IMP = IMPURITIES

Figure 2. Process in which phosphoric acid is the sole product. SULFURIC AClO

PHOSPHATE ROCK

B FILTRATION

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ROCK

IMP WATER

GYPSUM IMP WATER

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-

n SOLVENT STRIPPING

-

1

5 MCP OCP IMP SOLVENT

I

1

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16

SOLVENT

12 hP04 SOLVENT

*

.

G SOLVENT SEPARATION

MCP = MONOCALCIUM PHOSPHATE OCP = OlCALClUM PHOSPHATE IMP = IMPURITIES

Figure 3. Schematic diagram for process with solid product containing dicalcium phosphate and with recycle of low-impurity phosphoric acid.

calcium phosphate. In this case no dicalcium phosphate would be recycled. It is therefore clear that by the choice of recycle streams the product from the process could consist of only phosphoric acid, only dicalcium phosphate, or any proportion of the two. The process may be operated with dissociation of a monocalcium phosphate slurry in step E, as described above, or with dissociation of a solid monocalcium phosphate, in which case water is evaporated from the crude phosphoric acid after step B or from the monocalcium phosphate slurry after step C or both. The choice as to whether water should be evaporated will depend on several factors, such as type of dissociation equipment to be used, concentration and purity of product phosphoric acid desired, and the like. A large number of solvents may be used; the choice will depend on effectiveness in promoting dissociation, cost, ease of separation, ease of recovery, and purity of phosphoric acid desired. It is, in general, necessary that phosphoric acid have high solubility in the solvent and dicalcium phosphate have low solubility in the solvent. Some solvents that have been found effective for the

dissociation in various degrees are methanol, ethanol, acetone, 1-propanol, 1-butanol, 3-methylbutanol, l-hexanol, 1-octanol, 6-methyl-1-heptanol, 2-ethylhexanol, l-decanol, 8-methyl-1-nonanol, methyl n-butyl ketone, and tetrahydrofuran. An example of a process based on the solvent-dissociation reaction in which phosphoric acid is the sole product (Figure 2) shows that the dicalcium phosphate produced by the dissociation of monocalcium phosphate is recycled to the acidulation step (A). In this example, the crude monocalcium phosphate is dried (D)to about 47% Pz06 content before dissociation (E). After removal of the solvent (G),the product phwphoric acid will contain about 54% P206, which is the concentration of most commercial phosphoric acid. This product will be much lower in impurity content than most commercial phosphoric acid. These results are achieved without use of an acid concentration step per se; however, water removal is achieved during drying of crude monocalcium phosphate. An example of a process based on the solvent-dissociation reaction in which dicalcium phosphate is the sole product (Figure 3) shows that the phosphoric acid pro-

Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 410

duced by the dissociation of monocalcium phosphate is recycled and mixed with impure phosphoric acid and reacted with phosphate rock to produce monocalcium phosphate (C). The product stream shown in the flowsheet contains a substantial amount of undissociated monocalcium phosphate and therefore contains a significant amount of water-soluble P2OP This can be reduced to a lower level by the inclusion of additional dissociation steps in which the solid product is further treated with solvent. A further variation of a process for producing impure dicalcium phosphate as the sole product is elimination of the filtration step B in Figure 3, thereby allowing the gypsum and other impurities to remain in the reaction streams and eventually leave the process as constituents of the product. This modification will reduce cost of operating the process because gypsum filtration equipment and operation will be omitted. The product dicalcium phosphate will not be as concentrated in P2O5, but for some applications this would not be a great disadvantage.

Experimental Section Experiments were made to determine the suitability of various solvents and to determine optimum operating conditions for each of the steps in the process, included in the experiments were the several variations described above, using Florida and North Carolina concentrates. Preparation of Crude Phosphoric Acid. Pulverized Florida phosphate rock containing 34.3% Pz05,49.5% CaO, 0.25% MgO, 1.33% Fe2O3, 1.28% A1203,and 3.84% F was treated with sulfuric acid and wet-process phosphoric acid containing 25% P205(to simulate wet-process acid production processes) at 75-80 "C, and the reaction product was filtered. The resulting phosphoric acid contained 31.0% P2O5,0.13% CaO, 0.30% MgO, 1.18% Fe203, 0.66% Al2O3, and 1.91% % F. Another batch of phosphoric acid was prepared from a North Carolina phosphate rock containing 32.9% P2O5,54.0% CaO, 0.27% MgO, 0.80% Fe2O3, 0.80% A1203,and 3.99% F; this acid contained 26.9% P2O5,0.10% CaO, 0.52% MgO, 2.09% Fe2O3, 1.40% A1203, and 1.30% F. Preparation of Crude Monocalcium Phosphate. The crude phosphoric acid was reacted with additional finely ground phosphate rock in the proportion required to result in a final CaO:P205mole ratio equal to 1.00. The resulting product was a slurry of crude monocalcium phosphate. Some of the monocalcium phosphate slurry was dried to various degrees to give thick pastes and solid products. The slurry made from Florida rock contained 31.9% PZO5,13.1% CaO, 0.21% MgO, 1.20% Fe203,0.98% A1203,and 2.46% F. The solid product made from North Carolina rock contained (after drying) 46.6% PZO5,19.4% CaO, 0.80% MgO, 2.54% Fez03, 1.83% Al2O3, and 1.32% F. Dissociation of Crude Monocalcium Phosphate with Various Organic Solvents. Crude monocalcium phosphate made from Florida phosphate rock was treated with various organic solvents. The treatment time was 2-4 h, which was established as sufficient to result in equilibrium conditions. The reaction temperature was 70 "C except for methanol (55 "C) and acetone (50 "C). Other experiments at 25 "C showed little differences between those at the above temperatures. The product phosphoric acid was separated from the insoluble residue by filtration. The results (Table I) show a varied degree of monocalcium phosphate dissociation when different solvents are used. The degree of dissociation is expressed as percent Pz05 yield in the product acid (the filtrate) based on the total Pz05in the initial monocalcium phosphate. It is apparent from reaction 1that complete dissociation would result in

Table I. Dissociation of Monocalcium Phosphate with Various Solventsa p105

solvent methanol ethanol 1-propanol 1-butanol isoamylalcohol 1-hexanol 2-methylpentano1 1-0ctanol 2-ethylhexanol CO-898 Umbrex N 1-decanol 1-dodecanol acetone me thy1 bu ty 1 ketone tetrahy drofuran

% impurities in acid (basis

in acid (% of MCP P,O,) 35.8 28.7 30.7 25.9 15.7 12.2 1.7

CaO

Fe,O,

0.64