WO2005087661A1

WO2005087661A1 - A cyclic process for the continuous production of dicalcium phosphate

Info

Publication number: WO2005087661A1
Application number: PCT/IN2004/000062
Authority: WO
Inventors: Abburi Visweswara Rao
Original assignee: Abburi Visweswara Rao
Priority date: 2004-03-16
Filing date: 2004-03-16
Publication date: 2005-09-22

Abstract

The present invention relates to a cyclic process for continuous production of dicalcium phosphate from rock phosphate. Rock phosphate is treated with excess of spent acid or weak sulphuric acid under agitation. The slurry obtained is filtered and the filter cake is recycled for further leaching. Fresh rock phosphate is added to the filtrate under stirring followed by hydrated lime to increase the pH. This slurry is filtered and the filter cake is recycled to the first step. Hydrated lime is added to the filtrate which contains substantially pure mono calcium phosphate to convert the same to dicalcium phosphate. The filtrate obtained is recycled.

Description

A CYCLIC PROCESS FOR THE CONTINUOUS PRODUCTION OF DICALCIUM PHOSPHATE

This invention relates to a cyclic process for the continuous production of ^" dicalcium phosphate of substantial purity from rock phosphate. Substantially pure fluoride free feed grade dicalcium phosphate is produced continuously by the process.

Background of the invention: Dicalcium phosphate hereinafter also referred as DCP is a feed supplement for animals. Rock phosphate is a source for the commercial production of DCP. According to this invention rock phosphate is first treated with excess of weak sulphuric acid to produce a solution consisting of Ca, P₂O₅ and S0₄ ions. A further quantity of rock phosphate is then added to the leach and the pH of the leach is increased by adding hydrated lime or calcium phosphate. Substantially pure monocalcium phosphate is produced in solution thereby which is subsequently neutralized by the addition of lime to produce fluoride free DCP. Solids obtained is recycled to the first step for further leaching and the filtrate obtained after the removal of DCP is recycled to the first step as spent and gypsum is a by-product of this process.

Animal feed grade dicalcium phosphate is commercially produced by several processes. Prior art in this field mainly relates to acidulating rock phosphate with mineral acids such as sulphuric and hydrochloric acid to produce phosphoric acid which is then neutralized with calcium hydroxide or carbonate for producing DCP. This neutralization step is preceded by removal of fluoride ions from phosphoric acid solution. Prior art processes involve removal of almost the entire calcium content from rock phosphate. Required amount of calcium is then added for the preparation of DCP. Sulphuric acid is the preferred mineral acid to be used for treatment of rock phosphate and the acid produced in this reaction is phosphoric acid which is subsequently converted or hydrated lime after removal of fluorine. Removal of fluorine results in considerable loss of phosphoric acid in the form of phosphates. This prior art process suffers from low efficiency. For every kg of P₂0₅ produced by this method generates approximately 4.5 kg of gypsum. This process also consumes a very large quantity of hydrated lime.

Page no.2 pant 2

A process for the production of super phosphate with low free acid content is the subject of the PCT application no. PCT/IN03/00039. Mono calcium phosphate in the super phosphate is converted into DCP in this process. Yet another feature of the invention in PCT/IN03/00039 is in converting rock phosphate to monocalcium phosphate in the form of super phosphate and extraction of water soluble portion therefrom.^'

Objects of the invention: The present invention makes it possible to produce DCP using weak spent sulphuric acid without the cumbersome step of producing super phosphate (SSP). This invention also avoids the use of sulphuric acid in high concentration as against the prior art disclosure which requires 65 to 70% concentration of H₂S0₄. Spent acid concentration in the subject invention ranges from 10 to 35%o which is unsuitable for use in prior art processes unless the concentration is raised to the desired level. -Z- The objective of this invention is in using spent sulphuric acid of low concentration for leaching rock phosphate.

Yet another objective is to extract the maximum possible phospha te content from the rock phosphate by using excess of sulphuric acid. Page no.3 - pnra nos. 3,4 & 5

Another object of this invention is to minimize the use of calcium by using rock phosphate to produce mono calcium phosphate which is subsequently converted into dicalcium phosphate.

Cyclic and continuous process avoids loss of phosphate ions during removal o f fluorine in the course of the process.

This process enable to produce DCP from Rock Phosphate and Spent Sulpl ..uric Acid directly without lhe formation of super phosphate. Calcium content of the rock phosphate is utilized to the maximum and the consumption of lime is reduced to the minimum.

This process is carried out at a temperature range of 25 to 45°C which ensures minimum energy consumption.

Summary of the invention This invention relates to a cyclic process for continuous production of substantially pure dicalcium phosphate. This process comprises the steps of treating rock phosphate with excess of weak sulphuric acid or spent acid, filtering to remove filter cake therefrom adding a fresh batch of rock phosphate and sufficient calcium salt to raise the pH of the filtrate to 2.5 to 2.6 so as to produce monocalcium phosphate, filtering the slurry and neutralizing the filtrate with hydrated lime till the pH is increased to at least above 6 to precipitate dicalcium phosphate therefrom, separating the solids therefrom and recycling the filtrate to the first step. The solids separated from the slurry after the first treatment with weak sulphuric acid Ms recycled for further leaching with the spent acid obtained from the final step.

Brief description of figure: Figure 1 shows the process flow chart for producing continuously DCP- feed grade.

Description of the invention with reference to the flow chart

Page no.4 - para no.4

In step f, rock phosphate containing 27 to 37% of P₂θ5 content is ground to a fineness less than 90.% of standard 100 mesh. Even coarser rock phosphate may be used in this process. This rock phosphate is leached with a recycle solution from a subsequent process step or a dilute solution of sulphuric acid of 10 to 35% concentration. Acid addition is continued till the pH of the slurry is 0.8 to 1.2. Phosphate content in gypsum is kept about 0.5% and the P₂0₅ content is maintained after the completion of reaction at a level of 2.5 to 9%.

The slurry is filtered off and the cake is washed and leached with the solution obtained from the subsequent step to recover further water soluble P₂0₅. After completion of leaching the cake is-discarded as gypsum. The wash water is recycled to step I. The filtrate is treated with substantially equivalent quantity of rock phosphate as in step I and agitated till the pH reaches a level of 1.8 to 2.0. After achieving this pH the slurry is treated with hydrated lime to raise the pH to 2.5 to 2.6. At this pH, phosphate ions get converted into mono calcium phosphate and all the impurities are precipitated out. The filtrate obtained is further treated with hydrated lime to raise the pH to 6.2. DCP precipitated is filtered off and the filtrate is recycled to step I for supplementing acid wash. The settled solids from step 2 is further treated with spent sulphuric acid to raise the pH to 1.2 as in step I. Thus the process results in continuous precipitation of DCP at the end and gypsum produced at step I after repeated leaching is discarded.

Description of preferred embodiments: The following examples establishes the viability of this process. 1. 50 gms of rock phosphate of Egyptian origin, containing 32% of total P₂0₅ on dry basis and containing a moisture of 2.5% is ground to a fineness of 90% passing through 100 mesh and added with 120 gms of spent acid solution containing 37% of H₂S0₄ and with 175 gms of water. The contents are agitated thoroughly for 2 hours. The slurry attained a pH of 0.9. The slurry is filtered off. The filtrate weighed 184 gms with a P₂O₅ content of 5.49%. The filtrate showed a fluoride content of 0.172%. The Cake is washed with 175 gms of water. The wash solution weighed 180 gms and has a P₂0₅ content of 2.45%. The wet cake weighed 156 gms. The example showed that a P₂0₅ content of 1.09 grns is lost through the cake, and a recovery of 14.51 gms of P₂0₅ in both the solutions. This represents a recovery of 93%. The example shows that substantial recovery of phosphate is possible using spent sulphuric acid at a pH of 0.9. . 180 gms of the filtrate from above example is further treated with rock phosphate of 50 gms and agitated till a pH of 1.7 is attained. A quantity of 4 gms of hydrated lime with 68% available lime is made in to a slurry in 20 gms of water and added and agitated for about an hour. The slurry attained a pH of 2.52. The slurry is filtered by adding 40 gms of water to rinse the container of the slurry. The solution weighed 228 gms with a P₂0₅ content of 5.54% and a Fluorine content of 0.008%. The wet cake weighed 65 gms. The filtrate obtained is a substantially pure mono calcium phosphate.

3. The filtrate obtained in above example is treated with hydrated lime solution till a pH of 6.6, and filtered. The wet cake weighed 39.8 gms. The filtrate weighed 212 gms and exhibited a P₂0₅ content of 0.12%. The cake is dried to obtain DCP, and dried cake weighed 29.97 gms. The dry product has a P content of 18.14 and Fluorine of 0.062%o. This example shows that a substantially pure DCP could be obtained from the filtrate in Experiment 2.

4. The wet cake obtained in Experiment 2 is added with 120 gms of spent sulphuric acid as in Experiment 1 and the wash solution of 175 gms obtained in experiment 1 is also added. The contents arc agitated for 2 hours and the slurry attained a pH of 1.04. The slurry is filtered. The filtrate weighed 255 gms with a P₂0₅ content of 5.71%. The cake is further washed with 180 gms of filtrate obtained in Experiment 3 and filtered. The wet cake weighed 98 gms with a total P₂0₅ content of 0.75%. The wash solution weighed 187 gms with a P₂O₅ content of 1.10%. This experiment shows that the cake obtained in Experiment 2 can be treated again with spent sulphuric acid to recover substantially all the phosphates. . 250 gms of filtrate obtained in experiment 4 is further added with 70 gms of rock phosphate till a pH of 1.85 is obtained. 4 gms of hydrated lime solution containing 25 gms of water is added to the slurry to raise the pH to 2.46. The slurry is then filtered off. The filtrate weighed 250 gms with ^' a P₂0₅ content of 6.77% and" Fluorine content of 0.008%. The cake weighed 99 gms. This experiment shows that all the filtrate obtained in the process can be recycled and using rock phosphate, pure mono calcium phosphate solution can be obtained.

6. The filtrated obtained in the above experiment is again treated with hydrated lime to obtain DCP. 250 gms of filtrate from the experiment 5 is treated with hydrated lime to a pH of 6.5 and obtained 56.9 gms of wet cake of DCP. The product when dried exhibited a P content of 18.11% and a fluoride content of 0.05%.

7. The product obtained in Experiments 3 and 6 was further analysed and the product has a Ca content 22.9%, citrate solubility of 98.3%, acid insolublcs of 0.32% and loss on ignition of 21.2% thus confirming that the product obtained is a good quality feed phosphate.

The yield of P₂O₅ from this process is about 90 to 94% depending upon the quality of rock phosphate used. The process efficiency is also dependent on the acid strength with the minimum strength being as lov as 8%. Spent or weak sulphuric acid which is otherwise difficult to dispose of, is utilized this way.

Claims

CLAIMS:

1. A cyclic process for continuous production of substantially pure dicalcium phosphate comprising the steps of (1) leaching rock phosphate ^" with spent or weak sulphuric acid in excess under agitation; (la) separating the filter cake from the slurry obtained; (2) adding fresh rock phosphate to the filtrate obtained from step (la) under stimng, followed by the addition of calcium carbonate or hydrated lime to raise the pH to produce mono calcium phosphate; filtering the slurry and neutralizing the filtrate with hydrated lime to convert mono calcium phosphate to dicalcium phosphate, separating the precipitated dicalcium phosphate therefrom; recycling the filter cake obtained in step (la) and step (2) for further leaching in step (1), and recycling the filtrate obtained in step 3 to step (la) as cake wash.

2. The cyclic process as claimed in claim 1 wherein the rock phosphate is ground to a size less than 100 mesh.

3. The cyclic process as claimed in claims 1 and 2 wherein weak sulphuric acid or spent acid containing up to 35%> of H₂SO4 is added to rock phosphate and leached till the slurry obtained shows a pH of 0.9.

4. The cyclic process as claimed in claims 1 and 2 wherein the filter cake obtained from the slurry is washed and the wash water recycled to step (1).

5. The cyclic process as claimed in claims 1 to 4 wherein fresh rock phosphate added to the filtrate is equivalent to the initial quantity of rock phosphate added.

6. The cyclic process as claimed in claim 5 wherein calcium carbonate or hydrated lime is added to raise the pH to about 2.5 to 2.6.

7. The cyclic process as claimed in claim 6 wherein hydrated lime is added to the filtrate obtained in step 3 till the pH is raised to about 6.2.

8. Dicalcium phosphate produced by the cyclic process as claimed in any of the preceding claims.