IL99356A - Process for the treatment of a residue which is obtained by the decomposition of phosphate rock by a mineral acid - Google Patents

Description

I 9 1 O L 8 PROCESS FOR THE TREATMENT OF A RESIDUE WHICH IS OBTAI ED BY THE DECOMPOSITION OF PHOSPHATE ROCK BY A MINERAL ACID mav piTsn lY-apnnn lviNua ^la'ia'.? πη The Applicants: ROTFM AMFFRT NECEV 1,1*0. 23.1 OTEJDK Dtm P.O.Box 435 . 435 .T.n Dimona 86103. 86103 n. n The Inventors: D ' N ' Ji oo n 1. Alexander IOSEF 5/12, Hakanaim St. 5/12 D'Kjpn mm ARAD . my 2. Meir BEHAR T n D ·>κη .2 30, Freud St. 30 ima mm HAIFA no n 3. Menachem BAR -ON 1 iN-m on JO .3 53/15, Elazar Ben-Yair 53/15 - Ντ-Ί-ϊ ITP^ mm ARAD my The present Invention relates to a process for the recovery of valuable constituents from the residue resulted by the decomposition of phosphate rock with a mineral acid and after extraction of the phosphoric acid. More particularly, the invention relates to a process for the recovery of valuable constituents from the residue resulted by the decomposition of phosphate rock by a mineral acid whose calcium salts are water soluble and separating out the clarified dissolution liquor.

BACKGROUND OP THE INVENTION.

The manufacture of phosphoric acid from phosphorus-containing minerals and roost particularly from phosphate rock is a long established and commercially important Industry. Two general classes of processes are used to recover the phosphoric acids - In one class of process, the phosphorus content of the rock is reduced to elemental phosphorus in an electric furnace and the recovered phosphorus is burned to P~Oe which is absorbed in water to form phosphoric acid. This approach, although producin a very pure product and without waste for disposal, is very expensive and its manu cture is now used only in small quantities.

- In the second class, the process involves the use of a mineral acid to produce the phosphoric acid which is recovered either b filtration, in case that a water insoluble calcium salt results, or by solvent extraction from a clarified dissolution liquor in case that the mineral acid produces a water-soluble calcium salt. This approach is now widely used, in view of the many improvements suggested in the last twenty years which yield a phosphoric acid product of a very high purity.

One of the drawbacks of the processes in the second class is that after the separation of the phosphoric acid, the resulted residue r which comprises the common impurities present in the commercial phosphate deposits of phosphate rock together with some acidic constituents, has to be disposed, off* This is Indeed a very serious problem in these days, particularly from the ecological considerations. Moreover, this residue contains significant amounts o valuable constituents including phosphorus values which accordingly are lost.

In addition to the phosphorus values, the main constituents of the residue are: CaF2, CaSO^, KjSiFg, CaCl2, (when HC1 is used as mineral acid) along the other impurities normally present in phosphate rock including various cations such as; Fe, Si, K, Zn, Mg, Na, etc.

In the following Table 1, are presented analyses of some main constituents present in residues obtained in the decomposition of several sources of phosphates with hydrochloric acid, after the removal of the clarified solution containing the phosphoric acid and calcium chloride.

TABLE 1I Weight percentage of the main constituents in the solids present in the residua.

Ion constiPhosphate Calcined phos Calcined tuent rock phate without phosphate washing after washing Ca"H' 34.4 36.5 32.2 F~ 13.1 20.6 16.2 6.1 1.6 P2°5 Considering the fact that during the washing step there are additional losses of phosphate values, the above, results clearly show that significant losses of PgO^ are incurred with any source of starting phosphate.

Another difficulty involved with the above residue is the problem of its separation, y filtration and washing which is practically impossible, or by settling in a thickener from the clarified solution. This also increases the costs of phosphoric acid production by the acidulation of phosphate rock with a mineral acid which produces a soluble calcium salt.

An Interesting use for the residue was suggested in the Israeli patent No. 64166, which consists in its transpor- tation of aald residue in places where sealing of earth ponds, reservoirs or canals is required. However, as mentioned above, this residue contains significant amounts of P2°5» wnAcn are actually lost and thus will increase the costs of the phosphoric acid manufacture.

It is an object of the present invention to provide a process for the recovery of valuable constituents from the residue resulted after the decomposition of phosphate rock with a mineral acid whose calcium salt is water soluble and removing the dissolution liquor ..It is another object of the present invention to reduce significantly the amount of the residue to be disposed which results in the manufacture of phosphoric acid produced by solvent extraction. It is yet another object of the present invention to reduce significantly the acidic constituents in said residue.

BRIEF DESCRIPTION OF THE INVENTION.

The invention relates to a process for the recovery of valuable constituents from the residue resulted by the decomposition o phosphate rock with a mineral acid selected from hydrochloric acid and nitric acid and removing the dissolution liquor, which comprises the treatment of said residue which contains ea++, p2°5 nd solids, with sulfuric acid, optionally in the presence of hydrochloric acid, producing a filterable calcium sulfate, the resulted filtrate which contains up to 50 g/1 Ca* and hydrochloric acid or nitric acid, depending on the mineral acid used in aald decomposition, being reused* It was found that when the calcium ion content in said filtrate is below SO g/1 and preferably below 30 g/1, the resulted calcium sulfate can be filtered rapidl .Furthermore the resulted cake can be easil washed and thus it will be possible to recover any valuable soluble constituents present therein. In this manner, the hydrochloric acid which is co-produced with the calcium sulfate by said acidulation, can be recycled in the process for the decomposition of phosphate rock. This is a substantial benefit to the phosphoric acid production via the solvent extraction technique.

Among the main impurities which are mostly found in the phosphate rocks it should be mentioned silica and fluoride, the latter constituent being generally present in the form of calcium fluoride. A typical analysis of the residue, after the removal of the dissolution liquor, shows that it contains about 2056 by wt.F~ and 10% Si02 (on dry basis). Upon the acidulation of said residue with sulfuric acid, the fluoride constituent is transformed into hydrofluoric acid, which subsequently will react with silica producing fluosilicic acid.This can be easily recovered as alkali fluosllicates, in the form of a pure precipitate by addition of sodium or potassium chloride. Alkali fluosllicates are valuable compounds being used for many purpoeee such ast an additive in drinking water, starting reagent for many fluoride compounds, etc. In this manner, the present invention enables in addition to the recovery o the phosphoric acid present in said residue along the hydrochloric acid resulted by its acidula-tion with sulfuric acid, also the removal of fluoride and silica constituents, and their recovery as useful compounds* Moreover, the removal of the fluoride constituent from the filtrat is of great importance, since it enables the utilization of the resulted fluoride-free hydrochloric acid, for a further decomposition of phosphate rock.

One of the findings of the present Invention is the fact that the calcium sulfate, which is produced by the acidu-latlon of the residue resulted after the removal of the dissolution liquor with sulfuric acid, appears in a form which can be easily filtered and washed. In this manner all water-soluble valuable constituents in the resulted cake, i.e. hydrochloric acid, phosphoric acid and fluoride compounds can be recovered as useful compounds. There are well-known the serious problems encountered in the manufacture of wet-process phosphoric acid, i.e. acidu-lation of phosphate rock with sulfuric acid and filtering out the phosphoric acid and particularly the washing of the resulted calcium sulfate, to remove the adhered acid. There are known many patents claiming various additives and types of filtration systems to improve the filtration rate as well as the washing efficiency of the resulted calcium sulfate.

The sulfuric acid to be used in said acidulatlon may be of any concentration and even a solution of only 10% by Weight and even obtained as a by product from any source, could be successfully utilized. On the other hand, a more concentrated sulfuric acid is most desirable since in this manner a more concentrated hydrochloric acid would result and this could be easily utilized in the decompositio of phosphate rock. As known, a high concentration of hydrochloric acid is most beneficial in the phosphoric acid manufacture via solvent extraction, since it will increase the distribution coefficient of the P2°5 towar<*s the solvent.

Another beneficial result connected with the concentre-tlon e ect of the hydrochloric acid in the production of said calcium sulfate, is a consequence of the water of crystallization which are bound to the calcium sulfate in the form of dihydrate thus resulting a more concentrated solution of hydrochloric acid.

The temperature which was found to be useful for the separation of the resulted calcium sulfate, after the acldulation with sulfuric acid, is generally in the range of 30 to 65°C. The reaction of the sulfuric acid with the residue, occurs smoothly even at a lower temperature than 30°C. However at this temperature some difficulties might be encountered in the filtration and washing of the resulted cake. It is suggested to carry out the filtration and washing at a temperature, above 0°C and most preferably above 50°C. At this temperature, both the filtration rate and washing are significantly improved, so that the valuable constituents present in said cake can be easil recovered.

Summing up, the present invention solves successfully by a simple process, two main problems encountered in the manufacture of phosphoric acid via a mineral acid which produces a soluble calcium salt, i.e. hydrochloric acid and nitric acldt - First, it enables to recover most of the valuable constituents which are present in the resulted residue after the removal of the clarified dissolution liquor - second, it reduces substantially the problem of disposal of said residue, by transforming the heavy volume of the "mud" containing significant amounts of acidic constituents, into a cake with a reduced volume and substantially free of acidic components, which can be easily handled* In this manner, the process according to the present invention increases significantly the yield of the PjOg recovered fron the phosphate rock and thus greatly improves the economy of the phosphoric acid produced b solvent extraction.

The invention will be hereinafter illustrated by a number of Exampies, eing understood, that these Examples are presented only for a better understanding of the invention, without being limiting thereto. A person skilled in the art, after reading the present speci ication, will be in a position to insert modifications without being outside the scope of the invention as limited by the appended Claims.

The concentrations as given in the Examples are by weight unless otherwise stated.

An amount of 513 g of a residue containig 256,5 g solids (14.5% on dry basis) and 256.5 of adhered acidic solution was obtained after the decomposition of phosphate rock (Oron, Israel) with hydrochloric acid and separating out the clarified dissolution liquor.

The analysi of said adhered dissolution liquor was a follows: 110 g/1 ?205' 105 g 1 c ** and "* free acidity. To the above residue it was added 1000 ml of a mixture of acids containing 9.53% h*2S04 and 23.8% HC1, obtained as a by product in a sulfonation reaction.

After agitation (laboratory mixer, 250 rpm), the mixture was heated at 50°c and filtered. The filtration of the calcium sulfate took only 35 seconds. The resulted filtrate which contained 3.4 g/1 Ca4+, 280 g/1 HC1 and 41 g/1 &2°5* weB added *° tne hydrochloric acid to be used for the dissolution of phosphate rock.

Example 2.

An amount of 407, g of a residue containing 203.5 g solids (20% on dry basis) and 203.5 g of adhered acidic solution was obtained after the decomposition of phosphate rock (Oron, Israel) with hydrochloric acid and separating out the clarified dissolution liquor. The analysis of the adhered solution was the same as in Example 1.

The residue was reacted with 1000 ml of the mixture of acids as used in Example 1. The resulted slurry was heated at 40°C; the filtration of the resulted calcium sulfate in this case took 76 seconds.

The resulted filtrate contained 2.2 g/1 Ca"*, 275 g/1 HCl Example 3.

The experiment as in Example 1 was repeated using 293 g of residue (17% solids on dry basis) and 442 g of adhered acidic solution obtained after the decomposition of phoephate rock with hydrochloric acid and removal out the clarified dissolutio liquor.

To the residue ah amount of 442 g of sulfuric acid with a concentration of 21.3% was added. The reaction mixture was heated to 5°C. The filtration of the resulted calcium sulfate took 40 seconds.

The filtrate obtained contained 3.8% g/1 Ca++, 13,6% by weight HC1 and 3.5% by weight P2°5* Example 4.

An experiment was carried out in a reactor in a continuous manner using 290 g/h of a residue containing 14.3% solids and 2.35 g/1 Ca++.

The solids composition (on dry basis) was as follows: - 9.9% Si02 - 18.26% Ρ"· - 8% organic matter, and - 5% water of crystallyzation.

To the residue it was added 740 ml/h of a mixture of acids (d°l.193) containing 10,7% sulfuric acid and 23.6% hydrochloric acid. The residence time was about 1 hour.

The amount of solids in the resulted slurry was 14%.

The reactor was heated to about 50°C; the resulted calcium sulfate dihydrate filtered easily, the filtration rate being 7.7 t/m 2.d. The humidity of the resulted cake was 37.7 .

The analysis of the filtrate obtained was as. follows: 2.1 g/1 Ca"^ 41 g/1 P2QS 284.3 g/1 HC1, and 0.83 % F~ The cake was washed couhtercurrently with 70 g/h of water arid the resulted solution contained 36.7 g/1 P2Q5 and 285 g/1 HC1, The humidity of the cake was 32% (on dry basis) and contained only 0.16% P205 and 0.73% hydrochloric acid. In this manner, the total yields of ^2°5 and HC1 were 98.8% and 99.4% respectively.

After the removal of the fluoride from the filtrate of the above cake, the resulted solution as well as the solution obtained from washing said cake, were added to the hydrochloric acid used in the decomposition of phosphate rock.

Claims (8)

- 13 - 99356/2 C L A I M S.

1. A process for the recovery of valuable constituents from the residue resulted by the decompos tion of phosphate rock with a mineral acid selected from hydrochloric acid and nitric acid and removing out the clarified dissolution liquor, which comprises the treatment of said residue which contains Ca++, P2O5 and solids comprising silicates, fluorides and with sulfuric acid, optionally in the presence of hydrochloric acid, producing a filterable calcium sulfate, the resulted filtrate which contains hydrochloric acid or nitric acid, depending on the mineral acid used in said decomposition, and up to 50 g/1 Ca++ to be reused in the decomposition of phosphate rock.

2. The process according to Claim 1, wherein said filtrate contains below 30 g/1 Ca++.

3. The process according to Claims 1 or 2, wherein the concentration of the sulfuric acid used in the acidula-tion is above 10% by weight producing a concentrated solution of hydrochloric acid.

4. The process according to Claims 1 to 3, wherein the filtration of the resulted calcium sulfate is carried out at a temperature in the range of 25° to 65°C.

5. The process according to Claim 4, wherein said filtration is carried out at a temperature above 40°C. j

6. The process according to Claims 1 to 4, wherein the i fluoride and silica present in said residue are trans-* formed into fluosilieic acid , which can be removed in the [ form of alkali fluosilicate by precipitation with sodium or potassium chloride.

7. , The process according to Claim 6, wherein the F~~ free filtrate obtained after the removal of said fluosilicate is recycled in the decomposition of phosphate rock

8. A process for the recovery of valuable constituents from the residue resulted by the decomposition of hosphate rock with a mineral acid selected from hydrochloric acid and nitric acid and removing out the clarified dissolution liquor, substantially as described in the specification and in any one of the previous Claims. For the Applicants, Simon Lavie . Patent Attorney