GB2094282A

GB2094282A - Phosphoric acid manufacture

Info

Publication number: GB2094282A
Application number: GB8205334A
Authority: GB
Original assignee: DHARAMSI MORARJI CHEMICAL
Current assignee: DHARAMSI MORARJI CHEMICAL
Priority date: 1981-02-27
Filing date: 1982-02-23
Publication date: 1982-09-15
Also published as: GB2094282B; IN154778B

Abstract

Phosphoric acid is manufacured by the use of fluosilicic acid with or without phosphoric acid, at a temperature between ambient and 100 DEG C until the phosphate and fluorine content of the rock phosphate is solubilised, the slurry being filtered and the whole or a part of this filtrate of the slurry being concentrated until calcium silicofluoride is precipitated, the slurry being cooled and filtered to give product phosphoric acid and calcium siliofluoride as a residue.

Description

SPECIFICATION Process for the manufacture of phosphoric acid and cement grade gypsum This invention relates to a process for the manufacture of phosphoric acid and cement grade gypsum. The starting material for the process of this invention, as in conventional processes, for manufacture of phosphoric acid is rock phosphate. Rock phosphate obtained from foreign sources as well as indigenous sources has varying percentages of P2O5.

This invention provides a process for the preparation of phosphoric acid and cement grade gypsum from imported rock phosphate as well as virgin, unbeneficiated and coarsely ground indigenous rock phosphate, the latter containing a high percentage of silica and correspondingly low percentage of P205.

In the conventional processes to prepare phosphoric acid from rock phosphate, the preferred samples of rock phosphate have a low percentage of impurities such as silica and a correspondingly high pecentage of P205. A sample of such rock phosphate is finely ground and is reacted with sulphuric acid to give phosphoric acid and calcium sulphate either as hemihydrate (CaSO4. zFH20) or as dihydrate (CaSO4. 2H2O).

The conventional processes cannot be applied with advantage to rock phosphate which contains more than 10% silica.

The conventional processes require finely ground rock phosphate for the reaction. When rock phosphate contains a high percentage of silica, fine grinding involves higher power consumption and makes the process comparatively more expensive. This invention also provides a process for manufacturing phosphoric acid and cement grade gypsum from virgin, unbeneficiated coarsely ground rock phosphate having silica content higher than that permissible in the coventional process and correspondingly lower P205 content.

This invention provides a process for manufacture of phosphoric acid wherein fluosilicic acid, with or without phosphoric acid, is used two attack rock phosphate at a temperature between ambient and 1 00 C until the phosphate and fluorine content of rock phosphate is solubilised, the slurry being filtered and the whole or a part of this filtrate of the slurry being concentrated until calcium silico fluoride is precipitated, the slurry being cooled and filtered to give product phosphoric acid and calcium silico fluoride as residue.

This invention further provides a process for manufacture of phosphoric acid wherein fluosilicic acid has strength of 8/12%, preferably above 10%, H2SiF6 and phosphoric acid has strength of 18/22% P2Os, preferably 20% P205 and the reaction period is half an hour to four hours depending upon the nature of rock phosphate.

This invention further provides a process for the manufacture of phosphoric acid and cement grade gypsum wherein rock phosphate is reacted with a mixture of fluosilicic acid and phosphoric acid at a temperature between ambient and 1 00 C until the phosphate and fluorine contents of the rock phosphate are solubilised, the slurry being filtered to remove insoluble impurities and silica, the filtrates being divided into two unequal portions, A and B, the portion B being smaller than A, the portion B being concentrated until calcium silico-fluoride is precipitated, the resulting slurry being cooled and filtered to yield the product phosphoric acid and a cake of calcium silico-fluoride, which cake is fed into a crystalliser containing seed crystals of gypsum slurry along with the aforesaid portion A and sulphuric acid, the reaction being allowed to continue in the crystalliser until gypsum crystals are formed, the resulting slurry being filtered to give cement grade gypsum crystals as residue and the filtrate containing a mixture of fluosilicic acid and phosphoric acid, the filtrate being recycled to commence the process with a fresh quantity of rock phosphate. The principal object of this invention is to achieve solubilisation of the phosphate and fluorine contents of the rock phosphate being processed.

How the invention may be carried out will now be described, by way of example only, and with reference to the accompanying drawings in which the single figure is a flow-chart of the process according to the present invention.

In the process of this invention, rock phophate 1 is reacted in a reactor 3 with a mixture 1 7 of phosphoric acid (18 to 22%. preferably 20% P205) and fluosilicic acid (8 to 12%, preferably above 10% H2SiF6) at a temperature between ambient and 100"C but preferably around 70 C, for a period of time between a half to four hours but, preferably two hours.

The phosphoric acid is added as aforesaid, as a media. The reaction results in nearly complete dissolution of phosphate and fluorine contents of the rock phosphate. The reaction product is filtered by filter 4 to remove silica mud and other solid impurities 5. The filtered silica mud and other solid impurities are washed several times and each was is added to the filtrate. The filtrate comprised of phosphoric acid, calcium silico-fluoride, fluosilicic acid and small quantities of dissolved impurities. According to the invention, the filtrate from filter 4 is then divided into two unequal streams, 6 and 7, designated A and B, 7 being the minor stream. Stream 7 will contain P205 equivalent to the P205 put in earlier in the form of rock phosphate. The filtrate in the minor stream 7 is made to flow into a concentration unit 9.During the process of concentration, water vapour 10 leaves the unit and the phosphoric acid is upgraded from the initial strength of 18 to 22% W/W P205 to 45 to 52% W/W P205 and solid calcium silico fluoride is precipitated. The slurry in the concentration unit 9 is cooled in a heat exchanger 11 and filtered by filter 1 2 to obtain phosphoric acid 13, which is one of the products sought by this invention. The residue consisting of the wet cake of silico-fluoride 1 4 and the filtrate of the major stream 6 are fed into a crystalliser 8 containing seed crystals of gypsum. Sulphuric acid 2 of 98.5% W/W strength is simultaneously added into the crystalliser 8. The slurry is maintained at a temperature between 60 and 65"C, with agitation for adequate growth of gypsum crystals.In the course of the reaction, further gypsum crystals are formed and fluosilicic acid is regenerated. The gypsum crystals are removed from the slurry by filtration by a filter 1 5. The filtration, after several washing operations, gives a product of gypsum 1 6 which is the second product sought by this invention. The filtrate 1 7 contains a mixture of phosphoric acid from the stream 6 and regenerated fluosilicic acid. This acid mixture 1 7 is recycled to the reactor 3 for the re-initiation of the process of this invention.

The chemistry of the several reactions involved in this invention is expressed by the following equations: 1. REACTION: 3Ca3(PO4)2.CaF2 + 10 H2SiFS + H3PO4

10 CaSiF6 + 6 H3PO4 + 2HF + XH3PO4 SiO2 4HF

SiF4 + 2H20 SiF4 + 2 HF

H2SiF6 II.CONCENTRATION: H3PO4 + CaSiF6 + 2H20

H3PO4 + CaSiF6. 2H20 > l Ill. CRYSTALLISATION OF GYPSUM: PRECIPITATION: CaSiF6 + 2H20 + H3PO4 + H2SO4

CaS04. 2H2 + H2SiF6 + H3PO4 (Acid mixture for recycle) One of the most striking features of the process of this invention is the near complete dissolution of fluorine content of the rock phosphate, thus increasing the availability of fluorine by-products in substantial quantities for the preparation of the various chemical products like SFS, AIP3 and Cryolite.

Since the fluorine content of rock phosphate gets almost completely dissolved in the process of this invention, there is little evolution of gaseous fluorine as such during the reaction. In effect, the solubilised fluorine is totally recycled in the process of this invention.

This invention accordingly provides a process for manufacture of phosphoric acid and cement grade gypsum from rock phosphate comprising the following steps: (a) Treating in a reactor the rock phosphate as hereinbefore defined at a temperature between ambient and 1 00on, preferably around 70"C, with a mixture of phosphoric acid (18 to 22%, preferably 20% W/W P2Os) and fluosilicic acid (8 to 12% preferably above 10% W/W H2SiF6) for a period of time during which the phosphate and fluorine contents of rock phosphate are solubilised from a half hour to four hours, preferably two hours; (b) Filtering the product of reaction (a) to remove insoluble silica mud, and other insoluble impurities which are washed several time and washings being added to the filtrate;; (c) Dividing the filtrate and washings obtained at the end of step (b) into two unequal streams, designated A and B for defining this invention in the claims, B being the smaller of the two; (d) Concentrating by vacuum evaporation, the filtrate in stream B until the strength of P205 reaches 45% to 52% W/W P2O5; preferably 45% W/W P2O5, when practically all the calcium silicofluoride precipitates; (e) Cooling and filtering off the cake of wet crude calcium silicofluoride leaving phosphoric acid as the filtrate; ; (f) Feeding the cake of wet calcium silicofluoride obtained at the end of step (e) along with the major stream A into a crystalliser containing seed crystals of gypsum slurry from an earlier charge, adding simultaneously sulphuric acid of strength 98.5% at a temperature between 60"C and 65"C over a period of 3 hours with agitation, and filtering off the gypsum crystals formed; (g) Recycling the filtrate obtained at the end of step (f) which contains phosphoric acid and fluosilicic acid to react with a fresh supply of rock phosphate. This invention is now described and illustrated by the following examples: EXAMPLE 1.

200 gms. of unbeneficiated coarse rock phosphate (1) with partical size of - 8 to + 25 mesh was selected which, in analysis, showed the following composition: P205 33% CaO : 45.20% F . 3.20% SiO2 15% R203 : 2.40% CO2 : 1.00% NaCI 0.10% SO3 0.40% The rock phosphate sample was reacted with 2090 gms. of acid mixture containing 20.0% P205 W/W, 11.91% H2SiF6 W/W and 0.5% H2SO4 W/W at 70"C. for a period of 2 hours in a well stirred reactor (3). The slurry was filtered to remove 1 30 gms. of wet insoluble impurities comprising quartz, mud, earthy matter and gypsum. The filtrate and washings weighing 2260 gms. contained practically 98% phosphate and 95% fluorine content of the rock phosphate, in solution.Analysis of the filtrate gave P205 = 21.357% W/W Fluroine expressed as H2SiF6 = 11.354% W/W This mass of filtrate and washings was divided into two streams: Stream 6 of 1 860 gms. and stream 7 of 400 gms. The stream 7 was concentrated by vacuum evaporation at 10 mm of mercury pressure and 80"C. temperature. When the P205 reached a level of 45% W/W, crystalline calcium silico fluoride dihydrate, CsSiF6. 2H20 was thrown out. The slurry was cooled to room temperature and filtered. It gave 85 gms. of wet crude cake of crystalline calcium silicofluoride dihydrate and 1 40 gms. of product phosphoric acid containing 45% P205 W/W in the filtrate. A total quantity of 1 75 gms. of water, was distilled off in the above process.The recovery of P205 from rock phosphate to product acid was about 95%.

The 85 gms. cake of wet crude, crystalline, calcium silicofluoride dihydrate, CaSiF6. 2H20, was then added into a well stirred crystalliser 8 along with the major stream 6, containing 1 860 gms. of the aforesaid filtrate from the earlier stage 4 and was reacted with 1 55 gms. of 98.5% W/W H2SO4, which was simultaneously added to the same crystalliser 8 at 65"C temperature, during the course of three hours. On filtration and washing, this reaction gave 300 gms. of wet gypsum cake, CaSO4. 2H20. This gypsum was dazzline white in colour and contained a maximum of 0.3% P205 and 0.2% F impurities on a day basis. This is of a quality which is required for use in cement manufacture.This step also resulted in a filtrate comprising 2000 gms. of acid containing 20.9% W/W P205 and 12.54% W/W H2SiF6. This filtrate after suitable dilution with 90 ml wash water, resulted in 2090 gms. of recycled acid containing 20% W/W P205 12.0% W/W H2SiF6. The said mixture was recycled 1 7 to the reactor 3 for a fresh lot of 200 gms. of rock phosphate.

EXAMPLE 2.

In this example, the rock phosphate selected has a lower precentage of P205 and a higher percentage of silica than in the Example 1.

200 gms. of low grade unbeneficiated coarse rock phosphate with particle sizes of - 8 to + 25 mesh and containing high silica impurities and low P205 and other consitituents as specified in the analysis given below: P205 20.04% CaO 26.65% R203 4.77% F 2.00% SiO2 43.32% LO.I at 1000"C 2.89% was reacted with 1 263 gms. of recycled acid mixture containing 20% W/W P205, 11.96% W/W H2SiF6 and 0.5% W/W H2SO4 at 70"C for a period of two hours in well stirred reactor 3. The slurry was filtered to remove 1 70 gms. of wet insoluble impurities comprising quartz, mud, earthy matter and gypsum. The filtrate and washings weighing 1 393 gms. Contained practically 96% phosphate and 90% fluorine content of the rock phosphate in solution.The analysis of the filtrate gave: P206 . 20.90% W/W Fluorine expressed as H2SiF6 . 11.17% W/W This mass of filtrate and washings was divided into two streams: Stream 6 containing 11 53 gms. and stream 7 containing 240 gms. The stream 7 was then subjected to concentration by vacuum evaporation at 10 mm of mercury pressure and 80"C temperature. When the concentration of P205 in the stream 7 reached a level of 45% W/W, crystalline calcium silicofluoride dihydrate CaSiF6. 2H20 was thrown out. The slurry was cooled to room temperature and filtered off to give 50 gms. of wet crude cake of crystalline calcium silicofluoride dihydrate and 93 gms. of phosphoric acid containing 45% P205 W/W in the filtrate as the product sought by this invention.A total quantity of 107 gms. of water was distilled off in the above process. The recovery of P205 from rock phosphate to product phosphoric acid was about 93%. The 50 gms. cake of wet crude crystalline calcium silicofluoride dihydrate, CaSiF6. 2H20, was then added into a well stirred crystalliser 8 along with the major stream 6 containing 11 53 gms. of the aforesaid filtrate from the earlier stage and was reacted with 94 gms. of 98.5% W/W H2SO4 which was simultaneously added to the same crystalliser at 65"C temperature during the course of three hours. On filtration and washing, this reaction gave 1 80 gms. of wet gypsum cake, CaSO4. 2H2O. This gypsum was dazzling white in colour and contained a maximum of 0.3% P206 and 0.2% F as impurities on a dry basis. Such gypsum is of a quality required for use in cement manufacture. This last step also resulted in a filtrate comprising 1 238 gms. of acid containing 20.425% P206 and 12.207% W/W H2SiF6. The said filtrate after suitable dilution with 25 ml. of wash water resulted in 1 263 gms. of acid mixture containing 20.0% W/W P2Os and 11.96% W/W H2SiF6. The said mixture was recycled to the reactor 3 for a fresh lot of 200 gms. of rock phosphate.

Claims

1. A process for the manufacture of phophoric acid wherein fluosilicic acid, with or without phosphoric acid is used to attack rock phosphate at a temperature between ambient and 100"C until the phosphate and fluorine content of rock phlosphate is solubilised, the slurry being filtered and the whole or part of this filtrate of the slurry being concentrated until calcium silicofluoride is precipitated, the slurry being cooled and filtered to give product phosphoric acid and calcium silico-fluoride as residue.

2. A process, as claimed in claim 1, wherein the fluosilicic acid has strength of 8% to 12%, preferably above 10% H2SiF6 and phosphoric acid has strength of 18% to 22% P205 preferably 20% P205 and the reaction period is a half hour to four hours depending upon the nature of the rock phosphate.

3. A process as claimed in either claim 1 or 2, wherein rock phosphate is reacted with a mixture of fluosilicic acid and phosphoric acid at a temperature between ambient and 100"C until the phosphate and fluorine contents of rock phosphate are solubilised, the slurry being filtered to remove insoluble impurities and silica, the filtrate being divided into two unequal portions, A and B, the portion B being smaller than A, the portion B being concentrated until calcium silico-fluoride is precipitated, the resulting slurry being cooled and filtered to yield the product phosphoric acid and a cake of calcium silico-fluoride, which is fed into a crystalliser containing seed crystals of gypsum slurry along with the aforesaid portion A and sulphuric acid, the reaction continuing in the crystalliser until gypsym crystals are formed, the resulting slurry being filtered to give cement grade gypsum crystals as residue and the filtrate containing fluosilicic acid and phosphoric acid, the filtrate being recycled to commence the process with a fresh quantity of rock phosphate.

4. A process for manufacture of phophoric acid and cement grade gypsum from rock phosphate comprising the following steps: (a) treating in a reactor rock phosphate at a temperature between ambient and 100"C, preferably around 70"C, with a mixture of phosphoric acid (18% to 22%, preferably 20% W/W P205) and fluosilicic acid (8% to 12%, preferably above 10% W/W H2SiF6) for a period of time during which the phosphate and fluorine contents of rock phosphate are solubilised, from a half hour to four hours, preferably two hours; (b) Filtering the product of reaction (a) above to remove insoluble silica mud and other insoluble impurities which are washed several times and washings being added to the filtrate;; (c) dividing the filtrate and washings as obtained at the end of step (b) into two unequal streams, designated A and B for the sake of reference, B being the smaller of the two and containing P205 equivalent to P206 put in earlier in the form of rock phosphate; (d) concentrating by vacuum evaporation the filtrate in the stream B to strength of 45% to 52% P205, preferably 45% W/W P205; (e) cooling and filtering off the cake of wet crude calcium silicofluoride leaving phosphoric acid as the filtrate;; (f) feeding the filtered wet cake of calcium silicofluoride as obtained at the end of step (e) along with the major stream A into a crystalliser containing seed crystals of gypsum adding simultaneously sulphuric acid of strength 98.5% W/W, at a temperature between about 60"C to 65"C, with agitation over a period of three hours and filtering off the gypsum crystals formed; and (g) recycling the filtrate obtained at the end of step (f) containing mixture of phosphoric acid and fluosilicic acid to a fresh supply of rock phosphate.

5. A process for the manufacture of phosphoric acid and cement grade gypsum substantially as described in example 1 and with reference to the accompanying flow-chart.

6. A process for the manufacture of commerical phosphoric acid and cement grade gypsum substantially as described in example 2 with reference to the accompanying flow-chart.