Claims
- 1. A process for purifying crude wet-processed phosphoric acid containing 45-55% P.sub.2 O.sub.5 and several percent free sulphuric acid and further contaminants by adding to the phosphoric acid to be purified at an elevated temperature a calcium compound in equimolar proportions based on the sulphuric acid to precipitate the sulphuric acid in the form of calcium sulphate and avoid an excess of calcium; adding further to the phosphoric acid calcium sulphate seed crystals and at least one compound selected from the group consisting of alkali metal-, silicic acid and sulphidecompounds as precipitating agents for the further contaminants together with an adsorbent for organic contaminants; and separating the phosphoric acid so purified subsequently from solid matter present in the mixture obtained; adding to the crude wet-processed phophoric acid containing from 45 to 55 weight % of P.sub.2 O.sub.5 in a reaction zone, at a temperature of 80 to 100.degree. C, the calcium compound and the calcium sulphate seed crystals in the form of calcium sulphate semihydrate in the presence of the precipitating agents and active carbon as the adsorbent whereby easily filterable precipitates form; precipitating thereby the sulphuric acid in the form of calcium sulphate semihydrate together with the further contaminants with the resultant formation of a first suspension, passing off the said first suspension from the reaction zone; recycling a portion of the first suspension to the reaction zone; separating the balance of the first suspension into purified phosphoric acid and solid matter; preparing from the solid matter an aqueous second suspension containing up to 30 weight % of phosphoric acid and converting the calcium sulphate semihydrate in the aqueous second suspension at 20.degree. to 70.degree. C into calcium sulphate dihydrate; separating the aqueous phase of the aqueous second suspension from the solid matter therein, and water-washing the separated solid matter, recycling a portion of the separated aqueous phase and a portion of the wash water, from preparing the afore-mentioned aqueous second suspension containing the calcium sulphate semihydrate and combining the balance of the aqueous phase and wash water with the purified phosphoric acid whereby the sulphuric acid content of the purified phosphoric acid is minimized.
- 2. The process as claimed in claim 1, wherein a portion of the first suspension is recycled to the reaction zone after a period of 45 to 60 minutes.
- 3. The process as claimed in claim 1, wherein the calcium sulphate semihydrate in the aqueous second suspension is converted to calcium sulphate dihydrate at a temperature of 40.degree. to 50.degree. C.
- 4. The process as claimed in claim 1, wherein portions of the calcium compound, alkali metal compound(s), silicic acid, and sulphide compound(s), and the active carbon, which are employed are portions which provide a ratio by weight of calcium sulphate semihydrate to other solid matter in the first suspension, after precipitation, of approximately 7 : 1.
- 5. The process as claimed in claim 1, wherein phosphoric acid having been treated with hydrogen sulphide is used as the phosphoric acid to be purified.
- 6. The process as claimed in the claim 1, wherein the active carbon is added in a portion sufficient to establish in the first suspension, after precipitation, a ratio by weight of calcium sulphate semihydrate to active carbon within the range 10 : 1 to 15 : 1.
- 7. The process as claimed in claim 1, wherein the phosphoric acid to be purified contains fluorine in an amount of approximately 1 % by weight and H.sub.2 SO.sub.4 in an amount of at least 6.5 % by weight or alkali metal sulphate in a molar equivalent to the aforesaid amount of H.sub.2 SO.sub.4, all amounts being calculated on the P.sub.2 O.sub.5 -content of the phosphoric acid.
- 8. The process as claimed in claim 11, wherein the portions of the separated aqueous phase and of washwater are portions which provide the aqueous second suspension having a solids content of 10 to 20 weight %.
- 9. The process as claimed in claim 8, wherein the said portions are portions which provide suspension having a solids content of 12 weight %.
Priority Claims (1)
Number |
Date |
Country |
Kind |
2447390 |
Oct 1974 |
DEX |
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Parent Case Info
This application is a continuation of Ser. No. 618056, filed Sept. 30, 1975, now abandoned.
US Referenced Citations (10)
Foreign Referenced Citations (1)
Number |
Date |
Country |
37-3701 |
Jun 1962 |
JPX |
Continuations (1)
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Number |
Date |
Country |
Parent |
618056 |
Sep 1975 |
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