Abstract Phosphoric acids produced by subjecting phosphate ores to wetprocessing with mineral acids are purified by neutralizing them in a four step process by means of an alkali metal hydroxide and/or carbonate. In the first step, the crude phosphoric acids and the alkali metal hydroxide and/or carbonate are mixed together so as to produce a mixture with a pH-value of between 4 and 9, and resulting primary sludge consisting substantially of insoluble aluminum and iron phosphates is isolated from the purified phosphate solution. In the second step the sludge is mixed with a mixture consisting of alkali liquor (i.e. alkali metal hydroxide solution) and a trialkali phosphate solution and with a mixture consisting of water glass and a trialkali phosphate solution so as to produce a mash containing SiO2 and Al2O3 in a molar ratio of larger than 2:1, and dissolved alkali metal oxide and dissolved P2O5 in a molar ratio of between 2.9:1 and 3.3:1. In the third step the second step mash is heated to temperatures of between 80* and 100* C and sludge appearing in the mash is disintegrated. This results in the formation firstly of a solution and secondly of a solid residue which is isolated and discarded. In the fourth step, the third step solution containing between 0.1 and 1.0 weight percent fluorine, is cooled to effect crystallization of a trialkali phosphate/alkali fluoride double salt, which is isolated from its mother liquor, and successively used for purifying a portion of the crude phosphoric acid with the resultant formation firstly of a secondary neutralization sludge, which is discarded, and secondly of a purified alkali phosphate solution, which is combined with alkali phosphate solution purified in the first step.
The process as claimed in claim 1, wherein mother liquor coming from the fourth process step is the trialkali phosphate solution used in the second process step.
Claim 2
Dependent Claim
20 words
The process as claimed in claim 1, wherein the crystallization in the fourth process step is effected in two stages.
Claim 3
Dependent Claim
48 words
The process as claimed in claim 3, wherein the crystallization is effected in two stages, the crystallization in the first stage being effected at temperatures of between 37* and 70* C, and the crystallization in the second stage being effected at temperatures of between 10* and 37* C.
Claim 4
Dependent Claim
33 words
The process as claimed in claim 1, wherein the sludge is mixed in the second process step with alkali liquor diluted with trialkali phosphate solution in a ratio by weight of about 1:2.
Claim 5
Dependent Claim
33 words
The process as claimed in claim 1, wherein the sludge is mixed in the second process step with water glass diluted with trialkali phosphate solution in a ratio by weight of about 1:1.
Claim 6
Dependent Claim
64 words
The process as claimed in claim 1, wherein the mash produced by mixing the sludge with the alkali liquor or with the mixture of alkali liquor and trialkali phosphate solution is stirred in the second process step for a period of between 5 and 20 minutes, and the mixture of trialkali phosphate solution and water glass is successively added to the mash so stirred.
Claim 7
Dependent Claim
35 words
The process as claimed in claim 1, wherein the mixture produced in the second process step is heated to temperatures of between 80* and 100* C over a period of between 30 and 300 minutes.
Claim 8
Dependent Claim
41 words
The process as claimed in claim 1, wherein the heated mixture is diluted in the third process step with mother liquor coming from the fourth process step so as to establish a P2O5 concentration of between 7 and 10 weight percent.