Wet-process phosphoric acid obtained from Kola apatite. Purification from sulphates, fluorine, and metals

The purification of pre-concentrated up to 80 wt.% H3PO4 phosphoric acid (PA) obtained from Kola apatite in one of the Polish plants by the wet processing route was carried out by sulphate precipitation, desorption of volatile components (SiF4, HF) and liquid-liquid extraction method using 4-methyl-2-pentanone (MIBK). The experiment was carried out on a laboratory scale. The effects of the reagent grade Ca(H2PO4)(2) . H2O, CaHPO4 . 2H(2)O, Ca,(PO4)(2), and technical grade calcium oxide, the molar ratio of Ca2+ to SO42- (0.8-1.5), the temperature (343-363 K), and the duration of precipitation time (1800-7200 s) on the extent of purification from SO42- were determined. The most efficient precipitant was CaHPO4 . 2H(2)O. The precipitation using CaHPO4 . 2H(2)O purified phosphoric acid from the initial SO42- concentration (1.5-1.8%) to a level of 0.1-0.2 wt.%. The use of 100% excess of SiO2 over the stoichiometric ratio (in relation to SiF4), while air bubbling and very intensive stirring of the phosphoric acid at 403 K was carried out, allowed the fluorine concentration to decrease to a level below 0.005 wt.% of F. Purification from metals was carried out at room temperature using 1:1.22 mass ratio of PA to MIBK. The stripped phosphoric acid, with a concentration of about 50 wt.% of H3PO4 and 1.5 wt.% of MIBK, contained Fe and Al at a level of 0.01-0.005 wt.% each, Pb, Th < 1 ppm, Cr, Co, Ni < 0.1 ppm, As, Cd-not detected. (C) 2002 Elsevier Science B.V. All rights reserved.