Speciation and Conversion of Metastable Phosphate in the Acidic Leaching Solution of Converter Vanadium Slag

In the typical calcium roasting—acid leaching process of converter vanadium slag—the quality of the vanadium products obtained depends on the removal of impurities, particularly phosphorus which has complex species under different conditions. Owing to the formation of complex metastable phosphates, their removal from acidic leaching solutions is an enormous challenge. Therefore, the speciation and conversion of metastable phosphorus during the hydrometallurgical leaching process of vanadium were investigated. The results showed that at least five species of phosphates—orthophosphate, pyrophosphate, tripolyphosphate, trimetaphosphate, and P-heteropolyacid anions—were found in the acidic leaching solution. These phosphates were metastable and regularly converted depending on the solution concentration, standing time, and pH value. With an increase in the total phosphorous content, the chain-condensed phosphates—pyrophosphate and tripolyphosphate—were slightly hydrolyzed to orthophosphate, while the trimetaphosphate and heteropoly anions were more stable due to their ring or cage structures. When the leaching solution was set aside for 28 days, the orthophosphate content decreased from 60.8 to 34.4%, whereas the content of P-heteropolyacid anions increased from 6.2 to 31.0%. With an increase of acidity, the content of P-heteropolyacid anions increased significantly from 4.1 to 24.4%, whereas the orthophosphate content decreased from 63.7 to 43.9%, when the pH value decreased from 5.5 to 1.5. The conversion mechanism was discussed, and the results can provide theoretical guidelines to improve phosphorus removal in vanadium hydrometallurgy. Graphical Abstract