Phosphorus release from low-grade rock phosphates by low molecular weight organic acids

To understand phosphorus release from low-grade rock phosphates (PRs) by low-molecular weight organic acids (LMWOAs) and the factors influencing P release from PRs, we investigated the characteristics of P release from two low-grade PRs, selected from Baokang and Zhongxiang. Hubei province of Central China, using seven LMWOAs and other two inorganic acids. The amount of P release from PRs with different acids and equilibrium pH were measured and the different factors in P release were also compared. Laboratory studies showed that the average amounts of P released by all the organic acids from the two PRs increased with the concentrations of organic acids. The effectiveness of the organic acids in releasing P from PRs followed the order, i.e., oxalic acid > citric acid > tartaric acid > formic acid > malic acid > succinic acid > acetic acid. The P released from PRs was negatively correlated with the equilibrium solution pH. The kinetic of PRs dissolution showed that the dissolving process was described with different equations. The types and concentrations of the acids, liquid/solid ratio and PR powder particle size had great impact on the P release from PRs. In addition, the amount of P released from PRs was related to the nature of the PRs, leaching time and temperature.