Relative importance of protons and solution calcium concentration in phosphate rock dissolution by organic acids

A series of experiments was conducted to quantify the relative contribution of protons and other mechanisms to the dissolution of phosphate rocks (PRs ) from six countries in solutions of low-molecular-weight-aliphatic organic acids. The amounts of P and Ca released after 3d of incubation at 28 degre es C were determined in all the experiments. In the first experiment the so lubility of the PRs (< 500 mu m particle size) in 100 M (25 mL g(-1) PR) ox alic, tartaric, and citric acids was compared with that in three mineral ac ids and four chelating compounds. There were no differences in the amount o f P released by the mineral acids, but the organic acids released more P th an could be accounted for by protonation. The chelating compounds were the least effective. In the second experiment, 1 g each of Sri Lanka and Togo P Rs was incubated with 2.5 mmol of the organic acids using three acid concen tration (mM):acid volume ratios (250:10, 100:25, and 25:100). The amount of P dissolved from the PRs and the amount of acid remaining in the filtrate were about the same for all three treatments. A mixture of the organic acid s and the respective Na-salts was prepared at six different acid:salt ratio s in the third experiment. Total acid concentration in all the solutions wa s 100 mM. Higher proportion of the Na-salt in the mixture decreased PR diss olution, while the amount of the acid remaining in the filtrate increased. The Ca concentration in the filtrate showed an opposite trend. The results indicated that PR dissolution by the organic acids was initially dependent on protonation, which accounted for only 13-38% of the amount of P dissolve d. Most of the amount of P dissolved could therefore be attributed to other mechanisms. Removal of dissolved Ca from the solution appears to be the ma jor factor controlling the subsequent dissolution of the PRs. The ability o f tartaric and oxalic acids to dissolve effectively the PRs was attributed to the formation of an insoluble calcium compound which was precipitated fr om the solution. For the use of organic acids as a potential amendment for improving the P availability of PRs, the Ca binding power of the organic ac ids should also be considered.