| China, a large agricultural country, demands a huge number of chemical fertilizers. Other countries in the world currently use soluble potassium resources to product potassium fertilizers. China is poor in soluble potassium resources but rich in insoluble potassium resources such as potassium feldspar. Recently, large reserves of phosphorus-potassium associated ores consisting of potassium feldspar, fluorapatite, quartz, dolomite and iron pyrites were found in Yiling area of Yichang City, Hubei Province. In this kind of mid-lowgrade ore, the content of P2O5 is 6~12%, and the content of K2 O is 5~9%. Decomposing the phosphorus-potassium associated ores by hydrochloric acid could extract potassium at relatively low temperature and at the same time phosphorus, aluminum, and calcium could be extracted and effectively utilized, which is of great significance.In this work, the leaching of potassium from the phosphorus-potassium associated ores of Yiling area of Yichang City by using hydrochloric acid at low temperatures was studied. Among the investigated additives for assisting potassium leaching, calcium fluoride was found to the best one based on the results of leaching rate of potassium. The effects of grinding time, reaction temperature, content of hydrochloric acid, leaching time, feed ratio, hydrochloric acid dosage and speed of agitation on the leaching rate of potassium were investigated by single factor experiment. A response surface method with four factors of leaching temperature, reaction time, content of hydrochloric acid and dosage of hydrochloric acid and an evaluating indicator of leaching rate of potassium was used to optimize the leaching process. The optimum parameters were phosphorus-potassium ore of 20 g, grinding time of 15 min, hydrochloric acid content of 23%, reaction temperature of 92 °C, reaction time of 4h, feed ratio of 0.2, hydrochloric dosage of 80 m L, and agitate speed of 500 r/min. The leaching rates of potassium predicted by the model and obtained by the experiment under the optimum parameters were 92.82% and 91.3%, respectively; the deviation of the model was 1.64%.The leaching process and mechanism of the phosphorus-potassium associated ores were investigated by combining experimental results with instrument analytical results such as XRF, XRD, and SEM-EDS. The detailed steps of the ore reacting with calcium fluoride and hydrochloric acid was described as follows. Firstly, the easily decomposed components in the ore, including fluorapatite, dolomite and calcium fluoride, were dissolved rapidly by acid and many tiny channels were generated on the surface of ore particles. Then, the potassium feldspar on the surface of ore particles were decomposed by the joint effects of hydrogen ions and fluoride ions and at the same time, hydrogen fluoride diffused into the inner of the ore particles through the tiny channels and the potassium feldspar in the inner of the ore particles was gradually decomposed. Some potassium feldspar in the deep inner of the ore particles could not be decomposed because of the internal diffusion effect and the obstructed channels resulting from the nonreactive particles adsorbed on the channels.Both the mechanism analysis and calculation fitting results indicated that the leaching process of the phosphorus-potassium associated ore followed the nonreactive shrinking core model controlled jointly by chemical reaction and internal diffusion. The apparent activation energies of the phosphorus-potassium associated ores from Zhongjianping, Taiyangshan and Yinjiaping were: 34.01, 34.40, and 30.67 k J/mol; the apparent rate constant were: Km=379.2e-4090.6/T, Km=472.5e-4137.4/T, and Km=115.2e-3688.8/T. Finally, the kinetic equations of the model controlled jointly by chemical reaction and internal diffusion.Large-scale leaching of phosphorus-potassium associated ore of 200 g, 875 g and 42 kg was carried out based on the laboratory experiments(20 g), and the results indicated that there was no obvious scale-up effect. |
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