Utilization Of Aluminum Resources In The Process Of Chemical Fertilizer Production By Phosphate-potassium Ores

N, P, and K in chemical fertilizer of China is out of proportion and the phenomenon that the fertilizer contains much nitrogen, less phosphorus and extremely less potassium is very serious because China is extremely poor in soluble potassium resources and the majority of phosphate belong to the low taste. However, insoluble potassium resources such as potash feldspar are abundant and widely distributed in our country. Recently, large reserves of phosphorus-potassium ores consisting of potassium feldspar and fluorapatite were found in zhongjianping area of Yichang City, Hubei Province. Rationally developing and comprehensively utilizing the phosphorus-potassium ores in this region will enhance the comprehensive utilization of the resources and reduce potassium dependence on foreign countries. Raw aluminum hydroxide can be obtained by cleaning the acid-leaching solution of phosphorus-potassium ores for fertilizer preparation. Discarding the raw aluminum hydroxide is a waste of resource and can cause environment problem, while recycling and reusing it will improve the economic efficiency of utilizing phosphorus-potassium ores comprehensively.This work explored the synthesis of aluminum tripolyphosphate, an antirust pigment, by using purified aluminum hydroxide obtained from the raw aluminum hydroxide. The work included pretreating the acid-leaching solution of phosphorus-potassium ores to obtain raw aluminum hydroxide, purifying the raw aluminum hydroxide by alkali extraction to obtain purified aluminum hydroxide, and synthesizing aluminum tripolyphosphate by using purified aluminum hydroxide and phosphoric acid. The aluminum tripolyphosphate obtained under the optimum conditions was characterized by chemical analysis, X-ray diffraction(XRD) and Fourier transform infrared spectroscopy(FT-IR). The results obtained are as follows:(1)The preparation of fertilizer by using the acid-leaching solution of phosphorus-potassium ores involves ammonia neutralization, and the free impure ions and fluorosilicate in solution could react with ammonia and form precipitate entering into raw aluminum hydroxide, resulting in small mass ratio of alumina to silica(A/S) in the raw aluminum hydroxide and a lot of free chlorine ions in acid-leaching solution, which is counterproductive to recycle aluminum element and synthesize low chlorine NPK compound fertilizer. Therefore, we need to remove chlorine and silicon from the acid-leaching solution. Distillation was used to remove chlorine with the addition of sulfuric acid. The results showed that when stoichiometric amount of sulfuric acid was added into the acid-leaching solution of phosphate-potassium associated ores, reaction temperature was 70 °C, distill time was 50 min,vacuum degree was 0.09 MPa and the volume of acid infusion about 8% was remained,80% of chloride ions were removed from the acid-leaching solution. Silicon in the chlorine-removed acid-leaching solution could be removed by flocculation with Fe(OH)3. The results showed that after flocculation treatment, the content of Al2O3 in aluminum hydroxide increased from 17.68% to 23.04%, while the content of SiO2 decreased from 10.28% to 4.63% and A/S increased from 1.71 to 4.97.(2)The effects of leaching parameters on the leaching rate of alumina from raw aluminum hydroxide were investigated. The results showed that the leaching rate of alumina reached 72% with leaching time being 40 min,leaching temperature being 90 °C, concentration of aluminum hydroxide being 3 mol/L and the ratio of liquid to solid being five. CaO was used for deep desilication and the purified aluminum hydroxide was prepared respectively by NaAlO2-HNO3 and seed precipitation methods. The results showed that the purity of aluminum hydroxide prepared by both methods could reach 96%.(3) The effects of process parameters on the synthesis of aluminum tripolyphosphate by using aluminum hydroxide and phosphoric acid were investigated and the optimum parameters were as follows: the molar ratio of phosphoric acid to aluminum hydroxide was 3:1, neutralization reaction temperature was 70 °C, neutralization reaction time was 60 min, condensation temperature was 290 °C, and condensation time was 4 h. Chemical analysis, and FT-IR and XRD characterizations showed that the contents of Al2O3 and P2O5 in the aluminum tripolyphosphate obtained under the optimum parameters were 17.68% and 67.33%, respectively, which met the requirements of enterprise standard Q/HGY07-1999.