Study On Preparation Of NPK Compound Fertilizer And Calcium Sulfate Whisker By Using Acid-leaching Solution Of Phosphate-potassium Associated Ore Obtained At Low Temperature

The soluble potassium resources are severely deficient in China, which leads to the imbalanced ratio of potassium element in fertilizer and the extensive importation of potassium fertilizer. Insoluble potassium resources such as potash feldspar are rich in China and Yichang city of Hubei province is full of phosphorous-potassium associated ore, which mainly consists of quartz, potash feldspar and collophane. Using these insoluble potassium resources to prepare potassium fertilizer has great industrial prospect. Our group’s study showed that the decomposition of the phosphorous-potassium associated ore by hydrofluoric acid with the addition of fluorite at 90 °C for 4 h could produce a leaching rate of 90% for both potassium and calcium. In this work, the acid-leaching solution of the phosphorous-potassium associated ore was used to prepare low chlorine compound fertilizer by distillating hydrochloric acid in the acid-leaching solution with the addition of sulfuric acid, based on the study of the effects of different kinds of hydrochlorides on the distillation of hydrochloric acid with the addition of sulfuric acid. In addition, both gypsum, the by-product in the fertilizer industry, and the acid-leaching solution of phosphorous-potassium associated ore, were used to prepare calcium sulphate whisker. The effects of process parameters on the aspect ratio and yield of whisker were investigated, and the whisker obtained under optimal conditions was characterized. The main experimental results were as follows:(1)In the process of removing chlorine from the acid-leaching solution of phosphorous-potassium associated ore, the content of HCl in the distillate decreased in the order MgCl2>AlCl3>CaCl2>KCl. The content of HCl in the distillate increased and the effect of hydrochloride on the removal of chlorine decreased when sulfuric acid was added into the acid-leaching solution. When 80% of the acid-leaching solution of phosphate-potassium associated ore was distilled(boiling range 109.2-124 °C) by adding stoichiometric amount of sulfuric acid, 78% of chloride ions could be removed from the solution. Low chloring NPK compound fertilizer with each mass content of N, P and K more than 5%, total mass content of N, P and K more than 50%, and mass content of chlorine less than 1.5% could be obtained by using ammonia neutralization method or methanol precipitation method.(2)Gypsum, the by-product in the fertilizer industry, was used to prepare calcium sulphate whisker by using atmospheric acidification method. When the sulfuric acid concentration was 5.25 mol/L, supersaturation was 0.4, heating temperature was 85 °C, heating time was 1 h, solution was cooled naturally by truning off the heating system, aging temperature was 20 °C, and aging time was 3 h, calcium sulfate whisker with an aspect ratio of 71 could be obtained. FT-IR, XRD and SEM analyses showed that the as-prepared whisker was calcium sulfate dihydrate. The sulfuric acid could be repeatedly used in the process, and with the cycling of sulfuric acid, the yield of whiskers increased and the aspect ratio of whiskers slightly decreased. The method had the advantages of moderate reaction conditions, simple operation procedure, and easy control.(3)Calcium sulphate whisker was also prepared by a simple process by dropping sulfuric acid into the acid-leaching solution of phosphate-potassium associated ores. When the sulfuric acid concentration was 0.87 mol/L, heating temperature was 105 °C, the molar ratio of sulfate ion to calcium ion was 1.2, the dropping rate of sulfuric acid was 2.5 mL/min, and stirring rate was 166 r/min, calcium sulphate whisker with an aspect ratio of 32 could be obtained.