Study On The Production Of Potassium Dihydrogen Phosphate With Triple Superphosphate

Potassium dihydrogen phosphate is a very important high concentration compound fertilizer which contains phosphate and potassium nutrient. It is widely used in agriculture industry because of its high nutrient content, good stability and high solubility in water. The present production crafts of potassium dihydrogen phosphate are always expensive, and usually cause heavy erosion. A new production technology is developed in this paper in which crystal of potassium dihydrogen phosphate is produced through the reaction of triple superphosphate and potassium sulfate. The new technology has the advantages of short flow, simple equipments, low energy consumption, less investment; hence it has a wide application prospect.Operation parameters are firstly determined by preliminary experiments. The effects of theses parameters on the recovery rate of P₂O₅ and K₂O are investigated through three-factor quadratic regression orthogonal design. The regression equations are obtained and the optimal operation conditions are as the following: reaction temperature is 50℃, reaction time is 120 minutes, molar ratio of potassium sulfate to monocalcium phosphate is 1.1:1, and volume ratio of liquid to solid is 4:1. Under the above optimal conditions, P₂O₅ recovery exceeds 70% and K₂O recovery exceeds 98%. These results can be a reference for the production of potassium dihydrogen phosphate from triple superphosphate (TSP).The existence of impurities in mother liquid will degrade the product. The sedimentation performances of theses impurities can be adjusted by pH value. The experimental results show that the quality of the product can meet the demand when the pH value of the mother liquid is larger than 4.5 and the losing of P₂O₅ is less than 4.72%.The studies of preparation of large size potassium dihydrogen phosphate crystal are also carried out because large size crystal usually has high purity and no tendency for agglomeration. Metastable zone width of the reaction mother liquid is measured and compared with the pure mother liquid. The effects of additives on the metastable zone width are also investigated. The crystal growth rates under different temperature or supersaturation are also measured. The experimental results show that metastable zone width of the reaction mother liquid is larger than that of the pure mother liquid, only sodium citrate can broaden the metastable zone width, and crystal growth rate increases with supersaturation.By adopting the above optimal operation parameters, high purity and large size crystals of potassium dihydrogen phosphate are produced from continuous crystallization process. The quality of the crystal product can meet the demand of the national first grade standard.