Process Research For Producing KH₂PO₄ With Fertilizer CaHPO₄ And KCL

This paper analyzes the kinetic behavior in the reaction of CaHPO4 and KHSO4 (mixed by H2SO4 and K2SO4), and the process of producing KH2PO4 with KC1, H2SO4 with fertilizer CaHPO4 as the raw material.In this research, a reaction of H2SO4 with K2SO4 is carried out to decompose CaHPO4. Through the analyzing of the relation between sulfate used and decomposition time, we find for all the reaction, the reaction rate is fast at the beginning, but slow down following the reaction time, no matter how large the ratio of SO42- and Ca2+. So it shows that a "transparent solid-state membrane" occurs and the passivation of plain fertilizer is mainly caused by the overdose of H2SO4. The amount of used SO42- takes big effect on the passivation. When the amount of H2SO4 acid is excess for 30%, the reaction will be stopped uncompleted, that indicates that there is a limitation of overdose SO42- the same as for wet H3PO4 acid process. The key point to gain more phosphorus is to overcome the passivation of plain fertilizer. For the reaction of plain fertilizer and KHSO4, in the beginning of the reaction, transforms quickly, from 1 to 1.5 hour it could be transformed to 80%, after this period, in the end of the reaction, the speed of reaction will decrease, but still continues. The decomposition rate mainly depends on the time of reaction and the amount of used H2SO4.The result of the research indicates increasing the time and the temperature of the reaction can raise the decomposition rate. In the beginning of the reaction, the decomposing speed under 50℃ is faster than under 70℃, under 60℃ thedecomposing speed is the fastest. But in the end of the reaction, during the range of 50-70℃,the decomposition rate will increase as the increasing of temperature and speed of the reaction.Through the research the reaction of two kinds of particle sizes under different temperature, we consider the reaction speed of small particle plain fertilizer is faster than the big ones.The liquid-solid multiple-decomposition reaction of plain fertilizer and KHSO4 is a diffusion controlled reaction and its kinetic model can be written as follows:In this model, we do not consider the CaHPO4 particles spherical theoretically and believe that there are fine holes between their surface and interior. When fitting the - 2.2 power of the time into the equation, it is find that as the time goes on, the reaction is more and more difficult to complete, the reason is the resistance of the diffusion increases with the time.The process study of producing KH2PO4 finds that it is feasible to use the multiple-decomposition reaction to produce KH2PO4. The suitable operating conditions are:The amount of H2SO4 based on the ratio to Ca2+ 1:1-1.05 The ratio of phosphorus and potassium: 1:1 The solid -liquid ratio: 1:3-4 The reaction temperature: 70-85℃The lose of potassium is mainly related to the content of fluorine in the plain fertilizer, the lose of phosphorus is related to the amount of used H2SQ4 acid, reaction time and solid-liquid ratio in the system. The main parameters in the process are similar with that of wet H3PO4 acid process, so we can borrow part of the research conclusions from wet H3PO4 acid process.During the process of producing KH2PO4, based on the ratio of K:P and S:Ca, methanol is used to separate out K2SO4 as precipitate and leave H3PO4 inthe methanol in free acid type. By adjusting the ratio of K2SO4 and KH2PO4 in the product, P-K ratio in the product can be adjusted. In this way we can produce composition fertilizers with different P-K ratio.