Research On Fundamentals In The Technology Of Extracting Potash From Seawater By Ion Exchange Method

China is a biggest country on agriculture in the world, and its annual consumptionquantities of potassium fertilizer are more than 10 million tons. However, because ofthe lack of inland potassium minerals the domestic potassium fertilizer supply fallsshort of demand, up to now about 80% potassium fertilizers depend on import.Therefore it is imperative to exploit potassium resources in varieties of ways,especially the potassium of seawater in which the potassium reserves is about 500trillion tons. By the unremitting efforts of scientists of our country, the difficultproblems in the industrial circles of world, e.g. potassium is commercially extractedfrom seawater, have been broken, and the efficient and energy-saving technologies ofextracting potash from seawater by zoelite (such as clinoptilolite) have beendeveloped.The technology of extracting potash from seawater by ion exchange methodconsists of following steps: the modification of natural CLN (clinoptilolite) as ionexchanger;enriching potassium process from seawater by ion exchange;extractingpotash process from enriched potassium solution by extracting-cooling crystallization.In order to solve the engineering problem for scale up effect, the fundamentals in thenew technology of extracting potash from seawater were studied systematically bothin experiment and theory. The research results are as follows: Firstly, the research evolutions of technologies of extracting potassium fromseawater were summarized, the development trends of the new technologies werepredicted, and the aims of this work were made up. Secondly, the chemicalcomposition and structure as well as surface characteristics of natural CLN wereascertained, and the optimal method of modifying natural CLN was proposed. Thirdly,the ion exchange isotherms of Na⁺-K⁺, NH₄⁺-K⁺, Ca²⁺-K⁺ on modified CLN weredetermined, the selectivity of zeolites to cations and the effect of temperature on itwere found out, and the experiential models of cation ion exchange equilibrium andthe calculation equations for thermodynamic functions in the process of ion exchangeof zeolite have been proposed. Fourthly, the control mechanisms of kinetics of Na⁺-K⁺,NH₄⁺-K⁺, Ca²⁺-K⁺ ion exchange on modified CLN have been determined, that is, theprocesses were mainly controlled by film diffusion, and the kinetic curves and masstransfer coefficients have been obtained. On the basis of Whiteman film theory themass transport theoretical models of ion exchange process have been established forthe first time, and the maximum deviation between the simulation results and theexperiment data was not more than ±5%. Fifthly, the phase equilibrium data ofrelated water-salt and mixed solvent systems have been determined in detail, and thelaws of influence of extractive agents on solubility of potash were ascertained.Considering the inner interactive force in solution, the calculation models of solubilityof strong electrolyte in the mixed solvent system have been established, and thesimulation results showed that the mean relative deviations were less than 8%. Sixthly,crystal superstructure and laws of effect of varieties of dynamic factors on thecrystallization process of KNO3 were primarily found out. Seventhly, the results of200 t/a pilot test showed that the new technology of extracting potassium nitrate fromseawater was advanced and feasible, and would have stronger competitive power dueto its advantages of inexhaustible seawater and low cost of production. On thisresearch basis, 10,000 t/a project of extracting potassium nitrate from seawater hasbeen put into production.