Study On The Crystallization Process Of Potassium Chloride Prepared From Low-sodium Carnallite

Potassium chloride(KCl)is an important resource of salt lake resources,it is also a main raw material of potassium fertilizer.Its large-scale production is mostly obtained from the brine extraction of the chlorinated salt lake.And the main method is reverse flotation-cooling crystallization process.However,problems such as smaller particle size,lower recovery rate,and higher proportion of powdered potassium salts are pervasive and long-standing.In recently years,effects of supersaturation,residence time and stirring speed on the thermodynamics,kinetics,and morphology of crystals have been extensively studied.The establishment of related kinetic models provides a theoretical basis for the preparation of large particles of KCl.However,The particle size of the crystalline product still does not meet the potassium fertilizer standards.In this thesis,low-sodium carnallite is used as the raw material,and thermodynamics and kinetics of the crystallization process are used as the theoretical basis for preparing of KCl by cooling crystallization.At the same time,an evaluation model for the contribution of nucleation and three-dimensional growth to quality is established.The main research contents of this article are as follows:(1)The solubility of KCl in low-sodium carnallite-water system is measured by variable temperature dissolution method.The contents of KCl in the corresponding solution are measured and calculated by atomic absorption spectrometry.At the same time,a simplified solid-liquid equilibrium equation is used to fit the solubility data.The effects of temperature,stirring speed,and cooling rate on the width of the metastable zone and the induction period of the system are studied using FBRM and Optimax.The nucleation series,surface tensions and surface energy factors of KCl are calculated by applying the classical Nyvlt theoretical equation and the 3D nucleation principle,and combining the experimental metastable zone and induction period data.They provide the theoretical basis for the improvement of the crystallization process.(2)FBRM,PVM,SEM and other analytical methods are employed to study the effects of different stirring speeds,cooling rates,and seeds on the distribution of particle sizes and morphology of KCl crystals.The crystals are prepared by cooling crystallization in a low-sodium carnallite-water system.Atomic absorption spectrometry is employed todetermine the changes in the supersaturation of the solution system(expressed as KCl content)during the crystallization process under the conditions of rapid and slow cooling and with or without seeds.The crystalline products are analyzed using Occhio Nano500 XY.The growth mechanisms of KCl crystals are discussed from the aspects of morphology,quantity change,aspect ratio,box filling rate and sphericity.A model for evaluating the contribution of crystal nucleation and three-dimensional growth to quality is established.(3)Based on the above thermodynamic and kinetic mechanisms,four improvement paths are proposed from the perspectives of cooling rate,stirring speed and crystallization time.The crystallization process and crystallization products of the optimal solution are compared with the traditional slow cooling path(the cooling rate is 0.1 ?/min),and the kinetic verification is performed.The optimal process operating conditions for preparing KCl crystals by cooling in a low-sodium carnallite-water system were determined: the speed is 300r/min,the potency of seeds is 0.01 g/mL,and the cooling rate is accelerated from 0.1 ?/min to 0.4 ?/min(0.1 ?/min for 80 min,0.2 ?/min for 55 min,and finally 0.4 ?/min for 40min).The dynamics conjecture of the optimal path is confirmed.