KCl-MgCl_2-H_2O体系25℃盐析光卤石的等温热力学与结晶规律

Isothermal Thermodynamics and Crystallization Behavior of Carnallite Salt-out in KCl-Mg Cl_2-H_2O System at 25℃

光卤石可以采用盐析的方法生产,即在光卤石饱和液中加入老卤或水氯镁石使光卤石析出.本文选择KCl–Mg Cl2–H2O体系,对老卤和水氯镁石盐析光卤石的相关热力学和结晶规律进行研究.文中使用了盐析结晶和兑卤结晶两个概念,并将实际的光卤石盐析生产过程分为直接反应、兑卤结晶、盐析结晶以及由这些基本过程叠加而成的连续结晶过程.热力学研究表明:采用盐析结晶,钾盐的总回收率可达90%以上,而兑卤结晶不足56%.盐析结晶对光卤石的饱和度远大于兑卤结晶的过饱和度,而这种饱和度可通过分步进料或连续进料方式得到减小.结晶动力学研究考虑了兑卤加料速率、盐析加料方式、叠加操作周期等因素对光卤石粒度及其分布规律的影响.在连续结晶中,盐析过程对粒度增长的作用大于兑卤过程的作用,通过4个周期的叠加操作,光卤石的平均粒度可达到300,μm以上.

Carnallite could be produced by salt-out method such as mixing bischofite or bittern with carnallite saturated brine. In this research,the isothermal thermodynamics and crystallization behavior of carnallite salt-out by adding bischofite or bittern to the potassium contained brine were theoretically and experimentally studied on the basis of ternary system KCl-MgCl2-H2O at 25,℃. The terms of‘salt-out crystallization’ and‘mix-brine crystallization’ were employed,and the real indus-trial process of carnallite salt-out was considered as including three individual processes of instant reaction,mix-brine crys-tallization,salt-out crystallization,and a continuous process. The thermodynamic results show that over 90%of potassium could be recovered in the salt-out process,but less than 56%in the mix-brine process. The solubility index for carnallite in salt-out process is much higher than that in the mix-brine process,but it can be greatly reduced by multi-steps or continuous feeding methods. The particle size and distribution of carnallite in the crystallization process were studied by considering the influence of feed rate in the mix-brine process,feed method in salt-out process,and cycle times in continuous process.The results show that the contribution of salt-out process to the carnallite crystal growth is greater than that of the mix-brine proc-ess. The partical size of carnallite could easily get to 0.3 mm by 4 times’ cycle crystallization.