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Crystallization Process Investigation of Potassium Chloride from Carnallite Decomposition
Ling Zhou, Zuoliang Sha, Na Tang Tianjin Key Laboratory of Marine resources and Chemistry, Tianjin University of Science & Technology, Tianjin, 300457，China, zsha@tust.edu.cn

The crystallization process for producing potassium chloride from carnallite was studied in a continuous operated system. Thecrystallization mechanism was experimentally studied for making KCl crystal from solid carnallite, and the factors affecting the crystal size and size distribution were investigated. The effects of residence time and MgCl2 concentration in mother liquor on crystallization process and product crystal size were also studied. Furthermore, the crystallization kinetics of potassium chloride was reportedfor the system.

1. Introduction
The technology for production of potassium chloride depends on the resources of raw material, such as the solution from seawater or salt lake. If the raw material is relatively pure solution of KCl, cooling crystallization is commonly employed. However, if the production is based on the solution of salt lake or the brine from seawater, the potassium chloride isnormally produced from a solid form of carnallite (KCl·MgCl2·6H2O) (Yang Hongmei, 2003). The basic thermodynamic theory of separating KCl from carnallite is shown in Fig.1, which is a phase diagram of KCl-MgCl2-H2O system at 20 . If water is added with exactly required amount into pure carnallite (point G, solid), the system point in the diagram will move from G to point D. During the process ofwater addition, carnallite will be decomposited and the system will become a solid phase of KCl (point A) and solution (point P). The production of potassium chloride from carnallite includes many basic processes so that the size of product is difficult to control. On the other hand, the reported data on crystallization kinetics for potassium chloride were based on pure potassium chloridesolution (Hong Xianlan et al., 1994 and Xia Shuping et al., 1993). Crystallization kinetics is the vital factors affecting in the crystallization process (Randolph Alan D et al., 1988 and Mydlaz J. et al., 1993).But the kinetic data obtained from pure solution can’t be directly used for the crystallization process from solid carnallite. Moreover, the crystallization process of potassium chloride fromcarnallite is more complicated because the KCl crystals formed from a solid phase of carnallite. Therefore, the crystallization mechanism is very important for the process controlling. The aim of this work is to clarify the basic crystallization mechanism of potassium chloride from carnallite. Furthermore, the crystallization kinetics and the effect of operation conditions on particle sizedistribution were studied. The method for controlling the particle size distribution was also proposed.

Figure 1: Phase diagram of KCl-MgCl2-H2O system at 20

Figure2: Experimental set-up

2 ． The mechanism of KCl crystallization from carnallite decomposition
2.1 Experiments set up The KCl crystal forming can be thought as two mechanisms. The one was saying as that the KCl in carnallite crystalsshrinks into pure KCl crystals. It means that the KCl in a solid carnallite crystals directly become another solid phase. Another one is that KCl is first transferred to liquid phase from solid carnallite phase, and then was crystallized from solution. In order to clarify the mechanisms of KCl crystal forming from carnallite, the experimental was arranged as follows. The raw material of carnalliteused in the experiments was provided by Qinghai Saline Lake Industrial Groups. 300 ml of water was firstly added into a vessel, and temperature was controlled at a constant 20 during the whole process. Then, a weighted amount of solid carnallite was added into the water

and mixed for 30 minutes. The clear solution was sampled for determining the concentration of K+, Mg2+ and Cl- by titration....