China’s Panxi region is rich in titanium reserves, mainly distributed in vanadium and titanium magnetite [1,2]. Titanium resource accounts for 35% of the world’s primary ilmenite reserves and 93% of China’s primary ilmenite reserves [3,4,5]. However, the grade of titanium ore is relatively low and the content of calcium and magnesium impurities is high [6,7]. TiO2 content in titanium concentrate is only between 46% and 48% [8,9].
TiCl4, a key intermediate raw material in the preparation of titanium products, is mainly prepared by boiling and molten salt chlorination. However, the boiling chlorination process requires a high level of titanium raw material. This low-grade titanium ore with high calcium and magnesium content cannot be used in the boiling chlorination process to produce TiCl4 [10,11].
Titanium resources with high calcium and magnesium content are more applicable for TiCl4 production using molten salt chlorination technology [12,13,14]. However, the biggest problem of the molten salt chlorination process is the large amount of molten salt chloride slags generated [15,16]. When producing 1 t of titanium tetrachloride, about 200-500 kg of molten salt chloride slag can be generated . These slags often accumulate in large quantities and are not effectively utilized, causing serious environmental pollution . The disposal of molten salt chloride slags has become a serious problem in the molten salt chlorination process .
Currently, there is little research concerning the treatment and utilization of molten salt chloride slags (neither domestic nor overseas). The main treatment methods used are the pile burial method and the water-soluble method. The pile burial method refers to burying the molten salt chloride slags in the waste mine or using lime intervals to place them on the wasteland. This method can cause environmental pollution. In the literature, scholars first dissolved molten salt chlorinated slag in water and then processed the dissolved solution and filtered slags to recover useful substances [20,21,22,23]. However, the water-soluble method is a complex process with high secondary waste generation and fails to fundamentally solve the problem of molten salt chloride slags [22,23]. Moreover, NaCl, the main component of molten salt chloride slags, is necessary for the process of molten salt chlorination. The question concerning how to recover NaCl from molten salt chloride slags reprsents the key to the recycling of molten salt chloride slags.
The composition of molten salt chloride slag is complex. Here, calcium and magnesium impurities mainly exist in the form of chloride salts, with a CaCl2 content of 8% and MgCl2 content of 18% . Excessive magnesium chloride content will lead to higher density and viscosity of the molten salt [24,25,26]. This affects the surface wetting ability of solid particles in molten salt by influencing its fluidity and bubble rise rate, which is harmful to the molten salt chlorination process [27,28]. In this paper, a new process for treating molten salt chloride slag is proposed to remove MgCl2 from molten salt chloride slag by the high-temperature phase transition method. How to remove the MgCl2, the most abundant impurity, from molten salt chlorine slag represents the key issue of molten salt chlorine slag treatment.
In the available literature, scholars used the strongly oxidizing, alkaline waste brine from the purification of molten salt chlorination tail gas to treat the leachate of molten salt chloride slags and then recover NaCl and MgCl2 from it . However, this method is a complicated process and has a low recovery efficiency. In this paper, a new process of high-temperature phase transition method was used to treat molten salt chloride slag. When Na2SiO3 was added to the molten salt chloride slags at a high temperature, NaCl remained in the liquid state to produce the Mg-containing solid phase. The liquid NaCl was separated from the Mg-containing solid phase and reused in the molten salt chlorination process. Solid magnesium-containing silicate could be used as a constructal material, so that all magnesium chloride and sodium chloride were recycled.
The transition of Mg-containing phases and their separation of NaCl are the key parts of this work, and they are systematically studied in this paper. Thermodynamic analysis, phase transition behavior, and Mg removal behavior are all investigated.