Advances in the preparation process and mechanism study of high-purity anhydrous magnesium chloride from magnesium chloride hexahydrate

Abstract

In the extraction of potassium from salt lakes, Mg is abundant in the form of bischofite (MgCl₂·6H₂O), which is not utilized effectively, resulting in the waste of resources and environmental pressure. Anhydrous MgCl₂ prepared by the dehydration of bischofite is a high-quality raw material for the production of Mg. However, direct calcination of MgCl₂·6H₂O in industrial dehydration processes leads to a large amount of hydrolysis. The by-products are harmful to the electrolysis process of Mg, causing problems such as sludge formation, low current efficiency, and corrosion in the electrodes. To obtain high-purity anhydrous MgCl₂, different advanced dehydration processes have been proposed. In this review, we focus on the recent progress of the dehydration process. Firstly, we discuss the molecular structure of MgCl₂·6H₂O and explain the reason why much hydrolysis occurs in dehydration. Secondly, we introduce the specific dehydration processes, mainly divided into direct dehydration processes and indirect dehydration processes. The direct dehydration processes are classified into gas protection heating and molecular sieve dehydration process. Indirect dehydration processes are classified into thermal dehydration of ammonium carnallite (NH₄Cl·MgCl₂·6H₂O), thermal dehydration of potassium carnallite (KCl·MgCl₂·6H₂O), thermal decomposition of the [HAE]Cl·MgCl₂·6H₂O, organic solvent distillation, ionic liquid dehydration process and ammonia complexation process. In the meanwhile, purity of anhydrous MgCl₂ of each dehydration process, as well as the advantages and disadvantages, is discussed. The characteristics of different processes with a simple economic budget are also given in this paper. Finally, the main challenges are evaluated with suggested directions in the future, aiming to guide the synthesis of high-purity anhydrous MgCl₂.