Solid–liquid phase equilibria in the aqueous system containing the chlorides of potassium, ammonium, and calcium at 298.2, 323.2, and 348.2 K

Abstract

In order to obtain the crystalline forms of the salts of the potassium, ammonium, calcium coexisting chloride system, the phase equilibria relationship of quaternary system K+, NH4+, Ca2+//Cl−-H2O at 298.2, 323.2, and 348.2 K was studied by isothermal dissolution equilibrium method. The solubility and density of equilibrium liquid phases of the system were experimentally determined; X-ray powder diffractometer was used to determine the compositions of the equilibrium solid phase at the quaternary invariant point. It is found that the quaternary system is a complex system at these three temperatures. The phase diagram at 298.2 K consists of three invariant points, seven univariate curves and five crystalline phase regions, forming the solid solutions (NH4Cl)x(KCl)1−x and (KCl)x(NH4Cl)1−x; while at 323.2 and 348.2 K the phase diagram consists of five invariant points, eleven univariate curves and seven crystalline phase regions, the double salts (KCl·CaCl2) and (2NH4Cl·CaCl2·3H2O), solid solutions (KCl)x(NH4Cl)1−x and (NH4Cl)x(KCl)1−x were formed. Among them, the crystalline phase region of solid solution (KCl)x(NH4Cl)1−x is the largest at three temperatures, indicating that it is the easiest to crystallize in this system. Comparing the phase diagrams of the quaternary system at 298.2, 323.2, and 348.2 K, it can be seen that the crystalline form of CaCl2 changes with the increase of temperature: CaCl2·6H2O at 298.2 K, CaCl2·2H2O at 323.2 and 348.2 K. From 323.2 to 348.2 K, the crystalline phase regions of (KCl·CaCl2) and (2NH4Cl·CaCl2·3H2O) increased gradually.

1. Introduction

China is a large agricultural country, and also a big country of potassium salt consumption and import. The rational exploitation and utilization of potassium salt is directly related to national food security [1,2]. The global potash (potassium chloride equivalent) reserves are 14.9 billion tons, and the resources are 69 billion tons, mainly distributed in Russia, Canada, Belarus, and Turkmenistan. China's potash reserves are 170 million tons, accounting for only 5.15% of the world's total potash reserves [3]. The occurrence form of potassium resources in China has its particularity, 98% of the proven potassium resources are abundant in liquid brine potassium deposits, which mainly distributed in salt lake brines in Qinghai, Xinjiang and Xizang [4,5], as well as deep brine represented by Sichuan Basin, Jianghan Basin and Qaidam Basin [6-8]. Among them, the concentration of K+, Ca2+ and NH4+ in the Xuanhan deep brine in Northeast Sichuan is 25.955, 10.877, and 0.816 g·L−1 [6]. Nanyishan deep brine in the Qaidam Basin is a calcium chloride type with high salinity (118.2-287.9 g·L−1), high calcium (14.73-16.09 g·L−1), and the content of potassium is 5.21-7.66 g·L−1, ammonium is 0.143 g·L−1 [9], which has high development and utilization value.

In addition, due to the particularity of its occurrence state, deep brine has the characteristics of deep burial and high brine temperature. For example, the burial depth of brine in northeastern Sichuan is 1000-3260 m, the occurrence temperature of brine is mostly concentrated in 296.2-348.2 K. At the same time, the deep brine also contains ammonium and calcium, which is significantly different from the salt lake brine system and seawater system, it may form double salts or solid solutions with coexisting potassium, which makes the ion interaction relationship complex and increases the difficulty of salt separation. Therefore, to obtain the salt interaction relationship of the chlorides type solution containing potassium, ammonium and calcium at multi-temperature, the phase equilibria of system K+, NH4+, Ca2+//Cl−-H2O is necessary.