The 273.15-K-Isothermal Evaporation Experiment of Lithium Brine from the Zhabei Salt Lake, Tibet, and its Geochemical Significance

The Zhabei Salt Lake is located in the hinterland of the northern Qinghai-Tibet Plateau, China. The salt lake is of the sodium sulfate subtype and rich in lithium. In order to exploit lithium in the brine of the salt lake, a 273.15 K-isothermal evaporation experiment of the brine from the Zhabei Salt Lake was carried out for the first time. Based on the 273.15 K-isothermal phase diagram of the quaternary system Na+, Mg2+//Cl−, SO42−–H2O, the evaporation-crystallization path of the brine was drawn and the sequence of salt precipitation was confirmed. The experimental results show that due to the large amount of SO42− precipitated as mirabilite, the concentration of SO42− in the brine was stabilized at about 0.25 mol L−1, thus avoiding formation of Li2SO4·3Na2SO4·12H2O in the early stages of the low-temperature evaporation process. Lithium was effectively concentrated to 1.09 mol L−1 in residual mother liquor as the evaporation rate reached 96.76 %, and the molar ratio of Mg/Li decreased from 1.25 to 0.23. The enrichment behavior of lithium is described as a power function of Y = k(1 − X)−1, where Y = the concentration of lithium in the residual brine, X = the extent of evaporation, and k = the initial value of lithium concentration of the brine. In addition, the experiment suggests that the salts precipitated from the brine of the Zhabai Salt Lake contain not only the mineral assemblage with the characteristics of carbonate-type salt lakes, such as borax and lansfordite, but also the mineral assemblage with the characteristics of sulfate-type salt lakes, such as carnallite and lithium sulfate monohydrate. This finding, the data from the Zhabei Salt Lake’s brine in 1978, and the distribution characteristics of the hydrochemical zones of salt lakes on the Qinghai-Tibet Plateau, indicate that this salt lake is in a special stage of transition from a sulfate-type salt lake to a carbonate-type.