A New Process For Separating Lithium And Magnesium By The Carnallite-Phosphate Method

With the rapid development of new energy vehicles,especially for the lithium electric vehicles,the consumption of lithium has greatly increased in recent years.Although there are abundant lithium resources from salt lakes in China,the coexistence of few lithium ion with great magnesium ion,and the 35 times of magnesium-lithium ratio make it difficult to extract lithium from brines because of the similar ionic properties.That has become the bottleneck to make the best use of lithium resources in brines.In this study,a new two-stage process for separating magnesium and lithium from the brine evaporated from salt field based on a high magnesium-lithium ratio salt lake is proposed,which is called the carnallite-phosphate method.In the first stage,potassium chloride is added into the brine to make half of MgCl₂ and the new-added KC1 separate from the brine in the form of the carnallite?KCl·MgCl₂·6H₂O?crystal.In the second stage,Na₂HPO₄ is added in the filtrate to remove the residual Mg²⁺in the form of MgHPO₄ sediments and separate from lithium-ion.Then,the obtained MgHPO₄ is dissolved in hydrochloric acid and reacted with melamine to form a precipitate which is reacted with sodium hydroxide to form the Na₂HPO₄.Phosphate sediment and melamine can all be circulated-used.In the process of carnallite crystallization,the phase graph of KCl-MgCl₂-H₂O system?25°C?is analyzed,so the circulation of process route of carnallite removal by KC1 is determined.The first phase of magnesium removal is divided in two steps.The best experimental results are as follows.During the first step of magnesium removal,the dissolution temperature is 55?and the amount of KCl is 36.8%of the theoretical amount,the result is that the magnesium removal efficiency is 24%,and the lithium loss rate is 2.36%.During the second step,under the condition that the evaporation temperature is 80?,the amount of potassium chloride is 19.06%of theoretical dosage,the evaporation of water is 16.78%,the result shows that the magnesium removal rate is 29.1%,and the lithium loss rate is 3.04%.Therefore,the total magnesium removal rate in the first section was about 53%.In the second phase of magnesium removal by Na₂HPO₄,an in-depth thermodynamic analysis of the Li⁺-Mg²⁺-PO₄^3--HPO₄^2--H₂PO₄^-system is carried out,and the optimal thermodynamic conditions for magnesium removal by phosphate without Li₃PO_4?s?precipitation have been found.The result shows that the removal of magnesium is 99%,the loss rate of lithium is less than 1%,and the mass ratio of Mg/Li in the filtrate is 0.16 with the conditions of 1.0 times the theoretical dosage of saturated Na₂HPO₄ solution,reaction temperature of 40?,reaction time for 30 mins,aging time for 3 h and pH=6.5.In the tentative exploration of phosphate recovery,Na₂HPO₄ is recovered by melamine complex.The effects of adding methods and adding amount of melamine are studied.The results show that when the C₃H₆N₆:H₃PO₄?molar ratio?is 2:1,the method of adding melamine should be step by step.The adding process is as follows.Half amount of melamine is firstly added in to the flask,then the solution P is slowly added into the flask,followed by stirring for 30 mins.The other half of melamine are added into the solution divided into four equal parts.The reaction between phosphoric acid and melamine is complete and the residual phosphorus in the solution is only1.28%.Besides,the effects of washing times of the precipitate and adding amount of sodium hydroxide are studied.The result shows that the recovery rate of phosphorus is as high as 99.06%with the conditions that the washing times are four times and the molar ratio of phosphoric acid to sodium hydroxide is 1:3.The magnesium removal efficiency of recycled Na₂HPO₄ is basically the same as that of the fresh Na₂HPO₄.This process has been applied to extract lithium in the brine sampled from the West Taijinar salt lake in Qinghai Province.The result shows that the loss rate of lithium is only 6.4%,and the removal efficiency of magnesium is as high as 99.6%.Obtained MgHPO4_?s?can be recovered legally by melamine complexation,thus the cost of raw materials is greatly saved.Which give some guidance for industrial production.