Research On Recovery And Utilization Technology Of Slag From Lithium Extraction By Nanofiltration

ABSTRACT:The separation of magnesium and lithium resource is the prominent technical problems that existed in the salt lakes of our country. Nanofiltration is of great competitive in the separation of magnesium and lithium from salt lake because of its operation simplicity and high separation efficiency.In the lithium extraction process of nanofiltration membrane, the rich-lithium brine, which was obtained in the low-pressure side, still contained some magnesium. After demanesium initially by Na2CO3and deeply by NaOH, the brine can be used for preparing lithium carbonate. However, some lithium precipitated in the form of lithium carbonate with magnesium hydroxide during the demanesium process, which needed the recovery and utilization. This paper proposed to use the above mentioned magnesium slag as material to separate magnesium and lithium:(i) extracting lithium by the carbonation process,(ii) removing calcium of the magnesium carbide by the calcinination-hydration technique and then obtaining magnesia through a calcination process, and (iii) producing lithium carbonate by the thermal decomposition of LiHCO3solution.In the lithium leaching process, two carburizers (i.e. CO2and NH4HCO3) were used respectively to investigate the lithium leaching behavior. It can be found that the lithium leaching efficiency is higher by using NH4HCO3as the carburizer. The optimum carbonation conditions were as follows:added NH4HCO3as dropwise, NH4HCO3/Mg(OH)2molar ratio is1:0.8, reaction temperature is30℃, reaction time is2h. The lithium extraction efficiency can reach to97.3%during the optimum process. And lithium concentration is0.8g/L, which needs beneficiation. However, during the lithium enrichment process, the concentration of (NH4)2CO3increased with the increase of carbonation times, which resulted in the waste of ammonia. Hence, the CO2-NH4HCO3combined cycle carbonization technique was utilized to turn (NH4)2CO3into NH4HCO3. It is found that the lithium concentration is5.51g/L after10 times recycle at the reaction system’s terminal pH of9.50.The calcination-hydration technique was employed to remove the calcium carbonate. The effects of calcinations temperature, soaking time, heating rate, hydration time and hydration temperature on calcium removal efficiency was studied. It can be found that calcinations temperature and hydration temperature have a significant effect on calcium removal efficiency. A lower (less than920℃) or a higher temperature (more than950℃) can cause the fosted buring and dead buring of calcium carbonate respectively, and then led to the low hydration efficiency of calcium oxide, which meant the low calcium removal efficiency. The calcium removal efficiency remained at a high level at the temperature range of920℃to950℃. A higher hydration temperature could prompt the hydration of CaO and MgO. The Mg(OH)2cannot be detected after90min of hydration at20℃, while most of MgO has been hydrated into Mg(OH)2after30min of hydration at70℃. The optimum reaction conditions as follows:calcinations temperature of920℃, soaking time of40min, heating rate of10℃/min, hydration time of30min and hydration temperature of40℃. The calcium removal efficiency is97.2%in the optimum process. After the calcination process, the purity and apparent specific volume of the prepared MgO particles were99.2%and6.47cm3/g respectively, which met the quality standard request of light MgO.Lithium carbonate was obtained by the thermal decomposition of LiHCO3solution. The non-isothermal process of Li2CO3was studied, and decomposition temperature was obtained. In the isothermal process of Li2CO3, the influence of decomposition temperature, decomposition time, agitation and seed crystal on the crystallization of Li2CO3was investigated as well. It is found that the decomposition of LiHCO3was mainly conducted at the first30min, lithium concentration and pH were decreased with the increase of decomposition time. After the first30min, the decomposition of LiHCO3was almost completed, pH and lithium concentration remained at a stable level. A higher temperature can prompt the decomposition rate of LiHCO3and the yield of Li2CO3. The result showed that after60min of decomposition at80℃, the yield of Li2CO3is40.47%and the purity reach up to99.12%. Additionally, the crystallization kinetics of Li2CO3was characterized by secondary reaction kinetics. It was found that the crystallization of Li2CO3is dominated by the chemical reaction step.