Lithium recovery from lepidolite by sulfuric acid method

As an important element supporting the new energy strategy, lithium has been widely used in lithium high-energy batteries, nuclear power generation, glass ceramics, grease and other fields due to its unique physical and chemical characteristics. In 2019, the utilization rate of lithium in battery products has increased to 65%.

Predictably, the security of lithium supply has become a top priority for global technology companies. Since the late 1990s, salt lake brine has been the main resource for the production of lithium compounds because of its cost-effectiveness, and the development of lithium mines should improve amid the urgent demand for lithium products.

As a lithium-containing ore with abundant reserves, lepidolite can alleviate the urgent demand for lithium to a certain extent. Several methods for extracting lithium from lepidolite have been developed, such as chloride roasting, sulfate roasting, limestone roasting, and sulfuric acid roasting.

The principle of all these methods is based on displacement reaction by roasting with different additives (chloride, sulfate or limestone) or digestion with concentrated sulfuric acid. During the roasting process, the lepidolite ore is roasted at a high temperature with additives added to extract lithium from the stable aluminosilicate structure of the lepidolite, and then the roasted product is leached to obtain a lithium-containing solution.

Reaction of lepidolite with concentrated sulfuric acid (98%) at 130 °C for 15 min resulted in ~80% lithium yield. Zhang studied the reaction of lepidolite concentrate particles with 85% H2SO4 at 200 °C, and the lithium yield was 97.1%. Acid-insoluble sulfates KAl(SO 4 ) 2, Al(SO 4 )OH·5H 2 O and Li 2 SO 4 are generated during this process. Compared with the conventional roasting process, the reaction temperature required by the sulfuric acid method is lower, which can save a lot of energy, and has been widely used in lithium ore extraction.

However, lepidolite leach solutions contain large amounts of aluminum. Since there are various forms of aluminum ions such as Al 3+, AlO 2 -, Al(OH) 3, Al(OH) 2+, Al(OH) 2 +, etc. in the aqueous solution, the removal of aluminum is relatively difficult. Usually, the removal of aluminum from the liquid requires the addition of acid, alkali or extractant, which will have a certain impact on the subsequent lithium recovery and cannot achieve flexible operation with the lowest cost.

The traditional method of removing impurities from lepidolite leachate is chemical precipitation, which requires the addition of alkaline substances, and then gradually precipitates to remove ions other than lithium, so as to obtain a lithium-rich solution with less impurity ions. Therefore, the method of effectively separating Al/Li in lepidolite leachate is very important to reduce the cost of lithium extraction.

Nanofiltration (NF) is a membrane separation technology. The nanofiltration membrane has a typical pore size (1nm) and charged groups are immobilized on the surface of the membrane. The mass transfer process of NF relies on the combination of steric hindrance and charge effect, which can effectively separate monovalent and multivalent ions. NF has been widely used in wastewater treatment and purification, and lithium extraction from salt lake brine. Based on previous studies, the separation performance of four commercial nanofiltration membranes (DK, DL, NF270, and Duracid NF) was evaluated in a simulated lepidolite leaching solution. The results show that the DK membrane exhibits extremely high Li/Al separation efficiency, which can be attributed to its pore size, smooth membrane surface, and appropriate zeta potential. In the separation of actual lepidolite leachate with different pH values, nanofiltration showed excellent Al/Li separation efficiency, and the lower the pH value of the leachate, the better the separation effect of DK membrane.