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Lepidolite belongs to the mica family of minerals. The raw ore grade of lepidolite is low, generally 0.3%~0.6%, lower than 1%~1.5% of spodumene. The Li2O content of lepidolite concentrate after mining is usually 2.0%~3.5%. Mica exists in the form of silicate and has a relatively tight structure. It needs to be roasted and defluorinated at high temperature in the early stage to make the raw ore structure loose before the next step of grinding. The main methods for extracting lithium from lepidolite include lime roasting, sulfuric acid roasting, sulfate roasting, chlorination roasting and pressure cooking.
In the early 20th century, the limestone method was widely used in the process of extracting lithium from lepidolite, and the process was relatively mature. The technology mainly uses solid-liquid separation during the leaching process of roasted slag to produce lithium carbonate. The main process is as follows:
1) First, lepidolite and limestone are roasted at a mass ratio of 1:3 at 900℃ in a rotary kiln;
2) The roasted mixture is water-leached to separate the waste slag without lithium from the solid and liquid;
3) The obtained solution is evaporated and concentrated to precipitate lithium hydroxide, and finally industrial-grade lithium carbonate products are obtained after carbonization, purification and other steps.
The lime method has simple steps and strong applicability, but the recovery rate of lithium content in the leaching solution is low, the energy consumption of the roasting method is high, and the final lithium recovery rate is low. Compared with the sulfuric acid method and the sulfate method, the lime method is less technically economical and environmentally friendly, and is currently being gradually eliminated.
Similar to the chlorination roasting process of spodumene, the chlorination roasting of lepidolite also converts lithium and other valuable elements into easily volatile chlorides through roasting, and then obtains lithium salt products through adsorption, deposition and other methods. Chlorination roasting has a short time and high conversion rate, but it also has the problems of high corrosiveness and large raw material consumption, which need to be further optimized.
The pressure cooking method is to mix the roasted and defluorinated lithium mica with sodium carbonate and roast it under pressure (200℃), and use the principle of ion exchange to replace lithium carbonate. Its process is similar to the sulfuric acid method, with a short production process and low cost, and different replacement salts can be used to produce different lithium salt products according to demand.
The key to the pressure cooking method for treating lithium mica is roasting and defluorination; the leaching rate of lithium mica obtained by pressure cooking before roasting is mostly around 60%, while the leaching rate after roasting and defluorination can reach more than 90%, and the resource utilization rate is relatively high.
Similar to the chlorination roasting process of spodumene, the chlorination roasting of lithium mica also converts lithium and other valuable elements into easily volatile chlorides through roasting, and then obtains lithium salt products through adsorption, deposition and other methods. Chlorination roasting has a short time and high conversion rate, but it also has problems of high corrosiveness and large raw material consumption, which needs to be further optimized.
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