With the rapid growth of lithium consumption and the gradual depletion of spodumene minerals, lithium extraction from other mineral resources has developed rapidly in recent years. Lepidolite is rich in resources and has a relatively high lithium content, making it the second largest ore resource for lithium extraction.

Acid process

The sulfuric acid method is also the main method for lithium extraction from lepidolite. According to different acid treatment methods, it can be divided into acid roasting method, concentrated sulfuric acid room temperature pretreatment method, high temperature pretreatment method and sulfuric acid pressure leaching method. The sulfuric acid roasting method is to first acid roast lepidolite and concentrated sulfuric acid at low temperature (110-200℃), cool the acidified clinker, and leach it in water to obtain a lithium sulfate solution. After acid treatment, lithium and the associated rubidium and cesium become soluble salts.

The advantages of sulfuric acid method for lithium extraction are strong adaptability to raw materials, small material flow, low energy consumption, simple leaching process and mild reaction conditions. It can extract most of the lithium and other valuable metals in lithium mica and produce less waste slag; but its disadvantages are also very obvious. It has high corrosion resistance requirements for equipment, large amount of residual sulfuric acid, aluminum in lithium mica is dissolved, and a large amount of alkali is consumed for treatment, which affects the subsequent purification and impurity removal process. In addition, the generated Al(OH)3 colloid will adsorb Li in the solution, causing Li loss and reducing the recovery rate of Li2CO3 product; it has little competitive advantage in economic cost.

Alkaline process

There are many studies on the extraction of lithium from lithium mica by limestone roasting. 2MeAl2Si3O9(F·OH) is the molecular formula of lithium mica, and Me represents lithium, sodium, potassium, rubidium, and cesium. After mineral roasting, water leaching is carried out. The lithium concentration in the leaching solution is low (about 4 g·L-1 Li2O), resulting in a large volume of mother liquor, which is about 10 to 15 times that of spodumene using limestone roasting method to extract lithium. The evaporation amount of concentrated lithium recovery is large and the energy consumption is high. The leaching residue is mainly composed of calcium silicate, calcium fluoride, etc. The dry residue volume per ton of lithium hydroxide monohydrate is more than 40 tons, and the composition is complex and extremely difficult to use. Therefore, the pollution of solid residue is quite serious.

Sulfate method

The sulfate roasting method of lithium mica is currently a widely used lithium extraction method. It has a similar process to the sulfuric acid roasting method. The difference is that the roasting temperature of the sulfate roasting method is relatively high, usually between 800 and 950℃.

When calcined at high temperature, the structure of lithium mica is loose, and the cations in the sulfate (K+, Na+, Ca2+, etc.) exchange with Li+, occupy the original Li+ position, replace Li+, and form a Li2SO4 solution.

Lithium extraction process of lithium mica sulfate method

The advantage of the sulfate method is that it is highly applicable and can process lithium mica ores of different grades; compared with the sulfuric acid method, the probability of sulfate reacting with aluminum in lithium mica to form soluble aluminum salts is small, and the loss of lithium caused by aluminum removal in the subsequent chemical precipitation method is small; the roasting time is short and the lithium precipitation rate is high.

However, its disadvantages are also obvious, mainly the following problems: the roasting temperature requirements are relatively strict, which is easy to cause ring formation in the furnace; the process is energy-intensive; the generation of low-solubility LiKSO4 complex salt affects the concentrated lithium precipitation process; some rubidium and cesium still exist in the residue and are difficult to use; the waste gases such as HF and SO2/SO3 generated by roasting need to be treated to reduce environmental pollution; when K2SO4 is used as sulfate for roasting, the cost is high, and Na2SO4 is used to replace K2SO4 to reduce the cost, but when it reaches a certain amount, it will produce a glass phase that affects the normal operation of the process.

Chlorination roasting method

The chlorination roasting method mainly involves mixing lithium mica with chloride salts (sodium chloride, calcium chloride, ammonium chloride, etc.) according to a certain ratio, then ball-milling to a certain particle size, roasting at high temperature, and then leaching to obtain a lithium-containing solution. The required lithium carbonate product is obtained through subsequent purification, impurity removal, separation, lithium precipitation and other process flows.

The advantages of the chlorination roasting method are its short roasting time, simple process, high lithium recovery rate, high extraction rate of valuable metals Rb and Cs, and low amount of waste residue; but its disadvantages are that it causes serious corrosion to the equipment, high requirements for equipment anti-corrosion, and the waste gas generated pollutes the environment and needs to be treated.

Pressure Cooking Method

The pressure cooking method for lithium extraction from lithium mica is essentially a reaction in which the lithium mica concentrate is first defluorinated and roasted with water vapor at high temperature, and the mineral phase structure of the defluorinated lithium mica changes. It is then mixed with a mineral phase reconstructor (alkali, chloride, sulfate and carbonate, etc.) in a certain proportion and then leached under high pressure in a pressure cooking reactor to obtain a leaching mother liquor containing lithium and other valuable metal compounds; the leaching mother liquor is then purified, impurities removed, and lithium precipitated to obtain the desired lithium salt product. Defluorinated roasted lithium mica with low fluorine content has a good Li2O dissolution rate, but excessive defluorination requirements require a large amount of fuel consumption, thereby increasing process costs, forcing researchers to seek a more economical defluorination process.

The pressure cooking method has the advantages of simple process flow, high Li2O leaching rate, short pressure cooking time, small material flow, low corrosion to reaction equipment and good comprehensive utilization; however, the pressure cooking method needs to be defluorinated and roasted, which puts pressure on environmental protection. Because the reaction needs to be carried out under high temperature and high pressure, the reaction conditions are relatively harsh, there are safety hazards, and there are high requirements for equipment and operation processes; the defects of this method hinder its further application in industry.

Mechanical activation method

The mechanical activation method for extracting lithium is to cause the lattice structure and physical and chemical properties of lithium mica concentrate to change through mechanical force. The reaction is essentially divided into two stages: the first stage is to destroy the crystal structure of lithium mica through the action of mechanical activation force, so that the particle size of lithium mica particles is reduced, the particles are refined, and the specific surface area of the particles is increased. In addition, part of the mechanical energy is converted into chemical energy inside the crystal in the form of lattice defects, so that the lithium mica particles are in a high-energy activation state and are easy to participate in chemical reactions; the second stage is that a mechanical chemical reaction occurs during the ball milling process, and the added mechanical activator activates the Li in the lithium mica. Under the combined action of mechanical force and activator, the stability of Li-O bond structure is reduced, the solid phase reaction between lithium mica and activator is accelerated, and the efficient leaching of valuable metal components such as Li is enhanced.

Compared with the traditional lithium extraction process, the mechanical activation method does not require high-temperature roasting. It is through the mechanical activation between lithium mica and mechanical activator that Li is in a high-energy activation state, which is very easy to dissolve into the leachate through the leaching process. It has the advantages of green environmental protection, short process and high lithium extraction rate; however, this method requires a long time of ball milling, has certain requirements for ball milling equipment, and consumes more mechanical activators or a larger amount of acid solution to achieve a higher lithium extraction rate.