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Click to add WeChatIn the process of lithium resource development and utilization, the processing and purification of lithium-containing minerals such as spodumene and lepidolite have long been carried out. At present, among the more than 130 known lithium-containing minerals, the processing and purification of lithium-containing minerals such as spodumene and lepidolite has long been carried out, and only a few silicate and phosphate minerals have economic value. This article takes spodumene (chain silicate) and lepidolite (layered silicate) as examples to introduce their lithium extraction technology.
Spodumene (LiAL[Si2O6]) has a high Li content (Li2O6%~9%) and is the main mineral source of lithium. Spodumene is a chain silicate mineral belonging to the monoclinic pyroxene mineral group, and silicon dioxide exists in tetrahedral form.
Due to the high stability of Li2SO4 in aqueous systems and its solubility, sulfation is one of the most commonly used technologies for processing spodumene to recover lithium. Other impurities (AL, NA, Mg and K) that are usually present in spodumene also form soluble compounds during the sulfation process, but lithium is slightly soluble in carbonate media, which helps to precipitate Li2CO3 from Li2SO4 dissolved in water.
For example: spodumene mineral is crushed and ground into a suitable particle size, then mixed with alkali metal sulfate (NA2SO4/K2SO4) and treated at 850~1200℃ to maximize lithium recovery. During the calcination process, Li is replaced by NA in NA2SO4, and Li2SO4 is formed through an ion exchange reaction (1170℃). Since the melting point of K2SO4 is higher than that of NA2SO4, a higher temperature is required for the ion exchange reaction to occur, but at higher temperatures, silica will decompose, resulting in a reaction that is not conducive to lithium recovery.
In the carbonation treatment, spodumene is mixed with NA2CO3 and heated at 525~675℃, so that the NA atoms of NA2CO3 replace the Li atoms, forming lithium carbonate and sodium aluminum silicate. In the presence of CO2, the roasted product is leached with water to precipitate and separate lithium in the form of Li2CO3.
Before Li2CO3 is produced, the formation of LiHCO3 is an important step. Since the solubility of Li2CO3 in water is very low, CO2 gas needs to be injected to convert it into LiHCO3 with higher solubility. Lithium bicarbonate exists at pH 6-8. Autoclaving the system in the presence of CO2 at 250°C can increase the solubility of LiHCO3 in the solution, thereby improving the process efficiency.
Chlorination is also an effective purification method because CL2 can react with metal oxides and silicates to form water-soluble chlorides. The chlorination treatment of lithium-containing ores can selectively extract lithium at high temperatures, but the process is complex and requires highly corrosion-resistant equipment. When the Si-AL-Li-O-CL system reaches equilibrium during the chlorination process, the possibility of forming solid or liquid LiCL and other solid products of AL and Si in different phases. In addition, in a fixed bed reactor, the chlorination of spodumene concentrate (7.25% Li2O and 2% impurities Fe, CA, Mg) was carried out under the conditions of a total flow rate of 100ml/min, a CL2 partial pressure of 0.2~1.0Atm (about 0.02~0.1MPA) and 1000~1100℃.
Lithium recovery is carried out by leaching β-spodumene in liquid hydrofluoric acid. Under reasonable leaching conditions (temperature 75°C, 7% HF, stirring speed 330r/min, reaction time 10min), up to about 90% of Li can be recovered. Si and Al dissolved in HF are removed by reacting with NAOH to form NA2SiF6 and NA3ALF6 precipitates. At the same time, LiF is also converted into a soluble hydroxide form (LiOH) and evaporated until the lithium concentration reaches 20g/L. Finally, the solution is heated to 95°C for 20min to precipitate lithium in the form of carbonate, and the unwashed Li2CO3 precipitate is obtained with a purity of 98%.
Lepidolite, also known as lepidolite, is a layered silicate mineral with a monoclinic crystal structure. Its basic framework is two silicon-oxygen tetrahedral meshes filled with octahedral coordinated cations, and lithium is sandwiched between the two tetrahedral meshes in the form of octahedral coordination. If lithium is to be extracted from lepidolite by leaching, it is necessary to first release lithium from the encapsulation structure by roasting. Lepidolite can be roasted by a variety of reagents. At present, there are several methods, including sulfate roasting, carbonate roasting, and chloride roasting, i.e., defluorination lime pressure treatment.
As it is difficult to directly leach lithium from lithium mica, it is then leached and precipitated to extract a concentration of 1% to 2%. Concentrated sulfuric acid is used to treat the mineral at a temperature of 150 to 170°C, replacing Li+ with H+ to form Li2SO4. The digested Li2SO4 suspension is then selectively dissolved in cold water. After removing the dissolved AL by CA(OH)2 precipitation, NA2CO3 is used as a precipitant to precipitate lithium in the form of Li2CO3 at 90°C.
Lithium can also be recovered by leaching lepidolite with water after roasting with CACO3. In this process, the mineral ground to the required particle size is roasted with CACO3 at 1000°C until the material is completely solidified]. The melt is treated with hot water to dissolve Li, NA, Rb, CS, CA and a small amount of Fe and Mg. Therefore, impurities are removed before Li2CO3 precipitation, and CA is separated by precipitating CACO3 by passing CO2 into the solution, and neutralized with HCL to convert it into chloride.
In addition to the sulfate and carbonate processes, lithium can also be extracted by leaching after chloride roasting. During calcination with sodium chloride and calcium chloride, lithium forms soluble chloride complexes such as NACL, CACL2 and mixtures thereof.
When lepidolite and sodium chloride are calcined at a mass ratio of 1:2 and 1:1, the product combinations are LiCL, KCL, NAALSi3O8 and SiO2. When CACL2 and lepidolite are calcined at a mass ratio of 1:2, the product combinations are LiCL, KCL, NAALSi2O6, SiO2, CAF2, CASiO3 and CAAL2Si2O8. When the mass ratio of CACL2 to lepidolite increases to 1:1, the LiALSi2O6 phase disappears.
Due to the incomplete diffusion of chlorine in lepidolite, the extraction efficiency in both cases is only 62%. Further roasting with a mixture of NACL and CACL2 increases the fluidity of chloride and reduces the viscosity of the liquid phase, thereby improving the extraction rate of lithium, because the melting point of the mixture of NACL and CACL2 is lower than that of NACL and CACL2. Therefore, the mixed chloride roasting is easier to diffuse to the surface of lepidolite than pure NACL or CACL2, and the extraction efficiency of Li is increased to 92.00% when maintained at 60℃ for 30min. After the leaching solution is purified under appropriate pH conditions, lithium is recovered in the form of Li2CO3.
Lepidolite is treated with steam (H2O) at high temperature to form LiAL(SiO3)2 (aluminum silicate), KALSi2O6 (leucite), and HF (hydrofluoric acid). During the heat treatment, H2O dissociates into H+ and OH-. H+ reacts with fluoride to form HF, and hydroxyl reacts with Si—O—Si bonds of lepidolite to form Si—OH groups. Si—OH groups can react with OH- to form new phases, such as H2O, leucite, or aluminum silicate. Under the optimal steam roasting conditions (860℃, 30ｍiN), 42.3% of fluoride can be removed from lepidolite. The steam-treated lepidolite is further pressurized with NA2CO3 solution at 130℃ to obtain Li with a purity of about 99%. Based on the principle that the solubility of LiOH is lower than that of other alkali metal hydroxides, LiOH is recovered in the form of salt crystals. After redissolution, CO2 gas is introduced to generate LiHCO3, which is heated to 90~100℃ to precipitate Li2CO3.
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