Extraction system, extraction method for separating magnesium and extracting lithium and boron from magnesium-containing brine with complex solvent of secondary amide/alkyl alcohol as well as applications thereof

An extraction method for separating magnesium and extracting lithium and boron from magnesium-containing brine with a complex solvent of secondary amide/alkyl alcohol, wherein, it comprises the following steps:

• S1. The magnesium-containing brine is taken as a pre-extraction brine phase;
• wherein, in the magnesium-containing brine, the concentration of lithium ions is 0.1˜21 g/L, the concentration of magnesium ions is 80˜125 g/L, the concentration of chloride ions is 200˜400 g/L, the mass ratio of magnesium to lithium is 4.8˜1100:1, the concentration of boric acid and borate ions is 0.5˜20 g/L totally on the basis of B2O3, the density of brine at 20° C. is 1.25˜1.38 g/cm3, and the pH value of the brine is adjusted to 0˜7 with hydrochloric acid or sulfuric acid;
• S2. The complex solvent of secondary amide/alkyl alcohol is taken as a pre-extraction organic phase;
• S3. The pre-extraction organic phase and the pre-extraction brine phase are mixed at a volume ratio of 1˜10:1, and subjected to a single-stage extraction or a multi-stage countercurrent extraction, then subjected to a two-phase separation to get a loaded organic phase and a post-extraction brine phase;
• S4. With water as a stripping agent, the loaded organic phase is subjected to a single-stage stripping or a multi-stage countercurrent stripping, the ratio of stripping phases, i.e., the volume ratio of the stripping agent to the loaded organic phase, is 1:1˜20, then the two phases are separated to get a post-stripping organic phase and a post-stripping water phase;
• wherein, in the step S4, the stripping temperature is 0˜50° C.;
• the two phases are mixed with stirring;
• after the stripping, the two phases are separated by centrifugation or settlement;
• S5. The post-stripping organic phase is returned to the step S2 to realize the recycling of the complex solvent;
• S6. The post-stripping water phase is further purified and concentrated;
• boric acid is precipitated from the solution, washed and dried to produce a boric acid product;
• S7. An agent for removing impurities is added into the lithium-containing solution after the precipitation of boric acid to remove the sulfate radicals and remaining magnesium ions therein, to get a refined lithium chloride solution;
• the agent for removing impurities is one or two or more of calcium oxide, calcium hydroxide, calcium chloride, barium chloride, sodium carbonate, sodium oxalate or sodium hydroxide;
• S8. The refined lithium chloride solution is subjected to concentration, crystallization, separation and drying to produce a lithium chloride product.