Extraction system, extraction method for separating magnesium and extracting lithium from magnesium-containing brine with secondary amide-type solvent as well as applications thereof

An extraction method for separating magnesium and extracting lithium from magnesium and lithium-containing brine with a secondary amide-type solvent, wherein, it comprises the following steps:

• S1. the magnesium and lithium-containing brine is taken as a pre-extraction brine phase;
• wherein, in the magnesium and lithium-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 density of brine at 20° C. is 1.25˜1.38 g/cm3, and the pH value of the brine is between 1 and 7;
• S2. the secondary amide-type solvent is taken as a pre-extraction organic phase;
• wherein the secondary amide-type solvent contains secondary amides;
• the secondary amides consist of a single compound or a mixture of two or more compounds;
• wherein, the single compound has a structure as shown in Formula (I):
• wherein, R1 is selected from a C2˜C12 alkyl or a C3˜C12 cycloalkyl containing a single-ring structure, R2 is selected from a C1˜C11 alkyl or a C3˜C11 cycloalkyl containing a single-ring structure, and the sum of the number of carbon atoms contained in the two groups R1 and R2 is 11˜17, wherein the alkyl or the cycloalkyl comprises various isomers;
• when R1 and R2 are well-determined, the secondary amides is the single compound, while the mixture means a substance mixed by two or more compounds generated as a result of the variations of R1 and R2;
• and, the secondary amide-type solvent containing the secondary amide has a freezing point less than 0° C.;
• 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.