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 system for separating magnesium and extracting lithium from magnesium-containing brine with a secondary amide-type solvent, wherein, the extraction system 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 extraction system is a 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 extraction system containing the secondary amide has a freezing point less than 0° C.