Renewable magnesium removing agent and its use in preparation of low-magnesium lithium-rich brine

A method for removing Mg from a brine, comprising:

• providing a first solution, which is a brine from a salt lake, wherein the first solution is a chloride solution, and the first solution at least contains Mg2+ and Li+;
• adding a magnesium removing agent into the first solution, wherein the magnesium removing agent comprises a magnesium phosphate-containing double salt of one or more selected from the group consisting of ammonium magnesium phosphate, potassium magnesium phosphate, sodium potassium magnesium phosphate and sodium ammonium magnesium phosphate to produce a mixed reaction product comprising a solid-phase material and a liquid phase material, wherein the solid-phase material comprises a solid-phase reaction product obtained from a magnesium removing reaction between at least a portion of Mg2+ and the magnesium phosphate-containing double salt;
• carrying out a solid-liquid separation on the obtained mixed reaction product after the magnesium removing reaction is ended;
• obtaining the solid-phase material and the liquid-phase material after the solid-liquid separation is finished, wherein the solid-phase reaction product comprises a magnesium phosphate hydrate, and the liquid-phase material is a second solution;
• and separating the chloride of the alkali metal or of the ammonium from the second solution;
• and
• carrying out a regeneration reaction by reacting the solid-phase reaction product and the chloride of the alkali metal or of the ammonium, wherein the alkali metal is selected from Na and/or K, to produce new magnesium phosphate-containing double salt.