A renewable magnesium removing agent and its use in a preparation of a low-magnesium lithium-rich brine are provided. The magnesium removing agent includes a magnesium phosphate double salt of an alkali metal or ammonium. A regeneration of the magnesium removing agent is realized by adding the magnesium removing agent into Mg.sup.2+-containing chloride salt solution, wherein Mg.sup.2+ in the chloride salt solution and the magnesium removing agent are subjected to a magnesium removing reaction to form a solid-phase reaction product and carrying out a solid-liquid separation on an obtained mixed reaction product after the magnesium removing reaction is ended to separate the solid-phase material comprising a magnesium phosphate hydrate and then separating out a chlorine salt of the alkali metal or the ammonium from a remaining liquid-phase material, and finally carrying out a regeneration reaction on the magnesium phosphate hydrate and the chlorine salt of the alkali metal or the ammonium.

A renewable magnesium removing agent and its use in a preparation of a low-magnesium lithium-rich brine are provided. The magnesium removing agent includes a magnesium phosphate double salt of an alkali metal or ammonium. A regeneration of the magnesium removing agent is realized by adding the magnesium removing agent into Mg.sup.2+-containing chloride salt solution, wherein Mg.sup.2+in the chloride salt solution and the magnesium removing agent are subjected to a magnesium removing reaction to form a solid-phase reaction product and carrying out a solid-liquid separation on an obtained mixed reaction product after the magnesium removing reaction is ended to separate the solid-phase material comprising a magnesium phosphate hydrate and then separating out a chlorine salt of the alkali metal or the ammonium from a remaining liquid-phase material, and finally carrying out a regeneration reaction on the magnesium phosphate hydrate and the chlorine salt of the alkali metal or the ammonium.

Processes are provided which pyrolytically extract hydrochloric acid from a magnesium ion-rich salt mixture. In this regard, a supply of the magnesium ion-rich salt mixture (e.g., bittern) may be directed to a pyrolytic chamber where it is contacted with heated gas (e.g., combustion flue gas) at a sufficient temperature and for a sufficient time to form a vapor product stream comprised of hydrochloric acid and an insoluble pyrolyzed mixed salt stream comprised of magnesium hydroxide and sodium sulfate decahydrate. The solid pyrolyzed mixed salt stream may be separated into separate product streams comprising the insoluble magnesium hydroxide and remaining soluble salt fractions, while the vapor product stream of hydrochloric acid from the pyrolytic chamber may be condensed form an aqueous HCl solution. The magnesium ion rich salt mixture may be dehydrated prior to pyrolysis to achieve magnesium ions in a tetrahydrate state or lower (e.g., a monohydrate to a trihydrate state).