Purification of magnesium chloride from mixed brines via hydrogen chloride absorption with ambient temperature and pressure regeneration of super azeotropic hydrochloric acid

Abstract

Desalination is an important source of potable water for a large portion of the world that is expected to expand in response to population increase and climate change. Despite its benefits, desalination produces a hypersaline waste, termed ‘brine’, which has negative impacts on the marine ecosystem and is costly to dispose of safely. An integral step in making desalination more environmentally friendly is the elimination of liquid wastes, so-called ‘Zero Liquid Discharge’. To make Zero Liquid Discharge financially viable, it is critical to separate and purify the constituent salts into products. We report on the design and operation of a system for continuous production of high purity Mg-chloride from desalination brine. This system is part of a Zero Liquid Discharge process that makes desalination net negative in terms of CO2 emissions by converting magnesium chloride into magnesium carbonate. Precipitation of sodium and potassium chloride from brine was achieved by absorption of a super azeotropic HCl gas. The super azeotropic HCl gas was regenerated by mixing the acidified brine in a low-temperature, saturated magnesium chloride solution. The system recovered 89% of the Mg2+ content of desalination brine as 99.6 mol% purity magnesium chloride. This level of recovery and purity without the use of consumable chemicals is not found elsewhere in industry or the literature. Internal regeneration of HCl reached 66% at a HCl concentration of 54 wt%. Empirical results suggest a longer operation would yield 92% regeneration of 65 wt% HCl. Using lessons learned from this test program, a range of long-term continuous operation conditions were found that could regenerate >95% of HCl at a concentration of >90 wt%. Such performance may make this novel HCl regeneration process interesting to industries that require hydrochloric acid recycling, such as hydrometallurgy. With the recovery and purity of magnesium chloride achieved in the current system, desalination brine can be converted into product streams, including those applicable for atmospheric CO2 removal.