Ettringite Precipitation for Sulfate and TDS Reduction: A 10 GPM Pilot Plant Study

Sulfate-impacted mine waters are under regulatory scrutiny to reduce total dissolved solids (TDS). There are several options to reduce TDS. One option is transforming the soluble sulfate into insoluble ettringite. The Water Company LLC (TWC) patented a process for sulfate removal through ettringite formation by reacting directly with metallic aluminum. Freeport-McMoRan (FMI) and TWC have collaborated to design, build and operate a 10-gallon-per-minute (gpm) ettringite pilot plant to establish reagent consumption, plant availability and full-scale design criteria. Using metallic aluminum as the primary aluminum source for ettringite precipitation eliminates the addition of other ions, resulting in significant demineralization. The pilot plant achieved greater than 98% water recovery, greater than 90% reagent efficiencies and maintained reactor sulfate levels below 250ppm. BACKGROUND Ettringite is a hydrated calcium, aluminum, sulfate mineral with the following formula: Sulfate-impacted mine waters are under regulatory scrutiny to reduce total dissolved solids (TDS). There are several options to reduce TDS. One option is transforming the soluble sulfate into insoluble ettringite. The Water Company LLC (TWC) patented a process for sulfate removal through ettringite formation by reacting directly with metallic aluminum. Freeport-McMoRan (FMI) and TWC have collaborated to design, build and operate a 10-gallon-per-minute (gpm) ettringite pilot plant to establish reagent consumption, plant availability and full-scale design criteria. Using metallic aluminum as the primary aluminum source for ettringite precipitation eliminates the addition of other ions, resulting in significant demineralization. The pilot plant achieved greater than 98% water recovery, greater than 90% reagent efficiencies and maintained reactor sulfate levels below 250ppm. BACKGROUND Ettringite is a hydrated calcium, aluminum, sulfate mineral with the following formula: Ca6Al2(SO4)3(OH)12•26H2O Ettringite readily precipitates in high pH water and by doing so, removes sulfate from impacted waters. The SAVMIN process, piloted in South Africa, is one method to produce ettringite. In this process, aluminum hydroxide is the aluminum source. The Cost Effective Sulfate Removal (CESR) process is similar to SAVMIN, but uses a proprietary aluminum source instead of aluminum hydroxide. Outotec and Veolia also have their own versions of the process using their specialized process equipment. TWC patented a version of ettringite precipitation using metallic aluminum as the aluminum source for ettringite formation. The benefit of using aluminum metal is that no additional ions are added to the process. At high pH, the raw aluminum corrodes in water causing hydrogen gas generation as the water is chemically reduced. This reaction is shown below: