Mineral carbonation of red gypsum via pH-swing process: Effect of CO2 pressure on the efficiency and products characteristics

• Red gypsum can be used as a raw material for sequestering CO2 by mineralization.
• Ca extraction as the primary step of pH swing process affects the carbonation efficiency.
• A direct relationship between CO2 pressure and carbonation efficiency is observed.
• High purity calcium carbonate (CaCO3) is produced.

The objective of this study was to investigate the feasibility of indirect carbonation of red gypsum through pH swing process. The effect of CO2 pressure as one of the major controlling parameters in carbonation process was investigated in details. The dissolution of red gypsum was performed using 2 M H2SO4 at 70 °C for 60 min reaction time. The pH was regulated using NH4OH, which also precipitated out the impurities from the solution. Finally, the calcium rich solution was poured in an autoclave reactor and CO2 was injected into the reactor for carbonate precipitation. The experimental results showed that the Fe, the main impurity can be effectively separated prior to carbonation stage (92–95%). At low pH the Fe was separated while at high pH Ca was precipitated as carbonate. The carbonation efficiency was directly affected by the CO2 pressure and the maximum efficiency (100%) was achieved when 8 bar CO2 pressure was used. In addition, carbonation experiments resulted in CaCO3 production in the form of calcite, aragonite, and vaterite with 98% purity.