Abstract: The present invention relates to a process of purifying a residue from an industrial process using calcium ions to obtain a purified brine, comprising: (a) mixing the residue comprising sulfate salts from the industrial process with calcium salts; (b) separating insoluble species and/or precipitates from the suspension from (a); (c) adding one or more selected from C02, carbonates, bicarbonates, hydrogen carbonates or fluorinated salts into the filtrate from (b) to remove excess calcium ions; and (d) separating precipitates from the suspension from (c) to obtain a purified brine. Strontium and/or barium salts, which typically exist in the residue, may also be removed from the residue by using the present process. According to the present method, calcium ions are effectively removed from the brine. Thus, a deposition of calcium ions on devices, which are used in the further processes such as crystallization or electrolysis, can be avoided.