A cleaner production of sodium hydrogen carbonate: partial replacement of lime by steel slag milk in the ammonia recovery step of the Solvay process

Most soda ash originates from the Solvay process that involves the aqueous reaction of NaCl, CO₂, and NH₃ rendering, after five steps, sodium carbonate as well as solid and liquid wastes. In the final stage, the NH₄Cl originated in the first step reacts with Ca(OH)₂ (lime milk), recovering most of the NH₃, recycled in the process. In this work, we propose the use of steel slag, a low cost and abundant waste material of siderurgy, as source of CaO in the Solvay process. In our approach, the lime milk is partially replaced by steel slag in the NH₃ recycling, recovering up to 40 wt% of the total ammonia. It can decrease the amount of CaCO₃ and fossil fuels required and consequently decrease the amount of wastes originated in the procedure. In addition, we found that a simple thermal treatment of the mother liquor, immediately before the reaction with Ca(OH)₂, could recover about 17–26 wt% of the NH₃, reducing reagent demands and consequently effluent amounts. Another advantage is the lesser Ca(II) detected in the waste waters and the fewer caustic sludge produced in this new process. Despite it is a preliminary work, at the laboratory scale, our procedure suggests a synergic connection of the steel and soda ash productions that might reduce energy and costs, as well as the carbon dioxide emissions related to both the Na₂CO₃ and steel industries.