Energy-efficient mineral carbonation of blast furnace slag with high value-added products

Large quantities of CO2 and blast furnace (BF) slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO2 emission reduction and comprehensive utilization of the solid waste. An energy- and cost-efficient mineral carbonation technology combined with NH3 capturing CO2 and simultaneous power generation is presented in this article. In this process, blast furnace slag was leached with recyclable ammonium bisulphate (ABS) to extract Ca, Mg and Al. Gaseous ammonia, which is produced during ammonium sulphate (AS) roasting for ABS regeneration, was used to capture CO2 in flue gases using a CO2 mineralization cell (CMC) to simultaneously generate NH4HCO3 and electricity. The extracted CaSO4 and MgSO4 reacted with an NH4HCO3/NH3 solution to realize stable CO2 sequestration while the extracted Al was recovered in the form of aluminium ammonium sulphate, which is a high value-added product. The process parameters and efficiency of the leaching, mineralization and Al recovery were investigated in detail in the present study. The results showed that approximately 91% of Mg and 97% of Ca in the BF slag were converted into corresponding carbonates and 76% of aluminium was recovered in the form of NH4Al(SO4)212H2O with a purity of 99.6 wt.%. 3802 kg of blast furnace slag was required for sequestration of 1000 kg CO2. A preliminary energy analysis indicated that although the present process was somewhat energy intensive without a net reduction of CO2 emission, an improved process, which combined the NH3 capturing CO2 and simultaneous generating of electrical energy with a mixed ammonium sulphate (AS)+ blast furnace (BF) slag roasting instead of the separated ammonium sulphate roasting and ammonium bisulphate aqueous leaching of the blast furnace slag, was energy-efficient with a 567 kg net reduction of CO2 emission during the sequestration of 1000 kg of CO2. An economic analysis of the process demonstrated that the sales revenue of the primary products after deducting the cost of raw materials reached up to $427 for sequestration of 1000 kg of CO2.