Strong acid-mediated Ca2+ extraction–CO2 mineralization process for CO2 absorption and nano-sized CaCO3 production from cement kiln dust: Simultaneous treatment of CO2 and alkaline wastewater

Abstract

Increasing yearly stockpiles of cement kiln dust (CKD) and the associated high landfilling cost require innovative solutions to utilize CKD. This study proposes a strong acid-mediated extraction–mineralization process that utilizes CO2. The process comprises the extraction of Ca2+ from CKD using strong acids, mineralization of CO2 using extracted Ca2+ and alkaline wastewater, and production of Ca carbonate. High Ca2+ leaching efficiency, high Ca2+ extraction efficiency, and high-purity nano-sized CaCO3 production from CKD were achieved under normal operating conditions in an in-situ strong acid solution. The process feasibility was primarily tested via an individual extraction process using aqueous HCl and HNO3 solutions from typical CKD and mineral carbonation experiments using Ca(OH)2 and alkaline wastewater obtained from the extraction process. The process had a Ca2+ leaching efficiency of 94.73%, Ca2+ extraction efficiency of 93.54%, and CaCO3 yield of 1.45 ton/ton CKD in the aqueous HCl solution. Polymorphism and crystal structure analyses showed that CaCO3 obtained after CO2 mineralization was mainly present as calcite. Alkaline wastewater acted as a pH buffer, enhanced the reactivity with CO2, accelerated the conversion of CO2(g) to CO32-(aq), and acted as an accelerator that promoted the nucleation growth of CaCO3 by OH- ions during the mineralization process. The effect of the nucleation and growth of CaCO3 on the particle and crystallite size is highly dependent on the system pH. These results can be applied to CO2 utilization processes by industrial by-products.