Lithium silicates synthetized from iron and steel slags as high temperature CO2 adsorbent materials

Abstract

The use of solid wastes and industrial by-products to prepare CO2 adsorbents is an alternative to conventional reagent grade raw materials that has recently gained interest. Among waste materials, slag has a high content of silica and calcium and is the largest solid by-product from iron and steel industry, thus its use can reduce the production costs of CO2 adsorbent materials, such as lithium silicates, which are applied in capture processes at high temperatures. Li4SiO4 has potential applications in post-combustion CO2 capture as well as in H2 production by sorption enhanced steam reforming process. In this study, Li4SiO4 was prepared using solid-state reaction and two iron and steel slags as SiO2 sources to evaluate their characteristics and CO2 capture capacities. The slag-derived lithium silicates (S1-Li4SiO4 and S2-Li4SiO4) were characterized by XRD, adsorption-desorption N2 and SEM. Different capture tests at CO2 partial pressures ( $$P_{{{\text{CO}}_{2} }}$$ ) of 0.05, 0.10, 0.15 and 0.20 were performed using thermogravimetric (TG) and temperature programmed (TPC-TPDC) techniques. The kinetic parameters of the CO2 capture process were obtained by fitting the experimental results to the Avrami–Erofeev model. Finally, the cyclic behavior of S1-Li4SiO4 and S2-Li4SiO4 was analyzed in $$P_{{{\text{CO}}_{2} }}$$ of 0.2 and 0.05. XRD patterns showed that Li4SiO4 was the main crystal phase (60 wt%) present in S1-Li4SiO4 and S2-Li4SiO4 in addition to calcium phases such as Li2CaSiO4, Ca3SiO5 and CaO. According to the TG and TPC-TPDC tests, the derived lithium silicates showed CO2 uptake three times greater than the values recorded for Li4SiO4 (134 mgCO2/g sorbent for S1-Li4SiO4) produced from pure reagents, at $$P_{{{\text{CO}}_{2} }}$$ between 0.2 and 0.05 and 650 °C. Furthermore, these materials had kinetic constants at least one order of magnitude higher than those reported for Li4SiO4, at the aforementioned operating conditions. Both materials exhibited an excellent stability during 20 cycles of CO2 adsorption/desorption. These results showed that slags can be used as silica source to produced adsorbents with better performance and stability in the CO2 capture process at high temperature than the one of Li4SiO4 produced from pure reagents, at $$P_{{{\text{CO}}_{2} }}$$ of 0.2–0.05.