UCLA

Limestone (calcite, CaCO3) is an abundant and cost-effective source of calcium oxide (CaO) for cement and lime production. However, the thermochemical decomposition of limestone (∼800 °C, 1 bar) to produce lime (CaO) results in substantial carbon dioxide (CO2(g)) emissions and energy use, i.e., ∼1 tonne [t] of CO2 and ∼1.4 MWh per t of CaO produced. Here, we describe a new pathway to use CaCO3 as a Ca source to make hydrated lime (portlandite, Ca(OH)2) at ambient conditions (p, T)-while nearly eliminating process CO2(g) emissions (as low as 1.5 mol. % of the CO2 in the precursor CaCO3, equivalent to 9 kg of CO2(g) per t of Ca(OH)2)-within an aqueous flow-electrolysis/pH-swing process that coproduces hydrogen (H2(g)) and oxygen (O2(g)). Because Ca(OH)2 is a zero-carbon precursor for cement and lime production, this approach represents a significant advancement in the production of zero-carbon cement. The Zero CArbon Lime (ZeroCAL) process includes dissolution, separation/recovery, and electrolysis stages according to the following steps: (Step 1) chelator (e.g., ethylenediaminetetraacetic acid, EDTA)-promoted dissolution of CaCO3 and complexation of Ca2+ under basic (>pH 9) conditions, (Step 2a) Ca enrichment and separation using nanofiltration (NF), which allows separation of the Ca-EDTA complex from the accompanying bicarbonate (HCO3 -) species, (Step 2b) acidity-promoted decomplexation of Ca from EDTA, which allows near-complete chelator recovery and the formation of a Ca-enriched stream, and (Step 3) rapid precipitation of Ca(OH)2 from the Ca-enriched stream using electrolytically produced alkalinity. These reactions can be conducted in a seawater matrix yielding coproducts including hydrochloric acid (HCl) and sodium bicarbonate (NaHCO3), resulting from electrolysis and limestone dissolution, respectively. Careful analysis of the reaction stoichiometries and energy balances indicates that approximately 1.35 t of CaCO3, 1.09 t of water, 0.79 t of sodium chloride (NaCl), and ∼2 MWh of electrical energy are required to produce 1 t of Ca(OH)2, with significant opportunity for process intensification. This approach has major implications for decarbonizing cement production within a paradigm that emphasizes the use of existing cement plants and electrification of industrial operations, while also creating approaches for alkalinity production that enable cost-effective and scalable CO2 mineralization via Ca(OH)2 carbonation.