2020 Virtual AIChE Annual Meeting

(685h) Calcination-Free Production of Calcium Hydroxide at Sub-Boiling Temperature

Authors

Sara Vallejo Castaño - Presenter, University of California, Los Angeles
Erika Callagon La Plante, University of California, Los Angeles
Sho Shimoda, University of California, Los Angeles
Laurent Pilon, University of California, Los Angeles
Gaurav Sant, University of California, Los Angeles
Calcium hydroxide (Ca(OH)2), also known as portlandite, slaked lime, or hydrated-lime, is a widely used product in a variety of industries: as a soil stabilizer in road and building construction, as a bleaching agent in the Kraft paper process, and as a flocculent in the water treatment industry. According to the United States Geological Survey (USGS), global lime (Ca(OH)2 and CaO) production reached 420 million metric tons in 2018. Ca(OH)2 has drawn attention in the context of climate change mitigation because it can be used as a CO2 sorbent via the formation of a stable calcium carbonate (CaCO3) solid following the reaction: Ca(OH)2 + CO2 → CaCO3 + H2O a reaction which can offer cementation, e.g., in concrete. But, the production of Ca(OH)2 features a carbon intensity on the order of 0.77 tons of CO2 per ton of Ca(OH)2 produced.

This work establishes a new route to synthesize Ca(OH)2 by low-temperature precipitation from an alkaline solution saturated in the solid. This calcium-rich alkaline solution can be obtained by dissolving Ca-bearing wastes such as slags in water. The dissolution of slag releases calcium ions in a wide range of concentrations below the saturation level of Ca(OH)2 in water. Therefore, the slag leachate is concentrated using reverse osmosis (RO) by such that a less concentrated stream permeates through the membrane while dissolved salts and other impurities become concentrated in the retentate stream. Such concentration occurs on account of a combination of size exclusion, charge exclusion, and physico-chemical interactions between the ions in solution and the functionalized semipermeable membrane. Once the solution is concentrated, precipitation of Ca(OH)2 from the saturated leachate can be induced by a temperature-swing process since the solubility of Ca(OH)2 in water decreases with temperature. This allows for the production of Ca(OH)2 crystals via a low-temperature, aqueous route.

The proposed process presents several advantages compared to traditional Ca(OH)2 production. First, the raw materials constitute industrial by-products and waste, that are available at large scale, globally. Second, the process eliminates the CO2 generation associated with the calcination reaction. Finally, a detailed account of the mass and energy balances discloses that the operation of this process is particularly advantaged by the use of low-grade waste heat (e.g., T < 100°C), e.g., from industrial operations (e.g., power plants, steel plants, etc.) to operate the temperature swing process. This ensures that the CO2 emissions associated with the proposed Ca(OH)2 production process are very low; i.e., on the order of 0.27 tons of CO2 per ton of Ca(OH)2 produced. Taken together, this work highlights the opportunities, advantages, and knowledge gaps that need to be overcome to create low-temperature calcination-free pathways for the industrial-scale production of portlandite and other divalent hydroxides.