UCLA

Calcium hydroxide (Ca(OH)2) is a commodity chemical that finds use in diverse industries ranging from food to environmental remediation and construction. The commercial production of Ca(OH)2 by limestone calcination is an energy intensive and CO2 emitting process. Nevertheless, on account of its high specific CO2 uptake (0.59 g per g of Ca(OH)2), Ca(OH)2 could be a “CO2-negative” material if produced in a manner that obviates the need for the thermal decomposition of limestone. This dissertation aims to demonstrate and evaluate the feasibility of upscaling a novel aqueous-phase calcination-free process to recover Ca(OH)2 from industrial alkaline waste based on a three-step process comprised of: (i) calcium leaching from steel slag, (ii) leachate concentration by reverse osmosis (RO), and (iii) Ca(OH)2 precipitation from the concentrated solution through heating. The proof-of-concept was demonstrated on the laboratory scale by individually testing each step. Slag leaching, and reverse osmosis (RO) concentration were evaluated with bench-scale batch experiments. The results demonstrated that alkaline, Ca-containing solutions can be derived from leaching slag with DI water, and that RO could concentrate these solutions by a factor of 2 or higher. However, membrane scaling was evidenced when operating close to the saturation point of Ca(OH)2. Following concentration, Ca(OH)2 was precipitated by forcing a temperature excursion in excess of 65�C while harnessing the retrograde solubility of Ca(OH)2. Thereafter, a continuous, low-temperature (< 95�C), aqueous-phase pilot-process to produce Ca(OH)2 was designed and assembled. The quantification of the mass and energy balances revealed that increasing the calcium concentration of the feed solution and the precipitation temperature, decrease the energy demands of the RO step. The pilot system operated continuously for 24 hours and achieved a production rate of nearly 1 kg per day of Ca(OH)2 with a purity greater than 95 wt.%. The particle size of the precipitates depended on the residence time in the precipitation reactor, suggesting an ability to produce size-controlled particulates. Importantly, the process achieved full water recirculation, indicative of a low consumable water demand. Finally, the up-scale feasibility of the process was evaluated by means of an economic, CO2 footprint and geospatial distribution analysis considering the location and availability of slag, electricity, and waste heat sources. The study revealed that the Mid-West, Mid-Atlantic, and South-East regions of the U.S are potential areas to upscale the technology due to the proximity between feedstocks and waste heat sources. The economic analysis showed that RO’s electricity and membrane replacement requirements were the largest drivers of the operating cost. Finally, the CO2 footprint of the process could be 40% to 80% lower than the benchmark product if the electricity was sourced from natural gas and wind power, respectively. This is significant as Ca(OH)2 produced in this manner can uptake more CO2 than it associated with its own production, i.e., a CO2-negative material.