Calcination-free production of calcium hydroxide at sub-boiling temperatures

Abstract

Calcium hydroxide (Ca(OH)₂), a commodity chemical, finds use in diverse industries ranging from food, to environmental remediation and construction. However, the current thermal process of Ca(OH)₂ production via limestone calcination is energy- and CO₂-intensive. Herein, we demonstrate a novel aqueous-phase calcination-free process to precipitate Ca(OH)₂ from saturated solutions at sub-boiling temperatures in three steps. First, calcium was extracted from an archetypal alkaline industrial waste, a steel slag, to produce an alkaline leachate. Second, the leachate was concentrated using reverse osmosis (RO) processing. This elevated the Ca-abundance in the leachate to a level approaching Ca(OH)₂ saturation at ambient temperature. Thereafter, Ca(OH)₂ was precipitated from the concentrated leachate by forcing a temperature excursion in excess of 65 °C while exploiting the retrograde solubility of Ca(OH)₂. This nature of temperature swing can be forced using low-grade waste heat (≤100 °C) as is often available at power generation, and industrial facilities, or using solar thermal heat. Based on a detailed accounting of the mass and energy balances, this new process offers at least ≈65% lower CO₂ emissions than incumbent methods of Ca(OH)₂, and potentially, cement production.