A pilot-process for calcium hydroxide production from iron slag by low-temperature precipitation

The production of Ca(OH)2 via thermal decomposition of limestone is an energy intensive process resulting in significant CO2 emissions. However, if produced in a manner that obviates the need for the thermal decomposition of limestone, Ca(OH)2 could be a “CO2-negative” material. Herein, we design and demonstrate the operation of a continuous, low-temperature (< 100 °C), aqueous-phase pilot-process to produce Ca(OH)2 using calcium extracted from alkaline industrial wastes. The three-step process encompasses unit operations including: (i) calcium leaching from basic oxygen furnace BOF slag, (ii) leachate concentration by reverse osmosis (RO), and (iii) Ca(OH)2 precipitation by temperature swing. This process presents several advantages compared to the traditional route. First, it advances the recycling of waste from the steel industry. Second, it operates significantly below the temperature used in conventional lime production (900 °C) as it bypasses limestone calcination, responsible for ~65% of the CO2 emissions from the current process. Thus, the new process has a lower CO2 footprint and enables the use of industrial waste heat. The mass and energy balances were quantified to reveal that increasing the calcium concentration of the feed solution and the precipitation temperature, decrease the energy demands of the RO step, thereby reducing the process’s overall CO2 footprint. The pilot system operated continuously and achieved a production rate of nearly 1 kg per day of Ca(OH)2 with a purity greater than 95 wt%. The average particle size of the precipitates depended on the residence time in the precipitation reactor, demonstrating an ability to produce size-controlled particulates. Importantly, the process achieved full water recirculation/reutilization level indicative of a low-consumable water demand. The outcomes offer new insights and understanding relevant to developing and upscaling low-CO2 processes for cement, lime, and portlandite.