Pilot-scale Experimental Work on the Production of Precipitated Calcium Carbonate (PCC) from Steel Slag for CO2 Fixation

Abstract

The production of steel is a very energy intensive process and the industry contributes a significant amount to global carbon dioxide (CO2) emissions. Steel production also generates steel slag, a calcium-rich waste which has few useful applications and is partly landfilled. Producing precipitated calcium carbonate (PCC) from steelmaking slag (Slag2PCC) is a way to reduce CO2 emissions while at the same time turning slag waste into a valuable product. In the Slag2PCC process, a solution of ammonium chloride (NH4Cl) is used to extract calcium from steelmaking slag which is then bubbled with a CO2-containing gas in a process called carbonation to form PCC. This thesis has focussed on how the process conditions during carbonation affect the carbonation process as well as the quality parameters of the PCC produced. Carbonation tests were performed at laboratory scale (5L) and at a recently constructed pilot-scale Slag2PCC plant (200L). From the laboratory tests it was found that temperature, calcium concentration [Ca2+], NH4Cl solvent concentration [NH4Cl], CO2 flow and agitation speed have important effects on the carbonation process and the quality of the PCC produced. PCC particle size can be reduced by lower temperature, lower [Ca2+], lower [NH4Cl], lower CO2 flow and higher agitation speed. It was also found that increasing [NH4Cl] and CO2 flow is likely to increase particle agglomeration. Temperature, [Ca2+], [NH4Cl], and calcium to carbonate ratio [Ca2+]/[CO32-] appear to be the most significant factors determining the crystal morphology. Work with the pilot plant showed that the equipment should be modified to improve mixing and solid suspension performance. While the production of rhombohedral calcite and aragonite polymorphs was successfully demonstrated, attempts to make scalenohedral PCC in the pilot plant based on conventional Ca(OH)2 slurry carbonation conditions were not successful, believed to be due to the NH4Cl, [Ca2+], pH, or the difference in supersaturation conditions compared with conventional Ca(OH)2 slurry carbonation. The cost of PCC production from the Slag2PCC process was also estimated as 65 €/t based on the results of the pilot scale work. The CO2 emissions of the Slag2PCC process were estimated as -0.229 tCO2/tPCC, giving a CO2 capture cost of 284 €/t. However, if the PCC can be produced at high quality and sold at market price (120 €/t), the process delivers a profit of 55 €/tPCC and CO2 capture becomes profitable at 239 €/tCO2.