Residual heat-driven pressurized digestion of free lime in basic oxygen furnace steel slag: industrial-scale test and carbon mitigation potential

Global annual production of basic oxygen furnace (BOF) steel slag exceeds 150 million tons. The volumetric instability of this steel slag induced by free lime (f-CaO) severely limits its resource utilization. Water or steam is the best medium for digesting f-CaO, and high-pressure environment can accelerate the process. The paper demonstrates an industrial-scale residual heat-driven pressurized digestion process that innovatively harnesses slag’s inherent thermal energy to generate autogenous steam pressure (0.2 MPa) in a closed equipment. The process includes roll crushing and pressurized hydrothermal digestion. The relationship between water spraying time and steel slag temperature, and the relationship between number of roll crushing, initial temperature and particles size distribution were clarified during roll crushing step. The relationship between steel slag temperature, water spraying regulation and system pressure maintain time was optimized in the processes of pressurized hydrothermal digestion step. The results of 30 t industrial trials showed that heat-driven pressurized digestion could rapidly reduce f-CaO content from about 7.2% to 1.3% within 3 h without additional energy consumption. The treated slag exhibited a 20-day immersion expansion ratio of about 0.5%, satisfying China’s GB/T 20491 (<2%), Japan’s JIS A 5015 (<2%) and European Union’s EN 13242 (<3.5%). Compared with water steam aging, high-pressure water steam aging and hot water aging, the CO 2 emission reduction potential of our process treating 1 t steel slag is between 100 and 180 kg. And the annual global CO 2 emissions reduction could reach 7.5 million tons if 50% of the global BOF steel slag adopted this technology.