The bulk of steel production is met through the Basic Oxygen Furnace (BOF) route. In 2017, 71.6% (1206.9 million metric tons) of the world steel was produced through the BOF route. The impurities present in hot metal are removed in the form of slag upon addition of fluxes and oxygen blowing during the converting process in BOF. This slag is known as steel making slag or BOF/LD slag and is considered as a solid waste by the steel industries. About 150-200 kg of BOF slag is generated per ton of liquid steel depending on the raw material chemistry and the steel grade requirement.
Around 45% of the solid wastes generated in steel plants, such as Blast Furnace (BF) slag, BF sludge, BF flue dust, BOF slag, BOF sludge and ladle slag are dumped. Applying the same proportion to BOF slag, the amount that is dumped comes out to be ~ 95 mtpa. The chemical composition of the BOF slag varies depending on the raw material chemistry, steel grade produced and operating procedure of steel plants.
Steel plants are trying to use a part of BOF slag generated in BF to harness the free CaO and FeO present. Although the BOF slag contains large amounts of free CaO and FeO which are of great value, it needs to be processed to remove other unwanted phases to maximize its efficient utilization as an ingredient for sinter plant or for direct charging into BF. The presence of high levels of phosphorus (1-3 wt.% P2O5) in the BOF slag prevents its unlimited internal recycling (within steel plant) since it increases the phosphorus load in hot metal/steel. Increased phosphorus levels mean increased effort, treatment time and associated cost for removal of additional phosphorus from steel. Hence, phosphorus needs to be removed from the BOF slag first to turn this so-called ‘waste’ product into a valuable one. The present work depicts a practically implementable methodology consisting of chemical modification and physical separation of granulated BOF slag for P2O5 removal enabling its internal recycling.