Study on Binder of Cold-Bonded Pellets Containing Basic Oxygen Furnace Dust Based on Hydration Mechanism of Magnesium Potassium Phosphate Cementitious Material

Basic Oxygen Furnace (BOF) dust can be mixed with binders to prepare cold-bonded pellets and return to BOF for recycling, because of its high total iron and alkaline oxide content. Nevertheless, the lengthy digestion reaction of dead-burned MgO and CaO contained in BOF dust causes expansion and cracking of the pellets, which seriously affect the pellet strength. In this study, it was first proposed to use a hydration reaction between dead-burned MgO and KH₂PO₄ to form bonding phases based on the hydration mechanism of magnesium potassium phosphate (MKP) cementitious material, to solidify MgO contained in BOF dust and improve the pellet strength. The key issue of using MKP cementitious material as a binder to improve the pellet strength is to choose appropriate magnesium-to-phosphate molar ratio M/P and water-to-binder mass ratio w/b. Based on this, cold-bonded pellets were prepared by using different ratios of MKP binder. The compressive strength, hydration products, microstructure and thermal properties of the pellets were analyzed, and the bonding mechanism of MKP binder in the pellets was proposed. The results show that the pellets have the highest mechanical strength and the most K-Struvite crystals under M/P molar ratio of 4 and w/b mass ratio of 0.5. At high temperatures, K-Struvite crystals will not evaporate harmful phosphorus-containing substances due to the decomposition of the binder components. MKP binder is unevenly distributed in the pellets: in the binder concentrated areas, the massive and dense K-Struvite crystals wrap BOF dust particles; in the binder decentralized areas, the fine K-Struvite crystals are filled between the dust particles or adhere to the surface of dust matrix, connecting the dust particles into a whole.