Gas-refractory interactions of basic refractories with sulfur-bearing gases produced in converter furnaces during extraction of Platinum Group Metals in noferrous industry

摘要

The fundamental reaction mechanisms, when unused chrome-magnesite,udmagnesite-chrome and magnesite refractories are exposed to sulfur-bearing gas mixtures ofudSO2-O2, SO2-N2 and SO2 at 700°C to 1350°C, were investigated. The investigationsudcomprised chemical analyses, thermodynamic analyses and microstructural examination.udThe extent of the conversion reactions was measured as a function of temperature andudreaction time. Six basic refractories were investigated and characterized using ScanningudElectron Microscopy (SEM) and X-ray powder diffraction (XRPD) methods. Theudmicrostructural examination indicated that physical changes in the original microstructure ofudthe refractory could possibly influence the reactions with sulfur-bearing gases and theudchemical analyses of the product phases confirmed the formation of secondary sulfatesudwithin pores, along grain boundaries and cracks. The structural features in the originaludrefractory were determined by technological processes employed during original firing inudproduction. The factors attributed to the principal physical changes of the originaludmicrostructure were related to the diffusion processes, which affected the phase distribution,udpore size distribution and grain size. The fundamental reactions involved in the pore sizeuddistribution and microstructure were attributed to the differential diffusion mechanism ofudindividual components that resulted in the development of magnesia spinels.udThe effects on the microstructure demonstrated that the reactions were limited to the grain boundary region due to the product layer formed as the reactions progressed. The chemicaludproduct analyses indicated that in the sulfates formed, the sulfation reactions wereudsignificantly affected by the equilibrium thermodynamics of the oxygen and sulfur dioxideudgas combination in the products of 1/ 2udSO2 O2 P P . The phase changes led to the dissolution of theudrefractory components and crystallization of MgO and CaO. This influenced theudpermeability of the sulfur-bearing gases that reacted to form MgSO4 and CaSO4 found asudgrain boundary and pore space fillings. Resistance to sulfur-bearing gas infiltration andudsulfation in fine-grained direct bonded refractories were found to be better than that of theudcoarse-grained direct bonded refractories. The decreased sulfation of the fine-grainedudrefractory was related to the restricted diffusion processes due to the stable magnesia spineludphases and retention of the closed pore structure at different temperatures. However, theuddissolution and crystallization mechanism described in sulfation, demonstrated that MgSO4,udCaSO4, CaMg3(SO4)3 and compound mixtures of sulfate, silica, magnesia and sesquioxidesud(Fe2O3, Al2O3 and Cr2O3) were formed along the grain boundaries.