Copper is a kind of strategic resource to support low carbon transition and economic development. The main sources of raw materials for copper smelting are copper concentrate and recycled copper resources. The existing researches on the damage mechanism of refractories that are used in copper smelting mainly focus on the smelting furnace, converter, anode furnace and other furnace types for copper concentrate pyrometallurgical copper refining process. However, there is little research on the damage mechanism of refractory, used in recycled copper refining furnace. Nerin-Gutai-Lu (NGL) has a great significance for the recycled of waste copper and alleviates the shortage of copper concentrate resources. The copper smelting process is characterized by intense chemical reactions, fast speed, high heat intensity and complex furnace atmosphere (i.e., O₂ and SO₂) in the furnace, which can easily damage the furnace lining refractories. The magnesia-chrome refractories have extensive applications in coppers smelting because of its excellent slag corrosion resistance. In this study, the damage mechanism of magnesia-chrome brick used in NGL recycled copper refining furnace was investigated. The macroscopic morphology, phase composition, and microstructure of the used magnesia-chrome brick were analyzed.

The sample was selected from magnesia-chrome brick used in the NGL furnace of a copper plant in Jiangxi Province, China, and their corrosion behavior and mechanism were analyzed. The residual brick was horizontally cut and divided into different areas according to the degree of corrosion, which were the reaction layer, infiltrated zone and the similar original brick zone. The chemical and phase composition of different areas of the residual brick were characterized by X-ray fluorescence (XRF) and X-ray diffraction (XRD), and the bulk density and apparent porosity were measured based on the Archimedes principle using water as a medium. The degradation on mechanism and microstructure of the sample were analyzed by scanning electron microscopy (SEM), and the composition of the sample was identified by attached energy dispersive spectrometry (EDS).

The magnesia-chrome brick is destroyed through chemical damage by NGL slag and copper melt due to the forsterite phase and the dissolution of periclase grains. The crude copper and copper oxides exhibit an intense permeability due to the penetration of slag (containing gas) and dissolution of periclase particles. With a large number of crude copper and copper oxides melting, the direct combination of fused magnesia-chrome, and periclase is destroyed. This metal melt can penetrate into the internal structure of magnesia-chrome brick and reach over 260 mm, which distributes in the matrix around the chromite particles and the grain boundaries, and pores or cracks of magnesia aggregate. Subsequently, SO₂ and O₂ are diffused to the similar original brick layer on the cold face of the residual brick due to the dissociation of monticellite at the grain boundary. SO₂/SO₃ in gas-phase medium reacts with CaO in brick to form CaSO₄. The XRD patterns indicate that the main component of the reaction layer on the cold face of the residual brick is CaSO₄ phase. The related reaction under the gas diffusion of SO₂-O₂ can cause a volume expansion, leading to the structural looseness and exacerbating the melting corrosion of refractories.

The diffusion of metal elements in NGL refining slag, i.e., ferrum, nickel and zinc, led to the interaction between slag and periclase grains, resulting in the formation of forsterite (containing Fe) and (Mg, Fe, Ni)O solid solutions, which reacted with chromite to form (Mg, Fe, Ni)(Cr, Al, Fe)₂O₄ multiphase spinel. The reasons of chemical damage in magnesia-chrome refractories included the formation of forsterite phase and the dissolution of periclase grains. The corrosion of refractories by metal melts, such as Cu-Cu_xO, was mainly infiltrated. The gases containing SO₂-O₂ diffused into the similar original brick layer of the residual brick cold surface, generating low melting point alkaline earth metal sulfides mainly composed of MgSO₄ and CaSO₄. The relevant reactions under the action of SO₂-O₂ could cause a volume expansion, resulted in a loose structure of magnesia-chrome refractories and exacerbating the melting corrosion of refractories. The uniform microstructure of the fused magnesia-chrome block in the furnace lining was a mixture of magnesia-chrome spinel and periclase, which could effectively resist the infiltration of copper melt. In addition, the high-Fe spinel ring formed at the edge of chromite particles also enhanced the slag corrosion resistance.