Effect of Respectively Adding SiC and Fused Al_2O_3-ZrO_2 Clinker in Al_2O_3-SiO_2 Castables on Resistance to PbO-rich Slag

Abstract: Among several techniques, Kaldo furnace has been widely used in nonferrous metal smelting due to its good flexibility for starting materials, low equipment investment, low production cost and better environmental protection. However, the service lives of the so far used flue working linings in Kaldo furnace are poor in China, because of severe attack by PbO-rich slag. Therefore, improvements in refractory selection and properties, in particular resistance to the PbO-rich slag, are urgently necessary. Nevertheless, little published systematic work in this connection has been found.Considering silicon carbide and zirconia are of favorable constituents to enable improved properties of Al_2O_3-SiO_2 refractories, especially in slag resistance and thermal shock resistance, this work attempted to introduce SiC and ZrO_2, using black silicon carbide (SiC 98%) and fused corundum-zirconia clinker (ZrO_2 30%) respectively, into low cement (LC) Al_2O_3-SiO_2 castables to be used for Kaldo furnace flue to improve the resistance to PbO-rich slag. The LC Al_2O_3-SiO_2 castables was prepared with bauxite (Al_2O_3 87%) as the aggregates, and brown fused alumina,α-Al_2O_3 ultrafines, CA cement and microsilica as the matrix portion. The effects of respective additions of SiC up to 25% at 5% interval and the fused corundum-zirconia clinker up to 20% at 4% interval on the resistance to PbO-rich slag, thermal shock resistance (TSR) and physical properties in terms of bulk density (BD), apparent porosity (AP), cold compressive strength (CCS), cold modulus of rupture (CMOR) and permanent linear change (PLC) were investigated. The oxidation dynamics of the castables containing 25% SiC was investigated by TG analysis. Microstructures of the castables before and after slag resistance test by crucible approach were analyzed by means of SEM-EDS and XRD.Compared with the reference castables, adding SiC leads to obviously improved resistance to the PbO-rich slag. Meanwhile thermal shock resistance gets also improved along with increased SiC addition. SiC addition has little effect on physical properties including BD, AP, CCS, CMOR and PLC.Oxidation dynamics investigation in the temperature range of 1250℃~1350℃reveals that the relationship between the weight gain of unit area and time of the castables specimen containing 25% SiC, as a result of SiC’s oxidation, follows a parabolic equation, i.e. (ΔW_t/A_t)~2 = kDt. The reaction rate is controlled by oxygen diffusion rate, and the apparent activation energy is derived to be 308.7 kJ/mol.With increased corundum-zirconia addition, BD tends to decrease, while AP to increase after heating at different temperatures. CCS and CMOR the castables heated at 110℃for 24h and 1000℃for 3h tends to decrease, whereas those heated at 1300℃for 3h of the castables increase at first and then decrease as SiC addition increases; at 8% corundum-zirconia addition, CMOR of the castables reaches a maximum, where the slag resistance shows the best. No positive effect on TSR as a result of corundum-zirconia addition has been achieved under the conditions in this work.The slag corrosion mechanism is revealed as that the penetrated PbO in the slag reacts with SiO_2 in the Al_2O_3-SiO_2 castables, yielding low-melting point compounds, such as 2PbO·SiO_2, PbAl_2Si_2O_8, PbAl_2SiO_6, etc., leading to the degradation of the castables. On the other hand, SiC and ZrO_2 show better corrosion resistance to the PbO-rich slag.This work reveals that the resistance to PbO-rich slag can be significantly improved by adding SiC or somewhat improved by a proper addition of fused corundum-zirconia clinker in the Al_2O_3-SiO_2 castables. What achieved by this work deserves to be a reference and guidance to further work on the refractories used for Kaldo flue…
Key words: Slag resistance; PbO-rich slag; Kaldo furnace; LC castables; Al_2O_3-SiO_2 system; silicon carbide; fused corundum-zirconia clinker; properties

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