Study of the corrosion resistance on nano-ZrO 2 doped MgO ceramic composites by copper slag.

This study investigated the corrosion mechanism on 100 wt. % MgO and 95 wt. % MgO with 5 wt. % nano-ZrO 2 ceramic composites. First, MgO powder and powder mixtures (MgO + nano ZrO 2 ) were uniaxially and isostatically pressed and, then, they were sintered at 1650 °C. Corrosion by copper slag was studied with sintered samples. Physical properties, microstructure and penetration of the slag in the refractory were studied. Results reveal that ZrO 2 nanoparticles enhanced the densication of the samples, promoting grain growth due to diffusion of vacancies during sintering process. Additionally, magnesia bricks were severely corroded, if compared with those doped with nano-ZrO 2 , due to the dissolution of MgO grains during the chemical attack by copper slag.


Introduction
Copper metallurgy is mainly based on copper sulphides as chalcopyrite (CuFeS 2 ), bornite (Cu 5 FeS 4 ) and chalcocite (Cu 2 S) are the main ores. Thus, fusion-conversion process is the most widely used in the copper metallurgy to obtain the metal. The fusion process generates two immiscible phases: matte (mostly copper as sulphide) and slag (most of iron as fayalite). Copper in the slag will be as sul de (matte) that was dragged or trapped by the slag, or as oxide associated to other oxides of the slag.
Copper slags are disposed in dumps or sold as abrasives or gravel for mortar [1], when the copper content is < 2%. Likewise, matte and slag severely attack the refractory lining at a high temperature, being On the other hand, 35 Mt of copper are worldwide produced every year: 60% by pyrometallurgical processes and 40% from copper scrap [2]. Thus, copper industry is a great consumer of refractory materials as more than 25000 tons of refractory materials are used in its primary production (copper smelting, conversion and re ning in furnaces) [2]. Therefore, the partial or total replacement of the refractory means a signi cant investment for copper companies. This way, researchers focus on obtaining refractory bricks with exceptional properties using nanotechnology [3,4]. Within the exceptional properties, the resistance to the aggressive conditions of the immiscible phases involved in the copper production is included. This manuscript is aimed at studying the corrosion resistance of 100 wt. % MgO and 95 wt. % MgO with 5 wt. % nano-ZrO 2 ceramic composites by copper slag. 5 min. Finally, green specimens were sintered at 1650 °C for 4 hours. Physical properties were evaluated in terms of apparent porosity (AP) and bulk density (BD) by Archimedes method.
Copper slag was used to perform the corrosion test. The corrosion test consisted in determine the chemical resistance of the sintered refractory samples against the slag penetration. The corrosion experiments were done drilling a hole (~ 3.5 mm in diameter with 2.5 mm in depth) in the center of the upper face of the cylindrical sample and lling it with powder slag (5 g). The sintered samples analyzed in this chemical test were 25 mm in diameter and 6 mm in height. The experiment was carried out in an electric furnace at 1550 °C for 4 h at the maximum temperature. Both the heating and cooling rates were 5 °C/min. Afterward, samples were transversely cut using a diamond disc. After cutting, the surfaces of interest were polished with SiC paper for microscopy evaluation.
The cross-sections of polished sintered samples, before and after the chemical test were characterized by scanning electron microscope (SEM) (FEI-Model/Nova-NanoSEM200) with an energy disperse X-ray spectroscopy (EDX) detector, the measurements of the penetration distance were carried out through micrographs (SEM analysis) of the chemically attacked samples, likewise, Likewise, the slag concentration inside sample was detected through EDX microanalysis, detecting the variation in the chemical composition through the cross-sectional sample (from the base to the slag powder deposit).
Chemical composition of copper slag and MgO were determined by an X-ray uorescence (XRF) spectrometer (Axios, PANalytical) with a Rh-anode X-ray tube with a maximum power of 4 kW. The presence of phases of copper slag and raw material were determined by X-ray diffraction (XRD) method, using a Bruker D8 Advance diffractometer operated at the tube voltage and current of 40 kV and of 30 mA respectively, with Cu-Ka radiation. Table 1 collects the values of the chemical composition and phase percentage of the copper slag. Copper slag is mainly formed by fayalite (Fe 2 SiO 4 ), magnetite (Fe 3 O 4 ) and copper oxide and sul de (0.5-2 wt. % Cu) as secondary phases. Iron is the main element in copper slags, > 40% [5,6]. Data of XRF analysis collected in Table 1 indicate that the copper and iron contents (in wt.%) are 1.84 and 42.82, respectively, are in the above-mentioned ranges for copper slags  with copper slag. Figure 3(a) shows a micrograph corresponding to the upper-area of the sample (of the hole where the slag was deposited). Figure 3(b) shows the zone where the slag begins to in ltrate into the material. Likewise, in the Fig. 3(b), the slag advances of two different manners: rst, direct in ltration of slag particles that attacked the MgO grains (with a penetration depth of 25 µm from the hole, the grain is attacked), being this penetration depth less than in the 100 wt. % MgO sample; second, when the particles were inside the material, they dissolved in form of molten slag (liquid) through the grain boundaries (the slag circulates intergranularly). This kind of advancement of the slag was inhibited due to the ZrO 2 nanoparticles (phase of circular shape) since they act as barriers against the penetration of the intergranular liquid, observed as retention points (non-circular phase due to accumulation in different proportions of slag elements on the periphery of ZrO 2 nanoparticles) where the slag was retained around the ZrO 2 particles. Although the CaZrO 3 phase was not identi ed after chemical corrosion analysis, this phase also helped to hinder the penetration of the refractory and increased the viscosity of the low melting point phases of the molten slag, slowing down the intergranular path of the molten slag.

Conclusion
The corrosion resistance of 100 wt. % MgO and 95 wt. % MgO with 5 wt. % nano-ZrO 2 ceramic composites by copper slag was studied in this manuscript. An increase in the bulk density of the samples with the addition of 5 wt.% of nano-ZrO 2 was evident, obtaining 3.04 g/cm 2 . Also, it is observed that in the sample with 100 wt. % MgO, the penetration of the copper slag (3000 µm) is greater than in the sample of 95 wt. % MgO with 5 wt. % nano-ZrO 2 (~ 636 µm). In this last ones, ZrO 2 nanoparticles acted as retention points to stop intergranular in ltration of the copper slag blocking its path.