Phase chemistry study of the interactions between slag and refractory in coppermaking processes

The molten oxides (slag), matte and metal charges during smelting, converting and refining stages of the pyrometallurgical coppermaking processes are contained in refractory-lined vessels. The refractory materials are selected so as to provide resistance to corrosion by molten phases and thermal insulation to minimize heat losses while maintaining the charge in a molten state. However, high process temperature, highly agitated and chemically aggressive melts in furnaces can result in rapid degradation of the refractory and premature shutdown of the reactor for relining; imposing additional costs on processes in the form of planned and unplanned maintenance. The focus of the present study is on detailed characterization of the phase chemistry and slag interactions with refractories. The rate of reactions between refractories and liquids depends on the phase equilibria. Post-mortem analysis of the spent brick from Isa smelter was followed by isothermal finger laboratory test under controlled conditions. Electron probe X-ray microanalysis (EPMA) is used to measure the compositions of the phases across the samples. This information is linked to the phase equilibria. Thermodynamic modelling is carried out by FactSage to assist in interpretation of the results. Phase analysis of used refractory and laboratory tests for Isa smelter indicate formation of a protective spinel layer on the hot face slowing refractory dissolution

Mitigating Chemical Degradation of Magnesia-chromite Bricks in Contact With a PbO Based Slag

Vessel integrity is a vital aspect in the production of metals, determining the efficiency and feasibility of pyrometallurgical production processes. The linings in the reactors are made of refractory bricks, ceramic materials used for their excellent chemical and thermo-mechanical properties at the operational temperature. Nevertheless, failure of the lining occurs over time due to a combination of thermal, mechanical and chemical stresses, requiring a timely and costly replacement of the lining. Reducing the refractory wear would lead to less standstills and thus a more cost efficient production process. The objective of this work is to investigate the in-situ formation of phases by reaction with the slag as a method to limit the chemical refractory degradation caused by PbO-SiO2 based slags. The chemical degradation of porous magnesia-chromite bricks by PbO-SiO2 based slags is caused by (1) liquid infiltration into the porous refractory brick and (2) dissolution of MgO from the refractory into this liquid slag. Slag engineering is successfully used to form a protective layer between the liquid and the refractory brick sample, slowing down the dissolution rate as the refractory components have to diffuse through this new layer. For deep infiltration slags, the liquid composition, and thus the chemical corrosion, changes inside the refractory brick and the protective layer no longer forms. The formation of in-situ phases is used to seal off the open pores in the refractory. This has been tested for refractory brick under isothermal conditions and under a temperature gradient. The latter uses a newly designed experimental setup. The ability to seal off the pores before complete infiltration depends on the ratio between the growth rate of the new phases and the infiltration rate of the liquid.Voorwoord I Abstract III Samenvatting V List of minerals and symbols VII Table of content IX Chapter 1 General introduction 1 1.1 Research objectives 3 1.2 Outline of the text 3 Chapter 2 Magnesia-chromite bricks 7 2.1 Production of magnesia-chromite bricks 8 2.2 Behavior in use 13 2.2.1 Thermo-mechanical degradation 13 2.2.2 Chemical degradation 14 2.3 Identification of degradation 16 2.4 Conclusion 17 Chapter 3 Mitigating the chemical wear 23 3.1 Viscosity of the slag 23 3.2 Changing the solubility of refractory components in the slag 23 3.3 Direct-indirect dissolution 26 3.4 Infiltration 28 3.4.1 Fundamentals of infiltration 28 3.4.2 In-situ measurements 29 3.4.3 Changes in slag composition 30 3.4.4 Predicting the slag changes 30 3.4.5 Methods to stop infiltration 34 3.5 Conclusions 37 Chapter 4 Slag engineering for PbO slags 43 4.1 Introduction 43 4.2 Materials and methods 43 4.3 Results 45 4.3.1 Slag S1 47 4.3.2 Slag S2 51 4.4 Discussion 55 4.5 Conclusion 56 Chapter 5 The influence of slag compositional changes on the chemical degradation of magnesia-chromite refractories exposed to PbO-based non-ferrous slag saturated in spinel 57 5.1 Introduction 58 5.2 Experimental procedure 59 5.3 Results 61 5.3.1 Microstructure 61 5.3.2 Composition of the slag inside the sample 69 5.4 Discussion 71 5.5 Effect on the overall degradation of a refractory lining 74 5.6 Conclusions 77 5.7 Acknowledgement 79 Chapter 6 The effect of phase formation during use on the chemical corrosion of magnesia-chromite refractories in contact with a non-ferrous PbO-SiO2 based slag 85 6.1 Introduction 86 6.2 Experimental procedure 89 6.3 Results and discussion 90 6.3.1 Reacted brick 90 6.3.2 Slag-refractory interaction 98 6.4 Conclusion 108 6.5 Acknowledgement 109 Chapter 7 The effect of a temperature gradient on the phase formation inside a magnesia-chromite refractory brick in contact with a non-ferrous PbO-SiO2-MgO slag 113 7.1 Introduction 114 7.2 Experimental procedure 116 7.3 Results 119 7.3.1 Temperature measurements 119 7.3.2 Microstructure 121 7.3.3 Slag composition as a function of position 125 7.4 Discussion 129 7.4.1 Local temperature 130 7.4.2 Slag composition 130 7.4.3 Infiltration behavior 131 7.4.4 Effect of external cooling on chemical degradation 134 7.5 Conclusion 136 Chapter 8 Conclusions and future work 141 8.1 Conclusions 141 8.2 Future work 143nrpages: 147status: publishe

Parameter investigation of the experimental methodology of electrical conductivity measurements for PbO containing slags

During pyrometallurgical extraction, some metals are lost to the slag phase which limits the process efficiency and may also cause environmental harm if those slags are used afterwards for the concrete industry. One way to reduce metal losses is via slag cleaning in a submerged arc electric furnace (SAF) where heat is generated via the conversion of electricity via the slag's resistance and thus the slag's electrical conductivity is an important process parameter. To study this parameter, a four-electrode conductivity setup was constructed as it can provide equally fast results as a two-electrode configuration but without the need for a cable correction as this correction may introduce measurement errors. To make a comparison between both setups, a binary SiO2-PbO system was tested as this system has already been frequently studied via two-electrode setups in literature. Our experimental data and activation energy values agree with those from literature which confirms that our setup provides reliable data. In addition, specific focus was put on the experimental methodology in terms of reproducibility, measurement technique (Electrochemical Impedance spectroscopy (EIS) versus single frequency measurements) and crucible materials (Pt-Ir versus Al2O3). The standard deviation on our results is found to be within the same range as those in literature. EIS and the single frequency method yields the same results with the difference being less than 1%. Finally, the crucible material has a clear influence as the oxidic crucible increases the cell constant by a factor 3 compared to a metallic crucible. The resulting conductivity values for the liquid PbO-SiO2 slags are of similar order, but the measurements in Al2O3 are unstable over time as the conductivity decreases continuously during the experiment due to crucible dissolution

Influence of simultaneous infiltration and reaction on the chemical degradation of magnesia-chrome bricks in contact with a synthetic lead slag

Chemical degradation of a refractory lining is most commonly considered as a dissolution process whereby the refractory components dissolve into the liquid slag. Hence, the lining thickness is steadily reduced. The dissolution process is not merely limited to the slag-refractory contact surface, but due to capillary forces, also occurs inside the porous refractory brick. This paper describes how the interaction between slag and brick during infiltration, changes the slag composition as a function of infiltration depth. Knowledge about the selective filtering of certain slag components by a refractory brick is indispensible in predicting the chemical degradation, especially when significant changes in slag composition are present. In this case, often seen with non-ferrous slags, the situation in the interior of the brick cannot be adequately predicted based on the global slag and brick composition. To study this “filter effect”, a magnesia-chrome finger in contact with a PbO-SiO2-ZnO-Al2O3-CaO liquid slag at constant temperature is tested. Using SEM-EDS analyses the slag composition as a function of infiltration depth is measured. Based on the microstructure at different positions, the interactions leading to modified slag compositions are determined. They can be classified into two categories: (1) slag components that diffuse into the brick phases and (2) reaction between slag components and dissolved brick components forming new solid phases. Slow diffusion prevents the slag from reaching its equilibrium concentration before it infiltrates further in the brick. The slag composition, therefore, not merely depends on the occurring reactions but also on the relative diffusion rate in the refractory phases (compared to the slag infiltration rate).status: publishe

Chlorine Addition to Existing Zinc Fuming Processes: A Thermodynamic Study

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