Viscosity predictions of fayalite slags by modified quasichemical model and Einstein-Roscoe equation route

Slags are very important formations in controlling the metallurgical processes. They are used mainly for their three features such as being a heat shield, helping purifying the liquid metal and protecting liquid metal from re-oxidation. There are many different types of slags regarding their compositions. This compositional diversity comes from the impurity level of ores as well as usage of secondary raw materials or additions such as CaO, MgO and Al2O3. In copper metallurgy, slags that occur after smelting and converting stages contain considerable amount of copper together with mainly FeO and SiO2. This type of slags are classified under "fayalite type of slags". In order to gain this valuable copper content inside these fayalite slags, and for better environmental management, slag cleaning furnaces are used which have a reducing atmosphere. In the slag cleaning process as well as in other metallurgical processes, determination of the physical properties of the slags are very important. Particularly with the increasing secondary raw materials usage in copper metallurgy, the chemical composition of the slags changes accordingly. Therefore, it is beneficial to use computer based simulations prior to the production processes and determine the optimal furnace parameters for reducing the operational costs. Computational fluid dynamics (CFD) simulations require physicochemical properties of the slags to have better approximations to reality. Therefore, determination of the physicochemical properties of the slags are very important. Viscosity, density, thermal conductivity and electrical conductivity are main physicochemical properties that affect the CFD calculations. Among them, viscosity is considered to be the most important physicochemical property since it controls heat and mass transfer, kinetics and phase separation in metallurgical processes. Given that the wide range of compositional diversity in the slag systems, experimental viscosity data of an induvidial slag system at an induvidial temperature are not found very easily. Therefore, many different viscosity models were developed in order to determine viscosity values of the slags where no experimental data is available. These models could be classified as classical models and advanced models. Classical models use data regression for a certain slag system by using semi-empirical mathematical expressions and some basic structural parameters. Advanced models, on the other hand, use physics. Thermodynamic models are using databases which have the information on the thermodynamic properties of binary oxide systems. They are capable of calculating the local changes that happen in their thermodynamic properties due to mixing and correlating this structural information with viscosity since viscosity is strictly structure dependent. Thermodynamic models were found to be superior to other advanced models such as molecular dynamic (MD) simulations since they have been developed recently. FactSageTM is one of the thermodynamic softwares that uses so called "Modified Quasichemical Model" which is an upgraded version of the general quasichemical model. It is capable of calculating the phase equilibrium of a system at a certain temperature and oxygen partial pressure by using "equilib" module. It is also capable of calculating the viscosity of a fully liquid oxide system at a certain temperature. However, slags may not be fully liquid within the operational conditions in metallurgical processes. The amount of solid particles, in grams, could be calculated by using FactSageTM "equilib" module and the effective viscosity of the solid particle containing slag could be calculated by using Einstein-Roscoe equation accordingly. However, an investigation on the viscosity of the pure fayalite slags and the effect of NiO, CaO and MgO additions together with the literature comparison is needed in order to test the success of this modelling route. In this thesis, firstly the effect of SiO2 content, solid phase formations, temperature and oxygen partial pressure (Fe3+/Fe2+ ratio) on the viscosity of the pure FexO-SiO2 system was studied. The effect of SiO2 was studied in the range of 0 to 40 wt.% and at the temperature range of 1205 to 1400 oC. Only FeO and SiO2 constituents were used as input which was thought to be best representing the Ar atmosphere deoxidized by Ca that was used in the literature study. The overall viscosity values were in good agreement with the literature but viscosity maximum around the fayalite region was not observed for both temperatures between 1250 to 1400 oC which was in disagreement with literature study. However, at 1205 oC, viscosity maximum due to solid fayalite formation was observed. Also inside the low SiO2 region, monoxide (FexO) solid phase was present between 1205 to 1350 oC which increased the viscosity up to certain SiO2 concentrations. Also it was observed that the viscosity of the liquid slag increased linearly up to orthosilicate composition which is XSiO2=0.33 mole % then exponentially with increasing SiO2 content. For the effect of temperature, viscosity calculations were conducted at different temperatures for three different slags and compared to literature studies. It was found that our calculations were following Arrhenius type of relation with temperature, viscosity decreased with increasing temperature. As for the effect of Fe3+/Fe2+ ratio, in other words oxygen partial pressure, FactSageTM "equilib" module was used intensively to calculate the oxygen partial pressures corresponding to certain CO2/CO ratios. It was found that oxygen partial pressure increases with both increasing temperature and CO2/CO ratio which was also reported by literature studies. With the light of this information, "equilib" module calculations were conducted for three different slags having 27, 30 and 31.6 wt.% SiO2 content for different CO2/CO ratios and temperatures. It was approved by FactSageTM that Fe2O3/FeO ratio increased with increasing PO2 values. Also Fe2O3/FeO ratio decreased with the increasing SiO2 content for a fixed CO2/CO ratio which was in accordance with the literature. Viscosity increased slightly with increasing Fe2O3 content, Fe3+/Fe2+ ratio, for 27 wt.% SiO2 containing slag which was in accordance with literature. However, for 30 and 31.6 wt.% SiO2 containing slags, our calculations remained consistent with 27 wt.% SiO2 containing slag where literature results were indicating that viscosity decreased slightly. Overall, the effect of Fe3+/Fe2+ ratio was found to be "slight" when compared to the effect of SiO2 concentration and temperature which was in agreement with many literature works. Secondly, the effect of NiO, CaO, MgO and Al2O3 additions on the viscosity of the fayalite slags was investigated by using FactSageTM "viscosity" module + Einstein-Roscoe equation route and compared to different literature studies. Calculating and comparing the viscosity values of ternary oxide system with this route was relatively harder due to complexity of the system and the low amount of experimental studies that used similar compositions. The effect of NiO content was studied by using the solubility limits of the Ni from the crucible into the FeO-SiO2 system by using "equilib" module. It was found out that Ni has the solubility of 8.12 wt.% inside the slag containing 32 wt.% SiO2 at 1350 oC having the oxygen partial pressure of 10-7.17 atm. Given that these conditions, viscosity of the slag changed due to increasing NiO content. It was found that 8.12 wt.% NiO increment increased the viscosity of the slag by 17.8 %. This calculation was relatively in agreement with the literature study which indicated a very sharp increase in their viscosity due to oxidizing atmosphere and Ni crucible usage. As for CaO, the effect of temperature and CaO content were calculated between 1200 and 1490 oC for the slags containing fixed 35 and 50 wt.% FeO content. Oxygen partial pressure was 10-9 atm. CaO addition inside the liquid slag decreased its viscosity. Our calculations were in good agreement with literature and non-modified FactSageTM "viscosity" module-only calculations since almost all of the slags were fully liquid under these thermodynamic conditions. Nevertheless, calculations for the slags containing fixed 50 wt.% FeO was in better agreement with our calculations than 35 wt.% FeO. Finally, for MgO addition, again the effect of temperature and MgO content were investigated between 1200 and 1500 oC under the oxygen partial pressure of 10-9 atm. For "the effect of temperature", our calculations were in good agreement with the literature but some disagreements were present. For "the effect of MgO content" calculations, for fixed 45, 50, 55 wt.% SiO2 containing slags, it was found that viscosity increased with increasing MgO content as a general trend which were surprisingly in agreement with literature. However, FactSageTM "viscosity" module-only calculations proposed the vice versa. These calculations were very promising for postulating that our calculation route can make significant approximations when calculating the viscosity of solid containing slags. Finally for Al2O3, the effect of temperature and Al2O3 content were investigated between 1200 and 1500 oC under Ar atmosphere. For "the effect of temperature", our calculations were in good agreement with the literature but again some slight disagreements were present. For "the effect of Al2O3 content" calculations, for fixed 41 wt.% SiO2 containing slag, it was found out that viscosity increased with increasing Al2O3 content inside the liquid slag. The results were in good agreement with literature. In fact, the charge compensating effect of Al2O3 in FeO bearing slag systems was also observed in our calculations. The calculations for CaO, MgO and Al2O3 were done with either fixed FeO or SiO2 contents and with the addition of CaO, MgO or Al2O3, the content of the SiO2 and FeO was changing respectively in accordance with the total percentage. It was very hard to determine whether CaO, MgO or Al2O3 addition was actually affecting the viscosity or the "accordingly changing" constituent was responsible for this change. Therefore, some set of theoretical calculations were conducted for both CaO, MgO and Al2O3 additions into the slags having different fixed FeO/SiO2 ratios as 1.22, 1.85 and 3 for different temperatures. There were solid particle containing regions as well as fully liquid regions. Viscosity increased accordingly inside the solid particle containing regions where some other mechanisms were present within the fully liquid regions.