The computational thermodynamic modelling of the phase equilibria pertaining to the IiO₂ - Ti₂O₃ - FeO slag system

[摘要] ENGLISH ABSTRACT:During the production of pure Ti02 for the pigment industry, ilmenite, containing 35 -60 % Ti02, is reduced to high titania slag, containing 85 - 95 % Ti02 and pig iron.These ilmenite smelters are operated in very tight operating windows. Over reducingthe slag may lead to the formation of TiC and reducing much of the Ti02 to Th03.According to Namakwa Sands furnace operators, this does not only affect the grade ofthe product, but it can cause slag foaming and furnace eruptions. In under reducingconditions, the liquid slag is fluxed by the FeO and may corrode the furnace liningand consequently lead to run-outs. The reducing conditions in the furnace are notonly controlled by carbon addition, but also by temperature. Standard practise inindustrial ilmenite smelters is to operate the furnace with a slag freeze lining toprotect the refractory lining from chemical and physical attack by the slag. It istherefore clear that it is of great importance to be able to predict the slag liquidustemperature at different compositions. This can help the operator to avoid dangerousoperating conditions.Over the past few decades, a number of solution models have been developed todescribe non-ideal solutions. With the rapid increase in computer power, thesemodels became more valuable and practical to use in advanced control and decisionsupport.In this study, some of the better-known models are discussed and evaluatedfor the Ti02 - Th03 - FeO system, based on a critical review of properties andmeasurements published in literature.Two of these models, the modified quasi-chemical model and the cell model werechosen to be applied to the high-titania slag system. Both these models are based onstatistical thermodynamics with some differences in the initial assumptions. In thisstudy, the model parameters for the cell model were regressed from experimentaldata.The high-titania slag produced, consists mainly of titanium in different oxidationstates and FeO, placing its composition inside the Ti02 - Th03 - FeO ternary system.Reliable experimental data for this system are very limited. All three binary systems contained in the Ti02 - ThO) - FeO system were considered, namely FeO - Ti02,Ti02 - ThO) and FeO - ThO). Only liquidus data for these three binaries were usedto regress the model parameters. Accuracy of the models was determined bycalculating the root mean square (RMS) error between the experimental data pointand the value calculated using the model and the newly determined model parameters.These errors corresponded weil with the reported experimental error of the datasetsfor both the models and all the binary systems.Due to the fact that this study focussed on the liquidus surface of the system, theresults were also plotted in the form of binary phase diagrams and ternary liquidusisotherms. The cell model uses only binary interaction parameters to describe theternary system. These parameters are not expanded to higher order polynomials,which makes this model more robust, but also less accurate than other models such asthe modified quasi-chemical model.

[发布机构] Stellenbosch University