The ideal aluminium smelting cell should operate at a fixed temperature and superheat. However, spatial and temporal operating strategies cause changes in temperature, which usually result in variations in superheat as well. Contrasting, in the long term, for mature cells the aluminium fluoride consumption is fairly accurately reflected by the soda and calcium oxide contents of the primary alumina. Therefore the poor control of aluminium fluoride concentration reflects the poor understanding of the causes of variation in aluminium fluoride concentration and molten bath mass within the cell. The aims of this thesis were to i. Develop a better understanding of how the dynamics of the aluminium smelting process impact process conditions ??? hence bath chemistry ii. Subsequently develop and evaluate diagnostic models that may be used to minimise the variations in chemistry in individual operating cells The key control feature to minimise adverse effects is Superheat. The ideal aluminium smelting cell should operate at a fixed temperature and superheat. However, spatial and temporal operating strategies cause changes in temperature, which usually result in variations in superheat as well. In this thesis industrial aluminium reduction cells and their material handling and dry scrubbing operation were analysed in respect to their energy and material balance. A number of experiments were carried out to study the influence of process parameters and operations on the state and path function of a cell. Bath inventory measurements lead to a better understanding of the underlying process behaviour, and it was obvious that energy and mass balance cannot be controlled independently. With regard to the response of bath inventory, bath and liquidus temperature to pot operation, the following interesting phenomena were identified: - some cells are active or inactive with respect to their response to aluminium fluoride additions - positive and negative voltage steps cause non-proportional changes in liquidus and bath temperatures - the liquidus temperature, bath volume and composition can respond rapidly to changes due to alumina feeding Successful application of the results and understanding developed in this research resulted in an energy requirement reduction of 1 kWh/kg

The ideal aluminium smelting cell should operate at a fixed temperature and superheat. However, spatial and temporal operating strategies cause changes in temperature, which usually result in variations in superheat as well. Contrasting, in the long term, for mature cells the aluminium fluoride consumption is fairly accurately reflected by the soda and calcium oxide contents of the primary alumina. Therefore the poor control of aluminium fluoride concentration reflects the poor understanding of the causes of variation in aluminium fluoride concentration and molten bath mass within the cell. The aims of this thesis were to i. Develop a better understanding of how the dynamics of the aluminium smelting process impact process conditions ??? hence bath chemistry ii. Subsequently develop and evaluate diagnostic models that may be used to minimise the variations in chemistry in individual operating cells The key control feature to minimise adverse effects is Superheat. The ideal aluminium smelting cell should operate at a fixed temperature and superheat. However, spatial and temporal operating strategies cause changes in temperature, which usually result in variations in superheat as well. In this thesis industrial aluminium reduction cells and their material handling and dry scrubbing operation were analysed in respect to their energy and material balance. A number of experiments were carried out to study the influence of process parameters and operations on the state and path function of a cell. Bath inventory measurements lead to a better understanding of the underlying process behaviour, and it was obvious that energy and mass balance cannot be controlled independently. With regard to the response of bath inventory, bath and liquidus temperature to pot operation, the following interesting phenomena were identified: - some cells are active or inactive with respect to their response to aluminium fluoride additions - positive and negative voltage steps cause non-proportional changes in liquidus and bath temperatures - the liquidus temperature, bath volume and composition can respond rapidly to changes due to alumina feeding Successful application of the results and understanding developed in this research resulted in an energy requirement reduction of 1 kWh/kg

Aluminium smelting is a high temperature electrometallurgical process, which suffers considerable inefficiencies in power utilization and equipment maintenance. Aluminium smelting cell works in the extreme environments that contain extraordinarily aggressive gases, such as HF, CO and SO2. Mild steel used as a structural material in the aluminium industry, can be catastrophically corroded or oxidized in these conditions. This project was mainly concerned with extending the lifetime of metal structures installed immediately above the aluminium smelting cells. An aluminium-rich coating was developed on low carbon steel A06 using pack cementation technique. Yttria (Y2O3) was also used to improve the corrosion resistance of coating. Kinetics of the coating formation were studied. XRD, FESEM and FIB were employed to investigate the phase constitution and the surface morphology. Together with other potentially competitive materials, aluminium-rich coating was evaluated in simulated plant environments. Results from the long time (up to 2500h) isothermal oxidation of materials at high temperature (800??C) in air showed that the oxidation resistance of coated A06 is close to that of stainless steel 304 and even better than SS304 in cyclic oxidation tests. Coated A06 was also found to have the best sulfidation resistance among the materials tested in the gas mixture contains SO2 at 800??C. Related kinetics and mechanisms were also studied. The superior corrosion resistance of the coated A06 is attributed to the slow growing alpha-Al2O3 formed. Low temperature corrosion tests were undertaken in the gas mixtures containing air, H2O, HCl and SO2 at 400??C. Together with SS304 and 253MA, coated A06 showed excellent corrosion resistance in all the conditions. The ranking of the top three materials for corrosion resistance is: 253MA, coated A06 and SS304. It is believed that aluminised A06 is an ideal and economical replacement material in the severe corrosive aluminium smelting cell environment.

Aluminium smelting is a high temperature electrometallurgical process, which suffers considerable inefficiencies in power utilization and equipment maintenance. Aluminium smelting cell works in the extreme environments that contain extraordinarily aggressive gases, such as HF, CO and SO2. Mild steel used as a structural material in the aluminium industry, can be catastrophically corroded or oxidized in these conditions. This project was mainly concerned with extending the lifetime of metal structures installed immediately above the aluminium smelting cells. An aluminium-rich coating was developed on low carbon steel A06 using pack cementation technique. Yttria (Y2O3) was also used to improve the corrosion resistance of coating. Kinetics of the coating formation were studied. XRD, FESEM and FIB were employed to investigate the phase constitution and the surface morphology. Together with other potentially competitive materials, aluminium-rich coating was evaluated in simulated plant environments. Results from the long time (up to 2500h) isothermal oxidation of materials at high temperature (800??C) in air showed that the oxidation resistance of coated A06 is close to that of stainless steel 304 and even better than SS304 in cyclic oxidation tests. Coated A06 was also found to have the best sulfidation resistance among the materials tested in the gas mixture contains SO2 at 800??C. Related kinetics and mechanisms were also studied. The superior corrosion resistance of the coated A06 is attributed to the slow growing alpha-Al2O3 formed. Low temperature corrosion tests were undertaken in the gas mixtures containing air, H2O, HCl and SO2 at 400??C. Together with SS304 and 253MA, coated A06 showed excellent corrosion resistance in all the conditions. The ranking of the top three materials for corrosion resistance is: 253MA, coated A06 and SS304. It is believed that aluminised A06 is an ideal and economical replacement material in the severe corrosive aluminium smelting cell environment.