Gudrun Saevarsdottir

Dynamic Current Distribution in the Electrodes of Submerged Arc Furnace Using Scalar and Vector Potentials

This work presents computations of electric current distributions inside an industrial submerged arc furnace. A 3D model has been developed in ANSYS Fluent that solves Maxwell’s equations based on scalar and vector potentials approach that are treated as transport equations. In this paper, the approach is described in detail and numerical simulations are performed on an industrial three-phase submerged arc furnace. The current distributions within electrodes due to skin and proximity effects are presented. The results show that the proposed method adequately models these phenomena.

On the Use of Multivariate Statistical Methods to Detect, Diagnose and Mitigate Abnormal Events in Aluminium Smelters

This work aims to demonstrate how the use of latent variable methods can detect and diagnose the onset of an abnormal situation in aluminium reduction cells. Using recent data from Alcoa Fjardaal, Principal Component Analysis (PCA) was used to model typical variations occurring in 336 different pots. A drift in process operation was correctly identified sooner than with traditional statistical control technique. Additionally, concentration in low trace elements in the metal also corresponded to a drift in process operation. Such an early warning could help mitigate the impact of abnormal events.

Effect of Carbide Configuration on the Current Distribution in Submerged Arc Furnaces for Silicon Production—A Modelling Approach

Current distribution is critical for good operation of Submerged Arc Furnaces for silicon production. Control systems do not offer this information as it is not directly measureable, but metallurgists operate furnaces based on experienced interpretation of available data. A number of recent dig-outs of industrial furnaces has expanded available information on location dependent charge properties, thus enabling numerical models with reasonably realistic domain configurations. This has the potential to enhance understanding of critical process parameters allowing more accurate furnace control. This work presents computations of electric current distributions inside an industrial submerged arc furnace for silicon production. A 3D model has been developed in ANSYS Fluent using electric potential solver. Electrode, arc, crater, crater wall, and side arc that connects electrode and crater wall are considered for each phase. In this paper the current distributions in electrode, arc and crater wall for different configurations and thickness of the crater walls are presented. The side-arcs are modelled as either a single concentrated arc, or a smeared out arc, in order to capture extreme cases. The main result is that side arc configuration is more important for the fraction of the current passing through the crater wall than the carbide thickness. The current fraction bypassing the main arc through the charge is highly influenced by the ease of contact between electrode and conducting charge material. Qualitatively, the results are in a good agreement with previously published results from literature.

Waste Heat Recovery from Aluminum Production

Around half of the energy consumed in aluminum production is lost as waste heat. Approximately 30–45% of the total waste heat is carried away by the exhaust gas from the smelter and is the most easily accessible waste heat stream. Alcoa Fjarðaal in east Iceland produces 350 000 tons annually, emitting the 110 °C exhaust gas with 88.1 MW of heat, which contains 13.39 MW exergy. In this study, three scenarios, including organic Rankine cycle (ORC) system, heat supply system and combined heat and power (CHP) system, were proposed to recover waste heat from the exhaust gas. The electric power generation potential is estimated by ORC models. The maximum power output was found to be 2.57 MW for an evaporation temperature of 61.22 °C and R-123 as working fluid. 42.34 MW can be produced by the heat supply system with the same temperature drop of the exhaust gas in the ORC system. The heat requirement for local district heating can be fulfilled by the heat supply system, and there is a potential opportunity for agriculture, snow melting and other industrial applications. The CHP system is more comprehensive. 1.156 MW power and 23.55 MW heating capacity can be produced by CHP system. The highest energy efficiency is achieved by the heat supply system and the maximum power output can be obtained with the ORC system. The efficiency of energy utilization in aluminum production can be effectively improved by waste heat recovery as studied in this paper.

Investigation of Sodium Sulfate Additions into Cryolite-Alumina Melts

In the Hall-Heroult process, sulfur impurities may not only emit harmful gases but also reduce current efficiency. To better understand this process, the behaviour of sulfur compounds in a cryolite-alumina melt at 1253 K (980 °C) was investigated in a laboratory cell. Sodium sulfate (Na2SO4) was added into the molten bath as a sulfur source. Furnace off-gases were passed through a mass spectrometer for qualitative assessment. The stability of sodium sulfate in the cryolite melt was found to depend on the presence of carbon (t1/2 = 116 min) and carbon-aluminium (t1/2 = 29 min). It changed dramatically during electrolysis (t1/2 = 5–8 min). Detected sulfurous gases included SO2, COS, CS2, and H2S.

Effect of Current Density and Phosphorus Species on Current Efficiency in Aluminum Electrolysis at High Current Densities

Effect of phosphorus on current efficiency for aluminum deposition was measured in a laboratory cell with current densities of 0.8 and 1.5 A/cm2. Controlled amounts of AlPO4 were added to the bath at the beginning of the experiment, and the effect on the current efficiency was studied at current densities of 0.8 A/cm2 and 1.5 A/cm2. Phosphorus levels were monitored through sampling the bath using High Resolution Inductively Coupled Plasma Mass Spectrometry at regular intervals during the whole electrolysis. The deleterious effect of phosphorus on the current efficiency was found to be pronounced at low concentrations up to 220 ppm. The current efficiency was found to increase with increasing cathodic current density and have a maximum current efficiency of 95.5% at 1.5 A/cm2 for this particular cell design. A further increase of current density up to 2 A/cm2 resulted in a decrease of current efficiency.

Effect of Current Density and Phosphorus Impurities on the Current Efficiency for Aluminum Deposition in Cryolite-Alumina Melts in a Laboratory Cell

The current efficiency in industrial Hall-Heroult cells for aluminum production may be up to 96%. [1]. The back reaction between the dissolved metals (aluminum and sodium) and the anode product play the major part in the loss in current efficiency. Also impurities, such as phosphorous, which participate in cyclic red/ox reactions at the electrodes contribute significantly to reduced current efficiency. Phosphorous and other impurities are recycled with secondary alumina. Further potline amperage increase in industrial cells may require higher current densities. Thus the current efficiency dependence on current density and phosphorous content was studied at current density ranging from 0.85 – 1.5 A/cm2 in a laboratory cell. Current efficiencies from 90 – 97% were obtained and increased slightly by increasing cathodic current density. The current efficiency decreased by about 3.8% per 100 ppm of phosphorus in the electrolyte.

Electric and Thermal Operations of Furnaces for Ferroalloys Production

Almost all ferroalloys are produced by smelting in submerged arc furnaces, powered by either AC or DC sources. To understand the smelting process and furnace operations, it is important to manage the theory of electrical circuits, efficiently apply and control active and reactive parts of the system, optimize the power factor and the operation of the electric arc, and so on. These issues are outlined and discussed in this chapter, which provides necessary theoretical and practical knowledge for those who do not have a background in electrical engineering. They are complemented with discussions of heat balance and electrical control of the smelting furnaces and different emissions associated with the ferroalloys production processes.