Reducing PFCs with Local Anode Effect Detection and Independently Controlled Feeders in Aluminum Reduction Cells

Three dimensional steady finite element models have been developed to study the effect of the novel structure cathode (NSC) on the electromagnetic field in aluminum reduction cells. In addition to reduction cells, the finite element system includes the busbar, ferromagnetic shell and air. Numerical results show the current density, magnetic flux density, electric potential and electromagnetic force is different between NSC and traditional cells. The NSC with the convex block can divide the aluminum melt into a number of parts, where the electromagnetic force decreases. There are different small circulations formed around the convex blocks in aluminum melt, which not only reduce the fluctuation and the anode effect caused by lower alumina concentration, but also promote the dissolution of alumina. Compared to the traditional aluminum reduction cells, the NSC cells have the advantage of reasonable current density distribution and relatively uniform electromagnetic force distribution, so that the aluminum melt has smaller fluctuations and higher efficiency of current.Keywordsaluminum reduction cellsinnovation cathode structureelectromagnetic fieldnumerical simulation

The concentration of alumina in the electrolyte is of great significance during the production of aluminum; it may affect the stability of aluminum reduction cell and the current efficiency. However, the concentration of alumina is hard to be detected online because of the special circumstance in the aluminum reduction cell. At present, there is lack of fast and accurate soft sensing methods for alumina concentration and existing methods can not meet the needs for online measurement. In this paper, a novel soft sensing method based on the voltage between the anode and the cathode, the current of the anode rod and the modified extreme learning machine for online measurement of the alumina concentration is proposed. In order to achieve the soft sensing scheme, considering the problems on the work spot, such as anode effect, the paper mainly develops a system that can realize the online measurement of the voltage between the anode and the cathode. The hardware of the system is composed of three parts including protection circuit, filter circuit and data acquisition card. When the anode effect occurs, the protection circuit which is used to ensure the safe use of the acquisition device will switch off the connection between the voltage and the data collection card. When the voltage returns to normal level, it will switch on the connection. The modified ELM algorithm is based on the enhanced random search which is called incremental extreme learning machine in some references. It randomly chooses the input weights and analytically determines the output weights without manual intervention. The experimental results show that the approach can give accurate estimations of alumina concentration within 6 percent relative error.

Convection rates and patterns in the bath electrolyte and the cathode metal of 150 kA prebaked anode aluminum reduction cells have been studied by means of radioactive tracers. Gamma radiation probes have been used for a continuousin situ determination of the radiotracer activity in the bath and liquid metal. The measurements are compared with predicted convection patterns from model magnetodynamic calculations for both magnetically compensated and uncompensated reduction cells. The measurements give evidence of a strong coupling between the convection currents in the metal and bath phases. The recorded activity vs time data further allow an estimation of the degree of turbulence in the reduction cells. From these investigations attempts are made to correlate data from the convection studies with current efficiency, as determined by the method of isotope dilution. The same method has also been used to determine liquid bath volume and the melting-out of solid bath electrolyte during periods with anode effect.

Perfluorocarbon (PFC) emissions are the result of a phenomenon called anode effect (AE), when normal electrolysis is discontinued due to lack of alumina and another reaction takes place leading to generation of PFC gases. Anode effects have been characterized in the industry by measuring their frequency and duration when the cell voltage is above 8 volts. Efforts have been made by the Aluminium Industry to reduce AE frequency and duration leading to considerable reductions in PFC emissions. However, recent independent measurements reported sources of PFC emissions that are “non AE related”. Measurements have been carried out on all DUBAL cell technologies in 2010/2011 and similar occurrences have been observed. DUBAL has initiated a program to identify the possible mechanism of the “non AE related” PFC emissions and to develop new logics for treating it. In this paper, a study of low voltage PFC emissions at DUBAL will be presented.

Influencing factors on PFC (perfluorocarbon) emission were investigated in Chinese smelters. The formation mechanism of non-anode effect related PFC emissions (short as NAE-PFC) from aluminum reduction cells were discussed. Anodic overvoltage has an obvious influence on NAE-PFC formation, CF4 will emit when anodic overvoltage is higher than 2.55 volts, C2F6 will be generated when anodic overvoltage surpasses 2.68 volts. Alumina concentration in bath with anodic current density also has a clear influence on NAE-PFC formation. Alumina fluctuates in large cells from the metal-tapping-end to the flue-end. Alumina always distributes unevenly in the bath because of bath flow driven by magnetic fields, which will also cause NAE-PFC formation. NAE-PFC will emit when there is not enough alumina under certain anode’s bottom. The limit alumina concentration is 75% of normal level which is 2.0–3.5% in Chinese smelters. Cells will generate NAE-PFC when any anode’s current density is higher than 1.5 times of normal level which is 0.72–0.80A/cm2 in Chinese smelters.

Recent investigations on aluminum electrolysis cell demonstrated limitations to the commonly used tier-3 slope methodology to estimate perfluorocarbon (PFC) emissions from high-voltage anode effects (HVAEs). These limitations are greater for smelters with a reduced HVAE frequency. A novel approach is proposed to estimate the specific emissions using a tier 2 model resulting from individual HVAE instead of estimating monthly emissions for pot lines with the slope methodology. This approach considers the nonlinear behavior of PFC emissions as a function of the polarized anode effect duration but also integrates the change in behavior attributed to cell productivity. Validation was performed by comparing the new approach and the slope methodology with measurement campaigns from different smelters. The results demonstrate a good agreement between measured and estimated emissions as well as more accurately reflect individual HVAE dynamics occurring over time. Finally, the possible impact of this approach for the aluminum industry is discussed.

This paper puts forward a new aluminum reduction cell shell technology, specifically a pre-stressed shell. The technology was developed in order to solve the breakage of aluminum reduction shell caused by large deformation. The work is based on pre-stressing technology research and application in the field of steel structures. By pre-stressing the shell cradle, the technology significantly improves the cell structural performance, increases structure safety, reduces materials consumption and is beneficial to prolong the cell life. This paper introduces the design method (force analysis and experimental research), detailed measurements of the pre-stressed shell and gives the application example.

The paper presents a new nonstationary three-dimensional mathematical model of an aluminium reduction cell which makes it possible to perform coupled thermoelectric and magnetohydrodynamic calculations taking into account sideledge formation. The model considers the nonlinear dependence of material electrical conductivity and thermal conductivity coefficients on temperature, and the nonlinear dependence of magnetization on the magnetic field strength for ferromagnetic materials. Heat transfer coefficients on outer surfaces included the radiant and convective components of heat transfer and were functions of the ambient temperature and the local surface temperature. The energy equation took into account internal heat sources due to the electric current flow, exothermic reactions and additional thermal effects associated with the raw material loading and phase transitions. The control volume method was used to obtain a numerical solution. The developed mathematical model was experimentally tested at the S8BME aluminium reduction cell. The paper presents the calculated and experimental data of magnetic, electric, thermal and hydrodynamic fields. The comparison of calculation results with the results of industrial experiments showed that the developed model reflects physical processes taking place in the aluminium reduction cell with accuracy sufficient for engineering calculations. The calculated values of electrical voltage, magnetic induction and temperature practically coincide with the measured ones. Velocity directions in the metal and the sideledge profile shape obtained by calculation have insignificant differences from experimental values. The developed model can be used to estimate operation specifications and design parameters for new and modernized aluminium reduction cells. Further studies will be aimed at refining the calculated results by improving the developed mathematical model.

Development and application of soft sensor model for heterogeneous information of aluminum reduction cells

Most laboratory systems investigating the aluminium production process utilize a single anode set-up. When approaching alumina depletion under constant current conditions in such a system, the potential will increase to high levels (>10 V) and initiate an anode effect and perfluorocarbon generation. However, it has been discovered by industrial measurements that perfluorocarbon generation may also occur at normal cell voltages. With the use of a two-anode setup in parallel with an electronic load this phenomena was investigated in the laboratory. The results indicate that as long as the rest of the cell can acquire the extra load, partial passivation of one or more anodes is possible and can be accompanied by small amounts of PFC evolution (0–3 ppm mol CF4). Individual anode potentials can be highly elevated, albeit the changes get buried in the total cell voltage. Only when the total load becomes too large the voltage rises abruptly and substantial amounts of PFC can be produced (≫1000 ppm mol CF4).

Industrial test and numerical simulation were synchronously applied to analyze the side heat transfer process and enhance heat transfer in aluminum reduction cell. The 3D slice finite element model of aluminum reduction cell was developed, with which the sidewall temperature field of the cell was computed by using software ANSYS. The main influencing factors on heat dissipation were analyzed and some effective measures were proposed to enhance sidewall heat transfer. The results show that the shell temperature of the test cell and the common cell is respectively 312°C and 318°C and the ledge thickness is 16 cm and 15 cm when side coefficient of heat transfer between the shell and the surroundings is 70 W/(m2·K). With the increase of the side coefficient of heat transfer between the shell and the surroundings, the temperature of the shell decreases but the thickness of the side ledge increases when the electrolytic temperature, the ambient temperature, the coefficient of heat transfer between molten bath and ledge, the eutectic temperature and the thermo-resistance of the side lining are constant.

Alumina concentration is an important parameter in the production process of aluminum electrolysis. Due to the complex production environment in the industrial field and the complex physical and chemical reactions in the aluminum reduction cell, nowadays it is still unable to carry out online measurement and real-time monitoring. For solving this problem, a soft-sensing model of alumina concentration based on a deep belief network (DBN) is proposed. However, the soft-sensing model may have some limitations for different cells and different periodic working conditions such as local anode effect, pole changing, and bus lifting in the same cell. The empirical mode decomposition (EMD) and particle swarm optimization (PSO) with the DBN are combined, and an EMD–PSO–DBN method that can denoize and optimize the model structure is proposed. The simulation results show that the improved soft-sensing model improves the accuracy and universality of prediction.

To design a multi standard and a multi band transceiver, a quadrature voltage controlled oscillator (VCO), which has a wide tuning range, is essential, while providing low phase noise with low current consumption. In this paper, a dual band quadrature VCO in 0.18 mum CMOS process is presented. To accomplish dual band operation, reconfigurable LC tank is adopted. Series coupling transistors are attached to generate quadrature phase signal with low power consumption and low phase noise. To improve phase noise performance, current sources are eliminated and a filter inductor at the common source node is added. The measured phase noises of low frequency mode and high frequency mode are -101.81 dBc/Hz and -100.59 dBc/Hz at 10 kHz offset frequency; and -123.83 dBc/Hz and -115.06 dBc/Hz at 1 MHz offset frequency and figure of merit (FoM) at 1 MHz offset frequency of -181.8 dB and -180.5 dB with 4 mA current consumption with a 1.7 V supply.

Material Balance of Silicon in Aluminum Reduction Cells

A two-dimensional (2D) finite element model (FEM) of an anode immersed in an aluminum reduction cell was developed to study the initial current distribution in the anode as a function of anode geometry and electrical anode conductivity gradients. The numerical results of the initial state of the anode electrical current were used to describe analytically how this will affect the variation in the anode-to-cathode distance (ACD) in a steady-state scenario after several hours in the electrolysis bath. The electrical power loss in the anode has also been studied at different anode geometries and material properties. The slot positioning, slot depths, and stub hole dimensions have been considered in the FEM. The anode is implemented as an inhomogeneous orthotropic material with a defined six-parameter equation. The degree of initial inhomogeneous anode current density, which is expressed with a defined parameter k 0, can reach values to cause variations in the ACD typically measured in the aluminum industry. To avoid a variation in the ACD for this case, the defined bath conductivity relation n should be within certain limits for the analyzed industrial reduction cell. The lowest degree of initial inhomogeneous current in the anode is achieved with deeper slots closer to each other and with an electrical current entering the anode in the bottom of the anode stub hole.