Energy Balance Regulation and Flexible Production: A New Frontier for Aluminum Smelting

Abstract

Through a critical review of recent literature on aluminum smelting cell energy balance, this paper defines specific energy constraints which govern the feasibility of cell operation in practice. Using these constraints as a basis, the objective of reducing energy consumption per kilogram of aluminum produced was examined, again with reference to published data and modern cell developments over the last 5 years. Both incremental and quantum steps in cell design are considered in this analysis, in pursuit of a pathway to lower energy consumption in a process where energy efficiency has not yet risen above 50 pct. In Section V and VI of this work, a generic high amperage cell technology is examined using a computational model of the cell energy balance, in which the resultant electrolyte phases and their thermal, electrical, and physical states can be determined. Using a series of trial energy balances, a feasible operating point emerges, and the possibility of flexible cell amperage and production rate is tested in a preliminary way. The specific energy consumption and market implications of this new technology direction are examined. I. RECENT RESEARCH IN THE FIELD OF SMELTING CELL ENERGY BALANCEUNDERSTANDING and improvement of the energy balance of aluminum smelting cells continue to be a limiting factor in both cell design and operation and in final performance. Modern, high amperage cell designs still suffer habitually from localized sidewall failures due to lack of understanding or control of the rate of thermal convection to the walls. Cathode deposits build up on modern cell cathodes causing horizontal currents and electromagnetic instability, increasing energy usage, and reducing efficiency. Why this is the case now, after more than one hundred years of publications and excellent industry developments and training on this subject ? The answer can only be that the problem of achieving an acceptable energy balance is extremely difficult, especially in the context of the need to constantly increase production rate and reduce electrical energy consumption. The mind set of constantly increasing production is one which needs to be questioned in the context of international and local market signals, and this has been discussed in a recent market analysis.[1] This analysis concludes that structural oversupply and growing available stocks of aluminum will continue to drive lower prices despite healthy demand for the metal.Recent studies of cell energy balance [2,3] have focused on building superior three dimensional hydrodynamic field models to predict heat transfer coefficients for the electrolyte and metal phases.[2] This excellent computational work has confirmed previous experimental studies [4] in terms of the magnitude of heat transfer coefficients from the electrolyte, but unfortunately, the resultant electrolyte conditions (temperature field and superheat) were not determined in these models-rather they were inputs to the simulation. The result is an unrealistic sidewall heat flux and ledge profile becaus...

Technique for Low Amperage Potline Operation for Electricity Grid Storage

“…From the previous thermo-electrical cell analyses, [2] a proportion of 38 pct of Q original is exiting through the walls (side and end) of modern cells, and 48 pct via the top of the cell. These assumptions are discussed in the Section II-B below.…”

“…In addition, the major heat generation occurs in the bath due to the voltage drops there, and the bulk of the Energy Imbalance in Eq. [2] therefore impacts on the bath directly. The anode and cathode heat generation contributions to the imbalance are much smaller, and any imbalances there are also communicated rapidly by convection to the liquid bath or metal from the solid anodes and cathode, respectively.…”

“…[12] After this time, the reduction in the temperatures of the solid materials in the cell walls will act to reduce the heat losses there, creating a progressively greater error in Eqs. [1] and [2] over time.…”

“…In the present study, the heat loss through the anodes in Figure 1 is assumed to be 60 pct of the total top loss, based on steady-state thermo-electric simulations, [2] where values between 57.4 and 61.7 pct were obtained depending on anode immersion depth, and anode cover level. Total top heat loss has also been computed in these simulations and was found to be in the range from 46.7 to 52.5 pct of the cell heat loss depending on the depth and quality of anode cover applied.…”

“…The fundamental constraints in smelting cell energy balance are discussed in detail in a previous paper. [2] The outstanding issue which remains un-addressed in the literature, and in the industry is that approximately 85 pct of the heat loss from aluminum smelting cells is driven by the superheat and volume of the bath (and metal), and cannot be regulated independently of these key characteristics. Therefore, it is currently not possible to alter heat dissipation significantly to allow flexible power input and amperage, without unacceptable changes in bath superheat and volume.…”