Mark Dorreen

Increasing the Power Modulation Window of Aluminium Smelter Pots with Shell Heat Exchanger Technology

With power prices constantly rising, and varying aluminium prices requiring operating flexibility, the financial incentive for smelters to adopt a power modulation strategy is becoming larger. However, the power modulation window, in which a smelter can safely operate its reduction cells, is limited. The Light Metals Research Centre has developed the Shell Heat Exchanger (SHE) technology for controlling the heat dissipation from aluminium smelting pot shells. By varying the air flow through the SHE, the heat removal from the shell can be increased or decreased as desired, doubling the previous power modulation window or allowing power modulation with minimal disturbance to the pot thermal balance.

Response of Cryolite-Based Bath to a Shift in Heat Input/output Balance

A technology for low amperage potline operation is now recognized as a competitive advantage for the aluminum smelting industry in order to align smelter operations with the power and aluminum price markets. This study investigates the cryolite-based bath response to heat balance shifts when the heat extraction from the bath is adjusted to different levels in a laboratory analogue. In the analogue experiments, the heat balance shift is driven by a graphite ‘cold finger’ heat exchanger, which can control the heat extraction from the analogue, and a corresponding change in heat input from the furnace which maintains the control temperature of the lab “cell.” This paper reports the first experimental results from shifting the steady state of the lab cell heat balance, and investigates the effects on the frozen ledge and bath superheat. The lab cell energy balances are compared with energy balances in a published industrial cell model.

Investigation of the influence of heat balance shifts on the freeze microstructure and composition in aluminum smelting bath system: Cryolite-CaF2-AlF3-Al2O3

In an aluminum electrolysis cell, the side ledge forms on side walls to protect it from the corrosive cryolitic bath. In this study, a series of laboratory analogue experiments have been carried out to investigate the microstructure and composition of side ledge (freeze linings) at different heat balance steady states. Three distinct layers are found in the freeze linings formed in the designed Cryolite-CaF2-AlF3-Al2O3 electrolyte system: a closed (columnar) crystalline layer, an open crystalline layer, and a sealing layer. This layered structure changes when the heat balance is shifted between different steady states, by melting or freezing the open crystalline layer. Phase chemistry of the freeze lining is studied in this paper to understand the side ledge formation process upon heat balance shifts. Electron probe X-ray microanalysis (EPMA) is used to characterize the microstructure and compositions of distinct phases existing in the freeze linings, which are identified as cryolite, chiolite, Ca-cryolite, and alumina. A freeze formation mechanism is further developed based on these microstructural/compositional investigations and also thermodynamic calculations through the software—FactSage. It is found that entrapped liquid channels exist in the open crystalline layer, assisting with the mass transfer between solidified crystals and bulk molten bath.

Responses of Lithium-Modified Bath to a Shift in Heat Input/Output Balance and Observation of Freeze-Lining Formation During the Heat Balance Shift

In the aluminum electrolysis process, new industrial aluminum/electricity power markets demand a new cell technology to extend the cell heat balance and amperage operating window of smelters by shifting the steady states. The current work investigates the responses of lithium-modified bath system when the input/output balance is shifted in a laboratory analogue to the industrial heat balance shift. Li2CO3 is added to the cryolite-AlF3-CaF2-Al2O3 system as a bath modifier. A freeze deposit is formed on a ‘cold finger’ dipped into the bath and investigated by X-ray diffraction analysis and electron probe X-ray microanalysis. The macro- and micro-structure of the freeze lining varies with the bath superheat (bath temperature minus bath liquidus temperature) and an open crystalline layer with entrapped liquid dominates the freeze thickness. Compared with the cryolite-AlF3-CaF2-Al2O3 bath system, the lithium-modified bath freeze is more sensitive to the heat balance shift. This freeze investigation provides primary information to understand the variation of the side ledge in an industrial cell when the lithium-modified bath system is used.

Transforming the Way Electricity is Consumed During the Aluminium Smelting Process

This paper examines the development of the new EnPot heat exchanger technology for aluminium smelters and the potential impact it could have on the sustainability and economics of primary aluminium production. The EnPot technology can be used to help the aluminium smelting industry be part of the solution to accommodate increased intermittency in our future renewable energy generation, post COP 21. The EnPot system provides for the first time, dynamic control of the heat balance of aluminium smelting pots across the potline, so that energy consumption and aluminium production can be increased or decreased by as much as plus or minus 30% almost instantaneously. This enables a new way of thinking to emerge when considering the relationship the aluminium smelter plays in connection to the power grid. The EnPot technology provides smelters with the means to free up power back to the grid, transforming the smelter from only an end user of power into a ‘virtual battery’ for the electricity network. This fundamentally changes the way electricity is consumed by the smelter and is a concept already being tested by several smelters.

Role of Heat Transfer and Interfacial Phenomena for the Formation of Carbon Oxides in Smelting Cells

Thermochemically CO should be the dominant product and various theories have been proposed to explain the electrochemical dominance of CO2. Following publication of the proposed correlation between current efficiency and cell gas composition by Pearson and Waddington [1], smelting operators have considered the presence of high amount of CO to be a direct indicator of poor cell performance. However substantial deviations occasionally occur in the gas composition [2, 3] yet the rigour of correlations and reaction mechanisms interpretation have not been questioned. As a consequence of anode gas composition trends associated with large multi-electrode smelting cells, and aided by supplementary data, the mechanistic interpretation for the formation of CO and CO2 during aluminium electrowinning has been re-analysed. The data indicates interfacial heat transfer to satisfy the entropic energy plays an important role in determining the proportions of the gas.

Observations of freeze layer formation during heat balance shifts in cryolite-based smelting bath

Abstract The responses of a cryolite-based bath to shifts in heat input/output balance have been investigated in previous work. The effect of such heat balance shifts on bath superheat and on the frozen ledge, protecting the cell walls, is of most interest and has been investigated primarily in an experimental analogue to a smelting cell. In this paper, the microstructure and morphology of the freeze itself, as a function of the applied heat balance shift is investigated. Different layers were detected in the freeze, showing that both the freeze morphology and the phases present vary depending on bath conditions. The phase composition change was also analysed and compared with the bulk bath in different thermal states, created when the heat balance was shifted. Based on the investigation, a freeze growth mechanism is proposed.

Current Efficiency Studies in a Laboratory Aluminium Cell Using the Oxygen Balance Method

There have been many studies investigating the effects of various parameters on the current efficiency in aluminium smelting cells. One of the most important and most widely debated of these parameters is alumina concentration, because of its implications in feed strategies and cell management. This paper presents the results from a study investigating the effects of alumina concentration, bath chemistry, current density and anode-cathode spacing on current efficiency. Gas emissions from a laboratory scale cell were measured by on-line mass spectrometry, current efficiency was determined by an oxygen balance method.