Barry J. Welch

Impact of Alternative Processes for Aluminum Production on Energy Requirements

Increasing prices and the shortage of large blocks of electrical energy have given greater impetus to the search for viable alternative processes for aluminum production. These include electrolysis of aluminum chloride, sulfide, and nitride; carbothermal reduction of either the ore or alumina; and disproportioning reactions of either aluminum sulfide or the monochloride route. Common to all these processes are the starting material—an ore containing aluminum oxide—and the final product—the metal. Thus, the thermodynamic cycle will invariably dictate similar theoretical energy requirements for the three processes. In practice, however, the achievable efficiencies and, more noticeably, the proportion of electrical to carbothermal energy required for the various stages of operation can vary.

Interfacial processes and the performance of cathode linings in aluminum smelters

Measurements of the rate at which aluminum carbide dissolves in aluminum smelting electrolytes show that the dependence on electrolyte acidity (or excess aluminum fluoride concentration) follows a similar trend to that for the carbide saturation solubility, indicating mass transfer control. However, since the electrolyte supply at the aluminum/cathode carbon interface is limited, preferential corrosive wear will occur in areas where there is maximum accumulation of sludge and subsequent back feeding. The rate of the carbide corrosion can be reduced by adding Ti(IV) in relatively small concentrations (about 100 ppm); this also causes wetting of the carbon by the electrolyte. With further increases in the Ti(TV) concentrations—typically to about 350 ppm—apparent wetting with the metal also occurs, but under such conditions an electronically conducting electrolyte/carbide layer exists between the carbon and the wetted metal. Because of the higher viscosity of the titanium-rich metal phase when metal wetting occurs, the thickness of the film increases significantly. This mitigates titanium diboride formation on the carbon surface when the electrolyte contains both titanium oxide and boron oxide.

Technological developments for aluminum smeltinq as the industry enters the 21st century

The capital-intensive nature of aluminum smelting, with its low productivity per unit reactor and high electrical consumption rates, has motivated the search for alternative smelting processes to replace the aging Hall-Héroult technology. Optional routes include carbothermic reduction of alumina, chlorination followed by electrolysis of aluminum chloride, and electrolytic decomposition of alumina using inert electrodes. Still in need of some fundamental innovation, the alternative techniques are limited by unsatisfactory materials performance and reactor design constraints. There have been, however, significant advances in the process efficiencies and scale of both the Bayer process and the Hall-Héroult cells. As a result, the basic Hall-Héroult technology will continue as the dominant aluminum smelting process for at least the next 50 years.

Process Adaptations for Finer Dust Formulations: Mixing and Forming

Traditionally recipes have used 3000 Blaine dust in the carbon pastes due to limitations that existed previously in the processing equipment particularly in the classifying, weighing, preheating and mixing stages. The optimum processing conditions will change for different recipe conditions and different equipment capabilities. For a given paste plant design, the recipe should be chosen requiring optimum processing conditions as dictated by the plant equipment limitations.

Influence of Cell Operation on Cathode Life in Aluminum Reduction

There are many identifiable causes that can contribute to the ultimate failure of an aluminum reduction cell. They can be broadly classified within the bounds of cell design, materials of construction, method of construction and preheating, and lastly, cell operational history. This contribution correlates data obtained from a program of failed cell autopsies with observations of cell design and operation in relation to the ultimate cause of failure. The following aspects were determined to be of particular importance in achieving a good pot life; a sidewall design which can maintain stable heat transfer conditions over the life expectancy of the cell; heat-up and startup procedures which closely preserve the integrity of the lining materials; and operating parameters which minimize temperature disturbances or “sickness” incidents during the life of the cell.

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.

The Influence of Low Current Densities on Anode Performance

While the dominant amount of electrolysis occurs on the bottom surface of aluminium smelting anodes, the sides of the anodes contribute up to one-third of the electro-active area. This zone contributes to the electrolytic process, but has an ever decreasing current density, also it is subjected to more vigorous agitation as the waves of anode gas bubbles rise near the vertically oriented surfaces. Some studies have suggested that most of the dusting of anodes occurs on this face, although the explanations are based on carboxy reaction. By performing a laboratory scaled investigation of the consumption rate of different quality carbon anodes, it is evident that low current density contributes significantly to the development of a roughened surface profile. The consumption rate simultaneously accelerates although its extent is clearly dependent on carbon quality. The differential reactivity was greater for some coke types and low baking temperatures.

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.

A Study of Some Aspects of the Influence of Cell Operation on Cathode Life

There are many identifiable causes that can contribute to the ultimate failure of an aluminium reduction cell. They can be broadly classified within the bounds of cell design, materials of construction, method of construction and preheating and lastly, cell operational history. This contribution correlates data obtained from a programme of failed cell autopsies with observations of cell design and operation in relation to the ultimate cause of failure. The following aspects were determined to be of particular importance in achieving a good pot life: A sidewall design which can maintain stable heat transfer conditions over the life expectancy of the cell. Heat-up and start-up procedures which preserve as near as possible the integrity of the lining materials. Operating parameters which minimise temperature disturbances or ‘sickness’ incidents during the life of the cell.