Reduction Process

Optimising the reduction process in an aluminium plant

Summary

Aluminerie Lauralco Inc., a primary aluminium plant located near Quebec, aims to make the best possible use of energy resources. During the initial operation period, the staff began to optimise the current efficiency of the reduction process, which resulted in reduced energy consumption. Further efforts included measures to reduce anode effects and lower fluorocarbon emissions. The company has also set up an energy committee to focus on continuing efforts to further reduce the plant's energy consumption and lower fluorocarbon emissions.

Highlights

Achieves a high current efficiency of over 96%
Reduces energy consumption to below 13,000 kWh/t of aluminium
Reduces anode effects and fluorocarbon

Aim of the Project

Aluminerie Lauralco Inc. opened the primary aluminium plant in 1992 and work closely with staff to constantly refine the process by improving the current efficiency of the reduction process and reduce the anode effects.

The Principle

Electricity is the main energy source of this primary aluminium plant. Used widely for the reduction process, power is supplied at 315 kV and redistributed throughout the plant at 69 and 25 kV, after going through two large 450 MVA transformers. A continuous power output of over 350 MW is required to achieve the annual production of aluminium. Natural gas is used as a source of thermal energy.

From 1992 to 1995, the start-up and initial period, staff were mainly concerned with optimising plant operation, and were not able to concentrate on reducing the energy consumed by the aluminium process. However, the first three years of operation saw the successful completion of a vital energy optimisation project and the reduction of fluorocarbon emissions.

The Situation

The first major energy-saving project was to improve the current efficiency of the reduction process. It began during the initial period of consolidating productivity in 1993, and is still ongoing.

The reduction process is at the heart of the primary aluminium plant - this is where the main raw material, alumina, is transformed into aluminium. At this particular plant, 264 electrolytic pots (model AP-30 made by Pechiney) achieve this using a very modern technology. The power (315 kA at 69 kV) is used in series through the solution of dissolved alumina in each pot. The molten aluminium is then drawn off under a vacuum at regular intervals, and moved to the handling crucibles. This process uses over 90% of the electrical power consumed by the plant.

The current efficiency is one of the reference indexes for the energy performance of the electrolytic process. It is calculated as the ratio of the theoretical electrical power to actual electrical power consumed by the reduction process. Increasing the current efficiency therefore improves energy consumption.

During the start-up period it was not possible to achieve the performance predicted by the technology supplier. In view of the importance of this element for productivity and energy consumption, two improvements were made.

The first was to optimise the operational parameters of the process, such as the percentage of fluorine, solution, feed rates, ventilation rates, etc. This was carried out in 1993-94, giving results that were useful but not enough to achieve the required efficiency objective.

The second improvement was made in early 1995 and involved physical modification of the 264 electrolytic pots. At a rate of two pots per week, the modifications were completed in 1997.

Despite this major investment, the project is expected to become cost-effective within three years, and will result in a 5% increase in production capacity. These improvements generate annual energy savings of around 500 GWh, giving a Faraday efficiency of 96.2%, which reduces the energy consumption to below 13,000 kWh/t of aluminium.

A second major project concerns measures to reduce the anode effects of the process. These are 'inefficiencies' in the electrical current passing through the solution in the pot during electrolysis. This can be caused by poor positioning of the anodes, or irregular wear (e.g. cavities), and result in an abnormal increase in the temperature of the solution and the anodes, plus increased production of fluorocarbons. This in turn leads to significant energy losses, as well as emitting gases that are harmful to the ozone layer.

Reducing anode effects is another way to improve the current efficiency of the reduction process. The company already has an international reputation for operational performance in terms of anode effects, which have been reduced to 0.26/pot/ day. However, the company has now identified new measures to reduce anode effects even further.

In early 1996, the company set up an internal energy committee to continually reduce the plant's energy consumption and lower emissions of carbon dioxide and fluorocarbons (CF4 and C2F6).

The Company

Aluminerie Lauralco Inc. is a subsidiary of Alcoa. The plant is located 80 km west of Quebec along the north shore of the St Lawrence River. It produces 230,000 tons of aluminium per year in the form of T-ingots, each weighing 5.5 tons, using a semi-continuous vertical casting process. Over 560 staff are employed at the plant.

Economics

The physical modification of the electrolytic pots required an investment of around CAD 60 million, including modifications to the peripheral equipment (crucibles, traveling cranes, mobile equipment etc.). Despite this, the project is expected to become cost-effective within three years.