Globally, aluminium smelters produce in excess of 500,000 tonnes of spent pot lining (SPL) each year, and safely disposing of this unavoidable but hazardous waste product is a challenge faced by all aluminium smelters.
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In many cases, spent smelting pot lining is disposed of in landfills, but increasing concern about this practice is resulting in regulations in some countries banning this form of disposal. As a consequence, hundreds of thousands of tonnes of spent pot lining are being stockpiled in an increasing number of countries, awaiting the development of a successful and economic way of disposing of it.
Now, a group of Australian companies are set to give this global industrial waste headache its remedy following the development of a new process that renders the hazardous waste harmless, and at the same time produces two commercial by-products - aluminium fluoride and a safe ‘synthetic sand’.
The Project Team and Their Objectives
The award-winning Alcoa Portland spent smelting pot lining process is the result of a A$26 million collaborative research and development programme between Portland Aluminium, Alcoa, Ausmelt and CSIRO. Thanks to the determination and dedication of a small team, its aim - to develop a treatment process for spent pot lining that would destroy its hazardous condition of absorbed fluoride and traces of cyanide, recover and recycle its fluoride content, and dispose of the generated slag in an EPA approved method - has been realised.
‘The new process possesses the desired attributes for environmental solutions,’ says Ken Mansfield, Manager of Portland Aluminium’s Spent Pot Lining Project. ‘It recovers and reuses valuable minerals and thereby conserves natural resources, and has end products that are commercially useful.’
Project Origins and Successes
It has taken almost 13 years for the team to find, develop, and perfect its solution, which started with the assessment of existing and past processes in 1989. When this worldwide search failed to uncover an acceptable solution, trials began in 1991 to develop a new treatment system. Success came at the end of 2001 when development work by Ausmelt Ltd and CSIRO Minerals helped to successfully complete the two main areas of the process, the pyrometallurgical phase and the formation of aluminium fluoride.
The Treatment Process
Incorporating the SIROSMELT Technology (commercialised as the Ausmelt Technology) developed by former CSIRO researcher Dr John Floyd, the new treatment system comprises two major developments. Firstly, after suitable preparation, the spent pot lining is fed into an Ausmelt-designed top submerged lance furnace in which the cyanide-forming materials are destroyed at temperatures of up to 1,250°C and the contained fluorine is driven off as hydrogen fluoride in the off-gases.
In the second major step, a unique gas treatment process, developed by CSIRO Minerals and Portland Aluminium, converts the hydrogen fluoride in the off-gases to aluminium fluoride in a multi-stage fluidised bed reactor. The furnace also produces a granulated slag referred to as ‘synthetic sand’. The aluminium fluoride is recycled into the aluminium smelting process and the synthetic sand can be used in commercial applications such as road making and concrete products.
Advantages of the Process
It is the successful generation and reuse of aluminium fluoride in the smelting of aluminium that makes this process significantly different from competing processes around the world. However, during its development, the project’s team had to overcome many technical hurdles.
Environmentally Friendly By-Products
A key aim of the treatment process was to produce slag that was not harmful to the environment or to human health. It had to meet Australia’s Victorian EPA leachability criteria so it could be used commercially. Portland Aluminium installed a slag granulation process in June 2001 and modified the chemical composition of the slag in an attempt to achieve lower leachability. ‘Recommendations to change the slag’s composition by CSIRO’s Dr Shouyi Sun aided this aspect,’ says Mansfield. ‘Our synthetic sand has approval from the Victorian EPA for unrestricted environmental use as its leachability characteristics are less than 15 ppm. It is also being trialled for use as a replacement for sand in concrete.’
Reusable Process By-Products
Another challenge was to recover and recycle its fluoride content. ‘The formation of aluminium fluoride from the processing of spent pot lining and the reuse of this material in the aluminium smelting process was one of the most important challenges for the team,’ says Mansfield. Development work was undertaken with CSIRO Minerals on a small-scale experimental process to see if it was possible to convert low-concentration synthetically-produced hydrogen fluoride gas and smelting grade alumina to aluminium fluoride, Results from trials carried out at Alcoa’s Point Henry smelter in Geelong, Victoria, Australia, and Portland Aluminium’s pot rooms found no significant differences in operation efficiency and metal purity using spent pot lining aluminium fluoride compared to commercial aluminium fluoride.
This new spent pot lining treatment process eradicates the largest environmental problem in the aluminium industry and avoids landfill and the potential for future liabilities from leachate contamination of water resources, achievements that were praised by Australia's Victorian Premier Steve Bracks as he presented Portland Aluminium and the spent pot lining process with one of Australia’s highest environmental awards, the inaugural Victorian Premier’s Business Sustainability Award for 2002. ‘Instead of the aluminium industry stockpiling more and more spent pot lining, Portland Aluminium’s innovation means there is now a long-term solution that is economic, sustainable, environmentally and socially responsible,’ said Bracks at the awards ceremony in May.
Enquiries about the process have been received from aluminium smelters worldwide and a proposal for a feasibility study to implement the process in a large smelter in the Middle East is currently being processed. ‘The process is being continuously improved, and increased reliability in materials selection for the aggressive conditions created within the process are being evaluated,’ says Mansfield. However, he believes that many aluminium smelters will only alter their current practices when required to do so by legislation or community expectations towards sustainable development. Therefore, commercialisation of the process on a large scale could be a few years away yet.
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