Thermodynamic models of industrial processes are very useful in providing real increases in process productivity and efficiency.  The predictive capability of such models depends significantly on the accuracy of their underlying data.  In this context, accurate experimental measurements of liquor properties are essential.  Collaborative research work has resulted in substantial improvements in process simulators at realistic temperatures and liquor concentrations found in the Bayer process.  This work has led to significant increases in refinery productivity and efficiency.

Detailed collaborative studies of the formation of aluminium hydroxide or ‘hydrate’ particles in the Bayer precipitation circuit have enhanced understanding of how the agglomerates form and how their toughness may be improved.  Subsequent calcination of hydrates has directed understanding of what happens in calciners, especially the unwanted formation of fine particles as coarser ones fracture.  This has led to process changes to ensure production of tougher hydrates and less breakdown, producing alumina with superior properties for supply to aluminium smelters.

Other collaborative studies have examined the mechanisms of alumina particle breakdown in aluminium smelters and how process changes can reduce the impacts of fine alumina on smelter efficiency and environmental considerations.

Reduced scale formation

The formation of scale on the walls of tanks and pipework impacts on refinery productivity, and any reduction in the maintenance required to remove scale increases efficiency.  With industry awareness of the problems scale can cause, and this topic having been highlighted in a previous edition of the Technology Roadmap, a specialty chemical company was able to develop an additive to the liquor which significantly reduces scale formation without any deleterious effects on liquor productivity.

Improved thickener operation

Thickeners in alumina refineries separate the residue (red mud) from the pregnant Bayer liquor (rich in dissolved alumina).  Substantial collaborative research has been conducted on the operation of thickeners, in particular to:

• maximise the efficient use of flocculants added to enhance thickening
• maximise their throughput by utilising computational fluid dynamics (CFD) to determine the most effective design of rakes and overflow weirs.

Results have led to dramatic improvements in thickener operation.

Understanding sustainability of bauxite residue storage

The first Alumina Technology Roadmap identified a goal for bauxite residue:

“Develop methods to achieve a 1,000-year ecologically sustainable storage of red mud and other solid wastes in existing storages, and make substantial progress in storage for later reuse as well as achieve substantial progress in the reuse of the red mud.”

A subsequent collaborative project entitled Bauxite Residue Sustainability Measure of Improvement addressed the need for quantitative measures for the assessment of storage practices, using a quantitative, risk-based methodology within a sustainability framework.  It delivered:

• a risk-based methodology for the evaluation of current and proposed residue storage practices in a sustainability framework
• a basis for business cases for change where desirable, and a direction for technology development
• benchmarking of practices in other industries
• proposals for best practices on a flexible, situational basis
• improved understanding of the balance between costs and risks of current and future options, including whole of life costs
• inputs to the development of legislation and in responding to community concerns.

Managing naturally occurring radioactivity in context

Bauxite can contain naturally occurring radioactivity from the presence of U-238 and/or Th-232.  Some of the radioactivity is associated with minerals such as ilmenite, monazite or zircon in the bauxite.  Most of the radioactivity present in the ore is transferred to the residue tailings (red mud) from the Bayer caustic leaching process.  There is an increasing need to understand the nature of the radioactivity contained in ores, process intermediates and wastes as the result of changing international guidelines, Codes of Practice and regulations relating to the presence of naturally occurring radioactive material (NORM).  Strategic research is required to better characterise the occurrence and distribution of NORM in bauxite ores and residues in order to facilitate the future management of residues and the possible use of the residues in other industries.

A collaborative project was undertaken to deliver:

• A report on the available data on radioactivity and measured doses associated with bauxite ores and leach residues and solutions deposited in red mud tailings facilities. It examined the range of data in the alumina industry and placed it into context with respect to NORM concentrations and associated doses.
• A report on international and local regulations and trends, and the implications for bauxite mining and alumina producers. It interpreted the current regulatory position and the trends in international guidelines and regulations in the context of the broader minerals industry, which might be seen as affected by the presence of NORM, and the radioactivity concentration and dose data for the range of specific circumstances existing in the Australian alumina industry.

Ecotoxicity of bauxite residue trace elements

Bauxites and associated leachate contain a range of trace elements.  These trace elements may be associated with the residue slurries, or leached from the residue deposits over time and need to be managed accordingly.

A collaborative project identified and collated best practice testing protocols for a range of bauxite residue trace elements in terms of their ecotoxicity.  Of particular importance was an understanding of ecotoxicity with respect to:

• open circuit discharge
• long-term management of closed residue deposits where leachate will need to be collected, treated and possibly discharged
• re-use of residue where trace elements have the potential to leach into receiving environments.

The project achieved a good understanding of the way ecotoxicity might be assessed and how this assessment can be structured into guidelines/regulatory frameworks for managing water quality.  The study reviewed existing testing protocols and identified those likely to be considered best practice for bauxite residue type leachate and the specific receiving environments of interest.

Constructed wetlands to treat alkaline leachate

A collaborative project was conducted at the Aughinish alumina refinery to study constructed wetlands for treatment of alkaline leachate from bauxite residue disposal areas.

1. Selection of a standardised leach testing protocol for development of a risk-based by-product assessment methodology.
2. Utilisation of bauxite residue in the production and use of geopolymers.
3. The International Aluminium Institute commissioned a series of detailed reviews on the use of bauxite residue (BR) in the cement industry. These cover existing and potential use in:

• Portland Cement clinker production
• special cements
• supplementary cementitious materials.

To assist in expanding use of residue in these areas, an Excel-based calculation tool was developed to provide a simple assessment of the possible applications of BR in the cement production chain.  The main objective of this tool is to present a robust evaluation of an alternative use of BR, respecting the requirements of each industrial sector.  The tool is designed to be used by representatives of both the bauxite residue producers and cement sectors from a number of participating countries.

The specific objective of the tool is to provide a comparative analysis of a baseline operation from the cement industry with alternative scenarios including the addition of bauxite residue in its normal operation, as well as an economic analysis for the bauxite residue producer over the possible disposal of its residue.

The tool evaluates electricity use, fuel consumption, expected emissions and related costs per tonne of material produced, comparing all possible scenarios of addition of bauxite residue with the baseline of normal operation of the cement plant.  In addition, the tool assesses the logistical expenses and potential cost avoidances and revenues that are possible for the bauxite residue producer with commercialisation.