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Aluminium Production: A More Sustainable Future

Aluminium cans

Alcoa has designed a large-scale bioreactor that can degrade up to 40 tonnes per day of oxalate produced at one of its aluminium refineries.

By Naomi McSweeney

Bacteria that consume sodium oxalate have the potential to reduce the environmental footprint of aluminium production while saving the industry millions.

Aluminium is the most highly used and versatile non-iron-containing metal in the world. It is a light, durable and flexible metal with properties including high heat and electrical conductivity, radiant heat reflection and resistance to atmospheric corrosion.

The physical properties of this metal can be enhanced by alloying, heat-treating and mechanical working to produce stronger and more durable metals.

Due to its unique properties and versatility, aluminium makes up a major component of items such as iPods and MP3 players, laptops, computers and gaming consoles, and cars, bikes and planes. Aluminium is also heavily used in food and medicine, packaging, construction and building, and electronics and electricity transmission industries.

As the world’s population grows, the demand for this metal is also increasing. In terms of commercial use, aluminium is second only to steel and its combination of functionality and cost-effectiveness makes it the most versatile metal in the world.

Aluminium occurs in nature in the form of an ore known as bauxite. Bauxite is refined using the Bayer process to produce alumina, which is further processed to make aluminium metal and fabricated aluminium products like AlFoil.

During the production of alumina, organic compounds are also produced as a result of the breakdown of plant, tree and leaf matter associated with the ore. One of these compounds, sodium oxalate, precipitates out with alumina at the end of the Bayer process. This results in excess alumina dust and caustic losses, and ultimately increases the operating costs associated with Bayer processing.

As a result, it is necessary to remove sodium oxalate from the process. Traditionally, oxalate was treated by combustion, which resulted in the release of excess CO2 into the atmosphere. Better ways of using or treating this impurity are currently being researched.

Sodium oxalate is a very common chemical compound found everywhere in nature. Rhubarb (Rhuem rhabarbarum) is well-known for producing oxalate, which remains in the soil of the root system and makes the leaves of the plant poisonous to humans and animals when consumed in large amounts. Oxalate is also common in the diet of humans and other animals as a by-product of protein metabolism, and is often tested for in our urine as symptoms of diseases such as hyperoxaluria and oxalosis.

So it is little wonder that the bacteria that like to use carbon from oxalate as a source of energy for their growth are also common in nature. A suite of oxalate-degrading microorganisms (oxalotrophs) have been found in the roots of oxalate-producing plants, in the soil and in the gastrointestinal tract of humans and other plants. These micro­organisms have a large range of growth requirements and are found in a large number of taxonomic families. For example, the oxalate-degrading bacteria found in the gut of humans (Oxalobacter spp.) grow in the absence of oxygen and in acidic conditions, while oxalotrophic bacteria found in the soil (Ralstonia spp.) require oxygen for growth and prefer a pH that is closer to neutral.

Oxalotrophic bacteria have already been successfully used to remove oxalate produced by Bayer refining of bauxite at various alumina refineries. However, very little research has focused on the microbial ecology of these bioreactors or trying to define which bacteria were responsible for the removal of sodium oxalate within the removal process.

After many years of research, including lab-scale and pilot-scale studies, Alcoa designed and put into operation a large-scale bioreactor that showed the immediate ability of degrading up to 40 tonnes per day of oxalate produced at one of its refineries in the south-west of Western Australia. In comparison to the traditional combustion methods of treatment and construction and maintenance of storage facilities, the use of this biological removal process currently saves the alumina giant millions of dollars in both capital and energy costs and also the equivalent CO2 emissions of 2500 average-sized cars per year.

But what bacteria are responsible for the oxalate degradation? And where did they come from?

Using the DNA from the bioreactor’s microbial community to create a genetic fingerprint, research carried out at the University of Western Australia in collaboration with Alcoa and the CSIRO Light Metals Flagship showed that the bacteria responsible for “eating” the sodium oxalate were actually native to the environmental waters and soils of the refinery and were mostly relatives of already well-described soil and water bacteria.

The bioreactor is essentially a melting pot of many different microorganisms, all with their own role in the biological removal of oxalate. However, the research group was able to isolate the key microbe responsible for the biodegradation of sodium oxalate, and found that it belonged to a new group of unidentified bacteria belonging to the beta-Proteo­bacteria subgroup.

Along with this discovery, the group has also been able to isolate a new species of the genus Halomonas, a well-known group of organic carbon-eating, high pH-loving soil and water bacteria that are found everywhere in nature. The new group of beta-Proteo­bacteria bacteria and new species of Halomonas can consume the organic carbon in oxalate contained in the bioreactor and use the energy generated from its breakdown into CO2.

Sodium oxalate is typically a difficult fuel for bacterial growth as it releases a lot less energy when broken down compared with other organic acids. When bacteria are grown with sodium oxalate, an alternative carbon source is often required as a supplement to increase the amount of energy and carbon available for growth.

So the discovery of groups of bacteria that can use sodium oxalate as a sole source of carbon and energy for growth at high pH is unique.

Research is now focusing on the complete characterisation of the oxalotrophic bacteria. By defining the optimum growth conditions of the isolated oxalotrophic microorganisms and mixed cultures of bioreactor micro­organisms, we can strive to achieve maximum oxalate degradation at a lab-scale and implement this knowledge at the large-scale to increase the efficiency of the process.

Observing how variables such as temperature, pH and oxalate and nutrient concentrations affect the overall performance of the oxalate-degrading bioreactor and the activity of the individual micro­organisms is also crucial to the understanding of the biological process. By improving the knowledge of the microbial ecology of the biological removal of sodium oxalate, steps can be taken to modify it to remove stockpiled sodium oxalate and then to replicate the process with the described bacteria of interest at other alumina refineries, both nationally and internationally.

Western Australia is a prime location for the development of this natural removal process. Bauxite from the state is typically low-grade and uniquely associated with high concentrations of complex carbon compounds. The use of this more sustainable, completely natural oxalate removal alternative is a far more economically and environmentally sound treatment option for sodium oxalate.

Therefore this removal process has economic implications for the alumina and aluminium industries, and can reduce the environmental footprint of one of the biggest industries in the world. As the demand for aluminium grows to make new products, new methods to make its production more sustainable will continue to be sought, and this is one step in the right direction for the industry.

Naomi McSweeney is a PhD Student with CSIRO’s Light Metals Flagship.