Appendix H: CO2 for use in bauxite residue carbonation
The extraction of alumina from bauxite ore results in a highly alkaline bauxite residue slurry (known as ‘red mud’). The bauxite residue contains a mixture of minerals and some alkaline liquor (NaOH) from the Bayer extraction process. A new technology has been developed whereby concentrated CO2 is used as a means of treating the highly alkaline by-product (pH = 13) from the extraction of alumina. The process provides direct carbonation of the bauxite residue, locking up CO2 and reduces the pH of the slurry to a less hazardous level.
The new bauxite residue treatment technology has been operating on trial at Alcoa’s Kwinana refinery for several years. The plant uses waste CO2 transported by a pipeline from a nearby ammonia plant. The plant locks up 70,000 tonnes of CO2 per year and results in direct carbonation of Kwinana’s entire residue by-product, which is typically between 2 and 2.5 million dry tonnes per annum.
Alcoa’s patents on the technology have expired, but they are offering other alumina producers a ‘technology transfer’ package that includes their more detailed IP.
Alcoa have also recently patented an integrated carbon capture and residue carbonation process that would allow the use of flue gas from captive power generation plant emissions.
Alcoa plan to deploy the technology to nine of their alumina refineries worldwide. Deployment acrossalone is estimated to store 300,000 tonnes of CO2 permanently each year.
Red mud treated with sea water has a large theoretical capacity to absorb CO2 (up to 750kg CO2/t red mud). However, Alcoa only proposes a level of 30-35kg of CO2 per tonne of red mud (dry weight) as this is what is required to convert all of the alkalinity to carbonates.
Furthermore, Alcoa have raised concerns regarding the ability to retain the extra bicarbonate CO2 within the residue.
There is potential to implement the technology in Aluminium refineries around the world. There is also potential to neutralise the present excess stores of highly alkaline bauxite residue located in tailing ponds, to remove potential environmental risks and create a valued product. There is potential for neutralised bauxite residue to be used as road base, building materials or as soil amendment on acidic soils.
Size of market
Worldwide over 70 million dry metric tonnes of bauxite residues are generated annually when alumina is extracted from bauxite ore. Globally more than 200 million tonnes of bauxite residue has accumulated, The majority of which is stored in tailing ponds.
Potential for revenue generation
There is little potential for direct revenue generation as the resulting neutralised bauxite has little commercial value. The potential for economic feasibility should instead be based on the potential savings associated with the handling, transportation and storage of the neutralised product over the original alkaline residue.
At present, The highly alkaline bauxite residue is simply a by-product of the alumina production process and has no commercial use. A potential benefit is the use of the neutralised residue as a soil amendment. Although it is likely that the bauxite will have little commercial value (and essentially be offered free to the agricultural sector), The real commercial value will arise from the costs saved from negating the need to store the unwanted residue. A Bauxite Residue Sustainability report released by Alcoa suggests a benefit (reduced tailings management cost) equivalent to over AU$20/t of CO2 utilised.
Benefits of re-using CO2 to neutralise bauxite residue include:
- The residue with a pH level of about 13.5 has limited potential for reuse, presents an environmental risk and is required to be stored in lined storage areas. By mixing concentrated CO2 through the bauxite residue the pH is reduced to 10.5, presenting a significantly lower environmental risk and the potential for reuse as road base, building materials or to improve soils.
There are a number of barriers which will affect theof widespread implementation of the technology including:
- high purity of CO2 required;
- locality of CO2 source;
- no prospects for revenues as a result of production of useful by-product; and
- relatively low levels of CO2 storage.
Technology has not received any government funding/grants.