By using waste carbon dioxide (CO₂) to neutralize the alkalinity in bauxite residue, Alcoa’s Kwinana alumina refinery in Australia has lowered environmental risks, increased opportunities to reuse a large-volume solid waste, and reduced the need for significant ongoing management of bauxite residue storage areas.

In Australia, Alcoa produces alumina from low-grade bauxite deposits mined in the country’s Darling Range.  For every ton of alumina produced, approximately two tons of bauxite residue results.

During the refining process, a caustic soda solution is added to the bauxite to dissolve the alumina.  This allows the alumina to separate from the undissolved solids, which are essentially sand and mud.  Although the solids are washed to recover and recycle as much caustic soda as possible, the final bauxite residue still contains a residual level of alkalinity and has a pH of around 13.5.

The residue carbonation process, which took Alcoa over a decade to develop and pilot, mixes high concentration CO₂ with the bauxite residue prior to it being deposited onto drying beds.  The Kwinana refinery currently uses high purity CO₂ that is a waste from a nearby industrial facility

Residue carbonation’s benefits include the following:

• Shorter residue drying times.  These reduce the area needed for drying, in turn allowing deferral of construction of the next drying area by two to three years.
• Greenhouse gas benefit, since the CO₂ consumed equals at least 70,000 tons per year.
• Reduced dust from the residue drying areas.
• Creation of a more benign waste that brings with it prospects for improved remediation and long-term management opportunities as well as alternative use opportunities. 
• Reduced risk of ground and surface water contamination.

Alcoa has adopted residue carbonation as its preferred method of residue storage at Kwinana, and planning for installation of the technology is underway at a number of other Alcoa locations.

In 2004, the Kwinana refinery received an Alcoa EHS Achievement Award for its work in this area.