Investigation of closure techniques for the Rusal Aughinish Bauxite Residue Disposal Area (B.R.D.A.) and the impact on the surrounding environment post-closure

Rusal Aughinish operates an alumina refinery situated on Aughinish Island on the south side of the Shannon estuary. The company submitted an application to extend the Bauxite Residue Disposal Area (B.R.D.A.) by another 80 hectares in 2006. Space to store residue was estimated to run out in 2011, thus requiring the construction of the extra 80 hectares which needed planning permission and an extension of the licence. It was vital to get planning permission and licence extension. There is a large volume of residual alkalinity, held in a soluble and solid phase, retained in the bauxite. The E.P.A. had requested a Residuals Management Plan which included how the refinery shut down, decommissioned and the site management following its closure. This plan sought to ensure that the closure technique will have no impact on the environment. This plan sought to ensure that the leachate from the BRDA will have a pH lower than 9.0 in 5 years after closure and the residue would be covered with a sustainable vegetation. As such, residue neutralisation methods had to be be researched and investigated, and that all residue pumped to Phase 2 extension would have a pH of 9.0.

Rusal Aughinish has determined that the restoration of the B.R.D.A. surface will support a “nature conservation” end-use (AAL, 2005d). Vegetation trials since the 1990s, the research into neutralization methods of the residue, plot and Demonstration Cell trials guaranteed this commitment.

This theses involves small plot trials (2m x1m) and larger plot trials (10m x 2m) which were established in 2004 and the construction of the two Demonstration Cells which were amended with gypsum, process sand, and spent mushroom compost in 2006/ 2007. This amendment lowered the pH, the exchangeable sodium percentage (ESP) and the availability of Al and Fe in the residue and allowed for grass growing. The bauxite residue mixed with process sand, gypsum and organic waste was seeded. Results show that the establishment of vegetation was achievable. Additionally, investigations were carried out into the use of machinery on the residue to plough in the sand, gypsum, compost and the grass seed.

Two Demonstration Cells were constructed within the confines of the B.R.D.A. (0.6ha) in 2006 and filled in 2007.The sides and floor of the cells were lined, and a leachate collection system was installed on the floor of the cells. Monitoring of pH, electrical conductivity, and soda was conducted in run-off and leachate before and after vegetation growth on the residue in the cell. No reduction was noted in leachate or run-off pH since monitoring commenced in 2007.

Following research into neutralization methods the use of sulphuric acid was the deemed best option, but only partial neutralization can be achieved, due to the large volume of acid required for full neutralization. There was also the likelihood of creating pH conditions that would lead to H2S odour problems in the BRDA area. Carbonation would also be possible but would require the construction of a plant or the importation of liquid CO2. Seawater neutralisation using water from the Shannon estuary was prohibited by costs, mainly high-pressure pumps and the treatment of the return liquid to the Shannon River. An initial modelling project looked at groundwater flow within the B.R.D.A. in two dimensions and assumptions were made as to the physical stratification and structure of the B.R.D.A. Mud-Farming commenced in 2009 and was evaluated. Modifications have been made to the method of mud farming and results give a partial neutralization with a pH of around 10.0 to 10.5. Trials have been concluded with a pilot Wetlands project.

In conclusion, direct vegetation was found to be feasible, and so avoiding the high cost of topsoil. Soil construction and plant establishment was demonstrated. Demonstration Cells were constructed as per design and monitoring for pH, conductivity and soda of the leachate and run-off before and after vegetation growth. This monitoring is still on- going with further trials in the second Demonstration Cell and monitoring of the vegetation cover. Filling of the cell with residue was determined by the stacking angle of the residue.

Controlling percentage solids of the residue to the BRDA is very important in order to achieve proper stacking of the residue. From the trials and laboratory results leachate pH may take years to drop from 13 to 9.0 or below.

Recommendations to the company include further monitoring of Demonstration Cell leachate, run-off, and the vegetation cover on the residue. Finally, it is recommended to continue investigation into residue neutralisation methods, wetland trials, and Mud Farming.

The period of this study is from 2003 to 2011.