Authors: Taki, G; Grierson, PF; Saini, N; Brand, HEA; Murphy, DV; Santini, TC

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Taki, G, Grierson, PF, Saini, N, Brand, HEA, Murphy, DV & Santini, TC 2022, 'Assessing the performance of blended byproduct caps for revegetation and closure of tailings storage facilities', in AB Fourie, M Tibbett & G Boggs (eds), Mine Closure 2022: 15th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 941-948,

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Establishment of a vegetative cover during closure of tailings storage facilities is a critical component of the development of an environmentally sustainable landscape after mining. However, establishing vegetation on fresh bauxite residue (alumina refining tailings) is constrained by the high alkalinity, salinity, sodicity, elevated concentration of trace elements, and low plant available nutrients in residues. Currently, design of store and release vegetative covers for closure of tailings storage facilities in southwest Australia requires excavation of local soils and importing nutrients and mulch to apply on top of the tailings storage facility. Where the residues are mostly benign, in situ remediation (application of amendments directly into tailings to remediate the chemical and physical conditions) techniques may be a viable approach to create a plant growth medium for closure and revegetation. Nevertheless, using imported soils and blending products is expensive. Neutralisation of bauxite residue disposal areas (BRDAs) for capping offers a potential alternative and substantial cost savings, especially when coupled with incorporation of materials to develop an improved substrate for plant growth. In this study, a new technique called ‘blended byproduct capping’ was developed for closure of the South32 Worsley Alumina BRDA in southwest Australia. The blended byproduct cap uses bauxite processing residues that are blended with available byproducts readily and cheaply available onsite at the refinery. Three types of bauxite processing residue (bauxite residue fines, bauxite residue fines plus 10% bauxite residue sand, and bauxite residue sand) were blended with three byproducts (fly ash from power generation, eucalypt mulch from site clearing, and gypsum from other operations nearby) either alone or in combination to create 15 potential capping materials. These capping materials were leached under glasshouse conditions for 18 weeks (three wetting and drying cycles, three weeks each) to assess changes in pH, EC, total elements and nutrients. The three best performing capping materials in terms of chemo-physical properties were then selected for germination and growth experiments. Germination rates of barley (Hordeum vulgare), ryegrass (Lolium multiflorum) and clover (Trifolium spumosum) were assessed and then surviving plants grown for four weeks after the first visible leaf was observed. Root and shoot biomass were harvested at the end of the experiment. More than 90% of barley and ryegrass seeds germinated. Clover germination was less than 60% both blended byproduct caps and potting mix. However, biomass and growth rates were significantly lower in blended byproduct caps compared to potting mix for all three species. Overall, we conclude that blended byproducts caps show significant promise as a cost-effective alternative for BRDA closure and revegetation but require further optimisation.

Keywords: bauxite residue deposit area, red mud, bauxite residue sand, germination success, plant growth, annual grass, clover

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