Chemistry of bauxite residue neutralisation and aspects to implementation

Acid transformation of bauxite residue: Conversion of its alkaline characteristics

“…Residual sodium hydroxide (NaOH), sodium aluminate (NaAl(OH)4) and sodium carbonate (Na2CO3) that can't be completely separated by counter-current decantation (CCD) water washing remain in the residue and are responsible for the soluble alkalinity. The leaching of alkaline ions (OH -, Al(OH)4 -, CO3 2-) provides some buffering capacity around pH 11 to 8.3 [29,30].…”

“…Dosing with waste acids is effective to lower pH and reduce alkaline substances, but nevertheless, the acid transformation of bauxite residue is controlled by a complicated set of chemistry reactions that depend upon the interaction between the solution phase and multiple solids [15,29,50]. Unfortunately, dissolution behavior of alkaline substances at a specific pH is absent, but is pivotal if a valid acid transformation theory is to be understood.…”

Revegetation strategies for seawater neutralized bauxite residue at Rio Tinto Alcan using various amendments

“…Environmental risks from these storage areas include wind and water erosion and drainage of leachate . For example, the alkaline, saline and sodic leachate may need to be managed and treated during operation of the refinery and then for many decades after closure.Several remediation measures have been employed to reduce the alkalinity of the residues prior to their deposition in storage areas including neutralization with CO2, seawater and acid Menzies et al, 2004;Khaitan et al, 2009; Sushil et al, 2012;Kirwan et al, 2013).Australia is currently the world's largest producer of bauxite and in 2014, 81 million tonnes were produced (Statista, 2016 In north Queensland, bauxite from far northern Queensland is processed at two alumina refineries.The residue is mainly mud (< 150 µm) with a small amount (about 3% of total mass) of sand (> 150 µm) present. Residues are seawater neutralized in especially designed facilities before deposition in the residue storage areas.…”

“…As a result, some refineries now partially neutralize their residues before they are deposited in RDAs. This reduces the need for costly on-going intensive management of the RDA and its leachate after closure and also opens up opportunities for re-use of the residue which, to date, have been hindered because of the high pH (Kirwan et al, 2013).Neutralization may involve (a) direct carbonation whereby the residue slurry is treated with CO2 gas Dilmore et al, 2008), (b) addition of spent acids (Kirwan et al, 2013) or (c) reaction of residues with seawater Palmer et al, 2009). Seawater neutralization was pioneered at the Queensland Alumina Limited refinery where, in order to conserve fresh water, seawater was used as a medium to transport residue from the refinery to disposal lagoons ).…”

“…The alumina refineries in northern Queensland are located on the coast and the residue is seawater neutralized prior to deposition in storage areas. After neutralisation, the pH of bauxite residue is reduced from 11.0-13.0 down to about 9.0 through precipitation of soluble alkalinity as Ca, Mg hydroxides/hydroxycarbonates and hydrotalcite-like compounds Kirwan et al, 2013). The sodicity of the residue is also decreased because Ca, Mg and K in seawater replace Na on exchange sites.…”