Fractionation and mobility of metals in bauxite red mud

Abstract

Red mud (RM) is a strongly alkaline residue generated in enormous amounts worldwide from bauxite refining using the Bayer chemical process. RM is composed mainly of Fe, Ti and Al oxides and hydroxides, but it also contains an array of trace metals and metalloids at different concentrations. The purpose of this paper is to assess the potential mobility of metals in RM, with special emphasis on pH effect. The 'operational' distribution and leachability of metals within/from RM was studied by applying a sequential extraction procedure (SEP) and several leaching tests (rapid titration, equilibration acidification, batch leaching with acetic acid and also the toxicity characteristics leaching procedure (TCLP) and the DIN 38414-S4 procedures, used as reference methods) carried out at different pH, solid/liquid ratio, extraction period and type of acid (HCl or acetic acid). Chemical analysis showed that, in addition to the major metals Fe, Al and Ti, RM contains several trace metals, some of them (Cr, Cu and Ni) in concentrations exceeding the regulatory limits. SEP showed that a majority of the metals in the RM (between the 32.2±8.5 for Cd and 95.3±0.4% for Ni) were found in the residual fraction, suggesting that they are not readily mobile under normal environmental conditions. Leaching tests performed at different pH showed that a significant fraction of the metals is mobilised from RM only under very strong acid conditions (pH<2), whereas Al is released in considerable amounts at pH<5.3. Among the trace metals, Cr requires special attention because of its relative high concentration in RM and the higher concentrations of this metal mobilised at low pH. The leaching tests using acetic acid showed that the standard TCLP largely underestimates the release of trace metals from RM, and therefore it is not advisable to evaluate the actual potential leaching of trace metals from this residue.

Mobilisation of arsenic from bauxite residue (red mud) affected soils: Effect of pH and redox conditions

“…HCO À 3ðaqÞ þ H þ ðpH> $ 5Þ ð 3Þ M 2+ ions, such as Ca 2+ and Mg 2+ , can be exchanged from the soil by Na + ions present in the red mud, or alternatively sourced from the dissolution of Ca aluminosilicate minerals (e.g. cancrinite/ hydrogarnet (Hind et al, 1999;Rubinos and Barral, 2013)) present in red mud that are unstable below pH 12 (Castaldi et al, 2010). These M 2+ ions, can then precipitate carbonate phases (e.g., Ca 2+…”

Dissolution of sodium, aluminum and caustic compounds from bauxite residues

“…It is estimated that approximately 120 million tons of bauxite residue are produced in the world annually (Milacic et al, 2012;Power et al, 2011;Rubinos and Barral, 2013). Thus it is evident that bauxite residue is currently a major concern in the production of alumina .…”

Results of the clean-up operation to reduce pollution on flooded agricultural fields after the red mud spill in Hungary

“…Red mud is mainly composed of Fe, Ti, and Al oxides and hydroxides, but it also contains an array of trace metals and metalloids in different concentrations and NaOH, which has corrosive properties (Rubinos and Barral 2013). The main constituents and properties of red mud from Ajka are Al 2 O 5 10-15 % (w/w), Fe 2 O 3 40-45 % (w/w), CaO 6-10 % (w/w), MgO 0.5-1 % (w/w), SiO 2 10-15 % (w/w), Na 2 O (chemically bound and dissolved) 5-6 % (w/w), TiO 2 4-5 % (w/w), and V 2 O 3 0.1-0.2 % (w/w), pH; 10-12.…”