No abstract

During acid leaching of bauxite residue (red mud), the increase in dissolution of rare-earth elements (REEs) is associated with a substantial co-dissolution of iron; this poses problems in the downstream processing (i.e. solvent extraction or ion exchange). Six different slags generated by reductive smelting of the same bauxite residue sample were treated by high-pressure acid leaching (HPAL) with HCl and H 2 SO 4 to selectively extract REEs. Thus, up to 90 wt% of scandium was extracted from the slags using H 2 SO 4 at 150°C, while with HCl the extraction of scandium reached up to 80 wt% at 120°C. The extraction of yttrium, lanthanum and neodymium was above 95 wt% when HCl was used as a reagent, but it was much lower (< 20 wt%) with H 2 SO 4 , presumably due to the formation of a double sulfate (NaLn(SO 4 ) 2 ⋅nH 2 O) and/or due to the adsorption on the surface of silicon/aluminium-oxides compounds. In addition, HPAL of bauxite residue slags led to a significant co-dissolution of aluminium (> 90 wt%, 18 g L −1 ), while the concentration of the remaining iron (> 60 wt%) was of 3 g L −1 in the leachate. The co-dissolution of silicon and titanium was lower than 5 wt%.

Highlights  High CO2 concentrations reduce the bauxite residue alkalinity.  Cancrinite forms upon decomposition of sodalite with high CO2 concentration.  High stability of aluminosilicate compounds with high sodium dissolution.  Silica dissolution limits the recovery of metals by acidic leaching.  The recovery of scandium is restricted by its chemical association to major elements.

Nanoclay-based nanocomposites have been widely studied and produced since the late 1990s, and frequently end up in waste disposal plants. This work investigates the behavior of PA6/HNTs nanocomposites (nylon-6 incorporating halloysite nanotubes) during incineration. Incineration tests were performed at lab-scale using a specific tubular furnace modified in order to control the key incineration parameters within both the combustion and postcombustion zones. The combustion residues and combustion aerosol (particulate matter and gas phase) collected downstream of the incinerator furnace were characterized using various aerosol analysis techniques. Time tracking of the gas and particle-number concentrations revealed two-step char formation during combustion. HNTs transformed into other mineral structures which were found in both the aerosol and the residues. During combustion of the polymer, it appears that HNTs contribute to the formation of a cohesive char layer that protects the residual material.