Electroacoustic Isoelectric Point Determinations of Bauxite Refinery Residues: Different Neutralization Techniques and Minor Mineral Effects

Abstract

Bauxite refinery residue (BRR) is a highly caustic, iron hydroxide-rich byproduct from alumina production. Some chemical treatments of BRR reduce soluble alkalinity and lower residue pH (to values <10) and generate a modified BRR (MBRR). MBRR has excellent acid neutralizing (ANC) and trace-metal adsorption capacities, making it particularly useful in environmental remediation. However, soluble ANC makes standard acid-base isoelectric point (IEP) determination difficult. Consequently, the IEP of a BRR and five MBRR derivatives (sulfuric acid-, carbon dioxide-, seawater-, a hybrid neutralization, i.e, partial CO(2) neutralization followed by seawater, and an activated-seawater-neutralized MBRR) were determined using electroacoustic techniques. Residues showed three significantly different groups of IEPs (p < 0.05) based around the neutralization used. Where the primary mineral assemblage is effectively unchanged, the IEPs were not significantly different from BRR (pH 6.6-6.9). However, neutralizations generating neoformational minerals (alkalinity precipitation) significantly increased the IEP to pH 8.1, whereas activation (a removal of some primary mineralogy) significantly lowered the IEP to pH 6.2. Moreover, surface charging curves show that surfaces remain in the ±30 mV surface charge instability range, which provides an explanation as to why MBRRs remove trace metals and oxyanions over a broad pH range, often simultaneously. Importantly, this work shows that minor mineral components in complex mineral systems may have a disproportionate effect on the observable bulk IEP. Furthermore, this work shows the appropriateness of electroacoustic techniques in investigating samples with significant soluble mineral components (e.g., ANC).

Interaction between phosphate and acid-activated neutralized red mud during adsorption process

“…Therefore, the (Table 3), which was much higher than that of many other adsorbents for phosphate adsorption ( Table 4). The mineralogical changes and increased quantities of amorphous precipitates after acid activation might increase the specific surface area, and then substantially improved the phosphate adsorption capacity of AaN-RM [14].…”

“…Genç-Fuhrman et al [9] attributed the excellent adsorption performance of AaN-RM to the increased surface area by cleaning of adsorption sites during acid activation. However, Freire et al [14] concluded that the increased surface area was more likely attributed to the generation of amorphous and semi-amorphous Al and Fe. There are no sufficient evidences to reveal the governing mechanisms for phosphate adsorption, and the exact phosphate complexes on AaN-RM surface are unclear.…”

Acid transformation of bauxite residue: Conversion of its alkaline characteristics

“…Furthermore, with consumption of free OH -, Al(OH)4 -will precipitate as gibbsite (Al(OH)3) and possible dawsonite (Eqs. 11-13) [35,44,45]. Use of CO2 would also reduce the emission of industrial carbon dioxide, which could create an additional benefit.…”

“…Al(OH)4 -+CO2+Na + →NaAlCO3(OH)2+H2O (13) Interaction of waste acid can react with hydroxides and carbonates, even for oxides that may be leached [45,[47][48][49]. 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].…”

Comparison of several different neutralisations to a bauxite refinery residue: Potential effectiveness environmental ameliorants

“…) and bi-carbonate (HCO 3 À ) ions should be the dominant products of the CO 2 neutralisation process, without substantially adding to, or reducing total alkalinity of the decant liquor (Freire et al, 2012;Johnston et al, 2010). However, several authors claim that dawsonite precipitation is observed (Cooling et al, 2002;Guilfoyle et al, 2005;Jones et al, 2006) (Eqs.…”