Analysis of bauxite residue components responsible for copper removal and related reaction products

Development of Environmentally Clean Construction Materials Using Industrial Waste

The accumulated waste generated from industries severely affects environmental conditions. Using waste as a construction material or soil stabilization is an emerging area in the construction industry. Introducing new additive materials to strengthen local soils using industrial waste is an inexpensive and more effective method to improve the soil. In light of this, this study aims to develop environmentally clean construction materials for stabilizing natural loam (NL) using red mud (RM), blast furnace slag (BFS), and lime production waste (LPW). Nine different mixtures were prepared with four different combinations of RM (20, 30, and 40%), BFS (25, 30 and 35%), LPW (4, 6 and 8%), and various content of NL. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic absorption spectroscopy (AAS), and axial compressive strength were examined. The results indicated that the optimum strength was obtained from the sample containing 40% of RM, 35% of BFS, and 8% of LPW. The observed compressive strength of the sample for 90 days was 7.38 MPa, water resistance was 7.12 MPa, and frost resistance was 7.35 MP, with low linear expansion meeting the demands for first class construction materials of the Kazakh norms. The mineral composition analysis evidenced the lack of heavy metals contaminants and hazardous compounds. Based on strength and environmental performance, RM, BFS, LPW, and NL mix can be used as a road base material. This process is believed to reduce environmental pollution related to RM and BFS, and lower the road base cost.

Purification of Low-Concentration Carbonyl Sulfide by Red Mud-Based Adsorbent

Bauxite residue, also known as red mud (RM), is a kind of industrial solid waste with high alkali content, complex composition and difficult utilization. In this study, a new type of RM-based adsorbent was prepared by using polyethylene glycol modified RM and was used to remove low concentration of COS in flue gas. The optimum preparation conditions of adsorbent and the optimum technological parameters of COS adsorption purification were investigated. Under the optimal conditions, the adsorption efficiency of the new adsorbent exceeds 95%, and the COS adsorption capacity reaches 63.56 mg/m³. The characterization results showed that the main active components of the adsorbent were active alkali, FeOOH and Fe₃O₄, and the main products were Na₂S₂O₃, Na₂SO₄, FeS and FeS₂.

Removal of Cr (Ⅲ) from aqueous solution by using bauxite residue (red mud): Identification of active components and column tests

Bauxite residue is the by-product of the aluminium industry with an annual output of more than 200 million metric tons in China. Its treatment is still a big problem because more than 96% of that is stockpiled on land causing environmental pollution and threatening the human health. This study used bauxite residue to remove Cr (Ⅲ) from aqueous solution and analyzed the removal mechanism. The removal time was dependent on the initial concentrations of Cr (Ⅲ) and different active components acted on different reaction period. Reaction time increased from <5 min to >2 h with an increase of Cr (Ⅲ) concentration from 5 to 100 and 170 mg/L. The existing forms of adsorbed-Cr were iron oxide-bound Cr (40.80%-87.85%), sulfide-bound Cr (4.04%-20.28%) and residue (6.60%-33.72%). All the components started to react as soon as bauxite residue was added. Cr did not precipitate in the presence of high alkalinity bauxite residue due to the slow release of alkalinity maintaining pH < 6, thus producing Cr(OH)²⁺, Cr₂(OH)₂⁴⁺ and Cr₃(OH)₄⁵⁺ by hydrolysis without precipitation. Fe₂O₃ and Al-containing components were the main active phases for Cr (Ⅲ) removal, with the reaction time lasting more than 2 h and producing Ca₆Al₄Cr₂O₁₅, AlCr₂, (Si, Al)₂O₄, Fe(Cr, Al)₂O₄, FeCr₂Si₃O₁₂, MgCr_0·1Fe_1·9O₄ and MgCr_0·4Fe_1·6O₄. Finally, bauxite residue was granulated and used for column tests. Cr (Ⅲ) wastewater (1 and 50 mg/L) was treated and the effluent can meet the first level of the Shanghai standard (0.1 mg/L) defined by Integrated Wastewater Discharge Standard (DB 31/199-2009).

Effect of phosphogypsum and poultry manure on aggregate-associated alkaline characteristics in bauxite residue

Bauxite residue is a highly alkaline solid waste with poor physical structure which ultimately limits plant growth. Ecological reconstruction is an effective strategy to improve its environmental management, although soil formation process still requires further investigation. Here, an incubation experiment was used to investigate the effects of phosphogypsum and poultry manure, on aggregate size distribution and aggregate-associated exchangeable bases of bauxite residue. Phosphogypsum and poultry manure additions significantly increased the proportion of 2-1 mm residue aggregates and enhanced mean weight diameter (MWD) of residues in the 0-20 cm and 20-40 cm layers, although little effect was evident in the 40-60 cm layer. Phosphogypsum addition reduced pH and EC values to approximately 8.5 and 200 mS/cm in different size aggregates at 0-20 cm. Exchangeable Ca²⁺ concentration was improved, especially in 0.25-0.05 mm and <0.05 mm aggregates, following amendment additions. The relative contents of katoite and cancrinite in >0.25 mm aggregate fractions were relatively higher, which was consistent with changes in pH. Phosphogypsum and poultry manure changed the microstructure and surrounding pores of residue aggregates, whilst the concentration of Ca on microaggregate surfaces was higher than that on macroaggregates. These findings reveal that application of phosphogypsum and poultry manure directly alter the distribution of exchangeable bases and alkaline indicators within residue aggregates, resulting in aggregate size distribution and microstructure variations.

Improved bauxite residue dealkalization by combination of aerated washing and electrodialysis

Bauxite residue, a major by-product of the alumina-producing Bayer process, is a serious environmental pollutant due to its high alkalinity. Here, we reported an operation system designed in our laboratory that included washing and electrodialysis dealkalization systems with aeration pipes. Washing with aeration releases a substantial amount of free alkali and attached alkali into water and increases the dealkalization efficiency. The washing liquid was treated with five steps of batch-mode electrodialysis. The average removal of total dissolved solids (TDS) after the aeration and non-aeration electrodialysis processes were 61.30% and 39.61%, respectively. The average removal of OH^- under aeration conditions was 76.62%, a value that was greater than the value produced under non-aeration conditions (68.48%). This efficiency was also higher than that of some other reports (64.90-68.50%). Aeration decreased the energy consumption to a greater extent than the non-aeration condition. NaOH was recovered in terms of the concentration chamber, and the NaAl(OH)₄ present in the dilution chamber was separated for the electrodialysis treatment. Membrane scaling was generated to a lesser amount under aeration conditions than that of non-aeration conditions, which would improve the dealkalization efficiency. The high repeatability of the experiments was indicated by the intraclass correlation coefficient (P < 0.05).

Fabrication of a low-cost adsorbent supported zero-valent iron by using red mud for removing Pb(ii) and Cr(vi) from aqueous solutions

In this study, a granular red mud supported zero-valent iron (ZVI@GRM) was successfully prepared and was used to remove Pb(ii) and Cr(vi) from aqueous solution. Zero-valent iron (ZVI) was synthesized by direct reduction of iron oxide in red mud by maize straw as a reductant at 900 °C in an anoxic atmosphere. The technical characterization (SEM, EDS, XRD, FTIR and BET) revealed that ZVI@GRM was loaded with zero-valent iron and contained different size pores. The factors of adsorption experiments include initial concentration, contact time, pH and temperature. The Pb(ii) and Cr(vi) removal by ZVI@GRM well fitted the pseudo-second-order kinetics model and the removal of heavy metals was an endothermic process. Essentially, Pb(ii) was transformed to precipitate forms (Pb⁰, Pb (OH)₂, or 2PbCO₃·Pb (OH)₂) and Cr(vi) was converted to Cr (OH)₃ or Cr³⁺/Fe³⁺ hydroxides. The maximum removal capacity for Pb(ii) and Cr(vi) by ZVI@GRM was 149.42 and 37.14 mg g⁻¹. ZVI@GRM was a low-cost material and had outstanding performance and great potential in wastewater treatment.

In this study, a granular red mud supported zero-valent iron (ZVI@GRM) was successfully prepared and was used to remove Pb(ii) and Cr(vi) from aqueous solution.