Variation on leaching behavior of caustic compounds in bauxite residue during dealkalization process

A practical method for identifying key factors in the distribution and formation of heavy metal pollution at a smelting site

Smelting activities are the main pathway for the anthropogenic release of heavy metals (HMs) into the soil-groundwater environment. It is vital to identify the factors affecting HMs pollution to better prevent and manage soil pollution. The present study conducted a comprehensive investigation of HMs in soil from a large abandoned Zn smelting site. An integrated approach was proposed to classify and quantify the factors affecting HMs pollution in the site. Besides, the quantitative relationship between hydrogeological characteristics, pollution transmission pathways, smelting activities and HMs pollution was established. Results showed that the soils were highly contaminated by HMs with a pollution index trend of As > Zn > Cd > Pb > Hg. In identifying the pollution hotspots, we conclude that the pollution hotspots of Pb, As, Cd, and Hg present a concentrated distribution pattern. Geo-detector method results showed that the dominant driving factors for HMs distribution and accumulation were the potential pollution source and soil permeability. Additionally, the main drivers are variable for different HMs, and the interaction among factors also enhanced soil HMs contamination. Our analysis illustrates how the confounding influences from complex environmental factors can be distilled to identify key factors in pollution formation to guide future remediation strategies.

Rapid conversion of alkaline bauxite residue through co-pyrolysis with waste biomass and its revegetation potential

The extreme alkalinity of bauxite residue (BR) leads to difficulty with its reuse. Alkaline leachate and dust generation during the stacking process can pollute surrounding soil, air and water. In this work, co-pyrolysis of bauxite residue and sawdust was applied to rapidly produce a soil-like matrix that met the conditions for plant growth as demonstrated by ryegrass pot experiments. The present study aimed to characterize the detailed changes in physicochemical, mineral weathering, and microbial communities of the pyrolyzed BR with different ratios of saw dust after plant colonization for 2 months. With increasing sawdust addition during co-pyrolysis, the pH of BR decreased from 11.21 to 8.16, the fraction of macro-aggregates 0.25-2 mm in the water-stable agglomerates increased by 29.3%, and the organic carbon concentration increased from 12.5 to 320 mg/kg, whilst facilitating the degree of humification, which were all beneficial to its revegetation performance. The backscattered electron-scanning electron microscope-energy-dispersive X-ray spectrometry (BSE-SEM-EDS) results confirmed the occurrence of sodalite and calcite weathering on aggregate surfaces, and X-ray photoelectron spectroscopy (XPS) results of surface Al and Si compounds identified that some weathering products were clay minerals such as kaolinite. Furthermore, bacterial community composition and structure shifted towards typical soil taxonomic groups. These results demonstrate soil development of treated BR at an early stage. The technique is a combination of alkalinity regulation and agglomerate construction, which accelerates soil formation of BR, thus proving highly promising for potential application as an artificial soil substitute.

Red mud recycling by Fe and Al recovery through the hydrometallurgy method: a collaborative strategy for aluminum and iron industry

In this work, a collaborative strategy for the aluminum and iron industry based on red mud recycling through the hydrometallurgy method was proposed. In this method, Fe³⁺ and Al³⁺ were firstly separated from the red mud by using H₂SO₄ as a leaching agent, which was by-produced from the sintering process of an iron and steel industry. Multiple influence factors on the leaching process were investigated, with the H₂SO₄ addition amount showing the strongest influence on the leaching rates of Al and Fe. The main components of the filter residue were CaSO₄, TiO₂, and SiO₂, which could be reused as additives in the building materials. Subsequently, the final Fe recovery product was obtained through the co-precipitation, Fe/Al separation, and Fe(OH)₃ calcination. In the final product, the content of Fe₂O₃ reached 82.87%, and the iron grade was 58.01%, meeting the requirement being raw materials for sinter production.

Migration of Alkaline Constituents and Restoration Evaluation in Bauxite Residue Disposal Areas

Bauxite residue is a highly alkaline waste from alumina refining, and is mainly disposed by stacking with high environmental risks. Here, the migration of alkaline constituents and the restoration evaluation with phosphogypsum were discussed by soil column experiments to investigate the alkaline regulation in bauxite residue disposal areas (BRDAs). The pH, free alkali, exchangeable sodium in the top layer (0-25 cm depth) covered with BR and phosphogypsum mixtures were reduced from 10.89 ± 0.02, 285.45 ± 21.15 mmol/kg, 385.63 ± 30.34 mg/kg to 9.00 ± 0.50, 12.50 ± 1.50 mmol/kg, 97.00 ± 10.50 mg/kg. For the sublayers, including depths of 35, 45, 55 cm, these values dropped to 9.86, 10.06, 10.03; 38.23, 86.12, 148.00 mmol/kg; 152.90, 246.00, 305.00 mg/kg, respectively. These results indicated alkaline indicators for phosphogypsum amended BR declined dramatically, and the parameters for sublayers were also decreased due to the migration of alkaline constituents. The physicochemical properties for amended BR could meet the conditions for plant growth. This research provided a reference for alkalinity regulation in BRDAs by phosphogypsum.

Conversion of alkaline characteristics of bauxite residue by mechanical activated pretreatment: Implications for its dealkalization

Following the strict environmental policies of various countries, the strong alkalinity of bauxite residue (BR) has become a worldwide problem limiting the sustainable development of the global alumina industry. Continuous conversion of solid-phase alkalinity to free alkali is a major challenge for BR dealkalization to reduce its environmental impact. This work aimed to investigate the effect of mechanical grinding pretreatment on the transformation mechanisms of alkaline solids to free alkali at the BR interface under acids leaching, by monitoring the morphology, phase, and speciation transformations of Al and Si using primarily cross-section scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) elemental mapping, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The results indicated that particle grinding wrapped some of the alkaline minerals inside the particles to inhibit its release process. The leaching kinetics revealed the order of the buffering effect of minerals against acids leaching is firstly dissolved by minerals containing Na and Ca via the ion-exchange process, followed by Si and Al through the hydrolysis of the desilicated products. The mineral dissemination characteristics and surface compositions further confirmed the undissolved minerals block the interface reaction between embedded alkaline solids and acids to result in the difficult reaction dissolution of alkaline minerals, which is induced by ball milling. This novel approach provides new insight into the efficient dealkalization of BR on a large scale in the industry.

Integrating column leaching experiments and geochemical modelling to predict the long-term alkaline stability during erosion process for gypsum amended bauxite residue

Gypsum amendment is widely used to resolve alkalinity issues and implement sustainable management for bauxite residue disposal areas (BRDAs). Amended BRDAs under natural conditions suffer from long-term erosion processes. Nevertheless, the effect of erosion on amendment efficacy is rarely assessed. In this study, by integrating the geochemical modelling of PHREEQC and column leaching experiments, the dissolution of alkaline solids in bauxite residue (BR) and gypsum amendment, as well as their environmental behaviors, were determined through a 1-year simulated rainfall leaching experiment. The PHREEQC simulation results demonstrated that Na⁺ ion strength, CO₂ partial pressure and rainfall, all affected the saturation index (SI) of calcite significantly and accelerated its corrosion, leading to the dissolution of gypsum and calcite in a relatively stable state. However, Na⁺ ion strength and rainfall significantly acted on the SI of gypsum, which lead to loss of Ca²⁺ and reduction of alkaline stability. In addition to the effects of Na⁺ and Ca²⁺ on the saturation concentration of gypsum and calcite solution, Na⁺ and Ca²⁺ also exhibited significant effects on the equilibrium of chemical species reactions. The column results confirmed that stability of gypsum and calcite was consistent with the simulation results of PHREEQC in the BRDAs environment. Furthermore, multiple linear regressions revealed differences in combined contributions of rainwater and atmospheric CO₂ on the stability of calcite and gypsum. The PHREEQC simulation provides a new approach to predict long-term alkaline stability of BR as well as to establish sustainable remediation on BRDAs during erosion process.

Co-Disposal of Coal Gangue and Red Mud for Prevention of Acid Mine Drainage Generation from Self-Heating Gangue Dumps

The seepage and diffusion of acid mine drainage (AMD) generated from self-heating coal gangue tailings caused acid pollution to the surrounding soil and groundwater. Red mud derived from the alumina smelting process has a high alkali content. To explore the feasibility of co-disposal of coal gangue and red mud for prevention of AMD, coal gangue and red mud were sampled from Yangquan (Shanxi Province, China), and dynamic leaching tests were carried out through the automatic temperature-controlled leaching system under the conditions of different temperatures, mass ratios, and storage methods. Our findings indicated that the heating temperature had a significant effect on the release characteristics of acidic pollutants derived from coal gangue, and that the fastest rate of acid production corresponding to temperature was 150 °C. The co-disposal dynamic leaching tests indicated that red mud not only significantly alleviated the release of AMD but also that it had a long-term effect on the treatment of acid pollution. The mass ratio and stacking method were selected to be 12:1 (coal gangue: red mud) and one layer was alternated (coal gangue covered with red mud), respectively, to ensure that the acid-base pollution indices of leachate reached the WHO drinking-water quality for long-term discharge. The results of this study provided a theoretical basis and data support for the industrial field application of solid waste co-treatment.

Alkalinity neutralization and structure upgrade of bauxite residue waste via synergistic pyrolysis with biomass

Bauxite residues, a large volume solid waste, are in urgent need of effective disposal and management. Especially, strategies to alleviate the high alkalinity of bauxite residue remain a big challenge. Here, we developed a synergistic pyrolysis to neutralize the alkalinity of bauxite residue and upgrade the structure of biomass simultaneously. By cooperating the catalytic feature from bauxite residue, rice straw, a cellulose-enriched biomass, could prefer to produce acidic components under a hypothermal pyrolysis temperature (below 250 °C) and partial oxygen-contained atmosphere as evidenced by the synchronous TGA-FTIR analysis. In return, these in-situ produced acidic components neutralized the bauxite residue profoundly (pH decreased from 11.5 to 7.2) to obtain a neutral product with long-term water leaching stability. Also, a higher pyrolysis temperature led to neutral biochar-based products with well-defined carbonization characteristics. Thus, the biomass-driven pyrolysis strategy provides a potential to dispose the alkalinity issue of bauxite residue and further opportunities for the sustainable reuse and continuing management of bauxite residue.