Recovery of valuable metals from red mud: A comprehensive review.

As a bulk solid waste with high alkalinity, red mud (RM) not only occupies a large amount of land and requires high maintenance costs, but also unavoidably generates serious hazards to the surrounding ecological environment. The comprehensive treatment of RM has become an enormous challenge for the green, low-carbon and high-quality development of the global alumina industry. To minimize the RM destruction to the ecology and the waste of secondary resources, the sustainable utilization of RM was widely investigated in the past decades, especially for the recovery of valuable metals. This paper systematically summarized the research status of recycling valuable metals (Al, Fe, Na, Ti, Sc, Ga, V and RE) from RM in recent years. The recycling technology mainly includes physical beneficiation, hydrometallurgy, pyrometallurgy and electrodialysis. The technical principles and characteristics as well as the current problems of various recovery processes from RM were comprehensively introduced, and the future development directions of sustainable utilization were also prospected. The advantages and disadvantages based on the different aspects of recovery efficiency, energy consumption and environmental impact were also discussed. The proposal of new technologies for the harmless, high-value and full utilization of RM is beneficial to the future research on the comprehensive utilization of bulk industrial solid wastes.

Fabrication of Ti3Al-Based Intermetallic Alloy by Laser Powder Bed Fusion Using a Powder Mixture.

Due to their light weight and outstanding mechanical properties at high temperatures, Ti3Al-based intermetallic alloys have driven increasing interest from both academia and industry; however, when additive manufacturing (AM) is applied to them, the outcome is hardly satisfying. In this work, we report a crack-free Ti3Al-based alloy fabrication by laser powder bed fusion (LPBF) using a mixture of a commercial Ti-48Al-2Cr-2Nb powder and a pure Ti powder. With the aid of a high cooling rate during LPBF, the as-built sample shows a ductile ß phase with some partially-melted particles. After the heat treatment, partially-melted particles were dissolved, and the sample showed equiaxed α2 precipitates in the ß matrix. The hardness was 515 ± 38 HV in the as-built sample and 475 ± 37 HV in the heat-treated sample. This study shows a novel strategy to fabricate crack-free Ti3Al-based alloy using LPBF from powder blends.

A novel method for extraction of platinum from spent automotive catalyst: utilization of spent fluid catalytic cracking catalyst as flux.

A novel method of smelting of mixture of spent automotive catalyst (SAC) and spent fluid catalytic cracking catalyst (SFCC) to recover platinum and prepare glass slag was investigated. Compared to other metals collection processes for single hazardous waste solid, this method reduced the amount of fluxing materials addition and increased the processing types of hazardous solid waste simultaneously. The optimum SFCC addition, iron collector addition, Na2B2O4•10H2O addition, CaO/SiO2 mass ratio, temperature, and holding time for platinum recovery were 20, 11, 16 wt%, 0.6, 1550-1600°C, and 60 min, respectively. In this proposed combined process, more than 98% of platinum is efficiently recovered from SAC. Meanwhile, the concentration of platinum in glass slag was less than 7 g/t. The leaching characteristics of heavy metals in slag confirmed the obtained glass slag is a non-hazardous slag due to the low leaching rate of heavy metal ions. This article proposed an effective and environmentally friendly method for the recovery of platinum from SAC via a combined smelting process.

A Study on the Effect of Graphene in Enhancing the Electrochemical Properties of SnO2-Fe2O3 Anode Materials.

To enhance the conductivity and volume expansion during the charging and discharging of transition metal oxide anode materials, rGO-SnO2-Fe2O3 composite materials with different contents of rGO were prepared by the in situ hydrothermal synthesis method. The SEM morphology revealed a sphere-like fluffy structure, particles of the 0.4%rGO-10%SnO2-Fe2O3 composite were smaller and more compact with a specific surface area of 223.19 m2/g, the first discharge capacity of 1423.75 mAh/g, and the specific capacity could be maintained at 687.60 mAh/g even after 100 cycles. It exhibited a good ratio performance and electrochemical reversibility, smaller charge transfer resistance, and contact resistance, which aided in lithium-ion transport. Its superior electrochemical performance was due to the addition of graphene, which made the spherical particle size distribution more uniform, effectively lowering the volume expansion during the process of charging and discharging and improving the electrochemical cycle stability of the anode materials.

Recovery of nickel and preparation of ferronickel alloy from spent petroleum catalyst via cooperative smelting-vitrification process with coal fly ash.

Spent petroleum catalyst (SPC) is a highly toxic material since it contains heavy metals and hazardous substances. A novel recycling technology based on the cooperative smelting-vitrification process by using coal fly ash (CFA) as a fluxing material was proposed. The benefits of employing CFA in this cooperative smelting-vitrification process of SPC have been demonstrated via the results of lab-scale and scale-up experiments. The experimental results indicated that with a collector iron (Fe) addition of 26 wt%, a C/O molar ration of 1.4, and an H3BO3 addition of 14 wt%, the maximum nickel (Ni) recovery was ∼98% by controlling the CFA addition of 40-50 wt%, basicity of 0.4-0.5, smelting temperature of 1550°C, and smelting time of 60 min, respectively. In this process, a ferronickel (Ni-Fe) alloy with a high Ni grade of 10 wt% was successfully obtained, which could be directly further produced stainless steel. Meanwhile, a glass slag with a low Ni content (below 0.12 wt%) was also obtained, and its leaching characteristics further confirmed it is a non-hazardous slag because heavy metals were successfully encapsulated in glass slag, and thereby, this proposed method achieved the transformation from hazardous solid waste to general solid waste. The results of the 10 kg scale-up experiment indicated the possibility of industrialization of this new technology. Therefore, the process proposed in this study is a practical and promising process for Ni recovery from SPC and reutilization of CFA.

Compressive Behavior of Al-TiB2 Composite Foams Fabricated under Increased Pressure.

The application of increased pressure was used as a strategy to investigate the effect of different cell structures on the mechanical properties of Al-TiB2 composite foams. In situ Al-xTiB2 (x = 5, 10 wt.%) composites were foamed under three different pressures (0.1 MPa, 0.24 MPa, 0.4 MPa) through the liquid melt route. The macro-structure of the composite foams was analyzed in terms of cell size distribution measured by X-ray microcomputed tomography (micro-CT). It was found that the mean cell size decreases, and the cell size distribution range narrows with increasing pressure. Uniaxial compression tests revealed that the stress fluctuation (Rsd) of 10TiB2 foams is larger than that of 5TiB2 foams under the same pressure. Moreover, cell size refinement causes the simultaneous deformation of multi-layer cells, which leads to an enhancement in the energy absorption efficiency and specific energy absorption. The comparison of experimental data with theoretical predictions (G&A model) is discussed.

Metals smelting-collection method for recycling of platinum group metals from waste catalysts: A mini review.

Platinum group metals (PGMs) are widely applied in the field of catalysts due to their excellent catalyst activity and high-temperature stability. The rapid generation of the waste catalyst has become the significant characteristic of PGMs with the accelerating consumption of limited PGMs nature resources. It is necessary to recover/recycle PGMs from a waste catalyst for both economic and environmental benefits. This paper reviews the PGMs recovery from waste catalysts using a metals smelting-collection process, which belongs to the main pyrometallurgical process, in the presence of various metal collectors, such as lead, copper, iron, matte, print circuit board (PCB) or reactive metals of calcium and magnesium. The current status of recovery of PGMs from waste catalysts through the addition of various metals as the collector is discussed and existing advantages and challenges are highlighted in this paper. Meanwhile, in the view of the promising processes of PGMs recovery, the influencing factors such as the economic, environmentally friendly, sustainable recycling, commercial scale, and low-grade materials are considered.

Combined Effect of SiO2 and O2 on the Crystallization Behavior of Modified Ti-Bearing Blast Furnace Slag.

FactSage simulation, X-ray diffraction (XRD), and scanning electron microscopy (SEM) coupled to energy-dispersive spectroscopy (EDS) analyses were performed to explore the combined influence of SiO2 and O2 on the crystallization behavior of modified Ti-bearing blast furnace slag. The results of FactSage simulation showed that an increase in the added amount of SiO2 was beneficial to the conversion of Ti-bearing minerals into simple titanium oxides. An increase in oxidation time was conducive to the transformation of low-valent titanium oxides to tetravalent titanium oxide. Therefore, Ti components were precipitated only in the form of rutile under the optimal added amount of SiO2 (40 g) and the optimum oxidation time (126 s). The results of XRD and SEM analyses were well in agreement with the results of FactSage simulation. Moreover, the mechanism of formation of rutile and glassy phases was discussed.

The Microstructure of Nanocrystalline TiB2 Films Prepared by Chemical Vapor Deposition.

Nanocrystalline titanium diboride (TiB2) ceramics films were prepared on a high purity graphite substrate via chemical vapor deposition (CVD). The substrate was synthesized by a gas mixture of TiCl4, BCl3, and H2 under 1000 °C and 10 Pa. Properties and microstructures of TiB2 films were also examined. The as-deposited TiB2 films had a nano-sized grain structure and the grain size was around 60 nm, which was determined by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. Further research found that a gas flow ratio of TiCl4/BCl3 had an influence on the film properties and microstructures. The analyzed results illustrated that the grain size of the TiB2 film obtained with a TiCl4/BCl3 gas flow ratio of 1, was larger than the grain size of the as-prepared TiB2 film prepared with a stoichiometric TiCl4/BCl3 gas flow ratio of 0.5. In addition, the films deposited faster at excessive TiCl4. However, under the condition of different TiCl4/BCl3 gas flow ratios, all of the as-prepared TiB2 films have a preferential orientation growth in the (100) direction.