Search2020 Vol. 27, No. 8
The mining industry produces billions of tons of mine tailings annually. However, because of their lack of economic value, most of the tailings are discarded near the mining sites, typically under water. The primary environmental concerns of mine tailings are related to their heavy metal and sulfidic mineral content. Oxidation of sulfidic minerals can produce acid mine drainage that leaches heavy metals into the surrounding water. The management of tailing dams requires expensive construction and careful control, and there is the need for stable, sustainable, and economically viable management technologies. Alkali activation as a solidification/stabilization technology offers an attractive way to deal with mine tailings. Alkali activated materials are hardened, concrete-like structures that can be formed from raw materials that are rich in aluminum and silicon, which fortunately, are the main elements in mining residues. Furthermore, alkali activation can immobilize harmful heavy metals within the structure. This review describes the research on alkali activated mine tailings. The reactivity and chemistry of different minerals are discussed. Since many mine tailings are poorly reactive under alkaline conditions, different pretreatment methods and their effects on the mineralogy are reviewed. Possible applications for these materials are also discussed.
Mine closure is associated with many negative impacts on society and the environment. If these effects are not rationally addressed, they would pose risks of mine closure. Thus, a risk management method is needed to mitigate these adverse impacts and address mine-closure issues. An integral framework for mine-closure risk management that includes risk assessment and risk treatment was proposed. Given the fuzziness and randomness of the transformation between qualitative and quantitative knowledge in the risk assessment process, a novel risk assessment method based on the cloud model was presented, which fully considers the uncertainty in risks themselves and in the reasoning process. Closed mine reutilization is an effective risk treatment option in response to the identified high risks, but it requires selecting optimal reutilization strategies for the successful implementation of the reuse plan. To this end, a hybrid semi-quantitative decision method is proposed to optimize decision-making. The results of a case study showed that this risk management methodology can help budget planning for risk treatment and provide an instructional framework to effectively reduce the negative effects of closed mines.
Small-particle interlayers (lenticles) show some characteristic hydraulic properties and can affect the movement of unsaturated water. In this study, we developed a novel online capillary-water-absorption monitoring device and conducted three groups of comparison tests to simulate lenticle positions and thicknesses with respect to the capillary rise. The results show that the characteristic wetting front exhibits a fast rise in the early stage, a slow rise in the middle stage, and stability in the later stage. The motion of the capillary water in the lenticle is mainly transversal, with the upward curve being “flat,” and the longer is “flat,” the longer is the time needed for the water to move. The interlayer can form a capillary stagnation zone with moisture content close to saturation. The high interlayer may form a discontinuous corrugated capillary zone. Thus, when the wetting front reaches the “coarse-grain (lower)−fine-grain (upper)” interface, the “anti-capillary barrier effect” results in more moisture in the upper layer. Thus, when the wetting front of the capillary water reaches the “fine-grain (upper)−coarse-grain (lower)” interface, the “capillary barrier effect” causes the moisture content of the upper tailings to decreases sharply because of the horizontal movement of the water in the fine medium. It is clear that the presence of lenticles can retard the rise of capillary water by storing water.
Cemented paste backfill (CPB), a mixture of tailings, binder, and water, is widely and extensively used for the recovery of mineral resources, the prevention of ground subsidence, and the management of mine waste. When installed, the CPB is subjected to complex environmental conditions such as water content, temperature, and power, which have a significant impact on its efficiency. Thus, this study conducts a series of laboratory programs, including investigation of moisture, temperature, stress–strain relation, and microstructure to show the effect of curing humidity on the CPB behaviors. The results obtained indicate that ambient humidity can have a dramatic effect on CPB in terms of its macro performance of internal relative humidity, temperature and strength, as well as the micro expression. Typical examples of these effects on CPB include an increase in curing humidity, which favors binder hydration, and then an increase in hydration materials, temperature and peak stress in the CPB. The results obtained will lead to a better understanding of CPB’s responses to various environmental conditions.
In underground mining, there has been an increasing use of “cemented paste” for the backfilling of stopes. As this cemented paste backfill (CPB) enters the stope as a fluid, shotcrete barricades are often used to retain the fill material during and after the filling operations. However, failures of barricades have been reported around the world in recent years. This paper presents an analytical solution based on the elastic thin plate theory for calibrating the design of shotcrete barricades in underground mines using CPB. This solution was used to determine the quantitative relationships between the lateral loading from the paste and the barricade response during the backfilling process. The results show that the proposed solution agrees well with in situ data. According to the actual barricade responses, the acceptable tensile stress and an analysis method of cracks development are proposed. The proposed solution has practical significance for underground mines.
The composition of tailings particles in mines plays a key role in the flocculation settlement of slurries. To study the influence of coarse particle tailings (CPTs) on the flocculation settlement of tailings slurries (TSs), static flocculent settling tests, scanning electron microscopy observations, and laser particle size analyses were conducted using the tailings obtained from a copper mine. The results demonstrate that (i) in the accelerated and free settling process, CPTs did not directly settle at the bottom of graduated cylinders; instead, they were netted by the flocculent structures (FSs) and settled together more quickly. The CPTs accelerate the rapid settlement of TSs; the acceleration effect is more obvious when the CPTs content is greater than 50wt%. (ii) The most appropriate flocculant unit consumption (FUC) is 20 g·t−1, and no substantial increase is observed in the flocculant settling velocity with an increase in the flocculant because the effective FSs did not substantially change and thus did not lead to a notable increase in the settling velocity of the solid–liquid interface (SLI). (iii) In the effective settling space of the thickening facility, free water quickly flowed from the pores of FSs, which is reflected in the period from 0 to 1 min.
A process of biooxidation followed by thiosulfate leaching of gold from refractory gold concentrate was investigated. Mineralogical studies on the concentrate showed that very fine gold grains (<10 μm) were encapsulated in pyrite and arsenopyrite, while the proportion of monomer gold was only 21%. The gold-bearing sample was identified as a high-sulfur fine-sized wrapped-type refractory gold concentrate. The gold leaching efficiency obtained by direct cyanidation was only 59.86%. After biooxidation pretreatment, the sulfide minerals were almost completely decomposed, 92wt% of the mineral particles of the biooxidation residue were decreased to <38 μm, and the proportion of monomer gold in the biooxidation residue was over 86%. Meanwhile, the gold content in the biooxidation residue was enriched to 55.60 g/t, and the S, Fe, and As contents were reduced to approximately 19.8wt%, 6.97wt%, and 0.13wt%, respectively. Ammoniacal thiosulfate was used for gold extraction from the biooxidation residue of the refractory gold concentrate. The results showed that the optimal reagent conditions were 0.18 M thiosulfate, 0.02 M copper(II), 1.0 M ammonia, and 0.24 M sulfite. Under these conditions, a maximum gold leaching efficiency of 85.05% was obtained.
To investigate the interaction mechanism between 95Cr saw-wire steel and different refractories, we conducted laboratory experiments at 1873 K. Five crucible materials (SiO2, Al2O3, MgO·Al2O3, MgO, and MgO–CaO) were used. The results indicate that SiO2, Al2O3, and MgO·Al2O3 are not suitable for smelting low-oxygen, low-[Al]s 95Cr saw-wire steel, mainly because they react with the elements in the molten steel and pollute the steel samples. By contrast, MgO–CaO is an ideal choice to produce 95Cr saw-wire steel. It offers three advantages: (i) It does not decompose by itself at the steelmaking temperature of 1873 K because it exhibits good thermal stability; (ii) [C], [Si], and [Mn] in molten steel cannot react with it to increase the [O] content; and (iii) it not only desulfurizes and dephosphorizes but also removes Al2O3 inclusions from the steel simultaneously. As a result, the contents of the main elements ([C], [Si], [Mn], [Cr], N, T.O (total oxygen)) in the steel are not affected and the content of impurity elements ([Al]s, P, and S) can be perfectly controlled within the target range. Furthermore, the number and size of inclusions in the steel samples decrease sharply when the MgO–CaO crucible is used.
The effect of microstructure and passive film on the corrosion resistance of 2507 super duplex stainless steel (SDSS) in simulated marine environment was investigated by electrochemical measurements, periodic wet–dry cyclic corrosion test, scanning Kelvin probe force microscopy, atomic force microscopy, and X-ray photoelectron spectrometry. The results show that the occupation ratio of γ phase increases with the decrease in cooling rate, whereas the content of α phase reduces gradually. In addition, the σ precipitated phase only emerges in the annealed steel. The pitting sensitivity and corrosion rate of 2507 SDSS reduce first and then increase as the cooling rate decreases. The σ precipitated phase drastically reduces the protective ability of the passive film and facilitates micro-galvanic corrosion of the annealed steel. For various microstructures, the pits are preferentially distributed within the σ and γ phases. The corrosion resistance of 2507 SDSS prepared by different cooling methods is closely related to the microstructure and structure (stability and homogeneity) of the passive film. Normalized steel shows an optimal corrosion resistance, followed by the quenched and annealed steels.
Two dissimilar steel plates, structural steel and mild steel, were joined by explosion welding to form a composite. The composite was then heat-treated by quenching at 840°C for 30 min followed by tempering at 200°C for 3 h. The microstructure was investigated under an optical microscope and a scanning electron microscope. The mechanical properties were measured using Vickers microhardness and Charpy impact tests. The results show a deformed interface with typical wave features at the welding zone, but no defects were observed. Moreover, the ferrite in the parent plate in the weld zone was elongated due to the strong plastic deformation from the explosion. After heat treatment, the hardness of the flyer plate (structural steel) was over HV0.2 800, while that of the parent plate (mild steel) was HV0.2 200. The increase in hardness was due to the presence of martensite. Moreover, the average impact energy was increased from 18.5 to 44.0 J following heat treatment; this is because of the formation of recrystallized grains at the weld interface, which is due to the dynamic recovery and local recrystallization, and the strong elemental diffusion that occurred between the two plates.
Inconel 718 is a Ni–Fe-based superalloy widely used in aerospace engines because of its excellent mechanical properties. However, the inferior stability of the γ″ phase limits the application of Inconel 718, which coarsens rapidly at temperatures greater than 650°C. Further improving the temperature tolerance of Inconel 718 requires optimization of the phase configuration via modification of the alloy’s chemical composition. Given the aforementioned objective, this work was conducted to study the precipitation behavior and thermal stability of the strengthening phases with various structures in modified Inconel 718 alloys by tailoring the Al/Ti ratio. With increasing Al/Ti ratio, three particle configurations were formed: γ′/γ″ composite, isolated γ′, and γ′/γ″/γ′ composite particles. The results of aging tests demonstrate that the isolated γ′ and the γ′/γ″/γ′ composite structure exhibited better thermal stability at temperature as high as 800°C. The isolated γ′ exhibited a reduced coarsening rate compared with the γ′/γ″/γ′ composite particles because the isolated γ′ phase was rich in Al, Ti, and Nb. However, the γ′/γ″ composite particles coarsened and decomposed rapidly during aging at temperatures greater than 700°C because of the lower stability resulting from the larger number of γ″ particles. The obtained results provide necessary data for the compositional optimization of novel 718-type alloys.
Friction stir processing (FSP) can be used to improve surface composites. In this study, a modified method of FSP called friction stir vibration processing (FSVP) was applied to develop a surface composite on AZ91 magnesium alloy. In this technique, the workpiece is vibrated normal to the processing direction. The results illustrated that compared with the FSP method, the FSVP caused a better homogeneous distribution of SiC particles in the microstructure. The results also showed that matrix grains of friction stir vibration processed (FSV-processed) samples ((26.43 ± 2.00) μm) were finer than those of friction stir processed (FS-processed) specimens ((39.43 ± 2.00) μm). The results indicated that the ultimate tensile strength (UTS) of FSV-processed specimens (361.82 MPa) was higher than that of FS-processed specimens (324.97 MPa). The higher plastic strain in the material during FSVP, due to workpiece vibration, resulted in higher dynamic recrystallization, and consequently, finer grains were developed. The elongation and formability index of the FSV-processed specimen (16.88% and 6107.52 MPa·%, respectively) were higher than those of the FS-processed sample (15.24% and 4952.54 MPa·%, respectively). Moreover, the effects of FSVP were also found to intensify as the vibration frequency increased.
Recycling and reusing materials from waste have become a nexus in the development of sustainable materials, leading to more balanced technologies. In this study, we developed a composite coating by co-depositing recycled ceramic particles, pulverised fly ash (PFA) and medical ceramics (MC), into a nickel–phosphorus matrix using a typical electroless plating process. Scanning electron microscopy (SEM) images indicated well-dispersed particles in the Ni–P matrix. However, compared with the MC particles, the PFA particles were distributed scantily with a lower content in the matrix, which could be due to the less impingement effect during the co-deposition. A modified microstructure with refined grains was obtained for the PFA-incorporated composite coating, as seen in the SEM micrograph. The X-ray diffraction result of the MC-incorporated composite coating showed the formation of NixSiy phases in addition to the typical Ni3P phases for the heat-treated electroless Ni–P coatings. Upon heat treatment, the PFA-reinforced composite coating, due to a modified microstructure, exhibited a higher microhardness up to HK0.05 818, which is comparable to that of the traditional SiC particle-embedded composite coating (HK0.05 825). The findings can potentially open up a new strategy to further advance the green approach for industrial surface engineering.
Rutile titania (TiO2) was successfully prepared via hydrolysis of TiCl4 in the presence of AlCl3. The powders were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) surface area analysis. In the present system, AlCl3 functions as a nucleating agent and induces the formation of rutile TiO2. The influences of HCl and isopropanol concentrations on the purity and morphology of the rutile TiO2 were investigated. The purity of the rutile TiO2 increased with increasing concentration of HCl. Evenly dispersed rutile TiO2 particles with a spherical morphology were obtained when the HCl and isopropanol concentrations were 0.5 and 1 mol·L−l, respectively. Furthermore, the prepared TiO2 powders were used in adsorption tests of the heavy metal pollutant Cr(VI). Rutile TiO2 sample S-9 demonstrated greater adsorption performance and a removal efficiency that was greater than 99.95% after 60 min of adsorption when the Cr(VI) concentration was 200 mg·L−l. The maximum adsorption capacity on rutile TiO2 was 28.9 mg·g−1. This work provides an easy path to prepare a high-performance rutile TiO2 adsorbent with potential applications in water pollution treatment.
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