equipment use for mine gold from tailings

from tailings to treasure? miners make money reprocessing tailings

from tailings to treasure? miners make money reprocessing tailings

The environmental concerns associated with mining are well known. Mining operations produce waste that must be responsibly processed and disposed of to prevent environmental damage. As a previous blog, Mining and the Environment: What Happens When A Mine Closes? explains, tailingsmineral waste productsare a major pollutant. Tailings may be dumped in or near water or transported by wind or water to contaminate the surrounding area. Mine sites typically manage tailings by constructing ponds secured by dams.

According to the web site, another strategy is to produce thickenedtailings, which are pressed or have chemicals added to remove excess water. Thickenedtailingscan be mixed with cement and used in construction or as backfill in underground mines.

Now, some mining companies are figuring out ways to turn tailings to profits with novel reprocessing technologies to extract valuable metals from the waste. One example reported in Rapaport Magazine describes the efforts of De Beers Consolidated Mines (DBCM) to extract overlooked diamonds from 360 million tons of old tailings surrounding the Kimberley mines in South Africa. According to the company, thanks to advances in separating, sorting, and crushing equipment, very small diamonds can be recovered from the residue of the original diamond-bearing ore. De Beers recovered 815,036 carats of diamonds from 6,133,799 tons of tailings in 2013 and expects to continue operations beyond 2030.

Tailing may turn out to be a viable source of another valuable and highly sought-after resourcerare earth elements. An article appearing on ABC News, Old Mine Tailings: New Mother Lode for Rare Elements, reported findings from the U.S. Geological Survey indicating that discarded mine tailings may yield significant amounts of rare earth elements; modern extraction techniques would now permit their recovery. This is good news because rare earth elements, which are extremely difficult and costly to mine, are critical components in consumer electronics such as televisions, computers, cameras, and mobile phones, as well as catalytic converters and metal alloys.

Because China monopolizes the worlds supply and charges a premium, Western countries are looking for alternative sources. An article on The Wits Business School Journal website, Abandoned Mines Spark New Gold Rush, evaluates the situation in South Africa, where tailings reprocessing is gaining traction both for economic and environmental reasons. Examples include:

Australian Broadcasting Companyreports that Western Australian company Carbine Resources is investigating the technical viability of extracting an estimated million ounces of gold and 60,000 tons of copper left in the tailings at the old Mt. Morgan Mine site in central Queensland. Numerous other tailings projects are underway in Australia.

To accurately identify minerals within tailings, geologists turn to analytical technologies including both laboratory and portable X-ray fluorescence (XRF) instruments. Portable XRF analyzers provide fast, accurate analysis of tailings to quickly and easily gauge the efficiency of extraction and enrichment processes.The real-time assay data provided by a portable XRF analyzerallows for timely process adjustments, improving productivity and reducing the need for reprocessing. Happy Mothers (Lode) Day!

I bought 50 acres. Did research and found out my 50 acres was a copper mine ,original owners were eventually wanted for treason These people ( original grant land owners)were sent to Canada by King Henri VIII. They were also the first railroads builders. What should I do with all those very very very old tailings?

precious metals reclamation mining company

precious metals reclamation mining company

NOTICE: This site is for information purposes only. The information on this site is meant for people outside of the United States. Accessing this site means you acknowledge and agree with these statements.

Apache Mill Tailings USA, Inc. is a precious metals mining company specializing in gold, silver, copper and high value, rare earth minerals reclamation recovery. To maximize profits, accelerate project success and reduce risk, we work with above ground mine and ore mill tailings deposits.

Vast treasures are waiting to be taken from selected properties with already mined tailings piles. Old processing technologies focused only on gold recovery have left behind fortunes which can be easily recovered. No mining is required. These riches are above ground and "shovel ready". Multiple environment friendly, non-toxic processing technologies are available to quickly and profitably reclaim these precious metals. Additional Benefits - We plan to use all reclamation by-product to create mortarless, interlocking building blocks and bricks perfect for construction of affordable housing, retaining walls and civic buildings. Applying this patented technology will create a sustainable, seamless complete manufactured product loop.

Harvesting Fallen Gold. Specializing in the environment friendly reclamation recovery from above ground, previously mined and milled tailings deposits, we do not have to dig or build mines. Normal mining costs are about 50% of the income derived. Apache's reclamation costs are estimated to about 3% of income. Old time ore processing only looking for gold left behind vast treasures of waste tailings piled in above ground dumping sites. Old separation technology missed tremendous amounts of gold. Vast fortunes of precious metals and rare earth elements - Not even known of at the time - were discarded. These treasures are ready to be recovered by Apache using modern processing technology. Our shovel ready, high value tailings processing reclamation business strategy will produce fast revenues and high profits margins.

Advanced Reclamation and Nano Recovery. With several high yield processing processes available to us, we can customized each project operations to deliver maximum profits as fast as possible. Utilizing our specially designed truck mounted systems we can set up production quickly and scale up to multiply production outputs as needed. Loading trucks and shipping ore to vetted crushing and processing facilities will produce rapid project revenues. We also have the option of using on site crushing equipment and non-toxic leaching systems. Dry method heat systems and advanced air separation green technology will be used on future projects. An environment friendly company, Apache Mill Tailings will lead the way in the use of current and new technologies for high profit reclamation processing.

Deposits Worth Billions of Dollars. We select the best high grade, sweet spot mining claims in prime areas of historically known successful gold mining districts. The project sites have immense above ground tailings piles that can be readily processed. Large mining projects or major ore mill processing plants, where the best ore from 100's of miles around was shipped, operated on these sites. Additional projects are being investigated and negotiated for acquisitions at this time. Our targeted projects are located in the Western USA. Assay results verify easily recoverable gold, silver and high value precious metals deposits worth Billions of Dollars. The assets, revenues and profits from these projects alone would make Apache Mill Tailings USA, Inc. a mining industry leader. A conservative projection of $186 Million Monthly Revenue can be generated from targeted projects.

Nothing on this site is to be interpreted as a solicitation or offer of any kind for any purpose in any form or content. All contents of this site is for informational purposes only and is intended only to outline the basic information of potential precious metals reclamation projects and Apache Mill Tailings USA, Inc. potential acquisitions, ownership and future targets. Upon accessing this site, all visitors hereby acknowledge this Disclaimer.

Notice: The information on this site is presented for Discussion Purposes Only. As there are both distinct regulations, security and privacy issues regarding this industry, the enclosed information is most basic and introductory in nature. The information on this site does not constitute an offer to sell or solicit the purchase of any security, nor does it constitute an obligation to underwrite, place or otherwise distribute any security described herein. The Content is for informational purposes only, you should not construe any such information or other material as legal, tax, investment, financial, or other advice. Nothing contained on this site constitutes a solicitation, recommendation, endorsement, or offer by Apache Mill Tailings USA, Inc. or any third party service provider to buy or sell any securities or other financial instruments in this or in any other jurisdiction in which such solicitation or offer would be unlawful under the securities laws of such jurisdiction.

copper tailing - an overview | sciencedirect topics

copper tailing - an overview | sciencedirect topics

Copper tailings are a finely ground waste after copper mineral has been extracted from the ores during beneficiation. Similar to CS, the use of copper tailings as a geotechnical material has also been reported in the literature, but on a relatively smaller scale (Collins and Ciesielski, 1994; Gupta and Thomas, 2013; Nitish etal., 2013; Miller and Collins, 1976). Although copper tailings are not the main focus of this chapter, for completeness, the material properties of copper tailings and their use in geotechnical applications will be discussed here briefly.

The chemical composition of copper tailings, as provided in Gupta and Thomas (2013) and Miller and Collins (1976), shows that copper tailings have low calcium oxide (CaO) at 0.16%, indicating that it has no cementitious property. However, its high total silica, alumina and iron oxides (SiO2+Al2O3+Fe2O3) of more than 85% and low loss on ignition of less than 3% meet the chemical requirements of FA suggested in ASTM C618-12a (2012), which might suggest that copper tailings can be potentially suitable for use as a stabilising agent for soils.

As copper tailings appear in ground form, the grading of copper tailings reported by Gupta and Thomas (2013) and Nitish etal. (2013) has a sizeable silt fraction of 0.060.002mm. Copper tailings are considered to be finer than CS, which usually has a dominant sand fraction. Table 6.5 compares the geotechnical properties of copper tailings to those of CS, which have been reported in this chapter, as well as those of typical sand. In general, the following points have been observed:

The compaction characteristics of copper tailings are not clearly understandable from the existing data. In the standard Proctor test, copper tailings have a lower MDD and higher OMC than both CS and sand. However, in the modified Proctor test, the MDD and OMC of copper tailings are higher than those of both CS and sand. Given that copper tailings are generally finer than CS, it can be assumed that CS should have better compaction characteristics than copper tailings; nevertheless, further investigation is needed to establish clarity.

Although it might appear that the research in the area of copper tailings characteristics and their potential for use in geotechnical applications has not been as extensive as in the case of CS, in two significant reports of approximately 90 pages each, dealing with waste materials and their recycling as sustainable construction materials in geotechnical applications, prepared under the National Cooperative Highway Research Program (USA), Miller and Collins (1976) and Collins and Ciesielski (1994) have stated that more than 3 million tons of copper tailings were used in embankment construction in Utah, USA, during the 1970s. Although the details of their geotechnical properties as well as service performance are not clear in the reports, this revelation can be encouraging, as it shows that copper tailings may offer a viable use in real geotechnical applications.

Copper tailings are beginning to draw attention for their possible use as a sand component in concrete. A series of laboratory tests undertaken by Thomas etal. (2013), using up to 60% copper tailings as part of sand, showed that the performance of concrete mixes designed at water/cement ratios of 0.40, 0.45 and 0.50 improved with the use of copper tailings up to 30% for compressive strength, flexural strength, pull-off strength, drying shrinkage, water permeability and abrasion resistance. The durability of concrete in terms of chloride resistance, sulphate resistance and alkalinity was reported not to be affected by the use of copper tailings. As the particle size of copper tailings is generally fine, having a fineness modulus of about 1.60, the material may possibly be used as a filler in concrete and this could open up the scope for further research in this area.

The properties of copper tailings, a fine-grained residue waste produced from beneficiation of copper ore, and their effects on the properties of bituminous mix were studied by Oluwasola etal. (2014, 2015). The copper tailings were used at 20% content as a fine granite replacement for fraction size smaller than 1.18mm. The aggregate grading of the copper tailings mix was kept similar to the reference mix, thus the difference in aggregate packing between the two mixes was kept to a minimum. In the same study, the effects of bitumen characteristics were investigated; however, this is considered to be outside the scope of this section.

Marshall properties: The optimum bitumen content of the copper tailings mix was similar to the corresponding reference mix. As to be expected, the air void content and VMA of the two mixes were very close, because of their similar aggregate gradings. Although all the Marshall properties met the requirements provided by the Asphalt Institute (1997) the copper tailings mix showed slightly higher Marshall stability and lower flow than the corresponding reference mix.

Resilient modulus: The resilient modulus of the copper tailings mixes, measured at 25 and 40C in both aged and unaged conditions, tended to be higher than the reference mix, which suggests copper tailings may be more angular than the crushed granite.

Water sensitivity: The water sensitivity of both the mixes expressed as the tensile strength ratio (TSR) of the conditioned specimen to unconditioned specimen showed that the TSR of the copper tailings mix at about 92% was slightly lower than that of the corresponding reference mix at about 95%. Nevertheless, the TSR of the copper tailings mix still met the minimum requirement of 80% as specified by AASTHO T283.

Based on the field experience of using copper tailings in HMA applications, the Texas Transportation Agency (Transportation Research Board, 2013a) has suggested that adjustment of construction practice and proper training to field crews would be advisable when dealing with copper tailings.

Overall, although the research on the use of copper tailings is in its preliminary stages, with many aspects still requiring thorough investigation, the limited information available would suggest that copper tailings, like CS, could also be a viable material as a natural sand replacement in road pavement applications.

Rapid hardening Portland cement P. O 42.5R, fly ash, and silica fume were used as binding materials. Local river sand with a specific surface area of 0.101m2/g and copper tailings with a specific surface area of 0.141m2/g served as the fine aggregates. Highly efficient polycarboxylate-based superplasticizer with a water reducing rate of more than 30% and a solid content fraction of 37.2 % were adopted to achieve the required flowability for the mixture. Flowability should be controlled to ensure the fresh paste is smoothly and continuously transported from the storage system to the nozzle without blockage and disruption, therefore, realizing the compatibility between workability and printing process. Additionally, low shrinkage is essential as the free-form components are built without formwork. A small number of polypropylene fibers were employed to reduce the cracking produced by water evaporation. Table 8.1 shows the mixture proportions of the raw materials used for material preparation. The chemical compositions of tailing determined by X-ray fluorescence (XRF) analysis are listed in Table 8.2 and the particle size distribution parameter of tailings is presented in Table 8.3. After a series of attempts and trials, it was found that the most suitable mix for 3D concrete printing is comprised of a tailing to sand mass ratio of 2:3, so that 40% natural sand was replaced by mining tailings [23].

In the preparation process, polypropylene fibers and the dry powders (i.e., cement, fly ash, silica fume, natural sand, and tailings) are firstly blended for three minutes to obtain a uniform mixture. Then, one half of the total amount of water along with the superplasticizer was added and stirred for two minutes. Subsequently, the second half of the total amount of water together with superplasticizer is poured in and stirred for another 2minutes.

As mentioned in Chapter 3, CS is a non-hazardous material in itself and poses no health threat to the environment, as accepted by two independent organisations, the US Environmental Protection Agency (US EPA, 1991) and the Basel Convention of 1996 (Alter, 2005). The decision to exclude CS from the hazardous waste list under Subtitle C of the Resource Conservation and Recovery Act, by the US Environmental Protection Agency in 1991, was based on the investigation of several industrial processing wastes, including CS, copper tailings and calcium sulphate wastewater treatment plant sludge, from 91 plants located in 29 states. Five years later, CS was also characterised as non-hazardous by the United Nations Basel Convention on the Transboundary Movement of Hazardous Wastes and Their Disposal in 1996, based on the studies of slag samples sourced from Canada, Chile and the United States (Alter, 2004).

Figure 8.1 shows the results of leaching concentrations of heavy metals of 43 CS samples. The results were reported over a period of 32 years during 19832014 from 10 countries across the world, with India, Singapore and the United States accounting for more than half of the results. The test samples were separated based on the type of CS tested, namely, air-cooled (2), quenched (10), spent (16) and not identified (15). As spent CS is usually the quenched CS originally used as an abrasive material, the majority of the results plotted in Figure 8.1 are likely to be those of quenched CS.

Amongst the test methods adopted, the US Environmental Protection Agency (EPA) Method 1311 for toxicity characteristic leaching procedure (TCLP) is most commonly used (US EPA, 1992), which was developed to simulate leaching in a weakly acidic environment at municipal solid waste landfills. Another method worth mentioning is the multiple batch extraction method, which comprises a series of repetitions of single-stage leaching with fresh leachant, which was designed to monitor the leaching behaviour of a material over time. A detailed list of other test methods used can be found in Chapter 3 (Section 3.7.1).

It can be seen from Figure 8.1 that all the leached element concentrations of CS samples measured using TCLP are below the corresponding maximum allowable levels set by the US EPA (2012). Indeed, for several elements, the leached concentrations are well below the maximum allowable limits, as can be seen from Table 8.1. A similar observation is also found for the results obtained from multiple batch extraction and other test methods, except for one Pb leaching value, which exceeds the limit.

When subjected to aggressive conditions such as high acidic and low alkaline solutions, the leached element concentrations of CS do not show an alarming level. The leaching behaviour of CS appears not to be influenced by its type, air-cooled, quenched or spent.

Additionally, the effects of particle size of the test sample and pH condition used for the leaching test of CS were studied by Vitkova etal. (2011), and the results suggest that the leaching of Cu, Co and Zn is the highest for the sample with a high content of particles with size less than 1mm or tested at low pH of 4 and 5. The latter observation is also confirmed in a case study undertaken in Vietnam by Dung (2012). In a separate study, Harish etal. (2011) carried out an algal growth inhibition test under laboratory conditions to assess the leaching of CS from a copper smelter plant in India. The results show that the leachate of CS is harmless to the growth of the marine microalgae used. However, it has been suggested that further tests on the changes in the growth rate and lipid profile of algae might be needed for a complete understanding of the toxic nature of CS in microalgae.

Extraction of copper from its ore in copper industries produces two types of waste named as copper slag and copper tailings. The copper slag is produced during the process of smelting for sulphide ore and collected as the material that floats on the top of the molten copper in a furnace. This material ultimately turned into glassy solid after cooling and discarded as waste in dump yards. After suitable crushing, this waste can be utilized as filler in asphalt mixes. Copper slag has a finer gradation and higher specific gravity than limestone filler due to relative higher oxides of iron [72]. It consists of CaO and SiO2 in its composition and can display pozzolanic properties which could improve the performance of its mixes against moisture [81]. It also has a relatively higher CaO content than granite and silica aggregate and is expected to display superior performance in saturated and freezing-thawing conditions [72]. Despite this, there are very few studies that explore the potential of copper slag as filler in asphalt mixes. It was observed that replacement of conventional limestone filler with copper slag can produce asphalt mixes with superior fatigue and cracking resistance [72]. This was attributed to finer gradation of copper slag, which can produce stiffer mix at same filler content. Although copper slag had trace amounts of heavy metals like As, Cd, Cu, Cr, Pb and Zn in its composition, they were found to be stabilized due to their encapsulation by the bitumen present in asphalt mix [72].

Copper tailing is the waste rock remaining after ore has been processed to remove the copper. It is usually pulverized to the size of fine sand. It has relatively high specific gravity and primarily consists of silica in its composition. Due to presence of silica, asphalt mixes prepared with it displayed relatively lower yet satisfactory moisture resistance and adhesion than that of conventional OPC modified mixes [30]. It is relatively less porous, due to which it can produce economical mixes with lower OBC [30]. However, a recent study has suggested that asphalt mixes produced with copper tailings have marginally lower yet satisfactory Marshall stability and rutting resistance [30]. Unlike copper slag, copper tailings doesnt found to have any heavy metals in its composition.

The cementitious materials in the mine filling field mainly include pozzolanic materials (Portland cement, the most commonly used) as cementitious materials. Moreover, minerals with potential cementitious properties, such as blast furnace slag, fly ash, metakaolin, and copper tailings, have been used as a supplement to cement through physical, chemical, and heat treatments (Kinnunen etal., 2018; Vargas and Lopez, 2018). Among these materials, alkali-activated pozzolanic materials are widely used because of their relatively simple processes. These types of activators are usually aqueous sodium silicate, sodium carbonate, sodium hydroxide, potassium carbonate, and sodium metasilicate. Commonly used materials in the industry are gypsum, lime, cement clinker, etc.

Blast furnace slag is obtained by quenching molten iron slag (a byproduct of the iron- and steel-making process) from a blast furnace in water or steam to produce a glassy or granular product. The activation activity and mechanism of various types of slag have been studied (Jin etal., 2015; Zheng etal., 2019). Ferdi (Cihangir etal., 2012) reported that the acidic and neutral slags activated with aqueous sodium silicate and sodium hydroxide (ASSH and NSSH, respectively) were tested as alternative binders to the OPC in paste backfill and showed that these binders exhibit excellent mechanical properties with a less porous microstructure and good resistance to attacks by aggressive environments. These backfill materials not only have a good mechanical performance but also decrease the backfill costs and reduce the industrial waste emissions in the Shirengou iron mine and Jinchuan nickel mine (Du etal., 2012; Yang, Z.Q. etal., 2017).

Fly ash is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired boilers together with the flue gases. The fly ash produced from the burning of younger lignite or subbituminous coal, in addition to having pozzolanic properties, also has self-cementing properties. In the presence of water, fly ash (especially class C fly ash) hardens and becomes stronger over time. Fly ash has an enhancing effect on the long-term strength of the filling body and can improve the fluidity of the filling slurry, which is beneficial to increasing the filling concentration and enhancing the gravity conveying the performance of the pipeline. The Xinqiao pyrite mine replaces cement with fly ash, and the mechanical strength of the filling body at a Portland cement, fly ash and aggregate (tailings or river sand) ratio of 1:2:8 was 2MPa for 90 days, which reduced the cost of filling materials by 2030% (Kang, 2011).

Red mud (or bauxite tailings) is a highly alkaline waste product composed mainly of iron oxide that has a potential cementitious activity due to the production process of alumina. The filling material made of red mud, coal gangue and cement had an excellent mechanical performance (5.49MPaat 60 days) with a less porous microstructure and more hydration products based on X-ray diffraction analysis, Fourier transform infrared spectroscopy, thermogravimetry-differential scanning calorimetry, and scanning electron microscopy-spectral analysis (Chen, J.L. etal., 2017). Zhu etal. (2015) developed a new type of filling cement based on red mud, which is formulated with 50% red mud, 30% slag, 10% desulfurized gypsum and 10% cement clinker. The mechanical strength of the paste prepared from the cementitious material at a cement-sand ratio of 3:17 and a water-cement ratio of 1.2 was 80% and 40% higher than that of the ordinary Portland cement paste, respectively, at 1 and 3 days of curing. It should be reminded that red mud is a high alkaline material (pH>12.5 usually) containing considerable contaminants, such as radioactive minerals, alkaline compounds, fluoride, heavy metals, etc. when these contaminants are present in paste backfill body placed to underground or open-pit goaf, the environmental risks need to be considered. Leaching tests of paste materials containing red mud are usually conducted to assess environmental risks(Man etal., 2013).

Due to the relatively low reactivity of mine tailings, their use as a single geopolymerization precursor commonly leads to geopolymers having slow setting, low mechanical strength, and poor durability, strongly restricting their application. Thus, adding aluminosilicate materials with higher reactivity is a feasible strategy to improve the properties of the synthesized geopolymers. Such supplementary materials are divided into low/non-Ca- (e.g., fly ash and calcined clay) and high Ca-containing (e.g., blast furnace slag) additives.

Low and non-Ca-containing additives such as fly ash and metakaolin usually show higher reactivity than mine tailings during alkali activation. Hence, more reactive Si and Al can dissolve to form the geopolymer nanostructure, modifying the resulting strength (Tian etal., 2020). In this case, most of the mine tailings can serve just as an aggregate considering their extremely low contribution of reactive Si and Al.

The addition of low and non-Ca-containing materials commonly contributes to strength development. Wang etal. (2019) reported that the geopolymer prepared adding 20wt% metakaolin exhibited a compressive strength up 45MPa, while the one obtained using pure garnet tailings had a strength of only 15MPa. A similar phenomenon was observed by Wan etal. (2018) when utilizing metakaolin as the auxiliary material; with increasing the metakaolin content from 0 to 50wt%, the compressive strength increased from 2 to 15.5MPa due to the formation of more hydration gels (Wang etal., 2019). Sedira etal. (2018) drew a similar conclusion from mineralogical and microstructural characterization results, that is, the formation of more reaction products when adding red clay brick waste (RCBW). In particular, increasing the RCBW content from 10% to 50% resulted in a 136% increment of the compressive strength (from 25 to 59MPa) after curing for 28 days.

However, although using additives favors the strength development, the properties of the obtained geopolymer highly depend on the processing conditions, such as curing temperature, alkali concentration and type, and water/solid ratio. Manjarrez etal. (2019) used copper tailings and obtained the highest strength (23.5MPa) under the following conditions: 50wt% low Ca-containing slag, 10M NaOH, Na2SiO3/NaOH=1.0, and curing at 60C for 7 days. A higher NaOH concentration (15M) led to flash setting of the geopolymer pastes, resulting in lower strength.

Besides providing more active ingredients, the incorporation of supplementary materials can also modify the Si/Al ratio of the precursor. The Si/Al ratio is one of the main factors determining the geopolymer properties, including mechanical strength, immobilization capacity, and durability (Xu and van Deventer, 2003). Zhang etal. (2011) reduced the extremely high Si/Al ratio of copper tailings by adding various contents of fly ash down to its optimum range (13), obtaining a denser microstructure. Ren etal. (2015) used an aluminum sludge (AS) with high Al2O3 content to modify the Si/Al ratio of copper tailings. The addition of 10 and 20wt% AS decreased the Si/Al ratio from 3.38 to 2.71 and 2.17, respectively, and increased the compressive strength from 28.5MPa to 34.9 and 44.8MPa, correspondingly, at a Na/Al ratio of 1.1.

The properties of early-age geopolymers significantly depend on the calcium content; for example, Ca-containing systems exhibit a much faster setting (Reig etal., 2018) and higher compressive strength (Temuujin etal., 2009) compared with non-Ca-containing ones. This may be caused by the formation of additional hydration gels, i.e., calcium silicate hydrate (CSH) and calcium silicate aluminate hydrate, which have been reported for the Na2OCaOSiO2Al2O3H2O system; the precipitation of these phases can improve the aluminosilicate dissolution in alkaline solutions and, subsequently, the geopolymerization reaction (Temuujin etal., 2009).

Kiventer etal. (2016), when using gold tailings as a single precursor, observed the highest compressive strength (3.5MPa) at a high NaOH concentration (15M) after curing for 28 days. However, further increasing the NaOH concentration led to poor paste workability and difficult specimen molding. In contrast, when using only 5% BFS, activated by a 5M NaOH solution, the compressive strength was more than doubled. The author attributed the strength improvement to two factors: the increased glassy-state phase portion and the modified Si/Al ratio of the precursors when adding BFS.

Ahmari and Zhang (2013) selected cement kiln dust (CKD) as the supplementary material to modify the properties of copper tailings-based geopolymers when using NaOH as the activator. The CKD addition strongly improved the final compressive strength and durability but water absorption. A CKD content of 5% did not lead to obvious morphological changes (Fig.4) of the binder phases (labeled as A); however, when it increased to 10%, a monolithic binder between particles (labeled as C) appeared. The energy-dispersive X-ray spectroscopy results indicated that the Si/Al ratio of A increased along with the CKD content because of the enhanced dissolution of Si due to the alkalinity increment via CKD hydration. This could lead to more rigid geopolymer gels. X-ray diffractometry (XRD) and Fourier-transform infrared (FTIR) spectra revealed that adding CKD favored the generation of CaCO3 within the geopolymer gel. Besides, the Ca from CKD could act as a charge balancing cation and be incorporated into the geopolymer structure.

Fig.4. High-magnification scanning electron micrographs of geopolymer brick specimens prepared at 15M NaOH, 16% initial water content, and curing at 90C for 7 days by adding different cement kiln dust contents: (a) 0%, (b) 5%, (c) 10%, and (d) 10% combined with 7 days of immersion in water. A and C denote the binder phase, while B indicates the unreacted one (Ahmari and Zhang, 2013).

Nonetheless, Ca-containing additives cannot always contribute to the properties of mine tailings-based geopolymers. do Carmo e Silva Defveri etal. (2019) tested the application of iron tailings as the main geopolymerization precursor, attaining high mechanical strength; after curing for 7 days at 100C, the highest compressive and flexural strengths were 112.8MPa and 21.3MPa, respectively. Incorporating glass wool residue (GWR) as a co-precursor reduced the mechanical strength at every substitution ratio. The authors attributed this result to the brittle behavior of GWR. Quantitative XRD results revealed higher chantalite contents in the geopolymer prepared using GWR compared with that obtained only with the iron tailings; the authors attributed this to the additional Si and Al provided by GWR.

The combination of two or three technologies might be more efficient in terms of biorecovery. Combination of bioleaching with a biosorbent strategy yielded a >86% Cu recovery from waste PCBs using USCT-R010 isolates for bioleaching and the dead biomass of A. oryzae and Bakers Yeast as biosorbents (Sinha et al., 2018). The hybrid of ammonium thiosulfate (AT) and Lactobacillus acidophilus (LA) achieved 85% gold recovery from waste PCBs, the - interaction between AT and LA enhanced amide absorption bond and then lead to improvement of gold recovery (Sheel and Pant, 2018). MFC and bioleaching hybrid technologies also significantly improved the recovery rate of Cu, with 54% leaching and 78% recovery of Cu from the secondary copper tailings (Huang et al., 2019b). A similar concept was used for the recovery of Cu from copper sulfide minerals and enhanced Cu recovery, which was attributed to an MFC-mediated decrease in the pH value derived from the anodic sulfide/sulfur oxidation process (Huang et al., 2019d). The successful biorecovery of Cu provides a potential for recycling of critical metals from wastes via a combination of these technologies. Researchers investigated the integration of two stages, i.e., bioleaching and electrochemical extraction, for the recovery of Nd and La from monazite rock ore (Maes et al., 2017). The results showed that Nd was concentrated from 392mg/L in the lixiviants to 880mg/L after the electrochemical extraction process through utilization of a CEM under the effect of an electric field. Thus, the combination of these technologies might be a promising way to recover critical metals from wastes in the future. The exponential growth of emerging materials (e.g., nanomaterials) in a wide range of potential applications implies that various types of waste would become an additional secondary source of critical metals (Tan et al., 2017). The interactions between fungi and bacteria occurred in various environments (Deveau et al., 2018), new metal-reducing bacteria could be isolated through the utilization of fungi (Furuno et al., 2012). Furthermore, the co-culture of bacteria and fungi might be a potential way to deal with critical metals for high recovery rate (Deveau et al., 2018), such as REEs (Hopfe et al., 2017).

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