Mine tailings are the finely ground residue from ore extraction. The grain size of the tailings depends on the nature of the ore and the milling process. Size measurements (Robertson, 1994) of tailings from four mines in Ontario, Canada, indicated the tailings materials to be predominantly silt and fine to medium sand with <10% clay content. Tailings are transported from the mill and are discharged into an impoundment as a slurry containing 30 wt.% solids. The method of deposition affects the distribution of tailings particles within the impoundment. Discharge commonly takes place at elevated perimeter dams; hence there is potential for extensive hydraulic sorting, with coarser fractions settling near the discharge point and finer fractions settling in distal portions of the impoundment (Robertson, 1994). At some sites, tailings are thickened to >60 wt.% solids prior to deposition. Thickening the tailings allows a more rapid settling of the solids, which therefore reduces the potential for hydraulic sorting, resulting in a more uniform grain-size distribution than is observed in conventional tailings areas (Robinsky, 1978; Al, 1996).
During tailings disposal, water is continuously added to the impoundment and the water table remains near the impoundment surface. After tailings deposition ceases, precipitation becomes the dominant source of recharge to the impoundment. The water table falls to an equilibrium position controlled by the rate of precipitation, the rate of evapotranspiration, and the hydraulic properties of the tailings and the underlying materials (Dubrovsky et al., 1984; Blowes and Jambor, 1990).
The fine grain size of mine tailings results in a high moisture-retaining potential for these materials, which is a situation distinctly different from that in waste-rock piles. Whereas waste-rock piles commonly have a large open and free-draining porosity, mine tailings drain slowly, maintaining a large residual moisture content under gravity drainage. Measured moisture contents of conventional tailings impoundments vary from 10% to 100% saturation (Smyth, 1981; Blowes, 1990). The residual moisture content of thickened tailings is greater than that observed for conventional tailings (Robinsky et al., 1991; Al and Blowes, 1996). The high residual moisture content of mine tailings results in a low gas-filled porosity, and in rapid changes in hydraulic gradient in response to precipitation (Blowes and Gillham, 1988; Al and Blowes, 1996).
Precipitation that falls on the impoundment surface migrates downward and laterally through the tailings impoundment into underlying geological materials (Figure 9). Groundwater velocities in tailings impoundments are relatively low. Coggans et al. (1999) estimated that the groundwater vertical velocity ranged from 0.2ma1 to 1.0ma1 at the Inco Ltd. Copper Cliff Central Tailings area in Sudbury, Ontario, whereas horizontal velocities were on the order of 1015ma1. At the Nickel Rim tailings impoundment, also near Sudbury, Johnson et al. (2000) estimated groundwater vertical and horizontal velocities were in the range 0.10.5ma1 and 116ma1.
The surface areas of tailings impoundments vary from less than 10 ha to several square kilometers, and the thicknesses of the tailings deposits vary from a few meters to more than 50 m. The relatively low groundwater velocities and the large areal extent of tailings impoundments result in long time intervals between the time of groundwater infiltration and the time of groundwater discharge to an underlying aquifer or to the surface-water environment (Figure 10). These long travel times result in the delay of measurable environmental degradation at the groundwater discharge point until long into the life of the impoundment. The severity of the negative environmental effects associated with tailings impoundments may not be evident until long after mine closure and decommissioning of the impoundments. The subsequent prevention and remediation of low-quality drainage waters are more difficult than during the active mining. The long travel distances and low groundwater velocities result not only in the potential for prolonged release of contaminants from the tailings impoundment, but also in large long-term treatment costs. For example, Coggans (1992) combined estimates of the rate of sulfide oxidation with estimates of groundwater velocity at the Inco Copper Cliff Central Tailings area in Sudbury, Ontario, and predicted (i) that the peak release of sulfide oxidation products will occur 50 yr after the impoundment is decommissioned, and (ii) that high concentrations of oxidation products will persist for 400 yr thereafter.
In most tailings impoundments, gaseous diffusion is the most significant mechanism for oxygen transport. The rate of diffusion of oxygen gas is dependent on the concentration gradient and the diffusion coefficient of the tailings material. The diffusion coefficient of tailings is dependent on the air-filled porosity of the tailings; the coefficient increases as the air-filled porosity increases, and the coefficient decreases as the moisture content increases. Several empirical relationships have been developed to describe the dependence of the gas diffusion coefficient on the tailings moisture content (e.g., Reardon and Moddle, 1985). These relationships indicate a maximum diffusion coefficient at low moisture contents, with a gradual decline in diffusion coefficient as moisture content increases to 70% saturation, followed by a more rapid decline as the moisture content increases further. The relationship between moisture content and diffusion coefficient results in rapid oxygen diffusion in the shallow portion of the vadose zone of a tailings impoundment, where the moisture content is low. The rapid diffusion of oxygen in this zone replenishes oxygen consumed by the oxidation of sulfide minerals. As the sulfide minerals in the shallow portion of the tailings are depleted, the rate of sulfide oxidation decreases due to the longer diffusion distance and the higher moisture content of the deeper tailings.
In many tailings impoundments a variety of sulfide minerals is present. Jambor (1994) reported a general sequence of sulfide-mineral reactivity observed in several tailings impoundments, from the most readily attacked to the most resistant, to be pyrrhotitegalena-sphaleritepyrite-arsenopyritechalcopyritemagnetite. Blowes and Jambor (1990) observed systematic variations in sulfide-mineral alteration versus depth at the Waite Amulet tailings impoundment, Rouyn-Noranda, Qubec. On the basis of the observations, the degree of alteration was classified into a numerical scale as shown in Table 12. The sulfide alteration index indicates the relative degree of alteration of sulfides. Because pyrrhotite is the sulfide mineral most susceptible to alteration, the extent of its replacement forms the basis for the alteration index. When plotted versus depth on a vertical axis, the alteration index estimates made at the Sherridon Mine, Manitoba, correlated well with geochemical parameters measured in adjacent drill-holes, and with gas-phase O2 concentrations (Figure 11).
The microbially mediated oxidation of sulfide minerals within mine-tailings impoundments generates acidic conditions and releases high concentrations of dissolved metals. Mill tailings at the Heath Steele mine in New Brunswick contain up to 85 wt.% sulfide minerals (Blowes et al., 1991; Boorman and Watson, 1976). Pore-water pH values as low as 1.0, and concentrations of dissolved SO4 up to 8.5104mgL1 were observed in the shallow pore water of the tailings impoundment (Figure 12;Blowes et al., 1991). This water also contained up to 4.8104mgL1 Fe, 3,690mgL1 Zn, 70mgL1 Cu, and 10 mg L1 Pb. The shallow pore waters at the Waite Amulet tailings impoundment in northwestern Qubec contain 2.1104mgL1 SO4, 9.5103mgL1 Fe, 490 mgL1 Zn, 140 mgL1 Cu, and 80 mgL1 Pb. The pH of this water varies from 2.5 to 3.5 (Blowes and Jambor, 1990). High concentrations of dissolved zinc (48mgL1), copper (30mgL1), nickel (2.8mgL1), and cobalt (1.5mgL1) were observed in the shallow groundwater at the inactive Laver copper mine in northern Sweden (Holmstrm et al., 1999). These low-pH conditions and high concentrations of dissolved metals occur within the shallowest portions of the tailings impoundment. As this water is displaced downward through the tailings, or through adjacent aquifer materials, the pH gradually rises, and many metals are removed from solution by precipitation, co-precipitation or adsorption reactions. High concentrations of Fe(II) and SO4, however, move down through the tailings and aquifer sediments relatively unattenuated (Dubrovsky et al., 1984; Johnson et al., 2000). As this groundwater discharges from the tailings impoundment, Fe(II) oxidizes and precipitates as ferric (oxy)hydroxide and ferric hydroxysulfate minerals. These reactions release H+, generating acidic conditions within surface waters. The transport of Fe(II) along the groundwater flow path, therefore, provides the vehicle for transporting acidity long distances from the oxidation zone to the surface-water flow system.
Figure 12. Pore-water chemistry and saturation indices versus depth at the tailings site of the Heath Steele mine. IA represents saturation indices calculated using an ion-association model, and SII represents saturation indices calculated using a specific ion-interaction model (after Ptacek and Blowes, 2000).
Waters draining abandoned mines, mine spoils, and tailings deposits are often characterized by low pH and elevated concentrations of soluble metals (particularly iron) and sulfate (Table 3). These are generically referred to as AMD waters (or acid rock drainage in North America). Acidity in such waters derives from the presence of soluble aluminum, manganese, and iron (mineral acidity) as well as hydronium ions. Extremely acidic lakes may develop naturally in volcanic area, for example, Lake Kawah Idjen in Indonesia, which has a pH of 0.7. Acidic mining lakes, in contrast, are relics of opencast mining, where worked-out voids have not been backfilled, and become progressively filled with rising groundwater or river water. Where the surrounding bedrocks are rich in sulfide minerals (normally chiefly pyrite and marcasite) and contain small amounts of carbonates, the oxidative dissolution of the former can lead to the formation of extremely acidic mine lakes. Acid mine lakes are particularly abundant in central Europe, in parts of Germany, Poland, and the Czech Republic. In past times (up to the end of the twentieth century) the extensive reserves of lignite in these areas were extracted by opencast mining on enormous scales, leaving a legacy of a very large number of man-made lakes of varying sizes and chemistries. In the Lusatia district of eastern Germany alone there are an estimated 200 mining lakes of >1ha that have pH values of <3.
Source: Data are from Johnson DB (2006) Biohydrometallurgy and the environment: Intimate and important interplay. Hydrometallurgy 83:153166, and Nordstrom DK, Alpers CN, Ptacek CJ, and Blowes DW (2000) Negative pH and extremely acidic minewaters from Iron Mountain, California. Environmental Science and Technology 34: 254258.
Source: Data are from Johnson DB (2006) Biohydrometallurgy and the environment: Intimate and important interplay. Hydrometallurgy 83:153166, and Nordstrom DK, Alpers CN, Ptacek CJ, and Blowes DW (2000) Negative pH and extremely acidic minewaters from Iron Mountain, California. Environmental Science and Technology 34: 254258.
The microbiology of AMD streams has been the subject of a number of reviews in books and journals. Knowledge of how biodiverse these flowing waters can be has expanded considerably since At. ferrooxidans was first isolated from an AMD stream draining a bituminous coal mine in the United States in 1947. The most important factors in determining which microbial species are present in AMD appear to be pH, temperature, and concentrations of dissolved metals and other solutes. At the most extreme end of the AMD spectrum, the microbiology of mine waters within the Richmond mine at Iron Mountain, California (which can have negative pH values), has been studied extensively. Within this abandoned mine, pyrite is undergoing oxidative dissolution at a rate that is sufficient to maintain air temperatures of between 30 and 46C, and produce mine waters containing 200gl1 of dissolved metals. A novel iron-oxidizing archaeon, Ferroplasma acidarmanus, was found to be dominant in waters within the mine that had the lowest pH and highest ionic strengths, while L. ferriphilum and L. ferrodiazotrophum were also associated with exposed pyrite faces. Sulfobacillus spp. were more important in some of the warmer (43C) waters. At. ferrooxidans was rarely found in sites that were in contact with the ore body, though it was found in greater abundance in the cooler, higher pH waters that were peripheral to the ore body. In contrast, a microbiological survey of much cooler and higher pH mine waters at an abandoned subarctic copper mine in Norway showed that an At. ferrooxidans-like isolate (closely related to a psychrotolerant strain found subsequently in a mine in Siberia) was the dominant iron oxidizer present. L. ferrooxidans was only detected in enrichment cultures using mine water inocula. The Norwegian AMD waters also contained significant numbers of acidophilic heterotrophs related to some species (Acidiphilium, Acidocella, and Acidisphaera) that had previously been observed in acidic environments, and one (a Frateuria-like bacterium) that had not.
The importance of At. ferrooxidans-like bacteria in cooler (<20C) mine waters of pH 23 has also been supported at sites in other parts of the world. For example, biomolecular analysis (from clone libraries) of four AMD sites at the Dexing copper mine in the Jiangxi province of China found differences in the distribution of acidophiles with water pH. In the most acidic site (pH 1.5), Leptospirillum spp. (L. ferrooxidans, L. ferriphilum, and L. ferrodiazotrophum) were the dominant species in the clone library, while in pH 2.0 AMD L. ferrodiazotrophum was the single dominant species detected. In slightly higher pH (2.2) AMD, most clones recovered were related to At. ferrooxidans, while in the highest pH waters (3.0) most were related to the heterotrophic moderate acidophile Acidobacterium. Where mine waters have pH values of above 3, however, there is increasing evidence that moderately acidophilic iron oxidizers assume a more important role than At. ferrooxidans. The dominant iron oxidizer in AMD flowing from an underground coal mine in south Wales was found to be a Thiomonas-like bacterium, and similar strains (given the novel species designation Thiomonas arsenivorans) were isolated from an abandoned tin mine in Cornwall, England, and a disused gold mine (Cheni) in France. Other acidophilic Bacteria isolated from the Cornish site included Acidobacterium-like and Frateuria-like isolates, and an iron oxidizer related to Halothiobacillus neopolitanus. Further evidence of the importance of previously uncultured acidophiles in AMD has come from a study of acidic (pH 2.73.4) iron- and arsenic-rich water draining mine tailings at Carnouls in France. The dominant bacteria found in clone libraries were betaproteobacteria, many of which were related to a Gallionella-like sequence previously reported in a chalybeate spa in north Wales. The sole Gallionella sp. that has been characterized (Gallionella ferruginea) is a neutrophilic iron oxidizer that grows best under microaerophilic conditions, and the circumstantial evidence for the existence of an acidophilic (or acid-tolerant) species of Gallionella is intriguing. Researchers also found evidence of SRB distantly related to Desulfobacterium in AMD at Carnouls. SRB may also be found in sediments (and microbial mats) underlying AMD, though the pH in such sediments is frequently much higher than the AMD itself.
Microbiological studies of acid mine lakes in Germany have focused on phototrophic eukaryotes as well as acidophilic bacteria and have also examined how dissimilatory microbial reductive processes may be stimulated in order to ameliorate water acidity and immobilize metals. A survey of 14 acidic lakes in Lusatia (ranging in pH from 2.14 to 3.35, and conductivities from 690 to 4460Scm1) found a positive correlation between the relative numbers of the iron-oxidizing heterotroph Fm. acidiphilum and concentrations of aluminum. However, it was concluded that indicator groups of bacteria, rather than single species, were better correlated with different lake chemistries. Addition of organic carbon, nitrogen, and phosphorus to enclosed water columns in a pH 2.6 mine lake was shown to induce changes in both water chemistry and microbiology. Treatment of water resulted in increased microbial diversity, and SRB (Desulfobacter spp.) were among the microorganisms detected in the amended water columns.
One other important extremely acidic ecosystem that has been studied extensively is the Rio Tinto, a major river, some 92km in length, located in southwest Spain (Figure 3). The source of the river is the Pea de Hierro (Iron Mountain) in the Iberian Pyrite Belt, and from there it flows though a large and historic area of copper mining (the Riotinto mines), eventually reaching the Atlantic Ocean at Huelva. Interestingly, even above the Riotinto mines, the river is acidic and enriched with metals, but this is very much accentuated as it flows through the (now abandoned) mining district. The river has a mean pH of about 2.2 and its distinctive red coloration derives from its soluble ferric iron content (2gl1). Primary production in the river is carried out by both photosynthetic and chemoautotrophic acidophiles. A study of the indigenous prokaryotes showed that >80% were Bacteria, and that Archaea accounted for only a relatively small proportion of cells. A variety of different iron oxidizers (At. ferrooxidans, Leptospirillum spp., Fm. acidiphilum, and Fp. acidiphilum) as well as the iron-reducing heterotroph Acidiphilium were identified. A geomicrobiological model involving cyclical oxidation of ferrous iron and reduction of ferric iron has been proposed to account for the remarkable chemical stability of the river ecosystem.
The development of functional technosols from mine tailings is possible after an initial rapid weathering and addition of OM amendments (Li and Huang, 2014; Uzarowicz and Skiba, 2011). Subsequently in weathered tailings with a much reduced sulfide content (eg, <5%) and neutral pH conditions, the development of physical structure (ie, aggregates and pores) and of soil-like heterotrophic microbial communties is fundamental for the formation of technosols with soil-like biogeochemcial processes (Li and Huang, 2014).
Improving aggregation in base metal mine tailings is an important step in the engineered pedogenesis to rehabilitate functional root zones for direct revegetation with native plant communities. The organomineral association critical for microaggregation can be stimulated in OM amended tailings through the interaction between functional organic ligands in the OM and charged surfaces of minerals in the tailings. In a pot experiment that lasted 40 days, neutral Cu/Pb-Zn tailings were amended with sugarcane residue (SR) or its compost (SRC), which were planted with or without a pioneer plant species, red flinders (Iseilema vaginiflorum) (Yuan, 2014). It was found that the organic amendments (particularly the SRC) with functional organic ligands (eg, amine, carboxylic, hydroxylic, alcoholic, and phenolic hydroxyls) mainly stimulated microaggregation in the Cu/Pb-Zn tailings, while the role of plant roots in the tailings was mainly related to the formation of macroaggregates. Therefore, both organic amendments rich in functional groups and pioneer plant growth may be adopted to improve physical structure and stimulate the development of technosols in Cu/Pb-Zn tailings.
In weathered sulfidic tailings of base metal mines such as Pb-Zn-Ag mines, OM amendments or organic carbon sources from plant roots were useful to stimulate further weathering of unstable minerals and induce the shift of microbial communities toward heterotrophic ones in the root zones (Li etal., 2013b, 2014; Li and Huang, 2014). Community composition, which was compared in Pb-Zn-Cu tailings with and without revegetation under subtropical and semiarid climatic conditions, was characterized by using 16S rRNA gene based pyrosequencing with universal primers (Li etal., 2014). Bacterial diversity, as indicated by both the operational taxonomic units (OTU's) number and the Shannon index of the revegetated samples, was significantly higher than that of the sample from the pure tailings. At the phylum level, Proteobacteria and Bacteroidetes were remarkably higher in the revegetated samples compared to the pure tailings; this is possibly related to the change in the organic carbon pool. Phylotypes belonging to Thiobacillus were found thriving in the revegetated tailings (Li etal., 2013b).
Large quantities of radioactive waste and mine tailings are being generated annually worldwide from mining and processing uranium ores for nuclear energy. Radioactive tailings represent a large clean-up challenge to the mining and nuclear energy industries. Therefore in the first few sections of this chapter we detailed the physical, chemical, storage, transportation and disposal of radionuclides, and which ones are more dangerous to the environment and human health. The conclusions arrived at suggest that a common-sense approach to radioactive waste and its safe disposal involves the following four steps:
Assess all options for the management of radioactive waste. Most radioactive waste management assumes the need for off-site storage, but the option of storing waste where it is produced needs re-evaluation. Even if centralized facilities exist, waste is inevitably stored at the sites of production for too long, and on-site storage facilities must be adequately constructed and regulated.
Scientific and environmental strict criteria must be used in choosing operational and management options. Since technologically safe solutions are now available for radiation, we have no right to transmit unprocessed nuclear waste to future generations in an open-ended way.
Invest in phytoremediation of radionuclides, a technology already demonstrated on a pilot-scale; but it needs to be developed at an industrial and commercial level. Phytoremediation of radioactive contaminants is still in development, and more research is required to address plant species that will be most effective in different radioactive waste scenarios. Plants must be able to survive and thrive in contaminated waste, yet be able to mitigate the toxic radioactive pollutants.
To summarize, much of the technology for safe handling and storage of radionuclides is available. There is not one solution but rather several complementary solutions to the problem of managing radioactive waste. We must pursue research on radioactive safety encased in inert substances, continue to reprocess spent fuel and immediately begin work on underground storage. Public involvement in decisions and informed consent to proposals is also essential from a practical point of view, because there is a long history of communities successfully mobilizing to force the abandonment of nuclear projects.
The majority of this chapter, however, is devoted to the growing importance of phytoremediation of radioactive waste sites. A brief description of non-plant based remediation methods is outlined, followed by a detailed evaluation of plant based remediation methods, including hyperaccumulation, radioactive tolerance, uptake and distribution, aquatic phytoremediation and cost estimates. Phytoremediation has become a fast growing field of research and development for application to radionuclide waste. Phytoremediation, although still an emerging technology for radioactive contaminated sites, has become more attractive due to its low cost, high public acceptance and environmental (green) acceptability. It is not a method to be used for all radioactive waste problems, but it is well worth considering as a major supplement to existing technologies. A number of important areas of research and development in phytorermediation still need to be improved, and future research should focus on these. Topics to be better understood include the role of soil chelation and soil acidity on extraction of radionuclides. Increased growth and biomass of selected plants for remediation, and in particular trying to achieve greater rates of transpiration in plants that is the driving force for radionuclide hyperaccumulation is required. There will also need to be better identification of plant species with increased resistance to radiation, and careful selection for better adaptation of plants to radiation poisoning. Clearly, different approaches using different plants are going to be the normal way of dealing with different radionuclides.
Phytoremediation technology has been demonstrated in a number of different situations and for a number of different toxicants (including radionuclides), but has not yet been commercially exploited. More research is required for the development of plants tailored to remediation needs, and the use of transgenic fast-growing woody trees like willow and poplar, where genetic engineering is going to play an important role. The use of trees and transgenic technology is well advanced in some phytoremediation situations and superior seedlings of quick-growing trees are already available. The concept of manipulating plant genes for toxic metal (and perhaps radionuclide) uptake is today a cutting-edge research topic; however, these technologies are not likely to be a total substitute for more basic, site and field testing of various phytoremediation methods. Neverthless, transgenic plants would provide more suitable plants for increased secretion around roots to aid radionuclide uptake and binding, and better overall growth and biomass to absorb and remediate radioactive sites. The use of trees and forests leads to considerable savings in remediation of radioactive sites, as clearly demonstrated by removal of tritium in wastewater. The likelihood of public acceptance of genetically engineered plants for phytoremediation should be welcomed, since it has the potential to clean up the environment of toxicants; however, as in the whole public debate on radioactive waste, public involvement in decision making and good information is critical. Phytoremediation technology has attracted a great deal of attention in recent years and the expectation is that phytoremediation is likely to capture a significant share of the environmental remediation market. It is expected that phytoremediation of radionuclide wastes will become an integral part of the environmental management and risk reduction strategy for governments, industry and society.
Composts have also been added to soils to assist phytoremediation. In mine tailings restoration when clay loam soils were amended with composted biosolids, willows growing in such medium were very effective in phytoextraction of Mn, Cu, and Cd (Boyter etal., 2009). Compost amendments (obtained using wasted tea leaves as the main carbon source and swine manure as the nitrogen source by mixing at a mass ratio of 20 to 1) are found to be effective in assisting the growth of rape seeds, sunflowers, tomatoes, and soapworts in silt loams, and in performing the phytoextraction of Cu, Ni, and Cr from water-washed silt loams (Sung etal., 2011). Composts can not only supply nutrients to plants, but also can create loose and ventilated soils for plants growing in hostile soils. It is seen that both the CEC and the organic matter increase in the test soils after adding the compost (Sung etal., 2011).
The main sources of Cd inputs to rice soil include P fertilizers, biosolids, and mine tailings (Table 4.5). Cadmium input through P fertilizers can be reduced by either selective use of PRs with low Cd or treating the PRs to remove Cd. Superphosphate fertilizer manufacturers in many countries including New Zealand and Australia are introducing voluntary controls on the Cd content of P fertilizers. For example, the fertilizer industry in New Zealand achieved its objective of lowering the Cd content in P fertilizers from 340mgCdkg1P in the 1990s to 280mgCdkg1P by the year 2000 (Bolan etal., 2003a; Rys, 2011). The Cd content as determined by the PR source is the most difficult to control because supplies of PRs with low Cd contents are limited and sources with higher Cd contents continue to be used in many countries for practical reasons. A number of PRs (e.g. Jordan (El Hassa) PR and Morocco (Khouribga) PR) are low in Cd, and these can be used for the manufacture of superphosphates. Alternatively, since Cd has a low boiling point (BP=767C), it can be removed by calcining the PRs. Phosphoric acid used in the food industry is manufactured mostly only after the removal of Cd through calcination of the PRs. Calcination of PRs may not be a likely option in the fertilizer industry because it is expensive and calcination decreases the reactivity of PRs making them unsuitable for direct application as a source of P (Ando, 1987).
Chien etal. (2009) mentioned in a recent review that if a water-soluble P (WSP) fertilizer contains a high Cd content, granulation of WSP fertilizer with potassium chloride (KCl) may result in a higher Cd uptake by crops compared to the same, but bulk-blended PK fertilizer. They suggested that a possible explanation would be that in granulated PK fertilizers, KCl- and Cd-containing P fertilizers are in the same granule and thus are in close contact, thereby increasing the possibility of forming readily bioavailable CdCl20 and CdCl1+ complexes. They also added that it would be less likely that the complexes would form when KCl- and Cd-containing P granules are physically separated in bulk-blended PK fertilizers.
The above hypothesis was tested and confirmed by Chien etal. (2003) in a preliminary greenhouse study using upland rice and soybean (Glycine max (L.) Merr.). In their study, all P and K sources produced by either granulation or bulk blending had the same granule size (1.683.36mm diameter). The results showed that the agronomic effectiveness in increasing crop yield was the same with Cd-containing SSP and the reagent-grade monocalcium phosphate [(MCP) (0% Cd)], whether granulated or bulk blended with KCl. However, they noticed that concentrations of Cd in plant-tissue samples of all crops were much lower for MCP than for SSP. In all the plant-tissue samples, Cd concentrations obtained with granulated (SSP+KCl) fertilizers were higher than that with bulk-blended (SSP)+(KCl) fertilizers. Their results demonstrated that bulk blending of Cd-containing P fertilizers with KCl can reduce Cd uptake by crops compared to the same, but granulated, PK fertilizers (Fig. 4.2). Although PK sources, instead of NPK sources, were used in their study, they expected that inclusion of N will not affect the results, and, if proven true, the process of bulk blending, compared to granulation in decreasing Cd uptake, would also apply to NPK compound fertilizers.
Figure 4.2. Grain yield of upland rice and Cd concentrations in rice grain and straw. GL, granulated; SSP, single super phosphate; BB, bulk-blended; MCP, mono calcium phosphate (Chien etal., 2003). Means followed by the same letter within the treatments are not significantly different at p<0.05.
Another possible mechanism for producing a healthier plant is reduction of metal toxicity in contaminated soils and mine tailings that, under normal conditions, almost completely inhibits plant growth. Although the bacterium tolerate only moderate levels of metals and other toxic compounds (see previous reviews Bashan and Holguin, 1997; Bashan and Levanony, 1990; Bashan et al., 2004; also Kamnev et al., 2005, 2007), it apparently contributed mechanisms allowing plants to grow in mine tailings or contaminated soils. Cadmium causes severe inhibition of growth and nutrient uptake in barley. In the presence of CdCl2, inoculation with A. lipoferum partly decreased Cd toxicity, possibly through the improvement of mineral uptake. Additionally, inoculation slightly enhanced root length and biomass of barley seedling treated with Cd and the amount of nutrients absorbed by the inoculated plants increased significantly. There was only some protection against Cd toxicity, but no uptake of Cd, since Cd content in the inoculated plants was unchanged (Belimov and Dietz, 2000; Belimov et al., 2004). A. brasilense Sp245 associated with wheat changes the speciation, bioavailability, and plant uptake of arsenic. Plants inoculated with Azospirillum accumulated less arsenic than did uninoculated plants (Lyubun et al., 2006). Inoculation of the wild desert shrub quailbush (Atriplex lentiformis) growing in extremely stressed environment with A. brasilense strains Sp6 and Cd, such as acidic mine tailings having high metal content, resulted in a significant increase in production of plant biomass (L.E. de-Bashan et al., unpublished data). Similar results were obtained when wild yellow palo verde desert trees (Parkinsonia microphylla) were inoculated with A. brasilense Cd in rock phosphate tailings (Bashan et al., unpublished data).
Plants have been used to correct human error over the ages. A few species are capable of revegetating Roman lead and zinc mine tailing in Wales (Smith and Bradshaw, 1979). Of these, plants that can withstand toxic wastes after they have been taken up are of interest for phytoremediation. Two types of multi-cut species are usually considered for phytoremediation, with the cut material burnt to extract the heavy metals or to oxidize the organic wastes: herbaceous species such as B. juncea and Spartina spp. (cord grasses), which are most efficient at dealing with surface wastes and trees such as Populus spp., for dealing with deeper wastes (Pilon-Smits and Pilon, 2002)
Heavy metal tolerance has been brought into B. juncea (Indian mustard) from slow-growing Thlaspi by protoplast fusion (along with many other genes; Dushenkov et al., 2002). It was better yet to transgenically transfer genes leading to enhanced glutathione content (Bennett et al., 2003) to make the necessary phytochelatins that complex the heavy metals. A single cropping of B. juncea does not clean up a toxic site. Many growth cycles are required, with multiple harvests and natural reseeding. B. juncea, even more than its close relative B. napus (oilseed rape), is not fully domesticated, and the multiple cycles of cropping would allow the possibility of selecting for ferality. Thus, mitigation seems necessary to prevent volunteers from becoming feral. One gene that might specifically fulfill the need for a mitigator gene is over-expression of a cytokinin oxidase (Bilyeu et al., 2001), which reduces cytokinin levels. This in turn led to phenotypes with far reduced shoot systems (unfitness to compete) but with faster growing, more extensive root systems (Werner et al., 2003), all the better for extracting toxic wastes. Genes that delay or prevent flowering may also be useful with the Brassica species, allowing multiple cuts of larger vegetative plants and preventing gene flow.
Heavy metals and metalloids are the major contaminants that accumulate in soil through emissions from industrial areas, disposal of metal wastes, mine tailings, animal manures, pesticides, sewage sludge, coal combustion residues, atmospheric deposition, wastewater irrigation, and spillage of petrochemicals (Khan et al., 2008; Zhang et al., 2010). The heavy metals found mostly at contaminated sites include Zn, Cu, Cr, Pb, As, Cd, Ni, and Hg. Metals unlike organic contaminants do not undergo degradation by microbes and chemicals (Kirpichtchikova et al., 2006). After introduction into soil, their total concentration persists for a long time (Adriano, 2003). Contamination of soil by heavy metals poses a threat to the ecosystem and humans through the food chain, ingestion, or contact with soil, drinking of groundwater, land tenure problems, and food insecurity due to reduction in usable land for agricultural production (McLaughlin et al., 2000a,b; Ling et al., 2007).
A variety of approaches can be used for the remediation of contaminated soil. The technologies have been broadly classified by the US Environmental Protection Agency (EPA) into two categories: (1) containment remedies and (2) source control (Maslin and Maier, 2000; McLaughlin et al., 2000a,b). Containment remedies involve the construction of caps, liners, and vertically engineered barriers (VEB) for the prevention of contaminant migration. Source control includes ex situ and in situ treatment technologies. Ex situ treatment technologies involve the removal or excavation of contaminated soil from the site, whereas in in situ treatment technologies there is no need to excavate contaminated soil; it is treated at its original site.
The selection of any remediation technology depends on a number of factors, according to Wuana and Okieimen (2011), including: (1) long-term effectiveness, (2) cost, (3) general acceptance, (4) commercial availability, (5) applicability to mixed wastes/organics and heavy metals, (6) applicability to high metal concentrations, (7) volume reduction, (8) toxicity reduction, and (9) mobility reduction. Reliable methods to detect environmental pollutants, their dynamics and fate, are required to evaluate their impact on soil quality and living organisms. Techniques used to monitor volatile and semivolatile pollutants in soil include physicochemical techniques such as solid phase micro-extraction (SPME), followed by analysis by GC-MS, use of bioindicators, and use of sensing technology (e.g., electronic nose) (Cesare and Macagnano, 2013).
The increase in human population has raised the quantity of waste and introduced many different types of pollutants into water bodies; these were not considered pollutants earlier but are now seen as harmful to the environment and public health. The pollutants include pharmaceuticals, toxins, hormones, viruses, and endocrine-disrupting chemicals (Xagoraraki and Kuo, 2008). Heavy metals (e.g., Cd, Pb, Mn, Fe, Zn, Cu) are also major contaminants of water (Opaluwa et al., 2012). Human activity is the major source of most of the water pollutants, whereas some amount of them are added by natural activities such as volcanic eruptions.
The main anthropogenic activities that cause water pollution include agricultural waste, livestock waste, industrial chemical waste, pesticides, fertilizers, mine drainage, untreated municipal sewage, spillage of petroleum products, spent solvents, and so forth. Once pollutants are discharged into any of the surface water bodies or the groundwater, they enter the water cycle. Pollutants may also undergo physical, biological, and chemical transformations (Xagoraraki and Kuo, 2008). The contaminants in water bodies, such as heavy metals, are also bioaccumulated in the flora and fauna of that region and so enter into the food chain. The contaminated water, whether used for drinking, irrigation, or other purposes, may lead to many health issues. For example, chemical pollutants can damage functional systems (e.g., immune system and nervous system) and major organs (e.g., kidney and liver), and pathogenic microorganisms in the water lead to gastrointestinal problems.
Increased cancer risk is also a major threat posed by enhanced concentrations of pollutants in drinking water (Xagoraraki and Kuo, 2008). An atomic absorption spectrophotometer (AAS) is used to assess the presence and amount of heavy metals in polluted water (Opaluwa et al., 2012). Adsorbents, such as activated carbon, can be used to remove heavy metals from contaminated water; however, it is an expensive material. So, instead of using commercial activated carbon, researchers used materials (e.g., sawdust, chitosan, mango leaves, coconut shell) that were inexpensive, had a high-adsorption capacity, and were locally available (Renge et al., 2012).
Contaminants enter plants when they are grown in soil that has various types of them, such as heavy metals, or when irrigated with polluted water containing contaminants. The plants show growth reduction, altered metabolism, metal accumulation, and lower biomass production (Nagajyoti et al., 2010). Some metals (e.g., Mn, Cu, Zn, Co, and Cr) are important for plant metabolism in trace amounts. When these metals are present in bioavailable forms and in excess, they become toxic to plants. Few heavy metals are very toxic to metal-sensitive plants, so result in growth inhibition and may also cause death of the organisms. The uptake of heavy metals does not show a linear increase with an increasing metal concentration. A number of factors affect the uptake of heavy metals by plants, which includes the growing environment. Some examples are soil aeration, soil moisture, soil pH, temperature, competition between plant species, type and size of plants, plant root systems, type of leaves, the elements available in the soil, and plant energy supply to roots and leaves (Yamamato and Kozlowski, 1987).
Metal contamination affects various biochemical and physiological processes in plants such as carbon dioxide fixation, gaseous exchange, respiration, and nutrient absorption. The toxic effects of six heavy metalsMn, Cd, Cr, Hg, Co, and Pbwere studied on Zea mays by Ghani (2010). Cd was found to be the most toxic and Cr to be the least toxic metal. The phytotoxicity of the six heavy metals was found in this order: Cd>Co>Hg>Mn>Pb>Cr. Heavy metals in plants lead to production of reactive oxygen species (ROS) such as hydrogen peroxide, superoxide radicals, and hydroxyl radicals. The ROS can oxidize biological molecules, lead to major cellular damages, and ultimately cell death. Hydroxyl radicals produced in the DNA proximity can remove or add hydrogen atoms to the DNA backbone or bases, respectively (Pryor, 1988). This resulted in 104105 DNA base modifications in a cell in one day (Ames et al., 1991). Fe2+ ions, free in solution or coordinated with ring nitrogens or complexed to a phosphate residue, were involved in these DNA alterations mediated by the hydroxyl radical (Luo et al., 1994).
Metal ions also lead to oxidative modification of proteins and free amino acids (Stadtman, 1993). The oxidation in proteins most commonly occurs at arginine, histidine, methionine, proline, cysteine, and lysine residues. Transition metals (e.g., iron) and oxygen lead to lipid peroxidation and damage to biological membranes. Plants cope in a number of ways with metal toxicity. The ROS generated in leaf cells are removed by enzymes of the antioxidant system of plants such as ascorbate peroxidase (APX), superoxide dismutase (SOD), glutathione reductase (GR), and catalase (CAT). Proline is reported to detoxify active oxygen in Cajanus cajan and Brassica juncea under heavy metal stress (Alia et al., 1995).
Burning of coal in thermal power plants and disposal of fly ash, long-term mining and smelting of the sulfide ores, runoff from mine tailings, and application of pesticides and herbicides release huge amounts of arsenic in to the biosphere. Additionally, arsenic is also used in the production of semiconductors, lead-acid batteries, and pesticides and herbicides, in the glass industry and copper refining industry, and in the hardening of metal alloys. Use of arsenic in wood preservation is very common and has increased significantly in the last few decades . Wood may deteriorate by the attack of insects, fungi, bacteria, and animals, but can be protected by impregnating with CCA with the composition CuO (18.5%), Cr2O3 (47.5%), and As2O3 (18.5%). At one time, arsenic compounds such as lead arsenate, calcium arsenate, and sodium arsenate were used as pesticides for debarking trees, to control ticks, fleas, and lice, and in aquatic weed control. However, these applications have been banned due to the toxic effects of arsenic and later public awareness about food safety and environmental contamination .
A new study released by CRU Group has found more than 43 million tonnes of copper is sitting idle in waste dumps at mines across the globe. The metal is considered too difficult to extract economically using conventional mining methods. Thats the equivalent of more than a decades worth of mine supply and worth $2.4 trillion at current prices, Bloomberg reports.
Even Natural Resources Canada has estimated that there is about $10 billion in total metal value in Canadian gold mining waste alone. The Canadian Governments Natural Resources Clean Growth Program has awarded Jetti with funding to further research and encourage projects utilizing its more energy-efficient process to extract copper from regular- and lower-grade ores, as well as waste mining materials and tailings.
Jetti has developed a catalyst that can liberate copper from low-grade chalcopyrite ores, known to have copper grades of well below 1%. The process works by disrupting the sulphur metal bond of the mineral.
The company installed its first commercial plant last year at a mine in Arizona run by Capstone Mining Corp (TSX: CS). Capstone says that by processing millions of tons of waste rock, it hopes to produce an additional 350 million lb. of copper, thought to be worth more than $1.6 billion at current prices, in the next two decades.
The technology could unlock the processing of millions of tons of copper thats already been mined and it could help extend the life of existing mines while allowing new projects to process lower-quality ore from the start.
Jetti currently has a pipeline of 23 projects at various stages, including five pilots and three operations that its looking to transition to commercial status in the next year or so. By the middle of the decade, its plants could start having a material impact on global copper supply.
Were unlocking a colossal, stranded resource, but were not going to be doing it in an incredibly short period of time that swamps the industry with excessive production or at a dramatically lower cost than is currently done, said Outwin.
SEOUL A specially designed vessel developed by South Koreas state power company for the quick installation of offshore wind power platforms was launched at the southwestern port city of Gunsan. It can lift and transport a heavy and high structure weighing up to 1,500 tons. An offshore wind power platform consists of a turbine Read MoreShare this:TwitterFacebookLinkedIn
At IQIP, we are proud of our broad hammer range. However, there was still a gap. Therefore, we created the S-350 Hydrohammer. When a Hydrohammer S-280 is too light, but an S-500 is too heavy, we have found a great middle-ground at IQIP: the Hydrohammer S-350. Are you interested in our trustworthy Hydrohammer? Mail us Read MoreShare this:TwitterFacebookLinkedIn
Storm Geomatics have used a SL40 USV supplied by Thurn Group in a recent wide swath bathymetric river survey. In early spring 2021 Storm Geomatics were approached by an existing client to carry out topographic and bathymetric surveys on the River Aire, at the village of Newlay, northwest of Leeds, Yorkshire in the UK. The Read MoreShare this:TwitterFacebookLinkedIn
Following the receipt of equipment orders for four offshore wind service vessels announced in May 2020, MacGregor has received an additional order for another two vessels. The four Commissioning Service Operation Vessels (CSOV) will further expand the stensj Rederi Edda Wind fleet. Both vessels will be built at the Astilleros Gondn shipyard in Asturias, Spain Read MoreShare this:TwitterFacebookLinkedIn
Antwerp, Belgium, headquartered Euronav NV (NYSE: EURN & Euronext: EURN) reports that it is in a Joint Development Program (JDP) with the largest shipbuilder in the world, Hyundai Heavy Industries (HHI) and classification societies Lloyds Register and DNV, to help accelerate the development of dual fuel ammonia (NH3) fitted VLCC and Suezmax vessels. The initial Read MoreShare this:TwitterFacebookLinkedIn
Dongzhuang Hou, Lang Liu, Qixing Yang, Bo Zhang, Huafu Qiu, Shishan Ruan, Yue Chen, Hefu Li, "Decomposition of Cyanide from Gold Leaching Tailingsby Using Sodium Metabisulphite and Hydrogen Peroxide", Advances in Materials Science and Engineering, vol. 2020, Article ID 5640963, 7 pages, 2020. https://doi.org/10.1155/2020/5640963
Cyanidation is widely used by most gold mine worldwide and will remain prevail in years (or decades) to come, while cyanide is hazardous, toxic pollutants whose presence in wastewater and tailings can seriously affect human and its environment; hence, it is necessary to control these contaminants. The purpose of this study was to examine the effects through the investigation of changes in pH, concentration, and contact time, and the optimal conditions were obtained. It has been proven that the decomposition of cyanide in solution and tailings increased as the alkalinity in the presence of 0.5g/L Na2S2O5. An increase in H2O2 (30%) concentration (from 1 to 4mL/L) increased the decomposition in solution, while the effect on removing cyanide was better when pH was 9 than 8 and 10 in tailings. The cyanide in tailings decreased in the first 4h and increased after 4h. The effective and economic conditions for maximum decomposition of cyanide from leach tailings are first treated in 0.5g/L Na2S2O5 at pH 10 for 3hours and then 2mL/L H2O2 (30%) is added to the tailings at pH 9 for 4 hours through comparative study. The findings provide the basis to optimize the decomposition of cyanide from gold leaching tailings in mining or backfilling by using the synergetic effect of Na2S2O5 and H2O2.
Cyanide leaching (cyanidation), which converts the gold into a cyanide complex (Au(CN)2) that is soluble in water, is currently the most prevailing and effective process to extract gold from ores [1, 2]. This process requires excess cyanide to improve gold recovery and produces exceptionally large quantities of cyanide-bearing wastes in the form of tailings and waters. Free cyanide, which is the main byproduct that results from metallurgical processes , is considered the most toxic cyanide form as it causes harmful effects at relatively low concentration. Other cyanide species are easily dissolved to free cyanide under acid conditions . It will take a longtime for cyanide in the tailings to be reduced to biologically harmless through natural attenuation , so tailings containing cyanide should be treated before they are released into the environment to avoid detrimental effects on the receiving environment . The decomposition of cyanide from gold leaching tailings is one of the biggest challenges for the gold mine in the last decades, so the appropriate treatment of the cyanide in the tailings is required to avoid or minimize environmental and health issues.
Several biological, physical, and chemical techniques, electrolytic oxidation, and other methods to decompose or recycle cyanide have been developed for the treatment of cyanide solutions. Currently, as the widely used technologies for industrial production of the decomposition of cyanide are INCO process, alkali-chlorine process, activated carbon process, ozone oxidation process, hydrogen peroxide oxidation process , and recovery hydrogen cyanide . INCO and alkali-chlorine processes are used to remove high concentration cyanide, while they cannot completely accomplish the degradation of cyanide species [13, 14]. Activated carbon can absorb cyanide and make it gather, but cannot decompose it . Ozonation and hydrogen peroxide oxidation processes, which are not easy to produce secondary pollution in the treatment of cyanide, are used to treat low concentration cyanide [16, 17], but the cost is higher. Recovery hydrogen cyanide can reduce cyanide content and cost, but it cannot meet the requirements of tailings backfilling .
Backfilling technology is the best choice for the mine to break through the bottleneck of resources, environment, and safety , but harmful substances such as metal materials and solvent left in the gold leaching tailings should be treated before backfilling in order to prevent them from polluting groundwater. Many organizations and countries around the world have already issued effective cyanide management policies; some countries, include Costa Rica, Argentina, Germany, the Czech Republic, and Turkey, have an outright ban on the use of cyanide in gold extraction throughout the country [19, 20]. In 2018, China issued the technical specification for pollution control of cyanide leaching residue in gold industry (TSPC), in which ozonation and hydrogen peroxide oxidation are selected as a method of deep decyanation. The study on cyanide removal is mainly focused on industrial wastewater ; the study on cyanide removal in gold leaching tailings, which contains wastewater and tailings, is still limited. While the mineral composition of gold ore is complex, cyanide, powder of activated carbon, hydrogen peroxide (H2O2) and other chemical were also added in the mineral processing flowsheet, which made the composition of tailings more complex. Most of the residual cyanide in tailings are strongly adsorbed on the surface of minerals, only a small amount of free cyanides and hydrolytic complex cyanides are able to enter into the leaching solution .Therefore, it is necessary to explore more efficient methods to remove cyanide. The appropriate decomposition of CN is a complex process that requires several approaches are combined to improve the efficiency of the treatment and consider treatment economy, and combination methods were used to try to remove cyanide .
Based on the method of cyanide removal in solution, the effects of pH, concentration, and contact time of H2O2 for the decomposition of cyanide from leach tailings were analysed after Na2S2O5 reaction. The mechanism of cyanide removal was also discussed. The outcome of this study was to find the best conditions for decomposition of cyanide from leach tailings, lead to bulk utilization of tailings in gold metal mine, and alleviate the ground collapse and soil pollution caused by traditional mining.
The gold leaching tailings used in the experiment comes from Xiajiadian gold mine, Shaanxi province, China, which belongs to orogenic gold deposit, accounting for 52% in China . The concentration of gold leaching tailings, where the diameter of tailings is 100600m, is 35%, pH is 10.78, the initial cyanide concentration in the solution is 30.66mg/L, and the cyanide leached from the leach tailings is 11.78mg/L. The total carbon content of tailings is 2.476.67%, in which the organic carbon content is 1.474.29%. The solution temperature was controlled at 232C.
H2O2 (30%) and sodium metabisulphite (Na2S2O5) were employed as received. Copper employed as the decomposition catalysts [25, 26] was added as the copper sulfate for experiments. Concentrated (98%) H2SO4 and NaOH were employed for pH adjustment. All chemicals were of analytical reagent grade. Agitated air was used in this experiment in order to accelerate the decomposition of cyanide, and the inflation rate was 50mL/min.
0.5g/L of Na2S2O5 and 0.2g/L of CuSO4 (as catalyst) were added to 1000mL gold leaching tailings, and the solutions were air stirred at a constant rate of 50mL/min to accelerate cyanide oxidation. Individual experiments were carried out at pH values ranging from 8 to 11. After 3h at each pH, 10mL sample solutions, which were filtered with a 0.45m filter paper and a 100g solid which was leaching residue, were analysed for residual cyanide. Then, 1, 2, or 4mL/L of H2O2 (30%) was added to 1000mL gold leaching tailings, which had been treated with Na2S2O5 at pH 10 for 3h. Individual experiments were carried out at pH values ranging from 8 to 10, the solutions were air stirred at a constant rate of 50mL/min to accelerate cyanide oxidation. After 1h, 2h, 4h, and 8h at each pH, a 100g solid which was leaching residue and some 10mL sample cyanide wastewater were analysed for residual cyanide. NaOH or H2SO4 was added to maintain a pH of 811 throughout each test.
The steps of cyanide leaching of leach tailings follow the technical specification for pollution control of cyanide leaching residue in gold industry issued by China. Contaminated cyanide solution was prepared by mixing the pregnant solution and water (extractant) according to the ratio of 10L water to 1kg tailings and oscillated at 302 revolutions per min by flip oscillator with 232C for 182 hours referring to the technical specification for pollution control of cyanide leaching residue in gold industry.
Total cyanide (CNT) content was determined by isonicotinic acid-pyrazolone spectrophotometry referring to the water quality-determination of cyanide volumetric and spectrophotometry method. The cyanide reacts with chloramine T to give cyanogen chloride, which reacts with isonicotinic acid to give pentenedialdehyde, and finally condenses with pyrazolone to give blue dye; then, it is measured at 638nm by UV9100A ultraviolet-visible spectrophotometer (YQ00302), with a detection limit of 0.004mg/L.
Individual experiments were carried out at pH values ranging from 8 to 11. Figure 1 shows the effect of equilibrium pH on the cyanide of solution and lixiviant in the presence of 0.5g/L Na2S2O5 in 3 hours. It is evident that an increase in pH increased the extent of decomposition of cyanide in solution and lixiviant of tailings. This result showed that the decomposition of cyanide increased as the alkalinity of the solution increased, the cyanide in lixiviant of tailings was the least when the pH was 10 under the same conditions, and the effect of cyanide decomposition in tailings was equivalent to that of pH 9, 10, and 11. Considering the investment cost, the effect of decomposition of cyanide from solution and tailing by Na2S2O5 is the best when pH is 10.
The pH value had a significant effect on cyanide removal [21, 27]. The tailings were treated with 0.5g/L Na2S2O5 for 3 hours at pH 10, and then individual experiments were carried out at pH values ranging from 8 to 11. Figures 2(a)2(c) show the effect of equilibrium pH on the cyanide of lixiviant of tailings after Na2S2O5 treatment in the presence of H2O2, and it is evident that the effect on removing cyanide was better when pH was 9 than 8 and 10 in lixiviant. Figure 2(d) shows effect of equilibrium pH on the cyanide of solution added with 2mL/L H2O2 for 2hour, and it is evident that an increase in pH increased the extent of decomposition in solution, which is in agreement with those of Tu et al.  who observed that the decomposition of cyanide in mine wastewater added with H2O2 increased as the alkalinity of the solution increased.
Figure 2 shows that the effect of removing cyanide by H2O2 in solution is different from tailing under the same conditions. The reason for this phenomenon may be that the cyanide in the solution, which is free cyanide, is easy to be treated, while the diffusion rate of cyanide in the tailings to the solution is lower than the removal rate by H2O2 of the solution.
The ability of the decomposition of cyanide will be affected by the concentration of H2O2. Figure 3 shows the effect of H2O2 concentrations ranging from 1 to 4mL/L on the decomposition of cyanide. It is evident that an increase in H2O2 concentration increased the extent of decomposition in tailings; the most cyanide is leached from the tailings when 2mL/L H2O2 is added to the solution.
In order to remove cyanide in tailings as much as possible, excessive Na2S2O5 was added to the pretreatment, and the reaction occurs when H2O2 reached a certain concentration, resulting in the low efficiency of cyanide treatment at 2mL/L:
Contact time is also the influence factor of H2O2 for the decomposition of cyanide . Figure 4 shows the effect of contact time ranging from 1 to 8h on the rate of decomposition of cyanide of lixiviant in tailings. The figure showed that cyanide decreased in the first 4h and the content of cyanide increased after 4h. In the first four hours, H2O2 reacted with cyanide in the tailings, which caused the cyanide content in the solution to decrease, while most of the cyanides adsorbed on pyrite [28, 29] and chalcopyrite , which is widely distributed in gold ore, are driven by chemisorption of carbon. The remaining H2O2 could not continuously reduce the cyanide content in the tailings, and it might be that the unreacted cyanide in the tailings adsorbed by activated carbon reentered the tailings with the concentration of H2O2 decreased. It is possible that the cyanide adsorbed by pyrite and activated carbon is gradually converted into free cyanide, and the unreacted cyanide in pulp re-enters the tailings with the decrease of H2O2 concentration.
As explained above, the best conditions for decomposition of cyanide from leach tailings are first treated in 0.5g/L Na2S2O5 at pH 10 for 3 hours and then 2mL/L H2O2 is added to the tailings at pH 9 for 4hours.
The decomposition of cyanide increased as the alkalinity of the solution and tailings increased in the presence of 0.5g/L Na2S2O5. An increase in pH increased the extent of decomposition in solution in the presence of H2O2, while the effect on removing cyanide was best when pH was 9 in lixiviant of tailings. An increase in H2O2 concentration increased the extent of decomposition in solution. The cyanide decreased in the first 4h, and the content of cyanide increased after 4h by the adsorption effect of active carbon and pyrite.
All the leaching metal of gold leaching tailings treated by Na2S2O5 and H2O2 can meet the backfilling requirements (0.05mg/L) of TSPC. Considering fully with effectiveness and practicability, the best conditions for decomposition of cyanide from leach tailings are first treated in 0.5g/L Na2S2O5 at pH 10 for 3 hours and then 2mL/L H2O2 is added to the tailings at pH 9 for 4 hours.
Highlights. (i) Sodium and hydrogen peroxide for cyanide decomposition are applied. (ii) The factors affecting cyanide removal were explored. (iii) The optimal conditions for cyanide removal were examined. (iv) The lessons are useful for backfilling by gold tailings to develop efficient circularity.
This work was supported by the National Natural Science Foundation of China (no. 51904225), Science and Technology Program of Shaanxi (nos. 2020JQ-748 and 2019JM-409), and Doctoral Start-up Program of Xian University of Science and Technology (nos. 2016QDJ031 and 201610).
Copyright 2020 Dongzhuang Hou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
"This is no less than a step change in water management. It is the needed piece of the puzzle towards the only sustainable tailings management; dry stacked tailings maximizing the water recirculated in the process, states Director of Tailings Solutions in FLSmidth, Todd Wisdom.
As the need for dry stacked tailings has increased, so has the need for larger and larger dewatering equipment to accommodate the huge volumes of tailings requiring treatment. Some large mining operations produce well over 100,000 dry tonnes of tailings per day. Dewatering this massive volume of solids to the geotechnical standard needed for dry stacking requires large filtration areas which, using old technology, would require football fields of filters. These in turn would require an unsustainable amount of piping, valves, pumping, conveyance and maintenance. To reduce the number of filters, FLSmidth decided to develop an economically viable system for dewatering high tonnages to the scale of more than 120,000 tonne/day of tailings.
"As the ore grades decline, miners' revenue per tonne decreases accordingly, so we need to decrease the cost of tailings processing to make the operation economically viable and our solution cost competitive with alternate technologies," explains Todd Wisdom.
To optimize the capital and operating costs of plants operating at large tonnages, FLSmidth came up with the AFP-IV Colossal filter. The filter enables effective dewatering of large volumes of tailings with less than half the number of filters required with competing large filters. The new filter not only increases the amount of water that is returned to the plant for reuse, it is also able to decrease the amount of water that is lost to the environment through evaporation and seepage. These are two major hurdles that can be expensive and difficult to overcome in traditional wet tailings facilities.
To prove the operating and financial benefits of their large format tailings dewatering facility, FLSmidth partnered with a large mining company in Chile and installed a complete tailings filter system to process approximately 10,000 tonnes per day of whole tailings. A full scale implementation of this technology would allow mines to operate with a water make-up ratio of 0.2 m3/tonne, compared to traditional sand dams with a water ratio of 0.7 m3/tonne.
"With this breakthrough development in filtration technology, we have made the operating and capital costs of dry stacked tailings competitive with traditional technologies for large scale mines. A large copper concentrator with a tailings output of 100,000 tonnes per day, for example, could save 200 million m3 of water over ten years by using our dry stacked tailings technology combining thickener and filter with our RAHCO mobile stacking and conveying system to dispose of the tailings as dry filter cake," states Todd Wisdom. In Chile that would translate into $USD1 billion for desalination.
FLSmidth's AFP-IV Colossal filter is a filter press, which uses high pressure and fast filtering to achieve much larger single machine capacity and optimal cake moisture concentration. Where vacuum filters are not effective at higher altitudes, and belt presses and centrifuges are only effective for small tonnages, filter presses are able to use higher driving forces to achieve high dewatering rates and can operate effectively at high altitudes. This allows them to have the smallest installed footprint and low installation costs, alongside greater operational flexibility.
"Operational flexibility is needed as the tailings filtration characteristics change over the life of the mine. For these reasons, we believe that large filter presses like this one will become the equipment of choice for high tonnage tailings dewatering applications," says Todd Wisdom.
The feed to the filter press is pumped from a buffer tank, which has sufficient capacity to allow a constant flow from the tailings thickener during the batch filtration process. The pumping of the feed slurry under pressure into the chambers/cloths, provides the force to build a cake within the chamber. As the cakes form, the pressure to produce a properly compacted cake rises steadily. Most filter presses on tailings applications operate with up to a 15 bar driving force. Using FLSmidth technology, this driving force can easily be reached using just the feed pumps to the filter. After the cake is formed, high pressure air, in the range of 7 to 10 bar, can be blown through the cake to further reduce its moisture content and produce a non-saturated cake, if required.
The cake is formed by the capture of solids on filter cloths which encapsulate the filter chambers. The press is closed by means of hydraulic cylinders and multiple plates, which, on closure, form the chambers that capture the filter cake. The moisture of the discharged filter press cake is in the range of 10 to 25 wt % and can be either saturated or unsaturated depending on the filter design and geotechnical requirements.
The Colossal filter is the largest capacity filter in the industry. It is capable of discharging 20,000 tonnes of filter cake per day and can recover 600 m3 of process water per hour. That is the equivalent of six Olympic sized pools each day.
"To cover the need of a large mine with 150,000 t/d of tailings, ten filters would probably need to be in operation. In arid climates, the amount of water saved more than offsets the costs of this solution, which notably also saves vast amounts of energy. And it enables mining to continue in areas of the world where access to water is extremely scarce and problematic," Todd Wisdom concludes.
FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.
At CDE, we are proud to apply over 25 years of industry expertise to help increase the potential of your mine. Our industry knowledge allows us to transform even the most abrasive materials into saleable products.Due to the modularity of our offering, we can deliver small to large-scale plants in some of the most remote locations on earth.
We deliver a unique approach to the mining sector by using ground-breaking modular technologies in the applications of coarse ore beneficiation, ultra-fines recovery, and tailings dam rehabilitation.Engineered to sustain our planet, our energy-efficient processes enable maximum recovery of high-value minerals whilst minimising our footprint on the environment. Our modular solutions are engineered for sustainable mining, by maximising resources and minimising waste.
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The key to unlocking this value is a combination of washing, scrubbing, and classification or by isolating those difficult to remove fractions using our modular tailings recovery systems.Choosing CDE means extending the life of your reserve and investing in the sustainability of your mining operations.
Through the introduction of a customised CDE process improvement package we can facilitate the processing of lower-grade ores.By reducing the cut-off grade we are able to deliver significant efficiencies to your mining operation.