Commonwealth of Australia 2010
Fate of cyanide in tailings storage facilities
Download 2.29 Mb.
|
- Navigate this page:
- 23.6.2Processes for detoxifying or recovering cyanide in tailings
- 23.6.3Transformation in TSFs and migration
- 23.6.4Fate modelling and monitoring data
- 25.Volatilisation
- 26.Migration in seepage from TSFs
23.6Fate of cyanide in tailings storage facilities23.6.1Tailings constituentsDuring the gold/metal leaching process within the mill, NaCN is added to the ore and under the alkaline conditions used, the free cyanide produced when it initially dissolves is predominantly present as CN- (a greater proportion of HCN is present where suboptimal pH is used due to local ore or water conditions – see Sections 5.2.2 and 23.3). However, the reactive nature of this mixture leads to the formation of other compounds and products of the parent cyanide compound in the ore slurry (Moran and Brackett, 1998; Smith and Mudder, 1993; Lye, 2002). With reference to the mining industry, tailings contain the fine-grained waste material remaining after the economically recoverable metals and minerals have been extracted from the ore (Section 5.2.2). The physical and chemical properties of tailings vary with the nature of the material being processed and the process itself, which may also vary over time (MCMPR/MCA, 2003). In general, cyanide compounds and products in tailings may potentially include:
Table 6.. Microbial cyanide degradation pathways (adapted from Meehan, 2000)
A range of other compounds may also occur in tailings that are unrelated to cyanide. These may originate, for example, from the ore, from chemicals added during gold ore processing, or from abiotic and biotic reactions and products. 23.6.2Processes for detoxifying or recovering cyanide in tailingsThere are various processes available to detoxify cyanide (Botz, 2001; Young, 2001; Young and Jordan, 1995; USEPA, 1994c), some of them used within Australia to treat tailings, such as the following:
There have also been processes developed commercially for regeneration of free cyanide from the tailings water so it can be re-used (Botz, 2001; Fleming, 2001; Barter et al., 2001; AMMTEC, 2005). These include:
However, there are technical limitations and problems, and financial considerations that have so far limited the use of these techniques to a few sites worldwide. The Cyanisorb® process was developed in New Zealand in 1989 and used at the Golden Cross gold and silver mine in New Zealand. The SART process has been used at least at the pilot plant scale in Australia, including at the Telfer gold mine in WA, and has been operated in full scale plants overseas (Environment Australia, 1998). Eisler et al. (1999) commented that water hyacinth (Eichornia cressipes) has been proposed as the basis of a cyanide removal technology, as this water plant can survive for at least 72 h in a nutrient solution containing as much as 300 mg CN/L and can accumulate up to 6.7 g CN/kg DW plant material (citing *Low and Lee, 1981). However, Eisler et al. (1999) note that how to dispose of this plant material is then an issue, which may be one reason that large scale use of water hyacinths for this purpose has not yet been implemented. 23.6.3Transformation in TSFs and migrationAs indicated in Figure 6., the fate of cyanide compounds in TSFs is complex and a range of reactions may occur resulting in degradation and transformation, and atmospheric emissions occurring (e.g. Staunton, 1991c-d, Smith and Mudder, 1993; Ellis, 1997; Lye, 2002). The kinetics of cyanide reactions are site-specific, being influenced by the general chemistry and geochemistry, including variations in pH, redox potential (Eh), salinity, temperature, density of solids, type of minerals, the individual metal cyanide complexes and their concentration, and the presence of any free cyanide (Environment Australia, 1999a-d). Ellis (1997) concluded from cyanide fate modelling that the main factors affecting natural degradation of cyanide in TSFs were pH, temperature, TSF size, presence of metal cations and the effect of discharging slurry over a sloping entry section (beach). Botz et al. (1995) considered that the main cyanide degradation mechanisms occurring in TSFs are the dissociation of metal cyanide complexes and volatilisation of cyanide as HCN, and the principal factors affecting degradation are pH, temperature, photolysis and volatilisation. It is likely that techniques used to minimise the amount of water delivered to a TSF would reduce the proportion of cyanide that is volatilised as HCN, as would free cyanide destruction processes (Staunton, pers. comm. 2005). Figure 6.. General fate of cyanide in tailings storage facilities (Smith and Mudder, 1993) 
According to Smith and Mudder (1993), the main processes affecting cyanide in tailings in TSFs in the short term may be described on the basis of operational areas as follows:
In general, while a proportion of cyanide will remain within a TSF for an extended period of time before eventually degrading or stabilising (Smith and Mudder, 1993; Staunton, 1991c-d), volatilisation of HCN will also account for a proportion of the cyanide added to a TSF (Ellis, 1997; Smith and Mudder, 1993; Simovic and Snodgrass, 1985; Staunton et al., 2003). In addition, depending on factors such as discharge volume and concentration, climate and structural integrity, there is an inherent potential for migration of contaminants in seepage of process water to underlying stratigraphy at some TSFs (DEWHA, 2006; Environment Australia 1999a-d; WRC, 2000b). Often mining operations have water or cyanide reagent recovery systems in order to reclaim cyanide solutions from tailings, via a lined decant pond, for re-use in the gold ore beneficiation process. 23.6.4Fate modelling and monitoring data24.Trends in WAD CN over time and within a TSFThe Northern Territory Bird Usage of Tailings Storage Facilities Coordinating Group study (Donato, 1999) consistently reported lower WAD CN concentrations in TSF surface waters than at the points of discharge to the TSFs. Mudder and Goldstone (1989) monitored the fate of cyanide compounds in tailings decant for 4 weeks and in tailings pore waters for 12 weeks, finding that WAD CN concentrations declined by up to 95% during the period of the tests whereas the total cyanide concentration remained stable. Several studies, including AMIRA Projects P277 (1989-1991) and P497/497A (1997-2000) provide information on the fate of cyanide in TSFs. The development of a model to predict the deportment of cyanide in and around tailings structures was commenced by AMIRA during P497A and the work continued in AMIRA project P420B. Cyanide species deportment modelling was also evaluated in the Minerals and Energy Research Institute of Western Australia (MERIWA) project M398 discussed below (Adams et al., 2008b). However, Staunton et al. (2003) indicate that the application of mass-balance modelling to determine the fate of cyanide in TSFs is not appropriate due to substantial error inherent in the method and thus the potential for erroneous emissions and loss estimates. Detailed data are available for WA gold mine operations participating in the studies reported by Adams et al. (2008a,b,c), namely Granny Smith Gold Mine, Kanowna Belle Gold Mine, and St Ives Gold Mine (the latter has both mill and heap leach operations). Granny Smith is a saline site (14 000-50 000 mg/L TDS in process waters) and the other two sites hypersaline (>50 000 mg/L TDS). Cyanide sampling conducted during the project showed that the three TSFs experience daily fluctuations in WAD cyanide discharge concentrations within the range of 35 to 170 mg/L. Limited evaluations were also conducted at a second saline site. In all cases there was a significant drop in WAD cyanide from spigot to supernatant, but evaluations in Phase II of the project showed hypersaline sites generally showed significantly greater cyanide degradation in the flume (turbulent slurry flow over the beach zone) than the saline sites. This was expected by the investigators, as it has been shown that the solubility of hydrogen cyanide is lowered with increasing salinity, and in addition, the pH values of hypersaline solutions are lower which also favours loss of cyanide by evaporation (Section 23.3.2 and below). 25.VolatilisationThe rate of HCN release (i.e. volatilisation) from cyanide solutions in TSFs is largely governed by the pH of the solution. The lower the pH, the greater the rate of HCN formation and evolution to air. The loss of cyanide can be managed through regulation of pH in process streams (CMEWA, 2002). Mill tailings ideally have a pH 10 (Smith and Mudder, 1993), but a significant proportion of Australian operations are at a pH of ~9.0-9.2 (Section 5.2.2). In either case, conditions in TSFs are generally less alkaline and create an environment conducive to HCN formation and volatilisation, whereas volatilisation losses during the process are low (Section 5.2.2). AMIRA project 497A found that volatilisation of HCN is the main cyanide loss process from TSFs (Dilworth, 2000). Staunton et al. (2003) discussed the relevance of results reported by Schmidt et al. (1981), for an evaluation of cyanide loss in a barren solution bleed pond with no solid tails present. In that study, natural degradation mechanisms reduced the cyanide concentration by 99.9% over a four month period, with volatilisation of free cyanide as HCN evidently the most important mechanism and oxidation to cyanate accounting for ~11% of the total cyanide loss. They noted that this test situation differs from that occurring in typical gold mining TSFs operating in Australia, where solution and solids are deposited together in the TSF. The solid and liquid phases partly separate, with some of the solution rising to the surface to form a tailings or decant pond, while the remainder of the solution is trapped below, within the consolidated solids (see Figure 6.). The cyanide in the decant pond is available for volatilisation as HCN, and up to 90% of this cyanide may be lost as HCN. However, the solution trapped within the consolidated solids is not available for volatilisation and as discussed above, converts to more stable metal cyanides. Thus they concluded that the percentage of the cyanide entering a TSF which is evolved as HCN would be significantly less than 90%, but may possibly be as high as 30% or more. Staunton (1991c) monitored degradation of free cyanide in a TSF, indicating that volatilisation of HCN is more significant immediately following discharge of tailings; however, over time and in conjunction with burial by overlying tailings, volatilisation becomes less significant. More stable, metal cyanide complexes are more apparent. Due to their stability, these complexes may persist within the TSF environment for an extended period of time (i.e. years; Kjeldsen, 1999). Copper cyanides form the stable Cu(CN)32– species in tailings solutions that can significantly contribute to WAD cyanide, limiting volatilisation of HCN (Adams et al., 2008b,c). Synthetic copper cyanide degradation tests at different salinities conducted under the MERIWA project exemplified the stabilizing effect of copper on WAD cyanide, with cyanide degradation halting after about 24 h in all cases, with WADCN:Cu ratios remaining at ~0.8 from 24 h through to 120 h, consistent with the stable Cu(CN)32– remaining in solution for lengthy periods, confirming previous results. 26.Migration in seepage from TSFsIt is not possible to generalise regarding the potential for migration of cyanide and products from TSFs in seepage to groundwater, except that it may potentially occur, particularly below unlined TSFs, TSFs with poorly designed and constructed liners or where liner integrity is violated. The potential for, and rate of, seepage to underlying strata from TSFs is highly variable and varies site-specifically. Reviews by DEWHA (DEWHA, 2006; Environment Australia, 1999a-d) suggest that seepage rates of 0 to 10% have been quoted in the mining industry. Seepage is the rate of movement of process water to underlying stratigraphy relative to the inflow rate of process water. However, Staunton et al. (2003) indicated that a seepage rate of 10%, together with the assumption that the seepage will have a CN concentration equal to that of the TSF return water, will generally result in overestimation of cyanide migration as both figures are at the high end of the range. Evaluation and modelling of seepage in AMIRA Project 497A found that cyanide may be incorporated into buried tailings after tailings settlement, as the stable iron-cyanide complex, ferrocyanide, which is chemically stable and of lower toxicity. The research was undertaken on five different types of TSFs and suggests that the process may be widespread. Instability of iron-cyanide complexes may occur in the presence of sunlight (photolysis), resulting in the formation of free cyanide. This would probably volatilise at the surface to air and concentrations are unlikely to be high. Dissolution of metallocyanide complexes may also occur in the presence of acid rock drainage (Dilworth, 2000). Kjeldsen (1999) indicated that iron cyanide complexation may potentially account for a large proportion of the cyanide present, and that the stability and low solubility of these complexes under low pH (e.g pH 6) conditions has the potential to inhibit migration in seepage. However, strong alkaline conditions may increase cyanide dissolution and the relative potential for migration in seepage. Table 5. summarises the result of TSF pore water samples from sediment cores (0-2 m) collected from three TSF operating in the early 1990s (Staunton, 1991b-d). Cyanate and thiocyanate tailings concentrations ranged up to 320 and 610 mg/L, respectively, and up to 2100 mg/L in TSF pore waters (0-1.6 m depth). Free and WAD CN concentrations ranged up to 290 and 630 mg/L, respectively. No samples from greater depths were available. Staunton (pers. comm. 2005) commented on results of studies of the concentration of cyanide residues in a decommissioned site, at depths up to 6.5 m. Concentrations of WAD and total CN in a water extract from the entrained liquor (pore waters) tended to decrease with increasing depth, from a maximum of ~4-5 mg/L and ~32-38 mg/L, respectively, at 0.5-1 m, to a minimum at 2.5-3 m of ~1 mg/L and ~7 mg/L, respectively. They then increased to ~6 mg/L and ~27 mg/L, respectively, before again declining. In a separate study of an ‘old’ tailings facility, WAD and total CN concentrations in a water extract from the entrained liquor tended to fluctuate around 1 mg/L and 3-9 mg/L, respectively at 0.5-10.5 m, except for a marked peak of ~3 mg/L for WAD CN and 22 mg/L for total CN at 6.5 m. This presumably reflected different concentrations over time in the material deposited, as well as changes over time. Staunton et al. (2003) indicate that seepage is highly variable and may range by 5% of the cyanide added to a TSF. Smith et al. (1984) recorded total cyanide in seepage at a decommissioned TSF (20 years old) at various depths in the underlying stratigraphy (11 to 47 m) at concentrations up to 2.9 mg/L, but at the deeper locations (e.g. >30 m), concentrations were 1 mg/L. Staunton (1991c) reported on a study of groundwater quality down gradient from a TSF in Queensland, finding no evidence of migration of cyanide in seepage to groundwater. Groundwater monitoring data from a TSF in WA indicated relatively consistent concentration ranges for total, WAD and free cyanide over the 6 year monitoring period of 0.01–3.6, 0.01-0.66, 0.01-0.29 mg/L, with the majority of the cyanide associated with strong complexes (e.g. iron-cyanide complexes). Groundwater monitoring in the vicinity of TSFs is a regulatory requirement of state and territory agencies in Australia. Consistent with ANZECC/ARMCANZ Guidelines (Section 78.2.2), the goal in most cases is to protect the groundwater as a future resource and hence to ensure that its potential uses are not affected by contamination of water from TSFs and other potential CN sources on the site, rather than necessarily stipulating that no contamination of groundwater with cyanide should occur at all, as is the case in some jurisdictions. Thus, it is considered that there is less need to protect hypersaline water that has no value for stock or irrigation use than water which is higher in quality and may be used for purposes other than mining and industry. There is a common (though evidently not general) licence limit for WAD CN in groundwater of 0.5 mg/L. Where this is exceeded in hypersaline areas, rather than requiring reinstallation or adaptation of the TSFs to minimise seepage, recovery bores are installed to intercept the water and return it for use in the process stream. It should be recognised that there are also other important potential groundwater impacts to the environment from TSFs apart from cyanide. Tailings waters may contain many other components of potential concern, which depending on local conditions, may affect pH, raise salinity and may contaminate the water with heavy metals and other potentially toxic elements such as arsenic and selenium. Changes in the original depth of the groundwater may also have effects on water quality, additional to quality effects from the TSF waters themselves, and shallow watertables (or groundwater mounds) are themselves a problem. Tolerances for salinity and pH impacts also vary with the quality of the original groundwater (on the principal of protecting future use as a resource). Groundwater is often a source for the process water used at gold processing sites, and at WA gold processing facilities in the Kalgoorlie/Laverton area it is often saline or hypersaline (e.g. >50 000 mg/L) and is only considered useful for mining. Directory: data -> assets -> word doc -> 0017 0017 -> National Industrial Chemicals Notification and Assessment Scheme word doc -> Priority Existing Chemical word doc -> Rail safety news issue 6 – October 2011 word doc -> Review of Multiple Chemical Sensitivity: Identifying 0017 -> Rail Safety News Edition 8, December 2012 0017 -> Real-Time License Plate Localisation on fpga 0017 -> Careers network bulletin issue 6 July 16 2012 0017 -> Wiser science: Finding References inspec for Physics and Engineering 0017 -> Part A: Depth Study: The globalising world Popular culture and public policy Download 2.29 Mb. Share with your friends:
|