 Commonwealth of Australia 2010

18.Tailings storage facilities

Tailings are predominantly discharged to tailings storage facilities (TSFs). The term TSF is defined as an area used to confine tailings and it refers to the overall facility. It may include one or more tailings (or water) dams or other tailings impoundments (MCMPR/MCA, 2003). Types of tailings impoundments used to confine tailings include tailings dams, mined-out voids, valleys in overburden stripping or underground mined areas (WRC, 2000c). A tailings dam is defined as the actual artificial embankment used to retain tailings (MCMPR/MCA, 2003). The ultimate purpose of a tailings impoundment is to contain tailings, often with a secondary purpose of interim storage of water for reclamation in the mine or ore processing mill (MCMPR/MCA, 2003; WRC, 2000c).

There are important considerations in site selection for a tailings dam and in choosing a suitable storage facility design and construction technique for the site, including meteorology, topography, geotechnical and geochemical factors, groundwater, seismology, and tailings properties (EPA, 1995b; MPD, 2004; Williams and Jones, 2005; DITR, 2007). The structural stability of the dam is important because of the possible release of large volumes of water and semi-fluid tailings, should failure occur. Appropriate design and management plans for the TSF are also important to minimise, monitor and if necessary control seepage into groundwater and to control any surface water generated (e.g. seepage into drains, rainfall run-off and accidental spills). Groundwater monitoring is highly important even after site selection, design and management plans have been thoroughly considered, as there are limitations in the modelling and other techniques used in the site selection and design phases. As well, any TSF that is very large will have the potential for water that is contaminated to seep into the groundwater. Monitoring of the groundwater gives a good basis for ongoing management of the facility.

As will be discussed later (Section 88.3), appropriate TSF design and management strategies may also be important to minimise potential impacts on wildlife. Appropriate measures also need to be considered to protect and restore the environment when the facility is no longer required (e.g. containment/encapsulation, drainage systems, and ongoing monitoring and control of surface and groundwater). There are various state/territory control measures and legislative requirements for TSF design, operation and subsequent maintenance to ensure adequate safety of the structure, prevent and/or manage groundwater contamination and minimise other potential impacts. Guidance on dam design and operation is available from various sources (Section 78.2.3). Cyanide is not the only contaminant present and TSFs may contain various heavy metals that may be mobilised, and acid drainage may be generated from the oxidation of sulphides (USEPA, 1994c).

MPD (2004) notes that as tailings material must be securely stored for an indefinite period and present no hazard to public health and safety or the environment, the closure of a TSF and rehabilitation works must be inherently stable, resistant to degradation and consistent with the surrounding landscape. Planning and preparation for ultimate restoration start early, including setting aside of topsoil and other material for covering the closed TSF (Section 70.2).

MPD (2004) indicates that potential environmental impacts of decommissioned TSFs include groundwater contamination, acid drainage and erosion of material by water and wind. The threat of catastrophic failure is usually reduced due to the de-watered nature of the deposit, but under some circumstances remains an important consideration. The operator should design a post-closure monitoring program including revegetation performance, flood mitigation and drainage control, seepage, erosion control, control of pests, plants and animals, groundwater quality, and should demonstrate that completion criteria have been met and the site is safe and stable. Even the best designed facilities will require maintenance or care, and proponents and operators of a TSF should make provision for the long term costs associated with the TSF and its maintenance.

Information received on a number of gold mines in Australia shows that most have paddock type TSFs, typically rectangular structures with 1, 2 or 3 cells. Tailings is discharged into these facilities from spigots usually located at several points along one or more edges, with a central point or area at one end where the decant water is collected. A number of mills also use in-pit tailings facilities, placing tailings into mined-out pits, with suitable precautions to prevent seepage near the surface and manage drainage. At least one facility uses an alternative to the paddock system, with thickened tailings discharged from one or more points towards the centre of an approximately circular structure (Central Tailings Discharge - CTD) to produce an essentially conical formation, the decant water being collected into a dam at the lowest point on the edge of the shallowly sloped cone. These systems have various advantages and disadvantages in the way they need to be managed and in their performance. TSFs can be quite large, e.g. with each paddock cell ~50 ha, and one example with a three-cell paddock facility occupying 284 ha, and with heights up to ~10-50 m.

In general, mills seek to recover as much water as possible from the tailings before and/or after deposition to the TSF, to minimise the amount of water and toxic solutes which need to be disposed of, to maximise the use of residual free cyanide, and to maximise water use efficiency. Temporary pools of supernatant form before the water drains through or off the surface of the TSF. Typically, water drains from the freshly applied slurry and supernatant areas to a collection point, where it flows or is pumped into a decant pond. Drainage and surface seepage from elsewhere in the site or from run-off or overflow from the TSF are directed into the decant pond and/or other temporary storage ponds and if necessary pumped back to the TSF or into the decant or process water ponds. Water from the decant pond is pumped back to the mill for re-use in the process.

Conventional (unthickened) slurry from gold tailings typically contains <45% solids (Williams and Jones, 2005). Partially removing water prior to discharge produces a thickened discharge (~60% solids) which still flows as a slurry, but behaves differently when deposited in the TSF (e.g. forming a natural beach slope suitable for the CTD system – DMEWA, 1999). This helps limit the amount of free water available to pond on the surface of the TSF, hence reducing its area, depth and the time taken for it to drain away. Water extraction can be carried further to form a paste, which is then deposited by a technique known as dry stacking, with loss of water from the TSF by evaporation and little or no generation of decant water (DMEWA, 1999; Norman and Raforth, 1998). Water extraction can be carried further to produce a filter cake (>75% solids - Williams and Jones, 2005).

19.Concentrations of cyanide and related substances in TSF tailings

Concentrations of cyanate and thiocyanate have been investigated by Staunton (1991b-d) at three gold mine TSFs operating in the early 1990s (Table 5.). As indicated in Table 5., analyte concentrations in tailings, TSF pore waters from shallow cores (0-2 m) and decant and reclaim waters varied widely within and between operations. Measured cyanate and thiocyanate tailings concentrations ranged up to 320 and 610 mg/L, respectively. Free and WAD CN concentrations in tailings ranged up to 210 and 340 mg/L, respectively. However, generally cyanide addition is more carefully controlled in recent years, so these numbers may be larger than average numbers at the present time (Staunton, pers. comm. 2005).

Tailings constituents may vary temporally within the one operation. For example, monthly cyanide (free, WAD, total) monitoring data collected from the discharge point from a tank mill process leading to a TSF were available for this assessment from a gold mill in Western Australia. The samples were collected between 1995-2002. In summary, the data (73 samples) indicated free CN contributed between 3%-100% of the WAD CN concentration and WAD CN contributed between 34%-100% of the total CN concentration. Average tailings concentrations were 71 mg/L (range 2.9-190, SD31) for free CN, 94 mg/L (32-220, SD30) for WAD CN and 122 mg/L (42-230, SD31) for total CN. In 2002, free, WAD and total CN concentrations averaged 58 (5.1-100), 84 (35-120) and 151 (87-180) mg/L, respectively. However, data for free CN concentration should be considered with care, as there may be significant uncertainty in accuracy due to changes in the form of cyanide present after sampling and methodology difficulties (R. Schulz, Environmental Chemist, Consultant and Auditor, pers. comm. 2006).

Table 5.. Tailings, TSF pore water (0-2.0 m), decant and reclaim water concentrations (mg/L) of cyanate, thiocyanate and cyanide forms (after Staunton, 1991 b-d)

Mine/Material	Free CN	WAD CN	Total CN	Cyanate	Thiocyanate
Tailings pulp	29-76	320-340	510-700	51-320	540-610
Tailings disch. Pt.	120-210	130-230	130-250	7.5-110	5.4-9.3
Tailings disch. Pt.	65-72	78-87	83-97	44-48	13-16
Tailings (fresh) Surface 1 day Surface 1 week	120 9.5 50	130 6.5 ---	130 37 170	110 180 ---	9.3 21 75
TSF (pore water) cores (0-1.0 m)	<0.2-39	1.8-58	11-120	94-110	8.9-19
TSF (pore water) cores (0-1.6 m)	0.32-290	<0.3-630	0.32-1300	<30-2100	28-2100
TSF (pore water) cores (0-2.0 m)	0.8-38	1.6-34	8.4-580	24-150	<15-160
Decant solution	110	90	110	---	55
Reclaim dam	18	8.5	19	200	22
Return water	27	35	43	63	12

• 
  Sites using processes such as the Inco process (Section Error: Reference source not found) to ‘kill’ cyanide to very low levels before release of the tailings, resulting in WAD CN concentrations in the tailings and in dams typically <1 mg/L.
  
• 
  Sites which generally or always meet a 50 mg/L WAD CN limit for the protection of wildlife and livestock other than aquatic organisms, as may be required by state/territory agencies and as recommended under the International Cyanide Management Code (ICMC - Sections 86.1.1, 88.2.4). This may be achieved through monitoring of the process and tailings streams and steps such as adjusting the concentrations used in the process, dilution, or if necessary, treatment with detoxifying agents (e.g. metabisulphite, ferric sulphate or Caro’s acid). In some cases lower levels (e.g. 30 mg/L) were targeted to meet limits set by state/territory agencies (e.g. NSW – Section 84.1.1) or company policy, and these were achieved in decant water, if not also in tailings discharge.
  
• 
  Sites which frequently exceed the 50 mg/L WAD CN limit, usually having WAD CN in the 50-100 mg/L range and at times higher. These may be situations where detoxifying agents are not used and/or where ore and water characteristics make it more difficult, to achieve a 50 mg/L target (e.g. where the process is carried out at pH ~9.2 rather than 10.5 – Section 5.2.2, and/or where significant copper concentrations are present [Environment Australia, 1998]).
  
• 
  One particular site, where mean WAD CN at the tailings spigot and in the decant pond was well above 100 mg/L, due to high cyanide concentrations required for the type of ore treated. Rather than seeking to lower WAD CN to safer levels, the approach taken at this site has been to adopt other measures, including netting of the decant pond.
  
• 
  Sites where NaCN is not used - where gold-rich concentrate is produced for processing elsewhere and no cyanide is present in tailings.
  

The Western Australian Department of Industry and Resources (WA DoIR) indicated that in the past companies have usually discharged at 50-100 mg/L WAD CN, which reflected a balance between having sufficient CN in solution to extract the gold from the ore without discharging excessive CN to the TSF (this may have altered where the ICMC has been adopted). With current standards required by state/territory regulators and progress in adoption of the ICMC, it appears that an increasing number of mines are seeking to limit the concentration of WAD CN in discharge with a view to meeting limits of 50 mg/L WAD CN or lower, except for mine sites which are hypersaline (see Section 66.1.2).

It is important to be aware of where samples for analysis are taken, as data generally show a clear decline in WAD CN concentration away from the discharge point. The ICMC 50 mg/L limit does not apply to open-topped process tanks and vessels such as CIL tanks, but at the spigot discharge point or other areas where wildlife may seek access to tailings solutions. There may be exceptions for specific sites where scientific argument can be presented to argue for a higher limit to apply at certain points without adverse effects to wildlife, e.g. in the vicinity of the spigot discharge point (see Section 66.1.2).

19.1.1Flotation use in base metal ore processing

Sodium cyanide (or other cyanide salts, such as calcium cyanide) is used in sulphide mineral flotation circuits as a depressor for sulphide minerals, selectively suppressing them from floating in the froth when they would otherwise do so (Environment Australia, 1999a-d; Dr D. Muir, Parker Centre/CSIRO Minerals, pers. comm. 2005). For example, cyanide is used to suppress pyrite, pyrrhotite and marcasite (iron sulphide minerals) in the presence of other base metal sulphides (e.g. copper, zinc and lead), and arsenopyrite and enargite (arsenic-containing minerals) in the flotation of nickel minerals. In the case of iron sulphides, the cyanide acts by forming a passivating layer of ferrocyanide or ferricyanide on the iron sulphide surface, which inhibits xanthates adsorption in the subsequent flotation. Depression occurs at approximately neutral pH values of 6.5-7.0 (Furesteneau 2007b). Sodium cyanide is most commonly used but it is possible for other base metal complex cyanides (such as zinc and copper) to be used instead. Sodium cyanide may react with copper and zinc minerals first and then iron minerals.

Emission estimation technique manuals for the National Pollutant Inventory (Environment Australia, 1999a-d) indicate that the scale of use of inorganic cyanides (i.e. not necessarily NaCN only) at most facilities within the nickel concentrating, smelting and refining industry is unlikely to trigger the reporting threshold (i.e. 10 tonnes/annum), whereas reporting of cyanide emissions to the NPI will be triggered at most zinc, copper and lead concentrating, smelting and refining facilities (note that cyanide may instead be used for gold and potentially silver extraction from the ore, particularly where copper ores are processed). Emissions from these industries may potentially occur from chemical storage and mixing areas due to spills and leaks at process areas and from TSFs following process solution disposal.

Typical dosages for base metal flotation are 50-250 g/tonne added to the mill, and the amount added is normally consumed in the process (Muir, pers. comm. 2005). Hence dosing levels may be at similar levels to those used for gold cyanidation (of the order of 100-500 g/tonne of ore), but with cyanidation a large excess of cyanide is used, leaving a high percentage of unconsumed free cyanide. There may also be significant differences in the fate of the cyanide added between cyanidation and various flotation processes because the pH conditions and other characteristics of the processing streams differ significantly from those used for cyanidation (e.g. the waste stream may be acidified in steps subsequent to the use of cyanide as a suppressor).

Rates indicated by Von Michaelis (1984) for flotation use at Canadian base metal mines (zinc, zinc/copper, lead/zinc/copper, copper/molybdenum/others) were 0-25 g/tonne (0-0.05 lb/ton) in most cases, and 50 g/tonne (0.1 lb/ton) in one case. The author noted that cyanide used in selective flotation processes is carefully controlled by metering in weak solutions to selectively react with the surface of a small portion of the ore. Hence probable concentrations of complex cyanides in tailings were estimated to be 0-25 mg/L (ppm), or 35 mg/L at the zinc/copper mine with the highest addition rate (presumably this refers to total CN, as forms not included in WAD CN may be present, such as ferrocyanide). As some cyanides are oxidised to thiocyanates and cyanates in the flotation circuit, these estimates were considered likely to be high.

Higher concentrations of free cyanide remained in the tailings stream at a zinc-lead mine in South Africa, where free CN concentrations at the lead tailings thickener and concentrate thickener were, respectively, 53 mg/L and 60 mg/L (note that the recovered water was recirculated back to the ball mill and flotation plant). These concentrations were stated to be much higher than the amount required for depression of sphalerite (zinc blend, i.e. zinc iron sulphide), but there were difficulties at this site due to copper in the ore causing activation of the sphalerite (Coetzer et al., 2003; Seke, 2005).

Data for five lead-zinc mines in the USA showed the ‘raw waste load’ of cyanide was 0.013-0.109 g/tonne of ore milled, with the range of cyanide concentration (presumably free CN) in wastewater being 0.01-0.03 mg/L (USEPA, 1994b). USEPA (1994b) noted that another mine and associated mill had consistently exceeded the discharge limitations for cyanide, total soluble solids and heavy metals (the relevant concentrations were not stated). Discharge from the tailings pond at this site (which received all mine and mill waste waters and discharges from process area drains) had been shown to be toxic to aquatic life, but there is too little evidence to ascertain whether this was due to cyanide.

Thus material disposed of to the TSF evidently usually includes little or no residual free cyanide and consists of heavy metal cyanides and cyanide complexes such as ferrocyanide, though in some situations a significant proportion of free cyanide may be present. No actual Australian data were found for free, WAD or total cyanides in used process solution or in TSFs at such sites. Worst case concentrations of total CN would correspond to the input rate (i.e. ~0-250 mg/L), but these are likely to be somewhat reduced by degradation to forms such as cyanate and thiocyanate. The formation of HCN and volatilisation is also likely to be a more prevalent pathway for dissipation of CN from TSFs than that of gold processing TSFs. This is due to the pH at which depression of sulphide minerals occurs being lower than that for gold beneficiation. Although the subsequent flotation process may occur in a wide range of pH conditions (2-12), sulphidic mine wastes tend to become acidic with the oxidation of sulphide minerals primarily from pyrite and pyrrhotite forming sulphuric acid (Trefry et al. 2008).

The use of sodium cyanide in flotation in base metal ore processing presents similar issues to those presented by the use of sodium cyanide in gold mining, but in Australia the total amount of cyanide used for flotation is significantly less and other salts of cyanide may be used instead of NaCN. The information available suggests that the quantity of cyanide used at individual sites is relatively low compared to gold mine use and that concentrations of free or WAD cyanide in the tailings stream are also low compared to those commonly occurring at gold mining facilities.

19.1.2Electroplating, metal cleaning and metal surface treatment

20.Electroplating

Release of HCN from the electroplating solution during use is discouraged by the alkaline pH of the solution. Used electroplating solution is treated immediately with processes such as hypochlorite or chlorine to destroy residual cyanide. The pH is then lowered to precipitate metal hydroxide complexes and after flocculation the waste is filtered to separate the solid and liquid waste. Safety requirements for humans dictate various measures to avoid acidic solutions coming into contact with cyanide solutions prior to cyanide destruction, to avoid generation of HCN gas (NOHSC, 1989; World Bank Group, 1998). The solid waste is sent to landfill and/or recycled, and the liquid waste recirculated as far as possible or released to the sewer, under trade waste agreements with sewerage system utilities, landfill operators and licenses authorised by state and territory environment protection agencies.

21.Metal cleaning

22.Iron and steel surface treatment

Various heat treatment processes (carburising and carbonitriding [case hardening], nitrocarburising, and commercial processes based on these techniques, such as the Tufftride salt bath nitrocarburising processes) use molten salt baths containing NaCN and an alkaline earth salt (e.g. barium chloride) for the surface hardening of iron and steel (CHTA, 1996; Bull and Page, 2000). Depending on the type of process and end result required, this may occur at ‘low’ (560-720°C - nitrocarburising) or ‘high’ temperatures (800-940°C – carburising and carbonitriding), under various atmospheres, and with or without quenching or other steps in the overall process. There are alternative carburising, carbonitriding and nitriding treatments that do not use molten salt baths and cyanide.

These methods obtain their surface hardening effects through the diffusion of carbon and/or nitrogen into the surface of the metal to depths up to several millimetres. In the molten salt bath carburising process, cyanide is consumed through oxidation to cyanate, which dissociates at the steel surface to form CO and results in impregnation of the metal by C and N (Bull and Page, 2000):

4NaCNO  2NaCN + Na₂CO₃ + CO + 2N_Fe

2CO  CO₂ + C_Fe

There are both liquid and solid wastes from these processes. Solid residue from the baths is re-used and topped up, but ultimately needs to be disposed of due to the accumulation of sodium cyanate and residues from other components. Liquid wastes are generated from quenching of the treated metal and from washing out of vessels etc. Waste from these processes is generally collected by professional disposal companies, treated as necessary and disposed of or recycled under licenses authorised by state and territory environment protection agencies (J. Rea, Contract Heat Treaters’ Association of Australia (CHTAA), pers. comm.. 2005). The likely reason for collection rather than on-site destruction is the generally smaller scale of heat-treating operations compared to electroplating facilities (H. DiSouza, Orica Chemnet, pers. comm., 2005).

22.1.1Other uses

Relatively small volumes of NaCN, generally in small containers, are used for various purposes in laboratories. Advice to users of NaCN and other cyanides in laboratory situations emphasises various precautions because of the risk of HCN generation or potentially violent reactions, including avoiding contact with incompatible chemicals and using under a fume hood. Recommendations for dealing with spills, cleaning and disposal are based on steps such as washing with sodium hypochlorite solution, making solutions alkaline and adding excess ferrous sulphate solution to complex the cyanide into ferricyanide, or oxidising waste to the cyanate by adding potassium permanganate. Thus, according to laboratory protocols, cyanide residues in laboratory waste and spills should be treated prior to disposal. Disposal recommendations for the treated waste include discharge to the sewer (in ample water, for small quantities of waste and with any necessary permission from local Authorities), or by sealing in a container for storage then removal by a licensed waste contractor.

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