 Commonwealth of Australia 2010

6.Cyanidation and other techniques of beneficiation

• 
  cyanidation (the most commonly used process today - the original MacArthur-Forrest Process was patented in 1887);
  
• 
  flotation (chiefly used on ore that is finely disseminated and contains small quantities of gold in association with base metals – USEPA, 1994);
  
• 
  amalgamation with mercury (was used from the late 19th to the middle of the 20th century in Australia and is still used by artisanal or small scale miners in some countries, but is inferior to cyanidation for large scale mining and presents major environmental and health issues – Dhindsa et al., 2003; USEPA, 1994); and
  
• 
  gravity concentration (placer/nugget mining).
  

Various alternative lixiviants to cyanide have been used or investigated, as have microbiological beneficiation methods for removing gold from ore (e.g. Yestech, 2002). However, there are considerable technical difficulties to be overcome before even the most promising alternative lixiviant (thiosulphate) can become practicable, and significant changes to other processes, such as gold recovery from solution, may be needed (Ritchie et al., 2001; Nicol and O’Malley, 2001). Some alternatives are only suitable for certain types of ore, e.g. acid thiourea, which is unsuitable for use in soils with soluble iron and clays, as typical of Australia (Chemlink Consultants, 1997). The various alternatives also have their own significant environmental and health hazards. In several cases, alternative lixiviants are required in greater concentrations than cyanide, possibly making the resulting hazard greater despite lower absolute toxicity, in some cases potentially mobilising other metals at the pHs used, and some may leave long-lasting residues (McNulty, 2001; Young, 2001; Gos and Rubo, 2002; Environment Australia, 1998). Further investigation of the use of thiosulphate is continuing under a project at the Parker Centre in WA (AMIRA P420C Gold Processing technology)^a.

Cyanidation is the process of using cyanide solutions to dissolve gold. In this process, ore is exposed to cyanide solution, and the gold dissolves by forming the cyanoaurite ion. The gold cyanidation process may be represented by Elsner’s equation:

4Au + 8CN^- + O₂ + 2H₂O  4Au(CN)₂^- + 4OH^-

or alternatively (which of these equations is correct is not absolutely agreed) by Bodländer’s equation (AMIRA, 1991a, Habashi, 1998; Heath and Rumball, 1998):

2Au + 4CN^- + O₂ + 2H₂O  2Au(CN)₂^- + H₂O₂ + 2OH^-

As shown by these equations, the process requires oxygen. The reaction is generally considered most effective at a pH of 9.5-11, with the optimum being pH ~10.5 to maximise the proportion of free cyanide present as CN^- present, without reaching more basic conditions which tend to slow the process (USEPA, 1994). However, practices may vary according to mineral content of ores. For example, it is common in many Western Australian mines in the Eastern Goldfields to use a pH of ~9.0-9.2 due to the large amount of magnesium (Mg) present in these ores or hypersaline groundwater: buffering effects mean that elevating the pH above this would consume large amounts of lime to form Mg(OH)₂ before the pH could rise further (W. Staunton, Parker Centre/Division of Science and Engineering, Murdoch University, pers. comm. 2005).

In practice, a large excess of cyanide is used and the amount which needs to be added per tonne of ore varies widely. Adams et al. (2008a) state: ‘Cyanide dosing strategies will necessarily change over time as different parts of the ore body, containing different sulphide and gangue mineralogy, are mined. Ores containing high soluble copper generally require higher cyanide dosages to ensure that sufficient free cyanide is available in solution for efficient leaching to occur. Ores that are high in silver often also need higher cyanide dosing to account for the slower leaching silver minerals. … The nature of metallurgical process plants is … such that these dynamic systems are constantly varying, and this will influence the cyanide and other species and levels that are presented to tailings.’

Using data obtained from company reports for gold mines in various countries over the period 1991-2006, Mudd (2007) related cyanide consumption (kg CN/kg gold produced – presumably actually referring to NaCN) to gold grade (g Au/t ore), in a paper discussing sustainability issues in gold mining. Overall average cyanide consumption from the data obtained was 141 kg CN/kg gold. However, for high grade mines (> 6 g Au/t ore) cyanide consumption was commonly < 100 kg CN/kg gold, while for lower grade mines (< 2 g Au/t ore) the consumption of cyanide tended to increase as the grade declined, reaching as high as ~1000 kg CN/kg gold. Mudd (2007) fitted a power regression which showed a general trend of increasing cyanide consumption as gold grade of the ore decreased (y = 332.03x^-0.8725, r² = 0.5468, >50 data points). However, some caution is needed in interpreting this relationship, as inspection of the plotted data shows that the amount of cyanide used is clearly affected by factors other than the gold grade. At low grades: cyanide consumption at ~1 g Au/t ranged widely, from <50 kg CN/kg Au to ~1000 kg/kg Au; and cyanide consumption at a few individual sites at ~3-5 g Au/t ranged well above other sites, from ~400 kg CN/kg gold up to almost 1250 kg CN/kg gold. Mudd (2007) argued that as there is a declining trend in ore grades in the countries he reviewed (Brazil, South Africa, Canada and the United States – average grades in Australia in 2003 = ~2 g/t), average cyanide consumption per kg gold is likely to increase gradually in the medium term. Mudd (2007) also discussed the similar implications of ore grades for energy and water consumption and greenhouse gas production. He noted that many companies now publish sustainability reports which give details of water and energy use, but few also include cyanide consumption. He suggested that ‘to improve transparency in the gold mining sector and demonstrate continuing evolution in sustainability performance, there is a strong case for reporting of cyanide.’

Recovery processes used with cyanidation

There are two main types of processing technique which are currently used to recover gold when cyanidation is used for ore beneficiation:

• 
  cyanidation-zinc precipitation (or zinc cementation, the Merrill-Crowe process), where the gold is precipitated with zinc dust – this older technique has been replaced by carbon adsorption in many cases, but is better suited to gold ore containing large amounts of silver; and
  
• 
  cyanidation-carbon adsorption, where the gold is adsorbed onto activated carbon – this is the most commonly used method.
  

With cyanidation-zinc precipitation, zinc displaces gold in the sodium cyanoaurite and precipitates the gold. Steps involved in this include leaching, clarification (filtering), deaeration (oxygen inhibits gold recovery) and precipitation, followed by recovery by smelting the gold-containing material after filtering and sometimes other treatment. The barren solution that remains can be chemically treated (neutralised) or regenerated and returned to the leach circuit. Zinc precipitation wastes include filter cake material and spent leaching solution that is not returned to the leaching process. These may contain cyanide residues as well as zinc and lime and are typically disposed of with tailings (USEPA, 1994).

Cyanidation-carbon adsorption involves four steps: leaching, loading, elution, and recovery. The activated carbon can be regenerated and re-used numerous times by acid washing and reactivation in a kiln. Small amounts of cyanide residues are present on the carbon after the elution process, but this is likely to be released and converted to ammonia (NH₃) under the conditions in the reactivation kiln (Staunton et al., 2003). Up to 10% of the carbon may be lost during each cycle through abrasion to suboptimal size, ashing, or incidental loss. Waste carbon fines and acid wash may contain some residual base metals and cyanide. Most operations capture suboptimal size carbon particles for recovery of additional gold via incineration or concentrated cyanide leaching. The acid wash solution is treated and recirculated or disposed of in the general tailings stream (USEPA, 1994).

Ion exchange resin (Resin-in-Pulp) is a new processing technology which could be a future alternative to carbon adsorption. Other very uncommon techniques used to separate and recover gold after cyanidation include solvent extraction and direct electrowinning (USEPA, 1994).

In general, there are two basic types of cyanidation-carbon adsorption operations, tank (mill) leaching and heap leaching (Logsdon et al., 1999; USEPA, 1994). These are described further below.

Figure 4.2 presents a simplified and generalised flow diagram of gold ore beneficiation and processing technologies used with tank and heap leaching in Australia (adapted from DEWHA, 2006). As indicated in Figure 4.2 for tank leaching sodium cyanide is added during the cyanidation process to the process ore slurry to promote the dissolution and complexing of the gold to cyanide. The pH of the slurry is typically raised using lime to minimise cyanide losses due to volatilisation as HCN. Typical concentrations of cyanide used with tank leaching are 100-500 mg CN/L.

Tank leaching with activated carbon may include one of two distinct types of operations, Carbon-in-Pulp (CIP) or Carbon-in-Leach (CIL). In CIP operations, the ore pulp is leached in an initial set of tanks with carbon loading occurring in a second set of tanks. In CIL operations, leaching and carbon loading of the gold occurs simultaneously in the same set of tanks. Most gold mining operations in Australia currently use tank leaching with CIP or CIL.

Sodium cyanide is also added during the carbon loading (e.g. CIP or CIL) process. CIP involves the removal of complex gold ions from solution by adsorption onto activated carbon. The slurry that has undergone cyanidation is passed through a cascade of agitation tanks. As the slurry moves down the cascade, gold is adsorbed onto granular activated carbon, which is then retrieved and processed. The main difference between CIP and CIL is that in the CIL process the cyanidation and adsorption processes are not staged separately, so that gold dissolution and recovery from the slurry proceed simultaneously in each CIL tank.

Sodium cyanide is also added during the elution process, which involves the washing of gold-loaded charcoal in hot water, caustic soda and cyanide solution to remove the gold from the washing liquor. The resulting liquor is known as pregnant solution. Gold is recovered from the pregnant solution by electrowinning (electrodeposition on a steel wool or copper cathode) and subsequent smelting, or by zinc precipitation (as described above) (USEPA, 1994).

The residues from tank leach operations are deposited into tailings storage facilities (TSFs), as discussed in Section 17.1.3. The potential fate of cyanide in TSFs is described in Section 23.6.

Heap leaching

The heap leach process is an alternative cyanidation process for relatively low-grade gold bearing ores (also shown in Figure 4.2). Heap leaching is also used to extract copper from ore using sulphuric acid. Heap leaching has been used in relatively few gold mining operations in Australia, and then often as a secondary treatment (e.g. re-treating old tailings piles), whereas it has been extensively used in the USA. Reasons for low use in Australia include the following (Staunton, pers. comm. 2005):

• 
  recovers a lower proportion of the gold present and there is a prolonged delay (several weeks or months) before gold production starts;
  
• 
  Australian producers build tank leach plants relatively cheaply;
  
• 
  Australian ores are typically high in clay, making agglomeration treatment necessary to achieve satisfactory permeability;
  
• 
  in many cases the more amenable surface ores have been mined already and subsurface ores are more refractory.
  

Four types of leach pads are identified (Thiel and Smith, 2003):

• 
  conventional (flat) pads: constructed on relatively flat land, either graded smooth or with terrain contouring. Ore is stacked in thin (5-15 m) lifts;
  
• 
  dump leach pads: similar to conventional but constructed on undulating terrain. Ore is stacked in thicker lifts (~50 m);
  
• 
  valley fills: constructed in valleys with a buttress dam at the bottom of the valley or a levelling fill within the valley; and
  
• 
  on/off (dynamic) pads: constructed on flat ground using a robust liner and overliner system, thin lifts (4-10 m thick) are loaded and leached, with the leached (spent) ore (‘ripios’) removed for disposal and the pad recharged with fresh ore. Usually loading is automated using conveyors and stackers.
  

Lime is usually added to the cyanide solution to curtail the loss of cyanide through volatilisation of HCN. As the reaction is oxygen dependent, the solution is oxygenated prior to or during spraying. The solution slowly infiltrates through the ore heap, dissolving the gold into solution. The leaching process may continue for several weeks or months. The dissolved gold in solution collects in surface drains and sumps at the edges of the heap and is pumped to a lined holding pond or tank (pregnant pond) and from there onto gold recovery (Figure 4.2).

After the gold is extracted the remaining solution is moved into another lined pond, which is commonly called the barren pond. The cyanide concentration in this pond may then be increased so that the solution is again suitable for use in the leaching process, and the solution is used again on the ore heap. Gold is recovered from the pregnant solution generated from heap leaching using adsorption in a series of columns containing activated carbon (Carbon-in-Column), or by zinc precipitation (USEPA, 1994a; USGS, 1999).

After the leaching cycle for soluble gold residues has been completed, the spent ore and remaining cyanide solution become wastes. During the decommissioning process, the heap is typically rinsed with mine water or mill waste water to leach the heap of remaining soluble cyanide residues. This may continue for some time until testing shows acceptably low levels of cyanide, and after other reclamation steps the heap may then be left in place. Alternatively (e.g. where there are impermeable sections of ore or the leach pad area is to be reclaimed), the heap may be dismantled and the ore treated in batches (USEPA, 1994c).

To enhance percolation of the lixiviant through the heap, a process called ‘agglomeration’ may be used to aggregate small particles together and prevent fine particles blocking pores. This typically involves crushed ore being mixed with portland cement and/or lime, wetting the ore evenly with cyanide solution before the heap is built, and mechanically tumbling the ore mixture so fine particles adhere to the larger particles. This improves the efficiency of extracting gold from the heaped ore and may increase the rate of flow of cyanide solution through the heap by a large factor (e.g. 6000 fold), decreasing the overall leaching time needed (USEPA, 1994). It is also a means of reducing the duration and extent of surface ponding of cyanide solution on the heap. However, there are additional costs involved and potential problems if the treated material should de-agglomerate.

The potential fate of cyanide in heap leach operations is described in Section 26.1.

Figure 4.. General gold ore processing: cyanide inputs and potential emissions (adapted from DEWHA, 2006)
Sulfide conc.

ammonia

NaCN

Mixed ores

Cyanide in various forms

HCN

HCN

HCN

Sulfide ores



Calcine

Oxide ores



Sulfide

NaCN

7.Scale and distribution of the Australian gold mining industry

Gold production

Most of the gold produced in Australia in recent years has been obtained using sodium cyanide beneficiation. Data for the amount of gold mined therefore give a broad indication of the likely relative amount of sodium cyanide used, except for the few mines in that period where gold beneficiation occurred without the use of cyanide, and noting that the amount of cyanide which needs to be added per g of gold or per tonne of ore varies widely (see above). These data (Figure 4.) follow a broadly similar pattern to data for the numbers of gold mines and gold operations discussed below.
Figure 4.. Gold production in Australian states and territories during the years 1999 to 2008 (Source: ABARE, 2002-2009)

Number of gold mine operations and tailings dams

Estimates of the number of gold mines operating in Australia are not completely accurate and are subject to variation due to mine closures and openings. However, available data in 2005 (former GeoScience Australia website; K. Porritt and M. Huleatt, Minerals Resources and Advice, Geoscience Australia, pers. comm.. 2005) indicated a total of approximately 122 mines operating in Australia had gold as their most significant commodity (some of these also produced other minerals, such as silver or copper, or in a very few instances zinc, lead or molybdenum). The great majority of these mines were in Western Australia (95 in WA, 8 in Queensland, 7 in NSW, 6 in the Northern Territory, 3 in Victoria, 2 in Tasmania and 1 in South Australia). There were an additional 17 mines where gold was considered a secondary or very minor commodity (e.g. where copper is the principal commodity). The number of operating mines in WA has fallen significantly since then: Geoscience Australia (2009) indicates that in early 2009 there were 97 mines with gold as the most important commodity (69 in WA, 9 in Queensland, 6 in NSW, 6 in the Northern Territory, 4 in Victoria, 2 in Tasmania and 1 in South Australia). There were a further 12 mines where gold was a secondary commodity (4 in WA, 7 in Queensland and 1 in NSW), and 10 mines where it was only a minor commodity.

These data refer to individual open pits or underground mines, whereas a gold processing facility may treat ore from more than one pit successively or concurrently, or may re-treat existing tailings heaps. A few gold processing facilities do not use cyanide to produce gold, e.g. because they only use gravimetric or flotation processes to separate gold or produce a gold-rich concentrate. Mines where gold is a secondary commodity may or may not use the cyanide process – e.g. cyanide was used for the initial, oxide ore phase at Northparkes, but not in the second stage.

Thus the number of sites using sodium cyanide for ore beneficiation is significantly less than the number of mines indicated above. Estimations from GeoScience Australia (Huleatt, pers. comm. 2005) are that in the March Quarter of 2005, there were 51 primary gold processing facilities, and gold was produced as a by-product from another 12 operations.

Mines producing other metals, such as zinc, lead and nickel, may use NaCN in their flotation circuit, but in relatively minor quantities compared to gold processing facility use.

Depending on its size and age, a gold processing facility may have multiple active or decommissioned tailings storage facilities (TSFs) associated with it, typically 1 to 3. The Western Australia Department of Industry & Resources estimates that there are approximately 370 TSFs in the state (i.e. for all minerals, not just gold), of which ~200 are active, with 15 new proposals current at the time (2004). The Northern Territory Department of Business, Industry and Regional Development estimates that there are 50-150 tailings dams in the Northern Territory. Estimates of the number of TSFs which are active or recently active in other states total at least ~75. However, based on the estimated number of gold mining operations above, there would be ~50-150 TSFs receiving cyanidation wastes at present in Australia.

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