Table of Contents
Process Selection Trends
A general trend in precious metals extraction technology has been the treatment of ores with progressively lower grades. The lowest cost treatment, dump leaching of material at run-of-mine size, has been increasingly applied. For example, Zortman-Landusky (Montana) is successfully leaching material above a cutoff grade of 0.2 g/t (0.006 oz/ton) at an average rate of about 28,000 mtpd.
The trend to treat lower grade ores also extends to grinding applications. RTZ’s Ridgeway mine in South Carolina processes 15,000 mtpd of ore grading 1.0 g/t (0.029 oz/ton) above a 0.6 g/t (0.019 oz/ton) cut-off grade by grinding, agitated cyanide leaching and carbon adsorption (CIL). This is believed to be the lowest grade, stand-alone gold ore milling circuit in the world.
Both Homestake McLaughlin (California) and Round Mountain (Nevada) have installed semi-mobile gyratory crushers. Each crusher is fed by an inclined apron feeder. This allows the dump pocket to be at the same level as the crusher, reducing overall height, as shown in Figure 1. Portable crushing systems have been used successfully in a variety of applications and are particularly effective for crushing of ores in small heap leach operations.
Paralleling successful applications in the copper industry in North America, the Waterflush crusher is presently being utilized at two North American gold operations, both at Amax Gold properties: Sleeper (Nevada) and Hayden Hill (California). In both applications, a fine crush is achieved, typically less than 6 mm, and the crushers are efficient; replacing what would normally be a conventional rod mill Power consumption remains attractive, while wear costs are still being evaluated.
Gravity concentration continues to be installed in grinding circuits to treat ores that contain free, and particularly coarse, gold in an effort to recover gold as early as possible in a flowsheet, thereby reducing the demands on downstream processes and helping to maximize gold recovery. If the correct equipment is used, gold particles as fine as 10 microns can be recovered effectively by gravity concentration, as can gold associated with heavy sulfide minerals.
The Knelson concentrator, shown in Figure 2, uses centrifugal force to assist in the density separation. It has proven to be effective in a number of applications, including the treatment of sluice concentrates at Les Mines Camchib (Canada), and the recovery of gold from base metal flotation circuits by Noranda (tested at various operations in Canada).
Flotation of gold, either as free gold or in association with sulfides, is once again common and will continue to gain favor as a preconcentration method ahead of oxidation circuits. Homestake McLaughlin has installed a flotation circuit to concentrate gold and gold-bearing sulfides from their oxide circuit. The concentrates are stored and used to regulate the sulfide sulfur content of refractory material feeding to their autoclave circuit.
Column cells have been installed in gold recovery circuits at the Austin Gold Venture (Nevada), Asamera’s Cannon mine (Washington) and at Echo Bay McCoy (Nevada). In all cases the column cells were used to clean a rougher flotation concentrate.
Five years ago only a few roasting operations existed for treatment of refractory gold ores in North America, i.e. Giant Yellowknife (NWT), Campbell Red Lake (Ontario). The roasters at Campbell Red Lake have since been shut down and replaced with pressure oxidation to treat the arsenic-rich ore. At the Con Mine (NWT), a pressure oxidation circuit was recently commissioned to treat the arsenic trioxide product of earlier roasting operations. A major advantage of pressure oxidation is that the arsenic is precipitated as a stable ferric arsenate, provided the Fe:As ratio is kept sufficiently high above about 4:1. Unlike arsenic trioxide, this product can be disposed in tailings impoundents.
The higher oxygen content of the system improves temperature control, which in turn results in lower overall roasting temperatures and higher overall recoveries. The counter-current movement of the ore and the gas stream is greatly reduced as a result of the oxygen roast versus an air roast, which greatly improves the ability of the system to fix sulfur in the bed. Stack emissions are well below the regulatory standards.
The roaster at Cortez is the first circulating fluid-bed roaster for whole ore. Initially, roaster operations were hampered by problems in the dry grinding circuit and downstream from the roaster. Once the grinding problems were overcome, continuous roaster operations were adversely affected by a finer particle size distribution than originally expected. Plants which autoclave concentrate, such as Campbell Red Lake, are recycling autoclaved solids for dilution of sulfide sulfur to below 6% in order to prevent elemental sulfur formation in the autoclaves. For low-sulfide ores, heat conservation by slurry preheating is typically required, using the “Flash-Splash” technique. Temperature limitations on high-pressure diaphragms have been overcome at Mercur with the installation of check valves away from the pump, so that the pump is working on a lower temperature “tee” as shown in Figure 4.
Whole ore plants suffer from the high energy costs associated with oxygen dispersion. The authors remain bullish on the bio-oxidation of concentrates and if a commercial plant is operated in North America it is likely to be on concentrates. Successful bio-oxidation of concentrates is occurring in Brazil, South Africa and Australia. While it may be an easy explanation, it is difficult to believe the potential success of bio-oxidation is a phenomena associated with the southern hemisphere.
Agitated and Vat Leaching
Vat leaching is little used in North America, principally because of the favorable climate for heap leaching, but also reflecting a general trend in gold metallurgy worldwide. However, Homestake’s Lead operation in South Dakota still uses vats that have been in use for the best part of a century. In 1992, the Lead plant increased throughput by injecting oxygen into the vats and by installing vacuum-assisted drainage. The former increases gold dissolution rate while the latter reduces drain time. Both modifications are expected to significantly reduce batch leaching cycle times below the 150 hours previously experienced and increase throughput from the mid-1991 rate of 6,700 mtpd to a target of 7,200 mtpd.
Since the introduction at Coeur d’Alene’s Rochester Mine (Nevada), drip tube irrigation has become the accepted practice for most heap leach operations. The merits of drip irrigation include improved percolation rates, better solution flow control and distribution, and improved productivity and safety during winter operations. However, all this gain is not without some cost, since the drip emitters are more susceptible to scaling problems in the summer. The tubes act as solar collectors thus elevating the solution temperatures and enhancing scale formation. The emitters are also less forgiving of the fine silts and solids accumulation from ponds which can be stirred up due to fluctuating pond levels or rainfall events. Various drip emitter designs and systems, at varied costs, are available as our industry borrows from the agricultural industry; T-tape, recently introduced to the mining industry at Chimney Creek and Mesquite, is reducing material costs by almost 65% percent. The successful application of drip tube has also allowed leaching at very low application rates down to 0.0007 gpm/ft². This can be most beneficial for the treatment of high clay ores.
At Amax’s Wind Mountain operation in northern Nevada, low-grade ores were initially treated by crushing and heap leaching, although crushing was gradually phased out of the operations due to favorable leaching response of the run-of-mine ore. Towards the end of the life of the mine all of the material mined from the pit was placed on the leach pad. This was possible due to the close proximity of the leach pad to the pit and the ability to increase pad height, with no adverse effects on the structural integrity of the liner or on metallurgical recovery.
Solution Concentration and Purification
For both milling and heap leaching operations, carbon adsorption has been the preferred method of gold recovery over the last five years, with the notable exception of high-silver ores and a few other specific applications (see Zinc Precipitation section).
In slurry systems, there is a general trend towards the use of carbon-in-leach (CIL) as opposed to leaching and carbon-in-pulp (CIP). This is still a matter of debate in the industry, but many plants which have used CIL have found that lower capital costs from fewer slurry tanks more than offset additional carbon handling as a result of lower carbon loading. This choice must be considered on a case-by-case basis. Interstage screening has continued to receive much attention, with various equipment in use.
Ion-exchange resins have received considerable attention worldwide as a result of the successful application of this technology in place of CIP at Golden Jubilee (South Africa), masterminded by Chris Fleming while at Mintek. In addition, at least two large gold plants, the large Muruntau mine in Uzbekistan and Zod in Armenia, are known to use in-pulp ion-exchange resin systems for gold recovery. Although effective, resin-in-pulp (with the unfortunate acronym “RIP”) requires complex elution and regeneration procedures and, as employed in the former USSR, is unlikely to present any advantages over carbon systems. On the other hand, resins offer some potential technical and economic advantages over activated carbon and these are currently being pursued in the west.
Merrill-Crowe zinc precipitation is responsible for just 9% of gold recovered from grinding and leaching circuits, with the remaining 91% recovered by activated carbon systems. This compares with 40% on a worldwide basis. This reflects the recent development of the gold industry in the USA which has utilized more cost-effective carbon adsorption techniques. Nonetheless, Merrill-Crowe zinc precipitation is still preferred for the treatment of the following ores and solutions:
- Solutions with high-silver content, greater than 10:1 Ag:Au,
- High-grade solutions containing >10-12 mg/l gold equivalent,
- Small operations, recovery less than about 25 oz gold equivalent per day.
In a significant departure from standard electrowinning practice, the Williams Mine, located in the Hemlo district of Ontario, replaced the use of mild steel wool cathodes with knitted stainless steel wool cathodes. Previously, mild steel wool cathodes were smelted with fluxes about twice weekly, with high crucible wear resulting from the need to oxidize large amounts of iron in the furnace. This method was costly and labor intensive because of the need to replace steel wool routinely, the frequent crucible replacement, and the associated labor requirements.
Zinc Precipitation From Hot Carbon Eluates
Recovery of gold from hot, concentrated carbon eluates by zinc precipitation has proven to be popular in several North American operations, notably, Jerritt Canyon, Chimney Creek, Santa Gertrudis (Mexico), and Hayden Hill (California). The method is also widely applied in Canada, particularly in small plants, and has proven to be simple to operate and highly efficient for gold recovery from solution. The advantages over electrowinning are potentially better security, better control of mercury (if present) and high single-pass efficiency for equivalent capital cost.
Since its introduction at Key Lake (Saskatchewan, Canada in the uranium industry, sub-aerial tailings deposition has been applied increasingly in the North American mining industry. Several gold operations have utilized the system, e.g. Sonor, Gold Jamestown, Chimney Creek, Cyprus Copperstone and Bi. Springs (Nevada).
Detoxification of cyanide solutions has received much attention in recent years because of increasingly stringent environmental regulations. The USA and Canada are unsurpassed in their ability to promulgate such regulations, and the industry must react quickly to survive. Many operations in Canada and the USA have been faced with detoxification problems that required quick solutions to avoid serious legal proceedings, with the possibility of curtailed operations. In some cases, quite radical measures such as the installation of very costly reverse-osmosis plants or even more exotic and costly chemical treatment schemes have been considered especially in more strict states, e.g., South Dakota and California. The key to avoiding these problems is to plan for detoxification early in the project development if there is a chance that it may be needed. In addition, adequate emergency solution storage must be provided to allow accumulated solutions (possibly in excess of those projected by standard water balance calculations) to be contained, taking advantage of natural degradation processes.
Several operations are in the process of rinsing leach pads and performing final reclamation. At the REN mine, Relief Canyon and Borealis, all in Nevada, third party contractors have assumed responsibility for reclamation; a trend which may become more popular in the future. Leach pad rinse solutions are typically pumped through adsorption columns containing high activity carbon to recover precious metals and remove other heavy metals. Cyanide levels are effectively reduced by this method, particularly the weak acid dissociable (WAD) complexes of copper, zinc and nickel, and the solution can be returned to the pads for additional rinsing.