Barrick's flagship operation, Goldstrike, will pour its 20-millionth ounce of gold this month. Reaching this milestone required that the operation embrace leading technologies and best practices. The exploration, mining, processing, technical services, and environmental teams are all forward-thinking and resolved to keep this operation among the leading low-cost producers.
Last year Goldstrike produced a record-high 2,452,358 ounces of gold at a cash cost of US$170/oz.
Senior vice-president (United States operations) Greg Fauquier heads an operation that has just come off a banner year because of 1,800 dedicated people who had a vision and wanted to see it succeed. Indeed, the employees we met were enthusiastic and radiated pride in being part of a successful team.
After all, some say that the best job security is to work for a profitable company. Goldstrike certainly contributes toward that goal; this year's budget is 2.28 million ounces of gold.
High grades a di vidend for mining
Barrick purchased the Goldstrike property in 1986 when it was a small heap leach operation. Today the operation comprises the Betze-Post open pit, the Meikle underground mine and the Rodeo underground development project, two different mineral processing plants, and an award-winning reclamation project.
At the end of last year, reserves at the Betze-Post pit contained 18 million ounces of gold.
Moving 1.3 billion tons of material each year, 116.4 million tons of which is ore, has to be done with productivity in mind. Two programs are underway to ensure low mining costs. First is the purchase of larger haul trucks. Second is the backfilling of a portion of the pit, a strategy that shortens haulage distance.
By the end of 2002 the Betze-Post pit will boast a fleet of 32 Komatsu HaulPak 930E (330-ton) trucks, replacing 190-ton vehicles. Not only will there be less than half the number of units, the new trucks can reach speeds of 40 miles/hour, even when loaded. The smaller trucks were run with a trolley-assist grid, which improved their operation but also inflated the cost of running them. The trolley system is gone now, and only 20 of the smaller trucks were in use when CMJ visited in March.
The plan to backfill a portion of the pit benefits Goldstrike by shortening its hauls for 461 million tons of waste rock. The waste is being placed in the eastern end of the pit bottom and will eventually reach a depth of 200 feet in 2003. Currently, Barrick is placing 1.8 million tons of backfill in the pit to assist Newmont Mining Corp. with mining its adjacent Deep Post underground property.
This innovative level of co-operation was made possible because Barrick and Newmont concluded an asset swap two years ago. Both companies gained access to continuous mineralization that allowed them to rationalize their mining plans.
The Betze-Post pit is mined using conventional truck-and-shovel techniques. Blastholes are drilled with Ingersoll-Rand DM45 or DMM2 drills and loaded with Anfo, breaking up to 200,000 tons per shot. Excavation proceeds in 20-ft benches. Overburden is removed by P&H 2800 shovels with dippers of 41- and 44-yd3 dippers. There are two ore shovels, one O&K RH200 33-yd3 hydraulic excavator and one Hitachi EX3500 24-yd3 hydraulic excavator. A Letourneau L1800 loader is also available to rehandle overburden and to load ore. The truck haul to either the autoclave or roaster is about 3.5 miles.
A computerized communications system aids in maintaining productivity. Truck dispatch and tracking includes the type of material in the bed as well as the location of each unit. Vehicles can be sent to the appropriate ore or waste pile, for refuelling or maintenance. The shovels are fitted with global positioning systems (GPS) to ensure the success of selective mining. GPS is also coming to dozers for precision grading of the ramp and reclaim slopes.
Going for High-Grade Underground
North of the open pit, where deeper mineralization was outlined in 1989, the Meikle underground mine is a source of high-grade mill feed. Proven and probable reserves at Meikle average 0.540 ounces per ton (oz/t) Au (the highest of any Barrick mine) compared to the Betze-Post pit with an average grade of 0.155 oz/t. Mined at a rate of 2,750 tons daily, Meikle will yield nearly 670,000 ounces of gold this year.
Production began from the Meikle mine in 1996 using the longhole method. It is built in 75-ft levels with an internal ramp. The mine is home to Tamrock and Cubex drills. Ore is mucked with Tamrock, Wagner and EJC load-haul-dumpers (LHD) in sizes from 8- to 2-yd3. It is hauled by 20- and 30-ton Dux trucks. A KueKen Model 160 crusher on the 1875-ft level reduces ore to -6-inches, and the ore is skipped to the surface 10-tons at a time using a Canadian Ingersoll-Rand hoist. Men and materials enter the mine using a separate service shaft.
Ground conditions require special attention, hence the use of cemented backfill and the practice of shotcreting all permanent openings. An underground backfill plant operates on the 1,075-ft level. Fill is trucked to a raise and distributed by LHDs.
Another challenge of the Meikle mine (and the Rodeo development) is the elevated rock temperature. Northern Nevada is a geothermally active area. The underground aquifers contain hot water and bedrock temperatures are in the 110-140ºF range. To make the workplace comfortable, Goldstrike operates a large refrigeration plant. Cold air is downcast into the Rodeo mine, moved across the drifts to Meikle and exhausted.
Halfway between the Meikle mine and Betze-Post pit is the busiest spot at Goldstrike--the Rodeo underground development. In March, work was progressing on the underground shotcrete plant, shop, backfill system, fuel depot, and temporary waste pass. Development waste was being skipped at a rate of 1,600 tons per day up the 1,400-ft-deep Rodeo shaft. Ore was being trucked through the 1600-level drift to the Meikle mine for hoisting, and traffic was heavy. Men and equipment were hustling along the same roadway. Before production begins in the fourth quarter of this year, Dynatec will have to finish the haulage drift on the 3620 level.
The Rodeo deposit contains proven and probable reserves of 9.24 million tons grading 0.414 oz/t Au. There is also a resource of 7.34 million tons grading 0.333 oz/t Au. All told, the development may yield 6.275 million ounces of gold. Rodeo production is planned at 3,000 tons of ore per day or about 350,000 ounces of gold annually.
Development is proceeding with plans to practise longhole stoping and cemented fill. Stopes will be 20-ft wide between 60-ft levels. Blasting will be done with a bulk emulsion developed for the Meikle mine; the emulsion contains 20-30% polystyrene. Elphinstone 50-ton haul trucks will be used, and hoisting automated. In fact, all underground functions will be monitored from computers on surface. Ground conditions are poor; the mudstone host rock is very soft. Shotcrete is applied all the way to the face ahead of screening and bolting to protect the Maclean and Secoma bolters.
Meikle and Rodeo are inclusive names; there are several other zones to be exploited through their respective workings. Meikle includes the Main, South and Deep zones. Between the two mines are the East Griffin and West Griffin zones, which will be accessible once the haulage drift between Meikle and Rodeo is complete. Rodeo mining plans incorporate the Goldbug, North Betze and North Post zones. They may also be expanded to include the Barrel zone.
Barrick has proven at Goldstrike that it uses technology and best practices to successfully mine in diverse and difficult conditions. For its efforts, the company is rightfully proud of building not only the biggest gold mine in North America but the best.
The ores of Nevada's Carlin Trend are nothing if not challenging to treat. Goldstrike has developed three successive strategies for efficient, low-cost recovery. Originally, oxide ore was treated by heap leaching. The oxide ore was exhausted in 1998. Meanwhile, a conventional grinding and autoclave facility was built to treat refractory ore. It has grown to a daily capacity of 17,500 tons with six autoclaves. Last year the new US$330-million dry grinding/roaster plant was commissioned to treat "double refractory" ore, that which is both refractory and carbonaceous. The carbon content results in considerable "preg-robbing," and makes ore unsuitable for autoclave treatment.
The timeline of engineering and building the roaster was driven by the length of time needed to secure an air quality permit from the state. Application was made in August 1997, but construction could not begin until the permit was received on October 1, 1998. During that time Hatch Associates worked on detailed engineering based on a feasibility study prepared by Barrick and Kilborn SNC-Lavalin. Commissioning went smoothly and nameplate capacity was reached in July 2000.
The 12,000-tons/day roaster is the largest gold treatment plant of its type in the world. It employs crushing, dry grinding, fluid-bed roasting, and carbon-in-leach (CIL) gold recovery. Loaded carbon is transported to the existing refinery. Head grades are expected to be 0.14 oz/t Au from pit ore and 0.52 oz/t Au from underground ore.
Feed for the roaster is supplied mainly from the Betze-Post pit (11,400 tons/day), with the balance coming from underground. It is trucked to the two-stage crushing plant and blended. The first stage of crushing occurs in a 42- by 65-inch Svedala gyratory crusher where feed is reduced to -6-inches. It then passes over a Simplicity vibrating screen and through an MP 800 Nordberg cone crusher. The 1.5-inch product is then stockpiled. As the ore is reclaimed, lime is added.
Dry grinding was an attractive choice due to the low calculated heat values of the blended ore. At 200 to 300 Btu per pound, it would not generate enough heat to evaporate moisture from a conventional grinding slurry.
The stipulation was made that any dry grinding plant would be an environmentally clean plant, and a solution was found in Krupp Polysius double rotator mills. There are two dry grinding lines, each with a 19-ft-diameter by 69-ft-long, 10,000-hp Polysius two-stage mill. Feed enters a drying chamber followed by coarse and then fine grinding in separate sections. Material is classified and recirculated until the desired size is attained. (These mills are described in more detail in CMJ April 2001, p.11.) The mills' operation is practically dust-free because there are a minimum of transfer points, the system is under negative pressure, and there is a single exit point for dust and ground material.
All material leaves the mills from a central discharge ring. Particles finer than 0.25-inch are air-swept upwards to the Krupp Polysius Statopol static classifier. Particles too heavy to be carried by the air stream are collected in an air slide and travel by bucket elevator to a Krupp Polysius dynamic classifier. Fine material from the static classifiers is sent to four Wheelabrator baghouses for product collection. Other particles from the static classifiers report to the dynamic classifiers (along with larger particles discharged via the air slide). The dynamic classifier performs a further separation using a high-speed air stream moving counter to a rotating-vane element. Coarse material returns to the mill for further reduction, and the finer dust product is also collected in the baghouses.
Solids collected in the baghouse hoppers are recovered by air slides. They are transferred through gravity tipping valves to either of the two 2,000-ton roaster feed silos, supplied by FFE Minerals.
The roaster met its environmental compliance regulation in August 2000, only weeks after start-up. The entire plant is allowed a maximum total particulate release of 12.6 lb/hour. That represents a capture rate of 99.998%.
Oxygenated Fluid-Bed Roasters
The roaster was built to treat ore containing both sulphides (average 1.9% in the feed) and carbonaceous material (1.3% on average). The installation is based on Freeport-McMoRan (FMC) technology. Pilot-plant testing was conducted by Hazen Research. The 1,100-ton/day oxygen plant for the project was built by Air Products & Chemicals on a turnkey basis and is owned by Barrick and operated by Air Products.
The plant has a pair of Technequip Dorr-Oliver two-stage, oxygen-enriched vertical units. Each roaster is 110 feet high and 22 feet in diameter at the hearth. The stages are divided by an interstage windbox located in the middle of the unit. Primary and secondary cyclones on both stages capture solids and recycle them to the roaster. To avoid bridging problems, all transfer lines are vertical.
Feed and air are introduced into the first (upper) stage at the bottom of the bed, creating a residence time of about 20 minutes for the ore. The operating temperature is maintained at 1,025ºF. It can be lowered by an automatic water quench or raised with the addition of coal.
Partially oxidized material is transferred to the second stage. It is injected into the bottom of the bed along with 99.5% O2. In the highly oxidizing environment the reactions are completed during a residence time of 24 minutes. A temperature of 1,050ºF is maintained.
Calcine leaving the roasters is quenched, neutralized with lime and thickened in an Outokumpu SupaFlo thickener ahead of the CIL circuit.
Roaster off-gases are cooled and cleaned to remove solids, Hg, SO2, SO3, CO and NOx. Hot gas from each roaster's first stage secondary cyclone is independently water-quenched to cool it to 180ºF to saturate it and remove about 50% of the particulate matter. A venturi-type scrubber removes most of the remaining solids. The individual gas streams are further cooled to 94ºF in tube and shell heat exchangers and then combined.
The cooled gas stream is saturated by fresh water sprays and passed through a wet electrostatic precipitator. Particulate matter is reduced to the level required by the mercury removal system. Any solid-phase mercury and sulphuric acid mist is removed at the same time.
Vapour-phase mercury is removed with the Boliden-Nozinc process. It reacts with mercuric chloride (HgCl2) solution in a packed tower to produce calomel. A portion of the calomel is reacted with chlorine gas to create HgCl2 and recirculated; the remainder of the calomel is packed in drums and sold.
Sulphur dioxide (SO2) is removed in a dual alkali circuit, which produces clear solutions rather than lime slurries. Sodium sulphite is recycled through a seven-stage tray tower, and the SO2 precipitates as sodium bisulphite. Sodium sulphite is regenerated by adding slaked lime to a bleed stream. The resulting slurry is thickened, with the overflow returned to the tower, and the insoluble portion is discharged to tails.
Carbon monoxide (CO) is removed from the gas in a recuperative thermal oxidizer. Gas passes by a heat exchanger and then flows into a combustion chamber. Gas, with the CO oxidized to carbon dioxide (CO2), exits through the heat exchanger to be cooled ahead of NOx removal.
NOx concentration is lowered in a selective catalytic reduction unit. Aqueous ammonia is injected over a zeolite catalyst. The reaction creates nitrogen and water vapour. Clean gas is then discharged to the atmosphere.
Cooled calcine from the roasters is neutralized in two tanks and thickened in a 110-ft-diameter SupaFlo thickener ahead of the CIL circuit. There are six 48-ft-diameter CIL tanks. The first tank serves as the addition point for NaCN and also as the recovery point for the loaded coconut carbon. Normal loading is 125-175 ounces Au per ton of carbon. Loaded carbon containing 1,500 to 2,100 ounces of gold per 12-ton batch is sent to the refinery at the autoclave plant.
The roaster CIL plant employs Kemex rotary swept screens (also known as NKM screens) rather than vibrating screens to retain carbon in each tank. The Kemex screens were chosen for their low capital, operating and maintenance costs, and are performing as expected.
Tails are removed from the sixth CIL tank and treated with the Inco-SO2/air process to destroy residual cyanide. Ammonium bisulphite was chosen as a reagent because it is non-toxic and easy to handle. Copper sulphate is added as a catalyst, and weak acid dissociable cyanide levels of <2 ppm are achieved. Detoxified tails are pumped to a common tailings area used by both mineral processing plants.
SAG Grinding Sulphide Ore
Goldstrike's other, and older, mineral processing plant relies on SAG and autoclave technology to treat refractory (sulphide) ore. The first mill was commissioned in 1988 and the second mill was commissioned in 1992. The Phase V expansion involves installation of a new tertiary ball mill to maintain throughput as ore hardness increases. The autoclave facility expanded from one to six autoclaves from 1990 to 1993.
The mill and autoclave plants process 17,500 tons of ore daily through two parallel SBC (SAG-ball-crushing) lines and six autoclaves. Mill No.1 handles approximately 4,000 to 4,500 combined tons from underground and open pit operations. About 13,500 additional tons comes from the pit and is milled in Mill No.2. Pit ore has an average grade of 0.26 oz/t Au and that from underground runs 0.64 oz/t Au.
Run-of-mine ore is trucked to the plant, stockpiled and blended ahead of crushing. The crusher for Mill No.1 is a 50- by 60-inch Telesmith jaw. For Mill No.2, a 42- by 65-inch Allis-Chalmers gyratory crusher is used. Ore is reduced to -6-inches and is fed to the respective grinding circuits.
In Mill No.1 the SAG mill is an 8-ft by 22-ft, 2,500-hp Allis-Chalmers unit. After the ore is reduced to 2.5 inches, it is screened. Screen oversize passes through a 4-ft-3-inch Nordberg Omnicone crusher. Screen undersize is pumped to the cyclone sump. A bank of six Krebs D20 cyclones makes a split, sending overflow for thickening and underflow for further regrinding. There are two ball mills: a 13-ft-6-inch by 18-ft Dominion with an air clutch and a direct-drive 12-ft- 6-inch by 14-ft Allis-Chalmers.
Mill No.2 has a 12-ft by 24-ft, 4,000-hp Fuller SAG mill. SAG throughput is screened and the oversize reports to a 5-ft Nordberg Omnicone crusher. Screen undersize is pumped and cycloned in 12 Krebs Super D20 units. Overflow is thickened ahead of the autoclaves, and underflow is reground. The ball mill is a 16-ft- 6-inch by 30-ft-6-inch Fuller ball mill.
Both the SAG mills have variable speed drives with load commentator inverters (LCI). These were developed by Barrick and General Electric. They allow the motors to be brought up to speed at a fixed rate, then switched to variable speed operation. The LCI arrangement provides flexibility at less cost than starting fully variable speed motors.
The new ball mill for Phase V will be operational in August. It is a 3,200-hp, 16-ft by 23-ft-6-inch Svedala unit with rubber liners. It will be used in a tertiary configuration in Mill No.1, bringing throughput up to the designed rate of 6,000 tons/hour. Installation could be a challenge as the most practical method of placement may be to remove the mill building's roof and drop the half-shells into place.
To maximize grinding throughput rates and improve product size, Goldstrike has a Minnovex expert system on both grinding lines. The system can be used on all of the circuits or only parts of them while operators or DCS control is used on other parts. In practice, operators set the parameters and allow the system to run the circuits 97% of the time. The next step is to add a neural network, giving the system the ability to "learn" on its own.
The Art of the Autoclave
Cyclone overflow from both grinding lines is thickened in a pair of high-rate thickeners to raise the solids content to between 52% and 55% by weight. Slurry is transferred to the first of four rubber-lined acidulation tanks in series. The slurry is mixed with sulphuric acid to reduce carbonate to a level suitable for autoclave treatment. Each tank is mechanically agitated and is injected with compressed air to assist in removing carbon dioxide.
The next step is to preheat the slurry in splash towers. There are three splash towers (low-, medium- and high-pressure) ahead of the original autoclave and two (low- and high-pressure) ahead of each of the remaining five vessels. Slurry is injected horizontally at the top of each tower. It flows down over baffle plates as steam rises from below. To facilitate pumping, a pool of slurry is allowed to accumulate at the bottom. Slurry heated to 310ºF is pumped to the autoclave by a pair of Geho positive displacement pumps in parallel. The double pump arrangement is important in maintaining throughput as each one can handle 70% of the flow, should the other be down for any reason.
Goldstrike originally had a single 15-ft-outside-diameter by 75-ft-long horizontal autoclave. The first expansion added two more, larger (82-ft-long) vessels, and the second increase added three more. Other circuits and systems (such as the 1,400-tons/day oxygen plant) have grown the same way. The autoclave function was described in CMJ, November, 1989, pp. 63-65.
Zero Discharge Strategy
Goldstrike's tailings strategy is to maintain a zero discharge facility. The tails are subaerially deposited and solution (water) is recirculated to the processing plants.
The tailings area has a composite liner of clay and high-density polyethylene. The site also has a blanket and finger underdrain system, which facilitates dewatering and consolidation, and limits hydrostatic head development on the liner.
Surface water is diverted around the tails area, and ample freeboard is provided to contain the potentially worst-case rainstorm. Surface and groundwater are monitored for both chemistry and volume to verify that the tailings facility is operating properly. In addition, piezometers have been installed in the basin, embankments and drains to confirm operation in accordance with design parameters.
A closure and reclamation plan that includes reprofiling, soil cover and revegetation has been developed. A bond is in place covering these future expenditures.
The waste rock at Goldstrike is categorized by sulphide content, and the higher sulphide waste is encapsulated within other waste having buffering capacity during dump construction. The dump toes and other final grade areas are concurrently reclaimed as practical. A separate bond is in place to cover these costs.
Emission concentrations of the Goldstrike roaster
|Particulate||6.0 lb/hr||4.3 lb/hr|
|Mercury||0.20 lb/hr||0.022 lb/hr|
|Sulphur dioxide||44.9 lb/hr||16.1 lb/hr|
|Carbon monoxide||47.1 lb/hr||5.9 lb/hr|
|NOx||36.8 lb/hr||14.8 lb/hr|