Mining heavyweight Barrick deploys multipurpose mix, backfill operation at rich, but remote, Nevada deposit
The Barrick Cortez Hills gold mine is located approximately 35 miles from Battle Mountain, Nev., and represents a major expansion project for the world’s largest gold mining company. It consists of a reopened, open-pit mine and an all-new underground ramp-accessed mine. The former features major upgrades to all facilities and an upcoming mill renovation; the new underground operation requires a major multipurpose concrete and backfill batch plant.
The underground mine utilizes conventional concrete for structural conditions and water control, plus pavement as required. Shotcrete is produced for overhead rock stabilization and tunnel support, and backfill (unclassified waste rock material overburden/ore). Production of the concrete and fill materials is expected to exceed 1 million yd. annually, with the underground mine anticipated to yield 1.08 million to 1.12 million ounces of gold over the next 10 years.
Barrick enlisted specialty design-build contractor Plant Architects + Plant Outfitters (PA+PO) to develop an early conceptual design for the multipurpose batch plant as the mine was being staged for reopening. A year later, the company issued a pre-qualified bid call to three firms to supply the large-scale plant. PA+PO was chosen to design-build the entire facility based on a turnkey offer that included 100 percent of the structural steel and storage bin fabrication being completed onsite, at an area adjacent to the dual mine ramp portals.
Conceptual Design Considerations
Robert Ober, CEO of PA+PO, had been involved in the design and installation of several working and since retired multipurpose backfill plants in northern Nevada over the past two decades. In the past, he had utilized various plant and compulsory mixer manufacturers for these installations. For Barrick Cortez Hills, he teamed up with his design partner of the last several years, Lloyd Garcia, on a plant that would actually be manufactured on the mine site by PA+PO welders and fabricators, and installed by the crews of PA+PO ironworkers, millwrights and electricians—a total design-build project.
Garcia and Ober assembled a team consisting of an architectural designer, architectural engineer, and registered architect. Rounding it out were registered structural engineer and civil engineers, with several design engineers tending to the material handling systems.
The design team produced a rendering of the overall enclosure of the cold weather plant first, for a limited footprint located just aside the twin portals of the underground mine, and to assimilate the traffic pattern and access for the underground delivery 20-ton articulating dump bodies and underground mixer truck chassis.
Access to the plant for three aggregates (a blend sand to 3/8 in., a classified 1 in.-plus, and the unclassified waste rock for backfill) was to be from the hilltop above and adjacent to the portals. Three aggregate bins would total 1,000 tons, and the dual powder silos would each be sized at 250 tons—requiring a large-sized enclosure capable of protecting the plant equipment: a 10.5-yd. output twin-shaft compulsory mixer, 3.5 million-Btu hot water heater, dual-ramped wash water weir pits, admixture storage, batching hopper, and dual apron feeders for the offset two aggregates, with the sand gravity fed into the 12-yd. cumulative aggregate batcher to a 60-in., high-speed transfer belt.
The all-weather enclosure required a heavy snow load design and wind load capability—not to mention the fact that the plant was to be in close proximity to a fault line and carry attendant seismic design requirements. Garcia and Ober’s team chose a cladding of aluminum, powder-coated exterior and stainless steel light-colored reflective sandwich panel of 4-in. thickness. The plant was designed as cut into the hillside.
Onsite Fabrication, Infrastructure
Undertaking a large-scale installation such as the Cortez Hills plant required a crew of 55 working seven days per week for five months. The onsite fabrication is a trademark of the PA+PO crews and was used here in lieu of a factory-purchased option due to the totally custom nature of the design, coupled with savings afforded by not utilizing any bolt-together structures, but rather fully-welded steel structures and plate sub assemblies. They would have required some 35 truckloads if fabricated offsite and transported to site.
MSHA requirements for fabricating a complete plant called for a stellar safety plan and enforcement program at Barrick Cortez Hills. No lost-time accidents were experienced, and the manpower from PA+PO was supplemented by several subcontractors, whose crews logged perfect safety records as well.
The all-welded steel structure of the plant was designed to sit atop a floating monolithic concrete slab of four levels of 1-in. reinforcing steel with 2½-in.-diameter anchor bolt embeds, satisfying seismic design factors for both the mixer and powder silos tower structure, plus 700-ton aggregate structure. PA+PO construction crews together with selected excavation and concrete subcontractors completed the infrastructure in less than three weeks, with 14-hour shifts, seven days per week common. Concrete work was expedited prior to the early arriving winter weather in this high desert locale.
The plant was originally to be placed in high wall cut above the underground mine portals. Shortly after iron work was started, however, Barrick officials decided it best to have PA+PO fabricate a steel structural bridge as integral to the plant, with a concrete deck for the dual 988 CAT loader feed of aggregates from the topsides of the configuration. This required that PA+PO undertake a 16-ft.-deep (4.8-meter) concrete abutment for slope stabilization and bridge anchoring, and a 24-in.-thick (600-mm) concrete bridge deck. PA+PO fabricated a 75-ft. structural steel bridge, engineered in house with third-party certification. This loader access bridge was designed to also offer topsides access from the load-out level below via a series of stairs and catwalks. The topsides elevation is 68 feet above the plant discharge level below.
Mixing Technology, Power
The Cortez Hills multipurpose plant is capable of producing any type of concrete to varied specifications due to its design, which includes the world’s first 10.5-yd. (8-cubic-meter) output compulsory twin-shaft mixer, designed and manufactured for PA+PO engineers by OMG-SICOMA in Perugia, Italy. The specialty mixer manufacturer sought to provide a twin-shaft model that not only could provide 600-yd./hour theoretical output of low-slump concrete, but also produce any slump mix at high-outputs without de-rating for dry mixing or low-slump mixes. The mixer is fit with four, 100-hp drives in a quad/Janus style. The mixer is key to the full-range of materials the plant system produces.
Barrick’s engineering and electrical staff specified Allen-Bradley components and a non-proprietary programmable logic controller (PLC). The plant was wired in sealed cable and heavy-duty aluminum tray as designed, supplied and installed by PA+PO and a local subcontractor. The electrical motor control and automation system was installed in a converted, customized 40-ft. sea container insulated and lined in stainless steel and aluminum.
With the entire project completed under the auspices of the PA+PO ISO9001 Quality Assurance program, the commissioning plan for the plant system was undertaken during the conceptual design phase and refined as the project moved through its various stages of construction up to pretest and test running. A third-party commissioning engineer was hired to complete a checkout of the plant following the process and instrumentation diagram certified for construction by the electrical and automation personnel.
The organized commissioning of the entire plant allowed for an on-time completion and startup, with plant optimization occurring in only three days of test operation, during which all theoretical outputs and throughputs were achieved, and all contractual requirements met. Such an industrial approach to the concrete plant commissioning is an approach that mine management encouraged and PA+PO required of the entire team. Punch-listed items were completed on a three-level basis (safety, operational and aesthetic), each tracked systematically.
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Architects + Plant Outfitters, San Antonio.