Kinross Gold’s Ft. Knox gold mine in Alaska produces about 40% of its annual output from the Walter Creek valley fill heap leach
operation, pictured here. At full development, maximum heap height from toe to crest will be approximately 1,100 ft and maximum ore
thickness in the pad about 500 ft. Construction began in 2007 with the project scheduled to be built in seven stages extending to 2021.
(Photo: Knight Piésold)

However, in the same vein as the military maxim that “the enemy always gets a vote” in a battle’s outcome, a number of complicating factors often have votes in the outcome of a heap leaching campaign. These include local weather conditions, ore composition variability, leach pad design and maintenance, and the hardto- figure settlement and segregation behavior of rock fragments and particles, to name just a few. Regulatory and societal concerns involving the use of cyanide or sulphuric acid in large quantities as well as the potential for environmental damage by fluid leakage from pad linings or piping can also affect the operational ‘ballot.’

Yet, these risk factors generally don’t deter companies from planning and developing heap-leach operations where the mineralogy, topography and applicable technology are appropriate. Heap leaching often offers smaller or less-capitalized companies an affordable, relatively quick path to production. For companies that own an oxide ore-capped sulphide deposit, heap leaching of the oxides can generate revenue needed to buy and build the facilities needed to later process the sulphide ore.

Partly because of its relative economy of capital and quick startup capabilities, heap leaching technology is successfully being adapted to cope with increasingly riskier environments such as extended frigid weather conditions, enabling leach operations to function effectively in high-altitude locales or in Arctic or sub-Arctic regions. In a paper presented at the Heap Leach Solutions 2015 conference held in Reno, Nevada, USA, authors Krishna P. Sinha and Mark E. Smith noted that there are a surprisingly large number (>70) of cold-climate heap leach projects either being planned or currently operating at production rates ranging from less than 1 million mt/y to 30 million mt/y.

Although cold-climate heap leach mines face operational challenges, they can be as equally attractive as their moderate-climate counterparts for, say, larger producers in need of additional production from proven reserves. A case in point is Kaminak Gold Corp., former owner of the Coffee gold project in Canada’s Yukon province: Kaminak was acquired by Goldcorp for $520 million in 2016 after a feasibility study for the Coffee project indicated that the mine, which would employ heap leaching, could be paid for in just two years with initial production of more than 200,000 oz/y over the first five years at an estimated AISC of $550/oz.

The Coffee project would be part of a trio of cold-climate mines and advanced projects in the Yukon/northern Alaska region of North America; At latitude 65° north, Kinross’ long-running Fort Knox gold mine in Alaska is currently the northernmost heap-leach operation in North America, producing 40% of its total annual production of 390,000 oz of gold from the year-round Walter Creek heap-leach facility. Fort Knox poured its seven-millionth ounce of gold in 2016. And, Victoria Gold is advancing the Eagle Gold project in the Yukon, developing a deposit containing an estimated 2.66 million oz in reserves at 0.7 g/t. Eagle’s AISC are anticipated to be less than $700 per oz, with an annual production rate of 200,000 oz.

As the authors of the conference paper point out, cold-climate operations require unique design and operational considerations, including possibly seasonal operations, maintaining heat within or adding heat to the heap, permafrost management, leaching kinetics (especially for bioleach operations), managing ice and snow, and closure.

But leach operators in more moderate climatic locales usually can’t function in a set-it-and-forget-it mode, either: producers both large and small continually tweak the parameters of their existing leach setups to overcome problems arising from changes in external or pad-internal conditions; these countermeasures, for example, range from crushed-ore agglomeration, or applying liner ‘raincoats’ to exclude excess moisture from heavy precipitation, to drilling injection wells directly into the heap to improve leach-solution access, and even to exploring new methods to process ripios, or spent ore, for further metal extraction.

Kinross Gold’s Bald Mountain openpit heap-leach mine in eastern Nevada and its Round Mountain mine in the south-central part of the state provide good examples of typical modifications to improve leaching efficiency and overall performance: During a 2016 tour of Bald Mountain — one of the largest mine sites in the U.S., with a total land package of 600 km2 — the company noted that it was considering connecting two of its carbon-in-column process plants to maximize the utilization of existing carbon columns; investigating leaching via secondary wells; and using material from old heaps to build new heap phases.

Kinross was also looking at implementing overall best practices at Bald Mountain, taken from a program at its Round Mountain operation where heap leach improvement efforts have included carbon column optimization, new pad side liners, pH enhancement involving automated reagent controls at multiple pads, and solution pumping and piping optimization. It also was evaluating additional future opportunities such as adding barren solution through wells to increase pH and cyanide concentrations and enable the solution to find a different path to the ore.

Beyond physical measures such as these, companies are also investigating the potential advantages of applying increased automation and data analysis to heap leach processes — a trend that is being driven at least in part by the availability of more-sophisticated instrumentation and data-collection and reporting platforms.

Sepro Mineral Systems recently delivered three large agglomeration drums, similar to those shown here, to heap
leach operators in Indonesia, Mexico and Peru. The 3.6 x 10-m units are the largest models offered by Sepro, and
include customized features to meet specific customer application requirements

To solve its acid management challenges, the mine installed Emerson’s Smart Wireless solution consisting of 50 wireless differential pressure devices. The self-organizing IEC 62591 (WirelessHART) network covers an area stretching 2 km (1.5 miles) by 650 m (2,100 ft) wide and sends pressure and pH variables to the main control system. The pressure and pH data are available online for operation and control which enables managers to optimize maintenance resources and minimize the exposure time of operators in the field. Benefits attributed to the Smart Wireless solution include reduced risk of costly pad collapse, a copper production improvement of 7%, reduced environmental impact, and a reduction of 14% in use of sulphuric acid. Payback of the investment took less than six months, according to Emerson. Additional benefits gained by the use of wireless devices included mitigation of the risk of losing data transmission due to cable deterioration — and the wireless devices can be easily moved at low cost.

In another example, Barrick has included refinements to its heap-leach operations as an integral part of its overall ‘digital reinvention’ initiative announced in collaboration with network technology provider Cisco last September. The company’s Cortez gold mine in northern Nevada was chosen as the pilot site for a flagship digitization operation because it has a mix of “all the right ingredients,” according to Barrick, including an underground mine, an open pit, a processing plant that isn’t overly complex, and a mine that had already implemented some digital technology.

According to a February 2017 presentation by Michelle Ash, Barrick’s chief innovation officer, the big-picture framework for the company’s digitalization efforts will include programs to enhance predictive analytics, global task management, integrated planning and a company- wide ‘analytics hub’ to improve overall management awareness.

The company’s 2016 digital objectives for its heap leaching operations included improved carbon management through operation and analytics preventing ‘ounce loss,’ as well as automated carbon and reagent control to reduce downtime and reagent costs. Its list of objectives for 2017, according to Ash, includes automation of its heap leach operations to increase throughput and recovery.

At BHP Billiton’s massive Olympic Dam copper mine in South Australia, that company is also viewing adjustments to its heap leach operations in a widely inclusive format — in this case, a plan more attuned to maximizing its current physical assets to significantly increase Olympic Dam’s production rate from its current output of more than 200,000 mt/y. In July 2016, Jacqui Mc- Gill, asset president–Olympic Dam, told an audience at a South Australia Chamber of Mines event that: “...with some efficient investment of capital and further optimization, Olympic Dam could further increase capacity, potentially up to 280,000 mt of copper per year.”

McGill went on to say, “We also retain attractive options beyond 280,000 mt/y. Continuing expansion underground, and successful on-site application of heap leach technology could provide an opportunity for further growth. This would see heap leach working in parallel with our current concentrator and uranium leach plant, feeding into an expanded smelting and refining facility. This could allow us to produce up to 450,000 mt/y of copper, while continuing to use the current stope mining method with a significantly smaller footprint and capital cost than the prior open-cut design.

“Opening up the Southern Mine area will provide us with a much better grade in copper and copper sulphur ratio in order to maximize the metal units through the existing surface facility,” she continued. “For some time now, you would have heard us talk about the application of heap leach technology at Olympic Dam as a potential low cost processing option for the future. Many of you might also recall that we commenced column testing at Wingfield in 2010, which was followed by the 2013 commencement of the larger CRIB based testing.

“This test-work is moving into another exciting phase, with smelter pilot plant test work soon to commence at Outotec in Finland. This test work…is aiming to determine the effectiveness of smelting concentrate from the heap leach process with existing OD concentrate in a pilot flash matte smelter. It then aims to produce blister copper in a novel hybrid DBF smelter arrangement, also at pilot scale. The findings from the testing are being used to assess the smelting process and technology alternatives for the concentrate generated from heap leach. The test work will be using around 1.5 mt of Olympic Dam ripios concentrate which has been produced from the existing heap leach CRIB demonstration facilities at Wingfield, along with 2 mt of existing Olympic Dam concentrate.”

Back to the Not-so-Basics
However, not all improvements in heapleach technology start at the top; basic machinery used in the process continues to improve, as illustrated by the sidebar to this article and other innovations involving fundamental equipment, such as agglomerator drums used to add cement to crushed ore to minimize fines before placement on the heap. Even these seemingly elementary units now offer sophisticated control capabilities, according to Sepro Mineral Systems Corp., which points out that its line of heavy duty, tire-driven agglomeration drums are equipped with a variable frequency drive (VFD) package, allowing the equipment’s operation to be fine-tuned. Their Rotary Control Console (RCC) component features a touch-screen HMI (Human Machine Interface), which is used to control and monitor the operating parameters. When equipped with the company’s Autopac option, the drums include a jacking cradle, tire pressure sensors, and hardwired safety limit switches.

The tire drive system allows individual tires to be removed without complicated shaft, bearing and coupling arrangements, according to the company. The tires are mounted on purpose-built wheel assemblies, which are driven by a bevel/helical gearbox direct connected to the electric motor. Starting and speed control is done via an AC variable frequency drive.

Sepro said it recently completed delivery of three of these drums to mining customers in Indonesia, Mexico and Peru. The 3.6 x 10-m drums are currently the largest size manufactured by Sepro and although the units use mainly standard, off the shelf major components, customized features can be added to support client specific requirements: in this case, two of the three units included a newly developed, Sepro-manufactured liner that is offered as a more affordable option to traditional alternatives. Already used in Sepro’s Falcon gravity concentrators, the liner material has demonstrated longer wear resistance, reduced downtime and other opportunities for overall cost savings, according to the company.

In addition, the delivered units offer multiple angle settings to adjust drum slope, providing tighter control when monitoring speed and tonnage by enabling the drum to be adjusted to a shallower or steeper angle as needed. And, in one particular instance, a client had requested a redesigned discharge chute to work with their conveyor system. Sepro said its team was able to develop a unique solution that met the client’s requirements without compromising the performance of the machine, allowing it to work with their existing conveyor system.

Mobile Stacker for Heap Leach Applications
Southern Copper Corp. recently commissioned the world’s largest Mobile Stacking Conveyor (MSC) system, which can stack ore at 12,500 metric tons per hour (mt/h), at its Cananea copper mine in northern Mexico. Supplied by FLSmidth, the MSC is an automated system that uses GPS and a self-alignment system to stack ore accurately, which should optimize leaching efficiency.

“This was a significant leap for Cananea in terms of automated technology,” said Willem Fourie, product line manager in FLSmidth. “The automation improves machine performance as it stacks ore, producing the desired heap profile. The MSC can move backwards or forward in a linear fashion or rotate about any point. Ultimately, this will lead to better leaching efficiency from the heap.”

Cananea’s MSC is nearly 300 m long. “With the integrated GPS, the machine knows exactly where it is located on a real-time basis,” said Mark Mulligan, FLSmidth’s process line manager for hydromet technologies. “From a mechanical perspective, the machine auto-self-aligns as it progresses forward on a real-time basis. Because the machine is self-aligning, the heap is formed more accurately.”

An 80-m heap is a large heap by any standard. “To stack ore that high, the rock has to be quite competent to be placed in a single stack,” Fourie said. “Since the machine supports itself over its length, it is naturally good at handling that type of situation. It can also accommodate for movement in the terrain.”

The world’s largest MSC places ore on a 80-m heap using side stacking at the Cananea mine in northern Mexico. The top photo shows forward placement.
(Photos: FLSmidth)

FLSmidth has minimized maintenance requirements, Fourie explained. “The mine’s crews are quite familiar with the machines and they can attend to its needs quickly,” he said. “The machine also monitors itself, which makes fault-finding easier.”

The system is comprised of 40-m bridge systems, which were built off-site. They are supported by the hydraulic-driven crawler pads. “The sections arrive on site and we click them together and string them with a belt,” Fourie said. Cananea’s system uses a 6-ft belt.

Ramp-up for this systems was very quick. “We were at full production within weeks,” Fourie said. “We have supplied most of the world’s mobile stacking conveyors so I guess we’re getting good at it.”

For FLSmidth, Mulligan explained, “Being able to improve productivity for the miners is very important especially as far as availability and being able to stack ore accurately and consistently, which enables better leach efficiencies and ultimately better return on the investment.”

FLSmidth is also working on two copper heap leach projects in Myanmar, which will come online this year: “FLSmidth is supplying self-driven, 6,500- mt/h mobile grasshopper conveyors at one mine feeding an existing MSC, as well as a complete radial stacking system for another mine inclusive of grasshoppers, ramp conveyors, stacker feed bridge and the radial stacker at 1,200 mt/h throughput,” Fourie said.

The company is designing a number of large mobile stackers. “We are in the early stage of designing an MSC with double the output of the Cananea machine,” Fourie said. These machines work well for heap leach applications, Fourie explained, and they are also well-suited to stack overburden and dry tailings.