Adding a high-pressure grinding roll and removing a tertiary crusher not only extended Kıs¸ladag˘’s life of mine and improved recovery rates, it is saving Eldorado Gold energy as well, lowering the company's carbon footprint. Courtesy of Eldorado Gold

In July 2021, Eldorado Gold replaced the tertiary crushing circuit with a Weir Minerals Enduron high-pressure grinding roll (HPGR) at its Kışladağ gold mine in Turkey, a move that the company said would contribute to extending its “cornerstone” asset’s life to 15 years.

According to the company’s updated NI 43-101 on Kışladağ, a high-tonnage heap-leach gold operation, HPGR test-work yielded an average increased recovery of 3.9 percentage points at the highest operating pressure, reaching 54.8 per cent gold recovery in comparison to the base test of 50.9 per cent. The machine, which replaced five tertiary cone crushers, allowed Eldorado to increase its throughput rate to 12.6 million tonnes per year, up from 12 million.

The change-up also had an additional benefit, said Simon Hille, Eldorado’s senior vice-president of technical services: the HGPR is significantly more energy efficient than the previous crushing stage.

“Much more of the energy goes towards breaking the particle,” he said. “So we are looking at using this energy to unlock pathways for metal extraction. It’s certainly very key for high-tonnage heap-leach operations.”

Energy efficiency has taken on new importance in light of the company’s commitment to significantly reduce its carbon footprint. Eldorado said in February 2022 it would slash its greenhouse gas emissions by 30 per cent from 2020 levels by 2030, or approximately 65,000 tonnes of carbon dioxide equivalent. Hille noted that Kışladağ, along with Eldorado’s other assets in Turkey and Greece, are connected to “brown” grids powered by a large percentage of fossil fuels.

Mineral processing accounts for between three and six per cent of the world’s electricity consumption. At the site level, comminution represents 36 per cent of miners’ total energy usage, and flotation, filtering and drying makes up another four per cent, according to a 2021 report commissioned by Weir Group and written by Engeco, an Australia-based energy and climate change strategy consultancy. (Diesel use makes up the remaining 60 per cent.) The grinding mill is typically the single largest user of mine site energy, the report found.

Improving the efficiency of these processes could help miners with non-renewable power sources make significant dents in their carbon footprints.

Grinding down energy use

Making changes to an existing grinding circuit can be challenging, but Marc Allen, founder and technical director of Engeco and author of the 2021 report, said there are numerous ways for mineral processors to reduce the mill’s energy draw.

A grinding audit can help ensure that the grinding circuit is optimized for flotation or leaching to prevent against expending unnecessary energy on over- or under-grinding, and maximize throughput. Advanced process controls for the mill can help processors set and maintain an optimum grinding range.

Hille, meanwhile, emphasized the importance of a strong understanding of ore characterization, and making sure there’s been an “effective analysis made around the liberation requirements relative to recovery.”

He also noted that while mineral processors can push for the ultimate recovery at their plants, a small percentage increase often requires a disproportionate amount of additional grinding and energy use. There is also a risk of over-grinding and making the ore too small and difficult to float, when concentrating through flotation, he said.

Linking grind size and recovery at Eldorado’s Lamaque underground gold mine in Quebec helped the company get a larger throughput, of 2,500 tonnes per hour (TPH), out of its process plant for the same energy draw as the nameplate capacity of 1,850 TPH, Hille said. While Lamaque’s Triangle deposit already has a relatively consistent ore character, one of the key factors was finding the optimal leach time. Balancing leach time and grind size to find the optimal recovery is an important metallurgical skill and will prevent energy wastage through over-grinding.

Improving blast patterns could also reduce the amount of energy needed during the grinding phase, Allen said. “The more breakage we can do of the rocks before we hit the grinding circuit, the better.”

Brownfields versus greenfields

While replacing more inefficient equipment, such as the SAG mill, with newer technology has the potential to save on energy, Allen acknowledged it can be a costly endeavour, and is dependent on whether the ore is amenable.

But, he added, greenfield operations have a major opportunity to “make better energy decisions” and develop a more efficient flow sheet, incorporating equipment like HPGRs, high intensity grinding and stirred or vertical mills.

“Looking at different ways to approach flow-sheet design is key, because once you embed that energy, once the SAG mill is in place, there’s not much more you can do to make it more efficient,” said Hille.

The processing plant’s physical footprint – including the use of concrete and larger buildings with higher ceilings to accommodate large flotation circuits – also represents an opportunity for greenfield operations to reduce emissions, said Glenn Kosick, chief executive officer of Woodgrove Technologies.

Related: As the use of artificial intelligence and machine learning in mining is becoming more commonplace, research is underway to automate every part of a mining operation

Kosick gave the example of a conventional flotation plant with seven cells, which sit on concrete pads, explaining that most conventional flotation cells have a step height between cells of 0.6 metres to one metre. The top of the concrete pedestal for the first cell would be between 4.2 and seven metres high. The diameter of a large tank cell can be between eight metres and 11 metres. Using these dimensions, this means a minimum of 14 to 38 cubic metres of concrete for the first cell pedestal alone. Additionally, other factors such as earthquake proofing and soil conditions can require even larger or thicker pads, and consequently more concrete.

Manufacturing of one cubic yard of concrete generates about 400 pounds of carbon dioxide. The company’s staged flotation reactor (SFR) and direct flotation reactor (DFR) have footprints about two-thirds and one-third the size of a conventional plant, respectively, and the DFRs sit flat on the floor.

“That’s a big contributor, the construction of these facilities,” he said. “If we can reduce that, it’s an important thing — as well as the cost [savings].”

In terms of cost savings, Kosick claims a net operating energy savings of between one-third and one-half of conventional flotation cells for SFRs and DFRs respectively.

The geometallurgy opportunity

Allen said geometallurgy –  the integration of geological, mining and metallurgical information and practices to improve an ore body’s value and minimize technical risks –  holds a lot of potential to reduce energy use from the grinding mill by significantly improving the consistency of ore the processing plant receives. But he said it is not commonly done within the sector.

“There’s a tendency to operate geology, metallurgy and mining all as separate silos: geologists know what’s in the resource, miners mine whatever’s in the mine plan and metallurgists have to deal with it,” he said. “Breaking down those barriers… means you don’t have a plant that just takes what it’s given.”

The practice involves dividing the ore body into blocks and developing technical parameters for how amenable each block is to grinding, creating a predictive mineral-processing model and optimizing the flow sheet based on that.

“Ore bodies are highly variable, their characteristics change, so the deeper the understanding one can get of the ore body and the finer resolution and understanding [of] energy requirements throughout the blocks…you’re able to optimize the grinding design to minimize the power required,” said Kosick, whose previous company, MinnovEX Technologies, specialized in the use of geometallurgy in grinding circuit design.

While geometallurgy has been discussed for years, Allen said it can be done more effectively today thanks to advancements in computing technology and data manipulation capability that allow for higher fidelity block models.

Innovations underway

According to Allen, the industry’s conservative approach to technology has slowed the adoption of new energy-saving mineral processing technology.

“The industry is very good at being first to be second…[and] it becomes a little difficult to get this technology at the lab scale and the pilot scale, and convince someone to do it in the field,” he said. “There’s also this tendency to think, ‘this situation works quite well right now,’ which feeds into the hesitance to move.”

But Hille said he thinks that mentality is changing, with the push towards net zero prompting mining companies to start evaluating the energy footprint of major pieces of equipment upfront.

Numerous research projects are also under way to significantly change the way ore is processed. In 2018, federally funded agency Impact Canada launched the Crush It! Challenge, which aims to find innovative approaches to reducing comminution’s energy draw. While the pandemic delayed awarding a winner, six semi-finalists received $800,000 each to prove out their technology.

“That innovation space is thriving right now,” said Hille. “A lot of budding technology is coming down the pipeline.”

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