Reducing Mine Water and Waste Through Preconcentration

Preconcentration offers miners the opportunity to address some of the operational, environmental and CSR-based challenges that lie ahead

By Carly Leonida, European Editor

Preconcentration, as a concept, is not new. It is a commonly used process in base metals, coal, iron ore and phosphate operations, and selective ore sorting in the form of hand picking has been employed since the birth of mining to optimize processing.

At a commercial scale, processes such as gravity concentration, heavy media separation and ore sorting have been used, where cost effective, since the 1970s. However, there is renewed interest in preconcentration due to technological advances and shifts in the monetary and social costs of two key inputs to ore processing — energy and water. In line with this, there has been an increase in the number of patents registered for preconcentration technologies, particularly those targeted at reducing water consumption and improving tailings deposition as a result of benefication of valuable minerals at coarser particle sizes.

Preconcentration is vital to the future of the mining industry as it has the potential to lower the cost and risk associated with mineral transportation, processing and tailings disposal. It also cuts energy and water use and minimizes the environmental footprint of operations.

Reducing the amount of energy required for comminution by discarding waste material prior to grinding lowers greenhouse gas emissions and will be key in helping many mining companies realize their long-term goal of carbon neutrality. Lowering the quantity of fine material sent to tailings also reduces net water consumption and lessens the impact of operations on local communities, which is especially important in arid environments and remote areas, for example in high altitude South American locations.

“Preconcentration can potentially offer a higher rate of return, enabling new projects with low ‘in-ground’ grade to secure funding,” said Greg Lane, director at the Coalition for Energy-Efficient Comminution (CEEC). “For existing projects, there are opportunities to retrofit in order to improve processing efficiency. In some cases, resource recovery can increase if  the processing of previously sub-economic resource is enabled.”

The CEEC is an independent organization dedicated to driving advances in mineral processing.

“Our focus is on highlighting existing and emerging technologies and processes that are practical, reduce energy and/or water consumption, and have potential for a favorable economic outcome,” CEO Alison Keogh said. “Operators, researchers and collaborative groups are working with mining suppliers, services and software groups to develop and trial advanced preconcentration technologies. The rate of uptake will depend on the success of these technologies in the field.”

In 2019, the CEEC Medal for Technical Research was awarded to Laureate Professor Graeme Jameson and Dr. Cagri Emer for Coarse Chalcopyrite Recovery in a Universal Froth Flotation Machine. The paper documents a novel flotation device, the NovaCell (more on this later), which features a fluidized bed for coarse particle collection and a high shear aeration zone for ultra-fines separation.

CEEC Director Joe Pease explained: “Coarse flotation is an important development that links with and amplifies the economic impact of preconcentration. For example, HydroFloat technology is being trialed to evaluate the possibility of improving metals recovery at Rio Tinto’s Kennecott Copper operation in Utah, U.S., and to improve copper and gold recovery from tailings at Newcrest’s Cadia Valley operations in New South Wales, Australia. HydroFloat developer, Eriez, is also sponsoring The University of Queensland’s Julius Kruttschnitt Mineral Research Centre (JKMRC) in the formation of a Collaborative Consortium for Coarse Particle Processing Research.”

The development of sophisticated, precise sensors is opening up opportunities for preconcentration in commodities such as gold and base metal sulfides using time tested techniques such as dense media separation, gravity concentration and in bulk sorting — the latter is already being used in a semi-commercial application for sulfide ores at several sites, and there are a number of others implementing testing. CRC ORE is currently funding sensor development and demonstrations with groups including CSIRO, the University of Adelaide and the National Research Council in Canada.

Additionally, data science and integrated modelling approaches are enabling the assessment of which orebodies are amenable to upgrades using preconcentration and helping to quantify the benefits and impact of preconcentration.

“These types of collaboration help produce data that supports wider awareness and adoption of new preconcentration technologies,” Lane added.

Engineering Grade

CRC ORE is a Cooperative Research Centre focused on Optimizing Resource Extraction for the mining sector. It is jointly funded by participants including mining and METS companies, research organizations and the Australian government, and is an independent technology broker and facilitator.

“We are working to minimize the impact of declining grades and improve the productivity, energy and water signatures of mining operations,” CEO Ben Adair, explained. “We do this by enabling mining operations to reduce the amount of waste they process and identify increases in overall value. Operators can decrease their use of energy and water, leading to increased sustainability, greater profitability and a smaller environmental footprint.”

Preconcentration is core to the work that CRC ORE is doing. Grade Engineering, one of its signature innovations, employs a combination of techniques to reject low-value material early in the mining process.

Adair said, “Preconcentration is important to the future of the mining industry as it enables operations to work smarter by processing less barren waste and instead focus on maximizing the processing of target minerals. This is particularly important as deposits are becoming increasingly difficult to mine, which in turn drives up capital expenditure. Any improvements that can be made to optimize resource extraction are extremely beneficial.

“Collaboration across our participant cohorts has been critical to the development of Grade Engineering. It is only through collaboration that we have been able to design, test and assess improvements that have resulted in preconcentration outcomes at sites.”

Adding Value at San Cristóbal

A successful full-scale production trial of Grade Engineering was recently demonstrated at Sumitomo Corp.’s Minera San Cristóbal (MSC) operation in Bolivia.

The CRC ORE and MSC teams conducted site studies and analysis in 2017 to determine the level of opportunity available and a full-scale production trial using a Metso Lokotrack ST2.8 mobile screening plant began in 2018.

The trial focused on upgrading mineralized waste from the pit to determine if Grade Engineering could produce a new economic stream of valuable material that could then be combined with ROM feed through to the concentrator to produce a positive net smelter return.

Adair said the results are impressive: “The production trial resulted in a 75% rejection of barren material from sub-economic mineralized waste,” he said. “The ‘accepts’ fraction, representing 25% of the mass has been upgraded to more than twice the grade of the traditional feed stock.”

CRC ORE said this result is of significance to the operation, with work to date extending the life of mine by at least two years. Additional work is now being undertaken that will further extend this time considerably.

“The Minera San Cristóbal engagement has been an outstanding success where the intrinsic culture of the site has facilitated the rapid testing and deployment of the technologies,” Adair said.

“We currently are engaged at several site trials nationally and internationally on behalf of our participants. This includes a number of Western Australian gold and base metal operations. These trials are focused on a number of gangue rejection techniques and are showing highly promising results for those involved.”

High Voltage Pulse Enables Preconcentration

High Voltage Pulse (HVP) is a novel technology being developed at The University of Queensland’s Sustainable Minerals Institute (SMI) with the potential to transform ore preconcentration in the mining industry.

“HVP applies electrical energy directly to ore fragments, which has a tendency to break mineral-enriched ore,” explained Professor Frank Shi, technical director for the newly established High Voltage Pulse Collaborative Research Program. “This means that, by breaking fragments down into different sizes, the technology could enable ore preconcentration through size separation.”

Research conducted at SMI’s JKMRC has discovered that HVP energy is not distributed evenly to the particles in the processing zone. When subjected to nanosecond-short pulses, particles with high conductivity/permittivity minerals tend to induce a breakdown channel passing preferentially through the body of the particle, while the barren rock does not.

After this selective breakage, fragments from the mineral-enriched ore particles can fall through different sized holes in a screen below, but the larger barren rock does not. This produces a low-grade oversize stream and a high-grade undersize stream.

“The potential of HVP for preconcentration can be demonstrated using the laboratory results derived from gold ore with a feed size of 26.5-37.5 mm,” Shi said. “It indicates the feed, after being treated by HVP, can split into two streams at a cut size of 19 mm with gold grades of 1 ppm and 0.3 ppm, respectively.

“The process recovers 90% of gold in the high-grade stream and rejects 23% of the feed into the low-grade stream. The HVP treatment of this feed size requires specific energy of 2.8 kWh/mt. If the low-grade stream can be rejected as a coarse tailings without the need to be further ground to micron size, the economic benefit is significant.”

Cracks or micro-cracks have also been found to be generated in the high-grade fragments, while the particles in the low-grade stream appear more competent. This means that the high-grade product after HVP-enabled preconcentration has been selectively pre-weakened, resulting in further energy savings in downstream comminution.

“Because preconcentration is enabled by HVP selective breakage, which is strongly related to particle grade rather than inferred from particle surface properties used in other types of ore preconcentration technology, the HVP-enabled technology can sort the feed ore particles more accurately and reject a larger amount of coarse barren rock,” Shi said.

The Next Steps for HVP

Potential applications of the HVP preconcentration technology include the rejection of barren pebbles from grinding mill pebble streams to increase overall capacity of the existing equipment; coarse waste rejection at mine sites to reduce haulage; combined advantages of pre-weakening and preconcentration of mill feed; new engineering design of multigrade comminution and recovery circuits; and upgrading ore to allow the mined cut-off grade to be reduced, which has a huge impact on the total viable orebody.

Shi said that promising results have been achieved in laboratory-scale batch tests.

“Additional barriers remain for the industrial uptake of this technology, including the need to scale up the process and equipment, and demonstrate safe and effective performance in a continuous operation,” he said. “In addition, knowledge gaps around ore composition and amenability to HVP, and the optimum means of incorporating HVP to achieve maximum benefits, into mineral processing circuits exist.”

To address these gaps and sufficiently de-risk HVP in a pilot-scale continuous operation, further research is necessary. After detailed discussions with mining companies, the HVP Collaborative Research Program recently commenced at The University of Queensland, with major sponsorships from Newcrest Technology, Newmont USA and SMI’s Complex Orebodies Program.

JKMRC and the Huazhong University of Science and Technology in China are research providers for the program. “The aim is to collaborate with industry to address the challenges surrounding HVP comminution with the objective of delivering the next generation of comminution technology capable of resulting in more sustainable mining industry,” Shi concluded.

ARC Centre of Excellence Opens its Doors

The new ARC Centre of Excellence for Enabling Eco-Efficient Beneficiation of Minerals (CoE) at the University of Newcastle is working with seven other Australian universities to develop transformational technologies that enable a competitive and sustainable future for Australia’s minerals industry.

As preconcentration can help to address mine footprint, energy and water use and boost resources recovery — some of the most pressing challenges that mines face, not just in Australia, but globally — it is fundamental to many of the CoE’s projects.

Centre Director, Laureate Professor Kevin Galvin, explained how preconcentration could impact flowsheets in the next 10-20 years, and some of the CoE’s current projects.

“Preconcentration will increasingly be used in greenfield sites where there is the opportunity to deliver this kind of innovation,” he said. “In the future, preconcentration will be project specific, requiring more planning, more ore characterization and improved models, especially for the newer technologies. The reduction in the environmental footprint will be central to the license to operate.”

Galvin currently leads the CoE’s novel system hydrodynamics project — the center’s first program — along with Laureate Professor Graeme Jameson and Professor Bill Skinner. It is concerned with transforming the hydrodynamics of process systems at a macroscopic scale.

“We have a pipeline of technologies at different readiness levels,” Galvin explained. “The work is supported by two other programs that focus on the physical chemistry of foams and emulsions at the meso-scale to effect separations, and the molecular scale in the development of new, more selective reagents.”

Program one will investigate:

1. Characterization of ores using a broad range of techniques including X-ray CT scanning;

2. Different mechanisms in coarse particle separation;

3. Ultrafast methods of processing, utilizing hydrophobic interactions; and

4. Advances in solid-liquid separation.

“Our focus on coarse particle separations recognizes the need to promote hydrodynamic quiescence for supporting coarse particle flotation,” Galvin said. “Multiple fluidized bed technologies will be investigated, and methods of gravity separation including a novel dry separation mechanism will also be investigated.”

The Reflux Classifier

Galvin also developed the Reflux Classifier, a device used to achieve gravity separation of fine particles. An R&D agreement was reached between the University of Newcastle and Ludowici, and the technology is now being developed by FLSmidth. The machine uses a combination of a fluidized bed and inclined channels along with a laminar shear mechanism to drive density-based separations.

Early research into the Reflux Classifier was supported by ACARP and also the Australian Research Council to solve a previously intractable problem in separating fine metallurgical coal. In more recent years the technology has been applied across a range of commodities in more than 150 installations around the world. Galvin is currently working with FLSmidth on related technologies including the RCAir to achieve coarse particle flotation, and is due to commence a full-scale trial of an inverted device, the Reflux Flotation Cell, a new flotation technology offering an order of magnitude reduction in the cell footprint with extreme levels of cleaning.

For Galvin, industry collaboration is essential to developing approaches to preconcentration.

“Given the significant pressures on the sector to lower its environmental footprint, and reduce energy and water consumption, early adoption is needed. It is also important to connect capability in characterization, fundamental modelling, experimental validation, and end-user application to facilitate this development,” he said.

“The entire industry is under a common pressure to minimize its environmental footprint. If the industry adopted a ‘safety share’ philosophy, then it would be possible to communicate the impact of measures being taken by the different operations around the world.

“This would promote and encourage innovation across the industry. Every feed scenario is different, but there are common elements that could be used in certain applications.”

NovaCell — A New Way With Flotation

As mentioned earlier, Jameson, laureate professor at the University of Newcastle, and his colleague, Emer, received the 2019 CEEC Medal.

The duo showed that the use of the NovaCell in a typical base-metal concentrator could reduce the operating costs by 40%. In addition, because waste gangue from the NovaCell is so coarse, it is easier to de-water, allowing more process water to be returned to the mill. Coarse particles also create tailings dams that are more stable than those produced by older technologies.

Jameson described the NovaCell and the significance of fluidized bed technology.

“Undoubtedly the most interesting development in the field of flotation is the introduction of fluidized bed flotation,” he said. “A major problem with conventional mineral processing is the generation of large amounts of very fine ore suspended in water. Ore usually contains only a very small quantity of valuable material and all the rest goes to waste.

“Currently, ore that enters the concentrator is finely ground, because of the limitations of conventional flotation machines. There is an urgent need for a new flotation machine that can separate valuable materials from waste at an early stage, eliminating the need for very fine grinding.”

The NovaCell can be applied to a wide size range of particles including those beyond the size processed using conventional cells. It incorporates the quiescent conditions of a fluidized bed to ensure coarse particles remain attached to bubbles. The ore is ground to a top size that is much coarser than traditional flotation machines can handle (the top size is a function of the ore characteristics and could be as large as 5 mm). Using a combination of a high-shear aerator to capture the finest particles, and a fluidized bed to capture the coarse particles, high recoveries can be obtained across the whole size range.

“The flotation product at this stage may represent only 10-20 % of the feed material,” Jameson said. “It is re-ground to liberate the values to give a high-grade product, and the gangue is discharged to waste at a relatively coarse size.”

The NovaCell is now being commercialized with industry partners and the design for a 250-mt/h unit for a copper concentrator is currently being finalized.

Teck Opts for Sorting at Highland Valley

Teck Resources has been involved with modern sensor-based preconcentration since 2010, with initial efforts centered around understanding the available technology, equipment scale and value driving potential. This work has now advanced into select lab, field and commercial testing at a variety of base metal projects and operations.

“Currently, we are using shovel-based ore sorting in full-scale operation at Highland Valley Copper in British Columbia, and we have advanced trials under way at two of our base metal sites,” said Bryan Rairdan, technical director for processing at Teck Resources.

“We selected these locations as the technology was amenable to measuring the grades of these deposits, as well as a general understanding of the deposit heterogeneity that was available to leverage. We initially focused on shovel-based technology due to the fact that the sensing occurs as close as possible to the in-situ ore, preserving heterogeneity, as well as the minimal infrastructure required to integrate ore sorting into the operation.”

The company’s primary focus is on employing X-ray fluorescence (XRF) based technologies to reject gangue early in the mining process. Rairdan said that collaboration has been vital to Teck’s efforts.

“We have partnered with some early-stage technology providers to help them advance their commercially available equipment,” he said. “We are also working alongside industry partners to develop emerging sensor technologies that are advancing towards commercial deployment.

“Additionally, we are working with engineering firms to determine some of the ancillary equipment requirements and costs required to allow the sorting technologies to advance. The majority of our in-house efforts have been focused around deposit heterogeneity definition. Having a variety of different collaboration methods is important, as some of the challenges the discipline faces are common across the industry, while some are more deposit specific.

“We hope to see benefits across the mining value chain. More precise separation of waste and ore should result in reduced operating costs, reduced energy consumption, and reduced tailings storage requirements.”

For now, Teck is continuing to advance its application and understanding of the shovel-based systems in use today and will be working to better understand the applicability of prompt gamma neutron activation analysis (PGNAA) technology.

“We are also exerting considerable efforts to better define deposit heterogeneity at a scale that allows ore sorting to be considered and to be able to calculate the expected benefit of the technology,” Rairdan added.