Why be energy efficient?

Comminution (crushing and grinding), separation (froth flotation) and concentrate drying are the processes where most of the energy use and greenhouse gas emissions are derived in the mining and mineral processing sectors. Comminution is the most energy intensive of these processes. In some mining sub-sectors, such as gold, copper and nickel, comminution comprises over 70% of total energy use. Comminution energy costs are forecast to increase significantly over the next two decades due to the overall trend of declining ore grades. Some of these costs can be better managed through adopting more efficient comminution processes and technologies.

For information on mineral processing technologies, see Technology Background – Mineral Processing.

Opportunities

Many of the opportunities for improving the energy efficiency of mineral processing technologies are part of developing a holistic (whole-of-system) comminution energy efficiency strategy. There are also opportunities to improve the efficiency of mineral separation processes and of drying and dewatering.

For information on mineral processing technologies, see Technology background – Mineral processing.

Implement a holistic comminution energy efficiency strategy

The selection of appropriate comminution technologies, crushing and grinding mill configurations, depends on the feed-size distribution, the desired product size and other physical properties which define the material. To achieve the full potential of energy efficiency opportunities, a holistic (whole-of-system) approach can be adopted. Such a strategy should consider factors like:

  • resource characterisation
  • pre-concentration of ore grades
  • more efficient and fit-for-purpose technologies
  • flexible comminution circuits
  • efficiency of motor systems indirect energy use.

Some examples of opportunities in this area are outlined below.

Implement resource characterisation to target higher-grade ore

Ore concentration variability and other characteristics of rock types significantly influence mine-to-mill design and operational efforts to minimise total energy use. Typically, what geologists can predict about the ore body and processing performance from observations at the core scale is different to the experience of engineers.

Techniques in geometallurgy, i.e. the practice of combining geology or geostatistics with metallurgy, help to address these differing perspectives by better defining the concentration variability and characteristics of the ore body. This is achieved by performing many smaller volume (lower-cost) tests, then using the data obtained to construct a 3D geometallurgical model of the ore body.

The 3D model is used to inform a ‘smart blasting’ approach that targets the sections of the ore body with the highest ore-grade concentration. Leading companies, which have partnered with the CRC Optimising Resource Extraction (ORE) to apply these methods to their mines, have shown that this process can reduce business-as-usual trends in energy use per tonne of metal by 10–50%.

Geometallurgical mapping can also be combined with optimisation software to investigate a variety of solutions. For example, the Sustainable Minerals Institute (SMI) at the University of Queensland, in partnership with Anglo Platinum, has developed the Geology-Mine-Plant Management Tool. This application employs a 3D geometallurgical model to optimise the energy and water use, and greenhouse gas emissions across the whole geology-mine-plant extraction process.

The software also helps to optimise mine evaluation techniques, and enables both the optimal design of mine-to-mill circuits and the integration of energy efficiency into daily business.

Until recently, the geometallurgical approach has been limited by data and computing power. Recent progress in the area has been driven by:

  • rapid advances in geometallurgical measurement and analytical technologies, resulting in an increased ability to more accurately measure properties on smaller and smaller volumes of rock
  • exponential improvement in computing power.

For more information

Invest in pre-concentration of ore grades

An increase in ore grade can be achieved ahead of crushing. The CRC ORE at the University of Queensland has shown that it is possible to upgrade the concentration by as much as 2.5 fold in average ore concentration feed to the grinding mill through using smart blasting, ore sorting and gangue rejection at actual mines.

Figure 1. Selective blasting, ore sorting and gangue rejection (Source: CRC ORE, 2011)

These strategies are outlined in more detail below.

Use selective/smart blasting technologies

Conventional blasting explodes the entire block/region of a mine to achieve the top size that can be transported in haul trucks and processed through the primary crusher.  Selective/smart blast design technology raises the grade of ore being fed to the crusher and grinding mill by improving resource characterisation and reducing the net total energy consumed at the crushing and grinding stages. This is achieved in three ways:

  • less energy is required to crush the ore to the same product size if the feed-size to the primary crusher is decreased
  • additional macro- and micro-fracturing within individual fragments from the blasting makes fragments easier to fracture further, using less energy in the crushing and grinding phase
  • an increased percentage of relatively small mineral particles can bypass stages of crushing.

Software packages are available to assist in designing effective blasting techniques, including analysing and evaluating energy, scatter, vibration, damage and cost.

For more information

Use pre-concentration ore-sorting technologies

Automated ore-sorting applies optical sensors that can be coupled with electrical conductivity and magnetic susceptibility sensors. This controls the mechanical separation of ore into two or more categories. Depending on the type of ore, different sensor types can be used in sorting technologies, which include:

  • colour recognition sensors – recognise precious metals or gemstones by their hue
  • near infrared radiation sensors recognise minerals by their near infrared spectrum
  • x-ray fluorescence sensors – recognise diamonds by their fluorescence
  • x-ray transmission sensors – detect specific ores or coal via atomic density
  • radiometric sensors – identify elements by their radioactive decay
  • electromagnetic sensors – recognise metals through their conductivity.

Optical sensors determine if a particle contains valuable mineral or can be rejected as gangue. Upon detection, the system sends a signal to the computer which activates air jets that deflect the valuable mineral from the gangue, and thereby increase the overall ore grade.

Ore-sorting technologies assist energy-efficiency improvement by:

  • pre-concentrating ore underground or at remote sites to reduce haulage and hoisting costs
  • pre-concentrating mill feed into higher ore-grade concentrations, enabling the mill to process material at a very high concentration of ore grade, without low grade material and gangue driving down the average. 
  • providing real-time data to operators for process optimisation and work-index prediction.

For more information

Reject gangue using ore-sorting technologies

Gangue usually occurs in the ore body as large clumps that contain little or no valuable mineral. Much of it can be readily removed prior to comminution, significantly reducing energy use. Gangue is usually harder than the valuable minerals because it contains a high concentration of silicates. Ore bodies are typically 98% gangue mineral.  Once mined, gangue can be rejected by progressively processing the ore with a range of separation technologies. These include screens, density separators (such as heavy media circuits or drum separators) and magnetic separators.

Mineral characteristics can be determined by various methods, including optical, radiometric, x-ray and laser ore-sorting devices. These analyses can assist in early gangue rejection. Sorting coarse rock prior to comminution is usually the lowest energy-using separation and can be achieved at high-tonnage throughput rates.

The effectiveness of each device depends on the ore’s texture—defined by properties including mineral shape and grain size, and the association between different minerals.  A better understanding of ore texture is critical in the selection of an optimum ore-separation technology.

For more information

Apply more efficient comminution technologies

A wide range of comminution equipment is available for working with various materials and under differing conditions. The choice of equipment and design of circuits has a significant influence on energy use. Historically, semi-autogenous grinding (SAG) equipment  has been preferred for its ability to handle all ore types. However, modern improvements in high-pressure grinding rolls and stirred grinding mills mean these technologies can now be more widely used instead. Both are more energy-efficient than SAG technology.

Large-scale application of technologies for crushing, such as high-pressure grinding rolls (HPGR) and vertical shaft impactors (VSI), allow a finer crushed feed ahead of the energy intensive grinding circuits and reduces overall energy intensity. Studies show that in some mines the amount of input energy to the grinding process can be improved by as much as 40% by using the latest, most efficient equipment.

This energy-efficiency opportunity still exists for many mining companies as the application of HPGR and VSI devices has been very limited, despite many of the technologies being available to the industry for over 20 years.

Wear to the steel balls, media consumption and liner in a standard grinding circuit contributes 50–100% more overhead to the direct energy consumption used to produce replacement materials. Grinding media consumption and liner wear can be reduced by technologies like high pressure grinding rolls (HPGR) that do not use high embodied energy grinding media.

Advances in computer modelling, by CSIRO and the University of Queensland can be applied to optimising the design of fit-for-purpose comminution equipment to achieve still larger energy savings.

High-pressure grinding roll mills

For coarser grinding, high-pressure grinding rolls (HPGR) are being applied, often as a replacement for the conventional semi-autogenous grinding (SAG) mill, primarily to reduce energy consumption and costs within crushing and grinding circuits.  Studies show that, under various conditions, an HPGR circuit can achieve a 15–20% reduction in direct energy (kWh/t or energy cost) compared to traditional SAG mills. HPGR circuits reduce the need for grinding media, and can also provide around 25% reduction in operating costs. These results appear to be a generic feature of HPGR circuits irrespective of the operating conditions.

HPGR has the additional benefit of not using high embodied-energy grinding media and therefore helps reduce energy-use over the entire lifecycle.

High-intensity stirred grinding mills

Stirred milling technology has been firmly established in recent decades as being more energy efficient than conventional milling for regrinding applications. Ceramic media can expand this technology for use with coarser grind sizes and, due to recent developments, is now more economically viable to implement.

For more information

Use more flexible comminution circuits

Since the early 1980s, many mining companies have moved to using a single comminution circuit with very large semi-autogenous grinding (SAG) mills. This approach has enabled expansion into processing large, low-grade ore bodies and to treat large volumes. However, mineral resources are heterogeneous and have variable characteristics and properties. Also, ore processed in the first few years of a mine is usually not the same as that obtained in later years from the greater depths of the site. Typically, the ore changes in physical and mineralogical characteristics.

The disadvantage of the conventional approach is that comminution becomes less efficient as ore body changes. This is because the single operating-circuit had been optimised with initial ore grades and properties. To overcome this, many companies moved to comminution circuits with at least two (some use more than four) parallel milling circuits and high-pressure grinding roll (HPGR) technologies.

With this multiple-circuit approach, high- and low-grade ores are processed simultaneously but on separate circuits, enabling each grade to be ground closer to its optimum recovery size, which increases grinding efficiency and reduces energy use. HPGR technologies machine settings can be adjusted to react to ore variations whilst maintaining a high throughput. This makes them ideal technologies to add to existing and new comminution circuits.

For more information

Improve the energy efficiency of motor systems

Electric motors drive crushing and grinding mills, consuming a significant amount of energy in comminution. As comminution mills work to process more throughput, using the most appropriate combination of motor systems will directly improve performance, flexibility of operation, and total energy efficiency of the mill. This requires detailed investigation and selection of motors.

Variable speed drives enable a mill to run faster or slower, thereby finding its optimum operating point. This assists in maintaining a consistent particle output-size regardless of differences in ore grade or liner wear effect.

Recent developments have made motors with variable speed drives the smart technological option for comminution plants. Faster, more accurate controllers, improved components and cost reductions have revolutionised motor drive control methods.

The volume of installed AC variable speed drives is increasing. Variable speed drives enable significant energy savings (for more information, see Motors and Motor Systems). In this instance, they also provide enhanced flexibility in the comminution circuit process.

Barrick Gold Corporation achieved 4.4% energy efficiency improvements in the motor systems in their mills. For more information on opportunities in motor systems technologies, see Motors and Motor Systems .

For more information

Improve froth flotation efficiency

Froth flotation is a method of mineral separation in fluid which relies on the difference between the chemical composition of minerals compared to gangue. Flotation cells (tanks of fluid) are aerated to produce bubbles. The high-grade mineral particles attach to the bubbles, which rise to the surface and form a froth which can then be recovered. The froth produces a concentrate of the targeted mineral.

For some mineral processing sites, it is still possible to invest in more energy-efficient froth flotation cells, such as the Jameson Cell which produces smaller bubbles more consistently than earlier cell types. Additionally, mixing and adhesion occur more quickly and in a smaller space, and a higher percentage of mineral is recovered. The Jameson Cell requires a pump but has no need for a motor, air-compressor or moving parts.

Other actions for improving the efficiency of separation processes involve mixers and agitators. Mixer and agitator manufacturers are citing significant energy reductions (30–50%) through the use of new blade and tank designs. Analysis of cell efficiencies may reveal opportunities for further energy savings.

For more information

  • Froth Flotation Control 2011
    • International Journal of Mineral Processing

    This review looks at each of the four essential levels of process control (instrumentation, base level flotation control, advanced flotation control and optimising flotation control) and examines current and future trends within each sub-level.

    Note, this publication is free to access, but users must register first.

  • Dewatering and Drying in Mineral Processing Industry 2010
    • Drying Technology

    This article provides a global overview of the types of dewatering equipment and dryers currently used in mineral processing industries and identifies new concepts that may be applicable in the minerals industry.

    Note, this publication in free to access for members, but non-members must purchase the article.

Improve the efficiency of dewatering and drying

Drying and dewatering in mineral processing reduces the energy required for materials movement. However, at mine sites where concentrate is processed, drying is itself a significant energy-using activity. Therefore, where feasible, mining sites should use:

  • waste heat to reduce the energy required for drying
  •  solar drying of concentrate.

For more information

Key resources

  • Review of Existing Eco-Efficient Comminution Devices 2011
    • Coalition for Eco-Efficient Comminution

    This resource provides comprehensive information on the different technical efficiencies for various types of size-reduction devices. The review also considers attributes of each device from the viewpoint of eco-efficiency.

  • Efficiency, Economics, Energy and Emissions – Emerging Criteria for Comminution Circuit Decision Making 2011 (Opens in a new window)

    This paper presents the summarised results from the completion studies of circuit technology comparisons conducted in the past few years. An additional sustainable aspect incorporating the impact on environment is evaluated.

  • Eco-Efficient Comminution Resources 2012
    • Coalition for Eco-Efficient Comminution

    The Coalition for Eco-Efficient Comminution (CEEC) has been established and supported by a broad range of mining sector companies keen to accelerate awareness, knowledge transfer and, by implication, improve energy and cost outcomes in the substantive area of comminution. CEEC is a global initiative, providing access on the most current technical scientific papers and field studies to the worldwide mining industry.