There is significant potential to reduce energy costs in mining through an integrated approach to energy-efficiency investment.
Applying energy-efficiency strategies to comminution, the largest area of energy usage, usually offers the best scope for savings. Opportunities for energy savings exist in other areas including:
- blasting and sorting
- froth flotation separation
- materials movement
Examples of opportunities in these areas are outlined below.
Energy-mass balance assessment
An energy-mass balance (EMB) to model energy use is the basis for an effective energy management system. An EMB assists understanding of energy flows, mass flows and other factors to determine the efficiencies of processes and equipment.
EMBs can reveal significant energy and cost savings by identifying:
- how much energy is being used, wasted or lost through fixed energy and start-up/shut-down losses
- whether systems and equipment are operating according to design and work schedules
- energy use variability and its underlying causes
- if useable waste heat is being produced, or if processes could be powered by other sources.
See the Energy-mass balance for mining guide for more information on how to conduct a mining EMB.
Ore concentration variability of rock types significantly influences mine-to-mill design and efforts to minimise total energy usage. Geologist predictions about ore body and mineral processing performance from observations at the core scale are often different to the reality faced by engineers.
Geometallurgy helps to address this difference by performing many smaller volume (lower-cost) tests. The data is then used to construct a 3D model of the ore body. The model informs a smart-blasting approach that targets the highest ore grade concentrations.
3D models of the ore body can also enable best design of mine-to-mill circuits, and the integration of energy efficiency into measurements of energy use per unit of metal produced.
Blasting and sorting
Selective smart blasting, ore-sorting and waste removal can increase ore grade ahead of crushing or grinding by removing gangue.
Selective smart blasting
Conventional blasting affects the entire block/region of a mine to achieve the top size for transporting and processing. Selective smart blast design uses data to target relatively high ore concentration sections with greater blast energy.
This significantly improves the grade of ore fed to the mill. The net total energy consumed at the crushing and grinding stages is reduced.
Software is also available for designing blasting techniques, including analysing and evaluating energy, scatter, vibration, damage and cost.
Ore-sorting and waste removal
Gangue occurs in the ore body as large clumps. It is usually denser than the valuable minerals because it contains a high concentration of silicates.
Sorting can help the progressive upgrade of ore concentration. This enables the mill to process at a very high concentration, without gangue driving down the average. The sorting criteria should also be integrated with the mine plan and blast design to ensure only the right parts of the ore-body are sent for blasting and sorting.
Once mined, gangue can be rejected by processing through a series of separation devices. Optical, radiometric, X-ray and laser ore sorting can also be used for gangue rejection. The effectiveness of each device depends on the ore’s grain size, shape and the association between minerals.
Compressed air systems are used for blasting and to operate mucking machines and other equipment including power-tools.
Compressed air energy efficiency can be improved by:
- adhering to maintenance schedules
- identifying and fixing leaks
- using variable speed drives
- compressed air systems that can run closer to full load.
Where de-watering is required, pumping systems can be optimised by:
- more efficient motors and pumps
- smooth pipes with a large diameter
- running pumps continuously at low speed instead of short periods at high flow.
In open-cut mines, rather than pumping water from the bottom of the mine up to the top, the water can be put into dust-suppression water tankers at the bottom. Tankers can spray water while driving it uphill (tankers usually spray while driving downhill).
In underground mines, the energy used for de-watering can be partly offset by using a simple turbine. This will convert the potential energy of the down-flowing chilled water used for air-cooling or in the mining process.
Comminution (crushing and grinding) is responsible for at least 40% of total energy use in mining and mineral processing. Improving flow sheet design will:
- maximise gangue rejection ahead of the next downstream step
- ensure the most energy-efficient crushing and grinding technologies.
There are many specific energy-efficiency opportunities for comminution, which are outlined below. These are best applied to the design of greenfield comminution circuits when an increase in capacity is required or a change in ore hardness is expected for an operating circuit.
Electric motors are used to drive crushing and grinding mills and consume a significant percentage of energy in comminution. As comminution mills work to process more throughput, using the most appropriate combination of motor systems will directly improve performance. Getting this right requires detailed investigation and selection of motors and circuit design.
See the Motors and variable speed drives guide for more information.
Significant amounts of energy going into the grinding process can be saved by using more efficient equipment. Computer simulations using the discrete element method show that most rocks larger than the discharge grate size do not break in the first collision. Instead, these rocks accumulate damage in multiple collisions before breaking, which is an inefficient use of energy.
The choice of equipment and design of circuits has a significant influence on energy use.The practical energy efficiency of a milling process (the fraction of energy input used in breakage) is approximately 40% when using semi-autonomous grinding (SAG) equipment. Replacing this equipment with a high-pressure grinding roll (HPGR) circuit can improve the energy efficiency of the milling process to 80%.
In addition, the combined use of energy-efficient crushing and fine grinding equipment helps reduce energy use through the following:
- Reducing primary and secondary recirculating loads, for lower power requirements, less ore to handle and a switch to a smaller mill.
- Creating a steeper distribution of particle sizes for easier mineral liberation and more-efficient downstream processing.
- Reducing the need for grinding media that has high embodied energy. For example, HPGR circuits eliminate the need for high embodied energy grinding media.
The reduction ratios for each successive crushing and grinding process influence the distribution of particle sizes and the energy use in the process. Energy use is relatively low when particle sizes are consistent. Finer particles resist breaking and are displaced, causing energy to dissipate. Larger particles reduce grinding efficiency. Screens and filtering devices help achieve a more consistent particle size.
The target product size influences the size and energy use of a comminution circuit. As the product becomes finer, the internal flaws in each particle decrease.
An alternative approach for the selection of a target product-size for multi-mineral ores is the progressive liberation method. This involves liberating one mineral at a time by applying the following concepts:
- Multiple valuable minerals are grouped, increasing their effective concentration, and enabling liberation to be achieved at coarser target product sizes.
- Fully liberated particles (100% valuable mineral) are recoverable in a flotation process.
- Particles containing at least 15% valuable mineral by sectional area are recoverable in a flotation process using the appropriate conditions and reagents.
If minerals are sufficiently liberated or recoverable they can be separated from the ore before further comminution. This strategy can also remove gangue from the ore which leads to less grinding energy and more efficient separation in downstream processes.
Advanced and flexible comminution circuits
Using a single comminution circuit with very large SAG mills has enabled companies to expand into large, low-grade ore bodies. A disadvantage of this approach is comminution becomes less efficient as ore body concentrations decline with only one circuit operating.
Many companies address this problem by using two parallel milling circuits. This allows high and low grade ores to be processed simultaneously but on separate circuits, enabling each grade to be ground closer to optimal recovery size.
The discrete element method (DEM) is increasingly useful as a tool that can help provide fundamental insights into comminution processes and the behaviour of specific comminution machines.
DEM can contribute to:
- design and rapid manufacture of comminution equipment
- increased operational efficiency of all comminution unit processes
- greater understanding of particle flow and breakage processes within comminution equipment through DEM modelling.
Separation (froth flotation)
Froth flotation is a method of mineral separation which relies on the different chemistry between minerals and gangue. Energy savings are possible through using more advanced froth flotation technologies and control engineering.
Technologies such as the Jameson Cell produce smaller bubbles more consistently than previous flotation cells. Mixing and adhesion occur more quickly and in a smaller space compared to a traditional froth flotation cell.
A higher percentage of mineral is recovered, improving the economics of a mine. The Jameson Cell also has no need for a motor, air compressor or moving parts.
The inventor of the Jameson Cell, Professor Graeme Jameson, has developed a new fluidised bed flotation machine that can handle much larger particles than can be processed in conventional flotation machines. More information is in the Innovations section on this page.
Material movement is usually the largest area of energy use after comminution. It consumes more than half the total energy used in mining sub-sectors such as iron ore and bauxite.
Conventional hauling of mineral ore, overburden and waste using diesel powered trucks on gravel roads, increases rolling resistance.
Improve the fuel efficiency of haul trucks through fleet optimisation and upgrades. Examples include:
Driver training. Economical driving (eco-driving) practices incorporate a range of behaviours such as gentler acceleration and braking, and driving slower with less idling.
Better haul efficiency. Payload management ensures that each haul truck carries optimum tonnage for best fuel efficiency and can reduce the number of trucks required.
Measuring haul truck performance. Performance indicators use an equivalent flat haul calculation to account for elevation changes on a specific mine route. ‘Best truck ratio’ models evaluate and benchmark the efficiency of fleet operations across a single or multiple site operation.
Haulage system upgrades. Larger coal trucks will reduce the number of trucks in a circuit. Lightweight, hybrid diesel electric trucks are fuel-efficient and can recover energy through regenerative braking on descent into a mine. Trolley trucks on tram power lines can access or feed in electricity to enable energy to be recovered as the trolley trucks descend back into the mine.
Alternatives to haul trucks
There are alternative material movement methods to complement haul trucks. These include:
Conveyor belt systems. These are significantly more efficient in transporting materials than haul trucks, using about 20% of the energy required by heavy-duty trucks. There is also scope to improve their performance through optimisation using simulation models and better monitoring and management.
In-pit-crushing-conveyor. IPCC systems are the most energy-efficient means for hauling ore, overburden and waste from open cut mines. IPCC systems do have significantly larger upfront costs, however, compared to haul trucks.
Overburden slushers instead of electric draglines. An OS uses 2 winches to drag a large bucket across the overburden, then to the top of the mine. Existing draglines can be converted.
Draglines. Electric motors can be upgraded, the ropes and motors strengthened and the bucket and rigging configuration revised. This will decrease the weight of the system while increasing the weight it can carry.
Mineral ores are often transported long distances. Conveyor belts across horizontal distances as well as freight rail are both more energy efficient than trucking on gravel roads.. The design and operation of these systems can be made more energy efficient. Analysis of freight movements can also lead to fuel efficiencies through improved scheduling and a reduction in stop/start events.
For underground mines, ventilation is a necessary and significant use of energy.
The biggest savings in ventilation come from reducing the area cooled. Fan and pumping energy losses are often high due to the long distances air and chilled water must be transferred
Localised systems using the latest high-efficiency air conditioners, fans and pumps will be more efficient. Reducing air or water flows by even a few percent can offer disproportionately large energy savings, so consider the use of variable speed controls.
For coal mines, the economics of using ventilation exhaust air as an input to on-site electricity generation are constantly improving. Identifying areas of high-flow resistance (pinched ducts or pipes, sharp corners) can also add to savings.
Relatively easy savings can also be achieved through maintenance improvements. Fan impellers or blades should be regularly cleaned to avoid fouling, which causes static pressure losses.
Waste heat recovery
A waste heat recovery plant can use excess heat from the kiln to generate electricity. Kiln off gases generate steam which drives a turbine generator that powers the downstream physical and chemical separation stages of the plant. Waste heat can also be recovered to provide cooling (via absorption or adsorption chillers).
See the Waste heat recovery guide for more information.
The energy efficiency of offices, staff facilities and accommodation can be maximised by using LED lighting, HVAC and hot-water systems. Efficient office equipment and appropriate management of it can offer substantial savings.
Shading, management of air-leakage and light-coloured buildings can improve comfort and cut energy costs. Paying attention to thermal bridging (such as heat leaking around insulation via metal framing) can also bring large dividends. A thermal imaging camera can be used to identify heat leaks.
See the Commercial building owners and tenants guide for more information.
Fluidised bed flotation cell
Historically, mineral ores have been crushed and ground down to a size small enough to undergo froth flotation to separate out the valuable minerals. There is an upper limit on the size of the particles that can be treated so all material to be floated must be ground below this size.
A new fluidised bed flotation machine, the NovaCell, can handle particles that are much larger than can be processed in conventional flotation machines. The device was developed by University of Newcastle’s Laureate Professor, Graeme Jameson. NovaCell can drastically reduce the mining industry’s energy and water consumption, and greenhouse gas emissions world-wide.
The NovaCell process can reduce energy costs in a typical base metal mill by 40%. This translates to 10% of the total operating cost of a mine that produces a metal like copper, zinc or nickel. Another advantage of the NovaCell is that the rejected material is much coarser than found in current practice. The reject looks like beach sand, which drains easily.
Any water it holds can be recovered and re-used in the mineral concentrator. In one operation, the NovaCell addresses two issues of major importance to mining and mineral processing globally – reducing energy consumption and minimising water requirements. [Source: University of Newcastle]
Alternatives to underground diesel use
Diesel engines are widely used in underground mining vehicles, meaning ventilation systems need to work extra hard to dilute or dispel the fumes. Alternatives to diesel, such as hydrogen, liquefied natural gas, and lithium-ion batteries are advancing in development and cost-effectiveness.
Deep sea mining
Deep sea mining is a process that a number of nations and companies are exploring because, in certain parts of the ocean, minerals exist at higher ore grades than on land. Advances in underwater drilling are making operations more feasible than in the past.
Studies are needed to determine if overall deep sea mining uses less energy than land based mining. The high ore concentrations in the deep sea beds suggest it may take significantly less energy to recover the same amount of valuable minerals.
Mineral exploration in Australia (Geoscience Australia) Australian Government
Renewable energy in the mining sector (Australian Renewable Energy Agency) Australian Government
Mining, Drilling and Civil Infrastructure Australian Industry and Skills Committee
Sustainable Minerals Institute University of Queensland