Progress on energy efficiency is underpinned by corporate support and an energy management team to enable the design of a whole-of-mine energy management system. An energy management system provides a framework within which to undertake effective energy use measurement, analysis, as well as identification and implementation of energy efficiency opportunities.
Use an energy-mass-balance assessment approach
Developing an energy-mass-balance (EMB) to model energy use provides a starting point to underpin an effective energy management system. An EMB assists understanding of the energy flows, mass flows, and other factors influencing energy use, to determine the efficiencies of processes and equipment. Thorough EMBs can reveal significant energy and cost savings by identifying:
- how much energy is being used, wasted or lost through, for instance, fixed energy overheads (vehicle idle, ventilation, conveyors left on) and start-up/shut-down losses (truck cool-down)
- 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 processes could be powered by other energy sources
Achieving optimal energy-efficiency levels requires accurate information on the ore bodies and rock feed for mineral extraction and comminution. Identifying and characterising mineral ore bodies enables the highest concentration to be targeted for blasting and extraction.
Selective blast design, combined with ore sorting and gangue rejection, improves the grade fed to the crusher and grinding mill with large reductions in energy use.
The level of ore concentration variability and other characteristics of rock types significantly influence mine-to-mill design and operational efforts to minimise total energy usage. Typically, geologist predictions about the ore body and mineral processing performance from observations at the core scale are different to the reality faced by engineers.
Geometallurgy helps to address this difference by performing many smaller volume (lower-cost) tests, then using the data to construct a 3D geometallurgical model of the ore body. The model is informs a smart-blasting approach that targets the highest ore grade concentrations.
Leading companies, in partnership with the Cooperative Research Centre for Optimising Resource Extraction (CRC ORE), have shown that this process can reduce business-as-usual trends in energy use per tonne by 10–50%.
3D models of the ore body can also enable the best design of mine-to-mill circuits and the integration of energy efficiency into the measurement of energy use per unit of metal produced.
Selective smart blasting, ore-sorting and waste removal can increase ore grade ahead of crushing or grinding by removing gangue. The CRC ORE at the University of Queensland has shown it’s possible to achieve up to a 2.5 fold increase in average mineral ore concentration feed to the grinding mill in this way.
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 technology uses data to target relatively high ore concentration sections of 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.
Savings of up to 30% have been reported. Software is also available to assist in designing effective 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.
Ore 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. These devices include density separators and magnetic separators.
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 texture, defined by properties including grain size, shape and the association between minerals.
Comminution (crushing and grinding) is responsible for at least 40% of total energy usage in mining and mineral processing. Improving flow sheet design reduces the direct and indirect energy for comminution through:
- maximising gangue rejection ahead of the next downstream step
- ensuring the most energy-efficient crushing technologies ahead of the energy-intensive grinding step
- ensuring the most energy-efficient grinding technologies
There are many specific energy-efficiency strategies for comminution, which are outlined below. These methods are best applied to the design of greenfield comminution circuits, when an increase in capacity is required, or when a change in ore hardness is expected for an existing operating circuit.
Use more energy-efficient grinding technologies
Studies show that in some mines the energy going into the grinding process can be improved by as much as 40% 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.
A range of comminution equipment is available for many materials under different conditions. The choice of equipment and design of circuits has a significant influence on energy use. For example, the practical energy efficiency of a milling process, defined as the fraction of input energy that is utilised in breakage, is about 40% when using semi-autonomous grinding (SAG) equipment.
The input energy could be halved, giving an energy efficiency of about 80% more through an efficient high-pressure grinding roll (HPGR) circuit over the traditional SAG circuit.
In addition, the combined use of energy-efficient crushing and fine grinding equipment helps reduce energy use by:
- reducing primary and secondary recirculating loads, leading to lower power requirements, less ore to handle, and potentially a switch to a smaller mill
- creating a steeper distribution of particle sizes, leading to easier mineral liberation and more-efficient downstream processing
- reducing the need for grinding media which has high embodied energy. For example, HPGR circuits eliminate the need for high embodied energy grinding media.
Select the coarsest possible grind size
The target product size has a large influence on the size and energy use of a comminution circuit. As the product becomes finer, the internal flaws in each particle become fewer, the particles become stronger, and the grinding energy increases.
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 the desired level of 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 flotation conditions and reagents.
If minerals are sufficiently liberated or recoverable, then they can be separated from the ore before further comminution. This strategy can also be used to remove gangue from the ore, leading to less grinding energy and more efficient separation in downstream processes.
Optimise particle size
The reduction ratios for each successive crushing and grinding process influence the distribution of particle sizes and the energy use of 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. A consistent distribution of particle sizes produces superior flotation performance.
Use more 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 that comminution becomes less efficient as ore body concentrations decline but there is only one circuit operating.
Therefore, many companies have moved to using comminution circuits with at least 2 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.
Discrete element method (DEM) is increasingly useful as a tool that can help provide fundamental insights into comminution processes and into the behaviour of specific comminution machines. It can contribute to the design and rapid manufacture of comminution equipment, improvement of existing equipment, and increased operational efficiency of all comminution unit processes.
For example, the DEM modelling can now allow detailed exploration of particle flow and breakage processes within comminution equipment. It can also assist in developing a clearer and more comprehensive understanding of the detailed processes occurring within.
Improve the efficiency of separation processes
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.
For example, 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.
Improvements are also being made in control engineering of flotation systems.
After comminution, materials movement tends to be the next largest area of energy usage consuming more than half the total energy used in mining 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.
There are ways to improve the fuel efficiency of haul trucks through fleet optimisation and upgrades. There are also alternative material movement methods to complement haul trucks:
- pit mobile crushers
- conveyor systems
- overburden slushers
- electric draglines
- lighter haul trucks
- diesel-electric trolley haul trucks
Some examples of opportunities in this area are outlined below.
Optimise hauling efficiency in existing truck fleets and mines
Actions that can improve the energy efficiency of existing haul truck fleets in mining include:
Optimising payload management - payload management ensures that each haul truck carries the optimum tonnage of material to increase fuel efficiency. In some cases this approach can also reduce the number of trucks required to complete tasks. For example, Thiess implemented payload management systems, identifying energy- efficiency opportunities which save up to 117,300GJ and 8200tCO2e per year.
Implementing improved driver practices
Eco-driving incorporates a range of behaviours such as smoother driving, gentle acceleration and braking, and driving more slowly with less idling.
Fortescue Metals Group quantified the energy costs associated with stopping haul trucks unnecessarily, which equated to 361kL (13,935GJ) of diesel per year for the Caterpillar 777 fleet and 407kL (15,710GJ) of diesel per year for the Terex 3700AC fleet for a single stop sign per payload cycle.
Purchase larger haul trucks
The purchase of larger coal trucks at for the Jellinbah East Mine in central Queensland will reduce the number of trucks in a circuit. This will reduce fuel use by 151,893L or 5,863GJ per year.
Measure and analyse haul truck energy performance
Downer EDI Mining developed performance indicators that use an equivalent flat haul calculation to account for elevation changes on a specific mine route. The indicators provide a more consistent measure of true energy performance, enabling the company to track energy intensity over time. The Commodore open-cut coal mine in south east Queensland has been used as the pilot site for energy-efficiency innovations. Energy intensity of the mine improved by 18% over 5 years.
Benchmark and compare performance across the haul truck fleet
Leighton Contractors developed a ‘best truck ratio’ model to evaluate and benchmark the efficiency of fleet operations across a single site and multiple operations, where the nature of the work varied greatly.
This model provides an indication of how efficient their fleet is in comparison with what is realistically possible.
Consider energy efficiency when upgrading haulage systems
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. For example, RioTinto in Namibia, has adapted this idea to diesel electric trucks so they can save fuel and recover energy on their descent into the mine.
At this mine, Rio Tinto has invested in overhead wires so that haul trucks with diesel-electric units can draw power like a trolley bus. This reduces fuel consumption with a payload of 182t from 350L an hour to 25L an hour. Although it consumes electricity, overall energy savings of up to 30% can be achieved.
Also consider the benefits of complementing the use of haul trucks with the following:
Conveyor belt systems - Shown to be significantly more energy 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 improved 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.
Improve the efficiency of existing draglines - Electric motors can be upgraded, the ropes and motors strengthened and the bucket and rigging configuration revised to decrease the weight of the system while increasing the weight it can carry.
Improve the efficiency of product transport
Mineral ores are often transported long distances. Conveyor belts across horizontal distances are significantly more energy efficient than trucking on gravel roads. Freight rail also is more efficient than trucking. The design and operation of conveyor belts, trucking, and freight rail themselves can also all 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 significant area of energy usage. Energy savings can also be achieved through ensuring ventilation supply matches demand, minimising energy use in air and water flows and by reducing the area required to be cooled.
Often fan and pumping energy losses are high due to the long distances air and chilled water must be moved. Localised systems using the latest high-efficiency air conditioners, fans and pumps will be more efficient.
Maintain and optimise fan system operations
Relatively low-cost energy savings can be achieved through maintenance improvements. For example, fan impellers or blades should be regularly cleaned to avoid fouling in dusty environments, which causes static pressure losses.
Energy-efficiency savings can also be achieved by ensuring ventilation supply matches demand. As ventilation is a major health and safety issue, many mines run the systems harder than necessary. The systems are also subject to changing characteristic curves as the workings move. This means a system that is initially optimised will deviate from this optimum over time.
Minimise energy use in air and water flows
The largest savings in ventilation come from reducing the area cooled. Often fan and pumping energy losses are high due to the long distances air and chilled water must be moved.
Localised systems using the latest air conditioners, fans and pumps can be more efficient. Reducing air or water flows by even a few percent can offer disproportionately large energy savings, so variable speed controls can achieve large savings.
Researchers are working on lightweight personal cooling units, which may allow space conditioning to be reduced.
For coal mines, the economics of using ventilation exhaust air as an input to on-site electricity generation are also improving. Identifying areas of high-flow resistance (pinched ducts or pipes, sharp corners) can also add to savings.
Reducing on-site energy demand and recovering waste heat can bring additional cost-effective energy savings.
Active demand management can help to avoid the need for investment in generation capacity. For example, slowing pumps, dimming lights and cycling non-essential air conditioners and other equipment can limit peak demand. Insulation and shading of buildings and equipment can also cut peak cooling loads.
A lot of lighting is needed as mines are often 24-hour operations. Moving to more efficient lighting is a relatively simple step operationally that can bring rapid returns on investment to help fund other energy-efficiency investments.
The energy efficiency of offices, staff facilities and accommodation can be maximised by using high-efficiency lighting, HVAC and hot-water systems. Selection of efficient office equipment and appropriate management of it can offer substantial savings.
In hot regions, additional insulation, 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 pay large dividends. A thermal imaging camera can be used to identify heat leaks.
Waste heat recovery
Waste heat can be recovered to provide both electricity and steam. For example, Xstrata Copper has invested in waste heat recovery at its Mount Isa Mines copper smelter to produce steam and electricity to produce 77,000MWh thereby avoiding the consumption of 1.15PJ of natural gas.
Iluka Resources Limited’s South West synthetic rutile plant incorporates a waste heat recovery plant which uses the waste heat from the kiln to generate electricity for the rest of the operation. 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).
Further energy efficiency opportunities can be achieved by implementing improvements in specific technologies, such as motors, pumps and fans, lighting and air compressor systems. These systems consume a significant amount of energy in mining and mineral processing.
For example, to improve their pumps, Xstrata Copper replaced ‘two thickener underflow pumps at the Mount Isa Mines copper smelter to improve pumping efficiency thereby reducing power consumption by approximately 1325GJ per year. Xstrata also replaced compressor cooling water pumps at the Xstrata Copper Townsville Refinery to deliver an estimated annual energy saving of 630GJ.
Compressed air systems are used to blast coal and to operate stoppers, mucking machines and other equipment requiring pressured air flows. Compressed air energy efficiency can be improved by adhering to maintenance schedules, identifying and fixing leaks, using variable speed drives and selecting compressed air systems where they can run as close as possible to full load.
Investments in resource characterisation, smart blasting and ore concentration upgrade can save water as well as energy in the comminution and mineral processing stages.
Where de-watering is required, the energy efficiency of pumping systems can be optimised by using efficient motors and pumps, using smooth pipes with a large diameter, and running the 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, which tankers can use to spray water while driving 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 to convert the potential energy of the down-flowing chilled water used for air-cooling or down-flowing water used for the mining process. Xstrata has invested in an underground Pelton Wheel generator at the Xstrata Mount Isa Mines copper operations.