Overhauling the daily grind

Online Editor

Ata Douzdouzani discusses the power of water-cooled drives in sustainable grinding mills.

Right now, the global mining industry is caught between a rock and hard place. It’s expected to increase output and productivity but reduce costs and carbon emissions. Here, we explore how water-cooled drives in grinding applications can help solve this conundrum and play a pivotal role in the decarbonisation of the mining sector.

The pursuit of sustainability targets is fuelling a global rise in demand for elements obtained by mining. Lithium, nickel and cobalt; graphite, copper and aluminium – all are vital to the manufacture of clean energy technologies and enhanced energy efficiency. To meet this rising demand, the mining industry must increase its output.

But mining is not without its own sustainability goals; it too is expected to lower its carbon emissions in line with global sustainability targets, despite the predicted increase in the energy intensity of its operations. The only way forward is to improve energy efficiency across the value chain.

It’s worth noting that comminution, the process in which solid materials are reduced in particle size by crushing, grinding or cutting, accounts for a significant portion of energy consumption in mining operations. A mining energy consumption report, conducted by Engeco and commissioned by the Weir Group, revealed that the process consumes close to 40% of the total energy used in mining operations, with the grinding mill as the largest single consumer, making it a key area for energy conservation.

Enhancing energy efficiency

Generally, large-scale mining machinery – such as ball mills, rod mills and roller presses – is operated by medium voltage (MV) variable speed drives (VSDs), which are already renowned for their energy saving qualities. However, it would be prudent to carefully consider the optimal design of the complete VSD system, especially the choice between air-cooled and water-cooled drives. Especially because the latter boasts additional energy and cost benefits.

Let’s consider air-cooled drives first. An air-cooled MV drive setup comprises the VSDs and integrated transformer, together with upstream switchgear, often all housed in an electrical room, otherwise known as an ‘E-room’. All the heat losses from the equipment into the E-room must be handled by a large heating ventilation and air-conditioning (HVAC) system, with a considerable impact on both upfront investment and operating costs, including energy usage and maintenance. This comes with a relatively large installation footprint on sites where space can sometimes be at a premium. In general, the efficiency of this approach is typically lower than 96.5%.

Furthermore, with the arrangement described above (an air-cooled MV drive with integrated transformer), a separate, external step-down transformer is needed from the 33kV grid supply to the motor voltage of 11.6kV or 3.3kV. This incurs further energy losses of 1% while adding further to the financial burden.

In contrast, a growing number of mining sites are adopting water-cooled drives paired with an external ONAN- (oil natural and air natural) cooled transformer. Only the VSD panel has to be installed in the E-room, which is therefore much smaller. No step-down transformer is required. Direct connection at 33kV means that just a single, external transformer is used, which also acts as the converter transformer.

The cooling water for the drive is usually supplied by the plant’s common water system. If this is not available, then a closed-circuit cooling system is created using a chiller or fin-fan heat exchanger. Even in this case, the energy losses are much lower than when using an HVAC system to cool the air in the E-room. In some cases, it might be possible to recover heat from the cooling circuit for use in other parts of the process to further boost the overall system efficiency.

Water-cooled drive systems achieve an efficiency of greater than 97.5% That means they will have energy losses of some two percent lower than the equivalent air-cooled drive system. And because water-cooled drives generally run cooler than air-cooled versions there is less stress on their critical electronic components. This helps ensure a longer service life.

The combined impact of the reduced losses, along with the reduced cost of equipment and a lower installation footprint, makes water-cooled drives the optimal choice for grinding plant in terms of the total cost of ownership (TCO) throughout a 20-year anticipated service life.

Protecting the environment

Water-cooled drives offer significant environmental benefits compared to air-cooled drives. Not only do they help to reduce the energy required for cooling, resulting in a smaller carbon footprint, but they also minimise the release of heated air into the atmosphere, lessening the impact on local air quality. This is particularly important in mining operations where air pollution can have adverse effects on nearby communities and ecosystems.

Water-cooled drives tend to produce less noise compared to air-cooled drives, creating a more comfortable environment for operators and maintenance crew. Another comfort factor is the reduced level of airflow since – unlike air-cooled drives – the cooling system does not have to blast several cubic metres of air per second.

Sometimes the excess heat generated during the cooling process can be repurposed for various applications within the mining operation, such as heating nearby facilities or powering additional processes. This helps to maximise energy efficiency and reduce overall waste.

Defence against dust, particles, and electrical explosions

Air-cooled drives don’t usually have strong ingress protection against things such as dust and particles from entering the system, unless they have special ducting. Even then, some air can still get in. So air-cooled drives need filters in the HVAC system, to prevent dust and particles from entering. But in dusty environments such as mining sites, these filters need to be replaced frequently.

On the other hand, water-cooled drives can have a higher level of ingress protection. The standard rating for water-cooled drives is IP54, which protects against dust and even conductive particles, which is particularly important since it reduces the risk of electrical explosions
or arc flash.

Water-cooled drives are the ideal companion for HGPRs

High-pressure grinding rolls (HGPRs), also called roller presses, are fast gaining acceptance in hard-rock processing, mainly because of their energy efficiency when reducing mineral ore to into fragments. With energy, cost and water savings, a smaller installation footprint, and extended availability, it’s no wonder we are seeing a growing number of HGPRs within crushing and grinding circuits.

Drives play an important role in addressing one of their main challenges in the field. This is because wear and tear on the rolls may cause feed variance, which affects crushed ore quality.

An HPGR with a brand-new set of rollers starts operation with identical torque and speed. Depending on various operating factors, the rollers experience different wear rates over time. These factors include feed size, ore characteristics, and operating pressure.

Using a drive enables the grinding rolls to operate at optimal speed and provide accurate load sharing between the two rolls that can help reduce the impact of wear and tear. Water-cooled drives provide this functionality, and with their emphasis on efficiency and compactness, they are the perfect partner for HPGRs.

Case study: industry leading energy efficiency for Australian mine

The Iron Bridge Magnetite Project in Western Australia aims to produce 22 million wet tons of high-grade magnetite concentrate for use in steel production. To achieve energy efficiency, ABB-supplied drives with external transformers are used in eight switch rooms, reducing heat and room size by half. These drives connect to a 33kV network, eliminating the need for an intermediate switchboard and reducing costs.

Compact ABB high-voltage induction motors power equipment such as HPGRs, offering high efficiency in a smaller size. The project’s innovative dry crushing and grinding circuit enhances operational efficiency in terms of energy and water use, as well as overall cost.

Ata Douzdouzani is with ABB Motion.

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