Improving emissions for ore grinding: Australian technology to help lower energy consumption in mining operations

The largest consumer of energy on a mine site is the technology used to break the raw ore into smaller pieces for processing or transport. Last year, Perth-based mining services company Scanalyse was awarded an AusIndustry Climate Ready grant to the value of nearly $1.8 million to support their research into improving the energy efficiency of mine-site ore grinding.

The largest and most inefficient machine on a mine site is the SAG mill – a giant machine much like a large tumble dryer that breaks the raw ore into smaller pieces by literally bashing them together. What is an SAG mill? AG/SAG mills are used to grind raw ore or primary crusher product into either a finished size ready for processing, or an intermediate size for final grinding in another type of mill, such as a ball mill or pebble mill. AG/SAG milling is often chosen today for primary grinding because it can result in fewer grinding stages in the preparation of ore, compared to previous technologies, and therefore results in lower overall capital and maintenance costs. It is, however, highly energy inefficient. Autogenous grinding (AG) is the size reduction of material using a tumbling mill that utilises the feed material itself as a grinding medium. The more common semi-autogenous grinding (SAG) is the same use of a tumbling mill but with the addition of supplementary grinding media. The most common supplementary medium is large steel balls. Grinding can be wet or dry, with wet grinding being accomplished with a slurry of 50 to 80% solids.

Peter Clarke, CEO of Scanalyse Pty Ltd

These mills are built in the form of a rotating drum in which the ore and any supplementary grinding medium is lifted and dropped on itself until the pieces are reduced to the required size. The liner of the mill consists of lifters that lift the ore as the drum rotates. These mills are shorter in length than their diameter, and can be very large – often in excess of 10 m in diameter. For example, the SAG mill at the Ernest Henry Mine owned by Xstrata in Mount Isa, Queensland, is 10.4 m in diameter and 5.1 m long, and is capable of grinding over 1300 tph of copper and gold ore, and is by no means the largest of its kind. The steel balls used in SAG mills are typically 13 cm or more in diameter. The load, including any supplementary grinding media (the ‘ball charge’), usually occupies about 30% of the volume of the drum, while the steel grinding balls are typically 8% of this volume. The feed rate of the ore is limited by the power available to turn the mill and the maximum weight the hydrostatic trunnion bearings which support the turning mill shell can withstand.

Global carbon and energy impact

The power to spin SAG mills comes from large electric motors coupled to the mill shell through a clutch and gear system. The electricity consumed by the motors is the largest cost and inefficiency. Typical power plants for SAG mills are very high, with some consuming more than 20 MW.

Peter Clarke, CEO of Scanalyse, estimates that there are about 2500 mills of this type at mine sites around the world. “Typically, 60% of a mine site’s energy consumption goes into crushing and grinding, and SAG mills are typically only between 1% and 5% efficient,” he said. “Most of the power is converted to heat in the load and only a small percentage of the power contributes to ore breakage.”

It is not hard to see what these figures show - that ore grinding is a significant contributor to greenhouse gas pollution. The wasted energy in the process could be being wasted at a rate higher than 30 GW worldwide.

Spatial imaging technology

Initially, Scanalyse developed technology to measure SAG mill liner wear and damage. Although constructed of a highly wear-resistant steel, the liner inside the mill is subject to enormous impact and abrasive forces from the load and the supplementary medium. Understandably, it will wear down, usually unevenly, and will need to be repaired or replaced at regular intervals.

Unfortunately, there has not been any effective way for a mine operator to determine the amount of wear on a mill liner without regular physical inspection – which means shutting down the mill to allow staff to climb inside and take measurements. Not only is this process dangerous and difficult for staff, but it means that the starting and stopping of the mill for inspections increases the energy consumption of the mill and creates business losses through the reduction in milling output – typically $50,000 per shutdown. In addition, physical inspection does not allow for a comprehensive inspection of the entire liner. “Inside the mill it’s very dark, very damp and metal coloured. So if you’re inside looking for damage, it’s very easy to miss,” said Clarke.

The technology for mapping the inside of mills was originally developed by spatial sciences researchers from Western Australia’s Curtin University of Technology. This three-dimensional laser scanning and imaging technology, now commercialised as MillMapper by Scanalyse, creates a virtual mill with comprehensive liner profile data. Using a specially calibrated terrestrial laser scanner, MillMapper uses automated modelling software processing several million data points to provide a high-density 3D colour-coded thickness map of the mill, as shown in Figure 1. The advantages of this technology to date have been early detection of failure, reduced inspection duration, optimised maintenance scheduling and fewer mill restarts.

Figure 1: MillMapper 3D cutaway image with colour-coded thickness.

Early detection of liner failure

An important advantage of the MillMapper technology is the early detection of potentially catastrophic events such as liner failure. Laser scanning technology allows for the quick detection of chips and cracks, as well as the breakage and wear of the steel ball charge. Figure 2 depicts the end liner wear, showing new and worn liners as colour variations.

Figure 2: a) Feed end liners just after installation, blue indicating little wear, and b) showing high wear in red.

Reduced inspection duration

The rapid automatic collection of comprehensive data on the mill liner using laser scanning reduces the inspection time to as little as five minutes, and eliminates the need for staff to enter the mill. This can reduce the inspection time from two hours down to about 20-30 minutes because major OHS issues do not need to be taken into consideration.

Optimised maintenance scheduling

The numerical and visual analysis tools available with spatial imaging technology make it possible to accurately map the liner thickness over the entire mill surface and predict wear rates and patterns over the liner’s life cycle. Storing these changes over time provides comprehensive data to allow operators to decide on optimal relining schedules and to reduce the number and length of shutdown times, since ultimately the mill will not need to be inspected as often (see Figure 3).

Figure 3: Liner wear trend graphs.

Fewer mill restarts and reduced energy costs

The high consumption of electricity involved in rotating an SAG mill directly relates to not only high operating costs, but also high greenhouse emissions. MillMapper’s determination of liner shape allows for correlation between liner shape and energy consumption over time (see below). Also, every time an electrically driven machine is started and stopped, the energy consumed in overcoming inertia is far greater than the normal energy consumption during operation, while producing no useful work output. Reducing the number of times per year that an SAG mill must be stopped and started for inspections directly relates to lower energy consumption and lower greenhouse emissions.

Future efficiencies

Reducing maintenance overheads is one thing, but Scanalyse is now taking the quest for more efficient ore milling one large step further.

Optimising liner design

It has been discovered that liner shape has a dramatic effect on mill efficiency. For example, the most effective production rates often occur during the last period of liner life, and can be as much as 30% higher compared to the use of a new liner. Until the laser imaging technology of MillMapper was available, there was no comprehensive liner shape data available. Now it is possible to perform research to find the optimal liner design to maximise the efficiency of the mill.

Large-scale data correlation

In 2008, the Department of Industry, Innovation Science and Research awarded Scanalyse a Climate Ready Program Grant to the value of $1,779,952 to assist in the further research and development of services based around the MillMapper technology, the objective being to improve SAG mill efficiency worldwide.

The aim of the research is to collect and correlate data from mills, from both MillMapper and other sources, relating to:

• liner shape
• ball shape and condition
• mill speed
• mill power consumption
• size of the ore being ground
• the abrasive and hardness characteristics of the ore

In collaboration with the Julius Kruttschnitt Minerals Research Centre (JKRMC) at Queensland University, data correlation and analysis methodologies are being developed to determine the most efficient overall mill liner design given different combinations of mill size and ore type. The size, wear and breakage of the mill charge (steel balls) can also be analysed to provide a model of optimal quantity, size and shape to achieve greatest possible grinding efficiency. Mill owners will also be able to compare performance of their mill against other mills under similar conditions.

“By increasing the number of tonnes of ground ore per kilowatt hour, we are hoping to achieve an average of 10% efficiency improvement for mills where MillMapper is used, and a global reduction in ore-grinding related carbon emissions of 1%”, said Clarke.

“There is no easy fix for the inefficiencies of SAG mills – while we are breaking ore by smashing the rocks together, there will always be enormous amounts of energy lost in heat,” said Clarke.

“While new, more efficient technologies for breaking ore may be invented in the future, most of the SAG mills in the world represent an enormous capital investment for their owners (typically $10 million each), and still potentially have 20 years of working life left in them.

“So it is important that we get the best results from what we have – and while the improvement as a percentage seems small, the size of the energy consumption in mining means that we hope to enable miners to make a significant contribution to the lowering of greenhouse emissions,” he said.

Scanalyse Pty Ltd
www.scanalyse.com