The air apparent

More scientific approaches are helping solve the challenges of mine ventilation. Michael Sheffield writes for Australian Mining.

More scientific approaches are helping solve the challenges of mine ventilation. Michael Sheffield writes for Australian Mining.

The key to aligning mine operations with the ‘more scientific’ aspects of the product delivery process that lie upstream and downstream from it — which are anchored in the sciences of geology and metallurgy — is systems reengineering based on the generation and use of objective, scientific information about what is done and the environment in which it is done.

More scientific approaches to the problem, and tools which utilise developments in information science and computing are making this possible.

“To secure the future of the minerals industry in Australia, we need to solve the technical challenges that will be associated with Australian operations in the future. These include: limited or no outcrop; greater depths of operation; higher rock stresses; increased gas levels; lower grades; scarcer human resources; globally high standards of safety and health; and appropriately strict environmental regimes,” the CSIRO says in explaining the focus and direction of work currently being undertaken in its Minerals Down Under Research Flagship.

That means, among other things, an increased ventilation challenge with the potential to compromise operational viability not only at increased depths but also, with the increased use of declines both for access and rock haulage instead of shafts, nearer to the surface in large mines that pursue continuing increases in scale, particularly in hot tropical areas.

M J Howes put the challenge this way in a paper presented to the Eighth International Mine Ventilation Conference in Brisbane in July 2005: “Declines for both access and rock haulage are replacing shafts for mining relatively shallow deposits of down to 1,000 m below surface. Where the orebody does not outcrop and where surface access may be restricted, it is often necessary to drive long dead end headings before a connection can be made back to surface and a ventilation circuit established.

His paper, Ventilation and Cooling Design for Long Declines looked at a mine in tropical Western Australia that is to be developed to at least 2,100 m, and possibly 3,000 m depending on surface access and ground conditions, before ventilation can be established.

A window to this challenge is seen at Roxby Downs.

Roxby Downs, which takes ‘scale’ to new heights, is chosen by as an example of a sunrise opportunity for Australian mining in the 21st century by Donovan & Associates in A mining History of Australia, a National Mining Heritage research project conducted by them for the Australian Council of National Trusts in 1995.

“The Olympic Dam mine at Roxby Downs reflects something of the industry on the eve of the 21st century,” they say.

“The orebody was discovered in 1975 by advanced exploration strategies and the mine was officially opened on 5 November 1988 after extensive proving up of mineral reserves. Versatile runner-tyred mining equipment is used underground to recover the ore and within three years of the mine opening, more than 50 km of underground roadway had been completed, with expectations that this would increase by 8 to 10 km each year. With proved reserves in excess of 450 m tonnes of copper, gold, uranium, silver and lead ores, and a projected annual production rate up to 2.5 m tonnes, the mine is anticipated to continue for centuries”.

Accompanying that ongoing investment in underground roads, Roxby Downs will have an ongoing requirement for ventilation infrastructure.

“In its proposed expansion,” the Olympic Dam Expansion Environmental Impact Statement says, “the Whenan Shaft, the service decline, the Robinson Shaft and the new No.3 shaft would be maintained as air intakes. Approximately one new ventilation shaft would be required each year.”

Supplementing that would be a significant ongoing investment in the mine’s secondary ventilation system.

“In order to achieve and maintain the proposed level of production, it would be necessary to develop some 28 km/a of underground openings up to years 2000-01. After this initial period, the development requirement would be about 17 km/a.

“Nearly all of these openings would be developed as ‘dead ends’, which would require forced ventilation to remove blasting fumes and dust, diesel fumes, radiation products and heat from the rock and from machinery. For ventilation of development headings, an air volume of 25 m3/s per heading has been allocated based on operational experience of the existing mine, with up to four headings in each mine area,” the EIS says.

A dynamic challenge

The dynamic nature of the challenge facing the ventilation engineer in this future setting was underscored by C A Rawlins and H R Phillips in their paper ‘Underground Mine Ventilation Planning and Design With Regards to Heat Load and Cooling Mechanisms’ to that 2005 Brisbane conference.

“As mines extend their search for minerals of economic value deeper and deeper into the earth’s crust, ventilation and refrigeration aspects of mines become more prominent than before. Technology changes and improvements need to be incorporated into ventilation and refrigeration designs, where possible and advances in this specialised field explored,” they say.

“The ventilation engineer is obliged to include and evaluate all aspects of a mine design philosophy. Only after the different parameters that influence the air requirements have been analysed and evaluated can the total mine design air quality be determined.

“Heat ingress into underground excavations will differ depending on the location, mining method, ventilation strategy and various other parameters. Furthermore, heat flow from the surrounding rock surface into the underground excavation increases with a virgin rock temperature increase”.

Also, they add, “The ventilation design engineer needs to constantly update the design as new information becomes available. Whether it is a gold mine, copper mine, diamond mine, etc, heat is a contaminant to be controlled and managed in order to provide a productive environment. The associated heat load reduction methodologies of a project, from time to time, need re-evaluation and or scrutiny as technologies improve and change”.

Airflow

DJ Brake and T Nixon in a paper ‘Correctly Estimating Primary Airflow Requirements for Underground Metalliferous Mines’, delivered at the Tenth Underground Operators’ Conference in Launceston in April 2008, take this a step further.

Their message is that a ventilation solution can never be any better than the airflow requirement on which it is based, and airflow requirements are consistently underestimated.

“The primary ventilation system is a major contributor to the capital and operating cost of most mines. It also has a major bearing on the health and safety of the workforce. Probably the most important single design parameter for the primary ventilation system is the overall airflow requirement and errors in correctly establishing this value have a wide variety of methods of estimating primary airflow requirements in a mine,” they say.

One source of under estimation is the method used to arrive at these estimates.

Commonly, Brake and Nixon say, estimates are based “largely or entirely” on airflow requirements indicated by “the total diesel engine fleet capacity (kW) and a statutory requirement, such as 0.05 m3/s per kW or rated engine power”.

They list twelve other causes of underestimation. Failure to provide for leakage in the auxiliary ventilation ducts; leakage in the workings; essential anti-recirculation bypass flows; ramps and other underground fixed plant and infrastructure and travel-ways; likely changes in diesel technology; and increased mine resistance. Failure to recognise which is the critical contaminant to be diluted. Failure to understand the impact of both increased mine resistance and leakage on airflow requirements and fan performance and the incremental nature fixed-cost of primary ventilation. And lastly, failure to properly assess ventilation planning and implementation lead times.

Real-time monitoring

An exciting look at a future ability to reengineering ventilation and dust control with tools that make full use of emerging information and computing technologies was provided in paper ‘Mine Real-Time Personal Respirable Dust and Diesel Particulate Matter Monitoring’, by A D S Gillies, H N Wu and T Harvey, also delivered at the Launceston conference.

This paper evaluated two new developments in mine atmospheric monitoring, the first involving the use of a new personal dust monitor (PDM) to give real-time respirable dust reading and the use of real-time atmospheric diesel particulate matter (DPM) monitoring, in a regulatory setting that is moving away from prescriptive requirements towards risk reduction and ‘chain of responsibility’ thinking.

This is the exciting forefront at which science informs and supports decision making to allow productivity and safety issues to be managed dynamically, in real-time, in a way which incorporates ventilation and dust control as part of the total mine system.

Because PDMs are worn by mine workers, and because they report dust loading data on a continuous basis, miners and mine operators have an ability to view dust levels in real time throughout the mine. The advantages of being able to monitor dust levels in real-time from the points at which mine workers are operating, as they move from place to place, are clear.

Better control

Diesel Particle Monitors provide a better alternative to tag boards as a means of limiting the access of diesel vehicles to a particular ventilation split or mining sector to manage exhaust DPM and gases.

Under this system, diesel tags or tokens are used to control the number of vehicles entering an area to limit the level of pollution

As Gillies et al explain: “An alternative approach is to invest in underground continuous real-time monitoring of exhaust gases, DPM and section air quality and then integrate this information to determine whether an additional vehicle can enter, without exceeding the diesel token limit. This approach optimises the access of diesel vehicles and replaces the existing manual tag board system. This system would allow productivity to improve by detecting dirty engines and permit the maximum number of vehicles to be in use in a ventilation split based on real exhaust contamination.”

What is important (and different) here is the dramatic difference in the improvement in the quality of the information that is achieved by tying information to particular vehicles.

“Currently the predetermined ‘tag’ allowance may be excessively stringent for a well-maintained vehicle and so vehicles have to wait and waste time until another vehicle leaves the second ventilation split,” they say.

The alignment of this approach with the intention of ‘chain of responsibility’ and risk management legislation is evident from their concluding comments.

“These monitors give the potential to improve understanding of the mine environment and to empower and educate operators in the control of their environment. Both monitoring approaches have application for coal and metalliferous surface mining operations in addition to underground evaluations.”

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