Most longwall operations in Australia use a uni-directional cutting technique which provides simple operation, environmental benefits, minimal manning and horizon control benefits.

In the increasingly competitive market place, however, using the conventional or traditional mining sequences or cutting cycles may not allow the equipment to realise its production potential.

Andrew Rutherford

As companies strive for a competitive edge, the market leaders will find new methods to gain that small increment of productivity that may make the difference between being at the top or an also ran.

Longwall retreat mining has traditionally produced coal using two methods, bi-directional and uni-directional mining cycles. The use of bi-directional cutting has always been seen as more productive on longer faces, however as shearer power and haulage speeds have increased, and thicker seams have been targeted, and environmental considerations increase, uni-directional cutting has become more competitive and can be more productive than Bi-directional cutting.

As computer power on longwall faces has allowed support control systems to become more sophisticated, further alternatives have been developed and are now available to the longwall market, notably the half-web cutting cycle, used in one form at 20 mile in the United States.

The half-web system is basically a uni-directional system of cutting in mid face, with bi-directional gate sequences that allow faster shearer and cutting sequences because the shuffle required for the bi-directional cutting is not required and loading on the shearer drums is reduced.

The half-web is achieved by pushing the AFC 50% after the supports have advanced to provide a half web cut for the return shearer run. This provides a pre-splitting effect on the coal to reduce lumps and can equalise the coal flow in each cutting direction.

The half web system requires a modern support control system, such as the Joy RS20 or DBT PM4 system, to operate effectively, and support hydraulics should be highly efficient and controlled. If you have such a system, then significant cutting cycle benefits may be possible, over and above your current cutting rate.

In the last 12 months, trials have been completed at a number of mines using the half web system over a number of shifts and significant benefits have been seen in production cycle times (up to 40% improvement may be possible), lump/blockage reduction and coal handling. There is evidence that improvements may be possible in the areas of equipment loading and wear, environmental management of longwall faces and face management and control.

So why has there been such a poor response to the use of the system? One factor may be that the industry is so traditionally focused and slow to change, or perhaps there is a lack of understanding on how the system may operate in the Australian conditions.

Another area of concern may be the requirement to have a high quality and high availability support control system, needed to effectively operate the system. Traditionally support control systems have been used until their efficiency has reduced productivity to a significantly low level that the longwall is no longer cost effective.

Renewing the support control system may cost $3-4 million but allow production increases that offset this cost, but only providing the support structures and hydraulics are suitable to operate at higher production levels.

Mine operators should not shy away from purchasing new longwall equipment because it is only 10 years old. The longwall technology has advanced substantially in the last 10 years and the Australian industry must invest in this technology to remain competitive with open cuts and new developments overseas.

How many of the highly productive longwall mines in the US have equipment purchased in the 80s or even early 90s?

Whatever the reasons, it is time that the Australian industry began to look at its current operations more critically and consider embracing the new systems and electronics available to them.

Changing the cutting system may provide a solution to productivity shortfalls, and improve the environmental and face management practices. Nothing is gained by remaining with the same old system "because that is all we have ever used", when alternatives are available and may prove more efficient and cost effective.

The market conditions have started to change for the better. The time has come to question the current system and seek improvements.

Contributed by Andrew Rutherford, longwall mining and training consultant. A more detailed article by Rutherford, comparing the relative merits of bi-di versus uni-di, is planned to be published in the March 2001 edition of Australia’s Longwalls.