Economic Mining of the Lignite Deposit Zangtal Unterflöz as Example for Mastering Difficult Geological Goiditions in the Alps

Summary

Lignite is mined in open pit and underground in the lower seam of the Zangtal area. The geological hanging wall consists of gravel and clay, the foot wall of waterbearing limestone and dolomite. The coal is sold to households or steam power stations.

According to the original plan, drilling disclosed 6.5 Mio tons of coal. Out of this 4.5 Mt could be mined, the recovery of the rest being impossible because of the poor width of the seam, blocked by safety pillars or considered as mining losses.

The deposit should be divided into two sections, the northern part close to the surface to be mined in open pit up to an overburden — coal ratio 6: l; the other section should be mined underground mainly by a fully mechanized longwall method, slicewise from the hanging towards the foot wall.

The pillars remaining between the longwall areas and the edge should be mined by a mining method still to be developed. However, it became evident during the planning phase that this deposit could be economically mined, although with some mining losses.

For the time being longwall mining is the main production method representing 58 per cent of the total output. There are four longwall face sections in the hanging slice of the seam; currently the first longwall face is mined in the central slice. The longwall face section is fully mechanised and equipped with a double drum sectional cutter, an armoured conveyor and a hydraulic roof support. The length of the longwall face is 140 m, that of the total section 800 m.

Exploration of the deposit showed major difficulties in longwall mining. Due to the irregularity of the deposit the longwall face had to be developed with an inclination of 30 degrees in haulage direction and of 22 degrees in advance direction. Due to the steep inclination of the longwall face the chocks had to be protected against tipping and sliding, and the sliding of the armoured conveyor was avoided by installing a system of hydraulic cylinders. Becaase of the varying inclination of the seam in advance direction difficulties were encountered in the form of wall face blow-outs and the longwall face equipment was hard to handle.

In open pit mining the muddy surface water was very difficult to pump. Moreover, due to the steep inclination the exposed foot-wall of the seam endangered mining operations by land slides.

For mining of pillars, suitable sectional cutting machines were developed. According to this system galleries are driven into the section and coal is subsequently mined in retreating in the form of fishbone-galleries. In case the width of the coal seam exceeds the height of the sectional cutting machine, the coal on the top of the seam which cannot be mined by the sectional cutting machine because of the excessive width of the seam, has to be recovered by drilling and blasting methods.

The galleries are also driven with sectional cutting machines having to overcome an inclination of up to 25 degrees. Since the floor tends to swell, it is necessary to provide a closed ring support in four parts. For the Zangtal mining district a section of the ring support had to be especially designed for the use of this mining method together with a sectional cutting machine.

The very economic open pit mining method in the northern part of the deposit showed that the remainder of the seam could be mined in the same way. Therefore, underground mining could be reduced.

Additional boreholes, sunk from underground galleries, revealed larger coal quantities within inclines than originally assumed. Consequently a limitation of the underground mining operations by extending the open pit mining are was not necessary. Due to this fact the total coal production of this mine will amount to 5.3 Mt.

In case the present forecast concerning mining operations in Zangtal materializes, approximately 72 per cent of the T.74 Mt geological reserves can be mined economically.

This shows that underground longwall mining losses are twice as much assumed. But the open pit methods with no loss was considerably extended and losses underground could also be reduced by mining the above-mentioned pillars.