PNEUMATIC STOWING

PNEUMATIC & MECHANICAL STOWING

These methods of stowing are not practised in India. They are definitely costlier than hydraulic stowing, but will have to be adopted at some collieries in the near future for nearly complete extraction.

Pneumatic stowing

The stowing material may consist of washery refuse, boiler ashes, surplus pit rubbish, pickings from the screening plant, or shale bands from the coal seam. Sand alone is not used as it is heavy and abrasive. A mixture of sand and washery dirt may be suitable. Debris containing much clayey material is also unsuitable because it clogs the pipes. Very dry material should be dampened to allay dust. Damp material is less abrasive and gives tight packing in the goaf.

The crushed material passes from the crusher through 65 mm to 75 mm aperture wire screen to a storage bunker. Tubs are loaded at the storage bunker and then lowered underground when coal-winding is slack. There should be sufficient storage room underground to stock these tubs and adequate haulage facility so that during stowing shift the stowing operations are continuous and materials are supplied to the stowing plant without interruption. Underground, the loaded tubs are taken to a tippler which tipples the material on the feeding hopper of the stowing machine which is situated at a convenient point near the goaf to be packed.

The compressed air required for the stowing operations are supplied from the surface through pipes of 200 mm to 300 mm dia. Smaller size pipes are rarely used. The surface air compressor is multistage, has a large capacity of 55 to 85 m³ of free air/min, consuming 400 to 600 H.P., and compresses the air to nearly 6 kgf/ cm². The stowing pipes are of high carbon steel 10 mm thick, in lengths of 3 m. Roadway pipes may be with flange joints but pipes at the stowing range have quick release couplings (victaulic or hamecher type) with rubber seals. The compressed air ranges in the shaft and galleries are fitted with automatic water drainage devices and pressure gauges at intervals.

At the goaf the material used for boxing is combated sheets, hessain cloth, wooden planks, wire netting or paper reinforced with very thin wire netting.

Stowing Machines

The machines are of two types:—

(a) Chamber machines, e.g.

(1) Torkret single chamber.

(2) Double chamber or automatic.

(3) Three chamber or vollautomat.

(b) Compartment or pocket wheel machines:—

(1) Beien

(2) Brieden

(3) Blast slower.

Fig. 10.8 illustrates the Markham blastower. It consists of a rotating paddle wheel or drum driven at 40 to 50 r.p.m. by an electric motor of 15 to 20 H.P. The drum is divided into chambers which are filled with debris from the feed hopper above. As the drum rotates, each chamber in turn reaches a position where it forms a continuation of the compressed air inlet on one side and the delivery outlet on the other side. The compressed air thus flows through the chamber in the rotating drum, carrying the debris with it through the stowing pipes to the face. The-machine has a capacity of 1.5 m³/min with an air consumption of 70 to 85 m³/ min. Allowing for stoppages, stowing rate may average to 42 m³/hour. Pressure gauges are provided to indicate air pressure at the inlet to the machine and the back pressure at the outlet due to resistance set up in the stowing pipes. The air pressure at the machine is regulated down to 1.8 to 2.8 kgf/cm³.

At the discharge end of the stowing range a detachable deflecting nozzle is fitted to enable the operator to direct¹ stowing material as required. The air velocity in the pipes is about 60 m/sec. The average velocity of discharge of stowing material is about 15m/sec. The packing in the goaf is nearly 70% as compact as the solid coal. Convergence of the order of 20% takes place in course of time. The compactness of the packing depends on the type of material, its size, density, moisture content, and length of stowing pipe and the velocity of discharge.

In this method the face becomes dusty due to air-borne dust of stowing operations and there is much noise by the machines. Danger of electrostatic sparks due to high discharge of compressed air also exists. Proper earthing is therefore essential. The work can proceed during coal winning and loading shifts also.

The method is adopted in Germany, Britain and other foreign countries.