Room & Pillar

This method has evolved from the older system of breast stoping with timbered supports, combined with random 6-m sq pullers, which was the system used thought Mosaboni mine until the early 1970’s.

For room and Pillar stoping, a pilot raise is put up from one level to the next, following the hanging wall contact. A wooden chute is installed at the lower level, together with a 2 drum electric scraper winch. Leaving a still pillar of 6 m above the lower level, and crown pillar of 4 m below the upper level, the side walls of the raise are progressively stripped, using aitleg mounted jackhammers. After blasting, the roof is secured and roofbolted, using 1.5 m x 20 mm dia torsteel cement grouted bolts on a 1.2 mx 1.2 m staggered pattern, and the broken ore is scraped down to the chute. Where or thickness, and therefore stope height, is insufficient for the use of standard rockbolts, a two piece coupled bolt is employed.

Initially stope sizing was such that topes were stripped out to 10 to m span. Leaving 3 m wide rib pillars between one stope and the next. Rock mechanics studies, carried out partly with the aid of CMRS, have enabled stopes to be widened to 15 m as the standard span and there are plans to increase this yet again to 20 m in favorable districts. Once the stope has been widened to full width, any ore in the floor is progressively stripped until the footwall contact is reached. Room and pillar stoping is generally used where the ore is between 1m and 4m thick. Ore extraction is about 65%.

Horizontal cut & fill

Where the width of the orebody and roof conditions permit, the ore is mined bya standard horizontal mechanized cut and fill method. A stope is started with a pilot raise following the footwall contact, and an initial stope drive above a still pillar.

Drilling of the back within the stope may be carried out using arleg mounted jackhammers or using drills mounted on Teledyne stope wagons. Mechanises loading is undertaken using 0.76 cub m LHDs.

Stope back and the hanging walls are roof bolted at 1.5 m x 1.5 centres, the roof bolts grouted in over their whole length. There is a tendency for the chlorite schist hanging wall to slugh off for a few cms above the countact, and overall dilution of the ore runs at about 15%.

Deslimed mill tailings are used as backfill. At the Mosaboni mine these are sent underground via a 76 mm dia. borehole; thence to the stopes through a 88 mm ida. seamless pipe. Backfill for the Pathagora mine is also taken from he Mosaboni concentrator, while that for Surda comes from the  nearby South bank concentraror. Fill for the Rakha mine is supplied from the Rakha plant. AT present there is a shortfall of material for use in hand to assess the availablility of higher percentages of deslimed tailings and sand gathered from the Subbarnrekha bed to make up the balance required.

Stope orepasses and access ladderways are built up through the filled zone out of 10mm thick curved steel plantes welded together make continuous inclined cylinders of 1.6 m dia. These provide excellent smooth walled conditions for the ventilation air flow, and relatively low friction conditions to facilitate the sliding of ore down the ore passes at the comparatively flast angle necessary, as dictated by the configuration of the orebody. Where the ore is usually flat, the orepass may be put in the footwall.

Post Pillar stoping

This method is a development from horizontal cut and fill, and is used in cases where the width and / or inclination of the orebody, or the condition of the hanging wall, or the concentration of rock stresses, are such that the ordinary method of roof bolting and fill would not give sufficient support in the stopes.

In the post pillar stoping method, narrow (4 m sq) pillars are left in the ore at regular intervals (13 m along the strike and 9 m across) so that in plan view they appear like a normal chequer-pattern room and pillar working. The pillars thus give additional strength to prevent hanging wall to foot wall closure after stoping while the fill material surrounding the narrow pillars provides them with lateral support and prevent them failing through buckling under load.

In some areas of surda, Rakha and Mosaboni mines, where fractured conditions of the hanging wall are encountered, cable bolting using cables upto 20 m long is also employed to supplement roof bolting. 57 mm dia. holes, 10 m to 20 m long are drilled into the hanging and back of stope at 1.9-2 m intervals. Two 16 mm dia. tensile steel cables are inserted together with a plastic grouting tube and a small dia “breathing” tube. The grouting tube is slowly withdrawn as the grout is pumped in. In two of the post pillar stages larger (1.68 Cub. m) LHDs are in use.

Ores are drawn from stopes chutes is hand trammed to ore passes leading down to one of the main haulage levels

On the main haulage level, trains of wagons hauled b 4 te / 8 te battery or diesel locaomotives of various makes carry the ore to central grizzlies of the horizontal fixed-bar type, with 250 mm openings, beneath which are bunkers leading to skip pockets. Oversize, material is broken by Teledyne electro-hydraykuc rock breakers. Broken ore is hoisted to the surface by skips or cages. Most of the production from Moaboni mine is taken up the main inclined shaft system – from No. 2 subincline to No.1 sub-incline and finally up the Main Shaft the multiple handling required forming one of the main bottlenecks or increased production. The circular shaft takes care of man and material winding and about 25% of ore hoisting.