Notes on Shrinkage

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Notes on Shrinkage Stoping

This method is very similar to sublevel stoping. Here, the caved ore is not loaded in its entirety.Instead, it is used as a base for further mining activity.

Application

- steep orebody (dip greater than angle of repose) the best dip is in the 70-90 degree range. Ore support of the hanging wall is less for less steep dips. At a

dip equal to the angle of repose, support for the hanging wall is close to zero.

- firm and competent orebody- minimum thickness of about 1 m and maximum anywhere from 3-30 m- stable hanging wall and footwall as broken ore doesnt give much support- regular boundaries- must not be affected by storage in the stope. Must not emit toxic gases

for example, some sulfide ores decompose

Development

- elaborate development for loading the mined ore

develop a haulage drift parallel to the bottom of the stope drive cross-cuts from the haulage drift into the bottom of the stope undercut the stope, about 10 m above the crosscut level finger raises and cones to be developed from the undercut to the crosscuts

This can be further simplified as in sublevel stoping. Instead of finger raises and cones, one could alterthe undercut to allow loading from the crosscuts

Widely spaced drawpoints imply more ore left behind in the stope. This will be fine if the sill is minedlater.

- develop a raise, for access and ventilation, from the haulage drift to the main level at the topof the stope

- stope height usually no more than 75 100 mProduction

- overhead drilling and blasting- due to uneven floor, mechanization not possible

air-leg drills and stopers

Ore Handling- direct loading from chutes above into cars or use draw point system with LHDs

Comments

- capital is held up in the stope for a long period- labor intensive as mechanization impossible and, therefore, low production- dangerous working environment- however, viable for a small mine where investments are low


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Example: Mouska Gold Mine (Canada)

(Underground Mining Methods.., Hustrulid and Bullock, 2001)

Orebody

- lode type deposit, with mineralization in narrow quartz veins (about 30cms wide). The hostrock is diorite (UCS: 175Mpa, Poissons ratio: 0.15, RQD: 80). The sp.gr. of ore is 2.8

- cut-off 9.8 g/t of gold- Shrinkage stopes are 1.6m wide

Mine

- 100,000 tpy @ 5 days/week. Underground employment: 63- 72% of the production comes from shrinkage stoping, 20% longhole stoping and rest

development- Daily production: 400 tons of ore and 200 tons of waste- Access by a four compartment 485m shaft- Levels at 60m center and equipped with rail haulage. Mine advances from top to bottom.

Stope

- An ore drift (or sill) is driven into the ore prior to developmentof the haulage drift (see figure). This drift serves to explore theprofile of the vein. It also forms the bottom level of the stope.Establishment of the vein profile helps in not only reducingdilution, but in stope planning as well (location of drawpoint,haulage drifts etc)o The width of this drift is critical, especially in the first 4 lifts. Dilution is caused by too wide a drift. A

typical drift (and hence stope) is 1.5-1.6m wide.o The sill is no more than 30m ahead of the haulage drift.o Slushers are used to bring the ore to the neared drawpoint from where they are loaded by rubber mounted

mucking machines into trains.

- Drawpoints are 10m long, 2.7mX2.9m in cross-section, and 10m apart. The roof at theintersection of the drawpoint and haulage drift ismade higher to accommodate loading intotrains.

- Remote controlled muckers recover ore betweendrawpoints. The drawpoints include a small bayfor the remote mucker operator. Also, slasherholes are drilled (to be blasted if needed) ifremote mucking is necessary. The extra spaceallows for easy movement of muckers.

- Haulage drifts are in waste rock and 10m awayfrom the stopes

- Ave width of stopes is 1.6m. Each cut is 2.4mhigh.

- 10 stopes required to produced 400 tons per day, of which, 6 are mining stopes, 2 aredevelopment stopes and 2 are in the final phase.

- On a given lift (2.4m high), drilling is horizontal (breasting).o Easy to track the veino Unexploded holes are better taken care ofo Better controlled blastso Miners are not exposed to the ground they are actually drilling


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- Blasts are about 25 ton-

Stope Issues

- A slusher is used to ensure a level working surface.In some cases, wooden platforms are built for thesame purpose.

- The tonnage of ore drawn from each draw point iscarefully controlled to ensure a level working floor

- Safety protocols are followed when drawing orefrom the stope

- Hangups are common in the narrow stopes evenwhen the ore is finely broken. Mucking is stoppedif the surface level of the broken ore has not movedeven though about 50 tons of ore has been removed.The hangup is removed by a high pressure waterspray. The water hose is brought to the desiredlocation on a remote controlled carrier that rides ona wooden rail installed in the back of the stope.

- Ore recovery from the stope is about 95%. Theempty stope is not backfilled.

Ore Inventory

- 35% of the ore (swell) removed after a blast- Blasting stops when the blasted ore muck surface is within 5m of the top level- Ore can remain in the stope for almost 8 months. The quantity in the stope can be as much as

10,000 tons. At 15g/ton grade, the value of the trapped ore can be as much as $2.75 million@$550 /oz

Ground Control

- Problems caused by:o Diorite, as it is brittle under pressureo Intersection of major joints forms destabilizing blockso Significant changes in dip and strike as that results in

changes in stope profile as the stope chases the ore.

- Hanging walls, footwalls and backs are bolted in a1.2mX1.2m pattern using 1.2m long bolts

Service Raise

- Raises are excavated for supplying the stopes from theupper level with services such as compressed air, water,ventilation and access)

- It is driven in ore and connects the drawpoint of anupper level stope.

- The raise is 2.1mx1.8m and dips at 600-650- It consists of a ladder way for men. Ladder segments

are 7m in length and offset from each other for safety.- A wooden slide is also installed to allow miners to

operate a winch and bucket for transporting suppliessuch as drill rods, bolts, explosives etc.


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Manway

- A stope requires at least two access ways (for ventilation and safety). The service raise (inthe middle of the stope) constitutes one access way (access from the top). The other is themanway (access from the bottom) that is typically located at one end of the stope.

- The manway is connected to the drawpoint at the end of the stope by a side opening.- It has wood on one side and follows the rock profile on three sides. Sometimes, wood

becomes necessary on all sides.- It is raised one lift at a time along with the mining cycle.

Equipment and Haulage

- Primary drilling equipment consists of handheld drills. Rubber tired mucking machines loadore into rail cars

- 5.5 ton battery powered locomotives haul seven 5-tonne ore cars to ore passes- Ore is crushed and then taken to the surface on 6t skips. A new shaft is being excavated to

access deeper levels.-

Economics- Capital expenditure (long term development, construction, exploration drilling, and

equipment) has been about US$1.7 million per year.- Operating cost at the mine is about US$160 per ounce. Gold price in 2006: $550-650/oz

Comments

- Labor intensive. Workers tend to suffer from back problems- Equipment used in this type of mining has not changed in the past 40 years


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Courtesy: Natural Resources Canada Website

Courtesy: Natural Resources Canada Website

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Notes on Shrinkage