In Pit Crushing and Conveying Systems

In Pit Crushing and Conveying Systems

Belt conveyor systems, long the standard transportation technique in soft-rock open-pit applications, are encountering growing interest in the hard-rock open-pit sector as economic factors force companies to seek an alternative to costly truck haulage. A general review of mine conveyor systems is followed by detailed analysis of the mobile and semi-mobile in-pit crushing plant essential to successful use of mine conveyors.

Belt Conveyors Widespread in Soft-Rock Mining

In the soft-rock sector, positive experience with belt conveyors was gained in lignite mine-fields in the nineteen-fifties. In Germany, for example, 103.5 km of belt conveyor line were in use by 1962, 53.3 km of which were engaged in overburden handling; roughly 50% of these belt conveyor systems had belt widths of 1200 or 2000 ram. Since the nineteen-sixties, belt conveying has continued to make great strides in soft-rock open-pit mining, becoming the most important means of transport in this sector; belt conveyor systems in current use with bucket wheel excavators achieve performances per day of up to 240000 m³ bank.

If the situation in hard-rock open-pit mining is compared with that outlined above, it is apparent that the belt conveyor, despite its recognized advantages as a bulk transporter, has by no means achieved the same degree of acceptance as in the soft-rock sector. However, recent cost trends call urgently for fresh consideration of all possible forms of mine transportation, including belt conveyor systems.

Many of the general features indicated above are typical of large open-pit mining operations in North America. Mines commonly handle over 100000 t of ore and waste per day, moving the material through great horizontal and vertical distances. Transportation costs constitute a very high percentage of total mining costs. According to a recent estimate, operating costs for a 108-t haul truck may represent up to 40% of the current selling price of, for example, copper. Transportation costs of this magnitude offer an obvious starting-point for cost reduction programmes.

Truck Haulage Versus Belt Conveying

Although truck haulage remains the most widespread method of material transportation in open-pit mining, recent advances in truck technology (e.g. larger payload capacity, electric wheels and longer drive trains) have so far failed to offset the growing economic drawbacks outlined above. Moreover, the trend towards longer hauls and greater mining depths tends to expose inherent weaknesses of the shovel/truck concept. Increased haulage distances not only result in expansion of the truck fleet to maintain previous production levels, but impose increased manpower and support equipment requirements.

There are nonetheless a number of limitations on the use of conveyor systems in the hard-rock sector, which partly explain their relatively slow acceptance in this field. Flexibility is the argument most often cited in favour of truck-based systems. Truck transport can easily be adapted to constantly changing mining faces, with less exacting demands on long-term mine-planning strategy and sore scope for tactical improvisation.

In-Pit Crushers; the Key to Hard-Rock Conveying

The essential task of the in-pit crusher in a belt conveyor transport system is to reduce blasted material to a conveyable size. Crushers for this purpose are almost exclusively of the low crushing ratio type (up to 1 : 10), i.e. so- called primary or pre-crushers, designed as mobile or semi-mobile units.

Further processing to end-product quality takes place centrally outside the pit. For certain applications, reduction to millable quality (crushing ratio in the range from 1 : 10 to 1 : 80) may also be carried out in-pit, dispensing with the need for further crushing.

Developments in the use of movable crushers have typically been divided into three phases: prototype operation over a number of years, followed by gradual introduction once the system philosophy has been proven, and a subsequent rapid spread to other mines.

Recent increased use of movable crushers in hard and abrasive rock applications has led to growing popularity of gyratory crushers. Roll crushers are generally used for high-throughput overburden crushing.

The specific energy requirements of crusher units are interesting in relation to their individual system tasks:

– Reducing blasted material to conveyable size (crushing ratio below 1 : 10),
– Reducing blasted material to finer sizes than required for belt conveying (crushing ratio above 1 : 10).

Operational Units Prove Concept Feasibility

In 1982/83 EGAT (Electricity Generating Authority of Thailand) invited tenders for a sub-contract to remove 15 million m of overburden annually over a six-year period at its Mae Moh open-pit mine. The successful bidder was a consortium formed by Bangkok Motors and Sahakol Engineering.

In October 1983, a contract to supply three semi-mobile crushers for Mae Moh was awarded to Krupp Industrietechnik. Specifications included the following main parameters:

Performance : 3600 t/h
Material : overburden
Feeding lump size : 1000 x 1500 mm
Product size : 0 – 350 mm
Maximum frequency of moves : several per year
Specific gravity : 1.96 t/m³

Overburden is loaded on to trucks at the face and dumped into the feed bin. An apron feeder under the bin transfers the material via the feed box to the double-roll crusher. Any material passing through the apron feeder plates is caught by the spillage conveyor under the feeder and likewise led to the crusher.

The crusher reduces the material to its product size of 350 mm max. and drops it through the collecting hopper to the variable-height discharge conveyor.

In early 1983, Utah Mines Ltd. decided to modernize its copper ore mine on Vancouver Island. The costly truck fleet used for ore transport was to be replaced by a belt conveyor system combined with semi-mobile crushing plant. Cost savings in the order of 30% were anticipated for a system comprising in-pit truck haulage (truck capacity 154 t), and an in-pit semi-mobile crusher integrated with belt conveyor transport from the mine to the processing plant.

 

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