Jim Dowling - Mining Engineer

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SKIP OR "CAR AND CAGE" HOISTING

 

Skips can be used alongside any main transport system, car and cage only when the main transport is rail bound mine cars. In this case mine cars are loaded inbye from passes or stopes and hauled out to the shafts by locomotive or rope haulage. When skip hoisting is used, mine cars are tipped into feed bins at the crusher station. When cage hoisting is used, these same full mine cars are loaded directly into the cage, hoisted to surface and tipped there.

Underground skip loading equipment forms a permanent installation below the lowest haulage level, normally serving the entire mine life. All ore and waste produced above is dropped down via passes to the bottom level. A primary crusher is required for hard rock as part of the skip loading station, to reduce sizes down to c. -150 mm for ease of handling. For softer minerals such as coal crushing may not be necessary. Shaft spillage up to around 1% of hoisted tonnage is associated with skip hoisting, facilities must be in place to deal with this. Discharge at surface is into a hopper which feeds by gravity to an inclined conveyor delivering to a crushed ore stockpile. At metal mines, this often doubles as a blending stockpile, the conveyor slowly traverses left and right, layering the ore so that once recovered for processing, variations in grade are reduced. This ore has been through the primary crusher underground, so can be loaded straight onto a belt conveyor, with no further treatment, for delivery to the mill. Discharged waste goes straight to a dump.

With car and cage hoisting there is no need for a primary crusher or any special loading pockets underground. The shaft bottom will be just below the lowest shaft inset level, leaving space for arrestors to hold the cage and a few more metres for a water collection sump. Ore and waste produced on upper levels can be hoisted from there, as long as the winder has this capability as regards clutched drums and balance ropes. On surface, the decking level will be in the headgear, sufficiently high off the ground so tipped rock can gravitate to the next stage. This will be primary crushing for hard rock, that is, most metal ores. In this case, ore is tipped into a hopper and loaded into a dump truck for transit to the crusher, where it will be reduced to -150 mm for subsequent conveyor transport. It is unlikely the crusher will be built into the headframe. For smaller mines the primary crusher may well be a mobile, quarry type and may even be leased rather than purchased. Softer ores, where run of mine (ROM) rock is mostly -150 mm without primary crushing, is simply loaded straight onto a belt conveyor for onward transport, either to a stockpile or straight to the processing plant.

 

 

Examples of shaft cross section layouts

Examples of shaft cross section layouts

 

 

Isometric of a two deck cage, doors removed for clarity

Isometric of a two deck cage, doors removed for clarity

 

 

Popular type of skip, shown in loading position on the left and discharging on the right

Popular type of skip, shown in loading position on the left and discharging on the right

 

 

Typical crushing and skip loading station

Typical crushing and skip loading station. The right hand drawing in bold shows the usual enlarged shaft side position. The left hand drawing shows a crusher station remote from the shaft, connected to the skip loading bins via a small drive and belt conveyor, if weak ground conditions make this alternative necessary

 

 

Spillage handling facilities at the bottom of a skip hoisting shaft are essential

Spillage handling facilities at the bottom of a skip hoisting shaft are essential

 

 

Section showing the important features of skip hoisting (left) and car and cage hoisting (right)

Section showing the important features of skip hoisting (left) and car and cage hoisting (right)

 

 

SKIPS V CAR AND CAGE

Car and Cage hoisting has the following advantages compared to skips:

Much lower capital cost. Expensive underground crusher and skip loading facilities are avoided.
Shallower shaft. The saving in shaft sinking depth could be 40 – 80 m.
Smaller headframe. Skips are long and thin, cages are short and squat. So headframes with skips have to be taller, to permit discharge by gravity, than those using cages.
Shorter hoist. All hoisting is from bottom haulage level rather than skip loading elevation. In addition, rock from higher up can be hoisted from those higher levels.
Minimal shaft spillage.
At a new mine, production hoisting can begin much earlier.


The following disadvantages apply to car and cage compared to skip hoisting:

Higher operating cost. Car and cage hoisting cost per tonne (cpt) is at least twice that for skips.
Slower hoisting. Skips are 3 – 4 times faster in tonnes per hour (tph) in like for like situations.

In practice, skips are favoured for medium to large mines with a longer life. Car and cage hoisting can be the correct choice for smaller mines.

 

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