Mining Spreader

Overview

A mining spreader is the machine that disposes of overburden once it has been stripped off a coal or lignite seam. It sits on the spoil side of an open pit, receives a continuous stream of waste rock from a conveyor system, and throws it off the end of a long boom to build the spoil pile back up in an orderly way. A spreader is the mirror image of a bucket-wheel excavator: instead of digging, it places, and the two often work as a pair at opposite ends of the same conveyor line. Large spreaders carry a discharge boom well over a hundred metres long and ride on banks of crawlers to keep their weight from sinking into the loose ground they stand on.

The machine combines a Crawler Base for travel, a Slewing Superstructure for rotation, a Receiving Boom that takes the feed, a Discharge Boom that places it, a Luffing System to set the boom height, and a Counterweight Boom to balance it, all run from a Operator Cab and powered through the Electrical System system.

How it works

Overburden arrives at the spreader from a conveyor bridge or a shiftable bench conveyor and lands on the Receiving Boom. Its Receiving Belt carries the material to the Transfer Point over the slew axis, where it drops onto the Discharge Boom. The discharge belt runs out the long boom and throws the material off the tip onto the spoil pile. To spread the spoil evenly the operator slews the whole superstructure left and right and luffs the boom up and down, laying the material in layers rather than dumping it in one cone. As the pile fills, the spreader trams along on its crawlers to a fresh section. The motions are continuous, so a spreader places material at the same enormous hourly rate that the excavator on the other end digs it.

Travelling on soft ground

Spoil ground is loose and weak, so the Crawler Base is built to keep ground pressure low. Rather than two big tracks, a spreader runs many Crawler Unit crawler units, each with its own Crawler Track Chain chain, wide Track Shoe shoes, and a Travel Motor driving through a Gearbox Housing gearbox. The units are tied to a Portal Frame through Equalizer Beam beams that share the load evenly when the ground dips, and Steering Cylinder cylinders steer each unit so the machine can follow the curving dump line. Spreading load over many wide crawlers is what lets a multi-thousand-tonne machine stand and move on fresh spoil without sinking.

Slewing and the two booms

The Slewing Superstructure turns on a Slew Bearing, driven by several Slew Drive units so the booms can swing through a wide arc and spread material sideways without the machine moving. The Receiving Boom and Discharge Boom are lattice trusses carrying belts on Idler Set idlers. The discharge boom is the long cantilever, so its Belt Drive Station station and Discharge Chute throw the material clear of the machine and out over the pile. Material crosses from one boom to the other at the central Transfer Point, which sits on the slew axis so the feed path stays continuous no matter which way the booms point.

Luffing and balance

The operator sets dump height with the Luffing System. A Luffing Winch reels Luffing Rope ropes over Sheave Block blocks from a Luffing Mast, raising and lowering the discharge boom so the spoil can be laid in level lifts. A boom that long would tip the machine forward if it were not balanced, so the Counterweight Boom carries a Ballast Box of Ballast Fill on a short Counterweight Truss on the opposite side of the slew axis. The counterweight is sized so the loads on the slew bearing stay within limits through the boom's full range of luff and slew.

Power and control

The spreader runs on grid power through a Cable Reel that pays out a trailing cable as it moves. A Transformer steps the voltage down, a Drive Cabinet feeds the belts, slew, luffing, and travel through variable-speed drives, and a Control System coordinates the motions and watches structural loads so the operator cannot overstress the booms. From the Operator Cab the operator works two Joystick joysticks and reads LCD Panel displays of belt load, boom angle, and crawler position.

Place in the mine

A spreader closes the loop in a continuous-mining system. A bucket-wheel excavator and a conveyor bridge strip and move the overburden, and the spreader places it back on the worked-out side of the pit, so the hole advances steadily as a wave. Matching the spreader's placement rate to the excavator's dig rate and the conveyor's transfer rate is the whole design problem of such a system, and the spreader is sized in boom length and throughput to suit the depth of overburden and the geometry of the spoil side it has to fill.