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Zisco restructured.

The materials handling facilities designed and built by Bateman for the restructuring of Zimbabwe Iron and Steel Co (Zisco) at Redcliff near KweKwe, Zimbabwe, have been operating well since commissioning last year. They include stackers, reclaimers, conveyors and dust control systems. The work was carried out because the depleting Buchwa mine's iron ore reserves needing to be supplemented by ore from Ripple Creek. The complex comprises blast furnaces, coke ovens, steel mills and sinter and limestone plants.

Ore from the new Ripple Creek mine, south of the main plant, is delivered on an overland conveyor. The Ripple Creek plant has facilities for crushing, screening, sampling and blending. Run-of-mine ore can be up to 900 mm in size. The 1,000 t/h primary crushing and screening section produces a minus-150 mm product. Secondary crushing and screening further reduce this to minus-31.5 mm.

Blending to a constant grade is undertaken via two 60,000 t stockpiles in tandem. A 1,000 t/h Bateman/Schade slewing luffing stacker layers the ore onto the stockpiles such that it is suitable for blending. It is recovered by a 500 t/h Bateman/Schade double harrow bridge reclaimer.

Blended ore is carried on a 15.6 km, single flight, 500 t/h curved conveyor to the Redcliff plant. Here the minus-31.5 mm ore is transferred by conveyor onto conical stockpiles and mixed with ore from Buchwa. Variable speed Bateman belt feeders reclaim the ore and feed it to a tertiary crushing, screening and sampling plant to produce minus-8 mm material. This is fed to two parallel blending stockpiles.

Limestone produced from a pit adjacent to the Redcliff plant is recovered from a stockpile, crushed and screened to minus-3 mm. It is conveyed over the KweKwe River and stored in a concrete silo before being conveyed to the blending stockpiles. Batches of limestone, coke and reverts (the undersized fraction from previous processing) are transported from their respective storage areas by means of the same conveyor to the sinter plant.

The iron ore and limestone are blended to the consistent grade required for the sinter plant using a 700 t/h Bateman/Schade luffing stacker and a 500 t/h Bateman/Schade single harrow bridge reclaimer. A transfer car moves the reclaimer between adjacent stockpiles.

The sintered product is conveyed to a final screening facility from where only the oversize fraction is fed to the blast furnaces. The reverts are stockpiled, feeding back to the sinter plant for reprocessing via a feed bin and vibrating feeder. A wheel loader supplies the feed bin.

In addition to the 15.6 km overland conveyor, 46 conveyors with a total length of 6 km have been supplied to convey the limonite, coarse and crushed iron ore, dolomitic limestone, sinter and coke. Their lengths range from 11 to 525 m with belt widths from 750 to 1,800 ram. The maximum capacities are between 128 and 1,760 t/h.

The 15.6 km overland conveyor is, according to Bateman, the longest single flight, curved, steel cord conveyor in the world. An appropriate balance between speed and belt width keeps the cost of belt and idler replacement to a minimum. Sophisticated finite element simulation technology from Conveyor Dynamics was used to conduct dynamic analyses during the design phase to ensure that design parameters accounted for all the phenomena arising from the transmission and accumulation of dynamic shock waves.

The 750 mm wide Bridgestone belt is 31.37 km long with 56 splices. The curved portion of the installation has a radius of curvature of 6 km. Total installed power is 1,000 kW. The two belt drives at the Redcliff head end, one of which is a tandem driver, and the one at the tail, are all electronically controlled by means of a telemetry link between the head and tail ends of the conveyor. This provides a controlled start and an inspection speed for easy belt maintenance. The belt takes about eight minutes to be brought up to its full speed of 4.7 m/s, and three minutes to be brought to a halt. Emergency shutdown is controlled by means of a flywheel at the head end operating in conjunction with a disc brake at the tail.

The take-up tower at the tail houses the vertical take-up and belt storage and optimises tile dynamic behaviour of the installation. It is 51 m high and includes an 80 t counterweight. Belt storage capacity is 120 m.

More than 4,000 specially designed freestanding portal frames support the belt. This eliminates the transmission of harmonic vibrations. The entire belt length is covered to avoid heavy rains doubling the weight being carried. Belt flutter and vibrations are reduced by a unique design of idlers and idler spacing. The idler spacings minimise harmonic vibration and on the curved section are half the spacing of the straight sections. Idler and belt life are increased as tile belt on the return side is turned over to keep only the clean side of the belt in contact with the idlers.

Belt speed and slip are monitored and dynamic tension is measured using load cells. The safety system incorporates a long-line signalling system reporting to the central control room and comprises pull wire alarm, startup warning and belt alignment systems. In addition to the on-site SCADA system, a data capture system enables data to be sent to Bateman in Johannesburg for analysis. This feedback allows ongoing service needs to be monitored. In addition, Bateman provides a monthly condition monitoring service to advise on the integrity of the system.
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Title Annotation:Zimbabwe Iron and Steel Co
Publication:Mining Magazine
Date:Jul 1, 1998
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