Big toys on the coal front

The largest opencast mine in the country, the Rotowaro coalfield near Huntly, produces up to two million tonnes a year using the heaviest mining machinery operating in this country. By ALAN TITCHALL.

Rotowaro was mined underground until the 1960s, mostly dug out by hand and loaded into 1200 hundred weight (just over half a tonne) skips by men working in pairs, each shovelling an average of 20 tonnes of coal a day.

These days the words ‘manual labour’ in a coalmine may as well be a Spanish guitar player, and the machinery used in opencast operations just keeps getting bigger.

While production at Rotowaro is at its final stage (see feature in February-March issue of Q&M), mine owner Solid Energy estimates there’s still about nine million tonnes of recoverable coal remaining in different seams in the Awaroa 4 pit, or about eight more productive years left.

Consented in March 2004, the Awaroa Pit originally contained 11.4 million tonnes of coal and 130 million cubic metres of overburden. Removal starting in June 2004 and the pit is expected to reach 180 metres at its deepest point. The coal seams are, on average, between two and five metres deep (and some up to 20 metres deep).

Overburden removal and coal extraction at is contracted out to HWE Mining, owned by Australian mining giant Leighton Contractors. The operation is seven days a week, and 22 hours a day with 220 fulltime staff split into shifts (including Solid Energy staff and other contractors).

The machines used for removing overburden and digging up coals seams is carried out by an army of excavators ranging in size from 120-tonnes to the massive 400-tonne Komatsu PC4000 excavator. A small crew circulates around the site relieving drivers with a ‘hot seat’ change-over for breaks and lunches.

Hauling overburden and coal is the job of a fleet of heavy trucks, ranging from the 65 tonne, mechanical-drive Caterpillar 777D to the massive 140 tonne, electric-drive  Komatsu 730E hauler used to carry overburden. The 730E is the largest truck used in New Zealand, costing more than $4 million and capable of carrying 190 tonnes. Each of its wheels weigh three tonne and each tyre costs $26,500 to replace, and it only takes about 18 months to chew through a tyre.

Three Komatsu 730Es servicing two Komatsu PC4000s are used to strip overburden. The PC4000, costing almost $6 million, is the biggest digger used in New Zealand – the track alone is two metres high. With an eight metre bucket, it can pick up to 21- 23 cubic metres, or 50 tonne of dirt, in one scoop. These monsters chew through five litres of diesel a minute. Collectively, the pit machinery can consume 50,000 litres a day, which really hurt last year when diesel prices were high.

Over the next eight years, it is estimated that the heavy truck fleet will make 135,000 round-trips carrying 50 million cubic metres of overburden from the Awaroa 4 pit to backfill the former Township pit using road overpass 80 metres long, over nine metres high, and 12.5 metres wide, (the width of three 730E dump trucks), that reaches across the public Rotowaro Road. Specifically designed and constructed to take the weight of the 400 tonne loads, the elliptical, multi-plate steel structure is the biggest of its kind in the country and cost $5 million to build (contractor was Brian Perry Civil). 

From pit to stockpile

The mechanical process from pit to stockpile is: coal seam, digger, coal dump truck for three kilometres to a hopper and rotary breaker, electromagnet, washer, conveyor belt, screening and crushing plant, coal scanner, one of five stockpiles, front end loader, analyser, and truck hopper or rail hopper for loading into transport.

A variety of different sized trucks, up to the large Cat 777D, are used to haul coal from the pits seams. Coal is lighter than rock, so a load of pure coal can be hauled in a vehicle with higher sides than those used on the overburden. As the coal is excavated closer to the edges of the seam, where there’s more rock, the digger operator must match the size of the truck to the weight of the load.

The dumpsters haul the coal to a giant hopper on the edge of the mine site that feeds a New Zealand-built rotary breaker that features a large, revolving cylindrical screen that breaks the mined coal down into smaller pieces (50mm), and removes, rock, old wooden pit props, roofbolts and other debris from the old underground workings.

A electromagnet at this stage removes any scrap from the old workings. The coal is then crushed and sorted by its ash and sulphur content using a radioactive analyser and relayed by conveyor belted to stockpiles (ranging from 5000 to 6000 tonnes) at the blending plant and ‘loadout’.

A front-end loader is used to mix coals from different stockpiles to customer specification before being loaded by conveyor belt to a 1200 tonne hopper that fills either 50 tonne rail wagons (every 30 seconds) for the Glenbrook Steel Mill, or the 200 tonne truck bins for a six kilometre trip along a private road to the Huntly West Mine stockpile for the Huntly Power Station.

Much of the coal goes to stockpiles at the Huntly West Mine which used to supply the power station until it was closed in 2003.  The stockpile facility at the site is capable of storing up to 600,000 tonnes, made up of a working stockpile ‘A’ with a capacity of 150,000 tonne; a 90,000 tonne strategic pile ‘B’; and a relatively new pile ‘C’ with a capacity of 350,000 tonnes.

From here it is sent to the power station by the original Huntly West Mine conveyor belt that travels over four kilometres of farmland to another stockpile area at the rear of the power station.

The journey from pit to power station is one of less than 20 kilometres, but involves some of the country’s heaviest machinery and plant, working 24/7.