Rotowaro's final chapter

The end is in sight for the Rotowaro coalfield near Huntly as the last resources are mined out of the Awaroa 4 pit; the final chapter in a mining history that will have gone back 100 years.  BY ALAN TITCHALL.

Solid Energy’s coal production in the north island is largely made up of the huge Rotowaro Open Cast mine on the west side of the Waikato River at Huntly, and the Huntly East Underground Mine on the east side.

The Rotowaro coalfield was first mined in 1915, with open cast mining starting in 1958 and production starting in earnest with the commissioning of the Huntly Power Station and the Glenbrook steel mill. Both were specifically designed and calibrated for the Huntly region’s soft sub-bituminous coal. Able to use coal or gas, Huntly was fired on gas from the Kupe fields in Taranaki until 2003 when reserves started to decline and coal became cheaper to use.

These days, the majority of Rotowaro’s coal (at least 70 percent) travels by truck and overland conveyor to the power station operated by Genesis Energy. Another 25 percent is sent by rail to New Zealand Steel’s Glenbrook Mill on Auckland’s Manukau Harbour. The rest of the coal is sold to the industry and home heating markets in the North Island.

Over the winter of 2008 Huntly was pumping over-time to generate power to the South Island to compensate for low hydro lake levels, which also meant the Rotowaro Open Cast mine was in full production keeping up with coal demand.

Located 10 kilometres west of Huntly township, Rotowaro produces between 1.5 and 1.9 million tonnes a year (depending on demand from the power station), from different seams in the Awaroa 4 pit where production started in 2004.

Unlike the long, flat, easy to strip seams in Australia, coal recovery is hard work in New Zealand where the 40 million year-old seams are broken and divided by faults and comparatively rugged terrain. We work hard for our coal in New Zealand – making our skilled labour very sought after internationally. If can mine in New Zealand you can mine anywhere.

Overburden removal and coal extraction at Rotowaro is contracted to Aussie mining company HWE Mining, a subsidiary of Leighton Contractors. Overburden from the Awaroa 4 pit is trucked to the adjacent Township pit as back-fill. Exhausted pits are filled with back-fill, contoured to fit the surrounding topography and replanted for future use as pasture or forestry. The rehabilitation of the Weaver’s Opencast Mine (1958-1993) on the edge of Huntly Township was completed in 2006 and is now known as Puketirini (formerly Weaver’s Park). Featuring a 54-hectare, 50-metre deep lake bordered by wetlands and a mixture of pasture and recreational areas, the park was gifted to the Waikato District Council in 2007. Solid Energy’s environmental programmes in the area will be the subject of a future feature in Q&M magazine, as will the machines used for the job of removing and redistributing overburden and digging up coal seams.

Operations at Rotowaro are run 22 hours a day, seven days a week with 220 fulltime HWE Mining staff working around two shifts a day and another 50 contractors and 45 Solid Energy staff also involved in the operation.

Consented in March 2004, the Awaroa Pit contained 11.4 million tonnes of coal and 130 million bcm of overburden. To extract around 1.8 million bcm of coal every year, 20 million bcm of overburden has to be removed. This is a deep mine, expected to reach 180 metres at its deepest point, and the coals seams are, on average, between two and five metres deep (and some up to 20 metres deep). It is economical to mine any seam more than 750mm thick.

Looking ahead

Solid Energy estimates that Awaroa 4 has about nine million tonnes of recoverable coal left, or about seven to eight more years of production, depending on future demand from Genesis Energy. The pit will then be returned to a mixture of pasture and forest and the final void turned into a recreational lake.

Huntly has plenty of resource left. The Huntly East Underground Mine, for instance, has another 20 years left in at least says Solid Energy.

“We have an ongoing programme of exploration in the area to focus on resources to have continuous production and we have a couple of other opencast prospects,” says Craig Smith, general manager North Operations.

Solid Energy also has its sights on the Maramarua opencast mine about 80 kilometres north east of Huntly and adjacent to State Highway 2. Smith says mining operations are likely to resume in the Kopako  1 (K1) pit at Maramarua and possibly extend to K4 ­– a new pit in the planning.

Maramarua was originally commissioned to supply the Meremere power station that was built in the 1950s, mothballed in 1991 and eventually demolished. Out putting 210MW it was the country’s first thermal power station. Around 800,000 tonnes of coal from the Maramarua field supplied the plant with coal via a six-mile long aerial ropeway.

Following Meremere’s closure mining ceased and both K1 and K2 pits were allowed to flood. These days Glencoal, another coal producer, operates a new mine called K3 and Solid Energy has installed pumps to drain the flooded K1 and K2 pits. Once the pits dewatered, the company expects to start mining about 300,000 tonnes of coal a year, supplying both New Zealand Steel and Genesis Energy. A coal handling and processing plant, including a hopper, crusher, conveyor and screens, and contractor’s facilities are to be built at the site.

“Extracting the last coal from K1 will provide about a one million tonne resource, and the new adjacent area, K4, about another 10 million tonnes of resource.”

Smith concedes that accessing the new K4 resource is “complicated” because of major infrastructural challenges that involve high tension power lines, the site’s proximity to State Highway 2, and complex negotiations with land and mineral owners.

“So it has taken a while to get to full feasibility status, but it’s the next best prospect for Solid Energy.”

The smaller K1 resource has the necessary consents applications with Council for processing and land ownership issues have been settled with local residents, says Smith.

“The K1 operation can start on the back of our current operations, while K4 requires more investment and is a riskier prospect. We are not prepared to tick it off yet.”

Plans for Maramarua also involve a coal (and ash) private transport corridor built between the mine and main trunk railway line at Mercer.

“In the short term, while production is low, we intend to transport coal from the mine by truck via State Highway 2 – north and south. But if K4 gets going we would have to seriously look at a dedicated mine road to connect to the railhead at Mercer.”

The coal resource at Maramarua is of a slightly lower rank (reduced energy per tonne of approximately 10 percent) than that at Rotowaro, but is still a high quality coal, low in ash and sulphur. Solid Energy has another coalfield, Ohinewai, just north of Huntly and bordering on the southern west end of Lake Waikare.

“It’s a big resource of around 40 million tonnes and very similar in quality to Rotowaro. However, a mixture of private and crown ownership, peat material on top of the seams, and the proximity of the Waikato river and Lake Waikare creating possible hydrological problems, means that we view this resource a longer term prospect.”

Then there’s the licence Solid Energy still holds to mine Huntly West, last mined about eight years ago, and now an area used to stockpile around 650,000 tonnes of Rotowaro coal for the nearby Huntly Power Station.

“Huntly West is a very large resource that we do have plans to mine; most likely as part of our Huntly East (underground) Mine. The licences are pretty much adjacent.”

While the coal is not continuous between Huntly West and East, separated by a barren zone that will have to be crossed, Smith says tapping the western resource from the East mine is likely the most sensible and economical prospect.

“We haven’t got a good handle yet on whether the original West shaft and two drift accesses will be reinstated as part of a renewed operation.”

Huntly East has already been extended north west to access a further six million tonnes of coal to support a new five-year supply agreement with New Zealand Steel signed earlier this year, Smith adds.