Taharoa ironsand mining and ship loading
Engineering Site (eg Portland cement works, Maori fortifications)
The Taharoa ironsand mining project was commissioned in 1972 and has been a success story for the local community and New Zealand Steel Mining. (www.nzsteel.co.nz)
Taharoa, an isolated and exposed location on the North Island’s west coast, has the largest deposits of ironsand (titanomagnetite) in New Zealand. There was an estimated 300 million tons of concentrate available when the mining project commenced.
Titanomagnetite sand comes from volcanic deposits which have been eroded by coastal action and dispersed along the coast by littoral currents. Taharoa has both wind and waterborne deposits in dunes reaching up to 90 metres (m) in height.
The ironsand is mined by a floating cutter suction dredge. The material is then concentrated through cyclonic and magnetic separators, and stored in stockpile heaps. The area has no natural harbour and is subject to storms and waves of up to 11m high. As such, the ironsand concentrate is pumped as a slurry out to a bulk cargo ship tethered to a single point mooring buoy 3 kilometres (km) from the shore.
In terms of value, the titanomagnetite concentrate is a low grade iron ore with 57 per cent iron by weight and 7.6 per cent titanium dioxide. However, the ironsand is exported to places such as Australia, China and Japan.
Beginning the process
H. E. Fyfe of the Geological Survey and D. S. Nicholson, a mining engineer, identified the significant ironsand deposit at Taharoa in the 1940s for the Department of Scientific and Industrial Research. They confirmed this in 1957 when Kaiser Engineers Incorporated of United States of America wanted to investigate the possibility of supplying ironsand to a South Island steel mill.
However, the feasibility of the project depended on finding an economic method of transporting the ironsand to its place of use. Land transport to a suitable port, or shipping in small vessels from Kawhia, was uneconomic. An off-shore loading method was not developed at this time.
New Zealand Steel Limited identified this as a solution. They investigated the site in 1968 and commissioned McConnell Dowell to undertake pumping trials, research pumping abrasive slurry mixes and dewatering of the slurry in a ships hold.
The land and the people
Until 1968 Taharoa was a very isolated community with no road access. That soon changed when New Zealand Steel went forward with their mining project. The mine’s site is actually owned by a local Maori land trust, whose members are from Ngati Mahuta Ki Te Hauauru. The mining lease covers approximately 1600 hectares divided into sections: Northern, Central and Southern. Areas of cultural significance, such as urupa, are respected and excluded from being mined.
Taharoa village was also built by the company with 65 houses on leased tribal land for the families of workers, as well as a hall, Kohanga Reo pre-school, school, shop, and fire brigade and ambulance facilities.
Over 80 per cent of the mine’s workers are local Maori. The company has also had a wider social and economic role in the community by providing educational grants for travel and boarding to employees’ children, as well as school leaver apprenticeships at the mine. By 2013 the beneficiaries had invested over 50 million dollars in royalties into businesses and farms.
To mine the ironsand a small lake is formed and an electro-hydraulically operated cutter suction dredge excavates the sand to a floating trommel screen, which removes particles larger than 2.5 millimetres. Then the sand is pumped as slurry to the floating concentrator building where lighter material is removed before the denser fraction passes through magnetic separators. The concentrated iron slurry is then pumped up to 5km where it awaits the arrival of the bulk carrier ship.
In 2000 operations moved to the Central area after the Southern mining section was worked out. This involved shifting the 250 tonne dredge, the 450 tonne surge bin and the 1000 tonne concentration plant 2km overland to a new site. Moving the heavy concentrator plant by truck was the largest overland load moved in New Zealand. This was accomplished by using a specially constructed 20m wide roadway which afterwards became an airstrip.
After areas have been mined, the residue tailings are re-shaped to match the natural environment and planted with native grasses or commercial forests. Surface vegetation stripped from the land before mining is stockpiled and re-used during the rehabilitation operations. Once the land has stabilised it is returned to the local owners.
New Zealand Steel Mining has a policy of ensuring minimal impact to the environment, recycling water where possible and ensuring minimal effect on aquatic life. The owners have formed a Taharoa Lakes Trust whose responsibility it is to manage and control the use of Lake Taharoa and four smaller lakes. The trust is responsible for overseeing water, fishing and shooting rights, and boating. The Foreshore and Harbour Kaitiaki Ngati Mahuta Ki Te Haruru group take care of the local foreshore and southern area of Kawhia Harbour.
Once ironsand is mined at Taharoa the concentrated iron slurry is then pumped out to ships and exported. This requires a pumping station. At the station two pumping systems operate side by side, each with seven 380mm single stage Warman centrifugal slurry pumps in series. Twin pipes then each deliver 1,375 tonnes of material per hour, each at 760 kilopascals. These submarine pipes end on the sea bed under a buoy, and have massive vertical flexible hoses connected to them.
The single point mooring buoy is 11m in diameter and weighs 185 tonnes. The buoy is anchored 3 kilometres off shore in 35m deep water by six 350m long studlink chains, which each has a six tonne Vryhof Strevpris anchor.
Loading at the buoy is restricted by defined weather and swell conditions. Ships moor to the buoy’s axis which has a bearing and arm. This allows a vessel to weathervane around the buoy and always be lying into the wind and sea. The buoy’s rotating manifold is connected to flexible hoses rising from the seabed pipes, and then floating hoses connects to the ships deck cargo distribution system.
The ship is prepared to receive the slurry by pumping fresh water into the holds. This provides protection from the ironsand slurry hitting and damaging the hold floor. The percentage of concentrate is then increased until the designed density (50 per cent by weight) is reached.
New Zealand Steel also used a similar off-shore loading technique at Waipipi, near Waverly, in the 1970s. However, unlike Waipipi the geology of the seabed meant that burying a portion of the pipline was impractical at Taharoa. Therefore, the initial pipeline was laid and allowed to settle down through the sand while being restrained laterally by piles.
Transporting the ironsand
The initial shiploading system was built for vessels of 50,000 to 70,000 dead weight tonnage (DWT). By 1978 ships of 150,000 DWT were being used, which required the dual pipelines to increase loading rates. Over the years the pipeline has also been gradually extended further off-shore in order to accommodate these larger vessels.
The Taharoa Express 143,000 DWT was built in 1990 and reached the end of its economic life in mid 2012. On 25 May 2012 a new 175,000 DWT purpose-built bulk carrier Taharoa Destiny arrived at Taharoa and was blessed by the local Maori. By 2014 a further vessel should be in service. This extra capacity could see 24 hour seven day operation of the mine site.
In the ship’s holds the concentrated ironsand drains easily to a moisture content of less than 3 per cent by the time it reaches customers in China or Japan. This is similar to that of sand in sand hills. The speriodal shape of the sand grains allows the easy drainage allowing the free surface water to drain away and be pumped overboard. Unloading of the ironsand is by clamshell cranes.
Prepared by John La Roche with editing and comments by Murray Lye, December 2012
John Ingram, et. al. ‘Symposium on Taharoa ironsands project,’ New Zealand Engineering, Vol. 29:5 (May 1974), pp.129-133 (see attachment below)
Les Jones, ‘Pumping Ironsand and Slurry, Off shore Single Point Mooring Buoy,’ unpublished IPENZ report, 15 October 2012
New Zealand Steel Ltd, ‘New Zealand Steel Mining Taharoa Ironsand,’brochure, c.2012
New Zealand Steel Ltd, ‘Taharoa Destiny: Celebrating a new era in the history of Taharoa Ironsand,’ brochure, c.2012
New Zealand Steel Ltd, ‘New Zealand Steel Mining Taharoa: Rehabilitation Programme,’ Presentation, New Zealand Petroleum and Minerals, www.nzpam.govt.nz
H. E. Fyfe and D. S. Nicholson, ‘Borehole survey of the North Island ironsands from New Plymouth to Kaipara Harbour,’ New Zealand Journal of Geology and Geophysics, Vol.1:4 (November 1959)
Ross Galbreath, DSIR: Making science work for New Zealand, Wellington, Victoria University Press, 1998
New Zealand Engineering Vol.29.5 May 1974.pdf
Taharoa, near Kawhia, west coast of the North Island
The mining operations can be viewed from Taharoa Rd, just before the road ends at the Taharoa Aerodrome.
Nature of Engineering
Manufacturing and Industrial Processing
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Lat: -38.166786 Long: 174.706950