Onehunga Ironworks

The Onehunga Ironworks was a colonial-era iron smelting and rolling operation at Onehunga, on the Manukau Harbour, (now a suburb of Auckland, New Zealand). It was at one time claimed to be the largest ironworks in the Southern Hemisphere. It is significant, both as the first large scale attempt to exploit New Zealand's iron-sand by direct reduction, and as a precursor of the modern steel industry of New Zealand. The ironworks was located adjacent to the original Onehunga railway station. It operated—but not continuously—from 1883 to around 1895. It was partially demolished around 1903 but its brick chimney and some of its other structures were still standing in the late 1960s.

Historical context

New Zealand's iron-sand resource

Vast deposits of iron-sand exist over 480 kilometres of the North Island's coast from Kaipara Harbour down to Whanganui. These iron-sand deposits are rich in the mineral titanomagnetite that originates as crystals within volcanic rock. As the rock is eroded, rivers carry the heavy grains of titanomagnetite to the coast. Currents, wind, and wave action then move the minerals along the coastline, concentrating them in dark-coloured sands on the sea floor, on beaches and in dunes. Captain James Cook was probably the first European to record the 'black sands' of New Zealand's North Island, during his first voyage around New Zealand in 1769–70. In 1839, Ernst Dieffenbach, employed by the New Zealand Company to describe New Zealand's natural resources, noted the 'black titanic iron-sand' on beaches along the Taranaki coast.

Earlier smelting of iron-sand Smelting of iron-sand has been carried out successfully in Japan for centuries, The Japanese method is a type of direct-reduction smelting. Smelting occurred in a Tatara furnace. That process is slow and makes only small batches of metal (known as Tamahagane) that is used in the making of high-quality steel weapons. Although New Zealand's iron-sands are smelted today on a commercial scale, it took many years and many failed attempts before a successful process was developed that could smelt titanomagnetite iron-sand in commercially viable volumes. Before the establishment of the Onehunga Ironworks, other attempts had been made to smelt New Zealand iron-sands, but only "partial success was attained by smelting, in furnaces, bricks formed of the ore with calcareous clay and carbonaceous matter". The most notable of these earlier ventures was the New Zealand Titanic Steel and Iron Company, which was led by Edward Smith and had erected a blast furnace at Te Henui near New Plymouth. Attempts to smelt iron-sands in blast furnaces—the conventional means used for other iron ores—failed for two main reasons; the fine sand grains blocked the flow of hot air through the furnace—something that could be overcome, to an extent, by binding the sand into 'bricks' as mentioned above—and carbon from the coke combined with titanium in the iron-sand to produce a thick pasty layer of compounds that blocked up the tap holes used to draw off the molten iron and slag.

History

First period of operation 1883–1887

John Chambers It was the potential to exploit deposits of iron-sand near the heads of Manukau Harbour, which led to the establishment of the Onehunga Ironworks. John Chambers had visited England and America in 1876 trying to interest ironmakers in the iron-sand, without success, but while in America he became aware of a process by which it was claimed wrought iron could be made from iron-sand. Chambers and an American, Guy H. Gardner of New York, jointly purchased the New Zealand patent rights of the furnace design patented in 1873, by Joel Wilson of Dover, New Jersey. Wilson provided the services of William Henry Jones to come out to New Zealand to supervise the work. A full scale furnace using this design was erected at Onehunga during 1882. This first furnace was completed by early February 1883. A public demonstration of the furnace operation and smelting of iron-sand took place in early February 1883. The first billets of wrought iron smelted from iron-sand were made on 27 February 1883.

New Zealand Iron and Steel Company (Limited) The initial success led to the formation of a company, New Zealand Iron and Steel Company (Limited), to expand the operation. The company had a capital of £200,000 made up of 40,000 shares of £5 each. Of these shares only 9,103 were sold to the public, resulting in a paid-up capital of £45,515. The site on which the experimental furnace had been erected, 5 acres on the south-eastern side of Onehunga railway station, was purchased. This land had a water frontage onto the harbour—allowing raw material to be landed at the works—and a rail connection. Copious supplies of freshwater could be obtained from the Onehunga Springs. A lease—from where the iron-sand would be obtained—was taken over 6.5 miles of beach (some at South Head and some at the North Head of Manukau Harbour) and 1000 acres of land at the North Head. It was planned to erect ten new furnaces and a rolling mill was ordered.

Wilson's process for iron-sand smelting

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