Iron, especially in the form of steel, is the dominant metal used in all modern societies and a key factor in their industrial development. It is the fourth most abundant element in the earth’s crust and, after aluminium, the second most abundant metal.
Most of these metals are smelted from naturally occurring material known as ore. Iron is extracted from iron ores, commonly found in rock. The main purpose for extracting iron is to make steel.
New Zealand has only a small amount of one frequently used iron ore – limonite, which is found at Onekakā in north-west Nelson. However, it does have another, unique source of iron: beach sand composed of a type of iron oxide called titanomagnetite.
During his first voyage around New Zealand in 1769–70, Captain James Cook recorded ‘a black sandy bottom’ in his journal, as the Endeavour sailed southward along the coast near Raglan. In 1839 Ernst Dieffenbach, hired by the New Zealand Company to describe New Zealand’s natural resources, noted the ‘black titanic iron-sand’ on beaches along the Taranaki coast. In fact these sands stretch for 480 kilometres, from Kaipara Harbour down to Whanganui.
Beach sands containing iron minerals are common around the world. Many have been studied as potential sources of iron, but few are of commercial value. New Zealand’s ironsand deposits, among the largest in the world, are rich in the mineral titanomagnetite. This is a black, heavy, magnetic iron ore which originates as crystals within volcanic rocks. In New Zealand, it occurs in the darker rocks of the Taranaki volcanoes and the lighter-coloured rocks of the Taupō Volcanic Zone. As the rock is slowly eroded, rivers carry the grains of titanomagnetite to the coast. Ocean currents then move the minerals along the coastline, and the action of wind and waves concentrates them in dark-coloured sands on the sea floor, on beaches and in dunes.
These ironsands form the country’s greatest reserves of metal ore.
The British colonists who first arrived in New Zealand expected to find mineral wealth, and hoped that the country could become a manufacturing hub – a ‘Britain of the South’. In 1865, promotional material referred to a future in which ‘the forests and fertile plains of New Zealand will resound with the clang of the forge and the hum of the factory, and the midnight glare of the furnace illumine the surface of her lakes and rivers.’ 1
European settlers soon discovered this abundant resource, but the sands defied efforts to extract the valuable iron. It would take more than a century of perseverance and ingenuity for New Zealanders to develop a specialised technology to make steel from sand.
To make steel you must first have iron. Iron is produced by smelting – heating iron ore to separate out the iron. Iron ore contains iron oxides, and when it is heated with coke (essentially carbon), oxygen moves from the oxides to the carbon, reducing the ore to molten iron. In the 19th century Britain smelted iron from lumps of iron ore, using blast furnaces fuelled by coke. In this process, molten iron from the ore forms a pool at the base, and is poured into moulds. The impurities form a floating layer of slag (waste) on top, which is also drawn off.
The molten iron, mixed with steel scrap, is then heated in a furnace. Through chemical reactions with carbon, the mixture is alloyed into steel, which is stronger and more flexible than iron.
Between 1869 and 1914 at least four New Zealand companies built blast furnaces to smelt ironsands, but all encountered seemingly insurmountable difficulties. First, the fine sand grains blocked the flow of hot air through the furnace. Unsuccessful attempts were made to bind the sands into briquettes with materials such as clay and coal.
The second and more taxing problem was the titanium in the ironsands. When they were were fed into a blast furnace, carbon from the coke fire combined with the titanium to produce a thick pasty layer of compounds beneath the slag. This soon blocked up the tap holes used to draw off the molten metal and slag.
From the 1850s many methods for reducing iron ore to iron were patented worldwide. New Zealander John Chambers obtained rights to one method, and built a specialised furnace to smelt ironsands at Onehunga in 1883. The early trials were very successful, until Chambers’s American manager got into a fight over a card game at a hotel, shot a man, and was sentenced to 14 years for attempted murder. His skill in operating the furnace must have been crucial as no one could replicate his method, and Chambers’s company sank.
For a time, attention turned from ironsands to another iron ore, limonite, in north-west Nelson. The Onekaka Iron and Steel Company built a blast furnace and ironworks and from 1922 succeeded in smelting iron. However, despite government subsidies, the business was uneconomical. In addition, the size of the deposits proved to have been overestimated, and the plant closed when it ran out of ore.
Between 1900 and 1908 a New Zealand engineer, John Cull, experimented with innovative techniques for smelting ironsands in an electric arc furnace. With this method, electricity arced from electrodes to the ironsands, which were heated and became transformed into molten iron. Because the heat was produced with electricity rather than from coke, less carbon was released. There was just enough to reduce the sands to iron without producing the pasty titanium layer. Cull patented his methods, but they went largely unnoticed.
By the 1940s, however, more widespread expertise was available on the electric smelting of ores containing titanium. Laboratory work by a Norwegian company had shown that careful control of slag composition could achieve trouble-free smelting. The Department of Scientific and Industrial Research (DSIR) trialled these methods at Onekakā in 1949. There was still an old steel works there, and ironsands were brought in for smelting.
After a century of trial and error, the most feasible way of smelting the Taranaki ironsands proved to be in an electric arc furnace – as Cull had shown. However, electricity costs were high, so the idea was abandoned for some time.
In 1954, DSIR metallurgist Tom Marshall visited a Norwegian scrap-steel plant. He became convinced that New Zealand could make steel from scrap using an electric arc furnace, and that this economical method could also be used for smelting ironsands.
The Iron and Steel Industry Act 1959 led to the establishment of the NZ Steel Investigating Company. At last in 1962 a method for converting ironsands to steel in an electric furnace had positive results.
The process was refined over several years, and in the mid-1960s a steel mill was set up at Glenbrook, 60 kilometres south of Auckland. Exploiting its proximity to the abundant coal supply at Huntly, Glenbrook was one of the first steel operations in the world to use coal rather than coke.
Turning New Zealand’s ironsands into steel is a unique process that takes place at two sites in the North Island.
Since 1969, ironsands have been mined at Waikato North Head, at a plant close to the mouth of the Waikato River. Waikato North Head has more than one billion tonnes of ironsand reserves that contain at least 33.8% titanomagnetite, the main iron mineral in the sand, and are estimated at 74 million tonnes. These will yield 19.4 million tonnes of concentrate containing 59% iron.
The ironsands contain a fair amount of ordinary sand or silica. The iron-rich, magnetic titanomagnetite sand is concentrated from this by means of gravity and magnetic separation. As much as 70% of the sand is returned to the mined area, where it is re-contoured into dunes and planted in pine forest.
The concentrate is mixed with water to produce a slurry, for ease of transport.
From the Waikato North Head plant, it is pumped through an 18-kilometre underground pipeline (the first of its kind in the world) to the Glenbrook steel mill.
The water is removed from the slurry, and the remaining sand is blended with Huntly coal, which produces the required carbon reaction when heated. The mixture is reduced in rotary kilns to sponge iron, which contains 70% metallic iron. This is then melted in an electric arc furnace to produce molten pig iron, which contains about 93% iron.
The pig iron is poured into a steel-making vessel, along with recycled steel scrap. Oxygen is blown onto the molten mixture to convert the impurities to slag, and fluxes (substances that promote melting) are added to produce liquid steel. The slag is poured off and the molten steel is transferred to a treatment station where it is brought to its final composition. The molten steel is then poured into a mould where it solidifies to form a continuous slab. Electric furnaces are particularly suitable at this stage for producing stainless steel and other highly alloyed steels made to exacting specifications.
Although some iron is used as wrought iron and cast iron, much of it is converted to steel. As an alloy of iron and carbon, steel is much more ductile and malleable than iron alone. There are five main types: carbon steel, alloy steel, high-strength low-alloy steel, stainless steel and tool steel. More than 90% of all steels are carbon steels. Stainless steels contain chromium, nickel and other alloying elements that keep the metal bright and rust-resistant.
Today, 650,000 tonnes of steel are produced each year, contributing about 5% of the nation’s gross domestic product. Glenbrook remains the only steel manufacturer in the world to use titanomagnetite sand as its source of iron.
Prior to steel-making at Glenbrook, a slag that is rich in the metal vanadium is separated from the iron. It is a valuable by-product – in the 2000s, 12,000 tonnes per year were exported to China, representing 1% of the world’s vanadium production. Vanadium is used in producing rust-resistant steel, and steel for high-speed tools.
At Taharoa, further down the coast from Waikato North Head, beach and dune sands are dredged to produce a concentrate averaging 40% titanomagnetite. Since 1972 Taharoa has produced about 1.4 million tonnes of concentrate per year, with an annual value of about $30 million. The concentrate is not processed further, but pumped through a 3-kilometre pipeline directly to a vessel at sea, ready for export. Total exports to the end of 2000 were over 37 million tonnes, mainly to Japan, with small quantities to South Korea and China. The titanomagnetite is used there as an additive, rather than the main feedstock in blast furnaces.
At the Glenbrook plant, molten steel is cast as huge, 21-centimetre-thick slabs that weigh over 10 tonnes. After they have cooled, they are reheated and passed repeatedly through hot rollers that reduce their thickness. The steel is then rolled into coils and cooled by water.
The Glenbrook company operates two modern cold mills, which have highly accurate x-ray systems to control thickness. The coils are processed to remove the iron-oxide layer that forms after hot-rolling and cooling. They are then passed through cold rollers to their final thickness.
Some types of sheet steel are given a thin coating of metal which provides protection against rust and corrosion. Two different varieties are produced. Traditional galvanised steel (Galvsteel) is coated with zinc, whereas a newer product, Zincalume, has a coating of 55% aluminium and 45% zinc.
Pre-painted steel (Colorsteel) now accounts for a large proportion of the roofing used in New Zealand. A builder or owner can purchase sheet steel in the colour of choice, rather than having to paint the roof after a building is completed.
The agricultural industry is a major user of steel pipes. Stock-handling equipment, implement frames and agricultural trailers are some of the steel-pipe products made by New Zealand Steel. The recently completed Waikato water pipeline was made from hot-rolled coil, galvanised Zincalume, and Colorsteel.
Although no hot-rolled beams are manufactured for the New Zealand construction industry, the Glenbrook plant produces welded steel beams. These offer greater flexibility in design – for example, curved and sculptured beams.
Hot-rolled steel is produced for beams, flooring, storage tanks and flanges. Cold-rolled steel products include shelving, drums, saw blades, hinges and car parts.
Since 1991, New Zealand has exported more iron and steel than it imports. In 2004, 60% of Glenbrook steel, including very high-purity stainless steels, was exported.
Corrugated iron has been one of the characteristic building materials in New Zealand for over 150 years. It is technically light steel sheet that has been galvanised (treated with a coating of zinc on both sides) to prevent rusting, then rolled into corrugations at either 3 or 5 inches (76 or 127 millimetres). First produced in English steel mills in the 1830s, it was regarded as suitable only for temporary buildings.
In the goldfields of California, Australia and New Zealand there was a need for speedy construction, and corrugated iron was just the material: weatherproof, light, portable, and easy to put up. R. & T. Haworth started producing galvanised iron in Dunedin in 1864, from imported steel plate.
As the population became settled, more permanent buildings were built, and corrugated iron was generally used only for those parts not immediately visible. Many houses and commercial buildings had imposing wooden or stone frontages, but the sides and back were utilitarian – summed up as ‘Queen Anne in the front, and a meat safe at the back’. 1 In between there was usually a lot of corrugated iron.
Corrugated iron was widely used in rural areas as a general-purpose building material. Most farmers could readily build a frame for a shed or hay barn, and then clad it with corrugated iron. Because the sheets of iron had to be purchased and then transported to the farm, they were often re-used once a temporary building was abandoned.
Tramping huts were traditionally made of corrugated iron. Before the days of helicopter transport, it had to be carried to the site – often a slow trip with an awkward load.
Working in a remote part of the Buller Coalfield in 1938, geologist Harold Wellman saw a loose sheet of corrugated iron lying by the Stockton electric railway. Needing to make an outdoor chimney, he picked it up and started to walk back to camp with the sheet on his back. Unfortunately the iron touched the overhead power lines, and he got an electric shock that he never forgot.
From the early 20th century onwards corrugated iron was mainly used as a roofing material, traditionally painted red or green. Many immigrants recorded their surprise at seeing the colourful iron roofs when they first arrived. When the 1935 Labour government started an active programme of house construction, they discouraged the use of imported components, so tiles and other roofing materials gradually replaced iron.
In the 1970s New Zealand Steel started to produce a variety of flat and corrugated products that have been widely used for roofing. In the 1990s, these reappeared as a cladding material, as part of a local architectural style based on functionalism and nostalgia.
Because it is flexible and readily shaped, corrugated iron has enjoyed a renaissance as a material for modern sculpture.
Chapple, Geoff. Corrugated iron in New Zealand. Wellington: Reed, 1983.
Galbreath, Ross. DSIR: Making science work for New Zealand. Wellington: Victoria University Press, 1998.
Thomson, Stuart. Wrinkly tin: the story of corrugated iron in New Zealand. Wellington: Steele Roberts, 2005.