Smelting
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.
Making steel
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.
Difficulties smelting ironsands
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.
A man of steel
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.
Smelting limonite
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.
Attempts at electric smelting
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.
A unique industry develops
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.
