Turning New Zealand’s ironsands into steel is a unique process that takes place at two sites in the North Island.
Waikato North Head: sand to iron
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.
Glenbrook: iron to steel
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.
Types of steel
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.
Exporting vanadium and ironsands
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.