A significant characteristic of New Zealand ironsand is its relatively high titanium content.
Titanium-bearing iron ores are widely distributed throughout the world, notably in rock form in South Africa, Russia, Norway and North America, and as sand in Japan, South America and New Zealand. Many attempts have been made to smelt them into iron and steel.
Japanese and Indian titaniferous ores have been successfully exploited since ancient times to produce high-quality wrought-steel swords by low-temperature reduction in charcoal furnaces and hand forging. However, attempts using blast-furnace techniques were almost entirely unsuccessful.
Nineteenth-century iron makers had not understood initially that New Zealand iron sands contain titanium Ð an analysis presented to a select committee of the British House of Commons in 1844 contained no mention of the element. Later it was believed that the titanium content would yield high-quality steel comparable to those of Sweden and Russia and with some of the attributes of the famous Japanese swords.
However, as early as 1866, the Taranaki provincial government had received the opinion of Dr Abel, director of chemical works at London's Woolwich Arsenal, that titanium would reduce the fusibility of slag, making the ore unsuitable for blast furnace smelting.
Despite Abel's advice, John Parry, Edward Smith and D. Atkinson conducted blast furnace smelting trials at New Plymouth in 1868, using the Pioneer Steel furnace. Their first problem was the fineness of the ironsand, which choked the furnace and prevented the flow of gases. They solved this by feeding the furnace with consolidated balls of iron sand bonded with clay.
However, when they attempted to draw off the molten metal, the tap holes became clogged with accretions of a viscous pasty material. Similar problems led to the failure of the New Zealand Titanic Iron and Steel Company in 1878, after which the New Zealand Government received further British advice reiterating Abel's opinion that titanium in the sand was causing the thick slag.
Compared to the best non-titaniferous ores, ironsands provide a raw material of relatively low grade, containing only 58-60% iron by weight after physical concentration using magnetic and gravity methods. The iron-bearing mineral is titanomagnetite, a solid solution of titanium, magnesium, manganese and vanadium in magnetite (Fe3O4). Other substances, such as silica, phosphorus and lime, are also present as inclusions. A typical analysis of New Zealand concentrated titanomagnetite gives the composition by weight illustrated in the pie chart.
In a blast furnace, hot air is blown through a mixture of iron ore, coke and fluxes such as limestone, creating conditions suitable for the reduction of iron oxide to metallic iron. The principal reducing agent is carbon monoxide, formed by reaction of carbon in the coke with oxygen in the air blast.
For magnetite ore the reduction reactions are:-
Fe3O4 + CO ¯ 3FeO + CO2 Reaction 1
FeO + CO ¯ Fe + CO2 Reaction 2
At a temperature of about 1500'C, the molten metal forms a pool at the base of the furnace, upon which floats a layer of molten slag containing the impurities.
In a blast-furnace, the titanium (IV) oxide (TiO2) in titanomagnetite ore is reduced to titanium nitride (TiN), titanium carbide (TiC) and the titanium (II) oxide (TiO).* These stable products have very high melting points and, in large amounts, can produce highly viscous slags. As solids wetted by both molten pig iron and slag, they form accretions and pasty masses, which block the tap holes of the furnace when the metal or slag is drawn off.. BEW
* The Roman numerals IV and II denote the oxidation state of the metal, which specifies the number of units of positive charge resulting from loss of electrons.