Between 1900 and 1908 J. E. L. Cull used a different approach to the production of iron from Taranaki iron sands. After systematic analysis of earlier failures, he developed new techniques which took specific account of the nature of the iron sands.–
Cull began his investigations while working for the Dunedin engineering firm, Cutten Brothers, on the design and construction of gold dredges. In alluvial mining the high specific gravity of gold is used to separate it from sand. However some components of black sand from the South Island West Coast also have a high specific gravity and were often precipitated with the gold. Magnets had been used to separate small amounts of the magnetic iron-bearing grains from the gold and Cull had designed a machine to handle larger amounts. A Mr J. McPhee built a prototype separator, which worked well, except that not all of the heavy components of West Coast sand are magnetic. Cull concluded that McPhee’s separator would not pay.
In his paper in the New Zealand Journal of Science and Technology (Cull 1918) he described what happened next. "Possessed now of a magnetic separator, I and several others decided to follow up the treatment of black sand from the point of view of its iron content; and with that object in view Mr S. H. Jenkinson afterwards went to the United States of America to get experience in the iron and steel works of that country, the author remaining in New Zealand, and as opportunity offered during the years 1900 to 1908 carrying on experiments."
(The experiments were carried out in the back yard of his parent’s house in Dean Street in Christchurch)’
In 1902 Cull returned to Canterbury University College as a demonstrator in the mechanical engineering department and, until 1908, tackled the fundamental issues of extracting iron from black sands, attempting ’to produce, on a small scale, a furnace which on a large scale might be expected to smelt the sands commercially."
After magnetic concentration, the iron sand was mixed with powdered coal (a cheap reductant) in a combustion chamber. Direct reduction of the iron oxide was achieved at a temperature below the melting point of iron, yielding a porous metallic product known as sponge iron.
When, owing to its fineness, the sponge iron was found to re-oxidise during conveyance to a melting hearth, Cull experimented with methods of delivering it to an electric-arc furnace in the absence of air.
He then added lime as flux, struck the electric arc and successfully separated slag and metal. He noted correctly that the sands could be smelted by purely electrical means without the pre-reduction step, but that pre-reduction with coal led to considerably reduced consumption of energy.
Why Did It Work
Why, then, did Cull’s laboratory method work when attempts using blast furnaces had failed?
First, his direct reduction of solid-phase iron was achieved at temperatures too low for the formation of the titanium nitride, carbide and oxide compounds that hampered blast furnace smelting.
Secondly, at the electric-smelting stage, the formation of titanium carbide was minimised by close regulation of the carbon concentration and independent electrical control of temperature.
Thirdly, the absence of air minimised titanium nitride and oxide formation during smelting.
Cull’s experiments were an unheralded milestone in the history of steel production. Performed at the very beginning of the development of electric steel making, they pre-dated by far the widespread use of electric furnaces.
His combination of :
sponge-iron manufacture and
electric smelting .
anticipated processes adopted in New Zealand nearly half a century later.
Moreover, his 1908 patent, "Improved Electrical Process for the Manufacture of Iron, Steel and Other Metals from their Ores," incorporated two other features that would later be adopted – the hot transfer of sponge iron to the electric furnace and the co-generation of electricity using hot gases produced during pre-reduction.
Unfortunately, his work could not be applied commercially until the later development of electric pig iron smelting and bulk power generation in New Zealand.
Cull’s Electric Furnace
This specification drawing accompanied Cull’s 1908 patent for an "Improved electrical process for the manufacture of iron, steel and other metals from their ores."
Ironsand and powdered coal are fed into the reduction furnace (A) from the hopper (E) by a screw (G, F). Pre-heated air is blown into the upper part of the furnace along the duct (H).
The iron ore is reduced to sponge iron in the reduction zone (B). The air admission rate is adjusted to control the combustion rate of the coal. This ensures both that the desired reaction temperature is achieved and that a reducing atmosphere is maintained throughout the enclosure (C). The spent gas leaves through the duct (J).
The reduced sponge iron slides down into the melting zone (D). Here an arc struck between electrodes (K) and (L) provides the heat to melt the iron. This and the accompanying slag are run off periodically from the molten pool through the tapping hole (0). JSP