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The history of the five furnaces in Tasmania, three in Victoria, one in South Australia and seven in New South Wales that had a life of less than twenty years is one of exploratory or experimental attempts to demonstrate the feasibility of producing iron from native raw materials rather than an economic exercise in the production of iron. They all failed because no one was prepared to buy the iron at a price equal to or higher than the actual cost of production. These 16 furnaces were built and operated from 140 years ago during the 80 years from 1848 until 1928.

In Tasmania where ironstone was first discovered in Australia, the attempts at iron smelting were carried out in the four years from 1873 to 1877 mostly in the area now known as Beaconsfield. Beaconsfield in northern Tasmania is generally known for the large production of gold from one of the richest quartz reefs. Gold was discovered in 1847 and the reef mined from 1877 to 1914. Near Beaconsfield, west of the Tamar River, was the site of the earliest recorded mineral deposit discovered in Australia except for coal reported in 1793 at South Cape Bay by French explorers and by George Bass in New South Wales in 1797. Colonel Paterson of the military corps had established a settlement at Yorktown on the Tamar West Arm in 1804 and in preparing his ship Lady Nelson for a return voyage to England in 1805, took aboard a ballast of iron ore, it being the heaviest material to be found in the locality. First reports identifying the material as iron ore were made in 1822 then the first geological investigation and report was made in 1866. The iron ore was considered of equal or higher grade than many English ores and in 1872 two companies were formed to commence smelting, the Tasmanian Charcoal Iron Co to exploit the first discovered ore at Mt Vulcan and the Ilfracombe Iron Co utilising an ore deposit six miles to the south at the foot of Blue Peaked Hill or Sugar Loaf. The Ilfracombe blast furnace stood on a sandstone base and was contained in an iron shell approximately 10 feet in diameter and 45 feet high. The Tasmanian Charcoal Iron Co experimented with a hydrogen reduction reverberatory furnace, the hydrogen being produced by a charcoal fired retort. After two or three attempts, samples of iron were produced but failure to maintain furnace heat and allow fluid iron to be tapped from the furnace forced the enterprises to be abandoned. The remnants of the Ilfracombe furnace still remain.[1]

Ilfracombe Blast Furnace, 2017

Source: Paul A.C. Richards Collection 2017

Iron and Iron Smelting

People knew about the metal iron even in the Stone Age, though they couldn’t make it. Iron is the major constituent of meteorites, and ancient peoples found small pieces that had fallen to earth, and used them in jewellery. Perhaps they knew that it came from the sky, and believed it was a gift from their gods.

In the Neolithic, or new Stone Age, people became quite skilled in making pottery and pottery kilns, and it is likely they gained the ability to make copper and tin in their kilns. Around 5,300 years ago they found that mixing the two metals created bronze, which was much harder and could be made into tools and weapons. Thus began the Bronze Age.

About 4,000 years ago, the more efficient metalworking forges and pottery kilns were reaching higher temperatures, and someone in China discovered how to make the meteorite material, iron, in simple forges and kilns producing a “bloom” of metal and slag that was then hammered to remove the impurities. Perhaps it was related to their using ochres. Ochre is iron ore and was roasted to improve the colour, and also used to decorate pottery. Three thousand years ago this iron-producing technology reached Turkey.

Good quality iron was harder than bronze, yet still malleable (or able to be hammered into shape), and iron ores were much more common than copper and tin ores. And so the Iron Age began in Europe.

Early iron furnaces were preheated by burning charcoal, and when hot, iron ore and more charcoal were tipped in from the top, in a roughly one-to-one ratio. Inside the furnace chamber, carbon monoxide produced by partial combustion of charcoal in limited air, reduced the iron oxides in the ore to metallic iron, without needing to melt the ore. So they could operate at lower temperatures than the melting temperature of the ore.

Operating these iron furnaces was a very skilled occupation, as the temperature and ratio of charcoal to iron ore had to be carefully controlled to keep the iron from absorbing too much carbon and becoming brittle. The output was a spongy mass of iron and slag at the bottom of the furnace, called a “bloom”, which was scraped out. Though tricky to get working properly, “bloomeries” did not need any additional material such as limestone to work. The Romans learned how to reheat and hammer the iron in a process called tempering, which made the iron even better.

In the first century AD, the Chinese discovered that blowing air into the furnace from the bottom would raise the temperature and improve the quantity and quality of the iron. They could get reasonably pure molten iron, instead of a spongy bloom. They also found that the addition of other minerals as “fluxes” helped to separate the impurities, and around 350AD realised that they could use coal instead of charcoal, though they didn’t need to make the switch until their forests were disappearing in the 11^th century.

The forcing of air into the base of the furnace could be done by foot operated or waterwheel operated bellows. The air was blasted into the base of the furnace as hard as they could achieve, and the new type of kiln was therefore called a “blast furnace”.

This new blast technology took a long time to get to Europe, apparently coming in via Belgium around 1450 and into England in 1491. The fuel was always charcoal, and the smelting process took place in purpose designed furnaces.

The illustration that follows shows a drawing of a furnace from the Renaissance. Note the wood being roasted to make charcoal on the left, and the foot-operated bellows at the base. The men are tipping in iron ore, charcoal and smashed limestone in carefully controlled measures. Because air has to get through the mixture, the iron ore and limestone cannot be pulverised into dust. While that would make for a quicker reaction in theory, in practice it would block the passage of air and thus lower the furnace temperature.

In a blast furnace, fuel, ore and limestone (as a flux) are continuously supplied through the top of the furnace, while a blast of air is blown into the lower section of the furnace. The downward flow of the ore and flux (called the “charge”) in contact with an upflow of hot, carbon monoxide-rich combustion gases creates a chemical reaction throughout the furnace as the charge moves downward. The end products are molten iron and slag that collect at the bottom and are tapped off. They come out separately when the furnace is hot enough and working properly. The searingly hot flue gases exit from the top.

For those interested in chemistry, the reactions are quite simple, and the aim is to produce carbon monoxide, which then reacts with the iron ore to produce carbon dioxide and iron.

First the compressed air blasts into the charcoal or coke: 2C + O2 > 2CO. Then the iron ore reacts with the carbon monoxide to form iron oxide: 3Fe2O3 + CO > 2Fe3O4 + CO2, then Fe3O4 + CO > 3FeO + CO2. The iron oxide reacts with carbon monoxide to form iron: FeO + CO > Fe + CO2.

At the same time as this is happening, the roasted limestone forms quicklime: CaCO3 > CaO + CO2. Then the quicklime removes sand in the ore to form slag: CaO + SiO2 > CaSiO3, which is removed from the base of the furnace.

All the iron reactions require extreme heat and produce carbon dioxide and while the chemistry is simple, the practice is not. It takes an experienced person to maintain the reactions. Too much air and not enough charcoal, for example, will produce carbon dioxide rather than carbon monoxide and the reduction of the ore to iron will not occur.

It was discovered in England in 1709 that coke, made by roasting coal, could be used instead of charcoal to power their blast furnaces. Over the course of the 18^th century the disappearing forests led to coke becoming the preferred fuel, and iron production increased rapidly. The perfection of Watt’s steam engine in 1790 allowed a better blast to be produced by steam power rather than water or wind power, and production increased even more rapidly.

In 1827 the invention of superheating the air before the blast was another huge improvement and by 1870 seventeen out of twenty furnaces in Britain were hot blast. The superheating of the blast air was achieved by capturing the hot exhaust gases and using their energy to preheat the fresh, incoming air. Hot blast furnaces can use raw coal as a fuel, while cold blast furnaces can only use charcoal or coke.

Unlike England, Australia and Europe still had forests in the 19^th century, and while people had access to free and easily cut wood, they preferred to use charcoal to produce iron. There was also a prevailing view that it also gave a better quality product. With the limited experience of iron smelting here, there was a conscious decision to use the older, simpler and well-understood technology. Thus the first iron production in Australia would be by cold-blast, using charcoal as fuel.

In the early days, the Australian colonies did not have the manpower or investment capital to build blast furnaces. While iron had been scarce and expensive at first, once the colonies were exporting large quantities of wool, whale oil and other goods to Britain, ships often returned empty and would carry iron as ballast in their holds, making transport virtually free.

Despite this there was interest, and in 1848 a small and simple smelter was built at Mittagong, using hematite ore that had been discovered in 1833 by the NSW government surveyor. It was an open forge rather than a blast furnace, using charcoal and local limestone, though the lime was hardly needed due to the good quality ore. The ore was pre-roasted in open stacks and after smelting produced a bloom which had to be hammered. Unfortunately the hammer fractured in 1852 and the operation closed soon after.

The operation was re-floated in 1860 as the Fitzroy Iron Works Company, named after the Governor, and built a new cold blast furnace at the Mittagong site in 1864 with the intention of using coke and lime. There was smelting in 1865 but they encountered technical problems and shut down. An attempt to recapitalise in 1866 failed, though they had produced 3,000 tons of good quality iron by that time – an achievement not unnoticed by the Tasmanian promoters of our iron industry.

Interestingly, their coke-smelted pig iron production costs when working successfully at this time were said to be £5/17/6 as against an imported cost of £5 per ton for pig iron brought out essentially as ballast for returning wool ships. Technical problems aside, they were doomed by their cost of production, which, though not excessive, could not compete with ballast from England.

This situation changed with the railway building boom of the late 1860s, and rising iron prices, coinciding with enormous wealth generated by the goldfields, led to renewed interest in local production. The high-quality, easily accessed and extensive iron ore deposits around Beaconsfield were recalled and re-examined.

References

[1]http://ro.uow.edu.au/cgi/viewcontent.cgiarticle=1725&context=ihsbulletin