Draft:Water jacket furnace

A water jacket furnace is a type of blast furnace used to smelt non-ferrous metallic ores, most typically ores of copper or silver-lead. It takes its name from the water jacket arrangement used to cool the lower furnace casing and prolong the life of the furnace hearth. Nearly entirely replaced, by flash smelting of ore concentrates, the water jacket furnace is now virtually an obsolete technology.

History

Water jacket furnaces under construction

In the mid 19th Century, most non-ferrous smelting was done using reverberatory furnaces. Blast furnaces were used to smelt sulphide copper ore in the Harz Mountains of Germany. The mines at Burra in South Australia tried to adopt the technology, in 1847, but without success because the German furnace design, using horse-powered bellows to provide the air blast, was not well suited to their carbonate copper ore. However, the later water jacket design seems to have arisen in North America, and an alternative name was 'American water jacket furnace'. Water jacket furnaces began to be common in the later part of the century from the 1880s, particularly for smelting sulphide ores. Unlike reverberatory furnaces, water jacket furnaces could be made in a factory and then reassembled at site.

Not all situations were well-suited to water jacket furnace operation, and some attempts to use them were costly failures, such as at Burraga and at the Overflow Mine at Bobadah, both in New South Wales. However, the furnaces were hugely successful, when well applied, such as at the vast Anaconda Copper Mine, in Butte, Montana, and the Mt Lyell Mine in Tasmania, and at many other mines.

Water jacket furnaces only ever partially displaced reverberatory furnaces in the copper industry, until both furnace types were displaced, almost entirely, by flash smelting, between around 1949 and 1980.

Technology and application

Smelting

A water jacket furnace could be used to reduce non-ferrous oxide ores mixed with coke, to produce metal, but its main use was smelting the more common sulphide ores. The feedstock was the ore, coke and fluxes. When smelting lead sulphide ores, a water jacket furnace produces molten lead and slag. Lead and silver often occur in the same ore body. Separating silver metal from the crude lead produced by a furnace needed a second process of refining, such as the Parkes process.

The pyrometallurgical process of a water jacket furnace, when smelting copper sulphide ores, is fundamentally different to a conventional blast furnace used to make iron, or a water jacket furnace used to make lead. The conventional process produces molten metal by reducing the ore, and separating out the silica as slag. Water jacket furnaces, when smelting sulphide copper ores, produce molten copper matte, which must be further treated in a convertor (similar in concept to a Bessemer convertor) or reverberatory furnace to produce copper metal. The product of that conversion process is known as blister copper. The smelting of sulphide ores in a water jacket furnace can be viewed as concentrating the non-ferrous metallic portion of the ore, as matte, and separating out some impurities, such as silica and iron, in the slag, and much of the sulphur, as sulphur dioxide in the off-gas. The molten slag and matte separated, with the denser matte accumulating at the bottom of the furnace.

Small round water jacket furnace for silver-lead ore

Depending upon the composition of the ore being smelted, the choice of a suitable flux was particularly important. Fluxes used could be limestone, iron oxide, or silica (quartz), depending upon what was needed to create slag and to minimise the loss of copper with that slag. When both 'basic' (oxide or carbonate) ores and 'siliceous' ores were available, feeding the furnaces with a mixture of the two copper ore types reduced the amount of other fluxes needing to be added.

Advantages and disadvantages

Water jacket furnaces had some advantages over reverberatory furnaces. Fuel consumption was lower. Sulphide ores could be smelted without first roasting the ore. Production per furnace was generally higher. Low grade ore could be smelted, because the water jacket furnace could more readily discharge large amounts of molten slag. Because solidified slag is unsuitable to backfill the voids (stopes) created by underground mining, disposal of large volumes of slag, from smelting of low grade ores, was a significant problem. Some mines treated molten slag with water to create granulated slag, which could be used to backfill stopes. Otherwise, the molten slag was dumped and large slag dumps accumulated near the smelter, becoming a lasting legacy of smelting operations. Smelting of low grade ores became less prevalent, once the froth flotation process was used to produce ore concentrates.

Another advantage of the water jacket furnace was that, while out of service, the bottom of the furnace, if so designed, could be 'dropped' for cleaning it up or for repair. Over time, a significant amount of copper material would accumulate in the bottom of a reverberatory furnace which could not be accessed without effectively demolishing the furnace.

Differences from conventional blast furnace

Rectangular cross-section water jacket furnace.

An initial disadvantage of the water jacket furnace was its use of coke as fuel. It could not use the cheaper fuels such as firewood or fine raw coal that could be used to fire a reverberatory furnace. That disadvantage was offset by lower overall fuel consumption. In the first years of the 20th Century, the perfection of a technique known as pyritic smelting greatly reduced coke consumption, when smelting suitable ores such as chalcopyrite, by optimizing the use of the sulphur in the ore itself, as a fuel. Water jacket furnaces also needed blowers and a cooling water supply, and were more complex to build and operate than the reverberatory furnaces. Water jacket furnaces, like other blast furnaces, are best operated continuously, and smelters that used them had to work continuously too.

The design of water jacket furnaces differ from the conventional blast furnaces used for smelting iron ore, which use a hot blast. Water jacket furnaces typically used a cold air blast, typically provided by a positive-displacement blower, such as a Roots blower. Preheating of the air blast was used on some water jacket furnaces. The horizontal cross-section of water jacket furnaces was usually rectangular—although circular ones did exist—whereas conventional blast furnaces always have a circular horizontal cross-section. In some larger furnace designs, molten metal and molten slag were tapped at the opposite narrow ends of the rectangular base. Water jacket furnaces typically had a higher number of smaller tuyeres than a conventional blast furnace. Typically, the feedstock was fed into a water jacket furnace through a sliding door arrangement in the side of the upper furnace structure, but not via the top itself as in a blast furnace for iron. At the top of a water jacket furnace there was a fixed flue. The off-gas contained a large proportion of sulphur dioxide and was not suitable to be recycled, as done in a blast furnace making iron, which generates blast furnace gas. Dust carried in the flue gas was often collected, as it had a significant metallic content.

Other applications

A variant of the water jacket furnace was used to smelt lead-zinc ores using the Imperial Smelting Process. In that case, the furnace was completely sealed, to allow the zinc to be recovered, from flue gases, in its vapour phase.

Although rarely done, small water jacket furnaces have been used to recover gold from quartz rock—particularly if the ore was very rich in gold or sulphide ores of other metals were also present—as an alternative to crushing the rock and extracting the gold using other methods. However, it was a very inefficient method of extracting gold.

Where gold and silver were present in copper ores, the precious metals were present in the copper matte produced by a water jacket furnace. The precious metals could later be separated from blister copper, using electrolytic copper refining, and delivered in the form of dore bullion.

Demise

As the typical average ore grades of copper mines declined, ore smelting became uneconomic and was largely superseded by a process consisting of ore concentration, especially using froth flotation, and smelting of ore concentrates. The water jacket furnace was less well suited to that new regime—especially large ones that had been used to smelt low grade ores—and, after the introduction of flash smelting, from around 1949, had fallen out of favour by 1980. It is now a largely forgotten technology.

• Water jacket furnaces
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  Section through a water jacket furnace. This one is most likely a furnace at Mount Lyell in Tasmania.
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  Great Cobar mine - Water jacket furnaces, from outside the smelter building, showing furnace flues to main flue, and slag wagons (1912)
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  Lower part of a water jacket furnace used to smelt copper ore.
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  Great Cobar mine - Pouring molten copper matte from a water jacket furnace, viewed from inside the smelter building (June 1912)
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  Great Cobar mine, feeding floor of furnaces (1912)
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  Great Cobar mine, slag runoff into ladle wagons.
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  Anaconda Copper, Montana, charging equipment at the upper level of furnaces.
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  Surviving lower portion of a water jacket furnace, Zeehan, Tasmania

See also

• Blast furnace
• Reverberatory furnace
• Flash smelting
• ISASMELT
  

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