Reverberatory furnace

A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases. The term "reverberation" is used here in a generic sense of "rebounding" or reflecting, without the more common acoustic denotation.

Operation

Process chemistry determines the optimum relationship between the fuel and the material, among other variables. The reverberatory furnace can be contrasted on the one hand with the blast furnace, in which fuel and material are mixed in a single chamber, and, on the other hand, with crucible, muffling, or retort furnaces, in which the subject material is isolated from the fuel and all of the products of combustion including gases and flying ash. It has been stated in some contexts that the reverberatory furnace also typically separates the material from the hot gases, but this does not seem to be the case in general. Indeed, some applications require contact between the material and the hot gas. There are, however, a great many furnace designs, and the terminology of metallurgy has not been very consistently defined, so it is difficult to categorically contradict the other view.

Applications and comparison with blast furnace

The applications of these devices fall into two general categories, metallurgical melting furnaces, and lower temperature processing furnaces typically used for metallic ores and other minerals.

A reverberatory furnace is at a disadvantage from the standpoint of efficiency compared to a blast furnace due to the spatial separation of the burning fuel and the subject material, and it is necessary to effectively utilize both reflected radiant heat and direct contact with the exhaust gases (convection) to maximize heat transfer. Historically these furnaces have utilized solid fuel, and bituminous coal has proven to be the best choice. The brightly visible flames (due to the substantial volatile component) give more radiant heat transfer than anthracite coal or charcoal.

Contact with the products of combustion, which may add undesirable elements to the subject material, is used to advantage in some processes. Control of the fuel/air balance can alter the exhaust gas chemistry toward either an oxidizing or a reducing mixture, and thus alter the chemistry of the material being processed. For example cast iron can be "puddled" in an oxidizing atmosphere to convert it to the lower-carbon mild steel or bar iron.

Reverberatory furnaces (here usually called air furnaces) were formerly also used for melting brass, bronze, and pig iron for foundry work.

History

The first reverberatory furnaces were perhaps in the medieval period, and were used for melting bronze for casting bells. They were first applied to smelting metals in the late 17th century. Sir Clement Clerke and his son Talbot built cupolas or reverberatory furnaces in the Avon Gorge below Bristol in about 1678. In 1687, while obstructed from smelting lead (by litigation), they moved on to copper. In the following decades, reverberatory furnaces were widely adopted for smelting these metals and also tin. They had the advantage over older methods that the fuel was mineral coal, not charcoal or 'white coal' (chopped dried wood).

In the 1690s, they (or associates) applied the reverberatory furnace (in this case known as an air furnace) to melting pig iron for foundry purposes. This was used at Coalbrookdale and various other places, but became obsolete at the end of the 18th century with the introduction of the foundry cupola, which was a kind of small blast furnace, and a quite different species from the reverberatory furnace.

The puddling furnace, introduced by Henry Cort in the 1780s to replace the older finery process, was also a variety of reverberatory furnace.

Aluminium smelting

Today, reverberatory furnaces are widely used to smelt secondary aluminium scrap for eventual use by die-casting industries.

Aluminium scrap of all types may be melted in what is now known as a sloping hearth reverberatory. Even irony aluminium scrap can be smelted down and then the solid iron is raked out of the main bath leaving only the molten aluminium behind

The simplest reverberatory is nothing more than a steel box lined with alumina refractory brick with a flue at one end and a vertically lifting door at the other. Conventional oil or gas burners are placed usually either side of the furnace to heat the brick and the eventual bath of molten metal is then poured into a casting machine to produce ingot.

The static furnace is tapped at the bottom by simply removing a ceramic covered cone which then allows the molten aluminium to flow into a launder and then on to the casting machine itself.

The "tap" or cone controls the flow of aluminium by way of simply restricting the flow of metal and can be stopped completely if required at any time.

ee also

*Puddling furnace

References

*Encyclopædia Britannica, 14th ed.
*J. Day & R. F. Tylecote (eds.), "The Industrial Revolution in Metals" (1991)
*P. W. King, 'The Cupola at Bristol' "Somerset Araeology and Natural History" 140 (for 1997), 37-52.
*P. W. King, 'Sir Clement Clerke and the Adoption of coal in metallurgy' "Transactions of the Newcomen Society" 73(1) (2001-2), 33-53.