Furnace used for smelting, refining, or melting in which the fuel is not in direct contact with the contents but heats it by a flame blown over it from another chamber. Such furnaces are used in copper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum recycling. In steelmaking, this process (now largely obsolete) is called the open-hearth process. The heat passes over the hearth and then radiates back (reverberates) onto the contents. The roof is arched, with the highest point over the firebox. It slopes downward toward a bridge of flues that deflects the flame so that it reverberates.
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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.
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.
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.
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.
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.