Reverberatory furnace
A reverberatory furnace is characterized by the fact that the heat is reflected (reverberated) by its vault towards a chemical reaction zone, physically separated from the one where the reaction takes place of combustion that provides energy to the system. In this type of furnace, the fuel (coal, gas, fuel oil, etc.) is burned in a separate chamber from that of the materials to be treated. Thus, direct contact and undesirable interactions between the fuel and the materials to be transformed or refined are limited.
These furnaces, whose operating principle is very old, were subject to improvements in the 18th and 19th centuries, particularly to optimize the metallurgical transformation processes. One of the first applications was the puddling of pig iron, an essential process during the first Industrial Revolution. These furnaces have been and are used in the metallurgical industry, ceramic firing and in the chemical industry.
Reverberatory furnaces are low in height and long. At one end is the hearth where the fuel is burned, and at the opposite end the chimney. The flames and products of combustion pass through the furnace and are directed, through the vault in an appropriate way, towards the furnace floor, where the charge of metal to be melted is located. This charge is heated, not only by its contact with the flames and hot gases, but also by the radiation heat from the vault of the reverberatory furnace.
History
The use of the reverberatory furnace is linked to the need to replace charcoal with the abundant and much cheaper hard coal. However, the sulphurous vapors which bothered those who used coal were correctly associated with the embrittlement of iron by sulphur. The English metallurgist John Percy traces the use of the reverberatory furnace in metallurgy to a patent granted in 1613 by James I of England to Jean Rovenzon, who had recovered the rights to a patent from Simon Sturtevant, who had failed to materialize the idea:
It is alleged that Rovenzon "practiced satisfactorily" what Sturtevant had promised and had not fulfilled; but there is no evidence to support the fact. The new patent holder had composed a Metallurgy Treaty (Treatise of Metallica), "different to the one published by Simon Sturtevant on his patent." Rovenzon seems to be the inventor of the “big bolts; refined fires and deflection boilers [sic], in which “the materials to melt or forge may be kept separate from the fuel”; and he clearly describes the reverberate oven. He concludes his small treatise, announcing that “[...] this new invention [...] will produce “my results with mineral coal and its new furnaces, if there is a suitable factory to establish the furnace in it”. The invention he talks about was probably the use of the reverberum oven.John Percy (Full Treatment of Metallurgy; Volume 3, p. 14-15)
The term reverberatory furnace is mentioned in treatises on chemistry or sculpture from the XVII century to designate a furnace used to melt enamels or decompose substances in the laboratory, and in which the flame is not applied directly to the product to be heated but rather reverberates through a vault.
In metallurgy, the reverberatory furnace as it is understood today appears in Great Britain, and more precisely on the border of Wales and England, in the last third of the century XVII. From then on, it was characterized by the separation between the heating and the product to be heated and by a tall chimney that allowed the fireplace to be activated by natural draft. The preferred fuel was coal. It was used both for extractive metallurgy (reduction of copper or lead ores, refining lead into silver) and for remelting iron (especially old cannons) to produce new castings.
Used in France for lead reduction since the 1730s, probably fueled by wood; the lower temperatures obtained in this way probably explain why it was not then used for melting.
Since 1750, the reverberatory furnace was associated with blast furnaces using coke to produce large pieces of cast iron (especially for artillery pieces) in factories such as Carron (Scotland) or Bersham (Wales). In 1775, the French Navy hired a British iron master, William Wilkinson, to build a reverberatory furnace dedicated to the manufacture of cast iron artillery pieces, creating the d'Indret Foundry. Kilns of the same type were later built in Ruelle, in Angoumois and in Creusot (Burgundy).
Reverberatory furnaces have been used for the smelting of both ferrous and non-ferrous metals, such as copper, brass, bronze, and aluminum. It has also been used for smelting copper concentrate and separating slag, as well as for smelting ores and refining or melting low-melting metals such as aluminum. They have also been used in the production of copper, tin, and nickel, in the production of certain concretes and cements, and in the recycling of aluminum.
Today, reverberatory furnaces are not as widely used as in the past, due to the greater availability of fuels with a higher degree of purity and environmental regulations, which prohibit the use of the most polluting fuels. In addition, its energy efficiency is lower due to the losses that the ore and fuel are not in direct contact with. Even so, they continue to be used, especially in less developed countries and with more lax environmental protection regulations.
Settings
Reverberatory furnaces have taken various forms. The first were an improvement on the shaft furnaces. They allowed the use of fuel such as wood that produced flames. In the primitive reverberatory furnaces, the flames occupy the entire cavity of the furnace and the materials to be heated are placed in the middle. The improvement of the reverberatory furnace consisted in separating the fuels from the materials to be heated and in particular in carrying out the heating on one side of the furnace. In addition to improving heating, mixing of metallic materials with fuels is avoided and therefore the metal thus obtained is prevented from being contaminated or modified. The reverberatory furnace, therefore, consists of a hearth where the fuel is burned and a work chamber where the metals or minerals are placed.
Design of the reverberatory furnace in the 18th and 19th centuries
a: Grid
a1: Tronera
b: Altar
c: Working chamber
d: Rampa
e: Chimney
f: Door that allows to load the oven with the raw material to be treated
(Manuel théorique et pratique de la métallurgie du fer) by Adolf Ledebur, 1895)
A reverberatory furnace consists of the following elements:
- Home:
- Grid: where solid fuels such as coal or wood are placed. This grid is absent if gaseous or liquid fuels are used.
- Cinderette: located under the grid to collect the ashes.
- Tronera: opening provided with a door that allows to deposit the fuel on the grid.
- Working House:
- Altar: sometimes called bridge or main altar (Grüner)
- Solera: the lower part of the oven
- Door or hollow: provided with a detachable door, allows the introduction of metallic materials. It also allows us to intervene on these materials (gathering them mechanically as in the case of potential for example).
- Boveda:
- The vault covers the home and the work chamber, and its concave form makes it easier for the infrared radiation to move from the first to the second.
- Evacuation of gases:
- Rampa: Conduction located immediately after the work chamber that allows the circulation of gases.
- Computer: a kind of valve that sometimes equips the ramp, thus allowing to regulate the flow of the circulating gas.
- Fireplace: allows gas evacuation. Its dimensions are extremely important as they will condition the "tiro". Since most reverberum furnaces are usually not equipped with a fan, it is the firing of the fireplace which determines the efficiency of the oven. There may be an oven fireplace, but in workshops with several furnaces, there may be a common fireplace for all ovens.
The heat created by the combustion passes over the altar, passes through the work chamber following the vault and is then evacuated through the ramp and chimney.
Scheme from Manuel théorique et pratique de la métallurgie du fer (Adolf Ledebur, 1895)
Grid
The grate is present only in ovens that use solid fuels, such as coal or wood. Supports fuel. The spacing of the bars and their shapes allow the flow of ashes towards the lower part called the ashtray, where their removal is facilitated. This space also allows air circulation.
Grids can be horizontal, sloped or stepped. The horizontal grid has the great drawback of discontinuous manual loading. It is necessary for the operator to open the chamber to load the furnace. This opening causes the oven to cool down. Grüner mentions an experiment comparing the operation of a furnace by different operators, the result of which shows differences of up to 25% in fuel consumption. Inclined or stepped grates can be automatically fed by a device placed outside the oven, which allows the flow of charcoal by gravity or by the action of a person.
Altar
The altar is the wall that separates the hearth (and therefore, the materials to be treated) from the fuel. Sometimes there is some ambiguity about the exact definition of this part. Gaspard Monge in his book on the manufacture of cannons defines it as: ...the part of the furnace on which the mass of metal is placed and not as the separation. Grüner calls it in his treatise on metallurgy the bridge or the high altar. The altar (or the little bridge) is a small dike that separates the solera from the ramp. This separation exists in the event that the hearth is concave, thus preventing the molten material from flowing towards the ramp.
The altar is exposed to high temperatures, so it is made of a refractory material. Optionally it can be equipped with cooling means. Its height depends on its use. When you want to protect the metal from the flame and thus avoid a chemical action that would modify its nature, you build higher. But in exchange, the oven loses part of its heat efficiency.
Solera
The hearth is also made of refractory materials. Its nature, shape and dimensions closely depend on the use of the oven.
For puddling ovens, in 1818 the iron hearth cooled by circulating water was introduced. It is coated with basic slag, which improves the refining of the casting.
The hearth is flat when the oven is only used for heating. It is hollow (or concave) when the furnace is intended to melt a metal. The shape can be rectangular, if the operator has no action to perform during heating, or oval if he has to intervene (in the case of puddling ovens).
In some cases, waste heat from the hearth is reused by equipping the furnace with other hearths or by passing the flux through steam boilers.
Vault
The vault, also built with refractory material, has a double mission: firstly, it conducts the hot smoke produced in the hearth once it passes the altar towards the work chamber, where it gives off its heat; and secondly, its convex shape allows reflecting the infrared radiation generated by the fire in the home towards the work chamber, increasing its temperature.
Use in metallurgy
The reverberatory furnace can be used for various types of operations, such as roasting (heating solid materials such as minerals), reheating or melting.
Steel Industry
In the steel industry, the reverberatory furnace was used throughout the XIX century in the puddling process, which made it possible to transform the pig iron in puddled iron by working on a metal heated until it becomes pasty. The process was abandoned at the beginning of the 20th century, when converters allowed mass production of steel to begin.
At the beginning of the XX century, the hearth furnace appeared, which allows the controlled refining of pig iron to produce quality steel. In 1950, more than half of the world's steel was produced by this process, which disappeared in the 1980s when the oxygen converter became more widespread.
Metallurgy of non-ferrous metals
The reverberatory furnace is used for both roasting and smelting ores. In this last task, he competes with the blast furnace. In fact, it allows the fusion of very fine granulometry minerals, such as those from mineral enrichment plants, without being entrained by the gases. Since the end of the XX century, due to ecological constraints, the reverberatory furnace has been disappearing, being replaced by other processes such as the flash fusion process.
Roasting
The reverberatory furnace is especially indicated for roasting powdery materials, which would be dragged along by the gas flow if they were treated in a shaft furnace, where, in addition, their permeability to the passage of hot gases is insufficient. Although this method was dominant at the end of the 19th century century, it was already outdated at the beginning of the century XX. Indeed, since contact with the hot and oxidizing atmosphere is relatively low, roasting is slow and incomplete, even if the charge is renewed regularly.
Toasting in the reverberatory oven consists of spreading a layer of 8 to 10 cm thick on the bottom of the oven and stirring it (by hand in the century XIX, then automatically at turn of century XX, as in the Edwards oven). As only the top layer undergoes oxidative roasting, manual or mechanical shoveling is necessary, but this is often insufficient to achieve rapid and complete roasting. In addition, the gases resulting from roasting (such as sulfur gases or zinc oxides) are not forcefully expelled from the bed of material and tend to remain in the spaces between the grains (by adsorption), which penalizes the progression of oxidation.. However, this phenomenon is much less penalizing in reverberatory kilns than in other processes (tub kiln, cupola or stack roasting) since the gases leave the load more quickly. Thanks to its flexibility, the reverberatory furnace, which allows temperature adjustment regardless of the degree of oxidation, is a popular tool for mineral desulfurization.
Reverberatory kilns still have the disadvantage of their low thermal efficiency. To improve this and facilitate access to the charge, the bottom of roasters is long and narrow. In the 19th century, it was at least 10 meters long and at most 2 meters wide. There are two zones: the heating zone, near the fire bridge (or altar), and the working chamber, where the material is slowly heated, which is 15-20 times larger. Materials are pushed from the chimney into the hearth. Therefore, to facilitate its movement, the floor is sometimes sloped or designed for a cascading flow (oven with shelves). To obtain an oxidizing grid, the air inlets are increased. Cold, denser air circulates under the fumes, in contact with the load that it oxidizes. But if it is intended to treat the combustion gases (for example, to recover sulfur), the excess air must be limited. In this case, the frequent turning of the product spread on the floor must be done mechanically, because otherwise access to the cargo leads to recurrent air inlets. Many systems have been proposed, from kilns with inclined shelves in which the charge cascades (Hasenclever kiln) to kilns equipped with mechanical paleo systems.
Like most roasting processes, sintering should be avoided to facilitate material handling. But for lead ore roasting, it is common for sintering, or even melting, to take place in the same furnace.
_(p._840).png) Reverberated furnace used in 1883 to roast lead ore, then called "modern design" |  Reverbere furnace used to roast silver ore, marketed in 1897 by the Anaconda Mining Company. |
.png) Reverbero furnace with solera inclined to 18° (French horn), used for fine cascarilla calcination in the early twentieth century | _(p._831).png) Multiple solera reverberate oven (table linen), used at the end of the nineteenth century for the coughing of sulfurous lead minerals. The load passes successively from the upper home to the lower home, circulating in the opposite direction to the smokes |  View in section of the rotating sillet system of the Edwards kiln, as used in 1921 |
Copper or nickel ore casting in matte finish
At the turn of the 21st century, flash smelting provided more than 60% of copper production. It stands out compared to the reverberatory furnace, firstly for ecological considerations, and above all for the energy savings it allows. During the 1973 oil crisis, it replaced it permanently.
Since the reverberatory furnace burns primarily hydrocarbons, it achieves melting in an environment with little free oxygen. The matte is slightly oxidized, and therefore contains little copper, between 40 and 50%. The performance is low because since the bath is not agitated, the heat exchanges are less effective. On the other hand, the separation between the metal and the slurry is good: the slag, which contains less than 0.6% copper, is not reprocessed. Finally, the fumes from the combustion of hydrocarbons are abundant and contain less than 1% of SO
2: this concentration is too low to ensure economical desulfurization, and too high to be released into the atmosphere.
At the beginning of the XXI century, the reverberatory furnace only survives in a few factories. Thus, of the 30 reverberatory furnaces operating in 1984, only 10 were operating in 2002. It is used to smelt copper ores, possibly containing zinc, from matte. The kiln is approximately 40m in length and is primarily constructed of acid (silica-based) refractory material. It is equipped with a heat recovery unit capable of reusing 35 to 50% of the energy of the fumes that escape from the furnace at 1200 °C. The low thermal performance of the reverberatory furnace is compensated by the fact that it is content with practically any fuel. But "one major drawback makes the use of such furnaces impossible in many inhabited areas: it is the enormous volume of fumes produced both by combustion and by oxidation of sulphides. Dust extraction from flue gases is expensive and often incomplete due to its volume. They contain gas SO
2 contaminant, but at too low a concentration to produce sulfuric acid". Thus, the chimney of the reverberatory furnaces of the Hudson Bay Mining and Smelting Co, in Flin Flon, is 250 m high.
A typical configuration of a reverberatory furnace from the late XX century is the one used at the Onahama foundry (Japan): 2 kilns of 9.73 x 33.55 x 3.69 m and 11, 1 x 33.27 x 4.0 m, producing 1000 tm per day of matte with 43% copper and as many slags, with 0.6% copper. These furnaces are fed with concentrated copper ore and with slag from a Peirce-Smith converter in an equivalent quantity. The gas is desulfurized in a unit that produces gypsum.
Current and future developments
Flash melting at the turn of the century XXI concentrates 50% of world copper production and 30% of nickel production. It is a direct evolution of the reverberatory furnace.