Metallurgical Industrial Furnaces
Metallurgical Industrial Furnaces
An industrial furnace is essentially a thermal enclosure and is employed to process raw materials at high temperatures both in solid state and liquid state. The principle objectives of an industrial furnace are (i) to utilize heat efficiently so that losses are minimum, and (ii) to handle the different phases (solid, liquid or gaseous) moving at different velocities for different times and temperatures such that erosion and corrosion of the refractory are minimum.
Industrial furnaces which used in the metallurgical industry for carrying out various metallurgical processes are known as metallurgical industrial furnaces. Metallurgical furnaces are mostly used for (i) extraction of metals from ores, (ii) calcining and sintering of ores, (iii) melting, refining and alloying of metals, (iv) heating of metals, (v) carbonizing of coals, and (vi) heat treatment of metals etc. Energy sources for metallurgical furnaces are (i) combustion of fossil fuels, such as solid, liquid and gaseous fuels, (ii) electric energy such as resistance heating, induction heating or arc heating, and (iii) chemical energy such as exothermic reactions.
Process heating metallurgical furnaces are insulated enclosures designed to deliver heat to the charge. Melting ferrous metals needs very high temperatures (higher than 1250 deg C), and can involve erosive and corrosive conditions. Shaping operations use high temperatures (1050 deg C to 1250 deg C) to soften the materials for processing such as forging, rolling, pressing, bending, drawing, and extruding etc. Treating can use midrange temperatures (600 deg C to 1050 deg C) to physically change crystalline structures or chemically (metallurgically) alter surface compounds, including hardening or relieving strains in metals, or modifying their ductility. These include aging, annealing, austenitizing and carburizing, hardening, malleabilizing, martinizing, nitriding, sintering, spheroidizing, stress-relieving, and tempering. Processes which use low temperatures (less than 600 deg C) include drying, polymerizing, and other chemical changes.
Metallurgical furnaces which do not show colour, that is, in which the temperature is below 650 deg C, are generally called ‘ovens’. However, the dividing line between ovens and furnaces is not sharp. For example, coke ovens operate at high temperatures (higher than 900 deg C). Many of the furnaces are termed ovens, kilns, heaters, afterburners, incinerators, or destructors. The furnace of a boiler is its ‘firebox’ or ‘combustion chamber’, or a fire-tube boiler’s ‘Morrison tube.’
Industrial heating operations encompass a wide range of temperatures, which depend partly on the material being heated and partly on the purpose of the heating process and subsequent operations. In any heating process, the maximum furnace temperature always exceeds the temperature to which the furnace charge is to be heated.
Classification of metallurgical industrial furnaces
Metallurgical industrial furnaces are classified in several ways. These are described below.
Furnace classification by heat source – Heat is generated in the furnace to increase the furnace temperature to a level which is higher than the temperature needed for the process, either (i) by the combustion of a fuel, or (ii) by conversion of electric energy to heat. Some furnaces also utilize the waste heat from the metallurgical process. Fuel-fired furnaces are most widely used, but electrically heated furnaces are also used where they offer advantages which cannot always be measured in terms of fuel cost. In fuel-fired furnaces, the nature of the fuel can make a difference in the furnace design, but that is not much of a problem with modern industrial furnaces and combustion equipment. Additional basis for classification can relate to the place where combustion begins and the means for directing the products of combustion (POC).
Furnace classification by the method handling materials into, through, and out of the furnace – Furnaces can be batch type or continuous type.
Batch-type furnaces are often termed as ‘in-and-out furnaces. These furnaces have one temperature set point, but have usually 3 zones of control for maintaining uniform temperature throughout, because of a need for more heat at a door and at the ends. These furnaces can be charged manually or by a manipulator. After placing the charge in the furnace, both the furnace and its charge are brought upto the required temperature together, and depending on the process, the furnace may or may not be cooled before it is opened and the charge is removed from the furnace usually through a single charging and discharging door. Batch furnace configurations include box, slot, car-hearth, shuttle, bell, elevator, and bath (including immersion). For long solid loads, crosswise piers and top-left/bottom-right burner locations circulate for better uniformity. Bell and elevator kiln furnaces are often cylindrical. Furnaces for pot, kettle, and dip-tank containers can be fired tangentially with high velocity, low swirl flames instead of flat flame with very high swirl. There are many types of batch furnaces. Examples are crucible, pot, kettle, dip-tank furnaces, and movable hearth furnace etc.
Continuous furnaces move the charged material while it is being heated. Material passes over either a stationary hearth or the hearth itself moves. If the hearth is stationary, the material is pushed or pulled over skids or rolls, or is moved through the furnace by wire ropes or mechanical pushers. Except for delays, a continuous furnace operates at a constant heat input rate, burners being rarely shut off. A constantly moving (or frequently moving) conveyor or hearth eliminates the need to cool and reheat the furnace (as is the case with a batch furnace), thus it saves energy. Horizontal straight-line continuous furnaces are more common than rotary hearth furnaces, rotary drum furnaces, vertical shaft furnaces, or fluidized bed furnaces. Common examples of continuous furnaces are reheating furnace in rolling mill, continuous belt-conveyor type heat treat furnace, roller hearth furnace, and tunnel furnaces / tunnel kilns.
Alternatives to straight-line horizontal continuous furnaces are rotary hearth (disc or donut) furnaces, inclined rotary drum furnaces, tower furnaces, shaft furnaces, fluidized bed furnaces, and liquid heaters and boilers. Rotary hearth or rotating table furnaces are very useful for many purposes. Charges are placed on the merry-go-round-like hearth, and later removed after they have completed almost a whole revolution. The rotary hearth, disc or donut (with a hole in the middle), travels on a circular track. The rotary hearth or rotating table furnace is especially useful for cylindrical charges, which cannot be pushed through a furnace, and for shorter pieces which can be stood on end or laid end to end. The central column of the donut type helps to separate the control zones.
Multi hearth furnace is a variation of the rotary hearth furnace with many levels of round stationary hearths with rotating rabble arms which gradually plow granular or small lump materials radially across the hearths, causing them to eventually drop through ports to the next level.
Inclined rotary drum furnaces, kilns, incinerators, and dryers often use long type luminous flames. If drying is involved, substantially more excess air than normal can be justified to provide greater moisture pickup ability.
Tower furnaces conserve floor space by running long strip or strand materials vertically on tall furnaces for drying, coating, curing, or heat treating (especially annealing). In some cases, the load can be protected by a special atmosphere, and heated with radiant tubes or electrical means.
Shaft furnaces are usually refractory-lined vertical cylinders, in which gravity conveys solids and liquids to the bottom and by-product gases to the top. Examples are cupolas, blast furnaces, and lime kilns.
Fluidized bed furnaces utilize intense gas convection heat transfer and physical bombardment of solid heat receiver surfaces with millions of rapidly vibrating hot solid particles. The furnaces are of several types as given below.
- A refractory-lined container, with a fine grate bottom, filled with inert (normally refractory) balls, pellets, or granules which are heated by POC from a combustion chamber below the grate. Loads or boiler tubes are immersed in the fluidized bed above the grate for heat processing or to generate steam.
- Similar to above, but the granules are fuel particles or sewage sludge to be incinerated. The space below the grate is a pressurized air supply plenum. The fuel particles are ignited above the grate and burn in fluidized suspension while physically bombarding the water walls of the upper chamber and water tubes immersed in its fluidized bed.
- The fluidized bed is filled with cold granules of a coating material (e.g. polymer), and loads to be coated are heated in a separate oven to a temperature above the melting point of the granules. The hot loads are then dipped (by a conveyor) into the open-topped fluidized bed for coating.
Furnace classification by fuel – In fuel-fired furnaces, the nature of the fuel can make a difference in the furnace design, but this is not much of a problem with modern industrial furnaces and burners, except if solid fuels are involved. Similar bases for classification are air furnaces, oxygen (O2) furnaces, and atmosphere furnaces. Related bases for classification can be the position in the furnace where combustion begins, and the means for directing the POC. Examples are internal fan furnaces, high velocity furnaces, and baffled furnaces.
Electric furnaces for industrial process heating can use resistance or induction heating. Theoretically, if there is no gas or air exhaust, electric heating has no flue gas loss.
Resistance heating usually involves the high electricity costs, and can require circulating fans to assure the temperature uniformity achievable by the flow motion of the POC in a fuel-fired furnace. Silicon (Si) control rectifiers have made input modulation more economical with resistance heating. Various materials are used for electric furnace resistors. Most are of a nickel–chromium (Ni-Cr) alloy, in the form of rolled strip or wire, or of cast zigzag grids (mostly for convection). Other resistor materials are molten glass, granular carbon (C), solid C, graphite, or silicon carbide (SiC) (glow bars, mostly for radiation). It is sometimes possible to use the charge which is being heated as a resistor.
In induction heating, a current passes through a coil that surrounds the piece to be heated. The electric current frequency to be used depends on the mass of the piece being heated. The induction coil (or induction heads for specific charge shapes) is to be water cooled to protect them from overheating themselves. Although induction heating generally uses less electricity than resistance heating, some of that gain can be lost due to the cost of the cooling water and the heat which it carries down the drain. Induction heating is easily adapted to heating only localized areas of each piece and to mass production methods.
Many recent developments and suggested new methods of electric or electronic heating offer ways to accomplish industrial heat processing, using plasma arcs, lasers, radio frequency, microwave, and electromagnetic heating, and combinations of these with fuel firing.
Furnace classification by recirculation – For medium or low temperature furnaces/ovens/dryers operating below 750 deg C, a forced recirculation furnace or recirculating oven delivers better temperature uniformity and better fuel economy. The recirculation can be by a fan and duct arrangement, by ceiling plug fans, or by the jet momentum of burners (especially high-velocity low swirl flame burners). In these furnaces, the requirement is thoughtful circulation design and careful positioning relative to the furnace charges.
Furnace classification by direct-fired or indirect-fired – If the flame is developed in the heating chamber proper, or if the POC are circulated over the surface of the workload, then the furnace is said to be direct-fired. In most of the furnaces, ovens, and dryers, the charge is not harmed by contact with the POC. Indirect-fired furnaces are used for heating materials and products for which the quality of the finished products can be inferior if they have come in contact with flame or POC.
In such cases, the charge can be (a) heated in an enclosing muffle (conducting container) which is heated either from the outside by the POC from burners or heated by radiant tubes which enclose the flame and POC. In case of a double muffle arrangement, not only the charge is enclosed in a muffle but the POC are confined inside muffles called radiant tubes. This use of radiant tubes is to protect the inner cover from uneven heating is being replaced by direct-fired with flat (very high swirl) or high velocity (low swirl) flames to heat the inner cover, thereby improving thermal conversion efficiency and reducing heating time. The radiant tube furnace is for charges which require a special atmosphere for protection of the material from oxidation, decarburization, or for other purposes. The indirect-fired furnace is built with a gas-tight outer casing surrounding the refractory lining so that the whole furnace can be filled with a prepared atmosphere. Heat is supplied by fuel-fired radiant tubes or electric resistance elements.
Classification by furnace Use (including the shape of the material to be heated) – These are soaking pit or ingot-heating furnace, usually in a vertical position. There is forge furnace for heating whole pieces or for heating ends of bars for forging or welding. Slot forge furnace has a horizontal slot instead of a door for inserting the many bars which are to be heated at one time. The slot also often serves as the flue. Furnaces named for the material being heated include bolt heading furnaces, plate furnaces, wire furnaces, rivet furnaces, and sheet furnaces. Some furnaces are also classified by the process of which they are a part, such as hardening, tempering, annealing, melting, and polymerizing. In carburizing furnaces, the charge to be case-hardened is packed in a C-rich powder and heated in pots/boxes, or heated in rotating drums in a carburizing atmosphere.
Classification by type of heat recovery (if any) – Most heat recovery efforts are aimed at utilizing the ‘waste heat’ leaving through the flues. Some forms of heat recovery are air preheating, fuel preheating, charge preheating, recuperative, regenerative, and waste heat boilers. Preheating combustion air is accomplished by recuperators or regenerators. Recuperators are steady-state heat exchangers which exchange heat from hot flue gases to cold combustion air. Regenerators are non-steady state devices which temporarily store heat from the flue gas in many small masses of refractory or metal, each having considerable heat-absorbing surface. Then, the heat absorbing masses are moved into an incoming cold combustion air stream to give it their stored heat. Furnaces equipped with these devices are sometimes termed recuperative furnaces or regenerative furnaces.
Regenerative furnaces in the past have been very large, integrated refractory structures incorporating both a furnace and a checker work refractory regenerator, the latter often much larger than the furnace portion. However, most regeneration is now accomplished with integral regenerator/burner packages which are used in pairs.
Both preheating the charge and preheating combustion air are used together in steam generators, rotary drum calciners, metal heating furnaces, and tunnel kilns.
Other furnace type classifications – There are stationary furnaces, portable furnaces, and furnaces which are slowly rolled over a long row of loads. Many kinds of continuous ‘conveyor furnaces’ have the charge carried through the heating chamber by a conveying mechanism. Some forms of conveyors are wire ropes, rollers, rocker bars, and self-conveying catenary strips or strands.
Oxygen furnace is the furnace which uses O2 enriched air or near-pure O2. In many high-temperature furnaces, productivity can be increased with minimum capital investment by using O2 enrichment or 100 % O2 (oxy-fuel firing). Either method reduces the nitrogen (N2) concentration, lowering the percentage of diatomic molecules and increasing the percentage of triatomic molecules. This raises the heat transfer rate (for the same average gas blanket temperature and thickness) and thereby lowers the stack loss. O2 use reduces the concentration of N2 in a furnace atmosphere (by reducing the volume of combustion air needed), so it can reduce NOx emissions.
Important metallurgical furnaces
Important metallurgical furnaces used in various metallurgical processes (Fig 1) are (i) coal carbonization furnaces, (ii) rotary kilns, (iii) multiple hearth furnaces, (iv) shaft furnaces, (v) rotary hearth furnaces, (vi) smelting, melting, and refining in bath and flash smelting furnaces, and (vii) electro-thermal furnaces.
Fig 1 Important metallurgical process furnaces
Coal carbonization furnaces
Coal carbonization furnaces are popularly known as coke ovens, where the coal carbonization process takes place. The process consists of thermal decomposition of coals either in the absence of air or in controlled atmosphere to produce a carbonaceous residue known as coke. 3 types of coke ovens are used for coal carbonization. These are (i) beehive ovens, (ii) by-product ovens, and (iii) non- recovery ovens.
A beehive oven is a simple firebrick chamber built with an arched roof so that the shape inside is that of an old-fashioned beehive. Its dimensions are typically 4 m wide and 2.5 m high. Beehive ovens are usually built in rows, one oven beside another with common walls between neighboring ovens. Such a row of ovens is termed a battery. A battery usually consists of many ovens, sometimes hundreds, in a row.
The beehive oven is a simple domed brick structure into which coal can be charged through an opening at the top and then leveled through a side door to form on a bed of around 600 mm to 900 mm thick. Heat is supplied by burning the volatile matter (VM) released from the coal, and carbonization progresses from the top down through the charge. Around 5 tons to 6 tons of coal can be charged, and a period of 48 hours to 72 hours is needed for the carbonization.
By-product coke ovens are the chambers made of refractories to convert coal into coke by carbonizing coal in absence of air and there by distilling the VM out of coal. Byproduct coke ovens are also arranged in a battery containing number of coke ovens (can vary from 20 to 100 in each battery).
Modern by product coke ovens are comprised of chambers 15 metres (m) to 20 m long, 6 m to around 9 m high, 500 mm to 600 mm wide and having a wall thickness of around 100 mm. A number of these chambers alternating with similar cells that accommodate heating flues form as a battery. Crushed coal is loaded along the top of the ovens using a charging car on rails and is leveled by a retractable bar.
The operation of each oven is cyclic, but the battery contains a sufficiently large number of ovens to produce an essentially continuous flow of raw coke oven gas. The individual ovens are charged and emptied at approximately equal time intervals during the coking cycle. Coking proceeds for 15 hours to 18 hours to produce BF coke. During this period, VM of coal distills out as coke oven gas. The time of coking is determined by the coal blend, moisture content, rate of under firing, and the desired properties of the coke. Coking temperatures generally range from 900 deg C to 1100 deg C and are kept on the higher side of the range to produce blast furnace (BF) coke. Air is prevented from leaking into the ovens by maintaining a positive back pressure in the collecting main. The ovens are maintained under positive pressure by maintaining high hydraulic main pressure of around 10 mm water column in batteries. The gases and hydrocarbons which evolve during the thermal distillation are removed through the off take system and sent to the by-product plant for recovery.
Non-recovery ovens are generally of horizontal design and operate under negative pressure unlike by-products ovens which operate under positive pressure. Primary combustion air, introduced through ports in the oven doors, partially burns directly the volatiles (Including tar and benzol) in the oven space above the coal. This generates the heat needed for the process. The mixture of the crude and the waste gases is led through the vertical ducts in the side walls to the heating flue system under the oven sole. Secondary air is introduced into the sole flues, which runs in the serpentine fashion under the coal bed and completes the combustion of the gases. The design of the flues and the control of the air flow allow the coking rate at the top and bottom of the coal bed to be equalized. Due to the temperatures generated, all the hydro-carbons and by-products are burned within the oven. The time of coking varies from 48 hours to 72 hours depending upon the design of the non-recovery coke ovens. Hot gases pass in a waste tunnel to heat recovery steam generators (HRSG), where high pressure steam is produced which is normally utilized for power generation.
A rotary kiln is an inclined, rotating cylindrical reactor through which a charge moves continuously. The rotary kiln is used when thermal processing of solids which is more severe than drying is required. The furnace walls (normally lined) make intermittent contact with the flue gas and the charge. Heat required for the various physical and chemical processes is delivered to the charge by lifting and overturning the charge as it moves through the interior of the rotary kiln. The most widespread usage of rotary kiln is in the production of cement clinker, limestone calcining, production of calcined and dead burnt dolomite, calcined magnesite, and iron ore reduction for the production of direct reduced iron (DRI) etc.
The rotary kiln consists of a lined hollow cylinder, mounted in an inclined position on rolls and rotated slowly by a drive. The charge material moves from the feed end to the discharge end because of the rotary motion and gravity. The inclination is between 1.5 % and 5 %. Speed is between 0.2 rpm (rotations per minute) and 2 rpm. Variable-speed drives are normally used to control the residence. Kiln diameter is usually constant over the full length. Some rotary kilns have internals such as conveying or lifting flights, built in crossed-hanging link chains, or ring dams. In some processes, air-feed pipes or burner tubes for gas or oil are installed on the furnace shell. Air or other gases can also be introduced through ports in the lining.
Rotary kiln carries out several functions simultaneously. It is equipment for conveying, mixing, heat transfer, and reaction. These functions are to be in harmony. The charge in the kiln moves both radially and axially. Radial motion is determined by the degree of filling (percentage of cross-sectional area occupied by the charge) and the rotational speed. The angle of repose and the kiln inclination govern the axial motion.
The interior of the charge tends to have a higher bulk density than the exterior, and grain size increases toward the outside. This tendency can be counteracted by the internals, which also improve heat transfer into the charge. Dust production can be limited by pelletizing the feed.
Heat transfer occurs principally from the combustion gas (generated by a burner usually installed at the discharge end of the kiln) to the charge. The driving force is generally the temperature difference. The gas can move co- or counter-current to the longitudinal movement of the charge. Cocurrent gas flow is advantageous only when the charge temperature does not have to exceed a certain value. The counter-current arrangement is preferred because it involves increased total energy consumption.
Multiple hearth furnaces
Multiple hearth furnaces used to be in a dominant position as a roasting furnace for sulphide ores (mainly pyrites in sulphuric acid production). It has now been almost completely replaced by fluidized-bed roasting equipment since the 1960s. Fluidized-bed furnaces allow much higher throughputs than multiple hearth furnaces, with substantially better control of reaction temperature and O2 partial pressure in the roasting gas. However, the multiple hearth furnaces continue to find use in some special areas of process engineering.
A multiple hearth furnace consists of an internally lined steel cylinder with a number of horizontally mounted, lined platforms called hearths. The circular hearths are thinner near the centre, which has an opening for a vertical shaft. An adjustable-speed drive with overload protection turns the shaft at 0.2 rpm to 5 rpm. From 1 to 4 rabble arms per hearth are latched to the shaft in a gastight manner. These arms bear oblique stirring teeth to move the solids over the hearth. On one hearth, the motion is from centre to edge, on the next from edge to centre depending on the inclination of the stirring teeth. The openings in the hearths, through which the charge travels from the top of the furnace to the bottom, thus alternate from central to peripheral.
Since the temperature in the furnace is high, the shaft and rabble arms are air cooled. The shaft has double walls. Cold air supplied by a fan enters the outside space, passes through the shaft and arms, and leaves the furnace at 200 deg C to 300 deg C by way of the centre space. Each hearth has several doors, which allow monitoring of the reaction and replacement of the rabble arms. The doors can be sealed tightly or can have adjustable air slots to admit cooling or combustion air if a slight sub atmospheric pressure is maintained in the furnace.
Appropriate reactions for the multiple-hearth furnace are (i) slow reactions (since long residence times can be achieved), (ii) reactions between solids and quantities of gas which are too small to maintain a fluidized bed, (iii) processes in which the solid is inlet in slurry form and the slowest and most gentle drying possible is desired, (iv) processes in which solids are to be exposed to a stepwise varying reaction temperature during thermal processing, and (v) reactions in which the solid undergoes slight softening, agglomeration, or sintering so that fluidized-bed processes cannot be employed.
Further, since the roasting reactions are exothermic, the furnace is normally to be heated only at the start of the process. The material fed to the top most hearth is distributed by the teeth on the rabble arms, slowly transported to the centre of the hearth, and dried. Then the ore falls into the first roasting zone, where it is heated in contact with hot roasting gas until it ignites. The reaction goes to completion as the charge is transported further over the hearths. On the last hearth, roasting air drawn or blown into the furnace from the bottom is preheated by cooling the residue. The progress of the reaction is monitored by measuring the temperature on the individual hearths.
A shaft furnace is a furnace which has an upright working chamber of circular, elliptical, or rectangular cross section in which a fixed bed (or descending column) of solids is maintained , and through which an ascending stream of hot gas is forced . It is used to smelt or roast lumped materials. The heat required for smelting or roasting process is produced by the combustion of a fuel either directly in the furnace or in an external firebox from which the combustion products are supplied to the furnace. There is counter current movement of gases and the solids in the furnace.
Moderate velocities of the gaseous combustion products are characteristic of shaft furnaces. At such velocities, the bulk of the lumped materials (the charge) is not entrained by the ascending gas stream and, in contrast to the case of a fluidized- bed furnace, maintains aerodynamic stability. The counter current motion of the charge (from the top to the bottom) and of the gases forced through the charge (from the bottom to the top) and the direct contact between the charge and the hot gases result in good heat exchange and the generation of low-temperature exhaust gases. Thus, shaft furnaces are characterized by a high thermal efficiency and a relatively high output. Such furnaces are widely used to smelt iron ores (blast furnace and direct reduction furnace) as well as non-ferrous ores.
The shaft furnaces are designed for continuous operation. The main components of shaft furnace consist of (i) a top, through which the charge is loaded and the gaseous combustion products are discharged, (ii) a shaft equipped with tuyeres, through which either a blast for fuel combustion or hot gases are supplied, and (iii) an inside crucible with a refractory lining, where the liquid products collect. The furnace is tapped at intervals.
The shaft furnaces have a variety of uses in metallurgy. The Rachette furnace is employed in lead production. The distinguishing feature of the lead blast furnace is that the throat widens upward. The iron smelting blast furnace is the most important type of shaft furnace. The blast furnace has the form of two truncated cones set with their large bases together. The widening of the stack from top to bottom reduces the frictional resistance as the burden moves downward. The blast furnace is divided into the several sections namely throat, stack, bosh, and hearth. The shaft furnaces are also used for smelting of copper, melting of pig iron for production of castings (cupola).
Ores concentrates, or metals for shaft furnaces are to be lumpy or to be agglomerated by sintering, pelletizing, or briquetting. To some extent, fine ores or secondaries can be blown through the tuyeres directly into the raceway for very fast reduction.
Rotary hearth furnaces
Rotary hearth furnaces are also known as rotating table furnaces. They are very useful for many purposes. Besides being utilized for the heating of circular loads (for example in the pipe rolling mills), the rolling hearth furnaces are also being used for the reduction processes. In these furnaces materials are placed on the merry-go-round-like hearth. The materials travel on a circular track and undergo reduction reactions while travelling. The reduced product is later removed after the materials have completed almost a whole revolution. The furnaces have several zones.
Special features of the rotary hearth furnace are (i) they are of simple design and have good reliability, (ii) the separated zones allow accurate process control, (iii) less thermal and mechanical stressing of batch carriers, (iv) loading/offloading can take place at a single location, (v) allows high degrees of flexibility in configuring feed and take-off lines and no extra grates needed for emptying runs, and (vi) better net throughputs, resulting from the use of lighter base grates or ceramic batch carriers for individual parts handling.
Rotary hearth furnaces are utilized for heat treating of large pipes, carbon baking, calcining of coal and carbon products, direct reduction of ores, and processing of iron nuggets.
Smelting, melting, and refining in bath and flash smelting furnaces
Fine concentrates can be smelted without agglomeration in flash (after drying) or bath smelting furnaces. The heat of exothermic chemical reactions provides the energy for autogenous smelting. All these furnaces are operated continuously. A wide variety of such smelting furnaces and converters are being used. The smelting furnaces represent a modern approach of intensive metal smelting under environmentally ‘clean’ conditions.
In bath smelting furnaces and converters, O2 or O2-enriched air is blown into the liquid metal or matte baths via tuyeres, lances, or injectors to oxidize elements which are to be removed as impurities. Coal or reducing gases can also be blown into liquid slags via tuyeres, lances, and injectors for slag reduction. In the case of aluminum (Al) smelting, chlorine (Cl) is blown through pipes and stirrers into the liquid metal bath to remove alkaline and alkaline-earth elements. Bath smelting furnaces are used for smelting of copper (Cu), and lead (Pb). Bath smelting furnaces are in development for the direct steel production from iron ores
The first flash smelting furnace started operation in 1949. By now, several flash smelting furnaces treat Cu concentrates, and smelt nickel (Ni) concentrates. The flash smelting furnace consists of a circular reaction shaft for roasting and smelting of dry concentrates in suspension with highly enriched air, a settling hearth for collection of the droplets and separation of matte (metal) and slag, and an off-take shaft for waste gas and flue dust.
Bath smelting and refining furnaces are often used for melting, refining, and alloying metals. These furnaces are operated batch wise and are fed with solid and liquid metal. They are of the stationary, tilting, or rotary type. Their applications include (i) electric arc furnaces (EAFs) and induction furnaces (IFs) for steel and cast iron smelting and refining, stationary and tilting hearth furnaces and rotary for Al melting, refining, and alloying, (iii) stationary hearth furnaces for Cu matte and ferronickel smelting, rotary furnaces for anode Cu smelting and refining, stationary and tilting hearth furnaces for Cu scrap melting and refining, and (vi) rotary furnaces for secondary Pb smelting. For mixing the melt, such furnaces are stirred mechanically, inductively, or by gas.
Converters are mainly used for (i) conversion of hot metal (HM) together with scrap into steel, (ii) conversion of Cu matte into blister Cu and the refining of secondary black Cu, (iii) refining of secondary Al, and (iv) conversion of Ni matte into Ni.
Majority of global steel production is carries out in converters. HM together with steel scrap (for cooling) is transformed to steel in the converter. Accompanying elements, particularly C, are oxidized to a desired level by O2. In the past, air was used for converting in the Bessemer (since 1855) and Thomas (since 1877) converters. Nowadays only O2 converters are in use which employs lances for top blowing and injectors for bottom blowing or side blowing. The LD process was developed in 1952 in Linz and Donawitz, Austria. O2 is blown onto the bath of liquid iron from above through a water- cooled O2 lance. The AOD (argon-oxygen-decarburization) converter permits oxidation and degassing of steel before casting. Loading always takes place through the mouth of the converter. Steel is tapped via a taphole situated in the upper side by tilting the converter. Another development of melting and converting of scrap is the energy-optimizing furnace (EOF) which combines preheating, melting, side blowing, and direct heat recovery from the reactor gases.
The use of electricity for metallurgical applications started in at the beginning of the nineteenth century. As early as 1810, Davy performed experiments in which he produced alkali metals via fused salt electrolysis. In 1888 H´eroult patented a small electric furnace for the production of Al, which formed the basis of the present-day electrolytic recovery of Al from alumina (Al2O3). The use of induction as a heating method was patented by de Ferranti in 1887. Although the experimental use of an electric arc can be traced back to 1810, the development of the first EAF is attributed to Siemens, who developed a small arc furnace in 1878 – 1879. From this modest beginning, the use of an arc for melting and smelting in furnaces progressed from an arc furnace (developed by H´eroult in 1900) to the ultrahigh power (UHP) technology (developed in 1960 – 1962), with which very high melting rates can be attained.
The earliest recorded application of electro-slag refining dates back to 1892. By the time of Hopkins’ development of the electro-slag process in 1935, vacuum arc refining was making great headway. Vacuum melting was introduced as a plant-scale operation in 1917 when Rohn melted Ni-base alloys by resistance heating. In 1923, vacuum induction furnaces are being operated. By about 1956 the potential of vacuum arc remelting for steels, as well as Ni- and titanium (Ti) -base alloys had been used for the production of improved gas turbine disks, shafts, and casings.
The use of electron-beam technology for smelting and melting is as young as plasma metallurgy, although experiments with electron beams (at the time known as cathode rays) commenced as early as 1852. This technology was patented in 1907. Semi continuous electron-beam melting was first performed in a cold mould crucible in 1954. By 1957 facilities were available for processing Ti ingot.
Plasma metallurgy was merely a topic of science fiction up to the 1950s. The first investigations into carbo-thermic reduction of oxides were carried out in the late 1950s. In a 1962, the use of plasma torches for melting scrap was started.
The short historical overview shows that most of the basic principles of electro-thermal furnaces have been known for a considerable time. Electro-thermal furnaces are classified on the basis of these principles.
In resistance furnaces, heat transfer occurs either directly or indirectly according to Ohm’s and Joule’s laws. They can be operated in an AC (alternating current) or DC (direct current) mode. Furnaces which implement direct resistance include (i) resistance furnaces for solid-state reactions (production of graphite and carbides), (ii) resistance furnaces for the production of Cu, Ni, iron (Fe), tin (Sn), and zinc (Zn) or their intermediate products from oxides and sulphides and slags, (iii) electro slag refining (ESR) for the production of clean ferrous or nonferrous metals and alloys such as Ti, steels, and super alloys, (iv) furnaces which employ a combination of arc and resistance heating are used for the production of calcium carbide (CaC2), ferroalloys, liquid iron, phosphorus (P), and Si compounds by reduction, (v) fused-salt electrolysis cells for the production of Al and alkali metals can also be considered to be electrochemical thermal reactors, and (vi) salt-bath furnaces for the heat treatment of metals.
Indirect heating is mostly performed in a closed chamber in which heating elements, situated in the walls or suspended in the chamber. Heating elements transfer their heat via radiation, conduction, and convection to the object to be heated. This type of furnace is applied for heat treatment of metals, ceramics, and for the production of carbides.
In EAFs, the heating occurs because of the high temperature (higher than 6000 deg C) of gas plasma created by the arc. These furnaces can be operated either in an AC or DC mode. These furnaces are used for melting of steel scrap and cast iron scrap (in the past also Cu scrap). Scrap is melted in arc furnaces in which the arc is situated above the liquid slag. The slag acts as a heat distributor and a refining agent for the liquid metal.
In vacuum arc remelting (VAR) a consumable electrode, made from the metal to be melted / refined, is melted in a vacuum to produce high-purity metals. An example is the production of high-purity Ti from Ti sponge.
In IFs, the heat is generated according to Lenz’s, Ohm’s, and Joule’s laws. Two types of furnaces are generally used for melting operations. They are the crucible furnace and the channel furnace. Crucible furnaces are used mainly to melt particulate cast iron, (stainless) steel, base metals, Cu, and Al. Channel furnaces are applied in the ferrous industry as a holding furnace. In the nonferrous industry they are also used as a melting furnace for Al, Zn, and Cu.
In vacuum induction furnaces, volatile components are removed from less volatile metals and alloys. Examples are the refining of special steels, super alloys, and Cu. Silver (Ag) crusts from Pb refining are also enriched in Ag by volatilization of Zn.
Induction heating is also used during annealing, hardening), brazing, soldering, and longitudinal welding.
In electron beam furnaces, electron guns produce high energy electrons, which impart their energy to the furnace charge to affect its melting. Electron-beam furnaces are used to melt and/or refine (i) refractory metals such as vanadium (V), niobium (Nb), and tantalum (Ta), (ii) metals such as molybdenum (Mo) and tungsten (W), (iii) reactive metals such as zirconium (Zr) and hafnium (Hf), and (iv) ceramics such as Al2O3, zirconia (ZrO2), and uranium carbide (UC).
In plasma furnaces, heating is performed by either transferred or non-transferred arc plasma torches. In contrast to arc furnaces, where arcs are produced between a graphite electrode and the charge, plasma furnaces use gas-stabilized plasma arcs produced by water-cooled plasma torches with non-consumable electrodes. Consumable hollow carbon electrodes have also been applied. Plasma furnaces have been employed commercially, for example, to produce ferrochromium (FeCr), to melt steel scrap, and to recover valuable metals from steel flue dust.
Although ‘laser furnaces’ are nonexistent, many applications of laser heating have been reported in metallurgy, such as in cutting, welding, machining, and surface treatment of metals.
In addition to the heating methods mentioned above, infrared, dielectric, and microwave heating are also available. However, these are not presently used to effect a chemical reaction at high temperature on an industrial scale.
Electro thermal furnaces can be used for the production of metals from raw materials by reduction. They can also be applied during metal refining. If cheap electrical energy is available heating in electro-thermal furnaces has many advantages over heating with fossil fuels.
All resistance furnaces operate on the basis of Joule’s law, i.e., an electrical conductor emits heat when a current flows through it. The quantity of electric power which is converted into heat can be expressed by Joule’s law. Ohm’s law relates the resistance of the conductor and the current through it to the applied voltage. Direct resistance furnaces exploit these principles for metallurgical purposes.
In direct resistance furnaces the feed or charge is the conductor and its resistance to the current flowing through it creates the heat for melting. The current in these reactors flows between at least two or more electrodes, one of which may be submerged in the material to be heated / melted. Direct resistance furnaces can be divided into three categories on the basis of the reactions taking place in them. These are (i) reduction resistance furnaces, (ii) refining resistance furnaces, and (iii) solid state resistance furnaces.
Electro-thermal reduction furnaces use electric energy to heat the feed to the required operating temperature. In conventional reduction furnaces, heat liberated during the exothermic reaction between, for example, coke and O2 is utilized to heat the feed. In electro-thermal furnaces, coke is only needed for reduction.
Reduction resistance furnaces have a number of advantages over furnaces using fossil fuels. Electro-thermal processes permit higher productivities due to higher temperatures and energy densities. Higher grade products can be produced because ash and impurities (phosphorus and sulphur) derived from the fossil fuel are absent. Since the furnaces are usually covered, dross formation can be minimized because atmospheric O2 can be excluded. A smaller volume of off-gas volume is produced by resistance reactors with a higher carbon monoxide (CO) concentration because only reduction reactions take place. Electric furnaces can easily be enclosed, permitting tight control of the off-gas composition. Metallurgical coke can be replaced by, for example, charcoal which contains much less impurities. If the furnace is operated under vacuum conditions, reduction and degassing can take place in a single step. The disadvantage of electro-thermal furnaces are high energy costs which result from their energy-intensive operation.
Normally two liquid phases are present in a reduction furnace. A salt or oxide slag floats on the second phase (the produced metal or matte) which collects at the bottom of the furnace. Small amounts of solid feed are added continuously. This open-bath operation can be used for the production of FeSi (ferro silicon), Si, CaSi (calcium silicide) , and Ti slags. The electric power is normally transferred to the furnace by 3 or 6 electrodes which are submerged in the molten slag. Typically, 3-phase current circuits are used. The electric resistance of the slag produces heat which keeps the molten phases in a liquid state and supports the reduction reactions.
In some cases large amount of solids (e.g. the feed) can float on top of the slag (covered bath operation for the production of FeCr, FeMn (ferro manganese), SiMn (silico manganese), FeNi (ferro nickel), and CaC2). In addition to resistance heating, these solids are also heated by electric arcs which are created between the feed and the electrodes, and between the individual feed grains. Hence, these furnaces are also called submerged-arc or arc-resistance furnaces.
The electrodes used in reduction resistance furnaces are either graphite or the cheaper Soderberg type. The C paste for the Soderberg electrodes is filled into moulds above the furnace. The electrodes are then baked by the heat produced by the current flow and the furnace. Hence, they need not be pretreated in a separate furnace as is the case for graphite electrodes. Electrode consumption depends on the application.
The electrodes can be arranged on the vertices of an equilateral triangle or in a row (3 or 6). The best arrangement for a particular application depends on parameters such as furnace efficiency, type of feed, and investment costs.
The type of refractory used in the walls and the roof of reduction furnaces depends on the type of application and exposure it is required to withstand. They include magnesite, chrome magnesite, alumina, and graphite.
The metal itself has a very low resistance and hence produces very little heat. The resistivity of the slag, the slag depth, and the distance between the electrodes are the critical parameters controlling the amount of heat produced in the slag. The distance between the electrodes is chosen large enough to ensure that the current passes through the slag and metal and does not short- circuit between the electrodes.
The circulation of the slag, which can reach velocities between 20 centimeters per second (cm/s) and 30 cm/s close to the electrode, but less than 1 cm/s in colder sections of the furnace. This circulation is created by (i) hot slag being replaced by colder slag from the surface due to density differences, (ii) CO bubbles at the electrode surface, which drag the slag upwards, and (iii) electro-magnetic forces created by the current flowing between the electrodes (three phase furnaces create a downward movement of the slag, whereas DC furnaces create an upward movement if the bottom electrode is a cathode and an upward movement if the bottom electrode is an anode.
In slag – matte furnaces the slag (400 mm – 1500 mm deep) shows an upward circulation around the electrode. The movement is in the opposite direction in the matte (400 mm – 500 mm) due to the electro-magnetic forces (electro-dynamic effects) and because the matte and slag flow in the same direction at the interface. In addition to the above mentioned parameters affecting furnace operation, a number of operating parameters pertaining to the slag, can be modified to achieve optimal recovery of the valuable metals and compounds from the feed. These parameters are a function of the composition of the slag or material to be melted or smelted and temperature reached. They include (i) viscosity, which affects the mixing and settling of the reduced and melted metal drops, (ii) density differences between slag and metals affect the settling behaviour of the phases or metals to be recovered, (iii) resistivity, which has an effect on the amount of heat created and hence on the temperature via Joule’s law, and (iv) O2 permeability of the slag, a low O2 permeability allows the required reducing conditions to be maintained within the metal – matte phase as well as in the slag. An additional factor which affects the performance of these furnaces is the residence time of the material to be treated in the reactor.
Reduction furnaces are used in many different processes for primary and secondary production. These include (i) reduction of Pb and Zn from oxide materials (ores, roasted sulphide ores, secondary oxide materials), (ii) production of ferroalloys from oxide ores, (iii) Ilmenite reduction to iron and a Ti slag, (iv) iron and steel production from ores and pre-reduced pelletized iron ores, (v) matte production from sulphide concentrates of Cu and Ni, (vi) production of CaC2 and corundum, (viii) cleaning of slag (recovery of valuable elements) produced by primary and secondary smelting operations, and (viii) melting and cleaning of secondary metals (scrap).
The example of refining resistance furnaces is the electro-slag refining (ESR) process where the refining operation is carried out in the resistance furnace. The high resistance of the slags (CaF2–CaO–Al2O3) is used to produce enough heat to melt and refine high-grade steels, Ti, Ti alloys, and Ti compounds. The main components of the furnace are its consumable electrode (raw material), a water-cooled mould, and a bottom plate, which are connected to an alternating current source.
Initially the electrode is submerged in a high-melting liquid slag, the mould and bottom plate contain the slag. At sufficiently high voltages, the liquid slag starts to melt the consumable electrode at its surface. The liquid metal collects at the tip of the electrode and drops through the slag. During this process, it is refined due to its interaction with the slag. The liquid metal collects in the mould, and the bottom plate moves downwards at the same rate as that of metal production. The refined metal is cooled by the water cooled mould and solidifies. The mould and bottom plate are made of Cu and water cooled to prevent reactions and to ensure high heat transfer. This cooling system is responsible for high energy losses.
The feed material for the solid state resistance furnace has a high electric resistance. A current is applied to the feed via two electrodes and the heat generated permits the desired reactions to take place in the solid state. Two typical applications are the graphite furnace and the SiC furnace.
In graphite furnace, petroleum coke is converted into graphite which is used for electrodes in other electro-thermal furnaces. The feed (petroleum coke with tar as a binder) is mixed and usually extruded into the desired shape of the electrodes. The electrodes are heated upto 800 deg C to1300 deg C in fuel heated furnaces to carbonize the binder. Then the electrodes are charged to a resistance furnace, where they are stacked parallel to the face sides. The void areas are filled with crushed graphite to ensure that the electrodes are in good electric contact with one another. Electric current flows through the furnace and produces the operating temperatures (2400 deg C to 2800 deg C). The amorphous coke is transformed to graphite (hexagonal lattice) within several days. Since the conductivity of the amorphous coke is much lower than that of graphite, the furnace voltage is changed in order to maintain the required temperature.
In case of SiC furnace, two solid materials (sand and coke) are mixed and filled into a furnace which is similar to that used for graphite production. The reaction SiO2+3C = SiC+2CO takes place in the solid state. The sand – coke mixture with additions of sawdust and sodium chloride(NaCl) is charged into the furnace around a central core of graphitized coke. This central core is connected to electric power and the coke is used as a conductor, which reacts in a similar way as described in the case of graphite furnace. SiC formation starts at 1700 deg C, at 2400 deg C to 2500 deg C a coarse crystalline product is formed. NaCl is added to evaporate undesired elements such as Al, Fe, and Ti as chlorides.
Arc furnaces – An electric arc can be produced and sustained between two electrodes or an electrode and a liquid melt if the voltage is high enough. The transformation of electric energy to heat takes place through the current in the ionized plasma of the arc, in which the temperatures can reach 6000 deg C.
EAFs are classified according to whether transfer of heat from the arc to the furnace feed is indirect or direct. In indirectly heated furnaces, the arc burns between two electrodes without contact with the feed and the heat is transferred only by radiation and convection. This technique has been employed in single-phase rotary furnaces, which are, however, no longer in use because of economic reasons. In directly heated furnaces an arc burns between the electrode and the melt. Heat is transferred via the fire point produced by the arc by conduction, radiation, and convection to the melt. Directly heated arc furnaces can be used for melting and refining. The arc-resistance and submerged-arc furnaces are mainly used for reduction and their construction is similar to that of resistance furnaces.
The principal application of EAFs is for the production of steel. One-electrode furnaces generally operate in the DC mode with the electrode forming the cathode and the bottom of the furnace forming the anode. 3-electrode furnaces are mostly AC furnaces. The electrodes are switched into a 3-phase circuit and are usually placed on the vertices of an equilateral triangle in a round furnace. Maximum furnaces are equipped with removable roofs. The required atmosphere can be provided inside the furnace which can be tilted during tapping. Dolomite and magnesite refractories are used for lining the lower walls and the bottom of the steel shell of the arc furnaces.
The upper walls and the roof of the furnaces are water-cooled to provide good resistance against high temperature and temperature changes. The high temperature obtained in arc furnaces allows high production capacities. When scrap is melted down, crater-type holes are burnt into the scrap which protects the furnace roof and walls from overheating.
Modern furnaces can be tilted only 12 degrees during slagging off operations. Liquid steel tapping is done by eccentric bottom tapping. Sometimes the mixing in the furnace is improved by an induction stirring coil underneath the furnace or by bubbling gas through a porous plug.
The advantages of EAFs are (i) high temperatures can be reached within a short time, (ii) continuous temperature adjustment is possible via computer control systems which control the level of the arc, (iii) the atmosphere in the furnace can be tightly controlled, (iv) in contrast to conventionally heated furnaces, impurities are not brought into the steel by the energy input, (v) the furnace can easily be brought on- or offline with relatively little energy and time, furnace operation can be controlled to meet the production requirements of the continuous casting machines, (vi) oxidation, reduction, and alloying can be carried out in one charge with little loss, and (vii) desulphurization is possible.
Development of arc furnace technology has led to the construction of UHP furnaces. The outstanding electrical efficiency of these furnaces can be attributed to low Joule-effect losses and conductors with small cross sectional areas. The short, high-current arcs improve heat transfer. These furnaces are computer controlled.
Vacuum arc refining furnaces – Vacuum arc refining (VAR) furnaces are normally used for consolidation and refining reactions. The raw material (sponge or scrap) is pressed into an electrode shape, which is used as a consumable electrode in the arc furnace. Usually DC current is used to provide a stable arc between the consumable electrode and the counter electrode at the bottom of the furnace. After the arc is ignited the electrode starts to melt, volatile elements and entrapped gases are removed through the high temperature and the vacuum conditions. The refined metal drops to the water cooled Cu bottom where it solidifies. Cu is used as the mould material to prevent reactions with refractories.
VAR furnaces can also be used with a permanent electrode if the material is fine scrap. This is fed in separately. These furnaces are also used for alloying high-melting metals such as Nb, Ta, or Ti.
Liquid Ti is very reactive, as are Mo and Zr. Hence, these metals are to be melted in a controlled atmosphere or a vacuum. VAR furnaces are thus suitable for melting and/or recycling these highly reactive metals (also possible in electron-beam furnaces). As with ESR furnaces, metal sponge and scrap can be mixed and pressed into consumable electrodes and used as such in the VAR furnace. Often refining involves several steps to achieve a pure product, this explains the relatively high energy consumption for Ti. VAR are also used to degas certain steel grades during remelting.
Induction furnaces – The increasing demand for high-quality metals and alloys under ecologically clean conditions has increased the application of IFs in foundries for cast iron, steel, and nonferrous metals (mainly Al, Cu, Mg, Ni, precious metals and their alloys). There are two basic types of IF. These are the crucible IF and the channel IF. IFs are mainly used for melting and refining of metals and alloys. Special applications are zone refining and levitation melting. Inductive heating is also used for brazing and welding, different types of heat treatment (e.g. surface treatment, heating and annealing), metal transport and dosing, and electromagnetic casting.
Electromagnetic induction heating is a direct method for contactless heating with a high power density which implies that the material to be heated (melted) is not contaminated by heating gases or electrode materials. However, contact and reactions with the crucible (graphite or refractories) can take place. Advantages of these types of furnaces are excellent alloying and mixing conditions, good temperature control of the melt, a low slag formation, and low off-gas volume.
The heating takes place since an AC coil induces a potential in an electrical conductor (in IFs solids and/or melts) situated inside the coil due to the changing magnetic field, which creates eddy currents (Lenz’s law). The eddy currents or induced current produce heat according to Joule’s law.
The resistivity and relative magnetic permeability of the material to be heated and the frequency of the primary current circuit are the most important parameters of induction heating. The induced current within the conductor is not evenly distributed, but mostly located near the surface (skin effect).
Crucible induction furnaces have a cylindrical induction coil consisting of water- cooled Cu tubes. The crucible is located inside the coil and usually consists of rammed or brick refractories, graphite, or clay – graphite. In rare cases a more expensive prefabricated ceramic or steel crucible can be used. The inert cold-wall induction crucible is an interesting development. The crucible can and is to be covered to reduce heat losses. For special applications (metal distillation, refining, melting of reactive or high quality metals and alloys, etc.) the furnace is operated as a vacuum or controlled atmosphere IF.
There are two basic types of vacuum IF, the compact vacuum crucible IF and the vacuum chamber IF. The compact vacuum crucible IF is operated at a low vacuum. It has a simple design and lower investment costs than the vacuum chamber IF, but the load capacities can be higher. The coil, vacuum chamber, and crucible form an integrated unit. In contrast, the crucible and induction coil of a vacuum chamber IF are located within a sealed, water-cooled chamber. It allows a higher level of vacuum and permits the metal to be poured under vacuum within the sealed chamber.
Industrial crucible IFs have different sizes and are operated at different frequencies under air or vacuum. Laboratory scale IFs have capacities down to a few grams and frequencies reaching up to MHz range.
The biggest advantage of crucible IFs is the outstanding bath mixing behaviour. Interaction of the magnetic field and electrical currents create electromagnetic forces, which leads to the elevation of liquid metal in the centre of the bath and mixing. As a result of stirring, crucible IFs can melt loads of small pieces of material (e.g. chips), which are rapidly drawn into the bath. This facilitates operations such as alloying and refining, and ensures a uniform bath composition. The scale of mixing is inversely proportional to the applied frequency.
Crucible IFs are used for melting and refining numerous metals and alloys. They are usually applied in cast iron, steel, and nonferrous metal foundries (Al, Mg, Cu, brass, bronze, Zn, Ni, precious metals, and super alloys). They are unsuitable for slag metallurgical operations since only the metal is heated by induction and not the slag. In addition, the advantages of numerous crucible designs (e.g. rammed or brick lining; ceramic, graphite, steel, or Cu crucible) render them very attractive. They also permit various operations, such as alternative or continuous melting, continuous melting and pouring, quick alloy changes, and application of vacuum or controlled atmosphere. They are sometimes used as buffer furnaces or grading furnaces.
Line-frequency crucible IFs are mainly employed in cast iron, heavy metal, and Al foundries. They are sometimes used in (high-grade) steel foundries. High furnace capacities are possible. Medium-frequency crucible IFs are mainly used in operations which require frequent alloy changes for melting high-grade steel, alloyed cast iron, Cu, Al, and precious metals in medium-size furnaces. The low power input required for holding furnaces means that short induction coil furnaces can be used for these applications.
If possible, the crucibles have a rammed lining (acidic monolithic linings) due to their lower costs, which produces significant savings in larger furnaces. They are widely used in cast iron, steel, and nonferrous metal foundries, especially in furnaces with high capacities and little or no metal and alloy changes. The linings are partially infiltrated by the metals/alloys and hence cause contamination during an alloy change. Nonferrous metal foundries, which frequently change alloys often, employ more expensive pre-fabricated crucibles. Precious metal foundries need prefabricated crucibles to minimize metal losses in the linings. Mg and its alloys are melted solely in steel crucibles due to their chemical behaviour. Alloys which attack acidic linings require basic linings (Al2O3, MgO) or graphite crucibles.
Channel IFs – In the channel IFs the induction coil is located under or beside the crucible and has an iron core. The coil is enclosed in a channel. This construction acts as a transformer with a short circuit secondary coil producing the best power transfer efficiency (0.95 to 0.98 with furnace efficiency around 0.9). The metal in the channel is heated and pumped back to the metal reservoir in the crucible by the repulsive force of the coil and the induced currents. This means that the furnace is always to be charged with a certain amount of liquid metal.
Various designs of channel IFs are available. There are inclined and vertical channel furnaces, horizontal channel furnaces, and open channel furnaces. They are normally used as holding or casting furnaces and less frequently as melting furnaces. Due to large flow rates in the channel, they are more susceptible to erosion of the refractory within the channel. However, they permit high capacities and power efficiency is greater than 80 % which is larger than that for crucible IFs. Large holding furnaces can have more than one channel. The quantity of refractory materials used for channel furnaces is high. Careful maintenance of the channel is also necessary.
Due to the relatively low specific power requirements, channel IFs are mainly used for holding and superheating cast iron and nonferrous metals, but less often for steel. Channel IFs are often used in combination with other furnaces and act as buffer furnaces. They are sometimes used as melting or casting furnaces, these include automatic air pressure pouring systems.
Channel IFs are not as widely applied as crucible IFs in nonferrous industries. Clogging of the channel is a handicap, especially in the melting of Al. However, the different designs of channel inductors permit various applications, such as coating pots (zinc) or pouring furnaces.
There are also available special IFs. Reactions with the crucible or refractory material are undesirable for special high-grade metals and alloys. Use of special induction coil design and suitable frequencies and induction currents, allow metals or alloys to be melted and centered inside the coil by levitation melting. This melting method is only used on a laboratory scale for small amounts of pure metals and alloys. Induction zone melting is used to refine very pure metals or alloys, especially for the production of semiconductors. Germanium and silicon mono crystals are produced by this technique.
Plasma furnaces – Plasma is a partially ionized (upto 50 %) gas which contains electrons, ions, energized molecules, dissociated molecules, neutral molecules, and atoms. The plasma operates at atmospheric pressure and is sufficiently conducting to permit stable transfer of electric power between two or more electrodes.
The principal difference between plasma furnaces and arc furnaces is the use of a plasma torch instead of electrodes. In a plasma torch thermionic electrons are emitted from a cathode and accelerated towards the anode. They collide with gas molecules and ionize them. The positively charged gas ions are accelerated in the opposite direction towards the cathode with which they collide, releasing their energy and hence sustaining the thermionic emission. Depending on the type of torch and its construction materials, water cooling can be applied. Several types of AC and DC torches are available. They include transferred arc, non-transferred arc, and superimposed arc.
Furnace construction is similar to that of arc resistance and arc furnaces. Refractories and other components are selected to suite the specific application. Plasma furnaces are recent addition into primary and secondary metallurgy.
Electron beam furnaces – In electron beam furnaces the kinetic energy of highly accelerated electrons in a vacuum is transferred to the surface of a material on impact. This leads to local temperature increases and melting of the material. The principle component of an electron beam furnace is the electron gun which can be work-accelerated or self-accelerated. The self-accelerating gun is most commonly used. Here the voltage is applied between the cathode and an auxiliary hollow anode situated just below the cathode. In work-accelerated guns the anode is the material to be melted, this techniques is used to good advantage in zone melting.
The self-accelerating electron gun consists of an electron emitting cathode, an accelerating hollow anode, and electromagnetic focusing and deflection coils. In addition to the gun, the furnace comprises of a work chamber in which melting and refining is performed in water-cooled crucibles, vacuum equipment for the electron gun and work chamber, and a computerized control system which controls and monitors the operating parameters.
Various types of self-accelerating guns and gun configurations have been developed. These include annular, axial (or pierce), axial differential pumped, transverse and radial multifilament gun arrangements. Feeding of material to the furnace is facilitated by drip melting of vertically or horizontally fed consumable electrodes, or bath melting of granulated material and scrap. The melted product can be continuously cast into bars or moulds.
Electron-beam melting has higher investment costs per installed kilowatt hour than other melting techniques. However, it has distinct advantages. Success can be ascribed to its abilities to fuse any low-volatility material which undergoes a solid – liquid transition, and to maintain the fused condition for as long as is required to obtain the desired refining action. Production of high quality refractory metals is possible, which is not the case with the other methods. Although investment and maintenance costs are high, they are compensated by increased quality.
The refining action in the electron-beam furnace can be classified into (i) removal of sub-oxides with high vapour pressures from their respective metals, (ii) Removal of interstitials such as N2 and H2, (iii) removal of O2 by C as CO, (iv) removal of highly volatile metals from the metal to be melted and refined (e.g. removal of Cr and Fe from Zr), and (v) removal of insolubles from the melt material and of inclusions which float to the surface of the melt; since most materials to be processed are usually clean this contribution to refining is small
In addition to the usual refractory metals (V, Nb, Ta, Mo, W, Zr, Hf) which are normally melted and/or refined in electron-beam furnaces, Be, Co, U, Ni, and alloys such as special steels have also been melted. As an example, the refining action during the melting of Hf in an electron-beam furnace is far superior to that in vacuum arc melting.
For economic reasons Ti is mostly processed via vacuum arc melting, although electron-beam furnaces are also applied. The impurities O2, N2, C, and Fe remaining after refining lie in similar ranges for both processes, H2 values are, however, far lower for electron-beam melting. Ceramics (e.g. ZrO2 and Al2O3), high-quality steels, and carbides of uranium, Th, and Zr are also melted in electron-beam furnaces. Electron beams are also used for metallization, vacuum deposition, centrifugal casting, machining, and welding.
Fused-salt electrolysis cells – Fused salt electrolysis cell can be classified both as a resistance electro-thermal reactor and as an electro-chemical reactor. It is not, however, a purely electro-thermal reactor. Due to their low reduction potentials, elements such as Al, Mg, and Na cannot be recovered from aqueous solutions – H2 ions are reduced in preference to these elements to produce H2 gas. Recovery of these elements by electrolysis can only be carried out in stable, non-aqueous electrolytes. In order to illustrate the technology of fused-salt electrolysis, two examples are described. They are (i) recovery of Al from Al2O3 and Na from NaCl.
The feed material for the production of Al is Al2O3. Due to alumina’s high melting point (2050 deg C), fused-salt electrolysis of pure Al2O3 is not economically viable. The Hall –H´eroult process, developed at the end of the nineteenth century, permits electrolysis at 940 deg C to 980 deg C, this temperature is produced due to the resistance of the stable salt electrolyte based on the NaF – AlF3 – Al2O3 system to the electric current. This electrolyte system has a sufficiently low liquid temperature and high Al2O3 solubility. Generally around 2 % to 8 % Al2O3 is dissolved in the cryolite-based (Na3AlF6) electrolyte. The density difference between the liquid Al and the liquid electrolyte is sufficiently large to ensure good separation of the liquid electrolyte and liquid Al. The overall cell reaction for the electrolysis is given as Al2O3+ xC = 2Al + (3−x) CO2 + (2 x−3)CO where 1.5≤x≤3.
A typical electrochemical cell for the production of Al consists of (i) a C lined steel vessel (C blocks at the bottom of the furnace 400 mm deep), (ii) 2 rows containing a total of 16 to 24 consumable prebaked C anodes, (iii) liquid salt electrolyte (cryolite-based), (iv) a liquid Al cathode, and (v) a layer (200 mm to 400 mm) of insulation (fire clay refractory) at the bottom of the furnace, and (vi) anode and cathode bus bars. In modern cells small amounts of Al2O3 are fed between the two rows of suspended anodes at frequent intervals. Crust breakers between the two rows of anodes break the frozen bath crust, and a ventilation shaft removes the anode gas evolved.
High-purity Al is produced in a refining cell. This cell comprises (i) a C base, which acts as the anode, (ii) an anode alloy containing 30 % Cu above the anode, (iii) an AlF3 – NaF – BaF2 – CaF2 based electrolyte in the middle, and (iv) high-purity liquid Al. It is operated at a temperature of 750 deg C, maintained by heat liberated due to the resistance of the electrolyte to the electric current.
Na is recovered from liquid NaCl by fused-salt electrolysis in a Downs cell. The salt is maintained in the liquid state due to the resistance of the electrolyte to the electric current. The electrolyte is a liquid mixture of BaCl2, CaCl2, and NaCl. A cathode cylinder is concentric with a graphite anode, the inter-electrode gap is around 50 mm. A diaphragm in the gap between the anode and cathode is attached to collection bells, allowing separate recovery of Na and Cl.
Since Na has a lower density than the molten electrolyte, it drifts to the surface of the liquid electrolyte where it is recovered in the collection bell.
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