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4Metallurgical Furnacesexceed a certain value. The countercurrent arrangement is preferred because it involves an increased total energy consumption. The differenttypes of heat transfer in a rotary kiln are shown inFigure 3. Fundamental work is reported in [136],[137]. 60 mm because it delivers a more uniformproduct at a higher rate ( Lime and Limestone,Chap. 3.3.2.). This quicklime is particularly suitable for steelmaking and has been used in fluegas desulfurization since the 1980s [31], [138].Typical dimensions of rotary kilns with aquicklime capacity of 560 t/d areKiln without preheaterKiln with vertical preheaterFigure 3. Types of heat transfer in a rotary kilnA) Heat transfer to material by gas radiation and convection;B) Heat transfer to material by brick radiation; C) Conductive heat transfer to material from brick; D) Heat transfer tobrick by radiation, convection, and heat loss by shell radiation and convection145 m 3.45 m diameter56 m 4.0 m diameterThe rotary kiln also competes with the shaftfurnace in the production of sintered magnesite and dolomite ( Magnesium Compounds,Chap. 4.2.5.). As in limestone calcination, therotary kiln offers the advantage of handlingcharge material of finer size with a broadsize range (e.g., 2 – 60 mm) and can also handle throughputs at high calcination temperature(1600 – 1900 C). A rotary kiln 170 m long witha shell diameter of 4.5 m, used for the production of sintered magnesite, is reported to havea capacity of 530 – 600 t/d. The same authorindicates that the same mathematical relationships cannot be used for the design of cement,dolomite, and magnesite kilns [32].1.3. Applications1.3.1. Roasting and CalciningRotary kilns were employed for the exothermic roasting of sulfidic ores and for the endothermic removal of water of hydration andcarbon dioxide from fine-grained materials suchas ores, phosphates, alumina, ilmenite, and titanium dioxide. Today these processes are performed almost exclusively in fluidized-bed reactors ( Fluidized-Bed Reactors), which offerbetter heat- and mass-transport conditions.However, rotary kilns have advantages wheresoftening, sticking, or even partial melting of thematerial cannot be avoided.1.3.2. Calcination of Limestone, Dolomite,and MagnesiteWhereas coarsely crushed limestone can be converted to quicklime by heating in the shaft furnace, the rotary kiln is preferred for particle sizes1.3.3. Production of Cement ClinkerCement is produced almost exclusively in rotarykilns ( Cement and Concrete, Chap. 1.4.3.).World production was ca. 730 106 t in1976 [33] and 1363 106 t/a in 1994 (USA188 106 t/a, Europe 251 106 t/a). Developments in clinker production have, therefore, always spurred rotary kiln technology as a whole.Energy accounts for as much as 50 % of the totalproduction costs of cement clinker [34]. Energysaving approaches have led to thermally sophisticated, large- capacity units. The trend is alsotoward using coal as the energy source. Longrotary kilns with slurry feed (Fig. 4 A) have increasingly been replaced by “heat-exchanger”kilns (Figs. 4 B, C) in which the raw meal isheated in upstream suspension-type heat exchangers [36–38]. Development work with improved dust collection systems has led to energyconsumption in the order of 3.35 MJ per kilogram of clinker.

Metallurgical Furnaces5Figure 4. Kiln systems for cement clinker production of 2000 t/d [35]A) Wet process; B) Dry process; C) Process with precalcining1.3.4. Iron Ore ReductionThe rotary kiln should continue to grow in importance as a reduction apparatus. It can employa wide range of carbon carriers not suitable asreductants for shaft furnaces (e.g., the blast furnace): from anthracite and coke breeze to charcoal fines, lignite, and brown coal.The charge (ore and reductant) usually movesthrough the rotary kiln countercurrent to thehot gases. Coupled reactions – ore reduction bycarbon monoxide and reaction of carbon withcarbon dioxide with regeneration of carbonmonoxide – occur in the charge. The reactivityof carbon is critical for the process as a whole[39]. Some of the carbon monoxide formed es-

6Metallurgical Furnacescapes from the charge so that oxidizing gasesfrom the free kiln volume cannot permeate intoit.Reducing conditions depend on the temperature, reactivity, and quantity of the reductant;the residence time; and the charge holdup at thedischarge end of the kiln. The rate of reductioncan be controlled to meet a variety of objectives,from the formation of magnetite to the production of carburized molten pig iron. Details aresketched in Figure 5 and described briefly below.In magnetizing roasting, the iron content oflean ore is transformed to magnetite (Fe3 O4 ) at700 – 1000 C. The product is concentrated bygrinding and magnetic separation. The temperature profile and reducing conditions can be controlled with gas- or oil-fired shell heaters locatedalong the kiln; fuel is supplied to the rotating kilnthrough special seals. The reducing action canalso be aided by the addition of small amountsof coal.The most frequently used process for theproduction of sponge iron is the SL/RN process [40], [139], illustrated in Figure 6. Lumpore, pellets, titanomagnetite-containing seasand concentrates, zinc- and lead-containingresidues from iron and steel production, and pellets from leach residues are reduced in existingplants ( Iron, Chap. 2.6.4.).1.3.5. Volatilization ProcessesMetals and metal compounds with a high vapor pressure can be recovered by volatilizationfrom their raw materials in a rotary kiln. Thevolatilization may be enhanced by selective reduction. Examples include arsenic, antimony,lead, cadmium, mercury, silver, tin, and zinc inelemental form or as compounds, for example,zinc as the metal and lead as PbS or PbCl2 .The way in which the charge moves throughthe rotary kiln (a rolling motion called “wälzen”in German) has given its name to the Waelz process used for decades in the volatilization of zinc( Zinc) and lead ( Lead, Chap. 4.2.2.3.).Low-zinc ores and other raw materials are fedtogether with solid reductants to the kiln, wherethe metal is volatilized under reducing conditions. The vaporized metal is oxidized in thefree volume above the charge and recovered asdust (the so- called Waelz oxide) from the offgas [41]. Figure 7 is a schematic of a modernWaelz plant. The Waelz process has also provedsuitable for zinc- and lead- containing residuesfrom iron and steel production [42]. A flow sheetof this process is shown in Figure 8. Higher productivity with less energy consumption and lessmechanical dust formation can be achieved bypelletizing the feed. The Waelz kiln oxide canbe purified by washing with Na2 CO3 solution.1.3.6. Other ApplicationsOther applications of the rotary kiln include:1) Oxidation of ilmenite [43]2) Calcination of pellets after preliminary heattreatment on a traveling grate (e.g., see Iron, Chap. 2.1.1.5.)3) Calcinationofpetroleumcoke( Petroleum Coke, Chap. 3.2.) [44]4) Reduction of heavy spar5) Processing of gypsum to sulfuric acid andcement (gypsum – sulfuric acid process)6) Production of TiO2 pigment ( Pigments,Inorganic, Chap. 2.1.3.1.)7) Production of mercury ( Mercury, Mercury Alloys, and Mercury Compounds,Chap. 4.1.3.)8) Volatilization of zinc, lead, and copper withCaCl2 [45]Waste incineration and coal gasification maybe important future applications of the rotarykiln.2. Multiple-Hearth FurnacesFrom a dominant position as a roasting furnace for sulfide ores (chiefly pyrites in sulfuric acid production), the multiple-hearth furnace has been almost completely displacedby fluidized-bed roasting equipment since the1960s. Fluidized-bed devices ( Fluidized-BedReactors) permit much higher throughputs thanmultiple-hearth furnaces, with substantially better control of reaction temperature and oxygenpartial pressure in the roasting gas. Nonetheless,the multiple-hearth furnace will continue to finduse in some special areas of process engineering.

Metallurgical Furnaces7Figure 5. Characteristics and possibilities for the reduction of iron ores in rotary kilnsFigure 6. SL/RN process for sponge iron productiona) Bins for raw material; b) Waste-heat boiler; c) Aftercombustion chamber; d) Electrostatic precipitator; e) Rotary kiln; f) Coalbin; g) Fine-ore bin; h) Rotary cooler; i) Screening; j) Magnetic separation; k) Hot charging; l) Electric-arc furnace

8Metallurgical FurnacesFigure 7. Flow sheet of a modern Waelz planta) Plate feeder; b) Willemite bin; c) Coke and anthracite bins; d) Mill; e) Cooling tower; f) Settling chamber; g) Waelz kiln;h) Producer gas plant; i) Burner; j) Waelz slag pit and scraper; k) Cyclones and bag filters; l) Two electrostatic precipitators;m) Waste-gas fan and stack; n) Pelletizing disk; o) Waelz oxide bins2.1. DescriptionA multiple-hearth furnace (Fig. 9) consists ofan internally lined steel cylinder with a numberof horizontally mounted, lined platforms calledhearths. The circular hearths are thinner nearthe center, which has an opening for a verticalshaft. An adjustable-speed drive with overloadprotection turns the shaft at 0.2 – 5 rpm. Fromone to four rabble arms per hearth are latched tothe shaft in a gastight manner. These arms bearoblique stirring teeth to move the solids over thehearth; on one hearth the motion is from center to edge, on the next from edge to center depending on the inclination of the stirring teeth.

Metallurgical FurnacesThe openings in the hearths, through which thecharge travels from the top of the furnace to thebottom, therefore alternate from central to peripheral.93) Processes in which the solid is inlet in slurryform and the slowest and most gentle dryingpossible is desired4) Processes in which solids must be exposed toa stepwise varying reaction temperature during thermal processing (within certain limits,the temperature and the gas atmosphere canbe varied from hearth to hearth)5) Reactions in which the solid undergoes slightsoftening, agglomeration, or sintering so thatfluidized-bed processes cannot be employedFigure 8. Flow sheet of Waelz process with steel mill dustas feed and briquetting of productBecause of the high temperature in the furnace, the shaft and rabble arms are air cooled.The shaft has double walls; cool air supplied by afan enters the outside space, passes through theshaft and arms, and exits the furnace at 200 –300 C by way of the center space. Each hearthhas several doors, which allow monitoring of thereaction and replacement of the rabble arms. Thedoors can be sealed tightly or can have adjustableair slots to admit cooling or combustion air if aslight subatmospheric pressure is maintained inthe furnace.Multiple-hearth furnaces are built in varioussizing, ranging from 2 to 8 m in diameter andhaving 3 – 16 hearths. Residence time in the furnace is easily controlled by varying the shaft rotation speed or the number of rabble arms andteeth on each arm. Suitable reactions for themultiple-hearth furnace are1) Slow reactions (because long residencetimes can be achieved in the multiple-hearthfurnace)2) Reactions between solids and quantities ofgas that are too small to maintain a fluidizedbedFigure 9. Lurgi multiple-hearth furnace [50]a) Hopper; b) Circular hearth; c) Hollow shaft; d) Rabblearm with teeth; e) Discharge outlet; f ) Gas outlet2.2. Applications2.2.1. Roasting of Sulfide OresSince the development of fluidized-bed techniques, sulfide ores such as pyrite, pyrrhotite,zinc blende, and chalcopyrite have not beenroasted in multiple-hearth furnaces. Ores thatcan be roasted only with great difficulty, suchas molybdenum disulfide, are still treated inmultiple-hearth furnaces.

10Metallurgical FurnacesBecause roasting reactions are exothermic,the furnace usually has to be heated only at thestart of the process. The material fed to the topmost hearth is distributed by the teeth on the rabble arms, slowly transported to the center of thehearth, and dried. Then the ore falls into the firstroasting zone, where it is heated in contact withhot roasting gas until it ignites. The reaction goesto completion as the charge is transported furtherover the hearths. On the last hearths, roasting airdrawn or blown into the furnace from the bottomis preheated by cooling the residue. The progressof the reaction is monitored by measuring thetemperature on the individual hearths.The roasting of molybdenum disulfide(Eqs. 1 – 3) is carried out at ca. 630 C( Molybdenum and Molybdenum Compounds, Chap. 4.2.). The hearths on which theexothermic roasting reactions (1) and (2) occurare cooled by the admission of air or water.2 MoS2 7 O2 2 MoO3 4 SO2(1)MoS2 6 MoO3 7 MoO2 2 SO2(2)2 MoO2 O2 2 MoO3(3)After the roasting reaction has gone largely tocompletion, the molybdenum dioxide productis oxidized to the trioxide (Eq. 3). This reaction, however, is not sufficiently exothermic tokeep the temperature high enough to burn offthe residual sulfur. In a furnace with a total ofnine hearths, the seventh and eighth are thereforeheated with oil or gas.2.2.2. Use for Endothermic ReactionsFor endothermic reactions such as the calcination of a magnesite, dolomite, bauxite, clay, orzinc carbonate, the heat required is deliveredfrom outside into the furnace. If the feed materials can tolerate some overheating by flameradiation, one or more burners can be built intothe furnace shell at the hearths where heating isrequired (see Fig. 10). Otherwise, hot flue gasis admitted at these locations after generation inone or more combustion chambers. Heat lossesare higher and throughput capacities lower thanthose of fluidized-bed furnaces and rotary kilns,so the calcination reactions mentioned above arerarely carried out in multiple-hearth furnaces.The use of this type of equipment is restricted toresidues containing vanadium and tungsten.Figure 10. Heating of individual hearths with jacket burners[50]a) Multiple-hearth furnace; b) BurnerMost vanadium is produced from vanadiumcontaining slags such as those from steelmaking.Because most of the vanadium in slag is in thetri- and tetravalent forms, which are insoluble inwater, it must be converted to water-soluble pentavalent sodium vanadate by thermal oxidationand the addition of Na2 CO3 and NaCl. This process is carried out in the multiple-hearth furnace( Vanadium and Vanadium Compounds).The multiple-hearth furnace can also be usedto oxidize tungsten- containing slags or residuesfrom the metal fabrication industry (turningchips, grinding waste) in order to recycle thematerial in the form of WO3 to tungsten production.2.2.3. Clarifier Sludge TreatmentThe flexibility of the multiple-hearth furnaceled, in the 1980s, to its increasing use for theincineration of waste from a variety of sources.The main products treated by incineration aresludges from wastewater treatment and from cellulose and paper mills.

Metallurgical FurnacesToday, most sludge incinerators built aroundmultiple-hearth furnaces are designed so thatcombustion is self-sustaining, i.e., no fuel needsto be added. The sludge is therefore preparedby mechanical dewatering in centrifuges or filter presses.If the degree of dewatering is insufficientfor self-sustaining combustion, a thermal drying step is interposed. For sludge or waste incineration, the multiple-hearth furnace is equippedwith an afterburning chamber (f) and gas vapor recycle (g) (see Fig. 11). The afterburningzone is needed for the residue-free combustionof volatile components at 800 – 900 C. Recycling of vapors and flue gases by means of a fanthus helps by minimizing pollution and controlling temperature.11the burnup zone of the fluidized-bed. One advantage of this arrangement, compared to theconventional fluidized-bed furnace with slurryfeed, is that the air excess can be reduced to 30 –40 %, so neither supplemental fuel nor externalpredrying is usually necessary (given an appropriate degree of dewatering).Figure 12. Combined multiple-hearth – fluidized-bed furnace (MHFB)a) Fluidized bed; b) Startup burner; c) Drying hearths;d) Distribution hearths; e) Gas – vapor recycle; f) PreheaterFigure 11. Multiple hearth for sludge incinerationa) Combustion air; b) Cooling air for ash; c) Cooling air;d) Startup burner; e) Multiple hearth; f) Afterburning chamber; g) Recycle fanThe use of multiple-hearth furnaces forsludge incineration led to the developmentof combined multiple-hearth – fluidized-bed(MHFB) furnaces in 1980. Furnaces of thistype (Fig. 12) have the advantages of both themultiple-hearth furnace (slurry feed) and thefluidized-bed furnace for combustion processes.In principle, such a device is a fluidizedbed furnace with a few hearths added on top asneeded to predry the sludge. Flue gases ascending from the fluidized bed are drawn throughthe hearths in countercurrent to the slurry feed.About 50 % of the water is vaporized outside thefluidized bed, and the sludge is thus predried.Vapor produced by predrying can be recycled to3. Shaft FurnacesThe shaft furnaces have a variety of uses in metallurgy. The Rachette furnace is employed inlead production ( Lead, Chap. 4.2.2.2.). Figure 13 shows a chair jacket profile used today.The distinguishing feature of the lead blast furnace is that the throat widens upward. The products, lead bullion and slag, are obtained by reduction; the slag contains any zinc present asZnO. The Imperial Smelting furnace (Fig. 14)was developed for the simultaneous processingof lead and zinc concentrates ( Zinc). It differs from ordinary lead blast furnaces by havinga hot stack and a zinc splash condenser.In the following, the iron blast furnace( Iron, Chap. 2.5.) is described in more detail as it is the most important type of shaft fur-

12Metallurgical Furnacesnace. The blast furnace has the form of two truncated cones set with their large bases together.The widening of the stack from top to bottomreduces the frictional resistance as the burdenmoves downward.Figure 13. Profile of a lead blast furnaceA typical iron blast furnace is shownin Figure 15. Modern blast furnaces have ahearth diameter of 10 – 14 m, a volume of2000 – 4000 m3 , reach throat gas pressures ofup to 0.35 MPa, and have production capacities of (1.5 – 3.5) 106 t/a of pig iron. The blastfurnace is divided into the following sections:throat, stack, bosh, and hearth.The blast furnace is charged with burden,coke, and additives via the charging platform(throat). The blast-furnace gas or top gas is alsowithdrawn from the furnace through the throat.For design reasons, the throat diameter shouldbe kept as small as possible. The stack is conical and terminates in the throat at the top. It expands downward to the belly. The next sectiondown is the bosh, which narrows toward the bottom and forms the transition to the blast-furnacehearth. The narrowing of the bosh correspondsto the decrease in burden volume as the cokeis consumed. This cylindrical section has watercooled tuyeres through which the blast is introduced into the furnace. The hearth should havea relatively small diameter because the hot blastdoes not have a long penetration depth and otherwise would not reach the interior of the burden.The two main products of the furnace, slag andiron, collect in the hearth. Excess slag is withdrawn through slag holes, while the hot metalwhich has a higher density, is withdrawn throughtap holes.The main function of the iron blast furnaceis to reduce iron oxides to metallic iron. Thereducing melting of iron- containing substancestakes place in a countercurrent arrangement; theascending hot gas

c 2005Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim 10.1002/14356007.b04339 Metallurgical Furnaces 1 MetallurgicalFurnaces For other industrial reactors and their applications, see Stirred-Tanc and Loop Reactors, Tubular