GeneralWe encounter bulk solids every day in mineral and non-min-eral form – from, for example, mining (ores, coal) throughbuilding materials (cement) to foodstuffs (sugar, coffee,cereals) and chemical or pharma ceutical products (pig-ments, pills).The properties of bulk solids are determined and character-ized based on particle size, particle distribution, bulk densi-ty, angle of repose, moisture content and temperature.Amongst other things, these param eters determine the flowbehaviour and flowability of bulk solids, and therefore havea crucial influence on the process engineering and equip-ment used in the processing of these coarse-, fine- and ultra-fine-grained materials. The design of thermal processingplants for the heat treatment of bulk solids must also beadapted to the characteristic properties of the materials.Typical kiln or furnace applications can be found in theceramics, glass and binders industry as well as in the chem-icals industry, comprising powders, granular as well aslumpy, dry and wet materials.To ensure the effective mechanical and thermal treatmentof the multitude of different bulk solids, a cor res pondinglywide range of different industrial kiln and furnaces [1] withdifferent conveying systems are available as rugged pro-duction plants. In the following, the main types of kiln andfurnace for drying, debinding, calcining, and sintering ofbulk solids to application tem pera tures up to 1800 °C andin different atmospheres are examined in greater detail. Abasic differentiation is made between intermittently andcontinuously operating kilns and furnaces.

Rotary and pendulum kilnsThe key characteristic of rotary and pendulum kilns is thatthe bulk solids are kept in constant motion during heattreatment. The kilns are oper ated continuously, allowingdirect transport of the bulk solids through the kiln. In therotary kiln, the inclin ation of the rotary tube over its lengthand the rotational speed are adjusted to the bulk solid to betreated. Depending on the type of heating, a differentiationis made between directly and indirectly heated rotary kilns. Indirectly heated rotary kilns are used particularly when thefeed material is prone to severe dust generation, mustundergo a defined temperature profile or has to be treatedin special processing conditions, e.g. inert or protectiveatmosphere. The rotary tube is heated from the outside bymeans of burners or electric heaters. Depending on pro cessrequirements, heating is divided into several control groupsover the length of the rotary tube. Depending on theprocess requirements with regard to max. temperatures,

cfi/Ber. DKG 90 (2013) No. 4

ThermalProcessingPlants for HeatTreatment ofBulk Solids

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• Tube material: steel, ceramic,graph ite

• Protective gas operation: oxidiz-ing, inert, reducing

• Tube seal: mechanical seal, sealinggas

• Mechanical effect on the product:high

• Degree of automation: high, fullyautomatic material feed and dis-charge is possible.

Another model of kiln is the internal-ly heated rotary kiln, which basicallyhas a rotary or pendulum tube withan insulating refractory lining. Theinsulation is used primarily for heatprotection, but, however, also has tobe designed as wear protection or aninert protective barrier between thetube and bulk solid. Tubes of thistype are heated directly at the frontend with a burner. Generally, theburner gases or flue gases are fed ina counterflow to the direction ofproduct transport.Among the internally heated rotarykilns, there is another special kilndesign with a characteristic pendu-lum movement of the tube, which istherefore termed a pendulum kiln.The pendulum movement of thetube enables the burners or electricheating elements to be arrangedlengthwise over the top of the pen-dulum kiln. This allows the setting ofa selective temperature curve overthe length of the kiln, which is notpossible in the case of internallyheated rotary kilns. Fig. 1 shows a DRI rotary kiln withburner heating and temperaturetracking by means of pyrometers.The tube is mounted onto a supportframe for optimal adjustment of theangle of inclination to the bulk solidsto be transported. The tube is sealedat each end with so-called sliders,which also have suitable equipmentfor material feed and discharge. Asthe firing chamber and heat treat-ment chamber form one unit in therotary tube of this type of kiln, theprocess temperature profile is essen-tially defined by the selected burnerconfiguration and the form of theburner flame.Characteristics of directly heatedrotary kilns are:• Application temperature: 650–

1600 °C • Heating: fuel, direct • Kiln atmosphere adjusted by

means of the burner, oxidizing,inert, reducing

• Tube seal: mechanical seal, sealingair

• Mechanical effect on the product:high

• Throughput rate: medium to high,depending on the product

• Degree of automation: high,optionally available with fully auto-matic material feed and discharge.

For bulk solids that are, for example,prone to severe abrasion as a resultof the rotating flow in the tube orstart sticking during heat treatment,rotary kilns are generally not used,here tunnel kilns are favoured.

Tunnel kilnsTunnel kilns for thermal processingof bulk solids are used industrially indifferent models up to firing tem -peratures of 1800 °C. Typically, thekiln is named after the principle ofcontinuous conveying, for exampleby means of pusher plates, poweredroller conveyor or kiln cars.Tunnel kiln cars for the transport ofbulk solids through kilns are usedwhenever large throughput ratesmust be realized energy efficientlyand the feed material consists ofshaped products that may not beexposed to any additional mechan -ical load. Traditionally, bulk catalystsand ceramic grinding beads are sin-tered in this way. Fig. 2 shows a typ-ical car structure with deck. This isconfigured as a “setter tray” intowhich the green products are filledand then sent through the kiln. Theburners arranged at the side of thekiln fire below the deck and abovethe bulk solids. As a result, the bulksolids can be heat treated very se -lect ively and evenly. Especially in thecase of temperature-critical bulksolids, “dead burning” of the outerareas is avoided because tem pera -ture gradients depending on thebed height can be minimized. Tunnel kilns with car conveying aredesigned today primarily as auto-matic heat treatment plants with fill-ing and emptying equipment inte-grated in the car circuit. Characteristics of tunnel kilns withcar transport are:• Application temperature: up to

1800 °C • Heating: fuel, direct • Kiln atmosphere: adjusted by

means of the burner, oxidizing,inert, reducing

• Mechanical effect on the product:low

• Throughput rate: high• Degree of automation: high,

optionally available with fully auto-matic car loading and unloading.

Tunnel kilns with pusher plate trans-port are the first choice for the heattreatment of bulk solids especially

E 12 cfi/Ber. DKG 90 (2013) No. 4

atmo sphere and bulk solids proper-ties, metallic rotary tubes are usedup to 1150 °C. Ceramic tubes areused to protect the product againstcontamination and allow muchhigher temperatures than metaltubes. How ever, ceramic tubes arelimited with regard to the geometrywith which they can be fabricated.Especially for the use of protectivegas as the re act ive gas, tubes madeof graphite are used. In this case, thesealing of the rotary tube is especial-ly important to protect the kilnagainst leakage and minimize thenecessary process gas volume flow.Characteristics of indirectly heatedrotary kilns are:• Application temperature: 650–

1600 °C• Heating: electric or fuel, indirect

Process Engineering

Fig. 2 Tunnel kilnwith deck structurefor sintering bulk solids (factory photo: RiedhammerGmbH)

Fig. 3 Pusher plate kiln (factory photo: Riedhammer GmbH)

Fig. 1DRI rotary kiln (factory photo:RiedhammerGmbH)

when long kiln cycles with lowthroughput rates or reactive kilnatmospheres have to be realized for heat treatment. Fig. 3 shows atunnel kiln with pusher plate con-veying for debinding, calcining andsintering ceramic powders in ceram-ic setters up to 1650 °C. The kiln is operated with H2/N2 protectivegas [2]. With the sluice system at the entrance and exit, the consump-tion of the pro tect ive gas is mini-mized. The kiln atmosphere is introduced into the kiln in the transi-tional zone between high-tempera-ture sintering zone and cooling zoneand therefore supports both coolingand gas separation of the two zones.The bulk solid is transported in cover ed setters, which are stackedon the actual pusher plates. Appro-priate filling, cleaning and emptyingequipment for the setters is fre-quently integrated in the pusherplate circuit.Depending on the heat treatmentprocess, pusher plate kilns are pri-marily equipped with electric heat-ing in combination with inert kilnatmospheres, in which case no gastightness or lock system is generallyrequired. Characteristics of tunnel kilns withpusher plate conveying are:• Application temperature: up to

1650 °C • Heating: electric, fuel direct • Kiln atmosphere: protective gas or

adjusted by means of the burner,oxidizing, inert, reducing

• Mechanical effect on the product:low

• Throughput rate: low – medium• Degree of automation: high,

optionally available with fully auto-matic filling, emptying and clean-ing equipment.

If the heat treatment process, prod-uct and atmosphere control allowthe relatively heavy pusher plates tobe eliminated, tunnel kilns withpowered roller conveyors, so-calledroller kilns, can be used. This type ofkiln is used especially for short firingor sintering cycles and high through-put rates, and they boast very highenergy efficiency [3]. The bulk solidis filled into ceramic bowls or cru-cibles, which are then transportedthrough the kiln in one or more lay-ers by the roller conveyor. Depend-ing on the bulk weight of the prod-uct and the crucible size, usefulwidths of more than 2 m are gener-ally realized today. With the single-layer operation, veryhigh temperature accur acy is real-

ized over the useful cross-section,which leads to very homogeneousproduct quality especially in oxida-tion or reduction processes. In thegas-tight version, roller kilns are usedmainly for sintering metal powdersin N2/H2 protective atmospheres.Fig. 4 shows a roller kiln with auto-matic transport system for theceramic crucibles and setters. Withthe further development of theceramic roller materials, today heattreatment processes up to 1650 °Ccan be realized in roller kilns. In addi-tion flexible conveying systems canbe installed with appropriate drivepartition and control of the convey-ing speed, which allows completepro cess separation of the individualpro cess steps. Characteristics of roller kilns are:• Application temperature: up to

1650 °C• Heating: electric, fuel direct/indir -

ect• Kiln atmosphere: protective gas, or

adjusted by means of the burner,oxidizing, inert, reducing

• Mechanical effect on the product:low

• Throughput rate: medium-high• Degree of automation: high,

optionally with fully automatic fill-ing, emptying and cleaning equip-ment.

Intermittent kilnsThe big advantage of intermittentkilns is their flexibility. As a batchkiln, the intermittent kiln is used typ-ically where it is necessary to changefiring and atmosphere curves as wellas firing cycles as a result of differentproducts. The material to be firedand the insulated kiln chamberundergo the entire heat treatmentprocess together. These kilns are not usually protectivegas kilns. They are heated directlywith fuel by means of burners. Themost important kiln types are tophat kilns, elevator kilns, chamberkilns and shuttle kilns. The namegenerally depends on the way thebulk solids are fed into the kiln. Withthe integration of energy recupera-tion technology, especially forincreasing energy efficiency, it hasbeen pos sible to substantially reducethe trad itional disadvantage of inter-mittent kilns compared to continu-ous kilns. The development of theburner control and regulation sys-tems as well as the design programsfor simu lation of flue gas flows in theuseful chamber of the kiln have con-tributed to this.

cfi/Ber. DKG 90 (2013) No. 4 E 13

For heat treatment in shuttle kilns,the following technologies are avail-able today:• Burners with pulse firing• Modulating burner control • Individual burner control• Recuperator burner technology

(Reko).Moreover these kilns are generallyequipped with efficient heatexchangers for preheating of thecombustion air and with thermaland catalytic post-combustion sys-tems. Fig. 5 shows a shuttle kiln for multi-layer debinding and sintering of bulksolids and powders in crucibles orsetters with recuperator burner tech-nology and consequently at a 40-%lower energy consumption com-pared to conventionally equippedshuttle kilns.Characteristics of intermittent kilns(shuttle kilns) are:• Application temperature: up to

1800 °C • Heating: fuel direct • Kiln atmosphere: adjusted with the

burners, oxidizing, inert, reducing

Process Engineering

Fig. 4 Roller kiln for single-deck sinteringof bulk solids and powders in crucibles(factory photo: Riedhammer GmbH)

Fig. 5 Shuttle kiln with Reko burners (factory photo: Riedhammer GmbH)

and highly technical specificationsresult. To meet these specifications, a largenumber of suitable different thermalprocessing equipment is availabletoday. Firing tests in existing equip-ment or the specific development offiring technology in the test centre[3] have proven effective in enablingselection of the most appropriatethermal processing and plant engin -eering for energy-efficient heattreatment of bulk solids.

References[1] Becker, F.: Brennstoffbeheizte Kera -

mik öfen. Praxishandbuch Thermopro-zesstechnik. Bd. II, 2nd Ed. Essen 2011, 518 ff.

[2] Weber, H.: Einblicke in RiedhammerApplication Center (RAC) – Brenn-punkte der Ofen- und Prozessentwick-lung für Technische Keramik. cfi/Ber.DKG 89 (2012) [3] D8–D10

[3] Weber, H.: Energieeffizienz in der kera-mischen Industrie – Technische Ent-wicklungen im Ofenbau. Prozesswär-me – Energieeffizienz in der industriel-len Thermoprozesstechnik. Sonderpu-blikationen 2011, 204 ff.

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• Mechanical effect on the product:low

• Throughput rate: medium, cycle-dependent

• Degree of automation: high, op -tionally with fully automatic carloading equipment.

SummaryFor the heat treatment of bulk solids,the thermal processing equipmentmust be adapted to properties suchas flow behaviour, flowability andthe mechanical properties of thebulk solids. In addition, temperaturecurve and firing time as well asatmosphere control determine theprocess cycle. Accordingly, complex

Process Engineering

Hartmut Weber, Andreas Hajduk,Ralf FinkRiedhammer GmbH90411 Nuremberg, GermanyE-mail: mail@riedhammer.de www.riedhammer.de