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The sealing system is installed between the rigid housing components (feed and discharge cars) and the rotating tube and

the type of heating between directly heated rotary kilns (Fig. 1), and indirectly heated rotary kilns (Fig. 2).

Indirectly heated rotary kilns

In indirectly heated rotary kilns, the rotary tube is heated from outside. This can be ef-fected by means of burners or electric heat-ing. The indirectly heated model is used es-pecially when the treated product is prone to strong dust formation, has to go through a defined temperature profile or has to be treated in special process conditions, e.g. inert or protective atmosphere. As tube materials, depending on the process requirements with regard to maximum tem-perature, atmosphere and bulk solids prop-erties, metal tubes are used up to 1150 °C. Ceramic rotary tubes are used to protect the product against contamination and permit much higher temperatures (up to 1600 °C) than metal tubes. However, ceramic tubes are limited with regard to their geometry in manufacturing. This also applies to the ap-plication of tubes made of graphite.Fig. 3 shows the 3D model of a DRA rotary kiln. The actual rotary tube is supported on rollers by means of two riding rings. It is driven by means of a friction wheel or gear ring/chain. The product is fed from the left by means of an infeed screw conveyor or vibrofeeder. The tube is sealed at both ends by means of mechanical axial seals, which seal off the kiln chamber atmosphere in the tube (product chamber) against ambient air.

General

The key feature of rotary kilns is that during heat treatment the bulk solids are constant-ly moved through the kiln from the feed to the discharge end as a result of the rotation of the slightly inclined tube. The kilns are generally operated continuously and fully automatically. They allow direct transport of the bulk solids without further conveying equipment, which makes these kilns very energy-efficient [4].As the rotary kiln process has a significant mechanical impact on the bulk solids, rotary kilns must be specifically designed for the materials treated in them. As parameters characterizing the bulk solids behaviour, their flow behaviour, flowability, bulk den-sity, moisture content and process tempera-ture have to be taken into consideration and provision made for these in the design and engineering of the heat treatment equip-ment. Adaption to the specific bulk material is effected by variation of the inclination of the tube along its length, the rotary speed of the tube and, if required, additional com-ponents installed inside the tube.In the ceramics, glass and binder industries, rotary kilns are used today for the thermal treatment of powder, granular, lumpy and dry or wet mixes. The main applications are drying, debinding, presintering, calcining, foaming and sintering. In the following, key rotary tube kiln pro-cesses are presented with reference to typi-cal applications for different atmospheres. A basic differentiation is made depending on

Rotary Kilns – Processing Equipment for the Heat Treatment of Bulk Solids H. Weber

Rotary kilns are frequently used for the thermal treatment of bulk solids used in daily life. Their applications extend from chemical and pharmaceutical products, such as pills and pigments, building materi-als, such as cement, mining and smelting materials, such as ores, coal and coke, to catalyst substrates, abrasives and battery materials. In each case the heat treatment process must be adapted to the respec-tive type of bulk solids. Today, in addition to a wide range of different industrial furnaces, principally rotary kilns are used [1–3].

Hartmut Weber

Riedhammer GmbH

90411 Nürnberg

Germany

E-mail: hartmut.weber@riedhammer.de

www.riedhammer.de

Keywords: heat treatment, calcination,

sintering, rotary kiln, pendulum kiln,

protective gas atmosphere, pyrolise,

indirect heating, direct heating

Fig. 1 DRI rotary kiln heated directly with gas burners (Company photo: Riedhammer GmbH)

Fig. 2 DRA indirectly heated rotary kiln (Company photo: Riedhammer GmbH)

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designed to be gastight in these applica-tions. Another model of kiln is the directly heated rotary kiln.

Directly heated rotary kilns

Directly heated rotary kilns are heated with a burner firing directly into the rotary tube. For this reason, this type of kiln is termed an internally heated rotary kiln or DRI. Directly heated rotary kilns generally con-sist of insulated kiln feed and discharge cars and a rotary tube generally lined with refractory insulation. The insulation serves primarily as thermal protection, must, how-ever, be designed far more as wear protec-tion or as an inert protective layer between tube and bulk solids [4]. Rotary kilns of this type are heated directly with a burner at the front. Generally, the burner gases and flue gases are led in the opposite direction to the transport of the products. Fig. 6 shows a schematic of the structure of a directly heated rotary kiln. The material is fed from the left by a feeder in the feed car. At the other end is the burner in the discharge car. The flue gases flow towards the product coming along the length of the inclined rotary tube. Accessed via a shaft with chute, the cooling tube is arranged under the discharge car. Directly heated ro-tary tubes are also used when, for example, the product is fed as slip or slurry. In this case, as shown here, a predryer is installed upstream of the rotary tube with refractory

ers developed within the SACMI Group and therefore offer a very economic and highly energy-efficient alternative to the burners established on the market. After the heat treatment in the heated rotary tube, gen-erally a cooling process follows to cool the product to approximately room tempera-ture. Fig. 3 shows a rotary kiln in which the di-rectly heated rotary tube is connected with the cooling tube by means of a chute. Fig. 4 shows a rotary kiln with a cooling tube directly flanged onto it. Here, the di-rection of product transport is from right to left here. Such a kiln concept is used espe-cially when condensation on cold surfaces must be avoided during the heat treatment process. In multistage rotary kiln processes as generally applied in material recycling or pyrolysis processes, the kiln atmosphere contains high loads of highly and medium bituminous volatile constituents. To reliably avoid condensation, the insu-lated kiln feed and discharge cars are addi-tionally heated. Moreover, the product is fed and discharged via lock systems to reliably prevent the ingress of air into the tube and avoid entrainment of the process gases into the cooling zone with the risk of condensa-tion. In addition it is important to seal the rotary tube against leakages so that often the feeding and discharging equipment is

pressed against this accordingly. With the selection of the optimum sealing material, any frictional forces can be influenced to a large extent. For inert gas or reactive gas processes in the rotary tube, a sealing gas is generally introduced to support the sealing system. For processes in air atmosphere, a cheaper radial seal is frequently used.Inside the rotary tube, depending on the process necessities and product behaviour of the bulk solids, additional components can be installed [5]. These are used not only to support product transport, but especially to improve heat transfer and mixing of the product flow, for example, lifters, which en-sure cascading of the material. Additional baffle rings, for example, are used at the kiln exit to increase the filling level of the easy-flowing bulk solids in the kiln. The heating equipment is installed in an in-sulated casing surrounding the rotary tube. Heating is effected by means of burners, which, corresponding to process require-ments, are divided into several control groups over the tube length and flush the rotary tube with flue gases to obtain spe-cific temperature profiles [4].The use of recuperator burners is an option for indirectly heated rotary kilns to selec-tively use the energy of the flue gases for preheating the combustion air at the burn-ers. Riedhammer can install the RKO burn-

Fig. 3 DRA indirectly heated rotary kiln with separate cooling tube (3D model: Riedhammer GmbH)

Fig. 4 DRA indirectly heated rotary kiln with directly flanged cooling tube (3D model: Riedhammer GmbH)

Fig. 5 Discharge end IDO 10, DRA indirectly heated rotary kiln with the directly flanged cooling tube, maximum firing temperature: 1150 °C; feed-dependent throughput rate up to 1000 kg/h; kiln length: 10 m; heated length: 7 m (Source: IBU-tec advanced materials AG/DE)

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specific temperature curve can be set over the kiln length [2]. Fig. 8 shows a comparison of the tempera-ture profile with associated dwell time at maximum temperature for a directly heated rotary kiln and a corresponding pendulum kiln. In the directly heated rotary kilns, fir-ing chamber and heat treatment chamber form one unit, and the process temperature profile is essentially determined by the se-lected burner configuration and the form of the burner flame. In the pendulum kiln, this is not the case, the process tempera-ture profile is, as in the indirectly heated rotary kiln, selectively determined by means of individual heating groups, as a result of which a specific temperature plateau can be realized.

Summary

For the heat treatment of bulk solids, di-rectly and indirectly heated rotary kilns are

insulation. This can also be equipped with internal components, such as chains or scrapers, to avoid or remove any deposits formed. Fig. 7 shows the discharge end of an indi-rectly heated DRI rotary kiln with the burner system and the pyrometers necessary for monitoring the temperature [2].Directly heated rotary kilns for technical ce-ramics and advanced materials can be used for temperatures up to 1650 °C.

Pendulum kilns

Among the internally heated rotary kilns, there is an additional model characterized by an oscillating tube motion and therefore called a pendulum kiln (Fig. 8). The pendu-lum motion of the tube with a deflection of around 120° enables the burners or electric heating elements to be arranged over the length of the kiln ceiling. As a result, un-like in the internally heated rotary kilns, a

Fig. 7 DRI directly heated rotary kiln (Company photo: Riedhammer GmbH)

Fig. 8 Dwell time versus temperature in rotary and pendulum kilns (Schematic: Riedhammer GmbH)

Fig. 6 Directly heated DRI rotary kiln with flange-mounted predryer and separate cooling tube (Schematic: Riedhammer GmbH)

available, the process engineering of which can be adapted to the complex bulk solids properties, such as flow behaviour, flowa-bility and the mechanical properties of the bulk solids.

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[3] Hajduk, A.: Moderne Trends in der Wärmebehan-

dlung von hochentwickelten Materialien. cfi/Ber.

DKG 92 (2015) [4] D 15–18

[4] Weber, H.: Energieeffizienz in der keramische In-

dustrie – Technische Entwicklungen im Ofenbau.

Prozesswärme – Energieeffizienz in der indus-

triellen Thermoprozesstechnik. Sonderpublika-

tionen 2011, 204 ff.

[5] Weber, H.: Einblicke in Riedhammer Application

Center (RAC) – Brennpunkte der Ofen- und

Prozess-Entwicklung für Technische Keramik. cfi/

Ber. DKG 89 (2012) [3] D 8–10

and products. Finally, key features of rotary kilns are maximized availability, minimized space and labour requirement and excellent cost efficiency.

References

[1] Becker, F.: Brennstoffbeheizte Keramiköfen. Prax-

ishandbuch Thermoprozesstechnik. Band II, 2.

Aufl., Essen 2011, 518 ff.

[2] Weber, H; Hajduk, A; Fink, R.: Thermo-Prozessan-

lagen für die Wärmebehandlung von technis-

chen Schüttgütern. cfi/Ber. DKG 90 (2013) [4]

D 11–13

Rotary kilns are robust production plants with high throughput rates, which permit selective adjustment of temperature and dwell time and, in gastight design, selective control of the atmosphere. Rotary kilns are very energy-efficient as no additional transport aids, like setters, or handling equipment are necessary, they en-able excellent heat transfer, and very short dwell times. In ceramics, rotary kiln technology has re-peatedly proven itself as a starting technol-ogy for new and innovative applications

Fig. 9 DP pendulum kiln directly heated with burners with predryer and downstream cooling tube (Schematic: Riedhammer GmbH)

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