Pulp & paper


    In plants with a predryer, the predryed material in the form of filter cakes is fed into the kiln, where it is dried in a counterflow of hot kiln gas. Due to the kiln's geometry, refractory bricks and concretes are used, together with suitable thermal insulation.

    Kiln inlet cone
    The inlet cone prevents a reverse flow of the material, and is lined with abrasion-resistant concrete.

    Rotary kiln
    The rotary kiln is lined with refractory bricks. Refractory concretes are used for lining cones and installations such as chains, lifters or lifting flights.
    In plants without a predryer, the material is fed into the kiln as sludge with a high water content. Evaporation of the water is intensified in the chain zone, whereby the chains dip into the sludge and as they rise up again, the attached sludge is dried in the counterflow of hot gas. Moreover, this technology acts as a dust filter to a certain degree. Dust adheres to the moist material, and thereby becomes part of the clinker. For the purpose of better heat transfer, lifters or lifting flights raise the material and let it fall through the hot gas flow. These lifting devices can be metallic and/or ceramic. In order to keep the heat losses low, the lining in the rotary kiln usually consists of two layers. The actual refractory bricks are installed on a layer of insulating bricks. Hereby, the lining thickness mostly depends on the process, and also on the kiln diameter. Plants that are mechanically and/or thermally highly stressed rely on a single-layer lining without insulation.

    Preheating zone

    This zone serves to dry the material at temperatures between 600 and 800 °C. The length/diameter ratio of this zone is 4...5 x d. Fireclay bricks with low heat conductance are used here.

    Calcining zone
    Calcination (deacidification) of the material takes place in this zone along a length of 2...3 x d. About 98 % of the calcination process is carried out in this zone, where the temperature is about 1100 °C. The products required here must exhibit properties that are a balance between refractoriness, abrasion resistance, and low thermal conductance.

    Combustion zone
    Final calcination takes place in this zone at temperatures between 1100 and 1300 °C. The zone length is 4...6 x d. As this zone is subjected to the highest thermal stress, the products used here must exhibit high refractoriness and good thermochemical resistance. Hereby, the stresses in the zone can be in contradiction to the demand for low thermal conductance.

    Cooling zone
    The material remains in the kiln over a length of about 2 x d before being discharged. The temperature here is about 1000 °C.

    Kiln hood
    The burner is located in the kiln hood, which seals the kiln from the outside. Refractory concretes are used here, because of the geometry in this area. Depending on the installation method, either grouting or gunning concrete is used. The temperature here is about 1000 °C.

    The material is cooled in a counterflow of fresh air, whereby the heated air is used as combustion air in the kiln. As in the kiln hood, refractory concretes are also installed here. Abrasion resistance is an essential requirement for the refractory material used here, as well as good thermal shock resistance.
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