SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES

Refractory Lining of Cement Kiln Systems

2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES

With regard to the requirements on refractory materials, the kiln system can be subdivided into various zones according to specific operating conditions (Fig. 1). The designation of the various zones refers to the respective reactions in the burning process which, however, are anything but clearly defined as they overlap in both directions. The point, at which the change from one refractory quality to another is made can usually be determined only by observing the behaviour of the lining in operation.

Fig. 1

The main points of the following explanations are also summarised in Table 1.

Table 1

BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.1 Drying Zone (applies only for wet and semi-wet process kilns)

2.1 Drying Zone (applies only for wet and semi-wet process kilns)

In the drying zone, the water content of slurry or nodules is evaporated. This reaction is almost terminated at material temperatures of 100°C.

In wet process kiln, the drying zone is generally a part of the chain zone. In grate pre-heater kilns, drying takes place in the first pre-heater chamber.

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 2. SUBDIVISION OF THE BURNING PROCESS AND SELECTION OF REFRACTORY QUALITIES / 2.2 Chain Zone

2.2 Chain Zone

In wet process kilns, the chain zone can be subdivided into a drying and a preheating zone. The

material temperature at the outlet of the chain zone depends on arrangement and qualities of chains as well as type of system, i.e. nodule or dust kiln, and is in the order of magnitude of 200 to 400°C.

The length of the chain zone ranges from 7 to 9 kiln diameters in wet kilns and from 3 to 5 kiln diameters in long dry kilns.

The lining of the chain zone should be of a dense, low alumina firebrick with high abrasion resistance to withstand the abrasive action of the chains.

An air setting mortar should be used as there is insufficient temperature to vitrify a heat setting mortar. In wet kilns, furthermore, the mortar must be water resistant.

Since the arrangement of the chain hangers requires extensive modification work, sometimes dense fireclay castables are installed, which, however, should be carefully compacted in order to obtain high abrasion resistance.

2.3 Preheating Zone

In the preheating zone, hydrate-water is driven off and the raw material is heated up to approx. 700°C. The length of the preheating zone in long kilns is approx. 4 to 8 kiln diameters (excluding chain zone). In short pre-heater kilns the preheating zone is a part of the pre-heater.

Regarding the lining of preheaters, reference is made to paragraph 4.1: Lining of Suspension and Grate Preheaters.

The preheating zone of long kilns is usually lined with low alumina firebricks or, for better thermal insulation, with lightweight firebricks. With respect of lining stability, two layer lining is generally not recommended in rotary kilns of more than 3.5 m diameter.

2.4 Calcining Zone

The calcining reaction starts already at material temperatures below 600°C and is completed at approx. 1200°C. Since, however, the most part of calcination takes place between raw material temperatures of 700 to 900°C, usually this range is designated as calcining zone.

short pre-heater kilns calcining takes partially place in the pre-heater. There, the length of the calcining zone in the kiln amounts to approx. 4 to 6, in kilns with precalciners 1 to 3 kiln diameters.

From the refractory point of view, the calcining zone can still be lined with normal fireclay bricks or, for better insulation, with fireclay lightweight bricks. In case of alkali attack good operational results have been achieved with acidic light weight bricks with an SiO2 content above 65%. These bricks form with alkalis a vitreous layer of 2 to 3 mm thickness on the surface, which prevents the further alkali

infiltration.

Two layer linings are generally not recommended. There are, however, two layer bricks on the market, consisting of a dense working part and a porous insulating part. Such bricks are generally installed for heat saving purpose, if lightweight bricks show unsatisfactory operating results.

2.5 Transition Zone

The transition zones are located on both sides of the sintering zone. Since the length of the sintering zone varies with process fluctuations, the transition zones are characterised in particular by unstable coating formation.

Usually, the inlet-side transition zone is further subdivided into a colder section, so-called safety zone, and into a hot section, the proper transition zone.

The safety zone has a length of approx. 2 kiln diameters and is usually lined with alumina rich bricks with Al2O3 content of 50 to 60%. The bricks must have good thermal shock resistance and low porosity in order to have good resistivity against chemical attack. The application limit of alumina rich or high alumina bricks in the transition zone is generally determined by eutectic reactions in the system Al2O3 - CaO - SiO2 or by alkali-spalling. In such cases, lining lifetime can be improved only by installing basic bricks.

The transition zones are often exposed to considerably higher loads than the sintering zone itself. Quite often no or only unstable coating is formed. Thus, the bricks are exposed directly to the flame radiation and considerable temperature variations. The lengths of the transition zones vary from 2 to 4 kiln diameters.They are usually lined with chrome free magnesia-spinell bricks produced of very pure, synthetic materials or with magnesia-chrome bricks containing approx. 60 to 70% MgO.

2.6 Sintering Zone

Although this zone is often referred to as burning zone preference is given to the expression sintering zone on account that this better describes the mechanism of reactions taking place.

The sintering zone is generally covered with a more or less stable coating, formed with clinker and liquid phase. Liquid phase starts to form at raw material temperatures above approx. 1250°C. However, as the lining surface temperature is higher than the one of the raw material, coating formation starts already at raw material temperature above 1050 to 1150°C.

The term ‘sintering zone’ can also be explained as zone of increased material diffusion with formation of C3S modifications, the latter starting at temperatures above approx. 1100°C.

The maximum material temperature in the sintering zone is in the order of magnitude of 1400 to 1500°C at the beginning of the cooling zone.

The length of the sintering zone is generally between 3 to 5 kiln diameters and depends largely on the shape of the flame and type of fuel. Coal flames give generally short, oil flames medium and gas flames longer sintering zones. In kilns with precalciner, the sintering zone length amounts to 5 to 8 kiln diameters due to the higher specific material throughout.

The bricks in the sintering zone are exposed to chemical attack by the liquid phase of the clinker and by alkali sulphates, high temperatures and, depending on coating stability, high thermal shocks. These conditions are best met by basic bricks due to their high refractoriness and good resistance against chemical attack.

Thus, generally, chrome free magnesia-spinell bricks, magnesia-chrome or dolomite bricks are

installed. When using chrome-free magnesia-spinell bricks, qualities particularly developed to improve coating adhesion should be chosen (qualities based on natural sinter).

Dolomite bricks have generally good operating performance in zones with coating formation. The price for dolomite bricks is only approx. 60% of that of magnesite. A disadvantage of dolomite bricks is, however, its sensitivity to moisture. Thus, for longer storing times, these bricks are to be stored air-tight. During longer kiln stops the dolomite lining is to be protected against moisture by tightening the kiln tube and putting a hygroscopic agent (e.g. burned lime) in the kiln.

Raw material analysis and tests can give some indications on selection of brick quality to be installed. The final decision, however, is often to be based on operating experience, i.e. by trial and error.

Insulation of the burning zone with insulating back-lining is generally not recommended. By insulating, basically the hot face temperature of the bricks will be increased, resulting in reduced coating thickness and higher chemical and thermal load of the bricks. Furthermore, two layer lining is less stable and leads, particularly in big kilns, often to early failure due to relative movement and loosening of the lining.

In cases, where a coating does not form, insulation may be helpful in reducing heat losses and protecting the kiln shell, particularly in the tyre area. In such specific cases basic bricks with

back-linings of 40 mm hard fireclay slabs are sometimes installed. However, generally installation of two layer linings is not recommended due to reduced lining stability.

2.7 Cooling Zone

The cooling zone in the rotary kiln reaches approx. from the burner nozzle to the kiln outlet. In this zone, the clinker is cooled down from its maximum burning temperature of 1400 to 1500°C to approx. 1350°C in kilns with grate, rotary or shaft coolers and to approx. 1250°C in kilns with planetary coolers. The cooling zone in kilns with planetary coolers has a length of approx. 1.5 to 2 kiln diameters. It consists generally of a cam lining for better cooling efficiency and a dam ring for equalising of clinker distribution to the individual cooler inlet openings. For camlining and damring, dense high alumina bricks with approx. 80% Al2O3 and considerable thermal shock resistance are generally used. The backing of the damring can be made of hard fireclay bricks with adequate mechanical strength.

The length of the cooling zone of kilns with grate, rotary or shaft coolers is generally 0.5 to 1 kiln diameter. It is usually lined with dense, abrasion resistant high alumina bricks containing 80% Al2O3. In case of heavy chemical attack it can be necessary to line this zone with basic bricks, which however, should have high thermal shock resistance (magnesia-spinell bricks). Since the outlet zone is often free of coating, kiln shell temperature will increase due to the higher thermal conductivity of basic bricks. Due to the high thermal and mechanical load of the bricks in this zone, insulating back-lining is

not recommended.

The end of the cooling zone, the nose ring, is one of the most critical points of cement kiln linings. Its lifetime is often lower than the one of high loaded sintering zones. Shape and quality of nose ring bricks requires therefore careful consideration.

Basically, complicated special shapes should be avoided since special shaped bricks are often

‘hand-made’ and can have a much poorer quality than machine made bricks of the same composition. Nose ring design according to Fig. 2a and 2b would allow to use standard brick sizes with minor modifications and is to be preferred against the design according to Fig. 2c.

Fig. 2a Nose Ring Design Using Bricks

Fig. 2b Nose Ring Design Using Bricks

Process Technology / B05 - PT II / C03 - Refractory Linings / Refractory Lining of Cement Kiln Systems / 3. IMPORTANT FEATURES OF REFRACTORIES INSTALLATION

In document Cement Industry Process Technology - Holderbank Course (2 of 3) (Page 142-147)