A Rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns include: Cement. Lime.

1. 1
Koya University
Faculty of Engineering
Chemical Engineering Department
Chemical Industry
Rotary kiln
Preparation By:
Aree Salah
Alan mawlud

2. 2
List of content:
Abstract………………………………..……………………..3
The history of the rotary kiln...…………………………..….4
The history of cement industry …………………..……….5-6
Introduction ………………………………………...……..7-8
Rotary Kiln Processes …………………..…………………...9
Wet and Dry Processes………....................................10-11-12
The clinker cooler..................................................................13
Thermal profile and kiln subdivisions ……..……....14-15-16
Discussion..…………………………………........17-18-19-20
References………………………………………………......21

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ABSTRACT:
This work presents the simulation of a rotary kiln used to produce cement
clinker. The effort uses an original approach to kiln operation modeling. Thus,
the moving cement clinker is accurately simulated, including exothermal
reactions into the clicker and advanced heat transfer correlations. The
simulation includes the normal operation of a cement kiln, using coal in an air-
fired configuration. The results show the flame characteristics, fluid flow,
clinker and refractory characteristics. Two types of coal are employed, one with
medium-volatile and one with low-volatile content, with significant differences
noted in the kiln operation.A specific goal of this effort is to study the impact of
oxygen enrichment on the kiln operation. For this purpose, oxygen is lanced into
the kiln at a location between the load and the main burner, and the impact of
oxygen enrichment on the kiln operation is assessed. Different oxygen injection
schemes are also studied. Thus, varying the angle of the oxygen lance enables to
handle various problems as reducing conditions, overheating in the burning
zone or refractory wall. It is concluded that oxygen has a beneficial role in the
fuel combustion characteristics, and its impact on refractory temperature and
the clinker is negligible, in conditions of increased productivity and overall
efficiency.The paper presents the impact of dust insufflation into the kiln, such
as reduced temperature profile, resulting in a less stable combustion process.
The work shows the beneficial influence of oxygen enrichment on kiln operation
in the presence of dust, leading to an increase in the amount of dust capable of
being insufflated into the kiln.The paper presents the impact of dust insufflation
into the kiln, such as reduced temperature profile, resulting in a less stable
combustion process. The work shows the beneficial influence of oxygen
enrichment on kiln operation in the presence of dust, leading to an increase in
the amount of dust capable of being insufflated into the kiln.

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The history of the rotary kiln:
About 1900, various metallurgists were experimenting with the rotary kiln for
nodulizing flue-dust, fine iron ores, etc. Edison conducted experiments, for
example, on the fine concentrates obtained from his magnetic separators. Within
a few years plants were established for this purpose. The rotary kiln also
furnished a simple means of utilizing the soft clayey ores, such as that of the
Mayari field in Cuba. Practically all of the schemes tried for placing this ore in
satisfactory condition for the blast furnace were unsatisfactory until the rotary
kiln was tried. The plant in Cuba consisted of twelve kilns 30 m long and 3 m in
diameter and producing 1500 - 2000 tonnes per day. In 1914 application of the
rotary kiln for the partial roasting of copper sulfide concentrates containing
appreciable amounts of pyrite, to decrease the sulfur content before charging to
the reverberatory furnace was conducted in USA. The kiln was 2 - 2.5 m
diameter and 5 - 8 m long laid horizontally and operated batch-wise. In later
design the inclined kiln was used; the charge was introduced at the burner side
with provision of introducing secondary air through a pipe in the center of the
kiln (Fig. 8). At present, rotary kilns are used for drying ores and the production
of alumina by the dihydroxylation of Al(OH)3, reduction of iron oxide by the
Krupp–Renn process, in the TiO2 pigment manufacture, and other processes
(Fig. 9).

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The history of cement industry:
In 1885 a continuous reactor was needed to replace the shaft furnace which was
operated batch-wise. The shaft furnace was used for making cement clinker and
was borrowed from the limestone calcination industry, which was usually
known as lime kiln. Since the process was operated batch-wise, at the end of
heating the charge, the kiln was allowed to cool and the product raked out.
Naturally, this was a wasteful process due to the consumption of large amounts
of fuel. The rotary kiln was adopted by cement manufacturer in 1824 as soon as
Joseph Aspdin (1788-1855), the brick-layer and mason in Leeds, England
discovered what he called Portland cement1. Although Portland cement had
been gaining in popularity in Europe since 1850, it was not manufactured in the
US until the 1870s. The first plant to start production was that of David O.
Saylor at Coplay, Pennsylvania in 1871. In 1885, an English engineer, Frederick
Ransome, patented a slightly titled horizontal kiln which could be rotated so that
material moved gradually from one end to the other. Because this new type of
kiln had much greater capacity and was heated more thoroughly and uniformly,
it rapidly displaced the older type. In 1880, about 42 000 barrels of Portland
cement were produced in the United States; a decade later, the amount had
increased to 335 000 barrels. One factor in this tremendous increase was the
development of the rotary kiln.

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In 1888, Fredrik L‫و‬ssِe Smidth (1850–1899) (Fig.
4), Danish engineer and industrialist in
Copenhagen, in association with two other Danish
engineers, Alexander
Foss and Paul Larsen, delivered the first cement
plant to a manufacturer in Sweden. In 1898, he was
the first to introduce the rotary kiln in the cement
industry and
became later one of the major suppliers of rotary
kilns worldwide. Thomas A. Edison (1847-1931)
(Figs 5 and 6 ), the American inventor, was a
pioneer in the further development of the rotary kiln in his Edison Portland
Cement Works in New Village, New Jersey where he introduced the first long
kilns used in the industry 46 m long in contrast to the customary 18 to 24 m. In
1902, together with José Francisco de Navarro (1823–1909) (Fig. 7) founded the
Universal Atlas Portland Cement Company whose largest plant was in
Northampton, PA and won the enormous contract for supplying cement for the
Panama Canal. By 1904, Navarro became the largest cement manufacturer in the
world, producing 8 million barrels per year. Today, some kilns are more than
150 m long. The increased production of cement due to the use of efficient
rotary kilns has a parallel improvement in crushing and grinding equipment.

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Introduction:
Rotary kiln is a machine whose working temperature can reach the
temperature to calcine superfine kaolin. At present the rotary kiln
technology in our country is mature and advanced, which represents the
development direction of calcination technology of superfine kaolin. This
calcinations technology has low energy consumption and high output,
and after dehydration and decarburization and whitening, the products
have stable performance and can be used in such industrial fields as
paper making and coating.
The cement rotary kiln produced by Hongxing Machinery has simple and
solid structure, stable operation, convenient and reliable control of the
production process, fewer quick-wear parts, high quality of final products
and high running rate, so that it is the equipment for cement plants to
calcine high quality cement and it is also widely used in metallurgy,
chemistry and construction industry. In addition, Hongxing Machinery is
able to provide customers with highly efficient vertical-cylinder preheater
and five-star cyclone preheater.

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According to the types of materials to be processed, rotary kiln can be
divided into cement kiln, metallurgical chemical kiln and limestone rotary
kiln. Rotary cement kiln is mainly used for calcining cement clinker and it
can be divided into two types, namely dry type production cement kiln
and wet type production cement kiln. Metallurgical chemistry kiln is
mainly used for the magnetizing roasting of the lean iron ore and the
oxidizing roasting of the chromium and josephinite in the steel works in
the metallurgical industry, for the roasting of high alumina bauxite ore in
the refractory plant, for the roasting of clinker and aluminium hydroxide in
the aluminium manufacturing plant and for the roasting of chrome ore in
the chemical plant. Limestone kiln is mainly used for roasting active lime
and lightly calcined dolomite used in the steel works and ferroalloy
works.
The cement equipments with various types produced by Hongxing
Machinery including rotary cement kiln and rotary kiln have reasonable
price and high quality, and we can design the product manufacturing
scheme according to your specific needs. If you want to learn more
about cement equipments, feel free to contact Hongxing Machinery, and
we will serve you with heart and soul.

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Rotary Kiln Processes:
With the arrival of rotary kilns, cement manufacturing processes became
sharply defined according to the form in which the raw materials are fed to the
kiln. Raw materials were either ground with addition of water, to form a slurry
containing typically 30-45% water, or they were ground dry, to form a powder
or "raw meal".
1. In the Wet Process, the kiln system is fed with liquid slurry, the water
then being evaporated in the kiln.
2. In the Semi-Wet Process, raw material is prepared as a slurry, but a
substantial proportion (50-80%) of the water is mechanically removed,
usually by filtration, and the resulting "filter cake" is fed to the kiln
system.
3. In the Dry Process, the kiln system is fed with dry raw meal powder.
4. In the Semi-Dry Process, a limited amount of water (10-15%) is added to
dry raw meal so that it can be nodulised, and the damp nodules are fed
to the kiln system.

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Wet and Dry Processes:
With the arrival of rotary kilns, cement manufacturing processes became sharply
defined as wet process or dry process.
1. In the Wet Process, the kiln system is fed with a rawmix in the form of a
liquid slurry, typically containing 30-50% of water by mass.
2. In the Dry Process, the kiln system is fed with a rawmix in the form of a
dry powder.
The process selected depends to a certain extent upon the nature of the available
raw materials.
At the start of the twentieth century, both the American and the British
industries were highly concentrated geographically. The British industry was
concentrated in the Thames and Medway estuaries, and the epicentre of the
American industry was the Lehigh and Delaware valleys in eastern Pennsylvania
and north-west New Jersey. The Cambrian argillaceous limestones of the
Jacksonburg Formation in that area are hard rock, most readily processed by dry
grinding. This fact provided a further impetus to the development of rotary kilns,
since for shaft kilns, a powdered rawmix must be briquetted in a more-or-less
expensive pressing process, whereas untreated powder can easily be fed to a
rotary kiln. It is for this reason that all the original American rotary kilns used
the dry process. The wet process gradually developed, initially in more remote
wet raw material regions such as the marl belt of central Michigan. Later, the
wet process came to be used in Pennsylvania mainly because of the ease of wet
blending, but the majority of kilns continued to use the dry process throughout
the twentieth century.
In Britain the situation was quite different. In the Thames and Medway areas,
dry process raw material preparation was practically impossible. The wet chalk
(typically 40% water by volume) can't be ground to a powder until it has been
dried, but the un-ground chalk can't easily be dried because its spongy texture
tenaciously retains water. On the other hand, wet-grinding it with water is
trivially easy. Where chalk marl was available in the southern part of the
Medway valley, a dry process developed using brick-making techniques,
allowing shaft kilns to be used in the period 1900-1928, but this was a marginal
technology because of the poor homogeneity of the brick “pug”. So with the
arrival of rotary kilns, wet process was initially the universal choice.

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Wet Process: Dry Process:
In the parallel wet and dry processes in America, the dry process was marginally
more energy-efficient, but the differential was small due to the lack of good heat
exchange in the kiln – a dry kiln simply produced hotter exhaust gas. The early
short kilns (length : diameter ratio 12:1 or less) were troublesome on wet
process because the hot and over-fuelled conditions of operation necessary to
complete all burning stages in a short length led to high dust loss and emissions
of black smoke. It was early appreciated by the more scientific practitioners that,
at least in theory, the dry process ought to be more efficient. It is characteristic
that the first British dry process rotary kilns were installed by A. C.
Davis at Norman (1904), and Davis promoted the system with an evangelism
that flew in the face of the objective facts. Having started in the industry by
constructing Saxon (1901) with Schneider kilns fed with dry-ground and
briquetted Chalk Marl, his business strategy was to run flat out, selling at or
below cost price, and generally spreading alarm and despondency among the
“old fashioned” manufacturers by suggesting that his costs were half of theirs,
which they may indeed have been. With the arrival of rotary kilns, he naturally
continued the same behaviour, by publicising his use of dry process as an
economy that others could not match. To drive home the point, he installed no
less than five kilns at Norman – a larger installation than at any of the other
independent companies at this stage.

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The Norman installation was described in great detail in an article in The
Engineer. The kilns were 60 ft long, and of low LD ratio: only 9.61:1. They
were supplied byFellner & Ziegler who also supplied APCM. Whereas the marl
at Saxon was dried in a coal-fired Smidth drier, at Norman, it was dried in rotary
driers heated by the kiln exhaust gases. The article includes a lengthy
description of the raw meal mixer. This was a complex mechanical device with
action equivalent (in theory) to a blending silo operating in “overflow” mode. In
the light of later experience, it would in all probability have spent much of its
time blocked solid, and therefore allowing run-of-mill meal to go straight to the
kiln feed silo. The design makes it clear that rawmix blending was already
understood to be the major stumbling-block in the dry process, and that the
technical challenge was at this stage a long way from being solved. The
perceived success of the Norman kilns was sufficient to persuade several other
plants to embark on rotary kilns using the dry process, but as kilns developed,
the vast majority were wet process.

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The clinker cooler:
There are various types of cooler - we will consider only one, the 'grate cooler'.
Grate cooler: the hot clinker falls out of the kiln and moves along the cooler,
towards the foreground of the image.
The purpose of a cooler is, obviously, to cool the clinker. This is important for a
several reasons:
 From an engineering viewpoint, cooling is necessary to prevent damage
to clinker handling equipment such as conveyors.
 From both a process and chemical viewpoint,
it is beneficial to minimise clinker
temperature as it enters the cement mill. The
milling process generates heat and excessive
mill temperatures are undesirable. It is
clearly helpful, therefore, if the clinker is cool
as it enters the mill.
 From an environmental and a cost viewpoint,
the cooler reduces energy consumption by
extracting heat from the clinker, enabling it
to be used to heat the raw materials.
 From a cement performance viewpoint, faster
cooling of the clinker enhances silicate
reactivity.
The cooled clinker is then conveyed either to the clinker store or directly to the
clinker mill. The clinker store is usually capable of holding several weeks'
supply of clinker, so that deliveries to customers can be maintained when the
kiln is not operating.

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Thermal profile and kiln subdivisions:
The rotary kiln thermal profile varies throughout its length, depending on the
temperature and chemical reactions involved during the process (see in Table
1).
The rotary kiln can be subdivided into several zones or regions that are exposed
not only to thermal and chemical wear but also to mechanical stresses. The
influence of one or several of these factors, to minor or greater proportion
determines the refractory lining type required for each zone:
• Decarbonation zone: from 300ºC to 1000°C (+)
This stage can occur either inside of the old wet process rotary kilns or in the
preheater tower of modern units consisting of two steps: Firstly, between 300°C
and 650°C where the raw meal heating occurs, accompanied by a dehydration
reaction; Secondly, between 650°C and 1000°C, when the limestone
decarbonation takes place generating CO2 and CaO.
The first step is characterized by the following aspects:
• Presence of raw meal (there are no new mineral phases development);
• Erosion (due to raw meal flow at high velocities);
• low temperature;
• Evaporation and dehydration (of water) chemically bonded to the raw
material.
In this zone it is very important that the refractory products have the capability
to protect the rotary kiln drive (good insulation degree) and good resistance to
impacts of anomalous build-ups. Bricks with less than 45% Al2O3 content are
suitable. Besides that, when alkaline salts are present, a vitreous glassy layer
can develop with the alkali on the brick surface, preventing its propagation or
later infiltration.
In the second stage of these reactions, the development of new mineralogical
phases occurs:
- Formation of CaO and CO2;
- Formation of CA, C12A7 and C2S;
- Temperature variation;
- Alkali attack.
Usually, the use of bricks with a 70% Al2O3 content is recommended, which
offers a high mechanical resistance, low porosity, and low thermal conductivity.

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However, the risk of eutectic reactions formations on the Al2O3-CaO- SiO2 ,
system and alkali resistance is a limiting factor.
• Upper transition zone: from 1000ºC to 1238°C (+)
It is the most unstable and difficult area for refractory specification. Although
the temperature range varies from 1000°C to 1338°C, incidences of thermal
overloads are frequent. This fact is linked on the flame shape, to the fuel type
and to the design of the kiln main burner. Therefore, it is in this area where
coating starts to develop as soon as first drops of liquid phase appear. Coating
becomes very unstable if the operational conditions present high variability.
Table 1
• Sintering zone: from 1338ºC to 1450°C (+)
In this area a full development of coating at 1450ºC(+) is expected. The
presence of some liquid phase facilitates the dissolution of C2S in the same what
promotes the reaction that generates C3S. The highest temperature in the kiln is
reached at this area. Usually it should be around 1450ºC for ordinary Portland
Cements. Liquid phase is also around 25% at 1450ºC. If process is under
control, coating will be stable and able to protect the lining during the whole
campaign. However, if there is a big variability at ram meal control parameters
or uneven fuels types shifting, coating will be unstable and refractories
submitted to an enormous thermo-chemical wear. The refractory products must
resist high temperatures, infiltration of molten liquid calcium silicates, and/or
alkaline sulfates, and be able to hold a stable coating.
Usually at this kiln zone it is possible to find:
• Presence of incipient liquid phase from 18 to 32%, free lime and C2S;
• Development of C3S by the reaction of CaO and C2S.
• Clinker liquid phase infiltration and coating formation;

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• Chemical attacks by alkaline sulfates;
• High operational temperature.
• Lower transition zone from: 1400ºC to 1200°C (+)
This area usually operates between 1400°C and 1200°C. Around 1200ºC begins
the crystallization of the clinker the mineral phases, but not. Although the liquid
phase can still be present, it is a stage of low chemical activity, considering that
most of C3S has already been formed with a remaining amount of free lime
around 1%. Nevertheless, it is a zone submitted to temperature variations since
it is right under the influence of the secondary air temperature coming from the
cooler.
This area is characterized by the following aspects:
• Presence of the clinker liquid phase;
• Chemical attacks by alkaline sulfates;
• Frequent temperature variations when flame impinges over the brick;
• Continuous thermal shock;
• Redox atmosphere when using alternative fuels with poorly designed burner;
• Mechanical stress imposed by the tire station and kiln shell ovality.
In order to support the temperature variations under mechanical stress, this part
of the process requires the use of basic bricks with high structural flexibility,
low permeability to gas, high hot modules of rupture and abrasion resistance.
• Pre-cooling zone from: 1200ºC to 1000°C (+)
Originally, many kilns have been designed to promote the end of freezing and
crystallization of the just developed clinker phases. However, nowadays, the
existence of this zone into the kiln depends of the clinker cooler type and the
secondary air temperature entering into the kiln. With old grate coolers it was
around 700ºC, and for the modern high efficiency ones from 1150°C to 1100°C.
In this zone at that temperature range, there is high abrasion (clinker nodules),
accentuated discharge erosion (by the clinker dust carried by secondary and
tertiary airs) and mechanic stresses (nose ring plates and retention ring for
refractory products).
The main characteristics of this kiln zone are:
• High abrasion / erosion;
• Frequent thermal shocks;
• High mechanical stresses (compression/traction).
In most of the modern furnaces equipped with high efficiency coolers, this zone
is not inside the rotary kiln but in the first cooling compartment.

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Discussion:
Question-1: What is the maximum continuous shell temperature a kiln stands
without permanent damage to the shell?
Answer-1: The maximum recommended kiln shell temperature varies by plant,
by country and by kiln manufacturer, despite the fact that most kiln shells are
made of low alloy carbon steel. Age of the kiln shell, distance between the tires,
and structure of the shell are some important points should be considered before
deciding what the maximum allowable temperature for a kiln is. Let us explain
these points briefly:
1. Age and condition of the kiln shell: Old kilns shells have been exposed to
creep for a long time and are more prone to develop fatigue cracks than newer
shells.
2. Distance between tires: The longer the shell span, the less it will resist high
temperatures without sagging. Therefore, longer spans have more tendencies to
develop permanent deformation than shorter spans.
3. Kiln shell structure: Kiln shells are made with structural rolled steel plate,
such as A.S.T.M. A36. The tensile strength of this type of steel at room
temperature is 50,000 to 80,000 psi. As stated before shell strength is measured
at a room temperature. Figure-1 is showing how shell strength drops
considerably as its temperature is raised. It is interesting to notice that there is a
gain in strength between room temperature and 200 °C, followed by a sharp
loss in strength as the temperature goes up. At 430 °C the ultimate strength of
the steel drops from 75,000 psi to 50,000 psi (a hefty 33%) loss. Some
investigators report a 50% strength loss for the same temperature range.
Figure-1: Kiln shell strength as temperature raise.

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Question-2: What is the maximum red spot temperature on the shell force kiln to
stop?
Answer-2: The short answer is 550ºC if the spot is permanent and persistent.
This is a short answer, but when we talk about red spot, damaging of shell, long
kiln stoppage, and losing millions of Riyals or Dollars; this answer cannot be 3
acceptable. A number of factors are absolutely necessary to be considered in any
red spot before taking the decision of kiln stoppage:
1. Proximity of the red spot to the tires or gear: Red spots near tires and bull
gears require immediate action. These spots almost invariably force the kiln
down. Shutdown procedure must start immediately to avoid damaging the kiln
shell.
2. Extension of the red spot: The longer the circumferential extension of the red
spot, the greater the risk of shell permanent deformation or collapse. If there is
any persistent red spot covering more than 10% of the kiln circumference
(figure-4); Kiln should stop immediately.
3. Kiln alignment conditions: Misaligned kilns induce localized stresses along
the kiln length. If the red spot coincides with an area of stress concentration, the
shell sometimes elongates or twists beyond recovery.
4. Whether the red spot is exposed or under roof: If the kiln shell is directly
exposed to the elements and a heavy rainstorm hits the red spot, the shell may
develop cracks under sudden quenching. Sometimes the brick results severely
crushed in the red spot area.
5. The presence of shell cracks in the vicinity of the spot: The presence of cracks
in the vicinity of the hot spot calls for an immediate kiln shutdown to avoid shell
splitting.
Figure-4: Circumference red spot

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Question-3: Every year cement industry loses millions of dollars in unexpected
kiln shutdowns caused by rings build-up inside the kiln. What are the reasons
behind formation such type of build-up?
Answer-3: Kiln Build up (figure-11) or ring formation mechanism can be
divided depend on formation chemistry or formation location as the following:
a). Rings with regard to formation chemistry:
1. Sulphur Rings: Sulphur-induced rings are formed when the molal sulfur to
alkali ratio in the system is more than 1.2. In such cases, there is a considerable
amount of free SO3 circulating in the kiln. At a certain concentration level in the
kiln gas, sulfation of the free lime occurs with anhydrite formation (CaSO4). If
the kiln is burning under slightly reducing conditions, more volatile and lower
melting sulfur salts may form, therefore increasing the severity of the problem.
The salts, in molten state, coat the traveling clinker dust, forcing it to stick to the
kiln wall in the form of rings. Sometimes the chemical analysis of such rings
does not indicate high sulfur concentrations, proving that even a small amount
of free sulfur is sufficient to cause rings.
2. Spurrite Rings: Carbonate or spurrite rings are formed through CO2
desorption into the freshly formed free lime, or even through belite
recarbonation. These rings are hard, layered, and exhibit the same chemistry as
regular clinker. Spurrite is a form of carbonated belite. When the carbonate in
the spurrite is replaced with sulfur the new mineral is called sulfated spurrite.
Spurrite rings form whenever the partial pressure of CO2 above the bed of
material is high enough to invert the calcining reaction.
3. Alkali Rings: The third type of ring occurs whenever the sulfur-to-alkali
molal ratio is less than 0.83, usually in kilns with heavy chlorine loads. In such
cases, low-melting potassium salts provide the binder for clinker dust travelling
up the kiln. Through a "freeze-and-thaw" mechanism, these rings can assume
massive proportions. Alkali rings are far less common than other types because
sulfur and carbonates usually are in excess relative to potassium.
Figure-11: Kiln build-up

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b). Rings with regard to formation location:
1. Intermediate Rings: Intermediate rings are dense, hard and seldom fall off
during kiln operation. They are elongated, being some 10-15 meters long and
extending from 7 to 11 kiln diameters from the outlet. This ring is clinker-like in
colour indicating it being composed of well burnt material. They have a layered
structure, according the curvature of the kiln shell. Their chemical
composition is very similar to that of clinker. No increase in concentration of
S03 or alkalis takes place, and often the ring shows lower volatile element
values than for clinker. The alite of the inner layers may decompose into belite
and secondary free CaO, resulting from cooling down of the inner layers to a
temperature lower than the stability temperature of the alite (1260°C). The
mechanism of bonding is the freezing of the alumino-ferrite melt. The smallest
clinker particles of 150-450 mm are carried back by the gas stream, fall down
and are deposited on the kiln refractory lining, in a zone where temperatures of
below 1250°C exist. The clinker dust particles freeze in place, and because the
kiln charge is still fine, it does not possess sufficient abrasive action to remove
the growing ring.
2. Sinter Rings: These rings occur in the burning zone inlet, some 4-5 diameters
from the kiln outlet. They are greyish-black in colour, hard and formed by small
clinker nodules and clinker dust. Because of the presence of large pores and
voids, no layered structure is formed. Their chemical composition is that of the
clinker with no concentration of volatile elements. The alite of the inner layers
may decompose into belite and secondary free CaO. The bonding is created by
the freezing of the clinker liquid phase. This
phenomenon occurs especially in the burning zone inlet, where the liquid phase
is just starting to form, at approximately 1250°C. Due to the rotation of the kiln,
the material freezes with each kiln rotation and deposit of clinker particles
having less than 1 mm diameter may reach a large thickness.
3. Coal Ash Rings: In kilns fired with a high ash content coal, rings can form at
7-8.5 diameters from the kiln outlet. They are dense, with a layered structure
and sometimes glassy in appearance and built up from particles some 150-250
mm in size. They are rather less dense and have larger pores and voids than
intermediate rings. Their chemical and mineralogical composition is very
similar to that of clinker. As the ring grows up and the temperature of the inner
layers falls down the alite may decompose into belite and secondary free lime.
The bonding mechanism is the freezing of molten coal ash particles and perhaps
to a slight extent, the freezing of the clinker liquid phase. The molten coal ash
droplets adhere to the kiln refractory lining in a zone where the temperature is
high enough so that they are still partially sticky. When this layer passes under
the kiln charge, one ach kiln rotation, a portion of the still very fine kiln charge
adheres to it.

21. 21
References:
1-
Rotary Kilns: Transport Phenomena and Transport Processes
2-
http://www.cementkilns.co.uk/rotary_kilns.html
3-
http://www.dgengineering.de/Rotary-Kiln-Plants.html
4-
http://www.rotarykilnanddryer.com/index.html?aspxerrorpath=/
5-
http://www.a-cequipment.com/products/rotary-kilns
6-
http://combustion.fivesgroup.com/products/burners/rotary-kiln-precalciner-
burners.html
7-
http://www.merriam-webster.com/dictionary/rotary%20kiln
8-
http://www.khd.com/rotary-kilns.html