1

Koya University

Faculty of Engineering

Chemical Engineering Department

Chemical Industry

Rotary kiln

Preparation By:

Aree Salah

Alan mawlud

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

3

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.

4

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).

5

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.

6

In 1888, Fredrik Lوss ِ e 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.

7

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.

8

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.

9

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.

11

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.

13

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.

14

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.

15

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;

16

• 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.

17

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

19

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

20

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

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