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Encyclopedia > Cement kiln

Cement Kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement. Over a billion tonnes of cement are made per annum, and the cement kiln is the heart of this production process. Sampling fast set Portland cement Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and plaster. ... In the most general sense of the word, cement is a binder, a substance which sets and hardens independently, and can bind other materials together. ...

Contents

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The Manufacture of Cement Clinker

A typical process of Portland cement manufacture consists of three stages: Sampling fast set Portland cement Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and plaster. ...

  • grinding a mixture of limestone and clay or shale to make a fine "rawmix"
  • heating the rawmix to sintering temperature in a cement kiln
  • grinding the resulting clinker to make cement.

In the second stage, the rawmix is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel. Successive chemical reactions take place as the temperature of the rawmix rises: Limey shale overlaid by limestone. ... The Gay Head cliffs in Marthas Vineyard are made almost entirely of natural clays. ... Shale Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. ... In the most general sense of the word, cement is a binder, a substance which sets and hardens independently, and can bind other materials together. ...

  • 70-110°C - Free water is evaporated.
  • 400-600°C - clay-like minerals are decomposed into their constituent oxides; principally SiO2 and Al2O3. Dolomite (CaMg(CO3)2) decomposes to calcium carbonate, MgO and CO2.
  • 650-900°C - calcium carbonate reacts with SiO2 to form belite (Ca2SiO4).
  • 900-1050°C - the remaining calcium carbonate decomposes to calcium oxide and CO2.
  • 1300-1450 - partial (20-30%) melting takes place, and belite reacts with calcium oxide to form alite (Ca3O.SiO4).

Alite is the characteristic constituent of Portland cement. Typically, a peak temperature of 1400-1450°C is required to complete the reaction. The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1-10 mm. This is called clinker. The hot clinker next falls into a cooler which recovers most of its heat, and cools the clinker to around 100°C, at which temperature it can be conveniently conveyed to storage. The cement kiln system is designed to accomplish these processes efficiently. Dolomite crystals from Touissite, Morocco Dolomite is the name of both a carbonate rock and a mineral consisting of calcium magnesium carbonate (formula: CaMg(CO3)2) found in crystals. ... Sampling fast set Portland cement Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and plaster. ... Clinker has several meanings: In boat building, clinker is a method of constructing wooden boats by fixing planks to a frame so that the planks overlap each other gaining support from the frame and from adjacent planks. ... Clinker has several meanings: In boat building, clinker is a method of constructing wooden boats by fixing planks to a frame so that the planks overlap each other gaining support from the frame and from adjacent planks. ...

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Early History

Portland cement clinker was first made (in 1842) in a modified form of the traditional static lime kiln[1][2][3]. The basic, egg-cup shaped lime kiln was provided with a conical or bee-hive shaped extension to increase draught and thus obtain the higher temperature needed to make cement clinker. For nearly half a century, this design, and minor modifications, remained the only method of manufacture. The kiln was restricted in size by the strength of the chunks of rawmix: if the charge in the kiln collapsed under its own weight, the kiln would be extinguished. For this reason, beehive kilns never made more than 30 tonnes of clinker per batch. A batch took one week to turn around: a day to fill it, three days to burn off, two days to cool, and a day to unload. Thus, a kiln would produce about 1500 tonnes per year. Sampling fast set Portland cement Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and plaster. ...


Around 1885, experiments began on design of continuous kilns. One design was the shaft kiln, similar in design to a blast furnace. Rawmix in the form of lumps and fuel were continuously added at the top, and clinker was continually withdrawn at the bottom. Air was blown through under pressure from the base to combust the fuel. The shaft kiln had a brief period of use before it was eclipsed by the rotary kiln, but it had a limited renaissance from 1970 onward in China and elsewhere, when it was used for small-scale, low-tech plants in rural areas away from transport routes. Several thousand such kilns were constructed in China. A typical shaft kiln produces 100-200 tonnes per day.


From 1885, trials began on the development of the rotary kiln, which today accounts for more than 95% of world production.

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The Rotary Kiln

General Layout of a Rotary Kiln
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General Layout of a Rotary Kiln

The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1-4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it to gradually move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400-800°C before it enters the kiln, thus causing intense and rapid combustion of the fuel. Clinker has several meanings: In boat building, clinker is a method of constructing wooden boats by fixing planks to a frame so that the planks overlap each other gaining support from the frame and from adjacent planks. ...


The earliest successful rotary kilns were developed in Pennsylvania around 1890, and were about 1.5 m in diameter and 15 m in length. Such a kiln made about 20 tonnes of clinker per day. The fuel, initially, was oil, which was readily available in Pennsylvania at the time. It was particularly easy to get a good flame with this fuel. Within the next 10 years, the technique of firing by blowing in pulverized coal was developed, allowing the use of the cheapest available fuel. By 1905, the largest kilns were 2.7 x 60 m in size, and made 190 tonnes per day. At that date, after only 15 years of development, rotary kilns accounted for half of world production. Since then, the capacity of kilns has increased steadily, and the largest kilns today produce around 10,000 tonnes per day. In contrast to static kilns, the material passes through quickly: it takes from 3 hours (in some old wet process kilns) to as little as 10 minutes (in short precalciner kilns). Rotary kilns run 24 hours a day, and are typically stopped only for a few days once or twice a year for essential maintenance. This is an important discipline, because heating up and cooling down are long, wasteful and damaging processes. Uninterrupted runs as long as 18 months have been achieved.

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Dry and Wet Processes[4]

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% of North American Capacity using Wet Process
Mean Fuel Energy used in North American Kilns
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Mean Fuel Energy used in North American Kilns

From the earliest times, two different methods of rawmix preparation were used: the mineral components were either dry-ground to form a flour-like powder, or were wet-ground with added water to produce a fine slurry with the consistency of paint, and with a typical water content of 40-45%.


The wet process suffered the obvious disadvantage that, when the slurry was introduced into the kiln, a large amount of extra fuel was used in evaporating the water. Furthermore, a larger kiln was needed for a given clinker output, because much of the kiln's length was used up for the drying process. On the other hand, the wet process had a number of advantages. Wet grinding of hard minerals is usually much more efficient than dry grinding. When slurry is dried in the kiln, it forms a granular crumble that is ideal for subsequent heating in the kiln. On the other hand, in the dry process, it is very difficult to keep the fine powder rawmix in the kiln, because the fast-flowing combustion gases tend to blow it back out again. It became a practice to spray water into dry kilns in order to "damp down" the dry mix, and thus, for many years there was little difference in efficiency between the two processes, and the overwhelming majority of kilns used the wet process. By 1950, a typical large, wet process kiln, fitted with drying-zone heat exchangers, was 3.3 x 120 m in size, made 680 tonnes per day, and used about 0.25-0.30 tonnes of coal fuel for every tonne of clinker produced. Before the energy crisis of the 1970s put an end to new wet-process installations, kilns as large as 5.8 x 225 m in size were making 3000 tonnes per day.

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Preheaters

In the 1930s, significantly, in Germany, the first attempts were made to redesign the kiln system to minimize waste of fuel. This led to two significant developments:

  • the Grate Preheater
  • the Gas-suspension Preheater.
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Grate Preheaters

The grate preheater consists of a chamber containing a chain-like high-temperature steel moving grate, attached to the cold end of the rotary kiln. A dry-powder rawmix is turned into a hard pellets of 10-20 mm diameter in a nodulizing pan, with the addition of 10-15% water. The pellets are loaded onto the moving grate, and the hot combustion gases from the rear of the kiln are passed through the bed of pellets from beneath. This dries and partially calcines the rawmix very efficiently. The pellets then drop into the kiln. Very little powdery material is blow out of the kiln. Because the rawmix is damped in order to make pellets, this is referred to as a "semi-dry" process. The grate preheater is also applicable to the "semi-wet" process, in which the rawmix is made as a slurry, which is first de-watered with a high-pressure filter, and the resulting "filter-cake" is extruded into pellets, which are fed to the grate. In this case, the water content of the pellets is 17-20%. Grate preheaters were most popular in the 1950s and 60s, when a typical system would have a grate 28 m long and 4 m wide, and a rotary kiln of 3.9 x 60 m, making 1050 tonnes per day, using about 0.11-0.13 tonnes of coal fuel for every tonne of clinker produced. Systems up to 3000 tonnes per day were installed.

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Gas-suspension Preheaters

Cutaway view of cyclone showing air path
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Cutaway view of cyclone showing air path

The key component of the gas-suspension preheater is the cyclone. A cyclone is a conical vessel into which a dust-bearing gas-stream is passed tangentially. This produces a vortex within the vessel. The gas leaves the vessel through a co-axial "vortex-finder". The solids are thrown to the outside edge of the vessel by centrifugal action, and leave through a valve in the vertex of the cone. Cyclones were originally used to clean up the dust-laden gases leaving simple dry process kilns. If, instead, the entire feed of rawmix is encouraged to pass through the cyclone, it is found that a very efficient heat exchange takes place: the gas is efficiently cooled, hence producing less waste of heat to the atmosphere, and the rawmix is efficiently heated. This efficiency is further increased if a number of cyclones are connected in series.

4-Stage preheater, showing path of feed
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4-Stage preheater, showing path of feed

The number of cyclones used in practice varies from 1 to 6. Energy, in the form of fan-power, is required to draw the gases through the string of cyclones, and at a string of 6 cyclones, the cost of the added fan-power needed for an extra cyclone exceeds the efficiency advantage gained. It is normal to use the warm exhaust gas to dry the raw materials in the rawmix grinding mill, and if the raw materials are wet, hot gas from a less efficient preheater is desirable. For this reason, the most commonly encountered suspension preheaters have 4 cyclones. The hot feed that leaves the base of the preheater string is typically 20% calcined, so the kiln has less subsequent processing to do, and can therefore achieve a higher specific output. Typical large systems installed in the early 1970s had cyclones 6 m in diameter, a rotary kiln of 5 x 75 m, making 2500 tonnes per day, using about 0.11-0.12 tonnes of coal fuel for every tonne of clinker produced.

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Precalciners

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% of North American Capacity using Precalciners
Mean Daily Output (tonnes) of North American Kilns
Enlarge
Mean Daily Output (tonnes) of North American Kilns

In the 1970s the precalciner was pioneered in Japan, and has subsequently become the equipment of choice for new large installations world-wide. The precalciner is a development of the suspension preheater. The philosophy is this: the amount of fuel that can be burned in the kiln is directly related to the size of the kiln. If part of the fuel necessary to burn the rawmix is burned outside the kiln, the output of the system can be increased for a given kiln size. Users of suspension preheaters found that output could be increased by injecting extra fuel into the base of the preheater. The logical development was to install a specially designed combustion chamber at the base of the preheater, into which pulverized coal is injected. This is referred to as an "air-through" precalciner, because the combustion air for both the kiln fuel and the calciner fuel all passes through the kiln. This kind of precalciner can burn up to 30% (typically 20%) of its fuel in the calciner. If more fuel is injected in the calciner, the extra amount of air drawn through the kiln would cool the kiln flame excessively. The feed is 40-60% calcined before it enters the rotary kiln.


The ultimate development is the "air-separate" precalciner, in which the hot combustion air for the calciner arrives in a duct directly from the cooler, bypassing the kiln. Typically, 60-75% of the fuel is burned in the precalciner. In these systems, the feed entering the rotary kiln is 100% calcined. The kiln has only to raise the feed to sintering temperature. In theory the maximum efficiency would be achieved if all the fuel were burned in the preheater, but the sintering operation involves partial melting and nodulization to make clinker, and the rolling action of the rotary kiln remains the most efficient way of doing this. Large modern installations typically have two parallel strings of 4 or 5 cyclones, with one attached to the kiln and the other attached to the precalciner chamber. A rotary kiln of 6 x 100 m makes 8-10,000 tonnes per day, using about 0.10-0.11 tonnes of coal fuel for every tonne of clinker produced. The kiln is dwarfed by the massive preheater tower and cooler in these installations. Such a kiln produces 3 million tonnes of clinker per year, and consumes 300,000 tonnes of coal. 6 m diameter appears to be the limit of size of rotary kilns, because the flexibility of the steel shell becomes unmanageable at or above this size, and the firebrick lining tends to fail when the kiln flexes.

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Ancillary Equipment

Essential equipment in addition to the kiln tube and the preheater are:

  • Cooler
  • Fuel Mills
  • Fans
  • Exhaust Gas Cleaning Equipment.
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Coolers

Early systems used rotary coolers, which were rotating cylinders similar to the kiln, into which the hot clinker dropped. The combustion air was drawn up through the cooler as the clinker moved down, cascading through the air stream. In the 1920s, satellite coolers became common and remained in use until recently. These consist of a set (typically 7-9) of tubes attached to the kiln tube. They have the advantage that they are sealed to the kiln, and require no separate drive. From about 1930, the grate cooler was developed. This consists of a perforated grate through which cold air is blown, enclosed in a rectangular chamber. A bed of clinker up to 0.5 m deep moves along the grate. These coolers have two main advantages: they cool the clinker rapidly, which is desirable from a quality point of view, and, because they don't rotate, hot air can be ducted out of them for use in fuel drying, or for use as precalciner combustion air. The latter advantage means that they have become the only type used in modern systems.

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Fuel Mills

Fuel systems are divided into two categories:

  • Direct firing
  • Indirect firing

In direct firing, the fuel is fed at a controlled rate to the fuel mill, and the fine product is immediately blown into the kiln. The advantage of this system is that it is not necessary to store the hazardous ground fuel: it is used as soon as it is made. For this reason it was the system of choice for older kilns. A disadvantage is that the fuel mill has to run all the time: if it breaks down, the kiln has to stop if no backup system is available.


In indirect firing, the fuel is ground by an intermittently-run mill, and the fine product is stored in a silo of sufficient size to supply the kiln though fuel mill stoppage periods. The fine fuel is metered out of the silo at a controlled rate and blown into the kiln. This method is now favoured for precalciner systems, because both the kiln and the precalciner can be fed with fuel from the same system. Special techniques are required to store the fine fuel safely, and coals with high volatiles are normally milled in an inert atmosphere (e.g. CO2).

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Fans

A large volume of gases has to be moved through the kiln system. Particularly in suspension preheater systems a high degree of suction has to be developed at the exit of the system to drive this. Fans are also used to force air through the cooler bed, and to propel the fuel into the kiln. Fans account for most of the electric power consumed in the system, typically amounting to 10-15 kWh per tonne of clinker.

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Gas Cleaning

The exhaust gases from a modern kiln typically amount to 2 tonnes (or 1500 cubic metres at NTP) per tonne of clinker made. The gases carry a large amount of dust - typically 30 g per cubic metre. Environmental regulations specific to different countries require that this be reduced to (typically) 0.1 g per cubic metre, so dust capture needs to be at least 99.7% efficient. Methods of capture include electrostatic precipitators and bag-filters.

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Kiln Fuels

Fuels that have been used for primary firing include coal, petroleum coke, heavy fuel oil, natural gas, landfill off-gas and oil refinery flare gas. High carbon fuels such as coal are preferred for kiln firing, because they yield a luminous flame. The clinker is brought to its peak temperature mainly by radiant heat transfer, and a bright (i.e. high emissivity) flame is essential for this. Fuels burning with a less luminous flame, such as natural gas, tend to result in lower kiln output. In addition to these primary fuels, various combustible waste materials have been fed to kilns, notably used tyres, which are very difficult to dispose of by other means. In theory, cement kilns are an attractive way of disposing of hazardous materials, because of the temperatures which are much higher than in other combustion systems, because of the alkaline conditions afforded by the high-calcium rawmix, which can absorb acidic combustion products, and because the clinker can absorb heavy metals into its structure. However, in order to avoid emissions (e.g. of dioxins) it is necessary to control the system in a manner that is non-optimal for efficiency and output, and coarse combustibles such as tyres can cause major product quality problems.

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Notes

  1. ^ R G Blezard, The History of Calcareous Cements in P C Hewlett (Ed), Lea's Chemistry of Cement and Concrete, 4th Ed, Arnold, 1998, ISBN 0-340-56589-6
  2. ^ A C Davis, A Hundred Years of Portland Cement, 1824-1924, Concrete Publications Ltd, London, 1924
  3. ^ G R Redgrave & C Spackman, Calcareous Cements: their Nature, Manufacture and Uses, London, 1924
  4. ^ Trend charts are based on USGS Annual Reports (for detailed output) and Cembureau World Cement Reports (for process details).

  Results from FactBites:
 
Cement Kiln Dust Wastes | Wastes | US EPA (1591 words)
Cement kiln dust (CKD) is the fine-grained, solid, highly alkaline waste removed from cement kiln exhaust gas by air pollution control devices.
Cement kiln dust is one of the wastes exempted.
Cement kiln dust is included as one of the six special wastes.
Fcb ciment : cement industry, mineral grinding equipment, machinery manufacturer, cement plant supplier (886 words)
Fcb ciment is fully dedicated to the cement industry and to the mineral grinding applications, offering a wide range of equipment, from the spare part to the turnkey plant, thanks to a double expertise of Turnkey General Contractor and Process Mechanical Equipment Supplier.
cement manufacturing equipment Fcb ciment is fully dedicated to the cement industry and to the mineral grinding equipment.
From the spare part (revamping, rotary kiln alignment) to the turnkey cement plant, thanks to a double expertise of Turnkey General Contractor and Process Mechanical Equipment Supplier.
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