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Definition of Terms

Cement kilns

Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types

of hydraulic cement, in which calcium carbonatereacts with silica-bearing minerals to form a mixture

of calcium silicates. Over a billion tonnes of cement are made per year, and cement kilns are the heart of

this production process: their capacity usually defines the capacity of the cement plant. As the main

energy-consuming and greenhouse-gasemitting stage of cement manufacture, improvement of kiln

efficiency has been the central concern of cement manufacturing technology.

Cement Kiln Emissions

Emissions from cement works are determined both by continuous and discontinuous measuring

methods, which are described in corresponding national guidelines and standards. Continuous

measurement is primarily used for dust, NOx and SO2, while the remaining parameters relevant pursuant

to ambient pollution legislation are usually determined discontinuously by individual measurements.

Coolers

Coolers come in both direct and indirect form. They are typically used after a calciner,

incinerator or other high temperature-processing unit.

Dryers

Dryers are used to remove moisture from materials. Rotary kiln dryers, fluid bed dryers,

intrainment dryers and impact dryers use hot gases to heat the feed material and evaporate the water.

Electrostatic Precipitators

Or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are

highly efficient filtration devices that minimally impede the flow of gases through the device, and can

easily remove fine particulate matter such as dust and smoke from the air stream.

Fuel mills

Fuel systems are divided into two categories:

o Direct firing

o Indirect firing

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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

Grate Preheater

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 1020 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 blown 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%.

Gypsum

A very common mineral, hydrated calcium sulfate, CaSO 4 2H 2 O, occurring in crystals and in masses,soft enough to be scratched by the fingernail: used to make plaster of Paris, as an ornamental mat

erial, asa fertilizer, etc.

Limestone

Is a sedimentary rock composed largely of the minerals calcite and aragonite, which are

different crystal forms of calcium carbonate(CaCO3). Many limestones are composed from skeletal

fragments of marine organisms such as coral or foraminifera.

Limestone makes up about 10% of the total volume of all sedimentary rocks. The solubility of limestone

in water and weak acid solutions leads tokarst landscapes, in which water erodes the limestone over

thousands to millions of years. Most cave systems are through limestone bedrock.

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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 raw mix 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

were 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.

Pyro Processing

Pyro processing is used to increase the economic value of ores, minerals, waste and related

materials by changing their mechanical and/or chemical properties through the addition or removal of

heat.

Raw Mill

The equipment used to grind raw materials into "raw mix" during the manufacture of cement.

Raw mix is then fed to a cement kiln, which transforms it into clinker, which is then ground to make

cement in the cement mill. The raw milling stage of the process effectively defines the chemistry (and

therefore physical properties) of the finished cement, and has a large effect upon the efficiency of the

whole manufacturing process.

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Introduction

Portland cement is a fine powder, gray or white in color that consists of a mixture of hydraulic

cement materials comprising primarily calcium silicates, aluminates and aluminoferrites. More than 30

raw materials are known to be used in the manufacture of portland cement, and these materials can be

divided into four distinct categories: calcareous, siliceous, argillaceous, and ferrifrous. These materials

are chemically combined through pyroprocessing and subjected to subsequent mechanical processing

operations to form gray and white portland cement. Gray Portland cement is used for structural

applications and is the more common type of cement produced. White portland cement has lower iron

and manganese contents than gray portland cement and is used primarily for decorative purposes.

Portland cement manufacturing plants are part of hydraulic cement manufacturing, which also includes

natural, masonry, and pozzolanic cement. The six-digit Source Classification Code (SCC) for portland

cement plants with wet process kilns is 3-05-006, and the six-digit SCC for plants with dry process kilns is

3-05-007.

Portland cement accounts for 95 percent of the hydraulic cement production in the United

States. The balance of domestic cement production is primarily masonry cement. Both of these

materials are produced in portland cement manufacturing plants. A diagram of the process, which

encompasses production of both portland and masonry cement, is shown in the figure of flow diagram.

As shown in the figure, the process can be divided into the following primary components: raw materials

acquisition and handling, kiln feed preparation, pyroprocessing, and finished cement grinding. Each of

these process components is described briefly below. The primary focus of this discussion is on

pyroprocessing operations, which constitute the core of a portland cement plant.

Manufacturing Process for Portland Cement

Raw Materials

The initial production step in portland cement manufacturing is raw materials acquisition.

Calcium, the element of highest concentration in portland cement, is obtained from a variety of

calcareous raw materials, including limestone, chalk, marl, sea shells, aragonite, and an impure

limestone known as "natural cement rock". Typically, these raw materials are obtained from open-face

quarries, but underground mines or dredging operations are also used. Raw materials vary from facility

to facility. Some quarries produce relatively pure limestone that requires the use of additional raw

materials to provide the correct chemical blend in the raw mix. In other quarries, all or part of the

noncalcarious constituents are found naturally in the limestone. Occasionally, pockets of pyrite, which

can significantly increase emissions of sulfur dioxide (SO2), are found in deposits of limestone, clays, and

shales used as raw materials for portland cement. Because a large fraction (approximately one third) of

the mass of this primary material is lost as carbon dioxide (CO2) in the kiln, Portland cement plants are

located close to a calcareous raw material source whenever possible. Other elements included in the

raw mix are silicon, aluminum, and iron.

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These materials are obtained from ores and minerals such as sand, shale, clay, and iron ore. Again, these

materials are most commonly from open-pit quarries or mines, but they may be dredged or excavated

from underwater deposits.

Either gypsum or natural anhydrite, both of which are forms of calcium sulfate, is introduced to

the process during the finish grinding operations described below. These materials, also excavated from

quarries or mines, are generally purchased from an external source, rather than obtained directly from a

captive operation by the cement plant. The portland cement manufacturing industry is relying

increasingly on replacing virgin materials with waste materials or byproducts from other manufacturing

operations, to the extent that such replacement can be implemented without adversely affecting plant

operations, product quality or the environment. Materials that have been used include fly ash, mill

scale, and metal smelting slags.

Raw Materials Preparation

Blending and Drying

The second step in portland cement manufacture is preparing the raw mix, or kiln feed, for the

pyroprocessing operation. Raw material preparation includes a variety of blending and sizing operations

that are designed to provide a feed with appropriate chemical and physical properties. The raw material

processing operations differ somewhat for wet and dry processes, as described below.

Cement raw materials are received with an initial moisture content varying from 1 to more than

50 percent. If the facility uses dry process kilns, this moisture is usually reduced to less than 1 percent

before or during grinding. Drying alone can be accomplished in impact dryers, drum dryers, paddle-

equipped rapid dryers, air separators, or autogenous mills. However, drying can also be accomplished

during grinding in ball-and-tube mills or roller mills. While thermal energy for drying can be supplied by

exhaust gases from separate, direct-fired coal, oil, or gas burners, the most efficient and widely used

source of heat for drying is the hot exit gases from the pyroprocessing system.

Materials transport associated with dry raw milling systems can be accomplished by a variety of

mechanisms, including screw conveyors, belt conveyors, drag conveyors, bucket elevators, air slide

conveyors, and pneumatic conveying systems. The dry raw mix is pneumatically blended and stored in

specially constructed silos until it is fed to the pyroprocessing system.

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Slurry Mixing and Blending

In the wet process, water is added to the raw mill during the grinding of the raw materials in ball

or tube mills, thereby producing a pumpable slurry, or slip, of approximately 65 percent solids. The

slurry is agitated, blended, and stored in various kinds and sizes of cylindrical tanks or slurry basins until

it is fed to the pyroprocessing system.

The heart of the portland cement manufacturing process is the pyroprocessing system. This

system transforms the raw mix into clinkers, which are gray, glass-hard, spherically shaped nodules that

range from 0.32 to 5.1 centimeters (cm) (0.125 to 2.0 inches [in.]) in diameter. The chemical reactions

and physical processes that constitute the transformation are quite complex, but they can be viewed

conceptually as the following sequential events:

1. Evaporation of free water;

2. Evolution of combined water in the argillaceous components;

3. Calcination of the calcium carbonate (CaCO3) to calcium oxide (CaO);

4. Reaction of CaO with silica to form dicalcium silicate;

5. Reaction of CaO with the aluminum and iron-bearing constituents to form the liquid phase;

6. Formation of the clinker nodules;

7. Evaporation of volatile constituents (e. g., sodium, potassium, chlorides, and sulfates);

and

8. Reaction of excess CaO with dicalcium silicate to form tricalcium silicate.

Rotary Kiln

This sequence of events may be conveniently divided into four stages, as a function of location

and temperature of the materials in the rotary kiln.

1. Evaporation of uncombined water from raw materials, as material temperature increases to

100C (212F);

2. Dehydration, as the material temperature increases from 100C to approximately 430C

(800F) to form oxides of silicon, aluminum, and iron;

3. Calcination, during which carbon dioxide (CO2) is evolved, between 900C (1650F) and

982C (1800F), to form CaO; and

4. Reaction, of the oxides in the burning zone of the rotary kiln, to form cement clinker at

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temperatures of approximately 1510C (2750F).

Rotary kilns are long, cylindrical, slightly inclined furnaces that are lined with refractory to

protect the steel shell and retain heat within the kiln. The raw material mix enters the kiln at the

elevated end, and the combustion fuels generally are introduced into the lower end of the kiln in a

countercurrent manner. The materials are continuously and slowly moved to the lower end by rotation

of the kiln. As they move down the kiln, the raw materials are changed to cementitious or hydraulic

minerals as a result of the increasing temperature within the kiln. The most commonly used kiln fuels

are coal, natural gas, and occasionally oil.

In the wet process and long dry process, all of the pyroprocessing activity occurs in the rotary

kiln. Depending on the process type, kilns have length-to-diameter ratios in the range of 15:1 to 40:1.

While some wet process kilns may be as long as 210 m (700 ft), many wet process kilns and all dry

process kilns are shorter. Wet process and long dry process pyroprocessing systems consist solely of the

simple rotary kiln. Usually, a system of chains is provided at the feed end of the kiln in the drying or

preheat zones to improve heat transfer from the hot gases to the solid materials. As the kiln rotates, the

chains are raised and exposed to the hot gases. Further kiln rotation causes the hot chains to fall into

the cooler materials at the bottom of the kiln, thereby transferring the heat to the load.

Dry process pyroprocessing systems have been improved in thermal efficiency and productive

capacity through the addition of one or more cyclone-type preheater vessels in the gas stream exiting

the rotary kiln. This system is called the preheater process. The vessels are arranged vertically, in series,

and are supported by a structure known as the preheater tower. Hot exhaust gases from the rotary kiln

pass counter currently through the downward-moving raw materials in the preheater vessels. Compared

to the simple rotary kiln, the heat transfer rate is significantly increased, the degree of heat utilization is

greater, and the process time is markedly reduced by the intimate contact of the solid particles with the

hot gases. The improved heat transfer allows the length of the rotary kiln to be reduced. The hot gases

from the preheater tower are often used as a source of heat for drying raw materials in the raw mill.

Because the catch from the mechanical collectors, fabric filters, and/or electrostatic precipitators (ESP)

that follow the raw mill is returned to the process, these devices are considered to be production

machines as well as pollution control devices.

Preheater and precalciner kiln systems often have an alkali bypass system between the feed end

of the rotary kiln and the preheater tower to remove the undesirable volatile constituents. Otherwise,

the volatile constituents condense in the preheater tower and subsequently recirculate to the kiln.

Buildup of these condensed materials can restrict process and gas flows. The alkali content of portland

cement is often limited by product specifications because excessive alkali metals (i. e., sodium and

potassium) can cause deleterious reactions in concrete. In a bypass system, a portion of the kiln exit gas

stream is withdrawn and quickly cooled by air or water to condense the volatile constituents to fine

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particles. The solid particles, containing the undesirable volatile constituents, are removed from the gas

stream and thus the process by fabric filters and ESPs.

The semidry process is a variation of the dry process. In the semidry process, the water is added

to the dry raw mix in a pelletizer to form moist nodules or pellets. The pellets then are conveyed on a

moving grate preheater before being fed to the rotary kiln. The pellets are dried and partially calcined by

hot kiln exhaust gases passing through the moving grate.

Clinker Cooler

Regardless of the type of pyroprocess used, the last component of the pyroprocessing system is

the clinker cooler. This process step recoups up to 30 percent of the heat input to the kiln system locks

in desirable product qualities by freezing mineralogy, and makes it possible to handle the cooled clinker

with conventional conveying equipment. The more common types of clinker coolers are (1)

reciprocating grate, (2) planetary, and (3) rotary. In these coolers, the clinker is cooled from about

1100C to 93C (2000F to 200F) by ambient air that passes through the clinker and into the rotary kiln

for use as combustion air. However, in the reciprocating grate cooler, lower clinker discharge

temperatures are achieved by passing an additional quantity of air through the clinker. Because this

additional air cannot be utilized in the kiln for efficient combustion, it is vented to the atmosphere, used

for drying coal or raw materials, or used as a combustion air source for the precalciner.

Finish Grinding Mill and Air separator

The final step in portland cement manufacturing involves a sequence of blending and grinding

operations that transforms clinker to finished portland cement. Up to 5 percent gypsum or natural

anhydrite is added to the clinker during grinding to control the cement setting time, and other specialty

chemicals are added as needed to impart specific product properties. This finish milling is accomplished

almost exclusively in ball or tube mills. Typically, finishing is conducted in a closed-circuit system, with

product sizing by air separation.

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Machine used in the Process

Kiln

Kilns are an essential part of the manufacture of all ceramics, which require heat treatment,

often at high temperatures. During this process, chemical and physical reactions occur that permanently

alter the unfired body. In the case of pottery, clay materials are shaped, dried and then fired in a kiln.

The final characteristics are determined by the composition and preparation of the clay body, by the

temperature at which it is fired, and by the glazes that may be used. Although modern kilns often have

sophisticated electrical systems to control the firing temperatures, pyrometric devices are also

frequently used.

Clinker Cooling Machine

Structure comprises are set separately with the remaining heat generator is connected with the

hot blast pipeline of the i and the coal mill is connected with the hot blast pipeline of the ii. This utility

model can solve the deficiency of existing technology through adding into the coal grinding of the

pipeline of the wind amount it improves raw coal drying effect is. The whole production line this

invention claims a production and consumption reducing this invention claims a first condition that. The

utility model discloses an improved warm-air pipeline of a cement clinker grate refrigerator. The

improved warm-air pipeline comprises a warm-air pipeline I and a warm-air pipeline II, wherein the

warm-air pipeline I is connected with a waste heat generator, the warm-air pipeline II is connected with

a coal mill, and the warm-air pipeline I and the warm-air pipeline II are separated. The improved warm-

air pipeline can overcome the defects in the prior art. Drying effect of raw coal can be improved by

increasing air quantity of a coal mill pipeline. Prerequisite conditions are offered to production

improvement and consumption reduction of a whole production line.

Cement Silos

Cement silos are on-site storage containers used for the storage and distribution of various types of cement mixtures. Silos of this type come in a variety of sizes, making them ideal for use at many kinds of construction sites. A cement silo can be a permanent structure, or a portable model that can be relocated when necessary. Like many other types of silos, the cement silo usually is equipped with some type of blower to help expel the stored contents into a truck or other receptacle.

A cement storage silo can be structured to hold no more than a few tons of dry cement product, or be designed to efficiently hold several hundred tons. Generally, larger silos are permanent structures that cannot be moved. These are likely to be found at concrete plants, where the finished product is stored until it is time for shipment. Many building sites that utilize concrete in the construction process opt for portable cement silos that can be moved around the site as the need arises.

It is not unusual for construction companies to keep several portable cement silos available for different building projects. These simple storage devices can usually be set up in a matter of hours, then dismantled

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once the project is complete. Storage of the portable cement silo is relatively easy, since the components can be stored in a warehouse until the device is needed at another building site.

Cement Raw Mill

Cement raw mill is the equipment used to grind the hard, nodular clinker from the cement kiln

into the fine grey powder that is cement. Most cement is currently ground in the ball mill, a horizontal

cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis, imparting a

tumbling and cascading action to the balls. The gas temperature is controlled by cold-air bleeds to

ensure a dry product without overheating the mill. The product passes into an air separator, which

returns oversized particles to the mill inlet. Occasionally, the mill is preceded by a hot-air-swept hammer

mill which does most of the drying and produces millimeter-sized feed for the mill.

Belt Conveyor

Belt conveyor is adaptable to both stationary and mobile crushing plants, it is widely used in

mining, metallurgical and coal industry to transfer sandy or lump materials, or packaged materials. In

terms of transferring capacity, the good belt conveyor should feature strong transferring capacity, easy

maintenance and long conveying distance. According to different materials, we design different models

of belt conveyor. The conveying system can be one single or multi-conveyors or combined with other

conveying equipment according to various requirements.

Vertical Roller Mills

With the continual increasing demand for portland cement and constant pressure for reduced

energy consumption, producers are exploring a wide variety of cost-saving manufacturing options. One

option is vertical roller mill technology for finish grinding.

Traditionally, plants used ball mills to grind clinker and gypsum into cement. The result: the

majority (60%) of finish grinding in the world is still performed using the ubiquitous ball mill. Ball mills

are cylindrical steel shells with steel liners. These rotating drums contain grinding media that tumble

inside the cylinder. The grinding balls cascade and tumble onto the clinker and gypsum to produce

cement. Almost all ball mills use a form of closed circuit grinding that returns material that is too coarse

back to the ball mill inlet while material fine enough to meet product requirements is collected. The

separator or classifier determines which particles will be returned and which particles are sufficiently

fine. With an effort to increase production, ball mill physical size has increased almost to the physical

limitation dictated by the gas velocities and accompanying pressures necessary for the process. Ball mills

may not be the most efficient means of size reduction but their reputation for product consistency and

their simplicity of operation have made them an historic plant favorite.

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References:

W. L. Greer, et al., "Portland Cement", Air Pollution Engineering Manual, A. J. Buonicore

and W. T. Davis (eds.), Von Nostrand Reinhold, NY, 1992.

U. S. And Canadian Portland Cement Industry Plant Information Summary, December 31,

1990, Portland Cement Association, Washington, DC, August 1991.

Emissions From Wet Process Cement Kiln And Clinker Cooler At Maule Industries, Inc., ETB

Test No. 71-MM-01, U. S. Environmental Protection Agency, Research Triangle Park, NC,

March 1972.