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The rotary kiln

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 gradually to 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

pipe

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

to

800

°C

before

it

enters

the

kiln,

thus

causing

intense

and

rapid

combustion of the fuel.

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.

[

edit

] The wet process and the dry process

% of North American Capacity using Wet Process

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%

[4]

.

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. 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

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.

An interesting footnote on the wet process history is that some

manufacturers have in fact made very old wet process faciltities

profitable through the use of waste fuels. Plants that burn waste fuels

enjoy

a

negative

fuel

cost

(they

are

paid

by

industries

needing

to

dispose

of materials that have energy content and can be safely disposed of in

the cement kiln thanks to its high temperatures and longer retention

times).

As

a

result

the

inefficiency

of

the

wet

process

is

an

advantage

—

to

the manufacturer. By locating waste burning operations at older wet

process locations, higher fuel consumption actually equates to higher

profits for the manufacturer, although it produces correspondingly

greater emission of CO

2

. Manufacturers who think such emissions should

be reduced are abandoning the use of wet process.

Preheaters

In

the

1930s,

significantly,

in

Germany,

the

first

attempts

were

made

to

redesign the kiln system to minimize waste of fuel

[5]

. This led to two

significant developments:

?

?

the grate preheater

the gas- suspension preheater.

[

edit

] 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

[6]

. 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

blown

out

of

the

kiln. Because the rawmix is damped in order to make pellets, this is

referred to as a

applicable to the

slurry, which is first de-watered with a high-pressure filter, and the

resulting

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.

[

edit

] Gas-suspension preheaters

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

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,