■ Compact design

■ Suitable for grinding any type of coal

■ Long service life of grinding parts

ATOX Coal Mill

Proven technologyOver the last few decades, the air-sweptvertical roller mill has become the standardsolution for coal grinding installations.Continuous improvement of the ATOX coalmill has resulted in today's compactconstruction featuring high reliability and awide range of mill sizes. The ATOX grindsand dries all types of coal and is highlyeconomical in terms of specific energyconsumption.

Versatile systemThe ATOX coal mill is suited for variousinstallation types, whether inert or non-inertand direct or indirect firing systems. Aninert system designed for indirect firing isthe most common solution at cementplants. Equipped with the high-efficiencyRAKM dynamic separator, the ATOX coalmill will grind any type of coal to the neces-sary fineness. The ATOX mill, if providedwith a variable speed motor, will grind pet-coke and anthracite down to a finenessbelow 5% +90 µm.

Separator and nozzle ring are sized inde-pendently of the mill itself, and this makesit possible to select them according to theamount of gas needed for drying andconveying the material. The ATOX mill willgrind and dry coal containing more than 20per cent moisture in one operation.

Working principlesThe raw coal enters the mill via an airtightfeed screw and is discharged onto thecentre of the rotating grinding table. The

rotation of the table accelerates the flow ofmaterial towards the grinding track wherethe coal is ground between the table andthe three rollers. The coal then continuesover the dam ring and is entrained in thehot drying air that enters the mill housethrough the nozzle ring.

The air lifts the coarser particles back ontothe grinding table and sweeps the finerparticles up to the separator that lets thefinal product proceed to the mill outletwhile returning the coarse fraction to thetable for further grinding.

Having left the mill at the top, the finalproduct continues with the air to a cycloneor a bag filter where it is collected.

Continuous improvement

Nozzlering

100 %

100 %

200 -300 %

1500 -3000 %

100 -200 %

50 - 60 m/s

MAIN FEATURES

• High-efficiency millgrinding coal, lignitepetcoke, anthracite,etc.

• Dynamic rotary separator ensures high-grade endproduct.

• Compact design minimizes space requirement.

• Segmented roller and table wear parts prolong service lifeand facilitateinstallation.

• Large rollers permitcoarser feed andensure a thickgrinding bed.

• No auxiliary drive needed.

• High flexibilityensures stable opera-tion, even during low production load.

• Optimized air distribution reducespressure loss.

VE

RT

IC

AL

M

IL

L

SY

ST

EM

Material circulation inside the mill.

LayoutThe layout of a coal grinding system musttake into account the available heat sourcesfor drying the raw coal as well as the fireand explosion hazards of coal and coaldust. The entire grinding plant must there-fore be shock-resistant and equipped with anumber of explosion relief valves.

Non-inert operationWhen grinding low-explosive to moderatelyexplosive coal types, the system may oper-ate under non-inert conditions. Non-inertoperation allows using excess air from theclinker cooler for drying and conveying pur-poses. No recirculation of air nor waterinjection in the mill is required, becauseatmospheric air can be used to maintain thenecessary flow, independent of the amountof hot air needed for drying.

Inert operationA commonly adopted safety precaution isto operate the coal grinding system underinert conditions. At a cement plant this canbe achieved by using exit gases from thekiln preheater to dry and convey the materi-al through the mill. A variable amount ofcleaned air can be recycled from the grind-ing system filter to maintain the requiredflow for separation and conveying throughthe mill, independent of the amount of hotair needed for drying.

The ATOX coal mill has a water injectionsystem to compensate for a low water con-tent in the raw coal, which allows increas-ing the amount of hot air instead of recircu-

lating air at a level that would invalidate theinert condition of the grinding system.

For most types of coal, the grinding systemis arranged so that finish ground coal mealis collected in a bag filter or a cyclone fol-lowed by an electrostatic precipitator,before the cleaned air enters the mill fan.For more explosive types of coal such as lig-nite, the filter may be installed on the pres-sure side of the fan. This eliminates ingressof false air into the filter and keeps the levelof oxygen in the filter and the recirculatedair at a minimum.

Material flow Mill air flow

Raw coal feed

Raw coal feed

Raw coal feed

To coal meal silo

To coal meal silo

To coal meal silo

From preheater

From cooler

From preheater

Material + air flow

Water

Water

Tostack

Tostack

Tostack

a)

b)

c)

Layout of a) non-inert,b) inert and c) inertsystem for handlinghighly explosive coals.

ATOX Coal Mill

Main gear unit

Hydraulic cylinder, incl. accumulators

Tension rod

Grinding table

Buffer house for torque rod

Grinding roller assembly

Double feed screw

Reject cone

Guide vanes

Cage rotor

Mill outlet

Variable speed drive for separator

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

10

10

11

11

12

12

Both the grinding table and rollers are fittedwith segmented wear parts. Segmentationallows the use of very hard and wear-resist-ant material without running the risk ofthermal cracks occurring in the wear seg-ments. The cylindrical shape of the rollersmakes it possible to reverse the segments,enabling a high degree of material utiliza-tion even in the case of uneven wear. Using wear-resistant, high-chromium whitecast iron ensures long life of the grindingsegments.

Hydraulic cylinders anchored in the founda-tion block generate the grinding force forthe individual rollers. The hydraulic force istransferred to the ends of the roller shaftsvia tension rods. The joints of these rods aredesigned as prestressed bolted flangeswhich offer high resistance against fatiguefailure.

The roller assembly is kept concentric with the table by means of horizontal torque rods connecting each roller shaft end to buffer houses in the mill casing via shock absorbers.

Before starting the mill motor, the rollers are lifted off the grinding track. When the mill fan has been running for a short while, mill feed is started and the rollers are lowered onto the grinding bed.

Mill design for long life and simple operation

Rollers with wear segments

An oil circulation system effectively lubri-cates the bearings of the grinding rollers.Each roller is fed individually with condi-tioned oil from a common supply station inwhich a separate circulation system providesfiltration and temperature conditioning. Formost mill sizes a three-section flow dividerensures an equal flow of oil to each roller.Instead of the flow divider system, thelargest coal mills have separate feed andsuction pumps for each roller. High-tempe-rature grease is used to lubricate the bear-ings of the smallest mill types.

The simple and lightweight loading arrangement ensures the lowest possible inertial reactions to gear and foundation parts.

Feed arrangementThe raw coal enters the mill via a doublefeed screw that also serves as an air sluice.The double screw is specially designed tohandle sticky material.

RAKM rotary air separatorThe rotary air separator is flanged to thetop of the mill housing. The rotor shaft isdriven by a variable-speed AC motor via agear unit. The rotor runs inside a ring ofguide vanes. The material entrained in theair from the mill enters the rotor throughthe guide vanes. The rotor rejects thecoarse particles to be collected by the guidevanes and returned via the reject cone tothe grinding table for further grinding,while the air and the finished material leavethe separator via the outlet duct. The fine-ness of the ground product can be adjustedby varying the speed of the rotor.

High-performance gear unitThe standard main gear unit for an ATOXmill is of the sturdy bevel-helical or morecompact bevel-planetary type supplied byMAAG Gear.

The gears are designed for high dynamicloads with a generous service factor. Theaxial thrust bearing supporting the grindingtable and the grinding force is of the seg-mented design in which all thrust pads areimmersed in an oil bath.

The lubricant for the thrust pads and forthe internal gearings/bearings is conditionedand filtered in a separate pump station.

High-performance units

Bevel-helical gear type Bevel-planetary gear type

Changing of wear segmentsChanging of roller wear segments for small-er-size mills normally takes place outside themill. The whole roller assembly is pulled outon a special trolley and serviced on the plat-form in front of the mill. In the case of big-ger-size mills changing of roller and tablesegments is easily carried out inside the millwhich is provided with a small hoist for thatpurpose.

Sizing the Atox millThe specific energy consumption dependson the grindability of the raw coal and thecoal meal fineness required.

The grindability is usually specified accord-ing to the Hardgrove Grindability Index(HGI). The specific energy consumption ofan ATOX coal mill based on the Hardgroveindex is shown by the grindability diagram.

The specific energy consumption stated inthe grindability diagram is based on thecapacity including residual moisture in thecoal meal. This is an important considera-tion for coal types such as lignite that areoften produced with 8-12 per cent residualmoisture.

trolley

kWh/

t pro

duct

HGI8070605040

690

7

8

9

10

12

13

11

14

10% + 90 µm15% + 90 µm20% + 90 µm

5% + 90 µm

25% + 90 µm

Removal of rollerassembly in small-sizemills before changingsegments.

Changing of the wearsegmentsin large-size mills.

Atox coal mill sizing – Specific energy consumption.

RAKM

Separator

12.513.515.517.520.022.525.027.530.032.535.037.5

F

mm

185019702200245029203260360039504510490052005580

G

mm

48205140603064107240801089409610

10210110901200013230

H

mm

305033103880416045505170575063206960757083909120

Max.speedr/min

436406356316250223201183160147141132

Installedpower

kW

101216223141546995116140166

Atox

Mill

12.513.515.517.520.022.525.027.530.032.5

A

mm

2800326536104095445049505335560075008420

B

mm

1900210024002600290032503550390043604694

C

mm

2800310034503800500056006000600066007600

D

mm

5500610068007400750086009200100001050011300

E

mm

1600180020002250250030003000350035003800

Installedpower

kW

93-106113-129159-182215-246300-343403-461524-599666-761828-946

1011-1156

Tablespeedr/min

50.148.245.042.339.637.335.433.832.331.1

Data in this brochure is intended for preliminary project planning only. Manufacturer reserves the right to modify equipment details and/or specifications without notice.

USAF.L.Smidth Inc.2040 Avenue CBethlehem, PA 18017-2188Tel: +1 610-264-6011Tel: +1 800-523-9482Fax: +1 610-264-6170E-mail: info@flsmidth.com

INDIAFuller India LimitedCapital Towers180, Kodambakkam High RoadNungambakkamChennai 600 034IndiaTel: +91 44-827-6030/8228623Fax: +91 44-827-9393E-mail: fil@fullerindia.co.in

DENMARK F.L.Smidth A/SVigerslev Allé 77DK-2500 ValbyCopenhagenTel: +45 36 18 10 00Fax: +45 36 30 18 20E-mail: info@flsmidth.com

D

B

E

F

CA

G

H

Separator dimensions and characteristics

Mill dimensions and characteristics

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