Ball Mill TestingCement / Raw Mills

D.Baird /Sean Haseldine

OBJECTIVES

• To Establish Actual Performance Criteria

• To Identify the Problem Areas

• To resolve the Problems and Optimise the mill

Product Residues (Raw Meal)Surface Area (Blaine / L+N)

Feed Quality Clinker / Gypsum ratioClinker chemistryC3S / C2SFeed size

Monitoring mill operation“Establish the facts”

Mill Throughput Tonnes /h

Power Used kWh - Mill + Ancillaries

Temperature Degrees C

Air Flow Kg air /Kg cement

Water injection % on Feed rate

Motor power

Cos φφφφ from the motor nameplatenormally 0.85 - 0.9

Where V = Applied voltage

I = Motor current

Cos φφφφ = Motor power factor

3 = 1.732

W = x V x I x Cosφφφφ3

Effect of Gypsum, Limestone + Pozzolan On mill performance

• 1% SO3 = m2/kg Surface Area• 1% Gypsum = Production• 1% Gypsum = m2/kg Surface Area• 1% Limestone = % B.C.I Grindability • 1% Pozzolan = to % B.C.I.

Grindability

+30+5%

+12-2.5

-0.4 -0.67

Relationship between power consumption and Chemistry for two closed circuit mills

No 2 Mill SRC

Nº1 Mill (OPC)

Nº1 Mill (OPC)

Nº2 Mill (SRC)

Mill Only Kwh/t

L.S.F.

LSF

C2S

C3S

32.5 33.0 33.5

C3S

C2S

C3S

C2S

Mills under AutomaticTotal Feed

Control

Chemical effects on mill

performance

Max. T.P.H.

t.p.h.

Kwh/T

1986 1987

C2S

C3S

Min C2S

Max. C3S

Lowest cementgrindability

Highest mill output

Grindability

Axial Testing

Procedure1. OBSERVE STABLE MILL OPERATION BEFORE THE TEST.

MEASURE POWER CONSUMPTION AND PRODUCTION.TAKE SAMPLES OF FEED AND PRODUCT.TAKE SEPARATOR AND MILL CIRCUIT SAMPLES

2.- STOP BOTH MILL AND FEED SIMULTANEOUSLY.

3.- TAKE AXIAL SAMPLES (ALONG THE LENGTH OF THE MILL)

4.- DETERMINE FINENESS OF THE SAMPLESFOR EXAMPLE RESIDUES AT 2.36 mm, 1.18 mm, 300 mic. 90 mic, 45 mic.

- CAN CALCULATE SURFACE AREA IF REQUIRED

5.- MEASURE - VOLUME LOAD = CALCULATE CHARGE WEIGHT

6. - CALCULATE POWER USEDNETT kW = 0.2846 x D x A x W x N

7. - PLOT THE AXIAL GRAPH.

Taking axial samples

Sample below the surface of the material

and media

x x x x

xxx

Measure the heightabove charge in each chamber H1 +H2

H2H1

Material Flow

% V

olum

e lo

ad

RATIO H/D

Ball mill volume load =estimation

50

45

40

35

30

25

20

15

10

5

0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85

H=HEIGHT ABOVECHARGE

D= INTERNAL DIA.

H

VOLUME LOAD FORMULAVOLUME LOAD FORMULA

% Volume Load =400/PI*((0.25*Cos(2*(H/D-0.5)))

-((H/D-0.5)*((H/D-(H/D)^2)^0.5)))

H = HEIGHT ABOVE CHARGE (M)

D = MILL INTERNAL DIAMETER INSIDE LINING (M)

Changes in apparent volume load due to charge expansion

H1 = Height above charge -Mill Crash Stop

Apparent Charge density Case %Volume loadt/m3

3.91 H1 Crash Stop 38%H 1 H 2

H2 = Height above charge - Mill Run Out

4.50 H2 Without Material 33%

Charge Expansion 5%

Mill power calculation - DAWN Formula

NETT KW = 0.2846 DAWN

Where D = Internal Diameter of Mill

A = 1.073 - J

(Where J = Fractional Volume Load i.e. if VL = 30% then J = 0.3)

W = Charge weight in Tonnes

N = Mill speed in RPM

Net and Gross mill powerNormal values for the ratio Net/Gross power

0.90 Normally for old and inefficient mills0.92 Polysius Combiflex Drive0.93 Girth gear driven mill0.94 -0.95 Modern central drive mill

Unexpected values0.88 High power losses possibly due to mill

chambers running empty.0.97 - 1.00 Low power losses possibly due to charge

expansion and/or build up of clinker nibs.

If unexpected values occur : review the mill data

Target levels for material fineness before the intermediate diaphragm

Sieve size % Cumulative Residue

2.36 mm 1 %

1.18 mm 6 %

300 um 20%

Basis - sieve about 0.5 - 1 Kg of material if there are clinker nibs present.

Axial samples can help identify the problem

% Retainedon the sieve

InletOutlet

chamber 1 chamber 2

100

0

Coarse accumulation dueto nibs leaving chamber 1.

45 mic

2.36mm2.36mm

300 mic

Targets for cement mills with 6-8mm slots

Sample fineness hereExpect 20% max + 300 mic

1% + 2.36 mm (4% max)

Low reduction in 45 micron sieve residuedue to media coating and charge cushioning with chamber overfilling. Badly worn or incorrect gradings withbackmixing can have the same effect.

Coarse material accumulation

% Retained v.s.Position C em en t M ill E xam p le

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

1a 1b 1c 2a 2b 2c 3a 3b 3c 3d 3e 3f

P osition A long M ill

% R

etaine

d

19,000

9500

6700

2360

1180

600

300

150

90

+ 45

SSA v.s Position

P lot of SSA (m 2/kg) V .s. position along m ill

0.00

100.00

200.00

300.00

400.00

1a 1b 1c 2a 2b 2c 3a 3b 3c 3d 3e 3f

P osition

SSA(m

2/kg)

S S A

INSPECTION POINTS

LINING PLATESWATER INJECTION

AIR OUTLET

PRODUCTDIAPHRAGMS

GRINDINGMEDIA

FRESH AIR INLET

SEALS

(SIZES - SAMPLES -CONDITION) (SLOT SIZES - AVAILABLE SLOT AREA- VENTILATION GRID)

Sampling closed circuit mills

Fresh Feed

Separator rejects

Mill Product (Separator feed)

SeparatorFines

Finished Product

Filter Product

CEMENT MILL - AIR FLOW TESTSMEASURED AIRFLOW= 200 m³/min

TEMP 50 ºC

MILL EXIT = 115ºC

TO ELEVATOR

TOTAL FREE VOLUMEABOVE CHARGE = 40 m³

APPARENT AIR CHANGES/MIN = 200 = 540

(TARGET 2 - 3 OPEN CIRCUIT 5 - 7 CLOSED CIRCUIT)( 0.25 0.4 KG AIR/KG CEMENT )

COLD AIR20ºC

FRESH FEED ANDSEPARATOR REJECTS

FALSE AIR INLEAK

ACTUAL -FROM HEAT AND MASS BALANCE

= 1 · 5AIR CHANGES !!!!!

MEASURED AIRFLOW= 200 m³/min

TEMP 50 ºCFRESH FEED ANDSEPARATOR REJECTS

COLD AIR20ºC

30% OF TOTALAIRFLOW

COLD AIR20ºC MILL EXIT = 115ºC

TOTAL FREE VOLUMEABOVE CHARGE = 40 m³

APPARENT AIR CHANGES/MIN = 200 = 540

(TARGET 2 - 3 OPEN CIRCUIT 5 - 7 CLOSED CIRCUIT)( 0.25 0.4 KG AIR/KG CEMENT )

TO ELEVATOR

CEMENT MILL - AIR FLOW TESTS

MILL TESTING EXAMPLEor

Your Time To Think• A Closed Circuit Cement Mill has

recently been overhauled with new internals.

• Output is up by 15% and you are promoted to Works Director.

• After 6 months, output drops by 25%.

• What will YOU do to avoid being demoted to Tea Boy?Your Boss hears about

the 20% drop in output !

Checklist-Look for Changes in:-• Cement Grindability, chemistry, SO3,

Free lime, C2S, C3S, Gypsum, MAC %.• Mill Absorbed Power, Kw per chamber.• Mill Internals Inspection, Media size

grading, Liners -type/condition, Diaphragm slot sizes/areas + % blockage.

• Powder levels, degree of coating, charge expansion.

• Cement-Residue, SSA, SO3, R-R Slope.• Mill Ventilation flow at mill inlet and

filter, Inleaking air, Water Injection rates.• Ancillary plant limitations, B/Elevator etc.• Separator inspection, Efficiency, Bypass.MILL AXIAL SAMPLING TEST AND FULL

INTERNAL INSPECTION

Worn diaphragm

Media Blockage

1st ChamberLining

Worn Chamber Lining