Download Grinding Fundamentals PDF

TitleGrinding Fundamentals
Tags Industries Nature Bearing (Mechanical) Mill (Grinding)
File Size2.6 MB
Total Pages49
Document Text Contents
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- Mineral Liberation 6
- Feed Preparation 6
- Dry Grinding 6
- Power Considerations 7
- Bond Grindability Tests 7
- Efficiency Factors 7
- Operating Work Index 8
- Limitations of the Bond Test 9
- Single Particle Tests 9
- Bond Impact Test 9
- Drop Weight Test 10
- Mill Speed 10
- AG Mills 10
- SAG Mills 10
- Rod Mills 10
- Ball Mills 11
- Linear and Speed Effects 11
- Fines Correction 11
- Classification and CLR aspects 11
- Optimization 11


- Bond Grinding Indices 13
- Unconfined Compressive - Strength (UCS) 13
- Impact Crushing Test 13
- Autogenous Tumble Test 14
- JK Drop Weight Test 14
- Other Tests 14

• Macpherson Test 14
• Hopkinson Pressure Bar 15
• Starky Test 15
• Pilot Scale Testing 15
• Circuit Surveys 15


- Application information 17
- Test Requirements 17
- Typical Flowsheets 17
- Standard Sizes 18


- Application information 19
- Ball Charge Regimes 19
- Test Requirements 20
- Typical Flowsheets 20
- Standard Sizes 20
- Pebble Mills 21

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Mill Sizing Consideration

AG and SAG mills currently face limitation which
are electrical and mechanical in nature.

The largest mills designed to date are 12.2 meters
diameters with 20MW installed power. Even larger
units of up to 13.4 meters diameter are now being
considered. At these sizes, engineering
tolerances are very small, as stresses imposed on
mill shells, heads and drives are very high. Also,
as the limits of current international foundry
capabilities are pushed (because of the very large
size and weight of the components), there is more
risk associated with the delivery of such large mill
units of fully fabricated design, using shell
mounted bearings to reduce the peak stress

The largest single pinion drive on offer currently is
6.75 MW, with plans to go to 8 MW in the next few
years. Therefore, in order to provide the + 20 MW
power demanded by the world’s largest mills, it is
necessary to go to bigger and bigger gearless
drives which increase the capital cost of the mill

Breakage Rate/
Population Balance Model

Another method of modeling the grinding process
within AG and SAG mills uses population balances
of the various particle size fractions.

The population balance assumes the mill is in
steady state and therefore the amount of material
within a particular size fraction must be constant.
That is, the discharge from the mill must equal the
amount of material that has resulted from
breakage of larger size fractions, less any of the
contents selected for breakage into smaller size

Discharge rates from the mill depend on the mill
contents and the classification of the particles
through the grates and pebble ports. The ability of

the contents to pass through the grate becomes a
mass transfer problem.

The material selected for breakage depends on
the mill contents and also the breakage rate of
each fraction. Empirical correlations of the
breakage rates with varying operating conditions
and mill dimensions have been developed.

Material arising from the breakage of larger
particles is estimated in terms of an appearance
function. This defines the particle size distribution
resulting from the breakage of an individual
particle, through the combination of mechanisms
including impact and abrasion.

Modeling requires the fitting of the breakage rates
to plant sizing data around the mill. This can be a
tedious task, especially if the data used is not of
good quality.

Power Model

For SAG mills, mill power draw is not only a
function of mill size but charge density as well.
For ball and rod mills, the charge is predominantly
steel, but for SAG mills this is a mixture of steel
and rock media, as well as slurry.

Power draw theory is based upon a charge load in
the equilibrium, and relates to its center of gravity
Figure 14 describes this relationship visually.
The following formula relates these values to the
actual power draw:

N = c x W x Rg x n


N = the gross mill power, in kW

c = is a constant, nominally 1/1200 for SAG, AG
and pebble mills

W = the weight in kg of the charge

Rg = the distance in meters of the center of
gravity from the mill center

n = is the speed of the mill in rpm

The power model is such that the mill power is
more sensitive to diameters than length.
Diameters affects power draw exponentially,
whereas the relationship between length and
power is linear. Therefore, incremental changes in
diameter provide step changes in power draw.
The trade-off is that the capital cost of the mill also
climbs steeply with increasing diameter partly due
to manufacturing methods and also as a result of
increasing surface area and greater load on the
mill structure.

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Outokumpu utilizes unique software that enables
the mills to be accurately sized from a given power
draw, for both AG and SAG applications.

Application information

AG and SAG circuits are now widely accepted for
the following reasons:

• the process is well understood, and its
behavior can be predicted

• it is cost and power effective versus multi-
line crushing

• lower maintenance requirements than a
fine crushing circuit

• high availability
• well suited to high-clay or sticky ores
• very high throughput in a single line

SAG or AG milling usually offers superior net
returns on a project when compared to more than
two lines of conventional crushing or other grinding
solutions, but there are exceptions:

• slimes generation is problematic to
downstream processes such as flotation

• the ore is so competent that the mills

retains a disproportionate amount of lump
in the mill that cannot be dealt with by a
ball charge or pebble extraction and

• the ore is very non-abrasive, resulting in

low media and liner consumption rates in
all equipment

Outokumpu offers very large cone crushers and
the unique Water flush crusher which can be
applied with conventional ball mills to address
many of these difficult ores.


Ore characterization in the form of autogenous
and impact crushing tests will reveal an ore’s
tendencies toward critical size build-up of tough

pebbles (30 to 80 mm size range) that inevitably
results in poor grinding efficiency. To combat this
problem, it is common to install pebble relief ports
or slots, of width 50 to 90 mm. Depending on the
out come of testing. Critical size pebbles can then
be extracted, screened and conveyed to a pebble
crusher, which is a more effective means of
imparting efficient breakage. Crushed material at
–10 to 15 mm is usually recycled back to mill

feed for subsequent grinding by the ball or rock
charge. The resulting circuit, comprising a semi-
autogenous mill, ball and crusher is known as
SABC, while the equivalent autogenous circuit is
designated ABC (Figure 15). As a guideline, if the
impact work index of the critical size material
peaks at around 1.5 time the ball mill work index, a
crusher will show economic benefit in reducing
fSAG, sometimes to unity.

Consideration needs to be given to providing
affective means of removing steel scats and tramp
metallics from the pebble crusher feed, avoiding
damage to crusher liners. This consists of a
combination of a belt magnet, magnetics sensor
after the magnet and a diversion gate to allow
automatic bypass of the crusher in the event that
metal on the belt is sensed.

Shell supported ball mills supplied by Outokumpu
Mills A/S. Use of shell mounted pad bearings allows
the feed and discharge arrangements to be
configured to suit process parameters.

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Compañía : _____________________________________________
Dirección : _____________________________________________
Teléfono : _____________________________________________
Fax : _____________________________________________
E-Mail : _____________________________________________
Nombre de la Persona de Contacto :__________________________
Cargo : _____________________________________________
Fecha : _____________________________________________

Sírvase llenar en la máxima extensión posible el siguiente cuestionario y dejar en blanco
los datos que no se encuentren disponibles

Si ya existe un molino en operación en una etapa de molienda, similar al del nuevo molino
a ser instalado en un futuro, preste especial atención a las preguntas No. 23 y siguientes.

1. Material a ser procesado : ____________________________________
2. Características del Material : ____________________________________
3. % Humedad en la Alimentación : _______________________________
4. Work Index (Wi) : _____________________________________________
5. Gravedad Específica de la Roca : _______________________________
6. Densidad Aparente (Bulk) de la alimentación : ______________________
7. Capacidad (ton solidas / h) : ____________________________________
8. Tamaño de Alimentación (Max.size / F80) : ________________________
9. Tamaño del Producto (P80) : ____________________________________
10. Tipe de Circuito: Húmedo ___ Seco___ Abierto___ Cerrado___
11. Tipo de descarga necesaria: __ Overflow, __ Malla en Trommel __ Grate
12. Localización de la planta : _______________________________________
13. Altitud (msnm) : ______________________________________________
14. Temperatura Ambiente : _________ ( F / C)
15. Velocidad Crítica : ______________
16. Potencia (hp/kW) : ______________
17. Tipo y Material de revestimiento preferido : __________________________
18. Tamaño del Molino Requerido : Diam. (Interno): ________ Longitud (Long. Efectiva

de Molienda): ________
19. Instalación : Bajo Techo ____ Al aire libre____
20. Transmisión Preferida ________________________(Acople, Embrague, motor de

baja/alta velocidad, etc.)
21. Sistema Eléctrico: _______ Volts, _____Hertz, _______ Phase

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22. Existe ya un molino en operación?(Si/No) : _______

Si ya existe un molino, favor especifique los siguientes datos:

23. Molino existente : __________________
24. Dimensiones del molino : ________________
25. Tipo de Molino : __________
26. Tipo de descarga : _________
27. Potencia del motor instalado : _________
28. Tipo de circuito: ___ Abierto, ___ Cerrado, ___ En Seco, ___En Húmedo
29. Indique para el molino existente Tamaño de Alimentación (Max. size / F80):-

30. Indique para el molino existente Tamaño de Producto (P80) : ___________
31. Indique para el molino existente la potencia consumida y/o potencia consumida por

ton: _______

Fin del cuestionario

Sírvase devolver este cuestionario a Outokumpu Técnica Perú SAC
FAX: (51 1 ) 221 2633

Gracias por su tiempo

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