AMIT 135: Lesson 6 Grinding Circuit – Mining Mill Operator Training

Contents

Objectives

At the end of this lesson students should be able to:

Explain the grinding process
Distinguish between crushing and grinding
Compare and contrast different type of equipment and their components used for grinding
Identify key variables for process control
Design features of grinding equipment (SAG, BALL and ROD MILLS)
Explain typical flowsheets of grinding circuits involving single or combination of equipment

Reading & Lecture

Size reduction by crushing has a size limitation for the final products. If further reduction is required, below 5- 20mm, grinding processes should be used.

Grinding is a powdering or pulverizing process using the rock mechanical forces of impaction and attrition.

The two main objectives for a grinding process are:

To liberate individual minerals trapped in rock crystals (ores) and thereby open up for a subsequent enrichment in the form of separation.
To produce fines (or filler) from mineral fractions by increasing the specific surface.

Grinding takes place in more “open” space which makes the retention time longer and adjustable compared to crushers.

Chart showing theoretical size reduction and power ranges for different grinding mills — Theoretical size reduction and power ranges for different grinding mills
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AG/SAG Mills

Autogenous Grinding (AG) Mill

Wet or dry
Primary, coarse grinding (up to 400 mm feed size)
Grinding media is grinding feed
High capacity (short retention time)
Sensitive to feed composition (critical size material)

Semi-Autogenous Grinding (SAG) Mill

Wet or dry
Higher capacity than A-G mill grinding
Primary, coarse grinding (up to 400 mm feed size)
Grinding media is grinding feed plus 4-12% ball charge (ball dia.100- 125 mm)
High capacity (short retention time)
Less sensitive to feed composition (critical size material)

Semi-Autogenous Mill

An image of a very large ball mill — Note the size of the mill: Why does the diameter need to be so big?
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SAG Mill Circuit Example — Gold Processing

SAG mill circuit example for gold processing
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AG/SAG Mill

AG/SAG mills are normally used to grind run-off-mine ore or primary crusher product.
Wet grinding in an AG/SAG mill is accomplished in a slurry of 50 to 80 percent solids.

Image of a simulation of a charged ball mill — 2D and 3D simulations of particles in a SAG Mill
red=fastest, blue=slowest moving particles
[image: (135-6-4)]

The mill product can either be finished size ready for processing, or an intermediate size ready for final grinding in a rod mill, ball mill or pebble mill.
AG/SAG mills can accomplish the same size reduction work as two or three stages of crushing and screening, a rod mill, and some or all of the work of a ball mill.
Because of the range of mill sizes available, AG/SAG milling can often be accomplished with fewer lines than in a conventional rod mill/ball mill circuit.

A diagram of types of AG/SAG mills
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Rod Mills

Wet only
Coarse grind
Primary mill at plant capacities of less than 200 t/h
Coarse grinding with top size control without classification
Narrow particle size distribution

Mostly dry
Coarse grind high capacity
Special applications
End discharge: finer product
Centre discharge: rapid flow, less fines
Narrow particle distribution

Grate discharge is not available in Rod Mills

Rod Mill Comparison

Overflow

Wet only
Robust and simple
Finer grind (longer retention time)
High risk for over grinding
Ball charge 35-45%

Discharge

Discharge end more complicated
Coarser grind (shorter retention time)
Lower risk for over grinding
Can take about 5-10% more ball with corresponding higher throughput

Images of a ball mill exterior, interior, and grinding media — Ball mill exterior, interior, and grinding media
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Pebble Mill

Wet or dry
Always grate discharge
Secondary grinding
Grinding media :
- A size fraction screened out from feed
- Flint pebbles
- Porcelain balls
- Al203 balls
Larger than Ball mills at same power draw

Special Tumbling Mill

Conical Ball Mill

Wet or dry (air swept)
Overflow or partial grate
Conical shell for “graded” ball charge and optimal size reduction
Only available in small and intermediate sizes
Efficient “high reduction ratio grinding’

SSR Mill (Svedala Rubber Roller mill)

Wet or dry
Overflow and grate discharge
Light and fabricated construction
Ready assembled on steel frame
Easy to move
Limited in size (max. diameter 2.4 m)

Vertimill ® (Metso Minerals)

Wet grinding only
Top or bottom feed
Grinding by attrition/abrasion
Primary or regrinding mill
Ideal for “precision” grinding on finer products
Restriction in feed size (6mm)
Restriction in energy (1119 kW/ 15oo hp)
Ball size max. 30mm

Vertimill diagram — Vertimill by Metso Minerals
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Stirred Mills vs. Tumbling Mills

Advantages of Stirred Mills (VERTIMILL ®):

Lower installation cost
Lower operation cost
Higher efficiency
Less floor space
Simple foundation
Less noise
Few moving parts
Less overgrinding
Better operation safety

Typical Cost of Grinding

The main costs for grinding are energy, liners and grinding media.
They are different for different mill types. For tumbling mills:

Diagram of cost breakdown by mill type
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Basic Mill Linings

Use rubber linings wherever possible due to lifetime, low weight, easy to install and noise dampening.
When application is getting tougher use steel-capped rubber, still easier to handle than steel.
When these both options are overruled (by temperature, feed size or chemicals) use steel.
Ore-bed is a lining with rubber covered permanent magnets used for special applications like lining of VERTIMILLs, grinding of magnetite

Three images of different types of lining types — Various types of mill linings
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Size Grinding Mills

Fundamental to all mill sizing is determining the necessary specific power consumption for the grinding stage (primary, secondary, tertiary etc.) .

It can be established (in falling scale of accuracy) in one of the following ways :

Operating data from existing mill circuit (direct proportioning) .
Grinding tests in pilot scale, where the specific power consumption is determined (kWh/t dry solids) .
Laboratory tests in small batch mills to determine the specific energy consumption.

Energy and power calculations based on Bonds Work Index (Wi, normally expressed in kWh/ short ton).
Other established methods, for instance Hardgrove Index, population balance.

Scale Up in Sizing for Grinding Mills

Scale-up criterion is the net specific power consumption, i.e. the power consumed by the mill rotor itself minus all mechanical and electrical losses divided by the feed rate of solids.
For the full scale mill, specific power is multiplied by the feed rate to get the net mill power. This must then be increased by the anticipated mechanical inefficiencies (bearing/gear friction losses and possible speed reducer losses) as well as electrical losses, in order to arrive the gross mill power.
For all AG or SAG installations, batch lab tests are mandatory to determine whether this type of grinding is possible at all, as well as establishing the necessary specific power consumption.

Size Reduction Circuits

Single Stage AG Mill

For the rare cases where primary AG milling will inherently produce the required product size. (Wet and dry)

AG Mill + Crusher

For the cases where critical size fractions needs to be removed from the mill and crushed separately to prevent overgrinding. Resulting size must match product requirements.

Single stage AG mill and AG mill with crusher circuits
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AG Mill + Ball Mill + Crusher (ABC Circuit)

This can be used to correct a too coarse product from the primary mill.
Mostly operated wet, but also dry possible.

AG Mill + Pebble Mill

Two stage AG-grinding with the primary mill in open circuit and the secondary pebble mill in closed circuit.
The pebbles screened out from the primary mill or recirculated to the primary mill based on size.

Circuits diagramming AG mill and Ball Mill and Crusher as well as AG Mill, and Pebble Mill — Circuits diagramming AG mill + Ball Mill + Crusher as well as AG Mill + Pebble Mill
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AG Mill + Ball Mill / Vertimill

Pebble mill is replaced by a ball mill or a Vertimill in the previous circuit.
This is used when there is not enough pebbles available in the circuit, or all autogenous grinding produces too much fines

AG Mill + Ball Mill / Vertimill

Pebble mill is replaced by a ball mill or a Vertimill in the previous circuit.
This is used when there is not enough pebbles available in the circuit, or all autogenous grinding produces too much fines

Two circuits diagramming AG Mill and Ball Mill / Vertimill — Circuits diagramming AG Mill + Ball Mill / Vertimill
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Single Stage SAG Mill

Using SAG instead of AG increases the capacity as well as application range, but will also increase wear costs and still be dependent on “natural” product size being close to the desired.
Common circuit in the US and Canada

Closed Vertimill Circuit with Integral Classifier

Used for wet circuits with not too fine desired product and/or not stringent coarse end oversize of the product.
Max. feed size – 6 mm (1/4″).

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