AMIT 135: Lesson 6 Grinding Circuit

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
[image: (135-6-1)]
 

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?
[image: (135-6-2)]
 

SAG Mill Circuit Example – Gold Processing

SAG mill circuit example for gold processing
SAG mill circuit example for gold processing
[image: (135-6-3)]
 

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
A diagram of types of AG/SAG mills
[image: (135-6-5)]
 


 

Types of rod mills
A diagram of types of rod mills [image: (135-6-6)]

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 mills in situ
Rod mills in situ
[image: (135-6-7)]

Rod Mill Comparison

 

Rod mill diagrams of overflow and grate discharge
Rod mill diagrams of overflow and grate discharge
[image: (135-6-8)]

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
[image: (135-6-11)]

 

Pebble Mill

A pebble mill diagram
Pebble mill diagram
[image: (135-6-13)]
  • 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

Special tumbling mills
Special tumbling mills
[image: (135-6-14)]
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
[image: (135-6-16)]
 

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
Diagram of cost breakdown by mill type
[image: (135-6-17)]

 

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
[image: (135-6-18)]
 


 

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 :

  1. Operating data from existing mill circuit (direct proportioning) .
  2. Grinding tests in pilot scale, where the specific power consumption is determined (kWh/t dry solids) .
  3. Laboratory tests in small batch mills to determine the specific energy consumption.
  1. Energy and power calculations based on Bonds Work Index (Wi, normally expressed in kWh/ short ton).
  2. 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
[image: (135-6-19)]

 


 

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
[image: (135-6-21)]

 


 

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
[image: (135-6-23)]

 


 

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

 

Single stage SAG mill circuit
Single stage SAG mill circuit
[image: (135-6-25)]

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″).

 

 

Closed Vertimill circuit with integral classifier
Closed Vertimill circuit with integral classifier
[image: (135-6-26)]

 

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