The two main functions of a mill liner consist in protecting the mill shell against wear and in transmitting the energy of the mill motor to the grinding charge to carry out the required size reduction in the most efficient way.

For a mill liner, the design as well as the liner material are

to be considered.

The required movement of the mill charge to carry out effi-

ciently a certain type of size reduction (for instance coarse

grinding, fine grinding,...) must be promoted by the right

mill liner design. A wrong liner design choice leads to poor

grinding (excess energy usage), higher grinding media con-

sumption and even higher liner maintenance costs.

The liner choice should therefore always be seen as one

element influencing the total grinding cost of liner, grinding

media and energy, and not as a stand-alone cost. With re-

gards to liner life/cost for a specific design it is important

to choose the correct material/alloy for that application.

Grinding mechanisms:

The rules that guide the design of a liner depend on the

required grinding mechanism. Three main categories of

grinding mechanisms can be highlighted:

1. IMPACTThe mechanism of reducing the big rocks is "crushing"

which requires impacts. Either by impacting of grinding

balls on the rocks (SAG mill) or by interaction between

rocks on the one side and mill shell and toe of the grinding

charge on the other side (FAG mills). The trajectories of

grinding media and rocks promote size reduction by im-

pacts. This grinding mechanism is required in SAG and FAG

mill applications.

2. PERMEABILITYThe main size reduction in this mechanism is carried

out in the ball charge. The liner design needs to be such

that the charge is "opened-up " (permeability) to al-

low bigger grains to be drawn into the ball charge where

they will be reduced. Limited ball trajectories allow this

opening up of the charge. This mechanism is typically

needed in coarse grinding for instance primary ball mills).

3. ATTRITIONThis mechanism involves size reduction between moving

grinding balls. Adequate size reduction will require high

relative movement between the grinding balls while main-

taining the pressure within the ball load. Liner design is

such that trajectories should not occur. A compact charge

is required.

MINE LINERS ...

THE CHOICE DESIGN

1 2 3

• Mill type: SAG, FAG, Rod mill, primary ball mill, secondary

ball mill, regrind, …)

• Ore feed size (F80 ,F100) and required product size (P80)

determining the grinding mechanism to be favoured

• Mill size (diameter, length)

• Maximum ball top-up size

• Mill rotational speed (rpm, variable or fixed speed, single

or dual rotation)

• Ore type (abrasivity of the ore)

• Mill discharge type (grate discharge mill, overflow mill)

• Available drilling pattern

• Total filling degree (ore and balls)

FAG & SAG

Main design parameters:

• H (effective lift)

• S (spacing between lifters) and

• alpha (attack angle)

Iteration process to reach best-suited H,S and alpha.

FINE GRINDING

Reducing small particles by attrition requires a combina-

tion of force (F) and relative speed (v).

The selection of the liner material is linked to the applica-

tion (the main parameter also considered when choosing

the liner design) and the resulting working conditions.

The table below gives general resistance characteristics of

the most common liner alloys

ex overflow mill

ex SAG liner

ex BM3 - RGD

... THE RIGHT CHOICE

PARAMETERS EXAMPLES MATERIAL

ALLOYSAUST. MN STEELS

PEARLITIC STEELS

MART.HICR ALLOYS

Composition Mn Cr-Moly 12-27 Cr

Structure Matrix MatrixMatrix + Cr carb.

(30-35%)

Hardness200-450HB after work hardening

325-400HB 600-650HB

Resistance abrasion

- to + - ++

Resistance corrosion

- - ± to ++

Resistance repeated impacts

+++ ++ +