GRINDING MEDIA AND CASTINGS IN MINING AND OTHER RELATED INDUSTRIES

Introduction

Scaw limited supplies the mining and mining related industry with grinding media (mill balls) as well

as ferrous and manganese castings to be used in various size reduction equipment such as Crushers,

Semi Autogenous (SAG) and Ball mills. In the mining industry, Scaw has always been active in the field

of comminution as it is the first stage of mineral extraction/recovery.

Background

Comminution is a process of reduction of large solids/rocks (Ore) into smaller fragments. The size

reduction continues until the Ore is reduced to a very fine fraction (powder). This process serves as a

primary function in mineral processing industries. In mineral processing, the rock containing valuable

minerals must be broken sufficiently fine to liberate valuable minerals from waste constituents, which

can then be easily separated by an appropriate beneficiation method.

The role of grinding media and mill liners

Mill balls/grinding media are used in SAG and Ball mills to break/fragment (grinding) the ore into smaller particles. They can be used in either wet or dry grinding.

Liners are materials made of steel used on the inner surface of the SAG and ball mills to provide corrosion resistance (erosion from chemical action) and abrasion resistance (erosion by friction) and thus protect the barrel structure (shell) from rapid wear.

The grinding occurs by impact and attrition/abrasion of the ore by mill balls and liners. The liners also provide lifting action to enhance impact crushing.

Figure 1.0 Ball mill structure

Figure 2.0 SAG mill showing wet grinding

The efficiency of the mill balls is dependent on the surface hardness and toughness. The hardness of

the mill balls is dependent on the chemistry and micro structure. In the process of grinding, the mill

balls gradually wear out but the wear rate is much lower than that of the ore.

Figure 3.0 Mill ball surface and core hardness

Mill balls

Surface hardness core hardness

Mill liners

Water

As the balls wear out, they reduce in size, therefore, there is need for the mill balls to be replenished

as need arises. The method of replenishment is either continuous or batch depending on the type of

mills used. The rate at which the mill balls are replenished is determined by;

1. The level of mill balls in the mill

2. The amperage (power used to run the mill)

Figure 4.0 SAG/Ball Mill Operation

GRINDING MEDIA OFFERED AT SCAW LIMITED

Scaw limited manufactures Hammer Forged, Low and High Chrome cast Steel Mill Balls of various sizes.

HAMMER FORGING PROCESS

The production of Hammer forged mill balls starts by melting steel scrap and casting into ingots of

various sizes depending on the final size of the mill ball.

Figure 5.0 Ingots ready for rolling into round bars

The ingots are them rolled into round bars.

Figure 6.0 Round Bars ready for preheating before hammer forging

The round bars are cut according to the required size, preheated and finally hammered into forged

balls.

Figure 7.0 Round bars after preheating before hammer forging

Figure 8.0 Hammer forging

Figure 9.0 Forged ball temperature check before water quenching

Figure 10.0 Hammer forged ball before tempering

Figure 11.0 Hammer forged ball after tempering (Final Product)

HAMMER FORGED BALLS

There are various types of forged balls but Scaw limited manufactures Hammer forged balls. The

characteristics are in the table below:

SIZE (mm)

ENVIRONMENT SURFACE HARDNESS (HRC)

CORE HARDNESS (HRC)

CHROME (%Cr)

CARBON (%C)

SULPHUR (%S)

SILICON (%Si)

MANGANESE (%Mn)

100

Alkaline

≥ 60

≥ 50

0.30 Max

0.70 -1.00

0.05 Max

0.30 - 0.50

0.80 - 1.00

125

≥ 52

≥ 48

0.30 Max

0.60 - 0.70

0.05 Max

0.30 - 0.50

0.80 - 1.00

They undergo heat treatment characterized by water quenching and Tempering.

STEEL MILL BALL CASTING PROCESS

Figure 12.0 Molten metal being transferred into the ladle

Figure 13.0 Molten metal being poured into moulds

Figure 14.0 Cast mill balls

HIGH CHROME CAST STEEL BALLS

These are available as:

TYPE SIZE (mm)

ENVIRONMENT SURFACE HARDNESS (HRC)

CORE HARDNESS (HRC)

CHROME (%Cr)

CARBON (%C)

SULPHUR (%S)

SILICON (%Si)

MANGANESE (%Mn)

Special High Chrome

40 -80

both Acidic and Alkaline

≥ 60

≥ 58

15 -28%

2.50-3.50

0.06 max

1.00 max

1.00 max

High Chrome

40 -80

both Acidic and

Alkaline

≥ 60

≥ 58

15 -17%

0.70 -1.00 0.06 max

1.00 max

1.00 max

125

1.80 -2.50

Medium High chrome

40 -100

Alkaline

≥ 60

≥ 58

11 - 14%

2.00 -2.50

0.06 max

1.00 max

1.00 max

High chromium content and corrosion resistance undergo heat treatment characterized by Oil

Quenching and Tempering.

Figure 15.0 Scaw Limited High Chrome Steel Balls

LOW CHROME CAST STEEL BALLS

TYPE SIZE (mm)

ENVIRONMENT SURFACE HARDNESS (HRC)

CORE HARDNESS (HRC)

CHROME (%Cr)

CARBON (%C)

SULPHUR (%S)

SILICON (%Si)

MANGANESE (%Mn)

Low Chrome

40-125 Alkaline ≥45 ≥44 1.50-2.50 2.00-2.50 0.06 0.06 1.00 Max

They undergo heat treatment characterized by Air hardening (Air Quenching) and Tempering.

Figure 16.0 Scaw Limited Low Chrome Steel Ball

HIGH CHROME CYLPEBS

They are used in Regrind units to obtain a very fine grind. They are either cylindrical or spherical.

TYPE SIZE (mm) ENVIRONMENT SURFACE HARDNESS(HRC)

CORE HARDNESS(HRC)

CHROME (%Cr)

CARBON (%C)

MANGANESE (%Mn)

High Chrome

20 - 40 Both Acidic and Alkaline

≥ 65 ≥ 58 11 – 17 2.00 -3.00 1.20 max

They are also used to fill the gaps between mill balls and ore particles in order to control the grinding

in the mill. They undergo heat treatment characterized by Oil hardening (Oil Quenching) and

Tempering.

Figure 17.0 Scaw Limited High Chrome Cylpebs

LOW CHROME CYLPEBS

TYPE SIZE (mm) ENVIRONMENT SURFACE HARDNESS (HRC)

CORE HARDNESS (HRC)

CHROME (%Cr)

CARBON (%C)

MANGANESE (%Mn)

High Chrome 20 - 40 Alkaline ≥ 45 ≥ 44 1.50-2.50 2.00 -2.50 1.20 max

They undergo heat treatment characterized by Air hardening (Air Quenching) and Tempering.

Figure 18.0 Scaw Limited Low Chrome Cylpebs

CASTINGS OFFERED BY SCAW LIMITED

Casting is a manufacturing process in which a liquid metal is poured into a mould. Scaw Limited

manufactures high quality castings such as Jaw Crusher liners, Base plates, Mill liners (shell, end and

trunnion liners), Secondary and Tertiary crusher Bowl and Mantle liners. These castings are important

spare parts of mills and crushers. The main roles are:

• To protect the cylinder/shell from high wear rate

• Provide surface for impact and friction action

• Help to improve the mill grinding efficiency

• Increase Ore crushing/grinding rates and reduce metal wear rate.

Figure 19.0 Scaw Limited Grid Mill Liners Figure 20.0 Scaw Limited Solid Mill Shell Liner

Figure 21.0 Scaw Limited Mill End Liner Figure 22.0 Scaw Limited Casted Trunnion Liner

Figure 23.0 Scaw Limited Mantle Liner Figure 24.0 Scaw Limited Jaw Crusher plates