Mechanical Technology February 2015

Mechanical Technology — February 2015

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Materials handling and minerals processing

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ultotec’s Anthony Yell says

the company has manu-

facturing plants in South

Africa and Chile and can

also supply spare parts from Australia,

Chile and the US, which reduces the

supply chain dramatically and extends

its geographic footprint. “We have sup-

plied mill and scrubber trommel screens

to the four corners of the earth, from

South America to Papua New Guinea

and from Siberia to Kazakhstan. There

are not many places where they are not

prevalent as the Multotec product has

proved to be highly reliable.”

Yell explains that mill and scrubber

trommel screens fit onto the end of a

grinding mill or a scrubber by means of a

bolt on flange. Sizes range from 950 mm

by 1 200 mm up to a massive 6.0 m by

6.0 m. Mill trommel screens are supplied

typically into the platinum, gold and

copper sectors while scrubber trommel

screens are being supplied mainly into

the diamond sector.

Self-driven trommel screens have

their own drive arrangement and are

supplied mainly into the heavy mineral

sands sector to remove organic material

from beach sand, for example, as well as

to water treatment and coal briquetting

plants. Self-driven trommel screens are

supported on both ends, sitting on a ma-

chine tyre and with rollers at the bottom.

They are driven through the discharge

end by either electric or hydraulic drives.

“The latter has proved to be very reliable

and allows for the flexibility of fully vari-

able speed,” Yell says.

Trommel screens are designed using

software such as finite element analysis

(FEA) in order to calculate the stresses

in the structure for a full rotation. These

stresses must be under a maximum

permissible level in order to optimise the

fatigue life. Mill and scrubber trommel

screens have a design life in excess of five

years, while self-driven trommel screens

have a design life of ten

years. Yell explains that

7608:1993 for Fatigue

Design Assessment of

Steel Structures

is ad-

hered to in the design pro-

cess. Self-driven trommel

screens are also designed

using FEA and can also be supplied

with a feed and discharge arrangement

in accordance with specific customer

requirements,” Yell says.

FEA is also used to calculate the

requirement for special cross bracing to

counter torsional twisting. “When the

mill or scrubber starts up, the trommel

wants to turn in the opposite direction

for a split second. If the trommel is not

braced sufficiently, torsional twisting will

occur, which will weaken the trommel

structure and ultimately lead to prema-

ture failure. In addition, the load in a

trommel is shifting continuously, placing

ongoing stress on the trommel, which

complicates the structure,” Yell says. It

is such complexity that means that every

single trommel screen is unique.

“Multotec custom designs each prod-

uct to suit the specific application. We

size it; we check the load; we examine

the material lump size. When a client

requests a trommel screen, our first step

is to determine the main parameters.

There may be a lot of variation in the di-

ameter of the bolt on flange diameter, for

example, as it depends on the size of the

mill. Thus there is not a standard range

apart from the basic design parameters

that we use.”

Such is Multotec’s experience in this

regard that it prides itself on its ability to

turn customer quotes around in 24 hours.

“We use sophisticated programmes to

design the equipment needed, taking

into account the basic FEA requirements

at tender stage, whereupon an accurate

drawing, price and quote can be gener-

ated. The speed and efficiency with

Local trommel screens succeed internationally

A SAG mill trommel frame with cast polyurethane screen

panels and a pinless fastening system.

Multotec is a global leader in trommel screen design and

manufacture and has been supplying the market for more than

20 years. Says screening product manager, Anthony Yell: “We

deal with most of the major mill manufacturers globally. It is a

highly specialised segment that was developed in South Africa

and Multotec has built a reputation that now enables us to supply

internationally.”

which we can respond to a customer’s

enquiry put us streets ahead of the rest

of the market.”

Yell says the key to the success of

Multotec’s trommel screens lies in main-

taining the integrity of the rubber lining

through regular maintenance. “Trommel

screens are generally rubber lined. It is

critical to ensure the integrity of that

rubber lining. If it wears out it needs to

be repaired, as a trommel screen can fail

prematurely if its structure is eroded.”

The rubber lining protects the trommel

screen from corrosion and wear and is

fundamental to an extended trommel life.

“If they are maintained properly, trom-

mel screens will last for many years. We

have had trommel screens running for

over ten years that are still operational,”

Yell says. Customers can return trommel

screens in order for the rubber lining to be

stripped off and the structure subjected

to non-destructive testing to determine

the integrity of the trommel structure.

“If it is still within specification it can be

shot blasted and relined, whereupon it

will be good to go for many more years,”

Yell explains.

Multotec has the added advantage

of being able to supply polyurethane

panels for smaller feed sizes where the

main wear factor is sliding abrasion and

rubber panels where impact is the wear

mechanism. “We design and manufac-

ture both rubber and polyurethane panels

in house,” Yell says. He adds that a

recent trend in mineral processing has

been that mills are tending to get big-

ger and therefore trommel screens are

becoming larger as well. “This can pose