Mechanical Technology February 2015

Mechanical Technology — February 2015

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Materials handling and minerals processing

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Above:

Anthony Yell, screening product manager, Multotec, with a SAG mill

trommel with modular Injection moulded screen panels.

Right:

The 3A self-driven trommel installation at Grande Côte, Senegal.

challenges in terms of acceptable life

of screen media. However, we do have

solutions for this,” Yell says.

“Mill manufacturers often increase

throughputs by accelerating the dis-

charge rate out of mills by using so called

pulp lifters, which function a bit like a

pump impeller to propel the material out

of the mill at a higher velocity. The trom-

mel design implication is that we need to

stop the feed landing half way down the

trommel screen as this reduces trommel

efficiency and causes high wear to the

screen media. “In order to combat this

we have designed what we call a baffle

plate placed at the feed end of the trom-

mel that forces the feed down onto the

trommel surface to improve screening

efficiency,” Yell continues.

“If the trommel screen rotates in one

direction only, we recommend installing

an advancing spiral or scroll in order to

be able to control the retention time of the

material in the trommel. “The scroll pitch

can be designed to increase or reduce the

retention time accordingly. Furthermore,

single or twin start scrolls can be used

to further vary the retention time and im-

prove screening efficiency when required.

There is a great deal that can be done to

optimise output,” Yell says.

“One of the biggest problems we have

today with the larger trommel screens is

the high wear rates of the screen media

due to the high peripheral speed of the

trommel screen itself.” In addition, the

trend towards larger mills means higher

flow rates and velocities. “Double the

velocity on a polyurethane screen equates

to an increase in the wear rate by a factor

of five,” Yell notes.

“We have invested extensive research

and development into producing panels

that last longer, as with large copper pro-

cessing plants where the large tonnages

mean an average run time of 12 to 14

weeks without any stoppages for panel

change outs. It has been our objective to

develop panels that can last that length

of time,” says Yell.

This usually requires that trommel

screen panels become bigger and heavier.

“On the smaller trommel screens we use

modular injection moulded panels of

305 mm by 305 mm up to around 2.5 m

in diameter. For sizes above that it is more

cost effective to manufacture larger panels

in either rubber or polyurethane. The chal-

lenge is manufacturing panels that are

not too large or too heavy for two people

to lift and install. Otherwise specialised

lifting equipment is required which then

complicates the re-lining procedure.”

From a safety perspective, Yell notes

that all the panels are fastened from

inside the trommel screen. Everything

is installed and replaced from inside the

trommel, generally using polyurethane

pins and sleeves. Our solutions mean

that that the panels are easy to remove

and replace safely.

Another issue related to panel wear

is ‘pebble porting’ in semi-autogenous

(SAG) milling, which sometimes results

in hard rounded pebbles that cannot be

economically ground down any further

being ejected from the mill and sent

to secondary crushing. These rounded

pebbles, with diameters from 50 mm to

80 mm, can account for up to 20 % of

the trommel feed needed to be screened

out. “They tend to be abrasive and cause

excessive panel wear. Multotec has

developed a range of panels that can

handle these pebbles and still achieve

acceptable life,” Yell says.

“The most important factor for us is

to understand the trommel duty and the

load coming off the end, because we have

to design the front-end of the trommel

to be able to take this load.” Yell points

out. “We have focused investment in

developing designs for many years and

our trommel screen designs have proven

themselves to be exceptionally reliable,”

Yell says. “As long as they are maintained

properly in terms of the rubber lining and

panel replacement then they will last a

long time.”