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completed it was not long before the plant was up and

running and immediately achieved its designed maxi-

mum throughput of 250 tonnes per hour. In fact, it

would reach peaks of 270 tonnes if utilising virgin ma-

terial rather than recycling the 10 mm and 6 mm through

a vertical shaft impact crusher for increased sand yield.

A substantial boost in productivity was not the only

benefit sought by Smiths during the design process.

Extra flexibility in producing aggregates to particular

specifications, especially sand, was also required as was

a greater consistency in operation and energy

efficiency.

All these aims have been met by CDE who used their

experience and knowledge to bring together the most

suitable elements available to achieve these important

goals.

The inclusion of the VSI working alongside a

Hydrocyclone unit to produce two grades of sand which

may or may not be recombined after separation, de-

pending upon market demand. This was not possible

with the old plant and adds another option for customers

looking for more refined grades of material.

Energy savings are gained by the use of CDE’s pat-

ented design of screen boxes. These were developed by

the company after they had examined the causes of

failure of such major components and come to the con-

clusion that there were better ways of constructing them

to provide a higher degree of reliability.

The use of bolts in construction rather than welding,

cast cross members and a tri-wall design has eliminated

many of the weak points and allowed for a 30% saving

in weight which in turn translates to a 30% saving in

energy required to vibrate it.

In addition to the lighter box the use of Rosta sus-

pension mounts ensures that less vibration is transmitted

to the structure which reduces stress upon the installa-

tion as a whole and keeps the kinetic energy of vibration

where it is intended, within the screens.

The virgin material extracted by Smiths is a reason-

ably consistent mixture of sand and larger aggregates

found in the alluvial deposits typical of the Thames

Valley.

Even though pre screening will extract the majority

of the sand from the mix a large capacity log washer is

still required to remove grains that may still adhere to

the shingle.

The AggMax 151R deals with this task and the sand

yielded joins that from the pre-screen section before

both streams enter the Hydracyclone unit.

The cleaned aggregate ejected from the log wash is

then free to be separated into three different grades or

diverted to the crusher as required.

Processed stone emerging from the crusher is reu-

nited with the main flow of gravel before it enters the

final screening box, thus avoiding the need for it to pass

once more through the log wash,

where it would cause additional wear

and interfere with the cleaning

action.

The log wash itself is fitted with

variable drive, so that it can be started

under load, eliminating the need for

it to be cleaned out each time it is

closed down. Starting and stopping

the plant is a fully automated sequen-

tial procedure and is controlled from

a cabin situated at the finished prod-

uct end of the line.

[cw]

Contractors World UK & Ireland Vol 6 No 1

25

Contractors World UK & Ireland