Page 31 - HUB-4 Magazine Issue 79

Aggregate
Washing & Screening
Washing with Barrels
The Need for a Barrel.
The increasing demand to reclaim and recycle a bigger proportion of the aggregate extracted from the earth and likewise the need to clean extractions with more silt has ensured the washing barrel continues to play a major role in sand & gravel and quarry processing plants.
Furthermore concrete aggregate production requires good quality sand & aggregate in terms of being free of adhering silts.
Background:
Using a barrel to wash aggregates has been adopted since the very early days of washing plant and although the basic concept has not changed, the modern day washing barrel has become a far more efficient and user friendly machine than the older style machines. The modern machines can incorporate modern materials allowing shorter down time and easier maintenance. This coupled with the robust nature of the machine maintains the barrel as an important method of washing aggregates.
Where are Barrels Used?
Barrels are incorporated where adhering clays or fines require to be liberated from the aggregate to produce a clean product.
Types of barrels:
There are two types of barrels:
1. Washing Barrels turn at a relatively slow speed (approx. 25% of critical speed) and would normally be incorporated where material does not require intensive attrition to clean it or where the material might break up due to its fragile composition and hence create undesirable fines. For example, coal washing or Kaolin.
2. Scrubber Barrels turn at a more swift 50% of critical speed. The faster speed of the barrel creates far more attrition of material on itself allowing the fines and clays to literally be scrubbed off the aggregate.
Within the above types there also exists a variety of concepts which can allow a flexibility on the overall processing plant design depending on the material analysis.
These concepts essentially revolve around the sand content in the feed and the direction of the water flows within the barrel.
1. Uniflow. This concept as the term suggests is where all the material that is fed into the machine comes out at the discharge end. On sand & gravel plants traditionally the material feed would have been removed of most of the minus 5mm sand thus allowing only the fines adhered to the stone to be scrubbed. All of the water used in this concept would enter at the feed end of the barrel and would travel in the same direction as the material.
2. Contraflow. This concept is offered where customers wish to scrub all of the as dug feed including the sand. Usually, best results are achieved when the sand content does not exceed 50% of the feed due to the cushioning effect of the sand. Unlike the uniflow solution the water is introduced into both the feed end and the discharge end of the barrel. Perforated de-sanding meshes are incorporated into the feed end of the barrel and this design creates a flow of sand and water toward the feed end where the sand is removed for further processing. An addition to this concept is the incorporation of a perforated de-sanding discharge trommel to remove further sand and allow a certain amount of dewatering to take place before the material passes over the trommel for further processing.
Wileman Engineers Limited is a leading supplier of Scrubber Barrels to the Sand & Gravel and associated industries and has been supplying machines to the major UK aggregate companies since the 1970s. The main features of the Wileman design are as follows:
1. A barrel of robust design incorporating roller paths which are machined to produce a concentric and smooth drive.
2. The incorporation of truck tyres to support and drive the barrel again allows a smooth, quiet drive, eliminating excessive loading on the supporting structure.
3. The use of abrasive resistive rubber linings and polyurethane meshes provides a machine which can last indefinitely with regular maintenance.
4. The water used is introduced at relatively low pressures 25 to 30 psi and therefore is not reliant on high pressure expensive pumps.
www.hub-4.com March/April - Issue 79
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