Load & Haul

Dewatering sand: The fundamentals

{{image2-a:r-w:200}}The development of what is known today as a ?dewatering screen? can be traced back to the middle of last century. Screening, both dry and wet, had come close to reaching the physical limit of efficient screening. Fine screening was something of a final frontier, while wet screening fine material creates the biggest challenge of all.

Today, many manufacturers around the world provide a range of screens that fall into the category of dewatering screens. A variety of industries use them and their operations cover a diverse set of operating parameters.

{{image3-a:l-w:200}}The basic fundamentals of dewatering screens were previously covered in an article in Quarry in 2009 (see Quarry 17(6), June 2009), by John Bennington.

Eccentric shafts have been used, although vibrators are more prevalent, situated at an angle to the screen deck. The screen itself will typically be run at a slight elevation to horizontal. Variations are found in different manufacturers? philosophies on the amplitude and frequency used.

{{image4-a:r-w:200}}One constant is the throw or acceleration, which is typically five Gs (acceleration due to gravity) or higher. Essentially, a bed of sufficient depth must be formed to allow the water to be forced out by the vibrational forces, through the porosity of the bed. Hence, the coarser the feed is, the easier it is to dewater.

Dewatering screens have been successfully installed across Australia. The accompanying photos show a variety of operations. The Penrith Lakes Development Scheme had a number of dewatering screens on its fines plant. Dewatering screens were in operation in South Australia more than a decade ago. More recently, dewatering screens have been regularly installed in sand processing operations, to the point that some are now offered as a standard item, accompanying a typical cyclone wash plant.

{{image5-a:l-w:200}}Greystone?s work in dewatering screens led to its Aggre-Dry range, as shown in Figure 1. The Neilsen Group?s Brendale site, on the northern outskirts of Brisbane, operates a dewatering screen that is fed directly from a classifier. Figure 2 illustrates the ?throw? action of this dewatering screen, with water-laden suspension forming airborne droplets from the intense motion. Rocla?s Calga Quarry has operated a dewatering screen for some time on its main products.

A fine concrete sand is shown discharging from this unit in Figure 3. Dewatering screens have also been used in directly handling the product of horizontal classifiers (classifying tanks).

While there are issues with running dewatering screens, they are reasonably low cost and low maintenance. Once a unit is set up for the existing production, it should run well and give a consistent, drier product.

The Hawkesbury sandstone of the Somersby Plateau was formed in the Triassic period. The siliceous material was part of a massive fluvial system that deposited sand, along with some shale lenses and clay, throughout the extended Sydney basin more than 240 million years ago. With the formation of the sandstone and localised volcanic activity, there was uplifting and gentle folding, leading to erosion and the existing topography.4,5

{{image6-a:r-w:200}}Hanson?s Somersby operation ? Central Coast Sands ? in New South Wales has been in operation for nearly three decades. Hanson?s forerunner Pioneer took over the site just over 25 years ago.

Sandstone is ripped, loaded and hauled to a grizzly over a feeder. The +150mm material is returned to the pit for more dozer primary crushing. The throughs of the grizzly are fed to a trommel and then screened at 65mm, 14mm and 6mm, with essentially a -5mm material being processed as sand.

The sand is pumped through a series of 27? cyclones, with the overflow from the first two bins/sumps flowing to a fines recovery circuit. Here, an 18? cyclone removes a final silt component and attempts to dewater the product.

{{image7-a:l-w:200}}This final material recovered from the overall washing circuit is extremely fine, all -600 microns. Neil Catt, the quarry manager, has successfully installed a Joest 610mm x 1830mm dewatering screen to dewater these fines and retain as many as possible.

Figure 4 shows the discharge of the cyclone alone, a continuous stream of dilute slurry, while Figure 5 shows the effort required in attempting to move the very ?wet? slurry. Figure 6 illustrates the end result, the new dewatering screen at the base of the cyclone tower and the stackable free standing stockpile of sand, while Figure 7 illustrates the drier/stackable material falling off the end of the dewatering screen.

At times, the addition of equipment becomes just a ?band aid? on a failed process. With Hanson?s upgrade, the addition of one piece of equipment resolved a processing problem. This resulted in a better product that was more readily retained and easier to handle, while cleaning up the site and saving on costs.

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