Solving the Biggest Issues with Washing and Dewatering Sand
First published in the August 2020 issue of Quarry Management as Sorting out your Sand
Whether you are new to the wet processing side of aggregate production or have been operating sand plants for years, Dave Schellberg of McLanahan Corp. provides some useful advice on how to solve the biggest issues with washing and dewatering sand
Before examining the issues commonly faced when washing and dewatering sand – such as sand that is discharging to the product pile too wet, product-sized sand being sent to waste, sand that is not consistently in spec and too many hours spent cleaning out settling ponds – it is first necessary to address the types of equipment used to wash and dewater sand.
Several choices are available for washing and dewatering sand that is typically –5mm (4 mesh) and finer. These options range from the simple to the more complex and can vary in capital purchase price, operational cost (including wear parts) and electrical power consumption. Other factors, such as the moisture level in the washed product discharge when the sand goes on to the finished stockpile, should also be considered when setting up a sand washing plant.
Fine material screw washers
Among the simplest devices for washing sand and removing silts and clays is a fine material screw washer, often called a sand screw. This can be installed on the ground next to a wet vibrating screen, with a simple flume or chute used to feed the sand screw with a flow of water and sand slurry.
If properly sized by a local dealer or manufacturer, it should be possible to retain most of the required +75-micron (200 mesh) sand. For 75–100 tonnes/h of sand feed, around 10kW (15hp) of electric power will be needed to put a conveyable sand on a product pile via an elevating belt stacking conveyor.
Globally, hydrocyclones have been used widely for a similar fines removal and dewatering function. Having no moving parts, a hydrocyclone or group of hydrocyclones in a cluster can be installed in several ways. Some are mounted on towers or some type of structure and positioned away from the other processing equipment.
In order to achieve proper performance, hydrocyclones must typically be fed with a pumped slurry at a consistent percentage solids range and at a pressure determined by the hydrocyclone supplier. With these requirements, the capital and operating costs, including the necessary electric power, should be examined when considering hydrocyclones.
Recently, varying designs of dewatering screens have entered the market. A properly designed dewatering screen will discharge a drier washed sand product than any other commonly used dewatering device. Also, the space required is less than the other options.
Depending on the sand feed slurry and, specifically, the percentage of solids in the sand flow, these units often require a sump, pump and a hydrocyclone or two to partially dewater the slurry to allow the screen to perform adequately. The capital cost required for a dewatering screen system is often more than double that of the other choices, whilst the electricity cost is often up to three times more.
Bucket wheels are another option for dewatering a sand slurry. Some designs for retaining +75 micron (200 mesh) washed sand are more complex than others. While the cost of electricity to drive the wheel assembly is typically low, the capital cost of a bucket wheel can be higher than some of the other sand dewatering equipment choices.
Thickeners and filter presses
When operating a wash plant, environmental regulations typically require that the effluent of the silt/clay-laden dirty wash water is contained on site. As this wash water may contain 10% ultra-fine solids, it cannot be sent back to the plant for reuse unless the silt and/or clays can be separated from most of the water.
A thickener allows a wash plant to reclaim up to 85% of the water for immediate reuse in the washing plant. Concentrated mud discharging typically at a 40% solids content also reduces the space required for tailings containment. Various types of thickeners are widely used in aggregate washing plants.
Other devices that can further dewater a thickener solids discharge include filter presses, centrifuges and belt-filter presses. Some aggregate companies have totally eliminated the settling ponds associated with their washing plants by using these technologies.
Common issues with washing and dewatering sand
Having looked at the equipment typically found in sand washing plants, it is time to address some of the typical issues and questions that arise when washing and dewatering sand, and put forward some solutions.
My sand is wet when discharging on to the stockpile from the dewatering equipment:
Depending on what equipment you are using, sometimes all it takes is a simple adjustment or even the ‘addition’ of water at the right injection point to make the sand going on to the pile drier.
If you have sand screws, ensuring you have washback water to flush accumulated fines out of the dewatering zone channel can make a big difference, resulting in a drier sand discharging from the machine.
How much water is needed in the water connections?
These connections can range from ¾in (19mm) to 1.5in (38mm). While there are published tables from some manufacturers, the right answer is that you only need enough water to keep the trough or channel clear of fine sand.
Whatever your plant’s water pressure requirement may be, having a hand control valve on these water lines can easily allow an adjustment to add enough (not an excessive amount of) water.
If you operate vacuum-assisted hydrocyclones, sometimes called separators, a simple adjustment of the vacuum air-line that varies the opening of the rubber underflow regulator can result in a drier sand discharge.
A periodic visual observation of the units in your sand washing plant, be it a primary sand circuit or a secondary fine sand recovery system, can help in reducing the moisture content of the material discharging to the sand pile.
The positioning of weirs on sand screws and bucket wheels is used more for controlling fines retention, and the weirs can be adjusted to a lower position to provide more dewatering. This option is often overlooked as a potential adjustment to discharge a drier sand to stockpile.
I’m losing +75-micron (+200 mesh) product-sized fine sand:
Perhaps you have recently added more water to the wash screen that precedes your sand dewatering gear to make the rock cleaner. This can result in the loss of +75-micron (+200 mesh) sand to your settling pond. You cannot always cut back on the wash screen water, as the rock may be too dirty and out of spec.
If you have to maintain the same water volume to what may now be undersized equipment and you want to keep this fine sand in your washed sand, some type of capital equipment changes or additions will be needed.
If you use a bucket wheel or sand screw, the addition of a properly sized hydrocyclone to handle the dewatering unit’s overflow has successfully been used to recapture lost product-sized fines. This would also require the addition of a sump and pump to feed the hydrocyclone at the required inlet pressure. The mounting of a hydrocyclone over the discharge of a dewatering unit is commonplace.
If you are using a sand screw, another option is to replace the existing unit with a larger machine. The capacity of a larger fine material screw washer shaft might not be needed, but the bigger tub and higher water volume capacity will often keep the finer sand in the product pile.
It is worth talking to several suppliers to get ideas as to what might be the best solution to this issue. A lot of sand producers have had this problem.
I can’t consistently make spec C-33 sand:
There could be several reasons why an operation is consistently unable to make in-specification concrete sand, beginning, perhaps, with the material deposit. This can happen in either a crushed stone manufactured sand plant or with an alluvial sand deposit.
A change in the mesh screen aperture used on the wash screen before the sand processing equipment can make a difference. A slight variance in the screen mesh or type of screening media can make a significant difference to the sand gradation that goes on the washed sand pile.
Difficulties making a specification C-33 concrete sand may be because your equipment is not retaining sufficient –150-micron (100 mesh) fines. Having the right gear to retain the required fines, as discussed previously, may help you make a spec sand.
Lastly, you may need to consider the addition of a sand classifier before your washer/dewatering unit. There are two basic types of sand classifiers that can remove excess intermediate sand that is +150 micron (100 mesh). One is the V-box, multi-valve discharge station classifying tank; another is the up-current classifier, sometimes referred to as a hydrosizer.
These units allow the re-blending of coarse, intermediate and retained fine sands into two or three products, depending on the configuration, allowing for the production of a spec construction sand such as C-33 concrete sand.
My boss says we need to reduce the number of hours bailing out our settling ponds:
Some sand plant operators do not keep track of the number of man and equipment hours spent clearing out settling ponds. If you are filling in a worked-out, abandoned section of a stone quarry, you can keep filling the void you have worked out.
Other operators, however, must maintain containment ponds so as not to discharge ultra-fines into nearby streams or on to adjacent land. Some aggregate plants use many acres of land to contain ultra-fines and some operators have to limit their plant running hours to keep up with the constant clearing out of their settling ponds.
In the latter case, several equipment options are available to reduce or eliminate the need for these containment ponds. The use of hydrocyclones or a system with hydrocyclones and dewatering screens can recover ultra-fine material 38 micron (400 mesh) and larger. The use of this gear can reduce the volume of flow into a settling pond.
If your solids effluent fraction is more –38-micron clays and silts, the use of thickeners and filter presses is more commonly implemented. Thickeners separate solids from the liquid in a slurry to reduce the volume of material destined for settling ponds and to recycle clear process water for reuse in the plant.
Filter presses also separate solids from the liquid in a slurry, producing a drip-free cake from the solids and clean process water for reuse in the plant. With a filter press, settling ponds can be completely eliminated, and with that, the time required for cleaning out those ponds.
About the author
Dave Schellberg is a McLanahan product support specialist for the company’s wet processing/washing and mixing equipment range. He has been involved in the process selection and sales of the company’s feeding, crushing, screening and washing equipment product lines since 1975, first as a representative and, since 1985, as an employee.
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