Maximize washing operations with the right equipment, practices
Hydrosizers assist the reblending of coarse, intermediate and retained fine sands into two or three products. Photo: McLanahan Corp.
Whether you’re new to the wet-processing side of aggregate production or you’ve operated sand plants for years, there are a variety of challenges you will experience when washing and dewatering sand.
First, it is important to understand the types of equipment used in such applications. Several options are available for washing and dewatering sand that is typically 5-mm minus and finer, ranging from simple to more complex and varying in capital purchase price, operational cost and electric power consumption.
Equipment considerations
1. Fine material screw washers. A material screw washer, often called a sand screw, is among the simplest devices for washing sand and removing silts and clay. It can be installed on the ground next to a wet vibrating screen. Via a simple flume or chute, a slurry flow of water and sand can be used to feed the sand screw.
If properly sized by a dealer or manufacturer, you should be able to retain most of the required 200 mesh sand. For 75 to 100 tph of sand feed, you only need about 10 kilowatt, or 15 hp, of electric power to put a conveyable sand on a product pile via an elevating belt stacking conveyor.
2. Hydrocyclones. Hydrocyclones are widely used for similar fines removal and dewatering functions. Having no moving parts, a hydrocyclone can be installed in several ways. For example, some are mounted on structures such as towers and positioned away from other processing equipment.
In order to achieve proper performance, hydrocyclones must typically be fed via a pumped slurry in a consistent percentage-of-solids range, as well as at a pressure required by the hydrocyclone supplier. With these requirements, the capital and operating costs, including the necessary electric power, should be reviewed when considering hydrocyclones.
3. Dewatering screens. Varying designs of dewatering screens are on the market. A properly designed dewatering screen discharges the driest washed sand product of any dewatering device commonly used. Additionally, less space is required than other options.
Depending on the slurry of the sand feed and the percentage of solids in the sand flow, these units often require a sump, pump and one or two hydrocyclones to partially dewater the slurry and allow the screen to adequately perform. The capital cost of a dewatering screen system is often two-plus times more than other choices, and the electricity cost is often up to three times more.
4. Bucket wheels. Some bucket wheel designs are more complex when compared to others for retaining 200 mesh washed sand. While typically low in cost in terms of the electric power required to drive the wheel assembly, a bucket wheel’s capital cost can be higher than some sand-dewatering equipment choices.
5. Thickeners and filter presses. When operating a wash plant, environmental regulations typically require that you contain the effluent of the silt/clay-laden dirty washwater on your property. You will have a lot of washwater, but because it may contain 10 percent ultra-fine solids, it can’t be used back into your plant – unless you can somehow separate the silt and clays from most of the water.
A thickener allows a wash plant to reclaim water for immediate use back into the plant. Photo: McLanahan Corp.
A thickener allows a wash plant to reclaim up to 85 percent of the water for immediate use back into the wash plant. Concentrated mud discharging typically at 40 percent solids content also reduces the space required for tailings containment.
Addressing producer concerns
Even after selecting the right equipment for their washing and dewatering operations, producers may still encounter problems that need troubleshooting. Here are a few concerns manufacturers commonly hear from customers, along with potential solutions:
1. The sand is wet when discharging onto the stockpile from the dewatering equipment. Depending on the equipment being used, a simple adjustment or the addition of water at the right injection point is sometimes all it takes to create drier sand.
When using sand screws, ensuring there’s enough washback water to flush accumulated fines out of the dewatering zone channel can make a big change, resulting in a drier sand discharging from the machine.
Producers sometimes ask how much water is needed in water connections. These connections can range from 3/4 in. to 1.5 in. While there are published tables from some manufacturers, you only need enough water to keep the trough or channel clear of fine sand. Whatever your plant’s water pressure may be, having a hand-control valve on these water lines can easily allow an adjustment to add enough water.
If you operate vacuum-assisted hydrocyclones, a simple adjustment of the vacuum airline that varies the opening of the rubber underflow regulator can result in a drier sand.
Additionally, weir positioning 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 – an option often overlooked as a potential adjustment to discharge drier sand.
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