The basics of classifying tanks and screw washers

Sand classifying tanks are used in sand and gravel plants as well as in manufactured sand plants.

These units provide a simple way to remove excess water and slimes or undesirable grain sizes from natural, alluvial sand or manufactured crushed sand feeds.

Aggregate producers who need to wash sand, but not classify it, can consider a fine material screw washer, which is considered a low-cost, low-horsepower machine in comparison to alternatives.

Classifying tanks

Classifying tanks are widely used for processing sand to meet construction grade specifications. Uses include removing excess water, classifying sand by removal of excess intermediate mesh sizes, retaining finer mesh sizes and making multiple products from a single feed material.

Classifying tanks are effective, low-maintenance units that produce one or more specification products. With either slurry or a dry feed, they are capable of handling sand gradation swings in the average plant producing concrete, asphalt and mason sand products while minimizing waste.

Sand classification is based on the different settling rates of various grain sizes. As water and material enter the feed end, coarser grains settle first, and finer grains settle in successive sizes down the length of the tank. At the top of the tank, a series of hydraulic control mechanisms operates the discharge valves at the bottom of the tank. Depending on the type of control system and products produced, one, two or three discharge valves are located at each station.

Here are five questions that should be answered for equipment suppliers to properly size a classifying tank:

1. How many gallons of water are coming in with the feed?
2. What are the tons per hour this tank will be handling?
3. How many desired end products do you want?
4. What is the screen analysis of the feed material?
5. What are the specifications of the desired products?

Fine material screw washers

Consider, too, that a number of aggregate producers have a need to wash sand. Fine material screw washers are one such option to do it. Fine material screw washers are primarily used to dewater and wash minus 3/8-in. or 10-mm sand, or other fine material generally having a specific gravity of 2.5 to 2.7.

These machines can be used following sand classifying tanks, belt conveyors, other fine material screw washers or hydrocyclones. But they are often used after a wash screen when the sand is in a dilute slurry.

Producers tend to choose fine material screw washers because of their simple operation. They are generally inexpensive and deliver an immediately saleable product. Fine material screw washers require minimal process knowledge and can usually handle variability throughout the feed material in a pit.

Here are four questions that should be answered for equipment suppliers to properly size a screw washer:

1. What are the tons per hour of sand this screw washer will be handling?
2. How many gallons of water per minute are coming in with the sand feed?
3. What is the specification of the product you wish to produce?
4. What is the gradation of the feed material?

FinaL thoughts

When choosing and sizing a classifying tank or fine material screw washer, it is important that the machine size selected can handle all of your requirements.

By dismissing the sizing requirements, the machine selected may not be the correct one for your processing needs. Improper sizing and selection can lead to poor results and a loss in profitable sand retention.

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Information for this article courtesy of Eagle Iron Works.