You’ve decided on a Fine Material Washer for washing your fine aggregate — now what? Here are five factors you should consider before sizing a Fine Material Washer.

1. Tonnage

When sizing a Fine Material Washer for your application, consider how many tons per hour you’re looking to produce. Fine Material Washer capacities increase as the diameter of the screw shaft increases. A larger screw shaft equals more tons per hour (tph). For example, a 20” single shaft screw operating at 100% speed can handle 30 tph, while a 72” single screw shaft operating at 100% speed can handle 475 tph.

Adding an additional screw shaft of the same size doubles that capacity. For example, a 36” single screw shaft operating at 100% speed can handle 100 tph, whereas a 36” double screw shaft operating at 100% speed can handle 200 tph.

Single screw washers capacity chart.

Double screw washers capacity chart.

A screw nears capacity when the third flight out of the water has material overflowing the shaft.

2. Silt content

Undesirable silt is minus 200 mesh material, which generally is not allowed in a concrete sand. Knowing the amount of minus 200 mesh material contained in the feed is key to determining how much water is needed to:

• Ensure proper Fine Material Washer selection and operation
• Allow for maximum +200 mesh sand solids recovery
• Effectively remove the silt

In order to meet these silt dilution requirements, most manufacturers recommend at least 50 gallons per minute of water for every ton per hour of silt.

3. Determining how much water you need

To determine how much water you need to effectively remove the silt from your feed and ensure the most efficient Fine Material Washer operation, follow these steps.

First, determine how many tons of silt per hour is contained in the sand feed. To do this, take the percentage of silt from the input feed and multiply it by the tons per hour capacity of your Fine Material Washer. For example, if your feed contains 5% silt and you are operating a 36” single shaft Fine Material Washer, which produces 100 tph when operating at 100% speed, you are processing 5 tph of silt.

To determine how many gallons of water per minute you need, multiply the tons per hour of silt by the recommended 50 gpm for every ton per hour of silt. Using the results of the previous example, 5 tph of silt multiplied by 50 gpm equals 250 gpm of freshwater needed.

4. Fine sand retention requirements

The fourth factor you’ll want to consider when sizing a Fine Material Screw Washer is the size unit required to retain the finest sand you want after you’ve determined how much water is required to remove the silt. Sometimes the size selection for fine product retention determines Fine Material Washer selection and not the tons per hour capacity of the screw shaft.

The chart below can help with determining the amount of water needed to make common mesh splits of 100, 150 and 200. For example, according to the chart, a 44” single shaft will handle 760 gpm and retain 150 mesh fines.

Single screw washers water gallonage chart.

Double screw washers water gallonage chart.

The 150 mesh split is ideal for:

• Concrete sand
• Asphalt sand
• Golf sand

The 200 mesh split is commonly used for mason sand.

5. Product gradation

With the undesirable silt removed, the product gradation will determine the screw speed. Screw speed is based on the percent passing the 50 mesh sieve for the product gradation begin produced.

The recommended screw speed can be determined by taking 1,500 divided by the percent passing 50 mesh. If you have 15% or less passing 50 mesh, the recommended screw speed is 100%. If you have 30% or less passing the 50 mesh, the screw speed should be 50%. The slower screw speed in the second example allows the fine mesh particles more time to settle out in the washer tub.

When screw shaft speed reduction is required, the capacity is also reduced. When the screw speed is lowered to 50%, so is the capacity. Using a 36” Single Screw Fine Material Washer as an example, the capacity is reduced from 100 to 50 tph.

For new Fine Material Washer applications where product gradations do not exist, approximate screw speeds can be calculated based on theoretical product gradations. If you need assistance calculating theoretical product gradations, contact Eagle Iron Works.

Sizing a Fine Material Washer

All five of these factors — tonnage, silt content, water requirements, fine sand retention requirements and product gradation — should be taken into account before sizing a Fine Material Washer. Knowing these numbers ahead of time can help ensure you get the right machine for your application.