Introduction
Clean aggregate is necessary for use in construction projects. Aggregate, such as sand, stone and gravel, naturally contains clay, coatings, fines and other deleterious material that can be harmful to a construction project if not removed.
Aggregate producers have a number of equipment options for washing away this deleterious material from the sound aggregate, leaving a clean product that meets the standards for its various construction uses. One of these equipment options is a Coarse Material Screw Washer.
What is a Coarse Material Screw Washer?
Coarse Material Screw Washers are designed to wash crushed stone and gravel from 3/8” to 4”. They consist of a rotating screw shaft (or shafts) affixed with paddles at the feed end for scrubbing the material and screw flights along the remaining length of the shaft for conveying the material toward the discharge. The shaft features a drive at the discharge end of the machine and is housed inside a washer box that contains the aggregate material and water.
What is considered coarse material?
Aggregate ranging from 3/8” to 4” in size is considered to be coarse material. Anything smaller than 3/8” is considered sand. Crushed stone and gravel are two examples of coarse material.
What are Coarse Material Screw Washers used for?
Coarse Material Screw Washers remove light, loamy clays, dirt, crusher dust and coatings not removed by wet screening alone. They also remove lightweight, floating vegetation if it is not waterlogged, as well as soft aggregate.
How do Coarse Material Screw Washers work?
Material enters directly into the washer box of the Coarse Material Screw Washer, where it encounters a screw shaft comprised of a combination of paddles at the feed end and flights on the discharge end. The paddles provide a scrubbing action and facilitate particle-on-particle interaction for removing deleterious material from the sound aggregate.
The turbulent washing action, combined with rising current water introduced at the feed end, results in separation of the lighter, undesirable material (such as clay and floating vegetation) from the desirable material. This material floats to the top of the surface and overflows the weir located at the back of the box.
The washed aggregate is then conveyed out of the washing area and up the incline toward the discharge by the screw flights. Coarse Material Screw Washers are operated at a maximum slope of 18 degrees.
The clean aggregate should undergo a final washing via a rinsing screen directly following its discharge from the Coarse Material Screw Washer.
What is rising current?
A rising current of freshwater aids in separating unwanted material, excessive dirt or silt from the feed material. It reduces the density of the slurry, which minimizes material buildup in the washer box.
In what applications are Coarse Material Screw Washers used?
Coarse Material Screw Washers are typically found in aggregate applications to wash crushed stone and gravel to meet required specifications.
They are also used in C&D recycling and concrete reclamation applications to separate lightweight debris, fine sand and cement from coarse aggregate.
How to size a Coarse Material Screw Washer
The sizing and selection of a Coarse Material Screw Washer is based on:
- Type of material
- Desired capacity
- Maximum feed size
Each manufacturer will have a capacity chart that lists the sizes of machines it offers along with the maximum feed size, shaft speed, required horsepower and water gallonage needed to meet the required tons per hour.
Coarse Material Screw Washers typically come standard with a set number of paddles. For dirtier feeds, more paddles can be added to increase the washing action and retention time. Paddles do not convey material up the washer box as fast as screw flights, so adding more paddles means the material will be subjected to the scrubbing action facilitated by the paddles for a longer period of time.
As a general rule, when using additional paddles, the slope of the Coarse Material Screw Washer must be lowered and the power to the motor must be increased to help convey material to the discharge end.
Coarse Material Screw Washer maintenance
To keep Coarse Material Screw Washers operating optimally, it is important to establish a proper maintenance program and stick to it regularly. A minimum recommended maintenance program is usually listed in the manual.
Maintenance includes ensuring all applicable components are lubricated with the correct type and amount of lubricant once every shift. Bearings, motors and reducers should be inspected for excessive heat, and electrical cables, water lines and all connections should be checked for wear or damage. Belts should be checked for proper tension and alignment.
Once a week, all bolts and fasteners should be inspected for tightness, and all seals should be inspected for proper performance. The area around the motor and fan should be inspected for cleanliness.
Every three months, the wear shoes should be inspected for operating condition. Replace any wear shoe before it wears down to the same diameter as the screw flights.
What are the differences between a Coarse Material Screw Washer and a Blade Mill?
Blade Mills are another piece of equipment used for scrubbing deleterious material from stone and gravel feeds. They appear similar to Coarse Material Screw Washers in that they feature a screw shaft with paddles and flights inside a washer box, but where Coarse Material Screw Washers can only process the coarse aggregate, Blade Mills can accept an all-in feed; material does not need to be pre-screened before entering the Blade Mill.
Another noticeable difference is the paddles shafts. Coarse Material Screw Washers have paddles at the feed end of the shaft and screw flights along the remainder of the shaft. Blade Mills feature alternating paddles and flights on the screw shafts, and they have more paddles than Coarse Material Screw Washers.
Blade Mills are also run relatively flat in comparison to Coarse Material Screw Washers, which operate on a fairly steep incline to allow the dissolved deleterious material to overflow the back weir. Since Blade Mills do not have weirs, the dissolved deleterious material is discharged with the product. Therefore, the material must be further processed upon discharge.
Operational tips for Coarse Material Screw Washers
- Always start a screw washer with the feed box empty
- Always introduce feed material only when the machine is operating at full speed
- Always run the machine until empty, or as empty as possible, before shutting it down
- Screen the sand (minus ¼” material) out of the feed ahead of the Coarse Material Washer so as not to cushion the washing action
