Chapter 8: Washing & Classifying | P&Q University Handbook

Coarse Material Washers

Built primarily to wash crushed stone and gravel, coarse material screw washers effectively remove light, loamy clays, dirt, crusher dust and coatings not eliminated by wet screening alone. They can also be used to remove floating vegetation and soft aggregates from feed material.

Coarse material screw washer paddles work in conjunction with screw flights to provide scouring and agitation. The turbulent washing action, combined with rising current water introduced at the feed end at the bottom of the box, results in the separation of lighter fractions from sound aggregates.

Lighter fractions float to the surface due to the rising water in the box and then overflow the weir located at the back. The desired clean product is scrubbed and conveyed by paddles and flights to the discharge end.

Paddle configurations vary based on the design and length of the washer. More paddles increase washing action but decrease capacity because paddles do not convey material up the box as efficiently as flights. When additional paddles are used, it is necessary to lower the slope of the box and increase motor horsepower to help convey material to the discharge end. Reversing some paddles can also retain material in the box longer.

Proper maintenance of coarse material washers is essential for efficient, long-term operation. Consider the following practices:

• Perform regular inspections. Establishing a structured inspection schedule is fundamental to effective maintenance. Conduct daily, weekly, monthly, quarterly, six-month and yearly checks to ensure all components are in good working order. Regular inspection of paddles and wear shoes helps identify issues before they worsen, ensuring continuous and efficient operation.
• Monitor bearings and motors for heat and noise. Bearings and motors are critical components. Monitor them regularly for excessive heat and unusual noise, which may indicate misalignment or wear. Overheating can cause premature failure, while noise may signal problems requiring early intervention to prevent costly downtime.
• Check oil levels consistently. Proper lubrication reduces friction and wear on moving parts. Ensure oil is within the manufacturer’s recommended range and use the correct type for optimal performance.
• Inspect v-belts for tension and alignment. V-belts are vital for operation. Misaligned or loose belts can cause slippage, reduce efficiency and increase wear. Belts that are too tight may snap under tension. Adjust as needed to ensure smooth operation.
• Inspect the rubber cover protecting the spider and flanges. This cover prevents contamination and damage. Regularly check for wear or damage and replace if necessary to protect critical components from debris and moisture.
• Remove excess grease from the outboard bearing vent opening. Excess grease can plug the vent slot, leading to overheating and increased wear. Nine out of 10 failed bearings result from this issue. Regularly clean out excess grease to ensure ventilation and prevent bearing damage.

Fine Material Screw Washers

Fine material screw washers are used primarily to dewater, classify and wash minus 3/8–in. sand or other fine material. They are designed to accept feeds from sand classifying tanks, belt conveyors, other fine material screw washers or slurry feeds. 

In some cases, they may be installed adjacent to wet vibrating screens, with slurry delivered via a flume or chute.

Several factors must be determined to properly size a fine material screw washer, including tons per hour, the amount of water used with the sand feed, the specification of the desired product and the gradation of the feed material.

As material enters the feed box, heavier particles sink to the bottom while finer fractions float to the surface and overflow the back weir. Material that sinks is conveyed from the pool area up a slope toward the discharge end.

To maximize performance with a fine material screw washer, consider the following:

• Adjust screw speed for finer sands. Screw speed is determined by the percent passing 50 mesh. Finer material requires more time to settle in the washer tub, so slower speeds are necessary. Screw speed can be calculated by dividing 1,500 by the percent passing 50 mesh. 
  
  For example, if 15 percent of the material passes 50 mesh, the screw should operate at 100 percent speed. If 20 or 30 percent passes 50 mesh, the screw should run slower at 75 or 50 percent speed, respectively. 
  
  Excessive shaft speed can cause fine material to build up in the washer tub corners, eventually filling the pool area and causing product-sized fines to overflow to waste.

• Add processing steps when needed. Additional washing may be required when sand feed contains high amounts of minus 200 mesh (0.075-mm) material. Generally, when minus 200 mesh exceeds 12 to 15 percent, a two-step process should be considered. This can be achieved with two screws or a hydrocyclone feeding a screw.
• Ensure sufficient water supply. Adequate water is essential for washing aggregates. As a rule of thumb, fine material washers require 50 gallons per minute of water for each ton per hour of silt removed.
• Incorporate rising current. Rising current allows fine-tuning via water injection beneath the pool areas. The addition of clean water improves classification by keeping ultra-fines in suspension while product-sized particles settle out.
• Level the weirs. Adjusting weirs optimizes washer performance. To remove small amounts of excess fines, raise the side weirs and lower the back weir. This increases velocity over the back, carrying excess minus 200 mesh fines out of the washer.
• Use flush water to reduce buildup. Adding water to the dry deck area can result in drier sand discharging to the conveyor and product pile. Flush water clears fine sand accumulation, maintaining open channels for drainage.
• Maintain proper feed entry. A calm pool area maximizes fines retention. Excess turbulence in the washer tub can cause the loss of minus 200 mesh fines and some plus 200 mesh fines. A feed chute or flume with a velocity break box helps minimize turbulence.
• Lubricate equipment properly. Proper bearing lubrication is critical for reliable operation and reduced downtime. For a 40-hour-per-week operation, lubricate the rear outboard bearing every three months or 500 operating hours. Avoid over greasing, as this can damage seals and plug the drainage port.

Attrition Scrubbers & Bucket Wheels

Attrition scrubbers, also known as attrition cells, are used to liberate deleterious material and remove them from competent aggregates. 

They also liberate clays, reduce product turbidity and break apart loosely conglomerated clusters in frac sand plants. 

Attrition scrubbers can be used in glass sand, frac sand, clay, and sand and gravel production, as well as in preparation of flotation feeds and reagent washing.

Attrition scrubbers produce a high-shear environment where particles scrub against themselves to scour their surface and liberate deleterious materials. All internal parts are completely rubber lined to maximize wear life and minimize maintenance time and costs of replacement parts.

The cells are typically fed by separators, hydrosizers or similar equipment that will prepare material at a high density and achieve the best scrubbing action possible.

Bucket wheels are capable of capturing, desliming and dewatering sand, frac sand, waste fines, specialty sands and gravel from dredging and sand processing operations. They are typically engineered for high solids recovery with low power consumption.

Classifying Tanks

Classifying tanks are widely used to process sand to spec. They remove excess water, classify sand by removing intermediate mesh sizes, retain finer mesh sizes and make 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 handle sand-gradation swings in the average plant while producing concrete, asphalt and mason sand products with minimal waste.

Sand classification is based on grain size settling rates. As water and material enter the feed end, coarser grains settle first while finer grains settle progressively down the length of the tank. At the top, hydraulic control mechanisms operate discharge valves at the bottom. Depending on the control system and product requirements, one, two or three valves may be located at each station.

Proper sizing of a classifying tank requires considering several factors, including water volume with the feed, tonnage capacity, number of desired end products and product specifications. 

Consistency of feed material is also critical. Variations in raw composition can disrupt separation and affect product quality. Regular monitoring and adjustment of feed help maintain stable operations.

Many modern tanks feature advanced PLC control systems that automatically adjust valve operations based on sensing paddle feedback. These controls improve classification accuracy, reduce operator intervention and increase overall efficiency. 

Proper adjustment of sensing paddles and weirs is equally important. Leveling the weirs ensures accurate classification and silt removal. Raising side weirs and lowering the back weir allows finer particles to overflow, effectively removing minus 200-mesh material.

Paddle heights should be set high in the first stations to accommodate fast-flowing coarse sand and lower in later stations to ensure fine sand is discharged as a concentrated product rather than excessive water.

Routine maintenance is crucial for consistent operation. Valves and valve seats are among the most frequently replaced components. While many original parts are hard-cast iron alloys, polyurethane replacements often offer extended wear life. Valve rods, torque motor paddles and paddle rods should also be inspected regularly and replaced when needed.

Sand discharges from valves through downpipes into sectionalized collecting flumes, often fitted with elbows. Elbows handling coarse sand wear faster and require more frequent replacement than those handling finer material. While many tanks still use steel pipes and hard-iron elbows, thick-walled PVC pipes and polyurethane elbows are increasingly common due to their lighter weight, ease of replacement and durability.

Collecting flumes should be lined with AR steel, rubber or polyurethane to protect against abrasion. Liners should cover both the bottom and sides of the flume.

Valve openings are controlled by hydraulic cylinders connected to valve rods, powered by a self-contained hydraulic unit. Hydraulic fluid and filters should be replaced periodically according to manufacturer recommendations. For tanks idle during seasonal shutdowns, the hydraulic reservoir should be checked for condensation buildup, with water drained and fluid levels maintained.

Electrical systems, wiring and hydraulic lines should also be inspected to ensure proper function and to prevent hazards such as grounding issues or leaks. The control enclosure atop the sand tank, which houses hydraulic cylinders, torque motors, solenoid valves, wiring and hydraulic lines, should be kept clear of accumulated sand to prevent component failure.

If the tank is equipped with a rising current recirculating pump, the impeller should be inspected for wear and replaced when needed. V-belts driving the pump and bearing lubrication also require regular attention in line with manufacturer specifications.

Although classifying tanks may appear complex, they are straightforward to maintain with routine inspections and scheduled service. Consistent maintenance reduces downtime, ensures reliable product output and extends equipment life, making classifying tanks a cost-effective investment for aggregate producers.

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