8 Common Misconceptions About Screw Washers

By: Dave Schellberg October 22, 2019
Although Screw Washers are well known in the aggregate and mining industries, here are a few common misconceptions about what they do.

Screw Washers are available in two basic types: coarse material and fine material. 

Coarse Material Screw Washers are designed/suited for washing coarse aggregates and ores being nominally +5mm. 

Fine Material Screw Washers are typically used for washing and dewatering -5mm x 0 sand, fines or coarse tailings, and for removing silts and/or clays being nominally -75 micron or 200 mesh. These units are also referred to by other names such as screw classifiers, spiral classifiers and sand screws. 

Today, Fine Material Screw Washers are widely used to dewater and/or wash solids with a specific gravity of 2.6 and higher. The maximum particle size of the solids is often controlled by a washing/vibrating screen before entering the machine in a slurry. The typical maximum solids feed size is 10mm or 3/8” but can be finer. 

Although Screw Washers are well-known and respected in the aggregate and mining industries, there are some common misconceptions about what they are and how they work.

Misconception 1: A Coarse Material Screw Washer is the same as a Log Washer

A Coarse Material Screw Washer is not a Log Washer.

Coarse Material Screw Washers are selected for washing rock and ore that have a mud and soluble clay contaminant that cannot be removed by a wet washing/vibrating screen alone. 

Compared to Log Washers and Rotary Scrubbers, these low-cost units are often overlooked for consideration even though the dirt, mud and clay can be washed away by the moderate agitation these devices provide. 

Coarse Material Screw Washers are suitable for many construction aggregate applications as well as many construction and demolition (C&D) washing plants.

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For washing construction aggregate like limestone, gravel, granite and similar hard ores, Coarse Material Screw Washers are selected when mud and soluble clays cannot be effectively washed on wet vibrating screens that have spray bars. With the addition of the right volume of water, these relatively low-cost and low kW/HP units do a great job.

It is generally recommended that this machine be followed by a rinsing/washing screen to make sure any filmy residue is washed off the rock and ore feed. In some instances, this is not necessary.

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Misconception 2: Coarse Material Screws can't remove lightweight contaminants

While many suppliers offer washing plants with Log Washers for processing C&D materials, in a lot of circumstances these costly, high kW/HP units aren’t needed for floating lightweight, rock-sized contaminants. 

Coarse Material Screw Washers can be supplied with a special overflow weir that allows most lightweight, non-waterlogged material to float out. Heavier, specific gravity-sized solids settle and are augured out as competent rock-like material.

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Coarse Material Screw Washers can be used to remove lightweight materials in C&D wash plants. 

Misconception 3: Reversible paddles reduce scrubbing

Reversible paddles can actually offer more scrubbing. 

In instances where a Coarse Material Washer isn’t fully loaded, some of the paddles can be flipped to scrub the rock more. It depends on the paddle design of the manufacturer you’re considering. Make sure you check this out before buying the next machine you need for your plant. 

Misconception 4: Fine Material Screws are recommended for cutting at a coarse sand size

Fine Material Screw Washers generally are not recommended for removing fine solids larger than 150 micron or 100 mesh. 

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When a separation ranging from 150 to 800 micron (or 100 to 20 mesh) is required, other types of classifiers, like Hydrosizers™ (or other, similar up-current classifiers), are recommended for use prior to dewatering in a Fine Material Screw Washer.

Misconception 5: All Fine Material Screws are created and designed equally

Most competitive units have a close tolerance or spacing between the screw shaft and the curved dewatering plate of the washer box or tub. Because of this close spacing, the abrasive sand can wear holes in the tub. 

A wider gap between the rotating screw and the stationary tub allows for a liner of sand to develop that nearly totally eliminates abrasive wear on the mild steel tub. 

When you’re getting pricing on a new sand screw, ask the dealer about this spacing to compare it to others you are considering.

Misconception 6: Fine Material Washers typically have a reputation for losing product sized fine sand

While this may be true for some Fine Material Washers, units with larger pool settling areas and longer weirs provide more fines retention.  

There really is a difference in the design of the washer box that directly determines the + 75 micron or +200 mesh product sized fine sand retention. If this sand overflows with the silts and clays, you’re losing tons per hour of product

Longer weirs and a larger pool setting area put more sand in the product pile, not lost to the tailings pond. Check the manufacturer’s published water volume capacity chart. For the same sized unit, there’s typically a 20% difference between one manufacturer’s unit and another in the water volume capacity.

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Misconception 7: The length of the screw shaft doesn't matter

Longer screw shafts provide increased dewatering time, resulting in a drier product discharge. The additional length is all in the dry deck dewatering section of the washer box. 

This is applicable in both Fine and Coarse Material Washers.

Misconception 8: The lower bearing seal needs replaced every year or two on a Screw Washer

While this is a common practice with the some manufacturers' machines, it is not true for all Screw Washers. 

Screw Washers can be designed with bearing seals that last for years. Check out this detail very carefully. With a separate grease point for a submerged lower bearing assembly, seal life is substantially increased in many cases to six to eight years, which increases a machine’s availability and reduces downtime. 

Not every manufacturer has such a superior design. Due diligence in knowing what is being offered can pay big dividends to lowering your sand screw maintenance costs.  

Tags: Washing and Classifying, Aggregates

Dave is a Product Support Specialist at McLanahan Corporation for the wet processing/washing and mixing equipment lines. He has been involved in the process selection and sales of the company’s product line of Feeders, Crushers, Screens and Washing Equipment since 1975, first as a representative and then as an employee starting in 1985.