Sand screws – possibly one of the oldest mechanical means of washing sand – have been used for decades.
Their use, misuse and abuse abound. In Australia, there are a diverse range of views on the viability of sand screws with regards to washing sand for construction aggregates.
Sand screws are variously categorised as inefficient, unreliable mechanical nightmares, responsible for fine sand losses, and more recently compared to other production equipment, whereby the stated throughput of most manufacturers is questioned.
Several issues have led to “urban myths” about sand screws – in Australia, at the very least. While commenting on and refuting some of these myths, this article will also attempt to detail developments on the use of sand screws as a primary sand washing process.
What is a sand screw?
A sand screw is essentially a helix or helical surface around a central cylindrical shaft. The design is commonly attributed to Archimedes (based on the Archimedes pump or screw pump).
Sand screws are known by a variety of names in a range of industries, most related to dewatering or classification, and have been used in Australia to wash salt, wash and separate plastic, carry out particle separation with copper processing, wash and recycle glass, dewater mineral sands and coal fines, and wash sand, including manufactured sand.
{{quote-A:R-W:300-Q:Sand screws are the most efficient way to wash sand for construction aggregates.}}The basic functions of sand screws have been covered previously. However, some clarification and reinforcement of the basic operations might be useful for the industry. Essentially, the sand screw acts as an auger, separating the sand from the pool of water. The feed material is introduced, usually vertically, through the opening in the feed box, generally with water.
A correctly designed or engineered sand screw has a separation plate, to force the sand “down” in the pool of water. For fine material to escape or overflow the weirs, it has to go under this plate and then rise in the pool of water. The sand screws turn at about 60 peripheral metres per minute (200 fpm); depending on the diameter of the helix or spiral, this equates to 15 to 30 revolutions per minute for typical production sizes.
When sufficient sand drops to the bottom of the tub or pool of water, it is gently lifted by the auger action of the helix or spiral. Sitting at an angle of 18o, the sand screw begins to auger sand out of the pool of water. A curved plate typically exists under the helix/spiral. As the sand leaves the pool of water, the excess water drains through the sand and runs into the flushing trough.
The flushing trough is provided with water, at the upper end of the sand screw, with sufficient velocity to flush the fine sand particles that have flowed into the flushing trough back into the pool of water. This allows the dewatering effect to operate at the highest efficiency.
As the sand travels up the curved plate and tub wall, it continues to dewater. With a basic sand screw, most manufacturers will advise a likely output of 80 per cent solids or 20 per cent moisture. This should be sufficient to send material up a typical stacker. In comparison, a hydrocyclone, on its own, will discharge only about 60 to 65 per cent solids.
Urban myths
{{image2-a:r-w:300}}The problems and urban myths surrounding sand screws arise when poor copies are sold and operated, or sand screws are sold with limited knowledge on how they operate.
In Australia, there are poorly designed or poorly engineered sand screws that do not have the correct tub or weir size. There are ones without flushing troughs, and no port to inject water. There are sand screws that have been installed at the wrong angle. When not enough water is used, the pool of water begins to thicken in density or specific gravity.
The higher this density or specific gravity goes, the larger the particle size that will overflow the weir. If the sand screw is run at the wrong speed, particularly too fast, then more particle sizes stay suspended in the pool of water, and don’t settle. This allows more particle sizes to overflow the weir.
Sand screws are sized on the maximum amount of water they can process while retaining certain size particles. The speed of sand screws is determined by the amount of fine material in the raw feed. If you do not run a sand screw with the correct amount of water, or if you do not run it at the correct speed, you will lose fines and you may have a wetter product (wetter than expected).
If you don’t replace a worn feed box, you may break a shaft when the sand feed subsequently wears out the wall of the shaft. If you don’t replace worn or broken wear shoes, also known as flight shoes, then you will wear out the helix/spiral.
Numerous personnel in the field of selling sand washing equipment will recite problems with sand screws. However, the majority of those problems occur when the equipment is run with a lack of knowledge. Sand screws are the most efficient way to wash sand for construction aggregates.
Evolution
{{image3-a:r-w:300}}In the 100 or so years sand screws have been operating, new motors, new gearboxes and new bearings have been incorporated into the units.
Each manufacturer has a specific way of dealing with the lower end of outboard bearing. Similarly, variations on gearboxes and arrangements with motors are prevalent. A variety of wear materials are available for the wear shoes, as well.
However, one major enhancement in the past decade was the development of the AggreDry washer. Superior Industries supplies a range of sizes of these unique machines, with the patented AggreDry dewatering and underflow return, incorporated into the dewatering screens, integrated into the design of the sand screw. Figure 1 provides an illustration of this process.
Moisture content for the AggreDry washer can be as low as eight per cent, with producers advising they can load trucks immediately, with material coming straight off the stacker. This eliminates the need for stockpile “drying” to enable product to be loaded out.
Once the fundamentals of sand screw operation are understood, they retain fines, at the desired size, use less water and produce a drier product. In this time of rising electricity costs, they also use less power than alternatives.