Sand plays a critical role as a construction aggregate and deserves special attention when considering the means of process control. Unlike coarse aggregate where various types of crushers may be used to upgrade mineral quality, sand basically relies on the same techniques to address both mineral quality and sizing. These techniques are called particle exclusion. Whichever size the Producer decides to eliminate for quality reasons obviously also affects the sizing.
1. Natural sand
Good quality natural sand is readily available in many areas and may be easy to obtain and process. As with the gravels that they often accompany, the sand deposits may not have been laid uniformly, meaning a potential change in quality and size is possible. In some deposits, sand found below the water table differs in fines content and quality from that found above the water table. Subsurface drilling, sampling, and testing is necessary to know to what degree and where these differences occur. Standard operating procedures in the Quality Control Plan should address the process if differences in size and quality are encountered, as a uniformly graded product of predictable quality is required to be maintained.
2. Manufactured sand
Because of the angularity, manufactured sand is very beneficial for use in hot mix asphalt where stability is critical. Many quarries are high in clay content and often a large amount of dust ends up in the feed stock for manufactured sand. Care is required to be taken to select the appropriate classification equipment that removes the necessary amount of minus No.200, yet retains other fractions of the sand gradation that are needed. For some uses, particle shape is important. Particle shape is set primarily by the crushing operation for the coarse aggregate. Any changes in crushers or crushing techniques may affect the properties of the manufactured sand product and therefore affect the customer’s use of the product.
Very few sand products are produced by air classification or by direct nonwashed screening. Most sands are produced with wash water and water classification. The key to all rinsing and water classifying systems is adequate delivery of water. Inadequate water supply and poor maintenance are the two most common reasons for inconsistent sand gradations. The most common water classifier is a simple screw sand washer (Figure 1) which may make a single “cut” in gradation and float out a certain amount of fines. By altering the through-put and rate of water flow the cut point may be changed.
A variation of the screw sand washer is the wheel sand washer (Figure 2). This device also is capable only of making a cut in the feed stock but may be more finely tuned and may be the better choice when trying to retain as much No. 50 and No. 100 material as possible.
An even more sensitive method of cutting out fines is the wet cyclone (Figure 3). The sand slurry in the cyclone is spun at a prescribed velocity. Centrifugal force separates the coarser fraction from the water and fines which exit to the pond.
Any of these techniques could conceivably be used with others in tandem or in tandem with rinse screens. The material could then be back-blended to create a desired product. A simpler and probably more cost effective way to control a sand gradation on multiple sieves is the rising current, multiple cell classifier. This equipment has numerous cells, each having varying water pressures that for different sizes of material. Any number of cells may then be combined to create the final product. With this type of system a high degree of process control is possible.