Aggregates Manager Digital Magazine
Issue link: https://read.dmtmag.com/i/120734
EQUIPMENT MANAGEMENT A fine material screw washer may be among the simplest machines to maintain in an aggregate plant, but appropriate safety precautions must always be taken. • • • Natural wear can occur to your screw washer if it is not properly maintained. • and undesirable fines are level and are positioned to retain the fine sand desired while removing the unwanted excess fine sand. As an example, often times, if excess fines are in the product, raising the side weirs and dropping the back weir allows for a maximum removal of undesirable fines. • It is important to review the amount of material passing 200-mesh in the feed. In most applications, this material is considered to be unwanted fines and needs to be removed. With too little water, the operator will not be able to remove the desired amount of minus 200-mesh material. As a general rule of thumb, the operator needs 50 gallons per minute for every 1 short ton per hour of minus 200mesh material in the feed. Water that comes in with the feed must also be taken into account. A washer box can only process so much water while retaining material to a certain size. Most manufactures will provide a water volume chart specific to their equipment. This will determine the size of a washer box that must be used in order to retain-product sized material while overflowing (removing) unwanted fines. Rising current classification water beneath the pool area of the screw washer may also need to be adjusted. If product-sized fines are overflowing — the screw washer and any rising current water is being injected beneath the pool area — reduce the flow or shut it off completely. When desired product-sized fines are being lost and are overflowing the screw washer, sometimes correcting improper sand slurry feed entry into the screw washer helps. The pool area in back of the baffle plate near the overflow needs to be as calm as possible so turbulence is reduced to a minimum, allowing for the settling of fine, product-sized sand. Directing the sand slurry feed to the discharge side of the baffle plate will allow for minimum turbulence at the rear of a fine material screw washer for optimum fine-sand retention. Screw shaft speed, which in many instances is set by the manufacturer for the original application, may need to be adjusted, particularly if the screw is turning too fast for finer sand. Generally, for dewatering an average concrete sand having up to 20 percent of the product passing 50 mesh, the peripheral speed for a fine material screw washer should not exceed 150 feet per minute or 75 percent of the full speed. Keep in mind that the reduction in the speed that is often needed for fine mason sand will result in a capacity reduction. AGGREGATES MANAGER April 2013 29