Maximize washing operations with the right equipment, practices
A properly designed dewatering screen discharges the driest washed sand product of any dewatering device commonly used. Photo: McLanahan Corp.
2. I’m losing 200 mesh sand. Perhaps you’ve recently added more water to the wash screen that precedes your sand dewatering gear to make the rock cleaner. This can result in losing 200 mesh sand to your settling pond. And cutting back on the wash screen water is not an option because the rock is too dirty and out of spec.
Some capital equipment changes or additions may be needed if your equipment is undersized for the water volume at hand. If you use a bucket wheel or sand screw, the addition of a properly sized hydrocyclone to handle the dewatering unit’s overflow can recapture lost, product-sized fines. This would also require the addition of a sump and pump to feed the hydrocyclone at the required inlet pressure. Mounting the hydrocyclone over the discharge of your existing dewatering unit is common.
Another option when using a sand screw is to replace the existing unit with a larger machine. The capacity of a larger fine material screw washer shaft might not be needed, but the bigger tub and higher water volume capacity will often keep the finer sand in your product pile.
3. I can’t consistently make C-33 sand. There could be several reasons why an operation is unable to consistently meet specifications with concrete sand, beginning, perhaps, with its material deposit. This can happen in either a crushed stone manufactured sand plant or an alluvial sand deposit.
A slight variance in the screen mesh or type of screen media can make a significant difference in the sand gradation that goes on the washed sand pile.
Lastly, you may need to consider the addition of a sand classifier to be used before your washer/dewatering unit. There are two basic types of sand classifiers that can remove an excess intermediate sand that’s 100 mesh. One is the V-box, multi-valve discharge station classifying tank. Another is the up-current classifier, sometimes referred to as a hydrosizer.
These units assist the reblending of coarse, intermediate and retained fine sands into two or three products, depending on the configuration, allowing for the production of an in-spec construction sand.
4. We need to reduce the number of hours bailing out settling ponds. Some sand plant operators don’t keep track of the number of man and equipment hours spent bailing out settling ponds. If you’re filling in an abandoned section of a quarry, you can just keep filling the hole you’ve mined.
Several equipment options are available to wash and dewater sand that is typically 5-mm minus and finer. Photo: McLanahan Corp.
Other operators must maintain containment ponds so as not to discharge super fines into nearby streams or onto adjacent land. Some aggregate plants use many acres of land to contain super fines. Certain operators must limit their plant’s running hours to keep up with the constant mucking out of their settling ponds.
If the latter example resonates, several equipment options are available to reduce or eliminate the need for these containment ponds. Hydrocyclones, or a system with hydrocyclones and dewatering screens, can recover ultra-fine material 400 mesh and larger. Use of this gear can reduce the volume of flow into a settling pond.
Thickeners and filter presses may also be implemented. Thickeners separate solids from the liquid in a slurry to reduce the volume of material destined for settling ponds and to recycle clear process water for reuse in the plant.
Filter presses also separate solids from the liquid in a slurry, producing a drip-free cake from the solids and clear process water for reuse in the plant. With a filter press, settling ponds can be completely eliminated, thereby also eliminating the time required for cleaning out settling ponds.
Information for this article courtesy of McLanahan Corp.
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