You may see fine product-sized sand accumulating in your settling pond and say to yourself, "If I could save that stuff, I could sell it."
The fine sand may be +200 (75µm) or +325 mesh (44µm) that may be visible from your wash plant's initial discharge into a settling pond. Keep in mind that these coarse fines are likely a fraction of the effluent discharge that may also be unwanted silts and clay.
To determine the amount of fine sand being lost that can be recovered, you need to first determine the particle size distribution of the total solids flow. If you have a basic Screw Washer set up for washing and dewatering sand, it's pretty simple to gather a representative sample from the overflow.
If your sand processing plant has multiple pieces of equipment with varying configurations, such as that include a Classifying Tank, Hydrosizer™, Screw Washers, Dewatering Screens and Hydrocyclones, it may be best to sample the total flow discharging into your settling pond or Thickener.
How to take a sample
When considering sampling, safety and precaution must be taken into account before gathering a Screw Washer's overflow or plant's effluent discharge into a settling pond.
Keep in mind that whatever your sand plant's equipment entails, samples are collected over a brief period of time. The solids collected are dependent on the material being fed into your plant at the time of slurry sampling. Some plant feed sources vary, which can then affect what solids are discharging to the tailings pond. Some plant fee sections may have more clays or silts than others, which can vary day to day.
Sample gathering a Screw Washer's overflow can be quick and simple. Having two or three empty wide-mouth sports drink bottles (keep the screw-on cap) can be used for collecting and shipping effluent samples for lab analysis.
McLanahan's lab is well equipped for analyzing properly collected samples and can provide a report that includes a percent solids of the samples submitted and a particle size distribution down to 400 mesh (38 microns). There are many minerals processing labs that can also provide a proper report of the gradation of solids not saved in the Screw Washer product.
From a Screw Washer overflow weir
When gathering the Screw Washer overflow using the sports drink bottles, one only needs to simply sweep the wide-mouth bottle quickly across the back and side weirs in a couple of seconds without overflowing or boiling over the bottles. Continued sampling with a bottle once it is full can cause displacement, resulting in improper solids collection and an incorrect lab report of the solids gradation.
If there are fine sand solids overflowing the Screw Washer that can be allowed in the product pile, you could consider a couple of simple options. One would merely be a larger Screw Washer to handle the slurry flow and retain the product-sized fines being lost.
There are other options that typically include the use of Hydrocyclones to recover the fine sand. The Hydrocyclones could be mounted over the Screw Washer's discharge or the Screw Washer's overflow could be pumped to another location and, again with the use of the Hydrocyclones, recover fine sand and dewater the solids onto a stockpile.
From a sand plant
Collecting a sample of a more complex sand plant may best be accomplished at the common discharge point (if one exists) of a single pipe that flows into a settling pond. To gather a representative slurry of solids and water, the pipe cannot be below or submerged into the tailings pond but should have an elevated discharge where a small series of samples can be gathered.
This gathering may be over a period of three or four hours, providing a larger sample that can be sent to McLanahan or a minerals processing lab to determine the percent solids of the tailing's slurry and the gradation of the solids like previously described.
Additional sampling equipment may simply be that same wide-mouth sports drink bottle secured or heavily taped to a stick-like broom handle. If you can safely extend the sports drink bottle across the bottom of the slurry flow discharging from the pipe, quickly sweep the bottle to gather a slurry sample.
Carefully pour the bottle's sample in a larger bucket that has a snap-on lid. This bucket/lid can then be used to send the collected slurry sample to a lab for analysis.
Sweep the bottle two times every 20 minutes for the three- to four-hour period and you should have a pretty representative tailings flow sample of what was discharging from your plant over that period of operation.
For either of these sampling scenarios, you will also need to determine the volume of slurry flow, such as the gallons per minute or cubic meters per hour required to size the equipment and the various options that may be in your budget. McLanahan can help you with the water volume calculation too.
Considering to what extent processing equipment may be purchased, one needs to consider the value of the recoverable sand. Additionally, reduced operational costs may be realized in handling a lower volume of solids flowing to your settling pond, and the resultant reduction of equipment and man hours needed to dig out settling ponds.
McLanahan offers a wide range of equipment for recovering and dewatering fine sand and silts or clays. Customers have totally eliminated settling ponds with a turnkey system of fine solids dewatering, which returns clear process water for use back to your washing plant. Depending on your requirements, we can offer low-cost, simple solutions to total fines recovery and dewatering process options for your operation.