Are You Losing Good Sand? How To Sample A Weir Overflow

By: Dave Schellberg March 12, 2019
Sampling your weir overflows can help you determine if you are losing product-sized sand to your settling pond.

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 (75µ) or +325 mesh (44µ) 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. 

How to take a sample

When considering sampling

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.

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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.

Tags: Washing and Classifying, Aggregates, How To

Dave is a Product Support Specialist at McLanahan Corporation for the wet processing/washing and mixing equipment lines. He has been involved in the process selection and sales of the company’s product line of Feeders, Crushers, Screens and Washing Equipment since 1975, first as a representative and then as an employee starting in 1985.