Did you know that your rock and sand products often go through a wash cycle?
Why Do We Wash Sand and Gravel?
Gravel and other rock products are sent through a wash cycle so that the amount of fines—the tiny particles of rock dust that make gravel look “dirty”—is reduced. Sand products have finer particles of fine sand and silt that are washed away in the same process. These processes make products appear cleaner and less dusty.
Cleaning materials not only makes gravel and sand products look more appealing, it’s also important in reducing silt buildup. For example, streambed rehabilitation experts are careful to avoid introducing too much silt to streams, because it can impact the flow of water and affect the habitats of fish and other aquatic species. Our custom river rock mixes, also known as streambed cobbles or fish cobbles, must meet the low silt content requirements of each unique streambed restoration project for this reason.
What Happens to the Fines?
So what happens to all of the fines after they’re washed off of gravel and sand? The fines are pumped from a holding tank to man-made settlement ponds. Over time, the fines sink to the bottom of the pond, and the remaining water is reused over and over again.
As the ponds fill up with fines, they have to be dredged to create space for more material. Dredging means that a crane with a large bucket attached skims the bottom of the pond and pulls out the fines. The fines are loaded into a dump truck and stockpiled in a different area where the remaining water can evaporate.
In the case of fines from King Creek Pit (a sand and gravel pit), the silty fines can be used as a natural pond liner, which slows the absorption of water into the ground. This material is slightly coarser than clay.
Fines from Kapowsin Quarry (a basalt quarry) can be used in the manufacturing of cement.
Using the Ultra-Fines Recovery System to Save Water and Create Cleaner Materials
King Creek Pit recently installed a game-changing piece of equipment to increase water recovery and reduce the labor associated with dredging the settlement pond. Called an ultra-fines recovery (UFR) plant, the system conserves water by processing it more rapidly and efficiently than the old system.
This helps Washington Rock avoid having to import water in the summertime when water evaporates more quickly and the water table is low.
As explained by our quality control manager, Ryan Hogg, the UFR system recovers water in a 2-step process:
- A very fine, very high-frequency screen is combined with panels (a.k.a. baffles) that obstruct the flow of water to separate solids from water-sized particles.
- A cyclone-like action further separates ultra-fine solids from the water.
After the UFR system processes materials, the materials move to a tank where they are cleaned even further. The end result is that sand and gravel products are cleaner and more water is recovered. The settlement ponds also need to be dredged less frequently.
The Ultimate Byproduct: Ultrafine Sand
The UFR system makes it possible to separate ultrafine sand particles from coarser sand particles. Using a 50/50 ratio of ultrafine sand and silt, Washington Rock created a new ultrafine sand product.
“Ultrafine sand can be delivered with the right amount of moisture so it can be shaped and compacted on site,” Ryan explained.
Ultrafine sand works well as a sand base for swimming pools, a shaping sand for sandboxes and sandcastles, and a beach volleyball sand that is soft on the feet.
Learn more about ultrafine sand on the Ultrafine Beach and Pool Sand product page. Special thanks to Ryan Hogg for providing the technical information about the UFR process.