Understanding the Role of Fines Recovery in Sand & Gravel Operations
Most aggregate operations utilize washing to create clean, in-spec material. Washing inevitably produces a waste stream, comprised of the water used to wash material, silts, fines and clays.
This waste stream is usually disposed of in settling ponds, as they are cheap and easy solutions. However, settling ponds can become expensive and difficult to manage over time.
As settling ponds are filled, the settled solids must be removed, or surface water needs to be pumped out of the pond to create space for more waste. Not only is this a time-consuming task, but an expensive one as well.
Implementing a fines recovery process in your operation can help minimize the cost and labor needed to maintain settling ponds by reducing pond cleanout, minimizing the load on downstream equipment and creating a saleable product from waste.
Fines Recovery Systems
Fines recovery can be applied through various pieces of equipment.
A common solution is a fines recovery system, such as the Hawk Fines Recovery System, which features several equipment pieces that work together to recover ultra-fine particles and reduce the number of solids that go to a settling pond. A typical fines recovery system consists of a sump, pump, hydrocyclones and dewatering screen.
The waste stream enters the sump and is pumped up to the hydrocyclones. The hydrocyclones separate the waste stream into liquids and solids. Fine particles down to 400 mesh are captured by the hydrocyclone and discharged onto a dewatering screen for moisture removal.
The remaining water is discharged from the top of the hydrocyclones, where it can be sent to a settling pond or used in additional downstream processing equipment.
Other Fines Recovery Equipment
Along with fines recovery systems, thickeners and filter presses are also used for fines recovery. When used together, a fines recovery system typically precedes a thickener and/or a filter press. Fines recovery systems, thickeners and filter presses can be used in combination with each other or as individual pieces of equipment to achieve fines recovery.
Thickeners use chemical reagents to settle suspended solids in water. The fines are separated from the water, allowing for recycling of water and a lighter load for the settling pond.
Thickeners concentrate the waste stream up to 60% solids, which can then be sent to a small pond. Because thickeners concentrate the solids, the pond fills slower and requires less maintenance than an operation without a thickener.
Similarly, filter presses help separate waste slurry for fines recovery and tailings management.
A typical filter press is comprised of a series of plates with filter cloths. The plates are held together with a hydraulic mechanism and slurry enters the filter press at a high velocity. As the slurry fills the plates, the solids are captured on the filter cloths while the water exits the filter press.
Filter press cakes can be handled by a conveyer or front-end loader and a haul truck, eliminating the need for a settling pond. When used with a thickener, filter presses recover up to 98% of process water for reuse.
Benefits of Fines Recovery in Sand & Gravel Operations
Fines recovery can play an important role in sand and gravel operations, providing opportunities for cost savings, better access to reserves and more saleable product for your operation.
Reduced pond cleanout and costs
Clearing out your pond can become a frequent process without the use of fines recovery equipment. Without a fines recovery process, all parts of the waste stream are dumped in the pond, filling it faster and requiring more frequent cleanout.
More frequent cleanouts mean more money and time spent on the pond. Implementing a fines recovery process reduces the frequency of pond cleanout, saving time and money for your operation.
Reduced load on downstream processing equipment
If your operation also entails tailings management and water recycling equipment, a fines recovery process can help reduce the load on those pieces of downstream equipment, saving you money and time.
Fines recovery systems can better prepare the material that is typically considered waste for further downstream processing. Ensuring the feed has been properly prepared for further processing will minimize maintenance costs and downtime, as well as help to prevent wear and tear.
Making a saleable product
The material that is discharged from the fines recovery system is a drip-free solid that could potentially be sold as a new product. As fines recovery systems separate the fines from the liquid, those fines are dewatered and can be sold as pond lining or backfill, becoming a new revenue source.
If the material is not able to be sold, it can be used for site reclamation, helping restore the site back to its original condition.
Fines recovery can be a profitable addition to a new or existing site. Whether it be reduced pond cleanout or the creation of a saleable product, a fines recovery unit can provide numerous benefits for your sand and gravel processing operation.