Materials engineers constantly strive to improve concrete and bituminous mixes and road bases, and clean aggregate is a vital part of that effort.
Yet, aggregate producers can find it difficult to meet requirements for “cleanliness.” While hydraulic methods are most satisfactory for cleaning aggregate to achieve desired results, they are not always perfect. It is still necessary to accept materials on the basis of some allowable percent of deleterious matter.
In broad terms, construction aggregate is washed to make it meet specifications. Specifically, however, there is more to the function of water in processing aggregate than mere washing. Among these functions are:
• The removal of clay and silt
• The removal of shale, coal, soft stone, roots, twigs and other trash
• Sizing
• Classifying or separating
• Dewatering
Because no washing method is flawless and because some materials may require too much time, equipment and water to make them conform to specifications, it is not always economically practical to use such materials.
Therefore, it is important to test the source thoroughly beforehand to ensure the desired finished aggregate can be produced at a reasonable cost. Many manufacturers of washing equipment will examine and test samples to determine whether their equipment can do the job satisfactorily.
The ideal gradation is seldom, if ever, met in naturally occurring deposits. Yet, the quality and control of these gradations is essential to the workability and durability of the end use. Gradation, however, is a characteristic that can be changed or improved with simple processes, and it is the usual objective of aggregate preparation plants.
Crushing, screening and blending are methods used to affect aggregate gradations. However, even following these processes, the material may still require washing to meet a cleanliness specification.
Also, screening is impractical for aggregate smaller than No. 8 mesh, in which case, hydraulic separation, or classifying, becomes an important operation.
Washing and classifying of aggregate can be considered in two parts, depending on the size range of material.
• Coarse material: Generally above 3/8 in. (sometimes split at 1/4 in. or No. 4 mesh). In the washing process, it usually is desired to remove foreign, objectionable material, including fine particles.
• Fine aggregate: From 3/8 in. and smaller. In this case, it generally is necessary to remove dirt and silt while retaining sand down to 100 mesh, or even 200 mesh.
Classification system controls
Most classifying plants are operated via automated control systems.
To meet finished product specifications, the feed rate of incoming material, the vibration rate of the sorting screens and the flow rate of the water through the sand classifying tank must be carefully monitored and controlled. Automated control systems allow the operator to compensate easily for changes in the feed or slurry mix being fed to the plant, to closely monitor the manufacture of sand products for total tonnage and quality and to automatically make products to a closely defined, predetermined specification, with few (if any) operator calculations, minimizing the opportunity for human error.
In the processing and handling of sand, gravel or crushed stone, it is necessary to complete the separation or dewatering of the fine solid materials from the slurry containing them.
Wastewater treatment
Washing sand and aggregate results in the discharge of dirty water from wet screening decks, sand screws or sand classifiers.
The wastewater typically carries fines out to a series of settling ponds. While this is the most common method of treating wash water fines, it is not the most sustainable method. As such, many operations employ equipment such as filter presses, hydrocyclones and other equipment to more efficiently reclaim and reuse wash water – and to minimize settling pond use and maintenance.
Traditional settling ponds
The use of settling ponds is a traditional choice, but one that can be problematic. Consider the following:
• Water being recovered for washing may be lost to evaporation or percolation in the pond, which may require the operator to provide “make-up water” (which may be scarce) to the plant.
• The real estate for a properly sized settling pond may not exist at the plant site.
• The cost and time involved in cleaning the ponds with a dragline or excavator may be excessive. And, cleaning a pond – especially the fine material that flows downstream and settles very slowly – is extremely inefficient.
• Dirty water may get back to the plant, limiting production, producing washed material out of spec or even shutting down production.
• Ponds may present hazards to nearby equipment and to workers.
Still, some sites can be ideally suited to the use of settling ponds.
Some operations may access an exhausted portion of a large quarry where the dirty water can be deposited and the fines never need to be recovered. Clean water is recovered from the opposite end of the pond and the fines settle down in the deep quarry bottom, never to be dealt with again.
Hydrocyclones
Hydrocyclones are tapered cones that receive the dirty wash water at a high velocity. The water travels in a tight circle within the cone, and centrifugal force throws the largest particles to the outside of the cone where they slide down the cone and out the bottom.
Rather than discarding all the particles into a settling pond, the cones can recover the No. 200-mesh and larger material, which can be sold as aglime, mineral filler, lining for utility trenches, mortar, grout additives and other purposes.
Sustainability
Lastly, it is important to conduct periodic water audits to determine the best and most sustainable use of water within the aggregate washing process.
Operations should develop a maintenance program that routinely inspects all plumbing equipment and fixtures, water lines, spray systems, valves and pumps. Metering at strategic points in the facility helps to detect leaks and maintains minimum flow rates.
Above all, operations should employ the optimal reuse and recycling systems for aggregate washing.
Related: Prioritizing portable wash plant safety
Information for this article derived from Pit & Quarry University, an educational resource of Portable Plants’ sister publication.