Materials engineers constantly strive to improve concrete and bituminous mixes and road bases. Clean aggregate is a vital part of that effort.
Still, aggregate producers often find it difficult to meet all the requirements for “cleanliness.” While hydraulic methods are most satisfactory for cleaning aggregate to achieve desired results, they are not always perfect. So it remains necessary to accept materials on the basis of some allowable percent of deleterious matter.
In the broadest terms, construction aggregate is washed to make it meet specifications. Specifically, though, 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; and 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’s important to test the source thoroughly beforehand to ensure the desired finished aggregate can be produced at a reasonable cost.
A project materials engineer can be of immeasurable help in determining the economic suitability of material and, typically, must approve the source before production begins.
In addition, a number of washing equipment manufacturers will examine and test samples to determine whether their equipment can do a job satisfactorily. No reputable manufacturer wants to recommend equipment if it has reasonable doubt about satisfactory performance on the job.
The ideal gradation is seldom, if ever, met in naturally occurring deposits. Yet, the quality and control of these gradations is absolutely 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 is the usual objective of aggregate-preparation plants.
Crushing, screening and blending are methods used to affect aggregate gradations. But even following these processes, 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:
1. Coarse material. Generally above 3/8 in. (and sometimes split at 1/4 in. or #4 mesh). In the washing process, it usually is desired to remove foreign, objectionable material –including fine particles.
2. 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.
Of increased importance
With specifications being stringent, washing and classifying of aggregate materials is ever increasing.
As pits and quarries progress into their reserves, the more easily available material is depleted. This forces operations – particularly sand and gravel – to process material with a greater amount of clay and silt.
While some materials may only require rinsing to remove small silt particles, other materials may require scrubbing to remove clay and other deleterious materials.
Sand and gravel typically are mined in a moist or wet condition by open pit excavation or by dredging. Open pit excavation is often conducted with power shovels, front-end loaders, bucket-wheel excavators or draglines.
Alternatively, dredging involves mounting equipment on boats or barges and removing the sand and gravel from the bottom of a body of water by suction or bucket-type dredges. After mining, the materials are transported to the processing plant by suction pump, earthmovers or trucks, or by the method of automated belt conveyor systems.
Although significant amounts of sand and gravel are used for fill, bedding and base products without processing, most domestic sand and gravel is processed prior to use.
Processing sand and gravel products for a variety of specific market applications requires the use of different combinations of equipment, which may include portable and stationary washing and screening plants; sand classification tanks and systems; dewatering screens and screws; coarse and fine material washers; blade mill washers; log washers; rotary and vibrating screens; and more.
Manufactured sand is produced from the crushing of hard stone such as granite.
As an alternative to river sand, the use of manufactured sand is becoming more prevalent because of dwindling natural sand reserves and the constraints and expenses associated with mining natural sand and gravel near urban areas where material is needed most.
Washing and classifying equipment may be used in the processing of manufactured sand. The crushing process creates a significant amount of fines, which are undersized, fine particles that pass through the smallest screen openings (which are measured by mesh sizes).
Minus 100 and minus 200 mesh fines require washing to be removed. While these fines are allowable in the bottom end of asphalt sand products, it is common to wash manufactured sand in this application. Most concrete sand products require the removal of fines. Equipment choices vary in the removal of fines, from the use of a wet screen to the use of a classifying system.
Manufactured sand is widely accepted in asphalt mixes. In concrete mixes, manufactured sand is often blended with natural sand. Washing (versus air separation) is generally recommended when classifying material for concrete mixes.
Sand processing overview
Although processing operations vary, the material flow in a sand processing operation can be described in the following basic example:
The initial material feed that passes through the coarsest screen (i.e., the largest screen openings) is washed in a log washer before it is further screened. The name “log washer” comes from the early practice of putting short lengths of wood logs inside a rotating drum filled with sand and gravel to add to the scrubbing action.
A modern log washer consists of a slightly inclined horizontal trough with slowly rotating blades attached to a shaft that runs down the axis of the trough. The blades churn through the material as it passes through the trough to strip away any remaining clay or soft soil.
The larger gravel particles are separated out and screened into different sizes, while any smaller sand particles that had been attached to the gravel may be carried back and added to the flow of incoming material.
The material that comes off any intermediate screens may be stored and blended with either the coarser gravel or the finer sand to make various aggregate mixes.
The water and material that pass through the finest screen are pumped into a horizontal sand classifying tank. As the mixture flows from one end of the tank to the other, the sand sinks to the bottom, where it is trapped in a series of bins.
The larger, heavier sand particles drop out first, followed by the progressively smaller sand particles, while the lighter silt particles are carried off in the flow of water. The water and silt are then pumped out of the classifying tank and through a clarifier, where the silt settles to the bottom and is removed. The clear water is recirculated to the feeder to be used again.
Information for this article adopted from Pit & Quarry…
Read More:The importance of setting up your wash operation for success – Pit & Quarry