P&Q University Lesson 9: Washing & Classifying

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 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. It is still necessary to accept materials on the basis of some allowable percent of deleterious matter.

In broadest 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:

■ Removal of clay and silt.
■ 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.

The project materials engineer can be of immeasurable help in determining the economic suitability of the material, and typically, must approve the source before production begins.

Further, many manufacturers of washing equipment will examine and test samples to determine whether their equipment can do the job satisfactorily. No reputable equipment 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. 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 #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.

INCREASING IMPORTANCE

With specifications becoming increasingly stringent, washing and classifying of aggregate materials is ever increasing. As pits and quarries progress into their reserves, the more easily extracted material is less available, often forcing operations — particularly sand and gravel operations — to process material with a greater amount of clay and silt. While some materials may require only 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

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 or the constraints and expense of extracting natural sand and gravel near urban areas where material is most needed.

Washing and classifying also may be used in processing 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). The 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 recommended when classifying material for concrete mixes. When blending natural and manufactured sand, it is best to use a classifying tank.

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 (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 the intermediate screen(s) 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.

SAND CLASSIFYING SYSTEMS

It is important to take a closer look at sand classification systems. Sand classifying and re-blending systems allow material producers to modify a raw natural sand blend into a high-quality, industry-standard specification mix; or blend a natural sand and a manufactured sand to a desired specification product mix. Classifying systems are available in either stationary, portable or semi-portable configurations. The choice of a particular configuration depends on the physical characteristics of the sand mining site and any relocation needs.

Classifying and re-blending systems allow producers to do the following:
■ Scalp (remove) excess water from dredged material.
■ Reject deleterious (harmful) material.
■ Separate sand particles.
■ Re-blend sand particles to create more than one spec product simultaneously.

Sand classification systems use water and the principles of gravity and settling to separate and re-blend sand. For a traditional classifying tank, the industry-standard method is based on the principle that coarser sand particles are heavier and will settle near the feed end. Finer and lighter sand particles are carried progressively further down the length of the tank. During this process, the tank is constantly supplied with a feed of water that keeps the top of the tank overflowing. This constant movement of water over the edge of the tank (or weir) helps eliminate organics and other material that should not be present in the final sand blend.

As the sand particles of different sizes settle to the bottom of the tank, they begin to pile up around valves located there. These valves are typically installed in sets of two or three. Valves are grouped together in stations, and each station is equipped with a sensing paddle that turns at a slow but constant speed. As the sensing paddle turns, the sand builds up around it. Once enough sand piles up around the paddle, the paddle begins to “stall,” sending a signal to the classifying tank controller that this station’s area is filling up with sand. In turn, this signal causes the valve or valves in that station to open up to discharge sand.

The sand that is discharged from each station is sent into a flume at the bottom of the tank where the sand is remixed and dumped into one of several dewatering devices. Once the sand has been separated from the water, it is moved with conveyors to large stockpiles. Sand is stored in these stockpiles to dry and then shipped to customers for use in a wide variety of projects, including concrete mixes, masonry mixes, golf course bases, and sand traps and fill sand.

Each of these re-blended products is made of a different mixture or “recipe” — specification — of particle sizes. Specifications are created in a variety of ways, but all are measured by the amount of material that either passes through or is retained by different-sized sieves or screens. Within these specifications, the producer must determine sieve sizes, along with just how much material must be of a specified size. To remain competitive and profitable, operations typically rely upon skilled quality control teams to test, monitor and maintain all specification requirements of final products.

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. With that in mind, new solutions to the problem are often developed, such as recently introduced unit that is a combination of a fine material washer and a dewatering screen.

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, water clarifiers and flocculent systems 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.

However, 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.

HYDROCLONES

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 #200-mesh and larger material, which can be sold as aglime, mineral filler, lining for utility trenches, mortar, grout additives and more.

WATER CLARIFIERS

Water clarifiers are of value to a producer who wants to minimize settling pond maintenance and reclaim a large percentage of the water immediately. Also, for those producers who have an area that has already been mined and will ultimately be reclaimed, a good clarifier can pump the solids to that area, and it never needs to be conveyed, loaded or hauled again. While clarifiers may require a significant initial capital outlay, they require minimal maintenance and operating costs.

Clarifiers are combined with flocculent systems. Flocculants rapidly settle out virtually all of the suspended solids in a dirty water stream. Liquid flocculants require very little hardware and can be introduced via a small chemical metering pump with some dilution water added. However, liquid flocculants are not always environmentally friendly and often separate or stratify in the container before use. Dry flocculants are more environmentally friendly but require more sophisticated equipment to get them into the solution properly. It is imperative to work with a competent vendor to select the right flocculent for a given application.

FILTER PRESSES

Filter presses take the silt that a clarifier has collected and essentially squeeze the water out of it to form a cake that is discharged and can be conveyed or transported by a loader to a disposal area; or the cakes can be used as backfill material. The use of clarifiers and filter presses minimizes fines going into settling ponds, while making waste material easier to handle.

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. But, above all, operations should employ the optimal reuse and recycling systems for aggregate washing.

Wet processing equipment

Selecting equipment for washing and classifying can seem like a daunting task. There are countless solutions using a variety of equipment that can be put together based on a producer’s needs. Although some of this equipment was detailed earlier, following is a more-complete overview of wet processing equipment available to aggregate producers.

AGGREGATE CONDITIONERS

Aggregate conditioners are designed to help producers start to liberate light, loamy clay or dirt from either coarse rock or sand before further processing. They can be placed ahead of a wash screen or other washing equipment, such as sand classifying tanks or fine material screw washers. Using a combination of paddles and flights arranged in alternating format along the length of the shaft, aggregate conditioners begin to scour, abrade and break down deleterious material.

Although similar in appearance to coarse material screw washers, aggregate conditioners function very differently. For example, all material and water entering the aggregate conditioner must exit through the discharge opening at the bottom of the box opposite the feed end. There is no overflow. They sit on a slope of zero to 5 degrees, which gives them much greater capacities.

COARSE MATERIAL SCREW WASHERS

Built primarily to wash crushed stone and gravel, coarse material screw washers effectively remove light, loamy clays, dirt, crusher dust and coatings that cannot be removed by wet screening alone. They also can be used to remove floating vegetation and soft aggregate from the material feed.

Coarse material screw washer paddles are used in conjunction with screw flights to provide scouring and agitation. The turbulent washing action combined with rising current water, which is introduced at the feed-end at the bottom of the box, results in separation of the lighter fraction from the sound aggregate.

Lighter fractions float to the surface due to water rising in the box and then overflow the weir located at the back of the box. Desired clean product is then scrubbed and conveyed by the paddles and flights to the discharge end of the box.

Paddle configurations can vary based on the design and length of the coarse material screw washer. More paddles increase washing action but decrease capacity because paddles do not convey material up the washer box as fast as flights. When using additional paddles, it is necessary to lower the slope of the box and increase motor horsepower, which will help convey material to the discharge end. Additionally, reversing some of the paddles will retain material in the box longer.

FINE MATERIAL SCREW WASHERS

Fine material screw washers are used primarily to dewater, classify and wash minus-3/8–in. sand or other fine material. They are designed to accept feeds from sand classifying tanks, belt conveyors, other fine material screw washers or slurry feeds.

In a fine material screw washer, as material enters the feed box, heavier material sinks to the bottom of the box, while finer fractions float to the surface and over the back weir. Material that sinks is then conveyed from the pool area up an 18-degree slope toward the discharge end.

As material is conveyed, it enters the dry deck section of the washer box at which point the water begins to separate from the material. Curvature of the rotating screw flights conform to the curved section of the washer box to efficiently convey, wash and dewater.

Sizing and selection of fine material screw washers is based on the type of material, feed gradation, desired product specification, capacity required and water volume, if in a feed slurry.

LOG WASHERS

Log washers are used in aggregate processing to remove tough, plastic-type clays from natural and crushed gravel, stone and ore feeds. When in operation, log washers sit at a slope of between zero to 14 degrees, depending on the severity of the washing application. As the percentage of deleterious material increases, the slope must be raised to increase retention time. In some cases, lower slopes are able to increase capacity while decreasing horsepower and wear.

One limitation of log washers is that they must have a controlled top size. In general, 36–in.- and 38–in.-diameter units can accept feed material up to 4 in., while a 46–in.-diameter unit can accept feed material as large as 6 in. cubed. It is recommended that sand-sized fractions be removed prior to the log washer as finer material tends to cushion the washing action.

When selecting a log washer, the amount, type and percentage of deleterious material to be removed from the aggregate must be considered. As the percentage of deleterious material increases, consider longer-length units to increase the washing action. In some severe applications, two or more log washers in a series may be required.

SAND CLASSIFYING TANKS

Sand classifying tanks provide several results, including removing excess water, classifying material by removing excess of certain mesh sizes, retaining finer mesh sizes and making multiple products from a single feed material. They are effective, low-maintenance units that produce one or more specific products. With either slurry or a dry feed, they are capable of handling sand gradation swings in the average plant, while minimizing waste.

Sand classification is based on the different settling rates of various grain sizes. As water and aggregate material enter the feed end, coarser grains settle first, and finer grains settle in successive sizes down the length of the tank. At the top of the tank, a series of hydraulic control mechanisms operate the discharge valves at the bottom of the tank. Depending on the type of control system and product produced, one, two or three discharge valves are located at each station.

ATTRITION SCRUBBERS

Attrition scrubbers, also known as attrition cells, are used to liberate deleterious material and remove it from competent aggregate material. They are also proven to liberate clays, reducing product turbidity, and to break apart loosely conglomerated clusters in frac sand plants. Attrition scrubbers also can be used in glass sand, frac sand, clay and sand and gravel production, as well as in preparation of flotation feeds and reagent washing.

Attrition scrubbers produce a high-shear environment where particles scrub against themselves to scour their surface and liberate deleterious materials. All internal parts are completely rubber lined to maximize wear life and minimize maintenance time and costs of replacement parts.

The cells are typically fed by separators, hydrosizers or similar equipment that will prepare the material at a high density and achieve the best scrubbing action possible.

DEWATERING SCREENS

Dewatering screens have a wide variety of applications, including dewatering, desliming, degritting, rinsing, scrubbing and washing.

They are designed to produce a consistent drip-free product, which is typically significantly drier than other dewatering equipment, such as fine material screw washers. Materials such as sand, gravel, crushed aggregates, frac sand, industrial sands, mineral sands, etc., are all capable of being processed through a dewatering screen.

To dewater material effectively, slurry is fed onto a steep, downward-inclined screen surface at the feed end of the dewatering screen to achieve rapid drainage. A pool of water begins to form in the valley as material builds up on the slightly upward inclined surface.

Counter rotating motors create a linear motion, driving solids uphill, while liquid drains through the screen media. The uphill slope of the screen, along with a discharge weir, creates a deep bed that acts as a filter medium, allowing retention of material much finer than the screen openings.

Where cleaner aggregate materials are required, cross dams and sprays can be included to assist with the washing process, liberating fines that pass through the media.

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Sources:

Contributors to this chapter include the following, in alphabetical order:

John Bennington
Product Manager, Washing/Classifying
Superior Industries Inc.

KPI-JCI & Astec Mobile Screens

Rick Madara
Sales Manager, Aggregate Processing
McLanahan Corp.

Steve Slater
Sales Manager
EIW

Terex Washing Systems

W.S. Tyler

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Lesson 9 Quiz

1. What tools are typically needed to conduct open pit excavation?

2. What size material is considered fine aggregate?

3. Why is manufactured sand becoming more prevalent than natural sand in some cases?

4. What do classifying and re-blending systems allow aggregate producers to do?

5. What are the benefits of water clarifiers to producers?

6. What two systems minimize fines from going into settling ponds, making waste material easier to handle?

7. What materials do log washers remove?

8. What size material can hydrocyclones’ cones recover?
Click here for the quiz answers.