Selecting the Right Aggregate Washing & Classifying Equipment

aggregate washing equipmentAggregate producers need to wash concrete and asphalt sands for several reasons. They need to remove lightweight particles such as lignite, mica and shale. They need to remove other undesirable particles such as sand in a gravel pile. Producers often have to alter feed gradation to meet a design spec. And many times, producers need to blend multiple feeds when material is being supplied by multiple quarries.

At the center of all of those reasons for washing is one simple objective.

"Anything producers can do to make their inputs cleaner and more to spec, the more they will help the customer save money — and the more likely the customer will want to continue buying from them," says John Bennington, director of washing and classifying equipment at Superior Industries, a manufacturer of crushing, screening, washing and conveying solutions.

There is no disputing the importance of washing. The question is: What is the best way for an aggregate producer to go about doing it?

Equipment for Finer Materials (< 3/8 inch)

Fine material screw washer. This is a popular option when producers need to bring feed into spec by removing 150-mesh material. The material sinks, which allows the lighter material to be pushed out over the top. The heavier material is then augered up an incline which dewaters that material.

These machines do some light scrubbing, but it isn't very heavy because the material is rolled when augered up the incline.

Relatively low horsepower is needed to operate these machines.

Discharge moisture can vary somewhat widely; anywhere from 10-25 percent.

Cyclones. "These machines are popular for making very sharp cuts," Bennington says.

Material separation is determined by the base diameter. "If someone talks about a 24-inch cyclone, they are talking about the base diameter," Bennington points out. The exit diameter is referred to as the apex. The size of the apex determines the size of the cut.

"The bigger the apex, the finer the cut," Bennington explains. "On the other hand, if you restrict the flow by having a smaller apex, you force more material out the top so only the course material can get out the bottom."

Feed pressure is another important consideration. As material comes into a cyclone, it spins (hence its name). The heavier and larger particles sling to the outside, and then slide down the outside to the bottom and out the exit (apex).

"The faster you run the cyclone, the more material that gets slung to the outside," Bennington says. "Conversely, the slower you run it, the less separation you get. But the bigger material still gets slung to the outside while the finer material stays in the middle."

When using a cyclone, Bennington says you can make a fairly precise split with a homogenous material. But if you continue changing pressures and the volume goes up and down, you'll end up with different cuts.

"One of the most important things with a cyclone is to make sure the pump has a constant amount of material coming into it," Bennington says. "If you lose solids, you need to increase the amount of water you're pumping in. If you lose water, you need to increase the amount of solids. On that note, it's very important to have proper pump sizing with a cyclone."

There is some light scrubbing with a cyclone because particles rub together, particularly in the impeller of the pump.

Medium horsepower is required to operate a cyclone.

Discharge moisture is 30-40 percent, which is pretty thin. "That's why producers generally run right to a dewatering screen after operating a cyclone," Bennington relates.

aggregate dewateringCyclone with dewatering screen. Building on the above point, Bennington says the most common way cyclones are used is in tandem with a dewatering screen.

"The screen usually has urethane media with 1/4 to 1 mm slots," Bennington says. "You want to put so much material on the screen that the particles that are smaller than the slots get pumped back to the cyclone. For whatever is left on the screen, shaking breaks the surface tension of the water and allows it to drain out."

This type of equipment configuration is ideal for 150-mesh material.

Only light scrubbing takes place, much like with a straight cyclone.

This type of configuration requires more horsepower than others.

Discharge is very dry at roughly 10 percent.

Fine screw washer with dewatering screen. With this configuration, producers get the same advantages of using a sand screw. This time, however, the material is deposited on a dewatering screen after traveling up the incline.

This type of equipment is also ideal for 150-mesh material.

Like a standard fine screw washer, light scrubbing takes place.

More horsepower is required compared to a standard fine screw, though it is still relatively low.

Thanks to the dewatering step being built in, discharge is very dry at roughly 10 percent.

An added benefit is that the footprint is no different than a standard fine screw washer. Also, no extra pump is required.

mclanahan log washerLow-water washer. All of the previous machines accept slurry feeds. A low-water washer is different in that it accepts feed directly from a crushing circuit. "So there is no need to haul the material to a separate washing site," Bennington points out. "You can also wash the crusher dust to create a product you can sell."

With this type of machine, material is discharged into an agitator. When water is introduced, it slurries the material. It then does a bit of scrubbing before discharging the material onto a dewatering screen.

"Unlike with a cyclone or sand screw, this machine is not trying to lift the fines out," Bennington says. "We're literally pushing them through the dewatering screen with a high-pressure, low-volume spray bar that pushes the dirty water through the bottom. With this process, you can reduce the silt by at least 4:1."

A low-water washer is ideal for 100-mesh material.

Only light scrubbing is involved.

Low horsepower is required.

Discharge is very dry at 10-12 percent.

Bucket wheel. As its name implies, a bucket wheel scoops material from the water bath to separate sand from silt. Screen media in the back allows the water to drain through. Then the material is either dumped directly onto a conveyor or into a stockpile. If the producer chooses, the material could also be taken to a separate dewatering screen.

"This type of machine tends to make a coarser cut (100 mesh) than a sand screw or cyclone," Bennington points out. "Bucket wheels are very popular with dredging operations because of their high water capacity."

Bucket wheels use very low horsepower.

No scrubbing is involved.

Discharge is dry at 10-15 percent.

classification of fine aggregate

 

Equipment for Coarser Materials (> 3/8 inch)

Coarse material screw. Bennington says producers commonly use this type of machine to wash soils and crusher dusts out of material. They are well-suited to 1/4 to 4-inch capacities.

This machine also helps with screening inefficiencies. "They are designed to blow fine materials out," Bennington says. "They have a narrow back end and run much deeper water overflow to blow sands out."

Only light scrubbing is involved.

Coarse material screws require low horsepower.

Log washers. These machines can generally handle up to 4-inch material. They are designed to do heavy scrubbing in order to remove plastic clays.

"They can actually handle up to 10 percent plastic clay content," Bennington says. "Interlocking paddles literally grind rock against rock, and the clay in between is what gets pummeled. Then we put just enough water into the log washer to dissolve that clay and lift it out. It's very important to use the right amount of water."

Log washers are also capable of removing light, friable material.

Some de-sanding is required before putting material into a log washer. Then, at the end of a cycle, it's important to follow with a rinse screen.

"Yes, you've done a great job of cleaning the rock," Bennington says. "But you've just lifted that rock out of a vessel of dirty water. That's why I always recommend a quick rinse before putting the material into a stockpile."

Log washers use quite a bit of horsepower, but water usage is relatively low.

Blade mill (aka prep screw). Unlike everything else we've discussed thus far, this washing machine comes into play "in front" of the sizing screen.

"Blade mills take a dry, raw feed and slurry it," Bennington says. "They remove some light soils and soluble clays, and add water to the aggregate which discharges onto the screen."

There are two primary reasons why an aggregate producer would want one. "First, you have some material that just needs medium scrubbing and your spray bars aren't doing a good enough job," Bennington says. "The blade mill can start that process and get the aggregate wet. The other reason is that a blade mill increases screening efficiency by about 15 percent because the material can go through it faster."

Blade mills can scrub up to 4-inch material.

They use roughly the same low horsepower as a coarse material washer.

classification of coarse aggregate

Classifying Applications

Density separator. This type of equipment is primarily used in the frack and industrial sand industries. They are conical-shaped at the bottom and square at the top.

"You create a fluidized bed of material," Bennington says. "There's something like a sand bar floating in the middle of the machine. Water bubbles through it, and the light sands slowly but surely work their way to the top of that bed of material before getting pushed out over the weir. The bigger particles filter down to the bottom of the pile and into the cone. The cone has a valve so you can adjust it just like a cyclone; the bigger the opening, the smaller the material that comes out.

"The nice thing about density separators is that for 40 to 100 mesh, you can just turn the dial and make that split," Bennington adds. "A density separator acts like a screen in that sense."

Classifying tank. Material is slurried and fed into the front end of this machine. The sand is then separated by size and specific gravity; coarse sand falls at the feed end, fines fall at the end, and silt overflows the side of the tank and into a trough.

Clusters of valves (typically three) line the side of the tank at strategic points. A level sensor is in the middle of them. When sand builds up in a particular spot, a signal is sent to the control. The control then opens one of the valves to let the sand out.

There are also three troughs along the length of the tank. Each cluster of valves feeds into these troughs.

"This allows you to blend an entire tank together into different ratios and in different spots to make two different specification products and a third excess product," Bennington says. "However, most producers only make two, such as concrete or asphalt sand as the primary, and then using the leftover as mason sand or ice control sand. Whatever the case, the big thing to remember is that you must make one secondary product because you need somewhere to throw the material away."

Another thing about classifying tanks is that they require secondary dewatering. "The discharge from the bottom of the classifying tank is only about 60 percent solids," Bennington says.

This article is based on a presentation given by John Bennington at CONEXPO-CON/AGG 2020. Bennington is commonly recognized as the "dean of wet processing technologies" throughout the aggregates industry. He is the director of washing and classifying equipment at Superior Industries, a manufacturer of crushing, screening, washing and conveying solutions. Visit superior-ind.com for more information.