A New Economical Solution to Efficient Crusher Fines Management.
By Carol Wasson
|Superior’s Alliance Low Water Washer.|
Depleted natural sand reserves, environmental constraints and decade-long permitting processes are just a few of the drivers behind a fast-growing demand for manufactured sand – and a significant need for efficient crusher fines management.
To meet product specifications, operators are typically turning to costly air classifiers or high-water-use washing with cyclones or sand screws. While the latter options may work well for some operations, others are seeking alternative, affordable solutions especially if they operate dry crushing facilities, or portable plants; or if they are limited by their footprint or available water resources.
At issue is the amount of water required for conventional washing methods; and a welcomed access to a new, economical low-water washing solution. “A sand screw needs 50-gal.-per-minute of water for every ton of minus-200 mesh in the feed,” said John Bennington, director of washing and classifying equipment for Superior Industries. “Even if an operation is only running at 100 tph, and their minus-200 mesh is at 15 percent – that is 15 tons of fines requiring 750-gal.-per-minute of water to adequately lift the silt over the weirs of the screw. That’s a lot of water for most crushing operations,” he said.
As an industry veteran in washing and classifying, Bennington had been brainstorming a low-water washing solution for many years, and “with the resources of Superior Industries, we were able to design it, refine it, and prove it in the field,” he said, referring to the new Alliance Low Water Washer.
Bennington explained that the unit combines a specially designed agitator and a dewatering screen within one machine. It allows the operation to wash crusher dust right next to the crushing circuit, eliminating the cost of handling and hauling the fines to a separate washing circuit.
Notably, he said that the system accepts a dry feed directly from the crushing circuit and processes the material into a higher-value manufactured sand, while requiring up to 80 percent less water consumption.
|The low-water washer averages about 300 gal.-per-minute of water usage.|
Reno, Nev.-based Rilite Aggregate was facing a water management challenge. The company had been washing its concrete sand with a 54-in. twin screw that required 2,200 gal.-per-minute. “We had to fill a holding pond and utilize a large pump. With that, we could wash material for only three hours per day,” said Michael Rudolf, a long-term crushing subcontractor who has worked with Rilite for nearly 20 years.
He added that the facility must use an additional 60,000 gal.-per-day to fill the water trucks used to wet the quarry roads for airborne dust control. The water-management situation reached a “crisis-mode” when a specification change in utility bedding sand required the company to wash that product as well, lowering the minus-200 mesh from 15 percent to 7 percent or less.
Rudolf said that they needed an aggressive washing system that could run off available water. “With a drastic change in sand specifications, we were forced to add washing capacities, so we wanted a lower cost, lower maintenance solution that would limit water use. We have our own well but it only pumps 220 gal.-per-minute,” he said. The local Superior Industries dealer, Kimball Equipment, referred Rudolf to John Bennington. Soon thereafter, Rudolf and the local dealer flew to a quarry in Traverse City, Mich., to see the prototype of the Alliance Low Water Washer in action.
According to Bennington, the Michigan operation is processing about 100 to 120 tph of feed through the low-water washer, and generally runs 200 to 250 gal.-per-minute of water, pumped from a well. The raw feed averages about 22 percent minus-200 mesh. At that rate, the conventional sand screw would require 1,100 gal.-per-minute.
“After washing, the operation is achieving under a 6 percent minus-200 mesh, with a target of 4 percent after further refinements,” Bennington said.
At the Michigan site, Rudolf ran pre- and post-wash samples through the new washer, and found that the unit would meet Rilite’s specification requirements. This led to the installation of the Alliance Low Water Washer in February 2017.
“I had the site ready to go, so it was a matter of unloading the truck, plus one day of hookup, and we were in business washing bedding sand at 175 to 200 tph – and it handles that capacity in such a small footprint, allowing us to install the washer without making any modifications to the existing plant,” said Rudolf. He added that he’s placed a belt stacker in front of the washer that can swing to feed the washer or swing away to bypass the washer when needed.
|Aerial view of the water-saving equipment.|
Rudolf said that the low-water washer averages about 300 gal.-per-minute of water usage fed from a large fresh water pond that acts as a surge system for the well, which is used daily to fill the water trucks.
“Versus the previous use of twin sand screws at 2,200 gal.-per-minute, I now vary my flow rate to the low-water washer between 200 to 400 gal.-per-minute. This has dramatically lowered our washing costs. We can conserve water, and manage its use without jeopardizing our need for water trucks for dust control,” he stressed.
Importantly, Rudolf points to the moisture content of the material after washing. “It’s between 11 and 13 percent. With that, we can sell the product within one day. With our previous setup, we had 22-percent moisture content after washing and had to let the material sit and dry for about a week,” he said, summing up the biggest advantage – “low water use and large throughput.”
Efficient Fines Management
|After washing, the operation is achieving under a 6-percent minus-200 mesh.|
Bennington stressed that many operations stockpile material to be washed, and sometimes those stockpiles simply collect over time. When necessary, that material is hauled over to a separate screen/screw wash plant, meaning that the operation is handling the material multiple times. “This method typically incurs three ‘touches’ to the material, at a cost of an additional $1.50 per ton to wash the sand,” he said.
To avoid washing altogether, Bennington said that some operations will employ a common shortcut by trying to dry feed a sand screw. “That method doesn’t work very well, particularly in limestone. Ultimately, they find that they can only lower the minus-200 mesh by about 3 percent, which is almost nothing,” he said.
The new low-water washer, said Bennington is a far more economical and efficient method. Its agitator section is positioned at the front end of the screen where water is added to the dry feed and mixed, producing a thick slurry. The agitator is equipped with a spray bar along the length of the blade mill for thorough cleaning.
The slurry is fed to a robust dewatering screen that is outfitted with a series of spray bars that help to push the material down through the screen. “Using a sand screw requires ‘lifting the silt over the weirs of the screw with much water usage. Alternatively, when the new washer pushes the material down through the screen, much less water is needed,” Bennington said.
Conventional wash methods, high water use, and multiple material handling is much like the cliché of “throwing the baby out with the bath water.” New low-water washing systems ensure efficient crusher fines management, higher-quality products and lower operating costs per ton.
Carol Wasson is a Fort Wayne, Ind.-based freelancer.