Producer reduces downtime with double-deck screens

The Aggregates USA Midway Quarry near Knoxville, Tennessee, needed to find a more efficient method to produce dry manufactured sand. This was always a challenge, since the breaking characteristics of the limestone required that an approximate 1/8-in. (3 mm to 3.5 mm) top-size be targeted. Different types of tertiary crushers had been tried, but the plus/minus 3/16-in. particles had always been stubborn to break down.

Therefore, to make the manufactured sand and keep the 20 mesh and 50 mesh in spec, the top size of 1/8 in. was found to be the best. This made the dry screening difficult, since the limestone was powdery and sticky, and would build up on the wire screen media. High-carbon and oil-tempered wire would not work owing to the porous nature of the metal finish, so stainless steel wire was preferred, along with long slotted 3/32 in. (2.8 mm) holes. But even this did not consistently keep the wire clean.

Heated deck screens were used. Heating decks was, and still is, a viable option to reduce and minimize buildup on wire cloth. The technology is used not only with limestone, but with other materials being screened, including clay at brick plants. So the heat became the remedy chosen in 1997 by American Limestone – the previous name of Midway Quarry. And that technology worked to keep the wire cloth clean on the sand screens for 20 years.

Replacement

Photos by Mark White

In 2015 a decision was made to replace two of the sand screens, owing to age and other considerations. Quarry management was looking for a way to help employees more easily and safely replace screen media and do maintenance on the proposed replacement screening units.

The 10-ft.-long, one-piece, end-tensioned wire panels being used were considered prohibitive to install, due to the access clearance between the decks. This had been a long-term challenge the quarry was no longer willing to accept. It was also decided that a review of the application would be done, using the Vibrating Screen Manufacturers Association (VSMA) screen sizing calculations and field data available.

AggFlow software was used, since the VSMA screen deck sizing is core to it and embedded in the application review it provides. Actual field samples were taken of the raw feed to the sand screens, showing the tons per hour to each unit along with the sieve gradation to do the VSMA review.

The samples were consistent with what the quarry management felt were good average values. The AggFlow review showed that two 5-ft. x 12-ft. double-deck units could replace the two 5-ft. x 10-ft. triple-deck units. This would eliminate the middle deck, which was initially proposed as a relief or breaker deck.

The 5-ft. x 12-ft. size screen box would also fit onto the tower, since the original screens taken out in 1997 were that size. The principal advantage of the proposed 5 x 12-ft. double-deck screens was the improved crawl-through clearance provided to the maintenance personnel. Taking out the middle deck would give more height for media replacement and other tasks that were required.

Another consideration that was reviewed for the proposed application change was the screen slope, speed and stroke (amplitude). A 30-degree high-frequency unit was preferred, but the profile of the unit and existing chute setup eliminated this possibility. A standard 20-degree unit was chosen with a stroke and speed suitable for that design, taking into consideration optimizing the application.

The last and most vital decision was the wire-cloth screen media selection. What could be used to take the place of the heated deck screens used for two decades? This was key to the success of the application change. In this case, Major Wire non-woven stainless-steel wire panels were chosen, with a diamond-shaped hole of a similar size to that of a woven square hole. Also, three 48-in.-long panels were chosen for the 12-ft. length, in lieu of the single 10-ft. panel on the heated deck screens.

This decision, along with the additional crawl-through clearance, makes changing out the wire panels much easier, safer and cost effective. Also, by going to a non-woven product, a much smaller wire diameter could be used. If too small a wire diameter is used on woven panels, the wire can break itself up, and be installation sensitive. The weave becomes the weakness. A non-woven wire works best in this application.

Calculations

This photo shows the lack of deck clearance on the old screen decks at the Aggregates USA Midway Quarry near Knoxville, Tennessee. The company says the additional crawl-through clearance makes changing out the wire panels much easier, safer and cost effective.

Following is a summary of the savings Aggregates USA has experienced by replacing the two triple-deck heated deck screens with the two double-deck non-heated deck screens. It should be noted that only the middle and bottom decks were heated – four of the six decks.

Let’s start with the power needed to heat the wire cloth. The four 5-ft. x 10-ft. wire-cloth panels (50 sq. ft.) used a 40 kVA heating system. Power consumption varied from 20 and 40 kVA, depending on the woven screen cloth parameters, and the moisture content of the limestone material. It is estimated the electrical cost was $2 per hour for each deck, or $8 per hour for the four decks. Considering 2,500 hours per year of production, that is a savings on kWh of $20,000 per year by eliminating the heated decks.

But keeping in mind the real advantage of going from the triple decks to double decks was the crawl-through clearance, and the ability to replace screen cloth faster, easier and safer. According to the plant manager and maintenance personnel, Midway Quarry historically and routinely replaced two panels per month, taking three laborers three hours to do the work. The new units take two laborers about 30 minutes to replace one panel.

Heated triple decks
◾ 2 panels per month x 12 = 24 panels/year (24 x $500/panel = $12,000/year)
◾ 18 labor hours per month x 12 = 216 labor hours/year (216 x $30/labor hour = $6,460/year)
◾ Total: $18,460/year

Non-heated double decks
◾ 12 panels/year (complete change-out of both screen units) (12 x $750/panel = $9,000/year)
◾ 1 labor hour/month x 12 = 12 labor hours/year (12 x $30/labor hour = $360/year)
◾ Total: $9,360/year

The icing on the cake for this application change became the increase in production, and also the reduction of downtime caused by the 120-in.-long, end-tensioned, woven-wire cloth panels breaking during key production.

The production increase of the targeted 1/8-in.-minus manufactured sand is averaging 10 tph. Midway has routinely tracked its downtime related to these two screen units at an average of 8 percent. This reduction of production loss is an additional savings on top of the extra 10 tph.

A related benefit of the non-woven wire is the passing of blasting wire. This had been an ongoing problem at this site, and would completely blind up the woven wire. Panels would have to be removed owing to being completely covered in the wire. This problem has been completely eliminated.

Company history

In 1997, Midway Quarry was owned by ASARCO mining company, and was known as American Limestone. American Limestone was eventually sold to Rinker Materials, which then sold the site to Aggregates USA, the current owner.

Aggregates USA is a producer of construction aggregate in the Southeastern United States. Headquartered in Birmingham, Alabama, the company has more than 20 locations, with operations servicing Georgia, Florida, Tennessee and Virginia.

The company was formed with a focus on safety, operational excellence and value-adding acquisitions. Since its inception, the company has optimized operations and added new locations to better serve customers in its footprint.

Mark White of Paschal Associates is an independent engineering consultant with more than 35 years of experience in the aggregate industry.