Leveling the Playing Field
Most stone quarries are created by starting at ground level and excavating down into the earth. Nonetheless, there are a small number of quarries that are started high above ground level on a mountain or hill. On these sites, mining advances toward the base until it is level or below the level of the surrounding grade.
Such is the case at a quarry in Avondale, Pa., that is 30 miles west of Philadelphia and 12 miles northwest of Wilmington, Del. Avondale is a well-established residential community with 1,100 residents. Near the center of the town, there is a hill about 110 feet high and 20 to 25 acres wide at its base. While there were some minor excavating activities carried out in the 1980s and early 1990s, it was of little significance. One contractor stripped some overburden, and another quarried some rock.
In 2006, TechniVate Co. was contracted by the property owner, Limestone Properties, LLC, to remove a complete section of the hill to make it level with Avondale’s street grade for the purpose of building a residential area and a commercial buildings park on site. As a diversified site improvement company, TechniVate was up to the task. It employs 120 people and has a fleet of 100 pieces of major equipment. Services offered include preparing turnkey building sites and specific project activities that range from erosion control and underground utilities installation to road paving.
The Avondale project was started in April 2007, and by the beginning of November, 632,000 tons of rock and sand had been excavated and processed through two onsite portable crushing and screening systems. There is at least 2.7 million tons of rock yet to be quarried in order for the hill to be brought to the specified grade.
Challenges and opportunities
TechniVate subcontracted the rock drilling and blasting to a local contractor. Three Sandvik down-the-hole drill rigs are being used for blasthole drilling. The blasthole pattern is 8 feet by 10 feet by 30 feet, which is within the Pennsylvania Department of Environmental Protection (DEP) specified drilling-for-blasting parameters. TechniVate considers this blast design to be acceptable; however, the rock fragmentation has resulted in oversize rock jamming the jaw crushers.
Most of the rock excavated at the top fourth of the hill is mica and mica schist. Both rocks are relatively soft and are suitable for structural fill such as certain sections of berm construction as found on sanitary landfill projects. For construction purposes, the rock is classified as structural fill and is marketed as such to contractors, mostly for private construction applications, and to the Cherry Island Landfill facility in New Castle County, Del. Owned and operated by the Delaware Solid Waste Authority, the landfill is currently increasing capacity through added vertical waste containment. To that end, a berm is being built around the perimeter that is one mile in circumference and 60 feet high. It will require approximately 3 million cubic yards of fill. TechniVate anticipates supplying most of the project’s aggregates needs for the berm.
Fortunately, as the mining descends, better quality, more usable rock including sandstone, quartz, and schist is found. It is the sandstone (sedimentary) and the quartz (metamorphic) stone that has the best physical characteristics for a variety of construction applications. None of the rock types is overly abrasive. In fact, a recent Los Angeles abrasion test shows an average of 45 percent for the best rock. This degree of abrasiveness is within acceptable limits as outlined in Pennsylvania Department of Transportation (PennDOT) specifications. Both the sandstone and quartz are classified as granular fill so it can be used effectively for many different construction applications such as road base, where the structural fill cannot be used.
It is projected that the quarrying phase of this project should be completed within three years. That timetable is based, in part, on the projected quantities of both structural and granular crushed/screened fill that is to be shipped to the Cherry Island Landfill.
Versatile crushing and screening
Throughout the Avondale project, six portable crushing and screening plants — including three different brands — are being used. Having supplied four of the six plants, Extec Screens and Crushers Limited (which is now fully owned by the Sandvik Mining and Construction Group of Sandvik AB) is the primary equipment vendor.
There are two crushing and screening systems, each including a primary jaw crusher, a secondary cone crusher, and a double-deck screen linking the two crushers. The reason for having three different brands of equipment within the two systems is one of pragmatics. The cone crusher for System I was selected because it could meet TechniVate’s required short delivery schedule. Rounding out System I is an Extec C12 jaw crusher (48- by 28-inches) and an Extec S-5 screening plant (double-screen box assembly, 60- by 90-inches each). System I would be sufficient for all production needs if it were to be operated in double shifts on weekdays and possibly on Saturdays, however the DEP has placed time restrictions on when crushing and screening is allowed.
With a greater throughput capacity than System I, System II was added to meet the market’s demand for aggregates. It includes the Extec X44 SBS cone crusher, a 48- by 32-inch jaw crusher, and an Extec S-6 screening plant (double-screen box, 60- by 120-inches each). The bigger jaw crusher was added to this system because too many occurrences of oversize (30-inch plus) rock pieces were causing crusher jams.
Although the System I jaw crusher is designed to be used in rugged applications, greater-than-normal rock fragmentation generated more oversize than typically seen in such sites. Improving rock fragmentation wasn’t a viable alternative due to the site’s urban location and the DEP’s tight blast-design parameters. Although Will Mains, TechniVate’s quarry manager, and Alistair Parker, Extec’s dealer technical services manager, both said they thought the jaw could handle the oversize material, a bigger machine seemed to a good option. Under such blasting limitations, they reasoned that a larger capacity jaw crusher would help to minimize the occurrence of oversize rock jamming. “It reduces the occurrences of jamming to only a day,” Mains says. “Before, there were two to three jams taking place each day.”
With the bigger jaw crusher’s higher price tag, the company decided to rent it rather than buy it. However, TechniVate bought all of the Extec equipment outright, and it will remain in the fleet as the company builds its recently launched recycling venture. Future quarrying beyond the present site is dependent on the availability of other sites. “If we find another quarry like this one, all the equipment — including the big jaw crusher — will be used there,” says Donald Taylor, vice president for TechniVate. “Nevertheless, we are starting the asphalt paving and concrete recycling business, and we certainly need our Extec jaw crusher for doing onsite recycling.”
Despite its smaller size, the C12 jaw crusher is capable of crushing typical quarry-size rock, asphalt paving, and concrete pieces. According to Taylor, the difference in throughput between the two crushers is not exceptional. He says that while the big jaw is very good in processing oversize rock, it is too time-consuming and expensive to move it from recycling jobsite to jobsite. Simply put, it is not as versatile as the Extec jaw crusher is, he says.
Versatility is an issue because Taylor is considering setting up a crushing and screening yard where contractors can dump their concrete and asphalt paving. However, he realizes that many times the contractors will want the portable plants brought to their sites for crushing. The current trend is for onsite crushing and with good reason — fuel prices are high and continue to increase so the contractors do not want the expense of hauling the blasted rock to a crushing/screening yard and then hauling crushed stone back to the construction site.
Mains agrees with Taylor’s assessment as to what equipment is best assigned to the combined quarry and recycling activities. “In my opinion, the big jaw is very good if it is kept in a quarry. Otherwise, it costs too much to move it from one site to another,” he says. “Many recycling projects have small quantities to crush, and we could not justify the mobilization costs. For instance, the C12 plant just returned from crushing only 2,500 tons of concrete, which is a small project. Nevertheless, our price was competitive.”
According to Mains, the smaller jaw crusher requires only 1.5 man hours of preparation before transport, as well as the use of one road tractor. A lowboy is not needed because the plant has its own bogie. At the site, the same crew member can have the plant production-ready in less than 1.5 hours. Mains says by comparison, it takes three men a day to prep the big jaw crusher to make it transport-ready. The time to haul it to the site must be accounted for, and finally, three men and another day are needed to make it production-ready.
That is not all that is involved, however. It requires the use of either a crane or a large hydraulic excavator to help reduce the plant into three major transportable components and three tractors with lowboy trailers to transport the three components. Once delivered to the site, a crane or large hydraulic excavator is again needed to help reassemble the plant.
Central to both crushing and screening systems at the quarry are the portable screens. The double-box design provides high-throughput capacities despite each screen’s compact size and each plant’s small footprint. Each box screen can be independently adjusted to a specified slope. The vibration rate, amplitude, and speed also are adjustable. This enables the operator to control the rate of undersize material that is removed upon its first impact on the screen, so the near size goes to the secondary screen box that is set at a gentler slope. The gentler slope efficiently screens out the remaining undersize material.
Plant optimization options
An efficient production layout is used for the crushing-screening systems. The jaw is fed the blasted rock, which in turn sends the crushed rock to the screen where the undersize (sand) is stacked with the side-delivery conveyor. The main conveyor stacks the desired aggregate size while the oversize is sent to the closed-circuit cone crusher for re-crushing. Lastly, the cone’s crushed material is sent back to the screen.
The big jaw crusher (at System II only) is fed rock sizes 36-inch minus and the smaller jaw crusher (at System I or System II) is fed 30-inch minus. Occasionally, the smaller jaw crusher replaces the bigger jaw at System II. Despite the big jaw crusher’s superior capacity, the Extec jaw crusher’s throughput is only 50 tons per hour less. The smaller jaw crusher substitutes at System II when the big jaw crusher must be serviced. “We use the jaw crushers interchangeably with the S-6 screen and X44 SBS cone crusher because it is more important to keep the higher producing System II running,” Mains says.
With the S-6 screen’s bigger capacity, System II’s production is said to be outstanding using either jaw crusher heading the system. The screen can more than handle what it is being fed. To illustrate, the bigger jaw crusher’s throughput rate is 450 to 550 tons per hour and the smaller jaw crusher’s throughput rate is 450 to 500 tons per hour. The crushing rates fluctuate depending on what ratio of rock to sand is fed the jaw crusher. Crushed rock fed into the Extec X44 SBS cone crusher from the screen is 2 to 6 inches, which is in turn reduced to 2-inch minus. The throughput for that cone crusher is 200 to 250 tons per hour, and the screened-products throughput delivers 450 to 550 tons per hour, which include a 2-inch minus aggregate and 1/4-inch sand.
The System I throughput is not as high because the cone crusher production is only 100 to 140 tons per hour, even though the crushed rock fed it is 5-inch minus instead of the 6-inch minus fed the cone crusher in System II. “The reason we feed a smaller size rock to the [System I] cone is that the throughput rate was too low when feeding it 6-inch minus,” Mains says.
Despite, the bottleneck of the smaller cone in System I, it has a finished-products throughput ranging from 250 to 300 tons per hour. The Extec S-5 screen throughput of 375 to 475 tons per hour is fed from the jaw crusher crushed rock and the recirculated crushed rock fed from the cone crusher.
According to Taylor, the portable crushing and screening equipment used at the quarry has been quite satisfactory, although he says he prefers the quality service he receives from the Extec dealer, Extec Eastern of Trainer, Pa. “All the equipment has been performing well, but the services we receive from Extec Eastern have been outstanding. We will continue our business relationship with Extec Eastern because of the services experienced and their products,” he says. “We lose practically no production downtime using Extec equipment because the dealer responds immediately to any plant issues. The high-quality services offered by this company are as important to us as the quality product it is selling.”
Rodney Garrett, a freelance journalist and photographer, specializes in the construction and mining industries.