Hard, harder, hardest
April 1, 2007 By: Rodney E. Garrett Pit & QuarryDresser Traprock isn't easy to mine, but the market makes it worth the effort.
Some quarries have hard rock that tests at 35,000 to 45,000 psi. Rock pushing 45,000 psi or more certainly is hard. And as for 80,000 psi rock — that's hard redefined.
Very few quarries can boast having rock that has an unconfined compression test result of 80,000 psi, but Dresser (Wis.) Trap Rock Inc. can.
 Challenge; Solution; Tip |
Maybe boast is not the best word to describe a rock that is this hard, for it brings before the company a unique set of challenges not experienced by most others in the rock crushing-screening business. Nonetheless, while this trap rock is difficult to mine, crush and screen, it has attributes that make it a highly desirable rock for producing quality aggregates, construction applications and decorative uses.
Because it is found in the Dresser area, the rock's proper designation is Dresser Traprock. Dresser Trap Rock Inc. sells its aggregates starting at $12 per ton; decorative boulders are priced up to $60 per ton. So while the mining and production of the Dresser Traprock is a challenge for the company, the market demand for its extensive range of products is widespread. Currently, many of the products are marketed bulk or bagged through retail dealer outlets in five upper Midwestern states.
 Dresser Trap Rock Inc. and Director of Operations Rick Demulling improved their reliability in crushing Dresser Traprock. |
Crusher maintenance
Dresser Trap Rock Inc. has been mining its Dresser quarry since 1914 and thereby has a good idea as to what mining, crushing and screening methods are cost-effective for hard rock. Each facet of the quarry operations is a story in itself, so this report will focus on the proper crusher maintenance procedures necessary to ensure minimum downtime and efficient crushing production.
Dresser Traprock is not friendly to jaw crushers or cone crushers used to bring the mined rock into marketable products. Its extreme hardness resists fragmentation and in turn, that means the crusher's parts directly exposed to the rock during the crushing process need to withstand repeated severe impacts.
About 1 million tons of 50 different rock products are produced here annually. Producing 50 different products necessitates a plethora of crushers and screens at the primary, secondary and tertiary stations and beyond. At the primary, there are two jaw crushers: One is a 26-year-old 60x84 built by Allis Chalmers (AC), and the second is a 30-year-old 42x48 built by Hewitt-Robins. The 60x84 crushes 1,500 tph, while the 42x48 crushes 700 tph. These jaw crushers are operated from March to November and are shut down during the winter season.
 Because every quarry operation is unique, Demulling recommends trying different brand parts. |
Rock fragmentation from the blasting is consistent enough that it need not be scalped but fed directly to the jaw crushers. The size fed is 3-ft. minus. All oversize pieces are reduced to crushable sizes using a hydraulic excavator fitted with an impact breaker.
The AC jaw crusher is still a good performer, even after 26 years, according to Rick Demulling, director of operations for Dresser Trap Rock. However, there was a hurdle to be made so the crusher would be more reliable. The original equipment manufacturer (OEM) jaws were a four-section-per-side design, which had to have some of its mounting bolts replaced weekly.
"There were seams created by the four-part-per-side jaws design, and some of those seams would gradually spread apart from the constant rock crushing action," explained Demulling. "Eventually, the void between the sections was 1/2-in., and that enabled rock pieces to get wedged in between the sections. That in turn created lateral forces, which eventually would shear the mounting bolts."
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