Photos by Bill Weaver
Magnesita’s York, PA Quarry is not your ordinary quarry. Most quarries don’t do a careful chemical analysis of the rock in each and every blast hole, followed by more chemical analyses of the rock after that. Most quarries sell stone that has been mined for aggregates. What Magnesita sells is rock transformed by 3500 degree F. heat into something far different.
This York Quarry is located atop a native dolomite that has just the right chemical and physical properties that, when processed, it becomes high value refractory products. This is currently the only quarry in the US doing this, and has been since 1964. Their close attention to detail pays. As Geologist Liz Graybill puts it, “It’s the ultimate value-added product.”
Quarrying at Magnesita’s York Quarry is a very complex process. Although the quarry contains a dolomite of exceptionally high purity (about 99.4% pure), this unique rock is jumbled together in the quarry with other rocks of lesser value. Working this quarry is daunting.
“We have drilled cores on a 75 foot grid over the whole property, so we know what’s here,” stated David Hopkins, Resource Mining Operations Manager. “Surprises are not good in this business. We’re on very, very tight specs. Other quarries have high purity dolomite, but it doesn’t have the right physical and/or chemical characteristics for this use.” This limestone is processed into protective refractory linings that will withstand the high heat of, blast furnaces.
In the quarry
Blasting the shot rock is also a much more complex process than at most quarries. Sections to be blasted are in single rows only. “Because we have such tight chemical specifications, larger blasts could contaminate everything,” explained Hopkins. “We mine on a chemical grade, so every time we blast, we sample the drill holes for their chemistry. That way we can determine what the material will be used for before it’s shot down. We mark each shot, and give our operators color-coded maps and reports so they know the use of each shot. Each blast hole give us about 1,000 tons of material. We do 30,000 to 40,000 tons of material each time we blast, year round.” Such is the demand for their products!
At the face of the shot rock, operators equipped with color-keyed maps, collect and blend the blasted rock, right at the quarry face before loading it into haul trucks with their CAT 990 loader. They do this blending based on the breakdown of all the chemistries of the specific shot. From there, the material is transported to the primary crusher in Komatsu 605 70- ton haul trucks, with the material being kept separate by their respective chemical composition.
Located nearby, the primary crusher is an antique 1957 Universal Double Impeller Impactor, which crushes 700 tons an hour to -2 ½ inches. The crusher building carries a color-coded flag, to match the color keys on the maps. This allows the geologists driving past to check what material is currently being crushed.
To protect the rock from moisture, the crusher, conveyors, and rock storage are all under roof or covered. “We want to keep the rock dry,” explained Hopkins. “We’ve found that most of the impurities are in the fracture planes. If the rock gets wet, the impurities stick. If the rock remains dry, the impurities will mostly get screened off.”
The valuable, high purity Ledger Dolomite – primarily tan and oolitic- is divided into three grades by chemical composition. These three grades are kept separate from each other and from all other rock on their journey through the crushers.
“When we do a ‘campaign’ on a certain quality of dolomite,” explained Hopkins, “we scale down the hoppers to remove any rock residue from a previous crushing to prevent contamination.” From the primary crusher, the three grades of high purity dolomite stored separately in a long building behind the primary crusher. In short order, that rock is moved to the main plant’s secondary crushers, 2 Symons cone crushers, where it is reduced in size to -3/8”x10 mesh at 250 tons per hour “on a good day.”
Into the rotary kilns
From there it travels by conveyor or elevator to enter one of two rotary kilns, each heated to 3500 degrees F. “These are probably the hottest rotary kilns in the world,” noted Hopkins.
Still, the testing continues. “Every fifteen minutes, we take a sample of the feed stock before it enters the feed bins for the kiln,” explained Graybill, “just to make sure there have been no slip-ups along the way.” That is how important the precise chemical composition of the dolomite is to the whole process.
Once in the rotary kilns, the rock spends 6 to 7 hours changing it both physically and chemically in a process called ‘sintering.’ This intense heating produces chemical changes that reduces and densifies the dolomite, driving the carbon dioxide out of the rock, turning it into calcium oxide and magnesium oxide, which can withstand the heat in steel plant blast furnaces.
Their own two rotary kilns are lined with the refractory bricks made on site because the material can withstand the 3500 degrees F. heat. These kilns were commissioned in 1952 and 1959 respectively. The kilns are coal and coke-fueled.
Natural gas, which is used in the tunnel kilns in the making of refractory bricks, could not get the rotary kiln hot enough. To meet emission regulations, “only very low ash, low sulfur coal can be used,” said Hopkins.
The refractory-grade material emerging from the rotary kiln travels next to a plant where they crush it and size it for the various refractory products. Next, it is moved to an on-site building to be made into one of four materials, depending on its precise chemical composition. They produce refractory bricks; mortar with refractory qualities for use with the bricks; gunning mixes, which are used to patch damaged refractory linings in steel mills without having to shut down production; and monolithics, which are a loose refractory material that is rammed into place in tight spaces where bricks would not fit, or simply used in places where steel plant operators prefer monolithics over brick for their quick application.
Meanwhile, back at the quarry, there remain many other types of rock not chemically suitable for producing refractories. Nevertheless, the company finds a use for all of it. “At this quarry, nothing is wasted,” explained Hopkins.
Dolomite that has a higher percentage of impurities, for example, is dried, then pulverized in a KVS ball mill and sold in several states as a high quality agricultural lime by Baker Lime.
Baker Lime has developed a value-added version of its product, a pelletized lime made using a by-product of paper making as a binder. In addition, some of the screenings from the production of kiln feed are sold as bedding material for dairy cows and horses.
Other rock is quarried by Vulcan Materials. “We have an agreement with Vulcan to take all our waste rock for aggregate,” said Hopkins. Vulcan sells much of it for road building, and some of the aggregate is used in hot mix asphalt by a plant on site.
Beautiful white marble from the quarry is sold as decorative aggregate. Black limestone from ancient microbial reefs found within the Ledger Dolomite, is used for some asphalt products. Vulcan crushes a much “younger” red conglomerate that also crops up in sections of the quarry for use in road sub-base.
“The rocks with the greatest impurities tend to dynamite into larger pieces, possibly due to recrystallization,” Hopkins continued. Vulcan puts some of these through a Grizzly sizer at the quarry to be sold for riprap. They sell some of the large rocks for landscape boulders. Vulcan also sells manufactured sand from quarried materials.
As mining enlarges the quarry, which is approaching one mile in length, the clay overburden is used in landfill liners and at local dirt racetracks. Top soil is stored for use in the eventual restoration of the mined area. Truly, there is NO waste! “We take out about 1.2 million tons per year,” added Hopkins.
Hopkins has spent his entire career as a geologist and manager in this one quarry, working in part on figuring out the puzzle of which rock types are located where. “Geologists typically jump jobs more often,” he added, but he has been content with the considerable complexities of this job. Graybill on the other hand, has worked with the company for a year a half. She worked as a consultant out of grad school, and seems similarly interested in the geological challenges of the work. Together they make a great team.