Paving to the center of the earth


Asphalt adds safety and efficiency to hard rock mine

Over 93 percent of U.S. roads above the ground are paved in asphalt. But, recently a Montana mine chose to pave their tunnels in asphalt to protect their workers and profits.

In the world of precious metal mining, commodities such as platinum, gold, silver or copper are typically what come to mind. However, there is one metal that is not as well known, but invaluable to a variety of industries. That metal is palladium.

Palladium is the silver or steel-white colored metal of the platinum group metals discovered in 1803. It is the least dense of this family, and it does not tarnish in air. This unique metal is a known catalyst for hydrogenation and dehydrogenation processes.

Some of its other more common uses include jewelry (white gold is decolorized gold via the addition of palladium), dentistry, medicine, electronics, petroleum refining, auto catalysts, fuel cells, hydrogen purification and even watch making applications. Of these, the most common end product for palladium is auto catalysts (catalytic converters) at around 73 percent.

Active mining of palladium is currently found in only a very limited number of locations worldwide. Mines are producing the metal in Canada, South Africa, Russia and in the United States. The only active palladium mine in the U.S. is the Stillwater Mining Company operation near Nye, Montana. The mine is nestled at the base of the Beartooth Mountains about 100 miles southwest of Billings, Montana.

The two products targeted by Stillwater are platinum and palladium. In a typical year the operation will yield approximately 400,000 ounces of palladium and 120,000 ounces of platinum. Total revenue generation from all activities at Stillwater, raw material extraction and the recycling of catalytic converters, exceeded $1 billion in 2013.

Why asphalt?

Operations such as the Stillwater Mine fall under the category of hard rock mines. Hard rock mines are tunnel operations through solid rock for the purpose of retrieving desirable materials. A variety of inherent risks are associated with this type of operation, not the least of which is air quality for the workers and equipment.

Any industrial operation is constantly seeking means of improving their operating efficiency and increasing their profitability. It is with these considerations in mind that the engineers at Stillwater turned to asphalt to aid in worker safety and improve operations within the mine.

To better understand the benefits of asphalt envisioned by Stillwater Mining, we need to provide some perspective. The roads within the mine are the result of the mining process itself. In the case of Stillwater, the drilling, blasting and mucking of materials form tunnels through the bedrock. The loosened material is hauled to the surface to remove the platinum and palladium, leaving typical mine tailings. Currently there are over 85 miles of tunnels within the mine. These tunnels are excavated of solid rock, and are used to access the ore-bearing material and become the main travel ways in the mine. The material from these excavations is taken to the surface, some of which is crushed and classified to be used as road base for the main access tunnels. Many of these tunnels are subsequently converted into travel ways as the mining process continues.

The historical means of underground road construction at Stillwater is to return 3-inch minus crushed waste from the surface. This material is laid, graded and compacted with full-size heavy equipment in the tunnels. This process fills in the highly irregular surface produced in the blasting to create an easily navigable travel way. Roads constructed in this fashion have proven to be satisfactory but a change was envisioned in 2007 that would produce a more stable cross section and reduce the dust generated by the haul trucks at the same time. A total of five sections are to be constructed over a ten-year period.

Stillwater Mining turned to DOWL HKM Engineering to design an improved haul road with a compacted base and an emulsified-bound surface. A number of challenges faced the engineers that tackled this project. The environment in the mine needs to maintain a healthy atmosphere for the miners. Stillwater mine engineers chose asphalt, but DOWL HKM needed to select the flexible binder for the project.

Choosing the right process

The first options eliminated were traditional hot mix and warm mix asphalt. The only practical location at the mine to produce either of these options would have been above ground and then to haul the mixed material to the construction site. No practical solution was envisioned that would allow for the material to arrive on site and still be at an acceptable temperature to achieve compaction.

Another option that was investigated was to use a cutback binder. However, the very nature of cutback binders would mean that the asphalt base would be cut with some volatile thinning agent such as diesel, kerosene, naphthalene, etc. The typical curing of a cutback involves the volatilization of the cutting agent primarily from its exposure to the sun. This volatilization would have been quite problematic in the underground environment of the mine. Engineers rightly eliminated the cutback option, as a cutting agent would degrade the underground air quality.

That left an emulsified asphalt option. However, similar to cutbacks, asphalt emulsions typically lose their water component, at least in part, from evaporation via heating from the sun. This, of course, cannot occur where the sun cannot shine. However, with an engineered emulsion, an exothermic process can be designed into the binder to induce the breaking and setting of the emulsion absent the sun or other external heating.

In the case of Stillwater, the environment at the road site is a near constant 55°F and 30 percent humidity all the time. These environmental realities, as well as the fact the pavement will also serve as a drainage conduit for the natural water seepage into the mine meant that dependable moisture resistance is also a must. Moreover, the need for high early strength in the surface due to the very high loadings with each pass of the 35 metric ton haul vehicle while unloaded or 70 metric tons when loaded added an additional constraint to the formulation of the emulsion.

These constraints led DOWL HKM to a lab optimization process investigating various emulsion options from regionally available suppliers. Following typical recommendations as found in the “Basic Asphalt Recycling Manual,” engineers optimized both total moisture and asphalt content. All of this led to an emulsion rate of 6.0 percent, which provided a residual asphalt rate of 3.8 percent asphalt using PASS-R® engineered emulsion. An additional one percent of water was incorporated to reach the lab-determined optimum moisture content.

Specific challenges

Another significant challenge was the basic logistics of moving materials approximately one-mile vertically into the mountain. This movement was accomplished via elevator that moved the aggregate and the binding agents from the surface to the construction zone. Perhaps an even bigger issue than getting the material was getting the construction equipment to the construction level.

To begin with, the reclaimer utilized for the project was a Wirtgen 2000. This is a full-size machine that cannot travel through the tunnels of the mine to the working level due to the tight turns. Therefore, Wirtgen designed this reclaimer so that it could be separated into two parts; loaded onto the same elevator that hauls the materials and lowered to the working level where it was reassembled.

A test strip was constructed in the more forgiving environment outside the mine prior to its travel to the interior construction location. Two test strips were constructed so the crew could gain experience and confidence in the equipment and the process as adjustments to the reclaimer were needed based on the first test strip.

Yet another matter was the thickness design. Stillwater and DOWL HKM relied on the experience of other mining operations for their thickness design. Communications with others indicated that a minimum of four inches of base was needed for good performance. To ensure a conservative design, a design cross section was established at a minimum of six inches of 3-inch minus to provide a base to construct the 3/4-inch minus, emulsion-stabilized material to a depth of 8-12 inches. The paving aggregate was again crushed mine waste that were processed into three separate materials. The three bins of materials were combined above ground and transported into the mine for construction.

Getting to work

The paving crew for this project were selected mine employees. Wirtgen trained the crew on the operation of the reclaimer. DOWL HKM worked with Stillwater’s crew to establish the roller pattern for this process using nuclear density gauges to measure in-place densities. In addition to density, quality control testing was conducted recording moisture content, thicknesses, residual asphalt content and volumetric properties including air voids and VMA (voids in mineral aggregate.)

The construction process was not a rapid endeavor. Restrictions in space and limits on the ability to stockpile materials kept production constrained from volumes typically seen in highway scenarios. Much like a surface project, the base materials were spread with a motor grader and compacted at near optimum moisture content. Upon this the 3/4-inch paving material was spread to its desired depth again via the motor grader.

The reclaimer, following a nurse truck with the emulsion, progressed through this material. Minor handwork addressed the edges of a given pull. A Hamm 7-ton single drum vibratory roller accomplished compaction. Both the base and the emulsion-treated layers were compacted to 95 percent of their respective lab densities. In a representative day, 650-1000 linear feet by 14-16 foot wide surface were constructed.

Underground paving is certainly not the most common usage for asphalt. However, even in this non-typical location, asphalt is once more showing its value.

“The paving of these roads reduced dust generation which benefits our miners and the underground electronics in the mine. While the unpaved road would washboard needing watering and blading, the paved surfaces require far less maintenance. The paved surface also gives our trucks better traction. And last, but not least, asphalt is cool,” said Joel Debruycker, Stillwater Project Engineer.

Johnson is an Asphalt Institute Regional Engineer based in Billings, Montana.

Joel DeBruycker and Kurt Grabow contributed to this article. DeBruyker is the Project Engineer at Stillwater Mining Company. Grabow is Lab and Materials Manager at DOWL HKM Engineering.