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Stillwater doubling output.

John Chadwick examines mine planning for long-distance adit access and TBM development that will eventually lead to a doubling of current output to 1.3 Moz/y

Stillwater Mining Co (SMC) is developing a second underground palladium/platinum mine, East Boulder, some 21 km west of its existing operation in south central Montana, US (MM, January 1996, pp.10-18). SMC's operation is already the highest grade, and lowest cost producer of Platinum Group Metals (PGMs) in the world. Furthermore, it is the only significant source of PGMs outside of South Africa and Russia

SMC's expansion plans up to 2001 cover increasing output from the existing Stillwater mine from 2,000 to 3,000 t/d, bringing East Boulder into production, and expansion of its smelter and base metals refinery (BMR). The total cost of this is $385 million. A mill expansion at Stillwater was completed earlier this year, as was the tailings impoundment. Also at Stillwater, the paste backfill plant has been commissioned and underground development is ongoing. At a capital cost of $75 million this will raise total annual production from the existing mine to 725,000 oz. The capital cost of the East Boulder project is estimated at $270 million, to bring a 500,000 oz/y capacity operation on stream. The remaining $40 million of capital is being spent on the smelter, where a new TBRC and furnace arrived in the second quarter and are now being started up, and in the BMR where the copper circuit has been completed and two autoclaves are currently being installed.

There have been a few setbacks this year and the anticipated increase in output in the first half did not occur, owing to difficulties encountered by the large number of inexperienced employees, construction of mine infrastructure and contract inefficiencies. However, with improved production in the second half, through a new five-year workforce contract and several productivity initiatives, SMC still hopes to achieve production of 380,000-400,000 oz/y of palladium and 120,000-125,000 oz of platinum.

The US National Materials Advisory Board has estimated that the Stillwater Complex contains more than 225 Moz of PGMs. They occur in a thin reef, the J-M Reef in the banded zone of the complex. The reef averages about 3 m in thickness and contains 0.5 - 2.0% metallic sulphides and various PGMs. In fact mineralisation fluctuates in width both vertically and horizontally, going from a minimum of less than 25 mm to in excess of 20 m and consists of platinum and palladium at a ratio of 1:3.3, an unusually high palladium content. The average grade of the zone is 19 g/t of platinum and palladium. The average mining width is 3.05 m (10 ft). The orebody pinches and swells from less than 305 mm to more than 21 m along the horizontal strike length.

SMC holds 995 mineral claims covering a 45-km strike length of the J-M Reef 136 km southwest of the largest city in Montana, Billings. The dip of the narrow reef varies from almost vertical at the east end of the Stillwater mine to some 55[degrees] in the East Boulder area. It is the only known economic underground source of PGMs in the Americas at this time.

SMC has calculated its reserve/resource base to an elevation of some 460 m. The zone of mineralisation has been intersected at depth and appears to form a continuous layer with an indicated vertical continuity exceeding 1,800 m. Geological and geophysical evidence suggests that the reef extends downwards well beyond the limits of current drilling. Mapping and gravity surveys also suggest that the dip of the reef flattens gently and may extend 50km or more to the north, The capacity to expand future output at Stillwater will not be constrained by the reserve base. Every year for the past six, SMC has increased its reserves, which now stand at 37.1 Mt at 0.71 oz/t (24.35 g/t) platinum and palladium-sufficient for a continuing mine life greater than 50 years. Production has increased steadily in the second half of the 1990s.

Total production, with East Boulder starting to contribute ounces by year-end 2000, is projected to be 725,000 oz in 2000, 1 Moz in 2001, 1.2 Moz in 2002 and 1.3 Moz in 2003.

Franco-Nevada Mining Corp. owns a 5% Net Smelter Return (NSR) royalty covering approximately 82% of the existing reserves and 95% of the resources identified in the Stillwater Mining Complex. Franco-Nevada states on its website; "The management team at Stillwater is to be commended for perhaps the finest performance by any mining executive group in 1998." Those achievements included entering into long-term sales contracts with major end users (General Motors, Ford Motor Company, Mitsubishi, and Kemet Metals), effectively creating a hedge on 90% of palladium production for 1999 to 2003 with a floor price of $225/oz, and a cap of $400/oz on 30% of production. As Franco-Nevada points out: "This removed recession, substitution, commodity and Russian supply risks."

SMC put in place the required finances to enable the projects to proceed through an equity issue which raised $67 million and by securing a seven-year term bank loan of $175 million. Operating cash flow will finance the balance of the expansion costs.

Mine development

At the original Stillwater mine access to the reef is from drifts driven along strike in the footwall of the orebody, some 36 m off the ore zone. These access drifts are driven conventionally by drilling and blasting at 91 m vertical intervals. Mining is mainly by mechanised ramp and fill, sublevel longhole stoping and cut and fill.

The East Boulder project consists primarily of two adits collared on the East Fork of the Boulder River to access the orebody and ancillary facilities for the crushing and concentration of ore that will develop the western portion of the J-M Reef. The western portion of the J-M Reef was delineated by surface drilling and the development of the Frog Pond adit in 1980 and 1981. East Boulder is some 21 km northwest of Stillwater, along the strike of the orebody. Its surface facilities have been built in the nearest accessible area to Stillwater, the East Boulder River Valley, 5.6 km north of the ore zone and the same distance from Stillwater's surface facilities and underground production areas. East Boulder was a challenging mine to design because of the remoteness of this site; high mountains rise above the ore zone itself. In addition to SMC personnel, mine planning involved MRDI personnel from the Agra/Simons group, Kilborn International, Continental Conveyor, and Parsons Brinkerhoff Transit and Rail System . Kilborn International put a design team at the mine for several months.

The initial East Boulder access plan called for a single, 4.6 m diameter adit, and a suitable tunnel boring machine (TBM) was purchased from Construction and Tunneling Services (CTS) to drive this entry. However, the potential limitations of a single adit led to the decision to drive a second to prevent any transportation bottlenecks. The secondhand TBM for this project, a 4.6 m diameter Robbins machine, was purchased from BHP, being the one used on Magma Copper's Lower Kalamazoo project (MM, October 1994 pp.186-192). At Magma's San Manuel block cave mine it had driven about 4,200 m of 4.6 m diameter drift in a large loop around the orebody with a number of 150 m radius curves.

The CTS TBM began boring in August 1998, driving the first adit which will penetrate the reef and enter the footwall at the 6500 level (measured in feet above sea level, that is 1,981 m). This will be the first production level. The main workshop and parts and consumables storage complex is also being developed on 6500 level, along with the materials cassette transfer station. The 6500 level adit will provide high-speed access to the mine for personnel and materials.

The second adit will reach the 6400 level, 30.5 m below (1,950.5 m above sea level) the first. The adit itself is the dedicated conveyor drive for muck haulage from the mine to surface and will connect with the East Boulder mine's dedicated rail haulage level along strike on 6400 level. The first adit is expected to reach the reef in the first quarter of 2000, with the second reaching the reef in the second quarter of the year.

Muck handling

The main problems in planning East Boulder's muck handling systems were the long adits and the orebody's long strike length. Rail, slurry, conveyor and truck haulage were all considered. Considerations for the permanent system included the long, straight 5.6 km distance from the mine to the surface portal, the need to move as much as 6,350 t/d of ore and waste, the adits' diameter, the amount of underground infrastructure required to support the muck handling system and the ability to increase capacity.

Pumping the muck as a slurry would have required a major underground cavern to house grinding mills and the milling could have adversely affected performance in the flotation plant. So the slurry option was rejected. The circular profile of the adits would have limited truck capacities to too small a size to be economical over the long distances required, so truck haulage was rejected.

Rail haulage is being used to haul development muck from the first TBM to surface. However, this was not a long-term option for the mine because, like the truck haulage option, the required capacity could not be provided economically within the envelopes of the circular adits.

Thus, an extendable conveyor with a 914-mm belt will be installed in the second adit. This conveyor will be the permanent primary muck haulage from the mine as the tunnel is bored. While a conveyor system requires underground crushing, it reduces traffic and potential bottlenecking in men and materials handling in the first rail adit.

However, it was the advantages of conveyor haulage in driving the second adit that really led to its selection. Using a feeder-breaker to feed muck to the conveyor allows large quantities of development muck to be brought to surface and helped achieve a 'fast track' pre-production schedule. This conveyor will be used to haul all waste and ore once the adit reaches the East Boulder workings.

The permanent muck haul does, however, include some rail transport, in a dedicated, isolated drive on 6400 level. Trains will collect muck from ore and waste passes spaced at 914 m intervals. These passes will be fed from the mining areas by rubber-tyred LHDs or trucks. Trains will haul rock to dump pockets at a permanent crusher station. This will be equipped with a 1.5 m apron feeder, a 0.9 m x 1.2 m jaw crusher, a 1.3 m crusher discharge conveyor, a belt magnet and a 16 t overhead crane. Two transfer conveyors will carry material from here to the main conveyor. In the second adit, the 900-mm main conveyor will transport minus-150 mm material to the surface stockpiles.

Materials handling

Careful planning of materials handling was required because of the long distances from the surface facilities into the mine. A system of 2.4 x 2.4 x 6 m Connex shipping containers carries all the workshop and normal storage complex materials. In this way, containers can be loaded at a vendor or central warehouse, shipped to the mine and then underground.

A cassette system was chosen for the various items like rock bolts, drill steel, explosives and ventilation extension equipment that need to go directly to the development areas and stopes. They have not only to go 5.6 km underground, but also have to be distributed along the long strike length and vertical extent of the orebody. The system uses dedicated flatcars and rubber-tyred carriers to speed deliveries to underground work areas from surface.

The cassette handling transfer area close to the underground workshops is equipped with a 9-t crane to lift cassettes from the flatcars to cassette storage bays, from where they are collected by the rubber-tyred carriers. A 23-t standard flatcar is used to bring rubber-tyred equipment into the mine. Another 23 t flatcar, in this case a 'low-boy', is available for over height deliveries.

The underground workshops are accessed easily from all production areas. They are capable of all repairs, including full overhauls as the great distance involved precludes removing equipment to surface facilities. Thus, the underground facility includes fabrication, steam cleaning, fuelling and lubrication.

Other major underground infrastructure includes the main explosives magazine, two major electrical substations, a compressor plant, a tailings backfill plant and pumping stations.

Personnel movement

Again, the great distances involved were the main criteria in designing the personnel transportation system, which needed to be of high speed. Early studies by Parsons Brinkerhoff considered the sort of light rail vehicles used in public transportation. However, this involved high costs in vehicles, signalling systems and conversion of the rail track gauge. Further study showed that reducing the desired speed for personnel transport from up to 65 km/h to 32-40km/h would provide the greatest benefit at the most reasonable cost.

This rail vehicle is still being designed. However, the intention is that this personnel carrier will bring workers into a station in the orebody for rapid unloading. The station is located such that people can quickly transfer to rubber-tyred vehicles for travel along strike or to a hoist in a service shaft to travel to upper levels of the mine.

Tunnel boring

The 250 t CTS TBM, as well as driving the first adit, will be used for future mine development. The specific features of this machine include, according to Clayr Alexander [3], mine manager, "an assembled drive cartridge of 120 in [3.05 m] allowing it to be lowered down the Nye shaft without disassembly, and a 60 m turning radius by utilising a short front shield configuration with a floating gripper. All cylinder joints are ball sockets for easy re-assemble. Face and crown support shields are included for encounters with faulted and contact zones that do not stand. The cutterhead has a full diameter rotating rim bar with an extendable roof support behind the last gage cutter. The cutterhead face is smooth and held close to the cutter tip profile. Ground support and probing is provided for a variety of methods: probing/grouting, radial drilling, ring beam installation and crown strap installation."

The two Boart HD150 rock drills supplied with the TBM are interesting. Mounted on a rotating, sliding arm that comes off the front of the gripper shoes, they will drill holes for 1.5 m rock bolts. When grouting ahead of the TBM is required, the HD150s can be removed from their roof drill mounts and placed in the two grout drilling mounts on the top portion of the bridge. For this task, the HD150's will use 1.2 m sections of drill steel for extension drilling.

Construction of the CTS TBM was completed in July 1998 when it was delivered to site, reassembled and field tested. It completed 127 m of advance that month, 356 m in August and 414 m in September 1998. Since then, it has averaged an advance of 261 in/month. Its backup system was designed for muck haulage by rail and consists of 16 rolling decks, two ramped decks and one set of sliding rail points. Its main conveyor carries muck from the front of Deck 1 and discharges it at Deck 6. One section is equipped with double tracks to allow loading of one muck train while bringing in the next empty train for loading.

Fast-track development

The second, Robbins, TBM was purchased to fulfill the most critical elements of the accelerated project schedule in its early stages, quickly providing a second means of egress and the ventilation needed for underground development.

Mine planning included features to speed development of the upper levels of the orebody. An Alimak driven raise system consisting of four raises throughout the 6500 level infrastructure complex will allow access to the 6800, 7100 and 7400 levels long before ramps have been driven between levels. As soon as a large enough station can be opened up, jumbos and LHDs will be hoisted to the levels to develop the footwall laterals. These machines will be captive to the level to which they are assigned.

Development picks up

As the expansion of the Stillwater mine has progressed, production has not met company expectations since the beginning of the year. However, while production did not increase in the third quarter, being similar to the second quarter, a number of key areas critical to performance at expanded production levels showed significant improvement in the third quarter.

Through the first six months of this year, development driveage was not at a level to sustain the projected goal of a 3,000 t/d mining rate, averaging only 762 m/month through June 1999. However, in July and August, 1,000 m and 1,257 m were advanced, respectively, and the trend for more than 1,200 m/month of development was continuing. Additionally, the company has brought in contract development drilling.

To operate at a sustained level of 3,000 t/d from the Stillwater mine, it has been estimated that 70 working stopes are required. At the end of June, only 46 stopes were available for ore production. This situation has also improved strongly, with 52 stopes available at the end of August and 55 at the end of September.

During September, the average number of rounds blasted was about 49/day, the highest in the history of the mine, compared with 44/day for the first six months. This increase has lifted the total tonnage mined (ore and waste) from the 3,540 t/d average for the first six months to a third quarter average of more than 4,000 t/d. Mining bottlenecks encountered during the first half of the year have been overcome.

SMC further reports that: "Two key areas which have yet to show significant improvement are the amount of waste mined and productivity as measured by ounces per man hour throughout the mine. The waste issue is expected to show improvement as development and stopes available for mining increase, and planned changes in mining methods are implemented. A number of productivity improvement programmes are underway in one area of the mine as a prototype. In the first eight weeks, this section of the mine has shown a 38% improvement in ounces per man hour. These productivity improvement programmes will be implemented throughout the mine over the next several months."

Strong markets

The future looks bright for SMC. In July 1998, the European Union adopted a new two-to seven-year plan to cut 70% of the pollutants in car emissions. New cars must be fitted with platinum or palladium catalytic converters plus computers that warn of any performance failures of these devices. These changes will require car makers to invest an estimated $65 billion to meet the new standards.

Palladium is particularly well suited to eliminating the smog creating vehicle emissions and tighter emission regulations are driving manufacturers to switch from platinum to palladium catalysts. Palladium use in auto-catalysts rose 30% to 4.2 Moz in 1998, according to Johnson Matthey. Amplats, the world's largest platinum producer, said that if palladium demand in auto-catalysts continues to grow at 12.5% through 2001, then auto-catalysts alone will consume all the palladium mined each year.

Franco-Nevada reports that total world demand for palladium, at a record 8.2 Moz, is set to outstrip mine supplies by 1 Moz this year. World demand for palladium has more than doubled in the last five years. Russia's supplies fell 4% in 1998 to 4.6 Moz. It has been selling stockpiles to keep pace with demand for several years and Russia would need major capital investments to bring on new supplies.

Mining Journal's Precious Metals and Minerals Supplement (May 28) notes that in May last year palladium traded at a premium to platinum, for the first time ever, but this was short lived. Calendar 1998 palladium prices rose 63% to close at $332/oz and averaged $285/oz for the year. PGM prices now are well in excess of gold, with platinum ending 1998 at $364/oz. Automakers indicate, according to Johnson Matthey, that prices for palladium would have to exceed $1,200/oz before its cost would drive them to seek alternative technologies, and security of supply is of great concern to them.

SMC is not alone with its interest in the PGM potential of the area. Idaho Consolidated Metals Corporation (ICMC) is exploring three properties close to Stillwater in Park, Sweet Grass and Stillwater counties. The Chrome Mountain property of some 1,420 ha, for instance, covers 11.3 km of strike length of the ultramafic and banded series of the Stillwater Complex. The banded zone consists of several stratigraphic subdivisions for plagioclase bearing cumulates and hosts several PGM bearing layers, the best known being the J-M Reef. The property adjoins Stillwater's patented ground and is within 450 m of its Frog Pond adit.

ICMC's second property, the Picket Pin zone, is parallel and to the north of the J-M Reef and its third, the Black Butte claims, are located 2.4 km east of the main stem of the Stillwater River in the Stillwater Complex.


(1.) Sabala, James and Wilson, Gina, A new era of growth, Nesbitt Burns' Gold and Precious Minerals Conference, Lake Tahoe, March 1, 1999

(2.) Strickland, B. and Einarson, D., Design of Stillwater Mining Company's East Boulder mine, SME Annual Meeting, Denver, March 1-3, 1999.

(3.) Alexander, C., Development of Stillwater Mining Company's East Boulder Project using tunnel boring technology, SME Annual Meeting, Denver, March 1-3, 1999.
                        SMC's operating improvement
                 Production Cash costs
                  ('000 oz)     ($/oz)
1995                    220        215
1996                    255        184
1997                    355        174
1998                    444        151
1999 [e] [plus or minus]450    155-175
                          TBM main specifications
                              CTS Robbins
- diameter (m)                4.6     4.6
- operating thrust (MN)       8.5     7.3
- installed power (kW)      1,340   1,250
- speed (rpm)            11.6/3.8    12/4
- diameter (mm)               432     432
- number                       32      33
- capacity (kN/cutter)        267     222
Minimum turning
  radius (m)                   60     107
Boring stroke (mm)          1,220   1,550
- width (mm)                  610     762
- speed (m/min)               120     152
- capacity ([m.sup.3]/h)      490     552
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Publication:Mining Magazine
Geographic Code:1USA
Date:Nov 1, 1999
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