Mining in the Arctic - History

7-93_keen_Polaris PermaFrost.pdf

Zinc Mining in Permafrost at Polaris

I have come across this old technical paper on "Zinc Mining in Permafrost at Polaris", written by AJ Keen (General Manager, Mining Ventures, Cominco Ltd) for the International Symposium - World Zinc '93, Note, the mine was operated by Cominco Ltd. from 1981 to 2002, the mine has since closed!!!!!

• The paper provides a comprehensive overview of the Polaris mine, from its geology and mining methods to its surface facilities and management practices.
• The paper's main-focus is on the challenges of mining in permafrost, and how Polaris had successfully overcome these challenges.
• Key geological features of the Polaris Mine orebody: it is a Mississippi Valley-type deposit, located within permafrost, Ore minerals consist of sphalerite and galena with the gangue being dolomite with calcite and marcasite.
• it’s a high-grade zinc and lead deposit, producing over 200,000 tonnes of zinc concentrate when in operation.
• All aspects of the Polaris Operation are relevant to future underground & DSO development of the Storm Copper Project should it become economic to process.
• Given the Technical Paper was written in 1993 (30 years ago), mining & mineral processing technology such as robotics, AI, and Ore Sorting has all significantly evolved.

ABSTRACT
Cominco Ltd's Polaris Mine in Canada's High Arctic has been successfully mining zinc ore in permafrost since 1981. Mining methods using frozen backfill have been the key to success. The mine is now producing about 60 per cent of the ore from pillars while maintaining high productivities. Annual ore production is just over one million tonnes providing more than 200,000 tonnes of zinc concentrate each year. Cominco Ltd has adapted management and operating practices to suit the harsh environment and the isolated site and has shown that a profitable development can be carried out in the tough Arctic conditions, with minimum impact on the environment.

INTRODUCTION
Cominco's Polaris Mine is an underground lead/zinc operation located on Little Comwallis Island in the Canadian High Arctic. Polaris is the world's most northerly base metal mine, situated at latitude 75~ and longitude 970W (Figure 1).



The closest community is Resolute Bay 100km to the southeast.
The climate is typical of the Arctic, very dry with short, cool summersand long, cold winters. Temperatures range from a winter low of -45°C to asummer high of 15°C. There is continuous darkness from November throughJanuary, while continuous daylight prevails from April to August.Precipitation is light, about 13 cm of snow and rain peryear. Production at Polaris began in November 1981. Thecurrent mining rate is 1,000,000 tonnes per year. The mine employs240 people with 180 on-site at any time.

GEOLOGY
The orebody is a Mississippi Valley type. Ore minerals consist of sphalerite and galena with the gangue being predominantly dolomite with calcite and marcasite. The orebody is situated completely within permafrost, consequently thevoids frequently found in the ore are filled with ice. The orebody strikes north-south, dips approximately 20 degrees to the east and takes up an area about 400 m wide by 700 m long. The upper portion of the orebody, 60 m to 150 m belowsurface, lies in an area called the Panhandle Zone. The deeper portion of the orebody known as the Keel Zone extends to 300 m below surface (Figure 2).



The deposit is strongly zoned with a highgrade zinc/lead and iron-rich tabular core overlain and underlainby vein, stringer and breccia mineralisation. A number of major faults strike north-south across the orebody. These faults show minor relative movement, up to 20 m. The faults are steeply dipping to the east and show strong open contacts filled with ice or frozen mud. Four distinctjoint sets have been observed, steeply dipping and oriented up to 90° to the faults. The ore reserves at January 1993 were 9.5 million tonnes at 3.7 per cent Pb, 14.0 per cent Zn, 2.9 per cent Fe, after producing 10.1 million tonnesat 3.8 per cent Pb and 13.9 per cent Zn since November 1981.

PERMAFROST
The permafrost extends from surface to roughly 500 m below the surface. Rock temperatures range from -14°C near surface to 3°C at the lowest mining horizon in the Keel Zone (Figure 3).

The in-situ moisture content of the ore is two per cent by weight and in its frozen state the ground is very competent. In order to preserve the integrity of the rock the thermal equilibrium of the mine must be maintained. Calculations of the annual heat balance show 1.6 times more heat is removed from the mine than the heat input from various sources. The major sources of heat input are diesel engines, electrical equipment and fresh water used to consolidate backfill. CollectIvely their contribution is 82 per cent of the total heat generated. The major component of heat extraction is ventilation during the months of October to May. Thermistors have been installed to monitor the thermal activity of the rock. Thermistor data collected suggests that the mine's thermal gradient is 0.03'C increase per metre depth. In September 1983, two thaw-induced groundfalls caused by warm ventilating air prompted the installation of a mine air cooling system. A 450 tonne refrigeration plant with a rating of? kW capable of cooling 118 m3/s was installed and Commissioned for the 1984 summer. The collar temperatures of thermistors installed along the main decline have been reduced by an average of 3°C over the 1983 data.

MINING

Primary mining

The mine is accessed from surface by a decline to the six main levels and the crusher. Primary stoping in the Panhandle Zone was completed in 1985 and is currently being carried out in the Keel Zone. Sublevel longhole stoping is used in the primary stopes. Each stope is 15 m wide, 100 to 150 m long, with 18 m wide rib pillars left between stopes for secondary mining. Stopes are mined in sequence starting from the bottom working towards the hanging wall in 30 m lifts and from south to north (Figure 4).

The stope topcut and undercut consist of an 8 m wide by 5.5 m high pilot drift that runs from west to east along the centre-line of the stope. A full 15 m wide overcut and undercut is created by taking 3.5 m slashes on both sides. Electric hydraulic or pneumatic longhole machines drill 76 mm parallel. vertical downholes from the topcut on a 1.8 m spacing and 2.1 m burden. The hol.es are load.ed ~ith ~O and.three t~ four rings are blasted at a trrne. Mucking 15 carned out WIth 6 m remote controlled scooptrams and 24 tonne trucks. All drilling in the mine is dry with the exception of diamond drilling. Compressed air is used to flush out cuttings and dust collectors are mounted on all drilling equipment. A series of 1.8 m diameter raisebore holes for ventilation and backfill are located at 30 m intervals along the stope with the eastern-most hole used as the slot raise for initial blast relief. Over 3200 m of raise are bored each year using a Robbins 73RM-AC machine.

Fresh air enters the mine through the main portal at a volume of 280 m3 /s and is exhausted through a number of fans located over raisebore holes on surface. In the summer, mine ventilation is reduced to as low as 94 m3 /s when the mine air cooling plant is in operation. 1bis reduces underground operations to production activity only. Fans are typically 37 to 112 kW with diameters ranging from 1.2 m to 1.5 m. The ambient temperatures underground average -20°C in the winter months and _5°C in the summer.

Backfill

It is imperative to use a backfill material that does not affect the thermal equilibrium of the mine by warming the permafrost. Large quantities of silty limestone and shale, known as the Cape Phillips Formation, are found on surface overlying the orebody. This material when mixed with water and frozen provides a good backfill. Backfill test work at Polaris has shown that increasing the moisture content increases the uniaxial compressive strength of the frozen backfill. Laboratory and field tests have shown best results with a moisture content of 11 to 12 per cent water by weight. This results in strengths of four to six MPa. The strength will decrease at higher moisture contents when the backfill becomes too saturated. Increasing moisture content will also increase the energy associated with the latent heat of fusion resulting in a longer freezing time and increasing heat output. Further cooling of the frozen backfill also increases the uniaxial compressive strength. In the Keel Zone, winter rock temperatures of -13°C will produce frozen backfill strengths up to six MPa once equilibrium is achieved between the host rock and the frozen backfill. Uniaxial compressive strengths will also increase with increasing backfill dry density. This is a difficult variable to control in the mine because the backfill is dumped in 30 m lifts and relies on the weight of the fill for compaction. The average in situ dry density is 1740 kg/m3 which represents 80 per cent compaction relative to a standard Proctor test. At this average dry density the uniaxial compressive strength ranges from two to four MPa. The rate at which backfill freezes is of importance since it impacts on the mining schedule. Wmter fill requires up to one and a half years to freeze completely, while summer fill requires up to three years. Pillar mining sequencing is scheduled to take this into account. Various methods of placing backfill material in stopes have been tried. The most successful winter method is to dump dry fill from surface down a raisebore hole to be mixed with water underground. By mixing underground, good quality backflil can be produced consistently. A 0-8 dozer mixes, then pushes the fill over a bank into the mined out stope, gradually building 30 m lifts. During the summer months, the surface material has enough contained free water to permit dumping directly into the stope. The fluid consistency of the summer material allows complete tightfilling to the hangingwall. Prior to backfilling a stope, a frozen impermeable plug is constructed at the undercut stope access. The placement of backflil is a year-round activity. Annual backfill requirements are up to 300 000 m3 of surface quarried material and 45 000 m3 of underground mine waste.

Pillar mining

Pillar mining in the Panhandle Zone is almost complete. The first Keel Zone pillar was recovered in November 1987. In 1992 pillar production accounted for 56 per cent of the total mine output; this will increase to 82 per cent by 1995

The Keel Zone pillars are 18 m wide, 30 to 120 m high and up to 150 m long. Each pillar is divided into a number of 25 to 30 m long stages separated by a 5 m post pillar (Figures 4 and 5). Pillars are mined from east to west and from bottom to top. The original pillar mining method was based on mining a pillar to full height before filling. Although it has been possible to expose backfill up to 87 m high and 49 m wide in several pillars, there have been some flil failures resulting in lost ore. These failures were almost always due to low moisture content in the flil. The failed fill was placed before the current filling methods were established. Current practice in the higher pillars is to mine two lifts (60 to 70 m high) and backfill prior to blasting the top one or two lifts (Figure 5). In order to speed up the turn around time, and to provide a sufficient number of pillar production headings, dry backfill is used to flil the mined pillar voids. The 5 m post pillars are required to retain the dry fill in the mined pillar voids and provide some support to the frozen stope fill walls and hangingwall. The post pillars are not designed to carry loads from the hangingwall to footwall. The mining method requires the development of 4.5 m wide x 5 m high headings along the centre of the pillar at 30 m intervals. Similar to primary stoping, a slot is raisebored at the east end of each pillar stage and a series of backfill holes are raisebored from surface to allow free-dumping of fill upon completion of mining. Fanned downholes 76 mm in diameter are drilled on a 2 m burden using an electric-hydraulic longhole drill. Normally two or three rings are blasted at a time using ANFO. Pillar blasting is largely dependent upon the structural features of each stage and the type of stope fill. It is becoming more common to mass blast to an enlarged slot, especially in the top lift of each pillar stage. Muck removal is via remote controlled 6 m3 scoop trarns loading 24 tonne trucks in the undercuts for hauling to the orepass.

Ground support

Initial ground support in all development is with 1.8 m split sets on a 1.2 m square pattern installed with an electric hydraulic rock bolter. If the opening is more than 8 m wide, 2.7 m swellex bolts are used on a 1.5 m x 2.0 m pattern. The ultimate mining hangingwall is supported with 7.3 m swellex bolts on the same pattern. Swellex bolt holes are drilled with a modified single boom jumbo and a scissor lift truck is used for installation. Swellex expansion is achieved using glycol because of the cold working temperatures. The early stope mining method in the Keel Zone required development of drifts in the pillars, parallel to the stope drifts, to provide multiple access points for production mucking. About half of the pillars were developed in this marmer. These drifts were left for up to five years before they were used for pillar mining. Many became filled with backfill from the stope filling process, resulting in major rehabilitation work to remove frozen fill and ice prior for use in pillar mining. As pillar mining progresses from south to north the stress patterns are changing and these old pillar drifts are showing local signs of stress-induced fracturing and slabbing. The fracturing and slabbing results from high stresses, relative to the strength of the rock mass, being transmitted vertically from hangingwall to footwall through the pillar. As a result of the pillar drift deterioration ground support methods have been modified. Existing pillar drifts are now being re-supported with grouted cable bolts and re-bar. 1bis support is designed to knit the ground together around the drift openings. Four cable bolts 8 m long on a 2 m burden, plated and tensioned to eight tonnes, are installed in the back of the drift. The cable bolts are double 16 mm cables. The walls are bolted with 1.8 m grouted, plated re-bar. The cable bolts and re-bar appear to have been successful in controlling the poor ground conditions and allowing mining to proceed safely.

The use of cement grout in frozen rock presents a number of different problems. The grout tends to freeze before it has cured. Testwork has been carried out at Polaris with various grouts using hot and cold water and calcium chloride as an accelerator. The best results were obtained using grout mixtures containing hot water (>22°C) and no calcium chloride. Shotcrete has been tried for ground support but with limited success to-date. More trials are planned for 1993. A ground monitoring program is in place using extensometers (surface and underground) for regional monitoring. Surface subsidence is monitored using surface monuments. Convergence monitors have been installed in a number of pillar drifts to measure local movement.

CONCENTRATOR AND SERVICES

Concentrator The main crusher is located in the footwall just below the orebody. Run of mine ore is crushed underground to minus 15 cm and is stored in two underground storage bins with a capacity of 7000 tonnes. A series of five conveyors 2170 m long lift the ore 300 m to the secondary crusher in the concentrator. The concentrator consists of conventional zinc/lead circuits producing two products, a lead concentrate and a zinc concentrate. Two small column cells have been introduced into the lead circuit and a tower mill is used to re-grind the zinc scavenger concentrate and zinc fmal cleaner tails increasing recovery and grade. Both concentrates are dried in rotary dryers using diesel generator exhaust gases as a heat source. The plant was originally designed with a throughput of 2050 tonnes per day. Since 1986 it has been operating at a throughput rate of 2800 tonnes per day. The plant continues to maintain over 97 per cent annual operating time. Simplicity and flexibility have been the main keys to the high plant availability and high milling capacity.

Surface facilities

The Polaris mine is one of the most compact mining operations in Canada. Surface facilities include the process barge, the concentrate storage building, accommodation building, dock facilities including shiploader, and fuel oil storage. The barge houses the concentrator, powerhouse, vehicle repair shops, welding shop, machine shop, electrical and instrument shops, warehouses, changehouse, laboratories, and the general operating office. Fresh water supply for the operation is obtained from Frustration Lake, five kilometres from the barge. Diesel power is generated on-site. The power plant consists of four units each with a maximum capacity of 2300 kW and one unit at 1100 kW. Three 450-kW emergency diesel generators are located in the accommodation module to provide emergency power for all systems if necessary. Waste heat recovery plays an important role in reducing energy consumption. There are two waste heat recovery systems, one on the engine cooling system and one in the engine exhaust, both use glycol heat exchangers. Annual fuel oil consumption for power generation and mobile equipment is approximately 16 million litres. The accommodation building contains 240 rooms, in four residential modules, each module connected to the other by alternating service modules containing the dining room, commissary, pool and administration offices. Recreational facilities include a pool room, library, indoor swimming pool with sauna and whirlpool, gymnasium with a mezzanine running track, weight room, games room and TV room. A clinic, staffed by a registered nurse, is equipped with an emergency treatment room.

Maintenance

Polaris follows the centralised maintenance concept, the maintenance department has the responsibility to maintain all equipment at the site, and also operate and maintain the power plant. Preventive maintenance, work orders, equipment history and parts information systems are all computerised. Availabilities of over 97 per cent in the mill and 85 per cent in mobile equipment have consistently been achieved. Following is a list of the underground mobile equipment:

Production and development equipment

• 2 Atlas Copco 2 boom electric hydraulic jumbos, • 1 Atlas Copco 1 boom electric hydraulic jumbo, • 1 Tamrock electric hydraulic roof bolter, • 3 Tamrock electric hydraulic Solo drills, • 2 Air-trac drills, • 1 Quarry drill (down-the-hole), • 1 Robbins 73RM Raisebore drill, • 4 Wagner ST8B remote scooptrams, • 2 Wagner ST8A scooptrams, • 4 Trucks, IDT426, and • 2 Trucks, IDT413.

Service equipment • 1 Cat D8 dozer (for backfill), • 1 Cat 950B forklift/loader, • 7 Dux service trucks, • 1 Teledyne scaler, • 2 Mobile rockbreakers, and • various Toyota land cruisers and pick up trucks.

Logistics

An airstrip and dock are part of the surface facilities. The airstrip is 1200 m in length permitting the use of DC-3, Twin Otter and Hercules aircraft. A Twin Otter provides air service from the mine site to Resolute Bay. The dock is designed to accommodate bulk carriers up to 40,000 tonne capacity. Shiploading is carried out during a period of 9 to 12 weeks in late summer. Canarctic Shipping's MV Arctic, a 25,000 tonne Canadian vessel with Arctic Class 4 equivalency, provides ice breaking capability to handle the early and late season voyages. The concentrates are stored in an A-frame structure, the largest building in the Arctic. It is capable of storing 40,000 tonnes of lead concentrate and 175,000 tonnes of zinc concentrate. The bulk of the supplies required for the operation are brought in by sealift each summer, 3,000 tonnes arrive from Europe on concentrate back-haul, a further 1,600 tonnes is shipped by freighter from Montreal.

MANAGEMENT

Pesonnel policies

There are several rotation schedules at Polaris, employees from the south work a nominal nine week-in three week-out, schedule which provides 39 work weeks each year. Senior management work six weeks on-site followed by a two to three week break. Northern native employees from the Arctic (Inuit) may opt to work a six and four schedule designed to provide them with sufficient time off-site to pursue their traditional life styles. The Polaris work rotations are longer than those at most long distance commuting mines but have proven to be effective. The longer rotation allows more flexibility in scheduling for both the employee and the mine. Changing a few people at a time ensures continuity, better communication and keeps the team concept alive. Polaris has a socio-economic agreement with the Government of the Northwest Territories which provides for job opportunities for Northerners. The mine employs about 240 people for normal operations. This can increase to 275 in the summer period. There are 60 management, technical, supervisory and support staff employees. The workforce in the mine is 60, maintenance 60, mill and surface 37, and miscellaneous 23. These numbers include up to 30 per cent off-site at anyone time. Turnover of permanent employees has been low, in 1992 turnover was only two per cent.

Management approach

A major contributor to the success of the Polaris Mine has been the non-traditional management approach. The avoidance of regimentation and the encouragement of interaction at all times between everyone on-site has provided a comfortable working environment. The introduction of a Gainshare program in 1984 went a long way to encourage increased production and cost reduction in subsequent years. The Gainshare program provided an increased emphasis on teamwork, employee participation and communication.

CONCLUSION

Mining at Polaris has required innovative technology for operation in permafrost and the Arctic environment. Many innovations have ensured an efficient year-round operation. Adapting cold weather technology to assist the mining operation has provided safe and productive mining methods. Non-traditional management and personnel policies have also contributed to the success of the Polaris Mine. Cominco has proven that mining in this remote area of Canada can be carried out successfully in harmony with the people and the environment.