Lucky Friday Mine

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Mine Overview

The Lucky Friday mine was a cut and fill operation located in Mullan, Soshone County, Idaho. Using a slice cut and fill method with timber and hydraulic sand fill, it achieved an annual production of 167,829 tonnes[1]. The ore body was 396 to 488m long on the strike and 1421m on the dip, averaging a width of 2.3m and a dip of 70⁰[1]. The characteristic of the ore was fairly strong and the foot wall and hanging wall were highly fractured and sheared, considered to be very weak.

Table 1: Relevant Statistics about the Lucky Friday mine[1]

Annual production

167,829 tonnes

Depth (average)

1296 meters

Size of ore body

396-488m long, 1421m dip, 2.3 m width, dip 70⁰

Characteristic of the ore

Strong

Characteristic of hanging wall

Weak

Size of stope

~680 tonnes per stope

Productivity

12.1 per miner shift hour, 13,608 t per month

Mine access

Shaft

Number of employees

95 underground production, 38 maintenance, 120 as hoister, sander, nipper, cager, etc.


Mining Method Overview

Lucky Friday used a conventional stoping cycle that consisted of sampling, drilling, blasting, mucking, rockway for extraction, backfilling, loading and hauling, ground support, and stope ventilation. The reason cut and fill was chosen is because the slusher was the most efficient mucking technique for a narrow, steeply dipping orebody where high selectivity is essential.

Table 2: Select data for the Lucky Friday mine design[1]

Sampling

Channel sampling

Drilling

Percussion drills. Back stopping round 0.9-1.8m. breast stopping round 1.5m to 4m

Blasting

aqua gel slurry

Mucking

14.9kW (20hp) electric double drum slusher

Rockway for extraction

1.2 meter diameter rockway raised with the stope and encased in the sand fill

Backfilling

Hydraulic sand(Classified mill tailing) fill (65% solid)

Loading and hauling

Side dump cars pulled by 5.4 tonnes battery locomotive on rail

Ground support

Timber stull and post sets, stull only, or rockbolts with plates

Stope ventilation

2830-5664 L/s auxiliary axial flow fan


Use of Underhand Cut and Fill

Conversion from Cut and Fill

At 1420 m below surface, the mine began to encounter serious rockbursting problems culminating in fatalities in both 1984 and 1985[2].To mitigate the rockburst hazards that Lucky Friday was experiencing, Hecla began experimenting with underhand cut and fill, which it referred to as "Lucky Friday Underhand Longwall"[3]. The tests used a paste backfill with no free water and a bolt method which reduced backfilling preparation times. In March 1986, another rockburst-caused fatality in the overhand cut and fill section of the mine forced production to cease by April 1986[2]. Hecla converted the entire mine to mechanized underhand cut and fill mining and resumed production in October 1987[2].

Mining

Hecla uses backfill with the following properties for the mine.

Table 3: Backfill properties[3]
Backfill type Tailing paste fill with no free water
Cement Content 8 - 10%
UCS 2070 kPa
Curing Time 3 days
Delivery Rate 118 t/hr


Drilling is done with a jumbo and drillholes must be a minimum distance of 0.3 m from the backfill to prevent blasting damage[3]. 1.5 m3 LHDs are used for mucking[3]. Other than the prefill bolt patterns,rockbolting is rarely done because the chain-link fence provides adequate support to prevent groundfall. In some areas, spot bolting is used to provide additional support. The Gold Hunter 30 vein is an exception to this; 1.2 m split set bolts are used to anchor steel mats on the north wall and chain-link fencing on the south wall[3]


Safety

The backfill used in Lucky Friday Mine can withstand rockbursts, thereby reducing the number of groundfall injuries. The backfill was able to withstand a 3.5 Richter scale rockburst that was caused by blasting in the mine while the chain-link fence contained the resulting fly rock[3].

"Prior to converting to underhand mining, the Lucky Friday Mine experienced three rockburst-related fatalities in four years. However, in the 20 years since converting, a rockburst-related fatality has not occurred at the mine.[3]"

At the introduction of underhand cut and fill, the US Bureau of Mines and the National Institute for Occupational Safety and Health installed geotechnical monitoring systems to measure changes in the backfill as mining continued. The monitoring showed that up to 10 cm of deformations caused by wall closure occured as a result of mining beneath the fill[3]. The fill was supported for long enough that mining was several cuts down by the time the fill failed and thus failure was not a concern[3].

References

  1. 1.0 1.1 1.2 1.3 SME. (1998). Techniques in Underground Mining. Society for Mining, Metallurgy, and Exploration Inc.
  2. 2.0 2.1 2.2 R. Pakalnis et al, “Design Spans – Underhand Cut and Fill Mining,” in CIM-AHM, Toronto, CAN, 2005, pp. 1 – 9.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 T.J. Williams and T.M. Brady. Underhand Cut and Fill Mining as Practiced in Three Deep Hard Rock Mines in the United States [Online]. Available: http://www.cdc.gov/niosh/mining/UserFiles/works/pdfs/ucafm.pdf


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