The No. 12 orebody itself, a volcanogenic massive sulphide body sandwiched between two metamorphosed sedimentary and felsic volcanic rock units, is no longer the infinite resource it once seemed. Since its discovery in 1954 (one of the first discoveries using the then-embryonic technology of electromagnetics), the mine has been stacking up reserves. The company’s policy of spotting diamond drill holes 300 m ahead of the last known ore intersection has outlined the deposit over a strike length of about 1,000 m. The deposit dips steeply at about 75 degrees to the east. That drilling and a shaft-deepening project have finally reached what appear to be the limits of the orebody at a depth of about 1,125 m and along strike to the south, says Chief Geologist William Luff. That leaves only one direction for further exploration potential — along strike to the north. Successes have been recorded there, but because of the abstract nature of the geological reasoning being tested, some have dismissed the program as “dreaming.”
The year 1987 was a turning point for the mine. It was then that the deposit finally revealed itself to be finite — indeed depleting. As a consequence, Brunswick began mining more ore than it could add to its reserve base. Twenty-five years from now, it could be all mined out. Realizing this, the company has undertaken the latest available technology to mine the deposit as responsibly as possible. An example is an ongoing program to model the deposit in three dimensions on a powerful Intergraph computer workstation. That data base will eventually be linked with a displacement discontinuity computer modelling program already in use, predicts James Smith who is in charge of information services. Such computer tools help mine-planners extract the most ore without compromising mine stability.
In its expensive quest to modernize, Brunswick recently received a helping hand from two fronts — the metal markets and parent firm Noranda Inc. The recent turnaround in base metals prices couldn’t have come at a more opportune time. In the past year, 56 cents (us)-per-lb zinc has solidified Brunswick’s financial fortunes. Having recovered from two successive annual losses in 1985 and 1986, the company enjoyed profits of $19.7 million in 1987 and $31 million in 1988. (In the company’s 25-year history, only eight annual losses have been recorded, six in the period 1967-1972.) Thanks to Noranda’s 63% controlling position, Brunswick can tap into a staff with considerable mining and milling expertise. Noranda has interests in 13 mines in Canada (more than any other company), so it also offers the market muscle that a stand-alone company couldn’t possibly muster.
Brunswick has embarked on an ambitious capital spending program to make up for what looks, in hindsight, to be a couple of poor decisions during leaner years. One of those decisions was made when William James was president (he is now president of Falconbridge Ltd). In Brunswick’s 1977 annual report, James said:
“In order to maintain a stable financial position during this period of depressed zinc markets (24.9 cents us per lb), capital and operating expenditures have been reduced. To this end, the expansion project to increase the capacity of the mine was cut back in October and it was decided to complete the No. 3 shaft only to the 3,700-ft level, instead of the originally planned 4,500-ft level.”
Two years later, Brunswick would record its best-ever year with profits hitting $71.3 million. That shaft- deepening project will be completed this year at an estimated cost of $40 million.
The second damaging decision was more recent, and it too was probably based on poor metal prices. Brunswick decided in 1985 and 1986 not to replace underground capital equipment while the company was in the red.
As a result of these resolutions, the mine has been playing catch-up since 1987. “A lot of our equipment is eight to 10 years old,” says Mechanical Superintendent Jack Smith, “and it had reached the point where we couldn’t keep up with breakdowns.” The situation in the mill hasn’t been as critical. A 7-year plan to replace flotation cells is in its fifth year and will ultimately cost about $13 million. Another $26 million is being spent replacing equipment in the mill. In 1990, the bulk of Brunswick’s 10,500 tonnes of daily ore will come from two levels below 725 m. Today, about 8,000 tonnes are mined each day from below the 725-m level. Stopes on the 425-m and 575-m levels will be all mined out by the end of 1989 and 1990 respectively. Mining above the 725-m level will then be reduced to cleaning up remnant pillars, says Fred Hermann, chief engineer. The 850-m level will be mined at a rate of about 3,800 tonnes per day for the next 10 years and the 1,000-m level can sustain about 20 years of mining. For 10 of those years, mining rates will be in the order of 5,000 tonnes per day. The orebody is being developed on the bottom (1,125-m) level, where production should begin in early 1990. All in all, the company has a comprehensive plan for the next 10 years. How many years of economic mining remain depends on exploration successes to the north (see separate story). “We have more faith now in our 15-year plan than we ever did,” Vice- president John Moerman says.
At depths below 725 m, maximum principle stresses are about 75-80 MPa oriented in the horizontal plane. The compressive strength of the ore is about 206.8 MPa and the augen schist wallrocks have an unconfined strength of about 34.5-48.2 MPa. Using this information as input to a 2-dimensional displacement discontinuity software program called N-fold, Golder Associates (as consultants) and Brunswick started mathematically modelling the whole orebody as early as 1980. About five different approaches to mining wer e modelled. From that work were determined the maximum safe stope dimensions and mining sequence using mechanized cut-and- fill methods.
It was decided that the amount of ore that would have to be left behind in the sill pillars, as indicated by this computer modelling, was too great to justify the labor-intensive cut and fill method. So starting in 1985, the company began mining by a modified blasthole open stoping method on the 1,000-m level. About 6,500 tonnes are mined each day by cut-and-fill. This year, the first of the blast-hole stopes (149-8 stope) is being filled with cemented backfill. The new mining method will concentrate production in a smaller area of the orebody. Also the amount of ore in pillars will be minimized and the cemented backfill will allow adjacent pillars to be mined with acceptable dilution. The first stope to be mined next to the cemented backfill (5% cement by weight a
nd 95% crushed rhyolite) will expose the backfill some time later this year (probably October). How well it stands up will determine the cement-to-waste-rock ratio in future backfilled stopes. The company has already installed a rubber-lined Ross cement mixer (the Cadillac of mixers, according to Shift Boss Eric Payne) in an underground backfill plant. It is designed to produce 1,200-1,400 tonnes of backfill material per shift.
Senior ground control engineer Brent Kristof says Brunswick’s intention is to shed stresses to regional rib pillars rather than battle stresses by bolting the back on each lift (as is the case with cut-and-fill). Cable bolts are used to support the back of each drilling sub-level. The company installs 114,300 m of cable per year using manpower and unsophisticated drill rigs. Two rib pillars, 90 m wide and one pillar 30 or 60 m wide (the exact dimension has yet to be determined) are planned in the lower part of the mine. The vertical height of ore blasted in each blasthole stope is 30 m and each stope measures about 30 m along strike. A total of three 30-m sub-levels will be mined between the main levels, leaving a sill pillar of 25 m at the 725-m level, 30 m at the 850-m level and about 35 m at the 1,000-m level. To prevent sloughing from the hangingwall, the company is experimenting with an arbitrary cable- bolting pattern of 3 ft (vertical) by 5 ft (horizontal) on the 4 m of exposed hangingwall on each sub-level. Each cable bolt is 7 m long and all bolts are tied together on the face by steel strapping.
“This is the key ground control measure we have to take for this mining method,” Kristof says. “The idea is to treat the hangingwall as a beam which will fail as the stope is mined out. If we can support that beam at 30-m intervals from each sub-level, we’ll likely get about 8 m of waste rock sloughing off into the stope. Without the sub-level support, we might expect to get up to 38 m of waste rock as dilution.”
To monitor the small-scale release of strain energy in the mine (and thereby prevent what could become large-scale accidents), Brunswick has installed a computerized, mircoseismic monitoring system. Thirty- two geophones have been installed, and this year 13 hydrophones will be set up on the lower levels to monitor rock movements on the 1,000-m level. “We’ve decided to switch from geophones to hydrophones,” Kristof says, “because the full wave-form we can capture using the hydrophones improves source location to within 5 m or better.” Accuracy using geophones is about 10 m (50-60 m in areas where coverage is poor). There have not been any rockbursts at Brunswick, yet underground miners recognize the value of the microseismic network in monitoring the behavior of the ground as mining progresses.
Brunswick is proud of the fact that it has been involved in rock mechanics for about 10 years. “Noranda didn’t pick up on rock mechanics as fast as we did,” Kristof says. By spending $4.5 million last year, Brunswick began the Herculean task of replacing its fleet of underground equipment. About $ 10 million is budgeted for 1989.
Since June, 1988, Brunswick’s miners have been test-driving a number of new mining machines. “Production has suffered a little because of the learning curves involved,” says General Mine Foreman Bruce Jamieson (no relation to Mine Superintendent Brian Jamieson), “but the guys generally like the new machines.”
Production drillers have switched from using pneumatic wagon drills in the mechanized cut-and-fill stopes to using three different types of blasthole drills in the new open stopes. One new rig in particular (the SIMBA H274, manufactured by Atlas Copco of Sweden) is making waves with Brunswick’s operating crews. An electro-hydraulic top hammer drill, this unit can achieve 20 inches of advance per minute in a 4 1/2-in- diameter hole drilled in the hard Brunswick ore, reports Industrial Engineering Supervisor Robert Baker. While the unit costs more than the conventional pneumatic in-the-hole drill, Baker describes it as quieter and faster. Output per manshift and cost per tonne are Brunswick’s main criteria. Total mining costs in 1988 where about $19 per tonne of ore treated. Drilling holes 30 m long, the SIMBA has been as accurate as the in-the- hole blasthole drills. Brunswick’s in-the-hole drills include two rubber- tired drills, one track-mounted CD- 360 drill with booster compressor (both manufactured by Continuous Mining Systems of Sudbury, Ont.), two CMM 1s and one CMM machine (both models from Ingersoll-Rand of Montreal, Que.).
Other innovations in underground drilling being tested at Brunswick include the use of tube rods. These are being developed in Finland by Kometa, a division of Secoroc. These large-diameter rods have a thinner, harder steel wall but are no lighter than the thicker-walled conventional blasthole drill rods. Yet because of their larger diameter, they provide a more rigid drill string when in the hole. For this reason, they provide added stability while drilling, thereby improving accuracy of the hole. Rod life is one factor being evaluated in the development program. (The rods are also being tested in at least two other mines — Page-Williams and Golden Giant in Hemlo, Ont.)
Brunswick intends to experiment with 5-inch diameter holes and has invited the explosives-manufacturing unit of C-I-L Inc. to participate in experimenting with various blasting patterns and hole-loading designs. Their objective is to arrive at the most economic blasting technique for Brunswick’s unique ground conditions.
For mucking, Brunswick is going almost entirely with Toro load-haul- dump machines. Thirteen of this manufacturer’s big 500 CD units were on order at the time of our visit. At the drawpoints, operators dismount the machine and plug a remote control harness into the batteries of their miner’s lamps, then guide the machine into the stope using remote control. By late 1989, haulage distances will be 400-500 m, so mucking crews have been testing a Voest Alpine AL-60 drawpoint continuous loading machine. In a 97,000-tonne test over a 302-day period, Brunswick determined it could increase production by 25%-50% over LHD-truck combinations. A full report on its performance was given by project technologist Jerome McDonald at the ninth underground operators’ conference of the Canadian Institute of Mining and Metallurgy, held recently in Sudbury, Ont. “We were very satisfied with the machine,” McDonald said. “But a decision to buy will depend on cost benefits and whether 15 minor modifications can be made.” A detailed study examining the cost of owning and operating the new machine compared with the old is being conducted.
For haulage, the company has decided to purchase MT 433 trucks from Wagner Mining Equipment of Portland, Ore. “They’re expensive,” Baker says, “bu t they’re better than the other units we considered.”
The equipment replacement program is concurrent with both the extension of the main (No. 3) production shaft to the lower reaches of the orebody and the installation (below the 1,175-m level) of a new crusher station and materials-handling system. Later this year, the shaft extension will be connected to the existing shaft and a new, gyratory crusher station (rated at 850 tonnes per hour) will become operational. Two 30-tonne skips from the 1,250-m loading pocket station are also expected to begin operating later this year, at which time about 80%-85% of the mine’s production will come up the No. 3 shaft (the current amount is about 75%). It seems safe to say that, as new equipment arrives over the next two years, the No. 12 mine will find itself at the forefront of low-cost zinc producers.
But first the company has some bitter pills to swallow. It will lose about 240,000 tonnes of ore this year because of a 28-day partial shut-down scheduled for July. The purpose of the shut- down is to re-rope the conveyances in the No. 3 shaft and tie together the steel work in the two sections. At a time when zinc prices are expected to average 75 cents (us) per lb and better (according to estimates by Shearson Lehman Hutton of London,
England), that’s a pill Brunswick would just as soon not take. But the company has an ace up its sleeve. By resuming operations at the Heath Steele-Stratmat project, southwest of the No. 12, production for the year should actually increase. Brunswick bought into the project in 1986 and has been exploring there ever since. The Heath Steele mill, which has a capacity of about 4,000 tonnes per day, last produced concentrates in 1983. The plan is to run it at about 2,200 tonnes per day, treating ore from three deposits within a 6-km radius. This year, if the start-up goes smoothly, the mill could produce 22,000 tonnes of zinc concentrate, 7,000 tonnes of lead concentrates, 4,000 tonnes of copper concentrates and 2,400 tonnes of bulk concentrates. Production from one of the deposits will be split 50/50 by Brunswick and Noranda; the rest will be split 25%/75% between Brunswick and Noranda. However, Brunswick has an option agreement to increase that to 50%.
The re-roping project on No. 3 shaft will be an operational challenge in itself. But once it is complete with the two 30-tonne skips operating from the 1,250-m level loading pocket, and once the new crushing station is tied in to the ore-handling system, productivity is expected to jump. The recent change in mining methods to modified blasthole open-stoping from mechanized cut-and-fill should ensure this. Brunswick is expected to spend $20-$25 million over the next four years to upgrade the dewatering and loadout area of the mill and to replace obsolete flotating equipment. In addition, the company will spend $1-$1.5 million to expand the recently-installed Fisher Provox computer control system. The system may eventually control the entire mill. Only the flotation section is currently controlled by this system. A Bailey Network-90 process computer is now used in the dewatering circuit, together with numerous Allen-Bradley programmable “logic-controllers” to control the operation of equipment. This system will be capable of communicating with the Fisher Provox system.
“The idea is that the mill operators will use the computer system in one area of the mill, then go on to implement it in the other sections until the whole mill is on the system,” explains John Martin, concentrator superintendent. The Fisher-manufactured system is part of a larger program of improvements recently implemented by Noranda. By using the same computer control system in all of its mills in Canada, Noranda hopes to achieve two goals. The first is to get a commitment from the supplier to continue developing its product to Noranda specifications by offering it a large market for its product. The second is to facilitate the movement of mill operators from pant to plant. Having the same control systems in each mill should make retraining unnecessary. (A new Fisher control system is planned for the Heath Steele mill as well.)
“Ten years ago all of Noranda’s mines would have done things their own way,” says Martin, who is a member of Noranda’s steering committee on milling technology. “Things were not nearly as efficient as they are now.”
Yet another attempt to improve milling recoveries is the planned installation (at Heath Steele) of a Tri- flow heavy media separator to reject light-density material early in the milling circuit. The machine is being developed for Noranda.
Historically, the mill at Brunswick’s No. 12 mine has been complicated. Recoveries were low and costs were high, mainly as a result of the large amount of steam used in the process. In 1984, the company rented the pilot plant at Lakefield Research in Peterborough, Ont., and in three months a new flowsheet was designed. That simplified design knocked two to three dollars off each tonne of ore treated. “But the mill still does not work, metallurgically, as we’d like it to,” Martin says.
One change in the flowsheet was to pump the high-grade portion of the copper-lead rougher concentrate straight to the second cleaner, instead of regrinding all the cleaner feed. This has been so successful that the company is looking to apply the same strategy to other lead and zinc circuits.
The Brunswick mill produces four concentrates by selective flotation — a zinc, a lead, a copper and a bulk (combined lead/zinc) concentrate. Total payable recoveries are 82% for zinc, 64.5% for lead, 57% for copper and 56.5% for silver. Milling costs account for about one-third the total cost of treating ore at Brunswick. In 1988, milling costs were about $13 per tonne of ore treated.
A detailed mineralogical look at the mill by the Research and Productivity Council of New Brunswick has revealed that the company is grinding its ore too fine in some cases. “We’ve had to back off and discuss how we can avoid this over-grinding,” Martin says. It may involve some additional screening in the crushing circuit. Automation of the crushing circuit should help too. “Since the Brunswick ore is very fine-grained and relatively high-grade, extensive regrinding of various flotation products is required to obtain the desired liberation. The final products at Brunswick typically exceed 90% minus 37 microns and, hence, 30% of the grinding power available at Brunswick is used for regrinding,” Martin says.
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