Printer Friendly

Mount Isa - mining.

How the mines are cutting operating costs through benching and open stoping mining methods and the use of the latest technology

Australia's Mount Isa copper and silver-lead-zinc mining and mineral processing complex, including the Hilton mine nearby, is the core operation of MIM Holdings. It has always been among the world leaders in adopting and adapting to new technology and methods. At present many new developments are underway, with the current focus on reducing annual operating costs.

In October last year Mr. N. ColdhamFussell, MIM's managing director and chief executive officer, addressed the NSW Securities Institute and elaborated the strategy for Mount Isa. "Within the last ten years, MIM could claim to be one of the lowest cost copper producers in the world, and in fact the lowest cost lead-zinc producer. We cannot make these claims today. Clearly, we, like all Australians, have lost much of our competitive position through a combination of domestic inflation impacting our costs and a strong Australian dollar containing our prices/revenues, without any ability to pass on increased costs to a market where inflation has been at low levels since the 1982 recession.

It is essential for our growth and international objectives that we must reduce our position on the copper and lead-zinc cost curves at the Mount Isa and Hilton mining and smelting complex".

Aside from the inflation and overvalued Australian dollar, the other main reason for the deteriorating competitive position at Mount Isa is that its "large reserves of copper and lead-zinc are competing with new rich ore discoveries which can be mined by low-cost open cut mining methods.

MIM is taking direct action to reduce the impact of the economic environment in a number of ways.

First, we are aiming to reduce operating costs. At Mount Isa, this reduction will be in the order of 20% - a target of more than $A100 million per annum by 1993. Already the workforce at Mount Isa has been reduced by 500 through attrition and retrenchment, without loss of production, and we expect the workforce to decline further. Today, the workforce is down to 1960's levels, with treble the ore volumes of those days.

In addition to operating cost improvements, a very significant investment has been made in the development and application of technology aimed specifically at reducing costs. In the last financial year, almost $A500 million was committed to capital programmes to increase production capacity, improve productivity, and reduce costs. Significant success is being achieved".

Mining today

MIM aims to be re-positioned as a larger, lower cost producer of copper and lead-zinc, the base metals on which the mines are built. Isa and Hilton will remain core MIM operations for decades to come provided their unit costs are reduced to improve their competitive positions. A 40% improvement in productivity is targeted for achievement by 1995, and increases in production to 210,000 t of copper, 210,000 t of lead and 250,000 t of contained zinc by 1993.

In the 1991 financial year, the Mount Isa complex treated 5,772,349 t of copper ore and 5,528,720 t of silver-lead-zinc ore. Total contained metal outputs were 179,304 t Cu, 165,969 t Pb, 13,924,246 oz Ag and 238,830 t Zn; a record in lead and second highest in silver and zinc. In fact, were it not for certain technical problems encountered during the year lead output would have been even higher.

Mount Isa, in North-West Queensland, is one of the largest underground mines in the world (see also MM, November 1985, pp. 421-433). The first discovery of lead-carbonate ore was made in February 1923, but production did not start until June 1931, at an annual rate of some 660,000 t of silver-lead-zinc ore. During the Second World War, at the request of the Australian Government, Mount Isa made a complete production change from lead to copper. Copper production ceased again in 1946 and did not become a major part of the mine's production again until the beginning of 1953. Today the mines treat over 11 Mt of ore, and this production level is to rise still further.

Hilton, 20 km north of Isa, was first discovered in 1947. Trial mining began 40 years later and commercial production began in 1990. Hilton's silver-lead-zinc ore production reached 1,153,874 t in the 1991 financial year. Under original plans, Hilton would have started commercial production earlier; but it was delayed when additional reserves were discovered at Isa.

The Isa mine is located below the valley of the Leichhardt River. The copper and silver-lead-zinc deposits occur in a sedimentary formation about 1,000 m thick, the Urquhart Shale. This formation consists of alternating sedimentary beds, variously composed of dolomite, quartz, mica and feldspar which were laid down about 1,600 million years ago.

While the copper and silver-lead-zinc orebodies occur within the same formation, they exist as close but separate entities which are mined and treated separately. Because lead was historically the dominant metal in the silver-lead-zinc ore, Isa personnel normally just refer to that ore as lead; for the remainder of this article it will be referred to in that same manner. In fact, however, today's proven reserves contain more zinc than lead.

At present Isa's 1100 orebody at the south end of the mine contributes most of the copper production. However, the current development thrust is focused on the 3000/3500 orebodies, which will soon be contributing substantially to copper output. These lie to the northeast of and much deeper than the present copper producing area. The lead orebodies are in the northern mining area. Isa's current operations cover 5 km along strike.

Isa's mineable copper ore reserves published in the 1991 Annual Report amount to 70 Mt of proven ore at 3.2% Cu and 50 Mt of probable ore at 3.6% Cu. Mineable silver-lead-zinc ore reserves (proven) were 45 Mt containing 132 g/t Ag, 5.4% Pb and 6.6% Zn.

Integrated mine design

There are two important types of mineralisation; the stratiform silver-lead-zinc deposits at the Isa and Hilton mines and the disseminated silica-dolomite copper orebodies at Isa. The silver-lead-zinc orebodies have very sharp ore contacts. Sectional methods of interpretation and ore reserve calculations are employed successfully. Copper mineralisation is erratic with the high-grade core of the 1100 orebody surrounded above and to the sides with a lower grade halo, and the peripheries are hard to define manually. Geostatistics is used for ore reserve calculations in these orebodies and to define economic limits to the mineralisation.

Not surprisingly Mount Isa is a very 'computer literate' operation and its new computerised mine planning system is generating a wide range of benefits for planning and operating personnel. The Integrated Mine Planning System (IMPS) allows engineers and managers to call up plans, sections or three-dimensional views of mining areas on PCs or powerful Unix workstations bringing the most up-to-date information instantly to their screens.

IMPS is a CAD-based system for geological interpretation and modelling, mine design and underground surveying of current and future operations at both Isa and Hilton mines. It brings together the vital information from the geology, mine design and survey departments in a manner that is quickly and easily understood.

IMPS is built around the Microstation CAD software package, and Oracle database, which runs on a network of Intergraph UNIX workstations and MS-DOS PCs. The latest hardware additions to the IMPS network include the new Interact 6000 Series workstations which feature dual 19 in screens. The workstations can process 10 million instructions each second and display high resolution graphics in up to 256 different colours.

Staff from the Mine Planning, Geology and Information Systems Departments at Isa and Hilton mines started work on the joint project back in May 1989, after having determined that no suitable 'off-the-shelf' system existed.

Since then, many utilities have been developed by the project team to complement the Microstation CAD package and make it more suitable for use in a mining environment. These improvements include the ability to create solid three-dimensional surfaces of structure, orebody or stopes. Ore surfaces, for both lead/zinc and copper orebodies, can be generated from geostatistical data within minutes at any grade. This is an extremely powerful utility, particularly for the lead/zinc orebodies when different cut-off formulae can be rapidly evaluated in conjunction with the tonnes and grade estimation utilities.

Another feature is the enhanced presentation capabilities which allow production and planning personnel to use IMPS to evaluate several design options before committing to a mining plan. Indeed, the tools that are now available to mine planning provide great improvements in engineering and economics, while maintaining considerable flexibility. Designs can now be fully tested for their practicality with the placement of fixed and/or mobile equipment models within the mine model to test clearances, lines-of-sight at junctions and other critical operating considerations.

The introduction of IMPS has been very successful and development work continues. Existing rock stress and fill stability analyses and ventilation software are to be interfaced with the system. Production and resource scheduling will also be integrated, and the potential to integrate budgeting and financial forecasting activities has been recognised.

Aside from the speed and accessibility of IMPS, other benefits have been a great reduction in data transferal between departments in the form of paper plans, charts and tables. One significant advantage is the ability to produce plans at any scale with selected information without the inconvenience of conventional plan boundaries. This has facilitated a major reduction in the number of plans that have to be 'maintained at different scales'.

Bench stoping

A new mining method has been introduced and dramatically raised the efficiency of lead ore production. Until recently Isa recovered a lot of this ore from mechanised cut-and-fill stoping. Now, however, bench stoping is largely replacing cut-and-fill. Sub-level open stoping is the primary, and preferred, mining method, but where it is not possible benching is employed. The choice between open stoping or benching is largely determined by the width of the particular orebody to be mined. Benching is replacing cut-and-fill in the 4-15 m wide orebodies. The lead orebodies vary from 4 to 48 m in width, are up to 1,400 m along strike and can reach 800 m in height.

The benefits of bench stoping over cut-and-fill are increased productivity, improved safety, better utilisation of equipment and lower costs. There is a $A10/t difference in mining costs in favour of benching, due largely to its higher productivity and lower ground support requirements. It also improves the head grade of the ore, through reduced dilution. Development accounts for 30% of the production from bench stoping. The lead mining area converted from CAF to benching in about seven months.

Company operating planning engineers summed up the significance of the change to benching; the method is safer for our workforce as no one works in the stope during the extraction phase. Secondly, using virtually the same men and equipment, the rate of ore extraction in the narrower orebodies can be more than doubled, significantly reducing unit costs.

With the increasing emphasis on benching in the narrower lead/zinc orebodies, it became apparent that there was a need for a small, manoeuvrable, low capacity diamond drill to collect geological and planning data. To satisfy this need a JKS Boyles Bazooka rig was purchased recently. It can drill 35 mm diameter holes, 30 m long, rapidly. Trials are in progress also using the 46 mm coring system which is standard to the lead/zinc mining areas.

Benching has been under trial for some years and has gradually been refined, particularly in ground support and filling techniques. In the current financial year, output from the bench stopes will be more than 2 Mt. This will account for nearly 45% of the total annual lead ore production.

In cut-and-fill stopes a vertical slice up to 4 m high was extracted along the length of the orebody. Benching allows for much larger slices of ore, currently 12-20 m, to be extracted. The width of a bench is defined by the orebody width while the length and height will vary for each bench depending on the local ground conditions. The stopes may be up to 250 m in length.

To develop a bench stope, sill drives are mined at the top and bottom of the ore block. Rockbolts are then installed in the back and the walls. Cable bolting is also used as required. Parallel blastholes are drilled down into the bench, normally with Atlas Copco Simba H221 rigs. These holes are then progressively charged and blasted. Typically about two rows of holes are blasted at any one time.

The broken ore is loaded by LHDs. To remove all the ore the LHDs have to go into the stope, therefore remote control units are used.

As production progresses along the bench, more and more of the hanging wall is exposed. Often, especially if it is a long bench, fill is progressively introduced to provide some support and improve the stability of the exposed hanging wall. Underground development waste and hydraulic sand fill from the surface are used for fill.

One of the major areas converted to benching is a MECAF (mechanised cut-and-fill) area where mining started over six years ago, the Racecourse orebodies just above 19 Level. Lead planning manager at Mount Isa, Tim Horsley, said: "Conversion to benching in MECAF will significantly reduce operating costs and will allow an increase in production up to 1 Mt/y in this area. Even at these increased extraction rates, there are sufficient reserves in the Racecourse orebodies above 19 Level to sustain production there into the next century".

Bench stoping is a very flexible method. If mining conditions so dictate, production could revert to cut-and-fill almost immediately. Another advantage of benching is a considerable improvement in the working environment as a through-flow ventilation system can be developed.

Not all Mount Isa's lead orebodies are amenable to open stoping or benching. A small amount of cut-and-fill mining will continue for some years.

Specialised roofbolter

The usual ground support method for bench stoping is to use Split Set bolts in the hanging wall of the sills. Cone and anchor bolts are used to support the roof, followed by double-strand (4.5 m) cable bolts in the hanging wall and, if necessary, combination cable bolts in the back. The choice of support in each case depends on stress field, ground conditions, geological structures and orebody width. Split Sets are used in the footwall as required.

Bench stoping has reduced support costs compared with CAF techniques. It also provides greater safety for the miners as less work has to be done beneath an unsupported roof.

However, bench stoping required four distinct types of drilling unit, a face drilling jumbo, roofbolter, cablebolter and longhole drill rig. It was decided that a simpler option would be to use a single machine for drilling roof bolt holes, installing the Split Sets and drilling the cablebolt holes. To this end a tender for such a machine was put out in late 1989 and, as a result, Secoma produced its prototype Special Roof Bolter (SRB).

Power for this machine is either drawn from the mine's electricity supply or is generated by the on-board Perkins diesel engine. It utilises a Pluton 17 carrier and the boom is a shortened and strengthened B37B. The front-end Drill Positioning Unit (DPU) houses the hydraulic supply line control valves and supports and manoeuvres the three main units needed for drilling and bolting. These are the bolt carousel, the rod/bolt loading cylinders and the drilling and bolting turret. The TLH 28 turret is mounted with two drills; a Hydrastar 300LT drifter for long holes and an HH 200 Impactor for the Split Set bolts. The carousel can accommodate either 7 or 8 ft Split Sets for conventional bolting or extension drill steels for drilling up to 20 m long holes for later cable installation and grouting.

By late last year the SRB availability was 87.5% and it was achieving 110-120 m/shift, with a target of 150 m/shift. It had at that time been used mostly for cable bolting, drilling holes at all angles, horizontal holes up to 15 m and downholes up to 13 m long. Its versatility makes it suitable for use in both open stoping and bench stoping areas.

MIM LHDs

Isa and Hilton together have a large fleet of LHDs comprising 17 Eimco 913s, 12 Elphinstone units (five R1500s, two R2600s and five R2800s), two Toro 500DLs and six Toro 400Ds, and 40 UGL 400s. Early in the year one of Elphinstone's new R1700 units began operation and most recently a Wagner ST7 went underground on May 8, to begin work in the 1100 copper orebody. Both units are being monitored carefully as possible replacements for the fleet of UGL 400s.

The latter is a 'home-grown' machine. It is based on an old Australian-designed unit that was built in Melbourne. MIM decided to take on the manufacture and improvement of these units for two basic reasons. Firstly heavy-duty machines with a bucket capacity of 4 |m.sup.3~ (5 |yd.sup.3~) were required, and were not available at the time. Secondly the company found it cheaper to produce its own machines. As operating requirements change, improvements continue to be made.

The chassis, bogie, boom and bucket are all manufactured by subcontractors for MIM. The engine is a 255 hp Cummins V903 and the torque converter and transmission are supplied by Clark. Axles are supplied by local company Birrana Engineering with a Rockwell differential centre. The whole LHD fleet is fitted with Bridgestone slick tyres.

Isa built its first UGL 400 in November 1987 and the last new one to be built has now gone into service. No further new units are planned but old units are still being rebuilt at a rate of 10 each year. This rebuild programme still has about four years to go. Isa's diesel workshop engineers expect these units to have a life of at least 20,000 hours. By the end of 1991, the longest elapsed life of any unit was 9,000 hours so there are still many operating hours left in the UGL 400 fleet.

The production record for the mines' LHDs is held by a UGL 400 which filled 348 bucket loads in one shift. At the diesel workshop the productivity of these machines is judged on the work undertaken, bucket loads or tonnes moved, rather than purely on hours worked, and rightly so. Operating hours take no account of the actual work achieved and do not discriminate between light work or hard productive operation. Operating cost per ton is far more important than cost per hour.

Three of Hilton's LHDs and eight of Isa's are remote control machines. This is also an MIM design and the company makes its own transmitters and receivers. A recovery system has also been developed for 'lost' remotely controlled units, which are hooked out with a second machine.

Copper ore production

At present all copper ore production derives from sub-level open stoping. In the currently worked 1100 orebody stope dimensions vary according to the orebody configuration, but the typical stope is vertical over the full height of the orebody and 40 m by 40 m in plan. In the 3000 orebody, which is now being opened up, the open stopes are expected to be 60 m high and 30 m x 30 m in cross-section. Each stope will provide an average of 190,000 t.

In the current open stopes, development drives are usually mined 4 m high and 3.7 m wide. A cut off slot is developed at one extremity of the stope from a cut off raise, typically bored to a diameter of 1.8 m by a Robbins 61R raise-borer. Drawpoint troughs are developed beneath the stopes, through which the broken ore is drawn off and hauled to an orepass by LHDs.

Stope drilling is undertaken from the sub-levels, which are accessed via ramps in the footwall. In-the-hole hammer drilling, using Atlas Copco Simba and CMM drills, is the technology employed. Drill levels are vertically 40 m apart in 1100 and will be 60 m apart in 3000.

Mount Isa's ore handling system comprises two main shafts, R62 for lead ore and U62 for copper ore. R62 is 1,144 m deep, taking it below 21 Level (1,076 m) and the bottom of the known lead orebodies. U62 is slightly deeper at 1,154 m. The average hoisting rate in R62 is 930 t/h at a speed of 15 m/s; while the hoisting rate in U62 is 1,200 t/h at 16 m/s.

Copper ore is hauled exclusively on 19 Level (960 m), whilst lead ore is hauled on 19 and 15 Levels (728 m). All production ore haulage is on 1,067 mm gauge track. Trains on 15 Level consist of 12 7.1 |m.sup.3~ Granby cars drawn by 20t electric trolley locos. On 19 Level, 11.3 |m.sup.3~ capacity bottom-dump Hudson Rockflo cars are hauled by 20 t battery/pantograph locos operating in tandem on a computer-controlled loop haulage system. Three trains are in operation on 19 Level, commonly achieving 12,000 to 15,000 t/shift.

The rail haulage system delivers ore to underground crusher stations, Nos. 3 and 4 for copper ore, and No. 2 system crushing lead ore and waste. Each of the three installations consists of a 2.1 m by 1.5 m primary jaw crusher on 20 Level which reduces run-of-mine ore to minus-300 mm at a rate of up to 1,000 t/h. Each primary crusher used to feed secondary jaw crusher installations on 21C Sub-level. The Nos. 3 and 4 secondary crushers are not operating as the size reduction to -140 mm they produced is no longer required. No. 2 secondary crushing system has 1.07 m x 0.76 m crushers reducing the lead ore to -120 mm.

A portion of the 1100 copper orebody lies below 19C Sub-level which is the lowest extraction horizon from which the LHD units can tip rock directly into ore passes feeding the 19 Level haulage, approximately 30 m below. Consideration of several alternatives led to the decision to drive a conveyor decline from the main shaft area to a point beneath the 1100 orebody where a new primary crusher on 23C Sub-level handles ore which can be tipped directly into its own ore passes. Primary crushed rock is hauled to the central shaft area by a Cable Belt conveyor installation. Waste discharges into existing waste handling facilities and ore enters the existing copper ore stream via transfer conveyors ahead of the No. 3 and No. 4 secondary crushers.

The complete 1100 orebody ore handling system was the biggest single mine project undertaken over the last two decades, costing in excess of $A38 million. This development programme spanned more than four years and ensures the continuity of copper ore production until development of the deeper 3000 orebody is completed and production from there begins. The last phase, the P41 crusher station, was commissioned in June 1989.

The crusher chamber is 29 m long, 15 m wide and 20 m high. Mining the access to it and excavating the chamber at a depth of more than 1 km below surface took over two years. During the construction of the crusher station, more than 1,000 |m.sup.3~ of concrete were poured and over 600 t of structural steel were installed. Its heart is a Jaques 2000 x 1500 double toggle jaw crusher measuring 6.65 m long, 4.85 m wide and 4.12 m high. It weighs 210 t and has a design throughput of 1,200 t/h.

Work on another major underground facility for the development of the 3000/3500 orebodies began in March. The new 26B Level T64 bulk power supply substation is adjacent to the T62 haulage decline, some 1,400 m below the R62 shaft surface plant, and is due for completion in August. Installation work involves the placement of some 5 km of high voltage power, earthing, communication, instrument monitoring and control cables across various levels in the 3000 orebody between 20 Level and 26B Sublevel.

The substation will house 11 kV switchgear, 3.3 kV pump starter equipment, 440 V distribution equipment, high voltage transformers, instrument monitoring and communications and control equipment. It will be connected within a power system network to other substations on 20 and 23 Levels and will provide 11,000 V, 3,300 V, 1,000 V and 440 V power supplies for the mine's pumping and ventilation requirements, the Kiruna Truck haulage fleet, initial mine block production, 27 Level workshop/stores complex and future 3500 orebody mining development.

Mobile mining

The Cable Belt drive was the first development job in the world undertaken by a Robbins Mobile Miner (MM, November 1988, p.395). This machine was a joint development between MIM and the Robbins Co. The S60 decline is 6.5 m wide by 3.7 m high driven at a gradient of 1 in 7 for a total length of 1,156 m. The main Cable Belt conveyor was commissioned in mid-1987 and, with a length of 1,850 m, is believed to be the longest underground Cable Belt conveyor in the southern hemisphere.

Mount Isa's Mobile Miner has now been decommissioned but it was the work it undertook at the mine which largely proved the concept. A second Mobile Miner, MM130, has been developed by Pasminco and Robbins and was delivered to Broken Hill at the start of this year. The predicted mining rate of this 260 t machine is 200 t/h. Robbins has a development/management contract with Pasminco Mining to develop MM130 fully from a production prototype to a production machine by the end of 1992.

Electric truck haulage

Development for the deeper 3000 copper orebody at Mount Isa has started and production from these orebodies will eventually replace output from the 1100 orebody as its reserves diminish. Estimated capital costs to develop the area fully will be about $A200 million spread over a decade. Some $A60 million is budgeted for the initial phase to start production. A substantial part of the development cost is being spent on increased underground cooling capacity to combat virgin rock temperatures of 50|degrees~C and more.

The deep ore is being accessed by two main ramps. T62 is exclusively for truck haulage and U62 is a utility ramp. T62 is being developed at a gradient of 12% with passing bays mined at 1 km intervals. These ramps are 4.7 m high by 5.5 m wide. Recognising that good road maintenance is essential to efficient operation in these ramps, a Caterpillar 120G grader is constantly at work to ensure even road surfaces.

Ore will be hauled up the ramps by a fleet of Kiruna Electric K1050E trucks, operating on an overhead trolley line system. Two of these 50t units have already been delivered to the mine and are assisting with the development of the 3000 orebody. An additional two trucks are currently being assembled in Sweden for delivery in early 1993 when production is due to commence. At full production the plan is to have five of these trucks. They will haul ore 4.5 km from 28 Level bins to 19 Level and tip into the same bin system as the ore trains. They are expected to haul more than 1 Mt/y.

The truck haulage option was chosen in preference to a sub-vertical shaft because it would not have been possible to carry out exploration drilling and sub-level development while shaft sinking, and because of the high cost of shaft sinking. Undertaking sub-level and ramp development simultaneously means the payback on the truck haulage system is much quicker. ABB Drives and Kiruna Truck, the joint developers of this unit, claim that taking energy, manpower and ventilation costs into account, the operating costs of a Kiruna Electric unit can be less than 50% of those for diesel truck haulage. As well as requiring less ventilation, it requires less 'coolth' from the refrigeration system.

A three-phase, 1,000 V, trolley line powers the trucks. This voltage is converted to direct current via a Tyrak 8 thyristor converter installed on board. The converter supplies 0-1,200 V to two LJM 290 traction motors connected in series. Series connection allows power to be distributed evenly to all four wheels.

The truck's current collector is automatically connected to, or disconnected from, the overhead line. A special patented current collector system in combination with a built-in NiCd battery makes this possible. Disconnected from the trolley line, the truck can continue working for about 600 m on the battery alone, at a reduced speed of 5 km/h. In trolley operation the truck hauls at about 18 km/h and can climb gradients of 12-15%. Speed control via the accelerator is regulated by increasing the motor voltage from 0 to 1,200 V; the speed increases up to 2,000 rev/min and by decreasing field voltage the speed is increased to 2,000-4,000 rev/min.

Robbins 84R raise borers are currently developing the ventilation raises for T62 and the mine's Robbins 61R is mining the waste and dust extraction passes. The main exhaust shaft for 3000 orebody, Q60, was mined using the Horodiam method. Under the Horodiam method a pilot hole is raise bored and it is then widened by drilling and blasting longholes from a module suspended on a winch from the top of the raise. This 7.6 m diameter raise connects to another shaft going to surface.

In-the-hole hammer drilling by Atlas Copco Simba 269 rigs drilling 60 m downholes will provide production from 3000. All jumbos in 3000 section will derive their power from a 1,000 V alternating current supply. The advantage of this higher voltage in comparison with the 440 V supply used in older areas is that the voltage drop is 5% in up to 1 km of cable, whereas the same drop occurs after only about 250 m at 440 V. Hilton used a 1,000 V supply from the outset, and was one of the first mines in the world to do so.

It is unlikely that much new drilling equipment will need to be purchased for the initial 3000 production. However, automation will play a key role and high-pressure drilling is a technology that is being examined for the future. Electric LHDs are being seriously considered because of the high cooling and ventilation requirements of this area.

At present a great deal of exploration drilling into the 3000 orebody is being undertaken using Craelius Diamecs, a D250 and D700, and a Longyear LM37. It will be about another year before ore production commences.

Isa Refrigeration

3000 orebody will be one of the deepest copper mines in the world, with big stopes requiring efficient refrigeration. Isa has had a refrigeration plant for many years in some limited areas; high summer temperatures on surface, a steep geothermal gradient and extensive use of diesel equipment make this essential. The requirements expected of this facility have grown steadily over the years. In the 1960s that surface plant was converted to supply chilled water in closed circuit from surface to two high-pressure heat exchangers located 900 and 1,000 m below surface. The secondary water from the heat exchangers was circulated to cooling coils and used to cool the underground service areas and crushers. Early in the 1970s a third compressor (nominal duty 1.25 MW) was installed as a spare unit and 100 kW and 50 kW spot coolers were used in areas remote from intake air where additional cooling was required. Today Isa operates 13 main ventilation fans.

Early in the 1980s much consideration was given to future cooling requirements as the mine progressed to deeper levels like the 3000 orebody. Meanwhile, a fourth compressor with a refrigeration capacity of 1.25 MW was installed in the surface plant and a third underground heat exchanger was placed 1,000 m underground. Besides providing cooling for the initial development in the deeper ore zones, this expansion also supplied chilled water for cooling of the Mobile Miner operations. As well as bulk cooling the air to the face, in this application the chilled water was used in the machine's cooling water circuits and for dust suppression.

One of the options to accommodate the increased cooling needs of the deeper levels to be mined this decade was to expand the existing surface plant and install more heat exchangers underground. Ultimately it was decided to make a change to an open-circuit system with a new surface refrigeration plant, because it offered a lower capital cost and lower operating costs. These advantages are mainly associated with reduced pumping and maintenance as a result of the elimination of the high-pressure heat exchangers.

The new refrigeration plant has been fully operational since the end of 1989. It has an initial capacity of 11 MW, with the potential to double this. Each Howden compressor is a 5 MW unit, and their effective duty is 11.5-12 MW of refrigeration. These two screw compressors, plate evaporators and five-cell condensers are located on surface, together with a precooling tower and three storage dams. The Sulzer precooling tower cools the hot water returning from underground to within 1|degree~C of the surface wet-bulb temperature.

The complete system is shown in the diagram, p 17. The temperature, |T.sub.1~, of the intermediate dam determines the initial flow rate of variable speed pump |P.sub.1~, and the number of compressor sets operating. Originally the plant was set up such that water leaving it should not exceed 1|degree~C or it would be returned to the intermediate dam and the water flow rate to the refrigeration sets would be reduced. That minimum temperature requirement increased to 4|degrees~C in winter. It has since been found that for current cooling demands 8|degrees~C is a perfectly acceptable temperature for the chilled water, which is supplied underground in open circuit to the main 2 megalitre cold-water storage dam 1,020 m below surface.

An important aspect of the design was to minimise energy losses by insulating the main pipelines with vapour-impermeable foamed glass and the installation of a Pelton wheel for energy recovery.

The direct-contact heat exchangers are designed to maximise the return water temperature and to adjust to seasonal cooling requirements. For the same refrigeration load a return water temperature lower by 1|degree~C means that more chilled water must be recirculated at a cost (calculated at the time the plant was installed) equivalent to $A3,500/MW/y. At the height of summer the chilled water flow rate is 58-63 litres/s. Towards winter that rate drops to around 20 litres/s. At present only one of the refrigeration sets operates, but once production from 3000 orebody really gets going, all this new capacity (around 120 litres/s) will be required.

Another new mine environment facility is the U62 shaft (the copper ore hoisting shaft) chiller plant. This 11-12 MW bulk air cooler will complement the refrigeration plant with the chilled air (at around 7|degrees~C) it sends underground. This must be commissioned by September 1993 to assist with the necessary cooling for 3000 workings.

Automating Hilton

Mr. Dick Potts, Hilton's mine manager, is convinced that automation in all sectors is the key to future productivity and efficiency. He aims "to do more with less" while at the same time making mining operations "quicker, cheaper and smarter". Tonnage must be increased and operating costs brought down. The underground work-force has already been reduced to 380, from 500, since the end of January 1991.

Much has already been achieved. In the 1991/92 financial year Hilton will mill 1.75 Mt, up from just over 1.15 Mt last year, while costs will have been cut by 40%. Its concentrator has a capacity of 1 Mt/y, the balance is trucked 20 km south to Isa for processing. Hilton intends to match Isa's costs. In its favour are higher grades than Isa's lead-silver-zinc ore, but it has to contend with much more discontinuous ground and more difficult mineralogy.

Hilton's mineable reserves amount to 9 Mt proven (150 g/t Ag, 6.2% Pb and 8.5% Zn) and 13 Mt probable (129 g/t Ag, 5.9% Pb and 8.5% Zn). In the identified mineral resource category (in-situ) are a further 56 Mt measured (101 g/t Ag, 5.4% Pb and 11% Zn) and 46 Mt indicated (89 g/t Ag, 5.4% Pb and 10.8% Zn).

Hilton ceased all cut-and-fill stoping last year. Production is currently coming from 12 Level, 720 m below surface, and is now either from open stoping or benching, with benching accounting for about 25% of the total. The benches are of varying height and strike length due to faulting and shearing in and around the orebodies. By mid-1991, some 52,000 t had been extracted from two benches in 2 orebody.

Hilton's biggest bench will be 4N50 in 4 orebody. This bench will be 175 m long, up to 28 m high, and will yield 118,000t of ore.

A specific Hilton development for bench stoping is the use of 'teleloaders'. These are used because the line of sight between the remote LHD operator and the muckpile is not always clear due to the folding of the orebodies, which are narrow and twisting. To overcome this problem, Hilton introduced the use of TV cameras on the LHDs. Two cameras, mounted front and back, provide pictures for the operator sitting in a cabin, often over 100 m from where the LHD is operating. This technology has now been introduced at Isa.

Experience has shown that operating the machines in this manner is faster and more efficient than standard remote control, as the operators have a much better idea of the LHD position at any one time. As a result the 'teleloader' achieves better bucket loading and faster tramming speeds.

The first trials were undertaken in January 1991, then, after some modifications to make the system simpler and more rugged, a machine started operations in November last year. During its first two weeks of operation the teleloader extracted 11,176 t of ore from 4 South 50 bench stope, 12D sub-level. It was thus achieving production rates of up to and above 2,000t/d. The average tramming cycle for that bench covered 250 m and the operator remained in the control room some 100 m from the machine at all times.

Since then, trials have been carried out with colour video cameras. This provides better picture definition, is easier on the eyes and performs better in the low-light conditions experienced in the stope. In the future it is hoped that one operator will be able to operate more than one LHD using the video remote system. Hilton is also working on introducing condition monitoring into its remote control systems.

Mt. Isa's first teleloader went underground in March and started work in 41E2 bench in the lead/zinc mining area, one of the largest benches containing some 70,000 t of ore. A second unit is being built and a third is under consideration for Mt. Isa, where the majority of the benches in the Racecourse area will be mucked by teleloaders.

Automated drilling

Four jumbos are employed for development, three Atlas Copco units and one Secoma Pluton 17 used exclusively on the decline to Hilton North. Two Pluton 17 rockbolters are used for roofbolting. Production depends on two Tamrock machines, a Datasolo 111006 and a Solo H606RA. The latter drills cablebolt holes and undertakes some production drilling. The rig can be used with both T45 and T38 rod strings drilling 89 mm or 70 mm holes respectively. The Datasolo is very accurate over the 26 m holes most commonly drilled, and can drill holes up to 38 m long. In bench drilling applications it accomplishes the task with no deviation at all. The mine finds its automation function very effective. Award conditions require for Australian workplaces compulsory meal breaks but this unit can be left drilling while the operator takes a break over eight shift changes. Automation allows performance to increase from around 100 m/ms to over 120 m/ms.

Similarly, Hilton is the first mine in the world to use Robbins Company's raise drill automation package (MM, October 1991, p. 231). Typically a raise drill must be shut down during shift changes and then restarted. Hilton's 63R can continue reaming operations unattended. Now the mine is examining possibilities for the automation of its hoisting systems.

Hilton will soon commission a major pump station on 8 Level to drain a surface aquifer. Last year it was pumping 6,000 |m.sup.3~/d, that requirement has now risen to 13,000 |m.sup.3~/d.

Hilton North

The long-term future of the Isa complex in silver-lead-zinc ore production will come to depend more heavily on Hilton. Its reserves are contained in seven known orebodies, compared with 14 at Isa, but they are higher grade. Further into the future it is expected that Hilton North will play an important role. An access and exploration project is underway including a 1.75 km drive from the existing Hilton workings which should hit the Hilton North target in about 13 months. Because there is great confidence that Hilton North will prove to be an economic prospect all this development is geared for future production.

Hilton North was discovered in 1980/81, of similar size and potential to the Hilton ore block. It is particularly promising as early estimates suggested higher zinc grades than the Hilton Block.

References

J.V. Bower, P.C. Edwards-Davis, B.E. Hall and T.P. Horsley, 'The development of an integrated mine planning system at Mount Isa Mines', 2nd Australian Conference on Computer Applications in the Mineral Industry, July 1991.

M.J. Howes, 'Review of ventilation and refrigeration in deep, hot and mechanized mines in Australia.' Transactions of the Institution of Mining and Metallurgy, Vol.99, A73-84.

MINE FACTS

Name: Mount Isa and Hilton

Ownership: MIM Holdings 100%

Location: North-West Queensland, Australia

Reserves (mineable): Mount Isa - 70 Mt at 3.2% Cu (proven), 50 Mt at 3.6% Cu (probable) and 45 Mt at 132 g/t Ag, 5.4% Pb and 6.6% Zn (proven). Hilton - 9 Mt at 150 g/t Ag, 6.2% Pb and 8.5% Zn (proven) and 13 Mt at 129 g/t Ag, 5.9% Pb and 8.5% Zn (probable)

Start up: Mount Isa started milling in May 1931, Hilton began trial mining in 1987

Mine life: Well into the 21st century

Mining methods: Sub-level open stoping and bench stoping

Ore production: Mount Isa 11 Mt/y and Hilton 1.8 Mt/y

Treatment: Mt Isa - separate copper and lead concentrators, conventional and Isasmelt copper and lead smelting Hilton - 1 Mt/y concentrator

Finished products: Current annual combined contained metal outputs of 180,000 t copper, 166,000 t lead, 239,000 t zinc and 14M oz silver; rising to 210,000 t copper, 210,000 t lead and 250,000 t zinc by 1993
COPYRIGHT 1992 Aspermont Media UK
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Mount Isa Mines Holdings Ltd.
Author:Chadwick, John
Publication:Mining Magazine
Article Type:Cover Story
Date:Jul 1, 1992
Words:7259
Previous Article:Yemen's gold deposits attract interest.
Next Article:Mt. Isa - processing and metallurgy.
Topics:

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters