The high-shear colloidal mixer, pioneered over 60 years ago, is internationally recognized as the most efficient method of mixing cement based grouts. Only recently, however, has this technology been applied to the production of cemented rockfill (CRF) for use in underground mines in North America.
Traditionally, paddle type mixers have been used to produce the cement based slurries required for the preparation of cemented rockfills. While considerable success has been achieved with these mixers, tremendous advantages are attained with the colloidal mixer. The combined effect of the highly efficient mixing action and the ability to mix low water/solids ratios allows for reductions in the cement content for a given strength requirement. Cement can also be replaced by cheaper fillers such as flyash or PFA resulting in significant cost savings.
Although large scale dedicated CRF plants are typical, there is a growing need for mobile plants capable of operating anywhere underground. The compact size and rapid batching speed of the high-shear colloidal mixers has allowed them to be incorporated into skid mounted systems capable of producing and discharging cement based slurries onto rockfill aggregate for the production of CRF. In many cases run-of-mine development waste can be taken directly to a portable plant located near the dumping face and used as CRF aggregate thereby eliminating the cost of waste removal.
INTRODUCTION
Typically one of two possible methods for preparing cemented rockfill (CRF) is employed by hardrock mines. The first method uses concrete batching technology whereby all the mix ingredients i.e. the binder, water and aggregate are all added together into a batching mixer such as a rotary drum, pan, compulsory or ribbon mixer. The second and more common method is to prepare the cement based binder in a slurry form, usually with a paddle mixer, and subsequently spray this binder onto the fill aggregate. While this method has been used for many years little attention has been paid to the actual preparation of the cement based slurries for the fill. Paddle mixers do yield adequate results, as evidenced by the number of mines using this technology, however a more efficient type of slurry mixer is available - the colloidal mixer.
Keller Colcrete successfully pioneered the development of the high-shear colloidal mixer in 1937 and for over 60 years it has been internationally recognised as the most efficient method of mixing cement based grouts. These mixers are used by the British Nuclear Industry for the preparation of special grouts used for the encapsulation of medium and low level radioactive wastes. The U.S. Army Corp of Engineers also specifies the use of colloidal mixers in water cut-off grouting applications for hydro dams.
Colloidally mixed grouts are readily used in the construction industry for work involving ground anchors, slurry and diaphragm walls, soilcrete jet grouting, soil nailing, duct grouting and slab jacking. Offshore marine platforms utilise the mixers in fabric formwork, pipeline protection and support, underbase grouting of platforms, pile annulus grouting and underwater grouting. In tunnelling, these mixers are used for ground treatment, compensation grouting and for the preparation of bentonite lubrication for pipejacking and TBM tunnelling operations.
It has been only within the last three years however that these high-shear colloidal mixers have been adapted for use in the preparation of cemented rockfill within the North American mining industry.
PRINCIPLES OF HIGH-SHEAR COLLOIDAL MIXING
Colloidal Mixer Design
Colcrete high-shear colloidal mixers are exclusively used by Thiessen Team in their CRF slurry batching plant designs. This particular mixer will be discussed below. Readers are referred to Houlsby (1990) for a more detailed design comparison of Colcrete and other commercially available colloidal and paddle type mixers.
Colloidal Mill
The key element of the colloidal mixer, depicted in figure 1, is the colloidal mill. The mill is comprised of a high speed rotor (or discar) operating at 2000 rpm coupled with a close fitting chamber housing. The discar is free to float horizontally on its mounting shaft with the internal fluid pressures centralizing it in the housing. The clearance between the discar and the walls of the housing is approximately 3 mm. It is here that a violent turbulence and high shearing action is created which is capable of breaking down clusters of dry cement particles (agglomerates). This ensures that a maximum interdispersion of fluids and solids occurs.
The colloidal mill acts both as a mixer and a centrifugal pump. In a CRF plant the colloidal mixer can thus directly discharge a mixed slurry either into a holding hopper or directly onto the rockfill aggregate in a haul truck or LHD. The colloidal mill is capable of generating a maximum discharge pressure of 200 kPa and a flow rate of up to 850 l/min. It is possible to increase the mills efficiency as a pump (thus giving it a higher pressure capacity) but this would reduce its efficiency as a mixer. This lower pump efficiency translates into more work being done on the material being mixed. In other words more energy is being input into the mix ingredients resulting in better mixing. Depending on the required batch size of the mixer, from one to four colloidal mills will be used. Each mill requires either a 22 kW electric motor or appropriately rated diesel or air motor equivalent.
Mixing Tank
The mixing tank, besides being a convenient receptacle in which to dump the ingredients, also acts as a centrifugal separator. The centrifugal action of the circulating material spins the heavy, unmixed, thicker grout towards the outside of the tank whereas the lighter portions of the mix, i.e. water and partly mixed grout, move inwards towards the throat of the tank where the vortex feeds into the mixer housing. Once through the high-shear mixer the material is then discharged tangentially into the outer part of the vortex amongst the relatively thicker grout where it migrates inwards to the vortex throat under the influence of the strong centrifugal separation process. Multiple passes through the rotor produce thicker and thicker grout until the entire mix becomes uniform and the centrifugal action can no longer separate differing densities. At this point the surface of the vortex has a smooth, uniform appearance. The vortex action created inside the tank also helps to rapidly assimilate any admixtures into the mixer when first added. Depending on the size of the mixer the entire mixing process can take as little as 15 seconds.
Feed Box
The feed box to the rotor housing is of ample proportion to ensure large lumps of unmixed cement at the start of the mixing cycle are able to pass through to the rotor. These lumps can be quite sticky on the first pass before being broken up. They are adequately handled by the Colcrete mixers as the rotor is hard up against the drum.
Figure 1: Components of a high-shear colloidal mixer
The output from the colloidal mill is split into two paths. Either the slurry is redirected tangentially back into the drum, to help create the vortex action, or it is discharged. Simple pneumatic or manual pinch valves are used to control this flow.
All of these properties can be attained with the use of high-shear, high-speed colloidal mixers. Kravetz (1959) explains that the high-speed shearing action combined with the centrifugal action of colloidal mixers thoroughly breaks up cement clumps and separates air bubbles, both of which slow the wetting process of cement grains. As a result, each grain is rapidly and thoroughly wetted and put into suspension. The mixing action also continually breaks away the hydrates that form on the surface of wetted cement grains exposing new areas to water. The hydrate elements that form are of colloidal size and as the amount of these elements increases the mixture becomes more colloidal in character.
Colloidal Suspensions
While the term “colloidal” is often applied to high-shear mixers and the slurries they produce, strictly speaking, the term is incorrectly applied. “Semi-colloidal” or “near-colloidal” are more accurate descriptions. A colloid is defined as a solid, liquid, or gaseous substance made up of very small, insoluble, nondiffusible particles (as large molecules or masses of smaller molecules) that remain in suspension in a surrounding solid, liquid, or gaseous medium of different matter. An example of this is smoke. The solids are certainly too fine to be seen, too fine to be removed by filter paper and will remain in suspension indefinitely.
Mayer (1959) measured the effect of high-shear colloidal mixing on cement grain size, particularly grains under 20 μm in size. The percentage of grains 5 μm in size was shown to be twice as large after high-speed mixing than with ordinary mixers, which accounts for the fact that the suspensions obtained are much more stable.
Practical Benefits of Colloidally Mixed Products
With respect to cement based slurries on the other hand, it is possible to filter out the solids (though perhaps not all if the cement is microfine) and individual grains can readily be seen. Particles will settle out leading to grout bleed. Cement slurries are thus not true colloidal suspensions.
The practical benefits of colloidally mixed grouts and/or slurries include:
• The grout or slurry mix is nearly immiscible in water. This allows the mix to resist washout or contamination with groundwater. The effect of the colloidal mixer, however, is to aggressively shear and break down individual cement grains and to make cement hydrates form of colloidal size such that the slurry exhibits colloidal properties, i.e. the slurry forms a stable suspension.
• The mix is stable and fluid enough to allow it to be pumped considerable distances.
• The slurry permeates uniformly into voids.
• Segregation of sand, if incorporated in the mix, is virtually eliminated.
Properties of High Quality Grouts and Slurries
A high quality grout or slurry is regarded as having the following properties (Houlsby, 1990).
• The grout or slurry has less settlement, i.e. bleed of the cement when stationary.
• Every particle of cement in the mix is thoroughly wetted (by the high speed shearing action of the mixer). Individual grains are separate from each other without flocs or clumps.
To illustrate the benefits of colloidally mixed grouts a series of comparative tests were conducted using grout prepared from type 10 ordinary Portland cement (OPC) and water. A Colcrete SD4 colloidal mixer and a Thiessen Team TC3100 paddle mixer were used to prepare slurry samples ranging from a 0.5 to 1.4:1 water:cement ratio. The samples were mixed for one minute in the colloidal mixer whereas they were mixed for 15 minutes in the paddle mixer. Cylinders 7.6 cm in diameter by 15.2 cm high were used for sample preparation.
• Each cement grain is surrounded by a film of water which chemically activates each particle, giving the full hydration necessary for strength and durability.
• The cement is thoroughly mixed with any other constituents of the mix or admixtures.
• The grout or slurry is uniform throughout.
Figure 2 shows the results of grout bleed measurements taken from the cured samples after 28 days. The differences between the mixers is pronounced.
• The mix exhibits some colloidal characteristics due to the maximum gel formation of the cement. colloidally mixed samples formed stable suspensions and consequently very little settling of cement grains occurred. Grout bleed was minimal at less than four per cent in the thinnest grout. The paddle mixed samples however had grout bleeds approaching 40 per cent in the 1.4:1 water:cement ratio mix. The cement grains simply settled out before significant hydration could occur. Clearly the low-energy paddle mixer was not able to adequately break down individual cement grains.
The samples were also tested according to ASTM D-2938-86 procedures to determine their unconfined compressive strengths. From the results, shown in figure 4, two conclusions can be drawn. First, the data points for the colloidally mixed samples exhibit less scatter than the data points for the paddle mixer prepared samples.
Figure 2: Comparison of cement grout bleed for samples prepared with colloidal and paddle mixers
This indicates that the colloidal mixer is capable of producing a more consistent, more uniform product. Secondly, the strength of the colloidally mixed samples shows an average 10 MPa strength improvement for a given density. The ability of the colloidal mixer to break down and wet all the cement agglomerates yields an improved 28 day compressive strength.
The improved performance of the colloidally mixed product has tremendous cost saving implications. For example, to produce a 25 MPa grout strength a colloidally mixed product requires a cured density of some 1650 kg/m3 whereas a paddle mixer prepared product requires a higher density approaching 1810 kg/m3 (from figure 4). Based on theoretical densities this translates to a 13 per cent reduction in cement content for the product prepared in a high-shear colloidal mixer. By the same token a 5 MPa grout requires 36 per cent less cement if prepared in a colloidal mixer. Clearly there is the potential to save significantly on the most costly component of CRF - the cement.
This is also evident in figure 3 which visually compares the 0.5:1 water:cement ratio samples. The colloidally mixed sample exhibits a uniform colour and texture and appears completely homogeneous whereas the paddle mixed sample shows a definite grainy texture with individual cement clumps visible. Colour variations were evident in all the paddle mixed samples.
Admixtures and Colloidal Mixers
The ability of colloidal mixers to accept admixtures and sands are also important benefits. Sands, i.e. fines can be incorporated directly into the mixer up to a maximum sand:cement ratio of 4:1 by weight. A minimum water:cement ratio of 1.12:1 is recommended in order to maintain a fluid sand/cement/water mix at this high sand content. Conversely, if no sand is added, water:cement ratios as low as 0.33:1 can be handled by the mixer. This also makes the colloidal mixer ideally suited for the preparation of grout used in cablebolting where the optimal water:cement ratio should be in the range of 0.35 to 0.4:1 (Hutchison and Diederichs, 1996).
Figure 3: Texture of 0.5:1 water:cement ratio grouts
ratios as low as 0.33:1 can be handled by the mixer. This also makes the colloidal mixer ideally suited for the preparation of grout used in cablebolting where the optimal water:cement ratio should be in the range of 0.35 to 0.4:1 (Hutchison and Diederichs, 1996).
Figure 4: Comparison of 28 day strength for cement grout
samples prepared with colloidal and paddle mixers
If sands are to be directly incorporated into the mixer then the sand should be clean, sharp and natural with a maximum particle size of 5 mm. The sand should be well graded with a good distribution across the grading curve. If material up to 10 mm size is incorporated then some settling of the sand will occur.
Flyash, PFA and even silica fume can all be incorporated into the mixer along with the cement. For maximum advantage these materials should have a particle size less than or equal to the cement particle size. Flyash should show no more than 8 per cent loss on ignition. Admixtures, lime and even bentonite can all be mixed with the colloidal mixer.
Factors Contributing to CRF Performance
It has been shown that for a given strength requirement a savings in cement usage can be realised by using a high-shear colloidal mixer. The colloidal mixer gets the maximum benefit out of every cement grain thus higher strengths are achieved.
For CRF, all things being equal, a higher quality slurry can thus be expected to produce a stronger rockfill. This needs to be verified on a site specific basis, however, as no two mines produce cemented rockfill in precisely the same manner.
Besides the quality of the slurry, a number of other factors also contribute to the final in situ strength of placed CRF. These factors include (Yu and Counter, 1983, Yu, 1989, Reschke, 1993, Farsangi, Hayward and Hassani, 1996):
• Cement content
• Water:cement ratio of the slurry
• Nature and quality of admixtures such as flyash
• Degree of mixing between the cement slurry and fill aggregate
• Composition and quality of aggregate
• Aggregate size distribution and percentage of fines
• Segregation of material during placement
• Attrition of aggregate during transport and placement
• Aggregate temperature (i.e. freezing conditions)
There are a great number of factors contributing to the strength of a cemented rockfill. However, because of the obvious importance of the cement itself, the colloidal mixer with all its advantages offers a great opportunity for long term cement (and cost) savings in any CRF operation.
CURRENT COLLOIDAL MIXER BASED CRF PLANTS
The use of high-shear colloidal mixers for CRF slurry plants within the North American mining industry was pioneered by Thiessen Team, who have designed and built five plants for underground hard-rock mines in the past three years. These systems and their respective mines are highlighted. Lamefoot Mine, Republic, Washington (U.S.A.) Located in northeastern Washington state, the Kettle River Operations of Echo Bay Minerals was brought on-line in 1990. A centralized mill has been processing ores mined from a series of small deposits located in the area. Underground mining is currently under way on the fifth and highest grade deposit, Lamefoot.
Lamefoot is a 1350 tpd longhole open stoping operation. Due to the reasonably high gold grades, a cemented rockfill was selected early on in the mine design process to maximize extraction without losing reserves in irrecoverable pillars. The Lamefoot mine was the first operation in North America to utilise a colloidal mixer as the heart of its CRF slurry batching plant. Constructed in 1995, the batching plant is located underground near the main portal. The primary cement storage silo is situated just outside of this portal and uses a blower to charge a small 16.3 tonne surge silo underground which in turn feeds the slurry plant. Both the cement and water are weigh batched in separate hoppers that feed into a Colcrete SD1000 (1000 liter) colloidal mixer.
The entire system is PLC (programmable logic controller) based which is designed to operate the plant unattended. A 0.8:1 water:cement ratio batch of slurry is prepared in the plant using 680 kg cement and 545 kg water. This is initiated remotely with a pull-bob system by the haul truck operator upon approach to the plant. Total batching time takes 5 to 7 minutes.
The haul trucks have a capacity of 21.2 tonnes but typically hold 11.8 tonnes of rockfill aggregate. The prepared grout slurry is dispatched through a spray bar directly onto the aggregate in the truck box. The discharge is initiated by a second pull-bob located at the spraybar assembly. Discharge takes less than 1 minute after which the PLC initiates a cleaning cycle, flushing a small quantity of water through the mixer and discharge lines.
Run-of-mine waste is used as aggregate with a maximum size typically below 45 cm. While daily fill production has reached as high as 1100 tpd, the average daily fill rate is considerably lower. A monthly fill rate of 10 000 tonnes is typical.
Because the cemented rockfill is end dumped over the stope face, some segregation occurs as the aggregate falls. This has been seen in subsequent exposures. Nonetheless, excellent performance has been achieved. Backfill strength is monitored regularly through underground sampling. Cylinders 15 cm diameter are prepared from the dumped rockfill (excluding aggregate greater than 7.5 cm). Compressive strengths of 4.8 MPa are typical (Thompson, 1997).
This high in situ fill strength is evidenced in fill exposures as great as 25 m high by 40 m along strike which stand intact with minimal dilution. Part of the success is attributed to the quality of the slurry prepared by the colloidal mixer.
Myra Falls Mine, Campbell River, B.C. (Canada)
Westmin Resources Myra Falls mine is located at the geographical center of Vancouver Island. The mine has been in production since 1966 and during that period has produced approximately 17 million tonnes of polymetallic massive sulphide ore at an average grade of 2 gm/t gold, 58 gm/t silver, 1.9 per cent copper, and 5.2 per cent zinc (Pearson, 1977). Current mineable reserves are roughly 9.1 million tonnes at a scheduled production rate of 3650 tpd.
Historically, the mining methods used were primarily room and post-pillar along with traditional cut-and-fill. Since 1991 however, a move was made towards bulk mining methods in order to reduce costs. While cemented hydraulic fill is used extensively, the mine is currently in the process of developing a new high-density fill system.
Myra Falls Mine, Campbell River, B.C. (Canada)
Westmin Resources Myra Falls mine is located at the geographical center of Vancouver Island. The mine has been in production since 1966 and during that period has produced approximately 17 million tonnes of polymetallic massive sulphide ore at an average grade of 2 gm/t gold, 58 gm/t silver, 1.9 per cent copper, and 5.2 per cent zinc (Pearson, 1977). Current mineable reserves are roughly 9.1 million tonnes at a scheduled production rate of 3650 tpd.
Historically, the mining methods used were primarily room and post-pillar along with traditional cut-and-fill. Since 1991 however, a move was made towards bulk mining methods in order to reduce costs. While cemented hydraulic fill is used extensively, the mine is currently in the process of developing a new high-density fill system.
Within the past two years development has focused on newly delineated ore zones located some distance away from the existing underground infrastructure. Here mining methods called for longhole open stoping with a consolidated fill in primary stopes. Scheduling requirements dictated the need to be producing ore from secondary stopes relatively early on in the mine plan however no backfill was available. Unfortunately delays were experienced in the implementation of the new fill system.
To meet the short term requirements a small scale portable CRF slurry system was brought in.
This slurry batching plant is designed around a Colcrete SD24 (680 liter) colloidal mixer and is mounted on skids to be fully portable underground. As the mine is accessible by shaft only the major components can simply be unbolted into cageable sizes. Fully constructed the plant is under 4.2 m high.
A 6.5 tonne holding hopper contains the cement for the plant. This hopper is in turn filled by 0.9 tonne bulk bags of cement through a bag unloading bin and feed conveyor. The PLC monitors the cement content of the holding hopper by way of a radio frequency probe. The hopper is aerated to promote cement flow.
The key to the success of this design is the placement of the entire colloidal mixer platform atop load cells. The colloidal mixer hopper thus functions as a mixer and as a scale to accurately weigh the cement and water. This layout optimises the size of the system and increases the batching speed as the cement is constantly being mixed as it is augered into the mixer.
The PLC controls all facets of the plant operation including the batch recipes for either truck size or LHD size quantities of slurry. The slurry is discharged through a spray bar assembly. On completion the mixing tank partially fills with water and the colloidal mill is run to purge itself, the mixing hopper and all delivery lines. In this regard the system is able to operate with minimal human intervention and remain clean. By skipping the purge cycle the system is capable of discharging a full batch of slurry every two minutes in what is termed the “bumper-to-bumper” mode.
A five per cent CRF utilising run-of-mine development waste was produced and used in the extraction of one pillar block. Soon after the successful recovery of this pillar the plant was retired. The infrastructure was finally developed for the mines conventional cemented hydraulic fill and despite the positive financial return on the CRF venture, hydraulic fill was deemed to be more cost effective in the long term.
Polaris Mine, N.W.T. (Canada)
Cominco’s Polaris mine is an underground lead-zinc operation that has the distinction of being the world’s most northerly base metal mine. It is situated on Little Cornwallis Island in the Canadian High Arctic at a latitude of 75º 23’N.
Polaris Mine, N.W.T. (Canada)
Cominco’s Polaris mine is an underground lead-zinc operation that has the distinction of being the world’s most northerly base metal mine. It is situated on Little Cornwallis Island in the Canadian High Arctic at a latitude of 75º 23’N.
The primary mining method used is sub-level longhole open stoping with backfill. Stopes and pillars are 15 m and 18 m wide respectively and are laid out along the strike of the orebody. Overall sizes range from 100 to 150 m in length and from 60 to 110 m in height. Stopes are mined full length in 30 m lifts while the pillars are mined full length in 25 m stages. Raisebore holes 1.8 m in diameter are drilled from surface to the stopes and are used for ventilation, slotting and backfilling.
The existing fill system utilizes a 50/50 mix of quarried surface rock (dumped through raisebore holes) and underground waste rock. The fill is mixed with 11 per cent water by weight and pushed into the stopes where, after 1.5 to 2 years, a frozen 3 MPa fill is attained. During the summer months, a wetter, more flowable backfill mixture is dumped from surface into the tops of previously filled stopes to ensure a tight fill to the hangingwall. This backfill takes considerably longer to freeze because of the latent heat of fusion associated with the higher moisture content.
In 1995 mine personnel decided to examine the possible use of CRF to supplement the frozen rockfill system. Future pillar extraction plans called for a backfill that would offer superior stiffness properties (for improved ground control) and be capable of developing high early strengths (for increased cycle times) as compared to the existing frozen fill. Although the use of CRF in mining is relatively common, Polaris faced the added challenge of making a reliable cemented product in permafrost conditions where surface temperatures range from a low of -55 ºC in the winter to a high of +10 ºC in the two month ice free summer.
Due to the severe attrition associated with 250 m dump heights, the surface quarried aggregate is screened to a size range of 12.5 to 200 mm. After dumping, tests have shown this yields a placed aggregate with approximately 45 per cent passing 10 mm (Dismuke and Diment, 1996).
With vertical exposures required of up to 100 m, a minimum compressive strength of 2.5 MPa was deemed necessary for the cured rockfill. In order to achieve this strength in situ, a controlled lab strength of 5 MPa was targeted. Ultimately, a CRF product capable of developing 4.9 MPa in the lab was developed using:
• 5 per cent type 30 high early strength cement (by weight of aggregate)
• 2.5 per cent calcium chloride (by weight of cement)
• 0.7:1 water:cement ratio
The use of calcium chloride as an accelerator is limited, in most civil engineering applications, to two per cent by weight of cement. Higher dosages give decreased concrete strengths at later stages due to the break down of calcium chloride over time. However, at the colder temperatures found at the mine site, the break down occurs considerably slower and is negligible in terms of the required life span of the fill.
Aggregate heating is also critical to the success of the CRF fill as winter temperatures can result in the aggregate being as cold as -30 ºC. To meet the fill requirement of 3000 tpd, a 3 MW hot water aggregate heating circuit was designed, capable of warming the aggregate at a rate of 125 tph to a final temperature of +15 ºC.
To produce the required slurry, a 2000 liter capacity colloidal mixer was developed by Thiessen Team in conjunction with Keller Colcrete. The largest ever produced, this mixer incorporates four colloidal mills to provide rapid, efficient mixing of the water, cement and calcium chloride.
As the entire CRF plant is located on surface, the complete circuit is enclosed and fully heated. Heated aggregate is stored in a surge bin which conveys the rock to a fully enclosed and heated load out structure. The original design called for the aggregate to be dumped through a “ladder” mixing chute (a chute with angled baffle plates on the walls) with the slurry being sprayed on the aggregate as it enters the chute. It was believed this “mixer” would improve the mixing of the slurry and the aggregate. In practice however, freezing problems were encountered in the chute and now the slurry is dumped directly onto the aggregate in the truck box.
The CRF haul trucks have a dump box which is heated with engine exhaust. From the load out, the truck proceeds to the raisebore hole and dumps the CRF into the stope. The system became fully operational in March 1996 and has been successfully used to eliminate post-pillars in one zone of the mine. Seven pillar stages have been filled to date with some 265 000 tonnes of fill. Five full face exposures of the CRF have been made, the largest being 90 m high by 18 m wide. Results have been very favorable with fill dilution well below five per cent.
Part of the success has been attributed to the placement of the raisebore holes. Since the CRF is dumped from surface the compacted zone of fill (directly under the raisebore hole) typically has the highest strength. This is positioned immediately adjacent to the next mining face.
Due to the success to date, future plans call for a ten per cent reduction in the cement content of the CRF to 4 per cent in an attempt to reduce costs. This reduction is based strictly on favorable in situ observations of the placed CRF.
Meikle Mine, Carlin, Nevada (U.S.A.)
The Meikle mine is located in the northeastern part of Nevada in the famous gold-rich Carlin Trend. Owned and operated by Barrick Gold, the orebody was first discovered in 1989 and was brought into full production late in 1996 at a scheduled mining rate of 1900 tonnes/day. There are 10 years of reserves at a gold grade in excess of 24.3 gm/tonne. Although the orebody is relatively shallow, only 300 to 600 m below surface, the ambient rock temperature averages 60 ºC thus necessitating the installation of a 10 MW refrigerative plant (White and Kral, 1994).
Although some cut-and-fill mining may be required where geometry dictates, the predominant mining method is sublevel longhole open stoping with consolidated backfill. Cemented rockfill was selected as the optimal method of backfilling.
The primary backfill plant on site consists of a 7.6 m3 Besser ribbon mixer. Aggregate is produced from pit waste that is shipped directly to a crushing and sizing plant. A single product is produced consisting of less than 5 cm size particles with 40 per cent passing 9.5 mm. The aggregate is transferred dry to the underground backfill plant through three 305 mm I.D. vertical transfer pipes located in the ventilation shaft. A CRF strength of 4.1 MPa is targeted for primary stoping where the fill will not be undercut. A 6.9 MPa strength is required for underhand cut-and-fill mining areas.
A small scale CRF slurry system was acquired as a back-up to the primary system to ensure production requirements would be met. A Colcrete SD1000 colloidal mixer based system was selected. The mixer is located adjacent to the main CRF plant diverting the cement feed from the Besser mixer into a small aerated surge bin. The colloidal mixer is mounted on load cells to weigh the water and cement. The cement is mixed as it is augered into the colloidal mixer hopper thus reducing batch times.
A pulse jet dust collection system de-dusts the plant and feeds the reclaimed cement back into the feed auger to the mixer. The CRF slurry system went on-line in January 1997. Typical cement contents are 6.3 per cent by weight of fill. Spray bars are located on two separate levels within the mine. A batch of slurry is initiated remotely with call buttons and varies from 455 kg to 545 kg of water per 725 kg cement depending on the level. The slurry is sprayed onto approximately 11.5 tonne loads of run-of-mine waste rock in 14.5 tonne haul trucks. While typical daily production rates are lower, this back-up system has been called on to produce in excess of 1600 tonnes of fill in a single day.
Following the discharge of a batch of slurry the plant weighs the water to be used in the next batch and utilises this water to purge the discharge lines, scour the mixer and flush the spraybar. The discharge return lines actually pressure-wash the interior of the mixer through an axial spray nozzle. This allows the plant to operate virtually maintenance free.
Sullivan Mine, Kimberley, B.C. (Canada)
Cominco’s Sullivan mine is located in the southeast corner of British Columbia and is one of the largest and longest operating underground lead-zinc mines in the world. In excess of 135 million tonnes of ore averaging 6.5 per cent lead, 5.6 per cent zinc and 76.5 gm/t silver, has been produced since the discovery of the orebody in 1892.
The orebody is roughly circular in plan measuring 2000 m in diameter with an average dip of 30º and an ore thickness ranging from less than 1 m to over 100 m. Room-and-pillar mining methods were used in the past with pillar recovery beginning in the mid 1940’s. Mass pillar recovery techniques are currently employed along a caving or retreat front across the entire width of the orebody. The caving/retreat process is required to enable complete extraction of the pillars while maintaining an orderly and controlled cave of the hangingwall. The retreat is now 1200 m long and extends to 400 m below surface. The mines current production rate is 7100 tpd of which 85 per cent comes from this pillar recovery.
The remaining ore comes from a zone independent of the main mining area. This low dipping fringe area is currently being mined by mechanized room-and-pillar methods. Reserves were initially estimated at 1.8 million tonnes grading an average 9 gm/tonne silver, 4.3 per cent lead and 13.1 per cent zinc (Rodenstein and van Ooteghem, 1993). Because of the high grade within this area, an economic study concluded that the implementation of a CRF system could be justified to mine out one particular pillar area.
Due to the remote location of the mining area, a skid mounted CRF plant virtually identical to the Myra Falls Operation portable plant was selected (figure 5). This system was slightly more elaborate, incorporating a larger 1000 liter colloidal mixer as well as a bulk bag unloader and pulse jet dust collection system (not shown in figure 5). The bulk bag unloader is comprised of a holding frame, dump bin and screw conveyor which fills the 6.5 tonne cement hopper of the plant. The PLC controls bag flexors on the holding frame to fully discharge the cement from the bottom dump nozzle of the bulk bag. The dust collection system scavenges cement dust from the bulk bag unloader, the cement
Figure 5: The Thiessen Tornado 1000 - A PLC controlled grout slurry plant
with 6.5 tonne cement hopper and 1000 liter colloidal mixer
storage hopper and the mixing tank of the colloidal mixer. Reclaimed dusts are returned to the hopper feed auger. Other than placing bulk bags in the holding frame the entire slurry system operates unattended. The system is currently being commissioned and thus no performance data is yet available.
The mining strategy is to drive beneath the pillar area and develop drawpoints for the pillar extraction. Some 80 000 tonnes of CRF will be required for hangingwall support around the zone. The projected recovery is 85 per cent of the pillar area yielding approximately 125 000 tonnes of high grade ore. Run-of-mine development waste will be used for aggregate.
A 0.78:1 water:cement ratio slurry will be batched in the colloidal mixer using 160 kg water and 205 kg cement. This will be discharged into a LHD bucket carrying some 3.6 tonnes of waste. With a 5.6 per cent cement content in the fill it is expected that a high quality stiff product will be produced providing the necessary hangingwall support for pillar extraction.