Abstract:
An apparatus for producing high density slurry and paste backfills for use, for example, in mining operations. The high density slurry or paste is produced from a mill tailings mixture in a silo which includes a percolation and decant apparatuses for percolating water out of the mill tailings mixture and for decanting clarified water from atop the settled tailings. While settling occurs, air is introduced from the bottom of the silo to agitate the mixture to ensure substantially homogeneous settlement of the solids. Once settled, the resultant high density slurry or paste is fluidized by air in order to give the paste more readily flowable characteristics.

Description:
This application is a continuation-in-part of application Ser. No. 08/786,965 filed Jan. 24, 1997, now U.S. Pat. No. 5,888,026 and entitled “Backfill Paste Production Facility and Method and Apparatus for Producing High Density Slurry and Paste Backfills”. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for producing high density slurry and paste backfills for use in mining operations and to backfill paste production facilities which incorporate the aforesaid method and apparatus. 
     BACKGROUND OF THE INVENTION 
     There are many different reasons for using backfilling in mines and not only to the benefit of the environment. In some cases, creating an opening underground poses no problem other than with the disposal of waste materials. However, in most types of rock, when an opening is created, it causes stress realignment around the opening. This not only creates problems with spalling and rock falls during rockbursts but also limits the size of the opening that can be made. 
     Some of the reasons backfill is used in mines (not in any specific order): 
     (a) to keep highly stressed rock around an opening from spalling. Spalling not only dilutes the ore but it can cause hazards in stopes where people are working; 
     (b) to keep negatively stressed rock (in tension) around an opening from coming loose for the same reasons in (a) above; 
     (c) to absorb some of the excess stresses in order to minimize damage from rockbursting; 
     (d) to act as pillars in some types of mining to permit removal of more ore; 
     (e) as a working platform for personnel and equipment in undercut and fill operations; 
     (f) to prevent surface subsidence in shallow mines or soft rock mines; 
     (g) to alleviate environmental hazards associated with surface disposal of waste materials; and 
     (h) to dispose of large quantities of mining wastes underground. 
     Various traditional materials have been used for backfill, the choice of which is usually dependent upon the reason for backfilling and on cost. Some of the materials include but are not limited to: 
     (a) cemented rockfill (stiff fill); 
     (b) uncemented esker sand; 
     (c) gravel; 
     (d) uncemented classified mill tailings (cycloned to separate the very fine particles or slimes) and unclassified (total tailings) in hydraulic form (40% to 55% solids content); 
     (e) cemented classified and unclassified mill tailings in hydraulic form; and 
     (f) a combination of esker sands and classified and unclassified tailings. 
     With the growing environmental concern for tailings disposal on surface, there has been a growing interest in using the tailings for mine backfill whether or not backfill is necessary for ground control purposes. Mill tailings have traditionally been used for backfilling of mines but usually in a classified form. Once classified, the finer portion of the material (slimes) was then sent to the surface disposal site or tailings pond. However, in recent years, mine operators have seen the need to dispose of their total tailings in the form of backfill. 
     The use of hydraulic tailings fill entails considerable preparation before the filling of the opening can commence. First a bulkhead must be constructed which will hold back the hydraulic mass while allowing water to percolate out of the fill. In some larger mines, drainage pipes such as extruded plastic weeping tile are hung from the top of the stope and brought together under the bulkhead. If a large stope is being filled, a plug is often poured first with a higher cement content (up to 30%) to just above the bulkhead. When the plug is set, the remainder of the stope is filled with the weaker cemented fill (normally around 10% cement). Such extensive preparation is costly in terms of both time and capital. 
     Another major cost associated with the use of hydraulic backfill is the water that leaches out of the fill mass must be pumped back up to surface. If the mining method used calls for mining to progress against the backfill, sufficient time must be allowed for the fill to drain and cure which can typically be on the order of from 28 to 56 days. 
     As alternatives to hydraulic fill, high density slurry or paste backfills may be used. High density slurry is simply a thicker hydraulic fill. It has a solids content in the range of 60% to 70% rather than the 40% to 55% range of hydraulic fills. The lower water content requires less extensive bulkhead construction, exhibits faster percolation times, requires less water to be pumped back up to the surface, and requires less cement to achieve the same strength as hydraulic fill. 
     Paste backfill is an even higher density tailings material in the range of 76% to 84% solids content, depending on the size gradation of the material. By definition, a paste is a material that does not exude water after it has been placed. Such a material has many benefits when used as a backfill in that it does not require water to pumped back up to the surface, lighter or even no bulkhead construction, much less time to resume mining, and it only uses 1.5% to 3% cement. The main drawback in the use of pastefill is that due to its consistency, it is difficult to transport. 
     Prior Art High Density Slurry/Paste Fill Systems 
     Currently known systems used to thicken hydraulic tailings slurry to a higher density or paste employ commercially available drum filters, thickeners and/or blend the slurry with alluvial sand to produce paste. While the sand is used in the process to try to achieve a higher strength due to the particle size of the sand, excavation of alluvial sand raises environmental issues. 
     The thickener system starts out with the tailings in a storage silo. From the storage silo, the tailings are dumped into the thickener. Once the mixture has thickened sufficiently, it is dumped to a mixer where cement is added. After mixing, the resultant backfill material is either pumped or gravity fed to the underground stopes. This thickener system prepares paste in batches as it takes time to remove the water in the thickener. Therefore, if a continuous backfilling system is required, two tailings storage silos and two thickeners are required. The main drawbacks associated with this system are that the thickeners are very costly, i.e. up to $1,000,000 each; being mechanical devices, they require a high level of maintenance; due to the various factors which can affect the thickening process, the fill plant operator must be consistent in judging the release time of the fill from the thickener; thickeners are large and require considerable floor space; and the energy costs to operate such thickeners are relatively high. 
     The drum filter system begins thickening in the same way, with hydraulic tailings coming from a storage silo. The slurry is fed from the silo to one or more drum filters. These filters use a permeable membrane and air to remove most of the water from the slurry, leaving a cake of tailings (filter cake) on the drums. The actual paste production involves re-pulping the filter cake by adding a controlled amount of water and cement and mixing to obtain the proper consistency. The material is then poured or pumped from the mixer to the stopes. 
     While the initial cost of a drum filter system is much lower than that of a thickener, the drum filter system still suffers from some major disadvantages which include the large floor space required for the storage of the filter cakes; personnel required to handle the cake between filter, storage and the re-pulping process; the maintenance costs of the filters; and the fact that the operator has less control due to the handling required and the resulting extra personnel involved. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks and disadvantages by providing an integral containment vessel and high density slurry or paste producing unit which accepts the mill tailings and processes them into a backfill paste ready to be mixed if desired with a setting agent for use as a backfill in mining operations. 
     As the mine tailings slurry enters the vessel, it immediately begins a settling process. This process, by gravity settling alone, normally requires 24 to 48 hours. Due to the fine nature of the material it is very slow to settle and must be agitated to avoid uneven settling of particles and to produce a homogeneous end product. While it is known to use water jets to agitate slurries, with water jets, the settled tailings are fluidized in the bottom of the vessel, rat-holing frequently occurs due to plugging nozzles and insufficient local fluidization, and the addition of water is counter-productive and limits the maximum pulp density (solids content) of the end product. 
     In the present invention, while the slurry is being dewatered, air is injected into the slurry to fluidize the settled tailings. Solid particles are fully fluidized by the air from the bottom to the top of the vessel, resulting in a paste with uniform particle size distribution and viscosity. The fluidized paste is pumpable and the paste in the vessel can be made to flow under the influence of gravity in a smooth and stable manner. Furthermore, it has been found that it is possible to obtain even higher pulp densities by fluidizing tailings settled in the silo as compared with gravitational settling methods. 
     In normal operations, the output of tailings from the mill is approximately 30% solids, the remainder is water. To achieve a paste backfill, the slurry must be thickened to between 67% and 87% solids, depending on the particle size distribution and the specific gravity of the incoming material, thus requiring a substantial amount of the water to be removed. By speeding up the removal of water, the length of time for the overall process can be reduced substantially. 
     In order to increase the rate at which water is removed from the mill tailings, a combination of decanting and percolating of the water in the slurry material is effected. During initial settling of the material, particulate material gravitates to the bottom of the vessel leaving clearer water with low solids content at or near the top which is decanted. Due to hydraulic forces and the weight of solids above, further water in the material is exuded in the lower levels. This percolated water is also removed until a predetermined solids density has been attained. Left to nature, a graduated mass would form in the vessel since the coarser material tends to settle first. In order to achieve a homogeneous paste, the material is agitated with air introduced through nozzles located in the lower portion of the vessel. The air also acts to fluidize the material such that it may be readily removed from the vessel once it has become a uniformly-graded paste. 
     In order to achieve this, there is provided in one embodiment of the present invention a novel device which will be referred to herein as a percolation/decant insert. The percolation/decant insert, as the name implies, is a dual purpose unit which allows the water in the slurry to percolate into the device during settling and also decants the water left on top of the solids as a result of the settling. As the solids settle in the silo, clear water is left on the surface. Rather than manually inserting a suction hose to pump this water or waiting for the solids to discharge and pouring the water out later, it can be removed, i.e. decanted, during the process. In general, the percolation/decant insert comprises a screen portion through which water in the slurry can percolate, a variable height decant portion atop the screen portion through which the clearer water at the surface of the slurry can be decanted, and means for removal of the decanted and/or percolated water. 
     The use of the percolation/decant insert increases the dewatering rate significantly as compared with a similar vessel utilizing a combination of gravity settling and drawing off of clarified liquid. Not only is the thickening time considerably reduced, since the throughput of the vessel can thereby be significantly increased, vessel sizes can be comparatively smaller and still out-produce conventional gravity settling vessels in paste production. It has also been found that the final product can be made up to 5% more dense than with conventional methods. 
     While the operating containment vessels may be newly constructed, since most mining operations already have in place existing storage silos for receiving and dispensing the hydraulic tailings material the elements of the present invention may be readily adapted for implementation therein. 
     The primary advantages in using such a system is that the process can take less than 25% of the normal time required to densify tailings; no additional buildings or other such structures are required for the plant if holding silos already exist; due to the very few mechanical components, less maintenance is required; the system is compact and uses only slightly more floor space than required for the silo(s); the system is easily expandable, because of its modular nature. e.g., a 2 silo continuous system can have its capacity increased 50% by the addition of one more silo; the system is readily completely automated; there are comparatively very low manufacturing and installation costs as compared with present options; and the system requires very little energy to operate. 
     The present invention also relates to a method for producing high density slurry or paste backfill which method is embodied to a certain extent in the manner in which the aforementioned apparatus functions. The invention further includes a high density slurry or paste backfill production facility which incorporates the novel integral containment vessel and high density slurry or paste producing unit. 
     In another embodiment, the integral high density slurry or paste production unit is provided with separate decant and percolation apparatuses. The decant apparatus comprises a decant screen and an associated launder tray disposed along the periphery of a feed cone positioned at the upper end of the silo. The feed cone is intended to speed settling and provide clearer decant water. A greater area for decanting is provided by placing the decant screen around the circumference of the feed cone, thereby speeding the decanting. The feed cone incorporates a feed well which permits the introduction of slurry material centrally with respect to the silo. More importantly, the feed well introduces the fresh slurry below the surface of the settling slurry during decanting to minimize turbulence and mixing with the clarified water to be decanted, thereby further expediting that stage of the dewatering process. By providing a additive input line in association with the feed well, various chemical additives, flocculents, dispersants, etc. may be diffused into the slurry at the time of its input. 
     The percolation apparatus of the second embodiment is provided in openings in the walls of the vessel instead of centrally as in the first embodiment. This allows a much greater area for water percolation and facilitates maintenance. A plurality of percolation screens, each mounted in a frame and supported by an enclosure, are disposed spaced-apart in the silo walls. Water which percolates through the percolation screens is drained through a water discharge outlet located in the enclosure. 
     Both the percolation screens and the decant screens of the second embodiment are provided with means for removing particles that become lodged in the screen passages. Preferably, vibrating pneumatic hammers are provided which, when operated, shake the screen, thereby dislodging trapped particles. 
    
    
     These and other features of the invention will become apparent from the detailed description set out hereinbelow and the accompanying drawings in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of the integral containment vessel and high density slurry or paste producing unit according to a first embodiment of the invention, partially broken away to reveal the percolation/decant insert and the interior and exterior features; 
     FIGS.  2   a  and  2   b  are side elevations of the integral containment vessel and high density slurry or paste producing unit as shown in FIG.  1  and illustrating the operation of the percolation/decant insert; 
     FIG. 3 is a schematic diagram of a high density slurry or paste backfill production facility shown incorporating the first embodiment of the integral containment vessel and high density slurry or paste producing unit; 
     FIG. 4 is a schematic diagram of another high density slurry or paste backfill production facility which incorporates more than one of the first embodiment integral containment vessels and high density slurry or paste producing units; 
     FIG. 5 is a side perspective view of the integral containment vessel and high density slurry or paste producing unit according to a second embodiment of the invention, partially broken away to reveal the interior and exterior features; 
     FIG. 6 is a side perspective view of the feed cone, feed well and launder tray section of the second embodiment as shown in FIG.  5  and similarly partially broken away; 
     FIG. 7 is an enlarged, side perspective view of the feed well of the second embodiment, again being partially broken away for purposes of illustration; 
     FIG. 8 is an enlarged, cross-sectional view of the launder tray which is used to decant water from the containment vessel; 
     FIG. 9 is a magnified, perspective view of a portion of a screen useful in the present invention as a decant screen and/or a percolation screen; and 
     FIG. 10 is an enlarged, cross-sectional view of the percolation apparatus of the second embodiment of the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown an integral containment vessel and high density slurry or paste producing unit  10 , comprising in this case a silo  11  fitted with a percolation/decant insert  12  and an air fluidization/agitation system  14 . The silo  11  shown includes a conical bottom  13 , but may have a hemispherical or other shaped bottom. 
     The percolation portion of the device is a custom built water-well screen  16 . These screens preferably consist of a cylindrical, vertical, stainless steel framework with bands of stainless steel wrapped around the framework and machine welded. A specified gap is left between the bands (for example, a #8 screen has a 0.008″ gap). The porosity of the screen or screen number used depends on the size of the material being settled. 
     A conduit  18  at the bottom of the percolation screen  16  allows water which percolates through the screen to be removed from the vessel by means of gravity. Alternately, a stainless steel submersible pump could be placed in the bottom of the percolation screen  16  and connected to a current sensing switch. When there is sufficient water in the screen  16 , the pump will activate and pump the water out of the silo and back to the original process for reuse. 
     Above the screen  16  is the decant portion  20  of the device. In general, the decanter  20  is an adjustable length conduit which extends above the screen  16 . In the first embodiment, a light coil spring  22  is attached at one end to the top of the percolation screen  16  and the other end to an annular plate  24 . The plate  24  has an aperture through which the surface water may be decanted. The spring  22  is covered preferably with a ½″ thick, rubber-like material  23 , such as LINATEX™ brand. 
     The annular plate  24  may be raised and lowered by any form of suitable activation means. As shown in FIG. 1, a pneumatic or hydraulic cylinder  30  is used as the activation means for the decanter  20 . A pneumatically operated system is actually preferred since a source of compressed air is already needed for use with the agitation/fluidization system  14 . The pneumatic cylinder  30  is mounted by way of a mounting plate  26  to the upper closure  28  of the silo  11 . The cylinder piston  32  is attached to a series of connecting rods  34  through upper frame  36 . The connecting rods  32  extend through bushings  38  and through the upper closure  28  and mounting plate  26  to connect with the annular plate  24 . Thereby, activation of the cylinder  30  causes piston  32  to move and, hence, causes the annular plate  24  to be raised or lowered accordingly via the connecting rods  34 . Lowering of the height of the annular plate  24  will allow the clear water on top of the solids to decant into the percolation screen. This water is removed from the bottom of the percolation screen in the same manner as the percolation water. 
     To provide additional support and stability to the percolation/decant insert, support members  40  may be provided between the percolation screen  16  and the sides or bottom of the silo  11 . 
     Depending on the nature of the tailings being thickened, it may be necessary to periodically remove debris and other fine particles which accumulate on the screen  16 . To this end, there may be provided within the screen a plurality of air nozzles  42  which can blow air through the screen to dislodge any accumulation and to prevent the screen  16  from plugging with fine particles. Advantageously, the air for these air nozzles  42  can be provided from the same source that is used to drive the pneumatic cylinder  30 . The air is supplied to the air nozzles  42  through a plurality of air manifolds  44  that encircle the interior of the screen  16 . The air is alternately supplied to these manifolds through a rotating air valve (not shown). 
     At the bottom  13  of the silo  11 , there is located the agitation/fluidization system  14 . This system  14  includes a plurality of nozzles  50  disposed in the lower end of the silo  11  to provide air injection for both agitation of the settling tailings and fluidization of the thickened product to assist with its removal from the vessel In the arrangement shown in FIG. 1, the nozzles  50  extend into the silo  11  and are supplied air by exterior manifolds  52 ,  53 . An exterior arrangement of the agitation/fluidization system  14  facilitates maintenance and repair thereof without necessarily having to remove the contents of the silo. As shown in FIG. 1, the nozzles  50  are arranged in a series of concentric rings  54   a ,  54   b ,  54   c ,  54   d  and  54   e  to ensure complete agitation and fluidization of the contents of the silo. 
     FIGS.  2   a  and  2   b  illustrate the operation of the percolation/decant insert. The mill tailings slurry  60  (usually between 20-30% pulp density) enters the top of the silo  11  through inlet conduit  62  and begins to settle toward the bottom  13 . The stainless steel screen  16  allows water in the material  60  to percolate through the screen slots while holding back the fine particles. The water then exits by means of gravity through conduit means  18  or via the a pump, if provided. The consistency of the mill tailings  60  is similar to silt, therefore they tend to quickly plug the slots in the screen. The screen  16  is cleaned periodically by means of the plurality of air nozzles  42  attached to the plurality of air manifolds  44  that encircle the interior of the screen  16 . 
     When sufficient settling has taken place and there remains a layer of clear water  64  on top of the densified tailings  66  (see FIG.  2   b ), the pneumatic actuator  30  is retracted. This pulls down on upper frame  36 , which in turn exerts a downward pressure on connecting rods  34 , pushing them through bushings  38 . This results in the annular plate  24  being translated in a downward direction. With its downward travel, spring  22  is compressed, folding the rubber-like covering  23 , and allowing the clarified water  64  to decant through the aperture in the annular plate  24 , when the height of the plate  24  is below the surface level of the water  64  in the silo  11 . The water  65  which is decanted falls under influence of gravity to the bottom of the percolation screen  16  where it exits through the conduit means  18  or by way of the pump, if provided. 
     By simply letting a tailings material settle in a silo, the larger, heavier particles will settle first and the result is layers of progressively denser material. To ensure a homogenous mix, the tailings should be agitated until settling is completed. Agitation air is injected into the settling contents by means of the air nozzles  50  provided at the lower end of the silo  11 . 
     Once the mine tailings have been densified and dewatered in the silo to anywhere from 76% to 84% solids, it is very difficult to remove from the silo. However, it has been found that the continued agitation of the slurry at higher densities causes a fluidization of the slurry/paste to occur. The result is a homogeneous, dense slurry being produced which behaves as a time-dependent non-Newtonian fluid and possesses good flow characteristics. In most cases, the thus fluidized densified slurry/paste will flow out of the vessel under the influence of gravity when the valve  68  on the outflow conduit  70  is opened (see FIG.  1 ). 
     It has also been found that by adding a small quantity of water to the fluidized tailings upon exit, the ability of the high density slurry/paste to flow is enhanced without overdilution. The water acts as a lubricant between the silo-bottom wall and the material inside. The water may be introduced via the nozzles on rings  54   d ,  54   e  through manifold  53  by simply switching the fluid source through valving (shown in FIG.  3 ). If required, both manifolds  52 ,  53  can be used to introduce the necessary minimum amount of water to induce the flow of the fluidized slurry/paste. 
     Preferably, nozzles  50  are of a non-plugging type. In the subject environment, most commercially available nozzles tend to plug when the fluid is switched off. The extremely fine particles present in mine tailings enter the nozzle orifice and harden. Quite often, when the fluid is switched back on, even at higher pressures, the particles do not dislodge. In this regard, the type of nonplugging nozzle disclosed in U.S. Pat. No. 5,405,063, issued Apr. 11, 1995 and assigned to the same Applicant as the subject invention, incorporated herein by reference, has been found to be particularly useful The nozzles that are used for agitation can be exactly the same nozzles as the fluidization nozzles. 
     The percolation/decant insert is designed to considerably reduce the thickening or dewatering time, thereby reducing the size requirement of the silos. As an example, a 3,000 ton silo of nickel tailings will take approximately 24 hours to gravity settle from a density of 35% solids to a density of 75% solids. Once settled, the silo contains approximately 2,000 tons of densified material while the remainder is water, thus allowing about 85 tons/hr of material until the next silo is sufficiently settled. 
     Designing a plant using the percolation/decant insert to obtain 85 tons/hr would only require a 500 ton silo as compared to the 3,000 ton unit in the example above, as the densification time is reduced as well as the usable capacity of the silo. 
     FIG. 3 illustrates a high density slurry/paste production facility which employs the aforementioned integral containment vessel and high density slurry or paste producing unit  10 . A dilute tailings slurry from the mill enters the system through conduit means  80  to selector valve  82 . The selector valve  82  is used to direct the slurry to either the silo  11  or on to a surface disposal site via conduit means  84 . If a second silo (not shown in FIG. 3) is included in the system, valve  82  and conduit means  84  would direct the material to the second silo. The quantity of slurry entering the silo  11  is measured in terms of flow rate and total flow by means of a flow meter  86 . 
     Silo  11  has a preferred geometry: the height of the tank portion being about twice the diameter, with a conical bottom having a 45° slope. Once the tailings slurry (usually between 20-30% pulp density) enters the silo  11 , air from source A is introduced via valve  88  and metering valves  90  through the nozzle manifolds  52 ,  53  and enters the silo  11  through the plurality of nozzles  50  to agitate the settling material The placement, vertical and horizontal angles of nozzles has an effect on the agitation and fluidization processes, and their particulars depend on the size of the silo and the specific gravity of the tailings material. 
     While the tailings material is settling, percolation screen  16  allows percolated water to exit the silo via conduit means  18  (as shown in FIG.  2   a ). Air nozzles  42  blow air through the screen  16  to keep the screen from plugging with fine particles. Once the material has dewatered for a specified time to bring the pulp density to between 76% and 86% solids, again depending on the specific gravity and particle size distribution of the material as well as the desired consistency of the final product, the water that has collected on top of the paste is decanted by actuating the ram  30  on the percolation/decant insert and allowing the water to exit the silo by conduit means  18 . 
     Now that a paste or high density slurry exists in the silo  11 , it is necessary to fluidize the material to allow it to exit. Fluidization of tailings by air in a viscous medium is affected by a number of factors, such as particle size, water content in settled bed, flow velocity and fluidizing time. Air from source A and, if desired, water from source W are introduced through control valve  88  into supply manifolds  52 ,  53  and through the plurality of nozzles  50  (not shown in FIG. 3 because of the scale of the drawing) which urges the paste to exit the silo  11  via outflow conduit  70  once valve  68  has been opened. 
     On its voyage to screw mixer  92 , the pulp density of the material is monitored by density meter  94 . Flow meter  96  measures the quantity of material entering mixer  92  as well. By calculation, the information from density meter  94  and flow meter  96  is used to determine the feed rate for cement as controlled by weigh feeder  98  mounted directly below cement hopper  100 . 
     When the paste and cement reach the upper end of screw mixer  92  they have been sufficiently mixed to produce a cemented tailings backfill which is fed by gravity or pumped to underground workings through conduit means  102 . 
     The system, as shown in FIG. 3, constitutes a batch plant as it takes between 3 to 8 hours, depending on the incoming material. If a mine requires a continuous system another silo equipped with the same working components is added and with the paste being fed in a similar manner into the same screw mixer  92 . While the material in the first silo is being emptied, the material in the second silo is being thickened. The silos would obviously be sized to provide the require tons per hour output that the mine requires. 
     This system is designed to use total tailings but depends on the grind or fineness of the mill output. If the tailings are too fine, such as in the case of some gold mines, an optional agglomeration unit may be added to the system. 
     For varied reasons, total tailings as mine backfill material is not always desired. In mills that refine gold bearing ore, the ore must be ground to a very fine powder to allow complete extraction of the gold. The resulting tailings are extremely fine and the finer portions of this material is commonly referred to as “slimes” due to its consistency when wet. It requires more binder or cement to achieve the same strength as a coarser material. It also tends to leak out of an underground stope if the water content of the fill is too high. 
     It does however have a couple of advantages. If used in paste fill, it helps the paste to flow through the pipeline somewhat like a lubricant. If a total tailings is used, the paste usually has a lower moisture content. 
     When the finer portion of tailings is to be excluded from the backfill, a hydrocyclone or other such device may be used to separate or classify the finer and coarser materials. Most existing backfill plants will have a cyclone in place. 
     FIG. 4 illustrates another backfill paste production facility which includes a pair of the integral containment vessels and high density slurry or paste producing units  10  for continuous paste backfill production. Regardless of its continuous production capabilities, the type of system illustrated in FIG. 4 is particularly useful if for any reason, a mine cannot use total tailings from the mill as backfill This system employs a hydrocyclone  110  at the top of each silo  11  to separate the finer material. The finer material, or slimes, can then be optionally pumped to a tailings pond via conduit  112  or where sending this fine material to the tailings pond is not acceptable, the cyclone overflow can be sent to an agglomeration plant  114 . The agglomeration plant  114  is used to pelletize the fine materials and inject them into the mine backfill supply pipe  102  after the mixer  92  as will be explained in greater detail hereinbelow. The integration of the pelletized fine material with the paste or high density slurry results in a material having equal or better strength than the strength of the slurry or paste material alone. 
     In order to speed settling of the solids in the silos  11 , a small amount of flocculent from flocculent tank  116  can be injected into the slurry during or before agitation. Since the quantity of flocculent required to be injected is relative to quantity of slurry in the silos  11 , flocculent metering pumps  118  ensure that the right amount of flocculent is injected and on a continuous basis in accordance with the amount of slurry as detected by inflow meters  86 . It will be appreciated that injecting flocculent at the same time that the slurry is being pumped into the silo  11  will tend to reduce the required agitation time. 
     The paste or high density slurry is produced in the silos  11  as explained above and in a manner so as to provide continuous consumption of the tailings and/or continuous production of paste backfill. Once the paste in the one silo has been fluidized, the valve respective  68  is opened to permit the paste to flow to mixer  92 . Most types of mixers such as commercially available paddle type or passive mixers can be used in these systems although some consideration should be paid to the particle size distribution of the tailings material. 
     Cement or other binder(s) used for the cementation of the tailings can be stored in its own silo(s)  100  and delivered to the mixer  92  as required. The amount of binder to be added is determined by the strength requirements of the mine. Based thereon, the rate of addition of binder or setting agent is then dependent on the density and flow rate of the paste as measured by meters  96 ,  94 , respectively. In addition to the density meters  96  which monitor the density of the paste material prior to the mixer, it is preferable to also include a density meter  120  after the mixer  92  to provide a final product density and valuable feedback to ensure the proper proportions of setting agent and, more importantly, the amount of water, are carefully controlled. 
     Valves used in controlling the flow of either slurries or paste should be of a type that allow opening to the same diameter as the pipe to which they are attached. Butterfly valves, for example, have the flap in the centre of the valve and, therefore, even when open, tend to plug the line very easily. The most satisfactory type of valve has been found to be a pneumatically actuated pinch valve which are available in almost any size. 
     As explained briefly above, the very fine materials or slimes from the total tailings can be pre-separated via hydrocyclones  110  and passed to the agglomeration plant  114  to be processed into pellets. The agglomerated material or pellets are then introduced into the system after the mixer  92  by means of a pellet injection ‘Y’  122 . The agglomeration plant  114  takes in the slimes, adds binders while tumbling, thus producing pellets. The pellets may be stored until hardened or cured then fed into system or they may be flash cured by means of a dryer upon exiting the agglomeration plant  114  and directly injected into the system. In general, the pellet injection ‘Y’  122  uses gravity to feed the pellets into the mixer discharge line  102 . However, advantage can be taken of the vacuum created at the ‘Y’  122  when paste backfill is flowing down the line  102  under the influence of gravity. 
     A second embodiment of the integral containment vessel and high density slurry paste production apparatus is shown in FIG.  5  and generally denoted by reference numeral  210 . The apparatus  210  is similar to the unit  10  illustrated in FIG. 1 (and may be substituted for the unit(s)  10  in the high density slurry or paste backfill production facilities of FIGS.  3  and  4 ), with the primary difference being that the percolation/decant insert  12  of the FIG. 1 unit  10  is replaced by separate percolation and decant apparatuses  220 ,  216  which perform generally the same respective functions. In addition, a feed cone  221  is provided at the top of the silo  211  which includes a feed well  225  disposed generally centrally thereof for introducing the raw slurry material. Details of the feed cone  221  and the feed well  225  are illustrated in FIGS. 6 and 7. 
     As with the silo  11  of the unit shown in FIG. 1, the shape and configuration of the silo body  211  and the bottom discharge cone  213  can be the same. Preferably, the silo body  211  has 2:1 aspect ratio (height:diameter) and the angle of the discharge cone  213  is approximately 45°. The high density slurry or paste production apparatus  210  includes an air/water fluidization/agitation system  214  disposed in the discharge cone  213  comprising a plurality of nozzles  250  which are supplied through manifolds  252  and  253  with air (or water) in the same manner as with the first embodiment shown in FIGS. 1,  2   a  and  2   b . Similarly, a discharge valve  268  permits the densified slurry/paste to be removed from the containment vessel through outflow conduit  270 . 
     The purpose of the feed well  225  is to eliminate or minimize turbulence in the vessel, thereby allowing the solids particles to settle out of suspension more quickly. The feed well  225  is also designed to aid in mixing chemical additives such as dispersant, flocculent, etc. with the incoming slurry feed, to speed the reaction of the chemical with the feed material. 
     In general the feed well  225  is a conduit, such as a cylinder or pipe, that is open at both ends  227 ,  229 . The feed well  225  is mounted vertically in the centre of the feed cone  221 , for example, by a bridge that runs across the top of the system (not shown). Preferably, the feed well diameter is about {fraction (1/10)} th  the diameter of the silo body  211 . The length of the feed well  225  is approximately equal to the height of the feed cone  221 . A grate  231  made of steel or other acceptable material is mounted inside the feed well  225  at a distance of about ¼ the total distance from the top to the bottom. Positioning nearer the top  227  permits ready access for removal of any accumulated debris. The openings of the grate  231  are sized according to the particle size distribution of the material including any foreign objects able to be pumped to the system as determined by laboratory testing. 
     A feed pipe  262  enters the feed well  225  at or near its upper end  227  above the grate  231 . The feed pipe  262  must be sized according to the flow rate of the incoming material so as to ensure adequate throughput. 
     An optional chemical addition pipe  234  can enter the feed well  225  near the top  227  and into an additive diffuser  235 , the type to be determined by what type of chemical is added. The addition of the chemical is through a metering pump as specified by the company supplying a particular chemical. 
     The feed cone  221  provides a greater area for settling out of solids from the slurry and also allows a greater area for the decanting of the water which is effected though the decanting apparatus  216  disposed at the top of the feed cone  221 . Another advantage is that it allows the containment vessel to have more usable volume or height and thereby increases the consolidation of the solids. The walls  237  of the feed cone  221  are angled, preferably about 60° from horizontal, such that solid particles will flow down the walls into the silo body  211 . The feed cone  221  may be bolted or otherwise affixed to the top of the silo body  211 . Preferably, the feed cone  221  has an aspect ratio of 1:3 (height of feed cone:height of silo body). 
     The decanting apparatus  216  comprises a decant screen  239  and an associated launder tray  241  disposed about the periphery of the upper end of the feed cone  221 . The purpose of the launder tray is simply to collect the supernatant water and to channel it to a common discharge piping system  243  (see FIG.  5 ). The launder tray  241  is separated from the feed cone  221  by the decant screen  239 . Preferably, the launder  241  protrudes horizontally from the feed cone  221  at a ratio of about 1:15 (launder tray width:top of feed cone diameter). The height of the launder tray  241  is preferably about equal to its width. A plurality of outlets  245  are provided in the bottom of the launder tray  241  and attached together by an overflow discharge pipe  243  to remove the supernatant water from the vessel. The outlets  245  and the discharge pipe  243  should be sized to accept the maximum outflow of water from the system. 
     Around the inner circumference of the launder tray is a decant screen  239  that is preferably set at a 75° angle from the horizontal, sloping inward toward the centre of the feed cone  221 . The purpose of the decant screen  239  is to hold back solids particles while allowing the supernatant water to flow through to the launder tray  241 . The slightly downwardly facing screen  239  assists in preventing blockages since restrained particles will have the tendency to fall back into the feed cone  221  under the influence of gravity. 
     Preferably, the decant screen  239  is a stainless steel, vertical rib, water-well-type screen (see FIG. 9) that is mounted within a frame  247   a , 247   b  for ease of replacement. The slot size of the screen  239  is selected relative to the particle size distribution of the material to be handled by the system and as determined through laboratory testing. As shown in FIG. 9, the preferred screen comprises a plurality of vertical triangular ribs  251  welded to a framework of horizontal bars  253  to provide a series of vertically-extending channels  255  which diverge in the direction of flow F. The ribs  251  are affixed to the bars  253  along one of their apexes  257  such that the sides  259  of the ribs  251  opposite the apex  257  form a generally flat, upstream surface  261  which, together with the vertical divergent channels  255 , minimize the propensity of particles becoming lodged between the ribs  251 . 
     To further prevent blockages from occurring on the decant screen  239 , a plurality of pneumatic hammers  263 , numbering for example 3 to 8, are provided spaced about the launder tray frame  247   a  and attached to the decant screen  249  (see FIG.  8 ). The screen  239  is cleaned by intermittent operation of a pneumatic hammer type vibrator  249  that vibrates the screen  239  periodically to dislodge any entrained solids particles that can impede the flow of water through the screen  239 . 
     The percolation apparatus of this embodiment is shown generally in FIG.  5  and in detail in FIG.  10  and comprises a plurality of percolation screens  265  arranged around the inner circumference of the silo body  211 , near its top, in apertures in the silo body wall  267 . Preferably, the height of these screens  265  is approximately one third the total height of the silo body  211 . The percolation screens  265  are mounted in a frame  269  for ease of replacement. A tank-like enclosure  271  protrudes from the silo body  211  behind each screen  265  to contain the percolation water and direct it to an opening  273  in the bottom of the enclosure  271  for discharge. The enclosure  271  also provides a mounting for a pneumatic hammer-type vibrator  275  for dislodging of solids particles from the screen  265 . 
     Preferably, the percolation screen  265  is a stainless steel, vertical rib, water-well-type screen of the same type as used for the decant screen  239  (see FIG. 9) with the slots sized according to the particle size distribution of the solids particles. This slot sizing may be determined through laboratory testing. 
     During initial filling of the silo  211 , the percolation screens  265  act in the same way as the decant screen  239  by permitting water to escape therethrough. However, once the slurry has risen to approximately the top of the feed cone  221  where the decanting screens  239  become operational, the percolation screens  265  then have a higher solids concentration with which to deal. Their main purpose is to allow any water that is displaced from the pores or voids in the solids mass of the slurry/paste to escape from the vessel, thereby speeding the consolidation of the material. 
     Preferably, at least four percolation screens  265  are installed in each system but the number of screens  265  can be higher depending on the particle size distribution, specific gravity and settling density test results as determined through specific laboratory testing. 
     In operation, the mill or mine tailings are pumped in a slurry form (as they are when they leave the mineral processing circuit) directly from the mine&#39;s mill, to the backfill plant. The slurry enters the system  210  through the slurry feed line  262  into the feed well  225 . 
     The feed well  225  channels the slurry downward into the centre of the silo  211 . The introduction of the slurry material centrally of the silo  211  through the feed well  225  is less important during initial filling than when the silo/feed cone  211 , 221  is full of slurry and/or after dewatering has commenced. Once settling has commenced, if the inflow is just dumped indiscriminately into the silo  211 , it tends to stir up the entire mass and slows the settling of solids. However, by feeding the silo  211  through the feed well  225 , the slurry is deposited well below the surface (which is at the level of the decant screen  239 ), thereby speeding the settling of the solids and eliminating turbulence. 
     Should the materials properties be such that a flocculent, dispersant, or other chemical additive is necessary to speed the settling process, such additive can be introduced through the additive inlet  233  and dispersed in the feed well  225  by diffuser  235  (see FIG.  7 ). The additive may be dispensed by a metering pump, as specified by the chemical supplier. The chemical additive would mix with the incoming tailings slurry in the feed well  225  as the slurry rebounds from the diffuser grating  231 . 
     As the tailings slurry rises in the silo  211 , it will eventually reach the level of the percolation screens  265 . At this point, water will begin to flow through the percolation screens  265  while the solids will be retained in the silo  211 . The water is collected by a circumferential pipe  277  that is connected to the percolation discharge outlets  273  at the base of the percolation water collector or enclosure (see FIG.  5 ). 
     The channels  255  in percolation screens  265  tend to become blocked with agglomerated solids particles and occasionally need to be cleaned. This is accomplished through intermittent activation of the pneumatic vibrating hammer  275  that is attached to each percolation screen  265  (see FIG.  10 ). 
     When the slurry level reaches the level of the decant screen  239  at the top of the feed cone  221 , the water will begin to flow through the decant screen  239  in the same way as with the percolation screens  265 . At this point, the input of fresh slurry material is controlled to ensure the slurry material in the vessel does not overflow the decant screen  239 . As mentioned above, the decant screen  239  is angled inward at the top to help prevent solids particles from adhering thereto (see FIG.  8 ). However, if the decant screens  239  become blocked, the pneumatic vibrating hammers  263  are operated periodically to clear the screen  239  of any particles that may have adhered. 
     The water flows through the decant screens  239  into the launder tray  241 , then flows through the overflow discharge outlet  245  and into a circumferential collector pipe  243 . 
     While this system is being filled with tailings slurry, solids are continuously settling in the water and therefore the slurry becomes thicker and hence, coarser at the bottom of the silo  211 . The pressure caused by the weight of the water/solids mixture in the feed cone  221 , tends to compact the lower layer of thickening material, thereby squeezing water out of the pores or voids in the solids mass. This pore water or percolation water is forced through the percolation screens  265 . 
     Once the solids become sufficiently dense in the feed cone  221  (this varies according to the material properties), a valve (not shown) on the feed line  262  is closed and the flow of slurry is diverted. The supernatant water will continue to be decanted until the surface level of the material falls below the bottom of the decant screens  239 . The percolation screens  265  continue to allow water to escape from the material until a predetermined solids density has been attained. Then, to maintain a specific water/solids ratio, valves (not shown) which prevent further water from exiting the percolation discharge outlets  273  are closed. 
     The system is left in a static state, allowing the settling and consolidation process to continue. 
     As mentioned earlier, the coarser material tends to settle first, giving a graduated solids mass in the silo. To achieve an homogeneous paste from the system, pressurized air is released through the fluidization nozzles  250  located in the discharge cone  213 . This fluidization process continues for a predetermined period of time (again depending on the material properties), until the material becomes a uniformly-graded paste. The discharge valve  268  is throttled open to allow a predetermined rate of flow and the paste material makes its way, for example, to the mixer  92  (as shown in FIGS. 3 or  4 ). As soon as the flow rate from the discharge cone  213  is consistent, the fluidization air is turned off. 
     There has been shown and described above two embodiments of an integral containment vessel and high density slurry or paste producing unit, a method for producing high density slurry and/or paste backfi, and examples of associated plant facilities which incorporates both of these aspects of the invention. It will be appreciated that various modifications and/or substitutions can be effected within the present technology without departing from the spirit or scope of the claims as appended hereto.