Abstract:
A separation system is presented that partitions a slurry containing a plurality of particles that are influenced by a fluidization flow (which comprises teeter water and gas bubbles) and a fluidized bed. The separation system comprises a separation tank, a slurry feed distributor, a fluidization flow manifold and a gas introduction system. All of these components are arranged to create the fluidized bed in the separation tank by introducing the slurry through the slurry feed distributor and allowing the slurry to interact with the fluidization flow that enters the separation tank from the fluidization flow manifold. The gas introduction system is configured to optimize the gas bubble size distribution in the fluidization flow. The gas introduction system comprises a gas introduction conduit and a bypass conduit. The gas introduction system can be adjusted by modulating the flow of teeter water through the gas introduction conduit.

Description:
BACKGROUND 
     Fluidized-bed or teeter-bed separation systems are used for classification and density separation within the mining industry. The metallurgical performance and high capacity of these separation systems make them ideal for feed preparation prior to flotation circuits. It has been found that when this type of separation system implements a fluidization flow with the addition of air bubbles, performance can be improved beyond that achieved by systems using only water. This variety of separator is called an air-assisted separation system. These devices are typically controlled using two basic operating parameters: fluidization flow rate and fluidized bed level. What is presented are improvements to an air-assisted separation system, incorporating various novel features, that further enhance the separation process. 
     SUMMARY 
     What is presented is a separation system for partitioning a plurality of particles contained in a slurry. The particles are influenced by a fluidization flow, which comprises teeter water, gas bubbles, and a fluidized bed. The separation system comprises a separation tank, a slurry feed distributor, a fluidization flow manifold, a gas introduction system, and an underflow conduit all arranged to create the fluidized bed in the separation tank by introducing the slurry through the slurry feed distributor and allowing the slurry to interact with the fluidization flow from the fluidization flow manifold. The separation tank has a launder for capturing particles carried to the top of the separation tank. The gas introduction system is configured to optimize the gas bubble size distribution in the fluidization flow. The gas introduction system comprises a gas introduction conduit and a bypass conduit for a flow of teeter water to bypass the gas introduction conduit. The gas introduction system can be adjusted to optimize the gas bubble size distribution by modulating the flow of teeter water through the gas introduction conduit. The gas introduction conduit and the bypass conduit converge to create the fluidization flow. The volume of fluidization flow is controlled by modulating the flow through said gas introduction system. 
     In some embodiments of the separation system, a pressure reading apparatus is arranged and configured to measure the density of the fluidized bed. In some embodiments the pressure reading apparatus comprises two pressure sensors to measure the density of the fluidized bed, or a differential pressure transmitter configured to measure the density of the fluidized bed. In some embodiments a density indicating controller is used to control the gas introduction system and the underflow conduit and to adjust the density and level of the fluidized bed based on calculations performed by the density indicating controller based on signals from the pressure reading apparatus. 
     Some embodiments of the separation system comprise a slurry aeration system for aerating the feed slurry. Some of these embodiments comprise a sparging apparatus for aerating the fluidization water. Other embodiments of the separation system further comprise a chemical collector or a surfactant introduced into the fluidization flow to condition the particles in the slurry or to facilitate aeration of the fluidization flow. 
     Those skilled in the art will realize that this invention is capable of embodiments that are different from those shown and that details of the devices and methods can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and descriptions are to be regarded as including such equivalent embodiments as do not depart from the spirit and scope of this invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding and appreciation of this invention, and its many advantages, reference will be made to the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  shows a schematic view of the separation system; 
         FIG. 2  is a perspective view of a fluidized bed separation cell; 
         FIG. 3  is a cross-section of a separation tank showing the components of a typical fluidized bed; 
         FIG. 4A  is a cross-section of a separation tank showing the components of a less-dense fluidization bed; and 
         FIG. 4B  is a cross-section of a separation tank showing the components of a more-dense fluidization bed. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, some of the reference numerals are used to designate the same or corresponding parts through several of the embodiments and figures shown and described. Variations of corresponding parts in form or function that are depicted in the figures are described. It will be understood that variations in the embodiments can generally be interchanged without deviating from the invention. 
     Separation systems implementing fluidized beds (also called a teeter bed or a teeter water bed or a fluidized teeter bed) are commonly used in the minerals industry to partition a plurality of particulate mineral species contained in a liquid suspension or slurry. These slurries consist of a mixture of valuable and less valuable mineral species. Separation systems that implement an aerated fluidization flow (teeter water with gas introduced to form gas bubbles) and a fluidized bed are called air-assisted separation systems. An example of an air-assisted separation system as described herein is the HYDROFLOAT™, manufactured by Eriez Manufacturing Company of Erie, Pa. As shown in  FIGS. 1 through 3 , the air-assisted separation system  10  comprises a fluidized bed separation cell  12  with an associated gas introduction system  38 , slurry aeration system  62 , and pressure reading apparatus  70 , each discussed in more detail below. As best understood by comparing  FIGS. 1 and 2 , slurry is fed into a separation tank  14  through a slurry feed distributor  16 , generally located in the upper third of the separation tank  14 . The particulate mineral matter in the slurry moves downwards countercurrent to an upward flow of teeter water. The teeter water is fed into the separation tank  14  through a fluidization flow manifold  18  generally located around the center of the separation tank  14  and connected to an inflow conduit  17 . 
     Comparing  FIGS. 2 and 3 , as slurry is introduced into the upper section of the separation tank  14  through the slurry feed distributor  16 , the upward flow of teeter water and gas bubbles collide with the downward flowing slurry, causing the particles in the slurry to separate as a result of some of the particles in the slurry selectively attach to the gas bubbles. The particles that are fine/light are hydraulically carried upward by the flow of teeter water and those particles attached to the gas bubbles float to the top, staying within an overflow layer  20  to eventually be carried over the top of the separation tank  14 . After being carried over the top of the separation tank  14 , these particles flow into either an external overflow launder  22  or an internal overflow launder  24  and are carried out of the system by an overflow conduit  25  that drains both overflow launders  22  and  24 . 
     The particles that are more coarse/dense, and those that did not attach to the gas bubbles that have sufficient mass to settle against the upward flow of teeter water, fall downwardly through the separation tank  14  and form a fluidized bed  26  of suspended particles. The fluidized bed  26  acts as a dense medium zone within the separation tank  14 . Within the fluidized bed  26 , small interstices create high interstitial liquid velocities that resist the penetration of the particles that could settle against the upward flow of teeter water, but that are too fine/light to penetrate the already formed fluidized bed  26 . As a result, these particles will initially fall downward until they contact the fluidized bed  26  and are forced back upwardly to accumulate in the overflow layer  20 . These particles are eventually carried to the top of the separation tank  14  and end up in one of the overflow launders  22  or  24 . 
     The particles that are too coarse/dense to stay above the fluidized bed  26  and those that do not attach to a gas bubble will eventually pass down through the fluidized bed  26  and into an underflow layer  28 . Once in the underflow layer  28 , these particles are ultimately discharged from the underflow layer  28  through an underflow conduit  30 . An underflow valve  32  regulates the amount of coarse/dense and unattached particles discharged from the separation tank  14 . The type of underflow valve  32  is dependent on the application and can vary from a rubber pinch valve to an eccentric plug valve, but it should be understood that any under flow valve  32  that can adequately regulate the discharge of coarse/dense particles may work. 
     Hindered-bed separators segregate the particles that are fine/light from those that are course/dense based on their size and specific gravity. The separation effect is governed by hindered-settling principles, which has been described by numerous equations including the following: 
               U   t     =             gd   2     ⁡     (       ϕ   max     -   ϕ     )       β     ⁢     (       ρ   s     -     ρ   f       )         18   ⁢     η   ⁡     (     1   +     0.15   ⁢           ⁢     Re   0.687         )                 
where U t  is the hindered-settling velocity of a particle (m/sec), g is the acceleration due to gravity (9.8 m/sec 2 ), d is the particle size (m), ρ s  is the density of the solid particles (kg/m 3 ), ρ f  is the density of the fluidizing medium (kg/m 3 ), η is the apparent viscosity of the fluid (kg·m −1 ·s −1 ), φ is the volumetric concentration of solids, φ max  is the maximum concentration of solids obtainable for a given material, and β is a function of Reynolds number (Re). By inspection of this equation one having ordinary skill in the art can determine that the size and density of a particle greatly influences how that particle will settle within a hindered settling regime.
 
     One having ordinary skill in the art can also see that aerating the teeter water, by introducing gas (i.e., air) into the flow of the teeter water to create gas bubbles, will affect the settling characteristics of the particles that attach to these gas bubbles. The fluidization flow of the air-assisted separation system is aerated by introducing gas into the flow of teeter water prior to entering the separation tank  12 . Therefore, for known slurry compositions, the fluidization flow can be modulated to optimize gas bubble interactions with target particles and carry these target particles to the top of the separation tank  12  for removal. 
     As shown in  FIG. 1 , a gas introduction system  34  is used to optimize the gas bubble introduction to the fluidization flow. The gas introduction system  34  comprises two conduits arranged in parallel, a gas introduction conduit  36  and a bypass conduit  38 . Both conduits are located downstream from a teeter water supply line  40 , which provides the supply of teeter water to the gas introduction system  34 , and upstream from the inflow conduit  17  and fluidization flow manifold  18 . When the flow of teeter water enters the gas introduction system  34 , it splits apart so that a first portion of the flow of teeter water flows through the gas introduction conduit  36  and a second portion of teeter water flows through the bypass conduit  38 . 
     The first portion of the flow of teeter water is aerated in the gas introduction conduit  36 . A gas introduction point  44  introduces gas into the flow of teeter water to generate bubbles as the flow of teeter water passes through the gas introduction conduit  36 . A sparging apparatus  42  sparges, or breaks up, the generated gas bubbles into smaller gas bubbles. Any type of sparging apparatus that can sparge the bubbles sufficiently may be used, such as, but not limited to, an in-line static mixer or high shear sparging system. Generally, the sparging effect of the sparging apparatus  42  varies with the flow rate of teeter water through it. The gas introduction conduit  36  also comprises a flow meter  46  to monitor the rate of flow of teeter water through the gas introduction conduit  36 . Typically, this flow meter  46  is located upstream of the gas introduction point  44  to reduce the interference of gas bubbles on the operation of the flow meter  46 . 
     The gas introduction system  34  may combine other types of systems to introduce gas and sparge bubbles than have been shown. In  FIG. 1 , the gas introduction point  44  is shown to provide pressurized gas to the system. It will be understood that systems that do not need condensed gas to operate may be used instead, such as aspirators that utilize the Venturi effect to draw gas into the flow of teeter water. 
     The bypass conduit  38  allows the second portion of the flow of teeter water to bypass the gas introduction conduit  36 , without interfering with the efficient operation of the sparging apparatus  42 . The bypass conduit  38  comprises an automatic valve  47 , which controls the volume of flow passing through the bypass conduit  38 . At the end of the gas introduction system  38  when both the first and second portions of the flow of teeter water converge, the portions combine to create the fluidization flow that enters into the fluidized bed separation cell  12 . 
     When the separation system  10  is in use, the flow meter  46  communicates with a computing mechanism  49 , which communicates with and adjusts the automatic valve  47  to throttle the flow of teeter water passing through the bypass conduit  38 . This approach maintains a constant flow of teeter water through the gas introduction conduit  36 . The teeter water supply line  40  also incorporates a control system  48  which consists of a flow measurement device  78 , a flow control valve  80  and a density indicating controller  76 , discussed below. The control system  48  modulates the volume of flow of teeter water before entering the gas introduction system  34 , which will subsequently optimize the volume of fluidization flow entering into the fluidized bed separation cell  12 . 
     In certain applications, air-assisted separation systems use reagents, such as chemical collectors, to condition particles to improve attachment of target particles to the gas bubbles. Surfactants are also used to facilitate the general creation of gas bubbles. To introduce these reagents, prior art separation systems (not shown) typically incorporate a plurality of stirred-tank conditioners (not shown). The stirred-tank conditioners, however, consume a great deal of energy and occupy significant floor space. As such, there is an incentive within the field to achieve the goal of introducing reagents into separation systems while consuming less energy and space than would be needed to incorporate a plurality of stirred-tank conditioners. 
     Referring back to  FIG. 1 , it has been found that reagents can be introduced into the separation system  10  simply by being injected into the teeter water supply line  40  using a collector pump  58  or a surfactant pump  60 . As the reagent is introduced into the teeter water supply line  40 , it travels with the teeter water to the gas introduction system  34 . Injecting the reagents into the gas introduction system  34  causes them to directly and completely mix into the fluidization flow prior to entering the separation tank  14 . It has also been found that mixing the reagents and fluidization flow through the gas introduction system  34  in this manner causes a more evenly distributed and intimate mixture than one created through the use of a stir tank. 
     It has also been found that pre-aeration of the slurry within the slurry feed distributor  68  allows for contacting of the gas bubbles and particles entering the separation tank  12 . To accomplish pre-aeration, a slurry aeration system  62  is incorporated into the feed introduction system  16 . The slurry aeration system  62  introduces aerated water into the slurry while still traveling through the slurry feed piping  16  or directly into the slurry feed distributor  68 . The slurry aeration system  62  comprises two lines, a water introduction line  64  and an air introduction line  67 . The water and air pass through a sparging apparatus  42  and is subsequently discharged into the slurry feed piping  16  or the slurry feed distributor  68 . The addition of air into the feed slurry enhances the flotation kinetics by reducing the contacting time required in the separation tank  12 . 
     It has also been found that if the density of the fluidized bed  26  is manipulated, it is possible to influence the type of the particles that flow through the fluidized bed  26 . As shown in  FIGS. 4A and 4B , when the fluidized bed  26  becomes denser, particles that are coarser/denser can be held within the fluidized bed  26  without falling downward into the underflow layer  28 . The opposite effect occurs when the fluidized bed  26  is more dilute and less dense. As the fluidized bed  26  becomes less dense, particles that are fine/light will fall downward through the fluidized bed  26  and into the underflow layer  28 . Given that the separation system can make separations based on the size and/or density of the particles within the slurry, it is beneficial to adjust the density of the fluidized bed  26  so as to control the operation of the fluidized bed separation cell  12 . 
     Referring back to  FIG. 1 , to adjust the fluidized bed  26 , a pressure reading apparatus  70  is installed within the fluidized bed separation cell  12  to gauge the pressure within the fluidized bed  26  and relay that information to a computing mechanism (not shown), which calculates the density of the fluidized bed  26 . The computing mechanism is typically a programmable logic controller, but any apparatus able to calculate the density of the fluidized bed  26  may work. 
     At least two pressure transducers are placed within the separation tank  14 , an upper pressure transducer  72  and a lower pressure transducer  74 . The pressure transducers  72  and  74  are typically individual pressure sensors that have internal strain gauges used to measure the pressure created by the mixture of fluid and slurry surrounding the pressure sensors within the separation tank  14 . Both the upper pressure transducer  72  and a lower pressure transducer  74  are configured to read the density of the fluidized bed  26  immediately surrounding their position within the separation tank  14 . It should be noted that even though pressures transducers with internal strain gauges are commonly used, one of ordinary skill in the art will see that any device able to read and convey the pressure of the surrounding pressure of the fluidized bed may work, such as, but not limited to, a differential pressure transmitter configured to measure the discrete density of the fluidized bed or a single differential pressure transmitter. The readings from the transducers  72  and  74  is compiled and sent by the pressure reading apparatus  70  to the computing mechanism to be calculated. 
     The density of the fluidized bed  26 , ρ b , is calculated by the computing mechanism using the following equation: 
               ρ   b     =         Δ   ⁢           ⁢   P   ×   A       V   z       =       Δ   ⁢           ⁢   P     H             
where ΔP is the differential pressure reading calculated from the upper pressure transducer  72  and lower pressure transducer  74 , A is the cross-sectional area of the separator, V Z  is the volume of the zone between the two transducers  72  and  74 , and H is the elevation difference between these transducers  72  and  74 .
 
     The upper pressure transducer  72  and lower pressure transducer  74  are each installed at different elevations but in close proximity to one another. The typical elevation difference between the upper pressure transducer  72  and lower pressure transducer  74  is 12 inches (305 mm) to minimize any signal disturbances caused by turbulence of the fluidized bed  16 , but one of ordinary skill in the art will see that any distance between the transducers may work. 
     As the volume of fluidization flow being introduced into the separation tank  14  increases, it dilutes the fluidized bed  26  and causes the bed to expand, resulting in a lower density reading from the pressure transducers  72  and  74 . In contrast, as the volume of fluidization flow introduced into the separation tank  14  decreases, the fluidized bed  26  will contract and becomes denser, resulting in a higher density reading from the pressure transducers  72  and  74 . To control the volume of fluidization flow entering and leaving the separation tank  14 , a density indicating controller  76  monitors the readings from the two pressure transducers  72  and  74  and subsequently adjusts the flow rate of teeter water to the gas introduction system  34 . A density indicating controller  76  can also control the level of the fluidized bed  26  by monitoring the reading from only one of the two pressure transducers  72  and  74 , typically the lower pressure transducer  74 , and subsequently causing fine tuned adjustments based on that single reading. 
     A second density indicating controller  75  is also used to control the level of the fluidized bed  26  by monitoring the reading from only one of the two pressure transducers  72  and  74 , typically the lower pressure transducer  74 , and subsequently adjusting the discharge rate of material exiting the separation tank  14  via the underflow control valve  32 . 
     When incorporating the pressure transducers  72  and  74 , adjusting the volume of fluidization flow entering and leaving the separation tank  14  should typically be set to occur very slowly and in small increments, otherwise the changes in the volume of fluidization flow can cause large fluctuations in the two pressure transducers  72  and  74  that will create inaccuracies within the density calculations. It is advantageous to implement a time delay between the two pressure transducers  72  and  74  and the density indicating controller  76 . This time delay will allow for a more accurate reading of the fluidized bed  26  density because the density indicating controller  76  will make adjustments in flow rate of teeter water entering or exiting the separation tank  14  based upon a density reading of a fluidized bed  26  that has had time to settle between different adjustments. A calculation of an average reading, provided over a small period of time, may also accomplish a more accurate reading of the fluidized bed  26  density. 
     It can be advantageous to program the density indicating controller  76  to control the minimum and maximum volume of fluidization flow entering and exiting the separation tank  14 . For example, the lowest parameter of the volume of fluidization flow should be set to one that is approximately 10-20% less than the minimum actual volume of fluidization flow ideal for the specific type of slurry being used, this effect will limit the potential for sanding problems. The highest parameter of the volume of fluidization flow should be set to one that is approximately 10-20% more than the maximum actual of the volume of fluidization flow ideal for the specific type of slurry being used within the separation tank  14 , this effect will limit the misplacement of the particles that are more coarse/dense from accidentally entering into one of the launders  22  or  24 . 
     This invention has been described with reference to several preferred embodiments. Many modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such alterations and modifications in so far as they come within the scope of the appended claims or the equivalents of these claims.