P/00/012 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION INNOVATION PATENT Invention Title: "DENSITY CONTROL OF A FLUIDIZED BED" The following statement is a full description of this invention, including the best method of performing it known to me/us: TITLE DENSITY CONTROL OF A FLUIDIZED BED BACKGROUND 5 The teeter-bed separator (hereinafter "fluidized-bed separator") variety of hydraulic separators is a mainstay within the mining industry. These fluidized-bed separators are used for both classification and density separation. The high capacity and sizing characteristics of these fluidized-bed separators make them ideal for feed preparation prior to coarse flotation 10 circuits. Moreover, these fluidized-bed separators are typically easy to control with two basic operating parameters, including fluidization liquid rate and bed level. The fluidization liquid is typically water. What is presented is a new innovation to augment the control of the fluidization water rate and bed level. 15 SUMMARY OF THE INVENTION A control method for fluidized-bed separation systems for partitioning a slurry is presented. The slurry includes at least one species that has a low hindered settling velocity and is influenced by a fluid flow. The hydraulic separation system comprises a separation tank, a slurry feed point, a fluid 20 conduit, and an underflow conduit that are all arranged to create a fluidized bed within the separation tank. The fluidized bed is created by injecting slurry through the slurry feed point and allowing the slurry to interact with teeter water injected from the fluid conduit. The system also includes a pressure reading apparatus and density indicating controller. The pressure reading 25 apparatus is arranged and configured to measure the density of the fluidized bed. The density indicating controller controls the fluid conduit and underflow conduit, so as to adjust the density of the fluidized bed based on calculations that are relayed to it from the pressure reading apparatus. The hydraulic separation system could further comprise a gas sparger that creates a 1 mixture of gas bubbles and teeter water that is injected into the separation tank from the fluid conduit. A method of controlling the density of a fluidized bed of a fluidized bed separation system is also presented. The method comprises the steps of: 5 permitting a fluidized bed to form; determining a first fluid pressure from an upper pressure transducer; determining a second fluid pressure from a lower pressure transducer that is at a distance from the upper pressure transducer; calculating the density of the fluidized bed; and adjusting the density of the fluidized bed according to the calculation of the density of the 10 fluidized bed. The method could further comprise a step of programming a density indicating controller to make adjustments in the fluid flow rate entering into the separation tank. The method could further comprise a step of programming a density indicating controller to make adjustments in the 15 fluidized bed by adjusting an underflow conduit. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a fluidized bed separation cell; Fig. 2 shows a cross-section of a separation tank showing a typical 20 fluidization bed density; Fig. 3A shows a cross-section of a separation tank showing a less dense fluidization bed density; Fig. 3B shows a cross-section of a separation tank showing a more dense fluidization bed density; 25 Fig. 4A depicts a schematic view of the fluidized bed separation system implementing hydraulic separation; Fig. 4B depicts a schematic view of the fluidized bed separation system of Fig. 4A that implements a differential pressure transmitter to calculate the pressure of the fluidized bed; 2 Fig. 5A depicts a schematic view of the fluidized bed separation system incorporating a gas sparging system; Fig. 5B depicts a schematic view of the fluidized bed separation system of Fig. 5A that implements a differential pressure transmitter to 5 calculate the pressure of the fluidized 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 10 embodiments and figures shown and described. Corresponding parts are denoted in different embodiments with the addition of lower case letters. 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. 15 Hydraulic fluidized bed separation systems are commonly used in the minerals industry to separate mineral species in suspension in liquid slurries. Such mineral species are often suspended with a mixture of unwanted constituent species. Fluidized bed separation systems comprise a fluidized bed separation cell 10, as shown in Figs. 1 and 2, with an associated control 20 system that is discussed in more detail below. A typical fluidized bed separation cell 10 is a separation device that also acts as a three-phase settler. As best understood by comparing Figs. 1 and 2, slurry is fed into a separation tank 12 through one or more slurry feed points 14 located generally in the upper third of the separation tank 12. Particulate matter, 25 containing fine/light particles and coarse/denser particles, in the slurry moves downwards countercurrent to an upward flow of teeter water and fed into the separation tank 12 through series of fluid conduits 22 generally located around the center of the separation tank 12. As slurry is introduced into the upper section of the separation tank, 30 the flow of teeter water will cause the particles in the slurry to separate. 3 Finellight particles, with hindered settling velocities, are carried hydraulically upward, by the flow of teeter water, staying within an overflow layer 20 and eventually carried over the top of the separation tank 12 and into an overflow launder 24. After flowing into the overflow launder 24, the fine/light particles 5 are carried out of the system by an overflow conduit 26. Whereas other more coarse/denser particles, with higher settling velocities, that have sufficient mass to settle against the flow of teeter water, fall downwardly through the separation tank 12 and create a fluidized bed 16 of suspended particles. This fluidized bed 16 acts as a dense medium zone 10 within the hydraulic fluidized bed cell 10. The small interstices within the fluidized bed 16 create high interstitial liquid velocities that resist the penetration of the fine/light particles. As a result, fine/light particles, that are too large to immediately follow into the overflow launder 24, are forced back upward so they accumulate back into the overflow layer 20 and are 15 eventually carried over the top of the separation tank 12 and into the overflow launder 24. After a few passes, the coarse/denser particles too big to stay above the fluidized bed 16 will eventually pass down through the fluidized bed 16 and into an underflow layer 18. After passing through the fluidized bed 16, the coarse/denser particles 20 are ultimately discharged from the underflow layer 18 by an underflow conduit 28. An underflow valve 30 regulates the amount of coarse/denser particles discharged from the separation tank 12. The type of underflow valve 30 is dependent on the application and can vary from a rubber pinch valve to an eccentric plug valve, but one of ordinary skill in the art will see that any under 25 flow valve 30 that can adequately regulate the discharge of coarse/denser particles may work. The separation effect that is provided by a fluidized bed separator is governed by hindered-settling principles, which has been described by numerous equations including the following: 4 S 181l(I + 0.15 Re"") where Ut is the hindered-settling velocity of a particle (m/sec), g is the acceleration due to gravity (m/sec 2 ), d is the particle size (m), ps is the density of the solid particles (kg/m 3 ), pf is the density of the fluidizing medium (kg/m 3 ), rj is 5 the apparent viscosity of the fluid (kg-m- s 1 ), 4 is the volumetric concentration of solids, $max is the maximum concentration of solids obtainable for a given material, and P is a function of Reynolds number (Re). By inspection of this equation, one having ordinary skill in the art can determine that the size, density, and overall settling velocity of a particle both greatly influence how that particle 10 will settle within a hindered settling regime. As such, applying flotation fundamentals and introducing gas, i.e., compressed air, and dispersing this gas throughout the teeter bed to affect change in apparent settling velocity of a component via bubble attachment is also a variant of this teeter-bed device and obvious to one having ordinary skill in the art. Flotation separators are frequently 15 used in the minerals processing industry to segregate fine particles from their surface wettability characteristics, i.e., size, shape, density, etc. Test data has shown that if the density of the fluidized bed 16 is manipulated, it is possible to choose different varieties of coarse/denser particles to flow through the fluidized-bed 16. As shown in Figs. 3A and 3B, 20 when the fluidized bed 16 becomes denser, coarse/denser particles having higher settling velocities can be held within the fluidized bed 16. The opposite effect also occurs when the fluidized bed 16 is more dilute and less dense. As the fluidized bed 16 becomes less dense, fine/light particles having hindered settling velocities will fall through the fluidized-bed 16. Given that the 25 hydraulic separation systems of the above nature can make separations based on the settling velocities of the particles within the slurry, it is beneficial to adjust the density of the fluidized-bed 16 so as to control the gravity of particle that is separated by the hydraulic fluidized-bed cell 10. 5 A schematic showing how the density of the fluidized bed 16 can be controlled is shown in Figure 4A. To adjust the fluidized bed 16, a pressure reading apparatus 44 is installed within the hydraulic fluidized bed cell 10 to gauge the pressure within the fluidized bed 16 and relay that information to a 5 computing mechanism (not shown) that will calculate the density of the fluidized-bed 16. The computing mechanism is typically a programmable logic controller, but any apparatus able to calculate the density of the fluidized-bed 16 may work. In the embodiment of the pressure reading apparatus 44 shown, at 10 least two pressure transducers are placed within the separation tank 12, an upper pressure transducer 32 and a lower pressure transducer 34. The pressure transducers 32 and 34 are typically individual pressure sensors that have internal strain gauges to measure the pressure created by the mixture of fluid and slurry surrounding the pressure sensors within the separation 15 tank 12. Both the upper pressure transducer 32 and a lower pressure transducer 34 are configured to read the density of the fluidized-bed 16 immediately surrounding their position within the separation tank 12. It should be noted that even though pressures transducers that have intemal strain gauges are commonly used, one of ordinary skill in the art will see that any 20 device able to read and convey the pressure of the surrounding pressure of the fluidized bed may work. The readings from the transducers 32 and 34 is compiled and sent by the pressure reading apparatus 44 to the computing mechanism to be calculated. The density of the fluidized bed 16, Pb, is calculated by the computing 25 mechanism using the following equation: APxA AP Pb H where AP is the differential pressure reading calculated from the upper pressure transducer 32 and lower pressure transducer 34, A is the cross sectional area of the separator, Vz is the volume of the zone between the two 6 transducers 32 and 34, and H is the elevation difference between these transducers 32 and 34. The upper pressure transducer 32 and lower pressure transducer 34 are each installed at different elevations but in close proximity to one another. 5 The typical elevation difference between the upper pressure transducer 32 and lower pressure transducer 34 is at typically at 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 that signal disturbances will not cause problems in calculations 10 may work. Teeter water is injected into the separation tank 12 through the fluid conduit 22 by an injection system 38. The injection system 38 comprises a series of components that ensures that there is enough pressure to maintain a steady flow of teeter water entering into the separation tank 12. As more 15 teeter water is added into the separation tank 12, the fluidized bed 16 will expand, by diluting the slurry with teeter water, and result in a lower density reading from the pressure transducers 32 and 34. In contrast, as the amount of fluid injected into the separation tank 12 decreases, the fluidized bed 16 will contract and becomes denser, resulting in a higher density reading from 20 the pressure transducers 32 and 34. To control the flow rate of teeter water entering and leaving the separation tank 12, a density indicating controller 36 (DIC) monitors the readings from the two pressure transducers 32 and 34 and subsequently causes fine tuned adjustments in the flow rate of teeter water that enters the separation tank 12 through the fluid conduits 22 by 25 providing set points to a flow indicating controller 42 (FIC). A separate level indicating controller 40 (LIC) controls the fluid exit rate that leaves the separation tank 12 through the underflow conduit 28 via a second series of controllers 40 that controls the underflow valve 30. The fluidized bed 16 can be adjusted using this network of controllers 30 incorporating a programmable logic controller/distributed control system that 7 implements a cascading loop in order to keep the fluidized bed 16 density constant. However, one of ordinary skill in the art will see that any apparatus or system able to adjust and keep the fluidized bed 16 density constant will work. This configuration can be advantageous in situations where the 5 coarse/denser particles with higher settling velocities and/or fine/light particles with hindered setting velocities within the slurry having regularly changing characteristics (i.e., particle size, particle density, settling velocity, etc.). When incorporating the pressure transducers 32 and 34, fluidization 10 adjustments should typically be set to occur very slowly and in small increments, otherwise the changes in flow rate of teeter water can cause large fluctuations in the two pressure transducers 32 and 34 that will create inaccuracies within the density calculations. Time must be allowed for the increase or decrease in the fluidization flow to effect changes in the fluidized 15 bed 16. It is also advantageous to implement a time delay or an average reading provided over a small period of time between the at least two pressure transducers 32 and 34 and the density indicating controller 36. This time delay will allow for a more accurate reading of the fluidized bed 16 20 density because the density indicating controller 36 will make adjustments in flow rate of teeter water entering or exiting the separation tank 12 based upon a density reading of a fluidized bed 16 that has had time to settle between different adjustments. Although, not necessary, it could be advantageous to program the 25 density indicating controller 36 with both a minimum and maximum teeter water flow rate that enters into the separation tank 12. For example, the lowest parameter of the teeter water flow rate should be set to one that is approximately 10-20% less than the minimum actual fluid flow that is ideal for the specific type of slurry being used within the separation tank 12, this effect 30 will limit the potential for sanding problems. The highest parameter of the 8 teeter water flow rate should be set to one that is approximately 10-20% more than the maximum actual fluid flow that is ideal for the specific type of slurry being used within the separation tank 12, this effect will limit the misplacement of coarse/denser particles from accidentally entering into the 5 overflow launder 24. As shown in Figure 4B, the pressure reading apparatus 44a could also be a single differential pressure transmitter 46a. This differential pressure transmitter 46a directly calculates the pressure of the fluidized-bed through a reading from pressure sensors 32a and 34a that have internal strain gauges 10 to measure the pressure created by the mixture of fluid and slurry surrounding the pressure sensors within the separation tank 12. In this embodiment there is no need for a separate calculation of pressure differences. As shown in Figure 5A, the embodiment of the fluidized bed separating 15 system 8b incorporates a gas sparging system 38b as opposed to just the injection system of previously discussed embodiments. The gas sparging system is the injection system in combination with a gas sparger that continuously injects a plurality of gas bubbles into the teeter water prior to being injected through. With this gas sparging system 38b, a mixture of gas 20 bubbles and teeter water is simultaneously injected into the separation tank 12b through the fluid conduit 22b. Incorporating a mixture of gas bubbles and teeter water into the separation tank 12b of the fluidized bed separating cell 10b makes it easier to recover coarse particles that have been found to be difficult to recover using 25 the hydraulic flotation methods discussed above. As the mixture of gas bubbles and teeter water rises through the fluidized bed, the gas bubbles will selectively attach to the hydrophobic particles and form particle/bubble aggregates that segregate atop the fluidized bed due to their apparent hindered settling velocity. The settling velocity of the hydrophobic aggregates 30 more hindered than that of the fluidized bed. These hydrophobic aggregates 9 are eventually carried into the overflow launder 24b by the rising current of the teeter water. The hydrophilic particles will settle into the fluidized bed and ultimately discharged through the underflow conduit 30b. As more of the mixture of gas bubbles and teeter water is added into 5 the separation tank 12b, the fluidized bed will expand and result in a lower density reading from the pressure transducers 32b and 34b. In contrast, as the amount of fluid injected into the separation tank 12b decreases, the fluidized bed will contract and becomes denser, resulting in a higher density reading from the pressure transducers 32b and 34b. To control the fluid flow 10 entering and leaving the separation tank 12b, a density indicating controller 36b monitors the readings from the two pressure transducers 32b and 34b and subsequently causes fine tuned adjustments in the mixture of gas bubbles and teeter water flow rate that enters the separation tank 12b through the fluid conduits 22b by providing set points to a flow indicating 15 controller 42b. A separate level indicating controller 40b controls the exit rate of the mixture of gas bubbles and teeter water leaving the separation tank 12b through the underflow conduit 28b via a second series of controllers 40b that controls the underflow valve 30b. The fluidized bed can be adjusted using this network of controllers 20 incorporating a programmable logic controller/distributed control system that implements a cascading loop in order to keep the fluidized bed density constant. However, one of ordinary skill in the art will see that any apparatus or system able to adjust and keep the fluidized bed density constant will work. This configuration can be advantageous in situations where the 25 coarse/denser particles with higher settling velocities and/or fine/light particles with hindered settling velocities within the slurry having regularly changing characteristics (i.e., particle size, particle density, settling velocity, etc.). When incorporating the pressure transducers 32b and 34b, the 30 adjustments of the mixture of gas bubbles and teeter water entering the 10 separation tank should typically be set to occur very slowly and in small increments, otherwise the changes in the flow rate of the mixture of gas bubbles and teeter water can cause large fluctuations in the two pressure transducers 32b and 34b that will create inaccuracies within the density 5 calculations. Time must be allowed for the increase or decrease in the fluidization flow to effect changes in the fluidized bed. It is also advantageous to implement a time delay or an average reading provided over a small period of time between the at least two pressure transducers 32b and 34b and the density indicating controller 36b. 10 This time delay will allow for a more accurate reading of the fluidized bed density because the density indicating controller 36b will make adjustments in the flow rate of the mixture of gas bubbles and teeter water entering or exiting the separation tank 12b based upon a density reading of a fluidized bed that has had time to settle between different rate adjustments of the mixture of 15 gas bubbles and teeter water. Although, not necessary, it could be advantageous to program the density indicating controller 36b with both a minimum and maximum flow rate of the mixture of gas bubbles and teeter water that enters into the separation tank 12b. For example, the lowest parameter of the flow rate of the mixture of 20 gas bubbles and teeter water should be set to one that is approximately 10 20% less than the minimum actual flow rate of the mixture of gas bubbles and teeter water that is ideal for the specific type of slurry being used within the separation tank 12, this effect will limit the potential for sanding problems. The highest parameter of the flow rate of the mixture of gas bubbles and 25 teeter water should be set to one that is approximately 10-20% more than the maximum actual flow rate of the mixture of gas bubbles and teeter water that is ideal for the specific type of slurry being used within the separation tank 12b, this effect will limit the misplacement of coarse/denser particles from accidentally entering into the overflow launder 24b. 11 As shown in Figure 5A, the pressure reading apparatus 44c of the fluidized bed separating cell 10c incorporating a gas sparging system 38c could also be a single differential pressure transmitter, instead of implementing pressure transducers. This differential pressure transmitter 46c 5 directly calculates the pressure of the fluidized-bed through a reading from pressure sensors 32c and 34c that have internal strain gauges to measure the pressure created by the mixture of fluid and slurry surrounding the pressure sensors within the separation tank 12. In this embodiment there is no need for a separate calculation of pressure differences. 10 To control the density of the fluidized-bed, first, the fluidized-bed must be permitted to form within the separation tank 12c. Next, a first fluid pressure must be determined from the upper pressure transducer 32c, and then a second fluid pressure must be determined from the lower pressure transducer 34c. Once the two fluid pressures are determined, the density of 15 the fluidized-bed between the sensors is calculated and the density of the fluidized-bed is adjusted according to the calculations made. If one of ordinary skill in the art would find it beneficial, the density indicating controller 36c can be programmed to make adjustments in the flow rate of teeter water that enters and leaves the fluidized-bed based upon calculations of the 20 fluidization-bed that have already been made. This innovation 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 25 so far as they come within the scope of the appended claims or the equivalents of these claims. 12