P/00/012 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION INNOVATION PATENT Invention Title: "CAVITATION TUBE SPARGING SYSTEM" The following statement is a full description of this invention, including the best method of performing it known to me/us: Cavitation Tube Sparging System Background Flotation separators are used extensively throughout the minerals industry to partition and recover the constituent species within slurries. A 5 slurry is a mixture of liquids (usually water) with various species having varying degrees of hydrophobicity. The species could be insoluble particulate matter such as coal, metals, clay, sand, etc. or soluble elements or compounds in solution. Flotation separators work on the principle that the various species within the slurry interact differently with bubbles formed in the 10 slurry. Gas bubbles introduced into the slurry attach, either through physical or chemical means, to one or more of the hydrophobic species of the slurry. The bubble-hydrophobic species agglomerates are sufficiently buoyant to lift away from the remaining constituents and form a froth that is removed for further processing to concentrate and recover the adhered species. 15 Column flotation cells were introduced about thirty years ago as devices capable of producing flotation concentrates that were lower in impurities than those produced by other types of flotation systems. The ability to operate columns with deep froth beds and the ability to wash the froth were the main reasons cited for the improved metallurgical recovery performance. 20 In recent years, many producers have installed column flotation systems as a means of boosting production whilst reducing operating costs. Column flotation cells are flotation devices that act as three-phase settlers where particles move downwards countercurrent to a swarm of rising air bubbles that are generated by an air sparging system located at the 25 bottom of the cell. There are numerous air sparging systems including air lances, porous tubes, eductors, and static-mixers. The bubbles generated by the air sparging system are sized to provide the maximum amount of bubble surface area given a constant energy input. In other words, the designs of the 1 various sparging devices are engineered to provide the smallest size and largest number of bubbles possible. Summary of the Invention According to one aspect, the present invention is a flotation separation 5 system for partitioning a slurry, the slurry including a hydrophobic species which can adhere to gas bubbles formed in the slurry, said flotation separation system comprising: a flotation separation cell, said flotation separation cell including an air sparging system and a separation tank; 10 said air sparging system comprising a recirculation pump, a slurry inlet for drawing slurry from said separation tank, an air injection system, at least one in-line cavitation tube, and a slurry outlet to return slurry to said separation tank. Preferably, said air sparging system comprises a plurality of cavitation 15 tubes in parallel. According to another aspect, the present invention is a cavitation tube for a sparger system for a flotation separation cell, said cavitation tube comprising: an inlet and an outlet each of about the same diameter; 20 a cylindrical throat located between said inlet and said outlet having a smaller diameter than said inlet; a pressure recovery zone extending from said cylindrical throat to said outlet comprising a sloped wall with an increasing diameter. Preferably, the cavitation tube is manufactured of polyurethane, 25 ceramic, tungsten carbide, or hardened steel. Preferably, the ends of said cavitation tube further comprises any of threaded couplings, mechanical couplings, or flanges. 2 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. 5 FIG. 1 depicts a column flotation system that implements one embodiment of the invention disclosed herein; FIG. 2 depicts a cut out view of a cavitation tube according to one embodiment of the invention; FIG. 3 depicts a cross sectional view of the cavitation tube of FIG. 2; 10 FIG. 4 depicts a cross sectional view of a cavitation tube according to another embodiment of the invention; FIG. 5 illustrates the attachment of picobubbles to bubbles and particles; and FIG. 6 illustrates the buoyancy effect of attachment of picobubbles to 15 bubbles and particles. Detailed Description What is presented is a cavitation tube for an air sparging system for a flotation separation cell. The cavitation tube comprises an inlet and an outlet of about the same diameter, a cylindrical throat smaller in diameter than the 20 inlet, and a pressure recovery zone extending from the cylindrical throat to the outlet comprising a sloped wall with an increasing diameter. The cavitation tube may be manufactured of polyurethane, ceramic, tungsten carbide, or hardened steel but other construction materials are possible. The ends of the cavitation tube may be any of threaded couplings, mechanical 25 couplings, or flanges. Flotation separation systems for which the cavitation tube is used are typically for partitioning slurries that include a hydrophobic species which can 3 adhere to gas bubbles formed in the slurry. The flotation separation system comprises a flotation separation cell that includes an air sparging system and a separation tank. The air sparging system further comprises a recirculation pump, a slurry inlet for drawing slurry from the separation tank, an air 5 injection system, at least one in-line cavitation tube, and a slurry outlet to return slurry to the separation tank. In some flotation separation systems, the air sparging system could comprise a plurality of cavitation tubes in parallel. A cavitation tube could also be used for pre-aerating the feed as it enters the flotation separation cell. 10 Those skilled in the art will realize that this invention is capable of embodiments that are different from those shown and that the 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 15 depart from the spirit and scope of this invention. 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. Corresponding parts are denoted in different embodiments with the addition of lowercase letters. 20 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. Flotation separation is commonly used in the minerals industry to separate mineral species in suspension in liquid slurries. Such mineral 25 species are often suspended with a mixture of unwanted constituent species. A typical column flotation cell 10, as shown in FIG 1, is a flotation device that also acts as a three-phase settler. Slurry is fed into a separation tank 12 through one or more a feed points 14 (only one is shown in FIG. 1) located generally in the upper third of the separation tank 12. Particulate matter in the 4 slurry moves downwards countercurrent to a swarm of rising air bubbles that are generated by an air sparging system 16 located at the bottom of the separation tank 12. The rising bubbles form a froth layer at the top of the separation tank 12. Hydrophobic particles in the slurry collide with and attach 5 to the bubbles and rise to the top of the separation tank 12, eventually reaching the interface between the slurry (collection zone) and the froth layer (cleaning zone). The location of this interface, which can be adjusted by the operator, is held constant by means of an automatic control loop which regulates a valve (not shown) on the column flotation cell 10 tailings line 18. 10 Varying the location of the interface will increase or decrease the height of the froth zone. As more slurry is introduced into separation tank 12 and more bubbles are generated by the air sparging system 16, the froth overflows into an overflow launder 20 and is transported out of the column flotation cell 10 through a concentrate line 22. 15 Wash water, which is introduced at the top of the separation tank 12 through a froth washing system 24, filters down through the froth layer and removes entrained particles and also displaces any clay-laden slurry that may have travelled up into this zone. This action significantly improves the purity of the froth product by removing ultra-fine clay particles and also serves to 20 maximize recovery by stabilizing the froth structure. According to the present invention, the air sparging system 16 consists of a recycle pump 26, a slurry distribution manifold 28, and a series of cavitation tubes 30 designed specifically to induce cavitation and generate fine bubbles. A portion of slurry is withdrawn from the separation tank 12 and 25 recycled by the recycle pump 26, typically a centrifugal pump, to the slurry distribution manifold 28 where it is divided equally between pluralities of cavitation tubes 30. Process air is injected under pressure from an air distribution manifold 32 to the inlet of each cavitation tube 30 to provide additional air for flotation. The mixture of slurry and air passes though each 30 cavitation tube 30 sparger and is returned into the bottom of the separation 5 tank 12. While the example shown in the figures show the air sparging system 16 having multiple cavitation tubes 30, it is possible for some systems to have only a single cavitation tube 30 or a mixture of cavitation tubes 30 and other sparging technologies such as static mixers, air lances, etc. 5 The air rate used is selected according to the slurry feed rate and froth concentrate production requirements. This parameter typically has the largest effect on the operation of the column flotation cell 10 with respect to the grade and the recovery rate of product. The bubbles generated by the air sparging system 16 are sized to provide the maximum amount of bubble 10 surface area given a constant energy input. In other words, the air sparging system 16 is engineered to provide the smallest size and largest number of bubbles possible. The air sparging system 16 generates large amounts of very small bubbles for a given airflow. This is commonly reported in terms of the 15 superficial bubble surface area rate (Sb), and is defined as the total bubble surface area per unit of time passing through a given cross-section of a column flotation cell. Studies indicate that higher Sb values signify that more bubble surface area is being generated for a given air rate and is a direct result of a finer bubble size distribution. With higher Sb values, the probability 20 of bubble-particle collisions are greatly improved, which has a direct bearing on slurry processing capacity of the column flotation cell 10. Systems with higher Sb values are also better able to process slurries that contain very fine material. Such systems typically process material finer than 0.150-mm and typically have a high volume flow, reduced residence time, and a finer feed 25 size distribution. The cavitation tube 30 system presented herein seeks to maximize Sb using the principle of hydrodynamic cavitation. Hydrodynamic cavitation is the process of creation and growth of gas bubbles in a liquid due to the rupture of a liquid-liquid or a liquid-solid interface under the influence of external 6 forces. Hydrodynamic cavitation occurs when the pressure at a point in a liquid is momentarily reduced below its vapor pressure due to high flow velocity. Minute air or vapor-filled bubbles are carried on by the flow to regions of higher pressure. 5 As best understood by comparing FIGS. 2 and 3, a cavitation tube 30a comprises an inlet 34a and an outlet 36a each of about the same diameter. A cylindrical throat 38a is located between the inlet 34a and the outlet 36a and has a smaller diameter than the inlet 34a. A pressure recovery zone 40a comprising a sloped wall with a gradually increasing diameter extends from 10 the cylindrical throat 38a to the outlet 36a. The ends 42a of the cavitation tube 30a could be any appropriate coupling required for the particular application including threaded couplings, mechanical couplings, or flanges. In the embodiment shown in FIGS. 2 and 3, the ends 42a comprise flanges. The cavitation tubes 30a are designed to exploit cavitation and create 15 fine bubble dispersions. Liquid in the cylindrical throat 38a has a higher flow velocity and lower pressure than liquid in the inlet 34a, resulting in cavitation. Research has shown that the cavitation is directly proportional to the dissolved air content in liquid. Addition of chemicals such as frothers produces smaller and more copious cavities by stabilizing the cavity and 20 preventing cavity collapse and coalescence. Another embodiment of cavitation tube 30b is shown in FIG. 4. In this embodiment one end 42b is a threaded coupling while the other is a flat end of a mechanical coupling. Within the cavitation tube 30a, in addition to traditional sparging of air into bubbles in the slurry, two mechanisms occur almost simultaneously: 25 extremely tiny bubbles, referred to as picobubbles, are generated and the slurry is intensely mixed. Picobubbles are precipitated onto the surfaces of hydrophobic mineral particles as a result of cavitation and then immediately are subjected to intense mixing with air in the pressure recovery zone 40a. During the cavitation process, bubbles that are generated on a particle 7 surface by cavitation remain naturally attached to the particle, eliminating the collision and attachment process, which is often the rate-determining step for flotation. More efficient attachment of particles and improved flotation rates have been observed when picobubbles co-exist with larger air bubbles 5 commonly used in flotation separation cells. The combination of larger bubbles and picobubbles generated by the cavitation tubes 30a provides a synergistic effect of exploiting the effects of cavitation with traditional sparging techniques. The amount of time that a bubble and a particle touch is referred to as 10 the "sliding time". The time it takes for a bubble to rupture and attach to the surface of a particle is referred to as the "induction time". For any given bubble-particle combination, sliding time needs to be longer than induction time for attachment to occur. Smaller bubbles tend to rise slower within the slurry in the separation tank. Because picobubbles rise slower than the larger 15 bubbles formed with traditional sparging methods, they tend to spend more time in close proximity or touching particles or other bubbles. Picobubbles therefore have a greater probability of attachment to particles than larger bubbles. In addition, smaller bubbles like picobubbles tend to not bounce off the surface of a particle as larger bubbles tend to. This ultimately increases 20 the sliding time between picobubbles and particles. In addition, while it takes a certain amount of energy to get a larger bubble to attach to a particle. It takes far less energy for the larger bubble to attach or combine with a smaller bubble. Therefore, as shown in FIG. 5, picobubbles 44 attached to the surface of a much larger particle 46 activate flotation by promoting the 25 attachment of larger bubbles 48 to the bubble-particle complex as larger bubbles 48 tend to more favorably attach to picobubbles 44 than directly to particles 46. In other words, picobubbles 44 act as secondary collectors for particles 46, reducing the amount of flotation promotion chemicals needed, enhancing particle-bubble attachment probability, and reducing the particle 30 bubble detachment probability. This leads to substantially improved flotation 8 recovery of poorly floating fine and coarse particles and reduced reagent cost, which is often the largest single operating cost in commercial mineral flotation plants. Application of cavitation tube spargers to coal flotation resulted in an increase in flotation yield up to 15 wt%, a froth promotion 5 chemical dose reduction of 10%, and a collector chemical dose reduction of 90%. In-house research shows that hydrodynamic cavitation significantly increased flotation kinetics of silica and zinc sulfide precipitates. Cavitation also improves the flotation efficiency of coarse particles by reducing the particle-bubble detachment probability as particle-bubble 10 aggregates rise in slurry in the separation tank 12. This is best illustrated in FIG. 6 where the larger bubble 48 is attached to a particle 46 indirectly though its attachment to multiple picobubbles 44. Without cavitation generated picobubbles 44, particles 46 will detach from the larger bubble 48 surface when the capillary force and other attachment forces are exceeded 15 by detachment forces, such as the viscous or drag force (Fd), the gravitational force, and the hydrostatic pressure. As the drag force is directly proportional to the particle 46 diameter; coarser particles are more likely to detach from larger bubble 48 surfaces than finer particles. This has been recognized by many researchers as the main reason for low flotation 20 recovery of coarse particles. However with the picobubbles 44 present, each large bubble 48 and particle 46 is attached to a plurality of picobubbles 44 and the probability of detachment of either the large bubble 48 or the particle 46 to every picobubble 44 is low. In addition, the picobubbles 44, particularly those underneath the particle 46, will tend to push the particle 46 upward, 25 further facilitating particle recovery Cavitation tubes are manufactured using a variety of very durable materials such as polyurethane, ceramic, tungsten carbide, and hardened steel to provide long life under a wide variety of applications. The preferred material for abrasive applications is 27% Chrome Ni-Hard ASTM A532,3A. 9 A cavitation tube could also be used for pre-aerating the slurry feed before it enters the flotation separation tank 12 at a feed point 14. This invention has been described with reference to several preferred embodiments. Many modifications and alterations will occur to others upon 5 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. 10