Abstract
A mixing apparatus for mixing particles in a liquid and its use are disclosed. The mixing apparatus comprises a tank having a bottom and a substantially vertical side wall, an agitation means comprising a rotation shaft located vertically and centrally in the tank, and an impeller arranged at a height above the bottom at the end of the rotation shaft and the impeller being a downward pumping axial or mixed flow impeller. The bottom is equipped with a corrugated formation comprising alternate consecutive ridges and valleys, the ridges and valleys extending radially in relation to a center of the bottom, whereby the valleys concentrate and channel the mixing power near to the bottom to direct the flow of the liquid and to increase the velocity of the flow near to the bottom.
Claims
1. A mixing apparatus for mixing particles in a liquid, the mixing apparatus comprising a tank having a bottom and a substantially vertical side wall, an agitation means comprising a rotation shaft located vertically and centrally in the tank, and an impeller arranged at a height above the bottom at the end of the rotation shaft and the impeller being a downward pumping axial or mixed flow impeller, wherein the bottom is equipped with a corrugated formation comprising alternate consecutive ridges and valleys, the ridges and valleys extending radially in relation to a center of the bottom, whereby the valleys concentrate and channel the mixing power near to the bottom to direct flow of the liquid and to increase the velocity of the flow near to the bottom, wherein the height of at least one of the ridges decreases when moving from the side wall towards the center of the bottom of the tank and the highest point of the ridge is at a height of 0.1-1 times the radius of the bottom, preferably 0.35 times the radius of the bottom, and wherein the tank comprises at least one vertical baffle mounted to the side wall substantially above the highest point of the ridge for directing the flow of the liquid to be mixed.
2. The mixing apparatus according to claim 1, wherein the corrugated formation comprises at least two ridges and a corresponding number of valleys, preferably at least four ridges and corresponding number of valleys.
3. The mixing apparatus according to claim 1, wherein the length of each ridge and valley is at least ⅔ of the radius of the bottom.
4. The mixing apparatus according to claim 1, wherein the ridges and valleys extend from the side wall to the direction of the center of the bottom.
5. The mixing apparatus according to claim 1, wherein at least one of the ridges and valleys extends the whole length between the center of the bottom and the side wall.
6. The mixing apparatus according to claim 1, wherein a floor of at least one of the valleys length is horizontal for its entire length.
7. The mixing apparatus according to claim 1, wherein the floor of at least one of the valleys is at an angle to horizontal in the direction of the valley.
8. The mixing apparatus according to claim 1, wherein the height of the ridge is 0.1-1 times the radius of the bottom, preferably 0.35 times the radius of the bottom.
9. The mixing apparatus according to claim 1, wherein the height of the ridge decreases so, that the ridge has a radial profile of a straight line, a convex or a concave curve or a line with one or more bends.
10. The mixing apparatus according to claim 1, wherein the cross section of each ridge, when seen from the direction of the side wall towards the center of the tank, is a triangle, a triangle with at least one concave side, a triangle with at least one convex side, a triangle with a rounded tip, a semicircle, an arc or a combination of them.
11. The mixing apparatus according to claim 1, wherein the corrugated formation is an integral part of the bottom, a separate part on the bottom or forms the bottom.
12. (canceled)
13. The mixing apparatus according to claim 1, wherein the impeller is a hydrofoil impeller, a propeller or a pitched-blade turbine.
14. The mixing apparatus according to claim 1, wherein the tank is a cylinder or a right prism.
15. The mixing apparatus according to claim 1, wherein the mixing apparatus is made of metal, such as steel or titanium, fiber reinforced plastic, such as glass fiber, or the combination thereof.
16. The mixing apparatus according to claim 1, wherein the mixing apparatus is meant to be used in a hydrometallurgical process.
17. The mixing apparatus according claim 1, wherein the mixing apparatus is a flotation conditioner tank, filter feed tank or a gold cyanide leaching tank.
18. Use of the mixing apparatus according to claim 1, for mixing particles in a liquid.
19. The use according to claim 18, wherein the particles have a density of at least 1 kg/L, preferably at least 2 kg/L.
20. The use according to claim 18, wherein the particles have an average diameter of 20-5,000 μm, preferably 100-200 μm.
21. The use according to claim 18, wherein mixing particles in a liquid belongs to a hydrometallurgical process.
22. The use according to claim 18, wherein mixing particles in a liquid is flotation conditioning, filter feed slurry agitation or gold cyanidation.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:
[0042] FIG. 1A is an axonometric illustration of a mixing apparatus according to the present disclosure with four ridges and corresponding valleys.
[0043] FIG. 1B is a schematic view of the mixing apparatus of FIG. 1A as a longitudinal section in the direction of arrow a in FIG. 1A.
[0044] FIG. 1C is an axonometric illustration of a mixing apparatus according to the present disclosure with 3 ridges and corresponding valleys.
[0045] FIG. 1D is an axonometric illustration of a corrugated formation according to the present disclosure with 6 ridges and corresponding valleys.
[0046] FIGS. 2A-2F depict some possible shapes of the corrugated formation showing radial profiles of two ridges opposite to each other.
[0047] FIGS. 3A-3D are schematic presentations of some possible shapes of the corrugated formation shown from the direction of the tank wall (arrow a in FIG. 1A).
[0048] FIGS. 4A-4F depict CFD-models of the enhancement of flow speeds in a mixing apparatus according to the present disclosure presented as longitudinal sections of the tank.
[0049] FIGS. 5A-5F depict CFD-models of the enhancement of flow speeds in a mixing apparatus according to the present disclosure presented as cross sections above the bottom of the tank.
[0050] FIG. 6 illustrates the increase in the effective mixing volume in a mixing apparatus according to the present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0051] FIG. 1A presents an embodiment of the mixing apparatus according to the current disclosure as an axonometric illustration. In FIG. 1A, as in the following figures, structural details have been omitted for clarity. The mixing apparatus according to the current disclosure comprises a tank 1 having a bottom 2 and a wall 3. The bottom 2 is circular and the tank 1 is a right circular cylinder. The mixing apparatus further comprises agitation means 4 which comprises a rotation shaft 5 and an impeller 6. In FIG. 1A, the impeller 6 is a downward pumping hydrofoil impeller. The dimensions of the tank 1 and the agitation means 4 can vary according to the specific application for which the mixing apparatus is being used. At the bottom 2 of the tank 1, there is a corrugated formation 7. The corrugated formation 7 has four ridges 8 and corresponding valleys 9 positioned so that the angle between two neighboring ridges 8 and corresponding valleys 9 is the same for all ridges 8 and corresponding valleys 9. All the ridges 8 extend from the side wall 3 to the center of the bottom 2. They are sloped so, that the top 11 of the ridge 8 at the wall 3 is at a height which is ⅓ of the radius of the bottom 2. The slope has a constant angle for the entire length of the ridge 8, i.e. the top 11 of the ridge 8 forms a straight line. At the center of the bottom 2 where the ridges 8 meet, their height is zero. The cross-section of a ridge 8 has a profile of a triangle. The valley floor 10 is horizontal throughout its length and the length equals the radius of the tank bottom 2. The neighboring ridges 8 meet at the valley floor 10, so that the valley floor 10 remains narrow throughout its length. In this embodiment, the corrugated formation 7 is constructed so, that it lies directly over the flat bottom 2 of the tank 1. Therefore, the the valley floor 10 and the tank bottom 2 are substantially at the same level.
[0052] When particles in a liquid are mixed in a mixing apparatus displayed in FIG. 1A, the rotation shaft 5 rotates the impeller 6, which creates a liquid flow that is substantially towards the bottom 2 of the tank 1. Without limiting the invention to any specific theory, the corrugated formation 7 directs the flow of the liquid and increases the liquid flow velocities in the vicinity of the bottom 2 improving the suspension of the particles in the liquid. The corrugated formation 7 also increases the upward flow velocities along the tank wall 3, increasing the portion of the liquid volume having high enough velocity to keep the particles suspended. There are no baffles 12 drawn in FIG. 1A, but in most applications they are used. They, in addition to their possible other functions, further assist in directing the flow of the liquid upwards along the wall 3 of the tank 1.
[0053] FIG. 1B displays the tank 1 of FIG. 1A as a longitudinal section along arrow a in FIG. 1A. Features located that cannot be seen from this angle are depicted with dashed lines. In FIG. 1B, three ridges 8 (marked as 8a, 8b and 8c) are displayed. Two ridges, 8a and 8b, that face each other are shown along the top 11 of the ridges 8a and 8b. Ridge 8c which is at a straight angle to the two other ridges 8a, 8b is visible in the middle of the tank 1. There are two valleys 9 visible in FIG. 1B behind the two ridges 8a, 8b. The bottom 2 of the tank 1 is shown at the same level with the floor 10 of the valleys 9. The dashed lines show how the neighboring ridges, 8a, 8c, and 8b, 8c, meet at the floor 10 of the two valleys 9. Further, it is shown how the height of all ridges 8a, 8b, 8c decreases from the wall 3 towards the center of the tank 1 so, that the tops 11 of the ridges 8a, 8b, 8c meet at the center of the bottom 2 of the tank 1. The agitation means 4 is located at the center of the tank 1. The rotation shaft 5 extends from the top of the tank 1 downwards and the impeller 6 is mounted at a height of ½ times the radius of the tank 1 above the center of the bottom 2 of the tank 1.
[0054] FIG. 1C displays an embodiment of the mixing apparatus according to the current disclosure as an axonometric illustration. It is similar to the embodiment displayed in FIGS. 1A and 1B, but it comprises three ridges 8 and corresponding valleys 9. The ridges 8 and corresponding valleys 9 are radially symmetrically situated, i.e there is an angle of 120° between each neighboring ridge 8 and corresponding valley 9.
[0055] Further, the embodiment in FIG. 1C comprises baffles 12 mounted above each ridge 8. The baffles 12 are constructed as known in the art and the selection of their characteristics belongs to the knowledge of the skilled person. It is depicted in FIG. 1C how the baffles 12 are attached to the wall 3 of the tank 1 above the top 11 of each ridge 8. Without limiting the invention to any specific theory, the location of the baffles 12 helps to direct the flow of the liquid.
[0056] FIG. 1D depicts a corrugated formation 7 according to the present disclosure. It comprises six ridges 8 and corresponding valleys 9 extending radially in relation to the center of the bottom 2. All the ridges 8 slope downwards from the tank 1 wall 3 and meet at the center of the bottom 2. The corrugated formation 7 with, for example, 6, 8, 9 or 12 ridges 8 and corresponding valleys 9 might be preferable if it is to be used in an tank 1 with a large diameter of the bottom 2. It might be easier to construct such a corrugated formation 7 when individual parts remain smaller. FIG. 1D further displays a part of the bottom 2 of the tank 1. However, the bottom 2 is portrayed in FIG. 1D only to provide orientation. The corrugated formation 7 according to the present disclosure can be constructed separately from the bottom 2 of the tank 1. The corrugated formation 7 according to the present disclosure is retrofittable to existing mixing apparatuses or it can be introduced in the tank already during construction. There are a number of ways to construct a tank 1 bottom 2 with a corrugated formation 7 according to the present disclosure and the selection of a suitable method is within the knowledge of the skilled person based on this disclosure.
[0057] In all the embodiments of FIGS. 1A-1D, the ridges 8 have a straight sloping radial profile, i.e. the top 11 of the ridge 8 is a straight line that descends from the wall 3 end of the ridge 8 towards the center of the bottom 2. In FIGS. 1A-1D, the cross section of the ridge 8 is a triangle, i.e. also the sides of the ridge 8 were straight. However, this is not necessary and all embodiments can be implemented with other shapes as well.
[0058] FIGS. 2A-2F depict some possible shapes of the corrugated formation 7 according to the present disclosure. In FIGS. 2A-2F, a longitudinal section of the bottom part of a tank 1 with a corrugated formation 7 is displayed. The corrugated formation 7 comprises four ridges 8 and corresponding valleys 9. However, the different shapes depicted in FIGS. 2A-2F could be implemented in a corrugated formation 7 with any number of ridges 8 and corresponding valleys 9. In FIGS. 2A-2F, longer dashes are used for depicting the shape of the ridges 8 at both sides of the corrugated formation 7 and shorted dashes for depicting the shape of the ridge 8 at the center of the corrugated formation 7 (i.e. the one furthest away from the viewer).
[0059] In FIG. 2A, the top 11 of the ridge 8 has a concave shape when viewed from the side. The sides of the ridge 8 are not straight, but also have a slightly concave profile.
[0060] In FIG. 2B, the top 11 of the ridge 8 has a sharp bend, i.e. a change in inclination. The ridge 8 slopes towards the bottom 2 faster near the wall 3. On the side of the bend which is closer to the center of the tank 1 bottom 2, the ridge 8 slopes more slowly. The overall shape of the ridge 8 can be thought as approximating a concave shape. The bend is also present at the sides of the ridge 8, but the part of the ridge 8 that is at the wall 3 follows the contour of the wall evenly.
[0061] In FIG. 2C, the top 11 of the ridge 8 has a convex shape when viewed from the side. The sides of the ridge 8 have a concave profile.
[0062] In FIG. 2D, the top 11 of the ridge 8 has a sharp bend, i.e. a change in inclination. The ridge 8 slopes towards the bottom 2 slower at its wall 3 end. On the side of the bend which is closer to the center of the tank 1 bottom 2, the ridge 8 slopes more steeply. The overall shape of the ridge 8 can be thought as approximating a convex shape. The bend is also present at the sides of the ridge 8, but the part of the ridge 8 that is at the wall 3 follows the contour of the wall evenly.
[0063] In FIG. 2E, the shape of the ridges 8 is similar to that in FIG. 2C, but the ridges 8 do not reach all the way from the wall 3 to the center of the bottom 2. In this embodiment, they are approximately 85% of the radius of the bottom 2. The height of the ridges 8 at the wall 3 is similar to FIGS. 2A-2D, i.e. ⅓ of the radius of the bottom 2.
[0064] In FIG. 2F, the top 11 of the ridge 8 forms a straight line from the wall 3 to the center of the bottom 2. In this embodiment, the sides of the ridges 8 have a sharp bend.
[0065] In summary, the ridges 8 and corresponding valleys 9 can have many different shapes as long as sufficient guiding efficiency for the liquid is achieved. This depends on the specific application for which the corrugated formation 7 according to the present disclosure is used.
[0066] FIGS. 3A-3D depict schematic presentations of cross sectional profiles of the ridges 8 in a corrugated formation 7 according to the present disclosure. The ridges 8 and corresponding valleys 9 are viewed from the outside of the tank 1 wall 3, in the direction of the arrow a in FIG. 1A. FIGS. 3A-3D are not in perspective and present two neighboring ridges 8 as being on a plane.
[0067] In FIG. 3A, the cross section of the ridges 8 is a triangle and the floor 10 of the valley 9 is extremely narrow, as the neighboring ridges 8 meet at the floor 10 of the valley 9. The top 11 of the ridge 8 is sharp. In FIG. 3A, also the bottom 2 of the tank 1 is visualized and is at the same level with the floor 10 of the valley 9. It is omitted from FIGS. 3B-3D.
[0068] In FIG. 3B, the ridges 8 and corresponding valleys 9 have a curved cross sectional profile. The top 11 of the ridge 8 as well as the floor 10 of the valley 9 are not sharp as in FIG. 3A.
[0069] In FIG. 3C, the two ridges 8 are of different shape. This, although possible for the functioning of the corrugated formation 7 according to the present disclosure, is probably rare in practice. The main purpose of FIG. 3C is to present two further alternatives for the shape of the ridges 8 according to the present disclosure. First, as in FIG. 3A, the ridge 8 on the left has a cross sectional profile of a triangle. The ridge 8 on the right, has a cross sectional profile with a bend (cf. FIG. 2F where the bend is in another direction). Further, in FIG. 3C, the floor 10 of the valley 9 is broad and flat, as there is space between the two neighboring ridges 8.
[0070] FIG. 3D depicts two further embodiments of the ridges 8 according to the present disclosure. The sides of the ridge 8 can be concave, as in the ridge 8 on the left, or they can have a convex profile as in the ridge 8 on the right. The ridge 8 on the right has a rounded top 11.
[0071] The above-listed shapes are not meant to exhaust all the possible shapes in which the corrugated formation 7 according to the present disclosure can be embodied. They are only to provide examples of possible alternatives, and others can be envisaged.
[0072] FIGS. 4A-4F depict a CFD-model of the enhancement of the flow speeds in a mixing apparatus according to the present disclosure presented as a longitudinal section of the tank. In FIGS. 4A, 4C and 4E, the flow speeds are depicted in grayscale, a lighter color meaning faster flow speed. A scale bar is provided at the bottom of the figure. In the scale bar, negative values indicate flow speeds towards the bottom and positive values towards the surface of the liquid being mixed. In FIGS. 4B, 4D and 4F, the flow speeds are depicted with velocity arrows, a longer arrow meaning faster flow speed. A scale bar is provided at the bottom of the figure. In all figures, mixing parameters, such as impeller speed and properties, and slurry characteristics were equal. The impeller and the rotation shaft are visible at the center of each of FIGS. 4A-F. In FIG. 4F, two ridges according to the present disclosure are sketched at the bottom of the tank.
[0073] FIGS. 4A and 4B illustrate the flow speed in a flat-bottomed tank, FIGS. 4C and 4D in a dished-bottom tank and FIGS. 4E and 4F in a tank according to the present disclosure. It can be seen in FIGS. 4A-F that the volume of higher flow speeds is slightly larger in a dished-bottom tank than in the flat-bottomed tank. However, the volume of higher flow speeds increases further in a tank with a corrugated formation according to the present disclosure, especially in the upper half of the tank in the vicinity of the tank walls.
[0074] FIGS. 5A-5F depict a CFD-model of the enhancement of flow speeds in a mixing apparatus according to the present disclosure presented as a cross section at 85 mm above the bottom of the tank. The figures illustrate a tank with a diameter of 8500 mm and solution depth of 8500 mm. The impeller diameter is 3458 mm and rotation speed 32 rpm. In FIGS. 5A, 5C and 5E, the flow speeds are depicted in grayscale, a lighter color meaning faster flow speed. A scale bar is provided at the bottom of the figure. In FIGS. 5B, 5D and 5F, the flow speeds are depicted with equal speed contours. In all figures, mixing parameters, such as impeller speed and properties, and slurry characteristics are equal.
[0075] FIGS. 5A and 5B illustrate the flow speed in a flat-bottomed tank, FIGS. 5C and 5D in a dished-bottom tank and FIGS. 5E and 5F in a tank according to the present disclosure. It is evident from FIGS. 5A-F that the flow speeds at 85 mm above the bottom of the tank vary between different tank bottom configurations. The flow speeds are lowest in a tank with a flat bottom and they increase to some extent with a dished-bottom tank. However, with a corrugated formation according to the present disclosure, a clear positive difference to the other two is attainable: flow speeds above approximately 1 m/s do not form separate areas with this configuration, but instead the majority of the bottom is covered with flow velocities of 1 m/s and higher.
[0076] FIG. 6 demonstrates the increase in the effective volume when a corrugated formation according to the present disclosure is used at the bottom of a mixing tank. With effective volume is herein meant the volume in which the solids are suspended relative to the whole volume occupied by the liquid (i.e. the slurry volume).
[0077] The experiment was conducted with laboratory-scale equipment with an OKTOP 3200 axially downward pumping hydrofoil impeller with a diameter of 154 mm. Tank i) had a flat bottom, tank ii) a dished bottom and tank iii) was equipped with a corrugated formation according to the current disclosure. All tanks had a diameter of 362 mm and were loaded with 37.3 L water. Thus the liquid depth varied, being largest in tank iii) and smallest in tank i) with a flat bottom. The solution to be mixed contained 400 g/L quartz sand as the solid component. The particle diameter of the solid matter was 125-185 μm corresponding to typical particles in hydrometallurgical applications. The tank dimensions, impeller and its rotation speed and baffle configuration were kept constant.
[0078] As can be seen in FIG. 6, in tank i), the particles are suspended in only a portion of the liquid volume, the effective volume being about 70% of the total slurry volume. The effective volume remains approximately the same in tank ii) with a dished bottom. However, with the mixing conditions in the experiment, in tank iii), the effective volume increased to 94% of the slurry volume. Conversely, under the same conditions, it was observed that the impeller speed in which all particles are in motion and none of them remain on the bottom of the tank more than transiently (the so-called just-suspended speed, Njs), is significantly lower for tank iii) (285 rpm) than for the other two (330 rpm for i) and 390 rpm for ii)).
[0079] To summarize, the mixing apparatus according to the present disclosure can produce a more efficient mixing than prior art solutions.
[0080] It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead, they may vary within the scope of the claims.
