Flotation columns are used in the froth flotation concentration of chemical substances such as minerals, oils, and inks. In a typical mineral flotation process, finely graded ore, containing minerals and gangue, is suspended in a liquid. The liquid and the suspended ore particles are injected together with reagents into a flotation column or tank at a predetermined distance from the top of the column. The column or tank typically contains a plurality of vertically oriented baffles to assist in minimizing any fluid recirculation throughout the column which is detrimental to the flotation separation process. At the bottom of the column, air is injected to form small bubbles which then rise and carry entrained minerals to the surface of the column. The mineral content is removed via overflow at the top of the column.

Wash water may be applied at the top of the column to wash down gangue particles, which are then removed at the bottom of the column.

In order to generate gas or air bubbles in a flotation column, at least one an aeration stream is directed into the column. The aeration stream may be a gas-containing liquid or, alternatively, gas or air without water or other liquid.

To effect the injection of an aerated stream into the column, various devices such as spargers, injectors, aspirators, nozzles and bubble generators are commonly used. The air is released from the aeration stream as minute bubbles when pressure is reduced as the stream exits a discharge tube and enters the column. Air that is not properly dispersed may show up as undesirably large bubbles that rise rapidly to the surface where they burst with a resultant destruction of froth and the sinking of solids.

While the bubble size is generally related to the size of the particles to be separated from the ore in the column, highly uniform small diameter bubbles are required to efficiently float fine mineral particles for removal to the overflow at the top of the column.

Spargers are well known for use in the separation of minerals from gangue in froth separation processes. Typically, spargers include one or more distribution rings of nozzles to supply air into and across the column. In some instances, rather than supplying aerated water to the column, air is supplied to the column to form the required bubbles.

In many conventional sparging systems, the rate of gas introduction is varied depending on required flotation process parameters. However, varying the rate of gas introduction generally changes the sizes of the bubbles.

Increasing bubble size is generally associated with inefficient column operation.

Large bubbles generally increase mixing in the flotation column and decrease the probability that bubbles and particles will collide. In addition, large bubbles reduce the reaction rate.

SUMMARY OF THE INVENTION The present invention is directed in part to an improved nozzle assembly for a sparging system which may be incorporated into any aerated contacting reaction vessel including froth flotation columns, froth flotation cells, aerated pressurized vessels, and oxidation reactors. The sparger nozzle assembly comprises a nozzle having a converging-diverging section, that is, a section with a converging upstream portion and a diverging downstream portion. In other words, the nozzle has a passageway with a constriction or nozzle throat. The passageway walls preferably converge and diverge gradually.

In accordance with another feature of the present invention, the sparger nozzle assemblies all include Laval nozzles.

In a specific embodiment of the present invention, a sparger nozzle assembly comprises a housing having an inlet and an outlet for a gas-containing liquid, a check valve mounted to the housing in a flow passageway extending from the inlet to the outlet, and a velocity-increasing section of the passageway downstream of the valve.

In any case, a nozzle included in a nozzle assembly in accordance with the present invention is of a design for generating an outlet speed in excess of Mach 1, thereby maximizing the shear forces exerted on the gas at the nozzle tip and producing optimally small bubbles for a contacting reaction process such as the flotation of mineral values. The nozzle is exemplarily a Laval nozzle. Both the check valve and the Laval nozzle are advantageously constructed at least in part of wear resistant materials.

It is well established that flotation rate and, therefore, flotation recovery are directly related to the bubble surface area flux rate. Bubble surface area flux rate Sb, in turn, is directly proportional to the superficial gas velocity Vg and inversely proportion to the bubble diameter Db : Sb = 6Vg/Db.

A nozzle assembly in accordance with the present invention produces gas bubbles of an optimal size. The bubble size is fixed within a narrow range, regardless of the selected operating parameters of the flotation column. This fixing of the bubble size within an optimal range of values results in efficient operation of a flotation column incorporating such nozzle assemblies. The substantially uniform fixed bubble size is achieved by using a nozzle assembly designed to ensure that the gas is injected into the flotation column at speeds in excess of Mach 1. The gas (e. g., air) is injected at supersonic gas injection velocities regardless of the bubble surface area flux rate. The bubble surface area flux rate may be adjusted, for example, by varying the gas pressure upstream of the nozzle or by deactivating (turning off) one or more nozzles of a sparging system.

In a method for operating an aerated reaction vessel in accordance with the present invention, the vessel is provided with a sparging system including a plurality of nozzle assemblies. Each of the nozzle assemblies is operated to inject gas into the vessel at supersonic speeds, thereby generating bubbles having sizes within an optimal range.

A nozzle assembly in accordance with the present invention may be easily retrofit to existing aerated reaction vessels such as flotation columns. Where the sparger pipes are formed at their downstream ends with screw threads, the nozzle body is provided with a mating thread enabling simple attachment.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is partially a partial cross-sectional view of a froth flotation column and partially a side elevational view of a sparger nozzle, in accordance with the present invention.

Fig. 2 is a front elevational view of the nozzle of Fig. 1.

Fig. 3 is a longitudinal cross-sectional view taken along line III-III in Fig. 1.

Fig. 4 is a longitudinal cross-sectional view of a sparger nozzle assembly in accordance with the present invention.

Fig. 5 is a longitudinal cross-sectional view similar to Fig. 3, showing a modified sparger nozzle in accordance with the present invention.

Fig. 6 is a partial longitudinal cross-sectional view similar to a portion of Fig. 3, showing a related sparger nozzle modification in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in Fig. 1, a sparger nozzle 10 has an inlet end 12 connected via screw threads 14 to a free end of a sparger pipe 16 which traverses an opening 18 in a side wall 20 of a froth flotation column 22 (or other aerated reaction vessel). Sparger pipe 16 is mechanically connected to side wall 20 via a column connector assembly and is coupled to a compression fitting assembly and a valve assembly (none shown) outside flotation column wall 20, as illustrated and described in U. S. Patent No. 5, 676, 823, the disclosure of which is hereby incorporated by reference.

Flotation column 22 preferably incorporates a plurality of sparger pipes 16 angularly spaced from one another and extending radially in a plane near a lower end of the column. Each sparger pipe is provided with a respective nozzle 10.

As depicted in Fig. 3, nozzle 10 comprises a body member 24 having a flow passageway 26 provided with a flow-constriction or nozzle throat 28.

Because of flow constriction 28, passageway 26 decreases in cross-sectional area from inlet end 12 to a transverse section 30 of minimal cross-section and increases in cross-sectional area from transverse section 30 to an outlet end 32.

Flow constriction 28 induces an increase in the velocity of a pressurized gas such as air which enters passageway 26 via sparger pipe 16. For a wide range of inlet pressures, the gas velocity is increased from Mach 1 at flow-constriction or nozzle throat 28 to a substantially fixed value in excess of Mach 1. The supersonic outlet speed maximizes the shear forces exerted on the gas at nozzle outlet end 32 and thereby produces optimally small bubbles for flotation of mineral values.

Nozzle 10 is exemplarily a Laval nozzle made of wear-resistant materials, at least along passageway 26 and flow constriction 28. Passageway 26 may have any of a variety of profiles upstream and downstream of flow constriction 28.

For example, the profile may be straight on one or both sides, i. e., passageway 26 may have a conical converging section and/or a conical diverging section, as discussed below with reference to Fig. 5. Alternatively, passageway 26 may have a smoothly curving profile as illustrated in Fig. 3, which will enhance efficiencies.

The size of bubbles ejected from passageway 26 at outlet end 32 is essentially fixed within an optimal range, regardless of the selected operating parameters of the flotation column. In particular, the gas ejection rate and therefore the bubble size will stay at least approximately constant for different bubble surface area flux rates or gas flow rates within the column or reactor vessel. The bubble surface area flux rate may be adjusted, for example, by varying the gas pressure upstream of nozzle 10, within a range above a minimum value, or by deactivating (turning off) one or more nozzles of a sparging system.

The fixing of the bubble size at an optimal value thus results in efficient operation of flotation column 22.

As illustrated in Fig. 4, an alternative nozzle assembly 34 for a froth flotation column comprises a body member or housing 36 attached to and traversing a wall 37 of a flotation column. Body member or housing 36 has a gas inlet 38 at one end and an outlet opening or mouth 40 at another end. Between inlet 38 and outlet opening 40 is a flow passageway 42 including an upstream chamber 44 and a downstream portion 46. Downstream portion 46 defines a flow passageway 48 provided with a flow-constriction 50 so that the passageway decreases in cross-sectional area from an inlet or upstream end 52 to a transverse section 54 of minimal cross-section and increases in cross-sectional area from transverse section 54 to outlet opening 40. As discussed above with reference to Figs. 1-3, flow constriction 50 ensures that gas exits through outlet opening 40 at a supersonic speed regardless of variations (within limits) in the pressure head. Downstream portion 46 takes the form of a Laval nozzle which produces optimally small bubbles for flotation of mineral values. Body member or housing 36 is preferably made of wear-resistant materials, at least along passageway 48 and flow constriction 50.

Nozzle assembly 34 incorporates a spring-loaded valve mechanism 56 which acts as a check valve to prevent back flow of process slurry and/or fluid on loss of air. Valve mechanism 56 includes a rod 58 which extends longitudinally or axially through chamber 44. At one end rod 58 carries a dart valve member 60 made of wear-resistant material and seated in inlet or upstream end 52 of downstream portion 46. At an opposite end, rod 58 is provided with a flange 62 and a compression spring 64 seated between the flange and a transverse end wall 66 of housing or body member 36. Rod 58 traverses a bore 68 in end wall 66 and is provided with a bellows-type sealing element 70 connected on one side to rod 58 and on another side to wall 66. An end cap 72 screwed to housing or body member 36 surrounds spring 64.

The size and number of sparger nozzles 10 or 34 are selected based on maximum gas velocity that will provide a suitable environment for flotation (bubbly flow).

As shown in Fig. 5, a modified sparger nozzle 10'similar to nozzle 10 of Figs. 1-3 has an inlet end 12'with internal screw threads 14'for coupling to a free end of a sparger pipe not shown) which traverses an opening 18 in a side wall 20 of a froth flotation column 22. Nozzle 10'comprises a body member 24' having a flow passageway 26'provided with a flow-constriction or nozzle throat 28'located at a junction of a converging conical inlet portion 74 and a diverging conical outlet portion 76 of flow passageway 26'. Flow constriction 28'induces an increase in the velocity of a pressurized gas such as air which enters passageway 26'via the sparger pipe. As in all of the nozzles specifically disclosed herein, the magnitude of the velocity increase depends largely on the ratio of the cross-section area of a nozzle outlet 32'and the cross-sectional area of a transverse section 30'at constriction or throat 28'. For a wide range of inlet pressures, the gas velocity is increased from Mach 1 at flow-constriction or nozzle throat 28'to a substantially fixed value in excess of Mach 1. The supersonic outlet speed maximizes the shear forces exerted on the gas at nozzle outlet 32'and thereby produces optimally small bubbles for flotation of mineral values. The operation of nozzle 10'is essentially the same as that of nozzle 10, described above.

As illustrated in Fig. 6, an alternative nozzle assembly 34'for a froth flotation column or other aerated reaction vessel comprises a body member or housing 36'attached to and traversing a wall (not shown) of the flotation column or reaction vessel. Body member or housing 36'has an outlet opening or mouth 40'at an exit end. A downstream portion 46'of a flow passageway 42'includes a converging conical section 78 and a diverging conical section 48'. Conical sections 78 and 48'join one another at a flow-constriction or throat 50'so that the passageway 42'decreases in cross-sectional area from an inlet or upstream end 52'to a transverse section 54'and increases in cross-sectional area from transverse section 54'to outlet opening 40'. As discussed above with reference to Figs. 1-3, flow constriction 50'ensures that gas exits through outlet opening 40 at a supersonic speed regardless of variations (within limits) in the pressure head. As further discussed above, the speed of fluid, particularly gas, at outlet opening 40'depends essentially on the ratio of the area of outlet opening 40'to the area of transverse section 54'. The larger this ratio, the higher the velocity of the ejected gases, the greater the shear forces, and the smaller the bubbles injected into a flotation column or other aerated reaction vessel.

Downstream portion 46'is a type of Laval nozzle which produces optimally small bubbles for flotation of mineral values. Body member or housing 36'is preferably made of wear-resistant materials, at least along passageway section 42'and flow constriction 50'.

Like the embodiment of Fig. 4, nozzle assembly 34'incorporates a spring- loaded valve mechanism 56'which acts as a check valve to prevent back flow of process slurry and/or fluid on loss of air. Valve mechanism 56'includes a rod 58' which extends longitudinally or axially through an upstream chamber 44'. At one end rod 58'carries a dart valve member 60'made of wear-resistant material and seated in inlet or upstream end 52'of downstream portion 46'. The following table illustrates how a significant reduction in air bubble size is achievable when utilizing speed and shear enhancing converging-diverging nozzles 10, 10', 34, 34' in a sparging system. Mach No. Frother, ppm Average Diameter, mm 1.00 2 0.60 1. 00 10 0. 36 1. 70 2 0. 43 1. 70 10 0. 30 Gas injected into a flotation column or, more generally, a reaction vessel pursuant to the present invention may be conveyed to nozzle 10, 34, 10', 34'in gaseous form, as a mixture of gas and water, or dissolved in a liquid such as water. In any case, gas delivery is effectuated via a gas-containing fluid : the fluid is either a gas containing nothing but the gas or a liquid containing the gas.

Accordingly, the term"gas-containing fluid"as used herein designates both a pure gas or a liquid containing gas.

Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention.

For instance, it is to be understood that a sparging system and nozzle in accordance with the present invention may be used in any reaction vessel requiring the dispersal of small gas or air bubbles in a liquid or slurry. Such aerated reaction vessels include, but are not limited to, contacting reaction vessels, flotation columns, flotation cells of any type, pressurized aerated vessels, and oxidation vessels.

Nozzle 10 may be connected to sparger pipe 16 by means other than internal screw thread 14. Alternative connectors include an external screw thread, one or more set screws, brazing or welding, and other coupling well known to those skilled in the art.

Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.