Apparatus and method for forming emulsions for use in flotation

TECHNICAL FIELD

[0001] The present invention relates to an apparatus and method for forming emulsions for use in flotation.

BACKGROUND ART

[0002] Minerals and coal need to be beneficiated from ore/concentrates/crudes before being sold into the market. Among the present beneficiation methods, froth flotation is widely employed for fine and ultrafine size fractions of ore. Froth flotation is a physicochemical separation process utilising air bubbles to selectively pick up certain minerals and transport the aggregate to the upper froth zone while leaving other minerals behind in the lower pulp phase. Collectors, which are chemicals that assist in adhering the targeted products to the air bubbles, are typically added to the pulp to increase yield. Froth flotation is a widely practised process that is well known to person skilled in the art.

[0003] A number of flotation processes use oily or hydrophobic chemicals to increase flotation recovery. For example, diesel can be used in coal flotation to improve flotation recovery. Diesel, and other oily chemicals, are insoluble in water and therefore have a low efficiency in dispersing in coal slurry and adsorbing on coal surfaces, or dispersing in mineral slurries and adsorbing on mineral surfaces. It is well documented that breaking diesel oil into fine droplets can improve its dispersion and adsorption efficiency, and therefore will improve coal flotation and reduce reagent consumption. However, to the best knowledge of the inventors, there is presently no large scale application of diesel emulsification in full-scale flotation plants. Traditional studies in this area have mainly focussed on chemical emulsifiers, which are surfactants. The benefit of diesel emulsification on coal flotation was demonstrated by using chemical emulsifiers. However, the industry is normally reluctant to use chemical emulsifiers as they can cause significant foaming problems in the flotation plant. In the industry, frother, a very weak surfactant, is used and it can cause foaming problems even at 10 ppm.

[0004] Forming emulsions of diesel or other oily chemicals for use in flotation plants will require the use of inexpensive but robust and reliable equipment. Identification of off-the-shelf equipment that meets these requirements has been problematic. [0005] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

SUMMARY OF INVENTION

[0006] The present invention is directed to an apparatus and method for forming emulsions for use in flotation, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

[0007] With the foregoing in view, the present invention resides broadly in an apparatus and method for forming emulsions for use in flotation.

[0008] In the first aspect, the present invention provides an apparatus for forming an emulsion comprising an aqueous phase and a hydrophobic phase comprising diesel or an oily component or a non-water soluble component, the apparatus comprising a venturi mixer and a static mixer in series.

[0009] The present invention can be used to emulsify diesel and any other oily collector and any other non-water-soluble chemical reagent used in flotation plants. Some examples include fatty acids which are commonly used as collectors for non-metallic minerals and thionocarbamate used as collector in the flotation of copper, lead/zinc and precious minerals.

[0010] In one embodiment, the apparatus comprises the venturi mixer placed upstream of the static mixer. In this embodiment, an aqueous stream may pass through a nozzle of the venturi mixer and the diesel or the oily component or the non-water soluble component may be fed to a low pressure area in the vicinity of the nozzle of the venturi. This creates a mixture comprising an aqueous phase and droplets of the diesel or the oily component or the non-water soluble component mixed through the aqueous phase, and this mixture subsequently passes through the static mixer. The static mixer further reduces the droplet size of the diesel or the oily component or the non-water soluble component in the emulsion.

[0011] In another embodiment, the apparatus comprises the static mixer being located upstream of the venturi mixer. In this embodiment, a mixture of the aqueous phase and the diesel or oily component or the non-water soluble component is fed to the static mixer. This creates a mixture containing droplets of diesel or the oily component or the non-water soluble component in the aqueous phase. This mixture is then passed to the venturi mixer to further reduce the size of the droplets of the diesel or the oily component or the non-water soluble component in the mixture to create the desired emulsion.

[0012] In preferred embodiments of the present invention, the venturi mixer is placed upstream of the static mixer. This is advantageous in that the diesel or oily component or the non- water soluble component is supplied to the low-pressure region of the venturi mixer, meaning that high-pressure pumps are not required to supply the diesel or the oily component or the non-water soluble component. Rather, the low-pressure region of the venturi mixer draws the diesel or oily component or the non-water soluble component into the venturi mixer, where it is mixed with the aqueous stream passing through the nozzle of the venturi mixer. In contrast, if the static mixer is placed upstream of the venturi mixer, the diesel or oily component or the non- water soluble component must be supplied at the same pressure or even a higher pressure than the pressure at which the aqueous stream is being supplied to the static mixer. In most flotation plants, this will require provision of additional high-pressure pumps for the diesel or oily component or the non-water soluble component. A further advantage of placing the venturi mixer upstream of the static mixer is that this arrangement can surprisingly obtain finer droplet size than having the static mixer placed upstream of the venturi mixer.

[0013] The static mixer and the venturi mixer can be of a number of different sizes and configurations. The person skilled in the art will readily understand how to select an appropriately sized and engineered static mixer and an appropriately sized and engineered venturi mixer for use in the present invention. The main factors that need to be taken into account in selecting the static mixer and the venturi mixer are the desired final droplet size of the hydrophobic phase, the pressure of the liquid flowing through the static mixer in the Venturi mixer, the relative amounts of the aqueous phase and the hydrophobic phase and the pressure applied to the liquids passing through the mixers.

[0014] Static mixers typically comprise a pipe or tube having stationary mixing elements located or fixed inside the pipe or tube. The static mixing elements may take many different forms, including helical elements, baffles, blades, plates, bars or a grid of bars. The mixing action in static mixers is achieved by the continuous splitting, extension and transportation of the components. Static mixers are typically supplied as simple plain ended flanged pipe sections, although they may also be supplied as elbows or T-pieces. Static mixers have no moving parts and are virtually maintenance free.

[0015] The static mixer used in the experimental work conducted by the present inventors today is a 3/8 inch static mixer with 12 elements from Koflo, as described at: [0016] It will be appreciated, of course, that a large number of other static mixers may also be used in the present invention.

[0017] Venturi mixers are also well known in the art. A venturi mixer has a motive fluid inlet that provides motive fluid (in this case, the aqueous stream) to a nozzle. The nozzle opens into a chamber or housing that has another inlet through which the diesel or oily component enters the venturi mixer. As the aqueous stream passes through the nozzle, a low-pressure region is created and this draws the diesel or oily component into the venturi mixer. The mixture of the aqueous phase and the diesel oily component then passes through a converging inlet nozzle and then through a diverging outlet diffuser and then through the outlet of the venturi mixer.

[0018] In one embodiment of the present invention, the ratio of the diameter of the motive fluid inlet to the diameter of the outlet of the nozzle is between 3:1 and 5:1, or about 4: 1.

[0019] In one embodiment, the apparatus in accordance with the first embodiment of the present invention can provide an emulsion having a droplet size for the diesel or oily component of between 5 pm and 10 pm, or between 6 pm and 10 pm, or between 6.5 and 8 pm, or between 6.5 and 7.8, or between 6.5 and 7.0 pm. Throughout this specification, the droplet size is the Sauter mean size (D32) calculated from the droplet size distribution measured by Malvern mastersizer.

[0020] In one embodiment, the aqueous phase is provided to the Venturi mixer at a pressure of between 300 kPa and 1000 kPa, or between 400 and 900 kPa, or between 400 and 800 kPa, or about 750 kPa, or between 600 - 750kPa. The actual pressure at which the aqueous phase is provided to the Venturi mixer will depend, in part, upon the ratio of aqueous phase to the hydrophobic phase in the final emulsion, the viscosity of the component(s) making up the hydrophobic phase, and the salinity of the aqueous phase. Advantageously, the pressures that will commonly achieve the desired droplet size in the final emulsion are readily available in most flotation plants.

[0021] Venturi mixers may be purchased from a number of different suppliers. Alternatively, as in the experimental work conducted by the present inventors, the venturi mixer may be specifically engineered and manufactured for the particular application.

[0022] In one embodiment, the apparatus further comprises a particle removal apparatus located upstream of the venturi mixer. The particle removal apparatus is suitably arranged to remove any particles in the aqueous phase that are sized at or larger than the opening to the nozzle in the venturi mixer. It will be appreciated that if particles sized similar to or larger than the diameter of the nozzle in the venturi mixer are fed to the venturi mixer, those particles could block the nozzle. Therefore, it is desirable to remove those particles from the aqueous stream prior to feeding the aqueous stream to the venturi mixer in commercial embodiments of the present invention. In one embodiment, the particle removal apparatus comprises a filter, a strainer or a screen and the aqueous stream must pass through the strainer, filter or the screen (especially the strainer) before it enters the venturi mixer. In one embodiment, the apparatus of the present invention is arranged such that the strainer, filter or the screen (especially the strainer) can be back-washed to remove collected particles from the strainer, filter or screen.

[0023] In a second aspect, the present invention provides a method of forming an emulsion comprising an aqueous phase and a hydrophobic phase, the hydrophobic phase comprising diesel or an oily component or a non-water soluble component, the method comprising passing the aqueous phase and the hydrophobic phase through a venturi mixer and a static mixer to obtain an emulsion comprising an aqueous phase having droplets of the hydrophobic phase therein, wherein the droplets of the hydrophobic phase are of the size of between 5 pm and 10 pm.

[0024] In one embodiment, the aqueous phase is provided to the venturi mixer at a pressure of between 500 kPa and 1000 kPa, or between 600 and 900 kPa, or between 700 and 800 kPa, or about 750 kPa, or between 600 - 750kPa.

[0025] In one embodiment, the static mixer is located upstream of the venturi mixer.

[0026] In another embodiment, the venturi mixer is located upstream of the static mixer. For the reasons as noted in the discussion of the first aspect of the present invention, a number of advantages arise from having the venturi mixer located upstream of the static mixer and this is a preferred embodiment of the method of the second aspect of the present invention.

[0027] The static mixer and the venturi mixer may be as described with reference to the first aspect of the present invention. For brevity of description, this need not be repeated for the second aspect of the present invention.

[0028] In another embodiment, the method of the present invention may further comprise measuring or determining droplet size of the diesel or oily component or the non-water soluble component in the emulsion and adjusting one or more of fluid pressure, relative amount diesel or oily component or the non-water soluble component to aqueous phase, when droplet size deviates from a pre-set droplet size to bring droplet size back towards the pre-set droplet size. [0029] In one embodiment, the hydrophobic phase comprises from about 2% to about 15% by volume of the emulsion, or from about 2% to 10% by volume of the emulsion, or from about 5% to 10% by volume of the emulsion.

[0030] In one embodiment of the present invention, the emulsion may be formed without requiring addition of any emulsifiers or emulsifying chemicals. This has apparent benefits for reducing reagent costs. This may also minimise formation of excess froth or foam.

[0031] The emulsion formed in accordance with the present invention may be used in a flotation process. The emulsion has a droplet size of the diesel or oily phase that can be controlled to be optimal for flotation of the coal or mineral in the flotation plant. This can result in improvements in yield in the flotation plant and reduction in the amount of reagents that need to be used in the flotation plant, thereby improving the economics of the flotation plant. In embodiments where the optimum droplet size for the flotation process is smaller than 5 pm, the emulsion produced by the method of the second aspect of the present invention will not be optimal but as the droplet size of the diesel or oily phase is smaller than currently available in conventional flotation plants, the flotation process will still be improved when compared to conventional flotation plants.

[0032] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0033] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

BRIEF DESCRIPTION OF DRAWINGS

[0034] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0035] Figure 1 shows a schematic diagram of an experimental setup investigating droplet size that can be obtained using a static mixer only;

[0036] Figure 2 shows a graph of droplet size vs water pressure obtained using the experimental setup of figure 1 ;

[0037] Figure 3 shows a graph of droplet size vs water pressure at different water salinities, obtained using the experimental setup of figure 1;

[0038] Figure 4 shows a schematic diagram of a venturi mixer suitable for use in embodiments of the present invention;

[0039] Figure 5 shows a schematic diagram of an apparatus or arrangement in accordance with an embodiment of the present invention; and

[0040] Figure 6 shows a schematic diagram of the arrangement allowing backwash of the filter in the aqueous stream line of the flowsheet shown in figure 5.

DESCRIPTION OF EMBODIMENTS

[0041] The experimental work that led to the development of the present invention was based upon a study of optimum droplet size range for use in flotation, such as coal flotation. The droplet size is the most critical parameter in diesel/oily emulsification. The droplet size determines the benefits of emulsification in flotation and the cost of producing the emulsion. If the droplet size is too coarse, it will not provide the expected benefit in flotation. If the droplet size is too fine, it consumes a much higher energy to produce and also has lower benefits for flotation.

[0042] This experimental work led to the discovery that the optimum diesel droplet size for coal flotation ranges from 2.4 pm to 10 pm, depending on the type of coal and the salinity of process water. In these experiments, two plants were found to have an optimum droplet size of 6.7 pm and a third plant was found to have an optimum droplet size of 2.4 pm. This experimental work also demonstrated that further decreasing the droplet size consumes much higher energy without a further benefit to coal flotation.

[0043] In order to provide a mixing arrangement that is inexpensive, robust and requires little maintenance, the inventors first investigated using a static mixer to obtain an oil-in-water emulsion. The experimental setup that was used is shown in figure 1. In figure 1, water 10 is supplied by pump 11 to a first inlet 12 of static mixer 13. The static mixer 13 was a clear PVC static mixer from Koflo Corporation, model number 3/8-40C-4-12-2 having 3/8 inch pipe diameters, 12 mixing elements and a length of 11 inches. Oil 14 is supplied by pump 15 to a second inlet 16 of the static mixer 13. The mixture of water and oil then passes through static mixing elements 17, which mixes the oil and water together and breaks the oil phase into small droplets. An emulsion 18 containing droplets of oil in water was formed.

[0044] The emulsion 18 was analysed to determine droplet size of the oil phase. The droplet size was measured by Malvern mastersizer which analyses droplets using the laser diffraction technique. The results of this analysis are shown in figure 2. As can be seen, as the water pressure increases, the droplet size of the oil phase decreases. However, a law of diminishing returns was present and increasing water pressure beyond 500 kPa had little effect on further decreasing droplet size. The experimental work shown in figure 2 reveals that the static mixer can achieve a minimum droplet size of around 10 pm. The droplet size given in figure 2 is the Sauter mean size (D32). It is calculated from the droplet size distribution measured by Malvern mastersizer. D32 is a commonly used parameter for evaluating the quality of emulsions.

[0045] Figure 3 essentially repeated the experiments using the apparatus shown in figure 1, but with waters of different salinity. In this regard, most process water available in flotation plants in Australia comprises saline water of low to medium salinity. In the experimental work shown in figure 3, three different types of water were used as the aqueous feed, these being freshwater, saline water of low to medium salinity (conductivity of 3.2mS/cm), and saline water of high salinity (9.9mS/cm).

[0046] The results of these experiments are shown in figure 3. In general terms, increasing salinity results in an increase in the droplet size of the oil/diesel in the oil phase. However, for the low to medium salinity, this increase in droplet size was relatively small when compared to freshwater.

[0047] The results shown in figures 2 and 3 indicate that use of a static mixer alone will not be sufficient to obtain droplet size in the diesel/oily phase that is around the optimum droplet size for flotation, bearing in mind that two of the coal plants tested had an optimum droplet size of 6.7 pm. Using a static mixer alone and low to medium salinity water, the droplet size in the diesel/oily phase was found to be 11.5 pm at a water pressure of 750kPa. It will also be appreciated that the diesel/oil had to be supplied to the static mixer at the same pressure or a higher pressure than the water pressure to prevent blowback of water to the diesel/oil tank.

[0048] In order to further decrease the droplet size, the inventors gave consideration to using two static mixers in series. However, the inventors found that simply increasing the number of static mixing elements did not further decrease droplet size once more then 10 to 12 static mixing elements were in place. Therefore, placing a second static mixer in series with the first static mixer would not further lower the droplet size in the diesel/oily phase.

[0049] The inventors subsequently developed an experimental setup in which a venturi mixer was used in combination with the static mixer. A schematic diagram of the venturi mixer 32 is shown in figure 4. The venturi mixer 32 has an inlet 1 for motive fluid (in this case, the aqueous phase). Inlet 1 is in fluid communication with a nozzle 2. The inlet 1 has a diameter of about 1/2 inch and the nozzle has an opening with a diameter of about 4mm. The venturi mixer 32 has a second inlet 7, which provides an inlet for the diesel/oil component. As water leaves the opening in the nozzle 2, a low-pressure region is created near the nozzle 2 and this draws diesel/oil into the venturi mixer 32. The water leaving nozzle 2 is at high velocity and it breaks up the diesel/oil into small droplets. This mixture of water and droplets of oil pass through converging inlet nozzle 3, through diffuser throat 6 and out through diverging outlet diffuser 4. The mixture of water and droplets of diesel/oil leaves the venturi mixer through outlet 5. The venturi mixer 32 shown in figure 3 is essentially conventional in construction and will be well understood by persons skilled in the art.

[0050] Figure 5 shows a flow sheet detailing a possible mixer arrangement for forming a water/diesel emulsion that contains droplets of diesel sized around 6.5 pm. In the flowsheet shown in figure 5, a plant process water line 20 provides process water from a coal processing plant. Most Australian coal processing plants have process water that comprises saline water of low to medium salinity. The process water passes through an optional in-line filter 22.

[0051] The in-line filter 22 is an optional unit and is only required for processing plants which may have large debris or contaminants in their process water stream. Debris larger than 1 mm have a potential to block the venturi and static mixer. In this case, an in-line filter with automatic backwash can to be installed. An example design is shown in Figure 6. A screen filter will be installed in the pipe. Differential pressure will be used to detect the blockage of the screen filter. If a blockage identified, a set of valves will be automatically adjusted to backwash the filter. The apparatus controlling the filter 22, as shown in figure 6, is essentially conventional and will be well understood by persons skilled in the art. Commercially available Y-type strainer and T-type strainer can also be used as in-line filter for this purpose.

[0052] The process water then optionally passes a water conductivity probe 24. The water conductivity probe 24 can be used to monitor the water conductivity, which has a significant impact on diesel emulsification. This can be useful for processing plants which have a large variation in the conductivity of their process water. [0053] The process water then passes through a flow control valve 26. A water pressure transmitter 28 monitors water pressure and pressure signals are sent to a pressure controller 30. The flow control valve 26, water pressure transmitter 28 and pressure controller 30 enable automatic water pressure control. The water pressure will be adjusted to a set point by adjusting the flow control valve 26. This is important for the static mixer, since the water pressure is the most important parameter determining the droplet size.

[0054] The process water then passes into venturi 32, more specifically into inlet 1 of venturi mixer 32 (see figure 4).

[0055] The flowsheet shown in figure 5 also includes a plant diesel dosing line 34 that provides a supply of diesel for the emulsion. Some controls need to be in place when connecting the plant diesel line to the emulsification system. A valve 35 is present to connect the plant diesel dosing line 34, the diesel bypass line 42 and the line to venturi 32. A check valve 36 is added to avoid the backflow of water to the diesel line. A flow transmitter 38 is added to monitor the diesel flow rate. Diesel bypass controller 40 will be added to adjust the valve position to bypass the emulsification system when there is anything wrong with the emulsification system. For example, if there is a blockage in the static mixer, the diesel flow rate will drop, which will trigger the bypass controller and diesel will then flow along diesel bypass line 42. This will ensure the plant operation will not be interrupted when there is a malfunction of the emulsification system.

[0056] The emulsification system includes the venturi mixer 32 and a static mixer 44 in series with the venturi mixer 32. The static mixer 44 may be as described with reference to the static mixer 13 as shown in figure 1. The mixture of water and diesel, in the form of droplets of diesel mixed with the water, that leaves the venturi mixer 32, and passes into the static mixer 44, where further mixing and size reduction occurs. An emulsion containing an aqueous phase having droplets of diesel therein is removed from the static mixer via line 46.

[0057] A sampling port 48 is included to measure the droplet size during emulsification using FBRM (focused beam reflectance measurement). A control panel, shown schematically at 50, will be added to monitor and control key parameters, including water pressure, diesel flow rate, and water conductivity. The droplet size will be calculated based on these key parameters by using a developed algorithm. Droplet size is basically a function of water pressure, diesel flow rate, and water conductivity. The droplet size can also be automatically controlled to a set point by adjusting these parameters.

[0058] Emulsified diesel can then be provided to the flotation cell via diesel dosing point 52. [0059] For plant implementation, both the static mixer 44 and venturi mixer 32 are suitably made of stainless steel or other metal alloy to reduce the wear and corrosion and increase its lifetime.

[0060] The embodiment shown in figure 5, in which the venturi mixer 32 is positioned directly upstream of the static mixer 44, is the preferred embodiment of the present invention. In this regard, when placing the venturi mixer before the static mixer, the diesel feed pressure will be negative because of the venturi effect, which is more feasible for processing plants with limited diesel feed pressure. In contrast, if the static mixer is positioned directly upstream of the venturi mixer, the diesel feed pressure needs to be higher than the water line pressure. This might require provision of an additional higher pressure diesel pump.

[0061] The present inventors conducted tests to prepare diesel emulsions in water using a static mixer only, venturi mixer only, both the static mixer and venturi mixer with the static mixer before the venturi mixer, and both the static mixer and venturi mixer with the Venturi mixer before the static mixer. The diesel droplet size in the emulsion obtained using a feed water pressure of 750 kPa are set out in Table 1 below:

[0062] Table 1:

[0063] As shown in Table 1, the static mixer alone and the venturi mixer alone are not capable of achieving a droplet size of 6.7 pm, which is the optimum droplet size for coal flotation in two of the coal plants that were tested. Using the static mixer before the venturi mixer can get a diesel droplet size down to 7.8 pm, which is significantly closer to the optimum droplet size for two of the coal plants tested. Using the venturi mixer before the static mixer gives a droplet size of 6.5 pm, which is almost exactly what is required for optimum flotation performance in two of the coal plants tested.

[0064] The combination of the static mixer and the venturi mixer unexpectedly allows for significantly smaller diesel droplets to be obtained in the diesel/water emulsion. For some coal plants, the optimum droplet size for flotation performance is 6.7 pm and the apparatus shown in figure 5 can achieve this droplet size. For other plants that have a smaller optimum droplet size, use of the apparatus and method of the present invention can still result in significant improvements to flotation performance, even if droplet size is not fully optimised.

[0065] Using a venturi mixer in combination with a static mixer, as shown in figure 5, provides a mixing apparatus that has no moving parts, is robust and inexpensive, and will require little or no maintenance. The mixing apparatus only requires water pressure to operate and this water pressure is already present in the process water lines of most coal processing plants. Accordingly, implementation of the method and apparatus into a coal flotation plant will require a minimum of extra capital expenditure. As the flotation process can run at or closer to its optimum, improvements in flotation yield and reductions in reagent costs can also be achieved.

[0066] In the present specification and claims, the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0067] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment.

Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0068] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.