FIELD OF THE INVENTION

The present invention is directed to the processing of mined materials including organic compounds (such as coal) and inorganic compounds (such as minerals). In particular, but not exclusively, the present invention is directed to biodiesel compositions and uses thereof in methods for separation and/or recovery of a mined material such as coal.

BACKGROUND TO THE INVENTION

In the processing of mined materials, it is often necessary to separate a subset of components from a mixture in a process stream. The technique of froth flotation is often used for physically separating particles based on the differential abilities of the various particles in a process stream to adhere to air bubbles injected into a generally aqueous slurry. Particles which readily adhere to air bubbles are floated to the surface to form a froth phase, and are therefore partitioned away from the particles that remain completely wetted and remain in the lower slurry phase. Generally speaking, particles having hydrophobic characteristics are more likely to adhere to the air bubbles given their propensity to avoid water molecules.

Froth flotation is used in processing a broad range of mined materials, with a few examples being separation of fine coal particles from ash-forming minerals, separation of sulfide minerals from silica gangue and from other sulfide minerals; separation of potassium chloride from sodium chloride; separating silicate minerals from iron ores; and separating phosphate minerals from silicates.

Devising a froth flotation process is a complex undertaking, given the interplay between three major components: (i) equipment (including flotation cell design, agitation, and air flow), (ii) operation (including feed rate, particle size, pulp density and temperature), and (iii) chemistry (including collectors, frothers, activators, depressants and pH).

Collectors are important agents in froth flotation, functioning to selectively adsorb onto the surfaces of particles in a flotation process. The collector molecules form a thin hydrophobic film on the particle, which in turn increases the propensity for to the particle to adhere to a bubble.

Collectors can be generally classified according to their ionic charge: nonionic, anionic, or cationic. Nonionic collectors are typically simple hydrocarbon oils, and similar compounds, that have an affinity for surfaces that already having hydrophobic properties. Nonionic collectors selectively adsorb on these surfaces, and increase their hydrophobicity. The most commonly floated naturally-hydrophobic material is coal. The use of collectors such as fuel oil and kerosene significantly and selectively increases the hydrophobicity of the coal particles largely without affecting the surfaces of associated ash-forming minerals. The floated coal particles are recovered to thereby improve the overall recovery of coal.

While fuel oil and kerosene are generally effective and economical collectors, problems in their use nevertheless present. One problem is the variation in composition of these hydrocarbon preparations. Both preparations have variable proportions of different chain length hydrocarbons, this leading to a variation in performance in flotation applications. A further problem is that of recovery efficiency, which may be improved. Yet another problem relates to environmental and safety concerns surrounding the use of such hydrocarbon preparations. Froth floatation is also used for the complex separation of minerals. For the flotation of minerals having some degree of hydrophobicity, collectors may be used to improve hydrophobicity thereby promoting attachment to air bubbles. Prior art collectors may have similar problems to those used in coal floatation. It is an aspect of the invention to overcome one or more problems of the prior art by providing improved collectors for use in froth flotation applications. It is a further aspect to provide a useful alternative to prior art collectors.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. SUMMARY OF THE INVENTION

In a first aspect, but not necessarily the broadest aspect, the present invention provides a method for floating a particle in the processing of a liquid mixture (such as a slurry or a pulp), the method comprising the steps of: contacting a liquid mixture having a plurality of particles with a biodiesel so as to form a particle-biodiesel complex, contacting the particle-biodiesel complex with a plurality of gas bubbles so as to form a particle-biodiesel-bubble complex, and floating the particle-biodiesel-bubble complex to the surface of the liquid mixture. In one embodiment of the first aspect, the biodiesel is derived from an oil feedstock.

In one embodiment of the first aspect, the oil feedstock is plant-derived.

In one embodiment of the first aspect, the oil feedstock is a vegetable oil.

In one embodiment of the first aspect, the oil feedstock is derived from a plant of the family Asteraceae; optionally the subfamily: Helianthoideae; optionally the tribe Heliantheae; optionally the genus Helianthus, optionally the species H. annuus including any genetically modified variants thereof.

In one embodiment of the first aspect, the oil feedstock is derived from a plant of the order Poaceae; optionally the genus Oryza, optionally the species: O. sativa or O. glabberrima including any genetically modified variants thereof. In one embodiment of the first aspect, the oil feedstock is derived from a plant having an oil containing less than about 2% erucic acid.

In one embodiment of the first aspect, the oil feedstock is a canola oil. In one embodiment of the first aspect, the oil feedstock is derived from a plant of the order Brassicales; optionally the family Brassicaceae; optionally the geneus Brassica; optionally the species: B rapa or B napus or B juncea including any genetically modified variants thereof.

In one embodiment of the first aspect, the oil feedstock in not animal-derived. In one embodiment of the first aspect, the oil feedstock is synthetic.

In one embodiment of the first aspect, the biodiesel comprises a methyl ester having the formula: CH3-0-CO-R, wherein R is a hydrocarbon chain having a backbone of between 15 and 17 carbon atoms.

In one embodiment of the first aspect, the methyl ester is selected from the group consisting of methyl palmitate, methyl stearate, cis-9-oleic acid methyl ester, methyl linoleate, and methyl linolenate.

In one embodiment of the first aspect, the biodiesel comprises a combination of any two, three, four, or five of the following methyl esters: methyl palmitate, methyl stearate, cis-9-oleic acid methyl ester, methyl linoleate, methyl linolenate.

In one embodiment of the first aspect, the biodiesel comprises methyl palmitate and/or methyl lineolate.

In one embodiment of the first aspect, the biodiesel comprises methyl palmitate at a concentration of at least about 5% w/w of all esters contained in the biodiesel.

In one embodiment of the first aspect, the biodiesel comprises methyl linoleate at a concentration of at least about 5% w/w of all esters contained in the biodiesel. In one embodiment of the first aspect, the biodiesel comprises methyl palmitate and methyl linoleate at a combined concentration of at least about 10% w/w of all esters contained in the biodiesel.

In one embodiment of the first aspect, the method is devoid of the use of a petroleum-derived hydrocarbon as a collector in the formation of a particle-collector-bubble complex.

In one embodiment of the first aspect, the particle is a coal particle. In one embodiment of the first aspect, the coal particle is in the size range of from about 50 μπι to about 500 μπι.

In one embodiment of the first aspect, the method comprises the step of physically separating the floated particle from unfloated material in the liquid mixture.

In a second aspect, the present invention provides the use of a biodiesel as a collector in the flotation of a particle in a froth floatation process. In one embodiment of the second aspect, the biodiesel is derived from an oil feedstock.

In one embodiment of the second aspect, the oil feedstock is plant-derived.

In one embodiment of the second aspect, the oil feedstock is a vegetable oil.

In one embodiment of the second aspect, the oil feedstock is derived from a plant of the family Asteraceae; optionally the subfamily: Helianthoideae; optionally the tribe Heliantheae; optionally the genus Helianthus, optionally the species H. annuus including any genetically modified variants thereof.

In one embodiment of the second aspect, the oil feedstock is derived from a plant of the order Poaceae; optionally the genus Oryza, optionally the species: O. sativa or O. glabberrima including any genetically modified variants thereof. In one embodiment of the second aspect, the oil feedstock is derived from a plant having an oil containing less than about 2% erucic acid.

In one embodiment of the second aspect, the oil feedstock is a canola oil. In one embodiment of the second aspect, the oil feedstock is derived from a plant of the order Brassicales; optionally the family Brassicaceae; optionally the geneus Brassica; optionally the species: B rapa or B napus or B juncea including any genetically modified variants thereof.

In one embodiment of the second aspect, the oil feedstock in not animal-derived. In one embodiment of the second aspect, the oil feedstock is synthetic.

In one embodiment of the second aspect, the biodiesel comprises a methyl ester having the formula: CH3-0-CO-R, wherein R is a hydrocarbon chain having a backbone of between 15 and 17 carbon atoms.

In one embodiment of the second aspect, the methyl ester is selected from the group consisting of methyl palmitate, methyl stearate, cis-9-oleic acid methyl ester, methyl linoleate, and methyl linolenate.

In one embodiment of the second aspect, the biodiesel comprises a combination of any two, three, four, or five of the following methyl esters: methyl palmitate, methyl stearate, cis-9-oleic acid methyl ester, methyl linoleate, methyl linolenate.

In one embodiment of the second aspect, the biodiesel comprises methyl palmitate and/or methyl lineolate.

In one embodiment of the second aspect, the biodiesel comprises methyl palmitate at a concentration of at least about 5% w/w of all esters contained in the biodiesel.

In one embodiment of the second aspect, the biodiesel comprises methyl linoleate at a concentration of at least about 5% w/w of all esters contained in the biodiesel. In one embodiment of the second aspect, the biodiesel comprises methyl palmitate and methyl linoleate at a combined concentration of at least about 10% w/w of all esters contained in the biodiesel.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 A to ID are graphical representations of the composition of esters types in the various biodiesel compositions used in the coal floatation experiments described herein. The data represented in these figures are derived from Table 1. FIG. 2A is a graphical representation of the results of comparative coal flotation experiments described herein showing the composition of coal recovered by flotation. The data represented in this figure is derived from Table 2. For each collector, the bars each represent (form left to right): carbon, ash, and volatiles.

FIG. 2B is a graphical representation of the results of comparative coal flotation experiments described herein showing the recovery of coal by flotation. The data represented in this figure are derived from Table 2.

DETAILED DESCRIPTION OF THE INVENTION

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, appearances 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, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. Similarly it should be appreciated that the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and from different embodiments, as would be understood by those in the art. In the claims below and the description herein, any one of the terms "comprising", "comprised of or "which comprises" is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a method comprising step A and step B should not be limited to methods consisting only of methods A and B. Any one of the terms "including" or "which includes" or "that includes" as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, "including" is synonymous with and means "comprising".

Furthermore, it is not represented that all embodiments display all advantages of the invention, although some may. Some embodiments may display only one or several of the advantages. Some embodiments may display none of the advantages referred to herein.

The present invention is predicated at least in part on Applicant's finding that biodiesels derived from oil feedstock, and particularly plant-derived oil feedstock are useful as collector agents in froth floatation applications. It has been discovered that esters derived from oil feedstock are effective substitutes for prior art collectors such as kerosene and fuel oil. In some cases, the biodiesels provide advantages over prior art collectors as further described herein. Accordingly, in a first aspect the present invention provides a method for floating a particle in the processing of a liquid mixture (which is typically a slurry), the method comprising the steps of: contacting a liquid mixture having a plurality of particles with a biodiesel so as to form a particle-biodiesel complex, contacting the particle-biodiesel complex with a plurality of gas bubbles so as to form a particle-biodiesel-bubble complex, and floating the particle-biodiesel-bubble complex to the surface of the liquid mixture,

Particles for which the present methods are contemplated to applicable include any fine particles which a potentially floatable in a froth flotation method, and where the particles are floated with the assistance of a floatation agent which increases the hydrophobicity of the particle. In particular, the methods are applicable to mining applications, including the processing of minerals (such as sulphides, carbonates, oxides and phosphates) and non- minerals such as coal. In a froth flotation method, the material to be treated may be reduced to fine particles by crushing and grinding so that the various minerals exist as physically discrete grains. The particle sizes are typically less than 0.1 mm (100 μπι). In applications such as coal floatation, the target of flotation is the existing fine coal particles and therefore crushing is not a required step.

For the purpose of a brief overview and with reference to coal flotation, the present methods may be applied to the cleaning of fine coal particles. By this technique, fine particles of coal are separated from other components in the mined material by the selective attachment of air bubbles to the particles causing them to be buoyed to the surface of a coal-water suspension where there are collected. Since non-coal particles (which are typically hydrophilic, and therefore do not effectively complex with a hydrophobic collector) remain unattached to the bubbles, they are not recovered in the froth. This process cleans and separates the fine particles in a turbulent, aqueous environment where specific gravity is of lesser importance in the separation process as compared with the surface properties of the particles.

Froth floatation typically involves the thorough mixing of the fine coal particles with water. Frothers and other conditioning agents (including collectors) are added to the slurry and as air is bubbled up through the slurry the coal particles attach themselves to the bubbles and are carried to the surface. The resulting froth, containing primarily the hydrophobic components, is skimmed off, separating the coal from the mineral particles which remain in the water suspension.

The biodiesel used in the present methods may be any composition produced by the transesterification of lipids to produce fatty acid esters having 16-20 carbon atoms, 2 oxygen atoms and 28-38 hydrogen atoms. Biodiesel can be manufactured from a vegetable oil, an animal fat or a waste oil, or indeed a combination of feedstock oils. Biodiesel may furthermore be manufactured synthetically, albeit the cost would likely render it uneconomic for the large scale industrial methods proposed herein.

Methods for the manufacture of biodiesel from biological lipids are well known to the skilled artisan by the process of transesterification. By way of overview, in the transesterification process a glyceride reacts with an alcohol in the presence of a catalyst, forming a mixture of fatty acids esters and an alcohol. Using triglycerides results in the production of glycerol. Transesterification is a reversible reaction and is carried out by mixing the reactants. A strong base or a strong acid can be used as a catalyst. At the industrial scale, sodium or potassium methanolate is mostly used. The following reaction occurs:

Equation 1 Using methanol in the transesterification process has the advantage that the resulting glycerol can be separated simultaneously during the transesterification process. When using ethanol during the process the ethanol must be substantially free of water and the oil should have a low water content to achieve an easy glycerol separation. The end products of the transesterification process are raw biodiesel and raw glycerol. After a cleaning step biodiesel is produced.

Advantageously, biodiesel is substantially non-toxic or at least less toxic than prior art collectors such as Kerosene. Kerosene (being a prior art collector) is poisonous if inhaled or ingested and considered to be potentially carcinogenic. Furthermore, it has a low flash point 38°C and at ambient temperatures in a plant or in a storage facility may ignite or even explode. By contrast, biodiesel has a higher flashpoint (typically > 130°C). As will be appreciated, this combination of low toxicity and higher flashpoint presents significant occupational health and safety issues. Furthermore, biodiesel is biodegradable and does not present the disposal problems of kerosene and other petroleum-based collectors.

In one embodiment of the present methods the use of a biodiesel derived from an oil feedstock as a collector. As used herein, the term "oil feedstock" is intended to mean any feedstock which is a triglyceride and being in the liquid phase at 25°C. The oil will typically be derived from a plant source; including but not limited to a plant seed, stem, leaf, root, vegetative structure, non-vegetative structure, reproductive structure, or fruit. The use of oil feedstocks is generally preferable to the use of fat feedstocks. Fat feedstocks are typically obtained from animal-based materials such as beef tallow, pork lard and chicken fat. The use of animal fats as a feedstock is generally problematic. Animal fats frequently contain contaminants that should preferably be removed before the biodiesel is used in the present methods. Phospholipids (or gums) may form insoluble insoluble precipitates when they come into contact with water, as required in a generally aqueous slurry in a froth floatation process. Such precipitates can co-partition with the floated coal particles in the froth phase, and should therefore be removed from the biodiesel before use in the present methods.

Polymers that are formed naturally at the high temperatures of the rendering process used in the conversion of animal fats to biodiesel can contribute to a higher viscosity in biodiesel. The increase in viscosity that results can cause difficulty in dispersing the biodiesel in the aqueous slurry of the froth floatation cell.

A further problem with rendered fats of animal origin is polyethylene in the fat which comes from plastic bags, ear tags, or other plastic that is mixed with the animal byproducts. Finely divided polyethylene has been found to cause cloudiness in animals based biodiesel. These fine particles may co-partition with and contaminate the floated coal particles in the froth phase.

The high sulfur content of animal fat biodiesel may also confer an advantage in using oil- derived biodiesels. The sulfur is thought to originate from sulfur-containing amino acids associated with proteins that carry over from the rendering process. The sulfur decreases by about one-half during the conversion to biodiesel, however significant amounts remain. As will be appreciated, the presence of sulphurous compounds is generally undesired in coal products due to environmental concerns. The use of oils (and particularly plant-based oils) as feedstock allows for the ability to use well defined and controlled biodiesel compositions in the present methods. It is proposed that the higher level of control will improve recoveries and the purity of the floated coal particles. Process issues will be lowered, and environmental issues lessened. Methyl esters including methyl laurate, methyl myristate, methyl palmitate, methyl palmitoleate, methyl stearate, methyl oleate, methyl elaidate, methyl ricinoleate, methyl linoleate, methyl linolate, methyl arachidate, methyl gadoleate, methyl behenate, methyl erucate will be useful in coal floatation methods.

Applicant has shown empirically that useful recoveries of coal particles are found particularly with biodiesel having relatively high levels of methyl palmitate and/or methyl lineolate.

In one embodiment, the biodiesel of the present method comprises methyl palmitate at a concentration of at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% and 45% (w/w) of all esters contained in the biodiesel.

In another embodiment, the biodiesel of the present method comprises methyl lineolate at a concentration of at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% , 45% or 50% (w/w) of all esters contained in the biodiesel.

In a further embodiment, the biodiesel comprises methyl palmitate and methyl linoleate at a combined concentration of at least about 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50% (w/w) of all esters contained in the biodiesel.

The plant from which the plant-derived feedstock may be derived may be any organism of the kingdom plantae. Preferably, however, the plant is one which is easily and economically cultivatable by commercial agriculture methods. More preferably, the plant is one which generates a reasonable amount of oil such as that found in palm, soybean, rapeseed, sunflower seed, peanut, cottonseed, palm kernel, coconut olive, ground nut with shell, maize, sesame seed, linseed, safflower seed, and soybean. Other lesser-grown species such as Jatropha species, Camelina species (such as C. sativa), Acrocomia species (such as A. aculeata, Salicornia species (such as S. bigelovii), Cynara species (such as S cardimculus), Brassica species (such as B. carinata), Pongamia species (such as P pinnata), and Rizinus species (such as R. communis). Grasses may also be a useful and economic source of biofuel, including elephant grass, tall fescue, cock's foot, and canary reed grass. Algae are a particular rich source of oil having a yield of around 95,000 litres per hectare (compared with around 6,000 litres per hectare for palm oil). Algae can be easy to grow, and does not require additional fertilizers or pesticides. It can be grown in grey water or wastewater (in fact nitrogen-rich waste ponds are some of the better places to grow algae). It also can be grown on marginal land, does not therefore take away from land used in farming for food. The following species (with typical lipid content) are exemplary: Botryococcus braunii (20-42%); Neochloris oleoabundans (23%-40%), Nannochloropsis salina (37%), and Dunaliella tertiolecta (37%). Typically, the algae is firstly dried before extraction of lipids for input into the transesterification process to produce biodiesel.

The skilled person has significant knowledge in the area of froth floatation relating to hydrophobic particles and can apply that knowledge as required to implement the present invention in any circumstance, and up to an industrial scale. For example, in addition to coal, it will be possible to float other naturally-hydrophobic minerals such as molybdenite, elemental sulfur, and talc with nonionic collectors such as the biodiesel compositions described herein.

Nonionic collectors such as the biodiesel compositions described herein may be used as "extenders" for other collectors. Alternatively, other collectors may be used as extenders in forth flotation methods using biodiesel as the primary collector.

The skilled person further understands the attachment of the bubbles to the surface of a hydrophobic particle is determined by the interfacial energies between the solid, liquid, and gas phases. This is determined by the Young/Dupre Equation:

which relates the surface energy of the liquid/vapor interface, the surface energy of the solid/vapor interface, the surface energy of the solid/liquid interface, and the "contact angle" formed at the junction between vapor, solid, and liquid phases. If the contact angle is very small, then the bubble does not attach to the surface, while a very large contact angle results in very strong bubble attachment. A contact angle near 90° is sufficient for effective froth flotation in most cases.

Once the particles are rendered hydrophobic (or more hydrophobic), they are contacted with gas bubbles so that the bubbles can attach to the surface. Contact between particles and bubbles is typically accomplished in a flotation cell whereby a rotor draws slurry through a stator and expels it to the sides, creating a suction that draws air down the shaft of the stator. The air is then dispersed as bubbles through the slurry, and comes in contact with particles in the slurry that is drawn through the stator.

Particle/bubble collision is affected by the relative sizes of the particles. If the bubbles are large relative to the particles, then fluid flowing around the bubbles can sweep the particles past and without coming in contact. It is therefore generally preferred that the bubble diameter is roughly comparable to the particle diameter in order to ensure useful particle/bubble contact.

Means for the collection of particles in the froth phase are also understood. Once a particle and bubble have come in contact, the bubble must be large enough for its buoyancy to lift the particle to the surface. This is easier if the particles are low density (as is the case for coal) than if they are high-density (such as lead sulfide). The particle and bubble must remain attached while they move up into the froth phase at the top of the cell.

The froth layer must persist long enough to either flow over the discharge lip of the cell by gravity, or to be removed by mechanical froth scrapers. If the froth is insufficiently stable, the bubbles will break and drop the hydrophobic particles back into the slurry prematurely. However, the froth should not be so stable as to become persistent foam, as a foam is difficult to convey and pump through the plant.

The surface area of the bubbles in the froth is also important. Since particles are carried into the froth by attachment to bubble surfaces, increasing amounts of bubble surface area allows a more rapid flotation rate of particles. At the same time, increased surface area also carries more water into the froth as the film between the bubbles. Since fine particles that are not attached to air bubbles will be unselectively carried into the froth along with the water (entrainment), excessive amounts of water in the froth can result in significant contamination of the product with gangue minerals. The present methods may comprise the use of reagents other than collectors for certain applications. For example, frothers are compounds that act to stabilize air bubbles so that they will remain well-dispersed in the slurry, and will form a stable froth layer that can be removed before the bubbles burst. The most commonly used frothers are alcohols, particularly MIBC (Methyl Isobutyl Carbinol, or 4-methyl-2-pentanol, a branched-chain aliphatic alcohol) or any of a number of water-soluble polymers based on propylene oxide (PO) such as polypropylene glycols. The polypropylene glycols in particular are very versatile, and can be tailored to give a wide range of froth properties.

Prior artisans have found that use of different frothers produced changes in the flotation rate (K) and recovery (R) values in coal flotation, and reached the following conclusions. When frother dosage was held constant while collector dosage was increased, it was found that the flotation rate went through a maximum and then decreased. This was observed for all frother types and all particle size fractions. The difference between the frother families studied was that the collector dosage that produced the maximum value of K was different.

For all of the frother types, the finest (-88 μπι) and coarsest (+500 μπι) particles tended to float more slowly than the intermediate- size particles.

Changes in flotation rate were due to both changes in the coal particle size, and to frother/ collector dosage. While the contribution of particle size was generally more significant, the reagent dosage effect provides a useful means for adjusting K in the plant. With aliphatic alcohol frothers, the flotation rate maximum was much more pronounced than for the Propylene Oxide (PO) and combined Propylene Oxide/ Alcohol (PO- Alcohol Adduct) frothers.

Regardless of frother type, increasing the frother dosage to increase recovery always leads to less selective flotation.

The PO and PO-Alcohol Adduct frothers are more powerful recovery agents than alcohol frothers, and therefore should be used at lower dosages. Over-dosing with alcohol frothers leads to a slower flotation rate, because excesses of these frothers tend to destabilize the froth. This effect does not occur with the PO and PO-Alcohol frothers, and so overdosing with these frothers leads to high recovery with poor selectivity. PO frothers with molecular weights of 300 to 500 are optimal for coal recovery.

Alcohol frothers tend to be more effective for fine-particle recovery than for coarse particle recovery. To recover coarse particles, the alcohol frother and the hydrocarbon collector dosages should both be high. The alcohol will still provide reasonable selectivity at these high dosages.

The high-molecular-weight PO-based frothers are more effective for coarse particle flotation than the alcohol or low-molecular-weight PO frothers, but also have a lower selectivity. For both good coarse-particle recovery and good selectivity, the PO frothers should be used at low dosage, with low collector dosage as well. The PO-Alcohol Adduct frothers are even more effective for coarse-particle recovery, and need to be used at even lower dosages.

The optimal frother for high recovery with good selectivity will often be a blend of members of the various frother classes examined. It is reported that such frother blending will give enough benefit to be worth the effort in approximately half of all coal flotation operations.

None of the frothers in the three categories studied will change the shape of the grade/recovery curve. Changes in frother type and dosage simply move the flotation results along the curve. Similarly, changes in hydrocarbon collector dosage also mainly move the performance along the grade/recovery curve.

For medium and coarse coal size fractions, the total gangue recovered is linearly related to the total coal recovered. It is only for the finest particles that the gangue recovery increases non- linearly with increasing coal recovery. When floating coals with a broad particle size range, the majority of the gangue reaching the froth is from the finer particle size fractions.

As the rate of coal flotation increases, the rate of gangue flotation increases proportionately. This is typical of a froth entrainment process acting on the gangue. In some circumstance, the present methods may include the addition of a modifier. Modifiers are chemicals that influence the way that collectors attach to particle surfaces. They may either increase the adsorption of collector onto a given particle (activators), or prevent collector from adsorbing onto a mineral (depressants).

The simplest modifiers are pH control chemicals. The surface chemistry of most particles is affected by the pH. For example, in general minerals develop a positive surface charge under acidic conditions and a negative charge under alkaline conditions. Since each mineral changes from negatively-charged to positively-charged at some particular pH, it is possible to manipulate the attraction of collectors to their surfaces by pH adjustment. There are also other, more complex effects due to pHthat change the way that particular collectors adsorb on mineral surfaces. The present may further include the use a modifier which is an activator. Activators are specific compounds that make it possible for collectors to adsorb onto surfaces that they could not normally attach to. Depressants may also be used. Depressants have the opposite effect of activators, by preventing collectors from adsorbing onto particular mineral surfaces. Their typical use is to increase selectivity by preventing one mineral from floating, while allowing another mineral to float unimpeded.

The present methods may be used in relation to any or many types of floatation cell equipment. A floatation cell is a machine for mixing and dispersing air throughout the slurry while removing the froth product. Individual machines are connected to form a flotation circuit in order to fully clean the product.

Conventional flotation cells consist of a tank with an agitator designed to disperse air into the Slurry. These are relatively simple machines, with ample opportunity for particles to be carried into the froth along with the water making up the bubble films (entrainment), or for hydrophobic particles to break free from the froth and be removed along with the hydrophilic particles. It is therefore common for conventional flotation cells to be assembled in a multistage circuit, with "rougher", "cleaner", and "scavenger" cells, which can be arranged in various configurations. Another type of equipment that may be used in relation to the present methods is a flotation column. Flotation columns provide a means for improving the effectiveness of froth flotation. A column essentially performs as if it were a multistage flotation circuit arranged vertically, with slurry flowing downward while the air bubbles travel upward, producing a counter-current flow. The basic principle of column flotation is the use of counter-current flow of air bubbles and solid particles. This is achieved by injecting air at the base of the column, and feed near the midpoint. The particles then sink through a rising swarm of air bubbles.

Counter-current flow is accentuated in most columns by the addition of wash-water at the top of the column, which forces all of the water which entered with the feed downward, to the tailings outlet. This flow pattern is in direct contrast to that found in conventional cells, where both the air and the solid particles are driven in the same direction. The result is that columns provide improved hydrodynamic conditions for flotation, and thus produce a cleaner product while maintaining high recovery and low power consumption. The performance differences between columns and conventional cells may best be described in terms of the following factors: collection zone size, particle/bubble contact efficiency, and fines entrainment.

The collection zone is the volume where particle/bubble contact occurs, and it differs greatly in size between column and conventional flotation. In conventional cells, contact occurs primarily in the region surrounding the mechanical impeller. The remainder of the cell acts mainly as a storage volume for material which has not yet been through the collection zone. This creates a bottleneck which keeps the flotation rate down. In contrast, flotation columns have a collection zone which fills the entire volume of the machine, so that there are more opportunities for particle/bubble collisions. The reduced level of turbulence needed to achieve a good rate of recovery in columns also reduces the tendency of coarse particles to be torn away from the bubbles which they attach to, and therefore columns are more effective for floating coarser particles.

Columns exhibit higher particle/bubble contact efficiency than conventional machines, due to the particles colliding with the bubbles head-on. As a result, the energy intensity needed to promote contact is less, and so power consumption is reduced.

A second beneficial effect in certain types of flotation columns is the reduction of bubble diameter. As bubble diameter is reduced, the flotation rate of both the coarser and finer particles is improved. Coarse particles can attach to more than one bubble if the bubbles are small, and therefore the chances of the particle being torn loose and sinking again is reduced. For fine particles, the probability of collision with the bubble is improved if the bubble is small, as then the hydrodynamic forces tending to sweep the particle away from a collision are reduced. The reduction of bubble diameter has the added benefit of increasing the available bubble surface area for the same amount of injected air. It is therefore desirable to produce bubbles as fine as possible.

The entrainment of fine waste material in the froth product is a serious failing of conventional flotation machines. It results from the need for a certain amount of water to be carried into the froth as the film surrounding the air bubbles. As a result, fine suspended particles are swept into the froth with this water, even though they are not physically attached to the air bubbles. In most column flotation machines, the entrainment problem is addressed through the use of wash-water,

Where the conventional cell must allow a certain amount of feed water to enter the froth, the wash- water in the column cell displaces this feed water to the tailings, thus preventing entrained contaminants from reaching the froth. The net effect of the relatively gentle mixing, counter-current flow, and use of wash-water in columns is that there is a distance of several meters between the clean coal discharging in the froth and the concentrated gangue discharging at the tailings, with a gradual gradient of concentration between the two extremes. There is therefore a reduced possibility of coal being misplaced into the tails, or of gangue short-circuiting to the froth. The result is that a column is typically equivalent to between three and five stages of conventional flotation, depending on the column design.

A special bubble generator is typically provided in carrying out the present methods using floatation columns. The impeller-type mixers that are used in conventional cells are not well- suited for use in flotation columns, as they would either need excessively long shafts or rotating seals. In the original flotation column design, bubbles were produced using sintered ceramic air diffusers which produced very fine air bubbles. However, this was found to suffer from plugging problems, particularly in hard water, and so cloth and perforated rubber sheeting were adopted instead. These still required excessive maintenance, and so external bubble generators of various types have been adopted.

The external bubble generators combine a stream of water with air to produce a mixture of very fine bubbles in water. This mixture is injected into the column. This approach has a number of advantages, including: (1) The bubble generator is accessible for adjustment and maintenance; (2) There is no porous element inside the column to clog or become damaged, and so the dispersion of air in the column does not change; (3) The bubble generator can be designed to tolerate particulate matter in the water, and so recycle water can be used in the generators; and (4) External bubble generators can consistently produce very small bubbles in the column.

A key issue in flotation column operation is "axial mixing", which is mixing along the vertical axis of the column, as shown in Figure 16. As the air bubbles rise in the column, they carry a portion of the water up with them to the base of the froth layer. The water then descends again, setting up a strong mixing action. This tendency is greatest if the column is slightly tilted from vertical, as then the bubbles preferentially ascend on one side while the water descends on the other. This is why it is particularly crucial for flotation columns to be perfectly vertical. The tendency towards axial mixing is also increased if some large bubbles are present, and so performance is best if the bubbles are uniformly small in diameter.

Another approach to suppressing axial mixing is the introduction of horizontal baffles, made from perforated plates. These baffles interrupt the flow of the liquid as shown in Figure 16, preventing it from being rapidly carried to the surface, or from short circuiting directly to the tailings. Experiments with horizontal baffles in a flotation column have confirmed that they can greatly improve the operating characteristics of the column without sacrificing capacity, and a sufficiently open baffle design is highly resistant to plugging by coarse particles or debris. Overall, flotation columns generally have superior performance to conventional flotation cells. However, they fundamentally require automatic control, as they are not well-suited for use of the simple tailings overflow weirs which are commonly used for maintaining a constant pulp level in conventional flotation cells.

It is often useful to be able to determine the theoretical maximum amount of upgrading that can be achieved by froth flotation, given a specific ore and a particular set of reagents. One approach to this is "release analysis", which is carried out by progressively re-floating froth products to collect only the particles that are fully hydrophobic. Release analysis provides a means for comparing the performance of conventional flotation with column flotation. The product from release analysis is typically much higher-grade than the product from a single stage of conventional flotation. On the other hand, a correctly-operating flotation column will typically provide a product grade that is comparable to the grade of the product from the final stages of release analysis.

The empirical data provided herein resulted from experiments carried out at laboratory scale. However, the skilled person is amply enable to apply the teachings of the present specification to a larger scale commercial process. In order to compare froth flotation experiments in the laboratory with operations in the plant, it is necessary to take into account the differences in the way that the two types of cells are operated. This is most easily done by carrying out kinetic experiments to measure the recovery of material to the froth as a function of time. A useful model for flotation kinetics, which includes a term for both flotation rate and ultimate flotation recovery, is: r = R{ l-[l-exp(-Kt)]/Kt} where: r = total weight of component recovered at time t,

t = time,

K = rate constant,

R = ultimate theoretical weight recovery at "infinite" time

This model takes into account the fact that the hydrophobic particles vary in size and degree of hydrophobicity, and is therefore more appropriate than conventional reaction kinetics expressions that are intended to apply to systems of identical molecules. It is particularly useful for correlation of laboratory results with plant results. In conventional laboratory test work, it is common for the parameter R to be the most important in determining the flotation performance, because laboratory tests are often run until all floatable material is recovered. In the plant, it is common for the parameter K to be most important, because it is too expensive to provide enough cell volume to recover all material that does not float in a short time. Because of this difference in operation, the results of laboratory studies can be very poor predictors of plant performance. To correct this, it is best to run timed-flotation laboratory tests which can produce kinetic data, so that the R and K performance can both be determined. Then, based on the residence time of the plant-scale units, it can be determined whether the plant performance is being dominated by kinetics (K) or by the ultimate recovery (R).

Flotation cells for use in a plant must be selected based on laboratory and pilot-scale data. Laboratory tests are usually carried out as batch experiments and are generally quite straightforward, although it is necessary to keep a few points in mind: (a) The pulp must be agitated sufficiently to keep all particles in suspension; (b) It is often necessary to condition the reagents with the minerals for a period of time to ensure good coverage with collector; (c). In many cases, adding frother in stages along with makeup water may be necessary to keep the pulp level and froth depth constant; (d) The capacity of the cell increases as the percent solids increase, and so the best process economics are achieved at the highest percent solids practical. The most important information from this test work is:

(1) Optimum grind size of the ore. This depends not only on the liberation characteristics of the input material, but also on floatability. Excessively coarse particles will be too large to be levitated by attached bubbles, and excessively fine particles will float poorly due to not striking the bubble surfaces, oxidation effects, or other problems. The coarsest material that can be floated is normally around 300 μπι, while the finest particles are around 5 μπι.

(2) Quantity of reagents needed, and the appropriate points in the circuit to add each reagent.

(3) Optimum pulp density. This is necessary for determining size and number of flotation cells for a given capacity. (4) Flotation time needed to reach desired recovery. It is important to be aware that a longer time is needed for flotation in the plant than in the laboratory, mainly due to the increased time needed to move through the large equipment. For example, a typical floatation for coal in a laboratory scale cell may be 2-3 minutes while for an industrial rougher floatation cell the time may be 3-5 minutes. (5) Variability of the feed material. Since properties vary from point to point in the mine, it is preferred to run experiments not just with ore from one location, but from several locations.

(6) Corrosive and wear properties of the feed material, which are needed for selecting appropriate materials of construction for the plant equipment.

(7) Type of circuit, number cells per bank, number of flotation stages, and appropriate locations for recirculation of intermediate products. The number of cells per bank depends on the flotation characteristics of the material being floated. Typical practice can be as low as 3 cells or as many as 17 cells per bank. In the case of coal, it is typical to utilize 4 to 6 cells, however in some circumstances only two cells may be used per bank. The present invention will be now more fully described by reference to the following non- limiting examples.

EXAMPLES

The following experiments were performed using a laboratory scale floatation cell. Black coal having particle sizes covering the range of <75 μιη to >300 μιη with 27.9% ash content was used as the input material for coal floatation.

Example 1: production of biodiesel

In these experiments, biodiesel was made from four different oil feedstocks: vegetable oil, canola oil, sunflower oil and brown rice oil. Transesterification was carried out according to the Equation 1 herein.

The resultant biodiesel was washed with water to remove traces of KOH. The biodiesel water mixture was made in a laboratory separation funnel and mixed well therein. The mixture was then allowed sufficient time to separate into its component parts, at which time the biodiesel fraction was taken. This washing process was repeated until the separated water fraction had a neutral pH and was clear to the naked eye. The resultant biodiesel compositions were analysed for methyl ester content, with the results shown in Table 1 below:

Table 1. Analysis of methyl esters of biodiesel produced from a range of plant-derived feedstock oils.

Reference is made to FIG. 1 which represents the above information graphically. Example 2 Comparative Coal Floatation Studies

The biodiesel compositions as defined in Table 1 were used a collectors in the coal floatation process described above. As a comparator, kerosene was also assessed. All experiments were conducted using a single batch of fine black coal having particles sizes distributed in the range 75-500 μπι). A slurry of 10% solids by mass was prepared by mixing the coal sample with water.

Methyl isobutyl carbinol (MIBC) was added to the slurry as a frother to an equivalent of 0.1 kg/t. For control samples, an equivalent of 0.25 kg/t of kerosene was added as the collector. For test samples biodiesel an equivalent of 0.25 kg/t of kerosene was added as the collector.

Each slurry sample was placed into a laboratory scale flotation cell having a capacity of 3.6 litres (Essa™ Flotation Test Machine, manufactured by FLSmidth Pty Limited, Australia) The agitator of the floatation cell was turned on, and conditioning of the coal slurry/frother/collector mixture was allowed for 1 minute before the air valve was turned on to commence the injection of bubbles. Once a froth layer was well established, the froth was removed. The recovered coal particles were dried, weighed and analysed.

Recoveries of coal for each collector is shown in Table 2 below.

Table 2. Recovery (in grams) of coal particles using kerosene and various biodiesel compositions as collectors. Recoveries are shown according to particle size, and also total recovery for each collector.

As will be noted from the Table above, triplicate analyses of particle size were carried out for kerosene, and duplicate analyses for each of the biodiesel collectors. The coal recovered by floatation for each collector was further analysed for moisture content, ash content, carbon content. Results of the analyses is shown in Table 3 below. A recovery calculation is also shown in Table 3 below.

Table 3. Analysis of coal particles using kerosene and various biodiesel compositions as collectors. "Cone. " = concentrate. Reference is made to FIG. 2 which is a graphical representation of the carbon, ash and volatiles data presented for each collector in Table 3.

Reference is further made to FIG. 3 which is a graphical representation of the recovery data presented for each collector in Table 3.

It was found that the coal concentrate recovered using the present floatation method were all very similar, with the concentrates obtained by the use of the various biodiesels as collectors performing competitively to kerosene with regards to the grade. The ash content was reduced from 27.9% in the feed (i.e. input material) to around 10% in the concentrates. This is significant in that the use of biodiesel as a substitute collector for kerosene had no notable reduction in performance in reducing the amount of fly ash present in the feed coal, and thereby resulting in a cleaner concentrate.

Recovery is calculated by reference to the amount of coal present in the feed that is recovered in the concentrate collected by floatation. A high recovery rate is highly desirable in coal floatation so as to minimise loss of coal in the tailings. It is noted that biodiesel derived from sunflower oil, rice oil and vegetable oil demonstrated better recoveries that than seen for kerosene under identical conditions. The best recovery was noted where biodiesel derived was rice bran oil was used as a collector.

With the best recovery rate being 74%, it may be necessary (at least for economic reasons) to reprocess the tailings given the presence of an appreciable amount of coal therein. This can be achieved by way of a rough/clean flotation circuit, the implementation of which will be familiar to the skilled person having the benefit of the present specification.

According to gas chromatography analysis, biodiesel derived from rice bran oil demonstrated the highest proportion of methyl palmitate of all biodiesels tested. Given the biodiesel of rice bran oil demonstrated the highest recovery when used as a collector, it is proposed that the use of palm oils and other oils that will produce higher amounts of this methyl ester may produce even more effective biodiesel collectors for use in the present coal floatation method.

Another notable ester is methyl linoleate which is highest in biodiesel derived from sunflower oil and rice bran oil. Both biodiesels performed well as collectors, and so increasing the levels of this ester may provide improved recoveries of coals and decreased contaminant levels therein

These data together demonstrate the broad applicability of biodiesel as a collector in the floatation of fine particles. In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

In the following claims, any of the claimed embodiments can be used in any combination.