Coal flotation method

Field of the invention

The present invention relates to a method and apparatus for producing carbonaceous material which has an increased propensity to be separated in a flotation cell.

Background of the invention

Coal is an increasingly valuable resource due to its relative abundance and the depletion and loss of other natural energy resources such as petroleum, natural gas, oil shale and tar sands. As coal, along with other energy sources, has become more valuable, there has been a greater imperative to efficiently extract our available coal reserves. An important element of increasing the carbonaceous yield from coal deposits is to reduce the amount of carbonaceous material in the tailings stream of coal extraction plants.

Froth flotation separation techniques are used to separate a slurry of particulate matter into a lighter hydrophobic portion and a heavier hydrophilic portion. In this process air is introduced into the liquid slurry of particulate matter through a porous cell bottom or hollow impeller shaft, thereby producing a surface froth. Alternatively, a sparger device is used to form bubbles from shearing energy applied to input air.

These processes have been generally inefficient in transferring the carbonaceous rich particles into the froth phase due to the inefficient mechanisms used to lift the particulate matter into the surface froth phase. Although improvements in efficiencies are being made through the refinement of "microbubble" technology such as the "Microcel", the "Jameson" cell; and the "Ekof cell, the yield of carbonaceous material and the range of particulate matter which the flotation techniques may effectively and efficiently process has room for improvement.

Other efforts at minimising the amount of carbonaceous material have concentrated on improving reagents to collect the particular matter and form a stable froth phase; and further processing of the tailings stream. In particular, the tailings stream has been

further processed to produce a high carbonaceous cake material which may be made into a separate saleable product. However, these efforts have met with limited success in producing a cost effective solution to the recovery of a greater proportion of carbonaceous material from the raw material.

Therefore, there is a need for a flotation process which can process carbonaceous material over a wider particle size and density range, while minimising the loss of carbonaceous material.

Summary of the invention

In one aspectt of the present invention there is provided a method for the separation of carbonaceous material from inorganic matter comprising the steps of:

processing a feed stream of carbonaceous material in a first flotation cell to produce an overflow product stream and an underflow stream;

separating a further processing stream from the underflow stream; and

mechanically working the further processing stream to produce a flotation stream, and

either recycling the flotation stream for combination with the feedstream to the first flotation cell or processing the flotation stream in a second flotation cell.

In a second aspect of the invention there is provided an apparatus for the separation of carbonaceous material from inorganic matter including:

a first flotation cell for processing a feed stream of carbonaceous material to produce an overflow product stream and an underflow stream;

a separation means for separating a further processing stream from the underflow stream; and

a mechanical processing means for mechanically working the further processing stream to produce the flotation stream, the flotation stream providing a recycle stream for combination with the feedstream to the first flotation cell or a feed stream to a second flotation cell.

According to the first and second aspects, the flotation cell typically includes a liquid and a means for creating the froth phase on which floats carbonaceous rich material up to the surface of the liquid. The over flow stream from the flotation cell includes carbonaceous rich material which is removed from the cell. An underflow stream , includes material which could not be floated under the conditions in the flotation cell.

The apparatus is advantageously used to process a carbonaceous material which contains material of a size, density and/or hydrophobicity distribution which is not conducive to efficient flotation cell processing (i.e. low carbon yield). Thus, the apparatus enables the overall carbon extracted from a coal processing operation to increase, with the increased carbon yield providing an economic advantage over conventional processing techniques. Feed streams comprising carbonaceous matter may include material such as coal particulates in middlings streams from upstream separation operations. Alternatively, carbonaceous material from tailings dams may form the basis for the feed stream.

In one preferred form of the invention, the feed stream for the first flotation cell is derived from a source stream and is combined with the flotation stream and fed to the first flotation cell. The separation means which is preferably a sieve screen or other type of separator that separates particles on the basis of particle size, separates over size particles or particles larger than a particular size. These oversized particles constitute the further processing stream which are mechanically processed to reduce there size and then recycled as the flotation stream for combination with the feed stream to the first flotation cell. Optionally, the ^l mechanically worked further processing stream is separated to remove undersized or fine particles prior to being recycled as the flotation stream.

In another preferred form of this embodiment of the invention, the underflow from the first flotation cell is separated to produce a further processing stream, as above which in turn is mechanically processed to reduce the particle size. The resulting flotation stream is passed to a second flotation cell to undergo a flotation process. In this embodiment, the feed stream to the first flotation cell is derived from a source stream. The flotation conditions in the first flotation cell and the second flotation cell may be the same or different depending on the quality and nature of the coal to be recovered. In this preferred form, the overflow from the second flotation cell may be combined with the overflow from the first flotation cell to form the product stream.

The flotation cell will typically have an optimum particle size range for the transfer of hydrophobic carbonaceous material into the froth phase, with the size range being dependent upon the particle density. The mechanical processing means (eg. grinding or attrition) is able to reduce the large particles into the optimum particle size range, with the reduction in the particle size resulting in the liberation of inorganic material within the particles. With the liberation of the denser inorganic material, the proportion of lower density material (equating to cleaner coal) increases.

Carbonaceous material, such as coal, is soft and difficult to grind without excess fines (eg. less than 75μm) being produced. Therefore, it is counter-intuitive to attempt to improve the carbonaceous yield of a flotation cell, by subjecting the feed stream or underflow stream to a mechanical process which will increase the level of difficulty to recover fines.

As coal particles larger than about 300μm have been found to be less effectively removed during the flotation step, this portion of the feed stream or underflow stream is preferably separated and mechanical processed to liberate inorganic material from the composite particles; reduce the particles to a more optimal particle size for flotation removal; and remove the hydrophilic outer layer from the coal to produce a more hydrophobic particle.

The apparatus of the invention includes a separating means, prior to the mechanical processing means to separate a carbonaceous stream from a second separation means

or the underflow stream by size and/or density to form a further processing stream. This step preferably removes the high ash content material and finer particle material that does not easily float. Thus less energy is expended by the mechanical processing means, without substantial flotation yield loss; the flotation yield being the ratio of the mass of carbon in the outlet stream to the mass of carbon in the feed stream.

The further processing stream which contains the oversize material is then mechanically processed. After mechanical processing the stream preferably has a higher proportion of particles between 75μm and 300μm than the further processing stream. More preferably, the flotation stream has a higher proportion of particles between 100μm and 250μm than the underflow stream. The applicant has found that by mechanically working the particles in the further processing stream, the preferred particle range of the recycle stream may be achieved.

As carbonaceous material is generally less dense than inorganic matter (ash), it is preferable that the flotation stream is less dense than the tailing or underflow stream. The flotation stream preferably has a specific gravity of 1.25 to 2.4, more preferably 1.25 to 1.6 and most preferably 1.25 to 1.4.

To achieve the preferred density ranges, the apparatus may further include a density separating means to remove the denser particles from the flotation stream.

The propensity of the material in the flotation stream to be transferred into the froth phase may also be enhanced through increasing the hydrophobic nature of the flotation stream.

The rate of flotation of particles depends on their frequency of collision with bubbles and on the efficiencies of collision, attachment and stability between particles and bubbles. Most flotation machines produce maximum flotation for particles of optimum intermediate size with flotation decreasing for particle sizes each side of this optimum. The lower flotation of fine and coarser particles results mainly from their lower collision efficiency and lower stability efficiency (in the high turbulence of the flotation cell), respectively with bubbles. However, this maximum in flotation shifts to larger or smaller

particle sizes for minerals of lower or higher density. Apart from the dependency on particle size and turbulence, the flotation of particles increases with the hydrophobicity of the mineral surface (increase in bubble-particle attachment efficiency). Coal is naturally hydrophobic. However collectors such as surfactants or oil may be added to further increase this hydrophobicity, especially if the coal is oxidised which increases the number of hydrophilic surface hydroxide groups. The decrease in mineral hydrophobicity may also be the result of precipitation of metal hydroxides from solution on the mineral surface. Coating of the valuable minerals with hydrophilic gangue minerals such as silicates may also decrease surface hydrophobicity and therefore propensity of the particles to float.

The mechanical processing means is used in the invention to not only reduce the particle size of the material to form the flotation stream but also remove the surface layer. To remove or reduce the hydrophilic surface layer on the carbonaceous material, a stirred mill may be used to remove the surface layer through grinding attrition. In the process particle surfaces are abraded by applying intense shear and frictional forces between the grinding elements and particles. Whereas stirred mills have been traditionally used to reduce particles below 40μm, the applicant has found that a stirred mill may be used to decrease the particle size and enhance the hydrophobic nature of the oversize material in the recycle stream to produce particles within the preferred particle size range (75μm to 300μm).

The mechanical processing means may include a grinder (eg. ball or rod mill) for reducing the mean particle size of the further processing stream and liberate inorganic ash material; and an attritioner for increasing the hydrophobic nature of the recycle stream.

In a preferred aspect of this embodiment, the mechanical processing means may include two stirred or tumble mills may operate in series. The first stirred mill is set up to attrition the hydrophilic surface layer and the second stirred mill set up to reduce the particle size of the underflow stream. The product of the first (and second) stirred mill(s) may be separated by size and or density, with the coarser material (> 300μm) feed into the second stirred mill, and the ultra-fine (<75μm) material feed into a tailings stream.

The 75μm to 300μm fraction is fed into the flotation stream with its increased hydrophilic nature increasing its propensity to transfer into the froth phase and hence into the high carbonaceous product stream.

The second stirred or tumble mill operates to reduce the particle size to within the preferred particle size distribution. Preferably at least 80% of the output of the second stirred mill has a particle size of less than 300μm. The output of the second mill is also fed into the recycle stream with its enhanced particle and density distribution increasing its propensity to transfer into the froth phase and hence into the high carbonaceous product stream.

To achieve the desirable particle size reduction, liberation of inorganic matter and removal of the hydrophilic surface layer, the process parameters (e.g. solids volume concentration, grinding media/particle ratio, specific energy input, circumferential speed and grinding media filling ratio) of each of the stirred mills may be varied. In one preferred embodiment, one stirred mill may be set up to maximise the flotation yield through both attrition and comminution of the oversized material in the underflow stream. Preferably, the process parameters of the stirred mill are such that ultra-fine particulate production is minimised. Preferably, the net weight proportion of ultra-fine particles after mechanical processing is less than 30%, more preferably, less than 25%, even more preferably less than 20%.

The mechanical processing means may also be applied to other underflow, tailing or waste streams, such that the processed tailings may feed into the flotation tank. For instance, the tailings from gravity and/or size separators (eg. screens, teetered bed separators or spirals) may be processed to increase the carbonaceous yield of the overall process.

In a third of the invention there is provided a method for the separation of carbonaceous material including

processing a source stream of carbonaceous material,

separating the source stream in separator into an undersize stream for flotation and a further processing stream, and

mechanically working the further processing stream to produce a flotation stream, wherein the flotation stream is combined with the undersize stream for flotation from the separator to provide the feedstream to a first flotation cell.

In a fourth aspect of the invention, there is provided an apparatus for the separation of carbonaceous material including

a first flotation cell for processing a feed stream of carbonaceous material,

a separation means for separating a source stream into an undersize stream for flotation and a further processing stream, and

a mechanical processing means for processing the further processing stream to produce a flotation stream, wherein the flotation stream is combined with the undersize stream for flotation from the separation means to provide the feedstream to the first flotation cell.

According to the third and fourth aspects of the invention, the separator or separation means maybe at least a first separator and a second separator. The source stream which may have been subjected to earlier processing or separation is provided to the second separator.

The oversized material from second separator is separated in the first separator to produce the further processing stream. In this preferred form of the second aspect of the invention, the further processing stream is preferably the middlings stream from the first separator which is mechanically processed in the mechanical processing means to form the flotation stream. The flotation stream is then fed with the undersized material from the second separator into the first flotation cell. The overflow from the first flotation cell is then added to the product stream. Depending on the source of the feed to the second separator, the underflow stream may be combined with the tailings stream.

In fifth aspect of the present invention, there is provided a method for the separation of carbonaceous material from inorganic matter comprising the steps of:

mechanically working of a feed stream to produce mechanically worked product;

applying a separation means to the mechanically worked product of the preceding step to produce a low ash and a high ash portion, the low ash portion having, in comparison to the mechanically worked product of the preceding step, one of the characteristics selected from the group consisting of :

i. a higher portion of particles between a target range;

ii. a higher portion of particles having a specific gravity between 1.25 and 2.4; and

iii. a higher degree of hydrophobicity; and

feeding the low ash portion into a first flotation cell.

The feed stream preferably includes the underflow stream of a flotation cell.

In a further embodiment of the present invention, there is provided a method for attritioning/grinding a feed stream including the steps of:

feeding a predetermined portion of following items into a mill;

a) grinding/attritioning media;

b) fluid media; and

c) carbonaceous material, including an ash portion;

grinding/attritioning the carbonaceous material in the liquid media; and

separating the ground/attritioned carbonaceous material into a low ash portion and a high ash portion

wherein the low ash portion, in comparison to the feed stream, has at least one characteristic selected from group consisting of :

i. a higher portion of particles between a target range;

ii. a higher portion of particles having a specific gravity between 1.25 and 2.4; and

iii. a higher degree of hydrophobicity.

For all aspects of the invention, the target particle size range is preferably 63μm to 500μm, more preferably 75μm to 500μm, more preferably 63μm to 300μm and even more preferably 75μm to 300μm.

Brief description of the drawings

Figure 1a to 1d are schematic diagrams of various flotation methods for recovering carbonaceous material in accordance to several embodiments of the present invention.

Figure 2 is a graph of the partition number versus particle size of carbonaceous material for a conventional flotation cell.

Figure 3 is a graph of the regrinding sizing results for different grinding times and grinding media using the stirred mill of Figure 3a & b.

Figure 4 is a graph comparing the cumulative yield (%) versus the cumulative ash (%) for unground and ground material from the flotation underflow stream.

Figure 5 is a particle size distribution of a flotation feed sample and a particle size distribution of a flotation feed sample subjected to sonication for 1 minute.

Figure 6 is a backscattered electron image of a carbonaceous sample from the flotation feed sample of Figure 6.

Figure 7 is a graph of the flotation concentrates and tail fractions of the flotation feed sample of Figure 6.

Figure 8 is a graph of the flotation concentrates and tail fractions of the flotation feed sample subjected to sonication for 1 minute of Figure 6.

Figure 9 is a table summarising the testwork results in Example 3.

Figure 10 is a graph of the size distribution of coal tailings after milling after milling with balls, pebbles or rods.

Figure 11 is a graph of the size distribution of coal tailing after milling with cycles or balls.

Detailed description of the embodiments

Referring to Figure 1a, the flotation process of one embodiment of the present invention includes a first flotation cell 10. The flotation cell may be one of a variety of mechanically agitated and/or microbubble flotation cells available, including the "Microcel" tall vessel column in which air is admitted separately from the feed. Alternatively, 'shallow columns', with short retention times, in which air is admitted with the feed, such as the Jameson and Ekof type columns, may also be used.

The feed stream typically comprises a mixture of carbonaceous and composite carbonaceous/inorganic material from upstream separation processes such as spirals, teetered bed separators, reflux classifiers, coarse rejects and even the reprocessing of tailings from tailing dams.

In the first flotation cell 10, the carbonaceous material is subjected to a flotation process resulting in an overflow stream 11 and an underflow 12. The underflow which typically contain lumps of carbonaceous material higher in ash or otherwise denser than the

material which floated is passed to a size separator 13 such as sieve screens where fine material 14, 15 removed to a tailings stream 15. The oversize material which forms the further processing stream 16 is passed to a mechanical processing means which in this case is a stirred mill 17 where the material is size reduced made more suitable for flotation. The fine fraction of the mechanically processed material is removed in a size separator or the density separator and the flotation stream 19 combined with the feedstream 9 and fed into the first flotation cell 10.

It will be appreciated that other flotation process configurations may be used, as illustrated in Figures 1b to 1d, which fall within the scope of the present invention. In the embodiment of Figure 1b, a feedstream 21 is fed into the first flotation cell 20. As in the embodiment of Figure 1a, the underflow 22 is fed to a size separator 23 separates oversized material for further processing 24 and undersized material passes to a tailings stream. The further processing stream 24 is fed to a mechanical processing device which is an attrition/milling device 25 before being fed as flotation stream 26 to a second flotation cell 27 where a concentrate 28 is floated and the underflow sent to tailings. This concentrate 28 is combined with overflow 29 from the first flotation cell 20 to produce the product stream.

The embodiment of Figure 1c is similar to that of Figure 1c? except the underflow 31 from first flotation cell 30 being separated in separator 32 and further processed stream 33 is fed to mechanical processing means 33. The difference is that mechanical processing means is an attrition milling device 34. As the production of fines can be better controlled in this device the flotation stream 35 is fed directly back to be combined with feed stream.36 to the first flotation cell 30.

Figure 1d relates to a process configuration in which a middlings stream is mechanically worked, with the product of the mechanical working forming part of the feed stream to the first.

In this embodiment, a series of separators 41 , 42, 43 separate a feed stream by size. The primary separator produces an undersize stream 44 which is fed to a second separator 42. The second separator produces an undersize fraction 45 which is fed to

the first flotation cell 40. The oversize fraction 46 from the second separator 42 is fed to a third separator 43 which produces a middling fraction 47, a product fraction 48 and a purge stream 49. The middling stream 47 is passed to a mechanical processing means which is an attrition milling device 50 to produce the flotation stream 51. The flotation stream is added with the undersize feed stream 45 to the flotation cell 40 to undergo a flotation process. The product stream 52 is taken off as the overflow and the underflow is sent to tailings.

As illustrated in Figure 2, an optimal particle size range for a conventional flotation cell is in the range of 63μm to 300μm. With the use of a mechanical processing means and the flotation stream, a wider range of particle sizes may be effectively processed by the flotation cell. Thus, larger particle sizes of up to 500μm or above (depending upon the quality of the tailings steam) may be effectively processed, with the relatively low recovery rate of carbonaceous material during the first pass through the flotation cell compensated by higher recovery rates of the mechanically processed material fed into the flotation cell from the flotation stream.

As illustrated in Figure 1a, the underflow stream may include a separation means before and/or after the mechanical processing means, to enable the preferred particle size and/or density distribution to enter the mechanical processing means and/or the downstream flotation stream. The separation means may include spirals, teetered bed separators, reflux classifiers and may also include a preceding classification stage (eg. cyclone).

For the flotation cell of this embodiment, the preferred particle size range is between 63μm to 300μm, with a density range corresponding to the cleaner coal fractions (Figure 2). Clean coal has been classified as particles having a relative density range or specific gravity of 1.25 to 1.4, semi-clean coal 1.4 to 1.6, bony coal 1.6 to 2.4 and mineral stone > 2.4.

While separation and mechanical processing may be performed on the feed material, the use of a flotation recycling stream will more efficiently increase flotation yields, as

only the material which does not transfer into the froth phase, is required to be reprocessed, thus reducing capital and operating costs.

It will be appreciated that preferred particle size ranges will vary depending on the nature of the feed material and the partition number characteristics of the flotation cell used with a recycle flotation stream with mechanical processing. Developments in "microbubble" technology suggest that flotation cells may be able to effectively separate particle sizes above 500μm without a recycle stream. In these instances, the use of the present invention would further extend the particle size processing range and/or reduce the operating costs through reducing reagent consumption.

Reagents are added to the flotation cell to aid the particles to more readily float (collectors) and to create the froth phase which keeps the floated particles suspended on the top of the flotation medium, thus allowing these particulates to be removed in the outlet (high carbonaceous material).

Collector reagents are typically oil based products such as diesel. The coal which is lipophilic attaches to the surface of the oil droplets and floats upwardly along with the oil droplet utilising their buoyancy and hydrophilic nature. The mechanical reprocessing of the underflow material or material of a particular size increases the hydrophobic nature of coal coated with a hydrophilic outer layer (eg. oxidised organic material or inorganic material, such as clay). The Examples illustrates how the use of mechanical processing (stirred mill or sonication) increases the recovery yields due to increasing the hydrophobic nature of the coal particles.

The mechanical processing means is preferably a stirred or tumble mill, although other grinding/crushing equipment (eg. fluidised bed or high pressure griding rolls) may be used. A benefit of the stirred or tumble mill is that it grinds/attritions the coal particles under a liquid environment, thus minimising the extent of oxidation of the coal particle's surface. The re-oxidation of the coal's surface will increase the coal's affinity to water and hence lead to lower separation efficiency of the flotation cell. The cooling effect of the liquid (eg. water medium) also reduces the rate of potential oxidation.

There is preferably a short lag time between the mechanical processing operation and feeding the derived flotation stream into the flotation cell. The combination of a non- oxidising milling environment and short lag time ensures the hydrophobicity of the coal particles in the flotation steam is maximised.

The fluid media may also include a gaseous fluid, such as carbon dioxide or other non- oxidising gas. The use of carbon dioxide may be particularly advantageous when a fluidised bed is used to attrition/grind the underflow stream.

In addition, mechanical processing in a liquid medium enables the attrition mechanism to be more easily performed. The attrition process involves the collision of particles which result in shear and frictional stresses between the colliding particles. In contrast to the attrition process, crushing or grinding forces reduce the particle size through multiple fractures in the particle due to the higher compressive forces applied to the particle. Attrition may be advantageously employed to not only remove the fine impurity layer on the coal surface, but to fracture the particle along a weakened stress line. These weakened stress lines are more likely to coincide with the interface to inorganic matter. Therefore, the use of attrition may effectively liberate inorganic ash impurities. Also, as particle size reduction is able to occur with more gentle forces (lower energy costs), less fines are produced and hence greater carbonaceous yields may be achieved.

Depending upon the relatively size and hardness of the particles, the collisions may result in the removal of any surface layer (eg. cleaning process) and/or the reduction in particle size. Therefore, the stirred mill may be used to increase the propensity of the carbonaceous material to be transferred into the froth phase by:

• cleaning the surface of the coal particle to increase its hydrophobic nature;

• decrease the particle size of the material to enable the flotation bubbles to more readily elevate the particle into the froth phase; and

• liberate inorganic material during the process of decreasing the coal's particle size.

The relative size distribution of the grinding/attrition media to the feed of the stirred or tumble mill is an important parameter to control. If a cleaning process is required, then the particle size distribution is preferably similar to the particle's size distribution of the carbonaceous material. Similarly, if a particle size reduction process is required the mean size of the grinding media is preferably increased. In certain embodiments, the attritioning media are the particulates from the underflow stream, the collisions of the carbonaceous material able to clean their own surfaces and reduce the mean particle size of the underflow stream.

Example 1

Optical analysis of the flotation cell's underflow stream from two coal preparation plants indicated that there were significant amounts of coarse coal, both liberated and composite, being lost to tailings. It was unclear why these particles were reporting to underflow stream as clean coal particles, in the size range 63μm to 300μm, are known to have good flotation performance.

It was envisaged that these coarse particles could be recovered from the flotation underflow stream via a gravity pre-concentration step (Teetered Bed Separator, Spirals etc) and subjected to a recrushing/regrinding stage that would either liberate the coal from mineral matter, reduce the already liberated coal to a size that was more readily floatable or clean the coal surface to render it more hydrophobic.

Regrinding and flotation testwork was conducted to investigate whether these coarse coal particles could be recovered by employing a recrushing/regrinding and flotation step. Samples of middling particles were collected from the dewatering screens, in an upstream preparation plant, and used for regrinding and flotation testwork.

The samples were obtained by carefully collecting the top one to two centimetres of material on a dewatering screen with a scoop, as this material was noted to be significantly darker in colour and likely to be coal rich compared to the bulk of the flotation underflow material.

A stirred mill was developed to grind the samples of middling material.

Crushed river rock was used as the grinding media in the mill. The river rock was screened to give approximately 1mm to 4.75mm and 4.75mm to 8mm size fractions. Combinations of these two fractions were used in several grinding tests to investigate the effect of media size and grinding time. Results from these tests are detailed in Figure 3.

The grinding procedure involved placing a proportion of the river rock and required water in the mill container, turning on the stirrer then adding the middling sample, remainder of the water and enough river rock to give the surface of the grinding mass the appearance of a donut. Typical river rock charge was about 80Og, water about 200ml and middlings mass around about 20Og.

The results for the 20 second and 30 second run are contradictory and are probably due to experimental error as it was difficult to be precise with the start of the grinding period using the charging method described above. However, the "Stirred Mill" was clearly able to reduce the size distribution of the middlings sample with a low amount of grinding energy. The effect of adding coarser media could also be seen with a distinct reduction in coarse particles after the 4.75mm to 8mm river rocks were added (charge was 66% 1mm to 4.75mm and 34% 4.75mm to 8mm river rock)

The mixed media charge had the largest effect on coarse particle size reduction so this charge was used for subsequent flotation work. Results for the flotation testwork are detailed in Figure 4. Three lots of approximately 20Og (wet) were re-ground using the mixed media charge and a grind time of approximately 30s to produce approximately 50Og (dry) for a flotation test. The flotation cell size was 5L so 50Og of material was

required to give approximately 10% solids in the flotation cell. The 100% cumulative yield sample points represents the quality of the feed material used in the flotation test.

The flotation test procedure consisted of adding the middlings material to the flotation cell, adding enough tap water to fill the cell to 5L, conditioning for one minute with the impeller on, adding frother (eg. methyl iso-butyl carbinol (MIBC)) and conditioning for 30s then adding collector (eg. diesel) and conditioning for one minute before turning the air on. Timed concentrates were collected for 10s, 40s, 3min and 10min to enable yield- ash curves to be generated.

During the flotation tests on the reground material it was noted that the coal appeared to be readily floatable. A pre-concentrate was collected for the regrind test (for 10s) with the assumption that diesel would then be added. However, because the material remained readily floatable after the pre-concentrate was collected, diesel was not added to the flotation cell until after the second concentrate had been recovered for the reground material test. Clearly, regrinding the middlings material significantly enhanced its flotation response with approximately 70% being recovered before collector was added. Collector was always required at the start of the flotation tests on unground material. This demonstrates that the grinding operation had increased the hydrophobic nature of the coal particles relative to the unground samples.

It is envisaged that not just flotation streams could be retreated by recrushing/regrinding and flotation but any other coal process plant streams could be used. This would include spirals (product, middlings (mids), reject), Teetered Bed Separators, Reflux Classifiers, coarse rejects or even retreating material from tailings dams via a recrush/regrind, flotation route (with or without a pre-concentration stage). One example would be to upgrade the "spirals" product and recrush/regrind the spiral mids stream to place the material in the optimum size range for flotation. This would maintain good yields from the spirals circuit but at a lower ash content, thus allowing the upstream process to operate with a higher portion of material being processed through the flotation system, increasing yields.

Example 2

Samples (PD3X) from the flotation feed stream were assessed for the affect of 1 minute of sonication (sound wave energy which is defined for the purposes of this invention as being a subset of mechanical processing) on flotation performance. As illustrated in Figure 5, the sample subjected to sonication of 1 minute had an increased fines level. As illustrated in Figure 6, a portion of the coal particles are covered with clay particles. The introduction of sound wave energy to vibrate (mechanical processing) the clay particles free from the coal particles has resulted in an increase in particle sizes below 5μm.

The flotation performance of these two samples is provided in Figures 7 and 8. As illustrated, the samples which were subjected to sonication had increased flotation, particularly for particles greater than 200μm. Further, given the lower proportion of materials residing in the underflow stream, the use of sonication increased the yield of the flotation cell.

The proportion of higher particle size material in the underflow stream from the untreated sample, indicates that there would be benefits in removing the hydrophilic layer from the underflow stream and recycling this material into the flotation cell feed.

Example 3

Tests were performed to evaluate the suitability of using tumbling or stirred mills to effectively reduce the size of coal tailing particulates to between 75μm to 300μm, without an excessive increase in ultra-fine material (<75μm). The feed material had an ultra-fine (<75μm content of 1 % (2.8% in second campaign), with 80wt% of the particles being 702 μm or less (662μm in second campaign). The largest feed material was about 1.5mm, with about 99 wt% of the feed material being 1mm or less.

All grinding tests (unless otherwise stated) were carried out using 0.64kg of coal flotation tail and 0.64kg water (i.e 50% solids). Power consumption was measured using a power meter and logging the data to a computer throughout the test. The product sizing was done using a standard set of sieve screens.

Figure 9 is a table which summarises the results of the testwork.

Effect of grinding media

As illustrated in Figure 10, the different grinding media (rods (27mm), balls (25mm), pebbles (20 to 50mm) and cylpebs (25mm cylindrical grinding media)), in general, did not have a significant impact on the mill's performance. To produce a product in which 80 wt% of the product (P80) was 300μm or less, 2.5-3.0 kWh/t would be required. The corresponding percentage of ultra-fines (<75μm) is in the range of 25-28 wt %. The use of cylpebs produced significantly lower levels of ultra-fine particulates for the similar P80 sizes, as illustrated in Figure 11. Tests also indicated that the optimum % solids content is about 50%, with 60% solids being too high.

Stirred mill

As shown in Figure 9, the results from the pin stirred mill were very different form the cylpebs products, with P80 and ultra-fine levels being significantly poorer. This result is thought to be due to the small grinding media size (5mm). The ball size to particle size is generally around 20 for stirred mills to enable courser particles to effectively "nipped" between the grinding media.

Effect of liner material

The results from Figure 9 indicate that steel lined mills required less energy to achieve the same P80 sizes, with the rubber lined mills generally producing lower fines for the same P80 size.

The above observations indicate that the shape and the density of the grinding media may have an effect on the coal tailings grinding performance. Pebbles are advantageous over steel balls in coal grinding due to their lower density and therefore, reduced breakage force. A lower breakage force is considered to produce fewer fines. The density of the steel balls are 7.8-8 grams/cm ³ compared to a density of pebbles of about 2.5 grams/cm ³ . Therefore, to avoid excessive fines formation, the density of the griding media is preferably no greater than 3 grams/cm ³ and more preferably no greater than 2.5 grams/cm ³ . The ratio of the griding media size to the P80 size is preferably at least 10 and more preferably at least 20.

It will be appreciated that the above testwork was aimed at producing an increased amount of coal particulates between 75μm to 300μm. As previously discussed, with improved hydrophobicity, larger coal particle sizes (eg. 500μm or above) may be effectively recovered in a flotation cell. Therefore, the generation of ultra-fine particles may be expected to be further reduced (eg.< 15%) for high P80 sizes. Alternatively, the mills may process larger particles to achieve the broader target range, while avoiding the generation of excessive ultra-fine particulates.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

The will be understood that the term "stream" as used in this specification encompasses continuous, non-continuous, solids, semi-solids, slurries, gases and any other solid / liquid / gas mixture.