BENEFICIATION OF CARBONACEOUS FUELS

Abstract: Mineral particles are separated from particles of solid carbonaceous fuel by forming a mixture with water, ferromagnetic particles and hydrophobic oil at a weight ratio of carbonaceous fuel to magnetic particles of at least 25:1, shearing to flocculate the carbonaceous fuel particles, and then subjecting the product, to magnetic separation.

Description: BENEFICIATION OF CARBONACEOUS FUELS The present invention relates to the removal of undesirable non-combustible mineral matter from carbonaceous solid fuels. This process is sometimes known as beneficiation. US 3 926 789 (Schubert) discloses a process for separating one component of a mixture of solids by selectively coating the surface of the component with a magnetic fluid and then subjecting the mixture to- a magnetic separation process. The use of this process to separate mineral from coal is disclosed. A hydrocarbon base magnetic fluid was added to a coal-water slurry. Coal particles were attracted by a magnet while mineral particles were not attracted. Sladek and Cox of the Colorado School of Mines Research Institute have also discussed the beneficiation of coal in a paper entitled "Coal Beneficiation with Magnetic Fluids" Proc Conf Ind Appln of Magn Sep., Rindge, NH, USA, August 1978. This discusses the use of magnetic fluids in coal beneficiation. The paper explains that such fluids contain particles with a characteristic diameter of approximately 100 angstroms (10 nm). Sladek and Cox specifically refer to the difference in particle size between the particles of magnetic fluids and the magnetite commonly used in conventional coal washing. Sladek and Cox discuss the selective wetting of the coal particles in a coal/mineral mixture with a kerosine-based magnetic fluid. They also discuss the alternative approach of selectively wetting coal mineral rather than the organic material (coal). The surfaces of the minerals are hydrophilic and the possibility of wetting them with an aqueous-based magnetic fluid is mentioned. The magnetic particles in the aqueous medium are kept in dispersion by means of a dispersing agent. The magnetic fluids used by Sladek and Cox are not cheap; materials costs of several dollars per gallon are quoted. The expense of treating large volumes of coal from a typical mine will therefore be considerable. The magnetic fluids used in the processes of Shubert, and of Sladek and Cox are ultra-stable colloidal suspensions of magnetic particles. Because the magnetic particles are colloidal in size there will in general be at least one magnetic particle available for each particle to be removed by magnetic separation. There have been proposals to use larger size magnetic particles in magnetic separation. Thus International patent application WO 83/01397 discloses the selective removal of particulate mineral by treatment with magnetite. The magnetite is treated with silane and a suspension of magnetite particles in water was used to treat molybdenite ore in an ore.magnetite weight ratio of 2:1. In view of the very large amount of magnetite used and the high density of the molybdenite ore the ratio of numbers of magnetite particles to mineral particles will be high. Lockwood in some early US patents (US 996491, 1043850, 1043851) discloses the treatment of paints prepared from magnetite and an oily liquid. Examples of quantities used are 100 lbs of sulphide ore and 4| lb of magnetic paint containing a 2:1 weight ratio of magnetite to oil. However only a portion of the ore is intended to be magnetically separated, and the density of the ore will be close to that of the magnetite so that the number of magnetite particles is likely to be close to the number of target particles. When considering the removal of ash from coal one must remember that coal is a relatively low value product. The quantities of material used in any ash removal process must be kept low for economic reasons. Any non-combustible material used in removing ash from coal is itself ash and the quantity of such non-combustible material left in the coal should therefore be as low as possible. It is therefore __ C ?I clear that any non-combustible material such as that used in magnetic separations should be used in as small a quantity as possible, particularly if the coal rather than the mineral is to be removed magnetically. In order to provide a magnetic particle for each coal particle without using excessive amounts of magnetic particles it will be necessary to use very fine dispersions such as the colloidal dispersions taught by Schubert and Sladek and Cox. We have surprisingly now found that it is possible to obtain magnetic separation of coal from ash using small amounts of relatively large magnetic particles, and to use a cheap material which is readily available at many coal mines. According to the present invention the process for separating mineral particles from particles of solid carbonaceous fuel comprises forming a mixture of water and solid carbonaceous fuel particles, mineral particles, ferromagnetic particles having a weight mean particle size of at least 0.3 μm, and hydrophobic oil, wherein the weight ratio of carbonaceous fuel to magnetic particles is at least 25:1, and subjecting the resulting mixture to sufficiently high shear to cause flocculation of the particles of the solid carbonaceous fuel, and then subjecting the mixture to a magnetic separation so as to recover a magnetised fraction enriched in solid carbonaceous fuel from non-magnetised material. The particle size of carbonaceous fuel may for example less than 500 μm, more preferably less than 100 μm. The ferromagnetic particles may for example have a weight mean particle size in the range of 50 μm to 0.3 μm, more preferably 3 to 0.5 μm. Some writers classify materials into (1) ferromagnetic materials such as iron with a high magnetic s sceptibility; (2) paramagnetic materials with relatively low but positive susceptibilities and (3) diamagnetic materials with negative susceptibilities. Other writers divide materials into (a) ferromagnetic materials such as iron, (b) ferrimagnetic materials such as magnetite, (c) paramagnetic materials such as liquid oxygen, and (d) diamagnetic materials such as copper. In this specification "ferromagnetic" Includes both (a) ferromagnetic OMPI 4 and (b) ferrlmagnetic. The ferromagnetic particles are preferably particles of ferromagnetic metals, eg iron, cobalt» nickel. It is preferred to use ferromagnetic oxides of these elements, and It is particularly preferred to use magnetite. Magnetite is readily available at many coal mines as it is used commercially in dense medium separation processes. Magnetite particles in the preferred size range for use in the process may be obtained by grinding magnetite ore. Grinding is preferably carried out in the presence of oil. The aqueous mixture of carbonaceous fuel, ash, magnetic particles and oil may be formed by mixing the individual components together. It Is preferred however to form a dispersion of the magnetic particles and oil in water- and then to add the dispersion to the aqueous dispersion of carbonaceous fuel and ash. The nature of the mixing procedure used in the process of the present invention is believed to be critical If satisfactory results are to be obtained. A high shear mixing step is included In the preparation of the mixture. The use of a high shear mixing step appears to be required to produce agglomerates or floes of the carbonaceous.fuel. It Is difficult to quantify the amount of shear required to flocculate all the coal. Although mixers giving very high shear are not normally used for agitation, they are known to be available and mixers suitable for use in the process of the invention can readily be selected by a person skilled in agitation. Once such a person has been instructed that flocculation (and subsequent magnetic separation) can be achieved by using high shear mixing, the amount of shear required can be determined by simple tests. An example of a mixer which is capable of generating high shear comprises a rotor which is rotated at high speed adjacent to one or more stators which - may be in the form of perforated screens. The dispersion of ferromagnetic particles and hydrophobic oil is preferably prepared before the magnetic particles come into contact with the coal mineral slurry. The dispersion of magnetic particles in water may be prepared for example by mixing the components together in a high shear mixer. The hydrophobic oil may for example be a high boiling petroleum fraction such as those commercially available under the designation gas oil. The relative quantities of magnetic particles and hydrophobic oil which can be used may vary over a moderately wide range for example a weight ratio of magnetic particles to oil of 10:1 to 1:10, more preferably 1.5:1 to 1:1.5. The weight ratio of carbonaceous fuel (ie excluding the ash content) to magnetic particles is at least 25:1. Lower ratios greatly increase the cost and will give a significant ash content to the coal as a result of the presence of the magnetic particles. Preferably the weight ratio is not less than 50:1 preferably not less than 100:1. The weight ratio is preferably not greater than 1000:1, more preferably not greater than 300:1. Because of the low density of carbonaceous fuels such as coal in relation to that of most magnetic materials even the lowest weight ratio of 25:1 will correspond to a volume ratio of about 80:1 in the case of coal and magnetite. Methods and apparatus for carrying out magnetic separations are well known to those skilled in the art and it is not therefore necessary to discuss the magnetic separation in detail. The invention will now be described with reference to the following example. Example 1 Magnetite (0.5g) having a mean particle size of 1 μm was stirred in distilled water (450g) with gas oil (0.5g) using a high shear mixer for 30 minutes. Coal from the Horden Colliery (NCB) (50g) and water (150g) were mixed to form a slurry. The coal had an ash content of 45.8% mineral. The weight ratio of coal (excluding ash) to magnetite was 54.2:1. The slurry was added to the magnetite/gas oil suspension. Stirring with the high shear mixer was continued for a further 10 minutes. A sample (circa 20 ml) of this seeded slurry was then poured into a transparent (polymethyl methacrylate) cell fitted with a drain tap. The cell was located between the pole pieces of a commercially OMPI available laboratory scale wet magnetic separator sold by Boxmag Raplde Limited. With the field on maximum setting (about 0.8 Tesla)the coal was seen to migrate rapidly to the pole pieces of the magnet. The mineral matter remained unaffected and was run off from the cell through the tap. The magnetic field was then turned off and the magnetic material was washed out of the cell with clean water. The magnetic separation step on the retained material was then repeated once more. The magnetic material was then collected and dried. The ash content of the coal was reduced from 45.8% to 26% by weight. Example 2 Magnetite (0.5g) having a weight mean particle size of 2.5 μm was added to water (lOOg) contained in a 150 ml stoppered glass vessel. The vessel was then held in a sonic bath in which it was agitated by high frequency sound vibrations for 15 seconds. Gas oil (0.5g) was then added to the suspension. The suspension was further treated with high frequency sound for 15 seconds and then vigorously shaken by hand for 1 minute. Coal from the Horden Colliery (NCB) (50g) and water (300g) were mixed to form a slurry. The coal had an ash content of 45.4% wt and a weight mean particle size of 7 μm. * Numbers of particles present (calculated from Coulter counter weight per cent distribution) show 73 coal particles per 1 particle of magnetite in the slurry. The seed suspension was added to the coal slurry and stirred for 5 minutes with a Silverson standard model high speed mixer. The mixing head was fitted with a square hole, high shear screen. The mixer speed was set at maximum (about 8000 rpm). Flocculation of the coal particles took place. The slurry was transferred to a glass reservoir connected above a cell containing a wedge wire matrix sited between the poles of a Boxmag Rapid LHW magnetic separator. The cell was previously filled with water. A magnetic field of 1.0-Tesla was applied. The slurry was allowed to drain through the cell. The flow rate was approximately 50 ml per second. The mineral matter was seen to pass through the matrix unaf ected whilst the coal was retained within the matrix. The magnetic material was collected and dried. The ash content of the coal was 22.7% wt (61.7% wt of the feed). The non-magnetic material was collected and dried. The ash content of the nonmagnetics fraction was 82.7% (38.3% wt of the feed). Example 3 Example 2 was repeated except that: Magnetite having a weight mean particle size of 9.0 μm was used. Numbers of particles present (calculated from Coulter counter weight per cent distributions) show 424 coal particles per 1 particle of magnetite in the slurry. The mixture was passed through the magnetic separator at 65 ml/min. The magnetic material collected as in Example 1 had an ash content of 23.9% wt (42.9% wt of the feed). The non-magnetics fraction was collected and dried. The ash content of the non-magnetics was 61.1% wt (57.1% wt of the feed). Comparative Test A Example 3 was repeated except that the magnetite/gas oil seed suspension was added to the coal slurry and stirred for 5 minutes with an Anderman and Co Ltd Multirspeed paddle stirrer at 720 rpm. No flocculation of the coal particles took' place. The magnetic material was collected and dried as before. The ash content of the coal was 49.1% wt. (22.8% wt of the feed). The non-magnetics were collected and dried. The ash content of the non-magnetics fraction was 44.3% wt (77.2% wt). Example 3 The previous three experiments show that some mineral is being retained in the matrix. It is thought that this is due to the presence of magnetic minerals and/or entrapment of mineral within the floe. Coal from the Horden Colliery (50g) and water 300g were mixed to form a slurry. The coal had an ash content of 45.4% wt and a weight - O PI mean particle size of 7 μm. This slurry was then placed in the reservoir and allowed to drain through the matrix at a rate of 65 ml/min as described. Magnetic minerals are retained in the matrix whilst the coal and non-magnetic minerals pass through the matrix unaffected. The field strength was 1.5 Teslas. The ash content of the coal was 39.2% wt (81.1% wt of the feed). The coal slurry was treated as described in Example 1. The coal fraction was collected and redispersed in water (500 ml) and was then subjected to a further 1 minute mixing using the Silverson mixer. Magnetic separation was carried out as before. The magnetics material was collected and dried. The coal had an ash content of 9.1% wt (46.9% wt of the feed). A comparison of the results from Example 1 with those from Comparative Test A shows that, when using a paddle stlrrer, it is not sufficient to use oil and magnetite to obtain coal with decreased ash content. The ash content of the separated coal Is Comparative A was higher than in the original coal feed. This shows that it is necessary to use a high shear mixing step to obtain satisfactory separation. OMPI

Claims: Claims: 1. A process for separating mineral particles from particles of solid carbonaceous fuel which comprises forming a mixture of water, carbonaceous fuel particles, mineral particles, ferromagnetic particles having a mean particle size of at least 0.3 μm and a hydrophobic oil, wherein the weight ratio of carbonaceous fuel to magnetic particles is at least 25:1, and subjecting the resulting mixture to sufficiently high shear to cause flocculation of the particles of the solid carbonaceous fuel, and then subjecting the mixture to a magnetic separation so as to recover a magnetised fraction enriched in solid carbonaceous fuel from non-magnetised material. 2. A process according to claim 1 wherein the magnetic particles are magnetite particles. 3. A process according to claim 2 wherein the magnetite particles are obtained by grinding magnetite ore in oil. 4. A process according to. any one of the preceding claims wherein the mixture is formed by adding a dispersion of magnetic particles and oil in water to an aqueous dispersion of carbonaceous fuel and ash. 5. A process according to any one of the preceding claims wherein substantially all the carbonaceous fuel has a diameter less than 500 μm. 6. A process according to claim 5 wherein substantially all the carbonaceous fuel has a diameter less than 100 urn. 7. A process according to any one of the preceding claims wherein the magnetic particles have a mean particle size below 50 μm. 8. A process according to any one of the preceding claims wherein the weight ratio of carbonaceous fuel to magnetic particles is not less than 50:1.

Inventors: REESON STEPHEN JOHN, REESON, STEPHEN, JOHN
Applicants: BRITISH PETROLEUM CO PLC, THE BRITISH PETROLEUM COMPANY P.L.C.
Application year: 1984
Main classification: B03C1
IPC codes: B03B9, B03C1, B03D3