Patent Grant
7124968
The invention is in the field of physical separation of particulate matter. Specifically, the invention relates to the operation of a comminution device as a size reducing device, as concentrator, and as a separator of less friable particles liberated from the feed matrix in the grinding operation. More specifically, the invention is in the field of physical separation of particles removed from the comminution device to recover desirable particles from return to the comminution device.
Comminutors are employed to reduce the size of particles to a range which is desirable and to liberate impurities so that they can be removed downstream of comminution. The feed particles may range in size up to several inches while the product particles may range from inches down to microns in size. More comminution energy is required to bring a mixture of particles of widely ranging friabilities to the desired size consist than when the friable components alone are present. The invention relates to reducing comminution energy consumption and increasing the throughput of comminutors while improving the quality of the product of comminution by separating the friable and less friable components as they are liberated from the feed matrix within the grinding operation and before the hard component is overground. Specifically, the invention relates to modification and operation of comminuting devices and their classifiers, if they are used, so as to separate two streams from the comminution device. One is concentrated in the hard and less friable components liberated from the feed in the grinding operation. This may be either an impurity or a valuable component of the feed. The other is concentrated in the friable component of the feed. More specifically, the invention relates to combining the operation of a comminution device and a separation device so as to separate the hard components of the feed as they are liberated inside the comminutor but before they are overground. Separation methods based on gravity, size classification, dry magnetic separation, and triboelectric means are used to separate hard material form friable material found in a mill concentrated steam taken from the comminution device. Particulate matter of differing chemical and physical makeup can have different magnetic properties and may be electrically charged by contact and friction, tribocharging. By including triboelectric separation means, modified dry magnetic separators can be effective in recovering friable material of a great range of types from the mill concentrated fraction taken from the mill. By this combined pulverizer-separator operation, the MagMill™ can produce high quality comminution products without significant loss of the desired component. The friable material so separated is returned to the grinding zone for grinding to product fineness while the hard component is collected separately and not returned to the mill. By this means, both the quality and the recovery of the separated components are improved when compared to that of the state-of-the-art technology in which everything is reduced to the same size consist and then separated downstream of the comminution device.
The invention is distinguished from the state-of-the-art by processing a significant amount of the particles circulating inside the comminution device. In current technology, tramp iron exits are employed to separate very small amounts of hard and abrasive material before it destroys the inside of the grinding device. The rate of withdrawal of this undesirable material is typically less than 1/10% of the rate of feed to the comminutor. The desired goal is protection of the grinding device, not improvement of the quality of the product or increasing throughput and decreasing power draw. By contrast, the present invention preferentially extracts material from the inside of the comminutor which is concentrated in hard components of the feed. Indeed, if it is desired to improve the quality of the product, then the amount of material to be separated from the inside of the comminutor must be sufficient to have an effect. One tenth of one percent, generally, is not enough. The required amount is dependent upon the concentration of hard components in the withdrawn material and the recovery of more friable material from this stream which is subsequently returned to the comminutor for grinding to size specification. The present invention is unique in showing how and where to withdraw this material from the comminutor and in employing unique and powerful methods for recovery of the friable component inadvertently withdrawn from the internally circulating stream inside the comminutor.
Indeed, it has been discovered that particles of quality the same as or worse than that withdrawn from the tramp metal chutes can be withdrawn from the inside of a coal pulverizer at locations several feet above the throat area where air enters and tramp metal exits. This has been observed in grinding a blend of raw coals from North Central Pennsylvania in an ABB C.E. Raymond 633 bowl mill. For this mill, coal was withdrawn from the pyrite trap at the rate of 67 pounds per hour. This is small compared to the nominal 12–15 TPH fed to the pulverizer. The coal withdrawn from the pyrite traps had an ash level of 69.1 Wt. % and a sulfur level of 23.4 Wt. %. In the experimental tests, coal was withdrawn at the rate of 8.2 Lb/Hr from a sampling port located several feet above the top of the grinding bowl in the region which is open for air flow upward. While the particle size was smaller than that withdrawn from the pyrite traps, it had an ash level of 58.1 Wt. % and a sulfur level 33.6 Wt. %. This illustrates the potential for separation of refuse quality material from the flow of particles inside the pulverizer.
By way of example, coal is dry-milled to 200 mesh (74 micron) topsize at pulverized-coal fired power plants to promote good combustion characteristics. [See, for example, Steam, Its Generation and Use, Chapter 9, “Preparation and Utilization of Pulverized Coal,” Babcock & Wilcox, New York, N.Y., 1978, and Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation, Ed., Joseph G. Singer, Chapter 12, “Pulverizers and Pulverized-Coal Systems,” Combustion Engineering, Inc., Windsor, Conn. 1981, incorporated by reference herein]. The fine coal generated in the pulverizer is air-conveyed out of the mill directly to the burner. Coincidentally, grinding to 200 mesh is also effective in liberating fine minerals encased in the feed-coal particles even after the coal has been cleaned using conventional wet processing technology. However, other than tramp iron chutes called pyrite traps for removing small amounts of pyrites and other coarse debris, no means are employed in the coal pulverizers now used to separate minerals which are liberated there. Separation of hard minerals at the pulverizer would improve operation of the power plant by increasing the pulverizer throughput and reducing the power draw, by reducing abrasive wear, by reducing slagging, fouling, and water wall wastage in the furnace, by reducing emissions of sulfur and other hazardous air pollutants such as trace metals associated with minerals in the coal, including mercury, and arsenic which is deleterious to the operation of catalytic scrubbers used in post combustion separation of sulfur and nitrogen oxides.
The bulk of the hydrocarbon structure of bituminous coals is much softer than the minerals commonly found in coal. Consequently, hard minerals require more passes through the mill's grinding zone to reach product size specification (70% to 80% finer than 74 microns particle diameter) than does the soft coal. Because of this, the concentration of hard minerals is greater in the stream of oversize particles circulating inside the pulverizer (internal circulation) than it is in the feed coal. Iron pyrite, one of these minerals, is one of the hardest and most abrasive minerals commonly found in coal. Trace metals such as mercury, arsenic, and selenium are known to preferentially associate with iron sulfide minerals such as pyrites. Consequently, removal of refuse concentrated in the mill circulation can significantly lower the ash, sulfur, and trace metal levels in the mill product.
The logical place for fine coal cleaning is in the pulverizers, which are already used by the power plant. Indeed, EXPORTech Company, Inc. (Y. Feng, R. R. Oder, R. W. DeSollar, E. A Stephens, Jr., G. F. Teacher and T. L. Banfield, “Dry Coal Cleaning in a MagMill, appearing in the Proceedings of the 22nd International Technical Conference on Coal Utilization and Fuel Systems,” Clearwater, Fla., Mar. 16–19, 1997; See also R. R. Oder, R. E. Jamison, and E. D. Brandner, “Preliminary Results of Pre-Combustion Removal of Mercury, Arsenic, and Selenium from Coal by Dry Magnetic Separation,” appearing in the Proceedings of the 24th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, Fla., Mar. 8–11, 1999, pp. 151–158, incorporated by reference herein) has shown that refuse with high levels of ash and sulfur can be separated from the internal circulation of almost all commercial pulverizers used at power plants and that removal of this refuse from the mill can lower the ash and sulfur levels and reduce the levels of toxic trace elements in the pulverized product. ETCi has further demonstrated that dry magnetic separation can be used to recover clean coal from the refuse (R. R. Oder, R. E. Jamison, and J. R. Davis, “Coal Cleaning at Pulverized-Coal Fired Power Plants,” Proc. 11th Annual Pittsburgh Coal Conference—Coal: Energy and the Environment, Sep. 12–16, 1994, Pittsburgh, Pa., Ed., S-H Chiang, pp. 640–645 (1994), incorporated by reference herein). Additionally, ETCi has suggested that the combined process of pulverization, size and density classification in the mill, dry magnetic separation for recovery of clean coal from the mill refuse, plus return of the clean coal to the pulverizer for grinding to product fineness, is a novel method for efficient separation of ash forming minerals, sulfur, and hazardous pollutants from the coal fed to a pulverized-coal fired power plant. This novel method is not practiced in the electric power industry because of the significant engineering challenge associated with removal of a concentrated stream of refuse from the pulverizers. This obstacle has now been overcome and is the basis for the invention disclosed here.
It is important to note that others have used magnetic separation to separate hard gangue material from the feed to pulverizers, which is standard practice in the industry, and some also to recover the value component from the underflow in the pyrite traps or tramp metal chutes employed in most grinding mills. While this material may be blended with the product or returned to the mill for further grinding, these efforts have treated only a small amount of the material fed to the mill. The current invention is greatly different from these past efforts in two major ways. First, large amounts of material are extracted from the internal circulation of the mill from locations other than the tramp iron chutes. Secondly, powerful magnetic separation techniques are employed which have the capability for separation of materials ranging from strongly magnetic to diamagnetic. Indeed, with the addition of triboelectric phenomena (ElectriMag™ Separator co-pending application having Ser. No. 09/289,929 filed on Apr. 14, 1999, incorporated by reference herein), the method is now capable of separating particles based on both magnetic and surface charging characteristics. The present invention goes far beyond the present state-of-the-art. For this reason, the technology is not restricted to conventional applications to separation of strongly magnetic particles from inert materials. With the combined action of the pulverizer to liberate on the basis of differences in friability and the electric/magnetic separation mechanism employed, the technology can be applied to a wide range of important new applications.
Friability generally has to do with the ease with which small particles can be made in a comminutor. More friable particles produce a greater amount of finer particles than do less friable particles. Generally speaking, friability is related to the hardness of the material and to its ability to fracture which is related in a complex way to fundamental physical characteristics such as crack propagation in the solid. (Klaus Schonert, “Aspects of Very Fine Grinding,” Chapter 9 in Challenges in Mineral Processing, Proceedings of a Symposium honoring Douglas W. Fuerstenau on his 60th birthday, Editors, K. V. S. Sastry and M. C. Fuerstenau, Society of Mining Engineers, Inc., Littleton, Colo., 1989, incorporated by reference herein). Generally the energy to grind a solid to a specified particle size distribution has been related to an index called the Bond Work Index. This is widely used. Values of the Work Index range generally for 1.4 for calcined clay to 135 for mica. Coal of specific gravity 1.63 has a reported index of 11.4 (Chemical Engineers Handbook, Fifth Edition, Edited by Robert H. Perry, and Cecil H. Chilton, McGraw-Hill Book Company, New York, N.Y. 1973, page 8–11, incorporated by reference herein). Those solids with a large Work Index require more energy to grind to a given particle size. This means more time in the comminutor. Particles with lower Work Indices will require less time. The Work Index is a general measure of the tendency of hard to grind materials to concentrate in the internal circulation of comminution devices.
The value, 11.4, listed for coal is relatively high on this scale because coal of density 1.63 has a significant amount of mineral impurities which have higher work indices than does the mineral-free soft coal. Indeed, the grindability of coal is measured by an index which is different from the Bond Work Index. It is called the Hardgrove Grindability Index, (HGI) and is generally restricted to coal. HGI measurements are made when a specified particle size distribution of the coal is placed in a laboratory grinding machine of a standardized design and a specified amount of grinding energy is expended [See Steam, Its Generation and Use, Babcock & Wilcox, New York, N.Y., 1978, and Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation, Ed., Joseph G. Singer, Combustion Engineering, Inc., Windsor, Conn. 1981, incorporated by reference herein]. The amount of material in the product of grinding which is finer than 200 mesh (74 microns particle diameter) is compared with that of a standard coal whose HGI is taken as 100. On this scale, the higher the value, the more friable or easier the coal is to grind. The grindability of coal is a composite property made up of other properties, such as hardness, strength, and fracture for example. A general relationship exists between grindability and rank. Coals that are easiest to grind are found in the medium and low volatile groups. They are decidedly easier to grind than coal of the high volatile bituminous, sub-bituminous, and anthracite ranks. [See Coal Preparation, 4th Edition, Edited by Joseph W. Leonard, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. New York, 1979, Page 1–8, incorporated by reference herein].
The effects of coal grindability on pulverizer throughput are described in combustion technology handbooks [See for example Steam, Its Generation and Use, Chapter 9, “Preparation and Utilization of Pulverized Coal,” Babcock & Wilcox, New York, N.Y., 1978, and Combustion Fossil Power Systems, A Reference Book on Fuel Burning and Steam Generation, Ed., Joseph G. Singer, Chapter 12, “Pulverizers and Pulverized-Coal Systems,” Combustion Engineering, Inc., Windsor, Conn. 1981, incorporated by reference herein]. The grindabililty is a function of moisture in the coal, its rank, petrographic makeup, and mineral content and types. The effects of petrographic makeup and minerals have not been generally been recognized or used. (R. R. Oder and R. J. Gray, “The Effects of Coal Characteristics on Fine Grinding in a Pitt Mill, Chapter 48, in Comminution—Theory and Practice, S. Komar Kawatra, Editor, Society of Mining, Metallurgy, and Exploration, Inc. Littleton Colo., 1992, incorporated by reference herein). Table I shows the effects of ash and sulfur levels on the HGI of a blend of medium and high volatile bituminous rank raw coals being ground in an operating pulverizer in a pulverized coal fired power plant in North Central Pennsylvania. The “Pulverizer Concentrated Sample” is material withdrawn from the internal circulation of the pulverizer. It has significantly higher levels of ash and sulfur than the pulverizer feed and a significantly lower value of HGI. The increased sulfur in the “Pulverizer Concentrated Sample” is caused by increased concentration of iron pyrite in the sample. The “Magnetic Separator Reject” material is reject material taken from the “Pulverizer Concentrated Sample” by a dry magnetic separator of the type described in this patent. It is discarded from the pulverizer.
It is apparent that there are mineral impurities in the coal which can be removed from the internal stream circulating inside the pulverizer which have high concentrations of ash and sulfur and which adversely affect the grindability of the coal.
Effects of Hard Particles on Power Consumption and Throughput of a Coal Pulverizer
Separation of the particles of high levels of ash and sulfur from the internal circulation of a pulverizer will increase the effective grindability of the particles in the grinding zone. This has the effect of increasing the throughput and reducing the grinding energy of the pulverizer. This has been observed in grinding an Upper Freeport seam coal from North Central Pennsylvania. The coal was ground in a nominal 1½ ton per hour (TPH) pilot ring/roller pulverizer. A nominal 1½ TPH prototype MagMill™ was made by retrofitting an ElectriMag™ and ParaTrap Magnetic separator of the type described in this patent to the pilot mill. Mill concentrated material taken from the base of the pulverizer was processed. The throughput of the MagMill™ prototype increased to 120% and the grinding energy reduced to 70% of that of the unmodified pulverizer processing the same coal when the iron pyrite content in the product of the MagMill™ had been reduced by nominally 70% and the ash level by 40% with respect to the coal fed to the MagMill™.
Mill Wear
In general, the combination of hard materials, coarse particles, and high velocity are conducive to wear in mills. Extensive data on the wear and cost of various types of steels in ore grinding have been reported (Norman and Loeb, Trans. A.I.M.E., 183, 330, 1949, incorporated by reference herein). Mill wear or abrasion becomes critical on high-peripheral-speed equipment, particularly high-speed close-clearance hammer mills and coal pulverizers of the roller and bowl types low in the mill near the throat. An abrasion index in terms of kW-hr input/Lb of metal lost furnishes a useful indication. Rough values can be found in the Chemical Engineers Handbook, Page 8–10, 1973. Abrasive indices for the 38 materials shown range from 0.0001 for Sulfur, to 0.6905 for Quartzite. Coal is near the low end of the scale and most minerals in coal are near the high end.
Abrasive Wear In Coal Pulverizers. The abrasiveness of coal contributes to operating and maintenance costs at pulverized-coal fired power plants. Areas of high wear are areas inside the pulverizer where coarse particles of high concentrations of ash and sulfur are accelerated by high air velocity entering the mill. This is generally in the base. Abrasive wear is increased many times under the high contact pressure developed between coal and metal in pulverizers. It is important to recognize the relationship between high density coal components found in the lower portions of pulverizers and the wear they cause. This is illustrated in the results of abrasive wear tests made on Eastern US and a Western US raw bituminous rank coal and their specific-gravity fractions which are shown in Table II. These tests show the relationship between the ash levels, abrasive wear, and the specific gravity fractions of the coals. The abrasiveness of raw coals is almost completely due to mineral impurities. [Excerpted from Table 1–19, Coal Preparation, 4th Edition, Edited by Joseph W. Leonard, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. New York, 1979, Page 1–51, incorporated by reference herein.]
The abrasive wear attributable to coal is primarily related to the hardness of the minerals in the coal and especially to the quartz and iron sulfides, mainly iron pyrite. The hardness is measured by empirical tests but is closely related to fundamental properties. It is a function of the rank of the coal and varies greatly among the maceral components. The hardness of coal is generally in the range 10–70 kg/mm2 (Vickers Indentation Hardness Test). It has a maximum at 84% carbon (dry mineral free) and a minimum at 90% carbon (dry mineral free) and then increases again. By way of comparison, quartz and pyrite have Vickers hardness numbers of 1100–1260 and 840–1130 respectively and those of hard steels range from 600 to 700, [Coal Preparation, 4th Edition, Edited by Joseph W. Leonard, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. New York, 1979, Page 293, incorporated by reference herein.]
The present invention pertains to an apparatus for sorting particles. The apparatus comprises a first comminution mechanism for releasing particles encases in a solid matrix. The apparatus comprises a first separating mechanism for removing particles from the comminution mechanism. The apparatus comprises a size classification means for separating particles based on their size. The first separating mechanism is engaged with the comminution mechanism and the size classification mechanism. The apparatus comprises a first magnetic means for separating particles with a magnetic force. The apparatus comprises an electric mechanism for separating particles with an electrical force disposed adjacent to the magnet mechanism. The apparatus comprises a first providing mechanism for providing particles to the electric and magnetic separation means. The first providing mechanism is engaged with the size classification mechanism and the electric and magnetic separation means. The apparatus comprises a second magnet mechanism for separating the less magnetic particles separated in the first magnetic mechanism. The apparatus comprises a second mechanism for providing the particles to the second magnet mechanism. The second providing mechanism is engaged with the first and the second magnet mechanisms.
The present invention pertains to a method for sorting particles. The method comprises the steps of comminuting the particles to release particles encased in a solid matrix. Then there is the step of separating the particles from the comminuting device based on the hardness of the particles. Then there is the step of classifying these particles based on size. Then there is the step of charging the undersize particles by contact or by friction and of providing the particles to a first magnet mechanism and electric mechanism disposed adjacent to the magnet mechanism. Then there is the step of separating the particles with the magnetic force from the magnet mechanism and the electric force from the electric mechanism. The method also comprises the steps of providing the less magnetic particles separated in the first magnetic mechanism to a second magnet mechanism. Then there is the step of separating the less magnetic particles with the magnetic force from the second magnet mechanism.
In the accompanying drawings, the preferred embodiment of the invention and preferred methods of practicing the invention are illustrated in which:
Referring now to the drawings wherein like reference numerals refer to similar or identical parts throughout the several views, and more specifically to
The grinding chamber 200 inside of the pulverizer is shown in more detail in
Hot air 5 is blown into the base of the mill 207 through the air casing 18 shown in
The inside of the pulverizer at an elevation above the top of the gear train mechanism 211 is shown in
Separation mechanism 8 is a kick-out door. It opens to the inside of the pulverizer chamber at 209. The separation mechanism 8 can be located at any elevation from the top of the roller 202 up to the top of the grinding chamber at 210. It is preferentially at an elevation above or below the rotating arm 204 and at a location around the circumference of the grinding chamber which is away from the feed 4 and the mill drive shaft 212. More than one separation mechanism 8 may be used in the grinding chamber.
Separation mechanism 9 is a kick-out door. It opens into the region of the pulverizer above the top of the gear train mechanism 211 between the casing 400 and the inverted cone 401 of the classifier. It can be located at any elevation from the top of the gear train mechanism 211 up to an elevation below the entrance to the classifier at 402. More than one separation mechanism 9 may be used above the top of the gear train mechanism. It can be located anywhere around the circumference of the classifier.
The kick-out door mechanism can be as shown in
When there is no potential for explosion both kick-out door mechanisms 8 and 9 are attached directly to the pulverizer as shown in
A second mechanism 600 for removing particles from the inside of the pulverizer is shown in side view in
Particles removed from the pulverizer by separation devices 7, 8, 9, or 10 can be issued to reject stream 17 directly or conveyed 11 to feed hopper 20. The particles withdrawn from the internal circulation in the mill by any of the sampling mechanisms, 7, 8, 9, or 10 can be directed individually or in combinations to the reject stream 17. The conveyance mechanism 11 can be a screw conveyor or a conventional conveyor of the type manufactured by AFC of Clifton, N.J. The conveyance mechanism 11 and the separation mechanism 2 and return conveyance mechanism 16 and the reject conveyance mechanism 17 should be enclosed to prevent dusting. The capacity of the conveyors 11 ranges from 1/10 to the full rate at which particles are fed to the pulverizer and preferentially is in the range of ⅓ to ½ of the full rate of the feed. The capacity of the return conveyance devices 16 and the reject conveyance mechanism 17 ranges from ⅙ to the full rate of the feed to the pulverizer.
Particles are issued from feed hopper 20 to classifying screen 12. Oversize particles 15 generally coarser than 3 mm or 8 mesh taken from classifying screen 12 are conveyed to switch 19. The switch can divert the oversize particles back to the pulverizer through stream 16 or can discharge them to reject through stream 17, depending upon quality of the particles. The underflow at classifying screen 12 is conveyed to vibrating feeder 100 and thence to the electric and magnetic separator 13.
The following description refers to
A vertical section through the center of magnetic pulley 803 is shown in
Particles which migrate to the regions of high magnetic flux such as 906 and 907 and which are held there by magnetic forces at the surface of the belt are carried around the axis 908 of the pulley 803 by the belt 801 and drop free from underneath the belt at 804 as the belt pulls away from the cylindrical magnet surface. A doctor blade 807 is located at the back edge of the idler pulley 808 to remove particles adhering to the belt.
An electrode 809 is placed adjacent to the belt magnetic separator 13 as shown in
A catch basin 101 is located underneath the belt separator 13 to collect particles as they are thrown from the belt separator. The catch basin can be made from conducting or insulating materials. Non-conducting materials are preferred since there is less interaction with the applied electric field.
Particles collected in the receivers which are farthest from the magnet 803 are the least magnetic and have electric charge similar to that of the surface of the magnet. These particles are expelled to the weakly magnetic product through the chute 806. Particles which are charged oppositely to the surface of the magnet and which are the most magnetic will be collected at 818. They are expelled through chute 804. Particles with intermediate magnetism and with weak or no surface charging will be collected at 816. They constitute a middling fraction which is processed in the second magnet mechanism 14 for additional magnetic separation. These particles are expelled through chute 805 into magnetic separator 14.
Particles exiting the first electric and magnetic separator at 805 fall into a receiving bin 101 shown in end view in
The magnetic matrix 1200 is magnetized by the surrounding transverse access electromagnet 1300 and is an integral part of the magnet circuit. (The electromagnet 1300 is shown in top view in
The transverse access electromagnet shown in
A plan view of the top of the electromagnet 1300 with the magnetic matrix 1200 removed is shown in
Looking from above in
One end of the windings 1403 is shown in the left portion of the figure, the other 1404 in the right portion. Current connections are made at these ends to an external power supply (not shown) such as that supplied by Electronic Measurements, Inc., Neptune, N.J. The coil windings are made from copper or other suitable conductor and can be hollow to accommodate cooling. Connections are also made at these endings for cooling water supplied by a chiller (not shown) such as that supplied by Affinity Inc., Ossipee, N.H.
Referring now to
The weakly paramagnetic or diamagnetic particles in the stream of particles entering at 1806 are pushed by magnetic forces outward into the regions 1802 where the magnetic field strength is lowest. Paramagnetic particles are attracted and trapped in the regions 1803 near the pole tips. The magnetic force is sufficient to separate the particles but not strong enough for particles exiting the first electric and magnetic separator at 805 to stick to the poles in the magnetic separator 14. The particles which pass generally have magnetic susceptibilities less than about 5*10−9 m3/kg.
Shown in
In the bottom portion of
Referring now to
The particles collected in adjacent openings 1903 between splitters have different magnetic susceptibilities. Referring now to
Particles exiting at the bottom of splitter 1901 fall directly into chute mechanism 1904. Each segment 1906 of this mechanism has a ramp 1907 which directs the falling particles laterally out of the separator through holes 1905. The ramp in each adjacent segment is sloped oppositely so that all particles of like magnetism exit the separator on the same side. Paramagnetic particles will be on one side and diamagnetic particles will be on the other.
The composite stream of diamagnetic particles is discharged through chute 815 of
The weakly magnetic particles exiting the first electric and magnetic separator at 806, the diamagnetic particles exiting the second magnetic separator at 815, the paramagnetic particles exiting the second magnetic separator at 813, and the strongly magnetic particles exiting the first electric and magnetic separator at 804 can each be collected separately or can be combined as desired.
Referring now to
This pulverizer is different from the ring/roller in that a rotating bowl and stationary roller are employed as opposed to rotating rollers and a stationary ring. Further, this mill has a mechanism 2107 for discharging large pieces of very hard particles such as machine parts and railway spikes from the grinding zone in order to protect the mill. In grinding coal this is called a pyrite trap. Particles which are small enough to pass through the vane openings in the throat and which are heavy enough to fall under the air drag in that region will discharge underneath the bow and be swept by scrapers 2106 to the discharge chute 2107. When grinding coal, the rate at which particles emerge through the pyrite trap is very small, about 0.1% of the rate at which particles are fed to the pulverizer. This discharge mechanism is designed to protect the mill and not to improve the quality of the mill product.
Contrary to the pyrite traps, the MagMill™ is designed to remove large amounts of the particles circulating inside the mill. These particles are removed and processed external to the mill so as to improve the quality of the mill product. Withdrawal rates between 10% and 100% of the rate of feed to the mill and preferably 30% to 50% are employed.
A mechanism for removing mill concentrated material from the internal circulation of the mill is shown at 2101. This mechanism is of the types shown in
Particles withdrawn from the pulverizer by mechanisms 2101 can be either issued to reject stream 17 directly or conveyed 11 to separation mechanism 2 as shown in
Referring now to
This pulverizer is different from the ring/roller in that a rotating table and stationary tires are employed as opposed to rotating rollers and a stationary ring. Further, this mill has a mechanism 2207 called a pyrite trap for discharging large pieces of very hard particles such as machine parts and railway spikes from the grinding zone in order to protect the mill. Particles which are small enough to pass through the vane openings in the throat and which are heavy enough to fall under the air drag in that region will discharge underneath the table and be swept by plows 2206 to the discharge chute 2207. When grinding coal, the rate at which particles emerge through the pyrite trap is very small, about 0.1% of the rate at which particles are fed to the pulverizer. This discharge mechanism is designed to protect the mill and not to improve the quality of the mill product.
Contrary to the pyrite traps, the MagMill™ is designed to remove large amounts of the particles circulating inside the mill. These particles are removed and processed external to the mill so as to improve the quality of the mill product. Withdrawal rates between 10% and 100% of the rate of feed to the mill and preferably 30% to 50% are employed.
A mechanism for removing mill concentrated material from the internal circulation of the mill is shown at 2201. This mechanism is of the type shown in
Yet another mechanism 2299 for removing particles from the pulverizer is shown in the Figure. This mechanism is of the types shown in
Particles withdrawn from the pulverizer by mechanisms 2201 and 2299 can be conveyed directly to reject stream 17 or conveyed 11 to separation mechanism 2 as shown in
The MagMill™ shown in cut-away view in
A portion of the hard particles are withdrawn from the mill by particle sampling mechanisms 7, 8, 9, and 10. Some types of conventional pulverizers such as roller mills separate large and very hard debris such as iron spikes from the grinding zone through openings in the air flow passages in the bottom of the pulverizer (not shown here). These openings are generally called pyrite traps. They remove a very small amount of material from the pulverizer, generally less than 0.1% of the feed. The pyrite traps are intended to protect the pulverizer from damage. They are not used to improve the quality of the product of pulverizing. In a MagMill™, material is removed from the inside of the pulverizer through sampling mechanisms 7, 8, 9, & 10 at a very high rate. This can be as much as 100% of the rate at which particles enter the mill. Preferably, it is between 10% and 100% of the feed rate. More preferably it is between 30% and 50% of the feed rate. The purpose of removing this material is to improve the quality of the product. The advantage of processing this stream of particles taken from the internal circulation of the pulverizer is the extra mineral liberation in this stream. The particles are intermediate in size between the size of particles fed to the pulverizer and that issued in the product. Separation-of particles in this stream is more efficient than treating the feed. Further, this stream of particles has a high concentration of the hard material to be removed so that the separation mechanism 2 can be smaller than that required to treat the entire stream. The MagMill™ is a technically and economically advantageous method for improving the quality of the pulverizer product.
Particles which are removed from the internal circulation of the pulverizer through sampling mechanisms 7, 8, 9, & 10 can be either issued to reject stream 17 directly or fed to the hopper 20 and screening device 12 where oversize particles 15 are withdrawn. The particles withdrawn from the internal circulation in the mill by any of the sampling mechanisms 7, 8, 9, or 10 can be directed individually or in combinations to the reject stream 17 when the quality of the particles does not warrant processing through separation mechanism 2. Oversize particles are those which are too coarse for effective treatment in the separation mechanism 2. They are generally coarser than 8 mesh or about 3 mm. The top-size is dependent on the characteristics of particles to be processed in the separation mechanism. Generally, strongly magnetic particles can be processed efficiently at a coarser size consist than can feebly magnetic particles such as coal. When grinding coal, pulverizer concentrated particles are generally smaller than 8 mesh with only a few percent finer than 100 mesh. If the oversize particles coarser than 8 mesh are highly concentrated in hard impurity particles, they are rejected to stream 17. Otherwise the oversize particles are returned to the pulverizer for additional grinding through steam 16. Under size particles, generally finer than 8 mesh, are fed to the electric and magnetic means 2 where particles are separated on the basis of air drag, particle mass, surface charging, and magnetic characteristics. The less desirable hard particles separated by separator 2 are rejected from the MagMill™ in stream 17. The friable particles recovered by the magnetic separator are returned to the pulverizer for grinding to specification in stream 16. For coal, separation of mineral gangue results in a pulverized-coal product which has lower concentrations of ash, sulfur, and associated trace metals than the coal fed to the pulverizer.
The following description of the method is given in terms of pulverizing coal in a ring/roller mill. It illustrates the principles of separation in operation inside the mill and shows the function of the electric and magnetic separator. While the grinding mechanism illustrated is that of a ring/roller mill, mills and crushers of other types could have been used and products coarser than pulverized are possible. Further, the separation mechanism shown is not limiting. Means for particle size classification other than screening such as air classifiers, air tables, air cyclone, etc. can be used. Additionally, in some instances only the first stage ElectriMag Separator may be required.
There are many trace metals in coal. Each can range from parts per billion based on the weight of coal to thousands of parts per million. Of the trace metals, mercury, arsenic, and selenium are of particular interest because they are considered hazardous air pollution precursors (HAPS). Mercury is of particular interest because of the emissions restriction anticipated, less than 1 pound of mercury per 1013 Btu or approximately 1 pound of mercury 109 pounds of coal, and the difficult and cost of removal from flue gases, of the order of $20,000 per pound of mercury. With mercury levels typically 100 pounds per billion pounds of coal, very high efficiencies of removal will be required. Arsenic is of additional interest because this trace metal poisons catalytic reactors used for separation of nitrogen oxides from the combustion off gases. Catalyst replacement is very expensive.
Particles entering at 4 drop into the base of the pulverizer 207 where they are ground by being caught between the ring 201 and roller 202 mechanism. Particles are thrown in all directions by the energy release in the grinding event. The plow 206 revolves through the mass of particles in the base and moves these particles into the region between ring and roller. Large particles which strike the walls of the grinding chamber 200 fall back into the base of the mill where they are forced into the grinding mechanism again. Air 5 is blown into the base of the mill through air scroll casing 18. The upward swirl of air 2002 conveys particles in a swirling motion out of the grinding zone. Some particles are thrown outward against the inside wall of the pulverizer and fall back into the base of the mill where they undergo additional grinding. Small particles are conveyed upward in the pulverizer by the air flow 2002 and enter the classifier 2003 through vanes 402 at the top of the mill. The smallest particles are conveyed with the air flow out of the pulverizer at 6. Oversize particles which enter the classifier through vanes 402 are returned to the base of the mill through flap valves 403 at the base of the classifier.
A portion of the particles which are intermediate in size between that of the feed to the pulverizer and the product and which are in the base of the mill or are in motion above the grinding zone are removed from the inside of the mill through removal mechanisms 7, 8, 9, and 10. Mechanism 7 is a screw conveyor. Referring now to
Particles colliding with or moving near the walls of the grinding chamber 200 are removed from the pulverizer through separation mechanism 8 mounted on the wall of the grinding chamber. There may be more than one such separation mechanism and they may be mounted at various elevations above the top of the grinding zone in the base of the mill 207. The separation mechanism 8 opens into the mill through a hinged door which can be directed to catch particles which are rising, falling, or moving around the circumference of the mill in either clockwise or counterclockwise direction. An air-jet mechanism 615 can be used to prevent excess amount of fine material from being withdrawn from the mill. This is accomplished by directing the air jet into the mill through the opening for mechanism 8. The coarse particles which are deflected into the separation mechanism fall through an airlock mechanism which serves to isolate the atmosphere inside the mill. The mill can be of the overpressure or the under-pressure type. Particles exiting mechanism 8 can be discharged to the reject stream 17 directly when the quality of the particles does not warrant processing with separation mechanism 2 or conveyed to the separation mechanism 2 via conveyor 11. This conveyor can be a screw conveyor, a belt conveyor, elevator or any method for moving the particles in the minus 8 mesh size fraction.
Particles which are falling along the inside wall of the outside casing of the classifier are removed from the pulverizer circulation by separation mechanism 9. More than one such mechanism may be employed and they may be mounted at any elevation below the entrance to the classifier at the top of the mill. This mechanism may be arranged to catch particles rising, falling, or with a vortex motion in either direction around the inside wall of the classifier casing. Preferentially, it is arranged to catch particles falling back to the grinding zone. An air jet mechanism 615 similar to that described above can be used to prevent an excess of small particles from exiting the mill. The mechanism and the means to convey to the separation mechanism 2 or to the reject stream 2 are similar to that of separation mechanism 8.
A portion of the oversize particles exiting the bottom of the classifier cone through flap valves 403 are conveyed through screw conveyor 10. They can be discharged to the reject stream 17 directly when the quality of the particles does not warrant processing with separation mechanism 2 or to the conveyance mechanism 11. These particles are close to final particle size but have been recycled by the classifier because of their mass. They do not have excess ash and sulfur to the extent that oversize particles do which are low inside the pulverizer. The proportion of these particles which are separated from the mill will be dependent upon the necessity for complete treatment in the separation mechanism. Generally, this stream may be neglected or treated only in small quantity.
Particles withdrawn from the pulverizer are conveyed 11. They can be discharged to the reject stream 17 directly when the quality of the particles does not warrant processing with separation mechanism 2 or to the storage bin 20 at the input to the separation mechanism 2. Particles are discharged from bin 20 to the size classification means 12 which, for this example, is a screen. The undersize particles, generally smaller than 8 mesh, are discharged to vibratory feeder 100. The oversize particles 15 can be conveyed either to the pulverizer for additional grinding 16 or conveyed to reject 17 depending upon the quality of the particles. The product of the separation mechanism is returned to the pulverizer 16 for grinding to size specification. The reject from the separation mechanism is conveyed to refuse 17.
The vibratory feeder 100 serves to convey the undersize particles to the belt separator 801 and to electrically charge the particles by friction and contact. The material surface of the vibratory feeder is preferentially an electrical conductor which has a work function which is intermediate between the two major types of particles to be separated. For coal, copper is preferred. In contact with copper, the hydrocarbon component of coal will lose an electron to the copper and become positively charged. The inorganic particles to be separated will generally acquire the electron from the copper and become negatively charged. In addition to serving as an intermediary in the transfer of charge, the copper is an electrical conductor and this facilitates the charge transfer. The copper and inorganic particles do not have to be in direct contact to transfer the charge. The particles can also transfer charge by direct contact.
The vibratory tray is also used to set the rate at which particles are issued to the belt separator. This is controlled by the motor action of the tray feeder.
Particles issuing from the vibratory feeder fall onto belt 801 which carries the particles to the magnetic separator 803. Further electric charging can occur as the particles contact the belt. Electric charge can be imparted to the particles. The belt is electrically common with the surface of the permanent magnet 803. Triboelectric charging can occur when the particles skid on the surface of the belt. Skidding occurs when the horizontal component of the velocity of the particles falling from the vibratory tray feeder is different from that of the moving belt. The belt material may be made from insulating or conducting materials and may have iron fibers implanted which will enhance the magnetic field gradient at the surface of the magnetic separator 803. The preferred belt is an antistatic belt which prevents buildup of electric charge on the belt. This is a belt of the type manufactured by Taconic of Petersburgh, N.Y.
A scalping magnetic separator 802 is suspended just above the belt surface at the exit of the vibratory tray feeder. This separator removes very strongly ferromagnetic particles from the stream of particles and prevents these particles from entering the first magnetic separator 803.
An electrode 809 is suspended above the surface of the magnetic separator 803. A potential is applied to the electrode by source 810. The polarity of this electrode is chosen to be opposite of that of the surface of the magnetic separator which is grounded. For processing coal, the electrode is negative with respect to the surface of the magnetic separator. The electric field is directed from the surface of the magnet to the electrode. In this fashion, positively charged carbon rich particles are attracted to the electrode and repelled by the magnetic separator. Likewise, the minerals in coal are negatively charged and are attracted by both magnetic and electric forces to the surface of the magnetic separator. The electrode can be placed so as to support the separation. It can be at the end of the belt in front of the magnetic separator or can be at any angle with respect to the horizontal from 0 degrees to 90 degrees. It must be placed far enough away from the surface of the belt so that particles which are lifted off the belt and deflected outward can reach the appropriate collector without hitting the electrode. Particles which hit the electrode can be discharged and recharged so that they are driven back to the surface of the magnetic separator. This is undesirable. Potentials can be applied up to the breakdown strength of air.
Particles are thrown off of the belt separator 13 depending upon the balance of forces on the particles. First, any particle with an electric charge will have a mirror charge in the surface of the magnetic separator if the surface of the separator is an electrical conductor. This results in an electric field which can be directed away from the surface or toward it depending upon the sign of the charge on the particles. Nonetheless, the resulting attractive force is always toward the surface of the magnet. Next, the applied electric field at the surface of the separator is directed normal to the surface and away from it. The net electric field is the vector sum of the mirror field and the applied field. Negatively charged paramagnetic particles will be attracted to the belt while positively charged diamagnetic particles will be repelled if the applied field is greater than the mirror field.
The inertial force of rotation of the magnet is directed away from the magnet surface. Gravity is everywhere directed downward. A component of gravity directed toward the magnet surface will add to the attractive force in the upper 90 degrees of the motion around the end of the magnet and will subtract in the bottom portion of the arc. Once the particles leave the surface, air drag will tend to drag the particles into the magnetic fraction. The drag force will be greatest on the smallest particles in the stream. This is not expected to be important for particles generally greater than 100 mesh.
Negatively charged particles which are the most magnetic will travel around the arc of the first magnetic separator and will leave the belt underneath and away from the separator. They will generally have negative electric charges greater than −10−5 coulombs/kg and magnetic susceptibilities greater than 10–50*10−9 m3/kg. They will exit the first separator at 804.
Positively charged particles which are diamagnetic or the least magnetic will be thrown from the belt early in its travel around the magnet pulley. For coal, these particles will have magnetic susceptibilities generally less than 10*−9 m3/kg and may have electric charges generally greater than +10−5 coulombs/kg. They will exit the first separator at 806.
All other particles which have weak or no electric charge, generally between −10−5 and +10−5 coulombs/kg, and which are diamagnetic or very weakly paramagnetic with susceptibilities generally less than 10–30*10−9 m3/kg, will fall or be thrown from the belt near the leading edge of the magnetic pulley. They will exit the first magnetic separator at 805.
The particles exiting at 804 constitute the reject or refuse fraction. Those exiting at 806 are of product quality. The other particles exiting at 805 are of intermediate quality. They can be reprocessed in the second stage electromagnetic separator 14 depending upon needs.
Particles exiting the first electric and magnetic separation means at 805 fall into catch basin 101 which feeds the particles to the second magnetic separation device 14. Referring now to
The manifold plate 1100 has two columns of elongate rectangular openings for admitting coal from the basin 101 into the open space shown in
Referring now to
The weakly paramagnetic or diamagnetic particles in the stream of particles entering at 1806 are pushed by magnetic forces outward into the regions 1802 where the magnetic field strength is lowest. Paramagnetic particles are attracted and trapped in the regions 1803 near the pole tips. The magnetic force is sufficient to separate the particles but not strong enough for particles exiting the first electric and magnetic separator at 805 to stick to the poles in the magnetic separator 14. The particles which pass generally have magnetic susceptibilities less than about 5*10−9 m3/kg.
Shown in
By way of example, when the pole tip diameter is 25.4 mm and the width of pole gap is 8 mm, each pair of pole tip/convergers is capable of processing nominally 100 pounds of the weakly magnetic fraction of mill concentrated coal per hour. The matrix shown in
As the coal particles accelerate downward through the open region between poles shown in
In the bottom portion of
Referring now to
The particles collected in adjacent openings 1903 between splitters have different magnetic susceptibilities. Referring now to
Particles exiting at the bottom of splitter 1901 fall directly into chute mechanism 1904. Each segment 1906 of this mechanism has a ramp 1907 which directs the falling particles laterally out of the separator through holes 1905. The ramp in each adjacent segment is sloped oppositely so that all particles of like magnetism exit the separator on the same side. Paramagnetic particles will be on one side and diamagnetic particles will be on the other.
The weakly magnetic particles exiting the first electric and magnetic separator at 806, the diamagnetic particles exiting the second magnetic separator at 815, the paramagnetic particles exiting the second magnetic separator at 813, and the strongly magnetic particles exiting the first electric and magnetic separator at 804 can each be collected separately or can be combined as desired.
Although the invention has been described in detail in the foregoing embodiments for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be described by the following claims.
This application is a continuation of U.S. application Ser. No. 09/514,048 filed Feb. 25, 2000, now U.S. Pat. No. 6,820,829.
This invention was made with Government support under Grant DE-FG05-94ER81764 awarded by the Department of Energy. The Government has certain rights in this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3908912 | Irons et al. | Sep 1975 | A |
| 4113187 | Hoppen et al. | Sep 1978 | A |
| 4389019 | Adrian et al. | Jun 1983 | A |
| Number | Date | Country | |
|---|---|---|---|
| 20050145732 A1 | Jul 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09514048 | Feb 2000 | US |
| Child | 10988367 | US |
