METHOD OF PREPARING FINE-PARTICLE HIGH-LOADED COAL-WATER SLURRY

Abstract: Slurry (8) flowing out from a first stage mill (3a) is supplied to a second stage mill (3b) so as to be ground therein after almost all coarse particles (9b) have been substantially removed therefrom.

Description: DETAILED DESCRIPTION OF THE INVENTION: [Field of the Invention] The present invention relates to a method of preparing fine-particle high-loaded coal-water slurry which contains an extremely small amount of coarse particles. [Description of the Prior Art] High-loaded coal-water slurry is a fluid mixture of solid coal suspended in water, which is prepared in order hot only to facilitate the transportation and storage thereof but also to allow the direct combustion thereof to be performed like oil. Further, since this fluidification of coal makes it possible to incorporate an ash removing process into the preparing procedure, fuel having low ash and sulfur concentration can be obtained. As methods of preparing this high-loaded coal-water slurry, a wet method, a dry method, a method which is a combination of wet and dry methods, etc. are known. The wet method includes such methods as a method and a lower coal concentration higher coal concentration method. In the higher coal concentration method, wet grinding is conducted by utilizing a ball mill or the like while supplying crushed coal, water, and a dispersion agent comprising surfactant, thereby producing high-loaded coal-water slurry having the coal content of not less than 60% by weight. If required, a stabilization agent may also be added. When high-loaded coal-water slurry is burnt in combustion equipment such as a boiler, the required combustion period is determined in accordance with the coal fuel ratio, the ash content, the particle size of the coal, the oxygen concentration, the combustion temperature, etc. Therefore, when the period during which the slurry stays within the combustion equipment is insufficient to allow for the required combustion period, the combustion residue comprising fly ash, etc. which is discharged together with the combustion gases will contain unburnt carbon, thus resulting in a calorific loss. In addition, when the air ration is reduced and the combustion temperature is also lowered to a certain extent with the intention of reducing the generation of NOx in the exhaust gases as a result of the combustion, the combustion period will have to be increased. In particular, when burning high-loaded coal-water slurry containing relatively large amount of coarse particles, an increase in the amount of unburnt carbon in the residue comprising fly ash etc., or the emission of NOx due to an increase in the air ratio, are noticeable consequences. Accordingly, the method of preparing high-loaded coal-water slurry and equipment for preparing the same are required to be capable of preparing high loaded coal-water slurry in which coarse particles are minimized. It is also required that this method is economically feasible. Fig. 3 shows a conventional method of preparing fine-particle high-loaded coal-water slurry and conventional equipment for preparing the same. In Fig. 3, coal 1, which has been crushed previously, is supplied by a weighing feeder to a mill 3, for example a ball mill, together with water 4 and dispersion agent 5. The coal 1 is then ground and mixed into slurry within the mill 3 and flows out as fine-particle high loaded coal-water slurry 7, which is then transferred by means of a pump 6. The mill 3 has a length which is large compared to its diameter, the ratio of the length to the diameter being usually 4 to 8. The ratio of the length to the diameter of a mill employed in an ordinary method of preparing high-loaded coal-water slurry and ordinary equipment for preparing the same is 2 to 3. Increasing the ratio of length to diameter of the same makes it possible to increase the residence time of the coal-water mixture in the mill, thus promoting the grinding operation performed by the grinding medium, and thereby producing fine-particle high loaded coal-water slurry with fine particles. Fig. 4 shows another conventional method of preparing fine-particle high loaded coal-water slurry and another type of conventional equipment for preparing the same. In Fig. 4, reference numbers 3a, 3b and 9 respectively designate the first mill, the second mill, and a classifier. Reference numbers 1, 2, 4, and 5 respectively designate elements corresponding to those shown in Fig. 3. Slurry 8 from the first mill 3a is classified by the classifier 9, and most of the slurry 8 is made to flow as a lower flow 9a, while an upper flow 9b formed by the remaining part of the slurry is led to the second mill 3b wherein it is ground to form slurry 10. If required, ash in these parts of the slurry is removed by the coal cleaning device 11. Reference number 12 designates tailings. The slurry 9a and the slurry 10 thus introduced into a coal cleaning device 11 are low coal concentration slurry, each having a coal content of not more than 30% by weight. Slurry 13 from the coal cleaner 11 is introduced into a dehydrating device 14 which removes water 15 therefrom, and the resulting slurry is made into cake having a coal content of not less than 60% by weight. Subsequently, the cake is made into a fluid in a device 16. In this way, fine-particle high-loaded coal-water slurry 7 is produced and is transferred by means of the pump 6. However, both of the above conventional methods of preparing fine-particle high loaded coal-water slurry and equipment for preparing the same are not satisfactory in terms of simplicity in process and equipment, and reduction of coarse particles. To be specific, when use is made of a long mill which is so dimensioned that the ratio of length to diameter thereof is 4 to 8, fine-particle high loaded coal-water slurry may be prepared by its promoted grinding operation. With this arrangement, however, part of insufficiently ground coal are discharged from the mill before being subjected to an adequate amount of grinding action, thus resulting in the production of a slurry in which coarse particles still remain, even though it is relatively reduced compared to that of a shorter mill. Particularly in the preparing procedure of high loaded coal-water slurry having a coal content of not less than 60% by weight, this (coarse particles remaining) problem is aggravated, since the viscosity of the mixture rises as the grinding process proceeds. In the another conventional case, where two mills are used, most of the slurry from the first mill is sent directly to the subsequent process and only the remaining part is sent to the second mill. Since both grinding operations are preformed in a condition of diluted slurry, and a dehydrating device, a conditioning device, etc. are required for producing high loaded coal-water slurry, this arrangement suffers from the problem that the equipment and operation manner are unavoidably complexed. WO 85/01060 discloses a wet grinding method for preparing a carbonaceous slurry. In that method, coal is charged to a crusher and the crushed coal is fed to a first mill to which liquid (such as water) and diluted dispersing agent are also fed. The mill has sufficient coal and liquid fed to it so that it contains 60 to 82 volume percent of coal. Ground slurry from the first mill is passed through a sieve, which may be a 40 mesh sieve, which allows through underflow slurry of sufficient fineness (less than 420 microns); overflow particles which are greater than 420 microns are recycled back into the first mill where they are subjected to further grinding. A portion of the underflow slurry from the sieve flows through a first line to a viscometer, density meter, and a particle size distribution analyzer; the remaining Portion of the underflow slurry flows through a second line. If the viscometer indicates that the viscosity of the ground slurry is higher than desired, then the underflow slurry should be subjected to further tests (in the density meter and the particle size distribution analyzer). If the density meter indicates that the density of the underflow slurry is higher or lower than desired, the underflow slurry should then be subjected to further tests in the particle size analyzer to determine what the particle size distribution of the underflow slurry is and its attendant surface area and porosity. The particle size distribution analyzer analyzes the particle size distribution of the compact of the underflow slurry. From the data generated by the analyzer, the specific surface area and the porosity of the compact of underflow slurry can be determined. If the solids content, the viscosity, the specific surface area, and the porosity properties of the underflow slurry are as desired, then the underflow slurry is passed through a third line to a final trim tank. If, however, the solids content or the viscosity or the specific surface area or the porosity of the underflow slurry is not as desired, then a portion of the underflow slurry is passed through the second line to a second mill. Depending on how badly the underflow slurry is out of specification, from 1 to 30 volume percent of the underflow slurry is passed to the second mill and the remainder is passed to the final trim tank. Water is fed into the second mill so that the solids concentration of the ground slurry fed through the second line is adjusted to 30 to 60 volume percent. The diluted slurry in the second mill is then ground in the second mill until at least 95 volume percent of the particles in the slurry have diameters less than 20 microns and preferably, less than 15 microns. The slurry ground in the second mill is then passed to a high shear mixer. In general, the ground slurry can be mixed with water and dispersant in the high shear mixer for from 3 to 15 minutes. The mixture from the mixer is then passed through a viscometer, a density meter and a Particle size distribution analyzer. If the properties of the mixed slurry are not suitable, then the water flow to the second mill and/or the slurry flow to the second mill and/or the dispersant flow to the mixer are adjusted until the properties are suitable. If the properties of the mixed slurry from the mixer are suitable, then the mixed slurry is recycled to the final trim tank or to the first mill where it is mixed with and ground with crushed coal, water and dispersant. An object of the present invention is to provide a method of Preparing fine-particle high-loaded coal-water slurry which is excellent in the respect that the method is capable of preparing high-loaded coal-water slurry having minimal coarse particles, and thus the amount of unburnt carbon in the combustion residue and the NOx concentration in the exhaust gases are reduced substantially when the slurry is supplied to combustion equipment and burnt therein. The present invention provides a method of preparing fine-particle high-loaded coal-water slurry, the method comprising: supplying coal, water, and a dispersion agent to a first stage mill so as to grind the same therein, the weight percentage of the supplied coal being greater than 60% of the total weight of the supplied coal, water and dispersion agent; classifying the slurry in a classifier and removing those coarse particles contained in slurry flowing out from the first stage mill that are much larger than the mean particle size of said fine-particle high-loaded coal-water slurry; returning said coarse particles to said first stage mill for further grinding; and supplying the classified slurry from which said coarse particles have thus been removed to a second stage mill so as to grind the same therein; wherein: the slurry in the second mill has a weight percentage of coal greater than 60% of the total weight of the slurry; and the prepared fine-particle slurry having a weight percentage of coal greater than 60% of the total weight of the slurry is taken from the second stage mill. The grinding in the second stage mill may be such that a particle size reduction in the slurry to essentially less than 150 microns is effected and a substantial amount of fine particles of less than 74 microns therein obtained. The first stage mill and the second stage mill may each comprise a respective ball mill so dimensioned that the ratio of the length thereof to the diameter thereof is 2 to 4. The classifier may be a vibrating screen classifier, a cylinder-shaped classifier, or a horizontal disc-shaped classifier. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS: Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is the flow sheet of a method of preparing fine-particle high loaded coal-water slurry in accordance with the embodiment of the present invention; Fig. 2 is a diagram used to explain the particle size distribution obtained in accordance with the method of the invention and that of the prior arts; Figs. 3 and 4 are flow sheets of conventional methods of preparing fine-particle high loaded coal-water slurry. Fig. 1 shows the embodiment of the present invention. A first stage mill 3a is supplied with coal 1, water 4, and a dispersion agent 5 under the operation of a weighing feeder 2 in such a manner that slurry having a predetermined coal content can be obtained. The coal 1 is ground within the first stage mill 3a and the mixture flows out as slurry 8. The slurry 8 is then led to a classifier 9 wherein coarser particles 9b contained in the slurry 8 are removed from the mixture. A lower flow 9a is directly supplied to a second stage mill 3b wherein it is successively ground, flows out from the second stage mill 3b as fine-particle high loaded coal-water slurry 7 containing minimal coarse particles, and then is transferred by means of a pump 6. The coarser particles 9b may be sent back to the first stage mill 3a through suitable means and may be ground together with coal 1, water 4, and a dispersion agent 5. The slurry 8, the lower flow 9a, and the fine-particle high loaded coal-water slurry 7 have practically the same coal content which is not less than 60% by weight. Although at the first stage mill 3a, part of the coal which has been insufficiently ground is made to flow out from the first stage mill 3a as slurry containing coarse particles which has not been subjected to a sufficient degree of grinding action, the lower flow 9a containing lesser amount of coarse particles is supplied to the mill 3b where it is ground again. Therefore, since the coarse particles are substantially low in this stage of grinding, the coal particles can be adequately subjected to grinding action without any disturbance, thus promoting the grinding operation and thereby producing fine-particle high loaded coal-water slurry 7 which contains minimal coarse particles. In each of the first stage mill 3a and the second stage mill 3b, a ball mill which is so dimensioned that the ratio of length to diameter is 2 to 4 is used. Therefore, within the first stage and second stage mills 3a and 3b, grinding can so proceed in the state of high loaded coal-water slurry with adequately long residence time to prepare fine-particle high loaded coal-water slurry having a (specified) particle size distribution. This arrangement achieves a remarkably finer particle size distribution or arrangement in comparison to that of the case of a single mill having the length to diameter ratio of approximately 3. If mills having various improvements with respect to the structure and operating parameters are used as the first and second stage mills, this will enable realization of a method of preparing fine-particle high loaded coal-water slurry and equipment for preparing the same which exhibit excellent grinding efficiency (reduced power consumption) and the grindability. As for the classifier, a vibration screen having a screening surface, a structure employing cylindrical classifying members having slits, a structure employing rotatable horizontal disc classifying members having screening surfaces, a structure employing horizontally movable classifying members having screening surfaces which are movable in a continuous manner, or the like may be used. Classification is defined here as an operation of removing such particle from the original slurry that degrade grinding. In this embodiment of the invention, coarser particles contained in the slurry from the first stage mill are removed for the purpose of facilitating the grinding in the second stage mill. Consequently, the aperture size of the classifier 9 can be made much larger than the mean particle size of the fine-particle high loaded coal-water slurry 7, and this arrangement allows the slurry 9 to pass through the classifier 9 with ease even though the slurry 8 may have a high degree of viscosity. Therefore, the capability of the classifier 9 can be increased, and furthermore, even when preparing a large volume of fine-particle high loaded coal-water slurry, it is possible to handle the slurry without employing a classifier having an unreasonably large size. Experiments conducted in accordance with the above embodiment will be described below. The coal used in the experiments had the following properties: Fig. 2 shows the particle size of fine-particle high loaded coal-water slurry obtained in accordance with the above embodiment, as compared to the particle size of slurry in the prior arts. In Fig. 2, a curve group a indicates the particle size obtained by the present invention, a curve group b indicates that obtained by a conventional mill having the length to diameter ratio of 5, and a curve c indicates that obtained by a conventional mill having the length to diameter ratio of 3. The abscissa axis indicates the cumulative weight percentage under the size of 74 µm, which is an index of suitable particle size for combustion of fine-particle high loaded coal-water slurry, while the ordinate axis indicates the cumulative weight percentage over the size of 150 µm, which is an index of unsuitable particle size. Each of the curve groups collectively indicates the data values of a number of experiments. As will be clearly seen from Fig. 2, ordinate value (cumulative oversize of 150 µm corresponding to a abscissa value 85% (cumulative undersize of 74 µm) is much lower in curve group a (the present invention) than in curve groups b and c (prior arts). In addition, the combustion properties of fine-particle high-loaded coal and water slurry were found to be as follows. Bituminous coal was used as raw coal, and the combustion equipment used had an electrical output of 600 MW. As a result, it is clear that the present invention makes it possible to prepare fine-particle high loaded coal-water slurry containing minimal coarse particles, and also possible, when the slurry is supplied to combustion equipment to be burned therein, to reduce the amount of the unburnt carbon in the combustion residue, and to lower the NOx concentration in the exhaust gases. Incidentally, the arrangement of the mills and the classifier of the present invention is not limited to what has been described in the above embodiment. As described above in relation to the embodiments, according to the present invention, because the second stage mill is supplied with slurry from the first stage mill after coarser particles have been removed from the slurry by means of the classifier, and the slurry is then ground in the second stage mill, and also because the grinding is performed for an adequate period, fine-particle high loaded coal-water slurry containing minimal coarse particles can be prepared by relatively simple method and equipment without any complicated equipment or operation, and in addition, when the slurry is supplied to combustion equipment to be burnt therein, the amount of unburnt carbon in the combustion residue can be reduced and also the NOx concentration in the exhaust gases can be lowered. Further, according to the present invention, since the capability of the classifier can be increased, various advantages are attainable, such as the possibility of mass production of fine-particle high loaded coal-water slurry, and the possibility of excellent grinding efficiency (reduced power consumption) and grindability.

Claims: A method of preparing fine-particle high-loaded coal-water slurry, the method comprising: supplying coal (1), water (4), and a dispersion agent (5) to a first stage mill (3a) so as to grind the same therein, the weight percentage of the supplied coal (1) being greater than 60% of the total weight of the supplied coal (1), water (4) and dispersion agent (5); classifying the slurry (8) in a classifier (9) and removing those coarse particles (9b) contained in slurry (8) flowing out from the first stage mill (3a) that are much larger than the mean particle size of said fine-particle high-loaded coal-water slurry; returning said coarse particles (9b) to said first stage mill (3a) for further grinding; and supplying the classified slurry (8) from which said coarse particles (9b) have thus been removed to a second stage mill (3b) so as to grind the same therein; wherein: the slurry in the second mill (3b) has a weight percentage of coal (1) greater than 60% of the total weight of the slurry (9a); and the prepared fine-particle slurry (7) having a weight percentage of coal (1) greater than 60% of the total weight of the slurry (7) is taken from the second stage mill (3b). A method as claimed in claim 1, wherein the grinding in the second stage mill (3b) is such that a particle size reduction in the slurry (9a) to essentially less than 150 microns is effected and a substantial amount of fine particles of less than 74 microns therein obtained. A method as claimed in claim 1 or claim 2, wherein the first stage mill (3a) and the second stage mill (3b) each comprise a respective ball mill (3a, 3b) so dimensioned that the ratio of the length thereof to the diameter thereof is 2 to 4. A method as claimed in any one of claims 1 to 3, wherein the classifier (9) is a vibrating screen classifier. A method as claimed in any one of claims 1 to 3, wherein the classifier (9) is a cylinder-shaped classifier. A method as claimed in any one of claims 1 to 3, wherein the classifier (9) is a horizontal disc-shaped classifier.

Inventors: FUJIMOTO, HARUMI, HAGIWARA, TATSUO, KANAMORI, TOSHIAKI, KATAHATA, TADASHI, OZAKI, HIRONORI, SATO, NOBORU
Applicants: KAWASAKI JUKOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: F23K1
IPC codes: C10L1, F23K1, B02C17, C10L10