Method for mixing and granulating powder materials

Abstract: A method of mixing and granulating powder materials to prepare sintering mixes used in the production of sintered ores, for example, uses a multi-cylinder drum mixer having a plurality of concentric cylinders (2, 9) which are connected to one another and rotated around their common longitudinal axis which is downwardly inclined in the moving direction of materials. A plurality of sets of powder materials (13, 14) which may have different properties are separately fed into a plurality of sections ( a , b ) in the drum defined by the plurality of cylinders through hoppers (11, 12) at one end of the drum and moistened by water sprayed through water nozzles (17, 18). The plurality of sets of powder materials are granulated separately while moving down and rolling in the individual sections.

Description: The present invention relates to a method for mixing and granulating powder materials, such as in the preparation of sintering mixes which are used to produce sintered ores. More particularly, it is concerned with a mixing and granulating method and apparatus which are capable of granulating two or more classes or sets of powder materials Simultaneously and separately using a multi-cylinder drum mixer having a plurality of coaxially rotating cylinders. In the production of sintered ores, a sintering mix which primarily comprises a fine iron ore is initially prepared by mixing the fine iron ore with various additives such as limestone, coke breeze, and return fines and granulating (pelletizing) the mixture by aid of moisture. Some sintering mixes contain slaked lime or quicklime as a binder and various iron-containing dusts collected in a steelwork may be added to sintering mixes as an iron source. The granulated sintering mix is then fired to produce a sintered ore. Thus, a sintering mix used in the production of a sintered ore consists of a wide variety of powder materials, which are mixed and granulated together in a mixing and granulating apparatus such as a peg mill, pan pelletizer, drum mixer, or Eirich mixer. With the recent progress of sintering techniques, combined granulation processes in which two or more classes of materials are granulated separately to prepare different sintering mixes have been developed with the goal of improving the quality of sintered ores or decreasing the energy required for firing. For example, the double-layer sintering method comprises forming an upper layer of a coke-rich sintering mix and a lower layer of a coke-lean sintering mix on a sintering pallet to be placed into a firing furnace. The separated granulation method comprises granulating a high-CaO sintering mix and a low-CaO sintering mix separately and mixing the resulting granulated sintering mixes in preparation for firing them together. In the case of using two or more fine iron ores having different degrees of water absorption, it is proposed that the fine ores be separated into two or more classes depending on the degree of water absorption and each class of fine ores be granulated separately while water is added in an amount sufficient to form a sintering mix having an optimum moisture content. The resulting two or more granulated sintering mixes are then mixed and fired together. In any combined granulation process, each of the sintering mixes which are prepared separately requires at least one mixer or pelletizer (hereinafter collectively referred to as a mixer) in which raw powder materials are mixed and granulated to form the sintering mix. Thus, the number of mixers to be installed is equal to or larger than the number of sintering mixes to be prepared. According to the above-mentioned combined granulation processes, it is possible to improve the quality of sintered ores produced or decrease the energy required to fire the granulated sintering mixes together. However, these processes need the use of an increased number of mixers, leading to many problems such as an increase in plant investment, installation space, and energy costs for granulation. These problems interfere with wide applications of the combined granulation processes. When two or more sintering mixes which have been granulated separately are mixed in an already-installed mixer which is designed to prepare a single sintering mix by mixing all the powder materials together, the total amount of the sintering mixes to be mixed is often in excess of the maximum loading of the mixer. Therefore, it is difficult to perform the combined granulation processes using an already-installed mixer. As described above, many types of mixers including peg mills, pan pelletizers, drum mixers, and Eirich mixers have been used in the preparation of sintering mixes for the production of sintered ores. Drum mixers, which comprise an inclined rotating cylindrical drum, are the most popular due to their high production efficiency. In a drum mixer, while the fine iron ore and other materials charged are being tumbled in the rotating drum from the higher end to the opposite lower end, they are mixed uniformly and granulated. Figure 3(a) is a schematic vertical sectional view of a conventional drum mixer having an inclined rotating drum 2. Fine iron ore and other powder materials 6 are fed through a hopper 1 into the rotating drum 2 at one end thereof and an appropriate amount of water is added to the materials through a water nozzle 3 for moistening. While the moist powder materials are being tumbled in the rotating drum 2 toward the opposite outlet end 19, they are granulated into pellets and discharged from the drum through the outlet end. The drum 2 is inclined such that the outlet end 19 is slightly lower than the opposite inlet end. Figure 3(b) is a cross-section taken along line B-B in Figure 3(a), which shows the rolling of materials 6 in the rotating drum 2. Such a conventional drum mixer used to granulate sintering mixes in the production of sintered ores usually has a drum diameter of about 2 - 5 meters and an angle of drum inclination of about 1° - 5° and is operated at a rotational speed of about 5 - 10 rpm. During granulation, the space factor of the drum, i.e., the proportion of the space within the drum occupied by the materials being processed, is usually on the order of 10% since a larger space factor causes a loss in mixing and granulating efficiency in the drum. Therefore, the remaining approximately 90% of the space is not used at all. The granulating efficiency is improved with an increasing diameter of the drum since the rolling distance of the powder materials therein is increased. However, such a large drum requires an increased costs and space for installation and an increased power for operation. In US-A-2506618 an apparatus for sintering ores is disclosed having an outer cylinder and an inner cylinder which are coaxially arranged and rotate synchronously. Powder materials are fed into the cylinders by means of separate hoppers and moved toward the lower end of the cylinders where they are fed into a common cylinder where they are mixed together. The main purpose of the inner and the outer cylinders is to preheat the fed charges whereas granulations substantially take place in the mixing cylinder. It is an object of the present invention to provide a method and apparatus for mixing and granulating powder materials such as fine ores and other materials in a drum mixer which is capable of processing two or more classes or sets of powder materials separately and simultaneously in a single drum mixer. Another object of the invention is to provide a drum mixer-type apparatus for mixing and granulating powder materials in which the space factor and hence the granulation efficiency are increased, thereby making it possible to process an increased amount of powder materials in a single drum mixer having the same outer dimensions as conventional drum mixers. The present invention as characterized in claim 1 provides a method of mixing and granulating powder materials, using a multi-cylinder drum having a plurality of concentric cylinders connected to one another, the multi-cylinder drum being rotating around the common longitudinal axis of the concentric cylinders which is downwardly inclined with respect to the horizontal in the moving direction of materials, and separately feeding a plurality of sets of powder materials which may have different properties into a plurality of sections defined by the plurality of cylinders within the multi-cylinder drum through one end of the drum, thereby forcing the powder materials to move to the opposite end of the rotating drum while rolling therein to mix and granulate the plurality of sets of powder materials separately in the individual sections. The difference in properties between the plurality of sets of powder materials is the CaO contents. The multi-cylinder drum is a double-cylinder drum having an inner and an outer cylinders, and the difference in properties between the plurality of sets of powder materials are the CaO contents wherein a set of powder materials of a high-CaO content are fed into the inside of the inner cylinder and another set of powder materials of a low-CaO content are fed into the space between the inner and outer cylinders. A water nozzle for moistening the powder materials fed into the drum is preferably provided for each section within the multi-cylinder drum. In one embodiment, all of the plurality of concentric cylinders extend for the entire length of the drum. In another embodiment, the multi-cylinder drum has a front zone located on the material feeding side and a rear zone located on the material discharging side wherein the front zone of the drum is comprised of the plurality of concentric cylinders and the rear zone of the drum is comprised solely of the cylinder having the largest diameter among the plurality of cylinders. In other words, only the outermost cylinder extends for the entire length of the drum and the other cylinders do not extend to the rear zone. The present invention will now be described in detail referring to the accompanying drawings, in which Figures 1(a) and 1(b) schematically show a mixing and granulating apparatus according to the present invention wherein Figure 1(a) is a vertical sectional view and Figure 1(b) is a cross-section taken along line A-A in Figure 1(a); Figure 2 is a vertical sectional view schematically showing another mixing and granulating apparatus according to the present invention; and Figures 3(a) and 3(b) schematically show a conventional drum mixer wherein Figure 3(a) is a vertical sectional view and Figure 3(b) is a cross-section taken along line B-B in Figure 3(a). Figures 1(a) and 1(b) schematically show a mixing and granulating apparatus according to the present invention. The apparatus is a drum mixer having a double-cylinder structure, i.e., a double-cylinder drum which comprises an outer cylinder 2 and an inner cylinder 9 arranged such that the two cylinders are concentric, i.e., have a common longitudinal axis 5 and define a cross section of concentric circles. In this embodiment, both the cylinders 2, 9 extend for the entire length of the drum. The inner cylinder 9 is secured to the outer cylinder 2 by supporting rods 8. The inner cylinder 9 and the outer cylinder 2 define two sections in the drum, i.e., the section inside the inner cylinder 9 indicated by a (hereinafter referred to as inner section) and the section between the two cylinders indicated by b (hereinafter referred to as outer section). The end of the double-cylinder drum on the material feeding (inlet) side is provided with hoppers 11, 12, which are adapted to feed two sets of powder materials 13, 14 separately into the individual sections of the drum defined by the inner and outer cylinders, i.e., the inner section a and the outer section b , respectively. The powder materials 13, 14 fed into the inner and outer sections, respectively, are separately moistened by water sprayed through water nozzles 17, 18, which are located within the individual sections in the vicinity of the inlet end of the drum. Thus, each of the inner and outer sections is provided with a hopper and a water nozzle such that two sets of powder materials can be mixed and granulated in the inner and outer sections separately. The outer cylinder 2 is supported by a supporting roller 10 and a driving gear 4 and it is rotatable by rotating the driving gear 4 by means of a motor 7. As the concentric cylinders are rotated, the powder materials 13, 14 fed into the respective sections and moistened therein are forced to move toward the opposite ends of the inner and outer cylinders while being rolled. For this purpose, both the inner and outer cylinders 9, 2 are slightly inclined such that their common longitudinal axis 5 has a downward inclination toward the material discharging (outlet) ends 15, 16 of the cylinders. Figure 1(b) shows the rolling of the two sets of powder materials 13, 14 within the two sections a , b defined by the inner and outer cylinders 9, 2. When the outer cylinder 2 of the above-described double-cylinder drum mixer is rotated by the rotating action of the driving gear 4 which is actuated by a driving motor 7, the inner cylinder 9 secured to the outer cylinder 2 rotates at the same time. As shown in Figure 1(a), the powder materials 13, 14 fed through the hoppers 11, 12 at one end are forced to gradually move within the respective sections a , b toward the opposite outlet ends 15, 16 of the cylinders while rolling therein. The powder materials in each section are mixed and granulated separately from the powder materials in the other section during moving and rolling and the granulated materials are separately discharged from the drum. Therefore, by feeding two sets of powder materials which have different properties separately through the hoppers 11, 12, they can be granulated simultaneously and separately while the differences in their properties are maintained. For example, the two sets of powder materials may have different CaO contents, i.e., one being powder materials of a high CaO content and the other being powder materials of a low CaO content. In this case, it is preferable that the high-CaO powder materials be fed into the inner section a and the low-CaO powder materials be fed into the outer section b . In the double-cylinder drum, the outer cylinder 2 and the inner cylinder 9 are rotated synchronously and the rolling distance of material in the inner section a is shorter than that in the outer section b . This indicates that the granulation efficiency in the outer section b is higher than that in the inner section a . On the other hand, the granulation ability of powder materials tends to depend on the CaO content thereof. The granulation ability of the powder materials of a low-CaO formulation is relatively poor compared to that of the powder materials of a high-CaO formulation. Therefore, by feeding the high-CaO powder materials which have a higher granulation ability into the inner section a which has a lower granulation efficiency and the low-CaO powder materials which have a lower granulation ability into the outer section b which has a higher granulation efficiency, both sets of the powder materials can be granulated sufficiently and relatively evenly. It is conceivable that the powder materials fed into the outer section b might be brought into contact with the inner cylinder 9 during granulation, thereby adversely affecting the granulation of the materials. However, as described above, the space factor of the drum mixer is as low as about 10% when it is used to granulate powder materials and there is no appreciable adverse effect on granulation of the use of a double-cylinder drum. Instead of the above-described double-cylinder drum, a multi-cylinder drum having three or more cylinders can be used to mix and granulate three or more sets of powder materials separately at the same time. The use of a multi-cylinder drum mixer enables a central space in the drum which is idle in a conventional single-cylinder drum to be effectively used for granulation of powder materials, thereby increasing the granulation efficiency of the drum mixer without increasing the diameter thereof. The multi-cylinder drum mixer may be used in such a manner that the same powder materials are fed into each section of the drum defined by the plurality of cylinders. In such a case, there is the benefit over conventional single-cylinder drum mixers that the granulation efficiency of the powder materials and hence the productivity of sintered ores are improved. In another embodiment of the mixing and granulating apparatus according to the present invention, a multi-cylinder drum has a front zone located on the inlet side and a rear zone located on the outlet side and the rear zone is comprised of a single cylinder, which corresponds to the outermost cylinder having the largest diameter among the plurality of cylinders. Thus, only the outermost cylinder extends for the entire length of the drum. An example of such an apparatus is schematically shown in Figure 2. The apparatus is the same as that shown in Figures 1(a) and 1(b) except that a rear zone of the drum on the outlet side is comprised solely of the outer cylinder 2 rather than the double cylinders. Thus, only a front zone of the drum is comprised of the inner and outer cylinders 2, 9. The front zone of the drum mainly serves to mix and granulate the two sets of powder materials separately in the individual sections and the resulting two sets of granulates are mixed in the rear zone of the drum. Preferably, the ratio of length of the inner cylinder to the outer cylinder, i.e., the length of the front zone to the entire length of the drum is approximately 7 : 10. A lower ratio may result in insufficient granulation within the inner cylinder, while a higher ratio may result in insufficient mixing of the two sets of granulates. By using this type of an apparatus, it is possible to perform simultaneous and separate granulation of two sets of powder materials having different properties and subsequent mixing of the resulting granulates in a single mixer. Specifically, two sets of powder materials 21, 22 are separately fed into the inner and outer sections a , b , respectively. The granulation of the two materials proceeds within the respective sections defined by the two cylinders. The granulate formed from the powder material 21 in the inner section a is discharged from the outlet end 15 of the inner cylinder 9, thereby falling in the outer cylinder 2 and moving while rolling in that cylinder along with the granulate formed from the powder material 22 fed into the outer section b toward the outlet end 16 of the outer cylinder 2. While moving in the rear zone of the drum, the two sets of granulates are mixed to form a mixed granulated material 24, which is discharged from the outlet end 16 of the outer cylinder. Also in an apparatus of the type shown in Figure 2, the double-cylinder structure may be replaced by a multi-cylinder structure having three or more cylinders. Both the above-described drum mixers according to the present invention can be advantageously fabricated by relatively simple alterations of a conventional drum mixer. The method and apparatus of the invention can be applied to mixing and granulation of any powder materials which require separate granulation two or more different sets of powder materials, although the powder materials fed to the individual sections in the drum may be the same. The method and apparatus are particularly suitable for use in the preparation of granulated sintering mixes in the combined granulation processes. The powder materials used in the preparation of sintering mixes for the production of sintered ores comprise a fine iron ore and at least one powdery additive selected from limestone, coke breeze, slaked lime, quicklime, and iron-containing dust. For example, the apparatus shown in Figures 1(a) and 1(b) is particularly suitable for use in the preparation of sintering mixes for the double-layer sintering method and other methods in which the separately granulated sintering mixes are not mixed before sintering. It is useful in the separated granulation method and similar methods as well. The apparatus shown in Figure 2 is particularly suitable for use in the preparation of sintering mixes for the separated granulation method and other methods in which the separately granulated sintering mixes are mixed before sintering. EXAMPLE Three classes of powder materials for mixing having the formulations shown in Table 1 were subjected to mixing and granulation using the three types of drum mixers shown in Table 2. The resulting granulates were evaluated by a sintering test using a sinter pot measuring 300 mm in diameter and 500 mm in height. Among the mixing formulations shown in Table 1, Formulation C was a conventional formulation for a sintering mix used in the production of sintered ores and it contained limestone powder, return fines, and coke breeze as additives to a fine iron ore. Formulations A and B totally had the same formulation as Formulation C and each additive used in Formulation C was added to one of Formulations A and B so as to make a difference in the CaO content between these formulations. Thus, Formulation A gave powder materials of a high CaO content and Formulation B gave powder materials of a low CaO content. The operating conditions of each drum mixer were set such that the angle of inclination of the common longitudinal axis of the cylinders was 5°, the rotational speed was 6 rpm, and the total feed rate of powder materials was 800 ton/hour. The granulation period was about 4 minutes. The pot sintering tests were performed using 70 kg of a sintering mix for each test. The test results are shown in Table 3. Runs Nos. 1 and 2 illustrate granulation of Formulation C. A conventional Single-cylinder drum mixer was used in Run No. 1, while a double-cylinder drum mixer according to the present invention was used in Run No. 2 in which the powder materials were fed into the inner and outer sections of the drum at a weight ratio of inner to outer of 3 : 7. By using a double-cylinder drum mixer in Run No. 2, the granulation efficiency was improved over that in Run No. 1 where a conventional single-cylinder drum mixer was used, thereby resulting in improvement in productivity in the pot sintering test. Runs Nos. 3 to 6 illustrate granulation of two classes of powder materials having different CaO contents, i.e., the high-CaO Formulation A and low-CaO Formulation B. In Run No. 3, the two classes of powder materials (Formulations A and B) were sequentially granulated in a conventional single-cylinder drum mixer and the resulting two classes of granulates were mixed manually for 1 minute and subjected to the pot sintering test. In Runs Nos. 4 and 5, the two classes of powder materials were simultaneously granulated in a double-cylinder drum mixer as shown in Figures 1(a) and 1(b) by feeding the two classes of materials into the inner and outer sections of the drum separately. The powder materials of a high CaO content (Formulation A) were fed into the outer section in Run No. 4 and into the inner section in Run No. 5. The resulting two classes of granulates were then mixed manually for 1 minute and subjected to the pot sintering test. In Run No. 6, granulation of the two classes of powder materials and mixing the resulting two classes of granulates were performed in a double-cylinder drum mixer as shown in Figure 2 by feeding the powder material having a high CaO content into the inner section of the drum. In Runs Nos. 5 and 6 where different double-cylinder drum mixers according to the invention were used, the productivity of the sintered ore products and the resistance thereof to reduction degradation were improved over those in Run No. 3 where a conventional single-cylinder drum mixer was used. In Run No. 4, the productivity and resistance to reduction degradation of the sintered ore product were comparable to those in Run No. 3, but another benefit of the invention that the two classes of powder materials having different formulations could be granulated in a single drum mixer at the same time without installing a second mixer was obtained. Formulation A B C Fine iron ore 62% 20% 42% Limestone powder 10% 10% 0% Return fines 25% 0% 25% Coke breeze 3% 0% 3% Total 100% 30% 70% % CaO 9.0 20.7 4.0 % Moisture 6.0 8.0 5.1 (% by weight) Type Description of drum mixer Remarks 1 Single-cylinder drum measuring 5 m in diamter and 10 m in length Fig. 3 2 Double-cylinder drum consisting of an outer cylinder 5 m in diameter and an inner cylinder 3 m in diameter, both 10 m long Fig. 1 3 Double-cylinder drum having a 7 m-long front zone on the inlet side and a 3 m-long rear zone on the outlet side in which the front zone is a double-cylinder drum consisting of an outer cylinder 5 m in diameter and an inner cylinder 3 m in diameter and the rear zone is a single-cylinder drum 5 m in diameter; the entire length of the drum is 10 m Fig. 2 Run No. 1 2 3 (Conventional) (Present Invention) (Conventional) Type of drum mixer used Type 1 Type 2 Type 1 Formulations used Single cylinder C (70 kg) A (21 kg) B (49 kg) Double cylinder Outer section C (49 kg) Inner section C (21 kg) Productivity of sintered ore (ton/day-m 2 ) 28.2 31.0 30.5 Reduction degradation index (RDI, %) 37.1 36.9 34.6 Run No. 4 5 6 (Present Invention) (Present Invention) (Present Invention) Type of drum mixer used Type 2 Type 2 Type 3 Formulations used Double cylinder Outer section A (21 kg) B (49 kg) B (49 kg) Inner section B (49 kg) A (21 kg) A (21 kg) Productivity of sintered ore (ton/day-m 2 ) 30.5 31.3 33.2 Reduction degradation index (RDI, %) 34.6 32.8 33.0 In accordance with the present invention, it is possible to mix and granulate a plurality of sets of powder materials, such as those for sintering mixes used in the production of sintered ores, separately and simultaneously using a single drum mixer. The resulting granulates can be mixed in the mixer when the mixer is of the type shown in Figure 2. Since the space factor of the drum mixer can be increased, the granulation efficiency can be improved without increasing the diameter of the drum. The simultaneous and separate granulation of a plurality of powder materials can be readily achieved by using a multi-cylinder type of mixing and granulating apparatus of the present invention.

Claims: A method of mixing and granulating powder materials, using a multi-cylinder drum having a plurality of concentric cylinders connected to one another, the multi-cylinder drum being rotating around the common longitidunal axis of the concentric cylinders which is downwardly inclined with respect to the horizontal in the moving direction of materials, and separately feeding a plurality of sets of powder materials into a plurality of sections defined by the plurality of cylinders within the multi-cylinder drum through one end of the drum, characterized in that the plurality of sets of powder materials have different properties, wherein the difference in properties between the plurality of sets of powder materials is the CaO contents thereof, thereby forcing the powder materials to move to the opposite end of the rotating drum while rolling therein to mix and granulate the plurality of sets of powder materials separately in the individual sections. The method according to claim 1, wherein the multi-cylinder drum is a double-cylinder drum having an inner and an outer cylinder and a set of powder materials of a high CaO content are fed into the inside of the inner cylinder and another set of powder materials of a low CaO content are fed into the space between the inner and outer cylinders. The method according to claim 1 or 2, wherein the powder materials are materials for sintering mixes used in the production of sintered ores. The method according to claim 3, wherein the powder materials comprise a fine iron ore and at least one powdery additive selected from coke breeze, quicklime, and iron-containing dust. Method according to any of claims 1-4, characterized in that a water nozzle for moistening the powder materials fed into the drum is provided for each section within the multi-cylinder drum. Method according to any of claims 1-5, wherein all of the plurality of concentric cylinders extend for the entire length of the drum. Method according to any of claims 1-5, wherein the multi-cylinder drum has a front zone located on the material feeding side and a rear zone located on the material discharging side and wherein the front zone of the drum is comprised of the plurality of concentric cylinders and the rear zone of the drum is comprised solely of the cylinder having the largest diameter among the plurality of cylinders.

Inventors: KAWAGUCHI TAKAZO, MATSUMURA MASARU, KAWAGUCHI, TAKAZO, MATSUMURA, MASARU
Applicants: SUMITOMO METAL IND, SUMITOMO METAL INDUSTRIES, LTD.
Application year: 1992
Main classification: C22B1
IPC codes: C22B1, B01J2