GB2229940A

GB2229940A - Grinding process and a continuous high-capacity micronizing mill for its implementation

Info

Publication number: GB2229940A
Application number: GB8927959A
Authority: GB
Inventors: Tarcisio Orlandi
Original assignee: SnamProgetti SpA
Current assignee: SnamProgetti SpA
Priority date: 1988-12-16
Filing date: 1989-12-11
Publication date: 1990-10-10
Anticipated expiration: 2009-12-11
Also published as: AU4684089A; GB8927959D0; CN1025292C; CN1044419A; SU1776199A3; IT1228049B; DE3940906A1; GB2229940B; DE8914652U1; IT8822989A0; AU619369B2

Description

1 GRINDING PROCESS AND A CONTINUOUS HIGH-CAPACITY MICRONIZING MILL FOR ITS

IMPLEMENTATION CASE 2965 This invention relates to a high-capacity tubular micronizing mill operating on a continuous cycle. The dry or wet grinding of solid granular products for their size reduction is one of the most widespread industrial operations both for high-capacity production of low added value, typically in the mining and building industries, and for lowcapacity production of very high added value, typically in the fine chemical, pharmaceutical and cosmetics industries. To obtain ultrafine products with a less than 10 micron particle size, this second category is able to sustain high energy and processing costs, which however are unsustainable in the case of products of low added value. On an industrial scale, high-capacity grinding of the order of 10 t/h and above is conducted in rotary tubular mills partly.filled' with a grinding load consisting of impact-resistant regular_ solids, which are generally metal balls but can be of other shape and tvpe such as metal cylinders or bars, or regular stories. It is known thdt for a certain speed of the tubular mill, wll,i,-h is known as the critical speed and is expressed by the equation:

4 42.3 &)cr --- fD where W is the mill r.p.m. and D is its inner diameter in metres, the grinding load begins to be centrifuged. The grinding load produces its maximum work for a speed equal to about 85% of the critical speed. This type of mill can attain comminution ratios exceeding 100, but the best efficiencies are obtained for comminution ratios, ie particle size reductions, of about 25-30. Generally, continuous-cycle high-capacity tubular mills are able to provide a ground product with a particle size distribution of between 0 and 64 microns, but not finer. If finer products are required, then different grinders must be used, such as microsphere mills or compressed air micronizers, which are of much lower capacity not more than 1000 kg/h - and of very high energy consumption. Such grinders are used for example in the dyestuffs, pesticide or ink industries where ultrafine particle size distributions of 0-20 microns and sometimes 0-10 microns are required. The present invention enables the limitations of the equipment of the known art to be overcome by a dry or wet-operating continuous process able to produce with high unit capacity a ground productof particle size of less than 20 microns, with low energy consumption. The micronizing mill of the present. invention consists of a multichamber tubular mill, described hereinafter with reterence to Figure 1 which shows a typical embodiment thereof by way. of nonlimiting example. The described embodiment relates to the grinding of coal to obtain powder having a particle size suitable for its use in stable high- concentration aqueous suspensions directly usable as fuels in industrial burners. The micronizing mill according to the invention is in the form of a rotary drum 1 with a high ratio of length to inner diameter, this ratio being at least 5 and preferably 6 or more, its internal volume being divided by separator baffles 2 into a plurality of cylindrical grinding chambers 3, in which are placed grinding loads, the constituents of which are of decreasing size in progressing from the feed chamber to the discharge chamber. The shape of the separator baffles 2 is shown in greater detail in Figure 2. Feed is by means of a hopper device 4 with a rotary screw feeder 5, known in the art. The speed of rotation of the screw feeder 5 determines the throughput. Inside the cylindrical chambers 3 there are placed the grinding loads consisting of metal, eg steel, balls or rods.

The grinding load constituents are of decreasing size in progressing from the initial chamber which receives the feed, to the final chamber from which the micronized product is discharged. According to the present invention it has been found that optimum efficiency is obtained by placing in each chamber, and especially in the initial chambers, grinding loads consisting of bodies which are not all of the same size but of a size distribution such as to obtain the maximum number of possible collisions between the - 4 product and the grinding load, and having unit kinetic energies, at least for part of the grinding load, which are sufficient for comminuting the granules of largest size. The size distribution of the grinding loads has to be correlated with the particle size distribution of the feed. The walls of the grinding chambers 3 are provided with grooved armour cladding 6 which not only provides the necessary protection but also determines the mixing and advancement of the material being ground and rotates the grinding bodies which rise circularly along the grooved wall to a certain height, related directly to the speed of rotation, and then fall down through a parabolic trajectory onto the layer of granules, to effect their comminution. According to a preferred embodiment of the invention the rotary drum 1 is divided into three cylindrical chambers 3, of which the centre chamber is much longer than the other two. The purpose of the first grinding chamber is to reduce the particle size of the coarsest part of the feed, the more spacious central chamber performing most of the work, while the last chamber completes the comminution.

The micronized product is discharged from the last cylindrical chamber by the blades of the baffle and is conveyed to storage via the hopper 7, as known in the art. One of the essential components of the micronizing mill according to the invention is the separator baffle which acts both as a wall between the various chambers 3 and as a level controller for the product. It is shown in Figures 2 A and B. The sepdrator baffle consists of a outer ring 11 for its fixing to 0 z the tubular wall of the rotary drum 1 and two circular flat frontal walls 12 and 13 which face adjacent grinding chambers 3 between which the product is transferred proceeding from left to right.

In that wall 12 facing the upslream grinding chamber there are provided circular slots 14 through the inner circular band, whereas the peripheral circular band is without slots. At the centre of the baffle there is positioned a flared solid body such as the cone frustum 15, with its minor base facing the downstream grinding chamber. In the wall 13 facing the downstream grinding chamber there is a central circular hole coaxial to the conical body 15, to allow material discharge. Inside the hollow disc defined by the walls 12 and 13 there are located, in addition to the conical body 15, a plurality of blades 16 which transfer the product between the grinding chambers. The operation of the mill according to the invention is substantially the same for dry grinding as for wet grinding, in which the solid is in concentrated suspension in a liquid phase. As the mill rotates, the circular slots 14 in the circular sectors which have moved into a lower position allow passage of the turbid liquid in the case of wet grinding, or powder in the case of dry grindina, from the upstream grinding chamber into the inner recess of the baffle defined by the walls 12 and 13 and ring 11, and containing the blades 16, where it collects in accordance with the arrows. The sectors containing the turbid liquid or powder accumulated in the recesses continue to rotate and pass from the lower position to the upper position, the turbid liquid or powder.

retained by the blades 16 falling 1:v gravity onto the conical body and passing through the central circular hole in the wall 13 into the downstream grinding chamber located to the right of Figure 2B.

The flared body 15 can also be in the form of a truncated right pyramid of regular polygonal base.

The blade 16 can be formed with flat walls of C profile or with curved scoop-shaped walls. It can extend completely between the ring 11 and the flared body 15 to isolate the circular sectors from each other, or can leave by-pass gaps in the central zone as shown in Figure 2B or in the peripheral zone in proximity to the ring 11, so reducing the rate of effective transfer per revolution from one chamber to the next. In this respect, it should be noted that the required throughput of the mill is normally much less than the transfer capacity of the blades 16 if the circular sectors are completely isolated from each other. The mill throughput can be varied by varying a number of parameters. These are essentially the number, size and position of the slots 14, and in particular the height of the non-slotted peripheral band of the wall 12, and the number, shape and size of the blades 16 and their radial position in relation to the proportion of bypass and thus their transfer capacity. In a preferred embodiment of the invention the last chamber of the micronizing mill is separated from discharge by a separator baffle provided with wall 12 in which the non-slotted peripheral band is of substantially lesser height than in the other baffles so that 0 the ground product has a lesser level and is all contained within the grinding load, which acts a a filter and prevents discharge of particles outside the size range. To illustrate the advantages obtainable by the present invention, some coal wet-grinding tests carried out on a pilot micronizing mill constructed according to the present invention are described. EXAMPLE 1 The product to be ground was coal, grinding being effected with separate feeds of dry coal and water in a weight ratio of about 1:1. The pilot mill comprised 3 chambers of useful inner diameter, nett of the armour cladding, of 550 mm and a total useful length, nett of the baffles, of 3300 mm divided as follows: first chamber 760 mm, second chamber 1780 mm, third finishing chamber 760 mm. The separator baffles were of the shape shown in Figures 2A and B and had the following characteristics:

lst baffle: ratio of passage area to total area 3% height of slots 8 mm height of non-slotted circular band 86 mm No. of blades 4, C-shaped 2nd baffle: ratio of passage area to total area 2% height of slots 5 mm height of non-slotted circular band 86 mm No. of blades 4, C-shaped 3rd baffle (discharge):

ratio of passage area to total area 2.9% height of slots 5 mm height of non-slotted circular band 69 mm No. of blades 4, C-shaped Speed of rotation: 37 r.p.m. equivalent to 65% of the critical speed.

The grinding load was as follows:

1st chamber: steel balls with the following weight distribution:

mm dia. 13% mm dia. 20 mm dia. 15 mm dia.

25% 25% 37% 2nd chamber: steel balls with the following weight distribution:

mm dia. 24% mm dia. 76% 3rd chamber: 8 mm dia. steel balls or 8 x 8 mm rods.

The degree of filling maintained in the grinding chambers was as follows:

1st chamber 2nd chamber 3rd chamber 34% The obtained performance was as follows:

Particle size of coal feed Bond index Dry throughput Max product size Electricity consumption EXAMPLE 2 grinding load product 36% 29% 36% 35% 28% 0-6 mm 21 klvh/t 5 3 kg/h < 20 microns 100 kWh/t dry basis The same mill was used to micronize coal of finer particle size, fed in suspension. The first separator baffle was removed. The grinding load and the degree of filling were the same as in the second and third stages of the preceding example. The obtained performance was as follows: Particle size of coal feed Bond index Feed throughput 0-350 microns 21 kWh/t kg/h of turbid liquid containing 49% by weight Max ground product size < 20 microns Electricity consumption 65 k1WIl/t dry basis Speed of rotation 37 r.p.m. equivalent to 65% of critical speed.

Tests were also carried out on another pilot mill to determine the effect of the L/D ratio on the ground product, by reducing the useful length of the device. The tests were carried out using separate dry material and water feeds.

EXAMPLE 3

Inner diameter Useful length No. of chambers 2 Useful length lst chamber 560 mm Grindina bodies ist chamber Useful length 2nd chamber Grinding bodies 2nd chamber 0 Ltot/D = 4 Ltot/D = 6 600 mm 600 mm 2400 mm 3600 mm.

2 830 mm as lst chamber of Example 1 1840 mm 2770 mm as 2nd chamber of Example 1 Coal feed Particle size Bond index (k-Wll/t) Throughput, dry basis (kg/h) 20.8 Max. product size Energy consumption (kWIi/t) Unit production (kg/M3.h) Speed of rotation (r.p.m.) EXAMPLE 4

Inner diameter Useful length No. of chambers Useful length 1st chamber Useful length 2nd chamber Useful length 3rd chamber Grinding bodies Degree of filling Coal feed Particle size 0-6 mm 0-6 mm 21 21 39 < 20 microns < 20 microns 225 30.8 35.5 38.3 35.5 Ltot/D = 4 Ltot/D = 6 600 mm 600 mm 2400 mm 3600 mm 3 3 560 mm 830 mm 1280 mm 1940 mm 560 mm 830 mm as Ex. 1 as Ex. 1 as Ex. 1 as Ex. 1 0-6 mm Bond index (k1WIi/t) 21 Throughput, dry basis (kg/h) 27 C Max. product size < 20 microns < 20 microns Energy consumption (kbli/t) dry 160 100 Unit production (kg/M3.h) 39.8 64 Speed of rotation (r.p.m.) 35.5 35.5 From Examples 3 and 4 it can be seen that the surprising 0 0-6 mm 21 65 production incredse for fine particle sizes (< 20 microns) obtained by the increased length far exceeds the consequent increase in useful volume. There is also a considerable decrease in unit energy consumption 5 with increase in the number of grinding chambers. In the tests carried out it has also been found that maximum energy efficiency in the production of micronized material with a maximum size less than 20 microns is obtained within the speed range of 60-67% of the critical speed, the test range having been 40-80% The micronizing mill according to the invention can be used industrially both for wet and for dry grinding. Figure 3 shows a flow diagram for wet grinding. The granular coal is fed by the conveyor belt 20 to the mixer device 21 into which the suspension water is fed by the pump 22 and line 23. The suspension obtained is fed into the micronizing mill 1 according to the invention. It is discharged by the discharge device 24, consisting of a rotating structure with a perforated wall 25 which allows the micronized product suspension to pass and be removed via the line 26, while any undersized grinding bodies which have passed through the 0 separator baffles 2 are discharged from its end. They are collected in a hopper 27 and are periodically removed. The energy consumed during grinding results in a temperature increase of the aqueous suspension and a certain formation of steam. The steam extraction rate and the product suspension temperature are controlled by the regulator valve 28 connected between the extractor fan 29 and the dischdr-e device 24.

0 - 12 Figure 4 shows a process flow diagram for dry micronization coupled with a cyclone classifier. The granular. feed and the recycled coarse product fraction are fed to the feed hopper 31 and drawn into the micronizing mill 1 of the present invention by suction. It is kept under vacuum and if necessary under a controlled atmosphere, this latter being the case if the material to be ground can form dust or volatile products which are dangerous in the presence of air, such as coal. This atmosphere can consist of air and inert gas mixtures of composition outside explosive limits. By the effect of the suction, the micronized product is fluidized at the discharge and is fed through the line 32 to a first cyclone separator 33 which separates the coarser product fraction, this being recycled to the hopper 31 through the line 34. The finer fraction remains fluidized and is fed through the line 35 to a second cyclone separator 36 of higher efficiency, which separates the fine product fraction. The transport fluid leaves from the top of the cyclone 36 and is recycled to the hopper 31 by the suction fan 37, which compensates for the pressure drops in the overall circuit and the line 38. Part of the fluidizing gas is discharged to atmosphere through the line 39 after final dust removal in the filter 40. The product is discharged through the line 41. Part of the fluidizing transport gas has to be discharged to keep its composition within safety limits, because a certain infiltration of external air is inevitable from the feed devices and through the rotar couplings.

m Y Air is fed through the line 41 and inert cras through the line 42.

0 c) Operating the grinding process under vacuum prevents dust escaping into the atmosphere. To better emphasize the industrial advantages of the present invention, constructional and operational data are given below for a micronizing mill according to the invention designed for the wet grinding of coal, and in this case for processing granular fossil coal and petroleum coke, in accordance with the scheme of Figure 3. FEED Type Feed rate (t/h dry matter) Moisture content (% by weight) 5-10 Density (kg/dM3) 1.35 Grindability H.G.I. (hardness index) Bond index (kWh/t) Particle size distribution Mesh size mm.

2 1.5 1 0.7 0.5 0.35 0.25 mean diameter (mm) ILE'r FLUID (added water) Flow rate (t/h) Fossil coal Petroleum coke 20 6-11 1.4 52 21 n. d.

equal for both Total retained % by weight average 1 maximum 5 fl 5 of 14 91 15 ef 30 73 0.6 22.7 if 91 #I # 9 11 34 45 56 65 75 0.65 Temperature (OC) PH OUTLET FLUID Suspension flow rate (t/h) Steam flow rate (t/h) Temperature (OC) Concentration % by weight Viscosity (cp) pH 10 Suspended solid MILL CHARACTERISTICS Inner diameter nett of armour cladding Total length of cylinder Useful length of 1st chamber Useful length of 2nd chamber Useful length of 3rd chamber Grindina load:

1st chamber 2nd chamber 3rd chamber Type, distribution and degree of filling Installed power Separator baffles:

Overall thickness 1st and 2nd baffle Overall thickness 3rd baffle No. of sectors and blades 20-24 9-10 42.1 1.7 69 49-50 80-180 7 99.5% passing 20 microns all passing 32 microns 3.1 m 19. 0 m 4.0 m 9.5 m 4.3 m 51 t 119 t t as Example 1 2700 kW 500 mill 250 mm 14 Slot height Total slot area: 1st chamber 2nd chamber 3rd chamber OPERATING DATA residence time (minutes) Speed of rotation (r.p.m.) % critical speed Absorbed power (k-W) Energy consumption (kWh/t dry) as Example 1

0.252 M2 0. 154 M2 0. 232 & 36 15.5 65 2200 110

Claims

1. A continuous micronizing mill for the wet or dry micronization of granular material, comprising a rotary drum divided into several grinding chambers in which there are placed grinding media comprising hard bodies of regular shape and having an average body size which progressively decreases from the feed chamber to the discharge chamber. the ratio of the length to the diameter of the rotary drum being at least 5. said grinding chambers being separated from each other by baffles comprising two transverse perforated walls and transfer blades for moving the material within the baffles, the upstream wall being provided with through slots located in an intermediate circular band and the downstream wall being provided with a central aperture coaxial with a flared solid member connecting the two walls.

2. A mill as claimed in claim 1, wherein the grinding chambers are three in number, the central chamber being that of greatest volume.

A mill as claimed in claim 1 or 2, wherein the grinding media placed in one or more of the grinding chambers each consist of a mixture of grinding bodies of different sizes.

4. A mill as claimed in any of claims 1 to 3, wherein the separator baffle which separates the last grinding chamber from discharge has a peripheral nonslotted band of lesser height to maintain the level of the ground solid lower than in the other upstream grinding chambers.

5. A mill as claimed in any of claims 1 to 4, wherein the ratio of the length to the diameter of the rotary drum is at least 5.

6. A mill as claimed in claim 1. substantially as hereinbefore described with reference to, and as shown in, the drawings.

1 1

7. A method for the continuous micronizing of granular material by the mill as claimed in any of the preceding claims, wherein the speed of rotation of the mill is maintained at between 60% and 67% of the mill critical speed.

8. A method for the continuous micronizing of granular material by the mill as claimed in any of claims 1 to 6, wherein the grinding is carried out under vacuum, the ground product being discharged in the fluid phase and classified in a series of cyclone separators.

9. A method as claimed in claim 8, wherein the coarser fraction is recycled to the mill.

10. A method for the micronizing of granular is material by the mill as claimed in any of claims 1 to 6, wherein the mill throughput is varied by varying the shape and/or position of the blades within the separator baffles.

11. A method for the micronizing of granular material by the mill as claimed in any of claims 1 to 4, wherein in the last micronizing chamber. the ratio of grinding media to ground product is kept less than in the preceding chambers.

12. A method for the micronizing of granular material. substantially as hereinbefore described with reference to the drawings.

13. Granular material that has been micronized by a method as claimed in any of claims 7 to 12.

Paed 1990 atThe PatentOffice. State House.8671 High Holbo.-n.London WC1R4TP.Purther coplesmaybe Obtainedfrorn The Patent Office