Flowbed counter-blast mill for producing finest particles from feed material of low bulk density and method for doing so

Description

[0001] The invention relates to fluidized bed counterjet mills designed as classifier mills and concerns the structural design of the fluidized bed counterjet mill according to the preamble of the main claim and an associated method.

[0002] Fluidized bed counterjet mills consist of a housing with a vertical central axis. At the bottom is a grinding zone where the material to be ground forms a fluid bed. In this zone, the mill has several grinding nozzles evenly distributed around the circumference and pressurized with compressed air. The grinding nozzles are oriented in such a way that the material in the grinding chamber is drawn into the jets and accelerated by them. The impact of collisions between the material particles results in comminution and comminution. A classifying device is located above the grinding zone.The classifying device is generally designed as a centrifugal force classifier, whereby particles finer than the separation particle size are transported inwards into the rotating classifier wheel and separated, while particles coarser than the separation particle size are flung off the rotating classifier wheel and remain in the grinding container. The material to be ground is preferably fed into the grinding zone of the fluidized bed counterjet mill from above.

[0003] DE 31 40 294 A1 describes a fluidized bed counterjet mill. The feed material is metered into the mill's sump via a metering screw. DE 197 28 382 C2 discloses a fluidized bed counterjet mill in which the grinding gas jet is accelerated together with a portion of the material being ground and then introduced into the fluidized bed of material being ground in the mill. DE 10 2006 048 850 A1 describes, among other things, a process for producing amorphous particles for which a fluidized bed counterjet mill is used. The fluidized bed counterjet mill used is described in EP 0139279. As disclosed in EP 0139279, conventional fluidized bed counterjet mills have a product feed above the grinding chamber, so that the material being ground is fed into the grinding zone from above.

[0004] DE 10 045 160 A1 discloses a fluidized bed counterjet mill, wherein the feed material is fed from above and the mill has a vertically downward oriented feed section.

[0005] Fluidized bed counterjet mills process a wide variety of products. To achieve optimal grinding, not only the grinding process but also the mill itself is tailored to the material. With materials that have a low bulk density, or with materials whose ground products have a low bulk density, the problem arises that the particles primarily follow the gas flow and hardly settle. If the material is fed in above the grinding zone, it consequently sinks only insufficiently into the grinding zone and is instead presented to the classifier wheel for screening in an ungrounded or undispersed state. The coarse material rejected by the classifier wheel burdens the classifier and, against the upward flow, does not return to the grinding zone. This results in a significant increase in product volume during grinding, which in turn causes a sharp increase in pressure drop at the classifier and a decrease in throughput.The lower the product bulk density, the more pronounced this effect becomes. This problem occurs, for example, when grinding materials with bulk densities below 500 g / cm³, such as silica, but also with perlite or zeolites.

[0006] The invention is based on the objective of providing a fluidized bed counterjet mill and a method for operating a fluidized bed counterjet mill in order to optimize the production of fine particles from feed material with low bulk density. This is achieved taking into account increasing the throughput while maintaining stable process operation and ensuring the most energy-efficient process possible.

[0007] In a fluidized bed jet mill and associated method of the type described above, the problem is solved according to the invention by the characterizing feature of the main claim.

[0008] In the fluidized bed counterjet mill according to the invention, the feed material is metered from below into the sump of the mill as a gas-particle mixture, wherein a deflector hood is arranged above the feed material feed and below the grinding nozzle level and the grinding gas nozzles are designed flush with the wall.

[0009] The associated method according to the invention for operating the fluidized bed counterjet mill provides that the feed material is dosed as a gas-particle mixture into the sump of the fluidized bed counterjet mill below the grinding zone and is deflected into the grinding zone by a deflector hood arranged above the feed material feed.

[0010] By combining the features of the device as well as the process, the production of fine particles from feed material with low bulk density in fluidized bed counterjet mills could be significantly optimized compared to the state of the art with regard to throughput and process stability while maintaining good energy efficiency.

[0011] In tests, the inventors surprisingly discovered that a significantly higher throughput can be achieved by dosing the feed material from below into the sump of the fluidized bed jet mill than by feeding it laterally—above the grinding nozzles—into the grinding zone. By adding the material to the sump, it is forced to pass through the grinding zone and, after passing through, is already reduced to the target particle size and can pass the classifier wheel without causing any load. This operating method ensures that the fluid flows through the fluidized bed jet mill as straight and smoothly as possible, from bottom to top, in the direction of the mill's vertical central axis, i.e., in the direction of the gas flow.

[0012] Low-density feed materials, such as silica, are very fluid and therefore difficult to dose using a screw conveyor. The solution to this problem is to dose the fluidized feed material as a gas-particle mixture. For this purpose, a powder diaphragm pump is used, for example, to draw the feed material from a silo and feed it directly into the mill. This method of dosing is dust-free.

[0013] The feed material is fed into the fluidized bed counterjet mill from below, preferably at the lowest point of the mill, as a gas-particle mixture. There is a risk that feed material particles may pass through the grinding zone without being subjected to any stress. This can lead to spatter in the final product, meaning that excessively large and undispersed particles pass through the classifier wheel instead of being rejected. To prevent this unexposed passage through the grinding zone and the resulting spatter problem, a deflector hood is positioned just above the feed material inlet into the sump and well below the grinding nozzles. This hood prevents the feed material from being forced through the grinding zone and directs it into the grinding zone, where it is subjected to stress by the grinding jets and by particle impact.In its simplest form, the deflector hood is a circular disc with a suitable diameter, which is fixed perpendicular to the flow direction of the gas-particle mixture introduced by the powder diaphragm pump in the mill sump well below the grinding zone by a device, and slows down or deflects it.

[0014] The deflector hood can also be combined with other components in the fluidized bed jet mill.

[0015] In experiments, the inventors surprisingly discovered that, for materials with low bulk density, a flush-mounted positioning of the grinding nozzles is particularly effective in processing the feed material in the grinding zone. The processing of the feed material in the grinding zone by the grinding jets to produce the finest particles can involve comminution, but also deagglomeration or dispersion. When this patent application refers to comminution or grinding, it always includes deagglomeration or dispersion.

[0016] When processing low-density feed materials – such as silica – in the grinding zone, the process involves dispersion, which can be carried out particularly energy-efficiently with low grinding gas pressure. Simple cylindrical grinding nozzles are used for this purpose. Depending on the feed material being processed and the required grinding pressure, Laval nozzles of various designs are also employed.

[0017] The grinding jets can also be pulsating.

[0018] To optimize the process, water – or other additives – can be injected into the mill below the viewing zone as needed. Ideally, the water is injected directly after the grinding zone, either centrally or flush with the wall, into the mill hopper using two-fluid nozzles, along with air or another grinding gas used in the process.

[0019] Injecting water into the grinding chamber lowers the temperature of the gas-particle mixture. This serves two purposes: firstly, to protect the filter fabric, and secondly, to allow the use of smaller filters, as the reduced airflow due to the change in air density leads to a decrease in the required air volume flow. Furthermore, it achieves a targeted increase in particle weight. Water injection also reduces the electrostatic charge of the material, facilitating its removal from the machine and / or filter.

[0020] The grinding chamber of the fluidized bed jet mill is preferably cylindrical, but the diameter can also vary over the height.

[0021] The feed material has a bulk density of less than 500 g / cm³, preferably less than 250 g / cm³. The final product has a bulk density of less than 300 g / cm³, preferably less than 150 g / cm³, and particularly preferably less than 75 g / cm³.

[0022] The following feed materials with low bulk density and feed materials from which products with low bulk density are produced can be processed, among others, with the mill according to the invention: silica, expanded graphite, rice hull ash, perlite, zeolites and others.

[0023] The feed material in the fluidized bed jet mill, such as silica, generates a large product flow rate due to its low bulk density. At the classifier wheel, with its smaller orifices or free cross-sections compared to the mill chamber, this effect manifests as a significant pressure drop, as this is a constriction inherent to the mill's design. Furthermore, a co-rotating cloud of particles forms around the classifier wheel, which has not yet been ground to the target fineness.

[0024] To mitigate this effect, a classifier wheel with a particularly large surface area, i.e., free cross-sections, is to be used. The classifier wheel has an L / D ratio of > 1, preferably 1.2 to 1.3, where D is the diameter of the classifier wheel and L is the height relevant for classification (in the direction of the classifier wheel's central axis) of the flow channels, which are bounded by the classifier blades and the lower and upper cover plates of the classifier wheel.

[0025] Furthermore, a classifier wheel as described in DE 198 40 344 A1 is used. These classifier wheels can be used at low wheel speeds. Both effects (large free cross-section of the classifier wheel and the low speed) together reduce the resulting pressure loss, thus enabling a higher throughput.

[0026] When processing feed materials with low bulk density, or feed materials that produce low bulk density products such as silica, a significant pressure drop occurs due to the product cloud – particularly at the classifier wheel. A high-pressure blower overcomes this pressure drop and increases the throughput. Choosing a single-stage blower represents a still economically justifiable option.

[0027] The above-described constructive measures relating to the inventive flow bed counterjet mill enabled a significant increase in throughput for the same machine size compared to the prior art.

[0028] For the inventive method for operating the described fluidized bed jet mill, the feed material is dosed as a gas-particle mixture into the sump of the fluidized bed jet mill below the grinding zone and deflected into the grinding zone by a deflector hood (3) arranged above the feed material feed.

[0029] The pressure drop along the grinding gas flow from the grinding nozzles through the classifier wheel to the filter and blower is a key parameter in the process of generating fine particles in a moving bed jet mill for feed materials and / or products with low bulk density, such as silica, and is therefore ideally suited as a control parameter for the dosing rate to ensure stable operation. Adjusting the dosing rate based on the material weight in the grinding chamber is not feasible for these products due to their low bulk density, and optimizing the utilization of the classifier wheel by measuring its current draw with frequency converter operation is inherently impractical.

[0030] The control of the dosing rate based on pressure loss is implemented as follows: To determine the pressure loss, the relative pressure in the process chamber to the environment is measured and maintained at a constant level by controlling the blower speed. Simultaneously, a second relative pressure measurement is taken in the supply line to the filter or on the raw gas side within the filter. The differential pressure between the first and second relative pressure measurements is kept constant via the dosing rate. Alternatively, a differential pressure gauge can be used.

[0031] For an efficient grinding process, efficient generation of the grinding gas is also important; eliminating cooling or heating coils improves energy efficiency. The process therefore operates at the temperature generated at the air generator during compression.

[0032] Compressed air is preferably used as the grinding gas, but technical gases such as hydrogen, noble gases or superheated steam can also be used.

[0033] In the production of ultrafine particles of feed materials with low bulk density, the primary stress in a fluidized bed jet mill is deagglomeration or dispersion; the feed material agglomerates can be broken up with low jet power. For this reason, low grinding gas pressures are sufficient for the process and are also more efficient to generate. Furthermore, expensive screw compressors can be omitted. Rotary lobe blowers can be used for pressures up to 1 bar gauge, and rotary lobe compressors for pressures up to 1.5 bar gauge. Single-stage screw compressors are used for grinding pressures above 1.5 bar gauge up to 3 bar gauge.

[0034] The amount of grinding gas also significantly affects the pressure drop in the machine, especially at the classifier wheel, and therefore needs to be optimized. Too much air leads to a high pressure drop, while too little air reduces the throughput.

[0035] Water can be injected into the grinding chamber as needed. This allows the following objectives to be achieved: Lowering the temperature of the gas-particle mixture serves, firstly, to protect the filter fabric in the downstream filter and, secondly, to reduce the gas volume flow due to the change in air density. Increasing the specific weight of the material. Reducing the electrostatic charge of the material, which allows it to be carried away more effectively.

[0036] For fine material separation, a filter is installed downstream of the fluidized bed jet mill. A bottom-feed flow through the filter would significantly impede the discharge of the pulverized, extremely light, and bulky product. Therefore, a top-feed flow through the filter is used.

[0037] Products with low bulk density follow the gas flow and have too little weight to sediment themselves; therefore, the process and the machines are designed so that no sedimentation against the gas flow is required.

[0038] Since the finest particles produced, which have a low bulk density, often result in splashing, the amount of purge air at the gap between the classifier wheel and the fines outlet is increased.

[0039] A high cleaning pressure effectively prevents an increase in pressure loss across the filter membranes and ensures better discharge from the filter. The material expands during processing, resulting in bulk densities of 30-70 g / cm³. Therefore, it is essential to ensure that the product volume can be discharged through the feeder. This can be achieved by increasing the feeder's size or, within certain practical limits, by selecting faster cycle times.

[0040] The process is operated under negative pressure. A blower is used at the end of the process chain to maintain a slight negative pressure in the grinding chamber, classifier, and filter. This negative pressure also facilitates product transport from grinding to separation in the filter. Significantly higher throughputs can be achieved with negative pressure operation compared to positive pressure operation. While this results in increased power consumption for the blower, it yields a considerably higher throughput and reduces specific energy consumption.

[0041] Further details, features and advantages of the subject matter of the invention will become apparent from the dependent claims and from the following description of the associated drawings, in which - by way of example - a preferred embodiment of the invention is shown.

[0042] The Figure 1shows a fluid bed counterjet mill with the features according to the invention and the method according to the invention.

[0043] The fluidized bed jet mill (1) has a vertically oriented housing. The grinding chamber with the grinding zone is located in the lower section, and above it, at a defined distance, is the classification zone with the air classifier. The grinding chamber is preferably cylindrical. Grinding nozzles (2) are arranged around the circumference of the grinding chamber, through which fluid jets are guided into the grinding zone to process the material being ground. The material can be crushed, deagglomerated, and / or dispersed. A fluidized bed is formed. Gases, primarily air but also steam, can be used as the fluid. The grinding nozzles (2) are arranged evenly distributed around the circumference of the grinding chamber so that the grinding jets, or rather their central axes, intersect at a single point. In a preferred embodiment, three grinding nozzles (2) are arranged evenly around the circumference of the chamber, with their jets intersecting at a single point. For the grinding of materials, i.e.,For feed materials with low bulk density, the grinding nozzles (2) are installed in the grinding container so that they are flush with the walls. These grinding nozzles (2) are cylindrical and operate at low grinding pressures. The feed material is introduced from below into the sump of the fluidized bed jet mill (1). This occurs at the lowest point of the grinding container. The feed material is metered into the fluidized bed jet mill as a gas-particle mixture. A powder diaphragm pump (4) is preferably used for this purpose. To prevent the feed material from passing through the grinding zone to the classifier wheel (6) located above it, a deflector hood (3) is installed above the feed inlet and below the grinding nozzle inlets, i.e., below the grinding zone. In a preferred embodiment, this is designed as a circular disc and fixed below the grinding zone.It is arranged perpendicular to the flow direction of the gas-particle stream introduced into the sump and deflects or slows it down, so that the feed material is deflected laterally into the grinding zone.

[0044] Water can be injected into the grinding zone as needed; for this purpose, water nozzles (5) are arranged between the grinding zone and the inspection zone. These are two-component nozzles (5) with which water mixed with air is injected into the grinding zone to condition the grinding air and the material in the grinding zone. In a preferred embodiment, the two-component nozzle is located radially in the center of the grinding container above the grinding zone and radiates towards the grinding zone.

[0045] The air classifier, arranged above the grinding zone at a distance from it, has a centrifugal classifier wheel (6) with a vertical axis. The classifier wheel (6) has internal components in the flow channels bounded by the classifier wheel blades, as described in DE 198 40 344 A1. The classifier wheel (6) has a large surface area with an L / D ratio of > 1. To reduce pressure loss, the classifier wheel has a fines outlet with a large cross-section.

[0046] As from Figure 1 The fluidized bed jet mill (1) is fed with feed material from the feed hopper (7) into the mill sump via a powder diaphragm pump (4). Dosage is determined by pressure drop. The grinding nozzles (3) are supplied with compressed grinding gas, preferably compressed air from a compressor (8). Grinding takes place at temperatures corresponding to the gas outlet temperature at the gas-generating compressor.

[0047] For these feed materials with low bulk density, low-pressure grinding is preferred. The grinding pressure is ≤ 3 bar (gauge). Rotary lobe blowers can be used at pressures up to 1 bar (gauge), and rotary lobe compressors at pressures up to 1.5 bar (gauge). Above this pressure, single-stage screw compressors are used.

[0048] To optimize the grinding process, the pressure drop in the system, and specifically in the moving bed jet mill (1), must be reduced. This can be achieved by adjusting the amount of grinding gas. Simultaneously, to also reduce the amount of sprayed material, the amount of purge air at the classifier gap between the classifier wheel and the fines discharge is increased.

[0049] Following processing in the fluidized bed jet mill (1), the product is separated from the airflow in a filter (9). For light and bulky products, the airflow to the filter is from top to bottom, as a bottom-up flow would impede the discharge of the pulverized product. A high cleaning pressure effectively prevents an increase in pressure drop across the filter membranes and ensures better discharge from the filter. The very bulky product is discharged via a large indexing valve (10) at high cycle rates. A blower (11) is connected downstream of the filter. Its functions are to convey the bulky product and gas mixture through the system containing the fluidized bed jet mill according to the invention, to maintain a constant internal mill pressure, and to overcome the pressure drop caused by the product at the classifier wheel. This is a single-stage, high-pressure blower (11). Reference sign

[0050] Fluidized bed jet mill (1) Grinding nozzles (2) Deflector hood (3) Powder diaphragm pump (4) Water nozzles (5) Two-fluid nozzles (5) Centrifugal classifier wheel (6) Classifier wheel (6) Feed hopper (7) Compressor (8) Filter (9) Interval gate (10) Blower (11)

Claims

1. Fluidised bed opposed jet mill (1) to produce ultrafine particles from a feed material of low bulk density with a housing in vertical design, with a product feed point and a product discharge, with a grinding zone located in the lower area of the housing which has grinding nozzles (2) spaced evenly around the circumference whose jets intersect at one central point and with a classifying device installed in the upper area of the housing characterised in that the feed material is dosed as a gas-particle mixture from below into the sump of the fluidised bed opposed jet mill whereby a deflector hood (3) is fitted above the feed point and below the grinding nozzle level, and the grinding nozzles are designed to be flush with the wall.

2. Fluidised bed opposed jet mill (1) in accordance with Claim 1, characterised in that the classifier has a horizontally arranged classifying wheel (6).

3. Fluidised bed opposed jet mill (1) in accordance with Claim 1, characterised in that the feed material is dosed by means of a powder diaphragm pump.

4. Fluidised bed opposed jet mill (1) in accordance with Claim 1, characterised in that the classifying wheel (6) has fittings in its flow channels and an L / D ratio of >1, preferably between >1.2 and 1.3.

5. Fluidised bed opposed jet mill (1) in accordance with Claim 1, characterised in that the grinding nozzles (2) are cylindrical in design.

6. Fluidised bed opposed jet mill (1) in accordance with Claim 1, characterised in that nozzles (5) designed to dose additives, preferably water, are arranged above the grinding zone and below the classifying device.

7. Fluidised bed opposed jet mill (1) in accordance with Claim 1, characterised in that the process is operated with a one-stage fan (8) with a high pressure rating.

8. Process to operate a fluidised bed opposed jet mill (1) to produce ultrafine particles from a feed material of low bulk density in accordance with Claims 1 to 7, characterised in that the feed material is dosed as a gas-particle mixture into the sump of the fluidised bed opposed jet mill underneath the grinding zone and is deflected into the grinding zone by means of a deflector hood (3) located above the feed point.

9. Process in accordance with Claim 8, characterised in that water or another additive is injected into the fluidised bed opposed jet mill (1) during grinding.

10. Processin accordance with Claim 8, characterised in that the rate at which the feed material is dosed is regulated as a function of the pressure drop between the grinding chamber and filter.

11. Process in accordance with Claim 8, characterised in that the pressure of the grinding gas for the grinding nozzles is equal to or less than 3 bar (g).