Fluidized bed opposed jet mill for producing ultrafine particles from feed material of a low bulk density and a process for use thereof

Abstract

In consideration of increasing throughput rate of a stable operating process as well as making the process energy-efficient, the aim is to optimise a fluidised bed opposed jet mill and a dedicated process 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 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. This is achieved by the feed material dosed to the mill from the bottom into the mill sump as a gas-particle mixture, whereby a deflector hood is located above the feed point and below the grinding nozzle level, and the grinding gas nozzles designed to be flush with the walls.

Claims

1. A fluidized bed opposed jet mill to produce for producing ultrafine particles from a feed material of low bulk density having a housing in vertical design, with a product feed point and a product discharge, with a grinding zone located in a lower area of the housing which has grinding nozzles spaced evenly around a circumference thereof, with jets of the grinding nozzles intersecting at one central point and with a classifying device installed in an upper area of the housing wherein the feed material is dosed as a gas-particle mixture from below into a sump of the fluidized bed opposed jet mill, whereby a deflector hood is fitted above the product feed point and below a level of the grinding nozzles, and the grinding nozzles are configured to be flush with a wall of the grinding zone.

2. The fluidized bed opposed jet mill of claim 1, wherein the classifying device has a horizontally arranged classifying wheel.

3. The fluidized bed opposed jet mill of claim 2, wherein the classifying wheel has fittings in flow channels and an L/D ratio of >1.

4. The fluidized bed opposed jet mill of claim 3, wherein the L/D ratio is between 1.2 and 1.3.

5. The fluidized bed opposed jet mill of claim 1, wherein the feed material is dosed by a powder diaphragm pump.

6. The fluidized bed opposed jet mill of claim 1, wherein the grinding nozzles are cylindrical.

7. The fluidized bed opposed jet mill of claim 1, wherein water nozzles configured to dose water are arranged above the grinding zone and below the classifying device.

8. The fluidized bed opposed jet mill of claim 1, further comprising a one-stage fan with a high pressure rating.

9. A process for producing ultrafine particles from a feed material of low bulk density using the fluidized bed opposed jet mill of claim 1 comprising: dosing the feed material as a gas-particle mixture into the sump of the fluidized bed opposed jet mill underneath the grinding zone; deflecting the feed material into the grinding zone using a deflector hood located above the product feed point and subjecting the feed material to mechanical stress in the grinding zone, thereby producing the ultrafine particles from the feed material of low bulk density.

10. The process of claim 9, further comprising injecting water into the fluidized bed opposed jet mill during the subjecting the feed material to mechanical stress.

11. The process of claim 9, wherein a rate at which the feed material is dosed is regulated as a function of a pressure drop between a grinding chamber and a filter of the fluidized bed opposed jet mill.

12. The process of claim 9, wherein pressure of a grinding gas for injecting into grinding nozzles of the fluidized bed opposed jet mill is equal to or less than 3 bar.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other details, features and advantages of the disclosed subject matter arise from the claims and from the following description of the associated drawing in which a preferred embodiment is shown by way of example.

(2) The Figure shows a fluidised bed opposed jet mill with the disclosed-design features and the disclosed-design process.

DETAILED DESCRIPTION

(3) The housing of the fluidised bed opposed jet mill (1) has a vertical central axis. The grinding chamber and grinding zone are located in the lower area of the housing, and above them, at a defined distance, the classifying zone with the air classifier. The grinding chamber is preferably cylindrical in shape. Two grinding nozzles (2) are arranged around the circumference of the grinding chamber, through which the jets of fluid are injected into the grinding zone to subject the feed material to mechanical stress. The feed material can be comminuted, disintegrated and/or dispersed thereby. A fluidised bed develops here. As the fluid, gases—above all air but also steam—can be employed. The grinding nozzles (2) are spaced uniformly around the circumference of the grinding chamber so that the grinding jets or rather their central axes intersect at one point. In a preferred invention design, three grinding nozzles (2) are spaced uniformly around the circumference of the grinding chamber, whose jets intersect at one point. When grinding materials, i.e. feed material of low bulk density, the grinding nozzles (2) are inserted in the grinding chamber such that they are flush with the wall. These grinding nozzles (2) are cylindrical grinding nozzles (2) which are operated at low grinding pressures. The feed material is supplied to the fluidised bed opposed jet mill (1) from below into the mill sump. This is the lowest point of the grinding chamber. The feed material is dosed to the fluidised bed opposed jet mill as a gas-particle mixture. A powder diaphragm pump (4) is preferably used for this task. To prevent the feed material from flooding through the grinding zone up to the classifying wheel (6) fitted above, a deflector hood (3) is installed above the feed point and below the recessed grinding nozzles, i.e. underneath the grinding zone. In a preferred invention design, the deflector hood is a circular plate fixed underneath the grinding zone. It is arranged perpendicularly to the direction of flow of the gas-particle mixture introduced into the fluidised bed and deflects or brakes the flow so that the feed material is deflected to the side and into the grinding zone.

(4) If necessary, water can be injected into the grinding zone, to this end, water nozzles (5) are located between the grinding zone and the classifying zone. These nozzles are two-component nozzles (5) with which water and air is injected into the grinding zone in order to condition the grinding air and the material in the grinding zone. In a preferred invention design, the two-component water nozzle is located, when considered radially in the centre of the grinding chamber above the grinding zone and points towards the grinding zone.

(5) The air classifier located above and at a distance from the grinding zone has a centrifugal-force classifying wheel (6) with vertical axis. The classifying wheel (6) has fittings located in the flow channels delimited by the classifying wheel vanes as described in patent DE 198 40 344 A1. The classifying wheel (6) has a large surface area with an L/D ratio of >1. To reduce the pressure drop, the classifying wheel has a fines discharge with large cross-section.

(6) As can be seen in the Figure, the fluidized bed opposed jet mill (1) is charged by means of a powder diaphragm pump (4) with feed material out of supply bin (7) into the mill sump (12). Dosing is a function of the pressure drop. The grinding nozzles are supplied with compressed grinding gas, preferably compressed air from a compressor (8). The grinding is performed at temperatures which correspond to the outlet temperature of the gas at the gas-generating compressor.

(7) In the case of these feed materials of low bulk density, preferred is a low-pressure grinding process. The grinding pressure is 3 bar (g). At pressures of up to 1 bar (g), rotary piston fans can be employed, whereas rotary piston compressors are used for pressures up to 1.5 bar (g). Over and above this, single-stage screw-type compressors are also used.

(8) In order to improve the grinding process, the pressure drop across the system and especially the fluidised bed opposed jet mill (1) must be optimised. This can be done by setting a reduced grinding gas flow rate. In order to simultaneously reduce the spatter grain, the rinsing air flow rate at the classifying wheel gap between the classifying wheel and fines discharge is increased.

(9) Subsequently to being mechanically stressed in the fluidised bed opposed jet mill (1), the product is separated from the air volume flow in the filter (9). Because a flow direction in the filter from below would substantially hinder the discharge of the comminuted product, the flow direction for the light and voluminous products is from top to bottom. A dedusting pressure that is as high as possible effectively prevents an increase of the pressure drop at the filter membranes and makes for a better discharge from the filter. The extremely voluminous product is discharged by means of a large double flap valve (10) with high cycle times. Downstream of the filter is a fan (11) which has the task of conveying the voluminous product and gas mixture through the system with the invention-design fluidised bed opposed jet mill as well as keeping the pressure inside the mill at a constant level and overcoming the pressure drop that develops at the classifying wheel caused by the product. The fan (11) is a one-stage fan with a high pressure rating.

REFERENCE NUMERAL LISTING

(10) Fluidised bed opposed jet mill (1) Grinding nozzles (2) Deflector hood (3) Powder diaphragm pump (4) Water nozzles (5) Two-component nozzles (5) Centrifugal-force classifying wheel (6) Classifying wheel (6) Supply bin (7) Compressor (8) Filter (9) Double flap valve (10) Fan (11)

(11) Sump (12)