GB2111855A

GB2111855A - Fluid energy mill

Info

Publication number: GB2111855A
Application number: GB08232116A
Authority: GB
Inventors: Denis Ansbro; Walter Twist; Harold Edward Haigh; Trevor Carter
Original assignee: Tioxide Group Ltd
Current assignee: Tioxide Group Ltd
Priority date: 1981-12-11
Filing date: 1982-11-10
Publication date: 1983-07-13
Also published as: FR2517986A1; AU9068882A; AU554247B2; FI824246L; ZA828507B; ES518067A0; GB2111855B; ES8404876A1; DE3245446A1; FR2517986B1; FI824246A0; JPS58104642A; CA1199621A

Abstract

A fluid energy impact mill for comminuting e.g. titanium dioxide includes an injector to produce a stream of milling fluid containing particulate material to be milled and at least two milling surfaces 6,8 with which the particulate material is impacted and which are positioned in a reflective path for the stream of fluid. More than two impact surfaces may be used. The mill has associated therewith means viz an expansion chamber 10 and bag filter 11 to separate the milled material from the milling fluid which may be nitrogen. <IMAGE>

Description

SPECIFICATION Improved mill This invention relates to an improved mill and particularly to a fluid energy impact mill.

According to the present invention a fluid energy impact mill comprises an injector to produce in a milling chamber a stream of particulate material to be milled in a milling fluid, a first impact mill surface and a second impact mill surface in said chamber and arranged in a reflective path in the stream whereby particulate material impinges on the first impact mill surface and is reflected therefrom to the second surface, and separation means to separate the milled particulate material from the stream of milling fluid.

Afluid energy impact mill in accordance with the invention is useful for reducing the particle size of powders such as inorganic or organic pigment, drugs or other chemicals and products which are produced in a finely divided form. The invention is of particular use in milling inorganic products such as titanium dioxide pigments, titanium phosphate pigments, silica pigments, aluminium pigments, barium pigments, calcium pigments, carbon black, iron pigments, lead pigments and magnesium pigments, amongst others. The pigments may be colored inorganic or organic pigments and if desired the mill may be used for other opacifiers or fillers.

In the fluid energy mill there is produced a stream of the particulate material to be milled in a milling fluid. The stream having a sufficiently high velocity is directed to a first impact mill surface where impact effects the first milling stage and thence to a second impact mill surface where milling is further continued. To effect the transfer from the first impact mill surface to the second surface it is necessary for the two miling surfaces to be mounted in a reflective path for the stream carrying the particulate material so that the stream is reflected from the first milled surface to the second and thence further through the milling apparatus.If desired, the fluid energy impact mill constructed in accordance with the present invention may be provided with more than two impact mill surfaces providing that the surfaces are arranged in a continuous reflective path to effect transfer of the particulate material carried by the stream of milling fluid successively to the impact mill surfaces.

Normally however the impact mill constructed in accordance with the invention will include two impact mill surfaces mounted as described. The impact mill surface may be formed by a particular shaped portion of the wall of the milling chamber and may be formed of the same material as the wall of the milling chamber, or alternatively, the impact mill surface may be formed of a different material than the wall of the milling chamber but form part of the wall. If desired the impact mill surface need not form part of the wall and can be separate therefrom.

The particular material of construction of the impact mill surface will depend on the particular particulate material to be milled. Since it is possible that the impact mill surface will suffer erosion during use it would be undesirable to use a material of construction which would contaminate the milled particulate material and thus it is desirable that the impact mill surface is formed either of a material which will suffer extremely little abrasive erosion orto be formed of a material which when abraided does not objectionably contaminate the milled product. In suitable cases the impact mill surface may be formed of stainless steel or other suitable relatively hard metal or may be formed of a suitable ceramic material.

Usually the particulate material to be milled is fed cross-currently into a stream of milling fluid supplied to the mill through an injector at the required velocity and pressure. The velocity may be sonic and greater if desired. The milling fluid produces a stream of the particulate material in the milling chamber and after contact with the impact mill surfaces it is necessary to separate the milled product from the stream of milling fluid by any suitable means. For instance the stream of milled particulate material may be fed to an expansion chamber or a series of expansion chambers where the velocity of the stream is progressively reduced and thence to a bag filter or other suitable filtration device to collect the milled product from the stream of milling fluid.Alternatively the product after milling may be fed to a cyclone where separation of the constituent milled product occurs. If necessary a further supply of the milling fluid may be fed into the stream of ground particulate material prior to entry into an expansion chamber or series of chambers or a cyclone to dilute the stream and to assist removal of the particulate material from the product fluid stream.

The milling fluid used in the fluid energy impact mill of the present invention usually will be a gas or vapour which is substantially inert to the material to be milled. Suitable gaseous milling fluids are air, oxygen, nitrogen and other inert gases and steam in appropriate cases. If desired a treating agent for the particulate material may be introduced with the milling fluid to surface treat the particulate material during milling and such products can be surfactants or other surface modifying agents which interact with the surface of the particulate material at the appropriate milling temperatures.

Usually but not necessarily milling of the particulate material employing a fluid energy mill constructed in accordance with the invention is carried out at an elevated temperature and particularly when employing steam this will depend on the pressure of the steam used to produce the stream of milling fluid carrying the particulate material.

One form of fluid energy impact mill constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawing which is a diagrammatic representation of the mill.

The fluid energy impact mill 1 includes an elongated milling chamber 2 into which there is fed a stream of the particulate material to be milled from a hopper 3 in a milling fluid supplied by inlet 4 and venturi 5. The mill has a pair of impact mill surfaces 6 and 7 formed by shaping the walls of the mill 1 and in a separate section 8. An outlet 9 is provided which connects with an expansion chamber 10 of increasing diameter to which is attached a collection bag 11.

Between the end of the mill 1 and the first expansion chamber are inlets 12 for further supply of fluid compatible with the milling fluid to be used as indicated at 13.

In use milling fluid is fed to the mill through inlet 4and venturi 5 and forms a stream in the milling chamber 2 of the particulate material to be milled which is directed towards the impact surface 6. After contacting the surface 6 the material is reflected from the surface on to surface 7 and then to outlet 9.

Dilution of the stream is effected by supplying a further amount of compatible fluid 13 prior to passage of the stream of milled particulate material into expansion chamber 10. The milled material is collected in the bag 11 which is porous to the gaseous milling fluid used.

In the following Examples there was used a fluid energy impact mill constructed substantially as shown in the drawing in which chamber 2 had a diameter of 0.5 cms and impact surface 6 and 7 had an area 2.25 sq cms was supplied with a stream of titanium dioxide pigment in nitrogen.

Example 1 Nitrogen was fed to the mill through venturi 5 and the nitrogen initially had a temperature of 600"C and a pressure of 10 Kg/cm2. The amount of pigment fed to the milling chamber was 10 K grams per hour and the amount of nitrogen was 10 K grams per hour. An amount of cold airwas fed through inlets 12 in an amount of 10 K grams per hour and the milled titanium dioxide pigment collected in the bag 11.

The product obtained exhibited a satisfactory degree of milling.

Example 2 Example 1 was repeated except that 10.8 Kg/hr of titanium dioxide pigment was fed to the mill and nitrogen was simultaneously fed to the mill at a rate of 15.7 Kg/hr at a temperature of 200"C and a pressure of 15 Kg/cm2.

The milled titanium dioxide pigment, collected in bag 11, was found to have a satisfactory degree of milling.

Example 3 Example 1 was repeated except that 11.4 Kg/hr of titanium dioxide pigment was fed to the mill and nitrogen was simultaneously fed to the mill at a rate of 13.1 Kg/hr at a temperature of 400"C and a pressure of 15 Kg/cm2.

The milled titanium dioxide pigment was found to have a satisfactory degree of milling.

Example 4 Example 1 was repeated except that 9.3 Kg/hr of titanium dioxide pigment was fed to the milling chamber and nitrogen was simultaneously fed to the mill at a rate of 6.1 Kg/hr at a temperature of 600"C and a pressure of 7 Kg/cm2.

Claims

1. Afluid energy impact mill comprises an injectorto produce in a milling chamber a stream of particulate material to be milled in a milling fluid, a first impact mill surface and a second impact mill surface in said chamber and arranged in a reflective path in the stream whereby particulate material impinges on the first impact mill surface and is reflected therefrom to the second surface, and separation means to separate the milled particulate material from the stream of milling fluid.

2. Afluid energy impact mill according to claim 1 in which there is provided more than two impact mill surfaces arranged in a continuous reflective path.

3. Afluid energy impact mill according to claim 1 or 2 in which each impact mill surface is formed by a shaped portion of the wall of the milling chamber and of the same material as the wall.

4. Afluid energy impact mill according to claim 1 or 2 in which each impact mill surface is formed of a material different from that of the wall of the milling chamber.

5. Afluid energy impact mill according to claim 1 or 2 in which an impact mill surface is separate from the wall of the milling chamber.

6. Afluid energy impact mill according to any one ofthe preceding claims in which the impact mill surface is formed of stainless steel.

7. Afluid energy impact mill according to any one of claims 1 to 5 in which the impact mill surface is formed of a ceramic material.

8. Afluid energy impact mill according to any one of the preceding claims in which said separation means comprises one or more expansion chambers and a filter.

9. Afluid energy impact mill according to any one of claims 1 to 7 in which said separation means comprises a cyclone.

10. A method of milling a particulate material which comprises feeding a milling fluid through the injector of said fluid energy impact mill according to claim 1 to produce a stream of particulate material to be milled in said fluid and impacting said material with said first impact mill surface, reflecting said stream from said first surface to the second impact mill surface and subsequently separating the so milled material from said fluid.

11. A method according to claim 10 in which the milling fluid is fed through the injector to produce a stream of fluid having a velocity which is at least sonic velocity.

12. A method according to claim 10 or 11 in which the particulate material is pigmentarytitanium dioxide.

13. A method according to claim 10, 11 or 12 in which after milling the stream of milling fluid carrying the milled particulate material is fed to an expansion chamber orto a cyclone.

14. A method according to claim 13 in which a further supply of the milling fluid is fed into said stream prior to the expansion chamber or said cyclone.

15. A method according to any one of claims 10 to 14 in which said milling fluid is nitrogen.

16. A method according to any one of claims 10 to 14 in which said milling fluid is steam.

17. Afluid energy impact mill according to claim 1 constructed and arranged substantially as described herein and shown in the accompanying drawings.

18. A method of milling when employing apparatus according to claim 1 and substantially as described in any one of the foregoing Examples.