GB2273671A

GB2273671A - Method of simulataneously milling and drying

Info

Publication number: GB2273671A
Application number: GB9325202A
Authority: GB
Inventors: Andrew John Haddow
Original assignee: Tioxide Group Services Ltd
Current assignee: Tioxide Group Services Ltd
Priority date: 1992-12-24
Filing date: 1993-12-09
Publication date: 1994-06-29
Anticipated expiration: 2013-12-09
Also published as: GB2273671B; GB9226993D0; GB9325202D0

Abstract

A method of simultaneously drying and milling a wet particulate, material comprises passing the material in the form of a paste to be entrained by gas above ambient temperature through a venturi (2). The gas is introduced through a first jet nozzle (1) thus entraining the material which passes through first venturi (2), impacts upon a surface (5), is entrained in a stream of gas introduced through a second jet nozzle (6) and is passed through a second venturi (7) into a cylindrical separation chamber (10). The gas and particulate material are separated in the chamber (10), the gas exiting through exhaust port (12) and the solid exiting through (13). Titanium dioxide simultaneously milled and dried by this method has a quality equivalent to titanium dioxide which had been separately dried and milled. <IMAGE>

Description

METHOD OF SIMULTANEOUSLY MILLING AND DRYING This invention relates to a method of milling a wet particulate material and in particular to a method of simultaneously milling and drying said wet particulate material.

A number of processes, for example pigment manufacture, produce a wet particulate material which must be dried and also reduced in size to produce the finished product of the process.

Usually, the wet material is dried and then milled to reduce its particle size. However, it would be advantageous to combine the two steps of drying and milling since the processing time is thereby reduced and the cost of installing the necessary equipment is lower.

It is an object of the present invention to provide a method for simultaneously milling and drying a wet particulate material which method provides a finished product of equivalent quality to that produced by separate drying and milling.

Hence, according to the invention, a method of drying and milling a wet particulate material comprises passing a gas having a temperature above ambient temperature through a first jet nozzle while feeding said wet particulate material in the form of a paste through an inlet to be entrained by said gas, passing entrained material and gas through a first venturi axially in-line with said first nozzle and spaced therefrom by said inlet to impact on an impact mill surface mounted at a reflective angle to the axis of said first jet and said first venturi and to be reflected therefrom, feeding a gas to a second jet nozzle spaced from said impact mill surface and having a longitudinal axis transverse to the reflected line of the axis of said first jet nozzle and said first venturi to entrain material reflected from said impact mill surface, passing entrained reflected material and gas through a second venturi axially in line with second jet nozzle into a cylindrical separation chamber having a circumferential wall and having outlets for exhaust gas and particulate material and feeding means extending through said circumferential wall comprising said second venturi, separating the milled particulate material from said gas and discharging said separated milled particulate material and said gas separately from said separation chamber.

The particulate material which is to be dried and milled according to the method of the invention is introduced into a gas stream in the form of a paste. The proportion of solid material in the paste depends to some extent upon the nature of the particulate material but normally the solids content is greater than 60 per cent by weight. Preferably the solids content is from 65 per cent to 95 per cent by weight.

The method of the invention is of particular use in drying and grinding particulate material to a small controlled size range and particularly for those types of materials, such as pigments, where properties of the product can be changed by changing the particle size.

Inorganic pigments such as titanium dioxide, silica, silicates, aluminium oxide, antimony pigments, calcium pigments, carbon black, iron oxide, lead oxide, zinc oxide and zirconia are all suitable for drying and grinding according to the process. Other materials such as organic coloured pigments and pharmaceuticals can be dried and ground using the method and employing a suitable grinding gas.

The grinding gas which is employed has a temperature which is higher than ambient temperature. This temperature is adjusted to provide sufficient heat to dry the particulate material during the grinding process. Usually the gas has a temperature of from 300"C to 900"C and preferably from 300"C to 550"C.

Any suitable gas can be used to entrain and transport the particulate material to be dried and milled according to the process.

An inert gas can be used as can air. Preferably, however, the gas employed is superheated steam and the degree of superheating used is governed by the temperature at which the steam is injected.

Normally the gas supplied to the first jet nozzle and the second jet nozzle will have a pressure of at least 0.5 MPa and, preferably, a pressure of at least 1 MPa.

The method of the invention can be employed so as to produce any desired rate of output of dry, milled powder from a laboratory scale up to a full sized factory production rate. The particular sizes of the first and second jet nozzles, first and second venturis and cylindrical chamber employed depend on the desired output of milled powder as does the rate of feed or flow of carrier gas through the particular jet nozzles.

The first and second jet nozzles and associated venturi throats can have sizes chosen from within a wide size range and the gases fed through the first and second nozzles can be fed under a wide range of pressures chosen to match the particular jet sizes and product characteristics required. One particular preferred method of carrying out the invention utilises a ratio of throat area of the first venturi to the area of the first jet nozzle of about 11:1 and a ratio of the second venturi throat area to the area of the second jet nozzle of about 16:1 with a gas having a pressure of about 2 MPa.

Separate supplies of gas are fed to the first and second nozzles and in a particular method the rate of feed is such that the second nozzle is supplied with steam flowing at a rate of up to twice that flowing to the first nozzle.

If desired an additional supply of gas is introduced into the separation chamber through one or more inlets in the circumferential wall of the chamber. The total amount of gas fed to the separation chamber through these additional inlets through the circumferential wall will depend upon the desired amount of milling but normally the amount is less than or equal to that supplied to the mill through the first jet nozzle.

The mill used in accordance with the present invention can be constructed of any appropriate material such as stainless steel or various parts of the particular mill can be formed of ceramic material, if desired. An impact surface formed of suitable ceramic material is less liable to introduce unwanted contamination of the product by small amounts of iron.

One form of mill which is useful in the method of the invention will now be described by way of example only with reference to the accompanying drawings in which Figure 1 is a diagrammatic view showing part in sectional elevation and Figure 2 is a part sectional plan view.

As shown in Figure 1 the mill consists of a first jet nozzle 1 axially aligned but spaced from a first venturi 2. Between the nozzle 1 and venturi 2 is an inlet 3 for particulate material from a hopper 4. An impact surface 5 is mounted to receive material from the venturi 2 and to reflect the milled particulate material towards a second jet nozzle 6 supplied with a second venturi 7 axially aligned with the jet nozzle 6.

The second venturi 7 forms a powder feed device to feed powder through a powder inlet 8 in the wall 9 of a cylindrical chamber 10.

The cylindrical wall 9 of cylindrical chamber 10 is provided with a number of spaced gas inlets 11 directed to feed additional quantities of gas into the cylindrical chamber 10. The cylindrical chamber 10 is provided with a centrally located gas offtake 12 opposite an axially aligned milled dry powder offtake 13.

In operation the particulate material to be dried and ground is fed from hopper 4 through the feed inlet 3 and becomes entrained in gas supplied through jet nozzle 1. The gas, together with the entrained material is fed through venturi 2 and directed on to the impact surface 5 where milling takes place due to impact with the surface prior to being reflected towards the second jet nozzle 6. Gas flowing from the second jet nozzle 6 entrains the material reflected from the impact surface 5 and due to the influence of the second venturi 7 a reduction in pressure occurs together with a positive increase in the rate of flow of the powdered material to be ground from hopper 4 on to the impact surface 5. The impacted material after entrainment and passage through the second venturi is fed substantially tangentially into an inlet of the cylindrical chamber 10 through the feed inlet 8 where additional supplies of gas are introduced through the gas inlet 11 augmenting the flow of gas within the chamber 10 and increasing the milling effect occurring therein due to impact of the particles with each other. As the gaseous fluid and milled particles are transported towards the central regions of the chamber 10 the speed of the flowing gas becomes insufficient to support the milled particles which exit the chamber through the particle offtake 13 and exhaust gas, together with any very small particle size material, exhausts through the gas exhaust 12.

The milled particulate material obtained is substantially dry and of a similar quality to material which has been separately dried and subsequently. milled.

The invention is illustrated by the following example.

EXAMPLE A titanium dioxide filter cake obtained from a high pressure filter which formed part of the separation equipment in a titanium dioxide plant was fed to a grating machine. The filter cake contained 31% water and 69% titanium dioxide by weight and was broken up by the grating machine into pieces which were approximately 2.5 mm in diameter and 6 mm long. This material was fed at a rate of 160 kg per hour by means of a belt feeder into the hopper (4) of a mill as described above.

Steam at a pressure of 2 MPa and a temperature of 320"C was fed to the first and second jet nozzles at a rate of 163 kg per hour and 227 kg per hour respectively. The steam exiting the mill was maintained at 140cm.

The product obtained was substantially dry and had a quality equivalent to that of titanium dioxide which had been separately dried and micronised.

Claims

1. A method of drying and milling a wet particulate material comprising passing a gas having a temperature above ambient temperature through a first jet nozzle while feeding said wet particulate material in the form of a paste through an inlet to be entrained by said gas, passing entrained material and gas through a first venturi axially in-line with said first nozzle and spaced therefrom by said inlet to impact on an impact mill surface mounted at a reflective angle to the axis of said first jet and said first venturi and to be reflected therefrom, feeding a gas to a second jet nozzle spaced from said impact mill surface and having a longitudinal axis transverse to the reflected line of the axis of said first jet nozzle and said first venturi to entrain material reflected from said impact mill surface, passing entrained reflected material and gas through a second venturi axially in line with second jet nozzle into a cylindrical separation chamber having a circumferential wall and having outlets for exhaust gas and particulate material and feeding means extending through said circumferential wall comprising said second venturi, separating the milled particulate material from said gas and discharging said separated milled particulate material and said gas separately from said separation chamber.

2. A method according to claim 1 in which the particulate material is titanium dioxide, silica, a silicate, aluminium oxide, an antimony pigment, a calcium pigment, carbon black, iron oxide, lead oxide, zinc oxide or zircoma.

3. A method according to claim 1 in which the particulate material is an organic coloured pigment or a pharmaceutical.

4. A method according to any one of the preceding claims in which the paste has a solids content greater than 60 per cent by weight.

5. A method according to claim 4 in which the solids content is from 65 per cent to 95 per cent by weight.

6. A method according to any one of the preceding claims in which the gas has a temperature from 300"C to 900"C.

7. A method according to claim 6 in which the temperature of the gas is from 300"C to 550"C.

8. A method according to any one of the preceding claims in which the gas is an inert gas or air.

9. A method according to any one of claims 1 to 7 in which the gas is superheated steam.

10. A method according to any one of the preceding claims in which the gas is supplied to the first jet nozzle at a pressure of at least 0.5 MPa.

11. A method according to claim 10 in which the pressure of the gas is at least 1 MPa.

12. A method according to any one of the preceding claims in which the ratio of the throat area of the first venturi to the area of the first jet nozzle is about 11:1, the ratio of the second venturi throat area to the area of the second jet nozzle is about 16:1 and the gas used has a pressure of about 2 MPa.

13. A method according to any one of the preceding claims in which the second nozzle is supplied with steam flowing at a rate up to twice that flowing to the first nozzle.

14. A method according to any one of the preceding claims in which an additional supply of gas is introduced into the separation chamber through one or more inlets in the circumferential wall of the chamber.

15 A method according to claim 14 in which gas is supplied to the inlets in the wall of the chamber in an amount less than or equal to the amount of gas supplied to the first jet nozzle.

16. A method according to any one of the preceding claims in which the impact mill surface is formed from a ceramic material.

17. A method of drying and milling a wet particulate material substantially as hereinbefore described with reference to the Example.