EP0025279A2

EP0025279A2 - A method for the preparation of a uniform solid fuel-oil dispersion

Info

Publication number: EP0025279A2
Application number: EP80302760A
Authority: EP
Inventors: Christopher John Veal; Derek Richard Wall
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1979-08-15
Filing date: 1980-08-12
Publication date: 1981-03-18
Also published as: EP0025279A3; ZA804726B; ZA804725B; KR830003569A; EP0025279B1; KR830003567A; AU6133380A; JPS5628291A; AU6133280A; PL226237A1; EP0025278A3; EP0025278A2; JPS5636592A; PL226236A1; PL125850B1; DE3066980D1

Abstract

A uniform solid fuel-oil dispersion is prepared by grinding a solid fuel, e.g. coal, in an oil medium in a tumbling ball mill until the mean solid particle size is reduced to a value in the range 1 to 10 micron and the dispersion contains 15 to 55% by weight solid fuel expressed as a percentage by weight of the total dispersion.

Description

This invention relates to the preparation of dispersions of solid fuels in liquid fuels in tumbling ball mills and, more particularly, to the preparation of dispersions of coal in oil.
Our British Patent Specification No. 1523193 discloses a method for the preparation of coal oil dispersions which method comprises grinding coal in a medium consisting essentially of gas oil and/or a heavier petroleum fraction until the particle size is reduced to a value below 10 micron and the dispersion contains 15 to 55% by weight coal expressed as a percentage by weight of the total dispersion. 1523193 discloses that grinding can be carried out in vibratory or agitatory ball mills.
Tumbling ball mills are, of course, well known grinding machines and have been used in the past in both batch and continuous processes. They have been used primarily for the dry grinding of friable solids such as cement clinker. In certain applications, however, it is desirable to grind in the wet state, that is, when the solids are suspended in a liquid forming a slurry. The liquid concerned is normally water. An example of such a process is the beneficiation of metal bearing ores. In some cases, e.g. paint manufacture and metal powder processing, light hydrocarbon fractions are employed. The latter tend to be batch processes, however, carried out on a small scale.
Journal Soc. Chem. Ind. Vol 39, 1920, p.395 discloses that "colloidal fuel" may be prepared by grinding coal in oil in tube mills. The term "colloidal fuel" was used loosely at that time to describe relatively coarse suspensions of coal in oil. The most relevant fuel described contained coal particles ground so that at least 90% passed through a 100 mesh screen (B.S.S.) and 85% through a 200 mesh screen (B.S.S.). Such dimensions are far in excess of colloidal dimensions quoted by Glasstone as being in the range 0.2-0.005 micron. (Glasstone,Textbook of Physical Chemistry, Second Edition, Macmillan & Co. Ltd., London, 1953, page 1232.)
It was, in fact, believed that a tumbling ball mill would be incapable of wet milling solid particles such as coal to an average particle size of 10 micron or less, in the presence of a grinding medium such as fuel oil.
We have now surprisingly discovered that a tumbling ball mill can be used in the preparation of such fine dispersions.
Thus according to the present invention there is provided a method for the preparation of a uniform solid fuel-oil dispersion which method comprises grinding the solid fuel in an oil medium in a tumbling ball mill until the mean solid particle size is reduced to a value in the range 1 to 10 micron, preferably 2 to 5 micron, and the dispersion contains 15 to 55% by weight solid fuel, preferably 30 to 45%, expressed as a percentage by weight of the total dispersion.
Suitable friable solids include coals of various ranks, solvent refined coal, coal coke and petroleum coke. The preferable friable solid is bituminous coal.
A suitable oil is a petroleum fuel oil fraction having a viscosity of not more than 6000 seconds, preferably not more than 3500 seconds Redwood No. 1 at 100°F (37.8°C). The required viscosity may be achieved by "cutting back" if necessary with, for example, gas oil.
In the case of certain heavier fuel oil fractions it may be necessary to heat them in order to render them sufficiently mobil to permit use as a grinding liquid, initially in the case of a batch process or at the entry to the mill in the case of a continuous process. However, the action of grinding generates a considerable quantity of heat.
The tumbling ball mill is preferably operated under standard tumbling ball mill conditions, i.e. with the balls occupying 10-50%, most preferably 30-45%, of the internal volume, with the mixture of solid fuel and oil occupying 5 to 45%, and the speed of rotation of the mill about 60 to 95%, preferably 70 to 80%, of the critical speed which is defined as the lowest speed at which a single element of the charge would be held against the internal surface by centrifugal force.
Under these conditions with a viscous grinding liquid the contents of a tumbling ball mill will take up a kidney shaped configuration as the mill rotates. This cascades smoothly with little splashing or surging. Grinding takes place mostly in the interior of the kidney shaped mass and freshly ground surfaces are exposed to the gaseous atmosphere in the mill to a lesser extent than they would be in partially filled vibratory or agitatory ball mills, thus excluding the atmosphere from a substantial proportion of the grinding activity.
If no special precautions are taken, this atmosphere will normally be air. However, in order to reduce the risk of free oxygen contamination still further, in view of the adverse effect which this has on product stability, it is preferred to operate the tumbling ball mill with the contents under an inert blanket, e.g. of nitrogen. This has the further advantage of eliminating the risk of forming hazardous vapour mixtures. Inert gas blanketing is particularly advantageous when grinding at relatively high temperatures, e.g. close to or above the flash point of the oil.
Preferably the solid fuel supplied to the tumbling ball mill is preground to a particle size not greater than 250 micron. This may be achieved by conventional size reducing machinery such as atritors, hammer mills, etc.
Alternatively, lump coal can be supplied to the ball mill and ground down to its final size within the mill.
It is not necessary to premix the preground solid fuel with the oil since the tumbling ball mill acts as a mixer as well as a grinder.
When using a large tumbling ball mill often known as a tube mill, e.g., having a length to diameter ratio of greater than 3:1,it may be desirable to subdivide the mill into a number of compartments separated by screens through which the mixture can flow. In order to achieve more efficient grinding, the size of the balls is progressively reduced from the inlet chamber to the outlet chamber.
It is, of course, desirable to use balls made of a material which does not react with the solid fuel and the oil and which does not wear unduly either itself or the interior surface of the mill. Ball mills usually contain steel balls and these are suitable for the present purpose.
The dispersions will generally be prepared, used and stored at elevated temperature and under these conditions will be more stable than corresponding dispersions prepared in the presence of air.
The solid fuel-oil dispersions are suitable for use in blast furnaces, cement kilns and in industrial, marine and utility boilers.
The stability of the solid fuel-oil dispersion is a function of three important variables - the method of grinding, the final particle size and the concentration of solid in oil. If all three are chosen correctly then the dispersion is of enhanced stability at elevated and ambient temperatures.
At ambient temperature, the dispersion is in the form of a weak, thixotropic gel in which a physical network is formed by solid particles in oil. It is a uniform structure from which the solid particles cannot settle out because they form part of it. This is unlike coal oil dispersions prior to those disclosed in 1523193 which were merely slurries in which the coal particles were suspended in the oil from which they would eventually settle out, or colloidal suspensions.
At elevated temperature, although the gel-like structure is less apparent, the same interactions occur to confer enhanced stability.
If the solid particles are not ground in the oil in the absence of air, the solid particles will become oxidised and interact unfavourably. If the solid particle size is too great, forces will be insufficient to confer stability. The concentration of the solid particles is also critical. If it is too low, the dispersion will be unstable. If it is too high, the dispersion will become too solid like for pumping.
The invention is illustrated with reference to the accompanying drawing, which is a block diagram of a method for producing a coal-oil dispersion according to the present invention, and the Example.
With reference to the drawing:
The coal was passed through the pregrinding system described with reference to the drawing and passed to storage 6, with a maximum particle size of 212 microns.
The oil from storage at 80°C was passed through the heater 8 where its temperature was raised to 120°C.
Near the entry to the mill 9 which was approximately 2m diameter and 10m long coal was added to the oil to give a mixture containing 40% by wt coal. This material was then ground in the mill which was loaded to about 40% volume with steel balls.
The mill was operated at a speed of 24 rpm (80% critical).
Nitrogen was supplied to the interior of the mill and cooling water to the mill bearings.
The mean particle size of the coal after grinding was 3.7 micron as determined by an optical microscope technique.
A sample of the dispersion prepared above showed no signs of settling after standing for 24 hours at 100°C.
Lump coal.is supplied by line 1 to a screen 2 which passes lumps of size below 6.4mm to an atritor 3 which reduces the particle size to 70% wt below 200 mesh. Oversize lumps from the screen 2 are reduced in size in a jaw-crusher 4 and returned to the screen 2. Particles from the atritor 3 move to a classifier 5 from which particles below 212 microns in size are passed by an air lift to storage 6 and oversize particles are returned to the atritor 3 for further comminution.
Oil is supplied by line 7, passed through a heater 8 and powdered coal from storage 6 is added to the oil at the entry to a tumbling ball mill 9. The mill is divided into three chambers separated by screens 10 and 11. The mill supplied with nitrogen by means (not shown) to provide an inert atmosphere within the mill.
The coal-oil mixture passes continuously through the chambers of the mill and is removed by line 12.

Example

The coal was Pleasley Washed Singles with the following ultimate and initial particle size analysis (as received).
The oil was a mixed source heavy fuel oil with a viscosity of 3500 seconds Redwood No. 1 at 100°F (37.8°C). It had the following properties:

Claims

1. A method for the preparation of a uniform solid fuel-oil dispersion which method comprises grinding the solid fuel in an oil medium in a tumbling ball mill until the mean solid particle size is reduced to a value in the range 1 to 10 micron and the dispersion contains 15 to 55% by weight solid fuel expressed as a percentage by weight of the total dispersion.

2. A method according to claim 1 wherein the mean particle size of the solid fuel is reduced to a value in the range 2 to 5 micron.

3. A method according to either of the preceding claims wherein the dispersion contains 30 to 45% by weight solid fuel expressed as a percentage by weight of the total dispersion.

4. A method according to any of the preceding claims wherein the friable solid is coal, solvent refined coal, coal coke or petroleum coke.

5. A method according to claim 4 wherein the friable solid is bituminous coal.

6. A method according to any of the preceding claims wherein the solid fuel is preground to a particle size not greater than 250 micron before grinding in the oil medium.

7. A method according to any of the preceding claims wherein the oil is a petroleum fuel oil fraction having a viscosity of not more than 6000 seconds Redwood No. 1 at 37.8°C.

8. A method according to claim 6 wherein the oil is a petroleum fuel oil fraction having a viscosity of not more than 3500 seconds Redwood No. 1 at 37.8°C.

9. A method according to any of the preceding claims wherein the tumbling ball mill is operated with the contents under an inert gas atmosphere.

10. A method according to any of the preceding claims wherein grinding is carried out at elevated temperature.

11. A method according to any of the preceding claims wherein the mill is rotated at 60 to 95% of its critical speed.