Methods and apparatus for treating materials in liquids

The method for treating materials in liquids involves passing them with the liquid through a processing gap formed by a flow passage whose walls are closely spaced and move relative to one another transversely to the direction of flow, thereby producing "supra-kolmogoroff" mixing eddies in the gap, and at the same time applying ultrasonic longitudinal pressure oscillations that reverberate between the two closely spaced surfaces into the gap transversely to the direction of flow from transducers mounted on one wall, thereby producing "sub-Kolmogoroff" mixing eddies therein. The method is capable of rapidly producing relatively thick slurries of sub-micrometer particles that otherwise can take several days in conventional high shear mixers and ball or sand mills, or of rapidly dissolving difficultly soluble gases and powders into liquids. One type of apparatus consists of two circular coaxial plates, one stationary while the other is rotated, the opposed faces forming the processing gap being mirror finished; the rotational axis can be vertical or horizontal. Another type consists of an inner cylinder rotatable about a horizontal axis inside a stationary hollow outer cylinder with the facing walls closely spaced at their lowermost parts to form the processing gap. The ultrasonic transducers are mounted on the stationary member. The liquid/material mixture may be recirculated through a single mill or may be passed through a series of mills. The mixture may be pretreated in a high capacity reverbatory ultrasonic mixer before being fed to the mill or series of mills.

FIELD OF THE INVENTION 
The invention is concerned with methods and apparatus for treating 
materials in liquids, especially with methods and apparatus for mixing, or 
suspending, or dispersing, or dissolving, or deagglomerating, or 
comminuting materials, and more especially but not exclusively to such 
methods and apparatus employing finely divided ceramic materials in slurry 
suspensions thereof. 
REVIEW OF THE PRIOR ART 
Increasingly a number of manufacturing processes require the use of finely 
divided starting materials of, for example, particle size less than 5 
microns, frequently of particle size less than 1 micron, and increasingly 
of particle size as small as 0.1 micron. This is particularly the case 
with processes for ceramics, where the use of very finely-divided raw 
materials makes it possible to produce articles having improved 
properties, such as improved strength, mechanical and thermal shock 
resistance, and of maximum or near maximum theoretical density after 
firing or sintering. The particle size distribution is also an 
increasingly important criterion, and particularly the requirement that 
all of the particles are of a size within a narrow range about the nominal 
value. In industrial practice the achievement of such uniformity of 
particle size is extremely difficult and considerably increases the cost 
of production. 
For example, the manufacture of a ceramic part may require that the 
starting material be of average particle size 0.3 micron, with the 
expectation that the particle size distribution will have the typical 
bell-shape characteristic, i.e. the majority of the material (e.g. about 
70% by weight) is of about the specified particle size, while small 
portions (e.g. about 15% each) are oversize and undersize, the maximum 
particle size being about 1.0 micron. Even though the material was milled 
by its manufacturer to be of that average size, it is unlikely that as 
received by its ultimate user it is still in the same state of fine 
division, since with all particles, and particularly such fine particles, 
agglomeration begins immediately the powder leaves the grinding mill, and 
this continues during subsequent handling. The materials are frequently 
pelletized to facilitate their transport and handling, and must 
subsequently be de-pelletized by grinding before they can be used. The 
result is that at least a portion of the material is outside the specified 
particle size range, and there is a high probability it includes a large 
number of particles which are so big that their presence causes defects in 
the resultant sintered product. 
High speed stone (carborundum) and colloid mills are known for use in 
pigment dispersion in paints and consist essentially of two accurately 
shaped smooth stones working against each other, one of which is held 
stationary while the other is rotated at high speed (3600 to 5400 rpm) 
with a gap that is regarded by this industry as very small separating the 
two relatively movable surfaces. Thus, typically the spacing between the 
two faces is adjustable from positive contact to an appropriate distance, 
which with such mills is usually from a minimum of 25 micrometers to as 
much as 3,000 micrometers, but is usually of the order of 50-75 
micrometers. In the typical stone mill the charge feeds through a 
truncated conical gap to the grinding region, which has the shape of a 
flat annular ring, while in a colloid mill the grinding region itself has 
the shape of a truncated cone. The dispersion of the pigment in its liquid 
vehicle is produced by the viscous laminar flow that takes place between 
the parallel faces of the stones as the material is fed into the gap by 
gravity, or under pressure. A separation gap of 75 micrometers is said to 
produce a particle dispersion having an average particle size of 2-3 
micrometers, although the particle size distribution is not given, and 
substantially larger particles are certainly present. 
SUMMARY OF THE INVENTION 
It is a principal object of the invention to provide new methods and 
apparatus for the mixing, or for the suspension, or for the dispersion, or 
for the solution of gases and powdered materials in liquid vehicles, or 
for the deagglomeration, or for the comminution of powdered materials in 
slurry suspensions thereof. 
It is a more specific object to provide such methods and apparatus that are 
particularly suited for the deagglomeration or comminution of very finely 
divided ceramic raw materials in slurry suspensions thereof. 
In accordance with the present invention there is provided a new method for 
treating materials in liquids comprising the mixing of flowable materials, 
or the suspension or solution of gaseous or powdered materials in flowable 
liquid vehicles, or the deagglomeration and comminution of finely divided 
materials in flowable slurry suspensions thereof, the method comprising 
the steps of: 
passing the material to be treated through a processing passage between two 
closely spaced surfaces provided by respective mill members, the passage 
having an inlet thereto and an outlet therefrom establishing a 
corresponding flow path between them through the passage; 
while the material is passing in the processing passage moving at least one 
of the mill members so as to move the surfaces relative to one another in 
a direction transverse to the direction of the material flow in the flow 
path to subject the material to the effect of such relative movement; and 
at the same time applying longitudinal acoustic pressure oscillations so as 
to reverberate in the processing passage between the two closely spaced 
surfaces and so that the material in the passage is subjected to the 
effect of such oscillations and the reverberations. 
Preferably the spacing between the mill member surfaces and their speed of 
relative movement is such as to produce "supra-kolmogoroff" eddies having 
Reynolds numbers greater than unity in the material passing between them, 
while the longitudinal pressure oscillations simultaneously reduce in the 
material "sub-kolmogoroff" eddies whose size is smaller than the smallest 
of the "supra-kolmogoroff" eddies. 
Also in accordance with the invention there is provided new apparatus for 
treating materials in liquids comprising the mixing of flowable materials, 
or the suspension or solution of gaseous or powdered materials in flowable 
liquid vehicles, or the deagglomeration or comminution of finely divided 
materials in flowable slurry suspensions thereof, the apparatus 
comprising: 
an apparatus frame; 
first and second mill members mounted by the apparatus frame and providing 
respective first and second surfaces closely spaced from one another to 
form a processing flow passage between them for the flow therein of the 
material to be treated; 
the passage having an inlet thereto and an outlet therefrom establishing a 
corresponding flow path between them through the passage; 
motor means operatively connected to at least one of the mill members and 
moving the respective mill member so as to move the surfaces relative to 
one another in a direction transverse to the direction of flow of the 
material in the flow path to subject the material to the effect of such 
relative movement; and 
ultrasonic pressure generating means mounted on at least one of the mill 
members so as to apply ultrasonic pressure oscillations generated thereby 
into the processing passage transversely to the direction of flow of 
material in the flow path so as to reverberate in the passage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the system illustrated by FIG. 1, in which finely divided powder is to 
be uniformly dispersed in a liquid vehicle and ground (with any necessary 
deagglomeration) to a smaller particle size, powder from a powder supply 
hopper 10 is fed to a mixing tank 12, while a dispersion vehicle is fed 
from a supply tank 14, a preliminary rapid coarse dispersion being 
obtained by circulating the mixture continuously in a closed circuit 
comprising the reservoir 12, a pump 16, and a high flow capacity 
motionless reverbatory ultrasonic mixer 18. Preferably the mixer 18 is of 
the type described and claimed in my prior U.S. Pat. No. 4,071,225, the 
disclosure of which is incorporated herein by this reference. Briefly, 
such a mixer comprises an elongated chamber of thin rectangular cross 
section having the two parallel longer walls formed by two flat, very 
closely spaced plates 20, each of these plates having a plurality of 
ultrasonic generators 22 mounted on its exterior so as to direct the 
ultrasonic pressure oscillations into the chamber and towards the opposite 
wall, the oscillations from the opposed generators interfering with one 
another in a manner which produces intense small eddies that are 
particularly effective to produce mixing and dispersion of the powder into 
the medium. 
As is well known to those skilled in this art, the thorough dispersion of 
fine powders in a liquid dispersing vehicle using the conventional high 
shear mechanical stirring mixers, or ball and sand mills, is a lengthy and 
tedious process, often requiring several days to obtain an acceptable 
dispersion. There are a number of reasons for this, such as the increased 
surface area to be wetted resulting from the decrease in particle size, 
the inherent difficulty of wetting such fine particles, and the difficulty 
of deagglomerating the agglomerates that inevitably are present. A 
motionless reverbatory ultrasonic mixer such as that disclosed and briefly 
described above is able to produce acceptable dispersions in periods as 
short as 5-15 minutes, although with some processes it may be preferred to 
increase the mixing period to perhaps 30-45 minutes. 
Although in this specific system a single reverbatory ultrasonic mixer is 
employed, if a completely continuous system is preferred the single mixer 
can be replaced with a series or cascade of such mixers of the necessary 
continuous flow capacity. 
The dispersion vehicle, whether aqueous or non-aqueous, will most 
frequently include a dispersing agent or agents and usually will also 
include other functional additives, such as binders, plasticizers and 
lubricants. The relative proportions of the powder or powders, functional 
additives, and of the dispersion vehicle, are usually made such that the 
final dispersion is of minimum liquid content while giving a flowable 
slurry (which may be characterised as being "soup-like") capable of being 
circulated as described. 
Upon completion of this initial dispersion and mixing step the coarsely 
dispersed slurry is discharged to a holding tank 24 and fed from there via 
a pump 26 as a uniform continuous feed to a series or cascade of a 
plurality of plate mills 28 of the invention. A pump 26 is also provided 
between each successive pair of mills, so as to be able to control the 
rate and the pressure at which the slurry is fed to the respective mill. 
Referring now particularly to FIGS. 2-4, each mill comprises a baseplate 
30 supporting a stationary cylindrical base member 32. A circular 
vibratory face plate member 34 is securely mounted on a ring or annulus 36 
of resilient material, for example by being cemented thereto, and this 
annulus is in turn securely mounted in a counterbore 38, for example by 
being cemented therein, provided at the upper end of the cylinder 32, so 
that the plate is securely mounted on the base member. A small radial 
clearance is provided between the cylindrical edge of the face plate 34 
and the facing cylindrical wall of the counterbore, so that the plate can 
vibrate freely vertically, but is constrained against any appreciable 
transverse motion. The plate is vibrated by a plurality of ultrasonic 
generators 40 attached to its underside and uniformly circumferentially 
spaced about the plate center point 42, the generators being connected to 
a suitable electrical power source (not shown). 
A circular rotatable face plate member 44 is mounted above the plate 34 for 
rotation about a vertical axis 46 that passes through the center point 42 
by drive means comprising a vertical standard 48 attached to the base 
plate 30. A motorised drive head 50 is mounted on the standard and has a 
drive shaft 52 extending vertically downward therefrom, the plate member 
44 being attached to the lower end of the shaft at its respective center 
point, which also lies on the axis 46, so as to rotate therewith. The 
vertical height or spacing D (see FIG. 5) between the plate member 
surfaces, and consequently the vertical height of the flow passage, is 
accurately adjustable, either by moving the head 50 vertically on the 
standard, and/or by moving the shaft 52 vertically in the head, using any 
suitable micrometer system as will be well known to those skilled in the 
art. The plate member 44 is pressed strongly downwards, either by suitable 
spring or weight means applied via the drive head and the shaft 52, in 
order to maintain the small processing gap between the facing surfaces of 
the two plate members 34 and 44 at the desired value in the presence of 
the material flowing between them, as will be explained below. The 
operation of the mill generates sufficient heat that cooling greater than 
that which would be obtained by rotation of the plate 44 in air is 
desirable; to this end a cylindrical casing is attached to the 
circumference of the plate 44 and forms a coolant reservoir into which 
liquid coolant, such as cooled water, is delivered from a delivery pipe 
58, and from which the coolant is removed by a pump (not shown) via an 
outlet pipe 60. 
The predispersed slurry from the storage tank 24 is fed into the first mill 
28 in the series via a delivery pipe 62, which includes a flexible 
connection 64 so as not to interfere with the vibrations of the plate 34. 
The slurry enters between the plate members through a cylindrical hole 66 
in the center of the plate 34, this hole thus being the inlet to the 
processing flow passage 68 constituted by the corresponding circular 
space, the slurry flowing radially outwards in the treating zone 
constituted by the processing passage. Eventually the slurry reaches the 
circular outer edge of the plate 34 and the cylindrical gap between the 
adjacent plate edges constitutes the outlet from the passage; the slurry 
spills over the edge into a circular, upwardly open, downwardly inclined, 
collection trough 70, this trough completely surrounding the stationary 
base member 32 and delivering the slurry to the succeeding pump 26, and 
thus to the succeeding mill 28. 
During its flow in the passage 68 the slurry is subjected both to the 
effect of the relative rotation between the two plate members, and also to 
the effect of the longitudinal pressure oscillations or vibrations from 
the generators 40, these effects combining as will be discussed below to 
produce within a much reduced period of time a much more complete 
dispersion and wetting of the solid powdered material entrained in the 
slurry, together with the desired deagglomeration and comminution thereof, 
than has been possible with conventional high shear mixers. 
Typical fine powder materials that will be processed using the apparatus of 
the invention are alumina, silica and zirconia, all of which are available 
commercially as agglomerated primary particles of 5 micrometers or less, 
and particularly are available as agglomerated primary particles of the 
nominal size range 0.3-1 micrometer. The quantities of the powdered 
material and the functional additives that are introduced into the 
dispersion vehicle will of course depend upon the purpose of the slurry, 
but usually it is desired to keep the quantities of both the dispersing 
vehicle and the additives as low as possible to facilitate subsequent 
processing. Its consistency needs to be kept relatively "soup-like" to 
permit its free flow through the relatively narrow flow processing 
passages 68 of the mills, and a viscosity in the range of about 10-100 
centipoises will usually be satisfactory. 
In that this embodiment of the invention employs two relatively rotating 
plates as the mill members it is referred to herein as a plate mill, and 
in a particular preferred embodiment the two plate members are both of 25 
cm (10 ins) diameter and of 6.25 mm (0.25 in) thickness and are of silicon 
carbide, preferably diamond coated on their facing surfaces, both surfaces 
having a mirror finish and in this embodiment preferably being flat to a 
limit of 500 nanometers over 25 cms. Flatter surfaces are possible, but in 
this particular embodiment are not necessarily economical or essential. 
The range of flatness preferred for the apparatus of the invention, 
depending upon its particular application, is from 5 nanometers to 10 
micrometers per 25 cm. The diamond layer can be either crystalline or 
amorphous, and is applied by ion implantation, or by similar methods which 
by the nature of the process will replicate the original flatness of the 
base plate. 
The maximum height of the vertical gap D between the two plate surfaces is 
of course indefinite, since they will usually need to be separated for 
maintenance and inspection, while the minimum height during operation will 
be as small as 1 micrometer or less, which is the processing gap that will 
usually be required for processing the smallest particle size slurries, 
while permitting an adequate flow of slurry between the plates. In normal 
operation the processing gap size is correlated with the average particle 
size of the slurry, and in a series of mills will be progressively smaller 
from the first to the last mill. The range of gap sizes to be employed is 
from 1 to 500 micrometers, while the usual range of gap sizes for the 
processing of powdered materials is 1-10 micrometers; the preferred range, 
especially for the processing of ceramic raw powders is 1-5 micrometers. 
As described above, the processing gaps shown in the embodiments 
illustrated herein are not to scale, but are exaggerated for clarity of 
illustration. The processing of any particular slurry will usually involve 
a particular protocol which inter-relates the process time and the passage 
height of the successive mills; thus the process is initiated in the first 
mill in which the plates are relatively far apart in case any 
exceptionally large agglomerates are present, and the spacings in the 
subsequent mills are progressively reduced as the process continues and 
the particle size is reduced. It will usually be most effective to operate 
an individual mill with a relatively limited particle size range, and for 
example a mill with a feed in the range 0-100 micrometers will be employed 
to produce a product in the range 0-1 micrometer, (0-1,000 nanometers), 
while one with a feed in the range 0-1.0 micrometer will be employed to 
produce a product in the range 0-0.2 micrometer, (0-200 nanometers). 
Similarly, a mill with a feed in the range 0-0.2 micrometers will be 
employed to produce a product in the range 0-0.08 micrometer, (0-80 
nanometers). 
With such small gaps between the relatively moving members it is found that 
the viscosity of the flowable material is the controlling factor in the 
movement of the material through the processing gap. Thus the material 
clings to the two surfaces in the form of respective boundary layers, and 
they are so closely spaced that they engage one another without the 
presence of any intervening layer, the layer adhering to the moving plate 
is therefore dragged in contact with that contacting the stationary plate, 
and it is therefore this relative motion between the two surfaces that 
controls the flow of material in the processing passage. The thin layers 
between the plate members that are characteristic of the invention require 
the plate members to be relatively rigid and to be pressed strongly 
together in order to maintain them. The close spacing and thin layers also 
permit the quick and effective grinding of any very large particles that 
are present in the material, and this grinding action is also facilitated 
by the strong pressing of the plates toward each other. It is an advantage 
of the methods and apparatus of the invention that, owing to the much 
smaller processing gap, as compared for example to my own motionless 
reverberatory ultrasonic processor, or the high speed stone and colloid 
mills described above, it is no longer necessary to provide oscillators on 
both surfaces of the processing passage in order to obtain reverbatory 
action and sufficient intensity to obtain "sub-kolmogoroff" eddies. This 
permits simplification of the construction of the mill and avoids the need 
to provide oscillators and an electrical supply to the moving plate 
member. The size, number and spatial distribution of the ultrasonic 
generators 40 will of course be specific for the particular mill, and as a 
specific example only, in the mill described herein ten transducer 
generators are provided uniformly spaced in a single circle, each 
generator having an output of about 50 watts and operating with a range of 
frequencies 30 kHz to 50 kHz, which is the preferred range. The usual more 
extended range that will be used, depending upon the specific mill design, 
will be 8 kHz to 100 kHz. 
As was described above, it is well known to those skilled in the art of the 
production of slurries of ceramic materials that with small particles, 
even with high-power, high-shear mixers a relatively long period of 
"aging" is required to obtain complete dispersion, and this period is not 
shortened appreciably by increases in mixing power or in the shear 
velocity, the latter being produced by increasing the speed of rotation of 
the stirrer. A study by Dr. A.N. Kolmogoroff of such mixing processes gave 
what appears to be a possible explanation for the known fact that 
initially mixing proceeds rapidly, but then slows dramatically. He showed 
that the mixing depended upon the production of eddies, and that with 
conventional mixers using, for example, water as the dispersion vehicle 
and at a process temperature of 20.degree. C., it was impossible to obtain 
eddies of diameter less than about 10 to 20 micrometers. Liquid elements 
of smaller size than this became part of these smallest eddies and were 
shielded against the effect of turbulence, so that mass transfer would no 
longer be governed by convection but by the much slower molecular 
diffusion as a result of the concentration gradients. The smallest 
Kolmogoroff eddy that can be produced by these mixers is obtained when the 
Reynolds number approaches unity. This therefore explained the need for an 
"aging" period, during which this slower molecular diffusion could take 
place, and why it was not possible to reduce the overall time appreciably 
by increase in stirring power or shear velocity. 
There have been numerous proposals for the use of longitudinal pressure 
oscillations in various processes and apparatus, many of which have not 
proven to be commercially feasible owing to the high exponential 
attenuation (1/D, where D = vessel diameter or wall distance) of such 
oscillations. The eddies or vortices produced by such oscillations can be 
made to be much smaller than those produced by high shear mixers, 
increasing the rate of mass transfer with system elements in the micron 
and sub-micron ranges, but apparatus in which the oscillations are applied 
to a liquid or slurry moving in a channel with stationary walls, such as 
in the motionless reverbatory mixer 18, have been found to have their own 
problems, especially with small particle slurries. It has been found 
difficult to space the vibrating walls apart less than a few millimetres, 
and to maintain the opposed walls with uniform spacing as they are 
vibrated, but unless the walls are very closely spaced insufficient sound 
intensity is generated. Very close wall spacing in turn has been found 
frequently to produce oscillations which cause agglomeration instead of 
deagglomeration. Furthermore, high velocity eddies with Reynolds numbers 
larger than unity ("supra-kolmogoroff" eddies) of the type provided by 
mechanical high shear mixers were infrequent. It is further found that the 
walls tend to deform in shape with time, and that it is difficult to 
arrange for their adequate cooling without interfering with the placement 
and operation of the transducers or oscillators. An additional unexpected 
difficulty is that the moving fluid often "channels" in its flow through 
the passage, thus receiving non-uniform treatment. 
The methods and apparatus of the present invention make use of the 
discovery that fine particle fluids and slurries can be more efficiently 
treated by a combination of "macromixing" the flowable material between 
two relatively moving surfaces, which surfaces are sufficiently closely 
spaced and are moved relative to one another at sufficient speed to 
produce "supra-Kolmogoroff" eddies, and simultaneously "micromixing" by 
the application of the reverberatory longitudinal pressure oscillations 
between the moving closely spaced surfaces via at least one of the 
surfaces to simultaneously produce "sub-kolmogoroff" eddies which are 
smaller than, and are able to interact with, the smallest of the 
"supra-kolmogoroff" eddies for an unexpected synergistic and beneficial 
effect in mixing, dispersing, comminuting, deagglomeration, etc. Thus, in 
this embodiment the surfaces of the two discs 34 and 44 are moved relative 
to one another transverse to the direction of material flow at a distance 
apart sufficiently small, and a rotational speed sufficiently high, to 
produce these "supra-kolmogoroff" mixing eddies in the narrow passage 68, 
while at the same time reverberatory longitudinal pressure oscillations of 
the required high frequency and power are applied to produce the much 
smaller "sub-kolmogoroff" eddies required to penetrate and interact with 
the "supra-kolmogoroff" eddies in order to reach and affect the small 
particles entrained in the fluid. This close spacing and relative movement 
of the surfaces is also required to ensure that the longitudinal 
oscillations do not instead cause agglomeration of the particles, instead 
of the required deagglomeration. 
Thus, although it is well known that as a fluid flows in a passage the 
velocity gradient across the passage cross section is non-uniform, being 
smallest in the boundary layers at the surfaces and increasing towards the 
center of the cross section, it has not to my knowledge been realized that 
by causing relative movement of closely spaced passage wall as in a mill 
of the invention, controlled "supra-kolmogoroff" eddies can be generated 
extending transversely, circumferentially and radially in the otherwise 
laminar flow in the boundary layers, producing "macro" mixing of the 
fluid, into which can be added the "micro" mixing provided by the 
"sub-Kolmogoroff" eddies available by the action of the longitudinal 
pressure oscillations. 
Another effect obtained with the mill of the invention is a mechanical 
crushing of any particles larger than the passage height, the relative 
parallel movement of the walls ensuring that such particles cannot become 
jammed in the passage and eventually begin to block it, or prevent further 
closing of the plates together without damage to their surfaces. The 
mechanism by which the vertical pressure is applied can also operate to 
prevent jamming if grossly oversize particles are inadvertently present. A 
further effect of the relative movement is that it supplements the 
pressure applied to the slurry by the circulating pump to enable the mill 
to treat slurries that are thicker and of considerably greater viscosity 
than would be possible in its absence; this is especially important with 
ceramic slurries that are eventually to be molded and where the minimum 
amount of suspension vehicle is used. 
With the mill described, since the two plates rotate relative to one 
another, the relative circumferential linear transverse movement between 
them will vary progressively from zero on the rotational axis to a maximum 
at the circumferences, so that a preferred minimum threshold value for 
such movement will only be obtained at some radial distance from the axis. 
For the 25 cm (10 ins) diameter plates used in this embodiment the linear 
velocity of their operative surfaces relative to one another should be 
between 0.5 and 2000 meters per minute (20 and 80000 inches per minute) 
and with a rotary structure such as that described it will depend upon the 
rate of rotation of the upper plate; in this specific embodiment measured 
at a mean radius of 6 cm (2.5 ins) this should be between about 1 and 400 
revolutions per minute, while the preferred rate is between 50 and 200 
revolutions per minute. 
There is therefore the possibility of decreasing the cost of the plates 34 
and 44 by forming the highly polished and flat operative surfaces only at 
their annular outer portions, and an embodiment taking advantage of this 
is illustrated by the simplified FIG. 5, in which only essential elements 
are shown. The hole 66 in the plate 34 has been extended radially and the 
facing surfaces are only fully finished between the cylindrical plane 74 
and the outer circumferential plate edges. 
In another embodiment illustrated by FIG. 6 at least one of the facing 
surfaces of the plates (the surface of the plate 44 in this embodiment) is 
formed to provide the flow path gap 68 so that it decreases progressively 
in height from the center radially outwards. The annular shaped radially 
outer portion of the flow passage where the passage walls are sufficiently 
closely spaced therefore constitutes a treating zone in which the required 
action takes place. Such a mill in which the slurry is to be processed in 
a single pass will have the gap at the center of the maximum value 
required to process the material, while the gap at the circumference is 
the minimum value for this purpose. 
FIG. 7 illustrates an embodiment in which the two plate surfaces are 
conical with both pieces pointing downward; with such a structure the 
material must move upward against gravity in its flow through the passage, 
helping to ensure that the material is fully treated. 
Although the method and apparatus of the invention have been described in 
their application to the treating of ceramic slurries, it will be apparent 
that they are applicable generally to the mixing of materials, such as the 
mixing of two mutually non-soluble or difficultly soluble liquids, the 
solution of materials in liquids, particularly fine particle materials and 
materials that are of low solubility in the liquid, and the suspension of 
other materials in suspension vehicles, especially materials that are 
difficult to wet, and particularly fine particle materials. 
FIG. 8 illustrates the manner in which a single mill of the invention is 
used in a closed recirculating circuit to operate in a batch process. The 
premixed slurry is fed from the tank 12, as with the process of FIG. 1, to 
the holding tank 24 and is delivered by the pump 26 to the mill inlet pipe 
62. The mill outlet pipe 72 however discharges back to the tank 24, and 
the slurry is recirculated until the desired particle size distribution 
has been obtained. The batch process may be operated in accordance with a 
predetermined protocol whereby the mill plate members are spaced apart the 
maximum operative distance at the start, and are moved together, either 
progressively or stepwise, as the process proceeds until at its conclusion 
they are at the minimum operative spacing. 
FIG. 9 is a longitudinal cross section through another plate mill 
embodiment in which the two plate members are mounted for rotation about a 
horizontal axis 76. The stationary vibratory plate member 34 is securely 
fastened in the required orientation at the upper end of a standard 78 
mounted on the baseplate 30 and has a cylinder 80 of resilient material 
fastened to its cylindrical periphery. The inside surface of this 
resilient cylinder is in close rubbing contact with the corresponding 
cylindrical periphery of the rotatable plate member 44, so as to seal the 
cylindrical periphery of the flow path 68, except for a discharge nipple 
72 at its lowermost part, this nipple constituting the outlet from the 
path. The shaft 52 mounting the movable plate about the horizontal axis 76 
is mounted in a bearing 82 at the upper end of a standard 84 mounted on 
the baseplate 30 and is driven by a motor (which is not shown) via a 
coupling 85, which permits the necessary movement of the shaft and the 
plate along the axis 76 to vary the flow path height and to permit access 
to the space 68 as required. 
This embodiment has the advantage that there is less exposure to the air of 
the emerging processed slurry, in that it can flow from the nipple 72 
directly to the inlet of the next mill. 
FIG. 10 is a combined cumulative graph showing the particle size 
distribution of a slurry material prior to its processing in the mill of 
FIGS. 2-4, this particular characteristic being shown in solid lines. The 
material employed was a spray dried, partially stabilised zirconia of 
nominally 0.3 micrometer particle size that had been pelletized using a 
water soluble binder to prevent dusting and to permit its ready transport, 
the pellets being of 100-150 micrometer size. Fifty (50) grams of these 
pellets were predispersed for 30 minutes in 100 grams of water with a 
small amount of a surfactant (0.3% by weight of the zirconia) using an 
ultrasonic horn, which should have been sufficient to fully deagglomerate 
the raw powder. The characteristic shown as a solid line is that of the 
material after processing with the horn, but before processing in the 
plate mill of the invention, and it will be seen that only 82% is of a 
size smaller than 0.8 micrometers, there is virtually no material of size 
between 0.8 and 10 micrometers, and the remaining 18% is of size between 
10 and 80 micrometers. This is partly the result of agglomeration, but 
mainly the result of hardening of the pellets, making them difficult to 
restore to the original particle size without a complete expensive 
remilling of the material. The characteristic in broken lines is the 
result of the same test on material that has been processed in the plate 
mill of the invention for a period of 30 minutes; it will be seen that all 
of the material is below 0.8 micrometers, 99.25% is below 0.7 micrometers, 
and 96% is below 0.6 micrometers, and the material now shows an excellent 
typical symmetric bell curve distribution about a median value of about 
0.36 micrometers. 
In the embodiment of FIGS. 11-13 the stationary plate member 34 is replaced 
by a stationary outer hollow cylinder 86, while the rotary plate member 44 
is replaced by a solid inner cylinder 88 mounted for rotation about a 
horizontal axis 76 within the hollow cylinder, the flow path 68 being 
constituted by the annular space between their respective outer and inner 
surfaces. Such a mill is referred to herein as a "Roll" mill. The single 
ultrasonic generator 40 that is provided is mounted directly on the mill 
base 30, and supports the outer cylinder 86 via an intermediate coupling 
member 90. As much as possible of the remainder of the exterior of the 
outer cylinder is enclosed by a cover plate 92, and the space between the 
cover plate and the cylinder 86 is filled with wire mesh 94, thus forming 
a part annular enclosure for the passage of cooling water that enters 
through inlet 58 and leaves through outlet 60. The wire mesh increases the 
cooling efficiency of the enclosure by increasing the effective contact of 
the cooling vehicle with the cylinder outer wall. 
The interior of the outer cylinder is closed by two circular end cover 
plates 96 attached to respective flanges at the ends of the cylinder, one 
of the cover plates mounting the slurry feed pipe 62 at its lowermost 
point, while the other mounts the slurry discharge pipe 72 at its 
uppermost point. The two cover plates are provided with aligned vertically 
elongated holes 98 through which pass the shaft 52 on which the solid 
inner cylinder 88 is mounted, the holes thus permitting vertical movement 
of the shaft and the inner cylinder for adjustment of the gap between the 
lowermost part of the inner cylinder external surface and the 
corresponding lowermost part of the internal surface of the of the outer 
cylinder. Each cover plate carries a respective slotted guide member 99 
through which the shaft passes in order to permit the shaft to move 
vertically in order to vary the eccentricity of the relative rotation of 
the two cylinders, while constraining the shaft for such vertical 
movement. Two annular gasket seals 100 are sandwiched between their 
respective cover plate and the butting outer cylinder flange and closely 
embrace the shaft 52 to prevent escape of slurry through the elongated 
holes 98. The shaft is mounted for rotation about the horizontal axis 76 
by two taper roller bearings 102, each of which is mounted in a respective 
housing 104 mounted and constrained for vertical sliding movement only in 
a respective cage 106. Each bearing housing is urged to the bottom of its 
respective cage by compression springs 107, and each cage is mounted on 
the mill base 30 via a micrometer shaft 108, so as to permit the position 
of each cage above the base, and thus of the shaft 52, to be accurately 
adjusted as required. 
The inner cylinder 88 preferably is entirely of a sufficiently hard 
material, such as silicon carbide, with its external surface ground 
accurately and smoothly to the required limits. The outer cylinder can 
also be of the same material, but for economy can be of stainless steel 
with an insert 110 of the same material as the inner cylinder over its 
lowermost arc where the processing gap is formed and the grinding and 
milling action takes place, the inner milling surface of the insert being 
ground to the necessary profile and smoothness. In a specific embodiment 
the inner cylinder is of 15 cm (6 ins) length and diameter and is rotated 
at speeds in the range 200-2000 rpm, preferably 400-600 rpm. The 
circumferential extent of the insert 110 is about 2.5 cm (1 in) and the 
gap between its inner surface and the outer surface of the inner cylinder 
88 will vary in the range 1-500 micrometers, preferably in the range 1-100 
micrometers. The diametrically opposed gap at the uppermost parts of the 
two cylinders has a maximum value of about 5 mm (0.20 in). 
This embodiment also functions by surface action or "skin-drag" of the 
rotating outer surface of the inner cylinder 88, which captures the slurry 
as a boundary layer and drags it with it into engagement with the boundary 
layer that is present on the surface of the insert 110, this also 
producing the desired "supra-Kolmogoroff" eddies in the milling gap 
between the relatively moving cylinders, while the accompanying 
"sub-Kolmogoroff" eddies are produced by the ultrasonic transducers in the 
gap, so as to again permit the milling of the particles to sub-micron 
values. The rate of flow of the slurry through the mill is made such that 
all of it will be dragged by the rotating surface of the inner cylinder 
through the milling gap, despite the presence of the larger gap at the 
upper part of the mill, which may appear from the drawing as though it 
would short circuit the milling gap; however, as explained above, in this 
embodiment the maximum value of this gap is only 5 mm, and is more usually 
of the order of 1 mm, and this is sufficiently small to ensure that with 
the correct choice of flow rate the desired passage of all of the material 
through the processing gap will be achieved. If there is any doubt in this 
regard, or if a particular mill is to be operated with a flow rate 
sufficiently high for some bypass to be possible, then all that is needed 
is a circumferential seal intermediate the ends of the cylinders, either 
on the moving or the stationary cylinder, and extending into contact with 
the other cylinder. As with the other embodiments the milling surfaces are 
self-grinding and self-polishing and the "drum" structure is not only less 
expensive to produce but gives greater possibility of accurate control of 
the milling gap. It also may differ in its action from the plate mill in 
that the processing gap is usually sufficiently small for the two boundary 
layers to inter-engage with one another, while the opposite gap is usually 
large enough for an intervening layer to be present, so that a cycle is 
produced as the cylinder rotates of the establishment and removal of the 
intervening layer, this facilitating production of "supra-Kolmogoroff" 
eddies. It will usually be preferred to use a plate mill when the 
circumstances require the maximization of uniform "sub-Kolmogoroff" 
mixing, while it will be preferred to use a roll mill when the 
circumstances require the maximization of comminution. 
FIG. 14 shows apparatus according to the invention for carrying out 
otherwise difficult to perform chemical reactions and physical 
inter-actions, depending upon the formation of "sub-Kolmogoroff" eddies, 
such as the reaction of a gas with a liquid, or the rapid solution or 
reaction of a difficultly soluble gas in or with a liquid, or the solution 
of a difficultly soluble solid material in a liquid. This apparatus also 
consists of an inner cylinder 88 rotating about a horizontal axis within a 
hollow outer cylinder 86. The liquid to be reacted, or to act as the 
solvent, is fed through the reactor from a liquid inlet 62 at one end to a 
liquid outlet 72 at the other end, both the inlet and the outlet being 
disposed at the lowermost part of the outer cylinder, while the other 
component is fed into the action/reaction space between the two cylinders 
by an inlet 112, no separate outlet of course being required since it is 
being consumed by the liquid. The coupling member 90 is provided with 
passages 114 for cooling or heating liquid, depending upon whether the 
action/reaction taking place in the reaction gap is exothermic or 
endothermic, these passages being provided with heat exchange enhancing 
inserts, as disclosed for example in my U.S. Pat. No. 4,784,218, the 
disclosure of which is incorporated herein by this reference. The liquid 
component is fed at a rate to ensure that the pool formed is confined to 
the space between the relatively rotating members immediately adjacent to 
the ultrasonic transducers. 
The reaction gap can be of greater height than the grinding gap of the 
previously described embodiments and can be in the range from 1 micrometer 
to 5 mm, while the opposite gap can be in the range from 2 mm to 2 cm. The 
rate of relative movement of the two surfaces will also usually be much 
higher and, for example, with an inner cylinder of 15 cms (6 ins) diameter 
the rotational speed will usually be in the range 200 to 20,000 rpm, with 
a preferred range of 500-10,000 rpm. The highest possible speed is usually 
to be preferred, in that thinner more active films are produced on the 
inner cylinder outer surface, but an upper limit can be set by the 
possibility that the resultant centrifugal force completely disrupts the 
film. 
Although in the embodiments described the inner cylinder 88 is solid, it 
can instead consist of a hollow cylinder provided with a suitable internal 
structure by which it is mounted for rotation about the horizontal axis.