Treatment of particulate material

The invention provides a mill or classifier includes a rotor with a plurality of outwardly radiating beater elements which is mounted for rotation within a housing. The housing defines an upper level inlet for raw material and one or more outlets for treated material. An expansion chamber is provided into which material is thrown by the beater elements of the rotor in use.

FIELD OF THE INVENTION 
This invention relates to the treatment of particulate material by way of 
grinding, classification or the like. 
BACKGROUND OF THE INVENTION 
Conventional rotary machines such as impact or attrition mills and the like 
suffer from the following disadvantages. 
[1] Due to centrifugal force material tends to form a compacted layer on 
the inner periphery of the mill; 
[2] As a result of the condition in [1] above, excessive grinding of 
material producing unnecessary or excessive fines commonly takes place; 
[3] As a result of the condition [1] described above, difficulties are 
experienced in separating finer and coarser material; 
[4] Throughput of the mill is hampered as a result of the compaction 
described in [1] above and the difficulty described in [3] above; 
[5] Wear of certain parts of the mills is increased as a result of the 
conditions [2] and [3], described above. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a novel 
method and means for milling, classifying and the like which it is 
believed will overcome or at least minimize the difficulties set out 
above. 
A further object of the invention is to provide a versatile unit which can 
be used in different modes as follows to provide a wide range of 
classified products. 
(a) As an impact mill 
(b) As a differential impact mill 
(c) As an attrition mill 
(d) As a differential attrition mill 
(e) As an air classifier. 
It will be appreciated that different materials have physical 
characteristics which vary considerably particularly in respect of 
grindability and the invention has as an objective to provide a novel 
system which gives the necessary flexibility in performing the above 
functions on most dry friable materials within the basic framework and 
construction of the unit disclosed hereunder. 
According to the invention such a mill, classifier or the like comprises a 
rotor which carries a plurality of outwardly radiating beater elements and 
which is mounted for rotation within a housing, the latter defining an 
inlet located on an upper level of the housing for raw material and one or 
more outlets for treated material, characterised in an expansion chamber 
into which material is thrown by the rotor in use. Preferably the 
expansion chamber will be disposed at the upper level of the housing and 
the inlet will communicate with the expansion chamber. 
Further according to the invention an expansion chamber defines one or more 
partitions disposed generally parallel to the plane of the rotor. 
Where the apparatus of the invention is adapted to act as an impact or 
attrition mill, one or more radial or inclined plates generally parallel 
to the axis of rotation of the rotor will preferably also be provided to 
act as stationary impact plates. Where the apparatus of the invention is 
adapted to act as a differential mill or an air classifier one or more 
adjustable inclined or curved plates will preferably be provided as 
stationary guide plates in the expansion chamber to guide the unclassified 
material between the radial partitions in the direction of rotation of the 
rotor to minimize impact between the material, the rotor and the casing. 
Still further according to the invention the outlet is axially disposed and 
adapted to be coupled to suction means for drawing treated material 
through a rotating outlet. In a preferred arrangement the rotating outlet 
will be incorporated in the rotor of the device. Thus, in a preferred 
construction the rotor will define arm or plate members for carrying the 
beaters and which are flanked by front and rear side discs of the rotor 
define an air passage between adjacent arm members, the passages defining 
openings at the extremities of the arm members down which the sized 
material may be drawn. 
It is also envisaged that the beaters and beater support arms of the rotor 
may be radially disposed or alternatively could be arranged with forward 
or backwardly inclined at an angle to assist in withdrawing a larger 
classified maximum particle size or a small classifier maximum particle 
size respectively. 
In an alternative arrangement the front and rear side discs may be disposed 
with and the arm members be of tubular construction each defining an air 
passage through which sized material may be withdrawn. 
A feature of the invention provides for airflow through the mill or 
classifier to be such that sized material can readily be withdrawn as it 
is created. Thus the invention envisages that controlled upper level air 
inlets will be provided in the expansion chamber to direct air 
tangentially about the rotor, opposed axial inlets to direct the airflow 
from the axis of the rotor radially outwardly, and, in the case of the air 
classifier, one or more controlled peripheral inlets in the housing of the 
device to wash material away from the periphery of the housing. 
In all modes the factors that control the classification of the particles 
include the rotor speed and the air velocity down the air passages in the 
rotor. In all mill modes the rate of grinding of the feed material in the 
mill is a function of rotor speed and in all modes the air quantity 
passing through the mill must be sufficient to convey the product. In 
certain cases the product produced by the mill may be too fine yet the air 
quantity may not be easily increased sufficiently to rectify this and, at 
the same time, the mill rotor speed cannot be reduced without reducing the 
production rate due to the grindability of the material being treated. 
Thus further according to the invention the area of the air inlet openings 
at or towards the extremities of the arm members is preferably variable by 
providing removable fairings or the like at the zones. Since these inlet 
air velocities are usually high, these fairings will preferably provide a 
streamlined low resistance streamlined entry at these rotor air inlets. 
Where the apparatus of the invention is adapted to act as a differential 
mill or where it is adapted as an air classifier, an outlet for oversize, 
denser or harder material will also be provided at a lower level in the 
housing, the arrangement being one wherein the material is airwashed as it 
passes through the outlet. It is envisaged that control vanes will be 
provided at such an outlet, one control vane directing material towards 
the outlet while another will be adapted to direct airflow into the 
housing. 
According to whether the apparatus is used in impact mill mode, attrition 
mill mode or air classifier mode so the liners will be arranged for 
impact, attrition or for negligible reduction respectively. 
Also included within the scope of the invention is a method of milling 
comprising the steps of providing a mill having a rotor including a 
plurality of outwardly radiating beater elements rotatably mounted within 
a housing, the housing defining an expansion chamber located at an upper 
level of the housing, introducing raw material into the housing, and 
causing the beater elements to throw the raw material into the expansion 
chamber repeatedly during the milling process. 
The invention further includes within its scope a method of classifying 
comprising the steps of providing a mill having a rotor including a 
plurality of outwardly radiating beater elements rotatably mounted within 
a housing, the housing defining an expansion chamber located at an upper 
level of the housing, and introducing raw material into the housing along 
a pathway extending in the direction of rotation of the rotor. 
Still further according to the invention a method of milling or classifying 
includes the steps of withdrawing sized material from the casing 
co-axially with the axis of rotation of the rotor, such sized material 
being withdrawn along passages extending from the outer periphery of the 
rotor towards the axis thereof. 
Yet further according to the invention a method of milling or 
classification includes the steps of introducing air into the housing from 
the zone of the axis of rotation of the rotor for flow outwardly toward 
the periphery of the rotor. 
Further still according to the invention a method of milling or 
classification may include the step of introducing air into the casing at 
high level for circular flow in conjunction with the outer peripheral zone 
of the rotor. 
A method of classifying according to the invention may include the further 
step of introducing air into the casing at a lower level to wash material 
away from the lower peripheral zones of the casing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, the device, in accordance with the invention, 
for the treatment of particulate material is characterised in that it can 
be readily converted from an impact mill to a differential impact mill, to 
an attrition mill, to a differential attrition mill or to an air 
classifier. It can also be arranged to grind a coarse or an extremely fine 
product. 
Generally, in impact mill mode, the action is restricted to one of impact 
with a minimum of attrition so that the resulting product contains not 
only a sized maximum particle size through the classification action of 
the rotor but a maximum quantity of larger particle sizes and a minimum 
quantity of smaller particle sizes. Normally the speed of rotation of the 
rotor would be low and the centrifugal force on the particles small. 
In differential impact mill mode, the aim is the same but there is an 
additional action in the extraction of larger, denser or harder particles 
via the lower level outlet of the unit. 
Generally the action in impact mill mode is not so severe as in the 
attrition mill mode; less reduction is aimed for and usually the speed of 
the rotor would be lower. However, it will be appreciated that the 
inherent physical properties of the material being treated affect the 
product considerably. Generally also the impact mill mode will create the 
greatest production rate, the lowest power consumption per unit of 
production rate and the lowest wear of beaters and liners per unit mass of 
production. 
Generally, the action of the device in attrition mill mode is to provide a 
more severe attrition action so that a fine product is achieved below a 
chosen maximum particle size which is smaller than in compact mill mode. 
Production rate is generally lower with a higher power consumption per 
unit of production rate. In differential attrition mill mode, larger, 
denser or harder particles are separated out via the lower level outlet. 
This material is separated out before it is ground finer either because it 
is undesirable in the fine product or because it is useful; in any case, 
since it is usually more abrasive than the softer or lighter material, its 
extraction can increase the production rate of the fines and reduce wear 
considerably. 
It should be noted that, provided the feed material is not extremely 
friable when the apparatus is set to grind extremely fine, the product can 
still consist mainly of particles which are only just below the chosen 
classified maximum size. This is due to the expansion chambers and the air 
entry points enabling the sized particles to emerge as soon as they are 
created without undue retention. 
In air classifier mode, the apparatus will effectively split a feed of 
ground material into a larger fraction via the lower level outlet with the 
finer fraction classified out and conveyed in the airflow from the unit. 
The classification action of the rotor is such that a very accurate split 
a chosen particle size can be achieved. Factors affecting the split are 
the air velocity radially down the air paths in the rotor and the 
rotational speed of the rotor. 
In air classification mode, the aim is not normally a further reduction in 
the size of the particles of the feed material but to merely split it into 
the two fractions; in this case, therefor, steps are taken to reduce both 
impact and attrition and liners, for example, would normally be smooth 
faced. In this mode, feed rates can be considerable increased especially 
where the quantity of the larger fraction exceeds that of the smaller 
since it passes straight out through the lower level outlet. Power 
consumption is invariably low. 
With reference to FIGS. 1 to 21 a unit in accordance with the invention 
comprises a housing 1 preferably of a U-shape configuration in elevation 
as shown in the illustrations. The housing defining an upper level inlet 2 
for particulate material and an axial outlet 5 for treated material which 
is described in more detail below. Rotatably mounted within the housing 1 
is a rotor 8 which defines a plurality of generally radially extending arm 
members 8B with their leading faces mounting beater elements 8A of 
suitably wear resistant material, the beater elements 8A being secured to 
the arm members 8B by spigots 12 (see FIG. 5). It is a feature of the 
invention that the rotor 8 could be of hollow constructions between a 
front disc 8C and a rear disc 8D whereby passages 8F for treated material 
are defined between adjacent radially extending arm members 8B with an 
entry 8E for material being defined at the outer extremity of each arm 
member 8B. With this arrangement the axial outlet duct 5 described above 
will communicate with the interior central zone of the rotor 8 so that an 
outlet passage for treated material is defined from the outer extremities 
8E of rotor arms 8B down the spaces 8F and along the duct 5 to a suitable 
material collecting device such as a filter shown schematically at 18 in 
FIG. 20 or a cyclone collector 26 in closed circuit in FIG. 21. 
It will be appreciated that when a depression or negative air pressure 
exists in the outlet duct 5 by the action of fans or any prime mover 
downstream from the filter 18 as shown at 19 in FIGS. 20 and 21, air is 
drawn down through the entries 8E at the rotor and the resultant 
centripetal air drag on the material particles rotating around the rotor 
acts against the centrifugal force on the particles as shown in FIG. 5 
where particle P is at the point of classification with the arrows 
indicating the two forces. In impact or attrition mill modes, the 
particles are being reduced in size and since the air drag varies as the 
square of the particle diameter while the centrifugal force varies as the 
cube of the particle diameter it follows that the centrifugal forces on 
the diminishing particle sizes diminishes more rapidly than the 
centripetal air drag and, at the desired maximum particle size or less, 
the rotating particles move down the air paths 8F in the rotor and are 
conveyed out via the duct 5 to an external collector 18 or 26 in FIGS. 20 
or 21. 
FIG. 5 shows a typical rotor with beaters 8A against beater support plates 
8B with centrifugal force carried by cylindrical spigots 12. The area of 
the air entries 8E for a given air quantity governs the air velocity 
entering the entries 8E. 
FIG. 5 shows how the area of the gaps 8E can be decreased to provide a 
higher centripetal air velocity and hence a larger maximum particle size 
by the use of fairings 11. These may be fixed or, more conveniently, 
removable as shown. The outer peripheral surface of the fairings, if they 
are used, is protected by abrasion resistant material. 
Very fine particle sizes can be obtained by large gaps areas 8E between the 
beaters 8A and with the rotor running at high speed and with reduced air 
quantity if necessary. 
Large particle sizes, conversely, are classified out by using small areas 
8E, a slow running rotor and large air quantities. Variations of the rotor 
gaps and fairings 11 are shown in FIGS. 7, 8 and 9. 
A further feature of the invention provides for the housing 1 to define an 
upper level expansion chamber 1A which communicates with the inlet 2. 
Preferably the expansion chamber 1A will be divided into at least two 
chambers by means of a radial partition 9 as shown in FIGS. 3, 4 and 6, 
the partition being generally parallel with the plane of the rotor 8. 
Preferably both chambers will be divided along their peripheral lengths 
into sub chambers by means of generally inclined, curved or radially 
disposed impact or guide plates 10 which may be adjustable vertically or 
angularly and can be of different lengths to suit the characteristics of 
the materials being treated. These play an important part in the 
processes. See FIGS. 3 and 4, 6, 10, 11 and 12, and as described further 
or below. 
An important feature of the invention comprises the provision of a number 
of strategically placed air inlets into the housing which are designed to 
minimize compaction and over-grinding through excessive attrition 
resulting from undue retention of ground material in the housing 1 should 
a high percentage of fines not be required. The invention envisages that 
axial inlets 21 in FIGS. 1, 2, 4, 5 and 6 into the housing 1 will be 
provided at the axis of the rotor 8 to direct airflow from the center of 
the housing towards the periphery thereof. Further air inlets 3 and 4 are 
provided at the upper level in the expansion chamber zones 1A, see FIGS. 
1, 2, 3, 4, 5, 6, 10, 11 and 12, to permit constant and effective 
penetration of air through the moving material that has expanded into the 
zones 1A to avoid compaction. Preferably these inlets 3 and 4 will be 
adjustable by means of suitable damper plates. In addition air can enter 
via the feed inlet 2 which can be controlled by use of a tilting flap 
valve or if no air is required or if there is ricochet of particles 
emerging from the housing, by the use of a rotary seal at this inlet as at 
2A in FIGS. 20 and 21. 
Further air inlets 13 tangential to the rotation of the rotor are provided 
in the lower periphery of the housing 1. These can be adjusted for example 
by means of a flap 13A shown in FIG. 14 protected by an abrasion resistant 
surface. The flaps 13A not only admit air but deflect the rotating 
material to assist the ingress of the air when treating certain materials. 
The air inlets 13 are used in conjunction with the adjustments of the 
other controlled air inlets to obtain the most suitable penetration of the 
material being treated to prevent a high percentage of fines should this 
be undesirable or, particularly, when the unit is being used in air 
classifier mode to ensure the efficient removal of the finer fraction of 
the feed material before a coarser fraction is withdrawn via a lower level 
outlet 6. 
In the differential impact mill mode, the differential attrition mill mode 
and particularly the air classifier mode in all of which it is the aim to 
efficiently remove the larger, denser or harder material out via the low 
level outlet 6 it is important to maintain a rotation of the material 
keeping the larger, denser or harder material closer to the periphery. 
FIG. 11 shows the inclined impact plate 10 which may be preferred in the 
differential impact mill mode as opposed to the fiercer impact plate 10 of 
FIG. 10 which creates violent ricochet which might be preferred in the 
impact mill mode. It would invariably be used in the differential 
attrition mill mode. The curved guide plates 10 of FIG. 12 might be used 
in a differential attrition mill treating very friable material and would 
invariably be used in the air classifier mode. Thus the invention provides 
a variation of plates 10 to achieve efficiency in treating various 
materials. 
In the differential impact mill, differential attrition mill and air 
classifier mode the invention provides for the use of a low level outlet 6 
see FIGS. 3, 4, 6, 18 and 17, to remove oversize, dense or hard material 
from the housing 1. FIG. 6 shows how the material, which is indicated by 
arrows, enters the inlet 2 and is rotated in the housing by the rotor 
beaters, 8A. Since the housing 1 is under negative air pressure, air 
enters the housing and removes the classified material as previously 
described. The division plates 9 divide the upper zone of the housing into 
at least two expansion chambers 1A parallel to the plane of the rotor 8. 
Since the feed inlet 2 is at the back end of the housing 1 and the outlet 
6 is at the front end of the housing, the material moves in a spiral path 
from the back to the front end. The finer fraction is removed via the 
rotor air passages 8f and out via duct 5. The surviving coarser, denser, 
harder fraction survives the spiral path and the expansion into the upper 
chambers 1A until it reaches the outlet 6 where it either gravitates out 
or is assisting by the vane 6A (FIGS. 18 and 19). It may finally be 
airwashed by air entering via the outlet 6 controlled by further vanes 
such as 17A and 17B in an extension chute 17 (FIG. 19). It may, 
alternatively be removed without further air washing via a rotatary seal 
16. 
The interior surfaces of the housing 1 are protected by abrasion resistant 
liners. Those on the sides of the housing are generally flat without 
projections. The peripheral liners are indicated diagramatically in FIGS. 
15, 16 and 17 which show sectioned elevations and flat plan views of the 
liners. FIG. 15 indicates liners with projections 14 and a closed outlet 6 
for attrition mill mode and possibly for impact mill mode if the material 
being treated is hard. The air inlets 13 may be closed during this mode. 
FIG. 16 shows liners with projections 14 and outlet 6 open for the 
differential attrition mill mode with the air inlets 13 open or closed. 
FIG. 17 shows liners 15 without projections and outlet 6 open for the air 
classifier mode and differential impact mill mode with the air inlets 
optionally open. For the impact mill mode the arrangement could be as FIG. 
17 with the outlet 6 closed and air inlets 13 closed. 
FIG. 20 shows the apparatus according to the invention operating in any of 
the five modes described in a normal open circuit with the necessary 
auxilliary equipment. A feed bin 20 allows raw material to be fed via a 
vibrating feeder or any suitable feeder to the unit housing 1. A rotary 
seal 2A may optionally be used. The classified fine product is conveyed 
out via duct 5 to a collector, usually a filter collector 18 and 
gravitates out via a rotary seal 18A. A fan or fans in series or a 
positive displacement blower as required, 19, provides the necessary 
depression or negative air pressure in the system. Oversize, dense or hard 
material emerges, if desired, via a outlet 6. 
When running in air classifier mode the apparatus according to the 
invention may run in closed circuit as indicated in FIG. 21. Here all 
inlets and outlets are fitted with rotary seals. A high efficiency cyclone 
collector 26 may be used to collect the product. A fan 19 handles very 
fine particles escaping from a collector 26 and a bleed-off duct 22 with 
damper bleeds-off inwardly leaking and displacement air fed to a small 
filter 18. The system operates in a balanced condition. The fine fraction 
emerges at 23, the large fraction at 24 and a lesser quantity of very fine 
material at 25. An air bleed-in can be used if necessary. 
Doubtless many variations of the invention exist without departing from the 
principles set out in the consistory clauses.