Method and apparatus for grinding granular materials

The invention relates to a method of and apparatus for dry grinding a granular material in a grinding tube mill (1) having a final grinding compartment (2) and one or more preceding grinding compartments (3) containing grinding bodies. The material, after having passed through the preceding compartment or compartments (3), is discharged through openings (6) in the mill (1) and is divided into a fine and a coarse fraction in a separator (9). The coarse fraction is returned to the preceding compartment or compartments (3), and the fine fraction is fed to the final compartment (2). The ground material is discharged by flowing over a dam ring (12) from the final compartment (2). Any grinding bodies carried with the overflow are separated by a sieving diaphragm (13) from the material and returned to the final compartment (2). The invention also relates to the granular material ground according to the method of the invention.

TECHNICAL FIELD 
This invention relates to a method and an apparatus for dry grinding a 
granular material. The method is carried out in a tube mill having a final 
and one or more preceding grinding compartments containing grinding bodies 
in which the material, after having passed through the preceding 
compartment or compartments, is discharged through openings in the mill 
and is divided into a fine and a coarse fraction by a separation process 
from which the coarse fraction is returned to the preceding compartment or 
compartments, and the fine fraction is fed to the final compartment. 
BACKGROUND ART 
In known processes of the type contemplated in the present invention, 
granular material is admitted into a tube mill and is ground and passed 
through different compartments. After passing through the tube mill the 
material is discharged from the mill. The grinding in the final 
compartment takes place with the assistance of grinding bodies having an 
average piece weight between 20 and 40 grams (g). The minimum size is 
typically about 20 millimeters (mm). As a result of the free flow area 
required together with the strength and manufacturing requirements, small 
grinding bodies are not used since the slots in conventional outlet 
diaphragms used in the final compartment cannot be constructed 
sufficiently narrow so as to allow the use of smaller grinding bodies and 
ensure effective screening of the ground material. 
Although it has been widely recognized that in order to achieve optimum 
grinding economy, the size of grinding bodies used in the final grinding 
compartment of a mill should be far smaller than that presently in use, up 
until the present no method or apparatus has been devised in which such 
smaller grinding bodies may be used. 
We have invented a grinding method and apparatus according to which optimum 
grinding economy is achieved in a tube mill having two or more 
compartments. According to a significant feature of our invention, the 
tube mill utilizes grinding bodies which are particularly dimensioned in 
accordance with the size of the particles of materials required in the 
final product. 
DISCLOSURE OF THE INVENTION 
According to the present invention, a grinding method and apparatus are 
directed to achieving optimum grinding economy in a tube mill having two 
or more compartments by an arrangement which makes it possible to utilize 
grinding bodies of a size which is particularly related to the size of 
material required in the final product, preferably a very small size which 
produces a fine ground finished product. 
The present invention relates to a method of dry grinding a granular 
material in a grinding tube mill having a final and one or more preceding 
grinding compartments containing grinding bodies. The material, after 
having passed through the preceding compartment or compartments, is 
discharged through openings in the mill and is divided into a fine and a 
coarse fraction by a separation process. The coarse fraction is returned 
to the preceding compartment or compartments, and the fine fraction being 
fed to the final compartment. The ground material is discharged from the 
final compartment and grinding bodies carried with the material are 
separated from the material and returned to the final compartment. 
In particular, the present invention is directed to a method of dry 
grinding granular material to a finished ground material in a grinding 
tube mill. The tube mill has at least one opening, a final grinding 
compartment and at least one preceding grinding compartment containing 
grinding bodies. At least the preceding grinding compartment has an outlet 
sieving diaphragm. The method comprises the steps of passing the material 
through the preceding compartment or compartments, discharging the 
preground material through the openings in the tube mill, dividing the 
material into predetermined fine and coarse fractions, returning the 
coarse fraction to said at least one preceding compartment, feeding the 
fine fraction to the final compartment, discharging the ground material 
overflowing from the final compartment, separating the grinding bodies 
carried with the overflowing ground material and returning the grinding 
bodies to the final compartment. 
Thus the material fed to the final grinding compartment does not contain 
particles of material larger than the small grinding bodies can grind, and 
also the grinding bodies are prevented from leaving the mill together with 
the ground material without the risk that they may clog the outlet from 
the compartment. This can be achieved even when grinding bodies having an 
average piece weight about 1 gram are used. The maximum size of the 
particles to be ground by these bodies are 1 millimeter. 
Tests have shown that, in grinding cement, an economy of more than 14% can 
be achieved over long periods compared with conventional cement mill 
grinding to the same Blaine surface. The cement ground according to the 
present invention showed strengths superior to those of cement ground in 
conventional mills. These improved strengths are due to the steeper 
granulimetric analysis curves of the ground cement which can be attained 
and which, as experience shows, means improved strengths of cement ground 
to the same Blaine surface. This is an important advantage resulting from 
the use of small grinding bodies. Similar tests in which cement was ground 
to the same degree of strength development as conventionally ground cement 
showed improvements in grinding economy up to 27%. 
Preferably, the separation of the material discharged from the preceding 
compartment or compartments is effected at such a particle size that the 
fine fraction from this separation fed to the final grinding compartment 
is finished ground in one passage through this compartment. 
Preferably, the material is ground in a preceding and/or the final 
compartment by means of grinding bodies having an average piece weight 
below 10 grams, and preferably about 5 grams. The maximum size of the feed 
to the preceding and/or final compartment is equal to or below the width 
of the openings in the outlet sieve diaphragm of the respective 
compartment. In this case it is a question of using the optimum size of 
grinding bodies in a compartment for pregrinding the material. This 
measure contributes to the improvement of the grinding economy inasmuch as 
the initial coarse grinding is usually accomplished with grinding bodies 
having an average piece weight of about 1500 grams and which have an 
inferior grinding economy. Thus the grinding compartment used for this 
initial grinding can now be shortened in length. 
In certain cases, e.g., when grinding cement, it is preferable that the 
fine fraction be cooled before being fed to the final grinding 
compartment. 
In other cases, when grinding moist material, for example, cement raw 
materials, it is desirable that drying of the material take place 
simultaneously with the grinding and/or separation of the material by 
means of hot gases brought into contact with the material. 
In one exemplary embodiment, the material discharged from the preceding 
compartment or compartments is deprived of any already finished ground 
material before being subjected to the separation. 
Finally, it may also be useful to connect the final compartment to 
separator means including at least one or more cyclone separators, the 
separator being in a closed circuit arrangement therewith for precipating 
finished ground material. In this case part of the material may pass 
through the final compartment several times before it is finished ground. 
The invention also relates to an apparatus for dry grinding granular 
material comprising a grinding tube mill divided into a final and one or 
more preceding grinding compartments containing grinding bodies. The mill 
is provided with openings through which material may be discharged from 
the preceding compartment or compartments. The mill also comprises means 
for separating the material discharged from the mill openings into coarse 
and fine fractions, means to convey material discharged from the mill 
openings to the separator means and to convey the coarse fraction from the 
separator means to the feed end of the preceding compartment or 
compartments and the fine fraction to the feed end of the final 
compartment. At least one dam ring and sieving diaphragm are positioned in 
the outlet end portion of the final compartment. The sieving diaphragm is 
spaced apart from the dam ring to form a chamber and defines openings 
smaller than the size of the grinding bodies in the final compartment. 
Lifting means are provided in the chamber to return to the final 
compartment the grinding bodies that in use, pass over the dam ring with 
the ground material. 
In the apparatus according to the present invention, the sieving diaphragm 
is exposed to little wear. Therefore, it retains its original slit width 
and has no tendency to clog inasmuch as the dam ring relieves the pressure 
of the mill charge. 
As a further consequence, the free passage area of the sieving diaphragm 
can be made considerably greater than that of a conventional diaphragm and 
therefore offers less resistance to the flow of material and/or air or 
gases. 
The dam ring, which ensures the correct ratio of material and grinding 
bodies in the final compartment, is made of a special type of wear 
resistant steel to ensure long durability. 
In a preferred examplary embodiment, a preceding compartment is provided at 
each of its inlet and outlet ends, with a dam ring and a sieving diaphragm 
spaced apart therefrom to form a chamber from which grinding bodies that 
pass over the dam ring are returned to the compartment by lifting means 
provided in the chamber. The diaphragms at the inlet and outlet ends have 
openings which are of substantially the same size. Also, these openings 
are smaller than the size of the grinding bodies in that compartment which 
have an average piece weight of less than 10 grams. 
In the case of larger tube mills, for which central drives at the outlet 
end are preferred, it is useful to feed the material to the final 
compartment through openings in the mill and in such cases the final 
grinding compartment has a feed inlet chamber which communicates with the 
openings in the mill. The feed inlet chamber comprises a dam ring and 
lifting means for feeding the material into the compartment and for 
returning grinding bodies from the chamber to the compartment. 
In a preferred embodiment, the inlet chamber of the final compartment 
comprises a dam ring and a sieving diaphragm. 
In yet another examplary embodiment, the conveying means comprises means 
for conveying material from the outlets of both the final grinding 
compartment and a preceding grinding compartment to a preliminary 
separator for precipitating finished ground material. Further, the 
conveying means comprises means for conveying the non-precipitated 
material from the preliminary separator to a final separator which 
separates the material into the coarse and fine fractions. 
The separator from which the fine fraction is fed to the final grinding 
compartment preferably is a vibratory screen. However, an air separator 
may also be used, for example, when simultaneously grinding and drying 
material. The fractioning may take place at a particle size of up to about 
2 millimeters depending upon the grindability of the material to be 
ground. 
In many cases, for example, when grinding cement, it is important to 
effectively cool the material being ground. This cooling may take place by 
means of air or atomized water brought into contact with the material 
during the grinding or separation of the material. An additional cooling 
of the material may be obtained by providing a separate cooler in the path 
of conveyance for the material being fed to the final grinding 
compartment. 
In yet a further exemplary embodiment, the grinding bodies in the final 
grinding compartment are of an average weight of about 10 grams or less 
and more preferably of about 5 grams or less. The width of the openings of 
the diaphragm is preferably about between 2 and 5 millimeters. 
In still yet another exemplary embodiment, means are provided for drying by 
hot gases, the material in at least one preceding grinding compartment 
simultaneously while being ground in that compartment.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows a tube mill 1 having a final grinding compartment 2 and a 
preceding pregrinding compartment 3. These two compartments are separated 
by a solid wall 4. The final compartment 2 has outlet openings 5 in the 
mill shell and the compartment 3 has outlet openings 6 in the mill shell. 
The mill has trunnions 7 and 8. A vibratory sieve 9 is provided outside 
the mill 1. A conveyor 10 leads from a vibratory sieve 9 to a trunnion 8 
and another conveyor 11 leads to the trunnion 7. The final compartment 2 
is provided at its outlet end with a dam ring 12 and a sieving diaphragm 
13 spaced apart to form a chamber 14 in which there are provided lifting 
members 15 leading to the final compartment 2. 
The material to be ground is fed to the compartment 3 through the trunnion 
7 as indicated by arrow 16. This material is preground in the compartment 
3 by means of grinding bodies preferably having an average piece weight of 
about 1,500 grams. Sufficiently preground material passes from the 
compartment 3 through slots in the sieving diaphragm 17 to the outlets 6. 
The slots in the sieving diaphragm preferably have a width of between 
about 6 to 8 millimeters. 
An elevator 18 lifts the preground material from the outlets 6 to the sieve 
9. The size of the openings in the sieving plate of the sieve 9 are chosen 
so that the fine fraction passing through the sieve 9 and fed, by the 
conveyor 10, to the final compartment 2 can be finished ground in one 
passage through this compartment by means of grinding bodies preferably 
having an average piece weight of, for example, about 5 grams. The 
openings of the sieve 9 can have maximum dimensions of 1 to 2 millimeters, 
depending on the grindability of the material. 
The coarse fraction from the sieve 9 is fed to the preceding compartment 3 
by means of the conveyor 11 and is then subjected to a renewed grinding in 
the compartment 3. 
In the final compartment 2, the dam ring 12 ensures the correct ratio of 
grinding bodies and material to be ground. The finished ground material is 
discharged from the compartment by flowing over the dam ring 12. However, 
it is impossible to prevent a certain amount of the small grinding bodies 
from flowing over the dam ring 12 with the material. These grinding bodies 
would clog the openings in a sieving diaphragm 12 exposed directly to the 
pressure of the charge in the compartment. As is evident from FIG. 1, 
these grinding bodies are instead led to the sieving diaphragm 13 which is 
relieved from direct pressure by the dam ring 12. It is thereby possible 
to separate the bodies from the finished ground material without any 
clogging of the diaphragm 13 and to return the bodies to the compartment 2 
by means of the lifting members 15 which will be described in more detail 
below. The openings in the relieved diaphragm 13 may be as small as 1 to 2 
millimeters. The finished ground material leaving the openings 5 is 
carried away by a conveyor indicated by 19. 
The apparatus shown in FIG. 2 comprises a tube mill 21 having two preceding 
compartments 22 and 23 and a final compartment 24. The mill 21 has 
trunnions 25 and 26. The conveyor 11 from the sieve 9 leads to the 
trunnion 25 and the conveyor 10 leads to a stationary housing 27 
surrounding the mill 21. Dam rings 12 and sieving diaphragms 13 are 
provided at each end of the compartment 23 so as to form chambers 14 in 
which lifting members 15 are provided. Similarly, at the outlet end of the 
final compartment 24, a dam ring 12, a sieving diaphragm 13, and lifting 
members 15 are provided in the chamber 14. 
The final compartment 24 is provided with scoops 28 communicating with 
openings 29 in the mill shell. A dam ring 30 together with the solid wall 
4 forms an inlet chamber 31 to the final compartment 24. 
The material to be ground is fed to the compartment 22 through the trunnion 
25 as indicated by the arrow 16. In the compartment 22 this material is 
preground by means of grinding bodies having an average piece weight of, 
e.g., of 1,500 grams. Sufficiently preground material passes from the 
compartment 22 first through a heavy grate diaphragm 32, and then through 
a sieving diaphragm 13 having openings of about 5 to 6 mm. Further, the 
material passes through the chamber 14 having lifting members 15 and over 
the dam ring 12 into the compartment 23 where it is further preground by 
means of grinding bodies having an average piece weight, e.g., of 5 grams. 
The preground material passes out of the compartment 23 over the dam ring 
12 via the chamber 14 having lifting members 15 and through the sieving 
diaphragm 13 at the outlet end of the compartment 23. The outlet sieving 
diaphragm 13 has openings of the same size as that of the inlet sieving 
diaphragm 13 of the compartment 23 so that an accumulation of oversize 
unground particles will not take place in the compartment. Such particles 
will be returned to the compartment 22 via the sieve 9 as explained in 
connection with FIG. 1. 
The fine fraction from the sieve 9 is passed to the inlet housing 27 by 
means of the conveyor 10 and is fed into the final compartment 24 by the 
scoops 28. Due to the adjustment of the openings in the sieve 9 this fine 
fraction can be finished ground in one passage through the final 
compartment 24 by means of grinding bodies having an average piece weight, 
e.g., of 5 grams or even as small as 1 gram depending on the particle size 
fractioning of the sieve 9. The finished ground material is discharged by 
overflow through the trunnion 26 via dam ring 12, chamber 14 having 
lifting members 15, and the sieving diaphragm 13 which has openings of the 
order of 2 to 4 mm. 
In the apparatus shown in FIG. 2, the aim is to move as much of the 
grinding work as possible from the compartment 22 to the compartments 23 
and 24. Thus, the length of the compartment 22 which has the lowest 
grinding economy is shortened. 
The apparatus shown in FIG. 3 comprises a tube mill 33 having two 
pregrinding compartments 22 and 23 similar to those shown in FIG. 2, and a 
final grinding compartment 2 similar to that shown in FIG. 1. The material 
discharged from the compartment 23 is taken to the sieve 9 by the conveyor 
18. The coarse fraction from the sieve 9 is fed to the compartment 22 by 
the conveyor 11, whereas the fine fraction from the sieve 9 is taken by 
the conveyor 10 to an air separator 34. The material discharged from the 
final compartment 2 is fed to the same air separator 34 by means of a 
conveyor 35. The fine fraction 36 from the air separator 34 is finished 
ground material. The coarse fraction 37 from the air separator 34 is led 
to a cooler 38, of any known kind. In the cooler 38, this fraction is 
cooled before being fed to the inlet of the final compartment 2 as 
indicated by 39. The material, e.g., cement, can be cooled in all three 
compartments 2, 22, and 23 by means of air passed through the chambers and 
discharged through the openings in the mill shell. In this manner, fresh 
cooling air can be passed in through both ends of the mill 33 which is 
preferable to cooling by means of a single air stream passing through the 
whole length of the mill 33. Additional cooling can be provided by 
atomizing water into the compartments. However, due to the intense 
development of heat in a mill in which small grinding bodies are used to a 
large extent it is often useful to cool the material before it is fed to 
the final compartment in which there is the greatest risk of clogging the 
material on the grinding bodies. 
FIG. 4 shows an apparatus for simultaneously grinding and drying moist 
material, e.g., cement raw material. The apparatus comprises a tube mill 
40 having a drying compartment 41, a pregrinding compartment 42, and a 
final grinding compartment 43. The mill has trunnions 44 and 45 
communicating with feed hoppers 46 and 47. A diaphragm 48 having means for 
transportation of the predried material into the compartment 42 is 
provided between the compartments 41 and 42. Compartment 42 has an outlet 
sieving diaphragm 49 constructed together with an outlet sieving diaphragm 
50 for the final compartment 43. A dam ring 51 is spaced apart from the 
diaphragm 50 to form a chamber 52 wherein lifting members 53 are mounted. 
The outlet formed by the parts 50 to 53 functions in the same way as 
described in connection with the parts 12 to 15 of FIG. 1. 
The material, having passed through the diaphragms 49 and 50, leaves the 
mill through openings 54 in the mill shell. The mill shell is surrounded 
by a stationary casing 55 from the bottom of which a chute 56 leads to an 
inlet end of an elevator 57. The outlet end of this elevator is connected 
to an air separator 58 by means of a chute 59. The bottom of the air 
separator 58 is connected by a gas conduit 60 to the casing 55. From the 
top of the air separator 58, a conduit 61 leads to a cyclone 62. In turn, 
another conduit 63 passes from the top of the cyclone 62 to a fan and is 
followed by an electrostatic precipitator (not shown). A worm conveyor 64 
is provided at the bottom of the cyclone 62. 
The coarse fraction from the air separator 58 is passed through a pipe 65 
to a vibratory screen 66 from which the coarse fraction via a hopper 67, a 
worm conveyor 68, and a chute 69 is fed to the inlet hopper 46 and into 
the drying chamber 41. The fine fraction from the screen 66 is led through 
a chute 70 to the inlet hopper 47 and into the final compartment 43. Inlet 
conduits 71 and 72 for hot air or gas are provided in the inlet hoppers 46 
and 47. Moist material passes through pipe 73, hopper 46, and trunnion 44 
into the compartment 41 where it is predried by the hot gases admitted 
through conduit 71. The predried material is transported through the 
diaphragm 48 into the grinding compartment 42 where it is preground and 
simultaneously further dried by the hot gas. The preground material leaves 
the compartment 42 through the sieving diaphragm 49, passes through the 
openings 54, chute 56, elevator 57, and chute 59 to the air separator 58. 
The gas passes from the compartment 42 through the diaphragm 49, the 
casing 55, and conduit 60 to the air separator 58. From conduit 72, 
another stream of hot gas passes through the final compartment 43, the 
sieving diaphragm 50, casing 55, and conduit 60 to the air separator 58. 
The material discharged by overflow from the final compartment 43 in the 
manner previously described passes through the openings 54, chute 56, 
elevators 57, and chute 59 to the air separator 58, i.e., together with 
the preground material. 
From the air separator 58 finished ground material is carried away with the 
gas through the conduit 61 and is precipitated in the cyclone 62 from 
which it is taken away by the conveyor 64. The gas passes through the 
conduit 63 to the suction fan and electrostatic precipitator. The coarse 
fraction from the air separator 58 passes via the pipe 65 to the screen 66 
from which the coarse fraction via the hopper 67, conveyor 68 and chute 69 
is returned to the drying compartment 41. The fine fraction from the 
screen 66 passes through the pipe 70 and hopper 47 to the final 
compartment 43 and is ground in this compartment by means of grinding 
bodies having an average piece weight below 10 grams, preferably about 5 
grams, depending on the grindability of the material and the particle size 
at which the fractioning takes place in the screen 66. In order to avoid 
accumulation of oversize particles in the final compartment 43, the 
openings in the screen 66 are made smaller than the openings in the 
sieving diaphragm 50. The latter openings are preferably about 2 to 4 mm 
or even smaller. 
The grinding bodies used in the compartment 42 may have an average piece 
weight of about 1500 grams. The mill shown in FIG. 4 may also be provided, 
if desired, with two preceding compartments. 
According to FIG. 5, the dam rings 12 in both the grinding compartments 2 
and 23 are protected by heavy wear plates 75 which are normally made from 
a special steel alloy. The sieving diaphragms 13 in each compartment are 
thus protected against wear from the grinding charges in the chambers and 
are relieved of the pressure from the charges. Thus, small grinding bodies 
flowing with the material into the chambers 14 are not pressed into the 
openings of the respective diaphragm 13, which otherwise would have a 
clogging effect. 
Usually, one tube like lifting member 15 in each chamber 14 is sufficient 
to return small grinding bodies from the chambers to the grinding 
compartments 2, 23. 
The sieving diaphragm 13 may be made of perforated steel plates supported 
in a light frame fastened to the mill shell. The central parts 76 of the 
diaphragms 13 may be made of wire mesh. 
The diaphragm between the compartments 22 and 23 preferably consists of a 
wear resistant central grate 78 surrounded by heavy wear plates 77 spaced 
apart to form a coarse screen which retains the grinding bodies in the 
compartment 22. Lifters (not shown) are normally provided in the space 
between this coarse screen and the sieving diaphragm 13 for returning any 
coarse particles to the compartment 22. 
FIG. 5 shows stationary outlet casings 79 and 80 for the material 
discharged through the openings 5 and 6 in the mill shell. 
FIGS. 6 to 8 show scoops 28 mounted on the mill shell and communicating 
with the openings 29 in the mill shell. At the inlet end of the final 
compartment 24, and connected to the soild wall 4 and a cone 82 on same, 
scoops 81 are provided which open into a chamber 88, the downstream wall 
of which is formed by a sieving diaphragm 85 and a cone 87. A dam ring 30 
with wear plates 75 is spaced apart from the diaphragm 85 to form another 
chamber in which a second set of scoops 86 is mounted. These scoops 86 
open into the final compartment 24. 
A stationary casing 83 surrounding the mill shell receives the material 
discharged from the compartment 23. At the top of this casing 83 an outlet 
conduit 84 is provided for the discharge of any air or gas led through the 
preceding chambers 22 (FIG. 2) and 23. 
The material from the conveyor 10, illustrated in FIG. 2, is delivered into 
the casing 27 and is shovelled into the chamber 88 by the scoops 81. From 
the chamber 88 the material passes through the diaphragm 85 to the next 
chamber provided with the scoops 86 which deliver the material into the 
final compartment 24. The scoops 86 also return small grinding bodies 
which have passed over the dam ring 12 into the chamber containing the 
scoops 86. The openings in the diaphragm 85 are small enough to prevent 
the passage of the small grinding bodies but large enough to allow the 
material to be fed to the final compartment to pass through. Therefore, 
the particle size fractioning limit of the sieve 9 (FIG. 2) and the size 
of the small grinding bodies are adjusted in accordance with this 
requirement. 
In the tube mill shown in FIGS. 9 to 11, a dam ring 30 having wear plates 
75 is positioned apart from the solid wall 4 so as to form an inlet 
chamber in which are mounted scoops 90, the outer ends of which follow a 
cone 89. Besides the scoops 28 an additional scoop 91 is mounted on the 
mill shell. This scoop 91 projects close to the wall of the stationary 
casing 27 as can be seen in FIG. 11. 
FIG. 10 shows that the lifting member 15 for returning small grinding 
bodies to the compartment 23 is formed as a spiral. The material is fed 
tangentially into the casing 27 through a pipe 92 and against the 
direction of rotation of the mill and is caught by the scoops 28 which 
lead the material to the scoops 90. These scoops deliver the material into 
the final compartment 24. Any small grinding bodies which pass over the 
dam ring 30 into the casing 27 accumulate at the bottom of the casing 
beyond the path of the scoops 28 and are returned to the final compartment 
24 by means of the scoop 91.