Method and apparatus for crushing material for grinding

The invention relates to a method and to apparatus for crushing brittle material for grinding using a roller mill and a material bed roll mill for crushing the material for grinding falling downwards over the edge of the grinding track before it is delivered to the sifter. By the use of a material bed roll mill a considerable increase in the throughput capacity of the apparatus and a significant saving of energy can be achieved.

The invention relates to a method and apparatus for crushing brittle 
material for grinding. 
It is frequently desired to increase significantly the throughput capacity 
of apparatus of the aforementioned type (containing a roller mill, sifter 
and bucket elevator). 
The object of the invention, therefore, is to provide a method and 
apparatus improved in such a way that the throughput capacity of the 
apparatus is significantly increased with comparatively low expenditure on 
plant. 
This object is achieved according to the invention by the provision of a 
roll mill between the roller mill and bucket elevator. Advantageous 
embodiments also form parts of the invention as set forth hereinafter.

DETAILED DESCRIPTION 
The apparatus shown in FIG. 1 for crushing brittle material for grinding, 
for example cement clinker, contains a roller mill 1, a sifter 2 arranged 
above it, a material bed roll mill 3, a bucket elevator 4, a fines 
separator 5 and a plurality of further parts which will be described in 
connection with the functioning of the apparatus. 
The roller mill 1 which can be seen in detail in FIG. 2 has a horizontal 
grinding track 7 which runs around the vertical axis 6 and on which 
grinding rollers 8, 9 press. 
The grinding track 7 is driven by a drive motor 10 and a gear unit which is 
not shown in detail in the support of the roller mill 1. 
An adjustable nozzle ring 11 through which an air stream (arrow 12) is 
delivered to the grinding track 7 is arranged on the periphery of the 
grinding track 7. 
The material for grinding which is not yet sufficiently crushed falls 
downwards over the edge of the grinding track 7 (i.e. against the air 
stream) and passes via a chute 13 to the roll mill 3 (arrow 14). 
The dynamic sifter 2 arranged above the roller mill 1 contains a fixed 
(advantageously adjustable) guide vane ring 15 and sifting cage 18 which 
is driven by a motor 17 via a shaft 16. The shaft is mounted at its lower 
end by means of radial struts 19. 
The shaft 16 also has above the sifting cage 18 a distributor plate 20 to 
which the material to be sifted is delivered (arrow 23) from the bucket 
elevator 4 via short pipes 21, 22. 
Below the guide vane ring 15 the sifter 2 has a hopper 24 through which the 
tailings can be returned to the grinding track 7 of the roller mill 1. The 
raw material is delivered to the grinding track 7 through an opening 25 
(arrow 26). 
A tailings screw 27 which can be regulated in speed is also connected to 
the lower end of the hopper 24 and delivers an adjustable proportion of 
the tailings direct to the roll mill 3 (arrow 28 in FIG. 1). 
The air stream conducting the fines is drawn off from the sifter 2 via a 
pipe 29 (arrow 30). 
The apparatus functions using the method according to the invention as 
follows: 
The material for grinding which is first to be crushed is delivered to the 
roller mill 1 from a silo 31 via a proportioning belt scale 32 and an air 
excluding device 33. The material for grinding which has been sufficiently 
crushed on the grinding track is conveyed to the sifter 2 by the air 
stream. The fines leaving the sifter together with the air stream are 
precipitated in the separator 5. 
A proportion of the tailings precipitated in the sifter 2 is returned to 
the grinding track 7 of the roller mill 1. A further, adjustable 
proportion of the tailings passes via the tailings screw 27 direct to the 
roll mill 3. The material for grinding falling downwards over the edge of 
the grinding track 7 is also delivered to this material bed roll mill. In 
the roll mill 3, the rolls of which are pressed against one another with a 
high pressure, the material undergoes material bed crushing with 
noticeable formation of agglomerates. 
The material for grinding discharged from the roll mill 3 is conveyed 
upwards via the bucket elevator 4 and by means of a valve 34 is either 
delivered to the distributor plate 20 of the sifter 2 or to some other 
application (vehicle 35) (if required, this stream of material delivered 
via the bucket elevator 4 can also be divided). 
The agglomerates of material for grinding which are formed in the roll mill 
3 undergo disagglomeration on the rotating distributor plate 20 of the 
sifter 2. The agglomerates which are broken up in this way are thus 
delivered evenly to the sifting air. 
The finished material which is already present after passing through the 
roll mill 3 is thus directly sifted and no longer returned to the roller 
mill 1. In this way the roller mill 1 is substantially relieved of strain. 
As a result there is a considerable reduction in the pressure loss for the 
roller mill, which leads to a marked saving in energy in the pneumatic 
conveying of the material for grinding. 
It is also a particular advantage that the material recirculated by the 
bucket elevator is already largely dried, since in the roll mill 3 the 
material for grinding falling downwards over the edge of the grinding 
track 7 is ground together with tailings (delivered via the tailings screw 
27) which are already largely dried. 
The apparatus is controlled essentially by the use of the following control 
circuits: 
A first control circuit (regulator 36) regulates the 
quantity delivered to the roller mill 1 via the proportioning 
belt scale 32 as a function of the drive 
power of the roller mill 1. 
A second control circuit (regulator 37) keeps the 
electric current consumption of the material bed roll 
mill constant by appropriate regulation of the crosssection 
of the adjustable nozzle ring 11 and thus the 
gas speed in the nozzle ring (the proportion of 
material for grinding falling downwards against the 
air stream, and with it the quantity of material 
recirculated by the bucket elevator, alters with the 
gas speed in the nozzle ring). 
A third control circuit (regulator 38) keeps the 
quantity of gas drawn through the apparatus constant 
by influencing the speed of a fan 39 arranged after 
the fines separator 5. 
A fourth control circuit (regulator 40) controls a 
circulating air valve 41 and thus keeps the negative 
pressure prevailing in the air stream before the 
roller mill 1 constant. 
Only a proportion of the tailings precipitated in the sifter 2 is 
advantageously removed via the tailings screw 27 and delivered direct to 
the material bed roll mill 3. This part quantity is regulated by adjusting 
the speed of the tailings screw 27 which operates as an overflow screw. 
The quantity removed can be weighed and thus the speed of the screw can be 
controlled. 
The use of pneumatic means for conveying the material for grinding in a 
conventional grinding apparatus using a roller mill takes a comparatively 
large proportion of the total energy used for grinding. By means of the 
invention the material for grinding falling downwards through the nozzle 
ring 11 is subjected to further crushing by means of the material bed roll 
mill 3 and delivered via the bucket elevator 4 to the sifter 2. Up to 50% 
finished material is contained in this material for grinding which is 
delivered directly to the sifter (depending upon the type of material to 
be ground). This proportion of material is thus withdrawn from the 
pneumatic conveying. This results in a pressure loss for the grinding 
apparatus which is up to 40% lower, which results in a correspondingly 
high energy saving on the mill fan.