1.
HISTORICAL
The origin of the Roller
Grinding Mill can be found in the edge mill. This was used for comminution
even in antiquity. The grinding tools were stones. A single grinding roller,
or several joined together, were rolled over a circular grinding surface.
Grain was the material most usually ground, but also olives; and probably
even then this method was used for grinding minerals.
Fig 1: Stone
Age Mill
Fig. 1 shows a single-roller
mill of Stone Age design still in operating in Iraq in 1978. It is used
for grinding grain. The grinding surface is heated from below. The pulverizing
action of the roller is based on its weight.
2. RANGE OF APPLICATIONS
Roller grinding mills are
traditionally air-swept mills. They are used for fine and ultra-fine comminution
and simultaneous drying of minerals and crystalline materials such as limestone,
quick lime, cement raw materials, talcum, bauxite, magnesite, phosphate,
feldspar, heavy spar (barytes) and others, and for lignite, coal, graphite
and even for peat pellets. For some years roller grinding mills have also
been used increasingly for ultra-fine grinding of very hard and brittle,
and at the same time abrasive, materials such as slags and cement clinker.
The achievable product fineness
lies in the range between 50 % residue on 0.09mm (or 70 % residue on 0.09mm
for coal) and 10 % residue on 0.010mm.
3. DEFINITION OF A ROLLER
GRINDING MILL
Fig. 1 has in fact already
shown the basic design of a roller grinding mill. However, in the course
of almost 9 decades of the 20th century very varied forms of roller grinding
mills have been produced which, naturally, were first named after their
inventors.
Time and again possible
all-embracing generic terms have been coined but have not gained acceptance
because, in the final analysis, they were not comprehensive. One of the
first terms was,spring-loaded roller mill". Since August 1983 DIN 24100,
Part 2, ,,Mechanical Comminution; machine terminology" has provided the
standardized designation "Roller Grinding Mill". The definition reads:
"Machine in which the grinding
surface is annular. Grinding bodies (rollers or balls) roll on it. The
grinding bodies are pressed down on the grinding surface either by their
own weight, or by centrifugal force, by springs, or by hydraulic or pneumatic
systems. Both the grinding surface and the grinding bodies may be driven".
This last definition also
includes edge mills which normally operate on a batch basis and not continuously.
This article will only deal with those types of roller grinding mills where
the comminution is continuous. Normally the material being ground is dried
and transported by a stream of hot air or gas during comminution.
The industrial development
started at the beginning of the 20th century in the United States of America.
4. THE MAXECON SPRING-LOADED
ROLLER MILL
In 1906 Curt von Grueber
came back to Berlin from the USA. He founded the Curt von Grueber - Maschinenbauanstalt
in Teltow on the southern outskirts of Berlin. From the USA Curt von Grueber
had brought with him the licence to build the Maxecon mill.
Fig 2: Maxecon
Mill
This mill has a vertical
grinding ring which rotates about a horizontal axis. The ring is suspended
in the chamber with the aid of 3 convex rollers which are seated against
the concave inner track of the grinding ring. The rollers are offset by
120° to one another and rotate independently about their horizontal
axles. The rollers
are pressed against the inner surface of the ring by spring-loaded levers.
One of the rollers is driven through a pulley, which at the same time acts
as a flywheel, and also moves the grinding ring by friction.
The raw material is fed
to the mill via a suitable air lock and curved chute. The raw material
is introduced in front of the second roller (when seen from above) which
pre-crushes it. Centrifugal force then carries the partially pulverized
material to the next two rollers for further comminution. The pulverized
particles are finally ejected sideways so that they pass out of the roller
contact area. Originally the pulverized material was allowed to fall to
the bottom of the mill housing; later it was removed with a stream of air.
Over the years about 600
examples of the Maxecon mill were supplied to various industries. After
the patent expired many machine manufacturers copied the system, which
can be taken as proof of a successful design. The first critical test of
the Maxecon mill involved grinding coal at the Moabit power station of
the BEWAG in Berlin. As far as is known two machines were supplied, each
of which was designed for a raw coal feed of 5 t/h. They worked well and
were later supplemented with two more machines. The Moabit power station
was not destroyed during the second world war. The mills probably continued
to operate until the power station closed down.
The Maxecon mill was the
first externally-powered mill on the European continent. The achievable
production rates lay between 2 and 5 t/h. The design of the mill made any
increase in throughput unjustifiably expensive as the arrangement of rollers
inside a ring only permitted a small increase in roller diameter.
5. THE RAYMOND RING-ROLLER
NATURAL FORCE MILL
At that time the BEWAG were
interested in installing mills with higher throughput rates. Ernst Curt
Loesche, then partner and director of the Curt von Grueber Maschinenbauanstalt,
was approached with the proposition to acquire the licence for the Raymond
centrifugal ring-roller mill developed in America. The licence was granted
and the next BEWAG power station was fitted with this type of mill. Each
mill could grind between 10 and 12 t/h raw coal. Based on these mills,
this power station - Klingenberg in Berlin - had the largest raw coal processing
plant in Europe at that time.
The centrifugal ring-roller
mills are classified as natural force mills. This designation was based
on the fact that the grinding forces were produced by centrifugal forces
which acted on rollers which in turn circulate at a given speed.
Fig 3: Raymond
ring roller mill type Neuman&Esser, 50 t/h throughput
As Fig. 3 shows, each roller
has a vertical axle which is suspended freely from a flexible joint. Three
or more rollers hang from a rotating support. The rotation of this support
causes the rollers to move outwards like pendulums and press against the
vertical wall of the grinding ring. The raw material has to be introduced
into the grinding zone in front of the rollers with the aid of a plough
blade.
The advantage of this type
of mill over the Maxecon mill was that originally it could grind 12 and
then later up to 20 t/h coal. Apart from this the mill was also very suitable
as an airswept mill.
The ground material was
dried by a stream of hot air through the mill housing from below, and then
transported to the classifier positioned above the mill where it was separated
according to particle size.
These mills were not popular
in Germany for grinding coal. They had been designed for American soft
coal with a low ash content and good grindability, but in Germany the coal
is hard and has a high ash content. The harder coal requires a higher comminution
force. This could be achieved only by higher speeds of the suspended grinding
rollers, and thus higher centrifugal forces. This caused erratic running
and the resultant vibration damaged the mill foundations.
Advanced forms of the centrifugal
force ring-roller mill are still marketed today - in Germany, for example,
by Neuman & Esser - for comminution of a wide variety of minerals.
As with the Maxecon mill the roller diameters cannot be increased because
of the arrangement of the rollers within the surrounding grinding ring.
For this reason and because the comminution force (centrifugal force) is
dependent on the speed of the suspended rollers the ring-roller mill design
is limited to units of up to about 50 t/h throughput.
The experience which had
been gathered with both the Maxecon spring-loaded mill and with the Raymond
centrifugal force ring-roller mill led ultimately to the idea of combining
the advantages of the two types of machine.
6. THE MAXIMAL MILL
After considering the knowledge
collected in his own works Ernst Curt Loesche decided in 1925, when he
already had 100% ownership of the Curt von Grueber Maschinenbauanstalt,
that future mills should operate on exactly the reverse principle of the
Raymond centrifugal force ringroller mill: the grinding surface should
rotate so that the centrally-fed raw material is carried under rollers
by centrifugal force. The disadvantageous and severely limited centrifugal
force pendulum action of the rollers meant that spring-loaded rollers had
to be used to generate the grinding forces.
The roller axles were therefore
secured in stationary rocker arms which allowed them to pivot in one plane.
The end of each arm had an adjustable spring which generated the grinding
forces
.
The new mill design was
named the Maximal mill. Incidentally, this mill was the first to have a
grinding surface referred to as a bowl. This designation indicated the
very high edge of the rotating body in relation to its diameter.
Fig 4: Maximal
Mill 1925
This type of mill, which
is shown in Fig. 4, was no longer bound by the Combustion Engineering licence
for the Raymond mill. This signified a large step in development in both
financial and technological terms.
7. FROM THE MAXINAMAL
MILL TO THE LOESCHE MILL
1927 saw the next stage
of development. From its advantageous position the Curt von Grueber Company
under Ernst Curt Loesche was able to analyze the experience and the operating
values gathered with the different mill designs and to carry out a comparative
evaluation of the types of mill.
The Maximal mill still suffered
to some extent from the same problems as the ring-roller mill: the roller
diameters could only be increased to a limited extent because of the grinding
surface surrounding them. In addition to this the material being ground
was thrown off downwards over the grinding bowl without being deliberatly
carried upwards by the air stream in the mill as in the Raymond ring-roller
mill. These disadvantages were avoided in the next design change.
The wall of the grinding
bowl was inclined backwards and instead of being vertical the roller axles
were now inclined at 45° as shown in Fig. 5.
Fig 5: Mill
constructed 1927
At last it was possible
to increase the diameters of the rollers. This meant that the mill grinding
output relative to the bowl diameter could be raised above that of the
earlier systems. A curved surface for the roller tyre was also tested in
this mill. The number of rollers was still three based on the centrifugal
force ring-roller mill. The gas flow system was changed back to that of
the Raymond ring-roller mill. An annular gap was provided between the housing
and the edge of the bowl to improve the air flow.
8. THE FIRST LOESCHE
MILL
In 1928, only a year later,
a still flatter grinding surface was introduced. This was the logical consequence
of the idea of increasing the roller diameter still further and of controlling
the flow of the material on the grinding bowl by the inclination of the
bowl surface (Fig. 6).
Fig 6: First
LOESCHE Mill, constructed 1928
The rollers were now held in
rocker arms which had a strong resemblance to the rocker arms still used
today in small Loesche mills. The maximal mill had still shown a certain
relationship to the Raymond mill as far as the arrangement of the rollers
in relation to the bowl was concerned although the grinding principle was
reversed. Now, in the year 1928, a completely different and new grinding
system had been produced.
This new mill was also offered
to the BEWAG. They accepted the machine on the condition that it should
be called the "Loesche mill". This condition was intended to make it absolutely
clear that if it were a failure the inventor of the mill carrying his name
was responsible for the functioning of the machine and not the department
of BEWAG dealing with new construction.
Fig 7: LOESCE
Mill with 3-rollers (1928)
Fig. 7 shows the first Loesche
mill with integral drive and, even in 1928, a dynamic classifier!
It can be seen that the
number of rollers was originally adopted from the Raymond mill. At that
time many mills were fitted with 3 rollers. In
one case a mill was also made with 4 rollers. However, this was not very
successful. It was thought that the larger number of rollers were a disadvantage
because there was mutual interference between the rollers. Nowadays we
know that this was not the reason. Probably at that time it was not realized
that at retrogressive step had been taken. The smaller roller diameter
reduced the ability to draw in the material being ground. The small rollers
could not "grip" coarse pieces in the feed material. Shortly after this
Loesche mills were only made with 2 rollers.
After 1928 the inclination
of the grinding bowl was altered in stages to still flatter angles [1].
At the same time there was a decrease in the angle of the roller axle to
the horizontal. In the individual stages of the development the roller
axles of the conical rollers changed from an inclination of 90° to
45°, to 30°, to 22.5° and finally to 15°, the present value
which was being used by 1935. Cylindrical rollers with horizontal axles
were also tested. However, this solution was not followed up because of
a definite disproportion between high wear on the one hand and good grinding
performance on the other.
9. LOESCHE MILLS WITH
STEEL SPRING LOADING SYSTEMS
The spring loading system
also developed in stages alongside the grinding rollers. The Maximal mill
and the first models of the Loesche mill still had individual springs for
each grinding roller. These were open steel springs positioned between
the rocker arms and the mill housing. With increasingly flatter grinding
bowls, larger grinding rollers and a more vertical movement of the rollers
it became desirable to balance the roller forces between one another to
achieve a uniform loading on the thrust bearing which had to take the grinding
forces of the rollers. From then on the springs were no longer each allocated
to just one roller.
The rocker arms were instead
linked to one another using the spring loading system. In this way it was
possible to even out the grinding forces regardless of the position of
each roller on the grinding bed. With increasing mill sizes the springs
were placed in oil-filled tubes to assist the maintenance.
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Fig 8: LOESCHE
Mill, 1935
Fig. 8 shows a Loesche mill
of this design. The diagram also shows that in the strict sense of the
word the grinding bowl is no longer a bowl as the edge has become very
low when compared with the diameter. In this respect it is logical for
the standard description to speak only of roller grinding mills.
In the early days the Loesche
mill was driven though open gear wheels. This is illustrated in the previous
diagrams. Ease of maintenance required closed gear systems. These were
soon introduced at the beginning of the thirties. At the same time it became
clear that it is logical to accomodate the thrust bearing - which has to
take the grinding forces - inside the gear housing. This protected it well
against the penetration of dust, and it could be lubricated with the same
lubricating oil and cooled with the same cooling water as the gear wheels.
It was early practice to
provide the air-swept Loesche mill with a classifier on the mill housing.
Even in the twenties the different processes of
-
grinding
-
drying
-
classifying
-
transport of the ground product
were combined in a single mechanical
unit. The static centrifugal classifier was soon replaced by a rotating
basket classifier. A finer product of greater homogenity could be produced
with this rotary classifier than with the static classifier. Mills and
classifiers corresponding to this stage of development were produced with
great success until about 1960. The maximum throughput achieved with cement
raw materials was 50 t/h. The largest mill size was an LM 20 with an external
grinding surface diameter of 2.0 in and 2 rollers of 1.5 in average diameter.
10. THE FULLER-PETERS
MILL
In 1906 when Curt von Grueber
returned to Berlin from the USA Claudius Peters also came back from the
USA to Hamburg. Claudius Peters brought with him the licence for the Fuller
mill which had been invented in the United States by a Colonel Fuller.
It was a ring-ball roller grinding mill as shown in its present form in
Fig. 9.
Bild 9: Peters
Mill
The mill has the same structure
as an axial ball bearing. Balls roll over the material to be ground on
a horizontal troughshaped grinding surface. The balls are pressed down
onto the material by a spring - loaded thrust ring.
After the Fuller licence
had expired Claudius Peters brought the mill onto the market with some
further developments under the name of the Peters mill.
Like Ernst Curt Loesche,
Claudius Peters had also recognized that the grinding capacity of the mill
depended critically on the use of larger grinding bodies. The special feature
of the Peters mill - when compared with the original Fuller mill - lay
in the use of fewer balls of larger diameter. Nowadays Claudius Peters
AG is a sister company of Babcock & Wilcox International Group PLC,
Crawley/England. The Peters mill has proven itself in the market as a robust
machine. In the mill there are no roller bearings. Against this advantage
there is the disadvantage of rather rough running. During operation the
lower portion of the balls are cushioned on the grinding bed whilst the
upper portion is always in metallic contact with the pressure ring. The
balls running without a cage occasionally bump into each other horizontally
and attempt - as they are turning in the same direction - to climb up on
each other. This naturally leads to vibrations which increase with the
mass of the balls. This is one reason why the growth of the mill size is
restricted.
Unlike the Loesche mill
the grinding bodies revolve with the grinding bowl. In the Loesche mill
each grinding body is supported on an axle located in a fixed position
and during one rotation of the grinding body it rolls over a length of
the grinding surface corresponding to its circumference.
With free-rolling grinding
bodies, such as with the balls in the Peters mill, the grinding surface
must cover a greater distance to achieve the same length of roller path.
If the same comminution length is to be achieved as with stationary roller
bodies then the grinding surface must be run at higher speeds. However,
there are limits to this method due to the progressive increase in the
dynamic forces in the grinding bodies.
Without raising the grinding
speed the grinding output can only be increased by enlarging the mill diameter
-grinding bowl and housing - and increasing the number of rolling bodies.
This method is also limited because for a very large housing diameter it
is no longer possible to provide sufficient lift to the material being
ground using the flow of gas which is normally available. This is most
easily achieved with coal as coal has a significantly lower density than
most other materials which have to be ground. For this reason the Peters
mill has become known chiefly as a comminution machine for coal.
Furthermore the mill is
successfully used for the grinding of dry materials such as raw phosphate.
Another noteworthy application is its use as a calcinator for gypsum. It
can cope with gas entry temperatures of 600°C!
When the grinding elements
are worn, after opening the large mill doors, all the wear parts (grinding
balls and rings) can be exchanged with the help of a servicing device and
without further measures. The grinding elements have very long service
lives (operating hours).
In Great Britain a grinding
machine similar to the Peters mill is known as the Babcock-E-mill. The
E-mill is also a ring-ball mill but possesses a larger quantity of smaller
grinding balls compared to the Peters mill and is mainly used as a coal
mill for firing steam boilers.
11. THE BERZ MILL
Up to the end of the second
world war Max Berz worked as head of the design department in the Curt
von Grueber Maschinenbauanstalt, the predecessor of Loesche. He returned
home to Bavaria as any progress in his work in Berlin was out of the question
because of the political situation.
In 1947 Max Berz developed
the MB mill. This machine was intended to combine the advantages of the
Peters mill, i.e. the absence of roller bearings in the grinding bodies,
with those of the Loesche mill, i.e. the use of large rollers. This resulted
in a mill the principles of which can be seen in Fig. 10.
Fig 10: MB
Mill (Berz)
The Berz mill basically
has to operate with 3 rollers which are pressed down together from above
onto the grinding surface by a thrust ring with guide rails in accordance
with the principle of a statically defined three-point support. The grinding
surface has to have a tracking groove as a guide for the rollers. The guide
system operating in the dust stream is clearly subjected to just as much
wear as the spacers between the rollers.
Like the ring ball mill
the roller bodies are not held in one place. They are driven by the underlying
grinding surface, turn on their own and also travel - guided on the thrust
ring - with reduced speed over the grinding bowl. A relatively large increase
in mill diameter is needed to raise the grinding capacitiy.
This mill is a failure in
the cement industry. The Vereinigte Kesselwerke/Duesseldorf have helped
the MB mills built there under licence to a limited measure of success
as coal injection mills in power stations where they are used for the comminution
of coal which is relatively easy to grind.
12. THE MPS MILL
The licensing of the MB
mill by Gebr. Pfeiffer AG, Kaiserslautern/Germany was only effective for
a short time. In this company the MB mill concept was redesigned under
Siegfried Schauer. Siegfried Schauer, also a previous employee of the Curt
von Grueber Maschinenbauanstalt and a designer for Ernst Curt Loesche at
Berlin Teltow, joined Gebr. Pfeiffer AG after the war.
He combined the advantages
of the Loesche mill with the basic concept of the MB mill to form the MPS
mill:
The 3-roller system with
the tracking groove in the grinding surface was adopted from the MB mill,
as shown in Fig. 11. However, the rollers were fitted with roller bearings
on axles which were in turn held in loading members located in fixed positions.
The rollers are pressed down on the grinding bed by a thrust frame, but
now this takes place indirectly via a loading member which can adjust itself
through a pivot below the thrust frame. This principle eliminates the critical
wear points prevalent in the MB mill.
Fig: 11: Pfeiffer-MPS-Mill
In small mills the grinding
force is generated by a thrust ring and springs mounted above the thrust
frame. Preloading is achieved by a hydraulic tensioning device. In larger
mills the triangular thrust frame is coupled directly to tensioning rods
providing the grinding force. Preloading and spring effects are created
hydropneumatically.
The thrust frame is guided
in the upper portion of the hexagonal or cylindrical mill body. These guides
permit vertical movements. In order to locate the 3-roller-system within
the mill a damped torque reaction is provided by mounting the tensioning
rods at variable inclinations.
As in the Loesche mill the
rollers grind at full grinding surface speed. Redesigning the MB mill under
the leadership of Siegfried Schauer led to the mill design known since
the sixties as the Pfeiffer MPS mill. The success in the market confirmed
the correctness of the applied measures.
For changing the rollers
the "lift and swing system" is used more and more; the thrust frame is
pushed upwards via the tensioning rods thus maintaining the stable 3-point-support
of the system. The rollers can be swung outwards by means of pivot arms.
Large rollers are equipped with segmented tires. The segments can be made
as solid hard castings without the danger of breakage caused by temperature
stresses.
Unlike mills with freely-circulating
rolling bodies, such as the Peters mill, or those with grinding bodies
supported individually in rocker arms located in fixed positions, such
as the Loesche mill, the MPS mill functions with three rollers which provide
a statically determined 3-point support system.
An auxiliary drive is used
to start the mill. With its help the grinding bowl is first turned at very
low speed to flatten the grinding bed before the main motor is connected
and the bowl is accelerated to its rated speed. Normally in practice the
mill is not emptied completely and so upon restarting a grinding bed is
already present. Additionally the preloading of the roller can be reduced
during start-up of the mill. The auxiliary drive is also used as an "inching
drive" for the inspection of the rollers and the table.
The vertical movement of
each roller slightly affects the other rollers as all the rollers are sprung-loaded
against the grinding surface by a common thrust frame.
13. The Polysius roller
grinding mill
At the start of the sixties
Krupp acquired the Berz mill licence for the cement industry. As far as
is known, two mills were built for grinding cement raw materials, but these
disappeared from the market again after a short time because of the uncontrollable
dynamic forces involved.
The development of roller
grinding mills within Krupp was initiated by the Krupp-Polysius AG in Neubeckum/Germany.
This undoubtedly occurred because of their efforts to obtain their own
roller grinding mill to add to the range of machines for equipping cement
plants.
Under the designation "Polysius
roller mill" the comminution machine shown in Fig. 12 first became known
in the field of cement raw material grinding and then in the last few years
also for coal grinding in cement works.
Fig 12: Polysius
Roller Mill
The mill is a 4-roller grinding
unit. The mechanical design is characterized by two double rollers. Each
double roller has a roller carrier located in a fixed position. Roller
axles, on which the rollers run on roller bearings, are fixed in the roller
carriers.
The grinding rollers are
hemispherical. In conjunction with a double tracking groove in the grinding
surface it creates a stable grinding bed which in turn is important for
a quiet running and high availability operation. Two of these units stand
parallel to one another on the grinding bowl. Bolts at both ends of the
roller carrier (see Fig. 13) guide the double roller within the housing.
Fig 13: Roller
carrier with double roller (Polysius)
The guide system permits
vertical movements and tilting of the unit around the horizontal roller
axles. This compensates for variations in thickness of the grinding bed
between the inner an outer rollers; both rollers are always in contact.
The bolts restrict the tangential play within the housing by means of guide
consoles. The housing takes the tangential thrust which acts on the double
roller due to the rotation of the bowl. The unit consisting of roller carrier
and double roller is pulled onto the grinding bed by a hydraulic linkage.
When the mill is run empty
- i.e. for servicing purposes there can be metallic contact between the
rollers and the grinding bowl. The mill is started in a partially unloaded
state by reducing the hydraulic operating pressure in the spring loading
system. Large mill used for cement raw materials normally have auxiliary
motors. These are used for initial smoothing of the grinding bed at very
low bowl speeds as with the MPS mill - before the main drive is energized.
They are also useful during servicing work.
During the "springing motion",
i.e. the vertical displacement of the rollers, both rollers of a pair are
mutually selfsupporting because the complete unit can pivot to suit the
grinding bed. There is no mutual interaction between one double roller
and the other. Each double roller reaches its own setting individually.
The inner roller, i.e. the roller
positioned closer to the mill Centre, runs significantly more slowly than
the outer roller to match its smaller path diameter on the grinding surface.
The inner rollers therefore wear more slowly than the outer ones. The relative
velocity of the two rollers on the grinding path is small, however.
To change rollers the unit
consisting of roller carrier and double roller must be taken right out
of the mill housing using an overhead rail conveyor. This requires an assembly
area near the mill of almost the same size as the mill base, in case the
servicing work should be carried out here.
(Will be continued) |
