The Roller Grinding MiII - Its History and Current Situation - Part 2 
Dipl.-Ing. H. Brundiek, Duesseldorf
 

 
  

14. THE ATOX MILL 

For some years the Danish cement machinery manufacturer F.L. Smidth, Copenhagen built the Pfeiffer MPS mill under licence for grinding cement raw materials. A licence for building MPS coal mills for power stations was not available as Pfeiffer had awarded this licence to the Deutsche Babcock Werke AG, Oberhausen/Germany. After the expiry of the MPS licence, F.L. Smidth developed their own mill under the name Atox mill. As a rough approximation this roller-grinding mill can be regarded as a variant of the MPS mill. 
  

Fig 14 F.L.S-Atox mill
 
As Fig. 14 shows, the 3-roller system of the MB mill, which had already acted as godfather to the MPS development, was also adopted in the Atox roller grinding mill. Like earlier variants of the MB mill the rollers run in roller bearings on axles which are fixed in the centre of the mill in a starshaped or triangular mount. In the MB variants this mount was supported so that it could rotate on a kingpin in the centre of the grinding bowl. This allowed the 3-roller unit to rotate relative to the bowl around the bowl centre. 

However, the triangular mount of the Atox mill has no pivot pin. It just carries the three horizontal roller axles each offset by 120° from the others and guides the 3 cylindrical rollers. The axles pass right through the rollers and terminate as connecting pieces. This intrinsically rigid 3-roller star unit rests in a statically defined 3-point support system on the horizontal grinding surface of the bowl. As with the LOESCHE mill the horizontal grinding surface allows the use of very large rollers on the grinding bowl. The Atox mill has no need for a hemispherical grinding roller shape such as is needed in the MPS mill for tracking in a groove in the grinding surface because of pressure applied from above via articulated joints. The rigid 3-point system allows linear contact to take place between each roller and the grinding surface. 

The 3-roller unit is held in a fixed position in the grinding chamber. The rollers only rotate about their own axles but not around the centre of the bowl.  
  

Fig 15: Atox 3-roller-unit
  
Horizontal stay bars, as shown in Fig. 15, are anchored tangentially in the mill housing to support the turning moment against the housing. They are attached to the connecting pieces on the roller axles which project towards the housing from the rollers. The tie rods which run diagonally downwards and are part of the hydraulic linkage which draws the 3-roller unit against the grinding bed are also attached to the three axle connecting pieces. 

To start the mill the entire roller unit can - because of the rigid connection between the central star mount and the 3 rollers - be lifted by a few centimetres by reversing the hydraulic pressure in the cylinders of the hydropneumatic spring loading system. This eliminates the need for an auxiliary drive. 
The vertical movement of a roller when passing over the grinding bed affects the other two rollers. As all three rollers are connected rigidly to one another the grinding unit tilts about the line connecting the contact points of 2 rollers if the third roller is lifted. 

The Atox rollers work without levers and thrust pieces, which results in lower weights and correspondingly lower manufacturing costs. The rollers cannot, however, adjust themselves individually to suit the grinding bed. It is thus difficult to equalise the wear over the width of the roller. The mass of the rollers naturally increases with increasing mill size. It should be noted there that the dynamic forces, which occur during vertical acceleration, are developed not just by one roller but also - because of the rigid 3-roller system - from a certain proportion of the combined mass. 

To assist the changing of the grinding elements the roller tyres are segmented. The hydraulic lifting device for the roller unit can also be used as an aid to maintenance for supporting the statically defined 3-roller system. 
  
  
15. OTHER TYPES OF ROLLER GRINDING MILL 
  
The roller grinding mills described above may show some signs of their origins. They are, however, all characterized by the use of original ideas. Where they are made under licence the name of the licensee is also usually found linked to the mill designation. 

There are also a series of other types of roller grinding mills which represent a combination of known structural elements and, strictly speaking, are not original developments. These include the Japanese OK (Onoda) and CK (Kawasaki) roller grinding mills and the IHI mill roller grinding mill. Further references to these will be found in section 16. 

Apart from this, imitations or copies of well-known original developments are found in brochures, in the reference literature, and to a small extent in practice. The "disc" mill of Prerov/Czechoslovakia and the "conical-roller roller grinding mill with 4 grinding elements" of ZAB Dessau can be cited as examples. The latter corresponds to the LOESCHE 4-roller mill based on the Module concept. 
  

Fig 16: VR mill, combustion Engineering, USA
  
Several years ago a few examples of the VR mill (Vertical Roller Mill) were placed on the market in the USA by Combustion Engineering (see Fig. 16). 
  
  
16. GRINGING ELEMENTS FOR ROLLER GRINDING MILLS 
  
Now that the various roller grinding mills and their characteristic features have been introduced it should be of interest to consider the shapes of the grinding elements. There are often discussions about which shape of grinding element should be selected for optimum comminution in a mill. This ignores the origins of these shapes which are in fact linked with kinematic considerations and not with questions of comminution efficiency. 

When elastic forces were first applied to rollers through lever systems in addition to the dead weight of the roller itself it also became necessary to deal with the kinematic requirements of the lever systems. The object of all solutions must be to allow the rollers to act as perpendiculary as possible to the grinding plane. Fig. 17 shows the interrelationships in greater detail: 

Fig 17: Grinding element shapes; 
lever operated grinding bodies
  
  • In a ring-roller mill the rollers are suspended on vertical axles from above and press sideways against a grinding ring at right angles to this axle. The grinding plane must therefore be vertical (see upper diagram).
  • If the roller axle and its pivot point are set at an inclination then the roller describes a circular motion around this pivot which is now lower. To allow the rollers to act approximatly at right angles to the grinding surface the grinding plane must be inclined towards the rollers. This produces a conical grinding surface which approximately intersects the plane of the pivot point (see central diagram). This solution is used, for example, in the American Raymond-Bowl mill produced by Combustion Engineering / USA and in the EVT coal mill. EVT in Stuttgart/Germany is now partly owned by Alsthom S.A. and CE/USA.
  • If the roller axles are inclined even further and are allowed to pivot about a depressed pivot point then, on the same principle, a grinding surface positioned approximately at right angles to the roller movement automatically becomes a horizontal grinding surface. In this case the grinding surface also lies approximately in the plane of the pivot point of the lever system: LOESCHE mill (see lower diagram). A roller with an elevated pivot point would ruin a horizontal grinding surface, especially the edge of the bowl because it would no longer move at right angles to the grinding plane.
Fig 18: Grinding element shapes; 
track-guided bodies
Fig. 18 shows the kinematic situation in roller grinding mills with tracking grooves. 
 
  • If, instead of individual roller levers, a statically defined 3-roller system is selected which is loaded from above and in which the roller thrust members are located under a load star with articulated joints then on a level grinding surface the rotation of the bowl would cause the rollers to move outwards over the edge of the bowl. To prevent this and to provide the roller with a reaction against the upper joint a troughed groove is needed to act as a track in which the roller can align itself within the play in the tracking system. This results in the MPS system (see upper diagram). The upper limit to the bowl speed is set by the depth of the trough in order to prevent the roller from climbing up onto the retaining lip.
  • In principle the same kinematic system also applies to the MB mill.
  • The ring-ball mill shown in the lower diagram is a special form of the roller grinding mill. This machine works on the principle of an axial ball bearing. Like the MPS the grinding bodies must also be guided in tracking grooves; in this case both above in the thrust ring and underneath in the grinding ring.
  • The two pairs of hemi-Al spherical rollers with horizontal axles in the Polysius roller grinding mill locate themselves in the concave grooves of the grinding ring. The necessary kinematic limitation of the axial and radial play is achieved by guiding the roller carrier in the housing as described in section 13.
  • The Atox 3-roller system with no levers or joints works with horizontal roller axles. The stays for resisting the turning moment described in section 14 are sufficient to guide the 3-roller unit on the bowl. The combination of cylindrical rollers with a horizontal grinding surface is therefore logical. Hemispherical rollers and a groove in the grinding surface combined with turning moment anchors would lead to kinematic redundancy.
The roller grinding mills and their kinematic systems described were all developed in the USA and Europe. In all the models the governing kinematic conditions have led quite logically to grinding element shapes appropriate to them. In this context the development of roller grinding mills in Japan is interesting. In some models well-known design elements have been adopted from roller grinding mills developed in Europe. These elements were sometimes combined with one another without considering the kinematic requirements. The OK mill of Onoda/Japan shown in Fig. 19 can serve as an example. 
  
Fig 19: OK mill; Onoda
  
  
In this mill a rocker arm of the LOESCHE mill pattern was used with a hemispherical roller of the MPS mill pattern. By guiding the roller in a lever with fixed pivot axis on the one hand and by a hemispherical roller tyre in a grinding groove on the other the system has redundant constraint. The hemispherical roller attempts to align itself in the groove to match its loading and the bowl speed. However, as it is restrained in the rocker arm it exerts additional forces on the rocker arm and its support system. Because of the restraint in the arm the roller will itself gradually, by increased wear on the side of the dam ring, achieve the degree of freedom denied it by the groove. 

The same applies to the Kawasaki CK mill, which structurally is virtually the same as the OK mill, and to the IHI mill. The latter is shown in Fig. 20. 
  

Fig 20: IHI roller grinding mill
 

17. THE SIZE DEVELOPMENT OF THE ROLLER GRINDING MILL  
 
 
17.1 ROTARY KILN INITIATING FACTOR 
 
At the end of Section 8 on the LOESCHE roller grinding mill with steel spring loading system, mention was made of maximum cement raw material throughput rate of 50 t/h reached in 1960 with the LM 20 mill. At that time the LOESCHE mill was the only roller grinding mill available for use in the cement industry. From the 30s it was being used with gradually mounting success because of its advantages over the still widely used tube mills. The main advantages at that time were: 

  • significantly lower specific power consumption than the tube mill; however, this was less valuable than today because of the lower energy costs 
  •  smaller space requirements 
  •  less noise 
  •  more rapid exchange of grinding elements 

    •  
At the start of the 60s preheater rotary kilns were developed for the cement industry. The kiln capacities increased above 1000 t/d to about 1500 t/d. The output of cement raw meal required for the kiln gradually grew to 80 t/h, 100t/h and 120 t/h. The volumetric flow and the heat content of the kiln preheater exhaust gas was ideally suited to the operation of an ,,air-swept" roller grinding mill for transporting, drying and classifying the raw material so kiln development was the actual stimulant for developing the size of the roller grinding mill. 

In addition to large flow cross-sections in comparison to the bottle-neck constrictions of the tube mill trunnion bearing the rapid control behaviour of the roller grinding mill was also an advantage. This is related to the small entrained volume of raw material. 
 
 
17.2 DESIGN CONSEQUENCES 
 
Increasing product rates of the roller grinding mills had to be achieved by larger rollers on larger grinding bowls. Larger rollers also required stronger spring loading systems. This could only be achieved within very narrow limits using steel spring systems. The steel springs systems in themselves formed increasingly large undamped masses. Above a certain size of roller they were no longer manageable because of the space and forces involved. In LOESCHE the Hydropneumatic Spring-Loading system for the LOESCHE mill was developed in 1961 by Peter Gauer, who was then chief designer. This reduced the undamped mass of the "spring loading system" to the piston rod, piston and oil mass of the relatively small hydraulic cylinder. The Hydropneumatic Spring Loading system gained rapid acceptance. It was adopted by all manufacturers of roller grinding mills who came into the market later.

 18.  GRINDING PRINCIPLE, MODE OF OPERATION AND CONSTRUCTION  
 
The working and functional principles and the hydropneumatic spring loading system are explained using the example of LOESCHE mills as shown in Fig. 21: 
 
Fig 21: Modern LOESCHE mills 
 
A grinding surface with a vertical axis of rotation is driven through a gear system. Rollers located in fixed positions are pressed elastically against the grinding surface. The horizontal grinding surface combined with the roller axles positioned at shallow angles permits the use of large rollers on small areas, which is equivalent to a high output density. The left half of Fig. 21 shows the construction of the so-called Individual mills with 2 rollers. The right half shows large mills built on the Module system. The material to be ground is fed centrally from above onto the rotating grinding table either through the classifier positioned on top of the mill or from the side. The rollers pass over the feed material which is ground by the action of the forces exerted by the hydropneumatic roller spring loading system. When the grinding rollers run over the grinding bed on the table working pistons are lifted via rocker arms and guide rods. The oil in the upper cylinder chambers is forced into gas-filled hydraulic accumulators. The pulverized material is accelerated by centrifugal force into the region above the louvre ring which surrounds the grinding table where it is picked up by the upward stream of hot gas and carried to the classifier. 
  
Intimate contact with the hot gas causes spontaneous evaporation of the water contained in the material so the required mill outlet temperature of 70°C to 130°C is actually achieved within the grinding chamber. Temperatures of 150°C have already been used for special situations. 

The oversize material is rejected by the classifier in accordance with its setting and falls back onto the grinding table in the internal tailings recycle to be reground. The finished material passes through the classifier. 
 
The mill is driven by an electric motor through a special gear system. The roller forces are taken by a segmented thrust bearing in the gear system. The exchange of grinding elements is simple in concept. The system of guiding the rollers in rocker arms is designed to accommodate a swing-out device; by connecting up an auxiliary cylinder to the rocker arm the rocker-arm/roller unit can be pivoted completely clear of the mill housing. 
 
 
19. THE LOESCHE MODULE CONCEPT 
 
The requirements for the design of roller grinding mills were again increased at the start of the 70s by a technological leap in the field of cement kilns. These were equipped out as rotary kilns with precalciners which virtually doubled the previous outputs. The mill capacities required increased immediatly from about 120 t/h to 240 t/h. Gas flows and heat flows through the mills increased correspondingly. This gave LOESCHE the impetus for developing large mills on a Module system. It was a realization of the ideas of the author Horst Brundiek. The Module consists of a pedestal with rocker arm, roller and complete hydropneumatic spring loading system. 
 

Fig 22,23,24:left to right:
Roller in operating position
Roller in start position
Roller in service postion
 
Figs. 22, 23 and 24 show one such module in the operating position, in the starting position with hydraulically lifted roller and in the service position with the roller swung out. When the rocker-arm/roller unit is swung out either the annular roller tyre or a complete roller can be quickly and easily changed outside the mill. 
 
It is possible to start - even when the mill is full - with the rollers lifted clear of the grinding surface hydraulically. Unloaded the mill runs with about 40 % of the full load operating torque. It does not therefore require either a drive motor with increased starting torque or an auxiliary drive. 
  
Metallic contact of the rollers with the grinding surface is prevented either by mechanical spring buffers or by electronic control: an advantage which is made possible by the individual roller control in the rocker arms! 
 
The Module system permits mills to be built with 2, 3 or 4 rollers while retaining the unit described. Apart from manufacturing advantages and the saving in development technology this has decisive advantages for the user: 
  • the use of elements which have been tested in operation; 
  • the individual masses, which are of importance in the generation of dynamic forces, are restricted: the number of rollers, and not the individual roller mass, is increased:
Note: 
The experience obtained at the start of the LOESCHE mills which showed that 3 and 4 small rollers are less effective than 2 large rollers naturally no longer applies to large mills. The transition from 2 rollers to 3 and 4 rollers only takes place with rollers diameters from approximately 1600 mm, which are the components of the smallest Module. 
 
The roller grinding mill which is at present the largest in the world from the capacity point of view, i.e. an LM 50.4 LOESCHE 4-roller mill with an external grinding surface diameter of 5 m has a nominal finished material output rate of 550 t/h cement raw material for an average roller diameter of 2.360 mm. 
 
Fig 25: LOESCHE module in the basic and alternative position
 
By way of comparison it would be necessary to use a roller diameter of approximately 3000 mm to achieve the same grinding capacitiy with a LOESCHE roller grinding mill with 3 rollers. The mass of such a roller would be almost double that of the 4-roller mill. This shows very clearly the rise in the dynamic forces to be expected. 
  • Accurate and individual guidance of each roller in the rocker arm roller bearings allows the roller to work freely on the grinding bed without being affected by the other grinding bodies. In this way primary measures keep the dynamic forces low: 
  • The housing cross-section is easy to optimize by radial displacement of the module in relation to the table; this is illustrated in Fig. 25. 
  • The logical separation of the components directing the hot gas from those carrying forces permits the use of gas temperatures up to 700°C for drying, and at the same time avoids additional forces caused by thermal expansion. 
  • The specific investment costs DM/(t/h) are reduced, as shown qualitatively by the diagram in Fig. 26. 
Fig 26: Specific costs as a function 
of number of roller, LOESCHE 
 
A mechanical concept conforms to the market if the specific investment costs fall with increasing mill size. For a roller grinding mill the costs in DM per t/h should ideally fall hyperbolically with, for example, increasing table diameter. Hardly any design will be able to fulfill this requirement without reservations. 
The masses of the rollers of a roller grinding mill, for example, increase with the third power of the roller diameter, and the costs grow approximately in proportion. The specific costs of the mill therfore follow a parabola and not a hyperbola, i.e. up to a certain mill size the specific costs fall after which they increase again. 

To provide a better match with the hyperbolic profile the mills were then developed based on the LOESCHE Module system with 2, 3 or 4 rollers which form 3 interlocking parabolas and fit smoothly to the hyperbola within a limited band width. 
 
 
20. THE ADVANTAGES OF THE ROLLER GRINDING MILL 
 
When roller grinding mills are used as components of a process engineering plant they have a series of favourable characteristics which are a ' ssociated partly with pure machine technology and partly with questions of process engineering. The following list reflects the most important advantages of the roller grinding mill: 
 

  • Small ground area requirements due to vertical construction. 
  • Small specific space requirements due to compact construction. 
  • Low noise running of the grinding unit; the main sources of noise from a working roller grinding mill are the drive (motor and gear system) and any sealing-air fans and air seals. 
  • low-noise and low-vibration running when empty for roller grinding mills having individual control of the rollers in rocker arms which prevent metallic contact between the rollers and the grinding surface either by mechanical stops or hydraulic control. 
  • The ability to draw in coarse feed material with edge lengths of about 5 to 8 % of the average roller diameter saves energy in precrushing. 
  • Low specific wear - approximately 25 % less than with tube mills - combined with very short changing times for the roller tyres leads to lower operating costs. 
  • Long service life of the grinding elements through use of hardwearing materials, through wear-reducing methods and optimization measures with the increasing experience of the operator. 
  • Long service life of the housing lining by utilizing physical effects for the dust flows. 
  • Easy and rapid variation of the grinding forces with the aid of the hydropneumatic spring loading system to match the comminution work to the current physical properties of the material being ground. 
  • Simultaneous grinding, homogenizing, drying, classifying and transport of the material in one machine. 
  • Very effective drying during the grinding / classifying process due to high recycle of oversize material in the grinding and classifying space. 
  • Good controllability thanks to short residence time of the material in the grinding chamber; very suitable for fully automatic operation due to the short reaction time when there are changes in the process. 
  • Large control range of 4-roller mills with individual roller control for paired working pressure control for the rollers, i.e. 2-roller operation x 2 in one housing on one grinding table. (LOESCHE system) 
  • High availability of the roller grinding mill permits the most cost-effective combination in, for example, the cement industry of 1 kiln with 1 mill. 
  • 4-roller mills with rollers guided individually in the rocker arms (LOESCHE system) also permit 2-roller emergency operation in combination with sufficiently high volume flow at approximately 70 % of the full-load product rate, and only a slight reduction of the kiln output if the mill runs 24 h/d. 
  • Simultaneuos production of 2 or 3 particle size fractions by the use of special classifiers on the roller grinding mill. 
  • Low specific energy consumption e (kWh/t) by avoiding the circulation of loose grinding media as in tube mills. 
  • Low specific energy consumption e (kWh/t) as to a great extent there is rolling friction with little rolling resistance due to the use of grinding bodies with large diameters. 
  • Low specific energy consumption e (kWh/t) due to the comminution of relatively thin particle layers in the bed of material. 
  • Cost-effective comminution of the material bed through immediate break-up of agglomerates of pulverized particles with the integral classifying process in the same machine. 
  • Minimization of the total power consumption by minimizing the flow energy (mill resistance) with the aid of a very uniform flow of dust-laden gas from the optimized louvre ring with simultaneous maximization of the grinding output, which is not achieved with reduced volume flow and external material recycling. 
21. DESIGN EXAMPLE AND PROSPECTS 
 
A LOESCHE 4-roller mill with 3.5 in table diameter, which is in operation in southern Italy in Cemensud's Italcementi cement plant in Vibo Valentia, can be cited as an example for some of the advantages of roller grinding mills referred to. Fig. 27 shows the mill at Vibo Valentia.  
 
Fig 27: LOESCHE 4-roller mill
 
Ever since the time of inception of the LOESCHE mill it has been known that roller grinding mills are first-class gas stream dryers because the moist raw material is pulverized in a relatively thin bed of material. Even after a single roller pass almost all the pulverized material is subjected to high centrifugal forces due to the rotation of the table which accelerates it into the region above the louvre ring surrounding the table. In the turbulent zone formed above the louvre ring the majority of the water is evaporated spontaneously from the raw material. The classifier on top of the mill therefore separates the coarse material, which has already been substantially dried, from the finished material and guides it back to the table for recrushing. On the grinding surface predried material mixes with the fresh material so that the moisture content of the mixture in the grinding bed is significantly lower than in the raw feed. To a great extent this also prevents caking on the grinding surface. The requirements for simultaneous grinding and drying of moist raw material are a thin bed of material, high hot-gas temperatures and trouble-free introduction of the raw material in the mill. 
 
As has been proved in the LOESCHE research station, it is possible to cope with raw material - including cement raw materials - with feed moistures of 25 %. The positive test results prompted the Italian clients to order a LOESCHE roller grinding mill for grinding cement raw materials with a maximum mixed feed moisture content of 23 %. The moisture content of the marl, which has a CaC03 content of about 60 %, lies between 20 % and 28 %, the limestone has a moisture content of 11 % to 22 %. No previous industrial solution had pulverized such a moist material in a roller grinding mill without predrying. 
There was such faith in the functioning of the mill that the technical director of Italcementi, Professor Bucchi, converted the idea of „hornogeneous grinding" into practice as a new technology; no raw material homogenizing system was needed. 
 
Both the main components of the material to be ground are transported from the quarry by 2 conveyor belts with proportional control directly into the mill via an air-lock and fed centrally from above through the dynamic classifier. The LOESCHE mill dries and homogenizes the raw material in the gas stream during the grinding process. The exhaust gases from the rotary kiln are used f or drying, and the meal is supplied directly - after separation in an, electrostatic precipitator - to the burning process. Because of the high raw material moisture content the kiln waste gases are heated to 540°C in a muffle furnace; temperatures up to 700°C are possible. 
 
The meal separated in the filter is conveyed directly to the kiln without any intermediate silo. As the kiln is the command variable the mill must follow all the fluctuations of the kiln immediately. This means that if less raw meal is being sintered to clinker in the kiln the mill must produce proportionally less meal. At the same time the amount of kiln hot gas available to the mill falls with the clinker production. 
 
The mill is required to have a control capability between 50 % and 100 %, in this case 75 t/h to 150 t/h, during the kiln running time. The kiln running time is about 4 months without stop. A raw meal buffer in a silo with a capacity of only about 100 t is available for starting up the kiln and for minimal servicing work on the kiln and mill. 
 
The outlook for further use of roller grinding mills is encouraging from the technological aspect and extremely interesting from the economic aspect.