1 General information relating to grinding and combined drying and pulverization in HAZEMAG hammer mills
For crushing and simultaneously drying mineral raw materials – especially limestone, dolomite but also clays such as kaolin or bentonite are meant here – the single-rotor hammer mill series HUM, HUV or double-rotor HNM hammer mills are used. The RRSB grain size characteristic curves depicted in Fig. 1 describe the operating range of the HAZEMAG hammer mill that may be achieved in a simple pass. The grain size curves are average values for the processing of medium-hard limestone.
The products achieved by means of the HAZEMAG hammer mills’ impact system correspond to the respectively physico-technically possible grain size distributions in the product. The diagram illustrates the crushing effect on the grain size distribution which corresponds to a spectrum of basically displaceable in parallel grain size curves. Even though in many applications the generation of fines in the product is not desired they are unavoidable in grinding-technological respect because of the shape of the grain size characteristic curve.
2 Influences on the grain size properties
The granulometric composition of the finished product depends only in part of the feed material’s crushing behaviour. The configuration of the grain structure is mainly influenced by the following machine-specific factors:
The rotor circumferential speed, which may be chosen in the range of 25 to 100 m/s, depending on the desired grain structure
The gap width of the milling grates and/or perforated plates which are in the lower area of the mill’s working space
The configuration, i.e. the length of the grinding path(s), which are also arranged in the lower area of the mill’s working space
The rotor circumferential speed has the greatest influence on the fines produced in the finished product. The maximum admissible grain in the finished material is determined by the grate’s gap width or by the mesh aperture. It is advisable to use a frequency converter for the regulation of the rotor circumferential speed. By means of the combination of frequency converter and milling grates with different gap widths the HAZEMAG Hammer mills provide a very efficient output of the desired grain size distribution.
The HAZEMAG hammer mills are very versatile because their equipment may exactly be adapted to the respective application. HAZEMAG hammer mills have quickly rotating rotors in the working space, on which beaters (hammers) are flexibly mounted. Upon rotation the latter are radially oriented by centrifugal forces. The material to be crushed which is conveyed to the working space is comminuted through impact and beat as soon as the hammers strike it . During the impact procedure the material knocks against the moving beaters whereas in case of the stroke crushing is effected between two solid surfaces at a high stress speed. With a corresponding hammer mass and rotor circumferential speed the hammers maintain their extended position and only swivel out of this position in case of large feed sizes and hard or uncrushable foreign bodies .
The fixed grinding tools may either consist of stepped grinding paths, milling grates, punched sheets or a combination of these components. Depending on the machine type the symmetrically constructed mill casings may be opened manually or hydraulically. When opening the mill the snap ring grooves in which the grinding path tools lie, are laid bare, thus enabling the mounting parts to be easily accessible and exchangeable. The rotors are disk rotors and are mounted in heavy antifriction bearings, which – depending on the construction size – are lubricated with grease or oil.
The torsion-free mill casing is protected against wear through screwed-on lateral plates of wear-resistant material. The mill casing is mounted on a base plate, which reduces dynamic loads as far as possible. Due to the machine’s relatively low weight a trouble-free assembly of the complete unit even on lightweight basements and high steel platforms is possible.
4 Combined drying and pulverization systems
The drying and pulverization systems which may be combined with HAZEMAG hammer mills work according to the principle of short-time drying and are applied where a fine crushing at a simultaneous drying of the crushed material is required.
Drying is effected per convection, i.e. a hot gas hits the material to be crushed and effects the vaporization of water. Normally flue gases from a hot gas generator are used as hot gases; however, hot exhaust gases which are available on site are also usable. As fuels for the hot gas generator natural gas, heating oil or coal dust may be utilized. In combined drying and pulverization systems with HAZEMAG hammer mills no drying with a dormant material layer is effected. Grinding and simultaneous drying happens at a hot gas speed of 20 to 40 m/s; hence the drying period is within a range of only a few seconds. Drying is effected according to the direct current principle, where the material is permanently in motion and is only marginally heated-up, which is especially of great importance for the production of dry mortar, because too high material temperatures have a very negative effect on the subsequent process steps.
In order to achieve a successful drying the following points have to be complied with at the layout of a combined drying and pulverization system:
A high temperature gradient as possible between drying gas and material to be pulverized • A high difference in speed between hot gas and crushed material
A large specific surface area of the material to be crushed
In doing so thermal excessive stress of plant components and especially dew point undercuts have to be avoided at the coolingdown procedure in exhaust pipes and filters. Depending on size and design the specific heat requirement of combined drying and pulverization systems may amount to 800 to 2000 kcal/ kg water .
Crushing and drying complement one another very well at this combined procedure and support one another. The material stresses being produced by the heat input in the material to be crushed during the drying process support the crushing process. The enlarged product grain surface which results from the mechanic crushing in the hammer mill permits the quick evaporation of the water that adheres to the product grain matrix. The selection of the hammer mill within a combined drying and pulverization system is effected on the basis of the feed quantity and the water content of the material fed as well as of the required hot gas volume flow for drying.
5 Crushing ratio
The feed material which is conveyed to the processing chamber is crushed through impact as well as through attrition. Whilst on the inlet opening impact stress occurs, the crushed material is mainly comminuted through blows by the retracting effect of the rotor(s) in the lower part of the narrowing crushing room. It remains in the work space until it is discharged through the grate gaps or openings of the perforated plates. For the assessment of the crushing success generally certain data relating to the grain size structure of the crushed material before and after crushing suffice. If the largest grain size diameters each in the feed material and in the crushed material and the better accessible grain sizes d80 respectively are interrelated, the following characteristic value results :
Z80 = d80A/d80E
With the HAZEMAG hammer mill a very high crushing ratio is possible. Depending on the type series feed sizes of up to 250 mm are admissible. With medium-hard lime stone for example an end fineness of – 2 (6) mm with a fines share of approx. 50 % smaller than 0.09, may be achieved in one passage. Crushing ratios of 20:1 and higher are easily obtainable. Hence the achievable crushing ratio of the HAZEMAG hammer mills in relation to the throughput and demand of space is very high and second to none.
Due to the lateral feed of the double-rotor hammer mill type HNM the crushing degree may once more be increased by means of the double-stage grinding procedure. Here the lower part of the first working space is completely realized as grinding path and thus closed. The material to be ground leaves the grinding space only after having passed the second working space.
6 Power requirement of the hammer mill
For the correct dimensioning of the drive capacity the power requirement for the idling as well as for the actual crushing procedure of the material to be crushed is referred to. The power requirement in idling depends on the dimensions of the rotor mass. It nearly increases with the square of the rotor circumferential speed.
The power requirement determination for the respective application is effected via tests in the in-house HAZEMAG pilot plant station or is derived from empirically determined characteristic values.
Taking into account the mineralogical-petrographic rock properties and the desired crushing ratios the energy requirement of HAZEMAG hammer mills is in the range of 1…6 kWh/t. With good approximation the drive capacity depends at a linear rate from the throughput rate. Therefore the specific energy consumption is the smaller, the smaller the power requirement for the idling and the bigger the throughput rate is. The range of machines from HAZEMAG’s machine portfolio considers all product-specific, process-technical, market-economical and business requirements of the customer.