Solid material comminution or disintegration – Processes – With application of fluid or lubricant material
Patent
1995-10-16
1997-07-29
Rosenbaum, Mark
Solid material comminution or disintegration
Processes
With application of fluid or lubricant material
241 21, 241 241, 241184, B02C 1716
Patent
active
056515057
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of grinding cement, and more specifically a method of grinding cement manufactured in conventional grinding systems on an industrial scale in order to produce special qualities with a larger specific surface and/or with a steeper particle size distribution curve than what is economically feasable with conventional technique.
PRIOR ART
The manufacture of special qualities of cement with a larger specific surface compared with cement of standard quality is normally accomplished through longer residence times and a larger recirculation ratio for not completely ground goods in the same type of grinding systems that are used for the manufacture of standard cement.
When grinding in a rotating drum, see for instance GB-A-1 160 569, the movement of the grinding bodies is achieved within the mill by the friction against a casing provided with lifters. The rotational speed of the mill is determined by the so called critical velocity at which the peripheral speed becomes so high that the centrifugal force on the grinding bodies exceeds the gravity. The diameter has a great influence on the capacity, but it has to be limited when using small balls, which in turn is necessary for the fine grinding. Then the length has to be increased instead, which might be disadvantageous at grinding to extreme fineness, such as a low energy efficiency.
The conventional grinding systems are best suited for grinding cement to a specific area of 300-400 m.sup.2 /kg (Blaine), i.e. down to a k.sub.80 of about 30-40 .mu.m. The factor k.sub.80 is defined as the sieve opening through which 80% by weight of the goods passes. At a grinding driven further the energy consumption increases heavily and because of the increasingly less efficient use of energy it becomes disadvantageous to use this system. Hereby a relatively flattened particle size distribution curve is obtained, which is undesirable. Moreover it has become evident that the mineral components or additives that are most easily ground are recovered in the finest fractions.
It is well known that under similar conditions cement hardens quicker when the specific surface of the ground cement clinker and the hydraulic additives becomes larger.
The finest fraction usually have a higher content of e.g. gypsum, tricalcium silicate and alkali since these are more easily ground than the other clinker constituents, Moreover, one has to count With the fact that the finest fractions react to a larger extent with the atmospheric humidity and carbon dioxide in the air which is present at the grinding compared with the coarser fractions.
It has also been shown that substantially bigger increases of the pressure resistance can be obtained if, with the same specific surface, the particle size distribution curve is made steeper, i.e. with a relatively narrow distribution. This means that k.sub.80 with the same specific surface decreases slightly and in relation to the steeper inclination.
The hardening of the cement mortar follows the reaction of the cement with water. This is the reason why the grain size of the cement is of great importance. A finely ground cement grain gives a large contact surface to the water, which means that the reactions start quickly. Since the reaction penetrates to the same depth in all grains irrespective of their sizes the not hydrated part becomes comparatively smaller in finer cement particles compared to in coarser particles.
Investigations performed by the inventor clearly show that in fine grinding of mineral products it is not only the physical courses connected to mechanical degradation that occur. The structure of the material, the surface characteristics and the reactivity are changed too and in certain cases chemical reactions occur because of the great energy input at the grinding, both when it takes place in a wet and in a dry environment. The changes increase with the further fine-grinding and dry-grinding gives larger changes than wet grinding. However it should be mentioned that these effects from
Cementa AB
Rosenbaum Mark
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