Cement mill
The emergence of Portland cement in the 1840s made grinding considerably more difficult, because the clinker produced by the kiln is often as hard as the millstone material.
Fine grinding lessens this effect, and early cements had to be stored for several months to give the calcium oxide time to hydrate before it was fit for sale.
In Portland cement, a little calcium sulfate (typically 3-10%) is added in order to retard the hydration of tricalcium aluminate.
It is normal to add a certain amount of water, and small quantities of organic grinding aids and performance enhancers.
Grinding aids are typically chemicals added at a rate of 0.01-0.03% that coat the newly formed surfaces of broken mineral particles and prevent re-agglomeration.
Heat generated in the grinding process causes gypsum (CaSO4.2H2O) to lose water, forming bassanite (CaSO4.0.2-0.7H2O) or γ-anhydrite (CaSO4.~0.05H2O).
On the other hand, if milling temperature is too low, insufficient rapidly soluble sulfate is available and this causes "flash set" - an irreversible stiffening of the mix.
This helps keep the mill cool, and sweeps out evaporated moisture which would otherwise cause hydration and disrupt material flow.
Modern separators are capable of making a very precise size "cut" and contribute significantly to the reduction of energy consumption, and have the additional advantage that they cool both the product and the returned material, thus minimizing overheating.
This can be remedied by including 5% calcium carbonate in the cement: this soft mineral produces adequate ultra-fines on the first pass through the mill.
The toughest mineral is belite, because it is harder, and is somewhat plastic, so that crystals tend to flatten rather than shatter when impacted in the mill.
On the other hand, long burning at excess temperature, and slow cooling, lead to large, well-formed crystals that are hard to grind and un-reactive.
These have been used for many years for the less exacting raw-milling process, but recently roller mills, in combination with high-efficiency separators, have been used for cement grinding.
However, the narrowness of the particle size distribution of the cement is problematic, and the process has yet to receive wide acceptance.
However, particle size distribution is again a problem, and roll presses are now increasingly popular as a "pre-grind" process, with the cement finished in a single chamber ball mill.
This gives good cement performance, and reduces energy consumption by 20-40% compared with a standard ball mill system.
This is for two reasons: In addition to control of temperature (mentioned above), the main requirement is to obtain a consistent fineness of the product.
This used to be mainly a research tool, but with the advent of cheap, industrialized laser-diffraction analyzers, its use for routine control is becoming more frequent.
This may take the form of a desk-top analyzer fed with automatically gathered samples in a robotized laboratory, or, increasingly commonly, instruments attached directly to the output ducts of the mill.
