CEMENT MILL


cement mill

A cement mill (or finish mill ) is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills.

.A ball mill is a horizontal cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis, imparting a tumbling and cascading action to the balls.
The mill is usually divided into at least two chambers, allowing the use of different sizes of grinding media. Large balls are used at the inlet, to crush clinker nodules (which can be over 25 mm in diameter). Ball diameter here is in the range 60-80 mm. In a two-chamber mill, the media in the second chamber are typically in the range 15-40 mm, although media down to 5 mm are sometimes encountered. As a general rule, the size of media has to match the size of material being ground: large media can't produce the ultra-fine particles required in the finished cement, but small media can't break large clinker particles. Mills with as many as four chambers, allowing a tight segregation of media sizes, were once used, but this is now becoming rare. Alternatives to multi-chamber mills are:

pairs of mills, run in tandem, charged with different-sized media.
use of alternative technology (see Roll-presses below) to crush the clinker prior to fine-grinding in a ball mill.
A current of air is passed through the mill. This helps keep the mill cool, and sweeps out evaporated moisture which would otherwise cause hydration and disrupt material flow. The dusty exhaust air is cleaned, usually with bag filters.


Clinker hardness
The hardness of clinker is important for the energy cost of the grinding process. It depends both on the clinker's mineral composition and its thermal history. The easiest-ground clinker mineral is alite, so high-alite clinkers reduce grinding costs, although they are more expensive to make in the kiln. 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. The mode of burning of the clinker is also important. Clinker rapidly burned at the minimum temperature for combination, then rapidly cooled, contains small, defective crystals that grind easily. These crystals are usually also optimal for reactivity. 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. The effect of such a clinker can be to double milling costs.

Roller mills

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. The grinding action employs much greater stress on the material than in a ball mill, and is therefore more efficient.Energy consumption is typically half that of a ball mill. However, the narrowness of the particle size distribution of the cement is problematic, and the process has yet to receive wide acceptance.

High pressure roll presses

These consist of a pair of rollers set 8-30 mm apart and counter-rotating with surface speed around 0.9 - 1.8 m.s-1. The bearings of the rollers are designed to deliver a pressure of 50 MPa or more. The bed of material drawn between the rollers emerges as a slab-like agglomeration of highly-fractured particles. The energy efficiency of this process is comparatively high. Systems have been designed, including a de-agglomerator and separator, that will deliver material of cement fineness. 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.

Capacity of cement mills

The cement mills on a cement plant are usually sized for a clinker consumption considerably greater than the output of the plant's kilns. This is for two reasons:

The mills are sized to cope with peaks in market demand for cement. In temperate countries, the summer demand for cement is usually much higher than that in winter. Excess clinker produced in winter goes into storage in readiness for summer demand peaks. For this reason, plants with highly seasonal demand usually have very large clinker stores.
Cement milling is the largest user of electric power on a cement plant, and because they can easily be started and stopped, it often pays to operate cement mills only during "off-peak" periods when cheaper power is available. This is also favourable for electricity producers, who can negotiate power prices with major users in order to balance their generating capacity over 24 hours. More sophisticated arrangements such as "power shedding" are often employed. This consists of the cement manufacturer shutting down the plant at short notice when the power supplier expects a critical demand peak, in return for favourable prices. Clearly, plenty of excess cement milling capacity is needed in order to "catch up" after such interruptions.