Ball Mills Closed-circuit systems in details

The efficiency of the early stages of grinding in a ball mill is much greater than that for formation of ultra-fine particles, so ball mills operate most efficiently by making a coarse product, the fine fractions of this then being separated, and the coarse part being returned to the mill inlet.The proportion of the mill-exit material returned to the inlet may vary from 10-30% when ordinary cement is being ground, to 85-95% for extremely fine cement products.It is important for system efficiency that the minimum amount of material of finished-product fineness is returned to the inlet.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.

Efficient closed-circuit systems, due to their tight particle size control, result in cements with relatively narrow particle size distributions (i.e., they have fewer large and small particles for a given average particle size).This is advantageous because it maximizes the strength-producing potential of the clinker, since large particles are inert.As a rule of thumb, only the outer 7 µm "skin" of each particle is hydrated in concrete, so any particle greater than 14 µm in diameter always leaves an unreacted core.However, the lack of ultrafine particles can be a drawback.These particles usually fill the spaces between the larger particles in the grout, and if they are absent, the deficit is made up by additional water, resulting in a loss of strength.This can be solved by adding 5% calcium carbonate to the cement: this soft mineral produces enough ultra-fine powder on the first pass through the mill.

Energy consumption and output 

Clinker hardness

The hardness of the clinker is important for the energy costs of the grinding process.This depends on the mineral composition of the clinker and its thermal history.The easiest clinker mineral to grind is alite, so high alite clinkers reduce grinding costs, although they are more expensive to manufacture in kilns.The hardest mineral is belite because it is harder and somewhat plastic so when impacted in a mill the crystals tend to flatten rather than shatter.How the clinker is burned is also important.Clinker burns rapidly at the lowest combining temperature and then cools rapidly, containing small, defective crystals that are easily ground.These crystals are also usually the best for reactivity.On the other hand, prolonged combustion and slow cooling at excessively high temperatures can lead to large, well-structured crystals that are difficult to grind and non-reactive.This clinker effect could double the cost of milling.

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.