Cement mills grind clinker, a hard material produced by sintering aluminosilicate and limestone in a kiln, into fine particles. Ball mills are the most common type of cement mill, using steel balls to crush and grind the material. High-pressure roller mills are occasionally used but less widely accepted. The main constituent of Portland cement is clinker, which requires fine grinding before cement production. Ball mills subject the raw material to low levels of heat and pressure, with tightly controlled drum speeds and ball sizes. Other milling processes are less suitable for grinding clinker.
A cement mill is a plant used to grind the clinker component of concrete. Clinker is a hard, nodular material produced by sintering aluminosilicate and limestone in a cement kiln. There are several types of cement mills capable of grinding clinker of which the ball mill is the most common. Ball mills consist of cylinders partially filled with large numbers of steel balls which use their rolling action to crush the raw material as each cylinder is rotated. High-pressure roller mills are occasionally used to grind clinker, although their use has not yet gained widespread acceptance.
The main constituent of Portland cement is a sintered product of limestone and aluminosilicate known as clinker. This hard, pebble-like material is the ingredient in cement that causes it to “cure” and harden to form the mortar or concrete used so widely in construction. The clinker sintering process or heat combination produces a coarse, nodular material of particular hardness that requires fine grinding before cement production. Grain sizes of less than 45 µm are required which, due to the hardness of the clinker, requires a particularly efficient milling process. Although there are different types of mill capable of grinding clinker, the most commonly used cement grinding process is the ball mill.
Ball mills are highly effective grinding systems that subject the raw material to relatively low levels of heat and pressure. They consist of large diameter rotating cylinders or drums driven by electric motors. The drum is partially filled with high chromium steel balls or, occasionally, cubes. These roll freely within the cylinder when it is rotated. This rolling action crushes the clinker by impact and then grinds it fine due to the friction between the balls.
Ball mills often feature an intermediate partition that divides the barrel into two sections with different sized balls allowing for progressive grinding in a mill. The efficient grinding of clinker in a ball mill is subject to tightly controlled drum speeds. At rotational speeds above a mill’s critical speed, the contents are simply forced against the sides of the drum by centrifugal force with little or no grinding action. The size of the balls is also an important consideration in a cement mill. If the balls are too big, the raw material will not be ground finely enough.
Several other milling processes are capable of grinding cement clinker although none are as suitable as ball mills. The most common alternative cement mill process is the high pressure mill. This process uses sets of hardened steel rollers which grind the clinker. While these mills are effective and less expensive to operate, the small particle size they produce is sometimes problematic. Roller mills are, however, often used as a first stage process in ball mill plants as the initial crushing stage, thus reducing overall process time and increasing productivity.
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