Roll Mills

The roll mill is the simplest batch mixing equipment. The principal design consists of two horizontal rolls, usually of equal size, arranged side by side and rotating toward each other at different speeds. The ratio of the peripheral speeds of the rolls, known as friction ratio, ranges from 1 to 2 but is usually around 1.2. The higher friction ratio leads to a higher heat generation in the processed material. Friction, speed, and the sizes of the rolls influence the cooling of the material mass and the intensity of its treatment. Depending on the properties of the process material and its desired temperature, the rolls may be internally cooled or heated by using circulating cooling water or a suitable heating medium.

The basic mixing operation is illustrated in Fig. 4.5. The material is charged between the rolls in the form of lumps, pellets, chunks, or powder. As a result of rotation, adhesion, and friction, the material is entrained into the gap between the rolls, and upon discharge it sticks to one of the rolls, depending on their temperature difference and velocities. Another factor is the gap between the rolls. In batch mixing the material after loading passes through the gap between the rolls several times, and the mixing action is due to the different speeds of the rolls. Both the shearing action and entrainment of the material into the gap are very important in the mixing process and transporting of the material through the unit. The gap between the rolls can be adjusted by a massive screw mechanism.

During the operation, cutting of the sheet of the material (“blanket”), folding, and rolling are carried out, which increases the uniformity of the composition. If additives, often in the powder, liquid, or paste form, are to be incorporated, they are added into a rolling “bank” formed between the rolls.

Roll mills are also used for warming up materials for calendering or extrusion. In such cases the warm material is cut into strips that are fed into the calender or extruder often continuously. Another application of roll mills is to cool materials discharged from an internal batch mixer in a form of large lumps (see Section 4.2.3.1.2). The resulting blanket is then transported in the form of strips to the calender or extruder or removed from the rolls as thick sheets.

A thorough theoretical analysis of mill mixing is in Refs [22] and [23].

Two-rotor mixers with open mixing chambers have been developed and manufactured by Werner & Pfleiderer of Germany.

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Roll Mills

Roll Mills

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