Starting in the late 19th century, particularly for grinding in the cement industry, tumbling mills were built in which a short ball mill was attached to a tube mill to make a single mill with a division head separating it into two compartments. Large balls were charged to the coarse grinding compartment. Worn balls in the first compartment would pass through the division head and flow into the second compartment where they were needed for fine grinding. To maintain the volume of the grinding media in the fine- grinding compartment, it was necessary to periodically shut down the mill to add small balls to the second compartment. Balls were added to the first compartment with the mill feed. The design and manufacture of multiple-compartment ball/pebble mills became the heart of manufacturing tumbling mills to grind cement clinker and cement raw material.
To maintain the flow of material through very long mills, it was necessary to add a second division head to lift the material, increasing the flow gradient through the mill. This made the mill a three-compartment mill. In the 1920s and 1930s, many combinations were used for dry, multiple-compartment mills grinding cement clinker. To increase the amount of impact grinding, the diameter of the first compartment would sometimes be made larger than the diameter of the second compartment. This caused stress cracking in the tapered section of the shell joining the shells of the first and second compartments. With closed circuiting there were many combinations of open — and closed-circuit arrangements. Ground clinker was peripherally discharged from the first compartment and then fed to either screens or air classifiers, with the oversize returned to the first compartment. One example of this was to peripherally discharge the closed — circuit first compartment with the discharge going to a screen. The oversize went back to the feed end of the mill. The fines were fed to the second compartment either by a scoop feeder through openings in the shell into the division head between the two compartments of the mill or through a spout feeder at the discharge end trunnion. If the oversize from the screen was fed to the second compartment through the discharge diaphragm, there would be a separate peripheral discharge on the second compartment side of the division head. The second compartment could either be open — or close-circuited with an air separator. The design confusion was solved as multiple-compartment mills grinding cement clinker were standardized as closed-circuited mills with all of the mill discharge being fed to classification systems, and the oversize from the classifier being returned to the feed end of the mill. Closed circuiting of the first compartment was discontinued.
In the 1950s, the use of single-compartment, large-diameter ball mills in closed circuit with an air separator was developed. Still used today, this circuit is the same, in principle, as the single-stage ball-mill circuit used in mineral concentrators. The trend in large tumbling mills for grinding cement clinker is toward installing two-compartment tube mills in closed circuit with an air separator. The increase in energy use comes from the increase in mill length rather than an increase in mill diameter.
Work continues on improving the metallurgy of the wearing parts of the vertical roller mill so it can be used as a primary mill for grinding cement clinker. In the 1980s, for example, horizontal double-roll crushers with hydraulic high-pressure roll retainers proved to be successful in crushing cement clinker to make tube mill feed.
With the Krupp-Grusonwerk mill that was first used for grinding clinker to finished cement, the mill contents were fine and the product that passed through the circumferential screen was fine enough to give a consistently good cement when the clinker was soft. But when the clinker was hard, the product was coarser and the cement made from it was often unsatisfactory. The growing demand for good-quality cement highlighted the need for better equipment, and lawsuits in the 1890s emphasized the fact that ball mills were becoming the preferred machines for making very fine particles. Clinker grinding became a two-stage process carried out in a single-tube mill with two compartments (see Figure 7.15) separated by a partition that had four functions:
■ Retained large balls in the first compartment to break large lumps of clinker
■ Kept small balls in the second compartment to produce very fine particles
■ Allowed the air, which both dried and classified the limestone or clinker, to flow through the mill freely
■ Lifted the discharge from the first compartment to the horizontal center of the mill to be fed to the second compartment to give a gradient to the flow of material through the mill