The Raymond mill had rollers attached to vertical shafts that were suspended from a horizontal beam through joints that allowed them to swing. Rotation of the beam caused the rollers to swing outward and press against the grinding ring. The feed was introduced into the grinding zone between the rollers and the ring, and the product was extracted by air that was pumped into the grinding chamber through grates. Raymond mills worked well mechanically but were less successful at grinding the hard German coal than the soft American coal, because higher centrifugal forces were required and the vibrations damaged the mill foundations. Their proven performance in the United States on the softer minerals ensured that Loesche found a market for them in grinding limestone and phosphate rocks, but the original design had to be changed to work efficiently on coal at higher capacities.
The Fuller mill consisted of a U-shaped grinding ring with large steel balls rotating inside that were pressed down by a spring-loaded ring. The feed entered the grinding zone from the center of the ring and passed under the balls. The ground product was discharged from the other side of the ring.
By 1925, Ernst Curt Loesche had become part owner of von Grueber’s company, and he was given the contract to build mills for the new Klingenberg power station in Berlin that ground 10 to 12 tph. He had already obtained the license for Raymond ring-roller mills, which were being made in the United States, and he used these mills for the project.
Claudius Peters had a similar experience when he found that the capacity of the Fuller mill was limited by the diameter of the balls, so after his license to manufacture the mill expired, he changed its design to use fewer balls of larger diameter. He gained another increase in capacity by increasing the diameters of the housing and the bowl, which meant that he was able to use more grinding balls. The Peters mill was mainly used for grinding coal that had a low specific gravity. The large diameter of the housing, needed for the large bowl, reduced the velocity of the air that was used to lift the product from the mill to the classifier. Materials with a higher specific gravity were difficult to suspend in the low-velocity air. Low specific gravity coal was the most suitable feed for the Peters mill.
The Loesche and Peters mills were roller mills, which were similar in concept but different in design. Other companies also made similar mills with different designs. Each roller mill took some share of the market, because it was suitable for a specific application but
…for the most part the roller mills available on the market differ only in the comminution zone. There are differences in the geometric form of the grinding track and grinding elements, in the arrangement and number of grinding elements, and in the way they are held against the grinding bed. (Feige 1993)
Figure 6.11 summarizes four types of configurations of grinding tracks and grinding elements that have been widely used in roller mills.
In the United States, a ring-ball mill that was similar to the Peters mill was built by Babcock & Wilcox and first used in 1929 at Commonwealth Edison’s Powerton Station in the New York City metropolitan area. Known as the Babcock E mill, it contained a larger quantity of smaller balls than the Peters mill. These mills were used in England for pulverizing coal for boilers. About the same time, a horizontal three-roller mill with a screen discharge similar to the Griffin mill was built by the Bradley Hercules Company and used in a roller mill-tube mill circuit to grind cement clinker. The power consumed in grinding was low compared with two-stage or two-compartment ball mills, but the wear on the screens and side plates was severe, and there were high production losses resulting from the time required to repair the mills. At least one standby mill was required for every two operating mills installed.
Rod mills were also tested for grinding cement clinker. They consumed about the same amount of energy as a Bradley Hercules mill but placing rods in the mills required 1 hour or more every 2 days, because they had to be added one at a time through a small hole in the dust cover on the discharge end trunnion. This was not acceptable in cement plants.