The different types of roll mills that have been used for pastes include three-, and even five-roller mills. During the latter part of the 19th century, double-roll mills were adapted for use in the grain, ink, paint, and rubber industries, and they are still used for these purposes. Each application has specific objectives; for example, grains must be torn open so that the endosperm can be separated from the husk and the husk removed as waste; pigments must be dispersed completely in the paints or inks; and rubber must be shredded. The grain industry is, by far, the largest user of grinding rolls.
Roll mills were suitable for grinding materials with very high viscosity, which could be up to 5,000 poise for pastes and higher for rubber. Rolls were used for grinding in the
THE HIGH-PRESSURE COMMINUTION PROCESS
by Klaus Schonert
Extensive research in the late 1960s on single-particle breakage in the fine size range (100 |lm-3 mm) and model calculations of a multistep single-particle cascade, which can be considered as an ideal comminution process, have shown that ball milling has an efficiency of only 5%-10% (Schonert 1967). These results were proved with a lab-scale roller mill instrumented with a torque-meter and fed in such a way that the particles are broken in the roller gap without interfering with each other. The experimental procedure was designed to represent a multistep single-breakage cascade: After each run the product was sieved, and only the coarsest fraction fed again to the mill whose gap had been adjusted to a smaller width. In the last run the gap width measured 50 |lm (Schonert, Ohe, and Rumpf 1965). This work showed that powders of brittle materials could be produced with very low energy expenditure by single-particle breakage. However, the throughput of a roller mill is proportional to the gap width, and a huge number of mills are needed to achieve a reasonable capacity. An array of hundreds of roller mills performing single-particle breakage would not be a solution for low-energy comminution in practice.
One point was clear—saving energy in the milling of brittle materials needs to avoid particle interference during stressing and to discharge all fragments with the fineness of the desired product immediately after being produced. To meet the first demand, we investigated with a piston press the breakage of quartz and cement clinker in a mixture of particles and small steel balls. The ball diameter was varied between some millimeters and some hundred microns and the pressure kept below 10 MPa to avoid agglomeration. The results did not show a significant effect of ball diameter on comminution efficiency. By accident, some experiments were done without steel balls and the efficiency was the same. Our expectation that adding small steel balls would cause the singleparticle stressing condition to be approached in a particle bed failed.
After this disappointment we studied extensively the interparticle breakage in the particle bed. Again the pressure was kept small according to the general opinion that agglomeration has to be avoided because this worsens the comminution. From this research we learned a lot about interparticle breakage and improved our understanding of roller-table mills. The limitation with respect to agglomeration restricts the comminution effect at one stressing event; therefore, many stressing events with intermediary classification are needed to achieve a fine product. The internal circulating loads in such mills are quite high. For a better understanding, I decided to extend the pressure range up to several hundred megapascals. The briquettes produced were deglomerated by stirring in a liquid (water or methanol). These experiments provided us with the characteristics of interparticle breakage applying high pressures.
The next step was to investigate the deglomeration of the briquettes in a lab-scale ball and impact mill. With all these data we proposed an unusual comminution process consisting of high-pressure interparticle breakage followed by a deglomeration in a ball or impact mill. The astonishing results were that, in this way, only one-third to one-half of energy is needed compared to ball milling. In 1977, I applied for a patent claiming a two — step process consisting of stressing a particle bed with a high pressure above 50 MPa and a succeeding deglomeration.
High-pressure interparticle breakage can be performed in machines with different designs. With respect to the force flow, a two-roller machine is the simplest one. For this reason we continued our experimental and theoretical research on high-pressure roller mills. As far as the pressure is concerned, I want to mention the following:
■ Increasing pressure decreases in principle the comminution efficiency of interparticle breakage but raises the production of ready material, by which the recirculation is reduced.
■ The recirculation determines the size of mill, classifier, and transport equipments and, by that, the investment costs. Increasing pressure increases wear.
■ The comminution costs result from investment, energy expenditure, and wear. An economical optimization of a high-pressure comminution has to consider all three facts.
My patent (Schonert 1982) claims the high-pressure comminution process, not the high — pressure roller mill. In my understanding, only the idea to stress a particle bed by a pressure much higher than usual, and not to be afraid about agglomeration, is the essential point of the novel comminution method. One should remember the general principle that any agglomeration worsens the comminution effect and should be avoided. Contrary to that, in a high-pressure roller mill, the material is more or less briquetted. This argument was always essential for winning the big patent cases in Germany, the United States, and Denmark.
ink, paint, and rubber industries. In 1825, a roller mill was used in England for masticating rubber (Fischer 1944), and they are still used in the rubber industry. In the paint and pigment industry roller mills are now only used for inks. They work by applying shear and heat to the resin that contains the pigment; these soften the resin and the pigment is dispersed as the sheet of resin is passed repeatedly through the rolls.
CONCLUSION
The grinding component of size reduction uses the most energy and has the highest wear on the grinding surfaces. Roller mills, whether the ring-roller type or the double horizontal roll type, use the least energy but cost the most to maintain because of the high rate of wear on the surfaces. In the next chapter, we describe the tumbling mills (including ball mills) that were developed in the last half of the 19th century and saw dynamic growth in the 20th century, becoming the most widely used mills in the world.
Copyright © 2005 by the Society for Mining, Metallurgy, and Exploration.
All rights reserved. Electronic edition published 2009.