In those days all paints were ground in very small quantities by hand, and it was not unusual for an apprentice to work on a couple of ounces of paint for many months. In the age when few colours were known it was the colloidal grinding of paints that enabled the great artists of the Italian Renaissance to find expression and pass on their wonderful creations to prosperity. (Schotz 1931)
In the preparation of india ink by the Chinese, according to Wolfe, a mixture of glue, lampblack, egg white, cinnabar, and musk was beaten 30000 times in a mortar until the black was finely divided and deflocculated by intimate admixture with the protective colloids of the formula. (Fischer 1944)
For thousands of years the fine particles required for flour, pigments, ceramics, and minerals were produced in querns, saddlestones, and edge mills. During the 20th century, the uses of and requirements for ultrafine particles proliferated. In this chapter, we give a brief overview of how fine-grinding technology evolved, along with some of the different machines and processes in use.
By 1880, the Industrial Revolution was in full swing, and rapid growth was occurring in the cement, electricity, chemical, pharmaceutical, and other new industries that required fine particles. Better machines were required to produce them, and inventors applied their skills to the problem, patenting both low — and high-speed mills. The ball mills that were introduced about 1885 were low-speed mills suitable for grinding to about 100 pm but unsuitable for very fine grinding, because small particles were not broken efficiently by falling balls. Other methods for grinding with balls were devised, such as stirring the balls to create collisions. High-speed stirred mills use high energies per cubic meter of mill volume and give higher rates of breakage of small particles and higher rates of production of fine particles. They will be discussed in this chapter.
The nomenclature used in fine grinding is summarized as follows:
■ Ball mills: Separate grinding media are used, for both continuous flow grinding and batch grinding.
■ Stirred mills: The material being ground is kept in motion by stirring rather than tumbling.
■ Vertical high-speed stirred ball mills: defined as peg mills; horizontal high-speed stirred ball mills are defined as disc mills.
■ Colloid mills and pin mills: High-speed mills in which rotating pins pass each other with little clearance, creating intense turbulence and shear forces in the slurry close to the pins.
■ Vibrating and nutating mills: Very small balls are kept in rapid motion in a confined volume by moving the mill shell at very high speed.
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■ Jet mills: Stationary containers into which very high speed jets of particles are introduced so that they collide and cause the particles to break autogenously by impact and abrasion.
In this discussion, no attempt is made to define a “fine particle”; this has changed with time, and the size of particle that is considered to be fine in one industry may be coarse in another. Fifty years ago, particles of metalliferous ores that were less than 20 pm were regarded as fine; now the size is 5 pm. Historically, mineral pigments such as iron oxides were fine when they were ground to 10 to 20 pm; now they are ground to 1 to 2 pm. Synthetic pigments such as titanium dioxide are ground to 0.2 pm.
One of the problems with fine grinding has been in measuring size distributions of very fine particles so that the process can be controlled. Sieves have been available down to about 40 pm in size, but this size was not satisfactory for portland cement when it became a popular building material about 100 years ago, because the quality of the cement depended on its size distribution, and two-thirds of cement is less than 45 pm. The link between size distribution and surface area led to the Blaine number, which is related to surface area, being used in the cement industry as a measure of the quality of cement in terms of particle size.
In other industries that require very fine particles, a fineness of grind gage has been developed to determine the size of the largest particle in a product. This gage is a piece of steel into which a wedge has been machined from the surface to a depth of 10-200 pm, depending on the required particle size. A sample of the ground particles mixed with a liquid is placed at the deep end of the wedge and drawn toward the surface end with a blade. The depth at which the largest particles break the surface can be seen and measured.
The gage commonly used today is the Hegman gage shown in Figure 8.1. The wedge depth for various industries is
■ Paint and pharmaceuticals: 125 pm
■ Peanut butter: 20 pm
■ Chocolate: 105-185 pm
■ Ink: 10-30 pm
Fine products from grinding mills often have tight specifications on the amount of contaminates, such as iron, that is allowed in the product. This is why the grinding media are often natural silica pebbles or pebbles made from porcelains, most often an alumina porcelain.