Fred Bond was the engineer who did much to define the relationship between ore hardness, tonnage processed, size reduction achieved, and power required. Born in 1899 and raised in a rural community near Golden, Colorado, he attended and graduated with honors from the Colorado School of Mines (CSM) with a bachelor’s degree in metallurgy. He went to work as a metallurgist for New York & Honduras Rosario Mining Company at its mines in Honduras, where he learned about the pebble milling studies done there in the early part of the 20th century. He later returned to CSM for his master’s degree in metallurgy, then went to work at the U. S. Mint in Denver where he assayed the gold shipments. In 1928, he became a draftsman for the Tennessee Copper Company in Copperhill, Tennessee, where he worked with Mill Superintendent Jack Myers. Myers was a student of grinding and worked closely with Harlowe Hardinge, who was conducting research on grinding in the mills he had sold to Tennessee Copper.
A casualty of the economic downturn during the Great Depression, Bond was released by Tennessee Copper in 1930. Walter Maxson, who had been one of Bond’s professors at CSM, was managing the Mining Department at Allis-Chalmers, which sold
THE RITTINGER-KICK CONTROVERSY
The Rittinger-Kick controversy was prominent in the technical press from about 1900 to 1940. Much scientific thought was expended and many laboratory tests were made, but there were no definite conclusions. Some of the data indicated that Kick agreed better with crushing and Rittinger with grinding. …After about 1940 the controversy abated as being nonproductive. (SME-AIME 1985)
In a simple way the difference between the theories can be shown by considering the breakage of a cube of rock with 1-m sides. Assume that the original cube is broken into cubes with sides of 0.5 m, then 0.25 m, 0.125 m, and so on. After each breakage the volume of the cubes is one-eighth of the previous volume (i. e., the ratio of the volumes is constant), but the surface area doubles, from 6 to 12 to 24 units, and so on. Rittinger’s theory posited that the energy increased as the particle size decreased and surface area increased, but Kick’s theory postulated that the energy required for each step was constant. Experimental studies tended to support Rittinger’s theory, although Kick’s theory seemed to apply to coarser particles.
gold ore-processing systems at the time. Maxson hired Bond to design and build a grinding and metallurgical testing laboratory. In the 1930s, Bond traveled to Peru and Bolivia to start up gold ore-processing plants, and during World War II he was involved in the start-up of a radium ore mine at Great Slave Lake in northern Canada. All the while he continued his studies on grinding, and eventually he developed the rod and ball mill grindability tests that now bear his name. The method Bond developed for selecting ball mills (see sidebar) is still routinely used by mineral process design engineers.
Bond and Grindability Throughout the 20th century, grinding mills were selected based on the grindability of the material and the prediction of the power required per ton to grind the material to a known product size. Before Bond’s work, this estimate was based on experience and judgment. Metallurgists and process engineers who worked with mill manufacturers traveled extensively and gained wide exposure to plant data. Because it was generally accepted that they had a better knowledge of the relationship between grindability data and actual mill performance than the staff of mineral processing companies, whose experience was necessarily restricted to a few operations, engineers from mineral processing companies consulted with representatives of grinding mill manufacturers when mills and circuits were being selected for new mines and plant expansions.
To compete for the grinding mill business, manufacturers needed to increase their expertise in grinding technology, so they developed their own grindability tests and obtained operating data and corresponding ore samples from processing companies. From these, they could derive the relationship between grindability and mill performance. The early grindability tests were batch tests carried out in small-diameter ball or pebble mills, often called jar mills. The results were given in terms of revolutions of the test mill or time required to make the desired particle-size distribution and net weight of a specified mesh size produced per minute or per set number of revolutions in the test mill.
Each business that developed grindability tests also developed its own testing procedures, which were kept confidential. Both wet and dry test procedures were developed, but dry grinding was found to give more reliable and reproducible data. During the 1930s, instruments and techniques were developed to measure grindabilities, although the data from the tests were generally not used to directly select grinding energy. Instead, engineers searched for another, usually similar material that had the same grindability as the test material and for which plant production data were available; they then used these data to predict the energy required to grind the test material. The accuracy of the energy prediction depended on the knowledge and experience of the engineer making the selection.
Bond and the Work Index Equation The combination of physical data, particularly plant operating data recorded in the engineers’ “black books,” and the knowledge of how to use those data were important. The “old timers” carefully guarded their black books and shared as little as possible with associates as a protection against newcomers eager to take their jobs. Often they died or retired without disclosing how to use the black books, which rendered the books of little use. Bond’s research started a new era in grinding circuit design that did not involve looking for comparable operating data, and the need for the black books disappeared. The first half of the 20th century can be called the pre-Bond era, and the last half can be called the Bond era. The Bond grindability test procedure was completed in 1937.
During the late 1930s and 1940s, Bond devoted as much time as possible to understanding the energy-size reduction relationship. He began to develop the concept that the energy needed for grinding was the total energy needed to make the grinding mill product minus the energy needed to make the feed. Few companies kept accurate power data for their mills at the time, and the available data were in terms of amps, volts, and nameplate information on the rating of the motor. There was no power factor information to calculate motor power draw, so each mill manufacturer worked out a method to determine the mill power for designing mill drives.
Bond’s work in this area led to the concept of the “work index,” which is the kilowatt-hour per short ton required to reduce material from theoretically infinite feed size to 80% passing 100 pm. In establishing the work index equation, Bond assumed that the efficiency of classifiers was consistent, and he did not consider this to be a factor in determining the energy needed for grinding. In 1952, the Third Theory of Comminution, which contained the work index equation E = [ 1 / ,JX2 — 1 / Jx1 ], was published (Bond 1952). This equation related power required per ton to feed and product size and grind — ability. Bond’s method for determining grinding energy has become the standard. It is now used universally to quantitatively define the resistance of solid particles to breakage and is employed for grinding circuit design over a wide range of operating conditions. Although correction factors have been introduced, the procedure for design has remained unchanged. The work index equation is unlikely to be supplanted, although simulation techniques will augment it.
In the 1950s, demand was increasing for larger mills from both the mining and cement industries. Bond used the data he had collected to derive new power draw equations for rod and ball mills, and he was permitted to publish these in Crushing and Grinding Calculations (Bond 1961). There were small differences between manufacturers in how they rated the power draw for rod and ball mills, and this led to the rating of rod and ball mills by the power they were designed to draw.
Bond’s “computer” was a log-log slide rule that was 500 mm long and had slightly larger numbers than were standard on slide rules at that time. The accuracy of the slide rule was a limiting factor in the accuracy of calculations, and Bond’s larger slide rule improved the accuracy. He concluded that, if he plotted the natural size distributions of many materials on log-log plots, the sections of the graphs between the 80% and 20% passing sizes were essentially straight lines. Beyond these limits the graphs were irregular. So he selected the 80% passing size in microns as the most accurate and useful value to describe a size distribution. Sieves were used to give perfect classification in the closed-circuit grindability tests.
 |
FIGURE 2.1 Relationship between energy and particle size in breakage (Hukki 1961)
Finland’s R. T. Hukki reviewed data from many industrial operations and concluded that the three energy-size reduction theories referred to different regions on the curve relating energy used to particle size produced, as shown in Figure 2.1 (Hukki 1961).
This graph makes it clear why Bond’s work was so successful and why it will continue to be used to design circuits that grind to about 200 pm. His work covered a product range of 25,000 to 20 pm (25 mm to 0.02 mm), and most applications will continue to fall in this range. But extending this work to products less than 5 pm is incomplete.
After Bond retired in 1964, engineers at Allis-Chalmers continued to develop the Bond equation and publish their results. The book Design and Installation of Comminution Circuits, published by the Society of Mining Engineers in 1982, was dedicated to Bond, who died in 1976.