The ability to produce large quantities of fine powders was first realized in Denmark in 1880-1892, when Schmidt and Co. developed a ball mill with a long, horizontal, cylindrical drum, half-filled with either steel balls or silicon pebbles. Today, the ball mill is still virtually the only technology capable of producing fine-milled materials on commercial scale. However, the level of fineness that can be achieved with a ball mill is limited to about 10 to 50 microns. With today's requirements to achieve finer and finer particle sizes at higher efficiency levels, a new technology is clearly needed.
The concept is not new. The planetary mill method was first patented in the U.S. in 1889 (patent no. 405810) and in 1896 (patent no. 596828), then in France (patent no. 401833), the U.S. (patent no. 1144272) in 1909, and in Germany in 1939 (patent no. 660412). All of these patents describe periodic-action mills.
The first patent for a continuous-action mill was obtained by the French scientist A. Juasel in 1953, and then by Wilikinson in the U.S. in 1970 (patent no. 3529780).
In the former USSR, the first patent was obtained by S. I. Golosov in 1956 (no. 101984). This patent was renewed in 1970 (no. 265696 and 271289), 1972 (no. 345963), 1973 (no. 380350 and 401399), 1974 (no. 432925 and 447166), and so on.
Throughout the 1960-1980s, institutes such as Irgeredmet, Giprocement, Krasnoyarsk Polytechnical Institute, Institute of Geology and Geophysics and Yakut NIIProalmaz were all involved in developing and testing similar technologies.
Worldwide, some of the most significant developments were carried out by the Mining-ore Chamber in Johannesburg (SAR). In Russia, a major contribution into the development of this project was made by YakutNIIPromalmaz in 1972-1982. The result of these efforts was a mill with 40 tph capacity and a drive power of 800 kW for disintegrating diamond-bearing ore through a wet method without the use of grinding balls (issued in 1985 with Author's Certificate No. 1132977).
It is important to note that none of the aforementioned works succeeded commercially. All of the pilot designs had a limited service life of several tens of running hours, depending on the centrifugal loads-particularly the 40 tph wet grinding mill, which was capable of only 30 hours of operation in industrial tests.
The only positive results were the laboratory planetary centrifugal mills that are currently manufactured by a number of firms. However, the operating centrifugal loads on these mills are relatively low (12 g), and the grinding efficiency only approximately reflects the possibilities of this technology. Attempts to increase overloads to 20-25 g have resulted in a decrease in operating time to 10 or 20 hours.
Nevertheless, mills of this class are attractive to both science and industry because of their potential to achieve high-efficiency grinding with relatively low energy consumption. The challenges that have so far hindered the emergence of a production-scale planetary centrifugal mill are common for all developers and include (in order of significance):
1. The inability of standard roller bearings of the drums to operate at significant (>12 g) centrifugal loads.
2. The complexity of the design of continuous loading and discharging of material.
3. Excessive heat buildup in the drums and bearings.
4. The generation of dust during the grinding process.
Idle-state tests were carried out (without a ball load) for about 100 hours. The mill was disassembled, and no wear was found in the bearings. A series of grinding operations with quartz sand and metallurgical slag was then carried out. After the first grinding operation, a consistent fines fraction content (<5 microns) was observed (up to 50%), irrespective of the output and size of the initial material.
To achieve ultrafine powders, an aerodynamic classifier, a cyclone and a filter hose were added to the system. Disintegration in a closed-circuit mode was performed at an output of 300 kg/h, with a reduction in material size from 5 mm to less than 20 microns in one grinding cycle.
The test results proved that the new mill was capable of grinding even extremely hard, complex materials to an ultrafine powder with five to six times less energy consumption than conventional production mills. Additionally, with effective classification, the circular load will not exceed 80%, even when processing powders smaller than 5 microns.
*The Planetary-Centrifugal Mill™ (PCM™), supplied by CYCLOTEC/Leotec Group, St. Petersburg, Russia.
It has also been recognized that ultrafine powders and nanomaterials provide the ability to achieve unique electrical and conductive properties, making them useful in applications such as transparent conductors, high-dielectric ceramics, conductive pastes and inks, capacitors and electronic circuits. Other potential uses include magnetic materials, abrasives, catalysts and pigments.
With this new technology, ultrafine and nanomaterials can emerge from the lab and find roles in industrial applications.
For more information about the production-scale planetary ball mill, contact CYCLOTEC/Leotec Group, Shotlandskaya str. 8, 198035 St. Petersburg, Russia; +7 (812) 303-9158; fax +7 (812) 109-99-93; e-mail email@example.com, firstname.lastname@example.org, or email@example.com; or visit http://www.leotec.ru/main.php?show=en_m.