ONLINE EXCLUSIVE: Small Powders on a Large Scale

A new mill has been designed to efficiently and consistently produce ultrafine and nanoscale powders in production volumes.

In most industrialized countries, the process of disintegrating ores and other materials consumes enormous amounts of energy. By the end of the 1980s, in the former USSR alone, over 70 billion kW/h of electricity were used to crush and disintegrate more than 3 billion tons of mineral raw materials. At material handling plants, 50-70% of the total capital and operating costs are allocated to these processes. With energy costs on the rise, it's not surprising that companies are constantly searching for ways to improve these processes and equipment to minimize energy consumption while maximizing throughput.

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.

Superfine Disintegration Methods

Throughout the world, only a few technologies are used to produce superfine powders. These include:

• Desintegration in a ball mill through increased circulation (up to 1500%, depending on the hardness of the material) and increased grinding duration. Typical energy consumption can be as much as 2000 kW/ton. The minimum achievable particle size is about 10-50 microns.
• Disintegration in a vibrating mill. These mills tend to be unreliable in operation and possess a low grinding efficiency. Typical energy consumption is around 1000 kW/ton.
• Air-pressure mills. These mills also possess a low grinding efficiency and consume an average of 1500 kW/ton up to 8000 kW/ton of energy, depending of the initial fracture and hardness of the milled material.
• Various electromagnetic, ultrasonic, pulsed and other methods of superfine disintegration exist. However, these are generally not adaptable to commercial scale production and also tend to be unreliable.

Lab-Scale Planetary Centrifugal Mills

A search for a high-efficiency method of superfine disintegration resulted in the development of a new technology called the planetary centrifugal mill. The mill is based on the same operating principle as the ball mill. However, besides rotating on its own longitudinal axis, the grinding drum in a planetary centrifugal mill also moves on a circular platform, similar to the way planets rotate around the sun. This motion, known as "centripetal acceleration," replaces the gravity acceleration that acts on the balls in a ball mill, enabling the planetary centrifugal mill to exceed gravity acceleration tens to hundreds of times (10g - 300g).

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.

Production-Scale Planetary Centrifugal

Efforts to develop a new-generation planetary centrifugal mill began with tests of a new planetary gear system capable of operating under significant centrifugal overloads. The tests were performed under loads of 300 g over 120-hour milling cycles. After these tests were completed successfully, a pilot continuous-operation planetary-centrifugal mill* was designed and manufactured with the characteristics shown in Table 1.

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.

FOOTNOTE:

*The Planetary-Centrifugal Mill™ (PCM™), supplied by CYCLOTEC/Leotec Group, St. Petersburg, Russia.

Expanding Applications

Because the mill produces a high volume of consistent, ultrafine powders, it offers a number of potential benefits for ceramic manufacturers. For example, using ultrafine powders can allow the production of net-shaped or near-net-shaped ceramic parts using injection molding and other advanced technologies, thereby cutting production costs by reducing the need for expensive post-forming machining. Some examples include translucent aluminum oxide (Al2O3) for arc-tube envelopes, Al2O3 and titanium dioxide (TiO2) reinforcements for metal matrix composites, and Al2O3 and TiO2 porous membranes for gas filtration.

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 ezry.akkerman@leotec.ru, cyclotec@mail.spbnit.ru, or andrey.chumokhvalov@leotec.ru; or visit http://www.leotec.ru/main.php?show=en_m.