The use of vibration, in combination with grinding media, for material size reduction significantly increases the number of material impacts per minute, decreasing product-extraction time and creating a more desirable fractured product
By Ronald Fruit and Amy Donahue
The first vibratory mills to be developed for material grinding were relatively simple devices, utilizing an eccentric drive system mounted to a tube to create an elliptical motion suitable for attrition grinding. These devices were limited in throughput and efficiency. They also had complicated drive connections with very high power consumption per pound of material produced.
Vibratory grinding mills have been successfully used in tertiary grinding on primary minerals such as iron ores, for example. Feed materials have been ground to sizes below 200 mesh and some below 325 mesh.
General Kinematics began developing vibratory grinding mills in the 1980s, applying their knowledge of vibratory sand reclamation from the foundry industry to fine grinding applications for other products and materials. Since then, vibratory grinding mills have undergone an evolutionary process to improve efficiency, throughput and application. Past experience in utilizing vibration to tumble materials was combined with the energy-saving capabilities of two-mass drive technology to create a mechanical device with superior action and grinding efficiency.
When General Kinematics began developing vibratory grinding mills, the initial goal was to improve the mills’ tumbling action to help blend and homogenize the material while maximizing energy input to grinding. Testing determined the drive needed to create a linear action off-center of the grinding body that would create a rotational point, or node point, around which the mass would actually vibrate, thus providing a conveying action inside the tube and allowing the media to travel up the active side wall. When the media hit a point high enough on the tube, gravity would take effect and cause it to fall back down upon the media bed.
The net result of this testing was to create a tumbling action somewhat similar to a normal rotary mill, which blends and mixes media and material. The difference is that, simultaneously, individual impact grinding between all of the media pieces takes place with every stroke of the mill. A combined grinding and blending process is created.
The next step in developing vibratory grinding mills was to incorporate an energy-efficient drive system. The sub-resonant natural frequency two-mass drive system is a key feature of all General Kinematics equipment and was applied in place of a direct drive on the main vibrating working mass. The sub-resonant feature tends to produce more work with less energy, maximizing the use of the energy put into the system. Energy losses are much less than those of conventional grinding mills, resulting in better power efficiency. In most cases, the vibratory grinding mill will use one-third to two-thirds less kW per lb of material processed when compared with rotary and vertical mills.
The combined grinding actions generated by vibratory mills results in a very narrow output band of material discharge.
The action and the drive system are designed to create a grinding environment that produces a large number of impacts due to the vibratory action. Each vibration, or stroke, of the mill causes the media to expand and separate. This creates gaps in between the media pieces as well as between the liner and the media. Each stroke of the vibratory action provides an opening and closing of that media. Sufficient RPM is used to significantly increase the force being applied due to the G-forces (the actual dynamic energy that is being input to the media)—the net result can be described as “medium energy impact with high number of impacts per minute of operation.”
Standard rotary mills use higher energy, but fewer, impacts. The number of impacts in a rotary mill is directly related to how many times the rotation of the machine can pick up and drop the media to re-impact with other media and the material to be ground. A vibratory grinding mill will have several hundred cycles per minute of media opening and closing. The high number of impacts is combined with speed and stoke settings to produce up to 5 g’s of amplification on the media. Thus General Kinematics vibratory grinding mills create a higher energy grind due to the use of their kinetic energy.
Testing has concluded that the true advantage of this drive configuration is that enough energy is in place to achieve some crushing impacts where pieces will actually fracture, but also attrition grinding occurs in the form of chipping and cleaving edges. These combined grinding actions contribute a very narrow output band of material discharge.
Configuration of General Kinematics vibratory grinding mills is versatile as well, with the capability to perform in either dry or wet process grinding. The combined cleaving and attrition grinding tends to expose minerals and metals to be recovered at a faster rate than just grinding material down to a small particle size. Micro-fractures are generated that will expose the metal in the particulate without actually breaking down the encasing material.
Vibratory grinding mills are built to work with fine grinding and other applications. Add an in-solution process leaching agent, and successful grinding and leaching is achieved at the same time in the mill. This has been a successful solution for many mines and other applications where an in-solution process is necessary. These mills also can be heated and cooled via water-jacket. With a well enclosed system, fines are easily captured in an air sweep in the machine, with minimal leakage or dusting prevalent. In the case of liquid grinding applications, vibratory grinding mills are robust enough to contain liquids, and 2 in. of water pressure in the drums does not affect the grind.
The vibratory grinding action allows for use of almost any solid material as media. Several media types have been used to date based on the material to be ground. Examples of media materials used are steel, aluminum oxide, zirconia, tungsten carbide and rubber. A good example is a mill grinding sintered tungsten carbide using worn and rejected tungsten carbide inserts from drilling heads. This example also shows the flexibility of the vibratory grinding mills to use a variety of media shapes and sizes. The mills have used media shapes including round balls, short cylinders, cones and long rods.
|(Depth x Length, in. x ft.)||(1,200 rpm)||Length||Height||Width||(Cubic ft of material)|
|MD 2 x 6||24 x 6||10||07 ft. 3 in.||05 ft. 3 in.||04 ft. 6 in.||3|
|MD 3 x 10||36 x 10||15||11 ft. 3 in.||07 ft. 0 in.||06 ft. 6 in.||11|
|MD 4 x 15||48 x 15||30||15 ft. 5 in.||10 ft. 5 in.||12 ft. 0 in.||30|
|MD 6 x 18||72 x 18||100||21 ft. 9 in.||12 ft. 8 in.||11 ft. 6 in.||81|
General Kinematics vibratory grinding mills are easy to install. The two-mass drive system allows the unit to be heavily isolated to the support base. This means low dynamic forces to the installation floor and minimal foundation requirements. Large vibratory mills have been installed using epoxy anchor bolts on concrete floors 8–10 in. thick.
Vibratory grinding mills have had extensive success in tertiary grinding on primary minerals: granites, limestone, iron ores, hematite and magnetite, for example. Each of these materials has been ground to sizes below 200 mesh and some below 325 mesh. At these sizes, the value of base minerals is significantly increased.
Grinding carbides and tungsten, as well as zirconium oxide materials and other very hard to grind materials, fit well within the vibratory grinding mills process because of their moderate flow rates. With a feed size of –1/4, discharge variance of about 120 microns from largest to smallest piece can be generated. The lower overgrind enables the ability to achieve a very narrow discharge band, with less energy.
|Material||Liner Type||Wear Rate||Service Life|
|Zirconium oxide||Aluminum brick liner||1/8 in. / year||6–8 years|
|Tungsten carbide||Hardened steel liner||1/16 in. / year||7–9 years|
|Gold leaching process||Aluminum liner||<1/16 in. / year||8–10 years|
Finally, liner and media life experience different results within a vibratory grinding mill. Grinding and attrition mixed together means energy is being transmitted into the media, not the liner or machine. Because the action works media-to-media, there is very little media-to-liner grinding (see accompanying table above). Media life tends to be 30%–50% higher than most other grinding technologies, especially versus rotary mills in fine grinding applications.
Ronald Fruit is product industry manager in the technical sales group and Amy L. Donahue is marketing coordinator for General Kinematics Corp., 5050 Rickert Road, Crystal Lake, Ilinois, USA; Telephone: 815-455-3222; www.generalkinematics.com.