Solid material comminution or disintegration – Apparatus – Loose grinding body comminutor
Patent
1992-05-26
1995-01-24
Rosenbaum, Mark
Solid material comminution or disintegration
Apparatus
Loose grinding body comminutor
241176, 241179, B02C 1700, B02C 1704
Patent
active
053836154
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns ball milling and mechanical alloying. More particularly, it concerns an improved ball mill for use in grinding and in alloying (both low temperature alloying and high temperature alloying).
BACKGROUND TO THE INVENTION
Ball mills and attritors have been used for many years to produce fine powders. In ball mills, the energy input to the powder charge is provided by the rotation of the mill, a cylindrical cell or vial about a horizontal axis, so that hard balls within the mill are tumbled with, or onto, the charge in the mill. In attritors, metal arms are used to stir the ball charge.
As noted by Y S Benjamin in his article in Scientific American, volume 234, page 40 (1976), it was appreciated in the early 1970's that in addition to creating powders, ball milling could be used to produce solid state reactions which result in the synthesis of new alloys from elemental powders. It was also discovered that ball milling can modify an alloy structure. The first of these techniques (the synthesis of alloys) is known as "mechanical alloying"; the second technique has been termed "mechanical grinding".
When mechanical alloying is used to produce new materials, there is a combination of repeated welding, fracturing and rewelding of a mixture of powder particles having a fine microstructure together with a rapid interdiffusion process.
Both mechanical alloying and mechanical grinding have been effected using either the vibrating milling technique or the rotating milling technique. In vibrating-frame mills, hardened steel balls are caused to impact substantially vertically upon the powder charge. Local overheating of the particles can occur as a consequence of the mill structure. This local overheating is difficult to remove. In addition, the mixing of the particulates is very slow (and in some designs of mill, is almost non-existent). Thus rotating mills, in which the steel balls roll along a circular arc on the inner wall of the mill chamber or vial, are preferred for mechanical alloying.
In rotating mills, the powder charge is spread on the inner surface of the chamber. This ensures that heat generated within the chamber is removed by conduction through the cylindrical wall of the chamber and that there is effective mixing of the powder constituents. However, when using rotating mills, it is not possible to provide the impact energy of the balls that is achieved in vibrating-frame mills when a rotating ball mill is used.
DISCLOSURE OF THE PRESENT INVENTION
It is an objective of the present invention to provide an improved ball mill in which the impact energy of the vibrating-frame mill technique can be achieved while the cooling and powder mixing features of the rotating mill technique are maintained. It is a further objective of the present invention to provide a ball mill in which the energy or intensity of the milling process is variable and controllable.
These objectives are achieved by constructing the chamber of a rotating ball mill as a hollow cylinder or sphere of a paramagnetic material and mounting at least one magnet outside the chamber in a manner such that either (i) the magnet (or magnets) can be moved around the chamber along an arc centred on the axis of rotation of the chamber, or (ii) the location of the magnet (or magnets) can be varied between a number of mounting positions, each located on an arc centred on the axis of rotation of the chamber. Mounting a magnet (or magnets) in this manner creates a perturbation of the normal movement of the steel balls of the ball mill when the chamber is rotated. In particular, when a magnet is positioned vertically below the chamber, there is an increase in both the rotation of the balls in the chamber and their contact time with the powder charge of the chamber. As the magnet is moved to a position high on one side of the chamber, each ball is lifted by the magnet before being dropped on to the charge (including other balls) of the mill, to provide a high energy impact. At intermediate positions of the magnet arou
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Calka Andrzej
Ninham Barry W.
Husar John M.
Rosenbaum Mark
The Australian National University
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