Media milling

Details Media milling Media milling

2001-05-10

2003-08-12

Rosenbaum, Mark (Department: 3725)

Solid material comminution or disintegration

Processes

With application of fluid or lubricant material

Reexamination Certificate

active

06604698

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to media milling and in particular to media milling using two size distributions of milling media to obtain small particles of a solid material wherein the media are retained in the milling chamber of the media mill and the small particles are separated from the milling media.
BACKGROUND OF THE INVENTION
Size reduction of crystalline and amorphous solids by mechanical means using dry or wet milling techniques such as jet milling, ball milling, media milling, or homogenization is now widely used in a variety of industries. Diverse industrial applications include the production of paints; pigments; photographic materials; cosmetics; chemicals; metal powders useful as catalysts and supports; stationary phase particles useful in analytical and preparative chromatographic separations of chemical compounds and mixtures such as those encountered in forensic science, food, cosmetics, chemical, and pharmaceutical studies; powdered toners, both black and colored, useful in xerographic and printing applications including laser printing; and small particles of solid pharmaceutical agents including water-soluble, water-insoluble, and poorly water-soluble therapeutic and diagnostic imaging agents, medicinally active agents, medicaments, plant and herbal extracts, drugs, pro-drugs, drug formulations, diagnostic imaging agents, and the like. In pharmaceutical applications it is often desirable to prepare very small particles of an essentially water-insoluble or poorly water solid because the rate of dissolution of a particle and often the bioavailability of an essentially water-insoluble or poorly water-soluble drug can increase with increasing surface area, i.e., decreasing particle size.
Examples of mills used to accomplish particle size reduction include colloid mills, swinging mills, ball mills, media mills, attritor mills, jet mills, vibratory mills, and high pressure homogenizers. Size reduction methods are described, e.g., in U.S. Pat. Nos. 4,006,025, 4,294,916, 4,294,917, 4,940,654, 4,950,586 and 4,927,744, and UK 1,570,362.
In a communition or milling process, repeated collisions of milling media with a solid material being milled, i.e., the milled substrate, result in repeated fracture of the substrate and concomitant substrate particle size reduction. When a media milling process is used to reduce the size of particles of a substrate, the process is usually carried out in a mill comprising a milling chamber containing milling media, a solid material or substrate which is to be milled, and a liquid or gaseous fluid carrier in which the media and substrate are suspended. The contents of the milling chamber are stirred or agitated with an agitator which transfers energy to the milling media. The accelerated media collide with the substrate in energetic collisions that can crush, chip, fracture or otherwise reduce the size of the solid substrate material and lead to an overall reduction in substrate particle size and an overall reduction in substrate average or mean particle size distribution.
Milling media are generally selected from a variety of dense and hard materials, such as sand, steel, silicon carbide, ceramics, zirconium silicate, zirconium and yttrium oxide, glass, alumina, titanium, and certain polymers such as crosslinked polystyrene and methyl methacrylate. Polymeric media are sometimes preferable to conventional inorganic media because they do not degrade to deposit metal oxides and soluble salts in the milled substrate and pH fluctuations and chemical changes can be minimized during milling. Such changes may impair dispersion stability, hydrolyze certain solids, and alter milling performance. Media geometries may vary depending on the application, although spherical or cylindrical beads are most commonly used.
Milling media can be of various sizes and size distributions that include large milling media particles and smaller milling media particles. The size distribution of the milling media can be narrow in which case the media are substantially uniform or nearly uniform in size. Alternatively, more than one narrow size distribution of media can be used. If two substantially different media sizes are used wherein substantially all of the media can be classified as being of either one or the other size, then the size distribution of the milling media can be described as being bimodal. Bimodal size distributions of milling media are often used in a milling chamber containing a separator having openings smaller than the smallest size of media used. Such a separator or screen will not allow any size of media used in a bimodal or broad distribution of media sizes to pass out of the milling chamber. Alternatively, the milling media can be sufficiently small that substantially all of the milling media can pass through the openings in the separator or screen and thus pass out of the milling chamber. Alternatively, the size of the openings in the milling separator can be small enough to prohibit passage of one size distribution of media (i.e., a larger size) but permit the passage of another size distribution of media (i.e., a smaller size distribution of milling media).
Mills useful for reducing the particle size of a solid substrate can operate in a batchwise mode or in a continuous or semi-continuous mode. Mills operating in a continuous mode often incorporate a means such as a separator or screen for retaining milling media together with relatively large particles of the solid substrate being milled in the milling zone or milling chamber of the mill while allowing smaller particles of the substrate being milled, i.e., product substrate particles, to pass out of the milling chamber in either a recirculation or discrete pass mode. Recirculation is often in the form of a dispersion such as a slurry, suspension, dispersion, or colloid of the substrate suspended in a fluid carrier phase that moves from the milling chamber into an often stirred holding vessel and thence back to the milling chamber, frequently with the aid of a pump. A separator or screen is effectively located at the outlet port of the milling chamber. Such means for simultaneous milling and media separation are referred to as “dynamic media separation”.
In another method of continuous milling of a substrate, mills operating in a continuous mode can incorporate a means for retaining relatively large particles of the solid substrate being milled in the milling zone or milling chamber of the mill while allowing smaller particles of the substrate being milled, i.e., product substrate particles, as well as the milling media to pass out of the milling chamber in either a recirculation or discrete pass mode. In recirculation mode, the product substrate particles and the media suspended in a fluid carrier move from the milling chamber through the separator or screen into an often stirred holding vessel and thence back to the milling chamber, frequently with the aid of a pump.
In yet another method of continuous milling of a substrate, mills operating in a continuous mode can incorporate a means for retaining both relatively large particles of the solid substrate being milled and large size milling media in the milling chamber of the mill while allowing smaller particles of the substrate being milled, i.e., product substrate particles, as well as small size milling media to pass out of the milling chamber in either a recirculation or discrete pass mode. In recirculation mode, the product substrate particles and the small size media suspended in a fluid carrier move from the milling chamber through a separator or screen into an often stirred holding vessel and thence back to the milling chamber, frequently with the aid of a pump.
In a batch process, the milling media, the fluid carrier, and the substrate being milled remain in the vessel until the fractured substrate particles have been reduced to the desired size or to a minimum size achievable. The fluid carrier and the product substrate particles are then separated from the media particles with a separator or screen at the o

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