The manufacture of the precious metal inks starts with the production of a uniform particle size precious metal powder. The powder particles associate into clumps of particles or agglomerates, held together by a variety of associative forces. To produce suitable ink, these agglomerates must be separated into discrete particles and stabilized so that the particles do not reagglomerate. This function is carried out by a combination of product formulation, which provides the appropriate wetting and stabilizing system, and the use of a dispersion process to separate the particle clumps into individual particles.
Ball milling is generally characterized by long processing cycles and poor productivity, and precious metal dispersion is no exception to this rule. More importantly, ball mill dispersion of precious metal powders no longer provides particle size distributions that are narrow enough to meet the increasingly stringent requirements of the electronics industry without producing flattened or "flaked" particles, which are not acceptable in inks.
Unlike the ball mill, where the product formulation is placed into the mill prior to processing, media mills use an external mixer to produce the product blend and then pump it through the media mill for processing. The successful use of this process has been quite limited because the type of shear normally found in these mills will produce flattened particles within the dispersed and stabilized system.
However, media mill processes do produce the narrow particle size distribution desired for the product. The fine media mills used in these earlier tests were the typical peg or disc mills used by a broad range of industries. These mills are characterized by abrupt changes in product flow direction inside the mill. These directional flow changes are regarded as the primary cause of flaking particles produced during the dispersion process. If flaking of the metal particles is the goal, the disc design mill will achieve this requirement.
New dispersion process equipment has been developed* specifically to manufacture precious metal dispersions that are free of the flattened or "flake" particles that characterize other fine media mill dispersions and possess the narrow particle size distribution desired by the industry. The equipment was designed by examining the fine media mill dispersion process and defining those characteristics required by a specialized mill.
For example, two types of shear action exist in a typical fine media mill. The use of pegs or discs results in a significant pumping action involving abrupt changes in flow direction and rapid acceleration of the dispersion media/product mixture in the mill. However, those surfaces within a fine media mill that are parallel to the agitator shaft produce a swirling or stirring action rather than the pumping action of the pegs or discs. Studies have shown that this swirling action is much gentler than the pumping action and, therefore, less effective from a dispersion perspective.
Based on this analysis, the new media mill has been specifically designed to provide a minimum level of pumping action and a maximum level of swirling action in the dispersion process. In addition, because precious metal particles are quite heavy and prone to settling, the mill was designed to be free of corners where particles could accumulate, and the liquid volumes entering and leaving the mill were minimized to provide liquid flow velocities high enough to preclude particle settling.
To maximize the dispersing action in the mill without the pumping action that could flatten or "flake" the metal particles, the agitator was designed with a different relationship to the interior profile of the grinding chamber, when compared to traditional peg or disc mills. Even though the mill largely lacks the pumping action inherent in peg or disc mills, it is much more effective as a dispersion device than a ball mill, providing an equivalent particle size in a fraction of the residence time and providing a narrower particle size distribution.
Grinding media is loaded into the mill through the feed inlet tube, which can also be equipped with an engineered plastic insert that is used after the media has been charged to the mill to reduce its diameter and, therefore, the product holdup in the system.
The product is separated from the grinding media by a rotating screen mounted on the agitator shaft. This screen is designed to have a minimum internal volume to maximize product flow velocity and minimize particle settling. The separated product flows to an agitated minimum volume accumulation chamber and exits the mill through tubing designed to produce a flow velocity that precludes particle settling. The mill seal retains the product in the mill and protects the shaft bearing from contamination.
The mill is tailored to the stringent requirements of this industry while also providing significant increases in dispersion efficiency compared to conventional technology.