Milling Precious Metals
The rapid evolution of electronic data processing and its associated technologies has led to increasingly stringent requirements for the precious metal dispersions used for printed circuits in these end uses. In particular, precious metals dispersions have become increasingly finer in particle size and narrower in particle size distribution to print the smaller and smaller circuits required by the miniaturization of electronics technology. The precious metal inks are important to the electronics industry because of their excellent conductivity and extreme resistance to corrosion.
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.
Traditional Methods
Traditionally, precious metals dispersions have been made using ball mill dispersion equipment. Ball mills are basically hollow horizontal cylinders that are partially filled with ceramic balls and the precious metal ink compositional mixture. These mills are rotated at a speed that allows the balls to cascade through the mixture, separating the precious metal particle clumps into individual particles. While ball mills may be massive pieces of equipment for most industries, those used in precious metal dispersion are little larger than laboratory units.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.
Alternate Methods
Other dispersion processes exist that are potentially suitable for making precious metal dispersions. Of these, fine media milling has been selected and tried by a number of precious metal ink manufacturers. Fine media mills usually consist of a horizontal cylinder with a concentric internal agitator. The cylinder is normally filled from 75 to 90% with glass, ceramic or metal beads ranging from 0.25 to 2.0 millimeters in diameter, which, when stirred by rotation of the agitator, provide the level of shear necessary to disperse the particulate in the surrounding liquid. The agitator systems are usually either a pegged shaft or a disc shaft.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.
Meeting Stringent Requirements
The new mill is based on a proprietary jacketed cylindrical shell to eliminate any areas in which metal particles could accumulate. The cylindrical shell is normally stainless steel to provide heat transfer to the water jacket, and the rotating agitator is usually made from an engineered plastic to minimize metal contamination in the precious metal product. The agitator is mounted on a concentric shaft, using a bolt and a key to prevent it from slipping in operation.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.