Grinding small particles into a mixture or dissolving them in a solution, while one of the oldest manufacturing processes, continues to benefit from improvements. Equipment now, for instance, can produce smaller particles and provide greater productivity.
"Slurry milling technology has been evolving in the direction of more efficient mill designs and more efficient grinding media," notes Samuel November, senior milling engineer at Rohm and Haas, Bristol, Pa., which runs milling and dispersing lines to produce pigment dispersions for use in automotive coatings, inkjet applications and other colorants. "We have demonstrated improved product consistency by using the latest grinding media technology and good data collection, combined with our quality control parameters," he adds.
The Zeta Mill produces sub-micron particles via a rotor-stator arrangement of pins that creates high shear, shown above and in the cutoff diagram at left.
Milling's missionMilling and dispersing machines frequently are selected when solid particles must be suspended in a liquid carrier or mixture of components. When the particles must be processed to a specific size or size distribution, milling machines are often the first choice. Milling machines use grinding beads as media to crush particles. Most milling is done wet,"either with some liquid carrier or with the liquids in the final formulation such as paint. Dry impact milling,"with air jets propelling particles against each other,"suits certain friable materials or processes in which the end product needs to be air-classified.
Newer technologies feature higher speeds (10,000 rpm or more) and media whose diameter can be measured in microns. These mills find use in making pharmaceuticals and fine chemicals, electronics materials, and the inks and coatings used in desktop-publishing equipment. "In the past ten years, we've seen numerous applications where the top (maximum) particle size has gone from tens of microns to sub-micron," says John Sneeringer, technical director at Premier Mill, based in Reading, Pa.
However, well-established technology continues to play an important role. "Our ball mills are an older, slower technology, but in the right environment, they're the best choice," says David Mann, president of Patterson Process Equipment, Newark, Del. Patterson's horizontal ball mills generally run at speeds no higher than 20 rpm. "As long as the rotating parts are maintained, these units have tremendous longevity and are relatively inexpensive," he adds.
While the basic technology for milling and dispersing is well-established, selecting the right process and machinery is a challenge. "There's as much art as science to designing a process and selecting the appropriate equipment," notes Todd Kritzer, a vice president at Kady International, Scarborough, Maine.
Vertical KE50 mill suits autogenous grinding of highly abrasive materials.
Selection criteriaEquipment vendors and users agree that the first step in equipment selection is an evaluation of the process,"its raw materials and desired end products,"followed by testing on the actual milling and dispersing systems being considered. Many vendors offer lab-scale versions of their systems with capacities of around 1 l. "Our vendor, Premier Mill, was very helpful, not just in helping us understand the nature of the materials we process , but also in successfully scaling up from a 1-l test unit to a 200-l production unit," says one manager in agricultural chemicals processing. "The product specs for the production unit came within 3 percent of the laboratory results."
"We do fairly rigorous prequalification of new customer applications ... testing materials with a fairly quick turnaround," says Kritzer of Kady International. "When the prequalification is complete, we can provide a mobile pilot unit, or in some cases a free 21-day rental of a production unit. A high percentage of those units never come back,"the customer simply buys them."
Key variables for milling and dispersing systems, according to industry sources, are:
Hardness of particles, which is typically given on the Mohs scale. Some ceramics near the upper end of the scale now can be processed with wet media milling. Dry impact milling can be used for a wide range of hardnesses, although some materials can agglomerate in the impact chamber. Softer materials can be handled in shear mixer/dispersers, without milling.
Incoming and final particle-size distribution. The best efficiency in media milling generally occurs when the feed material is no larger than three to ten times the diameter of the media. So, feeds with large size distributions might require some type of pre-milling. Wet media milling is the preferred choice to achieve the finest final particle sizes.
Process temperature. Even simple, slow mixing generates heat; high-speed dispersion or milling can generate substantial amounts. To avoid product degradation or reactions in the dispersing zone, many systems come with water-cooled jacketing for the dispersing zone. Some processes run better with chilled or even cryogenically frozen feed, while others benefit from heating feeds through the jacketing or even steam sparging.
Process economics. Tradeoffs often center on processing time vs. energy consumption. Some large low-energy milling processes can require batch times that are measured in days. High-energy processes tend to be much faster and also considerably smaller. Media costs can be a significant factor in media milling. Non-media grinding processes such as jet milling or dispersing might require long residence times or multiple passes to achieve the desired particle-size reduction.