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.
"There is a pecking order, if you will, for this type of process equipment," says Stewart Rissley, a regional sales manager for Morehouse-Cowles, Fullerton, Calif. Mixers work with low- to medium-shear blending of liquids or slurries. Dispersers provide high shear and, when incorporating a rotor-stator design, particle-size reduction. Vertical media mills provide rapid particle-size reduction, while horizontal media mills achieve sub-micron particle-size reduction, he says.
To these can be added dry jet mills, which can handle harder materials and are very efficient when the overall process calls for air classification after the milling. "Sub-micron particles are produced using jet mills," says Harry Way, technical director at Netzsch, Exton, Pa., "but because of the low particle mass and the tendency for these particles to agglomerate, it is not an easy process ... We have seen figures where the energy costs are four or more times more than those for wet grinding," including spray drying or spin flash-drying the wet-milled material after grinding."
"The advantages of jet milling are low heat generation, narrow final particle-size distribution, and the ability to process abrasive materials that would chew up the liners of media mills," says Gregory Shemanski, president of Custom Processing Services, a Reading, Pa., toll processor. "Also, jet milling has fewer internal parts that have to be kept clean for feedstocks such as pharmaceuticals."
The key advantage of jet milling is that it generates very little heat, enabling temperature-sensitive materials to be processed, notes Tony Liuzzo, a product manager at Plastomer Products, Newton, Pa., which offers the Trost mill. It has a circular chamber and two opposing inlet jets. Feed material is jetted into the chamber and pulverized; the finer material flows to a central portion of the chamber and is removed, while heavier or larger particles recirculate. Other manufacturers of air-milling systems include Hosokawa Micron, Summit, N.J.; CCE Technologies, Cottage Grove, Minn.; and Stedman Machine Co., Aurora, Ind.
New look in beadsThe clearest trend is the drive toward smaller micron-size milling. Vendors cite a variety of reasons: the continually rising purity levels needed for microelectronics manufacturing; the clarity required for products like automotive topcoats; and the specific biological properties of controlled-release pharmaceuticals and agricultural chemicals. At the same time, exotic media such as yttrium-stabilized zirconia ceramics have become more generally available. They are capable of handling "materials we've never considered before," notes one vendor.
In addition, vendors are designing systems that simply work better. For example, CB Mills, Gurnee, Ill., offers a horizontal media mill called the Dyno-Mill ECM licensed from Willy A. Bachofen Machinenfabrik of Basel, Switzerland. The unit addresses a traditional problem of media milling, a hydraulic packing of the media that causes the media to concentrate in one section of the mill and thus compromises performance. The Dyno-Mill has a circular agitator element that controls distribution of the media, allowing the unit to handle a much larger throughput. One user in the coatings field notes that the ECM unit has cut processing time from 40 hr to 7 hr, "allowing us to handle a batch within one workshift and with a tight particle-size distribution." Depending upon the application, "you can cut energy costs by 75 percent," says Denny Stidham, CB Mills vice president.
Morehouse-Cowles has achieved a similar productivity improvement with its Zinger mill, (photo, above) according to Stewart Rissley. This unit features a vaned rotor that provides control over the flow of media inside the milling chamber. "Our largest 40-l mill will compete with 200- and 300-l conventional mills," he says.
Netzsch's Zeta mill (photo p. 39) a pin-mill design, is aimed at sub-micron applications. Pin mills feature a rotor-stator arrangement of pins that create the shear necessary to grind material while avoiding hydraulic packing. Netzsch units also include a screening component on the mill shaft called the dynamic cartridge media separator to enhance flow.
The company recently introduced another design, the KE 50 (photo, p. 41) a vertical unit for highly abrasive materials like silicon carbide that relies on the feed material itself as the grinding media (autogenous milling). Its vertical orientation lessens the abrasion by feed material on the vessel's walls while still achieving sub-micron milling, according to Netzsch's Harry Way.
The Zinger mill features a vaned rotor, allowing the flow of media to be controlled within the milling chamber.
Pre-mixing plusesTo get the highest productivity and product quality, the newer generations of high-speed bead mills benefit from a pre-dispersing or mixing phase that reduces size distribution and evens out a feed formulation. "We use high-speed dispersing agitators for mixing, followed by bead milling for size reduction and color development," notes Rohm & Haas' November.
One of the more widely used disperser designs features a disk shaped like a circular-saw blade mounted on a vertical shaft. The blade can be designed for high shear to help break up particles or for a high degree of mixing. Morehouse-Cowles' Rissley notes that his company will be introducing several new versions this fall.
For the past several years, Kady International has had considerable success with a concept it calls "premilling," in which a Kady mill replaces a conventional saw-tooth mixer. The unit employs a high-speed, high-energy rotor-stator arrangement. Feed material is hurled off the rotor tip at velocities of 9,000 fpm, breaking as it impacts the stator walls. In some cases, the Kady mill can eliminate the media mill entirely on products that otherwise would require only one media-mill pass, says Kritzer.
Meanwhile, Vortex Ventures, Houston, has developed an eductor design that produces extremely rapid wetting or mixing of powders, especially into water. The Lobestar Eductor now is commonly used in oilfield downhole applications. However, Gerald Lott, president of the company, notes that new uses are cropping up in chemical processing such as mixing cellulosic polymers.
Acme Soap Co., San Antonio, Texas, likes the machine's ability to wet powders that go into its cleaning ability formulations rapidly. "The eductor is located at the bottom of a 2,600-gal. mixer tank," says William Waring, production manager. "The rapid wetting allows us to run the machine with minimal manpower, with two crewmen able to load 2,000 lb. of citric acid powder into the mixer in 20 minutes."
Meanwhile, IKA Works, Wilmington, N.C., has just won a Vaaler Award for its Cone Mill (see p. 28)