special section | materials handling/powder processingMODERN Wet Milling and Material Processing for about a 65% (of total capacity) fill of product, which allows room for the grind- ing media to rise and fall for impact. Especially with smaller units, the labor and time required to batch and discharge the machine can become problematic and inefficient. Because of this inefficiency,a typ- ical ball mill will only be available for grind- ing around 75% of the time, which does not take into consideration both scheduled maintenance and emergency breakdowns. Continuous ball mills are in use around the world, but they tend to be very large units suitable only for high-volume slurries and non-critical milling applica- tions. Mills like this are commonly used in large ceramic tile factories to prepare slip for spray drying. Advancements in vertical ball mills have been made in recent years but also lack efficiency when compared to other methods. Another challenge involves finer and finer particle size demands. Tradi- tional ball mills are generally limited to a finished particle size of approximately➤ Processes in a variety of industries now routinely demand 5 microns, which can be achieved onlyparticle sizes with an absolute maximum of well below 1 micron. with great attention to media size.by Carl D. Yerger, President, Custom Milling & Consulting, Inc., Fleetwood, Pa. Grinding media for ball mills ranges in size from 10-90 mm for most tradi- he world of material process- Traditional Milling tional applications. Typical ball mills useT ing and size reduction demands faster and more efficient meth- ods to yield increasingly smallersizes and narrower particle size distribu-tions. Processes in a variety of industries The ceramic industry is no different from others in its search for modern manufac- turing techniques. The traditional ball mill has been the primary method used to mill ceramic materials for hundreds of high-alumina or porcelain balls for their density, but wear is relatively high. Mod- ern mono-size yttria-stabilized zirconium media is not a practical alternative due to the high cost per charge, as well as thenow routinely demand particle sizes with an years, and is still considered to be one of frequent re-charge rate. Simply put, it isabsolute maximum of well below 1 micron. the best mixers ever invented. difficult for the traditional ball mill to put Manufacturers are challenged to inte- Ball mills are generally batch units, enough energy into the media charge tograte a repeatable method of production which means that all ingredients are reduce particles to the sizes and distribu-that is capable of this level of size reduction charged into the chamber and the mill is tions now required.while controlling labor and energy costs. run unattended for a specified amount In all milling processes, the ultimateThe system must also be robust enough to of time (or number of revolutions). The performance of the mill is highly depen-handle the most abrasive of raw materials. design of a typical ball mill only allows dent on the media size and distribution.Above: The horizontal media mill has a closed chamber that receives the pumped pre-mixed slurry in one end and discharges the milled product fromthe opposite end. C E R A M I C I N D U ST RY ➤ S e p t e mb e r 2 0 1 1 29
MODERN WET MILLINGTable 1. Effect of different media sizes on the number of cycles required. 1.7-2.1 mm 1.0 mm 0.6 mm 0.3 mm Theoretical Passes Recirculation Time (Min) d50 (µ) d50 (µ) d50 (µ) d50 (µ) 0.0 0 2.606 2.606 2.606 2.606 1.2 15 2.310 1.959 0.978 0.427 2.4 30 1.985 1.413 0.550 0.336 3.6 45 1.637 1.059 0.552 0.316 4.7 60 1.411 0.820 0.454 0.297 5.9 75 1.271 0.655 0.440 0.273 7.1 90 1.094 0.477 0.367 0.260 8.3 105 0.950 0.423 0.291 0.251 9.5 120 0.862 0.381 0.284 0.244 10.7 135 0.778 0.376 0.277 0.232 11.8 150 0.673 0.346 0.268 0.221 13.0 165 0.602 0.329 0.260 0.218 14.2 180 0.573 0.307 0.258 0.214Table 1 shows the effect of different media particle impact and shear. At the exit end to two mills. When nano-sized particlessizes on the number of cycles required. of the chamber, media is retained in the are specified, such as those required in mill by a screen (also called a separator or ceramic inks used in inkjet applications,Horizontal Media Mills filter) that is of such size that slurry par- the mills are typically setup in high-flowThe horizontal media mill has been used ticles can pass through. It is important to recirculation.in the inks and coatings industries for over note that the screen plays no role in par- With the onset of digital technology30 years. Best described as a “high-energy ticle size distribution. in inkjet ceramic decoration, speciallycontinuous ball mill,” the machine’s processed inks are critical to success.design has undergone continuous changes Benefits The higher resolutions created by today’ssince its introduction to keep up with the Determined by product type and particle non-contact digital printing technol-demands for finer particle sizes and nar- size and distribution, these milling systems ogy offers unlimited image variation forrower particle distributions. are unique to each application so design ceramic tile production. The digital ink- The horizontal media mill has a closed and setup are crucial for success. Equip- jet printer is quickly becoming the stan-chamber that receives the pumped pre- ment manufacturers must be able to supply dard on the modern glaze line, using spe-mixed slurry in one end and discharges the latest in wear-resistant materials as part cially formulated and milled pigmentedthe milled product from the opposite of a successful installation. Selection of the ceramic inks in submicron particle sizes.end. The mill uses a mechanically sealed proper construction materials is based onshaft that is cantilevered from a heavy factors such as vehicle system, product end System Integrationbearing housing. This shaft can be fit- use and the abrasive nature of the raw mate- In today’s competitive environment, it’sted with a variety of agitators, includ- rials. Materials of construction for milling not just about selecting the right weting discs, pegs or disc/pin combinations, wearing parts might include various types grinding mill—integration to the overalldepending on the particular process. The of ceramics, urethane and urethane coated, system is important as well. Whereas achamber is typically charged with grind- high polymer plastics, and composites. ball mill installation is fairly straightfor-ing media to 60-90% of the net shell These milling systems are versatile ward and uncomplicated, the successfulvolume. The high energy imparted by and can be configured for one- or multi- installation of a high-energy attrition millthe agitators on the grinding beads cre- pass operation through outfitting with requires careful consideration of a num-ates frictional heat, which is removed by the proper tanks and pumps. In addi- ber of factors:chilled water circulating in the cooling tion, duplex systems with dual milling • Premix equipment selection for slurryjacket of the chamber. heads on a common frame combine the preparation As the milling shaft rotates, the media best of both by passing through a sec- • Properly sized chiller system to removeis fluidized by the agitators positioned on ond chamber on the same machine that BTU energy from the grinding processthe shaft. The peripheral agitator speed for can be charged with media of a different • Pump systems sized for optimum flowthis type of mill is typically 8-17 m/sec. The size than the first chamber. Duplex mill- and resistance to abrasive ceramicceramic slurry passes through the length ing (sometimes referred to as “cascad- materialsof the chamber under pump pressure ing”) offers the advantage of operating • Feed and receiving tanks equipped withwhile being milled through media-parti- two grinding chambers with one control mixers for adequate suspension andcle impact and shear, as well as particle- system in a reduced footprint compared batch turnover30 S e p t e mb e r 2 0 1 1 ➤ W W W. C E R A M I C I N D U S T RY. C O M
• Process plumbing designed to minimize waste and optimize product yield• Controls and instrumentation packages are crucial for process control and auto- mation, as well as data acquisition and product repeatability• If post milling filtration is required, the selection of a filter system is crucial to upstream mill efficiency• Moving materials to and away from milling system in the most efficient manner possible High-energy media (attrition) millsare commonly used in the manufactureof inks, coatings, agricultural, nutraceu-ticals and pharmaceuticals, electronics,and many other fine-particle products.Ceramic processes using media (attrition)mills include tape casting, specialty ceramiccoatings and the milling of digital ceramicinks. As material processing innovationsrequire finer and finer particle sizes, the Integration is important to the overall system.use of these systems will grow, especiallywhere high strength gains through the useof nano-sized materials are possible. The special requirements of ceramic OPF Enterprisesmaterials require the manufacturer to design P.O. Box 16898, Missoula, Montana 59808systems capable of withstanding the abra-sion and wear inherent in most processes. Fax and Phone: 406-493-1102If the materials of construction and systemsare designed correctly, there is a high prob- Bryan Geary has over 22 years of experience in differentability that high-energy media mills will manufacturing roles including production operator, laboratoryfind new homes in both advanced and tra- technician, laboratory manager, production supervisor, productionditional ceramics. Industries such as ceramic manager, assistant plant manager, plant manager, and director of manufacturing. He is an accomplished student and teacher oftile, brick, refractory coatings, oil and gas manufacturing methods and holds a green belt in lean Six Sigma.materials, glass, whitewares, pigments, elec- Cell Phone: (406) 370-8923 e-mail: email@example.com, polishing compounds, ceramic inks,and crucible manufacturing may use for-mulas requiring near- and sub-micron par- Carl Sorrell has spent 35 years in manufacturing roles includingticle sizes, opening new avenues to achieving production operator, front line supervisor, engineering manager,enhanced material properties. c maintenance manager, production manager, plant manager, director ofFor additional information, contact Custom research and development, and vice president of manufacturing. HeMilling & Consulting, Inc. at 1246 Maid- claims to be an engineer and can produce a Bachelor of Scienceencreek Rd., Fleetwood, PA 19522; or email diploma in Ceramic Engineering from the University of Missouri-Rollasales@cmcmilling.com. (now the Missouri University of Science and Technology). He holds a black belt in lean Six Sigma.AcknowledgmentThe author would like to thank OPF Cell Phone: (731) 499-0901 e-mail: firstname.lastname@example.orgEnterprises (www.ontheplantfloor.com)for their assistance with developing thisarticle. OPF is Custom Milling & Consult-ing, Inc.’s ceramic consultant.
n DOPF (On the Plant Floor) Enterprises has a richhistory with CMC and CMC equipment.We have worked together on successful projectsincluding an installation of Supermill Plus mediamills to produce high output of ceramic slurries insubmicron particle sizes.The OPF team has almost 60 years of hands-onexperience in the ceramic industries and is proudto partner with CMC to offer a complete packageto those dealing with ceramics and ceramic Product/Process Developmentprocesses. Materials are expensive. Time is priceless. ResultsOur special areas of expertise include can never arrive soon enough. CMC and OPF staff provide a winning combination with a proven track • Ceramic Engineering record in the industrial world. • Ultra-Fine grinding CMC’s state of the art laboratory and facilities give you additional means to achieve your goals. • Raw material selection, batching, mixing • Small scale milling to <0.2 microns • Product formulation • Scale-up estimates and trials • Ceramic Digital Inkjet • Formulation development and assistance • Refractory design, selection, sourcing • Equipment training and instruction • Bulk solids storage and transport • Raw material acquisition and storage • Scale up from R&D to pilot to full production • Finished goods storage • Factory and process design • Full process documentation to meet your ISO requirements FROM CONCEPT TOPRODUCTION WITH CMC AND • Process design OPF ENTERPRISES • Raw material evaluation Custom Milling & Consulting, Inc., 1246 Maidencreek Road, Fleetwood, PA 19522 (610) 926-0984 Fax (610) 926-0989 Website: http://www.cmcmilling.com OPF Enterprises, PO Box 16898, Missoula, MT 59808 Telephone and Fax 406-493-1102 Website: http://www.ontheplantfloor.com