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Ceramic Lining Crushes Jet Milling Problems
A custom processor uses a ceramic-lined jet mill to grind abrasives without the wear and tear.
By Debbie Weiner
CTSP Corp., a division of New Jersey-based fluoropolymer producer Ausimont USA, provides custom processing services for the plastics, additives, foods, and pharmaceuticals industries. CTSP employs 70 workers in two facilities -- its 70,000-square-foot headquarters in Belle Meade, N.J., and a 30,000-square-foot plant in Marshallton, Del. The Marshallton plant primarily supports Ausimont's polytetrafluoroethylene production with size reduction services. In Belle Meade, the custom processor performs various milling and grinding, blending, classifying, and finishing steps, such as packaging and warehousing, on a toll or contract basis.
A fully isolated processing room contains CTSP's stainless steel custom processing systems. The system shown here pneumatically loads the material, which a screw feeder then meters to the mill to be ground to the customer's specifications. The ground material is then collected in a baghouse and packaged according to the customer's instructions.
Since its 1991 inception, CTSP has grown from having two jet mills to having more than 20 mills capable of producing coarse to ultrafine powders. By continuing to acquire new milling equipment, the custom processor has increased its capabilities to handle more difficult processing requirements. The company often handles powders that require several processing steps before they become finished materials. CTSP's current annual processing volume is 6.5 million pounds.
CTSP sales and marketing manager Craig Davis says some customers use CTSP if they don't have the size reduction equipment and expertise necessary to complete a particular process. Others don't have the volume to justify the purchase of dedicated equipment. Or perhaps customers want to reduce certain materials off-site to avoid cross-contamination at their plants. For a customer developing new products, a custom processor provides an economical resource for milling and blending powders during the product refinement stage so that the customer can focus resources on existing products.
Another scenario occurred in 1998, when a chemical producer approached CTSP to grind between 250,000 and 300,000 pounds of the mineral wollastonite into a fine powder over an 18-month period. During this time, the customer sought to purchase and install its own milling equipment.
Trial run doesn't cut it
"As we do with all our customers, we conducted a trial processing run using a small amount of the material, in this case, 10 pounds of the wollastonite, to test the feasibility of achieving the customer's particular size requirements, which were between 1 and 1.5 microns," says Davis.
An operator adjusts the alumina-lined jet mill's parameters to achieve the customer's particle size specifications at the fastest possible throughrate.
CTSP used a 4-inch Micronizer laboratory jet mill for the trial run. The stainless steel mill has a shallow circular grinding chamber. Air, or another suitable gas, enters the chamber at an angle as a fine, high-velocity jet stream through six nozzles that are precisely aligned around the chamber's periphery. A screw feeder meters the powder while a venturi pneumatically conveys the powder into the chamber. As the powder enters the jet stream, high turbulence and energy are released, causing particle-on-particle impact and consequent size reduction. The turbulent flow also causes incidental impact against the chamber walls. Centrifugal force retains larger particles in the chamber's outer portion while centripetal force drags the smaller particles toward the center, where they are guided through a vortex finder to the outlet and then exit the mill. The powder then travels through a pipe for collection in a baghouse.
"It was apparent with the first pound that the wollastonite was too abrasive for successful operation," says Davis. "First, the normally white powder came out gray, indicating that it picked up metal from inside the grinding chamber. Second, the inside of the chamber showed significant signs of wear from contact with the abrasive powder."
A new mill with a twist
CTSP turned to their jet mill machinery supplier, Massachusetts-based Sturtevant Corp., for a solution to the abrasive grinding problem. Sturtevant manufactures a jet mill similar in construction to the one CTSP had already tried, but with a twist: Sturtevant's 24-inch jet mill is lined with alumina, an abrasion-resistant ceramic. Ceramic, one of the hardest materials available, resists wear associated with grinding ores, metals, and abrasive inorganics and chemicals. Alumina lines all product contact parts in the jet mill, including the chamber, venturi, and vortex finder.
After grinding, an operator packages and palletizes the material according to the customer's directions.
It took 3 months from the time of purchase to install the mill and begin the custom processing job on the wollastonite. The first run was 40,000 pounds with the same target particle size of between 1 and 1.5 microns.
The ceramic-lined jet mill is housed in an isolated processing room to prevent cross-contamination during processing. The wollastonite powder was vacuum-loaded into a stainless steel hopper above a volumetric screw feeder. The screw feeder metered the powder into the jet mill while the venturi pneumatically conveyed the powder into the chamber, which is approximately 3.5 inches deep.
As in the previous milling process, a fine stream of high-velocity air entered the chamber through nozzles, in this case 12, and caused particle-to-particle impact and size reduction. As the particles became smaller, they migrated toward the vortex finder in the mill top's center. As the particles passed through the vortex finder, they exited the mill and traveled through a 6-inch pipe for collection in a baghouse. The baghouse separated the powder from the process air using high-efficiency filters that allow air to escape but keep the powder trapped inside. The powder then dropped by gravity to the baghouse bottom. From here, a rotary valve discharged it into 200-pound fiber drums. CTSP marked the drums according to the customer's requirements, then shipped the drums to the customer for further processing.
"The results were very good," says Davis. "The powder was white, the mill showed no signs of wear, and the particle size was in the correct range."
With the purchase of the ceramic-lined jet mill, CTSP is now equipped to grind abrasive materials -- such as lithium carbonate, mica, and other materials used as additives -- into micron-size particles for several applications. The custom processor also uses the jet mill to grind nonabrasive materials, if needed.
Davis has found no problems with the ceramic-lined jet mill except for its somewhat delicate nature. "The ceramic lining is brittle, so we have to be careful when moving or opening the mill because replacing any of the ceramic lining would present significant time and expense issues," he says. "Otherwise, we've had no problems."
The customer has since purchased and installed its own ceramic-lined jet mill to process the wollastonite and is no longer a CTSP customer. "We always aim for long-term relationships with our customers," says Davis. "But of course, some jobs are temporary. However, being responsive and producing a high-quality powder that met customer specifications means that we'll always be in the running to support them with custom processing jobs in the future."
Finding the right grind
Companies who need custom processing services contact Davis at CTSP to discuss specific powders and applications, including safety and handling issues. Then Davis asks customers to send a small sample of the powder along with a material safety data sheet (MSDS). "All customer information is kept strictly confidential," notes Davis.
Once CTSP receives the sample and MSDS, the custom processor conducts a trial processing run, for which there is an hourly charge. Based on trial results and anticipated yearly volume requirements, Davis quotes a per-pound price. "However, if we're very familiar with a certain powder, I'll provide an immediate quote over the phone," he says. Processing costs depend on the material type and nature of the processing involved. Davis says CTSP's typical turnaround for a job is 1 to 2 weeks. If special equipment is required, it may take up to 3 or 4 weeks. "We don't require a certain amount of notice before beginning a job because we realize that for many of our customers, time is of the essence."
PBE
CTSP Corp., Belle Meade, NJ
908/359-7616 #351
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