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Raw material changes affect alumina powder production James L. Davis, Powder Processing Solutions Get your questions answered If you have a question about bulk solids handling or processing, send it to us and we'll find an expert to provide an answer. Email your questions to editor Jan Brenny, jbrenny@cscpub.com. We need your expertise If you're an independent consultant or academic and would like to share your expertise, please contact editor Jan Brenny, jbrenny@cscpub.com. Include information about your experience in the dry bulk solids industry. Q Our facility processes alu- mina powder and silane using an attritor. We've been experimenting with a finer, bi- modal alumina powder and a silane with a higher molecular weight. Pro- cessing takes longer and draw weights are down. Any thoughts on what might be causing this? A Grinding processes are very sensitive to changes in raw material feed, grind- ing speed, media, and other factors. In this case, an attritor — a stirred-me- dia-style mill — is being used to grind alumina and disperse silane as a surface coating. This is often done to change the bonding properties of the alumi- num oxide to promote good epoxy bonding to a substrate — although it's not clear whether that's the process in the current application. Aside from grinding the alumina, the mill is aiding the silane's chemical reac- tion with the surface of the alumina by dispersing the silane in the batch. In this case, two raw material changes have been made at once, which has changed the performance of the physi- cal finished product in the mill. (I as- sume that no changes were made to the mill itself.) The stated problem is a material flow problem when a drawdown is at- tempted. This is actually a symptom of the root problem, which is that the powder is most likely becoming more cohesive and less able to flow. A screen on the discharge prevents the media from being pulled out of the mill along with the finished product. The new fin- ished product is most likely holding up on that screen. The higher cohesiveness is probably a function of changes in the surface chemistry reaction between the silane and the alumina. Another indication that the reaction chemistry has changed is that the finished product's surface area has not changed even though a smaller alumina is being used. With a smaller alumina you would expect that the surface area of the finished product would increase. The cohesiveness of the finished products can be compared with a shear test, a uniaxial compres- sion failure test, or a funnel test. Each raw material has to be evaluated separately for its effect on the new raw material. First, a silane with a higher molecular weight has been incorpo- rated into the process. The surface chemistry of silane hydrolysis on alu- minum oxide surfaces is an inexact sci- ence. Still, it seems likely that a change in the type of silane could easily affect the physical properties of the finished product. I recommend running some tests with the older alumina and the new silane. Look specifically for changes in power draw or a temperature rise in the batch. If the mill has a cooling jacket, then check the delta T (temperature change) of the cooling fluid. Compare the power draw and temperature data throughout the batch in the old process and the pro- cess with the new silane. Higher or lower temperature changes may indi- cate more or less reaction between the silane and the alumina respectively. Higher power draw will indicate that a stickier powder is being generated. Next, evaluate whether the finer alu- mina is causing the problem. Run some additional batches with the old silane and the new alumina. Again, check power draw and temperature changes. Since the finished product particle size hasn't changed even with a smaller raw material, more heat may be being gen- erated by grinding the old alumina, which might aid the silane's surface re- action with alumina. If less reaction is occurring, then longer mixing times may be needed. You'll have to experi- ment to find out. Experimenting on the full batch scale could get expensive, so look for a test- ing facility with lab-scale attritors or lease a lab-scale unit for smaller test batches at your plant. James L. Davis, president Powder Processing Solutions Cincinnati, OH USA www.powderprocessing solutions.com Ask An ExpErt