A: Bill Noonan, Marion Process Solutions, says:
Material loss during powder mixing can reduce profitability. When you lose material, you lose financially. Beyond that, powders escaping can cause health hazards and make a mess. Cleaning a room dusted with powder is time-consuming and causes downtime during production. Six key techniques can help powder processors avoid material loss during processing, saving both time and money.
High-performance seals and gaskets prevent powder from escaping. The most common problems occur when the mixing equipment’s gaskets and seals wear. Standard tape gaskets lose their memory quickly, and then the doors won’t seal properly when closed. Sometimes frustrated equipment operators just remove the tape gaskets because they wear so quickly. Some mixers use an O-ring gasket that’s simple to replace. If you have easily replaceable gaskets, you can keep spares on-site and replace them as soon as you see signs of powder escaping from the mixer. An O-ring gasket with a pin retainer takes just moments to remove and install, so you can clean one set while using a second gasket set to keep the mixer in service. You can see these O-ring gaskets in action in this video.
Similarly, the main shaft seals are a point of friction and can wear out and allow powders to leak. Inspect main shaft seals daily to catch any wear and tear before they become a problem. Braided rope is a common and widely used packing material for seals. The rope is impregnated with oil, which attracts dust particles to prevent the particles from escaping through the seal. We use a patented double-lip seal gland that lasts longer than braided rope because it’s possible to remove and service the seals themselves. Regular seal cleaning and maintenance provide longer life and prevent premature deterioration of the seals. Proper airflow is very important, so check daily to ensure that the airflow to the seals is on. Having the airflow manifolds at eye level can also help you monitor what’s happening. Make sure to regularly clean and service the main shaft seals both to prevent material loss and increase the seals’ life, which will also save you money over the long term.
Proper inlet and outlet valve maintenance keeps materials from escaping between machines. Monitor equipment inlet and outlet valves, which can wear out and allow fine particles to escape at the connection between the valve and the equipment. Similarly, inlet valve fittings may have bolted or clamp connections. If you’re removing valves for cleaning, make sure the fit is still hermetic when you reaffix them to the equipment. When you detect wear, particularly in the fit to the equipment, replace the valves.
Improper equipment operation can also lead to inconsistent batches and material damage. In most powder applications, you need to achieve a precise mix, or the product becomes unusable. For example, nobody wants a spice mix that’s different every time they buy a package. With pharmaceutical and other chemical applications, precision is even more important, so proper equipment operation is critical.
Today, most mixers come with software controls that can help ensure perfect mixing. Beyond that, it’s important to train the staff not to rush or, conversely, take too much time and overmix.
Avoid overloading and underloading to improve performance and consistency. Another common cause of material loss is overloading or underloading the mixer. The ideal operating volume is 75 to 100 percent of capacity. Overloading the machine to try to save time results in inconsistent mixes and, potentially, unusable material. When the mixer is overloaded, the material above the paddle or ribbon in the 12 o’clock position doesn’t get incorporated properly into the material bed and causes the above material to become stagnant and results in improper blending and quality control problems. By having too little material inside the mixer, the orientation of the paddles or ribbon’s flighting doesn’t allow the material to move properly in the correct mixing action, which can cause inconsistent blending, longer mixing times, or both.
Carefully monitor temperatures to prevent damage. If your mix requires a thermal reaction, such as cooling, heating, or vacuum drying, you’ll need sensors to keep you informed about the process so you don’t overheat or underheat the mix. The most common error is overheating when operators are in a hurry to get material through the process. Overheating can cause burning or case hardening, so don’t let the operators deviate from the tested and verified process. If you have a chemical reaction that requires a precise temperature, talk to your supplier about getting equipment that will consistently stay within the required temperature range.
Every mix is different, which is one reason why we emphasize testing and consulting with experts. For example, flour is a lightweight material that floats to the top of a mix. Crystal-shaped materials such as sugar and salt have jagged edges that can wear off if the material is overmixed, creating smaller particles that can impact a mix’s consistency. The residual dust can also end up being left behind as residue in the mixer, or the dust may escape the mixer, creating a mess that needs to be cleaned.
Mixing for the right amount of time avoids separation and material degradation. When you’re mixing a powder with a combination of light and heavy powders, such as flour and sugar, it’s important not to overmix. At the optimal point, the mix will be homogeneous, but if you keep mixing powders with different weights and properties, they will start demixing, with the lighter material drifting to the top and the heavier material sinking to the bottom. You can think of the mixing time as a bell-shaped curve. If the perfect mix takes 4 minutes, at 5 minutes the material will have separated back down to where it was at minute 3. The most obvious loss with overmixing is that you could have started the next batch sooner. Sticking to the optimal mix time simply allows you to manufacture more product. Also, of course, if you have abrasive or crystalline materials, overmixing simply wears the trough and unnecessarily shortens your mixer’s life.
Keep your equipment well-serviced to maintain consistent quality in every batch. Finally, regular mixer maintenance keeps the equipment operating at optimal levels. As mentioned, the shafts, seals, gaskets, and valves are the most obvious areas to conduct maintenance. You’ll also want to make sure the trough is in good shape. I recommend having a service agreement with your equipment supplier to inspect the equipment regularly. An experienced supplier will set up an inspection schedule that makes sense for your application.
Bill Noonan, Marion Process Solutions, Marion, IA, manufactures and supplies industrial mixing and blending equipment.