You’ve prepared for months, maybe even years. A crowd gathers. People are chanting your name, and your stomach is doing backflips. Bob starts a drumroll. The swell of enthusiasm is deafening. You throw the Clark Griswold switch and wake up covered in sweat from the all-to-familiar nightmare: Selecting the wrong mixer for your application. So, pay attention; this is better than an uncle-in-business discount. Guaranteed.

1. Know your materials: Particle strength, sizing, weight, and flow characteristics are priority one. Without this info you’d be better off grabbing a wooden spoon. These details help determine whether you’re macro or micro mixing. Free-flowing materials with a mean particle size greater than 75 microns are typically best-suited for macro (low-shear) mixing. Due to minimal inter-particle forces these materials require low energy input, and as mixing occurs, the ingredients become randomly ordered and can quickly reach a desirable state of homogeneity. However, the final mixed product shouldn’t be considered stable as there is increased potential for the mixture to segregate during or post-discharge due to the size, shape, and densities of the particles. The two main areas of concern are percolation - the effect of gravity on packed powders and transportation - where separation can occur during discharge or conveyance. In other words, congrats on getting your mix together; now let’s make sure you keep it together.

In micro mixing, consistency and stability can be achieved with finer, more cohesive materials. Particle size is usually smaller than 75 microns and can include some form of a binder, liquid, etc. Stronger internal particle forces result in a more difficult mix that can produce agglomerations. For the particles to become randomly mixed, micro mixing equipment uses impact or shear to generate forces among the particles and break up agglomerates. Depending on the equipment configuration, this can be accomplished through a mixing motion that effectively scrubs particles against each other, or through using intensifiers operating at high speeds. 

2. Consider your conditions: Safety first. Separating the pros from the pretenders is hugely important when processing potentially dangerous materials. From toxicity to explosivity, mitigating dust, heat, pressure, and/or emissions is top priority. Once safety is satisfied, shift your focus to feeding the beast. The method for charging a mixer is critical to its success. Off-spec ratios always produce imperfect results, so it doesn’t matter if you’re slinging 50-lb bags or integrating highly accurate dosing systems. If executed poorly, both can balloon mix times while causing a host of other issues. When in doubt, remember this simple mixing formula: majors + minors + majors = a more harmonious outcome. Layering ingredients in this order will set you up to impress even the most discerning of your QC teammates. Lastly, does your mixing solution allow you to combine steps? Can you mix and dry, mix and coat, mix and heat/cool, mix and size reduce? Combining two or more steps can significantly streamline your process environment.  

3. Cleaning: It’s said that plant maintenance personnel can smell trouble, so help yourself out: choose simplicity of design. Whether you clean after every run or once a year, at some point you’ll deal with downtime. Mixer cleaning can be a dry process (air sweep, brushing, vacuum, ultrasonics) or wet (water, solvents, disinfectants, steam, ice), with options to automate by incorporating CIP skids with spray balls or lances inside the vessel. Fairly straightforward, but if it’s easy to clean, it’s more likely to get done. Cracks, crevices, and fasteners inside sanitary equipment are clearly bad, but don’t forget to pay attention to the small stuff. Case-in-point, tape gasketing. While it has many uses in industrial equipment, effectively sealing a mixer cover to eliminate dusting isn’t one of them. Not to mention tape residue isn’t much fun to remove. Instead look for easily replaceable and cleanable O-rings. No, not a channel gasket that can harbor materials; O-rings. And seals, yes there are some wonderful mechanical options available, but if your application doesn’t require them, start by asking this question the next time: Could my grandmother disassemble, clean, and reassemble the seal housing? If so, you’ve found the intersection of simple and easy.

Focus on these three key factors and dream away at your next “beaut” of a mixer, perfectly calibrated and providing optimal performance for years to come.

Ryan Murphy is senior vice president of sales and marketing at Marion Process Solutions. For more information, call 800-397-6371 or visit www.marionsolutions.com. 

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