Mixing and blending – a vital process step in a myriad of industrial applications. It is amazing to reflect upon how many bulk materials undergo mixing/blending operations to achieve homogeneity at a required quality level, whether the material is raw, and intermediate, or final product.
I often reflect upon the analogy of mixer choice to a tool set, whereby you have a toolbox with a variety of hand tools like a hammer, screw driver (several types/sizes), pliers, wrench, and wire cutters. Just as with requiring wire cutters to sever a wire vs. using a hammer to no avail, imagine selecting a tumble blender to achieve blending with a highly cohesive mixture, such as chocolate chips and cookie dough – not going to work!
How about the panacea of a “universal blender”? Even a Swiss Army tool has multiple implements to help you with your task at hand. There really is no universal blender, as even the advertised unit cannot deliver on its promise with single set up tooling. Imagine this, if you are a hammer, a lot of things start to look like a nail!
Let’s reflect upon some common industrial mixing/blending operations using a variety of technologies to reliably achieve blend uniformity in an efficient manner.
The food industry often relies upon ribbon mixers to blend together food powders, such as sugars, flours, and chemical additives like phosphates or stearates. These blender semi-fluidize the powder to rapidly incorporate the ingredients in a batch operation allowing manufacturing flexibility, speed, and accuracy.
The battery industry making cathode mixes usually use plow blenders, which have the ability to mix cohesive powders with liquids that are necessary to generate the chemical blend prior to compaction. The plow mixer easily works the mixture with a rapid blend time and ability to involve chopping of agglomerates through use of tulip high-speed effectors.
Imagine the challenges in the pharmaceutical industry trying to select an appropriate blender depending upon the process step you are designing. For instance, if you are handling a direct compression blend, you may use a tumble blender in twin-shell or double cone configuration. While if you are doing a wet granulation process, you are likely using a high-shear mixer that has choppers running at over 4,000 rpm to break particle clusters. Then, there is also tablet coating, where liquids are added to dry tablets to provide gentle tumble coating of the final dosage products. These examples are illustrative that the “hammer” is not the answer to all blending processes.
Blending in the plastics industry is paramount for custom compounded plastics. Look at how far long plastics have come with even soda bottlers. When was the last time you dropped a 2-liter bottle and it shattered, or the plastic became translucent? Likely not since the 80s. Not only have the chemical additives to HDPE and PP have improved the properties, but also the continuous mixing process for feeding compounding extruders has become commonplace in any film or extrusion plant making milk jugs, shampoo bottles, or toothpaste containers. The impact modifiers, clarifying agents, UVA/UVB inhibitors, and fire retardants are all effectively incorporated to the virgin plastic via effective use of continuous ribbon blenders feeding high throughput forming equipment.
Some industries are even able to use technology called “hopper blenders”, whereby a silo filled with an unblended solid can be mixed with off-spec. material through two to four recirculating passes just employing gravity flow alone equipment. This approach allows high throughput, efficient blending, and ability to store as well as mix in off-spec material. This technology has been used for decades in the plastics industry.
So, how do you know you made the right choice in mixer or blender selection? The answer is through effective sampling, of course. Knowing what you put in to a blender is not enough. Proper sampling of the blend is vital to know you have reached the blend end-state, and sampling of the stream at the end of discharge is critical too. This latter step is often overlooked as many believe if the mixer/blender shows uniformity, then it is guaranteed that the material going in to the next step in the process will be just as good. This is a false hope in most cases. Simply speaking, your work is not done once blending is done. In fact, the really hard work to avoid the effects of segregation are just beginning!
Eric Maynard is vice president, Jenike & Johanson. For more information, call 978-649-3300 or visit www.jenike.com.