Issue link: https://www.e-digitaleditions.com/i/1380119
6 / June 2021 powderbulk.com INDUSTRY PERSPECTIVE PBE Editor's note: This is one of a series of commentaries by Powder and Bulk Engineering's Editorial Advisory Board members and other experts. The editorials express the individual writer's views of trends affecting powder and bulk industries today. Powder and Bulk Engineering welcomes your response. Y ou've probably heard the idiom "that's the breaks," used to describe an unfortunate occur- rence about which nothing can be done. When it comes to particle size reduction, by using sound engi- neering and selecting the right technology, you can ensure a positive outcome rather than having to settle for unfortunate "breaks." Consider the following four focusing steps for guidance. First, quantify and qualify the bulk and particle properties of the material to be size-reduced. A repu- table size reduction equipment supplier will inquire about the following characteristics: particle size dis- tribution (feed versus desired product); particle shape; hardness on the Mohs scale; toughness or fragility (often based on unconfined compressive strength); flowability; combustibility or explosivity; abrasive- ness; range of bulk density and moisture content; shear requirements (for rubber block, for instance); and heat sensitivity (for materials such as plastics or foods). Second, define the desired reduction ratio (the ratio between feed particle size and product particle size, typically at 80 percent passing), as particle size reduction equipment can vary widely in its reduc- tion capabilities. For example, an impact crusher using kinetic energy to break hard rocks can often achieve a reduction ratio of 30, while a compression unit, such as a jaw crusher, may only reach a ratio of 3. Some tumbling mills, such as those used for grind- ing cement or ores, can yield a reduction ratio of 100 or even 1,000. A fluidized-bed jet mill may be able to achieve a reduction ratio of 100, but its maximum feed size may be only 1 to 2 millimeters! Third, consider whether you need a closed or open size reduction circuit. In an open circuit, after the par- ticle size reduction is complete, the material passes directly into the next process step without recircu- lating. This increases overall process efficiency but can also increase costs for additional equipment. In a closed circuit, the material discharges to a screening operation, and oversized particles are recirculated for further size reduction. This increases sizing accuracy and allows for a more compact system layout. Three important factors to consider at this stage are the required reduction ratio, the material's screenability, and the necessary throughput. Fourth, don't forget about sampling and mainte- nance! Sampling correctly is critical, especially if you're using process control techniques based on particle size reduction. Scrutinize your sampling method, tools, subsample volume reduction, and analytical methods to ensure that you don't introduce error into your process based on bad data. Maintenance should be obvious but is often overlooked due to other plant priorities. For instance, when was the last time main- tenance checked the condition of your hammermill's hammers and screens to ensure the mill is doing the job it was designed to do? Many particle size reduction equipment suppliers will be happy to assess your material and recommend methods and technologies to efficiently, accurately, and safely meet your needs. If you choose not to have your material evaluated and follow the steps I've described, well, then those will be the breaks that really will matter! Eric Maynard, vice president, Jenike & Johanson Particle size reduction — the breaks that matter