Crushing and screening of solids are essential to many solids-handling processes, but engineers must be mindful about how those actions will affect flow behavior of the bulk material
When it comes to processing bulk solids, (particle) size matters. Almost all industries rely on some sort of particle-size control within their processes, whether it is comminution (reduction of particle size by crushing, grinding or milling) or screening (separating materials based on particle size). For example, without crushing mined material, it would be impossible to extract valuable minerals and ores from the surrounding rocks. And effective screening is commonly used in the food industry to separate foreign materials and contaminants during food processing. However, crushing and screening can have unintended consequences, including causing process upsets in downstream equipment and creating potential safety issues. The good news is there are scientific approaches to evaluate these risks, and there are approaches for preventing poor process flow as a result of upstream particle-size control. This article touches on the background of comminution and screening, how both can impact processes, and how to evaluate flowability risks.
Size reduction and screening
Broadly speaking, comminution is defined as the act of reducing the average particle size of material by mechanical means. It serves as a critical process step in many industries, including mining, pharmaceutical and food industries. In general, comminution can be broken down into five methods: blasting, crushing, shredding, grinding and milling. Blasting is commonly done to reduce the size of large boulders, and of all the comminution methods, is typically used only for the largest particles. Blasting is also different from the other four methods, in that it typically occurs in the open, rather than within processing equipment. Shredding typically involves taking large, stringy materials (such as biomass, recycle waste and others) and shears the materials into smaller shreds. Grinding involves the chipping or “grinding” of a single material to create smaller particles.
Crushing can be done via three mechanisms: compression, shear and impact [1, 2]. Compression is typically used for hard, brittle particles and typically does not result in significant generation of fines. Shear crushing is similar to shredding, except that it is designed to handle sticky, cohesive bulk materials and powders. This method can lead to anisotropic shapes. Size reduction by impact is effective for hard, tough particles, and can lead to size reduction of 100 times smaller. However, impact crushing could result in shattering of the material. Milling typically handles size reduction via a fourth mechanism: attrition. Attrition mills are commonly used as a tertiary size-reduction step, and typically take longer than the previous three mechanisms described. Examples of each of these mechanisms are illustrated in Figure 1.
FIGURE 1: Several different particle-size reduction technologies are available for bulk solid materials
Screening is another key processing step found in almost all industries. Screening is a mechanical process used in material handling to separate or classify particles into different sizes. Proper screening is critical to meet material specifications, remove foreign material, and in certain applications, can even improve flowability of the material. Similar to crushing, there are several types of screening methods, and each has its own set of pros and cons. For example, vibrating screeners (Figure 2) are great for separating oversized and undersized materials (sometimes referred to as “scalping”) and generally have a higher capacity than other screeners. A single vibratory screener can have multiple decks to allow for additional material separation. Gyratory screeners (Figure 3) are like vibrating screeners, except they utilize a rotational motion to separate materials, and can be utilized for material classification. Centrifugal screening is a more aggressive method of screening that utilizes rotating paddles to allow for a high efficiency of screening in a small footprint. Tumbling screening is a gentler form of screening that combines vertical and gyratory motion in a large footprint and is common in the biomass industries (Figure 4).
FIGURE 2. Vibratory screeners, such as the one shown here, are effective at separating over- and under-sized material from a solid sample
FIGURE 3. A gyratory screener, like the one shown in the schematic diagram, uses a rotational motion to separate solid materials of differing particle sizes
FIGURE 4. This photo shows a tumbler screener, which is common in the biomass industries
Unintended effects of sizing
Comminution and screening are two crucial stages in the processing of materials across various industries. However, improper execution can have significant adverse effects on the plant operations. These effects may include reduced throughput, increased downtime, compromised product quality, and safety concerns. To assess the impact of these effects, we can categorize them into three distinct areas: flowability, process and safety.
Flowability effects. In the design and selection of comminution and screening equipment, solids flowability considerations often take a back seat to other parameters, such as reduction efficiency, throughput, screening efficiency, capital and operating costs, as well as the equipment footprint. However, this can be a critical mistake, because comminution and screening can have drastic impacts on the flowability of the bulk solids and powders, due to increased fines generation, separation of material, changes in particle shape, and more. These changes can lead to low-flow or no-flow conditions and material quality issues. Often, these issues are found in equipment immediately downstream of the crushing and screening operations — commonly in bins, hoppers and transfer chutes. Some of the common solids-flow problems include the following:
Cohesive bridging. This is a “no-flow” condition where material forms a stable arch over bin outlets, due to the material adhering onto itself. A similar sensation, plugging, can result in complete flow stoppages in chutes (Figure 5a).
FIGURE 5. Cohesive bridging (a) and ratholing (b) are two common no-flow conditions that can occur downstream of screening equipment
Mechanical interlocking. This is a “no-flow” condition where material forms a stable arch over bin outlets, due to the ability for coarse, anisotropic materials to interlock and form strong physical bridges.
Ratholing.This is a “no-flow” condition where a stable, open channel forms above the outlet and stagnant material outside of the channel does not empty (Figure 5b).
Buildup. This is a condition where material is allowed to build up within bins and chutes (Figure 6).
FIGURE 6. Fine material can be a problem if it builds up on the inside of bins and chutes
Limited discharge rate. This is a “not-enough-flow” condition when interactions between fine material and air restrict the solids discharge rate.
Flooding. This is a condition where interactions between fine material and air can cause the solid material to behave like a liquid, resulting in the material overwhelming downstream equipment (Figure 7).
FIGURE 7. Solids flooding occur when fine material interacts with air in such a way that the solids behave as a liquid
Segregation. This is a condition in which the material can segregate itself based on a number of different mechanisms. In comminution and screening applications, this can lead to reduced efficiency (Figure 8).
FIGURE 8. Segregation of fine and coarse particles can occur in gyratory screener
Caking. This is a condition in which stagnant material may agglomerate into lumps, potentially causing flow stoppages. The risk of this is increased when handling hygroscopic materials (Figure 9).
FIGURE 9. The material shown here is fine wheat that is exhibiting caking behavior
Dust generation. This is a mechanism by which additional release of fine particles can lead to overloading of dust-collection equipment.
The effects of comminution and screening are often determined by the quantity and distribution of the particles’ size following the sizing step. If a crushing operation results in a wider particle-size distribution, segregation of particles can occur, which can influence product quality and flowability. On the flip side, if a crushing operation leads to a narrow particle-size distribution, mechanical interlocking of similar sized particles can occur, which can result in flow obstructions, as well as screen blinding.
An example from the cement industry can help illustrate the concepts. Limestone is a critical ingredient in cement manufacturing and typically makes up approximately 80% of cement chemical composition. Blasting is conducted in the limestone quarry to extract limestone. Following blasting, large limestone rock is fed into a primary crushing operation, often a gyratory crusher. Following primary crushing, limestone is often transported via belt conveyor to a primary stockpile. Because the limestone has undergone significant size reduction, a large portion of the limestone is now fine. Fine limestone can be more cohesive than the original limestone rock, and thus, more prone to bridging, which can limit flowrate out of the stockpile. Ratholing can also occur within these stockpiles, and this can effectively reduce the live capacity of the stockpile significantly (often by up to 90%).
Bridging and ratholing are not the only negative side effects that can occur in this example. Segregation of fine and coarse limestone can occur, which will not only intensify cohesive bridging and ratholing, but can lead to separate “slugs” of fine and coarse limestone downstream. If a slug of fine material is discharged from the stockpile, dusting can occur, leading to material spillage, overloading of dust collection equipment, and inefficient use of crushing equipment, which can reduce equipment life. Likewise, if a slug of coarse limestone is sent downstream, it can mechanically interlock at hopper outlets, creating “no-flow” conditions, reducing crushing efficiency and blinding screening equipment.
Process effects. Negative flowability side effects are not the only concern of improper particle-size reduction. Improper crushing can have a direct impact on product quality, and can lead to either reprocessing or increased scrap product. Crushing can also reduce material integrity, leading to “weaker” material, resulting in less stable particles. Processes that include chemical reactions can be negatively impacted, because particle size can have a direct impact on reaction time and efficiency. The same can be said for drying processes, in which unsuccessful crushing can lead to either over- or under- dried material. Inefficient screening has a direct impact on product quality and could allow foreign material to enter the material stream.
Safety effects. As anyone in involved in manufacturing industries will tell you, safety is the number priority. Crushing and screening can have massive safety impacts. Obviously, machine entanglement is a major concern in the design of crushing and screening applications. But the potential safety hazards do not stop there. For materials that are explosive, dust explosivity risks can be drastically increased if crushing leads to increased dust generation. According to a study conducted by the U.S. Chemical Safety Board (CSB; Washington, D.C.; www.csb.gov), dust explosions pose a significant safety hazard, resulting in 718 injuries and 119 fatalities in the U.S. between 1980 and 2005 [3]. Additionally, increased dust generation can increase human exposure to chemical and toxic respiratory hazards.
Evaluating risks
Fortunately, flowability, process and safety risks can and should be identified before selecting comminution and screening equipment. Assessing these risks can be broken down into three steps:
1. Evaluate the material’s physical, chemical and flow properties
2. Conduct a flowability and size-reduction review
3. Perform preliminary safety assessments
Material characterization is paramount not only in selecting size-reduction and screening equipment, but also in the design of all solids handling equipment. Just as it is imperative that a car mechanic run diagnostics tests to understand how a car’s engine and transmission work together before beginning repair work, it is critical to fully evaluate the material being crushed or screened before selecting comminution and screening equipment. Unlike liquids and gases, you won’t find critical solids-flow property data in textbooks or manuals. Rather, flow property testing must be performed to establish design criteria. Common flow properties evaluated include the following:
• Cohesive strength – Used to determine hopper outlet sizing to prevent cohesive arches and ratholes from forming, as well as plugging, caking potential and buildup
• Wall friction – Used to evaluate solids flow along the inside surfaces of the hoppers, transfer chutes, and screens. Commonly used to determine critical hopper angles to avoid funnel flow pattern
• Compressibility – Used to establish the relationship between consolidating pressure and bulk density
Flow properties are not the only characteristics needed before selecting comminution, screening and other solids-handling equipment. As discussed earlier, most powders are explosive, and if conditions align, could create dust fires, deflagrations and explosions. Combustible-dust testing is critical in evaluating whether a material can form a combustible dust at the necessary concentration, and if so, how severe the explosion could be. Further, toxicity hazards should be established in this phase.
Once the material is properly characterized, comminution and screening equipment should be evaluated. Going back once again to the earlier cement-industry example, consider a very wet, cohesive limestone. Recall from that example that gyratory crushers are often used in primary crushing. This is highly effective at reducing large limestone rock, and could lead to 6:1 size reduction within the rock. However, if the limestone has a significant clay concentration, plugging of the gyratory crusher can occur, severely limiting throughput. Perhaps in this application, it makes more sense to utilize a shear crusher, which is better suited at handling sticky, cohesive materials. Likewise, the transfer chute following the crusher must be designed to prevent buildup, plugging and dusting of fine limestone particles. The same concept applies to all equipment downstream, including screeners, silos, dust collectors and feeders. Conducting a proper “flowability” review of the entire system before it goes online can reduce downtime, improve product quality, and result in significant operating cost savings.
Once equipment is selected and process flow diagrams begin to take shape, the most important step takes place. Designing safe processes is paramount, and final safety assessments should be performed before commissioning any comminution or screening equipment. In fact, the Occupational Safety and Health Administration (OSHA; Washington, D.C.; www.osha.gov) recommends beginning with a process hazards analysis (PHA) to identify, evaluate and develop control schemes for highly hazardous materials [4]. Likewise, a dust hazards analysis (DHA) should be performed to evaluate potential fire, deflagration and explosion hazards, before the equipment goes online. In fact, the National Fire Protection Association (NFPA) requires DHAs for many industries, including chemicals, food processing and woodworking [5]. PHAs and DHAs should be performed by qualified professionals who have knowledge in industrial safety and specific hazards associated with the materials being handled.
Concluding remarks
In most industries that handle solids, comminution and screening operations are crucial. There are various methods and technologies available for comminution and choosing the appropriate one can significantly affect the efficiency of energy usage, product quality, and the equipment lifespan. Similarly, screening methods can also have many of the same significant impacts. Both operations may lead to unintended consequences that can increase the risk of flow disruptions, downtime and safety hazards. However, by assessing the physical, chemical and flow properties of the materials, conducting a thorough flow processing review, and performing PHAs and DHAs when necessary, it is possible to mitigate these risks before implementing comminution and screening processes and before purchasing equipment. Following these steps will ensure that new operations will function correctly and safely right from the start.
Edited by Scott Jenkins
Editor’s note: all photos and diagrams in this article appear courtesy of Jenike & Johanson
References
1. Maynard, Eric. Fundamentals of Crusher Selection. World Cement, March 2010.
2. Maynard, E. (Presenter), and Cronin, K. (Moderator). Break it Down! How to Effectively Choose Size Reduction Equipment [Webinar]. Powder & Bulk Solids by Informa Markets, February 15, 2023.
3. Combustible dust safety. Chemical Safety Board (CSB). www.csb.gov/recommendations/mostwanted/combustibledust/
4. Occupational Safety and Health Administration. Process safety management: Hazards. Retrieved May 3, 2023, from www.osha.gov/process-safety-management/hazards
5. NFPA 652, “Standard for the Fundamentals of Combustible Dust”, 2016 ed.
Authors
Eric Maynard is a vice president for Jenike & Johanson (400 Business Park Drive, Tyngsboro, MA 01879; Email: [email protected]; Phone: Website: www.jenike.com), a world-renowned engineering consulting firm specializing in the storage, flow and processing of powder and bulk solids. During his 27 years at Jenike & Johanson, he has designed handling systems for bulk solids including iron ore, cement, coal, limestone, plastic powder, fertilizers, food products, and pharmaceuticals. He has gained valuable hands-on experience from working on over 750 solids handling and pneumatic conveying projects, and specializes in the cement and mining industries helping clients to reliably handle challenging bulk solids that are sticky, abrasive, and prone to segregate. Maynard has specialized knowledge in the areas of dust explosions, static electricity generation, and crushing technologies. Maynard’s working knowledge of the National Fire Protection Association (NFPA) standards for safe handling of combustible dusts helps him provide valuable advice to his clients to ensure safe handling of materials prone to explosions or fires. He sits on NFPA committees for standards 660, 652, 654, 655, and 91. His expertise also includes designing new pneumatic conveying systems and troubleshooting systems that are poorly operating and experiencing costly problems with abrasive wear, line blockages, throughput limitations, and particle attrition. Maynard has authored over 50 technical articles and is the company’s principal instructor for training on the storage, flow and pneumatic conveying of bulk solids presented through the American Society of Mechanical Engineers (ASME) and the American Institute of Chemical Engineers (AIChE). Additional subject areas of expertise include stockpile and feeder design, mixing/blending, particle segregation, sampling, and caking/agglomeration of bulk materials. Maynard frequently presents customized courses at individual companies and numerous conferences. Maynard received a bachelor of science degree in mechanical engineering from Villanova University and a master of science degree in mechanical engineering from Worcester Polytechnic Institute.
McKinnon Ray is a project engineer with Jenike & Johanson (same address; Email: [email protected]) specializing in the design of storage bins, stockpiles, transfer chutes, and conveying equipment for tricky powders and bulk solids. During his four years with the firm, has has worked on projects across several industries including cement, mining, food & beverage, agriculture, chemicals, and more. In addition to design work, he has experience with flow property and pneumatic conveying characteristics testing. Before Jenike & Johanson, Ray worked at Anheuser-Busch in the operations and logistics departments. McKinnon has a chemical engineering degree from Mississippi State University.