Bulk solids processing is perhaps the most fundamental aspect of modern industry. Each and every day, bulk solids are handled and processed by the ton to convert mined ores into refined metals, produce market-ready products from raw materials, and so much more – all to keep industries and economies turning.
Many aspects contribute to the design of a successful bulk solids operation, but perhaps none more important than the characteristics of the material itself; whether agglomerating, drying, heating, or simply handling a material in the form of a bulk solid, several material characteristics will influence system design. These characteristics are summarized here.
Material Characteristics to Consider During Bulk Solids Process Design
Several material characteristics can influence the design of a bulk solids processing operation. Further, each characteristic will have a unique influence in the application to which it is employed. The importance of each will also vary based on the way in which it will be processed and handled. In general, however, the characteristics listed below always influence the system to some extent and will need to be considered throughout the design process.
Particle Size
Particle size is one of the most basic and yet most important material characteristics when working with bulk solids, contributing to several factors that have ramifications on the size of equipment, maximum allowable capacity, pretreatment requirements, flowability, and more.
Particle size distribution, or PSD, is a common measurement used to indicate material sizing, referring to the distribution of material at each size within the given range of potential sizes.
If a material is to be granulated, particle size distribution typically must fall well below the desired end product size, so that the layering that occurs during the wet granulation process will not exceed the size of the desired end product. As such, in terms of agglomeration, a larger particle size often necessitates crushing, grinding, or other pretreatment methods in order to prepare the material for agglomeration.
While agglomeration requires material to fall within a specific size range, within that size range, it is often desirable to have a good distribution of material throughout the various sizes (PSD), as this allows smaller particles to fill in the spaces between larger ones, creating a more solid agglomerate.
Particle size also affects thermal processing techniques such as drying, cooling, and high-temperature treatments such as calcination.
In addition to potentially requiring pretreatment, feedstock particle size often has an influence on the retention time required to meet the specific objective. In drying, for example, larger particles typically require a longer drying time to fully reduce moisture. If the particle size distribution is too great, meaning that the range between the smallest and largest particles is too high, it is likely that small particles will be overdried, while larger particles will be underdried.
In the case of handling, particle size may influence how flowable a material is, the maximum allowable capacity of the equipment, and even the conveying speed. Operations working with a fine particle size may find they require additional measures to control dust, such as dust pick-off points at the loading zone.
Bulk Density
A material’s bulk density, or the weight of a material at a specified volume (typically pounds per cubic foot), contributes to the level of robustness required for equipment, particularly when it comes to the amount of horsepower a machine will need. Materials with a high bulk density will require more power to move than those with a comparably lower bulk density. The higher power requirement translates to more robust drive components and in some cases, other physical aspects of the equipment.
In a granulation line, the desired bulk density of the end product influences many aspects of the granulation process itself. When more dense agglomerates are desired, a pin mixer may be included in the process to promote early densification of granules. Binder concentration, spray location, frequency of sprays, and feed rate may also be manipulated to achieve the desired density.
Feedstock Uniformity
Feedstock uniformity is a critical factor in the design of any bulk solids handling or processing operation, no matter what the goal is.
Production lines and handling systems of all types are designed around a specific set of criteria assumed to be representative of the material. This means that when material characteristics fall outside of the given range, the equipment is no longer designed to handle it properly. This can cause any number of different issues, with many resulting in a process upset. Depending on the severity of the deviation, the process may not even be capable of running under the new conditions.
Abrasiveness
Abrasion occurs when material degrades a surface by mechanical action. In any process setting, the effects of an abrasive material are the same: equipment degradation. Care must be taken during the design process to choose abrasion-resistant materials for wear areas, or areas in frequent contact with the material. In selecting bucket elevator buckets, for example, nylon buckets may provide more protection. Reinforcement of high-wear areas may also be beneficial.
Or, in the case of a pugmill mixer, special abrasion-resistant paddle facings may be necessary.
Corrosiveness
Corrosion occurs when a material chemically acts upon a surface. Engineers must consider the material’s corrosivity when selecting materials of construction. Unlike abrasion, corrosion has the potential to affect any surface it comes into contact with – not just areas that are more susceptible to wear.
The potential for corrosion may also be considered in the fabrication techniques of the equipment, as crevices or areas where material is prone to cake or build up may need to be eliminated. For example, standard welding techniques may be suitable for most applications, but in the case of a corrosive material, could provide a point where material can catch and build up, accelerating the potential for corrosion.
Moisture Content
Moisture content has widespread effects in all types of equipment, affecting how material flows, discharges, and generally behaves. Materials will often act differently depending on their moisture content.
In a granulation process, moisture content must be carefully managed for proper granule formation, often through the removal of moisture prior to the granulation line, and the addition of moisture during the granulation process.
In a drying operation, reaching the desired moisture content is the primary focus. The level of moisture in the feedstock, therefore, determines how much moisture the bulk solids dryer will need to remove to reach the target percentage. This in turn factors into the sizing of the dryer, how much energy will be required, the necessary retention time, and more. The way a material carries its moisture content (internally or on the surface) will also influence dryer design.
While moisture content is not necessarily the focus of high-temperature thermal processing applications, it still plays a considerable role. In many cases, the material will need to fall within a specific moisture range prior to processing in the rotary kiln or other thermal device, requiring a pre-drying step. In other cases, the rotary kiln’s objective may be to remove chemically bound or crystalline water in the material (which could also require a pre-drying step).
Moisture content also has a significant influence on handling equipment design, particularly when it causes flowability challenges, making material difficult to feed and discharge from belt conveyors and bucket elevators.
Temperature
Many bulk solids are processed at room temperature or thereabouts, not requiring any particular consideration. In other settings, however, the material may be coming in hot, or may be only slightly warm, but hitting cold equipment where condensation could be an issue.
In high-temperature thermal processing applications, the material’s temperature is manipulated in order to cause or initiate a chemical or physical reaction in the material. The temperature must be tightly controlled to avoid undesirable responses, or avoid over processing the material. Similarly with drying, temperature aids in the removal of moisture, but must be carefully controlled to avoid overdrying the material.
Not surprisingly, temperature is also important in cooling applications. Rotary coolers, for example, are often positioned after rotary kilns in order to cool the material to a point that stops any ongoing reaction.
Temperature plays an important role in the design of bulk solids handling equipment as well. Belt conveyors may require a heat-resistant cover compound, or a chain elevator may be selected over a belt elevator. Temperature of the material also becomes a concern when the material will be handled in an outdoor environment where it is subject to extreme heat and cold. Precautions such as weather covers or gallery enclosures may be necessary, as well as minimizing the extent of temperature extremes. Similarly, special belting may be required for belt conveyors handling frozen materials in order to avoid issues with condensation.
Chemical Composition
The chemical composition of a material also has an impact on system design, particularly in the case of thermal treatment.
In drying or thermally processing a material, the chemical composition can help to predict combustion; if a material will combust at a certain temperature within the processing range, the rotary kiln will need to be properly sized to accommodate the excess heat. At the same time, analyzing the chemical composition will help to determine if a certain atmosphere will be necessary in the kiln. It also plays an important role in the design of the off-gas system.
The chemical composition is also important to know in designing a granulation process, as the presence of some compounds may cause the material to respond to agglomeration in a different way. In the case of calcium oxide, for example, a reaction occurs when CaO is combined with water. This reaction causes the material to become cementitious, which can actually aid in the agglomeration process, serving as a binding agent. Likewise, some compounds may hinder agglomeration.
In designing a handling system, the chemical composition is typically only a concern in terms of choosing materials of construction that are compatible with the material being handled.
Angle of Repose
A material’s angle of repose refers to how high the material can be piled before it begins to slide against itself off of the pile. This is frequently a consideration for engineers designing a material handling system, affecting how much material can be piled on the conveyor or in buckets; ultimately, it has implications on the maximum allowable capacity of handling equipment.
In general, angle of repose is less of a concern when it comes to agglomeration (though agglomeration can have a big impact on an end product’s angle of repose). It is, however, a concern when it comes to flighting in a dryer, cooler, or kiln in post-agglomeration treatment or otherwise.
Flights (material lifters) are used in rotary dryers and coolers in order to improve heat transfer between the material and the heating or cooling medium by picking up material and cascading it through the air stream. The material’s angle of repose will determine the amount of material flights can pick up and may require different flight designs.
Attrition
Attrition is the breakdown of material into fines. Attrition is highly undesirable, as it not only means material degradation, but it also introduces dust issues, which can be severe. The potential for attrition is therefore often a focus in the design of a bulk solids processing or handling operation.
In the case of granulation, attrition is typically not a concern with the raw material, but rather, the finished granule. Minimal attrition is often an important factor in the quality of granular products. Various process parameters are used during the granulation process to minimize the potential for attrition in the end product.
Operations handling friable granules or other bulk solids where attrition is a concern tend to employ more gentle handling techniques. This might translate to different flight design in the rotary dryer, fewer transfer points between conveyors, selection of a continuous bucket elevator over a centrifugal one, and more.
Special Qualities
The aforementioned properties cover the basic material-specific factors that influence the design of a bulk solids handling or processing operation, but of course, there are always a few outliers.
This could be the potential for a material to combust if aerated, the toxicity of a material, or even a material that is particularly fragile.
In dealing with such materials, engineers must design safeguards into the system according to the material needs, minimizing drop points, preventing flame exposure, and more.
De-Risking the Design Process
While testing is less of a concern for bulk solids handling operations, it is almost always important in developing a successful bulk solids processing operation.
All materials, and even the same material from different sources, present their own unique challenges to processors, requiring thorough testing to evaluate the specific material’s characteristics and behavior in response to the intended process.
FEECO routinely recommends testing in our Innovation Center as part of the process design journey. Here, customers can test drying, high-temperature thermal processing, and agglomeration/granulation processes in both batch- and pilot-scale equipment. Testing in a continuous process loop is also possible.
Through testing, clients can determine whether or not their intended process is feasible, and if so, the process parameters necessary to reach the desired results. Testing often reveals challenges specific to the material, as well as potential scale-up problems, ultimately derisking the scale-up process and paving a smoother road toward commercial-scale production. The data gathered during testing is then used in the equipment design process.
Conclusion
The design of any bulk solids operation is dependent upon a wide range of material qualities. While a number of different material properties can influence the design process, basic parameters such as particle size, bulk density, moisture content, and more, are almost always influential.
In business since 1951, FEECO serves industries ranging from fertilizers and chemicals, to mining and power generation. Our expertise in designing custom granulation, thermal processing, and material handling systems – from concept to completion – is relied upon by many of the world’s foremost companies. For more information, contact us today!