In the process industries, the vast majority of raw materials pass through a particulate stage. That is, the material being used or processed is in a powdered or granular form. Examples might be alumina, cement, sugar, flour, aggregates, coal, and plastics, to name a few. To handle these materials at industrial scale, we need conveying and handling systems that are able to store, feed, and convey these materials from one stage of a process to another.
When considering the handling of bulk particulate materials, the analogy of handling liquids is often considered. However, unlike liquids bulk materials have a wide range of bulk and particulate properties that vary enormously. The obvious properties are particle size and size distribution, particle density and, perhaps, particle shape. However, there are many more properties that have an enormous effect on the “handleability” of the material. For example, bulk density, permeability, cohesive bulk strength, moisture content, electrostatic charging, and the list goes on. These properties will significantly influence the performance of mechanical handling equipment.
The history of mechanical conveying of materials goes back a long way. It is known that the Archimedean screw was used over 2,000 years ago to elevate water and has been used to convey free-flowing particulates. This of course was the forerunner to the modern day screw conveyor whose design has been modified and developed to handle a wide range of bulk solid materials.
Click here for information about the upcoming International Powder & Bulk Solids Conference/Exhibition
Today, there is a wide range of mechanical conveying systems available that include belt conveyors, screw conveyors, bucket elevators, chain and flight conveyors, and many others. The question perhaps is why are there so many different types of conveyors and, with each type, so many variations in design? The follow-up question is, of course, how do we select the most appropriate type and design for our particular application?
The answer to the first question is that there are many different types of applications. For example, for a short conveying run horizontally in a straight line, a screw conveyor can be an excellent conveyor type to use. For longer distances, a belt conveyor might be considered. However, conventional screw conveyors and belt conveyors have limited ability to convey up an incline. Hence, if a significant increase in elevation is required a bucket elevator may be an option.
The many different applications and scale of operation has also led to many variations being developed from standard conveyors. For example, belt conveyors can vary in scale from conveying rates of several kg/hr to 10,000 tn/hr or more in extreme examples. Although the principles involved in each case are the same, the detailed design is significantly different. In addition, to overcome some of the limitations that exist with conventional belt conveyors, a range of novel designs have been developed such as the pipe conveyor, sandwich belts, and others that address the issues of inclined conveying or indeed vertically up conveying.
The answer to the question of how to select a conveyor depends significantly on the material properties of the material to be conveyed. The first step in any selection process is to understand the properties of the material to be conveyed by undertaking flow property tests. These tests essentially determine the bulk strength the material can develop under a variety of consolidation loads and the essential frictional properties of the material. These properties are essential to know when selecting both the type of conveyor and the particular design features that will be appropriate to ensure trouble-free operation.
Finally, many industries have extensive experience in using certain types of mechanical conveyors for their materials. Whist this experience sometimes limits the range of systems considered for particular applications (bulk materials handling tends to be a conservative business), there is significant innovation that takes place when modifying the design detail to match the needs of specific applications.
Professor Mark Jones, PhD, holds the chair in bulk solids handling at the University of Newcastle, Australia. He is also the director of the Centre for Bulk Solids and Particulate Technologies and director of TUNRA Bulk Solids Handling Research Associates. For more information, visit www.bulksolids.com.au.