Fibrous, needle-like, wiry strands, dendretic and other particulates that interlock, entangle or knit together and, to a certain extent flat flaky particles, fall into an entirely separate category of bulk materials to powders for flow evaluation, as do spongy, ‘wooly’, elastic, soft compressible products and readily deformed plastic materials. A third group of materials that are also outside conventional shear testing methods are those that sinter, fuse, bind, bond, merge or ‘cake’ at their points of co-ordination. In the first instance shear is strongly inhibited by the significant overlap, trapping and intermingling of the tendrils or extremities of the particles. Particle in the second class of bulk materials develop broad contact faces that strongly resist bulk shear because it is difficult to create a flat shear plane through the bed without deforming the whole particulate structure. ‘Caked’, sintered or bonded assemblies of particles in the third group have a bulk strength that demands failure of the bridging connections, which is usually of a totally different order of strength than the original shear strength of the particulate bed before this mechanism matured.

Bulk assemblies of all the above forms of particulate solids cannot be tested in conventional shear cell equipment and are not subject to the flow theory and design techniques of Jenike. They each require individual consideration. A recent four-year program investigating properties of bulk materials relating to quality in manufacturing has developed a ‘toolkit’ for measuring the strength of products that ‘cake’. One instrument used is the Ajax Tensile Tester, which has recently been redesigned to deliver more sensitive results and deal with products having much greater strength than normal cohesive compacts.

A distinction must be drawn between materials that elastically deform, but do not develop surface adhesion strength, and those that essentially merge on contact to form a continuous medium. For example, cork granules, rubber crumb and similar elastic products have high contact friction, develop high shear strength and resist flow by expanding, rather than separating, when compacting stresses are reduced. Such products are capable of sustaining arches of large dimensions if allowed to suffer normal storage stresses. The design of storage containers that are required to discharge these type of products by gravity, or even by feeders, requires special attention directed to minimise the overpressures that are created in the stored bulk material. A similar criterion applies to bulk materials that ‘fuse’ under contact load, such as soft or warm plastic granules and some detergent powders. Tests in these cases are focused on the crucial stress and ambient conditions under which the adverse behaviour circumstances develop, and then seek to avoid these conditions by the use of inserts, hopper design and management control and/ or treatment of the particles or control of ambient conditions.

The questioner has delved into a specialised area of solids handling; that of dealing with particularly awkward bulk materials. There is no universal answer or a central fount of knowledge, but certain specialist companies and organisations have explored these areas and developed techniques, methodologies and equipment to address these difficult bulk storage and handling problems. Ask the right questions of your suppliers to source requirements for such duties and, most importantly, separate equipment fabricators and makers of standard products from specialist equipment designers. ■

dear Mr. shock

We are producers of proportioning and storage facilities for fibrous powders as well as producers of shear testers for measuring the flowability of fibrous powders.

Please consult our website WWW.IPT-ONLINE.COM

Please send us your full address and we will send you some more information.

Best regards

Dr. Ivan Peschl ■