Practice Often Outpaces Theory When It Comes to Size Reduction
Unlike some engineering disciplines, the “sweet science of size reduction” is not governed by any general theory. Rather, it’s been mastered pragmatically over time, backed by knowledge of basic physical laws and a lot of trial and error, i.e., the scientific method.
Yet we’re still getting better at it all the time. Equipment is in service today suited to reduce every type of solid and semi-solid material. Data exists to be extrapolated for a multitude of applications and equipment specifications. Incremental innovation and design improvement is accomplished by engineers who have devoted their careers to understanding the needs of those accomplishing materials size reduction.
It’s basic general science that the mechanical properties of materials describe characteristics such as their strength and resistance to deformation. We have Hooke’s law and Young’s modulus. And there are laws of size reduction, such as those of Rettinger, Kick and Bond, each of which seems most applicable to certain material types.
Yet despite a world of scientific and engineering knowledge, the fact remains that productively accomplishing size reduction calls for experience as much as theory. For example, according to the theory of the solid state, only tension or shear, and never pure compression, can cause fracture. However, we know that forces can be applied as compression, tension, shear, impact and attrition. In practice, size reduction equipment applies more than one of these forces to the materials, although one may be predominant.
The Forces that Fracture
Tension is the cause of fracture in brittle materials, yet no practical size-reduction equipment applies a primarily tensile force. Brittle materials such as salt, limestone, glass or phenol formaldehyde, when subjected to compression in a double-roll or jaw crusher, apparently fracture in tension. But the results obtained in compression apparatus can differ from those of tensile tests.
Many ductile materials are tougher to reduce in applied compression than in tension. On the other hand, some materials that are brittle in tension, such as unmodified polystyrene, are tough applications for applied compression, with higher yield points and great elongation. Compression-type equipment is easily applied to brittle substances, but must be more carefully applied to ductile and soft materials to avoid flattening or compaction.
Attrition means to rub. But an attrition-type mill, such as double-disk burr mills, seems to work by shear. Particles are trapped in the voids of one attrition surface plate and sheared by the projections on the other surface. Many projections quickly reduce the particles trapped between the plates to the desired size. Various effects can be obtained in the finished product depending on the pattern of the attrition disk surface.
Shearing action is prominent in cutting and chipping machines such as rotary-knife cutters, saw-tooth crushers, guillotine cutters, toothed-roll crushers, wax chippers, wood chippers, dicers and slitting equipment. These machines have knives or blades that move, sometimes close to stationary bed knives, sometimes with a screen in close proximity to the knife path, to retain the material until proper size is reached. Cutting is regarded as shearing, even if the forces are compression applied at a line by an edged tool.
Impact force — e.g., in hammer mills — increases stress more than slower or static loading. The pin-disk mill and self-bombardment mill depend on impact forces to a great extent. In fact, impact is present to some extent in most size-reduction applications. It can be either useful or harmful.
As for practical considerations impacting overall productivity, it’s typically the case that the greater the material’s density the higher the production rate. Other factors influencing productivity include number of cuts or strikes per minute; effective open area of the system; and feeding efficiency. These in turn are functions of material properties such as strength, hardness, stickiness and ductility.
The fracture stress of solids varies with percentage and type of moisture in contact. Some materials that decrease in strength with increased moisture reverse the trend after a certain moisture percentage is reached, becoming stronger when wet. In general, however, increased moisture beyond a critical level lowers efficiency.
Soft, mush-like, clay-like and waxy materials can be reduced in size, including materials with high moisture content such as filter cake, by chopping the material without working it too much. For low average molecular weight materials, the mechanism of rupture seems to be based on viscous flow. In other materials, fracture in shear is found. Both natural and synthetic wax-like materials and paraffin are regularly handled in specialized equipment.
At end of day, given the “uneven” nature of the work performed, it’s not surprising that, based on theory alone, size-reduction operations can seem inefficient. What we do know is that engineers with both the scientific background and practical experience are very much capable of making size-reduction operations as productive as possible.