SOME ASPECTS ()F HANI)LING POWDERS IN MECHANICAL EQUIPMENT 697 limit the size at which residue clinging to a corner will be stable and usually some idea can be gained of the internal radius which would stay free of material. Similarly, there is a critical tube diameter which will pass weight a solid plug of cohesive material en masse as the surface area increases in direct proportion to diameter. The effect of slip on walls or working faces is easily appreciated. In some cases relative motion is essential for the equipment to work, e.g. scraper conveyors and helical screws. However, even in such cases, the full casing contact areas may not be slipping and one may be faced with a conveyed bulk in one state of turbulence and a relatively dead layer against the walls. The flow pattern of matehal with or around the moving blade may be complex (28, 29). Where the movement is relatively near to a fixed wall surface, "leakage" may take place and the flow or no-flow through the gap has similarities to the form of flow through a slot outlet of an asymmetric hopper. Apart from the obvious danger of particles jamming in the clearance. a form of structural arching may occur unless the gap is many times wider than the particle diameter, the actual ratio depending mainly on the blade angle, particle shape and powder state. Structural arches of this kind are statistical probabilities extremely sensitive to low ratios of particle/ orifice sizes (27, 30). The movement of a layer of material in the clearance space will depend on the strength of the material shear plane, area of failure and pressure normal to this shearing surface on the one hand and the cohesion to the wall, wall pressure and contact area on the other. The point may be vital with relation to contamination, degradation of the residue or flow pattern from a hopper {,31), in which case minimum clearance and a low friction surface minimises the stability of a dead layer and assists slip. (b) Completely undisturbed beds of material are rare in practice. Apart from spurious minor vibrations, there are usually small amounts of yielding or plastic deformation due to self-pressure. Solids in motion are relatively roughly handled and compaction is stimulated by vertical accelerations whereas segregation is favoured by vibration of a bed in a mild state of dilation. Motion en masse as in belt conveyors and mass flow hoppers, usually minimises segration and changes in powder state but loading and unloading techniques are major conditioning processes as dealt with below under chutes and "streams".

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Narrow failure planes are caused in some bulk handling equipment such as screw hopper dischargers, powder blenders and some types of scraper conveyor. Power requirements are sensitive to loading normal to the plane and starting loads are frequently many times greater than forces operating at working speed. Within this narrow band of failure, attrition is a function of load, and segregation due to percolation is a feature of dilation. Bulk shear takes place in screw conveyors and elevators, blenders, scraper conveyors and in "flowing material" in similar classes of equipment. Segregational effects are usually consequential to differing sizes of particles in the mass, the separation of these in the bulk being mainly due to a percolating or preferential locating mechanism for the smaller particles. Dilate conditions favour the ease of particle movement and as in bulk shear some re-orientation is taking place, the hazard of segregation tends to in- crease. The depth of bed section is an obvious limit to the depth of per- colation so a continuous "live" shallow stream will not incur the residual effects of batch plant operation or sustained flow over a "dead" layer. FREE SURFACE ACTIVITY In unconfined conditions the boundaries of the solids are acted on by gravity only and the surface will display behaviour which depends on the dilation of the solid. "Angle of repose" is only meaningful in specified conditions for most materials, and has no value other than its defined quality in specific circumstances (32). In dynamic conditions such as rolling down a slope or chute, the tendency is to entrain air due to particle separ- ation in their agitated condition. This effect leading to "flooding" may be pronounced with fine, less dense powders in conditions of initial turbulence. Segregation due to a preferential "capture" mechanism of small par- ticles operating on the surface layers is particularly effective in the building up of a pile of material (33, 34). CHUTES AND CHANNELS Chutes may be classified as "slow" or "fast" depending on whether or not the flow channel is fully occupied by the bulk material (35). Fast chutes operate with clearance between the top surface of the powder stream and the upper boundary wall, and although suitable for high rates of transit, such circumstances favour subsequent "flooding" and segre- gation. In principal, chutes form part of the flow path of the stream and