Powders behaviour

Although this guide is supposed to be concerned with only essentially dry solids, it should be noted that increased moisture levels eventually lead to a qualitative change in powder behaviour and properties when subjected to vibrations or shear. 

It is not surprising that the answer to testing of cohesivity may lie in packing properties the voids between the particles usually occupy a larger volume than the particles themselves and, in many processes involving powders, these voids determine the powder behaviour. 

Dyshlovenko, S., Pateyron, B., Pawlowski, L., and Murano, D. (2004a) Numerical simulation of hydroxyapatite powder behaviour in plasma jet. Surf. Coat. Technol., 179, 110-117. 

The average grainsize was determinedby means ofthe Sherrer s formula. Moreover, the nanopowders were investigated by means of transmission electron microscopy (TEM) (JEM 2010, JEOL). For the TEM studies the powders were dispersed in distilled water and a drop of suspension was placed on a copper grid with a transparent polymer and dried. Elemental analysis was performed by inductively coupled plasma (ICP) spectrometer, (Spectroflame Modula, Spectro). The in vitro test were performed leaving the nanopowders for 30 days in three different solutions distilled water, SBF (simulated body fluid [17]) and SBF added of citric acid (0.08 M), at 60°C. This temperature was chosen to better evaluate the powder behaviour under the most extreme conditions. 

To aid the process plant engineer, information must be available in terms of bulk powder behaviour under specific conditions. Powders should therefore be treated as a biphasic assembly and handled with knowledge of the interactions between a fluid and the solid external and internal surfaces. In this way, powder behaviour can be visualised as behaving in two separate manners either in terms of fluid-like behaviour or compacted solid-like behaviour. Both of these conceptual viewpoints will be considered in this section, but the emphasis will be on the fluid-like (flow) behaviour of powders. 

The relationship between mass and volume is density and the determination of this basic property is essential in the subsequent understanding of bulk powder behaviour. 

The largest category of bulk powder behaviour, expressed in the terms of the three-dimensional parameters, is either a mass-time or a volume-time relationship. This category measures the rate of discharge from orifices, as opposed to the bulk powder properties of flowability and/or floodability of powders (Table 1.1). 

The properties and phenomena associated with an assembly of particles and powder behaviour in processing are due to the combination of many individual particle and powdCT properties, some of which are listed below ... 

Figure 1.2 Micro-scale particle and macro-scale powder parameters contribution to bulk powder behaviour.

Both Molerus (1982) and Rietema (1984) extended the Geldart classification to a dimensionless correlation for bulk powder behaviour (Figure 1.8). A more sophisticated interpretation which takes into account particle interaction as well as particle forces has been proposed by Molerus (1980). 

McGee, E. (August 2006) Powder behaviour. Solid Bulk Handling, 12-14. 

Takafiimi Mikami, H. Kamiya. and M. Horio, Numerical simulation of cohesive powder behaviour in a fluidized bed. Chemical Engineering Science 53(10), pp. 1927 - 1940, 1998. 

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

Simons, S.J.R. and Fairbrother, R.J., 2000. Direct observation of liquid binder-particle interactions the role of wetting behaviour in agglomerate growth. Powder Technology, 110, 44-58. 

The thickness of amorphous alloys is dependent upon production methods. Rapid quenching from the liquid state, which is the most widely used method, produces generally thin amorphous alloy sheets of 10-30 tm thickness. This has been called melt spinning or the rotating wheel method. Amorphous alloy powder and wire are also produced by modifications of the melt spinning method. The corrosion behaviour of amorphous alloys has been studied mostly using melt-spun specimens. 

By the term particulate composites we are referring to composites reinforced with particles having dimensions of the same order of magnitude. Particulate composites are produced from a polymeric matrix, into which a suitable metal powder has been dispersed, and exhibit highly improved mechanical properties, better electrical and thermal conductivity than either phase, lower thermal expansivity, and improved dimensional stability and behaviour at elevated temperatures. 

Michrafy A, Ringenbacher D, Tchoreloff P. Modelling the compaction behaviour of powders Application to pharmaceutical powders. Powder Technol. 2002 127 257-66. 

Wu C-Y, Ruddy OM, Bentham AC, Hancock BC, Best SM, Elliot JA. Modelling the mechanical behaviour of pharmaceutical powders during compaction. Powder Technol 2005 152 107-17. 

Another filtration approach concerns the addition of a filter aid after reaction, like powered cellulose, celite or powdered graphite [6a]. The solution is filtered and the catalytic behaviour of both components, solution and filter aid, is then tested and compared with the catalytic behaviour before the filter aid addition. This methodology was first described by Maitlis and co-workers for distinguishing a heterogeneous component in a starting... 

Lithium burns violently in contact with sodium carbonate. This behaviour explains why use of extinguishing powders containing carbonates (and especially hydrogencarbonates) are forbidden for putting out lithium fires (and all fires of highly reducing metals). 

Finely powdered cobalt can detonate spontaneously in air. Its behaviour depends on its surface texture. Raney cobalt is much more dangerous than Raney nickel, which is more commonly used (see nickel). 

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