Particle Size Reduction and Separation PDF

10 Particle size reduction and size separation...

10 Particle size reduction and size separation Michael E. Aulton CHAPTER CONTENTS During raw material manufacture, the solid produced is often of a larger size than that Introduction to size reduction....... 159 required pharmaceutically, and thus size Influence of material properties on reduction is a key process prior to incorporation size reduction............... 159 of the material into a finished product. Crack propagation and toughness...... 159 Solid particles possess a range of mechanical Surface hardness............... 159 properties and consequently a number of Energy requirements of the size different mechanisms of size reduction exist, reduction process.............. 160 dependent upon these properties for the material in question. In turn, this influences the Influence of size reduction on size design of the commercial equipment for efficient distribution................ 161 size reduction. Size reduction methods.......... 162 The correct choice of the most efficient Cutting methods............... 162 commercial size reduction machinery will Compression methods............ 162 depend upon knowledge of these properties Impact methods............... 163 and mechanisms. Attrition methods............... 163 The method of size reduction not only affects Combined impact and attrition methods... 164 the mean particle size but also the distribution of the sizes of the powdered material. Methods Selection of the particle size reduction of determining and representing mean size and method.................. 165 size distribution are discussed. Introduction to size separation...... 165 It is often necessary to separate out from a Objectives of size separation......... 165 widely distributed range of sizes the single size Size separation efficiency........... 166 or narrow range of sizes that is best suited for the application in hand. This required size will Size separation methods......... 167 depend on the subsequent use of the material. Size separation by sieving.......... 167 For example, a drug used in a dry powder Size separation by sedimentation...... 168 inhalation formulation will need to be of a very Size separation by elutriation......... 169 different size than that required in an oral solid Copyright © 2017. Elsevier. All rights reserved. Size separation by cyclone methods..... 170 dosage form. Selection of a size separation process.. 170 Many different commercial size separation methodologies and apparatus exist – their Bibliography................ 171 designs are dependent on the final required particle size and the size of the commercial KEY POINTS batch. These methods include sedimentation, sieving elutriation and cyclone methods. The particle size of solid drugs and excipients As with size reduction, the pharmaceutical has a significant effect on many of their scientist must understand the parameters for properties, including the rate of dissolution and the selection of the best method for the material powder flow characteristics. in question. 158 Aulton's Pharmaceutics E-Book : Aulton's Pharmaceutics E-Book, edited by Kevin M. G. Taylor, and Michael E. Aulton, Elsevier, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/trinitycollege/detail.action?docID=5253018. Created from trinitycollege on 2025-01-27 11:39:39.  Particle size reduction and size separation CHAPTER 10 Introduction to size reduction 30 30 The significance of particle size in drug delivery has been discussed in Chapter 9, and some of the reasons 2r for performing a size reduction operation have already 60 245 245 60 been noted. In addition, the function of size reduction (also called comminution) may be to aid efficient processing of solid particles by facilitating powder mixing or the production of suspensions. There are also some special functions of size reduction, such 30 30 as exposing cells in plant tissue prior to extraction L of the active principles or reducing the bulk volume of a material to improve transportation efficiency. Fig. 10.1 Stress concentrations at the edges of a disc-shaped crack; r is the radius of curvature of the crack tip and L is the crack length. Influence of material properties on size reduction approaching 40% of the speed of sound in the solid. This crack propagation is so rapid that excess energy from strain relaxation is dissipated through the mate- Crack propagation and toughness rial and concentrates at other discontinuities, where new cracks are propagated. Thus a cascade effect Size reduction, or comminution, is carried out by a occurs and almost instantaneous brittle fracture process of crack propagation, whereby localized occurs. stresses produce strains in the particles that are large Not all materials exhibit this type of brittle enough to cause bond rupture and thus propagate behaviour, and some can resist fracture at much larger the crack. In general, cracks are propagated through stresses. This occurs because these tougher materials regions of a material that possess the most flaws or can undergo plastic flow, which allows strain energy discontinuities. Crack propagation is related to the relaxation without crack propagation. When plastic strain energy in specific regions according to Griffith’s flow occurs, atoms or molecules slip over one another, theory. The stress in a material is concentrated at and this process of deformation requires energy. Brittle the tip of a crack, and the stress multiplier can be materials can also exhibit plastic flow, and Irwin and calculated from an equation developed by Inglis: Orowan suggested a modification of Griffiths’ crack theory to take this into account. This relationship L σ K = 1 + 2   has a fracture stress, σ, which varies inversely with  2r  the square root of the crack length, L: (10.1) Ep where σK is the multiplier of the mean stress in a σ= material around a crack, L is the length of the crack L and r is the radius of curvature of the tip of the (10.2) crack. For a simple geometric structure such as a where Ep is the energy required to form unit area of circular discontinuity, L = 2r and the stress multiplier Copyright © 2017. Elsevier. All rights reserved. double surface. σK will have a value of 3. It can therefore be seen that the ease of comminu- In the case of a thin disc-shaped crack, shown in tion depends on the brittleness or plasticity of the cross section in Fig. 10.1, the crack is considered to material because of their relationship with crack have occurred at a molecular level between atomic initiation and crack propagation. surfaces separated by a distance of 2 × 10−10 m for a crack 3 µm long, which gives a stress multiplier of approximately 245. The stress concentration dimin- Surface hardness ishes towards the mean stress according to the distance from the crack tip (Fig. 10.1). Once a crack has been In addition to the toughness of the material described initiated, the crack tip propagates at a velocity in the previous section, size reduction may also be 159 Aulton's Pharmaceutics E-Book : Aulton's Pharmaceutics E-Book, edited by Kevin M. G. Taylor, and Michael E. Aulton, Elsevier, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/trinitycollege/detail.action?docID=5253018. Created from trinitycollege on 2025-01-27 11:39:39.  PART TWO Particle science and powder technology influenced by the hardness of the material. Hardness Energy requirements of the size can be described empirically by its position on a scale devised by a German mineralogist called Mohs. The reduction process Mohs scale is a table of minerals; at the top of the Only a very small amount of the energy put into a table is diamond, with Mohs hardness >7, and this comminution operation actually effects size reduction. has a surface that is so hard that it can scratch anything This has been estimated to be as little as 2% of the below it. At the bottom of the table is talc, with total energy consumption, the remainder being lost Mohs hardness