Particle Size Separation PDF

Particle Size Separation Dr. Isra Dmour Credit: Prof. Nizar Al-Zoubi 1 Particle Size Separation It is the classification of powder into separate particle size ranges. S...

Particle Size Separation Dr. Isra Dmour Credit: Prof. Nizar Al-Zoubi 1 Particle Size Separation It is the classification of powder into separate particle size ranges. Solid separation is the removal (separation) of powder particles from gases or liquids. 2 1  Size-Separation Methods Size separation by sieving  Principle of operation is similar to the use of sieving in particle size analysis with the following differences: – Sieves should be resistant to chemical action with the material to be sifted (It is more suitable to use stainless steel sieves than brass or bronze sieves). – The amount of material is larger in case of particle size separation than in particles size analysis, and therefore: requires larger sieves there are several techniques to encourage the particles to separate into their appropriate size fractions. Size separation can be done by dry or wet sieving. 3 Size-Separation Methods Size separation by sieving Sieves are described by : – Opening size – Number of opening per linear inch (mesh number) 4 2 Dry sieving techniques 1.Agitation methods: –In these methods size separation is induced by vibrations which facilitate passing of particle through pores. –Alternatively, by suitable way gyration of sieves can be used which causes the particles to spin, and thereby changing continuously their orientation. gyration –The output from gyratory sieves is often greater than that obtained by vibration sieving. 5 Dry sieving techniques 2. Brushing methods: – A brush is used (manually on small scale and mechanically on large scale) to reorientate particles on the surface of a sieve and prevent blocking of pores. 6 3 Dry sieving techniques 3. Centrifugal methods: Particles are thrown outwards onto a vertical cylindrical sieve under the action of high-speed rotor inside the cylinder. The current of air created by the rotor also assists in sieving. 7 Wet sieving – Wet sieving is generally more efficient than dry sieving. 8 4  Size separation by fluid classification Sedimentation methods Principles of operation Gravitational methods – Simple forms of sedimentation classification uses a chamber containing a suspension of solid particles in a liquid. – After predetermined time , particles less than a given diameter can be recovered by a pipette placed at a fixed distance below the liquid surface, or alternatively by a pump. Alternative technique – Continuous settling chamber: Particles in suspension enter a shallow container and they are acted by a driving force divided into two components: 1. horizontal due to particle velocity and 2. vertical due to gravity and corresponds to Stoke’s settling velocity. – The coarsest particles will sediment closest to the inlet, whereas the finest particles will sediment furthest from the inlet. 9 10 5  Size separation by fluid classification  Very fine particles will not sediment efficiently under the influence of gravity due to Brownian diffusion.  In order to increase the driving force of sedimentation, centrifugal methods can be used. Centrifugal methods – Single cylindrical centrifuges can be used to remove single size cuts from a fluid stream. – Multiple chamber centrifuges can separate a number of size ranges. 11 12 6  Size separation by fluid classification Elutriation methods Elutriation is a technique in which the fluids flows in an opposite direction to the sedimentation movement. Particles are divided into different size fractions depending on the velocity of the fluid. Comparison of (a) sedimentation and (b) elutriation. (a) sedimentation (b) elutriation 13 Size separation by fluid classification Elutriation methods The simple way is the separation of particles when they are suspended in a fluid moving up a column. In practice , the fluid velocity in a tube is not uniform. The highest velocity is found in the center and the lowest velocity at the tube walls. Particles can be seen to rise with the fluid and then to move outwards to the tube wall, where the velocity is lower and then tend to fall down. A separation into two sizes occur, but the size cut will not be clearly defined. 14 7  Size separation by fluid classification Elutriation methods Upward airflow elutriator – Particles are held on a supporting mesh through which air is drawn. – Classification occurs within a very short distance of the mesh. – Any particles remaining entrained in the air stream are accelerated to a collecting chamber by passage through a conical section. 15 Size separation by fluid classification Multistage elutriators These are gravitational elutriator used to separated powders into several size fractions. It is composed of columns with increasing diameters. As the diameter increase the fluid velocity decrease and smaller particles can be separated. 16 8  17 Size separation by fluid classification Cyclone methods Principle of operation: – The most common type of cyclone used to separate particles from fluid stream is the reverse-flow cyclone. – In this system, Particles in air or liquid suspension are introduced tangentially into the cylindrical upper section of the cyclone. – The relatively high fluid velocity produces vortex that throws solid particles out onto the walls of the cyclone. 18 9  Size separation by fluid classification Cyclone methods – The particles are forced down the conical section under the influence of the fluid flow. – At the tip of the conical section the vortex of fluid is above the critical velocity at which it can escape through the narrow outlet and form an inner vortex which travels back up the cyclone. – Coarser particles separate from fluid and fall out on the cyclone, while finer particles leave with the fluid through the vortex finder. 19 20 10