The Particle Separation in Mineral and Chemical - Essay Example

Leading header: The Particle separation in Mineral and Chemical     Your name   Course name             Professors’ name Date Introduction Mineral processing is a division in the science of extractive metallurgy, in other words this is a science and art of extraction of metals from their ores, then refining them and preparing the final mineral for use. Extractive metallurgy is divided into four divisions: Mineral Processing (or Beneficiation), Hydrometallurgy, Hydrometallurgy, and Electrometallurgy. In processing minerals, a number of units operations are used to prepare and classify mineral ores before the valuable constituents of minerals can be separated for further treatment. Most rocks contain mineral deposits but when the concentrations minerals is too low to justify mining, is usually considered a waste. Inside the metal ore, minerals are usually inside a gangue, and during mineral processing is to separate minerals from their ores (Choi 1982). The simplest method of separating mineral ores from gangue is picking out minerals crystals from rocks pebbles; this is always a very tedious process when the mineral particles are small in size. Another simple technique of mineral separation is to rely on the different densities of minerals and collecting them at different places. Mineral particles which are heavier in weight will drop out of suspension quickly, while minerals which are lighter in weight will be carried further by a steam of water in the processing device. In this modern age, techniques and machines have been employed to separate minerals from their ores. Mineral processing consists of two functions, firstly, it involves preparing and liberation of valuable minerals from waste materials and secondly, it involves separation of these minerals particles into valuable products called concentrations. Mineral particles will be exposed from their gangue; this process of size reduction of gangue is what we called comminution. The crushing and grinding processes of gangue will produce range of particles with varying degrees of liberation, any particle which does not exceed the target size for physical separation or chemical extraction will be returned into the crushing machine to be crushed until it has reached a required target. Meanwhile, in comminution or separation process, it begins during excavation or scrapers for softer material. Separation of mineral materials is carried out in sequential manners using crashers and screens followed by grinding mills and classifiers (Tarjá 1981). Reflux Classifier (RC) This is a new device which is being used by mining companies to separate mineral particles based on their mineral densities or minerals size. Water flow through a distributor plate at the base of reflux classifier and this will suspend mineral particle within the device. Inside the reflux classifier there are a set of inclined plates which amplifies the segregation rates of mineral particles. These inclined plates inside the RC will permit slower settling mineral particles to pass through the zone of inclined plates, while those fast moving mineral particles will return to the zone below. A sedimentation area inside the RC is increased by the inclined plates which are found inside the RC and this in return will permit higher throughputs. Particle Segregation Particle segregation is an important feature of non-uniform sheared granular flows, in which large particles rise to the top of the flow regime. By changing the flow’s spatial composition, segregation has significant consequences on the behavior of mixed particle systems. This process involves separation of mineral particles into various characteristics such as size, density, shape and other properties of particles of which they are composed (Tarjá 1981). Sifting segregation This is the process of separating the desired elements from undesired elements during mining, this separation of desired elements can be from the combination of solid to solid or solid to liquid materials. The most economical and easiest method of sifting segregation in mining is to utilize a series of sieves, and each sieve has a different holes size from the others. The mining products then shifted through series of sieves and the amount of product that passes through each hole is then measured. In the ancient history this method was widely used by farmers to separate their grains from chaff. However, in mining there are different materials with unique characteristics that sometimes inhibit particle separation; furthermore particles going through the sieve become smaller and even nano particles. Some materials are so small that makes it difficult for mining companies to separate. In this case, it calls it calls for sophisticated method such as ultrasonic vibration or vacuum sieve to separate these particles. After that Laser diffraction is used to separate these particles which are hard to separate mechanically (Choi 1982). This technique is used to separate materials with similar densities and sizes, the technique is suitable to separate mineral particles below 100 µm in size; the lower size minerals that can be used for separation using this type of technique is approximately 35 µm although particles which are smaller than µm in size can be separated by using this type of technique (Metso 2006 Angle of Repose The mixture of mining materials is poured into a quasi-two dimensional silo, water is then added into the mixture till it saturates. When the mixture reaches saturation, the angle of repose will increases while segregation decreases sharply in the mixture. The water added in the mixture has been observed to cause small particles to clump more than the large particles found in the mixture. From this point small particles which mainly consist of minerals are separated from the rest of the materials and then sorted out (McGlinchey 2008). Inclined Plane Flows Particle segregation on an incline is characteristic of a non-uniform, gravity driven granular flow. In systems with varying particle sizes, it has been shown that larger particles will generally segregate to the top of the flow regime, while smaller particles will remain at the bottom. Depending upon the relative size of the grains, it was found that the large particles would eventually return downward (with gravity) a short distance before being forced to the top again (McGlinchey 2008). Air elutriation analysis This is a simple technique that can be used to separate mineral particles into two or more groups; the mineral particles are usually separated by means of an elutriator machine (McGlinchey 2008). This machine is made up of a vertical tube up and fluid is passed at a measured velocity. When the mineral particles are put into the side tube of elutriator machine, the smaller mineral particles will be carried over in the fluid stream while those particles which are larger in side will settle against the upward current in the machine. Less dense materials will attain terminal velocities, and these particles will flow with stream. These particles will then be collected in the overflow chamber and hence it will be separated from the feed in the machine. The flow rate of the machine can be increased and this can be used to separate higher size ranges. If the overflows of materials pass from the first tube in the machine, the materials will go to a second tube of greater cross-section, and any number of tubes in elutriator machine can be arranged in series. Elutriator has its advantages, such as: bulk materials can be analyzed using centrifugal classification and the method is non destructive. It limitation is that a bulk of mineral- about ten grams- sample must be obtained; this technique is fairly time consuming analytically (McGlinchey 2008). Size Classification Size classification is the general terms for separation of mineral particles according to their size- exploit the differences in settling velocities exhibited by particles of different size. The simple method of mineral sizing is screening, in other words, this is the passing of the mineral particles to be sized through a number of screens. The equipment that are used in screening of mineral particles include grizzlies, vibratory screed, and wire mesh screens. Screening of minerals can be either static or a mechanism can be incorporated to shake or vibrate the screen (Metso 2006). The particle size distribution (PSD) is usually expressed by the method by which it is determined; the most common widely used method of determination in using this method is sieve analysis. In this method, powder consisting of minerals is separated on sieves of different sizes, thus, the particle size distribution (PSD) is defined in terms of discrete size which ranges between 45 μm and 53 μm". However, the notional “sieve” that will retain mineral particles above a certain size and allow other particles to pass below that size of sieve, is internationally used in presenting PSD data of all kinds. PSD method can be expressed as a “range” analysis in which the amount in each size range of the mineral particle is listed in order; sometimes this analysis can be presented in “cumulative” form- this is when the total particle sizes retained or passed by a notional sieve is given for a range of sizes. Range analysis is appropriate when a mid- range mineral particle size is wanted, while cumulative analysis is mostly used when the amount of under-size or over size of mineral particles must be controlled. The way in which size of a mineral is expressed will depend with different interpretations, a simple treatment is assumed that a mineral particle is sphere in shape and it will pass through a notional sieve, but in real situation a particle is irregular in shape and the way in which different particles will be characterized in the analysis will depend on the method of measurement used during separation. The advantage of this method is that is well adapted for bulk materials; large amount of material can be loaded into the sieve tray. However, this technique has its limitations, first many notional sieves that are used in this technique usually concerned with particles which are too small for separations; 37μm sieves is fragile and it will allow materials to pass with difficulties. Second, the energy used to sieve in this technique is arbitrarily determined but sometimes over energetic sieve technique will cause attrition of the particles and thus changes the particle size distribution (PSD), while insufficient energy used in this technique will fail to separate agglomerates. Although the use of manual sieve can be ineffective in mineral separation, automated sieving technologies can be employed and sometimes they come with image fragmentation analysis which makes it easier to separate minerals (Metso 2006). Density Separation This technique is based on the fact that different materials will have different densities, thus, a mixture of minerals with different densities will be placed in a liquid with an intermediate density, and minerals with less density compared with the liquid density will float, while those minerals with higher density than the liquid will sink. Typically mineral densities range from 2.2 g/cc to as much as 8 g/cc, but for silicate minerals their densities is between 2.5 and 3.5 g/cc. Suitable liquid that are widely used in density separation include bromoform (density = 2.84 g/cc) and diiodomethane (density = 3.31 g/cc). But you can dissolve sodium tungstate powder in water; it has a higher density when you compare it to bromoform and diiodomethane. The use of high density liquids is referred to as heavy liquids separation; they are generally done in separatory funnels. This procedure is very simple, the materials are placed in the separatory funnel and the heavy liquid- mixture of water and sodium tungstate- is added into the funnel. The funnel is then left to rest for sometimes to allow minerals with lighter densities to float and those with heavy densities to sink. When the minerals with two different densities have been separated in the funnel, the separatory funnel is opened and minerals with higher densities are transferred onto a piece of weighing paper in a funnel; this process is to allow the liquid to drain away, the minerals are separated then washed and examined optically for purity. The densities of bromoform and diiodomethane can be increased by adding acetone (density about 0.7 g/cc), thus, those minerals which have slightly difference densities will be separated when you increase the densities of heavy liquid until their densities lies between those of the minerals (Metso 2006), while insufficient energy used in this technique will fail to separate agglomerates. Although the use of manual sieve can be ineffective in mineral separation, automated sieving technologies can be employed and sometimes they come with image fragmentation analysis which makes it easier to separate minerals (Metso 2006). Froth flotation This method is widely used for concentrating minerals; it usually makes physiochemical separation. The mineral ore is crushed to produce slurry; the slur is then mixed with fine bubbles of air so that fine valuable minerals will be carried. This is because minerals which have been separated from non-precious materials will attach themselves to the bubbles and rise to the surface to form froth, this froth at the surface is then skimmed to form a product known as concentrate. Many minerals can be separated by using this technique when several changes will be done on the surfaces minerals with assistance of reagents which will make the mineral particle to be hydrophobic; stream of air is then added to move the hydrophobic particles to the surface of the slurry under agitation. The difference in density between the air bubbles and water will provide a buoyancy to lift the mineral particles to the surface where they will remain entrained in a froth which can be mechanically be removed, thus, making it possible to separate the minerals. This technique is used to separate materials with similar densities and sizes, the technique is suitable to separate mineral particles below 100 µm in size; the lower size minerals that can be used for separation using this type of technique is approximately 35 µm although particles which are smaller than µm in size can be separated by using this type of technique (Metso 2006). Electrostatic Separation Electrodynamic separators or electrostatic separators. Their work is similar but the forces which are applied on different particles are gravity and electrostatic attraction respectively. In the machine the two type of forces are usually charged by corona discharge, the charges will travel to a drum in a machine used to separate the mineral particles. Then, the particles will lose charge to the drum and are usually removed from the drum with centripetal acceleration force. In the technique will only work when a stream of mineral particles are passed past a charged anode in the machine, the conductors in the machine will lose electrons to the plates and are usually pulled away from the other particles which have the same attraction charges in the machine. The size of particles which are suitable to be used by this type of technique is between 75 and 250 micron and second the particles need to be dry and at same time should be uniform in their shape (McKay 1988). Magnetic Separation Some minerals have magnetic properties for example ilmenite and magnetite and so these minerals particles can be separated from non magnetic materials by using strong magnetic forces. There are a number of different techniques which can be involved in this process; these include HGMS, HIMS and LIM. In HGMS and HIMS are usually continuous processes and are used mostly to separate paramagnetic mineral particles while in LIMS process is mostly used to separate mineral particles which are ferromagnetic. Before these processes should be employed in separating mineral particles, it should be taken into account: the size of the mineral ore, the present of tramp minerals and the liberation of the mineral particles which are being separated by this process. This process work by moving mineral particles in the magnetic field, the force will be experienced in the magnetic field and is represented by an equation. f=m/k.H.dh/dx. Whereby K= magnetic constant, H-field strength, and dh/dx-gradient. With the assistance of this equation, the separation can be driven in two ways, through magnetic gradient in the field of the magnet or by the used of strength of magnetic field. The driving forces are either used with water or without water, for example, in spirals water is used to help in the separation of mineral particles, while increases the entrainment of the gangue in the concentrate (McKay 1988). The process of magnetic separation takes advantage of the magnetic properties on mineral particle. Most minerals particles falls into one of 3 magnetic properties: ferromagnetic, paramagnetic and diamagnetic. Mineral particles which are ferromagnetic can be attracted by a magnet and this makes them easily separated from other minerals by using a magnet. Paramagnetic and diamagnetic minerals cannot be attracted by a magnetic but they differ in how they interact with magnetic fields caused by a magnet, while diamagnetic minerals particles can be repelled by a magnetic field. Thus, if there is a mixture of paramagnetic and diamagnetic minerals particles, when a magnet is passed through these materials, paramagnetic materials will be attracted while those materials which are diamagnetic will be repelled by magnetic field. Furthermore, mineral particles which are paramagnetic can be separated using the same technique but this time it only depends on the degrees of paramagnetism. The device which is employed to separate mineral particles based on their magnetic properties is known as isodynamic magnetic separator; it consists of a large electromagnet through which mineral particles mixtures are passed on a metal trough. The strength of magnetic field is varied and slope of the separation trough is used to separate the mineral particles. This technique has its limitations, such as, it is impossible to completely eliminate impurities that can be found in the mixture. Dewatering This is an important process in mineral processing; it refers to removal of water which is contained in particles. This process is usually done for a number of reasons, specifically to make handling of mineral ores to be transported easily to the crushing factory, and allow further processing of mineral particles to occur and to be liberated from the gangue. The processes that are involved in dewatering include dewatering screens, sedimentation, filtering and terminal drying. This process in dewatering will increase in difficulty and the cost if the mineral particle decreases in size. This process is viable for mineral ores that are closely distributed in size because the aperture in the screen will only allow small mineral particles to pass through (McKay 1988). In sedimentation process, it is done by passing water into a large thickener; mineral particles will settle down in this device because of the effect of gravity or centripetal forces that will be experienced on the materials, to aid the process of sedimentation, coagulants are added into the mixture to reduce the repulsive forces that is found between the mineral particles. These repulsive forces come about due to the double layer which is formed on the surface of the mineral particles. Reference List Choi, W. (1982). Comminution and liberation studies of complex sulfide ores. Virginia: Virginia Polytechnic Institute and State University. McGlinchey, D. (2008). Bulk solids handling: equipment selection and operation. London: Blackwell Pub. McKay, J.D. (1988), “Column Flotation and Bubble Generation Studies at the Bureau of Mines”, Column Flotation ‘88, SME-AIME, Littleton, Colorado pp. 173-186. Metso (2006) Basics in Minerals Processing, Metso Minerals, Retrieved from http://www.metso.com. Tarjá, G. (1981). Mineral Processing: Fundamentals, comminution, sizing, and classification. Michigan: the University of Michigan. Read More