Mineral processing


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Mineral processing

  1. 1. Mineral Processing Page 1 of 25 MINERAL PROCESSING(MY-203) Group #7 Batch 2012-13 Group Members: S.M.Tanveer(MY-017) Ahmed Mazher(MY-011) Jahanzaib Sheikh(MY-010) Submitted To: Engineer Iftikhar Ahmed Channa
  2. 2. Mineral Processing Page 2 of 25 Table of Contents This Report Is Based Upon The Literature Work, Online Research Only, Any Industrial Experience, Self -Research Or Paper Work Is Not Included In This Oeuvre.
  3. 3. Mineral Processing Page 3 of 25 Mineral processing is the art of treating crude ores and mineral products in order to separate the valuable minerals from the waste rock, or gangue. It is the first process that most ores undergo after mining in order to provide a more concentrated material for the procedures of extractive metallurgy. The primary operations are comminution and concentration, but there are other important operations in a modern mineral processing plant, including sampling and analysis and dewatering. All these operations are discussed in this article. An ore is a type of rock that contains minerals with important elements including metals that can be extracted from the rock at a profit. The ores are extracted through mining; these are then refined (often via smelting) to extract the valuable element(s). The grade or concentration of an ore mineral, or metal, as well as its form of occurrence, will directly affect the costs associated with mining the ore. The cost of extraction must thus be weighed against the metal value contained in the rock to determine what ore can be processed and what ore is of too low a grade to be worth mining. An Iron Ore A Gold Ore
  4. 4. Mineral Processing Page 4 of 25 We feed ore into a series of crushers and grinding mills to reduce the size of the ore particles and expose the mineral. Water is also added which turns the ore into a slurry. We send this slurry to leaching tanks, where we add a weak cyanide solution to the slurry, which leaches gold and silver into the solution. This process removes up to 93 percent of the gold and 70 percent of the silver from the ore. Carbon granules are then added to the solution. The gold is pulled from the solution and attaches to the carbon. We then "strip" the gold from the carbon by washing it with a caustic cyanide solution. The carbon is later recycled. Next, we pump the bearing solution through electro-winning cells, which extract metals from the solution using an electrical current. After gold has been processed, the leftover waste material is called tailings. Tailings contain small amounts of cyanide and other hazardous chemicals, so they must be disposed of in an environmentally safe way. The tailings are stored in tailings dams, which are lined with impermeable layers. While the cyanide levels in the dam are safe, steps are taken to keep wildlife away from the dams. Over time, the chemicals break down and the solids settle to the bottom so that the water can be returned to the plant to be used in processing. We then smelt, which melts it in a furnace at about 2,100°F. We finally send the bars to a refinery for further processing. Routine sampling and analysis of the raw material being processed are undertaken in order to acquire information necessary for the economic appraisal of ores and concentrates. In addition, modern plants have fully automatic control systems that
  5. 5. Mineral Processing Page 5 of 25 conduct in-stream analysis of the material as it is being processed and make adjustments at any stage in order to produce the richest possible concentrate at the lowest possible operating cost. Sampling is the removal from a given lot of material a portion that is representative of the whole yet of convenient size for analysis. It is done either by hand or by machine. Hand sampling is usually expensive, slow, and inaccurate, so that it is generally applied only where the material is not suitable for machine sampling (slimy ore, for example) or where machinery is either not available or too expensive to install. Many different sampling devices are available, including shovels, pipe samplers, and automatic machine samplers. For these sampling machines to provide an accurate representation of the whole lot, the quantity of a single sample, the total number of samples, and the kind of samples taken are of decisive importance. A number of mathematical sampling models have been devised in order to arrive at the appropriate criteria for sampling. Ore Sampling After one or more samples are taken from an amount of ore passing through a material stream such as a conveyor belt, the samples are reduced to quantities suitable for further analysis. Analytical methods include chemical, mineralogical, and particle size.
  6. 6. Mineral Processing Page 6 of 25 Even before the 16th century, comprehensive schemes of assaying (measuring the value of) ores were known, using procedures that do not differ materially from those employed in modern times. Although conventional methods of chemical analysis are used today to detect and estimate quantities of elements in ores and minerals, they are slow and not sufficiently accurate, particularly at low concentrations, to be entirely suitable for process control. As a consequence, to achieve greater efficiency, sophisticated analytical instrumentation is being used to an increasing extent. In emission spectroscopy, an electric discharge is established between a pair of electrodes, one of which is made of the material being analyzed. The electric discharge vaporizes a portion of the sample and excites the elements in the sample to emit characteristic spectra. Detection and measurement of the wavelengths and intensities of the emission spectra reveal the identities and concentrations of the elements in the sample. In X-ray fluorescence spectroscopy, a sample bombarded with X rays gives off fluorescent X-radiation of wavelengths characteristic of its elements. The amount of emitted X-radiation is related to the concentration of individual elements in the sample. The sensitivity and precision of this method are poor for elements of low atomic number (i.e., few protons in the nucleus, such as boron and beryllium), but for slags, ores, sinters, and pellets where the majority of the elements are in the higher atomic number range, as in the case of gold and lead, the method has been generally suitable
  7. 7. Mineral Processing Page 7 of 25 Ore Being Chemical Analyzed A successful separation of a valuable mineral from its ore can be determined by heavy-liquid testing, in which a single-sized fraction of a ground ore is suspended in a liquid of high specific gravity. Particles of less density than the liquid remain afloat, while denser particles sink. Several different fractions of particles with the same density (and, hence, similar composition) can be produced, and the valuable mineral components can then be determined by chemical analysis or by microscopic analysis of polished sections. Some typical Illustrations
  8. 8. Mineral Processing Page 8 of 25 Coarsely ground minerals can be classified according to size by running them through special sieves or screens, for which various national and international standards have been accepted. One old standard (now obsolete) was the Tyler Series, in which wire screens were identified by mesh size, as measured in wires or openings per inch. Modern standards now classify sieves according to the size of the aperture, as measured in millimeters or micrometers (10 - 6 meters). Mineral particles smaller than 50 micrometers can be classified by different optical measurement methods, which employ light or laser beams of various frequencies. Mineral processing can involve four general types of unit operation: Communition Particle size reduction Sizing Separation of particle sizes by screening or classification Concentration by taking advantage of physical and surface chemical properties Dewatering Solid/liquid separation In all of these processes, the most important considerations are the economics of the processes and this is dictated by the grade and recovery of the final product. To do this, the mineralogy of the ore needs to be considered as this dictates the amount of liberation required and the processes that can occur. The smaller the particles processes, the greater the theoretical grade and recovery of the final product, but this however is difficult to do with fine particles as they prevent certain concentration processes from occurring.
  9. 9. Mineral Processing Page 9 of 25 Comminution is particle size reduction of materials. Comminution may be carried out on either dry materials or slurries. Crushing and grinding are the two primary comminution processes. Crushing is normally carried out on "run-of-mine" ore, while grinding (normally carried out after crushing) may be conducted on dry or slurried material. In comminution, the size reduction of particles is done by three types of forces-compression, impact and attrition. Compression and impact forces are extensively used in crushing operations while attrition is the dominant force in grinding. The primarily used equipment’s in crushing are-jaw crushers, gyratory crushers and cone crushers whereas rod mills and ball mills, closed circuited with a classifier unit, are generally employed for grinding purposes in a mineral processing plant. Crushing is a dry process whereas grinding is generally performed wet and hence is more energy intensive. The simplest sizing process is screening, or passing the particles to be sized through a screen or number of screens. Screening equipment can include grizzlies, bar screens,wedge wire screens, radial sieves ,banna screens, multi-deck screens, vibratory screen, fine screens, flip flop screens and wire mesh screens. Typical Examples of Screening Process
  10. 10. Mineral Processing Page 10 of 25 Sizing is the general term for separation of particles according to their size. The simplest sizing process is screening, or passing the particles to be sized through a screen or number of screens. Screening equipment can include grizzlies,bar screens,wedge wire screens, radial sieves, banna screens, multi-deck screens, vibratory screen, fine screens, flip flop screens and wire mesh screens. Bar Screen Grizzly Screen Multi-deck Screen Banna Screen
  11. 11. Mineral Processing Page 11 of 25 Screens can be static (typically the case for very coarse material), or they can incorporate mechanisms to shake or vibrate the screen. Some considerations in this process includes the screen material, the aperture size, shape and orientation, the amount of near sized particles, the addition of water, the amplitude and frequency of the vibrations, the angle of inclination, the presence of harmful materials, like steel and wood, and the size distribution of the particles. Classification refers to sizing operations that exploit the differences in settling velocities exhibited by particles of different size. Classification equipment may include ore sorters, gascyclones, hydrocyclones, rotating trommels, rake classifiers or fluidized classifiers. An important factor in both comminution and sizing operations is the determination of the particle size distribution of the materials being processed, commonly referred to as particle size analysis .Many techniques for analyzing particle size are used, and the techniques include both off-line analyses which require that a sample of the material be taken for analysis and on-line techniques that allow for analysis of the material as it flows through the process. Rotating Trommel Hydro-cyclone Gas Cyclone Apparatus Fluidized classifier.
  12. 12. Mineral Processing Page 12 of 25 There are a number of ways to increase the concentration of the wanted minerals: in any particular case the method chosen will depend on the relative physical and surface chemical properties of the mineral and the gangue. Concentration is defined as the number of moles of a solute in a volume of the solution. In case of mineral processing concentration means the increase of the percentage of the valuable mineral in the concentrate. Gravity separation is the separation of two or more minerals of different specific gravity by their relative movement in response to the force of gravity and one or more other forces (such as centrifugal forces, magnetic forces), one of which is resistance to motion (drag force) by a viscous medium such as heavy media or water. Gravity separation is one of the oldest technique in mineral processing but has seen a decline in its use since the introduction of methods like flotation, classification, magnetic separation and leaching. Gravity separation dates back to at least 3000 BC when Egyptians used the technique for separation of gold. It is necessary to determine the suitability of a gravity concentration process before it is employed for concentration of an ore. A criteria called as concentration criterion is commonly used for this purpose. It is defined as- Concentration Criterion (CC)= (SG of heavy mineral-SG of fluid)÷(SG of light mineral-SG of fluid), where SG=specific gravity for CC > 2.5, suitable for separation of particles above 75 micron in size for 1.75 <CC< 2.5, suitable for separation of particles above 150 micron in size for 1.50 < CC < 1.75, suitable for separation of particles above 1.7 mm in size for 1.25 < CC < 1.50, suitable for separation of particles above 6.35 mm in size for CC < 1.25, not suitable for any size.
  13. 13. Mineral Processing Page 13 of 25 Gravity Ore Concentration Process There are two main types of electrostatic separators. These work in similar ways, but the forces applied to the particles are different and these forces are gravity and electrostatic attraction. The two types are electro-dynamic separators (or high tension rollers) or electrostatic separators. In high tension rollers, particles are charged by a corona discharge. This charges the particles that subsequently travel on a drum. The conducting particles lose their charge to the drum and are removed from the drum with centripetal acceleration. Electrostatic plate separators work by passing a stream of particles past a charged anode. The conductors lose electrons to the plate and are pulled away from the other particles due to the induced attraction to the anode. These separators are used for particles between 75 and 250 micron and for efficient separation to occur, the particles need to be dry, have a close size distribution and uniform in shape. Of these considerations, one of the most important is the water content of the particles. This is important as a layer of moisture on the particles will render the non-conductors as conductors as the layer of the water is conductive. Electrostatic plate separators are usually used for streams that have small conductors and coarse non-conductors. The high tension rollers are usually used for streams that have coarse conductors and fine non-conductors. These separators are commonly used for separating mineral sands, an example of one of these mineral processing plants is the CRL processing plant at Pinkenba in Brisbane Queensland. In this plant, zircon, rutile and ilmenite are separated from the silica gangue. In this plant, the separation is performed in a number of stages with roughers, cleaners, scavengers and recleaners.
  14. 14. Mineral Processing Page 14 of 25 Electrostatic Separation Flowchart and Industrial Work Magnetic separation is a process in which magnetically susceptible material is extracted from a mixture using a magnetic force. This separation technique can be useful in mining iron as it is attracted to a magnet. In mines where wolframite was mixed with cassiterite, such as South Crofty and East Pool mine in Cornwall or with bismuth such as at the Shepherd and Murphy mine in Moina, Tasmania, magnetic separation was used to separate the ores. At these mines a device called a Wetherill's Magnetic Separator (invented by John Price Wetherill, 1844–1906)[1] was used. In this machine the raw ore, after calcination was fed onto a moving belt which passed underneath two pairs of electromagnets under which further belts ran at right angles to the feed belt. The first pair of electromagnets was weakly magnetised and served to draw off any iron ore present. The second pair were strongly magnetised and attracted the wolframite, which is weakly magnetic. These machines were capable of treating 10 tons of ore a day.This process of separating magnetic substances from the non-magnetic substances in a mixture with the help of a magnet is called magnetic separation. This process operates by moving particles in a magnetic field. The force experienced in the magnetic field is given by the equation f=m/k.H.dh/dx. with k=magnetic susceptibility, H-magnetic field strength, and dh/dx being the magnetic field gradient. As seen in this equation, the separation can be driven in two ways, either through a gradient in a magnetic field or the strength of a magnetic field.
  15. 15. Mineral Processing Page 15 of 25 The different driving forces are used in the different concentrators. These can be either with water or without. Like the spirals, washwater aids in the separation of the particles while increases the entrainment of the gangue in the concentrate. Magnetic Separation Flowchart and Practical Work Dewatering is an important process in mineral processing. The purpose of dewatering is to remove water absorbed by the particles which increases the pulp density. This is done for a number of reasons, specifically, to enable ore handling and concentrates to be transported easily, allow further processing to occur and to dispose of the gangue. The water extracted from the ore by dewatering is recirculated for plant operations after being sent to a water treatment plant. The main processes that are used in dewatering include dewatering screens, sedimentation, filtering, and thermal drying. These processes increase in difficulty and cost as the particle size decreases. Dewatering screens operate by passing particles over a screen. The particles pass over the screen while the water passes through the apertures in the screen. This process is only viable for coarse ores that have a close size distribution as the apertures can allow small particles to pass through. Sedimentation operates by passing water into a large thickener or clarifier. In these devices, the particles settle out of the slurry under the effects of gravity or centripetal forces. These are limited by the surface chemistry of the particles and the size of the particles. To aid in the sedimentation process, flocculants and
  16. 16. Mineral Processing Page 16 of 25 coagulants are added to reduce the repulsive forces between the particles. This repulsive force is due to the double layer formed on the surface of the particles. The flocculants work by binding multiple particles together while the coagulants work by reducing the thickness of the charged layer on the outside of the particle. Thermal drying is usually used for fine particles and to remove low water content in the particles. Some common processes include rotary dryers, fluidised beds, spray driers, hearth dryers and rotary tray dryers. This process is usually expensive to operate due to the fuel requirement of the dryers. Delkor Belt Filters For Dewatering Froth flotation is an important concentration process. This process can be used to separate any two different particles and operated by the surface chemistry of the particles. In flotation, bubbles are introduced into a pulp and the bubbles rise through the pulp. In the process, hydrophobic particles become bound to the surface of the bubbles. The driving force for this attachment is the change in the surface free energy when the attachment occurs. These bubbles rise through the slurry and are collected from the surface. To enable these particles to attach, careful consideration of the chemistry of the pulp needs to be made. These considerations include the pH, Eh and the presence of flotation reagents. The pH is important as it changes the charge of the particles surface and the Eh affects the chemisorption of collectors on the surface of the particles.
  17. 17. Mineral Processing Page 17 of 25
  18. 18. Mineral Processing Page 18 of 25
  19. 19. Mineral Processing Page 19 of 25 Figure 1.2b North American porphyry copper ore. Chalcopyrite (C) precipitated along fractures in quartz. Liberation of chalcopyrite is fairly difficult due to "chain-like" distribution. Fracture is, however, likely to occur into a low-grade concentrate by froth flotation
  20. 20. Mineral Processing Page 20 of 25
  21. 21. Mineral Processing Page 21 of 25 Valuable mineral
  22. 22. Mineral Processing Page 22 of 25 ∑weight=44.54100-0.1=99.9 99.9-2.9=97 97.0-9.5=87.5 100-undersize=oversize
  23. 23. Mineral Processing Page 23 of 25 Graph between sieve size or particle size & cumulative under size material (data extracted from Table 4.3)
  24. 24. Mineral Processing Page 24 of 25
  25. 25. Mineral Processing Page 25 of 25 Since the purpose of mineral processing is to increase the economic value of the ore, the importance of the recovery-grade relationship is in determining the most economic combination of recovery and grade which will produce the greatest financial return per tonne of ore treated in the plant. This will depend primarily on the current price of the valuable product, transportation costs to the smelter, refinery, or other further treatment plant, and the cost of such further treatment. A high grade concentrate will incur lower smelting costs, but the lower recovery means lower returns of final product. A low grade concentrate may achieve greater recovery of the values, but incur greater smelting and transportation costs due to the included gangue minerals. Also of importance are impurities in the concentrate which may be penalized by the smelter, although precious metals may produce a bonus. The net return from the smelter (NSR) can be calculated for any recovery-grade combination from: NSR = Payment for contained metal - (Smelter charges + Transport costs) This is summarized in Figure 1, which shows that the highest value of NSR is produced at an optimum concentrate grade. It is essential that the mill achieves a concentrate grade which is as close as possible to this target grade. Although the effect of moving slightly away from the optimum may only be of the order of a few pence per tonnes treated, this can amount to very large financial losses, particularly on high-capacity plants treating thousands of tonnes per day.