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Exercise 4-1 Dense-medium cyclone - Single stage One of the first investigations on the use of the dense-medium cyclone was reported by the Dutch State Mines in the 1960s. (Davies D. S., Dreissen H. H., and Oliver R. H. Advances in hydrocyclone heavy media separation technology for fine ores. Proc 6th International Mineral Procesing Congress, Cannes 1963 Roberts A., Editor. Pergamon Press pp 303 - 321). They were interested in separating magnesite from calcite and producing a low-silica magnesite concentrate using the dense-medium cyclone. In these two exercises we will investigate the possibility of using a two-stage dense-medium cyclone circuit to achieve the desired concentrate. Data for these examples is taken from the reference that is cited above. The target concentrate was specified as containing no more than 2% CaO and no more than 3.5% SiO2. Start a new job in MODSIM. Draw a flowsheet containing a single dense-medium cyclone and add a feed stream, an overflow stream and an underflow stream and add stream flyouts. Accept the flowsheet, specify a job name and edit the system data. Although we are interested ultimately in a two-stage circuit it is always a good idea to build up your flowsheet unit by unit so that you can test each individual model as you go. This exercise will enable you to extend your experience of specifying data for a multi-component ore. Specify 3 minerals, calcite, silica and magnesite with specific gravities 2.70, 2.67 and 3.00 respectively. Specify sp. gr. by mineral. Select 25 size classes with a largest size of 0.01 m. Select 6 grade classes and 1 S-class. Click "Set up grade classes" and specify the composition of each class according to the data in Table I. Compositions are specified by mass but please note that the laboratory data is given in terms of %CaO which must be converted to %CaCO3 to specify the mineral grade in the G-classes. (Calcite, CaCO3, contains 56% CaO). The class specific gravities need not be specified because these will be computed by MODSIM. How can you use the calculated specific gravities to check the experimental data given in Table 1? (The correct answer is worth a 2-point bonus for the first for-credit student who posts it) It is not necessary to specify a liberation model since no grinding mills will be used in the flowsheet. Double click on the plant feed stream and specify the particle size distribution from Table II. Set the feed rate at 100 tonnes/hr at 30% solids. Click "specify grade distributions" and specify the distribution among the grade classes from Table I. Note that the grade distribution does not vary with particle size so only one size range is required. This completes the specifications of the system data. Why is it necessary to specify the particle size distribution even though the washability data in Table 1 applies to all sizes? Another 2-pointer for for-credit students for this. Edit the unit parameters and check the help file for the dense-medium cyclone model DMCY. Select target specific gravity of 2.80. This will invoke the Gottfried-Jacobsen model to calculate the variation of both cut point and imperfection with the particle size. Run the simulation. View the flowsheet. In this plant we want to keep track of the CaO content of each stream and this can be displayed in the fourth quadrant of the flyouts. To set this up, edit the output format. In the "metals or elements" section, specify CaO in the first block. Specify 0.56 for the CaO content of calcite, 0 for the CaO content of silica and 0 for the CaO content of magnesite. You should now investigate how the grades of CaO and SiO2 in the underflow vary as the target specific gravity in the dense-medium cyclone is changed systematically over the range 2.80 to 3.1. This can be reported most effectively on a graph of grade vs target specific gravity. This will give you the opportunity to try out the the method for transfering output data from MODSIM directly to a spreadsheet or to your favorite graph plotting program. In this example this can be done mostly conveniently by running the simulation and choosing "Display simulation results" from the main View menu. Locate the underflow stream by its stream number, select that row in the table and copy (Use copy from the Edit menu or Ctrl-C) it to the clipboard. Open your spreadsheet and paste the row to a convenient location in the spread sheet. Go back to MODSIM, edit the unit parameter data and change the target specific gravity to the next value. Accept the new parameter data and run the simulation again and copy and paste the results. Do this until you have built up enough data to plot a graph of grade vs. target specific gravity for CaO and SiO2. You should also plot the recovery of magnesite vs CaO grade on a separate graph. From these graphs you can easily find the target specific gravity that is required to meet the product specification and also the yield of magnesite that can be achieved. (For the for-credit students, a 3-point grade bonus is available for anyone who can post a neat spreadsheet, including the graphs, before noon on Sunday June 22). If this seems to be a bit complicated you can download a working Excel spreadsheet or view a completed spread sheet to help you along. Remember that you get one row on your spreadsheet for each completed simulation run. You could also repeat the whole exercise using the alternative method of setting the cutpoint by specifying the medium specific gravity. If you decide to do this you will, you will gain some insight into these different modeling techniques for dense-medium cyclones. You will have to consult the textbook or the Technical Notes for detailed information. When you are comfortable with the simulation of a single stage dense-medium cyclone you are ready to add a second stage to the flowsheet. Save this job for future reference. |
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Table 1 Washability data for the magnesite ore |
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Table 2 Particle size distribution. |
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