Reverse column flotation of iron ore

Transcription

1 Reverse column flotation of iron ore T.C. Eisele and S.K. Kawatra Research scientist-engineer and professor, respectively, Department of Chemical Engineering, Michigan Technological University, Houghton, Michigan Abstract The use of column flotation was considered for the reverse flotation of iron ore, where the gangue (quartz) was recovered in the froth product. This use of the froth for carrying the gangue mineral rather than carrying the valuable concentrate significantly changes the operational requirements of the column. The objective of this work was to determine which of several column flotation technologies would be most suitable for this type of flotation operation. Studies were carried out using both the Jameson column and the Deister column. Comparative results are presented that show the capabilities of both column types in this application relative to conventional Wemco flotation cells. Columns were demonstrated to have grade-recovery performance superior to what could be achieved with conventional cells. Two-stage flotation with the columns could produce a concentrate that was up to 70.2% Fe at 87.5% iron recovery, compared to 70.4% Fe at only 77.7% iron recovery for four stages of flotation in conventional cells. It was shown to be particularly important to operate the columns to recover a maximum of the quartz gangue into the froth. It was noted that there are often problems in scale-up of laboratory columns to plant scale, because the increase in height and the decrease in the ratio of height to diameter at the larger scale allows more axial mixing and bubble coalescence, both of which degrade the column performance. Horizontal baffling was shown to improve column performance by reducing axial mixing and breaking up large bubbles, and so the use of such baffles in the reverse flotation application was investigated. Key words: Jameson Cell, Flotation Column, Iron ore, Magnetite, Quartz, Reverse flotation, Horizontal baffles Introduction Flotation columns can be used for improving the grade-recovery performance of froth flotation and are useful for reducing multistage conventional flotation circuits to only one or two stages of column flotation. Flotation columns have been used with great success in many applications, particularly as replacements for rougher and cleaner flotation. Based on their success in other applications, there is considerable potential for applying column flotation to iron ore flotation. Conventional froth flotation of iron ore requires many stages of flotation because of the relatively unselective collectors required for oxide mineral flotation (Smith and Akhtar, 1976). The greater inherent selectivity of column flotation would make it possible to reduce the number of stages needed, which would greatly simplify the flotation circuits. In this paper, the use of two different types of flotation columns is considered for the reverse flotation of silicate gangue from magnetite concentrate. Background Characteristics of reverse flotation of iron ore. In reverse flotation of iron ore, the silicate gangue minerals (primarily quartz) are recovered into the froth phase using cationic collectors such as amines at a ph of approximately 9 to 11. Because the concentrate in this case is the material that does not float, while the froth is the tailings product, a rougher/cleaner reverse flotation circuit would be considered a rougher/scavenger circuit in other flotation processes. Because of the nature of reverse flotation, the demands that it makes on flotation columns are somewhat different than for other flotation systems. In particular, production of a high-grade concentrate in reverse flotation requires that gangue mineral particles be very thoroughly collected from the bulk of the slurry, and the purity of the froth product is much less of an issue. To date, columns have been used with great success as cleaners, and as rougher/cleaners, where the froth grade is of greatest importance. In the present application, the purity of the froth is less important than the recovery of the non-floating iron oxide concentrate. Because of this basic difference, it was not completely certain whether column flotation would work well in this type of application. Implementation of flotation columns. It is known that there are two particular issues that cause problems in column scaleup that should be addressed before applying the preceding laboratory results on a full scale. These problems are axial mixing and bubble enlargement/coalescence. In most flotation Paper number MMP Original manuscript submitted online January 2006 and accepted for publication June Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. prior to Nov. 30, Copyright 2007, Society for Mining, Metallurgy, and Exploration, Inc. 61 Vol. 24, No. 1 May 2007

2 height to diameter (H:D) ratio than smaller columns. The axial mixing not only harms column performance, but also makes scale-up calculations more difficult (Finch and Dobby, 1990; Alford, 1992). Bubble enlargement and coalescence is also a problem because as the bubbles enlarge, their surface area decreases, reducing the quantity of floatable material they can carry. Very large bubbles also cause mixing and churning of the froth layer, which interferes with froth washing and increases entrainment. Because of these effects, it is desirable to keep the bubble diameter small. This effect becomes more pronounced as the column height increases, which is a result of the longer residence time of bubbles in the column and the greater difference in hydrostatic pressure between the column base and the froth. Because of these factors, laboratory and pilot-scale column flotation results are frequently very different from full-scale plant results. Typically, the performance at a small scale is noticeably superior. It would, therefore, be very useful if the design of flotation columns could ensure that the very large columns perform much more like the very small columns. Figure 1 Schematic of the Jameson flotation cell. Figure 2 Schematic of a Deister column. Air can be injected either through spargers or as an air-water mixture produced by a bubble generator. columns, there is no restriction to flow along the vertical axis of the column. This axial mixing is generally harmful, as it tends to reduce the product recovery and make the separation less effective. As columns are increased in size from laboratory or pilot scale to full scale, the degree of axial mixing tends to increase, as larger columns generally have a smaller Types of flotation column considered. For this work, three flotation column types were considered: the Jameson column, the more conventional Deister column and the horizontally baffled column. Jameson column: The Jameson cell is an Australian development and is a significant departure from conventional column designs (Kennedy, 1990). The bubble-particle contact and attachment are carried out rapidly outside of the main body of the column. This makes it possible for the Jameson cell to be much more compact than other columns, while still maintaining a high flotation rate and reasonable recovery. The Jameson cell consists of two main parts, as shown in Fig. 1. In the downcomer, the particles and air come in contact with each other, and an intimate mixture of air and slurry is produced. This mixture is accomplished by injecting the feed at high pressure through an orifice plate, producing a slurry jet that pulls air into the downcomer by aspiration (Manlapig et al., 1993; Atkinson et al., 1995). Due to the high mixing velocity and extensive bubble surface area in the downcomer, there is rapid contact and capture of the hydrophobic particles by the bubbles during their 5 to 10 second residence time in this zone. Because no additional air is introduced, any particles that do not contact air bubbles in the downcomer are unlikely to contact bubbles in the body of the cell. As a result, the recovery is often lower than for other column types. Deister column: The Deister column is a more traditional type of column, with feed injected at the midpoint, air introduced at the base and wash water introduced at the top to clean the froth product. A schematic of this type of column is shown in Fig. 2. Because the reverse flotation process requires very complete flotation of gangue particles from the nonfloating material, this type of column would be expected to give a better concentrate grade in reverse flotation than can be achieved with the Jameson cell. This is because the Deister column s greater height provides more opportunity for gangue particles to be collected and carried to the froth product. Baffled column: Both the axial mixing and bubble coalescence problems that complicate scale-up of flotation columns can be greatly reduced using the horizontally baffled column designed at Michigan Technological University (Kawatra et al., 1988; Kawatra and Eisele, 1993, 1994, 1995; Eisele and Kawatra, 1995). The purpose of these horizontal baffles is to allow full-scale plant columns to more closely approach the May 2007 Vol. 24 No. 1 62

3 performance seen in laboratory-scale columns. The horizontal baffles consist of simple perforated plates, with openings large enough to keep them from being plugged by solid particles, but small enough to break up vertical mixing currents as shown in Fig. 3. Materials and procedures Feed and reagents. The feed for all experiments was magnetite concentrate from plant magnetic separators, ground to approximately 80% passing 25 µm, and with an iron concentration of 64.8% to 66.4% Fe. The collector was amine (Arosurf MG 83 A), and the frother was 2-ethylhexanol. The ph and water chemistry were the same as were used in the flotation cells of the operating plant (ph 10.5). Table 2 Reagent dosages for two-stage column flotation experiments. Amine dosage, lb/st Test Stage 1 Stage Conventional flotation tests. Conventional flotation experiments were carried out using a laboratory-scale Wemco cell, floating magnetic concentrate. These were standardized experiments that are done routinely, using the magnetic separator concentrate as feed. These tests were carried out in multiple stages. For each stage, the procedure was as follows: Step 1: suspend pulp in cell, add reagents and condition; Step 2: float to remove silicates in the froth product; and Step 3: replace water removed and return to Step 1. Additional collector was added to each stage. Two tests were run. The first test was a four-stage test using collector at a dosage of kg/t (0.015 lb per long ton) per stage and frother at a dosage of kg/t (0.025 lb per long ton) per stage. The second test was a two-stage test adding kg/t (0.04 lb per long ton) of collector for each stage and 0.22 kg/t (0.05 lb per long ton) of frother to the first stage only. Jameson column studies. For these experiments, magnetic separator concentrate was fed to the column as slurry at 23% solids. The column was run continuously until it reached steady state. The collector dosage for all tests was kg/t (0.03 lb/st), and the frother dosage was kg/t (0.020 lb/st) for all tests except for Test 2, which used only kg/t (0.014 lb/st) frother. The settings used in the Jameson cell for each test are shown in Table 1. These operating conditions were not optimized specifically for the Jameson column and were mainly selected to be approximately comparable to the reagent dosages for the conventional flotation cells. Two-stage laboratory column studies. For these experiments, magnetic separator concentrate was fed to two columns, which were used in series as shown in Fig. 4. The columns were run continuously until they reached steady state. The laboratory column was 76 mm (3 in.) in diameter and 2.7 m (9 ft) tall, Figure 3 Effects of horizontal baffles on axial mixing and bubble size. Baffles interrupt flows along the vertical axis of the column and also break up bubbles that grow to an excessively large size. and it was equipped with air spargers. The reagent dosages used are shown in Table 2. Again, these reagent dosages were not fully optimized for the columns and were selected to be approximately comparable to the reagent dosages for the conventional flotation cells. Results and discussion The results for the Jameson Cell reverse flotation tests are shown in Table 3. From the data in Table 3 it appears that the Jameson Cell performance is very comparable to the performance of the Wemco laboratory cell. It should be noted that the Jameson Cell is primarily designed to quickly produce a high-grade froth. Because of the details of their air-injection system, single-stage Jameson Cell flotation typically has a lower recovery than other column types. Because this is a reverse-flotation application, to produce a high-grade concentrate, it is necessary to have a high recovery of the gangue minerals from the nonfloating Table 1 Test conditions for Jameson cell. The slurry orifice diameter was 6.0 mm, and the froth crowder was 152-mm (6 in.) in diameter. Downcomer Wash Froth pressure, Draft, Airflow, water flow depth, Froth Test psi in. L/min L/min in. % solids Vol. 24, No. 1 May 2007

4 Table 3 Comparison of Jameson Cell test results and conventional bench flotation with a Wemco cell for reverse flotation of silicates from magnetite. The Tail products are the silicate froths. Jameson Cell Test 4 was sampled twice during the run, providing two sets of results (4-1 and 4-2). Weight Fe Test Product Fe, % SiO 2, % recovery, % recovery, % Jameson Cell Test 1 Feed Tail Conc Jameson Cell Test 2 Feed Tail Conc Jameson Cell Test 3 Feed Tail Conc Jameson Cell Test 4-1 Feed Tail Conc Jameson Cell Test 4-2 Feed Tail Conc Four-stage Wemco flotation Feed Tail Tail Tail Tail Conc Figure 4 Two-stage configuration used for laboratory Deister columns. material. The Jameson Cell did not perform much better than a conventional flotation cell in a reverse-flotation application, although it might be possible to operate these cells at a higher throughput than conventional cells of comparable floor space and, therefore, increase plant capacity. The results for the two-stage column flotation tests are shown in Table 4. It is evident from these results that the two-stage flotation column performed much better than the two-stage Wemco flotation cell in this test series. This is a result of the ability of these columns to be operated to recover nearly all of the floatable material from the nonfloating product in very few stages of flotation. A composite graph of the grade/recovery performance of all of these experiments is shown in Fig. 5. From this graph, it can be seen that the two-stage Deister column can reach a higher recovery than either of the other two flotation options tested. However, at the recoveries that the Jameson cell can reach, the product grade from the Jameson cell appears to be slightly superior to what May 2007 Vol. 24 No. 1 64

5 Table 4 Comparison of two-stage Deister column reverse flotation of iron ore with two-stage flotation using a Wemco laboratory cell. Weight Fe Test Product Fe, % SiO 2, % recovery, % recovery, % Deister Column Test 1 Feed Tail Conc Deister Column Test 2 Feed Tail Conc Deister Column Test 3 Feed Tail Conc Deister Column Test 4 Feed Tail Conc Deister Column Test 5 Feed Tail Conc stage Wemco flotation Feed Tail Conc can be achieved using the two-stage Deister column. The laboratory tests with two-stage column flotation produced results superior to conventional flotation, but these results will not necessarily be reflected in a full-scale column. The basic problem in scale-up of laboratory column results is that as the column size increases, there are increases in both axial mixing and bubble enlargement/coalescence. If the fullscale column is expected to perform as well as the laboratory column, measures will need to be taken to minimize both of these problems. In earlier work by the authors (Kawatra and Eisele, 1993, 1994, 1995a, 1995b), horizontal baffling has been shown to be effective in improving the performance of columns that suffer from axial mixing and bubble coalescence problems. The effect of horizontal baffles on column operation is shown in Figs. 6 and 7. These experiments were conducted using coal as feedstock, and they have not yet been replicated for iron ore flotation. However, the conclusions that have been reached regarding the benefits of baffles in column scale-up are expected to remain valid for iron ore, because they are concerned with fundamental changes in the column hydrodynamics. The tests were conducted with identical feeds and operating conditions. Three tests were carried out without baffles and three tests with baffles. Each set of three tests was averaged to determine the variation from test to test. It can immediately be seen from the width of the error bars in Fig. 6 that there is much more variation between tests without baffles than with baffles. This difference is due to the baffles preventing the column operation from being disturbed either by axial mixing or by the formation of large air bubbles. The baffles also prevent feed water from being carried up into the froth. Therefore, they minimize entrainment effects. Similarly, the baffles prevent axial mixing from rapidly carrying material to the column underflow, as shown by the tracer results given in Fig. 7. By keeping material from Figure 5 Comparison of grade/recovery performance of the single-stage Jameson cell, the two-stage Deister cell and a multi-stage Wemco flotation cell. The two-stage column clearly gives highest Fe recovery, although the Jameson cell reaches a slightly higher grade at a given Fe recovery. short-circuiting to the tailings, the baffles ensure that all particles will have ample opportunity to contact air bubbles, and so will increase the recovery. Conclusions Reverse flotation of iron ore places different demands on a flotation column than do other flotation applications. High froth grade is less important than complete recovery of all floatable 65 Vol. 24, No. 1 May 2007

6 Figure 6 Effects of baffles on entrainment of feed water into the froth, as determined using dye tracer. This improvement was due to the baffles producing a more quiescent froth as a result of preventing axial mixing by coarse bubbles. Baffles also reduce the variation between tests and so make the column performance more reproducible. Figure 7 Effect of baffles on the rate of removal of material from a column, as determined using dye tracer. Baffles prevent material from being rapidly carried to the underflow by axial mixing, and so will tend to improve recovery. material from the nonfloatable product. Columns therefore perform best in this application in an arrangement that maximizes recovery, rather than one that maximizes froth grade. The Jameson Cell is primarily intended to rapidly produce a high-grade froth, and it has recovery performance that is roughly comparable to conventional flotation cells, although it does appear to require fewer stages to reach a high recovery. A two-stage column flotation process, using a standard type of flotation column, was shown to give better grade/recovery performance than up to four stages of conventional flotation. The arrangement used was designed to remove all of the floatable silicate gangue from the nonfloatable iron ore concentrate. A concern in scale-up of laboratory column results is that, during scale-up, the column height increases and the height: diameter ratio commonly decreases. These changes cause axial mixing and bubble coalescence to be much greater problems in the full-scale column than they were in the laboratory column. Horizontal baffles can be used to correct these problems, which is expected to allow a full-scale plant column to more closely approach the performance of laboratory columns. References Alford, R.A., 1992, Modeling of single flotation column stages and column circuits, International Journal of Mineral Processing, Vol. 36, pp Atkinson, B.W., Conway, C.J., and Jameson, G.J., 1995, High efficiency flotation of coarse and fine coal, Chapter 25, High Efficiency Coal Preparation, S.K. Kawatra, ed., Society for Mining, Metallurgy, and Exploration, Littleton, Colorado, pp Eisele, T.C., and Kawatra, S.K., 1995, On-Line Testing of a Horizontally-Baffled Flotation Column in an Operating Coal Cleaning Plant, Chapter 20, High Efficiency Coal Preparation, S.K. Kawatra, ed., Society for Mining, Metallurgy, and Exploration, Littleton, Colorado, pp Finch, J.A., and Dobby, G.S., 1990, Column Flotation, Pergamon Press, Oxford, England Kawatra, S.K., and Eisele, T.C., 1993, Use of horizontal baffles in column flotation, Proceedings of the XVIII International Mineral Processing Congress, Sydney, Australia, pp Kawatra, S.K., and Eisele, T.C., 1994, Use of horizontal baffles to reduce axial mixing in coal flotation columns, Proceedings of the 12th International Coal Preparation Congress, Cracow, Poland, pp Kawatra, S.K., and Eisele, T.C., 1995a, Laboratory Baffled-column flotation of mixed Lower/Middle Kittanning Seam bituminous coal, Minerals & Metallurgical Processing, Vol. 12, No. 2, pp Kawatra, S.K., and Eisele, T.C., 1995b, Baffled-column flotation of a coal plant fine-waste stream, Minerals & Metallurgical Processing, Vol. 12, No. 3, pp Kawatra, S.K., Eisele, T.C., and Johnson, H.J., 1988, Desulfurization of coal by column flotation, Chapter 7, Processing and Utilization of High Sulfur Coals II, Chugh and Caudle, eds., Elsevier, New York, pp Kennedy, A., 1990, The Jameson flotation cell, Mining Magazine, Vol. 163, No. 10, pp Manlapig, E.V., Jackson, B.R., Harbort, G.J., and Cheng, C.Y., 1993, Jameson Cell coal flotation, Proceedings of the 10th International Coal Preparation Exhibition and Conference, Kentucky, pp Smith, R.W., and Akhtar, S., 1976, Cationic flotation of oxides and silicates, A.M. Gaudin Memorial Flotation Symposium, M.C. Fuerstenau, ed., American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME), New York, New York, pp May 2007 Vol. 24 No. 1 66