Method for evaluation of upgrading by liberation and separation

Transcription

1 Method for evaluation of upgrading by liberation and separation Thomas Leißner 1, Thomas Mütze 1, Petya Atanasova 2, Kai Bachmann 2 and Urs A. Peuker 1 1. Institute of Mechanical Process Engineering and Mineral Processing, Technische Universität Bergakademie Freiberg, Germany 2. Helmholtz Institute Freiberg for Resource Technology, Germany ABSTRACT A method is presented for the evaluation of mineral processing by liberation and upgrading. The method bases on the plot of recovery of valuables versus the recovery of gangue (Fuerstenau upgrading curve). The locking curve of a feed material was plotted together with the upgrading curve in a Fuerstenau diagram. The assessment of liberation and upgrading is done by a comparison of surfaces formed by the curves and some characteristic lines of the diagram. This plot allows the calculation of two new quantitative measures for liberation and upgrading. Both parameters can be combined to a third parameter describing the quality of the whole technical setup for mineral processing by subtracting the upgrading parameter from the liberation parameter. The third parameter shows a positive value in cases of upgrading determined processes, a negative value in cases of liberation affected processes and about zero for processes affected equally by liberation and upgrading. Thus it is easily possible to distinguish between poor results in the mineral processing caused by insufficient liberation or poor results caused by insufficient upgrading. Preliminary results from two case studies, performed on two different ore types, are very promising illustrating the practical use of such an approach. 1

2 INTRODUCTION Separation processes account for the main process steps to recover valuables. They depend on upstream grinding steps (particle size and liberation), features of the minerals (separation feature), and the parameters of the separating machines. An exact evaluation of upgrading processes has to consider all those variables. Upgrading curves and parameters known from the literature (Drzymala, 27) display separability with regard to the separation feature or machine parameters without any direct consideration of the mineral liberation. Liberation results are rather presented as parameters (Bérubé and Marchand, 1984; Gaudin, 1939) or as a function of grade versus recovery (Miller et al., 29). If liberation results are to be related to upgrading curves the distribution of the separation feature (particle susceptibility, specific gravity of the particles, free surface of valuables ) has to be used to create mineralogical limiting curves (Leißner et al., 214) instead of the grade of the particles. METHODOLOGY Materials The liberation and separation characteristics of two different ores were studied. These are a greisen type ore from the Zinnwald deposit, Germany and a nepheline-syenite type ore from the Norra Kärr deposit, Sweden. The ore from the Zinnwald deposit comprises of approximately 67 % quartz, 2 % zinnwaldite, 8 % topaz, 3 % muscovite, and 2 % other minor minerals. The mineral of interest is the lithium bearing mica, zinnwaldite. The grain size of the zinnwaldite mica in the unbroken rock is about 1.2 ±.3 mm. In order to liberate the valuables comminution has to be done at least down to this size. A large sample, with the total weight of 2 tons, was obtained by local blasting operation. This sample was crushed to -5 mm particle size in the following sequence: 1. jaw crusher, 2. gyratory crusher and 3. cone crusher. This -5 mm material was ground further by three different techniques. The first grinding product was produced by a pin mill with 2 mm opening size. The second grinding product comes from a screen discharge ball mill with.9 mm screen size. The third grinding product was obtained by regrinding the coarse fraction (+ 1. mm) of the cone crusher product in a roller mill (1 mm gap). All three grinding products as well as the fines (- 1 mm) from the cone crusher were screened with.1/.2/.315/.5/.8/1./2. mm sieves. These fractions were analysed by the mineral liberation analyser (MLA) (for more information see Sandmann and Gutzmer (213)). Additionally, the pin mill fractions were separated magnetically at different field strengths using a ring-type separator and a Frantz isodynamic separator. Products obtained from these magnetic separations were analysed by ICP-OES to calculate the grades of zinnwaldite. The chemical dissolution of the ore was done with a mixture of hydrofluoric acid and nitric acid. The nepheline-syenite ore comprises of approximately 4 % feldspar, 17.5 % zeolite, 13.5 % aegirine, 9 % nepheline, 7.5 % eudialyte, 7 % leakeite, 1 % catapleiite, and 4.5 % others. The main rare earth elements (REE)-bearing mineral is eudialyte (for information on the geology and mineralogy of the Norra Kärr deposit see Sjöqvist et al. (213) and Bluemel et al. (213)). As suggested by Rudolph and Peuker (214), a suitable processing method for the recovery of REE is floatation. Preconcentration with magnetic separation could help to reduce the amount of barren particles undergoing flotation. A sample of 2

3 Valuables Recovery, R v (%) Perfect separation Valuables Recovery, R v (%) Perfect separation approximately 4 kg of ore was primarily crushed in a laboratory jaw crusher followed by a ball mill to mm particle size. In a next step a screen discharge ball mill with screens of 2./1./.5/ mm was used to grind the ore to different upper particle sizes. After grinding the material was screened to size fractions of.63/.125/.25/.5/1. mm. These size fractions were analyzed by MLA and ICP-OES. Samples of each size fraction were separated by a ring-type magnetic separator at increasing field strengths. The tailing from the separation at one field strength was used as feed for the separation at higher field strength. The products from magnetic separation were analyzed by ICP-OES to calculate the grades of the magnetic minerals (eudialyte, aegirine and leakeite). Method Using the advantages of MLA the model to assess the separation efficiency proposed by Helfricht (Helfricht, 1966) and Tolke (Tolke, 197) is further developed (Leißner et al., 213). A similar model to that of Helfricht and Tolke was developed by Mohanty (Mohanty et al., 1999). The plot of valuables recovery Rv versus gangue recovery Rg (Fuerstenau diagram) is used to compare liberation results as well as separation results on an equal basis (Figure 1). This is done by calculating parameters based on surfaces enclosed by different curves. and remixing, t(r g ) and remixing, t(r g ) A PL 2 Datenreihen4 Locking curve 2 Datenreihen3 Upgrading curve Gangue recovery, R g (%) A AL A AS Gangue recovery, R g (%) Figure 1 Fuerstenau diagram showing the theoretical limits, the locking curve and the upgrading curve Two limiting curves, r(rg) and t(rg) are of relevance. The curve r(rg) represents a perfect mixture which equals to processes with no upgrading. Products of such a process have the same composition. In the case of comminution each particle will exhibit the same composition (feed grade). Liberation would not have taken place. The curve t(rg) defines the perfect separation and remixing process. The perfect separation is characterized by fully liberated valuables, recovered into the concentrate. If % valuables are recovered then remixing of gangue particles into the concentrate takes place when the recovery of mass proceeds. The curves describing all upgrading processes are located in the triangle defined by these theoretical limits. 3

4 The liberation distribution of a particle population u(rg) (locking curve of the grinding product) represents the theoretical limit for all separation processes. A so called perfect separator (Finch and Gomez, 1989), which recovers particles with decreasing separation feature, is used to generate the locking curve as a mineralogical limit for separation. The locking curve is a function of the separation feature since the feature defines the kind of separation process. The separation feature can be replaced by the particle grade when two assumptions are valid. First, the ore can be reduced to a binary system because of a big difference in the separation feature of valuables and gangue minerals. Second, the separation feature is a function of the volumetric grade. If this is not applicable, the separation feature has to be used to generate the mineralogical limiting curve. The result of separation tests with varying parameters forms the upgrading curve s(rg). This curve can be used together with the mineralogical limiting curve to assess the upgrading as a combination of liberation and separation. Two parameters have to be calculated for this purpose. The liberation parameter Blib is the ratio of the surface representing the liberation achieved AAL and the surface representing perfect liberation APL (Figure 1, right). u(rg) drg 5 AAL B lib % A 5 % (1) PL The separation parameter Bsep is the ratio of the surface representing the separation achieved AAS and the surface representing the perfect separation which depends on the liberation achieved. Henceforth Blib will be referred to as the ratio of liberation and Bsep will be referred to as the ratio of separation. s(rg) drg 5 AAS B sep % % (2) AAL u(rg) drg 5 The upgrading parameter B based on the definition of Helfricht (Helfricht, 1966) equals the product of the ratio of liberation and the ratio of separation. AAS B % Blib Bsep (3) A PL A process parameter Z can be calculated based on the two ratios introduced above. This parameter describes whether the process is by insufficient liberation (Z < ) or affected by insufficient separation (Z > ). Z B B % (4) lib sep 4

5 Grade / % Ratio of Liberation / % RESULTS AND DISCUSSION Greisen-type ore Liberation The diagram in Figure 2b illustrates the ratio of liberation for different particle sizes. Grinding in a pin mill (impact grinding) liberates valuables more selective than using the other machines. The comminution in the roller mill and in the cone crusher showed similar results, as they both work primarily through compressive stresses. Liberation by grinding in the ball mill is more or less in the same range as in the cone crusher and in the roller mill but seems to be less selective with coarse particle sizes. A combination of the fines from the cone crusher and regrinding of the coarse particles in the roller mill is less efficient than the grinding in the pin mill but could have economic advantages due to the abrasive character of the ore (67 % quartz). (a) (b) Cone Crusher Roller Mill Ball Mill Pin Mill Feed Figure 2 Zinnwaldite grade (a) and ratio of liberation (b) versus particle size Observed enrichment of valuables in the fines of the cone crusher (Figure 2a) and a higher ratio of liberation in the coarsest fraction of the roller mill can be explained by special effects due to the grain shape of mica and by the experimental setup (-1 mm fraction of the -5 mm product from cone crusher). Assuming the very low mass flow during all tests, particle-particle contacts are avoided. For this reason big mica plates (+1 mm) pass through the gap of the rolls without being stressed. Isometric particles (quartz, topaz) however are broken to a size smaller than the gap. Separation Separation results from the isodynamic and the ring-type separator as well as the mineralogical limiting curve for the size fraction.1.2 mm are visualized in Figure 3. Both separators show equal results as zinnwaldite is easy to separate by strong magnetic fields. This is due to the magnetic properties of the other minerals associated with zinnwaldite. Quartz and topaz do not respond to the magnetic field. 5

6 Ratio / % Ratio / % Grade / % Valuables recovery / % Muscovite is less paramagnetic than zinnwaldite and accounts for only 3 % of the ore. Observed lower grades at small recoveries can be addressed to limonite containing particles. Limonite is a strongly paramagnetic mineral which results in high particle susceptibilities of low grade particles comprising limonite limiting Liberationcurve Isodynamic Separation (Isodynamic) Seperator Ring-Type Separation (Ring-Type) Separator 4 2 limiting Liberation curve Isodynamic Separation (Isodynamic) Seperator Ring-Type Separation (Ring-Type) Separator (a) Valuables recovery / % (b) Gangue recovery / % Figure 3 Halbich (a) and Fuerstenau (b) diagram for the magnetic separation of the pin mill product (size fraction.1.2 mm) Liberation Separation (Ring-Type) Upgrading (Ring-Type) 7 6 Liberation Separation (Isodynamic) Upgrading (Isodynamic) (a) (b) Figure 4 Evolution of the ratio of liberation, the ratio of separation and the upgrading parameter versus particle size (pin mill product, (a) ring-type magnetic separator and (b) isodynamic magnetic separator) The parameters B, Blib and Bsep show good results over a wide size range (Figure 4a and 4b). The upgrading works best in a size range of.1 to.5 mm. Small particles (-.1 mm) are well liberated but are difficult to enrich by dry separation technologies because of adhesion forces between particles or between particles and the machinery. The ratio of liberation slightly decreases with increasing particle size, the ratio of separation is nearly constant. This can be addressed to the good liberation of coarse particles. The coarse grain size and the relatively strong susceptibility of zinnwaldite in relation to the gangue minerals 6

7 Cummulative passing / % Magnetic grade / % Ratio of liberation / % allows for an efficient separation. The ratio of liberation exceeds the ratio of separation in all size classes except of the coarse one (ring-type separator, Figure 4a). Thus the calculation of the parameter Z would result in a negative value for the coarse particles and switch to positive values for fine particles. Therefore a processing of the greisen-type ore can be improved by grinding to sizes smaller than.8 mm and improving the separation of fine particles. The amount of fines smaller than.1 mm will have a more important influence on the overall result of the upgrading than the liberation of particles in the coarse grain sizes. Optimization has to focus on the reduction of the amount of fines produced while grinding. Nepheline-syenite ore Liberation The grinding of this ore in a ball mill at different screen sizes shows equal results in the evolution of liberation with particle size (Figure 5b). This result was expected as only the fineness of the grinding product (Figure 5a) was varied through changes in the screen size but no variation of the comminution principle (ball milling) took place. An enrichment of magnetic particles with decreasing particle size can be seen in Figure 6a. This effect just makes up a few percent and cannot really be addressed to selective comminution. Finer grinding products show decreasing grades of the fine fractions due to the amount of lower grade particles milled to these fractions. As it was described before the ore is fine grained. Thus high ratios of liberation can be achieved only with particle sizes smaller than.25 mm (a) mm 1. mm.5 mm PSD Grade (b) mm 1. mm.5 mm Figure 5 Particle size distribution together with grade versus particle size (a) and ratio of liberation versus particle size (b) of the products from ball milling with 2./1./.5 mm screen size Separation The upgrading by magnetic separation worked well for particles smaller than.25 mm (Figure 6a). Similar to the results of the greisen-type ore, the ratio of liberation exceeds the ratio of separation in all size classes. Therefore the upgrading can be matched to mainly affected by the separation step. The ratio of separation increases strongly for particles smaller than.5 mm. For the fine fraction ( mm) the ratio of separation is the best. Due to strong interactions of very fine particles (as described 7

8 Ratio / % Grade of fractions / % above), the size fractions -.63 mm were not tested. With increasing particle interactions the selectivity of the separation decreases thus decreasing the ratio of separation as well. The ratio of liberation increases more strongly than the ratio of separation below.25 mm. This trend can be addressed to two effects. The first one is the calculation of the mineralogical limiting curves which was done using the magnetic grade of the particles instead of the particle susceptibility. Failures can be caused by minerals of small content in the ore but with a magnetic susceptibility in the range of the valuables. If these minerals are missing in the calculation of the magnetic grade the gangue recovery will increase much faster due to the amount of magnetic minerals not classified as valuable. The second effect is the difference in susceptibility of valuables and gangue minerals. If the difference is small, poor separation will result even at good liberation Liberation 2 Separation Upgrading Aegirine Eudialyte (a) Figure 6 Evolution of the ratio of liberation, the ratio of separation and the upgrading parameter versus particle size (a) and grades of the fractions of magnetic separation for the size fraction mm (b) (b) For particle sizes between.125 to.5 mm the ratio of separation and the ratio of liberation show similar values. In that size range liberation and separation have the same influence on the upgrading. The upgrading parameter increases strongly with degreasing particle size. As the magnetic separation is considered as the process of preconcentration of valuables before flotation, the upgrading can be archived only with particles smaller than.5 mm. In most cases sufficient information about the magnetic susceptibility of the different minerals of the ore is missing. Therefore the grade of magnetic minerals is the only way to calculate limiting curves. Limiting curves calculated based on the separation feature will show a trend below the one based on the grades (Leißner et al., 214). This can be addressed to the non-binary character of ores comprising of more than two minerals. The calculation based on the separation feature leads to a reduced ratio of liberation combined with an increased ratio of separation. Investigations on the magnetic susceptibility of minerals are required to enable the calculation of limiting curves using the proper separation feature. Grades of aegirine and eudialyte (Figure 6b) indicate that aegirine is the mineral having the highest susceptibility. Aegirine containing particles are recovered into the concentrate at low magnetic field strengths. The grades of eudialyte show enrichment at intermediate magnetic field strength. This can be 8

9 due to the particle composition, the eudialyte susceptibility or to variations in the content of magnetic elements in the minerals. CONCLUSION Two new parameters were introduced to assess upgrading with respect to liberation (ratio of liberation) and separation (ratio of separation). The use of the parameters was proven on results of lab scale test work on the processing of a greisen-type ore and a nepheline-syenite type ore. Limiting curves have to be calculated based on the particle feature used to separate the valuables. If these particle features are unknown grades can be used instead with failures in accuracy. The upgrading of the ores showed good results at high ratios of separation. Particle sizes have to be coarse enough to overcome particle interactions. The upgrading designed as a combination of grinding and dry magnetic separation can be matched to mainly affected by separation for both ores. The optimization of the process designed as a combination of comminution and dry magnetic separation has to focus on improving the separation of coarse particles and reducing the amount of fines produced while grinding. It has to be mentioned that different upgrading curves can generate similar results in the calculated surfaces. This does not affect the proposed model. The model is to visualize the upgrading results as a function of particle size and their dependency to the liberation distribution as well as the performance of the separator. The surfaces are used to get integral parameters which are not dependent to any operating point. The model is useful to find the particle size range of the feed suitable for the favoured separation process. An operating point has to be found on non-classified material. ACKNOWLEDGEMENTS Research for this study was mainly performed at the research group of Mineral Processing, Institute of Mechanical Process Engineering and Mineral Processing, TU Bergakademie Freiberg. It was supported and benefited from numerous discussions by Thomas Zschoge and Albrecht Tolke. Furthermore, the authors are particularly indebted to Tasman Metals Ltd. for providing rock samples and useful information about the Norra Kärr deposit. The parts of this study dealing with the ore from the Zinnwald deposit have been funded by the Federal Ministry of Education and Research in the project Hybride Lithiumgewinnung (3WKP18A). REFERENCES Bérubé, M.A., Marchand, J.C., Evolution in the mineral liberation characteristics of an iron ore undergoing grinding. International Journal of Mineral Processing, 1984, 13, Bluemel, B., Leijd, M., Dunn, C., Hart, C.J.R., Saxon, M., Sadeghi, M., Biogeochemical expression of rare earth element and zirconium mineralization at Norra Kärr, Southern Sweden. Journal of Geochemical Exploration, 213, 133, Drzymala, J., Mineral Processing, Foundations of theory and practice of minerallurgy. 27, Oficyna Wydawnicza PWr. 9

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