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Assessment of alternative iron sources in the pressure leaching of zinc concentrates using a reactor model
hydrometallurgy
i
ELSEVIER
Hydrometallurgy 39 (1995) 147-162
Assessment of alternative iron sources in the pressure leaching of zinc concentrates using a reactor model Susan A. Baldwin, George P. Demopoulos * Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada Received 21 April 1995; accepted 8 June 1995
Abstract A mathematical model is used to simulate pressure leaching of six different zinc concentrate feeds: low pyrite, high pyrite, lead-zinc concentrate blend, Red Dog concentrate, Red Dog concentrate-zinc ferrite blend and Red Dog concentrate with hot acid leach liquor. In particular, the effect of iron content of the feeds on predicted performance for a specific autoclave configuration was investigated. For the Red Dog concentrate, which does not contain enough iron for successful zinc pressure leaching, two alternative iron sources were investigated. In the first case, zinc ferrite was blended with the concentrate feed. In the second case, aqueous solution, containing ferrous and ferric sulphate, from the hot acid leaching of zinc ferrite was added to the autoclave. Simulation results are presented in terms of autogenous operation, minerals conversions, aqueous solution compositions, oxygen consumption and jarosite production.
1. Introduction A comprehensive mathematical model and computer simulation package has been developed for pressure leaching, with oxygen, of sulphide minerals [ 1 ]. In this paper, we describe the use of such a tool in predicting the performance of the pressure leaching process for a variety of zinc concentrates and blends. As "ideal" zinc concentrates become more scarce, the zinc industry is looking at the feasibility of processing other than ideal concentrates and even concentrate blends using pressure leaching technology. Of particular importance in the leaching of sphalerite (ZnS) is the iron content of the feed. Iron is required for the production of ferric ion which oxidizes the zinc sulphide. Oxidation of zinc sulphide directly * Corresponding author.
0304-386X/95/$09,50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 3 8 6 X ( 95 )00027-5
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by oxygen is very slow. If the iron is incorporated in the zinc sulphide mineral matrix (marmatite: Zn~l _x)Fe~x)S)) then the leaching rate of the mineral is also significantly enhanced [ 2]. However, if present as a separate mineral, then the behaviour of that mineral during leaching greatly influences the effectiveness of the pressure leaching process for zinc concentrates. For example, pyrrhotite oxidizes quite readily and concentrates where the iron is present mostly as pyrrhotite are expected to leach easily. However, pyrite-containing concentrates are anticipated to be problematic, not only because of the refractory nature of pyrite, but also because oxidation of pyrite produces acid and is also very exothermic. In this paper, we have studied a number of scenarios where iron in the feed to the autoclave is present in very different forms. Firstly, we examined three different concentrates: a low pyrite, a high pyrite and a bulk concentrate [ 3 ] with much less marmatite and very significant amounts of pyrite and galena. Then, we investigated the Red Dog concentrate, which is being mined by Cominco Ltd. at their mine in Alaska [4] and contains very little iron. We first used the model to see what was predicted for leaching of Red Dog concentrate without an additional iron source. We then looked at possibilities for adding iron as (a) zinc ferrite or as (b) ferrous and ferric iron in solution from the hot acid leaching of ferfites. Model predictions for each of the six scenarios are presented and the predicted autoclave performance is discussed in terms of conversion, autothermal operation, acid consumption, oxygen consumption, jarosite precipitation and so on. But first, we give a brief outline of the pressure leaching model and describe use of the computer simulation program.
2. Background 2.1. Model description
A detailed description of the mathematical model on which the pressure leaching simulation program is based can be found in a separate publication [ 1 ]. The essential features are summarized below: 1. The model is based on the intrinsic kinetics for each reaction, which have been gleaned from experimental studies reported in the literature. For the mineral dissolution reactions, the kinetics are in the form of a particle shrinkage rate, in/zm min- 1, which is a function of the concentration of an aqueous oxidant species, like 02 or Fe 3÷, for example. 2. The autoclave is modelled as a series of perfectly mixed reactors. 3. Steady state material and energy balances are solved for each component in all three phases; gas, aqueous and solid. The population balance approach is used for the material balances for the minerals. 4. Iron precipitation is predicted by introducing an apparent equilibrium restriction on the ratio of ferric and acid concentrations. 5. The dependence of aqueous solution properties, such as density, vapour pressure, heat capacity and oxygen solubility on temperature and solution composition is taken into account. Highlights of this zinc pressure leaching model are also given in a recent publication [ 5 ] together with a description of another mathematical model for hot-acid-leaching (HAL) of zinc ferrites.
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149
2.2. Use of the simulation program
In this section, we give examples of the data required by the simulation program. We also describe how the program was used to predict the performance of a typical autoclave, based on the dimensions of the zinc pressure leaching autoclave at Cominco, in Trail, British Columbia, for a variety of feeds. Mineral data
The pressure leach simulation requires a data base of minerals with their thermodynamic and physical properties, as well as the stoichiometry and kinetics for the mineral oxidation reactions. For the concentrates studied in this work, data were required for the following minerals; marmatite (Zn(Fe)S), pyrrhotite (FeS), galena (PbS), pyrite (FeS2), chalcopyrite (CuFeS2) and zinc ferrite (ZnFe204). These are presented for the first four minerals in the previous publication [ 1]. Kinetic data for the leaching of chalcopyrite were obtained from an experimental study in the literature [6]. For the temperatures of interest in zinc pressure leaching (140--150°C) chalcopyrite is assumed to dissolve with the following stoichiometry to form mostly elemental sulphur. CuFeS2(s) + 2H2SO4(aq.) + O2(aq.) ~ CuSO4(aCl.) "~FeSO4(aq.)
( 1)
+ 2H20(aq.) + 2So) The rate of leaching was found to follow the, so-called, shrinking core model, 1 - ( 1 - x ) 1 / 3 =kl t
(2)
ro where x is the conversion, k~the particle shrinkage rate, in ~m min- 1, ro the initial particle size, in/zm and t the time, in min. The particle shrinkage rate was found to be a non-linear function of oxygen partial pressure, Po2; kl----APoJ( 1 +BPo~), where both A and B are temperature dependent constants. However, B is only significant for temperatures above 160°C and for our purposes we assumed the particle shrinkage rate to be a linear function of oxygen partial pressure. A depends on temperature in the well known Arrhenius function; kl = 9.39 × 1016po2exp ( - 151"0 kJ m ° l - ~) RT ixm min- 1
(3)
Po2 is the oxygen pressure in solution in atmospheres. This is, however, an oversimplification of the overall mechanism. The unrealistically high activation energy predicts a very strong temperature dependence, thus it should be emphasized that this equation is not appropriate for high temperatures, above 160°C. Other data required for chalcopyrite and its dissolution are presented in Table 1. As will be seen later, we have considered the possibility of blending zinc ferrite (ZnFe204) with Red Dog zinc concentrate so as to increase the iron content of the feed to the autoclave. From a recent study [8] it was observed that zinc ferrite dissolution is somewhat different when compared to the other sulphide minerals. Zinc ferrite particles are composed of equal-sized, sub-micron grains. The dissolution rate depends on the initial size of these grains and not the apparent particle size. It is important, then, for the simulation to
S.A. Baldwin, G.P. Demopoulos / HydroraetaUurgy 39 (1995)147-162
150 Table 1 Chalcopyrite data Heat capacity a Heat of reaction a
Cp = 87.0 + 0.0536 x T - 5.593 × 105 X T - 2 j mol- 1KzlH~x. = 710.45- 1.873 x ( T - 298) +0.0016X (T 2 - 2982) - 1.193 X 105 × ( 1 / T - 1/298) J mo1-1 Molecular weight MW= 183.1 g g-mole-t Density p = 5000.0 g l Volume shape factor b kv = 0.524 a
FromHSC [7].
b Volume of particle = kvL3/zm 3, where L is the particle characteristic length, in izm, in this case the diameter.
enter the grain size for the particle size distribution data. In addition, the rate depends on the activity of proton, which is a function of the solution composition. A correlation for proton activity as a function of total zinc sulphate, ferrous sulphate, ferric sulphate, sulphuric acid and temperature is given by Filippou [9, p. 156] was used in the present simulations. Zinc ferrite dissolution has the following stoichiometry: ZnFe204(s~ +
4H2SO4(aq.) ~ ZnSO4(aq.) + Fe2(SO4)3(aq.) + 4 H 2 0
(4)
And the grain shrinkage rate is given by { - 63.6 kJ mol- l) [ O/H+]0.57/zm min- 1 kI = 1.859 X 106exp k RT
(5)
where, OtH+is the proton activity. Thermodynamic and kinetic data are given in Table 2.
Reactor settings For this study, of the effect of different iron sources on the performance of the zinc pressure leaching process, we used the Cominco autoclave dimensions for our reactor settings. These are given in Table 3. The autoclave, shown in Fig. 1, has four compartments, the first being a little larger than the rest. The concentrate feed rate to the autoclave was kept constant at 240 kg min- 1 for all case studies. For temperature and acid control aqueous solution can be added to any compartment, where necessary. This was primarily recycled electrolyte, the composition of which is given in Table 3. In some cases, aqueous solution was required for cooling purposes but no more acid was needed. Here, water was blended with the recycled electrolyte. In one of the case studies, hot acid leach liquor was added together with recycled electrolyte so as to provide an iron source. The composition of the hot acid leach liquor was obtained from Filippou [9] and is presented in Table 3. Table 2 Zinc ferrite data Heat capacity Heat of reaction
Cp = 155.4 + 44.56 X 1 0 - 3 T - 15.98 × 105 X T-2 j mol- ~K- 1 AHr,, = --215.74 X 103--307.3 X (T--298) --261.2 X 10-3X (T2-2982) - 4 9 . 4 2 × 105X ( 1 / T - 1/298) J tool -1 Molecular weight MW= 241.0 g g-moleDensity p = 5330.0 g 1Volume shape factor kv = 0.524
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151
Table 3 Autoclave and feed settings
Autoclave Number of compartments Volume of compartment 1 Volume of compartment 2 Volume of compartment 3 Volume of compartment 4 Total pressure Oxygen purity
4 22.4 m3 17.4 m 3 17.2 m 3 17.4 m 3 12.5 atm 98.5%
Concentrate feed Flow rate of feed solids Feed slurry density Feed slurry temperature
240.0 kg min70.0% wt. 15°C
Aqueous feed streams
Concentrations (g 1- ' )
Fe: + Fe s + Zn 2+ H2SO4
Recycled electrolyte
Hot acid leach liquor
0.0 0.0 50.0 161.0
0.8 21.3 87.6 87.0
Concentrate compositions Four typical zinc concentrates and a zinc-lead bulk concentrate were studied and their mineralogies are presented in Table 4. The mineralogical compositions for the first three were obtained from an article by Chalkley et al. [ 3 ]. In the same article it was mentioned A Q U E O U S FEEDS
GAS VENT ZINC CONCENTRATE FEED
li
DISCHARGE
OXYGEN Fig. 1. Schematic of the autoclave configuration.
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that treatment of these three concentrates by pressure leaching has been demonstrated successfully. Those results are not, however, available in the literature. It would be very interesting to see whether the predictions reported in the present work match the findings of that study. For the low pyrite and high pyrite concentrates, Chalkley et al. report that pyrrhotite is not present as a separate mineral but is substituted for zinc in sphalerite. This gives marmatite minerals with 10.4% wt and 7.7% wt Fe, respectively. The kinetics for dissolution of marmatite, which is a function of the iron content, were determined from a correlation of the data in an experimental study by Palencia Perez and Dutrizac [2], presented in Baldwin et al. [ 1 ]. For the bulk concentrate, we assumed that low pyrite concentrate (10.4% wt Fe marmatite) had been blended with lead concentrate, which contained some pyrrhotite. The Red Dog mineralogical composition was obtained from Ashman and Jankola [4]. There, also assuming all FeS to be incorporated in marmatite, the iron content of the marmatite was 6.4% wt. For the Red Dog concentrate-zinc ferrite blend, the amount of zinc ferrite was determined as that required to bring the total iron content of the concentrate up to 10% wt Fe, the same iron content as the low pyrite feed.
Running simulations In this work, the objective was to predict the performance of pressure leaching of the concentrates given in Table 4. The strategy that was adopted was to control the temperatures and acid concentrations by adding aqueous solution. The acid concentration was maintained at 35 g 1-1 in the first two compartments, where most acid is required, by adding the appropriate amount of recycled electrolyte. In industrial zinc pressure leaching the temperature is maintained in the range 140--150°C [3]. For the first two compartments, a target temperature of 140°C was chosen. In most cases, in these two compartments, a large quantity of recycled electrolyte is required so as to provide sufficient acid. This has a cooling effect such that preheat of the aqueous feed is required so that the temperature in the compartment does not drop below 140°C. For optimum kinetics, it would be better to operate at as high a temperature as possible in all compartments. To achieve this, a more concentrated acid solution would have to be added, say by using plant acid instead of recycled electrolyte. This may not, however, be the best solution with respect to plant optimization. It is better to aim to make use of recycled streams rather than outside, and thus more expensive, reagents. Hence, we decided to use recycled electrolyte as the acid source and settle for the Table 4 Mineralogical composition of concentrates. Values are percent weight Mineral
Low pyrite
High pyrite
Bulk
Red Dog
Red Dog with ferrite
ZnS a FeS a FeS PbS CuFeS2 FeS2 ZnFe204 Reference
77.5 14.6 0.0 2.3 0.6 1.1 0.0 [ 31
77.5 9.6 0.0 2.3 0.6 7.9 0.0 [3 ]
47.7 6.1 7.0.0.0 17.3 1.4 20.0 0.0 [3 ]
87.1 6.3 0.0 1.2 0.0 0.0 0.0 [4]
74.7 5.4
a In marmatite.
1.0 0.0 0.0 14.2
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153
lower temperature of 140°C in the first two compartments. For the higher pyrite-containing feeds where cooling was required without further acid, water was added. The target temperature of 140°C was maintained for consistent comparison with the other concentrates. The temperature in the two final compartments were kept below 150°C by adding cooling water at 30°C, where necessary. Oxygen addition was determined as that required to maintain 95% purity oxygen in the dry off gas. This results in an oxygen utilization of 70%.
3. Results and discussion
3.1. Low pyrite concentrate This concentrate can probably be considered as t h e " ideal" zinc concentrate, in that there is sufficient iron ( = 10%) for good kinetics, most of which is incorporated in marmatite and leaches easily. Too much iron is undesirable because it then needs to be removed, usually as jarosite, which has to be ponded. Also, sulphate production should be minimized, since excess sulphate has to be removed with line addition. Pyrite, which oxidizes to sulphate and higher temperatures which promote sulphur oxidation to sulphate are, therefore, not desirable. The simulation results for the low pyrite concentrate are presented in Table 5. The temperature profile across the autoclave is from 140 to 150°C, and the acid concentration remains at = 35 ___2 g 1- ~. Most acid is required in the first compartment, 92 m3h - ~recycled electrolyte. In the second compartment, a blend of 13 m3h - 1 recycled electrolyte and 7 m3h- ~water was required. The water was needed for cooling the extra heat generated from pyrite dissolution, the extent of which was very small in the first compartment, but more significant, 44%, in the second. The final conversions of all minerals are very high. We wonder about the accuracy of the conversion predicted for pyrite, since practical experience has indicated that pyrite does not leach well. We expect conversions of about 55-60% [ 3 ]. One reason for this discrepancy could be because the kinetics presently used for pyrite do not take into account any galvanic effects between pyrite and the other sulphide minerals. Galvanic interactions have been postulated as being responsible for the retardation of pyrite dissolution in the presence of other sulphides, whose rates of dissolution are enhanced [ 10]. Once more appropriate experimental data are obtained for pyrite, a better kinetic expression can be input by the user into the model.
3.2. High pyrite concentrate Results for the high pyrite concentrate simulations are presented in Table 6. As expected, more heat is released due to pyrite oxidation and cooling water had to be added to compartments 3 and 4 to keep the temperature below 150°C. Acid concentration remains more or less constant at 35_+2 g 1-~ but Zn 2+ concentration is lower due to dilution by the cooling water. Also, as a consequence of adding cooling water, the residence time for the autoclave is somewhat shorter, 36 min versus 38 min for the low pyrite feed. Thus, minerals conversions are compromised but not significantly.
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S.A. Baldwin, G.P. Demopoulos l Hydrometallurgy 39 (1995) 147-162
Table 5 Simulation results: low pyrite concentrate Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2+ (gl -~) Fe3+ (gl -I ) Zn2+ (g 1-1 ) Cu2+ (g 1-1) H2SO4 (g 1-t) Flow ( m3h- 1)
Compartments 2
75.0 0.0 0.0 50.0 0.0 160.0 92.0
1
30.0 0.0 0.0 32.8 0.0 105.0 20.0
2
140 5.8 2.5 102.7 0.2 36.3 104.0
140 2.7 4.3 100.3 0.2 34.0 124.6
3
4
149 1.1 4.4 101.4 0.23 35.5 126.0
152 0.5 4.5 101.5 0.23 36.3 126.4
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
Zn(Fe)S PbS CuFeS2 FeS2 Jarosite (kg h- l)
0.770 0.638 0.043 0.123 0.0
2
3
4
0.935 0.864 0.674 0.563 1156.1
0.986 0.964 0.948 0.854 2085.6
0.997 0.991 0.979 0.938 2377.6
876.4 831.1
790.0 266.9
757.3 68.5
Oxygen usage
Po2 (kPa) Po2 (kg h- ~)
877,6 2564.1
In addition, less jarosite is produced in the high pyrite case, 1859.4 kg m i n - ~ versus 2377.6 kg m i n - 1 for the low pyrite case, despite the fact that both concentrates contain 10% wt Fe. But, iron is present in different forms in the two concentrates, mostly as marmatite in the low pyrite case and as marmatite and pyrite in the high pyrite case. According to the model predictions, the slow release o f iron from pyrite and the slower ferrous to ferric oxidation kinetics (compare [Fe 2÷ ] in Table 5 and Table 6) contributed to a lower yield o f j a r o s i t e in the autoclave. 3.3. Bulk concentrate
Simulation results for the bulk concentrate feed are given in Table 7. As expected, less preheat is required for autogenous operation o f the first compartment. For the subsequent compartments, quite a large quantity o f cooling water is needed, 124.0 m3h - 1 in total, to control the temperature. In the second compartment a mixture of 10 m3h - 1 recycled electrolyte and 70 m 3 h - 1 cooling water was added. With such high aqueous solution addition the residence time for the autoclave was only 31 rain. As a result the conversions are lower, particularly for pyrrhotite and g~ilena. Still the pressure leaching process is successful for
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155
Table 6 Simulation results: high pyrite concentrate Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fez+ (g 1-~) Fe3+ (g1-1 ) Zn2+ (g 1-1) Cu2+ (g1-1) H~SO4(g 1- l) Flow (m3h-~)
Compartments
2
3
63.8 0.0 0.0 50.0 0.0 160.0 78.5
30.0 0.0 0.0 25.0 0.0 80.0 45.0
4
30.0 0.0 0.0 0.0 0.0 0.0 12.5
30.0 0.0 0.0 0.0 0.0 0.0 5.0
1
2
3
4
140 5.4 1.8 105.4 0.02 33.3 90.6
140 3.1 4.4 88.4 0.15 35.0 136.9
150 1.5 4.5 82.3 0.21 36.0 151.1
150 0.8 4.6 80.3 0.21 36.0 156.2
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
Zn(Fe)S CuFeS2 FeSz Jarosite (kg h ~
2
0.740 0.048 0.14 0.0
0.891 0.527 0.404 256.0
3
4
0.970 0.945 0.816 1492.7
0.991 0.978 0.928 1859.4
778.0 694.6
777.8 184.5
Oxygen usage
Po2 (kPa) Po2 (kg h- ~)
876.9 2483.6
875.0 1389.6
treating this kind of concentrate blend, according to the model predictions, since the conversions are above 95%, not including pyrite. We are not certain of the predicted pyrite conversions, as mentioned previously. Large amounts of jarosite are produced since the bulk concentrate contains approximately twice as much iron, 18% wt, as the low and high pyrite feeds, which both have 10% wt iron. Pyrite conversion is approximately the same, but twice as much iron goes into solution, hence the higher amount ofjarosite production. In conclusion, from these predictions, despite the expected difficulties with processing high pyrite feeds; low conversions, high acid consumption and higher exothermic heat of reaction, the pressure leaching process is still very successful for processing these alternative zinc concentrates. It appears as if there are enough checks and balances in the process to prevent high pyrite feeds from being problematic. With very high pyrite containing feeds, like the bulk concentrate, much cooling water is required, which reduces the residence time. But, even with a residence time of only 30 rain sufficient conversion of the sulphide minerals results. The significant difference in processing these high pyrite concentrates, is that much higher aqueous solution flow rates result, increasing from 126 to 225 m3h- 1, approximately 80% more. Thus, although total iron and acid concentrations in the exit are approximately the same, there is still much more acid to be neutralized and iron to be precipitated and disposed. In addition, the zinc concentration becomes quite low, just 37 g i - ~ for the bulk
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Table 7 Simulationresults: bulkconcentrate Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2+ (g 1-1 ) Fe3+ (g 1-t) Zn2+ (g 1-1) Cu2~- (g 1-i) H2SO4 (g 1-1) Flow (m3h- 1)
Compartments
2
48.1 0.0 0.0 50.0 0.0 160.0 70.0
3
30.0 0.0 0.0 6.3 0.0 20.0 80.0
4
30.0 0.0 0.0 0.0 0.0 0.0 40.0
30.0 0.0 0.0 0.0 0.0 0.0 14.0
1
2
3
4
140 4.8 4.5 88.3 0.02 36.0 85.0
140 3.0 4.4 50.8 0.27 35.0 167.2
150 1.9 4.6 40.8 0.32 37.0 211.1
150 1.1 4.7 37.5 0.31 37.5 225.6
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
ZnS FeS PbS CuFeS2 FeS2 Jarosi~ (kg h -l)
0.837 0.684 0.684 0.022 0.126 584.1
2
3
4
0.906 0.746 0.746 0.294 0.147 1830.1
0.965 0.883 0.883 0.947 0.763 3135.3
0.988 0.954 0.954 0.984 0.934 3767.3
873.3 2161.4
774.6 1568.0
Oxygen usage
Po2 (kPa) Poe (kg h -l)
876.3 2504.4
774.4 381.8
concentrate. This means that a dilute product is being produced, and downstream processing will be more expensive. This is, of course, only a tentative conclusion based on performed simulations. Further optimization of pressure leaching of high pyrite zinc concentrates can be done by considering alternative autoclave designs and using the model to simulate the effect on the process.
3.4. Red Dog concentrate The Red Dog concentrate contains much less iron than the above concentrates, just 4% wt. In Table 8 we present simulation results for the first compartment only. This is because it was not possible to solve the material balance equations for the following compartments because the ferric concentration was too small. Even in the first compartment, there is only 0.3 g 1- ~ ferric, and the zinc sulphide conversions are quite low, 59% as compared with 79% conversion for the low pyrite concentrate feed. Thus, it is clearly necessary, for such a low iron content feed, to add an additional iron source. We investigated two possibilities: ( 1) adding zinc ferrite blended with the Red Dog concentrate to give the same feed rate to
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157
the autoclave of 240 kg m i n - t but with 10% wt iron, and (2) adding ferrous and ferric in solution from the zinc ferrite hot acid leach circuit. The results for the simulation with Red Dog concentrate-zinc ferrite blend are given in Table 9. They are quite similar to the results for the low pyrite feed in that the residence time is approximately 38 min. We were unable to find a steady state in the first compartment where both the temperature and acid concentrations were 140°C and 35 g 1- ~ respectively, thus the acid remains quite low,27 g 1-1 in the first compartment increasing to 34 g 1- ~ in the last compartment. The Zn 2 + concentration is higher ( 110 g 1- ~) than in the low pyrite case ( 100 g 1- ~) due to the extra zinc leached from the zinc ferrite. Zinc ferrite conversions are quite low, 42.5%, in the first compartment and approximately 11% remains undissolved at the autoclave exit. As has been reported previously [8], the kinetics of zinc ferrite leaching are adversely affected by the build-up of its products, ZnSO4 and Fe2SO4, which explains its incomplete conversion. Preheat of the aqueous feed was required in both the first and second compartments for autogenous operation at 140°C. These results can be compared with the simulation with hot acid leach liquor added to the first compartment instead of zinc ferrite. From Table 10, one can see that the model predicts more or less the same autoclave performance in terms of marmatite conversion and final solution composition, except for Zn 2 +, as for the case where zinc ferrite is fed directly. More aqueous feed is required for the Red Dog concentrate with hot acid leach liquor. The feed to the first compartment consists of 39 m3h- 1 of hot acid leach liquor with 87 g 1Table 8 Simulation results: Red Dog concentrate Temperatures and aqueous compositions
Temperature (°C) Fe2+ (gl -j ) Fe3+ (gl -~ ) Zn2+ (g 1- i) Cu2+ (gl -~ ) H2SO4 (g 1-1) Flow (rn3h- ~)
Aqueous feed
Compartments
1
1
78.0 0.0 0.0 50.0 0.0 160.0 69.7
Mineral conversions and jarosite precipitation (overall)
Minerals ZnS FeS PbS Jarosite (kg h- ~)
Compartment 1 0.591 0.734 0.734 0.00
Oxygen usage
Po2 (kPa) Po2 (kg h -~ )
876.7 1921
140 4.9 0.3 105.8 0.0 35.0 80.2
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Table 9 Simulationresults: Red Dog concentratewith zinc ferrite Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2÷ (g 1-j) Fe3÷ (gl -j) Zn2÷ (g 1- J) H2SO 4 (g 1- J) Flow (m3h- 1)
Compartments 2
80.0 0.0 0.0 50.0 160.0 82.0
1
65.0 0.0 0.0 50.0 160.0 20.0
2
140 4.8 3.1 108.2 27.1 93.1
140 2.3 4.3 108.5 34.0 113.8
3
4
146 1.0 4.2 110.7 33.8 114.8
148 0.5 4.3 111.5 34.0 115.1
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
Zn(Fe) S PbS ZnFe204 Jarosite (kg h J)
0.785 0.689 0.425 0.0
2
3
4
0.936 0.890 0.567 951.9
0.985 0.971 0.772 1976.4
0.997 0.992 0.894 2454.5
Oxygen usage Poe (kPa) Po2 (kg h-I)
875.6 2140.8
876.9 876.9
821.1 821.1
800.9 800.9
acid and 68 m3h - ~ of recycled electrolyte with 160.0 g 1-1 of acid. The total H2SO 4 requirement, in the first compartment, is 147 kmole h - ~ t , which is more or less the same as for the case with zinc ferrite; 148 kmole h - 1. In both cases, all the minerals, ZnS, FeS, PbS and ZnF%O4, each requires one mole of acid per mole of mineral leached. But, because the overall acid concentration in the hot acid leach liquor - - r e c y c l e d electrolyte blend is lower, 133.6 g 1-1, more aqueous solution is needed. Thus, the residence time is somewhat shorter, 33 min, and the exit mineral conversions slightly lower. Also, the Zn 2+ in the final compartment is lower, 94 g 1-1 versus 111.0 g 1-1. However, the model predicts that hot acid leach liquor is very suitable as an additional iron source for low iron concentrates such as the Red Dog concentrate. Zinc ferrite added directly also provides a sufficient iron source but is not all reacted in the autoclave and thus some zinc value will be lost in the exit solids, which may have to be recycled.
3.5. Comparison o f operating conditions between concentrates In summary, Figs. 2-5 compare the predicted performance of the autoclave for the five different feeds, discussed above, in terms of; ( 1 ) pre-heat temperatures, (2) aqueous feed Calculated from the concentration, 133 g 1-j, times the aqueous feed rate, 107 m3h-l, divided by the molecularweight of H.~SO4,98 kg kmole- ~.
S.A. Baldwin, G.P. Demopoulos/ Hydrometallurgy 39 (1995) 147-162
159
Table 10 Simulation results: Red Dog concentrate with HAL liquor feed Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2+ (g I -~) Fe3+ (gl -I ) Zn2+ (g 1- i) H2SO 4 (g 1- ~) Flow (m3h- ~)
Compartments 2
88.0 0.3 7.8 31.7 133.6 107.0
1
30.0 0.0 0.0 47.7 152.5 14.5
2
140 5.8 4.3 84.6 34.3 118.2
140 2.8 4.5 90.9 36.1 133.1
3
4
145 1.3 4.5 93.0 35.9 133.9
147 0.7 4.5 93.6 36.5 134.0
Mineral conversions (overall) and jarosite precipitation
Minerals
Compartment 1
Zn(Fe) S PbS Jarosite (kg h- ~)
0.772 0.646 128.3
2
3
4
0.932 0.875 1227.2
0.983 0.964 1957.3
0.996 0.989 2231.0
876.3 67.3
830.2 195.5
810.4 57.7
Oxygen usage Po2 (kPa) Po2 (kg h- ~)
875.6 2424.4
streams, (3) exit concentrations and (4) zinc extracted, oxygen consumed and jarosite produced. Fig. 2 shows that high pyrite feeds need less pre-heat, whereas feeds without pyrite, like Red Dog concentrate, need more preheating for autogenous operation of the first compartment at 140°C. The exothermic heat of reaction for pyrite oxidation ( A H ~ ,8 = 2729 kJ m o l - 1 ) is much greater than that for the other dissolution reactions ( A H ~ 8 = 3 0 - 425 kJ m o l - l ) . Better autogenous operation, without preheat, may be possible if the autoclave was redesigned with a double-size first compartment [ 10]. Also, preheating would not be necessary if the acid concentration of the aqueous feed was increased by adding concentrated acid (plant acid). However, preheat of the aqueous feed usually comes from steam produced when the autoclave solution is flashed to atmospheric pressure, and thus constitutes recycled heat. In any case, the pressure leaching model can be used to explore these options. Fig. 3 shows the total amounts of aqueous feed streams needed to maintain -~-35 g l acid concentration in the first two compartments and also for cooling, where necessary. The acid consumption was basically the same for all cases, 15-17 m3h - ~ H2SO4, except for the bulk concentrate with large amounts of pyrite, which needs a little less, 13 m3h - 1 H2SO4. One would have expected that the acid consumption to be significantly less for higher pyrite feeds since pyrite oxidation produces acid. However, since pyrite oxidation was predicted as being very small in the first compartment, the same amount of acid was required as for
160
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
90 80
70 to
•~ 60 ~-~ 50 a~
•
4°
Compartment 1
[ ] Compartment 2
g 3o 20 10
Low Pyrite
High Pyrite
Bulk Cone.
Red Dog w Ferrite
Red Dog w HAL Liquor
Fig. 2. Aqueous feed preheat temperatures (°C) for compartments 1 and 2, as predicted for the five different autoclave feeds.
120 100 80
•
60
Recycled Electrolyte
[ ] Water •
40
HAL Liquor
20 0 Low Pyrite
High Pyrite
Bulk Cone.
Red Dog w Ferrite
Red Dog w HAL Liquor
Fig. 3. Total required feed rates (m3h - ~) of recycled electrolyte, water and hot acid leach liquor predicted for the five different autoclave feeds. low pyrite feeds. O n e can see, however, that significantly more cooling water is needed for high pyrite feeds. This has two disadvantages; the residence time of the autoclave is reduced and a more dilute product is produced. This can be seen in Fig. 4, which compares the exit concentrations. The R e d Dog concentrate blended with zinc ferrite produced a solution
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
161
120 100 8O
•
Fe total
g
60 g
[]
zn
•
Acid
40 20
re__ Low Pyrite
High Pyrite
Bulk Conc.
Red Dog w Ferrite
Red Dog w HAL
Fig. 4. Exit aqueous concentrations of total iron, zinc and sulphuric acid (g 1-') predicted for the five different autoclave feeds.
9000 8000 7000 6000
• jarosite
"~5000 [ ] oxygen
4000
•
3000
Zn Extracted
2000 I000 0
Low Pyrite
High Pyrite
Bulk Conc.
Red Dog w Ferrite
Red Dog w HAL Liquor
Fig. 5. Total jarosite production, oxygen consumption and zinc extraction rates (kg h -1 ) predicted for the five different autoclave feeds,
product highest in zinc concentration, even though not all zinc ferrite was oxidized. Lastly, Fig. 5 gives the overall jarosite production, oxygen consumption and zinc extraction rates, in kg h - 1. Jarosite production is more or less the same for all cases, except for the bulk concentrate, which contains twice as much iron as the other feeds. Oxygen con-
162
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
sumption varies significantly, being greater for high pyrite feeds. Because pyrite is assumed to oxidize to sulphate, three and a half moles o f oxygen are required per mole of pyrite. Only half a mole of oxygen is consumed when the other minerals are oxidized.
4. Conclusions This work illustrates the use of a computer model for prediction of the performance of the pressure leach process for various alternative iron-containing zinc concentrates. W e examined in particular the effect of the iron source in the feed on pre-heat and aqueous solution requirements as well as temperature profiles, conversions, solution product concentrations-, oxygen consumption and jarosite production.
Acknowledgements This work was supported through an industry (Cominco Ltd., Sherritt Inc., Hudson Bay Mining and Smelting Co., N o r a n d a / C E Z Inc. and Air L i q u i d e ) - N S E R C IOR Grant. Additional support through a M c G i l l University postdoctoral fellowship awarded to Susan Baldwin is also kindly acknowledged.
References [ 1] Baldwin, S,A., Demopoulos, G.P. and Papangelakis, V., Mathematicalmodelling of the zinc pressure leach process. Metall. Mat. Trans. B, in press. [2] Palencia Perez, 1. and Dutrizac, J.E., Hydrometallurgy,26, 1991:211-232. [3] Chalkley, M.E., Collins, M.J. and Ozberk, E. In: I.G. Matthew (Editor), Proc. InternationalSymposium- World Zinc '93. AIMM, Parkville, Victoria, Australia, 1993, pp. 325-331. [4] Ashman, D.W. and Jankola, W.A. In: T.S. Mackey and R.D. Prengarnen (Editors), Lead-Zinc '90. TMSAIME, New York, 1990, pp. 253-275. [51 Demopoulos, G.P., Baldwin, S.A., Filippou, D. and Papangelakis, V.G. Presented at Zinc & Lead '95, Sendai, Japan, May 22-24, 1995. [6] Yu, P.H., Hansen, C.K. and Wadsworth, M.E. In: D.J.I. Evans and R.S. Shoemaker (Editors), International Symposium on Hydrometallurgy. AIME, New York, 1973, pp. 375--402. [7] H.S.C. Chemistry 1.10, Outokumpu Research Oy, Pod, Finland. [ 8 ] Filippou, D. and Demopoulos, G.P. Can. Metall. Quart., 31 (1), 1992:41-54. [9] Filippou, D, Ph.D. Thesis, McGill University, Montreal, 1994. [ 10] Peters, E. and F.M. Doyle. In: K.V.S. Sastry and M.C. Fnerstenan (Editors), Challenges in Mineral Processing. SME, Littleton, CO, 1989, pp. 509-526.
i
ELSEVIER
Hydrometallurgy 39 (1995) 147-162
Assessment of alternative iron sources in the pressure leaching of zinc concentrates using a reactor model Susan A. Baldwin, George P. Demopoulos * Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada Received 21 April 1995; accepted 8 June 1995
Abstract A mathematical model is used to simulate pressure leaching of six different zinc concentrate feeds: low pyrite, high pyrite, lead-zinc concentrate blend, Red Dog concentrate, Red Dog concentrate-zinc ferrite blend and Red Dog concentrate with hot acid leach liquor. In particular, the effect of iron content of the feeds on predicted performance for a specific autoclave configuration was investigated. For the Red Dog concentrate, which does not contain enough iron for successful zinc pressure leaching, two alternative iron sources were investigated. In the first case, zinc ferrite was blended with the concentrate feed. In the second case, aqueous solution, containing ferrous and ferric sulphate, from the hot acid leaching of zinc ferrite was added to the autoclave. Simulation results are presented in terms of autogenous operation, minerals conversions, aqueous solution compositions, oxygen consumption and jarosite production.
1. Introduction A comprehensive mathematical model and computer simulation package has been developed for pressure leaching, with oxygen, of sulphide minerals [ 1 ]. In this paper, we describe the use of such a tool in predicting the performance of the pressure leaching process for a variety of zinc concentrates and blends. As "ideal" zinc concentrates become more scarce, the zinc industry is looking at the feasibility of processing other than ideal concentrates and even concentrate blends using pressure leaching technology. Of particular importance in the leaching of sphalerite (ZnS) is the iron content of the feed. Iron is required for the production of ferric ion which oxidizes the zinc sulphide. Oxidation of zinc sulphide directly * Corresponding author.
0304-386X/95/$09,50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 3 8 6 X ( 95 )00027-5
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S.A. Baldwin, G.P. Demopoulos/ Hydrometallurgy 39 (1995) 147-162
by oxygen is very slow. If the iron is incorporated in the zinc sulphide mineral matrix (marmatite: Zn~l _x)Fe~x)S)) then the leaching rate of the mineral is also significantly enhanced [ 2]. However, if present as a separate mineral, then the behaviour of that mineral during leaching greatly influences the effectiveness of the pressure leaching process for zinc concentrates. For example, pyrrhotite oxidizes quite readily and concentrates where the iron is present mostly as pyrrhotite are expected to leach easily. However, pyrite-containing concentrates are anticipated to be problematic, not only because of the refractory nature of pyrite, but also because oxidation of pyrite produces acid and is also very exothermic. In this paper, we have studied a number of scenarios where iron in the feed to the autoclave is present in very different forms. Firstly, we examined three different concentrates: a low pyrite, a high pyrite and a bulk concentrate [ 3 ] with much less marmatite and very significant amounts of pyrite and galena. Then, we investigated the Red Dog concentrate, which is being mined by Cominco Ltd. at their mine in Alaska [4] and contains very little iron. We first used the model to see what was predicted for leaching of Red Dog concentrate without an additional iron source. We then looked at possibilities for adding iron as (a) zinc ferrite or as (b) ferrous and ferric iron in solution from the hot acid leaching of ferfites. Model predictions for each of the six scenarios are presented and the predicted autoclave performance is discussed in terms of conversion, autothermal operation, acid consumption, oxygen consumption, jarosite precipitation and so on. But first, we give a brief outline of the pressure leaching model and describe use of the computer simulation program.
2. Background 2.1. Model description
A detailed description of the mathematical model on which the pressure leaching simulation program is based can be found in a separate publication [ 1 ]. The essential features are summarized below: 1. The model is based on the intrinsic kinetics for each reaction, which have been gleaned from experimental studies reported in the literature. For the mineral dissolution reactions, the kinetics are in the form of a particle shrinkage rate, in/zm min- 1, which is a function of the concentration of an aqueous oxidant species, like 02 or Fe 3÷, for example. 2. The autoclave is modelled as a series of perfectly mixed reactors. 3. Steady state material and energy balances are solved for each component in all three phases; gas, aqueous and solid. The population balance approach is used for the material balances for the minerals. 4. Iron precipitation is predicted by introducing an apparent equilibrium restriction on the ratio of ferric and acid concentrations. 5. The dependence of aqueous solution properties, such as density, vapour pressure, heat capacity and oxygen solubility on temperature and solution composition is taken into account. Highlights of this zinc pressure leaching model are also given in a recent publication [ 5 ] together with a description of another mathematical model for hot-acid-leaching (HAL) of zinc ferrites.
S.A, Baldwin, G.P. Demopoulos/ HydrometaUurgy39 (1995)147-162
149
2.2. Use of the simulation program
In this section, we give examples of the data required by the simulation program. We also describe how the program was used to predict the performance of a typical autoclave, based on the dimensions of the zinc pressure leaching autoclave at Cominco, in Trail, British Columbia, for a variety of feeds. Mineral data
The pressure leach simulation requires a data base of minerals with their thermodynamic and physical properties, as well as the stoichiometry and kinetics for the mineral oxidation reactions. For the concentrates studied in this work, data were required for the following minerals; marmatite (Zn(Fe)S), pyrrhotite (FeS), galena (PbS), pyrite (FeS2), chalcopyrite (CuFeS2) and zinc ferrite (ZnFe204). These are presented for the first four minerals in the previous publication [ 1]. Kinetic data for the leaching of chalcopyrite were obtained from an experimental study in the literature [6]. For the temperatures of interest in zinc pressure leaching (140--150°C) chalcopyrite is assumed to dissolve with the following stoichiometry to form mostly elemental sulphur. CuFeS2(s) + 2H2SO4(aq.) + O2(aq.) ~ CuSO4(aCl.) "~FeSO4(aq.)
( 1)
+ 2H20(aq.) + 2So) The rate of leaching was found to follow the, so-called, shrinking core model, 1 - ( 1 - x ) 1 / 3 =kl t
(2)
ro where x is the conversion, k~the particle shrinkage rate, in ~m min- 1, ro the initial particle size, in/zm and t the time, in min. The particle shrinkage rate was found to be a non-linear function of oxygen partial pressure, Po2; kl----APoJ( 1 +BPo~), where both A and B are temperature dependent constants. However, B is only significant for temperatures above 160°C and for our purposes we assumed the particle shrinkage rate to be a linear function of oxygen partial pressure. A depends on temperature in the well known Arrhenius function; kl = 9.39 × 1016po2exp ( - 151"0 kJ m ° l - ~) RT ixm min- 1
(3)
Po2 is the oxygen pressure in solution in atmospheres. This is, however, an oversimplification of the overall mechanism. The unrealistically high activation energy predicts a very strong temperature dependence, thus it should be emphasized that this equation is not appropriate for high temperatures, above 160°C. Other data required for chalcopyrite and its dissolution are presented in Table 1. As will be seen later, we have considered the possibility of blending zinc ferrite (ZnFe204) with Red Dog zinc concentrate so as to increase the iron content of the feed to the autoclave. From a recent study [8] it was observed that zinc ferrite dissolution is somewhat different when compared to the other sulphide minerals. Zinc ferrite particles are composed of equal-sized, sub-micron grains. The dissolution rate depends on the initial size of these grains and not the apparent particle size. It is important, then, for the simulation to
S.A. Baldwin, G.P. Demopoulos / HydroraetaUurgy 39 (1995)147-162
150 Table 1 Chalcopyrite data Heat capacity a Heat of reaction a
Cp = 87.0 + 0.0536 x T - 5.593 × 105 X T - 2 j mol- 1KzlH~x. = 710.45- 1.873 x ( T - 298) +0.0016X (T 2 - 2982) - 1.193 X 105 × ( 1 / T - 1/298) J mo1-1 Molecular weight MW= 183.1 g g-mole-t Density p = 5000.0 g l Volume shape factor b kv = 0.524 a
FromHSC [7].
b Volume of particle = kvL3/zm 3, where L is the particle characteristic length, in izm, in this case the diameter.
enter the grain size for the particle size distribution data. In addition, the rate depends on the activity of proton, which is a function of the solution composition. A correlation for proton activity as a function of total zinc sulphate, ferrous sulphate, ferric sulphate, sulphuric acid and temperature is given by Filippou [9, p. 156] was used in the present simulations. Zinc ferrite dissolution has the following stoichiometry: ZnFe204(s~ +
4H2SO4(aq.) ~ ZnSO4(aq.) + Fe2(SO4)3(aq.) + 4 H 2 0
(4)
And the grain shrinkage rate is given by { - 63.6 kJ mol- l) [ O/H+]0.57/zm min- 1 kI = 1.859 X 106exp k RT
(5)
where, OtH+is the proton activity. Thermodynamic and kinetic data are given in Table 2.
Reactor settings For this study, of the effect of different iron sources on the performance of the zinc pressure leaching process, we used the Cominco autoclave dimensions for our reactor settings. These are given in Table 3. The autoclave, shown in Fig. 1, has four compartments, the first being a little larger than the rest. The concentrate feed rate to the autoclave was kept constant at 240 kg min- 1 for all case studies. For temperature and acid control aqueous solution can be added to any compartment, where necessary. This was primarily recycled electrolyte, the composition of which is given in Table 3. In some cases, aqueous solution was required for cooling purposes but no more acid was needed. Here, water was blended with the recycled electrolyte. In one of the case studies, hot acid leach liquor was added together with recycled electrolyte so as to provide an iron source. The composition of the hot acid leach liquor was obtained from Filippou [9] and is presented in Table 3. Table 2 Zinc ferrite data Heat capacity Heat of reaction
Cp = 155.4 + 44.56 X 1 0 - 3 T - 15.98 × 105 X T-2 j mol- ~K- 1 AHr,, = --215.74 X 103--307.3 X (T--298) --261.2 X 10-3X (T2-2982) - 4 9 . 4 2 × 105X ( 1 / T - 1/298) J tool -1 Molecular weight MW= 241.0 g g-moleDensity p = 5330.0 g 1Volume shape factor kv = 0.524
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
151
Table 3 Autoclave and feed settings
Autoclave Number of compartments Volume of compartment 1 Volume of compartment 2 Volume of compartment 3 Volume of compartment 4 Total pressure Oxygen purity
4 22.4 m3 17.4 m 3 17.2 m 3 17.4 m 3 12.5 atm 98.5%
Concentrate feed Flow rate of feed solids Feed slurry density Feed slurry temperature
240.0 kg min70.0% wt. 15°C
Aqueous feed streams
Concentrations (g 1- ' )
Fe: + Fe s + Zn 2+ H2SO4
Recycled electrolyte
Hot acid leach liquor
0.0 0.0 50.0 161.0
0.8 21.3 87.6 87.0
Concentrate compositions Four typical zinc concentrates and a zinc-lead bulk concentrate were studied and their mineralogies are presented in Table 4. The mineralogical compositions for the first three were obtained from an article by Chalkley et al. [ 3 ]. In the same article it was mentioned A Q U E O U S FEEDS
GAS VENT ZINC CONCENTRATE FEED
li
DISCHARGE
OXYGEN Fig. 1. Schematic of the autoclave configuration.
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S.A. Baldwin, G.P. Demopoulos/ Hydrometallurgy 39 (1995) 147-162
that treatment of these three concentrates by pressure leaching has been demonstrated successfully. Those results are not, however, available in the literature. It would be very interesting to see whether the predictions reported in the present work match the findings of that study. For the low pyrite and high pyrite concentrates, Chalkley et al. report that pyrrhotite is not present as a separate mineral but is substituted for zinc in sphalerite. This gives marmatite minerals with 10.4% wt and 7.7% wt Fe, respectively. The kinetics for dissolution of marmatite, which is a function of the iron content, were determined from a correlation of the data in an experimental study by Palencia Perez and Dutrizac [2], presented in Baldwin et al. [ 1 ]. For the bulk concentrate, we assumed that low pyrite concentrate (10.4% wt Fe marmatite) had been blended with lead concentrate, which contained some pyrrhotite. The Red Dog mineralogical composition was obtained from Ashman and Jankola [4]. There, also assuming all FeS to be incorporated in marmatite, the iron content of the marmatite was 6.4% wt. For the Red Dog concentrate-zinc ferrite blend, the amount of zinc ferrite was determined as that required to bring the total iron content of the concentrate up to 10% wt Fe, the same iron content as the low pyrite feed.
Running simulations In this work, the objective was to predict the performance of pressure leaching of the concentrates given in Table 4. The strategy that was adopted was to control the temperatures and acid concentrations by adding aqueous solution. The acid concentration was maintained at 35 g 1-1 in the first two compartments, where most acid is required, by adding the appropriate amount of recycled electrolyte. In industrial zinc pressure leaching the temperature is maintained in the range 140--150°C [3]. For the first two compartments, a target temperature of 140°C was chosen. In most cases, in these two compartments, a large quantity of recycled electrolyte is required so as to provide sufficient acid. This has a cooling effect such that preheat of the aqueous feed is required so that the temperature in the compartment does not drop below 140°C. For optimum kinetics, it would be better to operate at as high a temperature as possible in all compartments. To achieve this, a more concentrated acid solution would have to be added, say by using plant acid instead of recycled electrolyte. This may not, however, be the best solution with respect to plant optimization. It is better to aim to make use of recycled streams rather than outside, and thus more expensive, reagents. Hence, we decided to use recycled electrolyte as the acid source and settle for the Table 4 Mineralogical composition of concentrates. Values are percent weight Mineral
Low pyrite
High pyrite
Bulk
Red Dog
Red Dog with ferrite
ZnS a FeS a FeS PbS CuFeS2 FeS2 ZnFe204 Reference
77.5 14.6 0.0 2.3 0.6 1.1 0.0 [ 31
77.5 9.6 0.0 2.3 0.6 7.9 0.0 [3 ]
47.7 6.1 7.0.0.0 17.3 1.4 20.0 0.0 [3 ]
87.1 6.3 0.0 1.2 0.0 0.0 0.0 [4]
74.7 5.4
a In marmatite.
1.0 0.0 0.0 14.2
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995)147-162
153
lower temperature of 140°C in the first two compartments. For the higher pyrite-containing feeds where cooling was required without further acid, water was added. The target temperature of 140°C was maintained for consistent comparison with the other concentrates. The temperature in the two final compartments were kept below 150°C by adding cooling water at 30°C, where necessary. Oxygen addition was determined as that required to maintain 95% purity oxygen in the dry off gas. This results in an oxygen utilization of 70%.
3. Results and discussion
3.1. Low pyrite concentrate This concentrate can probably be considered as t h e " ideal" zinc concentrate, in that there is sufficient iron ( = 10%) for good kinetics, most of which is incorporated in marmatite and leaches easily. Too much iron is undesirable because it then needs to be removed, usually as jarosite, which has to be ponded. Also, sulphate production should be minimized, since excess sulphate has to be removed with line addition. Pyrite, which oxidizes to sulphate and higher temperatures which promote sulphur oxidation to sulphate are, therefore, not desirable. The simulation results for the low pyrite concentrate are presented in Table 5. The temperature profile across the autoclave is from 140 to 150°C, and the acid concentration remains at = 35 ___2 g 1- ~. Most acid is required in the first compartment, 92 m3h - ~recycled electrolyte. In the second compartment, a blend of 13 m3h - 1 recycled electrolyte and 7 m3h- ~water was required. The water was needed for cooling the extra heat generated from pyrite dissolution, the extent of which was very small in the first compartment, but more significant, 44%, in the second. The final conversions of all minerals are very high. We wonder about the accuracy of the conversion predicted for pyrite, since practical experience has indicated that pyrite does not leach well. We expect conversions of about 55-60% [ 3 ]. One reason for this discrepancy could be because the kinetics presently used for pyrite do not take into account any galvanic effects between pyrite and the other sulphide minerals. Galvanic interactions have been postulated as being responsible for the retardation of pyrite dissolution in the presence of other sulphides, whose rates of dissolution are enhanced [ 10]. Once more appropriate experimental data are obtained for pyrite, a better kinetic expression can be input by the user into the model.
3.2. High pyrite concentrate Results for the high pyrite concentrate simulations are presented in Table 6. As expected, more heat is released due to pyrite oxidation and cooling water had to be added to compartments 3 and 4 to keep the temperature below 150°C. Acid concentration remains more or less constant at 35_+2 g 1-~ but Zn 2+ concentration is lower due to dilution by the cooling water. Also, as a consequence of adding cooling water, the residence time for the autoclave is somewhat shorter, 36 min versus 38 min for the low pyrite feed. Thus, minerals conversions are compromised but not significantly.
154
S.A. Baldwin, G.P. Demopoulos l Hydrometallurgy 39 (1995) 147-162
Table 5 Simulation results: low pyrite concentrate Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2+ (gl -~) Fe3+ (gl -I ) Zn2+ (g 1-1 ) Cu2+ (g 1-1) H2SO4 (g 1-t) Flow ( m3h- 1)
Compartments 2
75.0 0.0 0.0 50.0 0.0 160.0 92.0
1
30.0 0.0 0.0 32.8 0.0 105.0 20.0
2
140 5.8 2.5 102.7 0.2 36.3 104.0
140 2.7 4.3 100.3 0.2 34.0 124.6
3
4
149 1.1 4.4 101.4 0.23 35.5 126.0
152 0.5 4.5 101.5 0.23 36.3 126.4
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
Zn(Fe)S PbS CuFeS2 FeS2 Jarosite (kg h- l)
0.770 0.638 0.043 0.123 0.0
2
3
4
0.935 0.864 0.674 0.563 1156.1
0.986 0.964 0.948 0.854 2085.6
0.997 0.991 0.979 0.938 2377.6
876.4 831.1
790.0 266.9
757.3 68.5
Oxygen usage
Po2 (kPa) Po2 (kg h- ~)
877,6 2564.1
In addition, less jarosite is produced in the high pyrite case, 1859.4 kg m i n - ~ versus 2377.6 kg m i n - 1 for the low pyrite case, despite the fact that both concentrates contain 10% wt Fe. But, iron is present in different forms in the two concentrates, mostly as marmatite in the low pyrite case and as marmatite and pyrite in the high pyrite case. According to the model predictions, the slow release o f iron from pyrite and the slower ferrous to ferric oxidation kinetics (compare [Fe 2÷ ] in Table 5 and Table 6) contributed to a lower yield o f j a r o s i t e in the autoclave. 3.3. Bulk concentrate
Simulation results for the bulk concentrate feed are given in Table 7. As expected, less preheat is required for autogenous operation o f the first compartment. For the subsequent compartments, quite a large quantity o f cooling water is needed, 124.0 m3h - 1 in total, to control the temperature. In the second compartment a mixture of 10 m3h - 1 recycled electrolyte and 70 m 3 h - 1 cooling water was added. With such high aqueous solution addition the residence time for the autoclave was only 31 rain. As a result the conversions are lower, particularly for pyrrhotite and g~ilena. Still the pressure leaching process is successful for
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
155
Table 6 Simulation results: high pyrite concentrate Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fez+ (g 1-~) Fe3+ (g1-1 ) Zn2+ (g 1-1) Cu2+ (g1-1) H~SO4(g 1- l) Flow (m3h-~)
Compartments
2
3
63.8 0.0 0.0 50.0 0.0 160.0 78.5
30.0 0.0 0.0 25.0 0.0 80.0 45.0
4
30.0 0.0 0.0 0.0 0.0 0.0 12.5
30.0 0.0 0.0 0.0 0.0 0.0 5.0
1
2
3
4
140 5.4 1.8 105.4 0.02 33.3 90.6
140 3.1 4.4 88.4 0.15 35.0 136.9
150 1.5 4.5 82.3 0.21 36.0 151.1
150 0.8 4.6 80.3 0.21 36.0 156.2
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
Zn(Fe)S CuFeS2 FeSz Jarosite (kg h ~
2
0.740 0.048 0.14 0.0
0.891 0.527 0.404 256.0
3
4
0.970 0.945 0.816 1492.7
0.991 0.978 0.928 1859.4
778.0 694.6
777.8 184.5
Oxygen usage
Po2 (kPa) Po2 (kg h- ~)
876.9 2483.6
875.0 1389.6
treating this kind of concentrate blend, according to the model predictions, since the conversions are above 95%, not including pyrite. We are not certain of the predicted pyrite conversions, as mentioned previously. Large amounts of jarosite are produced since the bulk concentrate contains approximately twice as much iron, 18% wt, as the low and high pyrite feeds, which both have 10% wt iron. Pyrite conversion is approximately the same, but twice as much iron goes into solution, hence the higher amount ofjarosite production. In conclusion, from these predictions, despite the expected difficulties with processing high pyrite feeds; low conversions, high acid consumption and higher exothermic heat of reaction, the pressure leaching process is still very successful for processing these alternative zinc concentrates. It appears as if there are enough checks and balances in the process to prevent high pyrite feeds from being problematic. With very high pyrite containing feeds, like the bulk concentrate, much cooling water is required, which reduces the residence time. But, even with a residence time of only 30 rain sufficient conversion of the sulphide minerals results. The significant difference in processing these high pyrite concentrates, is that much higher aqueous solution flow rates result, increasing from 126 to 225 m3h- 1, approximately 80% more. Thus, although total iron and acid concentrations in the exit are approximately the same, there is still much more acid to be neutralized and iron to be precipitated and disposed. In addition, the zinc concentration becomes quite low, just 37 g i - ~ for the bulk
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Table 7 Simulationresults: bulkconcentrate Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2+ (g 1-1 ) Fe3+ (g 1-t) Zn2+ (g 1-1) Cu2~- (g 1-i) H2SO4 (g 1-1) Flow (m3h- 1)
Compartments
2
48.1 0.0 0.0 50.0 0.0 160.0 70.0
3
30.0 0.0 0.0 6.3 0.0 20.0 80.0
4
30.0 0.0 0.0 0.0 0.0 0.0 40.0
30.0 0.0 0.0 0.0 0.0 0.0 14.0
1
2
3
4
140 4.8 4.5 88.3 0.02 36.0 85.0
140 3.0 4.4 50.8 0.27 35.0 167.2
150 1.9 4.6 40.8 0.32 37.0 211.1
150 1.1 4.7 37.5 0.31 37.5 225.6
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
ZnS FeS PbS CuFeS2 FeS2 Jarosi~ (kg h -l)
0.837 0.684 0.684 0.022 0.126 584.1
2
3
4
0.906 0.746 0.746 0.294 0.147 1830.1
0.965 0.883 0.883 0.947 0.763 3135.3
0.988 0.954 0.954 0.984 0.934 3767.3
873.3 2161.4
774.6 1568.0
Oxygen usage
Po2 (kPa) Poe (kg h -l)
876.3 2504.4
774.4 381.8
concentrate. This means that a dilute product is being produced, and downstream processing will be more expensive. This is, of course, only a tentative conclusion based on performed simulations. Further optimization of pressure leaching of high pyrite zinc concentrates can be done by considering alternative autoclave designs and using the model to simulate the effect on the process.
3.4. Red Dog concentrate The Red Dog concentrate contains much less iron than the above concentrates, just 4% wt. In Table 8 we present simulation results for the first compartment only. This is because it was not possible to solve the material balance equations for the following compartments because the ferric concentration was too small. Even in the first compartment, there is only 0.3 g 1- ~ ferric, and the zinc sulphide conversions are quite low, 59% as compared with 79% conversion for the low pyrite concentrate feed. Thus, it is clearly necessary, for such a low iron content feed, to add an additional iron source. We investigated two possibilities: ( 1) adding zinc ferrite blended with the Red Dog concentrate to give the same feed rate to
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
157
the autoclave of 240 kg m i n - t but with 10% wt iron, and (2) adding ferrous and ferric in solution from the zinc ferrite hot acid leach circuit. The results for the simulation with Red Dog concentrate-zinc ferrite blend are given in Table 9. They are quite similar to the results for the low pyrite feed in that the residence time is approximately 38 min. We were unable to find a steady state in the first compartment where both the temperature and acid concentrations were 140°C and 35 g 1- ~ respectively, thus the acid remains quite low,27 g 1-1 in the first compartment increasing to 34 g 1- ~ in the last compartment. The Zn 2 + concentration is higher ( 110 g 1- ~) than in the low pyrite case ( 100 g 1- ~) due to the extra zinc leached from the zinc ferrite. Zinc ferrite conversions are quite low, 42.5%, in the first compartment and approximately 11% remains undissolved at the autoclave exit. As has been reported previously [8], the kinetics of zinc ferrite leaching are adversely affected by the build-up of its products, ZnSO4 and Fe2SO4, which explains its incomplete conversion. Preheat of the aqueous feed was required in both the first and second compartments for autogenous operation at 140°C. These results can be compared with the simulation with hot acid leach liquor added to the first compartment instead of zinc ferrite. From Table 10, one can see that the model predicts more or less the same autoclave performance in terms of marmatite conversion and final solution composition, except for Zn 2 +, as for the case where zinc ferrite is fed directly. More aqueous feed is required for the Red Dog concentrate with hot acid leach liquor. The feed to the first compartment consists of 39 m3h- 1 of hot acid leach liquor with 87 g 1Table 8 Simulation results: Red Dog concentrate Temperatures and aqueous compositions
Temperature (°C) Fe2+ (gl -j ) Fe3+ (gl -~ ) Zn2+ (g 1- i) Cu2+ (gl -~ ) H2SO4 (g 1-1) Flow (rn3h- ~)
Aqueous feed
Compartments
1
1
78.0 0.0 0.0 50.0 0.0 160.0 69.7
Mineral conversions and jarosite precipitation (overall)
Minerals ZnS FeS PbS Jarosite (kg h- ~)
Compartment 1 0.591 0.734 0.734 0.00
Oxygen usage
Po2 (kPa) Po2 (kg h -~ )
876.7 1921
140 4.9 0.3 105.8 0.0 35.0 80.2
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Table 9 Simulationresults: Red Dog concentratewith zinc ferrite Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2÷ (g 1-j) Fe3÷ (gl -j) Zn2÷ (g 1- J) H2SO 4 (g 1- J) Flow (m3h- 1)
Compartments 2
80.0 0.0 0.0 50.0 160.0 82.0
1
65.0 0.0 0.0 50.0 160.0 20.0
2
140 4.8 3.1 108.2 27.1 93.1
140 2.3 4.3 108.5 34.0 113.8
3
4
146 1.0 4.2 110.7 33.8 114.8
148 0.5 4.3 111.5 34.0 115.1
Mineral conversions and jarosite precipitation (overall)
Minerals
Compartment 1
Zn(Fe) S PbS ZnFe204 Jarosite (kg h J)
0.785 0.689 0.425 0.0
2
3
4
0.936 0.890 0.567 951.9
0.985 0.971 0.772 1976.4
0.997 0.992 0.894 2454.5
Oxygen usage Poe (kPa) Po2 (kg h-I)
875.6 2140.8
876.9 876.9
821.1 821.1
800.9 800.9
acid and 68 m3h - ~ of recycled electrolyte with 160.0 g 1-1 of acid. The total H2SO 4 requirement, in the first compartment, is 147 kmole h - ~ t , which is more or less the same as for the case with zinc ferrite; 148 kmole h - 1. In both cases, all the minerals, ZnS, FeS, PbS and ZnF%O4, each requires one mole of acid per mole of mineral leached. But, because the overall acid concentration in the hot acid leach liquor - - r e c y c l e d electrolyte blend is lower, 133.6 g 1-1, more aqueous solution is needed. Thus, the residence time is somewhat shorter, 33 min, and the exit mineral conversions slightly lower. Also, the Zn 2+ in the final compartment is lower, 94 g 1-1 versus 111.0 g 1-1. However, the model predicts that hot acid leach liquor is very suitable as an additional iron source for low iron concentrates such as the Red Dog concentrate. Zinc ferrite added directly also provides a sufficient iron source but is not all reacted in the autoclave and thus some zinc value will be lost in the exit solids, which may have to be recycled.
3.5. Comparison o f operating conditions between concentrates In summary, Figs. 2-5 compare the predicted performance of the autoclave for the five different feeds, discussed above, in terms of; ( 1 ) pre-heat temperatures, (2) aqueous feed Calculated from the concentration, 133 g 1-j, times the aqueous feed rate, 107 m3h-l, divided by the molecularweight of H.~SO4,98 kg kmole- ~.
S.A. Baldwin, G.P. Demopoulos/ Hydrometallurgy 39 (1995) 147-162
159
Table 10 Simulation results: Red Dog concentrate with HAL liquor feed Temperatures and aqueous compositions
Aqueous feeds 1
Temperature (°C) Fe2+ (g I -~) Fe3+ (gl -I ) Zn2+ (g 1- i) H2SO 4 (g 1- ~) Flow (m3h- ~)
Compartments 2
88.0 0.3 7.8 31.7 133.6 107.0
1
30.0 0.0 0.0 47.7 152.5 14.5
2
140 5.8 4.3 84.6 34.3 118.2
140 2.8 4.5 90.9 36.1 133.1
3
4
145 1.3 4.5 93.0 35.9 133.9
147 0.7 4.5 93.6 36.5 134.0
Mineral conversions (overall) and jarosite precipitation
Minerals
Compartment 1
Zn(Fe) S PbS Jarosite (kg h- ~)
0.772 0.646 128.3
2
3
4
0.932 0.875 1227.2
0.983 0.964 1957.3
0.996 0.989 2231.0
876.3 67.3
830.2 195.5
810.4 57.7
Oxygen usage Po2 (kPa) Po2 (kg h- ~)
875.6 2424.4
streams, (3) exit concentrations and (4) zinc extracted, oxygen consumed and jarosite produced. Fig. 2 shows that high pyrite feeds need less pre-heat, whereas feeds without pyrite, like Red Dog concentrate, need more preheating for autogenous operation of the first compartment at 140°C. The exothermic heat of reaction for pyrite oxidation ( A H ~ ,8 = 2729 kJ m o l - 1 ) is much greater than that for the other dissolution reactions ( A H ~ 8 = 3 0 - 425 kJ m o l - l ) . Better autogenous operation, without preheat, may be possible if the autoclave was redesigned with a double-size first compartment [ 10]. Also, preheating would not be necessary if the acid concentration of the aqueous feed was increased by adding concentrated acid (plant acid). However, preheat of the aqueous feed usually comes from steam produced when the autoclave solution is flashed to atmospheric pressure, and thus constitutes recycled heat. In any case, the pressure leaching model can be used to explore these options. Fig. 3 shows the total amounts of aqueous feed streams needed to maintain -~-35 g l acid concentration in the first two compartments and also for cooling, where necessary. The acid consumption was basically the same for all cases, 15-17 m3h - ~ H2SO4, except for the bulk concentrate with large amounts of pyrite, which needs a little less, 13 m3h - 1 H2SO4. One would have expected that the acid consumption to be significantly less for higher pyrite feeds since pyrite oxidation produces acid. However, since pyrite oxidation was predicted as being very small in the first compartment, the same amount of acid was required as for
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S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
90 80
70 to
•~ 60 ~-~ 50 a~
•
4°
Compartment 1
[ ] Compartment 2
g 3o 20 10
Low Pyrite
High Pyrite
Bulk Cone.
Red Dog w Ferrite
Red Dog w HAL Liquor
Fig. 2. Aqueous feed preheat temperatures (°C) for compartments 1 and 2, as predicted for the five different autoclave feeds.
120 100 80
•
60
Recycled Electrolyte
[ ] Water •
40
HAL Liquor
20 0 Low Pyrite
High Pyrite
Bulk Cone.
Red Dog w Ferrite
Red Dog w HAL Liquor
Fig. 3. Total required feed rates (m3h - ~) of recycled electrolyte, water and hot acid leach liquor predicted for the five different autoclave feeds. low pyrite feeds. O n e can see, however, that significantly more cooling water is needed for high pyrite feeds. This has two disadvantages; the residence time of the autoclave is reduced and a more dilute product is produced. This can be seen in Fig. 4, which compares the exit concentrations. The R e d Dog concentrate blended with zinc ferrite produced a solution
S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
161
120 100 8O
•
Fe total
g
60 g
[]
zn
•
Acid
40 20
re__ Low Pyrite
High Pyrite
Bulk Conc.
Red Dog w Ferrite
Red Dog w HAL
Fig. 4. Exit aqueous concentrations of total iron, zinc and sulphuric acid (g 1-') predicted for the five different autoclave feeds.
9000 8000 7000 6000
• jarosite
"~5000 [ ] oxygen
4000
•
3000
Zn Extracted
2000 I000 0
Low Pyrite
High Pyrite
Bulk Conc.
Red Dog w Ferrite
Red Dog w HAL Liquor
Fig. 5. Total jarosite production, oxygen consumption and zinc extraction rates (kg h -1 ) predicted for the five different autoclave feeds,
product highest in zinc concentration, even though not all zinc ferrite was oxidized. Lastly, Fig. 5 gives the overall jarosite production, oxygen consumption and zinc extraction rates, in kg h - 1. Jarosite production is more or less the same for all cases, except for the bulk concentrate, which contains twice as much iron as the other feeds. Oxygen con-
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S.A. Baldwin, G.P. Demopoulos / Hydrometallurgy 39 (1995) 147-162
sumption varies significantly, being greater for high pyrite feeds. Because pyrite is assumed to oxidize to sulphate, three and a half moles o f oxygen are required per mole of pyrite. Only half a mole of oxygen is consumed when the other minerals are oxidized.
4. Conclusions This work illustrates the use of a computer model for prediction of the performance of the pressure leach process for various alternative iron-containing zinc concentrates. W e examined in particular the effect of the iron source in the feed on pre-heat and aqueous solution requirements as well as temperature profiles, conversions, solution product concentrations-, oxygen consumption and jarosite production.
Acknowledgements This work was supported through an industry (Cominco Ltd., Sherritt Inc., Hudson Bay Mining and Smelting Co., N o r a n d a / C E Z Inc. and Air L i q u i d e ) - N S E R C IOR Grant. Additional support through a M c G i l l University postdoctoral fellowship awarded to Susan Baldwin is also kindly acknowledged.
References [ 1] Baldwin, S,A., Demopoulos, G.P. and Papangelakis, V., Mathematicalmodelling of the zinc pressure leach process. Metall. Mat. Trans. B, in press. [2] Palencia Perez, 1. and Dutrizac, J.E., Hydrometallurgy,26, 1991:211-232. [3] Chalkley, M.E., Collins, M.J. and Ozberk, E. In: I.G. Matthew (Editor), Proc. InternationalSymposium- World Zinc '93. AIMM, Parkville, Victoria, Australia, 1993, pp. 325-331. [4] Ashman, D.W. and Jankola, W.A. In: T.S. Mackey and R.D. Prengarnen (Editors), Lead-Zinc '90. TMSAIME, New York, 1990, pp. 253-275. [51 Demopoulos, G.P., Baldwin, S.A., Filippou, D. and Papangelakis, V.G. Presented at Zinc & Lead '95, Sendai, Japan, May 22-24, 1995. [6] Yu, P.H., Hansen, C.K. and Wadsworth, M.E. In: D.J.I. Evans and R.S. Shoemaker (Editors), International Symposium on Hydrometallurgy. AIME, New York, 1973, pp. 375--402. [7] H.S.C. Chemistry 1.10, Outokumpu Research Oy, Pod, Finland. [ 8 ] Filippou, D. and Demopoulos, G.P. Can. Metall. Quart., 31 (1), 1992:41-54. [9] Filippou, D, Ph.D. Thesis, McGill University, Montreal, 1994. [ 10] Peters, E. and F.M. Doyle. In: K.V.S. Sastry and M.C. Fnerstenan (Editors), Challenges in Mineral Processing. SME, Littleton, CO, 1989, pp. 509-526.