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Ferric chloride leaching of mechanically activated chalcopyrite
Hydrometallurgy 49 Ž1998. 103–123
Ferric chloride leaching of mechanically activated chalcopyrite D. Maurice ) , J.A. Hawk Albany Research Center, US Department of Energy, Albany, OR 97321-2198, USA Received 17 July 1997; revised 29 December 1997; accepted 19 January 1998
Abstract Mechanical activation is a means of accelerating the leaching process. Contributing to the success of this technique are Ž1. increased surface area; Ž2. increased surface reactivity; and Ž3. the microstructural modifications stemming from the deformation. Previous studies have focused on gaining a fundamental understanding of these factors. This study represents a divergence from this approach, as we set out to determine the effectiveness of the technique when applied on a scale more reflective of how the process might actually be practised. To this end, chalcopyrite was autogenously milled in a horizontal mill, and leaching was conducted using a 5 M chloride leach solution. Leaching of as-received concentrate resulted in copper extractions of 75% in 5 h of leaching, whereas leaching mechanically activated concentrates resulted in copper extractions over 95% after 3 h of leaching. This study investigated the critical parameters affecting the efficiency of mechanical activation as a means of accelerating the oxidative leaching of sulfide minerals. The processes of attrition and fragmentation enhance reaction rates by increasing both the surface area and the density of defects. A small laboratory shaker mill and a small tumbling mill were used to mechanically process chalcopyrite; the former using steel grinding media, the latter using sized ore. The increase in surface area was determined using BET; and X-ray diffraction analysis was utilized to determine the full width at half maximum ŽFWHM. for chalcopyrite, which is subsequently used as a marker of the relative deformation. Leaching experiments were conducted with as-received concentrate and with processed concentrate. The contributions of the increased surface area and the deformed structure were correlated with the leaching kinetics. q 1998 Elsevier Science B.V. All rights reserved.
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D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
1. Introduction The use of mechanical activation as a means of accelerating leaching is not new. Gerlach and Gock w1x had a German patent granted in 1973 which included the application of mechanical activation to sulfide ores. Balaz ´ˇ and Ebert w2x referenced five studies, dated from 1977–1987, which demonstrate this effect on the leaching of zinc from sphalerite. In the past 10 years, a more focused effort has been made to quantify the kinetic enhancements due to mechanical activation w2–6x, which are attributed to increased specific surface area, enhanced surface reactivity, and to changes in the crystalline structure Že.g., amorphization.. In a study on the iron sulfate leaching of mechanically activated chalcopyrite, Tkacova ´ˇ ´ and Bala´ w3x found that the initial rate constant k 0 varied according to: S k0 s a q b Ž 1. X where S is the specific surface area and X is the content of crystalline phase that remains after mechanical activation. This equation predicts that, with a reduction in size or with an increase in amorphous content during milling, subsequent leaching is accelerated. Balaz ´ˇ and Ebert w2x ground sphalerite in a vibratory mill. After a period of rapid change in surface area and degree of amorphization, a transition point was reached, where the rates of surface area creation, and of crystalline-to-amorphous transition, were reduced substantially. Sphalerite samples, which had passed the transition, leached very rapidly, with over 65% of the Zn extracted in 2 h in a 4% peroxide solution. It is interesting to note that this was accomplished at room temperature, not at the elevated temperatures used in commercial leach processes. When the specific surface area exceeded 4 m2rg, the reaction rate increased substantially. When the rate constant divided by the effective surface Ž krS . was plotted as a function of the fraction of the sample that had amorphized, a linear relationship was obtained. However, it should be noted that the recovery after 4 h of milling was less than that after only 2.5 h. Consequently, it is unclear if the long milling times resulted in a particle size so small as to lead to complete coverage of the particles by reaction products during leaching Žwhich may change the controlling step from chemical reaction to diffusion. or to agglomeration. Subsequently, Balaz ´ˇ et al. w5x showed that in the leaching of mechanically activated stibnite, agglomeration may have counteracted the effects of grinding. Finally, Murr and Hiskey w6x found only a weak dependence of reaction rate on dislocation density over a range of 10 11 rm2 to 10 15 rm2 for shock-loaded chalcopyrite. However, they found surface effects related to their lixiviant, potassium dichromate, obscured their results at small particle sizes. The findings of Refs. w2,5,6x are in general agreement with Dutrizac w7x, who found that as chalcopyrite particle size decreases, the sulfur layer entirely coats the particle, preventing further dissolution. Dutrizac also found that accelerated attack can lead to complete coating, and if prolonged, will result in thickening of the sulfur layer and agglomeration of the chalcopyrite via sulfur linkages. Rice et al. w8x used a very concentrated solution, which may have reduced their overall leaching efficiency by this very mechanism.
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D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
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The vast majority of laboratory studies have used relatively mild lixiviants, which simplifies the process of characterizing the leaching results. Extraction curves tend to agree closely with either the core shrinkage model w9x or the diffusion barrier model w9x. Yet in practice, when large volumes of concentrate are to be leached, fairly concentrated solutions are used. For example, when ferric chloride solutions are used, the total chloride concentration is on the order of 5 M, achieved through the addition of sodium chloride and a small amount of hydrochloric acid Žused in this case to prevent hydrolysis of the iron.. It seems appropriate to test how well the laboratory findings on leaching mechanically activated concentrates translate to conditions more immediate to those that might be used in a pilot plant. In addition, most of these laboratory studies used small vibratory mills to mechanically activate the concentrate. It is evident that large mills, probably of the horizontal variety, would be used in any commercial scale process, so it seems pertinent to attempt to approximate milling conditions that might really be used. As an adjunct, the application of autogenous milling to reaction milling should be examined. Grinding media wear during comminution processes is substantial, and the media wear fragments become a contaminant that must be removed during refining. In autogenous milling, no separate media are used; rather, the material to be ground serves as its own grinding media, with breakage occurring as a result of ore chunks impacting other ore chunks. If the ore could be used to mechanically activate the concentrate, the process would become even more attractive, and it is possible that any wear of such ‘media’ would serve to enhance the economic value of the mix. However, it is conceivable that any wear debris would serve to contaminate or dilute the concentrate, since ore is predominantly gangue material. 2. Experimental Chalcopyrite concentrate and ores were supplied by a domestic copper producer. The ore was made up of two kinds of rock. One was granitic with dispersed chalcopyrite, bornite, and pyrite. The other showed a quartz vein with molybdenite, both with quartz and feldspar Žorthoclase.. The average composition of the concentrate is given in Table 1, while the constitution of the ores is found in Table 2. Table 1 Concentrate composition Constituent
Wt.%
Cu MoS 2 Fe S SiO 2 Al 2 O 3 CaO Fe 3 O4 H 2O
29.9 0.13 26.3 32.2 5.4 1.5 0.2 0.14 9.4
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
106 Table 2 Ore composition Mineral
Vol.%
Pyrite Chalcopyrite Bornite Molybdenite Hematite Chrysocolla Biotite
1–5% 0.5–4% 0–2% 0–3% Trace Superficial, trace 2–10%
The concentrate was wet-sieved and found to have the size distribution shown in Table 3. X-ray diffraction analysis indicated that the primary phrase present in the concentrate was chalcopyrite, with traces of pyrite and quartz. Leaching experiments used one of the following three size ranges Žy100 q 200 mesh, y200q 270 mesh, and y400 mesh. of concentrate. One gram samples were leached in 100 ml of a solution of ; 1.0 M FeCl 3 , 1.0 M NaCl, and 0.25 M HCl Žmeasured at 1.28 M Fe 3q, 4.68 M Cly .. Leaching times ranged from 3.6 ks to 18 ks. The leaching apparatus consisted of a 400 ml vessel with a magnetic stirring bar and with a rubber stopper, which was placed on a combination hot plate-stirrer with a temperature probe providing continuous temperature feedback to the hot plate. At the conclusion of leaching, the solution was filtered and analyzed by atomic absorption spectrophotometry using a Perkin-Elmer 3100 AA spectrophotometer. 1 The results of these leaching runs permitted the calculation of the activation energy and the kinetic constants for the as-received material. Having determined the size and temperature dependencies of the leaching behavior of the as-received concentrate, a series of milling runs was performed. Two mills were used: a horizontal ball mill of 0.6 m diameter, and a SPEX 8000 shaker mill. 1 All milling runs conducted in the horizontal mill used ore Žq4 mesh. as the grinding media. Most of the runs in the horizontal mill were performed using ‘fresh’ ore; a series of runs was also done using ore that had been comminuted in previous milling runs. All runs conducted in the horizontal mill used the same quantity of ore media. SPEX shaker mill runs used either ore Žy3 q 6 mesh. or stainless steel balls of 4.8 mm in diameter. Total media charge was chosen to give a constant charge ratio Ži.e., the mass of media divided by the mass of concentrate. and constant mill filling, for easy comparison between ore and steel media. Milled samples were agitated at 400 rpm and leached at 363 K for times up to 18 ks using the same apparatus and procedure as used for the as-received material. Specific area was determined by BET using a Leeds and Northrup Model 4200 Automatic Surface Area Analyzer. 1 Perhaps the simplest marker that reflects crystalline deformation is the full width at half maximum ŽFWHM. as determined by XRD patterns; this was determined for each sample by profile fitting using a Phillips APD3720 X-ray
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Reference to specific products does not imply endorsement by the US Department of Energy.
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Table 3 Concentrate size distribution Mesh
Mass %
q65 y65q100 y100q150 y150q200 y200q270 y270q325 y325q400 y400
1.03 2.63 6.30 8.00 13.00 7.37 4.68 57.00
diffraction system. 1 Metal extraction was determined by atomic absorption spectrophotometry using a Perkin-Elmer 3100 AA spectrophotometer. 1
3. Results and discussion 3.1. As-receiÕed material Initial leaching experiments were performed on the as-received material in order to determine Ž1. the activation energy unique to this batch of material; and Ž2. the dependency of leaching kinetics on surface area. Dutrizac w10x had previously noted that when concentrated leaching solutions are used, irregular leaching curves are sometimes produced. Typically, in chloride solutions of laboratory strength, the shrinking core model adequately describes the leaching behavior. Dutrizac w7x also noted that under accelerated attack, or with small particle sizes, surface passivation can result. Under such circumstances, it might be expected that the protective product layer might reasonably describe leaching kinetics. A key difference between the two is the effect of surface area, a variable of central interest in mechanically activated leaching. The fraction of Cu dissolved into solution was plotted vs. time of leaching for each set of conditions. Initial reaction rates were determined by fitting the data to an equation of the form:
a s a Ž 1 y eyb t .
Ž 2.
where a is the fraction of Cu extracted, t is the time, and a and b are constants, and then evaluating da dt
s abeyb t
Ž 3.
at the time t s 0. Fig. 1 illustrates the extraction curves obtained when y400 mesh chalcopyrite in the as-received condition was leached over a range of temperatures Ži.e., 333, 343, 353, and 363 K.. As expected, as the temperature of the leaching solution was increased, the fraction of copper extracted at any time also increased. From these data, the activation
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Fig. 1. Extraction curves for the as-received y400 mesh chalcopyrite at leaching temperatures of 333, 343, 353, and 363 K.
Fig. 2. Extraction curves for three particle sizes of as-received chalcopyrite at a leaching temperature of 363 K.
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
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energy for the as-received concentrate was determined by fitting the reaction rate constants to an equation of the form ln k s c q
Q T
Ž 4.
where Q is the activation energy and T is the absolute temperature. A value of 68 kJrmol K was obtained as the activation energy, which is in reasonable agreement with other published figures for Cu extraction from chalcopyrite w11x. Fig. 2 shows the extraction curves obtained when chalcopyrite of different mesh sizes was leached at 363 K. In general, the reaction rate constants were seen to vary inversely with particle diameter Žfor this we assumed diameters at the midpoints of the three ranges., indicating a lack of any protective product formation. It was also observed from XRD that no intermediate products were present, thereby supporting this assumption. 3.2. High-energy milling Having established a baseline of leaching behavior for the given combination of concentrate and concentrated solution, the effects of mechanical activation by milling were examined. A SPEX shaker mill was used for the initial runs, as the small loading results in a fairly uniform sample upon output. While small high-energy mills are certainly not representative of the mills that would be used in any large-scale, economically feasible operation, they do facilitate rapid, inexpensive laboratory testing. In addition, determining scaling factors between such mills and those such as might be used in large-scale operations was one goal of this work. Fig. 3 illustrates the temperature dependence on the extraction curves for y400 mesh chalcopyrite milled in a SPEX vial for 0.6 ks, using stainless steel ŽSS. milling media. Fig. 4 shows the effect of milling time in a SPEX mill on extraction behavior. Both of these figures are representative of the patterns exhibited by the remainder of the data from SPEX milled samples. From these SPEX shaker mill runs, it is clear that Cu extraction is significantly enhanced by the mechanical activation process. Almost full extraction of Cu was achieved by leaching for 18 ks at 363 K after 0.6 ks of milling, or by leaching for 9 ks at 363 K after 1.8 ks of milling. Table 4 summarizes the reaction rate constants k determined for each set of conditions. Fig. 5 shows a plot of the reaction rate constants for each temperature as a function of milling time. Since an objective of the research was to investigate the effect of autogenous milling on mechanical activation using a horizontal grinding mill, a series of SPEX mill runs were conducted using chalcopyrite ore of y3 q 6 mesh as the grinding media. Mill filling was kept constant with that of the previous SPEX mill runs, with approximately 15 g of ore used to fill a volume of the vial equal to that of the 50 g of steel balls. Fig. 6 illustrates the difference in extraction rates, which arise as a result of milling with different types of media. As expected, milling with SS media results in higher extraction rates compared to milling with ore. Table 5 summarizes the reaction rate constants determined for autogenous milling in the shaker mill. Fig. 7 plots the reaction rate constants as a function of milling time. It is
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Fig. 3. Extraction curves at 343, 353, and 363 K for y400 mesh chalcopyrite milled using steel media for 0.6 ks in a SPEX-type shaker mill.
Fig. 4. Extraction curves at 363 K as a function of residence time in a SPEX-type shaker mill using steel media.
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Table 4 SS milling in SPEX shaker mill Reaction rate constant k =10 4 Žsy1 .
Time of milling Žks.
Leaching temperature ŽK.
0
333 343 353 363
0.37 0.40 0.88 1.50
0.6
343 353 363
1.97 3.57 7.33
1.2
343 353 363
2.35 4.18 10.1
1.8
343 353 363
2.47 4.29 13.2
Fig. 5. Variation in reaction rate constant as a function of milling time using steel media at leaching temperatures of 343, 353, 363 K.
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Fig. 6. Extraction curves for a leaching temperature of 363 K as a function of grinding media and charge ratio, for a milling time of 1.2 ks and a leaching temperature of 363 K.
interesting to note that while the reaction rate constant increases with autogenous milling, total extraction does not Žcf. Figs. 2 and 6.. As the reaction rate constant is measured at t s 0, this implies that initial dissolution of the autogenously milled concentrate is more rapid with than with the as-received concentrate, but that dissolution is somehow retarded as leaching progresses. Fig. 8 shows that the intrinsic deformation of the autogenously milled concentrate is not much greater than that of the as-received concentrate, but Fig. 9 shows that the specific area is greatly increased with autogenous
Table 5 Autogenous milling in SPEX shaker mill Charge ratio
Milling time Žks.
Reaction rate constant k =10 4 Žsy1 .
5:1
0.6 1.2 1.8 2.4
3.02 4.02 4.63 4.13
10:1
0.6 1.2 1.8 2.4
4.13 4.03 4.66 5.37
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
Fig. 7. Reaction rate constants for autogenous milling in a SPEX-type shaker mill.
Fig. 8. Effects of grinding media material and charge ratio on FWHM.
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Fig. 9. Specific area as a function of milling conditions in SPEX-type shaker mill, using various charge ratios and milling media.
milling. This suggests that the autogenous milling resulted in a small enough particle size that surface passivation resulted. It is to be expected that milling with SS media results in the faster creation of defects. It should also result in less contamination of the concentrate as opposed to milling with ore, since wear debris from the ore effectively dilutes the concentrate. In Fig. 6, the effect of charge ratio between the two types of media is clearly shown by the extraction curves at 363 K. For the milling done with SS, the two extraction curves are nearly identical for the 5:1 and 10:1 charge ratios. This implies that the charge ratio is of minimal importance when milling with SS media. For laboratory purposes, this conclusion suggests that mill process conditions might as well be chosen on the basis of cost minimization. When using ore as the grinding media in the shaker mill, the lower Ž5:1. charge ratio, i.e., a lower ratio of grinding media mass to concentrate mass, resulted in slightly enhanced leaching kinetics. From this, it might be inferred that the additional energy per collision Žandror higher collision frequency. inherent in the higher charge ratio is of no benefit. This is supported by Fig. 8, where the degree of deformation, using FWHM as a marker, is practically identical when milling is done with ore for both ore charge ratios. On the other hand, when SS is used as the milling media, significantly more deformation and damage occurs than when using ore. Another conclusion from this series of experiments is that when milling with ore as the charge media, the higher charge ratio results in more dilution from media fragmentation. This is illustrated in Fig. 10, which shows the relative purity Ždefined as the mass of concentrate divided by the
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Fig. 10. Relative purity of discharge from SPEX-type shaker vial after milling, where ore was used as the grinding media. The relative purity is defined as the fraction of fines which is concentrate, i.e., the weight of the concentrate feedstock divided by the weight of the fines removed at the end of milling.
mass of concentrate plus ore debris. of the post-mill discharge. For every milling time, the relative purity of the resulting mixture is higher when the charge ratio is 5:1. 3.3. Low-energy milling All milling runs done in the horizontal mill used q4 mesh ore as the grinding media. As with the SPEX shaker mill, runs in the horizontal mill were conducted with charge ratios of 5:1 and 10:1. Unlike the high-energy runs done with in the SPEX shaker mill, a constant quantity of concentrate was used in the horizontal tumbling mill for each charge, resulting in a fairly constant mill filling factor Ž; 20%. for both charge ratios. It might seem that not maintaining constant filling when using the shaker mills would introduce a complication by altering mill energetics, but previous work w12x has demonstrated that this is not a concern in the range of filling factors used in this study. Fig. 11a,b illustrate that leaching kinetics were essentially insensitive to residence time in the tumbling mill. Table 6 summarizes the reaction rate constants determined for autogenous milling in the horizontal ball mill. Fig. 12 indicates that a higher charge ratio Ž10:1. results in improved kinetics, and the reaction rate constant is comparable to the best achieved during autogenous milling in the shaker mill. As residence times in tumbling mills are long, and because the milling is autogenous, dilution from media fragmentation is of primary interest. Fig. 13 illustrates the degree to
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Fig. 11. Ža. Fraction extracted after autogenous milling using the horizontal ball mill with a 5:1 oreto-concentrate charge ratio. Žb. Fraction extracted after autogenous milling using the horizontal ball mill with a 10:1 ore-to-concentrate charge ratio.
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Table 6 Autogenous milling in horizontal mill Charge ratio
Milling time Žks.
Reaction rate constant k =10 4 Žsy1 .
5:1
28.8 57.6 86.4 115.2
4.37 2.99 3.40 3.82
10:1
28.8 57.6 86.4
3.14 5.44 5.47
57.6 115.2
1.43 1.79
Pre-Used Ore 5:1
which ore debris contaminated the solid leaching charge. Analogous to the results from shaker milling, the relative purity is higher for the lower charge ratio Žafter 57.6 ks of milling.. Even a small absolute quantity of ore debris can become a substantial fraction of the material later directed to leaching operations. In industrial processing, mechanical activation might be done prior to the final concentration steps, somewhat mitigating this effect, but in our research, all leaching was done with full dilution by ore debris. In this
Fig. 12. Reaction rate constants for autogenous milling in a horizontal mill.
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Fig. 13. Relative purity of discharge from the horizontal mill as a function of milling time.
fashion any complications resulting from the contamination would be exposed, and a ‘worst case’ for leaching kinetics would be realized. The wear of the ore would be expected to contribute an additional small quantity of chalcopyrite for leaching Žsee Table 2., as well as the contamination from gangue fragments. For purposes of this work, however, extraction rates were calculated under the assumption that the copper extracted from chalcopyrite previously associated with ore, not concentrate, was minimal. A rough estimate of the maximum copper that might be extracted from ore debris validates this assumption; that is, if 50% of the solid charge used for leaching was ore debris Žwhich is higher than that seen in Fig. 10 or Fig. 13., and if this ore debris was 4% chalcopyrite Žsee Table 2., then the maximum error introduced is on the order of 2%. It is appropriate to define ‘usable ore’ to mean ore that was still q4 mesh at the conclusion of a typical milling run. In Fig. 14, it can be seen that under the conditions used in this research, media wear was comparatively rapid for the first 57.6 ks of milling, after which wear was minimal. This was promising, as it suggested that previously milled ore could be used as milling media in subsequent runs with minimal dilution of concentrate. Experiments were conducted using ore that had already been in use as media for times greater than 57.6 ks. Whereas the unused ore lost 7.9% of its weight in 57.6 ks of milling, previously used ore lost only 2.2% after 57.6 ks, which rose to only 2.3% after 115.2 ks. However, the dissolution of concentrate milled using this pre-used ore failed to exhibit any improvement over that of as-received concentrate.
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
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Fig. 14. Fraction of usable ore as a function of milling time, charge ratio, and pre-treatment conditions. The ‘fraction of usable ore’ is defined as the weight fraction of ore, which remains q4 mesh after milling in the horizontal mill. ŽCR s charge ratio..
Fig. 15. FWHM as a function of charge ratio in horizontal mill.
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Fig. 16. Changes in specific area as a function of charge ratio in horizontal mill.
The intrinsic deformation ŽFWHM. was comparable to that of concentrate that had been milled using fresh ore ŽFig. 15., but the increase in specific area was less ŽFig. 16.. It remains, then, to explore the relationships between concentrate deformation and specific surface area with the reaction rate constant. 3.4. Concentrate characteristics as functions of milling conditions The specific area Ž A s . is shown as a function of milling conditions in Fig. 9 ŽSPEX shaker mill. and 16 Žhorizontal mill.. As expected, A s increases with milling; however, what was unexpected was that changes in the charge ratio seemed to have minimal effect when SPEX shaker milling was done using ore as the grinding media or when a 5:1 charge ratio of SS milling media was used. A significant change in A s occurred when SS milling media was used at a 10:1 charge ratio and 2.4 ks. In the horizontal mill, the same general trend occurred, i.e., the milling done with the higher charge ratio led to an overall increase in A s , and this difference tended to increase with increasing milling time. It should be noted that the dilution of the concentrate with mill debris may well obscure changes in specific area that occur in the concentrate when using ore as the milling media. When leaching as-received concentrate, the reaction rate constant was seen to vary inversely with the particle diameter; equivalently, it varied linearly with the specific area. In Fig. 17, the data fall on two different curves with different slopes, with the line homologous to material shaker-milled using steel media being the steeper of the two.
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Fig. 17. Reaction rate constants as a function of specific area. There are two apparent regions to the data: the higher values correspond to milling with the SS media, the lower values correspond to milling with ore as the media. These data represent both the SPEX-type shaker mill and the horizontal ball mill Žhowever, they do not include data from autogenous milling in the SPEX-type shaker mill..
This would seem to suggest that as the concentrate becomes more deformed, it becomes more sensitive to changes in surface area. Figs. 8 and 15 show the changes that occur in the value of the FWHM as a function of milling conditions. Milling in the more energetic SPEX shaker mill ŽFig. 8. resulted in greater line broadening than that which took place in the horizontal mill ŽFig. 15.. When using steel media, this effect was even more pronounced. From Fig. 15, it can be seen that a higher charge ratio resulted in greater line broadening even in the horizontal mill. However, even at very long milling times, the value of the FWHM in the horizontal mill only approached those values of FWHM in the SPEX shaker mill at the shortest milling times. Copper extraction should be accelerated by the increase in specific surface area and by the incorporation of defects resulting from milling. Fig. 18 summarizes the results of measurements to determine the reaction rate constants as a function of FWHM values for all the milling experiments. The data all fall along one general curve, where the reaction rate constant increases with increasing values of FWHM, or increasing deformationrdefects in the milled concentrate. It is also observed that the autogenous milling Žeither in the SPEX shaker mill or in the horizontal mill. produced less relative deformation ŽFWHM values. and lower values of k relative to milling done with the SS media in the SPEX shaker mill. Indeed, as milling residence time increased in the SPEX
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Fig. 18. Reaction rate constant as a function of line broadening ŽFWHM..
shaker mill, the degree of deformation and the value of k increased along a single smooth curve regardless of the charge ratio. This suggests an interaction effect between charge ratio and mill residence time, which could be used to optimize Cu extraction by controlling the degree of deformationrreaction rate constant for a given ore and mill process conditions. Consideration of Figs. 15, 17 and 18 lead us to believe that milling with ore media tends to increase surface area, perhaps through attrition processes, without altering the natural crystal structure of the chalcopyrite to any great extent, while milling with SS media has a more pronounced effect on the chalcopyrite structure. This would seem to suggest that using SS in a SPEX-type shaker mill might as well be inappropriate for conducting preliminary studies for future translation to autogenous milling in a large horizontal mill, as results derived in this manner may be overly encouraging.
4. Summary Autogenous milling modifies concentrates primarily by increasing surface area, while minimally increasing defect density. Milling with SS media Žat least in a SPEX-type shaker mill. results in increased defect density, as well as increased surface area. The reaction rate constants for autogenous milling, and for SPEX shaker milling with SS media, vary along a smooth curve, increasing in magnitude with increasing values of
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FWHM. Changes in FWHM for a given set of mill processing conditions may enable scaling between the mills. Although different mechanisms of concentrate modification between the two sets of milling conditions exists, it may still be possible to predict the performance of a horizontal mill on the basis of milling done in a laboratory using a SPEX-type shaker mill with ore media. While autogenous milling eliminates the problem of expensive media purchase and wear to the grinding media and liners, the wear debris from the ore results in a diluted concentrate. In a SPEX-type shaker mill, this dilution was sufficient to retard the leaching relative to the leaching of unprocessed concentrate. That this retardation did not occur as a result of autogenous milling in a horizontal mill may complicate predicting the leaching behavior after autogenous milling in a shaker mill, on the basis of results from autogenous high-energy milling. Ore dilution can be minimized by using ore that has already been tumbled for some time; under the conditions used in this study, contamination was insignificant when using ore that had previously been used for 57.6 ks of milling. Whether fresh or used ore was used, the intrinsic rate constant was improved by autogenous milling in a horizontal mill.
Acknowledgements The authors would like to thank Mr. Mike Peck for his considerable help in conducting this research, Mr. Shain Thompson for his efforts to maintain and improve mill equipment, and to Mr. Dale Govier for performing the XRD analyses.
References w1x J.K. Gerlach, E.D. Gock, Ger. Pat. 2,138,143 ŽNIM Tr. 430.. w2x P. Balaz, ´ˇ I. Ebert, Oxidative leaching of mechanically activated sphalerite, Hydrometallurgy 27 Ž1991. 141–150. w3x K. Tkacova, ´ˇ ´ P. Balaz, ´ˇ Structural and temperature sensitivity of leaching of chalcopyrite with iron ŽIII. sulfate, Hydrometallurgy 21 Ž1988. 103–112. w4x K. Tkacova, V.E. Vigdergauz, V.A. Chanturiya, Selective leaching of zinc from ´ˇ ´ P. Balaz, ´ˇ B. Misura, ˇ mechanically activated complex Cu–Pb–Zn concentrate, Hydrometallurgy 33 Ž1993. 291–300. ˇ ˇ w5x P. Balaz, Non-oxidative leaching of mechanically ´ˇ J. Bruancin, ˇ V. Sepelak, ´ T. Havlık, ´ M. Skrobian, activated stibnite, Hydrometallurgy 31 Ž1992. 201–212. w6x L.E. Murr, J.B. Hiskey, Kinetic effects of particle size and crystal dislocation density on the dichromate leaching of chalcopyrite, Met. Trans. B 12B Ž1981. 255–267. w7x J.E. Dutrizac, Elemental sulfur formation during the ferric chloride leaching of chalcopyrite, Hydrometallurgy 23 Ž1990. 153–176. w8x D.A. Rice, J.R. Cobble, D.R. Brooks, USBM RI 9351. w9x M.E. Wadsworth, in: H. Eyring ŽEd.., Physical Chemistry, Vol. VII, Reactions in Condensed Phases, Academic Press, New York, NY, 1975, pp. 413–472. w10x J.E. Dutrizac, The dissolution of chalcopyrite in ferric sulfate and ferric chloride media, Met. Trans. B 12B Ž1981. 371–378. w11x J.E. Dutrizac, The leaching of sulfide minerals in chloride media, Hydrometallurgy 29 Ž1992. 1–45. w12x B. McDermott, MS Thesis, NC State Univ., May 1988.
Ferric chloride leaching of mechanically activated chalcopyrite D. Maurice ) , J.A. Hawk Albany Research Center, US Department of Energy, Albany, OR 97321-2198, USA Received 17 July 1997; revised 29 December 1997; accepted 19 January 1998
Abstract Mechanical activation is a means of accelerating the leaching process. Contributing to the success of this technique are Ž1. increased surface area; Ž2. increased surface reactivity; and Ž3. the microstructural modifications stemming from the deformation. Previous studies have focused on gaining a fundamental understanding of these factors. This study represents a divergence from this approach, as we set out to determine the effectiveness of the technique when applied on a scale more reflective of how the process might actually be practised. To this end, chalcopyrite was autogenously milled in a horizontal mill, and leaching was conducted using a 5 M chloride leach solution. Leaching of as-received concentrate resulted in copper extractions of 75% in 5 h of leaching, whereas leaching mechanically activated concentrates resulted in copper extractions over 95% after 3 h of leaching. This study investigated the critical parameters affecting the efficiency of mechanical activation as a means of accelerating the oxidative leaching of sulfide minerals. The processes of attrition and fragmentation enhance reaction rates by increasing both the surface area and the density of defects. A small laboratory shaker mill and a small tumbling mill were used to mechanically process chalcopyrite; the former using steel grinding media, the latter using sized ore. The increase in surface area was determined using BET; and X-ray diffraction analysis was utilized to determine the full width at half maximum ŽFWHM. for chalcopyrite, which is subsequently used as a marker of the relative deformation. Leaching experiments were conducted with as-received concentrate and with processed concentrate. The contributions of the increased surface area and the deformed structure were correlated with the leaching kinetics. q 1998 Elsevier Science B.V. All rights reserved.
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Corresponding author. E-mail: [email protected]; Fax: q1 541 967 5845.
0304-386Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 8 6 X Ž 9 8 . 0 0 0 1 3 - 9
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1. Introduction The use of mechanical activation as a means of accelerating leaching is not new. Gerlach and Gock w1x had a German patent granted in 1973 which included the application of mechanical activation to sulfide ores. Balaz ´ˇ and Ebert w2x referenced five studies, dated from 1977–1987, which demonstrate this effect on the leaching of zinc from sphalerite. In the past 10 years, a more focused effort has been made to quantify the kinetic enhancements due to mechanical activation w2–6x, which are attributed to increased specific surface area, enhanced surface reactivity, and to changes in the crystalline structure Že.g., amorphization.. In a study on the iron sulfate leaching of mechanically activated chalcopyrite, Tkacova ´ˇ ´ and Bala´ w3x found that the initial rate constant k 0 varied according to: S k0 s a q b Ž 1. X where S is the specific surface area and X is the content of crystalline phase that remains after mechanical activation. This equation predicts that, with a reduction in size or with an increase in amorphous content during milling, subsequent leaching is accelerated. Balaz ´ˇ and Ebert w2x ground sphalerite in a vibratory mill. After a period of rapid change in surface area and degree of amorphization, a transition point was reached, where the rates of surface area creation, and of crystalline-to-amorphous transition, were reduced substantially. Sphalerite samples, which had passed the transition, leached very rapidly, with over 65% of the Zn extracted in 2 h in a 4% peroxide solution. It is interesting to note that this was accomplished at room temperature, not at the elevated temperatures used in commercial leach processes. When the specific surface area exceeded 4 m2rg, the reaction rate increased substantially. When the rate constant divided by the effective surface Ž krS . was plotted as a function of the fraction of the sample that had amorphized, a linear relationship was obtained. However, it should be noted that the recovery after 4 h of milling was less than that after only 2.5 h. Consequently, it is unclear if the long milling times resulted in a particle size so small as to lead to complete coverage of the particles by reaction products during leaching Žwhich may change the controlling step from chemical reaction to diffusion. or to agglomeration. Subsequently, Balaz ´ˇ et al. w5x showed that in the leaching of mechanically activated stibnite, agglomeration may have counteracted the effects of grinding. Finally, Murr and Hiskey w6x found only a weak dependence of reaction rate on dislocation density over a range of 10 11 rm2 to 10 15 rm2 for shock-loaded chalcopyrite. However, they found surface effects related to their lixiviant, potassium dichromate, obscured their results at small particle sizes. The findings of Refs. w2,5,6x are in general agreement with Dutrizac w7x, who found that as chalcopyrite particle size decreases, the sulfur layer entirely coats the particle, preventing further dissolution. Dutrizac also found that accelerated attack can lead to complete coating, and if prolonged, will result in thickening of the sulfur layer and agglomeration of the chalcopyrite via sulfur linkages. Rice et al. w8x used a very concentrated solution, which may have reduced their overall leaching efficiency by this very mechanism.
ž /
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
105
The vast majority of laboratory studies have used relatively mild lixiviants, which simplifies the process of characterizing the leaching results. Extraction curves tend to agree closely with either the core shrinkage model w9x or the diffusion barrier model w9x. Yet in practice, when large volumes of concentrate are to be leached, fairly concentrated solutions are used. For example, when ferric chloride solutions are used, the total chloride concentration is on the order of 5 M, achieved through the addition of sodium chloride and a small amount of hydrochloric acid Žused in this case to prevent hydrolysis of the iron.. It seems appropriate to test how well the laboratory findings on leaching mechanically activated concentrates translate to conditions more immediate to those that might be used in a pilot plant. In addition, most of these laboratory studies used small vibratory mills to mechanically activate the concentrate. It is evident that large mills, probably of the horizontal variety, would be used in any commercial scale process, so it seems pertinent to attempt to approximate milling conditions that might really be used. As an adjunct, the application of autogenous milling to reaction milling should be examined. Grinding media wear during comminution processes is substantial, and the media wear fragments become a contaminant that must be removed during refining. In autogenous milling, no separate media are used; rather, the material to be ground serves as its own grinding media, with breakage occurring as a result of ore chunks impacting other ore chunks. If the ore could be used to mechanically activate the concentrate, the process would become even more attractive, and it is possible that any wear of such ‘media’ would serve to enhance the economic value of the mix. However, it is conceivable that any wear debris would serve to contaminate or dilute the concentrate, since ore is predominantly gangue material. 2. Experimental Chalcopyrite concentrate and ores were supplied by a domestic copper producer. The ore was made up of two kinds of rock. One was granitic with dispersed chalcopyrite, bornite, and pyrite. The other showed a quartz vein with molybdenite, both with quartz and feldspar Žorthoclase.. The average composition of the concentrate is given in Table 1, while the constitution of the ores is found in Table 2. Table 1 Concentrate composition Constituent
Wt.%
Cu MoS 2 Fe S SiO 2 Al 2 O 3 CaO Fe 3 O4 H 2O
29.9 0.13 26.3 32.2 5.4 1.5 0.2 0.14 9.4
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
106 Table 2 Ore composition Mineral
Vol.%
Pyrite Chalcopyrite Bornite Molybdenite Hematite Chrysocolla Biotite
1–5% 0.5–4% 0–2% 0–3% Trace Superficial, trace 2–10%
The concentrate was wet-sieved and found to have the size distribution shown in Table 3. X-ray diffraction analysis indicated that the primary phrase present in the concentrate was chalcopyrite, with traces of pyrite and quartz. Leaching experiments used one of the following three size ranges Žy100 q 200 mesh, y200q 270 mesh, and y400 mesh. of concentrate. One gram samples were leached in 100 ml of a solution of ; 1.0 M FeCl 3 , 1.0 M NaCl, and 0.25 M HCl Žmeasured at 1.28 M Fe 3q, 4.68 M Cly .. Leaching times ranged from 3.6 ks to 18 ks. The leaching apparatus consisted of a 400 ml vessel with a magnetic stirring bar and with a rubber stopper, which was placed on a combination hot plate-stirrer with a temperature probe providing continuous temperature feedback to the hot plate. At the conclusion of leaching, the solution was filtered and analyzed by atomic absorption spectrophotometry using a Perkin-Elmer 3100 AA spectrophotometer. 1 The results of these leaching runs permitted the calculation of the activation energy and the kinetic constants for the as-received material. Having determined the size and temperature dependencies of the leaching behavior of the as-received concentrate, a series of milling runs was performed. Two mills were used: a horizontal ball mill of 0.6 m diameter, and a SPEX 8000 shaker mill. 1 All milling runs conducted in the horizontal mill used ore Žq4 mesh. as the grinding media. Most of the runs in the horizontal mill were performed using ‘fresh’ ore; a series of runs was also done using ore that had been comminuted in previous milling runs. All runs conducted in the horizontal mill used the same quantity of ore media. SPEX shaker mill runs used either ore Žy3 q 6 mesh. or stainless steel balls of 4.8 mm in diameter. Total media charge was chosen to give a constant charge ratio Ži.e., the mass of media divided by the mass of concentrate. and constant mill filling, for easy comparison between ore and steel media. Milled samples were agitated at 400 rpm and leached at 363 K for times up to 18 ks using the same apparatus and procedure as used for the as-received material. Specific area was determined by BET using a Leeds and Northrup Model 4200 Automatic Surface Area Analyzer. 1 Perhaps the simplest marker that reflects crystalline deformation is the full width at half maximum ŽFWHM. as determined by XRD patterns; this was determined for each sample by profile fitting using a Phillips APD3720 X-ray
1
Reference to specific products does not imply endorsement by the US Department of Energy.
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
107
Table 3 Concentrate size distribution Mesh
Mass %
q65 y65q100 y100q150 y150q200 y200q270 y270q325 y325q400 y400
1.03 2.63 6.30 8.00 13.00 7.37 4.68 57.00
diffraction system. 1 Metal extraction was determined by atomic absorption spectrophotometry using a Perkin-Elmer 3100 AA spectrophotometer. 1
3. Results and discussion 3.1. As-receiÕed material Initial leaching experiments were performed on the as-received material in order to determine Ž1. the activation energy unique to this batch of material; and Ž2. the dependency of leaching kinetics on surface area. Dutrizac w10x had previously noted that when concentrated leaching solutions are used, irregular leaching curves are sometimes produced. Typically, in chloride solutions of laboratory strength, the shrinking core model adequately describes the leaching behavior. Dutrizac w7x also noted that under accelerated attack, or with small particle sizes, surface passivation can result. Under such circumstances, it might be expected that the protective product layer might reasonably describe leaching kinetics. A key difference between the two is the effect of surface area, a variable of central interest in mechanically activated leaching. The fraction of Cu dissolved into solution was plotted vs. time of leaching for each set of conditions. Initial reaction rates were determined by fitting the data to an equation of the form:
a s a Ž 1 y eyb t .
Ž 2.
where a is the fraction of Cu extracted, t is the time, and a and b are constants, and then evaluating da dt
s abeyb t
Ž 3.
at the time t s 0. Fig. 1 illustrates the extraction curves obtained when y400 mesh chalcopyrite in the as-received condition was leached over a range of temperatures Ži.e., 333, 343, 353, and 363 K.. As expected, as the temperature of the leaching solution was increased, the fraction of copper extracted at any time also increased. From these data, the activation
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Fig. 1. Extraction curves for the as-received y400 mesh chalcopyrite at leaching temperatures of 333, 343, 353, and 363 K.
Fig. 2. Extraction curves for three particle sizes of as-received chalcopyrite at a leaching temperature of 363 K.
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
109
energy for the as-received concentrate was determined by fitting the reaction rate constants to an equation of the form ln k s c q
Q T
Ž 4.
where Q is the activation energy and T is the absolute temperature. A value of 68 kJrmol K was obtained as the activation energy, which is in reasonable agreement with other published figures for Cu extraction from chalcopyrite w11x. Fig. 2 shows the extraction curves obtained when chalcopyrite of different mesh sizes was leached at 363 K. In general, the reaction rate constants were seen to vary inversely with particle diameter Žfor this we assumed diameters at the midpoints of the three ranges., indicating a lack of any protective product formation. It was also observed from XRD that no intermediate products were present, thereby supporting this assumption. 3.2. High-energy milling Having established a baseline of leaching behavior for the given combination of concentrate and concentrated solution, the effects of mechanical activation by milling were examined. A SPEX shaker mill was used for the initial runs, as the small loading results in a fairly uniform sample upon output. While small high-energy mills are certainly not representative of the mills that would be used in any large-scale, economically feasible operation, they do facilitate rapid, inexpensive laboratory testing. In addition, determining scaling factors between such mills and those such as might be used in large-scale operations was one goal of this work. Fig. 3 illustrates the temperature dependence on the extraction curves for y400 mesh chalcopyrite milled in a SPEX vial for 0.6 ks, using stainless steel ŽSS. milling media. Fig. 4 shows the effect of milling time in a SPEX mill on extraction behavior. Both of these figures are representative of the patterns exhibited by the remainder of the data from SPEX milled samples. From these SPEX shaker mill runs, it is clear that Cu extraction is significantly enhanced by the mechanical activation process. Almost full extraction of Cu was achieved by leaching for 18 ks at 363 K after 0.6 ks of milling, or by leaching for 9 ks at 363 K after 1.8 ks of milling. Table 4 summarizes the reaction rate constants k determined for each set of conditions. Fig. 5 shows a plot of the reaction rate constants for each temperature as a function of milling time. Since an objective of the research was to investigate the effect of autogenous milling on mechanical activation using a horizontal grinding mill, a series of SPEX mill runs were conducted using chalcopyrite ore of y3 q 6 mesh as the grinding media. Mill filling was kept constant with that of the previous SPEX mill runs, with approximately 15 g of ore used to fill a volume of the vial equal to that of the 50 g of steel balls. Fig. 6 illustrates the difference in extraction rates, which arise as a result of milling with different types of media. As expected, milling with SS media results in higher extraction rates compared to milling with ore. Table 5 summarizes the reaction rate constants determined for autogenous milling in the shaker mill. Fig. 7 plots the reaction rate constants as a function of milling time. It is
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Fig. 3. Extraction curves at 343, 353, and 363 K for y400 mesh chalcopyrite milled using steel media for 0.6 ks in a SPEX-type shaker mill.
Fig. 4. Extraction curves at 363 K as a function of residence time in a SPEX-type shaker mill using steel media.
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111
Table 4 SS milling in SPEX shaker mill Reaction rate constant k =10 4 Žsy1 .
Time of milling Žks.
Leaching temperature ŽK.
0
333 343 353 363
0.37 0.40 0.88 1.50
0.6
343 353 363
1.97 3.57 7.33
1.2
343 353 363
2.35 4.18 10.1
1.8
343 353 363
2.47 4.29 13.2
Fig. 5. Variation in reaction rate constant as a function of milling time using steel media at leaching temperatures of 343, 353, 363 K.
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Fig. 6. Extraction curves for a leaching temperature of 363 K as a function of grinding media and charge ratio, for a milling time of 1.2 ks and a leaching temperature of 363 K.
interesting to note that while the reaction rate constant increases with autogenous milling, total extraction does not Žcf. Figs. 2 and 6.. As the reaction rate constant is measured at t s 0, this implies that initial dissolution of the autogenously milled concentrate is more rapid with than with the as-received concentrate, but that dissolution is somehow retarded as leaching progresses. Fig. 8 shows that the intrinsic deformation of the autogenously milled concentrate is not much greater than that of the as-received concentrate, but Fig. 9 shows that the specific area is greatly increased with autogenous
Table 5 Autogenous milling in SPEX shaker mill Charge ratio
Milling time Žks.
Reaction rate constant k =10 4 Žsy1 .
5:1
0.6 1.2 1.8 2.4
3.02 4.02 4.63 4.13
10:1
0.6 1.2 1.8 2.4
4.13 4.03 4.66 5.37
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
Fig. 7. Reaction rate constants for autogenous milling in a SPEX-type shaker mill.
Fig. 8. Effects of grinding media material and charge ratio on FWHM.
113
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D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
Fig. 9. Specific area as a function of milling conditions in SPEX-type shaker mill, using various charge ratios and milling media.
milling. This suggests that the autogenous milling resulted in a small enough particle size that surface passivation resulted. It is to be expected that milling with SS media results in the faster creation of defects. It should also result in less contamination of the concentrate as opposed to milling with ore, since wear debris from the ore effectively dilutes the concentrate. In Fig. 6, the effect of charge ratio between the two types of media is clearly shown by the extraction curves at 363 K. For the milling done with SS, the two extraction curves are nearly identical for the 5:1 and 10:1 charge ratios. This implies that the charge ratio is of minimal importance when milling with SS media. For laboratory purposes, this conclusion suggests that mill process conditions might as well be chosen on the basis of cost minimization. When using ore as the grinding media in the shaker mill, the lower Ž5:1. charge ratio, i.e., a lower ratio of grinding media mass to concentrate mass, resulted in slightly enhanced leaching kinetics. From this, it might be inferred that the additional energy per collision Žandror higher collision frequency. inherent in the higher charge ratio is of no benefit. This is supported by Fig. 8, where the degree of deformation, using FWHM as a marker, is practically identical when milling is done with ore for both ore charge ratios. On the other hand, when SS is used as the milling media, significantly more deformation and damage occurs than when using ore. Another conclusion from this series of experiments is that when milling with ore as the charge media, the higher charge ratio results in more dilution from media fragmentation. This is illustrated in Fig. 10, which shows the relative purity Ždefined as the mass of concentrate divided by the
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
115
Fig. 10. Relative purity of discharge from SPEX-type shaker vial after milling, where ore was used as the grinding media. The relative purity is defined as the fraction of fines which is concentrate, i.e., the weight of the concentrate feedstock divided by the weight of the fines removed at the end of milling.
mass of concentrate plus ore debris. of the post-mill discharge. For every milling time, the relative purity of the resulting mixture is higher when the charge ratio is 5:1. 3.3. Low-energy milling All milling runs done in the horizontal mill used q4 mesh ore as the grinding media. As with the SPEX shaker mill, runs in the horizontal mill were conducted with charge ratios of 5:1 and 10:1. Unlike the high-energy runs done with in the SPEX shaker mill, a constant quantity of concentrate was used in the horizontal tumbling mill for each charge, resulting in a fairly constant mill filling factor Ž; 20%. for both charge ratios. It might seem that not maintaining constant filling when using the shaker mills would introduce a complication by altering mill energetics, but previous work w12x has demonstrated that this is not a concern in the range of filling factors used in this study. Fig. 11a,b illustrate that leaching kinetics were essentially insensitive to residence time in the tumbling mill. Table 6 summarizes the reaction rate constants determined for autogenous milling in the horizontal ball mill. Fig. 12 indicates that a higher charge ratio Ž10:1. results in improved kinetics, and the reaction rate constant is comparable to the best achieved during autogenous milling in the shaker mill. As residence times in tumbling mills are long, and because the milling is autogenous, dilution from media fragmentation is of primary interest. Fig. 13 illustrates the degree to
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Fig. 11. Ža. Fraction extracted after autogenous milling using the horizontal ball mill with a 5:1 oreto-concentrate charge ratio. Žb. Fraction extracted after autogenous milling using the horizontal ball mill with a 10:1 ore-to-concentrate charge ratio.
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117
Table 6 Autogenous milling in horizontal mill Charge ratio
Milling time Žks.
Reaction rate constant k =10 4 Žsy1 .
5:1
28.8 57.6 86.4 115.2
4.37 2.99 3.40 3.82
10:1
28.8 57.6 86.4
3.14 5.44 5.47
57.6 115.2
1.43 1.79
Pre-Used Ore 5:1
which ore debris contaminated the solid leaching charge. Analogous to the results from shaker milling, the relative purity is higher for the lower charge ratio Žafter 57.6 ks of milling.. Even a small absolute quantity of ore debris can become a substantial fraction of the material later directed to leaching operations. In industrial processing, mechanical activation might be done prior to the final concentration steps, somewhat mitigating this effect, but in our research, all leaching was done with full dilution by ore debris. In this
Fig. 12. Reaction rate constants for autogenous milling in a horizontal mill.
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Fig. 13. Relative purity of discharge from the horizontal mill as a function of milling time.
fashion any complications resulting from the contamination would be exposed, and a ‘worst case’ for leaching kinetics would be realized. The wear of the ore would be expected to contribute an additional small quantity of chalcopyrite for leaching Žsee Table 2., as well as the contamination from gangue fragments. For purposes of this work, however, extraction rates were calculated under the assumption that the copper extracted from chalcopyrite previously associated with ore, not concentrate, was minimal. A rough estimate of the maximum copper that might be extracted from ore debris validates this assumption; that is, if 50% of the solid charge used for leaching was ore debris Žwhich is higher than that seen in Fig. 10 or Fig. 13., and if this ore debris was 4% chalcopyrite Žsee Table 2., then the maximum error introduced is on the order of 2%. It is appropriate to define ‘usable ore’ to mean ore that was still q4 mesh at the conclusion of a typical milling run. In Fig. 14, it can be seen that under the conditions used in this research, media wear was comparatively rapid for the first 57.6 ks of milling, after which wear was minimal. This was promising, as it suggested that previously milled ore could be used as milling media in subsequent runs with minimal dilution of concentrate. Experiments were conducted using ore that had already been in use as media for times greater than 57.6 ks. Whereas the unused ore lost 7.9% of its weight in 57.6 ks of milling, previously used ore lost only 2.2% after 57.6 ks, which rose to only 2.3% after 115.2 ks. However, the dissolution of concentrate milled using this pre-used ore failed to exhibit any improvement over that of as-received concentrate.
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
119
Fig. 14. Fraction of usable ore as a function of milling time, charge ratio, and pre-treatment conditions. The ‘fraction of usable ore’ is defined as the weight fraction of ore, which remains q4 mesh after milling in the horizontal mill. ŽCR s charge ratio..
Fig. 15. FWHM as a function of charge ratio in horizontal mill.
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Fig. 16. Changes in specific area as a function of charge ratio in horizontal mill.
The intrinsic deformation ŽFWHM. was comparable to that of concentrate that had been milled using fresh ore ŽFig. 15., but the increase in specific area was less ŽFig. 16.. It remains, then, to explore the relationships between concentrate deformation and specific surface area with the reaction rate constant. 3.4. Concentrate characteristics as functions of milling conditions The specific area Ž A s . is shown as a function of milling conditions in Fig. 9 ŽSPEX shaker mill. and 16 Žhorizontal mill.. As expected, A s increases with milling; however, what was unexpected was that changes in the charge ratio seemed to have minimal effect when SPEX shaker milling was done using ore as the grinding media or when a 5:1 charge ratio of SS milling media was used. A significant change in A s occurred when SS milling media was used at a 10:1 charge ratio and 2.4 ks. In the horizontal mill, the same general trend occurred, i.e., the milling done with the higher charge ratio led to an overall increase in A s , and this difference tended to increase with increasing milling time. It should be noted that the dilution of the concentrate with mill debris may well obscure changes in specific area that occur in the concentrate when using ore as the milling media. When leaching as-received concentrate, the reaction rate constant was seen to vary inversely with the particle diameter; equivalently, it varied linearly with the specific area. In Fig. 17, the data fall on two different curves with different slopes, with the line homologous to material shaker-milled using steel media being the steeper of the two.
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
121
Fig. 17. Reaction rate constants as a function of specific area. There are two apparent regions to the data: the higher values correspond to milling with the SS media, the lower values correspond to milling with ore as the media. These data represent both the SPEX-type shaker mill and the horizontal ball mill Žhowever, they do not include data from autogenous milling in the SPEX-type shaker mill..
This would seem to suggest that as the concentrate becomes more deformed, it becomes more sensitive to changes in surface area. Figs. 8 and 15 show the changes that occur in the value of the FWHM as a function of milling conditions. Milling in the more energetic SPEX shaker mill ŽFig. 8. resulted in greater line broadening than that which took place in the horizontal mill ŽFig. 15.. When using steel media, this effect was even more pronounced. From Fig. 15, it can be seen that a higher charge ratio resulted in greater line broadening even in the horizontal mill. However, even at very long milling times, the value of the FWHM in the horizontal mill only approached those values of FWHM in the SPEX shaker mill at the shortest milling times. Copper extraction should be accelerated by the increase in specific surface area and by the incorporation of defects resulting from milling. Fig. 18 summarizes the results of measurements to determine the reaction rate constants as a function of FWHM values for all the milling experiments. The data all fall along one general curve, where the reaction rate constant increases with increasing values of FWHM, or increasing deformationrdefects in the milled concentrate. It is also observed that the autogenous milling Žeither in the SPEX shaker mill or in the horizontal mill. produced less relative deformation ŽFWHM values. and lower values of k relative to milling done with the SS media in the SPEX shaker mill. Indeed, as milling residence time increased in the SPEX
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D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
Fig. 18. Reaction rate constant as a function of line broadening ŽFWHM..
shaker mill, the degree of deformation and the value of k increased along a single smooth curve regardless of the charge ratio. This suggests an interaction effect between charge ratio and mill residence time, which could be used to optimize Cu extraction by controlling the degree of deformationrreaction rate constant for a given ore and mill process conditions. Consideration of Figs. 15, 17 and 18 lead us to believe that milling with ore media tends to increase surface area, perhaps through attrition processes, without altering the natural crystal structure of the chalcopyrite to any great extent, while milling with SS media has a more pronounced effect on the chalcopyrite structure. This would seem to suggest that using SS in a SPEX-type shaker mill might as well be inappropriate for conducting preliminary studies for future translation to autogenous milling in a large horizontal mill, as results derived in this manner may be overly encouraging.
4. Summary Autogenous milling modifies concentrates primarily by increasing surface area, while minimally increasing defect density. Milling with SS media Žat least in a SPEX-type shaker mill. results in increased defect density, as well as increased surface area. The reaction rate constants for autogenous milling, and for SPEX shaker milling with SS media, vary along a smooth curve, increasing in magnitude with increasing values of
D. Maurice, J.A. Hawk r Hydrometallurgy 49 (1998) 103–123
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FWHM. Changes in FWHM for a given set of mill processing conditions may enable scaling between the mills. Although different mechanisms of concentrate modification between the two sets of milling conditions exists, it may still be possible to predict the performance of a horizontal mill on the basis of milling done in a laboratory using a SPEX-type shaker mill with ore media. While autogenous milling eliminates the problem of expensive media purchase and wear to the grinding media and liners, the wear debris from the ore results in a diluted concentrate. In a SPEX-type shaker mill, this dilution was sufficient to retard the leaching relative to the leaching of unprocessed concentrate. That this retardation did not occur as a result of autogenous milling in a horizontal mill may complicate predicting the leaching behavior after autogenous milling in a shaker mill, on the basis of results from autogenous high-energy milling. Ore dilution can be minimized by using ore that has already been tumbled for some time; under the conditions used in this study, contamination was insignificant when using ore that had previously been used for 57.6 ks of milling. Whether fresh or used ore was used, the intrinsic rate constant was improved by autogenous milling in a horizontal mill.
Acknowledgements The authors would like to thank Mr. Mike Peck for his considerable help in conducting this research, Mr. Shain Thompson for his efforts to maintain and improve mill equipment, and to Mr. Dale Govier for performing the XRD analyses.
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