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The separation of arsenic from copper in a Northparkes copper–gold ore using controlled-potential flotation
Int. J. Miner. Process. 84 (2007) 15 – 24 www.elsevier.com/locate/ijminpro
The separation of arsenic from copper in a Northparkes copper–gold ore using controlled-potential flotation L.K. Smith, W.J. Bruckard ⁎ CSIRO Division of Minerals, Box 312, Clayton South, Victoria, 3169, Australia Received 18 January 2007; received in revised form 3 May 2007; accepted 16 May 2007 Available online 24 May 2007
Abstract In order for the high-arsenic regions of the Northparkes copper–gold orebody to be beneficiated economically, tennantite ((Cu,Fe)12As4S13) present in the ore needs to be rejected to enable copper concentrates to meet the typical smelter penalty level of 2000 ppm As. Using a composite sample of high-arsenic drill cores from Northparkes it was possible to selectively separate tennantite from chalcopyrite (CuFeS2) and bornite (Cu5FeS4) using controlled-potential flotation. The separation was made on a bulk copper– arsenic concentrate after reducing the pulp potential to about −150 mV SHE at pH 12 and floating the tennantite from the other copper minerals. The basis of the separation relies on findings that the lower limiting pulp potential threshold for tennantite is lower than that for chalcopyrite such that there is a potential window in the reducing region where tennantite is strongly floatable but chalcopyrite is not. Little or no selectivity between tennantite and chalcopyrite was found in the oxidising pulp potential region for the range examined. From the composite sample tested, which had a head grade of 0.11% As and 1.2% Cu, it was possible to produce a low-arsenic high-copper concentrate containing 52% of the non-tennantite copper and assaying 2600 ppm As. Computer simulations have shown that for a feed containing a more typical arsenic and copper level (200 ppm As and 1% Cu) the efficiency of separation should be sufficient to concentrate about 61% of the copper in a product assaying less than 2000 ppm As. A conceptual flowsheet for arsenic rejection from Northparkes copper–gold ore, based on the findings from this study, is presented and discussed in this paper. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Flotation; Tennantite; Copper; Controlled-potential flotation
1. Introduction Rio Tinto Limited operates the Northparkes copper– gold mine located in central NSW, Australia. The ore typically contains about 1% Cu and the main sulphide copper minerals present are chalcopyrite and bornite. However, some parts of the orebody are high in arsenic ⁎ Corresponding author. Tel.: +61 3 9545 8500; fax: +61 3 9562 8919. E-mail address: [email protected] (W.J. Bruckard).
content (above 200 ppm As) with the arsenic occurring mainly as the copper–arsenic mineral tennantite ((Cu, Fe)12As4S13). In order to produce copper concentrates from these sections of the orebody that meet smelter specifications arsenic will need to be rejected during beneficiation. Preliminary conventional flotation testwork conducted on the high-arsenic material has produced concentrates containing 5000–7000 ppm As. Arsenic is a penalty element for copper smelters where it can cause environmental problems when volatile arsenic
0301-7516/$ - see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2007.05.002
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compounds are emitted during smelting. Normally, high financial penalties are imposed by smelters to treat copper concentrates containing higher than 2000 ppm As (Wilson and Chanroux, 1993). Arsenic-bearing sulfide minerals like tennantite are difficult to separate from other non-arsenic sulphide copper minerals in conventional copper flotation circuits. The most successful approaches to effecting separation appear to be based on selective oxidation techniques (Fornasiero et al., 2001; Castro and Honores, 2000; Huch, 1993) or the exploitation of pulp potential effects (Menacho et al., 1993; Jaime and Cifuentes, 1995; Yen and Tajadod, 2000; Guo and Yen, 2005, 2006). Recently, as part of a project aimed at identifying options for an arsenic rejection circuit for the Tampakan (Philippines) copper flotation flowsheet, CSIRO Minerals developed methods for the selective flotation of enargite (Cu3AsS4), another arsenic-bearing sulphide mineral similar to tennantite, from other sulphide copper minerals based on the flotation response of various copper minerals and enargite to changes in pulp potential (Senior et al., 2006). One of the separation methods proposed was successfully tested on a sample of higharsenic Tampakan drill core, enabling the production of high-arsenic and low-arsenic copper concentrates. The low-arsenic copper concentrate was low enough in arsenic content to be sent to a conventional smelter for copper metal production while the high-arsenic copper concentrate would need to be treated by some other route to recover the entrained copper values. Given these controlled-potential flotation techniques had proved beneficial in the selective flotation of enargite from other sulphide copper minerals in the Tampkan ore, it was deemed worthwhile to determine whether the same methods would also be applicable to the selective flotation of tennantite from Northparkes ore. This paper details the results of a preliminary study on a sample of high-arsenic Northparkes drill core to assess whether controlled-potential flotation could be used to make a separation between tennantite and other sulphide copper minerals present to produce a lowarsenic copper concentrate suitable for smelting and a high-arsenic copper concentrate.
2.1. Drill core sample
2. Experimental
Table 1 Chemical analysis of drill core composite
Experimental work conducted included preparation of the high-arsenic drill core, flotation tests at various Eh and pH values, chemical and mineralogical analysis of flotation feed and product samples, and simulations to determine the performance of the proposed separation flowsheet on Northparkes ore of more typical head grade.
Head sample
The sample tested was a composite of several drill core intersections from a high-arsenic section of the Northparkes orebody. The intersections were stage crushed to pass 2 mm, blended to form a composite, and the composite divided into 1 kg lots for testing by standard means. Sub-samples were also split out for chemical and mineralogical analysis. The head assays of the composite are given in Table 1, which also contains the average calculated head assays from all the batch tests completed. These data show that the composite was particularly high in arsenic, assaying 0.11% As or 1100 ppm As. This is higher than the current estimate of an average arsenic head grade for the higharsenic section of the Northparkes orebody (200 ppm As). A drill core sample with high-arsenic head grade was specifically chosen for this study to increase the amount of arsenic mineral available for flotation testing in the arsenic rejection stage. 2.2. Grinding For each test, 1000 g of the drill core was mixed with Melbourne tap water and ground for 29 min in a cast iron rod mill with 15 rods at 60% solids to give a P80 (80% passing size) by weight of 90 μm. Before each test the mill and rod charge were cleaned by grinding a sample of quartz for 10 min. 2.3. Flotation 2.3.1. Reagents Northparkes supplied the reagents used in the rougher–scavenger. Solutions of sodium hydrosulfide (NaHS) and the frother, Interfroth 68, were added to the pulp without further dilution. The collectors, sodium isobutyl xanthate (SiBX) and AP 208 (a dithiophosphate), were made up fresh each day as 1% w/v solutions in distilled water. Make-up water was Melbourne tap water and the flotation gas was high purity bottled synthetic air (a synthetic
Assay head Calculated head b a b
Element (%) Cu
As
S
NTCu a
1.21 1.26
0.11 0.11
0.68 0.78
0.93 0.98
NTCu: Non-tennantite copper. Average of all flotation tests conducted.
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mixture of O2 and N2) during the rougher–scavenger stages and either the same synthetic air or high purity bottled nitrogen during the arsenic rejection stages. The reagents used to adjust the pulp potential during the arsenic rejection stage were either laboratory grade sodium hypochlorite (NaClO) or the same sodium hydrosulfide solution as was used in the rougher–scavenger. A 2.5% w/w suspension of laboratory grade lime was used to set the pH in the arsenic rejection stage. 2.3.2. Equipment The samples were floated in modified Denver style cells (Guy, 1992) in which the impellers were driven from below to allow the whole surface of the froth to be scraped with a paddle at constant depth and at constant time intervals. Two sizes of cells were used: a 3 dm3 cell for rougher–scavenger flotation and a 1 dm3 cell for further separation stages. The cells were fitted with a rubber diaphragm, sight tube and electronic sensor for automatic detection and control of pulp level. The pulp potential was measured continuously during testing using a high-impedance differential voltmeter with a polished platinum flag electrode and silver/silver chloride reference electrode. The performance of the electrode system was checked using a standard ferric– ferrous ion solution (Light, 1972). Measured potential values were converted to the standard hydrogen electrode (SHE) scale by the addition of 0.2 V. The pH of the pulp was also monitored continuously with a Radiometer glass/calomel electrode calibrated using standard pH 7 and pH 10 buffer solutions before each test. Radiometer TTT80 titrators and ABU80 burettes were used to add oxidant/reductant and acid/base to set and maintain the Eh and pH respectively. 2.3.3. Procedures The approach taken in this study was to produce a rougher–scavenger concentrate, using a standard laboratory procedure supplied by Northparkes, and then to use pulp potential control during a subsequent flotation stage on this concentrate to make an arsenic separation. The ground slurry was transferred to the 3 dm3 flotation cell and the pulp level set. The pH of the rougher– scavenger was the natural pH of the sample and, in these tests, was approximately pH 9.8. In the rougher stage, 100 g/t of sodium hydrosulfide, 10 g/t of xanthate and 30 g/t of AP208 were added and the pulp conditioned for 4 min. Frother (50 g/t) was added 1 min before flotation and concentrate collected for 6 min. In the scavenger stage, an additional 5 g/t of xanthate was added, the pulp conditioned for 1 min and concentrate collected for a further 6 min.
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For the arsenic rejection stage, the combined rougher– scavenger concentrate was transferred to the 1 dm3 flotation cell, the pH set to 12 with lime, and the pulp level set. Two regions of pulp potential were examined: an oxidizing region and a reducing region. If an oxidizing region was being tested, the pulp potential was adjusted to the required value using sodium hypochlorite, the pulp conditioned at that potential for 10 min and then concentrates collected for up to 10 min using synthetic air as the flotation gas. If a reducing region was being tested, the pulp potential was adjusted to the required value using sodium hydrosulfide, the pulp conditioned at that potential for 5 min and then concentrates collected for up to 10 min using nitrogen as the flotation gas. Small amounts of frother were added as required to maintain an active froth column. All flotation products were weighed wet, to allow calculation of water recoveries, dried and re-weighed, and then prepared for analysis in a standard manner. 2.4. Chemical analysis of products Sub-samples of the drill core composite and solid flotation products were assayed for copper, arsenic and sulfur by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using a standard method. Arsenic assays were used to determine tennantite (51.6% Cu; 20.3% As), and copper not accounted for in tennantite has been termed “non-tennantite copper”, or NTCu, and will include copper present in bornite and chalcopyrite. 2.5. Sizing analyses Sizing analyses was conducted using standard laboratory wet and dry screening methods. Where subsieve sizing was required a modified CSIRO Cyclosizing technique was used (Kelsall, Restarick, and Stewart, 1974). All size fractions were weighed and analysed by ICP-AES. 2.6. Mineralogical analyses The copper and arsenic mineralogy of the drill core sample and the textural associations of the minerals present were determined. The procedure used was to isolate the bulk of the sulphide minerals in a rougher– scavenger concentrate, to determine the minerals present by X-ray diffraction (XRD) and to analyse selected size fractions of the concentrate by scanning electron microscopy (SEM). The rougher–scavenger concentrate produced for the mineralogical analysis contained over 94% of the copper and arsenic in the feed.
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X-ray diffraction patterns were recorded on a Philips PW 1050 goniometer with a PW 1710 diffraction controller using CuKα radiation. Phases present were identified by comparison of the peak positions and intensities with data published by the International Centre for Diffraction Data (ICDD). Selected size fractions for SEM were mounted, polished, and carbon coated prior to be being analysed by SEM using a Joel superprobe fitted with wavelength dispersive detectors. The probe data obtained was analysed using CSIRO's in-house software package CHIMAGE. 2.7. Simulations Due to the higher than normal arsenic head grade of the drill core used for the flotation testing it was necessary to perform simulations to determine the performance of the proposed separation on ore of more typical head grades. Computer simulation packages developed by CSIRO were used to simulate the arsenic rejection flowsheet on ore of differing head grades and compare the results to a flowsheet without an arsenic rejection stage. For the simulations, the flotation feed was assumed to comprise four mineral components: tennantite, bornite, chalcopyrite and non-sulfide gangue (NSG). Bornite and chalcopyrite were assumed to be present in the feed in the ratio 2:1 based on the mineralogical investigation. The program then calculated the distribution of these components to various products and converted the results to grades and recoveries for Cu, As, NTCu and NSG. Distributions were calculated using the relevant, experimentally determined separation efficiencies. The model was used only in open circuit operation i.e., all results are for flowsheets without recycles. In practice, improved recoveries might reasonably be expected by recycling some streams. 3. Results and discussion The results are discussed in terms of characterisation of the drill core composite, the effectiveness of separating arsenic and copper minerals using controlledpotential flotation, and the assessment of simulations conducted to determine the performance of the proposed separation flowsheet on Northparkes ore of more typical head grade.
Northparkes orebody. Assuming all the arsenic present is in tennantite it follows that the NTCu assay for the composite (based on the feed assay) is 0.93% NTCu, meaning approximately 23% of the copper present in the composite occurs as tennantite. The important point here to note is that rejecting all the arsenic in any modified flowsheet will also reject 23% of the total copper present. The major sulfides detected by XRD analyses were bornite (Cu5FeS4), chalcopyrite (CuFeS2) and tennantite ((Cu,Fe)12As4S13). A small amount of pyrite was also detected. The non-sulfides identified were quartz, muscovite, dolomite and kaolinite. The electron microscopy analysis gave the same mineralogy except that a small amount of chalcocite (Cu2S) was also found. The data also revealed that the tennantite was largely liberated but, where it was locked, it tended to be locked with bornite. 3.2. Conventional flotation The results of a typical standard Northparkes laboratory flotation test on the drill core composite are shown in Table 2. Copper recovery in the rougher– scavenger was 93% and the concentrate assayed about 14% Cu and 1.3% As. After one stage of cleaning, the grade of the cleaner concentrate increased to 24% Cu at a copper recovery of 91%. The arsenic grade of the cleaner concentrate, however, was very high at 2.3% As. Under the conventional flotation scheme utilized here, both the copper and arsenic minerals floated very strongly. There was no selectivity between copper and arsenic. 3.3. Arsenic separation using pulp potential control For each test using pulp potential control as a means of rejecting arsenic, a rougher–scavenger concentrate was produced using the standard Northparkes procedure described previously. This concentrate was then subjected to a cleaning stage in which the pulp potential was controlled at either reducing or oxidizing potentials in an attempt to affect a separation of the arsenic minerals from the other copper minerals.
Table 2 Typical results for rougher–scavenger flotation and one stage of cleaning for the drill core composite using conventional flotation Flotation stage
3.1. Analysis of drill core composite The data in Table 1 show that the composite assayed 0.11% As or 1100 ppm As. This is higher than the current estimate of an average arsenic head grade for the
Rougher–scavenger Cleaner Calculated head a
Recovery (%) Cu
NTCu
92.6 90.6
91.3 89.0
NTCu: Non-tennantite copper.
a
Grade (%) As
Cu
NTCu a
As
96.7 96.0
14.1 23.6 1.24
10.7 17.9 0.95
1.33 2.27 0.11
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Fig. 1. Mineral recovery at 1 min as a function of pulp potential for enargite and chalcopyrite at pH 8 (Senior et al., 2006).
3.3.1. Reducing conditions If the flotation behavior of tennantite was similar to that of enargite, the most likely pulp potential region for making a separation between copper-iron sulfides (such as chalcopyrite and bornite) and tennantite is under reducing conditions. This is based on the findings of Senior et al. (2006) (see Fig. 1) that enargite is more floatable than chalcopyrite under reducing conditions. The data in Fig. 1 clearly show that a window of separation between enargite and chalcopyrite exists at pH 8 in the potential range − 25 and +50 mV SHE whereby it should be possible to float enargite selectively from chalcopyrite. While the data in Fig. 1 was obtained at pH 8, the dependence of enargite and chalcopyrite floatability on pulp potential has been shown to be largely independent of pH (Senior et al., 2006; Trahar, 1984) so a similar separation should be possible at higher pH values. Cleaner flotation at Northparkes is normally performed at pH 12 to assist with pyrite rejection so this was the pH used in the tests reported here. The results of the tests investigating the use of reducing conditions are shown in Fig. 2, a plot of component recovery (after 4 min flotation) as a function of pulp potential at pH 12. The results show that between − 200 and − 130 mV SHE there is a region where tennantite can be floated from the other non-tennantite copper minerals (NTCu). Recovery of arsenic in this region is between 80 and 90% while recovery of NTCu is about 30%. Below −200 mV SHE the flotability of tennantite
begins to decrease and above − 130 mV SHE the flotability of the other copper minerals begin to increase, and so selectivity between copper and arsenic is reduced. The recovery of NTCu in this region is higher than expected from entrainment alone. The mineralogy undertaken on the rougher–scavenger concentrate had indicated that the tennantite was largely liberated but where it was locked it tended to be with bornite. The additional recovery above that of entrainment for the NTCu may be a function of this non-liberation. Further mineralogical examination of the arsenic rich concentrate will be required
Fig. 2. Recovery of arsenic and non-tennantite copper (NTCu) after 4 min flotation as a function of pulp potential at pH 12 using Northparkes drill core composite and reducing conditions.
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to confirm this. Regrinding of the rougher–scavenger was not investigated in this study but may prove beneficial if locking of the tennantite with bornite is confirmed. Regrinding may increase tennantite liberation leading to a cleaner separation while regrinding in a steel mill would also help to provide the reducing environment needed for the separation. The pulp potential region at which the separation of tennantite from chalcopyrite and bornite was possible is lower than that expected from the single mineral data shown in Fig. 1 assuming that tennantite behaves in a similar manner to enargite and bornite in a similar manner to chalcopyrite. The threshold potential for the flotation of both tennantite and the non-tennantite minerals has been shifted about 100 mV in a more reducing direction. There could be many reasons for this shift in the pulp potential. It may be due to differences in the flotation behavior between tennantite and enargite on the one hand and between chalcopyrite and bornite on the other. The latter proposition is in accord with the findings of Richardson and Walker (1985), who saw a similar potential shift between bornite and chalcopyrite in tests conducted with pure minerals in an electrochemical flotation cell. The shift may also be due to hysteresis effects. In some systems, hysteresis in the threshold potential occurs depending upon whether the potential is being shifted to more reducing conditions or more oxidizing conditions (Heyes and Trahar, 1977). In the single mineral tests in Fig. 1 the potential was always shifted from a low potential to a higher one whereas in the testwork on Northparkes drill core the pulp potential in the arsenic rejection stage was shifted down from a high potential to a lower one because the bulk copper– arsenic concentrate was floated in air before the rejection step. Whatever the reason for the shift in pulp potential, it was still possible to make a separation of tennantite from chalcopyrite and bornite under reducing conditions. 3.3.2. Oxidizing conditions While the data in Fig. 1 indicate that there is not a region at high pulp potential (oxidizing conditions) where enargite and chalcopyrite could be separated, some copper minerals, such as chalcocite and cuprite, have been shown to have an upper limiting flotation threshold above which they do not float (Heyes and Trahar, 1979). While neither enargite nor chalcopyrite has shown an upper threshold in the tests in Fig. 1, it was considered prudent to conduct a few tests at high potential in case the behavior of tennantite at oxidizing conditions was significantly different from that of enargite. The results of tests investigating the use of oxidizing conditions are shown in Fig. 3, a plot of component
Fig. 3. Recovery of arsenic and non-tennantite copper (NTCu) after 4 min flotation as a function of pulp potential at pH 12 using Northparkes drill core composite and oxidising conditions.
recovery (after 4 min flotation) as a function of pulp potential at pH 12. These data show that the flotability of both tennantite and the non-tennantite copper minerals is strong up to +400 mV SHE and above +400 mV SHE the flotability of both tennantite and the nontennantite copper minerals decreases. There was no window where a separation of tennantite from the nontennantite copper minerals could be made using oxidizing conditions and so no further work was completed in this region. 3.4. Copper flotation after arsenic rejection After the non-tennantite copper minerals had been depressed and the tennantite removed by flotation under reducing conditions, the non-tennantite copper minerals were then floated by changing the flotation gas from nitrogen to synthetic air. Once the pulp potential had risen to the air-set potential (+ 120 mV SHE), the nontennantite copper minerals were removed to produce a low-arsenic copper concentrate as the froth product. Table 3 shows the results of two integrated flowsheet tests in which both a low-arsenic copper concentrate and high-arsenic copper concentrate were produced using the above methodology. The pulp potentials in the arsenic rejection stage for these two tests were − 130 mV SHE and − 200 mV SHE, i.e. in the region identified above where tennantite can be floated from the other non-tennantite copper minerals. Also included in Table 3 for comparison are the results of a baseline test with one stage of conventional cleaning (conducted at the natural Eh). The data in Table 3 confirm that at the reducing potentials tested high levels of arsenic rejection (N87%)
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Table 3 Integrated flowsheet and conventional cleaning results for the drill composite Arsenic separation potential
Product
Recovery (%)
Grade (%)
Cu
NTCu
a
As
Cu
NTCu a
As
− 130 mV SHE
Low-As product High-As product Cleaner tailing Rougher–scavenger tailing Calculated head
45.6 44.2 2.79 7.32
56.6 31.9 3.26 8.24
7.68 87.0 1.13 4.16
27.0 24.6 1.06 0.10 1.26
26.0 13.7 0.96 0.09 0.98
0.40 4.26 0.04 0.01 0.11
− 200 mV SHE
Low-As product High-As product Cleaner tailing Rougher–scavenger tailing Calculated head
40.5 47.7 2.89 8.95
52.0 34.0 3.37 10.6
4.08 91.0 1.36 3.56
27.4 26.2 0.73 0.12 1.22
26.7 14.2 0.65 0.11 0.93
0.26 4.70 0.03 0.00 0.12
Conventional cleaning (natural Eh)
Cleaner concentrate Cleaner tailing Rougher–scavenger tailing Calculated head
90.6 1.98 7.43
89.0 0.70 3.28
96.0 2.36 8.67
23.6 0.72 0.10 1.24
17.9 0.66 0.00 0.95
2.27 0.02 0.09 0.11
a
NTCu: Non-tennantite copper.
to a high-arsenic low-copper concentrate are readily achieved. Because the head grade of this drill core was so high it was necessary to reject over 90% of the arsenic in the controlled-potential flotation stage in order to achieve an arsenic grade in the low-arsenic highcopper product approaching the target of 2000 ppm As. The best result obtained was a low-arsenic copper concentrate containing 52% of the NTCu and assaying 2600 ppm As. The total copper recovery (to both the low-arsenic and high-arsenic products) in this test was 88.2%. By contrast, with one stage of conventional cleaning, 91% of the total copper is recovered but only 4% of the arsenic is rejected leaving a cleaner concentrate assaying 22,700 ppm As. 3.5. Simulations The arsenic head grade for the drill core tested was 0.11% As which is over five times that of the
current estimate of the average head grade of the ore to be mined. This high head grade was advantageous during testing of separation methods but also meant that it was unlikely that a concentrate assaying less than 2000 ppm As (the target arsenic assay in copper concentrate) could be produced from this drill core. Computer simulations were used to estimate the performance of the integrated arsenic rejection flowsheet treating a feed having a head assay of 1% Cu and 200 ppm As, that is, a feed more typical of that which is expected to be treated. The results of the simulations are shown in Table 4. The separation efficiencies from the test at − 130 mV SHE were used in the simulation of the arsenic rejection flowsheet. It is predicted that the low-arsenic copper concentrate would contain 54% of the total copper and 8% of the arsenic in a product that assayed 27% Cu and 730 ppm As, which is well below the arsenic limit of 2000 ppm As. The high-arsenic copper
Table 4 Predicted results for an arsenic rejection flowsheet and a one-stage conventional cleaning circuit treating a typical fresh feed of 200 ppm As and 1% Cu Flowsheet type
Product
Recovery (%)
Grade (%)
Cu
NTCu
a
As
Cu
NTCu a
As b
Arsenic rejection
Low-As product High-As product Calculated head
54.3 34.6
56.7 31.9
7.69 87.0
26.6 18.9 1.03
26.4 16.6 0.98
730 9219 200
Conventional cleaning
Cleaner concentrate Calculated head
88.8
88.5
94.6
26.0 1.03
24.7 0.98
5377 200
a b
NTCu: Non-tennantite copper. As grade in ppm.
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study, is presented in Fig. 5. Briefly, the process involves the following steps. • A bulk copper–arsenic flotation concentrate is produced using standard flotation conditions. • The bulk copper–arsenic concentrate is reground and becomes the feed to the arsenic rejection step. While
Fig. 4. Predicted grade-recovery curve for a low-arsenic product from an integrated flowsheet treating a copper ore with 200 ppm As and 1% Cu head grade.
concentrate would contain 35% of the total copper and 87% of the arsenic in a product assaying 19% Cu and 9200 ppm As. Included in Table 4 are the predicted assays and recoveries of a copper concentrate obtained by one stage of conventional cleaning of the rougher– scavenger concentrate. In this case the arsenic grade of the copper concentrate would be in excess of 5300 ppm As. As discussed previously, copper recovery to the lowarsenic concentrate will be dependent upon the amount lost to the high-arsenic product. Reducing the amount of arsenic rejected (and hence the NTCu loss in the arsenic separation step) would increase copper recovery to the low-arsenic product but with an increase in arsenic grade. Using the separation efficiencies from timed concentrates in the − 130 mV SHE test it was possible to estimate the arsenic grade and copper recovery of the low-arsenic concentrate if a less efficient arsenic rejection was made in the arsenic separation step. The results of the calculations are shown in Fig. 4, a plot of the arsenic grade of the low-arsenic product against the recovery of total copper to the low-arsenic product. It is estimated that copper recovery to the low-arsenic product could be increased by up to 7% (from 54 to 61%) while maintaining the arsenic grade of the same product below 2000 ppm As. There would be a corresponding decrease in copper recovery to the higharsenic product with an increase in the arsenic grade of this product from 9219 to 9438 ppm As. 3.6. Conceptual flowsheet A conceptual flowsheet for the rejection of arsenic from the Northparkes ore, based on the results of this
Fig. 5. Conceptual flowsheet for the treatment of a high-arsenic copper ore containing chalcopyrite, bornite and tennantite.
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it is unknown at present whether liberation is limiting the complete depression of the NTCu, the use of a regrind mill at this stage would be useful in generating the reducing conditions necessary for the arsenic– copper separation. • In the arsenic rejection stage, the pH is raised to 12 with lime and the pulp potential set to about − 150 mV SHE. At this potential the tennantite is floated while the chalcopyrite and bornite remain unfloated. The carrier gas in this step is nitrogen. The froth product is termed the high-arsenic copper concentrate and this product cannot be sent to a smelter. • The pulp potential is then raised to the air-set potential by changing the flotation gas to air and the chalcopyrite and bornite are floated. The pH remains at pH 12. The froth product is termed the low-arsenic copper concentrate and can be sent straight to a smelter to produce copper metal. The exact circuit configuration, reagent types and addition rates, and carrier gas requirements for the arsenic rejection step are yet to be determined or optimized and as such are beyond the scope of this study. However, it is noted that the bulk copper–arsenic concentrate, which is the feed to this circuit is a lowflow stream and hence the control of the pulp potential is likely to be easier and the addition rate of any potentialcontrolling reagents is likely to be lower. 3.7. Environmental considerations Finally, it should be remembered that the high-arsenic copper concentrate will have a significant copper content since tennantite itself contains 52% copper and the concentrate will also contain a small amount of chalcopyrite and bornite. The treatment of high-arsenic copper concentrates at present is problematic. However, an alternate approach to dealing with high-arsenic low-copper concentrates produced in the type of flotation circuit proposed here has recently been put forward (Jahanshahi et al., 2006). It is suggested that one way of treating this concentrate would be to selectively roast it, to fume off and capture the arsenic, and to then stabilize the arsenic product in a form that could be directly returned to the mine in a low-volume stream. This way little of the arsenic in the ore actually gets to the smelter and hence the dispersion of arsenic into the biosphere during smelting and refining is greatly diminished. The low-arsenic high-copper product would be sent directly to the smelter to produce copper metal. Not only does this treatment route have the advantage of closing the loop on arsenic deportment, but
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also any inefficiency of copper–arsenic separation in the arsenic rejection stage would not be as critical since the chalcopyrite and bornite lost to the high-arsenic product would be recovered in the calcines during roasting. The calcines would also be sent directly to the smelter to produce copper metal. This new approach would be worth testing on Northparkes high-arsenic copper ore. 4. Conclusions A flotation study undertaken on a sample of higharsenic Northparkes drill core has shown that it is possible to selectively separate tennantite from chalcopyrite and bornite using pulp potential control. The separation is made from a bulk copper–arsenic concentrate by reducing the pulp potential to about − 150 mV SHE at pH 12 and floating the tennantite from the other copper minerals. The pulp potential dependence of tennantite appears to be similar to that of enargite, another copper–arsenic sulphide, in that the lower limiting pulp potential threshold for tennantite is lower than that for chalcopyrite and bornite such that there is a potential window in the reducing region where tennantite is strongly floatable but chalcopyrite and bornite are not. In the oxidising pulp potential region there is little or no selectivity between tennantite and the other copper minerals for the pulp potential range examined. From the drill core tested, with a head grade of 0.11% As and 1.2% Cu, it was possible to produce a lowarsenic high-copper concentrate containing 52% of the non-tennantite copper and assaying 2600 ppm As. Computer simulations have shown that for a feed containing a more typical arsenic and copper level, that is, 200 ppm As and 1% Cu, the efficiency of separation should be sufficient to concentrate about 61% of the copper in a product assaying less than the present smelter penalty limit of 2000 ppm As. Acknowledgement North Ltd. (now part of Rio Tinto Limited) is thanked for supporting the work and for providing the drill core used in this study. The staff of the Analytical Services Group of CSIRO Minerals is thanked for conducting the chemical and mineralogical analyses associated with the testwork. References Castro, S.H., Honores, S., 2000. Surface properties and floatability of enargite. In: Massacci, P. (Ed.), Proc. of XXI Int. Miner. Process. Cong. p. B8b-47 (Rome—Italy).
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Fornasiero, D., Fullston, D., Li, C., Ralston, J., 2001. Separation of enargite and tennantite from non-arsenic copper sulphide minerals by selective oxidation or dissolution. Int. J. Miner. Process. 61, 109–119. Guo, H., Yen, W.T., 2005. Selective flotation of enargite from chalcopyrite by electrochemical control. Miner. Eng. 18 (6), 605–612. Guo, H., Yen, W.T., 2006. Electrochemical floatability of enargite and effects of depressants. In: Onal, G., Acarkan, N., Celik, S., Arslan, F., Atesok, G., Guney, A., Sirkeci, A.A., Yuce, E., Perek, K.T. (Eds.), Proc. of XXIII Int. Miner. Process. Cong., Instanbul, Turkey, vol. 1, pp. 504–509. Guy, P.J., 1992. The development of laboratory batch flotation equipment and practice at CSIRO, Australia. The AusIMM Annual Conference, Broken Hill. The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 75–79. Heyes, G.W., Trahar, W.J., 1977. The natural flotability of chalcopyrite. Int. J. Miner. Process. 4, 317–344. Heyes, G.W., Trahar, W.J., 1979. Oxidation–reduction effects in the flotation of chalcocite and cuprite. Int. J. Miner. Process. 6, 229–252. R.O. Huch, 1993. Method for achieving enhanced copper-containing mineral concentrate grade by oxidation and flotation. Patent WO 93/22060-PCT/US93/04271. Jahanshahi, S., Bruckard, W.J., Chen, C., Jorgensen, F.R.A., 2006. Management of minor elements in the production of base metals. Green Processing 2006, 3rd International Conference on the Sustainable Processing of Minerals, Newcastle, Australia. The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 25–30. Jaime, H.E., Cifuentes, G., 1995. Alternative treatment for high arsenic-bearing copper ores through the modification of pulp
electrochemical potential. In: Casali, A., Dobby, G.S., Moline, M., Thoburn, W.J. (Eds.), Proc. of Copper 95, Int. Conf. vol. II—Miner. Process. and Environ. Metal. Soc. of CIM, Canada, pp. 197–202. Kelsall, D.F., Restarick, C.J., Stewart, P.S.B., 1974. Technical note on an improved cyclosizing technique. Proc. Australas. Inst. Min. Metall, No. 251, pp. 9–10. Light, T.S., 1972. Standard solution for redox potential measurement. Anal. Chem.. 48, 1038–1039. Menacho, J.M., Aliaga, W., Valenuela, R., Ramos, V., Olivares, I., 1993. Selective flotation of enargite and chalcopyrite. Minerals, Vol. 48 (203), 33–39. Richardson, P.E., Walker, G.W., 1985. The flotation of chalcocite, bornite, chaclcopyrite, and pyrite in an electrochemical-flotation cell. In: Richardson, P.E., Woods, R. (Eds.), Proc. of XV Int. Miner. Process. Cong., vol. 2, pp 198–210, Edition Gedim, St. Etienne, France. Senior, G.D., Guy, P.J., Bruckard, W.J., 2006. The selective flotation of enargite from other copper minerals — a single mineral study in relation to beneficiation of the Tampakan deposit in the Philippines. Int. J. Miner. Process. 81, 15–26. Trahar, W.J., 1984. The influence of pulp potential in sulfide flotation. In: Jones, M.H., Woodcock, J.T. (Eds.), Principles of Mineral Flotation, The Wark Symposium. The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 117–135. Wilson, P.C., Chanroux, C., 1993. Revenue, calculation and marketing; copper. In: Noakes, M., Lanz, T. (Eds.), Cost Estimation Handbook for the Australian Mining Industry. The Australian Institute of Mining and Metallurgy, Parkville, pp. 325–368. Yen, W.T., Tajadod, J., 2000. Selective flotation of enargite and chalcopyrite. In: Massacci, P. (Ed.), Proc. of XXI Int. Miner. Process. Cong. p. B8a-49 (Rome—Italy).
The separation of arsenic from copper in a Northparkes copper–gold ore using controlled-potential flotation L.K. Smith, W.J. Bruckard ⁎ CSIRO Division of Minerals, Box 312, Clayton South, Victoria, 3169, Australia Received 18 January 2007; received in revised form 3 May 2007; accepted 16 May 2007 Available online 24 May 2007
Abstract In order for the high-arsenic regions of the Northparkes copper–gold orebody to be beneficiated economically, tennantite ((Cu,Fe)12As4S13) present in the ore needs to be rejected to enable copper concentrates to meet the typical smelter penalty level of 2000 ppm As. Using a composite sample of high-arsenic drill cores from Northparkes it was possible to selectively separate tennantite from chalcopyrite (CuFeS2) and bornite (Cu5FeS4) using controlled-potential flotation. The separation was made on a bulk copper– arsenic concentrate after reducing the pulp potential to about −150 mV SHE at pH 12 and floating the tennantite from the other copper minerals. The basis of the separation relies on findings that the lower limiting pulp potential threshold for tennantite is lower than that for chalcopyrite such that there is a potential window in the reducing region where tennantite is strongly floatable but chalcopyrite is not. Little or no selectivity between tennantite and chalcopyrite was found in the oxidising pulp potential region for the range examined. From the composite sample tested, which had a head grade of 0.11% As and 1.2% Cu, it was possible to produce a low-arsenic high-copper concentrate containing 52% of the non-tennantite copper and assaying 2600 ppm As. Computer simulations have shown that for a feed containing a more typical arsenic and copper level (200 ppm As and 1% Cu) the efficiency of separation should be sufficient to concentrate about 61% of the copper in a product assaying less than 2000 ppm As. A conceptual flowsheet for arsenic rejection from Northparkes copper–gold ore, based on the findings from this study, is presented and discussed in this paper. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Flotation; Tennantite; Copper; Controlled-potential flotation
1. Introduction Rio Tinto Limited operates the Northparkes copper– gold mine located in central NSW, Australia. The ore typically contains about 1% Cu and the main sulphide copper minerals present are chalcopyrite and bornite. However, some parts of the orebody are high in arsenic ⁎ Corresponding author. Tel.: +61 3 9545 8500; fax: +61 3 9562 8919. E-mail address: [email protected] (W.J. Bruckard).
content (above 200 ppm As) with the arsenic occurring mainly as the copper–arsenic mineral tennantite ((Cu, Fe)12As4S13). In order to produce copper concentrates from these sections of the orebody that meet smelter specifications arsenic will need to be rejected during beneficiation. Preliminary conventional flotation testwork conducted on the high-arsenic material has produced concentrates containing 5000–7000 ppm As. Arsenic is a penalty element for copper smelters where it can cause environmental problems when volatile arsenic
0301-7516/$ - see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2007.05.002
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compounds are emitted during smelting. Normally, high financial penalties are imposed by smelters to treat copper concentrates containing higher than 2000 ppm As (Wilson and Chanroux, 1993). Arsenic-bearing sulfide minerals like tennantite are difficult to separate from other non-arsenic sulphide copper minerals in conventional copper flotation circuits. The most successful approaches to effecting separation appear to be based on selective oxidation techniques (Fornasiero et al., 2001; Castro and Honores, 2000; Huch, 1993) or the exploitation of pulp potential effects (Menacho et al., 1993; Jaime and Cifuentes, 1995; Yen and Tajadod, 2000; Guo and Yen, 2005, 2006). Recently, as part of a project aimed at identifying options for an arsenic rejection circuit for the Tampakan (Philippines) copper flotation flowsheet, CSIRO Minerals developed methods for the selective flotation of enargite (Cu3AsS4), another arsenic-bearing sulphide mineral similar to tennantite, from other sulphide copper minerals based on the flotation response of various copper minerals and enargite to changes in pulp potential (Senior et al., 2006). One of the separation methods proposed was successfully tested on a sample of higharsenic Tampakan drill core, enabling the production of high-arsenic and low-arsenic copper concentrates. The low-arsenic copper concentrate was low enough in arsenic content to be sent to a conventional smelter for copper metal production while the high-arsenic copper concentrate would need to be treated by some other route to recover the entrained copper values. Given these controlled-potential flotation techniques had proved beneficial in the selective flotation of enargite from other sulphide copper minerals in the Tampkan ore, it was deemed worthwhile to determine whether the same methods would also be applicable to the selective flotation of tennantite from Northparkes ore. This paper details the results of a preliminary study on a sample of high-arsenic Northparkes drill core to assess whether controlled-potential flotation could be used to make a separation between tennantite and other sulphide copper minerals present to produce a lowarsenic copper concentrate suitable for smelting and a high-arsenic copper concentrate.
2.1. Drill core sample
2. Experimental
Table 1 Chemical analysis of drill core composite
Experimental work conducted included preparation of the high-arsenic drill core, flotation tests at various Eh and pH values, chemical and mineralogical analysis of flotation feed and product samples, and simulations to determine the performance of the proposed separation flowsheet on Northparkes ore of more typical head grade.
Head sample
The sample tested was a composite of several drill core intersections from a high-arsenic section of the Northparkes orebody. The intersections were stage crushed to pass 2 mm, blended to form a composite, and the composite divided into 1 kg lots for testing by standard means. Sub-samples were also split out for chemical and mineralogical analysis. The head assays of the composite are given in Table 1, which also contains the average calculated head assays from all the batch tests completed. These data show that the composite was particularly high in arsenic, assaying 0.11% As or 1100 ppm As. This is higher than the current estimate of an average arsenic head grade for the higharsenic section of the Northparkes orebody (200 ppm As). A drill core sample with high-arsenic head grade was specifically chosen for this study to increase the amount of arsenic mineral available for flotation testing in the arsenic rejection stage. 2.2. Grinding For each test, 1000 g of the drill core was mixed with Melbourne tap water and ground for 29 min in a cast iron rod mill with 15 rods at 60% solids to give a P80 (80% passing size) by weight of 90 μm. Before each test the mill and rod charge were cleaned by grinding a sample of quartz for 10 min. 2.3. Flotation 2.3.1. Reagents Northparkes supplied the reagents used in the rougher–scavenger. Solutions of sodium hydrosulfide (NaHS) and the frother, Interfroth 68, were added to the pulp without further dilution. The collectors, sodium isobutyl xanthate (SiBX) and AP 208 (a dithiophosphate), were made up fresh each day as 1% w/v solutions in distilled water. Make-up water was Melbourne tap water and the flotation gas was high purity bottled synthetic air (a synthetic
Assay head Calculated head b a b
Element (%) Cu
As
S
NTCu a
1.21 1.26
0.11 0.11
0.68 0.78
0.93 0.98
NTCu: Non-tennantite copper. Average of all flotation tests conducted.
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mixture of O2 and N2) during the rougher–scavenger stages and either the same synthetic air or high purity bottled nitrogen during the arsenic rejection stages. The reagents used to adjust the pulp potential during the arsenic rejection stage were either laboratory grade sodium hypochlorite (NaClO) or the same sodium hydrosulfide solution as was used in the rougher–scavenger. A 2.5% w/w suspension of laboratory grade lime was used to set the pH in the arsenic rejection stage. 2.3.2. Equipment The samples were floated in modified Denver style cells (Guy, 1992) in which the impellers were driven from below to allow the whole surface of the froth to be scraped with a paddle at constant depth and at constant time intervals. Two sizes of cells were used: a 3 dm3 cell for rougher–scavenger flotation and a 1 dm3 cell for further separation stages. The cells were fitted with a rubber diaphragm, sight tube and electronic sensor for automatic detection and control of pulp level. The pulp potential was measured continuously during testing using a high-impedance differential voltmeter with a polished platinum flag electrode and silver/silver chloride reference electrode. The performance of the electrode system was checked using a standard ferric– ferrous ion solution (Light, 1972). Measured potential values were converted to the standard hydrogen electrode (SHE) scale by the addition of 0.2 V. The pH of the pulp was also monitored continuously with a Radiometer glass/calomel electrode calibrated using standard pH 7 and pH 10 buffer solutions before each test. Radiometer TTT80 titrators and ABU80 burettes were used to add oxidant/reductant and acid/base to set and maintain the Eh and pH respectively. 2.3.3. Procedures The approach taken in this study was to produce a rougher–scavenger concentrate, using a standard laboratory procedure supplied by Northparkes, and then to use pulp potential control during a subsequent flotation stage on this concentrate to make an arsenic separation. The ground slurry was transferred to the 3 dm3 flotation cell and the pulp level set. The pH of the rougher– scavenger was the natural pH of the sample and, in these tests, was approximately pH 9.8. In the rougher stage, 100 g/t of sodium hydrosulfide, 10 g/t of xanthate and 30 g/t of AP208 were added and the pulp conditioned for 4 min. Frother (50 g/t) was added 1 min before flotation and concentrate collected for 6 min. In the scavenger stage, an additional 5 g/t of xanthate was added, the pulp conditioned for 1 min and concentrate collected for a further 6 min.
17
For the arsenic rejection stage, the combined rougher– scavenger concentrate was transferred to the 1 dm3 flotation cell, the pH set to 12 with lime, and the pulp level set. Two regions of pulp potential were examined: an oxidizing region and a reducing region. If an oxidizing region was being tested, the pulp potential was adjusted to the required value using sodium hypochlorite, the pulp conditioned at that potential for 10 min and then concentrates collected for up to 10 min using synthetic air as the flotation gas. If a reducing region was being tested, the pulp potential was adjusted to the required value using sodium hydrosulfide, the pulp conditioned at that potential for 5 min and then concentrates collected for up to 10 min using nitrogen as the flotation gas. Small amounts of frother were added as required to maintain an active froth column. All flotation products were weighed wet, to allow calculation of water recoveries, dried and re-weighed, and then prepared for analysis in a standard manner. 2.4. Chemical analysis of products Sub-samples of the drill core composite and solid flotation products were assayed for copper, arsenic and sulfur by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using a standard method. Arsenic assays were used to determine tennantite (51.6% Cu; 20.3% As), and copper not accounted for in tennantite has been termed “non-tennantite copper”, or NTCu, and will include copper present in bornite and chalcopyrite. 2.5. Sizing analyses Sizing analyses was conducted using standard laboratory wet and dry screening methods. Where subsieve sizing was required a modified CSIRO Cyclosizing technique was used (Kelsall, Restarick, and Stewart, 1974). All size fractions were weighed and analysed by ICP-AES. 2.6. Mineralogical analyses The copper and arsenic mineralogy of the drill core sample and the textural associations of the minerals present were determined. The procedure used was to isolate the bulk of the sulphide minerals in a rougher– scavenger concentrate, to determine the minerals present by X-ray diffraction (XRD) and to analyse selected size fractions of the concentrate by scanning electron microscopy (SEM). The rougher–scavenger concentrate produced for the mineralogical analysis contained over 94% of the copper and arsenic in the feed.
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X-ray diffraction patterns were recorded on a Philips PW 1050 goniometer with a PW 1710 diffraction controller using CuKα radiation. Phases present were identified by comparison of the peak positions and intensities with data published by the International Centre for Diffraction Data (ICDD). Selected size fractions for SEM were mounted, polished, and carbon coated prior to be being analysed by SEM using a Joel superprobe fitted with wavelength dispersive detectors. The probe data obtained was analysed using CSIRO's in-house software package CHIMAGE. 2.7. Simulations Due to the higher than normal arsenic head grade of the drill core used for the flotation testing it was necessary to perform simulations to determine the performance of the proposed separation on ore of more typical head grades. Computer simulation packages developed by CSIRO were used to simulate the arsenic rejection flowsheet on ore of differing head grades and compare the results to a flowsheet without an arsenic rejection stage. For the simulations, the flotation feed was assumed to comprise four mineral components: tennantite, bornite, chalcopyrite and non-sulfide gangue (NSG). Bornite and chalcopyrite were assumed to be present in the feed in the ratio 2:1 based on the mineralogical investigation. The program then calculated the distribution of these components to various products and converted the results to grades and recoveries for Cu, As, NTCu and NSG. Distributions were calculated using the relevant, experimentally determined separation efficiencies. The model was used only in open circuit operation i.e., all results are for flowsheets without recycles. In practice, improved recoveries might reasonably be expected by recycling some streams. 3. Results and discussion The results are discussed in terms of characterisation of the drill core composite, the effectiveness of separating arsenic and copper minerals using controlledpotential flotation, and the assessment of simulations conducted to determine the performance of the proposed separation flowsheet on Northparkes ore of more typical head grade.
Northparkes orebody. Assuming all the arsenic present is in tennantite it follows that the NTCu assay for the composite (based on the feed assay) is 0.93% NTCu, meaning approximately 23% of the copper present in the composite occurs as tennantite. The important point here to note is that rejecting all the arsenic in any modified flowsheet will also reject 23% of the total copper present. The major sulfides detected by XRD analyses were bornite (Cu5FeS4), chalcopyrite (CuFeS2) and tennantite ((Cu,Fe)12As4S13). A small amount of pyrite was also detected. The non-sulfides identified were quartz, muscovite, dolomite and kaolinite. The electron microscopy analysis gave the same mineralogy except that a small amount of chalcocite (Cu2S) was also found. The data also revealed that the tennantite was largely liberated but, where it was locked, it tended to be locked with bornite. 3.2. Conventional flotation The results of a typical standard Northparkes laboratory flotation test on the drill core composite are shown in Table 2. Copper recovery in the rougher– scavenger was 93% and the concentrate assayed about 14% Cu and 1.3% As. After one stage of cleaning, the grade of the cleaner concentrate increased to 24% Cu at a copper recovery of 91%. The arsenic grade of the cleaner concentrate, however, was very high at 2.3% As. Under the conventional flotation scheme utilized here, both the copper and arsenic minerals floated very strongly. There was no selectivity between copper and arsenic. 3.3. Arsenic separation using pulp potential control For each test using pulp potential control as a means of rejecting arsenic, a rougher–scavenger concentrate was produced using the standard Northparkes procedure described previously. This concentrate was then subjected to a cleaning stage in which the pulp potential was controlled at either reducing or oxidizing potentials in an attempt to affect a separation of the arsenic minerals from the other copper minerals.
Table 2 Typical results for rougher–scavenger flotation and one stage of cleaning for the drill core composite using conventional flotation Flotation stage
3.1. Analysis of drill core composite The data in Table 1 show that the composite assayed 0.11% As or 1100 ppm As. This is higher than the current estimate of an average arsenic head grade for the
Rougher–scavenger Cleaner Calculated head a
Recovery (%) Cu
NTCu
92.6 90.6
91.3 89.0
NTCu: Non-tennantite copper.
a
Grade (%) As
Cu
NTCu a
As
96.7 96.0
14.1 23.6 1.24
10.7 17.9 0.95
1.33 2.27 0.11
L.K. Smith, W.J. Bruckard / Int. J. Miner. Process. 84 (2007) 15–24
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Fig. 1. Mineral recovery at 1 min as a function of pulp potential for enargite and chalcopyrite at pH 8 (Senior et al., 2006).
3.3.1. Reducing conditions If the flotation behavior of tennantite was similar to that of enargite, the most likely pulp potential region for making a separation between copper-iron sulfides (such as chalcopyrite and bornite) and tennantite is under reducing conditions. This is based on the findings of Senior et al. (2006) (see Fig. 1) that enargite is more floatable than chalcopyrite under reducing conditions. The data in Fig. 1 clearly show that a window of separation between enargite and chalcopyrite exists at pH 8 in the potential range − 25 and +50 mV SHE whereby it should be possible to float enargite selectively from chalcopyrite. While the data in Fig. 1 was obtained at pH 8, the dependence of enargite and chalcopyrite floatability on pulp potential has been shown to be largely independent of pH (Senior et al., 2006; Trahar, 1984) so a similar separation should be possible at higher pH values. Cleaner flotation at Northparkes is normally performed at pH 12 to assist with pyrite rejection so this was the pH used in the tests reported here. The results of the tests investigating the use of reducing conditions are shown in Fig. 2, a plot of component recovery (after 4 min flotation) as a function of pulp potential at pH 12. The results show that between − 200 and − 130 mV SHE there is a region where tennantite can be floated from the other non-tennantite copper minerals (NTCu). Recovery of arsenic in this region is between 80 and 90% while recovery of NTCu is about 30%. Below −200 mV SHE the flotability of tennantite
begins to decrease and above − 130 mV SHE the flotability of the other copper minerals begin to increase, and so selectivity between copper and arsenic is reduced. The recovery of NTCu in this region is higher than expected from entrainment alone. The mineralogy undertaken on the rougher–scavenger concentrate had indicated that the tennantite was largely liberated but where it was locked it tended to be with bornite. The additional recovery above that of entrainment for the NTCu may be a function of this non-liberation. Further mineralogical examination of the arsenic rich concentrate will be required
Fig. 2. Recovery of arsenic and non-tennantite copper (NTCu) after 4 min flotation as a function of pulp potential at pH 12 using Northparkes drill core composite and reducing conditions.
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to confirm this. Regrinding of the rougher–scavenger was not investigated in this study but may prove beneficial if locking of the tennantite with bornite is confirmed. Regrinding may increase tennantite liberation leading to a cleaner separation while regrinding in a steel mill would also help to provide the reducing environment needed for the separation. The pulp potential region at which the separation of tennantite from chalcopyrite and bornite was possible is lower than that expected from the single mineral data shown in Fig. 1 assuming that tennantite behaves in a similar manner to enargite and bornite in a similar manner to chalcopyrite. The threshold potential for the flotation of both tennantite and the non-tennantite minerals has been shifted about 100 mV in a more reducing direction. There could be many reasons for this shift in the pulp potential. It may be due to differences in the flotation behavior between tennantite and enargite on the one hand and between chalcopyrite and bornite on the other. The latter proposition is in accord with the findings of Richardson and Walker (1985), who saw a similar potential shift between bornite and chalcopyrite in tests conducted with pure minerals in an electrochemical flotation cell. The shift may also be due to hysteresis effects. In some systems, hysteresis in the threshold potential occurs depending upon whether the potential is being shifted to more reducing conditions or more oxidizing conditions (Heyes and Trahar, 1977). In the single mineral tests in Fig. 1 the potential was always shifted from a low potential to a higher one whereas in the testwork on Northparkes drill core the pulp potential in the arsenic rejection stage was shifted down from a high potential to a lower one because the bulk copper– arsenic concentrate was floated in air before the rejection step. Whatever the reason for the shift in pulp potential, it was still possible to make a separation of tennantite from chalcopyrite and bornite under reducing conditions. 3.3.2. Oxidizing conditions While the data in Fig. 1 indicate that there is not a region at high pulp potential (oxidizing conditions) where enargite and chalcopyrite could be separated, some copper minerals, such as chalcocite and cuprite, have been shown to have an upper limiting flotation threshold above which they do not float (Heyes and Trahar, 1979). While neither enargite nor chalcopyrite has shown an upper threshold in the tests in Fig. 1, it was considered prudent to conduct a few tests at high potential in case the behavior of tennantite at oxidizing conditions was significantly different from that of enargite. The results of tests investigating the use of oxidizing conditions are shown in Fig. 3, a plot of component
Fig. 3. Recovery of arsenic and non-tennantite copper (NTCu) after 4 min flotation as a function of pulp potential at pH 12 using Northparkes drill core composite and oxidising conditions.
recovery (after 4 min flotation) as a function of pulp potential at pH 12. These data show that the flotability of both tennantite and the non-tennantite copper minerals is strong up to +400 mV SHE and above +400 mV SHE the flotability of both tennantite and the nontennantite copper minerals decreases. There was no window where a separation of tennantite from the nontennantite copper minerals could be made using oxidizing conditions and so no further work was completed in this region. 3.4. Copper flotation after arsenic rejection After the non-tennantite copper minerals had been depressed and the tennantite removed by flotation under reducing conditions, the non-tennantite copper minerals were then floated by changing the flotation gas from nitrogen to synthetic air. Once the pulp potential had risen to the air-set potential (+ 120 mV SHE), the nontennantite copper minerals were removed to produce a low-arsenic copper concentrate as the froth product. Table 3 shows the results of two integrated flowsheet tests in which both a low-arsenic copper concentrate and high-arsenic copper concentrate were produced using the above methodology. The pulp potentials in the arsenic rejection stage for these two tests were − 130 mV SHE and − 200 mV SHE, i.e. in the region identified above where tennantite can be floated from the other non-tennantite copper minerals. Also included in Table 3 for comparison are the results of a baseline test with one stage of conventional cleaning (conducted at the natural Eh). The data in Table 3 confirm that at the reducing potentials tested high levels of arsenic rejection (N87%)
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21
Table 3 Integrated flowsheet and conventional cleaning results for the drill composite Arsenic separation potential
Product
Recovery (%)
Grade (%)
Cu
NTCu
a
As
Cu
NTCu a
As
− 130 mV SHE
Low-As product High-As product Cleaner tailing Rougher–scavenger tailing Calculated head
45.6 44.2 2.79 7.32
56.6 31.9 3.26 8.24
7.68 87.0 1.13 4.16
27.0 24.6 1.06 0.10 1.26
26.0 13.7 0.96 0.09 0.98
0.40 4.26 0.04 0.01 0.11
− 200 mV SHE
Low-As product High-As product Cleaner tailing Rougher–scavenger tailing Calculated head
40.5 47.7 2.89 8.95
52.0 34.0 3.37 10.6
4.08 91.0 1.36 3.56
27.4 26.2 0.73 0.12 1.22
26.7 14.2 0.65 0.11 0.93
0.26 4.70 0.03 0.00 0.12
Conventional cleaning (natural Eh)
Cleaner concentrate Cleaner tailing Rougher–scavenger tailing Calculated head
90.6 1.98 7.43
89.0 0.70 3.28
96.0 2.36 8.67
23.6 0.72 0.10 1.24
17.9 0.66 0.00 0.95
2.27 0.02 0.09 0.11
a
NTCu: Non-tennantite copper.
to a high-arsenic low-copper concentrate are readily achieved. Because the head grade of this drill core was so high it was necessary to reject over 90% of the arsenic in the controlled-potential flotation stage in order to achieve an arsenic grade in the low-arsenic highcopper product approaching the target of 2000 ppm As. The best result obtained was a low-arsenic copper concentrate containing 52% of the NTCu and assaying 2600 ppm As. The total copper recovery (to both the low-arsenic and high-arsenic products) in this test was 88.2%. By contrast, with one stage of conventional cleaning, 91% of the total copper is recovered but only 4% of the arsenic is rejected leaving a cleaner concentrate assaying 22,700 ppm As. 3.5. Simulations The arsenic head grade for the drill core tested was 0.11% As which is over five times that of the
current estimate of the average head grade of the ore to be mined. This high head grade was advantageous during testing of separation methods but also meant that it was unlikely that a concentrate assaying less than 2000 ppm As (the target arsenic assay in copper concentrate) could be produced from this drill core. Computer simulations were used to estimate the performance of the integrated arsenic rejection flowsheet treating a feed having a head assay of 1% Cu and 200 ppm As, that is, a feed more typical of that which is expected to be treated. The results of the simulations are shown in Table 4. The separation efficiencies from the test at − 130 mV SHE were used in the simulation of the arsenic rejection flowsheet. It is predicted that the low-arsenic copper concentrate would contain 54% of the total copper and 8% of the arsenic in a product that assayed 27% Cu and 730 ppm As, which is well below the arsenic limit of 2000 ppm As. The high-arsenic copper
Table 4 Predicted results for an arsenic rejection flowsheet and a one-stage conventional cleaning circuit treating a typical fresh feed of 200 ppm As and 1% Cu Flowsheet type
Product
Recovery (%)
Grade (%)
Cu
NTCu
a
As
Cu
NTCu a
As b
Arsenic rejection
Low-As product High-As product Calculated head
54.3 34.6
56.7 31.9
7.69 87.0
26.6 18.9 1.03
26.4 16.6 0.98
730 9219 200
Conventional cleaning
Cleaner concentrate Calculated head
88.8
88.5
94.6
26.0 1.03
24.7 0.98
5377 200
a b
NTCu: Non-tennantite copper. As grade in ppm.
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study, is presented in Fig. 5. Briefly, the process involves the following steps. • A bulk copper–arsenic flotation concentrate is produced using standard flotation conditions. • The bulk copper–arsenic concentrate is reground and becomes the feed to the arsenic rejection step. While
Fig. 4. Predicted grade-recovery curve for a low-arsenic product from an integrated flowsheet treating a copper ore with 200 ppm As and 1% Cu head grade.
concentrate would contain 35% of the total copper and 87% of the arsenic in a product assaying 19% Cu and 9200 ppm As. Included in Table 4 are the predicted assays and recoveries of a copper concentrate obtained by one stage of conventional cleaning of the rougher– scavenger concentrate. In this case the arsenic grade of the copper concentrate would be in excess of 5300 ppm As. As discussed previously, copper recovery to the lowarsenic concentrate will be dependent upon the amount lost to the high-arsenic product. Reducing the amount of arsenic rejected (and hence the NTCu loss in the arsenic separation step) would increase copper recovery to the low-arsenic product but with an increase in arsenic grade. Using the separation efficiencies from timed concentrates in the − 130 mV SHE test it was possible to estimate the arsenic grade and copper recovery of the low-arsenic concentrate if a less efficient arsenic rejection was made in the arsenic separation step. The results of the calculations are shown in Fig. 4, a plot of the arsenic grade of the low-arsenic product against the recovery of total copper to the low-arsenic product. It is estimated that copper recovery to the low-arsenic product could be increased by up to 7% (from 54 to 61%) while maintaining the arsenic grade of the same product below 2000 ppm As. There would be a corresponding decrease in copper recovery to the higharsenic product with an increase in the arsenic grade of this product from 9219 to 9438 ppm As. 3.6. Conceptual flowsheet A conceptual flowsheet for the rejection of arsenic from the Northparkes ore, based on the results of this
Fig. 5. Conceptual flowsheet for the treatment of a high-arsenic copper ore containing chalcopyrite, bornite and tennantite.
L.K. Smith, W.J. Bruckard / Int. J. Miner. Process. 84 (2007) 15–24
it is unknown at present whether liberation is limiting the complete depression of the NTCu, the use of a regrind mill at this stage would be useful in generating the reducing conditions necessary for the arsenic– copper separation. • In the arsenic rejection stage, the pH is raised to 12 with lime and the pulp potential set to about − 150 mV SHE. At this potential the tennantite is floated while the chalcopyrite and bornite remain unfloated. The carrier gas in this step is nitrogen. The froth product is termed the high-arsenic copper concentrate and this product cannot be sent to a smelter. • The pulp potential is then raised to the air-set potential by changing the flotation gas to air and the chalcopyrite and bornite are floated. The pH remains at pH 12. The froth product is termed the low-arsenic copper concentrate and can be sent straight to a smelter to produce copper metal. The exact circuit configuration, reagent types and addition rates, and carrier gas requirements for the arsenic rejection step are yet to be determined or optimized and as such are beyond the scope of this study. However, it is noted that the bulk copper–arsenic concentrate, which is the feed to this circuit is a lowflow stream and hence the control of the pulp potential is likely to be easier and the addition rate of any potentialcontrolling reagents is likely to be lower. 3.7. Environmental considerations Finally, it should be remembered that the high-arsenic copper concentrate will have a significant copper content since tennantite itself contains 52% copper and the concentrate will also contain a small amount of chalcopyrite and bornite. The treatment of high-arsenic copper concentrates at present is problematic. However, an alternate approach to dealing with high-arsenic low-copper concentrates produced in the type of flotation circuit proposed here has recently been put forward (Jahanshahi et al., 2006). It is suggested that one way of treating this concentrate would be to selectively roast it, to fume off and capture the arsenic, and to then stabilize the arsenic product in a form that could be directly returned to the mine in a low-volume stream. This way little of the arsenic in the ore actually gets to the smelter and hence the dispersion of arsenic into the biosphere during smelting and refining is greatly diminished. The low-arsenic high-copper product would be sent directly to the smelter to produce copper metal. Not only does this treatment route have the advantage of closing the loop on arsenic deportment, but
23
also any inefficiency of copper–arsenic separation in the arsenic rejection stage would not be as critical since the chalcopyrite and bornite lost to the high-arsenic product would be recovered in the calcines during roasting. The calcines would also be sent directly to the smelter to produce copper metal. This new approach would be worth testing on Northparkes high-arsenic copper ore. 4. Conclusions A flotation study undertaken on a sample of higharsenic Northparkes drill core has shown that it is possible to selectively separate tennantite from chalcopyrite and bornite using pulp potential control. The separation is made from a bulk copper–arsenic concentrate by reducing the pulp potential to about − 150 mV SHE at pH 12 and floating the tennantite from the other copper minerals. The pulp potential dependence of tennantite appears to be similar to that of enargite, another copper–arsenic sulphide, in that the lower limiting pulp potential threshold for tennantite is lower than that for chalcopyrite and bornite such that there is a potential window in the reducing region where tennantite is strongly floatable but chalcopyrite and bornite are not. In the oxidising pulp potential region there is little or no selectivity between tennantite and the other copper minerals for the pulp potential range examined. From the drill core tested, with a head grade of 0.11% As and 1.2% Cu, it was possible to produce a lowarsenic high-copper concentrate containing 52% of the non-tennantite copper and assaying 2600 ppm As. Computer simulations have shown that for a feed containing a more typical arsenic and copper level, that is, 200 ppm As and 1% Cu, the efficiency of separation should be sufficient to concentrate about 61% of the copper in a product assaying less than the present smelter penalty limit of 2000 ppm As. Acknowledgement North Ltd. (now part of Rio Tinto Limited) is thanked for supporting the work and for providing the drill core used in this study. The staff of the Analytical Services Group of CSIRO Minerals is thanked for conducting the chemical and mineralogical analyses associated with the testwork. References Castro, S.H., Honores, S., 2000. Surface properties and floatability of enargite. In: Massacci, P. (Ed.), Proc. of XXI Int. Miner. Process. Cong. p. B8b-47 (Rome—Italy).
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