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Marmatite depression in galena flotation
Minerals Engineering 19 (2006) 860–869 This article is also available online at: www.elsevier.com/locate/mineng
Marmatite depression in galena flotation Keith Quast a
a,*
, Gavin Hobart
q
b,1
Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia b Onesteel-Whyalla Steelworks, Whyalla, SA 5600, Australia Received 13 July 2005; accepted 27 October 2005 Available online 19 December 2005
Abstract Laboratory flotation tests were conducted to investigate the role of pH, zinc sulphate and potassium metabisulphite in the flotation of galena and marmatite in the Broken Hill orebody. Marmatite was depressed under alkaline conditions alone, however the addition of the two depressants, either individually or combined, resulted in the greatest selectivity. The use of zinc sulphate under alkaline conditions resulted in good selectivity whilst maintaining a high lead recovery. The addition of potassium metabisulphite under alkaline conditions reduced zinc recovery, but high additions reduced lead recovery. The addition of both depressants at alkaline pH produced the best result, with a lead concentrate of 72.6% at 98% recovery with less than 4% zinc recovery. The overall aim of the study was to minimise the flotation of marmatite into the galena rougher concentrate, which was achieved under the conditions given above. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation reagents; Flotation depressants
1. Introduction The Broken Hill orebody was originally one of the worldÕs largest massive sulphide deposits. A number of distinct orebodies exist in the Broken Hill deposit, each with its own characteristic association of gangue minerals and its own distinguishing metal ratio. Galena, PbS (86.6% Pb) is the major lead mineral, with the zinc being present as marmatite (Zn,Fe)S, an iron rich sphalerite containing 9–11% iron. Silver is present as argentiferous galena and tetrahedrite, also as pyrargyrite and argentite. The major gangue minerals present are quartz, calcite, rhodonite, garnet, feldspar and fluorite (Higgins and Quast, 1992). Brief histories of the operation of the South Concentrator, previously owned by Pasminco Mining, now operated by Perilya q
Paper originally published in Proceedings Centenary of Flotation Symposium, Brisbane, Australia, 5–9 June, 2005 by the Australasian Institute of Mining and Metallurgy. * Corresponding author. Tel.: +618 8302 3816; fax: +618 8302 3683. E-mail addresses: [email protected] (K. Quast), HobartG @onesteel.com (G. Hobart). 1 Tel.: +618 8644 5508. 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.022
Limited, has been published by Tilyard (1991) and Watters (1993). The aim of this paper is to examine lead/zinc selectivity by manipulating pH in the range 7.5–9.5, the addition of zinc sulphate between 0 and 200 g/t, and the addition of potassium metabisulphite between 0 and 1000 g/t. 2. Literature review 2.1. Effect of pH Historically, ore flotation at the South Concentrator of Pasminco Mining was conducted at a natural pH of 7–8, although lime is often added to pH 8 (Burgess, 1992). The significance of pH as a flotation variable has become more apparent with the introduction of cemented backfilling techniques. Although laboratory testing showed no significant difference between cement and lime as a pH modifier, lead recovery suffered at pH values of 10 or higher (Gauci and Cusack, 1971). Wark and Cox (1934) showed that air–mineral contact for galena in the presence of 25 ppm ethyl xanthate was only possible for a pH less
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than 10.7. Cusack and Stump (1977) reported that galena losses in deleaded tailing only reached 10% above pH 11 in mill water, however this loss was noted at pH 9 using fresh water. Sutherland and Wark (1955) observed that the depressant action of lime was due to the formation of a finely dispersed hydrophilic film of lime on the surface of the sphalerite. Finkelstein and Allison (1976) state that galena floats selectively from sphalerite in a slightly alkaline environment (pH 8–10), with the sphalerite having to be activated (usually by copper ions) before it floats. Higgins and Quast (1992) showed that the addition of lime tended to slightly depress galena flotation, but substantially depress marmatite flotation. Bulatovic and Wyslouzil (1999) examined the addition of soda ash and lime on lead recovery from two complex Canadian lead–zinc ores. Depending on the ore, either soda ash or lime gave a maximum in lead recovery at about pH 9. 2.2. Addition of zinc sulphate Sphalerite tends to resist oxidation, thus very few zinc ions dissolve from the mineral. To ensure zinc sulphide activation is prevented, the activity of zinc ions should be 1000 times that of lead ions in solution. As little zinc ions are present in solution, zinc sulphate is added. Hence, by virtue of the equilibria involving basic lead carbonate and zinc hydroxide, the activity of zinc ions to lead ions is higher than the equilibrium ratio, and activity cannot occur. Solozhenkin and Vasyukevitch (cited by Finkelstein and Allison, 1976) concluded that the adsorption of zinc ions by marmatite, chalcopyrite and pyrite followed much the same path as was observed with sphalerite, except that adsorption was generally higher. In addition to thermodynamic considerations, there is considerable evidence that indicates that the colloids of zinc salts, formed under conditions where precipitation occurs, function as depressants for sphalerite. These include precipitates of zinc hydroxide, zinc carbonate, zinc sulphite and zinc cyanide. The depressant role of zinc hydroxide colloids was presented by Malinovsky in 1946 (cited by Klassen and Mokrousov, 1963). He established that zinc sulphate alone was unable to depress sphalerite, the depressant action of zinc sulphate only occurred in the presence of hydroxyl ions. Test work by Malinovsky indicated that an increase in the alkalinity of the pulp increased the depressing action of zinc sulphate, with zinc hydroxide first precipitating at a pH of 5.2. He proposed that the depressing action of zinc hydroxide was due to the zinc hydroxide being adsorbed on the surface of the sphalerite and preventing the adsorption of xanthate. These observations were later confirmed by Livshitz and Idelson (also cited by Klassen and Mokrousov (1963)) who demonstrated that the extent of depression of sphalerite and the concentration of colloidal zinc hydroxide occurring in the pulp were directly related.
861
Finkelstein and Allison (1976) summarised the possible explanations for the adsorption of these colloidal precipitates on the sphalerite surface as: (1) A precipitate is formed in the bulk solution and attaches itself by physical forces to the surface (i.e. agglomeration or coagulation). (2) A precipitate is formed, or reprecipitates, at the surface through the action of nucleating sites. (3) A monolayer of zinc hydroxy species chemisorbs at the sphalerite surface (e.g. through an oxygen bridge) thus providing a suitable surface for the attachment of or the reprecipitation of precipitates formed in the bulk. (4) A bulk precipitate chemisorbs at the surface. Grano et al. (1988) used zinc sulphate as a sphalerite depressant in the flotation of galena from the heavy medium slimes stream in the Mount Isa lead–zinc concentrator. The addition of zinc sulphate plus either soda ash or lime for pH control resulted in the depression of the sphalerite while not affecting the flotation of the galena. Plant flotation results showed that the addition of 0.9 kg/t of zinc sulphate at pH 8.5 halved the zinc grade of the lead concentrate at almost one third of the zinc recovery. El-Shall et al. (2000) examined the role of zinc sulphate in the depression of lead-activated sphalerite from a more fundamental point of view. They found that the activation of sphalerite by lead ions was due to a replacement reaction where lead replaces zinc on the surface. Depression of leadactivated sphalerite by zinc sulphate was thought due to a reversal of the activation reaction. 2.3. Addition of metabisulphite Misra et al. (1985) reported that sulphite can be used to depress sphalerite if added to the system prior to the addition of xanthate. Generally the sulphite is added in grinding or during conditioning, the amount varying from 1 to 2 kg/t of ore. These workers reported that sulphite has been used previously at Broken Hill as a zinc depressant and also as a cleaning reagent for oxidised galena. The conclusions from this study were: (1) Contact angle measurements and flotation experiments indicated that the addition of sulphite before xanthate resulted in effective sphalerite depression. (2) The homogeneous phase decomposition reaction of xanthate in the presence of sulphite was dependent on the SO2/xanthate mole ratio, dissolved oxygen concentration and pH. The rate of decomposition was very rapid at near neutral pH (6–6.5). (3) Xanthate adsorption on sphalerite was limited to an effective monolayer coverage. (4) The selective depression of sphalerite by sulphite was related to the relative rates of xanthate adsorption in comparison to xanthate decomposition.
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Several possible mechanisms have been suggested for the depressing action of sulphite on sphalerite and also as a flotation modifier during the flotation of sulphide ore. These mechanisms include: (1) Formation of a hydrophilic surface coating such as zinc sulphite or calcium sulphate (when lime is used as a pH regulator). (2) Formation of insoluble heavy metal sulphite salts and subsequent reduction in the heavy metal ion concentration in the pulp, thus preventing unintentional activation. (3) Xanthate decomposition due to sulphite promoted oxidation reaction. (4) Limited collector adsorption by control of the redox potential. Yamamoto (1980) stated that the simultaneous use of zinc ions and sulphite depressed the flotation of sphalerite, but the use of sulphite alone did not. He concluded that ethyl xanthate adsorbed on the surface of activated sphalerite was not desorbed by sulphite ion. Yamamoto (1980) supported the findings of Misra et al. (1985) in the fact that the presence of both oxygen and sul-
phite together results in the decomposition of xanthate to form perxanthate. In the alkaline pH range, the xanthate decomposition was slow, but this does not correspond to the depression of sphalerite which is possible in the presence of sulphite in the alkaline pH range. Depression of sphalerite at higher alkalinity is more probably due to the competition of other anions with xanthate for surface sites. Depression at lower pH values is probably due to the decomposition of xanthate. Pattison (1983) reported that for unactivated sphalerite, the likely formation of zinc sulphite species may be expected to result in depression, although the results of Mitrofanov and Kusnikova (1957) revealed little difference (in alkaline solution) in the xanthate uptake of unactivated sphalerite whether sulphite was present or not. PattisonÕs experiments produced only trace flotation of unactivated sphalerite with ethyl xanthate collector regardless of whether sulphite was used in the flotation system. He concluded that the mineral tested had only a limited number of surface sites available for collection possibly because of some degree of unintentional activation, or because of the iron content of the sphalerite, and that the sulphite did not prevent this limited collection. For copper-activated sphalerite, it was demonstrated that sodium sulphite 45
45
0 ZnSO4 100 ZnSO4 200 ZnSO4
pH 7.5 pH 8.5 pH 9.5
40
40
35
35
30
Zinc Recovery (%)
Zinc Recovery (%)
30
25
20
25
20
15
15
10
10
5
5
0 50
0 50
60
70
80
90
Lead Recovery (%)
Fig. 1. Effect of pH on lead/zinc selectivity.
100
60
70
80
90
100
Lead Recovery (%)
Fig. 2. Effect of zinc sulphate addition (in g/t) on lead/zinc selectivity at pH 7.5.
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was quite an effective depressant in alkaline solution, but that the rate of sphalerite flotation was actually enhanced near to neutral pH by the presence of sulphite. Grano et al. (1997) also investigated the effect of adding sodium metabisulphite (MBS) to the flotation of the Hilton lead–zinc ore. Previous work on the interaction of sodium metabisulphite and ethyl xanthate for the galena flotation from Hilton ore had been reported by Sheldon and Johnson (1988). They identified temperature and contact time as important parameters in the pilot scale processing of this ore. Grano et al. (1997) found that the simultaneous addition of xanthate and MBS decreased galena recovery due to ethyl xanthate decomposition by sulphite ion derived from MBS. The depression increased with increasing MBS addition and pulp temperature. Under these conditions there was an increase in the concentration of metastable ethyl perxanthate in solution, an intermediate product of xanthate decomposition by sulphite ion. Pulp chemical measurements in the Hilton Concentrator also showed increased ethyl perxanthate concentration when MBS was added at the same point in the flotation circuit as ethyl xanthate. In contrast, the application of an aeration stage after MBS addition, but prior to ethyl xanthate addition,
decreased the concentration of unreacted sulphite ion. This was demonstrated by a decreased ethyl perxanthate concentration in solution and restored galena flotation recovery. Under these conditions the dependency of galena flotation on MBS (and temperature) was less marked. Prevention of the solution decomposition of ethyl xanthate by sulphite allowed the depressant action of MBS on sphalerite (and iron sulphide) flotation to be optimised. 3. Materials and equipment The material examined was provided by the then operators of the Pasminco South concentrator. This was received as roughly 1.1 kg moist samples of primary cyclone underflow. The head grade of this ore was 8.5% lead, 60 g/t silver and 10.5% zinc. Adelaide tap water was used throughout the test work. The reagents used were sodium ethyl xanthate, methyl isobutyl carbinol (MIBC), zinc sulphate and potassium metabisulphite. The charges of cyclone underflow were ground in a Linatex lined laboratory rod mill charged with 12 stainless steel rods. Flotation tests were performed in a 2.5 l capacity Agitair flotation cell attached to a Galigher Agitair
45
10
0 ZnSO4 100 ZnSO4 200 ZnSO4
40
863
0 ZnSO4 100 ZnSO4 200 ZnSO4
8
35
6
Zinc Recovery (%)
Zinc Recovery (%)
30
25
20
4
15
10 2
5
0
0 50
60
70
80
90
100
Lead Recovery (%)
Fig. 3. Effect of zinc sulphate addition (in g/t) on lead/zinc selectivity at pH 8.5.
50
60
70
80
90
100
Lead recovery (%)
Fig. 4. Effect of zinc sulphate addition (in g/t) on lead/zinc selectivity at pH 9.5.
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flotation machine model LA-500. Values of pH were measured using a TPS Auto pH meter. 4. Procedure 4.1. Grinding The grind time required to match the rougher feed sizing on the plant (80% passing 200 lm) was determined by grinding the charges with 650 ml of Adelaide tap water.
A second reagent addition of sodium ethyl xanthate and MIBC was made, 10 g/t of each. The pulp was again conditioned for 2 min, then aerated and four more concentrates were floated over one minute intervals by scraping every 15 s. The six flotation concentrates and tailings from each test were filtered and dried. A sample of rougher tail was riffled out and this plus all concentrate products were pulverised and analysed by the Pasminco South Mine assay department. 5. Results
4.2. Flotation 5.1. Grinding test The ground ore was washed into the flotation cell and water added to raise the level of slurry to within 25 mm of the overflow weir. If depressants were used, they were added as dilute solutions and the pH adjusted using lime if required. The pulp was conditioned for 5 min prior to adding the collector and frother, at 50 g/t and 10 g/t respectively. The pulp was then conditioned for a further 2 min, then aerated at 3 l/min. Two concentrate products were removed into separate containers over one minute intervals by scraping every 15 s.
A 20 min grind produced a flotation feed with 80% passing 175 lm, and this was used throughout the test program. The flotation test results are presented as a series of graphs in Figs. 1–10 and a summary of final lead and zinc grades and recoveries presented in Table 1. In order to compare the effects of the reagent additions on lead and zinc flotation behaviour, the flotation data were subjected to analysis using the first order rate equation given below:
45 0 MBS 500 MBS 1000 MBS
45 0 MBS 500 MBS 1000 MBS
40
40
35 35
30
Zinc Recovery (%)
Zinc Recovery (%)
30
25
20
25
20
15
15
10
10
5
5
0
0 50
60
70
80
90
100
Lead Recovery (%)
Fig. 5. Effect of metabisulphite addition (in g/t) on lead/zinc selectivity at pH 7.5.
50
60
70
80
90
100
Lead Recovery (%)
Fig. 6. Effect of metabisulphite addition (in g/t) on lead/zinc selectivity at pH 8.5.
K. Quast, G. Hobart / Minerals Engineering 19 (2006) 860–869 10
10 0 MBS 500 MBS 1000 MBS
100/500 g/t 200/500 g/t 100/1000 g/t 200/1000 g/t
8
8
6
6 Zinc Recovery (%)
Zinc Recovery (%)
865
4
4
2
2
0 50
60
70
80
90
100
0 70
Lead Recovery (%)
80
90
100
Lead Recovery (%)
Fig. 7. Effect of metabisulphite addition on lead/zinc selectivity at pH 9.5. Fig. 8. Effect of combined depressant addition (ZnSO4/MBS in g/t) on lead/zinc selectivity at pH 7.5.
RðtÞ ¼ Rmax ð1 expðktÞ
ð1Þ
where R(t) is the recovery of mineral at time t, Rmax is the theoretical maximum recovery (at infinite time by curve fitting the experimental data) and k is the rate constant in units of time1. The data were fitted to the equation using a least squares fit program, giving the data listed in Table 2. The rate data were only reported to 1 decimal place and the Rmax data to the nearest integer in keeping with the accuracy of the experimental data. 6. Discussion The laboratory grinding size comparison to the plant flotation feed sizing, although not presented here, showed a slightly reduced amount of material coarser than 300 lm compared to the plant feed, possibly as a consequence of using a laboratory rod mill rather than a laboratory ball mill. The 80% passing size of the laboratory 20 min grind was 175 lm compared to the plant value of 200 lm. Under these conditions, liberation should be almost complete, as Cusack and Stump (1977) reported that grinding this ore to 92.5% passing 300 lm achieved 96% liberation of the lead and zinc sulphides.
Fig. 1 shows that lime depresses zinc, particularly at pH 9.5, where zinc recovery was only 6.6% compared to 30– 40% for pH 7.5 and 8.5. The selectivity fell away after the second and third stages of flotation at pH 7.5 and 8.5, possibly due to the fact that most of the fast floating galena had already been floated, so zinc began to float. Similar results were also reported by Higgins and Quast (1992) for ore from the same mine, who found that pH 9.5 resulted in the greatest lead/zinc selectivity. The flotation of the marmatite at the lower pH values could be greater in the laboratory than at the concentrator due to the use of Adelaide tap water that would not have the same concentration of zinc ions present in the circuit water used at Broken Hill. Figs. 2 and 3 show that the addition of small amounts of zinc sulphate greatly increased the lead/zinc selectivity at pH 7.5 and 8.5. At pH 9.5 the effect was not as pronounced (Fig. 3). From Table 1, at pH 7.5 and 100 g/t zinc sulphate, the zinc recovery was 6.3%, whereas at pH 9.5 and 200 g/t zinc sulphate addition, the zinc recovery was 4.7%. The addition of zinc sulphate had no effect on lead recovery, with higher lead grades corresponding to higher zinc
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10
100/500 g/t 200/500 g/t 100/1000 g/t 200/1000 g/t
8
8
6
6 Zinc Recovery (%)
Zinc Recovery (%)
100/500 g/t 200/500 g/t 100/1000 g/t 200/1000 g/t
4
4
2
2
0
0
70
70
80
90
100
80
90
100
Lead Recovery (%)
Lead Recovery (%)
Fig. 9. Effect of combined depressant addition (ZnSO4/MBS in g/t) on lead/zinc selectivity at pH 8.5.
sulphate additions at pH 9.5 (see Table 1). Similar results were also reported by Higgins and Quast (1992). The best results using pH and zinc sulphate additions corresponded to the addition of 200 g/t at pH 9.5, where a lead recovery of 98.7% at a grade of 70.1%, with a zinc recovery of only 4.7% (see Table 1). This result supports the work of both Malinovsky and Livshitz and Idelson (both cited by Klassen and Mokrousov, 1963) in that more alkaline conditions provided an increase in hydroxyl ions, and, along with the greater presence of zinc ions, more colloidal zinc hydroxide will be formed. This would precipitate on the surface of the marmatite and prevents the adsorption of the xanthate, and Livshitz and Idelson demonstrated that the extent of sphalerite depression was related to the concentration of colloidal zinc hydroxide in the pulp. The results in Figs. 5–7 show that the use of metabisulphite alone depressed marmatite, although Yamamoto (1980), Mitrofanov and Kusnikova (1957) and Pattison (1983) suggested that sulphite had little depressant activity on sphalerite in the absence of zinc ions. The addition of MBS was very effective in depressing the marmatite, espe-
Fig. 10. Effect of combined depressant addition (ZnSO4/MBS in g/t) on lead/zinc selectivity at pH 9.5.
cially at the higher addition (1000 g/t). The lower additions of MBS were not as effective as marmatite depressants as the zinc sulphate, however lead recovery remained very high at 98% (see Table 1). The higher addition resulted in very low zinc recoveries, but lead recovery was slightly reduced. The decision as to the optimum additions and conditions rest with the plant operation philosophy i.e. is a higher lead grade or higher recovery the most desirable. It must be remembered that these are roughing tests, so some of the marmatite that floats here could possibly drop out in the lead cleaning stages. The fact that this test work was performed in the alkaline region would appear to reject the proposals of Misra et al. (1985). Their concept of the depressant action of sulphites was due to xanthate decomposition at neutral pH, with very little decomposition of xanthate to perxanthate at pH 9.5. Pattison (1983) also reported that sulphite was a successful depressant for copper-activated sphalerite, but the presence of sulphite had little effect on unactivated sphalerite. This was not the case in the present study. By adding and conditioning the MBS ahead of xanthate addition seemed to avoid the xanthate decomposition by MBS noted by Grano et al. (1997).
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Table 1 Final grades and recoveries for lead and zinc Test no.
pH
Zinc sulph. (g/t)
Pot. meta. (g/t)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5
– – – 100 100 100 200 200 200 – – – – – – 100 100 100 200 200 200 100 100 100 200 200 200
– – – – – – – – – 500 500 500 1000 1000 1000 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000
Lead (%)
Zinc (%)
Gde.
Rec.
Gde.
Rec.
49.2 41.5 67.0 61.1 66.2 68.8 68.1 67.1 70.1 56.8 66.8 69.5 67.8 72.4 69.9 63.9 68.7 72.6 64.7 68.5 62.7 71.2 72.7 65.1 67.7 66.3 66.1
93.6 96.2 97.4 98.7 97.4 98.6 97.5 97.9 98.7 97.6 98.4 97.9 96.4 95.4 95.4 98.1 97.7 98.0 97.2 97.5 97.2 97.4 97.5 96.9 96.5 97.0 96.3
18.4 23.5 5.7 5.5 5.3 5.1 4.5 4.7 3.8 11.2 6.8 5.1 4.9 3.5 4.6 5.1 4.2 3.3 4.9 4.2 5.7 3.6 3.7 5.3 4.5 4.8 4.9
27.6 40.2 6.6 6.3 5.8 5.4 5.0 4.9 4.7 13.8 8.0 8.3 6.5 3.3 4.0 6.1 4.7 3.7 5.4 4.5 6.7 3.9 4.0 5.5 4.7 5.2 5.1
Table 2 Calculated values of rate constants (min1) and Rmax (%) for tests Test no.
pH
Zinc sulph. (g/t)
Pot. meta. (g/t)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5
– – – 100 100 100 200 200 200 – – – – – – 100 100 100 200 200 200 100 100 100 200 200 200
– – – – – – – – – 500 500 500 1000 1000 1000 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000
Lead
Zinc
k
Rmax
k
Rmax
0.9 1.0 1.0 1.6 1.6 1.6 2.0 1.6 1.4 1.1 1.8 2.0 1.7 2.0 1.9 2.1 1.5 1.8 1.9 1.9 2.1 2.0 1.6 2.0 1.5 2.0 2.0
100 96 98 97 97 98 96 97 99 99 97 96 95 94 94 97 96 97 95 96 95 96 96 95 94 96 95
0.4 0.1 0.5 0.5 0.5 0.6 0.8 0.5 0.4 0.2 0.5 0.6 0.4 0.8 0.6 0.4 0.4 0.7 0.5 0.3 0.6 0.5 0.5 0.5 0.4 0.4 0.6
31 100 7 6 6 5 5 5 5 20 6 7 7 3 4 7 5 3 4 6 6 4 4 5 5 6 4
868
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The effects of adding various combinations of zinc sulphate and MBS on lead/zinc selectivity are shown in Figs. 8–10. Comparing these figures reveals similar trends to those observed previously for the reagents added individually. It is obvious that pH plays a significant role in the effects of different concentrations of reagents. For the lower pH ranges, results indicate that the best selectivity occurred with the addition of 100 g/t zinc sulphate and 1000 g/t MBS, where at pH 8.5, lead recovery was 97.5% and zinc recovery 4.0%. The best overall result was pH 9.5, 100 g/t zinc sulphate and 500 g/t MBS, giving 98% lead recovery and 3.7% zinc recovery (see Table 1). The overall aim of this test program was to minimise the misreporting of zinc to the lead concentrate. The depression of the marmatite when both depressants are added is expected to be due to the formation of zinc sulphite. When this forms, it is adsorbed onto the surface of the marmatite, resulting in competition for surface sites between those anions and xanthate as proposed by Misra et al. (1985) under alkaline conditions. The result of the depressive action of zinc sulphate and MBS together is also supported by Yamamoto (1980) who observed that sphalerite flotation was depressed by the simultaneous use of zinc ions and sulphite. The kinetic data reported in Table 2 generally support the experimental data listed in Table 1. Values of the flotation rate constant for the galena are always greater than for the marmatite as would be expected. The high zinc recoveries reported for Tests 1, 2 and 10 in Table 1 are also evident in Table 2. Ignoring these zinc data, the comparative flotation rate data corresponding to the various conditions can be compared. From Table 1, the ‘‘best’’ test was Test 18, and the kinetic analysis showed a very low Rmax for zinc (3%), even though the rate constant for the zinc was relatively high. The corresponding rate constant for the lead was moderate and Rmax high, all supporting the values reported in Table 1 for this test. Unfortunately no replication testing was conducted at the time of this test program. The laboratory results were only to be used as a guide to possible improvements in plant performance, with the main aim of minimising the recovery of marmatite into the galena rougher concentrate. 7. Conclusions The addition of lime to increase the pH to 9.5 in the absence of the addition of any other zinc depressants was effective in reducing marmatite flotation. This was thought to be due to the formation of a thin film of lime on the surface of the marmatite. The addition of zinc sulphate resulted in marmatite depression and increased lead/zinc selectivity at higher pH and zinc sulphate additions. This was believed due to the formation of a zinc hydroxide precipitate on the marmatite surface, thus hindering xanthate adsorption. The addition of MBS also increased lead/zinc selectivity, but galena recoveries can be reduced at higher MBS additions. The best results observed in this laboratory
test program was the addition of 100 g/t zinc sulphate and 500 g/t MBS at pH 9.5. Under these conditions, lead recovery was 98% at 72.6% grade and zinc recovery into the lead concentrate was only 3.7% at 3.3% zinc grade. Kinetic analysis supported the experimental data, with the minimisation in zinc recovery to the overall lead concentrate being the main driver for the test program. Acknowledgements The authors are very grateful to the staff of the Metallurgy and Assay departments of the then Pasminco South Concentrator at Broken Hill at the time this study was undertaken. The authors also wish to thank the management of Perilya Limited, the current operator of this mine and concentrator, for permission to publish this paper. References Bulatovic, S.M., Wyslouzil, D.M., 1999. Development and application of new technology for the treatment of complex massive sulphide ores case study-Faro lead/zinc concentrator-Yukon. Miner. Eng. 12 (2), 129–145. Burgess, J., 1992. Pasminco Mining-Broken Hill metallurgical operations, Metallurgy Workshop, AusIMM Annual Conference, Broken Hill. The Australasian Institute of Mining and Metallurgy, Melbourne. Cusack, B.L., Stump, N.W., 1977. Australian lead/zinc mineral processing developments. In: Rausch, D.O. et al. (Eds.), Lead–Zinc Update. Society of Mining Engineers, AIME, pp. 183–209 (Chapter 10). El-Shall, H.E., Elgillani, D.A., Abdel-Khalek, N.A., 2000. Role of zinc sulphate in depression of lead-activated sphalerite. Int. J. Miner. Process. 58, 67–75. Finkelstein, N.P., Allison, S.A., 1976. The chemistry of activation, deactivation and depression in the flotation of zinc sulphide—a review. In: Fuerstenau, M.C. (Ed.), Flotation A.M. Gaudin Memorial Volume. AIME, pp. 414–457 (Chapter 14). Gauci, G., Cusack, B.L., 1971. Metallurgical effects associated with the use of cemented backfill at the Zinc Corporation and New Broken Hill Consolidated. Proc. Australas Inst. Min. Met. 237, 33–40. Grano, S.R., Ralston, J., Johnson, N.W., 1988. Characterisation and treatment of heavy medium slimes in the Mt. Isa lead–zinc concentrator. Miner. Eng. 1 (2), 137–150. Grano, S.R., Johnson, N.W., Ralston, J., 1997. Control of the solution interaction of metabisulphite and ethyl xanthate in the flotation of the Hilton Ore of Mount Isa Mines Limited, Australia. Miner. Eng. 10 (1), 17–39. Higgins, S., Quast, K.B., 1992. Zinc depression in the flotation of Broken Hill lead–silver concentrates. In: Proceedings, Metallurgy workshop, 1992 AusIMM Annual Conference, Broken Hill, The Australasian Institute of Mining and Metallurgy, Melbourne. Klassen, V.I., Mokrousov, V.A., 1963. An Introduction to the Theory of Flotation. Butterworths, p. 304. Misra, M., Miller, J.D., Song, Q.Y., 1985. The effect of SO2 in the flotation of sphalerite and chalcopyrite. In: Forssberg, K.S.E. (Ed.), Flotation of Sulphide Minerals. Elsevier, Amsterdam, pp. 175– 196. Mitrofanov, S.I., Kusnikova, V.G., 1957. Adsorption of diethyldithiophosphate and butyl xanthate by sulphides. Progress in Mineral Dressing. Almqvist and Wiksell, Stockholm, pp. 461–473. Pattison, I.G., 1983. Sodium sulphite as a depressant-theory and practice at the CSA Mine concentrator, Cobar, NSW. In: AusIMM Annual Conference, Broken Hill, NSW. The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 399–409.
K. Quast, G. Hobart / Minerals Engineering 19 (2006) 860–869 Sheldon, G.P., Johnson, N.W., 1988. Galena flotation with metabisulphite: solutions to problems from xanthate sulphoxy reactions. The AusIMM Bull. Proc. 283 (3), 49–52. Sutherland, K.L., Wark, I.W., 1955. Principles of Flotation. Australasian Institute of Mining and Metallurgy, Melbourne. Tilyard, P.A., 1991. Developments at Pasminco MiningÕs South concentrator. The AusIMM Proc. 296 (2), 9–16. Watters, T., 1993. Lead–zinc ore concentration practice at the South concentrator of Pasminco Mining-Broken Hill, Broken Hill, NSW. In: Woodcock, J.T., Hamilton, J.K. (Eds.), Australasian Mining and
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Metallurgy, the Sir Maurice Mawby Memorial Volume, vol. 1. The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 511– 515. Wark, I.W., Cox, A.B., 1934. Principles of flotation II an experimental study of the influence of cyanide, alkalis and copper sulphate on the effect of potassium ethyl xanthate at mineral surfaces. Trans. AIME 112, 245–266. Yamamoto, T., 1980. Mechanism of pyrite depression in the presence of sphalerite. In: Jones, M.J. (Ed.), Complex Sulphide Ores. The Institute of Mining and Metallurgy, London, pp. 71–78.
Marmatite depression in galena flotation Keith Quast a
a,*
, Gavin Hobart
q
b,1
Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia b Onesteel-Whyalla Steelworks, Whyalla, SA 5600, Australia Received 13 July 2005; accepted 27 October 2005 Available online 19 December 2005
Abstract Laboratory flotation tests were conducted to investigate the role of pH, zinc sulphate and potassium metabisulphite in the flotation of galena and marmatite in the Broken Hill orebody. Marmatite was depressed under alkaline conditions alone, however the addition of the two depressants, either individually or combined, resulted in the greatest selectivity. The use of zinc sulphate under alkaline conditions resulted in good selectivity whilst maintaining a high lead recovery. The addition of potassium metabisulphite under alkaline conditions reduced zinc recovery, but high additions reduced lead recovery. The addition of both depressants at alkaline pH produced the best result, with a lead concentrate of 72.6% at 98% recovery with less than 4% zinc recovery. The overall aim of the study was to minimise the flotation of marmatite into the galena rougher concentrate, which was achieved under the conditions given above. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation reagents; Flotation depressants
1. Introduction The Broken Hill orebody was originally one of the worldÕs largest massive sulphide deposits. A number of distinct orebodies exist in the Broken Hill deposit, each with its own characteristic association of gangue minerals and its own distinguishing metal ratio. Galena, PbS (86.6% Pb) is the major lead mineral, with the zinc being present as marmatite (Zn,Fe)S, an iron rich sphalerite containing 9–11% iron. Silver is present as argentiferous galena and tetrahedrite, also as pyrargyrite and argentite. The major gangue minerals present are quartz, calcite, rhodonite, garnet, feldspar and fluorite (Higgins and Quast, 1992). Brief histories of the operation of the South Concentrator, previously owned by Pasminco Mining, now operated by Perilya q
Paper originally published in Proceedings Centenary of Flotation Symposium, Brisbane, Australia, 5–9 June, 2005 by the Australasian Institute of Mining and Metallurgy. * Corresponding author. Tel.: +618 8302 3816; fax: +618 8302 3683. E-mail addresses: [email protected] (K. Quast), HobartG @onesteel.com (G. Hobart). 1 Tel.: +618 8644 5508. 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.022
Limited, has been published by Tilyard (1991) and Watters (1993). The aim of this paper is to examine lead/zinc selectivity by manipulating pH in the range 7.5–9.5, the addition of zinc sulphate between 0 and 200 g/t, and the addition of potassium metabisulphite between 0 and 1000 g/t. 2. Literature review 2.1. Effect of pH Historically, ore flotation at the South Concentrator of Pasminco Mining was conducted at a natural pH of 7–8, although lime is often added to pH 8 (Burgess, 1992). The significance of pH as a flotation variable has become more apparent with the introduction of cemented backfilling techniques. Although laboratory testing showed no significant difference between cement and lime as a pH modifier, lead recovery suffered at pH values of 10 or higher (Gauci and Cusack, 1971). Wark and Cox (1934) showed that air–mineral contact for galena in the presence of 25 ppm ethyl xanthate was only possible for a pH less
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than 10.7. Cusack and Stump (1977) reported that galena losses in deleaded tailing only reached 10% above pH 11 in mill water, however this loss was noted at pH 9 using fresh water. Sutherland and Wark (1955) observed that the depressant action of lime was due to the formation of a finely dispersed hydrophilic film of lime on the surface of the sphalerite. Finkelstein and Allison (1976) state that galena floats selectively from sphalerite in a slightly alkaline environment (pH 8–10), with the sphalerite having to be activated (usually by copper ions) before it floats. Higgins and Quast (1992) showed that the addition of lime tended to slightly depress galena flotation, but substantially depress marmatite flotation. Bulatovic and Wyslouzil (1999) examined the addition of soda ash and lime on lead recovery from two complex Canadian lead–zinc ores. Depending on the ore, either soda ash or lime gave a maximum in lead recovery at about pH 9. 2.2. Addition of zinc sulphate Sphalerite tends to resist oxidation, thus very few zinc ions dissolve from the mineral. To ensure zinc sulphide activation is prevented, the activity of zinc ions should be 1000 times that of lead ions in solution. As little zinc ions are present in solution, zinc sulphate is added. Hence, by virtue of the equilibria involving basic lead carbonate and zinc hydroxide, the activity of zinc ions to lead ions is higher than the equilibrium ratio, and activity cannot occur. Solozhenkin and Vasyukevitch (cited by Finkelstein and Allison, 1976) concluded that the adsorption of zinc ions by marmatite, chalcopyrite and pyrite followed much the same path as was observed with sphalerite, except that adsorption was generally higher. In addition to thermodynamic considerations, there is considerable evidence that indicates that the colloids of zinc salts, formed under conditions where precipitation occurs, function as depressants for sphalerite. These include precipitates of zinc hydroxide, zinc carbonate, zinc sulphite and zinc cyanide. The depressant role of zinc hydroxide colloids was presented by Malinovsky in 1946 (cited by Klassen and Mokrousov, 1963). He established that zinc sulphate alone was unable to depress sphalerite, the depressant action of zinc sulphate only occurred in the presence of hydroxyl ions. Test work by Malinovsky indicated that an increase in the alkalinity of the pulp increased the depressing action of zinc sulphate, with zinc hydroxide first precipitating at a pH of 5.2. He proposed that the depressing action of zinc hydroxide was due to the zinc hydroxide being adsorbed on the surface of the sphalerite and preventing the adsorption of xanthate. These observations were later confirmed by Livshitz and Idelson (also cited by Klassen and Mokrousov (1963)) who demonstrated that the extent of depression of sphalerite and the concentration of colloidal zinc hydroxide occurring in the pulp were directly related.
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Finkelstein and Allison (1976) summarised the possible explanations for the adsorption of these colloidal precipitates on the sphalerite surface as: (1) A precipitate is formed in the bulk solution and attaches itself by physical forces to the surface (i.e. agglomeration or coagulation). (2) A precipitate is formed, or reprecipitates, at the surface through the action of nucleating sites. (3) A monolayer of zinc hydroxy species chemisorbs at the sphalerite surface (e.g. through an oxygen bridge) thus providing a suitable surface for the attachment of or the reprecipitation of precipitates formed in the bulk. (4) A bulk precipitate chemisorbs at the surface. Grano et al. (1988) used zinc sulphate as a sphalerite depressant in the flotation of galena from the heavy medium slimes stream in the Mount Isa lead–zinc concentrator. The addition of zinc sulphate plus either soda ash or lime for pH control resulted in the depression of the sphalerite while not affecting the flotation of the galena. Plant flotation results showed that the addition of 0.9 kg/t of zinc sulphate at pH 8.5 halved the zinc grade of the lead concentrate at almost one third of the zinc recovery. El-Shall et al. (2000) examined the role of zinc sulphate in the depression of lead-activated sphalerite from a more fundamental point of view. They found that the activation of sphalerite by lead ions was due to a replacement reaction where lead replaces zinc on the surface. Depression of leadactivated sphalerite by zinc sulphate was thought due to a reversal of the activation reaction. 2.3. Addition of metabisulphite Misra et al. (1985) reported that sulphite can be used to depress sphalerite if added to the system prior to the addition of xanthate. Generally the sulphite is added in grinding or during conditioning, the amount varying from 1 to 2 kg/t of ore. These workers reported that sulphite has been used previously at Broken Hill as a zinc depressant and also as a cleaning reagent for oxidised galena. The conclusions from this study were: (1) Contact angle measurements and flotation experiments indicated that the addition of sulphite before xanthate resulted in effective sphalerite depression. (2) The homogeneous phase decomposition reaction of xanthate in the presence of sulphite was dependent on the SO2/xanthate mole ratio, dissolved oxygen concentration and pH. The rate of decomposition was very rapid at near neutral pH (6–6.5). (3) Xanthate adsorption on sphalerite was limited to an effective monolayer coverage. (4) The selective depression of sphalerite by sulphite was related to the relative rates of xanthate adsorption in comparison to xanthate decomposition.
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Several possible mechanisms have been suggested for the depressing action of sulphite on sphalerite and also as a flotation modifier during the flotation of sulphide ore. These mechanisms include: (1) Formation of a hydrophilic surface coating such as zinc sulphite or calcium sulphate (when lime is used as a pH regulator). (2) Formation of insoluble heavy metal sulphite salts and subsequent reduction in the heavy metal ion concentration in the pulp, thus preventing unintentional activation. (3) Xanthate decomposition due to sulphite promoted oxidation reaction. (4) Limited collector adsorption by control of the redox potential. Yamamoto (1980) stated that the simultaneous use of zinc ions and sulphite depressed the flotation of sphalerite, but the use of sulphite alone did not. He concluded that ethyl xanthate adsorbed on the surface of activated sphalerite was not desorbed by sulphite ion. Yamamoto (1980) supported the findings of Misra et al. (1985) in the fact that the presence of both oxygen and sul-
phite together results in the decomposition of xanthate to form perxanthate. In the alkaline pH range, the xanthate decomposition was slow, but this does not correspond to the depression of sphalerite which is possible in the presence of sulphite in the alkaline pH range. Depression of sphalerite at higher alkalinity is more probably due to the competition of other anions with xanthate for surface sites. Depression at lower pH values is probably due to the decomposition of xanthate. Pattison (1983) reported that for unactivated sphalerite, the likely formation of zinc sulphite species may be expected to result in depression, although the results of Mitrofanov and Kusnikova (1957) revealed little difference (in alkaline solution) in the xanthate uptake of unactivated sphalerite whether sulphite was present or not. PattisonÕs experiments produced only trace flotation of unactivated sphalerite with ethyl xanthate collector regardless of whether sulphite was used in the flotation system. He concluded that the mineral tested had only a limited number of surface sites available for collection possibly because of some degree of unintentional activation, or because of the iron content of the sphalerite, and that the sulphite did not prevent this limited collection. For copper-activated sphalerite, it was demonstrated that sodium sulphite 45
45
0 ZnSO4 100 ZnSO4 200 ZnSO4
pH 7.5 pH 8.5 pH 9.5
40
40
35
35
30
Zinc Recovery (%)
Zinc Recovery (%)
30
25
20
25
20
15
15
10
10
5
5
0 50
0 50
60
70
80
90
Lead Recovery (%)
Fig. 1. Effect of pH on lead/zinc selectivity.
100
60
70
80
90
100
Lead Recovery (%)
Fig. 2. Effect of zinc sulphate addition (in g/t) on lead/zinc selectivity at pH 7.5.
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was quite an effective depressant in alkaline solution, but that the rate of sphalerite flotation was actually enhanced near to neutral pH by the presence of sulphite. Grano et al. (1997) also investigated the effect of adding sodium metabisulphite (MBS) to the flotation of the Hilton lead–zinc ore. Previous work on the interaction of sodium metabisulphite and ethyl xanthate for the galena flotation from Hilton ore had been reported by Sheldon and Johnson (1988). They identified temperature and contact time as important parameters in the pilot scale processing of this ore. Grano et al. (1997) found that the simultaneous addition of xanthate and MBS decreased galena recovery due to ethyl xanthate decomposition by sulphite ion derived from MBS. The depression increased with increasing MBS addition and pulp temperature. Under these conditions there was an increase in the concentration of metastable ethyl perxanthate in solution, an intermediate product of xanthate decomposition by sulphite ion. Pulp chemical measurements in the Hilton Concentrator also showed increased ethyl perxanthate concentration when MBS was added at the same point in the flotation circuit as ethyl xanthate. In contrast, the application of an aeration stage after MBS addition, but prior to ethyl xanthate addition,
decreased the concentration of unreacted sulphite ion. This was demonstrated by a decreased ethyl perxanthate concentration in solution and restored galena flotation recovery. Under these conditions the dependency of galena flotation on MBS (and temperature) was less marked. Prevention of the solution decomposition of ethyl xanthate by sulphite allowed the depressant action of MBS on sphalerite (and iron sulphide) flotation to be optimised. 3. Materials and equipment The material examined was provided by the then operators of the Pasminco South concentrator. This was received as roughly 1.1 kg moist samples of primary cyclone underflow. The head grade of this ore was 8.5% lead, 60 g/t silver and 10.5% zinc. Adelaide tap water was used throughout the test work. The reagents used were sodium ethyl xanthate, methyl isobutyl carbinol (MIBC), zinc sulphate and potassium metabisulphite. The charges of cyclone underflow were ground in a Linatex lined laboratory rod mill charged with 12 stainless steel rods. Flotation tests were performed in a 2.5 l capacity Agitair flotation cell attached to a Galigher Agitair
45
10
0 ZnSO4 100 ZnSO4 200 ZnSO4
40
863
0 ZnSO4 100 ZnSO4 200 ZnSO4
8
35
6
Zinc Recovery (%)
Zinc Recovery (%)
30
25
20
4
15
10 2
5
0
0 50
60
70
80
90
100
Lead Recovery (%)
Fig. 3. Effect of zinc sulphate addition (in g/t) on lead/zinc selectivity at pH 8.5.
50
60
70
80
90
100
Lead recovery (%)
Fig. 4. Effect of zinc sulphate addition (in g/t) on lead/zinc selectivity at pH 9.5.
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flotation machine model LA-500. Values of pH were measured using a TPS Auto pH meter. 4. Procedure 4.1. Grinding The grind time required to match the rougher feed sizing on the plant (80% passing 200 lm) was determined by grinding the charges with 650 ml of Adelaide tap water.
A second reagent addition of sodium ethyl xanthate and MIBC was made, 10 g/t of each. The pulp was again conditioned for 2 min, then aerated and four more concentrates were floated over one minute intervals by scraping every 15 s. The six flotation concentrates and tailings from each test were filtered and dried. A sample of rougher tail was riffled out and this plus all concentrate products were pulverised and analysed by the Pasminco South Mine assay department. 5. Results
4.2. Flotation 5.1. Grinding test The ground ore was washed into the flotation cell and water added to raise the level of slurry to within 25 mm of the overflow weir. If depressants were used, they were added as dilute solutions and the pH adjusted using lime if required. The pulp was conditioned for 5 min prior to adding the collector and frother, at 50 g/t and 10 g/t respectively. The pulp was then conditioned for a further 2 min, then aerated at 3 l/min. Two concentrate products were removed into separate containers over one minute intervals by scraping every 15 s.
A 20 min grind produced a flotation feed with 80% passing 175 lm, and this was used throughout the test program. The flotation test results are presented as a series of graphs in Figs. 1–10 and a summary of final lead and zinc grades and recoveries presented in Table 1. In order to compare the effects of the reagent additions on lead and zinc flotation behaviour, the flotation data were subjected to analysis using the first order rate equation given below:
45 0 MBS 500 MBS 1000 MBS
45 0 MBS 500 MBS 1000 MBS
40
40
35 35
30
Zinc Recovery (%)
Zinc Recovery (%)
30
25
20
25
20
15
15
10
10
5
5
0
0 50
60
70
80
90
100
Lead Recovery (%)
Fig. 5. Effect of metabisulphite addition (in g/t) on lead/zinc selectivity at pH 7.5.
50
60
70
80
90
100
Lead Recovery (%)
Fig. 6. Effect of metabisulphite addition (in g/t) on lead/zinc selectivity at pH 8.5.
K. Quast, G. Hobart / Minerals Engineering 19 (2006) 860–869 10
10 0 MBS 500 MBS 1000 MBS
100/500 g/t 200/500 g/t 100/1000 g/t 200/1000 g/t
8
8
6
6 Zinc Recovery (%)
Zinc Recovery (%)
865
4
4
2
2
0 50
60
70
80
90
100
0 70
Lead Recovery (%)
80
90
100
Lead Recovery (%)
Fig. 7. Effect of metabisulphite addition on lead/zinc selectivity at pH 9.5. Fig. 8. Effect of combined depressant addition (ZnSO4/MBS in g/t) on lead/zinc selectivity at pH 7.5.
RðtÞ ¼ Rmax ð1 expðktÞ
ð1Þ
where R(t) is the recovery of mineral at time t, Rmax is the theoretical maximum recovery (at infinite time by curve fitting the experimental data) and k is the rate constant in units of time1. The data were fitted to the equation using a least squares fit program, giving the data listed in Table 2. The rate data were only reported to 1 decimal place and the Rmax data to the nearest integer in keeping with the accuracy of the experimental data. 6. Discussion The laboratory grinding size comparison to the plant flotation feed sizing, although not presented here, showed a slightly reduced amount of material coarser than 300 lm compared to the plant feed, possibly as a consequence of using a laboratory rod mill rather than a laboratory ball mill. The 80% passing size of the laboratory 20 min grind was 175 lm compared to the plant value of 200 lm. Under these conditions, liberation should be almost complete, as Cusack and Stump (1977) reported that grinding this ore to 92.5% passing 300 lm achieved 96% liberation of the lead and zinc sulphides.
Fig. 1 shows that lime depresses zinc, particularly at pH 9.5, where zinc recovery was only 6.6% compared to 30– 40% for pH 7.5 and 8.5. The selectivity fell away after the second and third stages of flotation at pH 7.5 and 8.5, possibly due to the fact that most of the fast floating galena had already been floated, so zinc began to float. Similar results were also reported by Higgins and Quast (1992) for ore from the same mine, who found that pH 9.5 resulted in the greatest lead/zinc selectivity. The flotation of the marmatite at the lower pH values could be greater in the laboratory than at the concentrator due to the use of Adelaide tap water that would not have the same concentration of zinc ions present in the circuit water used at Broken Hill. Figs. 2 and 3 show that the addition of small amounts of zinc sulphate greatly increased the lead/zinc selectivity at pH 7.5 and 8.5. At pH 9.5 the effect was not as pronounced (Fig. 3). From Table 1, at pH 7.5 and 100 g/t zinc sulphate, the zinc recovery was 6.3%, whereas at pH 9.5 and 200 g/t zinc sulphate addition, the zinc recovery was 4.7%. The addition of zinc sulphate had no effect on lead recovery, with higher lead grades corresponding to higher zinc
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10
100/500 g/t 200/500 g/t 100/1000 g/t 200/1000 g/t
8
8
6
6 Zinc Recovery (%)
Zinc Recovery (%)
100/500 g/t 200/500 g/t 100/1000 g/t 200/1000 g/t
4
4
2
2
0
0
70
70
80
90
100
80
90
100
Lead Recovery (%)
Lead Recovery (%)
Fig. 9. Effect of combined depressant addition (ZnSO4/MBS in g/t) on lead/zinc selectivity at pH 8.5.
sulphate additions at pH 9.5 (see Table 1). Similar results were also reported by Higgins and Quast (1992). The best results using pH and zinc sulphate additions corresponded to the addition of 200 g/t at pH 9.5, where a lead recovery of 98.7% at a grade of 70.1%, with a zinc recovery of only 4.7% (see Table 1). This result supports the work of both Malinovsky and Livshitz and Idelson (both cited by Klassen and Mokrousov, 1963) in that more alkaline conditions provided an increase in hydroxyl ions, and, along with the greater presence of zinc ions, more colloidal zinc hydroxide will be formed. This would precipitate on the surface of the marmatite and prevents the adsorption of the xanthate, and Livshitz and Idelson demonstrated that the extent of sphalerite depression was related to the concentration of colloidal zinc hydroxide in the pulp. The results in Figs. 5–7 show that the use of metabisulphite alone depressed marmatite, although Yamamoto (1980), Mitrofanov and Kusnikova (1957) and Pattison (1983) suggested that sulphite had little depressant activity on sphalerite in the absence of zinc ions. The addition of MBS was very effective in depressing the marmatite, espe-
Fig. 10. Effect of combined depressant addition (ZnSO4/MBS in g/t) on lead/zinc selectivity at pH 9.5.
cially at the higher addition (1000 g/t). The lower additions of MBS were not as effective as marmatite depressants as the zinc sulphate, however lead recovery remained very high at 98% (see Table 1). The higher addition resulted in very low zinc recoveries, but lead recovery was slightly reduced. The decision as to the optimum additions and conditions rest with the plant operation philosophy i.e. is a higher lead grade or higher recovery the most desirable. It must be remembered that these are roughing tests, so some of the marmatite that floats here could possibly drop out in the lead cleaning stages. The fact that this test work was performed in the alkaline region would appear to reject the proposals of Misra et al. (1985). Their concept of the depressant action of sulphites was due to xanthate decomposition at neutral pH, with very little decomposition of xanthate to perxanthate at pH 9.5. Pattison (1983) also reported that sulphite was a successful depressant for copper-activated sphalerite, but the presence of sulphite had little effect on unactivated sphalerite. This was not the case in the present study. By adding and conditioning the MBS ahead of xanthate addition seemed to avoid the xanthate decomposition by MBS noted by Grano et al. (1997).
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Table 1 Final grades and recoveries for lead and zinc Test no.
pH
Zinc sulph. (g/t)
Pot. meta. (g/t)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5
– – – 100 100 100 200 200 200 – – – – – – 100 100 100 200 200 200 100 100 100 200 200 200
– – – – – – – – – 500 500 500 1000 1000 1000 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000
Lead (%)
Zinc (%)
Gde.
Rec.
Gde.
Rec.
49.2 41.5 67.0 61.1 66.2 68.8 68.1 67.1 70.1 56.8 66.8 69.5 67.8 72.4 69.9 63.9 68.7 72.6 64.7 68.5 62.7 71.2 72.7 65.1 67.7 66.3 66.1
93.6 96.2 97.4 98.7 97.4 98.6 97.5 97.9 98.7 97.6 98.4 97.9 96.4 95.4 95.4 98.1 97.7 98.0 97.2 97.5 97.2 97.4 97.5 96.9 96.5 97.0 96.3
18.4 23.5 5.7 5.5 5.3 5.1 4.5 4.7 3.8 11.2 6.8 5.1 4.9 3.5 4.6 5.1 4.2 3.3 4.9 4.2 5.7 3.6 3.7 5.3 4.5 4.8 4.9
27.6 40.2 6.6 6.3 5.8 5.4 5.0 4.9 4.7 13.8 8.0 8.3 6.5 3.3 4.0 6.1 4.7 3.7 5.4 4.5 6.7 3.9 4.0 5.5 4.7 5.2 5.1
Table 2 Calculated values of rate constants (min1) and Rmax (%) for tests Test no.
pH
Zinc sulph. (g/t)
Pot. meta. (g/t)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5 7.5 8.5 9.5
– – – 100 100 100 200 200 200 – – – – – – 100 100 100 200 200 200 100 100 100 200 200 200
– – – – – – – – – 500 500 500 1000 1000 1000 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000
Lead
Zinc
k
Rmax
k
Rmax
0.9 1.0 1.0 1.6 1.6 1.6 2.0 1.6 1.4 1.1 1.8 2.0 1.7 2.0 1.9 2.1 1.5 1.8 1.9 1.9 2.1 2.0 1.6 2.0 1.5 2.0 2.0
100 96 98 97 97 98 96 97 99 99 97 96 95 94 94 97 96 97 95 96 95 96 96 95 94 96 95
0.4 0.1 0.5 0.5 0.5 0.6 0.8 0.5 0.4 0.2 0.5 0.6 0.4 0.8 0.6 0.4 0.4 0.7 0.5 0.3 0.6 0.5 0.5 0.5 0.4 0.4 0.6
31 100 7 6 6 5 5 5 5 20 6 7 7 3 4 7 5 3 4 6 6 4 4 5 5 6 4
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The effects of adding various combinations of zinc sulphate and MBS on lead/zinc selectivity are shown in Figs. 8–10. Comparing these figures reveals similar trends to those observed previously for the reagents added individually. It is obvious that pH plays a significant role in the effects of different concentrations of reagents. For the lower pH ranges, results indicate that the best selectivity occurred with the addition of 100 g/t zinc sulphate and 1000 g/t MBS, where at pH 8.5, lead recovery was 97.5% and zinc recovery 4.0%. The best overall result was pH 9.5, 100 g/t zinc sulphate and 500 g/t MBS, giving 98% lead recovery and 3.7% zinc recovery (see Table 1). The overall aim of this test program was to minimise the misreporting of zinc to the lead concentrate. The depression of the marmatite when both depressants are added is expected to be due to the formation of zinc sulphite. When this forms, it is adsorbed onto the surface of the marmatite, resulting in competition for surface sites between those anions and xanthate as proposed by Misra et al. (1985) under alkaline conditions. The result of the depressive action of zinc sulphate and MBS together is also supported by Yamamoto (1980) who observed that sphalerite flotation was depressed by the simultaneous use of zinc ions and sulphite. The kinetic data reported in Table 2 generally support the experimental data listed in Table 1. Values of the flotation rate constant for the galena are always greater than for the marmatite as would be expected. The high zinc recoveries reported for Tests 1, 2 and 10 in Table 1 are also evident in Table 2. Ignoring these zinc data, the comparative flotation rate data corresponding to the various conditions can be compared. From Table 1, the ‘‘best’’ test was Test 18, and the kinetic analysis showed a very low Rmax for zinc (3%), even though the rate constant for the zinc was relatively high. The corresponding rate constant for the lead was moderate and Rmax high, all supporting the values reported in Table 1 for this test. Unfortunately no replication testing was conducted at the time of this test program. The laboratory results were only to be used as a guide to possible improvements in plant performance, with the main aim of minimising the recovery of marmatite into the galena rougher concentrate. 7. Conclusions The addition of lime to increase the pH to 9.5 in the absence of the addition of any other zinc depressants was effective in reducing marmatite flotation. This was thought to be due to the formation of a thin film of lime on the surface of the marmatite. The addition of zinc sulphate resulted in marmatite depression and increased lead/zinc selectivity at higher pH and zinc sulphate additions. This was believed due to the formation of a zinc hydroxide precipitate on the marmatite surface, thus hindering xanthate adsorption. The addition of MBS also increased lead/zinc selectivity, but galena recoveries can be reduced at higher MBS additions. The best results observed in this laboratory
test program was the addition of 100 g/t zinc sulphate and 500 g/t MBS at pH 9.5. Under these conditions, lead recovery was 98% at 72.6% grade and zinc recovery into the lead concentrate was only 3.7% at 3.3% zinc grade. Kinetic analysis supported the experimental data, with the minimisation in zinc recovery to the overall lead concentrate being the main driver for the test program. Acknowledgements The authors are very grateful to the staff of the Metallurgy and Assay departments of the then Pasminco South Concentrator at Broken Hill at the time this study was undertaken. The authors also wish to thank the management of Perilya Limited, the current operator of this mine and concentrator, for permission to publish this paper. References Bulatovic, S.M., Wyslouzil, D.M., 1999. Development and application of new technology for the treatment of complex massive sulphide ores case study-Faro lead/zinc concentrator-Yukon. Miner. Eng. 12 (2), 129–145. Burgess, J., 1992. Pasminco Mining-Broken Hill metallurgical operations, Metallurgy Workshop, AusIMM Annual Conference, Broken Hill. The Australasian Institute of Mining and Metallurgy, Melbourne. Cusack, B.L., Stump, N.W., 1977. Australian lead/zinc mineral processing developments. In: Rausch, D.O. et al. (Eds.), Lead–Zinc Update. Society of Mining Engineers, AIME, pp. 183–209 (Chapter 10). El-Shall, H.E., Elgillani, D.A., Abdel-Khalek, N.A., 2000. Role of zinc sulphate in depression of lead-activated sphalerite. Int. J. Miner. Process. 58, 67–75. Finkelstein, N.P., Allison, S.A., 1976. The chemistry of activation, deactivation and depression in the flotation of zinc sulphide—a review. In: Fuerstenau, M.C. (Ed.), Flotation A.M. Gaudin Memorial Volume. AIME, pp. 414–457 (Chapter 14). Gauci, G., Cusack, B.L., 1971. Metallurgical effects associated with the use of cemented backfill at the Zinc Corporation and New Broken Hill Consolidated. Proc. Australas Inst. Min. Met. 237, 33–40. Grano, S.R., Ralston, J., Johnson, N.W., 1988. Characterisation and treatment of heavy medium slimes in the Mt. Isa lead–zinc concentrator. Miner. Eng. 1 (2), 137–150. Grano, S.R., Johnson, N.W., Ralston, J., 1997. Control of the solution interaction of metabisulphite and ethyl xanthate in the flotation of the Hilton Ore of Mount Isa Mines Limited, Australia. Miner. Eng. 10 (1), 17–39. Higgins, S., Quast, K.B., 1992. Zinc depression in the flotation of Broken Hill lead–silver concentrates. In: Proceedings, Metallurgy workshop, 1992 AusIMM Annual Conference, Broken Hill, The Australasian Institute of Mining and Metallurgy, Melbourne. Klassen, V.I., Mokrousov, V.A., 1963. An Introduction to the Theory of Flotation. Butterworths, p. 304. Misra, M., Miller, J.D., Song, Q.Y., 1985. The effect of SO2 in the flotation of sphalerite and chalcopyrite. In: Forssberg, K.S.E. (Ed.), Flotation of Sulphide Minerals. Elsevier, Amsterdam, pp. 175– 196. Mitrofanov, S.I., Kusnikova, V.G., 1957. Adsorption of diethyldithiophosphate and butyl xanthate by sulphides. Progress in Mineral Dressing. Almqvist and Wiksell, Stockholm, pp. 461–473. Pattison, I.G., 1983. Sodium sulphite as a depressant-theory and practice at the CSA Mine concentrator, Cobar, NSW. In: AusIMM Annual Conference, Broken Hill, NSW. The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 399–409.
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