The role of sodium sulphide in amine flotation of silicate zinc minerals

Minerals Engineering, Vol. 5, Nos. 3-5, pp. 411---419, 1992

0892-6875/92 $5.00+0.00 @ 1992 Pergamon Press plc

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THE ROLE OF SODIUM SULPHIDE IN AMINE FLOTATION OF SILICATE ZINC MINERALS M.J.G. SALUM, A.C. de ARAUJO and A.E.C. PERES Dept. of Mining Engng., Universidade Federal de Minas Gerais R. Espirito Santo 35-702, 30160 Belo Horizonte, MG, Brazil

ABSTRACT The role of sodium sulphide in amine flotation of the silicate zinc minerals willemite and hemimorphite is discussed. Data from the literature are compared with experimental results obtained by the authors in Hallimond tube floatability tests. The activating effect of sodium sulphide was investigated considering the dual role of this reagent, either as in pre-sulphidization or as flotation pH regulator. The practical conclusion of these fundamental studies is the possibility of partial replacement of sodium sulphide by sodium hydroxide, in amine flotation systems of zinc silicates, with significant benefits in terms of reagent costs and environmental impact. Keywords Froth flotation; zinc silicates; sodium sulphide; amine flotation; willemite; hemimorphite INTRODUCTION Sulphidization followed by flotation with xanthate is a well established industrial practice for oxidized copper and lead ores [1,2]. This reagent scheme is not used in the flotation of the zinc silicates, hemimorphite and willemite, due to the high consumption of sulphidizing agent, the need for activation with copper sulphate and the low recoveries obtained [2,3]. Amines have proved to be the most efficacious collectors for hemimorphite and willemite, when utilized after a sulphidization stage [2,4,5]. In the flotation of oxidized copper and lead minerals with xanthates there is agreement among the investigators that sulphidization renders the mineral surfaces similar (or even equal to) those of sulphides, which are able to adsorb xanthates [6,7]. Amines, on the other hand, are not suitable collectors for sulphides, which indicates that sulphidization plays a different role in silicate zinc minerals systems than with oxidized copper and lead ores. Sodium sulphide is the sulphidizing agent which yields the best performance in most flotation systems [2,3,4]. WILLEMITE AND HEMIMORPHITE FLOTATION WITH AMINES In the absence of sodium sulphide, amine adsorption density at the interface hemimorphite/willemite-aqueous solution is low and strongly dependant on pH [5,8,9]. The best flotation performance is achieved in the pH range above 10, indicating that free amine plays a significant role in these systems [2,3,4]. 411

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M . J . G, SALUMet at.

Hallimond tube floatability test results as a function of pH, with amine as collector, for willemite and hemimorphite are shown in figure 1. The curves (of similar shape for both minerals) show that willemite floats better than hemimorphite, especially in the less alkaline pH range [9]. The difference in behaviour could be explained by the chemical and structural composition of the minerals. Willemite (ZnzSiO4) is a typical nesolicate with the tetrahedral group (SiO44") linked by zinc cations while in hemimorphite (2ZnO.SiO2.H20) the zinc atoms are tetrahedrically coordinated with three oxygen atoms of the silica tetrahedron and with one hydroxyl ion. As a result, a lower mobility of Zn 2÷ cations in hemimorphite is observed, compared to that of witlemite [10]. More favourable conditions for amine adsorption on hemimorphite would require the loss of the hydration sheath.

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Fig.l Recovery of hemimorphite and willemite as a function of pH (amine concentration = 1 x 10 "s M) [9]. THE ROLE OF SODIUM SULPHIDE Activation effect

Sodium sulphide, a salt of a weak acid and a strong base, is a pH regulator for the alkaline region. Another effect is the enhancement of the floatability of willemite and hemimorphite with amine, as shown in figures 2 and 3 [2,5,8,9]. It can be seen that willemite is more sensitive than hemimorphite to the presence of sodium sulphide [9]. The replacement, even partial, of sodium sulphide by other pH regulating agents, such as sodium carbonate and sodium hydroxide, decreases the recovery and grade of the zinc concentrates, as shown in figures 4 and 5, for hemimorphite [11]. In the pH range of zinc silicates flotation, the predominant species of sodium sulphide dissociation is HS-. Most investigators consider that this species is adsorbed onto the zinc silicate surfaces [3,9,11 ]. Sodium sulphide is considered to render the surfaces more negative, favouring the electrostatic attraction mechanism between amines and the mineral surface [5,8]. Another proposition is that NazS seals off the surfaces of zinc silicates [3,9]. According to Rey et al [3], the sealing effect may be verified when both minerals are conditioned in slightly alkaline solutions of dithizone in the presence and absence of N a # . In the absence of NazS zinc dithizonate is precipitated from the solution. In the presence of N a # this precipitate was detected on the minerals surfaces.

Sodium sulphide in amine flotation of Zn silicates

413

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Fig.3 Recovery of hemimorphite as function of pH in the presence of NaOH and Na2S.9H20 (amine concentration I x 10 -5 M) [9]. Zinc ion determinations, in aqueous suspensions of willemite and hemimorphite, in the presence and absence of N a ~ , confirm its effect of fixing Zn z÷ ions on the surface, as shown in figures 6 and 7 [9]. These figures show that willemite is more soluble than hemimorphite and, therefore, more strongly affected by N a ~ . The solubility decreasing effect is not the only activating action of Na2S in zinc silicate systems. The presence of zinc/sulphur compounds, probably ZnS, is suggested. Marabini et al [6] propose the formation of ZnS on the surface of sulphidized smithsonite (a zinc carbonate). These investigators report that non-sulphidized smithsonite surfaces present

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hydrolized zinc compounds coexisting with amines. The hydrophobie character of the amine groups is then inhibited.

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Sodium sulphide in amine flotation of Zn silicates

415

amine adsorption [9]. Figures 8 and 9 show that longer conditioning times in the sulphidizing solution are required, the kinetics of HS" adsorption being slower than those of amine adsorption. This fact indicates that in plant operation the contact between the minerals and the Na2S solution should be performed in a pre-sulphidization stage. The utilization of Na2S as a pH regulator in the flotation stage is not as significant as is the use of the same reagent in the pre-sulphidization step.

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Fig.8 Recovery of hemimorphite as a function of conditioning time with Na#.9H20 (amine concentration = lxl0 "s M, Na2S.9HzO concentration = 0.04 g/l; pH = 10) [9].

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Fig.9 Recovery of willemite as a function of conditioning time with NazS.9H20 (amine concentration = 1 x 10 "s M, NazS.9HzO concentration = 0.04 g/l; pH ffi 10) [9].

Concentration and sulphidlzatlon pH An excess of sodium sulphide does not depress the floatability of willemite and hemimorphite with amines, an effect which is observed in the flotation of oxidized copper and lead minerals with xanthate [1,9,13]. Figure 10 shows results of floatability studies in the presence of amine as a function of NazS concentration in the pre-sulphidizing solution,

S o d i u m s u l p h i d e in a m i n e flotation o f Z n silicates

417

at flotation pH values of 9 and 10. In this case, the flotation pH was maintained at the sulphidization pH, the latter being controlled by the addition of NaOH (dotted lines) or HCI (full lines) [9]. These investigations show that: i. the recovery increases with increase in sodium sulphide concentration up to a certain limiting concentration, which depends on the flotation pH, remaining constant above this figure; ii. the recovery is enhanced by a larger N a ~ / N a O H ratio in the sulphidization solution.

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Fig.10 Recovery of willemite as a function of Na2S.9H20 concentration in pre-sulphidization at flotation pH levels of 9 and 10 (amine concentration = 1 x 10 -5 M; sulphidization pH ffi flotation pH) [9]. The influence of the sulphidization pH may be observed from figure 11, where two presulphidization conditions are illustrated, the flotation pH (9) and the amine concentration being kept constant. Curve 1 represents the condition where the sulphidization pH is set to the pre-sulphidization solution pH (determined by the Na2S solution itself). Under this condition willemite may be floated at pH values below or above the sulphidization pH. Curve 2 represents the condition where sulphidization pH ffi flotation pH (same as curve 2 from figure 10). These results show that at the same sodium sulphide concentration imparting a lower pH value to the solution than that of flotation (dotted portions of the curves) better floatabilities are observed on curve 2, which represents the condition of the sulphidization pH being increased with the addition of NaOH. An opposite effect is noticed on the full line portion of the curves where the sulphidization pH, for the same Na2S concentration, is larger for curve 1 than for curve 2. These findings probably also apply to the mineral hemimorphite due to its similar behaviour to willemite. These results seem to indicate the need for an alkaline environment for sulphidization, which may be justified by the predominance of HS" ions in the pH range between 7 and 13 [9]. The practical implication is the possibility of a reduction in N a ~ consumption in pre-

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sulphidization, with economical and environmental benefits, considering that N a ~ is more expensive and more toxic than other pH regulators.

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Fig.l I Recovery of willemite as a function of Na2S.9H20 concentration in pre-sulphidization at flotation pH = 9 [9]. Curve 1: sulphidization pH determined by the concentration of Na~S.gH20 in the sulphidizing solution. Curve 2: sulphidization pH ffi flotation pH (introduction of NaOH and HCI). CONCLUSIONS Sodium sulphide is an activator in the flotation of willemite and hemimorphite with amines. Due to chemical and structural differences between the two minerals, its effect on willemite is stronger than on hemimorphite. The sodium hydrosulphide ion, HS', is the predominant species in the pH range of sulphidization and flotation (10
Na2S addition in a stage prior to flotation (pre-sulphidization) is more efficient as a pH regulator than during flotation, due to adsorption kinetics differences between the collector and the activator (the former adsorbs faster than the latter). The increase in N a ~ concentration in pre-sulphidization enhances willemite floatability up to a limiting concentration, above which the effect becomes insignificant.

Sodium sulphide in amine flotation of Zn silicates

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The partial replacement of N a ~ by another pH regulator decreases its activating effect. Nevertheless, for the same Na2S concentration, a higher sulphidization pH produced by NaOH addition, increases the Hallimond tube floatability level of willemite. This is an important practical result because it suggests a decrease in Na2S consumption, which represents economical and environmental benefits, as Naz,S is more expensive and more toxic than other pH regulators. REFERENCES

.

.

3. 4. 5. 6.

.

8.

. 10. 11.

Castro, S., Goldfarb, J. and Laskowski, J., Sulphidizing reactions in the flotation of oxidised copper minerals. International Journal o f Mineral Processing. 141 - 161, (1974) Rey, M., Memoirs of milling and process metallurgy: 1 - Flotation of oxidised ores. Transactions Institution of Mining & Metallurgy, (1979) Rey, M., Sitia, G., Raffinot, P., and Formanek, V., The flotation of oxidised zinc ores. Trans. AIME, 199, (1954) Rey, M., and Raffinot, P., The flotation of oxidized zinc ores. Inst. Metall., London, (1953) Bustamantes, H.A., The flotation of oxide minerals with chelating agents. PhD Thesis Imperial College of Science and Technology, London. Marabini, A.M., Alesse, V., and Garbassi, F., Role of sodium sulphide, xanthate and amine in flotation of lead-zinc oxidized ores. Reagents in the Mineral Industry, Italy, p. 125-136 (1984) Gaudin, A.M., Flotation, New York, McGraw-Hill, (1957) Baltar, C.A.M., Aproveitamento de Min6rio oxidado de zinco corn baixo teor por flota~o. M.Sc. Thesis Universidade Federal do Rio de Janeiro, Brazil, in Portuguese. (1980) Salum, M.J.G., Estudo da flota~lo dos minerais silicatados de zinco corn amina em sistema sulfetizado. M.Sc. Thesis Universidade Federal de Minas Gerais, Brazil, in Portuguese. (1982) Evans, R.C., An Introduction to Crystal Chemistry. University Press, Cambridge. 2nd. ed., (1966) Billi, M., and Quai, V., Development and results obtained in the treatment of zinc ores at MMI mines. International Mineral Processing Congress, 6. Cannes, p.49-63. (1963)