Kinetics of sulfidization of malachite in hydrosulfide and tetrasulfide solutions

International Journal o f Mineral Processing, 37 ( 1993 ) 257-272

257

Elsevier Science Publishers B.V., Amsterdam

Kinetics of sutfidization of malachite in hydrosulfide and tetrasulfide solutions R. Zhou and S. Chander Mineral Processing Section, Mineral Engineering Department, The Pennsylvania State University, University Park, PA 16802, USA (Received 23 January 1992; accepted after revision 28 August 1992 )

ABSTRACT Zhou, R. and Chander, S., 1993. Kinetics ofsulfidization of malachite in hydrosulfide and tetrasulfide solutions. Int. J. Miner. Process., 37: 257-272. The kinetics of sulfidization of malachite in sodium hydrosulfide and tetrasulfide as the sulfidizing agents was investigated in this study. The sulfidization reaction occurred in two stages. The first stage involved the formation of a primary sulfidized layer and the second involved two types of secondary reactions. One was the precipitation of copper ions which diffused through the primary layer, and the other was the oxidation of the sulfidizing agent to oxysulfide species. The influence of the type and concentration of the sulfidizing reagent and solution pH was determined. A signifcant difference in the surface morphology was observed for malachite treated with these two reagents. With sodium tetrasulfide, a uniform sulfidized layer formed on the panicle surface, whereas for sodium hydrosulfide, the malachite surface was coated with loosely adherent precipitates, which might peel-off upon agitation. The results indicate that the extent of the reaction in the second stage was smaller in sodium tetrasulfide than in sodium sulfide.

INTRODUCTION

The recovery of oxidized copper minerals from predominant sulfide ore bodies is an important technological problem facing the mineral processing industry. Considerable work has been carried out in the past to increase the recovery of oxidized copper minerals by flotation (Soto and Laskowski, 1973; Castro et al., 1974a, b; Queirolo and Castro, 1976; DeCuyper, 1977; Aplan and Fuerstenau, 1984; Fuerstenau and Pradip, 1984). Based on these and other studies, the various methods available for flotation of oxidized copper ores are: ( 1 ) sulfidization followed by flotation with a xanthate collector; (2) carboxylic acid process; ( 3 ) leach-precipitation-flotation process; (4) direct flotation by using a variety of ionic, long chain sulfydryl, and chelating reagents. 0301-7516/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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R. ZHOU AND S. CHANDER

The first three of these methods are practiced in industry. However, the carboxylic acids method is not applicable for the ores containing carbonate gangue minerals (Gaudin, 1957 ). When significant quantities of sulfide minerals are present in the ore, the third method is not very effective due to loss of sulfide components in the leaching stage. Therefore, for the mixed sulfide/ oxide copper ores, still sulfidization-flotation process is the most effective methods for copper recovery. Oxidized copper minerals generally do not float with low molecular weight thiol collectors at the concentrations that are normally employed in the flotation of sulfide minerals. In such a system, the reaction products between the collector and the metal ions in the mineral do not adhere well to the surface (Wright and Prosser, 1965; Bustamante and Castro, 1975; Castro et al., 1976 ). The role of sulfidization, according to these researchers, is therefore to provide a copper sulfide layer on the mineral surface which stabilizes the collector coating. The following reaction was proposed by Bustamante and Castro ( 1975 ) for malachite sulfidized with hydrosulfide: CuCO

3

"Cu(OH)2 + H S - + O H - =Cu(OH)2 "CuS+CO~- +H20

( 1)

This reaction is not confined to the surface, but continues within the bulk of the particles to form copper sulfide coatings. The extent of reaction in excess of monolayer was also observed in sulfidization of tenorite by Castro et al. (1974a) who identified the reaction product as cupric sulfide by X-ray diffraction. Raghavan et al. (1984) also obtained an X-ray pattern for the sulfidized chrysocolla and suggested the formation of Cu2S coating. In the present study, discrete crystals of a new phase instead of a uniform coating were observed at the surface of malachite treated with sodium hydrosulfide. Even though sodium sulfide is one of the most widely used alkali metal sulfides in the sulfidization-flotation process, its effectiveness is not entirely satisfactory. Sulfidization treatment with such a reagent can enhance the hydrophobicity of the oxide minerals, but the sulfidized particles require quiescent conditions for flotation (Fuerstenau and Raghavan, 1986 ), possibly due to the fact that the sulfidized layer detaches readily, which has been confirmed as a part of this investigation. When an alkali sulfide is used as the reagent, a careful control of the sulfidization stage is critical to maximizing the flotation recovery (Malghan, 1986). Insufficient amount of sulfide gives poor recoveries because of inadequate sulfidization, while an excess sulfide causes poor flotation due to the depressant action of sulfide ions. A major difficulty in sulfidization process is to establish and maintain an o p t i m u m sulfide concentration. The microstructure of the oxidized mineral surface seems to play an important role in the sulfidization-flotation. The effect of sulfidization on the porous nature and the surface area of brochantite and chrysocolla was investigated by Raghavan et al. (1984). The surface area of brochantite was sub-

KINETICS OF MALACHITE SULFIDIZATION IN HYDROSULFIDE AND TETRASULFIDE SOLUTIONS

2 59

stantially reduced upon sulfidization and the flotation was improved. In contrast, it was observed that sulfidization neither affected the surface area nor the flotation response of chrysocolla, which had an extraordinarily large surface area of 126 m2/g for the 35 X 48 mesh size fraction. The effect of thermal treatment on the flotation behavior of chrysocolla was studied by several investigators (Parks and Kovacs, 1966; Castro et al., 1974b; Queirolo and Castro, 1976; Gonzfilez and Soto, 1978). Thermally activated chrysocolla was found to be more amenable to the sulfidization-flotation method. Raghavan and Fuerstenau (1977 ) studied the pore structure of chrysocolla and found that it contained a major fraction of micropores. The use of gas adsorption technique revealed that heat treatment of chrysocolla at 555 °C sintered pores whose radii were below 1.5 nm, and thus reduced the specific surface area. The thermal activation of chrysocolla flotation was ascribed to the reduction in the number of pores where precipitates could form. In this study, an alternate sulfidizing agent, namely sodium tetrasulfide, was tested. For the purpose of comparison, the sulfidization kinetics of malachite in both sodium sulfide and tetrasulfide solutions was investigated. In addition, the surface microstructures for malachite pretreated with sodium sulfide and sodium tetrasulfide were compared to determine the reasons for differences in the sulfidization characteristics of the two reagents. EXPERIMENTAL METHODS AND MATERIALS

Methods Sulfidization kinetics. The studies of rate of consumption of sulfidizing agents were carried out by adding one gram samples of 65 X 200 mesh malachite in 100 ml of sulfidizing solutions with the system open to air. This procedure was preferred so as to keep the conditioning stage similar to that likely to be used in practice. The particles were conditioned with the solution by gently stirring with a magnetic stirrer. The solutions were maintained at pH 10 for sodium tetrasulfide and at pH 9 for sodium sulfide, unless specified otherwise. The concentration of the sulfidizing agent in the solutions was monitored by UV spectrophotometry using a flow-cell with a path length of 10 mm. The samples for analysis of the liquid were taken with a 0.2/~m membrane filter. A diode array spectrophotometer Model HP 8451A was used to conduct quantitative analysis of amyl xanthate, sodium sulfide and sodium tetrasulfide by the methods previously described in the literature (Schawrzenbach and Fischer, 1960; Giggenbach, 1972 ). Scanning electron microscopy. After cutting, the malachite samples were wet polished first using 600-grit silicon carbide paper and then with 0.05/~m alumina suspension to produce a mirror surface. After polishing, the samples

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R. ZHOU AND S. CHANDER

were treated with sodium sulfide or sodium tetrasulfide solutions for various time periods. The samples thus treated were dried in a stream of purified nitrogen at room temperature. A gold coating was applied to the samples prior to analysis. The surface morphology of the treated malachite samples was investigated using an ISI ABT SX-40A scanning electron microscope.

Materials Reagents. The tetrasulfide stock solution was prepared by dissolving sodium tetrasulfide (96% purity, Alfa Product, Danver, MA) in de-oxygenated distilled water. The polysulfide ion content in the stock solution was determined by iodometric titration technique (Stahl, 1983 ). The sodium sulfide stock solution was prepared by adding sodium sulfide into oxygen-free distilled water. Care was taken to prevent oxidation of stock solutions. In all cases, they were prepared on the day of usage. The concentration of sulfide ions in the stock solution was determined by titration with a standard solution of lead perchlorate (Baumann, 1974). Samples. Lumps of malachite, obtained from Gila Co., Arizona, were crushed and concentrated by hand-sorting. The selected sample was dryground in a porcelain mill to minus 65 mesh and the 65 × 200 mesh size fraction was separated for use throughout the experiments. X-ray diffraction analysis indicated that the sample contained nearly pure malachite with small quantities of quartz. Chemical analysis of the sample showed that the sample contained approximately 3% quartz. RESULTS AND DISCUSSION

Sulfidization reactions In the present study, the sulfidization of malachite was carried out in mild alkaline solutions of pH 9 for sodium hydrosulfide and 10 for tetrasulfide. According to thermodynamics, most of the copper species should precipitate as copper sulfide in the presence of sulfide ions. The concentration of copper solution species in the sulfidizing residue should therefore be very small, if any. However, analysis of the sample solutions, which were taken using a 0.2 /tm membrane filter, showed the presence of a substantial amount of copper species in the sulfidizing residue. The results are given in the following discussion. A marked Tyndall effect was observed when a laser beam passed through the solution. These results indicated the presence of colloid particles in the sulfidizing residue. The formation of colloidal metal sulfides was also observed by Fuerstenau et al. ( 1985 ) during the sulfidization of lead oxides. Based on these observations, an alternate to Reaction 1 is proposed as the initial step in sulfidization:

KINETICS OF MALACHITE SULFIDIZAT1ON IN HYDROSULFIDE AND TETRASULFIDE SOLUTIONS

261

CuCO3 • Cu (OH)

(2 )

2 -I- H S -

= CuS +

C u 2 + --F 2 O H -

+ HCO~-

The CuS product in Reaction 2 is the primary sulfidization product which activates malachite. Reaction 2 differs from Reaction 1 with regard to the intermediate copper species. In Reaction 1, all the copper products are solid and remain on the malachite surface whereas in Reaction 2, a part of the malachite is considered to become soluble. The dissolved copper might subsequently hydrolyze to copper hydroxide or precipitate as copper sulfide depending upon solution pH and sulfide ion concentration. Based on thermodynamics, copper sulfide is expected to form readily when copper and sulfide ions are present together, and the hydroxyl ion concentration is small. This reaction, when occurring in solution to produce a bulk precipitate of copper sulfide, is referred to as the secondary reaction in this article and is further discussed in a later section. Similar to Reaction 2, the sulfidization of malachite with sodium tetrasulfide may be represented by the reaction: C u C O 3- C u ( O H ) 2 + S 2- =CuS

4 --~Cu 2+

+ 2 O H - + C O 2-

(3)

The CuS4 layer was believed to form on the malachite surface which promoted the flotation of malachite by xanthate. At pH's less than 9, precipitation of sulfur on the surface of malachite might also occur (Valensi et al., 1966; Teder, 1969 ): S2- + H + = H S - + 3 S °

(4)

The sulfidized layer on the malachite surface is likely to be composed of CuS4 and elemental sulfur with CuS4 dominating.

Sulfidization kinetics Effect of concentration of the sulfidizing agent. To determine the kinetics of sulfidization, the change in concentration of the sulfidizing species in the liquid phase was measured. The results are given for various initial concentrations of S 2- in Fig. 1. There is a rapid initial drop in concentration followed by a period in which the reaction followed first order kinetics. It is proposed that the rapid initial drop is associated with the formation of a primary sulfidized layer, and the first order kinetics with the formation of a secondary layer. Note that the first data point lies above the straight line portion corresponding to the first order kinetics for each concentration, implying that the primary reaction is very rapid. By extrapolation of the linear portion of the curves in Fig. l, the amount of tetrasulfide ions reacted in the initial period was estimated. This procedure required the assumption that all the S2- ions disappearing from the solution converted into the sulfidized layer. The results are given in Table 1.

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R. ZHOUAND S. CHANDER

i

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TINE. minutee Fig. 1. Residual concentration o f tetrasulfide ions vs. time in the malachite-tetrasulfide system for various initial tetrasulfide concentrations. Sulfidization pH: 10. 1 : 2 . 3 > ( 1 0 -3 M; 2: 1.15)< 10 -3 M; 3" 3.45)< 10 -4 M; 4: 1.15)< 10 -4 M; 5" 2.3)< 10 -5 M. TABLE 1 Specific rate constant vs. the initial concentration of tetrasulfide ions S~- concentration Initial conc., mole/l

Initial consumptiona mole/g malachite

2.3>(10-5 1.15>( 10 -4 3.45 >( 10 -4 1.15>( 10 -3 2.3 >( 10 -3

b 5.35>( 1.06 × 2.47X 4.10×

10 -6 10 -5 10 -5 10 -5

Calculated thickness of the primary sulfidized layer,/k

Specific rate constant, k l/sec

b

b 0.0185 0.00565 0.00142 0.00075

9 17 40 66

a Amount of S~- reacted in the initial period. b Data insufficient for extrapolation.

Using the specific surface area of malachite sample as 0.47 m2/g, which was obtained by the BET method, the thickness of the primary layer could be estimated. A cross-section area of 25/~2 and the vertical dimension of 5 A were assumed for the CuS4 molecule. The results are also included in Table 1. Even

KINETICS OF MALACHITE SULFIDIZATION IN HYDROSULF1DE AND TETRASULFIDE SOLUTIONS

263

though the magnetic stirring might result in grinding and detachment of the suliidized layer, the extent of such a process was considered to be small. In a separate experiment, it was observed that most of the particles were suspended near the wall of the beaker and did not accumulate beneath the magnetic bar when malachite was agitated with water under the conditions used for sulfidization. Thus, the error in calculation of the thickness of the coating layer is likely to be small. The thickness of the primary sulfidized layer increased with the initial concentration as can be seen in Table 1. The straight line portions of the kinetic curves in Fig. 1 correspond to the first order kinetics, for which dC/dt = - kC

(5)

which has the following integrated form: ln(C/Co)

(6)

= -kt

where Co is the initial bulk concentration, C is the bulk concentration at time t, and k is the specific rate constant. From the slope of the straight line portion of curves in Fig. l, the specific rate constant was calculated for each concentration of S2 - , and the results are given in Table 1. The results indicate that the reaction rates of tetrasulfide ions in the malachite suspension decreased with an increase in the initial concentration. A schematic description of the surface structure of the sulfidized malachite is presented in Fig. 2. Based on Reaction 3, copper ions were produced during the formation of the primary sulfidized layer. These copper ions would diffuse through the primary sulfidized layer and precipitate as CuS4 by the tetrasulfide ions to form a secondary layer on the particle surface. A model based

)

Na2S4

NariS

. ~ / / ~ unreacted malachite primary

sulfidized layer

secondary sulfidized layer

Fig. 2. Schematicrepresentation of the surface of the sulfidizedmalachite.

264

R. ZHOU AND S. CHANDER

on diffusion of sulfidizing species through the product layer was considered and found to be unsatisfactory. For the natural samples of malachite used in this study, the primary layer is expected to be defective consisting of cracks and crevices through which copper ions could diffuse. After formation of the primary layer, we consider that the diffusion of the copper ions through cracks and crevices in the primary layer is the rate determining step. It can be seen from Table 1 that the amount of tetrasulfide reacted in the initial period correlates with the reaction rate constant. As the initial concentration of $42- ions increases, the reaction rate constant decreases. Thus, the smaller rate constant indicates a greater extent of sulfidization in the initial period. For the purpose of comparison, the reaction kinetics of sodium sulfide on the malachite surface was also investigated. The results are given in Fig. 3. It is evident that the rate of consumption of hydrosulfide ions is much larger than that of tetrasulfide ions. The reaction rates are faster because the primary layer through which the copper ions diffuse is thinner. A schematic description of the surface structure of the malachite sulfidized with hydrosulfide is also included in Fig. 2 for the sake of comparison. This hypothesis was supported by the SEM study as presented later in this article. The rate of consumption decreased as the initial concentration of hydrosulfide ions increased. However, the rate of reaction did not follow the first-order kinetic model observed for tetrasulfide. The decrease in the hydrosulfide ion concentration was gradual. This difference in the kinetic behavior may be due to the 10 .3

i

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I=

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10 .6 0

TIME,

3

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minutes

Fig. 3. Residual concentration o f hydrosulfide ions as a function o f time for various initial concentrations o f hydrosulfide ions. Sulfidization pH: 9.1: 2.3X 10 -5 M; 2: 5.75× l0 -5 M; 3: 1.04 X 10 -4 M; 4:2.3 X 10 -4 M; 5:4.58 X 10 -4 M; 6:9.16 × 10 -4 M.

KINETICS OF MALACHITESULFIDIZATIONIN HYDROSULFIDEAND TETRASULFIDESOLUTIONS

265

larger extent of reactions in the second stage. This aspect is further discussed in a subsequent section. Effect ofsulfidization pH. The effect of sulfidization pH on the rate of consumption of sodium tetrasulfide ions is shown in Fig. 4. The rate of consumption was a strong function of solution pH. At pH 11.8, the reaction rate was small. Between pH 9.2 to 11.2, the reaction rates were independent of pH in the first 5 minutes or so. At a pH of 7.5 a rapid decrease in the tetrasulfide concentration was observed in the initial period of reaction. It was observed that when the solution pH was lower than 9, white colloids, believed to be that of elemental sulfur, formed in the liquid phase. At pH's greater than 9, such colloids were absent. The observation of elemental sulfur at pH's below 9.0 agreed with the review by Teder ( 1969 ). By extrapolation of the straight lines in Fig. 4, the amount of tetrasulfide ion consumed in the initial period can be estimated. The pH range can be divided into three regions based on the quantity oftetrasulfide ion reacted in the initial period. At pH above 11.2, the amount of tetrasulfide ion consumed in the initial period was small. The result implied that the extent of sulfidization was smaller at such pH's. Between pH 9.2 to 11.2, more tetrasulfide l@

g,1

0 I-I I-.9( rT

I"Z

LU

(.1 Z 0

0

¢ 0

lO.O

•

14.8

11.2

S

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TIME, minutes Fig. 4. Residual concentration o f tetrasulfide ions vs. time as a function o f pH in the presence o f malachite. Initial sulfur concentration: 3.45 × 10 -4 M.

266

R. Z H O U A N D S. C H A N D E R

ions were consumed compared to that at pH 11.8. In this pH range, the amounts of tetrasulfde reacted were almost the same, indicating a similar extent of sulfidization. At pH 7.5, most of the tetrasulfde ions converted to elemental sulfur in the initial period of reaction via Reaction 4, which contributed to the higher reaction rate.

Extent of secondary reactions In addition to the uptake of sulfur species by malachite surface, some oxys u l f d e species were produced in the liquid phase. The solution species were analyzed as thiosulfate and sulfate ions by ion chromatography. Such secondary reactions consumed part of the sulfidizing agent, diminishing the extent of sulfidization. To determine the uptake of sulfur, quantitative analysis of the remaining sulfidizing solution was performed after 10 minutes of sulfdization. The amount of the sulfidizing agent converted to the sulfdized layer was estimated by assuming that the thiosulfate and sulfate ions were the primary reaction products in solution. The sulfur uptake for various initial concentrations are shown in Fig. 5. With sodium tetrasulfide the percent conversion to the sulfidized layer varied between 80 to 90% whereas for sodium sulfide, it was only 25 to 55%. It is apparent that the amount of sulfur retained on the surface was much greater when sodium tetrasulfide was used. The sulfidization of malachite with sodium tetrasulfide was therefore much more effective. The large extent of secondary reactions in the sodium sulfide system might account for the deviation of reaction kinetics from first-order. It was observed in this study that copper species were also present in the liquid phase when malachite was placed in contact with the sulfidizing solu100

i

i

75 .< ,nr-

-.% 50

"9 ~)-x \

09

25

0

HS-

~'~.

<> S2 0

I

I

10

100

EQUIVALENT SULFUR CONCENTRATION, mg/I

Fig. 5. Percentage of sulfur uptake on the malachite surface vs. the initial concentration of sulfidizing agent. Sulfidization pH: 10 for tetrasulfide and 9 for hydrosulfide.

KINETICS OF MALACHITE SULFIDIZATION IN HYDROSULFIDE AND TETRASULFIDE SOLUTIONS

267

J

i' M I'-

iOi

-o- s,2-

//"~

- ~ - H$"

&j//

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./Jr

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EgUIVALENT. SULFUR CONCENTRATION, no/l

Fig. 6. Amount of copper species dissolved vs. the initial equivalent sulfur concentration of sodium sulfide and tetrasulfide.Analysiswas conducted at the end of sulfidization ( 10 minute conditioning). SulfidizationpH: 10 for tetrasulfide and 9 for hydrosulfide. tions. The amount of copper species released varied with the type and the initial concentration of sulfidizing agents. The samples for analysis were taken by a 0 . 2 / a n membrane filter. The copper in solution was analyzed by atomic absorption spectrophotometry and the results are given in Fig. 6. As the concentration of sulfidizing species increased, for example, from 15 to 44 mg/1, the amount of copper varied from 0.1 to 0.7 rag/1 for sodium tetrasulfide system, whereas in the case of hydrosulfide, the amount increased from 1 to 6 rag/1. In comparison with sodium tetrasulfide, more copper species were dissolved when malachite was conditioned in the hydrosulfide solution, which is expected on the basis of Eqs. 2 and 3. The copper ion produced in the sulfidization process may diffuse toward the solid/liquid interface and precipitate by the sulfidizing species. Some of the precipitates may remain on the surface as a secondary sulfidized layer. The others may disperse in the liquid phase as colloids.

Surface morphology The surface morphology of the malachite samples was examined by scanning electron microscopy and the photomicrographs are given in Figs. 7 and 8. The surface morphology of a polished sample of malachite without any contact with sulfidizing solutions is included for comparison. The natural sample contained a relatively flat surface with many cracks and crevices. When the samples were contacted with the hydrosulfide solution for various time periods, precipitates were formed on the surface as shown in Fig. 7b and c. The geometry and quantity of the precipitates changed with time for which

268

R. Z H O U A N D S. C H A N D E R

Fig. 7. Scanningelectronmicrographsof the malachitetreatedwith (a) distilledwateronly, (b) hydrosulfidesolutions for 0.5 minute, and (c) hydrosulfidesolutions for 10 minutes. Hydrosulfide ion concentration:4.2 × 10-2 M. pH: 9. the sample was immersed in the solution. For short sulfidization time, say 0.5 minute, the precipitates were rounded in shape. After 10 minutes, the precipitates grew into elongated particles and a layer of precipitate was formed. The precipitated particles are believed to be that of copper sulfide. The copper ions produced via Reaction 2 could diffuse through the primary sulfidized layer and were precipitated as copper sulfide by sulfide ions. In the case of sodium sulfide the thickness of the primary layer might be small. Under such conditions relatively large quantities of copper ions would diffuse outward from the bulk to combine with sulfide ions in solution. Such a process gives rise to relatively large rates of dissolution and precipitation. As a result, a substantial amount of sulfide ions was consumed. It is likely that the precipitate produced by such a mechanism might not attach firmly to the surface and would detach, especially under a strong agitation condition, such as those encountered in a flotation cell. In contrast, when the malachite sample was treated with sodium tetrasulfide solution a relatively bare surface with only a few spots of precipitates was

KINETICS OF MALACHITE SULFIDIZATION IN HYDROSULFIDE AND TETRASULFIDE SOLUTIONS

269

Fig. 8. Scanning electron micrographs of the malachite treated with sodium tetrasulfide solutions (4.2X l 0 -2 M ) for various times. (a): 0.5 minute; (b): 10 minutes (a typical micrograph); (c) l0 minutes (only observed at some spots), pH: 10.

observed (see Fig. 8a and b). The bare surface was hydrophobic, however, as was confirmed by Hallimond tube flotation and contact angle measurements. These measurements showed that the pretreatment with sodium tetrasulfide resulted in substantial increase in the hydrophobicity of malachite. Therefore, it is reasonable to assume that a uniform sulfidized layer is formed after the sample was treated with sodium tetrasulfide. Most likely, the precipitate formed on the surface at pH 10, shown in Fig. 8c, consist of CuS4. By comparing the micrographs of these precipitates with those obtained for sodium sulfide systems, it is found that the morphology of these precipitates is quite different. In the case of sodium sulfide, the precipitates were evenly distributed over the mineral surface. For the sample treated with sodium tetrasulfide, however, the precipitates were formed only in certain small regions of the surface. These precipitates appear to be coagulated. The formation of agglomerates in the latter case suggests that the precipitates might have been more hydrophobic than those obtained in the hydrosulfide solutions. A comparison of the extent of precipitation may imply that the rate of diffusion of

270

R. ZHOU AND S. CHANDER

copper ions is much smaller in the case of tetrasulfide than that in sulfide systems. The results obtained by SEM are consistent with the model proposed to explain reaction kinetics. These results strongly support the hypothesis that a primary sulfidized layer forms in the initial period of reaction and a secondary layer builds up by reactions between the copper ions, which diffuse through cracks and crevices in the primary layer and the tetrasulfide ions, which diffuse from the solution. The following assumption were made to calculate the effective diffusion coefficient of the copper ions in sodium tetrasulfide systems: (a) the primary sulfidized layer is uniformly distributed over the particle surface; (b) the number of moles of copper ions in the sulfidized layer is given by Reaction 3. It was further assumed that the copper ions are uniformly distributed within the primary layer; (c) the rate of diffusion of copper ions is the rate determining step in the course of reaction and the diffusion is through the solid layer. The calculated values of the effective diffusion coefficient was in the order of 10- ~ cm2/sec. The value is much too large for diffusion through a solid layer at room temperature and relatively small for diffusion through liquid. It is therefore considered that the diffusion of copper ions proceeded through the cracks and crevices in the primary layer. This hypothesis was also supported by the electron micrographs, in which patches of agglomerates were found generally near the cracks on surface. SUMMARY

The sulfidization of malachite occurs in two stages. In the first stage, which proceeds very rapidly, a primary sulfidized layer forms as a uniform coating. In the second stage, two types of secondary reactions are involved. One is the diffusion of copper ions through the cracks and crevices in the primary sulfidized layer to form a loosely adherent precipitate, and the other is the formation of oxysulfide species via oxidation of sulfidizing agents. Some of the precipitates formed as products of the secondary reactions are also present as colloids in the bulk solution. The extent of secondary reactions as determined by the amount of copper and oxysulfides in the liquid phase was much less for the sodium tetrasulfide when compared to hydrosulfide. Analysis of the sulfidizing residue show that part of the sulfidizing agent was oxidized to oxysulfide ions. For sodium tetrasulfide the percent conversion of the reagent to the sulfidized layer was substantially greater than that in hydrosulfide. The studies of surface morphology of the sulfidized malachite show that when sodium tetrasulfide was used as a sulfidizing agent, a uniform coating formed on the malachite surface; for sodium sulfide, the malachite surface was coated with loosely adherent precipitates, which might detach from the surface under strong agitation conditions such as those encountered in a flo-

KINETICSOF MALACHITESULFIDIZATIONIN HYDROSULFIDEANDTETRASULFIDESOLUTIONS

271

tation cell. Based on this study sodium tetrasulfide is expected to be a much more effective sulfidizing agent than hydrosulfide. ACKNOWLEDGEMENTS

The authors acknowledge the financial assistance from the National Science Foundation under Grant No. MSM-8413477 in support of this investigation. They also acknowledge additional assistance provided by the Mineral Processing Section, The Pennsylvania State University.

REFERENCES Aplan, F.F. and Fuerstenau, D.W., 1984. The flotation ofchrysocolla by mercaptan. Int. J. Miner. Process., 13:105-115. Baumann, E.W., 1974. Determination of parts per billion sulfide in water with the sulfide-selective electrode. Anal. Chem., 46:1345-1347. Bustamante, H. and Castro, S., 1975. Hydrophobic effects of sodium sulfide on malachite flotation. Trans. IMM, 84: C 167-C 171. Castro, S., Goldfarb, J. and Laskowski, J., 1974a. Sulfidizing reactions in the flotation of oxidized copper minerals, I. Chemical factors in the sulfidization of copper oxide. Int. J. Miner. Process., l: 141-149. Castro, S., Soto, H., Goldfarb, J. and Laskowski, J., 1974b. Sulfidizing reactions in the flotation of oxidized copper minerals, II. Role of the adsorption and oxidation of sodium sulfide in the flotation of chrysocolla and malachite. Int. J. Miner. Process., 1: 151-161. Castro, S., Gaytan, H., and Goldfarb, J., 1976. The stabilizing effect of NaES on the collector coating of chrysocolla. Int. J. Miner. Process., 3:71-82. DeCuyper, J., 1977. Flotation of copper oxide ores. Erzmetall., 30: 88-94. Fuerstenau, D.W. and Pradip, 1984. Mineral flotation with hydroxamate collectors. In: M.J. Jones and R. Oblatt (Editors), Reagents in the Minerals Industry. Institution of Mining & Metallurgy, London. Fuerstenau, D.W. and Raghavan, S., 1986. Surface chemical properties of oxide copper minerals. In: P. Somasundaran (Editor), Advances in Mineral Processing, Arbiter Symposium. The Society of Mining Engineers, Littleton, CO. Fuerstenau, D.W., Sotillo, F. and Valdivieso, A., 1985. Sulfidization and flotation behavior of anglesite, cerussite and galena. In" XVth International Mineral Processing Congress, Cannes. GEDIM, Vol. II, pp. 74-86. Gaudin, A.M., 1957. Flotation. McGraw-Hill, New York. Giggenbach, W., 1972. Optical spectra and equilibrium distribution of polysulfide ions in aqueous solution at 20°C. Inorg. Chem., 1 l: 1201-1207. Gonz~ilez, G. and Soto, H., 1978. The effect of thermal treatment on the flotation ofchrysocolla. Int. J. Miner. Process., 5: 153-162. Malghan, S.G., 1986. Role of sodium sulfide in the flotation of oxidized copper, lead, and zinc ores. Miner. Metall. Process., 3:158-163. Parks, G.A. and Kovacs, C., 1966. Thermal activation of chrysocotla for xanthate flotation. Trans. SME, 235: 349-354. Queirolo, C. and Castro, S., 1976. Effect of sulfidization of thermally activated chrysocolla. Trans. IMM, 85: C166-C168.

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Raghavan, S. and Fuerstenau, D.W., 1977. Characterization and pore structure analysis of a copper ore containing chrysocolla. Int. J. Miner. Process., 4:381-384. Raghavan, S., Adamec, E. and Lee, L., 1984. Sulfidization and flotation ofchrysocolla and brochantite. Int. J. Miner. Process., 12: 173-191. Schawrzenbach, G. and Fischer, A., 1960. Acidity of sulfanes and the composition of aqueous polysulfide solutions. Helv. Chim. Acta, 43:1365-1390. Soto, H. and Laskowski, J., 1973. Redox conditions in the flotation of malachite with a sulfidizing agent. Trans. IMM, 82: C 153-C 157. Stahl, J.W., 1983. Lewis acid-base chemistry and determination of polysulfides by thermometric enthalpy titration. Ph.D. Thesis, The Pennsylvania State University. Teder, A., 1969. Some aspects of the chemistry of polysulfide pulping. Svensk Papperstidn., 72: 294-303. Valensi, G., Muylder, J.V. and Pourbaix, M., 1966. Sulfur. In" M. Pourbaix (Editor), Atlas of Electrochemical Equilibria in Aqueous Solutions. Pergamon Press, New York, Section 19.2. Wright A.J. and Prosser, A.P., 1965. Study of the reactions and flotation of chrysocolla with alkali-metal xanthates and sulfides. Trans. IMM, 74: 259-279.