Influence of chain length on adsorption of xanthates on chalcopyrite

InlERnnlIOlmLJoUnnRL Of

mlnERR1 PROCEsSlne ELSEVIER

ht. J. Miner. Process. 52 (1998) 215-231

Influence of chain length on adsorption of xanthates on chalcopyrite J.A. Mielczarski

*,

Laboratoire “Encironnement et Mintralurgie”,

E. Mielczarski,

J.M. Cases

(IA 235 CNRS, INPL-ENSG, B.P. 40, Vandoeuure-lPs-Nanq 54501. France

Received 2 October 1996; revised 15 September 1997; accepted 10 October 1997

Abstract The direct characterization at a molecular level of the surface products formed by the interaction of aqueous solutions of ethyl and amyl xanthate with chalcopyrite polarized to different potentials were carried out by the infrared external reflection technique recently developed for detailed study of the adsorbed layer on mineral surfaces. The experimental spectroscopic data combined with the simulation of hypothetical adsorption layers have let us determine the type, structure and the surface distribution of the adsorbed species produced at different adsorption conditiorrs, and to propose mechanisms of the interaction between the aqueous solutions of collectors and the surface of chalcopyrite. The spectroscopic results show striking differences in the composition of the outermost layer of chalcopyrite contacted with ethyl or amyl xanthate solutions, which obviously will produce differences in the flotation behavior of the mineral and in consequence will influence its separation from other ore components. These observations explain why different xanthate homologues should be used in flotation practice. A detailed discussion of flotation pulp solution conditions to produce different types of hydrophobic surface species for either collector and collectorless flotation of chalcopyrite is also presented. 0 1998 Elsevier Science ELV. Keywords. xanthate; chalcopyrite; molecular chain length; surface adsorption

1. Intro’duction The flotation hydrophilic

behavior

species

is an aliphatic

* Corre:;ponding

present

of minerals

depends

at the surface

chain) are commonly

on the balance

of the minerals.

used for the selective

author. Tel.: + 33 83596383;

Fax: + 33 83575404;

between

Xanthates

hydrophobic

hydrophobization

of sulfide

E-mail: [email protected]

0301-7516/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO30 l-75 16(97)00074-4

and

(ROCS, , where R

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minerals. Improvement of selective separation of ore components is achieved by manipulation of the pH, E, (aeration, reducing or oxidizing reagents) and by adding modifiers, i.e. activators or depressants. It should be noted that one reagent could initiate several changes in the flotation system, as a variation of the mineral surface composition, pH change, E, etc. In practice it has been shown that different xanthate homologues from ethyl to amyl give optimal selectivity with different ores. The reason for this is not clear. Interaction of xanthates with the “fresh” surface of sulfide minerals is an electrochemical reaction. The following reasons could be considered: (1) differences in the adsorption potentials, adsorption of amyl xanthate starts at a much lower potential than that of ethyl xanthate; (2) differences in the adsorption kinetics; and (3) the difference in the nature and structure of the surface adsorption products. It is a common assumption that the same types of products (xanthate metal surface complexes and/or dixanthogen) are formed for different xanthate homologues in the same mineral system. The expected difference is a lowering of the potential of formation of the predicted xanthate products with an increase of the chain length. It is also noteworthy at this point that the open circuit potential (OCP) of sulfide minerals in amyl xanthate solution is much lower than in ethyl xanthate solution. In the spectroscopic and electrochemical studies (Mielczarski et al., 1996c, 1997a) where direct characterization at a molecular level of the xanthate surface products formed on chalcopyrite were carried out, significant differences between ethyl and amyl xanthates were observed. In this paper we will discuss in detail the implications of the differences observed in the interaction of ethyl and amyl xanthate with chalcopyrite polarized to various potentials, to flotation. Because this work is related to the flotation behavior of chalcopyrite, the electrochemical cell was open to air and the applied potential was between the OCPs of chalcopyrite in ethyl and amyl xanthate solutions (about 180 mV and 70 mV (SHE), respectively) and potentials about 300 mV above these values. The qualitative and quantitative evaluation of the xanthate adsorption layer was carried out on the basis of spectroscopic data obtained by the infrared external reflection method recently developed for mineral surface characterization (Mielczarski and Yoon, 1989; Mielczarski, 1993; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995). This method allows to carry out the detailed determination at a molecular level of the surface composition and structure (orientation, surface distribution) of the adsorbed layers on all types of solid surfaces including metallic, semiconductor and dielectric minerals as well as on transparent, opaque and nontransparent minerals. It is well known that the structure of the adsorbed layer at monolayer and submonolayer coverages has an important influence on the produced hydrophobicity of mineral, therefore, the possibility to monitor the surface phenomena, by the application of the new method especially suited for the study of complex natural minerals, could be important for a progress in selective separation.

2. Experimental 2.1. Materials The mineral sample of chalcopyrite (CuFeS,) was procured from the Iberian Pyrite Belt. This sample contains a small amount (below 1%) of other components, mainly

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sphalerite and pyrite. The slab sample of mineral with dimensions of 25 X 9 mm2 was cut from a massive rock and polished using emery paper and alumina powder and washed ,with water. The potassium amyl and ethyl xanthates used in these studies were synthesized from CS,, KOH and proper alcohol, and then purified by recrystallization from acetone and ether. 2.2. Adsorption

study at controlled potentials

A standard three-electrode cell and a Tacussel PRT 20-2X potentiostat were used in the electrochemical studies of the mineral sample. The mineral sample was the working electrode, while a platinum wire mesh separated from working electrode by frit, served as the counter electrode. All potentials are reported against the standard hydrogen electrodl: (SHE) scale. The electrochemically controlled adsorption was performed from 2.0 X 1W4 M xanthate solutions with 0.05 M Na,SO, as an electrolyte at pH 10 + 0.2 adjusted by adding KOH or H,SO, solutions. The mineral sample was immersed in xanthate. solutions, immediately after polishing. The electrochemical cell was open to air. In the standard experiment the mineral electrode was linearly swept from open circuit potential (OPC) to the desired potential and held at this potential for 10 min. Next the sample was immersed in water at pH 10 for 1 s and then immediately introduced to the spec trophotometer for infrared characterization. 2.3. Spectroscopic

analysis

The infrared spectra of mineral samples after adsorption were recorded on Bruker IFS88 or IFS55 FTIR spectrometers with an MCT detector cooled with liquid nitrogen. External reflection spectra were recorded by means of a special reflection attachment with a polarized incident beam at various angles of incidence. All the optical accessories were from Harrick Scientific Co. The spectra were taken at 4 cm-’ resolution by co-adding up to 500 scans. The unit of intensity was defined as - log (R/R,), where R, and R are the reflectivities of the systems without and with investigated medium (adsorption layer of xanthate), respectively. Other details of experimental procedure can be found in recent papers (Mielczarski et al., 1996c, 1997a).

3. Results 3.1. Optical consideration Detailed description of the infrared external reflection technique applied to the studies of adsa’rption layers of surfactants on mineral samples can be found in recent papers (Mielcn,arski, 1993; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995). The optical scheme of the experimental setup is shown in Fig. 1. These studies which provide detailed information about the nature and structure of the adsorbed layer at a molecular level were possible because of the use of spectral simulation and theoretical consideration of all the possible phases present in the mineral systems under investiga-

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If3 incident beam

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Process. 52 (1998) 215-231

reflected beam

air

multi-component adsorption layer chalcopyrite (electrode)

Fig. 1. Schematic diagram of multilayer system with two-component s-polarized electric field vectors are marked on incident beam.

adsorption

layer. Directions

of p- and

tion. Though the interpretation of the reflection spectra is not as straightforward as the transmission ones because of additional optical effects (Wang and Yen, 1988; Mielczarski and Yoon, 1989; Mielczarski, 1993; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995; Parikh and Allara, 1992; Hoffmann et al., 1995) the experimental spectra, recorded for different polarizations and angles of incidence beam, carry all the needed information about the chemical composition and structure (orientation, surface distribution, lateral interaction) of the adsorbed layer. This information could be extracted from the experimental results by a proper combination of experimental and simulated data which is demonstrated in several recent studies (Wang and Yen, 1988; Mielczarski and Yoon, 1989; Mielczarski, 1993; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995; Parikh and Allara, 1992; Hoffmann et al., 1995). A two-step procedure was developed (Mielczarski and Mielczarski, 1995) for the detailed evaluation of the composition and structure of adsorbed layers on nonmetallic (mineral) substrates on the basis of spectroscopic results. After completion of the compositional and structural determinations, the detailed quantitative evaluation could be performed. The spectral simulation permits also to optimize the experimental conditions (Mielczarski, 1993; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995; Hoffmann et al., 1995). It was found from the preliminary simulation of the experimental configuration that the best spectra, with highest signal/noise ratios, of xanthate submonolayer coverages on chalcopyrite could be recorded at an incident angle of 70” and p-polarization. Each of the recorded reflection spectrum at different angle of incidence carries out numbers of different types of information about the adsorbed layer. Fig. 2 is a good example of the complexity of the reflection spectra recorded from the same sample; these spectra are different because they are recorded at various incident angles (optical effects). It is like looking at the adsorbed molecules from different points of view, which allows to examine all the important features of the surface species: composition, orientation, surface distribution, lateral interaction, and adsorbed amount. If the molecules in the adsorption layer are oriented, each of the recorded reflection spectra will be differently modified (changes in the band shape, position and intensity) by the molecular

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b

a 1400

1000

Wavenumber, cm -1 Fig. 2. Infrared reflection spectra of the adsorbed layer on chalcopyrite contacted with 2X 10m4 M amyl xanthate solution for 10 min, at potential of 125 mV. The reflection spectra of the sample were recorded at incident angles of 60” (a), 70” (b) and 80” (c), and for p-polarization.

orientation. This allows to determine quantitatively the orientation of each part of the adsorbed molecules (molecular group) which shows a specific (positive or negative) absorbance band in the reflection spectra (Parikh and Allara, 1992; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995; Hoffmann et al., 1995). Infrared spectra are extremely characteristic for the investigated organic molecules (fingerprint) and very sensitive to the actual molecular state (physisorbed, chemisorbed, solid- or liquid-like form); th.erefore, there is only one possible solution, i.e. type(s) of adsorption product, its surface structure(s) and adsorbed amount(s) which could fit all the reflection spectra recorded at different angles and two polarizations. The appearance of the positive and negative (reverse) absorbance bands, depending on the orientation of the adsorbed molecules, is very specific for this method and it allows to determine the surface structure with relatively low error. All this makes the external reflection method very unique. The clomparison of the reflection spectra recorded from the same sample (Fig. 2) with the simulated spectra calculated for the hypothetical surface composition and structure (Fig. 3) is vitally important for detailed qualitative and quantitative interpretation of the experimental results. If the experimental and simulated spectra are not the same, the one or more assumptions made for simulation should be changed until a good fit is achieved.

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b I

L 1400

1300

1200

Wavenumber,

1100

1000

cm“

Fig. 3. Simulated spectra of two-component isotropic hypothetical adsorption layers on chalcopyrite produced by amyl and ethyl xanthates: (a) chalcopyrite covered by cuprous amyl xanthate (0.5 nm) and amyl dixanthogen (3 nm), position of negative band in parentheses; (b) chalcopyrite covered by cuprous ethyl xanthate (1 nm) and ethyl dixanthogen (1 nm).

Description of the simulation procedure can be found in previous works (Mielczarski, 1993; Mielczarski and Mielczarski, 1995; Mielczarski et al., 1995, 1996~). There is also a detailed discussion of the possible sources of the experimental errors of this method (Mielczarski and Mielczarski, 1995). In this work the adsorption layer was represented by two components, i.e. cuprous xanthate complex and dixanthogen. Fig. 3 represents spectra expected for an isotropic two-component adsorption layer with different thicknesses of each adsorbed product calculated for ethyl and amyl xanthates. The simulations were performed with the assumption that dixanthogen is present on the top of the cuprous xantbate complex (Fig. 3). 3.2. Spectroelectrochemical copyrite

studies of amyl and ethyl xanthate

interactions

with chal-

Application of different potentials to the mineral electrode makes it possible to control the surface composition and its structure, and as a consequence the hydrophobic property of the mineral surface. The electrochemical studies of chalcopyrite-ethyl xanthate system carried out in a nitrogen and air environment were already reported (Pang and Chander, 1990; Tolley et al., 1996). There are no significant differences between the features of the voltammograms presented recently (Pang and Chander, 1990; Tolley et al., 1996) and these obtained in the present work where xanthate

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solutions were air saturated. The mineral potential was controlled by potentiostat. Another way to change mineral potential is to add oxidizing or reducing chemicals. It has to be kept in mind that the latter potential regulation could result in side effects which overlap the changes caused by potential variation. For example, hydrogen peroxide used as a reagent to increase mineral potentials could produce microbubbles of oxygen on the mineral surface in a catalytic reaction of decomposition of H,O, at sulfide mineral surfaces, which modifies dramatically the flotation behavior of the minerals. In this case the catalytic properties of mineral components play a very important role in increasing hydrophobicity during their separation by flotation. The initial xanthate concentration 2 X 10m4 M used in these studies is similar to that in flotation plant conditions. It was found that at higher xanthate concentrations of lop3 or lo-* M, which are often used in electrochemical studies related to mineral flotation, the resuhs are very different from those obtained at typical flotation xanthate concentrations, i.e. 10e5, lop4 M. This also questions the validity of the conclusions drawn on

I

0.005

SHE (mv)

e

d C b

a 1400

Wavenumber,

1000

cm-’

Fig. 4. Infrared reflection spectra of adsorbed layer on chalcopyrite contacted with 2 X 10d4 M amyl xanthate solution for 10 min, at various potentials. The reflection spectra were recorded at incident angle of 70” and for p-polarizal:ion.

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the basis of those studies which are carried out at high concentrations to the observed flotation phenomena. Results of spectroscopic studies of the surface composition of chalcopyrite contacted with amyl xanthate solution for 10 min at various potentials are presented in Fig. 4. These spectra were recorded at the conditions which ensure the best spectroscopic sensitivity for the system (see discussion above). The absorbance bands of xanthate species, at about 1200 and 1034 cm-‘, are visible at a potential of 110 mV (Fig. 4b) indicating the formation of a cuprous amyl xanthate complex. At a potential only 10 mV higher (Fig. 4c) an additional band at 1269 cm -’ is observed showing the presence of amyl dixanthogen. While the amount of cuprous amyl xanthate complex does not increase with the further increase of potential, the amount of dixanthogen increases significantly. The additional small increase of potential to 150 mV (Fig. 4d) results in an about 6-times increase in absorbance of the dixanthogen band at 1269 cm-’ and the bands at 1034 and 1047 cm-t. An increase of the potential to 200 mV (Fig. 4e) involves a decrease, of about 30% in the observed intensity of dixanthogen bands, and the band at 1200 cm-’ characteristic for cuprous amyl xanthate doubles its intensity. This indicates that chalcopyrite undergoes oxidation at a potential of about 200 mV. Very different results were obtained when ethyl xanthate was used (Fig. 5). Adsorbed ethyl xanthate molecules are observed at a potential of 250 mV, which is about 70 mV higher (Fig. 5) than the OCP of chalcopyrite in the solution. The only adsorbed product

2

d

320

C

290

ba

250

g g 7

a

180 ow 1400

Wavenumber,

1000

cm -l

Fig. 5. Infrared reflection spectra of adsorbed layer on chalcopyrite contacted with 2 X 10m4 M ethyl xanthate solution for 10 min, at various potentials. The reflection spectra were recorded at incident angle of 70” and for p-polarization.

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is the cuprous ethyl xanthate complex with the absorbance bands at about 1200, 1125 and 10408 cm- ’ in a potential range up to 300 mV. The amount of produced cuprous xanthate complex on chalcopyrite surface increases to a multilayer coverage with an increase of potential. At a potential somewhat higher than 300 mV strong complex absorbance bands at 1270 cm- ’ and 1110 cm- ’, as well as an increase of intensities of the band;5 at about 1050 and 1040 cm-‘, were observed, indicating the formation of ethyl dix,anthogen.

4. Discussion Comparison of the reflection spectra of chalcopyrite contacted with amyl xanthate (Fig. 4) with the simulated one (Fig. 3a) shows a good agreement in the band positions and shapes except in the region of about 1020 cm- ’ where the calculated data show a negative band. Two conclusions can be drawn from this observation. First, that cuprous amyl xanthate is produced at the beginning of the adsorption followed by the formation of amyl dixanthogen. The latter is the major adsorption product of amyl xanthate on chalcopyrite at higher potentials. This is clearly seen from almost the same positions of the absorbance bands at about 1270, 1050 and 1030 cm-’ in the experimental and simulated spectra. The second finding is a lack of negative band at 1016 cm-i predicted by the spectral simulation, which indicates that the adsorbed molecules are organized in the surface layer. The detailed organization (orientation) of molecules at interface could be determined if the band assignment to the vibration of the particular molecular group is well known. Because at this time the assignment of all absorbance bands of amyl xanthate adsorption products are not well determined the detailed quantitative evaluation of molecular orientation was not performed. The detailed discussion of this problem will be the subject of the future paper. At the first approximation it can be assumed that the adsorbed amyl xanthate layer is isotropic. This allows to perform a semiquantitative estimation of the adsorbed products on chalcopyrite surface on the basis of the spectroscopic results by the simple comparison of the experimental spectra with the simulated spectra for the assumed adsorption layer thickness. The adsorption of xanthate at 110 mV (Fig. 4b) produces an about 0.6 nm (less than a statistic monolayer coverage) adsorption layer of cuprous amyl xanthate. At a potential of 120 mV (Fig. 4c), a 2.0 nm layer of amyl dixanthogen is additionally produced. The amount of dixanthogen increases considerably with an increase of the potential, for example at 150 mV a 10 nm layer was formed. At 200 mV, equivalents of 6 nm dixanthogen and 2.5 nm cuprous xanthate layers were produced. It should be noted here that if the quantitative evaluation of the adsorbed layer will be performed taking into account the real molecular organization, the estimated above thicknesses of the adsorbed layers could be only somewhat lower. Another method to estimate the adsorbed amount of xanthate employs the value of the electric charge passed through the system. As was reported (Mielczarski et al., 1996~) the estimation of the adsorbed layer thickness on the basis of electrochemical data gives several times higher values in comparison to the spectroscopic evaluation. The reasons of the significant overestimation, from the charge flow data, of the amount adsorbed have been discussed recently (Mielczarski et al., 1996~).

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For the oxidation X-=

0.5X,

reaction:

+ e-

(11

where X- is xanthate ion, the calculated reversible potential (El of the bulk phase formation of amyl dixanthogen on the basis of thermodynamic data (Kakovsky, 1957) is E = 86 mV for the solution conditions used in these studies. This indicates that the potential at which amyl dixanthogen was found (about 110 mV> is somewhat higher than that obtained from the thermodynamic estimation. Extension of the adsorption time to 2 h brings the experimental potential of dixanthogen formation (about 80 mV> to a very close agreement (Mielczarski et al., 1996~) with the thermodynamic value. Comparison of the experimental spectra of an ethyl xanthate adsorbed layer (Fig. 5d) with the simulated ones for an assumed two component layer (Fig. 3b) shows a very good agreement in both the position and the shape of the absorbance bands. This supports the explanation that the ethyl cuprous complex and ethyl dixanthogen are the two surface products of xanthate adsorption on chalcopyrite at higher potentials. However at lower and moderate potentials, the only adsorbed product is cuprous ethyl xanthate showing the absorbance bands at about 1200, 1125 and 1040 cm-‘. The calculation of the reversible potential of the bulk phase formation of ethyl dixanthogen on the basis of thermodynamic data (Kakovsky, 1957) for the reaction (1) gives a value of 181 mV. This indicates that ethyl dixanthogen was found at a potential about 140 mV higher than that predicted from the thermodynamic data. This observation is very different from that found for amyl xanthate. Another striking difference between these two xanthates is that the amount of cuprous ethyl xanthate adsorbed is not limited to about monolayer coverage at lower and moderate potentials as it was found for amyl xanthate. The spectral simulation of a hypothetical adsorption layer which fits the experimental spectrum presented in Fig. 5b allows to determine that at a potential of 250 mV the adsorbed cuprous ethyl xanthate forms a 0.7 nm thick layer (less than a statistical monolayer). The multilayer coverage is produced easily at a potential of 290 mV and the thickness of the adsorption layer is of 2.2 nm. Dixanthogen is adsorbed on the chalcopyrite surface already covered by a thick layer of cuprous ethyl xanthate. This can be easily seen from the results presented in Fig. 5c and d where an increase of potential by 30 mV results in the formation of a 5-nm layer of ethyl dixanthogen (bands at 1270, 1110, 1050 and 1040 cm-‘), while the amount of adsorbed cuprous xanthate remains almost the same (bands at 1200, 1125, 1050 and 1040 cm-‘). It is important to note that at higher potentials the multilayer coverage of cuprous xanthate complex and dixanthogen was found for both xanthates (Figs. 4 and 5); hence, the differences between the two xanthates during production of surface hydrophobic layer began to disappear at these conditions. These are the conditions where the oxidation of chalcopyrite itself starts to play an important role. To understand the observed differences between ethyl and amyl xanthate adsorption, several surface phenomena have to be considered. The first observation (Fig. 6) is that the OCPs of chalcopyrite in both xanthate solutions (about 180 and 70 mV for ethyl and amyl homologues, respectively) are very close to the reversible potentials of dixanthogen formation in the oxidation reaction (1). This would indicate that ethyl and amyl

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Ethiyl

225

OCP

pzJ

CuEtx

BCuEtX

+ EtX2

Amy1 OCP

m

(CuAmX +AmXd man

80

120 I

I

m

160 I

CuAmX,,,+

I

200 I

m

AmX Pmult

I

240 l

I

Potential Fig. 6. Pokntial at concentration

WAmX

280 I

+AmXJmult

320 I

SHE, mV

regions of the formation of different surface products from ethyl and amyl xanthate solutions of 2X 10m4 M, at pH 10.

dixanthogen could be easily produced on the chalcopyrite surface at somewhat higher potentials than their OCP. This is not the case. Ethyl xanthate adsorption product is present on the chalcopyrite surface at a potential 70 mV higher than OCP, and the surface product is not ethyl dixanthogen but cuprous ethyl xanthate which was found to be more stable than dixanthogen on the chalcopyrite surface. A substantial increase of the adsorption time at potentials close to OCP does not cause ethyl xanthate adsorption (Mielczarski et al., 1997a). However, in the case of amyl xanthate solution, with a long adsorption time (2 h) both adsorption products, cuprous amyl xanthate and amyl dixanthogen, are simultaneously formed at a potential very close to the reversible potential of Eq. (1) (Mielczarski et al., 1996~). This comparison indicates that in the basic ethyl xanthate solution (pH 10) the formation of the surface oxidized products of chalcopyrite, i.e. surface iron hydroxides, determined in recent XPS studies (Mielczarski et al., 1996a) is more favorable than surface adsorption products of ethyl xanthate. Only an increase of the chalcopyrite potential above OCP allows to form a surface cuprous ethyl xanthate complex (Fig. 5b). On the contrary, amyl xanthate forms a more stable surface cuprous amyl xanthate complex (lower surface solubility product) than the oxidation product (iron hydroxides) and, therefore, its adsorption is observed close to the OCP of chalcopyrite. The competition between the oxidation of chalcopyrite and adsorption of xanthate ions on chalcopyrite contacted with aerated aqueous basic solution (OH- ions, oxygen) could explain why ethyl xanthate is adsorbed on chalcopyrite surface at a potential higher than OCP.

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The second striking difference between adsorption of these two xanthates on chalcopyrite is that from ethyl xanthate solution, at moderate potentials, only cuprous xanthate is formed in the amount from submonolayer to multilayers, whereas from amyl xanthate solution the amount of cuprous amyl xanthate is limited to about monolayer coverage and is produced together with amyl dixanthogen. This could be explained by differences in kinetics between diffusion of xanthate from solution to surface and copper diffusion from the bulk of chalcopyrite to interface where the interaction between copper and xanthate takes place. It is well documented by different studies (Zachwieja et al., 1989; Pang and Chander, 1990; Mielczarski et al., 1996a) that chalcopyrite contacted with basic aqueous solutions shows a strong tendency for iron migration to interface with the formation of the outermost layer of iron oxides/hydroxides. Hence, copper concentration at chalcopyrite interface in the absence of xanthate in solution is very low. The presence of xanthate in solution is responsible for the changes in metal (copper and iron) atoms diffusions in the bulk of chalcopyrite and for the acceleration of copper versus iron migration to interface (Mielczarski et al., 1996b). If the amount of copper atoms at the interface meets a similar amount of xanthate molecules, the surface complex of cuprous xanthate is produced as the most stable adsorption product in an amount not limited to monolayer coverage. If the availability of copper at interface is much lower than that of xanthate ions, the dixanthogen is produced as the second surface product. These situations are observed for ethyl xanthate. At moderate potentials the formation of cuprous complex takes place whereas at higher potentials the formation of ethyl dixanthogen is observed. In the case of amyl xanthate the first adsorbed molecules seem to form a more compact (better organized) and thicker (longer aliphatic chain) surface barrier which slows significantly the interaction of copper atoms with xanthate ions from solution. This causes the amount of xanthate molecules at the interface usually to be higher than the amount of copper atoms; therefore, only a small amount of cuprous amyl xanthate is formed at the beginning of xanthate adsorption and amyl dixanthogen is also produced. When dixanthogen covers the mineral surface, the diffusion of copper atoms to solution interface is extremely limited which results in a constant amount of cuprous xanthate and a very strong increase of dixanthogen. This is observed for amyl xanthates (Fig. 41, and for ethyl xanthate at higher potentials (Fig. 5). It could be concluded that the kinetics of copper diffusion to chalcopyrite interface, or rather the relative diffusion of copper and iron to solid-solution interface, is the critical factor which controls the composition and structure of the produced surface layer. It is important to notice that the relative diffusion of copper and iron to chalcopyrite interface is very dependent on the solution composition and could be modified by proper changes of the solution composition and conditioning time. This finding is very important from the flotation point of view.

5. Comparison

and implication

for collector

and collectorless

flotations

Flotation of chalcopyrite at controlled potentials was reported in several works (Trahar, 1984; Richardson and Walker, 1985; Roos et al., 1990). The reason for a good chalcopyrite flotation even as a single mineral is not usually clear. This mineral can float

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with and without collector in basic solution. This observation clearly indicates that different changes in the surface composition of the mineral give the same effect, producing the outermost hydrophobic surface layer. Certainly the adsorption of collector is not the only possible way to make chalcopyrite hydrophobic. In this work we tried to apply the experimental conditions close to those employed in an industrial flotation. Alteration of chalcopyrite surface by its polishing during preparation for this work is more similar to autogenous grinding than with rod or ball mill conditions where a significant amount of iron is added to the system during grinding. Initial concentrations of xanthate collectors were very similar to those used in industrial conditions. Moreover, the potential of mineral sample was controlled in air saturated solution which imitate the typical flotation conditions. Under similar adsorption conditions to those presented in this paper (potential controlleNd by potentiostat), it was reported that chalcopyrite starts to float with ethyl xanthate at a potential of about 0 mV and reaches 100% of flotation at about 250 mV (Richardson and Walker, 1985). Ethyl xanthate concentration in that flotation test was of about 2 :< 10m5 M, which is one order of magnitude lower than that used in the present studies; hence, a 60 mV correction is required to compare the flotation results with the spectroscopic results of xanthate adsorption (Fig. 5). This comparison indicates that flotation of chalcopyrite in ethyl xanthate solution reaches a maximum at a potential about 60 mV lower than that at which about a monolayer of cuprous ethyl xanthate was observed on chalcopyrite surface (about 250 mV, Fig. 5b). In other flotation studies where an electrochemical Denver cell was applied (Roos et al., 19901, a maximum of chalcopyrite flotation was observed at about 200 mV at an ethyl xanthate concentration of 6.25 X 10e4 M. After the concentration correction there is an about 30 mV difference between the observed potentials at which the maximum of floatability is achieved (Roos et al., 1990) and the potential at which about a monolayer coverage by cuprous xanthate complex was found (Fig. 5a). Assuming that a good flotation of chalcopyrite requires a less than monolayer coverage by the cuprous ethyl xanthate which was obtained in this study at a potential somewhat lower than 250 mV, a good agreement between the earlier-mentioned flotation behavior and the spectroscopic findings in this work could be postulated. Taking into account that any even small difference in the mineral preparations and their treatment before spectroscopic and flotation studies could result in some differences in potential dependent surface phenomena (which are also kinetic dependent), the observed1 good correlation between flotation and spectroscopic data, obtained in different laboratories on different mineral samples, is very positive. It was already reported (Ralston, 1991; Das et al., 1992) that chalcopyrite samples procured from different sources show different flotation behavior even if all preparation and flotation conditions are practically the same. The same observation was made in our studies (Mielczarski et al., 1997b) where an about 50 mV shift was observed, in the adsorption layer formation and flotation, between two chalcopyrite samples procured from different deposits. It should be kept in mind that in the above comparison of the flotation and adsorption results the adsorption products were determined in separate experiments, not directly on the surface of the floated particles. This usually raises the possibility that the good correlation between the adsorption and flotation results could be also coincidental, and

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the reason for flotation of chalcopyrite is not necessary the formation of the adsorbed cuprous xanthate layer (see also discussion below). Because of the complexity of natural mineral samples (ores), this remark is valid for any direct comparison between flotation behavior and surface characterization data if the mineral samples used in these experiments are not the same. In other potential dependence flotation studies of chalcopyrite (Trahar, 1983, 1984) very similar flotation-potential relationships were found, i.e. an increase of recovery to 100% from a potential of about 0 to 200 mV, in the presence but also in the absence of ethyl xanthate in aqueous solution. In those studies the potential was regulated by addition of reducing (dithionate, Na,S,O,) or oxidizing (perchlorate, NaOCl) agents and the reported potential was measured by platinum electrode. Our observations indicate that the potential of platinum electrode is usually lower than that of the chalcopyrite electrode in the same copper mineral suspension by a value of about 50 mV, and in the cases when other sulfide minerals are present the potential of platinum electrode can vary significantly. These results (Trahar, 1983, 1984) showed that collector is not necessary to obtain 100% flotation of chalcopyrite in the potential region between 200 to 500 mV. The potential-collectorless flotation relationship was re-examined (Ralston, 1991) with the use of chalcopyrite samples from different sources. Very large differences were found (Gay and Trahar, 1985; Ralston, 1991) between collectorless flotation of those chalcopyrite samples, and some of the samples do not show reasonable flotation at all though the mineral potential ensured moderate oxidation conditions. This indicates that the internal properties of the mineral sample (mineralogical and composition variations, presence of trace elements, presence of other mineral components, etc.) have important effects on its floatability. The recently obtained results (Pang and Chander, 1990; Mielczarski et al., 1996a) allow us to make the following conclusions: to provide efficient collectorless flotation of chalcopyrite the following conditions should be fulfilled: (1) fast diffusion of iron atoms to chalcopyrite surface with the formation of a sulfur rich layer (metal deficient, polysulfides); (2) the formation of a well separated layer of iron hydroxides which could be easily mechanically and/or by dissolution removed from the surface in flotation agitation conditions; (3) slow diffusion of copper atoms to interface to conserve the sulfur-rich layer. Hence, to promote collectorless flotation the conditioning of chalcopyrite is required in non-xanthate solution at a pH about 10.5 and moreover, during flotation a strong agitation is needed. These conditions help to produce hydrophobic chalcopyrite particles with the outermost surface enriched in sulfur (Mielczarski et al., 1996a). If the small amount of xanthate is added to the solution, the copper atoms from the bulk of chalcopyrite are forced to migrate to interface with simultaneous decrease of diffusion of iron atoms. In consequence a mixed outermost layer, containing also cuprous xanthate complex, is formed which cannot be easily separated from the sulfur-rich layer. Moreover, the sulfur-rich layer is less delineated from the bulk of chalcopyrite and contains more iron and copper atoms (Mielczarski et al., 1996b). These together result in the surface phenomena which do not produce the hydrophobic sulfur-rich layer on chalcopyrite. If chalcopyrite cannot be floated without collector, this means that at least one of the above-mentioned conditions is not fully accomplished. In this case xanthate should be

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used to remove the hydrophilic species from chalcopyrite surface (iron hydroxides) and to produce hydrophobic coverage (cuprous xanthate complex and dixanthogen), which ensures flotation. If the surface is strongly oxidized, more xanthate is necessary to induce a good floatability. The oontact of “fresh” chalcopyrite with xanthate solution at the proper potential (see results above) provides the best conditions for chalcopyrite flotation. In this case the diffusion of copper atoms to interface is faster and that of iron atoms slower, as was recently shown (Mielczarski et al., 1996b) by XPS studies, which results in the formation of surface hydrophobic cuprous xanthate with a very limited amount of hydrophilic iron hydroxides. The absence of the iron hydroxide layer on chalcopyrite promotes the further diffusion of copper atoms to the interface, which results in the formation of hydrophobic cuprous xanthate. This ensures hydrophobic coverage strongly bonded to the chalcopyrite surface, which encourages adsorption of dixanthogen. Note that dixanthogen is observed on the mineral surface only when cuprous xanthate is already adsorbed. This discussion clearly shows that the worst conditions to induce flotation of chalcopyrite are when xanthate is added to solution after chalcopyrite is already covered by a layer of iron hydroxides. A very low xanthate concentration will show a similar effect. This is, unfortunately, very often the case in flotation practice. As was explained above, if the collectorless hydrophobization process is not completed by the removal of the oxidized layer, the addition of xanthate will make it difficult to produce the outermost hydrophobic sulfur-rich layer. Chalcopyrite covered by oxidation products will require more xanthate (higher collector consumption) to remove hydrophilic surface products and produce hydrophobic ones. This indicates that the point in which xanthate is added to flotation pulp is very important and is directed by the actual status of the mineral rsurface. For collector flotation the xanthate should be contacted with a “fresh” chalcopyrite surface directly after grinding in order to produce cuprous complex and dixanthogen as the hydrophobic species. The natural behavior of chalcopyrite in basic solution is the formation of hydrophilic iron hydroxides outermost layer, and this process could be minimized by an early contact of chalcopyrite with collector. If the iron hydroxide species dominate on chalcopyrite surface and are not removed from the surface (mechanically, by dissolution or interaction with collector), chalcopyrite shows a poor flotation.

6. Summary It is shown that the formation of different adsorption products of ethyl and amyl xanthates on chalcopyrite takes place at potential values which are not in full accord with those predicted by thermodynamics. There are two major products formed at the mineral interface: the surface cuprous xanthate complex, and dixanthogen. The spectroscopic results show striking differences in the composition of the outermost layer of chalcopyrite contacted with ethyl and amyl xanthate solutions. This obviously will produce differences in the flotation behavior of the mineral and in consequence will

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influence its separation from other ore components. It was found that at a gradually increased potential the ethyl xanthate produces at first the cuprous xanthate complex with a coverage of up to several monolayers, before ethyl dixanthogen appears as the second adsorption product at higher applied potentials. In the case of amyl xanthate, the cuprous xanthate complex is produced together with dixanthogen at the beginning of xanthate adsorption even at a monolayer coverage. Then, with a potential increase, the amount of amyl dixanthogen increases significantly to multilayer coverage whereas the amount of cuprous amyl complex remains limited to about a monolayer coverage. A significant increase of cuprous amyl xanthate is observed at very high potentials at which chalcopyrite begins to oxidize. Relative diffusion of metal (copper and iron) atoms from the bulk of chalcopyrite to interface is one of the reasons which causes the difference between the observed surface adsorption products. Relationships between the solution composition and the adsorbed layer composition and structure were described. These observations explain why the different xanthate homologues should be used in flotation practice. A good correlation was found between the recently reported flotation data and the formation of the surface products of ethyl xanthate determined in this work. Fig. 6 summarizes the results obtained and could be used in the determination of the optimal collector adsorption conditions for flotation separation of chalcopyrite. Collectorless hydrophobization of chalcopyrite is another way to increase its floatability. It was shown in this work that the best conditions for the formation of sulfur-rich layer are contrary to those required for the formation of xanthate hydrophobic layer. It is not possible to benefit by the formation at the same time of both hydrophobic surface products: sulfur-rich and xanthate adsorption layers. Therefore, the order and time at which chalcopyrite particles are contacted with different solutions (without or with collector) in real industrial flotation are very important and have tremendous effects on kinetics of chalcopyrite flotation as well as required dosage of collector. This work forms a fundamental basis for understanding the nature and structure of the outermost layer which determines the hydrophobic property (flotation behavior) of chalcopyrite, and hence its efficient selective separation from other sulfide mineral components.

Acknowledgements This work was supported by European Community (Projects MA2M-CT92-0062 and BRE2-CT94-0606). The authors thank Somincor for providing the mineral samples. The technical assistance of 0. Kassi is gratefully acknowledged.

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