FTIR and flotation investigation of the adsorption of ethyl xanthate on activated and non-activated sulfide minerals

International Journal of Mineral Processing, 30 ( 1990 ) 245-263

245

Elsevier Science Publishers B.V., Amsterdam

FTIR and flotation investigation of the adsorption of ethyl xanthate on activated and non-activated sulfide minerals J.O. Leppinen Technical Research Centre of Finland, Laboratory of Mineral Processing, SF-83500 Outokumpu, Finland (Received September 12, 1989; accepted after revision July 2, 1990

ABSTRACT Leppinen, O., 1990. FTIR and flotation investigation on the adsorption of ethyl xanthate on activated and non-activated sulfide minerals. Int. J. Miner. Process., 30: 245-263. The adsorption of ethyl xanthate on pyrite, pyrrhotite, chalcopyrite and sphalerite has been studied using FTIR-ATR techniques and microflotation. Non-activated minerals and minerals activated with copper sulfate have been investigated at different pH values and xanthate concentrations. Diethyl dixanthogen is formed on non-activated pyrite, pyrrhotite and chalcopyrite. Iron xanthate co-exists with diethyl dixanthogen as a monolayer form on pyrite and a copper xanthate surface compound coexists with diethyl dixanthogen on chalcopyrite. After copper sulfate activation a copper (I) type xanthate compound exists on all of the minerals studied. Acidic pH favours the adsorption of ethyl xanthate on non-activated minerals, whereas the neutral pH range is most favourable for xanthate adsorption on activated minerals.

INTRODUCTION

The interaction between xanthates and sulfide mineral surfaces plays a central role in the flotation process. This interaction has, however, proved to be very complex due to the different reaction mechanisms and reaction products depending on the prevailing conditions. The system is affected by factors such as pH, oxidation potential, and different solution species, in addition to the history of the mineral. Despite the development of analytical methods to investigate mineral-collector systems, many points are still not fully understood. For example, the surface species responsible for floatability have not been directly characterized in many cases. It is generally accepted that most sulfide minerals respond strongly to xanthates in flotation, with the exception of zinc sulfides which need to be activated by copper or other heavy metal ions prior to flotation (Gaudin, 1957; Finkelstein and Allison, 1976 ). The role of copper activation is to modify the 0301-7516/90/$03.50

© 1 9 9 0 - - Elsevier Science Publishers B.V.

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J.O. LEPPINEN

surface of zinc sulfide into a copper sulfide-like surface, thus making the adsorption of xanthate possible. Despite the fact that most sulfide minerals are floatable without activation, similar activation phenomena are known to occur in the flotation of other than zinc sulfides (Bushell et al., 1961; Abramov and Shtoik, 1970). The activation of zinc sulfides has been the subject of previous investigations (Baldwin et al., 1979; Termes and Richardson, 1986 ) but very little is known about the activation and adsorption of xanthate on other sulfide minerals. This work was carried out in order to characterize the surface species formed in the interaction between ethyl xanthate and the sulfide minerals pyrite, pyrrhotite, chalcopyrite, and sphalerite. The systems were investigated using FTIR spectroscopy employing the attenuated total reflection (ATR) technique. Sulfide minerals, both non-activated and activated with copper sulfate, were studied at different xanthate and copper ion concentrations at pH 4 to 12. Microflotation experiments were carried out and the results compared with the FTIR spectroscopic data. EXPERIMENTAL

A Nicolet 740 FTIR spectrometer with a liquid nitrogen-cooled MCT detector was employed to record the infrared spectra. The data collection time was typically 150 s for 500 spectra at 4 c m - ~resolution, and spectral averaging was used to gain an adequate signal-to-noise ratio. The internal reflection method was applied using Specac's 10-reflection ATR accessory and a specially constructed measurement cell as described in earlier papers (Leppinen, 1987; Leppinen et al., 1988a). A germanium reflection element with an angle of incidence of 45 ° was used. Computer subtraction of the absorption bands of water was applied to the spectra presented in this work. FTIR signal intensity was determined from absorbance mode spectra by measuring peak height at the particular wavenumber. Due to the complex nature of signal intensity in reflection spectroscopy the intensities in the figures are expressed only on a relative scale. This scale was kept constant in each pH-signal intensity diagram, facilitating direct comparison between different systems. The full length of the intensity axis corresponds to 1.0% absorption. The minerals were selected from samples obtained from Finnish metal mineral mines and from the Geological Survey of Finland. The pyrite sample was from Pyh~isalmi mine, the pyrrhotite sample from Enonkoski mine, and the sphalerite sample from Vihanti mine. The chalcopyrite was a geological sample from Paltamo, Finland. The pyrite and pyrrhotite samples were approximately 95% pure, but the sphalerite was only 65%, the impurities being mainly silicates. The pyrite contained some chalcopyrite and silicates as inclusions whereas the pyrrhotite sample contained small inclusions ofpentlandite, magnetite and silicates. The chalcopyrite was very pure (99.8%) con-

ADSORPTION OF ETHYL XANTHATE ON SULFIDE MINERALS

247

taining only a minor amount of sphalerite as inclusions. Most of the experiments on pyrite, however, were carried out using a high purity material (99%) from a single crystal of Spanish origin. The pyrrhotite used in this work was predominantly monoclinic Feo.svS. Prior to each test the mineral sample was crushed to about 2 mm particle size and then ground in a Retsch vibration ball mill with an agate cup and ball. The ground powder was manually dry-screened and the - 2 0 micron fraction was used for the adsorption and microflotation tests. In some flotation experiments the size fraction of 90-180 microns was used. Sample conditioning was started immediately after grinding and screening in order to minimize oxidation and other contamination effects in the sulfide sample. As the grinding parameters were kept constant the particle size and specific surface area of the powder did not vary by more than 10%. The average specific surface areas of pyrite, pyrrhotite, chalcopyrite, and sphalerite were 1.50, 0.98, 1.25, and 1.10 m2/g, respectively. Typically 0.3 g of mineral powder was conditioned in 200 ml of aqueous solution for 15 min. The pH was kept constant within 0.05 pH units through the addition of either sodium hydroxide or hydrochloric acid. The mineral slurry was transferred into the ATR measurement cell or into the microflotation tube, followed by recording the spectrum or flotation, respectively. The concentrations of copper(II) ions and ethyl xanthate ions in the solution were determined using an AMEL-433 polarographic analyzer applying the method described by Leppinen and Vahtila ( 1986 ). The solution analysis was used for each system to determine the relationship between FTIR signal intensity and the amount xanthate abstracted from solution. However, due to the difficulty caused by that part of xanthate which did not adsorb on minerals, but indicated abstraction of xanthate from solution, this could be successfully done only at coverages up to about one monolayer. The estimation of higher coverages is based on the rough assumption that signal intensity increases linearly with coverage. In the system lead sulfide-ethyl xanthate this was found to be true up to about two layers (Leppinen, 1987 ), while at higher coverages deviation from linearity occurred. A modified Hallimond tube (Siwek et al., 1981 ) was used for the microflotation tests with high purity nitrogen as the flotation gas at a flow rate of 4 ml/min. The flotation time was 2 minutes. Methyl isobutyl carbinol (MIBC) was used as the frother. Commercial grade 99% pure potassium ethyl xanthate (KEX) was further purified by dissolution in acetone and precipitation with ethyl ether. Ethyl dixanthogen was prepared from ethyl xanthate by oxidation with a stoichiometric amount of aqueous iodine (with potassium iodide) followed by extraction into ether and removal of the solvent by nitrogen flow. The metal ethyl xanthate compounds were precipitated from aqueous solutions of the metal sulfates by addition ofa stoichiometric amount of potassium ethyl xan-

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J.O.LEPPINEN

thate. In the case of copper xanthate, the co-precipitated dixanthogen was removed by dissolution in ether. All other reagents were of analytical grade. High-purity (Millipore) water was used throughout the work. RESULTS A N D D I S C U S S I O N

Figure 1 presents the FTIR spectra of ethyl xanthate (solid and aqueous solution) and ethyl dixanthogen (liquid) in the wavenumber range 900-1400 cm-~. Figure 2 presents the FTIR spectra of the ethyl xanthate compounds of copper(I), iron(Ill) and zinc(II). A group of three separate signals 1000-1300 c m - ~is typical of most of the ethyl xanthate compounds. The infrared absorption bands at the highest wavenumbers (1189-1290 cm -1) in the spectrum of the metal xanthate compounds and dixanthogen are designated to the asymmetric C - O - C stretching vibration, whereas the bands at the lowest wavenumbers ( 10061047 c m - 1) are due to the C=S vibration (Little et al., 1961; Arabinda et al., KEX so{id

KEX 0.1 M sotution

~

m

w

J

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(EX)2

o

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~2oo

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ADSORPTION OF ETHYL XANTHATE ON SULFIDE MINERALS

249

EuEX

Fe(EX) 3

l ~

o

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Fig. 2. F T I R reflection spectra of ethyl xanthate metal compounds.

1973 ). The middle bands ( 1108-1143 cm -t ) are designated to the symmetric C - O - C stretching vibration. In the spectrum of KEX the band at 1100 c m - ~is assumed to belong to the asymmetric C - O - C vibration and the band at 1138 c m - 1 to the symmetric C - O - C vibration,, whereas the lowest band at 1049 c m - ~ is designated to he C=S vibration.

Non-activated minerals Pyrite The FTIR spectra of the reaction products on pyrite, pyrrhotite and chalcopyrite after treatment with 1 × 1 0 - 4 M K E X solution at pH 5 are presented in Fig. 3. The FTIR signals of the xanthate species on pyrite occur at approximately the same position as those of bulk dixanthogen, with the main signals at 1288, 1262, 1239, 1107, and 1021 cm -~. A closer look at the system pyrite-KEX, however, reveals that dixanthogen occurs on pyrite not alone but together

250

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...o

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i~oo

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Fig. 3. F T I R reflection spectra of pyrite, pyrrhotite and chalcopyrite after 15 min treatment with 1 X 10 -4 M KEX solution at pH 5.0.

with another surface product with a large IR absorption signal at 1232 c m - ' (Fig. 4 ). When pyrite is treated with KEX at lower concentrations (2 X 10-6 to 3 × 10-5 M), the IR spectrum is distinctly different from that of xanthate on pyrite at higher concentrations, i.e. that of dixanthogen. Mielczarski ( 1986 ) has made similar observations in studies on the system marcasite and ethyl xanthate. The low-concentration surface species on pyrite does not exactly resemble any reference compound shown in Figs. 1 and 2. However, iron(Ill) ethyl xanthate has signals at 1026 and 1003 c m - 1compared to 1027 and 1006 c m in the surface compound. Also, the position of the signal at 1116 c m - 1 coincides very well with that of ferric ethyl xanthate. There exists, however, a significant shift from 1247 to 1232 c m - ' when the spectra of Fe (EX)3 and the surface species are compared. Several previous studies, however, show that this asymmetric C-O vibration band undergoes the greatest shift due to the bonding of the xanthate molecule (Poling, 1976; Mielczarski et al., 1981 ). The surface iron xanthate may also have some of the nature of iron (III) hydroxo xanthate mentioned by some authors on the basis of thermodynamic calculations (Wang, 1989 ). Chemisorbed dixanthogen is excluded due to the relative weakness of the bands at about 1020 cm-~ and also due to the fact that the dixanthogen band is unlikely to occur at a wavenumber as high as

ADSORPTION OF ETHYL XANTHATEON SULFIDE MINERALS

251

2x10-5M

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Fig. 4. FTIR reflectionspectra of pyrite after conditioningat two different KEX concentrations at pH 5.0. 1 1 1 6 c m - ~. The occurrence of the middle band at high wavenumbers is typical of metal xanthate compounds, however. Thus, it can be concluded that the surface product is an iron xanthate surface compound. At higher concentration (4 X 10- 5 M) (see Fig. 4) bands characteristic of diethyl dixanthogen occur in the FTIR spectrum, although at slightly lower wavenumbers than those of bulk (EX)2. The shift is due to overlapping of the signals of the iron xanthate surface c o m p o u n d and dixanthogen. Some shift caused by a weak chemisorption of (EX)2 species at low coverages cannot be completely excluded, however. Figure 5 shows that the signal intensity at about 1232 cm-1 increases up to about the one-monolayer level (0.23% absorption corresponds to 0.8 monolayers) at concentrations of 2 X 10- 6 M to 3 × 10- 5 M. A strong increase in the signal intensity is observed in the concentration range from 4 X 10-5 to 1 X 1 0 - 4 M , which indicates a multilayer formation of dixanthogen. The signal intensity at 5 X 1 0 - S M corresponds to approximately 3 layers, which means that one layer of (EX)2 is adsorbed on the chemisorbed iron xanthate layer. At higher concentrations (1 X 10 -3 M ) the amount of xanthate adsorbed increases to correspond to 6 to 7 layers (not shown in Fig. 5 ).

Pyrrhotite The adsorption product of KEX on pyrrhotite is much the same as that on pyrite (see Fig. 3 ). However, a clear shift occurs at the signals 1259, 1234 and 1 1 0 4 c m - ~. This downward shift indicates a chemisorption of ( EX ) 2 species. Similar shifting of IR bands has previously been observed with other neutral

252

J.O. LEPPINEN

PYRITE KEX, p H 5.0 s]gnol 1252 cm

.~

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//

,

k~

KEX Concentrotion

(M x 105 M)

Fig. 5. Dependence of the amount of ethyl xanthate adsorbed on pyrite on the concentration of ethyl xanthate at pH 5. The full length of the intensity axis corresponds to 1.2% absorption.

collector species such as thionocarbamates (Leppinen et al., 1988a). Comparing the FTIR spectrum of pyrrhotite at 1 X l0 -4 M KEX with that at 2 X 10- 5 M KEX, no difference in the signal positions was observed. This is contrary to the behaviour of pyrite and indicates that no iron xanthate species occurs on the surface of pyrrhotite, even at low xanthate concentrations.

Chalcopyrite At a 1 × 10-4M KEX concentration a mixture of two different surface products is generated. The signals of diethyl dixanthogen occur at 1239 and 1262 cm -1. The spectral subtraction of dixanthogen from the original spectrum reveals a c o m p o u n d with IR maxima at about 1200, 1117, 1028, and 1004 c m - ' (Fig. 6 ). Comparison with the reference spectra (Fig. 2 ) shows that this surface product is a metal xanthate c o m p o u n d closely resembling copper ( I ) xanthate. However, the signals at about 1200 c m - ' occur at higher wavenumbers than those of copper (I) xanthate and some downward shift is observed at the other signals. This may be due to chemisorption of copper xanthate, which typically causes shifting and broadening of signals. On the other hand, the shift at 1200 c m - ' is likely to be due to copper(II) xanthate, which is unstable as a bulk compound, but might exist as the surface species. Xanthate chemisorption on chalcopyrite can occur not only through copper but also partially through iron, which may also explain the difference from the copper xanthate spectrum. Previous electrochemical FTIR measurements have shown that dixanthogen is formed at low potentials followed by copper(I) xanthate at higher potentials at which copper ions are released from the surface of chalcopyrite (Leppinen et al., 1988b). Unlike the present study, no copper xanthate product was found in the previous investigation. The difference with respect to the copper xanthate species may be due to the fact that

A D S O R P T I O N OF E T H Y L X A N T H A T E O N S U L F I D E M I N E R A L S

253

a

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~

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-J

b

o,~

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i300

1200 i IOO WRVENUMSE~

]O00

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Fig. 6. FTIR reflection spectra of chalcopyrite after treatment with 1 X 10 -4 M KEX solution (a) and after subtraction of the spectrum of (EX)2 (b).

open circuit conditions were used in this work instead of the external potential control used in the previous investigation. The coverage of xanthate on chalcopyrite is generally much lower than that on pyrite or pyrrhotite. At pH 5 (CKEx 1 X 10 -4 M) the signal intensities on chalcopyrite refer to approximately one monolayer coverage of surface copper xanthate accompanied by some dixanthogen. The FTIR spectra indicated that the amount of dixanthogen is highest at low pH and decreases rapidly to pH 7, where practically no dixanthogen was present. Thus, at higher pH, copper xanthate surface species must be responsible for the floatability of chalcopyrite, in addition to the self-induced floatability, of course. Relationship to pH, non-activated minerals The pH-dependence of xanthate adsorption and the recovery in microfloration experiments was studied using the same particle size and pre-treatment in both techniques. A small particle size is essential for good sensitivity in FTIR measurements but, in some cases, causes problems in microflotation, especially at high pH, due to the extremely long thickening time leading to particle entrainment. This causes excessively high flotation recoveries at pH 9to 12. FTIR spectroscopy was used to estimate the amount of xanthate adsorbed on mineral surfaces. Compared to the values calculated from solution analysis data, the intensity of the FTIR signal showed to be a much more reliable

254

J.O. LEPPINEN

measure of the adsorbed xanthate. This is clue to the fact that part of the xanthate abstracted from solution does not adsorb on mineral particles but is converted to other compounds or exists as a separate phase. A particular discrepancy between the abstraction of xanthate from solution and the amount of xanthate directly measured on the surface, was regularly found in systems where large amounts of dixanthogen were formed. The adsorption of EX on pyrite has a m a x i m u m at pH 5 and decreases steadily to a m i n i m u m at pH 7 (Fig. 7 ). The adsorption decreases rapidly at pH values above 8, falling to about zero coverage at pH 10. The signal intensities at 2 X 10- 5M KEX show that the amount of the monolayer form of xanthate also decreases with increasing pH. Thus, the iron xanthate surface species seems to be essential for the formation of the dixanthogen layer as well. The microflotation results for pyrite follow the adsorption curve very nicely down to pH 9, after which the entrainment problems mentioned above cause excessive recoveries. This was confirmed by flotation experiments using coarser particles (90 to 180 microns), the results of which agree with the adsorption data at pH 9 to 12. The typical sinusoidal shape of the recovery-pH curve with a m i n i m u m at neutral pH values, observed by some authors (Fuerstenau et al., 1968; Fuerstenau and Mishra, 1981 ), is not very distinct in this study. However, the experiments carried out in this work indicate that this kind of behaviour is due to the special features of the mineral sample itself. The pyrite sample from Pyh~isalmi mine showed higher recoveries at pH 7 to 10 than the Spanish single crystal pyrite. When different pyrites were compared, there always existed a m i n i m u m in the flotation curve, but its position varied between pH 6 and 8, depending on the sample and the particle size. It must be noted that the particle size used in this study is smaller than that in the investigations in the literature. o _ / / ' ~

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PYRITE

KEX

1 × 10 -4 M

\

~

\ i

ommmo FTIR ~ Microflotation

\

:>~

(b

>

0 c) Q) O~

5

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o

4

5

6

7

8

9

10

11

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pH

Fig. 7. Dependence of FTIR signal intensity and microflotation recovery on pH. Pyrite, CKEX l X l0 -4 M (2 X 10-5M). The full length of the intensity axis corresponds to 1.0% absorption in the pH-recovery-intensity diagrams presented.

ADSORPTIONOFETHYLXANTHATEONSULFIDEMINERALS

255

The adsorption of KEX on pyrrhotite reaches a sharp m a x i m u m at pH 5.5 (Fig. 8 ). The adsorption is low at pH 7 to 9 and practically zero at pH 10. At pH 4 the adsorption of EX is low, too, probably due to dissolution and oxidation effects of pyrrhotite. The low tendency for generation of surface xanthates at higher pH must be due to the preferential formation of iron hydroxide surface complexes as a result of the oxidation of pyrrhotite. The recovery in the flotation experiments with pyrrhotite does not exactly follow the shape of the adsorption curve, except at high pH. A peculiar feature is that the floatability is high at low pH (pH 4), where the adsorption of xanthate is low. However, practically no flotation occurred at pH 4 without the presence of xanthate. Oxidation processes are known to generate elemental sulfur or make a sulfur-rich surface on pyrrhotite and other sulfide minerals, especially at low pH (Woods and Richardson, 1986). The high floatability may thus be due to a combined effect of xanthate and an excess of sulfur on the surface. Controversially, at pH 5.5 xanthate adsorption is very high and the floatability relatively poor. This must be due to the strong dissolution of pyrrhotite and possibly also the formation of hydroxides on the mineral surface, both of which reduce the hydrophobicity and make the multilayer coating of dixanthogen loosely and ineffective. Adsorption of ethyl xanthate on chalcopyrite (Fig. 9) is less than on pyrite and pyrrhotite (Figs. 7 and 8 ). The FTIR signal intensities indicate that the coverage does not easily exceed monolayer. The amount of ethyl xanthate adsorbed decreases steadily with increasing pH. It must be pointed out that at pH 4 to 6 some dixanthogen is adsorbed in addition to surface copper xanthate and that the signal intensity has been calculated using the signals of the copper xanthate surface species alone. The shape of the recovery curve for chalcopyrite follows the adsorption g PYRRHOTITE -

A o

/

\

eeee,

KEX 1 x 10 4 M FIR tcltion

'~

g

>

~

©

c

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4

5

6

7

8

9

10

11

12

1.3

pH Fig. 8. Dependence of FTIR signal intensity and microflotation recovery on pH. Pyrrhotite, CKEX 1 × 10-4M.

256

J.O. LEPPINEN

CHALCOPYRITE I o ~i~ -

-

KEX 1 x 10 -4 M

I

~ ~ \ l l x \

; ; ; ; ; FiR I l l l l l l Microflototion AAA~,* Microflotation. no KEX

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i

t

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5

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8

pH

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Fig, 9. Dependence of FTIR signal intensity and microflotation recovery on pH. Chalcopyrite, CKE X 1X

10-4M.

curve very closely, although relatively low coverages result in high flotation recoveries unlike in the case of pyrite and pyrrhotite. The experiments indicate that chalcopyrite possesses natural floatability, which leads to some floatability throughout the pH range (Luttrell and Yoon, 1984). Xanthate adsorption greatly enhances the self-induced floatability, however.

Activated minerals The FTIR spectra of four minerals activated with 5 × 10- 5M CuSO4 solution prior to conditioning with 5 X 10-5 M KEX solution at pH 7 are presented in Fig. 10. In all cases the spectra of the surface products closely resemble the spectrum of copper(I) ethyl xanthate (see Fig. 2) although slight variations occur at the signal positions. This variation depends on the coverage and the structural properties of the mineral surface to which ethyl xanthate is bound. The EX coverages calculated from signal intensities vary from about one monolayer coverage on pyrite and sphalerite to about 2-3 monolayers on pyrrhotite and chalcopyrite. The surface species formed on activated mineral surfaces depend strongly on the concentrations of copper sulfate and KEX. Fig. 11 presents the spectra of the surface of pyrite after activation with copper sulfate at four different concentrations ( 1 X 10 .5 to 3 × 10-aM) followed by treatment with 1 X 10 - 4 M KEX solution. The pyrite surface activated with 1 X 10- 5 M CuSO4 solution still behaves much like the non-activated pyrite, and thus dixanthogen is the main product on the surface. However, tiny signals ( 1190 cm - I ) of copper xanthate corresponding to a sub-monolayer coverage are also observed. When 3 × 10 - s M CuSO4 solution is used the amount of copper xanthate increases, although a considerable amount of dixanthogen is still formed. When

ADSORPTION

OF ETHYL XANTHATE ON SULFIDE MINERALS

257

FeS2

i

I

'

o~ I --~rm

Fel_xS

I

uJ u

~o oo

~

t.m .z- G, ..~ r ~ o o o o

CuFeS2

N (Zn, FelS

r-a

1~C30

130CI

1ZOO li00 WAVENUMBER

II

I[DO0

~OD

Fig. 10. FTIR reflection spectra of pyrite, pyrrhotite, chalcopyrite, and sphalerite after activation by 5 X l 0 - 5 M CuSO4 solution and treatment with 5 X l 0 - 5 M KEX solution at pH 7.

equal concentrations of copper sulfate and KEX are used, only copper xanthate is found on the surface. It must be noted that the pyrite sample was washed after the activation stage in order to remove residual copper sulfate from the solution. Thus, all the copper must have been on the solid surface of pyrite. Also, the spectrum of the surface species very closely resembles that of copper(I) xanthate, and no dixanthogen is present. This means that no free copper (II) species, leading, in the bulk phase, to copper (I) xanthate and dixanthogen, was present, and that the surface was fully converted into a copper sulfide-like surface. The surface probably resembles Cu2S rather than CuS. Thus, the reaction: FeS2 + 4 Cu 2+ + 6 e = F e 2+ +Cu2S

( 1)

proposed by Allison ( 1982 ) may be responsible for the activation of pyrite. Other reactions producing copper (I) species cannot be excluded, however. On increasing the copper sulfate concentration to 3 X 10 -4 M (Fig. 1 ld) there is no further increase in the amount of ethyl xanthate. However, a shoulder appears at 1218 c m - 1 in the IR spectrum. A probable explanation for the

258

J.O. LEPPINEN

a

\, ,,,' r~

b

~' ~'~JLQO'%',, / A V F--q,,q."".~ ~-, ,'%/ ,.., ~, I iL\, ,' V

uJ

Y ~

t~ uJ

~r-4

C

d

X :~ 3D

~5oo

12oo

i Joo

NRVENUMBER

~oo

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Fig. 11. FTIR reflection spectra of pyrite after conditioning in 1 X 10 - 4 M KEX solution preceded by activation by copper sulfate solution: 1X 10 -5 M (a); 3X 10 -5 M (b); 1X 10 - 4 M (c); 3X 1 0 - a M (d).

change in the copper xanthate signal is the generation of a copper(II) xanthate surface compound due to the copper (II) sulfide-like surface formed at high copper concentrations. Although not shown in the figure, the spectrum did not change at a copper sulfate concentration o f 1 X 10- 3 M but was the same as at 3 X 10 - 4 M. The other sulfides behave in the same way as pyrite with respect to concentrations of copper sulfate and ethyl xanthate. However, the concentration of copper sulfate needed for a complete conversion of the surface into copper sulfide varies, being 3 X 10 -4 M in the case of pyrrhotite.

Relationships to pH, activated minerals The amount o f ethyl xanthate adsorbed on activated pyrite is relatively low at pH 4 to 5 but increases steeply up to pH 8 (Fig. 12). Xanthate adsorption

259

ADSORPTION OF ETHYL XANTHATE ON SULFIDE MINERALS

PYRITE

-

CuS04

5

x

10 -5

M -

KEX

5

x

10 5 M

oooalal F[IR n u n n Micr0flotation

"~

c

o

._~

o

.~

o

E

5

> 0

°3

E c~

4

5

6

7

8 pH

9

10

11

12

5

Fig. 12. Dependence of FTIR signal intensity and microflotation recovery on pH. Pyrite activated by 5X 10 -s M CuSO4, CKEX5X 10-SM.

8 PYRRHOTITE

-

CuS04

5

x

10 -5 M

-

Iunn

KEX

5

x

10 -~ M

Microfl0tation

Q3

> 0

c

o

._~

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°3

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9

10

11

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Fig. 13. Dependence of FTIR signal intensity and microflotation recovery on pH. Pyrrhotite activated by 5 x 10- 5 M CuSO4, CKEX5 X I 0 - SM.

decreases at higher pH being practically zero at pH 12. The activation seems to have a greater effect at pH values at which xanthate does not adsorb on pyrite without activation. The amount copper ions consumed by the activation has been shown to increase with increasing pH (Wang, 1989) in the acidic range, which is also in agreement with the xanthate adsorption curve. The recovery curve for activated pyrite follows very closely the adsorption curve, reaching a maximum at pH 8. The recoveries at pH 4 to 6, however, are rather high as compared with the low adsorption densities of xanthate. This may be partly due to the formation at low pH values of a sulfur-rich surface which seems to require only a minor amount ofxanthate to render the mineral floatable. The FTIR spectra indicate that the maximum adsorption of ethyl xanthate on activated pyrrhotite occurs at pH 7 to 8 (Fig. 13 ). The curve closely fol-

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J.O. LEPPINEN

lows that of pyrite, except that the adsorption densities are too high at low pH. Some authors have observed an increase in copper adsorption on pyrrhotite at pH 3.3 to 7.8 (Wang, 1989) which agrees well with the present finding. The fact that xanthate adsorption is higher than suggested by the extent of copper adsorption may be due to the strong EX adsorption tendency of nonactivated pyrrhotite at low pH. The floatability of activated pyrrhotite at a 5 × 10- 5 M KEX concentration follows the adsorption curve obtained by FTIR measurements, with the exception that at pH 9 the floatability is higher than the adsorption curve suggests. The floatability reaches a maximum at pH 7 to 9 and the behaviour is much like that of pyrite, again showing, at high pH, greater floatability compared to the non-activated pyrrhotite. This is even-more distinct in the case of pyrrhotite, because practically no flotation occurs at neutral or alkaline pH without activation. The activation of copper-bearing sulfide minerals using copper (II) ions has been studied very little. However, the FTIR spectra clearly show that the surface product on activated chalcopyrite is different from that on the non-activated mineral (see Figs. 3 and 10). The adsorption of ethyl xanthate on activated chalcopyrite is greatest at pH 7 and shows a minimum at about pH 9 and another maximum at pH 10 (Fig. 14). This peculiar behaviour was checked by repeating the measurements several times at pH 8 to 10; the resuits confirmed the fact that there really is a minimum at pH 9. However, due to the very complicated system of hydroxo and carbonate complexes of iron and copper, this behaviour is difficult to explain. The flotation curve of activated chalcopyrite does not agree with the adsorption curve except at high pH. Actually, the flotation behaviour is very much like that of the non-activated chalcopyrite at low pH. The activation

8 CHALCOPYRITE

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ADSORPTION

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has a significant effect only at pH values from 8 to 12. It must be noted that the concentration of KEX is only 5 X 10- 5 M compared to 1 X 10- 4 M in the non-activated case, indicating that activation may lead to a lower collector consumption in the flotation of chalcopyrite. The adsorption curve of ethyl xanthate on activated sphalerite has a maxim u m at pH 8 to 9 (Fig. 15 ). Attempts to detect adsorption products on nonactivated sphalerite were not successful, indicating the great importance of the activation for the short-chain xanthate adsorption. The floatability of the sphalerite sample does not show any clear relationship to pH but is essentially constant throughout the pH range studied. The overall low recoveries are explained by the high silicate impurity level ( 35% ), which, if taken into account, can lead to 80-90% recoveries. SUMMARY AND CONCLUSIONS

The adsorption and flotation phenomena on the sulfide minerals, pyrite, pyrrhotite, chalcopyrite, and sphalerite, both non-activated and activated with copper sulfate have been studied using potassium ethyl xanthate as the collector. Different pH values and collector concentrations have been examined. The following conclusions can be drawn on the basis of the investigation. ( 1 ) The main adsorption product of ethyl xanthate on pyrite and pyrrhotire is diethyl dixanthogen. An iron xanthate surface c o m p o u n d is also observed on pyrite at a monolayer coverage. A mixture of copper xanthate compound and diethyl dixanthogen occurs on chalcopyrite, whereas no xanthate adsorption is observed on sphalerite. Multilayer coverages of (EX)2 are typical of pyrite and pyrrhotite at low pH values but the amount of surface products is limited to approximately one monolayer on chalcopyrite.

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( 2 ) T h e adsorption of ethyl xanthate on non-activated pyrite is favoured by low pH; however, the m i n i m u m is at neutral pH. EX adsorption on pyrrhotite shows a strong m a x i m u m at pH 5.5. The adsorption of EX on chalcopyrite is highest at low pH and decreases with increasing pH. ( 3 ) The adsorption product on copper-sulfate-activated sulfide minerals is a copper(I) xanthate surface compound. However, at low dosages of copper sulfate the minerals behave to some extent like non-activated ones, leading to the formation of copper xanthate and dixanthogen. The m a x i m u m coverages in the concentration range studied vary from less than one monolayer to 4 layers. (4) The neutral pH range (pH 7 to 9 ) is most favourable for the adsorption of ethyl xanthate on all of the activated minerals studied. ( 5 ) The floatability curves for non-activated pyrite and chalcopyrite agree with the pH-EX adsorption curves, whereas the highest flotation rate on pyrrhotite is observed at pH 4 instead of pH 5.5. The floatability curves for activated pyrite and pyrrhotite agree with the adsorption curves, whereas no clear m a x i m u m is observed in the floatability of activated chalcopyrite and sphalerite. (6) Activation with copper (II) ions has a great effect on all the sulfides studied, especially at high pH where adsorption and floatability are usually very low without activation. Thus, excessive dosages of copper sulfate may lead to undesirable activation and flotation of iron sulfides in the flotation of zinc sulfides, even at high pH. ACKNOWLEDGEMENTS The author wishes to thank the Technical Research Centre of Finland for its support and for permission to publish this work. Thanks are due to V. Hintikka, M.Sc. and M. Klemetti, M.A. for helpful discussions.

REFERENCES Abramov, A.A. and Shtoik, G.G., 1970. Activation of sulfide minerals during flotation. Tsvetn. Met., 19: 86-89. Allison, S.A., 1982. Interactions between Sulfide Minerals and Metal Ions in Activation, Deactivation and Depression of Mixed Sulfide Ores. MINTEK, Report M29, 35 pp. Arabinda, R., Sathyanarayana, D.N., Prasad, G.D. and Patel, C.C., 1973. Infrared spectral assignments of methyl and ethyl xanthato complexes of nickel ( II ). Spectrochim. Acta, 29A: 1579-1584. Baldwin, D.A., Manton, M.R., Pratt, J.M. and Storey, M.J., 1979. Studies of the flotation of sulphides. I. The effect of Cu (II) ions on the flotation of zinc sulphide. Int. J. Miner. Process., 6: 173-192. Bushell, C.H.G., Krauss, C.J. and Brown, G., 1961. Some reasons for selectivity in copper activation of minerals. Can. Min. Metall. Bull., 54: 244-251.

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Finkelstein, N.P. and Allison, S.A., 1976. The chemistry of activation, deactivation and depression in the flotation of zinc sulphide. In M.C. Fuerstenau (Editor), Flotation, A.M. Gaudin Memorial Volume. AIME, New York, N.Y., pp. 414-457. Fuerstenau, D.W. and Mishra, R.K., 1981. On the mechanism of pyrite flotation with xanthate collectors. In: H.H. Jones (Editor), Complex Sulphide Ores (Rome). IMM, London, pp. 271-278. Fuerstenau, M.C., Kuhn, M.C. and Elgillani, D.A., 1968. The role of dixanthogen in xanthate flotation of pyrite. Trans. AIME-SME, 241 : 148-156. Gaudin, A.M., 1957. Flotation. McGraw-Hill, New York, N.Y., 2nd ed., 310 pp. Leppinen, J.O., 1987. Infrared studies of thiol collectors on lead sulfide. AIME-SME Annual Meeting, Denver, Colo., Preprint NO. 87-73. Leppinen, J. and Vahtila, S., 1986. Differential pulse polarographic determination of thiol flotation collectors and sulphide in waters. Talanta, 33: 795-799. Leppinen, J.O., Basilio, C.I. and Yoon, R.H., 1988a. FTIR study of thionocarbamate adsorption on sulfide minerals. Colloids Surfaces, 32:113-125. Leppinen, J.O., Basilio, C.I. and Yoon, R.H., 1988b. In-situ FTIR spectroscopic study of the xanthate electrosorption on sulfide minerals. In: P.E. Richardson and R. Woods (Editors), Proc. 2nd Int. Symp. Electrochemistry in Mineral and Metal Processing, Atlanta, pp. 49-65. Little, L.H., Poling, G.W. and Leja, J., 1961. Infrared spectra of xanthate compounds, II. Assignment of vibrational frequencies. Can. J. Chem., 39: 745-754. Luttrell, G.H. and Yoon, R.H., 1984. Surface studies of the collectorless flotation of chalcopyrite. Colloids Surfaces, 12: 239-254. Mielczarski, J., 1986. In situ ATR-IR spectroscopic study of xanthate adsorption on marcasite. Colloids Surfaces, 17: 235-248. Mielczarski, J., Nowak, P., Strojek, J.W. and Pomianowski, A., 1981. Investigations of the products of ethyl xanthate sorption on sulphides by IR-ATR spectroscopy. In: J. Laskowski (Editor), Proc. XIllth Int. Mineral Processing Congr, Warsaw, Part A, pp. 110-131. Poling, G.W., 1976. Reactions between thiol reagents and sulfide minerals. In: M.C. Fuerstenau (Editor), Flotation, A.M. Gaudin Memorial Volume. A1ME, New York, N.Y., pp. 334-363. Siwek, B., Zembala, M. and Pomianowski, A., 1981. A method for determination of fine particle floatability. Int. J. Miner. Process., 8: 85-88. Termes, S.C. and Richardson, P.E., 1986. In Situ FT-IR Studies of Reactions of Activated Sphalerite with Aqueous Solutions of Potassium Ethylxanthate. U.S. Bureau of Mines, Report of Investigations 9019. Wang, X., 1989. The Chemistry of Flotation, Activation and Depression of Iron-Containing Sulphide Minerals. Ph.D. dissertation, Lule~ University of Technology, LuleL 8 pp. Woods, R. and Richardson, P.E., 1986. The flotation of sulfide minerals - electrochemical aspects. In: P. Somasundaran (Editor), Advances in Mineral Processing. SME, Littleton, Colo., pp. 154-170.