APPLICATIONS OF MIRAGE SPECTROSCOPY IN SURFACE CONCENTRATION MEASUREMENTS AND ADSORPTION KINETICS

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APPLICATIONS OF MIRAGE SPECTROSCOPY IN SURFACE CONCENTRATION MEASUREMENTS

AND ADSORPTION KINETICS

N. Rollat, V. Plichon

To cite this version:

N. Rollat, V. Plichon. APPLICATIONS OF MIRAGE SPECTROSCOPY IN SURFACE CONCEN-

TRATION MEASUREMENTS AND ADSORPTION KINETICS. Journal de Physique Colloques,

1983, 44 (C6), pp.C6-307-C6-312. �10.1051/jphyscol:1983649�. �jpa-00223208�

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JOURNAL DE PHYSIQUE

Colloque C6, supplement au nOIO, Tome 44, octobre 1983 page C6- 307

A P P L I C A T I O N S OF M I R A G E SPECTROSCOPY I N SURFACE CONCENTRATION MEASUREMENTS AND A D S O R P T I O N K I N E T I C S

N. Rollat and V. Plichon

Laboratoire de Chimie Analytique d e s Milieux RLactionneZs,ESPCI, 10, rue Vauquelin, 752.31 Paris Ceder 0 5 , France

~ 6 s u m G - I1 est montr6 clue la spectroscopie mirage (photothermal d&ction spectroscopy) permet des mesures analytiques parfaite- ment quantitatives. Sur l'exemple choisi (vert malachite adsorb6 sur silice) on obtient une prtcision meilleure q u l e n spectroscopie photoacoustique. Appl.iqu6e A lr6tude de la flottation de la blende cette technique nous a servi A mesurer la cinetique d'activation d r u n e blende, c r e s t dire la fixation de cations ~ g + , pb2+ ou cu2+ A sa surface.

Abstract - Experiments on malachite green adsorbed on silica show that Mirage spectroscopy (photothermal deflection spectroscopy) allows analytical quantitative measurements, still better than in photoacoustic spectroscopy. We have used this technique to measure the kinetics of activation of a sphalerite, that is the surface coverage of Z n S by A ~ + , pb2+ and cu2+ cations.

Mirage spectroscopy gives information on light absorbing species present at the surface of a solid (1). It seems a particularly conve- nient technique for solid-liquid interfaces studies for three reasons.

First, it is a non-destructive in-situ technique. Secondly, it can work whatever the surface roughness and the shape of the solid dipped into the liquid. Finally, due to its simplicity, expansive apparatus are not required.

We shall not recall the principles of Mirage spectroscopy which can be found in ref.(l). We shall point out that in our study only pcw- dered samples have been considered since our purpose is the study of the mechanisms of ores flotation which deals with small particules, typically 5 0 microns. This involves that the beam issued from the monochromator used to get a wide wavelength range (3.50-1200 nm) arrives downwards on the powdered sample

.

I - INSTRUMENTATION

The home made double beam spectrometer (half mirage - half ph0- toacoustic spectrometer) included a Xenon source, a large aperture (F/2) monochromator and a carbone black photoacoustic reference cell.

Fig. 1 represents a schemalic diagram of the spectrometer. The light beam of a Xenon arc lamp (450 W) is modulated by a mechanical chopper (5 to 400 Hz) electronically servo-controlled. The chopper is set up in front and not behind the monochromator in order to decrease damage of its optical surfaces(mirrors, grating) by the high flux of the lamp.

The monochromator (Jobin-Yvon H L - W ) has a concave holographic grating corrected from aberrations

( 9

⁼150mm, r = 335mm, aperture num- ber F/2, dispersion = 2 nm.mm-I). Though blazed at 350 nm, it has been used up to 1.2 microns. Wavelengths are scanned by a stepping motor.

At the exit of the monochromator, the light beam is focussed with an

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983649

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JOURNAL DE PHYSIQUE

P

'Mi /

_divider

a

Fig. 1 - Schematic diagram of the double-beam Mirage spectrometer.

elliptic mirror 1 on the sample cell 2.

A transparent slide 3 sends back a small part (about 1 0 % ) of the beam on a small photoacoustic reference cell4used to normalize the signal.

The probe beam is a Ilelium-Neon laser source, focussed on the sample by a lens (focal 80mm). The deflection of the beam is measured with a position detector 6 associated to an interferential filter.

The signals S1 and S from the position detector and the referen- cc cell microphone are itdependently pre-amplified, detected by a lock -in amplifier (ATNE model A D S 3 in A-B mode forthe position detector, model A D S 1 for the microphone), then divided by a home made divider.

The normalized signal is then recorded.

The sample cell is an ordinary spectroscopic cuvette cut at a height of 7mm, Fig. 2. Its support is mounted on to motorized Microcon- trole support allowing very small and smooth shifts by a stepping motor (not represented on the figure). When organic liquid was used a glass cover was added on the cell to slow the liquid evaporation. Reactants are added into the cell with a syringe.

Fiq. 2

-

Sample cell.

1-.monochromatic light beam 2-laser probe beam

3-deflection angle 4-syringe for reactant

3

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I - SOME EXAMPLES O F SPECTRA

Spectra of malachite green adsorbed on silica and zinc sulfide are illustrated on Fig. 3 and 4. They show that absorption spectra can be easily performed on powders dipped either in water or in organic solvent. For malachite green hexane and not water has been used as liquid because of a slight dissolution of the dye in water.In agreement with the dn/dt influence (I), signals were always higher in organic liquid than in water. These samples were used to perform some quantitative measurements.

-

^Figure^:³

Spectra of malachite green adsorbed on silica (17Hz,bandwidth:6nm,time constant of lock in amplifiers Is, 3 0 nm mn-',double-beam mode)

(1)0;(2)0.3;(3)0.6;(4)1.0;(5)1.3;

(6)2.5; ( 7 ) 3 . 4 x lo-' molm-2.

Fig. 4 - Spectra of pure ZnS and Z3S activated by C U ( N O ~ ) ~ 7 10- M aqueous solution.

ACu : wavelength of the ki.netic study.

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4 0 spo

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¹²⁰⁰

.

A/,,,,,

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C6-310 JOURNAL DE PHYSIQUE

I1 - Signal (S) and signal/noisratio (S/N) vs chopping frequency.

In photoacoustic spectroscopy (PAS) it is well know2 that the PA signal varies with the chopping frequency f following a f 1 law for a black body or a saturated sample, and a f -3'2 law for an unsaturated sample. These rules, established for bulky samples, are also valid for powders (2). In mirage effect, laws have not yet been published for powders.

Fig. 5 gives some experimental variations of the absolute signal for saturated powdered samples (PbS, sintered powder of reduced black Ti0 ) or unsaturated one (malachite green, Ro 0 ) . These curves show tha? same laws are experimentaly valid for powders in mirage effect 2 3 : slope of -1 for a saturated sample, -3/2 for an unsaturated one, and between -1 and -3/2 for a strongly absorbing compound like Ho203

Fig. 5 - Logarithm of the absolute Fig. 6 - Log (S/N) vs log f mirage signal vs the logarithm for a PbS powder sample dipped of the frequency(in Hz). Numberson in water (A= 484 nm).

curves indicate the slope.mmalachi- te green in hexane

(A=

604 nm)

O T i 0 2 in water

(A=

484 nm) 8 PbS in water ( A = 484 nm) V Ho203 in water

(A=

524 nm)

Normalization of the signal is performed by dividing the deflection signal of the mirage effect by the microphone response of the phoioacoustic cell containing a black body. However the deflection detector and the microphone have not the same response versus the chopping frequency f, chiefly at low frequencies. Fig. 6 represents the signal/noise ratio (S/N) of the normalized signal of a saturated sample as a function of f : best result is pbtained at 18 Hz ; it is the frequency chosen thereafter.

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I11 - CAI*IBRATION CURVES IN MIRAGE SPECTROSCOPY

Quantitativity of mirage spectroscopy was evaluated by comparison with photoacoustic spectroscopy, using the same samples and the same

light beam (same source, monochromator and mirrors) than in ref. (2).

Only malachite green dye adsorbed on silica could be studied because the methylene blue also used in ref. (2) shows a rapid degrada- tion (perhaps because of the 631 nm He-Ne laser probe, which corres- ponds to a strong absorption by the dye).

Calibration curves, that is the signal. versus the surface concentration of the dye, are shown on Fig. 7 for PAS and mirage spectroscopy.

A simple glance on both curves shows that accuracy of mirage is still better than PAS, anyway for that example.

Fia. 7 - Normalized siqnal ampli- Fig. 8

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Adsorption kinetics of tude (arbitrary units) vs. surfa-

~ g + , pb2+ and cu2+ on ZnS. Mira- ce concentration (in 10-~mo1 m-2) ge signal a t nm (Cu2+) of malachite green adsorbed on

silica at 4 2 0 nm. 520 nm ( ~ g + and pb2+) vs. time.

IV - KINETICS OF ZnS ACTIVATION BY Cu(II), Pb(I1) AND Ag(1)

Interest of spectrophotometric methods for studying ore flottation anti particularly sphalerite activation by copper has been briefly discussed in the paper by V. PLICHON et al. in this issue. Surface concentration measurements, mentionned as in important point in this paper, is presently under study by mirage spectroscopy and will not be discussed here. On the contrary we shall briefly focus on a more fundamental point, that is the kinetics of copper and other cations layer formation at the surface of ZnS. In this study, we did not use sphalerite, but chemical ZnS powder (Hopkin Williams).

Apparition of metal layers is followed by recording the mirage signal vs. time at a wavelength where tQe ca$ion abs rbs (for Cu e.g.

see Fig. 4). Fig. 8 shows results for Ag

,

Pb and Cuq+. After the cation injection, less than half minute is required for the cation to diffuse from the injection point to the ZnS surface (Fig. 2), then the mirage s<.qnal starts to increase.

Formation of the lead layer is achieved in less than 5 mn and the signal reaches a constant value. On the contrary, the copper layer is formed in two steps, a fast one (less than 5 mn) detailed on Fig.9, and a slow one (hours). Both steps obey a logarithmic kinetic law : S = a log t + b, with b increasing with the copper concentration in solution. Similar results have been already observed by others techniques(3)

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JOURNAL DE PHYSIQUE

Silver layer formation is faster and its kinetic can be measured with our instrumentation only for silver concentration below I O - ~ M .

It obeys a logarithmic law (Fig.

9 ) .

Quantitative determinations including measurements of the layer thickness and mechanistic interpretation are presently under study. But it may be already concluded from these preliminary experiments that mirage spectroscopy is a well suited in situ technique for minerals flotation studies.

I

log t

Fig. 9

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Kinetic of copper (on the left at pH=S and 980 nm) and silver (on the right at pH=S and 520 nm) layers formation (amplitude of the mirage signal has been corrected from the ZnS background and from the diffusion time of the cation between injection and signal increase).

ACKNOWLEDGMENT

The spectrometer has been constructed and the experiments performed in the Laboratoire dtOptique Physique de llESPCI (Dr. BADOZ, Pr. BOCCARA). This group is greatly acknowledged for its cordial wel- come, its technical help and the fruitful1 discussions.

REFERENCES

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-

BOCCARA A.

,

FOURNIER D

.

^{and BADOZ}^J

. ,

^Appl

.

^Phys

.

^Lett

.

36 (1980) 130.

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- EICHON v.,

LELIEVRE D., LELIBOUX M., FOURNIER D., CECILE J.L.

and BOISSAY S., Anal. Chim. Acta

138

(1982) 349.

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-

RALSTON J. and HEALY T.W., International J. of Mineral Processing 7 (1980) 175.