CE by: SK UK English pstop470648 on 17 June 2013

IOP PUBLISHING PHYSICA SCRIPTA

Phys. Scr. T152 (2013) 000000 (4pp) UNCORRECTED PROOF

Optical luminescence studies of thexanthate adsorption layer at thesolid–liquid interfaceR Todoran, D Todoran and Zs Szakacs

Technical University of Cluj Napoca, North University Centre of Baia Mare, Street V Babes, nr 62/A,RO-430083, Baia Mare, Maramures, Romania

E-mail: todoran radu@yahoo.com

Received 18 August 2012Accepted for publication 21 January 2013Published xxxOnline at stacks.iop.org/PhysScr/T152/000000

AbstractIn this paper, we propose optical luminescence as a method for the evaluation of the kineticsof adsorption processes to calculate the time required to achieve the dynamic balance of thethin layers formed at the mineral–xanthate solution interface. The method is based on themeasurement of the intensity for the integral optical radiation obtained from themineral–xanthate thin layer, which is stimulated with a monochromatic pulsating opticalsignal, as a function of time. The luminescence was studied for the galena, sphalerite andchalcopyrite minerals with sodium amyl and sodium isobutyl xanthates, for differentconcentrations of the solutions and different pH values under constant temperature. Using thesaid method, information was gained on the kinetics of the adsorption of xanthates. Goodcorrelation with the sequential (radiometric) investigation methods was noticed. A bettermeasurement of the time to achieve equilibrium in the formation of the adsorption layer wasalso obtained.

PACS numbers: 68.03.−g, 68.43.Mn, 68.43−h

(Some figures may appear in colour only in the online journal)

1. Introduction

In the study of the chemisorption and physical adsorptionmechanisms one must take into account the dynamicprocesses that take place at the interface level [1–3].

The boundary surfaces in liquids and solutions havehomogeneous characteristics for different regions. In thecase of solid surfaces, the position of the atoms isfixed and the surface has a complex, inhomogeneousstructure. This fact underlines the great interest in thestudy of adsorption processes at solid surfaces, especiallyfor industrial and technological processes, such as mineralflotation, based on this phenomenon at the solid–liquidinterface.

The physical and chemical adsorption processes of thexanthate molecules at the solid–liquid interface representa condition to modify the floatability of many non-ferrousminerals in the process of separating the metallic compoundsfrom the gangue [4–6].

In this paper, we propose optical luminescence as amethod in the evaluation of the kinetics of adsorptionprocesses, to calculate the time to achieve dynamic balancefor the thin layers that are formed.

Up to recent times, the study of solid–liquid interfaceswas mainly done to establish isothermal equations, buttrue thermodynamic equilibrium was infrequent. Thesewere statistical methods and were not applied for in situevaluations [7].

Knowledge of the kinetic mechanisms of adsorption–desorption is linked to understanding the related processes,which take place at the level of the liquid or solid phase, andplay an important role in the characterization of the interfacephenomena. There are a few models for the kinetics of theinterface processes with a different degree of generalization.In [8] a mathematical algorithm is proposed. It takes intoaccount the adsorption–desorption processes and the diffusionin the liquid phase, yet without a clear particularization tospecific cases of the solid–liquid interfaces.

0031-8949/13/000000+04$33.00 1 © 2013 The Royal Swedish Academy of Sciences Printed in the UK

Phys. Scr. T152 (2013) 000000 R Todoran et al

2. Method and samples

The optical luminescence method is based on the deter-mination of the integral optical radiation from the adsorptionlayer stimulated by a pulsing monochromatic signal.

The experimental determinations conducted wereaccompanied by radiometric ones. Radiometric analysisrequires that some molecules in the xanthate should bemarked with the radioisotope 35S. The main advantage ofthe optical luminescence is that it allows a continuous in situevaluation of the interface processes. The radiometric methodis also reliable, but special laboratory facilities are requiredto handle the radioisotopes. As it is a sequential method, thedeterminations should be made in time intervals or only at thestop of the interface process [9].

The optical luminescence determinations were madeduring an interval of 35 min, after which the equilibriumwas reached in the adsorption–desorption processes at theinterface level.

During the experimental determinations, we worked withthe following types of mineral: galena, sphalerite, activatedsphalerite and chalcopyrite. The types of xanthates used weresodium amyl xanthate (C6H16NaOS2) and sodium isobutylxanthate (C5H9NaOS2) in the following concentrations: 10,25, 50, 100 and 3000 mg l−1.

One must point out that, from the analysed minerals,sphalerite in its natural state has low adsorption capacity.That is the reason why, in most cases, it undergoes anactivation process before flotation in the industrial regime.The activation of sphalerite is a chemical procedure that isapplied to the surface of the mineral and facilitates the bondof the collector reagents on it. That is done by increasing thenumber of oriented adsorption centres of the collector reagentat the surface of the minerals by chemisorptions or exchangereactions. In most cases, the sphalerite is activated usingcopper sulphate. The mechanism of activation with coppersulphate, which facilitates the bond of the xanthogenate, is

ZnS

ZnS+ Cu2+

=ZnS

CuS+ Zn2+. (1)

After this treatment is applied to the surface of themineral, more powerful anodic regions are formed, facilitatingthe attachment of the polar group of the collector. Usingelectron spin resonance it was noticed that the interaction ofthe Cu2+ ion with the lattice of the sphalerite ZnS can takeplace through the capture of one lattice electron, followed bythe integration of the formed monovalent Cu+ in the lattice,based on the following mechanism [10]:

Zn|(−) . . . Zn2+ + Cu2+

→ ZnS|+ . . . − Cu+ + Zn2+. (2)

One can note the ionic exchange between the Cu2+ ionsand the Zn2+ ions from the lattice followed by the reduction ofCu2+ to Cu+ and its integration in the lattice. Thus, the numberof Cu2+ ions in the solution drops, while the number of Zn2+

ions is increased by the same amount [11].In the activation process the surface of sphalerite was

treated with a CuSO4 solution of 310 g l−1 for 30 min.The xanthate solutions used had the following pH values:

7, 8, 9 and 10. Different pH values were used due to its

important role in the stability of the xanthate, and especiallybecause of its direct influence on the development of theadsorption phenomena. The floatability is maximal in theabove-mentioned domain of the pH. For pH values lowerthan 5, floating and adsorption are almost non-existent, whileincreasing the pH to values higher than 10 induces very lowvariations in the processes.

The working temperature was set to an exact 18 ◦Cusing a thermostat. This temperature was chosen because itis the temperature most likely to be present in the industrialprocess [12].

To ensure a high measuring reproducibility, we fixedthe volume of each xanthate solution used in every kineticdetermination to 2 ml. To check the reproducibility ofdeterminations, multiple determinations were made for thesame mineral sample while the volume of the xanthatesolution remained fixed to the aforementioned value. Thesurface chemical abrasion was made with a spray of pickleliquor. An air spray was used to dry the surface. The mineralsamples were positioned with the polished surface in aperfectly horizontal position, and the xanthate volume wasplaced on this surface. The horizontal alignment of themineral surfaces was checked using two parallel beams oflight, originating from two coplanar horizontal He–Ne lasers.

The experimental installation for the in situ measure-ments of the optical luminescence radiation at themineral–xanthate solution interface is presented in figure 1.

The laser used in the experimental setup was a pulsingnitrogen laser of the LGI-21 type with a pulsing frequency of25 Hz and the wavelength of the emitted radiation being λ =

337.1 nm (3.68 eV). It has a power of 1.6 kW, and the lengthof a pulse is τ = 10−8 s. The laser radiation of this wavelengthshould stimulate a layer of at most 20–25 Å, a value resultingfrom the band absorption coefficients. The width of thedomain of laser measurements was 1 Hz; consequently, thefrequency band for the measurements was 25 ± 1 Hz.

The selective amplifier at 25 Hz of the type U2-8 was putto resonance with the signal from the photomultiplier. Theused electronic photomultiplier was of the FME-51 type. Thephotocathode is multi-alkaline, with a known sensitivity inthe spectral interval of 290–900 nm, and with low levels forthe induced noise signal. This essential characteristic is dueto the relatively large surface of the photocathode. The mirrorreflects the UV and total radiation and concentrates the lightbeam on the sample to the mineral–xanthate surface. The laserstabilizer gives good frequency stability to the laser radiationemitted in the pulsating system. The BS-8 optical filter blocksthe laser radiation from reaching the photomultiplier.

3. Results and discussion

The experimental determinations were made using the integralluminescence radiation on the full sensibility spectrum ofthe photocathode since work was undertaken with molecularspecies that have a broad frequency domain for it. We canstudy the formation of the xanthate adsorption layers on themineral surface from the voltage–time graphs obtained fromexperimental determinations, this phenomenon being stronglyconnected to the modification of the optical luminescence.

2

Phys. Scr. T152 (2013) 000000 R Todoran et al

Nitrogen laser LGI - 21

St - 100

St RS-22 U2-8

FME-51

1

2

3

4

5

6

7

8

9

10

11

12

Figure 1. Block scheme of the experimental installation for the in situ measurements of the optical luminescence radiation at themineral–xanthate solution interface. 1—laser stabilizer; 2—pulsing laser; 3—reflection mirror for the laser radiation; 4—mineral;5—xanthate solution; 6—laser radiation; 7—solid–liquid interface; 8—luminescence radiation; 9—optical filter; 10—photomultiplier;11—stabilizer for the photomultiplier; 12—selective amplifier.

Figure 2. Intensity for the luminescence radiation as a function of time for (a) amyl xanthate and (b) isobutyl xanthate and chalcopyrite forpH = 7 and solution concentrations of 10 mg l−1 (long-dashed green), 25 mg l−1 (dashed violet), 50 mg l−1 (square-dotted blue), 100 mg l−1

(circle-dotted orange) and 3000 mg l−1 (solid blue). The red circles are the rescaled result of the radiometric measurements.

During the adsorption process, we obtained the voltagecharacteristics proportional to the intensity of luminescenceradiations from the optically stimulated surfaces. Theparameters modified during the experimental determinationswere the type of mineral, the type of xanthate, theconcentration and the pH of the xanthate solution. In each setof determinations only one of these parameters was varied,while the other three were kept constant.

The experimental results were compared with theradiometric ones, which are extensively used in deter-minations of the xanthate content in the adsorbed layers onmineral surfaces. As usual 35S was the radioactive marker.The xanthate obtained from the solution was oxidized withalkaline permanganate to sulphate, and it was precipitated asbarium sulphate. The radioactivity of the dried precipitate wasmeasured knowing that 35S is a low-energy beta emitter andthat its half-life is 87.1 days. The good correlation betweenthe rescaled radiometric results and the optical determinationscan be observed in figure 2.

The luminescence curves indicate a slightly higher rate ofadsorption of the amyl xanthate than of the isobutyl xanthate,while, after the equilibrium is reached, the adsorbed quantitiesare almost equal.

The highest adsorption for the different minerals, as canbe observed in figure 3, is in the case of chalcopyrite, thengalena and activated sphalerite. The lowest adsorption wasnoticed for the natural, non-activated sphalerite [5, 13, 14].The concentration of xanthate collector reagents adsorbed onthe mineral surface increases rapidly in the first few minutesof contact and reaches a dynamic equilibrium after 25–30 min.Chalcopyrite presents the most intense adsorption propertiesfor these reagents; it is followed by galena and sphalerite. Theadsorption properties of sphalerite in its natural, non-activatedstate are very poor, as can be seen in figure 3.

The variation of the solution’s pH changes the adsorptionproperties of the minerals, as can be seen in figure 4. Thedecrease of the adsorption with the increase of the pH can beexplained by the high concentration of OH− ions. These ions

3

Phys. Scr. T152 (2013) 000000 R Todoran et al

Figure 3. Intensity for the luminescence radiation as a function oftime for amyl xanthate solution and chalcopyrite (solid green),galena (dotted violet), activated sphalerite (dashed blue) andsphalerite (long-dashed orange) with a xanthate-solutionconcentration of 50 mg l−1 and pH = 7.

Figure 4. Intensity for the luminescence radiation as a function oftime for amyl xanthate solution and chalcopyrite with a solutionconcentration of 50 mg l−1 for pH = 7 (solid green), pH = 8 (dottedviolet), pH = 9 (dashed blue) and pH = 10 (long-dashed orange).

have a depressing role; they compete with the ions from thecollector reagents in the adsorption process on the surface ofthe sulphurous minerals [15]. Sphalerite is the least influencedby these variations.

Results similar to the ones presented in this paperwere obtained by Tipman (Germany, 1975) [16] and by theteam led by Markovic (Yugoslavia, 1997) [17]. Markovicused amyl xanthate concentrations of 3 × 10−5 and 6 ×

10−5 mol l−1, where the pH was 7, 8, 9, 10 and 11. Themethod was UV–VIS and BET spectroscopy with sequential

Q1determinations, leading to the conclusion that the adsorptioncoefficient at the interface, expressed in mol m−2, dropswhen the pH increases. The samples used were chalcopyriteminerals dried in the oven; the pH was varied usingcalcium hydroxide. Tipman used a Beckman DU-55-UV–VISspectrophotometer to determine the concentration of theresidual xanthate in the solution after the adsorption processended on the chalcopyrite surface, with rigorously knowninitial concentrations of the xanthate solutions. The adsorptionratio was computed using the concentration difference of thexanthate solutions before and after the adsorption process. Theclear conclusion drawn by Tipman is that of the nonlineardependence between the increase in the xanthate solutionconcentration and the adsorption ratio, with a low increaseof adsorption at high concentration changes in the solution.

These results were obtained by sequential methods and canalso be deduced from our optical luminescence studies.

4. Conclusions

In this paper, optical luminescence was presented as an insitu evaluation method for the kinetics of the adsorptionprocesses at the mineral–xanthate solution interfaces. Theluminescence was studied for the galena, sphalerite andchalcopyrite minerals with sodium amyl xanthate and sodiumisobutyl xanthate. We noticed that amyl xanthate has amarginally higher adsorption rate than the isobutyl variation.The increase in concentration for the xanthate solutions doesinfluence in a nonlinear way the adsorption process: a highincrease of the solution concentration is reflected in a lowincrease of the adsorption coefficient. The highest adsorptionis obtained for chalcopyrite; that is followed closely bygalena and activated sphalerite, while natural sphalerite haslow adsorption properties. The influence of the pH of thexanthate solution was also dealt with; an increase in thepH level was associated with a decreasing tendency of theadsorption. The time to achieve dynamic equilibrium wasdetermined to be 25–30 min in all of our cases. A goodcorrelation with radiometric methods was also found. Thexanthate adsorption studies on sulphurous minerals led to abetter understanding of the dynamics of flotation processes;they had a direct technological influence on the optimizationof the time allocated to these processes.

References Q2

[1] Isobe Y I T and Senna M 1996 J. Phys. Chem. Solids57 373–9

[2] Sooklal K, Cullum B S, Angel S M and Murphy C J 1996 J.Phys. Chem. B 100 4551–5

[3] Huang J, Yang Y, Xue S, Yang B, Liu S and Shen J 1997 Appl.Phys. Lett. 70 2335–7

[4] Todoran R and Todoran D 2008 Phys. Mac. 57/59 35–40[5] Todoran R and Sharkany J 1999 Proc. SPIE 3687 26–8[6] Hough D B and Rendall H M 1983 Adsorption from Solution

at the Solid/Liquid Interface ed D G Parfit and C HRochester (London: Academic) pp 15–20

[7] Kralchevsky P A, Radkov S Y and Deukov D N 1993 J.Colloid Interface Sci. 161 361–5

[8] Shibata C T and Lenhoff A M 1992 J. Colloid Interface Sci.148 485–507

[9] Leja J 1982 Surface Chemistry of Froth Flotation (New York:Plenum) 205

[10] Barbus A, Sinko I and Cojocaru L N 1977 Rev. Roum. Phys.22 667

[11] Sinco I, Barbus A, Baican R and Cojocaru L N 1979 Studii siCercetari de Fizica vol 10 (Bucuresti: Acad. RSR)pp 1129–30

[12] Huber-Panu I 1930 Uber den Einfuss der Temperatur auf dieFlotation (Freiberg: E. Mauchisch) pp 107–8

[13] Todoran R and Todoran D 1998 Innovations in Mineral andCoal Processing ed S Atak, G Onal and M Sabri Celik(Rotterdam: A.A. Balkena) pp 183–7

[14] Todoran R, Todoran D and Stan O 1997 Balkan. Phys. Lett.5 2389–92

[15] Glembotchii O V, Mitrofanov S I and Davidova L M 1983Tsvetnie Metalli 11 116–7

[16] Tipmann R N and Leja J 1975 Colloid Polym. Sci 253 4–10[17] Markovic Z, Milosevic S, Mantonijevic M and Stanojlovic R

1997 Balkan Conf. on Mineral Processing Vatra Dornei volII 225–8 Q3

4

QUERY FORM

Journal: ps

Author: R Todoran et al

Title: Optical luminescence studies of the xanthate adsorption layer at the solid–liquid interface

Article ID: pstop470648

Page 4

Q1.Author: Please define BET.

Q2.Author: Please check the details for any journal references that do not have a blue link as they may contain some incorrectinformation. Pale purple links are used for references to arXiv e-prints.

Q3.Author: Marcovic Z has been changed to Markovic Z in ref. [17]. Please check and confirm the change.

Author: Please be aware that the colour figures in this article will only appear in colour in the Web version. If you requirecolour in the printed journal and have not previously arranged it, please contact the Production Editor now.