May, 1962
I N F R A R E D : ~ i S V E S T I G A T I O SO F X A S T H B T E ADSoRPTION BY LE.4D SULFIDE
879
AN INFRARED ISVESTIGATIOS OF XASTH9TE ADSORPTION BY LEAD SULFIDE BY ROBERT G. GREZNLER Research Division, Allis-Chdmers Mfg. Co., Milwaukee 1, Wisconsin Received November 1, 1961
Infrared spectroscopy is used to investigate the nature of compounds formed on the surface of PbS exposed to air, the effect on these surface compounds produced by the subsequent adsorption from potassium ethyl xanthate solution, and the nature of the resulting xanthate surface layer. PbS samples which have been exposed to air show infrared evidence of PbC03, PbSOa, PbSO,, and other lead-sulfur-oxygen compounds of unknown composition. Carbonate ions and, in some cascs, sulfur-oxygen ions are displaced by treatment with an aqueous solution of potassium ethyl xanthate but not by similar treatment with aTater alone. PbS samples which are heated in air to produce more oxidation of the sulfide ions adsorb more xanthate from solution than do less-oxidized oamples. A surface xanthate compound is formed on the PbS which has the infrared spectrum of lead ethyl xanthate. Part of the surface xanthate is readily removed by mashing in acetone. The amoudt of xanthate \r.hich remains on the surface after repeated washing in acetone corresponds approximately to a monolayer of xanthate ions on the PbS surface. h structure is suggested for the xanthate surface compound which is formed.
Introduction PbS,O,,, with n / m less than 4. He also reports8 The specific nature of the adsorption of xan- that the PbS,O, surface compounds are removed thates on sulfid.es has been debated at some lengt'h by washing in distilled water leaving an unoxidized in the literat'ure dealing with the flotation of surface. The work reported here investigates the existsulfide ores using xanthate coll.ectors. Reviewsl-s of the experiment~aldata and its significance in ence of surface compounds on PbS exposed to air, view of various theories of flotation still leave the the effects on these compounds of treatment with mechanism of adsorpt'ion uiidecided. There are KEX solution, and the nature of the surface many difficulties in trying to determine the nature xanthate which is formed. of surface layers by analyzing the mat'erial after Experimental Details it has been removed from the surface. The techPreparation of PbS.-The fact that PbS absorbs in the nique of studying surface layers by their infrared infrared limits the amount of this material which can be used spectra4 permits an examination of the surface in a transmission sample. As the particle size of the PbS reduced, the greater surface-to-volume ratio of the compounds or adsorbed layers without their i3 particles permits a greater ratio of surface compound absorpremoval and so has an obviouis advantage for this tion t o PbS absorption. In addition, the energy lost due problem. to scattering becomes less as the particle dimensions become Since much of the earlier work has been done on small compared to the infrared wave lengths employed. PbS was precipitated in hot basic solution by homothe flotation of galena by potassium ethyl xan- Fine geneous precipitation with thioacetamide and lead acetate. thate (KEX), the PbS-KEX system has been The precipitate was filtered, washed in distilled water, and chosen for this investigation. It is known that dried in a vacuum desiccator. Surface Area Determination .-The surface areas of the the xanthate ion is att,ached to the sulfide surface; samples were determined by a continuous-floiv nitrogen however, after reviewing the evidence for t,he PbS adsorption methodYin a modified apparatus which will be existence of lead xaiithat,e at the surface, Gaudiiij dexribed elsewhere.lo concludes : "These observations do not necessarily Xanthate Preparation and Treatment .-Potassium ethyl indicate that lea,d xanthate exists as a distinct vanthate from American Cyanamide Company i\-as purificd by washing with ether, dissolving in acetone, and exphase at a xanthated galena surface but they show traction with ether. The xanthate was used uithin three that the compoiieiitx of lead xanthate are avail- weeks after purification. For xanthate treatment, 500 mg. able there." :It generally is accepted5p6 t.hat a of PbS was placed in 50 ml. of distilled water containing xanthate ion cannot directly replace a sulfide 250 mg. of KEX, and stirred for 1 hr. The PbS was filtered out and enough material for a sample was air-dried on filter ion in lead sulfide, but probably exchanges with a paper. sulfate ion (or some other a'xidized sulfur ion), Infrared Spectra.-Samples were made into KRr pellets carbonate ion, or hydroxide ion formed on the by grinding 5 mg. of PbS with 700 mg. of KBr in a hand surface by exposure to the atmosphere. H a g i h ~ a , ~mortar, then pressing the powder in an evacuated die. Specm-ere scanned on a Perkin-Elmer Model 21 spectrometer examining the cleavage surface of galena crystal tra as soon as the various treatments were completed. by electron diffraction, identified PbS04 and PbzResults and Discussion SO5. He identified a crystalline carbonat'e only Spectrum of Adsorbed Xanthate.-Figure 1 after long ($&day) exposure to water. In no case did he observe any of the lower sulfoxides, shows the spectra of KEX, Pb(EX),, and PbS which has been treated with a KEX solution. 11) A. M.Gaudin, "Flotation," McGraw-Hill Book Co., Ino., New Recent investigatlors11,12disagree on the assignYork, A.' Y., 1957, Chapt. 9. ments of the xanthate bands in the 1000 t o 1200 (2) I -WE, 9
where E,
=
(2)
reversible work of adhesion.
(11 Ya. I. Fienkel, Zhur. Eksptl z Teoret Fzz , 18, 659 (1948). ( 2 ) Y a B Aron and Y a I. Frenkel, zbqd., 1 9 , 807 (1949).
In the entire study, however, no definitive experimental verification of eq. 1was given. I n the present study it has been shown that the previous conclusions can be expanded upon so as to permit estimation of tilt angles from readily measurable quantities. The equations which are developed also are experimentally verified. Theory If it is assumed that the entire initial resistance to sliding of the liquid drop is at the rear of the drop, then eq. 1 can be derived. Consider a liquid drop on an inclined plane surface (Fig. 1). As,wme the drop will begin to move a t an angle, a, where the accelerating force exceeds the retarding force at the rear of the drop. From Fig. 1 the work, dW, a t the rear of the drop due to the retarding , through a distance, ds, is given by force, f ~ acting dW = f~ ds (3) At the trailing edge of the liquid drop a new liquid surface and a new solid surface are formed and the interfacial surface is destroyed (or more rigorously there is exposure of liquid-vapor and solid-vapor interfaces and destruction of a solid-liquid interface). Consequently the work done a t the rear of the drop as an area dA of the solid surface becomes exposed can be given by dW
= (YLV
+
YSV
- rst)dd
(4)
Since dA = w ds where w is the width of the drop and since eq. 3 and 4 may be equated, the retarding force is given by
