T H E ELECTROSTATICS O F FLOTATION BY H. B. BULL
The purpose of this research was to investigate the electrostatic charge carried by finely ground galena particles in distilled water and in solutions of different concentrations of salts in hopes of being able to give a more logical and certain interpretation to the observed “toxic” effects of these salts on the flotation of galena. As is well known, there is quite a definite relationship between the concentration of a salt added to a flotation circuit and its toxic effect. This is clearly shown by the work of Gaudin and his associates’ and also by Taylor and The following graph shows the typical effects of a number of cations on the flotation of galena. 100
h
8 80 L!
T!
2 60
g F
$40
tt:9 20 3
k 80
FIG.I Showing “toxic” effect of electrolytes on the flotation of galena.
An attempt wm made to determine the electrobtatic charge on finely ground galena by the streaming potential method in an apparatus developed by B r i g g ~ . But ~ the resistance of the galena was so low that no difference of potential could be observed. The method finally adopted was based on a phenomenon discovered by Dorn4 in 1878 and bearing his name. He found that when a body falls through a liquid there is a difference of electrical potential established between the top and bottom of the liquid. Freundlich and Makeltj attributed the existence “Flotation Fundamentals” by A. M. Gaudin, Harvey Glover, and C. K. Orr Hansen. J. Phys. Chem., 32, 641-656 (1929). 3 J. Phys. Chem., 32, 641 (1928). Dorn: .Inn. Physik., (4) 3, 20 (1878); 9, 513; 10, 46 (1880). 5 Freundlich and Makelt: Z. Elecktroohemie, 15, 161 (1912).
657
THE ELECTROSTATICS O F FLOTATIOS
of this difference of potential to what is termed the zeta potential, that is the electrical potential existing across the immovable absorbed layer of ions and the boundary layer of movable molecules. The actual experimental technique was to allow ten grams of the purest crystallized galena, which had been ground to pass a I O O mesh sieve and retained on a 3 2 5 mesh sieve, to fall through a glass tube 65 cm. long and 3 cm. in diameter, which contained the liquid in question. Platinum wires fused into the malls of glass tube z cm. from each end were used as electrodes. h quadrant electrometer was used to register the difference of potential between the top and bottom of the tube. The observed deflection of t h e t250,
I
I
-50
I
0
10"10-'
20110"
J
Jo*fO-'
LONCENTCATfON OF 5ALT / N MILLIMOLS/U/LLIM€T€R
FIG. 2
quadrant electrometer, which was produced by the falling of the galena, was reproduced by an electromotive force whose strength could be accurately determined and thus the difference of potential produced by the falling galena was obtained. The galena was allowed to fall through varying concentrations of the nitrates of Th, Cr, Ag, P b and Co. Results I n Fig. z the observed difference of potential is plotted as the ordinate against the concentration of salt added in milliniols per milliliter as the abscissa. The sign of electrostatic charge on the galena is indicated.
Discussion It is quite clear that in this series of determinations the zeta potential has not been actually determined but we have reason to believe1 that while Freundlich and Makelt: Z. Elektrochemie, 15, 161 (1912).
6 j8
H. B. BULL
the difference of potential between the top and bottom of the liquid is not equal t,o the zeta potential it is, within certain limits, proportional to it,. A comparison between the toxic effect of lg, Pb, and Co and the effects that these ions produced on the electrostatic charge of the galena is highly suggestive. It indicates, perhaps, that a large part of the toxic effect of these ions is due to the raising of the electrical charge on the particles of galena and so preventing them from collecting on the air bubble thereby reducing the yield in flotation. This explanation becomes more convincing in light of E. F. Burton and J. E. Currie's' work who found that when small lead,shot were dropped through solutions of salts of different concentrations that there was a marked repulsion between the shot at certain salt concentration as evidenced by scattering of the shot as they fell and that these concentrations correspond to salt concentrations where there was a maximum difference of potential between the top and bottom of the liquid. The reason for the large effect of the silver ion as compared with the lead and cobalt ion is probably due to the fact that the silver ion is absorbed to a much greater degree than the lead and cobalt ion, this greater absorbability being due to the fact that silver sulfide is three times more insoluble than lead sulfide and eighty times more insoluble than cobalt sulfide. In as much as neither thorium nor chromium form stable sulfides and since the galena carries an initial positive charge, the nitrate ion in these cases plays a predominating role. There is apparently no connection between the extreme toxic effect of these ions on flotation and the electrostatic charge carried by the galena while in a solution of these salts. It is also highly suggestiw that finely ground quartz was found to carry a relatively large negative charge while it is well known that air bubbles in water are also negatively charged.' Summary There is an apparent connection bctwern the toxicity of Ag, Pb, and ('0on the flotation of galena and the electrostatic charge carried by finely ground galena in solutions of the nitrates of these cations. 2. There is apparent,ly no connection between the toxicity of T h and Cr on the flotation of galena and the electrostatic charge carried by finely ground galena in solutions of the nitrates of these cations. I.
Criiwisiiy o/ .1ffnriesutn,
Miiineapolis. S . A. SIcTaggart: Phil. Mag. (6), 27, 297 (1914). 'E. F. Burton and J. E Currie: Phil. Slag., ( 6 ) 49: 194 (19251.
