Table of Contents
It is generally accepted that the collector, potassium ethyl xanthate, will not float sphalerite. However, Gaudin showed that sphalerite readily floats if activated with certain metal cations before exposure to xanthate. Ralston and Hunter found that copper sulfate, is the best activator for sphalerite.
In recent years flotation reaction products formed on mineral surfaces have been analyzed by infrared techniques. Peck used the potassium bromide pellet technique to study oleic acid and sodium oleate adsorption on fluorite, barite, and calcite. Poling and Leja used the differential reflectance technique for studying the adsorption of xanthate on nickel surfaces. The Bureau of Mines used the internal reflection technique in conjunction with solvent extraction procedures to study the reaction of aqueous potassium ethyl xanthate on galena surfaces. In the internal reflection technique described by Harrick, a sample that selectively absorbs infrared radiation, when placed in contact with a reflecting prism, will produce an absorption spectrum that is characteristic of the material.
The objective of the research described in this paper was to determine the relationship between values of the pH of the collector solution and the amounts of the reaction products, cuprous ethyl xanthate and ethyl dixanthogen, adsorbed on sphalerite surfaces.
Experimental Materials and Techniques
Sphalerite sections, measuring approximately 45 x 20 x 10 millimeters, were embedded in resin at room temperature prior to grinding and polishing. Because grinding and polishing on motor-driven laps may yield slightly curved surfaces, the sections were rough-ground by hand on flat glass with 600-grit silicon carbide in water and were semipolished on a cloth saturated with an aqueous suspension of alumina.
Sphalerite powder was prepared by dry-grinding selected chunks of massive sphalerite for 36 hours and then by wet-grinding in ethyl alcohol for 12 hours in a ceramic mill using corundum pebbles as the grinding medium. The average particle size of the sphalerite powder was microns, as determined with a Sharples Micromerograph.
Precipitated zinc sulfide power with an average particle size of 2.5 microns was used.
The polished sections, powdered sphalerite, and precipitated zinc sulfide were activated in a copper sulfate solution for 2 hours, dried, and then immersed in the potassium ethyl xanthate solution for 2 hours. All reagents were reagent grade except potassium ethyl xanthate which was purified locally from technical-grade xanthate by dissolving in acetone and precipitating with petroleum, ether.
The flotation reaction products were identified by means of the Wilks multiple internal reflection (MIR) attachment or by the NaCl 1-millimeter liquid cell in the sample compartments of a Beckman Model 7 double beam spectrophotometer. The sphalerite sections were clamped to both sides of a thallium halide plate that fitted into the MIR attachment while the treated sphalerite powder, in the form of a Nujol mull, was spread on one side of this plate and placed in the MIR attachment.
Results and Discussion
Figure 1 shows the characteristic infrared absorption for the chemical compounds ethyl dixanthogen (5.3 x 10 -6 moles) and cuprous ethyl xanthate (6.21 x 10 -6 moles). These compounds are assumed to be formed during the reaction between aqueous potassium ethyl xanthate and sphalerite activated with aqueous copper sulfate. In figure 1 strong absorptions occur near 1,028, 1,242, and 1,260 cm-¹ for ethyl dixanthogen and near 1,034 and 1,200 cm-¹ for cuprous ethyl xanthate. From these spectra, the absorbance to mole ratios of each xanthate was determined to be 1.5 x 10 -5/mole for ethyl dixanthogen at the 1,242 cm-¹ peak, and 1.23 x 10 -5/mole for cuprous ethyl xanthate at the 1,200 cm-¹ peak. In the absorption spectral literature, assignments of absorption frequencies to C-O and CIS bonds in xanthate molecules show conflicting interpretations. Shankaranarayona and Patel suggest according to theoretical and experimental evidence, that strong absorptions around 1,200 and 1,030 cm-¹ in xanthate-related compounds are definitely due to the -C-O-C-group.
Treatment of Sphalerite Sections
Sphalerite sections were activated in 50 milliliters of 0.5 molar copper sulfate solutions and then treated in 50 milliliters of 0.00625 molar potassium ethyl xanthate with pH values 6.5, 8.5, 10.0, and 11.0. Figure 2 shows the infrared spectra of the activated sphalerite sections before and after washing in petroleum ether. The spectra before washing show slightly broadened absorption peaks centered at approximately 1,022, 1,240, and 1,260 cm-¹ indicating that ethyl di-xanthogen is present on the sphalerite surface. The spectra after washing show no ethyl dixanthogen absorptions. Hence, it is reasonable to suppose that only physically adsorbed ethyl dixanthogen is present because petroleum ether has very little solvation properties and would remove only physically adsorbed dixanthogen. However, the spectra of the washed surfaces show absorption at 1,032 and 1,190 to 1,200 cm-¹. These peaks correspond to those of cuprous ethyl xanthate shown in figure 1. The appearance of the absorption peaks of cuprous ethyl xanthate after washing does not indicate that this xanthate was chemically adsorbed because it is insoluble in petroleum ether. However, this xanthate is at least physically adsorbed on the sphalerite surface. In figure 2, the position of the absorption frequencies are at 1,022 to 1,023 cm-¹ and 1,032 to 1,034 cm-¹ for ethyl dixanthogen and cuprous ethyl xanthate, while in figure 1 similar absorptions occur at 1,028 and 1,034 cm-¹. This indicates also that these compounds are physically adsorbed on the sphalerite surface causing interactions between xanthate molecules and the sphalerite surface which weaken the bonds in the xanthate molecule and lower the absorption frequency.
The resolution of the spectrophotometer, 1 to 5 cm-¹, is sufficient to allow separation of absorption peaks for determining absorbances of both xanthate compounds. Figure 3 shows that the ratio of the absorbance of physically adsorbed cuprous ethyl xanthate to that of ethyl dixanthogen is directly related to the pH of the collector solution. Data for experiments at pH 11 were found to be less reproducible than data obtained at lower pH values and, therefore, were not included in figure 3.
In-order to use infrared spectroscopy techniques to identify reaction products on the xanthate-treated activated-sphalerite section, it required that the concentration of the copper sulfate solution be extremely high, 0.5 molar, compared to normal flotation practice. The reaction products were not detected at lower concentrations.
Treatment of Sphalerite Powder
Five grams of powdered sphalerite was activated in 45 milliliters of 0.01 molar copper sulfate solution and then treated in 250 milliliters of 0.0025 molar potassium ethyl xanthate solution with pH values 5.6, 8.5, and 11. Because of the alkalinity of sphalerite in water, the pH of xanthate solutions had a tendency to drift toward higher values. Figure 4 shows Nujol mull spectra of the xanthate-treated sphalerite and evaporated film spectra obtained from acetone washings of the treated powder. For pH values 5.6 and 8.5, in the Nujol mull spectra, absorptions at 1,038 and 1,200 cm-¹ show cuprous ethyl xanthate to be present on the sphalerite surface. In the evaporated film spectra, absorptions at 1,029, 1,240, and 1,260 cm-¹ show that ethyl dixanthogen was removed from the sphalerite surface. At pH 11.0 the presence of cuprous ethyl xanthate is shown by the shoulder peak at 1,035 cm-¹, while in the evaporated film spectra absorptions at 1,032 and 1,198 cm-¹ show removal of cuprous ethyl xanthate, and absorption at 1,265 cm-¹ shows some removal of ethyl dixanthogen. The fact that no ethyl dixanthogen was identified in the Nujol mull spectra indicates that the sensitivity of detection of ethyl dixanthogen is less than for cuprous ethyl xanthate.
Treatments of Zinc Sulfide Powder
To determine the difference in the amounts of adsorbed and unadsorbed ethyl dixanthogen, 10 grams of precipitated zinc sulfide was activated in 200 milliliters of 0.01 molar copper sulfate solution and then treated in 250 milliliters of 0.0025 molar potassium ethyl xanthate solution with pH values between 6.8 and 11.5. After the treatments, carbon disulfide extractions were made on the remaining xanthate solution, and the treated powder was washed in carbon disulfide to remove ethyl dixanthogen. The spectra of these carbon disulfide solutions were obtained, and the ethyl dixanthogen was determined by comparing absorbances at 1,240 and 1,260 cm-1 with the reference absorbance-concentration relationship shown in figure 5. Absorbance readings were obtained using the base-line tangent method of infrared quantitative analysis. Figure 6 shows the variation of the amount of ethyl dixanthogen removed from the activated powder system as a function of pH.
At the lower pH values, 6.8 and 8.0, less ethyl dixanthogen was found in the extracts than in the washings but at the pH values of 9.5 and 11.5 the converse was true.
Mechanism of the Sphalerite Activation
One hundred grams of powdered sphalerite were agitated in 600 milliliters of distilled water containing 6 x 10-³ moles of cupric ions. Copper sulfate was added until excess cupric ions were found in the activation solution. The powdered sphalerite washing and the remaining activator filtrate were analyzed by atomic spectrophotometry. The activator filtrate contained 4.8 x 10-³ moles of zinc ions and 2.55 x 10 -4 moles of cupric ions while the washing contained less than 10 -7 moles of cupric ions. Since the number of moles of zinc ions in the filtrate is slightly less, than the number of moles of cupric ions used for activation, an exchange of cupric ions with zinc ions may be assumed. The fact that only a few cupric ions were removed by the distilled water washings shows that these soluble ions are held firmly in the sphalerite lattice. When the concentration of the copper sulfate solution was increased 10 times, 3.9 x 10-³ moles of cupric ions were found in the washing. These excess ions might have been adsorbed on the surface.
During activation, cupric ions are exchanged with zinc ions in the sphalerite. When the cupric ions in the activated sphalerite react with potassium ethyl xanthate, ethyl dixanthogen and cuprous ethyl xanthate are known to be produced.
This investigation has shown that the ratio of the absorbance of cuprous ethyl xanthate to that of ethyl dixanthogen adsorbed on the surface of sphalerite sections is directly related to the pH of potassium ethyl xanthate solution for pH values from 6.5 to 10.