THE ADSORPTION OF COPPER SULFATE BY SPHALERITE AND ITS RELATION TO FLOTATION S. FREDERICK RAVITZ AND WILLIAM A. WALL Utah Engineering Experiment Station, University of Utah, Salt Lake City, Utah Received August 11, 1933
In the recovery of sphalerite (ZnS) by the flotation process, the mineral must almost invariably be activated, i.e., a soluble heavy-metal salt (in practice, copper sulfate) must first be added before the mineral can be successfully floated. The problem of the activation of sphalerite by copper sulfate has been the object of a great deal of attention, and the general consensus of opinion is that the copper sulfate reacts with the zinc sulfide to form the more insoluble cupric sulfide at the surface of the sphalerite (1). Although Taggart (2) seems to consider the film to be cuprous sulfide, and Cox and Wark (3) are skeptical as to the completely chemical nature of the activation, the formation of a film of cupric sulfide appears to be the most probable explanation from the experimental evidence which exists. If the formation of a film of cupric sulfide according to the reaction ZnS
+ C u + + = CuS + Zn++
be accepted as a working hypothesis, the thickness of the film is a matter of considerable interest. In order to obtain information concerning this, a series of tests was made to determine the amounts of copper adsorbed by various sizes of sphalerite. The mineral was carefully sized and deslimed as described by Kidd and Wall (4). Since cupric ion reacts with metallic if-on, it was very essential to remove all traces of the latter (which might have been introduced into the mineral in the crushing and grinding processes) from the sphalerite. Most of the iron was removed by means of a strong electromagnet, the mineral being cleaned with the magnet both before and after grinding and before and after sizing. To remove the final traces, the sized and deslimed mineral was leached for eighteen hours with 6 normal hydrochloric acid, and then thoroughly washed with copper-free distilled water and allowed to dry. A 50-g. sample was weighed out and treated with 1 normal hydrochloric acid to remove any oxide film which might have formed during drying. The acid was washed out as before, and 250 cc. of 0.02 M copper sulfate was added. The solution was very carefully analyzed iodimetrically with 13
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S. FREDERICK RAVITZ AND WILLIAM A. WALL
0.01 N thiosulfate before it had been added to the mineral and also after it had been in contact with the mineral for varying lengths of time. It was found, however, that practically all the adsorption took place during the first minute, there being no appreciable change in the concentration of the solution after that length of time. In column 4 of table 1 are given the amounts of copper adsorbed per gram of sphalerite for the various sizes studied. Some of the results obtained by Kidd and Wall (4) are also of interest in this problem. Using 0.15 lb. of potassium ethyl xanthate and 0.15 lb. of cresylic acid per ton of sphalerite, they found that best flotation recoveries were obtained with 0.1 Ib. of CuS01.5Hz0 per ton of mineral for particles larger than 104 microns, with 0.2 Ib. per ton for particles between 104 and 74 microns, and with 0.3 lb. per ton for particles finer TABLE 1
PARTICLE SIZE
MILLIGRAMS OF COPPER P E R GRAM OF ZINC S U L F I D E F O R MONOMOLECUL A R FILM
1
MILLIGRAMS OF COPPER P E R GRAM O F ZINC S U L F I D E FOR MAXIMUX RECOVERY
MILLIGRAMS OF COPPER PER ORAM O F ZINC B U L F I D E A D S O R B E D IN ADSORPTION TESTS
0.0127 0.0127 0.0127 0.0254 0.0381 0.0381 0.0381 0,0381
0.53 0.60 0.68 0.76 0.88 1.oo 1.20 1.40 1.60 1.80 2.20
"THICKNESS" OF FILM
microns
- 295+208 -2C8+147 - 147+104 - 104+74 -74+52 -52+37 -37+26 -26+18.5 -18.5+13 - 13+9.2 -9.2+6.5
0.01012 0.01433 0.02033 0.02867 0,04050 0.05733 0,08099 0.1147 0,1620 0.2298 0.3250
0.0381
0,0381 0.0381
51.3 41.8 33.5 26.5 21.7 17.5 14.8 12.2 9.9 7.8 6.8
than 74 microns. With greater or smaller amounts than these, the recoveries decreased. These results, expressed as milligrams of copper per gram of sphalerite, are listed in column 3 of table 1. In order to interpret the results of these two sets of experiments, it is first necessary to determine, for the various sizes, the amounts of copper required to form a monomolecular film on the surfaces of all the particles in one gram of sphalerite. This was done in the following manner, the -295+208 micron size being used as an example. For this size range, the average particle size is 0.0252 cm. If it be assumed that the particles are in the form of regular tetrahedra (the ideal crystal form of sphalerite) having edges of this length, the volume of each particle is 1.886 X cc. and the area is 1.10 X sq. cm. (For regular tetrahedra, I; = 0.11785 L3 and A = 1.732 L2, where L is the length of
ADSORPTION O F COPPER SULFATE B Y SPHALERITE
15
the edge.) Since the density of sphalerite is 4.1, the volume occupied by 1 g. is 0.244 cc. Dividing this by the volume of one particle gives 1.293 X lo5 as the number of particles per gram, which, multiplied by the surface area of one particle, gives 142.2 sq. cm. as the total surface area of 1 g. of the mineral. The lattice constant for sphalerite is 5.43 A.U. (5) and the unit cell contains two zinc atoms in one surface. Consequently there is one zinc atom in a surface area of 3 (5.43 X lo-* cm.)2, or 1.474 X 10-l5 sq. cm. Dividing this into the total surface area of 1 g. of the sphalerite gives 9.647 X 10l6as the number of atoms of zinc present in the surface of 1 g. of the -295+208 micron sphalerite. If, in accordance with the reaction given above, it be assumed that each of these zinc atoms reacts with a copper ion, this will also be the number of copper ions required to form a monomolecular film, and is equivalent to 0.01012 mg. of copper per gram of sphalerite. The corresponding amounts of copper for all the sizes studied are given in column 2 of table 1. DISCUSSION
A comparison of the second and third columns of table 1 shows that for particles larger than about 37 microns the amount of copper which gives the best flotation recovery is equal, within experimental error, to the amount required to form a monomolecular film. For smaller particles, however, maximum recoveries are obtained with considerably less copper than is required for a monomolecular film, and the difference increases rather rapidly with decrease in particle size, A probable explanation of these facts is that with the larger particles a practically complete monomolecular film is required in order t o provide sufficiently strong attachment to the air bubbles to enable the particles to remain attached throughout the flotation process; the reason that a greater amount of copper decreases the recovery may be due to reaction between the excess copper and xanthate, decreasing the concentration of the latter below its optimum value. (Kidd and Wall (4) found that 0.15 lb. of potassium ethyl xanthate per ton of sphalerite gave the optimum recoveries with particles larger than about 37 microns; with either more or less xanthate the yields were decreased.) With the smaller particles it seems quite likely that strong enough attachment could result with less than a complete monomolecular film, and since the weight of a particle decreases more rapidly than its area, the fraction of the area which would have to be filmed with copper would decrease with decrease in size. In column 5 of table 1 are given the “thicknesses” of the films of copper sulfide obtained in the adsorption experiments. These values were obtained by dividing the figures in the fourth column by the corresponding figures in the second, and are given purely for the purpose of comparison,
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8. FREDERICK RAVITZ AND WILLIAM A. WALL
since nothing is known of the arrangement of the molecules in the adsorption layer. Two facts, however, are immediately evident: first, the films formed on sphalerite in a solution of the strength used (many times that used in flotation practice) are apparently many molecules thick, and second, this “thickness” decreases as the particle size decreases. When the logarithm of the “thickness” is plotted against the logarithm of the average particle size, a straight line (figure 1) is obtained, which, when extrap-
Loyar/Yhmo f Average P a r f c / e Size
FIG. 1. VARIATIONOF APPARENTTHICKNESS OF ADSORBEDFILMWITH SIZE MICRONS)OF SPHALERITE PARTICLES
(IN
olated, indicates that a monomolecular film would result with particles 0.37 micron in size. The equation
where T is the apparent film thickness and d the particle size in microns, fits the curve very well. These facts are difficult to explain on the basis of familiar concepts. A possible explanation may lie in the change of surface forces with size; as the size decreases the initially formed layers of copper sulfide may become more compact, thus rendering diffusion more difficult, which would tend to decrease the thickness of the films. It is rather difficult, however, to conceive of any change in surface forces becoming appreciable in the larger sizes studied.
ADSORPTION O F COPPER SULFATE BY SPHALERITE
17
The recent theory of the mosaic or block structure of crystals leads to a more probable explanation. This theory, developed theoretically by Zwicky (6) and supported by the excellent experimental work of Goetz (7) on single bismuth crystals, states that all crystals have, besides their regular crystal structure, a much larger secondary structure manifested by periodic changes in density. Goetz found the resulting “blocks” to be approximately 1.4 microns in length. The work of Dean, Gross, Brighton, and St. Clair (8) indicates that in the case of galena the blocks have a length of less than 20 microns, and they consider it possible for certain molecules (e.g., H2S) to penetrate between these blocks. The results of the adsorption experiments can be explained by assuming that the copper ions penetrate to a certain extent between the elementary blocks of the sphalerite crystals, as well as forming a monomolecular film on the surfaces. When the individual blocks are liberated, only the amount of copper required to form a monomolecular film will be adsorbed, and from figure 1 it appears that for sphalerite the size of these blocks is 0.37 micron. It is obvious that the larger the particles, the more crevasses between blocks there will be for copper ions to diffuse into, and accordingly, the greater will be the apparent film thickness. This explanation does not necessarily conflict with that offered for the results of the flotation experiments. In the latter, the solutions were very dilute in copper and the total amount present was never appreciably greater than the amount required for a monomolecular film. It seems quite reasonable to suppose that the copper ions would react with zinc atoms in the surfaces exposed in preference to penetrating between the blocks. SUMMARY
1. With sphalerite particles larger than about 37 microns, the amount of copper sulfate which gives the best flotation recoveries is equal to the amount required to form a monomolecular film of cupric sulfide. 2. With smaller particles, maximum recovery is obtained with less than the amount of copper sulfate required for a monomolecular film, the difference between the amount required for maximum recovery and that required for a monomolecular film increasing with decrease in particle size. 3. A simple explanation is offered to account for these flotation results. 4. The amounts of copper adsorbed by sphalerite of various sizes from a 0.02 M copper sulfate solution are equivalent to surface films of copper sulfide many molecules thick. These “thicknesses,” however, decrease as the particles become smaller. 5. An explanation of the results of the adsorption tests by change of surface forces with size appears improbable. The theory of the mosaic structure of crystals, however, provides an explanation which agrees very satisfactorily with the results obtained.
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S. FREDERICK RAVITZ AND WILLIAM A. WALL
6 . The adsorption tests indicate that the unit blocks of sphalerite are approximately 0.37 micron in length. REFERENCES
(1) GATESAND JACOBSEN: Utah Eng. Expt. Station, Bull. 16, p. 53 (1925). TUCKER, GATES,AND HEAD:Trans. Am. Inst. Mining Met. Engrs. 73,354 (1926). GAUDIN,HAYNES,AND HAAS:Utah Eng. Expt. Station, Tech. Paper 7, p. 28 (1930). RALSTON, KING,AND TARTARON: Trans. Am. Inst. Mining Met. Engrs., Milling Methods, p. 389 (1930). RALSTON AND HUNTER: ibid., p. 401. GAUDIN:ibid., p. 417. (2) TAGGART: J. Phys. Chem. 36, 141 (1932). (3) C o x AND WARK:J. Phys. Chem. 37, 799 (1933). (4) KIDDAND WALL:Mining and Met. 14, 421 (1933). (5) WYCKOFF: The Structure of Crystals, 2nd edition, p. 229. Chemical Catalog Co., New York (1931). (6) ZWICKY: Proc. Nat. Acad. Sei. 16, 816 (1929). (7) GOETZ:Proc. Nat. Acad. Sci. 16, 99 (1930). (8) DEAN,GROSS,BRIGHTON, AND ST. CLAIR:Studies in Mineral Physics, U. S. Bureau of Mines 544-B, p. 6 (1933).
