GUEST AUTHOR John B. Brown1
Battelle Memorial Institute c ~I, Ohio
Textbook errors, 26
I
~Solubility ~ ~of
I t is not surprising that many chemists have the erroneous impression that gold is rather soluble in liquid mercury. Of all the textbooks which have been'e~amined,~ only one explicitly states that the amalgamation process for gold recovery depends on the wetting of gold by mercury and not on the solubility of gold in mercury. The amalgamation process is now becoming obsolete, but when it is discussed, many authors either state or imply that it works because the gold dissolves in mercury. A commercial application of the fact that gold does not dissolve in mercury is to be found in the "goldadhesion" filter.3 In this device a ring of gold alloy surrounds an aperture through which mercury passes. Floating contaniinants (oxide scum, water, oils, etc.) are prevented from working down between the mercury and the sides of the container and out the aperture by the adhesion of mercury to the easily "wet" gold ring. (12). Purity approaching that of triple distilled mercury is claimed when this filtration is preceded by vigorous aeration. At least two previous publications (9, K?) have pointed out that textbook explanations of the gold amalgamation process have been misleading. However, continued promulgation of this error in the most recent texts would seem to justify repetition in THIS JOURNAL of the accurate solubility data. These textbook discussions probably result from the statements about the gold amalgamation process in several of the more frequently consulted reference works. One of them states that "gold is miscible in mercury in all proportions." Similar statements are to he found in other treatises on inorganic chemistry and even in metallurgical reference works. The solubility figures given in one case (7) actually are values for the solubility of mercury in solid gold (2), not for gold in mercury! The opacity of mercury suspensions (or solutions) of metals precludes an easy observation of homogeneity. This fact alone is probably the chief reason for the assumption that gold is soluble in mercury. Visual observation will disclose nothing about the homogeneity Suggestions of material suitable far this column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as many details as possible, and particularly with references to modern textbooks, to Karol J. Mysels, Department of Chemistry, University cf Southern California, Los Angeles 7, California. ' On leave from Denison University, Granville, Ohio. a Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. To be presented, the error must occur in at least two independent standard books. Available from The Bethlehem Apparatus Company, Inc., Hellertown, Pa.
Gold in ~ ~ Mercury
of the system. Several authors (5, 4, 13) have discussed the definition of solubility and particle size in relation to the problem of determining whether a given mercury-metal system is a saturated solution or a suspension of small metal particles in the mercury. Most investigations of metal-in-mercury solubility have used some sort of filtration process to separate the solution from suspended particles. Because of the variability of pore sizes for the different filtering media, it has been suggested that reported values for metal solubilities in mercury tend to be too high rather than too low. As early as 1855 Henry (8) had noted that the solubility of gold in mercury was 0.14y0 and in 1878 Kasantseff (10) observed the gold content of saturated liquid amalgam was only 0.11, 0.126, and 0.65% a t 0, 20, and 100°C, respectively. In 1903, Richards (19) made reference to three different amalgams of gold. The first is a saturated solution of gold in liquid mercury containing about O.lyogold. His second amalgam is a solid gold mercuride. (Richards suggested the formula Au3Hg,. Later evidence would indicate that this second amalgam is probably a solid solution of mercury in Au3Hg.) Richards' third form is "nuggets of gold superficially coated with and cemented together by the other two forms of amalgam." Very careful determinations of the solubility of gold in mercury were made in Professor Sunier's laboratory at University of Rochester (1, 11, 22, 25, 24) and by Plaksin in Russia (14, 16, 18) about 30 years ago. More recently, Strachan and Harris (21) have studied the solubility of a large number of metals in mercury. The results of these studies are in agreement with the solubility values given above and indicate that gold should be considered only very slightly soluble in mercury. Table 1. Selected Values of Low Temperature Solubility of Gold in Mercury Temperature ("C) 0 6.96 20. 00
"Room" 20.00 39.98 60.32 80.8 101.2
Solubility (atom %)
Reference
011 0.1006 0126 0.128 0.1290 0.2045 0.3152 0 467 0.697
(10) ($9) (10)
(el) ($9) (83) ($3) (9%
(W)
The accompanying table presents selected values of the low temperature solubility of gold in mercury. The gold-mercury phase diagram shown in the figure is drawn principally from data of Plaksin (18) except that the liquidus curve from 100 to 400°C is drawn from Volume 37, Number 8, August 1960 / 415
amounts of gold (16, 17, 80). The pressed amalgam is then retorted to distill off the mercury dissolved in (and chemically bound to) the gold. The ingots cast from the retorting residue usually contain more than 80% gold, perhaps 16y0 silver, and small amounts of base metals such as copper and iron (5,19). The process outlined above works because gold does not dissolve in the mercury. If gold were soluble in mercury to any great extent, very little gold would be recovered from retorting the pressed amalgam. Literalure Cited (1) ANDERSON, J. T.,J . Phps. Chem., 36, 2145-65 (1932). (2) DAY, H. M., AND MATHEWSON, C. H., Am. Insl. Mining Met. Engr., Inst. Metals Diu. Tech. Publn. No. 884, 20 pp. (1938); C. A., 32, 1551'. (3) DERIGHT, R. E., J . Phys. Chem., 37,405-16 (1933). (4) DEWET,J. F., AND HAUL, R. A. W., 2. anmg. U. allgem. Chem., 277, 96-112 (1954). (5) DORR,J. V. N., AND BOSQUI,F. L., "Cyanidation and Con-
Figure 1.
Gold-mercury phore diagram
Sunier's solubility data (I), and the solid solution of mercury in gold is drawn according to Day and Mathewson (8). The eutectic temperature is about 0.1' below the freezing point of mercury and corresponds to almost pure mercury. Polymorphic transformations of AuHg, are represented by horizontal lines a t -36 and 122". The dashed line in the upper left corner of the diagram has been sket,ched in to show the vaporization of mercury. Perhaps these pages are an appropriate place to direct textbook authors' attention to Gmelin's statement: "Die Goldamalgamation ist ein komplizierter, noch nicht vollkommen geklarter Vorgang" (6). ("The amalgamation of gold is a complicated, and as yet incompletely explained process.") Mechanical mixing, chemical bonding, wetting, solution and alloying all ent,er into the process. The gold in the crushed ore is wet by the mercury and held on the amalgamating plate (Hudson (9) suggests a fly paper analogy); chemical reaction of gold with mercury then proceeds very slowly. Periodically, the amalgam is scraped from the plates, cleaned, and then squeezed through chamois skin or fine canvas to give cakes containing 40 to 50% gold (5). The mercury recovered from this operation is usually used over again in the mill, since chemically pure mercury is not as effectivea wett.ing agent for gold as is that which contains small
416
Journol o f Chemical Education
centration of Gold and Silver Ores," 2nd ed., McGrawHill Book Co., h e . , New York, 1950, pp. 1364, 177-81. (6) "Gmclins Handbuch der Anorganischen Chemie," System Na. 62, 8th ed., Verlag Chemie, Berlin, 1954, p. 289. (7) HAYWARD, C. R., "An Outline of Metallurgical Practice," 2nd ed.. D . Van Nostrand Co., Inc.. New York. 1940. p. 399. (8) HENRY,T. H., Phil. Mag., [11, 9, 458 (B55). (9) HaosoN, D. R., Metallurgia, 29, 299-303 (1944). (10) KASANTSEFF, M., Bull. soc. ehim., [2], 30, 20; B ~ i tAbstr., . 1- R7X. . - Q- -R .7.~ (11) MEES,G., J . Am. Chem. Soc., 60, 870-1 (1938). (12) NIEMAN, H. W., AND NIEMAN, C. W., U.S. Patent 2,583,438, Jan. 22, 1952. (131 . . PARKS.W. G.. AND PRIME.G. E.. J . Am. Chem. Soc... 58.. 141i-14 (1986). (14) PLAKSIN,I. N., J . RUSS.Phgs.-Chen~.Soc., 61, 521-34 (1929); C. A,, 24, 290. 115) PLAKSIN, I. N.,Z. Melallkunde, 24, 89 (1932). (16) PLAKSIN,I. N., Swelskaya Zololoprom., No. 9-10, 20-5 (1933); C. A , , 28. 57W2. (17) PLAKSIN,I. N., AND KOZE~UKHOVA, M. A,, Ann. secteur platine, Inst. ehim. g&. ( U . 8 . S. R.), 13, 95-111 (1936); C. A , 31, 1335'. (18) PLAKSIN,I. N., Ann. secteur anal. phys.-chim., inst. ehim. gdn. ( U . S. S. R.), 10, 129-59 (1938); C. A., 32, 3245*. (19) RIcnAans, R. H., "Ore Dressing," The Engineering and Mining Jmcmal, New York, 1903, vol. 11, p. 731. (20) RICHARDS,R. H., AND LOCKE,C. E., "Textbook of Ore
.
Dressing,'' 3rd ed., McGraw-Hill Book Co., Inc., New York, 1940, pp. 52ff. (21) STRKHAN,N. F., AND HARRIS,N. L., J . Inst. Metals, 85, 17-24 (1956). (22) SUNIER,A. A., AND GRAMKEE, B. E., J . Am. Chem. Soe., 51, 1703-8 (1929). (23) SUNIER,A. A., AND WHITE,C. M., J. Am. Chem. Soe., 52, 184'2-50 (1930). (24) SUNIER,A. A., AND WEINER,L. G., J . Am. Chem. Soc., 53, 1714-21 (1931).
