Ag2s Solubility in Sulfide Solutions up to 250°C in Order to Estimate Possible Silver Sulfide Complexes in Ore Solutions, the S
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Geochemical Journal, Vol. 21, pp. 291 to 305, 1987 Ag2S solubility in sulfide solutions up to 250°C ASAHIKO SUGAKI', STEVEN D. SCOTT', KENICHIRO HAYASHI', and ARASHI KITAKAZE1 Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Sendai 980, Japan' and Department of Geology, University of Toronto, Toronto M5S 1A1, Canada' (Received December 2, 1987: Accepted February 22, 1988) In order to estimatepossible silver sulfide complexes in ore solutions, the solubilityof Ag2Swas measuredbetween 25° and 250°Cin NaOH-H2S-H20solutions of 0.0 to 4.1m NaHSconcentration. Solu bilitychanges as a functionof temperature,total reducedsulfur concentration ES (mHS + mHS-)and pH. A maximumsolubility of 2140ppmAg was obtained at 250°Cin 4.1m NaHSconcentration under PH 2Sof 29.1 atm. From these solubility data, reactions to form silver sulfide complexes are estimated as follows: Ag2S + H2S = Ag2S(H2S), Ag2S + H2S + HS = Ag2S(H2S) (HS)-, Ag2S + H2S + 2HS = Ag2S (H2S) (HS)22 and Ag2S + 2HS = Ag2S (HS)22-. The equilibrium constants for these reactions are given in Table 2. Assuming that such silver sulfide complexes as above are in an ore solution , Ag2S (argentite or acanthite) is precipitated in response to changes of temperature, pH and total sulfur concentration (ES). Decrease of pH and ES is more effective than that of temperature. Silver sulfide complexes are more important than silver chloride complexes in ore solution of high ES, neutral to slightly alkaline pH and geologically reasonable chloride concentrations. INTRODUCTION to the experimental study by Seward (1973), Thermochemical data for aqueous metal bisulfide complexes are significant for the trans species at elevated temperatures and pressures port of gold in ore solutions under epithermal are indispensable for understanding transport of conditions, even if chloride complexes have the metals in hydrothermal solution. With these ability to transport fairly large amount of gold data, the physico-chemical conditions that are at higher temperatures (Henley, 1973). In light necessary if ore solutions are to transport of the above, there arises the possibility that enough metal to form ore deposits can be evalu silver is also transported as sulfide complexes ated. Furthermore, processes of ore deposition under epithermal conditions. The stability of such as cooling, pressure change, reaction with chloride complexes of silver up to 350°C have wallrocks, mixing of solutions (including dilu been determined by Seward (1976), but the tion) and boiling (Skinner and Barton, 1973) thermodynamic data for silver sulfide complexes can be estimated quantitatively. are scarce, particularly at temperatures more Chloride is the important complexing ligand than 100°C. Also there is disagreement about because it is usually the most abundant anion in the stoichiometry and stability of silver sulfide ore solutions, although numerous other candi complexes reported at 25°C. Cloke (1963) dates as complexing ligands have been proposed measured the solubility of acanthite at 25°C in (Barnes, 1979; Barnes and Czamanske, 1967). weakly to strong alkaline sodium sulfide solu Bisulfide ion is also known to be strong metal tions, and determined that silver was dissolved complexing agents at relatively low temperatures as polysulfide complexes such as Ag(S4)2'-, such as in epithermal environments. Silver is an AgS5S43 and Ag(HS)S42-. Anderson (1962) important ore metal in epithermal deposits, summarized the data of Treadwell and Hepen commonly occurring with electrum. According strick (1949) and Schwarzenbach et al. (1958) 291 292 A. Sugaki et alt which showed that silver was dissolved as the evacuated glass tube method (Sugaki and Ag2S(H2S)2 in weakly acid solution saturated Shima, 1965; Scott, 1974) at 400°C from with H2S at 25°C. From the results of radio elemental silver (99.999% pure) and sulfur chemical measurements at 25°C in the presence (99.99% pure). of excess sulfide (0.02m), Schwarzenbach and The procedures in the solubility experiments Widmer (1966) suggested that Ag2S dissolved were as follows. Ag2S (massive blocks) of 10 as AgHS at pH<4, Ag(HS)2 at pH 5-9 and to 25g and the required calculated amount Ag2S(HS)22 at pH> 9. The solubility of Ag2S of NaOH were loaded in the reaction vessel. was also measured between 100° and 180°C After attaching the valve assemblage, the vessel in H2S-saturated neutral to acid solution by and valve assemblage were evaculated with Ol'shanskii et al. (1959) and Melent'yev et al. a rotary vacuum pump to between 5 and 7 (1970) using a radioactive tracer technique, but X 10-3 torr. H2S gas was bled into the vessel they could not determine stoichiometry of the from a small H2S cylinder connected to the valve silver complexes. Recently, Gammons and assemblage. The amount of H2S gas transfered Barnes (1987) measured solubility of Ag2S in was controlled by warming the cylinder with near-neutral solutions buffered by H2S(aq) and water and by fixing the elapsed time during, HS in the temperature range 25° to 300°C. which the valve was open to the reaction vessel. They estimated that silver was dissolved to form The amount of H2S gas loaded was measured by sulfide complex as Ag(HS)2-. weighing the small cylinder before and after the In this study, we have measured the solu H2S gas injection. After the valve assemblage bility of Ag2S (acanthite, which converts to was again evacuated, H20 was loaded from a argentite above 177'C) in NaOH-H2S-H2O separatory funnel into the reaction vessel using solutions from 25° to 250°C, and have deter a hydraulic pump. Distilled and deionized water mined the stoichiometries and stabilities of the was used, the was boiled for a half hour just silver sulfide complexes. before the experiment in order to remove dis solved gases. The amounts of NaOH, H2S and H20 in the reaction vessel for each experiment SoLUrnm EXPERIMENTS are listed in Table 1. Samples of the solution in Experimental method the reaction vessel were extracted under run Our solubility experiments were carried out conditions by means of the sample tube method at Pennsylvania State University (500-series described by Barnes (1963, 1971), Crerar and experiments) and Tohoku University (100-series experiments) using Barnes volumetric hydro Table 1. Amount of NaOH, H2S and H20 in the reaction thermal systems with 1.11 chrome or teflon vessel for each experiment. lined rocking autoclaves. A detailed description NaOH H20 H20 Run No. of the apparatus and reaction vessels are in (g) (g) (ml) Barnes (1963, 1971, 1981). 102 83.997 74.969 700 Temperature was controlled with solid state 103 4.000 24.734 660 104 1.000 21.827 590 proportional controllers and was measured by 105 0.529 22.815 610 potentiometer within an accuracy of ± 0.1 ° C 106 0.138 25.104 670 107 0.119 24.742 660 using chromel-alumel thermocouples which were 108 19.972 26.646 745 inserted into wells in the top and bottom ends 109 27.050 25.860 733 of the reaction vessel. The temperature gradient 501 0.000 44.575 600 502 104.770 105.840 592 between two ends was maintained at less than 503 104.800 88.330 594 1°C. Total pressure was measured by Heise 504 24.000 62.990 589 505 24.070 22.170 583 Bourdon gauges (600kg/cm2 and 4000psi) 506 24.050 20.360 578 within ± 0.1 kg/cm2. Ag2S was synthesized by Ag2S solubility in sulfide solutions up to 250°C 293 250 0 CD0 0 C/0 CD CC) pH = neutral H2S(aq)/ HS 200 o 00 0 0 coo 00 0 U 0 a) 0 150 0 Cm CD o 0 lm 0 0 CE) L a) CL E a) I 100 0 CD 00 CD 0 m CE) 50 0 0 0000 00 2 3 4 5 6 7 8 9 I0 II pH Fig. 1. Temperature pH conditions of solubility experiments. Predominance regions of H2S(aq) and HS and the neutral pH curve are also shown. Barnes (1976) and Giordano and Barnes (1979) sulfur. The solution was then acidified with or directly by glass syringe connected with HNO3, and was analyzed for Ag by atomic capillary tube to the valve assemblage. Sampled absorption spectrophotometry. XRF was also solutions were passed through a teflon filter used to know approximate concentration of Ag (pore size, 2µm) attached to valve assemblage. before atomic absorption analyses. Generally, two or three samples were extracted at each temperature after a reaction period of Experimental results more than 50h. Each sampling temperature at The measured Ag2S solubilities are shown in 25° or 50°C intervals was approached from both appendix 2 together with estimated activities of lower and higher temperatures. According to H2S and HS-, total concentrations of Na and S Anderson (1962) and Giordano and Barnes and pH calculated by the method given below (1979), reaction rates for forming sulfide com and in the appendix 1. The experiments are plexes are very high, so equilibrium should have arranged in the order of increasing pH in the been achieved in our experiments as was, in fact, table of appendix 2. The measured concentra demonstrated by the reversals. The temper tion of Ag in the solutions range from 0.03 to ature and pH taken out samples in the experi 2140ppm. Maximum solubility was obtained in ments are in Fig. 1. run No. 502 at 250°C with 4.1 m NaHS and Sampled solutions were treated with excess 29.4atm PH2s. In general, the Ag2S solubility NaOH solution followed by 30% H202 in order increases in proportion to temperature, particu to convert reduced sulfur species to sulfate. The larly in the runs at high sulfur concentration.