The solubility of amorphousilica in water at high temperatures and high pressures

Transcription

1 Americn Minerlogis!, Volume 62, pges , 1977 The solubility of morphousilic in wter t high tempertures nd high pressures Rosnnr O. FounNrnn. S. Geologicl Suruey, Menlo Prk, Clifurni nno Jncr J. Rowe, S. Geologicl Suruey, Reston, Virgini Abstrct The solubility of morphous silic in wter t high tempertures nd high pressures ws investigted, using commercil gel nd vitreous silic (fused qurtz) s strting mterils. The experiments were crried out in n Inconel X bomb t tempertures from 180o to 382"C nd pressures from 200 to 1379 brs. Periodiclly, smll mounts of solution were withdrwn from the bottom of the bomb through stinlessteel filter into high-pressure stinlessteel cpillry smpling line, quenched in n ice bth, nd immeditely diluted with previously weighed silic-free wter. The diluted smples were nlyzed for colorimetriclly-rective nd totl silic. The silic gel gve the sme colorimetric silic s the vitreous silic. The solubility of morphousilic t the vpor pressure of the solution, from 0o to 250" C, is given by the eqution log C : -731/T , where C is the silic concentrtion in mglkg nd Z is bsolute temperture. The mximum solubility t the vpor pressure of solution is 1660 mg/kg t 340'C, nd the extrpolted solubility t the criticl point is 890 mglkg. At constnt pressure of 1034 brs, the solubility of morphousilic from 0o to 380oC is given by the eqution log C = The differentil het of solution, AF, is kcl,/mole, nd the differentil entropy of solution, AS, is 13.9 * 0.05 cllmole. Introduction tion nd then clculted solubilities t vpor pressure Much hs been published on the solubility of by extrpoltion of the high-pressure results. The morphous silic t tempertures below 100'C but curves of Figure I show results obtined by vrious reltively little of its solubility bove 100oCl Morey et workers for the solubility of morphou silic t the l. (1964\ concluded tht the combined results of vpor pressure of the solution compred with the Kithr (1960) nd Hitchen (1935) best described solubility of qurtz under similr conditions. the solubility from 0o to 335'C t the vpor pressure of the solution. In 1966 we (Fournier nd Rowe) Experimentl pprtus nd procedure published curve showing solubility vlues t tempertures bove bout 250"C significntly lower thn of Inconel X with n internl cpcity of 89 cms, The experiments were crried out in bomb mde those obtined by Hitchen. Becuse our 1966 results using experimentl pprtus similr to tht described nd illustrted by Morey nd Hesselgesser were probbly in error, we decided to study the solubility of morphous silic t high tempertures nd (1951). Becuse noble-metliners were not used, high pressures using technique tht would llow there my hve been some corrosion of the bomb nd quenching the solution to room temperture in few genertion of hydrogen gs during the experiments. seconds rther thn in minutes. In this work we used The finl solutions were not nlyzed to determine if pressures greter thn the vpor pressure of the solu- ny dissolved constituents were presentht could hve been contributed by corrosion of Inconel X or I Heitmnn (1965) stinlessteel. Becuse distilled wter ws used s the mesured the solubility of geltinous silic over wide rnge of tempertures (20" to 265'C) nd pressures (0.3 solvent, ny corrosion of the bomb would be expected to be smll, giving little interference. The to 400 kg/cm'z). However, most of his results pply to the trnsport of dissolved silic in suderheted stem. bomb ws suspended verticlly in doubly-wound I 052

2 FORNIER AND ROIIE: SOLBILITY OF AMORPHOS SILICA 1053 electric-resistnce furnce. Tempertures were monitored with thermocouples plced in wells t ech end 2000 of the bomb nd inside the bomb in stinlessteel o/ sheth projecting to the center of the smple chmber. The therml grdient long the length of the,o l bomb ws less thn 5oC, nd the therml grdient within the smple chmber ws probbly much less 6r20o J."o thn tht. The temperture vrition with time t ny 2'- 800 given point long the bomb during run ws +2C. -Ou' (I Periodiclly, smll mounts of solution were ^(Q'' -' withdrwn for 400 Fs"r"" nlysis through stinlessteel cpillry /^t\l n\)v' tube nd vlve t the bottom of the bomb. A stinless steel filter with men pore opening of 20 microns o r ws plced inside the bomb t the tip of the cpillry rsmpernrurr 'c smpling tube. Ner the exit end of the cpillry tube, Fig. l. Solubility in wter of morphous silic reltive to qurtz six-inch-long segment ws immersed in n ice bth t the vpor pressure of the solution. Along the qurtz curve, the to bring the temperture solid circles re from Morey nd others (1962), the hlf-filled of the solution below l5oc circles from Kennedy (1950). Along the morphousilic line, the before it emerged t tmospheric pressure. During hlf-filled circles re from Kithr (1960), the hollow circles from smpling, the pressure in the bomb ws mintined Hitchen (1935), the solid circles from Fournier nd Rowe (1966); nerly constnt by mens of n utomticlly ctuted pump tht trnsferred silic-free distilled w- dt plotted in Fig. 4. the dimonds represent results extrpolted from higher-pressure ter into the top of the smple chmber when the pressure dropped below preselected vlue. An externl high-pressure wter reservoir with cpcity known mount of silic-free wter. This procedure rectly into two previously weighed flsks contining of 2 liters prevented lrge pressure drops nd surges reduced the concentrtion of dissolved silic to during smpling. vlue below the solubility of morphous silic in wter t 25oC, nd thereby prevented the polymerizlion of silic fter the solutions were brought to Strting mteril Commercilly vilble vitreous nd geltinousilic were used s strting mterils. The vitreou silic or more dys, second smpling ws crried out t room temperture tmospheric pressure. After one ws Generl Electric Compny type 204 cler fused the sme temperture nd pressure to verify the ttinment of stedy stte, presumbly equilibrium. qurtz rod (0.5 cm in dimeter broken into 3 to 5 cm lengths). Approximtely 57-grm portions were used The temperture nd,/or pressure ws then chnged to fill the bomb ech time renewl of the chrge ws to different vlue nd the process repeted. required. Initilly, the vitreous silic ws nhydrous, One portion of ech of the diluted solutions ws but undoubtedly the surfces quickly hydrted t the then nlyzed immedihtely for colorimetriclly-rective silic, using modifiction of the molybdenum high temperture nd high wter pressures of the experiments. The geltinousilic ws mesh blue method of Shpiro nd Brnnock (1956); second portion ws nlyzed for totl silic by tretment certified high-purity mteril from the Fisher Chemicl Compny, grde 923. Approximtely 35-grm with NOH to convert ll polymerized species to portions were used in the experiments. colorimetriclly-rective species (Morey et l., 1962). All dilutions were done by weight, nd replicte nlyses generlly greed within 20 mg/kg for the initil Smpling nd nlyticl procedure The bomb nd chrge were mintined t temper. colorimetric silic nd within 100 mglkg for the totl ture nd pressure for time sufficient to ttir, silic. The colorimetric silic vlues for successive chemicl equilibrium, usully of the order of 2 to 5 smples collected within few minutes of ech other dys. Ech time the bomb ws smpled, the first 2 hd men verge difference of 0.94 percent, with milliliters of solution were discrded in order to rinse stndrd devition of 1.39 nd stndrd error of the the smpling tube nd to ssure tht the next portions men of In the sme smples the totl silic of solution to be nlyzed cme directly from the vlues hd men verge difference of 2.46 percent, min chmber of the bomb. Two smples of pproximtely 0.5 gm were then successively collected di- of the men of with stndrd devition of 3.62 nd stndrd error 0.38.

3 1054 FORNIER AND ROWE,' SOLBILITY OF AMORPHOS SILICA r0,000 I r I,OOO O J roo 25" 50' roo' r5o' 2oo' 3oo" 4oo'c 30 25?O IOOO/ T (.K) Fig 2 The solubility of morphous silic t 1034 brs compred with the solubility t the vpor pressure of the solution, plotted on logrithmic scle reltive to the reciprocl of bsolute temperture. Along the vpor pressure line, the smll circles re from Kithr (1960) below l00oc nd Hitchen (1935) bove 100'C The squres re from Fournier nd Rowe (1966); the dimonds represent results extrpolted from high-pressure dt plotted in Fig. 4 Along the 1034 br line, the circles show colorimetriclly-rective silic in solution using fused qurtz s the strting mteril, the tringles the colorimetriclly-rective silic using geltinous silic s the strting mteril. The verticl lines extending bove the tringles show the rnge in totl silic vlues obtined from geltinous silic strting mteril. r5 Results The experimentl results'z re plotted in Figures 2, 3, nd 4. The experimentl uncertinties re bout * I percent in silic, t2 percent in pressure, nd t3'c in temperture. Within the nlyticl precision the sme mounts of colorimetric nd totl silic were obtined using vitreous silic s the strting mteril. The geltinous silic gve the sme colorimetric silic s the vitreous silic, but higher nd errtic mounts of totl silic (Fig. 2). The errticlly high totl-silic vlues were probbly produced by smll mounts of very fine-grined solid prticles of silic tht disggregted from the geltinous strting mteril nd pssed through the 20-micron filter during smpling. In Figure 2, the stright line drwn through the results for runs t 1034 brs ws determined by lest-squres fit of the dt. The eqution of the line 1S logc: -810/T+4.82, 'To receive tble listing the experimentl results, order document AM from the Business Office, Minerlogicl Society of Americ, Suite 1000 lower level, 1909 K Street, N W, Wshington. D C Plese remit$l 00 in dvnceforthemicrofiche. (l) (n G (D cr l t o- r SILICA, PPM Fig. 3. The solubility of morphous silic (colorimetriclly-rective silic in solution) t vrious constnt tempertures nd vrible pressure. The circles show the distribution ofthe experimentl results, the tringles clculted solubilities using reltions shown in Figs.2 nd 4 The dotted line shows the solubility results t the vpor pressure of the solution

4 FORNIER AND ROWE: SOLBILITY OF AMORPHOS YLICA 1055 where C is the colorimetric silic concentrtion in mg/kg nd T is bsolute temperture. The R2 vlue for the lest-squres fit is The errors in the slope nd intercept of the regressed line re *6 nd *0.01 respectively. The lest-squres stright line through the dt for runs t the vpor pressure of the solution from 0o to 250oC is given by the eqution logc: -731/T+4.52, (2) nd the R2 vlue gin is The errors in the slope nd intercept of the regressed line re t2l nd t0.01 respectively. Following the ssumption of Morey et l. (1962), the prtil moll or differentil het of solution, Ail, of morphous silic t pressure of 1034 brs cn be clculted from eqution l; AF equls kcl/mole ( cl : I joule). This compres to LH of kcl/mole for qurtz t pproximtely 1000 brs. The chnge in free energy, AG, obtined by dissolving morphous silic t 1034 brs pressure is pproximtely given by the eqution LG = -RTloEC,:810R - 304, (3) where Cr is the concentrtion of dissolved silic in moles/kg of wter nd R is the universl gs constnt. The prtil moll entropy of solution, A,S, cn be obtined from the eqution,s : -(6AG/67)p.x: 3.04R, (4) nd AS equls 13.9 t 0.05 cllmole. In Figure 3, the solubility results t constnt temperture nd vrible pressure lie long stright line t 200oC. At higher temperture, the results deprt from stright line, nd progressively more curvture is shown. Interpoltion nd extrpoltion of the dt re difficult using this type of digrm. When the solubility vlues re plotted reltive to the density of the pure solvent, however, s in Figure 4, the dt t constnt temperture fll long nerly prllel stright lines over the entire rnge of the tempertures mesured, 200' to 382oC, t pressures between 1034 brs nd the vpor pressure ofthe solution. Discussion The similr mounts of colorimetric silic found in solution using vitreous silic nd geltinous silic suggest tht most morphous silic mterils will hve bout the sme solubility irrespective of the mnner of preprtion. However, very fine-grined or highly-stressed mteril could give higher solubility vlues. Evidently the fused qvrtz rods used in this study initilly hd highly stressed zones running E F = 70 F =64 o 20 loo,,r,o.,jo^lo lo'ooo Fig.4. The solubility in wter of morphous silic (colorimetriclly-rective silic) s function of the density ofthe pure solvent. The tringles re from Kithr (1935) nd Hitchen (1960) The circles show results using fused qurtz nd the squres geltinous silic s strting mterils. The dimonds indicte where isotherml lines, emnting from given tempertures long the 1034 br curve nd drwn prllel to other nerby isotherml lines, ttin densities equl to the vpor pressure of pure wter t the given tempertures. These dimonds define the morphous silic solubility curve t the vpor pressure of the solution bove 300oC, ssuming the density nd vpor pressure of the dilute solutions to be bout equl to pure wter. lengthwise through their centers tht dissolved more rpidly thn the surrounding glss; t the end ofech series of experiments, ech of the rods hd centrl cluster of l0 to 20 tiny "pin holes" of chnnels extending from one end to the other. At tempertures bove 300'C, the solubility vlues obtined by extrpolting higher-pressure results to the vpor pressure of the solution re significntly lower thn Hitchen's vlues (1935) nd higher thn our 1966 results (Fournier nd Rowe, 1966). It is likely tht Hitchen's grvimetric determintions of dissolved silic re too high bove 300oC, for cristoblite probbly nucleted nd grew in his solutions while morphous mteril continued to supply nutrient. This behvior ws observed in some of the experiments performed s prt of the work reported here.

5 1056 FORNIER AND ROWE: SOLBILITY OF AMORPHOS SILICA Our 1966 results re probbly too low, s three to five minutes were required to cool nd open the bombs used for those experiments. During tht reltively slow quench, some silic could hve precipitted onto the wlls of the continer or onto the morphous silic strting mteril. Four experiments reported by Heitmnn (1965) re in the temperture nd pressure rnge of our experiments. His pprtus ws bsed on continuous flow principle, nd he used both grvimetric nd colorimetric methods of nlysis for dissolved silic. nfortuntely specific nlyticl procedures re not given. Presumbly he determined totl silic crried by the solution flowing from the utoclve. His results re similr to the vlues we obtined for totl silic using geltinous silic strting mterils nd bout l0 to 15 percent higher thn our colorimetric silic vlues. At the vpor pressure of the solution, tking the solubility vlues extrpolted from higher pressure dt s more relible thn previous direct mesurements, the mximum solubility is 1660 mg/kg t 340'C nd 890 mg/ke t the criticl point. These dt cn be used to set the mximum limit for dissolved silic tht is likely to be found in hydrotherml systems, nd therefore my be useful in solving problems of scling in the utiliztion of geotherml energy. References Fournier, R. O. nd J. J. Rowe (1966) Estimtion of underground tempertures from the silic content of wter from hot springs nd wet stem wells. Am. J. Sci., 264, '1. Heitmnn, H. G. (1965) Solubility of silic in wter nd wter vpor. Glstech. Ber., 38, 4l-54. Hitchen, C. S. (1935) A method for experimentl investigtion of hydrotherml solutions, with notes on its ppliction to the solubility of silic. Trns. Inst. Mining nd Metllurgy, 44, 25s-280. Kennedy, G. C, (1950) A portion of the system silic-wter. Econ. Geol., Kithr, S. (1960) The polymeriztion of silicic cid obtined by the hydrotherml tretment of qurtz nd the solubility of morphous silic. Reu. Phys. Chem. Jpn, 30, l3l-137. Morey, G. W. nd J. M. Hesselgesser ( 195l) The solubility of some minerls in superheted stem t high pressures. Econ. Geol.,48, 82 l , R. O. Fournier nd J. J. Rowe (1962) The solubility of qurtz in wter in the temperture intervl from 29o to 300"C. Geochim. Cosmochim. Act snd - (196) The solubility of morphous silic t 25"C. J. Geophys. Res.,69, Shpiro, L. nd W. W. Brnnock (1956) Rpid nlysis of silicte rocks.. S. Geol. Suru. Bull C, Mnuscript receiued, Mrch 28, 1977; ccepted for publiction, My 16, 1977.