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1.
Reactive surface area is one of the key parameters for studying the kinetics of mineral dissolution. The common practice in experimental kinetics is to normalize the dissolution rate to the surface area measured by the BET method. The relationship between BET surface area and the reactive surface area is not trivial in minerals such as smectites, which possess both internal and external surface areas, and in which the dissolution is controlled by the chemical attack on the edge surface. The present study examines two proxies for the reactive surface area of the Clay Mineral Society reference smectite SAz-1: BET surface area and the edge surface area measured using AFM.Since smectites are very microporous, their BET surface area is strongly influenced by the degassing procedure. It is demonstrated that outgassing the smectite powder at 135°C in a 15 mL min−1 N2 gas flow for at least 24 hours minimizes contribution from micropores to less than 11% of the BET surface area.Following dissolution experiments in solutions with a low electrolyte concentration, the BET surface area increased from 34 ± 2 m2 g−1 in raw SAz-1 to 127 ± 13 m2 g−1 in SAz-1 sample recovered from dissolution experiments. This increase in BET surface area is explained by a decrease in the average size of the smectite aggregates, and by an increase in microporosity due to the depletion in the major interlayer cation, i.e., Ca2+. As the BET surface area of the raw smectite sample includes considerably less microporosity compared to the BET surface area of the smectite recovered from dissolution experiments, the former is a better approximation of the external surface area of the dried sample powder.AFM measurements show that there is no correlation between the specific external surface area of the sample and its specific edge surface area. This observation is explained by the platy morphology of the smectite particle in which the specific external surface area depends linearly on the height reciprocal, whereas the specific edge surface area is independent of the particles height and depends linearly on the sum of the reciprocals of the length of the axes. Therefore, there is no reason to expect a correlation between the BET and the edge surface area. Our results show that the edge surface area (4.9 ± 0.7 m2 g−1) of the smectite particles cannot be predicted based on its external surface area (136 ± 20 m2 g−1). Therefore, the BET surface area cannot serve as a proxy for the reactive surface area. We suggest using AFM measurements of the specific edge surface area as an alternative proxy for the reactive surface area of smectite.  相似文献   

2.
Surface coatings are very common on mineral grains in soils but most laboratory dissolution experiments are carried out on pristine, uncoated mineral grains. An experiment designed to unambiguously isolate the effect of surface coatings on mineral dissolution from any influence of solution saturation state is reported. Two aliquots of 53 to 63 μm anorthite feldspar powder were used. One was dissolved in pH 2.6 HCl, the other in pH 2.6 FeCl3 solution, both for ∼6000 h in flow-through reactors. An amorphous Fe-rich, Al-, Ca- and Si-free orange precipitate coated the anorthite dissolved in the FeCl3 solution. BET surface area of the anorthite increased from 0.16 to 1.65 m2 g−1 in the HCl experiment and to 3.89 m2 g−1 in the FeCl3 experiment. The increase in surface area in the HCl experiment was due to the formation of etch pits on the anorthite grain surface whilst the additional increase in the FeCl3 experiment was due to the micro- and meso-porous nature of the orange precipitate. This precipitate did not inhibit or slow the dissolution of the anorthite. Steady state dissolution rates for the anorthite dissolved in the HCl and FeCl3 were ∼2.5 and 3.2 × 10−10 molfeldspar m−2 s−1 respectively. These rates are not significantly different after the cumulative uncertainty of 17% in their value due to uncertainty in the inputs parameters used in their calculation is taken into account. Results from this experiment support previous theoretical and inference-based conclusions that porous coatings should not inhibit mineral dissolution.  相似文献   

3.
Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10−10 molfeldspar m−2 s−1, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10−10 molfeldspar m−2 s−1 and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 molfeldspar g−1 s−1. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10−12 molbiotite m−2 s−1, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10−12 molbiotite m−2 s−1, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10−12 molbiotite m−2 s−1 and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10−12 molbiotite g−1 s−1. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly.  相似文献   

4.
The dissolution kinetics of shallow water marine carbonates (low-Mg calcite, aragonite and Mg-calcites) were investigated in seawater (S = 35) at 25°C and a PCO2 of 10?2.5 atm. using the pH-stat method. Carbonate dissoluton rates (μmoles g?1 hr?1) fit the empirical kinetic expression, R = k(1 - Ω)n, where R = dissolution rate, k = rate constant, Ω = saturation state, and n = order of reaction. Reaction orders were near 2.9 for low-Mg calcites, 2.5 for aragonites and 3.4 for Mg-calcites.The rate constant, k, expressed as μmoles g?1 hr?1, varied by nearly a factor of ten for the different samples, reflecting differences in amount of reactive surface area. Reactive surface area of the biogenic phases ranged from 0.3% to 66% of the total surface area determined by the BET gas adsorption method. The discrepancy between reactive and total surface area was greatest for samples with high BET surface areas (> 1 m2 g?1) and delicate microstructures.Relative dissolution rates of the various biogenic carbonates as a function of seawater calcium carbonate ion molal product (IMP) were related to both mineral stability and grain microstructure. In seawater undersaturated with respect to aragonite, finely crystalline aragonites dissolved more rapidly than thermodynamically less stable high Mg-calcites (15–18 mole% MgCO3) with lower reactive surface areas. Therefore, under certain conditions, differences in grain microstructural complexity can override thermodynamic constraints and lead to selective dissolution of a thermodynamically more stable mineral phase.  相似文献   

5.
Hematite concentrate was mechanically treated using different milling machines and experimental conditions in air atmosphere. The changes in phase constitution, particles size, specific surface area, lattice parameters and X-ray amorphous phase fraction of activated hematite were determined. It was found that the agglomeration of the particles take place during extended milling with accessible pores for Nitrogen gas. The higher media surface brought about the largest specific surface area whatever milling devices used. After 9 h of grinding with higher media surface, the maximum and minimum specific surface area resulted from the grinding in the tumbling and vibratory mills, accounting for 6.83 m2/g and 18.42 m2/g, respectively. For the same grinding condition, tumbling mill produced the lowest X-ray amorphous phase. The maximum X-ray amorphous material estimated around 85% from the grinding in the planetary mill with higher media surface for 9 h of milling.  相似文献   

6.
Dissolution rates of sediments obtained from the Oued Cherf reservoir were measured in closed-system batch reactors at 25 °C in fluids sampled concurrently from the same locations as the sediments. The BET surface areas of the sediments ranged from 16 to 45 m2/g and consisted primarily of quartz, calcite, and clay minerals. After a brief initial period, release rates of Si, Mg, Ca, Cl, SO4, and NO3 from these sediments are approximately linear with time over the course of the experiments, which lasted from 3 to 5 months. BET surface area normalized Si release rates ranged from 10–17.4 to 10–18.4 mol/cm2/s. These release rates match closely Si release rates from quartz and clay minerals determined from laboratory dissolution rates reported in the literature. This coherence suggests that laboratory measured silicate dissolution rates can be used with confidence to predict the dissolution behavior of sediments in natural surface waters.  相似文献   

7.
Four porous, glass-dominated rhyolites from Kozushima Island, different in age and extent of weathering, were studied. Because the four rhyolites are homogeneously weathered to considerable depth, and because their initial chemical compositions were equal, the different rock characteristics can provide information about rates of rhyolite dissolution and clay mineral formation over ∼52,000 yr. Because glass surfaces retreat without surface roughening, surface area (measured by Brunauer-Emmett-Teller method; BET) was assumed to be approximately constant over time. The field dissolution rate, as inferred from the rate of loss of Si, was ∼6 × 10−19 mol cm−2 s−1. The estimated clay mineral formation rate was ∼1 × 10−19 mol cm−2 s−1. About 20% of dissolved Si precipitated as clays. In order to investigate the factors affecting the field dissolution rate, dissolution experiments that used powdered and block rhyolite samples were conducted. Under relevant field conditions (20°C and pH 6∼7), the rates were ∼5 × 10−17 and ∼5 × 10−18 mol cm−2 s−1 for powdered rhyolite and blocks, respectively. The dissolution rates obtained in this study decrease in the order powder > block > field. Because all surface areas were directly measured by BET, the differences are not attributable to the errors in surface area. The most plausible explanations of the slower rates are the lower degree of flushing and resultant high-solution saturation states in the pores (both in the field and in the rhyolite blocks used in experiments) plus the formation of alteration/hydrated layers at the glass surface.  相似文献   

8.
The effect of dry milling in a vibratory mill on the structural changes and microstructural characteristics of hematite using different methods was investigated. We have described the line profile analysis (LPA) to extract the size of coherently diffracting domains and the lattice strain of activated hematite in a vibratory mill. The Warren–Averbach and Williamson–Hall methods were used as the main tools for characterization. The changes in the particle size, surface area and new phase formation of hematite concentrate were also investigated. It was concluded that the breakage and agglomeration of particles take place mainly at lower and higher levels of specific energy input, respectively. The pores in agglomerates remain accessible for the nitrogen gas. Milling of hematite increased specific surface area up to 18.4 m2/g. The hematite milled under various levels of specific energy input did not undergo a significant reaction or phase transformation during milling. The Williamson–Hall method confirms its merit for a rapid overview of the line broadening effects and possible understanding of the main causes. The anisotropic character of line broadening for deformed hematite as a function of specific energy input was revealed. Higher level of specific energy input favors the generation of small crystallite size, higher microstrain, BET surface area, amorphization and line breadth. The Warren–Averbach method suggested that the nanocrystalline hematite with grain sizes of 73.5–12.2 nm was formed by mechanical treatment using different milling intensities in the vibratory mill. The root mean square strain (RMSS) at L = 10 nm varies between 1.7 × 10− 3 and 4.0 × 10− 3 depending on the level of energy input. Limits in the applicability of Williamson–Hall method and reliability of the results are discussed in detail.  相似文献   

9.
The dissolution of chlorite with intermediate Fe-content was studied macroscopically via mixed flow experiments as well as microscopically via atomic force microscopy (AFM). BET surface area normalized steady state dissolution rates at 25 °C for pH 2 to 5 vary between 10−12 and 10−13 mol/m2.s. The order of the dissolution reaction with respect to protons was calculated to be about 0.29. For pH 2 to 4, chlorite was found to dissolve non-stoichiometrically, with a preferred release of the octahedrally coordinated cations. The additional release of octahedrally coordinated cations may be due to the transformation of chlorite to interstratified chlorite/vermiculite from the grain edges inward.In-situ atomic force microscopy performed on the basal surfaces of a chlorite sample, which has been preconditioned at pH 2 for several months, indicated a defect controlled dissolution mechanism. Molecular steps with height differences which correspond to the different subunits of chlorite, e.g. TOT sheet and brucite like layer, originated at surface defects such or compositional inhomogenities or cracks, which may be due to the deformation history of the chlorite sample. In contrast to other sheet silicates, at pH 2 nanoscale etch pits occur on the chlorite basal surfaces within flat terraces terminated by a TOT-sheet as well as within the brucite like layer. The chlorite basal surface dissolves layer by layer, because most of the surface defects are only expressed through single TOT or brucite-like layers. The defect controlled dissolution mechanism favours dissolution of molecular steps on the basal surfaces compared to dissolution of the grain edges. At pH 2 the dissolution of the chlorite basal surface is dominated by the retreat of 14 Å steps, representing one chlorite unit cell.The macroscopic and microscopic chlorite dissolution rates can be linked via the reactive surface area as identified by AFM. The reactive surface area with respect to dissolution consists of only 0.2% of the BET-surface area. A dissolution rate of 2.5 × 10−9 mol/m2s was calculated from macroscopic and microscopic dissolution experiments at pH 2, when normalized to the reactive surface area.  相似文献   

10.
《Applied Geochemistry》2001,16(13):1499-1512
The oxidative dissolution rate of metacinnabar by dissolved O2 was measured at pH ∼5 in batch and column reactors. In the batch reactors, the dissolution rate varied from 3.15 (±0.40) to 5.87 (±0.39) × 10−2 μmol/m2/day (I=0.01 M, 23°C) and increased with stirring speed, a characteristic normally associated with a transport-controlled reaction. However, theoretical calculations, a measured activation energy of 77 (±8) kJ/mol (I=0.01 M), and the mineral dissolution literature indicate reaction rates this slow are unlikely to be transport controlled. This phenomenon was attributed to the tendency of the hydrophobic source powder to aggregate and minimize the effective outer surface area. However, in a column experiment, the steady-state dissolution rate ranged from 1.34 (±0.11) to 2.27 (±0.11) x 10−2 μmol/m2/day (I=0.01 M, 23°C) and was also influenced by flow rate, suggesting hydrodynamic conditions may influence weathering rates observed in the field. The rate of Hg release to solution, under a range of hydrogeochemical conditions that more closely approximated those in the subsurface, was 1 to 3 orders of magnitude lower than the dissolution rate due to the adsorption of released Hg(II) to the metacinnabar surface. The measured dissolution rates under all conditions were slow compared to the dissolution rates of minerals typically considered stable in the environment, and the adsorption of Hg(II) to the metacinnabar surface further lowered the Hg release rate.  相似文献   

11.
《Applied Geochemistry》2006,21(2):269-288
Reliable quantification of mineral weathering rates is a key to assess many environmental problems. In this study, the authors address the applicability of pure mineral laboratory rate laws for dissolution of mill tailings samples. Mass-normalised sulfide and aluminosilicate mineral dissolution rates, determined in oxygenated batch experiments, were found to be different between two samples from the same ∼50-year-old, carbonate-depleted mill tailings deposit. Consideration of difference in particle surface area and mineralogy between the samples resolved most of this discrepancy in rates. While the mineral surface area normalised dissolution rates of pyrite in a freshly crushed pure pyrite specimen and a sulfide concentrate derived from the tailings were within the range of abiotic literature rates of oxidation by dissolved molecular O2, as were rates of sphalerite and chalcopyrite dissolution in the tailings, dissolution rates of pyrite and aluminosilicates in the tailings generally differed from literature values. This discrepancy, obtained using a consistent experimental method and scale, is suggested to be related to difficulties in quantifying individual mineral reactive surface area in a mixture of minerals of greatly varying particle size, possibly due to factors such as dependence of surface area-normalised mineral dissolution rates on particle size and time, or to non-proportionality between rates and BET surface area.  相似文献   

12.
The dissolution rate and mechanism of three different cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain, were studied in detail by vertical scanning interferometry techniques. A total of 37 different regions (each about 124 × 156 μm in size) on the three sample surfaces were monitored as a function of time during dissolution at 25°C and pH 3. Dissolution produced shallow etch pits with widths reaching 20 μm during 8 h of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface.On the basis of etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 × 10−11 mol cm−2 s−1 over 5 h. The time variation is different for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface can be computed by using a reference surface. The different surfaces yield an “average” rate of 1.08 × 10−11 mol cm−2 s−1 with a standard deviation of 0.3 × 10−11 mol cm−2 s−1 based on about 60 analyses. The mean absolute rate of the dolomite surface is about 10 times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data that used BET surface areas yield rates that are at least 30 to 60 times faster than our directly measured mean dissolution rate for the same pH and temperature.A conceptual model for mineral dissolution has been inferred from the surface topography obtained by the interferometry investigations. In this model, mineral dissolution is not dominated by etch pit formation itself but rather by extensive dissolution stepwaves that originate at the outskirts of the etch pits. These stepwaves control the overall dissolution as well as the dependence on temperature and saturation state.  相似文献   

13.
In this study, combining interpretations of conservative dissolved ions and environmental isotopes in water were used to investigate the main factors and mechanisms controlling groundwater salinization and hydrogeochemical processes in the Eastern Nile Delta, Egypt. Hydrogeochemical and isotopic study has been carried out for 61 water samples from the study area. Total dissolved solid (TDS) contents of groundwater are highly variable rising along flowpath from the south (410 mg/L) to the north (14,784 mg/L), implying significant deterioration and salinization of groundwater. Based on TDS and ionic ratios, groundwater samples were classified into three groups. In low-saline groups, water chemistry is greatly influenced by cation exchange, mineral dissolution/precipitation, anthropogenic pollutants and mixing with surface water. Whilst, in high-saline groups, water chemistry is affected by salt-water intrusion, reverse cation exchange and evaporation. The chemical constituents originating from saline water sources, reverse ion exchange and mineral dissolution are successfully differentiated using ionic delta and saturation index approaches. The δ18O–δ2H relationship plots on a typical evaporation line, suggesting potential evaporation of the recharging water prior to infiltration. Isotope evidence concludes that the groundwater have been considerably formed by mixing between depleted meteoric water recharged under different climatic conditions and recently infiltrating enriched surface water and excess of irrigation water. The δ18O data in conjunction with chloride concentrations provide firm evidence for impact of dissolution of marine-origin evaporite deposits, during past geologic periods, on groundwater salinity in the northern region. Moreover, the relation between 14C activities and Cl? concentration confirms this hypothesis.  相似文献   

14.
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15.
Apatite dissolution experiments were conducted using both a fluidized bed and stirred tank reactor over a range of pH, temperature, solution saturation state, and on non-carbonated and carbonated apatite compositions: igneous fluorapatite (FAP) and sedimentary carbonate fluorapatite (CFA), respectively. From 2 <pH <6, the rate of release from dissolution of all apatite components [calcium (Ca), phosphorus (P), and fluoride (F)] increased with decreasing pH for FAP. From 6 < pH < 8.5, the FAP dissolution rate is pH independent. Measuring apatite dissolution rates at pH > 8.5 were not possible due to detection limits of the analytical techniques used in this study and the high insolubility of FAP. For the CFA compositions studied, the dissolution rate decreased with increasing pH from 4 < pH < 7. During early stages of the dissolution reaction for both FAP and CFA, mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P and Ca:F being greater than mineral stoichiometric ratios, suggesting that Ca was preferentially released compared to P and F from the mineral structure during the early stages of dissolution. An increase in reacted solution pH accompanies this early elevated release of Ca. As the dissolution reaction proceeded to steady state, dissolution became congruent. When normalized to BET measured surface area, FAP dissolved faster from 4 < pH < 7 compared to CFA. The apparent Arrhenius activation energy (Ea) of FAP dissolution over the temperature range of 25-55°C at pH = 3.0, I = 0.1, and pCO2 = 0 is 8.3 ± 0.2 kcal mol−1. Both the apparent exchange of solution H+ for solid-bound Ca at low pH in the early stage of dissolution and the Ea of dissolution suggest a surface and not a diffusion controlled dissolution reaction for FAP and CFA. The degree of undersaturation of the solution, ΔGR, with respect to FAP was important in determining the dissolution rate. At pH = 3.0, I = 0.1, and pCO2 = 0, the dissolution rate of FAP was ∼ 5× greater in the far-from-equilibrium region compared to the near-equilibrium slope region.A simple apatite weathering model incorporating the experimental results from this study was constructed, and numerical calculations suggest that during the Phanerozoic both the surface area of igneous rock available for weathering and the average global temperature were important factors in determining the P weathering flux from apatite dissolution. It is possible that elevated global temperatures coupled with relatively high surface area of igneous rock during the early- to mid-Paleozoic resulted in elevated P weathering fluxes, which along with climatic evolutionary pressures of the Neoproterozoic, facilitated the radiation of multicellular organisms, large-scale phosphorite deposition, and abundance of calcium phosphate shelled organisms during the early Cambrian.  相似文献   

16.
The experiments of the dissolution kinetics of fluorite were performed in aqueous HCl solutions over the temperature range of 25–100 °C using a flow-through experimental apparatus. With a constant input of aqueous HCl solution through the reactor, output concentrations of the dissolved species Ca, F, Cl vary with flow rate, as well as with the surface compositions. Measured output concentrations of dissolved species and the pH can be used to determine a rate law for fluorite dissolution. Fluorite dissolution rates are found to be pH dependent. Usually, dissolution rates of fluorite decreases with increasing dissolved Ca in the output solution at 25 and 100 °C. Dissolution rate can be expressed as
(1a)
where k is the rate constant and α is the order with respect to the hydrogen ion activity vs. the activity of dissolved Ca. The α was obtained from kinetic experiments. For the fluorite sample passed through 18–35 mesh, α =1.198 at 100 °C and k = 10−0.983, while fluorite dissolved in HCl–H2O solution at pH 2.57 of input solution. Adsorption of a proton and Cl−1onto the fluorite surface, surface cation exchange and the formation of the surface complex Ca(F, Cl)2 and/or (H2x, Ca1−x)(F, Cl)2 control dissolution rates. Investigation of the fluorite surface before and after dissolution by using X-ray photoelectron spectroscopy (XPS) indicate that surface modifications affect reaction rates.  相似文献   

17.
Fe/C-based magnetically activated carbon （MAC） was obtained by carbonizing and activating its precursor, that was prepared by co-precipitation of anthracite coal impregnated in ferric chloride solution. The effect of the concentrations of FeC13 and pH of solution on BET surface area, pore volume and magnetic properties of the MAC was studied by BET N2 adsorption and VSM method. The results indicated that the magnetization of MAC gradually increases with increasing concentration of FeC13 and pH value of solution, and BET surface area was inclined to fluctuation. The largest BET surface area and magnetization of MAC were 1327.5 m2/g and 35.56 emu/g, respec- tively. The form of magnetic matter in the magnetically activated carbon was mainly Fe3C by X-ray powder diffrac- tion （XRD） and magnetic attraction test.  相似文献   

18.
The intensification of the thiosulphate leaching of silver from a silver-bearing complex sulphide concentrate (Casapalca deposit) using mechanochemical alkaline leaching as the pretreatment step was investigated. The leaching of “as-received” concentrate with the thiosulphate solution afforded only 6% recovery of Ag into leach. The mechanochemical pretreatment resulted 85% amorphization of tetrahedrite as a silver-bearing mineral in the concentrate as well as an increase in specific surface area from the original value of 0.26 m2 g−1 to a maximum value of 15.7 m2 g−1. The pretreatment was performed in a stirred ball mill using variable milling times and sample weights. The physico-chemical changes in the concentrate as a consequence of mechanochemical pretreatment had a pronounced influence on the subsequent silver extraction. The optimum results from mechanochemical pretreatment and subsequent leaching of the concentrate with ammonium thiosulphate were achieved by using a milling time of 60 min and a weight of sample of 60 g. In this case, 99% recovery of Ag was achieved after only 3 min of leaching.  相似文献   

19.
The paucity of weathering rates for quartz in the natural environment stems both from the slow rate at which quartz dissolves and the difficulty in differentiating solute Si contributed by quartz from that derived from other silicate minerals. This study, a first effort in quantifying natural rates of quartz dissolution, takes advantage of extremely rapid tropical weathering, simple regolith mineralogy, and detailed information on hydrologic and chemical transport. Quartz abundances and grain sizes are relatively constant with depth in a thick saprolite. Limited quartz dissolution is indicated by solution rounding of primary angularity and by the formation of etch pits. A low correlation of surface area (0.14 and 0.42 m2 g−1) with grain size indicates that internal microfractures and pitting are the principal contributors to total surface area.Pore water silica concentration increases linearly with depth. On a molar basis, between one and three quarters of pore water silica is derived from quartz with the remainder contributed from biotite weathering. Average solute Si remains thermodynamically undersaturated with respect to recently revised estimates of quartz solubility (<180 μM) but exceeds estimated critical saturation concentrations controlling the initiation of etch pit formation (>17–81 μM). Etch pitting is more abundant on grains in the upper saprolite and is associated with pore waters lower in dissolved silica. Rate constants describing quartz dissolution increase with decreasing depth (from 10−14.5–10−15.1 mol m−2 s−1), which correlate with both greater thermodynamic undersaturation and increasing etch pit densities. Unlike for many aluminosilicates, the calculated natural weathering rates of quartz fall slightly below the rate constants previously reported for experimental studies (10−12.4–10−14.2 mol m−2 s−1). This agreement reflects the structural simplicity of quartz, dilute solutes, and near-hydrologic saturation.  相似文献   

20.
Mineral carbonation is one of several alternatives for CO2 sequestration and storage. The reaction rates of appropriate minerals with CO2, for instance olivine and serpentine with vast resources, are relatively slow in a CO2 sequestration context and the rates have to be increased to make mineral carbonation a good storage alternative. Increasing the dissolution rate of olivine has been the focus of this paper. Olivine was milled with very high energy intensity using a laboratory planetary mill to investigate the effect of mechanical activation on the Mg extraction potential of olivine in 0.01 M HCl solution at room temperature and pressure. Approximately 30–40% of each sample was dissolved and water samples were taken at the end of each experiment. The pH change was used to calculate time series of the Mg concentrations, which also were compared to the final Mg concentrations in the water samples. Percentage dissolved and the specific reaction rates were estimated from the Mg concentration time series. The measured particle size distributions could not explain the rate constants found, but the specific surface area gave a good trend versus dissolution for samples milled wet and the samples milled with a small addition of water. The samples milled dry had the lowest measured specific surface areas (<4 m2/g), but had the highest rate constants. The crystallinity calculated from X-ray diffractograms, was the material parameter with the best fit for the observed differences in the rate constants. Geochemical modelling of mechanically activated materials indicated that factors describing the changes in the material properties related to the activation must be included. The mechanically activated samples in general reacted faster than predicted by the theoretical models. Mechanical activation as a pre-treatment method was found to enhance the initial specific reaction rates by approximately three orders of magnitude for a sample milled dry for 60 min in a planetary mono mill compared to an unactivated sample. Wet milling in the planetary mill did not produce samples with the same maximum reaction rate as dry milling, but wet milling in general might be easier to implement into a wet carbonation process. Mechanical activation in a planetary mill is likely to consume too much energy for CO2 sequestration purposes, but the increase in obtained olivine rate constants illustrates a potential for using milling as a pre-treatment method.  相似文献   

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