外文翻译---厌氧的硫代硫酸盐浸出法-就地浸金工艺的研究.docx

附 录anaerobic thiosulfate leaching: development of in situ gold leaching systemsj.a. heath, m.i. jeffrey *, h.g. zhang, j.a. rumballabstractferric edta and ferric oxalate complexes are both effective oxidants for the aerobic and anaerobic dissolution of gold in thiosulfate solutions, and therefore are potential candidates for the development of an in situ leaching system. the thiosulfate and polythionates were quantified during leaching using hplc with perchlorate eluent and an anion exchange column, and it was found that both the iron edta and oxalate complexes have a low reactivity with thiosulfate, and they do not react with thiourea when it is added as a leaching catalyst. anaerobic leaching experiments showed that both systems were still active after seven days leaching, and when 1 mm thiourea was present, there was significant gold dissolution. however in the absence of thiourea, the gold leaching was very slow, and hence the addition of thiourea as a gold oxidation catalyst is required for the iron(iii) leaching systems. when anaerobic leaching was carried out in the presence of finely ground pyrite, the iron(iii) complex was rapidly reduced to iron(ii) as a result of the pyrite catalysed oxidation of thiosulfate. pyrrhotite was also found to be problematic as it directly reduced the iron(iii) complex, and therefore the quantity of gold leached was significantly lower in the presence of both these sulfide minerals. these problems associated need to be overcome if such a system is to be used in an in situ leach environment.1. introductionin situ leaching has been in use since the mid 1970s inthe united states and the former soviet union for producing refined uranium (mudd, 2001a, b). it has recently been implemented at beverley (2000), and is soon to be used at honeymoon well in south australia. it has also been utilised to recover copper (dandrea et al., 1977), and soluble salts such as halite, trona, and boron (bartlett, 1992), and potash from phosphate rock (habashi and awadalla,1988). the famous frasch process for mining sulfur with superheated water may also be consider as an in situ leaching process. however, in situ leaching technology has not been adopted for the recovery of gold, even though there are a number of deposits which have favourable characteristics, including the victorian deep leads.the victorian deep leads are buried alluvial gold bearing gravels, deposited in ancient valleys about 3060 million years ago. since that time the valleys have filled with sand, gravel, water, clay and other minerals, and the leads now lie up to 100 m below the surface. they are below the water table, however the water is slow moving at only a few meters per year (anon, 1982). the resource is very extensive at least 700 km are known to exist in victoria around the bendigo, ballarat and avoca areas. as it is an alluvial gold deposit, the content is highly variable, but averages approximately 4 g/m3. the thickness of the leads varies up to 5 m, and the width up to 1 km (anon, 1982). the sulfur content is typically low, varying from 1% to 5% (phillips and hughes, 1996), with marcasite being the major sulfide present with some pyrrhotite. a further complication is the presence of lignite, which has the potential ofpre- robbing gold from cyanide solutions. during the early 1980, cra limited undertook extensive research into the use of in situ leaching with cyanide to mine gold from the victorian deep leads. this work was unsuccessful for both technical reasons (availability of oxidant) and due to environmental concerns with pumping cyanide underground. any future development of the leads would need to be compa- tible with the current use of the groundwater resource in farming, as well as having strategies for environmental monitoring and rehabilitation in place. this leads to the visualisation of a process which uses a relatively benign gold leaching system, potentially based on thiosulfate as a ligand.the copper-ammonia-thiosulfate system suffers from several drawbacks, most notably, the reduction of copper(ii) ammine by thiosulfate, producing copper(i) thiosulfate and tetrathionate (byerley et al., 1973). this causes the gold leach rate to decrease as the reaction proceeds (breuerand jeffrey, 2000), and hence dissolved oxygen is required to oxidise copper(i) thiosulfate back to a copper(ii)ammine. this has been demonstrated by a number of authors, and a good summary of thiosulfate leaching experiments undertaken in the presence and absence of air sparging are presented by senanayake (2007). since the addition or control of oxygen underground is likely to be difficult, the copper-ammonia system is unlikely to be effective for an in situ leaching environment. however two promising iron(iii) oxidants have been independently developed; one using ethylenediaminetetraacetic acid(forming fe (edta)_) (zhang et al., 2005), and the other using oxalate (forming fe(c2o4)33_) (chandra and jeffrey,2005). both of these complexes are very stable, with logb(fe(edta)_) = 25.1 (smith and martell, 1989) and logb(fe(c2o4)33) = 18.6(smith and martell, 1977). the main advantage in using iron(iii) based oxidants is that they have been claimed to unreactive towards thiosulfate(compared to copper(ii) ammine therefore, they may potentially be used in an anaerobic environment, since oxygen may not be required to regenerate the oxidant. another advantage of the iron(iii) oxidants is that ammonia is not required to stabilise the oxidant. this enables their potential implementation into environments where legislation is strict on the use and release of ammonia into the environment.the focus of this work was to evaluate alternative oxidants for the in situ, thiosulfate leaching of gold. emphasis was placed on evaluation of iron(iii) based oxidants in anaerobic or low-oxygen environments, to simulate conditions likely to be found in deposits such as the victorian deep leads.2. experimental methods2.1. aerobic leachingthe aerobic leaching experiments were carried out using a standard glass electro- chemical cell. the main cell compartment had a luggin capillary with a reference electrode attached through the bottom by a water-proof screw fitting.the gold sample used was a rotating disc electrode(rde), 17 mm , 99.99% purity). the reference electrode used was an activon single junction saturated calomel reference electrode (0.244 v vs. she). the mixed potential was monitored using a radiometer copen- hagen pgz301 potentiostat, and the working electrode was connected to pine rotator with a variable speed drive.fig. 1. chromatograms at 200 nm for solutions containing 5 mmthiourea, 5 mm iron(iii), and either 5 mm edta (top) or 12.5 mmoxalate (bottomfig. 2. chromatograms at 200 nm for solution containing 1 mmthiourea,6 mm thiosulfate, 2 mm trithionate, 2.6 mm tetrathionate, and 0.9 mmpentathionatethe experiments were conducted at ambient laboratory temperature (22 2 _c), and the rotation speed was maintained at 300 rpm. the working electrode surface was prepared by polishing using struers waterproof silicon carbide paper (fepa p#2400). the electrode was then rinsed with deionised water. all aerobic leach solutions were prepared from deionised water and analytical grade reagents. the order of mixing the reagents is important to prevent the oxidation of thiosulfate by iron(iii);the thiosulfate was added to the solution already containing the ferric chloride and ligand. the ph was measured using a tps ph meter(wp-80), with attached double junction ph probe (glass membrane, tps) and resistance temperature detector(rtd). the ph was adju- sted manually using dilute solutions of sodium hydroxide and sulfuric acid (_0.1 m).2.2. anaerobic leachinganaerobic leaching studies were carried out in a custom built, sealable glovebox. the glovebox contained one large main compartment, and a smaller side compartment separated by an air-lock door. gloves were incorporated into the side of the main compartment to manipulate the interior contents whilst sealed. the glovebox internal atmosphere was controlled by bleeding gas into the main compartment. the gas flow rate was maintained such that the pressure inside the glovebox was slightly greater than atmospheric pressure (_0.1 l/min). oxygen concentrations inside the glovebox atmosphere and dissolved in the leach solutions were monitored using a syland 4000 dissolved oxygen meter and probe.fig. 3. comparison of feedta (closed symbol, solid line) and feox(open symbol, dashed line) for aerobic gold leaching in the ofthiourea at ph 7 and 5.5, respectively.a six-position magnetic stirrer mat and control panel was positioned inside the glovebox to spin teflon coated magnetic stirrer bars in each beaker (250 ml). gold sheet (12.5 _ 13 _ 1.5 mm, 99.99% purity) was suspended in the leach solution using a polypropylene crossbar and nylon thread. prior to each experiment all gold sheet surfaces were polished using struers waterproof silicon carbide paper (fepa p#1200), followed by rinsing several times with deionised water. rotation of the teflon stirrer bars was maintained at 200 rpm. leach solutions were prepared in the same manner as for aerobic studies, and then placed in the glovebox on the magnetic stirrer mat. gas was bled into the glovebox overnight to obtain the desired equilibrium internal atmosphere composition. after this equilibration, the leach solution ph was checked and adjusted if required. if required, a small volume of slurry of active sulfide mineral (freshlyground in a ceramic ball mill overnight to give a p80 of10 lm) was also pipe- tted into each beaker to give a concentration of 5 g/l mineral. the gold sheet was then manually suspended in the leach solution, and this was taken as initiation of the experiment. at specific times, samples were withdrawn from the leach solutions and a sub-sample from these was stabilised for gold analysis by addition of a small,known volume of alkaline cyanide solution. after completing the experiment the sub-samples were collectively analysed by icp-oes. a volume change correction was applied to the sample times, in accordance with the procedure outlined by choo et al. (2006).2.3. solution speciationsolution speciation was carried out using a high performance liquid chromatog- raphy (hplc) system.the hplcsystem comprised a dionex as16 strong base anionexchange column (4 _ 250 mm) with ag16 guard, coupled with a waters 2695 separation module. detection of uvactive components was accomplished using a waters 2996 uv photodiode array (pda) detector (k = 190400 nm). an isocratic elution with perchlorate at 1 ml/min flow rate was used to separate the anionic sample components.waters empower software was used for analysis of component peak areas, and the concentration determined based on comparison to calibration standards.3. results and discussion3.1. hplc based solution speciation for leach solutionsinitially the use of 200 mm perchlorate as the eluent,which has previously been used for the analysis of copper-ammonia-thiosulfate leach solutions (jeffrey and brunt, 2007), was tested for quantification of solutions containing thiourea (tu), ammonium thiosulfate (ats),polythionates, and either the ferric oxalate (feox) or ferric edta (feedta) complexes. it was found that although the negatively charged iron(iii) complexes eluted between the thiosulfate and thiourea, there was considerable overlapping of the peaks. however when the perchlorate concentration was reduced to 125 mm, a much better separation of the thiourea and the iron(iii) complexes was obtained. this is shown in fig. 1; the chromatogram at 200 nm for a 1 ll injection of solution containing 5 mm thiourea, 5 mm iron(iii), and either 12.5 mm oxalate or 5 mm edta. therefore 125 mm perchlorate was adopted as the eluent in the analysis of solutions from both aerobic and anaerobic leaching experiments.the chromatogram at 200 nm for a 5 ll injection of a standard solution containing 1 mm thiourea, 6 mm thiosulfate,2 mm trithionate, 2.6 mm tetrathionate and 0.9 mm pentathionate is shown in fig. 2 for the 125 mm perchlorate eluent. all these species are eluted within 13 min, and the peaks were identified by their uv spectrum,and also by comparison to the retention time of individual standards. if greater sample throughput is required, the run time for each sample can be decreased to 8 min by increasing the eluent flowrate from 1.0 to 1.6 ml/min. in terms of quantification, the pda detector produces a 3 d matrix of absorbance vs. wavelength vs. time, and hence the chromatograph can be displayed for any wavelength.to obtain maximum sensitivity, thiosulfate, tetrathionate and pentathionate were quantified at 214 nm, trithionate quantified at 192 nm, and thiourea quantified at 235 nm.3.2. aerobic leaching of goldinitial testwork was undertaken to compare the leaching kinetics of gold in thiosulfate solutions containing thiourea and either the feedta or feox complexes. for these experiments (and the anaerobic testwork), ammonium thiosulfate was chosen as the lixiviant, as the ammonium salt is significantly cheaper than the sodium salt. electrochemical studies have also shown that there is little difference in leaching between the sodium and ammonium salts when thiourea is present, so they can be interchanged (chandra and jeffrey, 2004). by using a rotating disk, the mass transfer and geometric surface area are both constant between and during experiments. fig. 3 shows the kinetic plot of gold measured in solution vs. the corrected time(based on volume correction due to sampling) (choo et al., 2006). it is clear from fig. 3 that the leaching of gold was more rapid for the oxalate system than for the edta system. two variations of each system were tested: (1) high reagent concentrations, with 50 mm thiosulfate, 5 mm thiourea, 5 mm iron, and either 5.5 mm edta or 12.5 mm oxalate; and (2) low reagent concentrations, with 25 mm thiosulfate, 2 mm thiourea, 2 mm iron, and either 2.2 mm edta or 4 mm oxalate. the ph of the edta and the oxalate systems were 7.0 and 5.5, respectively.not surprisingly, the use of high reagent concentrations resulted in more rapid leaching, although it is interesting to note that the gold leach rate in the low reagent oxalate system was similar to the high reagent edta system.table 1 calculated initial gold leach rates, and the rate between 5 and 7 h. alsoshown is the measured mixed potential (em)table 1 shows the initial gold leach rate (010 min) and the gold leach rate between 5 and 7 h for each of the experiments.for comparison, the oxygen diffusion limiting rate for gold leaching in air saturated cyanide solutions at 300 rpm is 55 lmol/m2 s (jeffrey and ritchie, 2000). for each of the leaching systems, it can be seen from both fig. 3 and table 1 that the initial leaching was more rapid, most likely as a result of the higher initial surface reactivity created by polishing. as the leaching proceeded, the leach rate decreased, approaching a constant value. table 1 also shows the mixed potentials measured during the leaching experiments, with those for the two oxalate systems being more positive than those for the two edta systems. this is consistent with the higher leach rates measured with the oxalate systems. it is interesting to note though that the mixed potential was similar for the two oxalate systems,and also for the two edta systems, even though the leach rate was higher for the high reagent systems. this is consistent with the low reagent systems having a lower thiourea concentration, and hence the gold oxidation half reaction was less active for the low reagent systems.fig. 4. formation of trithionate, tetrathionate, and pentathionate duringthe aerobic leaching experiments shown in fig. 3: j low reagent feedta;h low reagent feox; _ high reagent feedta; e high reagent feox.another important aspect of the ferric leaching systems that makes them ideal for an in situ process is the low reactivity of the iron(iii) complex with thiosulfate. this is illustrated in fig. 4, which shows the formation of polythionates during the aerobic leaching experiments. it is clear that the quantity of polythionates formed in each of the experiments was very low, verifying the claims of low reactivity of the iron(iii) complexes with thiosulfate in previous publications (zhang et al., 2005; chandra andtable 2 calculated change in thiosulfate concentration due to its oxidation, andthe corresponding change in iron(iii) concentration for each of theaerobic leach testsjeffrey,2005). this was consistent with the observation that the solutions remaining air saturated during the leaching experiments. the thiourea concentration was also measured for each sample, and this was found to be unchanged over the duration of the experiment. for the high reagent feedta system, trithionate was the main polythionate formed, whereas for the high reagent feox system, tetrathionate was the major product.using a sulfur balance, the loss of thiosulfate can be calculated for each of the systems, and then from an electron balance, the quantity of iron(iii) reduced can be determined. the equations used for these types of calculations are outlined in a previous publication (jeffrey and brunt, 2007), and the data is shown in table 2. it is interesting to note that the thiosulfate and iron(iii) loss was the highest for the high reagent feedta system, and lowest for the low reagent feedta system. for the feox system,the calculated iron(iii) loss was actually higher for the low reagent system than the high reagent system. this was most likely because a lower oxalate to iron ratio(2:1) was adopted for the low reagent system, and hence the concentration of the more reactive fe(c2o4)+ complex will be higher (chandra and jeffrey, 2005).3.3. anaerobic leachingthe anaerobic leaching of gold was initiall