The dissolution of gold in the thiosulfate solution aerobic and anaerobic, the ferric EDTA (FeEDTA) and ferric oxalate (FeOx) complexes are effective oxidizing agent, thus They are all possible candidates for developing a leaching process. The amount of thiosulfate and polythionate during the leaching process was determined using an anion exchanger and high performance liquid chromatography (HPLC) using a perchlorate solution as a scrubber. It has been found that the complexes of EDTA and oxalate of ferric iron have low reactivity with thiosulfate, and they do not react with thiourea when thiourea is added as a leaching catalyst. The results of the anaerobic leaching test were negative, and the two complexes were still very effective after 7 d of leaching, and a considerable amount of gold was dissolved in the presence of thiourea. However, in the absence of thiourea, gold leaching is very slow. Therefore, for both Fe(II) leaching systems, it is necessary to add some thiourea as a catalyst for oxidized gold. When anaerobic leaching is carried out in the presence of finely ground pyrite, the complex of ferric [Fe(III)] is rapidly reduced to two due to the catalytic oxidation of thiosulfate by pyrite. Valence iron [Fe(II)]. It has been found that the presence of pyrrhotite also causes some trouble because they directly reduce the Fe(III) complex. Therefore, in the presence of these two sulfide minerals, the amount of gold leached is significantly reduced. If this process is to be used for in situ leaching of gold ore, these problems associated with the presence of sulfide minerals must be addressed first.
I. Overview
Since the mid-1970s, the in-situ leaching process has been used to produce refined uranium in countries such as the United States and the former Soviet Union. It has recently been applied at the Beveley mine and will soon be used in the Honeymoon Well mine in South Australia. It has also been used to recover copper and certain soluble salts, such as rock salt, trona and boron, and recovering from potassium phosphate rock. The famous Frasch method, which uses superheated water to extract sulfur, can also be considered as an in-situ leaching process. However, the in-situ leaching process has not been used to recover gold, although many deposits, including the Victorian Deep veins, have some Conducive to the characteristics of local leaching.
The Victorian Deep vein is buried with gold-bearing gravel, which was deposited in some ancient river valleys about 3000-6000 years ago. Since then, these valleys have been filled with large amounts of sand, gravel, water, clay and other minerals, and these veins are now below the surface of the earth, reaching a depth of 100 m. They are all below the water table, but the water only flows slowly at a few meters per year. This resource is very widely distributed and is known to extend at least 700km in the Bendigo, Ballarat and Avoca areas around the Victorian.
Because it is a gold deposit, the grade of gold fluctuates greatly, but the average grade is about 4 g/m 3 . The thickness of the vein can reach 5m, the width can reach 1km, the sulfur content is usually low, from 1% to 5%. The pyrite (FeS 2 ) is the main sulfide mineral and contains some pyrrhotite (Fe 7 S). 8 ). To further complicate matters, there are still a number of brown coal, which are possible from the cyanide solution "robbing the gold." In the early 1980s, CRA Mining began a detailed study of the process of recovering gold from the Victorian Deep vein with cyanide, but for technical reasons (source of oxidant) and cyanide for underground pumps The solution was environmentally friendly and failed to succeed. The future mining problems of these veins must be consistent with the current situation of groundwater resources used for farmland irrigation, as well as local environmental monitoring and remediation strategies. This requires finding an environmentally safe immersion gold process based on the use of thiosulfate as a ligand.
The copper-ammonia-thiosulfate system is hindered by several disadvantages. The most obvious is that the Cu(II) ammonia complex will be reduced by thiosulfate to produce Cu(I) thiosulfate and even four. Sulfate. This causes the leaching rate of gold to decrease as the reaction progresses, and thus requires dissolved oxygen to oxidize and restore the Cu(II) thiosulfate to the Cu(II) ammonia complex. This has been confirmed by many authors' research, and Senanayake published a review in 2007, which is a good summary of the thiosulfate leaching test with or without jet air.
Since it may be difficult to introduce oxygen or control oxygen levels underground, the key-ammonia leaching process is unlikely to be effective for in situ leaching. However, two promising oxidants have been found separately: one using ethylenediaminetetraacetic acid (EDTA) [forming Fe(EDTA) - ] and the other using oxalate [forming Fe ( C 2 O 4 ) 3 3 - ] complex. Both of these complexes are stable, logβ[Fe(EDTA)-]=25.1, LOgβ[Fe(C 2 O 4 ) 3 3 - ]=18.6. The main advantage of using Fe(II) based oxidants is that They are said to be less reactive with thiosulfate [compared to Cu(II) ammonia complexes]. So they are likely to be used in anaerobic environments because they may not require oxygen to regenerate the oxidant. Another advantage of Fe(III) based oxidants is that ammonia is not required to stabilize the oxidant. This makes it possible to apply them in some environmentally sensitive areas where strict control is applied to the use and release of ammonia into the environment.
The purpose of this research work was to evaluate several oxidants that were selected for the in situ leaching of gold from thiosulfate. Emphasis is placed on evaluating the performance of Fe(III)-based oxidants in anaerobic or low oxygen environments to simulate oxygen-containing conditions that may be present in deposits such as the Victorian Deep vein.
Second, the test method
(1) Aerobic leaching
The aerobic leaching test was carried out using a standard set of glass electrochemical reactors. The reaction compartment has a Luggin Capillary and a reference electrode in the main compartment, which is secured to the bottom by a waterproof bolt. The gold sample used was a rotating disk electrode (17 mm diameter, 99.99% purity). The reference electrode used was an Activon type single injection saturated calomel electrode (0.244 V relative to the standard hydrogen electrode potential). The mixed potential is monitored using a Radiometer Copenhagen PGZ 301 potentiostat, and the working electrode is connected to a variable speed drive rotor.
The tests were carried out at a laboratory temperature (22 ° 2 ° C) with a rotational speed of 300 r/min. The surface of the working electrode was polished and cleaned using a waterproof type silicon carbide sandpaper (FEPA P#2400). The electrode is then rinsed with deionized water. All aerobic leaching solutions were prepared with deionized water and analytically pure reagents. The order in which the reagents are mixed is important in order to prevent oxidation of the thiosulfate by Fe(III). The thiosulfate is added to a solution already containing ferric chloride and a ligand. The pH was measured using a TPS type pH meter (WP-80) equipped with a dual injection probe (glass membrane) and a resistance temperature indicator. The pH is manually adjusted using a dilute solution of sodium hydroxide and sulfuric acid (~0.1 mol/L).
(2) Anaerobic leaching
The anaerobic leaching test was carried out in a custom-made glove box with a large main compartment and a smaller secondary compartment separated by a ballast. Gloves are placed on the side of the main compartment to manipulate the contents in a closed state. The internal atmosphere of the glove box is controlled by discharging gas into the main compartment. The flow rate of the gas is maintained such that the pressure inside the glove box is slightly above atmospheric pressure (~0.1 L/min). The atmosphere inside the glove box and the concentration of oxygen dissolved in the leaching solution were monitored using a Syland 4000 dissolved oxygen meter and probe.
A six-speed magnetic stirrer and control panel were placed in the glove box to rotate the Teflon-coated stir bar in each beaker (250 mL volume). Gold sheet with a size of 12.5 × 13 × 1.5 mm and a purity of 99.99%, suspended in a leaching solution using a polypropylene crossbar and nylon thread. The surface of the gold sheet was all Strueys waterproof SiC sand before each test. The paper (FE-PAP #1200) was polished and then rinsed several times with deionized water. The speed of the Teflon stir bar was maintained at 200 r/min.
The leaching solutions were prepared in the same manner as in the oxygen-containing experimental study and then placed in a glove box. Gas was introduced into the glove box for one night to achieve the desired internal equilibrium atmosphere composition. After this equilibrium is reached, the pH of the leaching solution is determined and, if necessary, adjusted. If necessary, a small amount of reactive sulfide mineral slurry (finely ground in a porcelain ball mill for one night to achieve a P80 = 10 μm particle size) can be added to each beaker with a drip tube to achieve a concentration of 0.5 g mineral/L. The gold piece was then manually suspended in the leaching solution and this time was taken as the starting point for the test. Several samples were taken from the leaching solution for several specific times, and a small sample taken from these samples was stabilized, and gold was analyzed by adding a known amount of a small amount of alkaline cyanide solution. content. After the test was completed, several samples were collected and analyzed by inductively coupled plasma emission spectroscopy. The volume change of the sampling time is calibrated according to the method proposed by Choo et al.
(3) Determination of solution components
The determination of the solution components is carried out using a high performance liquid chromatograph. The high performance liquid chromatograph includes a Dionex AS16 strong alkaline anion exchange column (4 x 250 mm) and a set of AG16 guards with a Water2695 separation module. The detection of the UV-active component was performed using a Waters 2996 UV photodiode detector (λ = 190-400 nm). The perchlorate solution was eluted at a flow rate of 1 mL/min to separate the anion sample components. Waters Empower software was used to analyze the peak area of ​​the components. The concentration is determined based on comparison with the calibration standard.
Third, the results and discussion
(1) Determination of solution components in leaching solution by high performance liquid chromatography
Initially tested with 200 mmol/L perchlorate solution as an eluent for oxalate (FeOX) containing sulfur veins (Tu), ammonium thiosulfate (ATS), polythionates and ferric iron Or a solution of ferric EDTA complex (FeEDTA) for quantitative analysis. This perchlorate stripping agent has previously been used to analyze copper-ammonia-thiosulfate leaching solutions. Although the negatively charged Fe(III) complex is eluted between thiosulfate and thiourea, there is considerable overlap in the peak regions. However, when the concentration of the perchlorate solution was lowered to 125 mmol/L, the thiourea and Fe(III) complexes were well separated. This result has been shown in Figure 1. That is, at 200 nm, for 1 μL of a solution containing 5 mmol/L thiourea, 5 mmol/L Fe(III), and 12.5 mmol/L oxalate or 5 mmol/L EDTA. Therefore, in aerobic and When the solution obtained in the anaerobic leaching test was analyzed, a 125 mmol/L perchlorate solution was used as the eluting agent.
Figure 1 contains 5mmol / L thiourea, 5mmol / LFe (III) and 5mmol / L EDTA (above)
Or a chromatogram obtained by analyzing a solution of 12.5 mmol/L oxalate (bottom) at 200 nm
At 200 nm for 5 μL of injection containing 1 mmol/L thiourea, 6 mmol/L thiosulfate, 2 mmol/L trisulphate, 2.6 mmol/L sulphate and 0.9 mmol/L sulphate The chromatogram obtained when the concentration of the perchlorate eluent was 125 mmol/L is shown in Fig. 2. All of these components were eluted within 13 min and peaked by their UV spectra and by comparison to their respective standard residence times. To achieve greater sample throughput, the collection time for each sample can be reduced to 8 minutes by increasing the flow rate of the scrubber from 1.0 to 1.6 mL/min. In terms of quantitative representation, the Waters 2996 Ultraviolet Photodiode (PDA) side detector produces a 3D matrix of absorbance versus wavelength and time. Therefore, the chromatogram can be used to display any wavelength. In order to achieve the highest sensitivity, thiosulfate, tetrathionate and hypopentasulfate are quantified at 214 nm, while trithionate is quantified at 192 nm and thiourea at 235 nm.
Figure 2 For a solution containing mmol/L thiourea, 6mmol/L thiosulfate, 2mmol/L trisulphate,
Chromatogram of a solution of 2.6 mmol/L tetrahydrosulfate and 0.9 mmol/L pentane sulphate at 200 nm
(2) Aerobic leaching gold
A preliminary test was conducted to compare the leaching kinetics of gold in a thiosulfate solution containing thiourea and FeEDTA (or FeOX) complexes. For these tests, including the anaerobic immersion gold test, ammonium thiosulfate was chosen as the leaching agent because the ammonium salt is much less expensive than the sodium salt. The results of electrochemical studies have shown that when thiourea is present, the leaching results using the sodium salt and the ammonium salt are almost indistinguishable, so they are interchangeable. When tested using a rotating disc, the mass transfer and geometric surface area between the two and during the test were fixed. Figure 3 shows the relationship between the leaching kinetics of gold measured in solution and the calibration time (based on calibration of the volume due to sampling). As is clear from Figure 3, gold is more rapidly leached in the oxalate system than in the EDTA system. Two variables were tested in each system: (1) high reagent concentration, using 50 mmol/L thiosulfate, 5 mmol/L thiourea 5 mmol/L Fe(III), and 5.5 mol/L EDTA ( Or 12.5mmol/L oxalate); (2) low test concentration, using 25mmol/L thiosulfate, 2mmol/L thiourea, 2mmmol/LFe(III), and 2.2mmol/L EDTA (or 4 Mmmol/L oxalate). The pH values ​​of the EDTA and oxalate systems were 7.0 and 5.5, respectively. Not surprisingly, a faster leaching speed is achieved with high test concentrations, although it is worth noting that in oxalate systems at low reagent concentrations, the gold leaching rate is similar to at high reagent concentrations. The leaching rate of gold in the EDTA system.
Figure 3 shows the results of aerobic leaching of FeEDTA and FeOX in the presence of thiourea at pH values ​​of 7 and 5.5, respectively.
-50 mmol/L ammonium thiosulfate 5 mmol/L thiourea, 5.5 mmol/L ED-TA;
-25 mmol/L ammonium thiosulfate, 2 mmol/L thiourea; 2.2 mmol/LEDTA;
-50 mmol/L ammonium thiosulfate, 5 mmOl/L thiourea, 5 mmOl/LFe.12. 5 mmol/L oxalate;
-25 mmol/L thiosulfate; 2 mmol/L thiourea, 2 mmol/L Fe. 4 mmol/L oxalate
Table 1 shows the gold leaching rate for each initial (0-10 min) for each test, and the gold leaching rate between 5 and 7 h. For comparison, the limit rate of oxygen diffusion was 55 μmol/m 2 ·s for gold leaching at 300 r/min in an air-saturated sodium cyanide solution. For each leaching system, it can be seen from Figure 3 and Table 1 that the initial leaching is relatively fast, most likely due to the higher reactivity of the initial surface after sanding by sanding. As the leaching process progresses, the leaching rate is gradually reduced and reaches a stable value. Table 1 also shows the mixed potentials that were flanked during the leaching test. For both oxalate systems, the measured mixed potentials were higher than those in the EDTA system. This is consistent with the more distant leaching rate measured in the oxalate system. It is worth noting that the mixed potentials for the two different concentrations of the oxalate system are similar, and the mixing potentials for the two different concentrations of the EDTA system are similar, although the leaching rate is higher in systems with higher reagent concentrations. some. This is consistent with a lower concentration of thiourea in the low concentration reagent system. Therefore, for systems with lower reagent concentrations, the half-reaction of gold oxidation is less effective.
Table 1 calculates the initial gold leaching rate and the leaching rate between 5 and 7 h, also shows the measured mixed potential (EM)
Another important issue in making the Fe(III) leaching system an ideal in-situ leaching process is the low reactivity of the Fe(III) complex with thiosulfate. This is well illustrated in Figure 4. Figure 4 demonstrates the formation of polythionic sulfate during the aerobic leaching test. It is clear that the amount of polythionate produced during each test is very small, confirming that the Fe(III) complexes obtained in previous papers are very reactive with thiosulfate. Low assertion. This is consistent with the fact that the solution is still saturated with air during the leaching test. The concentration of thiourea was also determined for each sample, and it was found that the concentration of thiourea did not change throughout the test. For the high reagent concentration FeEDTA system, trisulphate is a major polythionate formed, while for high reagent concentration FeOX system, tetrahydrosulfate is a major product.
Figure 4 shows the formation of trisulphate, sulphate and sulphate during the aerobic leaching test shown in Figure 3.
â– -FeEDTA system with low reagent concentration; â–¡-FeOx system with low reagent concentration;
â—†-High reagent concentration FeDETA system; â—‡-high reagent concentration Feox system
Table 2 shows the variation of thioate concentration and the corresponding Fe(III) concentration in each aerobic leaching test calculated by oxidation.
Using the balance of sulfur, the amount of thiosulfate loss in each system can be calculated, and then the amount of reduced Fe(III) can be determined based on the electron balance. The equations used for this type of calculation have been summarized in previous papers and the relevant data are listed in Table 2. Interestingly, the loss of thiosulfate and Fe(III) was highest for the high reagent concentration FeEDTA system and the lowest for the low reagent concentration FeEDTA system. For the FeOX system, the calculated amount of Fe(III) lost in a system with a low reagent concentration is higher than the amount lost in a high reagent concentration system. This is most likely because a lower ratio of oxalate to iron (2:1) is used in systems with lower reagent concentrations, so the concentration of Fe(C 2 O 4 )+ complexes with higher activity is higher. Will be higher.
(3) Anaerobic leaching
The anaerobic leaching test was initially carried out in the absence of thiourea in an attempt to develop a more acceptable in situ leaching method for social and environmental aspects. Thiourea has been classified as a carcinogen according to the relevant material safety record sheet, and its adverse effects on aquatic organisms and underground organisms are well recognized. Because the in-situ leaching test work was more concerned with long-term leaching kinetics, the freshly prepared solution was aged in the glove box for 18 h before inserting the gold piece. Figure 5 shows the gold concentration in the leaching solution as a function of leaching time after calibration in a test using 50 mmol/L thiosulfate and different concentrations of Fe(III). Similar to the case of Figure 3, more gold was leached in the oxalate system than in the EDTA system at the same Fe(III) concentration. However, for all experiments, the gold leaching rate was very low, and the highest gold concentration after leaching for 7 days was only 0.52 mg/L. This corresponds to an average leaching rate of 2 × 10 -3 μmol/m 2 • s, significantly lower than the rate measured in an aerobic leaching system in the presence of thiourea. This result is consistent with the results of electrochemical studies. Electrochemical studies have demonstrated that the oxidation of gold in thiosulfate solutions is very slow in the absence of thiourea or copper ammine complexes.
Fig. 5 Effect of Fe(III) concentration on anaerobic leaching of gold in FeEDTA and FeOX systems in the absence of thiourea
â– -50mmol / L thiosulfate, 11 mmol / L EDTA, 10 mmol / L Fe, PH7;
â—†-50 mmol/L thiosulfate, 4.4 mmol/L EDTA; 4 mmol/LFe, pH 7;
â–²-50 mmol / L thiosulfate. 1.1 mmol/L EDTA. 1mmol/LFe, pH 7;
â–¡-50 mmol/L thiosulfate, 30 mmol/L OX, 10 mmol/L Fe, pH 5.5;
â—‡-50 mmol/L thiosulfate, 12 mol/Lox, 4 mol/L Fe, pH 5.5;
â–³-50 mmol/L thiosulfate, 30 mol/Lox, 1 mmOl/L Fe. pH 5.5.
In order to increase the gold leaching rate in the system containing no thiourea, the concentration of thiosulfate was increased to 100 mmol/L, and the results obtained are shown in Fig. 6. For the EDTA system, at higher thiosulfate concentrations, significantly more gold was leached, and the gold concentration reached 3.26 mg/L after 6 days of leaching. This is equivalent to a leaching rate of . 0.014 μmol/m 2 ·s, but still relatively low. In the case of the oxalate system, when the concentration of the thiosulfate is increased, more iron hydroxide precipitates, so when the thiosulfate salt concentration is 10 mmol/L, the leaching is performed. Not as effective as in the EDTA system.
Figure 6 in the presence of thiourea in FeEDTA and FOOX systems
Effect of thiosulfate concentration on anaerobic leaching gold
â– -50 mmol/L thiosulfate, 4.4 mmol/L EDTA, 4 mmol/L Fe, pH 7;
â—†-100 mmol/L thiosulfate, 4.4 mmol/L EDTA, 4 mmol/L Fe, pH 7;
â–²-100mmol/L thiosulfate, 1mmol/L ED-TA, 10 mmol/L Fe, pH7;
â–¡-50mmol / L thiosulfate, 12mmol / LOX, 4 mmol / L Fe, pH 5.5;
â—‡-100 mmol/L thiosulfate, 12 mol1/L OX, 4 mol/L Fe, pH 5.5;
â–³-100 mmol/L thiosulfuric acid. Salt, 3 mol/L OX, 10 mmol/L Fe, pH 5.5.
An anaerobic leaching test was also conducted in the presence of thiourea in an attempt to increase the gold leaching rate. The selected thiourea concentration was 1 mmol/L, which was lower than the concentration used in the aerobic test. The test results are shown in Fig. 7. For FeEDTA and FeOX systems, the leaching rate of gold can be significantly increased after the addition of thiourea. Initial leaching in the oxalate system using 100 mmol/L thiosulfate was the fastest, but the same as when using 100 mmol/L thiosulfate and thiourea-free leaching (Figure 6). Due to the continuous formation of iron hydroxide, the leaching rate decreases with time. The concentration of gold after leaching for 6 days using an oxalate system of 50 mmol/L thiosulfate (59.4 mg/L Au) is similar to the EDTA system (54.4 mg/L Au) using 100 mmol/L thiosulfate. of. The corresponding average leaching rates were 0.25 and 0.22 μmol/m 2 ·s, respectively, which was an order of magnitude higher than the highest anaerobic leaching rate measured without thiourea. Therefore, in terms of leaching kinetics, FeEDTA and FeOX systems containing 1 mmol/L thiourea are likely to be used for leaching of gold under field conditions. There is not much understanding of the mechanism of thiourea-catalyzed gold oxidation half-reaction, and further research is needed. These research efforts may also find catalysts that increase the leaching rate of gold in these systems.
Figure 7 Adding thiourea for use in FeEDTA and FeOX systems 50 or
Effect of 100mmol/L thiosulfate on anaerobic leaching of gold
â– -100 mmol/L thiosulfate, 4.4 mmol/L EDTA, 4 mmol/LFe, pH 7.1 mmol/L thiourea;
â—†-50 mmol/L thiosulfate, 4.4 mmol/L EDTA, 4 mmol/L Fe, pH 7.1 mmol/L thiourea;
â–²-100mmol/L thiosulfate, 4.4 mmol/L EDTA, 4 mmol/L Fe, pH 7;
â–¡-100 mmol/L thiosulfate. 12 mmol/L OX, 4 mmol/L Fe, pH 5.5, 1 mmol/L thiourea;
â—‡-50mnol/L thiosulfate, 12mol/LOX, 4 mol/LFe, pH 5.5, 1 mmol/L thiourea;
â–³-100 mmol/L. thiosulfate, 12 mol/L OX. 4 mmol/L Fe. pH 5.5.
(4) Effect of sulfide minerals on anaerobic leaching
During the in-situ leaching process, the solution is pumped through a permeable ore body. This ore body may contain many different minerals with different reactivity. The most common reactive minerals are iron sulfide minerals. In order to evaluate the possible occurrence of Fe(III) oxidant leaching systems in the presence of these minerals, a small amount of freshly ground (P80 = 10 μm) pyrite (FeS 2 ) or pyrrhotite (Fe 7 S) 8 ) The pulp is added to the leaching solution just before the addition of the gold flakes. Figure 8 shows the results obtained when leaching with a solution containing 100 mmol/L thiosulfate, 10 mmol/L Fe(III) complex and 1 mmol/L sulfur vein. It is clear that for all tests, the gold concentration obtained was significantly lower than that measured in the absence of sulfide minerals. These results confirm that Fe (III) is rapidly reduced in the presence of pyrite and pyrrhotite. As shown in Figure 9, this has been confirmed by measuring the potential of a platinum electrode (commonly referred to as EH) in a leaching solution.
Figure 8 Using 100 mmol/L thiosulfate in FeEDTA and FeOX systems
And the anaerobic leaching of gold in the form of pyrite and magnetite when leaching with 1 mmol/L thiourea
â– -100 mmol/L thiosulfate, 11 mmol/L EDTA. 10 mmol/LFe, pH 7, 1 mmol/L thiourea. 1g pyrite;
â—†-100 mmol/L thiosulfate, 11 mmol/L EDTA, 10 mmol/L Fe, pH 7, 1 mmol/L thiourea, 1 g pyrite;
â–¡-100mmol/L thiosulfate, 30 mol/L OX, 10 mol/LFe. pH 5.5, 1 mmol/L thiourea, lg pyrite;
â—‡-100mmol/L thiosulfate, 30 mmol/L OX, 10 mmol/L Fe, pH 5.5, 1 mmol/L thiourea, 1 g pyrrhotite
Figure 9 Using 100 mmol/L thiosulfate in FeEDTA and FeOX systems and
The effect of pyrite and pyrrhotite on the measured EH potential level during the anaerobic leaching of gold at 1 mmol/L thiourea.
â– -100 mmol/L thiosulfate, 11 mmol/L EDTA, 10 mmol/LFe, pH 7, 1 mmol/L thiourea;
â—†-100 mmol/L thiosulfate, 11 mmol/L EDTA, 10 mmol/L Fe, pH 7.1 mmol/L thiourea, 1 g pyrite;
â–²100 mmol/L thiosulfate, 11 mmol/L EDTA, 10 mmol/LFe, pH 7, 1 g of pyrrhotite;
â–¡-100 mmol/L thiosulfate, 30 mmol/LOX, 10 mmol/L Fe, pH 5.5.1 mmol/L thiourea;
â—‡-100 mmol/L thiosulfate, 30 mol1/L OX, 10 mol/L Fe, pH 5.5. 1 mmol/L thiourea, 1 g pyrite;
â–³-100mmol/L thiosulfate, 30mo1/LOX, 10mmol/L Fe. pH 5.5.1mmol/L thiourea, 1g pyrrhotite
For comparison, an EH profile of FeEDTA and FeOX systems in the absence of sulfide minerals is also shown. Obviously, for the FeEDTA and FeOX systems, the EH values ​​were above 200 mV throughout the test in the absence of sulfide minerals. In contrast, when pyrite or pyrrhotite was added, the EH value was significantly lower, indicating a loss of Fe(III). It is therefore not surprising that the leaching effect is very poor in the presence of these minerals, and the ongoing research work aims to reduce the effect of reactive sulfide minerals on the anaerobic leaching of thiosulfate.
In order to find out the reason for the rapid decrease of the EH potential of the solution in the presence of sulfide minerals, the thiosulfate and polythionate in the leaching solution were also analyzed at the end of the experimental work. The results in Figure 10 show that the amount of polythionate formed in the presence of reactive sulfide minerals is significantly greater than in the absence of sulfide minerals (the results of the aerobic leaching test are shown in Figure 4). It can also be seen from Fig. 10 that the amount of polythionic sulfate formed in the presence of pyrite is greater than that produced in the presence of pyrrhotite, and for the EDTA system, the produced polysulfate The amount of salt is more than that produced in the oxalate system.
Figure 10 Condensate concentration produced during anaerobic leaching in a solution containing reactive sulfide minerals
It is well known that pyrite can catalyze the oxidation of thiosulfate by dissolving oxygen, so it is likely that a reaction of Fe(III) with thiosulfate is also present, a similar catalytic effect. In order to further understand the mechanism for the formation of polythionate in the presence of sulfide minerals, the sulfur balance and electron balance can be carried out in a similar manner to the results shown in Table 2. If it is assumed that the polysulfate is produced by the catalytic oxidation of thiosulfate by sulfide minerals by Fe(III), then the corresponding amount of reduced Fe(III) can be calculated and the results obtained together with several The total sulfur content present in the polysulfate form is shown in Figure 11. For some experiments using FeEDTA and pyrite, it can be seen that the calculated amount of reduced Fe(III) is very close to the initial concentration of Fe(III). This is consistent with the fact that pyrite can catalyze the reaction between FeEDTA and thiosulfate. This situation is quite normal because it cannot be assumed that pyrite itself is reactive with FeEDTA complexes. For some experiments using FeOX and pyrite, the amount of reduced Fe(III) was lower than the initial concentration, especially for the two tests using 10 mmol/L Fe(III). This may be due to the increased precipitation of iron in the presence of pyrite, resulting in additional loss of Fe(III).
For the test containing fine-grained pyrrhotite, Figure 11 shows the calculated amount of Fe(III) reduced by thiosulfate, which is significantly lower than the initial iron concentration. Therefore, in the presence of pyrrhotite, surface-catalyzed oxidation of thiosulfate is not the primary reaction mechanism for Fe(III) loss. However, since pyrrhotite is considered to be a reactive sulfide mineral, it can be considered that the sulfide mineral (S2-) in the pyrrhotite can be oxidized to poly-sulphate. In this case, 1 mol of S per oxidized requires a greater amount of electrons than oxidized thiosulfate. And Figure 11 also shows the amount of reduced Fe(III) calculated by oxidation of sulfide minerals to polythionic sulfate. For experiments using EDTA, this calculated value is closer to the initial Fe(III) concentration, and the difference is most likely due to the oxidation of pyrrhotite to some thiosulfate and polythionate. Since the solution initially contains 100 mmol/L thiosulfate, it is difficult to accurately extract a small amount (~0.1 mmol/L) of thiosulfate. Therefore, the generated thiosulfate is not included in the calculation process. For the oxalate system, the calculated amount of Fe(III) reduced by the presence of pyrrhotite is again lower than that of the EDTA system, probably due to the precipitation of iron, since FeOX is not as stable as the FeEDTA complex.
Figure 11 In a solution containing the reactive sulfide mineral shown in Figure 4,
The amount of sulfur present in the form of polythionate formed during anaerobic leaching.
Also shown is the amount of iron reduced based on oxidation of thiosulfate or oxidation of sulfide minerals to polysulfate
Fourth, the conclusion
High performance chromatographic analysis of thiosulfate and polythionate using anion exchange column and perchlorate eluent has been improved and can be used to analyze sulfur-containing veins and FeED-TA or FeOX in leaching solutions. This will determine the levels of iron, thiourea and polythionate in the leachate.
It has been shown that the direct reaction between the Fe(III) complex and the thiosulfate is very slow, so the formation of polythionate can only be determined at a very low rate. Fe(III) complexation The substance could not oxidize thiourea. It was found that the concentration of thiourea remained unchanged during the leaching process. The results of both aerobic and anaerobic leaching tests have demonstrated that Fe(III) complexes can readily leach gold when some thiourea is added as a catalyst for gold oxidation.
However, in the absence of thiourea, the anaerobic leaching process is rather slow. It has also been found that the addition of fine pyrite can rapidly reduce the Fe(III) complex to Fe(II) and correspondingly form some polysulfate. This is due to the fact that pyrite catalyzes the oxidation of thiosulfate by the Fe(III) complex. It has also been found that pyrrhotite also causes some trouble because it can directly reduce the complex of Fe(III), so in the presence of any of the above two sulfide minerals, the gold is significantly reduced. Leach rate.
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