Gypsum

Example 1

The effect of Tu on the electrochemical behavior of a chalcopyrite electrode was studied in a conventional 3-electrode glass-jacketed cell. A CuFeS₂ electrode was using as working electrode, a saturated calomel electrode (SCE) was used as reference, and a graphite bar was used as counter-electrode. The CuFeS₂ electrode was polished using 600 and 1200 grit carbide paper. All experiments were conducted at 25° C. using a controlled temperature water bath. The electrolyte composition was 500 mM H₂SO₄, 20 mM Fe₂SO₄ and 0-100 mM Tu. Before starting any measurement, solutions were bubbled with N₂ for 30 minutes to reduce the concentration of dissolved 02. Open circuit potential (OCP) was recorded until changes of no more than 0.1 mV/min were observed. After a steady OCP value was observed, electrochemical impedance spectroscopy (EIS) was conducted at OCP using a 5 mV a.c. sinusoidal perturbation from 10 kHz to 10 mHz. Linear polarization resistance (LPR) tests were also conducted using a scan rate of 0.05 mV/s at ±15 mV from OCP.

Linear potential scans were conducted at electrode potentials ±15 mV from the OCP measured at each Tu concentration. All scans showed a linear behavior within the electrode potential range analyzed. An increase in the slope of the experimental plots was observed with increasing Tu concentration. The slope of these curves was used to estimate the value of the polarization resistance (Ret) at each concentration. These values were then used to estimate the values of the dissolution current density using equation 1:

$\begin{array}{cc}{i}_{\mathrm{dissol}}\approx \frac{\mathrm{RT}}{{\mathrm{nFR}}_{\mathrm{ct}}}& \mathrm{Eq}.\left(1\right)\end{array}$

FIG. 3 shows the effect of Tu on the dissolution current density and mixed potential of the CuFeS₂ electrode, and indicates that a maximum dissolution current density was achieved when Tu concentration is 30 mM. Increasing Tu concentration to 100 mM resulted in a decrease in the current density and mixed potential of the CuFeS₂ electrode. Moreover, after immersing the CuFeS₂ electrode in the 100 mM Tu solution, a copper-like film was observed on the surface of the electrode, which film could only be removed by polishing the electrode with carbide paper.

FIG. 4 is a bar graph showing the effect of initial Tu or FDS concentration on the electrochemical dissolution of a chalcopyrite electrode in sulfuric acid solution at pH 2 and 25° C. A concentration of 10 mM Tu in the leach solution resulted in a six fold increase in dissolution rate compared to no Tu, and a concentration of 5 mM FDS resulted in a six fold increase relative to 10 mM Tu. A concentration of 10 mM Tu in leach solution also containing 40 mM Fe(III) resulted in a thirty fold increase in dissolution rate compared to 40 mM Fe(III) alone.

A column leach of different acid-cured copper ores was conducted with Tu added to the leach solution. A schematic description of the column setup is shown in FIG. 5. The column diameter was 8.84 cm, the column height was 21.6 cm, and the column stack height was 15.9 cm. The irrigation rate was 0.77 mL/min or 8 L/m²/h. The pregnant leach solution emitted from these columns was sampled for copper every 2 or 3 days using Atomic Absorption Spectroscopy (AAS).

The specific mineralogical composition of these ores are provided in Table 1. The Cu contents of Ore A, Ore B, and Ore C were 0.52%, 1.03%, and 1.22% w/w, respectively. Prior to leaching, ore was “acid cured” to neutralize the acid-consuming material present in the ore.

That is, the ore was mixed with a concentrated sulfuric acid solution composed of 80% concentrated sulfuric acid and 20% de-ionized water and allowed to sit for 72 hours. For one treatment using Ore C, Tu was added to the sulfuric acid curing solutions.

The initial composition of the leaching solutions included 2.2 g/L Fe (i.e. 40 mM, provided as ferric sulfate) and pH 2 for the control experiment, with or without 0.76 g/L Tu (i.e. 10 mM). The initial load of mineral in each column was 1.6 to 1.8 kg of ore. The superficial velocity of solution through the ore column was 7.4 L m⁻² h⁻¹. The pH was adjusted using diluted sulfuric acid. These two columns were maintained in an open-loop or open cycle configuration (i.e. no solution recycle) for the entire leaching period.

The results of leaching tests on the Ore A, Ore B and Ore C are shown in FIGS. 6, 7, and 8, respectively. The presence of Tu in the lixiviant clearly has a positive effect on the leaching of copper from the chalcopyrite. On average, the leaching rate in the presence of Tu was increased by a factor of 1.5 to 2.4 compared to the control tests in which the leach solutions did not contain Tu. As of the last time points depicted in FIGS. 6 to 8, copper extractions for columns containing Ore A, Ore B, and Ore C leached with a solution containing sulfuric acid and ferric sulfate alone, without added Tu, were 21.2% (after 198 days), 12.4% (after 50 days), and 40.6% (after 322 days), respectively. With 10 mM of added Tu, these extractions were 37.9%, 32.0%, and 72.3%, respectively.

Referring to FIG. 8, 2 mM Tu was added to the leach solution originally containing no Tu from day 322 onward, after which the leach rate increased sharply. From day 332 to day 448, the copper leached from this column increased from 40% to 58%, and rapid leaching was maintained throughout that period.

The averages for the last 7 days reported in FIG. 9 indicate that the leaching rate for acid-cured Ore C leached in the presence of 10 mM Tu is 3.3 times higher than for acid-cured Ore C leached in the absence of Tu, and 4.0 times higher than acid-cured and Tu-cured Ore C leached in the absence of Tu.

FIG. 10 shows the effect of Tu on solution potential. All potentials are reported against a Ag/AgCl (saturated) reference electrode. The solution potential of the leach solutions containing Tu was generally between 75 and 100 mV lower than the solution potential of leach solution that did not include Tu. Lower solution potentials are consistent with Tu working to prevent the passivation of chalcopyrite.

“Bottle roll” leaching experiments in the presence of various concentrations of Tu were conducted for coarse Ore A and Ore B. The tests were conducted using coarsely crushed (100% passing ½ inch) ore.

Prior to leaching, the ore was cured using a procedure similar to what was performed on the ore used in the column leaching experiments. The ore was mixed with a concentrated sulfuric acid solution composed of 80% concentrated sulfuric acid and 20% de-ionized water and allowed to settle for 72 hours to neutralize the acid-consuming material present in the ore. For several experiments, different concentrations of Tu were added to the ore using the sulfuric acid curing solutions.

The bottles used for the experiments were 20 cm long and 12.5 cm in diameter. Each bottle was loaded with 180 g of cured ore and 420 g of leaching solution, filling up to around one third of the bottle's volume.

The leaching solution from each bottle was sampled at 2, 4, 6 and 8 hours, and then every 24 hours thereafter. Samples were analyzed using atomic absorption spectroscopy (AAS) for their copper content.

The conditions for the bottle roll experiments are listed in Table 2. Experiments #1 to #6 were conducted using only the original addition of Tu into the bottles. For experiments #7 to #11, Tu was added every 24 hours to re-establish the Tu concentration.

A positive effect of Tu on copper leaching was observed. For the coarse ore experiments, a plateau was not observed until after 80 to 120 hours. Tu was added periodically to the coarse ore experiments, yielding positive results on copper dissolution.

The effect of different concentrations of Tu in the leach solution on the leaching of coarse ore (experiments #1 to #11 as described in Table 2) is shown in FIGS. 11 and 10.

For ore B, Tu was periodically added every 24 hours to re-establish the thioruea concentration in the system and thus better emulate the conditions in the column leach experiments. As may be observed from FIG. 9, 8 mM and 10 mM Tu yielded higher copper dissolution results than the other Tu concentrations tested for ore A. A plateau in dissolution is not observed until after approximately 120 hours, which varied with Tu concentration as shown in FIG. 11.

As may be observed from FIG. 12, 5 mM Tu yielded higher copper dissolution results than the other Tu concentrations tested for ore B. As with ore A, a plateau in dissolution is not observed until after approximately 80 to 120 hours, which varied with Tu concentration as shown in FIG. 12. Periodic addition of Tu resulted in increased copper dissolutions and produced a delay in the dissolution plateau.

Interestingly, solutions containing 100 mM Tu did not appear to be much more effective on copper extraction than those containing no Tu, and even worse at some time points. This is consistent with the results of Deschenes and Ghali, which reported that solutions containing 200 mM Tu (i.e. 15 g/L) did not improve copper extraction from chalcopyrite. Tu is less stable at high concentrations and decomposes. Accordingly, it is possible that, when initial Tu concentrations are somewhat higher than 30 mM, sufficient elemental sulfur may be produced by decomposition of Tu to form a film on the chalcopyrite mineral and thereby assist in its passivation. It is also possible that, at high Tu dosages, some copper precipitates from solution (e.g. see FIG. 17) to account for some of the low extraction results.