Application of Carrier Micro Encapsulation in Coal Preparation and Sulfide mineral processing

Transcription

1 Application of Carrier Micro Encapsulation in Coal Preparation and Sulfide mineral processing Rani Kumari Thakur Jha Candidate for the Degree of Doctor in Engineering Supervisor: Prof. Naoki HIROYOSHI Division of Solid Waste Resources and Resource Recycling 1. Introduction Pyrite (FeS 2 ) is a common gangue mineral that often associated with coal and other leaner metal sulfide ore. Separation of pyrite, FeS 2, from coal is environmentally important in order to avoid SO 2 emission during coal combustion in electricity plant, which may cause the acid rain formation. The separation of pyrite from metal sulfide ore is also recommended to economically extract copper or other metal. Therefore, pyrite is rejected as a gangue mineral from the valuable minerals by physical separation techniques e.g. froth flotation and disposed into tailing pond. Froth flotation is an effective method for removing pyrite from coal and valuable sulfide minerals, but the natural hydrophobicity of pyrite makes the separation difficult. Formation of acid mine drainage due to pyrite oxidation in tailing pond is also an important problem relating to pyrite in coal preparation. The present study proposed carriermicroencapsulation (CME) as a new method for suppressing both floatability and oxidation of pyrite. CME is a method to induce a thin layer of stable oxide or hydroxide (MO n or M(OH) n ) on pyrite using a watersoluble organic carrier combined with a cation M n+. In the present work, the pyrite is coated with a thin layer of Si oxide (SiO 2 ) or Si hydroxide (Si(OH) 4 ) by using a water soluble organic carrier-catechol combined with Si ions, namely, Si-catechol complex Si(cat) 3. The layer converts pyrite surface from hydrophobic to hydrophilic to suppress pyrite floatability as well as acts as a protective barrier against pyrite oxidation. The CME model is schematically presented in Figure.1. Figure.1: Schematic illustration of carrier-microencapsulation (CME) The SiO 2 or Si(OH) 4 layer formation by CME treatment was confirmed by spectroscopic and electrochemical studies for the pyrite without and with CME treatment and the electrochemical model of coating was proposed. The effect of CME treatment using Sicatechol complex, Si(cat) 3, on the wettability of coal and pyrite was investigated through Bubble pick up experiments, the effect of CME treatment on coal-pyrite flotation and for four different metal sulfide minerals were investigated by Hallimond tube floatation test. The effect of CME on pyrite oxidation was investigated through shaking flask leaching model experiments without and with CME treatment. 2. Materials and Methods A ground pyrite sample was prepared by crushing a specimen-grade pyrite (origin: Peru) in a jaw crusher (Retsch BB51) and grinding in a ball mill. The ground sample was dry-sieved to obtain a µm size fraction. Just before the experiments, the sieved pyrite was washed and dried: Ultrasonic washing was conducted for 30s using ethanol as medium to remove fine and oxidized particles during grinding; the ethanol was decanted and the pyrite was washed subsequently with 1 N HNO 3 to create fresh pyrite surfaces, distilled water for two times to wash off the nitric acid, and acetone for four times to facilitate drying; finally, the washed pyrite was vacuum-dried for twenty four hours [Sasaki et al. 1995]. Chalcopyrite from Osarizawa mine Japan, Galena from Morocco, and the sphalerite from Kamioka mine, Japan, were also crushed, dry-sieved and washed following the procedure described above for pyrite. The Coal from Kushiro coal mine, Japan was handpicked, crushed and dry-sieved to obtain the µm size fraction and stored in the nitrogen environment until being used for the experiments. For some experiments, the pyrite cube sample was sectioned from the specimen-grade pyrite crystal (origin: Peru) and polished to the size of ~ 1c.m. x 1.2c.m.x1cm. The surface of the cubic sample was washed as mentioned above. A stock solution containing known concentration of Si-catechol complex, Si(cat) 3, was prepared by adding Na 2 SiO 3.9H 2 O and benzene 1,diol (catechol, H 2 Cat) into distilled water. CME treatment of pyrite samples was done in a 50 cm 3 Erlenmeyer flask containing 1 g of the ground pyrite sample and 10 cm 3 of Si(cat) 3 solution. The flask was shaken in a water bath shaker under aerobic conditions at 25 o C for a required treatment time. The shaking amplitude of the water bath shaker was 4 cm and the frequency was 120 min -1. NaCl and HNO 3 were added to maintain the ph wherever required. After the required time of treatment, the flask was removed from the shaker and the filtrate and the CME treated pyrite sample were collected by filtration

2 using 5C filter paper. The pyrite was then washed gently with distilled water to be used for surface analysis. For the flotation tests, after the CME treatment, the pyrite along with the Si(cat) 3 solution were transferred to the bubble pick up optical cell or to the Hallimond tube to test the floatability. For the shaking flask leaching experiments, after the CME treatment, the filtration was done and the pyrite residue was gently washed. Afterwards, the CME treated pyrite was shaken for the desired time in a 50 cm 3 Erlenmeyer flask containing 2 g of the CME treated pyrite sample and 20 cm 3 of distilled water. To evaluate the bacterial effect, leaching experiments with iron oxidizing bacteria - acidithiobacillus ferrooxidans were added in to the flask. After the desired time of interval of leaching experiment, solid-liquid filtration was carried out and the filtrate thus obtained was analyzed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP- AES) to measure the Fe, and S content in it. The surface morphologies of the untreated and CME treated pyrite were analyzed by SEM- EDX. 3. Spectroscopic and electrochemical analysis of pyrite after CME treatment To establish the CME using Si-catechol, the CME treated pyrite sample was analyzed by Scanning Electron Microscopy, SEM-EDX (SSX-550, Shimadzu Co. Ltd., Japan), X-ray photoelectron spectroscopy, XPS (AXIS-His, Shimadzu/Kratos) and Fourier Transform Infrared Spectroscopy, FTIR (FT/IR-600, JASCO Co. Ltd., Japan). For the reference all the analysis were carried out with the untreated pyrite samples. Based on the results of the surface analyzing techniques and the electrochemical observations, the detail discussion of the mechanism for the CME using Si-catechol complex is discussed. Surface analysis of CME treated pyrite by SEM- EDX Figure 2: SEM EDX images and EDX spectra of pyrite treated with 100 mol m -3 of Si(Cat) 3 solution for 1 h. The pyrite treated with 100 mol m -3 Si(Cat) 3 solution for 1 h was analyzed by SEM-EDX; Figure 2 shows the SEM image with elemental mapping for Si, the EDX spectra is also shown in the same figure. The 2 peaks correspond to Si are detected together with the peaks of Fe and S in the EDX spectrum, and it was confirmed that Si covers pyrite surface homogeneously in EDX elemental mapping. Surface analysis of pyrite with/out CME treatment by FTIR Figure3: FTIR spectra of pyrite with and without CME treatment. CME treatment was done with 0.5 mol m -3 of Si(Cat) 3 solution. The treatment time was 1 h. Figure 3 shows the FTIR spectra obtained for the pyrite before and after treatment with 0.5 mol m -3 of Si(cat) 3 solution. The strongest adsorption peak was detected at around 1100 ~ 1200 cm -1 for the CME treated pyrite: this may correspond to Si-O-Si bend at 1137cm -1 reported by Park et.al.[2007] in contrast no peak in the same region was obtained for the non treated pyrite. The presence of this peak imply that silica oxide thin layer exist on the CME treated pyrite surface. Electrochemical study of the behavior of Si(cat) 3 solution on pyrite electrode The Electrochemical measurements were performed using a computer controlled electrochemical measurement system (Analytical SI 1280B, Solartron) on a conventional three electrode cell consisting of a saturated Ag/AgCl standard calomel reference electrode ( mv vs. standard hydrogen electrode), a platinum counter electrode and with the self prepared pyrite working electrode at 298K. The schematic diagram of the set up and cross section of the pyrite electrode is shown in Figure 4a and b respectively. The pyrite electrode for this study was made up of natural pyrite, sectioned and polished to a cube of the size of ~ 1c.m. x 1.2c.m. x 1cm. With the help of conducting silver paste (Dotite, Fujikura Kesaei Co.Ltd,Japan), the pyrite cube was attached with copper wire. A clear plastic tube as a body support was used and the electrode i.e., copper wire attached with pyrite cube, was inserted, the void place was then filled with an insulating methyl metacrylate-based cold curing agent (Technivit 3040, Heraeus Kulzer, Germany). The electrode surface was polished with water proof abrasive paper and rinsed with water to remove the residue before the experiments. The electrodes were immersed in the 150 cm 3 of 5 mol m -3 of Si(cat) 3 solution, and the nitrogen gas was purged for 10 min in the cell set up before each measurement to eliminate the excess oxygen from the system.

3 absence of Si(cat) 3 i.e., with distilled water as shown in Figure 5a. Therefore the anodic current peaks at around 0.2V corresponds to the oxidative decomposition of the Si(cat) 3 on the pyrite electrode. Based on the spectroscopic and electrochemical analysis of CME treated pyrite the most probable model was proposed. In the first step, Si(Cat) 3 adsorbs from the aqueous phase to the anode site of pyrite surface and then it is oxidatively decomposed into uinine and Si 4+ according to equation 1: Figure 4a: The three- electrode electrochemical set up used for the electrochemical measurements Figure 4b: Cross section of pyrite electrode The cyclic voltammetry were performed with pyrite electrode in distilled water and 0.5 mol m -3 Si(cat) 3 solution (initial potential: v; starting with anodic direction potential sweep; potential sweep rate: 50 mv/s; highest limit of potential sweep, 0.8 V vs. SCE; Lowest limit of potential sweep, -0.2 V vs. SCE in three cycles at room temperature in the nitrogen environment) and the obtained cyclic voltammogram is presented in figure 5 a and b respectively. The electrons released are consumed by dissolved oxygen at the cathode site of the pyrite as in equation 2: 3O H e - = 6H 2 O (2) In the next step, the released Si 4+ forms into Si (OH) 4 or SiO 2 on pyrite surface as follows: Si H 2 O = Si (OH) H +...(3) Si(OH) 4 = SiO H 2 O...(4) After summing up the reactions, the overall reactions will be like this (equations 5 and 6) Figure 5a: Cyclic voltammogram (-0.2V to 0.9V, 50mV/S) recorded for distilled water on pyrite electrode Figure 5b: Cyclic voltammogram (-0.2V to 0.9V, 50mV/S) recorded for 5 mol m -3 Si(cat) 3 on pyrite electrode As shown in Figures 5 a and b, notable increase in anodic current, commencing at a potential at 0.1 V and the highest anodic current was obtained at the applied potential of around 0.2 V for pyrite electrode with 5mol m -3 Si(cat) 3 solution (Figure 5b). In the negative sweep, cathodic current peaks were obtained at around 0V. The anodic current peak was not observed in the cyclic voltammogram for pyrite electrode in the 3 4. Effect of CME on coal-pyrite Flotation The SiO 2 /Si(OH) 4 coating made by CME, was evaluated for its effect on the coal and pyrite floatability. For this purpose, Bubble pick up and Hallimond tube flotation test were carried out. 4.1 Dynamic bubble pick-up experiments Dynamic bubble pick-up experiments [Yoon and Yordan, 1991] for the CME-treated pyrite or coal ( µm) were conducted using an electronic induction timer (MCT-100). The CME-treated mineral along with the solution was transferred from the Erlenmeyer flask to a rectangular optical glass cell to form a bed of particles. Inside the cell, an air bubble was formed at the tip of the capillary glass, and the bubble was brought into contact

4 with the mineral particles at various contact times. The experiments were carried out at constant amplitude of the capillary motion for bubble pick-up, and the contact time between particles and air bubble was 1,000 ms unless specified. Ten bubble pick-up trials were conducted on different spots of the bed. The attachment of the mineral particles to the air bubble was observed with the help of a microscope connected to a computer display. The probability of attachment of the particles to an air bubble was calculated as the number of times the particle attached to the air bubble over the total number of bubble pick-up trials. The effect of Si(cat) 3 concentration and ph on the attachment of the mineral particles to an air bubble was investigated. out using 41 wt% sodium tungstate solution of specific gravity 1.5 to separate coal and pyrite from the float and tailing part to calculate the percentage recovery of coal in the froth. Figure 6: Effect of concentration of Si(cat) 3 on attachment probability of pyrite and coal particle to air bubble in dynamic bubble pick up experiments. CME treatment time was 1 hr. Figure 6 shows the attachment probability of pyrite to an air bubble decreased drastically with increasing Si(cat) 3 concentration, and it became almost zero when the concentration was over 0.5 mol m -3. The decrease in the probability of attachment was only observed for the coal at Si(cat) 3 concentrations greater than 0.5 mol m -3 ; whereas the probabilities of attachment were over 0.9 at mol m -3 of Si(cat) 3 concentrations, thus the Si(cat) 3 concentration for CME treatment was decided to be 0.5 mol m -3 to get the desired result: suppress the pyrite floatability without any suppressive effect for coal. 4.2 Hallimond tube flotation experiments In Hallimond tube flotation test, after the CME treatment of pyrite for 60 min, the pyrite along with Si(cat) 3 complex solution in the flask was transferred to a beaker containing 100 cm 3 distilled water. A 1.1 cm 3 of 0.01 kmol m -3 potassium ethyl xanthate or cm 3 of the kerosene-water emulsion (kerosene content was 625ppm) solution was added as collector and the mixture was stirred over a magnetic stirrer for 5 minutes. The mixture was transferred to the Hallimond tube. Air was supplied to the Hallimond tube at a flow rate of 100 cm 3 min -1 for 5 minutes. The froth and tailings parts were recovered by filtration using 5B Advantec filter papers, washed with distilled water, transferred into drying bottles, and dried at 105 o C. From the weight of the dried sample, the percentage recovery of pyrite in the froth was calculated. The recovery in froth was calculated by the weight of pyrite in the froth divided by the weight of pyrite together in froth and tailing. In the case of coal pyrite mixture, after the flotation test the dense medium separation was carried 4 Figure 7: Effect of CME treatment on flotation of the coal-pyrite mixture in the presence of cm 3 of 625 ppm kerosene. The Si(cat) 3 concentration was 0.5 mol m -3, ph: 7-9, and CME treatment time was 1 h. Figure 7 shows the effect of CME treatment on the flotation of coal-pyrite mixture in the presence of kerosene. Coal floatability was unaffected by the CME treatment while pyrite floatability was significantly suppressed: coal recovery in the froth was more than 90% with or without CME treatment while pyrite recovery in the froth was 60% without CME treatment and 18% with CME treatment. More than forty percent suppression was observed for the pyrite suppression. This indicates that CME has the selectivity feature to suppress pyrite floatability selectively to enhance the separation of pyrite from coal in coal-pyrite flotation. 5. Effect of CME on sulfide mineral Flotation The SiO 2 /Si(OH) 4 coating made by CME, was evaluated for its effect on the floatability of four different metal sulfide minerals: chalcopyrite, pyrite, sphalerite, and galena. Hallimond tube flotation test were performed for four sulfide minerals without and with CME treatment, and the effect of ph, the Si (cat) 3 concentration, and the presence of flotation collectors (potassium amyl xanthate) are evaluated. Figure 8: The effect of different concentration of Si(cat) 3 on the sulfide mineral flotation with potassium amyl xanthate as a flotation collector. The ph was 8 to 8.5.

5 Figure 8 shows the result of flotation test by using Hallimond tube for the four metal sulfide mineral treated with the varied concentration of Si(cat) 3.For the reference, the result of control experiment i.e., treatment with 0 mol m -3 of Si(cat) 3, is also plotted. Without CME treatment, i.e., control, with 0 mol m -3 Si(cat) 3, all the four metal sulfide minerals were floated up and the percentage recovery in the froth was more than 95% for all the minerals. With the increasing concentration of the Si(cat) 3 for treatment, the floatability of ZnS was greatly suppressed, with the 5 mol m -3 Si(cat) 3 the recovery was 5% while the CME treatment with 50 mol m -3 Si(cat) 3 suppressed the ZnS floatability to no recovery in the froth. Great suppression of floatability was observed for PbS after the treatment with 0.5mol m -3 Si(cat) 3 and the treatment with 5 mol m -3 Si(cat) 3 made PbS hydrophilic and the recovery in the froth was 0.1%. The FeS 2 was also displayed adequate hydrophilic property and less than 1% recovery in froth was observed with 5 mol m -3 Si(cat) 3. The CuFeS 2 recovery in the froth was also affected by the increasing concentration of Si(cat) 3, however with the 0.5 mol m -3 Si(cat) 3 CME treatment, the recovery of CuFeS 2 was around 90% while the other three metal sulfide minerals were suppressed with the same condition. The CuFeS 2 floatability was suppressed with the highest concentration of Si(cat) 3 and the recovery was less than 1% after the treatment with 50 mol m -3 of Si(cat) 3. However after the treatment with such a high concentration of Si(cat) 3, 50 mol m -3, floatability of all the four minerals were suppressed. The floatability of sulfide minerals is also influenced by the affinity between the collectors and sulfide minerals; and CuFeS 2 has strongest affinity for potassium amyl xanthate to among these four minerals that made it float with 5 mol m -3 Si(cat) 3 concentration at slightly elevated ph. 6. Effect of CME on pyrite oxidation The effect of CME on pyrite oxidation, using [Si(cat) 3 ] tris-catecholato complex is evaluated. The shaking flask leaching experiments were designed and carried out to verify the effect of CME on pyrite oxidation. The effect of CME on pyrite oxidation with different concentration of [Si(cat) 3 ], different ph, and in the presence of iron oxidizing bacteria: Acidithiobacillus ferrooxidans was analyzed. After the CME treatment, the solid liquid filtration was done and the pyrite were washed gently with distilled water, afterwards 2g of CME treated pyrite shaken in 20cm 3 of distilled water under aerobic condition using a water bath shaker at 25 o C. The shaking amplitude of the water bath shaker was 4 cm; and the frequency was 120min -1. At fix intervals of 1 week, the 2 cm 3 of leachate were sampled from the flask to measure the ph and ORP and the Fe and S contents of the leachate by ICP-AES. To analyse the effect of iron oxidizing bacteria, during the shaking flask experiments Acidithiobacillus ferrooxidans were added and in the fix intervals of 1 week, the 2 cm 3 of leachate were sampled from the flask to measure the ph and ORP and the Fe and S contents of the leachate by ICP-AES. Figure 9: (A) ph profile of the leachates, (B) amount of Fe (in ppm) and (C) amount of S (in ppm) in the leachates of pyrite without and with CME treatment, obtained from shaking flask experiments for one month at one week interval. Figure 9 shows the ph, Fe and S contents in the leachate obtained at one week intervals for one month shaking flask experiment. In Figure 9(A) the ph trend with leaching time is presented. The result shows that the ph of the leachate of the pyrite with CME treatment is higher than that of the leachate of the untreated pyrite. The ph drop for un-treated pyrite was significant and after four weeks it was recorded 1.7. On the other hand, the ph of the leachate of the pyrite with CME treatment was 3.2 over the one month experiment. Figures 9 (B) and (C) show the Fe and S concentrations (in ppm) in the leachate, respectively. The amount of Fe and S leached from the CME treated pyrite is significantly less in comparison to that of from untreated pyrite after one month leaching. Figure 9(B) depicts that after 30 days of leaching 160ppm of Fe ion was present in the leachate of non-treated pyrite, on the other hand only 60ppm of Fe ion was present in the leachate of CME treated pyrite, indicating that suppression in Fe leaching behavior is achieved by CME treatment of pyrite. Suppression of sulphur leaching is also observed for CME treated pyrite: Figure 9(C) depicts that after 30 days leaching 20ppm of S ion was present in the leachate of non-treated pyrite, while the leached amount of S ion for CME treated pyrite leachate sample was only 2ppm. These results imply that the SiO 2 /Si(OH) 4 layer formed on the pyrite surface by the CME treatment confirmed itself as a barrier layer 5

6 against pyrite oxidation by bringing a halt in ph drop and by controlling the leaching of Fe and S. and without CME treatment, the ph values of the leachate of CME treated pyrite were over 2.3 throughout the investigated time period, where the ph of the leachate of the un-treated pyrite were lower and drops down to 1.5 at the fifth week. In Figure 10(B) and (C) show the amount of Fe and S leached from the pyrite with and without CME treatment as a function of time respectively. In Figure 10(B) the amount of Fe present in the leachate for non treated pyrite after five week is almost twice as of in the CME treated pyrite leachate: the leached amount of Fe from the un-treated pyrite increased with passing weeks and was 900ppm, but only 450ppm from the CME treated pyrite at five weeks. In Figure 10(C) the behavior of S leaching from pyrite with and without CME treatment is depicted. The sulfur leaching for non treated pyrite is almost double than that of CME treated pyrite after five weeks. Continuous extraction of S was observed from the un-treated pyrite and S concentration in the leachate reached 26ppm at five weeks contrary to 12ppm in the leachate of the CME treated pyrite. These results show that the Fe and S leaching was successfully suppressed by the CME treatment even in the presence of ample amount of iron oxidizing bacteria. Therefore it can be stated that the presence of bacteria would not hinder suppression of pyrite oxidation by the CME. Figure 10: (A) ph profile of leachate of pyrite with and without CME (B) amount of Fe and (C) amount of S present in the leachates of pyrite with and without CME treatment shaken in the presence of iron oxidizing bacteria for five weeks. Si(cat) 3 concentration was 5 mol m -3. Inoculated bacteria density was: 1.0 x10 8 cells /ml. The presence of iron oxidizing bacteria also enhances the spontaneous pyrite oxidation, in this context, the effectiveness of CME coating was investigated in the presence of iron oxidizing bacteria. One month shaking flask experiment on the CME treated and un-treated pyrite was done in presence of Acidithiobacillus ferrooxidan. The results of the ph profile and amounts of Fe and S leached at 1 week intervals in one month are presented in Figures 10 (A) (B) (C). Figure 10 (A) shows the ph profiles of the leachates of pyrite with and without CME in the presence of bacteria. It shows that CME could control the ph drop even in the presence of iron-oxidizing bacteria; the final ph values were almost same throughout the investigated time period. Comparing the results of with 6 7. Conclusions Carrier-microencapsulation (CME) was proposed as a method to prevent pyrite floatability and control pyrite oxidation in coal and in metal sulfide mineral processing. In CME, pyrite is coated with a thin layer of Si oxide or Si hydroxide using a water-soluble organic carrier combined with Si ions. Pyrite floatability was effectively suppressed by CME using Si(cat) 3 solution as a pretreatment for pyrite. Pyrite oxidation was also significantly suppressed by CME even in the presence of iron oxidizing bacteria. The selective formation of Si(OH) 4 or SiO 2 coating on pyrite is assumed to have had suppressed the floatability of pyrite and facilitated the separation of pyrite from coal and other metal sulfide minerals. References 1. Park E.S., Ro H.W. Nguyen C.V., Jaffe R.L., and Yoon D.Y. Infrared Spectroscopy Study of Microstructures of Poly(silsesquioxane)s, Chem. Mater /cm071575z 2. Sasaki K., Tsunekawa M., Ohtsuka T., and Konno H. Confirmation of a sulfur-rich layer on pyrite after oxidative dissolution by Fe(III) ions around ph 2. Geochimica et Cosmochimica Acta, 59, , Yoon R. and Yordan J.L. Induction time measurements for the quartz-amine flotation system. Journal of Colloid and Interface Science, 141:2, , 1991