Thermal Behavior of Pyrite in the CO2 and N2 Atmosphere for Obtaining Pyrrhotite: A Magnetic Material

In the southern of Brazil the coal is composed of a large amount of pyrite (FeS2), an environmental problem for this region because the pyrite turns a waste. This work investigated the behavior of pyrite in CO2 and N2 atmospheres, aiming to identify better thermal treatment conditions of the pyrite waste that favor obtaining a material with magnetic properties. The results show that the samples treated in both conditions presented hysteresis and some magnetic properties. The best results were obtained from N2 because it is an inert gas, avoiding the pyrite oxidation and, consequently, favoring a major amount of pyrrhotite. The X-ray diffraction analyses showed that partial thermal decomposition of the pyrite occurs at 600 oC. The total decomposition was reached at 800 oC and the pyrrhotite phase was obtained. Such findings are relative new and can help enhance the utilization of pyrite, contributing to the environmental sustainability of the coal mining industry.


Introduction
In the coal mining, after the extraction of ROM (Runof-Mine / not classified ore) and the breaking of the largest blocks, the processing starts with the reduction of the ore sizes using an open crushing circuit and vibratory sieves, aiming a particle size near 1¼" (~ 3.2 cm). Following, the ROM is conduced to jigging. In this step, are obtained the processing coal (floated) and three sunken fractions: R1 -primary (rich in pyrite); R2 -secondary (rich in clays minerals) and R3 -tertiary (rich in carbon).
The processed coal is sieved in a 0.8 mm aperture sieve, resulting in a coarse product, which remains retained, destined to the thermoelectric. The passant is conduced to the fine concentration circuit, that include cyclones, tables and / or vibrating screens and flotation 1 .
The Brazilian mineral coal has an elevated amount of impurities, 2% to 4% of sulphur and 40 to 60% of ashes, and the process is necessary to match the quality of products for thermoelectric plants. Thus, 60% of the ROM turns waste 2 and the fractions R1, R2 and R3 destined to the waste disposal cells 1 .
This final destination causes one of the great problems related to the coal mining, the acid mine drainage (AMD), an effluent with elevate acidity levels which is a result from oxidation of the metal sulphides, such as pyrite, with the action of water and atmospheric air 3 . AMD contaminates the aquifers with bio accumulative heavy metals, making the water of the region improper for domestic and farming uses 4 .
Studies show that the AMD can affect approximately 23000 km of water courses 5 and that the remediation for a typical abandoned mine needs the annual treatment of almost 3 million of cubic meters 6 .
The disposal of wastes, besides AMD, also damage de landscape at the regions of coal mining, because the deposits need a large area 7 .
Alternatives for the remediation of AMD are frequently studied [8][9][10] . However, the environmental problems can be avoided by the use of the mining wastes as raw material for other industrial activities 7 . The use of the fraction reach in pyrite and carbon would be reducing in 80 to 90% the potential of generation of acidity of the waste 11 .
The decomposition reaction of pyrite varies according to the environment and the conditions in which they occur (atmosphere, temperature, particle size, flow conditions). These factors allow pyrite to transform into hematite, magnetite and/or pyrrhotite 12 .
The phase changes promoted by thermal treatment of pyrite up to 800 ºC substantially increase its magnetization. These changes include the formation of magnetic phases such as pyrrhotite. In the thermal decomposition process, pyrite is transformed into pyrrhotite, and the FeS 2 grains become porous because of the loss of sulfur gas 13  Materials with magnetic properties have magnetic moments that manifest very large and permanent magnetizations with the absence of an external field 14 .
At ambient temperature, pyrite has paramagnetic properties, but when it is thermally treated and when the Curie temperature (Tc) is reached (between 280 and 320 ºC), pyrite is transformed into pyrrhotite, which can be ferromagnetic 15 .
Technological devices are based on magnetism and magnetic materials, and there are major applications of materials with ferromagnetic properties. Examples of these applications include energy system distribution and generation, electromechanical conversion (cars and planes), electronics and telecommunication, transducers, sensing, medicine and biomedical engineering, computer science and industrial automation, among others 14 .
In this context, the present work provides results regarding better reaction conditions for the coal mining waste composed of pyrite in N 2 and CO 2 atmospheres, aiming to obtain pyrrhotite with magnetic properties. This study can enhance the utilization of pyrite, contributing to the environmental sustainability of the coal mining industry.

Materials
The pyrite used in this study was collected in Treviso, in Santa Catarina (Brazil). The material was passed through a milling process with a jaw crusher and a disk mill, and subsequently, an eccentric mill with alumina balls was used. The pyrite reached particle size after passing through a 60-mesh sieve.

Leaching
To reduce the amount of iron sulfates, the samples were leached in distilled water three consecutive times. Fifty grams of pyrite waste was immersed in 2 L of water at 80 ºC, and the system was subjected to magnetic agitation (Fitasom, model 752A, Brazil) for 20 min. The pyrite was separated by vacuum filtration (Prismatec, model 131, Brazil) and dried in a vacuum oven at 100 ºC for 24 h.

Thermal treatment
The thermal treatment of the leached pyrite was conducted in a horizontal and tubular kiln, and a refractory support was used to place the samples. The treatments were separately performed in CO 2 and N 2 atmospheres (White Martins). The temperatures and times of the treatments are shown in Table 1. The temperatures were chosen based on work Kopp and Kerr (1958), who evaluated by thermal analysis that the oxidation of pyrite begins at 538 ºC 16 .

X-ray diffraction -XRD
The crystalline phases present in the natural pyrite sample and in the samples subjected to the thermal treatments were identified with an XRD system (Shimadzu, Japan) equipped with a theta-theta goniometer and Cukα radiation at 1.5406 A. The step size used in the analysis was 2 º.min -1 . The measurement ranged between 10 and 80 º at 30 kV and 30 mA.

Infrared spectrometry (FTIR)
A FTIR spectrometer (Shimadzu, Japan) was used, and the analysis was performed by transmittance, with a 0.2 cm.s -1 speed and with a 4 cm -1 resolution ranging between 400 and 4000 cm -1 . For the analysis, the samples were pressed at an approximate proportion of 95% potassium bromide (KBr) and 5% sample.

Magnetometric analysis
The magnetic properties were evaluated with a Vibrating Sample Magnetometer -VSM (MicroSense, USA). The samples were subjected to an external magnetic field for the magnetic flux density measurement. The saturation magnetization values (Ms) and the remnant magnetization were defined in relation to the maximum value of the obtained field. Figures 1 and 2 show the XRD results for the treated samples in the CO 2 and N 2 atmospheres, respectively. Both treatment samples are compared with the natural pyrite (A). Figures 1 and 2 show the presence of one phase for the natural sample (A): pyrite (JCPDS: 42-1340). For the two evaluated atmospheres, the treatment performed at 400 ºC (B and C) did not promote a phase change of the material. Increasing the temperature to 600 ºC resulted in the partial decomposition of the pyrite, which refers to the pyrrhotite phase (JCPDS: 29-0724) identified by the X-ray patterns. The total decomposition of pyrite into pyrrhotite was observed at  Figures 3 and 4 show the infrared spectra of the samples treated in the CO 2 and N 2 atmospheres, respectively, compared to the natural pyrite (A).

Infrared spectrometry (FTIR)
In Figure 3, peaks in the region around 422 cm -1 are characteristic of iron disulfide 17 . For the natural pyrite (A), there was a peak at 412 cm -1 , and for the samples treated in the CO 2 atmosphere, peaks at 416 cm -1 and 419 cm -1 were present at 400 ºC after 1 h (B) and 3 h (C), respectively.
For the pyrite at 600 ºC (D), which is partially decomposed, there was still a peak relative to iron disulfide at 419 cm -1 . For the samples treated at 800 ºC (E and F), the characteristic peaks of pyrite were not observed (total decomposition), and iron oxides peaks were present (433 cm -1 ) 18 .
Peaks at 797 cm -1 and 800 cm -1 correspond to quartz 4 . The spectrum indicates the presence of iron sulfates in the samples, with bands near and between 900 and 1200 cm -1 17 .
For the natural pyrite, there were absorption bands associated with the presence of carboxylic groups at approximately 1400 cm -1 (1423 cm -1 - Figure 3A) 19 .
The peak identified in the natural pyrite around 3400 cm -1 is attributed to the presence of OH radicals groups 19 .
The absorption regions of 2368 cm -1 ( Figure 3D) and 2392 cm -1 ( Figure 4B) corresponds to the presence of carbon dioxide 20 .
For all samples, except for the sample treated at 400 ºC for 3 h (C) and for 800 ºC in 1 h (E), Figure 4 verifies peaks relative to iron disulfide (409 cm -1 , 412 cm -1 , and 417 cm -1 ) 17 . The samples that did not show that characteristic can be oxidized, as is the case of (C), for which the peak at 433 cm -1 corresponds to iron oxides 18 .
The presence of peaks associated with FeS 2 at high temperatures suggests that the N 2 atmosphere is inert, while the CO 2 atmosphere is oxidant. Figure 5 shows the obtained result for the natural pyrite. Figure 5 does not show hysteresis, proving that the natural pyrite presents low magnetic activity.    The average saturation point, considering the increase and decrease of the applied magnetic field, was 4.06 × 10 -3 emu (Ms). The average magnetization in the maximum applied field was 3.81 × 10 -3 emu.

Magnetometric analysis
The figure shows a negative remanence (Mr) of -388.40 × 10 -3 emu and a positive coercivity of 120007 Oe. Figure 6 and Table 2 show the results of the samples treated in the CO 2 atmosphere.
Hysteresis was not verified in Figures 6B and 6C, proving that the treated samples at 400 ºC in the CO 2 atmosphere, for which the XRD analysis did not detect the presence of pyrrhotite, have low magnetic activity.
With increasing temperature, the partial and total decomposition of the pyrite (forming the ferromagnetic pyrrhotite phase; Figures 6D, 6E and 6F) and the presence of magnetic hysteresis cycle can be observed.
For the sample treated at 600 ºC for 2 h ( Figure 6D), from saturation and based of the decrease of the H field by the reversion of the magnetic field direction, the curve did not return to its original shape at 0.48 Oe, producing the hysteresis effect (delay) in the magnetic moment relative to the H, decreasing to 0.07 Oe.
For the pyrite treated at 800 ºC for 1 h (Figure 6E), from saturation, the H field is reduced by the reversion of the magnetic field direction, and the curve did not return to its original shape at 0.14 Oe. At this point, there is a hysteresis effect in the magnetic moment relative to H, decreasing to 0.09 Oe.
For the sample treated at 800 ºC for 3 h (Figure 6F), from saturation, the H field is reduced by the reversion of the magnetic field direction, and the curve did not return to its original shape at 0.17 Oe, producing the hysteresis effect at the magnetic moment relative to H, decreasing to 0.05 Oe. Figure 7 and Table 3 present the results of the samples treated in the N 2 atmosphere.
In the N 2 atmosphere, it can be observed that the realized treatments at 400 ºC ( Figures 7B and 7C) also resulted in a material with low magnetic activity, and with increasing temperatures ( Figures 7D, 7E and 7F), the presence of magnetic hysteresis cycle was verified.
For the sample treated at 600 ºC for 2 h ( Figure 7D), from saturation, based on the decrease in the H field by the reversion of the magnetic field direction, the curve did not return to its original shape at 0.28 Oe, producing the hysteresis effect of the magnetic moment relative to H, decreasing to 0.003 Oe.
For the pyrite treated at 800 ºC for 1 h (Figure 7E), the measurement of the hysteresis showed the result of 0.39 Oe, which decreased to 0.00 Oe.
For the results of the sample treated at 800 ºC by 3 h (Figure 7F), the hysteresis was measured at 0.19 Oe, and it decreased to 0.08 Oe.
Comparing the XRD of the samples in the two atmospheres (Figures 1 and 2), it is observed that for D condition (600 ºC and 2 h) pyrrhotite peaks are more intense for CO 2 , showing   When the magnetometric analysis results are compared, for 800 ºC and for 3 h, the samples treated in both conditions presented hysteresis and some magnetic properties. Both conditions showed good results for obtaining a material with magnetic properties. The best results were obtained from N 2 because it is an inert gas, avoiding the pyrite oxidation and, consequently, favoring a major amount of pyrrhotite. Reaction 2 indicates the mechanism for the formation of iron oxide (Fe 3 O 4 ) in CO 2 atmosphere 23 . (2)

Conclusion
Aiming to identify better reaction conditions for pyrite waste from Santa Catarina in order to obtain magnetic properties, it was observed that: 1. The natural pyrite and the samples subjected to 400 ºC thermal treatment showed low magnetic activity;   " + + + 2. Thermal treatments in both N 2 and CO 2 atmospheres at temperatures of 600 ºC and 800 ºC resulted in the formation of pyrrhotite. The samples treated in these conditions showed magnetic hysteresis cycle in the magnetometric analysis; 3. The pyrite decomposition into CO 2 is faster than in N 2 . But the inert atmosphere avoids the iron oxides formation, and, consequently, ensures a greater amount of pyrrotitite. The coal mining at southern Brazil, specifically in Santa Catarina State, contributed with the environmental problems of the region. Pyrite that is present in the waste of coal mining can be used for several industrial processes, including for the production of magnetic materials.