Characterization of oil shale residue and rejects from irati formation by electron paramagnetic resonance

Characterization of oil shale residue and rejects from irati formation by electron paramagnetic resonance^*

S.L. Cogo^I; A.M. Brinatti^I; S.C. Saab^I,†; M.L. Simões^II; L. Martin-Neto^II; J.A. Rosa^III; Y.P. Mascarenhas^IV

^IUniversidade Estadual de Ponta Grossa, Departamento de Física, Av. Carlos Cavalcanti, 4748, 84030-900, PR, Brazil

^IIEmbrapa Instrumentação Agropecuária, P.O. Box. 741, 13560-970, São Carlos, SP, Brazil

^IIIIAPAR - Unidade Regional de Pesquisa Centro-Sul BR 376 - Km 496, PO Box 129, 84001-970, Ponta Grossa, PR, Brazil

^IVInstituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, CEP 13560-970, São Carlos, SP, Brazil

ABSTRACT

In this study, sedimentary organic matter of oil shale rejects, calschist, shale fine and the so called retorted shale from Irati formation was characterized. EPR was used to analyse the samples regarding loss of signal in g = 2.003 associated to the organic free radical with the calcined samples and washing with hydrogen peroxide. The radical signal was detected in all samples, however, for the calschist and shale fine samples another signal was identified at g = 2.000 which disappeared when the sample was heated at 400 ºC. Hydrogen peroxide washing was also performed and it was noted that after washing the signal appeared around g = 2.000 for all samples, including retorted shale, which might be due to the quartz E₁ defect.

Keywords: Oil shale; EPR; Quartz.

1. INTRODUCTION

The pyrobituminous shale, as it is best known, is an oily sedimentary rock, which contains bitumen and kerogen, an organic complex that when heated decays and produces several products, residues and rejects.

According to conventional Geology, the Formation is a sedimentary Bay that marked the Permian Period and its deposition might have occurred in shallow sea water, in largely confined bays, in a climate suitable to calcarium precipitation and physico-chemical conditions favorable to the accumulation of organic matter, which generate pyrobitumen [1]. The Irati formation presents defined layers, which vary in thickness from 5.5 m to 3.5 m with 15% organic matter content intercalated by deposits rich in carbonates. In the region of São Mateus do Sul, Parana' State, southern Brazil, there are oil shale formations, called Irati Formation.

In 1970 a mineral (shale) exploration program was established, the Petrosix [1,2]. The mill which processes 1660 tons of "raw shale", firstly mined and crushed, then taken to the retorter, where it is pyrolized, cooked at about 500 ºC releasing the contained organic matter and leaving behind 1376 tons of the residue called retorted shale. Besides the residue, retorted shale, the Petrosix extraction process generates a large amount of rejects, mainly shale fines (less than 0.25 inch) with an oil content of about 15%, and calschist.

In Brazil, the oil shale has been produced since the end of XVIII century, however, very little is known about its residue (retorted shale) and rejects (calschist and shale fine), which together represent 82% of the total. They constitute a double problem, both contaminating the environment and contributing to higher oil and gas extraction costs [2].

The literature presents some works with shale and also with the retorted residue, however, there are no works with calschist and shale fine. Adams et al. (2005) [3] analyzing oil shale showed that Multivariate calibration modelling has proved to be effective and efficient tool combined with DRIFTS for predicting oil content from oil shale.

Sousa et al. (1997) [4] showed that the oil shale free radical transformation (production and decomposition) mechanism can be described as: A_i ⇒ B_i ⇒ C_i where B_i are paramagnetic species and A_i e C_i are diamagnetic species. Working with Electron Paramagnetic Resonance (EPR) on retorted shale samples Sousa et al. (1994) [5] analyzed the effects produced on the retorted shale samples at temperatures from 100 a 1000 ºC. They showed that the resonance signal in g = 4.2 associated to the ion Fe³⁺ with 5/2 spin, presents a decrease in the samples line width of the samples heated at 900 ºC e 1000 ºC, indicating the appearance of a glass phase.

The Electron Paramagnetic Resonance (EPR) is used in this work [6] to characterize the sedimentary organic matter obtained from the Irati Formation, observing the organic radical signal of shale residue in the Petrosix process, also called retorted shale, as well as the rejects called shale fine and calschist which did not undergo the process.

2. MATERIAL AND METHODS

The samples of the rejects and residue were the region of São Mateus do Sul, Parana' State, Brazil, Irati Formation. The retorted shale samples (RS), calschist (CS) and shale fine (SF) were milled and calcined in oven for 2 hours at 200 ºC and then at 400 ºC and finally at 700 ºC. They were also washed with hydrogen peroxide to eliminate part of the organic matter of the samples. During the washing 1 g of sample for 10 mL of H₂O₂ was used, each sample underwent 40 washings. The samples were put in quartz tube with about 20 mg of non-calcined samples, calcined samples and washed with Hydrogen peroxide.

EPR spectra were recorded for freeze-dried at room temperature in X Band (9.4 GHz). Experimental conditions were: modulation frequency of 100 kHz; microwave power (0.2 mW) in order to avoid semiquinone-type free radicals signal saturation, adequate modulation amplitude (0.1 mT) and time constant (2.56 ms) that was 1/4 of the conversion time to guarantee no signal deformation by increasing line width. The central field was 338 mT, with 5 mT scanning and modulation amplitude de 0.1 mT peak to peak (H_pp). A patterned sample Cr³⁺ with g = 1.9797 was added to the sample in the resonant cavity, in order to determine the g value for each sample [7].

The X-Ray Diffraction (XRD) analyses were carried out using an automatic horizontal diffractometer, rotating anode RIGAKU ROTAFLEX generator, Ru-200B model, with CuKα radiation (λk_α1 = 1.54433, Å λk_α2 = 1.54051 Å ) operating at 50 kV and 100 mA, in the continuous scanning mode with speed of 2 ^°/min., within the range of 5º < 2θ < = 100º, with divergence slit (DS): 1.00º, reception slit (RS): 0.30º, scattering slit (SS): 1.00º and the samples were powdered and sieved (53 µm aperture) and were placed on the sample holder and lightly pressed.

3. RESULTS AND DISCUSSION

Figure 1 presents the CS sample spectra without calcination and with the calcined samples at 200 ºC and 400 ºC (similar signals occurred for the SF and RS). The appearance of resonance signal in g = 2 of Fe³⁺ with large line width was noticed due to the dipolar interaction between the ferromagnetic structures when the sample was calcined at 400 ºC. Such result confirms what was verified by Fonseca et al. (1984) [8] and Sousa et al. (1994) [6] when they worked with RS samples, those authors attributed the Fe³⁺ signal to the oxidation of Fe²⁺ found in (Fe₂S₃) pyrite and the generation of Fe³⁺ in hematite (Fe₂O₃). The Fe³⁺ signal can be observed in g = 4.3 associated to the Fe³⁺ ion with 5/2 spin which can occupy either tetrahedral or octahedral sites of silicate structures. [5]

The spectrum also shows (NC and 200ºC) a sextet of lines centered at g = 2.00 with a hyperfine coupling constant (A) of 9 mT generated by Mn²⁺ ions with 5/2 nuclear spin [2].

The organic radical spectra for the non-calcined samples, for the three samples: calschist, shale fine and retorted shale, are shown in Figure 2a. The signal for the retorted shale presents a characteristic similar to the organic radical with value in g = 2.003 [8] and line width about 5.5 G. The signal for calschist and shale fine present the signal in g = 2.003, however, there is another signal in g = 2.000. There were no changes in the spectra obtained from samples heated at 200 ºC (Figure 2b) emphasizing that the organic matter related to the organic radical is stable at this temperature once the free radical signal in g = 2.003 increased in relation to the spectra at room temperature. Figure 2c shows the spectra after the samples were heated at 400ºC. It is possible to observe that the signal around g = 2.000 disappears, the organic radical signal becomes noisier highlighting decrease in the signal to noise ratio. When samples were heated at 700ºC, organic radical signal was not observed (data not shown) indicating loss of organic radical due to samples calcination.

The spectra of samples washed with hydrogen peroxide are shown in Figure 3 together with the non-calcined samples. The signal around g=2.000 is observed to increase in calschist and shale fine samples and also appeared in the retorted shale sample. Souza et. al (1994) [5] working with retorted shale did not identify the signal in g = 2.000 since this signal only appears when the part of organic matter that contains the organic radical in g = 2.003 is extracted. This is only possible when the sample is washed with hydrogen peroxide as shown in Figure 3.

The signal around g = 2.000 can be due to the quartz as it was also proposed by Bayer et al. (2000) [9] when studying soil physical fractions (oxisol) and identifying the organic signal due to the semiquinone radical and a signal in g=2.000. S. Botis (2005) [10] employing the EPR data reveal the presence of six paramagnetic defects: one oxygen vacancy center (E1), three silicon vacancy hole centers [/H^- (I). /H⁺ (II). /M⁺ ]and two peroxy centers, in the Athabasca quartz.

Figure 4 shows the XRD of retorted shale (RS) identifying the quartz as principal mineral, in accordance with the most intense peak and others, all indicated with hkl indexes [11], such result was also presented by Pimental et al. (2006) [ 12]. In this analysis was possible to verify three more intense diffraction peaks associated with the kaolin group and three more intense diffraction peaks associated with pyrite, all indicated with label and hkl indexes [11]. The signal in the region g = 2.000 is (using the Cr³⁺ pattern) 2.00043 which can be due to the unpaired electron in a vacancy of oxygen in which the electron occupies a silicon hybrid orbital sp³ which is linked to three oxygen atoms in quartz sample as proposed by Botis S. (2005) [10].

4. CONCLUSIONS

The spectra of EPR of the samples of CS, SF and RS showed the sign of Fe³⁺, Mn²⁺ and organic free radical. It was identified one signal characteristic similar of organic radical with value in g = 2.003. The samples de CS and SF were observed the presence of another signal in g = 2.00043, in the samples non-calcined and calcined. Through washes with H₂O₂ the presence of the other signal in g = 2.00043 was observed in all the samples including XR and this signal is probably due to quartz (vacancy of Oxygen) in agreement with XRD analysis.

(Received on 26 June, 2008)

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*

This work is part of the project "Use of subproducts of the processing of the pyrobituminous shale in the agriculture", agreement Petrobras-IAPAR

Publication Dates

• Publication in this collection
  15 June 2009
• Date of issue
  Mar 2009

History

• Received
  26 June 2008