Introduction

Because of environmental issues and high oil price, many countries are extensively developing hybrid electric vehicles. The success of these electric vehicles depends primarily on the performance of the battery. Due to the material low cost, many researchers have been performed on electrochemical properties of various metal sulfides such as FeS2 (Kim et al. 2007). Iron disulfide has been used in commercial lithium primary cells (Montoro et al. 2004) to power cameras, computers and watches; it has also been studied for applications in solar cells and electric vehicle batteries (Montoro and Rosolen 2003; Rezig et al. 1992; Siyu et al. 2009). Nanostructured FeS2 used as the cathode material in thermal batteries resulted in improved electrochemical performance (Masset and Guidotti 2008; Strauss et al. 2003; Shao-Horn et al. 2002). It was found that the performance of nanoFeS2 electrodes was superior to that of microFeS2 electrodes (Shao-Horn et al. 2002). In addition to size, purity of pyrite influences Performance of battery, too. Presence of impurity such as sulfur decreases efficiency. Briefly, a thermal battery has a solid electrolyte which is converted to liquid phase in operational mode. In this situation, electrodes must have good resistance against heat and mustn’t be decomposed or transformed to liquid phase. Sulfur decreases heat resistance of cathode. Also, evaporation of sulfur causes some problem and decreases electricity efficiency (Shao-Horn and Horn 2001). It is concluded that purification of pyrite is an important issue and must be studied extensively.

Many techniques of preparation were investigated in order to obtain nano pyrite. One of them is wet chemical synthesis. There are basically two pathways to produce pyrite in aqueous solution, i.e. (a) solvothermal and (b) hydrothermal production (Xuefeng et al. 2001; Kar and Chaudhuri 2004; Kar et al. 2004). Zheng et al. reported that they have prepared pyrite using FeSO4, Na2S2O3 and sulfur by a single stage hydrothermal method in the temperature range 90–280 °C for 24 h with average grain size of about 500 nm (Wu et al. 2004).

In this paper, nano pyrite was produced via hydrothermal process in a new way, using FeSO4 and Na2S2O3 without adding sulfur in a stirred reactor. Applying a stirred reactor is another innovation of this paper. In previous studies, a stainless-steel cylindrical impeller-free chamber was used to produce pyrite. Finally, the most appropriate method to purify pyrite was chosen.

Experiments

Materials

Iron sulfate (FeSO4, E-Merck) and sodium thiosulfate (Na2S2O3, E-Merck) were used as iron and sulfur sources without further purification. Carbon disulfide (CS2, E-Merck) was used as sulfur-extracting solvent. Other solvents were purchased from Merck Company and were used without further purification. The distilled water was prepared in double water distiller.

Pyrite production

Experiments were carried out in 2,000 ml stainless steel stirred reactor equipped with a temperature controller and a barometer (Fig. 1). Iron sulfate (11 g) and sodium thiosulfate (45 g) were dissolved separately in 500 ml distilled water, and were mixed in the reactor. In all experiment molar ratio of Na2S2O3 to FeSO4 was 4:1. The solution volume into reactor was kept constant at 1.00 ± 0.02 l. The Teflon-lined reactor was sealed and maintained at 180 ± 1 °C. All reactions were carried out in 4 h and agitation speed 1,000 rpm. After finishing the reaction, products which included pyrite (6 g) and sulfur (1.05 g) were filtered, washed two times with boiling distilled water, and then dried in oven for 4 h at 60 °C. Finally, the sample was purified by considered purification method.

Fig. 1
figure 1

A schematic of the reactor set-up constructed and utilized in this research. Inner diameter of reactor (D) = 10 cm, height of reactor (H) = 25 cm, impeller diameter (d) = 4.5 cm, distance between impeller and reactor bottom (t) = 4 cm, width of impeller (w) = 0.5 cm

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Purification of the produced pyrite

Solvent extraction

Several polar and non polar organic solvents were investigated for sulfur removal. The extraction was conducted at room temperature and sulfur was added to solvent slowly until the sulfur deposition was observed. Ultimately, solubilities of sulfur in different solvents were obtained. Results are given in Table 1.

Table 1 Solubility of sulfur in different solvents
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Reaction with sodium sulfite

It was reported that sodium thiosulfate can be produced by reaction between sulfur and sodium sulfite (Howard 1930):

$$ {\text{Na}}_{ 2} {\text{SO}}_{ 3} \left( {\text{s}} \right)+{\text{S}}\left({\text{s}} \right) \to {\text{Na}}_{ 2} {\text{S}}_{ 2} {\text{O}}_{ 3} $$
(1)

In other words, an alternative route to purify pyrite is the addition of sodium sulfite into reactor after finishing pyrite production process. This way was examined. Typically, 24 g sodium sulfite (Na2SO3) was dissolved in 500 ml hot water. After that, 12 g of sulfur was added to the solution. The suspension was maintained in room temperature and moderate agitation for 2 h. Then solid product was filtered, dried and weighted. In this process, 6 g of sulfur was consumed. Also, the reaction between Na2SO3 and FeSO4 was investigated in a separate experiment to find out effect of Na2SO3 on main reaction (reaction between Na2S2O3 and FeSO4).

After initial tests, the purification process was optimized to find out the required amount of Na2SO3 and optimum temperature. For this purpose, in each experiment, after finishing main reaction (reaction between Na2S2O3 and FeSO4), effluent was filtered and washed with distilled water. Next, solid product and sodium sulfite were taken into the reactor with 1,000 ml distilled water. The reaction was carried out at temperature range of 50–100 °C and stirred rate of 1,000 rpm. After that, effluent was filtered and extracted with 100 ml of CS2 for 15 min under magnetic stirring at room temperature. Finally, the mixture was filtered and solvent was evaporated to measure the quantity of unreacted sulfur. Operation conditions and results are given in Table 2.

Table 2 Operation conditions and results of reaction between Na2SO3 and sulfur in various conditions
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Thermal treatment

The powder obtained in main reaction was thermally treated in a furnace to remove sulfur. Treatment program was; heating from ambient temperature to defined temperature with heating rate of 10 °C/min under air atmosphere. After treatment, the amount of unreacted sulfur was measured by solvent extraction method. Results are presented in Table 3.

Table 3 Results of thermal treatment process at different temperature (initial mass of pyrite: 4 g)
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Characterization

The products were analyzed by XRD and SEM analysis. SEM images were collected on a Leica/Cambridge Instruments S360 SEM. XRD analysis was carried out using a Philips PW170 based diffractometer (CuKα radiation, 35 kV, 40 mA). XRD patterns, in the range of 3–90° 2θ, were collected under air using the following settings: 0.1 mm receiving slit, 0.4 s/0.02° 2θ counting time.

Results and discussion

Characterization of product obtained in hydrothermal synthesis step

The related X-ray diffraction pattern of the powder produced in main reaction is shown in Fig. 2. This figure shows that the powder consists of pyrite (FeS2) with a large amount of sulfur.

Fig. 2
figure 2

X-ray diffraction pattern of the product before washing with solvent (P pyrite, S sulfur)

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Solvent extraction process

Abilities of some solvents were investigated to remove sulfur existed in solid product. A summary of the results is shown in Table 1. It can be seen from that carbon disulfide (CS2) has the highest ability to solve sulfur. X-ray diffraction pattern of solid product after extraction is given in Fig. 3. A comparison between Figs. 2 and 3, it was found that sulfur has completely removed by CS2 extraction. However, because of low miscibility of CS2 in water (0.29 g/100 g at 20 °C), a pasty product was formed during washing with CS2 and consequently sulfur removal wasn’t performed well. For this reason, the product must be dried before CS2 extraction. In other words, CS2 extraction adds an extra drying step to the process.

Fig. 3
figure 3

X-ray diffraction pattern of the powder after washing with CS2 (P pyrite)

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Reaction with sodium sulfite

Effect of sodium sulfite on main reaction

As said in “Reaction with sodium sulfite”, an experiment was performed to find out whether sodium sulfite can be added to main reaction. In this experiment an orange-brown precipitate was obtained. The powder was investigated by XRD analysis (Fig. 4). The XRD pattern shows that the powder includes Fe2NaOH(SO3)2·H2O and goethite (FeO(OH)). Therefore, this method must be performed after filtration of products obtained in main reaction.

Fig. 4
figure 4

XRD pattern of the product obtained in reaction between Na2SO3 and FeSO4 (F Fe2NaOH(SO3)2·H2O, G goethite)

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Optimization of the purification process

Sulfur removal by reaction with sodium sulfite was investigated in various conditions. Results were summarized in Table 2. As seen in this table, sulfur was completely removed at 95 °C after 12 min. Therefore, sulfur can be removed by reaction with sodium sulfite. However, this method increases steps of pyrite production process. In other words, number of steps becomes twice.

Thermal treatment

Performance of thermal treatment method was investigated. For this purpose, the powder produced in hydrothermal synthesis process was heated in a furnace at different temperatures. Results are given in Table 3. The initial and final mass of all samples were measured. Figure 5 shows importance of the treatment temperature (Waters et al. 2008). At temperatures higher than 300 °C, pyrite is decomposed. This reaction can be represented as following (Boyabat et al. 2003):

$$ {\text{FeS}}_{2} \left( {\text{s}} \right) \to {\text{FeS}}_{1 - x} \left( {\text{s}} \right) + \left( {1 - x} \right)/2{\text{ S}}_{2} \left( {\text{g}} \right) $$
(2)
Fig. 5
figure 5

Mineral phases detected (through XRD analysis) after thermal treatment of pyrite for 1 h at various temperatures up to 1000 °C (Boyabat et al. 2003)

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Therefore, to prevent pyrite decomposition, it is necessary to keep treatment temperature below 300 °C.

During thermal treatment, sulfur is oxidized to sulfur dioxide and leaves pyrite:

$$ {\text{S}} + {\text{O}}_{2} \to {\text{SO}}_{2} $$
(3)

According to Table 3, it seems that thermal treatment at temperature 300 °C for 2.5 h is the most appropriate condition to purify pyrite (Fig. 6).

Fig. 6
figure 6

The XRD pattern of the powder after thermal treatment at 300 °C for 2.5 h

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Investigation of the product morphology after purification

Scanning electron microscopy (SEM) analysis was carried out on the product after each treatment (Fig. 7). This figure shows fine and spherical particles in all treatments. Also, particles have nano structures.

Fig. 7
figure 7

The SEM images of FeS2 produced at 180 °C after a solvent extraction with CS2, b reaction with Na2SO3 and c thermal treatment

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Conclusion

In this study, purification of pyrite was investigated via three routes. It was found that carbon disulfide is the most appropriate solvent to extract sulfur. However, this method had some disadvantages such as safety. Second route, reaction with Na2SO3, was a complicated method and increased steps of pyrite production process. Ultimately, it was suggested Thermal treatment as the best purification method.