Pyrolysis of Waste Tire Rubber in the Presence of Sn-bearing Iron Concentrates and its Effect on the Tin Removal from this Iron Concentrate

Abstract

The Sn-bearing iron concentrate had a great influence on the pyrolysis of waste tire rubbers, and simultaneously the Sn in this concentrate could be removed efficiently during the pyrolysis process. With the additive of Sn-bearing iron concentrates, the gas yield from the pyrolysis of waste tire rubbers increased while the solid yield decreased. This was mainly due to the reduction of Fe₃O₄ by the pyrolysis char. A CaO–MgO containing complex was formed during the pyrolysis of waste tire rubbers with Sn-bearing iron concentrates, which could prevent the formation of stable chemical structures in hydrocarbons and decrease the activation energy of degradation reactions. The derived-oil yield was increased with it. The ‘S’ in the waste tire rubber could be transformed into Sn-bearing iron concentrates through the formation of FeS and ZnS, and then be oxidized to SO₂ (g) by Fe₃O₄ or reduced to COS (g) by CO (g), causing the sulfur content in the derived-oil to be decreased. Simultaneously, in the presence of these generated SO₂ (g) and COS (g), the Sn in the Sn-bearing iron concentrate could be sulfurized and removed. The Sn residual content in this Sn-bearing iron concentrate was decreased to 0.062 wt.% at pyrolysis temperature of 1000°C for 60 min in a high purity N₂ flow rate of 100 ml/min.

1. Introduction

The Sn-bearing iron ore with a reserve of 0.5 billion tons in China cannot be used as a raw material for ironmaking due to its high Sn content.^1,2) Though with a treatment of traditional mineral processing, the Sn content in the obtained Sn-bearing iron concentrate still reaches 0.3–0.8 wt%, which is much higher than the Sn limitation content of 0.08 wt% in the ironmaking raw materials. For the utilization of this Sn-bearing iron concentrate, a sulfurization roasting method has been proposed to remove the excessive Sn as reported in previous researches.^3,4,5,6,7) The high sulfur coal,³⁾ pyrite,^4,5) ferrous sulfate⁶⁾ or waste tire rubber⁷⁾ were used as curing agents and the intermediate products of S, H₂S (g), SO₂ (g) and COS (g) played major roles for the Sn sulfurization in these sulfurization roasting processes. The tin contents in Sn-bearing iron concentrates could be decreased to lower than 0.08% in all these researches. Compared with roasting with high sulfur coal, pyrite or ferrous sulfate for Sn removal, the roasting temperature required could be reduced to 1000°C using the waste tire rubber as a curing agent, which is due to a lower temperature of the release of S-containing gases of H₂S (g), SO₂ (g) and COS (g). The waste tire rubber shows a better commercial competitiveness for removing Sn from Sn-bearing iron concentrates and further resource utilizing of this concentrate from the perspective of energy consumption.

Nearly 1 billion of waste tire rubber is accumulated each year in the world, and it breaks down hardly in the natural environment.^8,9,10) The disposal of it is difficult to be realized, and it will bring about a series of environment problem if with an inappropriate management. A pyrolysis process with the purpose of fuel production has been seen as an economically and environmentally acceptable way for disposing the waste tire rubber.^{11,12,13,14,15)} It is generally carried out at temperature ranging from 300°C to 800°C in inert atmosphere,^16,17) with the main products being as the pyrolysis oil and char. The reactor type, temperature, particle size of the waste tire rubber, fluidizing gas type and residence time are main parameters affecting on the yields of pyrolysis products, and generally a higher temperature and longer residence time increases the aromaticity of the oil fraction as well as the quality of char. However, the derived-oil presents some disadvantages over fossil fuel related to its high viscosity, high oxygen content and poor ignition performance. Hence, a zeolite catalyst has been added during the pyrolysis to produce single ring aromatic compounds in the derived-oil to upgrade the oil properties in some researches.¹⁸⁾ The interest of this oil as a fuel lies in its high heating value (35 to 45 MJ kg⁻¹),¹⁹⁾ which is much higher than that of the original tire rubber. However, the sulfur content in it of around 1 wt.% limits its utilization.^20,21) Based on this, during the pyrolytic distillation process, additives such as slaked lime (Ca(OH)₂), lime (CaO) and sodium hydroxide (NaOH) have been used to transfer the sulfur from the waste tire rubber through forming CaS or Na₂S and finally reduce the sulfur content in the derived-oil in previous researches.^22,23) The possible desulfurization reactions are presented below, in which the ‘S’ is transformed from gaseous of H₂S and COS to sulphide compounds (Na₂S or CaS).   

$$\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{S}\to \mathrm{C}\mathrm{a}\mathrm{S}+{\mathrm{H}}_2\mathrm{O}$$(1)

  

$$\mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}\mathrm{O}\mathrm{S}\to \mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{C}{\mathrm{O}}_2$$(2)

  

$$\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2+{\mathrm{H}}_2\mathrm{S}\to \mathrm{C}\mathrm{a}\mathrm{S}+2{\mathrm{H}}_2\mathrm{O}$$(3)

  

$$\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2+\mathrm{C}\mathrm{O}\mathrm{S}\to \mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$(4)

  

$$2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{S}\to \mathrm{N}{\mathrm{a}}_2\mathrm{S}+2{\mathrm{H}}_2\mathrm{O}$$(5)

  

$$2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+\mathrm{C}\mathrm{O}\mathrm{S}\to \mathrm{N}{\mathrm{a}}_2\mathrm{S}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$(6)

We note that the sulphide compound of FeS could also be formed while the waste tire rubber was roasted with Sn-bearing iron concentrates as reported by Yu et al.,⁷⁾ which might play an active role on decreasing the sulfur content in the derived-oil. To confirm it, the pyrolysis behavior of waste tire rubbers in the presence of Sn-bearing iron concentrates was researched in this paper, and simultaneously the tin sulfurization and removal from Sn-bearing iron concentrates was also focused.

2. Experiment and Methods

2.1. Materials

In this research, the Sn-bearing iron concentrate was obtained from an iron mine which locates in Yunnan province of China. Table 1 shows that 65.45 wt.% Fe and 0.41 wt.% Sn are contained in this concentrate, and they exist mainly in forms of Fe₃O₄ and SnO₂ respectively as reported in previous researches.^5,6) This concentrate can’t be used as a raw material for ironmaking due to its over high content (Sn>0.08 wt.%).

Table 1. Chemical composition of the Sn-bearing iron concentrate (wt.%).

Components	Fe	SiO2	MgO	CaO	Al2O3	Sn	Zn	Cu	others
Contents	65.45	4.11	1.27	3.85	0.75	0.41	0.11	0.12	24.3

The waste tire rubber used was collected from Shanghai City of China. Proximate analysis and ultimate analysis of it were performed according to the Coal Analysis Standard of GB/T212-2008 and GB/T476-2001 respectively, and the results are shown in Table 2. Table 2 shows that the contents of ash and fixed carbon are 8.08 wt.% and 82.54 wt.% respectively. XRF analysis was carrier out to detect the composition of the ash and the result is shown in Table 3. Table 3 shows that the sulfur content in the ash is 2.15 wt.%. Besides, the purity of N₂ used in this study was higher than 99.99 vol%.

Table 2. Properties of the waste tire rubber (wt.%).

Sample	Proximate analysis				Ultimate analysis
	Mad	Ad	Vdaf	FCad	Cdaf	Hdaf	Odaf	Ndaf	Sdaf
Rubber	0.49	8.08	62.42	29.01	82.54	7.62	7.03	1.03	1.59

Table 3. Chemical composition of the waste tire rubber ash (wt.%).

Element	Zn	Si	Ca	Fe	S	Al	K	Cu
Contents	21.78	19.82	3.55	2.54	2.15	1.88	0.51	0.28

2.2. Experimental Apparatus and Procedure

All the experiments were carried out in a horizontal alumina tube with a ceramic boat to hold the samples (Fig. 1). The horizontal alumina tube was heated by six silicon carbide bars, and its temperature was measured by a Pt-Rh thermocouple and controlled by an intelligent temperature controller (accuracy ± 1°C). The amounts of waste tire rubbers and Sn-bearing iron concentrates used were set as 3 g and 15 g and the particle size of them was set as below 0.074 mm in all experiments. For the experimental procedure, the alumina tube was firstly heated to a proper temperature and then the high purity N₂ with a flow rate of 100 ml/min was passed through it for 10 min to clean the air. After that, the right furnace cover (“3” in Fig. 1) was opened and the mixed samples of Sn-bearing iron concentrates and waste tire rubbers were pushed into the horizontal furnace tube simultaneously with a steel stick and then hold for an appropriate time. Then the residues were cooled down to room temperature under the high purity N₂ atmosphere and kept in a closed vessel for analysis. Meanwhile, the condensable products were collected via three ice-salt condensers and the gas was collected using an air bag during the experimental process. For decreasing the heat transfer difference between the pyrolysis of waste tire rubber with Sn-bearing iron concentrates and the pyrolysis of mere waste tire rubber, the additive of ceramic balls with particle size below 0.074 mm was used as a heat transfer medium in this research for comparative study. In addition, the ceramic balls have little effect on the pyrolysis of waste tire rubber from a chemical point of view. The corresponding experimental procedure for pyrolysis of waste tire rubber with ceramic balls was same as that using Sn-bearing iron concentrates above.

Fig. 1.

Schematic diagram of the experimental system. (Online version in color.)

2.3. Characterization

The yields of char and derived-oil were measured by weighting. The mass of the char was obtained by first separating it from the residues through vibration in a beaker loading proper water using an electrodynamic vibrator, in which the char was floating on the water due to its low density. Then the char was collected, dried and weighed. The yield of gas products was calculated based on the gas concentration and molecular mass.

The composition of the gas products was analyzed by a gas chromatograph (GC7890B-RGA, Agilent) coupled with a thermal conductivity detector (TCD) and flame ionization detector (FID), and further the composition of the sulfur-containing gas was detected by a gas chromatograph (GC9790II, Fuli). The composition of the derived-oil was detected with an elemental analyzer (Germany, Vario EL III). Chemical composition and mineralogy of the residues were characterized by chemical analysis, X-ray photoelectron spectroscopy (XPS), and electron probe microanalysis (EPMA-JAX8230, JEOL, Japan). Phase compositions of residues were detected by X-ray diffraction (Rigaku D/max-3B) with Cu–Ka radiation. Spectra were recorded between 10° and 90° with a step of 8° at the rate of one step per minute.

3. Results and Discussion

3.1. Pyrolysis of Waste Tire Rubber

As reported in previous researches, the Sn in Sn-bearing iron concentrates could be removed effectively with a proper addition of waste tire rubber at a temperature of 1000°C.⁷⁾ To realize the Sn removal simultaneously, the pyrolysis of waste tire rubber was studied at the temperature of 950°C, 975°C, 1000°C, 1025°C and 1050°C respectively. In addition, the ceramic balls were used as an inert additive for comparative study.

In Fig. 2, the rise of temperature increases the gas yield while simultaneously decreases the yields of solid and oil, which is attributed to a higher rate of devolatilization at the primary pyrolysis stage and a more prone to the secondary pyrolysis at a higher temperature.¹⁴⁾ For the primary pyrolysis stage, the pyrolysis reaction occurs in the core of the waste tire rubber powder, and the thermal decomposition products include liquid hydrocarbons, solid char and a small amount of vapors.¹⁴⁾ After the diffusion of the vapors through the waste tire rubber particles and into the gaseous phase, the homogeneous and heterogeneous secondary reactions are carried out inside the particles and in the gaseous phase simultaneously.²⁴⁾ The pyrolysis process enters the secondary pyrolysis stage. For the secondary pyrolysis stage, some of the primary pyrolysis products would be cracked into low molecular hydrocarbons in the presence of the ceramic balls or Sn-bearing iron concentrates. There is no obvious boundary between these two stages, and generally they happen simultaneously.¹⁴⁾ Compared Figs. 2(a) to 2(b), it can be found that the gas and oil yields with the additive of Sn-bearing iron concentrates are significantly higher than that with the inert additive of ceramic balls. Furthermore, Fig. 3(a) shows that there is an obvious increase in the yields of CO (g) and CO₂ (g) while a decrease in the yields of CH₄ (g) and H₂ (g) with the pyrolysis carried out in the presence of Sn-bearing iron concentrates compared to that with ceramic balls. Simultaneously, Fig. 4 shows that the Fe₃O₄ from the Sn-bearing iron concentrate is reduced to FeO. Then we can conclude that the reduction of Fe₃O₄ by the gas products of CH₄ (g) and H₂ (g) and solid product of char through Eqs. (7), (8), (9), (10), (11), (12) causes the gas yield increases and the solid yield decreases.   

$$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4(\mathrm{s})+{\mathrm{H}}_2(\mathrm{g})=3\mathrm{F}\mathrm{e}\mathrm{O}(\mathrm{s})+{\mathrm{H}}_2\mathrm{O}(\mathrm{g})$$(7)

  

$$4\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4(\mathrm{s})+\mathrm{C}{\mathrm{H}}_4(\mathrm{g})=12\mathrm{F}\mathrm{e}\mathrm{O}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}(\mathrm{g})$$(8)

  

$$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4(\mathrm{s})+\mathrm{C}{\mathrm{H}}_4(\mathrm{g})=3\mathrm{F}\mathrm{e}\mathrm{O}(\mathrm{s})+\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2(\mathrm{g})$$(9)

  

$$\mathrm{C}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})=\mathrm{C}\mathrm{O}(\mathrm{g})$$(10)

  

$$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4(\mathrm{s})+\mathrm{C}\mathrm{O}(\mathrm{g})=3\mathrm{F}\mathrm{e}\mathrm{O}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$$(11)

  

$$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4(\mathrm{s})+\mathrm{C}=3\mathrm{F}\mathrm{e}\mathrm{O}(\mathrm{s})+\mathrm{C}\mathrm{O}(\mathrm{g})$$(12)

Fig. 2.

Pyrolysis product distribution as a function of temperature with the additive of ceramic balls and Snbearing iron concentrates respectively in a high purity N₂ atmosphere for 60 min. (Online version in color.)

Fig. 3.

The changes in gas volumes of C-containing, H-containing (a) and S-containing (b) as a function of temperature with the additive of ceramic balls and Sn-bearing iron concentrates respectively in a high purity N₂ atmosphere for 60 min. (Online version in color.)

Fig. 4.

XRD patterns of the residues with the additive of Sn-bearing concentrates at temperatures of 950, 1000 and 1050°C respectively in a high purity N₂ atmosphere for 60 min.

When the waste tire rubber pyrolyzed with ceramic balls, Fig. 5(a) shows that the sulfur in the residues exists in the form of ZnS. With the pyrolysis of waste tire rubber carried out in presence of Sn-bearing iron concentrates, the ‘S’ also could be combined with ‘Fe’ forming FeS in the residues except for ZnS seen from Fig. 5(b). More sulfur might be transformed from the waste tire rubber to the residues during the pyrolysis process due to this extra-formation of FeS. As followed, the formed ZnS and FeS could be oxidize by Fe₃O₄ through Eqs. (13) and (14) and reduced by CO (g) through Eqs. (15) and (16),^7,25) causing the SO₂ (g) and H₂S (g) formation amounts being higher than that with the inert additive of ceramic balls as shown in Fig. 3(b). Consequently, the sulfur contents in the residues and derived-oil both decrease compared to that with the additive of ceramic balls as shown in Tables 4 and 5 respectively.   

$$\mathrm{Z}\mathrm{n}\mathrm{S}+3\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4=\mathrm{Z}\mathrm{n}\mathrm{O}+9\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{S}{\mathrm{O}}_2(\mathrm{g})$$(13)

  

$$\mathrm{F}\mathrm{e}\mathrm{S}+3\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4=10\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{S}{\mathrm{O}}_2(\mathrm{g})$$(14)

  

$$\mathrm{Z}\mathrm{n}\mathrm{S}+\mathrm{C}\mathrm{O}(\mathrm{g})=\mathrm{Z}\mathrm{n}+\mathrm{C}\mathrm{O}\mathrm{S}(\mathrm{g})$$(15)

  

$$\mathrm{F}\mathrm{e}\mathrm{S}+\mathrm{C}\mathrm{O}(\mathrm{g})=\mathrm{F}\mathrm{e}+\mathrm{C}\mathrm{O}\mathrm{S}(\mathrm{g})$$(16)

Fig. 5.

XPS spectra of S 2p from the surface of residues with ceramic balls (a) and Sn-bearing iron concentrates (b) at 1000°C for 60 min in a high purity N₂ atmosphere. (Online version in color.)

Table 4. The S contents in residues at different temperatures for 60 min in a high purity N₂ atmosphere with ceramic balls and Sn-bearing iron concentrates respectively.

S residual content/wt.%	Temperature
	950°C	1000°C	1050°C
Pyrolysis with ceramic balls	0.31	0.28	0.25
Pyrolysis with Sn-bearing iron concentrates	0.16	0.13	0.11

Table 5. Physico-chemical properties of the derived-oil obtained from the pyrolysis of waste tire rubber at different temperatures for 60 min in a high purity N₂ atmosphere with ceramic balls and Sn-bearing iron concentrates respectively.

Elements analysis	Pyrolysis with ceramic balls			Pyrolysis with Sn-bearing iron concentrates
	950°C	1000°C	1050°C	950°C	1000°C	1050°C
C/%	73.79	85.47	89.40	68.74	78.19	82.63
H/%	8.44	6.73	5.31	10.07	7.50	6.36
N/%	2.60	2.30	1.53	1.00	1.38	1.43
S/%	0.71	0.84	0.73	0.57	0.62	0.54
O + others/%	14.46	4.66	3.26	20.62	12.31	9.04
C/H ratio	0.74	1.06	1.41	0.56	0.88	1.09
LHV/MJ kg−1	35.46	40.42	42.67	30.45	34.14	36.12
TCV/J	24.75	24.78	25.73	24.91	25.60	28.10

As the temperature increases, the reactions of carbon chain rupture, olefin epoxidation and aromatization are promoted, causing more H to be moved toward the gas products and the H content in the gas products increases as shown in Fig. 3(a). Consequently, in the obtained derived-oil, the H content decreases, C content increases, and C/H atom ratio increases as shown in Table 5. When the temperature reaches 1050°C, the derived-oil obtains the lowest H content, the highest C content, and the largest low heat value (LHV) and total calorific value (TCV). When the waste tire rubber pyrolyzed with Sn-bearing iron concentrates, a CaO–MgO containing complex is formed seen from the composition of point ‘1’ in Fig. 6. According to previous researches, it can prevent the formation of stable chemical structures in hydrocarbons and decrease the activation energy of degradation reactions,^13,14) as a result of which the derived-oil yield is higher than that pyrolysis with the inert additive of ceramic balls seen from Fig. 3(a).

Fig. 6.

EPMA image of the residue with the additive of Sn-bearing iron concentrates at 1000°C for 60 min in a high purity N₂ atmosphere. (Online version in color.)

In Fig. 7(a), for the pyrolysis of waste tire rubber with ceramic balls, the oil and gas yields increase and the char yield decreases with the time from 1 to 5 min, and then all of them changes little with the time exceeding 5 min. It indicates that the pyrolysis is basically finished at 5 min. The pyrolysis of waste tire rubber in the presence of Sn-bearing iron concentrates might also be finished at 5 min due to the little change of the derived-oil yield with the time longer than 5 min as shown in Fig. 7(b). Increasing the time over 5 min, the gas yield increases and the char yield decreases, which is mainly related to the reduction of Fe₃O₄ by the char (Eqs. (10), (11), (12)). In addition, the yields of CH₄, H₂, C₂ + Hm and C₃ + Hm at 5 min in the presence of ceramic balls or Sn-bearing iron concentrates in Fig. 8 (a) are both closed to that obtained at 60 min in Fig. 3(a), which also confirms that the total waste tire rubber pyrolysis is achieved at 5 min.

Fig. 7.

Pyrolysis product distribution as a function of time at 1000°C in a high purity N₂ atmosphere with ceramic balls and Sn-bearing iron concentrates respectively. (Online version in color.)

Fig. 8.

The change in gas volumes of C-containing, H-containing (a) and S-containing (b) as a function of time at 1000°C in a high purity N₂ atmosphere with the additive of ceramic balls and Sn-bearing iron concentrates respectively. (Online version in color.)

In Fig. 8(b), for the pyrolysis of waste tire rubber with ceramic balls, the formation amounts of S-containing gases, including SO₂, H₂S and COS, increase with the time from 1 to 5 min and then change little with the residence time prolonged further. While the waste tire rubber pyrolyzed with Sn-bearing iron concentrates, Eqs. (13), (14), (15), (16) are promoted at a longer residence time, as a result of which the amounts of SO₂ and COS increase with the time exceeding 5 min as shown in Fig. 8(b).

As mentioned above, the Sn-bearing iron concentrate has a great influence on the pyrolysis of waste tire rubbers. The derived-oil yield could be increased and in additional the sulfur content in this derived-oil could be reduced. Simultaneously, more CO (g) and CO₂ (g), less CH₄ (g) and H₂ (g), and more SO₂ (g) and COS (g) would be released into the gas products during the pyrolysis process.

3.2. Sn Removal from the Sn-bearing Iron Concentrate

As mentioned above, the waste tire rubber releases massive CO (g), CH₄ (g), H₂ (g), SO₂ (g) and COS (g) during its pyrolysis with Sn-bearing iron concentrates, which can reduce and sulfurize SnO₂ from Sn-bearing iron concentrates based on the thermodynamics.^3,4,5,6,7) The Sn might be removed simultaneously from Sn-bearing iron concentrates during the pyrolysis process.

During the pyrolysis of waste tire rubber with Sn-bearing iron concentrates at 1000°C, the Sn, S and C contents in the residues at different times are shown in Fig. 9. Figure 9 shows that the S residual content decreases little with the pyrolysis time from 1 to 5 min, and it exists mainly in forms of FeS and ZnS as presented in Fig. 5(b). Simultaneously, the Sn content in the residues also decreases little in this time range, and most of the SnO₂ from the Sn-bearing iron concentrate cannot been reduced effectively seen from Fig. 10. Figure 10 shows that the residual Sn exists mainly in the form of SnO₂ at the pyrolysis time of 5 min. Increasing the pyrolysis time further, the reduction of SnO₂, the formation of SO₂ (g) and COS (g) through reactions (13)–(16), and finally the sulfurization of Sn through Eqs. (17) and (18) are all promoted. As a result, the C, S and Sn residual contents decrease obviously as presented in Fig. 9. Figure 9 shows that the Sn residual content is decreased to 0.062 wt.% at pyrolysis time of 60 min. Then combined with the fact that the total pyrolysis of the waste tire rubber is finished at residence time of 5 min as reported in Fig. 7, we can conclude that the pyrolysis solid product of char plays a major role on the reduction of SnO₂ through Eqs. (10), (19) and (20).

Fig. 9.

Effect of residence time on the Sn, S and C residual contents at 1000°C in a high purity N₂ flow rate of 100 ml/min.

Fig. 10.

BSE images of the residue with Sn-bearing iron concentrates at 1000°C for 5 min in a high purity N₂ flow rate of 100 ml/min. (Online version in color.)

During the pyrolysis process of waste tire rubbers with Sn-bearing iron concentrates at 1000°C for 60 min in a high purity N₂ flow rate of 100 ml/min, the Sn residual content in Sn-bearing iron concentrates decreases to 0.062 wt.%, which meets the limitation content of Sn in the iron concentrate (Sn<0.08 wt.%).   

$$\mathrm{S}\mathrm{n}(\mathrm{l})+2\mathrm{C}\mathrm{O}(\mathrm{g})+\mathrm{S}{\mathrm{O}}_2(\mathrm{g})=\mathrm{S}\mathrm{n}\mathrm{S}(\mathrm{g})+2\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$$(17)

  

$$\mathrm{S}\mathrm{n}(\mathrm{l})+\mathrm{C}\mathrm{O}\mathrm{S}(\mathrm{g})=\mathrm{S}\mathrm{n}\mathrm{S}(\mathrm{g})+\mathrm{C}\mathrm{O}(\mathrm{g})$$(18)

  

$$\mathrm{S}\mathrm{n}{\mathrm{O}}_2(\mathrm{s})+2\mathrm{C}\mathrm{O}(\mathrm{g})=\mathrm{S}\mathrm{n}(\mathrm{l})+2\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$$(19)

  

$$\mathrm{S}\mathrm{n}{\mathrm{O}}_2(\mathrm{s})+2\mathrm{C}=\mathrm{S}\mathrm{n}(\mathrm{l})+2\mathrm{C}\mathrm{O}(\mathrm{g})$$(20)

4. Conclusions

The Sn-bearing iron concentrate had a great influence on the pyrolysis of waste tire rubber, which could increase the derived-oil yield and decrease the sulfur content in this derived-oil. In addition, the tin in the Sn-bearing iron concentrate could be removed simultaneously.

A CaO–MgO containing complex was formed in the pyrolysis process of waste tire rubber with Sn-bearing iron concentrates, which could prevent the formation of stable chemical structures in hydrocarbons and decrease the activation energy of degradation reactions. Due to it, a higher derived-oil yield was obtained compared to that with an inert additive of ceramic balls. In addition, more sulfur could be transferred from the waste tire rubber to the Sn-bearing iron concentrate through the extra-formation of FeS. The ‘S’ in FeS could be oxidized to SO₂ (g) by Fe₃O₄ and reduced to COS (g) by CO (g), and then entered into the gas products. It caused the S content in the derived-oil to be decreased. Simultaneously, the Sn could be removed from the Sn-bearing iron concentrate. The pyrolysis solid product of char played a major role on the reduction of SnO₂, and the S-containing gas products of SO₂ (g) and COS (g) could sulfurize the Sn efficiently. At 1000°C for 60 min in a high purity N₂ flow rate of 100 ml/min, the Sn residual content in the residue decreased to 0.062 wt.%, which met the limitation content of Sn in the iron concentrate (Sn<0.08 wt.%).

Acknowledgments

The authors wish to express their thanks to the National Science Fund for General Projects (51874153) for the financial support of this research.

References

• 1)   P. L.  Li and  Y. L.  Xie: Conserv. Util. Miner. Resour., 4 (2003), 13. https://doi.org/10.3969/j.issn.1001-0076.2003.02.004
• 2)   F. F.  Wu,  Z. F.  Cao,  S.  Wang and  H.  Zhong: J. Alloy. Compd., 722 (2017), 651. https://doi.org/10.1016/j.jallcom.2017.06.142
• 3)   Y.  Yu,  L.  Li and  X. L.  Sang: ISIJ Int., 56 (2016), 57. https://doi.org/10.2355/isijinternational.ISIJINT-2015-428
• 4)   Y.  Yu,  L.  Li,  J. Y.  Wang,  K. Z.  Li and  H.  Wang: J. Alloy. Compd., 750 (2018), 8. https://doi.org/10.1016/j.jallcom.2018.03.404
• 5)   R. J.  Zhang,  L.  Li and  Y.  Yu: ISIJ Int., 56 (2016), 953. https://doi.org/10.2355/isijinternational.ISIJINT-2015-717
• 6)   R. J.  Zhang,  L.  Li,  K. Z.  Li,  Y.  Yu and  H.  Wang: ISIJ Int., 58 (2018), 453. https://doi.org/10.2355/isijinternational.ISIJINT-2017-568
• 7)   Y.  Yu,  L.  Li,  J. Y.  Wang,  J. C.  Wang and  K. Z.  Li: J. Hazard. Mater., 371 (2019), 440. https://doi.org/10.1016/j.jhazmat.2019.03.034
• 8)   B. S.  Thomas and  R. C.  Gupta: Renew. Sustain. Energy Rev., 54 (2016), 1323. https://doi.org/10.1016/j.rser.2015.10.092
• 9)   W. P.  Lu,  Y. C.  Guo and  B.  Zhang: J. Anal. Appl. Pyrolysis, 135 (2018), 327. https://doi.org/10.1016/j.jaap.2018.08.020
• 10)   R. K.  Singh,  B.  Ruj,  A.  Jana,  S.  Mondal,  B.  Jana,  A. K.  Sadhukhan and  P.  Gupta: J. Anal. Appl. Pyrolysis, 135 (2018), 379. https://doi.org/10.1016/j.jaap.2018.08.011
• 11)   J. D.  Martínez,  N.  Puy,  R.  Murillo,  T.  García,  M. V.  Navarro and  A. M.  Mastral: Renew. Sustain. Energy Rev., 23 (2013), 179. https://doi.org/10.1016/j.rser.2013.02.038
• 12)   P. T.  Williams: Waste Manag., 33 (2013), 1714. https://doi.org/10.1016/j.wasman.2013.05.003
• 13)   J.  Shah,  M. R.  Jan and  F.  Mabood: Iran. J. Chem. Chem. Eng., 27 (2008), 103. https://doi.org/10.30492/IJCCE.2008.7003
• 14)   S. Y.  Luo and  Y.  Feng: Energy Convers. Manag., 136 (2017), 27. https://doi.org/10.1016/j.enconman.2016.12.076
• 15)   M. F.  Laresgoiti,  I.  de Marco,  A.  Torres,  B.  Caballero,  M. A.  Cabrero and  M. J.  Chomón: J. Anal. Appl. Pyrolysis, 55 (2000), 43. https://doi.org/10.1016/S0165-2370(99)00073-X
• 16)   L.  Lombardi,  E.  Carnevale and  A.  Corti: Waste Manag., 37 (2015), 26. https://doi.org/10.1016/j.wasman.2014.11.010
• 17)   G.  López,  M.  Olazar,  R.  Aguado and  J.  Bilbao: Fuel, 89 (2010), 1946. https://doi.org/10.1016/j.fuel.2010.03.029
• 18)   N. A.  Dũng,  R.  Klaewkla,  S.  Wongkasemjit and  S.  Jitkarnka: J. Anal. Appl. Pyrolysis, 86 (2009), 281. https://doi.org/10.1016/j.jaap.2009.07.006
• 19)   S. Q.  Li,  Q.  Yao,  Y. B.  Chi,  J. H.  Yan and  K. F.  Cen: Ind. Eng. Chem. Res., 43 (2004), 5133. https://doi.org/10.1021/ie030115m
• 20)   F.  Murena: J. Anal. Appl. Pyrolysis, 56 (2000), 195. https://doi.org/10.1016/S0165-2370(00)00091-7
• 21)   H.  Pakdel,  D. M.  Pantea and  C.  Roy: J. Anal. Appl. Pyrolysis, 57 (2001), 91. https://doi.org/10.1016/S0165-2370(00)00136-4
• 22)   H.  Aydın and  C.  İlkılıç: Fuel, 102 (2012), 605. https://doi.org/10.1016/j.fuel.2012.06.067
• 23)   C.  İlkılıç and  H.  Aydın: Fuel Process. Technol., 92 (2011), 1129. https://doi.org/10.1016/j.fuproc.2011.01.009
• 24)   W. J.  Duan,  Q. B.  Yu,  T. W.  Wu,  F.  Yang and  Q.  Qin: Int. J. Hydrog. Energy, 41 (2016), 18995. https://doi.org/10.1016/j.ijhydene.2016.07.187
• 25)   Y.  Yu and  L.  Li: ISIJ Int., 60 (2020), 2291. https://doi.org/10.2355/isijinternational.ISIJINT-2020-095