Utilization of polymeric wastes in steel refining process: Carbon dissolution into liquid steel at high temperature

Abstract

High amount of polymeric food packaging wastes had been increasing during the Covid-19 epidemic, especially polystyrene (PS) and polypropylene (PP) packaging. This paper investigates the utilization of polymeric wastes as a liquid steel recarburizer in ladle refining process. PP was blended with PS into the ratios of up to 60 wt%, namely Blend#1 – Blend#6. The blends were pyrolyzed at 1550 °C for 15 min under argon atmosphere. The chars had high carbon content ranges between 86 and 91.47 wt%, and the crystallite size ranges between 0.27 and 2.45 nm. The chars were brought into contact with an electrolytic pure iron at 1550 °C under argon atmosphere for carbon dissolution experiment. It was found that overall carbon dissolution rates (K) for the chars were 1.46 × 10⁻³ - 8.4 × 10⁻³ s⁻¹, which occurred within the first 4–10 min and then keep pace with the maximum carbon content of 4.08–4.97 wt%. Sulphur transfer into liquid steel was slow for all cases with the content was in between 0.01 and 0.025 wt%. The rate controlling mechanism for carbon dissolution from polymeric chars was the dissociation of carbon atom from its host lattice. CaH₂O₂ is a filler in the PS, was found to retard the carbon dissolution, however it can be decomposed at steelmaking temperature. The chars produced from PS and PP can be replaced a commercial recarburizer without negative effect on steel quality.

1. Introduction

Home isolation, social distancing and work from home have been the preventive measures of many governments worldwide during the Covid-19 pandemic since 2020 up to now. These lead to the rapidly growth of food delivery business, especially in Southeast Asia. By this, the generation of polymeric food packaging wastes had been increasing vastly, such as polyethylene terephthalate (PET), high- and low-density polyethylene (HDPE and LDPE), polystyrene (PS) and polypropylene (PP). Almost 300 million tons of plastic waste are produced annually with the growing rate of 9% each year [1]. Among this, USA is the world's top waste plastic generator. In the past six decades, only 9% of plastic waste was recyclable, while 12% has been incinerated and 79% has been landfilled [1]. Of all polymeric food packaging in waste stream, the recycling rate of PS and PP was minimal. PS is a brittle plastic that resist to alkalis and acids. It can be found as fast-food packaging, disposable cutlery, Styrofoam food boxes and consumer goods. PP is a plastic suitable for packaging. It is durable and useable in the microwave without side effects. PP can be found in the forms of bottles, bottle cap films, straws and tray [2]. Due to its low recycling rate, the method to enhance the recycling rate is essential.

The chemical utilization of plastics waste like PS and PP can be found in many products. Mostly, it was turn into solid carbon and oil by pyrolysis or cracking method with or without catalyst. Pyrolysis is an alternative technique in chemical recycling of plastic waste, leading to thermal cracking and condensation of the polymers. The resulting products are liquids, gases, and solid. Pyrolysis temperature, time and heating rate are the important parameters for the process [3]. Pyrolysis at high temperatures and high heating rates (fast pyrolysis) will result in high yield of bio-oil, while slow pyrolysis with slow heating rates yields the primary product of char [4,5].

Miskolczi et al. [6] studied the pyrolysis of plastics waste in a pilot-scale reactor. It was reported that the wastes were decomposed at 520 °C with a feed rate of 9 kg. The pyrolysis with catalyst leads to an increase of gasoline and light oil, and a decrease of heavy oil concentration. Pol [7] reported the process to convert waste plastics like low density polyethylene (LDPE), high density polyethylene (HDPE), polyethylene terephthalate (PET), polystyrene (PS) and their mixtures into carbon microspheres (CMSs). The pyrolysis was conducted in a closed reactor under autogenic pressure (about 1000psi) at 700 and 800 °C to produce dry and pure powder of CMSs. It was found that this method can be used to produce CMSs for each individual plastic and their mixtures. Liu et al. [8] synthesized hydrogen and carbon nanotubes from polypropylene (PP) by using a two-stage pyrolysis with catalyst. It was reported that most of the resulting gas products were hydrogen (H₂) and methane (CH₄). The derived carbon nanotubes had high crystals. The concentration of hydrogen increases with the increase in decomposition temperature. As the pyrolysis temperature increases, the gas product pyrolysis increases while the liquid yield decreases. Miandad et al. [9]. studied influence of temperature and reaction time on the pyrolysis of PS in pilot scale reactor. It was reported that 76.0–78.7% of the liquid oil was produced by pyrolysis 1 kg of PS at 400–500 °C. Reaction time has insignificantly influenced the oil yield (79.0–80.7% for 60–120 min), but decreases of char and increases of gas yields. Park et al. [10] conducted continuous two-stage pyrolysis of waste polyethylene (PE): 30–300 °C and 653–736 °C. The product obtained from the first stage was fed into a fluidized bed reactor to pyrolyze at the second stage. The obtained product was 22.63–35.92% of the pyrolysis oil, consisting of 80–90% aromatic hydrocarbons. Fraczak et al. [11] investigated continuous pyrolysis of polyolefins and PS in different proportions and with the addition of PET and polyvinyl chloride (PVC), at the pilot scale. It was reported that the overall product yield had decreased with the addition of 5% PS, PET and PVC. These result in the formation of a lower-boiling product and the increase in amount of polyethylene conversion. By adding 10% of PS, the conversion of PP had increased, leading to a higher product yield and no significant change in the distribution of boiling temperatures [11].

Coal, coke and anthracite are the fossil fuels that used as the carbon resources in many steelmaking processes, such as in scrap melting and ladle refining processes. In electric arc furnace (EAF), carbon was charged with scrap metals as a fuel, and injected into the molten slag for slag foaming practice. In ladle furnace, carbon was used as a recarburizer to adjust the melt carbon content. Large quantity of fossil fuels consumption in iron and steel industries leads to the release of greenhouse gases and carbon footprint products. Therefore, it is essential to deduct amount of fossil fuels consumption in the industries to move toward sustainability. In this work, we offer an idea to simply utilize the polymeric wastes by converting them into valuable carbon source for liquid steel recarburizing in ladle refining process.

Several previous studies have shown that waste polymers can be utilize as a carbon resource in steelmaking processes by partially blending with metallurgical coke (up to 30 wt%) and used for various studies, such as carbon dissolution, slag foaming, FeO and MnO reductions in molten slag [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]. Most of these works had focused on the utilization of HDPE, PET, PP, PU, PC, Bakelite, melamine and rubber tire. For PS and PP, the study of carbon dissolution into liquid steel and the kinetics dissolution rate has not been clearly investigated.

Carbon dissolution into liquid iron is important for EAF steel making processes. This phenomenon can be found in both EAF melting and ladle refining processes. For refining process, carbon was added into the ladle furnace to adjust carbon content in liquid steel via carbon dissolution mechanism. This is called recarburizing process and the carbon is called recarburizer. Several researchers had studied carbon dissolution into liquid steel from both graphitic and nongraphitic carbon [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]]. It had been reported that carbon dissolution into liquid steel from graphitic carbon occur at a very fast rate with the melt carbon content reached about 4–5 wt% within 5 min after contact with liquid steel. On the other hand, carbon dissolution from nongraphitic carbon occur at a slower rate with the melt carbon content reached take longer time to reach a saturation limit, 5.28 wt% for plain carbon steel at 1550 °C. Carbon dissolution occurs in two steps: (i) the dissolution of carbon atoms from its host lattice and (ii) the transfer of carbon atoms across the interface [[23], [24], [25], [26], [27], [28], [29], [30], [31]]. Rate controlling mechanism for graphite is the first step, while it is the second step for nongraphitic carbon [[32], [33], [34], [35]]. Significant factors that affecting carbon dissolution into liquid steel are structure of the carbon that used in the process and ash content in the carbon and its chemistry.

The present study aims to evaluate the possibility for utilizing polymeric food packaging wastes (PS and PP) as a liquid steel recarburizer in ladle refining process. The novelty of this work is to utilize wastes PS and PP without mixing with the conventional fossil fuels to reduce effect of ash, which had not been investigated previously. The polymeric wastes will be converted into a carbonaceous char and then used for carbon dissolution experiment by reacting with liquid steel at 1550 °C for up to 30 min. Behavior of carbon dissolution into liquid steel for the chars and their kinetics dissolution rates will be investigated. The results will be compared with the dissolution behavior of the commercial recarburizer (anthracite). This could be one of the novel methods for the polymeric food packaging wastes management and the development of steelmaking industry toward sustainability.

2. Materials and methods

2.1. Sample preparation

Polymeric food packaging wastes (PS and PP) were collected from waste recycler in Samut Sakhon province, Thailand. PS and PP in the form of scraps were cleaned and air dried before using in the experiments. The polymers were then crushed into small pieces (<5 mm). PS was homogeneously blended with PP into 6 different ratios using a rolling mill for 30 min, see Table 1. The blending of PS and PP reveal the mixture of the polymers in waste stream. PS alone was employed in the present study for a sake of comparison. The blends were put in refractory crucible and then pyrolyzed with iron catalyst in a horizontal tube furnace at 1550 °C for 15 min under argon atmosphere. Small piece of iron chip was used as the iron catalyst in order to enhance the decomposition of polymers and the formation of carbon.

Table 1.

Blend ratios and chemical composition of the chars.

Samples	PS/PP Ratios	Ultimate Analysis (wt%)
	(wt%)	C	H	N	O	S
PS	100/0	87.77	0.23	0.27	<0.03	0.42
Blend#1	90/10	86.04	0.57	0.38	<0.03	0.46
Blend#2	80/20	87.65	0.53	0.27	<0.03	0.44
Blend#3	70/30	85.97	0.72	0.23	<0.04	0.42
Blend#4	60/40	89.23	0.55	0.30	<0.03	0.43
Blend#5	50/50	90.57	0.64	0.30	<0.03	0.46
Blend#6	40/60	91.47	0.77	0.27	<0.03	0.43

The obtained chars were ground using a ring mill and sieved into a powder of <106 μm for further experimental. Chemical composition of the chars was analyzed using a LECO CHN628 analyzer and performed in the center of scientific equipment for advanced research (TU-CSEAR), Thammasat University, Thailand and given in Table 1. LECO is a commercial company for chemical analyzer, LECO Corporation, MI, USA. Carbon content of the char that derived from the polymers ranging between 87.77 and 91.47 wt%. The increasing in PP concentration in the blends leads to the increase in carbon content of the chars. Structural and phase analysis of the carbon was conducted using X-rays diffractometer (XRD) and Raman spectroscopy techniques.

2.2. Carbon dissolution experiment

Approximately 0.2 g of the char powder was put in a refractory crucible and then pressed to smooth the top surface. Approximately 0.5 g of an electrolytic pure iron was placed on top of the char surface. The interactions were investigated through experiments in a horizontal tube furnace, while high purity argon (99.99%) flowing through at the rate of 1 L/min. The prepared crucible was inserted into the cold zone of the furnace where the temperature was about 300 °C for 5 min to prevent thermal shock. It was then inserted into the hot zone where the temperature was 1550 °C. The reaction time was noted to start when the iron chips had melted and from a spherical droplet (∼1 min) [[12], [13], [14]]. The crucible was quenched at 1, 2, 4, 8, 10, 15 and 30 min. The metal droplets were washed in ethanol using ultrasonic cleaning bath for 2 min to remove all debris on the metallic surface, and then air dried before carbon and sulphur measurement. Carbon picked up and sulphur transfer into liquid steel were measured using a LECO CS744 carbon-sulphur analyzer. Carbon picked up by liquid steel as a function of time for the polymeric chars can be determined. The overview of samples preparation and experimental procedure are given in Fig. 1.

Fig. 1.

Overview of samples preparation and experimental procedure.

Carbon dissolution behavior can be explained using Fick's law. A first order kinetic or equation (1) was used to describe the carbon dissolution into liquid steel, based on the variation of carbon content in the melt. Equation (2) is the integrated form of the first order kinetic equation.

$$\frac{d{C}_t}{dt}=\frac{Ak}{V}*\left(C_s-C_t\right)$$ (1)

$$\mathit{ln}\frac{\left(C_s-C_t\right)}{\left(C_s-C_o\right)}=-K*t$$ (2)

$$C_s=1.34+\left(2.57x{10}^{-3}\right)*T$$ (3)

k is the first order kinetic dissolution rate constant (m/s), while V represents the volume of the liquid steel droplet and A represents contact area between solid carbon and liquid steel droplet. K is equal to Ak/V, which is represents the overall kinetic dissolution rate constant (s⁻¹). C₀ is the initial carbon content (wt%) in liquid steel. Although the dissolution of carbon into liquid steel occurs so fast since the metal has partially melted, C₀ was set to be zero for all cases since an electrolytic pure iron was used in the present study. C_t is carbon content (wt%) in liquid steel at time (t) in second. C_s is the saturation limit of carbon in liquid steel. It can be calculated using equation (3), which is the empirical correlation proposed by Chipman et al. [36], where T is temperature in degree Celsius. At 1550 °C, C_s was calculated to be 5.28 wt%.

3. Results and discussion

3.1. Carbon dissolution into liquid steel

The variation in carbon dissolution into liquid steel at 1550 °C for the polymeric chars are presented in Fig. 2. Kinetics dissolution of carbon in the form of the ln((C_s-C_t)/(C_s–C₀))–vs.–time plots for the chars are showed in Fig. 3. The overall dissolution rates for the chars were calculated and shown in Table 2. Anthracite is the commercial recarburizer used in ladle refining process. It was supported by UMC Metal Co. LTD, steel mill in Thailand. Carbon dissolution behavior of the anthracite was reported in our previous work [37]. It was represented here for a sake of comparison. Carbon dissolution into liquid steel in the case of anthracite occurs slowly within the first 4 min of reaction with the kinetics rate of 0.88 × 10⁻³ s⁻¹ and then keeps pace with the rate of 0.36 × 10⁻³ s⁻¹. The melt carbon content reached ∼2.81 wt% after 30 min [37]. On the other hand, carbon dissolution for the PS char occurs rapidly within 4 min with the rate of 8.4 × 10⁻³ s⁻¹, which is 10 times faster than that of anthracite. The maximum melt carbon content in the case of PS was 4.75 wt%. For Blend#1 – Blend#6 chars, carbon dissolution occurs at the rates of 1.46–4.42 × 10⁻³ s⁻¹ within the first 4–10 min of reaction and then keep pace with the maximum carbon content of 4.08–4.97 wt%. The blending of PS with PP had insignificant effect on retarding the dissolution of carbon. All polymeric chars show greater carbon dissolution behavior than that of the commercial recarburizer. “The changes in kinetic rates of carbon dissolution can be explained in term of interfacial phenomena at solid carbon and liquid steel interface. During the first few minutes, liquid steel could contact with a fresh carbon surface with no interfacial ash layer, leading to the rapid rate of carbon dissolution into liquid steel. Thereafter, interfacial ash layer was formed at the metal/carbon interface hindering fresh carbon surface to be contacted with liquid steel, leading to the slower rate of carbon dissolution. The consumption of dissolved carbon atoms in liquid steel via interfacial reactions, such as desulphurization and the reduction of reducible oxides could also explain the two stages kinetic dissolution rates.”

Fig. 2.

Carbon dissolution into liquid steel for the chars and anthracite.

Fig. 3.

Plots of ln((C_s-C_t)/(C_s–C₀))–vs.–time for the chars and anthracite.

Table 2.

Kinetics rate of carbon dissolution into liquid steel at 1550 °C.

Samples	Stage I	Stage II	Times for Stage I
	K x 10−3 s−1	K x 10−3 s−1	min
Anthracite	0.88	0.36	4
PS	8.40	0.45	4
Blend#1	2.32	0.90	8
Blend#2	3.52	0.07	4
Blend#3	4.36	0.00	10
Blend#4	1.46	0.09	8
Blend#5	3.38	0.32	8
Blend#6	4.42	0.09	4

3.2. Sulphur transfer into liquid steel

Sulphur in liquid steel can decrease the diffusivity of carbon in liquid steel, thus hindering the carbon dissolution. The transfer of sulphur into liquid steel occurs concurrently with the dissolution of carbon. Variations of sulphur transfers into liquid steel from the polymeric chars are presented in Fig. 4. The sulphur transfers into liquid steel for polymeric chars were small with the sulphur level of 0.01–0.025 wt% for 30 min of reaction. This was about 2 times smaller than that in case of the commercial recarburizer. Sulphur transfer for the anthracite ranges between 0.02 and 0.07 wt%. The blending of PS with PP was found to have insignificant effect on the transfer of sulphur into liquid steel.

Fig. 4.

Sulphur transfer into liquid steel for the chars and anthracite.

3.3. Factor effecting carbon dissolution behavior

3.3.1. Effect of carbon structure

XRD patterns of the polymeric chars are presented in Fig. 5. The crystallite size (L_C) of the chars had been calculated using Debye–Scherrer equation (L_C = Kλ/βcosθ) and given in Table 3. K represents the shape factor, which is approximately 0.89. λ represents the X-ray wavelength for copper Kα, which is 1.5406 Å. β represents the value of full-width at half-maximum (FWHM) of the diffraction peak in radian. θ represents the diffraction angle of the diffraction peak. The 002-carbon peak occurs at 2θ angle of 25.4–25.6°. Small peaks of ash were seen, which expected to be CaO from the filler in the polymer. The PS char shows highest degree of crystallinity with the L_C of 2.45 nm, which is 2 times higher than that of the anthracite (1.19 nm). The data for anthracite was presented here for a sake of comparison [37]. The blending of PS with PP was found to decrease the crystallite size of the chars produced. The L_C values are in between 0.27 and 1.08 nm. The char derived from PS and PP is characterized as a semi crystalline in nature. This is supported by Raman spectra results provided in Fig. 6.

Fig. 5.

XRD patterns of the polymeric chars pyrolyzed at 1550 °C for 15 min.

Table 3.

Crystallite size (LC) of the chars and anthracite.

Samples	FWHM (2θ)	Center (2θ)	LC (nm)
Anthracite	6.7608	24.18	1.1893
PS	3.2899	25.40	2.4497
Blend#1	7.4564	25.58	1.0813
Blend#2	9.4532	25.56	0.8528
Blend#3	7.8340	25.59	1.0291
Blend#4	8.7368	25.57	0.9228
Blend#5	29.5225	25.40	0.2730
Blend#6	14.2236	25.60	0.5668

Fig. 6.

Raman spectra of the polymeric chars pyrolyzed at 1550 for 15 min.

Raman spectra for all polymeric chars show the disorder band (D-band) at the wave number of approximately 1347 cm⁻¹, which considered as the amorphous region. While, the order band (G-band) occur at the wave number of approximately 1584 cm⁻¹, which considered as the graphitic region. Intensity of D-band (I_D) and G-Band (I_G) was seem to be approximately equal, and thus the I_d/I_G ratio was expected to be about 1.0. The blending of PS with PP was found to decrease the intensity of D-band and G-Band of the polymeric chars. The devolatilization of PS/PP was expected to consist of volatiles matter, especially hydrocarbon compounds. The occurrence of graphitic region in the Raman spectra was due to the pyrolysis of PS and PP with Fe catalyst at 1550 °C, which promote the decomposition of CH₄ in the system. The presence of fresh Fe surface in the system leads to the decomposition of CH₄ and form C and H₂. The derived carbon atom is graphitic in nature, see equation (4) [38]. Standard free energy change (ΔG°) for equation (4) is −110.82 kJ at 1550 °C [38]. Thus, the chars derived should be the mixture of solid amorphous carbon and graphitic carbon.

CH₄(g) = C(s) + 2H₂(g) (4)

Carbon dissolution from graphitic material (high L_c) into liquid steel occur at a very fast rate due to its stack layer structure, which allow carbon atom to dissociate easily from its host lattice [[23], [24], [25], [26], [27], [28], [29], [30], [31]]. For non-graphitic materials (low L_c), the dissolution of carbon occurs at a slower rate than that of graphite due to its complex amorphous structure [[32], [33], [34], [35]]. In the present study, carbon dissolution for the PS char occurs at a rate of 10 times faster than that of the anthracite. This could be explained in term of the higher degree of crystallinity of PS char compares to the anthracite. For Blend#1 – Blend#6 chars, although the L_c value was lower than that of anthracite, the dissolution of carbon into liquid steel occurs faster. Theses indicate that not only carbon structure affect the carbon dissolution behavior, but also there are other factors affecting the dissolution of carbon from the polymeric chars and anthracite.

3.3.2. Effect of impurity in polymeric wastes

Impurities in polymers is mostly in the form of filler or plasticizer. The impurities could have positive or negative effect when reacting with liquid steel [12]. During the pyrolysis, the polymers will decompose and turn into both solid carbon and volatile carbon. The filler will turn into ash in the carbon derived. Ash oxides in the carbon were known to govern the transfer of carbon atoms from interfacial layer into liquid iron [[31], [32], [33], [34], [35]]. During the contact of carbon and liquid iron, ash oxides in the carbon will form a viscous ash layer at solid/liquid interface. This could retard the carbon dissolution process. Chemistry of ash oxides play a vital role on the forming of interfacial ash layer, which has both negative and positive effect on the dissolution of carbon into liquid steel. Al₂O₃ in the ash can form the rigid and rough interfacial layer, while SiO₂ can consume solute carbon atoms in the melt via silica reduction, equations (5-7) [34,35]. CaO can react as a fluxing agent to decrease melting temperature of the interfacial ash layer and can consume solute carbon atoms via desulphurization reaction, equation (8) [12,34].

SiO₂(s) + C = SiO(s) + CO(g) (5)

SiO(s) + C = Si + CO(g) (6)

SiO(g) + 2C = SiC(s) + CO(g) (7)

CaO(s) + C + S = CaS(s) + CO (g) (8)

Many previous studies [[6], [7], [8], [9], [10], [11]] converted plastics into various forms of solid carbon, such as carbon sphere and carbon nanorod, by pyrolysis at temperatures from 300 to 900 °C. However, effect of plastics fillers had not been reported. The PS used in the present study contains Ca(OH)₂ as a filler, which might affect the carbon dissolution into liquid steel. According to previous studies [[6], [7], [8], [9], [10], [11]], we have pyrolyzed PS and Blend#1 – Blend#6 with iron catalyst at 800 °C for 15 min under argon atmosphere. XRD patterns of the chars derived are presented in Fig. 7. It was found that the broad 002-carbon peak occurs at 2θ angle of 25–26°. Ca(OH)₂ peak was detected along with CaO peaks, which probably came from thermal decomposition of the filler. This reveals that the pyrolysis of PS and PS/PP blends at low temperature cannot eliminate the filler. The chars pyrolyzed at 800 °C were employed for carbon dissolution experiment, and the carbons picked up by liquid steel at 15 min of reaction are showed in Fig. 8. It was found that carbon dissolution occurred at a very low level with the melt carbon content ranges between 0.68 and 0.81 wt% for all cases. This is vastly different from that of the chars pyrolyzed at 1550 °C (4.08–4.97 wt% of carbon). From Fig. 5, the pyrolysis at steelmaking temperature (1550 °C) can surely remove Ca(OH)₂ compound by turning it into CaO. Low level of CaO (small CaO peaks) in the chars was not expected to retard the dissolution of carbon into liquid steel [12]. However, the presence of Ca(OH)₂ in the chars was expected to hinder the dissolution of carbon into liquid steel, and could responsible for the low level of carbon dissolution into liquid steel for the chars that pyrolyzed at 800 °C. Fig. 9 shows SEM micrograph and EDS spectra for the Fe droplet surface after interacted with the PS char (pyrolyzed at 1550 °C) for 15 min. The clear metal surface was observed with dendritic shape. There was no ash oxides layer covering the metal surface due to the very low level of ash content in the PS char. The presence of small CaS peak could be due to desulphurization reaction of CaO in the char and S in liquid steel according to equation (8). The Fe droplet surfaces after contacted with the chars of Blend#1 – Blend#6 were expected to be similar to that in the case of the PS char due to the low level of ash content.

Fig. 7.

XRD patterns of the polymeric chars pyrolyzed at 800 °C for 15 min.

Fig. 8.

Carbon picked up by liquid steel droplets from the polymeric chars pyrolyzed for 15 min at (a) 800 °C and (b) 1550 °C.

Fig. 9.

SEM micrograph and EDS spectra for Fe droplet surface after contacted with PS char (pyrolyzed at 1550 °C) for 15 min.

Table 4 shows kinetics rates of carbon dissolution and the maximum carbon picked up by liquid steel from various carbonaceous materials obtained from previous works compare to that from the present studies. It was found that the kinetics rates and the maximum carbon picked up values from the present studies are comparable to that of the previous works. The difference in kinetics rates were expected to be due to the different in characteristic of carbonaceous materials, such as carbon structure, carbon, sulphur and ash contents. For the present studies, rate controlling mechanism for carbon dissolution from polymeric chars was the dissociation of carbon atom from its host lattice, because there was no interfacial ash layer formed at the Fe/C interface. The present studies have shown that the chars produced from food packaging wastes (PS and PP) can be replaced a commercial recarburizer and no negative effect on steel quality. This is one of the novel methods for the polymeric wastes management and the development of steelmaking industry toward sustainability.

Table 4.

Kinetics rate and maximum carbon picked up by liquid steel from various carbon obtained from selected previous works compare to the present study.

Samples	k rates (x 10−3 s−1)	Max C in Fe (wt%)	Literatures
Char coal	–	<0.1 at 60 min	McCarthy et al. (2005) [34]
Coke	14.7	>5.0 at 30 min	Cham et al. (2004) [32]
Coke	1.1	>5.0 at 30 min	Cham et al. (2004) [32]
Coke	–	4.67 at 120 min	Chapman et al. (2008) [35]
Coke	0.7	0.8 at 60 min	Kongkarat et at. (2017) [14]
Anthracite	0.88	2.81 at 30 min	Kongkarat (2019) [37]
Melamine	0.55	5.65 at 45 min	Nath et al. (2012) [15]
Polycarbonate	19.2	4.60 at 15 min	Mansuri et al. (2013) [16]
Polystyrene	8.40	4.21 at 30 min	Present study
Blend#1	2.32	4.86 at 30 min	Present study
Blend#2	3.52	4.08 at 30 min	Present study
Blend#3	4.36	4.62 at 30 min	Present study
Blend#4	1.46	4.42 at 30 min	Present study
Blend#5	3.38	4.97 at 30 min	Present study
Blend#6	4.42	4.20 at 30 min	Present study

4. Conclusions

Utilization of polymeric food packaging wastes (PS/PP) as a liquid steel recarburizer is possible. In this study, the polymeric wastes were converted into chars by pyrolysis with iron catalyst at 1550 °C for 15 min. Carbon dissolution into liquid steel at 1550 °C for the chars were successfully investigated. The experimental results can be concluded as below.

• •The polymeric chars derived from food packaging wastes (PS/PP) were found to be semi-crystalline structure with the crystallite size (L_C) ranges between 0.27 and 2.45 nm. The crystallite size decreased with decreasing PS concentration in the blends.
• •Carbon contents in the polymeric chars was ranging between 86 and 91.47 wt% with small amount of hydrogen, nitrogen and sulphur (<1 wt%).
• •The polymeric chars were brought into contact with an electrolytic pure iron at 1550 °C for up to 30 min under argon atmosphere. The maximum carbon picked up by liquid steel for the chars was ranging between of 4.08–4.97 wt%, which almost two times higher than that of the commercial recarburizer (anthracite).
• •Carbon dissolution from the chars into liquid steel occur rapidly for all cases with the overall dissolution rates (K) of 1.46 × 10⁻³ - 8.4 × 10⁻³ s⁻¹ within the first 4–10 min and then keep pace until 30 min of reaction. The maximum was observed in the case of PS char (K = 8.4 × 10⁻³ s⁻¹), which ten times faster than that of the anthracite (K = 0.88 × 10⁻³ s⁻¹).
• •Among the PS/PP blends, Blend#3 shows the optimum carbon dissolution behavior with the fastest dissolution rate (K) of 4.42 × 10⁻³ s⁻¹.
• •Sulphur transfer into liquid steel was small for all cases with the maximum content ranges between 0.01 and 0.025 wt%, which does not affect steel quality.
• •Rate controlling mechanism for carbon dissolution from polymeric chars was the dissociation of carbon atom from its host lattice.
• •Ca(OH)₂ was found in PS as an impurity, which could hinder the dissolution of carbon into liquid steel. However, it can be eliminated by pyrolysis at steelmaking temperature (1550 °C or above). This is one of the advantages of utilizing polymeric food packaging in steelmaking process.

Author contribution statement

Sajjaporn Singsai: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Somyote Kongkarat: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This work was supported by National Research Council of Thailand (NRCT).

Data availability statement

Data will be made available on request.

Additional information

No additional information is available for this paper.

Declaration of competing interest

The authors declare no conflict of interest.