Kinetics of Reduction of Iron Ore Powder by Industrial Lignin from Pulping and Papermaking Waste Biomass Energy

Abstract

To explore the application of industrial lignin, a waste biomass resource, in the field of metallurgy, the kinetic behavior of iron ore powder reduction by Shenmu bituminous coal, Lu’an anthracite, and industrial lignin under different carbon–oxygen molar ratios (n_c:n_o = 0.5, 0.7, 1.0, 1.2, and 1.5) was studied using a thermogravimetric analyzer and reduction furnace. The results show that the reduction process of iron ore powder with three reducing agents conforms to the D1 model, that is, the kinetic equation is $G\left(\alpha \right)={\alpha }^2$. Under the same carbon–oxygen molar ratio, the activation energy of the iron ore powder reduced by industrial lignin is lower than that of pulverized coal. The activation energy increases first, then decreases and then increases with the increase in the carbon–oxygen molar ratio. When the molar ratio of carbon to oxygen is 1.2, the reaction activation energy is the lowest. At this time, the reaction activation energy of industrial lignin reduction iron ore powder is 14.21 kJ·mol⁻¹, that of Shenmu bituminous coal is 16.81 kJ·mol⁻¹, and that of Lu’an anthracite is 37.13 kJ·mol⁻¹.

1. Introduction

With the rapid development of the global economy and the continuous improvement in environmental awareness, the iron and steel industry, as an important field of energy consumption and carbon emissions, has become the focus of global attention on its green development path. According to the statistics of the World Steel Association, the steel industry accounts for 7% to 9% of global carbon dioxide emissions [1]. In 2022, the world’s crude steel production is 1.89 billion tons, of which 71.1% is produced by the traditional blast furnace-converter long process, and the average carbon dioxide emission intensity of this process is 2.33 tons/(ton crude steel) [1]. In 2022, China’s crude steel production is 1.019 billion tons, accounting for 53.9% of the world’s crude steel production, and the pig iron production is 860 million tons, accounting for 66.4% of the world ’s pig iron production [1]. In the iron and steel manufacturing process, the ironmaking process is not only the main source of energy consumption and emissions but also one of the units with the highest manufacturing cost [2,3]. Therefore, China’s ironmaking industry is facing great pressure regarding energy conservation and emission reduction. Under the background of the country’s vigorous development of green economy and low-carbon economy, exploring the green and low-carbon technology of ironmaking process has become the key to the sustainable development of the iron and steel industry.

As a renewable green carbon source, biomass has the advantages of wide distribution, rich resources, low content of harmful elements, and low pyrolysis temperature. It is expected to replace a part of coal and coke in the ironmaking process and achieve the goal of energy conservation and emission reduction [4,5]. Luo et al. [6] studied the mechanism of adding biomass into pellets and the limiting links in the reduction process of pellets containing biomass in the atmosphere of biomass syngas. The results showed that the reaction activation energy of composite pellets with biomass was lower than that of pellets without biomass. Srivastava et al. [7] studied the effects of roasting temperature and roasting time on biomass iron oxide composite pellets. The results showed that biomass composite pellets could obtain higher apparent density and metallization rate of direct reduced iron at lower temperature and shorter roasting time than pulverized coal as a reductant. Ueda et al. [8] studied the reduction behavior of biomass iron ore composite pellets using carbonized cryptomeria fortunei as a reducing agent. The results showed that the reactivity of biomass composite pellets was significantly faster than that of composite pellets using coke. Fu et al. [9] produced direct reduced iron from carbonized rice husk, coconut husk, and bamboo. When the molar ratio of carbon to oxygen is 1, the metallization rates of direct reduced iron from bamboo charcoal, coconut husk charcoal, coke, and rice husk charcoal are 95.56%, 95.42%, 88.07%, and 49.3%, respectively.

Currently, the world produces about 100 million tons of lignin per year through pulping and biorefinery processes (bioethanol preparation), worth USD 732.7 million [10]. However, about 95% of industrial lignin is used as fuel for heat and power generation, while only 5% is used effectively [11,12]. It is widely accepted that the biosynthesis of lignin stems from the polymerization of three types of phenylpropane units, also referred to as monolignols. These units are coniferyl, sinapyl, and p-coumaryl alcohol [13]. At present, the research on industrial lignin mainly focuses on its purification and modification. The modified industrial lignin can be used as flocculant, lubricant, adsorbent, dispersant, coating material, polymer additive, and so on [14,15,16,17,18]. However, the structure of lignin is different due to the extraction process and the presence of different functional groups, so the subsequent treatment cost is high [19,20,21,22,23].

China’s pulp import volume in the past decade is shown in Figure 1 [24]. In 2023, the domestic pulp output reached the highest value of 88.23 million tons [25]. It can be seen that China’s demand for pulp is very large. By 2023, the import volume of pulp has exceeded 36 million tons, which indicates that China’s annual self-produced pulp has been unable to meet China’s papermaking needs. China will also produce a large number of industrial lignin by-products every year. Therefore, the reduction, recycling, and harmless treatment of industrial lignin solid waste resources is one of the important tasks of the pulping industry. The elemental analysis of industrial lignin from different sources is shown in

Table 1. Element analysis of industrial lignin from different sources.

Types of Industrial Lignin	C (%)	H (%)	O (%)	N (%)	S (%)
wheat straw alkaline lignin [27]	54.75	5.407	35.35	3.75	0.743
pine alkali lignin [27]	65.36	5.836	25.928	0.08	2.796
enzymatic hydrolysis lignin [27]	56.06	5.599	36.939	1.15	0.252
hydrolysis lignin [28]	61	6.1	31.9	0.69	0.12
alkali lignin [29]	62.40 ± 0.14	6.14 ± 0.00	29.43 ± 0.23	0.26 ± 0.03	1.77 ± 0.06
lignin (isolated from lignocellulosic bioethanol residues) [30]	62.36	5.89	31.22	0.49	0.03
lignin (isolated from hardwood kraft black liquor) [30]	59.51	5.79	32.03	0.13	2.54
kraft softwood lignin [30]	66.10	6.37	25.30	0.67	1.57
soda non-wood lignin [30]	65.41	6.53	27.09	0.59	0.38

. It can be seen that the carbon content in industrial lignin is very high, and it has great potential as a reducing agent. In addition, industrial lignin contains a certain amount of sulfur, which may have an impact on blast furnace ironmaking. We will use hydrothermal treatment [26] to remove sulfur from industrial lignin in our subsequent work.

There are few studies on the use of industrial lignin as a reducing agent. Don et al. [28] studied investigated lignin as a reducing agent instead of fossil carbon for the reduction of zinc oxide and zinc ferrite contained in steelmaking dusts. Wei et al. [31] showed that lignin has great superiority compared with the reduction of iron oxide by coal. At the same time, Wei et al. [32], aiming to unveil a novel application of biomass, investigated and demonstrated coupled biomass gasification and iron ore reduction through the pyrolysis/gasification of iron ore–lignin pellets (ILPs) at 1093–1333 K. However, these studies on the reduction kinetics of iron ore powder are not comprehensive enough.

In this paper, the temperature range and kinetic behavior of industrial lignin and pulverized coal reducing iron ore powder were compared and studied, and the potential of industrial lignin in the ironmaking industry was discussed. We hope to find a way that can not only solve the problem of difficult treatment of industrial lignin but also reduce the cost of steel production, reduce CO₂ emissions, and turn waste into treasure.

2. Materials and Methods

2.1. Materials

The reducing agents used in the experiment include an industrial lignin (IL), Lu’an anthracite (LA), and Shenmu bituminous coal (SM). LA, SM, and iron ore powder were provided by an iron and steel company, while IL was provided by Huawei Youbang Chemical Co., Ltd. (Tumen, China). Before the experiment, the iron ore powder was placed in an oven and dried at 378 K for 24 h. The industrial analysis and elemental analysis of pulverized coal and IL are shown in

Table 2. Proximate analysis and elemental analysis of pulverized coal and industrial lignin (mass, %).

Samples	Proximate Analysis (ad.)				Elemental Analysis (ad.)
	Fixed Carbon	Ash	Volatile	Moisture	C	H	O	N	S
IL	18.01	12.32	57.82	11.85	32.57	4.86	42.42	0.45	9.39
LA	75.90	10.01	13.38	0.71	82.00	3.41	8.50	1.79	0.20
SM	62.41	7.46	28.28	1.85	75.90	4.14	16.50	1.38	0.63

ad.: air-dried basis.

. The chemical composition of iron ore powder is shown in

Table 3. Composition analysis of iron ore powder (wt%).

TFe	FeO	SiO2	CaO	Al2O3	MgO	Others
68.04	24.88	5.50	0.14	0.28	0.36	0.8

, and its main phase composition is Fe₃O₄ and SiO₂, as shown in Figure 2.

2.2. Methods

According to the method of GB/T 212-2008, the proximate analysis of industrial lignin is carried out. The analysis of O, N, and H elements was determined using ONH836 (LECO, Laboratory Equipment Corporation, San Jose, CA, USA), and S and C elements were determined using SC-144DR (Laboratory Equipment Corporation, USA) analyzer. The composition analysis of iron ore powder was provided by the steel company. The phase of iron ore powder was analyzed using the D8ADVANCE X-ray diffractometer (XRD, Bruker, Bremen, Germany), the scanning range was 10–90 degree, and the scanning rate was 5 degree/min. The reduction characteristics of iron ore powder with different reducing agents under nitrogen atmosphere were studied on a thermogravimetric analyzer (STA409CD, Netzsch Group, Hanau, Germany). In this work, the mixing ratio of the reducing agent and iron ore powder was determined according to the molar ratio of carbon to oxygen of 0.5, 0.7, 1.0. and 1.5, respectively, that is, n_c:n_o = 0.5, 0.7, 1.0, and 1.5, where n_c is the mole fraction of total carbon in the reducing agent; n_o is the mole fraction of oxygen in Fe₃O₄ in iron ore powder. For each test, the samples (about 10 ± 0.5 mg) were heated from 30 to 1100 °C at heating rates of 10 °C/min under an N₂ flow at 30 mL/min. A separate blank run was conducted using an empty pan under identical conditions, and these data were used for baseline correction during the evaluation of the sample thermal gravimetric analysis (TGA) profile.

The gas changes produced by the reduction of iron ore powder by three reducing agents when n_c/n_o = 1.0 were studied using a reduction furnace. The mixed sample (about 5 g) was placed in the hanging basket, which is suspended under the electronic balance. The N₂ with a flow rate of 3 L/h was introduced from the bottom of the reactor, and it passed through the sample and flowed out from the top. In this process, a layer of alumina balls with a thickness of about 100 mm was paved at the bottom of the reactor to ensure a steady flow of gas through the sample and reduce the experimental error. The equipment can be controlled by computer, and the maximum temperature can reach 1500 °C. The reduction temperature range is from room temperature to 1100 °C, and the heating rate is 10 °C/min. The gas analyzer is connected to the outlet of the reduction furnace, and the gas composition is automatically recorded every minute. The equipment is shown in Figure 3.

3. Results and Discussion

3.1. Comparison of Reducing of Iron Ore Powder with Different Reducing Agents

The experimental results of the reduction of iron ore powder with three reducing agents are shown in Figure 4. It can be seen from Figure 4 that when the carbon–oxygen molar ratio is 1.2, the TG curve is at the bottom, which indicates that the industrial lignin completely reduces the iron ore powder the most at this time. This is mainly because when the molar ratio of carbon to oxygen is less than 1.2, the content of reducing agent is too low, and the content of reducing gas containing C and H produced in the pyrolysis process is also low, resulting in the iron ore powder not being able to be completely reduced. When the molar ratio of carbon to oxygen is greater than 1.2, the content of reducing agent is too high, and more tar will be produced during the pyrolysis process, which will hinder the diffusion of reducing gas, and ultimately lead to the iron ore powder not being able to be completely reduced.

It can be seen from the DTG curve in Figure 4 that there are three main peaks during the reduction process. The first peak, that is, when the temperature is low, is a drying and degassing process. The weight loss of pulverized coal in this stage is mainly due to the release of water, some gasses (CH₄, CO₂, and N₂) in the pores, and the thermal decomposition of some chemical bonds with a weak bond energy. The weight loss of industrial lignin is due to the volatilization of free water and bound water, as well as the volatilization of a small amount of carboxylic acid during glass transition [29].

The second peak is mainly due to two factors, one is due to the precipitation of volatile substances in the reductant, and the other is due to the partial reduction of iron ore powder by the reducing gas generated during the pyrolysis of reductant. The third peak is mainly due to the reduction of iron ore powder by the char produced after the pyrolysis of the reductant, including two processes, Fe₃O₄ → FeO and FeO → Fe, which also include a small amount of carbon melting loss reaction.

3.2. Determination of Reduction Temperature Range

In order to determine the temperature range of reducing iron ore powder with the reducing agent, the reduction curve of the mixed sample of the reducing agent and iron ore powder (n_c/n_o = 1.0) was analyzed, as shown in Figure 5. It can be seen that the DTG curve will suddenly rise at a higher temperature, which is the violent period of iron ore powder reduction. According to the temperature corresponding to the rapid rise in the DTG curve in the high temperature zone, the starting temperatures of LA, SM, and IL reduced iron ore powder were determined to be 850, 850, and 800 °C, respectively. In order to further determine the starting temperature of iron ore powder reduction, the change in carbon monoxide gas content released during the reduction of three mixed samples with a molar ratio of carbon to oxygen of 1 (n_c/n_o = 1.0) was analyzed, as shown in Figure 6. It can be seen that the temperature at which CO is generated during the reduction of iron ore powder by pulverized coal is approximately 800 °C, and that of industrial lignin is about 700 °C. This indicates that the temperature at which CO is generated during the reduction of iron ore powder by the three reductants is consistent with the temperature at which DTG curve rises rapidly in the high temperature section of Figure 5, so it can be proven that the temperature range of iron ore powder reduced by the three reductants is correctly divided. It can be seen from Figure 4c that when the molar ratio of IL to iron ore powder is 1.2, the TG curve will not change when the reduction temperature is higher than 1020 °C, so the reduction temperature range of the sample is divided into 800~1020 °C. Based on the above analysis, the division results of reduction temperature range of iron ore powder reduced by three reductants are shown in

Table 4. Division of reduction temperature interval.

Samples	Reduction Temperature Range (°C)
LA + Iron ore powder (nc:no = 0.5, 0.7, 1.0, 1.2, 1.5)	850~1100
SM + Iron ore powder (nc:no = 0.5, 0.7, 1.0, 1.2, 1.5)	850~1100
IL + Iron ore powder (nc:no = 0.5, 0.7, 1.0, 1.2, 1.5)	800~1020

.

3.3. Kinetics of Iron Ore Powder Reduction

To explore the reasons for the differences in the reduction of iron ore powder by pulverized coal and IL, the kinetics of the reduction of iron ore powder by three reducing agents were studied. Since the reduction mainly occurs in the temperature range determined in Section 3.2, the kinetics of this temperature range is mainly studied.

This article uses the Coats–Redfern integration method [33] to study the process of reducing iron ore powder with different reducing agents. The equation derived by this method is shown in Equation (1).

$$\mathit{ln}\left[{\displaystyle \frac{G\left(\alpha \right)}{T^2}}\right]=\mathit{ln}\left({\displaystyle \frac{AR}{\beta E}}\right)-{\displaystyle \frac{E}{RT}}$$

(1)

where G(α) is the integral form of the kinetic mechanism function of solid reaction; A is the frequency factor, min⁻¹; E is the activation energy, J·mol⁻¹; R is the gas constant, 8.314 J·mol⁻¹·k⁻¹; T is the absolute temperature, K; β is the heating rate is, k·min⁻¹; α is the conversion rate, %. The expression of conversion rate α is as follows:

$$\alpha ={\displaystyle \frac{w_0-w_t}{w_0-w_{\infty }}}$$

(2)

where w₀, w_t, and w_∞ are the initial mass of the sample, the mass at time t, and the mass at the end of the reaction, respectively, in mg.

When the correct reaction mechanism G(α) is determined, the relationship between $ln\left[{\displaystyle \frac{G\left(\alpha \right)}{T^2}}\right]$ and 1/T must be in a straight line. The activation energy can be obtained by the slope of the straight line, and the frequency factor can be obtained by intercept.

The forms of commonly used solid reaction kinetic mechanism functions are shown in

Table 5. Expressions of G(α) and f(α) for the kinetic model functions usually employed for the solid-state reaction.

Shorthand	Reaction Model	Integral Formula G(α)	Differential Formula f(α)
Nucleation models
P1	Mampel power	α	1
P2	Mampel power	α1/2	2α1/2
P3	Mampel power	α1/3	3α2/3
P4	Mampel power	α1/4	4α3/4
A1	Avaramie Erofeev	[−ln(1 − α)]3/2	${\displaystyle \frac{2}{3}}(1-\alpha )[-\mathit{ln}(1-\alpha ){]}^{{\displaystyle \frac{1}{2}}}$
A2	Avaramie Erofeev	[−ln(1 − α)]2	${\displaystyle \frac{1}{2}}(1-\alpha )[-\mathit{ln}(1-\alpha ){]}^{-1}$
A3	Avaramie Erofeev	[−ln(1 − α)]3	${\displaystyle \frac{1}{3}}(1-\alpha )[-\mathit{ln}(1-\alpha ){]}^{-2}$
A4	Avaramie Erofeev	[−ln(1 − α)]4	${\displaystyle \frac{1}{4}}(1-\alpha )[-\mathit{ln}(1-\alpha ){]}^{-3}$
Chemical reaction
F1	First order	−ln(1 − α)	1 − α
F2	Second order	(1 − α)−1 − 1	(1 − α)2
F3	Third order	[(1 − α)−2 − 1]/2	(1 − α)3
Geometrical contractions
R2	Contracting area	1 − (1 − α)1/2	2(1 − α)1/2
R3	Contracting volume	1 − (1 − α)1/3	3(1 − α)2/3
Jandere quation
D1	One-dimensional	α2	$-{\displaystyle \frac{1}{2}}{\alpha }^{-1}$
D2	Two-dimensional	[1 − (1 − α)1/2]1/2	$4(1-\alpha {)}^{{\displaystyle \frac{1}{2}}}[1-(1-\alpha {)}^{{\displaystyle \frac{1}{2}}}{]}^{{\displaystyle \frac{1}{2}}}$
D3	Three-dimensional	[1 − (1 − α)1/3]1/2	$6(1-\alpha {)}^{{\displaystyle \frac{2}{3}}}[1-(1-\alpha {)}^{{\displaystyle \frac{1}{3}}}{]}^{{\displaystyle \frac{1}{2}}}$

[34]. Through calculation, we found that the correlation coefficient R² is optimal when G(α) = α².

The kinetic curves of iron ore powder reduction by three reducing agents are shown in Figure 7. The kinetic fitting equation and activation energy calculated from the kinetic curves are shown in

Table 6. Kinetic parameters of reducing iron ore powder with LA.

Reduction Temperature Range (°C)	nc/no	Fitting Formula	Activation Energy (kJ·mol−1)	Correlation Coefficient
850~1100	0.5	y = −13438.68x − 4.65	111.73	0.9495
850~1100	0.7	y = −13857.81x − 4.43	115.21	0.9521
850~1100	1.0	y = −14806.28x − 3.74	123.10	0.9579
850~1100	1.2	y = −4465.76x − 11.22	37.13	0.9768
850~1100	1.5	y = −13906.33x − 4.55	115.62	0.9446

,

Table 7. Kinetic parameters of reducing iron ore powder with SM.

Reduction Temperature Range (°C)	nc/no	Fitting Formula	Activation Energy (kJ·mol−1)	Correlation Coefficient
850~1100	0.5	y = −9275.81x − 7.65	77.12	0.9453
850~1100	0.7	y = −9246.24x − 7.81	76.87	0.9675
850~1100	1.0	y = −9972.79x − 7.40	82.91	0.9360
850~1100	1.2	y = −2022.47x − 12.90	16.81	0.9232
850~1100	1.5	y = −7633.14x − 9.09	63.46	0.9297

and

Table 8. Kinetic parameters of reducing iron ore powder with IL.

Reduction Temperature Range (°C)	nc/no	Fitting Formula	Activation Energy (kJ·mol−1)	Correlation Coefficient
800~1020	0.5	y = −4901.83x − 10.40	40.75	0.9373
800~1020	0.7	y = −4824.54x − 10.54	40.11	0.9760
800~1020	1.0	y = −4434.55x − 11.00	36.87	0.9546
800~1020	1.2	y = −1709.18x − 13.03	14.21	0.9105
800~1020	1.5	y = −3310.17x − 11.82	27.52	0.9203

. The change in activation energy under different carbon-oxygen molar ratios is shown in Figure 8. Higher n_c/n_o will lead to the change in the interaction between the gas (such as CO and CO₂) and solid products (such as char and slag) produced in the reduction process. This change will affect the balance of the reaction, which will reduce the correlation coefficient.

It can be seen from Figure 8 that under the same carbon–oxygen molar ratio, the activation energy of iron ore powder reduced by pulverized coal is higher than that of IL. The main reasons for the above phenomenon are as follows: (1) the biomass char after pyrolysis has better reactivity [35,36,37]; (2) the volatile content of industrial lignin (57.82%, In Table 1) is higher, which will produce more reducing gasses containing C and h during pyrolysis, which is more conducive to the reduction of iron ore powder; (3) according to the previous research results, the specific surface area of biomass char is larger than that of pulverized coal char [34], which increases the contact area between carbon and iron ore powder, thus improving the reduction reaction efficiency; (4) the iron produced by the reduction of iron ore powder has a catalytic effect on the pyrolysis of industrial lignin [31,38,39], which further promotes the reduction reaction. The reduction reaction mechanism of industrial lignin on iron ore powder can be explained by the following process.

$$\mathrm{I}\mathrm{L}\stackrel{{\mathrm{F}\mathrm{e}}_x{\mathrm{O}}_y}{\to }{\mathrm{H}}_2+\mathrm{C}\mathrm{O}+{\mathrm{C}}_{\mathrm{F}\mathrm{i}\mathrm{x}\mathrm{e}\mathrm{d}\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}}+\mathrm{O}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{s}$$

(3)

$$\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_y+y{\mathrm{H}}_2\to y{\mathrm{H}}_2\mathrm{O}+x\mathrm{F}\mathrm{e}$$

(4)

$$\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_y+y\mathrm{C}\mathrm{O}\to y\mathrm{C}{\mathrm{O}}_2+x\mathrm{F}\mathrm{e}$$

(5)

Fe_xO_y + 2y/3C_{Fixed carbon} → y/3CO₂ + y/3CO + xFe

(6)

The overall reaction is expressed as follows:

$$\mathrm{I}\mathrm{L}+3{\mathrm{Fe}}_x{\mathrm{O}}_y\to {\mathrm{H}}_2\mathrm{O}+\mathrm{CO}+{\mathrm{CO}}_2+3x\mathrm{Fe}+\mathrm{O}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{s}$$

(7)

It is well known that natural lignin is a complex organic compound with many functional groups. The industrial lignin used in this study was obtained by the alkaline pulping process, which is closer to the characteristics of natural lignin. Industrial lignin is mainly composed of the ether bond (R–O–R) with low bond energy (about 380–420 kJ·mol⁻¹) and is easy to break during pyrolysis, so it is easier to produce reducing gasses such as CO and H₂. Pulverized coal is mainly composed of polycyclic aromatic hydrocarbons connected by the carbon–carbon bond with large bond energy (about 1000 kJ·mol⁻¹), which is not easy to break during heating [40]. In addition, Hu et al. showed that the content of CO and CO₂ generated from coal pyrolysis increased linearly with the increase in the o/c atomic ratio in coal [41]. In this study, the O/C atomic ratio of IL is 1.30, which is much higher than that of LA (0.1) and SM (0.22), resulting in a higher content of CO and CO₂ produced by the pyrolysis of IL, which is conducive to the reduction of iron ore powder.

This study also found that when the molar ratio of carbon to oxygen is 1.2, the activation energy of the reduction reaction of iron ore powder is the smallest. This is because when the molar ratio of carbon to oxygen is small, the content of the reducing agent is insufficient, resulting in a small amount of reducing gas and char generated after pyrolysis, which is not enough to completely reduce the iron ore powder. When the molar ratio of carbon to oxygen is too high, the reducing agent will generate a large amount of tar during the pyrolysis process, which hinders the reduction of iron ore powder. Therefore, based on the results of this experiment, the optimum carbon–oxygen molar ratio should be 1.2.

4. Conclusions

In the face of increasingly stringent environmental protection policies, as well as the country’s determination to vigorously develop a green economy and a low-carbon economy. In this paper, the kinetic behavior of industrial lignin produced in the pulp and paper industry as a reducing agent to reduce iron ore powder was studied. The following conclusions are obtained:

(1)

Under the same carbon–oxygen molar ratio, the activation energy of iron ore powder reduction by pulverized coal (16.81~123.10 kJ·mol⁻¹) is higher than that of industrial lignin (14.21~40.75 kJ·mol⁻¹). Industrial lignin is expected to be used as a reducing agent in the ironmaking process. The process of iron ore powder reduction by industrial lignin and pulverized coal conforms to the one-dimensional Jander model, and the kinetic equation is $G\left(\alpha \right)={\alpha }^2$.

(2)

According to the thermogravimetric curves of iron ore powder reduced by different reductants (LA, SM, and IL) and the change in CO content in the reduction process, the reduction temperature ranges of LA, SM, and IL reduced iron ore powder were determined to be 850~1100 °C, 850~1100 °C, and 800~1020 °C, respectively.

(3)

Under the condition of ensuring both sufficient reduction and reduction efficiency, the optimum carbon–oxygen molar ratio for the reduction of iron ore powder by industrial lignin is 1.2.