Production of Refractory Bricks through Combustion Synthesis from Metallurgical Wastes and the Thermo-Physical Properties of the Products

Abstract

Industrial symbiosis is one of the key approaches to meet sustainable and low carbon production targets. Thus, through circular approaches, it is possible to reduce the use of natural crude materials and make production processes waste-free in the metallurgical industry. The purpose of this study was to study the possibility of using various metallurgical waste and low-grade semi-finished products, which do not have a direct application area, in the production of heat-resistant carbon-containing refractory bricks through the combustion synthesis (CS) method. In the experiments, used metallurgical wastes were wet filter cake (FC), sludge (S), and refractory magnesite scrap (MS) while semi-products were rich and poor dust of chrome spinel (Cr-S). Simultaneously with the experiments, thermochemical simulation studies were carried out using the HSC Chemistry 6.12 to predict the thermodynamic properties of the reactions and possible reaction products. Thermal conductivity coefficients were determined in products in terms of thermal properties of composite samples, they were between 0.511 and 1.020 Wm/K. The phase compositions of the produced samples were determined via XRD technique. The TG-DTA technique was used to characterize thermal behavior of products. In addition, mechanical properties were determined by compression strength test. As a result of experiments, it was observed that Cr-S-rich-based samples showed a promising result in comparison to others: increasing amount of useful carbide phases were formed and demonstrated a high value of mechanical properties. Compression strength was increased from 2.7 MPa (sample №4) to 15.8 MPa (sample №1) with increasing chromite-containing phases in the green samples.

1. Introduction

As a result of population growth and industrialization, increasing amounts of industrial wastes cause serious social and environmental problems. The management of solid wastes is one of the important environmental issues in various developing countries. Solid waste is a by-product of human activities such as construction, manufacturing, and transportation. The use of industrial waste in the production of other materials (e.g., construction materials) is important for preventing environmental pollution, reducing production costs, and saving energy. Thus, the development of many technologies for the processing of such wastes is of great importance [1,2,3,4]. Any metallurgical production is accompanied by the formation of many intermediate products and wastes. Disposal or additional processing of these products is difficult and not economically feasible. Currently, metallurgical plants have accumulated huge amounts of metallurgical wastes such as slag and fine dust of gas cleaning systems. Generally, they are dumped and stored in open areas and all harmful impurities with atmospheric precipitation, rain, and snow penetrate the soil and poison groundwater. In addition, fine dust-like wastes are carried by the wind over the nearby territory, causing irreparable harm to the environment. Practice has showed that these wastes contain not only harmful impurities, but also components useful from the point of view of the refractory industry and, in terms of their mineralogical and chemical composition, can serve as raw materials to produce carbon-containing magnesia-chromite refractories [5]. According to the Ministry of Ecology of Kazakhstan, about 700 million tons of industrial waste is generated annually in the country. According to their data, the country has accumulated more than 22 billion tons of waste [6]. It is known that the composition of waste from metallurgical production includes several valuable elements, especially transition metals such as chromium, iron, manganese, vanadium, titanium, etc. The content of some of the listed elements reaches such a level that dumps can be considered as secondary minerals for wide application in various industries, including the production of metal products and refractory materials based on them, which contributes to resource saving, as well as reducing water use and water pollution [7].

Refractory ceramic materials are special materials which are used in the form of lining in high temperature furnaces for metal smelting and combustion purposes. They can be in monolithic or composite form and in the form of powder or brick with special geometries [8]. The development of metallurgical processes, associated with new developments in this area, imposes high requirements for improving the resistance of refractory materials of equipment due to changes in thermal conditions and charge components. Improvement the quality of the produced metals has also depended on the development of new compositions of carbon-containing refractories which have been resistant to melts and slags over the last 30 years. The main advantage of carbon-containing refractories in comparison to other types is, in addition to high thermal stability, their low wettability by melts and slags, and, accordingly, their increased chemical resistance. Recent studies have shown the promise of using carbon-containing refractories in high-temperature metallurgy processes, which make it possible to almost double the service life of products and equipment [9,10].

Combustion synthesis (CS) is a synthesis method and it utilizes metallothermic reduction-based reactions which are highly exothermic. When some metals which have higher affinity to oxygen (e.g., Al, Si, Mg) are added into the charge mixtures, the exothermic reactions between those metals and metal oxides in the reactants make the synthesis reactions occur without any additional energy. In this context, metallothermic reactions and metallothermic-based synthesis reactions can be a remarkable alternative to conventional metal production technologies with a low energy requirement. There are several subgroups of CS processes: self-propagating high-temperature synthesis (SHS), volume combustion synthesis (VCS), and solution combustion synthesis (SCS). Solid reactants are used in both SHS and VCS methods while reactants are liquid in SCS reactions. When SHS and VCS reactions are compared, it can be stated that SHS reactions are fully self-sustainable whilst VCS reactions are not. It means, once you start a SHS reaction (by a flame, resistant wire passing electricity or in a furnace, etc.), SHS reactions occur, and reactions are completed in a self-sustainable mode without any additional energy. But, in VCS reactions, it can be necessary to provide energy (despite being quite low in comparison to carbothermic reactions) to continue the reactions [11,12].

CS processes are extremely simple and economical methods to produce materials (alloys, intermetallics, ceramics etc.). Chemical reactions that are sufficiently exothermic can spontaneously convert to products when initiated and will propagate through the reactant mixture as a combustion wave [11,12].

Grains of electrode graphite are used as a carbon-containing material, which has a high erosion resistance to metal melts and slags due to its low wettability. Finely ground graphite, which is added to the exothermic mixture, serves as a source of carbon in the formation of carbides in reactions with the reduced metal [12,13].

In the process of CS at temperatures of 1200 °C–1500 °C, the reduced metals interact with carbon to form refractory carbides, which prevent the oxidation of graphite grains, for example [14]:

MgCr₂O₄ + 2Al → MgAl₂O₄ + 2Cr

(1)

7Cr + 3C→ Cr₇C₃

(2)

2Mg₂SiO₄ + 2Al → MgAl₂O₄ + 3Mg + 2SiO₂

(3)

MgAl₂O₄ + MgO + SiO₂ → Mg₂SiO₄ + Al₂O₃

(4)

Si + C → SiC

(5)

In recent years, carbon-containing (periclase-carbon) refractories have been widely introduced into metallurgical production, which has made it possible to increase the resistance of the main thermal units several times. This is due to a complex of their unique properties—high refractoriness, thermal conductivity, electrical conductivity, chemical resistance to melts based on most metals, both in oxidized and reduced form, low coefficient of thermal expansion, etc. At the same time, carbon, in the form of crystalline graphite and/or carbon black, as well as the coke residue of organic binders, is part of the matrix of the refractory material. It provides low wettability when interacting with slags and metals, high corrosion resistance, and thermal stability due to excellent thermal conductivity and low modulus of elasticity. In this regard, in recent years, there has been a steady increase in demand for high-quality carbon-containing refractory materials. The development of scientific bases for obtaining new domestic carbon-containing refractory materials using fundamentally new techniques and methods with the involvement of local mineral raw materials and waste from the metallurgical industry in refractory production is one of the most urgent problems of applied science [7,8]. Excess of 50% of total produced refractories comprise carbon, as it must meet requirements for thermomechanical and chemical properties [15,16].

Use of metallurgical wastes in the construction industry has been the widely used solution instead of storing in dumping areas. More efficient construction in the European Union would affect 42% of final energy consumption and more than 50% of all materials mined. It can also help save up to 30% on water consumption [16,17]. Other valorization fields of metallurgical wastes are the cement industry and it was also reported in the literature that particularly EAF slag has a potential to produce iron-based products such as steel [18].

One of the main aims of the present study was to find new valorization fields for metallurgical wastes. Therefore, we tried to identify an appropriate material for use in metallurgical refractories by defining the mechanical and thermal properties of refractories produced from metallurgical waste (a complex combination of metallurgical waste), which have not been studied in detail before. Thus, a promising route was defined to produce a currently mostly used class of carbon-containing refractory materials from industrial wastes with respect to a sustainable approach.

2. Materials and Methods

2.1. Materials

The following initial powders were used as ‘‘green components’’ for preparing refractory bricks: Filter cake (FC), sludge (S), refractory magnesite scrap (MS), and rich and poor dust of chrome spinel (Cr-S). The samples were obtained from a ferro chromium producer in Kazakhstan (ERG). Their chemical contents are given in

Table 1. Chemical contents of green components (wt.%).

Green Components	FC	S	MS	Cr-S-Rich	Cr-S-Poor
Mg2SiO4	62.7	65.0	-	30.5	25.2
MgCr2O4	30.5	-	-	59.5	20.0
MgAl2O4	-	20.9	-	-	-
FeMgAlSiO2	-	-	-	-	7.5
MgO	-	12.6	90.55	2.5	43.5
SiO2	2.6	1.5	2.05	2.5	2.3
Al2O3	-	-	3.8	-	-
TiO2	-	-	1.0	-	-
CaCO3	-	-	-	3.5	-
CaO	-	-	1.8	-	-
Fe2O3	-	-	0.8	-	-
Fe	4.2	-	-	1.5	1.5

, which were obtained by means of XRD-Rietveld analysis and chemical analysis (AAS, ICP).

2.2. Methods

Experimental studies included the preparation of mixtures from green components, pressing them to obtain green compacts, CS reactions in a muffle furnace and characterizations. Block diagram of experimental studies were given in Figure 1. The green components were dried at 100 °C for 6 h and stirred for 30 min by using an aqueous magnesium sulfate binder in a ratio of 15 wt.% to charge. The samples were pressed with a load of 88 MPa with cylindrical shape. The samples had a diameter of 40 mm. Four different types of initial samples (

Table 2. Compositions of samples (wt.%).

№	1	2	3	4
Components
Grained graphite (0.5–2 mm)	20	30	30	30
Milled graphite (<100 µm)	5	5	5	5
Al powder	18	14	16	16
Si powder	4	4	4	4
FC	26.5	23.5	29.67	29.67
Cr-S-rich	26.5	-	-	-
Cr-S-poor	-	23.5	-	-
S	-	-	15.33	-
MS	-	-	-	15.33

) were prepared with different compositions for detailed investigation of refractory bricks. After that, samples were put in a furnace at 950 °C. It took approximately 15 min to start the CS process. The reaction start temperatures of the samples were measured using a pyrometer in the furnace and they were as follows: ~1290 °C for sample №1, ~1170 °C for sample №2, ~1200 °C for sample №3, and 1150 °C for sample №4. Although the green samples contained many components, it was predicted that Equations (1)–(5) were the main reactions during CS processes.

The following methods were used for the simulation of the probable phases in CS reactions and for the quantitative and qualitative characterizations of the obtained refractory samples.

2.2.1. X-ray Diffraction Analysis

X-ray diffraction patterns of the samples were obtained by means of a DRON-4 general-purpose X-ray diffractometer using copper radiation. Based on them, the phase compositions of carbon-containing composites were determined. Preliminary processing of X-ray patterns to determine the angular position and intensities of reflections was carried out using the search-match program. Semi quantitative analysis was carried out by means of X’pert Highscore plus software with ICDD and ICSD databases.

2.2.2. Thermal Conductivity Analysis

The thermal conductivity of carbon composite samples was measured using a Laser comp HFM Fox-50 setup under conditions specified in ASTM C518, EN 12664, and ISO 8301. The principle of operation of this device is shown in Figure 2.

In the HFM test, the sample is put between two plates at various temperatures. These plates are kept at a constant temperature by means of the Peltier effect. At this time, the heat flux along the sample and the temperatures are measured by a heat flux meter and a thermocouple located on the upper and lower plates of the instrument. HFM provides thermal equilibrium and a uniform temperature gradient, thermal conductivity is determined. The thermal conductivity measured by the HFM is determined by the Fourier thermal Equation:

q = −kA ΔT/Δx

(6)

In this Equation, q (W/m²) is heat flow, k (W/mK) is thermal conductivity, A (m²) is area, ΔT (K) is temperature difference, and x (m) is sample thickness. HFM works by measuring, recording, and printing using this equation and calibrated software. Before starting experiments, the instrument was calibrated with pyrex-7740, whose thermal conductivity is known for certain temperatures, and with a Pyrex calibration file [19]. The reproducibility of the device was reported as ±2% [20].

2.2.3. Thermochemical Simulation Analysis

HSC Chemistry 6.12 software was used for the simulation of possible products to increase the reaction temperature to 1400 °C. Plots were generated using the “Equilibrium Compositions” module to identify the molar ratios of possible products under temperature increase under adiabatic conditions. The Gibbs minimization method was used as the calculation module.

2.2.4. Compression Strengths’ Tests

ISO 844 standard was used for the compressive strength of the specimens. The standard contains a compressive stress at 10% relative deformation. The experimental samples were cut into small cubes of approximately 30 mm × 30 mm × 30 mm.

2.2.5. TG/DTA Analysis

TG is a thermal analysis method in which the mass of a sample is measured over time as the temperature changes. DTG is the differential thermogravimetric ratio of mass measurement Dm (rate of mass loss or increase) at heating/cooling/isotherm, interpretation of Dm by T or time (-dm/dt). The used TG/DTA device was TG-DTA PERKIN ELMER Diamond.

3. Results and Discussion

3.1. X-ray Diffraction Analysis of Refractory Bricks based on Metallurgical Waste

The XRD-Rietveld analysis results of the refractory materials are shown in

Table 3. XRD-Rietveld analysis results for the produced samples (wt.%).

№	1	2	3	4
Chemical Phases, wt. %
MgAl2O4	38.3	69.3	76.4	71.7
C	27.6	16.6	-	3.1
SiC	15.9	7.4	10.7	11.3
Al4.59Si1.41O9.7	9.4	-	-	-
Cr7C3	4.8	-	-	-
Al2Ca3(SiO4)3	3.9	-	-	-
MgCr2O4	-	3.6	-	-
Ca(Mg0.93Fe0.07)SiO4	-	-	5.9	9.7
Cr	-	-	4.0	4.2
Si	-	3.1	3.0	-

. The silicon carbide and chromium carbide were the import phases, which generally improve refractory characteristics such as porosity, refractoriness, cold crush strength, and thermal shock resistance.

3.2. Thermal Conductivity Analysis Results

The thermal conductivity analysis results of refractory samples are given in

Table 4. Thermal conductivity properties.

Sample №	Thermal Conductivity Constant (W/mK)	Heat Flux (W/m2)
1	0.841	267
2	0.929	309
3	1.020	308
4	0.511	160

. The results showed that refractory composites formed from different wastes have different thermal conductivity, and this difference was primarily determined by the chemical and physical properties of the material. It has also been noted that the good thermal conductivity of a composite or mixture of refractory bricks is related to the thermal conductivity of its components. The obtained data showed that there is a direct relationship between thermal conductivity and heat flux as expected. With high chromite content in raw blends (MgCr₂O₄) such as Samples 1 and 2, high thermal conductivity constants were measured. In opposite, the use of magnesite scrap in the raw blend (containing mainly MgO) decreased the thermal conductivity constant remarkably (Sample 4). The thermal conductivity constant of Sample 3, which contained Mg₂SiO₄ in its raw materials, was high as well. Measured thermal conductivity constants showed that physical compaction had an important role for increasing thermal conductivity, which can be seen as a result of compression strength tests (please see Section 3.4). Physical compaction of Samples 1, 2, and 3 was greater than that of Sample 4. Synthesized carbon-containing materials in the process of exposure to specific heat fluxes within the studied limits retained the structure and properties of products without changing the physical and mechanical properties, which shows their suitability for use in thermal units at significant temperature differences.

3.3. Thermochemical Modeling Results

The samples were heated at 950 °C and, because of exothermic nature of our reactions, the reaction temperatures increased. The maximum temperature was over 1400 °C. The Y-axis shows the number of products in molar ratio, and the X-axis shows temperature. The yield of product amounts (in moles) was simulated by using the HSC Chemistry 6.12 software on the ratio of raw materials in the mixture.

3.3.1. Modeling Results for the Sample №1

For the reactant content of the sample №1, because of a reaction at around 1200 °C, we obtained mainly MgO, MgO.Al₂O₃, MgCr₂O₃, and SiC with free C in our products (Figure 3). As the reaction temperature increased, we observed that the amount of SiC in the products decreased, while the amount of Al and C phases increased. The number of other phases has not significantly changed.

3.3.2. Modeling Results for the Sample №2

As a result of a reaction at around 1200 °C (Figure 4), in our products, we obtained mainly C, MgO, MgO.Al₂O₃, and SiC phases. As the reaction temperature increased, we observed that the amount of MgO in the products decreased, while the Al phase increased. The amounts of other phases (for example, MgO Al₂O₃) did not noticeably change.

3.3.3. Modeling Results for the Sample №3

In Figure 5, modeling results for the change of reaction products with increasing temperature were given. C, SiC, MgO, and MgO.Al₂O₃ were the main reaction products. Apart from free C, any other remarkable rise on probable phases was not observed with increasing temperature.

3.3.4. Modeling Results for the Sample №4

For the modeling results of the sample coded №4 (Figure 6), C, MgO, SiC, and a moderate amount of MgO.Al₂O₃ were observed as main probable phases to form with a remarkable amount of the Al. Any rise in their quantities was not detected. MgO was the only phase in a decrease with increasing temperature.

3.4. Compression Strengths’ Tests Results

The compressive strength tests of the samples were carried out in accordance with the requirements of the ISO 844 standard (

Table 5. The results of compression strengths’ tests.

Sample №	Fm (N)	Sm (MPa)	em (%)
1	21,796.4	15.8	6.63
2	21,811.19	15.7	4.34
3	21,758.02	15.7	9.89
4	3805.322	2.7	5.72

). Compressive strength is a critical quality indicator for refractory materials to ensure the safety of existing structures. The first three samples showed the acceptable results consistent with the compression strength tests of refractory materials, except sample №4.

3.5. TG/DTA Analysis Results

Measurements were performed when samples were heated at a rate of 10 °C/min in the temperature range of ~25–1050 °C (Figure 7, Figure 8, Figure 9 and Figure 10). All TG/DTA analyses were conducted under dry air atmosphere. All samples showed a weight loss at the temperatures greater than 500 °C. Weight loss values varied between ~15% (№2) and ~26% (№4). A weight loss beginning temperature of 500 °C is very identical for TG curves of carbon under air atmosphere due to the combustion with the oxygen in the air. Moreover, this idea is supported by the DTA results of the samples on the right end axis of the Figures. In DTA results, there are negative heat flow values (mW) which point out an energy release, in other words an exothermic reaction, with the beginning of weight loss. Those exothermic reactions might be a result of the combustion of carbon-based constituents, which were not reacted with other metals to form carbides, in the samples. Although DTA curves of all samples were similar to each other, a slight shift was observed between chromite-containing samples (1, 2) and magnesium-silicate- and magnesium-oxide-containing samples (3, 4) in their raw blends. It is clear to see from the TG results that a reaction started at around 800 °C for all samples resulting in weight gain (№1: ~850 °C, №2: ~780 °C, №3: ~840 °C and №4: ~860 °C). Moreover, a slight amount of heat adsorption was observed at around 580 °C, which was almost identical for the samples 1, 3, and 4. It was thought that those peaks might likely occur because of an instantaneous melting of any complex compound in the samples with a very slight amount.

4. Conclusions

A new valorization field of different metallurgical wastes (such as sludge, refractory magnesite scrap, chromium spinel dust, and filter cake) to produce carbon-containing and chemically resistant refractories was investigated through combustion synthesis (CS) based on the formation of refractory carbides and oxycarbides via exothermic reactions while preventing the oxidation of carbon particles. Experiments were carried out to obtain carbon-containing refractories from various combinations of wastes. Thermochemical modeling was carried out for various raw mixes containing metallurgical waste components using the HSC Chemistry 6.12 software. The resulting samples consisted of mainly MgAl₂O₄, SiC, and C with less amounts of other oxides and carbides. They had high compressive strength (up to 15.8 MPa) and thermal conductivity constant (between 0.511 and 1.020 W/mK) and meet the requirements for their industrial use such as oxidation resistance, they significantly exceed the analogues used.