Abstract
This article presents the results of the research on real coal charges of different compositions intended for coking with loading into the chamber by the stamping method. The results of the study established that with an increase in the content of coal at a low stage of metamorphism and a decrease in the content of coal at a high stage of metamorphism in the charges, a decrease in the quality indicators of the coke obtained from them leads to a decrease in the yield of coke. At the same time, there is a gradual decrease in the compaction of the charges from 22.5 to 21.1 kPa; their expansion pressure decreases from 6.8 to 5.9 kPa; and the work of stamping decreases from 8966 to 6822 J. It was also discovered that with the increase in the degree of grinding of the charge, and, accordingly, with the decrease in the average diameter of its particles, the work of stamping coal charges decreases from 7407 to 6238 J. The results can be used in the manufacturing of blast furnace coke by stamping coal charges.
Keywords: Coal concentrates, Charge, Preparation of coal charge, Stamped coal cake, Stamping work, Strength of stamped coal cake, Coke quality
Subject terms: Fossil fuels, Chemical engineering
Introduction
The problem of coke quality plays a key role in the operation of blast furnaces the main recipient and consumer of coke1,2. Coke in the blast furnace process performs the following functions: energy (provides heat), chemical (a source of reducing gases and carbon for iron recovery) and physical (supporting the column of charge materials, ensuring the flow of gas through the furnace and the flow of liquid metal into the lower parts of the furnace). The dynamic technological development of the steel industry, which was observed in the last decade, has led to an increase in the importance of the physical role of coke and, as a result, increased requirements for its strength parameters3,4. Coke quality parameters are influenced by a number of factors, including the quality of coal raw materials (including the degree of metamorphism, chemical composition, coking properties, and rheological, petrological and mineralogical characteristics) and the parameters of the preparation of the coal charge for coking (for example, bulk density, particle size composition and moisture)5,6. The quality of coke also depends on the conditions of the coking process (duration of coking and the final temperature of the process). Achieving the necessary indicators of coke quality by producers requires the use of a large amount of coking coal of the highest quality in the coke charge, which significantly increases the cost of production per unit of production. Approximately 75% of these costs are the cost of preparing the coal charge7. In general, with regard to quality and price parameters, it should be noted that there is a shortage of coking coal of the best quality on the world market, and its supply is characterized by large fluctuations in quality parameters. Coking coal is at least 50% more expensive than high-volatile coal8.
Therefore, to meet the high-quality requirements of blast furnace coke, which determines its competitiveness on the world market, there is a growing need to improve the technology of coke production to limit the possible choice of raw materials. In view of the current and forecasted situation, and considering the strategic importance of steel production, according to the decision of the European Commission dated September 3, 2020, coking coal is included in the updated list of critical raw materials strategic from the point of view of the functioning and economic development of the European Union9. Thus, there is a need to develop methods for modifying coal and coal charges using world trends and modern technological approaches. This will make it possible to increase the proportion of low-coking coal in the charge and obtain high-quality coke. One of the solutions is the use of appropriate charge preparation technologies to increase the density of coke (through drying, lubrication, partial briquetting or compaction of the charge), which increases the quality of the produced coke and/or allows an increase in the share of cheaper low-coking coal in the coal charge while maintaining the quality of the coke10.
The application of the above methods also has a positive effect on the environmental friendliness of coke chemical production; due to the increase in the productivity of coke batteries, the specific emission of substances into the environment decreases. Therefore, in recent years, there has been an increase in interest in this technology, which is expressed in the construction of coke batteries with the technology of stamping coal charges, both in Europe (Germany, Poland, the Czech Republic, Ukraine) and in the Eastern region (China and India)11,12.
Previous studies have established that the use of stamping technology allows one to produce higher-quality blast furnace coke (BFC), both in terms of mechanical strength (M25 and M10) and in terms of reactivity and postreaction strength (CRI and CSR)13–16.
A limiting factor for converting coke batteries from gravity loading to stamping technology is the difficulty of ensuring the stability of the stamped cake. When the height is increased without significantly increasing the width, it is difficult to stabilize the stamped coal cake. This, in turn, largely depends on the physical and mechanical properties of coal17–19. Previous studies have shown that with increasing metamorphism, the hardness of the coal material increases, and the crushing rate first increases and then decreases, which is associated with a decrease in the yield of volatile matter. Thus, low metamorphosed coal is moderately strong but more viscous, medium metamorphosed coal (due to well-developed cracking) is the weakest, and highly metamorphosed anthracite is the hardest and strongest20,21.
Additionally, preliminary experimental results show that a number of changes occur with an increase in the number of cycles of loading and unloading. Thus, the results of the calculations show that the relationship between the scattering energy of the fault rock and the number of cycles of loading and unloading corresponds to an exponential function22. These conclusions are consistent with our previously obtained data on the relationship between coal quality indicators and mechanical strength23–26. We also conducted an analysis of methods for determining the mechanical strength of coal27 and coal charges28,29, which has a significant impact on the possibility of stamping.
Researchers have also used computed tomography (CT) to highlight the internal crack network of coal samples30. The results of this study have important reference value for future research on the accurate and effective selection of multiple cracks that have a significant impact on the mechanical properties of the surrounding rock mass structure in the coal industry.
The mechanical response of coal during compression is influenced not only by the mechanical properties of the coal matrix and the spatial distribution and mechanical characteristics of structurally weak surfaces but also by the size of the test sample and loading conditions. To investigate the influence of these factors on the postpeak characteristics, different height-to-width models were constructed by varying the height of the numerical model, and the loading rate was achieved by adjusting the calculation time steps. The analysis showed that the deformation modulus, DFN density, height to width, and loading rate significantly affect the postpeak behavior of coal31,32.
From a practical point of view, one of the main decisive factors affecting the stability of the compacted cake is the time or work involved in compacting the stamped coal cake. This work, in turn, largely depends on all the abovementioned physical and mechanical properties of the coal. The study of stamping work, which must be applied to achieve the necessary density and stability of the stamped coal cake from a technological point of view, was set by the authors as the goal of this study. As part of this work, the following tasks will be solved:
to develop the influence of the petrographic characteristics of the charge (composition, average arbitrary vitrinite reflectance index R0, vitrinite content of the charge Vt, content of the sum of the fusinized components of the charge ∑FC, value of volatile matter of the charge Vdaf) on its stamping;
to establish the influence of the degree of grinding of the charge (average diameter of the particles of the charge ds, class content < 3 mm) on the work of stamping;
to find out the effect of the composition of coal charges (caking ability of the charge Сch and coking ability of the charge Kch) on coke quality indicators (coke yield Yc, mechanical strength M25, abrasion resistance I10, reactivity CRI and postreaction strength CSR).
Methods and materials
Methods
Methods of determining the characteristics of coal, coal charges, and coke
To determine the quality indicators of the coals, coal charges and obtained blast furnace coke, we used the following standard methods:
ISO 17246:2010 Coal - Proximate analysis;
ISO 18283:2022 Coal and coke - Manual sampling;
ISO 17247:2020 Coal and coke - Ultimate analysis;
ISO 334:2020 Coal and coke - Determination of total sulfur;
ISO 1170:2020 Coal and coke - Calculation of analyses to different bases;
ISO 7404-5:2009 Methods for the petrographic analysis of coals - Part 5: Method of determining microscopically the reflectance of vitrinite;
ISO 7404-3:2009 Methods for the petrographic analysis of coals - Part 3: Method of determining maceral group composition;
ISO 1953:2015 Hard coal - Size analysis by sieving;
ISO 10329:2018 Coal - Determination of plastic properties. Constant-torque Gieseler plastometer method;
DSTU 7611:2014 Hard coal. Method of determination of oxidation and degree of oxidation;
DSTU 7722:2015 Hard Coal. Method for determination of plastometric indices;
ISO 18894:2018 Coke - Determination of coke reactivity index (CRI) and coke strength after reaction (CSR).
The chemical composition of the ash was determined by the standard DSTU 9045:2020 Solid fuel. A method of determining the chemical composition of ash. The basicity index (Bb) and the base/acid ratio (Ib) were calculated by the equations:
 |
1 |
 |
2 |
where Ad is the ash content of coal in the dry state, %; Vdaf is the volatile matter in the dry ash-free state, %.
The caking ability of the coal charge (Сch) is the quantity of the specified components of vitrinite in terms of the organic mass of coal:33
 |
3 |
where 
is the percentage of vitrinite components with a reflectance ranging from 0.90 to 1.39% and Vt is the percentage of vitrinite group macerals, %.
The coking ability of the charge (Kch) is the ratio of the total content of components of its vitrinite with a reflection index from 0.9 to 1.39%, which shows high causticity and has the ability to accept descriptive additives and macerals of the liptinite group, to the sum of descriptive macerals and noncaustic components of vitrinite > 1.70%:
 |
4 |
where 
is the content of vitrinite components with a reflectance ranging from 0.90 to 1.39%; Vt is the content of vitrinite group macerals, %; L is the content of macerals of liptinite groups, %; ∑FC is the sum of fusinized components (I + 2/3Sv), %; and 
is the content of vitrinite components with a reflectance of 1.70% or more.
Determination of the strength of the stamped coal cake were carried out at the installation of the SE “UKHIN” (Fig. 1).
Fig. 1.
Apparatus for Compaction determining (the strength of a stamped coal cake per shear (σsh)): 1 – base, 2 – table, 3 – stamped coal cake, 4 – metal plate, 5 – container guide, 6 – container, 7 – mobile device of the bunker, 8 – hopper with lead fraction, 9 – on/off tap.
The stamped coal cake is placed on Table 2 of the apparatus such that the shorter rear face of the stamped coal cake 5 is in close contact with the limiting edge of the table evenly along the entire length of the edge, and exactly half of its longer side cantilever hangs from the table. In all experiments, the stamped coal cake is installed with the upper face up, that is, without turning it over when removing it from the stamped matrix. Holding the stamped stamped coal cake with one hand, placing pad 4 on it with the other hand, passing it through two pins 3 of the device, and lightly pressing the stamped coal cake with the pad using nuts. At the same time, the nuts were tightened to the stop with a small pressing force. After the stamped coal cake is pressed, it should not move freely on the table, but it should not be pressed by the overlay either because excessive pressure can lead to a violation of the internal structure of the stamped coal cake and reduce its strength.
Table 2.
Petrographic characteristics of the studied coal.
| Supplier, country | Petrographic composition (without mineral impurities), % |
Index of reflection vitrinite, % |
Stages of vitrinite metamorphism, % | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Vt | Sv | I | L | ∑FC | R 0 | 0.50–0.79 | 0.80- 0.89 |
0.90–1.19 | 1.20–1.49 | 1.50–1.69 | 1.70–2.59 | |
| EF “Pavlogradska”, Ukraine | 66 | 0 | 31 | 3 | 31 | 0.60 | 99 | 1 | 0 | 0 | 0 | 0 |
| EF “Dobropilska”, Ukraine | 72 | 0 | 25 | 3 | 25 | 0.77 | 58 | 34 | 8 | 0 | 0 | 0 |
| Wellmore, USA | 77 | 0 | 20 | 3 | 20 | 1.02 | 5 | 11 | 81 | 3 | 0 | 0 |
| Arch Coal Premium, USA | 72 | 0 | 24 | 4 | 24 | 0.98 | 3 | 10 | 84 | 3 | 0 | 0 |
| EF “Svyato-Varvarynska”, Ukraine | 84 | 1 | 14 | 1 | 15 | 1.23 | 0 | 0 | 41 | 59 | 0 | 0 |
| Illawarra, Australia | 91 | 0 | 9 | 0 | 9 | 1.27 | 0 | 0 | 31 | 65 | 3 | 0 |
After fixing the stamped coal cake on the table, metal plate 6 is placed on its console, and container 9 is placed on the plate in such a way that it touches the side end of pad 4. The thickness of this plate is ~ 2 times less than the thickness of the pad. Then, the lead fraction was slowly poured into the container from a container with a spout while continuously watching the container. As soon as the stamped coal cake is cut, the supply of the shot is stopped immediately. After that, the container with the shot and plate was weighed to the nearest 1 g.
The strength of the stamped coal cake per section is calculated as the ratio of the weight of the cargo to the area of the section in kPa, according to the following formula:
 |
5 |
where P is the total weight of the container with the fraction and the plate, kg;
S – cross-sectional area of the sample, cm2; g – acceleration of gravity, m/s2.
The cross-sectional area S (cm2) is calculated by the following formula:
 |
6 |
where b is the width of the stamped coal cake, cm, and h is the height of the stamped coal cake, cm.
The mass of the stamped coal cake (the mass of loading the charge into the collapsible matrix of the installation) was 1.1 kg, the mass of the impact load was 42 kg, the height of the load was 0.45 m, and the number of impacts was 3. The total work of stamping was 56.7 kg·m2, and the specific work was 51.54 kg·m/kg, which is 505 J/kg. This stamping operation is close to the stamping operation in an industrial SLEM (stamper loading ejector machine) when forming a stamped coal cake. The strength of the stamped coal cake is determined in parallel samples.
Method of coking stamped coal cake
The process of preparation for coking took place as follows: Coal charges of four variants were assembled (their detailed analysis is given below in Sect. 3), after which each of the charges was stamped.
The stamping of coal charges took place in the collapsible matrix (Fig. 2) in such a way that upon reaching the specified density of the stamped coal cake, the work done was recorded. The internal length of the matrix is 300 mm, width − 130 mm, height − 300 mm.
Fig. 2.
Сollapsible matrix for stamping.
The results were calculated according to formula (7):
 |
7 |
where A is the work of stamping the coal charge, J; n – the number of falls of the hammer when stamping the coal charge, units; F - the force of the fall of the hammer when stamping the coal charge; N; s is the height of the fall of the hammer when stamping the coal charge, 0.51 m.
In turn, the F indicator is calculated according to formula (8):
 |
8 |
where m is the mass of the hammer, 7.8 kg, and g is the acceleration of free fall, 9.81 m/s2.
Given that stamping occurs due to the application of the falling force of the hammer, which has a fixed mass and falling height, only the number of hammer blows and, accordingly, the work and/or time of stamping remain variable.
The length and width of the stamped coal cake are always equal to the internal dimensions of the matrix, and its height is determined by the given density of the stamped coal cake. In this case, at a density of 1.15 t/m3 in the wet state, the height was 140 mm.
The prepared charges were coked in a 5-kg laboratory furnace designed by the State Enterprise “UKHIN” (Fig. 3).
Fig. 3.
(left) 5-kg laboratory furnace designed by the Ukhin State Enterprise; (right) retort for loading the charge.
The essence of the methodology is as follows. A metal chamber with the following dimensions was inserted into an electric furnace preheated to 1100 °C: width – 150 mm, length – 270 mm, and height – 300 mm. The chamber was loaded with 4.5–5.0 kg of the tested mixture of coal in a specified grinding class of less than 3 mm with a mass fraction of total moisture of 11 ± 0.5%; the loading density was ~ 1150 kg/m3. When the temperature reached 950 ± 10 °C in the loading center, the research was stopped. The coking rate was ~ 25 mm/hour.
 |
9 |
where 
- is a rate of coking, mm/hour; 
– is a 1/2 width of coking chamber, mm; 
– is time of coking, hour.
The duration of the experiment was 2 h 50 min − 3 h. The coke was cooled by naturally way. That is, upon reaching the specified period and the final coking temperature, the retort with coke was removed from the furnace, and at room temperature, the coke was cooled to the temperature of the surrounding medium for 18 h.
Methods of determining coke characteristics
The coke was weighed, and the yield of dry gross coke from dry coal loading was determined:
 |
10 |
where 
is the yield of dry coke (%); 
– mass of dry coke (g); 
- mass of dry coal charge (g).
The obtained coke was subjected to destructive forces for the realization of cracks by dropping it 4 times from a height of 1 m onto a metal plate, after which the coke was subjected to screening (calibration) on sieves with round holes with diameters of 70, 60, 50, 40, 25 and 10 mm. Based on the screening results, the yields of the individual size classes were calculated.
Coke larger than 25 mm after dumping was subjected to destruction in a closed 4-section drum with a number of revolutions of 300, a speed of rotation of 45 rpm, a diameter of each section of 450 mm.
To do this, all the coke was divided into 4 equal portions. The weight for each portion was calculated by taking into account the ratio of size classes in coke larger than 25 mm after the reset. The weight of each portion ranged from 790 to 800 g. Each portion was loaded into a separate section of the drum and tested simultaneously with the others. After scrolling 300 complete revolutions of the drum at a rotation speed of 45 ± 1 rpm, the drum was unloaded. Coke from each section was screened on round sieves with hole diameters of 25 and 10 mm. The yield of coke greater than 25 mm, which characterizes the mechanical strength (M25), and the grade yield of less than 10 mm, which characterizes the abrasion resistance (I10) of coke, were determined.
Materials
We investigated coal from these 6 suppliers: EF “Pavlogradska”, Ukraine; EF “Dobropilska”, Ukraine; Wellmore, USA; Arch Coal Premium, USA; EF “Svyato-Varvarynska”, Ukraine; and Illawarra, Australia. The results of the research are given in Tables 1, 2, 3 and 4. Table 5 shows the chemical composition of the ash of the studied coal and the basicity indices calculated according to the obtained data.
Table 1.
Technological properties of the studied coal.
| Supplier, country | Proximate analysis, % | Plastometric indices, mm | Oxidation rate, oC | Expansion pressure, kPa | |||
|---|---|---|---|---|---|---|---|
| Ad | Sdt | Vdaf | X | Y | Δt | Р10 | |
| EF “Pavlogradska”, Ukraine | 10.1 | 1.32 | 43.6 | 56 | 9 | 5.0 | 0.0 |
| EF “Dobropilska”, Ukraine | 8.5 | 1.32 | 38.5 | 42 | 14 | 3.4 | 0.0 |
| Wellmore, USA | 9.7 | 0.81 | 32.0 | 9 | 19 | 3.8 | 3.8 |
| Arch Coal Premium, USA | 8.9 | 0.76 | 35.0 | 23 | 31 | 1.3 | 2.5 |
| EF “Svyato-Varvarynska”, Ukraine | 9.1 | 0.64 | 26.4 | 14 | 13 | 0.9 | 16.8 |
| Illawarra, Australia | 9.1 | 0.45 | 26.3 | 27 | 14 | 5.1 | 6.8 |
Table 3.
Elemental composition of the studied coal.
| Supplier, country | Ultimate analysis (dry, ash-free state), % | ||||
|---|---|---|---|---|---|
| Cdaf | Hdaf | N daf | Std | Oddaf | |
| EF “Pavlogradska”, Ukraine | 83.65 | 6.16 | 1.43 | 1.32 | 7.44 |
| EF “Dobropilska”, Ukraine | 83.89 | 5.91 | 1.83 | 1.32 | 7.05 |
| Wellmore, USA | 86.93 | 5.56 | 1.70 | 0.81 | 5.00 |
| Arch Coal Premium, USA | 86.76 | 5.80 | 1.63 | 0.76 | 5.05 |
| EF “Svyato-Varvarynska”, Ukraine | 87.63 | 5.34 | 1.72 | 0.64 | 4.67 |
| Illawarra, Australia | 85.55 | 5.42 | 1.68 | 0.45 | 6.90 |
Table 4.
Granulometric composition of the studied coal.
| Supplier, country | Granulometric composition (mm), % | Average particle diameter, mm | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| > 50 | 50–25 | 13–25 | 6–13 | 3–6 | 1–3 | 0.5–1 | < 0.5 | < 3 | ds | |
| EF “Pavlogradska”, Ukraine | 8.2 | 11.1 | 14.7 | 20.1 | 15.0 | 12.9 | 6.0 | 11.9 | 30.9 | 16.03 |
| EF “Dobropilska”, Ukraine | 6.0 | 11.4 | 19.4 | 21.1 | 14.4 | 12.5 | 5.2 | 10.0 | 27.7 | 15.41 |
| Wellmore, USA | 0.0 | 4.0 | 8.6 | 19.8 | 20.6 | 22.1 | 8.6 | 16.3 | 47.0 | 6.50 |
| Arch Coal Premium, USA | 0.0 | 0.0 | 1.4 | 3.0 | 9.1 | 21.8 | 16.9 | 47.8 | 86.5 | 1.64 |
| EF “Svyato-Varvarynska”, Ukraine | 0.0 | 7.6 | 4.1 | 12.6 | 15.3 | 17.9 | 8.0 | 34.5 | 60.3 | 6.02 |
| Illawarra, Australia | 0.0 | 2.5 | 6.1 | 15.5 | 15.4 | 14.9 | 8.6 | 37.0 | 60.5 | 4.71 |
Table 5.
Chemical composition of the ash of the studied coal.
| Supplier, country | Chemical composition of ash, % | Basicity indices | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| SiO2 | Al2O3 | Fe2O3 | MgO | CaO | Na2O | K2O | SO3 | Іb | Bb | |
| EF “Pavlogradska”, Ukraine | 48.90 | 21.27 | 14.96 | 1.01 | 5.01 | 1.75 | 2.01 | 3.50 | 0.353 | 5.92 |
| EF “Dobropilska”, Ukraine | 41.81 | 25.50 | 18.95 | 1.26 | 4.45 | 1.01 | 1.45 | 3.37 | 0.403 | 5.59 |
| Wellmore, USA | 46.80 | 26.06 | 19.45 | 1.25 | 1.59 | 0.63 | 2.01 | 1.18 | 0.342 | 4.86 |
| Arch Coal Premium, USA | 42.85 | 28.05 | 15.96 | 1.26 | 4.21 | 0.54 | 1.12 | 5.14 | 0.330 | 4.52 |
| EF “Svyato-Varvarynska”, Ukraine | 51.38 | 27.90 | 8.48 | 1.51 | 1.05 | 2.13 | 3.23 | 1.27 | 0.307 | 2.56 |
| Illawarra, Australia | 51.61 | 26.14 | 9.97 | 1.51 | 1.40 | 2.19 | 2.95 | 2.43 | 0.230 | 2.83 |
The investigated coal is not oxidized (Δt ≤ 6 °C). The coals of a low stage of metamorphism (R0 < 0.9%) of EF “Pavlogradska” and EF “Dobropilska” are characterized by volatile matter of 43.6 and 38.5%, respectively, plastic layer thicknesses of 9 and 14 mm, fused component contents of 31 and 25%, and average arbitrary reflectance of 0.60 and 0.77%, respectively.
Moreover, the Arch Coal Premium is characterized by similar vitrinite contents of 77% and 72%; the sum of the fusinized components is 20% and 24%; the vitrinite reflectance indices are 1.02% and 0.98%; the sum of the components of vitrinite in the fatty coal stage is 81% and 84%; and the sulfur content is 0.81% and 0.76%, respectively. However, there are certain disagreements regarding the thickness of the plastic layer. The plastic layer thickness of the wellmore concentrate is 19 mm, and that of the arch coal premium is 31 mm.
Two samples of coking coal (1.2% < R0 < 1.5%) from EF “Svyato-Varvarynska” and Illawarra are as similar as possible and are characterized by volatile matter of 26.3–26.%, a sulfur content of 0.45–0.64, a plastic layer thickness of 13–14 mm, an average arbitrary reflectance of vitrinite of 1.23 and 1.27%, and an ash content of 9.1%.
For the granulometric composition of the studied coal (Table 4), the course is coal of a low stage of metamorphism. Only the presence of a class > 50 mm (from 6.0 to 8.2%) was observed. The average diameter of the coal particles in this group is 15.41–16.03 mm, the fineness is 27.7–30.9%, and the content of the < 0.5 mm class is 10.0–11.9%. Arch Coal Premium coal from the middle stage of metamorphism is the smallest of the tested coals and does not require additional crushing (ds = 1.64 mm; <3 mm = 86.5%; <0.5 mm = 47.8%).
The carbon and oxygen contents (Table 3) are correlated with the degree of metamorphism of the studied coal. As the degree of metamorphism increases, the carbon content increases from 83.65 to 87.63%, and accordingly, the oxygen content decreases from 7.44 to 4.67%.
Table 6 shows the results of studies of the plastic properties of coal according to the Gieseler method (DSTU ISO 10329:2018) for determining the value of one or another coal as a raw material for obtaining coke of a given quality from the point of view of its refractoriness.
Table 6.
Plastic properties of the investigated coal according to Gieseler’s method.
| Supplier, country | Plastic properties of coal according to Gieseler’s method | ||||
|---|---|---|---|---|---|
| Tp, °С | Tmax, °С | T3, °С | ΔT, °С | Fmax, ddpm | |
| EF “Pavlogradska”, Ukraine | 402 | 427 | 450 | 48 | 10 |
| EF “Dobropilska”, Ukraine | 404 | 427 | 470 | 66 | 1412 |
| Wellmore, USA | 407 | 443 | 479 | 72 | 837 |
| Arch Coal Premium, USA | 349 | 451 | 484 | 105 | 40,184 |
| EF “Svyato-Varvarynska”, Ukraine | 422 | 455 | 482 | 60 | 29 |
| Illawarra, Australia | 403 | 454 | 490 | 87 | 505 |
Analyzing the presented data, it should be noted that the Arch Coal Premium coal is characterized by the maximum dilatation properties, which has the highest maximum plasticity Fmax – 40,184 ddpm, the lowest initial softening temperature – 349 °С, and a plasticity interval ΔT = 105 °С. The Illawarra coal has the highest solidification 490 °С, ΔT = 87 °С.
Results and discussions
Influence of the composition of coal charges on the indicators of stamping
To determine the influence of the composition of coal charges on the indicators of their stamping ability and the quality of the obtained coke, 4 variants of coal charges were compiled (Table 7).
Table 7.
Grade and component compositions of coal charges.
| Supplier, country | Share in the charge, % | |||
|---|---|---|---|---|
| 1 | 2 | 3 | 4 | |
| EF “Pavlogradska”, Ukraine | 31 | 34 | 37 | 40 |
| EF “Dobropilska”, Ukraine | 6 | 7 | 8 | 9 |
| Total coal of a low stage of metamorphism (R0 < 0.9%) | 37 | 41 | 45 | 49 |
| Wellmore, USA | 3 | 3 | 3 | 3 |
| Arch Coal Premium, USA | 3 | 3 | 3 | 3 |
| Total coal of the middle stage of metamorphism (0.9% < R0 < 1.2%) | 6 | 6 | 6 | 6 |
| EF “Svyato-Varvarynska”, Ukraine | 52 | 48 | 44 | 40 |
| Illawarra, Australia | 5 | 5 | 5 | 5 |
| Total coal of a high stage of metamorphism (1.2% < R0 < 1.5%) | 57 | 53 | 49 | 45 |
| In total | 100 | 100 | 100 | 100 |
Thus, the content of coal in the low stage of metamorphism increased from the 1st to the 4th variant from 37 to 49% due to the gradual decrease in coal in the high stage of metamorphism (1.2% < R0 < 1.5%) from 57 to 45%. It should be noted that variant 2 corresponds to the planned composition of the charge of the working coke chemical plant of ArcelorMittal PJSC “ArcelorMittal Kryvyi Rih” for coking.
The technological properties and petrographic characteristics of the coal charges that were used for laboratory coking are given in Tables 8 and 9. Analyzing the presented data, it can be asserted that the obtained values of the technological properties of the investigated coal charges correspond to the planned ones and change in accordance with the change in their stages of metamorphism.
Table 8.
Technological properties of the experimental coal charges.
| Variant | Proximate analysis, % | Plastometric indices, mm | ||||
|---|---|---|---|---|---|---|
| Wa | Ad | Sdt | Vdaf | X | Y | |
| 1 | 1.4 | 9.4 | 0.92 | 31.4 | 30 | 15 |
| 2 | 1.4 | 9.4 | 0.94 | 32.4 | 29 | 15 |
| 3 | 1.3 | 9.5 | 0.95 | 32.7 | 35 | 14 |
| 4 | 1.6 | 9.5 | 0.95 | 33.7 | 33 | 14 |
Table 9.
Petrographic characteristics of coal charges.
| Variant | Petrographic composition (without mineral impurities), % |
Index of reflection vitrinite, % |
Stages of vitrinite metamorphism, % | Cch, % |
Kch | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Vt | Sv | I | L | ∑FC | R 0 | 0.50–0.79 | 0.80- 0.89 |
0.90–1.19 | 1.20–1.49 | 1.50–1.69 | 1.70–2.59 | |||
| 1 | 75 | 1 | 21 | 3 | 22 | 0.93 | 35 | 3 | 29 | 33 | 0 | 0 | 46.5 | 2.3 |
| 2 | 75 | 1 | 22 | 2 | 23 | 0.91 | 39 | 3 | 27 | 31 | 0 | 0 | 43.5 | 2.0 |
| 3 | 74 | 0 | 23 | 3 | 23 | 0.89 | 42 | 4 | 26 | 28 | 0 | 0 | 40.0 | 1.9 |
| 4 | 73 | 0 | 24 | 3 | 24 | 0.86 | 46 | 4 | 24 | 26 | 0 | 0 | 36.5 | 1.6 |
With an increase in the content of coal in the low stage of metamorphism in coal from 37 to 49%, the Vdaf increases from 31.4 to 33.7%, and the Y indicator decreases from 15 to 14 mm. For the petrographic characteristics, with an increase in the content of coal in a low stage of metamorphism, R0 decreases from 0.93 to 0.86%, Cch decreases from 46.5 to 36.5%, and Kch decreases from 2.3 to 1.6. The moisture content of all coal charges was the same and amounted to ~ 11.5%, the content of classes 0–3 mm ~ 90%, density ~ 1.017 t/m3 (dry state) or ~ 1.15 t/m3 (wet state).
Table 10 presents the results of determining the particle size composition of coal charges. It should be noted that variant 3 of the coal charge was also prepared with a specified grinding of 88.0, 89.5, 90.5 (actual value – 90.4%) and 92.0% (actual value – 91.8%). The influence of the granulometric composition of coal charges on their work of stamping is considered in detail in Sect. 3.3.
Table 10.
Granulometric composition of coal charges.
| Variant | Granulometric composition (mm), % | Average particle diameter, mm | |||||
|---|---|---|---|---|---|---|---|
| > 6 | 3–6 | 1–3 | 0.5–1 | < 0.5 | < 3 | ds | |
| 1 | 0.5 | 9.1 | 28.6 | 21.4 | 40.3 | 90.3 | 1.30 |
| 2 | 0.9 | 8.9 | 31.1 | 18.8 | 40.4 | 90.3 | 1.35 |
| 3 | 0.8 | 9.0 | 35.2 | 17.0 | 38.0 | 90.2 | 1.40 |
| 3.1 | 1.8 | 10.2 | 28.7 | 19.8 | 39.5 | 88.0 | 1.45 |
| 3.2 | 1.2 | 9.3 | 29.4 | 21.6 | 38.5 | 89.5 | 1.38 |
| 3.3 | 0.8 | 8.8 | 25.7 | 23.5 | 41.2 | 90.4 | 1.26 |
| 3.4 | 0.4 | 7.8 | 25.0 | 28.2 | 38.6 | 91.8 | 1.20 |
| 4 | 0.8 | 9.1 | 36.8 | 16.0 | 37.3 | 90.1 | 1.44 |
According to the methods described in Sect. 2.1 (Figs. 1 and 2), we determined the parameters of compaction, density, expansion pressure, and the work required to achieve the specified density of 1.15 t/m3 in the wet state of the experimental coal charges. The obtained values are given in Table 11.
Table 11.
Indicators of compaction, density, and expansion pressure of coal charges.
| Variant | Moisture content, % | Compaction, kPa |
Density, g/cm3 |
Expansion pressure, kPa |
Work of stamping, J | |
|---|---|---|---|---|---|---|
| Wrt | σsh | γwet | γdry | Р10 | А | |
| 1 | 11.5 | 22.5 | 1.192 | 1.055 | 6.8 | 8966 |
| 2 | 11.3 | 22.1 | 1.183 | 1.049 | 6.6 | 7407 |
| 3 | 11.2 | 21.4 | 1.178 | 1.046 | 6.3 | 7017 |
| 3.1 | 11.2 | 22.5 | 1.150 | 1.021 | - | 7407 |
| 3.2 | 11.1 | 20.6 | 1.146 | 1.019 | - | 7017 |
| 3.3 | 11.0 | 14.4 | 1.110 | 0.988 | - | 6627 |
| 3.4 | 11.1 | 18.9 | 1.138 | 1.013 | - | 6238 |
| 4 | 11.4 | 21.1 | 1.166 | 1.033 | 5.9 | 6822 |
From the data in Table 11, it can be seen that with an increase in the content of coal of a low stage of metamorphism in the charges and a decrease in the content of coal of a high stage of metamorphism, a decrease in their vitrinite content Vt and the average arbitrary index of its reflection R0, a gradual decrease in the compaction of the charges from 22.5 to 21,1 kPa; their expansion pressure - from 6.8 to 5.9 kPa; and the number of hammer blows, respectively, and work to achieve the specified density of the stamped coal cake of 1.15 t/m3 per wet state of the charge.
Influence of the petrographic characteristics of charges on the stamping process
To determine the influence of coal charges on stamping, we constructed graphic dependencies of the work expended to determine the given density of the stamped coal cake from the average arbitrary reflection index of the vitrinite of the charges R0 (Fig. 4), from the content of vitrinite Vt (Fig. 5), from the content, the sum of the fusinized components of the charge ∑FC (Fig. 6), and from the indicator of volatile matter of the charge Vdaf (Fig. 7).
Fig. 4.
Dependence of the work of stamping on the average arbitrary reflectance index of the vitrinite charge R0.
Fig. 5.
Dependence of the work of stamping on the vitrinite content of the charge Vt.
Fig. 6.
Dependence of the work of stamping on the content of the sum of the fusinized components of the charge ∑FC.
Fig. 7.
Dependence of the work of stamping on the content of volatile matter in the charge Vdaf.
As shown in the figures, the dependence of the stamping operation on the sum of the fusinized components of the charged ∑FC and the volatile matter the charge Vdaf (Figs. 6 and 7) has an inverse effect; instead, the average arbitrary reflectance indices of the vitrinite of the charge R0 and the vitrinite content Vt (Figs. 4 and 5) are directly proportional. The influence of the composition of coal charges, in particular, a detailed consideration of the petrographic characteristics on the characteristics of the obtained coke, can be found34,35. Instead, they investigated the parameters of vitrinite reflection of the charges on the final indicators of coke quality, bypassing the indicators of compaction, density, and expansion pressure of coal charges.
The influence of the grinding degree on the work of stamping
Additionally, under laboratory conditions, we performed additional studies to determine the effect grinding degree on the work of stamping. With this designation, the charge corresponds to the grade-component composition of the charge of variant 3 of Table 7 (45% coal of a low stage of metamorphism (R0 < 0.9%); 6% coal of a middle stage of metamorphism (0.9% < R0 < 1.2%); 49% of coal of a high stage of metamorphism (1.2% < R0 < 1.5%). The component was refined to 88, 89, 90 and 91% of the content of the class < 3 mm (according to variants 3.1; 3.2; 3.3; 3.4 of Tables 10 and 11) in the variants of the charges, and the indicators of compaction (the strength of a stamped coal cake per shear (σsh) and work of stamping (A)) were determined.
Figures 8 and 9 present graphs of the dependence of the coal screen tamping operation on the specified density of the stamped coal cake on the average diameter of the particles of width ds and the content of the class < 3 mm in the charge.
Fig. 8.
Dependence of the work of stamping on the average diameter of the charged particles ds.
Fig. 9.
Dependence of the work of stamping on the content of the class < 3 mm in charge.
As seen, with the increase in the grinding degree of the charge and, accordingly, with the decrease in the average diameter of its particles, the work of stamping coal charges to achieve the given density of the stamped coal cake decreases. Burat et al.36 investigated the influence of grain size composition and degree of saturation on coal compaction processes in coke production. This confirms the influence of the granulometric composition of the charge on the density and shear strength of the stamped coal cake. However, in contrast to our study, where the parameters of stamped coal cake strength were studied at constant moisture content during charging, the authors of this article showed that the highest compressive strength was achieved at a degree of saturation of approximately 0.25. With a further increase in moisture, the strength of the stamped coal cake deteriorated very easily due to the presence of excess water.
In addition, another study37 showed that further increases in the mechanical quality indicators of coke up to I10 ≥ 5.0% from low-metamorphized noncoking coal are possible only at significantly higher values of compressive strength σsh ≥ 30 KPa, which are already achieved during briquetting of coal and require greater capital investments.
The influence of the composition of coal charges on coke quality indicators
Table 12 presents the analysis of the granulometric composition of the laboratory coke.
Table 12.
Granulometric composition of the laboratory coke.
| Variant | Granulometric composition (mm), % | Average particle diameter, mm | |||||
|---|---|---|---|---|---|---|---|
| > 80 | 60–80 | 40–60 | 25–40 | 10–25 | < 10 | ds | |
| 1 | 40.0 | 34.2 | 13.8 | 1.8 | 3.5 | 6.7 | 72.3 |
| 2 | 37.7 | 33.6 | 18.0 | 0.8 | 2.9 | 7.0 | 71.5 |
| 3 | 39.6 | 34.3 | 13.0 | 1.3 | 3.4 | 8.4 | 71.3 |
| 4 | 38.2 | 32.5 | 15.7 | 1.8 | 3.0 | 8.8 | 70.2 |
The analysis of the data presented in the Table 12 shows a decrease from the first to the fourth variant of coke size according to the main size classes by which industrial metallurgical coke is evaluated. In our opinion, this is fully explained by the gradual decrease in the vitrinite content of the charges from 75 to 73%, the vitrinite reflection index from 0.93 to 0.86%, as well as the caking ability of the charge (Сch) from 46.5 to 36.6% and the coking of the charge (Kch) from 2.3 to 1.6 (see data in Table 9).
Table 13 shows the quality indicators of the results obtained by the method collected in Sect. 2.1 (Fig. 3) from the investigated coal charges.
Table 13.
Quality indicators of the laboratory coke.
| Variant | Proximate analysis, % | Coke yield, % |
Mechanical strength, % |
Reactivity and postreaction strength, % | ||||
|---|---|---|---|---|---|---|---|---|
| Ad | Sdt | Vdaf | Yc | M25 | І10 | CRI | CSR | |
| 1 | 12.2 | 0.72 | 0.7 | 75.2 | 91.0 | 7.3 | 37.7 | 55.2 |
| 2 | 12.2 | 0.79 | 0.6 | 74.4 | 91.5 | 7.6 | 38.2 | 52.0 |
| 3 | 12.5 | 0.84 | 0.8 | 74.3 | 91.2 | 7.8 | 39.0 | 50.6 |
| 4 | 12.6 | 0.83 | 0.4 | 73.6 | 91.2 | 8.0 | 40.8 | 49.1 |
From the obtained data, it can be seen that with an increase in the content of coal at a low stage of metamorphism in the charges, the quality indicators of the coke decrease, as well as the grain size of coke. Thus, although the mechanical strength indicators of the studied cokes are close (91.0–91.5%), the abrasion resistance of the cokes which is characterized by the number of small classes of coke formed after tests in a rotating drum, deteriorated from 7.3% (1st variant) to 7.8 and 8.0% at cokes 3rd and 4th, respectively. Coke variant 3 (CRI = 39.0%, CSR = 50.6%) and 4 (CRI = 40.8%, CSR = 49.1%) are also characterized by the worst indicators of “hot” strength. Predictably, with an increase in the content of coal at a low stage of metamorphism, the yield of coke gradually decreased from 75.2% (variant 1) to 73.6% (variant 4). Similar results were obtained from the study of the effect of the carbon structure on the quality of metallurgical coke formation38. The worst coke in terms of quality indicators (I10 = 10.0%; CSR = 42.0%) was obtained from a sample of minimally carbonized coal (76.7%) with R0 = 0.97%. Instead, the best indicators (I10 = 6.2%; CSR = 62.0%) were obtained with Сdaf=89.9%; R0 = 1.28%. Similarly, a previous study35 revealed a high degree of correlation between the reflectivity parameters of the original coal or mixed vitrinite and the technological indicators CRI and CSR of cokes.
Figures 10 and 11 present graphical dependences of the index of coke abrasion resistance I10 on the caking of Cch and coking Kch charges, respectively.
Fig. 10.
The dependence of the index of coke abrasion I10 on the caking of the charge Cch.
Fig. 11.
The graph of the dependence of the coke abrasion index I10 on the coking of the Kch charge.
From these graphs, we observe that the dependences I10 = f (Сch) and I10 = f (Кch) are inverse and linear.
Conclusions
Based on the conducted research, the following can be formulated:
With an increase in the content of coal at a low stage of metamorphism in the charges from 37 to 49% and a decrease in the content of coal at a high stage of metamorphism from 57 to 45%, a gradual decrease in compaction of the charges from 22.5 to 21.1 kPa is observed; their expansion pressure decreases from 6.8 to 5.9 kPa, as well as stamping work to achieve the specified density of the stamped coal cake of 1.15 t/m3 per wet state of the charge from 8966 to 6822 J.
-
The work and/or time of stamping is inversely proportional to the content of coal in the charge of a low stage of metamorphism, the content of the sum of the fusinized components of the charge ∑FC, and the content of volatile matter of the charge Vdaf; instead, it is directly proportional to the average arbitrary reflectance index of vitrinite of the charge R0 and vitrinite content Vt.
With an increase in the degree of grinding of the charge and, accordingly, with a decrease in the average diameter of its particles, the work of stamping coal charges to achieve the given density of the stamped coal cake decreases from 7407 to 6238 J.
An increase in the content of coal at a low stage of metamorphism in the charges leads to a decrease in the size and quality indicators of the coke obtained from them. The mechanical strength of the studied cokes was similar (91.0–91.5%), and the indicator of abrasion of the cokes deteriorated from 7.3% to 7.8 and 8.0% in the cokes of the 3rd and 4th variants, respectively. The dependencies I10 = f (Сch) and I10 = f (Кch) are inverse and linear in nature. Predictably, with an increase in the content of low-metamorphose coal in the charges, the coke yield gradually decreased.
The practical effect of our findings are: higher specific density of the stamped coal cake can enhance the coke’s mechanical properties, making it more suitable for blast furnace operations; by increasing the density, it is possible to use cheaper, lower-quality coals without sacrificing coke quality, reducing overall production costs; enhanced preparation techniques can lead to more efficient coke production processes, reducing emissions and improving environmental performance of coke production facilities.
Author contributions
D.M., V.K., O.B., N.M. wrote the main manuscript and I.A. prepared all figures and tables. All authors reviewed the manuscript.
Data availability
Tha data and materials presented in this study are available on request from the corresponding author.
Declarations
Competing interests
The authors declare no competing interests.
Conflict of interest
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Data Availability Statement
Tha data and materials presented in this study are available on request from the corresponding author.