Selection of Binder Materials for the Production of Extruded Briquettes

Abstract

This study presents the results of a comprehensive study on various agglomeration methods and binder types for producing briquettes from raw materials. Also, this research focuses on one of the major issues in the production of extrusion briquettes, namely their mechanical strength during handling, transportation, and water exposure. Laboratory experiments were conducted to identify the most suitable binding agents, followed by industrial-scale trials of several formulations. The paper also includes the results of pilot-scale tests. Four types of binders were examined: bentonite, TD 021.005.BS, TD 000.411.BS, and TD 000.414.BS. The strength characteristics of the briquettes were evaluated in accordance with the relevant GOST standards: GOST 21289-75 for hot strength, GOST 25471-82 for drop strength, and GOST 15137-77 for impact and abrasion resistance. The findings indicate that, from a technological perspective, the most efficient binder for briquette production is TD 021.005.BS, when applied within the range of 2.5–3%. Notably, briquettes produced with this binder demonstrate superior moisture resistance compared to other formulations. After 24 h of immersion in water, they retained their original shape and structural integrity, confirming the binder’s high effectiveness for industrial applications.

1. Introduction

The production of ferrous and non-ferrous metals is accompanied by the generation of large volumes of waste, leading to the pollution of air and water resources [1,2,3]. Therefore, it is particularly important for the entire industry to focus on the implementation of environmentally friendly and resource-efficient technologies aimed at reducing the negative environmental impact of metallurgical processes.

To identify the most effective solutions for the processing of dispersed materials, a comparative analysis of existing technologies was carried out. The analysis of industrial methods for agglomerating ore materials revealed that each technology has its own advantages and limitations.

The pelletizing method requires advanced equipment that includes energy-intensive high-temperature firing. In addition, this process is not well suited for the processing of ultrafine fractions while producing pellets with a size of 10–12 mm [4,5].

The sintering method is widely used in the European Union and CIS countries and is characterized by high productivity and the ability to process waste materials. However, the main disadvantage of this agglomeration technology is its significant environmental impact. A sintering plant at an integrated metallurgical facility is responsible for more than 50% of the plant’s total dust and gas emissions [6,7].

The roll briquetting technology is characterized by process simplicity and relatively low capital investment requirements. However, its main disadvantages include the low mechanical strength of the final product, the need to use only dry raw materials within a specific particle size range, and the generation of a significant proportion of fine fractions during transportation to smelting units [8,9].

Moisture limitations of the feed mixture in roll briquetting necessitate the installation of additional dewatering and/or drying units, which also restricts the use of cement-based binders. The production of roll briquettes typically results in up to 30% of recyclable waste in the form of fine fractions of the briquetted mixture [7]. At the same time, achieving the required briquette strength often demands increased binder consumption.

A promising approach is the formation of briquettes under the simultaneous application of vibration and pressure (vibropressing). This method ensures dense particle compaction without the development of residual stresses and promotes the formation of structural transitions of the “solid–liquid–solid” type. The advantages of this method include low energy consumption, environmental friendliness, a short production cycle, and the potential for automation with high productivity (up to 30 t/h).

In addition to the briquette forming process itself, the vibropressing briquetting technology also incorporates technical measures to maintain the integrity of the weak, raw briquettes by transporting them on pallets into a heat–moisture curing chamber. This, however, necessitates a rather complex transportation system for moving briquettes from the vibrating table to the curing chamber, then to the product storage area, and returning the pallets back to the vibrating table. Such complexity increases production costs and reduces the overall reliability of the briquetting line [7].

A drawback of the vibropressing technology is the direct proportionality between the line’s productivity and the size of the briquettes, as well as the inverse proportionality between their cost and reducibility and their dimensions. Large briquettes with a hexagonal cross-section (60 × 60 × 60 mm and larger) pose difficulties during discharge from hoppers and when being loaded into furnaces due to mutual interlocking.

The rigid extrusion method is characterized by high productivity and the ability to process ultrafine fractions without the need for firing. This technology allows for the processing of both dry raw materials and materials with a moisture content of up to 18% [10,11].

According to a comparative analysis of the technical and economic indicators of the three industrial briquetting technologies presented in [7], the extrusion method can be reasonably selected as the primary briquetting technology for both industrial and natural raw materials used in ferroalloy metallurgy.

Table 1. Evaluation of the efficiency of various briquetting units [7].

Process Characteristics and Briquette Properties	Briquetting Units and Their Characteristics
	Vibrating Table	Roll Press	Extruder
Maximum productivity	30 t/h	50 t/h	100 t/h
Operational lifetime (cost of replacement components, USD per to	1 year (n.a.)	1 year (1.5)	1.5 years (1.0)
Cement content in the briquettes, %	8–10	15–16	4–6
Heat treatment of raw briquettes	80 °C (10–12 h)	not required	not required
Recirculation cycle	absent	30% of production	absent
Briquette shape	prism, cylinder	pillow-shaped	rod of any cross-section
Briquette dimensions, mm (max)	80 × 80	30 × 40 × 50	diameter 5–35 mm
Moisture content of the feed mixture, %	less than 5%	less than 10%	less than 12–18%
Possibility of stacking raw briquettes	not possible	possible	possible
Resource consumption:
Electricity	42.6 kWh/t	23.0 kWh/t	33.0 kWh/t
Natural gas	47 m3/t	-	-
Heat	0.3 Gcal/t	-	-
Compressed air	90 m3/t	-	-

presents an evaluation of the efficiency of various briquetting units.

As noted above, in recent years, the rigid extrusion technology has found increasingly wide application in the energy and metallurgical industries for the agglomeration of various dispersed materials.

At the Aktobe Ferroalloy Plant (AktFP), the processing of dry gas-cleaning dusts for their subsequent return to the metallurgical cycle is carried out at the briquette production unit (BPU), which is equipped with a STEELE 25 extruder. The core of the technological process is the method of rigid vacuum extrusion, used for processing dispersed materials and homogeneous plastic masses.

Although this technology for secondary raw material processing is original in its design, several issues have been identified during its implementation: low strength of the produced brex, significant fines formation during handling, and increased residual moisture in the product.

The products manufactured at the BPU are used as a charge additive (supplement to chromite ore feed) in the production of high-carbon ferrochrome, which imposes strict requirements on their mechanical strength. During transportation and loading-unloading operations, the brex are subjected to substantial mechanical stresses.

One of the well-known methods for agglomerating fine-dispersed raw materials for subsequent metallurgical processing is the so-called cold briquetting. Numerous techniques have been developed for producing briquettes from fine ores and concentrates, metallurgical waste, and carbon-containing dusts [12,13,14,15].

It is well established that the use of dispersed materials in an agglomerated form reduces dust emissions and ensures sufficient gas permeability of the charge column in metallurgical furnaces. Therefore, agglomerated materials must exhibit high cold and hot strength values [16,17]. In other words, the agglomerated feedstock must retain its strength characteristics at all stages of the metallurgical process.

For a long time, inorganic binders such as cement or bentonite were widely used in ferrous metallurgy for agglomeration. However, in recent years, polymeric organic binders have gained popularity and may eventually completely replace inorganic ones [16,18,19]. As a rule, polymer binders decompose at elevated temperatures without releasing harmful decomposition products and volatilize completely.

The primary mechanism responsible for the binding properties of polymer binders is the formation of adhesive films on the surface of the particles being agglomerated, which ensures strong adhesion between them. This observation is consistent with the findings reported in [20]. Figure 1. shows a micrograph of the structure of a chromite ore briquette produced with a polymer binder.

Some types of polymer binders, in addition to forming adhesive films, also create so-called polymer chains (fibers), which act as reinforcing agents, thereby increasing the strength and maintaining the integrity of the briquettes. There are also polymer binders known to improve the water resistance and impact strength of briquetted products

In the present study, various binding materials intended for the agglomeration of dispersed materials were selected and evaluated. Based on the results of laboratory experiments, the most promising formulations were identified and subsequently tested on an industrial scale.

2. Materials and Methods

For the pilot-scale industrial tests (PIT), four types of binding materials were selected: bentonite (“Tagan” LLP, Oskemen, Kazakhstan), TD 021.005.BS, TD 000.411.BS, and TD 000.414.BS (“Poliplast-UralSib“ LLC, Pervouralsk, Russia).

The dosage of the components in the extruded mixture is presented in

Table 2. Chemical composition of the raw dust, % (for the variants of extrudable mixtures.

Type of Binder	Consumption, %	Material	Moisture, %	Cr2O3	SiO2	CaO	MgO	Al2O3	FeO	C
Bentonite	8–10	APSM+ KShP-01+KShP-02	-	29.13	10.85	1.69	22	7.56	9.14	9.88
TD 021.005.BS	2.5		1.27	25.38	10.29	0.85	27.18	7.25	7.98	4.1
TD 021.005.BS	3		1.92	31.14	11.25	0.72	19.04	7.62	9.64	14.7
TD 000.411.БS	7		6.14	29.43	11.48	1.44	19.06	7.86	9.54	13.7
TD 000.414.БS	6		-	30.92	11.04	2.24	19.38	7.6	9.48	12.9

.

The briquette (brex) production process during the PIT differed from the current industrial technology only by the substitution of the binding material. The moisture content of the initial dust varied from 1.27% to 6.14%. The initial dust used in the PIT consisted of: KhShP-02 (high-grade), KhShP-01 (low-grade), and APSM. The chemical compositions of the initial dusts for each variant are presented in Table 2 and

Table 3. Chemical composition of the obtained briquettes.

Type of Binder	Consumption, %	Moisture, %	Cr2O3	SiO2	CaO	MgO	Al2O3	FeO	C
Bentonite	8–10	23.84	21.81	10.98	1.78	21.13	7.51	9.23	12.78
TD 021.005.BS	2.5	24.77	24.42	12.67	1.43	21.68	7.78	9.29	10.44
TD 021.005.BS	3	17.34	27.34	10.7	0.91	20.31	7.48	9.63	14.7
TD 000.411.БS	7	16.82	27.04	11.48	2.96	18.9	7.77	9.26	13.4
TD 000.414.БS	6	17.34	29.13	11.2	3.32	19.07	7.49	9.49	12.85

.

Analysis of the data in Table 2 shows significant fluctuations in the chemical composition with respect to MgO and carbon content. An increase in MgO indicates a predominance of MgO-poor KShP-01 dust in the composition, whereas samples with high carbon content are dominated by APSM.

Differences in chemical composition are also observed in the briquette samples (Table 3). In these samples, fluctuations in carbon and MgO content are less pronounced compared to the composition of the raw material. The likely reason for the stabilization of chemical composition across the variants is the averaging of the dust during passage through the hopper-feeder and the cascade of clay mixers.

The method for determining hot strength was performed as follows (GOST 21289-75): Briquettes, subjected to a top load (simulating their position on the furnace burden under a layer of charge), were placed in a Nabertherm N 7/H furnace and heated together with the furnace to a temperature of 1100 °C [21]. After reaching the target temperature, the condition of the briquettes was assessed for shrinkage or weakening. While still hot, they were tested for splitting strength using an RB-1000 testing press (R.B. Automazione S.r.l., Genoa, Italy). The average splitting strength was 22 kg/briquette. Industrial tests were conducted on existing equipment from J.C Steele & Sons. Strength tests were performed in accordance with GOST standards, namely GOST 25471-82 for drop strength determination and GOST 15137-77 for impact and abrasion strength determination [22,23].

3. Results and Discussion

The following section presents the results of the tests and their discussion, aimed at evaluating the changes in the strength characteristics of briquettes during the curing period. For each briquette variant, samples were taken separately to measure the strength characteristics after 2, 4, and 7 days of curing. The measurement results are presented in

Table 4. Strength characteristics of briquettes.

Type of Binder	Consumption, %	Days	Splitting Strength, kg/Briquette	Impact and Abrasion Resistance, %		Drop Strength, %	Fractions, %	Residual Moisture, %
				Impact	Abrasion		+10 mm
Bentonite	10	2	20.4	54.8	9.5	95.65	92	13.12
		4	41	60.7	7.8	93.7	93.7	19.4
		7	24	45.04	21.32	90.16	98	11.1
TD 021.005.BS	2.5	2	18	89.52	0.12	98.76	99	12.5
		4	29.6	85.0	8.0	97.76	99.5	10.86
		7	46.3	80.63	12.74	98.5	100	16
TD 021.005.BS	3	2	28.5	86.37	6.1	99.5	97	15
		4	43.4	89.2	0.23	99.2	99.6	11.4
		7	40.2	84.5	10.3	99.5	100	14.4
TD 000.411.БS	7	2	<20	58.84	22.47	92.4	95.3	11.3
		4	26	46.7	24.6	91.2	95.3	8.3
		7	40.40	46.3	27.0	92	93.6	7.6

.

The data in Table 4 show that briquettes with the binder TD 021.005.BS exhibited the highest results across all criteria. It is also noteworthy that, unlike the others, briquettes with this binder are water-resistant. After being immersed in water for one day, the briquettes retained their original shape and did not disintegrate. The plasticizing ability of TD 021.005.BS is also high, resulting in the longest and most plastic briquettes from the extruder, which is advantageous when they drop onto the side of a dump truck from the conveyor and are subsequently discharged in bulk into storage bins (Figure 2). As seen in Figure 2, briquettes with this binder are longer compared to those made with other additives.

The binder consumption was 2.5–3%, which is slightly higher than the initial dosage according to the pilot-industrial tests program. This increase in binder consumption is due to the carbon content varying from 4% to 15%. According to literature and practical experience, carbon-containing materials are poorly wetted; that is, the higher the carbon content in the dust, the lower the wettability, which negatively affects moldability and, consequently, the strength properties of the briquettes.

Pilot-industrial test results showed that using up to 3% of the binder TD 021.005.BS allows for compensating fluctuations in carbon content in the dust and producing briquettes of good quality. Unlike bentonite, this additive is a polymeric organic binder that burns out during melting in furnaces without diluting the briquettes in terms of the main element and does not affect the quantity or composition of the slag. Additionally, hot strength tests were conducted on briquettes with TD 021.005.BS (Figure 3).

Briquettes with the binder TD 021.005.BS retained their original shape. It should also be noted that binder dosages of 2.5% and 3% were selected for the pilot-scale experiments to evaluate the lower and upper limits of additive incorporation. Given the relatively high cost of the material, even a 0.5% variation can considerably affect the economic efficiency of the production process.

Briquettes produced with the binder TD 000.411.BS exhibited the lowest performance. During the pilot-industrial tests (PIT), frequent stoppages occurred due to the mixture sticking after exiting clay mixer No. 3. Increasing the binder dosage to 8% did not improve the results; the briquettes were short, brittle, and overly plastic, and the extruder die was often clogged. These PIT results indicate that the plasticizing ability of this material is low, making it unsuitable for the extrusion process.

Briquettes with the bentonite additive from “Tagbent” LLP also displayed low strength characteristics, though slightly better than those with TD 000.411.BS in terms of impact and abrasion resistance.

Regarding the binder TD 000.414.BS, it is clear that this additive is unsuitable for extrusion: the briquettes exiting the die were loose, exhibited transverse cracks, and were excessively plastic. Increasing the binder dosage did not improve performance. Water resistance of briquettes with either bentonite or TD 000.414.BS was poor; after one day of immersion, the briquettes disintegrated.

4. Conclusions

Laboratory and pilot-scale experiments were conducted to examine different agglomeration techniques and binder types for briquette production from the original raw material. Based on the analysis, among the three binders tested, the material TD 021.005.BS demonstrated superior strength and water resistance, identifying it as the most promising option for industrial-scale implementation. In contrast, the additive TD 000.414.BS exhibited extremely low mechanical strength under industrial conditions, rendering it unsuitable for further use in the production process. Furthermore, briquettes produced with bentonite as a binding material disintegrated during comparable tests, failing to meet the required performance specifications.

The practical significance of this study lies in the fact that the proposed technology employing the polymer binder TD 021.005.BS enables the production of briquettes with higher strength and improved moisture resistance, thereby enhancing the quality of the final product and reducing losses during transportation and storage. Furthermore, the determination of the optimal binder content range (2.5–3%) allows for the rational use of the material and consideration of the economic aspects of production, given its relatively high cost. As a result of the pilot-industrial tests (PIT) on dust extrusion using the binder TD 021.005.BS, the following results were achieved:

• splitting strength of the dry product: 46.3 kg/briquette (max); 31.3 kg/briquette average; 18 kg/briquette min. (standard: 30 kg/briquette);
• +5 mm fraction after triple drop from a height of 2 m: 100% max.; 99.75% average; 99.5% min. (standard: 90%);
• +5 mm fraction in the impact test: 89% max; 85% average; 80.63% min. (standard: 85%);
• −0.5 mm fraction in the abrasion test: 12.74% max; 6.43% average; 0.23% min. (standard: not more than 15%).

At the same time, the study has several limitations. Specifically, granulation and sintering tests were not performed due to the unavailability of suitable industrial equipment, which limits the ability to directly compare the obtained results with those from alternative technologies.

Future work will focus on optimizing the composition and dosage of polymer binders and expanding the experimental database through testing on different types of equipment. These efforts will enable a more comprehensive evaluation of the proposed technology’s potential and support its adaptation for large-scale industrial applications.