Briquetting of manganese ore fines with cement as binder

Abstract

This study develops briquettes from fines generated in the processing of manganese ore from a strategic manganeferous district of Minas Gerais-Brazil. The briquettes were produced based on the following variables: compaction pressure (20 and 25MPa), curing time (7 and 28 days), percentage by mass of binder (5% and 10%), and replacement with tailings (0% and 10%). Technological tests evaluated the compressive, abrasion, impact strength and water absorption. The best conditions for briquette included 10% Portland cement, 10% Mn tailings, compaction pressure of 25MPa and 7 days of curing, as well as 10% Portland cement, compaction pressure of 25MPa and 28 days of curing. The briquettes primarily contained the spessartine, todorokite, quartz, and pyrolusite minerals. The storage time tests indicated that the briquettes responded satisfactorily to storage for three months in a dry environment and at room temperature. Through crackling tests, the briquettes have potential to be commercially used.

keywords:
mineral processing; manganese ferroalloys; experimental factorial design

1. Introduction

About 90% of the manganese ores in the world are destined for the steel industry, and the addition of manganese confers malleability, toughness, and hardness to steel (Olsen et al., 2007; Tangstad, 2013). A common problem faced by the mining-metallurgical sector is the generation of massive fines during the exploitation, processing, and transportation of manganese ores, reaching up to 30% of all material. In addition to the storage difficulties of this fine material containing the useful resource, it cannot be used as a load for the electric furnace in alloy production. An alternative for using ore fines is for agglomerates like briquettes (Tangstad & Olsen, 1995; Junca et al., 2011; Grillo, et al., 2013; De Jesus & Tangstad, 2020).

Briquetting is an agglomeration technique that allows fine particles to acquire large shapes through compaction pressure. Despite briquetting not being common in the agglomeration of conventional concentrates, it is highly attractive for manganese ore fines because the pelletizing and sintering processes require a high implementation investment and large energy expenditures for steps, such as comminution and burning.

In contrast, briquetting can be performed at ambient temperatures, without requiring the material grinding, as well as requiring a lower initial investment than pelletizing and sintering, thus amassing great interest in terms of research and industrial application for small and medium-sized plants, having a great worldwide representation (Junca et al., 2011; Grillo, et al., 2013; Tangstad, 2013; By, 2017; Singh et al., 2021).

In the manganeferous district of Minas Gerais state in Brazil, an average of 12,000t/y of manganese ore fines and 8,000t/y of tailings, on average, are generated. The recovery of these materials reduces the environmental impact caused by the residue disposal, enables a greater use of the already exploited resource, results in a longer mine life, and is a greater generation of income. Briquetting can be considered a sustainable solution for utilizing these resources due to its environmental, economic, and social benefits (Singh et al., 2021).

Therefore, this article evaluates the use of fines obtained from the processing of manganese ore in the agglomeration for cement briquetting. In this study, cement-bonded briquettes with appropriate cold strength and metallurgical properties are fabricated for their application in a submerged arc furnace (SAF).

2. Material and method

2.1 Characterization of raw materials

The raw materials for the briquettes were collected from one of the mines of the manganeferous district of São João Del Rei-Minas Gerais. For both samples, the size distribution was determined via wet sieving. The chemical analysis of the elements and/or major compounds was performed by wet chemistry. The mineral constituents in the samples were determined in a semi-quantitative way through X-ray diffractometry (XRD) and total powder method (PANalytical X-ray diffractometer X'Pert³ Powder model with copper tube and 2θ interval, from 5º to 90º to identify the relatively abundant minerals in the sample).

2.2 Briquette production and quality tests

The briquettes were produced under different conditions, and the MINITAB 17^® software was used for the factorial planning of experiments. Portland cement was selected as the binder for briquetting.

The conditions for production were defined from the evaluation of previous briquetting studies (Richards, 1990; Thoms et al., 1999; By, 2017) and by preliminary tests. These conditions are presented in Table 1.

Table 1
Experimental variables and levels investigated in this study.

Levels	Factors
	Compaction pressure	Curing time	Percentage of binder	Replacement by tailings
-1	20 MPa	7 days	5%	0%
+1	25 MPa	28 days	10%	10%

The mixture procedure was carried out mechanically for 2min. Initially, the dry raw materials were mixed, followed by the addition of water to achieve the suitable moisture level. These moisture values ranged from 6% to 8%, and post-production, they were evenly transferred to a cubic mold of 5 cm × 5 cm × 5 cm. After compaction, the briquettes remained in the mold for 24 hours, followed by the curing stage in a dry environment at room temperature. The evaluation of the quality of manganese ore briquettes has limited literature references. In addition, to reach the SAF interior without disintegrating, the agglomerates must have sufficient mechanical strength (De Jesus & Tangstad, 2020).

The following response variables were evaluated: compressive strength (CS), impact strength index (ISI), abrasion strength index (ASI), water absorption index (WAI). The compressive strength was tested on the hydraulic press CI100 tons-press force (SOLOCAP), based on the standard ASTM C170. The impact strength of the materials was evaluated using a methodology based on ASTM D440-07, in which the briquettes were subjected to free falls from a height of 0.3 m. The abrasion strength consisted of subjecting the previously weighed briquette to vibratory sieving for 15 minutes. The evaluation was carried out by quantifying the loss of mass in particles smaller than 3.3 mm (test mesh) after carrying out the test. In the water absorption tests, the briquettes, weighed before, were immersed in cold water for 30 minutes, and then the percentage of water absorbed by the material was measured.

The chemical-structural characterization was constituted by the chemical analyses of the elements and/or major compounds. The morphology of the solid samples were examined in a scanning electron microscope (SEM), model JEOL JSM 6610. Storage and crackling time tests were also performed based on the previously reported studies (Faria et al., 2012; By, 2017; NBR ISO 8371, 2020).

3. Results and discussion

3.1 Characterization of raw materials

The fines product sample stood out for exhibiting higher manganese grade, lower grades of SiO₂ and Fe when compared to the tailings sample (Table 2).

Table 2
The chemical composition of raw materials.

Material	%Mn	%Fe	%SiO2	%Al2O3	%CaO	%MgO	%P
Fine product	27.88	5.99	25.80	8.97	1.91	0.70	0.07
Tailing	21.32	7.12	34.80	9.49	2.04	0.57	0.06

Table 3 presents the physical characterization results of the raw materials. The size analysis (Figure 1) showed that the fine product sample demonstrated a wide range of size distribution, a significant factor for mixture composition. The coarser particles, granulated around 1 to 3 mm, act as nucleation points through which fine particles, granulation less than 0.2 mm, interconnect by filling the voids, thus ensuring a better compaction efficiency of the aggregate (Dehont, 2006; Pan et al., 2016).

Table 3
Summary of the physical characterization of raw materials.

Sample	Property
	d80(mm)	d50(mm)	Density (g/cm3)
Fine product	4.45	2.32	3.15
Tailing	0.19	0.05	2.99

Figure 1
Size distribution of raw materials.

Diffractogram patterns of the fines product (Figure 2a) and tailings (Figure 2b) demonstrate that spessartine Mn₃Al₂(SiO₄)³, todorokite ((Ca,Na,K)(Mn⁺²Mn⁺⁴)₆O₁₂.xH₂O), pyrolusite (MnO₂), quartz (SiO₂) are the main crystalline phases in both raw materials.

Figure 2
Diffractogram pattern of raw materials (a) Fines product (b) Tailings.

3.2 Evaluation of the quality of briquettes

3.2.1 Compressive strength

The CS values for the briquettes ranged from 0.78 to 6.41MPa, which can be considered optimal, as observed by Thoms et al., 1999. Figure 3(a) shows the main variable influencing the CS of the briquettes was the percentage of binder, followed by compaction pressure and replacement with tailings. Therefore, the high values of these three variables provided the best answers to this quality parameter (Figure 3(b)). Portland cement is a matrix binder, which covers the surface of particles to connect them (Pietsch, 2002). Thus, using a higher binder percentage ensured higher compressive strength values for the briquettes (Ordiales et al., 2016). However, a large dosage of Portland cement as the binder may contribute to a significant amount of silica in the alloy (Eisele & Kawatra, 2003).

Figure 3
Compressive strength of the briquettes (a) Pareto chart (b) Main effects chart.

3.2.2 Impact strength

The ISI values for briquettes were observed between 1.03% and 13.24%. The variable that influenced the most was the percentage of binder, followed by compaction pressure, curing time, and tailings replacement, according to the Pareto graph (Figure 4(a)). From Figure 4(b), using the upper level of these three variables favored the generation of a stronger connection between particles and a better compaction, producing a more cohesive and resistant aggregate (Lemos et al., 2015). Conversely, the lower value of the curing time variable yielded the best results, which could be associated with how the briquette was cured. The goal of curing is to keep the aggregate saturated until the empty spaces initially occupied by water are occupied by cement hydration products. When premature water loss occurs, the aggregates have highly porous surface layers, and with less strength and durability. Thus, the briquettes are weakened over time (Tangstad et al., 2004; Mindess & Young, 1981).

Figure 4
Impact strength index of the briquettes (a) Pareto chart (b) Main effects chart.

3.2.3 Abrasion strength

The best ASI value was 12.91% for the composition produced with the lower level of the replacement by the tailings variable and higher levels for other variables. Some briquette compositions became quite friable, a factor contributing to lower ASI results, which may also be associated with the way the briquettes were cured. According to the Pareto graph (Figure 5(a)), the percentage of binder and replacement with tailings and the interaction between them have the most significant influence. From Figure 5(b), higher values of the four variables used in this study yielded the best results.

Figure 5
Abrasion strength index of the briquettes (a) Pareto chart (b) Main effects chart.

3.2.4 Water absorption

WAI values were among 89.5% and 93.07%, and all the rehearsed briquettes remained intact during the 30 min of submersion in cold tap water. From Figure 6(a), the variable curing time, percentage of binder, and interactions among compaction pressure, percentage of binder, and replacement with tailings present the most significant response to this parameter. According to Figure 6(b), except the curing time variable, all higher variable values yielded the best results for this parameter, producing more cohesive and less-porous briquettes. The higher the porosity, the greater the water absorption by the aggregate, therefore, generating briquettes with better responses to WAI (Tangstad et al., 2004).

Figure 6
Water absorption index of the briquettes (a) Pareto chart (b) Main effects chart.

3.2.5 Best compositions for the briquettes

According to the quality tests, no significant difference was observed among the different compositions (Table 4). According to the statistical analyses, the variable that most influenced the response was the percentage of binder, followed by compaction pressure and replacement by tailings. The briquettes produced with the higher levels of these variables obtained, yielded considerably better results among the quality tests. Conversely, the curing time variable exerted the least influence on the quality of the briquettes. These results seem promising from the industrial point of view, since post-production, the briquette could be used within the shortest possible time.

Table 4
The best compositions for the as-prepared briquettes.

Briquette	% Binder	Pressure (MPa)	Replace. by tailings (%)	Curing (days)	CS (MPa)	ISI (%)	ASI (%)	WAI (%)
BMC1	10	25	10	7	6.4	1.0	13.2	93.1
BMC2	10	25	0	28	5.0	1.8	12.9	91.3

3.3 Tests for briquettes with the best compositions

3.3.1 Chemical and morphological characterization

According to the chemical analysis of the briquettes with the best compositions (Table 5), regarding the Mn grade, the two compositions meet the specifications for manufacturing manganese alloys, and both have high Fe grades with a Mn/Fe ratio above 4. However, the ratio value did not reach six for any of these compositions, which is the minimum specification to produce the manganese iron alloys (Olsen et al., 2007). The Mn/P ratio was significantly higher than the specified minimum value of 233. Although briquettes do not acquire all chemical specifications in the manufacture of some alloys, they can be used in electric furnaces, by adjusting their grades through different blends.

Table 5
Chemical composition of the briquettes with the best compositions.

Briquette	%Mn	%Fe	%P	%SiO2	%Al2O3
BMC1	24.07	5.25	0.06	25.40	9.95
BMC2	28.77	5.65	0.09	25.20	10.06

From the SEM images obtained by the secondary electrons (Figure 7 (a) and (b)), the heterogeneity of the particles can be observed for the two compositions, along with a greater number of pores in BMC2 than BMC1, associated with a higher proportion of particles of lower granulation in BMC1 due to the addition of manganese ore tailings for the production of the mixture of briquettes of this composition.

Figure 7
Photomicrographs of the secondary electrons of the two best briquette compositions. (a) The photomicrograph of BMC1 (magnification of 40x); (b) The photomicrography of BMC2 (magnification of 43x).

3.3.2 Storage time

Tests were performed to evaluate the storage conditions of the briquettes produced with the two best conditions for 90 days, in a dry environment and at room temperature. The results for the briquettes post-curing and trial repetitions showed that the briquettes remained intact after three free falls from a height of 0.3 m, with a mass loss of less than 5% for particles smaller than 3.3 mm (Table 6); the results are consistent with those by By (2017), and can be considered satisfactory.

Table 6
The results of the storage time tests for the briquettes with the best conditions.

Briquette	Mass loss of particles smaller than 3.3 mm (%)
	After curing	After 30 days	After 60 days	After 90 days
BMC1	1.03	1.35	2.43	2.50
BMC2	1.81	2.55	4.11	4.80

3.3.3 Crackling

The results of the crackling tests for the two best briquette compositions showed that crackling in BMC1 was derisory, and in BMC2, this value did not exceed 1.20%. Ores with considerable fractions of oxidized manganese minerals exhibited higher crackling rates and were mostly associated with the decomposition of these oxides (Faria et al., 2012). As the composition of briquettes mainly contained todorokite and spessartine, the crackling was significantly low, reaching insignificant values for BMC1. However, the crackling may be associated with the elimination of structural water from the hydrated phases.

4. Conclusions

Based on the results of quality tests and statistical analyses, the best conditions for the manufacture of briquettes were with 10% Portland cement, 10% of Mn tailings, compaction pressure of 25 MPa, and 7 days of cure (BMC1), and with 10% Portland cement, compaction pressure of 25 MPa, and 28 days of cure (BMC2). The two compositions showed excellent results for the crackling index and a three-month storage in a dry environment at room temperature. These briquettes can be commercially used, using the fines material that would otherwise be stocked or discarded due to the impossibility of its use. Some advantages of this synthesis could be the reduction of the environmental impact, the greater use of the resource already exploited, and consequently, a greater generation of income. Notably, the new compositions of briquettes produced from the manganese ore fines from other mines belonging to this manganeferous district are being studied currently.

Acknowledgments

The authors are grateful to UFOP, CNPq, CAPES, FAPEMIG and the Multi-User Laboratory of High-Resolution Microscopy of the Institute of Physics of the UFG.

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Publication Dates

• Publication in this collection
  15 July 2024
• Date of issue
  2024

History

• Received
  19 Apr 2023
• Accepted
  11 Dec 2023