The technological development of solid-phase iron’s direct reduction, where strong iron-ore pellets represent the main charge, is a progressive field for contemporary metallurgical production.

Requirements of metal plating processes are mainly caused by the physicochemical changes, which occur in the iron-ore material during these processes. These requirements were comprehensively analyzed by the authors from [1–5].

Raw material has the following general requirements for metal plating shaft furnaces:

—high iron content at low sulfur contents, phosphorus, alkali, and nonferrous metal impurities, which largely affect the steel quality and performance indicators of its casting in electrical furnaces;

—high reducibility;

—high hot strength, which determines the pellet ability to retain integrity upon reduction;

—low susceptibility to sinter formation, swelling, and strain upon direct reduction of iron in solid-phase processes, which determines high gas permeability of charge in furnace;

—and increased onset fusion temperature [1, 6, 7].

A narrow particle size distribution of the agglomerated raw material possessing sufficient strength in initial state and upon reduction with high onset fusion temperature and low content of fine fraction (less than 5 mm) is necessary for metal plating furnaces with the aim to develop optimal gas-dynamic conditions, which provide high performance and high degree of metal plating. According to the experimental data, the following data is shown: the 9–16-mm pellets at content less than 5-mm fraction of less than 5% and the compression strength higher than 250 kg/pellet in the cold state.

With the incorporation of metal plating shaft furnaces, additional external and internal friction forces arose upon hot discharge due to large strain of reduced pellets, which determine the form of burden yield in furnace. With an increase in the internal friction coefficient, the motion of charge uniformity in the shaft furnace is distorted, which leads to gas-dynamic model violation of reduction and destabilization of the effective product quality. Friction coefficients are determined by the surface’s state and structure, as well as the degree of pellet strain in charge bottom lifts with a high degree of reduction [8, 9].

Many years fabricating iron-ore pellets showed that the addition of various fluxing or flux-strengthening components is one of the most widespread approaches to control physical and metallurgical characteristics of pellets [10–18].

The addition of dolomite to the pellet charge instead of limestone prevents the formation of fusible eutectics, which allows for a temperature increase to reduce pellets in metal plating furnaces. However, the compression strength of pellets decreases with the retention of temperature–time calcination conditions (from 242 to 160 kg/pellet), which is caused by the lack of liquid-phase binder upon high-temperature strengthening on a roast machine. To obtain high strength of roasted pellets with the addition of magnesium oxide, it is necessary to increase temperature in the roast machine area up to 1300–1320°C [6, 7, 10, 15].

The addition of MgO results in the decrease in the reduction ability of the pellets, as well as improvement of strength characteristics of the pellets upon recovery. The pellets with the basicity of 1.2, where limestone represents a fluxing additive (0.6% MgO), possess the highest reducing ability. In spite of the decrease in the reducing ability of the pellets upon the addition of MgO, it remains higher than at the basicity of 0.5 fluxed in the presence of CaO. Basicity is usually 0.5–0.8 upon the MIDREX process, while it varies from 0.2 to 1.2 in the HyL-III process [12–16].

The pellets containing 3–5% MgO, where dolomite is a fluxing additive, possess the highest metallurgical characteristics. Magnesite, olivine, and dunite can act as magnesium-containing additives to the charge of pellets. The studies showed that addition of 1–2% of olivine (Mg,Fe)2SiO4 to super-rich Olenegorskii concentrate (0.4–0.6% SiO2) increases both cold (by the factor of 1.5–1.7) and hot (by the factor of 1.5–3.0) strength of pellets [8].

At present, researchers in the field of iron-ore raw material production are focused on magnesium hydroxide Mg(OH)2, which represents an insoluble base.

An increase in the magnesium oxide content in pellets results in the formation of the pyroxene phase (Mg,Fe,Ca)2Si2O6 possessing high melting point (1540–1550°C) [13].

Natural minerals of magnesium hydroxide, which constitutes the main content of brucite rocks, was named after American mineralogist A. Bruce, and its chemical formula is Mg(OH)2. Its pure mineral composition is 69.12% MgO and 30.88% H2O.

Brucite is a mineral that mainly consists of magnesium hydroxide, as well as possesses lower porosity at heating and lower dissolving ability in the sinter burden materials as compared to magnesite.

Brucite contains less inclusions, which form the basis of slurries. It contains more magnesium oxide and possesses lower losses upon sintering as compared to magnesite. This is one of the advantages, because low-porous materials avoid fast cohesion of magnesium oxide in the metal upon heating.

Brucite generally has less inclusions. The silica content is lower than that in serpentinite, while the calcium oxide content is less than that in dolomite. If a magnesium-containing additive contains a large SiO2 content, a “thick drop structure” in the slaggy phase forms and, as a result, the agglomerate strength decreases [11, 16, 17].

Laboratory experiments using magnesium fluxing material represented by Flumag M based on brucite for the preparation of iron-ore pellets were carried out in the Stary Oskol Technological Institute, National University of Science and Technology “MISiS” [18].

Flumag M based on brucite contains at least 55% of magnesium oxide, at least 6% of silica, less than 1% of Fe2O3, and less than 0.03% of sulfur according to chemical analysis; losses on ignition are less than 35%.

A series of experiments on charge dosage into pellets were performed. At the Institute under laboratory conditions, limestone, bentonite, and Flumag M fluxing additive were dispersed on a disc grinder and the material was sieved through 0.071 mm sieve. Magnetite concentrate chosen for studies contained 97.65% of the particles with the size less than 0.045 mm.

Qualitative characteristics of charge components are given in Table 1.

Table 1.   Quality indicators of charge materials
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In order to evaluate the Flumag M fluxing additive as flux-strengthening components in the fabrication of iron-ore pellets, the pellets fabricated using limestone at analogous quantities were used for comparative analysis.

The bentonite dosage was constant in all experiments and corresponded to 0.6%.

A total of 2 kg of magnetite concentrate was taken for the preparation of pellets in order to provide the necessary amount of the test specimens.

Raw pellets were prepared in a laboratory pelletizer (Fig. 1). Then, they were exposed to particle size distribution analysis and compression and discharge strength and moisture content.

Fig. 1.
figure 1

Charge pelletizing.

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The discharge strength n, times, was calculated using the following equation:

$$n = \frac{{\sum {{{n}_{i}}} }}{{10}},$$

where ni is the number of discharges of one pellet until loss of integrity.

The test results were calculated exactly to the integer value. The charge pelletizing ability was identified as the material ability to form pellets with particular strength characteristics.

There is no standard procedure on the determination of pelletizing ability of fine-dispersed material. Thus, the researchers suggest various approaches to the determination of charge pelletizing ability. The staff of the Division of the Ugarov Stary Oskol Technological Institute, National University of Science and Technology “MISiS” suggested the procedure of pelletizing ability tests for the pelletizing study of iron-ore concentrates, which is described in detail in [1]. In accordance with this procedure, the pelletizing ability is performed according to the yield of the grains larger than 5 mm and the duration of pelletizing process corresponds to 20 min.

The moisture content and strength of raw pellets was determined according to the procedures accepted in industrial enterprises [19]. The laboratory study results of raw pellets are given in Table 2.

Table 2.   Quality indicators of experimental raw pellets with addition of Flumag M flux
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The weight fraction of moisture (W) was calculated as follows:

$$W = \frac{{{{m}_{1}} - {{m}_{2}}}}{{{{m}_{1}} - m}}\,\, \times \,\,100\% ,$$

where m1 is the container mass with shot before drying, g; m2 is the container mass with shot after drying, g; and m is the mass of the empty container, g.

Calculations were performed exactly up to the second decimal.

Divergence between the results of parallel measurements was less than 0.3%. Divergence between the results of three measurements was less than 0.4%.

The test results were calculated exactly up to the whole integer.

Heat treatment of the pellets was carried out in a laboratory furnace according to the developed temperature–time mode, which was maintained constant in all experiments. The maximum temperature of annealing corresponded to 1280°C (Fig. 2).

Fig. 2.
figure 2

Pellets roasting: temperature 900°С (а); temperature 1280°С (b).

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The compression strength of the pellets was determined according to the procedure in accordance with GOST [20] using a hydraulic press, which applies a maximum load on the pellet.

The compression strength of pellet Pav was calculated as follows

$${{P}_{{{\text{av}}}}} = \frac{{\sum {{{P}_{i}}} }}{n},$$

where Pi is the compression strength of one pellet, kg (N), and n is the number of pellets for the determination of strength, units.

The results were rounded up to one decimal value, lg/pellet. The dispersion and abrasive strength [21] were evaluated in a laboratory drum and determined as follows:

$${{\Pi }_{{ + 5}}} = \frac{{{{m}_{1}}}}{{{{m}_{1}} + {{m}_{2}} + {{m}_{3}}}}\,\, \times \,\,100,$$

where m1 is the mass of >5 mm fraction after testing in drum, kg; m2 is the mass of <5 mm and >0.5 mm fraction after testing in drum, kg; and m3 is the mass of the <0.5 mm fraction after testing in drum, kg.

The abrasion wear resistance of the pellets was calculated using the following equation:

$${{{\text{A}}}_{{ - 5}}} = \frac{{{{m}_{3}}}}{{{{m}_{1}} + {{m}_{2}} + {{m}_{3}}}}\,\, \times \,\,100.$$

The impact and abrasion wear strength were evaluated on two shots. The mean value of two parallel measurements was taken as the final result.

Table 3.   Quality indicators of experimental roasted pellets with addition of Flumag M flux
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Analysis of the test results showed that the Flumag M fluxing additive can be used in the production of iron-ore pellets. According to the concentrate features of various deposits under different process conditions, the fluxing additive dosage should be optimized at each particular process condition and can differ from the results above.

CONCLUSIONS

The Flumag M fluxing additive does not prevent the pelletizing process with the addition to the iron-ore concentrate as a charge component.

The Flumag M (brucite) is a technologically pure supplier of magnesium oxide into iron-ore charge and provides raw pellets upon palletization without worsening their qualitative characteristics.

Analysis of the roasted pellet characteristics shows that the compression strength of the roasted pellets slightly decreases with the addition of the Flumag M fluxing additive, which can be rationalized by the decrease in the amount of the liquid-phase binder when using magnesium oxide as a fluxing additive. This factor confirms the necessity for the increase in temperature of pellet annealing when using magnesial fluxing materials up to the temperatures of 1300–1330°C.

The impact and abrasion wear strength of the pellets is slightly higher with the addition of the Flumag M fluxing agent than those using limestone. The largest difference in abrasion wear strength characteristics was recorded at the content of 2% of Flumag M.

With an increase in the dosage of the Flumag M fluxing agent, the compression strength of pellets tends to decrease; however, this can be controlled by the change of the temperature–time mode of heat treatment.