EFFECT OF BENTONITE ON THE PROPERTIES OF IRON ORE PELLETS

V. M. Vitogin, T. G. Leont’eva, V. A. Trofimov

(Presented by the scientific seminar of the Department of General Chemical Technology)

In the practice of producing iron ore pellets, the question of the amount of bentonite introduced into the charge before pelletizing and of its effect on the mechanical and thermal properties of green pellets remains controversial. Thus, in particular, in the opinion of V. I. Korotich [1], the role of bentonite in a pelletized iron-ore charge is reduced solely to the plasticization of a moist dispersed material. On the other hand, it has been established experimentally that additions of bentonite determine the strength of pellets. P. N. Dokuchaev, for example, proposes an empirical equation for calculating the mechanical strength of pellets as a function of the swelling capacity of bentonites [2].

In the present work, the influence of the amount of additions of bentonite from the Makharadze deposit (swelling capacity 2.8, full swelling moisture content 117%, swelling rate 0.023 mm/min, total composition of the exchange-absorption complex 89.63 mg-eq.) on the mechanical strength and thermal stability of green iron ore pellets was investigated.

As the main object of study, production samples of magnetite concentrate with a particle size of less than 0.1 mm, from the pelletizing plant of the Sokolov-Sarbai mining and beneficiation combine, with a total iron content of 66.7%, were used. The experimental charges consisted of mixtures of magnetite concentrate and bentonite in amounts from 0 to 5%. In order to obtain the most homogeneous mixture possible, the components were mixed in the dry state, and then the charge was moistened to the optimal working pelletizing moisture content, calculated by the formula \(W_p^{\mathrm{op}} = \dfrac{\mathrm{MMB}}{K}\) [3]. Pelletizing of the green charge was carried out on a laboratory disk granulator with a diameter of

dish diameter 1000 mm, rim height 100 mm, with a dish inclination angle of 45° and a circumferential speed of 0.8 m/sec. The freshly prepared pellets were tested for thermal stability and mechanical strength, for crushing and for the number of drops from a height of 300 mm, according to the methods adopted at pelletizing plants. The pellets fired at a temperature of 1200°C were characterized by their crushing strength (in kg/pellet). Thermal stability was determined from the temperature at which cracking began and from the temperature of complete explosive destruction of the pellet structure—“shock.”

For this purpose, batches of fresh pellets were placed in a muffle furnace heated to a specified (constant) temperature and held in the furnace for 7 min. After holding in the furnace, the pellets were examined under eightfold magnification to establish the presence of cracks. The furnace temperature was then increased successively at intervals of 25°C until cracking and “shock” phenomena appeared. For each successive test, a fresh batch of pellets was used.

The results of the investigations are presented in Table 1 and in Figs. 1 and 2.

Fig. 1. Effect of bentonite additions on the strength properties of green pellets: 1—number of drops (pcs.); 2—crushing strength (g/pellet)

Fig. 2. Effect of the amount of bentonite on the thermal stability of iron-ore pellets: 1—change in “shock” temperature; 2—change in cracking temperature

From Fig. 1-1 it is seen that, as the bentonite addition is increased, the mechanical crushing strength of green pellets rises to a certain limit and then decreases sharply. The maximum value of the crushing load falls within the addition interval of 1–2%. The number of drops (until cracks appear), with an increase in added bentonite (Fig. 1-2), increases continuously. The pellets become more plastic and, when dropped, undergo only plastic deformations without noticeable disruption of the structure.

From Fig. 2-2 it is seen that the curve for the cracking temperature decreases monotonically as the bentonite addition is increased. The “shock” temperature (Fig. 2-1) increases somewhat with an increase in the amount of bentonite in the burden, but only up to a limit of 1–2%, and then decreases. The maximum of the “shock” temperature falls within the bentonite addition interval of 1–2%.

As can be seen from Table 1, the crushing strength of fired pellets made from pure concentrate is the max—

Table 1

Calculated optimum moisture content for pelletizing mixtures
and strength of fired pellets

... at the same time, the strength of the green pellets, as can be seen from Fig. 1, is the lowest. With an increase in the amount of bentonite added to the mixture, the strength of the fired pellets decreases.

Discussion of the results

The extreme character of the increase in the mechanical crushing strength of green pellets can be explained as follows. At small amounts (up to 2%), the addition of bentonite promotes strengthening of the pellets due to the formation and development of spatial coagulation structures in pore suspensions. At the same time, both the plasticity of the structure and the shear resistance increase. The increase in pellet strength with a further increase in the bentonite addition is limited by the deficiency of free, capillary water in the pellet structure and the corresponding incomplete swelling of the bentonite. Obviously, the bentonite addition in quantitative terms must be strictly coordinated with the moisture content of the iron-ore concentrate. It is precisely for this reason that the range of the optimum amount of bentonite added to the concentrate fluctuates within rather wide limits (1—2%). A substantial role here is played by the degree of swelling and the swelling dynamics of the bentonite. In other words, the optimum amount of bentonite can be established only with mandatory allowance for the amount of free (capillary) water in the mixture being pelletized, the degree of swelling, and the swelling rate of the bentonite. The latter factor is especially important, since the contact time of the bentonite with the moist concentrate is strictly limited by the technological regime and equipment design of the process of pelletizing iron-ore concentrates, while the swelling rate of bentonite depends both on its quality and on the ionic composition of the pore liquid.

In direct interrelation with mechanical strength is also the “shock” temperature, the curve of which (Fig. 2-1) has a similar character, although with a less pronounced maximum.

The gradual decrease in the crack-formation temperature (curve in Fig. 2-1) of green pellets with an increase in the amount of added bentonite from 0 to 5% is due to the known fact of increasing shrinkage deformations of the capillary-porous structure of pellets with increasing hydrophilicity and dispersity of the pelletized system.

The decrease in the crushing strength of fired pellets with an increase in the amount of bentonite introduced (Table 1) can be explained

because, when they enter places of possible contact between concentrate particles, bentonite particles hinder the formation of strong hematite bonds in these regions.

References

1. V. I. Korotich, Theoretical Foundations of Pelletizing Iron-Ore Concentrates. Metallurgizdat, 1965.

2. P. N. Dokuchaev, Investigation of the Influence of Bentonite and Lime Additions on the Pelletizing of Fine-Grained Iron-Ore Concentrates. Candidate’s dissertation, Rudny, 1968.

3. V. M. Vityugin, P. N. Dokuchaev, A. S. Bogma. Calculation of the optimum moisture content of materials before granulation.—Izv. vuzov, “Ferrous Metallurgy,” No. 8, 1969.