Effect of Overmoistening on the Properties of Iron-Ore Pellets during Convective Drying

T. G. Leont’eva, V. P. Gusev, V. M. Vityugin

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

In the process of bed convective drying of iron-ore pellets on the grate of an indurating machine, moisture is redistributed over the height of the bed. The surface rows of pellets, which are in contact with the hottest heat-transfer gas, are heated first. In this process, in accordance with the theory of A. V. Lykov, a partial transfer of moisture in the liquid state occurs from the hotter regions of the pellet bed to the less heated ones [1]. This transfer of moisture is possible when the pellets possess through capillarity. When the pellet bed is in contact, through capillarity is to some extent preserved along the contact planes. As a result of moisture transfer, overmoistening of the pellets in the lower part of the bed is observed. In addition, moisture accumulation in the colder regions of the pellet bed is promoted by condensation of moisture from the steam–air mixture.

The transfer of moisture in liquid form from some pellets to others weakens the bond strength between particles, “washes out” contacts, which leads to weakening and, in some cases, to complete destruction of the pellet structure. The destruction of pellets causes compaction of the bed, leading to disruption of the hydrodynamic and temperature regimes of operation of the entire indurating machine, to a reduction in machine productivity and in the quality of the fired product.

The effect of overmoistening of the lower regions of the pellet bed is observed only at the first stage of drying [2].

To determine the quantitative relationship between the magnitude of the moisture content and the properties of raw iron-ore pellets, laboratory investigations were carried out.

The objects of study were charge materials used in the production of iron-ore pellets at the pelletizing plant of the Sokolov–Sarbai Mining and Processing Combine: iron-ore concentrate, limestone from the Kzyl-Zhar deposit, and bentonite (Makharadze), with proportions in the charge of 91%, 8%, and 1%, respectively.

Pelletizing of the moist charge was carried out on a laboratory disk pelletizer with a diameter of 1 m and a rim height of 0.1 m, at a disk inclination angle of 45° and a circumferential speed of 0.8 m/min. The pelletizing time was 7 min. The initial working moisture content of the charge was 8.5%; the moisture content of the green pellets varied within the range 6.9–7.1%.

Convective layer drying of the pellets was carried out in an agglomeration bowl consisting of a quartz tube 120 mm in diameter and 400 mm long, with holes along the length of the tube and around its circumference every 10–20 mm for thermocouples. A wire mesh served as the bottom of the tube. A layer of freshly prepared pellets 300–320 mm high was charged into the tube. Control pellets were placed in wire baskets in different zones. The heat-transfer gas (hot air) at a temperature of 450°C was supplied from above downward through the layer at a velocity of 0.6 m/s. After a specified drying time, samples containing the control pellets were removed to determine moisture content and mechanical strength in compression and dropping. The moisture content of the pellets was determined by drying in an electric drying oven at 105°C for 2.5 h.

The results of studies on the changes in moisture content and mechanical strength of pellets from both the upper rows and the lower zones of the layer are given in Fig. 1 “A,” “B,” and “C.”

In the initial period of drying, in the second to fourth minutes, an increase in the moisture content of pellets located in the lower zones of the layer is observed, indicating the formation of an overmoistening zone, while the moisture content of pellets in the upper row decreases continuously. At the moment when the lower zones of the layer begin to become overmoistened, dehydration of the surface pellets intensifies. In Fig. 1 “C,” this moment is characterized by bends in the curves.

Fig. 1. Change in the properties of iron-ore pellets as a function of the time of layer convective drying: A — compressive strength; B — drop strength; C — moisture content. Lower lines are for surface pellets. Upper lines are for pellets in the lower zones of the layer.

The course of the curves in Fig. 1 “A” and “B,” characterizing the change in the pellets’ crushing and drop strengths, indicates that an increase in pellet moisture content relative to the initial moisture content in the initial period of drying causes a decrease in the mechanical strength of the pellets. At the same time, the mechanical strength of the pellets in the upper row gradually increases.

With increasing intensity of dehydration (the upper curve in Fig. 1 “C”), the degree of strengthening of the pellets correspondingly increases. The maximum increase in the moisture content of pellets located in the overmoistening zone (Fig. 1 “C,” upper line) corresponds to the maximum decrease in their mechanical strength (Fig. 1 “A,” “B,” lower curves).

When the pellets were examined at eightfold magnification after four minutes of layer convective drying, it was found that on pellets located in the lower zones—

as a layer, cracks appear. The strength of such pellets is low and, as a rule, does not exceed 0.5 kg/pellet.

Thus, as a result of the studies carried out, it has been established that overmoistening of the lower portions of the pellet bed during convective drying occurs in the second to fourth minutes.

A decrease in the moisture content of the pellets causes an increase in their mechanical strength and, conversely, overmoistening leads to an irreversible loss of strength. As the intensity of overmoistening increases, the weakening of the pellet structure becomes more pronounced.

REFERENCES

1. A. V. Lykov. Theory of Drying. “Energiya,” 1968.

2. N. E. Ruchkin. Investigation of the process of drying pellets during firing.—Transactions of the Uralmekhanobr Institute, issue II, Sverdlovsk, 1964.