INVESTIGATION OF THE DESTRUCTION OF THE STRUCTURE OF WET IRON ORE PELLETS DURING DRYING

V. M. Vityugin, T. G. Leont’eva

The drying of iron ore pellets must be optimized so as to ensure the maximum rate of the process while preserving the high mechanical strength and thermal stability of the dry product. The principal obstacle to intensifying drying is the cracking of the pellets, which lowers their mechanical strength, and complete, explosion-like destruction—“shock”—as a result of which fines are formed. At the same time, the gas permeability of the bed decreases sharply and, as a consequence, the productivity of the drying and firing process and the quality of the finished pellets decrease.

Analysis of the state of pellets in the drying zone over the height of the bed on industrial traveling-grate firing machines shows that, with an initially uniform moisture content, its redistribution is observed, with overwetting of the lower and middle layers. In the overwetted zones, during the first period of drying, crack formation occurs in the pellet structure. Under production conditions this phenomenon is observed in the zone of the first three drying chambers. In the second drying period, in the last chambers, explosion-like destruction of pellets into fine particles—“shock”—is observed.

To determine the causes of pellet destruction, several series of experiments were carried out. Pellets 15–20 mm in size were prepared from the production burden of the Sokolovsko-Sarbaisky Mining and Processing Combine (SSGOK) of the following composition: iron ore concentrate—91%, bentonite—1%, limestone—8%.

Both individual pellets and a pellet bed 100 mm thick were subjected to drying in a muffle furnace heated to a specified temperature. In all experiments, the holding time of the pellets in the furnace was 7 minutes. The temperature interval for the preliminary experiments was 100°C, and for the corrective experiments, 20°C.

To determine the influence of the gas phase on crack formation and “shock” of the pellets, balling was carried out with distilled water, technical—tap—water, and water saturated with CO₂. The main results are summarized in Table 1.

Raw iron ore pellets constitute a three-phase system: solid—liquid—gas. The solid part of the pellets is a typical polycapillary porous body capable of retaining water

molecular and capillary forces. The gas phase is a steam–air mixture.

The nature of the interaction of the phases determines the mechanical and thermophysical properties of the pellets. The latter are functionally dependent on the indices of maximum molecular moisture capacity \((W_{\mathrm{mmv}})\), maximum capillary moisture capacity \((W_{\mathrm{mkv}})\), the working moisture content of the pellets \((W_{\mathrm{opt}}^{p})\), and, as a consequence, on the index of lumpiness

Table 1

Results of observations of pellet destruction during drying

\[ K=\frac{W_{\mathrm{mmv}}}{W_{\mathrm{mkv}}-W_{\mathrm{mmv}}} \]

In the process of drying pellets, evaporation of capillary moisture occurs first; this is accompanied by vigorous, but, as a rule, surface vapor formation. With increasing temperature, the peripheral layers of water held by molecular forces gradually pass into capillary-bound water, the removal of which is evidently the main cause of crack formation. From the data given in the table it is clear that crack formation in the pellet structure is observed both during the drying of single pellets and of their layer. When pellets are dried in a layer, the crack-formation temperature is lower; moreover, cracks arise predominantly on pellets located in the zone of overmoistening. This can be explained by the fact that, in the case of overmoistening of pellets, washout occurs, contacts are weakened, and ultimately the structure is destroyed. “Shock” occurs at the end of the second drying period as a result of removal of molecularly bound water. Especially dangerous is the presence of trapped air in the structure of raw pellets. In the case of blockage of individual pores, arising during shrinkage, removal of the steam–air mixture from the pellet occurs explosively and causes complete destruction of the structure—“shock.”

From the analysis of the studies carried out, the following conclusions may be drawn.

1. Crack formation occurs at comparatively low temperatures at the beginning of the drying process, evidently during the period when capillary water is removed. The process of crack formation is intensified by the redistribution of moisture during heating of the pellet layer.

2. “Shock” occurs at considerably higher drying temperatures after cracks have formed, when evaporation of molecularly bound water takes place.

3. The presence of an entrapped gaseous phase promotes the intensive destruction of pellets during drying.