On the Methodology for Determining the "Shock" Temperature of Iron Ore Pellets
Abstract Generated abstract
The paper examines why laboratory methods for determining the “shock” temperature of green iron ore pellets produced values higher than those observed in industrial drying. It argues that the discrepancy arises from inadequate modeling of heat exchange in a pellet bed, since production behavior depends on gas permeability, mixed convective and conductive heat transfer, moisture migration, and the formation of overwetted zones that weaken pellet structure. Experiments with pellets made according to SSGOK burden and technology show that individual pellets under convective conditions give much higher shock temperatures, while pellet layers with minimized convection and contact with an overwetted zone yield values closer to production data. The authors propose determining shock temperature on a pellet layer of specified dimensions under thermal and hydraulic drying conditions corresponding to industrial practice.
Full Text
ON A METHOD FOR DETERMINING THE “SHOCK” TEMPERATURE OF IRON-ORE PELLETS
A. S. BOGMA, V. M. VITYUGIN, T. G. LEONT'EVA, I. P. CHASHCHIN
(Presented by the scientific seminar of the Department of General Chemical Technology)
In communication [1] an experimental setup was described for determining the “shock” temperature of pellets, developed at the Department of General Chemical Technology. However, when the setup was tested, contradictory results were often obtained. The use of other known methods for determining the “shock” temperature of pellets [2, 3] led to similar results. As a rule, the “shock” temperature determined by the above-mentioned methods proved to be overestimated in comparison with production data. For green pellets prepared in the laboratory from burden material and according to the technology of SSGOK, it was 650–720°. According to production data from SSGOK, the “shock” temperature of pellets does not exceed 450–500°.
The main reason for such a discrepancy is evidently the insufficient modeling of the character of heat exchange in laboratory installations. Under production conditions, the efficiency of the drying process is determined by the magnitude and stability of the gas permeability of the pellet bed.
With low and nonuniform gas permeability, the character of heat exchange in the bed becomes dual. In some regions of the bed convective heat exchange predominates; in others, where the gas permeability is low, thermal conductivity has a substantial effect on heat exchange. Such a character of heat exchange causes the transfer of moisture in the liquid state from hot regions to cold ones owing to thermal moisture conductivity.
Since the strength of the pellets being dried is ensured by the molecular forces of cohesive-adhesive interaction only at the points of contact of the solid particles, the displacement of moisture into the open pores and capillaries of the pellets, given the hydrophilicity of the solid particles, leads to weakening of the pellet structure and to their partial or complete destruction. This causes an increase in the nonuniformity of the gas permeability of the bed and a disturbance of the processes of heat and mass exchange, i.e., it leads to complete disruption of the drying process.
In an experimental setup for determining the “shock” temperature, the different character of heat exchange will be determined mainly by the size of the pellets.
In the case of determining the “shock” temperature for individual pellets, convective heat exchange predominates throughout the entire time of heat treatment, which leads to an overestimation of the shock temperature. When determining the “shock” temperature for a bed of pellets, the value-
thermal and moisture conductivity increases substantially, while the value of the “shock” temperature decreases somewhat.
Finally, in the absence of movement of the drying agent in the layer of pellets, heat exchange is determined predominantly by thermal conductivity. Such conditions favor the formation of an overwetting zone in part of the pellet layer, and the shock temperatures become minimal, approaching production values. Thus, the “shock” temperature must be determined not for individual pellets, but for a layer, with convective heat exchange reduced to a minimum.
The above propositions are satisfactorily confirmed by the experimental data presented in Table 1.
Table 1
| No. | Heat-exchange conditions | Number of pellets, pcs. | “Shock” temperature, °C | Notes |
|---|---|---|---|---|
| 1 | natural convection | 1 | 750 | • |
| 2 | absence of convection | 11 | 720 | |
| 3 | » | 11 | 420—450 | The lower pellets have contact with the overwetted zone |
For the investigations, pellets prepared from a burden according to the recipe and technology of SSGOK were used.
The burden consisted of 91% concentrate from Sokolovsko-Sarbaisk ores, of size 1—0 mm (iron content—67.6%), 8% limestone, of size 0.25—0 mm, and 1% bentonite, of size 0.25—0 mm.
For the investigations, pellets similar to industrial ones were selected, i.e., having the following characteristics:
crushing strength—0.7—1.2 kg per pellet,
porosity—27—30%,
pellet diameter 14—16 mm.
According to the experimental data obtained, the assumption was confirmed that the “shock” of iron-ore pellets depends mainly on the character of heat exchange in the layer.
The onset of “shock” is inevitably connected with the appearance of overwetting zones and the accompanying movement of water in the liquid state.
Thus, the method for determining the “shock” temperature, developed as a result of the investigations carried out, consists in taking as the test specimen a layer of pellets 100 mm high and 50 mm in diameter (or width). The lower pellets are in contact with the overwetted burden. The parameters of the thermal and hydraulic drying regime are set within limits corresponding to the industrial technological regime.
LITERATURE
- A. S. Botma, V. M. Vityugin. Installation for determining the thermal stability of raw granules. Izv. TPI, 148, 96, 1967.
- Ya. L. Serebryannyi. Tsvetnye metally, No. 2, 22, 1966.
- I. E. Ruchkin. Trudy Instituta Uralmekhanobr., Issue 11, 100, 1964.