Study of shrinkage phenomena during the drying of iron ore pellets
Abstract Generated abstract
This study examines shrinkage and deformation during drying of iron ore pellets and related charge materials, focusing on the effects of drying temperature, material composition, dispersion, and initial moisture content. Laboratory pellets made from magnetite concentrate, limestone, bentonite, and mixtures were dried at fixed temperatures from 105 to 400°C, and changes in linear dimensions were measured. The results indicate that shrinkage under mild drying conditions is greatest in materials with high optimum moisture content, specific surface area, and molecular moisture capacity, especially bentonite, while iron ore concentrate shows minimal shrinkage. At higher drying temperatures some pellets exhibit positive dimensional change, attributed to plastic deformation under vapor-gas pressure gradients, with shrinkage behavior governed jointly by material structure and drying regime.
Full Text
INVESTIGATION OF SHRINKAGE PHENOMENA DURING THE DRYING OF IRON-ORE PELLETS
V. M. Vityugin, T. G. Leont’eva, V. P. Gusev
(Presented by the scientific seminar of the Department of General Chemical Technology)
In the process of drying bodies formed from dispersed materials, a change occurs in their geometric dimensions—shrinkage. This phenomenon is associated with the removal of moisture from the structure of the bodies. The character and quantitative indices of shrinkage are determined by the amount of moisture, the nature of the dispersed materials, and also by the intensity and method of drying.
In accordance with the classification of A. V. Lykov [1], iron-ore pellets, depending on the composition of the charge, may be assigned to types of bodies between capillary-porous and colloidal capillary-porous bodies. For each of these types, shrinkage phenomena are distinctive. Naturally, at low drying intensities the shrinkage phenomena will be determined mainly by the type of body being dried. Thus, under a mild drying regime, shrinkage phenomena are practically not manifested in capillary-porous bodies. For colloidal capillary-porous bodies, shrinkage under these conditions is inevitable and occurs to a greater degree the higher the degree of colloidality. For capillary-porous bodies with a small content of colloidal particles, the most dangerous stage, from the standpoint of shrinkage, is the initial one, when capillary water evaporates from the surface layers of the body at a comparatively high rate. The moisture and temperature gradients that arise in this case cause cracking of the surface zone of the body. When the drying process is intensified, shrinkage phenomena increase. The character of shrinkage in this case may change depending on the type of capillary-porous body.
The present work sets forth the results of a study to determine the influence of the temperature regime, charge composition, and initial moisture content on the character and magnitude of shrinkage.
For the investigations, charge materials from the production of iron-ore pellets at the pelletizing plant of the Sokolov-Sarbai—
of the Sokolovsko-Sarbaisk Mining and Processing Combine (SSGOK): magnetite concentrate, limestone, and bentonite. The pelletizing of raw materials and mixtures was carried out on a laboratory disc pelletizer with a disc diameter of 1 m and a rim height of 0.1 m, at a disc inclination angle of 45° and a circumferential speed of 0.8 m/s; the initial moisture content was equal to the optimum working moisture for pelletizing; the pelletizing time was 7 min; the pellet diameter was 14–16 mm. The shrinkage was measured using a KM-3 cathetometer. The pellets were dried in a muffle furnace heated to a constant temperature. The results of the investigations are given in Table 1.
Table 1
Magnitude of shrinkage of materials as a function of natural properties and temperature regime
| Materials and mixtures | Specific surface, cm²/g | Moisture content of pellets, % | MMW, % | Porosity, % | Shrinkage, % 105°C | Shrinkage, % 200°C | Shrinkage, % 300°C | Shrinkage, % 400°C |
|---|---|---|---|---|---|---|---|---|
| Iron-ore concentrate (SSK) | 2100 | 7 | 5.6 | 26.8 | 0.7 | 0.3 | 0.1 | 0 |
| Limestone . . . | 4860 | 16 | 14.5 | 34.9 | 2.4 | 1.8 | 0.7 | +0.1 |
| Bentonite . . . | 562000 | 50 | 40 | 50 | 11 | 10.7 | crumbled | crumbled |
| SSK-95 Bentonite-5 . . . . . | — | 9.8 | 7.2 | 32.3 | 5 | 4.7 | 0.1 | +1.1 |
| SSK-91 Bentonite-1. Limestone-8 . | — | 8.7 | 6.5 | 29.3 | 2.9 | 1.8 | 0 | +0.8 |
The greatest shrinkage is characteristic of materials with maximum values of the optimum working moisture content. Thus, among the materials listed in the table, the greatest shrinkage is characteristic of pellets made of pure bentonite. Pellets made of iron-ore concentrate have the smallest shrinkage. The magnitude of shrinkage is associated with the occurrence of surface cracks on the pellets. The most intense crack formation is characteristic of bentonite specimens.
For pellets made of all the materials listed in Table 1, shrinkage (with a decrease in size) is observed under comparatively mild temperature regimes (up to 300°C). Under high-temperature drying regimes, an increase in the linear dimensions of the pellets is observed. It should be noted that the greatest values of positive shrinkage (with an increase in size) under severe drying regimes are characteristic of materials that exhibit the greatest shrinkage values under mild drying regimes (negative shrinkage). This fact can be explained by the plasticity of the pellet structure. Under the action of significant pressure gradients of the vapor-gas mixture between the center of the pellet and its surface, plastic deformation of the structure occurs without destruction. The mobility of the coagulation structure at contact points, most highly developed in pellets made from mixtures with bentonite additions, ensures the integrity of the pellets. On the other hand, the considerable strength of coagulation structures counteracts shrinkage stresses, which reduces the possibility of crack formation. However, the amount of bentonite does not indefinitely increase the resistance of the structure to shrinkage stresses. Thus, for specimens made of pure bentonite, for which maximally developed coagulation structures are characteristic, crack formation is maximal. In our view, this phenomenon is explained by the presence of a considerable amount of contact water, which can be indirectly assessed by the MMW index (maximum molecular
moisture capacity), and the magnitude of the contacts (water contacts) is commensurate with the sizes of the particles, which corresponds to the magnitude of shrinkage.
Consequently, the magnitude of shrinkage depends primarily on the nature of the material, on the degree of dispersion, the magnitude of the specific surface area, the maximum molecular moisture capacity, etc. The character of shrinkage is determined both by the nature of the material and by the temperature regime of drying.
REFERENCES
- A. V. Lykov. Theory of Drying. “Energiya,” Moscow, 1968.