EXPERIMENTAL STUDY OF DRYING PROCESSES OF PEAT-ORE BRIQUETTES FROM A MONO-CHARGE OF WET MOLDING

V. G. ZAVRIN, I. L. DUNIN, V. F. MESHKOV

(Presented by the Department of Industrial Thermal Power Engineering and by the scientific seminar of the Thermal Power Engineering Faculty)

It is known that careful preparation of the initial materials for smelting and the creation of a mono-charge make it possible to conduct the blast-furnace process more smoothly and improve the technical-and-economic indicators of blast-furnace smelting. The basic requirement for briquettes produced on the basis of peat and iron ore is increased strength and the lowest possible moisture content, which is dictated by the requirements of the technological regime in the blast furnace. Since, in the production of machine-formed briquettes, questions of their drying play a decisive role, experimental work was carried out in the thermotechnical laboratory of the Department of Industrial Thermal Power Engineering to determine optimal drying regimes, with the aim of obtaining briquettes with the best mechanical characteristics.

The experimental work was carried out in two stages. The purpose of the first series of experiments was:

a) determination of the optimal moisture content from which it is expedient to carry out artificial drying of briquettes at various temperatures of the drying agent;

b) determination of a drying regime under which the briquettes retain their shape and the necessary mechanical strength;

c) determination of the drying duration as a function of the initial and final moisture content of the briquettes and the drying regime. Drying of the briquettes was carried out in a drying chamber of the SЭSh-3 type (Fig. 1), equipped with a rotating table. The operating temperature in the chamber was provided by an electric heater and was maintained constant by means of a thermoregulator consisting of a mercury contact thermometer of the TK-6 type, a step-down transformer, and a relay. The heater consisted of two parallel sections with powers of 850 and 1100 watts. The 850-watt section was switched on by an additional switch for accelerated heating of the dryer and for operation at a drying-agent temperature above 120°C. The 1100-watt section was included in the thermoregulator circuit. Air exchange in the dryer was effected by a fan driven by an AOLЬ-011 electric motor. Drying of the briquettes was carried out in mesh aluminum trays. The parameters of the drying agent were monitored by means of mercury psychrometers. The accuracy of temperature regulation in the drying chamber was ±2°C. The loss of weight during the drying process was determined by means of second-class analytical balances of the ADV-200 type. The briquettes to be dried were prepared according to the method of the problem laboratory-

…for the comprehensive utilization of peat. The moisture content and its change during the drying process were determined by the standard method.

The experiments were divided into three groups.

The first group of experiments was carried out with compacts of increased (up to 75%) moisture content at high temperatures of the drying

Fig. 1. Diagram of the experimental installation

agent, equal in individual experiments to 120, 130, and 150°C. As a result of the experiments, it was established that, with such an organization of the drying process, the briquettes not only fail to meet the requirements imposed on them, but are also completely destroyed.

The second group of experiments was carried out at reduced temperatures of the drying agent, equal to 80 and 90°C, with compacts of various initial moisture contents (from 55% to 65%). It was established that even at low temperatures, compacts with high initial moisture content do not meet the requirements imposed on them.

The third group of experiments consisted of control tests. The experiments were carried out with briquettes of various initial moisture contents (from 40% to 60%) and at various temperatures (from 150 to 90°C). From an analysis of the tests it was concluded that only at a temperature of 90°C and an initial moisture content of 40% do the dried briquettes meet the requirements imposed on them for blast-furnace smelting. The drying curve of briquettes at a drying-agent temperature of 90°C and an initial moisture content of 40% is given in Fig. 2.

As a result of the first series of experiments, the following conclusions may be drawn:

a) artificial drying of peat-ore briquettes from a moisture content of more than 40% is inadvisable, since under all drying regimes a deterioration of the mechanical properties of the compacts is observed; the cause is the large and nonuniform shrinkage of the material, which leads to the formation of cracks and even to complete destruction of the briquettes;

b) the best result, with respect to the mechanical strength of air-dry briquettes, was obtained when drying compacts with an initial moisture content of 40% at a drying-agent temperature of 90°C; the drying time was 3 hours 4 minutes, which is considerably less than the drying time…

under natural conditions, which, according to experimental data, is almost three days.

Analysis of the drying processes of peat-ore briquettes under natural and artificial conditions made it possible to suggest that the cause of briquette destruction is severe drying conditions. Therefore, an attempt was made to investigate the influence of the moisture content of the drying agent on the drying of peat-ore moldings.

Fig. 2. Drying curves of peat-ore briquettes from a mono-charge

These experiments were carried out on the same installation, to which a device for humidifying the air had been added. It was a steam evaporator of variable capacity. The evaporator capacity was varied by means of a laboratory autotransformer. Briquettes with an initial moisture content of 40% were subjected to drying. The parameters of the drying agent (temperature and humidity) were varied over time in order to approximate the drying process as closely as possible to the real conditions of a convective dryer. The state of the drying agent at various points in time was monitored using an \(I—d\) diagram. The experimental data were processed by the standard method. As a result of the experiments performed, drying curves were constructed; they are shown in Fig. 2. The characteristics of the regimes of practical interest are given in Table 1. From the second series of experiments the following conclusions can be drawn:

Table 1

Characteristics of the drying regimes of the second series of experiments

a) intact briquettes were obtained for the first time; isolated cracks were found on the surface of briquettes dried at a lower moisture content of the drying agent;

b) increasing the moisture content of the drying agent made it possible to increase its temperature, which, in turn, intensified the drying process; the drying time was reduced to one hour, while the strength of the briquettes was higher than that obtained even under the low-temperature regime in the first series of experiments;

c) as the moisture content of the drying agent increases, the strength of the briquettes increases;

d) moistening the material during the heating period has a positive effect on the mechanical properties of the briquette and reduces surface cracking;

e) the drying rate under a mild regime is higher than under a severe one; this can be explained by the fact that, under a severe regime, moisture rapidly evaporates from the surface and, owing to the low thermal conductivity of peat, the heating process of the material is prolonged. In addition, drying processes under a moist regime are carried out at higher temperatures.