DEVICE FOR DETERMINING THE THERMAL STABILITY OF GREEN PELLETS

A. S. Bogma, V. M. Vityugin

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

The effect of treating granulated materials in various technological processes depends substantially on the moisture content and strength of the granules. As a rule, green pellets have insufficient mechanical strength and elevated moisture content and therefore require preliminary drying.

The change in the moisture content and strength of granulated materials during drying is determined by the drying method. The drying rate and the quality of dried pellets, for example in iron-ore production, largely predetermine the choice of the technological scheme and the regime of the entire pelletizing process. The quality of dried pellets is of especially great importance when selecting the firing method required to impart the necessary strength to them. The low strength of dry pellets practically rules out firing them in shaft and rotary furnaces.

Increasing the drying rate may lead to cracking or complete destruction of pellets as a result of intense steam formation. The temperature at which this phenomenon occurs is generally called the “shock” temperature. It is especially important to take this factor into account when firing pellets on a belt machine, when green pellets are exposed to gases at high temperature.

The results of heat treatment of green granules also depend substantially on the height of their bed. Moisture removed from the hotter layers condenses in the colder layers, which sharply reduces the thermal stability of the granules and creates drying conditions that are unequal over the height of the bed [1]. In this case, obviously, the higher the moisture content of the granules in the lower layers, the more dangerous an increased rate of their drying will be. From the foregoing it is clear that choosing the correct heat-treatment regime for granulated materials is of great practical importance. However, the actual absence of a method for determining the “shock” temperature with allowance for the real conditions of heat treatment in production processes leads to contradictory results [2, 3]. Therefore, the need has arisen to create such an installation that would make it possible to model fully the principal parameters of the heat-treatment process for granulated materials as applied to real conditions.

The principal parameters for modeling the heat-treatment process are temperature, relative humidity, drying rate,

of the binding agent and the drying time. Taking all these factors into account, we have proposed a laboratory experimental setup for determining the thermal stability of raw pelletized materials; the description of this setup is the purpose of the present article.

The working space of the setup (Fig. 1) is represented by two chambers 1, 2, in which different temperature conditions can be created. The heat-transfer medium is air, which can be heated both by heater 3 and, additionally, by a tubular electric furnace 4 of the T-40/600 type. Inside the refractory tube of heater 3 there are two nichrome coils 5, each rated at 1.0 kW, one of which is connected—

Fig. 1. Schematic of the laboratory experimental setup. 1, 2—working chambers; 3—heater; 4—electric furnace of the T-40/600 type; 5—nichrome coils; 6, 10—autotransformer (LATR-9A); 7, 19—gate valve; 8—fan; 9—steam generator; 11—regulating millivoltmeter of the MR1-02 type; 12—XA thermocouple; 13—starter of the P322M type; 14—radiant-energy reflector; 15—calcium-chloride tube; 16—vacuum pump; 17—drum-type gas meter of the GSB-400 type; 18—thermometers; 20—panel with electrical instruments.

connected directly to the electrical network, and the second through laboratory autotransformer 6 (LATR-9A). With gate valve 7 closed, the air forced by fan 8 passes only through the first working chamber and can be heated to 300–350°C, while its humidity is brought, with the aid of steam generator 9, to complete saturation (at a temperature not above 75–80°C). The coil of the steam generator is supplied through autotransformer 10.

Constancy of the specified temperature in furnace 2 is ensured by regulating millivoltmeter 11 of a magnetoelectric system with a photoelectric two-position regulating device (type MR-02), together with thermocouple 12 and starter 13 of the P 322 M type. The effect of radiant energy on the thermocouple is prevented by porcelain reflector 14. The working chambers are easily detached from the setup. Individual pellets are supported inside the chamber on an elastic thin wire with an annular base.

The moisture content of the air in chamber 1 is determined by the gravimetric method using calcium-chloride tube 15, filled with phosphorus anhydride; air is drawn off by vacuum pump 16, and the flow rate by drum-type gas meter 17 of the GSB-400 type.

The described installation makes it possible to carry out heat treatment of granulated materials under various temperature regimes in a stream of heat-transfer medium, while modeling the conditions, in terms of its principal parameters, as applied to real processes.

Conclusions

1. A laboratory experimental installation has been proposed for determining the thermal stability of pellets by the “shock” temperature.

2. It is necessary to develop a universal method for assessing the thermal stability of pellets by the “shock” temperature.

References

1. A. M. Parfenov. Agglomeration of iron ores, Moscow, Metallurgizdat, 136–137, 1954.

2. V. S. Abramov. Preparation of ores for smelting. Collection: Ferrous Metallurgy of the Capitalist Countries, Part II, Moscow, Metallurgizdat, 88, 1957.

3. A. I. Machkovskii, A. E. Seleznev. Nodulizing of iron-ore concentrates, Moscow, Metallurgizdat, 50, 57, 59, 1961.