Magnetizing roasting of iron ores

Abstract

While on a visit to the Czechoslovak Republic, we had an opportunity to acquaint ourselves with the work of research institutes and industrial establishments engaged on ore beneficiat ion. The magnet iz ing-roas t ing benef ic ia t ion shop of the iron and steel works in the town of Trznets processes par t ia l ly oxidized largeinclus ion carbonate iron ores from 9 different mines. The average iron content in ore mixture is about 27~ the iron content in the form of carbonate consitutes 70-90% of the total iron, Nevertheless, during the roasting without the addit ion of solid reducing agent, these considerable variations in the composit ion of the ore do not affect the final result. According to the investigations of Czechoslovakscient is ts for a successful magnet iz ing-roas t ing of ores in which the hemat i te is the main i ron-containing constituent, it is necessary to add 2% (by weight of ore) of pulverized brown or ord inaryeoal to the ore. The ratio o f (CaO + MgO) to $iO2 in the ore is 0.5; the moisture content of the ore is 7%. The par t ic le size of the ore for roasting is below 25 ram; the <3ram fraction constitutes about 80%. The shop contains four roasting units (Fig.) each of which consists of a 47.5 x 2.4 m (181 m S working volume) roasting furnace and a 25.0 x 1.5 m (40 m s) cooler. The del ivery end of the furnace and the charging end of the cooler are located in a stationary chamber with a guiding trough on which the roasted ore from the furnace passes to the cooler. At the same t ime, the ore is sprayed with water so that the cooling process is speeded up and the steam produced establishes a positive pressure in the chamber, thus preventing the leakage of air into the chamber. The joint between the chamber furnace and the cooler is made tight with the use of spring rings. Air is not al lowed to enter the chamber because it would oxid ize the ore aM affect its magnet ic properties. The steam is drawn off into a separate tube which is connected to the del ivery end of the cooler; in this way, the ore is cooled in a steam atmosphere. The temperature ofroast ingis 800-850 ~ C; in the furnace itself, the ore cools down to 700 ~ C and in the cooler to 100" C. The temperature of waste gases is 250 ~ C. The furnace is fired by means of an end burner with a mixture of blast-furnace, coke-oven and natural gases of calor i f ic value approximate ly 3300 k c a l / m s. The use of gas of a lower calor i f ic value causes a rapid fal l in the throughput capac i ty of the furnace, and on the other hand, with a higher calor i f ic value of the gas the ore becomes overheated and the roasting process is upset. It is recommended to use gas of a higher calor i f ic value outside the furnace and allow on ly the combustion products into the furnace. At the meta l lu rg ica l combine in Rudniany, tai l ings of 5000 k c a l / k g calor i f ic value from coal washing plants are used as fuel for furnaces for the magnet iz ing roasting of the same ores, and now it is intended to change over to fuel oi l which was used previously. The t echnological results and heat consumption in the roasting process are the same for al l three types of fuel (gas, fuel oil and tailings)o The furnaces for the magnet iz ing roasting have forced draft, and the cool ing drums have natural draft. Each furnace is equipped with an exhaust fan of 24,000 mS/hr capaci ty . The cross sect ion of the tube for s team draw-off from the cooler is 2.2 t imes smal ler than the cross section of the cooler. With the above operat ing conditions the raw ore throughput of one unit constitutes 700-750 ton/24 hrs