Rotating hearths

There is no limit to the possible configurations of electric furnaces. They exist as single-phase or polyphase, a-c or d-c, and one to six or more electrodes. The furnace cross sections can be round, heart-shaped, oval, rectangular, or variations of each. Some furnaces, especially those for silicon production, have a rotating hearth and shell. 

The rotary-hearth furnace consists of a heating chamber lined with refractory brick within which is an annular-shaped refractory-lined rotating hearth. Around the periphery of the rotating hearth, sand or circulating hquid seals are employed to prevent air infiltration. It can be made semicontinuous in operation. The hearth speed can be... 

In the proprietary rotary hearth method of calcining coke, green coke is fed to the perimeter of the rotating circular table and gently moved toward the center of the hearth in a circular path by stationary rabbles. A combustion zone above the coke bed, formed by liberated volatiles, supplies the heat necessary for calcination. A rotating hearth furnace can also be equipped with energy efficient features, such as steam generation and air preheat. 

Incineration rotating hearth with single, fixed plough. Combustion air enters tangentially and exits the top. 

Hearth. The hearth of a channel induction furnace must be designed to satisfy restraints that are imposed by the operating inductor, ie, the inductor channels must be full of metal when power is required, and it is also necessary to provide a sufficient level of metal above the channels to overcome the inward electromagnetic pressure on the metal in the channel when power is appHed. Once these requirements are satisfied, the hearth can then be tailored to the specific appHcation (13). Sizes range from stationary furnaces hoi ding a few hundred kilograms of aluminum to rotating dmm furnaces with a useful capacity of 1500 t of Hquid iron. 

Mu/tihearth Furnace. Multihearth furnaces are most often used for incineration of municipal and industrial sludges, and for generation and reactivation of char. The main components of the multihearth are a refractory-lined shell, a central rotating shaft, a series of soHd flat hearths, a series of rabble arms having teeth for each hearth, an afterburner (possibly above the top hearth), an exhaust blower, fuel burners, an ash removal system, and a feed system. 

The feed is normally introduced to the top hearth where the rabble arms and teeth attached to the central shaft rotate and spiral soflds across the hearth to the center, where an opening is provided and the soflds drop to the next hearth. The teeth of the rabble arms on the hearth spiral the soflds toward the outside to ports that let the soflds drop down to the next hearth. Soflds continue downward, traversing each hearth until they reach the bottom and the ash is discharged. The primary advantage of this system is the long residence time in the furnace controlled by the speed of the central shaft and pitch of the teeth. 

In the EASTMET process iron oxide fines (minus 0.1 mm), pulverized coal, and binder are mixed together and pehetized. The green pehets are heated in a dryer to remove moisture and fed to a rotary hearth furnace, where the pehets are placed on a flat rotating surface (hearth) in an even layer one to two pehets deep. As the hearth rotates the pehets are heated to 1250—1350°C, and the iron oxide is reduced to metallic iron in 6 to 10 minutes. 

Roasting . Roasting has been largely abandoned ia modem copper smelters, ia which this function is combiaed with the smelting furnace. In older systems, the multiple-hearth roaster is a brick-lined tower having horizontal brick hearths. The concentrate is iatroduced at the top hearth, where rotating arms with rabble blades turn it over and move it to holes ia the hearth. The concentrate is transferred successively to lower hearths and finally... 

The bottom of the clinker hearth has fixed ribs, which tend to hold the crushed clinker, which during the rotation of the hearth is carried round until it is brought against the fixed vane 0, this lifts it out of the water. 

Oxidation of sulfide ores (MeS + 1.502 =>MeO + S02, where Me is Fe, Mo, Pb, Cu, or Ni). Iron sulfide (pyrite) is burned with air for recovery of sulfur and to make the iron oxide from which the metal is more easily recovered. Sulfides of other metals also are roasted. A multiple-hearth furnace, as shown in Fig. 19-24, is used. In some designs, the plates rotate in others, the scraper arms rotate or oscillate and discharge the material to lower plates. Material charged at the top drops to successively lower plates while reactant and combustion... 

Sodium sulfate. A single-hearth furnace like that shown in Fig. 19-24g is used. Sodium chloride and sulfuric acid are charged continuously to the center of the pan, and the rotating scrapers gradually work the reacting mass to the periphery, where the sodium sulfate is discharged at 540°C (1000°). Pans are 3.3 to 5.5 m (11 to 18 ft) in diameter and can handle 5500 to 9000 kg/d (12,000 to 20,000 lbm/d) of salt. Rotary kilns also are used for this purpose. Such a unit 1.5 m (4.9 ft) in diameter by 6.7 m (22 ft) has a capacity of... 

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