Rotary hearth calciner

Allred, V.D., "Rotary Hearth Calcining of Petroleum Coke", paper presented at the 100th National Meeting of the American Institute of Metallurgical Engineers, New York, NY, March 1-4 (1971). ... 

Allred, V. Dean "Rotary Hearth Calcining of Petroleum Coke" presented at the meeting of the American Institute of Mining and Metallurgical Engineers (AIME), Paper No. A71-26. 

The yield of coke calcined in a kiln is usually slightly above 80% of the dry raw coke. Higher yields are achieved in rotary hearths because very Htde of the fines are burned or carried away by the combustion gas. 

Removal of volatile matter to about 0.5 wt% can be accomplished by calcining in a rotary kiln, rotary hearth, or vertical shaft calciner All of these processes heat green coke to temperatures in excess of 1000°C where shrinkage and subsequent densification take place. The volatile components are comprised primarily of methane, ethane, hydrogen, and hydrogen sulfide gases which can be employed as fuel for process heat. 

When petroleum coke is utilized for anode and electrode production and some specialty applications, it is necessary to calcine it to remove moisture and hydrocarbon VCM. Product qualities, along with production rate, are based on feedstock composition, kiln temperature profile, kiln residence time and cooling procedures. The two methods available for calcining coke commercially are the rotary kiln (5 ) shown in Figure 8 and the rotary hearth (6J shown in Figure 9. 

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. 

The so-called green petroleum coke produced by delayed coking still contains volatile components, which escape at high temperatures, leading to excessive porosity and cracking. In addition form stability cannot be ensured. To avoid this it is first subjected to calcination in gas fired rotary kilns or rotary hearth kilns at 1300 to 1400°C. This is also necessary with anthracite, which can be calcined at even higher temperatures. 

Green coke has a carbon content of around 92%. To produce carbon products, the green coke, which contains up to 15% volatiles, is calcined, reducing the volatile content to around 0.1%. Calcination takes place in rotary hearth or in rotary kiln calciners at temperatures from 1250 to 1450 °C the volatile components are used as fuel. 

Waelz process. In this process, after the calcine is leached, the zinc ferrite residue is filtered, washed, dried, and heated with coke in a rotary hearth furnace. The zinc is reduced and fumed off as metallic vapor, reoxidized, and collected as a pure zinc oxide in the bag-house dust and normally leached in a separate step. Part of the lead and silver is recovered, and the iron remains in the slag. 

Batch Furnaces This type of furnace is employed mainly for the heat treatment of metals and for the drying and calcination or ceramic articles. In the chemical process industry, batch furnaces may be used for the same purposes as batch-tray and truck dryers when the drying or process temperature exceeds 600 K (620°F). They are employed also for small-batch calcinations, thermal decompositions, and other chemical reactions which, on a larger scale, are performed in rotary Idlns, hearth furnaces, and shaft furnaces. 

The production of chemical-grade magnesia or light-burned MgO requires careful control of the calcination temperature to achieve the required specific surface area of the finished product. A furnace well suited to this requirement is the multiple-hearths Herreshoff-type. For the production of dead-burned magnesia typically shaft or rotary kilns are employed. See Chapter 5 for furnaces used in MgO production. 

A variety of furnaces are used in the magnesia industry to calcine either magnesite or magnesium hydroxide filter cake. The most commonly used are the multiple-hearth furnace (MHF) and rotary and shaft kilns. MHFs are primarily used to produce reactive lightly calcined magnesia, while rotary and shaft kilns produce hard and dead-burned grades of magnesia. 

Many sugarbeet processing plants operate lime kilns on-site to produce both the quicklime and the carbon dioxide required for the purification process. Shaft kilns are generally used, burning lump limestone. A few plants calcine the dried carbonation sludge, using rotary and circular multiple hearth kilns [30.6]. 

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