Production of Direct Reduced Iron in Rotary Hearth Furnace
Direct Reduced Iron(DRI) DRI: Iron produced by reducing an iron ore (65-70% iron) by reducing gas (H2+CO) at a temperature (approx. 800-1050 °C) high enough to burn off its carbon and oxygen content. Reduction is done below the m.pt of iron cause the produced iron may react with water and air under the evolution of heat.
Direct Reduced Iron(DRI) contd. Output: In the form of pellets 90 to 97% pure iron & rest-carbon with other impurities. Also called sponge iron due to its porous nature. 
Advantages of DRI over Pig Iron Rich in iron(up to 97%). Uses pelletized iron ore or natural lump ore(no melting). Used to produce Hot Briquetted Iron(HBI) at temp. >600 C, for ease of shipping, handling, and storage. Not cooled before using in steel making electric arc furnaces thereby saving energy. 
Rotary Hearth Furnace(RHF) A donut-shaped refractory-lined vessel. Contains rotating bottom or rotating hearth. Uses ore and carbonaceous reductant in the form of a single or a multilayer bed. The temperature is controlled by means of burners positioned along the walls and on the roof of the furnace.
Rotary Hearth Furnace contd. Rotary Hearth Furnace for Iron Ore Reduction American Iron and Steel Institute steeltrp.com/PDFs/9810.pdf [3*]
Rotary Hearth Furnace contd. Heat transportation by radiation. Pellets are heated up to the reduction temperature. Heat radiation to lower layers of the bed decreases because of shielding by the upper layers. Motion of iron oxide ore bed in RHF American Iron and Steel Institute steeltrp.com/PDFs/9810.pdf [3*]
Rotary Hearth Furnace contd. The productivity of the RHF is limited by heat transport. A number of stationary adjustable blades over the rotary table. Hot solids are pushed toward the center of the table by one blade width by each blade. Concentric path followed by removal through an opening at the centre of the hearth. 
A typical RHF (Rotary Hearth Furnace) http://www.businessdictionary.com/definition/direct-reduced-iron-DRI.html [1*]
Production of DRI in RHF Feed: agglomerates made from a mixture of iron oxides (virgin ore or by-products) carbon source(coal or charcoal) Hearth and reducing gas rotate in opposite directions. Reactions: Natural gas is partially oxidized (with heat and catalyst): 2 CH4 + O2 -> 2 CO + 4 H2 These gases are then treated with iron ore, producing solid sponge iron: Fe2O3 + CO + 2 H2 -> 2 Fe + CO2 + 2 H2O
Production of DRI in RHF contd. Also Iron oxides are reduced by ’C’ producing ‘CO’ Heat transfer by convection to reduction zone is undesirable, since re-oxidation occurs by the arrival of oxidizing gas(from heating burners).
Production of DRI in RHF contd. Combustion of: CO provides 85% & fuel(burners) provides 15% of heat. The iron oxides are reduced over a period of 6–12 min. Waste gases are removed from the charging side, and their heat is used in heat exchangers. The waste gases are subsequently burned in a steam boiler, cleaned of dust, and discharged through a smokestack.
Handling of DRI Sponge iron reacts with water and air under evolution of H2 and heat. If the heat generated can not be transferred to surroundings, self heating occurs which leads to complete oxidation. So keep DRI in an inert atm. with O2 less than 5 vol% Otherwise passivate it. Example: decreasing the reactive surface by immersion in sodium silicate soln followed by drying.
Benefits Energy-efficient feedstock for steel manufacturers which rely upon scrap metal. Reduced CO2 emission per ton compared to that produced by blast furnace. Lowered capital investment and operating costs in Rotary Hearth Furnace iron ore reduction. ZnO also undergoes reduction and zinc metal vapor - converted back to ZnO by waste gases. The captured ZnO in filters is a raw material for use in nonferrous metallurgy. [2*]
Current State in Iron Production The world’s first commercial RHF, with a diameter of 16.7 m, was built by the company INMETCO in Elwood (in the U.S.) for recycling wastes containing nickel, chromium, and iron. The Direct Reduced Iron unit has a capacity of 200,000 tons a year and was built in 1997. The company Iron Dynamics Inc. (U.S.) began operation of an IDP (Iron Dynamics Process) unit in 1999. This furnace has a capacity of 500,000 tons of sponge iron a year and is the largest such furnace in the world. At present India is the largest producer of sponge iron at 17 million tones/annum.[4*]
References  S. Halder and R.J. Fruehan Reduction of Iron-Oxide-Carbon Composites: Part I. Estimation of the Rate Constants Metallurgical and Materials Transactions B, 2008, Volume 39, Number 6, Pages 784-795  I. F. Kurunov Metallurgist, The Direct Production of Iron and Alternatives to the Blast Furnace in Iron Metallurgy for the 21st Century Vol. 54, Nos. 5–6, 2010  Patent Weiss, Hans J. (Frankfurt am Main, DE) 1984 Rotary hearth furnace plant United States MetallgesellschaftAktiengesellschaft (Frankfurt am Main, DE) 4431406  I. Sohn and R. J. Fruehan, The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part II. The reduction of iron oxide/carbon composites Metallurgical and Materials Transactions B, 2006, Volume 37, Number 2, Pages 223-229  Fu-ming Zhang, “Progress of rotary hearth furnace reduction technology, ibid” pp. 1347–1352  Hattwig, Martin; Steen, Henrikus (2004), Handbook of explosion prevention and protection, Wiley-VCH, pp. 269–270, ISBN 9783527307180. Bibliography [1*]http://www.businessdictionary.com/definition/direct-reduced-iron-DRI.html [2*]www.p2pays.org/ref/08/07543.pdf [3*]American Iron and Steel Institute: steeltrp.com/PDFs/9810.pdf [4*] http://www.spongeironindia.in/Sima%20April%20Issue%202010.pdf