Study of Organic and Inorganic Binders on Strength of Iron Oxide Pellets
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INTRODUCTION
RUN-OF-MINE iron ore contains approximately 25 pct iron. This is too low to be directly used in blast furnace and direct reduced iron (DRI) operations. Therefore, it must be upgraded to a fine particle concentrate. Fine iron ore concentrates are converted into suitable pellet feed through pelletization with the aid of a binder. The pellets are typically balled in a rotating drum or disk and are fired in a furnace to increase their compression strength. The process of firing is called ‘‘induration or sintering,’’ and these fired pellets are called ‘‘indurated or sintered pellets.’’ It is important that finished (indurated) pellets achieve compressive strength of the pellets 1780 N (400 pound force) or above to be able to survive during shipping, handling and ironmaking operation.[1] During pelletization, as a part of quality control, pellets are tested for their mechanical strength at different stages in the process. Common measurements include drop test of green pellets (wet pellets), compressive strength of dry pellets, and compressive strength of indurated pellets. Generally, pellets are 9 to 12 mm in diameter and must meet a minimum of 400 lbf (1780N) of compressive strength.[1,2] A binder is required to combine the iron oxide particles and hold them together during the green pellet production. Bentonite is a standard and most common binder used for iron ore pelletization. It is a low-cost binder and is available in huge amounts. Usually 0.5 to 1.5 pct by weight dosage of bentonite is used as a binder and the URVASHI SRIVASTAVA, formerly Graduate Student with the Michigan Technological University, Houghton, MI 49931, is now Metallurgist - Mineral Processing with the Research & Development Technology Center, Midrex Technologies, Pineville, NC 28134. S. KOMAR KAWATRA, Professor, and TIMOTHY C. EISELE, Assistant Professor, are with the Department of Chemical Engineering, Michigan Technological University, Houghton, MI 49931. Contact e-mail: [email protected] Manuscript submitted September 4, 2012. Article published online April 11, 2013. 1000—VOLUME 44B, AUGUST 2013
chemical formula for bentonite is (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2.n(H2O).[3] Due to its excellent hydrocolloidal property, it readily absorbs water to expand and spreads between the iron oxide particles to glue them together. During the firing process, the calcium and sodium cations acts as fluxing agent and fuses iron oxide particles together.[4,5] Bentonite, as a binder, provides strength to iron ore pellets at every step of pelletization, as it can withstand high temperature and does not burn away during the induration process.[5] The disadvantage with bentonite is that it contains approximately 45 to 65 pct silica (SiO2) impurity removed during upgrading process. An addition of 1 pct silica can increase the energy and flux cost to $2.50 per ton of hot metal.[4] Also, approximately 30 kWh additional energy is consumed with the addition of 1 pct of SiO2 in case of direct reduced pellets.[6] Due to these disadvantages with bentonite, many differe
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