PYROMETALLURGY OF IRON

Transcription

1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Pyrometallurgy of Iron Common sources of iron-bearing ores: PYROMETALLURGY OF IRON Oxide Ores Stoicheometry Iron Content (wt%) Magnetite Fe O 4 7.6% Hematite Fe O 69.94% Ilmenite FeTiO 6.8% Limonite FeO(OH) / HFeO 6.85% Quality of an ore depends on: location, iron content, water content, impurity content Reduction of iron oxides to the metal is accomplished in a Blast Furnace (BF). Iron Smelting (pellets, sinter, etc.) Iron and Steelmaking Processes (from coking ovens) Ore (sinter) Coke Limestone Blast Furnace Ore Beneficiation Ores must be processed to achieve desired chemical and physical characteristics Physical Form of Ore Charge Sinter Pellets Processing is called beneficiation and includes: Crushing Screening Blending Grinding Concentrating Classifying Agglomerating Nodules Briquette

2 Fluxes and Slag Most rocks are composed of silica (SiO ) and silicates (SiO - ). Blast Furnace Silica and silicates have high melting temperatures and would eventually clog the furnace. Fluxes are added to the charge to remove silica and silicates to produce slag. Slag is collected at bottom of furnace & doesn t dissolve in the molten metal. Indirect Reduction of Iron Oxidization C O CO ΔH = -0.5 kj FeO CO Fe CO 4 4 FeO4 CO Fe CO Coke + Ore Sec. red. rxn. C H O CO H Blast with hot air ΔH = +.4 kj FeO H Fe H O 4 4 FeO4 H Fe H O Fe O 4 (s) + 4CO(g) Fe(s) + 4CO (g), H = -5 kj Direct Reduction of Iron Oxidization C O CO ΔH = -0.5 kj FeO C Fe CO Coke + Ore Pri. red. rxn. C H O CO H Blast with hot air ΔH = +.4 kj FeO H Fe HO Fe O 4 (s) + 4H (g) Fe(s) + 4H O(g), H = +50 kj 9 0 Blast Furnace Operation Exhaust Gas Furnace Charge (ore, coke, limestone) Blast Furnace Design Dust Removal Heat Exchanger Preheated Charge FeO CO Fe CO FeO H Fe H O FeO4 CO Fe CO FeO4 H Fe H O FeO & Fe T max = 00 o C Temp. air blast = 00 o C Vel. air blast = 00mph Secondary reduction (~950ºC) Hot Blast Air CO H FeO+C Fe+CO Primary reduction (~650ºC) C+H O CO+H C+ --O CO

3 Temperatures and reactions in the iron blast furnace. Evaporation of Moisture Starts as soon as charge enter the BF Temperature at the top of furnace is ~00 o C. Evolution of Volatile Matter Not all volatile substances (H, N, and others) are removed from the coke in the process of coking This process is continued coming to the end in the high temperature zones 4. Decomposition of Carbonates Carbonates introduced into the BF = CaCO, MnCO, and MgCO. Decomposition of carbonates is endothermic. Avoid introducing carbonates into the burden 4. Reduction of Iron Fe O => Fe O 4 => FeO => Fe Fe O + CO = Fe O 4 + CO Fe O 4 + CO = FeO + CO FeO + CO = Fe + CO Coke carbon is the reducer: FeO + C = Fe + CO 5. Combustion of Fuel Coke carbon burns according to the reaction: C + O = CO Coke then reacts with CO as follows: C + CO = CO The gas above the tuyere is composed of CO and N. N is introduced with the blast and does not take part in the reactions Reduction of Silicon SiO + C = Si + CO (Endothermic reaction) What is the effect of increased Si content in the iron?? High silica in the slag (acid slag) is beneficial as regards to the reduction of silicon 7. Reduction of Manganese MnO => Mn O => Mn O 4 => MnO => Mn MnO + C = Mn + CO 8. Reduction of Phosphorus All phosphorus fed to the blast furnace is reduced and penetrates into molten iron High temperature is necessary to reduce phosphorus In normal operation, phosphorus does not go into Slag, which renders the removal from iron impossible 7 8

4 Carburization of Iron Slag Formation Ca, Mg, and Al oxides charged into the furnace pass entirely into slag Silica almost entirely enters slag. Only a small proportion of silicon is reduced from slag and appears in the iron Almost all of the iron is reduced and enters pig iron. Hence slag is low ferrous oxides. 9 0 Desulphurization of Iron Sulphur is oxidized in the hearth by oxygen air producing SO gas, followed by reaction with coke producing free sulphur: SO + C = S + CO Free sulphur reacts with iron giving FeS which readily dissolves in iron, resisting passage into slag Sulphur can be eliminated from iron by transferring it into insoluble compound in iron CaO (slag) + FeS (iron) = CaS (insoluble) + FeO FeO + C = Fe + CO Effect of Variables on BF Operations Effect of hot-blast temperature Can be used to increase heat input into the hearth without increasing coke rate The amount of CO remains the same and the proportion of direct and indirect reductions about the same Effect on Silicon?? Effect of Variables on BF Operations Effect of blast moisture Introduction of steam is done at the tuyeres Steam react with coke at the tuyeres to give H and CO Steam addition lowers the N content of the gas and H becomes beneficial for the ore reduction Effect of Variables on BF Operations Effect of hydrocarbon injection CH O = CO + H ~ 40% of the carbon saved is as a result of the higher hot-blast temperature ~ 60% of the carbon saved is as a result of direct replacement of coke carbon by natural gas carbon 4 4

5 Increasing temperature Blast furnace reactions Flow Chart for Carbon Reactions During the Formation of Steel Blast Furnace Iron Fe O (s) + CO(g) Fe(s) + CO (g) Fe(s) Fe(l) Fe(l) + C(s) Fe-C(liquid solution) C(s) + O (g) CO(g) Element Typical Pig Iron (wt%) Typical Steel (wt%) C Mn Si <0.5 P <0. <0.05 S <0.05 Refining Fe-C(liquid solution) + O (g) Fe(s) + CO (g) 5