Midrex the most widely accepted direct reduction (DR) process of ironmaking in the world was developed by Midland Ross Corporation of Cleveland, USA in 1967 , has the following distinctive features:
Recommended Natural Gas Composition for MIDREX Plants
Components | Vol % | Effects |
CH4 C2H6 C3H8 C4H10 +C4 (Hydrocarbon) CO2 N2 S | 75 - 100 0 - 25 0 - 4 0 - 2 0 - 0.5 20 max 20 max 20 ppm. (max Wt.) | -- -- Above 4% C3H8, water vapour content should be increased. -- Above 20% CO2, export fuel is produced. For every 10% of N2, fuel consumption increases by 2%. Above 20 ppm, carbon deposition on catalyst. |
MIDREX Process - Some Features
=> It allows the production of highly metalized DRI (exceeding 92%, see adjacent Table showing typical composition of Midrex DRI) and the carbon content of can be controlled in the range of about 1.0 - 2.0%.
=> Although originally developed for use with high grade pellets, the Midrex shaft furnace is now able to use some amount of lump ores. Optimum process conditions are often obtained by mixing 30-50% of an appropriate type of lump ore with high grade pellets. See adjacent Table showing Physical Characteristics of Pellets and Lump Ores used in the MIDREX Process.
=> Fuel utilization in Midrex process has steadily decreased from an average of 12.5 - 14 GJ/t of DRI to 9.5 - 10.5 GJ/t. This improvement in energy efficiency has been the result of higher reduction temperatures, enrichment of reduction gas with methane, utilization of in-situ reforming, and pre-heating of the process gas utilizing waste heat from the reformer.
=> Following the advent of in-situ reforming, oxygen carriers from an external source are now not required in the production of reformed gas. Therefore, the investment cost and operating costs of Midrex units have been reduced.
=> The DRI produced is relatively active towards re-oxidation, particularly when moisture is present. Hence it must be deactivated if it is to be stored or transported over a long distance.
Physical Characteristics of Oxide Feeds
(Pellets and Lump Ores) used in the MIDREX Plants
Pellets | Lump Ores | |
Screen analysis (wt %) 50 - 31.75 mm 31.75 - 6.3 mm + 15 mm 8 - 15 mm – 8 mm – 6.3 mm Bulk Density (t/m3) Compressive Strength (kg/pellet) ISO Tumbler Test (wt%) + 6.3 mm – 0.5 mm | -- -- 10% max 85% max 5% max -- 2.0 - 2.1 270 min 95% min 4% max | 5% max. 93% max. -- -- -- 7% max 2.0 - 2.6 -- -- -- |
Typical Product Composition of Midrex DRI
Content | Wt % |
Fe (total) Fe (metallic) Metallization SiO2 Al2O3 CaO MgO S P | 92 -93 84 - 88 93 - 95 2.0 - 3.5 0.5 - 1.5 0.2 - 1.6 0.3 - 1.1 0.005 - 0.015 0.02 - 0.04 |
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