Theories of Re-Oxidation of DRI

Re-Oxidation of DRI is related to the theory of corrosion caused by atmospheric oxidation in presence of oxidizing gas and moisture. The reasons for the high tendency of DRI to oxidize in the presence of air, and atmospheric moisture, rain are as follows..

HIGH POROSITY
Due to reduction of iron oxide of the Iron ore in the solid state, the porosity of the mass increases 50-70%. Thus, the surface to volume ratio of the iron ore becomes very high, as a result of which re-oxidation of DRI takes place at a faster rate.

ROUGH SURFACE
The ruptured surface of the DRI permits dust to accumulate and become solidly lodged. This cause rapid condensation of moisture, and thus accelerates corrosion

POOR THERMAL CONDUCTIVITY 
Sponge iron produced either by coal or gas based route posses poor thermal conductivity as a result, heat generated by exothermic re-oxidation reaction within the pile cannot be dissipated away and lead to temperature build-up in DRI, and thus, leads to accelerates the corrosion.

RE-OXIDATION REACTION

In Presence of Air

Hot sponge iron will react with oxygen present in air to form ferric oxide (Fe₂O₃) and Ferrosoferric Oxide (Fe₃O₄).

4Fe + 3O₂ = 2Fe₂O₃                                   -1760 kcal/kg Fe

The reaction is quite sluggish in dry air, at ambient temperature.

3Fe + 2O₂ = Fe₃O₄                                                 -1590 kcal/kg Fe

In presence of Moisture

The reaction continues as long as sponge iron remains hot and sufficient oxygen is available. Moisture pressure enhance the reaction rate.

2Fe + 3H2O = Fe₂O₃  + 3H2

This is a corrosion reaction.

In the presence of dissolved oxygen 

This reaction is accelerated by dissolved oxygen, which depolarized the cathodic area and produces ferrous hydroxide.

 2Fe + 3H2O + O₂ = 2Fe(OH)₂ 

----------

Proximate and Ultimate analysis of coal

Proximate and Ultimate analysis of coal

Ultimate Analysis Of Coal

The "ultimate" analysis" gives the composition of the biomass in wt% of carbon, hydrogen and oxygen (the major components) as well as sulfur and nitrogen (if any). The carbon determination includes that present in the organic coal substance and any originally present as mineral carbonate. The hydrogen determination includes that in the organic materials in coal and in all water associated with the coal. All nitrogen determined is assumed to be part of the organic materials in coal. 

Proximate Analysis Of Coal

The "proximate" analysis gives moisture content, volatile content,consisting of gases and vapors driven off during pyrolysis (when heated to 950 C), the fixed carbon and the ash,the inorganic residue remaining after combustion in the sample and the high heating value (HHV) based on the complete combustion of the sample to carbon dioxide and liquid water. Proximate analysis is the most often used analysis for characterizing coals in connection with their utilization

Some of Required Properties Of Iron Ore for Sponge Iron Operation.

Iron, along with its generic products, is currently the most widely utilised metal in the various sectors of the world’s economy. Many factors contribute to this, ranging from the good mechanical properties it possesses to the low cost associated with its production. Iron is mainly produced through two methods; the blast furnace, BF, route (pig iron), and the direct reduction, DR, route (sponge iron).

Iron ore can be used directly in its natural form as a raw material for processing iron or it can be upgraded through beneficiation before it’s charged into the BF or DR furnaces. The feedstock is evaluated for physical and metallurgical properties. Physical properties give an indication of the material behaviour during handling and descent in the furnace. Metallurgical properties on the other hand indicate the materials’ behaviour during the reduction process. In selecting iron ore for iron and steel industries, some of the properties which need to be considered include (i) tumbler, abrasion and shatter indices, (ii) porosity, (iii) chemical composition, (iv) loss on ignition, (v) reduction behavior, and (vi) thermal degradation.

The strength and resistance of the iron ore to degradation (Physical Properties) is represented by Tumbler Index, Abrasion Index, Shatter Index and Thermal Degradation Index as well as their apparent porosity.

(i) Tumbler Index 

The Tumbler Index is a relative measure of the resistance of the material to breakage or degradation by impact. 

(ii) Abrasion Index: 

It is a relative measure the degradation of iron ore by abrasion. The Tumbler Index and Abrasion Index are determined in a tumbler test apparatus as per the Bureau of Indian Standard No. IS:6495 . 

A tumble strength test measures two mechanisms of feedstock degradation, that is, the Tumble Index (TI) and the Abrasion Index (AI). It was carried out following the International Standard ISO 3271:1995(E) for determination of Tumble Strength for iron ore . Precisely, a 15 kg test block sample was tumbled in a circular drum rotating at 25 rpm for 200 revolutions. Subsequently, the ore was screened and fractions +6.3 mm and −0.5 mm were obtained. The percentage of the fractions in proportion to the feed weight is the value of the TI (+6.3 mm) and AI (−0.5 mm). The test was repeated four times and the average values for these tests represent the final TI and AI data.

(iii) Shatter Index 

It is a measure of resistance to free fall of the material . it is determined by the quantity of material which after being tested , retains a size over specified dimension. This test can be carried out as per Bureau Of Indian Standard No.IS:9963.

For estimation of a Shatter Index,  a dried lump iron ore sample (10 kg) of size −40 + 10 mm was dropped 4 times from a height of 2 m onto a cast iron floor (0.5 × 0.5 × 0.03 m). Thereafter, the iron ore was screened and the shatter index expressed as the wt% passing through a 5 mm sized screen (i.e., −5 mm fraction).

 
(iv) Thermal Degradation Index: 

It is a measure of the tendency of the iron ore bearing materials to undergo size degradation as a result of either thermal shock or reduction or both .The term decrepitation or low temperature breakdown (LTB) are also used in place of degradation in technical literature. 

(v) The apparent porosity

It  was determined using the GeoPyc 1360 pycnometer. A quantity of helium was placed in the sample chamber and its volume was measured. Thereafter, a 2.0 g iron ore piece was placed in the chamber together with the helium gas and the equipment registered the new volume values. The difference in the new and original helium volume gave the sample’s envelope and skeletal volumes. The difference in the envelope and skeletal volumes indicates the percentage of porosity of the sample.

Process Description and flow diagram of Coal Based DRI Plant

Process Description and flow diagram of Coal Based DRI Plant

Description of Sponge Iron Manufacturing Process 

Most of the plants in India use DRI process—a solid state direct reduction process by which iron ore is reduced to sponge without phase change. Raw material mix-iron ore, dolomite and coal are fed to one end of rotary Kiln as well as some fine coal also fed from discharge end and product sponge iron along with char is taken out from the other end.

 Apart from this, primary and secondary airs are supplied to the Kiln to initiate the combustion and reaction processes. The reaction takes place at high temp (1050˚C to 1065˚C). Coal plays a dual role in the Kiln. Part of coal is used as fuel to supply the desired heat so as to take the raw materials to the desired temp. But main role of coal is to supply carbon in the reduction process. Dolomite is used as sulphur scavenger which finally comes out with the char. Char contains ash of coal and other impurities of iron ore. 

The reactions inside the kiln are given below

 C + O2 = CO2 

CO2 + C = 2CO 

3Fe2O3 + CO = 2Fe3O4 + CO2 

Fe3O4 + CO = 3FeO + CO2 

FeO + CO = Fe (product) + CO2 

Plants in India use wide variety of raw material and coal which has direct bearing on the process. Again some plants don’t use iron ore directly. They make iron pellets before feeding it to rotary kiln. 

Few highlighting points of the selected plant are given below to understand the process better:

 • Other than oxides of Fe, iron ore also contains impurities like gangue material, sulphur, phosphorous and moisture. The quality of ore is determined by the iron percentage as it has direct bearing on the yield. The best quality feed has the following characteristics

• Fe (total): 65% or more; 
• LOI 1% 
• SiO2 + Al2O3 (Gangue material): 5% 
• Sulphur: 0.01% 
• Phosphorous: 0.05% 
• Moisture: 1% 
• Size: 3 - 18 mm (no crushing required, can be fed to kiln directly)

• As mentioned earlier Dolomite is used here as sulphur scavenger—it doesn’t have any role in the main stoichiometric reaction. Dolomite typically contains MgO (20%), CaO (28%) and acid soluble (7%). The size of dolomite is 2 - 8 mm hence no crushing is required. 

• The reducing agent is here non-coking coal. Coal size is 0-22 mm  hence it requires crushing before feeding into kiln. the good quality should be as bellow to get better result. 

Coal Properties	Indian Coal	South African
Coal Fixed Carbon %	43	55
Volatile Matter %	31	30
Moisture	8	12
Ash	26	15
GCV (Kcal/Kg)	5360	6540

Because of the huge length of the kiln and to maintain the temperature profile, 45-55% coal is injected from the discharge end. 

• Sponge iron produced by the process will be the high grade which contains 80% Fe, 12% FeO and 8% gangue material. After separation of sponge iron in the magnetic screen/pulley, the remains are collected as char. Char has good amount of fixed carbon (25% - 30%) and moderate GCV (2500 - 2600 Kcal/Kg). Char is sold outside as low grade fuel which is subsequently used in brick kiln or large boiler. Out of the total kiln discharge, 15% to 20% is the char generation.

What is DRI (Direct Reduced Iron)

Direct-reduced iron (DRI), also called Sponge iron, is produced from direct reduction Iron (in the form of lumps, pellets or fines) by a reducing gas produced from natural gas or coal. ‘Reduced iron’ derives its name from the chemical change that iron ore undergoes when it is heated in a furnace at high temperatures in the presence of hydrocarbon-rich gasses. Direct reduction refers to processes which reduce iron oxides to metallic iron below the melting point of iron. The product of such solid state processes are called direct reduced iron. The reducing gas is a mixture of gasses, primarily hydrogen (H₂) and carbon monoxide (CO). The process temperature is typically 800 to 1050 °C.