Direct Reduced Iron
Direct Reduced Iron
Direct reduced iron (DRI) is the alternative iron source for steelmaking. Its advantages as a virgin iron product and diluent for impurities, introduced with scrap, are well documented. DRI is also known as sponge iron. It refers to porous iron produced by the direct reduction (DR) process. It is the second most viable source of virgin iron used in steelmaking after hot metal which is produced in the blast furnaces (BF). The term DRI has a generic meaning which covers a number of products with a variety of properties and hazards.
DRI is produced directly in the solid phase by the reduction of iron ore (in the form of lumps or pellets) by either non-coking coal in a rotary kiln or by a reducing gas (produced by reforming of natural gas) in a vertical shaft kiln. The reducing gas can also be produced by the gasification of coal. The reducing gas is normally a mixture of gases. The majority gases in this mixture are hydrogen (H2) and carbon mono-oxide (CO).
International Organization for Standardization (ISO) has issued six standards for DRI. These standards are (i) ISO 4701:2019 Iron ores and direct reduced iron – Determination of size distribution by sieving, ISO 9686:2006 Direct reduced iron – Determination of carbon and/or sulphur – High frequency combustion method with infrared measurement, (iii) ISO 10835:2007 Direct reduced iron and hot briquetted iron – Sampling and sample preparation, (iv) ISO 11323:2010 Iron ore and direct reduced iron – Vocabulary, (v) ISO 15967:2007 Direct reduced iron – determination of tumble and abrasion indices of hot briquetted iron (HBI), and (vi) ISO15968:2000 Direct reduced iron – Determination of apparent density and water absorption of hot briquetted iron (HBI).
The DR process is a solid-state reaction process (i.e., solid–solid or solid–gas reaction) in which removable oxygen is removed from the iron oxide, below the melting and fusion point of the lump ore or agglomerates of fine ore. The external shape of the ore remains unchanged. Because of the removal of oxygen around 27 % to 30 % reduction in weight occurs, and a honey-combed microstructure with minute pores develops suggesting the name ‘sponge iron’ (solid porous iron i.e. lumps / pellets with many voids filled with air). DRI has a network of connecting pores. These pores results in a large internal surface area which is around 10,000 times higher than the internal surface area of solid iron.
Since there is no melting during the production of DRI, the external shape is retained. The true density ranges from around 3.5 g/cc (grams per cubic centimeter) to 4.4 g/cc. The true density of pure iron is 7.8 g/cc. Hence, there is around 45 % to 56 % reduction in true volume and this is manifested in the formation of pores throughout the interior of DRI pieces.
In the DR process, the solid metallic iron (Fe) is obtained directly from solid iron ore without subjecting the ore or the metal to fusion. Direct reduction can be defined as reduction in the solid state at O2 potentials which allow reduction of iron oxides, but not of other oxides (MnO, and SiO2 etc.), to the corresponding elements. Since reduction is in the solid state, there is very little chance of these elements dissolving (at low thermodynamic activity) in the reduced iron, so the oxides which are more stable than iron remain essentially unreduced.
The reduction process is a metallurgical process which is conducted at high temperature but substantially below the melting point of iron. Since the reduction reaction takes place in the solid state, the lumps or pellets retain their original shape, but are considerably lighter due to the removal of the oxygen. Hence, in addition to the metallic iron content, DRI contains some unreduced iron oxides and gangue. Since there is no separation of impurities in the DR process, all the gangue present in the original ore (silica, alumina, and oxides of phosphorous etc.) goes into the product DRI. In fact, concentration of these oxides increases to a level of plus 90 % because of the removal of oxygen. The colour can range from grey to almost black. DRI which is stored in the open develops a rusty surface.
DRI is a high-quality metallic iron product produced in a DRI kiln (either rotary or vertical shaft kiln) by removing oxygen from iron ore by means of heating and chemical reaction at temperatures below the melting point of iron. A relatively high grade of iron ore is needed as input, as the process retains the impurities. To produce 1 ton of DRI, around 1.5 ton of iron ore is needed as input. During the production process the iron ore, is heated to around 850 deg C. DRI is normally discharged at a temperature of around 50 deg C from the kiln.
The IMSBC (International Maritime Solid Bulk Cargoes) Code introduces new carriage requirements for DRI, which has been divided into the three sub-categories, based on the increasing hazardous nature of each group. The first category DRI (A), is the less reactive, high-density variety of DRI which is known as HBI (hot briquetted iron), or hot moulded briquettes (HMBs). DRI (A) is the least dangerous, as the process reduces the reactivity of DRI and the carriage requirements remain essentially the same as the previous BC Code wording.
The second category DRI (B), made up of lumps, pellets or cold moulded briquettes (CBI), is considered to be highly reactive and low density DRI. DRI (B) is highly reactive to moisture and the amendments allow carriage only under inert conditions with enhanced monitoring of atmosphere and changes in emergency procedures to reflect current best practices. For DRI (B), the solid BC (bulk cargo) Code already makes carriage compulsory for the entire voyage in an inert atmosphere containing less than 5 % oxygen. The IMSBC Code repeats this requirement, but specifies how this is to be performed. It states that an adequate means of maintaining the inert atmosphere in the hold(s) for the entire duration of the voyage is to be provided. An example of this is a vessel fitted with a nitrogen generating plant.
The third category DRI (C) is described as by-product fines and is intended to include all the materials generated as by-products in the manufacture and handling process of DRI (A) and / or DRI (B). DRI (C) comprises by-products, such as fines and small particles, which are not dealt with in the BC Code. DRI (C) has very similar properties to DRI (B), but finely divided DRI (C) is considered to be more reactive than DRI (B). This new schedule has been added to reflect the increase in carriage of fines and small particles, which are by-products of the manufacture of DRI (A) and DRI (B). This new schedule closely reflects the increased carriage requirements for DRI (B).
Characteristics of DRI
The main chemical characteristics of DRI are (i) metallization, (ii) carbon content, (iii) gangue content, and (iv) impurities (sulphur, and phosphorus etc.) and residual elements.
Metallization – The degree of reduction of DRI is normally expressed as the percent metallization of the product. It is the ratio of the metallic iron present in DRI divided by the total iron present in DRI. The degree of metallization depends largely on the type of reduction process being used. The degree of metallization varies from 85 % to 95 % depending on the process adopted for DRI production. Low degree of metallization leads to economic disruption such as higher energy consumption, higher slag volume, more heat time, and lower yield during steelmaking.
Iron content in the DRI is in two forms. One is in metallic form which is known as metallic iron, Fe (M), and the second form of iron which is present in residual iron oxides, Fe (O). The total iron, Fe (T), in DRI is the sum of these two iron components. Metallic iron is the aggregate quantity of iron, either free or combined with carbon (as cementite) present in DRI. Metallization of DRI is a measure of the conversion of iron oxides into metallic iron (either free or in combination with carbon as cementite) by removal of O2 due to the action of the reductant used. Degree of metallization of DRI is the extent of conversion of iron oxide into metallic iron during reduction. It is defined in percentage of the mass of metallic iron divided by the mass of total iron.
Carbon content – The control of the carbon content of DRI depends on the DR process being used. Those based on solid reductants normally give low carbon content (less than 0.5%), and this is inherent to the process itself. However, with the gaseous reduction processes, the carbon level can be adjusted, within limits, to any desired value. The carbon content of gas-based DRI varies between around 1.5 % and 4 %, corresponding to a degree of metallization of 85 % to 95 % respectively. The carbon as iron carbide (Fe3C) is more desirable than loose carbon fines or soot which is not useful to the process. Oxygen present in the DRI is in the form of FeO, which reacts vigorously with carbon in DRI within the liquid bath during steelmaking and improves heat transfer, slag metal contact, and homogeneity of the liquid bath. Thus, higher percentage of carbon is needed in DRI, and hence steelmaking operators prefer gas-based DRI (which contains 1.5 % C to 4 % C) instead of coal-based DRI (0.15 % C to 0.25 % C). Any unreduced iron oxide present in DRI, during steelmaking, enters the slag.
In general, when the percentage of carbon in DRI increases, the quantity of free carbon also increases. Free carbon poses disadvantages such as less DRI stability and lower yield / energy efficiency in the electric steelmaking furnace. During the production of DRI, parameters are to be so adjusted so that higher percentage of total carbon is present as Fe3C. Subject to the technology being used for the production of DRI and type of the DRI product being produced, DRI can have carbon more than 50 % of the total weight as Fe3C.
The carbon content of the DRI is dependent on the production process used for DRI production and the process parameters. Similarly the FeO content in the DRI is likewise affected by the production process used for DRI production and the process parameters. The shipping, storage techniques, and shelf time can also affect the value.
Gangue content – The gangue in DRI is substitutes for normal slag-producing agents and hence does not penalize the steelmaking operation. It is normally between 2 % and 4 %. However, the actual quantity depends on the proportion of silica in the gangue which is to be fluxed and on the percentage of iron units in the charge which is derived from DRI. All the gangue content of the iron ore fed to the DR kiln is finally lands to the DRI. Since DRI has become competitive with steel scrap, the gangue content of the DRI has to be reduced appreciably. The iron ore feeds for the DR processes normally contain less than 2 % silica and 1 % alumina. Moreover, certain additions of either limestone or dolomite are made to improve the behaviour of the iron ore pellets during reduction. Hence, some lime and magnesia are found in DRI.
Impurities and residual elements – The residual elements normally found in steel scrap (copper, zinc, lead, tin, arsenic, chromium, nickel, and molybdenum) are not frequently present in any noticeable quantities in iron ore deposits, and hence, these elements are found only as traces (less than 0.02 %) in DRI. The impurities which can be found in DRI are the alkalis (sodium oxide, potassium oxide), titanium oxide, sulphur, and phosphorus. The first three are normally present in quantities, which are small enough not to have any bearing on the steelmaking practice (e.g., sodium oxide + potassium oxide is less than 0.1 % and TiO2 is less than 0.1%).
The sulphur content of DRI is also relatively low, depending on the quantity of sulphur in the fuels and reductants used. Sulphur content of DRI varies from less than 0.005 % in the gas based DR processes employing sulphur-free gas to around 0.02 % in the coal based DR processes employing sulphur bearing coal and limestone together with iron ore in the charge mix.
Phosphorus is not eliminated during the DR process and hence the quantities found in DRI are directly a function of those contained in the iron ore. Phosphorus content normally found in DRI ranges between 0.01 % and 0.04 %.
Quality of DRI – The portions of iron oxide and gangue in DRI above certain minimums increase the power requirements in the electric furnace steelmaking compared to an equivalent quantity of scrap. A portion of the iron oxide reacts with carbon in the electric furnace steelmaking to produce metallic iron and carbon mono-oxide according to the reaction FeO + C = Fe + CO, delta H at 298 K = 153,888 kJ per mol. of C. This is an endothermic reaction. Also, the gangue and the associated flux need energy for melting.
However, a metallization which is too high decreases both the fuel efficiency and the productivity of the DR process. The metallization of DRI normally ranges between 85 % and 95 % depending on the process and on the reducibility of the original iron oxide. Based on the level of metallization, the carbon content of the DRI is controlled in some DR processes in the range of 1.5 % and 4 % to facilitate reduction of FeO by carbon during melting. This increases recovery of Fe and the CO generated promotes foamy slag practice in the EAF (electric arc furnace). Another impact on the operation of the electric steelmaking operations pertains to fines in the DRI which affect Fe recovery, increase the dust loading, and contaminate electrical parts.
A major advantage of DRI in electric steelmaking operations is the absence of contaminating residuals such as copper and tin. Phosphorus, manganese, and vanadium contained in the ore remain in the gangue in the gas-based processes and are normally not a factor in electric steelmaking operations.
Cold briquetted iron (CBI) is made from reduced iron fines combined with a small quantity of lime and sodium silicate (binders) which is then cold pressed in a hydraulic roll press to produce highly compacted pillow shaped material (briquette). Fines which are normally produced during the production of DRI can be screened out of the product for cold pressing, direct injection into an EAF, or sale to a secondary user such as a concrete plant. HBI / CBI are nothing but densified form of DRI with substantial increase in apparent density.
DRI is produced in several forms. These are lumps, pellets, HBI, DRI fines, and CBI. Majority of the commercially produced DRI is produced as lumps, pellets, or briquettes. Some fines (size minus 4 mm) are also generated during the production of DRI and can be mixed with the lumps, pellets, or briquettes. Normally the term DRI is used to refer to lumps and pellets, while HBI is used for the briquettes. Hot briquetting is normally done for the DRI produced in the vertical shaft kilns. Fig 1 shows different types of DRI.
Fig 1 Types of DRI
As compared with DRI, the most important specific characteristics of HBI are (i) no change in the chemical analysis by the briquetting process, (ii) only minimal loss of metallization even after long time storage, (iii) open air storage both at the producer and user sites does not cause problems (no need for inertized silos as with DRI), (iv) minimum risk of overheating during storage and transport (e.g. IMO allows ‘open storage prior to loading’), (v) special inertization of ship holds is not needed (no specially equipped ships), (vi) little production of fines during handling, (vii) handling similar to scrap (open air storage, transport, and handling with front-end loaders, magnets, belts, redler chains, etc.) with an additional advantage of the relatively small and uniform product size (charging of furnaces), (viii) high apparent and bulk density, (ix) low moisture saturation (maximum around 3 % as compared with 12 % to 15 % for DRI, and (x) efficient preheating for the EAF possible.
Majority of the DRI is produced as cold DRI (CDRI). After reduction, the DRI is cooled to around 50 deg C. This material is typically used in a nearby EAF or electric induction furnace (EIF). It is to be kept dry to prevent reoxidation and loss of metallization. CDRI is ideal for continuous charging to the EAF or EIF. Tab1 gives the properties of different forms of DRI.
|Tab 1 Properties of different forms of DRI|
|Elements||Unit||Coal based||Gas based|
|DRI lumps||DRI fines||HBI||CDRI||HDRI|
|Fe Metallic||%||80 – 82||80 – 82||83 – 90||83 – 90||83 – 90|
|Fe Total||%||90 – 92||90 – 92||89 -94||89 – 94||89 – 94|
|Metallization||%||88 – 90||88 – 90||92 – 96||92-96||92-96|
|P||%||0.06 max||0.06 max||0.005 – 0.09||0.005 – 0.09||0.005 – 0.09|
|S||%||0.03 max||0.04 max||0.001-0.03||0.001-0.03||0.001-0.03|
|C||%||0.1 – 0.25||0.25 – 0.3||1.5 – 4.0||1.5 – 4.0||1.5 – 4.0|
|Char||%||0.8 max||1 max|
|Al2O3 + SiO2||%||6||6||2.8 – 6.0||2.8 – 6.0||2.8 – 6.0|
|CaO + MgO||%||1||1|
|Bulk density||t/cum||1.8||1.75||2.4 – 2.8||1.6 -1.9||1.6 – 1.9|
|Apparent density||g/cc||3.4 – 3.6||3.4 – 3.6||5.0 – 5.5||3.4 – 3.6||3.4 -3.6|
|Product temperature||deg C||50||50||100||50||600 – 700|
For the production of HBI, hot DRI (HDRI) in the form of pellets or lumps, and fines at a temperature of around 650 deg C to 700 deg C is passed through hot briquetting machine where the product is pressed between the rolls at extremely high pressure to produce highly compacted pillow shaped material called hot briquetted iron (HBI). This closes several of the pores and limits the contact area which is available for reaction with air. It also increases the thermal conductivity. The typical size of HBI is 30 mm x 50 mm x 110 mm and a density of 5 g/cc minimum. No binder is used to make HBI. Fig 2 gives a comparison of the structures of DRI pallet and the structure of HBI.
Fig 2 Structures of DRI pellets and HBI
DRI / HBI can be used in the EAF, EIF, BF, and BOF (basic oxygen furnace). DRI / HBI can be used in the BF for the production of hot metal. The introduction of such a raw material is consistent with the production process of the BF, since even during a traditional process, performed by the introduction of coke and iron ores, the metal direct reduction takes place in the lowest part of the divergent section.
Just before the melting period, the BF is interested by the presence of DRI and hence, the charging of DRI / HBI aims at avoiding the reduction processes needed to reduce a fraction of the charged raw material. This implies (i) a decrease of the coke consumption, since a fraction of the charged raw material has been already reduced in the form of DRI / HBI, (ii) a decrease of CO2 (carbon di-oxide) released in the atmosphere, because of a lower intensive use of coke, (iii) a decrease of the sulphur concentration in the hot metal, because of a lower quantity of the introduced coke, and (iv) a possible increase of the hot metal (HM) productivity which can be expected at a maximum level of 10 %.
The charging DRI into a BF is expected to bring about positive effects such as increased production, a decreased reductant ratio, and a decrease in agglomerates and reduction of CO2. A laboratory study has shown that the upper limit of DRI which can be charged into a BF can be as high as 100 %. In actual operation at one of the steel plant in USA, the quantity of DRI recorded as charged into their BF reached a monthly average of 227 kg/tHM, indicating that up to around 20 % can be charged without causing any problem. Tab 2 gives a comparison of hot metal and DRI.
|Tab 2 Comparison of hot metal and DRI|
|Melting of iron||Yes||No|
|Primary fuel||Coke||Natural gas / coal|
|Total iron||%||Around 96||Around 92|
|Iron oxide||%||1 max.||Around 7 to 8|
|Gangue and impurities||%||Around 1||Around 5|
|ensity / porosity||High /low||Low / high|
|Carbon||%||4 % min.||1.5 – 4 for gas based DRI, 0.1 – 0.25 for coal based DRI|
|Material handling||Easy||Challenging (pyrophoric and fines)|
|Melting temperature||deg C||Around 1,250||Around 1,300|
|Power to melt in EAF (with energy from carbon)||kWh||Around 150||Around 400|
A study on the use of DRI along the main steelmaking processes has been performed The important points which emerged from the study are (i) the use of DRI / HBI in the BF allows a decrease of the equivalent charged coke, of the CO2 emission, and of sulphur concentration in the tapped hot metal, (ii) in the BOF and in the EAF a properly carburized metal bath allows the recovery of the iron oxides in the DRI / HBI, (iii) the interaction between carburizing materials in the metal bath, such as hot metal, and the DRI / HBI has to be achieved in order to maximize the metallurgical yield of the conversion and of the melting process, and (iv) in EAF, DRI / HBI has not to be charged by first bucket since such a raw material has to be added into a molten bath enriched by carbon.
Depending on the raw material and the reduction process used, apparent product density of DRI is around 2 g/cc associated with a very high specific surface area. The latter is typically around 1 square meter per gram (sqm/g). Due to the large specific surface area, DRI produced in the vertical kilns reacts very easily with water (particularly sea water) and / or oxygen. Since the reaction is exothermic, heat is produced. Owing to its spongy structure, DRI is also an excellent insulator. Hence, the excess heat produced in a DRI storage pile by, for example, the reoxidation with water does not easily dissipate. This can cause overheating and melt-down of DRI in piles, silos, or most dangerously in ship holds. The reaction with water also produces hydrogen which yields explosive mixtures with air.
Since the beginning of direct reduction, methods for passivation of the DRI have been developed and tested. Hot briquetting has become the most reliable process for this task. By applying this technique, DRI is densified immediately after reduction at high temperatures and with very high pressures. Hot briquetting of DRI closes internal pores, lowers the accessible surface, increases the apparent density, and improves thermal conductivity, all of which reduce reactivity.
HBI is produced from the DRI of the vertical shaft kiln and is the preferred DRI product for the merchant metallics market since it is much denser than CDRI, which reduces the reoxidation rate. This enables HBI to be stored and transported without special precautions under the International Maritime Organization (IMO) code for shipping solid bulk cargoes. Reoxidation and overheating of HBI is very unlikely. This results in considerable improvements of storage and transport characteristics. Additional advantages, such as higher density, improved handling, uniform product shape and size, as well as reduced fines production, are results of the physical characteristics of the HBI.
DRI from fine ores, reduced in fluid bed processes, is even more reactive and, even after cooling, cannot be safely handled in the open. In this case, hot briquetting not only passivates the product but also solves the inherent handling problems associated with the fine particulate material.
Hot DRI (HDRI) produced in the vertical shaft kiln can be transported to an adjacent EAF at up to 650 deg C to take advantage of the sensible heat, which allows increase productivity during the steelmaking and reduction in the production cost. There are four alternatives which are being commercially available for the transport of HDRI. These are (i) transport in a hot transport vessel, (ii) gravity transport of HDRI, (iii) pneumatic transport of HDRI, and (iv) hot transport conveyor system. Each of these alternatives has its best application, depending on such factors as transport distance, component arrangement, and conveying capacities.
Reoxidation, storage, and transportation of DRI
DRI is produced, in the solid state, by gaseous or solid reduction of iron ore. Because of the removal of oxygen, a honey-combed structure remains which has a very large surface area per unit weight. Hence, DRI has an inherent tendency to reoxidize back to its native stage. The main reasons for such behaviour are (i) extremely high surface area to volume ratio, and (ii) poor thermal conductivity (around 2.092 kJ/mhK) because of porosity and gangue content of DRI. Freshly prepared DRI is highly prone to oxidation whenever it comes into contact with air. The heat generated in the oxidation reaction increases the susceptibility of oxidation, thereby starting a sort of chain reactions and ultimately leading to the burning of DRI. Hence, storage and handling of DRI is a major concern of DRI.
Reoxidation of DRI – The reoxidation of DRI, caused by the influence of water and oxygen, is an exothermic reaction. Hence, an oxygen pickup of 0.1 % means a rise in temperature of around 35 deg C under adiabatic conditions. This makes DRI highly reactive in oxygen containing atmosphere. It is even more prone to reoxidation in the presence of moisture. Because of the poor thermal conductivity of DRI, the heat generated during the reoxidation reaction cannot be dissipated away, ultimately leading to the development of hot spots or even auto ignition of the DRI piles.
DRI is chemically reactive and this makes it dangerous to store or ship in bulk. In case of some stockpiles which have wet DRI buried deeply, high-temperature reoxidation reaction has occurred, the heat generated by the exothermic reoxidation reaction within the pile does not have an opportunity to dissipate. As a result, increase in temperature causes a corresponding increase in reaction rates, leading to auto ignition. Hence, overseas transportation of DRI has frequently resulted in corroding the cargo, even sometimes to the extent of setting ships ablaze, because of the initial reaction between DRI and seawater yielding hydrogen and heat. Tab 3 gives heat effects during reoxidation of DRI.
|Tab 3 Heat effects during reoxidation of DRI|
|Reaction||Heat of formation at 25 deg C and 1 atm.||Quantity of heat generated per kg DRI|
|0.95 Fe + 1/2 O2 = Fe(0.95)O||-1264.43||-4970.59|
|3Fe + 2O2 = Fe3O4||-1161.71||-6648.34|
|2Ee+ 3/2O2 = F2O3||-821.32||-7334.55|
|Fe + H2O + 1/2O2 = Fe(OH)2||-558.98||-9983.02|
|Fe + H2O + 1/2O2 = Fe(OH)2||-568.19||-10146.2|
|Fe +3H2O + 3/4O2 = Fe(OH)3||-825.08||-14731.86|
|3Fe + 4H2O = Fe3O4 + 4 H2||+27.61||+164.43|
|Fe + 2H2O = Fe(OH)2 + H2||+3.35||+59.83|
Chemical reactions involved in the reoxidation of DRI are (i) Oxidation in dry air 4Fe + 3O2 = 2Fe2O3, (ii) Oxidation in the presence of moisture 2Fe + 3H2O = Fe2O3 + 3H2, and (iii) Oxidation in the presence of dissolved oxygen 2Fe + 2H2O + O2 = 2Fe(OH)2. Fig 3 shows re-oxidation tendency of the DRI produced in a gas based DRI plant.
Fig 3 Re-oxidation tendency of the DRI produced in a gas based DRI plant
Reoxidation of HBI – HBI experiences metallization loss when it is stored in the open yard. It reoxidize slowly in storage in the same way as scrap. Since an DR product reacts with water, HBI is to be stored in an area with adequate drainage to avoid standing water. However, it is not necessary to cover storage piles since the relatively inert characteristics of HBI prevent rapid reoxidation. The HBI weatherability test shown in Fig 4 was conducted over an 8-month period in tropical conditions (27 deg C and 70 % relative humidity). The metallization loss was measured at the depth of 200 mm, 500 mm, and 600 mm of the pile surface.
Fig 4 Metallization loss for HBI stored in the open yard
Metallization losses can be minimized when stockpiled in an open yard by taking certain actions such as (i) building tent-shaped piles up to 6 meters high, and (ii) building piles slightly above ground level to provide a slope for better water drainage.
Steaming – When rain falls on HBI piles, the material absorbs some water (around 3 %) because of the capillary effects on the exposed surfaces and releases water vapour. This effect is called ‘steaming’. At a temperature above 50 deg C, HBI re-oxidizes when heavily wetted and create heat, which in turn results in evaporation of the water. HBI piles steam until the water is evaporated, the reoxidation reaction stops, and the material cools to ambient temperature. Hence, water is not to be sprayed on steaming piles of HBI.
Water effects– When water is present during or after reoxidation, corrosion (i.e., rusting) occurs. This effect is stronger when seawater is in contact with HBI. Hydrogen gas can evolve when water is present during oxidation reactions. This gas is highly explosive and detrimental to breathing in enclosed spaces.
Auto ignition – HBI is classified in the IMO IMSBC Code as MHB (material hazardous only in bulk). An MHB classification is given to a substance which is neither Class 4.2 (substances which can self-heat or are liable to spontaneous combustion) nor Class 4.3 (substances which emit flammable gases after contact with water) and hence, not considered dangerous.
However, HBI piles can reach the ignition point under certain conditions namely (i) sustained re-oxidation, (ii) excessive fines content in the pile, (iii) briquetting density below 5.0 g/cc, (iv) accumulated hot product, and (v) presence of excess water. Under such conditions, the pile ignites locally if the temperature of the pile exceeds 200 deg C (ignition temperature). The auto ignition tendency is to be monitored closely since hydrogen can be generated under wet conditions, especially in the presence of seawater, and there is no flame present.
Oxidation and passivation of DRI
As DRI is produced by reducing gases, it has a porous, sponge like form. Hence, it has a large surface area relative to its mass, which enhances its reactivity. If it becomes wet, it oxidizes and liberates hydrogen gas from the water. This particularly is true if the water contains dissolved salts such as sodium chloride (e.g. sea water). As part of the reaction, it heats up considerably, which further stimulates the oxidation of the still dry lumps or pellets, resulting in chain reaction which spreads rapidly throughout the DRI pile. When sufficient oxygen is available, temperature can reach as high as 1,500 deg C. If stored in a closed environment, such as the hold of a ship, the hydrogen liberated crates a potentially explosive mixture. Further, ventilation to remove the hydrogen enhances the oxidation, and hence the overheating.
Freshly produced DRI is very reactive to humidity and oxygen. The process of slowing down of the oxidation rate is called ‘passivation of DRI’. There are three methods which are used for slowing down of the oxidation rate of DRI. These methods are described below.
The first and simplest passivation technique is to allow DRI some time to age. Exposure of DRI to air creates a very thin layer of oxide on the surface of DRI. This layer retards the rate of further oxidation. Sometimes passivation is carried out first circulating nitrogen mixed with small oxygen through the storage area for a few days followed by ageing in an enclosed ware house for two to three weeks. This passivation technique is normally used when the DRI is used at adjacent steel melting shop or is to be transported to nearby locations.
The second method of retarding the oxidation rate is to coat the DRI with a chemical (e.g. sodium silicate, water glass) which protects it from the air. This has proven effective against over-heating in case of contact with fresh water but is not effective against sea water.
The third and most effective means of slowing oxidation is by addressing directly the source of the problem i.e. by lowering the surface area of the DRI and by increasing its thermal conductivity. This is carried out by compaction of the DRI as soon as it is produced at temperatures at or above 650 deg C, into briquettes of minimum density of 5.0 g/cc, so as to close majority of the pores in the material. The product is known as HBI. HBI is a relatively safe form of DRI which is suitable for bulk shipment and outdoor storage.
The chemical and physical stability of the DRI is dependent on several factors as described below.
Mineralogy or the origin of the iron oxide – The mineralogy or the origin of the iron oxide determines the reactivity and reducibility of the ore.
Reduction temperature and residence time – The stability of DRI to re-oxidation and self-ignition can be improved by increasing the working temperature and residence time during reduction. As the reduction temperature and residence time increases, the particles start to soften and grains start to fuse. This consequently reduces the internal surface area by producing larger but fewer pores.
Reducing agent composition – The reducing gas composition determines the efficiency of the reduction and the formation of stable carbides.
Surface area (degree of porosity) exposed to weathering conditions – The oxidation tendency of the DRI is proportional to the surface area exposed to weathering conditions. The DRI is normally briquetted into HBI or CBI to reduce the surface area being exposed to weathering conditions. HBI is produced from DRI fines and contains less carbon than DRI (average 1.5 % to 1.8 % C) since it is difficult to recover or replace the carbon burnt off in the briquetting machine while maintaining the needed temperature.
CBI is produced from DRI fines which are too small to be used in the granular form of hot briquetting. It has lower iron content than DRI and HBI. During briquetting, coke, alloying materials, and deoxidizing agents are used as binders. CBI is mostly used in BFs to increase the throughput, to continue operation if the coke oven is down, to improve the hot metal quality, and to reduce the coke requirement of the BF.
Age – The re-oxidizing tendency of DRI decreases with increasing age. The freshly produced DRI has a higher tendency to re-oxidize than the aged DRI. After the formation of oxidized coating on the DRI at first exposure to the oxidizing atmosphere, further oxidation is inhibited. To prevent first oxidation, there has been a considerable number of studies have been done to find coatings which can be applied to the DRI, without affecting the steelmaking process.
Previous history of handling and exposure – The quantity of dust formed during previous handling is an important factor for accelerating the rusting and self-ignition. The metal dust does not ignite as easily as the fuel dust. However, under certain conditions (high density dust and heat supply), it ignites faster than the compacted DRI. In addition, DRI dust rusts easily due to its fine size.
DRI is a solid, highly metallized structure, which still contains slag. The slag separation from the metal cannot be accomplished in direct reduction processes since there is no melting. DRI can be utilized in electric steelmaking processes, ferrous foundry operations, and blast furnaces. Increasing demand on electric steelmaking processes, and the need for steelmaking feed materials with well-known chemical composition, has enabled the DR processes to gain importance over the past century.
Storage and transportation / shipment of DRI / HBI
DRI, as produced, is very reactive to free water and oxygen. Pellets and briquettes are always passivated and cooled before being shipped on the seas, rails, or highways. DRI pellets can be subject to a high degree of reoxidation. Self-ignition can occur if there is a natural air draft through the pile, the pellets buried inside are wet and the volume of material is large enough to insulate against heat losses.
Fires result when dry DRI pellets are placed on top of wet material. The best way to stop a DRI fire is to spread out the hot material with a bulldozer to a height of half a meter. A second method is to bury the pile under sand or slag. In the situation of a fire inside a storage silo, the pile of DRI can be flooded with extremely large quantities of water. If the water is not sufficient to flood the burning pile of DRI, hydrogen gas evolves and hence all unnecessary personnel are to be evacuated from the area surrounding the fire.
HBI has a much more dense structure and lower surface area to volume ratio as compared to pellets. HBI on the surface of a storage pile has a 70 % lower metallization loss as compared to DRI pellets over the same time period. At 600 mm below the surface, the metallization loss for either DRI or HBI becomes negligible.
Upon receipt at the steel melting shop, the DRI or HBI is to be kept dry. This can be as simple as spreading a tarpaulin on top of the material or as complex as building a storage silo connected to a continuous feeding conveyor. The storage pile is to minimize the surface area to volume ratio. While the HBI is substantially less reactive than DRI pellets it is still to be kept dry since it can contains lump or fines which oxidize. The storage area is required to have a level bottom with good drainage. Due to the possibility of oxygen depletion in confined spaces, personnel are always to check the atmosphere before entering a storage silo containing DRI or HBI. Minimal handling is to be done to prevent the production of more fines.
The International Maritime Organization (IMO) Sub-Committee on Dangerous Goods, Solid Cargoes and Containers (DSC) recommended certain amendments to the entry for DRI in the IMO Code of Safe Practice for Solid Bulk Cargoes (the BC Code). The new amendments were agreed in September 2008 and were adopted by Maritime Safety Committee (MSC) resolution in December 2008 as part of the new International Maritime Solid Bulk Cargoes (IMSBC) Code, which was published in 2009. The code was recommended on January 1, 2009, but became mandatory on January 1, 2011.
DRI (normally in the form of pellets or lumps) has a hugely porous structure, which makes the material extremely reactive and prone to reoxidation on contact with air and/or moisture. These oxidation reactions cause self-heating in the stow, which can lead to auto-oxidation where cargo temperatures in excess of 900 deg C can be generated. Moreover, contact with moisture evolves hydrogen, an extremely flammable and sensitive gas which had caused explosions in the holds of several ships following its ignition.
DRI is highly susceptible to oxidation and rusting if left unprotected and is normally quickly processed further to steel. The bulk DRI can also catch fire since it is pyrophoric. The material, if wetted, can oxidize rapidly and generate heat over a period of time. In addition, hydrogen can be generated, which can possibly form an explosive atmosphere.
Molded briquettes and specially processed materials minimize certain risks. However, every effort is to be made to prevent the ingress of water into cargo compartments. Flammable gases are to be prevented from entering adjacent enclosed spaces. Prior to shipment, DRI is to be aged for at least 72 hours or treated with some passivation technique to reduce its activity to at least the same level of the aged product. Holds are to be maintained under inert atmosphere (less than 5 % oxygen and less than 1 % hydrogen) throughout the voyage.
The hazards of DRI – DRI is classified as a Group B cargo according to the BC Code, that is, a cargo which possesses a chemical hazard which can give rise to a dangerous situation on a ship. The principal hazards of all cargoes of DRI and its derivatives are twofold. First, they react with the oxygen present in the air, thereby producing heat and depleting the atmosphere of oxygen which is necessary for life. With DRI (B), the former effect can run away in spectacular fashion, leading to auto-oxidation of the iron in which the stow becomes incandescent as the temperatures approach 1,000 deg C. The cargo is then said to be on fire and burning much in the way that charcoal behaves.
While self-heating is dangerous and alarming, it is a gradual and progressive event which can frequently be diagnosed early, affording ship masters time to consult the association, obtain advice from suitably qualified experts, and institute suitable safety measures. Self-heating to dangerously high temperatures can be successfully prevented in most practical applications by compressing the DRI into HBI at the manufacturing stage, DRI (A). Association is not aware of any fine cargoes which have undergone self-heating to the auto-oxidation stage, although there is no theoretical reason why 4 mm pieces of DRI do not self-heat to auto-oxidation.
The second hazard is again related to the reactivity of iron, this time with moisture or water. The result is the generation of hydrogen gas, which is explosive over a very wide range of concentrations and, in practical situations, displays an alarming readiness to be ignited. Explosions of hydrogen in air are extremely violent and rapid. Flame speeds can exceed 1,000 m/sec and pressures exceeding 8 atmospheres can be generated in a fraction of a second. An unfortunate ship master has no time in which to react to a hydrogen (or any other gas) explosion.
All three types of materials evolve hydrogen to a greater or lesser extent on contact with water. Seawater is more aggressive than freshwater, producing a greater degree of self-heating and higher evolution rates of hydrogen.
Hazards associated with HBI – HBI is not a pyrophoric material, but neither it is an inert material. Hence, hazardous situations, such as overheating and hydrogen gas evolution, can occur if proper precautions are not taken. The schedule for HBI or DRI (A) in the IMO IMSBC Code under the heading ‘Hazards’ lists (i) temporary self-heating of around 30 deg C can be expected after material is handled in bulk, (ii) material can slowly evolve hydrogen after contact with water (hydrogen is a flammable gas which can cause explosions when mixed with air in concentrations above 4 %), (iii) liable to cause oxygen depletion in cargo spaces, and (iv) this cargo is non-combustible or has a low fire risk.
Advantages of DRI utilization
The advantages of the DRI utilization can be summarized as below.
Unlike coke (utilized for hot metal production in BF), DR processes utilize low-cost, widely available fuel and or reducing agent feed stocks like hydrocarbon gases (hydrogen, carbon mono-oxide, carbon di-oxide, methane) and coal-bearing materials (coal, coke breeze).
DRI is a manufactured product and hence it has a uniform size and composition. It has low sulphur and phosphorus content as compared to scrap. The utilization of DRI in electric steelmaking instead of scrap allows easier and cheaper production of high-quality steel. This is because of the consistent well-known chemical composition and low content of metallic residuals (copper, nickel, chromium, molybdenum, and tin) of DRI. Since DRI practically has no tramp elements, it allows steelmaking to dilute metallic residuals in scrap. The melting of DRI needs less energy and more uniform heating cycle than scrap in the electric steelmaking furnace. DRI has higher bulk density than most of the scrap utilized in in the electric furnace steelmaking. DRI has higher carbon than scrap, it reduces the quantity of carbon injection during steel making in EAF. Further, hot charging and continuous charging is feasible in electric steelmaking furnace with DRI.
During melting, several operational advantages can be obtained by the use of DRI instead of scrap. Examples are protection of wall and roof of EAF (by foamy slag formation), and increase in the thermal and kinetic efficiency because of bath self-stirring. The DRI can be charged continuously to the electric steelmaking furnace while the scrap has to be charged with batch operations. The utilization of DRI in foundry operations as inoculants improves the properties of the cast iron.
The efficiency and throughput of the BF can be improved by the utilization of DRI. The coke and flux consumption of the BF process can also be reduced by the utilization of DRI. The DRI has a more predictable price structure when compared with steel scrap.
Comments on Post (1)
Raj Kumar Choubey
Nice and elaborated informative.