Direct Reduced Iron and its Production Processes


Direct Reduced Iron and its Production Processes

Direct reduced iron (DRI) is technically defined as iron ore which has been reduced to metal without melting it. Hot briquetted iron (HBI) is a densified form of DRI to facilitate its handling and transport.
History
The first patent was in 1792 in United Kingdom presumably utilizing a rotary kiln but the development of the modern direct reduction (DR) process began in the middle of 19th century. Since 1920 more than 100 DR have been invented and operated. Most of them have not survived. The modern era of DR production began on December 5, 1957 when the HYL process plant first started production at Hylsa. The  first plant using Midrex process came into operation in May 17, 1969 at Oregon Steel mills in Portland, Oregon.

DRI Production process
A DRI production process is one in which the solid metallic iron is obtained directly from solid iron ore without subjecting the ore or the metal to fusion. The process principle is shown in Fig. 1.

dri hbi prodution principle

Fig 1 DRI process principle

Major DRI production processes are either gas based or coal based. Feed material in a DRI process is either iron ore sized to 10 to 30mm or iron ore pellets produced in an iron ore pellet plant.
In the gas based plant the reactor, the reduction reaction takes place is a shaft furnace. The shaft furnace works on counter current principle where the iron ore feed material moves downward in the furnace by gravity and gets reduced by the up flowing reducing gases. The pressure and temperature in shaft furnace in HYL process is 5-6 bars and 800-850 deg C. The same in Midrex process is 1-1.5 bar and 800-850 deg. C.
In a coal based plant the reactor for the reduction reaction is a inclined horizontal rotary kiln. In this kiln both coal and the iron ore feed material is charged from the same end of the kiln. During the movement of feed material forward the oxidation reaction of carbon in coal and reduction reaction of CO gas is carefully balanced. A temperature profile ranging from 800-1050 deg C is maintained along the length of the kiln at different zones and as the material flows down due to gravity the ore is reduced.
In a gas based process gaseous fuels are used. These fuels should be able to reform or crack to produce a mixture of H2 and CO gas. High methane containing natural gas is the most commonly used gas. Natural gas is reformed to enrich with H2 and CO mixture and this enriched and reformed gas mixture is preheated. In a coal based process non coking thermal coal is used. The carbon of the coal acts as a reducing agent in the process.
The reduction reactions in a gas based DRI process is as follows
With H2 gas
3Fe2O3+H2=2Fe3O4+H2O
Fe3O4+H2=3FeO+H2O
FeO+H2=Fe+H2O
With CO gas
3Fe2O3+CO=2Fe3O4+CO2
Fe3O4+CO=3FeO+CO2
FeO+CO=Fe+CO2
The basic reduction reactions in a coal based DRI process is as follows
C+ O2 = CO2
CO2 +C = 2 CO
3Fe2O3 + CO = 2Fe3O4 + CO2
Fe3O4 + CO = 3FeO + CO2
FeO + CO = Fe + CO2

Gas based DRI is not subjected for any magnetic separation since no contamination with non magnetic is possible. The gas based DRI is either cooled indirectly or used in hot condition. It is also briquetted in hot condition to produce HBI.
In the coal based process the hot reduced DRI along with the semi burnt coal is cooled in water cooled cylindrical rotary cooler to a temperature of 100 to 200 deg C. The cooler discharge consisting of DRI, char and other contaminants is passed through magnetic separators for separating DRI from other impurities.
The separated DRI is screened into two fractions +3 mm and -3 mm. -3 mm fractions is sometimes briquetted by using hydrated lime and molasses as binders.
Process flow in HYL DR process is in Fig.2

hyl process

Fig.2 HYL process flow

Process flow in Midrex DR process is in Fig.3

midrex process flow

Fig 3 Midrex DR process flow

Process flow in Coal based DR process is in Fig 4.

rotary kiln plant

Fig 4 DR based process flow

Properties of DRI
The comparison of coal based and gas based DRI is given in Table 1

Table 1 Comparison of coal based and Gas based DRI

Sl.No.  Subject                         Unit         Coal based         Gas based

1             Carbon content          %              0.2-0.25              1.2-2.5
2             Product size                                  varying                uniform
3             Material state                               stable                   Prone to re-oxidation
4             Bulk density         tons/Sqm     1.6-2.0                1.5-1.9
5             Non metalliks            %                0.3-0.5               Nil
6             Metallization             %                86-92                  85-93
7             HBI production                          Not feasible       Feasible

Since DRI is produced by removing oxygen from iron ore, its structure is just like sponge with a network of connecting pores. These pores results in a large internal surface area which is about 10,000 times greater than the internal surface area of solid iron. Due to it the DRI is also known as sponge iron.
HBI is produced by compacting DRI under very high pressure at temperatures in excess of 650 deg C. This closes many of the pores and limits the contact area which is available for reaction with air. It also increases its thermal conductivity.
The composition comparison of coal based and gas based DRI is at Table 2

Table 2 Comparison of composition of coal based and Gas based DRI

Sl. No.            Subject              Unit          Coal based            Gas based

1               Metallic iron            %                80 to 84                83 to 86
2              Oxide                           %                6 to 9                      5 to 8
3              Carbon                        %                0.2 to 0.25          1.2 to 2.5
4              Gangue                       %                 3 to 4                      2 to 6
5              Fluxes                         %                 1 to 3                      0 to3
6              Sulphur                      %                 0.02 to 0.03       0.05 to 0.25
7             Phosphorus              %                 0.04 to 0.07        0.03 to 0.08
8              Residuals                  %                  0.3 to 0.5               0.02 to 0.05

Advantage of DRI

• It allows steel making to dilute metallic residuals in scrap
• Since it is manufactured it has a uniform composition
• It has a uniform size
• DRI has low sulphur and phosphorus content as compared to scrap
• It allows improvement in BF productivity