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Midrex Process for Direct Reduction of Iron Ore

• satyendra
• April 9, 2017
• 1 Comment
• C reduction, carburizing, CDRI, DRI, H2 reduction, HBI, HDRI, Hot link, Midrex process, Reformer, Shaft furnace,

Midrex Process for Direct Reduction of Iron Ore

Midrex is an ironmaking process, developed for the production of direct reduced iron (DRI). It is a gas-based shaft furnace process is a solid state reduction process which reduces iron ore pellets or lump ore into DRI without their melting using reducing gas generally formed from natural gas. The principle of the reduction process using reducing gas is shown in Fig 1.

Fig 1 Principle of reduction process using reducing gas

The history of the Midrex process goes back to 1966 when Donald Beggs of the Surface Combustion Corporation conceives the idea for the Midrex direct reduction process.  The original process was developed by the Midland-Ross Co., which later became Midrex Technologies, Inc. It is now a wholly owned subsidiary of Kobe Steel. A pilot plant was built in Toledo, Ohio in 1967. The first commercial plant, having a production capacity of 150,000 tons per year, was built in Portland, Oregon, in 1969. The genius of the Midrex process is its simplicity. Donald Beggs’ concept of combining stoichiometric natural gas reforming with shaft furnace direct reduction of iron ore was a breakthrough innovation which has stood the test of time.

Since 1969, DRI production through Midrex process has crossed 500 million tons. Production from many of the Midrex plants exceeds their design capacity. Each year since 1987, DRI production through Midrex process is over 60 % of the total global production of DRI.

The process was immature in 1978, when Kobe Steel began the construction of a plant with a production capacity of 400,000 tons/year in the State of Qatar. Kobe Steel significantly modified the design, exploiting the company’s technologies developed through blast furnace operation, and stabilized the then new process. On the other hand, Midrex Technologies also carried out various improvements to the plants they built in various countries. These were all integrated in the early 1980s, making the process nearly complete.

The maximum production capacity of a Midrex unit in 1984 was 600,000 tons per year. Midrex shaft furnaces with 5.0 m, 5.5 m, and 6.5 m shaft diameters having annual production capacities of 800,000 tons, 1 million tons and 1.6 million tons of DRI respectively constitutes different development stages. Later with the improvements made, the capacity has increased to a level of 1.8 million tons per year in 2007, which is comparable to that of a fairly good size blast furnace. Super Megamod module having a capacity of 2.2 million tons of DRI per year has a shaft furnace with an internal diameter of 7.5 m and capability to produce more than 275 tons per hour. Today the Super Megamod module can have a capacity of even 2.7 million tons per year.

In addition, the process has been significantly improved since the commissioning of the first plant in 1969. The productivity gains of the Midrex process is due to (i) larger capacity shaft furnaces due to scale-up of the process equipment, (ii) continual refinement of the process which includes increased heat recovery, (iii) improved catalysts, (iv) hot briquetting, and (v) incorporation of new technologies such as double bustle, in-situ reforming, oxide coating, thin wall refractory, oxygen injection etc.

Process description

Iron burden for the shaft furnace can be iron ore pellet or sized lump ore or a combination of the two. However, the ore burden normally consists of blend of pellets and lump ore. The ore burden materials are transported and loaded into the designated bins which are earmarked for pellets or the lump ore. The capacities of the bins depend on the plant capacity. The bins earmarked for lump ore is equipped with a special ‘ladder’ to prevent fine generation due to the falling height. The normal blending ratio of pellet to lump ore is 80:20. Constant feeders below the bin control the blending ratio. The mix of ore burden is fed to the shaft furnace through the ore screens meant for removing oversize (+45 mm) and undersize (-6 mm) materials.

Midrex process is simple to operate and involves three major unit operations namely (i) iron ore reduction, (ii) gas preheating, and (iii) natural gas reforming. The heart of the Midrex process is its shaft furnace. It is a cylindrical, refractory-lined vessel and is a key component of the direct reduction process.  It is flexible as well as a versatile reactor. It can use natural gas, a syngas from coal, coke oven gas, or exhaust gas from Corex process as the reducing gas. It operates at slightly above atmospheric pressure and at operating temperatures which are around 950 deg C. The shaft furnace availability ensures Midrex plant to be operated for more than 8000 hours per year.

The shaft furnace is designed on the principle of counter flowing gas and solids to maximize reduction efficiency. The furnace assures uniform solids flow by effectively distributing the furnace burden and avoiding material bridging and gas channeling. Control is exercised with respect to the flow of gases between the various furnace zones. Shaft furnace also prevents the reducing gas from coming into contact with air. It prohibits gas flows from fluidizing the furnace burden. A uniform temperature profile is maintained across the cross-section of the furnace. The stoppage of furnace burden flow is avoided. The furnace design eliminates the need for water-cooled discharge cone.

These days standard Midrex shaft furnace has the features like (i) thin wall refractories at the reduction zone of the furnace to reduce the pressure drop of the burden and to increase the furnace volume, (ii) a double bustle port for injection of reducing gas since this allows the gas to better penetrate into the centre of the burden in the reduction zone and thus improves the metallization in the centre of the shaft furnace, and (ii) flow aid inserts of modified shape  for equalizing of the burden descending speed between the centre and wall side of the shaft furnace reduction zone.

The process has got capability to produce cold DRI (CDRI), hot DRI (HDRI) and/or hot briquetted iron (HBI). The process flow sheet is en givat Fig 2.

Fig 2 Midrex process flow sheet

The shaft furnace is a packed bed reactor with counter-current flow of the reactant. This type of reactor is generally a very efficient reactor for the processing of solid materials. There are three primary reasons for this efficiency namely (i) since the reactor is filled completely with ore burden, the volumetric productivity is usually very high, (ii) a moving packed bed ensures that each piece of the ore burden experiences the same temperature profile, gas composition, and residence time as every other piece, and (iii) counter-current flow provides the ore burden a very big driving force for the reaction and fast reaction times.

Operation of the shaft furnace is simple and straight-forward. Iron burden material is introduced at the top of the furnace through a proportioning hopper and descends downward by gravity flow. In the furnace it is contacted by upward flowing high temperature reducing gas, heated and converted to DRI. The reducing gas, which is primarily hydrogen (H2) and carbon mono oxide (CO), reacts with the iron oxide (Fe2O3) to reduce i.e. to remove the oxygen (O2) content and carburize the material prior to discharge. For the production of CDRI, the reduced iron is cooled and carburized by the counter-flowing cooling gas at the lower portion of the shaft furnace. The DRI can also be discharged hot either as HDRI or fed to a briquetting machine to produce HBI. Hence, the product of the furnace can be discharged as CDRI, HDRI, HBI or any combination simultaneously.

The reactions taking place in the shaft furnace are given below.

Reduction by CO

Fe2O3 + 3CO = 2Fe + 3CO2 (Overall reaction)

3Fe2O3 + CO = 2Fe3O4 + CO2 (Exothermic reaction)

Fe3O4 + CO = 3FeO + CO2 (Endothermic reaction)

FeO + CO = Fe + CO2 (Exothermic reaction)

Reduction by H2

Fe2O3 + 3H2 = 2 Fe + 3H2O (Overall reaction)

3Fe2O3 + H2 = 2Fe3O4 + H2O (Exothermic reaction)

Fe3O4 + H2 = 3FeO + H2O (Endothermic reaction)

FeO + H2 = Fe + H2O (Endothermic reaction)

Carburizing reactions

3Fe + CH4 = Fe3C + 2H2 (Endothermic reaction)

3Fe + 2CO = Fe3C + CO2 (Exothermic reaction)

3Fe + CO + H2 = Fe3C + H2O (Exothermic reaction)

The exhaust gas (top gas) emitted from the top of the shaft furnace is cleaned and cooled by a wet scrubber (top gas scrubber) and recirculated for reuse. The top gas containing CO2 and H2O is pressurized by a compressor, mixed with natural gas, preheated and fed into a reformer furnace.

Reducing gas consisting mainly of H2 and CO can be generated from a wide variety of energy sources. Natural gas mainly contains methane which can vary from 83 % to 96 %. Other constituents of natural gas are higher hydro-carbons. Natural gas can be reformed in a reformer which is a gas tight refractory lined furnace containing alloy steel tubes. The feed gas to the reformer is the fresh natural gas blended with the off gas for the shaft furnace which is being recycled. This blended mixed gas is heated and passed through catalyst filled tubes. Reformed gas is produced due the catalytic reactions taking place inside the catalyst filled tubes. The newly reformed gas containing around 90 % to 92 % of H2 + CO (on dry basis) is then fed hot directly to the shaft reduction furnace as the reducing gas.

Midrex process uses a solid catalyst for the gas phase reaction. Alumina or magnesia is the carrier material which gives the catalyst its shape and strength. The active ingredient of the catalyst, which increases the speed of the reaction, is normally nickel. Cobalt has also been used in some cases. Sulphur and halogens are the most common reforming catalyst poisons.

The reactions which are taking place in a reformer are given in Tab 1.

The reformer and catalyst design is to be such that it promotes the reforming reactions without permitting the carbon deposition reactions to take place. Generally steam reformers are used for reforming natural gas. For steam reformer, natural gas is to be desulphurized. Midrex process uses stoichiometric reformer. In this reformer stoichiometric ratio is an important parameter. The stoichiometric ratio is simply the molar or volume ratio of oxidants, CO2 and H2O to hydrocarbons which would result in the consumption of the hydrocarbon with no oxidant left over if the reaction proceeded to completion. Another version of the stoichiometric ratio is the ratio of the oxidants to carbon in the hydrocarbon in the reformer feed gas. In steam reformer it is the steam to carbon ratio. In Midrex reformer, the stoichiometric ratio is the actual ratio of the oxidants to hydrocarbon divided by the stoichiometric ratio of the oxidants to hydrocarbon. It is thus a measure of the excess oxidants in the reformer feed gas. It is thus more properly called the oxidant to carbon ratio.

The characteristics of the Midrex reformer include (i) no steam system is needed for reforming, (ii) no CO2 removal system is required for operation, (iii) hot reducing gas can be directly used in the shaft furnace without quenching and reheating, (iv) no O2 needed for reforming, (v) by using CO2 for reforming, less natural gas is required , and (vi) enables Midrex process to be a simple closed loop system minimizing energy consumption and the number of moving parts within the plant.

The Midrex reformer furnace is provided with several hundreds of reformer tubes filled with nickel catalyst. Passing through these tubes, the mixture of top gas and natural gas is reformed to produce reductant gas consisting of CO and H2. The reactions which occur in the Midrex reformer tubes are as follow.

CH4 + CO2 = 2CO + 2H2

CH4 + H2O = CO + 3H2

2CH4 + O2 = 2CO + 4H2

CO + H2O = CO2 + H2

CH4 = C(S) + 2H2

The Midrex reformer differs from steam reformer in many ways. It (i) reforms both carbon dioxide and water vapour, (ii) operates at an oxidant/carbon (Midrex stoichiometric) ratio of around 1.4, (iii) operates with sulphur present in the reformer feed gas, (iv) operates at low pressure, and (v) requires a unique catalyst design.

The thermal efficiency of the Midrex reformer is greatly enhanced by the heat recovery system. Sensible heat is recovered from the reformer flue gas to preheat the feed gas mixture and the burner combustion air. In addition, depending on the economics, the fuel gas can also be preheated.

Further for generation of reducing gas, coal of any type or ash content can be gasified. Coke oven gas can be reformed using the Midrex ‘Thermal Reactor System’. The export syngas from a Corex unit also makes a high quality reducing gas which can be used in a closely linked Midrex shaft furnace to produce DRI.

There are four discharge options (Fig 1) are available for Midrex process. These are cold DRI, HBI, hot DRI through hot link (HOTLINK), and hot DRI through hot transport conveyor or transport vessel.

Hot link process uses primarily gravity transport and use the same technology as used for gravity feed of HDRI for HBI production. The HDRI from the DRI shaft kiln is discharged into a surge bin outside and above the steel melting shop. Midrex modules with hot link are equipped to handle any upset conditions via the surge bin. This system supply HDRI to the electric arc furnace (EAF) as per the demand of the EAF. HOTLINK process is used when the distance between the DRI shaft kiln and the EAF is less than 40 meters.

Hot transport conveyor system is used where the steel melting shop is not adjacent to the DRI shaft kiln (more than 40 m but less than 100 m), an insulated mechanical conveyor is used for the transport of HDRI to the steel melting shop. In this case, DRI is discharged from the DRI shaft kiln onto a fully enclosed and insulated conveyor, designed to minimize temperature loss and prevent deoxidation. The conveyor has specially formed pans that have a similar form to buckets. The closed hood of the conveyor contains an inerting system. The conveyor provides reliable operation at reasonable costs.

Transport by hot transport vessels is used when the distance between the DRI shaft kiln and EAF is more than 100 meters or one DRI shaft kiln is to feed two steel melting shops or more. The transport of HDRI is done with the use of insulated vessels, normally having a capacity of 60 tons to 90 tons. From the DRI vertical kiln, the vessel is filled through a pipe with an air tight seal. After one vessel is filled, the pipe is closed and another vessel begins to fill, the filled vessel is transported to steel melting shop either on rails or on trucks.

Process improvements

A large number of process improvements have been carried out since the commissioning of the first plant in 1969. The early practice followed in the beginning was to utilize 100 % pellet feed and low reducing gas temperatures (around 780 deg C) due to the sintering tendencies of the pellet burden materials. In the mid-1970s, lump ore was first used and the practice was widely adopted in the 1980s. Use of lump ore has provided an additional benefit of preventing the sintering of the shaft furnace burden. This resulted into increase in the reducing gas temperatures from 780 deg C to 850 deg C. This resulted into around 13 % increase in the system productivity.

Further development in the operation practice took place in mid-1990s which consisted of the introduction of in-plant coating of iron oxide feed materials with CaO or CaO/MgO. This has resulted into further increase of the reducing gas temperatures (slightly more than 900 deg C). With this, the process productivity improved by another 11 %. All the developments upto mid-1990s has resulted into the temperature increase of the burden by around 40 deg C.

The developments till mid-1990s were towards increase of the reducing gas temperature without touching the quality of the reducing gas temperature. Further development efforts led to increase in the reducing gas temperatures at the cost of the reducing gas quality. This development of late 1990s led to increase in the reducing gas temperatures at the cost of the quality of the reducing gas. This was achieved through oxygen (O2) combustion of the gas. The higher reducing gas temperature along with the loss of the reducing gas quality provided a clear production advantage. The introduction of O2 injection resulted into combustion of a portion of the reducing gas CO+H2 by O2 and helped in the achievement of this effect successfully. O2 injection designs these days consist of the introduction of high purity O2 into the flowing hot reducing gas stream through a multiple nozzle arrangement. The O2 injection practice has resulted in increase in the reducing gas temperatures to more than 1000 deg C and further increase in the burden temperature upto 70 deg C. Although a portion of H2+CO is consumed by combustion with O2, raising the temperature of the reducing gas improves the productivity of the shaft furnace by 10 % to 20 %. Typical consumption of oxygen for this improvement is in the range of 12 N cum/ton to 15 N cum/ton. The overall productivity increase over the productivity of the first Midrex unit of 1969 due to use of lump ore, iron oxide coating and O2 injection is around 37 %.

The O2 injection, described above, has evolved into an improved technology, called OXY+, which was made possible by the introduction of a partial combustion technique. The OXY+ employs a combustor in addition to the reformer. The combustor partially burns fuel gas with O2 to produce H2+CO, which are added to the reducing gas generated by the reformer. The OXY+ system generates a reducing gas by reacting O2 and fuel gas at a stoichiometric ratio of around 0.5. The burner for OXY+ is installed in the reducing gas duct after the reformer. The heart of the system is the OXY+ reactor where fuel gas and O2 are mixed and burned in two stages. By proper staging, Oxy+ system provides (i) stable combustion, (ii) elimination of the soot generation, (iii) conversion of fuel gas to H2+CO, and (iv) protection of the material of construction from extreme temperatures. Important to the success of Oxy+ system is its control mechanism. It accurately meters O2 and fuel gas to each stage of the reactor. The close control of the combustion mixing of O2 and fuel gas helps in maintaining consistent gas quality and temperature. This serves to minimize the temperature increase of the gas entering the shaft furnace and provides for additional opportunity to increase production. The application of OXY+ results in a potential increase of 21 % in shaft furnace productivity.

The optimum productivity is achieved by maximizing the reducing temperature of the burden and the quality of the reducing gas entering the shaft furnace. These two factors are the keys to optimizing the production of the shaft furnace and its related gas generating equipment. By utilizing a combination of the two operating practices, oxygen injection and OXY+ system, as well as by maintaining the natural gas in the reducing gas stream, it is possible to independently control the shaft furnace burden temperature and the reducing gas temperature. This permits the plant operator to maximize the performance of the shaft furnace by maximizing the utilization of the reducing gases within the furnace. This practice offers the potential for a production increase of around 5 %.

Midrex double bustle design to distribute the reducing gas to the shaft furnace consists of two rings of ports around the circumference of the shaft furnace. Double bustle allows better distribution of the reducing gas when compared with a single bustle. Double bustle also allows higher flows of the reducing gas to the furnace without local fluidization of the DRI. These advantages help in increasing the shaft furnace productivity.

Injection of natural gas into the transition zone of the furnace has been introduced to achieve higher product carbon levels and also higher production rates. The transition zone is the part of the furnace which is below the reduction zone and above the cooling zone. As the DRI descend from the reduction zone into the transition zone, it is very hot. At this point, the excess heat is required to be removed before the DRI is discharge from the furnace. By injecting, natural gas in the transition zone, some of the available heat gets utilized in cracking of the hydrocarbons. This cracking of the hydrocarbon deposits carbon in the DRI product and releases H2, which flows upwards and being a reductant provides for additional reduction.

The preheating of the natural gas which goes to the transition zone is being explored. The benefit of the preheating of the natural gas is higher product carbon and higher production rates since larger amount of the transitional natural gas can be added. Presently, the flow of transition zone natural gas is limited by its cooling effect. The preheating of natural gas can ensure injection of higher amount of gas without quenching of the reduction zone.

Operating parameters and specific consumptions

Typical burden of 1.0 million tons per year Midrex unit at Comsigua, Venezuela consists of 80 % iron ore pellet and 20 % iron ore lump. Typical analysis of feed materials for this plant is given in Tab 2 and typical analysis of the pr0duct is given at Tab 3.

Typical operating parameters of the Midrex process is given in Tab 4

Typical composition and temperature of gas at the reformer input and output are at Tab 5.

Typical environmental control parameters are at Tab 6.