Process for Manufacturing of Iron Carbide
Process for Manufacturing of Iron Carbide
Iron carbide (Fe3C) is a high melting point, non-pyrophoric, strongly magnetic synthetic compound obtained in granular form. It consists of around 90 % total iron (Fe) and around 7 % total carbon (C). The primary use of the product is as a metallic charge during steelmaking for substitution of hot metal (HM), direct reduced iron (DRI), or steel scrap. The iron carbide process involves conversion of preheated fine iron ore particles to iron carbide. It reduces iron ore to iron carbide in a fluidized bed reactor, by contacting the iron ore with process gas consisting primarily of methane (CH4) and hydrogen (H2).
The process for the manufacturing iron carbide was originally designed and developed at Hazen Research Inc. in Golden, Colorado, USA by the technical vice president Dr. Frank M. Stephens. The process involves reduction of preheated fine iron ore particles (0.1 mm to 1.0 mm) in a closed circuit fluidized bed reactor by preheated process gas containing CH4, H2, CO (carbon mono oxide), CO2 (carbon di oxide) and water vapour(H2O) at 600 deg C. A 50 mm diameter batch reactor was used for the laboratory tests. This was followed by continuous tests on a 600 mm diameter reactor. Iron ore samples from several countries were tested at Hazen. The product was successfully converted to steel by MEFOS in Sweden in a basic oxygen furnace (BOF) in 1979.
After the initial laboratory tests at Hazen Research, Inc., Dr. Stephens applied for a patent and was issued on October 11, 1977 ‘US Patent No. 4,053.301’ by the Patent office of the United States. In 1985 Dr. Stephens retired and acquired the rights to the patent on the iron carbide. He formed a company by name ‘Iron Carbide Development Corporation’ (ICDC) and started marketing the process. In 1988, ICDC and Australian company of the PACT Resources, Pty. Ltd. joined hand to form ‘Iron Carbide Holdings, Limited’ (ICH). Additional development work was subsequently carried out in a pilot plant constructed in 1989 from an idle vanadium plant located in Wundowie, Western Australia. The reactor at the pilot plant was 1830 mm in diameter, operated with a bed depth of around 3700 mm. It produced around 25 tons per day.
During 1989, ICH produced 310 tons of iron carbide at the pilot plant. The iron carbide was sold to seven customers. Five companies (Nucor, North Star Steel, Mitsubishi, Qualitech Steel and Cleveland Cliffs) bought the license or the option to use this technology. Nucor Corporation converted iron carbide from the pilot plant at their steel mill in Darlington, South Carolina, USA using a 32 ton electric arc furnace (EAF). Iron carbide was injected at rates up to 90 kg/min in the EAF using an existing gunite tank.
Nucor acquired a license in 1992. It authorized PLS Engineering in Denver, Colorado, (now part of the Harris Group) to build a plant at Point Lisas, Trinidad to produce 300,000 tons of iron carbide per annum and by the end of 1994 the plant was in operation. Nucor started the construction in 1993, but unfortunately limited the funds to PLS. PLS depleted those funds before it completed the design of the plant. Nucor finished the design. In addition, Nucor minimized expenses for some of the equipment of the plant, including the heat exchangers.
The plant was started at the end of September 1994. However, the company had problems with the compressors for the process gas and shutdown plant. It left the cooling water running. During this period, a technician removed the level sensors in the columns of the packed tower for recalibration and hence the alarms for the liquid level were blocked. He informed the control room. One of the valves installed on the packed tower, despite being in a closed position, leaked and the leak was not noticed for several days. Once discovered, the column was drained immediately, but the damage had been done. The water had passed through the pipes and it was mixed with the iron oxide dust and had severely contaminated the heat exchangers. Nucor spent more than a year attempting to clean the exchangers but without success.
Nucor also faced several other operational problems at the plant. These problems were (i) undersized pipes for the process gas which limited the flow of the gas to a value of 65 % maximum of the design capacity, (ii) frequent breaking of the gas seal for the tuyere plate of the fluidized bed reactor, (iii) abandoning of the ore heating system since it failed due to abrasion which has caused compromising of the chemistry in the reactor and hence further limiting the quality and quantity of product, (iv) undersized collection tank for the scrubber, (v) unreliable system for regulating the flow of product through the product cooler which necessitated high maintenance, (vi) unreliable pneumatic lift system for the product which required frequent maintenance, (vii) very small size of screw feeder supplying ore feed to the plant, and (viii) inadequate packing glands of the screw feeder which leaked severely.
Nucor spent four years working with these problems, but in 1998, due to the drop of steel prices, they shut down the plant. The plant was subsequently demolished in 2002. In spite of all the above difficulties faced, 357,712 tons of iron carbide was produced in the plant which had shown that the process was technically feasible process. Mechanical failures encountered had stressed the requirement of adequate and reliable facilities.
Second attempt to produce iron carbide on a commercial scale was by Qualitech Steel Corporation in Corpus Christi, Texas, where it built a new plant. Qualitech acquired a license to produce iron carbide from ICH, but changed the process, when Mitsubishi Corporation funded the project and wanted to use two reactors and a pipe grid style, process gas distribution system in the fluidized bed reactor.
The Texas plant was just being commissioned when the parent company went bankrupt during March 1999, and the plant only produced a few thousand tons of the iron carbide before being shutdown in 1999 and demolished in 2004.
Frank A. Stephens, the son of Dr. Frank M. Stephens, Jr. acquired the exclusive ownership of the rights to the iron carbide process in 2010. During the early months of 2011, he formed International Iron Carbide LLC. The company owns the rights to 35 patents. The new company thoroughly analyzed the problems encountered in the plants at Trinidad and Corpus Christi and prepared documented solutions for the problems. Many of the solutions are, however, considered to be proprietary. Some of the solutions are (i) robust design of shell and tube heat exchangers, (ii) changed design of process gas system for achieving full capacity, (iii) improvement in the design of the fluidized bed reactor and the gas distribution system making the reactor more tolerant to plant shutdowns, (iv) new design for the gas seal for the tuyere plate in the fluidized bed reactor making it a dual gas seal, (v) introduction of a flash heating system for the ore feed which includes pneumatic transfer of hot material, minimal solids inventory for fast starts and stops, elimination of angular offsets to avoid abrasion, and modular construction for facilitating maintenance, (vi) changes in the scrubber design making the scrubber adequate to achieve full capacity, (vii) simplification in the product handling system which eliminates the lift system for the product entering the product cooler, and (viii) use of eductors to move solids in place of mechanical conveyors.
As on today there is no working plant for the production of iron carbide. However, International Iron Carbide has used their experience (both the positive as well as negative) obtained from the two first generation plants in Trinidad and Texas and has developed a design for a second generation plant which is based on the many lessons learnt. International Iron Carbide is actively searching entrepreneurs for collaboration to build the second generation plants.
The iron carbide manufacturing process is clean and simple. The three main process steps of the process (Fig 1) include (i) heating of iron ore to around 700 deg C, (ii) contacting the hot iron ore with pressurized H2 and CH4 gas mixture at a temperature of around 600 deg C and an absolute pressure of around 4.5 kg/sq cm in a fluidized-bed reactor for conversion of iron oxide to iron carbide with strong reducing gases, and (iii) cooling the product to around 65 deg C.
Fig 1 Three process steps of the process
The iron ore is usually hematite, which normally has a composition of 62 % to 65 % Fe, 1 % to 5% of gangue and 1 to 6 % of moisture. The stored ore does not usually need covering, weather permitting. The factors influencing the decision of whether or not to cover the mineral are the cost of fuel, the contents of natural moisture, and the climate.
From the ore storage, the ore is transported to a silo (day bin). The silo stores enough ore to operate the plant for around 24 hours. A variable speed conveyor measures and controls the amount of ore which is fed to the ore heating system.
The ore heater is a ‘flash heater’ which is comprised of three cyclones in series. The ore heater heats the ore up to 710 deg C by contacting the ore with the hot oxidizing gas leaving the firebox or burner. Increasing the temperature of the iron ore is useful for the process. It is helpful to the process because (i) it removes the moisture from the ore feed, and (ii) it partially oxidizes magnetite if present in the ore to hematite. This helps the reactions in the reactor, since hematite converts faster to iron carbide than magnetite. The ore heater discharges the hot iron ore into the feed lock hoppers, where it is held before being fed to the fluidized bed reactor.
The hot ore supply to the reactor is fed through two lock hoppers which operate in parallel. The lock hoppers are lined with refractory material. They have a pyramidal or conical bottom (hopper). Typically, one of the hoppers feeds the reactor for around one hour. Hoppers work in such a way that while one is feeding the reactor, the other hopper is filled.
The feeder hoppers also prevent the entry of the oxidizing gases into the reactor. After that the hot ore is purged with the N2 and the pressure is increased, the hot ore is continuously fed to the reactor at a rate which allows one hopper to empty and to be depressurized by the time the other hopper is filled.
Fluidized bed reactor and process reactions
The fluidized bed reactor uses iron ore fines which limits the need for pre-treatments such as sintering or pelletizing. The ideal feed material for the reactor is the hematite iron ore fines with its size in the range of 0.1 mm to 1.0 mm.
The inside diameter of the reactor is around 12 meters and operates at around 600 deg C. It receives the process gas consisting essentially of H2 and CH4. H2 is introduced to maintain the pressure in the reactor freeboard at 4.5 kg/sq cm absolute pressure. The compressors recycle the process gas to obtain a superficial velocity of 0.92 m/sec.
In the fluidized bed reactor, H2 and CH4 convert the heated iron ore into iron carbide. Oxygen combines with H2 to form water and carbon (C) combines with iron to form iron carbide. The general chemistry of the process is described by the equation 3 Fe2O3 + 2 CH4 + 5 H2 = 2 Fe3C + 9 H2O. This reaction is a kind of overall summary of all the reactions taking place in the process. The reaction proceeds in a relatively slow rate, and the residence in the reactor is much more when compared with the process in the steel making furnace. This time can, however, be reduced by changing the temperature and pressure.
In the gaseous phase part of the reactions there are three basic components namely (i) H2, (ii) O2 (oxygen), and (iii) C. These three elements interact with each other to form H2, H2O, CO, CO2 and CH4. The concentration of each of these compounds depends on the several factors such as (i) ratio of the masses of the individual elements, (ii) temperature of the system, (iii) system pressure, and (iv) to some extent the time in which the elements are in contact.
One of the two main gaseous reactions is steam reforming reaction CH4 + H2O = CO + 3 H2. In this reaction, natural gas reacts with steam to form H2 gas needed for the process. The second important gaseous reaction is that of water gas shift as described in equation CO + H2O = CO2 + H2.
When the reactants are placed all together at a high temperature a gas mixture is obtained containing H2, H2O, CO, CO2 and CH4. This mixture also contains a small amount of N2.
The gaseous reactions tend to be catalyzed by the presence of metallic iron and/or iron carbide. The C and H2 for the above reactions come from the reagents added in the process. The O2 is extracted from the ore by means of the reaction Fe2O3 + 3H2 = 2Fe + 3H2O. By controlling the relative concentrations of C and H2 in the process gas, it is possible to promote the removal of O2 from the ore and the addition of C to form iron carbide.
The reactions to form the iron carbide are slightly endothermic, so as to maintain the temperature to around 600 deg C, and it is necessary to heat the process gas to 633 deg C.
The reactor operates as a dense phase fluidized bed reactor. The gas bubbles are formed from the process gas in the fluidized bed. The reactor receives the ore on one side and discharges the product from the other size. The internal baffles of the reactor convey the solids within the reactor. The baffles minimize short circuiting of the solids in the reactor and create a more uniform residence time distribution for the solids. The retention time of solids is around 16 hours.
The process gas composition, temperature and pressure are continuously monitored by the process instrumentation. The process produces a non-pyrophoric product, which can then be stored and transported.
The reactor produces around 42 tons per hour of the product (iron carbide). The product is continuously discharged through lock hoppers from both the reactor and the off gas cyclones before being passed through product coolers.
One of the earlier process problems was the unintended production of free C (soot), coming from the Boudouard reaction, which form C and CO2 from CO (2CO = C + CO2). However, the International Iron Carbide has identified process conditions which do not allow the formation of free C, and still producing iron carbide of high quality. The specifics of these conditions are of proprietary nature.
The reactor discharges the iron carbide product via two lock hoppers which are (similar to those used to feed the hot iron ore to the reactor. The lock hoppers release the product to atmospheric pressure. In fact, while one receives the hot product at around 3.5 kg/sq cm of pressure, the other discharges the product to the cooling device (the product cooler) at atmospheric pressure. The iron carbide leaves the lock hoppers at around 590 deg C and is cooled to around 65 deg C by passing through the product coolers. The quantity of product in the product coolers is controlled with a variable speed discharge conveyor belt which conveys the iron carbide to the dry magnetic separator. The magnetic separator removes a significant amount of gangue liberated (typically 50 %), which, however, depends on the quality of the iron ore fed to the process.
A schematic flow sheet of the iron carbide process is given in Fig 2.
Fig 2 Schematic flow diagram of iron carbide manufacturing process
Gas treatment system
The process gas leaves the reactor at around 590 deg C. It passes through four parallel cyclones (the reactor cyclones) which are lined with refractory material. These cyclones remove most of the entrained solids from the process gas leaving the reactor. The fines collected by the cyclones (around 36 tons per hour) flow by gravity into a surge bin before being recycled to the reactor. Any additional solid pass through a series of lock hoppers before entering a product cooler, which cools the solids below 65 deg C (normally around 40 deg C), and finally add the solids to the product from the reactor cooled upstream of the magnetic separator.
The process gas leaving the reactor cyclones passes through four parallel heat exchangers (the process gas heat exchangers) which reduce the temperature of the process gas to 150 deg C. A venturi scrubber and a packed bed column (the packed tower) further cool the process gas to around 30 deg C to remove the water produced in the reactor and remove the remaining fine particles suspended in the process gas which has escaped the cyclones. This ensures that the quantity of fines in the process gas is low enough for avoiding the damage to the ‘process gas recycle compressors’.
A small stream of process gas is removed from the recirculating process gas (the bleed gas) leaving the top of the packed tower and before entering the recycle gas compressors, to prevent the accumulation of N2 in the circuit. The amount of bleeding depends on the N2 content of the natural gas, the quantity of N2 which gets into the system with the hot iron ore feed entering the reactor.
The make-up reagent gas, normally consisting of H2 and natural gas, enters the recirculating process before the compressors. The pressure of the freeboard in the fluidized bed reactor governs the exact amount of H2 to be introduced into the system. The amount of natural gas to be added to the process gas is determined by the concentration of CH4 in the process gas.
Two centrifugal compressors (the recycle gas compressors), one operating and the other standby, recycle the process gas with a pressure of around 4.20 kg/sq cm at the intake and around 5.30 kg/sq cm at the discharge. The differential pressure across the compressors regulates the flow of process gas to the reactor.
The four gas-gas heat exchangers, which cool the process gas coming from a fluid bed reactor, heat the process gas coming from the compressors, to a temperature of around 520 deg C. A gas heater (the process gas heater) further increases the process temperature to around 630 deg C, using the bleed gas and natural gas as fuel. The exhaust gas coming from the heater provides energy to heat combustion air supplied to the ore heater.
In the process, there are two water cooling systems. One system for direct contact with the process gas (the direct contact cooling water”), which provides cooling water to the ‘venturi scrubber’ and the ‘packed tower’, where in these devices the water comes in direct contacted with the process gas. The other water cooling system is the indirect water cooling system which provides cooling water to (i) the cooling system of the products (product coolers), (ii) the H2 reformer, and (iii) other minor heat exchangers used for the process.
The hot water leaving the packed tower returns directly to the contact cooling tower for the removal of the heat, while the water leaving the venturi scrubber goes through a thickener first, where it is filtered to remove the solids which are present in the contaminated water.
The process typically produces a product which has been converted 93 % to iron carbide. The typical mineralogical and elemental composition of the product consists of Fe3C- 91 % to 96 %, Fe (total) – 89 % to 93 %, Fe (met) – 0.5 % to 1 %, SiO2 + A12O3 – 2 % to 5 %, Fe3O4 -2 % to 5 %, C (as Fe3C) – 6 % to 6.5 %, and O (as Fe3O4) – 0.5 % to1.5 %. Iron carbide is magnetic, so if the gangue is be physically liberated, either before, during or after the process of carburation, then a dry magnetic separation can be carried out to lower the gangue content in the final product and therefore increase its iron grade.
Although the analysis of the product can vary depending on the type of ore used, there is no significant sulphur present in any case. Phosphorous level depends on the type of ore used and is usually present in the product as P2O5. But most of the phosphorous gets transferred to the furnace slag, not in the product.
The residual elements in the ore are normally present in the product as oxides, but since most iron-ores have very low levels of copper, nickel, chromium, molybdenum or tin, there are no significant amounts of these elements in the final product. As a result, iron carbide produced is very clean and provides an effective method of diluting the tramp residual metals during steelmaking, while avoiding the sulphur which generally comes with some virgin iron sources.
The typical consumptions of materials, energy, and at the battery limit per ton of iron carbide are iron ore – 1.42 tons, natural gas – 350 M cal, electricity – 400 kWh, water – 1.2 cum, nitrogen – 10 cum and labour – 0.22 worker hours.
Iron carbide also is very environmental friendly and provides large environmental advantages. The process achieves the lowest C emission of all virgin-iron steelmaking processes, producing only 1.09 kg of CO2 for each kg of steel produced. This is far less than the 2.01 kg for the conventional blast furnace -basic oxygen furnace route of steelmaking, 3.09 kg for coal based DRI, and 1.87 kg for natural gas based DRI – EAF route of steelmaking. Only steel totally made from scrap achieves a lower emission.
Advantages of iron carbide and its production
Advantages of iron carbide and its production process consist of the following.
- It is the better charge material than other materials for the EAF since it contains around 6.0 % to 6.5 % C and is produced from virgin iron ore and hence it contains negligible tramp elements. Use of iron carbide in steelmaking processes results into a low content of N2 and H2 in the steel.
- It is not pyrophoric and hence it is safe and easy to handle.
- It is a dense, granular powder which that dissolves easily in liquid steel. It can be easily injected into a BOF and/or an EAF, where it dissolves instantly.
- The process of producing iron carbide is environmentally friendly since there is no requirement of agglomerating iron ore.
- The only by-products of the process are water and CO2, and the amount of CO2 generated is much less than the amount generated during the production of steel by other processes. Further, a large quantity of CO2 leaves the reformer in a flow of gas that is concentrated, which is easy to sequester and/or re-use for other purposes.
- The process uses iron ore fines, which are less costly than pellets and iron ore lumps.
- The necessity of briquetting the product is not there.
- The process operates at low temperature and it is a thermally efficient process.
- The process is a closed loop process which uses 100 % of the reagents added.
- The process is simple, consisting of a single stage reactor, which is easy to control.