Title of Invention

A PROCESS FOR PRODUCING IRON CARBIDE FROM IRON ORE SLIME IN THERMAL PLASMA

Abstract

A process for producing iron carbide from iron ore slime in thermal plasma A process has been developed to prepare iron carbide (Fe3C) with 97 wt% purity from iron ore slime. DC extended arc thermal plasma was employed to carry out the reduction and carburization reaction in the temperature range 1200-1600ºC. The process takes 10-20 minutes time to complete the reaction in kg scale of operation which is significantly less compared to few hours taken by conventional fluidized bed process. From x-ray diffraction it was identified that the compound phase impurities present in the product were FeC, Fe2C, Fe5C2 (maxm. 2 wt% in total) and Fe203 (maxm. 1 wt%). Chemical analysis showed the critical elements to exist in the following range : C 6-9 wt%, S <0.01 wt%, P 0.11-0.14 wt%. Microhardness of the product was recorded to lie in the range 570-660 VHN0.1. The recovery of the Fe3C at different power input levels was studied and the highest yield of 92.2% was observed at the power input of 2.64 kWh/kg.

Full Text

The present invention relates to a process for producing iron carbide from iron one slime in thermal plasma. Slimes are generated during washing of iron ores and are carried away by water into tailing pond. Progressively , more and more quantities of iron ore slimes are generated over the years due to introduction of mechanised mining and exploration of low grade iron ores. In India alone, about 10 million tonnes of iron ore slimes are produced every year. The slimes contain 48-60% Fe and are thrown away without any productive use. Such rejection of huge quantities of powdery ores not only creates problem for waste disposal but also causes appreciable loss of mineral wealth and pollutes the environment. As long the high grade lumpy iron ore is plentifully available in the world, no one thinks about the utilization of slimes in terms of conventional iron extraction. In this situation, it is appropriate to make effort for using the iron ore slimes to prepare an alternate industrial product like iron carbide which would make significant value addition to the industrial waste and at the same time save the iron ore industry from the problem of waste disposal. Iron carbide (Fe3C) is industrially prepared from iron oxide or blue dust, a powdery ore of iron containing 98-99% Fe2O3, through fluidized bed reaction using a gaseous mixture of a reducing gas (e.g. H2) and a carburizing gas (e.g. methane, natural gas and the like). The gas solid reaction is carried out at around 600°C at 1.8 atmosphere pressure. The conversion of iron oxide to iron carbide takes place as per the following reaction : 3Fe203 + 5 H2 + 2CH4 = 2Fe3C + 9H2O (A.Ganguly, R.H.G.Rau, V.Shah, 'Hot metal and iron carbide: Competitive materials inputs to EAF' , IIM Metal News Vol. 19, No.2, 1997, pp 13-18) Iron carbide is also prepared when carbon in solid form (coke/graphite) reacts with liquid iron at high temperature (>1540°C) as discussed by Yu. M. Lakhtin in his book 'Engineering physical metallurgy and heat treatment', Mir Publ., Moscow, First Edn., Third Printing, 1986, pp 14. The following literature and works are significant to highlight the state of art: 1. A. Chatterjee, 'Alternative iron making processes' Proc. 'Conf. on Prod. Liquid Iron using coal', Bhubaneswar, Aug. 24-25, 1994, pp 30-33, Ed. H.S. Ray, D.N. Dey, R.K. Paramguru and A.K. Jouhari. 2. Oho Stellin, 'Carbon oxide reduction of iron ore', J. Metals, Jan. 1958 pp. 209-295. 3. R.J. Ochlberg, 'FIOR process for direct reduction of iron ore', Iron & Steel Engineer, Vol. 51, No.4,1974, pp. 58-60. 4. J. Werther, 'Metechniken fur Gas / Festoff - Wirbelschichtreaktoren', Chemie Ingenieur Technik, Vol. 62, No.8,1990, pp. 605-612. 5. A.W. Swanson, 'Iron Carbide, a possible replacement for premium quality scrap' SME Annual Meeting, 15-18 Feb. 1993, Rano, Nevada, USA, pp 93-28 (pre-print). US published Patent No.6,063,155 of May 16, 2000 (Masso et al.) describes a fluidized bed process for conversion of iron oxide to iron carbide which involves the following steps : Providing a fluidized bed reactor having metallising zone and a carburizing zone; feeding iron oxide to the said reactor; feeding a reducing gas to said reactor so as to provide reduced iron in the metallising zone; and feeding a carburizing gas to said carburizing zone so as to produce iron carbide in the carburizing zone having between about 2.2 wt% and about 6.5 wt% carbon and at least 80 wt% iron. The temperature was maintained between 720 to 840°C and pressure between 10 to 20 bars inside the reactor. US Patent No.6,328,946 of Dec. 11,2001 (Stephens, Jr.) describes a two step process to convert iron oxide to metallic iron in the first step and metallic iron to iron carbide in the second step. Hydrogen is used as the reducing gas in the first step and methane/carbon monoxide is used as carburizers of metallic iron in the second step. The temperature of carburising reaction is abut 800 K. UK patent application GB 2314075 of 17 Dec 1997 describes a process for converting iron oxide to iron carbide using hydrogen as reducing gas. Water produced by the reaction is reacted with CRj to produce CO and / or CC>2 which is subsequently employed to carburize the iron to iron carbide. European Patent Application EP 1036762 A1 of 24 Nov. 1998 describes a process for producing iron carbide from partially reduced iron oxide (FeO 1/3) using carbon and CH4/CO as the reductant. Various prior art references e.g. US Pat. No. 6,165,249, US Pat. No.6,328,783, US Pat. No. 6,264,911, US Pat No. 6,261,531, US Pat. No.6,071,468, US Pat. No.6,004,373, Patent No. GB 2314075A, GB 2204326A, GB2204327A, EP 0179490B1, DE 4320359, DE 4426623, JP 52-26211, JP 1-40765, JP 6-501983 describe several production methods of iron carbide from iron oxide and iron oxide containing ores. However, so far no work has been reported on the preparation of iron carbide from iron ore slime by thermal plasma process. The main object of this process is to prepare iron carbide from iron ore slime in thermal plasma. It is also the object of this invention to provide a value-added application to iron ore slime. Accordingly the present invention relates to a process for producing iron carbide from iron ore slime in thermal plasma which comprises: heating a mixture of the iron ore slime and carbon in a wt. ratio of slime to carbon in the range of 3:1 to 10:1 in an electrically conducting crucible that forms the anode and employing carbon as cathode by methane and hydrogen as plasmagen gas in the volume ratio of 1:1 to 1:4 into the arc zone of a plasma reactor at temperature ranging between 1200-1600°C in a time of 10-20 min. and recovering the solid iron carbide product after cooling for 3 hours inside the furnace/reactor. In an embodiment of the present invention wherein the slim composition used as follows: In another embodiment of the present invention wherein carbon is used in the form of graphite. In yet another embodiment of the present invention wherein wt. ratio of carbon to slime concentrate is maintained ratio ranging from 1 :3 to 1 : 1 0. — In another embodiment of the present invention wherein extended arc plasma uses an arc current in the range 200 - 500A, arc voltage in the range 20-60V, plasmagen gas flow in the range 0.5-2.5 litres per minute. In still another embodiment of the present invention wherein the volume ratio o to H2 in the plasmagen gas is maintained ranging between 1 : 1 to 1 :4. In another embodiment of the present invention wherein the purity of FesC in the product is 97 wt% and more. Iron ore slimes contains around 68 to 85% Fe2O3 in the Indian ores. The present invention used the beneflciated Indian iron ore slime (called slime concentrate) with the following composition as shown in Table 1 . About 80% of particles in the slime lie below 120fj.m size. In the conventional fluidized bed reactors the reaction kinetics of reduction and carburization of iron oxide is slow due to involvement of neutral gas molecules at elevated temperature (~600°C). On the other hand, in a thermal plasma furnace/reactor, about 3-4% molecules (by vol.) are continuously ionised from the plasma generating gas and are characterized with high ion temperature (8000- 10,000°C). Ions and other activated species like electrons, excited atoms etc. participate in the reaction and contribute to speed up the kinetics of reaction in a plasma process. Reduction and carburization time in plasma based reaction is thus appreciably reduced from few hours to less than half-an-hour in kg scale of charge processing. This leads to result in increased throughput and out-put of plasma furnace which in turn improve process economics of iron carbide production. The main findings of the process are : 1. For the first time iron carbide (FesC) has been prepared from iron ore slime by thermal plasma process using CFLt+Hj plasma. 2. The process makes use of low cost dc extended arc plasma formed by arcing between two graphite electrodes and the heat generated by the arc plasma causes reduction and carburization of the said slime to produce iron carbide (FesC) 3. The process makes direct use of the powdery raw material i.e. the slime concentrate without making agglomeration or pelletization, thus saving a considerable energy and cost. 4. The process saves at least one hour and more time in preparing FesC at kg and below scale of processing 5. The process produces FesC with 97% and better purity along with the presence of small amounts of other carbides of iron such as FeC, Fe2C, Fe5C2 (total up to 2 wt%) and Fe2O3 (up to 1 wt%) The invention provides a process for preparation of iron carbide (FesC) by thermal plasma which involves heating a mixture of beneficiated iron ore slime and carbon in an electrically conducting non-metallic crucible that forms the anode wherein the carbon cathode having provision for vertical movement and passing a mixture of methane and hydrogen gas at 1:1 - 1:4 volume ratio through a central axial hole is made to contact the anode and then slowly withdrawn to form the arc plasma in nontransferred mode which subsequently changes to transferred mode of operation soon after the charge becomes conducting at elevated temperature (~800°C) and the reduction and carburization is carried out at 1200-1600°C furnace temperature corresponding to 8,000- 10,000°C ion temperature generated by arc plasma and the reduction of iron oxide (slime) to iron carbide is completed within a short time ( min.) resulting in the production of FejC. The following plasma conditions were maintained in the extended arc type dc plasma to prepare iron carbide. Arc voltage : 20-60 V Arc current : 200-500 A Plasma generating gas (CH4+H2) : 0.5 - 2.5 lit/min at 1.1 - 1:4 volume ratio Plasma processing time : 10-20 min. Characterization of the plasma produced iron carbide product by x-ray diffraction (XRD) identified the product to be Fe3C with the presence of FeC, Fe2C, Fe5C2 (total 2 wt%) and Fe2C»3 (1 wt%). Microhardness of the product obtained in the form of solid lumps exhibited value in the range 570-660 VHN Q.I. Chemical analysis of the product showed results in the following range : C 6-9%, S wt.). The recovery of FesC at different inputs of power was studied and it was found that the recovery was the highest (92.2% by wt.) at the power input of 2.64 kWh/kg. • The following typical examples will illustrate how the process of the present investigation is carried out in actual practice and should not be construed to limit the scope of investigation. Example 1 The hearth of the plasma furnace/reactor consists of a graphite crucible vertically supported on its base. A graphite electrode (anode) introduced through the central hole of the crucible base maintained electrical connection. Another graphite electrode (cathode) with axial hole (5mm dia) was disposed from the upper end of the crucible having provision for vertical movement by rack and pinion arrangement. Carbon in the form of graphite lumps (1-2 mm size) was added to the slime (beneficiated ) at three different wt% such as 10, 15 and 20 and was thoroughly mixed. Anode and cathode were brought in very close to each other inside the reactor hearth (at a separation gap of 1-2 mm) and then lOOg of the charge (slime+carbon) was filled into the reactor followed by light ramming. A mixture of CfLt and H2 gas at 1:1 volume ratio and 0.5 litre per minute flow rate was passed into the arc zone through the axial hole of the cathode. Arc was struck in non-transferred mode and was allowed to continue for 2 minutes to heat up the surrounding charge for conducting electricity. The arc length was then gradually increased and fixed at 5-6 cm. After another 3 minutes, the entire charge became conducting and the plasma arc started working transferred mode. The high temperature 1500°C of the furnace due to high ion temperature (8000-10,000°C at arc point) and high enthalpy (105 w/cm2 )of the plasma .caused the iron ore slime (in the form of Fe2O3) undergo simultaneous reduction (by H2 and carbon) and carburization (by CH4 and carbon), thus producing FesC. The time taken for completing the reduction and carburisation reaction was 10 minutes. Voltage and current in this operation were 25V dc and 300 A respectively at full arcing condition. The product was cooled in the furnace up to room temperature and collected in the form of lumps. The fin.. Product was dark gray in colour. X-ray diffraction of the product identified the product to be FesC with presence of small amounts (1.5 wt% in total) of FeC, Fe2C , Fe5C2 and Fe2O3 (0.5 wt%). Yield of the product was in the range 91-92% (by wt.%). Chemical analysis of the critical elements in the final product showed the following : C 6.92 %, S (by wt.). Microhardness of the product studied in the grains were found in the range 600-660 VHN o.i . Example 2 1 kg of beneficiated iron ore slime was taken and carbon in the form of graphite lumps (1-2 mm size) was thoroughly mixed with it at 10,15 and 20 wt%. The resulting mixture was then charged into the plasma furnace following the procedure described in example 1 and was lightly rammed. A mixture of CH4 and H2 gas in the volume ratio 1:2.5 and at a flow rate of 2.5 litre per minute was passed into the arc zone through the axial hole of cathode. Arc was struck between the electrodes and operated in the non-transferred mode for first 5 minutes and then switched over to transferred mode as the charge attained high temperature (~800°C) to conduct electricity. Arc length in the plasma was maintained at 5-6 cm. In the full arcing condition the following voltage and current were maintained : arc voltage : 50V, arc current : 450 A. Reduction of Fe2Os in the slime concentrate to FesC took place at high reactor temperature (1450°C) due to high ion temperature (8000-10,000°C at arc point) and high enthalpy (105 w/cm2) of the CH4+H2 plasma. The time taken to complete reduction and'carburisation reaction was 15 minutes.' The product was cooled in the furnace up to room temperature and collected in the form of lumps. The colour of the final product was dark gray. X-ray diffraction identified the product to exist in FesC phase along with small quantities of FeC, FesC, FesCj (2 wt%) and Fe2O3 (0.7 wt%). Yield of the final product varied in the range 88-91 wt%. The level of C, S and P in the product were analysed by chemical analysis and were recorded as follows : C 6.71 %, S grains in the product exhibited values in the range 620-650 VHN Q.I. Example 3 700 g of the charge was processed in the dc arc plasma furnace as described in experiment 1. Reduction and carburization was carried out by repeating the procedure of experiment 1 but with the following changes in experimental conditions : Maxm. arc voltage : 40 Maxm. arc current : 300 A CH4+H2 ratio and flow rate : 1:2 (volume ratio) and 1.5 litre per minute Furnace hearth temperature : 1550°C The colour of the furnace cooled final product was dark gray. The best yield of the product was found above 90 wt%. X-ray diffraction of the identified FesC to be the major phase along with the presence of small amounts of FeC, Fe2C, FesC2 (1.5 wt% in total) and Fe2Os (0.2 wt%). Chemical element analysis noted the levels of initial elements as follows : C 6.89 %, S The main advantages are 1. Iron ore slime which is a major industrial waste can be used in beneficiated form to prepare an industrially useful carbide product (FesC) which replaces more costly iron scrap in electric arc furnaces and induction furnaces for steel making. 2. The process accepts both powdery as well as agglomerated charges to produce iron carbide. Thus, iron ore slime concentrates which are in fines form can be used directly for producing iron carbide, bypassing polluting agglomeration process like sintering/pelletising. 3. The process makes use of low cost extended arc thermal plasma formed byarcing between two graphite electrodes (non-transferred mode) and graphite electrode and charge (transferred mode) while mixture of CRt and Ha is continuously fed through the cathode. 4. The said process reduces the reduction and carburization time from one hour and more to 20 minutes and less (at 0.1-1 kg scale charge) as compared to conventional fluidized bed reactor process. 5. The process produces a single phase FesC compound (97%) with the presence of 2% other iron carbides which would be ideally suited from purity point of view to replace iron scrap in electric arc furnace and induction process for steel making.- 6. The process produces iron carbide which shows microhardness in the range 570-660 VHN o.i, thus making the product amenable to easy powder making for injection into steel making furnaces. We claim: 1. A process for producing iron carbide from iron ore slime in thermal plasma which comprises: heating a mixture of the iron ore slime and carbon in a wt. ratio of slime to carbon in the range of 3:1 to 10:1 in an electrically conducting crucible that forms the anode and employing carbon as cathode by methane and hydrogen as plasmagen gas in the volume ratio of 1:1 to 1:4 into the arc zone of a plasma reactor at temperature ranging between 1200-1600°C in a time of 10-20 min. and recovering the solid iron carbide product after cooling for 3 hours inside the fumace/reactor. 2. A process as claimed in claim 1, wherein the iron ore slime composition on wt. basis is Fe2O3 .. 93.2, SiO2 -1.94, AI2O3-4.25, MnO2 -0.87 3. A process as claimed in claim ,1 wherein carbon is used in the form of graphite. 4. A process as claimed in claims 1 to 2, wherein in the plasma reactor, arc plasma uses an arc current in the range 200 - 500A, arc voltage in the range 20-60V, plasmagen gas flow in the range 0.5-2.5 litres per minute. 5. A process as claimed in claims 1 to 4, wherein the purity of Fe3C in the product is 97 wt% and more. 6. A process for producing iron carbide from iron ore slime in thermal plasma substantially as herein described with reference to the examples.

Documents:

279-DEL-2003-Abstract-(22-10-2008).pdf

279-del-2003-abstract.pdf

279-DEL-2003-Claims-(22-10-2008).pdf

279-del-2003-claims.pdf

279-DEL-2003-Correspondence-Others-(22-10-2008).pdf

279-del-2003-correspondence-others.pdf

279-del-2003-correspondence-po.pdf

279-DEL-2003-Description (Complete)-(22-10-2008).pdf

279-del-2003-description (complete).pdf

279-del-2003-form-1.pdf

279-del-2003-form-18.pdf

279-del-2003-form-2.pdf

279-DEL-2003-Form-3-(22-10-2008).pdf

279-del-2003-form-3.pdf