|Title of Invention||
"A PROCESS FOR PRODUCING SPHEROIDAL GRAPHITE IRON FROM BENEFICIATED IRON ORE SLIME CONCENTRATE USING A PLASMA FURNACE"
|Abstract||A single step batch process has been developed to prepare spheroidal graphite iron from beneficiated iron ore slime concentrate by plasma smelting using LECO (a coke produced from lignite where fixed carbon is 74.03%) as the carbonaceous reductant. A dc extended arc plasma furnace (35 kWh) was employed to carry out the smelting at 0.5-2 Kg scale. Basicity of slag in the charge was varied between 1 and 2 over five different compositions to achieve the maximum metal yield with good fluidity of melt. The maximum yield was recorded at a slag basicity 1.5-1.8 without sacrificing melt fluidity. The pig iron melt produced after smelting was cast into a graphite mould where Fe-40% Si-9% Mg alloy was added to spheroidize graphite in the cast iron matrix. The SG product was recovered from the mould after cooling for 30-60 min in air. XRD analysis of the said product identified Fe to be the major phase along with the occurrence of graphite as minor phase. Small quantities of Fe2C, Fe3C and Fe304 were also detected. On examination under optical microscope, the product revealed nodular graphite particles with a density 1400/mm2 and less. The diameter of the graphite spherulite was found to be 16 urn and less. Chemical analysis of the SG casting showed the critical impurities to lie in the following levels: C 4.4-6, S 0.11- 0.16, P 0.05-0.069, Si 2.1-2.3, Mg 0.02-0.05 (wt.%). Fe content was found in the range 90-92 wt. %.|
|Full Text||The present invention relates to a process for producing spheroidal graphite iron in a plasma furnace.
SG (spheroidal graphite) iron is a type of cast iron which, unlike gray cast iron, is characterized by high strength and ductility. Due to improved mechanical properties comparable to cast carbon steel and favourable cast iron properties such as good machinability, good casting property, capacity to damp vibration, high wear resistance etc., SG iron finds wide application in engineering industries to make crankshafts, cylinder heads, mill rolls, press crowns and hammer anvils. In the ordinary cast iron, graphite occurs in the form of flake which acts as stress raiser to weaken the matrix for initiation and propagation of crack. In spheroidal graphite iron, graphite precipitates in the form of spheroids, spherulites or nodules which distribute the applied stress (when undergo mechanical loading) over the entire surface areas of spheroids to reduce stress concentration at any particular point and thus improves tensile strength and ductility of matrix. The spheroidization of graphite is generally done by addition of certain alkali earth metals like Mg (0.03 - 0.07 wt%), Ce (0.0009 - 0.02 wt%) (Ref: l.Yu.M.Lakhtin, 'Engineering physical metallurgy and heat treatment, Mir Publ. Moscow, Third Printing, 1986, pp 179, 2. P.K. Biswas, N. Girija, T. Ramachandran & B.C. Pai, 'Critical issue of rare earth applications in SG iron', IIM Metal News, Vol. 3(1), 2000, pp.3.)
Spheroidal graphite iron is industrially prepared from steel scrap, special grade pig iron and foundry returns by melting in water-cooled cupula furnace (heated by coal)
or arc furnace and followed by ladle addition of Mg in the form of Fe-Si-Mg or Ni-Mg alloy. Critical elements like C, Si, S and P generally remain within the following range in the cast iron matrix : C 3.3%, Si 2.2 - 2.5%, S 0.14%, P 0.2% (all by wt.). Yu M.Lakhtin and Biswas et al., in the above references have discussed in detail about the preparation of Spheroidal graphite iron and tolerable impurity contents in the starting raw materials.
The following literature and works are significant to illustrate the state of art:
1. Metals Hand Book, Desk Edn., 'Ductile iron', American Society for Metals, Ed: H.E. Boyer and T.L. Gall, Second Printing, 1985, pp 5.7
2. B. Chalmers, 'Physical Metallurgy', John Wiley & Sons, Inc., 1962, pp 267
3. R.D. Forrest, Foundry Trade Journal, Vol. 155, 1983, pp 399
4. I. Otto, Foundry Trade Journal, Vol. 162, 1988, pp 292
5. K. Nagai, K. Kishitake & T. Owadano, Mater. Sci & Technol., Vol.7, 1991, pp 464
6. A. Fontes, M. Jeandin, O. Uteza, M. Sentis & M. Frainais, Mater. Marufact. Proc, Vol.9, 1994, 415
7. Q. Chen, E.W. Langer & P.N. Hansen, J. Mater. Sci., Vol.32(7), 1997, pp 1825
US published Patent No.3,001,869 of Sept. 26, 1961 (Ford Motor Co., Delware) describes a process of Spheroidal graphite iron preparation which involves addition of the alloy 8% Mg-40% Si-Fe in the ladle stage to molten cast iron. The low yield of
Mg by this method had been improved by addition Mg metal clippings instead of the alloy to ladle prior to filling of the ladle with molten cast iron (prepared by electric furnace, melt temp. 2800°F) and protecting the Mg metal by a layer of ferrous metal.
UK Patent No.GB 938892 of Oct. 9, 1963 (National Castings Company, Ohio, USA) describes a process for producing nodular graphite cast iron castings from any molten iron by treating the molten mix with a pre-treatment slag that increases the responsiveness of the mix to nodularising agent such as magnesium.
UK Patent No. GB 946642 of Jan. 15, 1964 (Crane Co., Illinoise, USA) describes a process to produce iron castings in which flaky graphite carbon is converted to nodular form by introuducing hydride of yttrium or a rare earth metal to a bath of molten cast iron mix.
Relatively recent US Patents such as Patent No. 4619713 of 1986, 4762555 of 1988, 4889688 of 1989 and very recent Patent No. 6024804 of 2000 and Published Patent Applications No. 20010024622 of 2001, No.20010039853 of 2001 describe several production methods of SG iron from molten cast iron / ferrous metal by addition of Mg, SiC, CaC etc. to nodularize the flaky graphite.
Various prior art references e.g. JP 47-8338, JP 4801578, JP 57-89423, DE 2853871, PL 173194, EP 1126037 Al also describe the preparation methods of SG iron.
However, so far no work has been reported on the preparation of spheroidal graphite iron from beneficiated iron ore slime concentrate using thermal plasma process. The
main object of this process is to prepare spheroidal graphite iron from beneficiated iron ore slime concentrate by smelting in a thermal plasma furnace using carbon as the reductant.
It is also the object in this process to achieve nodularization of graphite by addition of Mg in the casting stage of smelted liquid iron (pig) without requiring remelting of the pig iron. . Another object of this invention is to provide a value-added application to a waste material like iron ore slime concentrate.
The beneficiated Indian iron ore slime concentrate used in this invention was produced by wet magnetic separation method. The chemical analysis of the slime concentrate is shown in Table 1.
In the slime concentrate 50% of the particles remain between 120 to 53 |im, 20% below 53 pm and the rest 30% is widely distributed between 355 to 120 \im.
Because of the small contents of both S and P and high Fe203 (hematite) content, the iron ore slime concentrate is considered to be ideally suitable for producing spheroidal graphite iron making.
In the conventional processes, coal fired cupula, electric arc furnace or induction furnace is used to melt iron scrap, pig iron or cast iron for adding Mg in the ladle stage to nodularize graphite in the casting. The present invention, in contrast, smelts a waste material like iron ore slime (in beneficiated concentrate form) which is cheap compared to the starting raw materials in the conventional processes and adds Mg to the smelted liquid iron in the casting stage for saving a considerable amount of energy in the whole process of reduction and nodularization.
Plasma furnace is used in smelting the charge in order to save considerable time (few hours to half an hour in case of Kg scale charge), increase throughput and yield of product. It also requires relatively low investment in the extended arc mode adopted here.
The main findings of the process are :
1. For the first time spheroidal graphite iron has been directly prepared from beneficiated iron ore slime concentrates by smelting in a plasma furnace.
2. The process makes use of low cost dc extended arc plasma formed by arcing between two graphite electrodes while using argon as the plasma forming gas. The heat generated by the arc argon plasma causes
which converts to spheroidal graphite iron by addition of Mg in the form Fe - 40% Si - 9% Mg alloy in casting stage.
3. The process makes use of LECO (a form of coke produced from lignite by low temperature carbonisation, Fixed carbon : 74.03%), a cheaper form of carbon as the carbonaceous reductant for smelting of Fe203.
4. The process makes direct use of powdery raw material, i.e. the slime concentrate without making agglomeration or pelletization, thus saving a considerable energy and cost.
5. The process uses direct smelting for producing SG iron from Fe203 which saves 3-4 kwh energy per kg of product compared to conventional process which consists of pig iron production from Fe2C>3 and remelting of pig iron for nodularization of graphite by adding Mg in casting stage.
6. The process saves at least one hour and more time in preparing SG iron at Kg and below scale processing in comparison to conventional processes.
7. The process produces spheroidal graphite iron with the following composition : 4.4- 6% C, 0.11-0.16% S, 0.05-0.06% P, 2.1-2.3% Si, 0.02-0.05% Mg and 90-92% Fe (all in wt%).
Accordingly the invention provides a process for preparation of spheroidal graphite iron from beneficiated iron ore slime concentrate in a thermal plasma furnace which involves heating a mixture of beneficiated iron ore slime concentrate and carbon in the form of LECO in an electrically conducting non-metallic crucible that forms the anode wherein the carbon cathode having provision for vertical movement and passing argon gas through a central axial hole is made to contact the anode and then slowly withdrawn to form the arc plasma (argon) in non-transferred mode which
slowly withdrawn to form the arc plasma (argon) in non-transferred mode which subsequently changes to transferred mode of operation soon after the charge becomes conducting at elevated temperature (~800°C) and the reduction and carburisation is carried out at 1500-1600°C furnace temperature corresponding to 7,000- 10,000°C ion temperature generated in the arc plasma and the reduction of iron oxide (Fe203) to iron is completed within 20-30 minutes, after which the arc is switched off and the iron melt is cast into a graphite mould while intoducing Fe-40% Si-9% Mg alloy to the mould just before pouring the melt into it. Spheroidal graphite iron is produced in the form of a cast product upon cooling the melt in the mould in air for 30-40 minutes. The following arc conditions were maintained in the furnace :
Arc voltage : 25-50 V DC
Arc current : 200-550 A
Arc length : 7 cm
Plasma forming gas : Ar
Rate of flow of Ar : 0.5-1.5 lit/min
By varying the basicity of slag [(CaO + MgO) / (Si02 + A1203)] in the charge stage between 1 to 2, the product yield was studied and found to vary between 65 to 93%. Maximum yield was recorded to be 93% at 1.5 - 1.8 slag basicity. The level of C in the SG iron was found to lie above 4.4% while S and P level remained in the range 0.09 - 0.2% and 0.06 - 0.069% respectively. Si content was observed to vary in the range 1.1 - 1.5% in the pig stage which improved to >2% upon addition of Fe-40% Si-9% Mg alloy. XRD analysis of the plasma smelted Spheroidal graphite iron thus produced exhibited Fe as the major phase along with the presence of minor phases
such as Fe2C, Fe3C, Fe304. Graphite was identified as a prominent minor phase to exist in the matrix. Spheroidization of graphite particles in the iron matrix was confirmed by observing under optical microscope. Typical morphologies of the flaky graphites and spheroids in pig and Spheroidal graphite iron produced from the beneficiated iron ore slime concentrate by the thermal plasma process are shown in Fig.l and 2 respectively. Electrical energy consumption was recorded to be 6-6.5 kwh for producing 1 kg of SG iron from the beneficiated slime concentrate by the plasma method. Graphite nodule density was found to vary between 1100-1400 per mm2.
The following 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.
The dc extended arc plasma furnace (35 kW) used for smelting the beneficiated iron ore slime concentrate consists of a hearth where a graphite crucible is vertically placed on its base being surrounded by bubble alumina insulation. A graphite electrode is fixed to the base of the graphite crucible at the lower end so as to provide electrical connection. In this way, the graphite crucible also acts as an electrode (anode). Another graphite electrode (cathode) with axial hole (5 mm dia) is disposed from the upper end of the crucible having provision for vertical movement by rack and pinion arrangement. Carbon in the form of LECO (1-2 mm size) was added to the said slime concentrate 25% (by wt.) excess over the stoichiometric requirement. CaO and MgO were then added to the above mixture in appropriate amount to maintain desired slag basicity of 1.5. 500g of the final mixture which constituted the charge
was taken and introduced into the graphite crucible kept in the furnace hearth under closed arc condition. A mild ramming was done to avoid the voids in the charge. Argon gas was introduced into the furnace at the rate of 0.5 lit/min through the hole in cathode. The cathode was then separated by about 1-2 mm and electric power was switched on to establish the arc. At this position the electrode was allowed to remain for 3-4 minutes for heating the surrounding charge to become electrically conducting. It was slowly further separated more and more from the anode and stabilised at an arc length of 7 cm. The furnace temperature was measured by pyrometer and found to be between 1550-1600°C. The high temperature and high enthalpy (1056 w/cm2) of plasma caused the iron ore slime concentrate (in the form of Fe2C>3) to undergo carbothermic reduction and produce pig iron. The time required to complete the plasma smelting was recorded to be 20-25 minutes. Voltage and current in the arc varied in the range 25-40 V and 250-400 A respectively during 20-25 minutes operation. After completion of smelting, the electric power was switched off and the melt was taken out of the furnace hearth to pour into a graphite mould in which 5g of 1-2 mm size Fe-40% Si-9% Mg alloy particles were spread out on its base. At the melt was poured and cooled in the mould, the Mg vapour diffused through the melt undergoing cooling and turned the morphology of graphite grains to sperullite structure. The casting was cooled in the graphite mould for 30 minutes and brought out for examination. XRD of the cast sample showed the presence of Fe as the major phase. Graphite was identified to occur as a minor phase besides small amounts of Fe304, Fe2C, Fe3C. On examination of polished sample under optical microscope, spheroidal structures of graphite was distinctly marked. Density of graphite nodules was found in the range 1200-1310 per mm2 with nodule size varying between 12 to 16 urn. Chemical analysis of the Spheroidal graphite product found the various
impurities in the following levels (wt%) : C 4.5, S 0.12, P 0.06, Si 2.24 and Mg 0.02. Yield of the product was determined to be 93%.
1 Kg of the beneficiated iron ore slime concentrate was taken and thoroughly mixed with LECO by 25% excess over stoichiometric requirement. CaO and MgO were then added in calculated quantities to vary the slag basicity from 1 to 2 and study the yield at five different slag basicity. Thus five numbers of samples, one Kg each with varying basicity in the above mentioned range, were prepared. Following the procedure described in example 1, the charges were smelted and nodularization of graphite was carried out. Argon flow rate was maintained at 1 lit / min and the maximum arc length at which arc was stabilised was 5 cm. The arc current and voltage were found to vary in the range 300-500 A and 30-50 V respectively. Smelting time was recorded to be 45 minutes to complete the reduction reaction. The melt temperature at the time of melting was monitored (by pyrometric technique) and determined to be 1600-1650°C. lOg of Fe-40% Si-9% Mg alloy was added to the melt at the time of casting in a similar way as described in the first example. The cast SG iron product was cooled in the graphite mould for 50-60 minutes and taken out for examination and characterization. XRD analysis established Fe to occur as the major phase while graphite,Fe2C, Fe3C and Fe304 were detected as minor phases. Optical microscopy of the polished specimens confirmed the spheroidized structure of graphite with nodule density in the range 1150-1300 per mm2 and size of graphite spherulite was observed in the range 10-14 ujn. Chemical analysis of the Spheroidal graphite product found the impurities in the following percentage (wt%) range: C
4.45-6, S 0.11-0.16, P 0.05-0.069, Si 2.1-2.3 and Mg 0.025- 0.05. Yield of the product varied between 65 to 93%.
2 Kg of charge was processed at 1.8 slag basicity following the procedure described in example 1. The following experimental conditions were maintained :
Ar flow rate : 1.5 lit/min
Arc voltage : 30-50 V
Arc current : 350-550 A
Maxm arc length : 5 cm
Melt temperature : 1600-1700°C
Smelting time : 50-60 min
Cooling time in mould : 60 min
25 g of Fe-40% Si-9% Mg alloy was added to the melt in the graphite mould to nodularize graphite. XRD analysis identified Fe to be the major phase along with the occurrence of minor phases like graphite, Fe2C, Fe3C, Fe3C, Fe3C>4. From optical microscopic study the spheroidization of graphite was confirmed. Nodule density was found in the range 1100-1200 per mm2. Chemical analysis of the product showed the impurity contents at the following levels (wt.): C 5.7, S 0.14, P 0.068, Si 2.5 and Mg 0.05. Yield was found to be equal to the yield obtained at 1.5 slag basicity (93%).
The main advantages are
1. Spheroidal graphite iron can be directly prepared from beneficiated iron ore slime concentrate by smelting in an extended arc plasma furnace and adding Mg to the melt in the form of Fe-Si-Mg alloy in the mould.
2. The process is a single step process which uses a mineral waste to produce a high valued industrial product like SG iron.
3. Compared to the total energy consumed in the production of pig iron / cast iron from iron ores and their subsequent conversion to SG quality, the said process reduces energy by 3-4 kWh per kg of SG product.
4. The process makes use of low cost extended arc thermal plasma formed by arcing between two graphite electrodes (non transferred mode) and graphite electrode and charge (transferred mode) while argon gas is continuously fed through the cathode.
5. The said process accepts both powdery as well as agglomerated charge in the furnace. Thus, beneficiated iron ore slime concentrates which are in fines form can be directly charged into the furnace, thereby saving cost due to pelletization.
6. The process saves smelting time to the extent of one hour and more (up to 2 kg charge processing) compared to conventional coal fired furnace.
7. The process produces SG iron with high density of graphite nodules (>1100 per mm2) and size ranging between 10-16 \xm.
8. The process produces SG iron which does not require heat treatment after casting.
We claim :
1. A process for preparation of spheroidal graphite iron from beneficiated iron ore slime concentrate which comprises smelting a charge consisting of a mixture of the said slime concentrate, carbon in the form of LECO, CaO and MgO, in an electrically conducting crucible that forms the anode, employing carbon cathode and argon gas as plasma forming gas in a plasma furnace at a temperature 1550-1700°C in a time of 20-60 minutes and casting the melt in a graphite mould while adding Fe-40% Si-9% Mg alloy to the melt in the mould stage to nodularize graphite and recovering the solid SG iron product after cooling the mould in air for 30-60 minutes.
2. A process as claimed in clam 1 wherein carbon is used in the form of LECO (coke produced from lignite by low temperature carbonisation).
3. A process as claimed in claims 1 to 2 wherein graphite nodule density in the SG product is in the range 1400/mm2 and less and graphite spheroid size varies in the range 16 nm and less.
4. A process as claimed in claims 1 to 3 wherein the cast SG product can be directly used in various applications without any heat treatment.
5. A process as claimed in claims 1 to 4 wherein the critical impurity contents are maintained at following levels : C 4.4-6, S 0.11-0.16, P 0.05-0.069, Si 2.1-2.3, Mg 0.02-0.05 (wt%).
6. A process for production of spheroidal graphite iron from beneficiated iron ore slime concentrate substantially as herein described with reference to the examples.
|Indian Patent Application Number||258/DEL/2003|
|PG Journal Number||25/2009|
|Date of Filing||10-Mar-2003|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110001, INDIA.|
|PCT International Classification Number||C01G 49/00|
|PCT International Application Number||N/A|
|PCT International Filing date|