Title of Invention

"AN IMPROVED PROCESS FOR DEPHOSPHORIZATION OF HIGH CARBON FERROMANGANESE"

Abstract

An improved process for dephosphorization of high carbon ferro manganese which comprises : a. melting high carbon ferromanganese in a graphite crucible at a temperature in the range of 1250-1300°C b. adding iron oxide to the melt, c. removing silica containing slag from the resulting melt by known methods such as skimming, d. adding the flux composition such as herein described, into the molten metal at the temperature in the range of 1300 - 1350°C e. removing the resultant (Phosphorus-rich) slag to obtain high carbon ferro manganese by known density separation method.

Full Text

This invention relates to an improved process for dephosphorization of high carbon ferromanganese. This invention particularly relates to an improved process for dephosphorization of high carbon ferromanganese using a novel synergistic flux. Ferromanganese is used as deoxidising and alloying agent in steel making for improvement in strength toughness, wear resistance, hardness and hardenability. It improves rolling and forging quality of steel. It is often added as an additive in the form of ferroalloys during tapping of liquid steel. However, the phosphorous contained therein, nearly completely enters into the steel. Phosphorous is known to make steel brittle and susceptible to hot corrosion cracking and stress corrosion cracking. It is, therefore necessary to restrict the levels of phosphorous during the production of ferroalloy itself. In a hitherto known process, Fujita et. Al. (Tetsu-to-Hagane vol 74 1988 p.816) have used different fluxes such a BaCO3, BaCO3l Li2CO3, Na2CO3/NaF, CaCO3l CaCI2 for removal of phosphorus from liquid Fe-Mn-C-P alloy. The extent of dephosphorization achieved was reportedly very low with CaCO3 based fluxes. They carried out experiments on a 70 kg scale using a flux to metal (weight) ratio 0.04. For Mn contents in the alloy ranging from 5 to 60%, they could achieve a phosphorus distribution between slag and metal (mass pet p)/ [mass pet p] of 15 to 20 under favourable experimental conditions. Amongst the various fluxes investigated the BaCO3 based flux showed a higher extent of dephosphorisation than with Li2Cox, Na2CO3 based fluxes did. They also reported that for BaCO3 based fluxes, the extent of dephosphorisation was higher without adding BaCI2 than with BaCI2. Even though they have not reported the MnO content of the slag, a significant loss of Mn is suspected resulting into a high MnO content (>25%) in the slag which may be responsible for the lower degree of dephosphorisation. Lee et.al. have disclosed a process which is covered in U.S. Patent Number 4,752,327, 1987 and uses BaCO3, BaCO3/BaCl2 fluxes for treatment of de-siliconised ferromanganese. They demonstrated that phosphorous can be removed in the form of phosphate from ferromanganese under oxidizing conditions. The extent of dephosphorisation was less than 50% for a silicon content of 0.069%. It appears that due to dissociation of BaCO3 Mn gets excessively oxidized and decreased the phosphate capacity of the slag. Reference may be made to a recent investigation reported in the proceedings of the 6 International Iron and Steel congress, 1990 Nagoya, ISIj, where in Less examined the effect of substitut9on of BaCO3 with Na2CO3, CaCO3, BaF2, BaCI2, CaF2 and Si02 in different proportions. He found that the degree of dephosphorization is enhanced by the addition of BaF2 but deteriorated with the additions of BaCI2, CaF2 and Si02. The maximum extent of removal of phosphorus was about 50% for silicon content less than 0.2% in the alloy. He measured oxygen potential (PO2) at different silicon contents of the alloy and reported that dephosphorisation does not take place when the initial silicon content of the alloy is 0.6%. At this value, PO2 corresponding to Mn-MnO equilibria reduces to 1.10"18 atm which represents the oxygen potential where phosphorus exists in the form of phosphate as well as phosphide. The lower degree of dephosphorisation when using flux of high phosphate capacity may be due to the presence of very high (>55%) MnO content in the slag. This is due to significant loss of Mn (7%) to the slag. In another investigation (Met Trans vol 20B, April 1993, p 339) Watanable et. Al. have reported that the BaO-MnO flux may not be useful for removal of phosphorus from conventional high carbon ferromanganese because (a) it is difficult to melt the flux near hot metal temperature and (b) the equilibrium partition ratio of phosphorus between slag and metal decreases at high Manganese content of the alloy and high temperature. Considering all these known information it can be summarised that the fluxes used so far for the dephosphorisation of ferromanganese suffer from the following drawbacks: a) Lower Phosphate capacity due to improper composition of the flux used. b) Presence of higher MnO content in the slag resulting in lowering of phosphate capacity due to excessive oxidation of Manganese under oxidizing condition. c) Difficulty in melting BaO-MnO Flux at moderate temperatures. ( At present high carbon ferromanganese produced contains about 0.4% phosphorus. There are no methods currently known for reducing the phosphorus levels in high carbon ferromanganese. Previous attempts at phosphorus removal from liquid ferromanganese by conventional methods under oxidizing conditions could not attain the desired level of phosphorus ( The main object of the present invention is therefore to provide an improved process for dephosphorization of high carbon ferromanganese . Another object of the present invention is therefore to provide an improved process for dephosphorization of high carbon ferromanganese using a synergistic flux composition which would melt at moderate temperatures ( Accordingly to another feature of this invention, there is an improved process for dephosphorisation of high carbon ferromanganese which comprises : a. melting high carbon ferromanganese in a graphite crucible at a temperature in the range of 1250-1300°C, b. adding iron oxide to the melt, c. removing silica containing slag from the resulting melt by known methods such as skimming, d. adding the flux composition such as herein described, into the molten metal at the temperature in the range of 1300 - 1350°C, e. removing the resultant (Phosphorus-rich) slag to obtain high carbon ferro manganese by known density separation method. The present invention uses a Barium oxide based flux which is a novel synergistic flux composition useful for dephosphorization of high carbon ferro manganese which comprises a mixture of Barium carbonate, Barium fluoride / Barium chloride, MnO in the range of 50-80, 10-30, 15-25 weight percent respectively. Various compositions of BaO-BaF2 MnO slag were tried in order to melt the flux near the temperature of FeMn alloy. At a specific composition range (BaO=35-60%, BaF2 = 15-25%, MnO=10-25%), it was possible to melt the flux in the temperature range 1300-1350°C Thus reaction between metal and slag progressed smoothly, resulting into successful transfer of phosphorous from metal to slag phase. It was observed that the in situ oxidation of Mn helps in the reduction of melting temperature of the BaO based flux. In the present work, the MnO content of the slag was kept at 15-25% whereas Lee reported MnO content in the range of 55-80% in slag. At a given content of BaO in the fluxes, an increase in MnO content lowers the BaF2 content in the slag; as a result the degree of dephosphorisation decreases. Also the presence of carbon from graphite crucible at a particular MnO content in the range of 15-25%) helps in the melting of flux at lower temperature. Addition of BaF2 not only helps in reducing the viscosity of the slag but it also increases the activity of manganese oxide which is required for successful transfer of phosphorus into the slag phase. The composition of the flux in the present invention is so selected that MnO content of slag phase is just sufficient for providing oxygen potential for selective removal of phosphorus and bringing down the melting temperature of the salg to the desired level. If the oxygen potential is high, the formation of manganese oxide will be more than desired. High MnO content (>25%) increases the activity of phosphorus pentoxide and reduces the phosphate capacity of the BaO based fluxes. The composition of Fe Mn alloy is also important for deciding the composition and quantity of flux. The effectiveness of the flux increases at low Mn content of the alloy as the latter increases the equilibrium phosphorus distribution ratio between the slag and the metal phases. Accordingly, the consumption of the flux will be less. If the silicon content of the alloy is more than 0.2%, silica content of the slag will increase which may consume highly basic BaO baed flux without effecting dephosphorisation. Accordingly, the present invention provides a synergistic flux composition useful for the dephosphorisation of high carbon ferromanganese which comprises a mixture of Barium carbonate, Bariumfluoride or Barium chloride in the proportion in the range of 50-80, 10-30 weight percent respectively. High carbon ferromanganese used may be selected from the following composition range Mn=60-80%, Si=0.6-0.8% P=0.3-0.5%, C=5-7% Fe= balance Mill scale (Iron oxide) is added in the rang eof 5-7% of th ealloy weight for the purpose of desiliconization. The flux developed for dephosphorisation will be effective only when the silicon content of the alloy is less than 0.2%. The flux used contains BaCO3. BaF2 / BaCI2 and may be used in the range of of 10-20% of alloy weight. The degree of dephosphorization is known to be dependent on the initial composition of the ferromanganese to be treated, the quantity as well as the chemical composition of the flux / reagent used and the experimental conditions maintained for the chemical reaction between the liquid alloy and the flux. On melting, the flux forms a slag whose phosphate capacity increases sharply with rising basicity and a decreasing temperature. The table 1 provides dataon suitable flux composition, which on melting cause in - situ generation of BaO-BaX2-MnO) slags and an effective sink for absorbing phosphorus from the liquid alloy. TABLE 1 (Table Removed) The following examples are given by way of illustrations and should not be construed to limit the scope of the invention. EXAMPLE 1 One kg. of ferromanganese alloy was melted in a graphite crucible of induction furnace. 50 gm mill scale was added at a temperature around 1300°C. The siliconized slag removed with the help of a graphite rod. A metal sample was sucked into a silica tube of internal diameter 4 mm. The bath temperature was maintained at 1300°C. A flux consisting of 120 gm BaCO3 and 60 gm BaCI2 was thoroughly mixed and added within 5 minutes of removal of siliconized slag. The Melt was stirred with a graphite rod. The melt was maintained at 1300-1350°C. Metal and slag samples were collected within 30 mts of addition of flux for final analysis. The results are shown in table 2 . EXAMPLE 2 One kg. ferromanganese was melted in a graphite crucible for Induction furnace. 50 gm Mill scale ws added at a temperature of about 1300°C, The siliconized slag removed after 10 mts with the help of a graphite rod. A metal sample collected. A flux consisting of 120 gm BaCO3 and 60 gm BaF2 was thoroughly mixed and added to the melt. The temperature of melt was maintained at 1300°C. The melt was stirred with a graphite rod. A metal sample collected after 30 mts of addition of flux for final analysis. Results are shown in table 2. The degree of dephosphorization increased to 68% when BaF2 was used in place of BaCI2 was used in place of BaCI2 alongwith BaCO3. EXAMPLE 3 One kg. desiliconized ferromanganese was melted in a graphite crucible in a induction furnace. A flux consisting of 180 gm BaCO3 and 30 gm BaF2 was thoroughly mixed and added to the melt. The melt was stirred with a graphite rod the metal and slag samples collected after 15 mts. of addition of flux for final analysis. Results are shown in Table 2. Within 15 minutes of addition of flux 60% degree of dephosphorization has been achieved. Table 2shows the typical test results and the reagents used. TABLE 2 (Table Removed) The main advantages of the present invention are : 1 Ability to melt at moderate temperature (1300-1350°C) 2. High degree of dephosphorization (68%) as compared with other fluxes hitherto known 3. Significantly lower loss of Mn (1-5%) compared with 7% in other known processes 4. The cost of treatment is lower than its value addition 5. Causes no environmental problem. We have described and claimed a novel synergistic flux composition useful for dephosphorization of high carbon ferro manganese which comprises a mixture of Barium carbonate, Barium fluoride / Barium chloride, MnO in the range of 50-80, 10-30, 15-25 weight percent respectively, in our copending patent appln. No. 2459/del/95. We claim; 1. An improved process for dephosphorization of high carbon ferro manganese which comprises : a. melting high carbon ferromanganese in a graphite crucible at a temperature in the range of 1250-1300°C, b. adding iron oxide to the melt, c,. removing silica containing slag from the resulting melt by known methods such as skimming, d . adding the flux composition such as herein described, into the molten metal at the temperature in the range of 1300 - 1350°C e. removing the resultant (Phosphorus-rich) slag to obtain high carbon ferro manganese by known density separation method. 2. An improved process as claimed in claim 3 wherein iron oxide used is added in the range 5-7%alloy weight. 3. An improved process as claimed in claims 3 & 4 wherein the flux added is in the range of 10%-20% of the alloy weight. 4. An improved process for dephosphorization of high carbon ferro manganese substantially as herein described with reference to the examples 1 to 3.

Documents:

181-del-2004-abstract.pdf

181-del-2004-claims.pdf

181-del-2004-complete specification (granded).pdf

181-del-2004-correspondence-others.pdf

181-del-2004-correspondence-po.pdf

181-del-2004-description (complete).pdf

181-del-2004-form-1.pdf

181-del-2004-form-19.pdf

181-del-2004-form-2.pdf

181-del-2004-form-3.pdf

181-del-2004-form-5.pdf

181-del-2004-petition-138.pdf