Process for Dephosphorization of Molten Metal During a Refining Process

Abstract

Process for dephosphorization of molten metal during a refining process using a lime composition in the form of compacted particles having a Shatter Test Index of less than 20%, leading to a refined metal reduced in phosphorus components to the extent that the refined metal reduced in phosphorus is showing a phosphorus content lower than 0.02 w % based on the total weight of the refined metal reduced in phosphorus.

Claims

1. Process for dephosphorization of molten metal during a refining process comprising the steps of charging a vessel with hot metal and optionally scrap charging said vessel with a first lime composition. blowing oxygen into said vessel forming slag with said first lime composition charged into said vessel dephosphorization of hot metal to form a refined metal reduced in phosphorus components, and discharging said refined metal reduced in phosphorus components characterized in that said first lime composition comprises at least OM first calcium-magnesium compound fitting the formula
aCaCO.sub.3.Math.bMgCO.sub.3.Math.xCaO.Math.yMgO.Math.uI, wherein I represents impurities, a, b and u each being mass fractions ≧0 and ≦50%, x and y each being mass fractions ≧0 and ≦100%, with x+y≧50% by weight, based on the total weight of said at least one calcium-magnesium compound, said at least one calcium-magnesium compound being in the form of particles, said first lime composition having a cumulative calcium and magnesium content in the form of oxides greater than or equal to 20% by weight based on the total weight of the first lime composition, and being in the form of compacts, each compact being formed with compacted and shaped particles of calcium-magnesium compounds, said compacts having a Shatter Test Index of less than 20%, and in that said dephosphorization step of hot metal leads to a refined metal reduced in phosphorus component to the extent that the refined metal reduced in phosphorus is showing a phosphorus content lower than 0.02 w % based on the total weight of the refined metal reduced in phosphorus.

2. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said first lime composition comprises one second compound selected from the group consisting of B.sub.2O.sub.3, TIO.sub.2, calcium aluminate, calcium ferrite such as Ca.sub.2Fe.sub.2O.sub.5 or CaFe.sub.2O.sub.4, metallic iron, CaF.sub.2, and one or more oxides, said oxides being selected from the group consisting of oxides based on aluminum, an oxide based on iron, an oxide based on manganese and their mixture.

3. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said charging step of said vessel with first lime composition is performed simultaneously or separately with a charging step of said vessel with a second lime composition.

4. Process for dephosphorization of molten metal during a refining process according to claim 3, wherein said second lime composition comprises at least one compound selected from the group consisting of compound i), compound ii) and/or compound iii): i) calcium-magnesium compound under the form of pebble lime having a cumulative calcium and magnesium content in the form of oxides greater than or equal to 70% by weight based on the total weight of said calcium-magnesium compound, ii) calcium-magnesium compound fitting the formula
aCaCO.sub.3.Math.bMgCO.sub.3.Math.xCaO.Math.yMgO.Math.uI, wherein I represents impurities, a, b and u each being mass fractions ≧0 and ≦50%, x and y each being mass fractions ≧0 and ≦100%, with x+y≧50% by weight, based on the total weight of said at least one calcium-magnesium compound, said at least one calcium-magnesium compound being in the form of particles, said first lime composition having a cumulative calcium and magnesium content in the form of oxides greater than or equal to 20% by weight based on the total weight of the second lime composition, and being in the form of compacts, each compact being formed with compacted and shaped particles of calcium-magnesium compounds, said compacts having a Shatter Test Index of less than 10%, and iii) a first calcium-magnesium compound fitting the formula
aCaCO.sub.3.Math.bMgCO.sub.3.Math.xCaO.Math.yMgO.Math.uI, wherein I represents impurities, a, b and u each being mass fractions ≧0 and ≦50%, x and y each being mass fractions ≧0 and 100%, with x+y≧50% by weight, based on the total weight of said at least one calcium-magnesium compound, said at least one calcium-magnesium compound being in the form of particles, and one second compound chosen in the group consisting of B.sub.2O.sub.3, NaO.sub.3, TiO.sub.2, calcium aluminate, calcium ferrite such as Ca.sub.2Fe.sub.2O.sub.4, metallic iron, CaF.sub.2, C, and one or more oxides, said oxides being selected from the group consisting of oxides based on aluminum, an oxide based on iron, an oxide based on manganese and their mixture, said second lime composition having an cumulative calcium and magnesium content in the form of oxides greater than or equal to 20% by weight based on the total weight of the second lime composition, and being in the form of compacts, each compact being formed with compacted and shaped particles of calcium-magnesium compounds, said compacts having a Shatter Test Index of less than 20%.

5. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said refined metal reduced in phosphorus is showing a phosphorus content lower than 0.015 w % based on the total weight of the refined metal reduced in phosphorus.

6. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said compacts of first and/or second lime composition under the form of compacts have a Shatter Test Index of less than 8%.

7. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein the compacted first and/or second lime composition under the form of compacts presents a Shatter Test Index of less than 20% after an Accelerated Ageing Test of Level 1 at 30° C. under 75% of relative humidity (that is, 22.8 g/m.sup.3 of absolute humidity for 2 h.

8. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein the compacted first and/or second lime composition under the form of compacts presents a Shatter Test Index of less than 20% after an Accelerated Ageing Test of Level 2 at 40° C. under 50% of relative humidity (that is, 25.6 g/m.sup.3 of absolute humidity, for 2 h.

9. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein the compacted first and/or second lime composition under the form of compacts presents a Shatter Test Index of less than 20% after an Accelerated Ageing Test of Level 3 at 40° C. under 60% of relative humidity (that is, 30.7 g/m.sup.3 of absolute humidity, for 2 h.

10. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein the compacted first and/or second lime composition under the form of compacts presents a Shatter Test index of less than 20%, after an Accelerated Ageing Test of Level 4 at 40° C. under 70% of relative humidity (that is, 35.8 g/m.sup.3 of absolute humidity, for 2 h.

11. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said particles have a size of less than or equal to 7 mm, observable by optical microscopy or by scanning electron microscopy and before compaction having a size of particles d.sub.100 of less than or equal to 7 mm.

12. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said particles of said at least one calcium-magnesium compound before compaction have a d.sub.90 of less than or equal to 3 mm.

13. Process for dephosphorization of molten metal during a refining process according to claims 1, wherein said particles of said at least one calcium-magnesium compound before compaction have a d.sub.50 of less than or equal to 1 mm.

14. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said compacts are of a regular and homogeneous shape which is typical of products from methods for shaping fines via a dry route, such shapes being selected from the group consisting of lozenges, tablets, compressed tablets, briquettes, platelets and balls and have a size comprised between 10 and 100 mm.

15. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said compacts have an average, weight per compact of at least 1 g.

16. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said compacts have an average weight per compact of less than or equal to 200 g.

17. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said compacts have an apparent density comprised between 1.5 g/cm.sup.3 and 3 g/cm.sup.3.

18. Process for dephosphorization of molten metal during a refining process according to claim 1, wherein said compact includes a through-orifice.

Description

EXAMPLES

Comparative Example 1

[0110] A 6 tons universal converter was configured as standard BOF with one bottom tuyere and a water cooled oxygen lance with one oxygen nozzle was used.

[0111] The nozzle was positioned 160 cm above bath level from 0-50 Nm.sup.3, 150 cm above bath level from 51-100 Nm.sup.3 and 140 cm above bath level from 101 Nm.sup.3 until the end of blow. The flow rate was 17.0 Nm.sup.3/minute oxygen.

[0112] The flow rate of the bottom tuyere was kept constant at 433 Ndm.sup.3/minute nitrogen.

[0113] The converter was charged with 615 kg scrap material (Analysis: 1.14 w % manganese, 0.25 w % carbon, 0.26 w % silicon, 0.023 w % phosphorus, 0.24 w % copper, 0.17 w % nickel, 0.22 w % chromium, 97.5 w % iron, 0.014 w % sulfur, 0.04 w % titanium, 0.01 w % vanadium and 0.052 w % molybdenum with respect to the total weight of the scrap material), and with 174 kg pebble lime 10-50 mm (95 w % CaO, 1 w % MgO, 0.2 w % Al.sub.2O.sub.3, 0.7 w % SiO.sub.2, 0.3 w % Fe.sub.2O.sub.3, 0.2 w % SO.sub.3, 0.01 w % P.sub.2O.sub.5 with respect to the total weight of pebble lime) and 4970 kg of hot-metal (Analysis: 3.52 w % carbon, 0.024 w % sulfur, 0.25 w % silicon, 0.53 w % manganese and 0.078 w %=780 ppm phosphorus with respect to the total weight of the hot metal). 232 Nm.sup.3 oxygen were blown onto this mixture in 14 minutes with constant bottom stirring.

[0114] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % compared to the total volume of waste gas.

[0115] After obtaining the “after blow” slag- and steel-sample in tilted position after end of blow, the converter was raised again for post-stirring with 433 Ndm.sup.3/minute nitrogen for five minutes.

[0116] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0117] The phosphorus content of the steel-sample after stirring was 0.020 w % (200 ppm) with respect to the total weight of the steel sample at 1646° C. steel temperature.

Comparative Example 2

[0118] Heat was processed under the same operating conditions as in the comparative example 1. The converter described above was charged with 621 kg scrap with the same chemical composition as in comparative example 1, 174 kg of pebble lime 10-50 mm with the same chemical composition as in comparative example 1 and 4950 kg of hot-metal (Analysis: 3.70 w % carbon, 0.017 w % sulfur, 0.37 w % silicon, 0.47 w % manganese and 0.078 w % phosphorus with respect to the total weight of the hot metal). 241 Nm.sup.3 oxygen were blown onto the metal bath in 14 minutes.

[0119] The oxygen flow rate was 17.0 Nm.sup.3/min and the same lance program was used as in comparative example 1.

[0120] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % with respect to the total volume of waste gas.

[0121] After obtaining the “after blow” slag- and steel-samples in tilted position after end of blow, the converter was raised again for post-stirring with 433 Ndm.sup.3/minute nitrogen for four minutes.

[0122] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0123] The phosphorus content of the steel-sample after stirring was 0.020 w % with respect to the total weight of the steel sample (200 ppm) at 1646° C. steel temperature.

Example 1

[0124] The converter according to comparative example 1 was charged with 508 kg scrap material (Analysis: 1.14 w % manganese, 0.25 w % carbon, 0.26 w % silicon, 0.023 w % phosphorus, 0.24 w % copper, 0.17 w % nickel, 0.22 w % chromium, 97.5 w % iron, 0.014 w % sulfur, 0.04 w % titanium, 0.01 w % vanadium and 0.052 w % molybdenum with respect to the total weight of the scrap material), and 174 kg of a first lime composition comprising fines lime particles compacted together and showing a shatter test of 2.8%, presenting a diameter of 21 mm and an average thickness of 15 mm and having the same chemical composition than the pebble lime mentioned above and 4900 kg of hot-metal (Analysis: 3.74 w % carbon, 0.015 w % sulfur, 0.36 w % silicon, 0.32 w % manganese and 0.075 w % phosphorus with respect to the total weight of the hot metal).

[0125] 227 Nm.sup.3 oxygen were blown onto this mixture in 14 minutes with constant bottom stirring.

[0126] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % with respect to the total volume of waste gas.

[0127] After obtaining the “after blow” slag- and steel-samples in tilted position after end of blow, the converter was raised again for post-stirring with 433 Ndm.sup.3/minute nitrogen for five minutes.

[0128] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0129] The phosphorus content of the steel-sample after stirring was 0.014 w % based on the total weight of the steel sample (140 ppm) at 1662° C. steel temperature.

Example 2

[0130] The converter described in the references was charged with 700 kg scrap material with the same chemical composition as in comparative example 1, a mixture of 58 kg of a first lime composition made from compacted lime particles with the same composition as in example 1 and 117 kg pebble lime as in comparative example 1 and 4950 kg of hot-metal (Analysis: 3.72 w % carbon, 0.015 w % sulfur, 0.28 w % silicon, 0.38 w % manganese and 0.075 w % phosphorus based on the total weight of the hot metal). 255 Nm.sup.3 oxygen were blown onto the metal bath in 15 minutes.

[0131] The oxygen flow rate was 17.0 Nm.sup.3/min and the same lance program was used as in comparative example 1.

[0132] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % with respect to the total volume of waste gas.

[0133] After obtaining the “after blow” slag- and steel-sample in tilted position after end of blow, the converter was raised again for post-stirring with 509 Ndm.sup.3/minute nitrogen for six minutes.

[0134] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0135] The phosphorus content of the steel-sample after stirring was 0.014 w % based on the total weight of the steel sample (140 ppm) at 1680° C. steel temperature.

[0136] The mixture of 33% compacted lime composition as in example 1 and 66% pebble lime as in comparative examples 1 and 2 shows the same improvement as example 1.

Example 3

[0137] The converter described in the references was charged with 700 kg scrap material with the same chemical composition as in comparative example 1, a mixture of 87 kg of the first lime composition made from compacted fine lime particles as mentioned in example 1 and 97 kg of a second lime composition comprising lime particles and iron oxide compacted together and thermally treated at 1100° C. leading to a conversion of 80% of the iron oxide into calcium ferrite (mostly in the form of Ca.sub.2Fe.sub.2O.sub.5) and showing a shatter test of 1.0% and presenting a diameter of 21 mm and a thickness of 15 mm respectively (85 w % CaO, 1 w % MgO, 0.2 w % Al.sub.2O.sub.3, 0.7 w % SiO.sub.2, 10.5 w % Fe.sub.2O.sub.3, 0.2 w % SO.sub.3, 0.01 w % P.sub.2O.sub.5 based on the total weight of the second lime composition) and 4930 kg of hot-metal (Analysis: 3.70 w % carbon, 0.016 w % sulfur, 0.23 w % silicon, 0.340 w % manganese and 0.076 w % phosphorus based on the total weight of the hot metal). 250 Nm3 oxygen were blown onto the metal bath in 15 minutes.

[0138] The oxygen flow rate was 17.0 Nm min and the same lance program was used as described in the references.

[0139] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % with respect to the total volume of waste gas.

[0140] After obtaining the “after blow” slag- and steel-samples in tilted position after end of blow, the converter was raised again for post-stirring with 519 Ndm.sup.3/min nitrogen for four minutes.

[0141] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0142] The phosphorus content of the steel-sample after stirring was 0.014 w % based on the total weight of the steel sample (140 ppm) at 1672° C. steel temperature.

[0143] The mixture of 50% first lime composition (without fluxes) and 50% second lime composition doped with iron oxide and thermally treated shows the same improvement as example 1 and 2. The presence of iron doped second lime composition allows forming slag earlier in the process. The process behavior was improved in a way that the process was less noisy and that less sculling occurred during the process compared to comparative examples 1 and 2.

Example 4

[0144] The converter as in comparative example 1 was charged with 573 kg scrap material with the same chemical composition as in comparative example 1, 202 kg a first lime composition made from quicklime particles doped with manganese oxide and iron oxide compacted together and showing a shatter test of 2.9% presenting a diameter of 21 mm and a thickness of 15 mm respectively (82 w % CaO, 1 w % MgO, 0.2 w % Al.sub.2O.sub.3, 0.7 w % SiO.sub.2, 10.0 w % Fe.sub.2O.sub.3, 2.0 w % MnO, 0.2 w % SO.sub.3, 0.01 w % P.sub.2O.sub.5 based on the total weight of the first lime composition) and 4960 kg of hot-metal (Analysis: 3.60 w % carbon, 0.011 w % sulfur, 0.46 w % silicon, 0.45 w % manganese and 0.076 w % phosphorus with respect to the total weight of the hot metal). After four minutes of blowing, 20 kg the first lime composition doped with iron oxide and manganese oxide were added into the converter to compensate the high silicon-content of the hot-metal. 251 Nm.sup.3 oxygen were blown onto the metal bath in 15 minutes.

[0145] The oxygen flow rate was 17.0 Nm.sup.3/min and the same lance program was as in comparative example 1.

[0146] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % with respect to the total volume of waste gas.

[0147] After obtaining the “after blow” slag- and steel-samples in tilted position after end of blow, the converter was raised again for post-stirring with 520 Ndm.sup.3/minute nitrogen for seven minutes.

[0148] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0149] The phosphorus content of the steel-sample after stirring was 0.014 w % based on the total weight of the steel sample (140 ppm) at 1678° C. steel temperature.

[0150] The use of Fe-Mn-doped lime composition of compacted particles shows the same improvement as example 1 to 3.

[0151] The process behavior was improved in a way that the process was less noisy and that less sculling occurred during the process compared to comparative examples 1 and 2.

Example 5

[0152] The converter as in comparative example 1 was charged with 520 kg scrap material with the same chemical composition as in comparative example 1, 195 kg of the second lime composition used in example 3 and 4980 kg of hot-metal (Analysis: 3.74 w % carbon, 0.014 w % sulfur, 0.38 w % silicon, 0.44 w % manganese and 0.074 w % phosphorus with respect to the total weight of the hot metal). 258 Nm.sup.3 oxygen were blown onto the metal bath in 15 minutes. The oxygen flow rate was 17.0 Nm.sup.3/min and the same lance program was used as described in the reference.

[0153] The blowing of oxygen was controlled by off-gas analysis. The blowing-process was stopped when the CO.sub.2-content of the waste-gas dropped below 4 vol % with respect to the total volume of waste gas.

[0154] After obtaining the “after blow” slag- and steel-samples in tilted position after end of blow, the converter was raised again for post-stirring with 292 Ndm.sup.3/minute nitrogen for seven minutes.

[0155] The converter was tilted again, the “after stirring” slag- and steel-samples were obtained.

[0156] The phosphorus content of the steel-sample after stirring was 0.015 w % based on the total weight of the steel sample (150 ppm) at 1681° C. steel temperature.

[0157] The use of compacted lime particles doped with iron oxide and thermally treated under the form of a compacted first lime composition shows the same improvement as example 1 to 4.

Example 6

[0158] The converter as in comparative example 1 was charged similarly as in example 5 except that this time the lime composition comprising a compacted mixture of lime particles and iron oxide was not thermally treated and showed a shatter test of 2.5%. Such approach offers a performance similar than in example 4.

[0159] It will be understood by the man skilled in the art that the results according to the present invention have been obtained in a pilot scale and cannot be compared with industrial processes where particular optimization has already been developed. What can be deduced from those examples is that the compact lime composition used in the process according to the invention allows reducing the final phosphorous content in the steel from 200 ppm (references) to 150-140 ppm (examples 1 to 6). In other words, the present invention allows a 30% reduction of the final phosphorous content in the steel which is quite outstanding.

[0160] Moreover, the results of examples 1 to 6 have been obtained at higher temperatures (1660-1680° C.) than the references (1646° C.). The person skilled in the art knows that it is harder to dephosphorize at higher temperature. Thus, for a same steel temperature, the improvement in the dephosphorization process obtained with the compacts of the invention, compared to the pebble lime reference, would be even higher than 30%. Therefore the examples according to the present have been performed in the worst case scenario but however still offer good results.

[0161] It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the appended claims.

[0162] For example, one can contemplate of course to add the compacts according to the present invention to conventional products used already in steel making such a sintered briquettes.

[0163] Alternatively, the compacts according to the present invention can also be used in a so-called “two-slag-process”. Such method consists of applying a second consecutive dephosphorization process to the refined metal in order to further reduce the phosphorus content. In this case, additional steps of removing the slag from the refined metal, followed by a second charging of the first lime composition, are performed before the discharging of the refined metal reduced in phosphorous components. The compacts of the present invention allow reducing drastically the time needed to perform such kind of process, due to an optimized slag formation.