PROCESS FOR MANUFACTURING A SLAG CONDITIONING AGENT FOR STEEL DESULFURIZATION

Abstract

Process for manufacturing a slag conditioning agent for steel desulfurization wherein a dried slag material obtained from secondary steelmaking process is mixed with quicklime particles to produce a slag conditioning agent.

Claims

1. Process for manufacturing a slag conditioning agent for steel desulfurization comprising the steps of: providing a slag material obtained from secondary steelmaking process, in particular a slag material obtained after an Al-killing steel process, said slag material comprising at least calcium, aluminum iron, moisture and a phase of calcium aluminate; mixing said slag material with quicklime particles having a predetermined maximum particle size, and drying said slag material by having at least a part of the quicklime particles reacting at least partially with a moisture contained in the slag material thereby obtaining a first blend comprising slag material, hydrated lime and optionally unreacted quicklime, sieving said first blend at a cut-size superior or equal to said predetermined maximum particle size of said quicklime to remove a passing fraction of said first blend from a retained dried fraction of said first blend, said passing fraction having a maximum particle size lower than said cut-size and comprising a majority of said hydrated lime; and mixing said retained dried fraction with a composition having an Al.sub.2O.sub.3 mass fraction of at least 80 wt % relative to the weight of the composition, and collecting a slag conditioning agent having an equivalent mass ratio CaO/Al.sub.2O.sub.3 comprised between 0.55 and 1.5.

2. Process according to claim 1 wherein the said slag material comprising at least calcium, aluminum iron, moisture and a phase of calcium aluminate comprises: an amount of calcium measured by X-ray fluorescence expressed in equivalent CaO comprised between 20 and 45 wt % relative to the weight of the slag material; an amount of aluminum measured by XRF expressed in equivalent Al.sub.2O.sub.3 comprised between 10 and 45 wt % relative to the weight of the slag material; and an amount of iron measured by XRF expressed in equivalent Fe.sub.2O.sub.3 of at least 2 wt % relative to the weight of the slag material, and wherein at least a part of said amount of calcium and at least a part of said amount of aluminum is present in said calcium aluminate phase

3. Process according to claim 1, wherein the said slag material comprises up to 20 wt % of iron expressed in equivalent Fe.sub.2O.sub.3 relative to the weight of the slag material.

4. Process according to claim 1, wherein the moisture of said slag material is present at a moisture content less than 10 wt % relative to the weight of the slag material.

5. Process according to claim 1, wherein the said quicklime particles comprises at least particles having a reactivity with water t.sub.60 according to the EN 459-2:2010E, less than 2 min.

6. Process according to claim 1, wherein the said quicklime particles comprises at least lime kiln dust particles.

7. Process according to claim 1, wherein the said quicklime particles have a BET specific surface area greater than 0.8 m.sup.2/g measured by nitrogen adsorption manometry after vacuum degassing at 190° C. for at least 2 hours, calculated by the multiple-point BET method as described in standard ISO 9277:2010 E.

8. Process according to claim 1, wherein the said slag material is crushed and optionally sieved to have a maximum particle size below a maximum value (b).

9. Process according to claim 1, wherein the said slag material is sieved to have a minimum particle size above a minimum value (a).

10. Slag conditioning agent for a process of steel desulfurization, comprising phases of calcium aluminate and an equivalent mass ratio CaO/Al.sub.2O.sub.3 comprised between 0.55 and 1.5, said slag conditioning agent having an iron content measured by XRF and expressed in equivalent Fe.sub.2O.sub.3 of at least 2 wt %, and an aluminum content expressed in equivalent Al.sub.2O.sub.3 of at least 30 wt %.

11. Slag conditioning agent according to claim 10, comprising an amount of calcium ferrites measured by XRD of at least 1 wt % relative to the weight of the slag conditioning agent.

12. Slag conditioning agent according to claim 10, having a particle size distribution comprised between a minimum value and a maximum value, said minimum value being of at least 1 mm.

13. Slag conditioning agent according to claim 10, having a particle size distribution comprised between a minimum value and a maximum value, said maximum value being of maximum 20 mm.

14. (canceled)

15. Use of slag conditioning agent according to claim 10 in a process of steel desulfurization, at an amount of 2 to 16 kg of slag conditioning agent/ton of steel, in combination with elemental aluminum in amounts less than 100 g/t of steel, and an amount of quicklime of less than 12 kg/t steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

Examples—

[0088] An example of a process for manufacturing a slag conditioning agent according to the present invention is described herein.

[0089] 25.2 ton of a slag obtained from secondary steelmaking process, in particular the slag obtained after Al-killing steel process, is crushed to obtain slag particles under 14 mm. After the step of crushing, a first sieving can be performed for removing the coarse particles that couldn't be crushed under 14 mm and those coarse particles can be re-injected in the crusher used for a further crushing in another production batch. 4 t of crushed slag particles are discarded. Then a second step of sieving is performed on the 21.2 t of crushed slag material particles to remove the fraction of crushed slag particles below 3 mm. 5.2 t of crushed slag particles under 3 mm are discarded and valorized for slag conditioning in steelmaking applications (passing fraction).

[0090] The elemental composition of the fraction (retained fraction) of slag material having a particle size in the range comprised between 3 and 14 mm is measured by XRF on samples dried at 105° C. This fraction comprises 24.6 wt % of Aluminum expressed in equivalent Al.sub.2O.sub.3, 41 wt % of calcium expressed in equivalent CaO, 16 wt % of iron expressed under equivalent Fe.sub.2O.sub.3, 5.5 wt % of magnesium expressed in equivalent MgO, 3 wt % of manganese expressed in Mn.sub.2O.sub.3, 7.1 wt % of silicon expressed in equivalent SiO.sub.2 and other impurities.

[0091] This fraction has a loss on drying (LOD) at 105° C. of 1.84%, corresponding to the moisture of the fraction and a loss on ignition (LOI) at 900° C. of 3.3 wt % which may correspond to the loss of hydroxides and/or carbonates in the phases present in the fraction. XRD analysis shows various phases of compounds including calcium aluminates and calcium ferrites.

[0092] 16 t of the fraction of slag particles between 3 and 14 mm is then dried by mixing with 1.6 t of high reactive quicklime having a particle size under 1 mm, a BET specific surface area superior to 0.8 m.sup.2/g and a reactivity t.sub.60 with water below 1 minute. The blend of slag particles and hydrated lime thereby obtained is sieved to remove the passing fraction of particles under 3 mm of particle size.

[0093] About 1.8 t of particles under 3 mm particle size is discarded. This latter fraction of discarded particles is valorized in applications of agriculture, sintering and slag conditions for steelmaking.

[0094] The elemental composition of the retained fraction at 3 mm (15.8 ton) of the blend of slag particles with quicklime having a particle size in the range comprised between 3 and 14 mm is measured by XRF on samples dried at 105° C. This fraction comprises 20.7 wt % of Aluminum expressed in equivalent Al.sub.2O.sub.3, 46.1 wt % of calcium expressed in equivalent CaO, 15.6 wt % of iron expressed under equivalent Fe.sub.2O.sub.3, 6.4 wt % of magnesium expressed in equivalent MgO, 2.7 wt % of manganese expressed in Mn.sub.2O.sub.3, 6.4 wt % of silicon expressed in equivalent SiO.sub.2 and other impurities. This fraction has a loss on drying (LOD) at 105° C. inferior to the level of detection which means that the fraction is well dried and a loss on ignition (LOI) at 900° C. of 4.58 wt % which may correspond to the loss of hydroxides and/or carbonates in the phases present in the fraction or in the added quicklime.

[0095] The same analyses are performed on the fraction of the blend of slag particles mixed with quicklime having a particle size below 3 mm. A sample of this fraction comprises 7.5 wt % of Aluminum expressed in equivalent Al.sub.2O.sub.3, 70.4 wt % of calcium expressed in equivalent CaO, 6.9 wt % of iron expressed under equivalent Fe.sub.2O.sub.3, 6.2 wt % of magnesium expressed in equivalent MgO, 0.9 wt % of manganese expressed in Mn.sub.2O.sub.3, 6.4 wt % of silicon expressed in equivalent SiO.sub.2 and other impurities. This fraction has a loss on drying (LOD) at 105° C. of 0.07% and a loss on ignition (LOI) at 900° C. of 24 wt % which to correspond to the loss of water molecules from hydrated lime. These measurements show that the moisture has been well removed.

[0096] 15.8 t of the fraction of the blend of slag particles with quicklime having a particle size in the range comprised between 3 and 14 mm is then mixed with 6.8 t of an alumina composition previously sieved to the same range of particle size between 3 and 14 mm, to obtain the slag conditioning agent according to the invention. The XRF analysis of the alumina composition shows that it comprises 86 wt % of aluminum expressed in equivalent Al.sub.2O.sub.3, 5.3 wt % of magnesium expressed in equivalent MgO, 5.5 wt % of silicon expressed in SiO.sub.2 equivalent, 2 wt % of zirconium expressed in ZrO.sub.2 equivalent, and other impurities.

[0097] The final elemental composition of the slag conditioning agent is measured by XRF analysis and contains 42.9 wt % of aluminum expressed in equivalent Al.sub.2O.sub.3, 28.9 wt % of calcium expressed in equivalent CaO, 12.8 wt % of iron expressed under equivalent Fe.sub.2O.sub.3, 4.1 wt % of magnesium expressed in equivalent MgO, 2.2 wt % of manganese expressed in equivalent Mn.sub.2O.sub.3, 7.2 wt % of silicon expressed in equivalent SiO.sub.2 and other impurities. The equivalent mass ratio CaO/Al.sub.2O.sub.3 is of 0.67. XRD analysis still shows various phases of compounds including calcium aluminates and calcium ferrites in less amounts than in the slag material because of the dilution effect with the alumina composition.

[0098] In a steel desulfurization process, the slag conditioning agent obtained according to the process of the present invention is used in an amount of 3 to 5 kg/t of steel, in combination with elemental aluminum in amounts of 600 steel and a determined amount of quicklime such as 10 kg/t steel. This steel desulfurization process is compared with a typical process wherein substantially pure calcium aluminate with an equivalent mass ratio CaO/Al.sub.2O.sub.3 of 0.5 is used in an amount of 5 kg/t of steel in combination with elemental aluminum in amounts of 40 g/t of steel and a determined amount of quicklime 10 kg/t steel.

[0099] Both processes show the same efficiency in term of desulfurization. Despite the steel desulfurization process using the slag conditioning agent obtained according to the invention requires slightly more elemental aluminum for reducing iron, the additional price of elemental aluminum is compensated by the cost-effective price of the slag conditioning agent compared to the price of substantially pure calcium aluminate.

[0100] Additional benefits of the process for manufacturing the slag conditioning agent according to the present invention is that slag material from secondary steelmaking process, in particular the slag obtained after Al-killing steel process, can be recycled. Also, some of the by-products obtained during the process can be valorized in other applications. In the process according to the invention, the step of drying the slag material with quicklime followed by the removal of hydrated lime dispense the use of a furnace for drying the slag material and thereby reduces the operation costs and the size of the production plant.

[0101] 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.