DESULFURIZATION COMPOSITION

Abstract

A desulfurization composition includes calcium oxide, silicon dioxide and aluminum oxide. The desulfurization composition is free of an alkali metal oxide. Aluminum oxide is present in an amount ranging from 20 to 26 wt % based on the total weight of the desulfurization composition, and the weight ratio of calcium oxide to aluminum oxide is within a range of 2.19 to 3. A method for desulfurizing molten steel using the desulfurization composition is also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Patent Application No. 104103462, filed on Feb. 2, 2015, the entire disclosure of which is hereby incorporated by reference.

FIELD

The disclosure relates to a desulfurization composition, more particularly to a desulfurization composition for desulfurizing molten steel.

BACKGROUND

During the manufacture of steel in a steel mill, sulfur impurities may be removed via a desulfurization process so as to improve steel quality. Specifically, a desulfurization agent with a low melting point and low viscosity is added to molten steel. The temperature and the partial pressure of oxygen during this desulfurization process are typically set at about 1600° C. and under 20 ppm, respectively.

A conventional method used to increase the desulfurization efficiency of the desulfurization agent and to improve steel quality involves the addition of quicklime (calcium oxide, CaO). As the amount of added quicklime increases, the viscosity of the desulfurization agent may also increase to a point that impedes the desulfurization process. To counter this effect, calcium fluoride (CaF₂) may be added to decrease the viscosity of the desulfurization agent. However, calcium fluoride may erode a furnace lining of a furnace used in the manufacture of steel, and the fluorine evaporated during the desulfurization process would greatly harm the human body and environment.

CN 103882183 A discloses a calcium fluoride-free desulfurization refining slag containing 50-60 wt % of calcium oxide, 7-12 wt % of silicon dioxide (SiO₂), 28-33 wt % of aluminum oxide (Al₂O₃) and 4-8 wt % of magnesium oxide (MgO). The weight ratio of calcium oxide to aluminum oxide ranges from about 1.52 to 2.14, preferably from about 1.7 to 1.9. After desulfurization, the melted refining slag mainly contains 12CaO.Al₂O₃. However, the desulfurization efficiency of the desulfurization refining slag disclosed therein is quite low.

Therefore, the applicants have endeavored to find a calcium fluoride-free desulfurization composition that is associated with improved desulfurization efficiency.

SUMMARY

Therefore, a first object of the disclosure is to provide a desulfurization composition that can alleviate at least one of the drawbacks of the prior art.

According to the disclosure, the desulfurization composition includes calcium oxide, silicon dioxide and aluminum oxide. The desulfurization composition is free of an alkali metal oxide. Aluminum oxide is present in an amount ranging from 20 to 26 wt % based on the total weight of the desulfurization composition, and the weight ratio of calcium oxide to aluminum oxide is within a range of 2.19 to 3.

A second object of the disclosure is to provide a method for desulfurizing molten steel comprising mixing the aforesaid desulfurization composition with molten steel.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawings, of which:

FIG. 1 is an x-ray diffraction pattern of a desulfurization composition of Example 1 that is heated to 1600° C. and then cooled to below 30° C.

FIG. 2 shows a differential thermal analysis (DTA) curve of the desulfurization composition of Example 1;

FIG. 3 shows a DTA curve of a desulfurization composition of Comparative Example 1;

FIG. 4 shows the viscosity of the desulfurization composition of Example 1 at different temperatures;

FIG. 5 shows the viscosity of the desulfurization composition of Comparative Example 1 at different temperatures;

FIG. 6 shows the outer appearance of a magnesia carbon brick after immersion in the melted desulfurization composition of Example 1;

FIG. 7 shows an end surface of the magnesia carbon brick shown in FIG. 6;

FIG. 8 shows the outer appearance of a magnesia carbon brick after immersion in the melted desulfurization composition of Comparative Example 2;

FIG. 9 shows an end surface of the magnesia carbon brick shown in FIG. 8;

FIG. 10 shows the outer appearance of a magnesia carbon brick after immersion in the melted desulfurization composition of Comparative Example 5; and

FIG. 11 shows an end surface of the magnesia carbon brick shown in FIG. 10.

DETAILED DESCRIPTION

An embodiment of a desulfurization composition of this disclosure includes calcium oxide, silicon dioxide and aluminum oxide and is free of an alkali metal oxide. Based on the total weight of the desulfurization composition, aluminum oxide is present in an amount ranging from 20 to 26 wt %. The weight ratio of calcium oxide to aluminum oxide is within a range of 2.19 to 3.

In certain embodiments, the weight ratio of calcium oxide to aluminum oxide is within a range of 2.2 to 2.5.

In certain embodiments, aluminum oxide is present in an amount ranging from 24 to 26 wt %.

In certain embodiments, calcium oxide is present in an amount ranging from 57 to 60 wt % based on the total weight of the desulfurization composition. In certain embodiments, calcium oxide is present in an amount ranging from 57 to 59 wt %.

In certain embodiments, silicon dioxide is present in an amount ranging from 7 to 17 wt % based on the total weight of the desulfurization composition. In certain embodiments, silicon dioxide is present in an amount ranging from 10 to 14 wt %.

The desulfurization composition of the disclosure may further include magnesium oxide. Magnesium oxide may be present in an amount ranging from 1 to 10 wt % based on the total weight of the desulfurization composition. In certain embodiments, magnesium oxide is present in an amount ranging from 3 to 7 wt %.

The desulfurization composition of the disclosure may further include other components, including but not limited to dolomite [CaMg(CO₃)₂] or bauxite (aluminous soil).

The desulfurization composition of this disclosure can be mixed with molten steel under heating (at, e.g., 1600° C.) to perform desulfurization of molten steel. The desulfurization composition after heating includes Ca₃SiO₅ and 3CaO.Al₂O₃. Since Ca₃SiO₅ and 3CaO.Al₂O₃ have relatively low melting points and similar viscosities to that of CaF₂, the desulfurization composition of the disclosure exhibits better desulfurization efficiency for molten steel.

The disclosure will be further described by way of the following examples. However, it should be understood that the following examples are solely intended for the purpose of illustration and should not be construed as limiting the disclosure in practice.

EXAMPLES

Preparation of the Desulfurization Composition

Example 1 (E1)

Calcium oxide (Cao, 95% purity), aluminum oxide, silicon dioxide and magnesium oxide powders were mixed to form a desulfurization composition. The amounts of the aforesaid components based on the total weight of the desulfurization composition are listed in Table 1. It should be noted that calcium oxide may be obtained by heating calcium carbonate, and thus calcium oxide having a purity of 95% can be replaced by calcium carbonate having a purity of 98%.

	TABLE 1
	Amount (wt %)	CaO/Al2O3
	CaO	Al2O3	SiO2	MgO	(wt %/wt %)
	E1	58	25	12	5	2.32

Comparative Examples 1 to 5 (CE1-5)

Each of the desulfurization compositions of Comparative Examples 1 to 5 was prepared by mixing components at particular weight percentages as shown in Table 2.

TABLE 2
	Amount (wt %)	CaO/Al2O3
	CaO	Al2O3	SiO2	MgO	CaF2	Na2O	(wt %/wt %)
CE1	52	28	5	5	10	—	1.86
CE2	60	20	12	5	5	—	3
CE3	50	15	30	5	—	—	3.33
CE4	50	35	15	0	—	—	1.43
CE5	60	20	10	5	—	5	3
—: not added

X-ray Diffraction (XRD) Analysis

The desulfurization composition of Example 1 was placed in a graphite crucible and was heated to 1600° C. under 1 atm in a frequency furnace to evenly melt the desulfurization composition. The melted desulfurization composition was poured out from the graphite crucible, and was cooled to a temperature below 30° C. so as to obtain a treated desulfurization composition. The treated desulfurization composition was subjected to XRD analysis. The XRD pattern is shown in FIG. 1. FIG. 1 indicates that the treated desulfurization composition mainly includes Ca₃SiO₅ and 3CaO.Al₂O₃.

Differential Thermal Analysis (DTA)

The desulfurization compositions of Example 1 and Comparative Example 1 were subjected to differential thermal analysis using Linseis STA PT 1600. The heating and cooling rate was 2° C./min, and the results are shown in FIG. 2 (Example 1)and FIG. 3 (Comparative Example 1).

As shown in FIGS. 2 and 3, the melting point of the desulfurization composition of Example 1 is 1493° C., which is less than the melting point (1535° C.) of Comparative Example 1. These results demonstrate that a low melting point of the desulfurization composition can be obtained by adjusting the weight percentages of the components thereof without the addition of calcium fluoride.

Viscosity Analysis

The viscosities of the desulfurization compositions of Example 1 and Comparative Example 1 were measured by a viscometer(Brookfield-DV DI RV). The results are shown in FIG. 4 (Example 1) and FIG. 5 (Comparative Example 1).

As shown in FIGS. 4 and 5, the viscosities of the desulfurization composition of Example 1 at 1600° C. and 1550° C. (116 cP and 122 cP respectively) were less than those of Comparative Example 1 (122 cP and 129 cP). These results demonstrate that a low viscosity of the desulfurization composition can be obtained by adjusting the weight percentages of the components thereof without the addition of calcium fluoride.

Desulfurization Rate Analysis

Thermo-Calc software, coupled with TCS Steels/Fe-Alloys Database v7.0 and TCS Fe-containing Slag Database v3.2, was used to determine the desulfurization rate of the desulfurization compositions of Example 1 and Comparative Examples 1 to 3. The parameters listed in Table 3 were inputted into the software. The amount of the desulfurization composition was set to be 2 wt % based on the total weight of a mixture containing the desulfurization composition and molten steel.

                            TABLE 3
                            Inputted parameter
Main component of molten    Iron
steel
Initial amounts of carbon   Carbon: 0.05
and sulfur in molten steel  Sulfur: 0.025
(wt %)
Initial amounts of Ca, Si,  Initial amounts of Ca, Si, Al and Mg in the
Al and Mg in the            desulfurization composition were
desulfurization composition determined by the components of each of
(wt %)                      the desulfurization compositions of Example
                            1 and Comparative Examples 1 to 3
Initial amount of oxygen in Initial amount of oxygen in the mixture was
the mixture (wt %)          determined by the components of each of
                            the desulfurization compositions of Example
                            1 and Comparative Examples 1 to 3
Temperature                 1580~1620° C.

The graph obtained from the software shows the amounts of C, S, O, Ca, Si, Al and Mg included in the mixture after desulfurization. The amount of sulfur (wt %) at 1600° C. was used to calculate the desulfurization rate. The amount of oxygen (wt %) at 1600° C. was used to calculate a partial pressure (ppm) of oxygen by unit conversion.

The desulfurization rate of the desulfurization composition was calculated according to the following equation:

A=(B−C/B)×100%

• • where A=desulfurization rate (%)
    • B=initial amount of sulfur in molten steel(wt %)
    • C=the amount of sulfur in molten steel (wt %) at 1600° C.

<Results of Desulfurization Rate Analysis>

It should be noted that the partial pressure of oxygen is an important factor for the desulfurization rate. In the desulfurization process, the lower the partial pressure of oxygen is, the higher the desulfurization rate will be (i.e., the better the desulfurization effect will be).

a) Comparison between Example 1 and Comparative Example 1:

The amounts of sulfur after desulfurization and the desulfurization rates of Example 1 and Comparative Example 1 are listed in Table 4.

TABLE 4
			CaO/	Partial	Amounts of	Desul-
	Amount	Al2O3	pressure	sulfur after	furiza-
	(wt %)	(wt %/	of oxygen	desulfurization	tion rate
	CaF2	Al2O3	wt %)	(ppm)	(wt %)	(%)
E1	0	25	2.32	10.3	0.009	64
CE1	10	28	1.86	11	0.0083	66.8

As shown in Table 4, the desulfurization rates of Example 1 and Comparative Example 1 are quite close under conditions in which the temperature is 1600° C. and partial pressure of oxygen of the two examples are similar.

The results demonstrate that, without calcium fluoride, the desulfurization composition containing the components with particular

weight percentages still exhibits a high desulfurization rate.

(b) Comparison between Example 1 and Comparative Examples 3 and 4:

The amounts of sulfur after desulfurization and desulfurization rates of the desulfurization compositions of Example 1 and Comparative Examples 3 and 4 are listed in Table 5.

	TABLE 5
				Amounts of
		CaO/	Partial	sulfur
		Al2O3	pressure of	after	Desulfurization
	Al2O3	(wt %/	oxygen	desulfurization	rate
	(wt %)	wt %)	(ppm)	(wt %)	(%)
E1	25	2.32	10.3	0.009	64.0
			8.8	0.00786	68.6
			3.77	0.00408	83.7
			2.53	0.00272	89.1
			2.28	0.00213	91.5
			1.74	0.0016	93.6
CE3	15	3.33	9.7	0.0188	24.8
			4.2	0.0123	50.8
			2	0.0104	58.4
CE4	35	1.43	9.7	0.014	44
			4.97	0.0091	63.6
			2.6	0.0047	81.2

As shown in Comparative Example 3 of Table 5, in which the amount of aluminum oxide was less than 20 wt % and the weight ratio of calcium oxide to aluminum oxide was greater than 3, the desulfurization rate was 58.4 wt % when the partial pressure of oxygen was 2 ppm. The desulfurization rate in Comparative Example 3 was less than the desulfurization rate of Example 1 (83.7 wt %) when the partial pressure of oxygen was 3.77 ppm. Furthermore, in Comparative Example 4 in which the amount of aluminum oxide was greater than 26wt % and the weight ratio of calcium oxide to aluminum oxide was less than 2.19, the desulfurization rate was 81.2 wt % when the partial pressure of oxygen was 2.6 ppm. The desulfurization composition of Example 1 exhibits a better desulfurization rate under a relatively high partial pressure of oxygen (3.77 ppm). It can be predicted that the desulfurization composition of the disclosure would have a higher desulfurization rate than that of Comparative Examples at the same partial pressure of oxygen. The results reveal that the desulfurization rate can be effectively enhanced depending on the particular weight percentages of the components within the desulfurization composition of the disclosure.

Erosion Rate Analysis

It should be noted that the manufacture of steel is conducted at a relatively high temperature (e.g., 1600° C.), and a furnace lining of a high temperature furnace used in the manufacture of steel has to be made of a refractory material. The commonly used lining material is magnesia carbon brick. The following experiment was performed to evaluate the effect of the desulfurization compositions of Example 1 and Comparative Examples 2 and 5 on the erosion rate of the magnesia carbon brick.

Each of the desulfurization compositions of Example 1, Comparative Example 2 and Comparative Example 5 was placed in a graphite crucible, and then heated to 1600° C. in a frequency furnace until melted. A square columnar magnesia carbon brick was then immersed in a respective one of the resultant melted desulfurization compositions for sixty minutes. Before and after immersion, a projected area of an end surface of the magnesia carbon brick on a plane perpendicular to an axis of the columnar magnesia carbon brick was calculated by multiplying a width and a length of the end surface. Erosion rate (%) was calculated according to the following equation:

D=(E−F/E)×100%

• • where D=erosion rate
    • E=projected area of the end surface of the magnesia carbon bricks before immersion in the melted desulfurization composition
    • F=projected area of the end surface of the magnesia carbon bricks after immersion in the melted desulfurization composition for 60 minutes

The length, width and projected area of the end surface of the magnesia carbon brick before and after immersion as well as the calculated erosion rate are summarized in Table 7. It should be noted that a lower erosion rate indicates that the desulfurization composition causes less erosion to the magnesia carbon brick. The outer appearances of the magnesia carbon bricks after immersion in the melted desulfurization compositions of Example 1, Comparative Example 2 and Comparative Example 5 are respectively shown in FIGS. 6-7, 8-9, and 10-11. The white portion covering a peripheral region of the magnesia carbon brick is the desulfurization composition.

	TABLE 6
	Magnesia carbon brick
		Length	Width	Projected	Erosion
	Immersion	(mm)	(mm)	area (mm2)	rate (%)
E1	before	14.47	13.36	193.32	4.6
(w/o Na2O	after	14.10	13.08	184.43
and CaF2)
CE2	before	15.12	12.86	194.44	6.5
(with CaF2)	after	14.54	12.50	181.75
CE5	before	15.53	14.42	223.94	6.6
(with Na2O)	after	15.22	13.74	209.12

As shown in FIGS. 6-11, the magnesia carbon brick immersed in the melted desulfurization composition with CaF₂ (Comparative Example 2) or Na₂O (Comparative Example 5) is more seriously eroded as compared to the magnesia carbon brick immersed in the desulfurization composition without Na₂O and CaF₂ (Example 1). Table 6 also reveals that the desulfurization compositions of Comparative Examples 2 and 5 exhibit higher erosion rates than that of Example 1. These data demonstrate that the desulfurization composition of this disclosure exhibits less erosive effects on the magnesia carbon brick (lining material), thus extending the service life of the high temperature furnace used for the manufacture of steel and increasing the use frequency of the furnace.

To sum up, by adjusting the weight percentage of aluminum oxide and the weight ratio of calcium oxide to aluminum oxide, the desulfurization composition according to this disclosure could obtain a desired melting point and viscosity, and exhibit higher desulfurization efficiency and less erosion rate to the furnace lining.

While the disclosure has been described in connection with what are considered the exemplary embodiments, it is understood that this disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.

Claims

1. A desulfurization composition comprising calcium oxide, silicon dioxide and aluminum oxide, wherein said desulfurization composition is free of an alkali metal oxide, andwherein aluminum oxide is present in an amount ranging from 20 to 26 wt % based on the total weight of said desulfurization composition, and the weight ratio of calcium oxide to aluminum oxide is within a range of 2.19 to 3.

2. The desulfurization composition as claimed in claim 1, wherein calcium oxide is present in an amount ranging from 57 to 60 wt % based on the total weight of said desulfurization composition.

3. The desulfurization composition as claimed in claim 1, wherein silicon dioxide is present in an amount ranging from 7 to 17 wt % based on the total weight of said desulfurization composition.

4. The desulfurization composition as claimed in claim 1, further comprising magnesium oxide.

5. The desulfurization composition as claimed in claim 4, wherein magnesium oxide is present in an amount ranging from 1 to 10 wt % based on the total weight of said desulfurization composition.

6. A method for desulfurizing molten steel comprising mixing the desulfurization composition as claimed in claim 1 with molten steel.