Method for removing phosphorus from phosphorus-containing substance

Abstract

Proposed is a method for removing phosphorus from a phosphorus-containing substance which is applicable in an industrial scale so as to effectively reduce phosphorus contained in the phosphorus-containing substance. In this method, the phosphorus-containing substance used as a raw material for metal smelting or metal refining is reacted with a nitrogen-containing gas at a treatment temperature T (° C.) which is lower than a melting temperature (T.sub.m) of the substance, so that phosphorus is removed preferably in the form of phosphorus nitride (PN). In this regard, a nitrogen partial pressure and an oxygen partial pressure in the nitrogen-containing gas are preferably controlled, thereby reducing a load of dephosphorization process, for example.

Claims

1. A method for removing phosphorus from a phosphorus-containing substance, characterized in that the phosphorus-containing substance used as a raw material for metal smelting or metal refining is reacted with a nitrogen-containing gas having an oxygen partial pressure P.sub.O2 of less than air at a treatment temperature T(° C.) which is lower than a melting temperature (T.sub.m) of the phosphorus-containing substance.

2. The method for removing phosphorus from a phosphorus-containing substance according to claim 1, wherein phosphorus nitride (PN) is formed by a reaction of the phosphorus-containing substance with the nitrogen-containing gas and removed.

3. The method for removing phosphorus from a phosphorus-containing substance according to claim 2, wherein the reaction of the phosphorus-containing substance with the nitrogen-containing gas is performed under control of a nitrogen partial pressure P.sub.N2 and the oxygen partial pressure P.sub.O2 in the nitrogen-containing gas.

4. The method for removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the nitrogen partial pressure P.sub.N2 (atm) in the nitrogen-containing gas is controlled to satisfy a condition represented by the following formula (1); [Formula 1]
0.2≤P.sub.N2≤0.9  (1).

5. The method of removing phosphorus from a phosphorus-containing substance according to claim 4, wherein the treatment temperature T (° C.) is controlled to satisfy a condition of the following formula (2) and the oxygen partial pressure P.sub.O2 (atm) in the nitrogen-containing gas is controlled to satisfy a condition of the following formula (3);
750≤T≤0.95×T.sub.m  (2) [Formula 2] wherein T.sub.m is a melting point (° C.) of the phosphorus-containing substance; [Formula 3]
log P.sub.O2≤−0.000025×T.sup.2+0.0723×T−60.9  (3).

6. The method for removing phosphorus from a phosphorus-containing substance according to claim 5, wherein the nitrogen-containing gas includes carbon monoxide (CO).

7. The method for removing phosphorus from a phosphorus-containing substance according to claim 6, wherein the nitrogen-containing gas includes carbon monoxide (CO) and carbon dioxide (CO.sub.2) and the oxygen partial pressure Poe is controlled by a partial pressure ratio P.sub.CO/P.sub.CO2.

8. The method for removing phosphorus from a phosphorus-containing substance according to claim 4, wherein the nitrogen-containing gas includes carbon monoxide (CO).

9. The method for removing phosphorus from a phosphorus-containing substance according to claim 4, wherein the nitrogen-containing gas includes carbon monoxide (CO) and carbon dioxide (CO.sub.2) and the oxygen partial pressure Poe is controlled by a partial pressure ratio P.sub.CO/P.sub.CO2.

10. The method of removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the treatment temperature T (° C.) is controlled to satisfy a condition of the following formula (2) and the oxygen partial pressure P.sub.O2 (atm) in the nitrogen-containing gas is controlled to satisfy a condition of the following formula (3);
750≤T≤0.95×T.sub.m  (2) [Formula 2] wherein T.sub.m is a melting point (° C.) of the phosphorus-containing substance; [Formula 3]
log P.sub.O2≤−0.000025×T.sup.2+0.0723×T−60.9  (3).

11. The method for removing phosphorus from a phosphorus-containing substance according to claim 10, wherein the nitrogen-containing gas includes carbon monoxide (CO).

12. The method for removing phosphorus from a phosphorus-containing substance according to claim 5, wherein the nitrogen-containing gas includes carbon monoxide (CO) and carbon dioxide (CO.sub.2) and the oxygen partial pressure Poe is controlled by a partial pressure ratio P.sub.CO/P.sub.CO2.

13. The method for removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the nitrogen-containing gas includes carbon monoxide (CO) and carbon dioxide (CO.sub.2) and the oxygen partial pressure Poe is controlled by a partial pressure ratio P.sub.CO/P.sub.CO2.

14. The method for removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the nitrogen partial pressure P.sub.N2 (atm) in the nitrogen-containing gas is controlled to satisfy a condition represented by the following formula (1); [Formula 1]
0.2≤P.sub.N2≤0.9  (1).

15. The method for removing phosphorus from a phosphorus-containing substance according to claim 14, wherein the nitrogen-containing gas includes carbon monoxide (CO).

16. The method of removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the treatment temperature T (° C.) is controlled to satisfy a condition of the following formula (2) and the oxygen partial pressure P.sub.O2 (atm) in the nitrogen-containing gas is controlled to satisfy a condition of the following formula (3);
750≤T≤0.95×T.sub.m  (2) [Formula 2] wherein T.sub.m is a melting point (° C.) of the phosphorus-containing substance; [Formula 3]
log P.sub.O2≤−0.000025×T.sup.2+0.0723×T−60.9  (3).

17. The method for removing phosphorus from a phosphorus-containing substance according to claim 16, wherein the nitrogen-containing gas includes carbon monoxide (CO).

18. The method for removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the nitrogen-containing gas includes carbon monoxide (CO).

19. The method for removing phosphorus from a phosphorus-containing substance according to claim 3, wherein the nitrogen-containing gas includes carbon monoxide (CO) and carbon dioxide (CO.sub.2) and the oxygen partial pressure P.sub.O2 is controlled by a partial pressure ratio P.sub.CO/P.sub.CO2.

20. The method for removing phosphorus from a phosphorus-containing substance according to claim 1, wherein the nitrogen-containing gas includes carbon monoxide (CO).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing a relation between a treatment temperature T (° C.) and an oxygen partial pressure (log P.sub.O2) when both equilibrium reactions of (a): reaction for removing phosphorus as a gas of PN and (d): equilibrium reaction between solid carbon and carbon monoxide gas are satisfied.

(2) FIG. 2 is a graph showing a relation between a nitrogen partial pressure (P.sub.N2) and a phosphorus removal ratio (ΔP) of iron ore at a treatment temperature T=1000° C.

(3) FIG. 3 is a graph showing a relation between a treatment temperature T (° C.) and a phosphorus removal ratio (ΔP) of iron ore at P.sub.CO=0.1 atm and P.sub.N2=0.9 atm.

(4) FIG. 4 is a graph showing an influence of a treatment temperature T (° C.) and an oxygen partial pressure (log P.sub.O2) upon a phosphorus removal ratio shown in Table 2.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(5) In developing aspects of the present invention, the inventors have focused on a substance that is inexpensive and has a high phosphorus concentration as a main raw material and an auxiliary raw material for metal smelting and metal refining, and have studied a method for preliminarily removing phosphorus from such a phosphorus-containing substance.

(6) The phosphorus-containing substance mentioned above used as a raw material (main raw material and auxiliary raw material) for metal smelting and metal refining usually contains phosphorus as an oxide such as P.sub.2O.sub.5 mainly and further contains a metal oxide such as CaO, SiO.sub.2, MgO, Al.sub.2O.sub.3, MnO, Mn.sub.2O.sub.3, FeO, Fe.sub.2O.sub.3 and so on. The raw material for metal smelting and metal refining, especially raw material for iron- and steel-making includes, for example, iron ore, manganese ore, steelmaking slag and so on, typical components of which are shown in Table 1.

(7) TABLE-US-00001 TABLE 1 (mass %) CaO SiO.sub.2 MgO Al.sub.2O.sub.3 T•Mn T•Fe P.sub.2O.sub.5 Iron ore — 3.5 — 1.4 — 63 0.2 Manganese ore 0.4 4.1 0.2 8.1 50.1 0.8 0.2 Steelmaking slag 41 13.8 6.1 5.6 1.6 18.7 1.6

(8) As mentioned above, the main raw material and the auxiliary raw material for metal smelting and metal refining (hereinafter, an explanation will be made taking “a raw material for iron- and steel-making” as an example) comprises various metal oxides. Since phosphorus has a weak affinity with oxygen compared to calcium (Ca) and silicon (Si), it is known that P.sub.2O.sub.5 in the phosphorus-containing substance is easily reduced in a reduction of the phosphorus-containing substance by carbon, silicon, aluminum and so on. On the other hand, iron is included in various raw materials for iron- and steel-making as an oxide in the form of FeO or Fe.sub.2O.sub.3 (hereinafter, abbreviated as “FexO”). Since the affinity of these iron oxides with oxygen is comparable to that of phosphorus, FexO is also reduced when the phosphorus-containing substance is reduced by carbon, silicon, aluminum and so on. In this regard, manganese is included as an oxide in the form of MnO, Mn.sub.2O.sub.3 or MnO.sub.2 (hereinafter, abbreviated as “MnxO”). Since the oxide of manganese is strong in affinity with oxygen compared to that with phosphorus but weak compared to that with carbon, silicon, aluminum and so on, MnxO is also reduced together with phosphorus when the phosphorus-containing substance is reduced by these substances.

(9) Phosphorus has a high solubility into iron or manganese, and especially, phosphorus formed by reduction is quickly dissolved into iron or manganese that are formed through reduction, thus forming a high phosphorus-containing iron or a high phosphorus-containing manganese. Therefore, the method for removing phosphorus formed by reduction has a problem that a phosphorus removal ratio is low because phosphorus is absorbed and dissolved into iron and manganese which are valuable components.

(10) As a result of diligent research to solve the problem, the inventors have found out that it is possible to perform a treatment under a temperature and oxygen partial pressure at which a metal iron and a metal manganese are not formed by removing phosphorus as a gas of phosphorus nitride (PN), and whereby absorption of phosphorus into iron and manganese can be suppressed.

(11) That is, the inventors have confirmed, by a thermodynamic consideration, that a reaction (a) represented by the following chemical equation 1 that removes phosphorus existing as P.sub.2O.sub.5 in the phosphorus-containing gaseous substance of phosphorus nitride (PN) is more stable than reactions (b) and (c) described in the following chemical equations 2 and 3, respectively, in which iron oxide or manganese oxide included in the phosphorus-containing substance are reduced to form a metal iron or a metal manganese, respectively.
[Chemical equation 1]
2P.sub.2O.sub.5(l)+2N.sub.2 (g)=4PN (g)+5O.sub.2 (g)  (a)
[Chemical equation 2]
2FeO (s)=2Fe (s)+O.sub.2(g)  (b)
[Chemical equation 3]
2MnO (s)=2Mn (s)+O.sub.2 (g)  (c)

(12) FIG. 1 shows a relation between a temperature and an oxygen partial pressure when equilibrium is established in the reaction (a) represented by the chemical equation 1. Moreover, FIG. 1 also shows a relation between a temperature and an oxygen partial pressure that can be achieved by equilibrium between a solid carbon and a carbon monoxide gas (reaction (d) represented by the chemical equation 4) for comparison. Here, it is assumed that an activity of P.sub.2O.sub.5 is 0.001; N.sub.2 partial pressure is 0.9 atm; PN partial pressure is 0.001 atm; an activity of C is 1; and CO partial pressure is 1 atm.
[Chemical equation 4]
2CO (g)=2C (s)+O.sub.2(g)  (d)

(13) In FIG. 1, in a region where a temperature and an oxygen partial pressure are beneath respective lines of the reactions (a) and (d), the reaction progresses to the right side in (a) and (d). That is, in order to achieve a nitriding removal of phosphorus in the reaction (a), it is necessary to control the oxygen partial pressure to not more than 2.2×10.sup.−19 atm at 800° C., not more than 1.45×10.sup.−14 atm at 1000° C. and not more than 4.66×10.sup.−11 atm at 1200° C.

(14) Here, in order to reduce the oxygen partial pressure, it is effective that an element such as a single element of Ca, Mg, Al, Ti, Si, C or the like, which is stable when formed into an oxide, is coexistent. However, the single metallic element is expensive. Accordingly, in accordance with aspects of the present invention, it is preferable to reduce the oxygen partial pressure by using carbon (C), from a viewpoint of reducing the treatment cost. It can be also understood by the fact that, as seen in FIG. 1, the oxygen partial pressure achieved by a solid carbon shows an sufficient value for proceeding the reaction (a) of a nitriding removal of phosphorus at a temperature of not lower than 724° C.

(15) Then, based on the research results mentioned above, an experiment is conducted to examine whether the nitriding removal of phosphorus is performed. In this experiment, 10 g of iron ore controlled to have a particle size of 1 to 3 mm is used as the phosphorus-containing substance, and 5 g of a reagent carbon (particle size of under 0.25 mm) is used as the solid carbon. Then, they are put on different boats made of alumina and kept stably in a small electric resistant furnace. In the furnace, the atmosphere is heated up to a predetermined temperature (600 to 1400° C.) while Ar gas is supplied at 1 liter/min. Thereafter, the supply of Ar gas is stopped, a mixture gas of carbon monoxide (CO) and nitrogen (N.sub.2) is supplied at 3 liter/min instead of Ar gas, and it is maintained at a constant temperature for 60 minutes. In this case, the ratio of the mixture gas of carbon monoxide and nitrogen is varied so that a nitrogen partial pressure P.sub.N2 is changed within a range of 0 to 1 atm. The supply of the mixture gas of carbon monoxide and nitrogen is stopped after a lapse of a predetermined time, and Ar gas is supplied at 1 liter/min instead, and iron ore is collected after the temperature is decreased to a room temperature. In this examination, the reagent carbon reacts with carbon monoxide gas first by supplying the gas in such a manner that the side where the reagent carbon stands is an upstream side.

(16) FIG. 2 shows a relation between a phosphorus removal ratio (ΔP={(P concentration before examination)−(P concentration after examination)]/(P concentration before examination)) (%) calculated from component analysis results of iron ore before and after the treatment mentioned above is performed at 1000° C. and a nitrogen partial pressure (P.sub.N2) (atm). As seen from FIG. 2, except in the case a nitrogen partial pressure (P.sub.N2) is 0 or 1 atm, phosphorus is removed from the phosphorus-containing substance, and a high phosphorus removal ratio of not less than 60% is obtained especially in the range of 0.2 to 0.9 atm. The reason why the phosphorus removal ratio is low at the nitrogen partial pressure of less than 0.2 atm is considered that the nitrogen partial pressure is too low and the phosphorus removal by the reaction (a) does not sufficiently proceed in a predetermined treatment time. It is also considered that the supply amount of CO gas is small when the nitrogen partial pressure exceeds 0.9 atm and the oxygen partial pressure is increased by oxygen formed by the thermal decomposition of iron oxide in iron ore to suppress the reaction (a) of nitriding removal of phosphorus. This is also understood from a fact that phosphorus is not removed by a supply of 100% nitrogen gas (P.sub.N2=1 atm).

(17) FIG. 3 shows a relation between a phosphorus removal ratio (ΔP %) and a treatment temperature T (° C.), which is obtained from component analysis results of iron ore before and after the experiment in which the treatment is performed by using an mixture gas of CO=10 vol % (P.sub.CO=0.1 atm) and N.sub.2=90 vol % (P.sub.N2=0.9 atm). As seen from FIG. 3, a high phosphorus removal ratio is obtained in a range of 750 to 1300° C., which is preferable for a nitriding removal of phosphorus. The reason why the phosphorus removal ratio is low at a temperature of lower than 750° C. is considered partly because, as shown in FIG. 1, the oxygen partial pressure necessary for a nitriding removal of phosphorus cannot be achieved by a solid carbon at a temperature of not higher than 724° C. Moreover, at a temperature of 1350° C. and 1400° C., iron ore is in a state from a semi-molten state to a molten state, and collected sample is aggregated. As a result, a gap and a pore between iron ore particles are vanished and an interfacial area contacting with gas is largely reduced. In this regard, the melting point (T.sub.m) of iron ore measured by a differential thermal analysis is 1370° C., and a high phosphorus removal ratio is obtained at a temperature of 1300° C. which is 0.95 times thereof. Therefore, it considered preferable that the treatment temperature is not higher than “0.95×T.sub.m (° C.)” in order to maintain a reaction interfacial area for removal of phosphorus.

(18) Experiments are made to manganese ore and steelmaking slag having different particle sizes by using the same method. As a result, it can be seen in all conditions that a high phosphorus removal ratio is obtained when the nitrogen partial pressure (P.sub.N2) is in the range of “0.2 to 0.9 atm” and the treatment temperature T (° C.) is in the range of “not lower than 750° C. but not higher than 0.95×T.sub.m (° C.)” (here, T.sub.m is a melting point of manganese ore or steelmaking slag).

(19) As explained above, in order to remove phosphorus in the phosphorus-containing substance by nitriding, it is considered necessary to perform the treatment at a predetermined temperature and supply nitrogen at an environment of low oxygen partial pressure. An equipment for performing such a treatment may include an equipment capable of heating and atmosphere control such as electric furnace, rotary hearth furnace, kiln furnace, fluidized bed heating furnace, sintering machine and so on.

(20) Moreover, a method for the reducing oxygen partial pressure may be any of the following methods, as long as the predetermined oxygen partial pressure can be obtained:

(21) (1) a method of contacting a solid reducing agent and a nitrogen gas at a high temperature;

(22) (2) a method of mixing a reducing gas such as carbon monoxide, hydrogen, hydrocarbon and so on with a nitrogen gas; and

(23) (3) a method of removing oxygen by introducing a nitrogen gas into a solid electrolyte to which a voltage is applied.

(24) Then, iron ore is charged into a rotary hearth furnace having a scale of 5 ton/hr and subjected to a nitriding treatment, in which the treatment temperature, oxygen partial pressure and nitrogen partial pressure are controlled by adjusting amounts of fuel and oxygen which are supplied into a heating burner, a ratio thereof, and an amount of nitrogen gas supplied. An operation condition of this equipment is set such that a time from charging to discharging is 30 minutes, and a temperature measurement and a gas component analysis are performed where a charged sample is located after a treatment is performed for 15 minutes. The concentration of carbon monoxide (CO) (vol %) and the concentration of carbon dioxide (CO.sub.2) (vol %) in the gas are measured by an infrared gas analyzer, and the residue is treated as a concentration of nitrogen (vol %). Moreover, the oxygen partial pressure P.sub.O2 is calculated from the measurement value of the P.sub.CO/P.sub.CO2 ratio based on the following equation 4, wherein ΔG° is a standard free energy change of formation; T is a reaction temperature (K); K is an equilibrium constant (−); R is a gas constant (cal/(mol.Math.K)); P.sub.CO is a partial pressure of carbon monoxide (atm); P.sub.CO2 is a partial pressure of carbon dioxide (atm); and P.sub.O2 is a partial pressure of oxygen (atm).

(25) $\begin{array}{cc}\left[\mathrm{Equation}4\right]& \\ 2{\mathrm{CO}}_2\left(g\right)=2\mathrm{CO}\left(g\right)+O_2\left(g\right)\Delta G^{\circ }=134,300-40.74\times T\left(\mathrm{cal}/\mathrm{mol}\right)K=\exp \left(-\frac{\Delta G^{\circ }}{RT}\right)={\left(\frac{P_{\mathrm{CO}}}{P_{{\mathrm{CO}}_2}}\right)}^2.Math.P_{O_2}& \left(4\right)\end{array}$

(26) Tables 2 to 6 show the treatment conditions and examination results with respect to respective nitrogen partial pressures P.sub.N2. Nitrogen partial pressures P.sub.N2 of Tables 2, 3, 4, 5, and 6 are 0.2, 0.5, 0.9, 0.15 and 0.95 atm, respectively.

(27) TABLE-US-00002 TABLE 2 (P.sub.N2: 0.2 atm) Oxygen partial Gas composition pressure Appearance Temperature ° C. CO vol % CO.sub.2 vol % N.sub.2 vol % $\frac{P_{\mathrm{CO}}}{P_{\mathrm{CO}2}}$ P.sub.O2 atm logP.sub.O2 atm ΔP % after treatment Invention 750 53.15 26.85 20 1.98 4.15E−21 −20.38 60 Granular Example 1 Invention 750 72.84 7.16 20 10.17 1.57E−22 −21.80 60 Granular Example 2 Invention 800 53.42 26.58 20 2.01 8.76E−20 −19.06 62 Granular Example 3 Invention 800 72.79 7.21 20 10.10 3.47E−21 −20.46 62 Granular Example 4 Invention 1000 39.80 40.20 20 0.99 7.16E−15 −14.15 70 Granular Example 5 Invention 1000 72.83 7.17 20 10.16 6.82E−17 −16.17 70 Granular Example 6 Invention 1200 26.31 53.69 20 0.49 3.95E−11 −10.40 71 Granular Example 7 Invention 1200 66.55 13.45 20 4.95 3.87E−13 −12.41 71 Granular Example 8 Invention 1300 27.02 52.98 20 0.51 6.74E−10 −9.17 67 Granular Example 9 Invention 1300 66.46 13.54 20 4.91 7.27E−12 −11.14 67 Granular Example 10 Comparative 700 26.67 53.33 20 0.50 2.18E−21 −20.66 0 Granular Example 1 Comparative 700 53.42 26.58 20 2.01 1.35E−22 −21.87 3 Granular Example 2 Comparative 700 72.75 7.25 20 10.03 5.42E−24 −23.27 3 Granular Example 3 Comparative 750 26.67 53.33 20 0.50 6.51E−20 −19.19 1 Granular Example 4 Comparative 800 27.02 52.98 20 0.51 1.36E−18 −17.87 1 Granular Example 5 Comparative 1000 3.81 76.19 20 0.05 2.81E−12 −11.55 1 Granular Example 6 Comparative 1200 3.81 76.19 20 0.05 3.80E−09 −8.42 1 Granular Example 7 Comparative 1300 3.81 76.19 20 0.05 7.02E−08 −7.15 1 Granular Example 8 Comparative 1400 3.81 76.19 20 0.05 9.15E−07 −6.04 1 Melt Example 9 Comparative 1400 26.67 53.33 20 0.50 9.15E−09 −8.04 1 Melt Example 10 Comparative 1400 66.73 13.27 20 5.03 9.04E−11 −10.04 1 Melt Example 11

(28) TABLE-US-00003 TABLE 3 (P.sub.N2: 0.5 atm) Oxygen partial Gas composition pressure Appearance Temperature ° C. CO vol % CO.sub.2 vol % N.sub.2 vol % $\frac{P_{\mathrm{CO}}}{P_{\mathrm{CO}2}}$ P.sub.O2 atm logP.sub.O2 atm ΔP % after treatment Invention 750 33.50 16.50 50 2.03 3.95E−21 −20.40 67 Granular Example 11 Invention 750 45.50 4.50 50 10.11 1.59E−22 −21.80 67 Granular Example 12 Invention 800 33.44 16.56 50 2.02 8.67E−20 −19.06 69 Granular Example 13 Invention 800 45.53 4.47 50 10.19 3.41E−21 −20.47 69 Granular Example 14 Invention 1000 25.25 24.75 50 1.02 6.75E−15 −14.17 78 Granular Example 15 Invention 1000 45.52 4.48 50 10.16 6.79E−17 −16.17 78 Granular Example 16 Invention 1200 16.89 33.11 50 0.51 3.65E−11 −10.44 79 Granular Example 17 Invention 1200 41.67 8.33 50 5.00 3.80E−13 −12.42 79 Granular Example 18 Invention 1300 16.67 33.33 50 0.50 7.02E−10 −9.15 72 Granular Example 19 Invention 1300 41.60 8.40 50 4.95 7.16E−12 −11.15 72 Granular Example 20 Comparative 700 16.44 33.56 50 0.49 2.27E−21 −20.64 0 Granular Example 12 Comparative 700 33.39 16.61 50 2.01 1.35E−22 −21.87 4 Granular Example 13 Comparative 700 45.41 4.59 50 9.89 5.57E−24 −23.25 4 Granular Example 14 Comparative 750 16.44 33.56 50 0.49 6.78E−20 −19.17 1 Granular Example 15 Comparative 800 16.67 33.33 50 0.50 1.41E−18 −17.85 1 Granular Example 16 Comparative 1000 2.38 47.62 50 0.05 2.81E−12 −11.55 1 Granular Example 17 Comparative 1200 2.38 47.62 50 0.05 3.80E−09 −8.42 1 Granular Example 18 Comparative 1300 2.38 47.62 50 0.05 7.02E−08 −7.15 1 Granular Example 19 Comparative 1400 2.38 47.62 50 0.05 9.15E−07 −6.04 0 Melt Example 20 Comparative 1400 16.67 33.33 50 0.50 9.15E−09 −8.04 3 Melt Example 21 Comparative 1400 41.69 8.31 50 5.02 9.08E−11 −10.04 3 Melt Example 22

(29) TABLE-US-00004 TABLE 4 (P.sub.N2: 0.9 atm) Oxygen partial Gas composition pressure Appearance Temperature ° C. CO vol % CO.sub.2 vol % N.sub.2 vol % $\frac{P_{\mathrm{CO}}}{P_{\mathrm{CO}2}}$ P.sub.O2 atm logP.sub.O2 atm ΔP % after treatment Invention 750 6.69 3.31 90 2.02 3.99E−21 −20.40 70 Granular Example 21 Invention 750 9.08 0.92 90 9.83 1.68E−22 −21.77 70 Granular Example 22 Invention 800 6.70 3.30 90 2.03 8.58E−20 −19.07 73 Granular Example 23 Invention 800 9.10 0.90 90 10.13 3.45E−21 −20.46 73 Granular Example 24 Invention 1000 5.02 4.98 90 1.01 6.88E−15 −14.16 81 Granular Example 25 Invention 1000 9.08 0.92 90 9.90 7.16E−17 −16.15 81 Granular Example 26 Invention 1200 3.33 6.67 90 0.50 3.80E−11 −10.42 82 Granular Example 27 Invention 1200 8.32 1.68 90 4.96 3.86E−13 −12.41 82 Granular Example 28 Invention 1300 3.29 6.71 90 0.49 7.30E−10 −9.14 75 Granular Example 29 Invention 1300 8.33 1.67 90 4.98 7.07E−12 −11.15 75 Granular Example 30 Comparative 700 3.38 6.62 90 0.51 2.10E−21 −20.68 0 Granular Example 23 Comparative 700 6.71 3.29 90 2.04 1.31E−22 −21.88 5 Granular Example 24 Comparative 700 9.08 0.92 90 9.85 5.63E−24 −23.25 5 Granular Example 25 Comparative 750 3.33 6.67 90 0.50 6.51E−20 −19.19 1 Granular Example 26 Comparative 800 3.33 6.67 90 0.50 1.41E−18 −17.85 1 Granular Example 27 Comparative 1000 0.48 9.52 90 0.05 2.81E−12 −11.55 1 Granular Example 28 Comparative 1200 0.48 9.52 90 0.05 3.80E−09 −8.42 1 Granular Example 29 Comparative 1300 0.48 9.52 90 0.05 7.02E−08 −7.15 1 Granular Example 30 Comparative 1400 0.48 9.52 90 0.05 9.15E−07 −6.04 0 Melt Example 31 Comparative 1400 3.33 6.67 90 0.50 9.15E−09 −8.04 1 Melt Example 32 Comparative 1400 8.32 1.68 90 4.94 9.37E−11 −10.03 1 Melt Example 33

(30) TABLE-US-00005 TABLE 5 (P.sub.N2: 0.15 atm) Oxygen partial Gas composition pressure Appearance Temperature ° C. CO vol % CO.sub.2 vol % N.sub.2 vol % $\frac{P_{\mathrm{CO}}}{P_{\mathrm{CO}2}}$ P.sub.O2 atm logP.sub.O2 atm ΔP % after treatment Comparative 700 27.95 57.05 15 0.49 2.27E−21 −20.64 1 Granular Example 34 Comparative 700 56.67 28.33 15 2.00 1.37E−22 −21.86 1 Granular Example 35 Comparative 700 77.28 7.72 15 10.01 5.45E−24 −23.26 1 Granular Example 36 Comparative 750 28.33 56.67 15 0.50 6.51E−20 −19.19 20 Granular Example 37 Comparative 750 56.85 28.15 15 2.02 3.99E−21 −20.40 20 Granular Example 38 Comparative 750 77.27 7.73 15 9.99 1.63E−22 −21.79 20 Granular Example 39 Comparative 800 28.33 56.67 15 0.50 1.41E−18 −17.85 22 Granular Example 40 Comparative 800 56.57 28.43 15 1.99 8.93E−20 −19.05 22 Granular Example 41 Comparative 800 77.17 7.83 15 9.86 3.64E−21 −20.44 22 Granular Example 42 Comparative 1000 4.05 80.95 15 0.05 2.81E−12 −11.55 30 Granular Example 43 Comparative 1000 42.29 42.71 15 0.99 7.16E−15 −14.15 30 Granular Example 44 Comparative 1000 77.24 7.76 15 9.95 7.09E−17 −16.15 30 Granular Example 45 Comparative 1200 4.05 80.95 15 0.05 3.80E−09 −8.42 30 Granular Example 46 Comparative 1200 28.33 56.67 15 0.50 3.80E−11 −10.42 30 Granular Example 47 Comparative 1200 70.67 14.33 15 4.93 3.90E−13 −12.41 30 Granular Example 48 Comparative 1300 4.05 80.95 15 0.05 7.02E−08 −7.15 28 Granular Example 49 Comparative 1300 27.95 57.05 15 0.49 7.30E−10 −9.14 28 Granular Example 50 Comparative 1300 70.95 14.05 15 5.05 6.88E−12 −11.16 28 Granular Example 51 Comparative 1400 4.05 80.95 15 0.05 9.15E−07 −6.04 1 Melt Example 52 Comparative 1400 28.33 56.67 15 0.50 9.15E−09 −8.04 1 Melt Example 53 Comparative 1400 71.00 14.00 15 5.07 8.90E−11 −10.05 1 Melt Example 54

(31) TABLE-US-00006 TABLE 6 (P.sub.N2: 0.95 atm) Oxygen partial Gas composition pressure Appearance Temperature ° C. CO vol % CO.sub.2 vol % N.sub.2 vol % $\frac{P_{\mathrm{CO}}}{P_{\mathrm{CO}2}}$ P.sub.O2 atm logP.sub.O2 atm ΔP % after treatment Comparative 700 0.01 4.99 95 0.00 1.36E−16 −15.87 0 Granular Example 55 Comparative 700 0.01 4.99 95 0.00 1.36E−16 −15.87 0 Granular Example 56 Comparative 700 0.01 4.99 95 0.00 1.36E−16 −15.87 0 Granular Example 57 Comparative 750 0.01 4.99 95 0.00 4.05E−15 −14.39 0 Granular Example 58 Comparative 750 0.01 4.99 95 0.00 4.05E−15 −14.39 0 Granular Example 59 Comparative 750 0.01 4.99 95 0.00 4.05E−15 −14.39 0 Granular Example 60 Comparative 800 0.01 4.99 95 0.00 8.81E−14 −13.06 0 Granular Example 61 Comparative 800 0.01 4.99 95 0.00 8.81E−14 −13.06 0 Granular Example 62 Comparative 800 0.01 4.99 95 0.00 8.81E−14 −13.06 0 Granular Example 63 Comparative 1000 0.01 4.99 95 0.00 1.75E−09 −8.76 0 Granular Example 64 Comparative 1000 0.01 4.99 95 0.00 1.75E−09 −8.76 0 Granular Example 65 Comparative 1000 0.01 4.99 95 0.00 1.75E−09 −8.76 0 Granular Example 66 Comparative 1200 0.01 4.99 95 0.00 2.36E−06 −5.63 0 Granular Example 67 Comparative 1200 0.01 4.99 95 0.00 2.36E−06 −5.63 0 Granular Example 68 Comparative 1200 0.01 4.99 95 0.00 2.36E−06 −5.63 0 Granular Example 69 Comparative 1300 0.01 4.99 95 0.00 4.37E−05 −4.36 0 Granular Example 70 Comparative 1300 0.01 4.99 95 0.00 4.37E−05 −4.36 0 Granular Example 71 Comparative 1300 0.01 4.99 95 0.00 4.37E−05 −4.36 0 Granular Example 72 Comparative 1400 0.01 4.99 95 0.00 5.70E−04 −3.24 0 Melt Example 73 Comparative 1400 0.01 4.99 95 0.00 5.70E−04 −3.24 0 Melt Example 74 Comparative 1400 0.01 4.99 95 0.00 5.70E−04 −3.24 0 Melt Example 75

(32) As apparent especially from Table 5 among the Tables, the phosphorus removal ratio is 30% at a maximum when the nitrogen partial pressure P.sub.N2 is 0.15 atm. That is, it is considered because the supply of nitrogen in an atmosphere gas is insufficient when the nitrogen partial pressure P.sub.N2 is 0.15 atm and the nitriding reaction (a) of phosphorus proceeds slowly and phosphorus is not sufficiently removed within 30 minutes which is a treatment time in this case.

(33) Moreover, as apparent from Table 6, a phosphorus removal is not confirmed at all when the nitrogen partial pressure P.sub.N2 is 0.95 atm. The reason thereof is considered as follows. Since the amount of the CO gas in the atmosphere is insufficient, oxygen formed by the thermal decomposition of iron ore and oxygen included in an involving air from a charging inlet for iron ore or a gap in the equipment are not completely removed. As a result, it is not possible to reduce to the oxygen partial pressure P.sub.O2 which is necessary for nitriding removal. It coincides with the fact that CO gas is barely detected in a gas analysis.

(34) On the other hand, in Invention Examples 1 to 30 which are conformity with the method according to aspects of the present invention described in Tables 2 to 4, the phosphorus removal ratio is as high as not less than 60%. From this fact, it can be seen that the nitrogen partial pressure P.sub.N2 (atm) satisfies not less than 0.2 but not more than 0.9 as a preferable condition so as to obtain a high phosphorus removal ratio.

(35) FIG. 4 is a graph showing a relation between a treatment temperature T and an oxygen partial pressure P.sub.O2 indicated in Table 2. Here, results that a phosphorus removal ratio is not less than 60% (invention examples 1 to 10) are plotted by ∘, and results that a phosphorus removal ratio is less than 10% (comparative examples 1 to 11) are plotted by X.

(36) As apparent from FIG. 4, a high phosphorus removal ratio is obtained when the following equations (2) and (3) are satisfied. Here, T is a treatment temperature (° C.) and T.sub.m is a melting point of the sample (iron ore: 1370° C.).
[Equation 2]
750≤T≤0.95×T.sub.m  (2)
[Equation 3]
log P.sub.O2≤−0.000025×T.sup.2+0.0723×T−60.9  (3)

(37) The reason why the phosphorus removal ratio is low when equations 2 and 3 are not satisfied is considered as follows. In Comparative Examples 1 to 3, the treatment is performed at a temperature of not higher than 700° C., and thus the low oxygen partial pressure necessary for removal of phosphorus by nitriding is not achieved by the oxygen partial pressure determined from CO—CO.sub.2 equilibrium. Moreover, in Comparative Examples 9 to 11, the treatment is performed at a temperature of 1400° C., which is not lower than a melting point of 1370° C. of iron ore as the sample, so that it is considered that the sample is melted and an inside pore and a gap between particles are vanished, resulting in large reduction of the interfacial area. In Comparative Examples 4 to 8, the temperature range represented by equation 1 is satisfied while the oxygen partial pressure does not satisfy equation 2, so that the low oxygen partial pressure necessary for removal of phosphorus by nitriding is not achieved.

(38) As a result of performing the same evaluation with respect to Invention Examples 11 to 30 and Comparative Examples 12 to 33 described in Tables 3 and 4, the same results as that of the above are obtained. Therefore, it is understood that a high phosphorus removal ratio of not less than 60% can be obtained when the conditions in equations 2 and 3 are satisfied.

(39) Even in the case that the treatment time is changed while the same equipment is used, it is confirmed that a high phosphorus removal ratio is obtained when a treatment temperature T, a nitrogen partial pressure P.sub.N2 and an oxygen partial pressure P.sub.O2 satisfy the above conditions.

INDUSTRIAL APPLICABILITY

(40) According to the technique disclosed in accordance with aspects of the present invention, it is possible to recycle the removed phosphorus nitride (PN) of an exhaust gas into phosphate fertilizer and so on such as a dust including P.sub.2O.sub.5 or the like.