JP7670960B2 - Method for producing phosphate fertilizer - Google Patents

Description

The present invention relates to a method for producing a phosphate fertilizer having an excellent available phosphate rate.

Conventionally, dephosphorization slag produced by dephosphorization of molten iron containing a high concentration of P has been used as a raw material for phosphate fertilizer because it contains a large amount of phosphoric acid (P ₂ O ₅ ). Patent Document 1 discloses a raw material for phosphate fertilizer in which the slag contains, in mass % of a ternary system of CaO, P ₂ O ₅ , and SiO ₂ , 50 to 65% CaO, 15% or more P ₂ O ₅ , and less than 10% SiO ₂ . Patent Document 2 discloses a method for producing a phosphate fertilizer raw material in which, during the dephosphorization process, the amount of CaO-based dephosphorization agent sprayed together with oxygen gas is adjusted to produce slag with a basicity of 1.5 or more, resulting in a composition containing 15% or more _{of P2O5 ,} and further, a silicon source is added from the top during the middle to final stages of the blowing process in the dephosphorization process to adjust the composition to a ternary system of CaO, _P2O5 , and _SiO2 containing 10% or more of _SiO2 .

Patent Document 3 discloses _a phosphate fertilizer raw material which contains a total of 60 mass% or more of CaO, _SiO2 , _P2O5 , and iron oxide (calculated as Fe), has a basicity α expressed as the mass concentration ratio of CaO to _SiO2 of 1.5 to 3.0, contains 8 mass% or more ( ^-4α2 + 23α- ₄ ) mass% or less _{of P2O5} , and contains 5 mass% or more and 25 mass% or _less of iron oxide calculated as Fe, and in which the total concentration of one or _more of _Ca3 ( _PO4 ) ₂ - _Ca2SiO4 solid solution, _{5CaO.SiO2.P2O5} , and _{7CaO.2SiO2.P2O5} is _{28 mass} % or more. Patent Document 4 discloses a method for producing a phosphate fertilizer that produces slag with _SiO2 / _P2O5 ≧ _0.4 , CaO/ _P2O5 ≧ _2.0 , _{P2O5 ≧} 10%, and T.Fe: 5% to 15%.

JP 2015-140294 A JP 2015-140473 A JP 2016-88757 A JP 2017-128747 A

However, as in the techniques described in Patent Documents 1 to 4 above, the composition ranges proposed for phosphate fertilizers are varied and wide-ranging, and the current situation is that the fertilizer effect cannot be fully guaranteed.

In view of the above problems, an object of the present invention is to provide a method for producing a phosphate fertilizer having a stable and high fertilizer effect.

Conventionally, the soluble phosphate ratio has been commonly used as an index for evaluating the effectiveness of phosphate fertilizers. The soluble phosphate ratio is the ratio of the amount of phosphate in the fertilizer that dissolves in a 2% citric acid solution. Citric acid is a strong acid with a pH of 2 to 3, so the pH is lower than that of the actual soil environment. The inventors of the present invention therefore believe that the reason why the fertilizer effect of conventional phosphate fertilizers cannot be fully guaranteed is because the phosphate content is evaluated based on the amount of phosphate that dissolves in a citric acid solution, and have considered using the soluble phosphate ratio as an index of fertilizer effect. The soluble phosphate ratio is the ratio of the amount of phosphate in the fertilizer that dissolves in a 2% ammonium citrate solution, and ammonium citrate has a pH of 6 to 7, which is closer to the soil environment. Therefore, its solubility can be said to be an index that directly represents the ease of dissolving phosphate into the actual soil.

Furthermore, the inventors noticed that solid solutions of silicocarnotite and nagelschmittite exhibit high fertilizer effects, and discovered in more detail the region with high soluble phosphate content in the ternary system CaO--SiO ₂ --P ₂ O ₅ , thereby arriving at the present invention.

The present invention is as follows.
(1)
A method for producing a _phosphate fertilizer in which the _P2O5 concentration is 10% or more by mass , the concentration ratio of CaO _{to P2O5} (CaO)/(P2O5 ₎ is 2.3 to 4.2, and _the concentration ratio of _{SiO2 to} P2O5 (SiO2 ₎ /( _{P2O5 )} is 0.6 to 1.3 _,
performing a dephosphorization process on the molten iron so that the P concentration in the molten iron after the dephosphorization process is 0.10% or more by mass% and the value Δ[P] obtained by converting the amount of P in the dephosphorization slag at the end of the dephosphorization process into the P concentration in the molten iron satisfies the condition of the following formula (1);
Cooling and pulverizing the dephosphorization slag obtained by the dephosphorization treatment;
A method for producing a phosphate fertilizer, comprising the steps of:
0.72×[Si] _in ≦Δ[P]≦1.56×[Si] _in ...(1)
Here, [Si] _in represents the total amount of Si in the dephosphorization slag and the molten iron during the dephosphorization treatment, converted into the Si concentration in the molten iron (mass %).
(2)
The method for producing a phosphate fertilizer described above in (1) is characterized in that the amount of the dephosphorization slag is calculated from the target composition of the dephosphorization slag and the [Si] _in , and the type and amount of a dephosphorization flux containing at least CaO is determined.

The present invention provides a phosphate fertilizer with a stable and high fertilizer effect and a method for producing the same.

FIG. 1 is a diagram for explaining the difference in soluble phosphate ratio in a ternary system of CaO—SiO ₂ —P ₂ O ₅ . FIG. 2 is a graph showing the relationship between the phosphoric acid concentration in dephosphorization slag at equilibrium during dephosphorization treatment and the P concentration in molten iron.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
In order to increase the soluble phosphate content in dephosphorization slag, it is necessary for phosphoric acid (P ₂ O ₅ ) to be present as a solid solution in which it is soluble, i.e., silicocarnotite (5CaO-SiO ₂ -P ₂ O ₅ ) or nagelschmittite (7CaO-2SiO ₂ -P ₂ O ₅ ). Here, in order for all the phosphoric acid in the slag to exist as silicocarnotite, the molar ratio of CaO to _P2O5 must be 5 times, that is _, the concentration ratio of CaO to _P2O5 (CaO)/ _{( P2O5 )} when converted into a weight ratio must be 1.97 or more, and further _, the molar ratio of _SiO2 to _P2O5 must be 1 or more, that is _, the concentration ratio of _SiO2 to _P2O5 ( _SiO2 )/( _P2O5 ) when converted into _a weight ratio must be 0.42 or more.

Fig. 1 is a diagram for explaining the difference in soluble phosphate ratio in _the ternary system of CaO- _SiO2 - _P2O5 . As shown in Fig. 1, if the slag composition is present in a triangular region where the ratio (CaO)/ _{( P2O5} ) is 1.97 and _the ratio ( _SiO2 )/( _{P2O5 )} is 0.42, the necessary condition for all _P2O5 to become solid _solution silicocarnotite is satisfied. Similarly, the necessary condition for _{all P2O5} to become solid solution nagelschmittite is said to be within a triangular region where the ratio (CaO)/( _P2O5 ) is 2.76 and the ratio ( _SiO2 )/ _{( P2O5} ) is 0.85, but this region is completely included in the region of silicocarnotite.

However, when the slag has a composition where the ratio (CaO)/(P ₂ O ₅ ) is around 1.97 or the ratio (SiO ₂ )/(P ₂ O ₅ ) is around 0.42, the composition varies and locally there is a shortage of CaO or SiO ₂ relative to P ₂ O _5. As a result, insoluble β-3CaO-P ₂ O ₅ is likely to precipitate instead of a solid solution, and the soluble phosphoric acid rate drops sharply.

On the other hand, it is believed that there are upper limits for the ratios (CaO)/(P ₂ O ₅ ) and (SiO ₂ )/(P ₂ O ₅ ) to show a high soluble phosphate ratio. When the ratio (CaO)/(P ₂ O ₅ ) becomes high, in addition to the solid solution described above, 2CaO-SiO ₂ and 3CaO-SiO ₂ with low phosphate concentration and insoluble 3CaO-P ₂ O ₅ with high phosphate concentration are precipitated. Similarly, when the ratio (SiO ₂ )/(P ₂ O ₅ ) becomes high, CaO-SiO ₂ and SiO ₂ with low phosphate concentration and insoluble 3CaO-P ₂ O ₅ with high phosphate concentration are precipitated. As a result, it is believed that if the ratio (CaO)/(P ₂ O ₅ ) or the ratio (SiO ₂ )/(P ₂ O ₅ ) increases excessively, the soluble phosphate percentage gradually decreases.

The inventors conducted experiments based on the above conditions and found that the soluble phosphate rate peaks near the composition ratio of Nagelschmittite, and as shown in Figure 1, it was found that in dephosphorization slag, a soluble phosphate rate of 80% or more can be obtained by setting the ratio (CaO)/(P ₂ O ₅ ) to 2.3 to 4.2 and the ratio (SiO ₂ )/(P ₂ O ₅ ) to 0.6 to 1.3, in mass percentages.

When the ratio (CaO)/(P ₂ O ₅ ) is less than 2.3 or the ratio (SiO ₂ )/(P ₂ O ₅ ) is less than 0.6, insoluble β-3CaO-P ₂ O ₅ may precipitate, which reduces the soluble phosphoric acid percentage. When the ratio (CaO)/(P ₂ O ₅ ) is more than 4.2 or the ratio (SiO ₂ )/(P ₂ O ₅ ) is more than 1.3, in addition to the above-mentioned solid solution, 3CaO-SiO ₂ or 2CaO-SiO ₂ with low phosphoric acid concentration and insoluble 3CaO-P ₂ O ₅ with high phosphoric acid concentration, or CaO-SiO ₂ or SiO ₂ with low phosphoric acid concentration and insoluble 3CaO-P ₂ O ₅ with high phosphoric acid concentration are precipitated, which reduces the soluble phosphoric acid percentage.

Next, a method for producing phosphate fertilizer according to the present invention will be described. First, molten pig iron with a high concentration of P is charged into a converter, dephosphorization flux is added, and oxygen is blown from a top blowing lance to perform the dephosphorization process. Here, in order to ensure that the dephosphorization slag contains a high concentration of phosphorus as a result of the dephosphorization process, the P concentration in the molten pig iron after the dephosphorization process is set to 0.10 mass% or more. If the P concentration in the molten pig iron after the dephosphorization process is less than 0.10 mass%, the phosphate concentration in the equilibrium dephosphorization slag will drop sharply.

Fig. 2 is a diagram showing the relationship between the phosphoric acid concentration in the dephosphorization slag at equilibrium during dephosphorization and the P concentration in the hot _metal . As mentioned above, there is an optimum region in the ternary system of CaO- _SiO2 - _P2O5 , and in this optimum region, the lower limit of the ratio (CaO)/( _SiO2 ) is about 1.8. In order to ensure a phosphoric acid concentration of 10 mass% or more even under such low basicity conditions, it is clear that the P concentration in the hot metal after dephosphorization needs to be 0.10 mass% or more, as shown in Fig. 2.

The amount of dephosphorization slag generated by the dephosphorization process varies depending on the Si concentration in the molten iron before the dephosphorization process, and in order to obtain a composition in the optimum range in the ternary system of CaO- _SiO2 - _P2O5 , _the amount of dephosphorization can be calculated as a function of the Si concentration in the molten iron before the dephosphorization process. Here, the slag amount SV (kg/t) of the dephosphorization slag generated by the dephosphorization process can be calculated from the P balance and Si balance as shown in the following formula (2).
SV=Δ[P]×10×(142/62)/((P ₂ O ₅ )/100)
= [Si] _in ×10×(60/28)/((SiO ₂ )/100)...(2)
Δ[P] = [P] _in - [P] _f ...(3)

Here, Δ[P] is the amount of dephosphorization, i.e., the amount of P in the dephosphorization slag at the end of the dephosphorization process converted into the P concentration in the molten pig iron (mass%), [Si] _in is the total amount of Si in the dephosphorization slag and the molten pig iron during the dephosphorization process converted into the Si concentration in the molten pig iron (mass%), [P] _in is the total amount of P in the dephosphorization slag and the molten pig iron during the dephosphorization process converted into the P concentration in the molten pig iron (mass%), and [P] _f is the P concentration in the molten pig iron at the end of the dephosphorization process (mass%). Substituting the values to control the ratio ( _SiO2 )/( _P2O5 ) to 0.6-1.3, it is necessary to perform the dephosphorization process so that Δ[P] falls within the range of _the following formula (4).
0.72×[Si] _in ≦Δ[P]≦1.56×[Si] _in ...(4)

As described above, the slag volume SV of the dephosphorization slag generated in the dephosphorization process can be calculated from the target composition of the dephosphorization slag and the initial Si concentration [Si] _in , and the type and amount of the dephosphorization flux can be determined. The type of dephosphorization flux contains at least CaO, and may contain _SiO2 , MgO, _P2O5 , _etc. for the purpose of adjusting the basicity and further improving the soluble phosphate rate. Note that the _SiO2 and _P2O5 added as the dephosphorization flux _are converted _into the Si concentration [Si] _in and P concentration [P]in in the molten iron during the dephosphorization process and used in the calculation.

The dephosphorized slag produced by the above steps can then be cooled and crushed to produce phosphate fertilizer.

Next, an embodiment of the present invention will be described. However, the conditions are merely examples of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this embodiment. The present invention can be implemented in various ways that achieve the object of the present invention without departing from the gist of the present invention.

(First embodiment)
The hot metal (P concentration: 0.60 mass%, Si concentration: 0.10 mass%) obtained by reducing steelmaking slag on the hot metal was charged into a 1 ton refining furnace, dephosphorization flux was added, and oxygen was supplied from the top lance at a total of ¹⁹ Nm3/t to perform dephosphorization. The added dephosphorization flux contained 18.3 kg/t of quicklime (CaO), 9.5 kg of lightly burnt dolomite, and 8.4 kg/t of silica sand ( _SiO2 ), with total CaO of 23.6 kg/t and total MgO of 3.4 kg/t. As a result of adding the above dephosphorization flux, the total amount of Si in the dephosphorization slag and the hot metal during the dephosphorization process converted into the Si concentration in the hot metal ([Si] _in ) was 0.49 mass%. On the other hand, since the dephosphorization flux did not contain phosphoric acid, the total amount of P in the dephosphorization slag and the molten iron during the dephosphorization treatment, converted into the P concentration in the molten iron ([P] _in ), was 0.60 mass%, which was the same as the P concentration in the molten iron before the dephosphorization treatment.

After the dephosphorization treatment, the molten iron temperature was 1420° C., the P concentration in the molten iron after the treatment [P] _f was 0.18 mass%, and the slag volume SV of the dephosphorization slag after the dephosphorization treatment was 61.9 kg/t.

The dephosphorization slag obtained by the dephosphorization treatment was then recovered from the converter, cooled, and analyzed for its components, which were found to contain 38.1% by mass of CaO, 16.9% by mass of _SiO2 , 5.5% by mass of MgO, 2.2% by mass of MnO, and 15.7% by mass _{of P2O5} . In other words, the basicity C/S was 2.25, the ratio (CaO)/( _P2O5 ) was 2.43, and _the ratio ( _SiO2 )/( _P2O5 ) was 1.08. From these results, the amount of P in the dephosphorization slag at the end of the dephosphorization treatment was converted into the P concentration in _the molten iron (Δ[P]), which was calculated to be 0.42% by mass.

The dephosphorized slag obtained was then crushed to under 150 μm and magnetically separated. Analysis revealed that the soluble phosphate (phosphate that dissolves in ammonium citrate solution) was 13.7% by mass, with a soluble phosphate rate of 87.0%. A planting test was then conducted using the obtained fertilizer sample, and it was found that the yield was equal to or greater than that of commercially available fertilizer.

(Second Example)
The dephosphorization process was carried out by charging molten pig iron into a converter, adding dephosphorization flux, and supplying oxygen from the top lance at a total rate of ¹⁹ Nm3/t. At this time, molten pig iron with various P and Si concentrations was prepared, and the amount of dephosphorization was controlled by using various dephosphorization fluxes, to obtain the dephosphorization slag and molten pig iron shown in Table 1. The obtained dephosphorization slag was then cooled and pulverized to under 150 μm, magnetically separated, and the soluble phosphoric acid was analyzed to calculate the soluble phosphoric acid ratio. It was evaluated that the effect of the present invention was achieved when the soluble phosphoric acid ratio was 80% or more. For reference, the citrate-soluble phosphoric acid ratio was also analyzed.

The underlines in Table 1 indicate values outside the scope of the present invention. As shown in Table 1, in Examples 1 to 4, the phosphoric acid concentration (P ₂ O ₅ ) in the dephosphorization slag was 10 mass % or more, the ratio (CaO)/(P ₂ O ₅ ) was 2.3 to 4.2, and the ratio (SiO ₂ )/(P ₂ O ₅ ) was 0.6 to 1.3, so that the soluble phosphoric acid rate was high.

In contrast, in Comparative Example 1, the soluble phosphate rate was high, but since the P concentration [P] _f in the molten iron after dephosphorization was less than 0.10 mass%, the phosphate concentration (P ₂ O ₅ ) in the dephosphorization slag was less than 10 mass%, and the fertilizer effect was insufficient. Also, in Comparative Examples 2 to 5, Δ[P] deviated from the condition of the above formula (4) in relation to the initial Si concentration in the molten iron, and therefore at least one of the ratios (CaO)/(P ₂ O ₅ ) and (SiO ₂ )/(P ₂ O ₅ ) deviated from the condition of the present invention, and the soluble phosphate rate was low.

Claims (2)

A method for producing a _phosphate fertilizer in which the _P2O5 concentration is 10% or more by mass , the concentration ratio of CaO _{to P2O5} (CaO)/(P2O5 ₎ is 2.3 to 4.2, and _the concentration ratio of _{SiO2 to} P2O5 (SiO2 ₎ /( _{P2O5 )} is 0.6 to 1.3 _,
performing a dephosphorization process on the molten iron so that the P concentration in the molten iron after the dephosphorization process is 0.10% or more by mass% and the value Δ[P] obtained by converting the amount of P in the dephosphorization slag at the end of the dephosphorization process into the P concentration in the molten iron satisfies the condition of the following formula (1);
Cooling and pulverizing the dephosphorization slag obtained by the dephosphorization treatment;
A method for producing a phosphate fertilizer, comprising the steps of:
0.72×[Si] _in ≦Δ[P]≦1.56×[Si] _in ...(1)
Here, [Si] _in represents the total amount of Si in the dephosphorization slag and the molten iron during the dephosphorization treatment, converted into the Si concentration in the molten iron (mass %). 2. The method for producing a phosphate fertilizer according to claim 1 , characterized in that the amount of the dephosphorization slag is calculated from the target composition of the dephosphorization slag and the [Si] _in , and the type and amount of the dephosphorization flux containing at least CaO is determined.

Images