METHOD FOR STABILIZATION OF REDUCING SLAG

Abstract

A method for stabilizing a reducing slag includes the steps of: subjecting a reducing slag which contains an alkaline earth metal oxide to a drying treatment, so as to obtain a dried reducing slag; mixing a urease-producing bacterium, a fermentation medium, and a nutrient, followed by conducting a fermentation treatment, so as to obtain a fermentation product containing urease; subjecting the fermentation product to a foam fractionation treatment, so as to obtain a foam containing urease; mixing the dried reducing slag, the foam containing urease, and urea, followed by conducting a urea hydrolysis reaction and a precipitation reaction in sequence, so as to obtain a product containing a stabilized reducing slag and a residual liquid; and subjecting the product to a solid-liquid separation treatment, so as to separate the stabilized reducing slag and the residual liquid from the product.

Claims

1. A method for stabilizing a reducing slag, comprising the steps of: (a) subjecting a reducing slag which contains an alkaline earth metal oxide to a drying treatment, so as to obtain a dried reducing slag, the alkaline earth metal oxide being selected from the group consisting of calcium oxide, magnesium oxide, and a combination thereof; (b) mixing a urease-producing bacterium, a fermentation medium, and a nutrient, followed by conducting a fermentation treatment, so as to obtain a fermentation product containing urease; (c) subjecting the fermentation product containing urease obtained in step (b) to a foam fractionation treatment, so as to obtain a foam containing urease, a concentration of the urease in the foam being greater than a concentration of the urease in the fermentation product; (d) mixing the dried reducing slag obtained in step (a), the foam containing urease obtained in step (c), and urea, followed by conducting a urea hydrolysis reaction and a precipitation reaction in sequence, so as to obtain a product containing a stabilized reducing slag and a residual liquid; and (e) subjecting the product obtained in step (d) to a solid-liquid separation treatment, so as to separate the stabilized reducing slag and the residual liquid from the product.

2. The method as claimed in claim 1, wherein in step (b), the urease-producing bacterium is an aerobic bacterium selected from the group consisting of Sporosarcina pasteurii, Bacillus subtilis, and a combination thereof.

3. The method as claimed in claim 1, wherein in step (b), the urease-producing bacterium is an anaerobic bacterium selected from the group consisting of Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus fermentum, and combinations thereof.

4. The method as claimed in claim 1, wherein in step (c), the foam fractionation treatment is conducted by introducing a gas into the fermentation product containing urease, so as to generate the foam containing urease.

5. The method as claimed in claim 4, wherein the gas is air.

6. The method as claimed in claim 1, wherein in step (a), the reducing slag is subjected to a sterilization treatment before or simultaneously with the drying treatment, the sterilization treatment being conducted using ultraviolet light.

7. The method as claimed in claim 1, wherein in step (a), the dried reducing slag is further subjected to a sterilization treatment after the drying treatment, the sterilization treatment being conducted using ultraviolet light.

8. The method as claimed in claim 1, wherein in step (a), the drying treatment is conducted using sunlight.

9. The method as claimed in claim 1, wherein in step (b), the fermentation treatment is conducted at a temperature ranging from 15 C. to 40 C. for a time period ranging from 16 hours to 120 hours.

10. The method as claimed in claim 1, wherein in step (c), the foam fractionation treatment is conducted at a temperature ranging from 15 C. to 40 C.

11. The method as claimed in claim 1, wherein in step (d), the urea hydrolysis reaction and the precipitation reaction are conducted at a temperature ranging from 15 C. to 40 C.

Description

DETAILED DESCRIPTION

[0014] For the purpose of this specification, it will be clearly understood that the word comprising means including but not limited to, and that the word comprises has a corresponding meaning.

[0015] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Taiwan or any other country.

[0016] Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which the present disclosure belongs. One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present disclosure. Indeed, the present disclosure is in no way limited to the methods and materials described.

[0017] The present disclosure provides a method for stabilizing a reducing slag, which includes the following steps (a) to (e).

[0018] In step (a), a reducing slag which contains an alkaline earth metal oxide is subjected to a drying treatment, so as to obtain a dried reducing slag. The alkaline earth metal oxide is selected from the group consisting of calcium oxide, magnesium oxide, and a combination thereof.

[0019] In step (b), a urease-producing bacterium, a fermentation medium, and a nutrient are mixed, followed by conducting a fermentation treatment, so as to obtain a fermentation product containing urease.

[0020] In step (c), the fermentation product containing urease obtained in step (b) is subjected to a foam fractionation treatment, so as to obtain a foam containing urease. A concentration of the urease in the foam is greater than a concentration of the urease in the fermentation product.

[0021] In step (d), the dried reducing slag obtained in step (a), the foam containing urease obtained in step (c), and urea are mixed, followed by conducting a urea hydrolysis reaction and a precipitation reaction in sequence, so as to obtain a product containing a stabilized reducing slag and a residual liquid.

[0022] In step (e), the product obtained in step (d) is subjected to a solid-liquid separation treatment, so as to separate the stabilized reducing slag and the residual liquid from the product.

<Step (a)>

[0023] Examples of the reducing slag may include, but are not limited to, a blast furnace slag, a basic oxygen furnace slag, and an electric arc furnace slag. In certain embodiments, the reducing slag includes silicon dioxide (SiO.sub.2) in an amount ranging from 22 wt % to 29 wt %, calcium oxide (CaO) in an amount ranging from 48 wt % to 50 wt %, magnesium oxide (MgO) in an amount ranging from 6 wt % to 10 wt %, aluminum oxide (Al.sub.2O.sub.3) in an amount ranging from 14 wt % to 18 wt %, and ferric oxide (Fe.sub.2O.sub.3) in an amount ranging from 1 wt % to 2 wt %.

[0024] According to the present disclosure, the drying treatment is conducted using sunlight to heat the reducing slag, thereby removing moisture from the reducing slag. During the drying treatment, the reducing slag is stirred using a mixing machine so as to accelerate progression of the drying treatment. In certain embodiments, by virtue of the drying treatment, the moisture content in the reducing slag is reduced to not greater than 10%. In certain embodiments, the drying treatment is conducted at a temperature ranging from 15 C. to 40 C.

[0025] In certain embodiments, in step (a), the reducing slag is subjected to a sterilization treatment before or simultaneously with the drying treatment. In certain embodiments, in step (a), the dried reducing slag is further subjected to a sterilization treatment after the drying treatment. A sequence of the drying treatment and the sterilization treatment is not limited. In an exemplary embodiment, the drying treatment is conducted first, followed by conducting the sterilization treatment.

[0026] In certain embodiments, the sterilization treatment is conducted at a temperature ranging from 15 C. to 40 C. In certain embodiments, the sterilization treatment is conducted using ultraviolet light to irradiate the reducing slag or the dried reducing slag. During the sterilization treatment, the reducing slag or the dried reducing slag is tumbled using a mixing machine with a conveyor belt and a roller shovel so as to accelerate progression of the sterilization treatment.

<Step (b)>

[0027] In certain embodiments, the urease-producing bacterium is an aerobic bacterium. Examples of the aerobic bacterium may include, but are not limited to, Sporosarcina pasteurii, Bacillus subtilis, and a combination thereof. In an exemplary embodiment, Sporosarcina pasteurii is deposited at the Bioresource Collection and Research Center (BCRC) of the Food Industry Research and Development Institute (FIRDI) (No. 331, Shih-Pin Rd., Hsinchu City 300, Taiwan) under an accession number BCRC 11596, and is also deposited at the American Type Culture Collection (ATCC) (10801 University Boulevard, Manassas, Va. 20110-2209., USA) in accordance with the Budapest Treaty under an accession number ATCC 11859, and Bacillus subtilis is deposited at the BCRC of the FIRDI under an accession number BCRC 16048, and is also deposited at the ATCC in accordance with the Budapest Treaty under an accession number ATCC 21332. Both strains are known and readily available to the public.

[0028] In certain embodiments, the urease-producing bacterium is an anaerobic bacterium. Examples of the anaerobic bacterium may include, but are not limited to, Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus fermentum, and combinations thereof. In an exemplary embodiment, the anaerobic bacterium is selected from the group consisting of Lactobacillus plantarum GKM3 which is deposited at the BCRC of the FIRDI under an accession number BCRC 910787, and is also deposited at the ATCC in accordance with the Budapest Treaty under an accession number ATCC 14917, Lactobacillus rhamnosus GKG6 which is deposited at the BCRC of the FIRDI under an accession number 81178, and is also deposited at the ATCC in accordance with the Budapest Treaty under an accession number ATCC 7469, Lactobacillus fermentum GKF3 which is deposited at the BCRC of the FIRDI under an accession number BCRC 910824, and combinations thereof. These strains are known and readily available to the public.

[0029] According to the present disclosure, the types and components of the fermentation medium may vary based on the urease-producing bacterium to be used. Examples of the fermentation medium may include, but are not limited to, M9 minimal medium, lysogeny broth, Tris-buffered basal medium containing yeast extract, sodium bicarbonate basal medium, and phosphate-buffered basal medium. In certain embodiments, the fermentation medium includes a nitrogen source and a carbon source. Examples of the nitrogen source may include, but are not limited to, tryptone and yeast paste. Examples of the carbon source may include, but are not limited to, glucose and glycerol. In certain embodiments, the fermentation medium further includes an inorganic salt. An example of the inorganic salt may include, but is not limited to, sodium chloride.

[0030] According to the present disclosure, the types of the nutrient may vary based on the urease-producing bacterium to be used. Examples of the nutrient may include, but are not limited to, a carbon source, a nitrogen source, and an inorganic salt. Examples of the carbon source may include, but are not limited to, glucose, glycerol, and muscovado sugar. Examples of the nitrogen source may include, but are not limited to, tryptone, yeast paste, and ammonium sulfide. An example of the inorganic salt may include, but is not limited to, sodium chloride.

[0031] According to the present disclosure, the procedures and conditions of the fermentation treatment are within the expertise and routine skills of those skilled in the art.

[0032] According to the present disclosure, the fermentation treatment is conducted at a temperature ranging from 15 C. to 40 C. for a time period ranging from 16 hours to 120 hours. In certain embodiments, the fermentation treatment is conducted at a pH value ranging from 5 to 9. In certain embodiments, the fermentation treatment is conducted at an aeration rate ranging from 0.15 vvm to 1.00 vvm (volume of sparged air to volume of culture medium per minute). In certain embodiments, the fermentation treatment includes a first fermentation process and a second fermentation process. In the first fermentation process, a mixture, which is formed by mixing the urease-producing bacterium, the fermentation medium, and the nutrient, is subjected to the fermentation treatment until an optical density (OD) of the mixture is measured to be not less than 1.0. Thereafter, in the second fermentation process, the mixture continues to be subjected to the fermentation treatment for a certain time period so as to produce a surfactant, such that the thus obtained fermentation product further contains the surfactant in addition to the urease. The surfactant is capable of facilitating foam formation during the foam fractionation treatment in step (c), thereby enabling the foam to carry the urease and assisting in the execution of step (d). Examples of the surfactant may include, but are not limited to, a protein-based surfactant and a peptide-based surfactant. In certain embodiments, the first fermentation process is conducted at a temperature ranging from 15 C. to 40 C. for a time period ranging from 16 hours to 48 hours. In certain embodiments, the second fermentation process is conducted at a temperature ranging from 15 C. to 40 C. for a time period of 72 hours.

<Step (c)>

[0033] In certain embodiments, the foam fractionation treatment is conducted by means of a foam fractionation column. The foam at different vertical heights within the foam fractionation column has a different concentration of the urease. In other words, as the vertical height increases, the concentration of the urease in the foam formed from the fermentation product also increases, so that the foam with different concentration gradient of urease is fractionated. Therefore, a length of the foam fractionation column affects a degree of urease enrichment in the foam. According to the present disclosure, in step (c), the foam fractionation treatment is conducted by introducing a gas into the fermentation product containing urease, so as to generate the foam containing urease. In certain embodiments, a flow rate of the gas ranges from 0.1 L/min to 1.0 L/min. Different gas flow rates result in different foam sizes which are categorized based on a magnitude of the gas flow rate to obtain the foams with different degrees of moisture content, thereby facilitating obtainment of the foams with different concentrations of the urease. An example of the gas may include, but is not limited to, air. In certain embodiments, the foam fractionation treatment is conducted at a temperature ranging from 15 C. to 40 C. for a time period ranging from 2 hours to 5 hours. In certain embodiments, the gas is introduced into the fermentation product containing the urease using an air disperser with multiple pores. A size of the pores may be adjusted according to a required size of an air bubble. Examples of the air disperser may include, but are not limited to, a sintered glass sparger, and a sintered metal sparger.

<Step (d)>

[0034] By virtue of conducting the urea hydrolysis reaction, the urease in the foam hydrolyzes the urea to produce carbonate ions which subsequently undergoes the precipitation reaction with the alkaline earth metal oxide in the dried or sterilized reducing slag, resulting in a formation of a carbonate salt precipitate, thereby converting the dried or sterilized reducing slag into the stabilized reducing slag. In certain embodiments, the urea hydrolysis reaction and the precipitation reaction are conducted at a temperature ranging from 15 C. to 40 C.

[0035] The stabilized reducing slag can be used as an aggregate in concrete. In addition, the stabilized reducing slag has a low content of the alkaline earth metal oxide, and hence does not exhibit expansion when in contact with water, which prevents serious damage such as cracking or crumbling of a cured material formed by the concrete, thereby achieving a goal of recycling the reducing slag for resource utilization.

<Step (e)>

[0036] The solid-liquid separation treatment is not particularly limited and can be conducted using any conventional solid-liquid separation technique well known to those skilled in the art. Examples of the solid-liquid separation treatment may include, but are not limited to, a sedimentation separation treatment and a filtration separation treatment.

[0037] 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

Example 1 (EX1)

[0038] A method for stabilizing a reducing slag of EX1 includes the following steps (a) to (e).

<Step (a)>

[0039] An electric arc furnace slag (serving as a reducing slag, ingredient: 25 wt % of SiO.sub.2, 49 wt % of CaO, 8 wt % of MgO, 17 wt % of Al.sub.2O.sub.3, and 1 wt % of Fe.sub.2O.sub.3) was introduced into a mixing machine, and was subjected to a stirring treatment and a drying treatment using sunlight at a temperature of 30 C. for a time period ranging from 12 hours to 48 hours so as to reduce an amount of moisture in the electric arc furnace slag to 10%, thereby obtaining a dried reducing slag. Thereafter, the dried reducing slag was introduced into a mixing machine with a roller shovel, and was subjected to a sterilization treatment using ultraviolet light at a temperature of 30 C., so as to obtain a sterilized reducing slag.

<Step (b)>

[0040] 1000 L of a fermentation medium (ingredient: 20 g/L of tryptone, 5 g/L of sodium chloride, and 20 g/L of urea), 10 g of a nutritional composition (ingredient: 20 g/L of glucose, 20 g/L of muscovado sugar, and 20 g/L of tryptone), and Sporosarcina pasteurii (serving as a urease-producing bacterium, obtained as a commercial product BCRC 11596 from the Bioresource Collection and Research Center (BCRC) of the Food Industry Research and Development Institute (FIRDI)) were mixed, so as to form a mixture. Thereafter, the mixture was subjected to a fermentation treatment at a temperature of 30 C. for 24 hours to achieve an optical density (OD) value of 1.0, followed by continuously subjecting the mixture to the fermentation treatment at a temperature of 30 C. for 72 hours, so as to obtain a fermentation product containing 0.006 wt % of urease.

<Step (c)>

[0041] The fermentation product containing 0.006 wt % of the urease obtained in step (b) was subjected to a foam fractionation treatment by introducing air at a flow rate of 0.5 L/min into the fermentation product at a temperature of 30 C. for a time period of 3 hours, so as to obtain a foam containing 0.5 wt % of urease.

<Step (d)>

[0042] 100 g of the sterilized reducing slag obtained in step (a), 60 mL of the foam containing 0.5 wt % of the urease obtained in step (c), and 12.02 g of urea were mixed, followed by conducting a urea hydrolysis reaction and a precipitation reaction in sequence at a temperature of 30 C. for a total time period of 6 hours, so as to obtain a product containing a stabilized reducing slag and a residual liquid.

<Step (e)>

[0043] The product obtained in step (d) was subjected to a solid-liquid separation treatment by gravity settling to separate the stabilized reducing slag and the residual liquid from the product, such that the stabilized reducing slag was located at the bottom and the residual liquid was located at the top.

Examples 2 and 3 (E2 and E3)

[0044] The procedures and conditions in a method for stabilizing a reducing slag of the respective one of EX2 and EX3 were similar to those of EX1, except that the total time periods of the urea hydrolysis reaction and the precipitation reaction in step (d) were varied as shown in Table 1 below. In addition, in step (b), the OD value of the mixture determined in each of the methods of EX2 and EX3 was 1.00.2.

Example 4 (EX4)

[0045] A method for stabilizing a reducing slag of EX4 included a first treatment of steps (a) to (e), a second treatment of steps (a), (d), and (e), and a third treatment of steps (a), (d), and (e) as follows.

[0046] In the first treatment, steps (a) to (e) of the method of EX4 were equivalent to steps (a) to (e) of the method of EX1.

[0047] In the second treatment, steps (a), (d), and (e) of the method of EX4 were similar to those of EX1, except that in step (d), the foam containing the urease was displaced by the residual liquid separated in step (e) of the first treatment, and the total time period of the urea hydrolysis reaction and the precipitation reaction was 72 hours.

[0048] In the third treatment, steps (a), (d), and (e) of the method of EX4 were similar to those of EX1, except that in step (d), the foam containing the urease was displaced by the residual liquid separated in step (e) of the second treatment, and the total time period of the urea hydrolysis reaction and the precipitation reaction was 72 hours.

Comparative Example 1 (CE1)

[0049] A method for stabilizing a reducing slag of CE1 includes the following steps (a) to (d).

<Step (a)>

[0050] An electric arc furnace slag (serving as a reducing slag, ingredient: 25 wt % of SiO.sub.2, 49 wt % of CaO, 8 wt % of MgO, 17 wt % of Al.sub.2O.sub.3, and 1 wt % of Fe.sub.2O.sub.3) was introduced into the mixing machine, and was subjected to a stirring treatment and a drying treatment using sunlight at a temperature of 30 C. for a time period ranging from 12 hours to 48 hours so as to reduce an amount of moisture in the electric arc furnace slag to 10%, thereby obtaining a dried reducing slag. Thereafter, the dried reducing slag was introduced into the mixing machine with the roller shovel, and was subjected to a sterilization treatment using ultraviolet light at a temperature ranging from 15 C. to 40 C., so as to obtain a sterilized reducing slag.

<Step (b)>

[0051] 1000 L of a fermentation medium (ingredient: 20 g/L of tryptone, 5 g/L of sodium chloride, and 20 g/L of urea), 10 g of a nutritional composition (ingredient: 20 g/L of glucose, 20 g/L of muscovado sugar, and 20 g/L of tryptone), and Sporosarcina pasteurii (serving as a urease-producing bacterium) were mixed, so as to form a mixture. Thereafter, the mixture was subjected to a fermentation treatment at a temperature of 30 C. for 24 hours to achieve an optical density (OD) value of 1.0, followed by continuously subjecting the mixture to the fermentation treatment at a temperature of 30 C. for 72 hours, so as to obtain a fermentation product containing 0.006 wt % of urease.

<Step (c)>

[0052] 100 g of the sterilized reducing slag obtained in step (a), 60 mL of the fermentation product containing 0.006 wt % of urease obtained in step (b), and 12.02 g of urea were mixed, followed by conducting a urea hydrolysis reaction and a precipitation reaction in sequence at a temperature ranging from 15 C. to 40 C. for a total time period of 12 hours, so as to obtain a product containing a stabilized reducing slag and a residual liquid.

<Step (d)>

[0053] The product obtained in step (c) was subjected to a solid-liquid separation treatment by gravity settling to separate 1041 g of the stabilized reducing slag and the residual liquid from the product, such that the stabilized reducing slag was located at the bottom and the residual liquid was located at the top.

Comparative Examples 2 to 6 (CE2 to CE6)

[0054] The procedures and conditions in a method for stabilizing a reducing slag of the respective one of CE2 to CE6 were similar to those of CE1, except that the total time periods of the urea hydrolysis reaction and the precipitation reaction in step (c) were varied as shown in Table 1 below. In addition, in step (b), the OD value of the mixture determined in each of the methods of CE2 to CE6 was 1.00.2.

Comparative Example 7 (CE7)

[0055] A method for stabilizing a reducing slag of CE7 included a first treatment of steps (a) to (d), a second treatment of steps (a), (c), and (d), and a third treatment of steps (a), (c), and (d) as follows.

[0056] In the first treatment, steps (a) to (d) of the method of CE7 were equivalent to steps (a) to (d) of the method of CE6.

[0057] In the second treatment, steps (a), (c), and (d) of the method of CE7 were similar to those of CE6, except that in step (c), the fermentation product containing the urease was displaced by the residual liquid separated in step (d) of the first treatment.

[0058] In the third treatment, steps (a), (c), and (d) of the method of CE7 were similar to those of CE6, except that in step (c), the fermentation product containing the urease was displaced by the residual liquid separated in step (d) of the second treatment.

[0059] The materials, the amounts thereof and the total time period for the urea hydrolysis reaction and the precipitation reaction in step (d) of the respective one of the methods of EX1 to EX3 and in step (c) of the respective one of the methods of CE1 to CE6 are shown in Table 1 below. The materials, the amounts thereof and the total time period for the urea hydrolysis reaction and the precipitation reaction in step (d) of the first treatment, in step (d) of the second treatment, and in step (d) of the third treatment of the method of EX4 and in step (c) of the first treatment, in step (c) of the second treatment, and in step (c) of the third treatment of the method of CE7 are shown in Table 2 below.

TABLE-US-00001 TABLE 1 Total time period for urea Fermentation hydrolysis product Foam Sterilized reaction and containing containing reducing precipitation urease urease slag Urea reaction (mL) (mL) (g) (g) (hour) EX1 0 60 100 12.02 6 EX2 0 60 100 12.02 12 EX3 0 60 100 12.02 24 CE1 60 0 100 12.02 12 CE2 60 0 100 12.02 24 CE3 60 0 100 12.0 36 CE4 60 0 100 12.0 48 CE5 60 0 100 12.02 60 CE6 60 0 100 12.02 72

TABLE-US-00002 TABLE 2 Total time Fermentation period for product Foam Sterilized urea hydrolysis containing containing Residual reducing reaction and urease urease liquid slag Urea precipitation (mL) (mL) (mL) (g) (g) reaction (hour) EX4 First treatment 0 60 0 100 12.02 6 Second treatment 0 0 60 100 12.02 72 Third treatment 0 0 60 100 12.02 72 CE7 First treatment 60 0 0 100 12.02 72 Second treatment 0 0 60 100 12.02 72 Third treatment 0 0 60 100 12.02 72

Property Evaluation

A. Measurement of Free Calcium Oxide Content in Stabilized Reducing Slag

[0060] The stabilized reducing slag of the respective one of the methods of EX1 to EX4 and CE1 to CE7 was subjected to determination of free calcium oxide content using an ethylenediaminetetraacetic acid (EDTA) titration method. According to the Chinese standard method GB/T 20491 (published in 2017) titled Steel slag powder used for cement and concrete, a free calcium oxide content in a slag is required to be less than 2.5 wt % to comply with the usage standards. The results are shown in Table 3 below.

Results

TABLE-US-00003 TABLE 3 Free calcium oxide content Compliance with (wt %) GB/T 20491 or not EX1 2.40 Yes EX2 2.02 Yes EX3 1.64 Yes CE1 3.06 No CE2 2.50 NO CE3 2.78 No CE4 2.62 No CE5 2.58 No CE6 2.45 Yes EX4 First 2.40 Yes treatment Second 2.45 Yes treatment Third 2.48 Yes treatment CE7 First 2.45 Yes treatment Second 3.98 No treatment Third 4.02 No treatment

[0061] Referring to Table 1 in conjunction with Table 3, in the respective one of the methods of EX1 to EX3, by virtue of the foam containing urease being subjected to the urea hydrolysis reaction and the precipitation reaction with the sterilized reducing slag and the urea for a total time period ranging from 6 hours to 24 hours in step (d), the free calcium oxide content measured in the stabilized reducing slag separated in step (e) was not greater than 2.40 wt %. In contrast, in the respective one of the methods of CE1 to CE6, by virtue of the fermentation product containing urease being subjected to the urea hydrolysis reaction and the precipitation reaction with the sterilized reducing slag and the urea for a total time period ranging from 12 hours to 72 hours in step (c), the free calcium oxide content measured in the stabilized reducing slag separated in step (d) ranges from 2.45 wt % to 3.06 wt %. To be specific, in the methods of CE1 to CE6, the total time period for the urea hydrolysis reaction and the precipitation reaction required 72 hours to make the resultant stabilized reducing slag separated in step (d) comply with the usage standards. These results indicate that stabilized reducing slag that complies with the usage standards can indeed be rapidly obtained by the method for stabilizing the reducing slag of the present disclosure.

[0062] Referring to Table 2 in conjunction with Table 3, in the method of EX4, by virtue of conducting the foam fractionation treatment in step (c) of the first treatment so as to obtain the foam containing relatively high concentration of the urease, the residual liquid obtained in step (d) of the first treatment accordingly contained a relatively high concentration of the urease. Therefore, the resultant residual liquid separated in step (e) of the first treatment could displace the foam and be reused in step (d) of the second treatment, and the resultant residual liquid separated in step (e) of the second treatment could displace the foam and be reused in step (d) of the third treatment, so that the free calcium oxide contents measured in the stabilized reducing slags separated in step (e) of the second treatment and in step (e) of the third treatment were 2.45 wt % and 2.48 wt %, respectively. These results demonstrate that since the residual liquid obtained in the first treatment has a sufficient concentration of the urease to displace the foam containing urease, steps (b) and (c) are not required in the second treatment, steps (b) and (c) are not required in the third treatment, and the stabilized reducing slags thus obtained in the second and third treatments can still comply with the usage standards.

[0063] In contrast, in the method of CE7, due to omitting the foam fractionation treatment in the first treatment, the residual liquid obtained in step (c) of the first treatment contained a relatively low concentration of the urease. Therefore, when the resultant residual liquid separated in step (d) of the first treatment was used to displace the fermentation product and was reused in step (c) of the second treatment, and the resultant residual liquid separated in step (d) of the second treatment was used to displace the fermentation product and was reused in step (c) of the third treatment, the free calcium oxide contents measured in the stabilized reducing slags separated in step (d) of the second treatment and in step (d) of the third treatment were 3.98 wt % and 4.02 wt %, respectively. These results demonstrate that since the fermentation product obtained in the first treatment contains a relatively low concentration of the urease, the residual liquid thus obtained in the first treatment accordingly contains a relatively low concentration of the urease. Therefore, the resultant residual liquid is not conducive to be reused and to displace the fermentation product in the urea hydrolysis reaction and the precipitation reaction, and the stabilized reducing slags thus obtained in the second and third treatments cannot comply with the usage standards.

[0064] Summarizing the above test results, it is clear that by virtue of conducting the foam fractionation treatment in step (c) of the method of the present disclosure, the concentration of the urease in the foam obtained in step (c) is greater than the concentration of the urease in the fermentation product obtained in step (b). Therefore, in step (d) of the method of the present disclosure, the urease with a relatively high concentration in the foam can rapidly hydrolyze the urea to produce carbonate ions with a relatively high concentration, and such carbonate ions subsequently undergo the precipitation reaction rapidly with the alkaline earth metal oxide in the dried or sterilized reducing slag, resulting in a formation of a carbonate salt precipitate, thereby converting the dried or sterilized reducing slag into the stabilized reducing slag in a relatively short period of time. In addition, because the foam obtained in step (c) of the method of the present disclosure contains a relatively high concentration of the urease, the residual liquid thus obtained in step (d) of the method of the present disclosure accordingly contains a relatively high concentration of the urease, so that the residual liquid separated in step (e) of the method of the present disclosure has a sufficient concentration of the urease and can be reused, and hence not only reduces production costs but also minimizes waste discharge.

[0065] In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiment(s). It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to one embodiment, an embodiment, an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects; such does not mean that every one of these features needs to be practiced with the presence of all the other features. In other words, in any described embodiment, when implementation of one or more features or specific details does not affect implementation of another one or more features or specific details, the one or more features may be singled out and practiced alone without the another one or more features or specific details. It should be further noted that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure.

[0066] While the disclosure has been described in connection with what is(are) considered the exemplary embodiment(s), it is understood that this disclosure is not limited to the disclosed embodiment(s) 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.