KR20220087914A - Waste sulfuric acid multistage neutralization with stainless steel annealing acid and its sludge recycling method - Google Patents

Abstract

The present invention comprises the steps of preparing a solution of spent sulfuric acid; a second or more neutralization step of neutralizing the spent sulfuric acid solution; and a moisture content reduction step of reducing the moisture content of the neutralized sludge generated in the neutralization step to 20% by weight or less; It relates to a waste sulfuric acid recycling method comprising a.

Description

Waste sulfuric acid multistage neutralization with stainless steel annealing acid and its sludge recycling method

The present invention relates to a multi-stage neutralization treatment method for stainless steel annealing and pickling waste sulfuric acid. More specifically, it relates to a recycling method for recycling neutralized sludge generated in a multi-stage neutralization treatment process.

Pickling is performed to remove surface impurities and oxide layer of stainless steel. The type of acid used differs depending on the process, but mainly sulfuric acid, hydrofluoric acid and nitric acid are used. As the waste acid treatment technology generated at this time, a method of neutralizing mixed acid by mixing waste acid was used in the past. However, recently, a technology for separating and treating waste acid without mixing treatment is being developed to convert waste acid and generated sludge into resources. A technology is being developed that neutralizes the waste mixed acid mixed with hydrofluoric acid and nitric acid and uses the generated sludge as a substitute for fluorite. .

However, since the neutralized sludge generated after neutralization treatment of waste sulfuric acid is often disposed of in landfill, it is necessary to increase the value-added resource through improvement of the neutralization treatment method or development of the use of the neutralized sludge.

On the other hand, AC electrolytic pickling technology is applied to improve the pickling performance of stainless steel. It is also used in combination.

In the neutralization treatment of the waste sulfuric acid, neutralized sludge is generated by neutralizing the diluted waste sulfuric acid solution with slaked lime as a neutralizing material. Therefore, the use of the sludge is limited, and since it is landfilled, it is necessary to develop a resource-saving technology through future process improvement.

About 5% of gypsum is used as an essential additive among raw materials for cement composition, and natural anhydrous gypsum, neutralized gypsum, and flue gas desulfurization gypsum are used. Gypsum is used to delay the setting of cement and as a reaction accelerator. The KS standard for gypsum for cement requires at least 25% of SO3 content.

Recently, regulations on the elution of heavy metals from cement are being strengthened, and in particular, the problem of Cr6+ elution is further strengthened. In order to reduce ^Cr6 + ^elution in cement, cement companies use a mixture of reducing agents such as iron sulfate. Efforts are ongoing.

An object of the present invention is to provide a method for reducing the redness of neutralized sludge generated during the neutralization treatment of waste sulfuric acid from annealing of stainless steel.

An object of the present invention is to provide a resource recycling method for reducing the moisture content of neutralized sludge generated during neutralization treatment of waste sulfuric acid from annealing of stainless steel.

A waste sulfuric acid recycling method according to an embodiment of the present invention comprises the steps of preparing a waste sulfuric acid solution; a second or more neutralization step of neutralizing the spent sulfuric acid solution; and a water content reduction step of reducing the water content of the neutralized sludge generated in the neutralization step to 20% by weight or less.

The moisture content reduction step is a step of controlling the moisture content of the neutralized sludge using waste heat.

The moisture content reduction step is a step of controlling the moisture content of the neutralized sludge by mixing a material containing CaO with the neutralized sludge.

The water content reduction step is a step of controlling the water content of the neutralized sludge by centrifugally dewatering the neutralized sludge.

Controlling the water content of the neutralized sludge by centrifugal dehydration of the neutralized sludge; includes a neutralized sludge flocculation step of growing the neutralized sludge average particle size to 50 μm or more, wherein the neutralized sludge flocculation step comprises 3 to 10 wt% of the neutralized sludge This is the step of inputting the particle growth nuclei.

The second or more neutralization step includes a first neutralization step, wherein the first neutralization step is neutralized to a pH of 2.5 to 4.5 by mixing a neutralizing agent with a waste sulfuric acid solution; and a filtration step of separating the first neutralized solution into a first neutralized filtrate and a first neutralized sludge.

The second or more neutralization step includes a second neutralization step that proceeds after the first neutralization step, and the second neutralization step mixes a neutralizing agent with the first neutralization filtrate generated in the first neutralization step to pH 8 neutralizing to 10 to 10; and a filtration step of separating the second neutralized solution into a second neutralized filtrate and a second neutralized sludge.

The second or more neutralization step is a step of neutralizing by mixing a neutralizing agent with spent sulfuric acid, wherein the neutralizing agent is limestone, quicklime, slaked lime, fine steel slag powder, steel additive dust, oil refinery desulfurization gypsum and stainless steel reduced slag. It is at least one Ca-based neutralizing agent selected from the group consisting of.

Preparing the waste sulfuric acid solution; comprises the step of separating phenolic acid and spent mixed acid from the pickling waste generated after stainless steel pickling, wherein the spent mixed acid is a hydrofluoric acid and nitric acid mixed solution.

The primary neutralized sludge has a color difference ((100-L) ² + a ² + b ² ) of 35 or less.

The first neutralized sludge has an SO ³ content of 25% by weight or more.

According to one embodiment of the present invention, it is possible to reduce the redness of the neutralized sludge by controlling the pH and neutralizing the spent sulfuric acid by dividing it into two steps.

According to one embodiment of the present invention, the SO ³ content of the neutralized sludge is increased, so that the quality of the gypsum produced by recycling it can be improved.

According to one embodiment of the present invention, when the stainless steel annealing pickling sulfuric acid sludge containing a high amount of moisture content, high drying cost, and a large amount of components that deteriorate gypsum quality such as Fe, is treated with the process technology of the present invention, most of the landfill treatment It is possible to manufacture the neutralized sludge into a gypsum product so that it can be economically turned into a resource.

According to one embodiment of the present invention, it is possible to produce gypsum with low CO ₂ generation.

1 is a flowchart illustrating a multi-stage neutralization treatment method for spent sulfuric acid according to an embodiment of the present invention.
FIG. 2 is a diagram showing the precipitation diagram of metal hydroxide (at 25° C.) showing the relationship of cation saturation concentration according to the pH of the spent sulfuric acid solution.

The terms first, second and third etc. are used to describe, but are not limited to, various parts, components, regions, layers and/or sections. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. As used herein, the meaning of "comprising" specifies a particular characteristic, region, integer, step, operation, element and/or component, and the presence or absence of another characteristic, region, integer, step, operation, element and/or component It does not exclude additions.

When a part is referred to as being “on” or “on” another part, it may be directly on or on the other part, or the other part may be involved in between. In contrast, when a part is referred to as being "directly above" another part, the other part is not interposed therebetween.

In addition, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

Although not defined otherwise, all terms including technical terms and scientific terms used herein have the same meaning as those commonly understood by those of ordinary skill in the art to which the present invention belongs. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, are not interpreted in an ideal or very formal meaning.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art can easily implement them. However, the present invention may be embodied in several different forms and is not limited to the embodiments described herein.

Hereinafter, each step will be described in detail.

A waste sulfuric acid recycling method according to an embodiment of the present invention comprises the steps of preparing a waste sulfuric acid solution; a second or more neutralization step of neutralizing the spent sulfuric acid solution; and a moisture content reduction step of reducing the moisture content of the neutralized sludge generated in the neutralization step to 20% by weight or less; including (FIG. 1).

In the present invention, the waste sulfuric acid generated after the general annealing and pickling treatment and the waste sulfuric acid generated after the AC electrolysis and pickling and the acid recovery facility (APU) to improve the pickling performance may be used. .

Preparing the waste sulfuric acid solution; includes the step of separating the waste sulfuric acid and the spent mixed acid from the pickling waste generated after stainless steel pickling. In order to use the waste sulfuric acid generated in the stainless steel annealing pickling plant as a resource, the process of separating and sorting waste sulfuric acid, waste hydrofluoric acid, and waste mixed acid containing waste nitric acid as shown in FIG. 1 should be preceded. When used without separating the waste mixed acid in which the waste sulfuric acid, hydrofluoric acid and nitric acid are mixed, it is difficult to achieve an SO ³ content of 25 wt% or more of the gypsum recycled from the neutralized sludge.

The waste mixed acid mixed with the waste hydrofluoric acid and waste nitric acid separated from the waste sulfuric acid is neutralized using a neutralizing agent such as slaked lime, and the neutralized sludge generated therefrom is dried and molded to be used as a substitute for fluorite, a raw material for an electric furnace for stainless steel production. .

The second or more neutralization step is a step of increasing the pH by mixing a neutralizing agent in the solution. Specifically, the neutralization step; may include a first neutralization step and a second neutralization step. The first neutralization step is a step of neutralizing the waste sulfuric acid solution by mixing a neutralizing agent. The second neutralization step is a step of further neutralizing by mixing a neutralizing agent with the neutralization filtrate generated in the first neutralization step.

The spent sulfuric acid solution according to the present invention contains Fe, Cr, and Ni, and Fe is mainly in the form of Fe ²⁺ , some of which is oxidized to form Fe ³⁺ , and Cr and Ni are Cr ³⁺ and Ni ²⁺ , etc. has a cationic form. On the other hand, since gypsum (CaSO ₄ ) has low solubility in weak acidity, it may be precipitated first in the form of sludge even when the pH is low during the neutralization reaction. Accordingly, the present invention devised a multi-stage neutralization method of spent sulfuric acid in order to selectively precipitate gypsum first. Table 1 below shows the main precipitates according to the pH range upon neutralization with Ca(OH) ₂ . Each of the pH ranges ① to ④ in Table 1 is the area shown in FIG. 2 .

pH range main sediment ① CaSO ₄ ·2H ₂ O, Fe(OH) ₃ ② CaSO ₄ ·2H ₂ O, Fe(OH) ₃ , Cr(OH) ₃ ③ CaSO ₄ ·2H ₂ O, Fe(OH) _3, Cr(OH) _3, Fe(OH) _2, Ni(OH) ₂ ④ CaSO ₄ ·2H ₂ O, Fe(OH) _3, Cr(OH) _3, Fe(OH) _{2 ,} Ni(OH) ₂ , Mn(OH) ₂ , etc.

The waste sulfuric acid recycling method of the present invention includes a second or more neutralization step. Fe, Cr, and Ni components are contained as little as possible in the primary neutralized sludge, and the SO ³ content is increased, so that the gypsum recycled from the primary neutralized sludge is Quality can be improved. On the other hand, in the secondary neutralized sludge, the SO ^3- content is greatly reduced, and Fe ₂ O ₃ and Cr ₂ O ₃ components are concentrated. That is, the secondary neutralized sludge can be used as an additive in the production of stainless steel by mixing with dust generated during the production of stainless steel due to the concentration of valuable components such as Fe, Cr, and Ni.

In order to recycle the waste sulfuric acid neutralized sludge into cement gypsum, the redness of the neutralized sludge must be reduced, and for this purpose, the Fe content in the sludge must be reduced. Accordingly, the present invention is to minimize the Fe content through the pH control during the waste sulfuric acid neutralization treatment. Specifically, the present invention intends to reduce the Fe content in the sludge through the multi-stage neutralization treatment of spent sulfuric acid.

The first neutralization step; neutralizing to pH 2.5 to 4.5 by mixing a neutralizing agent in the spent sulfuric acid solution; and a filtration step of separating the first neutralized solution into a first neutralized filtrate and a first neutralized sludge. When the pH of the primary neutralization is 2.5 or less, the pH of the primary neutralized sludge is low and when it is used as a gypsum for cement, a problem of lowering the strength of the cement may occur. On the other hand, when the pH is 4.5 or higher during the first neutralization, it is not preferable because there is a risk that the Fe component is precipitated in the first neutralized sludge and the color of the first neutralized sludge is red. Specifically, the pH of the first neutralization step may be 3.0 to 4.5 or 3.0 to 3.5. The first neutralized sludge obtained through the filtration step of the first neutralization step is subjected to a water content reduction step. In the filtering step, a general filter such as a filter press may be used.

The second neutralization step; neutralizing the neutralizing agent to a pH of 8 to 10 by mixing a neutralizing agent with the first neutralization filtrate generated in the first neutralization step; and a filtration step of separating the second neutralized solution into a second neutralized filtrate and a second neutralized sludge. In the case of secondary neutralization, when the pH is 8.0 or less, as shown in FIG. 2, Fe ²⁺ , Cr ³⁺ , Ni ²⁺ components contained in the waste sulfuric acid are not completely precipitated and there is a possibility that they may be contained in the wastewater. Above 10, the amount of sludge generated may increase due to excessive addition of the neutralizing agent, which is not preferable. Specifically, the pH of the second neutralization step may be 8.5 to 10 or 8.5 to 9.0. The second neutralized sludge obtained through the filtration step of the second neutralization step is subjected to a water content reduction step. In the filtering step, a general filter such as a filter press may be used.

The secondary neutralization filtrate may be treated with wastewater by adding acid (eg, hydrochloric acid) to reverse neutralization to adjust it to a neutral region.

The neutralizing material used in the neutralization step is selected from the group consisting of limestone (CaCO ₃ ), quicklime (CaO), slaked lime (Ca(OH) ₂ ), fine steel slag powder, steel auxiliary raw material dust, oil refinery desulfurization gypsum and stainless steel reduced slag It may be one or more Ca-based neutralizing agents. Specifically, the neutralizing agent is preferably a Ca-based neutralizing agent, not a CO ₂ , in order to reduce the generation of CO ₂ . The fine powder of steelmaking slag, dust from steelmaking auxiliary raw materials, desulfurization gypsum of oil refinery and reduced stainless steel slag contain a large amount of CaO, so it can be used as a neutralizing agent for cost reduction.

The first neutralized sludge and the second neutralized sludge obtained in the neutralization step may each undergo a moisture content reduction step. The neutralized sludge from each neutralization step can be obtained using a general filter such as a filter press. The neutralized sludge thus obtained has an average moisture content of 50 to 60% by weight, and a drying step is essential for recycling, and there is a problem in that the energy cost is high during drying. Accordingly, the present inventors came to devise an energy-saving water content reduction step.

The step of reducing the moisture content according to the present invention; may be a step in which the moisture content of each of the first neutralized sludge and the secondary neutralized sludge is 20% by weight or less. When the moisture content is 20% or more, it is not preferable because energy costs for secondary drying are high. On the other hand, if the moisture content is less than 20%, environmental problems such as scattering dust occur when handling semi-dried sludge, which is undesirable.

The moisture content reduction step; may be a step of controlling the moisture content of the neutralized sludge using waste heat. That is, the neutralized sludge can be semi-dried by using the waste heat of medium and low temperature generated in the steel mill process to reduce the drying cost.

The step of reducing the moisture content; may be a step of controlling the moisture content of the neutralized sludge by mixing a material containing CaO with the neutralized sludge. The CaO-containing material may be quicklime, steel additive dust, desulfurized gypsum, and the like. When a material containing CaO is mixed with the neutralized sludge, the CaO component hydrates and ages with moisture contained in the neutralized sludge to form Ca(OH) ₂ . At this time, the hydration reaction is an exothermic reaction, Moisture may be evaporated, and in the aging process, CaO component is hydrated to Ca(OH) ₂ , and moisture contained therein is consumed as crystal water, thereby reducing the moisture content. In addition, since CaO is hydrated to Ca(OH) ₂ during the mixing process, there is no adverse effect on the quality of the gypsum when the neutralized sludge is used as gypsum for cement.

In this case, the CaO content in the CaO-containing material used is preferably 50% or more. When the CaO content is 50% or less, the above-described exothermic reaction does not occur well, and the amount of water consumed in the crystal water in Ca(OH) ₂ is small, which is not preferable.

In addition, in order to reduce the water content of the neutralized sludge, in the present invention, a centrifugal dehydrator can be used without using a conventional filter press. However, when the neutralized sludge is subjected to simple filtration with a centrifugal dehydrator, it is not preferable because the neutralized sludge has fine particles with an average of 20 μm or less, so that the filtered neutralized sludge has a high water content. Therefore, in the process of coagulation of the neutralized sludge, 3 to 10% by weight of the primary and secondary neutralized sludge is introduced into the coagulation tank as grain growth nuclei (seed) to act as nuclei for the growth of crystal grains of the primary and secondary neutralized sludge. When the amount of the grain growth nuclei input is 3 wt% or less, the effect on crystal growth is small because the amount of grain growth nuclei is small. By inputting the particle growth nuclei, the aggregated particle size may be grown to 50 μm or more. The rotation speed of the centrifugal dehydrator may be 3500 rpm or more.

The primary neutralized sludge according to the waste sulfuric acid recycling method according to an embodiment of the present invention may have a color difference ((100-L) ² + a ² + b ² ) of 35 or less. That is, the color of the primary neutralized sludge does not turn red, so it is suitable for recycling as gypsum for cement.

In addition, the primary neutralized sludge may have an SO ³ content of 25 wt% or more. That is, as a result of controlling the pH by going through the neutralization step in two or more steps, the primary neutralized sludge may contain a lot of SO ³ , and accordingly, the gypsum produced therefrom is excellent in quality.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be embodied in various different forms, and is not limited to the embodiments described herein.

Experimental example

In this experimental example, the S content was measured through a C/S analyzer to measure the SO ^3- content in the neutralized sludge with controlled moisture content, and the SO ^3- converted value was used. In addition, to determine whether gypsum was formed, the crystal phase of the gypsum was measured through XRD analysis.

In addition, as for the method of measuring the moisture content of the neutralized sludge with controlled moisture content, the moisture content of the neutralized sludge dried at 45° C. until it became a constant weight was measured, and preferably, the moisture content of 20% or less was used as a quality control standard.

In addition, in order to measure the color of the neutralized sludge with controlled moisture content, the dried neutralized sludge powder was measured using a colorimeter. The degree of redness was quantitatively evaluated, and the degree of redness was compared by visually observing the surface of mortar or concrete prepared by substituting dry powder of neutralized sludge with a substitute for gypsum. In addition, by measuring the strength of the mortar and concrete prepared at this time, the possibility and quality of replacing the gypsum of the neutralized sludge prepared in the present invention were evaluated.

(1) Waste sulfuric acid and neutral fire preparation stage

The spent sulfuric acid used in this experimental example is annealed in a stainless steel annealing and pickling process, that is, a stainless steel sheet is annealed at a line speed of 5 to 50 mpm at an annealing temperature of 1050 to 1150 ° C. After the annealing, the strip is subjected to molten salt treatment and the molten salt treatment Waste sulfuric acid was separated and used in the waste acid wash solution generated in the step of immersion in a sulfuric acid tank and a mixed acid tank containing 10 to 30 g/L of hydrofluoric acid and 80 to 120 g/L of nitric acid.

The waste sulfuric acid obtained at this time was the concentration of the general waste sulfuric acid generated in the annealing pickling process, that is, the waste sulfuric acid having a sulfuric acid concentration of 55.5 g/L and an Fe concentration of 42 g/L. In addition, in order to improve the annealing and pickling performance, waste sulfuric acid generated after AC electrolytic pickling and acid recovery facilities, that is, waste sulfuric acid having a sulfuric acid concentration of 20 g/L and an Fe concentration of 42 g/L may be used.

In this experimental example, slaked lime powder was used as a neutralizing agent.

(2) Examples

Multi-stage neutralization was performed using 500 ml of spent sulfuric acid and slaked lime. In the first neutralization step, slaked lime was added until pH 4.3. The first neutralized solution was filtered and separated into a first neutralized filtrate and a first neutralized sludge. Slaked lime was added to the first neutralization filtrate until pH 9.4 again. The second neutralized solution was filtered and separated into a second neutralized filtrate and a second neutralized sludge.

The SO ^3- content of the primary neutralized sludge prepared in this practice was 50.4%, Fe ₂ O ₃ 4.4%, Cr ₂ O ₃ 1.5%, and the color difference was 32.3. The SO ^3- content of the secondary neutralized sludge was 26.8%, Fe ₂ O ₃ 32.1%, Cr ₂ O ₃ 6.5%, and the color difference was 65.3.

Afterwards, each of the obtained primary neutralized sludge and secondary neutralized sludge was controlled to have a moisture content of 20 wt% using a centrifugal dehydrator.

As a result, the obtained primary neutralized sludge is SO ^3- As the content increased, it was found that the quality of the gypsum produced therefrom was improved, and redness was also resolved when the color difference value was low.

On the other hand, the secondary neutralized sludge is SO ^3- It was confirmed that neutralized sludge was obtained with a significantly reduced content and enriched with valuable components such as Fe ₂ O ₃ and Cr ₂ O ₃ , which could be used as an additive in the production of stainless steel by mixing with dust generated during the production of stainless steel.

(3) Comparative Example

63 g of slaked lime was added to 500 ml of spent sulfuric acid, and neutralized sludge was prepared by performing one-stage neutralization at pH 9.5. After drying the neutralized sludge, the analysis results showed that SO ^3- 37.3%, Fe ₂ O ₃ 20.5%, Cr ₂ O ₃ 4.5%, and the colors of the neutralized sludge were L 52.1, a 14.6, b 29.2 when measured with a colorimeter, and the color difference ((100- L) ² + a ² + b ² ) was 58.0. There is no standard for color difference for using the neutralized sludge as plaster for cement, but it can be said that as the color difference increases, the cement applicability is inferior. As such, when the waste sulfuric acid is neutralized in one stage as in the conventional method, the color of the prepared neutralized sludge is red, which is not preferable because there is a limit to its use as a plaster for cement.

In the method for controlling the water content of neutralized sludge by mixing a material containing CaO with the neutralized sludge, when a material having a CaO content lower than 50% is mixed, the quality of the prepared mixed gypsum is lowered and the water content of the neutralized sludge is reduced was also not desirable. Therefore, it is undesirable to mix and use such low-quality gypsum with cement, as it causes a decrease in the strength of the cement.

The present invention is not limited to the embodiments, but can be manufactured in various different forms, and those of ordinary skill in the art to which the present invention pertains can change the technical spirit or essential features of the present invention to other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that this may be practiced. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (11)

preparing a spent sulfuric acid solution;
a second or more neutralization step of neutralizing the spent sulfuric acid solution; and
a moisture content reduction step of reducing the moisture content of the neutralized sludge generated in the neutralization step to 20% by weight or less;
A method of recycling waste sulfuric acid, comprising: According to claim 1,
The moisture content reduction step;
A method of recycling waste sulfuric acid, which is a step of controlling the moisture content of neutralized sludge using waste heat. According to claim 1,
The moisture content reduction step;
A method of recycling waste sulfuric acid, which is a step of controlling the moisture content of the neutralized sludge by mixing a material containing CaO with the neutralized sludge. According to claim 1,
The moisture content reduction step;
A method of recycling waste sulfuric acid, which is a step of centrifugally dewatering the neutralized sludge to control the water content of the neutralized sludge. 5. The method of claim 4,
Controlling the water content of the neutralized sludge by centrifugal dehydration of the neutralized sludge;
A neutralized sludge coagulation step of growing the average particle size of the neutralized sludge to 50 μm or more,
The neutralized sludge coagulation step is a step of inputting 3 to 10 wt% of the neutralized sludge into the particle growth nuclei, the waste sulfuric acid recycling method. According to claim 1,
The neutralization step that proceeds more than the second; includes the first neutralization step,
The first neutralization step is
Neutralizing to pH 2.5 to 4.5 by mixing a neutralizing agent with the spent sulfuric acid solution; and
A filtration step of separating the first neutralized solution into a first neutralized filtrate and a first neutralized sludge; Containing, Waste sulfuric acid recycling method. According to claim 1,
The neutralization step that proceeds more than the second; includes a second neutralization step that proceeds after the first neutralization step,
The second neutralization step is
Neutralizing to pH 8 to 10 by mixing a neutralizing agent with the first neutralization filtrate generated in the first neutralization step; and
Filtration step of separating the secondary neutralized solution into secondary neutralized filtrate and secondary neutralized sludge; Containing, Waste sulfuric acid recycling method. According to claim 1,
Neutralization step that proceeds more than the second;
A step of neutralizing by mixing a neutralizing agent with spent sulfuric acid,
The neutralizing agent is at least one Ca-based neutralizing agent selected from the group consisting of limestone, quicklime, slaked lime, steelmaking slag fine powder, steel auxiliary raw material dust, oil refinery desulfurization gypsum and stainless steel reduced slag, the waste sulfuric acid recycling method. According to claim 1,
Preparing the spent sulfuric acid solution;
Separating phenolic acid and spent mixed acid from the pickling waste generated after stainless steel pickling,
The waste mixed acid is a mixed solution of hydrofluoric acid and nitric acid, the waste sulfuric acid recycling method. 7. The method of claim 6,
The primary neutralized sludge has a color difference ((100-L) ² + a ² + b ² ) of 35 or less, a waste sulfuric acid recycling method. 7. The method of claim 6,
The primary neutralized sludge has an SO ³ content of 25 wt% or more, a waste sulfuric acid recycling method.

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