Preparation method for zinc-containing liquid, preparation method for electrogalvanization bath, and production method for electrogalvanized steel sheet

Abstract

Provided is a zinc-containing liquid that is produced from ironmaking dust and can be used to produce an electrogalvanized steel sheet that has a bright and beautiful appearance. The present invention is characterized by including a zinc-leaching step for bringing ironmaking dust into contact with an acidic aqueous solution to leach the zinc included in the ironmaking dust into the acidic aqueous solution, a first impurity precipitation step for precipitating impurities while the pH of a first treated liquid obtained at the zinc-leaching step is at 4.0–6.0, a second impurity precipitation step for adding at least 0.10 g / L of a chelating agent to a second treated liquid obtained at the first impurity precipitation step while the pH is at 4.0–6.0 to precipitate impurities, and a solid / liquid separation step for performing a solid / liquid separation on a third treated liquid obtained at the second impurity precipitation step to obtain a zinc-containing liquid.

Description

Method for preparing zinc-containing liquid, method for preparing electrogalvanizing bath, and method for manufacturing electrogalvanized steel sheet

[0001] The present invention relates to a method for preparing a zinc-containing liquid, a method for preparing an electrogalvanizing bath, and a method for producing an electrogalvanized steel sheet.

[0002] Zinc, a typical base metal, is used in a variety of industrial fields. In particular, in the steel industry, zinc has a low electrical potential relative to iron and exhibits sacrificial corrosion protection, providing excellent corrosion resistance. For this reason, zinc is used as a plating material for the surface of steel sheets, and is widely used in automobiles, home appliances, building materials, and other applications.

[0003] Commonly used methods for producing galvanized steel sheets include the hot-dip galvanizing method, in which a zinc coating layer is formed on the surface of the steel sheet by immersing the steel sheet in a hot-dip galvanizing bath, and the electrogalvanizing method, in which a steel sheet is immersed in an acidic solution containing zinc ions and a zinc coating layer is deposited on the surface of the steel sheet through an electrochemical reaction. In both methods, metallic zinc is often used as the zinc raw material, but in recent years, the rise of emerging countries has changed the supply and demand balance, causing the price of metallic zinc to rise.

[0004] In response to this situation, attempts have been made to separate and recover industrial waste and steelworks by-products composed of metals containing zinc instead of metallic zinc as a zinc source. For example, steelmaking dust such as blast furnace dust, converter dust, and electric furnace dust contains zinc derived from galvanized steel sheet scrap and iron ore, and techniques have been reported for separating and recovering zinc from the steelmaking dust as a zinc source. For example, Patent Document 1 describes a method for separating steelmaking dust into coarse dust and fine dust, contacting the fine dust with acid to leach zinc into the acid, and then repeatedly adding alkali and performing solid-liquid separation to separate and recover zinc.

[0005] Japanese Patent Application Laid-Open No. 2022-105980

[0006] Among zinc-coated steel sheets, electrogalvanized steel sheets are often used without painting after chemical conversion treatment, and therefore require excellent surface appearance. Whiteness is an important factor, and lightness (L value) is typically used as an indicator of whiteness. As a result of studies by the present inventors, it was found that when zinc separated and recovered by the method described in Patent Document 1 is used as the zinc source for an electrogalvanized bath, the L value of the plated surface decreases due to the influence of impurity metals (e.g., Pb and Cd) believed to be derived from steelmaking dust, and the plated appearance becomes darker than that of conventional electrogalvanized steel sheets. Therefore, when electrogalvanized steel sheets are produced using a zinc-containing solution containing zinc separated and recovered from steelmaking dust by the method described in Patent Document 1, the desired appearance cannot be obtained.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a zinc-containing liquid made from steel dust, which can be used to produce electrogalvanized steel sheets having a beautiful bright appearance.

[0008] The present inventors have conducted extensive research into ways to solve the above-mentioned problems. As a result, they have found that by contacting steel dust with an acidic aqueous solution to leach zinc contained in the steel dust into the acidic aqueous solution, adjusting the pH of the resulting treatment solution (first treatment solution) to between 4.0 and 6.0, and then adding a predetermined amount of a chelating agent to the resulting treatment solution (second treatment solution), it is possible to precipitate impurity metals as hydroxides and chelate compounds while maintaining the zinc concentration in the treatment solution. The present inventors have also found that using the resulting zinc-containing solution as a zinc source in an electrogalvanizing process makes it possible to produce electrogalvanized steel sheets with a beautiful, bright appearance, and have completed the present invention.

[0009] That is, the gist of the present invention is as follows: [1] A method for producing a zinc-containing liquid, comprising: a zinc leaching step of contacting iron manufacturing dust with an acidic aqueous solution to leach zinc contained in the iron manufacturing dust into the acidic aqueous solution, a first impurity precipitation step of precipitating impurities in a first treated liquid obtained in the zinc leaching step at a pH of 4.0 to 6.0, a second impurity precipitation step of adding 0.10 g / L or more of a chelating agent to a second treated liquid obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate impurities, and a solid-liquid separation step of performing solid-liquid separation on the third treated liquid obtained in the second impurity precipitation step to obtain a zinc-containing liquid.

[0010] [2] The method for producing a zinc-containing liquid described in [1] above further comprises a classification step of separating ironmaking dust into coarse dust and fine dust prior to the zinc leaching step, and the fine dust obtained in the classification step is used as the ironmaking dust to be used in the zinc leaching step.

[0011] [3] A method for preparing a zinc-containing liquid described in [1] or [2], wherein in the second impurity precipitation process, at least one of the chelating agents added to the second treatment liquid is a chelating agent that forms a chelate compound with at least one of Cd and Pb.

[0012] [4] The method for preparing a zinc-containing solution according to any one of [1] to [3], wherein at least one of the chelating agents added to the second treatment solution in the second impurity precipitation step is a compound having a structure represented by the following general formula (I) in a part thereof. １ and R ２ represents an optionally substituted alkyl group; M ＋ indicates a monovalent cation.

[0013] [5] A method for preparing an electrogalvanizing bath, comprising: adding a zinc-containing liquid prepared by the method for preparing a zinc-containing liquid according to any one of [1] to [4] above to an electrogalvanizing bath; and the pH of the electrogalvanizing bath is 0.5 or more and 3.5 or less.

[0014] [6] A method for producing an electrogalvanized steel sheet, characterized in that the electrogalvanized steel sheet is produced by electrogalvanizing the surface of a steel sheet using an electrogalvanized bath prepared by the method for preparing an electrogalvanized bath described in [5] above.

[0015] According to the present invention, it is possible to provide a zinc-containing liquid made from steelmaking dust as a raw material, which can be used to produce electrogalvanized steel sheets having a beautiful bright appearance.

[0016] 1 is a flowchart showing an example of a method for producing a zinc-containing liquid according to the present invention. 2 is a flowchart showing a preferred example of a method for producing a zinc-containing liquid according to the present invention.

[0017] (Method for producing zinc-containing liquid) Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments. In addition, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0018] 1 shows a flowchart of an example of a method for producing a zinc-containing liquid according to the present invention. The method for producing a zinc-containing liquid according to the present invention includes a zinc leaching step (step S1) in which ironmaking dust is brought into contact with an acidic aqueous solution to leach zinc contained in the ironmaking dust into the acidic aqueous solution, a first impurity precipitation step (step S2) in which impurities are precipitated from a first treated solution obtained in the zinc leaching step at a pH of 4.0 to 6.0, a second impurity precipitation step (step S3) in which 0.10 g / L or more of a chelating agent is added to the second treated solution obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate impurities, and a solid-liquid separation step (step S4) in which the third treated solution obtained in the second impurity precipitation step is subjected to solid-liquid separation to obtain a zinc-containing liquid.

[0019] <Zinc Leaching Step> First, in step S1, iron manufacturing dust is brought into contact with an acidic aqueous solution to leach zinc contained in the iron manufacturing dust into the acidic aqueous solution (zinc leaching step).

[0020] As the raw material iron-making dust, blast furnace dust, converter dust, electric furnace dust, etc. can be used.

[0021] The acidic aqueous solution used in this zinc leaching step can be a common acidic aqueous solution such as sulfuric acid or hydrochloric acid. From the viewpoint of using the finally recovered zinc-containing solution in an electrogalvanizing bath, it is desirable to use an acid containing the same type of anion as the electrogalvanizing bath to be used. When a sulfuric acid bath is used as the electrogalvanizing bath, it is preferable to use sulfuric acid as the acidic aqueous solution, and when a chloride bath is used as the electrogalvanizing bath, it is preferable to use hydrochloric acid as the acidic aqueous solution.

[0022] Although there is no particular limitation on the lower limit of the pH of the acidic aqueous solution used for zinc leaching, a pH below 1.0 increases the consumption of chemicals and increases the processing cost. Therefore, the pH of the acidic aqueous solution used for zinc leaching is preferably 1.0 or higher, and more preferably 2.0 or higher. On the other hand, if the pH of the acidic aqueous solution used for zinc leaching exceeds 5.0, zinc becomes difficult to leach. Therefore, the pH of the acidic aqueous solution used for zinc leaching is preferably 5.0 or lower, more preferably 4.0 or lower, and even more preferably 3.5 or lower.

[0023] The zinc leaching time is preferably 10 minutes or longer. By setting the zinc leaching time to 10 minutes or longer, the zinc can be sufficiently leached into the acidic aqueous solution. On the other hand, although there is no particular upper limit to the zinc leaching time, it is preferably 120 minutes or shorter in order to avoid prolonging the treatment time.

[0024] The reaction temperature during zinc leaching can be set arbitrarily within a temperature range in which the acidic aqueous solution does not solidify or evaporate. Considering the ability to maintain a constant temperature, the reaction temperature during zinc leaching is preferably 30°C or higher, and is preferably 90°C or lower because an excessively high temperature increases the amount of evaporation of the acidic aqueous solution. It is also preferable to stir the acidic aqueous solution during the leaching process. While there are no particular limitations on the method for stirring the acidic aqueous solution, examples include methods using a stirrer mixer, a propeller mixer, a pump, or the like.

[0025] <First Impurity Precipitation Step> Next, in step S2, impurities are precipitated in the first treatment solution obtained in the zinc leaching step while the pH of the first treatment solution is kept at 4.0 or more and 6.0 or less (first impurity precipitation step).

[0026] The first treatment solution obtained in the zinc leaching process is thought to contain impurities in the form of metal ions that inevitably leach into the acidic aqueous solution along with zinc from steelmaking dust. These impurities are thought to precipitate on the steel sheet together with zinc during electroplating, lowering the L value of the galvanized steel sheet. Therefore, reducing these impurities is essential to obtaining electrogalvanized steel sheets with a beautiful, bright appearance. On the other hand, if zinc is also precipitated when precipitating impurities, the recycling rate as a raw material for galvanizing will also decrease. Therefore, it is necessary to precipitate impurities while maintaining the zinc concentration in the first treatment solution.

[0027] The present inventors conducted extensive research into conditions for precipitating and separating only impurities while maintaining the zinc concentration in the first treatment solution. As a result, they found that impurity elements other than zinc can be precipitated as hydroxides and chelate compounds by adjusting the pH of the first treatment solution to 4.0 or more and 6.0 or less (first impurity precipitation step) and adding a predetermined amount of a chelating agent to the resulting treatment solution (second treatment solution) to precipitate the impurities (second impurity precipitation step). This results in a zinc-containing solution suitable for producing electrogalvanized steel sheets with a beautiful, bright appearance.

[0028] Therefore, in this first impurity precipitation step, the pH of the first treatment solution obtained in the zinc leaching step is adjusted to 4.0 or more and 6.0 or less, thereby precipitating impurities (mainly Fe) as hydroxides while maintaining the zinc concentration contained in the first treatment solution. By adjusting the pH of the first treatment solution to 4.0 or more, Fe can be efficiently precipitated, so the pH of the first treatment solution is set to 4.0 or more. The lower limit of the pH of the first treatment solution is preferably 4.3, more preferably 4.5. Furthermore, by adjusting the pH of the first treatment solution to 6.0 or less, zinc precipitation can be suppressed and the zinc concentration in the solution can be maintained, so the pH of the first treatment solution is set to 6.0 or less. The upper limit of the pH of the first treatment solution is preferably 5.8, more preferably 5.5.

[0029] When the pH of the first treatment solution obtained in the zinc leaching step is less than 4.0, it is necessary to adjust the pH of the first treatment solution to 4.0 or more and 6.0 or less, and examples of a method for adjusting the pH include adding a general alkaline aqueous solution of sodium hydroxide, calcium hydroxide, etc. On the other hand, when the pH of the first treatment solution obtained in the zinc leaching step is within the range of 4.0 or more and 6.0 or less, it is not necessary to adjust the pH.

[0030] The temperature of the first treatment liquid in the first impurity precipitation step can be set arbitrarily within a temperature range in which the first treatment liquid does not solidify or evaporate. The temperature of the first treatment liquid is preferably 30°C or higher in consideration of maintaining a constant temperature, and is preferably 90°C or lower because an excessively high temperature increases the amount of evaporation of the first treatment liquid. Furthermore, the reaction time of the first impurity precipitation step is preferably 10 minutes or more and 120 minutes or less in consideration of the impurity precipitation time and treatment efficiency.

[0031] <Second Impurity Precipitation Step> Subsequently, in step S3, 0.10 g / L or more of a chelating agent is added to the second treatment liquid obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate impurities (second impurity precipitation step).

[0032] Although some of the impurities (mainly Fe) can be precipitated as hydroxides in the first impurity precipitation step, heavy metal elements such as Cd and Pb remain in the solution without being precipitated. Therefore, in the second impurity precipitation step, a chelating agent is added to the second treatment solution obtained in the first impurity precipitation step, thereby precipitating heavy metal elements such as Cd and Pb as chelate compounds.

[0033] For the above-mentioned purpose, it is preferable that at least one of the chelating agents added to the second treatment solution in the second impurity precipitation step is a chelating agent that forms a chelate compound with at least one of Cd and Pb. By using such a chelating agent, heavy metal elements such as Cd and Pb can be efficiently precipitated as a chelate compound.

[0034] In the present invention, it is more preferable that at least one of the chelating agents added to the second treatment liquid in the second impurity precipitation step is a compound partially having a structure represented by the following general formula (I): １ and R ２ represents an alkyl group which may be substituted. ＋ represents a monovalent cation, such as a proton, an alkali metal ion, an ammonium ion, etc. The compound may contain two or more structures represented by general formula (I).

[0035]

[0036] The structure represented by the above general formula (I) is thought to easily form a stable complex with a heavy metal element such as Cd or Pb, since the compound contains a total of three nitrogen atoms and sulfur atoms each having a lone pair of electrons necessary for forming a coordinate bond with a metal.

[0037] Examples of compounds having such a structure include dimethyldithiocarbamates, diethyldithiocarbamates, dipotassium piperazine-1,4-dicarbodithioate, potassium diethylamine-N-carbodithioate, and ammonium 1-pyrrolidinecarbodithioate.

[0038] The amount of chelating agent added to the second treatment solution is preferably 0.10 g / L or more in order to sufficiently reduce impurity elements. The amount of chelating agent added is more preferably 0.2 g / L or more, and even more preferably 0.50 g / L or more. There is no particular upper limit to the amount of chelating agent added, but from the viewpoint of treatment costs, it is preferably 5.00 g / L or less.

[0039] In this second impurity precipitation step, a chelating agent is added to the second treatment solution obtained in the first impurity precipitation step at a pH of 4.0 or higher and 6.0 or lower. By maintaining the pH of the second treatment solution at 4.0 or higher when the chelating agent is added, decomposition of the chelating agent added in the second impurity precipitation step is suppressed, and the chelating agent can stably form complexes with heavy metal elements and efficiently remove them. The lower limit of the pH of the second treatment solution is preferably 4.3, more preferably 4.5. Furthermore, since zinc precipitates at a pH above 6.0 and the zinc concentration in the solution decreases, the pH of the second treatment solution must be maintained at 6.0 or lower. The upper limit of the pH of the second treatment solution is preferably 5.8, more preferably 5.5.

[0040] The pH of the second treatment liquid obtained in the first impurity precipitation step is preferably maintained in the range of 4.0 to 6.0 until the chelating agent is added. As described above, the pH of the second treatment liquid needs to be maintained at 6.0 or less, but the pH of the second treatment liquid may temporarily become less than 4.0 as long as the pH is in the range of 4.0 to 6.0 when the chelating agent is added.

[0041] The reaction time of the second impurity precipitation step is preferably 10 minutes or more. By setting the reaction time to 10 minutes or more, impurities contained in the second treatment liquid can be sufficiently precipitated. The upper limit of the reaction time is not particularly limited, but it is preferably 120 minutes or less to avoid prolonging the treatment time. The temperature of the second treatment liquid in the second impurity precipitation step can be set arbitrarily within a temperature range in which the second treatment liquid does not solidify or evaporate. Considering the ability to maintain a constant temperature, the temperature of the second treatment liquid is preferably 30°C or more, and is preferably 90°C or less because an excessively high temperature increases the amount of evaporation of the second treatment liquid.

[0042] <Solid-Liquid Separation Step> Finally, in step S4, the third treated liquid obtained in the second impurity precipitation step is subjected to solid-liquid separation to obtain a zinc-containing liquid (solid-liquid separation step).

[0043] This solid-liquid separation step is a step of separating the third treated liquid obtained in the second impurity precipitation step into a precipitated solid (hydroxides and chelate compounds of impurities) and a zinc-containing liquid. The method of solid-liquid separation is not particularly limited, and any method can be selected, such as gravity sedimentation, filtration, centrifugation, or a filter press.

[0044] In this way, a zinc-containing solution can be prepared from steelmaking dust. The zinc-containing solution obtained by the present invention can be used as part of an electrogalvanizing bath. It can also be used as part of a plating bath when electroplating a zinc-containing metal, such as Fe—Zn electroplating or Ni—Zn electroplating, and can also be used in the production of electroplated steel sheets having such an electroplated layer on at least one side.

[0045] <Classification step> Fig. 2 shows a flowchart of a preferred example of the method for producing a zinc-containing solution according to the present invention. As shown in Fig. 2, in the present invention, it is preferable to separate the ironmaking dust into coarse dust and fine dust in step S5, which is a step prior to the zinc leaching step (classification step). It is preferable to use the fine dust obtained in this classification step as the ironmaking dust to be used in the zinc leaching step, which is a step subsequent to this classification step.

[0046] Zinc volatilized during the steelmaking process generally exists as fine particles of a few micrometers in size and tends to have a lower specific gravity than iron. Therefore, fine dust contains more zinc than coarse dust. Furthermore, because fine dust has a smaller average particle size than coarse dust, it is more reactive with the acidic aqueous solution in the subsequent zinc leaching process, which increases the processing efficiency of the zinc leaching reaction.

[0047] In the present invention, when the fine dust obtained in the classification step is used as the ironmaking dust to be used in the zinc leaching step, the average particle size of the fine dust is not particularly limited. However, from the viewpoint of a high zinc content and high reactivity in the zinc leaching step, the average particle size of the fine dust is preferably 100 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less.

[0048] There are no particular limitations on the method used in this classification step. For example, the ironworks dust can be separated into coarse dust and fine dust by using a sieve or by utilizing the difference in the settling velocity or movement velocity of particles in a fluid (fluid classification).

[0049] (Method for Preparing Electrogalvanizing Bath) The method for preparing an electrogalvanizing bath according to the present invention is characterized in that the zinc-containing liquid prepared by the method for preparing a zinc-containing liquid according to the present invention described above is used as at least a part of the zinc source of the electrogalvanizing bath, and the pH of the electrogalvanizing bath is 0.5 or more and 3.5 or less.

[0050] In addition to the zinc-containing solution of the present invention, other zinc sources that can be used in the electrogalvanizing bath used in the present invention include, for example, zinc granules, zinc sulfate (heptahydrate), zinc chloride, and zinc carbonate. Furthermore, it is preferable to use an acidic bath, which has excellent productivity, as the type of electrogalvanizing bath, and a sulfate bath, a chloride bath, or a mixture thereof can be used. However, since chloride baths require treatment of chlorine gas generated when an insoluble anode is used, it is more preferable to use a sulfate bath.

[0051] The pH of the electrogalvanizing bath is adjusted to a range of 0.5 to 3.5, preferably 1.0 to 3.0. By adjusting the pH of the electrogalvanizing bath to a range of 0.5 to 3.5, active generation of hydrogen and a decrease in current efficiency can be suppressed, while the occurrence of plating burn on the plating layer can be suppressed, resulting in a beautiful plating appearance. The pH is preferably adjusted using an acid appropriate for the type of electrogalvanizing bath; for example, when the type of electrogalvanizing bath is a sulfuric acid bath, the pH is preferably adjusted with sulfuric acid.

[0052] The method of using the zinc-containing liquid according to the present invention as at least a part of the zinc source of an electrogalvanizing bath is not particularly limited, and possible methods include adding the zinc-containing liquid to an acidic bath prepared using a zinc source other than the zinc-containing liquid and then adjusting the pH, or adding a zinc source other than the zinc-containing liquid to the zinc-containing liquid and then adjusting the pH.

[0053] The proportion of the zinc-containing solution contained as a zinc source in the electrogalvanizing bath is preferably 1 mass% or more in terms of the total zinc source. This is because a proportion of the zinc-containing solution of 1 mass% or more ensures sufficient cost benefits according to the present invention. On the other hand, there is no upper limit to the proportion of the zinc-containing solution, and the entire amount of the zinc-containing solution may be used as the electrogalvanizing bath.

[0054] The zinc concentration in the electrogalvanizing bath is preferably 0.5 mol / L or more. If the zinc concentration is 0.5 mol / L or more, there is no shortage of zinc ions near the steel sheet to be plated during electrolysis, and plating burn can be suppressed. There is no upper limit to the zinc concentration, but from the viewpoint of processing costs, it is preferably 2 mol / L or less.

[0055] In addition, the temperature of the electrogalvanizing bath is preferably 30° C. or higher in consideration of maintaining a constant temperature. Although there is no particular upper limit to the temperature of the electrogalvanizing bath, a temperature of up to about 90° C. is practical and preferable because an increase in temperature increases the amount of evaporation of the plating bath.

[0056] Furthermore, if necessary, a conductivity aid can be added to the plating bath. This is because it can improve the conductivity of the plating bath. Examples of the conductivity aid include sodium sulfate, ammonium sulfate, and potassium sulfate. However, if the amount added is too large, the whiteness of the plating may decrease when electrolytic treatment is performed at a high current density. Therefore, the amount of the conductivity aid added is preferably 0.5 mol / L or less.

[0057] The preferred ranges for the concentrations of impurity elements (Pb, Cd, and Fe) contained in the plating bath are 1.0 ppm or less for Pb and Cd. If the concentrations of these impurity elements exceed 1.0 ppm, the L value of the plating surface may decrease. For the same reason, the Fe concentration is preferably 1000 ppm or less.

[0058] On the other hand, electrogalvanized steel sheets with a more beautiful surface can be obtained by adding inorganic components containing Sn, In, Bi, or Sb, or organic components having a 2-benzothiazolylthio group, as needed, to the plating bath. These inorganic and organic components are preferably controlled to a range of 0.01 to 3 ppm. The addition form of these components is not particularly limited. The inorganic components may be added in the metallic state or in the form of sulfates, chlorides, carbonates, oxides, etc. However, when using salts, it is preferable to use salts appropriate for the type of electrogalvanized bath. As the organic component having a 2-benzothiazolylthio group, 2-mercaptobenzothiazole (MBT) or its salts can be used.

[0059] (Method for manufacturing electrogalvanized steel sheet) The method for manufacturing electrogalvanized steel sheet according to the present invention is characterized in that electrogalvanized steel sheet is manufactured by electrogalvanizing using an electrogalvanized steel bath prepared by the method for preparing an electrogalvanized steel bath according to the present invention described above. When manufacturing electrogalvanized steel sheet, a zinc-containing solution may be prepared in advance by the method for preparing a zinc-containing solution according to the present invention, and a zinc source other than the zinc-containing solution, such as zinc particles or zinc sulfate, may be added as needed to prepare an electrogalvanized steel sheet, followed by electroplating. Alternatively, a zinc source containing the zinc-containing solution prepared by the method for preparing a zinc-containing solution according to the present invention may be added to the electrogalvanized steel sheet production line.

[0060] The current density during electrolysis is 10 to 130 A / dm ２ It is preferable to set the range to 10 to 100 A / dm ２ It is more preferable to set the current density during electrolysis to 10 A / dm ２ By setting the current density at 130 A / dm or more, it is possible to prevent a decrease in current efficiency and an increase in the time required to secure a predetermined plating coating weight. ２ By setting the following, it is possible to suppress the occurrence of plating burn.

[0061] The type of counter electrode (anode) is not particularly limited, but an iridium oxide electrode is preferred in consideration of the dissolution of impurities into the plating bath. Furthermore, a plating bath flow rate of 1.0 m / sec or more is preferred in terms of thinning the diffusion layer at the electrolytic interface.

[0062] In this way, electrogalvanized steel sheets can be produced using the zinc-containing liquid prepared by the method for preparing a zinc-containing liquid according to the present invention.

[0063] A surface-treated steel sheet can also be obtained by forming a chemical conversion coating on the surface of the electrogalvanized steel sheet produced as described above. The chemical conversion coating can be formed, for example, by a chromium-free chemical conversion treatment in which a chromium-free chemical conversion treatment solution is applied and then heated and dried at a steel sheet temperature of 80 to 300°C without rinsing with water. The chemical conversion coating may be a single layer or a multi-layer, and in the case of a multi-layer, multiple chemical conversion treatments can be performed sequentially.

[0064] Furthermore, depending on the intended use of the electrogalvanized steel sheet, a single-layer or multi-layer coating containing an organic resin can be formed on the surface of the plating layer or chemical conversion coating of the electrogalvanized steel sheet produced as described above to produce a surface-treated steel sheet. Examples of such coatings include polyester-based resin coatings, epoxy-based resin coatings, acrylic-based resin coatings, urethane-based resin coatings, and fluorine-based resin coatings. Also applicable are coatings in which part of the above resins is modified with another resin, such as epoxy-modified polyester-based resin coatings. Furthermore, the above resins can be supplemented with curing agents, curing catalysts, pigments, additives, and the like, as needed.

[0065] The coating method for forming the coating film is not particularly limited, but examples of the coating method include roll coater coating, curtain flow coating, spray coating, etc. For example, a coating material containing an organic resin can be applied to the surface of the plating layer or chemical conversion coating of an electrogalvanized steel sheet, and then heated and dried by means of hot air drying, infrared heating, induction heating, etc. to form a coating film.

[0066] The above-described method for producing a surface-treated steel sheet is an example, and the present invention is not limited to this.

[0067] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0068] First, electric furnace dust (Zn content: 40% by mass, average particle size: 10 μm) as steelmaking dust was added to a sulfuric acid aqueous solution adjusted to pH 2.0 and stirred for 1 hour to leach zinc into the sulfuric acid aqueous solution (zinc leaching process). Next, the resulting solution was subjected to bath adjustment under various conditions and impurity precipitation processes (first impurity precipitation process and second impurity precipitation process). Table 1 shows the type of chelating agent, and Table 2 shows the amount of electric furnace dust added, the type of alkaline aqueous solution used to adjust the pH in the first impurity precipitation process, the pH of the solution after pH adjustment, and the type and amount of chelating agent added in the second impurity precipitation process. The structural formulas of A1 to A4 shown in Table 1 are also shown. In both the first impurity precipitation process and the second impurity precipitation process, the impurity precipitation process was performed at a solution temperature of 50°C with stirring for 1 hour after bath adjustment. Subsequently, solid-liquid separation was performed using a suction filter (solid-liquid separation process) to obtain a zinc-containing solution. The concentrations of Zn, Fe, Cd, and Pb contained in each zinc-containing solution were measured using an ICP (Inductively Coupled Plasma) mass spectrometer, and the results are shown in Table 2.

[0069]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075] Electrogalvanizing treatment was carried out under the conditions shown below using electrogalvanizing baths in which zinc sulfate heptahydrate was added to each of the zinc-containing solutions obtained as described above to adjust the zinc concentration in the bath to the concentrations shown in Table 2, to produce electrogalvanized steel sheets. A cold-rolled steel sheet with a thickness of 0.7 mm, produced by a conventional method, was used as the base sheet for plating. Test pieces (150 mm x 70 mm) for batch electrogalvanizing were cut out from this steel sheet, and after degreasing and pickling, electrogalvanized. The zinc coating weight per side was 20 g / m in all cases. ２ It was decided.

[0076] Electrolysis conditions Current density: 50A / dm ２ Bath temperature: 50°C Electrode: iridium oxide Flow rate: 2.0 m / sec

[0077] Furthermore, the plated surface of the obtained electrogalvanized steel sheet was coated with an epoxy resin (trade name: jER1009, manufactured by Japan Epoxy Resins Co., Ltd.) to prepare a chromium-free chemical conversion treated steel sheet.

[0078] The lightness (L value) of the obtained electrogalvanized steel sheets and chromium-free chemical conversion treated steel sheets was measured using a spectrophotometer (SD5000 manufactured by Nippon Denshoku Industries Co., Ltd.) with SCE (specular reflection excluded) in accordance with JIS Z8722:2009. For electrogalvanized steel sheets, an L value of 80 or more was evaluated as ⊚, an L value of 76 or more but less than 80 was evaluated as ◯, and an L value of less than 76 was evaluated as x. For chromium-free chemical conversion treated steel sheets, an L value of 60 or more was evaluated as ⊚, an L value of 56 or more but less than 60 was evaluated as ◯, and an L value of less than 56 was evaluated as x. ⊚, ◯, and x indicate better performance in this order.

[0079] No. 1 in Table 2 is a comparative example of an electrogalvanized steel sheet and a chromium-free chemical conversion treated steel sheet that were electrogalvanized without performing either the first or second impurity precipitation step. It can be seen that the zinc-containing solution contained many impurities, resulting in a low L value. No. 2 is a comparative example in which only the first impurity precipitation step was performed. Although the Fe concentration in the zinc-containing solution was reduced, Cd and Pb still remained, resulting in a low L value. No. 3 is a comparative example in which only the second impurity precipitation step was performed. However, due to the low pH and decomposition of the chelating agent, the impurity concentration in the zinc-containing bath was high, similar to No. 1, and the L value was also low. No. 4 is a comparative example in which the pH was outside the lower limit of the first impurity precipitation step. Although the Fe concentration was reduced, due to the low pH and decomposition of the chelating agent, Cd and Pb remained, resulting in a low L value. No. 5 is a comparative example in which the pH in the first impurity precipitation step was outside the upper limit, and the Zn concentration was significantly reduced, making it unsuitable as a zinc source for electrogalvanization. No. 6 is a comparative example in which the amount of chelate compound added was outside the lower limit, but the Cd and Pb concentrations were not sufficiently reduced, and the L value was also low. On the other hand, in the inventive examples Nos. 7 to 22, it was possible to reduce the Fe, Cd, and Pb concentrations in the zinc-containing solution without significantly reducing the Zn concentration, and the L value was also high.

[0080] Nos. 23 and 24 are examples in which the zinc source content of the prepared zinc-containing solution in the electrogalvanizing bath was 1.1 mass%, near the lower limit of the preferred range of the present invention. No. 23 is a comparative example in which a zinc-containing solution prepared without performing either the first or second impurity precipitation process was used. Although the Fe, Cd, and Pb concentrations in the electrogalvanizing bath were reduced, the L value was low because the impurities were outside the allowable range. On the other hand, in Example No. 24, in which both the first and second impurity precipitation processes were performed, the Fe, Cd, and Pb concentrations in the electrogalvanizing bath were reduced, and the L value was also high. Furthermore, No. 25 is an example in which only a zinc-containing solution was used, but the Fe, Cd, and Pb concentrations in the electrogalvanizing bath were reduced, and the L value was also high. Furthermore, No. 26 is an example in which only a zinc-containing solution was used, but the Fe, Cd, and Pb concentrations in the electrogalvanizing bath were reduced, and the L value was also high. It can be seen that Example No. 10, which is an invention example to which MBT was added, has an increased L value compared to Example No. 10, which does not contain MBT.

[0081] According to the present invention, it is possible to provide a zinc-containing liquid made from steelmaking dust as a raw material, which can be used to produce electrogalvanized steel sheets having a beautiful bright appearance.

Claims

1. a zinc leaching step of contacting ironworks dust with an acidic aqueous solution to leach zinc contained in the ironworks dust into the acidic aqueous solution; a first impurity precipitation step in which impurities are precipitated in the first treatment solution obtained in the zinc leaching step at a pH of 4.0 or more and 6.0 or less; a second impurity precipitation step of adding 0.10 g / L or more of a chelating agent to the second treatment liquid obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate impurities; a solid-liquid separation step of subjecting the third treated liquid obtained in the second impurity precipitation step to solid-liquid separation to obtain a zinc-containing liquid; A method for preparing a zinc-containing liquid, comprising:

2. 2. The method for producing a zinc-containing liquid according to claim 1, further comprising a classification step of separating ironworking dust into coarse dust and fine dust prior to the zinc leaching step, and using the fine dust obtained in the classification step as the ironworking dust used in the zinc leaching step.

3. 2. A method for preparing a zinc-containing liquid according to claim 1, wherein in the second impurity precipitation step, at least one of the chelating agents added to the second treatment liquid is a chelating agent that forms a chelate compound with at least one of Cd and Pb.

4. A method for preparing a zinc-containing liquid as described in claim 2, wherein at least one of the chelating agents added to the second treatment liquid in the second impurity precipitation step is a chelating agent that forms a chelate compound with at least one of Cd and Pb.

5. The method for producing a zinc-containing solution according to any one of claims 1 to 4, wherein at least one of the chelating agents added to the second treatment solution in the second impurity precipitation step is a compound having a structure represented by the following general formula (I) in a part thereof. １ and R ２ represents an optionally substituted alkyl group; M ＋ indicates a monovalent cation. 【Chemistry 1】

6. A method for preparing an electrogalvanizing bath, comprising: a zinc-containing liquid prepared by the method for preparing a zinc-containing liquid according to any one of claims 1 to 4; and an electrogalvanizing bath having a pH of 0.5 or more and 3.5 or less.

7. A method for preparing an electro-galvanized bath, comprising: containing a zinc-containing liquid prepared by the method for preparing a zinc-containing liquid described in claim 5 in an electro-galvanized bath; and wherein the pH of the electro-galvanized bath is 0.5 or higher and 3.5 or lower.

8. A method for producing an electrogalvanized steel sheet, comprising the steps of: electrogalvanizing a surface of a steel sheet using an electrogalvanized bath prepared by the method for preparing an electrogalvanized bath according to claim 6, thereby producing an electrogalvanized steel sheet.

9. A method for producing an electro-galvanized steel sheet, characterized in that the electro-galvanized steel sheet is produced by electro-galvanizing the surface of a steel sheet using an electro-galvanized bath prepared by the method for preparing an electro-galvanized bath described in claim 7.

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