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

Abstract

Provided is a zinc-containing liquid using iron-making dust as a starting material, which can be used for producing an electrogalvanized steel sheet having a beautiful and bright appearance. The present invention is characterized by comprising: a zinc leaching step in which iron-making dust is brought into contact with an acidic aqueous solution, and zinc contained in the iron-making dust is leached into the acidic aqueous solution; a first impurity precipitation step in which impurities are precipitated in a state in which the pH of the first treatment liquid obtained in the zinc leaching step is 4.0-6.0; a second impurity precipitation step for adding 0.10 g / L or more of a chelating agent to the second treatment liquid obtained in the first impurity precipitation step in a pH of 4.0-6.0 to precipitate impurities; and a solid-liquid separation step in which the third treated liquid obtained in the second impurity precipitation step is subjected to solid-liquid separation to obtain a zinc-containing liquid.

Description

Technical Field

[0001] This invention relates to a method for preparing zinc-containing liquid, a method for preparing an electroplating zinc bath, and a method for manufacturing electroplated zinc steel sheets. Background Art

[0002] Zinc, as a representative base metal, is used in a wide variety of industries. Particularly in the steel industry, zinc exhibits excellent corrosion resistance due to its low potential relative to iron, providing sacrificial corrosion protection. Therefore, zinc is widely used as a coating material for steel sheets in automotive, home appliance, and building materials applications.

[0003] Common methods for manufacturing galvanized steel sheets include hot-dip galvanizing, which forms a zinc coating on the steel sheet surface by immersing it in a hot-dip galvanizing bath, and electroplating, which immerses the steel sheet in an acidic solution containing zinc ions and deposits a zinc coating on the steel sheet surface through an electrochemical reaction. Both methods typically use metallic zinc as the raw material. However, in recent years, with the rise of emerging countries, the supply and demand balance has changed, leading to an increase in the price of metallic zinc.

[0004] In such cases, attempts are being made to separate and recover industrial waste and ironmaking byproducts composed of other metals containing zinc as a zinc source, replacing metallic zinc. For example, ironmaking dust such as blast furnace dust, converter dust, and electric furnace dust contains zinc from galvanized steel scrap and iron ore. Therefore, technologies for recovering and separating zinc from ironmaking dust as a zinc source have been reported. For example, Patent Document 1 describes a method that separates ironmaking dust into coarse and fine dust particles, contacts the fine dust with acid to leach zinc into the acid, and then recovers the zinc by repeatedly adding alkali and performing solid-liquid separation.

[0005] Existing technical documents

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2022-105980 Summary of the Invention

[0008] In galvanized steel sheets, electro-galvanized steel sheets are typically used directly without coating after chemical conversion treatment, requiring excellent surface appearance. Whiteness is a crucial factor, usually expressed as brightness (L value). The inventors' research indicates that when zinc recovered from ironmaking dust using the method described in Patent Document 1 is used as the zinc raw material for the electro-galvanizing bath, the L value of the plated surface decreases due to the influence of impurity metals (e.g., Pb, Cd) believed to originate from ironmaking dust, resulting in a darker appearance compared to conventional electro-galvanized steel sheets. Therefore, when using a zinc-containing liquid containing zinc recovered from ironmaking dust using the method described in Patent Document 1 to produce electro-galvanized steel sheets, the desired appearance cannot be obtained.

[0009] Therefore, the object of the present invention is to provide a zinc-containing liquid made from iron smelting dust that can be used to manufacture electro-galvanized steel sheets with a beautiful and bright appearance.

[0010] The inventors conducted in-depth research on methods to solve the aforementioned problems. Their findings revealed that by contacting iron smelting dust with an acidic aqueous solution, the zinc contained in the iron smelting dust is leached into the acidic aqueous solution. After the pH of the resulting treatment solution (first treatment solution) reaches 4.0 to 6.0, a predetermined amount of chelating agent is added to the resulting treatment solution (second treatment solution). This maintains the zinc concentration in the treatment solution while allowing impurity metals to precipitate as hydroxides and chelates. Furthermore, the inventors discovered that by using the obtained zinc-containing liquid as a zinc source in the electroplating zinc process, electroplated galvanized steel sheets with a beautiful and bright appearance can be manufactured, thus completing the present invention.

[0011] That is, the essence of the present invention is as follows.

[0012] [1] A method for preparing a zinc-containing liquid, characterized by comprising the following steps:

[0013] In the zinc leaching process, iron smelting dust is brought into contact with an acidic aqueous solution, so that the zinc contained in the iron smelting dust is leached into the acidic aqueous solution.

[0014] In the first impurity precipitation step, impurities are precipitated when the pH of the first treatment solution obtained in the zinc leaching step is 4.0 to 6.0.

[0015] In the second impurity precipitation step, a chelating agent of 0.10 g / L or more is added to the second processed solution obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate impurities; and

[0016] In the solid-liquid separation process, the third processing liquid obtained in the second impurity precipitation process is subjected to solid-liquid separation to obtain a zinc-containing liquid.

[0017] [2] According to the method for producing zinc-containing liquid described in [1] above, in the pre-process of zinc leaching, a grading process is further included to separate iron smelting dust into coarse dust and fine dust, and the fine dust obtained in the grading process is used as the iron smelting dust used in the zinc leaching process.

[0018] [3] According to the method for preparing zinc-containing liquid described in [1] or [2] above, in the second impurity precipitation step above, at least one of the chelating agents added to the second treatment liquid above is a chelating agent that forms a chelate with at least one of Cd and Pb.

[0019] [4] The method for producing a zinc-containing liquid according to any one of [1] to [3] above, wherein, in the second impurity precipitation step above, at least one of the chelating agents added to the second treatment liquid is a compound having the structure shown in the following general formula (I). Wherein, R 1 and R 2 M indicates a substituted alkyl group. + It represents a monovalent cation.

[0020]

[0021] [5] A method for preparing an electroplating zinc bath, characterized in that the electroplating zinc bath contains a zinc-containing liquid prepared by any one of the zinc-containing liquid preparation methods described in any one of [1] to [4] above, and the pH of the electroplating zinc bath is 0.5 to 3.5.

[0022] [6] A method for manufacturing an electro-galvanized steel sheet, characterized in that an electro-galvanizing bath prepared by the method described in [5] above is used to electro-galvanize the surface of the steel sheet, thereby manufacturing an electro-galvanized steel sheet.

[0023] According to the present invention, a zinc-containing liquid made from iron smelting dust can be provided for use in manufacturing electroplated galvanized steel sheets with a beautiful and bright appearance. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating an example of a method for preparing a zinc-containing liquid according to the present invention.

[0025] Figure 2 This is a flowchart illustrating a preferred example of a method for preparing the zinc-containing liquid according to the present invention. Detailed Implementation

[0026] (Preparation method of zinc-containing liquid)

[0027] The embodiments of the present invention will now be described. It should be noted that the present invention is not limited to the following embodiments. Furthermore, in this specification, the numerical range indicated by “~” refers to the range including the values ​​described before and after “~” as both the lower and upper limits.

[0028] Figure 1 The diagram illustrates a flow chart of an example of a method for producing the zinc-containing liquid according to the present invention. The method for producing the zinc-containing liquid according to the present invention is characterized by comprising the following steps: a zinc leaching step (step S1), in which iron smelting dust is contacted with an acidic aqueous solution to leach zinc contained in the iron smelting dust into the acidic aqueous solution; a first impurity precipitation step (step S2), in which the pH of the first treated solution obtained in the zinc leaching step is set to 4.0–6.0 to precipitate impurities; a second impurity precipitation step (step S3), in which chelating agent of 0.10 g / L or more is added to the second treated solution obtained in the first impurity precipitation step at a pH of 4.0–6.0 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 the zinc-containing liquid.

[0029] <Zinc Leaching Process>

[0030] First, in step S1, the ironmaking dust is brought into contact with an acidic aqueous solution, so that the zinc contained in the ironmaking dust is leached into the acidic aqueous solution (zinc leaching process).

[0031] Ironmaking dust used as raw materials can include blast furnace dust, converter dust, electric furnace dust, etc.

[0032] In this zinc leaching process, common acidic aqueous solutions such as sulfuric acid and hydrochloric acid can be used as the acidic aqueous solution. It should be noted that, from the viewpoint of using the ultimately recovered zinc-containing liquid in an electroplating bath, it is preferable to use an acid containing anions of the same type as those in the electroplating bath. When using a sulfuric acid bath as the electroplating bath, sulfuric acid is preferred as the acidic aqueous solution; when using a chloride bath as the electroplating bath, hydrochloric acid is preferred as the acidic aqueous solution.

[0033] There is no particular lower limit to the pH of the acidic aqueous solution used for zinc leaching, but if the pH is below 1.0, the amount of reagent consumed increases, leading to higher processing costs. Therefore, the pH of the acidic aqueous solution used for zinc leaching is preferably 1.0 or higher, more preferably 2.0 or higher. On the other hand, if the pH of the acidic aqueous solution used for zinc leaching is higher than 5.0, zinc is 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.

[0034] Furthermore, the zinc leaching time is preferably 10 minutes or more. By setting the zinc leaching time to 10 minutes or more, the zinc can be fully leached into the acidic aqueous solution. On the other hand, there is no particular upper limit to the zinc leaching time, but to avoid prolonging the treatment time, it is preferable to set it to 120 minutes or less.

[0035] It should be noted that the reaction temperature during zinc leaching can be arbitrarily set within the temperature range where the acidic aqueous solution will not solidify or evaporate. Considering temperature stability, the reaction temperature during zinc leaching is preferably above 30°C. If the temperature is too high, the evaporation rate of the acidic aqueous solution will increase; therefore, it is preferably below 90°C. Furthermore, it is preferable to stir the acidic aqueous solution during the leaching process. The method of stirring the acidic aqueous solution is not particularly limited; for example, methods using a stir bar mixer, a propeller mixer, or a pump can be cited.

[0036] <First Impurity Precipitation Process>

[0037] Next, in step S2, impurities are precipitated in the first treatment solution obtained in the zinc leaching process at a pH of 4.0 to 6.0 (first impurity precipitation step).

[0038] It can be assumed that the first treatment solution obtained in the zinc leaching process inevitably contains impurities in the form of metal ions, which are leached from ironmaking dust into the acidic aqueous solution along with zinc. Furthermore, it is believed that these impurities precipitate onto the steel sheet along with zinc during electroplating, thus reducing the L-value of the galvanized steel sheet. Therefore, to obtain electroplated galvanized steel sheets with a beautiful and bright appearance, it is crucial to reduce these impurities. On the other hand, if zinc also precipitates during impurity precipitation, the resource reuse rate as a galvanizing raw material will also decrease. Therefore, it is necessary to maintain the zinc concentration in the aforementioned first treatment solution while simultaneously precipitating the impurities.

[0039] The inventors conducted in-depth research on the conditions for separating and precipitating impurities while maintaining the zinc concentration in the first treatment solution. The results showed that by maintaining the pH of the first treatment solution at 4.0–6.0 (first impurity precipitation step), and adding a predetermined amount of chelating agent to the resulting treatment solution (second treatment solution) to precipitate impurities (second impurity precipitation step), impurity elements other than zinc can precipitate as hydroxides and chelates. Therefore, a zinc-containing liquid suitable for manufacturing electroplated galvanized steel sheets with a beautiful and bright appearance can be obtained.

[0040] Therefore, in this first impurity precipitation step, by maintaining the pH of the first treatment solution obtained in the zinc leaching step at 4.0 to 6.0, impurities (mainly Fe) are precipitated as hydroxides while maintaining the zinc concentration in the first treatment solution. By maintaining the pH of the first treatment solution at 4.0 or higher, Fe can be precipitated efficiently; therefore, the pH of the first treatment solution is set to 4.0 or higher. The lower limit of the pH of the first treatment solution is preferably 4.3, more preferably 4.5. Furthermore, by maintaining the pH of the first treatment solution at 6.0 or lower, zinc precipitation can be suppressed, and the zinc concentration in the solution can be maintained; therefore, the pH of the first treatment solution is set to 6.0 or lower. The upper limit of the pH of the first treatment solution is preferably 5.8, more preferably 5.5.

[0041] When the pH of the first treatment solution obtained in the zinc leaching process is less than 4.0, it is necessary to adjust the pH of the first treatment solution to 4.0 to 6.0. Methods for adjusting the pH include, for example, adding common alkaline aqueous solutions such as sodium hydroxide or calcium hydroxide. Conversely, when the pH of the first treatment solution obtained in the zinc leaching process is within the range of 4.0 to 6.0, pH adjustment may not be necessary.

[0042] The temperature of the first processing solution in the first impurity precipitation step can be arbitrarily set within the temperature range where the first processing solution does not solidify or evaporate. Considering temperature stability, the temperature of the first processing solution is preferably 30°C or higher. If the temperature is too high, the evaporation rate of the first processing solution will increase, so it is preferably below 90°C. Furthermore, considering the precipitation time and processing efficiency of impurities, the reaction time of the first impurity precipitation step is preferably 10 to 120 minutes.

[0043] <Second Impurity Precipitation Process>

[0044] Next, in step S3, a chelating agent of 0.10 g / L or more is added to the second treatment solution obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate the impurities (second impurity precipitation step).

[0045] The first impurity precipitation step allows some impurities (mainly Fe) to precipitate as hydroxides, but heavy metals such as Cd and Pb do not precipitate and remain in the solution. Therefore, in the second impurity precipitation step, a chelating agent is added to the second treated solution obtained in the first impurity precipitation step, causing heavy metals such as Cd and Pb to precipitate as chelates.

[0046] For the above purposes, in the second impurity precipitation step, at least one of the chelating agents added to the second treatment solution is preferably a chelating agent that forms a chelate 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 chelates.

[0047] In this invention, in the second impurity precipitation step, at least one chelating agent added to the second treatment liquid is more preferably a compound having the structure shown in general formula (I). Wherein, R 1 and R 2 This indicates a substituted alkyl group. Additionally, M... + This indicates a monovalent cation, such as a proton, alkali metal ion, or ammonium ion. It should be noted that a compound may contain more than two structures represented by general formula (I).

[0048]

[0049] The structure represented by the above general formula (I) contains a total of three nitrogen and sulfur atoms with lone electrons required for coordination bonding with metals, and is therefore considered to be likely to form stable complexes with heavy metal elements such as Cd and Pb.

[0050] Examples of compounds with this structure include dimethyl dithiocarbamates, diethyl dithiocarbamates, piperazine-1,4-dithiocarbamate dipotassium salt, diethylamine-N-dithiocarbamate potassium salt, and pyrrolidine-1-dithiocarbamate ammonium salt.

[0051] Furthermore, the amount of chelating agent added to the second treatment solution is preferably 0.10 g / L or more to sufficiently reduce impurity elements. More preferably, the amount of chelating agent added is 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 cost, it is preferably 5.00 g / L or less.

[0052] In the second impurity precipitation step, a chelating agent is added to the second processed solution obtained in the first impurity precipitation step at a pH of 4.0 to 6.0. By ensuring that the pH of the second processed solution at the time of chelating agent addition is 4.0 or higher, the decomposition of the chelating agent added in the second impurity precipitation step can be suppressed, allowing it to stably form complexes with heavy metal elements and efficiently remove them. The lower limit of the pH of the second processed solution is preferably 4.3, and more preferably 4.5. Furthermore, when the pH exceeds 6.0, zinc precipitates, and the zinc concentration in the solution decreases; therefore, the pH of the second processed solution needs to be maintained below 6.0. The upper limit of the pH of the second processed solution is preferably 5.8, and more preferably 5.5.

[0053] The pH of the second treatment solution obtained in the first impurity precipitation step is preferably maintained at 4.0 to 6.0 before the addition of the chelating agent. As mentioned above, the pH of the second treatment solution needs to be maintained below 6.0, but as long as the pH is 4.0 to 6.0 when the chelating agent is added, the pH of the second treatment solution can be temporarily lower than 4.0.

[0054] Furthermore, the reaction time for the second impurity precipitation step is preferably 10 minutes or more. By setting the reaction time to 10 minutes or more, the impurities contained in the second treatment solution can be sufficiently precipitated. There is no particular upper limit to the reaction time, but to avoid prolonging the treatment time, it is preferably 120 minutes or less. It should be noted that the temperature of the second treatment solution in the second impurity precipitation step can be arbitrarily set within the temperature range where the second treatment solution will not solidify or evaporate. Considering temperature stability, the temperature of the second treatment solution is preferably 30°C or higher; if the temperature is too high, the evaporation rate of the second treatment solution will increase, therefore, 90°C or lower is preferred.

[0055] <Solid-Liquid Separation Process>

[0056] Finally, in step S4, the third processing liquid obtained in the second impurity precipitation process is subjected to solid-liquid separation to obtain a zinc-containing liquid (solid-liquid separation process).

[0057] This solid-liquid separation process separates the third processed liquid obtained in the second impurity precipitation process into a precipitated solid component (hydroxides and chelates of impurities) and a zinc-containing liquid. There are no particular limitations on the solid-liquid separation method; for example, gravity sedimentation, filtration, centrifugation, filter press, or any other method can be used.

[0058] This allows zinc-containing liquid to be produced from iron smelting dust.

[0059] The zinc-containing liquid obtained according to the present invention can be used as part of an electroplating zinc bath. Additionally, it can be used as part of a plating bath in zinc-containing metal electroplating processes such as Fe-Zn and Ni-Zn plating, and can also be used to manufacture electroplated steel sheets with these plating layers on at least one side.

[0060] <Grading Process>

[0061] Figure 2 The flowchart illustrates a preferred example of the method for preparing the zinc-containing liquid of the present invention. For example... Figure 2 As shown, in the present invention, in step S5, the initial stage of the zinc leaching process, it is preferable to separate the iron smelting dust into coarse dust and fine dust (grading process). Furthermore, the fine dust obtained in this grading process is preferably used as the iron smelting dust in the zinc leaching process, the subsequent stage of this grading process.

[0062] Zinc volatilized during the ironmaking process typically exists as fine particles around a few micrometers in size and tends to be less dense than iron. Therefore, fine dust contains more zinc than coarse dust. Furthermore, fine dust has a smaller average particle size than coarse dust, resulting in higher reactivity with the acidic aqueous solution in the subsequent zinc leaching process, thus improving the efficiency of the zinc leaching reaction.

[0063] In this invention, when using fine dust obtained in the classification process as the iron smelting dust in the zinc leaching process, the average particle size of the fine dust is not particularly limited. However, from the viewpoint that a higher zinc content results in higher reactivity in the zinc leaching process, 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.

[0064] There are no particular restrictions on the methods used in this classification process. For example, sieves can be used, or the difference in the settling velocity and moving velocity of particles in the fluid (fluid classification) can be used to separate ironmaking dust into coarse and fine dust.

[0065] (Method for preparing an electroplating zinc bath)

[0066] The method for preparing an electroplating zinc bath according to the present invention is characterized in that the zinc-containing liquid prepared by the above-described method for preparing the zinc-containing liquid of the present invention is used as at least a portion of the zinc source of the electroplating zinc bath, and the pH of the electroplating zinc bath is 0.5 to 3.5.

[0067] As the zinc source for the electroplating zinc bath used in this invention, in addition to the zinc-containing liquid of this invention, zinc granules, zinc sulfate (heptahydrate), zinc chloride, and zinc carbonate can be used, for example. Furthermore, as the type of electroplating zinc bath, an acidic bath with excellent productivity is preferred; sulfuric acid baths, chloride baths, and mixtures thereof can be used. Among these, in the chloride bath, since it is necessary to treat the chlorine gas generated when using insoluble anodes, a sulfuric acid bath is more preferred.

[0068] The pH of the electroplating zinc bath should be adjusted to a range of 0.5–3.5, preferably 1.0–3.0. Adjusting the pH to this range suppresses the active generation of hydrogen, which reduces current efficiency, and also inhibits plating burn-in, resulting in a beautiful plating appearance. It is preferable to use an acid appropriate for the type of zinc plating bath used for pH adjustment; for example, if the zinc plating bath is a sulfuric acid bath, sulfuric acid is preferred for pH adjustment.

[0069] The method of using the zinc-containing liquid of the present invention as at least a portion of the zinc source in an electroplating zinc bath is not particularly limited. For example, it is possible to add the zinc-containing liquid to an acidic bath that has been adjusted using a zinc source other than the zinc-containing liquid and then adjust the pH, or to add a zinc source other than the zinc-containing liquid to the zinc-containing liquid and then adjust the pH.

[0070] The proportion of zinc-containing liquid in the zinc source of the electroplating zinc bath is preferably 1% by mass or more, calculated based on the total zinc source. This is because by ensuring that the proportion of zinc-containing liquid is 1% by mass or more, the cost advantage of the present invention can be fully realized. On the other hand, there is no upper limit to the concentration of the zinc-containing liquid, and the entire amount of zinc-containing liquid can be used as the electroplating zinc bath.

[0071] The zinc concentration in the electroplating zinc bath is preferably 0.5 mol / L or higher. If the zinc concentration is 0.5 mol / L or higher, there will be no zinc ion deficiency near the steel sheet being plated during electrolysis, which can suppress the formation of charred coating. There is no upper limit to the zinc concentration, but from the viewpoint of processing cost, it is preferably below 2 mol / L.

[0072] In addition, considering the need for temperature stability, the temperature of the electroplating zinc bath is preferably above 30°C. There is no specific upper limit to the temperature of the electroplating zinc bath, but since the evaporation rate of the bath increases with higher temperatures, it is practically preferable to keep it around 90°C.

[0073] Furthermore, conductive additives can be added to the plating bath as needed. This is because it can improve the conductivity of the plating bath. Examples of such conductive additives include sodium sulfate, ammonium sulfate, and potassium sulfate. However, if the amount added increases, the whiteness of the coating may decrease when electrolyzing at a high current density. Therefore, the amount of the conductive additive added is preferably 0.5 mol / L or less.

[0074] As a preferred range for the concentration of impurity elements (Pb, Cd, Fe) in the plating bath, the concentrations of Pb and Cd are preferably 1.0 ppm or less. If the concentration of these impurity elements exceeds 1.0 ppm, the L value of the plating surface may decrease. Additionally, for the same reason, the concentration of Fe is preferably 1000 ppm or less.

[0075] On the other hand, by including inorganic components containing Sn, In, Bi, or Sb, or organic components having a 2-benzothiazole thio group, in the plating bath as needed, electroplated zinc-coated steel sheets with a more aesthetically pleasing surface can be obtained. It is preferable to control these inorganic and organic components within the range of 0.01 to 3 ppm. The form in which these components are added is not particularly limited. The aforementioned inorganic components can be added in a metallic state, or in the form of sulfates, chloride salts, carbonates, oxides, etc., but when using salts, it is preferable to use salts appropriate for the type of electroplating zinc bath. As an organic component having a 2-benzothiazole thio group, 2-mercaptobenzothiazole (MBT) or its salts can be used.

[0076] (Manufacturing method of electro-galvanized steel sheet)

[0077] The method for manufacturing electro-galvanized steel sheets according to the present invention is characterized in that electro-galvanized steel sheets are manufactured by electro-galvanizing using an electro-galvanizing bath prepared by the above-described method for preparing an electro-galvanizing bath according to the present invention. When manufacturing electro-galvanized steel sheets, a zinc-containing liquid can be prepared in advance using the method for preparing a zinc-containing liquid according to the present invention, and zinc sources other than zinc-containing liquids, such as zinc granules or zinc sulfate, can be added as needed. Electro-galvanizing is then performed after the electro-galvanizing bath is prepared. Furthermore, a zinc source, including the zinc-containing liquid prepared by the method for preparing a zinc-containing liquid according to the present invention, can be added to the electro-galvanizing bath in the electro-galvanizing steel sheet production line.

[0078] The preferred current density during electrolysis is 10–130 A / dm³. 2 The range is more preferably 10–100 A / dm 2 By setting the current density in electrolysis to 10 A / dm³ 2 The above measures prevent a decrease in current efficiency, which could lead to an increase in the time required to ensure the specified plating adhesion. On the other hand, by setting the current density during electrolysis to 130 A / dm³... 2 The following measures can suppress the burning of the coating.

[0079] There are no particular limitations on the type of electrode (anode), but considering the dissolution of impurities in the plating bath, an iridium oxide electrode is preferred. Furthermore, from the viewpoint of reducing the thickness of the diffusion layer at the electrolytic interface, the plating bath flow rate is preferably 1.0 m / s or higher.

[0080] Thus, zinc-containing liquid prepared by the method of the present invention is used to manufacture electro-galvanized steel sheets.

[0081] Surface-treated steel sheets can also be produced by forming a chemical conversion coating on the surface of the electro-galvanized steel sheet manufactured as described above. This chemical conversion coating can be formed, for example, by a chromium-free chemical conversion process, i.e., by coating with a chromium-free chemical conversion solution and then heating and drying the steel sheet at a temperature of 80–300°C without washing. It should be noted that the chemical conversion coating can be a single layer or multiple layers; in the case of multiple layers, multiple chemical conversion treatments can be performed sequentially.

[0082] Furthermore, depending on the intended use of the electro-galvanized steel sheet, a single or multiple layers of coating containing organic resin can be formed on the surface of the plating or chemical conversion treatment film of the electro-galvanized steel sheet manufactured as described above, thus producing a surface-treated steel sheet. Examples of such coatings include polyester resin coatings, epoxy resin coatings, acrylic resin coatings, polyurethane resin coatings, and fluorine resin coatings. Additionally, epoxy-modified polyester resin coatings, obtained by modifying a portion of the aforementioned resins with other resins, can also be used. Furthermore, curing agents, curing catalysts, pigments, additives, etc., can be added to the aforementioned resins as needed.

[0083] There are no specific provisions regarding the coating method used to form the aforementioned coating film, but examples of coating methods include roller coating, curtain coating, and spray coating. For instance, after applying a coating containing organic resin to the surface of an electro-galvanized steel sheet or a chemically converted coating, the coating film can be formed by heating and drying using methods such as hot air drying, infrared heating, or induction heating.

[0084] It should be noted that the above-described method for manufacturing surface-treated steel sheets is an example and is not limited thereto.

[0085] Example

[0086] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0087] First, zinc is leached into the sulfuric acid solution by adding electric furnace dust (Zn content: 40% by mass, average particle size: 10 μm) as ironmaking dust to a sulfuric acid aqueous solution adjusted to pH 2.0 and stirring for 1 hour (zinc leaching step). Next, the resulting solution is subjected to bath conditioning under various conditions, and impurity precipitation treatment is performed (first impurity precipitation step and second impurity precipitation step). Table 1 shows the types of chelating agents, 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 step, the pH of the pH-adjusted solution, and the type and amount of chelating agent added in the second impurity precipitation step. The structural formulas of A1 to A4 shown in Table 1 are also shown. It should be noted that in the impurity precipitation treatment, both the first and second impurity precipitation steps are carried out for 1 hour with stirring after bath conditioning at a solution temperature of 50°C. Then, solid-liquid separation is performed using a suction filter (solid-liquid separation step) to obtain a zinc-containing liquid. The concentrations of Zn, Fe, Cd, and Pb in each zinc-containing liquid were determined using an ICP (Inductively Coupled Plasma) mass analyzer, and the results are shown in Table 2.

[0088] [Table 1]

[0089] Table 1

[0090] No Compound Name A1 Sodium dimethyl dithiocarbamate A2 Sodium diethyldithiocarbamate A3 Dipotassium piperazine-1,4-dithiocarbamate A4 pyrrolidine-1-dithiocarbamate ammonium

[0091]

[0092]

[0093] Using electroplating baths prepared by adding zinc sulfate heptahydrate to the zinc-containing liquids obtained as described above and adjusting the zinc concentration in the bath to the concentrations shown in Table 2, electroplating was performed under the conditions described below to manufacture electroplated steel sheets. At this time, cold-rolled steel sheets with a thickness of 0.7 mm, manufactured using conventional methods, were used as the base sheet for galvanizing. Test pieces (150 mm × 70 mm) for batch galvanizing were cut from this steel sheet and subjected to degreasing and pickling treatments before electroplating. It should be noted that the zinc coating adhesion per single side was 20 g / m². 2 .

[0094] Electrolysis conditions

[0095] Current density: 50A / dm 2

[0096] Bath temperature: 50℃

[0097] Electrode: Iridium oxide

[0098] Flow velocity: 2.0 m / sec

[0099] In addition, chromium-free chemical conversion treated steel sheets are produced by coating the plating surface of the obtained electro-galvanized steel sheets with epoxy resin (manufactured by JAPAN EPOXY RESIN (Co., Ltd., trade name: jER1009).

[0100] The obtained electro-galvanized steel sheets and chromium-free chemical conversion treated steel sheets were tested for brightness (L value) according to JIS Z8722:2009 using a spectrophotometer (NDS SD5000, manufactured by Nippon Denshoku Kogyo Co., Ltd.) via SCE (stripping of positive reflections). For electro-galvanized steel sheets, L values ​​of 80 or higher were rated ◎, 76 or higher but less than 80 were rated 〇, and less than 76 were rated ×. For chromium-free chemical conversion treated steel sheets, L values ​​of 60 or higher were rated ◎, 56 or higher but less than 60 were rated 〇, and less than 56 were rated ×. It should be noted that ◎, 〇, and × represent the degree of goodness, respectively.

[0101] Table 2 shows a comparison between electro-galvanized steel sheets treated without the first and second impurity precipitation processes and chromium-free chemical conversion treated steel sheets. It can be seen that the zinc-containing liquid contains a large number of impurities, resulting in a low L value. Table 2 shows a comparison with only the first impurity precipitation process. Although the Fe concentration in the zinc-containing liquid is reduced, Cd and Pb remain, resulting in a low L value. Table 2 shows a comparison with only the second impurity precipitation process. Due to the low pH, the chelating agent decomposes, resulting in the same high impurity concentration in the zinc bath as Table 2, and a low L value. Table 2 shows a comparison with the pH below the lower limit in the first impurity precipitation process. Although the Fe concentration is reduced, the low pH causes the chelating agent to decompose, leaving Cd and Pb residues, resulting in a low L value. Table 2 shows a comparison with the pH exceeding the upper limit in the first impurity precipitation process. The Zn concentration is significantly reduced, making it unsuitable as a zinc source for electro-galvanizing. Example No. 6 is a comparative example where the amount of chelate added is below the lower limit, but it can be seen that the concentrations of Cd and Pb are not sufficiently reduced, and the L value is also low.

[0102] On the other hand, as can be seen from the invention examples No.7 to 22, it is possible to reduce the concentrations of Fe, Cd, and Pb without significantly reducing the Zn concentration in zinc-containing liquids, and the L value shows a high value.

[0103] Examples No. 23 and No. 24 are embodiments where the proportion of zinc in the electroplating bath as a zinc source for the prepared zinc-containing liquid is 1.1% by mass, which is close to the lower limit of the preferred range of the present invention. No. 23 is a comparative example using a zinc-containing liquid prepared without the first and second impurity precipitation processes. It is known that although the concentrations of Fe, Cd, and Pb in the electroplating bath are reduced, they exceed the allowable range of impurities, resulting in a low L value. On the other hand, regarding the inventive example No. 24, which implements the first and second impurity precipitation processes, it is known that the concentrations of Fe, Cd, and Pb in the electroplating bath can be reduced, and the L value also shows a high value. Furthermore, it is known that No. 25 is an inventive example using only a zinc-containing liquid, which can reduce the concentrations of Fe, Cd, and Pb in the electroplating bath, and the L value also shows a high value. In addition, it is known that No. 26 is an inventive example of No. 10 with the addition of MBT, and compared with No. 10 without MBT, the L value is increased.

[0104] Industrial availability

[0105] According to the present invention, a zinc-containing liquid made from iron smelting dust can be provided for use in manufacturing electroplated galvanized steel sheets with a beautiful and bright appearance.

Claims

1. A method for preparing a zinc-containing liquid, characterized in that, It includes the following processes: The zinc leaching process involves contacting iron smelting dust with an acidic aqueous solution, thereby leaching the zinc contained in the iron smelting dust into the acidic aqueous solution. In the first impurity precipitation step, impurities are precipitated when the pH of the first treatment solution obtained in the zinc leaching step is 4.0 to 6.

0. In the second impurity precipitation step, a chelating agent of 0.10 g / L or more is added to the second processed solution obtained in the first impurity precipitation step at a pH of 4.0 to 6.0 to precipitate the impurities. as well as In the solid-liquid separation process, the third processing liquid obtained in the second impurity precipitation process is subjected to solid-liquid separation to obtain a zinc-containing liquid.

2. The method for preparing zinc-containing liquid according to claim 1, wherein, The zinc leaching process further includes a grading process that separates iron smelting dust into coarse and fine particles, and uses the fine particles obtained in the grading process as the iron smelting dust used in the zinc leaching process.

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

4. The method for preparing the zinc-containing liquid according to any one of claims 1 to 3, wherein, In the second impurity precipitation step, at least one of the chelating agents added to the second treatment solution is a compound having the structure shown in general formula (I), wherein R 1 and R 2 M indicates a substituted alkyl group. + Indicates a monovalent cation.

5. A method for preparing an electroplating zinc bath, characterized in that, The electroplating zinc bath contains a zinc-containing liquid prepared by the method of any one of claims 1 to 4, and the pH of the electroplating zinc bath is 0.5 to 3.

5.

6. A method for manufacturing an electro-galvanized steel sheet, characterized in that, Electro-galvanized steel sheet is manufactured by electro-galvanizing the surface of a steel sheet using an electro-galvanizing bath prepared by the method described in claim 5.

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