A method for predicting the lead content on the surface of a tinplate

Abstract

This invention discloses a method for predicting the lead content on the surface of tin-plated sheets. Based on equipment and process parameters, it calculates the lead concentration in the pickling tank, the lead precipitation value after pickling, the cumulative lead content in the electroplating tank, and the lead precipitation value after electroplating. This allows for real-time prediction of the lead content on the tin-plated sheet surface, enabling parameter adjustments to reduce the lead content and bring it up to standard. This invention is simple to operate and effectively predicts the lead content of tin-plated sheets.

Description

Technical Field

[0001] This invention relates to the field of tin plating technology, and in particular to a method for predicting the lead content on the surface of tin-plated plates suitable for the tin plating process. Background Technology

[0002] Tinplate is widely used in food packaging, and its usage is gradually increasing with the improvement of people's living standards. However, tinplate contains lead, a harmful chemical element and a carcinogen. Therefore, food packaging strictly limits the lead content in tinplate. The EU standard EN10333-2005 stipulates that the lead content of tin ingots and tin plating layers must be less than 0.01% (i.e., 100 mg / kg). The Chinese national standard GB / T 2520-2017, "Cold-rolled Electroplated Tinplate and Strip," also stipulates that the lead content of tinplate and raw materials used in food packaging must not exceed 0.01%, which is equivalent to 0.0001 g / cm³. 2 Therefore, reducing the lead content in tinplate has become a key technological challenge for steel companies.

[0003] The main sources of lead in tin-plated sheets are tin raw materials, lead-tin alloys used in pre-pickling treatment, and the tin plating solution. Lead and tin belong to the same group of metals with similar chemical properties, so they often form symbiotic minerals. Therefore, lead is present in the tin ingots used in electroplating. Furthermore, lead and tin have very similar electrode potentials, making them prone to depositing together on the substrate surface, leading to excessive lead content. Pre-treatment before electroplating is necessary, including an electrolytic pickling step. The purpose is to remove oxides from the steel sheet surface using bubbles generated during electrolysis. Lead-tin electrodes are used in pickling, with the steel sheet acting as the induction electrode. Electrolytic pickling is an intermediate conductor method, involving polarity reversal, which causes free lead to dissolve. Producing tin-plated sheets with low lead content (less than 35 ppm) is costly; therefore, this method mainly studies the tin plating solution during electroplating and the lead deposited on the tin-plated sheet during electrolytic pickling. Current density has a significant impact on both electrolytic pickling and tin plating processes. During pickling, the free lead content in the pickling solution increases with increasing current density. During the electroplating process, as the current density increases, lead in the plating solution is more likely to deposit on the plating layer.

[0004] Therefore, to address the issue of excessive lead deposition on the substrate during pickling and electroplating processes due to factors such as excessive current density, which leads to excessive lead content in the final tin-plated sheet, this study aims to predict the lead content of tin-plated sheets by analyzing the external factors affecting them. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to provide a method suitable for predicting the change of lead content on the surface of tin plate after tin electroplating.

[0006] The technical solution of the present invention is a method for predicting the lead content on the surface of tin-plated sheets, the prediction method comprising the following steps:

[0007] (1) Before tin plating the steel plate, perform pretreatment by electrolytic pickling, and collect the steel strip in the pickling tank along the length L. sx m; pickling speed of strip steel v sx m / s; calculate the pickling time t sx ,s;

[0008] (2) Collect the current density D during acid washing sx A / dm 2 Electrochemical equivalent C of lead pb , g / (A·h); contact area S between strip steel and pickling solution sx cm 2 The volume V of the pickling solution sx L; pole spacing dj, mm; steel plate thickness d gb mm; Current efficiency η sx The free lead content ρ in the pickling solution was calculated. sx g / L;

[0009] (3) The electrochemical equivalent C of collected lead pb ,g / (A·h); pickling time t sx s; Current density D during pickling sx A / dm 2 The density of lead ρ pb g / cm 3 Current efficiency η sx The lead concentration ρ1, g / cm³ deposited on the steel strip was calculated. 2 :

[0010]

[0011] (4) After pickling, the strip steel enters the electroplating tank for electroplating. The electroplating raw material, tin granules, is added from the tin granule adding device. As the tin granules dissolve in the tin dissolving tank, they are carried into the plating solution in the electroplating solution circulation tank. During the electroplating process, tin and lead from the tin granules are deposited on the surface of the strip steel. The concentration of lead deposited on the surface of the strip steel is collected as the electrochemical equivalent C of lead. pb ,g / (A·h); Current density D during electroplating dd A / dm 2 Current efficiency η dd The density of lead ρ pb g / cm 3 ; Required plating thickness d (mm), calculate plating time t dd The lead concentration deposited on the strip surface is calculated according to the formula described in step (3);

[0012] (5) Then, the lead-tin ratio μ in the incoming tin granules and the tin concentration ρ in the plating solution are detected. sn , g / L; calculate the lead concentration ρ in the plating bath. dy , g / L; then collect the current density D during electroplating. dd A / dm 2 Electrochemical equivalent C of lead pb ,g / (A·h); Current efficiency η dd The density of lead ρ pb g / cm 3 The contact area S, cm² between the strip steel and the plating solution. 2 The thickness d of the steel plate gb mm; plating solution volume V dy L; interpolar spacing d j Calculate the lead concentration ρ2, g / cm³ deposited on the steel strip. 2 The total lead concentration is obtained by adding the lead concentration deposited on the strip after pickling; this is used to predict the lead content on the surface of the tinplate.

[0013] .

[0014] ρ z =ρ1+ρ2

[0015] In step (1), such as Figure 1 In the pickling tank, the lead at the anode dissolves, becoming free lead ions that enter the anode. Figure 1 To analyze the free lead concentration in the pickling solution within the pickling tank, it is necessary to first collect samples of the strip steel along a length L of the pickling tank. sx m; pickling speed of strip steel v sx The pickling time t can be calculated from the m / s. sx ,s.

[0016] In step (2), during the pickling process, the pickling current density, the contact area between the substrate and the pickling solution, and the volume of the pickling solution will all affect the free lead concentration in the pickling solution. Therefore, parameters such as the current density during pickling are collected.

[0017] In step (3), such as Figure 1 In the pickling solution, free lead ions undergo a reduction reaction on the substrate during the pickling process, and the resulting metallic lead is adsorbed onto the tin-plated substrate, which cannot be removed by water washing. To calculate the lead content adsorbed onto the tin-plated substrate, parameters such as the electrochemical equivalent of lead, pickling time, current density during pickling, lead density, and current efficiency need to be collected to calculate the lead concentration deposited on the tin plating.

[0018] In step (4), such as Figure 2As shown, the substrate after pickling enters the electroplating tank 8 for electroplating. The raw material for electroplating is tin granules, which are added from the tin granule adding device 10. The tin granules contain lead, which is carried into the plating solution in the electroplating solution circulation tank 12 as the tin granules dissolve in the tin dissolving tank 11. Because the electrode potentials of lead and tin are very similar, they are easily deposited on the surface of the strip steel together with tin during the electroplating process.

[0019] According to a method for predicting the lead content on the surface of a tin-plated sheet according to the present invention, preferably, a lead-tin alloy is used as the anode in the electrolytic pickling process.

[0020] According to a method for predicting the lead content on the surface of a tin-plated sheet based on the present invention, preferably, in step (1),

[0021] According to a method for predicting the lead content on the surface of a tin-plated sheet based on the present invention, preferably, the free lead content ρ in step (2) is... sx for:

[0022]

[0023] According to the method for predicting the lead content on the surface of a tin-plated sheet of the present invention, preferably, the electroplating time t in step (4) is... dd for:

[0024]

[0025] According to the method for predicting the lead content on the surface of a tin-plated sheet of the present invention, preferably, the lead concentration ρ in the plating solution in step (5) is... dy for:.

[0026] ρ dy =ρ sn μ

[0027] According to the present invention, a method for predicting the lead content on the surface of a tin-plated sheet, preferably, after step (5), the concentration of free lead accumulated in the plating solution is determined based on the density ρ of tin. sn g / cm 3 The contact area S, cm² between the strip steel and the plating solution. 2 Lead-tin ratio μ; plating thickness d, mm; plating solution volume V dy , L; Calculate the cumulative lead concentration ρ in the plating solution after adding tin granules during electroplating. lj g / L:

[0028]

[0029] The present invention aims to predict the lead content on the surface of the tin-plated plate, which is ultimately calculated through step (5). To predict the lead content on the surface of the tin-plated plate after the next electroplating, a step (6) can be added after step (5). This step calculates the lead in the plating solution minus the lead consumed in this electroplating, plus the lead in the tin particles added for the next electroplating, thus preparing for the prediction of the lead content in the next electroplating. The lead content on the surface of the tin-plated plate can be reduced by changing the process parameters of the electrolytic pickling section and the tin plating section. The lead content in the electrolytic pickling section can be reduced by decreasing the pickling time t. sx Reduce the electrode spacing (dj) between the pickling strip and the electrode plate, and lower the pickling current density (D). sx This can reduce the lead content in the pickling stage. In the tin plating stage, the lead content can be reduced by decreasing the plating time. dd Reduce the current density D during electroplating dd Reduce the electrode spacing d between the pickling strip and the electrode plate. j To reduce the lead content in the electroplating section, specific adjustments can be made based on the actual site conditions.

[0030] The beneficial effects of this invention are:

[0031] This invention calculates the lead concentration in the pickling tank, the lead precipitation value after pickling, the cumulative lead value in the electroplating tank, and the lead precipitation value after electroplating based on equipment and process parameters, thereby achieving real-time prediction of the lead content on the surface of tin-plated sheets and reducing the lead content on the tin-plated sheet surface. This invention is simple to operate, allows for real-time monitoring of lead content, prevents products from exceeding lead content standards, and helps companies ensure their products meet relevant environmental standards, avoiding legal risks and market access barriers due to excessive lead content. The electroplating tin production process can be optimized in a timely manner based on the prediction. This helps improve production efficiency, reduce unnecessary resource waste, and lower production costs. Attached Figure Description

[0032] Appendix Figure 1 This is a schematic diagram of electrolytic acid washing.

[0033] Appendix Figure 2 This is a schematic diagram of tin electroplating.

[0034] Appendix Figure 3 This is a calculation flowchart suitable for predicting lead content during the tin plating process.

[0035] In the figure, 1-conductive roller, 2-squeeze roller, 3-pressure roller, 4-stabilizing roller, 5-cathode plate, 6-anode plate, 7-submerged roller, 8-electroplating tank, 9-tin-lead plate, 10-tin granule adding device, 11-tin dissolving tank, 12-electroplating solution circulation tank. Detailed Implementation

[0036] The following example uses the electrolytic pickling and tin plating process at a steel mill. Figure 1 , Figure 2 The application of the lead content prediction method for the tin plating process described in this invention will be further illustrated by the examples.

[0037] Example 1:

[0038] (A): In order to analyze the free lead concentration in the pickling solution, it is necessary to first collect the strip steel in the pickling tank for a length of 20m; the pickling speed of the strip steel is 10m / s, and the pickling time is calculated to be 2s.

[0039] (B): The current density during pickling is 5A / dm³. 2 The electrochemical equivalent of lead is 0.0376 g / (A·h); the contact area between the substrate and the pickling solution is 480,000 cm². 2 The pickling solution has a volume of 30,000 L; the electrode spacing is 400 mm; the steel plate thickness is 0.35 mm; the current efficiency is 0.8; and the free lead content in the pickling solution is calculated to be 0.0152 g / L.

[0040] (C): To calculate the lead content adsorbed onto the tin-plated substrate, the electrochemical equivalent of lead (0.0376 g / (A·h)) needs to be collected; the pickling time is 2 s; and the current density during pickling is 5 A / dm³. 2 The density of lead is 11.34 g / cm³. 3 The steel plate thickness is 0.35mm, and the current efficiency is 0.8. The calculated lead concentration deposited on the substrate is 2.29*10⁻⁶. -6 g / cm 2 .

[0041] (D): To calculate the lead concentration deposited on the steel strip surface, the electrochemical equivalent of lead needs to be collected: 0.0376 g / (A·h); the current density during electroplating is 20 A / dm³. 2 The current efficiency is 0.8; the density of lead is 11.34 g / cm³. 3 The required plating thickness is 0.003 mm. The calculated electroplating time is 0.03 min.

[0042] (E): The lead-to-tin ratio in the incoming tin granules was measured to be 60 / 1000000, and the tin concentration in the plating solution was 20 g / L; the lead concentration in the plating solution was calculated to be 0.0012 g / L. Then, the current density during electroplating was collected at 20 A / dm³. 2 The electrochemical equivalent of lead is 0.0376 g / (A·h); the current efficiency is 0.8; and the density of lead is 11.34 g / cm³. 3 The contact area between the substrate and the plating solution is 480,000 cm². 2 The steel plate thickness is 0.35 mm; the plating solution volume is 30,000 L; the electrode spacing is 300 mm. Calculate the lead concentration deposited on the tinplate to be 4.87 * 10⁻⁶. -7 g / cm 2The total lead concentration, when added to the lead concentration deposited on the substrate after acid pickling, is 2.771 × 10⁻⁶. -6 g / cm 2 According to the Chinese national standard GB / T2520-2017 "Cold-rolled electroplated tinplate and strip", the lead content in the tin plating layer of tinplate used to manufacture containers (with or without organic coating) that come into direct contact with food, medicine, and beverages should not exceed 0.0100% (i.e., 100 μg / g, based on the mass fraction of the plating layer), which is equivalent to 0.0001 g / cm³. 2 The lead content on the surface of the final tin-plated sheet did not exceed the standard.

[0043] (F): Based on the density of tin, 7.28 g / cm³ 3 The contact area between the substrate and the plating solution is 480,000 cm². 2 Lead-tin ratio 60 / 1000000; plating thickness 0.003mm; plating solution volume 30000L; calculated cumulative lead concentration in the plating solution after adding tin granules after electroplating: 0.001194g / L.

[0044] Example 2:

[0045] (A): In order to analyze the free lead concentration in the pickling solution, it is necessary to first collect the strip steel in the pickling tank for a length of 20m; the pickling speed of the strip steel is 15m / s, and the pickling time is calculated to be 1.33s.

[0046] (B): The current density during pickling is 30 A / dm³. 2 The electrochemical equivalent of lead is 0.0376 g / (A·h); the contact area between the substrate and the pickling solution is 360,000 cm². 2 The pickling solution has a volume of 20,000 L; the electrode spacing is 400 mm; the steel plate thickness is 0.2 mm; the current efficiency is 0.8; and the free lead content in the pickling solution is calculated to be 0.166425 g / L.

[0047] (C): To calculate the lead content adsorbed onto the tin-plated substrate, the electrochemical equivalent of lead (0.0376 g / (A·h)) needs to be collected; the pickling time is 1.33 s; and the current density during pickling is 30 A / dm³. 2 The density of lead is 11.34 g / cm³. 3 The steel plate thickness is 0.2 mm, and the current efficiency is 0.8. The calculated lead concentration deposited on the substrate is 1.95 × 10⁻⁶. -4 g / cm 2 .

[0048] (D): To calculate the lead concentration deposited on the steel strip surface, the electrochemical equivalent of lead needs to be collected: 0.0376 g / (A·h); the current density during electroplating is 30 A / dm³. 2The current efficiency is 0.8; the density of lead is 11.34 g / cm³. 3 The required plating thickness is 0.002 mm. The calculated electroplating time is 0.015 min.

[0049] (E): The lead-to-tin ratio in the incoming tin granules was measured to be 60 / 1000000, and the tin concentration in the plating solution was 25 g / L; the lead concentration in the plating solution was calculated to be 0.0015 g / L. Then, the current density during electroplating was collected at 30 A / dm³. 2 The electrochemical equivalent of lead is 0.0376 g / (A·h); the current efficiency is 0.8; and the density of lead is 11.34 g / cm³. 3 The contact area between the substrate and the plating solution is 360,000 cm². 2 The steel plate thickness is 0.2 mm; the plating solution volume is 25000 L; the electrode spacing is 300 mm. Calculate the lead concentration deposited on the tinplate to be 1.5542 * 10⁻⁶. -6 g / cm 2 The total lead concentration, when added to the lead concentration deposited on the substrate after acid pickling, is 1.965542 g / cm³. 2 .

[0050] (F): Based on the density of tin, 7.28 g / cm³ 3 The contact area between the substrate and the plating solution is 360,000 cm². 2 Lead-tin ratio 60 / 1000000; plating thickness 0.002mm; plating solution volume 25000L; calculated cumulative lead concentration in the plating solution after adding tin granules after electroplating: 0.00148g / L.

[0051] According to the Chinese national standard GB / T 2520-2017 "Cold-rolled electroplated tinplate and strip", the lead content in the tin plating layer of tinplate used to manufacture containers (with or without organic coating) that come into direct contact with food, medicine, and beverages should not exceed 0.0100% (i.e., 100 μg / g, based on the mass fraction of the plating layer), which is equivalent to 0.0001 g / cm³. 2 The final tinplate surface had a lead content exceeding the standard. This was observed in the electrolytic pickling section, where a high lead content was deposited on the strip. Therefore, the lead content could be reduced by decreasing the current density during electrolytic pickling from 30 A / dm³. 2 Reduced to 20A / dm 2 The calculated lead content deposited on the tinplate is 8.822*10. -5 It conforms to national standards.

Claims

1. A method for predicting the lead content on the surface of tin-plated sheets, characterized in that: The forecasting method includes the following steps: (1) Pretreatment of electrolytic pickling before tinning of steel sheet, collecting the length L of the strip in the pickling tank sx , m; pickling speed v of the strip sx , m / s; calculating the pickling time t sx , s; (2) Collecting the current density D at pickling sx , A / dm 2 ; electrochemical equivalent of lead C pb , g / (A.h); contact area S of the strip with the pickling solution sx , cm 2 ; volume V of the pickling solution sx , L; inter-electrode distance dj, mm; thickness d of the steel sheet gb , mm; current efficiency η sx , calculating the free lead content ρ in the pickling solution sx , g / L; (3) Electrochemical equivalent of lead C pb , g / (A.h); pickling time t sx , s; current density D during pickling sx , A / dm 2 ; density of lead p pb , g / cm 3 ; current efficiency η sx , calculated lead concentration p1 deposited on the strip, g / cm 2 : (4) After pickling, the strip steel enters the electroplating tank for electroplating. The electroplating raw material, tin granules, is added from the tin granule adding device. As the tin granules dissolve in the tin dissolving tank, they are carried into the plating solution in the electroplating solution circulation tank. During the electroplating process, tin and lead from the tin granules are deposited on the surface of the strip steel. The concentration of lead deposited on the surface of the strip steel is collected as the electrochemical equivalent C of lead. pb ,g / (A·h); Current density D during electroplating dd A / dm 2 Current efficiency η dd The density of lead ρ pb g / cm 3 ; Required plating thickness d (mm), calculate plating time t dd The lead concentration deposited on the strip surface is calculated according to the formula described in step (3); (5) Then, the lead-tin ratio μ in the incoming tin granules and the tin concentration ρ in the plating solution are detected. sn , g / L; calculate the lead concentration ρ in the plating solution. dy , g / L; then collect the current density D during electroplating. dd A / dm 2 Electrochemical equivalent C of lead pb ,g / (A·h); Current efficiency η dd The density of lead ρ pb g / cm 3 The contact area S, cm² between the strip steel and the plating solution. 2 The thickness d of the steel plate gb mm; plating solution volume V dy L; interpolar spacing d j Calculate the lead concentration ρ2, g / cm³ deposited on the steel strip. 2 The total lead concentration is obtained by adding the lead concentration deposited on the strip after pickling; this is used to predict the lead content on the surface of the tinplate. r z =ρ1+ρ2.

2. The method for predicting the lead content on the surface of tin-plated sheets according to claim 1, characterized in that: In the electrolytic pickling process, a lead-tin alloy is used as the anode.

3. The method for predicting the lead content on the surface of tin-plated sheets according to claim 1, characterized in that: In step (1), 4. The method for predicting the lead content on the surface of tin-plated sheets according to claim 1, characterized in that: The free lead content ρ in step (2) sx for:

5. The method for predicting the lead content on the surface of a tin-plated sheet according to claim 1, characterized in that: The electroplating time t in step (4) dd for:

6. The method for predicting the lead content on the surface of a tin-plated sheet according to claim 1, characterized in that: The lead concentration ρ in the plating solution mentioned in step (5) dy for: r dy =ρ sn m.

7. The method for predicting the lead content on the surface of tin-plated sheets according to claim 1, characterized in that: After step (5), the concentration of free lead accumulated in the plating bath is determined by the density ρ of tin. sn g / cm 3 The contact area S, cm² between the strip steel and the plating solution. 2 Lead-tin ratio μ; plating thickness d, mm; plating solution volume V dy , L; Calculate the cumulative lead concentration ρ in the plating solution after adding tin granules during electroplating. lj g / L:

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