RU2685840C1 - Method of regenerating chromate zinc passivation solution - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention relates to methods of electrochemical recovery of chromate solutions used for passivation of zinc coatings, and can be used in electroplating shops and sections to prevent formation of liquid and solid wastes containing compounds of zinc, hexa- and trivalent chromium. Regeneration is carried out in anode chamber of two-chamber electrolytic cell with anode made of platinized titanium or niobium. Cathode chamber is separated from the anode chamber by a cation-exchange membrane, and electrolysis is carried out in two stages. At first step, cathode chamber comprises sulphuric acid solution 50–100 g/l and through electrolytic cell passes through 40–80 A⋅h/l of electricity at cathode and anode current density of 3–5 A/dm, then catholyte pH is increased to 3–4 and 10–20 A⋅h/l of electricity is passed at cathode and anode current density of 0.5–2 A/dm.EFFECT: method enables to remove zinc ions from catholyte with simultaneous elimination of growth of zinc dendrites.1 cl, 3 ex

Description

The invention relates to methods for the electrochemical regeneration of chromate solutions for the passivation of zinc coatings and can be used in the manufacture of products comprising galvanized parts.

Known electrochemical method of processing technological solutions based on chromic acid and its salts and the corresponding wash water in the trapping baths, which can significantly reduce the amount of chromates in the waste waters of the site, as well as the consumption of chemicals used in the preparation of fresh solutions and in the disposal of waste. [S.S. Kruglikov, V.A. Kolesnikov, V.A. Brodski, A.F / Gubin, N.E. Nekrasova, E.S. Reclaim Tank through Immersed Electyrochemical Modules, Kruglikova, Regeneration of Process Solutions and Water Purification // Galvanotechnik. - 2018. - Bd. 109, Nr. 2. - S. 246-252.].

The closest in technical essence and the achieved result is a method of electrochemical regeneration of chromate solutions used to passivate zinc coatings by electrochemical processing in the anode chamber of a two-chamber electrolyzer with a cation-exchange membrane, in which part of the trivalent chromium ions is oxidized at the anode to chromate ions, and some with hydrogen ions and zinc is transferred through a cation-exchange membrane in the catholyte - a solution of sulfuric acid. At the same time, water molecules are transferred from the anolyte to the catholyte, which leads to the formation of excess volumes of the latter. [Kruglikov S.S. Removal of Metal Cations from Chromate-based Solutions by Membrane Electrolysis // Metal Finishing. - 2009. - Vol. 107, No. 11. - P. 13-15.]. These excess volumes are liquid waste generated during the regeneration process and contain free sulfuric acid and zinc and chromium sulphate (III), as well as chromium (II) resulting from the cathode reduction of chromium (III).

Attempts were made to reduce the amount of waste generated by removing zinc ions from catholyte by electrodeposition on the cathode. To this end, the catholyte pH increased to 2-4. However, when discharging zinc ions, a rapid growth of zinc dendrites started on the cathode, which germinated through the membrane, forming through holes in it. Therefore, in the process of regeneration in catholyte always maintain a pH around zero, thus preventing the discharge of zinc ions and the growth of dendrites. As a result, at the end of the regeneration cycle, liquid waste is generated - excess catholyte volumes containing free sulfuric acid and zinc and chromium sulphates. The joint presence in the catholyte of zinc and chromium ions makes it impossible to eliminate the loss of chromium compounds and return them to the passivation solution for subsequent anodic oxidation to chromate.

The objective of the proposed method is to remove zinc ions from catholyte while simultaneously eliminating the growth of zinc dendrites that destroy the membrane.

The problem is solved by the method of regeneration of chromate solutions of zinc passivation by treating the regenerated solution in the anode chamber of a two-chamber electrolyzer consisting of an anode chamber with an anode of platinized titanium or niobium and a cation-exchange membrane of the cathode chamber separated from it, while electrolysis is carried out in two stages and in the first stage the cathode chamber contains a solution of sulfuric acid 50-100 g / l and 40-80 Ah / l is passed through the catholyte at a cathode and anodic current density of 3-5 A / dm ² , and in the second stage the pH of the cathode Litas are increased to 3-4 and 10-20 Ah / l are passed through the catholyte at a cathode current density of 0.5-2 A / dm ² .

The proposed method has the following advantages compared with the known:

1. The consumption of chromic acid or its salt is reduced, since all chromium ions transferred by the electric field from the anolyte to catholyte can be returned back to the anolyte, where they will be oxidized at the anode to chromate ions. Thus, the consumption of chromate is reduced and the formation of chromium-containing liquid or solid waste is prevented.

2. The formation of wastes containing zinc compounds is eliminated, since zinc is extracted from excess catholyte volumes in the form of metal sediment and can be returned to the zinc bath by anodic dissolution

The implementation of the proposed method is illustrated by the examples below.

EXAMPLE 1.

Regeneration was carried out for the passivation solution of galvanized parts of the following nominal composition:

HNO ₃ 5-10 g / l. Na ₂ SO ₄ 8-15 g / l. Na ₂ Cr ₂ O ₇ 10-25 g / l. (in terms of chromium (VI) 3.9-9.9 g / l).

The solution, which was in operation and subject to regeneration, contained:

Cr (VI) 8.2 g / l Cr (III) 3.8 g / l Zn 6.0 g / l

450 ml of this solution was placed in the anode chamber of a two-chamber electrolyzer, in which there was a platinized titanium anode. In the cathode chamber with a stainless steel cathode and the cation-exchange membrane MK40-L separated from the anode chamber, there was 450 ml of a solution of sulfuric acid with a concentration of 50 g / l.

The first stage of electrolysis was carried out at a cathodic and anodic current density of 3 A / dm ² . The amount of electricity passed through the catholyte was 40 Ah / l. Before starting the second stage of electrolysis, the pH of the catholyte was increased to 3 by adding sodium hydroxide, and the 2nd stage was started, after which the electrolysis was continued at a cathodic and anodic current density of 0.5 A / dm ² . The amount of electricity passed through the catholyte during the second stage was 10 Ah / l. During the second stage, zinc was released on the cathode in the form of a compact layer, without developed dendrites. The weight of the cathode sediment was 2.4 g. At the end of the second stage, sodium hydroxide was added to the catholyte and the precipitated chromium hydroxide precipitate was filtered and dissolved in the anolyte.

EXAMPLE 2.

Regeneration was carried out for a solution of the composition specified in Example 1. The solution, which was in operation and subject to regeneration, contained:

Cr (VI) 8.2 g / l Cr (III) 3.8 g / l Zn 6.0 g / l

450 ml of this solution was placed in the anode chamber of a two-chamber electrolyzer described in EXAMPLE 1, in which platinized niobium served as the anode and titanium as the cathode. Catholyte is a solution of sulfuric acid with a concentration of 100 g / l.

The first stage of electrolysis was carried out at an anodic and cathodic current density of 5 A / dm ² , passing 80 Ah / l of electricity through the catholyte. Before the beginning of the second stage, sodium hydroxide was added to the catholyte to pH 4. In the second stage, 20 Ah / l was passed through the catholyte at a cathodic and anodic current density of 2 A / dm ² .

At the end of the second stage, a compact zinc layer without dendrites was obtained at the cathode. The weight of the zinc metal sediment is 2.3 g. At the end of the second stage, sodium hydroxide was added to the catholyte and the precipitated chromium hydroxide precipitate was filtered and dissolved in the anolyte.

EXAMPLE 3.

Regeneration was carried out for a solution of the composition specified in Example 1. The solution, which was in operation and in need of regeneration, contained:

Cr (VI) 8.2 g / l Cr (III) 3.8 g / l. Zn 6.0

450 ml of this solution was placed in the anodic chamber of a two-chamber electrolyzer described in Example 1, in which platinized niobium served as the anode and titanium as the cathode.

450 ml of this solution was placed in the anode chamber of the two-chamber electrolyzer described in Example 1. Catholyte is a solution of sulfuric acid with a concentration of 75 g / l.

The first stage of electrolysis was carried out at an anodic and cathodic current density of 4 A / dm ² , passing 60 Ah / L of electricity through the catholyte. Before the beginning of the second stage, the pH of the catholyte was increased to 3.5 by the addition of sodium hydroxide. In the second stage, 40 Ah / l of electricity at anode and cathode current density of 1 A / dm ^{2 was} passed.

At the end of the second stage, a compact precipitate of zinc sponge without dendrites was obtained, weighing 2.3 g. At the end of the second stage, sodium hydroxide was added to the catholyte and the precipitated chromium hydroxide precipitate was filtered and dissolved in the anolyte.

The proposed method has the following advantages compared with the known method described in the prototype:

1 The formation of a zinc dendrite that destroys the membrane has been prevented, since metallic zinc is deposited on the cathode in the form of a compact metallic precipitate.

2. The formation of liquid and solid wastes containing compounds of zinc and chromium is completely eliminated, since after removing zinc, the chromium (III) and (II) ions remaining in the catholyte are precipitated by adding an alkaline reagent and dissolved in the anolyte as hydroxides, neutralizing the excess acidity of the latter.

Claims (1)

Method of regenerating chromate solution of zinc passivation by treating the regenerated solution in the anode chamber of a two-chamber electrolyzer consisting of an anode chamber with an anode of platinized titanium or niobium and a cathode chamber separated from it by a cation-exchange membrane, characterized in that the electrolysis is carried out in two stages, with the first step cathode chamber comprises a sulfuric acid solution of 50-100 g / l, and is passed through the catholyte 40-80 Ah / l of electricity at the anode and cathode current density of 3-5 A / dm ^2, then in the second step pH in the cathode chamber is raised to pH 3-4 and passed through catholyte 10-20 Ah / l of electricity at the cathode and anode current density of 0.5-2 A / dm ^2.