RU2433212C2 - Method of cleaning chromium plating electrolyte from impurities of iron and copper cations (versions) - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention discloses methods based on achieving in a chromium plating electrolyte, pH for formation of iron (III) hydroxide and pH for precipitation of chromates and hydroxochromates of iron (III) and copper (II) in the interval of 1.5-4.0. These conditions are achieved by adding to the chromium plating electrolyte or to its diluted solutions such reagents as aqueous solutions of hydrazine, hydroxyamine or hydrogen peroxide, as well as due to hydrolysis at temperature 70-120°C in a strongly diluted chromium plating electrolyte which is contaminated with iron and copper cations.

EFFECT: low concentration of harmful impurities.

3 cl, 2 ex

Description

Usage: in galvanic production to restore the performance of chromium electrolytes based on hexavalent chromium compounds.

The invention relates to methods for purifying a chromium-plating electrolyte based on chromic anhydride and sulfuric acid from harmful impurities — iron and copper cations. The proposed methods make it possible to purify highly contaminated chromium electrolytes containing from 5-10 g / l or more iron and copper cations, as well as highly contaminated chromium electrolyte concentrates, for example, containing, g / l: CrO ₃ 650, Fe ³⁺ 45, Cu ²⁺ 40.

The technical result of the proposed methods is to reduce the concentration of harmful impurities - iron and copper cations to 1.0-5.0 g / l, depending on the method and its implementation.

The essence of the invention: there are various methods for purification of chromium electrolyte: using reagents [1], ion-exchange resins or membrane electrolysis [2]. Each of the methods has its drawbacks. When cleaning the chromium electrolyte with a reagent method, a large amount of solid waste is generated. In the ion-exchange method, the disposal of large volumes of solutions arising in the process of restoring the performance of ion-exchange resins is required. Membrane electrolysis is characterized by a slow cleaning rate and the need to use chemically resistant ion-exchange membranes and direct current sources.

It is known that compounds formed by weak base cations, for example, ferric cations, and medium strength acids, such as chromic acid, undergo hydrolysis in dilute aqueous solutions, while hydrolysis increases with temperature [3], which is associated with an increase in the intrinsic constant dissociation of water with increasing temperature [4].

Upon dissolution of chromic anhydride in water, a mixture of acids is formed: chromic and dichromic. At a high concentration of chromic anhydride, polychromic acids are formed. The bichromat anion is formed from two hydrochromatic anions, and the chromic acid dissociation constant for the formation of hydrochromatic anions is greater than the constant of the dichromat anion formation from two hydrochromatic anions [5], therefore, in dilute solutions of chromic anhydride, primarily chromic acid is formed, and with an increase in the concentration of chromic anhydride, leading to an increase in the concentration of hydrochlorate anions, dichromic acid is formed. When acidifying solutions containing chromate anions, bichromate anions are formed, and this reaction is reversible [3].

Chromates, hydroxochromats and hydroxides of heavy metals are insoluble in water [4], and the pH of the hydroxide formation of ferric iron is much lower than that of cations of divalent copper and ferric chromium, ceteris paribus. PH of hydroxide formation of metal cations (in parentheses is given the concentration of metal cation, mol / L): ferric iron: 1.5 (1M), 2.3 (0.01 M), 4.1 (<0.001 M), trivalent chromium: 4 , 0 (1 M), 4.9 (0.01 M), 6.8 (<0.001 M), divalent copper: 4.2 (1 M), 5.2 (0.01 M), 5.7 (<0.001 M) [4].

Copper cations can be precipitated when the pH of the solution is significantly lower than the pH of their hydroxide formation due to the formation of another difficult soluble compound, for example, copper chromate, PR CuCrO ₄ = 3.6 · 10 ^-6 [4].

The following methods are proposed for purification of chromium electrolyte:

1) The contaminated electrolyte is mixed with distilled water in such a ratio that a solution is formed with a concentration of chromic anhydride of 2-10 g / l. The resulting solution is heated to undergo a hydrolysis of ferric iron compounds and precipitation of ferric hydroxide, chromates and hydroxides of iron and copper. The hot solution is filtered. As a result of this operation, the maximum possible amount of copper compounds and most of the iron compounds are extracted from the solution. For a more complete extraction of iron compounds, the filtrate with a concentration of chromic anhydride of 2-10 g / l is additionally diluted with distilled water to a concentration of chromic anhydride of 0.5-2 g / l, reheated to proceed the hydrolysis of ferric compounds and the hot solution is filtered from the precipitate . After concentrating the diluted solution of chromic acid, purified by the above method, to a concentration of chromic anhydride of 250 g / l, the solution will contain not more than 1.0 g / l Fe ³⁺ and not more than 3.4 g / l Cu ²⁺ .

Example 1

6 ml of contaminated chromium plating electrolyte containing CrO ₃ = 250 g / l, Fe ³⁺ = 18.0 g / l and Cu ²⁺ = 14.8 g / l are added to a heat-resistant volumetric flask with a capacity of 1 l, to which distilled water is added to a total volume of 0.5 l and mix. Heat the contents of the flask to a boil and incubate at a boiling point of 0.5 h, adding distilled water as it evaporates. The hot solution is quickly filtered through a filter. 300 ml of the filtrate are taken and mixed with 600 ml of distilled water in another heat-resistant 1-liter volumetric flask, the contents of the flask are heated to boiling and maintained at a boiling temperature of 0.5 h, adding distilled water as it evaporates. The hot solution is quickly filtered through a filter. The filtrate contains CrO ₃ = 0.95 g / L, Fe ³⁺ = 0.0035 g / L and Cu ²⁺ = 0.0128 g / L. After concentrating the filtrate by evaporating water to a concentration of CrO ₃ equal to 250 g / l, the concentration of impurity cations became equal to: Fe ³⁺ = 0.92 g / l and Cu ²⁺ = 3.36 g / l.

Dilution of the chromium electrolyte with distilled water immediately to a concentration of chromic anhydride of 0.5-2 g / l, subsequent heating and filtering from the precipitate, will lead to the maximum possible purification from iron cations and, at the same time, to some deterioration in the purification of the chromium electrolyte from copper cations, t. to. with an increase in dilution, the residual concentration of iron cations (during subsequent hydrolysis after dilution and concentration of the diluted solution of chromic acid to a concentration of chromic anhydride of 250 g / l) will continuously decrease and tend to a certain minimum value (about 1.0 g / l), and copper - first decrease, then pass a minimum, and then increase again, which is associated with a noticeable solubility of copper chromate in water (PR CuCrO ₄ = 3.6 · 10 ^-6 [4]).

It is recommended that prior to purification of the chromium electrolyte, sulfuric acid must first be removed from the contaminated chromium electrolyte by the standard method using the calculated amount of barium carbonate. Removal of a strong acid - sulfuric acid - contributes to a further deeper course of hydrolysis.

The degree of purification of the solution from cations of iron and copper depends on the temperature of the solution, ceteris paribus. The hydrolysis of dilute aqueous solutions of ferric iron compounds proceeds at a noticeable rate at a temperature of 70 ° C. With increasing temperature of the solution, the hydrolysis rate increases significantly, therefore, the cleaning process is best carried out at the boiling point of the resulting aqueous solution, which is close to the boiling point of pure water. Since the increase in temperature favorably affects the cleaning results, and the process temperature is limited by the boiling point of water, it is necessary to use increased overpressure to increase the boiling temperature of the aqueous solution.

The exposure time of the solution to complete the hydrolysis and establish equilibrium at the selected temperature is 1-30 minutes.

After hydrolysis, the purified, highly diluted chromic acid solution must be concentrated to a concentration of chromic anhydride of 50 g / l (this solution is used to adjust the working chromium bath) or 250 g / l (this solution is suitable for creating a new chromium bath). For concentration, use some of the known methods with the appropriate hardware design, for example:

a) evaporation at room or elevated temperature with or without ventilation (by passing a stream of air, including pre-heated, above the surface of the liquid for faster evaporation of water),

b) evaporation at boiling point at atmospheric pressure,

c) evaporation at boiling point under reduced pressure P, P = 2 ÷ 10 kPa,

g) freezing,

e) reverse osmosis.

In most of the above methods for concentrating a dilute solution of chromic anhydride, distilled water is formed, which must be reused to hydrolyze a new portion of the contaminated chromium electrolyte, which leads to a significant reduction in the energy consumption and installation size.

For continuous purification of chromium electrolyte from impurities of iron and copper cations under industrial conditions, it is necessary to mix the contaminated chromium electrolyte in the required proportion with distilled water coming from the distillate collector, and send the resulting solution using a pump to the first titanium heater, which is a coil made of titanium or other chemically resistant material, operable at elevated temperature and pressure. The titanium coil is heated by superheated steam or electric heating elements. The length, inner diameter of the coil and wall thickness of the titanium tube, the feed rate of the solution and the location of the fuel elements are selected so that the liquid in a short time heats up to the required temperature (70-120 ° C) and is kept at this temperature for at least 1-5 minutes. The hot solution after hydrolysis from the coil enters the first filter cartridge, where a solid precipitate is separated. Next, the hot filtrate for more complete removal of iron cations is mixed in the required ratio with distilled water coming from the distilled water collector, and pumped to the second titanium coil, similar in design to the first. The hot solution from the second titanium coil is fed to the second filter cartridge. Next, the hot filtrate enters the evaporator, where the concentration of the chromic acid solution occurs. In order to save energy, the evaporation of a solution of chromic acid is carried out under reduced pressure using a heat pump. Distillate - warm distilled water without additional cooling is sent directly to the distilled water collector, and the concentrate - a solution of chromic acid with a concentration of 50 g / l is used to adjust the existing chromium bath or, with further concentration to a concentration of 250 g / l according to chromic anhydride, - preparing a new chromium electrolyte.

2) Concentration of a solution containing 1.0 g / l of chromic anhydride requires a significant energy consumption. To reduce energy consumption, it is necessary to achieve a pH value, the formation of a precipitate of ferric hydroxide, chromates, iron and copper hydroxochromates with less dilution. For this purpose, the electrolyte is diluted with distilled water to a concentration of chromic anhydride of 50-10 g / l, heated to a temperature of 70-100 ° C and kept at the selected temperature for 1-30 minutes. The hot solution is filtered. In a filtered solution cooled to room temperature, a solution of hydrogen peroxide 3-30 wt.% Is introduced in portions with stirring until a pH value is achieved that provides the desired degree of purification of the solution from iron and copper cations. Hydrogen peroxide reacts with chromic acid according to the equation:

5

5

During the reaction, a chromic acid salt is formed from chromic acid - chromium chromate and the pH of the solution increase, a precipitate of ferric hydroxide, chromates, iron and copper hydroxochromates precipitates. To coagulate the precipitate and decompose the residue of hydrogen peroxide, the solution is heated to boiling and filtered while hot. Next, the solution is concentrated to a concentration of total (tri- and hexavalent) chromium corresponding to that found in the standard chromium electrolyte (250 g / l in terms of chromic anhydride). In a concentrated solution, the concentration of trivalent chromium differs from that required for the chromium plating process. If it is more necessary for the chromium plating process, then to reduce the concentration of trivalent chromium, excess trivalent chromium is oxidized into hexavalent chromium in the standard way - anode processing (by electrolysis at a certain ratio of the areas of the cathode and anode). It should be noted that electrolysis will completely remove possible residual amounts of hydrogen peroxide, because hydrogen peroxide is reduced to water at the cathode and oxidized to oxygen and hydrogen cations at the insoluble anode.

In the course of reaction (1), additional introduction of hydrogen cations is not required, i.e. there is no need to introduce an additional amount of a strong mineral acid, for example, sulfuric, the introduction of which will lead to the subsequent mandatory removal of sulfate ions to the required concentration, which is available in standard chromium electrolytes based on chromic anhydride and sulfuric acid. Also, alkali or alkaline earth metal cations are not introduced. Hydrogen peroxide reduction products - oxygen and water do not require any special methods for their removal from solution. When using a hydrogen peroxide solution, it must be taken into account that traces of iron and copper cations cause accelerated spontaneous decomposition of hydrogen peroxide according to the reaction equation:

Example 2

15 ml of contaminated chromium plating electrolyte containing CrO ₃ = 250 g / l, Fe ³⁺ = 18.0 g / l and Cu ²⁺ = 14.8 g / l, are introduced into a heat-resistant volumetric flask with a capacity of 0.5 l, where 235 ml of distilled water and stirred. Heat the contents of the flask to a boil and incubate at a boiling point of 0.5 h, adding distilled water as it evaporates. The hot solution is quickly filtered through a filter. After the filtrate was cooled to room temperature, 100 ml of the filtrate was collected and mixed with 20 ml of 30% by weight hydrogen peroxide solution, which was added in portions, in another 0.25 L heat-resistant volumetric flask. After the introduction of the entire amount of hydrogen peroxide solution, the contents of the flask are heated to a boil and kept at a boiling temperature of 0.5 h, adding distilled water as it evaporates. The hot solution is quickly filtered through a filter. The filtrate contains CrO ₃ = 10.7 g / l, Fe ³⁺ = 0.033 g / l and Cu ²⁺ = 0.16 g / l. After concentrating the filtrate by evaporating water to a concentration of CrO ₃ equal to 250 g / l, the concentration of impurity cations became equal to: Fe ³⁺ = 0.77 g / l and Cu ²⁺ = 3.73 g / l.

3) It should also be noted that with an increase in the activity of hydrogen cations (a decrease in the pH of the solution), the value of the standard redox potential for a system containing hexavalent chromium increases to a greater extent than for a system containing hydrogen peroxide, ceteris paribus. With an increase in the concentration of chromic anhydride in the solution, its pH decreases, therefore, to restore hexavalent chromium with the formation of chromium chromate according to reaction (1), it is better to add hydrogen peroxide directly to the chromium electrolyte without preliminary dilution.

If the content of iron and copper cations in the contaminated electrolyte is high, then the added hydrogen peroxide will be consumed in large quantities spontaneously by reaction (2), and not enter into the target reaction (1). In this case, hydrogen peroxide, as mentioned above, is added after preliminary dilution of the electrolyte and removal of some cations of iron and copper, i.e. the concentration of iron and copper cations catalytically decomposing hydrogen peroxide will be significantly reduced before the addition of hydrogen peroxide.

4) The use of hydrazine or its aqueous solutions with a concentration of 1-60 wt.% Is more economical from the point of consumption of the reducing agent to increase the pH of the chromium electrolyte. Hydrazine is a strong reducing agent and reacts with chromic acid according to the equation with the release of a large amount of heat:

As well as when using hydrogen peroxide, the products of hydrazine oxidation - nitrogen and water do not require any special methods for their extraction. The system does not introduce additional quantities of metal cations: sodium, potassium, calcium, magnesium and barium or anions. For the progress of reaction (3), it is not necessary to introduce an additional amount of strong mineral acid from the outside to increase the redox potential of the system containing hexavalent chromium. The presence of iron and copper cations in an acidic medium in the presence of a strong oxidizing agent, hexavalent chromium, does not prevent the reaction from occurring (3). After reaching the required pH value of the solution from the range of 1.5-4.0, the precipitate containing ferric hydroxide, chromate, ferric hydroxide and ferric copper, is separated by filtration of the solution. To complete the precipitation and coagulation of the precipitate, the electrolyte is heated. To destroy (oxidize) a possible residue of unreacted toxic hydrazine and its unstable intermediate products of incomplete oxidation, 3-30 wt.% Is introduced into the electrolyte after filtration, a solution of hydrogen peroxide in an amount of 1-5 ml per liter of electrolyte, after which the electrolyte is heated and filtered . In the filtrate, the concentration of trivalent chromium is reduced to the required level in a standard way — by anodic processing of the electrolyte at a certain ratio of the cathodic and anodic areas of the electrodes. In contrast to hydrogen peroxide, when a concentrated, for example, 50 wt.%, Aqueous solution of hydrazine is used, the chromium electrolyte is diluted slightly and therefore practically does not need to evaporate the excess water introduced with hydrazine.

Further addition of hydrazine, after reaction (3), leads to the formation of trivalent chromium hydroxide or hydroxochromate. Since the pH of hydrate formation of ferric iron is much lower than the pH of hydrate formation of ferric chromium (ceteris paribus), the precipitation of ferric hydroxide will be almost complete before the end of the formation of chromium chromate. Therefore, to remove iron cations from the chromium electrolyte, it is necessary to restore no more than 2/5 (or 40%) of the chromic anhydride present in the chromium electrolyte. In practice, this value, depending on the degree of contamination of the chromium electrolyte with cations of iron and copper, lies in the range of 10-30%.

Hydroxyamine possesses similar strong reducing properties, which reacts according to the equation:

Hydroxylamine, as well as hydrazine for reaction (4), does not require the introduction of an additional amount of any strong mineral acid, for example, sulfuric, into the chromium electrolyte, and the products of hydroxylamine oxidation, nitrogen and water, do not require any special methods for their extraction.

Information sources

[one]. The method of regeneration of spent chromium electrolytes. Stepanenko E.K., Smirnov A.L. RF patent No. 2061802, published on June 10, 1996.

[2]. Flexible automated galvanic lines. Directory. V.L. Zubchenko and others. M .: "Engineering". 1989, 379 p.

[3]. General chemistry. Glinka N.L. L., 1985, ed. 24, p. 249-254, p. 636.

[four]. A quick reference to chemistry. Ed. A.T. Pilipenko. Kiev: Naukova Dumka, 1987, 828 p.

[5]. A quick reference to chemistry. Under the total. ed. Kurylenko O.D. Kiev: Naukova Dumka, 1974.p. 345.

Claims (3)

1. The method of purification of a chromium electrolyte based on chromic anhydride and sulfuric acid from cations of iron and copper, characterized in that the electrolyte is diluted with distilled water to a concentration of chromic anhydride of 2-10 g / l, the resulting solution is heated to a temperature of 70-120 ° C and maintained in this temperature range 1-30 min for the reaction of hydrolysis of ferric iron compounds and precipitation of chromates and hydroxochromates of iron and copper, then the hot solution is filtered and the filtrate is diluted with distilled water to the concentration of chromic anhydride is 0.5-2 g / l, heated to a temperature of 70-120 ° C and kept in this temperature range for 1-30 minutes for the hydrolysis of ferric compounds to proceed and the hot solution is filtered from the precipitate, after which the hot purified diluted the chromic acid solution is concentrated in an evaporator under reduced pressure using the heat pump. 2. The method of purification of chromium electrolyte based on chromic anhydride and sulfuric acid from cations of iron and copper, characterized in that the electrolyte is diluted with distilled water to a concentration of chromic anhydride of 50-10 g / l, heated to a temperature of 70-100 ° C and maintained at the selected at a temperature of 1-30 min, the hot solution is filtered, the hydrogen peroxide solution is added portionwise with stirring to a filtered solution cooled to room temperature, until a pH in the range of 1.5-4.0 is reached, providing the required degree of the source of the solution from iron and copper cations, the solution is heated to boiling to coagulate the precipitate and decompose the remaining hydrogen peroxide and filtered while hot, the purified diluted chromic acid solution is concentrated in an evaporator under reduced pressure using the heat pump, the concentrated solution is then subjected to anode designing for the oxidation of excess trivalent chromium to hexavalent chromium. 3. The method of purification of chromium electrolyte based on chromic anhydride and sulfuric acid from cations of iron and copper, characterized in that a solution of hydrazine or hydroxylamine is introduced in portions with stirring until a pH in the range of 1.5-4.0 is achieved, providing the required degree purification of the solution from iron and copper cations, after which the solution is heated to complete the reaction and coagulate the precipitate of ferric hydroxide, chromates and hydroxochromates of ferric iron and ferrous copper and filtered, ki of unreacted hydrazine or hydroxylamine is removed with a 3-30 wt.% hydrogen peroxide solution, which is added in an amount equal to 1-5 ml / l, the electrolyte is heated and filtered, the purified chromic acid solution is subjected to anodic treatment to oxidize the excess trivalent chromium to hexavalent chromium .