RU2061802C1 - Method for regeneration of used chromium electrolytes - Google Patents

Abstract

FIELD: electroplating processes. SUBSTANCE: method for regeneration of used chromium electrolytes includes introduction of rated amount of phosphoric acid and/or its salt as reagent for formation of complexes with ions of metals-impurities. Then, mixture is neutralized. Concrete value of pH is determined proceeding from the composition of impurity metals in used electrolyte. It should equal the pH value of formation of the most easily soluble metal phosphate in acid. After neutralization, mixture is set until full maturation of the sediment, electrolyte is separated and corrected by acid content and special additives, and returned to the process cycle; and the sediment is used as pigment. EFFECT: higher efficiency.

Description

 The invention relates to the field of technology of galvanic processes and can be used in machine-building and radio-electronic industries for the regeneration of chromium electrolytes contaminated with impurities of metal ions.

 It is known to remove impurity metals from chromium electrolytes using strongly acidic cation exchange resins in the H-form to bind metals and release an equivalent amount of hydrogen ions into the solution (Flexible automated galvanic lines. Reference book. V. L. Zubchenko et al. M. Engineering. 1989 p. 378). However, this method is applicable only for the regeneration of diluted electrolytes with a concentration of up to 100 g / l of chromic acid.

 Closest to the proposed is a method of eliminating the influence of metal impurities in the chromium electrolyte by binding them to stable complexes, for example, the binding of excess iron in the surface activation solution of parts is achieved by introducing into this solution organic derivatives of phosphonic acids in an amount of up to 1% with respect to sulfuric acid (Flexible automated galvanic lines. Handbook. V. L. Zubchenko and others M. "Engineering". 1989, p.323, prototype).

 The degree of solution regeneration is 95-98%. However, organic derivatives of phosphonic acid are completely unsuitable for solutions containing chromium salts, due to their high oxidizing ability.

 The essence of the proposed method for the regeneration of spent chromium electrolytes, including the introduction of a reagent for the formation of complexes with metal impurities according to the invention, consists in the fact that the calculated amount of phosphoric acid or its salt is introduced as a reagent, the resulting mixture is neutralized to a pH value equal to the pH of formation the most readily soluble in acid metal phosphate from a mixture of metal impurities, defend until the mixture fully ripens, and then the electrolyte is separated from the precipitate, adjusted for content acids and special additives and returned to the process.

 With an increase in the content of impurity metals in chromium electrolytes, the technological mode of chromium plating is violated, therefore, the task arises of regenerating or replacing a part of the spent electrolyte volume with fresh chromium solution. All known methods for the recovery of chromium electrolytes are suitable for solutions with a concentration of chromic anhydride less than 100 g / l, therefore, often all chromium electrolyte from a plating bath is discharged due to the lack of an effective method for the regeneration of spent highly concentrated chromium solutions.

 The technical result obtained by the implementation of the proposed method is to enable the regeneration of an electrolyte with any concentration of chromium.

 The causal relationship of the proposed cleaning method between a set of sequential actions and the resulting technical result is as follows. When phosphoric acid or its salts are introduced, the phosphate ions present in them, combining with metal cations of impurities, form stable inorganic metal complexes in the chromium electrolyte. To ensure complete or, if necessary, partial formation of complexes, a calculated or stoichiometric amount of phosphoric acid or its salt is introduced. As a result of further neutralization of the electrolyte, for example, with alkali, their monophosphates are formed from metal complexes, which precipitate. The specific pH to which the electrolyte is neutralized is determined based on the composition of the impurity metals in the electrolyte being cleaned. Together with the sedimentation of the mixture until the sediment is fully ripe, these operations provide the necessary degree of precipitation of the metal monophosphate precipitate from the electrolyte. After a simple separation of the electrolyte from the precipitate and its correction, it is again suitable for chromium plating.

In addition, the invention:
allows you to restore the efficiency of spent chromium electrolytes with any degree of regeneration, that is, to update part of its volume or the entire volume;
prevents volley discharge of concentrated chromic solutions, violating the treatment plant operation mode;
eliminates the irreparable loss of valuable chromic acid compounds;
allows you to utilize the resulting precipitate in the form of a pigment with increased adhesive and anti-corrosion properties;
the process of restoration of chromic solutions and the chromium proper become completely waste-free.

 The possibility of implementing the proposed method for the purification of chromium electrolytes and its regeneration is confirmed by specific examples.

 Example 1

A tub for chrome plating zinc products in a rainbow color of 600 l is discharged into the factory sewer for disposal at the factory sewage treatment plant 4 times a year. Processing of the solution at the station is carried out using a solution of iron sulfate and a suspension of calcium hydroxide, after which the resulting precipitate is sent for disposal. The spent electrolyte contains, g / l: Na ₂ Cr ₂ O ₇ 36-42; H ₂ SO ₄ 18-30; Fe - up to 10 g / l; Zn up to 10; Cr up to 3. A sample of the solution from the bath at the time of selection contained the indicated components metals, respectively, g / l: 3.32; 7.68; 0.68, as well as 1.38 g / l Cu, pH 1.8.

To completely remove metals from a solution of sodium dichromate, it is necessary to add a stoichiometric amount of H ₃ PO ₄ , i.e. 15.7 g / l Precipitation of metal phosphates occurs at the following pH values: Fe 2.3-2.5; Zn 4.0-4.5; Cr 2.8-3.5; Cu 3.6-3.8. The mixture was neutralized with an alkali solution to pH 4.5. The treatment was carried out without heating, the mixture was left to stand for 19 hours, after which it was filtered, the precipitate was washed with water and dried at 150 ^o С. The color of the precipitate was bluish-green, weight 33.3 g from 1 liter of solution (about 80 kg per year).

 Chemical analysis of the purified electrolyte did not reveal the presence of metal impurities. Based on the analyzes, a calculation was made of the amount of additives of sulfuric acid and sodium dichromate to adjust the composition of the electrolyte, after which it acquired the ability to produce an iridescent film on samples subjected to chromium plating.

 Example 2

The spent chromium electrolyte for copper removal (copper etching) contains 200 g / l CrO ₃ , 2-4 g / l H ₂ SO ₄ , about 20 g / l Cu and up to 10 g / l Fe. A 20 liter bath is drained every month. A sample of a solution with a volume of 0.5 l with a content of: Cu 22.4 g / l; Fe 1.5 g / l; CrO ₃ 179.3 g / l, pH 1.25, was processed as follows: a stoichiometric amount of phosphoric acid equal to 6.5 ml was added (pl. 1.69), heated to 70 ^° C, neutralized to pH 4.0, kept in a thermostat for 1 hour, the precipitate was filtered on a vacuum filter, washed with 70 ml of water, dried at 150 ^o C. The purified solution was analyzed for the content of all components. No metals were found. The CrO ₃ content was 146.2 g / l. When 53.8 g of CrO _{3 was} added to the solution, the pH of the solution became 1.3. The electrolyte regained its working properties: within 5 minutes, the part placed in it after copper plating again lost its copper layer. The mass of sediment obtained from one liter of electrolyte was 42 g, which would be about 20 kg of pigment per year on a plant-wide scale.

 Example 3

The spent electrolyte for passivation of stainless steel has a temperature of 70-80 ^o C and contains 150-170 g / l CrO ₃ , up to 10 g / l Fe, Ni and Cr; 2.5 g / l H ₂ SO ₄ . A sample of a solution of 1.0 l contained: 162 g / l CrO ₃ ; 8.2 g / l Fe; 0.5 g / l Ni and 1.2 g / l Cr, pH 0.15. The stoichiometric amount of PO ₄ ion for the precipitation of metal phosphates is 16.4 g. Na ₃ PO ₄ salt was taken as precipitant.

The required amount of phosphate ion is contained in the canopy of this salt, equal to 28.3 g. After introducing the salt into the solution with stirring, the pH of the mixture increased to 4.7. After the mixture had matured for 1.5 hours in a thermostat at 70 ^° C, the precipitate was filtered off and acid was added to the electrolyte to pH 0.15. The content of CrO ₃ remained within the indicated limits. No impurity metals were found in the purified solution.

 Example 4

The universal standard chromium electrolyte after working out contains 200-250 g / l CrO ₃ , 2-2.5 g / l H ₂ SO ₄ and impurities Cu, Fe, Al, Zn up to 10-20 g / l, pH 0.25. A 600 liter bath is drained once every 3 years.

A sample of a 1-liter electrolyte after analysis of the composition of impurities was treated with a stoichiometric amount of a 50% solution of salt (NH ₄ ) ₂ HPO ₄ at a temperature of 70 ^° C. The pH value of the mixture was 4.3. After maturation of the precipitate for 2 hours at a temperature of 70 ^o C, it was filtered and washed. The filtrate and washings combined. The initial electrolyte was diluted by about 20. Metals precipitated by 100. The electrolyte was adjusted with CrO ₃ to 200 g / l and evaporated to remove NH ₄ ions; The pH of the electrolyte is 0.25. The chrome plating of a part in a Hull cell showed that the electrolyte restored its working properties.

 Thus, the purification of chromium electrolytes from impurity metals by the proposed method allows you to return them back to the chromium plating process, i.e. to regenerate. The concentration of chromic acid does not affect the cleaning process.

Claims (1)

A method of regenerating spent chromium electrolytes, including introducing a complexing agent into the electrolyte and neutralizing it to transfer metal impurities to the precipitate, separating the precipitate from the electrolyte and adjusting the composition of the purified electrolyte, characterized in that the calculated amount of phosphoric acid or its salt is taken as the complexing agent, and neutralization while maintaining the temperature of the mixture 65-70 ^o C and the pH value corresponding to the formation of the most readily soluble in acid metal phosphate from the amount of metal fishing impurities, while the duration of aging of the mixture under these conditions is 1.0-1.5 hours