RU1788087C - Method of chrome (iii) oxide production - Google Patents

Abstract

The method is designed to recover Cr (l 11) from solutions containing Cr (VI). SUMMARY OF THE INVENTION: A solution of hexavalent chromium is fed to an electrolyzer. The voltage on the cell is 2.5-3.5 V. The ratio of the areas of the cathode and anode (2-4): 1. A solution containing Cr (1M) is obtained. Trivalent chromium precipitated with hydroxy. ammonia house. The precipitate is dried at a temperature not lower than 200 ° C. 4 tab.

Description

The invention relates to electroplating, and more particularly to technological processes in which concentrated chromium-containing solutions are used, for example, in the passivation of zinc and cadmium, the etching of copper and its alloys, and the deposition of chromium coatings.

Due to the saturation during operation of concentrated chromic acid solutions with salts of iron, zinc, copper and other metals or contamination with nitrate ions, they lose their performance and must be cleaned or disposed of. Moreover, in some cases, for economic and technical reasons, mutation is preferred.

A method is known for producing chromium (III) oxide, including the electrochemical reduction of hexavalent chromium ions by electrolysis in a diaphragmless electrolyzer with insoluble anodes.

A disadvantage of the known method is the low content of chromium oxide in the main product.

The proposed method for the treatment of concentrated chromic acid solutions was tested during disposal of the spent solid chromium electrolyte with the composition: CrOz - 130 g / l; Cr (lll) - 26 g / l; Fe (lll) - 11 g / l; H2 $ 04-5g / l ..

The first stage of processing (electrochemical reduction) was carried out in a 1 liter electrolyzer with constant stirring using a graphite cathode with an area of 1.5 dm and a titanium-nickel (55% TI, 45% Ni) anode with an area of 0.5 dm2. The voltage at the electrodes was maintained equal to 3 V. After the current dropped to zero (end of electrochemical reduction) and the electrodes were unloaded, the solution was poured into a metal container (neutralizer) and the calculated amount of ammonium hydroxide in the form of ammonia water was introduced there. The calculation was carried out according to the formula:

Yo

VI

00

00

about

00 XI

Vnkoh

 ° 6 -tCrCVI) Mmn; he 100 3 - MN dON -dNH40H

- 1

Here: Cr (Vlj is the content of hexavalent chromium in the initial electrolyte according to the analysis results (geq / l); Ue is the volume of the processed electrolyte (l) ;. MNHVOH 35; (, МЩОН, VNH it is. Density, concentration (%) and ammonia volume

The catalyst with the resulting pasty mixture was loaded into an oven, where the product was dried for 8 hours at t ° 210 ° C. Drying the product at temperatures lower than 200 ° C does not allow decomposition of ammonium dichromate, therefore, heating is carried out at any temperature exceeding 200 ° C.

The proposed method allows to completely neutralize concentrated chromic acid solutions, because the content of the toxic component (Cr) here after treatment is zero. The prototype processing method provides a reduction in the Cr (VI) content only to 0.66 g / l.

The optimum mode of electrochemical reduction of hexavalent chromium is realized when graphite is used as the cathode, and a titanium-nickel alloy with a ratio of 3: 1 and a voltage of 3 V is used as an anode. Compared to a graphite electrode on cathodes from other materials, the limiting reduction rate is Cr (Vl) significantly lower (table 2).

And the use of a pair: cathode-graphite, anode

-titanium-nickel, provides a complete conversion of hexavalent chromium to trivalent. here (Cr (VI) / Cr (lII) ratio here after 20

hours of electrolysis is 0/65).

With a StoSa ratio exceeding 4: 1 and less than 2: 1, the energy consumption noticeably increases (Table 3). Use of voltage in the range 2.5 V U 3.5 V

increases the processing time due to a decrease at low U of the working current densities and hydrogen evolution at large U (Table 4). Thus, the claimed method makes it possible to isolate trivalent chromium almost completely.

Claims (1)

The formula has been invented and A method for producing chromium oxide, comprising the electrochemical reduction of bivalent chromium ions by electrolysis in a diaphragmless electrolytic cell with insoluble anodes, characterized in that, in order to increase the content of chromium oxide in the main product, electrolysis is carried out at a voltage of 2.5- 3.5 V using a graphite cathode and a titanium-nickel anode with a ratio of the areas of the cathode and anode (2-4): 1 with the precipitation of trivalent chromium ions by ammonium hydroxide and drying the product at a temperature not lower than 200 ° C.  Table 1 The effect of the voltage of the electrochemical treatment (Do) is the CrCO3 content in the solution after treatment (g / l), the current efficiency of the Cr (VI) reaction (lll) and the chromium (III) oxide content in the product (% Cr20z). The initial composition of the processed solution (g / l): CrOz-40; Fe3 + - 5; N2804 - 5. The cathode is graphite, the anode alloy is titanium-nickel, SniSa 3: 1. Prototype Table 2 The effect of the electrode material on the ratio of the Cr (VI) / Cr (lll) content after 20 hours of electrochemical treatment of the chromic acid solution and the maximum value of the cathodic current density (DK, max), excluding the evolution of hydrogen . Table 3 The effect of the ratio of the areas of the cathode and anode (graphite: titanium-nickel) on the time of electrochemical treatment of the chromic acid solution and the energy consumption () Table 4 The effect of voltage on the electrodes (graphite: titanium-nickel) during the electromechanical treatment of a chromic acid solution and energy consumption (5k: 5a 3: 1)