US3444060A - Method of electrolytically precipitating chromium metal from aqueous chromium (vi) oxide solutions - Google Patents

Description

3,444,060 METHOD OF ELECTROLYTICALLY PRECIPITAT- ING CHROMIUM METAL FROM AQUEOUS CHROMIUM (VI) OE SOLUTIONS Hans Rothmann and Werner Keil, Nuremberg, and Heinz Richter, Furth, Bavaria, Germany, assignors to Gesellschaft fur Elektrometallurgie m.b.H., Dusseldorf,

Germany No Drawing. Filed July 28, 1964, Ser. No. 385,777 Int. Cl. C22d 1/24, N

US. Cl. 204-105 4 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method of electrolytically precipitating chromium metal from aqueous chromium (VI) oxide solutions.

As known, metallic chromium is electrodeposited on metal objects from chromium (VI) oxide (CrO baths. Recently methods have been described of electrolytically recovering chromium metal from chromium (VI) oxide baths for metallurgical purposes. Several different kinds of procedure can be distinguished. For instance, according to one proposal known to the art, a single addition of chromium (1H) sulphate and of chromium (III) oxide (Cr O in certain proportions is introduced into a chromium (VI) oxide electrolyte for the purpose of depositing thick layers of chromium. According to another proposal sulphuric acid, hydrofluoric acid or fluosilicic acid is added to a chromium (VI) oxide electrolyte from which varying yields of metallic chromium with varying contents of oxygen are deposited. In the production of metallic chromium for metallurgical purposes from aqueous chromium (VI) oxide solutions the nature of other acids used as catalysts in the electrolyte is a matter of considerable importance. For example, if the chromium (VI) oxide electrolyte contains sulphuric acid or sulphate ions, then a low current yield of a sulphur-containing and, according to the conditions of electrolysis, particularly according to temperature, of an oxygen-containing chromium metal are obtained. However, if the chromic acid electrolyte is free from sulphate ions and the catalyst is exclusively fluosilicic or hydrofluoric acid, then a much better yield of a very pure metal is deposited from such solutions than that obtained from sulphate-ion-containing electrolytes. Hence, whereas in the technique of electrodeposition for forming metallic chromium coatings which are only a few microns thick the nature of the additional acid does not greatly matter, it does matter very considerably if a very pure chromium metal is to be recovered for metallurgical purposes. Chromium trioxide solutions of high purity must be used. The market price of chromium trioxide is high; it is also toxic and corrosive. Furthermore, from the method used for producing it, it still contains sulphate. Before it can be used as an electrolyte it must therefore be purified because the sulphate ions would otherwise accumulate and the electrolyte would be poisoned.

Purification of chromium trioxide by recrystallisation is difficult and expensive owing to its solubility in water and its high corrosive activity. The precipitation of the sulphate ions with barium is likewise complicated and involves the risk of introducing barium into the electrolyte.

It has now been found that substantially pure chromium can be produced from an aqueous solution of chromium trioxide and the above described difficulties overcome if, according to the invention, solid chromic oxide is added to the chromium trioxide solution. The method can be continuously performed by adding the chromic oxide at 'the same rate as chromium metal is deposited at the cathode. Unlike chromium (VI) oxide, chromium (III) oxide is cheap, non-toxic and has no corrosive effect. The chromic oxide dissolves in the original chromium trioxide solution and is oxidised to chromium trioxide at the anode.

It is advisable to use readily filtrable water-containing chromic oxide Which is sufliciently reactive to be oxidised at the anode to chromium trioxide and is also sufficiently pure to permit continuous addition thereof, according to the invention, to function as a chromium supplier to the aqueous chromium trioxide electrolyte without poisoning the electrolyte. The addition is made at the same rate as metallic chromium is deposited at the cathode. Preferably, the chromic oxide is the product of precipitation from alkali chromate or alkali dichromate solutions.

According to a further feature of the invention, the chromic oxide is the precipitation product of alkali chromate and alkali dichromate solutions containing Cr in concentrations between 20 and 200 grams per liter reacted with organic reducing compounds, preferably wood meal, for 1 to 3 hours at temperatures between 200 and 300 C. and preferably at about 250 C. The reaction may be carried out in an autoclave and forms a readily filtrable chromic oxide. Since commercial alkali chromate or alkali dichromate may contain up to 0.5% sulphur in the form of the sulphate, such a high sulphur content would lead to the production of a chromic oxide containing between 0.02 to 0.04% of sulphur. This sul phur content is objectionable when the oxide is to be used as a chromium supplier in the electrolysis of the chromium trioxide electrolyte. It is therefore advisable to reduce the sulphur content by precipitating the sulphur with an equivalent proportion of barium salt-preferably barium chlorideat a pH of 2 from the alkali chromate or dichromate solutions prior to their reduction. This precipitation may be performed either at elevated temperatures or at room temperatures. The loss of chromium in the precipitated barium sulphate is under 1%. An alkali-containing chromic oxide obtained from a desulphurated alkali dichromate solution has a residual sulphur content below 0.002%.

The sulphur-free chromic oxide thus obtained still contains 1 to 5% of Na O, probably in the form of sodium chromite, after having been filtered and washed with water. This high alkali content would also cause trouble when the oxide is used as a chromium supplier in the electrolysis of chromium trioxide. According to the invention this alkali content of the chromic oxide can be neutralised with or without the application of heat with a chromium trioxide solution which is free from sulphate and Washed out in the form of sodium chromate or dichromate, the chromic oxide at the same time absorbing a quantity of chromium trioxide roughly equivalent to the alkali oxide, a circumstance which substantially assists in promoting the subsequent oxidation of the chromic oxide to chromium trioxide in the electrolyte. A chromic oxide which has been thus treated has an alkali content below 0.1%. However, in performing the washing process care must be taken to ensure that the chromic acid contains no sulphate ions because if a chromic acid containing sulphate were used the alkali would of course be washed out of the chromic oxide and the extremely reactive trioxide absorbed, but at the same time sulphate ions would also be taken up from the sulphatecontaining solution of chromic acid. It is therefore a matter of importance to ensure that a chromium trioxide solution containing no sulphate is used for removing the alkali. The best course to take is to use the sulphatefree chromium (VI) oxide electrolyte.

The dilute alkali chromate or dichromate solutions obtained by washing out the alkali are reused for producing alkali chromate or dichromate solutions, so that chromium losses cannot occur during the performance of the method.

In order that the method proposed by the present invention can more readily be understood a number of examples will be described in greater detail.

EXAMPLE I 5.74 kg. of crystallised, commercial sodium dichromate containing 0.3% of sulphur were dissolved in 20 litres of water. As soon as the salt had dissolved the pH of the solution containing 100 g. of chromium per litre was adjusted to pH 2 with the aid of a pH meter. About 1.5 millilitres per litre of hydrochloric acid (1.19) were need ed. The sulphate ions were precipitated by the addition of 110 g. of crystallised barium chloride, dissolved in 200 millilitres of water. When precipitation was performed at room temperature and whilst stirring the solution, 99% of the sulphur was precipitated in the form of barium sulphate in 192 hours. The chromium which was carried down by the precipitate amounted to less than 1% of the total chromium introduced. Experiments have disclosed that the completeness of the precipitation in the stated conditions depends upon time. For instance at the end of 60 hours only 94% of the sulphur had been precipitated as barium sulphate and the chromium loss was The duration of the precipitation period could be reduced by heating the suspension. When the temperature was raised to 80 99% of the sulphate were precipitated as barium sulphate in 24 hours, the chromium loss being under 1%. After precipitation of the sulphur the clear solution was drawn off and poured into an autoclave of 30 litres capacity. 1 kg. of wood meal was added, the autoclave being sealed, the contents kept for one hour at 250 whilst being stirred and then cooled. A 99% yield-related to the chromium introduced into the solutionof a brilliant green, readily filtrable granular hydrated chromic oxide was obtained which was removed from the liquid on a suction filter. The oxide was washed with pure water until all alkalinity of the filtrate had disappeared (quantity of wash water used being about 100 litres). Filtrate and wash water were discarded. After having been dried for 2 hours at 105 the oxide had the following composition:

Percent (31 203 Na O 2.5-3.5 Fe O 0.008 S 0.002

Remainder water.

For removing the alkali the moist chromic oxide on the suction filter was wetted with 3.3 litres of dilute sulphate-free chromic acid electrolyte (80 g. of Cr per litre) by adding the latter in portions. As soon as the entire washing solution had been absorbed by the chromic oxide the latter was rewashed with litres of pure water with the application of gentle suction. The filtrate thus obtained contained about half the chromium trioxide from the wash solution, the remainder of the chromium trioxide solution having been retained in the chromic oxide.

Percent Cr O 71-75 Na O 0.05 F6203 0.00s s 0.002

Remainder Water.

EXAMPLE II Production of the chromium (VI) oxide electrolyte with chromium (III) oxide addition In order to provide the initial solution 100 g. of pure chromium trioxide and 6 g. of fluosilicic acid were dissolved in 1 litre of pure water and poured into a lead vessel connected to operate as an anode. A steel rod (dia. 1 cm.) served as the cathode. Whilst moist chromic oxide which had been prepared as described in Example I was continuously added its oxidation was effected with a current of 20 amps at 4 volts. At an electrolysing temperature of C. the chromium (VI) ion concentration in the solution rose by 7 to 8 grammes per hour in the solution. The current yield at the anode was 60 to 80%. A slight metallic deposit formed at the cathode. Oxidation was continued until 10 litres of a chromium (VI) electrolyte having the desired concentration of 156 g./ litre of chromium had been obtained. 2 litres of this solution were used for removing the alkali from the chromic oxide. 50 g. of fiuosilicic acid were added to the remaining 8 litres, the cathode was replaced by a steel cathode (dia. 6 cm.) and electrolysis was performed with a current density at the cathode of 100 amps/cm. at a voltage of 7 volts at C.

During the process 80 g. (=28 g. of Cr) of chromic oxide, moist from the suction filter and free from sulphur and alkali, were added per hour. The total quantity of chromium added in the course of 29 hours was 800 grammes. 840 grammes of chromium metal were recovered. When electrolysis was stopped the electrolyte contained 151 g./litre of chromium in the form of chromium trioxide. The added chromic oxide had been fully dissolved and oxidised to the trioxide during electrolysis. The chromium metal recovered in this way contained 0.04% of O and -60 millilitres of H g. The sulphur content was less than 0.01% and the total of other metallic impurities amounted to less than 0.0005

We claim:

1. A method of continuously producing pure chromium by electrolytic deposition from an aqueous solution of chromium (VI) oxide in the presence of fluosilicic acid or hydrofluoric acid, comprising the step of adding to the solution a solid pure activated hydrated chromium (II-I) oxide at a rate substantially corresponding to the rate of deposition of metallic chromium on the cathode and recovering the deposited chromium from the cathode, the said solid pure activated hydrated chromium (III) oxide being obtained by reducing a solution of substantially sulphur-free alkali chromate or alkali dichromate with an organic reducing agent at an elevated temperature in an autoclave, and filtering off the resulting chromium (III) oxide.

2. In a method of continuously precipitating and recovering chromium for metallurgical purposes by electrolytic deposition of the chromium on a cathode from an aqueous solution of chromium (VI) oxide in the presence of fluosilicic acid or hydrofluoric acid, the improvement comprising adding to the solution solid substantially pure readily filtrable activated, hydrated chromium (III) oxide, which is substantially sulphur-free, at a rate substantially corresponding to the rate of deposition of metallic chromium on the cathode and recovering the deposited chromium from the cathode, whereby the chromium (III) References Cited oxide dissolves in the solution of chromium (VI) oxide UNITED STATES PATENTS and is oxidized at the anode to form chromium (VI) oxide and functions as a chromium supplier to the aqueous 662 5 5 solution. 5

3. The process of claim 2 wherein the said chromium 3259560 7/1966 Brandes et 204-105 (III) oxide is the precipitation product from alkali chro- FOREIGN PATENTS mate and dichromate solutions which chromates have 829,429 3/1960 Great Britain.

been reduced.

4. The process of claim 2 wherein the chromium (III) 10 HOWARD WILLIAMS Pr'mary Exammeroxide is substantially alkali-free. H. M. FLOURNOY, Assistant Examiner.