US1928294A - Method of refining chromium and chromium alloys - Google Patents

Description

Patented Sept. 26, 1933 UNITED STATES METHOD OF REFINING OHROMIUM AND CHROMIUM ALLOYS Erik Liebreich, Berlin, Germany No Drawing. Application November 24, 1931, Serial No. 577,155, and in Germany November 9' Claims. (01. 204-1) My invention relates to improvements in the method of refining chromium and chromium alloys, in the production of chromic acid and the separation of iron compounds from chromic acid 5 or other dissolved chromium compounds- More particularly my invention relates to the treatment of chromium and chromium alloys such as ferrochromium of any content of chromium made by metallurgical or alumino-thermic processes.

The object of the improvements is to provide a process in which electrolytic baths containing succinic acid are used.

I have discovered that in a watery solution of succinic acid the chromium is dissolved in a hexavalent state, and that in-baths containing succinic acid and chromic acid and iron compounds a high proportion of iron is precipitated by heat. Further, I have discovered that if in addition to the said substances a soluble succinate, such for example as sodium succinate, is added the precipitation of the iron without heating or preferably while heating, may be supported so far that almost the whole amount of iron is precipitated. In this process sodium chromate or bichromate is formed, which however does not interfere with the cathodic separation of chr0- mium if the substances are used at the proper ratio.

The process may be conducted so that in the anodic dissolution of iron-chromium alloys the iron is precipitated on the bottom of the container in the form of a dense slime, while the pure chromic acid stands above the said slime from which, with the correct ratio of concentration of the succinic acid and the chromic acid, chromium may be separated on the cathode.

The slight acidity of the bath needed for the separation of the chromium may be obtained by high concentration of succinic acid which may be about 30% relatively to CrOa, or by substitut ing a part of the succinic acid by another active acid. An acid which is particularly suitable as a substitute for the succinic acid is the hydrofluoric acid which simultaneously increases the anodic potential. A similar result have the perchloric acid, the chloric acid, the persulfuric acid, and Caros acid. Therefore it is preferred additionally to add the said acids to the sulfuric acid for supporting the separation of the chromium, because the sulfuric acid spoils the anodic potential and tends to produce the formation of chromic chromate on the anode.

In lieu of the acids the salts thereof may be' used as far as they have the same action in the process.

At the beginning of the process the bath must be heated to about 40 to 50 C., in order to keep the succinic acid in solution. Ordinarily in the process the heat developed by the current is s'ufliflient for maintaining the succinic acid in soluion.

In lieu of causing the increase of the chromic acid in the bath by dissolution of the anode, the chromic acid may be added from the beginning.

This procedure is preferably used for the reason that thereby the conductivity of the bath is from beginning such that at the beginning the voltage of the bath may be from 4 to 5 volts. If no chromic acid is added to the bath, and the bath is a pure watery solution of succinic acid, at first voltages of from 40 to 50 volts are needed for causing a current of a density of from 1 to 2 amperes per square decimeter to pass through the bath, the necessary voltage being rapidly reduced by the increase of the conductivity. In. the process it is preferred to keep the cathodic current density at about 5 to 10 amperes per square decimeter or more, and to keep the anodic current density substantially on the same value, with a voltage of the bath of about 4 to 5 volts. However the anodic current densitymay be higher or lower without any detrimental efiect. In a modification of the process I use two cathodes one of which is loaded with a higher current density, that is with a density of 5 amperes per square decimeter and more, while the other one is loaded with a lower current density, say about 1 ampere per square decimeter and less.

. On the first-named cathode at first metallic iron is precipitated, and thereafter iron and chromium and finally chromium, while on the last-named cathode or near the same the compounds of the iron are precipitated. If it is desired to free only chromic acid or another chromic salt solution from iron, it is preferred to use a single cathode 9 which is loaded with the aforesaid low current density, and thereafter to electrolyze with an unsoluble anode. r

The manner of carrying out the process depends on \the result aimed at. 'Therefore the process should be different if pure chromic acid is aimed at, and if technical chromium 'or an iron-chromium alloy are intended to be refined.

Example 1 or several pieces of chromium of any desired origin placed on a sieve or within a net of stain-- less steel. coated with lead dioxide or chromate of lead, or another suitable material may be used. The anodic and cathodic current density are 'not bound to definite limits, but if it is desired to avoid cathodic precipitation of chromium the cathodic current density must be so low that no chromium is precipitated on the cathode, and it should be less than 2 amperes per square decimeter.

The bath is operated with a voltage of from about 3 to 4 volts, while it is kept at a temperature suflicient for keeping the succinic acid in solution. With the aforesaid concentration of the succinic acid in the bath a temperature of from 30 to 40 C. is sufficient which is ordinarily maintained by the heat of the current.

Example 2 Refining of technical chromium:The bath described with reference to Example 1 is used, but the cathodic .current density is kept so that chromium may be separated.

If a bath is used which contains only one half of the amount of succinic acid referred to in Example 1, about 1 gram of concentrated sulfuric acid and 1 gram of perchloric acid per liter are added when the chromic acid concentration has risen to about 200 grammes per liter, or in lieu of one of the said acids 1 gram of hydrofluoric acid is added, the anodic voltage acting on the chromium being regulated by means of lead anodes connected in shunt, so that nearly so much chromium is separated as is dissolved.

Example 3 Refining of iron chromium alloysz-While heating 250 grammes of succinic acid and 300 grammes of sodium succinate are dissolved in one liter of water, which solution is used as the electrolyte. As an anode I may use ferrochromium of any content in chromium in the manner described with reference to Example 1, and as a cathode I may use sheet iron or another metal bent into cylindrical form, from which the chromium is readily cracked off. Initially the voltage is from 40 to 50 volts, and it rapidly falls to 5 or 6 volts. The anodic current density may have any desired value, and the cathodic density should be from 5 to 8 amperes per square decimeter, if only one cathode is used. If however two cathodes are used one of the cathodes is made so small that it is loaded with 10 amperes per square decimeter, while the other one is so large that it is loaded with 0.01 ampere per'square decimeter. Of course the same regulation may be effected by regulable shunted resistances. After about 100 ampere-hours the first precipitation of iron will be observed, which is further increased in the course of the electrolysis. The best way of avoiding the deposition of slime on the cathode con- In lieu of the stainless steel lead to the bath, say 100 grammes per liter, regulation' of the voltage is avoided. Otherwise the process is-carried out in the same way.

Example 4 Freeing of a chromic acid bath from iron:- While slightly heating, about 125 grammes of succinic acid and 150 grammes of sodium succinate are added to the chromic acid, and the electrolysis is carried out with an unsoluble anode by loading the cathode, which consists of any suitable material, with about 0,01 amper per square decimeter. In some cases I add a little hydrofluoric acid, or a little sulfuric acid, or a little sulfuric acid and perchloric acid, in which case however the amount of the acid as compared to the chromic acid of the bath, should not exceed the known limits which must be observed for subsequent use of the solution for the purpose of the separation of the chromium.

I claim:

1. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic compound.

2. In an electrolytic process of separating chro-' mium from iron, electrolyzing in baths containing a succinic salt.

3. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic acid.

4. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic compound together with another compound having an acid radical.

5. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic compound, current being caused to flow to one cathode at a high current density to deposit metal thereon, and to flow at a low current density to another cathode to precipitate iron compounds thereat.

6. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic compound, the anode being substantially insoluble.

7. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic compound at a cathode current density ranging from 5 to more than 5 amperes per square decimeter.

8. In an electrolytic process of separating chromium from iron, electrolyzing in baths containing a succinic compound at a cathode current density of less than 5 amperes per square decimeter.

9. In an electrolytic process of separating chromium from iron, comprising electrolyzing in baths containing chromic acid and a succinic compound.

ERIK LIEBREICH.