Chemical Reduction

In chemical reduction processes (Ⅲ) oxide is always the starting material, for which it has to be as pure as possible particularly with regard to sulfur content (nickel alloys otherwise form nickel sulfide at grain boundaries). The reduction can be carried out with silicon and, in particular, aluminum and carbon. The reaction:



is insufficiently exothermic to be self-sustaining. Strong oxidizing agents such as potassium dichromate, chromium(Ⅵ) oxide, potassium perchlorate or barium peroxide are therefore added. The reaction is carried out batchwise in vessels lined with refractory materials. The metal is obtained in ca. 85 to 90% yield with a purity of 97 to 99% (by weight). The main impurities are silicon, aluminum and iron. Operators of this process include Gesellschaft fur Elektrometallurgie and Shieldalloy.

Reduction with carbon is achieved by reacting briquettes of chromium(Ⅲ) oxide and carbon at 1275 to 1400°C in a vacuum of 0.4 mbar in a slow reaction:



This "Simplex process" is operated by UCC.

Electrochemical Reduction of Chrome Alum

Electrochemical reduction of chrome alum to chromium metal proceeds as follows:
(1)dissolution of ferrochrome in sulfuric acid/ammonium sulfate,
(2)separation of iron as iron(Ⅲ) alum. Crystallization of chrome alum, NH₄Cr(SO₄)₂^.12H₂O,
(3)dissolution of chrome alum and electrolytic deposition of metallic chromium.

Electrolysis is carried out in a diaphragm cell with a stainless steel cathode. A 3 to 6 mm thick layer of chromium is obtained, which is chipped off the cathode at the end of the electrolysis. The energy consumption amounts to 18.5 kWh/kg chromium. After degassing (hydrogen removal), the chromium is 99.3% pure (by weight).

 Electrochemical Reduction of Chromium(Ⅵ) Oxide ("")

The electrochemical deposition of chromium upon metals is usually carried out in chromium(Ⅵ) oxide-containing baths to which ca. 1% sulfuric acid has been added. The material to be coated is used as the cathode, the anode usually being lead. A diaphragm is not necessary. The baths contain ca. 300 g/L of chromium(Ⅵ) oxide, in addition to a number of other components. The yield on the basis of electricity consumed is very poor, due to hydrogen being preferentially produced at the cathode. The energy consumption amounts to ca. 75 kWh/kg chromium. The chromium obtained is relatively pure.

This process is suitable both for the production of metallic chromium and for the coating of objects with thin, decorative or thicker hard chromium layers (chromium electroplating).