Introduction

Peroxygens can be used both to oxidise and reduce chromium species in aqueous solutions depending upon the acidity of the medium.  This property can be employed in the extraction of chromium for superalloy scrap, and in the detoxification  of waste chromic acid pickle liquors.

1. Extraction of chromium

Hydrogen peroxide will oxidise chromium (III) to chromium (VI) when the  oxidant is added to a solution of chromium (III) at pH values >7.0.  The reaction time required is of the order of 30 minutes.

An alternative method of oxidising chromium (III) to chromium (VI) is by the addition of Caro’s acid, (H2SO5) under acid conditions to a hot solution of the chromium compound (85oC)

2. Detoxification of chromium contaminated effluents

Although hydrogen peroxide will oxidise chromium (III) to chromium (VI) under alkaline conditions, it acts as a reductant in acid media.  Reduction is carried out at pH values lower than 7.

Neutralisation of the product with alkali then results in quantitative precipitation of chromium (III) as Cr(OH)3. 

Use is made of this in the disposal of waste chromic acid pickling liquors, where hexavalent chromium must be reduced to the trivalent state to enable precipitation of chromium as the hydroxide.