Ferrous sulfate is a commonly used reagent in gold processing plants for tailings treatment. Gold mine tailings often contain high levels of cyanide, which is highly toxic and hazardous. Therefore, even wastewater with relatively low cyanide content needs to be treated before discharge or reuse. Ferrous sulfate complexation is effective in cyanide removal.
The ferrous ions in ferrous sulfate react with free cyanide to form ferrocyanide complexes (Fe2+ + 6CN- → Fe(CN)6^4-). However, when an excess of ferrous sulfate is added to a cyanide-containing solution, cyanide can convert into an insoluble precipitate called ferric ferrocyanide [Fe4[Fe(CN)6]3], commonly known as Prussian blue. In some cases, insoluble ferrous sulfide or white insoluble ferrocyanide of iron and copper can be formed. This compound rapidly absorbs oxygen from the air and turns into a deep blue color, forming iron cyanide ferricyanide. However, the reaction is not as straightforward due to the various forms Prussian blue can take under different solution conditions. One of these forms is “soluble Prussian blue,” represented by MFeⅢ[FeⅡ(CN)6] (M = K or Na), which forms a colloidal solution with water. Additionally, precipitation and oxidation reactions involving ferrous hydroxide play a role. Therefore, the optimal conditions for using FeSO4 to remove cyanides from a solution involve determining the process of generating soluble and insoluble compounds through experimentation.
Wastewater treated with ferrous sulfate does not require separation and precipitation steps before proceeding to the next treatment, saving reaction units. Additionally, the concentration of free cyanide in ferrous blue is reduced, contributing to the quality enhancement of the product. Hence, using ferrous sulfate for treating cyanide-containing wastewater presents a cost-effective approach.