Flow Sheet of Gold Processing Using Aqua Regia
Aqua regia, a highly corrosive mixture of nitric acid and hydrochloric acid, is widely used in gold refining due to its ability to dissolve gold and other precious metals. Below is a detailed flow sheet outlining the key steps involved in gold processing using aqua regia.
1. Feed Material Preparation
The process begins with the preparation of gold-bearing materials, which may include scrap jewelry, electronic waste (e-waste), or ore concentrates. The material is crushed and ground to a fine powder to increase surface area for efficient dissolution. Impurities such as organic matter are removed through roasting or washing.
2. Dissolution in Aqua Regia
The prepared material is introduced into a reactor containing aqua regia (typically in a 1:3 ratio of nitric acid to hydrochloric acid). The mixture is heated to 60–80°C to accelerate the reaction. Gold dissolves into chloroauric acid (HAuCl₄) according to the following reaction:
\[ \text{Au} + \text{HNO}_3 + 4\text{HCl} \rightarrow \text{HAuCl}_4 + \text{NO} + 2\text{H}_2\text{O} \]
Other metals like platinum and palladium may also dissolve, requiring subsequent separation steps.
3. Filtration and Clarification
After dissolution, the solution is filtered to remove undissolved solids such as silica or silver chloride (AgCl). Activated carbon may be added to adsorb organic impurities before proceeding to metal recovery.
4. Precipitation of Gold
Gold is selectively precipitated from the solution using reducing agents such as sodium metabisulfite (Na₂S₂O₅) or ferrous sulfate (FeSO₄). The reaction reduces chloroauric acid to metallic gold:
\[ 2\text{HAuCl}_4 + 3\text{Na}_2\text{S}_2\text{O}_5 + 3\text{H}_2\text{O} \rightarrow 2\text{Au} + 6\text{HCl} + 3\text{Na}_2\text{SO}_4 + 3\text{SO}_2 \]
The precipitated gold appears as a fine brown powder, which is then washed thoroughly with distilled water to remove residual acids and salts.
5. Smelting and Casting
The washed gold powder is dried and mixed with fluxing