Caro’s acid is not totally stable and its decomposition is influenced by temperature. The natural solution is to cool the solution, but the efficiency of Caro’s acid is reduced due to the increased viscosity of chilled Caro’s acid. For this reason it is important to know how to add Caro’s acid into the slurry or barren solution. Some plants consider important to dilute with water the final product as it exits the static mixer before it mixes with tailing slurry. Basically, the water stops the decomposition reactions, suppressing acid mist and reducing the excessive viscosity.
For example, Caro’s acid diluted with 20 parts of water is able to reduce the WAD cyanide as effectively as a 25% solution at equal molar concentrations of Caro’s acid and free cyanide. In some occasions, dilute Caro’s acid has destroyed cyanide better than concentrate acid. According to some laboratory tests, Caro’s acid is decomposed at a rate of 0.3-0.5% /sec at 72-76 oC and 0.7-0.9%/sec at 90-93 oC on a 100% basis. The Caro’s acid funnel mixer is usually sized for each gold operation in order to improve the mixing efficiency and minimize the decomposition of the acid. After review the data of several gold operations, it has noted that in a properly sized equipment the residence time is 2.5 to 4.0 seconds when Caro’s acid is formed until it is mixed with dilution water. The same conclusion is obtained Caro’s acid is in contact with slurries without adding water. In this way, the short residence time reaches maximum Caro’s acid losses in 4 seconds of approximately 0.9 to 2.5% on a 100% basis at 76-90 oC and this reduction of Caro’s acid are less than 40-45% difference in the proportion of hydrogen peroxide converted to Car’s acid in hot and cold processes.
It is important to consider the right preparation system in order to reduce costs and avoid excessive losses of Caro’s acid during the preparation. If the cooling system must be selected, it is important to select equipment based on technical and cost considerations. It is possible to get potential savings of $150,000 to $200,000 in capital investment.