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CyPlus® Cold Caro’s Acid Systems as an Important Technology for Cyanide Effluent Treatment

Abstract

This paper discusses two recent applications of CyPlus® Cold Caro’s Acid systems and how the higher yield of Caro’s Acid can lead to this process becoming a process of choice regarding Cyanide Effluent Treatment (CET). It has become an important technology for Cyanide Effluent Treatment (CET) over the last years, having found numerous applications world-wide.

These two applications replaced conventional “Hot” Caro’s Acid systems, thus also enabling a direct comparison of reagent savings with the CyPlus® Cold Caro’s Acid systems. An insight into the chemistry of the Caro’s Acid processes is presented to understand the reasons for the higher efficiency better.

Background

CyPlus® has developed several Cyanide Effluent Treatment (CET) processes suitable for the treatment of cyanide-containing ore pulps, since conventional treatment methods, like H2O2 are mostly not feasible due to prohibitively high reagent consumption because of product decomposition. After collaborating in
the field of Cyanide Effluent Treatment with Inco Tech, Canada for many years, CyPlus GmbH developed the CombinOx® process together with them and then acquired the know-how for designing and operating the SO2/AIR process.

Feasibility studies conducted by CyPlus® in its laboratories in Germany compare various process options for the treatment of cyanide-containing ore pulps, including the following processes:
  • CyPlus® Hot Caro’s Acid process
  • CyPlus® Cold Caro’s Acid process
  • CyPlus® SO2/AIR process
  • CombinOx® process
  • CyPlus® Cyanide Recovery process
The feasibility study is performed on the effluent or generated effluent after leaching on representative samples. The feasibility study clearly illustrates which of the processes can reach the desired discharge limits for cyanide most effectively. It also compares the operating costs, mainly the consumption of
treatment reagents as well as the investment costs. Ultimately, the most economic and suitable process is recommended.

The feasibility study is performed in continuous mode to generate the process parameters that will enable meaningful scale-up for engineering. It is also important to use representative chemicals as applied on a large scale to obtain representative results. After the feasibility study has been performed, a basic engineering package is prepared on the process of choice to accommodate a reliable up-scale to the sitespecific conditions.

Finally, tailor-made equipment (e.g. CyPlus® Caro’s Acid G,enerator) is constructed, installed and commissioned on site. The design incorporates state-of-the-art safety features and modern reliable material of construction.

The CyPlus® processes are especially suitable for treating cyanide-containing effluents from ore processing operations in order to comply with stringent limits such as those set by the International Cyanide Management Code, the World Bank, EU Mines Waste Directive and local authorities.

The CyPlus® Cold Caro’s Acid, the CyPlus® SO2/AIR and CombinOx® processes are usually by far the most viable processes of the five processes above. Two recently commissioned large scale applications of the CyPlus® Cold Caro’s Acid process are examples of where this process can outperform the other
effluent-treatment systems.

The interesting thing about these two applications is that they replaced conventional “Hot” Caro’s Acid systems, enabling a direct comparison of reagent savings with the CyPlus® Cold Caro’s Acid systems. It was possible to reduce reagent consumption (hydrogen peroxide and sulphuric acid) by more than 30%, achieving the same low cyanide levels of 10 ppm CNtotal or better at both applications. In one application, the addition of copper sulphate was necessary to achieve the low levels, since the ferrocyanide had to be precipitated.

Another advantage of the CyPlus® Cold Caro’s system is that the heat from the reaction can be controlled and kept at low levels, thus ensuring safe operation.

Process Chemistry

Caro’s Acid is a powerful oxidant that is produced in situ by adding hydrogen peroxide to concentrated sulphuric acid. Caro’s acid, or peroxymonosulphuric acid, is used in a wide range of industrial applications. More recently, Caro’s Acid has been applied at precious metal mines to treat cyanidecontaining effluents. Applying decades of experience in the manufacturing of persulphate compounds, CyPlus, engineered the CyPlus® line of Caro’s Acid generators using proprietary technologies designed to generate peroxymonosulphuric acid safely, efficiently and economically at the customer’s site.

As shown in the chemical equation below, an equilibrium is reached:



For efficiency reasons, it is recommended that the peroxide be in the form of 70% strength and that the sulphuric acid be at least 93%. Once mixed, great care must be taken to accommodate the heat that is generated. The resulting solution containing the H2SO5, along with unreacted peroxide, excess sulphuric acid and water, can then be used as a powerful oxidiser, for example, to oxidise cyanide to cyanate:



CyPlus® Caro’s Acid Generators

CyPlus® custom design generators for specific applications, constructing either a conventional “Hot” CyPlus® Caro’s Acid generator or more frequently the proprietary high yield CyPlus® “Cold” Caro’s Acid generator. Generally, the difference between these two generators is the amount of H2SO5 yielded fromthe reactants, H2O2 and H2SO4.

The “product yield” is defined as the amount of active oxygen, AO, present as H2SO5 in the acidic solution after production:
Product Yield = (AO from H2SO5/Total AO) 100%

The high yield CyPlus® Cold Caro’s Acid generator, as the name indicates, produces a more concentrated H2SO5 solution than the conventional CyPlus® “Hot” Caro’s acid generator. This is accomplished by controlling the temperature during and after the reaction. A yield of > 80% is achieved using the CyPlus® Cold Caro’s Acid generator compared to a yield of only 30-40% using the conventional “Hot” CyPlus® Caro’s acid generator. This results in an overall reduction in reagent consumption of over 30% at both application examples presented above.

Fig. II: The higher the yield of H2SO5, the more efficiently the cyanide is destroyed. Please refer to the following graph (Residual CNWAD Results vs. Peroxide Addition Performed on a Slurry Sample):



Conclusions

The CyPlus® Cold Caro’s Acid process has become an important technology for Cyanide Effluent Treatment (CET) over the last years, having found numerous applications world-wide. CyPlus® Cold Caro’s Acid systems enable a higher yield of Caro’s Acid compared to conventional “Hot” systems and have lead this process to becoming a viable process alternative for Cyanide Effluent Treatment (CET).

It is possible to reduce reagent consumption (hydrogen peroxide and sulphuric acid) by more than 30% with the CyPlus® Cold Caro’s Acid systems, compared to the conventional “Hot” Caro’s Acid systems based on a direct comparison of reagent savings. Another advantage of the CyPlus® Cold Caro’s system is that the heat from the reaction can be controlled and kept at low levels, thus ensuring safe operation.

Moreover, it has been proven that the economic success and a compliant operation of Cyanide Effluent Treatment plant in the mining industry strongly depend on a reliable feasibility study comparing different treatment technologies, a reliable scale-up and the application of state-of-the-art and tailor-made equipment.

CyPlus is a worldwide leading company in cyanides. The wholly owned Evonik group subsidiary supplies products to customers in the mining, chemical, pharmaceutical, and surface treatment industries. The company meets the extensive and strict requirements of the international gold mining industry as a supplier of cyanides in accordance with the provisions of the "International Cyanide Management Code" (ICMC). 60 employees worldwide, production sites in Europe and sales and distribution offices on all continents provide customers swift and reliable service. CyPlus regards itself as the pacesetter of progress and sustainability in the industry.