Impacts of cyanide in gold mining and cyanide removal methodologies in liquid waste from gold processing

2.1. Cyanide use in industry

Cyanide is defined by an organic molecule that has a functional group of triple-bonded carbon nitrogen (-C≡N), while inorganic cyanide is usually a salt of the anion CN^-. Cyanide compounds or complexes are commonly used in mining industries in gold and silver extraction processes, but it can also be applied to other types of processes such as metal recovery of copper, zinc and lead in flotation processes, that uses cyanide as a reagent. In addition, its use is also applied in galvanoplasty industries, paints, plastics, agriculture, pharmaceuticals, food, dyes, among others, as it is illustrated in Table 1 (Botz, 2001Botz, M.M. (2001). Overview of Cyanide Treatment Methods. Mining Environmental Management. Mining Journal Ltd., 28-30. Available in: http://www.thevespiary.org/rhodium/Rhodium/pdf/cyanide.destruction.overview.pdf.; Kumar et al., 2017Kumar, V., Kumar, V., Bhalla, T.C. (2017). Microbial Remediation of Cyanides, in Chapter 4. Bioremediation Current Research and Applications, IK International, Denmark, pp. 88-110.; Sharma et al., 2019Sharma, M., Akhter, Y., Chatterjee, S. (2019). A Review on Remediation of Cyanide Containing Industrial Wastes Using Biological Systems with Special Reference to Enzymatic Degradation. J. Microbiol. Biotechnol. 35, 70. https://doi.org/10.1007/s11274-019-2643-8.).

When cyanide is in aqueous medium in its form of ion, CN^-, or in the form of HCN_aq, it is classified as free cyanide. When associated in the form of metal complexes, cyanide complexes are classified into two classes, namely: WAD (Weak Acid Dissociable), which means it is dissociable in weak acids, and SAD (Strong Acid Dissociable), those that are dissociable in strong acids. WAD complexes are usually with metals such as Cu, Ni, Zn, AG, Hg and Cd, while SAD complexes are usually with metals such as Fe, Pt, Co, Au and Pd. The species in the free or complex form are called total cyanide (Botz, 2001Botz, M.M. (2001). Overview of Cyanide Treatment Methods. Mining Environmental Management. Mining Journal Ltd., 28-30. Available in: http://www.thevespiary.org/rhodium/Rhodium/pdf/cyanide.destruction.overview.pdf.; Jiménez-Prieto et al., 2020Jiménez-Prieto, Y.J., Esperanza-Pérez, G., Ramírez-González, S., Alomas-Vicente, I. (2020). Assessment of Technological Alternatives for Cyanide Waste Waters Management in Gold Ores Processing Plant. Revista Cubana de Química 32 (2), 218-231. Available in: https://cubanaquimica.uo.edu.cu/index.php/cq/article/view/5136.).

It is noteworthy that when the cyanide ion is present in aqueous medium, its hydrolysis forming hydrocyanic acid (HCN), which has high vapor pressure and can generate highly toxic vapor. For this reason, processes that use cyanide should be conducted at controlled alkaline pH. The graph represented in Fig. 1 indicates this distribution of species in the system HCN/CN^-, in which at pH less than 9.2, hydrocyanic acid predominates and at pH above 9.2, the dissociated form predominates (Ciminelli and Gomes, 2002Ciminelli, V.S.T., Gomes, A.D. (2002). Princípios da Cianetação. Extração de Ouro: Princípios, Tecnologia e Meio Ambiente. Chap.3. 59-94. CETEM/MCT. Rio de Janeiro. Available in: http://mineralis.cetem.gov.br/handle/cetem/1220.).

2.2. Use of cyanide in the beneficiation of gold ore

Gold production around the world has double in the last four decades, as extraction processes were developed consequently, including the hydrometallurgical process involving cyanide and other reagents such as chloride and sulfur. The extraction of gold in Brazil began with the searches for precious metals in the time of colony, being initially in the region of Minas Gerais. At this time, gold was removed from superficial deposits and alluvion, being enriched and found in the form of large particles (Gökelma et al., 2016Gökelma, M., Birich, A., Stopic, S. Friedrich, B. (2016). A Review on Alternative Gold Recovery Reagents to Cyanide. J. Mater. Sci. Chem. Eng. 4 (8), 10 pages. https://doi.org/10.4236/msce.2016.48002.; Júnior, 2018Júnior, G.A.S. (2018). Flotação de Minério Sulfetado de Ouro para Diferentes Produtos Obtidos em um Equipamento Ore Sorting. Monografia. Universidade Federal de Ouro Preto. Ouro Preto. Available in: http://www.monografias.ufop.br/handle/35400000/1703.; ANM, 2023ANM (2023). Agência Nacional de Mineração. Anuário Mineral Brasileiro - Principais Substâncias Metálicas. Brasília. Available in: https://www.gov.br/anm/pt-br/centrais-de-conteudo/publicacoes/serie-estatisticas-e-economia-mineral/anuario-mineral/anuario-mineral-brasileiro/PreviaAMB2022.pdf.).

Gold occurs in nature in its elementary shape or in alloys with silver, copper and other materials, or in the form of associations with sulfide minerals, such as arsenopyrite (FeSsS), pyrite (FeS₂), among others. It is these forms of gold that determine the method of extraction of the mineral (Costa, 2022Costa, J.H.B. (2022). Panorama dos Estudos de Aproveitamento de Rejeitos de Mineração do Estado do Pará de 2010 a 2020. Trabalho de Conclusão de Curso, Universidade Federal de Alfenas. Alfenas. Available in: https://www.unifal-mg.edu.br/ceem/wp-content/uploads/sites/195/2023/03/Jaime-Henrique-Barbosa-da-Costa.pdf.).

For classification regarding leaching behavior, gold ores can be classified into complexes, free-milling, refractory or placer. Complex ores are associated with sulfides, which imply high cyanide expenditure for leaching. In the case of free-milling ore, 90% of gold can be recovered with conventional leaching. For refractory ore, it is necessary to use reagents or complex processes to achieve the recovery necessary for operational demand. Finally, the placer ores present free gold due to weathering action, so density concentrating processing methods are used to extract the gold (Costa, 2022Costa, J.H.B. (2022). Panorama dos Estudos de Aproveitamento de Rejeitos de Mineração do Estado do Pará de 2010 a 2020. Trabalho de Conclusão de Curso, Universidade Federal de Alfenas. Alfenas. Available in: https://www.unifal-mg.edu.br/ceem/wp-content/uploads/sites/195/2023/03/Jaime-Henrique-Barbosa-da-Costa.pdf.).

In the process of extraction of gold by conventional leaching, cyanide salts such as sodium (NaCN) and potassium (KCN) are used. It occurs the formation of stable complexes of the released cyanide ion (CN^-) with the gold particles. The reaction of formation of this complex, called dicyanoaurate, occurs in the presence of oxygen in an aqueous solution, as described in the equation below. This process requires excess cyanide dosage to increase gold recovery (Ciminelli and Gomes, 2002Ciminelli, V.S.T., Gomes, A.D. (2002). Princípios da Cianetação. Extração de Ouro: Princípios, Tecnologia e Meio Ambiente. Chap.3. 59-94. CETEM/MCT. Rio de Janeiro. Available in: http://mineralis.cetem.gov.br/handle/cetem/1220.; Hou et al., 2020Hou, D., Liu, L., Yang, Q., Zhang, B., Qiu, H., Ruan, S., Chen, Y., Li, H. (2020). Decomposition of Cyanide from Gold Leaching Tailings by Using Sodium Metabisulphite and Hydrogen Peroxide. Adv. Mater. Sci. Eng. 7, ID 5640963. https://doi.org/10.1155/2020/5640963.).

After leaching, gold recovery from the pregnant solution can be done by extraction techniques such as precipitation, solvent extraction, electrowinning and adsorption. Usually, mining companies use processes that involve gold extraction by adsorption with the addition of activated carbon in the solution, processes known as carbon-in-pulp (CIP), adding carbon to the pulp after leaching; carbon-in-leach (CIL), adding carbon to the leaching process; or carbon-in-columns (CIC), using columns stuffed with carbon. After contact time, carbon is removed from the solution to desorb gold. The liquid solution that no longer contains gold is the residue of the process (Altinkaya et al., 2020Altinkaya, P., Wang, Z., Korolev, I., Hamuyuni, J., Haapalainen, M., Kolehmainen, E., Yliniemi, K., Lundström, M. (2020). Leaching and Recovery of Gold from Ore in Cyanide-Free Glycine Media. Miner. Eng. 158, 10610. https://doi.org/10.1016/j.mineng.2020.106610.).

This residue generated may contain other toxic components, which can also form strong complexes with gold, such as ferricyanide (Fe(CN)₆ ^3-) and ferrocyanide (Fe(CN)₆ ^4-), due to the presence of different metals in the mineralogical composition of the ore. Cyanide consumption in leaching can reach up to 2500 g/t of processed material, which can generate effluents with high concentrations of cyanide complexes (Moura, 2005Moura, W. (2005). Especiação de Cianeto para Redução do Consumo no Circuito de Lixiviação de Calcinado da Usina do Queiróz. Dissertação de Mestrado. Universidade Federal de Minas Gerais. Belo Horizonte. Available in: http://hdl.handle.net/1843/BUOS-8DJHXK.; Vicente, 2014Vicente, C.M. (2014). Ensaios Exploratórios de Lixiviação de Minérios de Ouro. Dissertação de Mestrado. Universidade do Porto. Porto.; Chaguezac, 2018Chaguezac, D.F.C. (2018). Estudo de Degradação do Complexo Cianometálico Ferricianeto [Fe(CN)63-] dos Efluentes da Mineração Aurífera por Meio de Fotocatálise com TiO₂. Dissertação de Mestrado. Escola de Engenharia da Universidade Federal do Rio Grande do Sul. Porto Alegre. Available in: http://hdl.handle.net/10183/184612.; Han et al., 2022Han, W-W., Yang, H-Y., Tong, L. (2022). Cyanide Removal for Ultrafine Gold Cyanide Residues by Chemical Oxidation Methods. Trans. Nonferrous Met. Soc. China 32 (12), 4129-4138. https://doi.org/10.1016/S1003-6326(22)66083-7.)

Compounds containing cyanide are extremely toxic and can cause serious damage to animal life and the environment. The biggest concern about impacts is in relation to aquatic life. Thus, restricted standards were proposed for the disposal of effluents containing cyanide in several countries (Riani et al., 2007Riani, J.C., Pina, P.S., Leão, V.A. (2007). Tecnologia Limpa para Redução de Impacto Ambiental do Cianeto na Mineração de Ouro. Revista da Escola de Minas 60 (1), 21-28, https://doi.org/10.1590/S0370-44672007000100004.; Shin et al., 2019Shin, D., Park, J., Park, H., Lee, J., Kim, M., Lee, J. (2019). Key Microbes and Metabolic Potentials Contributing to Cyanide Biodegradation in Stirred-Tank Bioreactors Treating Gold Mining Effluent. Miner. Process. Extr. Metall. Rev. 41 (2), 85-95. https://doi.org/10.1080/08827508.2019.1575213.).

According to the federal legislation in Brazil, effluents containing cyanide cannot be discarded in a water body in concentration values above 1.0 mg·l^-1 of total cyanide and 0.20 mg·l^-1 of free cyanide (CONAMA, 2011CONAMA (2011). Conselho Nacional do Meio Ambiente. Resolução 430, de 13 de maio de 2011. Dispõe Sobre as Condições e Padrões de Lançamento de Efluentes, Complementa e Altera a Resolução No. 357, de 17 de março de 2005. ).

Cyanide removal treatments are therefore extremely important for maintaining the life of mining companies and to maintain the water quality of the environment and animal life. For the industrial scenario, it is essential that technologies for treatment of cyanide containing are economically beneficial and environmentally friendly (Shin et al., 2019Shin, D., Park, J., Park, H., Lee, J., Kim, M., Lee, J. (2019). Key Microbes and Metabolic Potentials Contributing to Cyanide Biodegradation in Stirred-Tank Bioreactors Treating Gold Mining Effluent. Miner. Process. Extr. Metall. Rev. 41 (2), 85-95. https://doi.org/10.1080/08827508.2019.1575213.).

Golden mining effluents are commonly found in pH alkaline ranges and high concentration of salts, sulfates (SO₄ ^2-), iron (Fe) and other toxic metals such as mercury (Hg), chromium (Cr), zinc (Zn), copper (Cu), nickel (Ni), arsenic (As), among others, because the composition and concentration of the effluent depends on the composition present in the gold ore, which is complex (Yaraghi et al., 2020Yaraghi, N., Ronkanen, A., Haghighi, A.T., Aminikhah, M., Kujala, K., Klove, B. (2020). Impacts of Gold Mine Effluent on Water Quality in a Pristine Sub-Arctic River. J. Hydrol. 589, 125170. https://doi.org/10.1016/j.jhydrol.2020.125170.; Alvillo-Rivera et al., 2021Alvillo-Rivera, A., Garrido-Hoyos, S., Buitrón, G., Thangarasu-Sarasvathi, P., Rosano-Ortega, G. (2021). Biological Treatment for The Degradation of Cyanide: A Review. J. Mater. Res. Technol. 12, 1418-1433. https://doi.org/10.1016/j.jmrt.2021.03.030.).

In the case of Kinross, in Paracatu - Minas Gerais, because of the ore high sulfide content, between 3% and 5%, the tailings have significant sulfide concentrations, and studies were developed for desulfurization. In addition, there are gold recovery processes in the cyanide tailings, which are impacted by the cyanide content, due to its high depressor potential in the flotation of the auriferous pyrite (Brasil, 2012Brasil, T.F.M. (2012). Estudo de Remoção de Sulfetos por Flotação de um Minério de Baixo Teor de Ouro. Projeto de Final de Curso. Universidade Federal do Rio de Janeiro. Rio de Janeiro. Available in: http://hdl.handle.net/11422/19195.).

For the effluent stocked in the dam of Geominera Center Enterprises in Cuba, the presence of heavy metals such as zinc, copper, cobalt, arsenic and nickel were evidenced (Jimenéz-Prieto, 2020Jiménez-Prieto, Y.J., Esperanza-Pérez, G., Ramírez-González, S., Alomas-Vicente, I. (2020). Assessment of Technological Alternatives for Cyanide Waste Waters Management in Gold Ores Processing Plant. Revista Cubana de Química 32 (2), 218-231. Available in: https://cubanaquimica.uo.edu.cu/index.php/cq/article/view/5136.).

In the case of La Quinta and Cepromet Minera S.A.C beneficiation plants in Peru, a characterization of their water was developed by Vuono et al. (2021)Vuono, D.C., Vanneste, J., Figueroa, L.A., Hammer, V., Aguilar-Huaylla, F.N., Malone, A., Smith, N.M., Garcia-Chevesich, P.A., Bolaños-Sosa, H.G., Alejo-Zapata, F., Polanco-Cornejo, H.G., Bellona, C. (2021). Photocatalytic Advanced Oxidation Processes for Neutralizing Free Cyanide in Gold Processing Effluents in Arequipa, Southern Peru. Sustainability 13 (17), 9873. https://doi.org/10.3390/su13179873.. In their study it was found total cyanide and a variety of pollutants including heavy metals. Table 2 shows part of the major components found in the water and their concentration as an example of what can occur in gold plants effluents.

Within the scenario of water resources scarcity and environmental requirements, a practice that has been used in mining industries is to recycle the water within the processes, usually from dams, thickening, flowing, filtering, and other processes, returning it to the process of mining and processing. However, the characteristic of this water can be little known to understand the impacts of its application on the process and the increase in recirculation implies the increase in concentration of ions and salinity of this water, requiring additional treatments before recycling (Sampaio et al., 2018Sampaio, J.A., Luz, A.B., Andrade, M.C., França, S.C.A. (2018). Água No Processamento Mineral. Tratamento de Minérios. Cap. 17. Ed. 6. CETEM. ).

3.1. Hydrogen Peroxide

The treatment of effluents containing cyanide with hydrogen peroxide (H₂O₂) is based on the oxidative potential of the reagent, which causes the cyanide to degrade into cyanate (CNO^-), a less toxic agent, and, in an alkaline environment, the cyanate oxidizes into ammonia (NH₃), according to the equations below (Chaguezac, 2018Chaguezac, D.F.C. (2018). Estudo de Degradação do Complexo Cianometálico Ferricianeto [Fe(CN)63-] dos Efluentes da Mineração Aurífera por Meio de Fotocatálise com TiO₂. Dissertação de Mestrado. Escola de Engenharia da Universidade Federal do Rio Grande do Sul. Porto Alegre. Available in: http://hdl.handle.net/10183/184612.).

This reaction is optimized at pH between 9.5 and 10, but it can stabilize if the pH is in the range of 12. The use of H₂O₂ is generally applied in cases where the cyanide concentration is low, and the oxidant can remove free ions and WAD cyanide. Its dosage tends to be up to 600% greater than the stoichiometry of the oxidation reaction (Botz, 2001Botz, M.M. (2001). Overview of Cyanide Treatment Methods. Mining Environmental Management. Mining Journal Ltd., 28-30. Available in: http://www.thevespiary.org/rhodium/Rhodium/pdf/cyanide.destruction.overview.pdf.; Chaguezac, 2018Chaguezac, D.F.C. (2018). Estudo de Degradação do Complexo Cianometálico Ferricianeto [Fe(CN)63-] dos Efluentes da Mineração Aurífera por Meio de Fotocatálise com TiO₂. Dissertação de Mestrado. Escola de Engenharia da Universidade Federal do Rio Grande do Sul. Porto Alegre. Available in: http://hdl.handle.net/10183/184612.).

Examples of use are in the studies by Kamrani et al. (2019)Kamrani, M.S., Seifpanahi-Shabani, K., Seyed-Hakimi, A., Al, G.A.M., Agarwa, Sh., Gupta, V.K. (2019). Degradation of Cyanide from Gold Processing Effluent by H₂O₂, NaClO and Ca(ClO)₂ Combined with Sequential Catalytic Process. Bulg. Chem. Commun. 51 (3), 384 -393. https://doi.org/10.34049/bcc.51.3.5052., which used pure H₂O₂, with dosages of 0,04 to 4,78 g·L^-1 in a solution with 40 mg·L^-1 of cyanide at pH 10, indicating the need for a stoichiometric ratio of 300% and suggesting that in the presence of catalysts, such as copper, the reaction is favored. In the study by Tu et al. (2019)Tu, Y., Han, P., Wei, L., Zhang, X., Yu, B., Qian, P., Ye, S. (2019). Removal of Cyanide Adsorbed on Pyrite by H₂O₂ Oxidation Under Alkaline Conditions. J. Environ. Sci. 78, 287-292. https://doi.org/10.1016/j.jes.2018.10.013., oxidation was achieved using 0,010 g·L^-1 of H₂O₂, and the pH was evaluated indicating that removal increases above pH 7 and is constant at pH 12.

Cyanide oxidation with hydrogen peroxide is catalyzed when copper is added. That occurs because of the reduction of the copper ion (Cu²⁺) to the tricyanocuprate complex (Cu(CN)₃)^2- in the presence of excess cyanide. However, the proportion of copper dosed must be evaluated to increase the effectiveness of cyanide degradation, since when tricyanocuprate becomes the dominant specie in the medium, the reaction is less efficient (Beattie and Polyblank, 1995Beattie, J.K., Polyblank, G.A. (1995). Copper-Catalyzed Oxidation of Cyanide by Peroxide in Alkaline Aqueous Solution. Aust. J. Chem. 48 (4), 861-868. https://doi.org/10.1071/CH9950861.).

3.2. Inco Process

The Inco Process is called after the name of the company (Inco) that first tried this solution to effluent treatment. The process is based on the use of sulfur dioxide and air in the presence of a copper catalyst, to degrade cyanide into cyanate, according to the equation below. This process is more effective for solutions containing free cyanide and WAD cyanide. However, precipitation of iron and cyanide complexes may occur due to the presence of copper. The reagent dosage in this process is above the reaction stoichiometry, reaching up to 200% (Botz, 2001Botz, M.M. (2001). Overview of Cyanide Treatment Methods. Mining Environmental Management. Mining Journal Ltd., 28-30. Available in: http://www.thevespiary.org/rhodium/Rhodium/pdf/cyanide.destruction.overview.pdf.).

A currently applied variation of the Inco Process is the use of sodium metabisulfite (Na₂S₂O₅), which provides the sulfur dioxide necessary for cyanide oxidation to occur (Botz, 2001Botz, M.M. (2001). Overview of Cyanide Treatment Methods. Mining Environmental Management. Mining Journal Ltd., 28-30. Available in: http://www.thevespiary.org/rhodium/Rhodium/pdf/cyanide.destruction.overview.pdf.; Hou et al., 2020Hou, D., Liu, L., Yang, Q., Zhang, B., Qiu, H., Ruan, S., Chen, Y., Li, H. (2020). Decomposition of Cyanide from Gold Leaching Tailings by Using Sodium Metabisulphite and Hydrogen Peroxide. Adv. Mater. Sci. Eng. 7, ID 5640963. https://doi.org/10.1155/2020/5640963.).

To achieve adequate cyanide concentration levels for effluent disposal, a combination of methods can also be applied, such as using Na₂S₂O₅ with H₂O₂ pretreatment. In this reaction, sulfuric acid (H₂SO₄) is formed and must be controlled as the optimum oxidation pH is in the alkaline range (Hou et al., 2020Hou, D., Liu, L., Yang, Q., Zhang, B., Qiu, H., Ruan, S., Chen, Y., Li, H. (2020). Decomposition of Cyanide from Gold Leaching Tailings by Using Sodium Metabisulphite and Hydrogen Peroxide. Adv. Mater. Sci. Eng. 7, ID 5640963. https://doi.org/10.1155/2020/5640963.).

According to studies by Han et al. (2022)Han, W-W., Yang, H-Y., Tong, L. (2022). Cyanide Removal for Ultrafine Gold Cyanide Residues by Chemical Oxidation Methods. Trans. Nonferrous Met. Soc. China 32 (12), 4129-4138. https://doi.org/10.1016/S1003-6326(22)66083-7., using 2.0 g·L^-1 of Na₂SO₃, equivalent to 400% of stoichiometry, working at pH 9, it is possible to remove cyanide from effluents starting from a concentration of 136 mg·L^-1 to values below 5 mg·L^-1. With the addition of air, the process becomes faster and more efficient, reaching values close to 2 mg·L^-1.

3.3. Alkaline Chlorination

Alkaline chlorination consists of the degradation of cyanide by the action of chlorine and involves two steps. In the first, occurs the conversion of cyanide to cyanogen chloride (CNCl) and in the second, through hydrolysis, the conversion to cyanate occurs (Schneider et al., 2019Schneider, C.L., Matiolo, E., Neumann, R., Gomes, O.F.M. (2019). Beneficiamento de Minérios. Recursos Minerais no Brasil: Problemas e Desafios. Academia Brasileira de Ciências, pp. 257-262. Rio de Janeiro. Available in: https://www.abc.org.br/IMG/pdf/doc-7006.pdf.).

The free and WAD cyanide forms can react with the chlorine gas used in this treatment method. As the name describes, alkaline chlorination reactions occur in pH ranges between 10.5 and 11.5 (Alvarez Rosario, 2017Alvarez Rosario, C.G. (2017). Avaliação da Degradação Bacteriana de Cianeto Usando Cepas Isoladas de Rejeito de Mineração. Tese de Doutorado. Universidade de São Paulo. São Paulo. https://doi.org/10.11606/T.3.2018.tde-15012018-141540.).

However, this is a less used method due to the formation of organochlorine compounds in its reaction, as cyanogen chloride can volatilize at a pH below 8. Furthermore, other oxidizing agents can be used to assist in the reaction, such as hypochlorite of sodium and calcium, which generate chloramines during the reaction, compounds that are extremely toxic to aquatic environments (Alvarez Rosario, 2017Alvarez Rosario, C.G. (2017). Avaliação da Degradação Bacteriana de Cianeto Usando Cepas Isoladas de Rejeito de Mineração. Tese de Doutorado. Universidade de São Paulo. São Paulo. https://doi.org/10.11606/T.3.2018.tde-15012018-141540.; Jiménez-Prieto et al., 2020Jiménez-Prieto, Y.J., Esperanza-Pérez, G., Ramírez-González, S., Alomas-Vicente, I. (2020). Assessment of Technological Alternatives for Cyanide Waste Waters Management in Gold Ores Processing Plant. Revista Cubana de Química 32 (2), 218-231. Available in: https://cubanaquimica.uo.edu.cu/index.php/cq/article/view/5136.).

According to Teixeira et al. (2013)Teixeira, L.A.C., Arellano, M.T.C., Marquez-Sarmiento, C.O., Yokoyama, L., Araujo, F.V.F. (2013). Tratamento de Efluentes Contendo Cianeto por Oxigênio Singlete Gerado Através da Reação de Peróxido de Hidrogênio e Hipoclorito de Sódio. XXV Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa & VIII Meeting of the Southern Hemisphere on Mineral Technology. Goiânia., the use of alkaline chlorination with sodium hypochlorite by itself can achieve good results of 87% of removal, but it is less effective and slower than other advanced oxidation processes, in the range of pH from 9 to 11 and concentration of cyanide of 1000 mg·L^-1. To analyze that, Teixeira et al. (2013)Teixeira, L.A.C., Arellano, M.T.C., Marquez-Sarmiento, C.O., Yokoyama, L., Araujo, F.V.F. (2013). Tratamento de Efluentes Contendo Cianeto por Oxigênio Singlete Gerado Através da Reação de Peróxido de Hidrogênio e Hipoclorito de Sódio. XXV Encontro Nacional de Tratamento de Minérios e Metalurgia Extrativa & VIII Meeting of the Southern Hemisphere on Mineral Technology. Goiânia. tried the use of sodium hypochlorite and hydrogen peroxide reaching a final removal of cyanide of 99.8%.

3.4. Ozonation

The use of ozone as a cyanide degrader is based on its high oxidizing capacity, being capable of transforming cyanide into cyanate, according to the equation below (Schneider et al., 2019Schneider, C.L., Matiolo, E., Neumann, R., Gomes, O.F.M. (2019). Beneficiamento de Minérios. Recursos Minerais no Brasil: Problemas e Desafios. Academia Brasileira de Ciências, pp. 257-262. Rio de Janeiro. Available in: https://www.abc.org.br/IMG/pdf/doc-7006.pdf.).

However, to use this method, it is interesting that the ozone is produced in the effluent treatment plant itself, which further increases the cost of this reagent due to the need to implement the production system (Schneider et al., 2019Schneider, C.L., Matiolo, E., Neumann, R., Gomes, O.F.M. (2019). Beneficiamento de Minérios. Recursos Minerais no Brasil: Problemas e Desafios. Academia Brasileira de Ciências, pp. 257-262. Rio de Janeiro. Available in: https://www.abc.org.br/IMG/pdf/doc-7006.pdf.).

For the studies of Barriga-Ordonez et al. (2006)Barriga-Ordonez, F., Nava-Alonso, F., Uribe-Salas, A. (2006). Cyanide Oxidation by Ozone in a Steady-State Flow Bubble Column. Miner. Eng. 19 (2), 117-122. https://doi.org/10.1016/j.mineng.2005.09.001., oxidation with ozone in a countercurrent bubble column can reach up to 90% of removal efficiency when dosing 1.2 mol of O₃ to each mol of CN or in higher dosages, indicating high consumption for this specific technique.

3.5. Complexation and precipitation with iron

Another way to remove cyanide complexes is to use iron ions to precipitate these contaminants. Stable iron ions are formed with cyanide, from the Prussian blue family, which have low solubility and can be separated from the effluent. In pH ranges from 2 to 11, decomposition of WAD complexes is possible. However, this process is not very efficient for a single treatment as it cannot remove high amounts of cyanocomplexes and the reactions are slow (Riani et al., 2007Riani, J.C., Pina, P.S., Leão, V.A. (2007). Tecnologia Limpa para Redução de Impacto Ambiental do Cianeto na Mineração de Ouro. Revista da Escola de Minas 60 (1), 21-28, https://doi.org/10.1590/S0370-44672007000100004.; Tang et al., 2019Tang, J., Shi, X., Sun, C., Zhuang, H., Ning, K., Zhang, C. (2019). Comparison Treatment of Cyanide by Chemical Precipitation, Fenton and Fluidized-Bed Fenton Process with Suspended Carrier Coated Iron Oxide: Parameter Optimization and Mechanism. Desalin. Water Treat. 158, 245-255. https://doi.org/10.5004/dwt.2019.24182.).

The studies of Ghosh et al. (1999)Ghosh, R.S., Dzombak, D.A., Luthy, R.G., Smith, J.R. (1999). In Situ Treatment of Cyanide-Contaminated Groundwater by Iron Cyanide Precipitation. Water Environ. Res. 71 (6), 1217-1228. https://doi.org/10.2175/106143096X122456. evaluated precipitation of iron cyanide in situ and in column tests and it was concluded that in neutral pH and low flow rates, concentrations of total cyanide of 0.5 mg·L^-1 were achieved. The maximum removal efficiency was reached when dosing 10% by weight of iron, not resulting in better efficiency when increasing the dosage of iron. It was also analyzed that, when dissolved iron is in excess, coprecipitation of hydrous ferric oxide can occur.

3.6. Biological treatment

Although chemical treatments are efficient, they are not always able to fully degrade cyanide complexes that are more stable and this may require more expensive and specific equipment and reagents, in addition to the generation of secondary compounds in the reactions, which must be treated later. As a result, biological treatment has become a very attractive alternative for industries (Alvillo-Rivera et al., 2021Alvillo-Rivera, A., Garrido-Hoyos, S., Buitrón, G., Thangarasu-Sarasvathi, P., Rosano-Ortega, G. (2021). Biological Treatment for The Degradation of Cyanide: A Review. J. Mater. Res. Technol. 12, 1418-1433. https://doi.org/10.1016/j.jmrt.2021.03.030.).

This treatment is based on the use of microorganisms and enzymes that degrade cyanide into carbon dioxide (CO₂), ammonia (NH₄), among other less toxic components. The metals released in this decomposition come from weak and strong cyanide complexes and can be removed through precipitation and absorption methods by the biofilm. Enzymes can carry out hydrolytic, reductive, oxidative or substitution and transfer degradation routes, also using oxygen and considering the pH of the medium. Normally, more than one route can be used for complete treatment, and in aerobic treatments, packed bed reactors, batch reactors, biological filters, facultative lagoons and activated sludge are generally used, among others (Sharma et al., 2019Sharma, M., Akhter, Y., Chatterjee, S. (2019). A Review on Remediation of Cyanide Containing Industrial Wastes Using Biological Systems with Special Reference to Enzymatic Degradation. J. Microbiol. Biotechnol. 35, 70. https://doi.org/10.1007/s11274-019-2643-8.; Shin et al., 2019Shin, D., Park, J., Park, H., Lee, J., Kim, M., Lee, J. (2019). Key Microbes and Metabolic Potentials Contributing to Cyanide Biodegradation in Stirred-Tank Bioreactors Treating Gold Mining Effluent. Miner. Process. Extr. Metall. Rev. 41 (2), 85-95. https://doi.org/10.1080/08827508.2019.1575213.).

In addition to being a highly effective technique, biological degradation can be accelerated and recover energy and health resources through techniques such as electro-biodegradation, anaerobic biodegradation systems and microbial fuel cells, which generate methane, energy and can produce biocompounds (Malmir et al., 2022Malmir, N., Fard, N.A., Aminzadeh, S., Moghaddassi-Jahromi, Z., Mekuto, L. (2022). An Overview of Emerging Cyanide Bioremediation Methods. Processes 10 (9), 1724. https://doi.org/10.3390/pr10091724.).

3.7. Comparison between methods

The different methods of treating effluents containing cyanide involve different degradation and transformation techniques into less toxic agents, with cyanate being commonly obtained and, in some methodologies, cyanate decomposition into other agents.

In most cases, a high dosage of reagent in relation to stoichiometry is necessary to remove excess cyanide present in the effluents and balance the oxidation equation. Regarding cyanide species, some methods are more effective for free cyanide and require additional steps or combinations to remove total cyanide, while other methods can remove free and complex cyanide at the same time. Table 3 shows the comparison in relation to the methods evaluated in this study.