19 Cyanide Treatment Technology: Overview George M. Wong-Chong, Rajat S. Ghosh, and David A. Dzombak CONTENTS 19.1 Technology Selection 387 19.1.1 Waste Characteristics 389 19.1.2 Cyanide Content of the Waste 389 19.1.3 Waste Matrix 389 19.1.4 Other Constituents of Concern 390 19.1.5 Treated Waste Quality Requirement 390 19.1.6 Cost 390 19.2 Summary and Conclusions 390 References 391 Anthropogenic cyanide has long been released to the environment through industrial effluent discharges (e.g., chemical manufacturing, coke plants, gold mining, gas plants, and electroplat- ing effluents) and unregulated disposal of contaminated solid wastes (e.g., aluminum manufacturing spent pot liner, and manufactured gas plants wastes). The latter practice has resulted in contaminated groundwater and soil. Today in the United States, environmental regulations have essentially curbed all uncontrolled discharge practices with industry applying, for the most part, best management practices and best available treatment technologies for the treatment of their wastes. Some areas of earlier waste disposal practice remain problematic (e.g., former gas plant spent oxide box waste and aluminum smelting spent pot liner disposal sites). For the treatment of cyanide in groundwater, wastewater, sludges, and contaminated soil there is available an array of technologies, some of which have been in commercial practice, while others these technologies, the waste matrix to which they are most applicable, and application status (bench- scale, pilot-scale, or commercial practice). Thetable alsoprovides comments as to industries inwhich individual technologies are in commercial practice. Fundamental details of these technologies and 19.1 TECHNOLOGY SELECTION Selection of an appropriate and economical treatment technology for a given waste depends on a number of factors, including: • Waste chemical and physical characteristics • Quantity of waste to be treated 387 © 2006 by Taylor & Francis Group, LLC example commercial applications are presented in subsequent chapters (Chapters 20 to 27). have been extensively evaluated but not used in commercial practice. Table 19.1 presents a list of 388 Cyanide in Water and Soil TABLE 19.1 Cyanide Treatment Technology: Overview Applicable cyanide species Waste matrix Technology status Technology Free WAD a FeCN b Wastewater Groundwater Sludge Soil c Bench Pilot Comm Comments Biological Microbial Gold mining wastewater; Coke plant wastewater Phytological Chemical oxidation (low temp) Alkaline Chlorination Electroplating wastewater Ozonation w/o UV UV d — Oxidation Other oxidation processes Kastone Air/SO 2 w/CO catalyst Gold mining waste Separation technologies Precipitation Coke plant wastewater with metal-CN; SPL e contaminated groundwater Carbon adsorption CN recovery in gold mining Ion exchange Membrane concentration Air/steam stripping CN recovery in gold mining Evaporation Thermal technologies High temperature alkaline Aluminum industry — SPL e chlorination treatment High pressure and temperature Aluminum industry — SPL e alkaline hydrolysis treatment Incineration Aluminum industry — SPL e treatment Wet air oxidation a WA D = weak acid dissociable cyanide. b FeCN = dissolved iron cyanide complexes. c Contaminated soil with iron-complexed cyanide. d UV = ultra violet irradiation. e SPL = spent pot liner. © 2006 by Taylor & Francis Group, LLC Cyanide Treatment Technology 389 • Waste matrix • Other constituents of concern in the waste • Treated waste quality required • Cost Brief discussions of these factors in the context of a specific cyanide waste follows. More detailed discussions of these factors in the context of an overall plant- or site-wide waste management strategy 19.1.1 WASTE CHARACTERISTICS • Water soluble forms: hydrogen cyanide and CN − ion (measured as free cyanide), weak metal–cyanide complexes (e.g., cyanide complexes with cadmium, copper, nickel, and zinc), and strong metal–cyanide complexes (e.g., cyanide complexes with cobalt and iron). Note that the analytical measurement “weak acid dissociable (WAD) cyanide” includes free cyanide and any weak metal–cyanide complexes, while “total cyanide” includes WAD and any strong metal–cyanide complexes. • Water insoluble forms: transition metal–metal cyanide complex compounds, for example, ferric ferrocyanide or Prussian Blue, Fe 4 (Fe(CN) 6 ) 3 (s). When these solid form com- pounds are placed in water, they dissolve to varying extents, ultimately to their equilibrium solubility limits, yielding low dissolved concentrations of metal–cyanide complexes under natural environmental conditions. This solubility may pose certain treatment challenges and regulatory concerns. 19.1.2 CYANIDE CONTENT OF THE WASTE The cyanide content of the waste (concentration and quantity) and treatment requirements directly impact the selection, size, and cost of the treatment system. For relatively small quantities of waste, as is sometimes the case with contaminated soils, off-site management (e.g., landfill disposal or incineration) would be more cost effective than on-site treatment. Vice versa, with large continuously generated wastewater streams, on-site treatment tends to be more cost effective. 19.1.3 WASTE MATRIX Cyanide occurs in numerous waste matrices, including: • Wastewaters generated from manufacturing operations, which can contain many chemical constituents, and both soluble and insoluble entities. • Contaminated groundwater, which can sometimes contain immiscible organic liquids, also known as “free product” (e.g., light nonaqueous phase liquids, or LNAPL, and dense nonaqueous phase liquids, or DNAPL). • Contaminated soil and soil slurries. • Contaminated sediment (e.g., dredge spoils). • Manufacturing waste sludges. These various waste matrices greatly influence treatment technology selection. In some cases, ancillary treatment processes may be required to ensure the proper operation of the “primary” treatment technology and overall compliance with requisite regulatory limits. The following two © 2006 by Taylor & Francis Group, LLC are presented in Chapters 26 and 27. In a given cyanide waste, cyanide can exist in numerous forms/species (discussed in Chapters 2, 5, 7, and 8), all of which impact the selection of a treatment technology. These forms include: 390 Cyanide in Water and Soil examples illustrate the differences in treatment schemes needed for similar wastes (e.g., groundwater contaminated with dissolved iron cyanide) in different matrices: 1. Matrix A, groundwater only: effective treatment can be achieved with iron cyanide precipitation only. 2. Matrix B, groundwater with LNAPL: effective treatment requires the separation of the LNAPL prior to iron cyanide precipitation. 19.1.4 OTHER CONSTITUENTS OF CONCERN The presence of other constituents of concern in the waste stream to be treated affects the treatment technology selection and often leads to a train of treatment processes to produce desired treated waste quality. Two examples illustrate the differences in treatment schemes needed for different types of wastes with different regulatory concerns. • Gold mining tailings: These wastes can contain WAD cyanides, strong metal-cyanide complexes, thiocyanate, and trace metals. Biological treatment has proven effective in treating these wastes. The WAD cyanides and thiocyanate are degraded to carbon dioxide, ammonia and sulfate; the strong cyanide complexes and trace metals are adsorbed on to the biological materials; and ammonia is biologically oxidized to nitrate [1]. • Coke plant wastewaters: These wastewaters contain ammonia, WAD cyanide, strong metal-cyanide complexes, phenols, other organics, thiocyanate, sulfide, and trace ele- ments. Treatment of these wastewaters requires a complex treatment train if all of these constituents are of regulatory concern. The treatment train can include: steam stripping to remove ammonia, WAD cyanide and sulfide; biological treatment for ammonia, resid- ual WAD cyanide, phenols, organics, thiocyanate, and sulfide; chemical precipitation for fluoride and strong metal cyanide complexes [2]. 19.1.5 TREATED WASTE QUALITY REQUIREMENT This regulatory factor is the primary driving force for treatment and it dictates the degree of treatment that must be achieved. Regulatory requirements and concerns with these requirements are discussed 19.1.6 COST The cost of treatment for a given waste stream can vary greatly depending on site-specific condi- tions and circumstances. Certain publications have provided cost information [1,3–5] specifically focused on treatment of cyanide-bearing wastes. However these cost data are somewhat outdated and should be adjusted for the time since publication. Even with inflation adjustment they should be considered as preliminary guides at best because of the changes in technology markets and global economics. In subsequent chapters, some additional treatment costs and guidelines are provided for specific technologies. 19.2 SUMMARY AND CONCLUSIONS • An array of technologies exists for treatment of cyanide species in wastewaters, groundwaters, soils, and sludges. • Selection of appropriatetechnology for a given waste depends on various factors, including physico-chemical characteristics of the waste matrix, treatment volume, required treatment level, and cost. © 2006 by Taylor & Francis Group, LLC in detail in Chapter 18. Cyanide Treatment Technology 391 • Compilations of cost information for certain cyanide treatment technologies exist but are somewhat outdated. Technology cost information from these compilations should be used only for preliminary assessments. REFERENCES 1. Whitlock, J.L. and Mudder, T., The Homestake wastewater treatment process: biological removal of toxic parameters from cyanidation wastewaters and bioassay effluent evaluation, in Cyanide Monograph, Mudder, T., Ed., Mining Journal Books, Ltd., London, 1998. 2. Wong-Chong, G.M., Sommerfield, F.J., and Velegol, D.J., Coke plant wastewater treatment at Bao Shan Steel, Shanghai, China, in Proceedings of WEFTEC Latin America, Rio de Janiero, 2000. 3. Smith, A. and Mudder, T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books, Ltd., London, 1991. 4. Mudder, T., Ed., Cyanide Monograph, Mining Journal Books, Ltd., London, 1998. 5. Palmer, S.A.K., Breton, M.A., Nunno, T.J., Sullivan, D.M., and Suprenant, N.F., Metal/Cyanide Containing Wastes: Treatment Technologies, Noyes Datacorp., Park Ridge, NJ, 1988. © 2006 by Taylor & Francis Group, LLC . presented in subsequent chapters (Chapters 20 to 27). have been extensively evaluated but not used in commercial practice. Table 19. 1 presents a list of 388 Cyanide in Water and Soil TABLE 19. 1 Cyanide. following two © 2006 by Taylor & Francis Group, LLC are presented in Chapters 26 and 27. In a given cyanide waste, cyanide can exist in numerous forms/species (discussed in Chapters 2, 5, 7, and. plant- or site-wide waste management strategy 19. 1.1 WASTE CHARACTERISTICS • Water soluble forms: hydrogen cyanide and CN − ion (measured as free cyanide) , weak metal cyanide complexes (e.g., cyanide