Commonwealth of Australia 2010
Fate of cyanide in heap leach ore heaps
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- 26.1.2Migration in seepage from heap leach operations
26.1Fate of cyanide in heap leach ore heaps26.1.1Changes in concentration and compositionThe reactions of cyanide in a heap leach process initially involve reactions of free cyanide, but will later also involve secondary cyanide compounds (refer Figure 6.2; Smith and Struhsacker, 1988). Loss of free cyanide may occur in the ore heap, the drainage channels or drainage collection ponds due to a range of factors such as volatilisation, iron complexation and precipitation processes within the heap. Figure 6.. Prevailing geochemical conditions and typical cyanide reactions in the heap leach environment (Smith and Struhsacker, 1988; Hallock 1990) 
Concentrations of cyanide compounds in porewaters and drainage vary temporally and site-specifically, and concentrations will be much higher during the operation phase than in the post-operational phase (Staunton, 1991d). Ore heaps may take some time to fully leach. Modelling technology is being developed and applied to predict the seepage lifetime of ore heaps (Mining Life-Cycle Center, 2003). The extent of puddling observed on 24 active heaps at Nevada gold mines evaluated by Henny et al. (1994) was severe in at least one heap at 9 of the mines and slight at another 8. The degree of puddling seemed to be influenced by the amount of clay in the ore, the quality of distribution system maintenance, the application rate of the cyanide solution, the degree to which compaction due to trucks and heavy machinery had occurred, and the duration of time that solution had been applied to the heap. Measured WAD CN concentrations in puddles at the top of the heaps ranged from 2 to 1120 mg/L. In some cases this was elevated above the stated concentration of NaCN used (e.g. WAD CN of 612 mg/L where NaCN was used at 265 mg CN/L), and in the case of the 1120 mg/L figure the application rate information was not available. Smith and Mudder (1993) reported concentrations of WAD cyanide in drainage channels of ~130 mg/L. ERA Environmental Services (1995) reported WAD cyanide concentrations in a pregnant liquor pond of 200 mg/L, and ~30 mg/L in a drainage channel. Johnson et al. (2002) investigated the drainage from a disused ore heap from the former Standard Hill gold mine, California, which had previously been subjected to cyanide solution application. Drainage effluent samples were collected from three locations including a drainage hole at the toe of the heap, an open receiving channel downstream of the heap, and from a holding pond into which the drainage discharged. The results are presented in Table 6.. Table 6.. Cyanide products (mg/L) in drainage from an inactive ore heap
The most abundant cyanide species in the drainage waters sampled by Johnson et al. (2002) were the strong metallocyanide complexes (96% or 4040 g/L), predominantly of iron and cobalt, with a smaller fraction (4% or 180-190 g/L) of WAD cyanide in the effluent. Lower (80% less) cyanide concentrations were detected in the pond surface water samples than sampled earlier in the drainage path, apparently due to the greater potential for losses from photolysis of metallocyanide complexes and HCN volatilisation or oxidation, changing pH conditions and biodegradation. Cyanate concentrations were marginally higher in the pond waters. WAD cyanide species contributed a smaller fraction (1%) to the total cyanide content of pond waters, with strong metallocomplexes predominating (Johnson et al., 2002). The open channel effluent displayed regular diurnal changes in cyanide speciation. In the channel effluent, WAD cyanide varied from 0.7 mg/L (day) to 0.2 mg/L (night), whereas Fe(CN)6x- (ferrocyanide + ferricyanide) showed the reverse effect (0.8 mg/L day and 1.9 mg/L night). However, there was also a net loss of free cyanide associated with this diurnal fluctuation, probably due to volatilisation. Iron and manganese concentrations in solution were higher at night, but silver, gold, copper and vanadium concentrations were higher during the day. Nitrite concentration was highest during the day and ammonia highest during the night. These diurnal changes were not the result of processes occurring in the heap, but were apparently due to photolysis of the strong metallocyanide complexes to WAD cyanide (mainly as free cyanide at measured rates in this instance of 0.09-9 mg CN/L/hour) when exposed to sunlight in the open channel and subsequent reactions of by-products. Staunton et al. (2003) indicated no reasonable estimates of relative cyanide volatilisation or migration in seepage from heap leach operations are available. 26.1.2Migration in seepage from heap leach operationsAs for TSFs, it is not possible to generalise regarding the potential for migration of cyanide and products from ore heaps in seepage to groundwater, except that it may potentially occur, particularly below unlined heaps, heaps with poorly designed and constructed liners or where liner integrity is violated. White and Markwiese (1994) reported a 375 m long plume of cyanide-contaminated groundwater underlying a disused heap leach residue pile in New Mexico, with groundwater total cyanide concentrations up to 0.7 mg/L. Staunton et al. (2003) indicated that the potential cyanide concentration in seepage is likely to be significantly lower than the concentration in solution added to the ore pile, and the presence of standard engineering controls (e.g. liner, under-drainage) would greatly reduce this potential. A review of current international practices by Thiel and Smith (2003) indicates that seepage from leach pad operations can be avoided by construction of a liner (usually geomembrane). Typically, Australian heap leach operations construct geomembrane liners on ore heap pads to avoid risk of groundwater contamination. Thiel and Smith (2003) identify geomembrane puncture due to rocks or geomembrane susceptibility to chemical corrosion as key concerns at heap leach operations; however, chemical corrosion is unlikely with cyanide solutions. A range of guidance and regulatory requirements are established to manage the risk of groundwater contamination at mine sites within a risk-based approach (refer Section 78.2.2). Directory: data -> assets -> word doc -> 0017 0017 -> National Industrial Chemicals Notification and Assessment Scheme word doc -> Priority Existing Chemical word doc -> Rail safety news issue 6 – October 2011 word doc -> Review of Multiple Chemical Sensitivity: Identifying 0017 -> Rail Safety News Edition 8, December 2012 0017 -> Real-Time License Plate Localisation on fpga 0017 -> Careers network bulletin issue 6 July 16 2012 0017 -> Wiser science: Finding References inspec for Physics and Engineering 0017 -> Part A: Depth Study: The globalising world Popular culture and public policy Download 2.29 Mb. Share with your friends:
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