Weak Acid Dissociable Research Articles

Fate and toxicity of ferric ferrocyanide with cyanogenic and non-cyanogenic plant species

Iron cyanide complexes are common contaminants at former manufactured gas plant sites. Although combined forms of cyanide have low toxicity, iron cyanide complexes can be decomposed to HCN through photolysis. This experiment was designed to evaluate the efficiency of phytoremediation and investigate potential toxic effect for iron cyanide complexes (i.e., ferric ferrocyanide) using the cyanogenic and non-cyanogenic plants in soil. Results showed that Sorghum (a cyanogenic plant) had the highest cyanide degradation in the root zone with the removal of approximately 32% while Switchgrass (a non-cyanogenic plant) and Flax (a cyanogenic plant) had 21% and 17% removal, respectively, with 4% dissipation in unvegetated soil. It was found that roots had higher cyanide concentrations than plant shoots. Water-soluble cyanide and the weak acid dissociable (WAD) fraction in the soil had increased at the end of the experiment, suggesting that plant may enhance the mobility and bioavailability of cyanide by root exudation. The risk associated with the leaching of cyanide compounds was assessed with the toxicity assay (Microtox). However, results indicated that the leachate from the pots was not toxic.

The effect of zinc cyanide on the flotation of gold from pyritic ore

In gold processing plants, the process water contains a considerable amount of weak acid dissociable (WAD) cyanide species. The cyanide-containing water recycled to flotation circuits has a deleterious effect on mineral flotation. Zinc cyanide can be a major constituent of the process water due to the cyanide leaching of zinc minerals and the zinc cementation applied to precipitate gold and silver. In the present study, the effect of zinc cyanide on the flotation of gold from a pyritic ore was evaluated and the competitive adsorption between zinc cyanide species and the collector, potassium amyl xanthate (PAX) or Aerophine promoter (3418A), on gold was studied by surface enhanced Raman spectroscopy (SERS). It was found that free cyanide did not depress the flotation of liberated gold, but zinc cyanide with a CN/Zn ratio of 2 did. The depression of gold flotation by zinc cyanide was due to the preferential adsorption of Zn(CN)2 on gold surface over either 3418A or amyl xanthate in the solution, rendering gold surface hydrophilic.

Gold dissolution and copper suppression during leaching of copper–gold gravity concentrates in caustic soda-low free cyanide solutions

In the cyanidation process, copper–gold deposits containing significant amounts of cyanide soluble copper can lead to high cyanide consumption with low gold extraction. The significant levels of weak acid dissociable (WAD) cyanide must then be destroyed prior to tailings disposal. This increases the cost to the gold mining companies to cover both the extra cyanide consumption in the leaching stage and the additional cost for cyanide recovery/destruction. Therefore, in this study, a selective cyanide leaching process of gold over copper from copper–gold concentrate (also see Oraby and Eksteen, 2014) containing 490g/t Au and 0.97% Cu present as metallic copper, oxides and sulfides has been investigated. To decrease the cyanide consumption, cyanide is added into the leach solutions to maintain a ratio of cyanide to total reactive copper (CN/Cu) below 2. At low CN/Cu ratio, increasing the pH of solution can lead to the precipitation of solubilized copper as CuO/Cu(OH)2, releasing cyanide ions for further both gold and copper dissolution. A comparison of leaching gold in cyanide–caustic, cyanide–ammonia and conventional cyanidation processes has been made. Higher gold extraction and lower copper concentration in the final leach was achieved in the cyanide–caustic system than in either the cyanide–ammonia or conventional cyanidation processes. The effects of the caustic soda (pH) concentration or cyanide concentration on gold extraction and copper suppression have also been studied. The results show that at high pH (>12), the gold dissolution rate increases significantly in solutions containing caustic soda and cyanide at zero, or very low free cyanide concentration.

Gold leaching in cyanide-starved copper solutions in the presence of glycine

In the cyanidation of copper–gold ores, the presence of copper minerals can lead to soluble gold losses, the production of weak acid dissociable (WAD) cyanide, as well as a number of operational challenges in CIP/CIL circuits with regard to competitive adsorption, and subsequent difficulties associated with elution, electrowinning and smelting. In addition, copper minerals are significant cyanide consumers, leading to higher cost in ore treatment. This paper presents a process to enhance the dissolution of gold using copper–cyanide solutions in the presence of glycine where the solution is cyanide starved. The effect of glycine addition on gold leaching kinetics in copper–cyanide solutions under different leaching conditions was studied. The results show that, in the presence of glycine, gold dissolution rate increases significantly in solutions containing copper–cyanide species at a very low, or zero, free cyanide concentration. In the presence and absence of glycine, gold dissolution rates in solutions containing 10mM Cu(CN)32− were 11.1μmol/m2·s and 0.65μmol/m2·s, respectively. It is shown that the average gold dissolution rate in a Cu–CN−-glycine system is about 6.5 times higher than the gold dissolution rate in the conventional cyanidation, the presence of cyanide being similar in each system. Kinetic and electrochemical studies were conducted to evaluate the effects of glycine concentration, pH, CN/Cu ratio, and initial copper concentrations on the dissolution of gold. It has also been shown that the gold dissolution rate increases by increasing the glycine concentration up to 1g/L and any further glycine addition has a negative effect on the dissolution of gold. Increasing leaching pH up to 12 enhances gold dissolution in the copper–cyanide–glycine solutions. However, increasing leaching pH to 13 has an adverse effect on the dissolution of gold.

A fundamental investigation of the Caro's acid cyanide destruction process

Cyanide destruction by Caro's acid is a process commonly employed by gold operations to meet regulatory requirements and/or the International Cyanide Management Code compliance for weak acid dissociable (WAD) cyanide concentrations in cyanidation tails or waste water from such operations. Typically small-scale bench tests to evaluate and assess the suitability of the Caro's acid destruction process are conducted before implementation. Additionally, the experimental procedure used during these preliminary investigations may also differ from that implemented at the full scale. This paper presents the results from a fundamental investigation of the Caro's acid process to establish a more detailed understanding of the impact that the Caro's acid source, alkali used and experimental procedure has on the destruction of cyanide. In particular, it was found that the control of pH was important to avoid additional (greater than stoichiometry) cyanide and copper loss (most likely as copper(I) cyanide precipitation) at low pH and maximise WAD cyanide destruction. Residual peroxide in the preparation of Caro's acid has the potential for subsequent cyanide destruction and could be exploited.

자외선 LED와 백금으로 박막된 TiO2 광촉매를 이용한 중금속과 결합한 시안화합물의 광촉매 산화

Cyanide can be leached out from the cyanidation method which has been used to extract high-purity gold and silver from ores, and it becomes a variety of cyanide complexes associated with heavy metals contained in ores. Such cyanide complexes are considered as persistent and non-degradable pollutants which cause adverse effects on humans and surrounding environments. Based on binding force between heavy metals and cyanide, cyanide complexes can be categorized weak acid dissociable (WAD) and strong acid dissociable (SAD). This study comparatively evaluated the performance of photo-catalytic process with regard to forms of cyanide complexes. In particular, both effects of UV LED wavelength and surface modification of photo-catalyst on the removal efficiency of cyanide complexes were investigated in detail. The results indicate that the performance of photo-catalytic oxidation is significantly affected by the form of cyanide complexes. In addition, the effect of UV LED wavelength on the removal efficiency was quite different between free cyanide and cyanide complexes associated with heavy metals. The results support that the surface modification of photo-catalyst, such as doping can improve overall performance of photo-catalytic oxidation of cyanide complexes.

High levels of activity of bats at gold mining water bodies: implications for compliance with the International Cyanide Management Code

Wildlife and livestock are known to visit and interact with tailings dam and other wastewater impoundments at gold mines. When cyanide concentrations within these water bodies exceed a critical toxicity threshold, significant cyanide-related mortality events can occur in wildlife. Highly mobile taxa such as birds are particularly susceptible to cyanide toxicosis. Nocturnally active bats have similar access to uncovered wastewater impoundments as birds; however, cyanide toxicosis risks to bats remain ambiguous. This study investigated activity of bats in the airspace above two water bodies at an Australian gold mine, to assess the extent to which bats use these water bodies and hence are at potential risk of exposure to cyanide. Bat activity was present on most nights sampled during the 16-month survey period, although it was highly variable across nights and months. Therefore, despite the artificial nature of wastewater impoundments at gold mines, these structures present attractive habitats to bats. As tailings slurry and supernatant pooling within the tailings dam were consistently well below the industry protective concentration limit of 50mg/L weak acid dissociable (WAD) cyanide, wastewater solutions stored within the tailings dam posed a minimal risk of cyanide toxicosis for wildlife, including bats. This study showed that passively recorded bat echolocation call data provides evidence of the presence and relative activity of bats above water bodies at mine sites. Furthermore, echolocation buzz calls recorded in the airspace directly above water provide indirect evidence of foraging and/or drinking. Both echolocation monitoring and systematic sampling of cyanide concentration in open wastewater impoundments can be incorporated into a gold mine risk-assessment model in order to evaluate the risk of bat exposure to cyanide. In relation to risk minimisation management practices, the most effective mechanism for preventing cyanide toxicosis to wildlife, including bats, is capping the concentration of cyanide in tailings discharged to open impoundments at 50mg/L WAD.

Ultratrace determination of total and available cyanides in industrial wastewaters through a rapid headspace-based sample preparation and gas chromatography with nitrogen phosphorous detection analysis

A new analytical method for the determination of both available (free and weak acid dissociable, WAD) and total cyanides in industrial wastewaters has been developed. It is based on the static headspace (HS) sampling procedure followed by a GC separation and the selective nitrogen-phosphorous detection (NPD), in which different thermal treatment allows the speciation of total and available cyanides. Detection limits (0.5μg/L), recovery (84.7–114.6% for free and 76.8–121.5% for total cyanides) and precision (5% at 5μg/L), evaluated on both real and synthetic samples, were fit-for-purpose for the legal requirement (5μg/L) enforced in the Venice lagoon, without significant interfering species. In addition, analytical results of the HS-GC-NPD method have been compared with those obtained using the 4500 CN and EN ISO 14403 official methods for the determination of total and free cyanides, respectively. The new method has been successfully applied for the determination of cyanide concentrations in main influent and final effluent to the Venice lagoon to verify the efficiency of the industrial wastewater treatment plant of Porto Marghera (Venice, Italy). The capability of the proposed method to detect the WAD cyanides has been tested by studying the acid dissociation of K2[Ni(CN)4]. An unexpected speciation picture was obtained for this complex, which suggests that the present definition and analytical strategy of this cyanide class should be reconsidered.

Online ligand exchange in the determination of weak acid dissociable cyanide by gas diffusion-flow injection analysis

A novel ligand exchange method for on-line gas-diffusion flow injection (GD-FI) determination of weak acid dissociable (WAD) cyanide has been successfully developed using a binary ligand combination of 0.10% thiourea and 0.10% pentaethylenehexamine. Complete WAD cyanide recovery with insignificant contribution from the dissociation of stable metal cyanide complexes (non-WAD complexes) has been achieved in the concentration range from 3μg L−1 to 10mgL−1 cyanide. The proposed GD-FI method offers considerable advantages over the existing standard methods for determining WAD cyanide, as it provides an analysis rate of 60 samples h−1 with a satisfactory reproducibility (average RSD=1.0% at n=3) and can be fully automated. A very good agreement between the proposed method and the standard distillation method has been observed when both methods have been applied to the determination of WAD cyanide in samples from the gold mining industry.

Cyanide detoxification of gold cyanidation tails and process streams

Cyanide detoxification by chemical means (sulphite, hydrogen peroxide or Caro’s acid) has typically been adopted by the gold industry to meet the International Cyanide Management Institute (ICMI) code and/or regulatory compliance for discharge into tailings storage facilities (TSFs), treatment of TSF return water or the discharge of excess water from the mining operation. This paper presents and discusses results from a number of different evaluations that have been conducted to reduce the cyanide concentration in cyanidation tails and process streams by chemical means. Preoxidation before cyanidation and the ore mineralogy were both found to potentially have a significant impact on reagent requirements in the treatment of cyanidation tails. The solution speciation provided important insights into the reagent consumption, which becomes very high in targeting residual weak acid dissociable (WAD) cyanide concentrations below 1 mg L−1. Preliminary evaluations of polishing and alternative processes for reducing the WAD cyanide showed potential to achieve these low concentrations and reduce the treatment costs.

Photo/photochemical oxidation of cyanide and metal–cyanide complexes: ultraviolet A versus ultraviolet C

Degradation of free cyanide (CN−), weak-acid dissociable (WAD) (, ) and strong-acid dissociable (SAD) cyanide complexes by photo and photochemical oxidation with ultraviolet (UV) light and H2O2 was investigated. The experiments were performed in batch reactors under ultraviolet A (UVA; 395 nm) and ultraviolet C (UVC; 254 nm) light; the degradation efficiency was followed in terms of free cyanide, complex and metal concentrations. UVC and UVA photo-oxidations were found to be equally effective in CN− and WAD degradation, while the degradation of the SAD complex was more difficult for both UV wavelengths, and UVC was more effective. The initial pH of the solution has influenced the degradation of all cyanide species and the optimum initial pH was evaluated as 10.5 for CN− and ; 12.0 for and 9.0 for degradation. Photochemical oxidation using H2O2 provided higher degradation at shorter durations with both UVA and UVC. Time-dependent variations in free cyanide and metal concentrations have indicated that metal–cyanide complexes are firstly degraded into metal and CN− ions, followed by oxidation of CN− ions, while metals in the system were partially removed as hydroxide precipitates. Therefore, depending upon the effluent requirements, the studied UV photo/photochemical oxidations were offered as either a pre-treatment method for the separation of metal and the cyanide, or as an oxidation technology to degrade especially WAD complexes and CN−. Estimated operational cost of photo-oxidation by UVC was 1.6–2.5-fold higher than UVA degradation, although degradation times were close. In the photochemical oxidation with H2O2, the operational costs of UVC and UVA degradation were closer, owing to peroxide costs, but UVC was still more expensive.

Factors influencing the risk of wildlife cyanide poisoning on a tailings storage facility in the Eastern Goldfields of Western Australia

Patterns of wildlife visitation and interaction with cyanide-bearing tailings slurry and solutions at the Fimiston tailings storage facility (TSF) have been reported in a previously published ecological study. The above-mentioned findings are extended in this paper by the examination of additional wildlife survey data, along with process water chemistry data collected during the same study period. Analysis of the combined results revealed that the primary wildlife protective mechanism in operation was effective management of tailings cyanide concentration. Nevertheless, tailings discharge concentration exceeded the industry standard wildlife protective limit of 50 mg/L weak acid dissociable (WAD) cyanide episodically during the study period. Wildlife that interacted with habitats close to the spigot outlet during brief periods of increased discharge concentration were likely to have been exposed to bioavailable cyanide at concentrations greater than the industry standard protective limit. However, no wildlife deaths were recorded. These results appear to support the hypothesis that hypersalinity of process solutions (unique to the Kalgoorlie district of Western Australia) and a lack of aquatic food resources represent secondary protective mechanisms that operated to prevent cyanide-related wildlife mortality during the project. The proposed protective mechanisms are discussed in the context of their potential application as proactive management procedures to minimise wildlife exposure to cyanide.

Birds associated with a tailings storage facility and surrounding areas from a South African gold mine

AbstractAs part of the ‘International Cyanide management code for the manufacture, transport, and use of cyanide in the production of gold’, companies voluntarily commit towards keeping the weak‐acid‐dissociable (WAD) cyanide concentrations in tailings below 50 mg l−1. Day and night‐time observations were made in winter at the tailings storage facility (TSF) of the Mponeng mine of AngloGold Ashanti, and extensive surveys of the surroundings were made for mortalities. The cyanide levels never exceeded 50 mg l−1 WAD. No dead birds or mammals were found. Twenty‐five bird species were recorded in and around the TSF and the associated return water dams (RWDs), with 20 occurring only in the surrounding grasslands or RWDs, and five (four wader species and a sand martin) on the TSF. The wading birds were most likely feeding on dead insects from the pond surface, as they adjusted their locations according to wind direction. Other birds actively avoided the TSF water, suggesting a locally learned behaviour. Whether this avoidance will also hold for migratory birds or natal dispersal of local birds in summer necessitates further investigation. A better understanding of the TSF–wildlife interactions would lead to improved measures to reduce environmental risks.

A new generation of cyanide ion-selective membranes for flow injection application: Part III. A simple approach to the determination of toxic metal–cyanide complexes without preliminary separation

A new flow injection approach to total weak acid-dissociable (WAD) metal–cyanide complexes is proposed, which eliminates the need of a separation step (such as gas diffusion or pervaporation) prior to the detection. The cornerstone of the new methodology is based on the highly selective flow-injection potentiometric detection (FIPD) system that makes use of thin-layer electroplated silver chalcogenide ion-selective membranes of non-trivial composition and surface morphology: Ag 2 + δ Se 1 − x Te x and Ag 2 + δ Se. An inherent feature of the FIP-detectors is their specific response to the sum of simple CN − + Zn(CN) 4 2− + Cd(CN) 4 2−. For total WAD cyanide determination, ligand exchange (LE) and a newly developed electrochemical pre-treatment procedure for release of the bound cyanide were used. The LE pre-treatment ensures complete recovery only when the sample does not contain Hg(CN) 4 2−. This limitation is overcome by implementing electrochemical pre-treatment which liberates completely the bound WAD cyanide through cathodic reduction of the complexed metal ions. A complete recovery of toxic WAD cyanide is achieved in the concentration range from 156 μg L −1 up to 13 mg L −1. A three-step protocol for individual and group WAD cyanide speciation is proposed for the first time. The speciation protocol comprises three successive measurements: (i) of non-treated, (ii) LE-exchange pre-treated; (iii) electrochemically pre-treated sample. In the presence of all WAD complexes this procedure provides complete recovery of the total bound cyanide along with its quantitative differentiation into the following groups: (1) Hg(CN) 4 2−; (2) CN − + Cd(CN) 4 2− + Zn(CN) 4 2−; (3) Cu(CN) 4 3− + Ni(CN) 4 2− + Ag(CN) 2 −. The presence of a 100-fold excess in total of the following ions: CO 3 2−, SCN −, NH 4 +, SO 4 2− and Cl − does not interferes. Thus the proposed approach offers a step ahead to meeting the ever increasing demand for cyanide-species-specific methods. The equipment simplicity makes the procedure a good candidate for implementing in portable devices for in-field cyanide monitoring.

Cyanide recovery by tailings washing and pond stripping

Modern gold producers are experiencing the operational dichotomy of minimizing wildlife exposure to toxic levels of cyanide in tailings and process waters, while decreasing operating costs for exploitation of lower-grade ores. The widely accepted tailings weak-acid dissociable (WAD) cyanide level of <50 mg/L is typically achieved by oxidation of cyanide using reagents such as SO 2/air, H 2SO 5 or H 2O 2. Consumption rates and costs of these processes can be substantial; moreover, destruction of the reagent that is often the single largest operating cost centre for the process plant represents an additional economic downside. Technical and economic aspects of an alternative process for achieving these eco-efficiency objectives with minimal additional hazardous chemicals are considered and compared against other process alternatives for a selected scenario. The WPS (washing – pond stripping) process comprises tailings slurry washing using counter-current high-rate thickeners and pond or tank stripping of cyanide from the thickener overflow. Water balance issues are avoided by use of cyanide-stripped water along with any make-up water requirements in the washing stage. The cyanide-rich caustic scrub solution is recycled to the leach. Metals and other species that may build up in plant process waters even in the absence of tailings thickening can be treated by a variety of conventional measures. The WPS process is based on proven unit operations and hence utilizes existing plant infrastructure and other resources wherever possible. Single tailings thickeners are used in some gold plants for the recycle of water, along with a portion of the cyanide and soluble gold losses; this equipment can be better utilized by washing with recycle water to raise recoveries to high levels. Moreover, removal of cyanide from the overflow water improves management of the water balance and reduces cyanide consumption in the mill. The existing process water pond or tank is augmented with a covering commonly used in the water treatment industry for biogas production and odour removal. This low-pressure system provides a long retention time for the HCN stripping process, and the positive effect of pond solar heating on the Henry’s Law constant can also improve the thermodynamic driving force for stripping. The covered pond eliminates wildlife deaths in this area of the site; by also venting the carbon-in-leach (CIL) tanks in-circuit cyanide losses are further minimized, resulting in lower lime consumption by operating at lower pH conditions. Scrubbing systems are increasingly being designed in to new and existing gold circuits for the removal of cyanide, ammonia, volatile organic compounds (VOCs) and mercury from carbon regeneration kiln and gold room off-gas streams. Moreover, the occupational health and safety issues regarding elevated HCN levels above leach tanks employing hypersaline waters may require venting and scrubbing systems to be introduced. The additional soluble gold that is recovered in the WPS process can be a significant economic driver along with the advantages of cyanide recovery over destruction. The environmental risk of ongoing cyanide tailings management is minimized, while providing economic benefits from the recovery of cyanide, water and gold.

Cyanide Speciation at Four Gold Leach Operations Undergoing Remediation

Analyses have been made of 81 effluents from four gold leach operations in various stages of remediation to identify the most -persistent cyanide species. Total cyanide and weak acid-dissociable (WAD) cyanide were measured using improved methods, and metals known to form stable cyanocomplexes were also measured. Typically, total cyanide greatly exceeded WAD indicating that cyanide was predominantly in strong cyanometallic complexes. Iron was generally too low to accommodate the strongly complexed cyanide as Fe(CN)6s3- or Fe(CN)6(4-), but cobalt was abundant enough to implicate Co(CN)6(3-) or its dissociation products (Co(CN)(6-x)(H2O)x((3-x)-)). Supporting evidenceforcobalt-cyanide complexationwas found in tight correlations between cobalt and cyanide in some sample suites. Also, abundant free cyanide was produced upon UV illumination. Iron and cobalt cyanocomplexes both photodissociate; however, the iron concentration was insufficient to have carried the liberated cyanide, while the cobalt concentration was sufficient. Cobalt cyanocomplexes have not previously been recognized in cyanidation wastes. Their identification atfour separate operations, which had treated ores that were not especially rich in cobalt, suggests that cobalt complexation may be a common source of cyanide persistence. There is a need for more information on the importance and behavior of cobalt cyanocomplexes in ore-processing wastes at gold mines.

Ligand displacement, headspace single-drop microextraction, and capillary electrophoresis for the determination of weak acid dissociable cyanide

A new method involving ligand displacement, headspace single-drop microextraction (SDME) with in-drop derivatization and capillary electrophoresis (CE) was developed for the determination of weak acid dissociable (WAD) cyanide. WAD metal–cyanide complexes (Ag(CN) 2 −, Cd(CN) 4 2−, Cu(CN) 3 2−, Hg(CN) 2, Hg(CN) 4 2−, Ni(CN) 4 2− and Zn(CN) 4 2−) are decomposed with ligand-displacing reagent and the released hydrogen cyanide is extracted from neutral solution (pH 6.5) with an aqueous microdrop (5 μl) containing Ni(II)–NH 3 as derivatization agent. The hydrogen cyanide extracted reacts with Ni 2+ to form a stable and highly UV absorbing tetracyanonickelate anion which is then determined by CE. Among the three different ligand-displacing reagents (i.e., ethylenediamine, dithizone and polyethileneimine) studied none of the reagents used alone releases cyanide completely from all WAD cyanide complexes. Complete recoveries were obtained by the extraction of WAD cyanide from 0.4 mol l −1 ethylenediamine chloride buffer (pH 6.5) containing 0.001% (wt) dithizone. Proposed system was applied to determine WAD cyanide in industrial wastewater and river waters samples with spiked recoveries in the range of 95.8–104.7%.

Refinement of Weak Acid Dissociable (WAD) Method for Measuring Weak Metal Cyanide Complexes in Aqueous Samples

The primary objective of this study was to correct deficiencies that had been identified in the current weakacid dissociable (WAD) cyanide analytical method. These deficiencies became apparent when the method was applied to groundwater and surface water samples dominated by iron–cyanide complexes (>50% of the total cyanide content); the analytical method, as is, is fine for determining WAD cyanide in any aqueous sample where weak metal cyanide complexes and free cyanide ions dominate the cyanide speciation (i.e., iron–cyanide complexes are <50% of the total cyanide content). It was also determined that the means required to correct these deficiencies depended upon which iron–cyanide complex was present (i.e., ferricyanide/ferrocyanide or an iron–pentacyano methylamino species, [Fe(CN)5NHCH3]4−). For the samples dominated by the ferri- and ferrocyanide complexes, addition of the prescribed amount of zinc acetate under mildly acidic conditions was sufficient to entirely remove these complexes from solution via formation of an iron cyanide precipitate. However, the high-temperature distillation step of the analytical method was found to destabilize the iron–cyanide precipitate thus formed, resulting in the recovery of some of this strong acid dissociable cyanide complex as WAD cyanide. To rectify the method deficiency, an intermediate filtration step using a 0.45-micron filter was introduced to remove the precipitate before distillation is performed. A slightly different modified method was required when the iron–cyanide complexes were dominated by the iron–pentacyano methylamino complex, that is, [Fe(CN)5NHCH3]4−. In this instance, it was necessary to add three times the prescribed amount of zinc acetate to remove the complex from the solution. Also, the precipitate was comprised of smaller size particles, and hence, a smaller filter size (0.2 micron) was required to remove the precipitate before distillation. These modifications to the current WAD method provided a more accurate determination of the WAD cyanide content of the sample that is dominated by iron–cyanide complexes, as verified by the EPA-approved available cyanide method OIA-1677. However, for samples dominated by weak metal–cyanide complexes, the current WAD cyanide analytical method is deemed appropriate.

Destruction of cyanide by hydrogen peroxide in tailings slurries from low bearing sulphidic gold ores

The main objective of this work was to determine the effectiveness and kinetics of hydrogen peroxide in destroying cyanide in the tailings slurry from a gold mine with low sulphide and heavy metal content. The impacts of catalyst (Cu) and hydrogen peroxide concentrations, temperature and pH on the extent and rate of weak acid dissociable (WAD) cyanide destruction were investigated. Experiments were conducted using the variable-dose completely mixed batch reactor bottle-point method. Both the rate and extent of CN WAD destruction generally increased with increasing peroxide doses for either absence or presence of Cu catalyst. Catalyst addition was very effective in terms of not only enhancing the cyanide destruction rate but also significantly reducing the required peroxide dosages to achieve CN WAD concentrations of about 1 mg/l, independent of the temperatures tested (10, 20 and 30 °C). The initial cyanide destruction rates increased between 1.2 and 3 folds with the addition of 30 mg/l of Cu. Kinetic experiments showed that in most cases little CN WAD destruction occurred after a reaction time of 2–4 h. The impact of slurry pH on cyanide destruction varied depending upon the dosages of Cu catalyst. Relatively lower peroxide dose/CN WAD ratios required to achieve less than 1 mg/l of CN WAD may be due to lower heavy metals and sulphide content of the ore, resulting in lower peroxide requirement for metal bound cyanides. During cyanide destruction, nitrate was initially formed as a by-product and then possibly converted to other some volatile nitrogen-containing species, as supported by the mass balance calculations.

Evaluation and testing of analytical methods for cyanide species in municipal and industrial contaminated waters.

Total cyanide analysis by distillation is used most commonly to assess cyanide content of water samples. This manual method is robust but slow and provides no information about cyanide speciation, a significant limitation in that cyanide species have substantially different toxicity characteristics. Seven alternative methods for the analysis of cyanide species or groups of species were evaluated in reagent water and five different contaminated water matrices, including five species-specific methods--weak acid dissociable (WAD) cyanide, free cyanide by microdiffusion, available cyanide, automated WAD cyanide by thin film distillation, metal cyanides by ion chromatography--and two automated techniques for total cyanide--total cyanide bythin film distillation and total cyanide by low-power UV digestion. The species-specific cyanide analytical techniques achieved low, ppb-level detection limits and exhibited satisfactory accuracy and precision for most contaminated waters. Analysis of low concentrations of cyanide species in raw wastewater was problematical for the available cyanide and ion chromatography methods, which experienced significant interference problems and/or low recoveries. There was recovery of significant diffusible cyanide in microdiffusion tests with nickel-cyanide-spiked samples, reflecting dissociation of this weak metal-cyanide complex during the test and demonstrating that the test can recover some fraction of WAD cyanide in addition to free cyanide. The automated total cyanide methods, which involve UV digestion, achieved low detection limits for most waters but exhibited low recoveries for some waters.