Gold Mill Effluent Research Articles

Pilot test operation of biological nitrogen removal of gold mill effluents

Effluents from gold mills are a potential environmental hazard, unless toxic substances such as nitrogen compounds (sodium cyanide, thiocyanate and ammonia) and heavy metals are removed. In northern Sweden, there are gold mines that are either already in operation, or to be opened, among them is the proposed Lappland Goldminers AB gold mill facility. The environmental permit for this new activity requires stringent removal levels of nitrogen. It was decided to operate a pilot plant to study the biological nitrogen removal performance. It has been based on a continuous activated sludge plant suited for biological nutrient removal. The plant has been operated at a neighbouring gold mill plant with an identical gold extraction process. The pilot study focused primarily on nitrification; however, also the denitrification performance and cyanide removal were monitored. The test results have been encouraging, showing up to a 100% nitrification capacity. The specific nitrification rate has been in the range (0.5) of 1.1–2.6g Nox/kg VSS/h, at temperatures ranging from 7.6 to 20°C. Different unintended process disturbances have been allowed for conclusions on the process stability and recovery potential. Low levels of total cyanide were recorded in the inlet, but virtually no traces were found in the treated water.

Use of quaternary ammonium salts to remove copper–cyanide complexes by solvent extraction

Cyanidation is one of the most common methods for the extraction of precious metals. In this process, effluents frequently contain relatively high concentrations of copper, which may react with cyanide to form cuprocyanide complexes adversely affecting the process. In this preliminary work, the use of solvent extraction to remove the copper–cyanide species from a synthetic solution similar to that of gold mill effluents was studied in order to permit the recycling of the solution into the process. For the extraction of these anions, the quaternary ammonium salts Quartamin TPR, Adogen 464 and Aliquat 336 were studied as extractants. The experimental results showed that for a synthetic solution of 710 mg/L copper and 1100 mg/L cyanide, it is possible to obtain a copper extraction of 99% when using 0.033 mol/L of the extractant Adogen 464 (organic/aqueous volume ratio (O/A) = 1) in the range of pH of 9–11. Up to 99% of the copper can be stripped from the organic solution after three contact times (5 min each) with 50 mL of sodium hydroxide 0.5 M (O/A = 1).

Biodegradation of cyanide containing effluents by Scenedesmus obliquus

Biological degradation of cyanide has been shown a viable and robust process for degrading cyanide in mining process wastewaters. Several algal cultures can effectively degrade cyanide as carbon and/or nitrogen source for their growth. In this study, cyanide effluent degradation by Scenedesmus obliquus was examined. Gold mill effluents containing WAD cyanide concentration of 77.9 mg/L was fed to batch unit to examine the ability of S. obliquus for degrading cyanide. Cyanide was reduced down to 6 mg/L in 77 h. Microbial growth and metal uptake of Zn, Fe and Cu was examined during cyanide degradation. The cells well adapted to high pH and the effluent contained cyanide and the metals. It is important that Zn level reduced down 50%, of the starting concentration. pH was kept at 10.3 to prevent loss of cyanide as HCN, due its volatile nature. The bio treatment process was considered to be successful in degrading cyanide in the mine process water.

Ferrate(VI) and ferrate(V) oxidation of cyanide, thiocyanate, and copper(I) cyanide

Cyanide (CN −), thiocyanate (SCN −), and copper(I) cyanide (Cu(CN) 4 3−) are common constituents in the wastes of many industrial processes such as metal finishing and gold mining, and their treatment is required before the safe discharge of effluent. The oxidation of CN −, SCN −, and Cu(CN) 4 3− by ferrate(VI) (Fe VIO 4 2−; Fe(VI)) and ferrate(V) (Fe VO 4 3−; Fe(V)) has been studied using stopped-flow and premix pulse radiolysis techniques. The rate laws for the oxidation of cyanides were found to be first-order with respect to each reactant. The second-order rate constants decreased with increasing pH because the deprotonated species, FeO 4 2−, is less reactive than the protonated Fe(VI) species, HFeO 4 −. Cyanides react 10 3–10 5 times faster with Fe(V) than with Fe(VI). The Fe(V) reaction with CN − proceeds by sequential one-electron reductions from Fe(V) to Fe(IV) to Fe(III). However, a two-electron transfer process from Fe(V) to Fe(III) occurs in the reaction of Fe(V) with SCN − and Cu(CN) 4 3−. The toxic CN − species of cyanide wastes is converted into relatively non-toxic cyanate (NCO −). Results indicate that Fe(VI) is highly efficient in removing cyanides from electroplating rinse water and gold mill effluent.

Iron(VI) and Iron(V) Oxidation of Copper(I) Cyanide

Copper(Il) cyanide (Cu(CN)4(3-)) in the gold mine industry presentsthe biggest concern in cyanide management because it is much more stable than free cyanide. Cu(CN)4(3-) is highlytoxic to aquatic life; therefore, environmentally friendly techniques are required for the removal of Cu(CN)4(3-) from gold mine effluent. The oxidation of Cu(CN)4(3-) by iron-(VI) (FeVIO4(2-), Fe(VI)) and iron(V) (FeVO4(3-), Fe(V)) was studied using stopped-flow and premix pulse radiolysis techniques. The stoichiometry with Fe(VI) was determined to be 5HFeO(4-) + Cu(CN)4(3-) + 8H2O - > 5Fe(OH)3 + Cu2+ + 4CNO- +3/202 + 6OH-. The rate law for the oxidation of Cu(CN)4(3-) by Fe(VI) was found to be first-order with each reactant. The rates decreased with increasing pH and were mostly related to a decrease in concentration of reactive protonated Fe(VI) species, HFeO4-. A mechanism is proposed that agrees with the observed reaction stoichiometry and rate law. The rate constant for the oxidation of Cu(CN)4(3-) by Fe(V) was determined at pH 12.0 as 1.35 +/- 0.02 x 10(7) M(-1) s(-1), which is approximately 3 orders of magnitude larger than Fe(VI). Results indicate that Fe(VI) is highly efficient for removal of cyanides in gold mill effluent.

Destruction of cyanide in gold mill effluents: biological versus chemical treatments

In gold mining, cyanide has been the preferred lixiviant worldwide since 1887. Although cyanide can be destroyed and recovered by several processes, it is still widely discussed and examined due to its potential toxicity and environmental impact. Biological treatment of cyanide is a well-established process and has been commercially used at gold mining operations in North America. Biological treatment processes facilitate growth of microorganisms that are essential for the treatment. The present review describes the advances in the use of biological treatment for the destruction of cyanide in gold mill effluents.

The role of calcium and sodium in toxicity of an effluent to mysid shrimp (Mysidopsis bahia)

AbstractThe source of acute toxicity of an aged gold mill effluent to juvenile mysid shrimp (Mysidopsis bahia [Americamysis bahia]) was identified. Effluent osmolality was equivalent to that of 12 ppt seawater. At five effluent concentrations ranging from 4 to 100% (v/v), using 12 ppt seawater for dilution, the onset of responses was most rapid at 37% effluent. Simulated effluent was created by adding Na+, Ca2+, Mg2+, K+, Cl−, and SO42− to distilled water at concentrations equal to those measured in the effluent. The unusual finding of a more rapid onset of responses at 37% than at 100% effluent was duplicated with simulated effluent, demonstrating that responses of M. bahia to effluent were attributable to one or more of the six ions that were included in simulated effluent. Proportionally, excess Ca2+ and Na+ deficiency were among the greatest differences in the ionic composition of effluent relative to seawater. Manipulation of Ca2+ and Na+ concentrations in effluent and in simulated effluent, along with the results of the previous experiments, demonstrated that excess Ca2+ was the sole source of effluent toxicity and that Na+ deficiency, relative to the proportion in seawater, reduced Ca2+ toxicity.

Relative sensitivity of marine species to an effluent with elevated total dissolved solids

AbstractThe acute toxicity of aged gold mill effluent was compared for two reference species (juvenile mysid shrimp [Mysidopsis bahia (Americamysis bahia)] and larval sheepshead minnows [Cyprinodon variegatus]) and larvae of three species that are indigenous to the southern coast of Alaska (red king crab [Paralithodes camtschaticus], northern shrimp [Pandalus borealis], and Pacific herring [Clupea harengus pallasi]). Effluent osmolality was equivalent to that of 12 ppt seawater. Concentration–response relationships for the effluent, adjusted to equal the osmolality and pH of 31 ppt seawater, were determined after 24 h of exposure for immobility, paralysis, and death. The magnitude of differences and rank of sensitivity of the indigenous species depended on the response that was used for the comparison. For all responses, M. bahia and C. variegatus were the most and least sensitive species, respectively.

Recent Applications of Peroxygen Reagents in Hydrometallurgy

The paper reviews development made over the past decade in the application of peroxygnn reagents in hydrometallurgical processes. The reagents are hydrogen peroxide, H202, Caro’s acid, H2MS05, and calcium peroxide, CaO2. Their chemical properties and methods for on-site storage or production are outlined. Most progress has been made in the gold extraction industry, where hydrogen peroxide and calcium peroxide have been used to enhance oxygen availability in the cyanide leaching of gold ores, in both agitated and heap leach systems. Properly controlled, the use of these reagents can lead to increased rates of gold extraction and savings in cyanide consumption. Hydrogen peroxide is also widely applied for the oxidative destruction of cyanide in gold mill effluent solutions and tailings ponds. Caro’s acid can also be used for this purpose, with advantage in certain situations, which arc discussed. Caro’s acid has found industrial application in the leaching of uranium ores in Australia at the Nabarlek mine a...

A thermoanalytical study of the decomposition of a cyanide precipitate in inert and reducing atmospheres

The thermal decomposition of a complex cyanide precipitate from a gold mill effluent treatment process [CANMET acidification-volatilization-reneutralization (AVR) Technology] was studied using thermogravimetry (TG)-Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The precipitate contained KZn cyanoferrates and copper thiocyanate. The TG/FTIR experiments were conducted in He and H 2-He atmospheres. The decomposition was found to occur in three stages. In the first stage, the dehydration of the cyanoferrates and the decomposition of the thiocyanate occurred. In the second stage, the decomposition proceeded with the formation of metallic copper and metal sulphides and cyanides. In the third stage the product contained metallic copper, iron and iron carbide. Iron sulphide was also detected when the decomposition occurred in He alone.

Use of Low Pressure Mercury Lamps for the Photodecomposition of Iron Cyanide in Gold Mill Effluents

Abstract The use of ultraviolet light (UV) from low pressure mercury lamps for destroying iron cyanide in synthetic and actual gold mill effluents was evaluated in this study. For the light intensities used in this study, UV irradiation was not able to efficiently destroy cyanide. However, it converted iron cyanide to a weak acid dissociable form which was destroyed by chlorine. Data from several bench-scale tests and one pilot scale test were used to estimate quantum efficiencies (moles iron cyanide destroyed/einstein). These efficiencies ranged from 0.2% to 1%; approximately 30% to 90% lower than those reported in the literature for potassium ferricyanide. The data collected during the study demonstrated the technical feasibility of using UV in conjunction with chlorination for destroying iron cyanide in gold mill effluents. However, low pressure mercury lamps do not appear to be a practical UV source for this purpose. Irradiation with high intensity lamps may be more practical and is recommended for experimental evaluation.

Removal of Iron Cyanide from Gold Mill Effluents by Ion Exchange

Using a strongly basic anion exchanger (Rohm & Haas Amberlite IRA-958) laboratory ion exchange tests were conducted on the removal of iron cyanide from synthetic and actual gold mill effluents. Variables examined included the effects of pH, concentration and hydraulic loading of the feed on the resin exchange capacity and resin column utilization. The results showed ion exchange to be an effective method of removing iron cyanide to concentrations of less than 3 mg/L. Tests with synthetic gold mill effluents gave resin exchange capacities ranging from 13 to 34 mg CNFe/mL resin depending on the concentration of iron, copper and zinc cyanides in these solutions. When raw barren bleed was tested, an exchange capacity for the ferro/ferricyanides was determined to be ~6 to 8 mg CNFe/mL resin. For chlorinated barren bleed, a resin exchange capacity of 14 mg CNFe/mL resin was observed. A capacity of 19 mg CNFe/mL resin was achieved for tailings pond decant. Regeneration with 15% NaCl provided mixed results with recoveries ranging from 40 to 100% of the iron cyanide exchanged by the resin. Upon multiple exchange-regeneration cycles, the resin lost 25% of the exchange capacity for total cyanide during the first cycle. Subsequently, the loss in exchange capacity was approximately 1% per cycle. Pilot scale tests were considered essential prior to full scale system design.