Removal of cyanide in aqueous solution by oxidation with hydrogen peroxide in presence of activated carbon prepared from olive stones

This work is dedicated to the removal of the very toxic free cyanide from aqueous solution by oxidation with hydrogen peroxide H₂O₂ in the presence of activated carbon prepared from olive stones. Effects of the initial molar ratio [H₂O₂]₀/[CN^?]₀, the initial cyanide concentration, the activated carbon concentration and the temperature on cyanide removal have been examined. The removal of free cyanide in absence of activated carbon showed very slow kinetics. The presence of activated carbon has increased the reaction rate showing thus a catalytic activity. The kinetics of cyanide removal has been found to be of pseudo-first-order with respect to cyanide and the rate constants have been determined for different values of the aforementioned parameters. The apparent activation energy has been determined from tests carried out at three different temperatures. It was found equal to 46.2 kJ/mol in the presence of activated carbon, which is about half of the 82.7 kJ/mol found for the oxidation in absence of the activated carbon.This process can be interesting for the cyanide removal from processed solutions because it does not use soluble metal catalyst and it consumes only hydrogen peroxide as chemical product.     

Oxidation of cyanide in effluents by Caro’s Acid

Caro’s Acid (peroxymonosulphuric acid: H₂SO₅) is a powerful liquid oxidant made from hydrogen peroxide that has been adopted for the detoxification of effluents containing cyanides in gold extraction plants in recent years.The present work reports the findings of a study on the kinetics of aqueous cyanide oxidation with Caro’s Acid. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology employed involved a sequence of two 2³ factorial designs using three factors: initial [CN⁻]: 100–400 mg/L; H₂SO₅:CN⁻ molar ratio: 1–1.5–3–4.5; pH: 9–11; each one conducted at one level of Caro’s Acid strength which is obtained with the H₂SO₄:H₂O₂ molar ratio used in Caro’s Acid preparation of 3:1 and 1:1. The objective was the evaluation of the effect of those factors on the reaction kinetics at room temperature. Statistical analysis showed that the three investigated variables were found to be significant, with the variables which affected the most being the initial [CN⁻] and the H₂SO₅:CN⁻ molar ratio. The highest reaction rates were obtained for the following conditions: H₂SO₅:CN⁻ molar ratio = 4.5:1; pH = 9; and Caro’s Acid strength produced from the mixture of 3 mol of H₂SO₄ with 1 mol of H₂O₂. These conditions led to a reduction of [CN⁻] from an initial value of 400 mg/L to [CN⁻] = 1.0 mg/L after 10 min of batch reaction time at room temperature. An empirical kinetic model incorporating the weight of the contributions and the interrelation of the relevant process variables has been derived as: −d[CN⁻]/dt = k [CN⁻]^1.8 [H₂SO₅]^1.1 [H⁺]^0.06, with k = 3.8 (±2.7) × 10⁻⁶ L/mg min, at 25 °C.     

Ultrasonic elution of gold from activated carbon

The feasibility of using ultrasound to enhance the elution of gold from activated carbon was investigated. A laboratory-scale elution column immersed in an ultrasonic bath was used to investigate three eluant systems. The kinetic activity of the activated carbon was improved in the elution with sonication. Ultrasonic radiation improved the recoveries and kinetics of elution considerably. This could possibly be attributed to an increase in diffusive transport caused by acoustic vortex microstreaming and microjets, as well as the prevention of activated carbon fouling which can lead to the blockage of active sites. The rate of elution was significantly increased by the addition of ethanol to the aqueous eluant, especially in the presence of ultrasound. It could be attributed to the increased activity of smaller ions in preference to gold cyanide in the presence of ethanol. Ethanol, by capturing the primary radicals (H^* and OH^*), generated secondary radicals (C₂H₄OH*) which could be responsible for the increased efficiency. The introduction of ultrasound into the conventional elution systems made it possible to elute gold cyanide from activated carbon under less severe conditions.     

The use of ion exchange resins for the treatment of cyanidation tailings part 1––process development of selective base metal elution

This work investigates the use of oxidative acid eluents for the elution of base metals from strong base ion exchange resins. Eluents composed of a mixture of H₂O₂ and H₂SO₄ were tested for eluting base metals from resins loaded with mixtures of base and precious metal cyanides. This process removed 100% of Cu and Zn loaded on the resin, without affecting the precious metal loading. It was found that copper could be removed separately from the other base metals. The elution technique was not effective for removing iron from the resin. Cyanide associated with base metals was recovered as NaCN. Some oxidation of cyanide was noticed, subject to the elution conditions.This oxidative acid elution process could be used in commercial operations for the selective elution of base metals from a strong base ion exchange resin bed operating in alternative adsorption/base metal elution cycles. Thus, virtually all metal cyanide species could be recovered from cyanide leached solutions or slurries to give relatively clean tailings without compromising precious metal recovery efficiency. The process also caters for cyanide recovery and recycling.     

Extraction of hydrogen cyanide from waste solutions of cyaniding circuit for sulfide flotation concentrates

The process for extraction of hydrogen cyanide to decontaminate solutions produced at cyaniding of sulfide flotation concentrates is developed. The centrifugal-bubbling apparatus is employed as a reactor. The regularities of HCN formation in an acid medium are established in investigation into kinetics of SCN⁻ thiocyanate oxidation by hydrogen peroxide H₂O₂ in presence of Fe²⁺, Fe³⁺ and pH ≤ 3.5. In the process proposed the evolved HCN is adsorbed by NaOH solution and returned to the circuit of leaching of gold and silver as NaCN, and the waste cyaniding solution is discharged into a waste dump, where it is mixed with industrial water to be utilized to transport flotation tailings. __________ Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No. 1, pp. 98–105, January–February, 2009.     

Formation of hydrogen peroxide by pyrite and its influence on flotation

Formation of hydrogen peroxide (H₂O₂), an oxidising agent stronger than oxygen, by pyrite (FeS₂), the most abundant metal sulphide on Earth, during grinding was investigated. It was found that pyrite generated H₂O₂ in pulp liquid during wet grinding and also the solids when placed in water immediately after dry grinding. Type of grinding medium on formation of hydrogen peroxide revealed that the mild steel produced more H₂O₂ than stainless steel grinding medium, where Fe²⁺ and/or Fe³⁺ ions played a key role in producing higher amounts of H₂O₂. The effect of grinding atmosphere of air and N₂ gas showed that nitrogen environment free from oxygen generated more H₂O₂ than air atmosphere suggesting that the oxygen in hydrogen peroxide is derived from water molecules. In addition, the solids after dry grinding producing more H₂O₂ than wet grinding indicate the role of pyrite surface or its catalytic activity in producing H₂O₂ from water. This study highlights the necessity of relooking into the electrochemical and/or galvanic interaction mechanisms between the grinding medium and pyrite in terms of its flotation behaviour.     

An amperometric method for measuring cyanide in CIP/CIL circuits

A silver rotating disc electrode has been used to measure the ionic cyanide (CN⁻_(aq)) concentration in gold leaching circuits amperometrically. When the electrode is held at 0.1 V vs SHE the current flow is proportional to the cyanide concentration. In laboratory solutions the correlation coefficient ranged from 0.9999 at 30 °C to 0.9995 at 50 °C. The method is free from interference from the anions expected at gold plants, except sulphide, which, if present, can be removed by precipitation with lead(II).Cyanide bound up in stable metal complexes decreases the available CN⁻, and is not measured by the method. Other than by complexation, the metals ions studied did not interfere with the method. CN⁻_(aq) converted to HCN_(aq) at low pHs is also not measured except on addition of a pH buffer. The results are consistent with those from a silver nitrate titration, that also does not measure HCN_(aq) or strongly complexed cyanide.Laboratory and field trials have shown that the method will work in leaching slurries without filtration, but regular electrode cleaning is required to ensure accurate results.     

Degradation of thiocyanate in aqueous solution by persulfate activated ferric ion

Thiocyanate formation from cyanidation of gold bearing ores is becoming a more common problem during gold processing. In this work, the application of an advanced oxidation process based on the use of persulfate (S₂O₈²⁻) as an environmentally friendly oxidant in the presence of ferric ion for destruction of a persistent and non-volatile inorganic contaminant, such as thiocyanate, in aqueous solutions is reported for the first time. The influence of various reaction parameters like ferric ion and persulfate dosage, initial thiocyanate concentration and the influence of radical scavenger are examined. An accelerated reaction using S₂O₈²⁻ to destroy thiocyanate can be achieved via chemical activation with Fe³⁺ to generate highly reactive sulfate anion-radicals (SO₄⁻). The results showed that degradation efficiency was negligible when persulfate was used alone, ferric ions significantly improved the degradation efficiency of thiocyanate at ambient temperature. Under the optimum molar ratios ([S₂O₈²⁻]:[SCN⁻] = 5:1 and [S₂O₈²⁻]:[Fe³⁺] = 1:0.2), 99% of thiocyanate present in aqueous solution at the initial concentration range of 1.72–17.2 mM was degraded within 60 min of reaction time. To evaluate the contribution of reactive free radicals generated through Fe(III)-mediated activation of persulfate to thiocyanate degradation, quenching experiments using methanol as the radical quenching agent were carried out. The obvious decrease in thiocyanate oxidation efficiency in the presence of methanol confirmed that the radical-based pathway was the dominant mechanism in Fe³⁺/S₂O₈²⁻ system. The degradation of thiocyanate was accompanied by the formation of cyanide as the main final product of the reaction. Thus the catalytic oxidation of thiocyanate makes it possible to return NaCN into the production process for leaching of precious metals. The work presents an efficient and environmentally acceptable wastewater treatment process applicable in mining facilities utilizing cyanidation of sulfide ores and/or concentrates.     

Management of thiosalts in mill effluents by chemical oxidation or buffering in the lime neutralization process

Laboratory studies were conducted to investigate the removal or management of thiosalts within the lime-neutralization process, to prevent or minimize the adverse effects of thiosalts that cause delayed acidity to downstream environment. The oxidizing reagent hydrogen peroxide (H₂O₂) and the pH stabilizing (buffering) reagents carbon dioxide (CO₂), sodium bicarbonate (NaHCO₃) and sodium carbonate (Na₂CO₃) were examined for removal and management of thiosalts, respectively. Chemical oxygen demand (COD) was determined to be a proxy for thiosalts and was employed for their rapid assessment. The Target Level of thiosalts harmless to aquatic life was found to be 30 mg/L or less. The optimized lime-neutralization process required a pH level of 9.5–10 and aeration. Over-liming to pH levels >11 did not provide excess alkalinity, hardness, or a decrease in thiosalt levels.Addition of H₂O₂ to either the acid or lime-neutralized water at a molar H₂O₂:S₂O₃ ratio of 1–1.5 removed thiosalts to safe levels. About 10–15 min. at room temperature was ample time low temperatures slowed down the process but the dosages were not affected. Removal of thiosalts from 170 to 30 mg/L caused a decrease in pH from 9.6 to 6.5. Among the buffering reagents studied, both NaHCO₃ and Na₂CO₃ provided adequate buffering and a stable pH of 7 to the lime-neutralized water; whereas CO₂ resulted in poor buffering and an unstable pH that remained below 6. In cold temperatures, NaHCO₃ and Na₂CO₃ also outperformed CO₂ with higher alkalinity and hardness. Na₂CO₃ addition to lime neutralized water at pH 9.5 was found to be the most cost-effective option. Other methods could have niche applications, depending on seasonal variations and temperature.     

高锰酸钾改性对颗粒活性炭吸附Cu2+的影响

采用KMnO₄溶液在回流状态下对颗粒活性炭进行了改性。考察了KMnO₄ 溶液浓度对Cu²⁺吸附率-pH曲线及pH的影响,并研究了KMnO₄ 改性对颗粒活性炭吸附和解吸Cu²⁺性能的影响。结果表明,KMnO₄ 改性使颗粒活性炭对 Cu²⁺吸附率-pH曲线及pH向低pH偏移。当KMnO₄溶液浓度为0.03 mol/L时,改性颗粒活性炭效果最好;KMnO₄ 改性提高了颗粒活性炭对Cu²⁺的吸附速率,使其投加量节省约3倍;Cu²⁺在改性及未改性活性炭上的吸附遵循Freundlich等温吸附方程,在相同Cu²⁺平衡浓度下,KMnO₄改性活性炭对 Cu²⁺的吸附量提高约 2.7倍;使用 0.35 mol/L的HCl溶液作解吸液,Cu²⁺从KMnO₄改性活性炭上的解吸率为 91.1%,而Cu²⁺从未改性活性炭上的解吸率仅为8.8%。KMnO₄ 改性提高了颗粒活性炭从水中吸附重金属离子的能力,并促进了其解吸再生性能。     

Mechanical activation of MoS2 + Na2O2 mixtures

A study of the mechanochemical activation of molybdenum ore concentrate (MoS₂) with sodium peroxide (Na₂O₂) shows that sodium molybdate dihydrate (Na₂MoO₄ · 2H₂O) is the final crystalline product. The mechanochemical formation of sodium molybdate dihydrate is evidenced by XRD, ²³Na MAS NMR and the increasing solubility of the molybdenum in water as the oxidative reaction proceeds.     

Preliminary Studies on Precipitating Manganese Using Caro’s Acid and Hydrogen Peroxide

We report preliminary studies on the precipitation of manganese compounds by oxidation with Caro’s Acid (peroxomonosulphuric acid, H₂SO₅) or hydrogen peroxide (H₂O₂), from solutions with [Mn] = 1.2 g/L to achieve residual [Mn] <1 mg/L at a pH range of 5–9. It was found that with the addition of sodium carbonate and either Caro’s acid or hydrogen peroxide, it was possible to reduce manganese from 1.2 g/L to less than 1 mg/L in 60 min (batch reaction) at 25°C (at pH ≥ 5 using H₂SO₅, and pH = 9 using H₂O₂). By comparison, simple hydroxide precipitation under the same conditions at pH = 9 would only lower [Mn] to 165 mg/L.     

催化协同臭氧氧化滴状黄药废水的研究

以铁矿浮选中含有的黄药为处理的目标物质,采用活性炭和紫外光与臭氧联用氧化雾化后的黄药。试验考察了pH值、臭氧通入量、活性炭、雾化、自由基捕获剂对黄药去除率的影响,并比较了曝气/O₃/UV/GAC和雾化/O₃/UV/GAC两种方法。结果表明,将废水雾化,不调节pH值,反应35 min,臭氧通入量为300 mg/L, 黄药的去除率达到96.3%,比曝气/O₃/UV/GAC联用黄药的去除率提高了15%。该工艺可以作为含有黄药废水的深度处理手段。     

A kinetic and electrochemical study of the ammonia cyanide process for leaching gold in solutions containing copper

One of the biggest challenges for the gold industry in the 21st century is the presence of copper in gold containing ore bodies. This is because copper consumes large quantities of cyanide. In addition, copper cyanide species are more stable than free cyanide, and hence are problematic in events of tailings spillage. One of the methods which has been suggested for treating copper containing ores is to leach with an ammonia cyanide solution. The effect of copper and ammonia addition on gold leaching kinetics was studied in the present paper. It will be shown that when the solutions do not contain copper, the addition of ammonia decreases the rate of gold leaching. When copper is added to solution, the leach rate does decrease due to the formation of the copper cyanide complexes. However it will be shown that under conditions of zero free cyanide, gold does leach readily via the Cu(CN)₃²⁻ complex. It was found that the addition of ammonia had little effect on the leaching of gold by Cu(CN)₃²⁻, but did increase the leaching kinetics when the major cyanide species present is Cu(CN)₂⁻. Under these conditions, leaching in the absence of ammonia is very slow. The effect of copper(II) addition was also studied, and it was found that in the absence of free cyanide the presence of copper(II) increases the leach rate, provided there is enough ammonia to stabilise it against reaction with the copper(I) cyanide complexes.     

Ferrates performance in thiocyanates and ammonia degradation in gold mine effluents

Available technologies for cyanides (CN⁻) treatment in gold mine effluents marginally degrade the thiocyanates (SCN⁻). Commonly, they convert the CN⁻ into a less toxic compound, such as cyanates (OCN⁻), which afterwards are oxidized and generate ammonia nitrogen (NH₃-N). Thus, the pretreated effluents require additional steps for complete degradation of SCN⁻ and NH₃-N. Recent research shows that ferrates [Fe(VI)] might represent a sustainable option for the efficient treatment of gold mine effluents. In this context, the objective of the present study was to assess the performance of Fe(VI) in SCN⁻ and/or NH₃-N treatment, as well as in their by-products degradation. The performance of Fe(VI) was evaluated using three different synthetic effluents (solutions) and two gold mine effluents contaminated by SCN⁻ and/or NH₃-N. Results indicated that more than 97% of SCN⁻ were degraded with Fe(VI), while the NH₃-N increased up to 50%, after SCN⁻ oxidation of the presence of NH₃-N, within one hour of reaction time. Consequently, for effluents that contain SCN⁻ and NH₃-N, longer reaction time is required or complementary treatment technologies, such as nitrification – denitrification, should be evaluated for complete degradation of N-compounds.     

鄂西某高磷鲕状赤铁矿磁化焙烧及浸出除磷试验

针对鄂西某高磷鲕状赤铁矿(铁品位43.50%),在实验室条件下采用磁化焙烧—磁选工艺获取铁精矿,并对该铁精矿进行酸浸、生物浸出除磷试验。研究结果表明,在焙烧温度850℃,焙烧时间25min,还原剂用量为矿石质量的5%,磨矿时间4min,磁场强度120kA/m条件下,得到铁精矿铁品位为54.92%,铁回收率为86.78%,P含量为0.83%;酸浸试验中矿浆浓度2%,分别用0.1mol/L的H2SO4,HNO3,HCl,草酸(C2H2O4),柠檬酸(C6H8O7)除磷,其中H2SO4除磷提铁效果最佳,铁精矿品位为57.98%,回收率为96.47%,除磷率为95.30%;生物浸出试验中矿浆浓度2%,用嗜酸氧化亚铁硫杆菌(At.f菌)对铁精矿作用后,磷含量为0.23%,用黑曲霉菌滤液对铁精矿作用后,磷含量为0.20%。     

Potential use of carbon felt in gold hydrometallurgy

The use of carbon felt as a three-dimensional electrode appears to be very promising for the recovery of heavy metals, and toxic compounds removal from dilute solutions, considering its favourable physico-chemical properties: high specific surface area, good fluid permeability and compressibility, chemical inertness and good electrical conductivity.This work presents the contribution of the carbon felt electrode in two different steps of the gold cyanidation recovery process

• &#x02022;- firstly as a cathode for the electrowinning of gold solution obtained after the elution of loaded carbon;
• &#x02022;- secondly as an anode for the electro-oxidation of cyanide ions present at low concentration (200–300mg/l) in waste streams.

In the first case, more than 10 kg of gold per m² of felt (2000 kg of gold per m³ of felt) can be loaded at 400 A/m² from dilute gold solutions (30 mg/l Au) with classic Faradic yields (6–12 %) and high overall extraction efficiency (>90 %). The felt homogeneously loaded with adhesive gold deposit can be smelted in an electric furnace without addition of fluxes since the carbon felt decomposes in gaseous products without formation of ashes.In the second case, the cyanide ion concentration can be lowered to as low as 10 mg/l by electro-oxidation to cyanate form (CNO⁻) at 400 A/m² in the anodic compartment of a divided cell. The presence of copper ion significantly improves the current efficiency of this electro-oxidation and sharply reduces the oxidation of the carbon felt. Copper ions accelerate the oxidation rate of free cyanide ions through the formation of easily oxidizable complexes owing to a mechanism in which the Cull Cu^ll redox couple acts as an electron transfer mediator.     

超声波辅助去除铅锌选矿外排废水中的硫化物

针对广东韶关某大型铅锌矿选矿外排废水中超标的硫化物,采用超声波辅助下的氧化-混凝沉淀法对其进行去除试验,探讨了氧化剂种类和用量、混凝剂种类和用量及超声波辅助反应时间对S^2-去除效果的影响。结果表明：在常温下向该废水中加入用量为20 mg/L的氧化剂H₂O₂,用量为150 mg/L的混凝剂FeSO₄和用量为1.0mg/L的助凝剂聚丙烯酰胺,在超声波辅助下搅拌反应20 min,S^2-离子的去除率可达到95%以上,含量可降到0.14 mg/L以下,废水水质达到国家排放标准。     

氧气传质对湿法炼锌过程中针铁矿法除铁的影响研究

湿法炼锌采用还原-氧化法(V.M.针铁矿法),对硫化锌精矿氧压浸出液中的铁离子进行去除,探究不同通气方式和氧气流量对除铁过程的影响。研究发现氧气的传质对氧化除铁过程有显著的影响,强化搅拌对氧气的传质影响较小,但曝气可以大大提高氧气的分散度,增加氧气传质,加快Fe_²⁺氧化速率,从而提高铁的去除效率率。研究结果表明,氧气流量≥1l/min时,氧气流量对铁的去除影响较小;在不添加晶种时,曝气氧化除铁形成大量非晶态的物质,该物质可能是铁水化合物(5Fe₂O₃.9H₂O),因吸附其他离子,阻碍沉淀物向针铁矿转变。研究结果将对湿法炼锌过程中的还原-氧化法(V.M.针铁矿法)除铁具有一定的参考价值。     

Effect of carbon coatings on gold dissolution in the presence of sulphide and lead

The role of sulphide and lead in gold dissolution in a cyanide medium has been investigated for pure gold with elemental carbon coatings. Sulphide reduced the dissolution of gold with or without carbon coatings to almost the same extent and this effect became more pronounced at a higher sulphide concentration. The carbon coating slightly reduced the negative effect of sulphide on gold dissolution at around 0.2–5 mg S²⁻/L. The negative effect of sulphide on gold dissolution decreased with increasing cyanide concentration. Lead increased the dissolution of gold with or without carbon coatings at a concentration up to 2 mg Pb²⁺/L with the beneficial effect declining over this concentration range. The carbon coating reduced the beneficial effect of lead on gold dissolution. Gold dissolution was retarded at around 5 mg Pb²⁺/L. The positive effect of lead on gold dissolution became more prominent with increasing cyanide concentration.SEM analysis revealed much less erosion of gold occurring with the addition of sulphide. The gold surface showed highly corroded spots scattered across relatively less corroded areas during leaching with the addition of lead, while the gold surface was smooth after leaching with a carbon coating. An XPS investigation indicated the formation of AuS_x at the gold surface with the addition of sulphide. Metallic lead or AuPb alloys and lead hydroxide were detected at the gold surface after leaching with the addition of lead. The carbon coating hindered the diffusion of lead to the gold surface and hence reduced the beneficial effect of lead on gold dissolution.