Cyanide Alternatives in Gold Leaching: Thiosulfate and Glycine-Based Systems

Introduction and Context

The search for effective and environmentally acceptable cyanide alternatives in gold leaching has intensified in recent years. Regulatory pressure, community concerns, and the need to operate in remote or sensitive areas drive formulators and R&D chemists to evaluate thiosulfate- and glycine-based lixiviants. From a hydrometallurgical perspective, these systems differ fundamentally from cyanide in redox behavior, gold complex stability, and required process conditions. Thiosulfate relies on oxidative dissolution, often requiring higher potential and sometimes elevated oxygen pressure, whereas glycine systems can combine complexation with redox activity, sometimes allowing milder conditions. This post details the key chemistries, practical dosage ranges, performance data, and formulation considerations for thiosulfate and glycine-based systems, with a focus on real-world applicability for process chemists and procurement engineers.

Thiosulfate-Based Leaching Systems

Thiosulfate (S2O3 2−) can dissolve gold under oxidizing conditions, forming predominantly Au(S2O3)2 3− complexes. The reaction is pH-dependent and requires sufficient oxidant potential, typically provided by dissolved oxygen, hydrogen peroxide, or hypochlorite. In acidic to neutral pH (approximately 9–11 for optimal stability), thiosulfate can compete with cyanide, though generally with slower kinetics and lower robustness toward sulfide and carbonaceous interference.

Key reaction pathways include:

• Oxidation of thiosulfate to tetrathionate by oxygen or oxidants, which can reduce available complexing species.
• Gold dissolution forming Au(S2O3)2 3−, with competing reactions leading to polythiosulfate and colloidal sulfur under aggressive conditions.
• Sensitivity to sulfide minerals, which can precipitate metal sulfides and passivate surfaces, and to carbonaceous material that competes for gold.

Typical dosage ranges for thiosulfate-based leaching are in the order of 0.1–0.4 mol/L (approximately 19–75 g/L as Na2S2O3·5H2O) depending on ore grade, particle size, and target recovery. Oxidant supplementation is often required at 1–5 g/L of dissolved oxygen equivalent, or equivalent oxidant mass. Operating pH is generally maintained between 9.5 and 11.0 to minimize thiosulfate decomposition while preserving gold complex stability. Residence times may range from several hours to multiple days, depending on kinetics and target recovery.

Glycine-Based Lixiviants and Redox Behavior

Glycine (NH2CH2COOH) offers a distinct mechanism: it can complex gold(III) strongly while also participating in redox shuttling under mildly oxidizing conditions. In the presence of chloride and suitable oxidants (e.g., oxygen, peroxide), glycine-based systems can achieve dissolution rates competitive with dilute cyanide, especially for ores with sulfide interference. The zwitterionic nature of glycine provides buffering capacity, allowing operation near pH 7–9 in some formulations, which can reduce corrosion concerns compared with alkaline cyanide circuits.

Critical formulation variables include glycine concentration, chloride activator concentration, oxidant type and dosage, and potential modifiers such as bromide or iodine. Typical glycine dosage ranges from 0.2 to 0.8 mol/L (approximately 15–60 g/L), with chloride added at 0.05–0.2 mol/L to suppress competing iron hydrolysis and improve gold dissolution kinetics. Dissolved oxygen or low-concentration peroxide (0.5–3 g/L) is commonly used to maintain the redox potential in the range of approximately 600–700 mV vs. SHE, optimizing gold dissolution while limiting unwanted side reactions.

Performance Data and Kinetic Comparison

Quantitative performance is best evaluated through column leach tests or batch kinetic studies under representative ore characteristics. The following table summarizes typical dissolution kinetics and recoveries under standardized conditions (ore assay ~5 g/t Au, particle size 75–150 µm, solid-to-liquid ratio 1:10, ambient temperature, oxidant as needed).

Lixiviant	Dosage (mol/L)	Typical Gold Dissolution Rate (g/L/h)	Time to ~90% Recovery (h)	Key Operational Notes
NaCN	0.01–0.02	0.5–2.0	12–48	Highly efficient, pH 10–11, sensitive to sulfides
Thiosulfate	0.15–0.30	0.1–0.5	48–120	Requires oxidant, pH 9.5–11, sensitive to S2−
Glycine (Cl−)	0.30–0.60	0.3–1.2	24–72	pH 7–9, chloride activator, moderate oxidant
Glycine (no Cl−)	0.30–0.60	0.1–0.4	72–160	Slower, pH buffering beneficial, less robust

These values are indicative and highly dependent on ore mineralogy, sulfide content, and oxidant regime. For thiosulfate, higher oxidant demand and potential thiosulfate decomposition at elevated temperatures can reduce efficiency. For glycine, chloride presence often markedly improves kinetics by preventing iron hydroxide passivation and stabilizing dissolved gold species.

Practical Formulation Guidance

When developing a thiosulfate or glycine-based leach circuit, consider the following formulation and operational guidelines:

• pH Control: Maintain pH within the optimal window for each lixiviant. Thiosulfate performs best at pH 9.5–11; glycine systems can operate effectively near neutral pH with appropriate buffering.
• Oxidant Management: Ensure sufficient dissolved oxidant to sustain gold dissolution without excessive oxidant consumption. Monitor redox potential to avoid over-oxidation of thiosulfate or formation of unwanted byproducts.
• Interference Mitigation: Pre-cleaning steps such as sulfide oxidation or carbon removal are critical for thiosulfate systems. Glycine is more tolerant of sulfides but may require chloride activators to achieve target kinetics.
• Compatibility with Downstream Processing: Evaluate compatibility with carbon-in-leach (CIL) or carbon-in-pulp (CIP) if applicable. Glycine can sometimes interfere with carbon adsorption; thiosulfate generally has lower carbon competition but may form soluble complexes with certain metals.
• Reagent Purity and Stability: Use high-purity thiosulfate and glycine to minimize impurities that could promote side reactions. Assess storage stability under process conditions, particularly for glycine in chloride-rich media.

Scale-Up Considerations and Monitoring

Scale-up from lab to pilot or production requires attention to mass transfer, mixing efficiency, and reagent consumption under realistic retention times. Implement continuous monitoring of gold in leach and tail streams, redox potential, and pH. For thiosulfate, periodic analysis of tetrathionate and sulfur byproducts is advisable to manage accumulation. For glycine, track chloride levels and iron hydrolysis to ensure activator efficacy. Pilot tests with actual ore are strongly recommended to validate kinetics and reagent economics before full deployment.

Summary

Thiosulfate and glycine-based lixiviants represent credible cyanide alternatives for gold leaching, each with distinct chemistries and operational requirements. Thiosulfate offers robust gold dissolution under oxidizing conditions but is sensitive to sulfide interference and requires careful oxidant management. Glycine-based systems, especially with chloride activation, can deliver competitive kinetics near neutral pH, with potential benefits for corrosion control and reagent handling. Formulators and procurement engineers should conduct targeted pilot testing to define dosage ranges, oxidant regimes, and compatibility with existing circuits. Understanding these parameters enables informed selection and stable operation of non-cyanide gold leaching technologies.

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