extraction of gold by chlorination

Extraction of Gold by Chlorination: An Overview

The chlorination process for gold extraction refers to a hydrometallurgical method where chlorine gas or chlorine-containing compounds are used to dissolve gold from its ores or concentrates, forming soluble gold chloride complexes. This technique, historically significant as one of the first chemical leaching methods developed in the late 19th century, operates on the principle that elemental gold can be oxidized and complexed by chlorine in an aqueous medium. While largely superseded by the more dominant cyanidation process for free-milling ores, chlorination remains relevant for specific refractory ore types and secondary materials where cyanide efficiency is low. The process typically involves roasting (if necessary to remove sulfides or organic carbon), chlorination leaching under acidic conditions, and subsequent recovery of gold from the pregnant solution via precipitation or adsorption.

Process Mechanism and Comparison with Cyanidation

The core reaction involves the oxidation of metallic gold (Au⁰) by chlorine (Cl₂) in an acidic environment to form the soluble auric chloride complex [AuCl₄]⁻:
2Au + 3Cl₂ + 2HCl → 2H[AuCl₄]
This reaction is rapid and can achieve high gold dissolution rates, even for some finely disseminated ores.

The key operational differences between chlorination and the standard cyanidation process are substantial, as summarized below:

Feature	Chlorination Process	Cyanidation Process
Reagent	Chlorine gas (Cl₂), hypochlorite, or other chlorides.	Sodium cyanide (NaCN) or potassium cyanide (KCN).
Typical Environment	Strongly acidic (HCl or H₂SO₄ medium).	Highly alkaline (pH >10.5, maintained with lime).
Kinetics	Very fast dissolution rate.	Comparatively slower dissolution rate.
Gold Complex Formed	[AuCl₄]⁻ (auric chloride).	[Au(CN)₂]⁻ (dicyanoaurate).
Sensitivity to Impurities	Highly sensitive to sulfide minerals and organic matter; pre-roasting often required. Less effective in presence of silver halides.	Sensitive to "preg-robbing" carbonaceous material and copper minerals.
Environmental & Safety Concerns	Handling of toxic, corrosive chlorine gas; highly corrosive process solutions; acid generation potential.	Extreme toxicity of cyanide; potential for catastrophic water pollution; requires stringent tailings management.
Primary Application Niche	Historically for free-milling ores; now more for refractory sulfides after roasting, electronic scrap recycling, and specific concentrates.	The industry standard for processing most free-milling oxide and sulfide ores globally.

As evidenced by the table, chlorination offers speed but introduces significant materials handling and corrosion challenges, limiting its widespread adoption compared to the more controllable, albeit toxic, cyanidation process.

Real-World Case Study: The Plattner Process and Refractory Ore Treatment

The most historically significant real-world application was the Plattner Chlorination Process, widely used in the late 1800s before cyanidation's rise. A concrete example was its use at mines processing telluride-bearing ores in Kalgoorlie, Australia, and Cripple Creek, Colorado, USA.

• Process Flow: After crushing, ore was roasted in hearth furnaces to oxidize sulfide minerals and remove arsenic/sulfur. The calcine was then placed in brick-lined vats or rotating barrels.
• Chlorination: Chlorine gas, generated on-site by reacting pyrolusite (MnO₂) with hydrochloric acid (HCl), was introduced into the moistened calcine.
• Gold Recovery: The soluble gold chloride was subsequently washed out with water. Gold was recovered from this solution by chemical reduction using ferrous sulfate (FeSO₄) or hydrogen sulfide (H₂S), precipitating metallic gold.

While this method is now obsolete for primary ore treatment due to costs and corrosion issues, modern adaptations persist in niche areas. For instance, chlorination leaching is effectively employed today in some facilities for recovering gold from calcined pyrite or arsenopyrite concentrates, where roasting has already rendered the material amenable to chlorine attack. Furthermore, it is a key stage in certain electronic waste (e-waste) recycling flowsheets, where aqua regia (a mixture of nitric and hydrochloric acids) or chlorine-based solutions dissolve gold from printed circuit boards after initial processing.

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Frequently Asked Questions (FAQs)

1. Why did the chlorination process decline in favor of cyanidation?
The decline was primarily economic and operational. Cyanidation proved simpler to operate on a large scale without extreme corrosion issues. It did not require expensive acid-resistant equipment or complex chlorine gas handling infrastructure. Furthermore, cyanidation could effectively treat a wider range of lower-grade ores without mandatory roasting at a lower overall cost per ton processed.

2. Is chlorination still used anywhere in modern gold extraction?
Yes, but selectively. Its main modern applications are:

• Treating specific refractory calcines where gold is encapsulated.
• Recycling secondary sources like electronic scrap catalysts.
• As part of niche hydrometallurgical flowsheets for complex concentrates.
• Used occasionally as an intensive pre-treatment ("chlorine wash") for carbonaceous ores to passivate organic carbon before cyanidation.

3. What are the main environmental risks of chlorination compared to cyanide?
While both involve toxic reagents, their risk profiles differ significantly:

• Chlorine: Presents acute inhalation hazards and forms corrosive acids that can lead to persistent ecosystem acidification if not neutralized.
• Cyanide: Presents acute systemic toxicity to aquatic life and mammals but degrades naturally under controlled conditions into less toxic compounds like ammonia/cyanate.
  Both processes require rigorous containment systems within modern plants.

4 Can chlorination recover platinum group metals (PGMs)?
Yes.This is one of its notable advantages over standard cyanidation.Chlorine under oxidizing acidic conditions can also dissolve platinum palladium,and other PGMs which form stable chloro-complexes making it potentially suitable for treating PGM-bearing materials alongside gold

References

1 Habashi Fathi A Textbook of Hydrometallurgy second edition Métallurgie Extractive Québec 1999 pp 364-370
2 Marsden John O House Iain The Chemistry of Gold Extraction second edition Society for Mining Metallurgy & Exploration Inc Littleton Colorado 2006 pp 379-383
3 Historical overviews documented by institutions such as The Minerals Council of Australia regarding early Kalgoorlie operations