Cyanide Process

Cyanide, also known as cyanidation, is a highly efficient and widely used technique for extracting gold from ore. First developed in the late 19th century, this chemical method revolutionized the gold mining industry by enabling miners to recover gold from low-grade ores with minimal waste. The process relies on sodium cyanide or potassium cyanide, which forms a soluble complex with gold, allowing it to be separated from the ore. In this article, students will understand how the cyanide process works is crucial for navigating the evolving landscape of precious metal extraction.

• 1783: Chemist Carl Wilhelm Scheele discovered that gold dissolves in cyanide, laying the groundwork for cyanide extraction.
• 1844–1847: Scientists like Bagration, Elsner, and Faraday further explored gold-cyanide interactions.
• 1887: Scottish chemists John S. MacArthur, along with Robert and William Forrest, developed the MacArthur-Forrest Process, making it possible to extract gold from low-grade ores using cyanide solutions.

1.0Gold Recovery Through Cyanide Process

Gold, found in low concentrations in ores, must be separated before extraction. Since it’s water-insoluble, sodium cyanide is used to dissolve it, forming a stable complex.

Steps in the Cyanide Process

Ore Processing:

• The process begins by crushing and grinding the ore into fine particles.
• If other metals or sulfide minerals are present, additional treatment is needed before leaching.

Leaching Process:

• A sodium cyanide solution is added to dissolve gold, forming soluble complexes (Elsner’s equation).
• Lime is used to maintain a stable pH (10–11) for an efficient reaction.

Gold Recovery:

• The gold-cyanide solution is treated with activated carbon or zinc to extract gold.
• Cementation Process: A zinc electrode with carbon paste is placed in the solution to aid gold recovery.

2.0Chemical Reactions in the Cyanide Process

During the cyanide process, the following reactions take place:

At the Cathode (Gold Reduction):

Gold is reduced either by gaining electrons or by decreasing its oxidation number.

e⁻+  [Au(CN)₂]⁻ → Au + 2CN⁻

In this reaction, gold is reduced by gaining electrons, decreasing its oxidation number.

At the Anode (Zinc Oxidation):

Zinc undergoes oxidation, losing electrons and increasing its oxidation number.

After the gold leaching process, cyanide remains in the tailings. This means that the slurry produced must either be treated for recycling or safely neutralized to prevent environmental harm.

Zn + 2OH⁻ → Zn(OH)₂ + 2e⁻

Zn(OH)₂+4CN⁻→ [Zn(CN)₄]²⁻+ 2OH⁻

Zinc hydroxide reacts with cyanide ions, forming a soluble zinc cyanide complex.

The Elsner equation represents the dissolution of gold in cyanide solution, an essential step in the cyanide process:

4Au + 8NaCN + O₂ + 2H₂O→4Na[Au(CN)₂]+4NaOH

Here’s a breakdown of the reaction:

• Gold (Au) reacts with sodium cyanide (NaCN) in the presence of oxygen (O₂) and water (H₂O).
• This forms the soluble gold cyanide complex sodium dicyanoaurate (Na[Au(CN)₂]) and sodium hydroxide (NaOH) as a byproduct.

While sodium cyanide (NaCN) is commonly used, other cyanide salts can also facilitate gold dissolution:

• Potassium cyanide (KCN) can replace NaCN, forming potassium dicyanoaurate (K[Au(CN)₂]).
• Calcium cyanide (Ca(CN)₂) is sometimes used due to its cost-effectiveness, especially in large-scale mining.

3.0Environmental Impact of the Cyanide Process

Cyanide, widely used in industries like mining and agriculture (particularly in processing crops such as cassava), can have severe environmental consequences. Below are some key effects:

• Cyanide Contamination: When released into the soil, cyanide disrupts its natural balance, reducing beneficial bacteria and fungi essential for soil health. It also alters pH levels and other crucial chemical properties.
• Microbial Imbalance: Cyanide presence can lead to the proliferation of harmful microorganisms, increasing the risk of plant and animal diseases.
• Water Pollution: Cyanide can seep into nearby water bodies through runoff or direct industrial discharge, contaminating drinking water and endangering aquatic life.
• Harm to Aquatic Life: Fish and other water organisms are highly sensitive to cyanide. High concentrations can cause mass die-offs, severely disrupting ecosystems.
• Human Health Risks: Communities near cyanide-processing industries face exposure through contaminated air, water, or food. Cyanide poisoning symptoms include headaches, dizziness, and, in severe cases, fatal consequences.
• Food Safety Concerns: Improper handling of cyanide in cassava processing can lead to its presence in food products. Consuming cassava with high cyanide levels poses health risks to both humans and animals.
• Ecosystem Disruption: Long-term exposure to cyanide can negatively impact ecosystems, reducing biodiversity and altering food chains by affecting plant growth and animal populations.
• Resource Degradation: Soil and water contamination from cyanide pollution can reduce agricultural productivity, ultimately affecting food security and local economies.