Cyanide Process

Introduction

Imagine discovering a treasure deep inside Earth's belly—gold. This, in all eras of humankind, has been a highly valued metal, one that stirred quests, wars, and conquests. But extracting gold from its ore is not that simple, certainly not as easy as digging it up. The cyanide process, or so-called gold cyanidation, is by far the most effective and widely used method of extracting gold, also referred to as the MacArthur-Forrest process. All in all, it is a chemical procedure that came into being during the discovery of gold mining in the late 19th century.

The cyanide process plays a very significant role in real life at the present time, particularly in the gold mining industry. It allows effective recoveries of gold from low-grade ores in gold-bearing ore processing with a solution containing sodium cyanide. Subsequently, particles of gold selectively bind to it, enabling their separation from the rest of the material. The gains in yield and profitability associated with gold mining operations are significant with the introduction of this method; nevertheless, the high toxicity of cyanide gives rise to important environmental and safety concerns.

The science behind these cyanidation processes shall be critically probed in this article by explaining the chemical reactions that give rise to them. We will learn how it was developed, its operation steps, and its economic importance. We shall then shed light on the environmental effects and mitigation measures taken to circumvent such effects, thus setting us upfront to understand this very important technique in extracting gold.

The cyanide process, also called the Macarthur-Forrest cyanide Process, method of extracting silver and gold from their ores by dissolving them in a dilute solution of sodium cyanide or potassium cyanide. The process was invented in 1887 by the Scottish chemists John S. MacArthur, Robert W. Forrest, and William Forrest. The method includes three steps:

• Contacting the finely ground ore with the cyanide solution.
• Separating the solids from the clear solution.
• Recovering the precious metals from the solution by precipitation with zinc dust.

$\begin{array}{c}\mathrm{Ag}+\mathrm{NaCN}+{\mathrm{H}}_2\mathrm{O}+\frac{1}{2}{\mathrm{O}}_2\to 2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+2\mathrm{NaOH}\\ 2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+\mathrm{Zn}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{CN}{)}_4]+2\mathrm{Ag}\downarrow \\ \text{more electropositive}\\ \text{metal}\\ 2\mathrm{Na}[\mathrm{Au}(\mathrm{CN}{)}_2]+\mathrm{Zn}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{CN}{)}_4]+2\mathrm{Au}\downarrow \end{array}$

NOTE: It is a metallurgical process/technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex.

Gold Cyanidation: The MacArthur-Forrest Process

The Process: Steps and Reactions

Contacting the Finely Ground Ore with the Cyanide Solution

At the very beginning of the cyanide process, the ore is finely ground to increase the surface area for the cyanide solution to work on. The cyanide solution is in contact with the ground ore. In this phase, the complexation of cyanide ions with gold and silver existing in the ore dissolves these metals.

$$\mathrm{Ag}+\mathrm{NaCN}+{\mathrm{H}}_2\mathrm{O}+\frac{1}{2}{\mathrm{O}}_2\to 2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+2\mathrm{NaOH}$$

Separation of Solids from the Clear Solution

A solution that contains the precious metals is just a clear cyanide solution, from which solid particles are to be removed. Conventionally, it should be done through filtration or settling processes to ensure that the solution with the dissolved gold and silver has no solid impurities.

Precipitation of Precious Metals from Solution by Zinc Dust

Finally, gold and silver are recovered from the cyanide solution by adding zinc dust that precipitates metals out of the solution. Being more electropositive, it would replace gold and silver in complex to form their precipitates.

$$2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+\mathrm{Zn}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{CN}{)}_4]+2\mathrm{Ag}\downarrow$$

Significance and Uses of the Reaction Arlington

Methods

Everyday Uses of the Chemicals Involved in the Reaction

The cyanide process is of very special importance in gold mining because it allows economic recovery of gold from low-grade ores that could not previously be economically mined. It is done on a large scale for mining gold at various locations across the world. It facilitates recovering far greater amounts of gold from ore than was previously possible.

The cyanide process is part of college courses on metallurgy and chemical engineering. It is a very great example of industrial-scale chemical reactions and processes applicable directly and illustratively, for example, in classes concerned with the formation of complexes, solubility, and electrochemistry.

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SOME SOLVED EXAMPLES

Example 1

Question:

Which of the following metals is obtained by leaching its ore with dilute cyanide solution?

1) Silver (correct)

2) Titanium

3) Vanadium

4) Zinc

Solution:

Silver is obtained by leaching its ore (Argentite, Ag2S or horn silver, AgCl) with dilute cyanide solution by which it is extracted in the form of a soluble complex.

$${\mathrm{Ag}}_2\mathrm{S}+4\mathrm{NaCN}\rightleftharpoons 2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+{\mathrm{Na}}_2\mathrm{S}$$

Silver is recovered from a solution of sodium dicyanoargentate by treating it with zinc scrap.

$$2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+\mathrm{Zn}⟶{\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{CN}{)}_2]+2\mathrm{Ag}$$

Hence, the answer is option (1).

Example 2

Question:

Gold is extracted by hydrometallurgical process, based on its property:

1) Of being less reactive

2) Of being electropositive

3) To form salts which are water soluble

4) To form complexes which are water soluble (correct)

Solution:

Gold is extracted by a hydrometallurgical process, specifically the cyanide process, which is based on its property to form soluble complexes.

$[2\mathrm{Na}[\mathrm{Au}(\mathrm{CN}{)}_2]+\mathrm{Zn}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{CN}{)}_4]+2\mathrm{Au}\downarrow ]$

Hence, the answer is option (4).

Example 3

Question:

Cyanide process is used in the extraction of:

1) Au

2) Ag

3) Both (1) and (2) (correct)

4) Cu

Solution:

The cyanide process, also known as the MacArthur-Forrest process, is used to extract both gold (Au) and silver (Ag) from their ores by dissolving them in a dilute solution of sodium cyanide or potassium cyanide. The process includes three steps: contacting the finely ground ore with the cyanide solution, separating the solids from the clear solution, and recovering the precious metals from the solution by precipitation with zinc dust.

$[2\mathrm{Na}[\mathrm{Ag}(\mathrm{CN}{)}_2]+\mathrm{Zn}\to {\mathrm{Na}}_2[\mathrm{Zn}(\mathrm{CN}{)}_4]+2\mathrm{Ag}\downarrow ]$

Hence, the answer is option (3).

Summary

Cyanidation, otherwise known as the MacArthur-Forrest process, is an important metallurgical technique applied in the extraction of gold and silver from low-grade ores. The cyaniding process, invented in 1887, consists of contacting the ore with a cyanide solution, separating the solids from the solution, and recovering the metals by precipitation with zinc dust. This revolutionized gold mining by economic welding of gold from ores of too low a grade in metal content. This extends into the fields of academics as well, wherein it is a popular case study in processing.