Coinage metals (Copper, Silver, Gold)

Copper (Cu), Silver (Ag), and Gold (Au) are commonly known as coinage metals. They are also called currency metals due to their extensive usage in producing expensive coins. These metals have been known to people for a long time for this particular use. These metals are located at the end of the series of transition elements in the group (1B) of the periodic table with atomic numbers 29 (Cu), 47 (Ag), and 79 (Au).

Occurrence of coinage metals

Coinage metals occur in native or combined states. For example, copper (Cu) is found as a native state on a larger scale in Michigan in the USA. In Pakistan, copper is found in the Chaghi District of Balochistan. Similarly, it can be found in India, such as Karnataka and Khetri copper belt in Rajasthan.

Silver (Ag) occurs in both states, separated and combined. During the mining of gold and lead, silver is obtained as a byproduct. Nevada, Idaho, and Arizona are the three states that produced silver at a large scale in the USA. In Pakistan, it is found in Chiniot by the Chinese group the metallurgical co-operation of China.

Gold (Au) occurs freely in nature. It can be found in quartz veins or alluvial sands or gravel. Particles of gold are also found in the sulfides of copper, silver, zinc, and lead. South Africa is the largest gold-occurring country in the world. Pakistan has a large number of gold minerals in Reko Diq in Baluchistan. It has over 1600 million tons of gold reserves. The United States of America is in the fourth position and India is in the eighth position in the production of gold.

Properties of Coinage metals

These are the physical and chemical properties of coinage metals:

Physical properties of coinage metals

Copper has a red-brownish color that turns dull during oxide formation. Silver has white while gold is soft, heavy, and yellow in color.
Gold and silver are called noble metals because they are unaffected under atmospheric oxygen.
All three coinage metals (Cu, Ag, Au) are ductile and malleable.
Coinage metals are good conductors of heat and electricity.
All of them are metals.

Chemical properties of coinage metals

The order of reactivity of coinage metals decreases with an increase in their atomic number. The order of reactivity is: Cu > Ag > Au
Air reacts with red cuprous oxide and converts it into black cupric oxide. Silver and gold are not reacted with air. They are more stable.
Alkalies do not react with coinage metals.
When coinage metals react with chlorine, the resultant product is chlorides.

Cu + Cl₂ ➝ CuCl₂

2Ag + Cl₂ ➝ 2AgCl

2Au + 3Cl₂ ➝ 2AuCl₃

The behavior of coinage metals with acids:
- When coinage metals react with acid, they cannot displace hydrogen atoms from acidic solutions.
- Dilute hydrochloric acid (HCl) and dilute sulfuric acid (H₂SO₄) cannot react with coinage metals in the absence of oxygen or oxidizing agent. Only copper can react with dilute acid in the presence of oxygen.
- Hot concentrated sulfuric acid can react with copper and silver.
- Nitric acid (HNO₃) reacts with copper and silver to give oxides.
- Gold is inert to every acid. It is only dissolved in the aqua regia.

Comparison to Alkali metals

When comparing the coinage metals with alkali metals, it can be seen that both have one electron in their outermost shell. However, the penultimate/valence orbit of coinage metals has 18 electrons while alkali metals have 8 electrons.

When the removal of valance electrons from alkali metal takes place, it gives a stable inert gas electronic configuration (eight electrons). On the other hand, coinage metals can lose more than one electron due to (n – 1) d¹⁰ns characteristic electronic configuration. The energy band difference between the (ns) and (n – 1) is not very large, for which reason, coinage metals show variable valency.

Ores of coinage metals

Ores are naturally occurring rocks that are composed of one or more minerals. Coinage metals in combined states are found in the form of ores.

Ores of copper (Cu)

These are the ores of copper (Cu):

Cuprite or ruby copper (Cu₂O)
Copper pyrite or chalcopyrite (CuFeS₂)
Copper glance of chalococite (Cu₂S)
Malachite (CuCO₂.Cu(OH)₂)
Azurite (2CuCO₂ . Cu(OH)₂)

Ores of silver (Ag)

These are the major ores of silver (Ag):

Silver glance or Arentite (Ag₂S)
Horn silver or ceragyrite (AgCl)
Argentiferrous galena (PbS)
Ruby silver or pyrargyrite (Ag₂S. Sb₂S₃)

Ores of gold (Au)

Gold has the following ores that are mentioned below:

Calaverite (AuTe₂)
Aurostibite (AuSb₂)
Maldonite (Au₂Bi)
Auricupride (Cu₃Au)
Syyanite (AgAuTe₂)

Extraction of Coinage metals (Cu, Ag, Au)

Coinage metals are extracted from their ores by different techniques. The initial steps are simple and the same for each of them.

Extraction of Copper (Cu)

These are the following steps to extract copper from sulfide ores.

Concentration

The first step is to concentrate the amount of copper mineral. For this purpose, the ore is passed through a froth flotation process which removes most of the impurities or rock waste. Then it is converted into powder form and mixed with water. After that, chemicals are added to it, and then apply the compressed air on the mixture. This enhanced the contact of particles with each other.

Copper minerals stick with the air bubbles and rise to the surface of the mixture in the form of a layer. Later, this layer of copper mineral is separated from the surface which is 25-30% concentrated.

Roasting

In the second step, volatile impurities such as arsenic, sulfur, and antimony are removed by converting them into oxides called roasting. This process is carried out in a furnace that has charged multi-floor.

These are the reactions that occur in the furnace:

CuFeS₂ + O₂ ➝ Cu₂S + 2FeS + SO₂

2Cu₂S + 3O₂ ➝ 2Cu₂O + 2SO₂

2FeS + 3O₂ ➝ 2FeO + 2SO₂

Smelting

Smelting is used to remove silica, alumina, and iron from the mixture. This process is carried out in the blast furnace. Before transferring the mixture from the roasting furnace into the smelting furnace, first, it is mixed with sand (flux). The temperature of the blast furnace is high enough to melt the mixture. This results, in iron oxide and alumina reacting with silica to form iron and aluminum silicates. These silicates are called slag.

For example,

Al₂O₃ + 3SiO₃ ➝ Al₂(SiO₃)₃

Fe₂O₃ + 3SiO₆ ➝ Fe₂(SiO₃)₃

Slag is float on the surface of the molten mixture and it can be removed by separation. After separation, the molten residue has copper sulfide and iron sulfide. It contains 45% of copper. Collectively this molten residue is called matter.

Bessemerisation

In this process, a Bessemer converter is used which is internally lined with refractory material like magnesite. The matter is injected into the Bessemer where the air is blown through the molten charge. This oxidizes the sulfur, iron, and other metal traces. These oxides are removed from the converter as gases, whereas iron oxide formed a layer of slag.

After preceding these steps, matter or molten copper is run into the metal moulds. Where it cools down and freezes the dissolved gases in the copper during solidification. Some of the gases such as sulfur dioxide have lower solubilities in solid copper as compared to molten copper and start evolving. Due to this reason, gas bubbles appeared on the surface of copper and give a blistered appearance.

Refining of copper (Cu)

Refining of copper gives a blister-free appearance. These are two methods used to refine copper.

Poling or fire refining

Blistered copper is first melted at a high temperature in the presence of excess air. This gives oxides of lead, sulfur, zinc, arsenic, and antimony that are removed as impurities whereas iron, cobalt, nickel, manganese, and bismuth are removed during the separation of the slug.

However, cuprous oxide (Cu₂O) is also obtained in this process which is further reduced to copper by heating with green wood poles. This process is called poling and copper is obtained known as tough coke copper.

Electrolytic refining

Electrolytic refining is composed of copper rods. These rods act as anodes and are dipped in the electrolytic solution of copper sulfate and sulfuric acid. However, cathodes are formed by pure copper sheets. During electrolysis, impurities are dissolved in the solution while metals such as gold, silver, and platinum form deposits on the anode.

Copper obtained by electrolytic refining is very brittle. Therefore to make it better, it is remelted and oxidized with hydrogen and again poling. This process gives 99.98% copper content.

Extraction of Silver (Ag)

Silver (Ag) is one of the best coinage metals and its extraction can be done by cyanide process. This process is also known as the Macarthur-Forrest process, in which silver ores are dissolved in the dilute solution of sodium cyanide and potassium cyanide. In this way, the extraction of silver minerals takes place.

These are the following step to extract silver (Ag) from their ores:

Grinding and Crushing

First of all, silver ore is converted into small particles and powdered form for further process. For this purpose, silver ore is crushed in a crusher machine that makes silver ore small pieces. These pieces are then converted into a powdered form in ball mills.

Concentration

The froth-flotation process is the best method to remove impurities from the crushed silver ore. After removing impurities like dust, rocks, and other metals, silver becomes concentrated.

Adding Solution (sodium cyanide solution)

After concentrating the silver ore, a dilute solution of sodium cyanide (0.2-1%) is added to it. During the process, the air evolved and bubbled in the mixture. The resultant product of the reaction is the conversion of silver from the ore into sodium argentocyanide.

Ag₂S + 4NaCN ➝ 2Na[Ag(CN)₂] + Na₂S

In the above reaction, 2Na[Ag(CN)₂] is sodium argentocyanide. This reaction is reversible and to move it forward direction, the air present in the reaction chamber converts sodium sulfide (Na₂S) into sodium sulfate and thiosulphate.

In the above process, there are some other reactions that are:

Na₂S + NaCN + H₂O + (O) ➝ 2NaOH + NaSCN

The addition of lead acetate converts the sodium sulfide into the precipitation of lead sulfide.

Na₂S + (CH₃COO)₂ Pb ➝ PbS + 2CH₃COONa

Precipitation of silver

Addition of zinc and caustic soda in the solution of sodium argentocyanide 2Na[Ag(CN₂)], results in the precipitation of silver.

2Na[Ag(CN₂)] + Zn + NaOH ➝ Na₂ZnO₂ + 2Ag + 2H₂O + 4NaCN

In the above reaction, Na₂ZnO₂ is the sodium zincate and Ag is the precipitated silver.

Refining of silver (Ag)

Precipitated silver is separated with the help of filtration and then refine. First, it is melted, and then it is converted into blocks and bars. To remove impurities from the silver, the electrolytic method is used.

Electrolytic method

In this method, impurities such as copper, gold, and other metals are removed by passing an electric current. An anode is made with impure silver or silver-containing impurities and a cathode is a pure silver. The electrolyte is a solution of 10% nitric acid and silver nitrate.

When current is passing through the cathode to the anode. The impurities and silver from the anode are dissolved in the solution and silver is start depositing on the cathode.

Extraction of Gold (Au)

Gold is one of the best coinage metal which is very expensive and precious. The extraction of gold passes through various steps and processes.

Washing process

The extracted gold from the alluvial sand or gravel passes through the washing process. It has the following steps below:

Planning

This process is used to separate the gold from sand or impurities. A circular pan with a one-foot diameter in width is used that is made with iron or zinc metal. Alluvial sand or gravel is placed in the circular pan and added water into it, then covered the pan. When applying the swirling motion to the circular pan, larger and heavy particles of gold remain in the pan while smaller particles of sand and impurities leave the pan.

Placer mining

Gold is also found in the sand at the edges of rivers, placer mining is used to separate or extract this kind of gold. For this purpose, a rod-like channel that is made with wood is used. It has pockets (sluices) at the bottom that collect gold. When water flows through the channel, the heavier particles of gold remain in the sluices, and impurities or sand leave the channel with water.

Amalgamation process

This process is used for further processing of extracted gold from the auriferous quartz. In the first step, the larger gold-containing rocks are crushed in a chamber of a stamp mill. A stamp is act like a hammer that crashed the gold into small particles. The final product is a pulp or slurry material.

In the second step, the pulp or slurry of gold-containing material is run over the amalgamated copper and mercury plates. The amount of free gold in the pulp makes a contact with mercury and forms an amalgam or blend mixture on the surface of the plate and then it is removed by scraping. In this way, about 70-80% of gold is obtained by scraping the amalgamated plates and 30-20% from the tailing of amalgamated plates.

Mac-Arthur-Forrest Cyanide Process

The crushed form of gold ore from the stamp mill or amalgamation process is reacted with a 0.25-1% solution of potassium or sodium cyanide. This process carries out in a large chamber made with wood or cement. An excess amount of air is also involved in the reaction. The final product of this reaction is potassium aurocyanide 4K[Au(CN)₂].

4Au + 8KCN + 2H₂O + O₂ ➝ 4K[Au(CN)₂] + 4KOH

Potassium aurocyanide is filtered from the solution and then reacts with the zinc. This gives precipitation of a complex (K₂[Zn(CN)]K₂Au) that contains gold, zinc, and potassium.

2K[Au(CN)₂] + Zn ➝ K₂[Zn(CN)]K₂Au

When the above-precipitated complex is reacted with dilute sulfuric acid (H₂SO₄), it dissolves the other metals like zinc and potassium. In this way, a large amount of gold is obtained which is further proceeded for melting and refining.

Refining of gold (Au)

Refining gold is an important step, it removes all the impurities and other metals such as copper, lead, zinc, etc from the gold. It is the final step in the extraction of gold.

These are the process of refining gold:

Cupellation

In this step, the gold with impurities is melted in a porous chamber called a cupel. The catalysts are borax and silica. Air is also passed through the chamber which oxidizes the impurities like zinc, and lead and removed them.

Quartation process

In this process, the separation of gold and silver takes place called parting. Concentrated sulfuric acid (H₂SO₄) is added. This results in dissolving the silver and other impurities but not gold. It is because gold has no reaction with sulfuric acid if silver is less than 70% and gold is more than 30%.

Miller’s process

Miller’s process is used to remove impurities from gold by using chlorine parting. Impure gold is reacted with borax and then chlorine gas is passed through the chamber. The temperature of the reaction is not very high. Due to low-temperature gold chloride is unaffected whereas other metal chlorides impurities such as Hg, Zn, and As become volatile. Silver chloride appears on the surface and is removed. After removing these impurities pure form of gold remains.

Electrolytic process

Electrolytic refining is the best method to obtain more pure form of gold. Impure alloy is taken as anode and pure gold is taken as a cathode. Solution of gold chloride is used as electrolyte with 10-12% hydrochloric acid. When current is passed through the electrodes, copper dissolve in the solution, silver precipitate as silver chloride on the surface of the solution whereas gold accumulates on the cathode.

Cleaning of coinage metals

These are the steps to clean coin or coinage metals:

Coinage metals can be cleaned with soft erasers, toothpicks, and ink eraser.
They can also be cleaned with brushed.
Wet cleaning can be used to clean silver and copper coins.
Cleaning with vitriol copper soap is the best method to clean dirty coins.
Immersion of coins into hydrogen peroxide solution, 10% ammonia solution, and lemon juice are the alternative ways to clean coinage metals.
Dilute Aqua Regia is the best method to clean gold coins.

Brightening of Coinage metals

These are the steps to brighten the coinage metals:

Silver coins can be brightened by giving them silver immersion baths such as ripolin and land’s precious metal bath. This results in the removal of silver sulfide and the coin remain unharmed.
Electrochemical reduction method can be used to brighten the silver coins.
Food products like carbonated drinks and sauce is an effective method to clean silver coins and metals.
Copper coins can be brightened by the use of copper soap.
Cotton pads with chemicals can also be used for the brightening and cleaning of copper and silver coins.
Gold coins can be brightened by washing with very dilute aqua regia. Concentrated aqua regia dissolves the gold coins.

Uses of coinage metals

These are the most important uses of coinage metals:

Uses of Copper (Cu)

Copper is used in the manufacturing of electrical conductors.
It is used in utensils, ornaments, kettles, metal coins, calorimeters, etc.
The most important use of copper is in electroplating and electrotyping.
Copper is used in alloys.

Uses of Silver (Ag)

Due to the softness of the silver, it is used in ornaments formation with copper (80% silver and 20% copper).
Silvering of mirror is required silver metal.
It is used in silver plating.
Silver is also used in photography but first, it is converted into silver bromide.

Uses of Gold (Au)

Gold is used as a currency.
It is used for making jewelry and coins by mixing with copper or silver.
Gold is also used in electroplating of copper, etc.
Compounds of gold are used in photography.
It is used in electronic products, like laptop processors, etc.

Concepts Berg

Why are coinage metals so-called?

Coinage metals are called coinage because they are used in the production of coins. They are also mixed with other metals for strengthening purposes.

What is the family of coinage metals?

Elements present in the family of coinage metals are copper (Cu), silver (Ag), and gold (Au).

What are coinage alloys?

When coinage metals are mixed with other metals are called coinage alloys.

Why are coinage metals used?

They are very expensive and efficient. They have many applications in the field of chemistry as well as other fields.

Which three metals are not suitable for coinage, and why?

Poisonous metals can not be used in coinage like mercury, lead, arsenic, etc.

Why were metals used to make coins in ancient times?

Some metals are very expensive and rare. So ancient people use them for currency.

References

Chemistry by Arun Mittal
Coinage metals (ncbi.nlm.nih.gov)
Cleaning and brightening of coinage metals (metaldetectingworld.com)