Extraction of Crude Metal from concentrated Ore- Calcination, Roasting, Smelting, Practice problems, FAQs

Metals are found on the earth crust in the form of its compounds.Whenever scientist found that certain place is rich in particular metal content, they start the process of mining to extract the metal. Once they remove all the dusty and rocky impurities, we are left with concentrated metal ore, which is nothing but complex compounds of metals. So these complex compounds have to be reduced to simplest form to extract metal from them. Metals occur in form of sulphides, oxides which are reduced to simpler oxides by process like calcination, roasting, smelting reduction. So lets study these process of extraction in detail.

Table of contents

• Calcination
• Roasting
• Smelting
• Reduction of Metal oxide
• Practice problems
• Frequently Asked Questions-FAQs 

Calcination

Calcination is the process of heating concentrated ore at a temperature slightly below its melting point in a limited supply of air or in the absence of air. In most cases, this is done in a reverberatory furnace. It renders the ore porous, allowing it to work up easily in future steps. Calcination is used in the following cases:

1. It is used to convert carbonates of metals into their corresponding oxides.

2. Removes water from crystallisation present in the hydrated oxide in the form of moisture. 

3. Other volatile and organic impurities can be removed by calcination.

Roasting

Roasting is the process of rapidly heating a concentrated ore below its melting point in an excess supply of air. Sulphide ores are typically converted to metallic oxides using this method. Roasting is used in the following cases:

Roasting at moderate temperature:

1. It converts sulphides of metals into oxides.

2. Removes volatile impurities of arsenic, antimony, sulphur and phosphorus by forming their oxides.

Roasting at high temperature:

Self reduction occurs when sulphide ore containing and is intensely heated in a free supply of air and reduced directly to metal.

(Copper galena)

(Galena)

(Cinnabar)

Smelting

The elimination of earthy impurities and reduction of the ore to metal are the two phases in extracting metals from calcined or roasted ore.The various processes for concentration of ores are only useful for separating a portion of the gangue or matrix, and the ore still contains some earthly impurities.

Certain compounds combine with the gangue, known as flux, to generate an easily fusible material,Slag is not soluble in molten metal when mixed with furnace charge and heated. The metal oxides so obtained by removing the fusible slag is reduced to metal by fusing it with a reducing agent. Smelting is a term used to describe this entire process.

Reduction of a Metal oxide 

Metals are found in conjunction with other elements in the ore. Metals are extracted from ore using a reduction process.

Reducing agents include carbon, hydrogen, and metals such as sodium, aluminium, and magnesium. The technique of reduction and the nature of the reducing agent to be utilised are determined by the metals' activity. Ellingham diagram showing the energy changes is useful for the reducing agent selection

 Metals in the lower portions of the electrochemical series, such as iron, zinc, tin, lead, and cadmium, are made via carbon reduction of their oxides at high temperatures, resulting in the metal in a molten state. Reduction of calcined or roasted ores into the free metals can be done in the following ways:

1. Reduction with Carbon: Metal is strongly heated with carbon. 

2. Reduction with Aluminium (GoldSchmidt or Aluminothermic process)

When the metal oxides of metal like and have a higher affinity for oxygen than carbon, their metallic oxides cannot be reduced by carbon. So aluminium powder is used to reduce them.

The oxide of the metal is thoroughly mixed with aluminium powder and a little , which acts as an oxidising agent and is placed in a crucible. Magnesium wire packed with magnesium powder and powder is used to ignite the charge. The reaction is quite exothermic, and the heat generated is enough to melt the metal.

1. Reduction with CO

1. Self reduction or auto reduction or air reduction When sulphide ore of metals such as , , , and , are heated in the air, a portion of the ore is converted to oxide or sulphate. And the remaining component of the sulphide ore reacts with its metal oxide to produce metal and sulphur dioxide. This process is called Self-reduction.Example:

Conversion of Lead sulphide to lead oxide Conversion of lead sulphide to lead sulphate Conversion of Mercury sulphide to Mercury oxide Conversion of Copper sulphide to Copperoxide

1. Reduction by Mg : In the Kroll Process, highly reactive metals like Magnesium is used for the reduction of Titanium chloride to Titanium metal.
2. Electrolytic Reduction: This is a form of electrolysis in which electric current passes through an ionic substance in a molten or dissolved state causing the electrodes to react chemically and the materials to decompose. By using this process, the hydroxides, oxides and chlorides of metals in the combined state are electrically reduced. At the cathode, the metals are collected. Some metals such as K, Na, Al are obtained through the process of electrolytic reduction.

Example: Al3+ ion (aluminium ions) from molten Al2O3 (Molten aluminium oxide) obtains three electrons from the cathode and turns into Al (aluminium metal.

Al3+ + 3e → Al

1. Reduction with hydrogen Hydrogen is an excellent reducing agent for metal oxides. In small trays, the roasted ore is heated in a hydrogen current. Metals such as tungsten and indium are reduced using this method.
2. Hydrometallurgy

This is the process of extraction of metal like gold, copper, and silver from an aqueous solution. The ore is treated with complexing agents to make them dissolve in aqueous solution and recovered from water solution by adding a reducing agent.

Gold for example is leached with a sodium cyanide aqueous solution when soluble gold cyanide is formed. The solution on treating with Zinc gives pure gold.

Practice problems

Q1. Sulphur is removed from the ores by the process known as:

(A) Calcination

(B) Smelting

(C) Roasting

(D) None of the above

Answer: (C)

Solution: Roasting is the process of converting sulphides ores into corresponding metal oxides in the presence of excess oxygen.

Q2. Metals that under auto reduction are:

(A) Mercury

(B) Copper 

(C) Antimony 

(D) All of the above

Answer: (D)

Solution: Sulphides of less electropositive metals like , , , and undergoes auto or self reduction to give corresponding pure metals.

Q3. Which of the following statement is true?

(A) The role of flux is to remove the non-fusible gangue as fusible slag.

(B) Flux are of two types, acidic and basic. 

(C) Both (A) and (B)

(D) None of the above

Answer: (C)

Solution: When mixed with furnace charge and heated, certain chemicals react with the gangue to generate an easily fusible material that is not soluble in molten metal. Species that react with gangue is known as flux. There are of two types: acidic and basic. Depending upon the nature of impurities, either one combines with it to form easily fusible slag.

Q4. Consider two chemical reactions (I) and (II) that occur during metallurgical process:

(I) 

(II)  

The correct statement regarding (I) and (II) are: 

(A) (I) reaction is roasting. 

(B) (II) reaction is calcination.

(C) Both (A) and (B)

(D) None of the above

Answer: (D)

Solution: Copper oxide when heated in excess supply of oxygen to give copper sulfide. This process of reducing metal sulfides to metal oxides is known as roasting. 

Hydrated ferrous oxide when heated strongly, leads to removal of water of crystalisation to give anhydrous ferrous oxide. This process of removal of water of crystallization in limited supply of oxygen is known as calcination. 

Therefore both (I) and (II) statements are correct.

Frequently Asked Questions-FAQs

Question 1. Why do we need to convert sulphides to oxides for extraction of pure metal?

Because it is easy to reduce metal oxide over metal sulphides to obtain pure metal.

Question 2. What is the difference between calcination and roasting?

Question 3. Why Carbon is used more often for the reduction of metals, over hydrogen?

Carbon can reduce a given metal over a wide range of temperature. Also, carbon is easily available element as compared to hydrogen gas. Production and storage of hydrogen gas is more difficult than carbon. Therefore to reduce the cost of extraction of metals, carbon is used as a reducing agent.

Question 4. In which furnace calcination and roasting are done?

Calcination and roasting are done in reverberatory furnace depending upon the conditions required in each case.

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