Extractions of Crude Metal from Concentrated Ore

All matter found on Earth is made up of different elements. These elements are distributed in the three main parts of the Earth, the atmosphere, the hydrosphere, and the lithosphere. The main source of most of these elements is the lithosphere. Elements are broadly divided into metals and non-metals based on their physical and chemical properties, metals are generally hard solids that have important properties such as lustrous luster (brilliance), ductility and ductility, high thermal and electrical conductivities. On the other hand, nonmetals do not have these characteristics. They are usually brittle, non-glossy, and have low thermal and electrical conductivity. The periodic table is made up of metals because about 80% of all the elements are known to be food. Different compounds may contain a particular element. Various principles of chemistry are involved in the joint extraction and isolation of an element.

The whole scientific and technical process used to separate a metal from its ores is called metallurgy.

Extraction of crude metal from concentrated ores

The process of working a concentrated ore to extract a metal depends on the nature of the ore as well as the nature of the impurities present in the ore. This includes oxidation and reduction reactions. The concentrate should be converted into a form that is suitable for a reduction. Typically, sulfide ores are converted to oxides before reduction because the oxides are easily reduced. The main operations for ore working include two steps:

1. Modification of the concentrated ore to its oxide form (oxidation de-electronation)
2. Modification of the oxide to the metal (reduction or electronation)

Modification of the concentrated ore to its oxide form (oxidation de-electronation)

The concentrated ore is either a hydrated oxide, a carbonate, or a sulfide. It can be converted to its oxide form by the following two methods:

• Calcination: Calcination is the process of heating an ore far below its melting point, either in a limited supply of air or in the absence of air. During calcination, the following changes occur:
  • Moisture is deflected.
  • The volatile impurities are deflected.
  • The ore becomes porous and therefore easily workable in later stages.
  • Water is removed from hydrated oxides.
  • Carbonates decompose into oxides.

For example, hydrated oxide and carbonate ores are usually calcined to give oxides. The zinc in calamine (ZnCO₂) is in the form of zinc carbonate. The ore is calcined which is heated strongly in the absence of air to convert it to zinc oxide. During calcination, carbon dioxide is expelled. Aluminium occurs in its bauxite ore as Al₂O₃.2H₂O. When bauxite ore is calcined, water vapour is released and anhydrous aluminium oxide is obtained. The water of hydration is removed from the limonite (Fe₂O₃.3H₂O).

• Roasting: Roasting is the process of strongly heating the ore in the presence of an excess of air to a temperature below the melting point of the metal. This method is mainly employed in the case of sulphide ores. Moisture and volatile impurities are removed as a result of roasting and the ores are converted into oxides. 

For example, impurities of sulphur, arsenic, and phosphorus were removed as their volatile oxides, SiO₂, As, O, P₂O₅ respectively. Metal ores are converted into their oxides. The SO produced is used for the manufacture of H₂SO_4. Both calcination and roasting are typically directed in a reflector furnace. In this type of furnace, the fuel does not come into direct contact with the charge. The flames are on a charge from the ceiling of the furnace and the air supply can be controlled by vents. Fierce flames collided with the charge. There are tap holes along the walls for raw metal and slag. This type of furnace can melt about 10 kg of charge per day. In roasting, the air vents are kept open while in calcination the air vents are partially or completely closed. A reflection furnace can be used for both oxidation and reduction.

Flux: If the calcined or roasted ore still does not contain impurities of the earthly substance, an additional substance called flux is also added to the ore during reduction. It mixes impurities to form an easily soluble product known as slag. Slag is not soluble in molten metal. Being lightweight, it can be easily skied from molten metal surfaces. That’s why it is necessary to remove the gang. Thus,

Flux is a substance that chemically combines with gangue (earth impurities) that may still be present in calcined or roasted ore to form an easily soluble material called slag.

Types of fluxes

There are two types of fluxes- basic flux and acidic flux. The choice of flux depends on the nature of the impurities present in the reaction. for example,

1. Basic Flux: If the ore contains acidic impurities such as SiO₂, P₂O, etc., then basic fluxes like lime (Cao), limestone (CaCO₃), magnesite hematite (Fe₂O₃ etc. are used.
2. Acid Flux: If the basic impurities are present in the ore such as CaO, FeO, MgCO₃, etc., then acidic fluxes like silica (SiO₂) or borax (Na₂B₄O₇), are used.

Modification of the oxide to the metal (reduction or electronation)

Metal oxides are usually reduced to free metals using a suitable reducing agent such as carbon, carbon monoxide, or another metal. The reduction process involves electron gain or electronification.

The process of extraction of metal by heating a metal oxide with a suitable reducing agent is called thermal reduction pyrometallurgy.

Some metals are easily reduced (reduction at low temperatures) while others are reduced with difficulty (ie reduction occurs at high temperatures). However, it can be noted that reduction always requires heating. The choice of the element that would be suitable as a reducing agent for given metal oxide and the optimum temperature at which reduction would proceed smoothly can be understood in terms of the basic thermodynamic concepts discussed later. Depending on the nature of the oxide and the metal, the metal can be extracted by the following reducing agents:

• Reduction with C or CO: In the metallurgy of Fe, Cu, Pb, Sn, Zn, Mg, Co, etc.
• Reduction with Na, Al, Mg, or hydrogen: In the metallurgy of Mn, Cr, Ti, Mo, W, etc.
• Reduction with water gas (CO, H₂): In the metallurgy of Ni.
• Self-reduction or auto-reduction in metallurgy of Pb, Hg₂Cu, etc.

Sample Questions

Question 1: The metal is not found in nature in the form of nitrate. Why?

Answer:

Nitrates of all metals are soluble in water. Hence, if metal nitrates are extant in the Earth’s crust, these will lightly and gradually be washed into the ocean by rainwater. Therefore, metals usually do not occur in the form of nitrates.

Question 2: Graphite is commonly used as anode but not from S to Zn Diamond. Give reason.

Answer:

Graphite is a good conductor of electricity due to the presence of free electrons within its layers. But no free electrons are present and hence, it is a poor conductor of electricity. Therefore, diamond is not used as a mode.

Question 3: Why is it advantageous to roast sulphide ores in oxides previous to reduction?

Answer:

The standard free energy of formation (Δ_fG⁰) of most of the sulphides is larger than those of CS₂ and H₂S. Therefore, these sulfides are more stable than sulfides containing carbon or hydrogen. Therefore, neither carbon nor hydrogen can convert metal sulfide to metal. Conversely, the standard free energy oxides have much lower immersion than 80, and the conversion of metal sulfides to metal oxides is thermodynamics. Therefore, it is common practice to roast the sulfide in oxide ore before reduction.

Question 4: What is the role of flux in metallurgical processes?

Answer:

Flux is used to make the melted mass more conductive. The flux combines with the impurities to form an easily pliable product called a separation.

Question 5: How are metals refined as semiconductors? What is the principle of the method used?

Answer:

Semiconductor metals are manufactured by the field purification method which is based on the principle that impurities are more soluble in the melt than in the solid-state of the metals.

Question 6: Give two requirements for vapour phase purification. 

Answer:

• The metal should form a volatile compound with the available reagent.
• The volatile compound must be volatile and easily decomposable so that recovery is easy.