Question

What is meant by the following terms: (a) calcination, (b) leaching, (c) smelting, (d) slag?

Short answer

(a) Calcination is a thermal treatment process that decomposes a solid substance, removes volatile components, or causes phase transition by heating it at high temperatures in the absence of air. It's mainly used for converting ores into oxides for easier metal reduction. (b) Leaching is a process that extracts metals from ores by dissolving the metal-containing part in a suitable solvent, leaving behind the gangue, which is separated afterward. (c) Smelting is a pyrometallurgical process that extracts or produces metals from ores by heating them at high temperatures with fluxes and reducing agents. The molten metal is then further refined to obtain the pure metal. (d) Slag is a byproduct of smelting, formed by combining metal oxide impurities with fluxes. It floats on the molten metal surface and can be separated and removed. Slag has various uses in construction, road building, and fertilizers.

Step-by-step solution

(a) Calcination Definition

Calcination is a thermal treatment process in which a solid substance, like an ore or mineral, is heated at a high temperature in the absence of air to cause the decomposition of the substance, removal of volatile components, or phase transition. This process is generally used for the conversion of ores into their oxides, which are more easily reduced to metal.

(b) Leaching Definition

Leaching is a process used for the extraction of metals from ores by treating them with a suitable solvent, which is capable of selectively dissolving the metal-containing part of the ore. The solvent, also called lixiviant or lixivium, passes through the ore and dissolves the desired metal, leaving behind the remaining gangue, which can be later separated through filtration, sedimentation, or precipitation.

(c) Smelting Definition

Smelting is a pyrometallurgical process, which is used for the extraction or production of metals from their ores. This process involves heating the ore, usually mixed with suitable fluxes and reducing agents, at high temperatures to separate the metal from its oxide or sulfide compounds by progressive reduction or formation of a molten metal phase. The product of smelting is a mixture of metal and impurities, which is further refined to obtain the pure metal.

(d) Slag Definition

Slag is a byproduct generated during the smelting process, formed by the combination of metal oxide impurities present in the ore with various fluxes added to lower their melting point. The slag, being less dense than the molten metal, floats on the surface and can be easily separated and removed. Slag usually consists of a mixture of silicates, aluminates, and other metal oxides, and has various uses in construction materials, road building, and fertilizers.

Key concepts

Calcination

Calcination is a thermal process used primarily in metallurgy and involves heating a substance, such as an ore or mineral, to high temperatures in the absence of air or oxygen. This high heat treatment typically leads to the decomposition of the material or the removal of volatile substances. One of the main goals of calcination is to convert ores into their oxide forms because oxides are easier to reduce to metal compared to other chemical forms.

Key points about calcination:

• The process occurs without air, so oxidation doesn't take place.
• It helps in the removal of impurities and volatile substances.
• Calcination is often used to prepare ores such as limestone (calcium carbonate) which, when calcinated, form lime (calcium oxide).
• This technique is essential for the transition of complex ores to simpler and more reactive metal oxides.

Leaching

Leaching refers to a method in the extractive metallurgy field where a solvent, called a lixiviant, is used to dissolve desired metals from an ore. This process enables the separating of the metal from the rest of the ore and impurities, known as gangue. Leaching is particularly useful when dealing with low-grade ores that would not be feasible to process via other mining methods.

Important aspects of leaching include:

• The selection of an appropriate solvent that dissolves only the targeted metal substance.
• The remaining gangue is separated through various methods such as filtration.
• Leaching is advantageous for its ability to selectively extract metals, minimizing the amount of unwanted material processed.
• Common examples include cyanide leaching in gold mining or sulfuric acid leaching in copper extraction.

Smelting

Smelting is a fundamental process in metallurgy used to extract metals from their ores. Through high-temperature treatment and the use of reducing agents, smelting separates the metal from the ore's oxide or sulfide compounds. The process often involves mixing the ore with various fluxing agents to aid in the separation.

Characteristics of smelting include:

• Involves heating ores to temperatures where metals attain a molten state.
• A reduced metal phase is formed by separating metal oxides or sulfides.
• The use of fluxes aids in removing impurities by combining with them to form slag.
• Produces crude metal which requires further purification to obtain the desired metal purity.
• Examples include iron smelting in a blast furnace or copper smelting in a reverberatory furnace.

Slag

Slag is a by-product formed during metallurgical processes such as smelting. It comprises impurities like metal oxides which are extracted from the ore and mix with fluxes used in smelting to reduce their melting point. The resulting slag is lighter than the molten metal, allowing it to float on top where it can be removed easily.

Key facts about slag include:

• Composed of silicates, aluminates, and other metal oxides.
• Serves as a protective layer preventing oxidation of the molten metal underneath during processing.
• Commonly used in infrastructure, such as road construction and as aggregates in concrete.
• Utilized in environmental applications, like soil stabilization and fertilizers.