Smelting in Metallurgy and Metal Extraction

Smelting is a process in which the base metal is extracted from the ore by applying heat to it. It is one kind of extractive metallurgy. It is used for the extraction of many metals like copper, aluminium, iron, silver, lead and many other base metals from their respective ores. For the decomposition of the ores, smelting uses heat and chemical reducing agents that eliminate other elements in the form of gases or slag that leaves behind the metal base. 

The reducing agents for smelting are commonly the fossil fuels of carbon such as coke or charcoal that were used in earlier times. Due to the low potential energy of the bonds in carbon dioxide during high temperatures, the oxygen in the ore binds to carbon. Iron smelting is usually carried out in a blast furnace to produce pig iron, which is then converted into Steel.

Carbon is applied as a chemical reactant in order to remove oxygen from the ore that earrings the purified metal element as a product. The source of carbon is oxidised in two different stages. In the first stage, carbon monoxide is produced when the carbon is combusted with oxygen in the air. After many successive interactions of carbon monoxide with oxygen, which is present in the ore, the entire oxygen would be removed leaving behind the raw base metal. 

In the second stage, the carbon monoxide reacts with the ore, where it attaches itself to another oxygen atom that finally releases carbon dioxide. It becomes necessary to use flux such as limestone as most of the earth is in its impure form in order to remove the accompanying rock gangue and slag. This is known as a calcination reaction that often removes carbon dioxide. Also, the process of electrolytic reduction of aluminium is generally referred to as aluminium smelting.

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Smelting Process

Usually, smelting means the extraction of metal from its ore by the process that involves heating and melting at the same time. The process of smelting is much more than just melting the metal out of its ore. Most of the ores are the chemical compounds of metals and other elements such as oxygen, where the ore is present as an oxide, sulphur, where the ore is present in the form of sulphide or carbon, and oxygen together results in the formation of carbonate. 

The worker has to make the compounds undergo a chemical smelting reaction to extract these metals. Therefore all the smelting processes such as iron smelting, aluminium smelting, copper smelting and other such base metal smelting use suitable reduction substances that combine with the oxidising elements to ultimately free the metal. The process usually takes place in three steps which are roasting, reduction, and fluxes.

1. Roasting: Roasting is a process followed in the case of sulfides and carbonates that removes the unwanted carbon or sulphur and leaves behind an oxide that can directly be reduced. Roasting is usually carried out in an oxidising environment. A few examples of roasting are as follows:

• A common ore of copper known as Malachite is primarily copper carbonate hydroxide Cu2(CO3)(OH)2. Between 250 degrees centigrade and  350 degrees centigrade, the mineral ore undergoes decomposition to copper oxide (2CuO), carbon dioxide (CO2) and water (H2O) in several stages. Later the water and carbon dioxide is expelled into the atmosphere leaving behind the copper (ll) oxide, which is then directly reduced to copper by the reduction process.

• The most common ore of lead is known as Galena which is primary lead sulphide (PbS). Lead sulphide is initially converted into lead sulphite (PbSO3) by oxidation reaction, which then thermally decomposes into lead oxide, and Sulphur dioxide gas is released. After the sulphur dioxide gases are expelled, the lead oxide is reduced, as mentioned below in the reduction process.

2. Reduction: The final step in the smelting process is applied to convert the oxides to elemental metals. A reducing environment that is made by the combustion in the furnace that is air starved is given by carbon monoxide to take out the oxygen atom finally from the molecule of the raw metal. In terms of the absolute temperature and in terms of the melting point of the base metal, the required temperature varies over a large range.  

After the reduction step is completed, Flux and slag can provide a secondary service. They are said to be providing a molten cover over the purified metal that prevents the contact of the metal from the oxygen as the metal still remains hot to easily get oxidised. This also prevents the impurities from developing in the metal. For instance:-

• At roughly 1250 °C, iron oxide becomes the metal iron where the absolute temperature is almost 300 degrees below the iron’s melting point, which is equal to 1538 °C (2800.4 °F or 1811.15 K).

• At roughly 550 °C, mercury oxide becomes vapour mercury where the absolute temperature is almost 600 degrees above the melting point, which is equal to  -38 °C (-36.4 °F or 235.15 K).

3. Flux: Flux is used by metal workers in smelting for various purposes. The most common of them is catalysing a reaction and chemically binding the impurities and the reaction product with the metal. Calcium that undergoes oxidation in the form of lime is often used for this purpose. As it has the capability to react with the carbon dioxide and sulfur dioxide that is produced during the stage of roasting and smelting so as to keep them out of the working environment. 

Environmental Impacts

The smelting industry has a serious effect on the environment that produces a lot of slag and wastewater is released into the environment along with toxic metals such as copper, silver, iron, Cobalt and selenium into the atmosphere. This melting industry is also known to be releasing toxic gas such as Sulphur dioxide that contributes to acid rain which in turn acidifies the soil and water.  Some of the environmental impacts are as follows:-

1. A staggering amount of toxic air pollution is emitted by the copper smelters in the United States. More than 50 tons of lead, 30 tons of arsenic, and 20 terms of selenium are emitted each year into the atmosphere. In the towns of Winkleman and Hayden of Arizona, the ambient arsenic level is more than 150 times higher than Arizona's health guidelines, where only two smelters are operating currently. There the cancer risk for the people living in those towns has been estimated to be one in hundred in hundred by EPA.

2. The gasification product suggests benzene cyanide ammonia, naphthalene anthracene phenol, and cresols are the toxic gasification products discharged by iron and steel, which forms wastewater pollutants. The above-mentioned elements and the range of more complex organic compounds collectively known as polycyclic aromatic hydrocarbons make extremely toxic wastewater pollutants. The pollutants that are generated by other smelting processes are very much with the base metal ores. For example, aluminium smelting typically generates fluoride, benzo(a)pyrene, antimony and nickel along with aluminium. Copper smelting usually discharges cadmium, zinc, lead, arsenic and nickel in addition to copper.

3. The labourers who are working in the smelting industries are reported to be developing respiratory illness that inhibits their ability to perform physical work ok in the smelting industry that is demanded by their jobs.