What is Lead?- Its Compounds, Manufacturing Process and Uses

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What is Lead?

Lead is a chemical element in the periodic table’s group-14 metals, with the symbol Pb and atomic number 82. It’s now utilized to make storage batteries and an anti-knock compound (lead tetraethyl) for pollution management.

Its density is 11.34g/cm3 and is found in a cubic close-packed crystal structure. Lead’s low melting point indicates that none of its four valence electrons engage in metallic bonding. Lead usually has a +2 oxidation number rather than a +4.

Since prehistoric times, lead has been utilized since it is relatively easy to extract (for a metal). In the Roman Empire, lead was widely available, and it was used in dishes, plumbing, coins, and statues. People utilized it for thousands of years until it was discovered to be deadly near the end of the nineteenth century.

Lead is classified as a post-transition metal except in powdered form. It is not as reactive as many other metals. It has a brittle metallic quality and frequently forms covalent connections with other elements. Rings, chains, and polyhedrons are easily formed when the element attaches to itself. Lead, unlike most metals, is soft, dull, and poorly conducts electricity.

lead metal

History of Lead:

The Egyptians were probably the first to extract lead, which they utilized to create miniature sculptures. Lead compounds have also been discovered in Egyptian ceramic glazes. By 2000 BC, lead was being used to make coins in China.

Lead’s corrosion-resistant characteristics were initially recognized by the Greeks, who used it as a protective covering on ship hulls. It is a current application in which lead compounds are still employed. As a result, the Romans began mining massive amounts of lead for their extensive water systems.

Roman lead output was estimated to reach around 80,000 tonnes per year by the first century AD. Baths were lined with lead sheets, and lead piping was made by wrapping lead metal sheets around a rod and welding the edges together. Lead plumbing, used until the twentieth century, helped prevent corrosion and caused widespread lead poisoning.

Because of its fire resistance, lead was utilized as a roofing material in several parts of Europe by the Middle Ages. The lead roofs of Westminster Abbey and St. Paul’s Cathedral in London date back hundreds of years. Pewter (a tin-lead alloy) was later employed to manufacture mugs, plates, and flatware.

Lead’s high density was found as an appropriate material for bullets – or lead shot – following the advent of weapons. In the mid-17th century, lead shot was initially made by allowing molten lead droplets to fall into the water and form a spherical shape.

Ores of Lead:

Galena, a Lead sulfide mineral (PbS), is the most important ore mineral. It has the potential to contain 86 percent lead. Its cubic, fully cleavable masses and metallic, Lead-gray cubic crystals (isometric system) are unusual and characteristic. The specific gravity is 7.4-7.6, and the hardness is 2.5.

Galena is a common mineral that forms enormous, irregular masses in dolomitized limestone, metamorphism zones, and volcanic rocks due to hydrothermal solutions. It is frequently rich in silver and can be exploited as a silver ore.

Compounds of Lead:

1. Lead Oxide:

Dissolving Pb3O4 in a weak nitric acid solution yields Pb (II) oxide, or PbO2. At temperatures above 300 °C, it decomposes to PbO, making it a potent oxidizing agent. PbO is a metal oxide in two varieties: litharge and massicot. In a reverberatory furnace, molten Pb is oxidized in the air at 600 °C to produce the litharge form. Pb is heated in air to make a massicot.

PbO is a basic metal that reacts with acids to generate Pb (II) salts. It features a unique layer structure, with four oxygen atoms forming the square pyramid’s base and Pb metal at the vertex.

2. Lead Halides:

The yellow solid PbF4 is the only stable tetrahalide of Pb. When heated, it decomposes into PbF2 and F2. Fluorine or HF can be used to make it by reacting with lead compounds. PbI4 does not occur because other tetrahalides are less stable.

Lead dihalides are crystalline solids that are marginally soluble in cold water but significantly more so in hot water. Photosensitive elements PbCl2 and PbBr2 deposit electromagnetic spectrum radiation.

3. Lead Carbonate:

In nature, lead (II) carbonate is found as cerussite, with the chemical formula PbCO3. NaHCO3 precipitates it from Pb (NO3)2 solution at low temperatures. Under the name white-Pb, it has been used as a white pigment. However, due to TiO2’s non-toxic nature, it is rapidly being replaced.

4. Lead Acetate:

Pb (CH3COO)2 or lead acetate is a white crystalline chemical compound with a sweet taste. Pb, acetic acid, and hydrogen peroxide are boiled together to make it. It is used to clean the eyes in medicine. It’s also utilized in the dyeing process to make various lead compounds and mordant.

Manufacturing Process of Lead:

The primary manufacture of lead required three phases:

1. Ore Concentration:

Ores of lead and zinc (typically sphalerite, ZnS) are frequently found combined and can contain silver, copper, and gold. The ore must first be separated from clays and other silicates (‘gangue’), followed by lead and zinc ore separation.

The method utilized is froth flotation, which involves a series of phases that progressively give a higher lead ore concentration. The ore, which contains gangue (normally 3–8% lead), is processed with water to a particle size of 0.25 mm, equivalent to fine sand. It is then combined with water and a foaming agent (a detergent) and agitated vigorously by air to produce a fine suspension with a froth of bubbles on top.

A succession of tanks is used to carry out the process. The lead and zinc minerals attach to the air bubbles transported to the surface because they are less easily wetted than the gangue. The lead and zinc ores are skimmed off as the rock particles sink.

The zinc ore is then separated from the lead ore. The zinc ore sinks, and the lead ore is skimmed out after a chemical called a depressant is applied that is soluble in water (for example, zinc sulfate). After adding a reagent like copper (II) sulfate, the zinc ore floats and may be skimmed off. The flotation tanks’ lead ore concentrate now contains around 50 percent lead and about 0.1 percent silver, a little useful proportion.

2. Smelting:

Although single-stage technologies with lower energy use and emissions are also utilized, smelting is normally two-stage. After combining with limestone, the filtered, concentrated ore is roasted on a moving belt in the air or oxygen-enriched air. Sulfide is mostly transformed to lead(II) oxide: Sulfur dioxide gas can be purified and utilized to manufacture sulfuric acid.

2PbS(s) + 3O2 (g) = 2PbO(s) +2SO(s) + 2SO2(g)

Sintering is the process of heating lead (II) oxide and forming lumps. The lumps (sinter) are crushed and sorted to an appropriate size for treatment in a blast furnace, similar to an iron blast furnace but smaller.

Each lump of graded sinter is mixed with roughly 7% of its mass in coke and limestone. The coke is added for two purposes: a reducing agent and a heat source when it reacts with the air pushed into the furnace, similar to how iron is made.

The limestone provides material for the slag flux, which contains impurities. The lead (ll) oxide is converted to molten lead as the mixture is delivered to the top of the blast furnace. The reducing agents are carbon and carbon monoxide, created from coke: The molten lead is tapped from the furnace’s bottom and cast into 4-tonne ingots or placed in a ‘holding kettle,’ which maintains the metal molten during the refining process.

PbO(s) + C(s) = Pb (l) + CO (g)

Pb(s) + CO (g) = Pb (l) + CO2 (g)

3. Refining:

The bullion is heated just to the point of melting. Copper sulfide and solid copper rise to the surface and are skimmed off. The arsenic is eliminated by blasting the lead with air and skimming off the slag, including arsenic oxides. The Parkes procedure is used to remove silver from a surface.

A silver-rich zinc crust forms and is removed after approximately 2% zinc is added to the lead. At 740 K (its melting point is 693 K), more zinc is added, and the bath is cooled to just over its melting point. A solid silver/zinc crust separates during cooling, rises to the surface, and is continuously removed.

Transferring the lead to a ‘dezincing kettle’ at 863 K removes the zinc. The desilvered lead has around 0.6 percent zinc and 0.0004 percent silver. When the zinc vaporizes, vacuum distillation at 860 K is used to dezise it.

Finally, by mixing sodium hydroxide into the molten lead at 760 K, all residues of antimony and zinc are eliminated (its melting point is 600 K). The sodium zincate and antimonate form a skin on the molten lead’s surface scraped off. The refined lead has a purity of 99.99 percent.

Properties of Lead:

  • Lead (Pb) is a highly malleable white shiny metal with a soft texture.
  • In addition to being corrosion resistant, the metal is a good conductor of electricity.
  • When burned in air, the powdered metal generates a bluish-white flame.
  • Lead fluoride is formed when fluorine is combined with oxygen at ambient temperature.
  • The melting point of lead is 327°C, which is quite low.

Uses of Lead:

  • The most common use for lead is in batteries and lead-acid accumulators, in which lead is present in both electrodes. The anode, or negative electrode, is formed of a lead grid, while the cathode, or positive electrode, is made of the oxide PbO2, with sulphuric acid as the electrolyte.
  • Lead shot is manufactured from antimony and arsenic alloyed lead.
  • In the form of tetraethyl lead Pb (C2H5)4, lead is used as an anti-knock addition in gasoline. This application now accounts for barely 1% of global consumption; only a few decades ago, its proportion of the lead market was as high as 20%.
  • Lead sheets and Terne plate (a steel sheet covered with a Pb–15 to 20% Sn alloy) are used as roofing materials in the construction sector.
  • Lead sheet (as Cu-alloyed Pb) is also employed as an anti-acid lining in the chemical industry, such as in the sulphuric acid production process known as “lead chambers.”
  • Also found in crystal glass and achromatic lenses made from flint glasses with a high refraction index.
  • Some soldiers contain lead, which is also utilized in soundproofing and radiation protection.

Advantages of Lead:

  • Lead is less purchased due to its toxicity. Less all over the world, this naturally leads to a reduction in its expense.
  • Because lead melts at a low temperature, it’s easy to work. It’s ideal for soldering.

Disadvantages of Lead:

Because lead is extremely hazardous, its usage can pollute the earth and, more crucially, water supplies. It can contaminate drinking water and cause illness. risks to human health as well as ecosystem disruption

Conclusion:

Lead is a chemical element found in nature as one of the basic chemical building blocks. Lead, a bluish-white metal, is found in trace amounts in soil, rocks, and water and is completely safe.

Toxic lead levels are now congregated near present and former lead facilities and mines due to centuries of mining and smelting. In many parts of the world, its widespread use has caused severe environmental damage, human exposure, and public health issues.

References:

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