Extraction of Metals

Introduction

     There are 109 elements known at present. 92 elements occur in nature, while the rest are man-made. All materials on the Earth are made of these naturally occurring elements. These are classified into metals, non-metals, and metalloids. The occurrence and extraction of some lighter metals is described here.

Occurrence of Elements in Nature

     A large number of elements occur inside or around the earth. Less reactive elements, like platinum, gold, nitrogen, oxygen etc., occur in free (native) state. Chemically active elements always occur in the combined state i.e. in the form of compounds containing that element. For instance, fluorine occurs as fluorides, sulphur occurs as sulphides etc. Metals that occur in the combined form exist as their oxides, carbonates, sulphides and silicates.

Earth as a Source of Elements

     Most elements are formed due to fusion of simple nuclei to give heavier nuclei i.e., nucleosynthesis or nuclear fusion reactions taking place in the stars. Atoms of these elements began to fall together about six billion years ago to form a system, presently called the solar system. A part of these elements formed in space and such a condensed phase of matter, formed the Earth.

Structure of the Earth

     During the formation of the Earth, cooling of gaseous mixtures took place and molten earth condensed to form different earth zones or layers. Each layer that formed on top of the other had a definite chemical composition and thus a distinct distribution of elements within it. The innermost layer or core was metallic and molten, the intermediate core was sulphide and a silicate layer formed on top, which was exposed to the Earth's atmosphere layer, creating the Earth's solid crust. This structure formation was suggested by Goldschmidt.

Rocks of the Earth

     Rocks are polycrystalline solid portions of the earth's crust having a definite texture and mineral content. Usually two or more minerals combine to form different rocks.

Weathering of Rocks

     Igneous rocks are composed of alumino-silicates of metals such as iron, calcium, magnesium and sodium. These rocks are continuously exposed to water, CO₂ and humic acids causing carrying away of Na⁺, Ca²⁺ ,Fe²⁺ and Mg²⁺ions. The extracted salts move away to seawater and the insoluble residue consisting of TiO₂, SiO₂ and Fe₂O₃ is left behind. This process is called weathering of rocks or selective extraction from the rocks. The ease of extraction depends upon the charge-to-size ratio of the ions. Thus, the weathering process produces oxides, carbonates etc., and the soluble salts get extracted to enter the sea water.

Accumulation of Elements by Biological Systems

     Some living organisms accumulate specific element(s) important for their biological activity. Typical examples are,
     * Carbon and Hydrogen are essential and major ingredient for our body.
     * Iodine in seaweeds.
     * Magnesium in chlorophyll.

Elements in Sea

     Apart from the salts which flow down to the sea waters due to weathering of the rocks, a variety of sources, such as land and sea volcanic activity, seismic activity, deposit large amounts of minerals and metals in the ocean. Some of these dissolve in the water and may rest at the ocean bed.

Chemistry of Salt Formation in Seawater

     The chemistry of seawater is very complex. Many multistep equilibrium operate simultaneously.

Occurrence of Metals

     Metals occur in nature in the free as well as in the combined states. The most unreactive metals i.e. which are not affected by air and water, like silver, gold and platinum are generally found in the free state. Most metals however, are found in the combined form as minerals.

Ores of Some Common Metals

     Metallic elements occur in the Earth's crust combined with non-metallic elements. After suitable treatment, the compounds are reduced to produce the metals.

Metallurgical Processes

     The process of extracting a metal from its ore and refining it, is called metallurgical process or simply as metallurgy.

Extraction of Metals

     Once the ore is calcined or roasted, the process of reduction obtains the metal. The extraction of metal from the calcined or roasted ore consists of two steps viz.,
     * Removal of the Earthly impurities.
     * Reduction of the ore to the metal.

Chemical Reduction Methods

     In chemical reduction process, the choice of reducing agent depends upon the chemical reactivity of the metal.

Electrolytic Reduction Method

     Certain metals can be obtained by affecting the reduction of the corresponding cations electrolytically. For example, sodium metal is obtained by electrolyzing molten sodium chloride.

Amalgamation Process

     Noble metals dissolve in mercury to form amalgams. In this process a mixture of finely powdered ore and water, known as slurry, is allowed to flow over copper or brass plates coated with mercury and arranged in a slanting position. The metal particles from the ore form an amalgam with Hg and are retained on these plates. The amalgam is scrapped off from the plates, and distilled in iron retorts, when mercury distills over leaving the free metal behind.

Refining of Metals

     The metals so extracted are further refined by suitable methods. The electrolytic method is the most suitable method for refining the non-ferrous metals. The metals produced by any method given above are not very pure. These need further refining.

Mineral Wealth of India

     India is favourably placed in its geographical and geochemical conditions for mineral wealth and has a very bright potential future for minerals. Indias coal, iron bauxite and silica deposits are well known. Off shore oil explorations in the last two decades have yielded good results and efforts are going on to tap the mineral wealth buried under the sea. In the past also, India has been famous for diamonds and silver deposits.