Silicon from silicate minerals

The iron formed in a blast furnace, called pig iron, contains impurities that make the metal brittle. These include phosphorus and silicon from silicate and phosphate minerals that contaminated the original ore, as well as carbon and sulfur from the coke. This iron is refined in a converter furnace. Here, a stream of O2 gas blows through molten impure iron. Oxygen reacts with the nonmetal impurities, converting them to oxides. As in the blast furnace, CaO is added to convert Si02 into liquid calcium silicate, in which the other oxides dissolve. The molten iron is analyzed at intervals until its impurities have been reduced to satisfactory levels. Then the liquid metal, now in the form called steel, is poured from the converter and allowed to solidify. 

The minerals in the earth s crust are those resources which we use for constructing our technical world. The element silicon, the sister element of carbon, is the second most abundant element after oxygen in the earth s crust. Therefore silicon is an ubiquitous element and an interesting resource for new applications. However, the most abundant silicon-containing minerals are sand and silicates. The preparation of silicon from these minerals uses high energy—consuming processes to produce very pure elemental silicon. To achieve a sustainable production line of silicon in this important field of chemistry, alternative routes have to be developed. 

Some of the diversity that characterizes the properties and compositions of the silicate minerals stems from the ability of the aluminum ion (Al ) to substitute for silicon in the tetrahedral unit. When silicate tetrahedra in a mineral are replaced by aluminum-containing tetrahedra, concomitant changes occur in the size of the tetrahedron (usual Si—O bond length = 0.160 nm. A1—O bond length = 0.178 nm) and in the cations or protons that balance the tetrahedral unit charge. Regular substitutions with distinct chemistries and structures lead to the formation of groups of discrete minerals called aluminosilicates. 

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

The primary source of aluminum is the mineral bauxite, which is found in large deposits in Australia, Jamaica, and Sumatra. Transportation costs are high, and many steps are required to process the ore. Because bauxite ore is usually contaminated with the oxides of iron, titanium, and silicon, pretreatment is required before extraction of the metal can begin. The crude bauxite must be crushed, washed, and dried in a kiln to free it from silicate clays, which interfere with later purification steps. 

Silicon is an abundant element that makes up 23% of the earth s crust, primarily in the form of silicate minerals and Si02 (sand, quartz, etc.). The name silicon is derived from the Latin names si lex and silicus, which refer to flint. The element is a brittle solid that has the diamond... 

Silicon is second only to oxygen in weight percentage of the earth s crust ( 28%) and is found in an enormous diversity of silicate minerals. Germanium, Sn, and Pb are relatively rare elements ( 103 wt%), but they are well known because of their technical importance and the relative ease with which Sn and Pb are obtained from natural sources. 

Silicon differs from carbon in that all of the element existing in nature is in an oxidized form, as SiCb or silicate minerals. The first step in the synthesis of sihcones then becomes the reduction of silica to an active reduced form of sihcon. The usual starting point is the electrothermal reduction of Si02 with carbon (equation 1). The elemental sihcon can be converted to reactive SiCLi by chlorination (equation 2) or to hydrochlorosilanes by reaction with HCl (equation 3). The importance of the latter materials in silicone technology will be described later. 

Silicon (from L. silex, flint), the fourteenth element in the periodic table, is a congener of carbon, in group IV. Silicon plays an important part in the inorganic world, similar to that played by carbon in the organic world. Most of the rocks that constitute the earth s crust are composed of the silicate minerals, of which silicon is the most important elementary constituent. 

The importance of carbon in organic chemistry results from its ability to form carbon-carbon bonds, permitting complex molecules, with the most varied properties, to exist. The importance of silicon in the inorganic world results from a different property of the element —a few coiiipounds are known in which silicon atoms are connected to one another by covalent bonds, but these compounds are relatively unimportant. The characteristic feature of the silicate minerals is the existence of chains and more complex structures (layers, three-dimen sional frameworks) in which the silicon atoms are not bonded directly to one another but are connected by oxygen atoms. 1 he nature of these structures is described briefly in later sections of this chapter. 

In view of the importance of alkoxysilanes and alkoxysiloxanes as precursors for glasses and ceramic materials, a process of obtaining these from portland cement and silicate minerals appears to be of industrial importance (55). If mild conditions are employed, the specific silicon-oxygen framework in the original mineral is often retained in the final alkoxysilane or alkoxysiloxane obtained, for example. 

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at midocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals. 

The outermost layer of our planet, the crust, contains the accessible mineral wealth of the planet. The eight most abundant elements in the crust (Table 1) make up 98.5% of the mass of the crust [10], The most common metal, silicon, is never found in its elemental form in nature. Instead, silicon is combined in silicate minerals, which make up more than 90% of the mass of the Earth s crust [11], Depending on the composition and formation conditions, silicate minerals have structures that range from individual clusters (orthosilicates) to three-dimensional networks (tecto-silicates) [11], These minerals can be contained in relatively pure single mineral deposits or, more commonly, in rocks such as granite that are made up of one or more mineral species. 

---