Manufacture Of Cement

Introduction

The right side of the periodic table having group number 13, 14, 15, 16, 17 and 18 are known as p – block elements. These elements have 3, 4, 5, 6, 7 and 8 electrons in their outer most shell, respectively. The last electron of these groups’ elements occupies the position in p – sub shell that is why are added as p – block elements. Their general configuration is ns2np1-6.

Some important properties of P-block

(1) Electron affinity

Electron affinity increase from left to right along the period amongst the p – block elements and it decreases from top to bottom. But group 15 is having exceptionally low values of electron affinity and is due to extra stability because of the presence of exactly half filled orbital in their valence shell. Similarly, elements of group 18 (noble gases) have zero affinities due to presence of complete octet which provides then stability.

(2) Metallic Character

The metallic character is governed by

(i) Size of atoms and

(ii) Ionization energy.

The elements having bigger size and low ionization energy has a greater metallic character. After combining both above mentioned factors we observes that the elements with above two properties are located in left corner of p – block and strong non – metallic elements are located at right corner and a diagonal strip of elements separates thus two, having in between properties are called as metalloids.

(3) Oxidation state

The p – block elements shows variety of oxidation states both positive and negative. Some of the p – block elements shows different oxidation state due to inert – pair effect, where their lower oxidation state is more predominant.

(4) Diagonal relationship

On moving diagonally across the periodic table the element show certain similarities

Elements of 2_nd period differ from their own group elements in some of the properties. This is due to the following reason:

(a) Small size

(b) Absence of vacant d-orbital

(c) High IP

Some important characteristics of p – block in tabular form

Boron Family

Group 13 elements are boron (B), aluminium (Al), Gallium (Ga), Indium (In) and thallium (Tl).

Boron is the only non – metal in this group others are metal. Non – metallic character of boron is due to its small size and high ionization energy. The general valence shell electronic configuration of these elements is ns²np¹.

General Introduction of Boron family

Boron

The name boron comes form the Arabic and Persian words for borax, its principal ore. It was first isolated in 1808 by Gay – Lussac and Thenard and independently by Sir Hymphry Day.

The pure element is shiny and black. It is very hard and in extremely pure form is nearly as hard as diamond, but too much brittle for practical use. At high temperatures it is a good conductor but at room temperature and below is an insulator.

Aluminium

Aluminium ranks thirds on the list of the ten most abundant elements in the earth’s crust, while its oxide is fourth among the ten most common compounds in the crust. It is the most abundant metal on the planet. Its name is taken from the Latin alumen for alum. It is soft, light weight and silvery, its existence was proposed by Lavoisier in 1787, it was named by Davy in 1807 and finally isolated by Orsted in 1825.

In its purest form the metal is bluish – white and very ductile. It is an excellent conductor of heat and electricity and finds use in some wiring. When pure it is too soft for construction purposes but addition of small amounts of silicon and iron hardened it significantly. Aluminium is the most abundant element in earth crust among this family

Gallium

Gallium is one of the elements originally predicted by Mendeleev in 1871 as aluminium, indicating that it should have the properties similar to aluminium. The actual metal was isolated and named by Paul – Emile Lecog de Boisbaudran in 1875.

At room temperature gallium is soft as lead and can be cut with a knife. Its melting point is abnormally low and it will begin to melt in the palm of a warm hand. Gallium is from one of the small numbers of metals that expands on freezing.

Indium

The element indium (Latin indicum, for the colour indigo) was discovered in 1863 by Reich and Ritcher. It is a rare metal, with an abundance similar to that of silver. It is generally found in deposits with zinc and refineries which produce this more common metal often sell indium as well.

The pure metal is so soft that you can “wipe” it onto other material in much the same way as lead. It is corrosion resistant.

Thallium

Sir William Crookes discovered thallium in 1861, positively identifying it by a green line in its spectrum (hence the name, which is from the Greek, thallos for “green twig”)

Thallium compounds are quite toxic and some have been used as rat poisons. A few compounds are used in glasses for special infra – red lenses.

General Trends in Physical properties

1. Density

Generally increases down the group but aluminium has an exceptionally low density.

2. Melting point and Boiling point

B to Ga decrease then Ga to Tl increases Ga has lowest M.P (29.8°C) and therefore liquid at room temperature.

3. Atomic radii and Ionic radii

On moving from B to Tl the size increases due to addition of new energy shells at each step down the group but Ga is smaller than Al.

4. Ionization Energy

Generally IE decreases down the group but Ga has higher IE than Al exceptionally due to smaller in size as compared to Al.

5. Metallic character

Electropositive character increases down the group hence metallic character also increase down the group but aluminium is having high metallic character than Gallium due to low IE than Ga.

6. Oxidation state

The elements of B group have three valence electron i.e .two in s – subshell. The most oxidation state should be +3 but due to small size of boron it can not lose its valence electrons to B3+ ion, and combines with other atoms through covalent bonds. Except boron, other elements also exhibit +1 oxidation state and down the group +1 state becomes more stable.

7. Reducing property

Down the group the reducing property decreases. Al > Ga > In > Tl

Group trends in chemical properties

1. Hydrides

None of the element from 13 group reacts directly to hydrogen. However a number of hydrides of these elements have been prepared by indirect methods. Boron hydrides are called boron.

Two types of boranes:

(a) BnH_n+4, called nidoboranes

(b) BnH_n+6, called arachnoboranes

The simplest borone is diborane B2H6

Other elements of this group forms only a few stable hydrides of NH₃ types. AlH₃ is colour less polymeric solid of formula (AlH3)x and contains Al ⎯ H ⎯ Al bridges. A complex hydride of aluminium is a very good reducing agent and used as a regent in lab is Li[AlH₄], Lithium aluminium hydrides. It is a white crystalline solid.

Gallium also form Li[GaH₄]

2. Oxides and hydroxides

The 13 group elements forms oxides and hydroxides of composition M2O3 and M(OH)3 respectively. As we move down the group the acidic character in oxides and hydroxides decreases and basic character increases due to decrease in strength of M ⎯ O bond due to which basicity increases.

B₂O₃ > Al₂O₃ > Ga₂O₃ > In₂O₃ > Tl₂O₃

B(OH)₃ > Al(OH)₃ > Ga(OH)₃ > In(OH)₃ > Tl(OH)₃

Trend in acidity

Basic Character increases

Oxides and hydroxide of Al and Ga shows amphoteric nature.

2Al(OH)₃₍s) + 3H₂SO₄(aq) → 3H₂SO₄ (aq) → Al₂ (SO₄)₃(aq) + 6H₂O

Al(OH)₃(s) + NaOH(aq) → Na[Al(OH)₄](aq)

Preparation of oxides of B and Al

B₂O₃ is a good dehydrating agent and reacts with water to form orthoboric acid (H₃BO₃).

B2O3 is an acidic oxide dissolves in alkalies to give borates as well as metaborates.

It is amphoteric in nature.

Crystalline alumina, Al₂O₃, exists in many forms, one of these called corundum, is very hard and is used as an abrasive.

3. Halides

Brorn and aluminium combine with halogens to form trihalides having the general formula MX3.

Halides of boron are covalent, boron trihalides exist as monomer having planar triangular geometry. BX3 acts as lewis acid.

Among boron trihalides the order for strong lewis acid is

BF3 < BCl₃ < BBr₃

Aluminium

Aluminium although is a reactive metal according to the electrochemical series, it is revered unreactive due to the formation of a oxide film on its surface.

Occurrence

It does not occur in native form. The important ores of aluminium are

(i) Bauxite (Al₂O₃.2H₂O)

(ii) Cryolite (Na₃AlF₆)

(iii) Feldspar (KAlSi₃O₈)

(iv) Mica KAlSiO₁₀(OH)₂

Extraction

Aluminium is extracted from bauxite in a two stage process.

(i) Stage – I

It involves the extraction of alumina (Al₂O₃) from bauxite.

(ii) Stage – II

Involves extraction of pure aluminium from Al₂O₃ by its electrolysis in molten cryolite [Na3AlF6].

(a) Purification of Bauxite

Bauxite contains SiO₂, iron oxide and titanium (IV) oxide as impurities. The bauxite ore is digested with a hot concentrated solution of NaOH at about 473 – 523 K and 35 – 36 bar pressure.

Aluminium oxide and silica dissolves to form sodium aluminate and sodium silicate respectively leaving behind iron oxide and TiO₂ which are filtered off.

The filtrate containing sodium aluminate and sodium silicate is diluted and seeded with freshly precipitated aluiminium hydroxide which induces the precipitation of aluminium hydroxide leaving behind sodium silicate in solution.

The auminium hydroxide is filled dried and calcinated at 1473 K to yield pure alumina (Al2O3)

(b) Electrolysis of pure alumina (Hall – Heroult process)

Alumina is obtained from alumina by electrolysis. Pure alumina is dissolved in molten cryolite (Na3AlF6) and is electrolysed in an iron tank lined by carbon internally. Carbon lining serves as cathode, while the number of dipping carbon rods in the fused electrolyte serves as anode.

The electrolyte composition ranges are Na3AlF6(80 − 85%), CaF2(5 – 7%), AlF3(5 – 7%) Al2O3
(2 – 8%).

Cryolite improves the electrical conductivity of the cell as alumina is a poor conductor. Also, cryolite lowers the melting point of the mixture to about 1250K. When electric supply is given aluminium is liberated at cathode and gets collected at the bottom of the tank from where it is removed through outlet. Oxygen liberates at anode and combines with the carbon of the anode to produce carbon monoxide which either burns or escape out.

At cathode: Al³⁺ (melt) +3e− → Al

At anode: O⁻² → O + 2e−

C(s) + O → CO(g)

CO + O → CO (g)

CO + O → CO₂ (g)

Properties of Aluminium

Physical properties

Aluminium is a soft light silvery – white metal which soon looses its shine due to formation of a layer of oxide on it. It is malleable and ductile. It m.p. is 932K. Electrical conductivity is twice than copper on weight to weight basis.

Chemical properties

1. Action of water

It does not react with pure water but it is corroded by water containing salts.

2. Action of air

It is not affected by dry air but highly electropositive and redily reacts with moist air and forms a hard protective layer of Al2O3, which renders it positive

3. Action of Acids

Aluminium dissolves in dilute mineral acids and produces hydrogen.

4. Reaction with strong alkali

Aluminium gives aluminate with liberating hydrogen too.

Uses of Aluminium

Aluminium is extensively used in industries as well as in every day life.

1. It is used in making many alloys, utensils, construction aerospace, window angles.

2. It is a good conductor of electricity, thicker cables of aluminium are used for transmission of electricity.

3. Aluminium foil is used in wrapping cigarettes, confectionary etc.

4. Aluminium is also used in the aluminothermic process for production of chromium and manganese from their ores.

Carbon Family

The carbon family or group 14 consists of carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn) and Lead (Pb). Carbon and silicon are non metals; germanium is a semi – metal or metalloid whereas tin and lead are metals. All the elements of group 14 have four electrons in their valence shells. Their general electronic configuration is ns²np². Carbon and silicon are the most abundant elements in the group in earth crust.

General introduction to the elements of carbon family Carbon

Carbon is the sixth most abundant element in the known universe but not nearly as common on the earth, despite the fact the living organisms contain significant amounts of the elements. Carbon commonly occurs in environment as methane (CH₄) and carbon dioxide (CO₂). Carbon exists in several forms called allotropes. Diamond is one of very strong crystal lattice, known as a precious gem. Graphite is another allotrope in which carbon atoms are arranged in planes which are loosely attracted to one another (hence used as lubricant). The recently discovered fullerenes are yet other form of carbon. Carbon has a very high melting and boiling point and rapidly combines with oxygen at elevated temperature. An important (but rare) radioactive isotope of carbons, C – 14 is used to date ancient objects of organic origin.