Manufacture of Cement

At the present time, Cement is one of the most important building materials.

When a strongly heated mixture of limestone and clay is mixed with a small amount of water, and this mixture allowed to set for a few hours,  this mixture resembles a hard stone-like substance. After setting, the stone-like mass looks like as famous Portland rock of England, and hence it was named Portland cement.

Chemically cement is defined as the varying composition of a finely ground mixture of calcium aluminates and silicates, and this mixture is hydrated by mixing with water, therefore, form a rigid solid structure with good compressive strength.

Cement is a mixture of the following compounds:

Composition of cement

Average chemical composition of cement:

Average chemical composition of cement

Raw Materials

The essentials raw materials for the manufacture of cement are limestone and clay which supply all the four principal ingredients, such as CaO, Al₂O₃, SiO₂, and Fe₂O₃. Calcium oxide and iron oxide, these both substance are obtained from limestone, while silica and alumina are obtained from the clay. Thus raw materials are two types---

1. Calcareous materials: limestone, calcium carbonate sludge, Marl, Chalk, and Alkali waste. These supply mainly CaO.

2. siliceous materials: Clay, Blast furnace slag, siliceous stone, Shale, Slate, etc. These supply silica, iron oxide, and alumina.

Cement making process

There are two methods of manufacturing cement.

1. Wet process.

2. Dry process.

The choice between two processes is usually governed by the following factors---

1. Physical conditions of raw materials.

2. Climate surrounding the place of manufacture.

3. Cost of fuel.

If limestone and clay are soft, the climate is fairly moist and the cost of fuel is cheap, the wet process is preferred.

In the wet process, the limestone is crushed in a suitable mill to prepared particles of suitable size, and clay is washed with water in a wash mill to remove flint and a slurry containing about 60% water is obtained. Crushed limestone and clay slurry are mixed in a special type of ball mill. The resulting slurry called raw slurry which further ground in tube mills and then stored in the collecting tanks, where after analysis addition is made to adjust the proportion. The raw slurry from the wet process is now introduced into the hopper provide on the upper part of a rotary kiln, which consists of an inclined steel cylinder which is about 200-300 ft long and 7-12 ft in diameter and the lower end provided with a fire hood, containing a short rotating cylinder. The charge moves forward slowly, due to the rotary motion (30-60 turns per hour) given to the kiln. The upper portion of the kiln is lined with ordinary brick, the middle portion is lined with thick fire brick and the lower firing zone is lined with fire clay bricks. A blast of burning coal dust and the air is blown from the lower end for a long time. The hot air obtained by cooling the hot clinker is also introduced into the kiln from the lower end. During its passage, the slurry first loses water in the upper portion and the remaining water is eliminated at about 750⁰C region when the charge centre in the middle portion of the kiln, the temperature rises to about 1000⁰C. At this high temperature, limestone is decomposed into CaO and CO₂. When the charge moves into the lowest portion, the hottest zone of the kiln, the temperature rises to about 1400⁰C to 1600⁰C. At this temperature, the mixture is partly fused and the chemical reaction between CaO and aluminium silicate (clay) takes place resulting in the formation of calcium silicates and aluminates.

Reactions

 CaCO₃ ----> CaO + CO₂

2CaO + SiO₂ ---> 2CaO.SiO₂ (Dicalcium silicate)

3CaO + SiO₂ ---> 3CaO.SiO₂ (Tricalcium silicate)

2CaO + Al₂O₃ ---> 2CaO.Al₂O₃ 

(Dicalcium aluminate)

3CaO + Al₂O₃ ---> 3CaO.Al₂O₃ 

(Tricalcium aluminate)

4CaO + Al₂O₃ + Fe₂O₃ ---> 4CaO.Al₂O₃.Fe₂O₃ 

(Tetracalcium alumino ferrite)

The resulting mass is either greenish-black coloured or grey coloured, and is called clinker. These are cooled in a rotary cooler and crushed. Generally, (2-3)% gypsum is added and finely ground to an exceedingly fine powder in grinding machines. The cement is filled in airtight bags to exclude moisture.

 

Flow sheet for manufacture of cement

        

 Synthesis of Acetanilide Paracetamol Phenacetin

Acetanilide, Paracetamol, and Phenacetin are the class of antipyretic analgesics drugs. Antipyretic analgesics are the class of drug that relieves mild to moderate pain, such as headache, myalgia, and arthralgia, and also lower the body temperature in pyrexia, that is in situations when the body temperatures have been raised above normal. In therapeutic doses, they do not have any effect on normal temperature. These drugs exert their action on the heat-regulating centre in the hypothalamus. The common group of compounds that used as antipyretic analgesics drugs are---

1. Aniline and para aminophenol derivatives such as acetanilide, paracetamol, phenacetin etc.

2. Salicylic acid derivatives such as aspirin, salol, sodium salicylate etc.

3. Quinoline derivatives such as cinchophen, neocinchophen etc.

4. Pyrazolones and pyrazolodiones derivatives such as phenazone, aminophenazone, phenylbutazone etc.

5. N-arylanthranilic acids derivatives such as mefenamic acid, meclofenamate sodium etc.

Synthesis of acetanilide

Aniline is the starting material for the synthesis of acetanilide. When aniline is treated with acetic anhydride in the presence of sodium acetate acetanilide is obtained.

Synthesis of acetanilide

Synthesis of paracetamol

The reduction of nitrobenzene by zinc and ammonium chloride gives phenylhydroxylamine. When phenylhydroxylamine is treated with sulfuric acid it gives para aminophenol, para aminophenol can be acetylated by a mixture of acetic anhydride and glacial acetic acid to give paracetamol (acetaminophen).

Synthesis of paracetamol

Paracetamol causes antipyresis by exerting its action on the hypothalamic heat-regulating centre, and analgesia by enhancing the pain threshold profile appreciably. It is effective in a wide variety of arthritic and rheumatic condition involving musculoskeletal pain as well as the pain of headache, neuralgias, myalgias, and dysmenorrhea. It is particularly useful in aspirin-sensitive patients.

Synthesis of phenacetin

Para nitrophenol on reaction with ethyl bromide in the presence of sodium hydroxide gives para nitrophenetole, this para nitrophenetole is reduced by Fe in presence of HCl to give para phenetidine, the resulting para phenetidine is acetylated with acetic anhydride to yield the compound phenacetin.

Synthesis of phenacetin

It is an analgesic and an antipyretic with similar effectiveness as aspirin. It has greater potential for toxicity (hemolytic anemia and methemoglobinemia) than paracetamol. Phenacetin is mainly used for mild to moderate pain.

 Vulcanization of Rubber

The wide application of rubber is due to its peculiar property, called elasticity and that is why the rubber is said to be elastoplastic or elastomer. However crude rubber is of little use as such, because it has very undesirable properties. It possesses elasticity only over a limited range of temperature and does not regain its original shape after being stretched. It becomes soften strictly on heating and brittle on cooling. It has low tensile strength and poor resistance to abrasion. By vulcanization, all these defects are removed, and a material of high tensile strength, sufficient toughness, high elasticity, and a non-sensitivity to a wide change of temperature is obtained. Vulcanization is carried out by heating crude rubber in presence of sulfur or dipping it in a solution of S₂Cl₂ in CS₂. Vulcanization depends upon the amount of sulfur used (by increasing the amount of sulfur the rubber can be hardened), temperature, and duration of heating.

Vulcanization is a progressed reaction and therefore it is only allowed to a definite stage. The time of vulcanization and temperature is reduced by adding accelerators and activators. When the amount of sulfur is increased, the rubber is hardening and by increasing the percentage of sulfur to 40-45%, a non-elastic substance, called ebonite is obtained.

Vulcanization is a free radical initiated chain reaction. The free radical is furnished by the accelerator by removing the hydrogen ion from the isoprene monomer forming an active center. The rubber on vulcanization undergoes a great reduction in plasticity, whereas elasticity is largely maintained. Actually, in the vulcanization process, sulfur adds at the unsaturated bonds forming bridges and cross-linking the linear molecules into a practically infinite three-dimensional structure. After the molecular chains are cross-linked, the rubber loses plasticity and acquires elastic properties.

In the vulcanization process, the raw rubber is heated with sulfur, and cross-linking of the polyisoprene chains with short chains of sulfur atoms give it extra strength without destroying the elasticity. Nowadays, a vulcanizing initiator, usually a thiol or a simple disulfide, is added as well.

Vulcanization Initiators

The thiols give sulfur radicals and the disulfides cleave easily as the S-S bond is weak. The initiator (RS˙) either attack the rubber directly or attack sulfur to open the S₈ ring.

Attack of initiators

As a result of this attack, the released sulfur radical can bite back on to the sulfur chain and form a closed ring of 5-7 sulfur atoms and forms a short chain of sulfur atoms attached to the initiator and terminating in a sulfur radical.

Formation of sulfur radicals

Now the process for the attack on rubber can start. The vulcanized rubber has many E-alkenes, whereas unvulcanized rubber is all Z-alkenes. The sulfur radicals do not add to the alkenes but rather abstract allylic hydrogen atoms. Writing only a small section of rubber:

Abstract of an allylic hydrogen

The new form allylic radical captured one of the sulfur ring presents (S₅-S₈).

Attack on the sulfur ring

The sulfur radicals can attack another chain to give a cross-link or bite back to give a link within the same chain. Many types of different sulfur links are formed and the next diagram summarizes a part of the vulcanized rubber structure.

Vulcanized rubber structure

 Carbonic anhydrase

The enzyme Carbonic anhydrase catalyzes the reaction:

CO₂ (aq) + OH^- <=====> HCO₃^-

Without the catalyst, the reaction occurs above P^H = 9, but in presence of the enzyme Carbonic anhydrase the reaction occurs at P^H = 7. At P^H = 7, the hydration of CO₂ takes very slowly at room temperature in the absence of any catalyst:

CO₂ (aq) + OH^- <=====> HCO₃^- + H⁺

(p_CO2 =1 atm; k= 10^-1s^-1)

 

The enzyme Carbonic anhydrase enhances the rate of hydration of CO₂ by a factor of about 10⁷ or more. The enzyme Carbonic anhydrase can also catalyze the hydrolysis of esters and aldehydes.

Carbonic anhydrase catalyzes the interconversion of carbon dioxide and carbonates. Carbonic anhydrase has one zinc atom per molecule and it coordinates to three histidine residues (His 94, His 96, His 119) and a water molecule or hydroxide ion, it is represented as L₃Zn²⁺OH^-, where L = imidazole N from histidine. Experimentally, the enzyme Carbonic anhydrase loses activity below P^H =7, it is known that the product of the hydration of CO₂ is HCO₃^-, as would be expected in neutral or basic solution.

Calculation have shown that water bound to the zinc ion can lose a proton readily, but imidazole bound to the zinc ion cannot. There is still an unsettled question about the first ionization from the zinc-bound water molecule. This reaction seems to be much too fast and is dependent on buffer concentration. The role of the buffer is still unknown, but in some fashion, it assists in the reaction. The sequence of reactions usually used to describe the reaction is as follows:

L₃Zn²⁺OH^- + CO₂ -------> L₃Zn²⁺OH^-•CO₂     ---(1)

L₃Zn²⁺OH^-•CO₂ ------> L₃Zn²⁺HOCO₂^-           ---(2)

L₃Zn²⁺HOCO₂^- ------> L₃Zn²⁺OCOOH^-            ---(3)

L₃Zn²⁺OCOOH^- + H₂O --> L₃Zn²⁺(OCOOH^-) (H₂O)  

----(4)

L₃Zn²⁺(OCOOH^-) (H₂O) ------> L₃Zn²⁺OH₂ + HCO₃^-  

-----(5)

L₃Zn²⁺OH₂ -------> L₃Zn²⁺OH^- + H⁺ 

(Which may be on a histidine N)  ---(6)

The complex formed in (1) is loosely bound, moving to the more tightly bound product of (2). The transition state of reaction (3) may be a bidentate hydrogen carbonate, or there may be a proton transfer between the bound oxygen atom and one of the unbound oxygen atoms. In either case, the result is probably a bound hydrogen carbonate ion that has the OH group at as great a distance from the Zn as possible. Whether reaction (5) has a 5-coordinate Zn with the addition of the water molecule is uncertain; it may just be part of the transition state.

The catalytic action of the zinc-site is initiated by the conversion of a coordinated water molecule to give a Zn-OH bond through the neighbouring histidine and a buffer medium participation. This is followed by a nucleophilic attack on the carbon atom of CO₂. The OH group is properly oriented for the attack through a hydrogen bond with a threonine residue in the protein chain (Thr 199). Dehydration of HC¹⁸O₃^- leaves an ¹⁸O on the zinc, suggesting that during the reaction time, the HC¹⁸O₃^- ion must have been directly coordinated to the metal.

The catalytic path for hydration of carbon dioxide by Carbonic anhydrase

In the red blood cells, the enzyme Carbonic anhydrase performs the important role of receiving carbon dioxide from tissues such as active muscle and releasing it in the alveoli of the lungs. Each molecule of enzyme Carbonic anhydrase can hydrate about one million of carbon dioxide per second at body temperature (37⁰C). Different closely related forms of the enzyme Carbonic anhydrase are found in mammals, each having the molecular weight 30,000 and a roughly ellipsoidal shape.