LITERATURE REVIEW

2.1 Lubrication and Lubricants.

Lubrication is the branch of science that deals with the application of lubricants to moving machine parts. The functions of lubricants are as follows;

To reduce friction between moving parts, and so minimize the frictional effect of metal-tometal contact.

To act as a cooling medium by absorbing and dissipating excessive heat generation. For example, it has been estimated that the big-end bearing of an internal combustion engine requires approximately 90 times as much lubricant to keep it cool as it requires for the actual reduction of friction (Culpin, 1981).

To act as a seal for pistons in internal combustion engines, so as to make the combustion chamber gas tight.

To remove dirt from engine parts, thus keeping them clean and efficient.

These elements move in relation to each other, either by sliding, rolling, approaching, receding, reciprocating, and angular or by combination of these motions. Improper, inadequate or excessive, lubrication results in premature failure of machine elements and consequent high maintenance costs and lower profitability in industries. In fluid dynamics, lubrication theory describes the flow of fluids (liquids or gases) in a geometry in which one dimension is significantly smaller than the others. Interior flows are the elements of machines that require lubrication are those in relative motion namely: gears, bearings, slides, guidesways, pistons/cylinders, cam/cam-followers, flexible couplings, pulley/wire ropes etc. those where the boundaries of the fluid volume are known, and include those inside bearings. Here a key goal of lubrication theory is to determine the pressure distribution in the fluid volume, and hence the forces on the bearing components. The working fluid in this case is often termed a lubricant.

Free film lubrication theory is concerned with the case in which one of the surfaces containing the fluid is a free surface. In that case the position of the free surface is itself unknown, and one goal of lubrication theory is then to determine this. Surface tension may then be significant, or even dominant. Issues of wetting and dewetting then arise. For very thin films (thickness less than one micrometre), additional intermolecular forces, such as disjoining forces, may become significant.

Lubricants are introduced to reduce friction between moving surfaces and may also have the function of transporting foreign particles. The property of reducing friction is known as lubricity.

A good lubricant possesses the following characteristics:

High boiling point.

Low freezing point.

High viscosity index.

Thermal stability.

Corrosion prevention.

High resistance to oxidation.

There are four main types of lubricants. These are solids (e.g. graphite), liquids (e.g. oils), plastics (e.g. grease) and gases (e.g. air).

2.2 Lubricating Grease

Lubricating grease is generally defined as “a solid to semi-fluid product or dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may also be included” (NLGI, 1987). Grease from the early Egyptian or Roman eras is thought to have been prepared by combining lime with olive oil. The lime saponifies some of the triglyceride that comprises oil to give calcium grease. In the middle of the 19th century, soaps were intentionally added as thickeners to oils (Thorsten et al, 2005). Over the centuries, all manner of materials have been employed as greases. Black slugs Arion ater were used as axle-grease to lubricate wooden axle-trees or carts in Sweden (Svanberg, 2006)

2.2.1 Fundamentals of Grease Formulation

Lubricating grease is a complex mixture of alkali, fatty acid (fats and oil), and lubricating oil. First the alkali reacts with the organic acid to form soap as shown in equations (1.1) and (1.2). The soap is heated to dehydrate and melt so as to have an intimate mixing with the lubricating oil to form grease. Soap production started around 2500 BC with boiling of fats with ashes. The formula for soap consisting of water, alkali and cassia oil was written on a Babylonian clay tablet around 2200 BC (Willcox, 2000). Soaps are salts of fatty acids and it may be hard or soft soap depending on the type of ingredients used (Okeke, 2009). Soaps are made by the hydrolysis of fats with caustic soda (Sodium hydroxide), thus converting the glycosides of stearic, oleic and palmitic acids into sodium salts and glycerol. Soaps have a cleansing action because they contain negative ions composed of a long hydrocarbon chains attached to a carboxyl group (Okeke, 2009). The hydrocarbon chain has an affinity for grease and oil and the carboxyl group has an affinity for water. In other words,

Soap has two dissimilar ends: a hydrocarbon chain, that is non-polar, and a carboxylate salt end, which is polar as shown in equation (2.3)

O … (2.3)

Sodium stearate, soap

The non-polar hydrocarbon chain is soluble in fats and oil while the polar end is soluble in water (hydrophilic). So in grease formation, the non-polar hydrocarbon chain causes the soap to dissolve in the lubricating oil, thereby thickening it. Although the gelling action of all thickening agent is not fully understood (Nwagbara, 2005), most of the soap types are fibrous crystallites. Oil is believed to be held in the fibrous structure by capillary forces adsorption on the gel-formation molecules and physical entrapment within the interlacing fiber structure.

Awoyale et al (2011) also asserted that base oil is trapped in the fibre network of the soap.

The relative importance of each of these mechanisms depends on the type and degree of dispersion of the thickener, the type and solvency of action of the oil, and the influence of any stabilizing agents and additives. Unfortunately, neither the chemical formula for grease nor the chemical equation that leads to its formation has been developed yet. Chemical formula and equation that leads to soap formation has earlier been given in this work.

2.2.2 Fat and Oil

Fats and oils are naturally occurring esters used as energy-storing compounds by plants and animals. They are derived from propane 1,2,3-triol, CH2OH–CHOH–CH2OH (commonly known as glycerol or glycerine). This molecule has the capacity to combine with one, two or three molecules of carboxylic acid. In practice, most fats are triesters derived from propane 1,2,3,-triol and a variety of long-chain carboxylic acids, sometimes called fatty acids. For example, a simple fat molecule is that derived from a mole of propane – 1,2,3-triol and three molecules of octadecanoic acid according to the equation (2.4)

CH2OH HOOC(CH2)16CH3 CH2OOC(CH)16CH3

CHOH + HOOC(CH2)16CH3 CHOOC(CH2)16CH3 + 3H2O … (2.4)

CH2OH HOOC(CH2)16CH3 CH2OOC(CH2)16CH3

1 Mole of Propane 1,2,3-triol - 3-decanoic acid Moles of octa 1triyl Mole of Propane trioctadecanoic – acid

In naturally occurring fats each molecule is derived from two or three different fatty acids. Table 2.1 gives some common fatty acids: fifty or so are found in nature, the vast majority having an even number of carbon atoms in their molecule (Hill and Holman, 1979).

Table 2.1: Some common fatty acids

Structure

Systematic name

Common name

Occurrence

CH2(CH2)16COOH

Octadecanoic acid

Stearic acid

Mainly in animal fats

CH2(CH2)10COOH

Dodecanoic acid

Lauric acid

Coconut oil, palmkernoloic

CH2(CH2)14COOH

Hexadecanoic acid

Palmitic acid

Most fats especially palm oil

CH2(CH2)7CH=CH

(CH2)7COOH

Octadec-9-enoic acid

Oleic acid

Most fats especially olive ore

Source: (Hill, and Holman 1979)

Fats containing large proportion of unsaturated acids tend to have low melting points: many are liquid at room temperature and these are called oils. They can be converted to solid fats by hydrogenation.

2.2.3 Liquid Lubricant

Lubricants commonly used in grease production are either derived from mineral oils or are classified as synthetic. The synthetic oils available are based on esters derived from animal or vegetable oils and other groups are derived from hydrocarbons (Liyedahl et al 1979).

However, lubricants that are derived from mineral oils predominate.

Mineral lubricants are obtained from crude petroleum and change very little on exposure to the air. A great variety of lubricants are produced, ranging from light to heavy oils. Crude oil is a complex mixture of hydrocarbons, mainly alkanes, cycloalkanes and aromatic. The residual oil from the primary distillation boils above 3500C and is a highly complex mixture of involatile hydrocarbons. Most of it is used as fuel oil in large furnaces such as those in power station or big ships. A proportion of it, however, is used to make lubricating oil and waxes. Both these materials contain C26 – C28 hydrocarbons; when pure these hydrocarbon are solid, but lubricating oil contains a complex mixture, each member of which depresses the melting point of the others so that the mixture is a liquid.

2.2.4 Types of Grease

There are seven types of grease available, each manufactured for special purposes. These include wheel bearing grease, universal joint grease, chassis grease, extended lubrication interval (ELI), multipurpose grease, extreme pressure grease (EP), and spindle grease. Generally speaking, greases are those lubricants, which at ordinary temperatures are in a solid, plastic state. Greases that are in a more fluid state are sometimes called semi fluid oils.

2.2.5 Classification

Greases may be classified according to their method and mode of manufacture; those obtained by saponification with a metallic base like sodium hydroxide and then mixed with mineral oil, those prepared by a cold process with calcium hydroxide and resin, and those obtained from residues composed of mixtures of fatty or mineral oils and the residual of petroleum distillation. In the chemical sense, the last mentioned is a solid mixture of oils and should not be considered grease because they are not formulated as grease (Billet, 1979). The use of only the first category of grease is recommended. Unlike petroleum jelly or any other type of grease, their soap content provided them with special qualities, namely, long persistence upon a surface and reduced viscosity during use. Their viscosity depends upon the viscosity of the dispersion medium, that is, the oil used in the manufacture of the grease. The base used for saponification has an important effect upon the lubricant‟s properties, and accordingly, may constitute another criterion of classification. For example, grease with bases of calcium, sodium, aluminum, or lithium have different qualities as explained below.

1. Aluminum Soap Grease

Aluminum base greases are resistant at high temperatures and possess particularly adhesive properties, which minimize leakage. It is smooth, transparent grease with poor shear stability but excellent oxidation and water resistance, but tends to have poor mechanical stability and so is not suitable for rolling bearings (Mohammed, M.A.R 2013)

2. Calcium Soap Grease

It is one of the earliest known greases and is water resistant and mechanically stable. Calcium soap grease usually has a low dropping point; typically 95 °C. High temperatures cause a loss of water and a consequent weakening of soap structure, and therefore the use of this grease is limited to a maximum temperature of about 60 °C

3. Sodium Soap Grease

It is fibrous in structure and is resistant to moderately high temperature but not to water. Sodium soap grease has a high dropping point (175 °C) than calcium grease. In applications where high temperatures, dynamic stress, or centrifugal effects are significant, sodium-base greases are more efficient than calcium-base greases. However, they are emulsified by water.

4. Lithium Soap Grease

It is normally smooth in appearance but may exhibit a grain structure. Lithium soap grease offers both the water resistance of calcium soap grease and high-temperature properties of sodium soap grease. They are water resistant and can be used at temperatures up to approximately 120○C this makes them useful in a wide range of applications and for this reason they are called multi-purpose greases. The vast majority of bearing used in industries have running temperatures well within the capability of the lithium soap thickened greases.

5. Mixed Soap Grease

It is generally manufactured by saponifying the fatty material with mixed alkalis derived from metals. One of the soaps usually predominates and determines the general character of the greases while the other modifies the structure in some way. This results, for example, in changes in texture and improved mechanical stability.

6. Complex Soap Grease

It is formed when two dissimilar acids are attached to the same metal molecules, thus restricting complexes to only polyvalent metals. There are several types of complex grease, such as, calcium complex grease, aluminum complex grease, and lithium complex grease. Calcium-complex greases commonly include a minor proportion of calcium acetate with fatty acid soap thereby forming multipurpose greases with dropping points above 260 0C.

7. Non Soap Grease

Many non-soap greases are present.

A. Polyurea

It is the most important organic non soap thickener. It is a low-molecular weight organic polymer produced by reacting amines with isocyanates, which results in an oil soluble chemical thickener. The polymer-thickened greases are used when semi-fluid type grease is required, for example in certain gearbox applications where leakage is a problem (Billet,

1979).

B. Organo–Clay

It is the most commonly used inorganic thickener. Its thickener is modified clay, insoluble in oil in its normal form, but through complex chemical processes, converts to platelets that attract and hold oil. Organo–Clay thickener structures are amorphous and gel-like rather than the fibrous, crystalline structures of soap thickeners. This grease has excellent heat resistance since clay does not melt. The clay-thickened greases are very useful in special high temperature, low speed applications, such as the lubrication of oven chain and kiln car wheel bearings. However, the presence of the mineral oil limits their upper temperature use to approximately 200○C.

Others

Sometimes, for special applications, fluids other than mineral oils are used. The silicone greases lubricate bearings that have to operate under extremes of heat and cold. Carbon black is used as a thickener in some high temperature petroleum and synthetic greases. Organic powders and pigments, which are stable at elevated temperatures, are being used increasingly (Billet, 1979). Tables 2.2 and 2.3 show the properties and compatibilities of different grease soaps respectively

2.4 Prospect of Using Plantain Peel Ash as the Source of Alkali.

It has been stated earlier that ashes of agricultural materials can be a source of industrial alkali. The present work is devoted to investigating the suitability of plantain peel ash as a source of alkali for lubricating grease production. Research has shown that plantain peel ash has been used to produce soap of good quality. Hence, it is believed that soap produced from the ash can equally be a good grease thickener.

2.5 The Plantain

The plantain, scientifically named musa Paradisiaca is a very starchy banana that is usually used in cooking. It is a fast - growing plant 3-5m high with herbaceous stem. The fruits grow in bunches of up to 200 fingers each. Plantains are widely grown across the world‟s tropical regions. They are major food crops in developing countries, and are also an important export crop to industrial countries. They are easy to grow and production is relatively stable. In addition, the fruits are highly nutritious, containing large amounts of carbohydrates and minerals such as phosphorus, calcium, and potassium as well as vitamins A and C. They are also important sources of revenue for many small-scale farmers. About 90% of the world‟s plantains are grown on small farms and consumed locally. An estimated 20 million people eat plantain as their major source of dietary carbohydrate. They are particularly important in East Africa, where they constitute the main staple food for about 50% of the population. In that part of the world, the annual consumption reaches over 400 kg per person. ). Almost 75% of the world‟s plantains were grown in Africa. Uganda is the largest producer of plantains and produces about 9 million tons per annum (www.fao.org). The fruits can be fried, baked, or roasted, and are also sold in pulp form, as chips, and in confectionery. In some countries, they are used to produce alcohol. The leaves and pseudo stem are also often used, for example, for wrapping food, for thatching, and in textile manufacture. The fruits can also be used as animal feed. The major pests are the banana weevil and parasitic nematodes. Studies have revealed that if plantains are infected with both weevils and nematodes, yield losses may reach 85%. Another major constraint of plantain is that the fruits are highly perishable. At ambient tropical temperatures, plantain has an average market life of 1 - 10 days, compared to several weeks for yam, for example. In Nigeria, plantains are grown at the following states of the federation; Rivers, Cross River, Imo, Anambra, Ondo, Lagos, Ogun, Oyo, kwara, Benue, Plateau, Kogi, Abia, Enugu, Edo and Delta with annual production of 1,855,000 Metric tons

2.6 Potassium Hydroxide

The major constituent of plantain peel ash is potassium hydroxide (Onyeagbado et al 2002). Potassium hydroxide is a chemical compound with formula KOH. Pure potassium hydroxide forms white, deliquescent crystals. For commercial and laboratory use, it is usually in the form of white pellets. A strong base, it dissolves readily in water, giving off much heat and forming a strongly alkaline, caustic solution. It is commonly called caustic potash. It closely resembles sodium hydroxide in its chemical properties and has similar uses, for example, in making soap, in bleaching, and in manufacturing chemical, but is less widely used because of its higher cost. Potassium Hydroxide is prepared chiefly by electrolysis of potassium chloride; commercial grades of it sometimes contain the chloride as well as other impurities. It is a major industrial chemical used as a base in a wide variety of chemical processes. It is used as a catalyst in reactions like the production of biodiesel. The advantage of using potassium hydroxide (KOH) and not sodium hydroxide (NaOH) is that NaOH “clumps” and KOH does not. Some uses of KOH include acrylate ester copolymer coating, defoaming agents used in the manufacture of paper, formulation aid for food, PH control agent, polyethylene resins, and textile processing. Other uses include in veterinary medicine, in disbudding calves horns and to dissolve scales and hair; in human medicine, to diagnose fungal infections, and as a wart and cuticle solvent; in manufacture of cleansers. This chemical is also used in washing powders, some denture cleaners, non-phosphate detergents, and drain or pipe cleaners. A very significant use of KOH in terms of significance to the average home consumer is that alkaline batteries use an aqueous solution of KOH as an electrolyte. Thus, potassium hydroxide helps to power flashlights, smoke detectors, and other battery powered household items.

2.7 Bio-Alkali

Bio-alkali is the alkali derived from the ashes of burnt biomaterials. Agricultural materials contain a good percentage of mineral salts. These include calcium, phosphorous, iron, sodium, potassium and so on, as shown in Table 2.3 When these materials are burnt in air, carbohydrates, fats, proteins and vitamins will all burn away. The resulting ashes contain oxides of these minerals. Some of these are basic oxides of potassium and sodium, which when dissolved in water yield their corresponding hydroxides (alkali) according to the equations (1.1) and (1.2)

Table 2.5 Mineral Salt composition of some raw foods mg/100g edible portion

Food Product

Calcium

P

Fc

Na

K

Plantain

7

30

0.7

5

385

Rice

32

221

1.6

9

214

Soya bean

226

4

8.4

0.5

1677

Wheat (whole grain)

1308

1016

0.6

532

1745

Yam tuber

20

69

0.6

-

600

Cowpea (mature dry)

74

426

5.8

35

1024

Groundnut (peanut)

59

409

2.0

5

674

Sorghum (whole grain)

23

263

3.6

-

-

Source: Enwere (2012)

Alkali is derived from the Arabic word “al-kali” which means the dust or ashes (Bajah and Godman 1976). This was based on its earlier mode of extraction. Soda (Sodium carbonate) was made by burning marine plants to ashes and extracting the soda from them, while potash (Potassium carbonate) was made by burning wood in iron pots and extracting the potash from the ashes. The name “Potash” actually came from the compound word “pot-ash” (Bajah and Godman 1976) The method of making these “mild” alkalis into “caustic” alkalis by treatment with lime was practiced in the time of Pliny (www.simplestarpage.com) in connection with the manufacture of soap, and it was known that the ashes of shore-plants yielded a hard soap and those of land plants a soft one. But the two substances were generally confounded as

“fixed alkali” (carbonate of ammonia being “volatile alkali”), till Duhamel du Monceau in 1736 established the fact that common salt and the ashes of sea-plants contain the same base as is found in natural deposits of soda salts (“mineral alkali”), and that its body is different from the “vegetable alkali” obtained by incinerating land-plants or wood in iron pot and extracting the potash (hence the name, pot-ash, and hence a derivation of potassium).

Later, Martin Heinrich Klaproth (Bajah and Godman, 1976) finding vegetable alkali in certain minerals, such as Lucite, proposed to distinguish it as potash, and at the same time assigned to the mineral alkali the name natrium, which survives in the symbol, Na, now used for sodium. The word alkali supplied the symbol for potassium, K (kalium). Analysis of alkalis derived from vegetable matter ashes by Nwoko (1980) and others (Onyekwere, 1996, Kuye and Okolie, 1990), showed that the extract was chiefly potassium hydroxide with some quantities of sodium hydroxide. Other metallic ions present constituting as a whole about 2% of the metallic ions present, were Ca++, Cr++, B++, Zn++, Fe++, Pb++ and Ni++. Tests have shown that bio-alkali produces harder soap than those produced from pure potassium hydroxide, and this is because of the presence of the above metallic ion and notably sodium (Onyeagbado et al 2002).

Generally speaking, alkalis are soluble bases. A base is a metallic oxide, or hydroxide, which neutralizes an acid to form a salt and water only. In solution, the bases form hydroxides. Common alkalis and their formulae are Sodium hydroxide (Caustic soda) NaOH, Potassium hydroxide (Caustic potash) KOH, Ammonia solution NH4OH and Calcium hydroxide (Lime water) Ca(OH)2. Alkalis have very low concentration of hydrogen ions when dissolved in water, and their pH values are above 7 (Sambal‟s Science Web 2006). Sodium carbonate

(correctly speaking, is a salt) is included as an alkali in showing the uses of alkalis (Baja and

Godman 1979). Ammonia solution also strictly speaking is not a hydroxide (Holderness and Lambert, 1976). A solution of sodium carbonate has an alkaline reaction, and it can be used in place of an alkali to neutralize an acid. The reaction is not neutralization by definition, as carbon dioxide is evolved. Both sodium and potassium carbonate have alkaline reactions with indicators, and they are called mild alkalis. The remaining alkalis are called caustic alkalis because they have a corrosive action when concentrated.

2.7.1 Properties and Uses of Alkalis

(a) Properties

The properties of alkalis are as follows;

They neutralize acids to form salt and water

They change red litmus to blue, methyl orange to yellow and phenolphthalein to pink (iii) They give off ammonia gas (not ammonia solution) when they are warmed with an ammonium salt

They have soapy feel

They have bitter taste

(b) Uses

Alkalis are used in large quantities in most chemical industries. Bases are less important commercially. Some of the more important uses of alkalis are as follows;

Caustic soda and potash are used in the production of soap, paper, artificial silk, mercerized cotton and lubricating grease.

Sodium carbonate is used in water softening and also in the manufacture of glass and soap

Lime is used in the manufacture of fertilizer

Ammonia is used in making household cleaners

2.7.2 Extraction of Bio Alkali

Caustic solution is also known as “lye water”. There are various methods of extracting bio alkali. Two are described here.

Method 1

The method described here was originally prepared and printed as a booklet, at the request of Christians in Burma, to help in situations where normal supplies of soaps are not readily available; and where caustic soda is hard to come by. This information was needed because soap can be powerful in stopping the spread of certain diseases (Peter, 1986).

(a) Ashes

Dried palm bunches, dried out banana peels, cocoa pods, kapok tree wood, and oak wood, (or for really white soap, apple tree wood) make the best lye ashes. Ordinary wood used in cooking will do also. Whatever wood is used, it should be burnt in a very hot fire to make very white ashes. When cold, these are stored in a covered plastic bucket or wooden barrel, or stainless steel container. If these are not available, a clay pot-jar which has been fired in a pottery-making kiln (not just dried in the sun) will do.

(b) Soft Water

Water from a spring or from showers of rain is called “soft water”, because it does not have metallic or acidic chemicals in it. This makes it useful for soap making, as there are no other chemicals in it which would get in the way of making soap. Ordinary bore well or river water can be used for making soap, but this will sometimes need a washing soda or baking soda added to it. Otherwise, some of the chemicals in the water will get in the way of making the soap. If ordinary water is to be used, it will be necessary to test if soda needs to be added by simply trying to make soap bubble up (foam) in it. If the soap easily foams up, the water is probably ok as it is. Otherwise adding a little bit of soda at a time, stirring it to make it disappear, until the water will foam the soap up, will be necessary.

(c) Safe Containers

Any of the types of containers, buckets, barrels or jars described in the Ashes or Soft Water sections are called “Safe containers”

(d) Making Lye Water

A large barrel or drum to be used to make the lye water can have a tap or hole at a level a little above the bottom, and some kind of filter placed on the inside, around the opening The barrel will be filled with white ashes to about 10cm or 0.1m below the top, and boiled soft water poured over the ashes. More cold (soft) water should be added slowly until liquid drips out of the barrel top. The tap should be closed or the hole blocked. More ashes could be added to top the barrel up again, and more soft water but so much water should not be added that the ashes swim. This should be left to stand for four or more hours (or even over night if there is time). Then the lye water should be allowed to drip into safe containers by opening the tap or unplugging the hole. When the brown lye water stops coming out of the barrel, or ash container, then more soft water should be poured through the ashes, collecting the lye which comes out in a separate “safe” container (as this lye may be weaker than the first lot).This process is repeated until no more brown liquid comes out of the ashes. The lye is stored in a safe container and the ashes dug into the vegetable garden. It should never be stored in aluminum or tin container. They are badly corroded by the caustic solution.

(e) Lye Water Strength

If an egg or potato floats just below half way, or a chicken feather starts to dissolve in it, the lye water is at the right strength for local soap production. If the egg will not float, then the lye water could be boiled down to make it stronger. If the egg seems to pop up too far, the lye water is too strong, and a little bit of soft water (a cup at a time) would be added stirring the lye water, until the egg floats so that its head pops up.

Method 2

This method was reported by Onyegbado et al (2002) of the Department of Chemical Engineering, University of Port Harcourt, Nigeria, to improve the quality of ash-derived alkali soaps in order to make them amenable to all the uses previously listed for potassium based soaps. The lye water produced can also find other application where alkali uses are required. The apparatus and materials used included an oven, weighing balance, a large

shallow tray called “combustion pan”, a sieve set, a spectrophotometer and laboratory glassware.

Unripe plantain peels were collected from Choba village, near Port Harcourt, Nigeria. They were dried in an oven at 100 °C for two days to constant weight. The peels were thus said to be “bone-dried”. The bone-dried peels were placed in an open “combustion pan” and heated till the peels ignited. The ignition temperature, which was presumably very high, could not be measured with a mercury-in-glass thermometer. A metallic rod with a wooden handle was used to turn the burning peels, thus ensuring uniform combustion. The ashing lasted three hours. Another sample of the “bone-dried” peels was not ashed but ground into a fine powder. No alkali was detected when this sample was leached with distilled and deionized water. The ash sample was homogenized by crushing by hand and then sieved to remove large particles. Kuye and Okorie (1990) had shown that a particle size of 1.06 x 10-4 gave the highest concentration of potassium hydroxide when slurry of the ashed sample containing 0.15kg of the ashes in 2.5dm³ of distilled and deionized water was kept for eight hours at 60ºC. In their study, the slurry prepared under the above conditions was kept for forty-eight hours in a further attempt to ensure maximum extraction of the alkali. Subsequently, the slurry was filtered to obtain the extract. Spectrophotometric analysis of the extract for metallic ions was done using an atomic absorption spectrophotometer (AAS) available at NAFCON, Onne, Rivers State, Nigeria. The percentage compositions of the metal ions in the extract were approximately as follows;

Sodium ion = 15.86%, Potassium ion = 84.14%. Other ions were insignificant

2.7.3 Industrial Manufacture of Alkali

Sodium hydroxide and potassium hydroxide are obtained from electrolysis of sodium chloride and potassium chloride respectively. Since both alkalis are extracted the same way, this work will only discuss the standard electrolytic process for the manufacture of sodium hydroxide. This is described in several textbooks.

Sodium hydroxide is obtained when a solution of sodium chloride is electrolysed. In solution, sodium chloride ionizes thus;

Nacl Na+ + cl- … (2.4)

The water molecules also ionize slightly

H2o H+ + OH ˉ … (2.5)

At the cathode, the hydrogen ions are discharged in preference to sodium, while at the anode; the chloride ions are discharged in preference to the hydroxyl ions.

Three types of cells have been devised for the production of sodium hydroxide

electrolytically: These are the diaphragm cell, the mercury cathode cell, membrane Cell

The Diaphragm Cell:

The cell consists essentially of a porous diaphragm, closely covered on the outside with steel gauze, which acts as the cathode shown in Fig.(2.1). The anode, which is made of graphite, dips into the concentrated solution of sodium chloride (brine) in the porous diaphragm. The brine gradually percolates through the diaphragm. During the electrolysis, chlorine is liberated at the anode and hydrogen at the cathode. Sodium ions collect at the cathode. The discharge of hydrogen ions (from water in the brine) leaves hydroxyl ions at the cathode; sodium and hydroxyl ions, together with water, form sodium hydroxide solution. Sodium hydroxide solution drips from the diaphragm and collects in the outer compartment of the diaphragm. On heating the concentrated solution in iron pans, fused sodium hydroxide is obtained, from which pellets or flakes are made.

The Mercury Cathode Cell:

The cell consists essentially of graphite anodes dipping into a concentrated solution of brine in a container as shown in Fig.(2.2). A layer of mercury covers the floor of the cell and is kept flowing slowly through the cell. Chlorine is librated at the anode, but sodium ions, being more easily discharged than hydrogen at the mercury elctrode, form a solution of sodium in mercury (sodium amalgam). This amalgam flows to a trough in which it reacts with water to give sodium hydroxide solution and hydrogen. Mercury, which is also regenerated in this reaction, is returned to the electrolytic cell to pass though the process again.

2H20+2Na/Hg = 2NaOH + H2 + 2Hg … (2.6)

Solid sodium hydroxide is obtained as before. Sodium amalgam is represented by the symbol Na/Hg, as it is not a compound of definite atomic proportions.

Membrane Cell

The Membrane Process

A membrane cell is a diaphragm cell with an improved diaphragm called a „membrane‟. This is made from polytetrafluoroethylene (PTFE), making it a plastic membrane, which has been modified to include anionic groups to act as an ion exchange membrane. The Membrane Cell is replacing the older techniques which used the Castner-Kellner Mercury cell (problems with mercury pollution) and the Gibbs Diaphragm cell

2.7.4 Alkali Metals

The Alkali Metals are the series of elements in Group 1 (IUPAC STYLE) of the periodic table, excluding hydrogen. These are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). They are highly reactive and are never found in elemental form in nature. See Table.2.4

The Alkali Metals are silver-colored (caesium has a golden tinge), soft, low-density metals, which react readily with halogens to form ionic salts, and with water to form strongly alkaline

(basic) hydroxides. These elements all have one electron in their outermost shell, so the energetically preferred state of achieving a filled electron shell is to lose one electron to form a singly charged positive ion.

Eg. Na + eˉ Na … (3.9)

Hydrogen, with a solitary electron, is sometimes placed at the top of Group 1, but it is not an alkali metal (except under extreme circumstances as metallic hydrogen); rather, it exists naturally as a diatomic gas. Removal of its single electron requires considerably more energy than removal of the outer electron for the alkali metals. As in the halogens, only one additional electron is required to fill in the outermost shell of the hydrogen atom, so hydrogen can in some circumstances behave like a halogen, forming the negative hydride ion. Binary compounds of hydride with the alkali metals and some transition metals have been prepared.

Under extremely high pressure, such as is found at the core of Jupiter, hydrogen does become metallic and behaves like an alkali metal.

Table 2.6 Elements of Group 1 in the periodic table.

Period

Element

Symbol

Atomic No

2

Lithium

Li

3

3

Sodium

Na

11

4

Potassium

K

19

5

Rubidium

Rb

37

6

Caesium

Cs

55

7

Francium

Fr

87

Source: http://sambal.co.uk/?page_id=57

The first ionization energy is much lower than the second. In the case of Na, it is nine times easier to remove the first electron than the second. The first electron, in an s-orbital, can be removed from the element easily, but the second electron must be removed from a noble gas core, which is closer to the nucleus, and therefore requires much energy. Thus, sodium readily forms Na+ ions, but never forms Na²+ ions. This means that sodium and the other elements in Group 1 have only the one oxidation state of + 1 in their compounds.