CHAPTER ONE1.0 INTRODUCTION1.1 STATEMENT OF PROBLEMCoal is a combustible sedimentary rock that is mostly made up of carbon and hydrocarbons. It is a non-renewable source of energy and is known as the most widely available fossil fuel in the world. It is commonly used for generating electricity; about 55 percent of electricity produced in the world is generated from coal (Biswal, 2012). Although it is very beneficial in terms of energy generation, its usage releases high amounts of waste materials, which can be detrimental to the environment. This is because the run-of-mine coal consists of many impurities such as sulfur, shale, clay etc. Coal cleaning, therefore, is required to remove excessive impurities for efficient and environmentally safe utilization of coal as the society expects cleaner energy with less pollution and an increasing emphasis on environmental sustainability. To meet the challenge of environmental sustainability and thus remain a part of a sustainable energy future, the coal industry is working on developing coal-based power plants that will reduce its greenhouse gas emissions. These plants are part of what is known as clean coal technology in the coal industry and are expected to be at least 90 percent emission free. In the design of the new technology coal-based power plants, coal washing techniques are incorporated to remove the impurities that cause the negative effects of coal use on the environment. However, for proper selection of coal washing devices, the knowledge of the difficulty or ease of cleaning coals is required. This can be achieved by using coal washability curves. Correct interpretation of these curves supplies a basis for reasonably accurate prediction of the behaviour of a raw coal or of a sized fraction thereof on gravity-concentration machines and froth flotation cells.Nigeria is blessed with an extensive coal resource. According to a report written by the Nigerian Coal Corporation (2004), the inferred coal reserves of Nigeria have been estimated at a range of 1.5 billion metric tonnes to 2.75 billion metric tonnes. The proven reserves of coal in Nigeria are about 639 million tonnes (Ogunsola, 1990; Nigerian Coal Corporation, 2004). Despite the extensive and long history of coal resources in the country, they are not being fully explored or even marginally developed. This is because of the oil and gas boom in the country, which has overshadowed the solid minerals sector. The problem of inadequate power supply in Nigeria has however made the country to have a second thought on its coal resources. Inadequate power supply has been a major problem facing the country. The Nigerian electric power sector operates well below its 5.9 gigawatts (GW) estimated capacity, and thus power outages frequently occur in the country (CPE, 2009). About 40 percent of Nigerian populace has access to grid electricity supply, which is generally unreliable (World Bank, 2005). All sectors (residential, commercial, and industrial) are increasingly in the habit of using privately operated diesel generators to supply electricity whenever there is power outages, which is very frequent. Worse still, the demand for electricity continues to increase as the population and economy grow. The country's electricity consumption is projected to increase from 7 GW in 2010 to about 15 GW in 2025 (CPE, 2009). Nigeria thus is looking for ways to meet to the power demand in the country. According to Oji et al (2012), recent years have seen governments desire to increase electricity generation through the utilization of its abundant coal deposits. This implies building clean technology coal-based power plants.For proper design of clean technology coal-based power plants that will help increase power supply in Nigeria, cleanability studies is required to determine the ease or difficult of cleaning Nigerian coals. Data collected from such studies will help in the design and construction of washing plants that will be attached to the coal-based power plants to remove impurities that cause the negative effects of coal use on the environment. Enugu coal being one of the coal resources in Nigeria has been studied in this work to determine its cleanability and use for electricity generation.

1.2 AIM AND OBJECTIVES OF THE STUDYThe main aim of this work is to evaluate the washability of Enugu coal for use in electricity generation in Nigeria. The specific objectives of the work are to: obtain the washability curves of Enugu coal by conducting sink-and-float analysis and froth flotation release tree analysis on the coal samples; establish the ease or difficulty of cleaning a Enugu from the generated washability curves; establish the organic efficiency of a hypothetical washing plant.1.3 PROJECT OUTLINEThe outline of this project work is summarized as follows:1. Chapter 1 Introduction, comprising statement of problem, aim and objectives of the study.2. Chapter 2 Literature review, comprising discussions of related research and existing knowledge on coal, various methods used for washing coal, a brief history of Nigerian coal reserve, and geology of Enugu coal.3. Chapter 3 Materials and methodology.4. Chapter 4 Results and discussion.5. Chapter 5 Conclusion of the project work and recommendations.

CHAPTER TWOLITERATURE REVIEW2.0 DEFINITION

Coal is a combustible black or brownish-black sedimentary rock that usually occurs in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen (Blander, 2011). Throughout history, coal has been a useful resource for human consumption. It is primarily burned as a fossil fuel for the production of electricity and/or heat, and is also used for industrial purposes such as refining metals. Coal forms when dead plant matter is converted into peat, which in turn is converted into lignite, then anthracite. This involves biological and geological processes that take place over a long period of time.Coal, a fossil fuel, is the largest source of energy for the generation of electricity worldwide (see Fig. 2.1), as well as one of the largest worldwide anthropogenic sources of carbon dioxide releases. Gross carbon dioxide emissions from coal usage are slightly more than those from petroleum and about double the amount from natural gas (Energy Information Administration, 2005). Coal is extracted from the ground by mining, either underground by shaft mining through the seams or in open pits.

2.1 COAL FORMATIONCoal has been identified as a combustible rock which had its origin in accumulation and partial decomposition of vegetation. Coal originates in continental basins with swampy edges where there is very dense vegetation. It originates as partly decomposed and macerated vegetable matter mainly as vascular land plants. The following parts can be easily recognized, resin, gums, waxes, pigments, oils and fats.Under suitable conditions for the accumulation and preservation of their remains, plants have contributed the essential ingredients to coal since their first appearance in the geological record. Not much is known about the plant life which existed during the early period of geological history, but microscopic examination of recognizable plant remains from the Carboniferous and succeeding periods has confirmed the presence of organs and tissues comparable with those of present day vegetation.About 300 million years ago, the earth had dense forests in low-lying wetland areas. Due to natural processes, like flooding, these forests got buried under the soil. As more and more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. For the process to continue, the plant matter was protected from biodegradation and oxidization, usually by mud or acidic water. This trapped the carbon in immense peat bogs that are eventually covered and deeply buried by sediments. Under high pressure and high temperature dead vegetation got slowly converted to coal (Fig. 2.1). As coal contains mainly carbon, the conversion of dead vegetation into coal is called carbonization, (Taylor et al, 2009). Fig. 2.1 Diagrammatic representation of how coal was formed (The NEED project, 2011)In terms of geological age, ideal conditions for coal formation were provided by the wide shallow seas of the carboniferous period. However, coal formation is also known from most of the geological periods. The exception is the coal gap in the Lower Triassic, where coal is rare: presumably a result of the mass extinction which prefaced this era.

Fig. 2.2 Approximate position of the proto-continents toward the end of the Carboniferous periodFig 2.2, it shows the approximate position of the proto-continents toward the end of the carboniferous period; the light blue portion representing shallow seas where many of todays coal deposits are found.2.2 Classification of coalsIt is considered that all ordinary coals are of plant, non-marine origin, and that the common banded coals were formed insitu. There is some diversity of opinion as to whether the different kinds of coals originate because of different kinds of vegetation or because of different degrees or stages of coalification. The generally accepted view is that all coals originated from swamps and went through the Peat to Antracite.There are many classifications of coals, but the most widely accepted all over the world as geological processes apply pressure to dead biotic material over time, under suitable conditions it is transformed successively into: Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water. It is also used as a conditioner for soil to make it more able to retain and slow release water. Lignite or brown coal is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Upper Paleolithic. Sub-bituminous coal, having properties that range from those of lignite to those of bituminous coal, is used primarily as fuel for steam-electric power generation and is an important source of light aromatic hydrocarbons for the chemical synthesis industry. Bituminous coal is dense sedimentary rock, black but sometimes dark brown often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities used for heat and power applications in manufacturing and to make coke Steam coal is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea-coal in the U.S, (Funk and Wagnalls, 1989). Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating Anthracite, the highest rank of coal is a harder, glossy, black coal used primarily for residential and commercial space heating. It may be divided further into metamorphically altered bituminous coal and petrified oil, as from the deposits in Pennsylvania Graphite, technically the highest rank is difficult to ignite and is not commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant.The classification of coal is generally based on the content of volatiles. However, the exact classification varies between countries.

2.3 USES OF COALThe recent increase in the price of crude and natural gas as sources of fuel for industrial heating has lead to a high demand for coal. Coal as an exhaustible energy source has been estimated to meet its future demand if used at present rate. The preference for coal is due to its very high linkage effect with thermal power, railways locomotive, fertilizers industry, cement, steel, electric power and a number of other industries (Gupta, 1990). The diverse usage of coal is majorly due to the presence of coal combustion products and a high calorific value of its organic constituents. These uses are discussed below. 2.3.1 Coal Combustion ProductsCoal combustion products (CCPs) can play an important role in concrete production. CCPs are the by-products generated from burning coal in coal-fired power plants. These by-products include: fly ash bottom ash boiler slag flue gas desulphurisation gypsum others types of material such as fluidised bed combustion ash, cenospheres, and scrubber residuesFly ash can be used to replace or supplement cement in concrete. In the USA, for example, more than half of the concrete produced is blended with fly ash.Among the most significant environmental benefits of using fly ash over conventional cement is that greenhouse gas (GHG) emissions can be significantly reduced. For every tonne of fly ash used for a tonne of portland cement (the most common type of cement in general use around the world) approximately one tonne of carbon dioxide is prevented from entering the earths atmosphere. Fly ash does not require the energy-intensive kilning process required by portland cement.Using CCPs reduces GHG emissions, reduces the need for landfill space, and eliminates the need to use primary raw materials. Fly ash produces a concrete that is strong and durable, with resistance to corrosion, alkali-aggregate expansion, sulphate and other forms of chemical attack.Coal combustion products are expected to continue to play a major role in the concrete market. Their use in other building products is also expected to grow as sustainable construction becomes more prominent, and more architects and building owners understand the benefits of using CCPs.2.3.2 Coal For Electricity GenerationModern life is unimaginable without electricity. It lights houses, buildings, streets, provides domestic and industrial heat, and powers most equipment used in homes, offices and machinery in factories. Improving access to electricity worldwide is critical to alleviating poverty.Coal plays a vital role in electricity generation worldwide. Coal-fired power plants currently fuel 41% of global electricity (Fig. 2.3).

Fig. 2.3 Total World Electricity Generation by Fuel (2006), IEA 2008Other includes solar, wind, combustible renewables, geothermal & waste.In some countries, coal fuels a higher percentage of electricity (Table 2.2). Table 2.1: Percentage electricity generated from coal (IEA 2010)South Africa 93%Poland 92%China 79%

Australia 77%Kazakhstan 70%India 69%

Israel 63%Czech Rep 60%Morocco 55%

Greece 52%USA 49%Germany 46%

Conversion of Coal to ElectricitySteam coal, also known as thermal coal, is used in power stations to generate electricity.Coal is first milled to a fine powder, which increases the surface area and allows it to burn more quickly. In these pulverized coal combustion (PCC) systems, the powdered coal is blown into the combustion chamber of a boiler where it is burnt at high temperature (Fig 2.4). The hot gases and heat energy produced converts water in tubes lining the boiler into steam.

Fig. 2.4 The use of coal in a boiler for generating electricityThe high pressure steam is passed into a turbine containing thousands of propeller-like blades. The steam pushes these blades causing the turbine shaft to rotate at high speed. A generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils. Electricity is generated when these are rapidly rotated in a strong magnetic field. After passing through the turbine, the steam is condensed and returned to the boiler to be heated once again.The electricity generated is transformed into the higher voltages (up to 400,000 volts) used for economic, efficient transmission via power line grids. When it nears the point of consumption, such as our homes, the electricity is transformed down to the safer 100-250 voltage systems used in the domestic market.

2.3.3 COAL & STEELSteel is an essential material for modern life. The manufacture of steels delivers the goods and services that our societies need healthcare, telecommunications, improved agricultural practices, better transport networks, clean water and access to reliable and affordable energy.Global steel production is dependent on coal. 70% of the steel produced today uses coal. Metallurgical coal or coking coal is a vital ingredient in the steel making process.Steel ProductionSteel is an alloy of iron and carbon. Steel is produced via two main routes Integrated smelting involving blast furnace (BF) iron-making followed by basic oxygen furnace (BOF) Electric arc furnaces (EAF).Raw MaterialsSteel is an alloy based primarily on iron. As iron occurs only as iron oxides in the earths crust, the ores must be converted, or reduced, using carbon. The primary source of this carbon is coking coal. This can be summed up as shown below; Iron Ore e.g. haematite ore (Iron(III)oxide), Fe3O2 or Magnetite ore, Fe3O4 Coke (carbon, C) Hot air (for the oxygen in it), O2 Limestone (calcium carbonate), CaCO3Coke MakingCoking coal is converted to coke by driving off impurities to leave almost pure carbon. The physical properties of coking coal cause the coal to soften, liquefy and then resolidify into hard but porous lumps when heated in the absence of air. Coking coal must also have low sulphur and phosphorous contents. Almost all metallurgical coal is used in coke ovens.The coking process consists of heating coking coal to around 1000-1100C in the absence of oxygen to drive off the volatile compounds (pyrolysis). This process results in a hard porous material - coke. Coke is produced in a coke battery which is composed of many coke ovens stacked in rows into which coal is loaded.The coking process takes place over long periods of time between 12-36 hours in the coke ovens. Once pushed out of the vessel the hot coke is then quenched with either water or air to cool it before storage or is transferred directly to the blast furnace for use in iron making.Iron MakingIron ore is mined in around 50 countries the largest producers are Australia, Brazil and China. Around 98% of iron ore is used in steel-making. During the iron-making process, a blast furnace (Fig. 2.5) is fed with the iron ore, coke and small quantities of fluxes (minerals, such as limestone, which are used to collect impurities). Air which is heated to about 1200C is blown into the furnace through nozzles in the lower section. The air causes the coke to burn, producing carbon monoxide which reacts with the iron ore, as well as heat to melt the iron. Finally, the tap hole at the bottom of the furnace is opened and molten iron and slag (impurities) are drained off. The Blast furnace reaction is shown below; the coke functions as as a fuel C(S) + O2(g) CO2(g) as a reducing agent CO2(g) + C 2CO(g)The above reaction frees the iron, which is molten at the high blast furnace temperature, and trickles down to the base of the blast furnace and run off. The main reduction is shown as;for haematite: Fe2O3(s) +3C(s) 2Fe(l) + 3CO(g)or 2Fe2O3(s) +3C(s) 4Fe(l) +3CO2(g)for magnetite: Fe3O4(s) + 4C(s) 3Fe(l) + 4CO(g)or Fe3O4(s) + 2C(s) 3Fe(l) + 2CO2(g).the limestone react with acidic mineral impurities, such as silicon dioxide, contained in the ore to form slag, as the reaction shows below;CaCO3 + SiO2 CaSiO3 + CO2

Basic Oxygen FurnaceThe most commonly applied process for steel-making is the integrated steel-making process via the Blast Furnace Basic Oxygen Furnace. In the basic oxygen furnace, the iron is combined with varying amounts of steel scrap (less than 30%) and small amounts of flux (Fig 2.5). A lance is introduced in the vessel and blows 99% pure oxygen causing a temperature rise to 1700C. The scrap melts, impurities are oxidized, and the carbon content is reduced by 90%, resulting in liquid steel.Fig. 2.5: Use of coal in steel-makingOther processes can follow secondary steel-making processes where the properties of steel are determined by the addition of other elements, such as boron, chromium and molybdenum, amongst others, ensuring the exact specification can be met.Optimal operation of the blast furnace demands the highest quality of raw materials the carbon content of coke therefore plays a crucial role in terms of its effect in the furnace and on the hot metal quality. A blast furnace fed with high quality coke requires less coke input, results in higher quality hot metal and better productivity. Overall costs may be lower, as fewer impurities in the coke mean smaller amounts of flux must be used.Basic Oxygen Furnaces currently produce about 70% of the worlds steel. A further 29% of steel is produced in Electric Arc Furnaces.

Electric Arc FurnaceThe Electric Arc Furnace process, or mini-mill, does not involve iron-making. It reuses existing steel, avoiding the need for raw materials and their processing. The furnace is charged with steel scrap, it can also include some direct reduced iron (DRI) or pig iron for chemical balance.The EAF operates on the basis of an electrical charge between two electrodes providing the heat for the process. The power is supplied through the electrodes placed in the furnace, which produce an arc of electricity through the scrap steel (around 35 million watts), which raises the temperature to 1600C, melting the scrap. Any impurities may be removed through the use of fluxes and draining off slag through the taphole.Electric Arc Furnaces do not use coal as a raw material, but many are reliant on the electricity generated by coal-fired power plant elsewhere in the grid.2.4 COAL PREPARATIONCoal preparation is a series of unit operations interconnected by a material handling system. The main unit operations are: comminution, sizing, concentrating, and dewatering. Comminution, or size reduction (crushing and grinding) is the reduction of coal from run-of-mine size to a plant feed size or sized for use. Sizing (screening or classification) is the separation into various size fractions for processing. Concentrating (cleaning, preparation, or beneficiation) is the separating of a raw coal into clean coal and refuse. Dewatering or drying is the removal of excess water from the clean coal and/or refuse (Albrecht and Matoney, 1982). A typical coal preparation plant is made up of several circuits as shown in Fig. 2.6. The coal clean devices that form part of a coal preparation plant are summarized below.Fig. 2.6 A typical coal preparation flow sheet (Albrecht and Matoney, 1982)

Jigs The first coal jigs were direct copies of ore jigs, in which a basket loaded with mixed particles was moved up and down in a tank of water. Thus agitated, the particles became rearranged in layers of increasing density from bottom to top. The same principle is used in modern coal jigs to stratify and separate the coal and refuse products. Fig 2.7 below shows a Baum Jig.

Fig. 2.7 A Baum Jig (Albrecht and Matoney, 1982)Heavy Medium VesselsCoal is slurried in a medium with a specific gravity close to that of the desired separation. The lighter coal floats and the heavier refuse sinks. The two fractions are then mechanically separated. While other media have been used, most coal cleaned by the heavy media process is separated in suspensions of magnetite in water. This suspension is achieved using very finely ground magnetite. By varying the amount of magnetite in the suspension, the specific gravity of the medium is changed, which changes the gravity of the separation. The process is versatile, offering easy changes in specific gravity to meet varying market requirements. Heavy medium separation provides more accurate separation and higher recovery of salable coal than jigging. A typical heavy medium vessel is shown in Fig 2.8.

Fig. 2.8 Heavy medium vessel (Albrecht and Matoney, 1982)Heavy Medium CyclonesIn its operation, slurry of coal and medium (magnetite dispersed in water) is admitted at a tangent near the top of a cylindrical section that is affixed to a cone-shaped lower section. The slurry forms a strong vertical flow; and under gravimetric forces, the refuse with its higher specific gravity moves along the wall of the cone and is discharged at the apex as shown in Fig 2.9 below. The coal particles of lesser specific gravity move toward the longitudinal axis of the cyclone and finally through the centrally positioned vortex finder to the discharged outlet as clean coal. The heavy medium cyclone functions efficiently even with large amounts of near gravity material in the feed.

Fig. 2.9 Heavy medium cyclone (Albrecht and Matoney, 1982)Water-only CycloneThe water-only cyclone performs a specific gravity separation employing only water and inertia. Its design feature which permits the use of water-only is the wide angle or angles in its conical bottom. This promotes the formation of a hindered settling bed, as the dense particles move down the side wall. Less dense particles cannot penetrate this heavy bed and move back into the main hydraulic current to be discharged out the top of the unit through the vortex finder. Applied in easier cleaning situations than heavy medium devices, water-only cyclones have been used to wash coals with a top size range of 1-3/4 inch to 28 mesh. A water only cyclone is portrayed in Fig 2.10.

Fig. 2.10 A Water-only cyclone (Albrecht and Matoney, 1982)TablesTables (Fig. 2.10) have been in use for over 60 years in treating 1-1/2 inch x 28 mesh coal. The most generally accepted explanation of the action of a concentrating table is that as the material to be treated is fanned out over the table deck by the differential motion and gravitational flow, the particles become stratified in layers behind the rifles. This stratification is followed by the removal of successive layers from the top downward by cross-flowing water as the stratified bed travels toward the outer end of the table. The cross-flowing water in made-up partly of water introduced with the feed and partly of wash water fed separately through troughs along the upper side of the table. The progressive removal of material from the top toward the bottom of the bed is the result of the taper of the table riffles, which allows successively deeper layer of material to be carried away by the cross-flowing water as the outer end of the table is approached. By the time the end of the table is reached, only a thin layer, probably not thicker than one or two particles, remains in the surface of the deck, this being finally discharged at the end of the table.

Fig. 2.10 Concentrating table (Albrecht and Matoney, 1982)Froth FlotationFroth flotation is a physicochemical method of concentrating fine coal. Coal is hydrophobic in nature, its gangue materials are hydrophilic; by the addition of chemicals in water these properties can be enhanced. The process involves chemical treatment of a raw coal pulp to create conditions favourable for the attachment of coal particles to air bubbles. Air bubbles are created by the rapid motion of the agitator mechanism which draws air down the hollow shaft and disperses the air into the pulp. The air bubbles carry the coal to the surface of the pulp and form a stabilized froth which is skimmed off while the ash containing particles remain submerged in the pulp. Flotation cells used for coal separation are shown in Fig. 2.11.

Fig. 2.11 Flotation cells (Albrecht and Matoney, 1982)2.5 COAL WASHABILITYWashability is a coal-cleaning jargon to designate amenability of a coal to gravity concentration. It is determined by sink-float testing as shown in Fig. 2.12. The test is performed by placing a sample of coal in progressively heavier specific gravity baths and scooping off the material that floats. This test shows how the quality and weight of the coal varies as the specific gravity changes. Washability test results are summarized in tabular and graphical form. The graphs obtained are known as "washability curves" (Fig. 2.13). The cumulative float at any float specific gravity is what a "perfect" separation would produce in the way of clean coal yield and quality, with the cumulative sink being the refuse from the perfect separation.

Fig. 2.12 Sink-float analysis (Albrecht and Matoney, 1982)

Fig. 2.13 Washability curve2.6 COAL FLOATABILITY Coal washability process works for only coarse coal particles. For fine coal particles, froth flotation based cleanability processes are applied. It is worth noting in coal floatability, a single batch flotation experiment provides only concentrate yield and concentrate ash content and no indication as to whether concentrates of different quality could be obtained. A yield/ash curve could be constructed be running a number of batch tests varying the reagent dosage conditions.Various floatability tests which include, test-tube floatability, film flotation, frothless flotation, contact angle measurements, and Hallimond tube flotation are already known, which use properties such as critical surface tension and contact angle to test the floatability of a mineral. These tests are good procedures for finding a minerals general flotation behavior but they do not necessarily predict how a mineral will behave in an industrial flotation circuit. Tests such as release analysis, tree analysis, locked-cycle tests, and the dissolved-air flotation microcell test are generally considered better for characterizing a minerals flotation behavior in an industrial environment. These floatability methods will be discussed below.The locked-cycle Flotation TestThe locked-cycle flotation test is one of the better procedures available since it is capable of providing a lot of information about a continuous flotation circuit (Nishimura, et. al, 1989). The test consists of a series of batch tests conducted with a small scale laboratory apparatus, where the products generated in one test (nth cycle) are added to the next test ((n+1) th cycle). A flowsheet of the test is shown in Fig. 2.14. Note that the test consists of five batch tests or stages which include a rougher, 2 scavenger, and 2-6 cleaner stages. The test can be arranged in any circuit configuration, but Fig. 2.14 shows the ideal 2 stage Rougher-Cleaner-Scavenger circuit as presented by Meloy (1983). In order to obtain acceptable information from a locked-cycle test, steady-state must be reached. Unfortunately, this leads to the major disadvantage of this procedure which is the fact that it is very time consuming. It has been shown that the test normally requires at least five cycles to reach steady-state, even in a simple circuit, and the time to reach steady-state increases as the amount of recycle material increases (Nishimura et al., 1989). The procedures main advantage is that the results provide good agreement with tests conducted on continuous flotation circuits. In other words the locked-cycle test is a good simulation of a plant circuit because it accurately reproduces the conditions of an industrial flotation circuit (Agar, 1985). Therefore, the locked-cycle test is as good a characterizing procedure as any found within the literature.Release AnalysisThe first release analysis procedure was developed by C. C. Dell in 1953 and is known as timed release analysis (Dell, 1953). In this procedure time is used to separate a sample into fractions of varying floatability. The flotation procedure is carried out in a laboratory batch cell and an excess of reagent is used throughout.

Fig. 2.14 A Locked- Cycle Flotation TestA serious drawback of this method is that water has to be decanted off the collection pan before a concentrate can be reintroduced to the cell in subsequent flotation stages to avoid pulp overflow and entrainment of gangue material. In the case of coal flotation, decantation is not very practical since low-density coal tends to remain in suspension and is often removed when water is decanted.In 1965, the decantation problem was partially solved by adjusting the flotation times associated with the procedure (Cavallaro and Deurbrouck, 1965). The procedure was additionally modified to allow for varying mechanical flotation conditions. Instead of only varying the flotation time for each concentrate, the impeller speed was also varied. This supposedly improved the separation because the fractionation was not just based on time but also on the physical flotation conditions.The only major problem associated with timed release analysis is that it is time consuming and rather tedious. However, it has been found to be reproducible, independent of cell type, reagent dosage, pulp density, and operator interaction. A flow sheet of the procedure is shown in Fig. 2.15 below.

Fig. 2.15 Schematic diagram of a Timed Release analysis procedureIn 1964, Dell outlined an experimentally simpler version of the original timed release analysis procedure referred to here as simplified release analysis (Dell, 1964). In the simplified release analysis procedure, the sample is fractionated by operating conditions as opposed to time used in the original procedure. Again the experiment is carried out in a standard laboratory flotation cell which allows for independent control of impeller speed and aeration rate. Excess of reagent is also used in this version just like the original procedure.The simplified release analysis procedure consists of two stages. The first stage objective is to completely separate the floatable material from the non-floatable material by repeated flotation of the concentrate under intense flotation conditions (i.e., high aeration rate and impeller speed). The second stage objective is to separate the floatable material from the first stage into fractions beginning with the low-recovery, high-grade material and proceeding to the high-recovery, low-grade fraction. The concentrate from the first stage is floated at several impeller speeds and aeration rates of steadily increasing intensity. Under these flotation conditions, concentrates of decreasing quality are collected and a release curve is generated. Therefore, the fractionation is dependent on the flotation intensity and not the timing of the concentrate collection. A flow sheet of this procedure is shown in Fig. 2.16.Although the simplified procedure is generally considered simpler than the timed release analysis technique, it may be more sensitive to operator bias. An example of this is found in the first stage of the experiment when the floatable material is separated from the non-floatable material. The number of floats used and the way in which the froth is removed can greatly affect this separation. If any floatable material is lost during this part of the experiment, the release curve can be impacted significantly. During the second stage, it is often difficult to control the rate of flotation by simply adjusting aeration rate and impeller speed. In the case of a highly floatable coal, for example, the valuable material may tend to float within the first two fractions leaving very little material for the latter fractions. This results in the data being grouped at either the highyield end or low-yield end of the release curve, depending on the floatability of the material.

Fig. 2.16 Schematic diagram of a Simplified Release analysis procedureAlthough the simplified procedure is generally considered simpler than the timed release analysis technique, it may be more sensitive to operator bias. An example of this is found in the first stage of the experiment when the floatable material is separated from the non-floatable material. The number of floats used and the way in which the froth is removed can greatly affect this separation. If any floatable material is lost during this part of the experiment, the release curve can be impacted significantly. During the second stage, it is often difficult to control the rate of flotation by simply adjusting aeration rate and impeller speed. In the case of a highly floatable coal, for example, the valuable material may tend to float within the first two fractions leaving very little material for the latter fractions. This results in the data being grouped at either the highyield end or low-yield end of the release curve, depending on the floatability of the material.The problem may be avoided by using flotation time to fractionate the sample in the second half of the test (Pratten et al, 1989); however, this procedure deviates from Dells original approach, and there is some question as to how the release curve is impacted by time fractionation in the second stage.Tree AnalysisAs a result of the possible experimental biases associated with release analysis, an alternative technique, known as tree analysis, was developed in 1983 (Nicol et al., 1983). The procedure was based on the concept of fractionating a sample by progressively refloating the concentrate and tailings such that the test branches out in the form of a tree.According to the tree analysis procedure, a sample is initially floated at some arbitrary set of flotation conditions (i.e. impeller speed, aeration rate, and flotation time) and is split into a concentrate and a tailings fraction. These fractions are then refloated under the same flotation conditions, and their subsequent concentrate and tailings fractions are refloated. This procedure is continued until the final products contain less than 2% of the initial feed mass or until the desired flotation level is reached. The final products are then analyzed and sorted in order of decreasing grade. These results are then used to construct a release curve. In order to produce data points in the high-grade region of the curve, collector dosages are generally kept low. A flow sheet of this procedure is shown in Fig. 2.17.

Fig. 2.17 Flow sheet of a Tree analysisTree analysis is probably the simplest of the release analysis procedures inconcept but it is also the most tedious since it may require anywhere from 16-32 fractions (i.e. 4-5 levels) to span the appropriate grade-recovery range. In theory, the tree procedure should provide the best separation since both the tailings and concentrate are refloated numerous times to ensure that all of the material is properly fractionated according to grade. One of the most recent forms of release analysis is a column-based procedure described by McClintock, Walsh and Rao (1995) which uses a flotation column in conjunction with a slightly modified version of Dells simplified release analysis procedure. The procedure uses a laboratory column with feed rates, wash water flow rates, and aeration rates appropriate for the size of the column being used. In the specific application presented, a glass column with an inside diameter of 25 mm and a height of 2.4 m was used. The aeration rate ranged from 50-250 ml/min, wash water rate from 15- 20 ml/min, and feed rate from 40-50 g/min. The column was operated in a continuous mode and feed was passed through the column to produce a concentrate and tailings fraction. After the initial concentrate was set aside, the tailings were pumped back through the column to produce a second concentrate. The procedure was repeated until the froth was barren. After this first stage of separation in which the floatable material was separated from the non-floatable material, the final tailings were set aside for analysis and the concentrates were combined for separation in the second stage of the experiment. To begin the second stage, the combined concentrates were pumped through the column and a low aeration rate was used to produce a high-grade fraction. Again the concentrate was saved for analysis and the tailings were pumped back through the column at a slightly higher aeration rate. The procedure was repeated at higher and higher aeration rates until the froth was barren. All of the resulting concentrates and the final tailings were then analyzed and the results used to construct the release curve.The main difference between column-based release analysis and Dells simplified release analysis technique is that the column-based procedure uses repeated flotation of the tailings in the first stage rather than the concentrate. This is possible because the wash water used in a flotation column prevents entrainment, so additional cleaning of the froth product is not necessary. Repeated flotation of the tailings also ensures that all floatable material reports to the second stage for fractionation.In spite of the work that has been conducted to define and develop the various release analysis procedures, these techniques are by no means considered standards for characterizing flotation. In fact, it is still unclear as to which technique comes closest to representing the ideal flotation separation. It is also unclear as to whether the separation obtained by release analysis can be surpassed using a novel cell design. In a recent comparison of column flotation technologies, Honaker, Mohanty and Ho (1995) reported that on several occasions, the separation performance of the flotation columns exceeded the separation results obtained using Dells simplified release analysis procedure.Furthermore, the Packed Column was found to give consistently better results under high-grade, low-recovery conditions than would be indicated by simplified release analysis. Clearly, additional work is needed in the effort to define a method of characterizing flotation performance that is comparable to float-sink analysis in gravity separation.2.6 ASH CONTENTAsh is the residue remaining after the combustion of coal under specified conditions (ASTM D-3174, ISO 1171) and is composed primarily of oxides and sulfates. Thus, ash is formed as the result of chemical changes that take place in the mineral matter during the ashing process. The quantity of ash depends on the nature of the mineral matter and the chemical changes that take place in the ashing. A fundamental practice in the coal preparation process is the analysis of the ash content of coal. And the knowledge of the composition of ash is useful in predicting the behavior of ashes and slags in combustion chambers. The amount and composition of ash is important in determining the best cleaning methods for coals, in selecting coals to be used in the production of coke, and in the utilization of the ash by-products of coal combustion. The release of certain trace elements into the environment as combustion products or in the disposal of ash is a concern for coal-burning facilities.2.7 NIGERIAN COAL RESERVE2.7.1 Coal Reserves and Brief HistoryCoal was the first fossil fuel to be discovered and used in Nigeria. The Federal Government of Nigeria owns the coal resources. Around 1919, the Geological Survey of Nigeria (GSN) was established to take over coal exploration activities. The Minerals Ordinance of 1946 and the Coal Ordinance No. 29 of 1950 provided the initial legal framework for the development of solid minerals in Nigeria (Behre Dolbear, 2008). Also in 1950, the Nigerian Government, through the Coal Ordinance No. 29, created the Nigerian Coal Corporation (NCC). Under this Ordinance, the NCC had the exclusive rights to mine coal in Nigeria until 1999, when a new mineral exploration and exploitation policy and legislation were established by the FGN. This policy opened up the sector to private industry participation and joint ventures.Nigeria has extensive coal resource with inferred reserves estimated at ranging from 1.5 billion metric tonnes to 2.75 billion metric tonnes (Nigerian Coal Corporation, 2004; U.S. Environmental Protection Agency, 2007) and proven reserves of 639 million tonnes, (Ogunsola, 1990; Nigerian Coal Corporation, 2004). Studies conducted Behre Dolbear (2005, 2008) for the Nigerian Ministry of Solid Minerals Development (MSMD) indicates that the Nigerian coal resources are far larger than previously reported. There are over 13 coal mines and more than 21 coal blocks located across Nigeria (Nigerian Coal Corporation, 2004). Table 2.2 shows the name, location, reserve, coal type, and mining method of thirteen existing coal mines in Nigeria with potential.Table 2.2 Existing potential coal mine sites with reserves in Nigeria

Nigerias coal resources are not being fully explored or even marginal ly developed despite its long history. Nigerian coals are of cretaceous or early tertiary age and are mostly located in the eastern part of the country, as shown on the map in Fig. 2.18. They are mostly lignite and sub-bituminous, with some deposits of high volatile bituminous (Ogunsola, 1990). Nigeria has more than 1.5 billion metric tons of sub - bituminous to bituminous coals and unquantifiable lignite deposits (NCC, 2007).Coal occurs in several areas in Nigeria and ranges from bituminous to lignite. The coal deposits of the Anambra Basin, located in southeastern Nigeria, appear to contain the largest and most economically viable coal resources. This basin covers an area of approximately 1.5 million hectares and is constrained by the Niger River on the west, the Benue River on the north and the Enugu Escarpment on the east. The coal is predominantly in one seam that outcrops along the eastern side of the basin at the base of the Enugu Escarpment and dips gently toward the center of the basin as we can see in Fig. 2.19 and Fig. 2.20.2.7.2 Enugu DistrictThe Enugu Coal District, covering 270,000 hectares of the coal basin, is centered around Enugu City, south of the Benue District. It has supported the largest amount of commercial mining in the past as Fig 2.21 shows. In addition to two underground mines, there are a total of 36 drill holes drilled in the area. Previous studies have estimated the demonstrated coal resource to be 49 million tones averaging 2.2 meters thick. An additional 111 million non- reportable tonnes of in-place coal are inferred to exist west of the old mine workings.

Fig. 2.19 Map of Nigeria showing coal and lignite areasEnugu is situated on the western edge of the Cross river plain and is dominated by the Enugu escarpment just west of the town. For the first (122 152) m, the escarpment is steep, but it then rises more gently to about 427 m above sea level and about 4 27 m above sea level and about 183 m above Enugu. Further west, several large but low hills attain an elevation of nearly 518 m. The plain on which Enugu stands is underlain by the Enugu shales. The lower coal measures outcrop on the lower and middle slopes of the escarpment, while the less resistant false bedded sandstones form the gentle upper slopes and crest (De Swardt and Casey, 1961). The estimated reserve off Onyeama mine is 150 million tonnes.

Fig. 2.20 Generalized geological Map of Nigeria

Fig. 2.21 Sketch Map of the Anambra coal basin showing explored areas (Drilled holes in orange colour).CHAPTER THREE3.0 MATERIALS AND METHODOLOGY3.1 MATERIALSThe coal used for this study was procured from Onyeama coal mine in Enugu state. There are five coal fields in Enugu; Onyeama, Okpara, Inyi, Ezinmo, and Amansiodo. According to Engr. Amanoh (2012), these coal fields have the same geology and chemical composition. Coal sample from any of the fields can be used as a representative sample for studies on Enugu coal. Onyeama coal was chosen for this study because of ease of accessibility of the coal field. The chemical composition of Enugu coal is given in Table 3.1. The raw coal collected from Onyeama mine was manually crushed to reduce the size for the sink-and-float analysis and the tree analysis. The particle size distribution of the reduced coal is shown in Fig. 3.1 and the d50 of the coal samples is 5950 m. 3.2 CHEMICAL REAGENTSThe chemical reagents required for the washability study of the coal samples are: Distilled water Zinc chloride Kerosene

TABLE 3.1 Chemical Composition of Coal in Onyeama MineQUALITY PARAMETERONYEAMA MINE

Moisture (%a.r.)7.5

Ash (%a.r.)8.4

Volatile Matter (%a.r.)38.26

Gross Calorific Value Kcal/Kg (a.r.)6.628

Fixed Carbon (%a.r.)45.38

Hydrogen (%a.r.)4.11

Total Sulphur (%a.r.)0.54

Nitrogen (%d b)1.44

Sulphate Sulphur (%b)0.01

Pyrite Sulphur (%b)0.05

Organic Sulphur (%b)0.69

Swelling No (FSI)1.00

Hard Groe Index(HGI)37.8

Dilatometer Test: Softening Point (oC)356

Reconsolidation Point ( oC)477

Fig. 3.1 Particle size distribution of the reduced coal 3.3 SPECIAL APPARATUSThe special apparatuses used in carrying out this study are: Hammer Beaker Hydrometer Desiccators Ball mill Denver Lab floatation cell and Muffle furnace

3.4 EXPERIMENTAL PROCEDURE3.4.1 Sample PreparationThe samples collected were manually crushed to obtain different size fractions suitable for the sink-float analysis and the froth flotation release tree analysis. Particle sizing analysis was carried out on the crushed coal product to obtain the size distribution curve. Shown in Fig. 3.2 and 3.3 are sieves and mechanical shaker used in carrying out the particle size analysis. For the sink-float analysis, coal samples in the size range of 13300 5600 m were used for the analysis. Reduced coal in the size range of 4000 600 m were further reduced and the 150 m coal samples used for the release tree analysis.

Fig. 3.2 Vertical arrangements of sieves

Fig. 3.3 Sieves mounted on mechanical shaker3.4.2 Sink and Float AnalysisCoal sample of 5.6mm to 9.5mm size range weighing 157g was used in the sink and float test. Solutions of different specific gravity ranging from 1.2 to 1.7 were prepared using ZnCl2 solution. The five solutions were put into five beakers respectively and arranged in the order of increasing specific gravities (1.3, 1.4,,1.7). The sample was first placed in the lowest specific gravity liquid. The coal particles with specific gravity lesser than that of the solution floated, while the ones greater than the specific gravity of solution sank (Fig. 3.4). The float from solution of S.G = 1.2 was collected, dried and weighed while the sink was immersed in the solution of S.G = 1.3, the coal particles with S.G less than 1.3 floated while the particles with greater S.G sank. The float from solution of S.G = 1.3 was collected, dried and weighed while the sink was immersed in the solution of S.G = 1.4. This process was repeated until we got to the solution with S.G of 1.7. The floats from different specific gravities were collected, dried and also weighed. These analysis were done making sure that we minimized the quantity of coal particles lost in the process of collecting the floats and sinks. The ash content of coal particles of the floats and sinks were determined and washability curves were plotted.

Fig. 3.4 Sink-float test3.4.3 Froth Floatation (Tree) AnalysisA three-level tree analysis was selected to characterize the coals being studied in this investigation. The analysis was done using a Denver laboratory flotation cell (Fig. 3.5). The impeller rate was set at 1500 rpm and the aeration rate at 50% of maximum, while the collection time was selected to be 2 minutes per float. These mechanical flotation conditions were held constant throughout the test. The collector used was kerosene.

Fig. 3.5 Denver lab flotation cell used for the tree analysisThe initial feed was then floated and its product and tailings fractions were then refloated separately. This procedure was repeated as needed to produce a three-level tree. A total of 8 products (four concentrates and four tailings) were obtained, and they were subsequently filtered, dried, weighed, and assayed. Fig. 3.6 shows the diagrammatic procedure of carrying out a three (3) level tree analysis. Data obtained from the analysis were used in plotting the coals floatability curve.

Figure 3.6 Schematic diagram of the 3-level tree analysis procedure3.4.4 Ash Content AnalysisAsh is the non-combustible residue that remains after the complete combustion of coal. It is the mineral constituent of coal that remains after carbon, oxygen, sulphur and water has been removed during combustion. The analysis is fairly straightforward with the coal thoroughly burnt and the ash materials expressed as a percentage of the original weight (sawtell, 1906). According to the ASTM method D3174 (3), the coal sample is placed in a cold furnace and heated gradually so that the temperature reaches 450o C to 500oC in one hour and 700oC to 750oC at the end of the second hour.In this project, the sample (yield) from both analysis (i.e. tree analysis and sink & float analysis) were dried and weighed after which it was placed in a crucible and heated in muffle furnace to a temperature of 730oC for two hours. After heating, the samples in the crucible were allowed to cool in the furnace and stored in desiccators to avoid any moisture content in the samples after which the samples in the crucible was weighed. The weight of the empty crucibles was determined before heating the samples in the muffle furnace. The following calculations were made to get the ash content of the samples (yield).X Weight of empty crucible.Y Weight of crucible + weight of sample (before heating)Z Weight of crucible + weight of sample (after heating)%Ash Content of sample = (Z X)/(Y X) * 100Note: a) Z X = weight of ash (ash). b) Y X = weight of sample (yield).Therefore; %Ash Content of sample = (ash)/ (yield) * 100

CHAPTER FOUR4.0 RESULTS AND DISCUSSIONS4.1 RESULTS OF SINK-FLOAT ANALYSISThe results obtained from the sink-and-float analysis of Enugu coal are presented in Tables 4.1 and 4.2. Table 4.1 Sink -Float test Data 1: S.G = Specific gravity; Weight = Yield1234567Cumulative8Cumulative0.1 Density distribution

S. G RangeS.GWeight (g)Weight (%)Ash(%)Ash as % totalWeight(%)Ash(%)9S.G10Cum weight %

Float on 1.31.39158.4143.9625.6858.4125.68

1.3 - 1.41.426.80317.251.1910.1875.6135.861.375.61

1.4 - 1.51.54.783.0764.081.9778.6837.831.420.27

1.5 - 1.61.66.8334.3862.232.7383.0640.561.57.45

1.6 - 1.71.71.20.7749.330.3883.8340.941.65.15

Sink in 1.7Sink25.20416.1787.0314.0710055.01

Total155.82100

Table 4.2 Sink-Float test Data 21234Ash5Distribution6Cumulative7Coal8Distribution9Cumulative

S.GMass(%)Assay (Ash %)(Mass in g)(%)(%)(Mass in g)(%) (%)

1.358.4143.9625.6846.6846.6832.7372.7572.75

1.417.259.1910.1818.5165.197.0215.683.35

1.53.0764.081.973.5868.771.12.4490.79

1.64.3862.232.734.9673.731.653.6794.46

1.70.7749.330.380.6974.420.390.8795.33

Sink16.1787.0314.0725.581002.14.87100

Total10085.70755.0110044.99100

Shown in Fig. 4.1 and Fig. 4.2 are the washability curves of Enugu coal generated from data obtained from Tables 4.1 and 4.2. The interpretations of these curves are given below.

Fig 4.1 Washability curves obtained from Table 4.1

Fig 4.2 Washability curves obtained from Table 4.2

4.1.1 INTERPRETATION OF SINK-FLOAT ANALYSIS RESULTSEase of Cleaning Enugu CoalThe ease or difficulty of cleaning a coal can be determined by using either the cumulative float curve, elementary curve, or distribution curve of the analyzed coal. For the cumulative float curve and elementary curve, the shape of the curves are compared to that of an ideal separation curve (Fig. 4.3-a), which is L in shape. If the curves approximate to the L shape of the ideal curve, it implies that the coal is an easy to clean coal (Fig. 4.3-b). If the curves do not approximate to the L shape, then the coal is difficult to clean. If the curves are vertical, it means that it is impossible to clean the coal (Fig. 4.3-c).As can be seen from Fig. 4.1, the cumulative float curve of Enugu coal does not approximate to the L shape of an ideal separation curve, this shows that Enugu coal is difficult to clean. To use the distribution curve to determine the ease or difficulty of cleaning a coal, the distribution curve of the coal must show a well-defined or horizontal plateau for the coal to be easy to clean. If no plateau is reached, the coal is not easy to clean. As can be seen from Fig. 4.1, plateau is not reached in the distribution curve of Enugu coal. This also helps to confirm that Enugu coal is difficult to clean.

Fig. 4.2 Primary washability curves for (a) ideal separation, (b) easy-to-clean coal, (c) difficult-to-clean, and (d) impossible separation (Laskowski,2001)Another method of determining the ease or difficulty of washing of coals is to estimate the difference between the recovery range obtained for a given range of specific gravities (say 1.4 1.6) from the washability curve, and comparing it with the standard range given in Table 4.3 below. From the Graph of Fig.4.1, the recovery at S.G of 1.4 is 73% and the recovery at S.G of 1.6 is 84%. The difference between the recoveries is 11 which falls under the category of Difficult as obtained from near gravity material table above. This also helps to confirm that Enugu coal is difficult to clean.Table 4.3 Near gravity material as measure of degree of coal cleaning difficultyPercentage of near gravity materialDegree of difficulty

0 56 1011 2020 or moreEasyModerateDifficultExtremely difficult

The difficulty of cleaning Enugu coal is as a result of the presence of near gravity materials (NGMs) in Enugu coal. NGMs are materials that have a relative density that is close to that of the separating liquid or dense medium used to separate coal and impurities contained in raw coal. They make the separation more difficult because they settle very slowly in the dense-medium due to the fact that their density is close to that of the medium. This increases the probability that material that should have reported to floats wrongly reports to sinks and misplaced material.With the difficulty in cleaning of Enugu coal, to use the coal for power generation, a low NGM coal should be blended with Enugu. This will help increase the ease of washing the coal for use in power generation. The distribution curve can also be used to design the washing plant that will be attached to the coal-based power plant that will utilize Enugu coal. The curve should be used to determine the specific gravity range at which the difference in % recovery will fall within the 0 7% range (easy to wash range). As shown in Fig. 4.1, this should be at specific gravity values greater than 1.5. Fig. 4.2 shows that if a dense-medium of specific gravity 1.5 is used, a recovery of about 80% will be obtained. Yield and Ash at Any Given Specific GravityFrom Fig. 4.1 and Fig. 4.2, we can directly find out the yield and ash of Enugu coal at a certain specific gravity of washing. This will help an operator to know the right specific gravity of the dense-medium to be used in the design of the washing plant required to achieve the given yield. For instance, using Fig. 4.1, at a specific gravity of 1.5, the coal yield that will be recovered will be about 80%, containing about 39% of ash (Fig. 4.4).Efficiency of a Washing PlantThe efficiency of a washing plant can be determined by using the washability curves plotted in Fig 4.1 and Fig. 4.2. This will help an operator to know if the washing plant is performing or not. For example, the washability curve from Fig 4.1 has a recovery of 80% coal yield at S.G of 1.5; assuming a separating equipment recovers 75% yield at the same specific gravity (1.5), it means that the separating efficiency () of the equipment is; = (actual yield) * 100 / (theoretical yield) = (75 * 100) / 80 = 93.75%This efficiency is also known as organic efficiency of the washing plant.

Fig. 4.4 Enugu coal washability curves showing the yield and ash content at specific gravity of 1.5.

4.2 FROTH FLOTATION ANALYSISThe result obtained in the froth flotation (tree) analysis is given in the Table 4.4.Table 4.4 Tree Analysis DataTest DataCalculated Data

1Sample2Mass3Assay4Mass5Recovery6RecoveryMass of Concentrate per 100

Units of Ash

(gms)Ash(%)Ash(%)Cum (%)7%8Cum (%)

Concentrate 19.7451.144.9830.5330.5359.7259.72

Concentrate 34.2146.641.9612.0242.5523.8183.53

Tailing 45.2646.612.4515.0257.5732.25115.78

Concentrate 23.145.51.418.6566.2219.01134.79

Concentrate 42.4842.111.046.3772.5915.21150

Tailing 14.8636.951.811.0483.6329.8179.8

Tailing 26.0634.892.1112.9496.5737.16216.96

Tailing 32.1226.550.563.4310013229.96

Total37.83330.3916.31100

Fig.4.4 Release tree analysis curveShown in Fig. 4.4 is the release tree analysis curve of Enugu coal. This curve is like the grade-recovery curve and can be used to determine the combustible recovery that will be obtained from Enugu coal at any given ash content. This curve can be used to analyze the cleanability of fine coal particles.Since Enugu coal is difficult to clean, instead of using the physical gravity separation process to separate the coal from ash, froth flotation technique can be adopted. This is because the separation technique is based on the surface properties of coal and its impurities. However, the size of coal concentrate can be a limiting factor when considering coal-based power plants as froth flotation works well only when the coal particles are fine, while the coal-based power plants may require large lumps of coal to operate. Therefore the fine coal particles are agglomerated or sintered for the coal base plant.

CHAPTER FIVE5.0 CONCLUSION AND RECOMMENDATION5.1 CONCLUSIONBased on the findings of this study, the following conclusions have been drawn:1) The washability characteristic curves of Enugu coal show that Enugu coal is difficult to clean. 2) The difficulty in the washability of Enugu coal is as a result of the high ash content of the coal and the presence of large amount of near gravity materials (NGMs) in the coal.3) To use Enugu coal for power generation, coals having low NGM should be blended with the coal. This will help increase the ease of washing the coal.4) The distribution curve of the coal can also be used to design the washing plant that will be attached to the coal-based power plant that will utilize Enugu coal. The curve should be used to determine the specific gravity range at which the difference in % recovery will fall within the 0 7% range (easy to wash range).5) Froth flotation technique has been recommended as the best technique for washing Enugu coal. This is because the separation technique is based on the surface properties of coal and its impurities, and not on the physical gravity properties of the coal. However, the size of coal concentrate can be a limiting factor when considering coal-based power plants as froth flotation works well only when the coal particles are fine, while the coal-based power plants may require large lumps of coal to operate. 5.2 RECCOMMENDATIONFor further work, the authors recommend the following:1) Washability studies of other coal deposits in Nigeria should be done. This will help determine the easy to wash coals in Nigeria that can be blended with Enugu coal.2) Washability studies of Enugu coal, blended with other low NGM coals should be done to determine if the difficulty in washing Enugu coal will be reduced.3) The sink-float analysis of the present study was carried out at a specific gravity range of 1.3 1.7. This gave fewer readings. We recommend a specific gravity range of 1.3 2.0 for further studies.

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