Studies On The Treatment Of Coal And Brewery Wastewater Using Adsorption And Coagulation– Flocculation Techniques

CHAPTER TWO

3.0 LITERATURE REVIEW

2.1 COAL Coal

Consists of a complex mixture of organic chemical substances containing carbon, hydrogen and oxygen, together with smaller amount of nitrogen, sulphur and some trace elements. Strictly speaking, coal (black or brownish black) is not a hydrocarbon, but a fossil, or an organic sedimentary rock, formed mainly by the action of temperature and pressure due to overlaying with layers (Onwu, 1999;Menkiti,2007).

2.1.1. COAL FORMATION

The principal initiating requirement for coal formation are swampy or marshy environment, climatic condition favorable for rapid plant growth with enough depth of water to exclude or severely restrict oxygen supply during breakdown of original plant material when it dies and falls into the water. (Shkla, 1979; Cairns, 1974; Menkiti, 2007). It is also essential that the swampy area should continue to subside in order to allow further growth and accumulation. Such subsidence creates pressure and temperature necessary for bacterially assisted partial oxidation and hydrolysis which degrade the cellulose, lignin and proteins ultimately into different rank of coal.

The quality (ranking) of coal depend also in the geological time of formation (Macrae, 2004). Decomposition occurs gradually under high pressure and in the absence of air. Carbon (IV) oxide, methane, and water were liberated leaving behind a material that contained a very high percentage of carbon. During this process of carbonization, the vegetable materials was converted in stage into lignite (brown carbon), bituminous (soft coal) and finally anthracite (hard coal) which is about 45% pure carbon. Impurities present include nitrogen, sulphur and phosphorus (Menkiti, 2007).

In due course, sub- bituminous coals
Pressure, time, heat Bituminous coal, semibituminous
coals
Anthracite
Fig 1: Schematic diagram of coal Genesis. (Source : Onwu, 1999; Menkiti, 2007).

2.1.1 TESTING OF COAL

The classical analytical method for coal analysis is called “ultimate” and “proximate” method of analysis.

a) Ultimate Analysis: The ultimate analysis precisely determines the proportion of the main chemical elements contained in the coal substance. These elements include hydrogen, nitrogen, sulphur and oxygen. The oxygen may be determined chemically but is usually estimated by difference. The result of the analysis may be presented as – received basis, a dry basis or most commonly, a dry mineral matter free basis. (Forney and Mcgee,1972;Menkiti, 2007).
b) The Proximate Analysis: This is so called because it is less precise, though generally it is very useful. It involves the determination of moisture, ash, volatile matter and fixed carbon contents. The results depend on the methods used, which mainly affects the volatile matter. The heating condition is particularly of interest in this type of analysis. On a dry ash-free basis, the fixed carbon plus volatile matter equals 100%. (Eddinger and Friedman,1996;Menkiti, 2007).

2.1.3 PROPERTIES OF COAL.

Coal properties are generally divided into two, namely: Physical and chemical a) Physical Properties: These are properties that determine the physical nature and character of coal..Such properties include the following: strength, caking power, coke ability, porosity, calorific value and reflectance. (Vullis,1985). b) Chemical Properties: These properties account mainly for the chemical behaviour of the coal. Such properties include the following: volatile matter, moisture content and fixed carbon (Menkiti, 2007).

2.1.4 USES OF COAL

Coal can be used in the following ways: a) Coal is used as a source of energy for cooking. b) Coal is used for making various drugs. c) It is also used for making various chemicals. d) Coal is used as a fuel to generate power for steam engines, factories and electric plants.

2.1.5 COAL WASTE WATER

The main categories of waste water emanating from coal processing are:
a) Acid-mine drainage waste
b) Coal washery effluent.

a) Acid-Mine Drainage: The results from the passage of water- runoff through mine where iron disulphide are exposed to oxidation. The mine sulphuric materials are exposed to air and moisture leading to the formation of ferrous sulphates and sulphuric acid as shown below: 2Fe S2 + 702 + H20 2Fes04 + 2H2S04 The ferrous sulphates are washed off in the run-off as acid mine drainage (Gillenwater, 2001). b). Coal Washery Effluent: This is large quantity of waste water resulting from the physical cleaning operation of coal. (Gillenwater, 2001). This operation is carried out after coal is mined and brought to the surface. The washery effluent also known as coal preparation waste, has to be treated to reduce toxicity and typically is composed of 40% solids. (Parton, 1996; Menkiti, 2007). Maiti (2004) Characterized coal washery effluent and came up with result shown below: