EFFECT OF SULPHATE IN THE ENVIRONMENT

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Thermal decomposition reactions of solids

Thermal decomposition of solids means the breakdown of one or more constituents of the reactants into simpler atomic groupings upon heating. The thermal decomposition of a solid may be associated with physical transformations, such as melting, sublimation and recrystallization. The recrystallization of a solid may result in the production of a higher temperature lattice modification, which permits increased freedom of motion of one or more lattice constituents. The reactivity and chemical properties of solids are strongly influenced by the relative immobility of the constituent ions or molecules in the lattice of the reactant phase. The reactivity of identical chemical groupings in a solid reactant may vary with their position in the solid, as the structure may contain imperfections. In regions of local distortion, the forces of lattice stabilization may be relatively diminished, with a consequent increase in the probability of reaction. This contrasts with the homogeneous behaviour of similar groups in the liquid or gaseous phase. In rate processes of solids it is often observed that there are localized regions or sites of preferred onset of reaction. Such initiation usually occurs at a surface, leading to the development of a zone of preferred chemical transformation, which thereafter progressively advances into adjoining volumes of unreacted material. This restricted zone of the solid is called the reaction interface (Bamford and Tipper, 1980).
The occurrence of reaction is usually regarded as being exclusively restricted to the reactant-product interface, at which local conditions markedly enhance the ease of the chemical transformation. The kinetic characteristics of the overall process are determined by the velocity of the advance of this interface into unchanged reactant and the variation of its effective area with time (Bamford and Tipper, 1980). The following general kinetic tenets have been used as a widely accepted basis for the interpretation of the kinetic behaviour of the decomposition reactions of solids (Bamford and Tipper, 1980):
1) the rate of reaction of a solid is proportional to the aggregate effective area of the reactant product interface,
2) the rate of interface advance is constant through an isotropic reactant under isothermal conditions and
3) the temperature dependence of the rate coefficient obeys the Arrhenius equation.
These tenets are applicable only where the reactant undergoes no melting. If no melting occurs, the shape of the fraction decomposed (α) against time (t) curve for an isothermal reaction can be related to the geometry of formation and advance of the reaction interface.

SULPHUR PRODUCTION PROCESS USING HYDROGEN GAS

Hydrogen sulphide (H2S) is a highly toxic, corrosive and malodorous gas. Besides its other bad habits, it also deactivates industrial catalysts. H2S is commonly found in natural gas and is also a by-product at oil refineries. If water comes into contact with gas streams containing hydrogen sulphide it turns sour (Cadena and Peters, 1988). In water, sulphide (S2-) has an oxygen demand of 2 mol O2/mol S2- and thus would consume oxygen and have an adverse effect on aquatic life if discharged into surface water (Kobayashi et al., 1983). Because H2S is such an obnoxious substance, it is converted to non-toxic and useful elemental sulphur at most locations that produce it.
Removal of H2S from gas streams is a familiar industrial requirement, whose economic importance will grow with the increasing utilization of fuels with higher sulphur content. Among the removal processes for H2S, conversion to elemental sulphur is advantageous because sulphur can be used for the treatment of gases in an environmentally permissible procedure (Astarita et al., 1983; Kohl and Riesenfeld, 1985). It can also be applied to the treatment of gases with relatively low concentrations of H2S in the presence of CO2. The conventional chemical processes for H2S abatement and sulphur recovery (e.g. the Claus process) have some drawbacks, such as deactivation, loss of absorbent or catalyst poisoning or side reactions, unfavourable selectivity, corrosiveness, toxicity and the need to operate at a high pressure or temperature (Cork et al., 1986).

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CHAPTER 1 INTRODUCTION
1.1 WASTE MATERIALS
1.2 SLUDGE DISPOSAL PROCESSES
1.3 RECOVERY PROCESS
CHAPTER 2  LITERATURE REVIEW
2.1 OCCURRENCE OF SULPHATE
2. 2 EFFECT OF SULPHATE IN THE ENVIRONMENT
2.3 TREATMENT OF SULPHATE RICH WATER
2.4 THERMAL ANALYSIS
2.5 THERMAL DECOMPOSITION OF GYPSUM TO CALCIUM SULPHIDE
2.6 SULPHUR PRODUCTION PROCESS USING HYDROGEN GAS
CHAPTER 3 EXPERIMENTAL TECHNIQUES
3.1 THERMOGRAVIMETRY
3.2 X-RAY ANALYSIS
3.3 TUBE FURNACE
3.4 MUFFLE FURNACE
CHAPTER 4 AIM
4.1 THERMAL STUDIES (A)
4.2 SOLUBILITY OF CAS
4.3 SULPHIDE STRIPPING AND ABSORPTION (B)
4.4 H2S GAS ABSORPTION AND SULPHUR FORMATION (C)
CHAPTER 5 MATERIALS AND METHODS
5.1 THERMAL STUDIES
5.2 SOLUBILITY OF CaS
5.3 SULPHIDE STRIPPING AND SULPHUR PRODUCTION
CHAPTER 6 RESULTS AND DISCUSSION
6.1 THERMAL STUDIES
6.2 SOLUBILITY OF CaS
6.3 REACTION MECHANISM FOR SULPHIDE STRIPPING
6.4 SULPHIDE STRIPPING USING A PRESSURISED UNIT
6.5 H2S GAS ABSORPTION AND SULPHUR FORMATION
CHAPTER 7 CONCLUSIONS
7.1 THERMAL STUDIES
7.2 SOLUBILITY OF CAS
7.3 REACTION MECHANISM FOR SULPHIDE STRIPPING
7.4 SULPHIDE STRIPPING USING A PRESSURISED UNIT
7.5 SULPHUR FORMATION
7.6 RECOMMENDATIONS
7.7 PROPOSED PROCESS DESCRIPTION
CHAPTER 8 REFERENCES

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