Dissolution Study of a South African Industrial Calcium based Waste

Transcription

1 Dissolution Study of a South African Industrial Calcium based Waste Nelson Masilela, and Hilary Rutto Abstract-- Wet Flue gas desulphurization (FGD) systems are commonly used to remove sulphur dioxide by contacting it with industrial calcium based waste (limestone) in aqueous phase in which it reacts with industrial calcium based waste to form either calcium sulfite or calcium sulfate. In this study, the dissolution of a industrial calcium based waste in HCl acid was investigated over a wide range of parameters. The effects of ph, stirring speed; solid to liquid ratio and acid concentration on the dissolution of industrial calcium based waste was studied. It was found out that the dissolution rate of industrial calcium based waste increased with increase in acid concentration, temperature, stirring speed and it decreased with an increase in solid to liquid ratio and ph. An activation energy of kj/mol was obtianed, this indicate that chemical recation was the rate limiting step Keywords Activation Energy, Coal Fly Ash, Desulphurization, Dissolution. I. INTRODUCTION MMISIONS of sulphur dioxide into the atmosphere have Eincreased steadily with industrial development, the combustion of coal produces sulphur dioxide. Legislation to curb the emission of sulphur dioxide was introduced in early 1990 in South Africa and there is an urgent need to conduct research in this field [1,2]. Flue gas desulphurization (FGD) system has been identified as a process for better performance and reliability so as to ensure that sulphur dioxide emissions are within the obligatory limits. There are several ways of removing sulphur dioxide from the flue gas. These include dry, semidry and wet flue gas desulphurization (FGD) process methods. Petrochemical industries produce polyvinyl chloride using ethylene and chloride. Chlorine is manufactured from soda ash. The reaction process produces mega tonnes of calcium based material inform of calcium carbonate with small amounts of Mg(OH) 2. It is estimated that amount of waste are between tonnes per month depending on the demands. Landfill is the usual method for disposing the calcium and magnesium alkaline waste. Land fill is no longer a viable solution due to space and growing concern on environmental Nelson Masilela is with Chemical Engineering Department, Vaal University of Technology, Vanderbijlpark Campus, Private Bag X021, Vanderbijlpark, South Africa. Hilary. Rutto, is with Chemical Engineering Department, Vaal University of Technology, Vanderbijlpark Campus, Private Bag X021, Vanderbijlpark, South Africa hilaryr@vut.ac.za). impact. Moreover with more electricity in demand in South Africa, more coal power station has been built which require desulphurisation to reduce the levels of SO 2 in the atmosphere. The use of calcium industrial waste as a sorbent in wet flue gas desulphurisation will lead to feasible way to address the issue of landfill and environmental pollution. Calcium sulphate (Gysum) can be generated in flue gas desulphurisation which nontoxic and has several application in the building and agriculture sector. Therefore a study on dissolution kinetics of calcium compound is very important in wet flue gas desulphurisation [3,4,5]. The dissolution on calcium and magenesium based material have been studied [6,7,8,9,10]. Several other studies have focused on dissolution rates have been carried out. Raschman [11] studied the dissolution of calcined magnesite in ammonium chloride solution at constant ph. The effect of acetic acid solution in the dissolution of magnesite in acetic acid solutions has been studied [12], it was established that the dissolution rate was controlled by chemical reaction. Bakan et al., [13] studied the dissolution kinetics of natural magnesite in lactic acid solutions and determined that the dissolution rate is controlled by surface chemical reaction. The main objective is to study the feasibility of using calcium industrial waste to produce sorbent for flue gas desulphurization, especially in regions where this waste are produced. In this study, the effect of experimental parameters including the HCl concentration, reaction temperature, solidto-liquid ratio, particle size and ph on dissolution rate of calcium industrial waste was investigated using a ph-stat apparatus II. MATERIALS AND METHOD A. Materials Calcium industrial based waste was obtained from a petrochemical industry that manufactures poly vinyl chloride. The waste material was crushed using a ball mill and sieved to different particle sizes using shaking screen sieves. The chemicals used were HCl and calcium ions standard and were obtained from local chemical suppliers] B. Method and experimental design A predetermined amount of industrial calcium based waste was added to the reactor vessel and the temperature, solid to liquid weight ratio, particle size, acid concentration, agitation and ph was varied according to the experimental design. The reaction temperature was controlled by a hot plate stirrer. The 55

2 ph of the reaction mixture was determined using ph electrode inserted in the solution and connected to a ph 200 1/8DIN ph controller. The controller has specific ph range of ph When the ph exceeds the set value, the pump is activated to add acid to the reaction vessel and lower the ph value to the set point. A sample was then be removed, filtered and analyzed for calcium using Atomic Absorption Spectrophotometer (AAS). The dissolution fraction was evaluated as: III. RESULTS AND DISCUSSION A. Mechanism for dissolution of industrial calcium waste The mechanism for dissolution of in industrial calcium based waste in the presence of HCl is as follows: HCI acid dissociation: + HCl aq) CI + H (1) ( The experimental setup for the experiment is illustrated in Figure 1. dissolution by hydrogen complexation: CaCO + H Ca + CO + H ( s) 2 2 O (2) ph CONTROLLER STIRRER COMPUTER ph ELECTRODE REACTION VESSEL PERISTALTIC PUMP ACID SOLUTION BEAKER CABLE HEATING PLATE WEIGHING BALANCE Fig. 2 The effect of stirring speed on the dissolution rate of calcium based industrial waste Fig. 1 Schematic diagram showing the experimental set up for dissolution of industrial calcium based waste using HCl acid.the effect of temperature, agitation, solid to liquid weight ratio, acid concentration and ph was determined using a one factor at time approach. This is shown in Tables I. TABLE I ONE FACTOR AT A TIME EXPERIMENTAL DESIGN FOR DISSOLUTION OF INDUSTRIAL CALCIUM BASED WASTE Parameter values Temperature ( C) b 75 ph b 5.5 Solid to liquid ratio (%) b Stirring speed (rpm) b Acid concentration (M) b 0.15 b The process parameter that will be kept constant during the experiment. Fig. 3 The effect of solid to liquid ratio on the dissolution of industrial calcium based waste 56

3 Fig. 6 The effect of ph on the dissolution of industrial calcium based waste Fig. 4 The effect of acid concentration on the dissolution of the calcium based industrial waste B. The effect stirring speed on the rate of dissolution of the calcium industrial waste. The experiment was carried out by varing the stirring speed as follows: 100 rpm, 200 rpm, 300 rpm, 400 rpm, to evaluate the effect of agitation on the rate of dissoluton. Figure 2 illustrates that the rate of dissolution increases with the increase in stirring speed. Stirring also increases the kinetic enegy of the molecules providing more chance of contact with the product layer. C. The effect solid to liquid on the rate of dissolution of the calcium industrial waste. The effect of solid to liquid ratio was carried out in the experiment between the ranges of (5 %-20%) to determine the effect of varying the amount of particles. Figure 3 illustrates that the increase in solid to liquid ratio has an inverse effect on the rate of dissolution. This is attributed by the increase in the solid weight per constant amount of product fluid available. D. The effect acid concentration on the rate of dissolution of the calcium industrial waste. Acid concentration was varied as follws, 0.05M, 0.075M, 0.1M, 0.15M to determine the effect on the rate of dissolution. As shown in figure 4, an increase in acid concentration increases the rate of dissolution. This is because there is more H + ions to dissolute the calcium industrial waste. Fig. 5 The effect of particle size on the dissolution of calcium based industrial waste E. The effect particle size on the rate of dissolution of the calcium industrial waste. Different size fractions were used in the experiment ot determine the effect of particle sizes during dissolution. Particles were varied as follows, <63µm, 63µm - 75µm, 75µm - 150µm, 150µm - 300µm. Figure 5 illustrates that the 57

4 disollution rate of calcium industrial waste, the dissolution rate increases with a decrease in particle size. The finer the particles are, the more surface area increases and that means more particles exposed for leaching. F. The effect ph on the rate of dissolution of the calcium industrial waste. Different ph of 5.5, 6, 6.5, and 7 were used in the experiment to analyse the effect of ph on the rate of dissolution. From figure 6, the dissolution rate decreases with an increase in ph. This could be because the no of H + decreases as the ph increases. G. The effect of Temperature on the rate of dissolution calcium industrial waste. The experiment was carried out in the following temperature splits, 30, 50, 70, 90 to investigate the influence of temperature on the dissolution rate. Figure 7 depicts that an increase in the temperature with time increases the dissolution rate of the calcium industrial waste. This is as a result of high kinetic energy available for the reacting molecules. According to figure 8, the activation energy was determines to be kj/mol this indicates chemical reaction was the rate limiting step [14,15,16]. IV. CONCLUSION The work demonstrated that it was feasible to use an industrial calcium based waste to produce sorbent that can be used in wet flue gas desulphurization. In this study, The effects of ph, stirring speed, solid to liquid ratio and acid concentration were studied in order to determine the dissolution of industrial calcium based waste. It was found out that the dissolution rate of industrial calcium based waste increased with increase in acid concentration, temperature, stirring speed and it decreased with an increase in solid to liquid ratio and ph. An activation energy of 20,26 kj/mol was obtianed, this indicate that chemical reaction was the rate limiting step. Fig. 7 The effect of temperature on the dissolution of industrial based wast Fig. 8 The Arrhenius plot for the dissolution of the calcium industrial waste REFERENCES [1] S.F. Randall, Matibe D.K, Electricity and externalities in South Africa, Energy Policy, vol. 31,pp ,2003. [2] J. Kaminski, Technologies and costs of SO 2-emissions reduction for energy sector, Applied Energy, vol. 75,pp ,2003 [3] S. Uchida, H. Moriguchi, H. Maejima, K. Koide, S. Kageyama, Absorption of sulfur dioxide into industrial calcium based waste slurry in a stirred tank reactor, Canadian Journal of Chemical Engineering. vol. 56, pp, , [4] L. Eisenlohr, K. Meteva, F. Gabrovsek, W. Dreybrodt, The inhibiting action of intrinsic impurities in natural calcium carbonate minerals to their dissolution kinetics in aqueous H2O CO2 solutions, Geochimica et Cosmochimica Acta, vol.63, pp , [5] L. Plan, Factors controlling carbonate dissolution rates quantified in a field test in the Austrian, Alps Geomo,vol. 68 pp , [6] R. H. Limo and C. Enweremadu, "The dissolution study of a South African magnesium-based material from different sources using a phstat," Chemical Industry and Chemical Engineering Quarterly,vol. 17, pp , [7] H. L. Rutto, "Effect of Addition of Ammonium Compounds on the Dissolution Rate of a South African Magnesium-Based Material," J. Chem. Eng. Japan, vol. 44, pp , [8] H. Rutto and C. Enweremadu, "Dissolution of a South African calcium based material using urea: An optimized process," Korean Journal of Chemical Engineering, vol. 29, pp. 1-8, [9] H. Rutto and C. Enweremadu, "A study on dissolution of a south african calcium based material in acetic acid solution for flue gas desulphurisation," in Electrical and Control Engineering (ICECE), 2011 International Conference on, 2011, pp [10] H. Rutto, Z. Siagi and M. Mbarawa, "Effect of ammonium compounds on dissolution rate of South African calcium-based material," J. Hazard. Mater., vol. 168, pp , [11] P.Raschman, Leaching of calcined magnesite using ammonium chloride at constant ph, Hydrometallurgy., vol. 56, pp ,

5 [12] O. Laçin, B. Dönmez and F. Demir, Dissolution kinetics of natural magnesite in acetic acid solutions, International Journal of Mineral Processing., vol. 75, pp , [13] F. Bakan, O. Laçin, B. Bayrak, H.Saraç, Dissolution kinetics of natural magnesite in lactic acid solutions, International Journal of Mineral Processing., vol. 80, pp [14] J. Cama, C Ayora, and A.C. Lasaga, "The deviation-from-equilibrium effect on dissolution rate and on apparent variations in activation energy," Geochimica et Cosmochimica Acta, vol. 63, pp ,1999. [15] G. Xianga, G. Rui-tanga, D. Hong-leia, L. Zhong-yangaCen, and C. Ke-faa, Dissolution rate of industrial calcium based waste for wet flue gas desulfurization in the presence of sulphite, Journal of Hazardous Materials, vol.168 pp ,2009. [16] S. Shin-Min, L. Jyh-Ping, S. Gwo-Yuan, Dissolution rates of industrial calcium based wastes of different sources, Journal of Hazardous Materials, volume.79 pp ,