Experiment-8-sedimentation.docx

EXPERIMENT 8 Sedimentation Introduction: Sedimentation is a phase separation process in which the solid particles settle out from the slurry under the action of gravity force. In a sedimentor a suspension or a slurry is separated into a supernatant clear liquid and a thick sludge. The process is termed thickening if the concentrated sludge is the main desired product and it is called clarification if the recovery of the clear liquid is the prime requirement. A slurry taken in a graduated cylinder can be observed to study the hindered settling phenomenon of aggregated flocs of fine particles. The well mixed slurry in the cylinder up to a given height will develop a clear liquid zone at the top having a clear boundary with the slurry below it. This is because the hindered settling of fine particles as flocs displaces the liquid upward to form a clear liquid zone. The continuous down-ward movement of solid flocs and upward movement of the displaced liquid make the interface boundary to descend down continuously. In other words, the effective settling height varies with time and hence the settling rate also varies with settling time. Therefore, the measurement of the height of the interface at different time intervals can be used as a pointer to follow the settling process. Objective: 1. To analyze the settling characteristics of a given solid-liquid slurry. Procedure: 1. A slurry of barium sulphate having a concentration of 40 g/lit is prepared by reacting the required amount of barium chloride with 10 % sulphuric acid solution and is taken in a measuring cylinder to a height about 30 cm. 2. The well stirred slurry is allowed to settle and the time taken for every cm fall of the interface is noted. 3. The readings are taken up to 20 cm fall of the interface between the clear liquid and the slurry and are tabulated. BaCl2 2H2O + H2SO4  BaSO4  + 2HCl + 2H2O 4. The same slurry is taken into a large diameter beaker and the experiment is repeated. The results are compared to find the wall effect. Tabulation: S. No 1 2 3 4 5 6 7 8 9 10

Settling time ,min

Height of the slurry, cm

S.No

Settling rate, slope cm/min

Slurry concentration, cm

1 2 3 4 5 6 7 8 9 10 Calculation: 1. A curve connecting the height of the interface from the bottom of the slurry on the ordinate and the settling time on the abscissa is plotted. 2. From this graph, using arbitrarily chosen values of settling time, the corresponding tangents to the settling curve and the values of slope and intercept (y-axis) are determined and tabulated. The slope of the tangent gives the settling rate in cm/min. The corresponding slurry concentration (C) values are calculated from the intercept values as follows: C Z C 0 0 ZI where: C0 = initial concentration of the slurry g/ lit Z0 = initial height of the slurry cm ZI = intercept of the tangent on the ordinate cm Using these values another curve connecting the settling rate (cm/min) with concentration of the slurry (g/lit) is plotted. Results and Discussion The settling characteristics of the given slurry was studied by plotting the curves connecting: 1. settling time vs. interface height. 2. Settling rate vs. slurry concentration.