Abstract: The concept of sedimentation involved in the processes of thickening and clarification was tested in an experiment involving 5 suspensions of limestone in water at different concentrations one with the addition of flocculant. The particle and bulk densities of limestone were calculated prior to the suspensions being mixed and allowed to settle in front of a video camera and type 1 sedimentation (3 different zones of concentration) was observed therefore deeming Kynch’s theory invalid. The change in height of the interface between the layers of different concentrations over time was measured to determine that the velocity of the interface or the batch settling rate decreased with higher concentration and also decreased when the suspension contained flocculant.
Transcript
Abstract:
The concept of sedimentation involved in the processes of thickening and clarification was tested in an experiment involving 5 suspensions of limestone in water at different concentrations one with the addition of flocculant. The particle and bulk densities of limestone were calculated prior to the suspensions being mixed and allowed to settle in front of a video camera and type 1 sedimentation (3 different zones of concentration) was observed therefore deeming Kynch’s theory invalid. The change in height of the interface between the layers of different concentrations over time was measured to determine that the velocity of the interface or the batch settling rate decreased with higher concentration and also decreased when the suspension contained flocculant.
The operations of thickening and clarification are very important then the objectives of increasing the concentration of a slurry (thickening) or attaining a clear fluid stream from a dilute slurry (clarification) are considered. Both of these processes are designed using the concept and theory of sedimentation or in other words the batch settling of multiple particles in suspension. The effect of particle concentrations on the settling rates of the slurries is important in predicting the behavioural characteristics of thickeners and clarifiers in the industry and so correlations must be investigated experimentally to better the accuracy of the designs of these processes.
Objectives:
1. The first objective is to define and calculate the bulk and particle densities of limestone.
2. The second objective was to measure batch settling rates for increasing starting concentrations and to evaluate the effect of the concentrations on these rates.
3. The third objective was to test whether Kynch’s theory was valid for the types of sedimentation experienced in the experiment.
4. The final objective was to compare batch settling rates with and without the addition of a flocculant to the slurry.
Approach:
Particle density was measured using a dry equi-mixture of two different sizes of limestone particles. The mixture was weighed and poured into a volumetric cylinder. From these two quantities the density could be calculated. A similar process using now a pre measured amount of limestone and water was added to a flask and the total density was calculated obtaining the bulk density.
Three suspension concentrations were prepared low, medium and high and were allowed to settle with the height being visually measured at a set interval for half an hour. From a height time curve of each concentration the settling rate or settling velocity could be calculated and compared.
Kynch’s theory is valid for Type 2 sedimentation so purely from observation the type of sedimentation could be defined and if the theory was valid then flux plots could be drawn using the experimental height time curves with Kynch’s theory and could be compared to the experiment for validity.
Two more suspension concentrations were prepared now adding flocculant to the one and using the same method of measuring the height at set time intervals the rates could be calculated and compared using the height time curves.
Experimental Setup:
The limestone consisted of two separate containers of 2 and 5 micron powders. Specific amounts were measured using plastic weigh boats and a scale and the limestone was added to the flasks using a paper funnel. All of the suspensions were made to a volume of 1 litre, this was measured out from a large container of de-mineralized water and poured into the various volumetric flasks all of which had tape measures attached to read of the height of the suspension in centimetres. The flasks were placed against a black background for the monitoring of the settling by the students and using the video camera. A stop watch was used for the time intervals.
Methodology: A 120g approximate mixture of equal amounts of 2 and 5 micron limestone powders
were weighed on the scale and added to an empty volumetric cylinder. The cylinder was twisted around until the powder was as flat as possible without compressing the limestone and the volume was read off.
160ml of water was measured out as well as a 40g mixture of equal amounts of 2 and 5 micron limestone powders. Part of the water as added to a volumetric cylinder followed firstly by the powder then the rest of the water to ensure all the powder was in contact with water so as not to fault the level of the mixture. The volume was then read off in order to calculate bulk density.
The same technique off adding a known mass of limestone between two volumes of water was then used to measure out three different concentrations low, medium and high all of a volume of 1 litre into volumetric cylinders. These cylinders were well mixed using stoppers at the open end and were placed simultaneously in front of the black screen and allowed to settle. The initial height was noted for each and the heights of the interface between the clear zone and the zone of initial concentration were noted every minute for a 30.5 minute period. This timeframe was also captured by a video camera directed on the three cylinders.
This same process was then carried out for two medium concentrations one with no flocculant and one with about 7 drops of flocculant.
Results and Discussion:
Particle and bulk densities:
TABLE 1 EXPERIMENTAL PARTICLE AND BULK DENSITIES OF LIMESTONE
2 microns(g) 5 microns(g) V(ml)
ρparticles (kg/m^3)
ρbulk (kg/m^3)
dry particles no water 59.92 59.39 130 917.7692308 1134.40678water 160 particles with water 20.85 19.94 177
The densities obtained are summarised in the table above. The particle densities obtained from literature re as follows: crushed: 1522kg/m^3 and dust: 1089kg/m^3 (Densities of some common materials, 2015); ground: 945kg/m^3 and pulverised 1089kg/m^3 (Bulk density chart, 2015). From these values our experimental value of 918 kg/m^3 seems to be quite close to the value of ground limestone density of 945kg/m^3. The other values and specifications of limestone powder size vary quite a bit from crushed to pulverised etc. and so an accurate comparison without the knowledge of the exact diameter of the limestone in literature is hard to discuss in this experiment.
Repeatability of the experiment:
This experiment is independent of the environmental conditions such as gravitational acceleration and atmospheric pressure, viscosity of fluids is affected by temperature but under the conditions of room temperature this should not affect results from experiment to experiment. Densities cannot change for these specific substances and so particle and bulk densities should remain constant making this a repeatable aspect of the experiment. Hence due to the density and gravitational pull being constant and with little affect on viscosity by temperature change the settling rates of these suspensions should be repeatable as long as the methodology is followed closely. The only real external factor that could affect this is the diameter of the volumetric cylinder as the closer the particle diameter comes to the diameter of the tube hindrance of settling is increased but if the size of the particles are maintained at very small diameters such as ours this should not affect the repeatability of the experiment.
Comparing the video data to our experimental data:
Height vs time curves were obtained from the video camera at each of the concentrations. The curves obtained from the experiment were then superimposed on these video data curves as seen below to see how closely they fit:
FIGURE 1 LOW CONCENTRATION CURVES FIGURE 2 MEDIUM CONCENTRATION CURVES
FIGURE 3 HIGH CONCENTRATION CURVES FIGURE 4 MEDIUM 2 NO FLOCCULANT
CURVES
FIGURE 5 MEDIUM 2 WITH FLOCCULANT CURVES
The solid lines representing the experimental curves fit quite closely to the line separating the blue and yellow regions representing the level of the initial concentration zone on each of the videos. The line is always slightly lower which is understandable due to the delay in
response between the visual reading of the height and the recording of the time. The error becomes greater in the medium 2 curves which was most likely caused by the change over which could have resulted in the incorrect setting up of the camera. All in all though this comparison is close enough to validate the experimental results.
Effect of starting concentrations on batch settling rates:
TABLE 2 STARTING CONCENTRATIONS AND THE INTERFACE VELOCITIES
concentrationconcentration (g/l)
velocity (m/s) down
High 0.031 0.00034716
7
Medium 0.025 0.00036116
7Low 0.017 0.0004305
From table 2 above it is obvious that with decreasing concentration the interface velocity (the velocity of the top of the initial concentration layer as it drops into the sedimentation zone) increases. When we use the term batch settling rates this velocity is the rate at which the suspension settles so this relationship is what we’ve determined. This relationship is proven from the following statements from previous work on batch settling rates: From an experiment in the stokes range for clay particles in concentrations of 10 to 100kg/l used at all size classes every test proved that higher initial concentrations reduced settling velocities. Errors only occurred when tube diameter was<45mm (Lovell et al, 2006). It is proven once again in a similar experiment to our own by (Shannon et al, 1964) on Batch and continuous thickening where height time curves and flux plots were drafted for different concentrations and the slopes of the height vs time curves became increasingly steeper with increase in concentration. The same behaviour is seen in the figure below of our experimental height vs time curves:
0 5 10 15 20 25 30 350
10
20
30
40
50
60
height vs time curves for low,medium and high conentrations
low medium highlow medium highLogarithmic (high)
time (min)
heig
ht (c
m)
FIGURE 6 COMPARISON OF HEIGHT TIME GRADIENTS FOR EACH CONCENTRATION
Kynch’s theory validity:
For all of the runs made in the experiment only 3 different layers of different concentrations were observed – clear fluid, initial concentration and sediment hence pointing to type 1 sedimentation thus making Kynch’s theory invalid to model this data. This observation is validated from the low concentration flux plot below. The curve has a positive gradient and so is in the region of type 1 sedimentation
When comparing the curves of the second medium concentrations below:
0 200 400 600 800 1000 1200 1400 1600 1800 20000
5
10
15
20
25
30
35
40
med 2
time (s)
heig
ht (c
m)
FIGURE 8 MEDIUM 2 CURVE WITHOUT FLOCCULANT
0 200 400 600 800 1000 1200 1400 1600 1800 20000
5
10
15
20
25
30
35
40
med 2 + F
time (s)
heig
ht (c
m)
FIGURE 9 MEDIUM 2 WITH FLOCCULANT CURVE
The curve with flocculant clearly has a smaller gradient as the linear part of this curve ends after the curve with no flocculant and from the calculated velocities of 0.000385 m/s for figure 7 and 0.00017 for figure 8 this is proven analytically and the conclusion that flocculant hinders the settling rate can be drawn.
Conclusions:
Particle and bulk densities fell within the range of the values specified in literature yet due to the fact that in literature values for the exact particle size could not be found the validity of our experimental results are inconclusive and the accuracy of the method used is not credible enough to make a valid comparison.
The experiment was deemed to be repeatable due to the method being unaffected by environmental changes and being independent of interfering external factors. The resulting height vs time curves obtained were deemed valid as they closely matched those taken from video data with the small error resulting from human error which can be expected and assumed negligible.
A clear conclusion from the settling velocities obtained at different concentrations that the higher the initial concentration of the suspension the slower the batch settling rate. This was a constant throughout the experiment and throughout all the literature sources describing previous experiments of similar conditions.
Kynch’s theory was not tested as the assumption that the sedimentation of the suspension must be of type 2 for this theory to be valid rules out this experiment as all suspensions were observed to be of the three zone type 1 sedimentation.
The addition of flocculant decreased the settling velocity of the suspensions prepared in this experiment.
References:
Densities of some Common Materials. 2015. Densities of some Common Materials. [ONLINE] Available at: http://www.engineeringtoolbox.com/density-materials-d_1652.html. [Accessed 20 March 2015].
Bulk Density Chart. 2015. . [ONLINE] Available at: http://www.anval.net/Downloads/Bulk%20Density%20Chart.pdf. [Accessed 20 March 2015].
Lovell C J, Rose W C. 2006. The effects of sediment concentration and tube-diameter on particle settling velocity measured beyond Stokes' range; experiment and theory. American Geological Institute
Shannon P T,' Robert D. Dehaas R D, Elwood Z, Stroupe P, Tory E M, 1964. Batch and Continuous Thickening. Independant. Enineering. Chemical Fundamentals. Volume 3 (3), pp 250–260
Appendices:Sample calculations:
Particle density:
ρp=mpVp
[1]
mp = mass of particles = 59.92g + 59.39g = 119.31g; Vp = Volume particles = 130ml
ρp=119.31 g130ml
=0.9178 gml
= 917.8kg/m3
Bulk density:
ρb=mtotVtot
[2]
Mtot = mass of particles + mass of water ; Vtot = volume of suspension
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