Particle Technology

hindered settling

Problems 6.9

Question 1.2
A 3% v/v suspension is to be settled in a batch settling tank prior to water reuse within a process.
A sample of the suspension settled with a clear interface when placed in a 1000 ml measuring cylinder.
The batch settling curve was as follows:

Time (mins): 20 40 60 80 100 120 140 160 200 240 260 280
Height (cm): 28 24.2 20.4 16.5 13.6 11.2 9.4 8.1 6.8 5.4 4.4 4.0


The plot the settling curve; i.e. the height of the interface against time graph is shown below, with two constructions lines in red and blue (do not consider these just yet). Batch settling curve
It is proposed to operate the settling tank on a three day cycle, in which 1000 m3 of suspension is run into the tank each day, allowed to settle and some of the liquid recycled back to the process. At the start of the third day 85% of the liquid from the first two days has been recycled. The total solid volume in the tank after addition of suspension on day 3 is 90 m3. Prior to removal on day 3, the total liquid volume in the tank is (m3):

a) 2910
b) 3000
c) 1260
d) 437

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