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Precipitation Unit

mixing tank stirred tank

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#1 Tamilselvi


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Posted 02 March 2013 - 06:30 AM



I am a final year chemical engineering student and I'm working on a tannery waste water treatment design project.


I am supposed to design and size a precipitation unit. The reactants are calcium hydroxide and chromium(III) sulphate ( precipitating chromium out). Can you please advise me if a mixing tank or stirred tank is sufficient for this process. And Right after this unit i have a settling tank to remove the precipitate.


Inlet flow : 1,727,741 kg/day

Outlet flow: 1,728,395 kg/day 

Lime added: 654 kg/day 

Inlet temperature: room temp

Outlet temperature: 297 K

Enthalpy : -8,196,232 kJ/day (endothermic)  


Please advice me of any books that has info to design this. 

P.S. I have tried Coulson and Richardson and David Hendrick's book on water treatment unit processes.

Thank you!! :)



#2 kkala


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Posted 02 March 2013 - 09:16 AM

So you actually need the residence time of the mixing tank(s), as I understand. I am not aware of "tannery wastewater treatment". One article probably of some usefulness can be <http://www.academia....GE_OF_POLLUTION>. Total time of chromate precipitation + settling (reaction to Cr(OH)3 slowly continues in settling) is about 3.5 h, but agitation lasts a few minutes (para 2 and Fig 3) (solubility of Cr(OH)3 less than Ca(OH)2, <http://faculty.ncc.e...PY4=&tabid=1903>).

Googling "tannery wastewater chromates treatment", or similar, returns a lot of data, probably containing some more precise info, if looked into.

#3 Tamilselvi


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Posted 02 March 2013 - 09:55 PM



Thank you for the links. I have found them before. :)But I'm just curious if a mixing tank is sufficient or would I need to design a reactor? 

Thanks again!



#4 kkala


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Posted 03 March 2013 - 09:54 AM

"Extrapolating" precipitation unit from alumina plants (alumina crystallization), I  think of a well agitated atmospheric tank, so that its content is almost homogeneous. This is actually a sort of  stirred tank reactor. Residence time (say half an hour) is the critical parameter for tank size. There are not high temperatures during formation of Cr(OH)3. Then settling occurs.

Precipitation and mixing may not be clearly separated in actual industrial practice, see fig 3 of infogate's report <http://www.gate-inte.../w019e_2002.pdf>. However better to show design steps clearly in a new project, equipment "merging" can be considered later.


Note: Attached reference may be useful.






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