I'm doing experiments to see if I can use lime to correct our effluent's acidic pH while managing to precipitate sulphates as gypsum sludge and other metal hydroxide sludge Me(OH)2 or Me(OH)3. So far my experiments show that the pH correction can be done perfectly well. I'm managing to reduce sulphates as well using ferric chloride as coagulant (dosing 10ml/l) in the stirred jar/beaker as my reactor. So I want to know if I'm dosing the correct amount of lime Ca(OH)2, this is how I calculate it:
I measure sulphates in the effluent, convert to g, then to mols and with 1:1 stochiometric ratio I then use those mols to convert them to g of lime needed to produce gypsum. My concern is: Can I just dose any amount and keep varying it per litre? or I'm doing it right to stick to stoichiometric amounts? Hope you understand my question.
Dear Hopewell,
I think your dosing calculation must be right because you are dosing as per stoichiometric calculations (ratio 1:1).
Further amount of dosing depends on how much reduction, you desire. if you can share influent and effluent levels, you probably get a better answer.
Regards
Pravin
Dear Pravin
Please see the attachment, I have attached a rough document showing results of one of the experiments. You'll see the properties of the influent and effluent.
Did you consider the solubility of gypsum? Ca(OH)2 is quite soluble, but it will be converted to CaCO3 as time goes on. CaCO3 is much less soluble than gypsum. That means the amount of SO4(-2) you can precipitate is control by the solubility of gypsum.
You cannot ignore the disparate ion effect. High ionic strength will increase the solubility of the calcium sulfate.
Yea I did consider the solubility product of gypsum which is a function of ions of Ca and SO4 and ionic strength of the solution. I'm trying to control this by raising pH conditions to 12 and higher. At these higher pH I've noticed better sulphate removal showing that at these pH Gypsum is precipitates better, insoluble.
I am considering the use of lime or caustic to adjust the pH of the raw water to our Water Treatment clarifier. I would like to know what factors should I consider to select whether to use Caustic or Lime and I want know how much quantity of caustic and lime should I dose to adjust the pH. The minimum pH that we observe on the samples taken from the envisaged raw water source (crocodile west river) is 7.36 and the average value is 8.25. The target pH value is 9.1. I would also like to know the optimum pH that the clarifiers are normally/ generally operating at.
About predicting the pH of Ca(OH)2 aq. solutions (saturated or not) ― cf. my posts (§ III., IV, V.) at:
https://www.researchgate.net/post/How-to-increase-pH-for-a-solution-without-making-a-chemical-reaction-with-CaCl2
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