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|Comparison basis||Hydrated lime|
|Product form||powder – Ca(OH)2 available as slurry to 35% solids||powder – Na2CO3 available as 15% solution||50% solution – NaOH||59% solids slurry – Mg(OH)2|
|Raw material cost (ton per ton basis)||least expensive||about half the cost of magnesium hydroxide||fluctuates widely with supply; often more expensive than Mg(OH)2||generally competitive with caustic soda; pricing consistent|
|Capital investment||high due to solids handling and slaking; also regular maintenance of system due to calcium deposits||least expensive; costs associated with solubilizing the powder, controlling foaming, and metals carryover||cost associated with safety considerations; no agitated tank required; freeze protection needed for 50% caustic||agitated storage tank required|
|Residence time||moderately fast acting to complete neutralization||moderately fast acting to complete neutralization||extremely fast acting to complete neutralization||moderately fast acting to 95% of neutralization; more controllable process|
|Maximum pH in overtreatment||12||>11||14||9|
|Sludge produced||heavy sludge containing calcium sulfate (gypsum) if neutralizing sulfuric acid||high volume, gel-like with heavy metals||high volume, gel-like with heavy metals||dense, low volume with heavy metals; easier to dewater|
|Safety||moderately hazardous material||moderately hazardous material||hazardous material requiring rigorous safety procedures||nonhazardous material|
|Environmental||can overshoot desired pH||sodium ion discharge||sodium ion discharge|
can easily overshoot desired pH
|buffers at pH 9|
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