Calcium Hydroxide [Ca(OH)2]. Also commonly referred to as slaked lime or hydrated lime; calcium hydroxide is formed as a result of hydrating lime (calcium oxide, CaO). Lime is by far the most economically favorable alkaline reagent to use for acid neutralization. Lime is significantly cheaper than caustic (NaOH), but is much more difficult to handle.
As with magnesium hydroxide, Lime is not very soluble in water. Although the reaction times of lime are substantially less than magnesium hydroxide, lime is difficult to handle because it is handled as a slurry.
Ca(OH)2 is divalent, yielding two moles of (OH)2 for every one mole of Ca(OH)2. When compared to caustic (NaOH), which is monovalent, twice the neutralizing power is available for a given molar volume of lime, thus contributing to the economy of lime. As with magnesium hydroxide, lime is normally delivered in dry crystalline form. This must then be mixed with water to form a slurry to be delivered to the process. The ease with which caustic (sodium hydroxide) can be handled makes it far more favorable than lime, at least for low volume applications.
Lime is a slurry that will rapidly separate from solution. The storage tank must be constantly agitated and chemical delivery lines must be kept in motion. Typically recirculation loops are employed with a metering valve, inline, for chemical delivery. Static lines are not acceptable because the slurry will separate, and lines will plug, over time.