Calcium Hydroxide -Lime-
There are many chemicals available on the market today that are suitable for use as neutralization chemicals. The most commonly used chemicals are discussed in an article available here: Neutralization Chemicals.
Calcium Hydroxide Ca(OH)2
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.
Lime forms a Two Normal solution in water in that each mole of Ca(OH)2 yields two moles of hydroxide - OH2 as follows in the simple neutralization of hydrochloric acid.
2HCl + Ca(OH)2 → CaCl2 + 2H20
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 and settle creating a sludge mass that may be very difficult to suspend. The storage tank must be constantly agitated and chemical delivery lines must be kept in motion. Typically recirculation loops are employed with an inline metering valve for chemical delivery. Static lines (not recirculated) are not acceptable because the slurry will separate, and lines will quickly plug.
Lime offers advantages when the precipitation of metals or fluorides is the goal. Calcium salts are normally quite insoluble, and due to the fact that lime is divalent, sludge densities are normally much higher than those formed with caustic (NaOH).
Lime is an excellent choice for acid neutralization. If volumes are relatively low, and precipitation of metal or fluoride ions is not paramount, then caustic (NaOH) may be the better choice because it is easier to handle.