Table 7392 Common Neutralization Reagents

Acid Reagents

Base Reagents

Concentrated (66°Be) sulfuric acid Concentrated (20 or 22°Be) hydrochloric acid Carbon dioxide Sulfur dioxide Nitric acid

Caustic soda (NaOH)

Ammonia

Soda ash (Na2CO3)

Hydrated chemical lime (Ca[OH]J

Limestone (CaCO3)

excess lime result in longer reaction times. This consideration becomes important when environmental engineers size neutralization vessels. Due to the continuing reaction of lime, pH measured at the neutralization tank discharge may not be the final effluent pH downstream of the discharge, if the nominal time constant of the neutralization vessel is too short.

The solubility of lime (CaO) in aqueous solutions decreases with increasing temperature, e.g., 0.131 gm per 100 cc at 10°C and 0.07 gm per 100 cc at 80°C. Figure 7.39.4 plots the pH range of lime solutions up to saturation at 25°C. Lime as a neutralizing reagent is normally supplied in slurries or as a dry feed along with water to a small, agitated, holding container that overflows to the neutralization process. Normally, 20% or less lime slurries are used. The lower the lime concentration, the easier the handling in terms of abrasion and clogging. Quicklime is available in bulk or bag form in reasonably standard sizes (see Table 7.39.4).

In addition to the dry form, a hydrated lime produced as a by-product in the manufacture of acetylene is available in slurry form having a nominal 35 wt % solids content. This lime slurry is delivered in tankwagons having a nominal capacity of 4500 gal. Table 7.39.5 gives a typical chemical analysis—particle size distribution—of this lime slurry. The cost of this lime form depends on the transportation cost.

Chemical reactions associated with hydrated limes depend on the type of acid(s) being neutralized. The reaction between sulfuric acid and hydrated, high-calcium and normal, dolomitic quicklimes shown in the following equations is typical:

FIG. 7.39.2 Chemical lime production process. (Reprinted from National Lime Association, Chemical lime facts, Bulletin 214, Washington, D.C.)
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