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Effluent Treatment - Lime Neutralization
May 21, 2003
Neutralization End Products
Lime is one of the most common alkalis used for neutralizing acids. Lime reacts readily with all types of acids, both inorganic and organic and weak or strong. There are many different types of lime which vary in their neutralizing power, reactivity and sludge forming characteristics.
Lime is derived from limestone which is a naturally occurring substance. As such, its composition and amount of impurities will vary depending on its source. The following are the general classifications and definitions for limestone and lime:
|High Calcium Lime||Contains less than 5%
magnesium oxide (MgO)
|Magnesium Lime||Contains between 5 to
35% magnesium oxide (MgO)
|Dolomitic Lime||Greater than 35%
magnesium oxide (MgO)
|Quicklime||Manufactured by calcining limestone at about 1315°C (2400°F) which drives off the chemically bound carbon dioxide.||CaCO3
+ heat = CaO + CO2
CaCO3•MgCO3 + heat = CaO•MgO + 2CO2
|Slaked Lime||Slaked lime is a
slurry formed by reacting quicklime with water. Slaking is an highly
exothermic reaction with considerable amounts of heat evolved.
Dolomite lime does not completely hydrate, only the CaO component.
|CaO + H2O
+ Ca(OH)2 + heat
CaO•MgO + H2O = Ca(OH)2•MgO + heat
|Hydrated Lime||Lime manufacturer hydrates quicklime, drying the hydrated lime and selling it as a product. Hydrated lime is an ultra-fine white powder.|
|Hydrated Dolomite Lime||A highly hydrated dolomite lime made using steam and under pressure.||CaO•MgO + 2H2O = Ca(OH)2•Mg(OH)2 + heat|
The reactivity of the lime is influenced by the physical structure of the limestone from which it is derived, chemical impurities and the severity of the calcination process. Soft-burned limes are generally more reactive and slake with explosive force to yield a very reactive hydrate or slaked lime slurry. Medium to hard-burned limes are less reactive and slake more slowly and to a lesser degree. A highly reactive lime will require less residence time to neutralize the acid which translates into smaller equipment. Thus it is important to know the type of lime available before designing an effluent treatment facility.
Hydrated limes are only slightly soluble in water. Solubility increases with declining temperature and decreases as the temperature rises. At 0°C, the solubility of CaO is only about 1.4 g/l. The lime that is in solution immediately ionizes to form Ca++, Mg++ and OH-. In the presence of an acid the ions react with their corresponding acid ions to from calcium or calcium-magnesium salts and water. As the ions react, the lime that is in suspension dissolves, ionizes and reacts with the acid. The process continues until all the acid is neutralized or all the lime is consumed.
The end product of acid neutralization is generally a slurry containing the precipitated salts suspended in water. It is rare that all the products of the neutralization reaction will be soluble leaving only a liquid effluent. The solids are generally sludge like and are difficult to separate from the liquid. Sludge settling rate are low resulting in large detention tanks or lagoons. Sludges are generally dewatered using a filter or centrifuge. Once the solids are separated from the solids, the liquid stream may undergo further treatment to remove other contaminants prior to being discharge to the environment.
The solid stream must be disposed of in an environmentally acceptable manner. The leaching characteristics and stability of the sludge will determine how and where it will be disposed. Total solubles in the sludge must be considered including toxic substances such as heavy metals. Over-neutralization and the use of high calcium or dolomitic lime as the neutralizer will produce the most stable sludges.