27Jan 27, 2012
Interview with Patrick Esor, Sodimate, Inc. General Manager
What are the main differences between hydrated lime and quick lime?
The main differences between hydrated lime & quicklime are their reactivity & their chemical composition. Hydrated lime and quick lime are both calcium compounds. In its hydrated state, calcium is called calcium hydroxide, and in its pure state it is called calcium oxide, or quicklime. Calcium oxide, the “natural” state of calcium that comes out directly from the mine, has a heavy density (65lb/ft³) and is more reactive than hydrated lime.
To simplify, hydrated lime is the result of adding water to powdered quick lime, putting it in a kiln or oven, and then hydrating/pulverizing it with water. The resulting lime has a density of 35lb/ft³, and is called calcium hydroxide because it has been hydrated.
It is necessary for calcium oxide (quick lime) to be slaked in a controlled environment because it can create heat that reaches up to 120 degrees Fahrenheit. Calcium hydroxide, or hydrated lime, is already neutralized, so it will not undergo oxidation and can be used with water, for water ph control, lime slurry addition, lime slurry mixes, soil rehabilitation and much more.
How do you determine whether to use hydrated lime or quick lime?
If dry, the process feed rate determines the choice between using hydrated or quick lime. Just remember that quick lime is more “reactive” than hydrated lime. However, in some processes, hydrated lime is not suitable even if we inject it dry, and vice versa with quick lime.
The best example is flue gas treatment, also known as Flue Gas Desulphurization, used in coal fire plants, cement industries, glass industries, and incinerators to reduce their HCl, Sox and Nox emissions. Some of these systems require hydrated lime to filter or catalyze the particles emitted after combustion, while other systems use quick lime. The principles are exactly the same, and the feed rates will be quite similar. However, the process and chemistry of the flue gas dictates when to use quick lime or hydrated lime.
If the system requires a lime slurry, the main way to determine if hydrated lime would be more suitable than quick lime is to know the amount that a given process requires. In systems that demand large quantities of lime, quick lime would be the preferred material as the density is twice the density of hydrated lime, which reduces the storage and transportation costs. However, quick lime’s hydrophobic reaction with water requires alime slaker to be used in the process. The quick lime is generally received in pebbles of about one quarter to one eighth of an inch, or in powder form(<300µ). The slaking of the pebble lime and powdered quick lime has to be engineered in respect to their exothermic reactions. The lime slaker mixes quick lime with water to create calcium hydroxide in a solution which is called lime slurry. Slakers are good for a high volume consumption, or high demand, of calcium. However, when a smaller or medium lime solution is requested, hydrated lime is more efficient because the equipment required to use the hydrated lime is simpler and does not need to be designed to handle an exothermic reaction. In this case, the powder is fed with screw conveyors directly into the slurry tank equipped with a slurry mixer, and water is added to match the requested lime slurry concentration (%).
What kind of system is recommended for a feed rate of 100lbs /hr of lime?
If this is for water treatment, we would recommend the use of hydrated lime because the feed rate is low. Depending on the budget, the lime could be stored within silos, hoppers, or even big bags or super sacs. The use of volumetric screw conveyors to feed and convey is likely the best & most cost effective solution of lime feeders, while in use with a lime silo.
However, if the lime is used for flue gas treatment or other industrial systems, it may be necessary to use quick lime. In these cases the lime needs to be put into a slurry. Even at a feed rate of 100lbs/hr a small lime slaker should be considered.
What equipment is mainly used with hydrated lime?
The volume of lime you need will determine what container capacity is necessary. The main containers used are silos, hoppers, and big bags. Discharging mechanisms include mechanical bin activators, air injection systems or simple vibrators pads. The lime can be fed and conveyed by different means, including rotary air locks, screw feeders, conveyor belts, drag chain conveyors or pneumatic conveyors. If the lime has to be put into lime slurry, a lime slurry tank can be requested and equipped with a slurry agitator and a water supply control valve.
To transport the lime slurry solution we use pumps. We generally recommend peristaltic pumps or progressive cavity pumps, or any type of lime slurry pumps our customer feels confident using.
The pumps needed with quick lime are exactly the same as with a hydrated lime solutions. However if the lime is injected into a solution, we will also need a lime slaker. The lime slaker is generally fit with a mixer and agitator, all made of stainless steel, in addition to a vacuum system (lime scrubber) that washes down and evacuates the heat and steam created from the reaction between the calcium oxide and water. Besides that, quick lime works exactly the same as hydrated lime, including storage, metering, and conveying. Quick lime and hydrated lime treatment processes only differ when the quick lime is used as a slurry solution.