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Hydrated Lime vs Quicklime: What’s the Difference and How to Use Them

Though hydrated lime and quicklime are both calcium compounds, they differ in many ways. The main differences between hydrated lime and quicklime are their reactivity & their chemical composition.

The chemical formula of Hydrated lime- Ca(OH)₂

The chemical formula of Quicklime- CaO

Hydrated Lime

  • Also called calcium hydroxide.
  • Has a density of 0.5 g / cm³.
  • Is a colorless crystal.


  • Also called calcium oxide.
  • Has a heavy density of 1 g / cm³.
  • Is a white, caustic, alkaline, and crystalline solid at room temperature.

It is necessary for calcium oxide (quicklime) to be slaked in a controlled environment because it can create heat that reaches up to 50 degrees Celsius. Calcium hydroxide, or hydrated lime, is already neutralized, so it will not undergo oxidation and can be mixed with water in our system, for water ph control, lime slurry addition, lime slurry mixes, soil rehabilitation and more.

The fact of adding water to the powdered quicklime, putting it in a kiln or oven, and then pulverizing it with water, lead to the reaction of CaO with H2O bringing the new chemical Ca(OH)2.

CaO + H2O Ca(OH)2

How do you determine whether to use hydrated lime or quicklime?

Usually, lime is used in water and wastewater treatment as well as Flue gas Desulphurization. The choice of using hydrated lime or quicklime will depend on the feed rate that the process requires. Also the ” reactivity ” of the powder will be a factor of choice.

For instance, in the sludge liming for sludge stabilization process, the Quicklime is more desired for its exothermic reaction where the increase of temperature will help to kill the pathogen existing in the sludge.

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 quicklime ..

To determine if hydrated lime or quicklime is more profitable, check the amount of powder needed for the process. For big consumptions of powder, quicklime will be the best powder to choose as the density (1g / cm³) is twice the density of the hydrated lime (0,5 g / cm³) which reduces the storage and transportation costs.

On the other hand, quicklime requires a lime slaker due to its hydrophobic reaction with water. The aim of using a lime slaker is to convert the quicklime into hydrated lime by mixing it with water. The lime slaker has the ability to control the exothermic reaction during the transformation of the CaO into Ca (OH) 2. The result will be a lime slurry ready to be injected into the process.

On the contrary, when the demand of lime consumption is low or medium, Hydrated lime is used. In fact, the equipment is simpler because this doesn’t require a lime slaker because there is no exothermic reaction. The scope can be reduced to a screw conveyor that feeds the powder directly into the preparation tank equipped with a slurry mixer, where water is added to match the requested lime slurry concentration (%).

What equipment is mainly used with hydrated lime?

SODIMATE scope of supply starts with the storage device that can be silo, big bag emptying station or hopper, a mechanical bin activator, an 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 needs to be converted into a lime slurry , a lime slurry tank can be requested and equipped with an agitator and a water supply control valve.

To transport the lime slurry solution we use pumps. We generally recommend peristaltic pumps.

What equipment is mainly used with quicklime?

The pumps needed with quicklime are the same as those used with hydrated lime, which includes an injector, sludge mixer, and mechanical bin activator. 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, used to eliminate the heat and steam created from the reaction between the calcium oxide and water. Besides that, quicklime works the same as hydrated lime, including storage, metering, and conveying.

In the mechanical bin activator unit ZDM guarantees flow control and accurate feeding of quicklime. Besides that, quicklime works the same as hydrated lime, including storage, metering, and conveying.

Industrial uses of quicklime

Quicklime has a wide range of uses, including in paper and pulp production, the mining industry, treatment of water and flue gases.

What kind of system is recommended for a feed rate of 45 Kg /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 in a silo, hopper, or even big bag station. The use of volumetric screw conveyors to feed and convey is likely the best & most cost effective solution of lime feeders.

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