HOW LIME IS MADE: The Lime Cycle
The word “lime” refers to products derived from heating (calcining) limestone.
In the Beginning
Limestone is a naturally occurring and abundant sedimentary rock consisting of high levels of calcium and/or magnesium carbonate, and/or dolomite (calcium and magnesium carbonate), along with minerals. Lime production begins by extracting limestone from quarries and mines.
Sizing . . .
Limestone enters a primary crusher to break the rock. Depending on the size of the feedstone required, limestone may go through a secondary or tertiary crusher to further reduce its size. The stone is then screened into various sizes ranging from several inches to dust-sized particles. The sized stone is then washed.
Calcining . . .
Processed stone is transported by conveyor belt to the lime kilns. To cook or “calcine” limestone, there must be a significant transfer of heat to the limestone. In general, the heat transfer from the fuel source to limestone can be divided into three stages:
- Preheating – limestone is heated by direct contact with kiln exhaust gases that enter the preheater kiln.
- Calcining – the kiln fuel is burned in the preheated air from the cooling zone and, as the limestone moves down the kiln the heat turns the limestone into quicklime and carbon dioxide (CO2).
- Cooling – quicklime leaving the calcining zone is cooled by direct contact with “cooling air.”
While there are multiple kiln types in use, the rotary kiln is the most common kiln found in the U.S. A rotary kiln consists of a rotating cylinder that sits horizontal on an incline. Limestone is fed into the upper or back end of the kiln, while fuel and combustion air are fired into the lower “front end” of the kiln. The limestone is heated as it moves down the kiln toward the lower end. As the preheated limestone moves through the kiln, it is “calcined” into lime. The lime is discharged from the kiln into a cooler where it is used to preheat the combustion air. Lime can either be sold as is or crushed to make hydrated lime.
Quicklime can be processed into hydrated lime by crushing the quicklime, adding water to the crushed lime (water accounts for approximately 1% of raw hydrate), and then classifying the hydrated lime to ensure it meets customer specifications before it is transported.