Lime kilns convert limestone into lime, an essential ingredient in construction and agriculture. Traditional kilns were small brick huts, while modern ones are continuous towers that produce a better product. Periodic kilns are permanent structures for lime harvesting, while heap furnaces are primitive and imprecise. Continuous kilns are free-standing structures that produce pure lime and manage fumes and dust emissions. The lime kiln process emits carbon dioxide gas, which can be toxic, but modern industrial kilns have sophisticated means to capture dust and diffuse emissions for safety and environmental health.
A lime kiln is a small scientific building or contraption in which limestone is converted into calcium oxide or lime. Lime is an essential ingredient in mortar, which is widely used in construction. It is also commonly used as a fertilizer and can help increase yields on many different agricultural crops. Traditional lime kilns were small brick huts in which the limestone was burned over a grate. The more modern iterations are the inner tower chambers where the limestone can be continuously heated and collected.
Lime has been an important part of most cultures since primitive times. The ruins of the furnaces can be found on almost every continent. There are three main types of lime kiln: a pile kiln, a periodic kiln and a continuous kiln. Most of the lime on the market today is produced in a continuous kiln. Such a kiln is more expensive and sophisticated than a stack kiln or periodic kiln, but produces a better product and generates more predictable and controlled results.
Heap furnaces are now rarely used. Most were designed to be temporary and were extremely primitive in form, often little more than limestone installed in a burn pile designed for lime mining once upon a time. The stones were usually placed on a grate over an open flame and the finished lime scraped off the ashes once the fire was extinguished. This process was imprecise and prone to the inadvertent inclusion of ash and other contaminants. It was mainly used on site in building construction and just outside the limestone quarries.
Periodic lime kilns are much more common. These are permanent structures specially designed for lime harvesting. Nearly all are made of brick, often built several layers deep to provide insulation. Inside the oven is a place for a wood fire. Above, small pieces of limestone are stacked in the shape of a dome. There is usually room for one or two people to stand and tend the fire and monitor the functioning of the lime kiln, although once the flames are lit the room is usually too hot to be inside.
A small hole, called an eye, is at the base of the kiln, and is where the finished lime accumulates and is collected. The whole process usually takes several days. First, the stone must be heated, then it must be processed, forming lime. Once the lime has been isolated and channeled into the eye, it must cool before it can be handled and harvested. Using a periodic lime kiln is generally more accurate than a pile kiln, but is still prone to impurities and cross-contamination.
One of the only ways to ensure a pure lime by-product is to use a continuous lime kiln. Such a furnace is a free-standing structure, but unlike its brick predecessors, it need not stand outside. Most are permanent parts of manufacturing plant science labs.
Continuous kilns are typically shaped like tall cylinders and are heated with oil through a central kiln. The limestone must be loaded at the top, heated as it passes through the center, then expelled as pure calcium oxide at the bottom end. The use of oil reduces the possibility of ash or soot buildup and the kiln can be fired continuously as long as there is raw limestone to work.
Another advantage of a continuous kiln is the management of fumes and lime dust. Lime dust is particularly corrosive, which is why outdoor structures were normally built some distance away from villages and other structures. The lime kiln process also emits carbon dioxide gas, which can be toxic. Most industrial kilns today have sophisticated means to capture dust and diffuse carbon dioxide emissions so as to promote both user safety and environmental health.
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