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Transforming stone into lime or dolime
Kilns form the heart of the transformation process, turning a raw carbonate material into an oxide – lime or dolime – which offers an expansive range of properties to be utilized by customers.
Chemical transformation requires substantial amounts of energy and is achieved in large industrial installations. Different types of technologies are used, mainly in rotary and vertical kilns.
More about rotary kilns
A rotary kiln is a long inclined tube, about 100 m long, which is fed with stone sizes ranging from 50 mm to less than 3 mm.
The rotation and the inclination of the kiln’s shell allow the stone charge to roll from the top of the kiln to the bottom. The charge occupies only 10% of the cross-sectional area of the shell; the remaining area is filled by gas flow.
The charge stays in the rotary kiln for 6 to 8 hours. Inside, it undergoes a progressive rise in temperature before reaching a decarbonation zone, where it is subjected to a high temperature. This takes place near the burner where the fuel is injected. Rotary kilns usually use multi-fuel burners that allow several types of fuel to be injected simultaneously. The flame profile can be adapted from sharp to long by adjusting the burner’s settings.
Rotary kilns generate lime with controlled reactivity and ensure total decarbonation with a very low residual CO2 value. These kilns are used to meet distinct product specifications such as low sulfur lime or special reactive lime/dolime and help to improve resource efficiency.
More about vertical kilns
Mixed-feed shaft kilns
Mixed-feed shaft kilns are usually constructed as a group of kilns in which the limestone (or dolomite) is mixed with a charge of solid fuel. Cooling air for the lime is blown in through the base to serve as combustion air for the fuel mixed with the stone at the head of the kiln. These kilns can generate extremely high temperature peaks. The charge is subjected to over-burning conditions. These kilns are used to produce lime with a very low reactivity, possibly even dead-burnt dolomite.
In other shaft kilns, several levels of lateral lances allow fuel to be introduced into the burning zone, thus generating an intense increase in temperature.
Annular shaft kilns
Annular kilns are an evolution of shaft kilns. In order to use alternative types of fuel, the kiln is composed of external combustion chambers at two levels. There are 4 or 5 combustion chambers on each level, depending on the capacity of the kiln. With these kilns, an internal cylinder creates an annular zone through which the charge passes, hence the name ‘annular kiln’.
Recirculating gas flow in the empty central cylinder means that the length of the effective decarbonation zone can be extended further down to the level of the lower burners. This allows higher reactive lime to be produced which is not possible with a classic shaft kiln.
Parallel flow regenerative (PFR) kilns
The PFR kiln is a very efficient shaft kiln composed of 2 or 3 shafts interconnected by a central crossover channel. Energy is supplied via lances that are plunged into the charge. Some kilns are fitted with 19 lances, whereas modern kilns are fitted with 33 lances in each shaft. PFR kilns operate in alternating modes; a burning cycle then a preheating cycle. When a shaft is in burning mode, it receives energy from the tips of the lances while the other shaft is in preheating mode. Approximately every 12 minutes, fuel injection is halted and the activity in each shaft is reversed; the shaft that was in burning mode switches to preheating mode and vice versa. The temperature of the bed increases cyclically as it moves down through the kiln during the preheating phase until it reaches the level of the lance tip. This is followed by a calcination phase at an intermediate temperature (less than 1,100°C) for a period of 9 hours, which leads to the production of reactive limes. Cyclic preheating combined with the reutilization of gas in the upper parts of the shafts give these kilns excellent thermal efficiency.
A comparison of kiln technologies
Capacities of industrial calcining tools vary: Shaft kilns can yield around 60 tons of finished product per day; the most recent PFR kilns (such as those introduced by Lhoist in Brazil in 2008) can deliver up to 800 tons per day; modern rotary kilns with vertical preheaters can produce up to 1,250 tons per day.
The energy efficiency of modern industrial kilns also differs, ranging from 55% for rotary kilns equipped with preheaters up to more than 80% for PFR kilns.
Size of charge
The size of the kiln feed stone charge depends on the type of technology. Shaft kilns cannot calcine fine grain sizes and are generally fed with stone sizes from 25 mm up to 180 mm. Rotary kilns can be charged with lower grain sizes to ensure more optimized, sustainable quarry operations.
The choice of technology employed improves resource efficiency and is based on the product qualities required by the markets as well as on well-balanced quarry development and improve resource efficiency. To select a kiln technology, an understanding of the characteristics of the calcined product required by customers is essential, as is knowledge of the stone properties available in our deposits.