Manufacturing of lime

Lime Manufacturing 

Process Description 

Lime is the high-temperature product of the calcination of limestone. Although limestone deposits are found in every state, only a small portion is pure enough for industrial lime manufacturing. To be classified as limestone, the rock must contain at least 50 percent calcium carbonate. 

When the rock contains 30 to 45 percent magnesium carbonate, it is referred to as dolomite or dolomitic limestone. Lime can also be produced from aragonite, chalk, coral, marble, and seashells. 

The Standard Industry Classification (SIC) code for lime manufacturing is 3274. 

The six-digit Source Classification Code (SCC) for lime manufacturing is 3-05-016. 

Lime is manufactured in various kinds of kilns by one of the following reactions: 

CaCO3 + heat → CO2 + CaO (high calcium lime) 

CaCO3 MgCO3 + heat → 2CO2 + CaO MgO (dolomitic lime) 

In some lime plants, the resulting lime is reacted (slaked) with water to form hydrated lime. 

The basic processes in the production of lime are: 

(1) quarrying raw limestone; 

(2) preparing limestone for the kilns by crushing and sizing; 

(3) calcining limestone; 

(4) processing the lime further by hydrating; and 

(5) miscellaneous transfer, storage, and handling operations. 

The heart of a lime plant is the kiln. The prevalent type of kiln is the rotary kiln. This kiln is a long, cylindrical, slightly inclined, refractory-lined furnace, through which the limestone and hot combustion gases pass counter currently. Coal, oil, and natural gas may all be fired in rotary kilns. Product coolers and kiln feed preheaters of various types are commonly used to recover heat from the hot lime product and hot exhaust gases, respectively. 

The next most common type of kiln is the vertical, or shaft, kiln. This kiln can be described as an upright heavy steel cylinder lined with refractory material. The limestone is charged at the top and is calcined as it descends slowly to discharge at the bottom of the kiln. 

A primary advantage of vertical kilns over rotary kilns is higher average fuel efficiency. The primary disadvantages of vertical kilns are their relatively low production rates and the fact that coal cannot be used without degrading the quality of the lime produced

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