At the end of this module, you will be able to:
- Identify the role of the Recaust system in a kraft pulp mill
- Describe the role of lime and the lime kiln in the recausticizing process
- Describe the calcining process
- Identify the major components and sections of a rotary lime kiln
- List the factors that affect the efficiency of the calcining process
- Describe exhaust gas treatment and monitoring
- Describe common problems in rotary lime kilns
- List key operating variables
The following key questions are answered in this module:
What is the most important chemical reaction that occurs in a lime kiln called?
The reaction that occurs in lime kiln is called calcining, the conversion of calcium carbonate to calcium oxide.
What is recausticizing?
Recausticizing is the process of converting the chemicals in spent cooking liquor back to their original form so that they can be re-used in the kraft pulping process.
What are the three major process sections of a rotary lime kiln?
The three major sections are defined by the processes which occur within them. They are the evaporation, heating and calcining sections.
What kinds of pollutants may be a in the exhaust gas from kiln?
Exhaust gas from from kiln typically has high levels of particulate which must be removed. It may also contain sulfur-compounds which need to be removed.
What can happen if poorly washed lime mud enters the kiln?
Poor washing will introduce sodium compounds into the kiln which may lead to the formation of rings within the kiln.
Below is a transcript of the video sample provided for this module:
A lime kiln uses heat, motion, and airflow to convert lime mud to lime, or in chemical terms, calcium carbonate to calcium oxide. This process is referred to as "calcining". The most common lime kiln designed found at kraft pulp mills is the rotary lime kiln. A rotary lime kiln is basically a long tube or cylinder set at a slight incline that slowly rotates. Lime mud is continuously fed in at the higher end of the cylinder, and the rotation and incline of the kiln cause the mud to slowly move down toward the opposite end. A burner at the lower end of the kiln provides the heat needed for the calcining reaction, and an induced draft fan at the lime mud feed end pulls the hot gasses through the kiln. The rotation of the kiln stirs the lime mud as it travels to fully expose it to the heat. By the time the lime mud complete its journey to the lower end of the kiln, it is about 98% calcined. The lime is now ready to be reused in the recaust process. The design and operation of a rotary lime kiln will be described next.
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