Lime Mud Dewatering

One of India’s largest paper-mill reduces the moisture content in lime-mud, achieves a significant reduction in the total cost, and improves plant operations.

Context

Mills use close to 200 liters of furnace oil to produce one ton of lime, one of the highest expenses in its operations. The presence of moisture in lime increases the consumption of furnace-oil, and disrupts the smooth operation of the lime kiln. A consistent quality of re-burned lime from the lime kiln is critical to the entire pulping process.

Large amounts of furnace oil consumption are in the caustic recovery process. Reducing moisture content in lime-mud is an effective way to reduce heat loss. It also saves significantly on furnace-oil consumption. Additionally, the yield of the lime kiln increases along with a reduction in scaling problems, thus improving the overall efficiency of the operations.

Program

HABER’s lime mud dewatering program was established at one of India’s largest paper mills, resulting in a significant reduction in the total cost of operations for the customer and improvement plant operations. The mill uses the Kraft pulping process, which is the most widely used pulping process in the paper-making industry due to its advantages over other pulping processes. The process involves the digestion of wood chips at high temperature and pressure, with black liquor being the by-product of the process. The black liquor is processed in the chemical recovery plant, where inorganic components are recovered and reused in pulping. Recovered organic components from the recovery plant will be used as fuel in the boiler. This process is highly efficient and ensures significant reuse (>95%) of chemicals used in the process.

Green liquor collected from the recovery boiler is sent to the causticizing plant to be causticized using lime. The causticized liquor (white liquor) is returned to the digester. The lime mud generated in this process is sent to the lime kiln for reprocessing. The lime kiln converts the lime mud generated in the causticizing plant into re-burned lime and is reused in the causticizing plant resulting in full recovery and reuse. The conversion is accomplished through calcination reaction at temperatures greater than 800 degree Celsius requiring large quantities of furnace oil. Hence, dry lime-mud going into lime-kiln would result in a decrease in furnace oil consumption. Moreover, variations in the quality of the re-burned lime can cause problems in the causticizing plant. Inefficient processing in lime-kiln may increase the emission of undesired noxious TRS (Total Reduced Sulphur) gases.

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Results

Reducing the moisture content in lime-mud has a direct impact on the fuel economy of the lime kiln. Since heat is required to remove the excess moisture, there is a strong correlation between lime mud moisture content and lime kiln fuel efficiency – the fuel consumption of the lime kiln improves by 2% for every 1% reduction in the moisture of lime mud. High moisture content can also result in other kiln operational issues like a decrease in the efficiency of calcination, ring and ball formation, and scaling on the surfaces. On the other hand, a very dry lime-mud can cause excessive dusting, and overheating, which results in failures of critical parts. Therefore, the optimum moisture content is maintained for both economic and operational reasons. 

Conclusion

Lime mud dewatering efficiency depends not just on the mud and liquor properties but also on the equipment design and operational parameters. And these properties change continuously and cause variations in performance and variations in fuel consumption. A program that monitors and reacts in real-time to these variations is thus highly beneficial to reduce variability, optimize operations and lower costs. After implementing HABER’s lime mud dewatering program, the mill saw a more than 10% increase in lime mud solids resulting in a significant reduction of fuel consumption in the lime kiln. The decreased use of fuel resulted in an overall reduction of the customer’s carbon footprint by nearly 1,000 tons per year. Lime production also increased by >10% improving the plant’s throughput significantly. Moreover, the operation is now experiencing fewer scaling and operational issues. HABER’s approach to lime-mud dewatering ensures optimum moisture levels are maintained, resulting in both economic gains as well as smooth operations. HABER’s solution is highly standardized and requires little to no customization resulting in quicker implementation and value realization.

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