Lime mud density control – global supplier put their experience to work at Swedish pulp mills

Lime mud density control – global supplier put their experience to work at Swedish pulp mills.


Spent cooking chemicals when wood chips become pulp must be recycled for both the environment and the mill economy. The last step in that recycling process is the long rotating lime kiln where lime mud, the sludge remaining after filtering the white liquor, is fed into the one end and comes out as burnt lime in the other. This calcinated lime is then used again for causticizing green liquor into white liquor (causticizing – to caustic soda).

The lime calcination closes the so-called liquor cycle, but it is a big fuel consumer with potential for optimization. This article features modernizations made as part of a project to stabilize the lime kiln temperature zones.

Situation today

Process control in this chemical recovery is difficult with abrasive and coating-prone media. To control lime mud density during pumping, externally mounted radiometric gamma density has been used for decades. This involves bureaucracy as in legislative permits and radiation safety compared to normal instrumentation. Calibration must also be performed in situ before the gamma meter can be used and then on regular basis owing to the decay of the gamma isotope.

Since lime mud is calcium carbonate (CaCO3) it is a slurry that the Swiss field instrument producer Endress + Hauser has extensive experience in as supplier to one of the global leading CaCO3 producers. Together the companies have developed a good understanding of material selection and best practices for maintenance.

Alternative solution

As of 2020 several of our Promass meters are installed and being evaluated on lime mud in Sweden says Ulf Johansson at Endress+Hauser. Sizing and tube selection is essential for a successful inline installation when you go from gamma to coriolis meters like Promass. We have also been able to compare how straight and bent tube coriolis meters are affected differently by scaling since we have both types installed.

The advantage over gamma density is that Promass comes pre-calibrated straight out of the box ready to measure and can be obtained with an accuracy of 1 kg / m3. No permit application or radiation safety necessary.

Promass is multivariable and delivers in addition to density also flow, temperature and concentration. Unknown to many is its optional viscosity measurement for which a special version is patented.

With wetted parts in SS316, 904L, Hastelloy, Titan, Duplex, Zirconium or Tantalum it is suitable for a wide range of media as hot tall oil, black liquor, green liquor or sulfuric acid. For an integrated pulp and paper mill it is a good alternative on chemical dosing for the paper machine's coating kitchen since it is available down to 1 mm in size which offer excellent low dosing accuracy. Promass is not disturbed by cationic liquids and can handle fast pulsating flows, something that pose a challenge for commonly used electromagnetic flowmeters.

Endress + Hauser also have gamma density on its program to continue offer a 1:1 replacement for such systems.

Contact Ulf Johansson to learn more about the technology and the collective experience in Sweden.

Ulf Johansson
Endress+Hauser AB
Phone: +4611-4446871