Inline green liquor monitoring

In paper and pulp production, both the acid sulfite and alkaline sulfate processes are used to pulp the wood. n this process, the wood fibers are dissolved from the natural material and separated from the other constituents, such as lignin. For ecological and economic reasons, the used chemicals must be recycled.

The goal hereby is to reconstruct caustic soda (NaOH) and sodium sulfide (Na2S) and generate energy, heat and steam, while reducing environmental damage as much as possible.

The lignin-containing cooking liquor (black liquor) produced during digestion is thickened and burned. In addition to energy and steam recovery, a melt is produced that settles at the bottom of the boiler and is regenerated in several recycling stages. The resulting green liquor from water and melt is converted into NaOH-containing white liquor by adding lime in the recausticizing stage, which is again used for splitting lignin.

By using the LiquiSonic® measuring technology, the green liquor treatment can be monitored inline at various points and the concentration for the following recausticizing can be determined.

Applications in the pulp and paper industry place high demands on all system. Our application specialists design the sensors to meet the expected conditions. The robust measuring method, which works without moving parts and seals, convinces with unsurpassed running times. Depending on the design, the sound velocity sensors withstand temperatures of up to 130°C. Highly resistant materials and coatings are used to ensure a long lifetime.

Online analysis of the green liquor concentrations by using LiquiSonic® measurement technology enables lime to be added in a targeted manner. This means that the individual process stages, such as dilution of the melt to form the green liquor and its storage and clarification, can be permanently monitored and automatically controlled. Time delays caused by sampling and laboratory analyses are eliminated. This efficient process control results in an increase in quality.

Measurement without LiquiSonic®

Measuring green liquor is not an easy task, due to the harsh environment and difficult measurement conditions. High temperatures of up to 100°C prevail in the plants, and aggressive and corrosive liquids are used. To make matters worse, the media to be measured have high viscosity and stickiness. A high proportion of solid particles and impurities, as well as suspended solids that tend to crystallize and thereby hinder adequate measurement.

Without continuous concentration measurement by LiquiSonic® only snapshots of the concentration values are available. Manual sampling supports errors caused by human hand. In addition, the time lag between sampling and the availability of results makes for an insufficiently controllable process. As a result, quality deficiencies frequently.

Measurement with LiquiSonic®

With LiquiSonic®, the solution from SensoTech, product quality is increased as the relevant key figures in the green liquor process are known precisely at all times. Measurements are not taken selectively, as in the case of sampling, but at one-second intervals over the entire course of the process. Resulting in no time lag between the measurement and the result, which means that deviations can be reacted to very quickly, e.g. in the form of re-dosing.


In addition to saving time, inline measurement has another advantage: the influence of human error is reduced to a minimum, since the measured values can be integrated into the process control system. The post-metering of the lime is more accurate due to automation, leading to an even more efficient process. This reduces lime consumption, which leads to a reduction in costs. Multiple sampling and waiting for concentration measurement results are in the past.


Additionally, the LiquiSonic® sensor is not affected by deposited particles or impurities. The LiquiSonic® sensors are particularly robust due to the special polymer coating, which allowes working with aggressive and corrosive media. After installation and adaptation of the measuring system to the process, no cleaning, maintenance or calibration is required!


The starting material in caustic recycling is the black liquor in the recovery boiler. Melt formed during combustion sediments and must be diluted by water or thin white liquor. The resulting green liquor is containing Na2S and Na2CO3, hot and often stored in a separate tank. It is then clarified and stored in a green liquor collection tank. The prepared green liquor is converted to NaOH with quicklime in the causticizing unit.

By analyzing the LiquiSonic® sensors within seconds, the individual process stages, such as dilution of the melt to  green liquor and its storage and clarification, can be permanently monitored and controlled. This efficient process control thus serves both to increase quality and to economically regulate the addition of lime.

Installation of LiquiSonic® measurement technology in the green liquor process

The LiquiSonic® pipe sensors are installed directly into the green liquor transporting pipelines. The PFA coating ensures many years of measurement accuracy despite alkaline, hot process liquids and deposit tendency. The LiquiSonic® Controller 30 can be connected with up to 4 sensors to monitor the green liquor  at several measuring points simultaneously:  

  • after the solution tank
  • before clarification
  • after clarification or before recausticizing

Typical measuring ranges:
Concentration: 0 - 30 m% or 950 - 1200g/l
Temperature: 5 - 95 °C

Customer benefits

The LiquiSonic® measuring system offers numerous advantages in paper and pulp production, especially in green liquor treatment:

  • stable and maintenance-free function despite the tendency of the green liquor to deposit
  • reduction of maintenance and cleaning costs, no cleaning accessories necessary
  • precise concentration determination in green liquor, as well as black and white liquor
  • traceable monitoring with data recording
  • increase of operational safety

Downloads for inline green liquor monitoring

Application report

Green liquor tratment

Pulp and paper industry