SUSPENDED SOLIDS IN WHITE LIQUOR

INTRODUCTION

The pulping process within a mill has always been a concern for operations and maintenance. It is the area in which chemical reactions are very critical to the overall process including the efficiency of the causticizing area.

APPLICATION

Our main concern in this application is to monitor for suspended solids in the clarified white liquor. By monitoring the solids, the operators are able to divert a potentially high suspended solids level from entering the digesters. This is a key measurement since the suspended solids level can dramatically affect the efficiency of the process.

It is also important to maintain low levels of solids after the clarifier or pressure filter, in order to reduce the pitch and scale on screens and thereby reduce downtime.

SOLUTION

The Monitek acoustic suspended solids monitor will continuously check the white liquor and immediately detect the presence of solids. For mills with white liquor clarifiers, the carryover of TSS in the white liquor would signify that they have overlimed in the recausticizing process. (Overliming usually causes poor settling of lime mud solids in white liquor.) The corrective action need then be to reduce the addition rate of lime (CaO) to the slaker.

A high reading of TSS in white liquor as it exits the pressure filter would indicate failure of one of the socks or discs. Corrective action could be taken immediately.

In the process where polymer is used, the system could be implemented for feed forward control for polymer addition into the clarifier and feedback control in the case of high-suspended solids in the clarified white liquor.

By continuously monitoring these parameters, we can reduce the level of downstream contaminants from entering the digesters.

In-line monitoring has been a problem in the past, due to the nature of the process liquid. The white liquor is harsh and very fouling, however Monitek offers an alternative to traditional optical methods of detection. Monitek’s patented acoustical system allows maintenance free operation in the harshest environments.

ADDITIONAL COMMENTS

The current monitoring method in most plants is by lab sampling, but large amounts of contaminated material can travel through the process before being detected. Therefore, continuous in-line monitoring is preferred for immediate action. Note: Similar monitoring can be done for suspended solids in green liquor. (For more information see Application Note 4.004.A0)