New Investigations in Brownstock Washing

K-Patents is a partner company in an ongoing research study that seeks ways to improve brownstock washing in the chemical pulping. The aim is to find a new more holistic mill level solution that minimizes washing loss and consumes less bleaching chemicals.

The first part of the study has revealed that K-Patents Process Refractometer is the most feasible instrument for determining real-time the washing loss because it measures exactly the concentration of washable liquid substances.

Other methods compared were

  1. The amount of sodium
  2. COD (Chemical Oxygen Demand) analysis
  3. Conductivity
  4. Total dissolved solids dry matter content

The amount of sodium describes the yieldied sodium, but ignores all other compounds. The COD analysis determines the oxidisible compounds, but not the unoxidisible compounds, and it is an off-line method with long delays. It is also heavily influenced by methanol, which does not have any significance for the washing loss. Conductivity meter has a short response time and it is suitable for very low concentrations. However it does not follow the dry solids at the high concentrations (see the graph below). Total dissolved solids dry matter content is an old off-line method that has long delays.

PURPOSE OF PULP WASHING

Kraft pulping is a series of water based chemical processes. The purpose of pulp washing is to remove black liquor from the pulp suspension. Pulp is washed so that the dissolved material of black liquor can be utilized in energy production and for the recycling of chemicals.

An efficient washing requires the control of the volume of washing fluid that is added to the system. Using large amounts of washing fluid a cleaner pulp is obtained, but an efficient operation of the recovery system requires minimum dilution of the black liquor, in order to reduce the consumption of energy during the evaporation.

On the other hand, in the case of insufficient washing, there is an excessive loss of black liquor, which affects the thermal balance of the line and of the chemical products in the recovery section, as well as leads to a greater consumption of oxidizing agents during bleaching. The parameters that describe the efficiency of washing (Efficiency Factor E) can be divided into two: wash loss and dilution factor. Wash loss is the amount of washable compounds in the pulp suspension which could have been removed in washing. Dilution factor is the amount of water that is used for washing. It is given as the amount of washing water that exceeds what is ideally required for a total displacement.

IMPROVED WASHING EFFICIENCY

The key finding of the research study was that neither the traditional lab sampling every 12 to 24 hours, nor the conventional on-line methods can reveal all process variations that take place. Refractive index measurements showed remarkable concentration changes in the bleaching pulp line. These variations have considerable implications on the Efficiency Factor, but they have been neglected until now. The process control has relied on faulty assumptions.

Refractive index measurements revealed that 10-20 l/s too much wash water is used for one washer. This results in additional evaporation costs of 300.000 - 600.000 EUR in a year.