Black Liquor Testing
Several properties of black liquor – including residual alkali, temperature, and solids content – are regularly monitored in order to monitor pulping, washing, and evaporation processes at kraft pulp mills. Residual alkali is monitored during cooking to ensure that sufficient residual alkali is present and prevent lignin redeposition. Weak black liquor, strong black liquor, and heavy black liquor solids are observed to monitor washer, evaporator, and concentrator performance, and ensure consistent solids contents to the evaporators and recovery boiler. Other analyses, including heating value, elemental, inorganic, and organic analyses, are used for research, design, and troubleshooting purposes.
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Learning Objectives
Describe the basic stages and flow path of black liquor in the kraft pulping liquor cycle List the typical components of black liquor Describe what happens to the inorganic compounds, organic compounds, and water in heavy black liquor in a recovery boiler Define “viscosity,” “density,” “specific heat,” “thermal conductivity” and “boiling point rise” List the properties of black liquor that are regularly monitored at kraft pulp mills Describe the importance of black liquor solids testing and monitoring Recognize how more advanced analyses – including heating value, elemental, inorganic, and organic analyses – are used for research, design, and troubleshooting purposes
Specs
| Course Level | Intermediate |
| Languages | English, French |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is in black liquor at a kraft pulp mill?
Black liquor contains inorganic cooking chemicals, dissolved organic compounds from wood, and a small amount of fibers and fiber fines in a large amount of water.
Why are concentrators often positioned downstream of multiple effect evaporators?
Concentrators are sometimes used to further concentrate black liquor, as traditional evaporators cannot handle such viscous liquids. Some newer evaporator installations are able to produce high enough black liquor solids content for burning without a separate final evaporation step.
Why is the residual alkali content of the black liquor monitored after cooking?
To ensure that sufficient residual alkali is present. If it is too low (indicated by a pH below 12), some of the dissolved lignin can redeposit on the fibers. Low pH can also lead to scaling problems and lignin precipitation in the evaporators.
Why would someone bother to measure the solids content of the weak black liquor coming off the brown stock washers?
Weak black liquor solids are typically used to monitor the efficiency and effectiveness of the brown stock washing process.
Why is it mandatory to monitor the black liquor solids of the liquor being burned at the recovery boiler?
Black liquor solids are monitored at the recovery boiler for safety reasons. If the solids were to fall too low, and water were to reach the smelt bed, a severe explosion could result.
Sample Video Transcript
It is important to always sample from the same locations and to follow a standardized sampling, and testing procedure. Weak black liquor should be sampled on the line from the washers that goes to the evaporators. Strong black liquor should be sampled on the line exiting the last evaporator effect. Heavy black liquor should be sampled on the line exiting the concentrator. And at the recovery boiler, the heavy black liquor should be sampled either on the liquor line to the recovery boiler or on the liquor header itself.
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