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Sulfur compounds in white and green liquor: Comparison of analytical methods for the determination of reduction degree

Luomala, Milja LU (2024) KEMK10 20242
Department of Chemistry
Abstract
Introduction:
The forest industry might not be the first thing to come to mind when thinking of chemistry, but the analysis of cooking chemicals in wood pulping involves many laboratory methods.
Background:
The chemical recovery cycle is an integral part of the Kraft process, which makes laboratory analyses of different steps of the process necessary. In the recovery boiler, sulfur is reduced to sulfide, a main active ion in the process. The inorganic chemical mixture is dissolved in white wash, and the resulting green liquor can be analyzed. Many methods use harmful chemicals, like lead perchlorate, which has led to a demand for alternatives.
Aims:
Comparing the current method for the determination of reduction degree in white... (More)
Introduction:
The forest industry might not be the first thing to come to mind when thinking of chemistry, but the analysis of cooking chemicals in wood pulping involves many laboratory methods.
Background:
The chemical recovery cycle is an integral part of the Kraft process, which makes laboratory analyses of different steps of the process necessary. In the recovery boiler, sulfur is reduced to sulfide, a main active ion in the process. The inorganic chemical mixture is dissolved in white wash, and the resulting green liquor can be analyzed. Many methods use harmful chemicals, like lead perchlorate, which has led to a demand for alternatives.
Aims:
Comparing the current method for the determination of reduction degree in white and
green liquor with a thermometric titration, as well as investigating the discrepancies between an online recovery analyzer and the laboratory method.
Methods:
The current total sulfur method includes two potentiometric titrations, one with silver nitrate to determine the sulfide content, and another with lead perchlorate for the total sulfur content. An adjacent method with formaldehyde was used to yield the sulfate content, for easier comparison with the alternative method. Green liquor results were also compared with the online recovery analyzer, which uses an ABC titration and an optical titration. Samples were collected and analyzed using all described methods for approximately 4 weeks.
Results:
Inconsistent results from the formaldehyde method indicate that it isn’t a reliable reference point for sulfate content comparison. The adequacy of the new method was therefore investigated by comparing the reduction degrees with the current method and Alkali R, which showed that the thermometric titration is very closely equivalent to the Alkali R, while the total sulfur deviates somewhat consistently from both. This indicates the occurrence of sulfur compounds other than sulfide and sulfate.
Conclusion:
The thermometric titration is statistically equivalent to Alkali R which makes it adequate to replace the current method. To still track the other sulfur compounds, the total sulfur titration can be performed less frequently. (Less)
Popular Abstract
Analysis of sulfur compounds in the wood pulping industry

The forest industry is one of the biggest exporters and employers in Sweden. One important process in this industry is the Kraft process, which is used in wood pulping to produce paper and cardboard. An integral part of this process is the recycling of the chemicals that the wood chips are cooked in. This mixture, called white liquor, contains sodium hydroxide and -sulfide. The focus of this project was to determine the amount of sulfide and other sulfur compounds throughout the recovery cycle. After use, the white liquor becomes black liquor, which is burned in the recovery boiler. In this step, most of the sulfur is reduced back to sulfide, the active form sulfur in white... (More)
Analysis of sulfur compounds in the wood pulping industry

The forest industry is one of the biggest exporters and employers in Sweden. One important process in this industry is the Kraft process, which is used in wood pulping to produce paper and cardboard. An integral part of this process is the recycling of the chemicals that the wood chips are cooked in. This mixture, called white liquor, contains sodium hydroxide and -sulfide. The focus of this project was to determine the amount of sulfide and other sulfur compounds throughout the recovery cycle. After use, the white liquor becomes black liquor, which is burned in the recovery boiler. In this step, most of the sulfur is reduced back to sulfide, the active form sulfur in white liquor. The inorganic chemicals from the recovery boiler are then dissolved and become green liquor.
Due to the nature of sulfur in this process, it is important to analyze white and green liquor frequently to ensure that a sufficient amount of sulfur is reduced to sulfide, so that when the new white liquor is used for cooking, the process keeps running efficiently. The current analytical method for the determination of reduction degree (the fraction of sulfide out of the total amount of sulfur) is two potentiometric titrations, with silver nitrate for the determination of sulfide and
with lead perchlorate for total sulfur. The latter solution is harmful if swallowed, reproductively toxic, and very toxic to aquatic life with long term effects, which is why there is a demand to replace this method with one that uses no or less harmful chemicals. The alternative being studied in this project is a thermometric titration with barium acetate, which, instead of electric
potential, measures changes in temperature due to a heat-releasing reaction with sulfate (the oxidized form of sulfur). The main aim of this project was to determine if the thermometric titration would be an adequate replacement for the current total sulfur method. The comparison was done by using a formaldehyde method similar to the total sulfur method, which gives the sulfate content, making it easier to compare with the thermometric titration results. The reduction degrees calculated though the different methods were also compared. The sulfate content of green liquor is also analyzed by an automatic online recovery liquor analyzer, Alkali R, which measures a titration with barium chloride to reach an optical inflection point. The second aim of this project was to investigate the level and reasons for the discrepancies between the analyzer and laboratory results.
The experimental work of the project had three main parts: Validation of the formaldehyde method and thermometric titration with standard solutions, method comparison with white and green liquor samples, and green liquor comparison with samples directly from Alkali R.
The formaldehyde method was very accurate when using standard solutions of sodium sulfate, but when liquor samples were analyzed, the results were inconsistent. When the thermometric titration was compared to Alkali R, however, the results were statistically equivalent, which would imply that the proposed method is reliable. By comparing the total sulfur content between all tried methods, it was concluded that the discrepancy between the current method and Alkali R are caused by other sulfur compounds in the liquor that aren’t accounted for in the Alkali R
method. (Less)
Please use this url to cite or link to this publication:
author
Luomala, Milja LU
supervisor
organization
course
KEMK10 20242
year
type
M2 - Bachelor Degree
subject
keywords
analytical chemistry, green liquor, kraft process, method comparison, reduction degree, thermometric titration
language
English
id
9178133
date added to LUP
2024-12-05 15:36:52
date last changed
2024-12-05 15:36:52
@misc{9178133,
  abstract     = {{Introduction: 
The forest industry might not be the first thing to come to mind when thinking of chemistry, but the analysis of cooking chemicals in wood pulping involves many laboratory methods.
Background: 
The chemical recovery cycle is an integral part of the Kraft process, which makes laboratory analyses of different steps of the process necessary. In the recovery boiler, sulfur is reduced to sulfide, a main active ion in the process. The inorganic chemical mixture is dissolved in white wash, and the resulting green liquor can be analyzed. Many methods use harmful chemicals, like lead perchlorate, which has led to a demand for alternatives. 
Aims: 
Comparing the current method for the determination of reduction degree in white and 
green liquor with a thermometric titration, as well as investigating the discrepancies between an online recovery analyzer and the laboratory method.
Methods: 
The current total sulfur method includes two potentiometric titrations, one with silver nitrate to determine the sulfide content, and another with lead perchlorate for the total sulfur content. An adjacent method with formaldehyde was used to yield the sulfate content, for easier comparison with the alternative method. Green liquor results were also compared with the online recovery analyzer, which uses an ABC titration and an optical titration. Samples were collected and analyzed using all described methods for approximately 4 weeks.
Results: 
Inconsistent results from the formaldehyde method indicate that it isn’t a reliable reference point for sulfate content comparison. The adequacy of the new method was therefore investigated by comparing the reduction degrees with the current method and Alkali R, which showed that the thermometric titration is very closely equivalent to the Alkali R, while the total sulfur deviates somewhat consistently from both. This indicates the occurrence of sulfur compounds other than sulfide and sulfate.
Conclusion: 
The thermometric titration is statistically equivalent to Alkali R which makes it adequate to replace the current method. To still track the other sulfur compounds, the total sulfur titration can be performed less frequently.}},
  author       = {{Luomala, Milja}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Sulfur compounds in white and green liquor: Comparison of analytical methods for the determination of reduction degree}},
  year         = {{2024}},
}