Novel treatment methods for green liquor dregs and enhancing circular economy in kraft pulp mills
Golmaei, Seyedmohammad (2018-12-17)
Väitöskirja
Golmaei, Seyedmohammad
17.12.2018
Lappeenranta University of Technology
Acta Universitatis Lappeenrantaensis
School of Engineering Science
School of Engineering Science, Kemiantekniikka
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Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-323-7
https://urn.fi/URN:ISBN:978-952-335-323-7
Tiivistelmä
Green liquor dregs (GLD) are known to be the largest fraction of inorganic residue in the kraft pulping process, or even in virgin pulp production. GLD originates from the chemical recovery cycle where effective cooking chemicals sodium hydroxide and sodium sulfide are regenerated from black liquor. Black liquor is the mixture of spent cooking chemicals and dissolved wood components which is resulted from delignification process where lignin is removed from wood chips. Unfortunately, GLD is still landfilled, while converting this residue to sustainable products would be considered as an achievement as regards material utilization in circular economy.
The main obstacle in converting this inorganic residue to sustainable products, i.e. fertilizing products, is the level of some metals with environmental pollution effects. Of the inorganic residues generated in the kraft pulping process, GLD is not the most enriched one by nutrient elements, but it could be suitable for soil fertilizing usages. However, removal of environmentally hazardous metals from GLD still remains as a challenge for currently applied separation techniques.
The present study is divided to two parts: in the first part the filtration characteristics of GLD are studied, and in the second part, practical methods for the separation of target hazardous trace metals such as Cd, Ni, Pb and Zn are studied. The results of the first part reveal that the filterability of GLD sludge can be improved by optimized filtration parameters without a need for filter aids such as a lime mud precoat.
In the second part of the study, the separation of hazardous metals by the use of chelating agents as the extractant and mechanical classification are investigated. The chelating agent EDTA is utilized successfully in the extraction of the target hazardous metals, especially Cd from GLD, while keeping most of Ca, which is the main mineral nutrient present in GLD. Hydrocyclone classification of GLD reveals that the target hazardous metals are mainly accumulated in the finer fraction of GLD separated into the overflow. The coarser underflow fractions containing a larger share of GLD and its Ca content are assessed regarding categorizing them as CE-marked fertilizing products. The results prove that the concentration of hazardous metals in GLD is reduced in the underflow fractions to a level lower than the maximum allowed concentrations in CE-marked fertilizing products.
This study is the first in which chemical and mechanical treatment methods are implemented successfully by novel methods in the separation of hazardous metals from GLD. The outcome of this thesis can be used for developing novel treatment processes in kraft pulp mills to reduce the amount of inorganic solid wastes, and to separate toxic metals from GLD while keeping its main nutrients.
The main obstacle in converting this inorganic residue to sustainable products, i.e. fertilizing products, is the level of some metals with environmental pollution effects. Of the inorganic residues generated in the kraft pulping process, GLD is not the most enriched one by nutrient elements, but it could be suitable for soil fertilizing usages. However, removal of environmentally hazardous metals from GLD still remains as a challenge for currently applied separation techniques.
The present study is divided to two parts: in the first part the filtration characteristics of GLD are studied, and in the second part, practical methods for the separation of target hazardous trace metals such as Cd, Ni, Pb and Zn are studied. The results of the first part reveal that the filterability of GLD sludge can be improved by optimized filtration parameters without a need for filter aids such as a lime mud precoat.
In the second part of the study, the separation of hazardous metals by the use of chelating agents as the extractant and mechanical classification are investigated. The chelating agent EDTA is utilized successfully in the extraction of the target hazardous metals, especially Cd from GLD, while keeping most of Ca, which is the main mineral nutrient present in GLD. Hydrocyclone classification of GLD reveals that the target hazardous metals are mainly accumulated in the finer fraction of GLD separated into the overflow. The coarser underflow fractions containing a larger share of GLD and its Ca content are assessed regarding categorizing them as CE-marked fertilizing products. The results prove that the concentration of hazardous metals in GLD is reduced in the underflow fractions to a level lower than the maximum allowed concentrations in CE-marked fertilizing products.
This study is the first in which chemical and mechanical treatment methods are implemented successfully by novel methods in the separation of hazardous metals from GLD. The outcome of this thesis can be used for developing novel treatment processes in kraft pulp mills to reduce the amount of inorganic solid wastes, and to separate toxic metals from GLD while keeping its main nutrients.
Kokoelmat
- Väitöskirjat [1037]