Split of sodium and sulfur in a Kraft mill and internal production of sulfuric acid and sodium hydroxide

Abstract: The removal of lignin in a Kraft pulp mill, with the aim to utilize the lignin as more value added green product than just firing lignin in black liquor, is possible with a LignoBoost plant. The LignoBoost plant uses sulfuric acid in the process and this results in an increased net input of sulfur to the pulp mills recovery cycle. The sodium/sulfur balance in a Kraft pulp mill is an important factor to be able to run a mill optimal. The increased input of sulfur into the mill when implementing a LignoBoost plant is therefore an issue to address. A too high sulfur/sodium ratio in the Kraft pulp mill is often solved by purging electrostatic precipitator dust from the recovery boiler. The major component of the ESP dust is sodium sulfate. When purging ESP dust from the recovery boiler the mill loose sodium and the need of sodium make-up increases. A large extent of the ESP dust that is not purged is returned to the recovery cycle of the mill via the evaporation plant. If the recycled sodium sulfate could be split and returned to the recovery cycle as one controlled sodium- and one controlled sulfur component or at least split into two flows where sulfur is enriched in one flow and sodium in the other flow, the sodium/sulfur balance would be easier controlled. In this master thesis the split of sodium and sulfur in sodium sulfate is addressed. The aim is to study opportunities to: • Enrich sodium and sulfur in two flows from the dissolved ESP dust, which is normally recycled to the evaporation plant. • Produce one sulfur component and one sodium component that can be utilized in the Kraft pulp mill, especially in an integrated LignoBoost process. • Accomplish this by using an electrochemical split of the sodium sulfate from the ESP dust to generate sodium hydroxide and sulfuric acid.   To be able to produce one sulfur component and one sodium component from the dissolved ESP dust an electrodialysis with or without bipolar membranes is the method to use decided after contact with Eka Chemicals research and development department and literature studies. An electrodialysis cell produces sodium hydroxide and sulfuric acid, from the sodium sulfate solution, that can be used in the Kraft pulp mill. The difficulty by using an electrochemical cell with ion selective membranes is the need of a pure feed to the cell. If a high content of contaminations, such as multivalent ions, is present in the feed solution to the cell scaling can be formed. Scaling leads to shorter membrane life that result in higher operational cost for the cell stack. Due to the multivalent ions in the electrostatic dust a pre-treatment such as carbonate- and hydroxide precipitation removal of the ions is suggested, which results in a decrease of the multivalent ions in the feed solution.   In previous work concerning electrochemical split of sodium sulfate the lack of utilization for the produced acid became negative in an economical point of view. The need of sulfuric acid to the LignoBoost plant is an advantage for the economical study. In this master thesis is:   • An economical case study for the implementation of an electrochemical cell, electrodialysis with or without a bipolar membrane, in a Kraft pulp mill performed. • A sensitivity analysis performed and evaluated in the aim of addressing the change in payback time due to alternating: Sodium price Membrane life Utilization of the acid produced from the electrochemical cell. The economical case study concerns a Kraft pulp mill with a LignoBoost plant. Utilization of the acid to the LignoBoost- and tall oil plant is varied, as is the membrane life for the cell stack. The membrane life is varied due to the difficulty of predicting the ESP-feed solutions affect on the membranes. The feed solution has to be tested in a cell to decide the real life for the membrane in this case.   The electrodialysis cell with bipolar membranes indicates promising economical gain for future implementation in a mill with LignoBoost lignin removal compared to the electrodialysis cell that indicates no economical gain for future implementation in a mill. For a mill with both a LignoBoost plant and a tall oil plant, i.e. optimized utilization of acid from the electrodialysis with bipolar membrane, and a five years membrane life in the cell, a payback of one and a half year can be reached. The same case but for an electrodialysis results in nine and a half payback years. The sensitivity analysis show that compared to the electrodialysis with bipolar membrane, the electrodialysis cell is more vulnerable to changes for the acid utilization, sodium hydroxide price and membrane life. The BME cell is most affected by changes in the sodium hydroxide price and the ED cell affects most by changes in the membrane life.