3.01.01 Kraft (Sulfate) Pulp: Continuous Digester
3.01.02 Kraft (Sulfate) Pulp: Brownstock Washing
3.01.03 Kraft (Sulfate) Pulp: Evaporation
3.01.04 Kraft (Sulfate) Pulp: Recovery Boiler
3.01.05 Kraft (Sulfate) Pulp: Causticizing
Overview of Process Control
Ref. 3.01.00 Kraft (Sulfate) Pulp Process (pdf)
After cooking, the pulp passes through a blow line to the blow tank and then onto a washing section. The diffuser washers wash black liquor from the fibres. The washed pulp is then screened before the bleaching process. The weak black liquor passes from the washing section to the chemical recovery area. K-Patents SAFE-DRIVE® Process Refractometer PR-23-SD is used in the blow line, extraction and washing stages to measure the efficiency of the digester. In washing, the black liquor is displaced by water. To optimize the pulp chemical consumption and water usage, the black liquor concentrations have to be measured before and after washing. The material balances can be calculated and the digester efficiency and washing process can be optimized and controlled using K-Patents SAFE-DRIVE® Refractometer.
Ref. 3.01.01 Kraft (Sulfate) Pulp: Continuous Digester (pdf)
Due to the economic and environmental pressures, there is a requirement for efficient fresh water usage in chemical pulping and bleaching. Lower levels of wash water consumption have a positive effect on the energy balance of the mill, and will decrease the impact on water resources. However, too low a level of wash water usage can adversely affect washing efficiency and increase chemical consumption in the bleaching process. To address this dilemma and to achieve an effective trade-off between washing efficiency, chemical consumption and energy efficiency, effective tools for the precise control of brown stock washing are needed. Total dissolved solids (TDS) changes in the incoming flow to the washer can be detected immediately and reliably through K-Patents Process Refractometer measurements. During brown stock bleaching, TDS changes also have a clear effect on chemical consumption through washing loss. Real-time measurement enables precise control of the washing line. This instant information allows the most effective control of the fresh water flow to the washer. With such control systems water usage can be optimized and thus excessive usage is avoided. In conclusion, by utilizing K-Patents refractometers and data-analysis tools it is possible to reveal inefficiencies in the washing line and continuously evaluate the washing results, thereby allowing improvements in washing efficiency and a subsequent reduction in water consumption.
Ref. 3.01.02 Kraft (Sulfate) Pulp: Brownstock Washing (pdf)
A resultant by-product of the kraft process is black liquor. It is a combination of the removed lignin, water and chemicals used in the extraction process. The black liquor, which originates from the washing process, is concentrated through evaporation and is then used to fire a boiler generating high-pressure steam for the mill processes. When softwood, such as conifers, is used in pulping, a soap-like substance is collected after the process. This soap is then acidified and used to produce tall oil. Tall oil is a source of fatty acids, resin acids and various other chemicals.
Ref. 3.01.03 Kraft (Sulfate) Pulp: Evaporation (pdf)
The recovery boiler plays a central role in the chemical cycle of a modern pulp mill. The recovery boiler is a chemical reactor, which is used for recovering chemicals from spent kraft liquor and generating energy at the same time. In the recovery boiler, the organic matter is burned. The dry solids liquor content required for firing is at least 60% but preferably more than 65%. Black liquor is concentrated by evaporating water from the liquor. When the concentration of black liquor is maximized, so is the energy production. Before entering the burners, glauber salt is added to cover chemical losses.
Ref. 3.01.04 Kraft (Sulfate) Pulp: Recovery Boiler Comparison (pdf)
The purpose of the chemical recovery process is to recover and regenerate the pulping chemicals, and to burn the organic material dissolved from wood to generate steam. The chemical recovery process consists of evaporation plant, recovery boiler and causticizing plant. The object of the causticizing process is to convert the inactive sodium carbonate (Na2CO3) into the active cooking chemical, sodium hydroxide, as efficiently as possible. The process can be divided into four parts: green liquor clarification/filtration, slaking, causticizing and white liquor clarification/filtration.
Ref. 3.01.05 Kraft (Sulfate) Pulp: Causticizing (pdf)
Overview of process control
Ref. 3.02.00 Sulfite Pulp Process (pdf)
Overview of process control
Ref. 3.03.00 Paper Sizing Process (pdf)
The liquid is usually cooked in continuous cookers. The starch is cooked by introducing steam directly into the starch slurry. The cooked starch is cooled and diluted to the desired concentration by adding water to it before it enters the storage tank.
Ref. 3.03.01 Starch Cooking (pdf)
The size liquid is commonly added to the paper with a size press or a blade coater. From the storage tank, the sizing liquid is pumped into the size press circulation system. There is a constant flow from the machine tank to the size press. At the size press, the sizing liquid is sprayed onto the paper. The paper absorbs a portion of the sizing liquid with the surplus being recirculated through a screen to the machine tank. At the size press, the paper absorbs a larger quantity of water than the sizing material. Therefore, it is extremely important to measure the size’s concentration levels precisely in the size press circulation system. Water must be added into the circulation to maintain the dilution levels.
Ref. 3.03.02 Size Press (pdf)