What Is Chemical Pulping In The Paper Industry?

What is chemical pulping in the paper industry?

Chemical pulping: refers to the process of using chemical agents to treat plant fiber raw materials under specific conditions, so that most of the lignin in them is dissolved and the fibers are separated from each other.

Cooking: The process of treating plant fiber raw materials with chemical agents
Cooking agent: The chemical agent used when steaming

Requirements for chemical pulping: Remove as much lignin as possible, while requiring the least fiber dissolution and proper retention of hemicellulose.
Remove as much intercellular lignin as possible from the wood raw material that binds the fibers together, separate the fiber cells or make them easy to separate, and also reduce the lignin content in the fiber cells appropriately. In short, remove as much lignin as possible, while requiring the least fiber dissolution and proper retention of hemicellulose.
The retention of hemicellulose is determined according to the requirements of pulp quality.

Classification of chemical pulping

Alkali pulping

Alkali pulping, also known as alkaline cooking, is a process of treating plant fiber raw materials with an aqueous solution of alkaline chemicals at a certain temperature to dissolve most of the lignin in the raw materials and separate the fibers in the raw materials into pulp.

• Alkali pulping can be divided into the following categories according to the different cooking agents used:
• Caustic soda method
• Sulfate method
• Sodium polysulfide method
• Prehydrolysis sulfate method
• Oxygen alkali method
• Others

The most commonly used are caustic soda method and sulfate method.

Sulfate method: Alkali cooking has a wide range of application for raw materials, especially the sulfate method, which is applicable to almost all kinds of plant fiber raw materials.

In addition, it can also be used for waste firewood of poor quality, wood processing scraps, sawdust, and wood with high resin content, etc.

Caustic soda method:
Caustic soda method is suitable for non-wood fiber raw materials such as cotton, hemp, and grass. It is also used for cooking hardwood, but rarely for cooking coniferous wood.

Sulfite pulping

According to the main components of the cooking liquor and the pH value, it can be divided into:

• Acidic bisulfite method
• Bisulfite method
• Slightly acidic bisulfite method
• Neutral sulfite method
• Alkaline sulfite method

The sulfite method we often talk about mainly refers to the acidic bisulfite method and the bisulfite method.

Composition and properties of cooking liquor

Alkali method:

Caustic soda method: NaOH
Sulfate method: NaOH +Na₂S

The main components of the caustic soda method are: NaOH, in addition, there is also Na2CO3, so the properties of the cooking liquid are mainly the properties of NaOH. NaOH acts mainly as a strong base during cooking (PH=14).
The main components of the sulfate method are: NaOH+Na2S, in addition, there are Na3CO3, Na2SO4, Na2SO3 and 2S2O3 from the alkali recovery system, and even a small amount of sodium polysulfide.

Sulfite method：

In the sulfate pulping process, in addition to the role of the strong alkali NaOH, the sulfide ions after the ionization of sodium sulfide and the product of hydrolysis, the hydrogen sulfide ion HS-, also play an important role.
In addition, Na2CO3, Na2SO3, and even sodium polysulfide will also play a certain role.
For sulfite cooking liquor, the composition is mainly SO2 and the corresponding salt base, such as Ca2+, Mg2+;
Different sulfite cooking methods have different cooking liquor compositions, and the existence form of sulfur dioxide changes with the pH value.
For example, in the acidic bisulfite cooking liquor, it exists mainly in the form of bisulfite ions and SO2, while in the neutral sulfite method, it exists in the form of sulfite.

There are some terms involved in the chemical method, and here we mainly introduce the terms involved in the alkaline method.

Total alkali

• For caustic soda cooking method, total alkali refers to NaOH+Na2CO3;
• For sulfate method, total alkali refers to NaOH+Na2CO3+Na2S+Na2SO3+Na2SO4+Na2S2O3;

Total titratable alkali

It refers to the titratable alkali in the alkali solution.

• Caustic soda cooking method: NaOH+Na2CO3
• Sulfate method: NaOH+Na2CO3

Active alkali

• Caustic soda cooking method: NaOH
• Sulfate method: NaOH+Na2S

Effective alkali

• Caustic soda cooking method: NaOH
• Sulfate method: NaOH+ ½Na2S

Sulfidation degree

• The sulfidation degree of white liquor refers to the percentage of NA2S to active alkali;
• The sulfidation degree of green liquor refers to the percentage of NA2S to total titratable alkali;

Alkali amount

Refers to the ratio of active alkali (mass) to the mass of dry raw materials during cooking;
Alkali amount = active alkali amount | absolute dry raw material mass *100%;

Dry raw materials and air-dried raw materials

• Oven dry: Raw materials refer to plant fiber raw materials without moisture;
• Air-dry: if it is not specified, it generally refers to plant fiber raw materials with a moisture content of 10%;

Liquid ratio(Liquor to wood ratio/Liquor to chip ratio)

Refers to the ratio of the mass of dry raw materials (kg or t) to the total volume of cooking liquid (L or M3) during cooking.

Pulping yield¹

• Coarse pulp yield: The percentage of the mass of dry raw pulp obtained after cooking to the weight of dry raw materials before cooking;
• Fine pulp yield: The percentage of the mass of dry fine pulp obtained after screening to the mass of dry raw materials before cooking;

Pulping hardness

It is an indicator of the relative content of lignin and other reducing substances remaining in the pulp after cooking or bleaching.

It can be measured with oxidants such as potassium permanganate, chlorine or hypochlorite, among which the most common is the chlorine acid and alkali. The hardness of paper can be expressed by potassium permanganate value, Kappa value², and Beckmann value.

Black liquor³

Waste liquor produced by alkaline cooking.

Because the color is dark brown, it is called black liquor. Black liquor contains a large amount of suspended solids, organic pollutants and harmful substances. Direct discharge into the water will cause serious pollution, so it needs to be treated. The current common treatment method is alkali recovery. This method can not only recover inorganic matter in the black liquor, but also convert organic matter in the black liquor into heat energy for utilization.

Green liquor⁴

When the black liquor is alkali recovered, the black liquor is evaporated and concentrated and then sent to the combustion furnace for combustion. The molten material flowing out of the combustion furnace is dissolved in dilute white liquor or water to form a solution.

Because the solution is green, it is called green liquor.

White liquor⁵

The solution obtained after the green liquor is causticized with calcium hydroxide.
The main components of caustic soda white liquor are NaOH, and there is a small amount of uncausticized Na2CO3.
The main components of sulfate white liquor are NaOH and Na2S, and there may be unreacted Na2CO3+Na2SO4+Na2SO3+Na2S2O3+Na2SN, etc.
After adding some chemicals, the white liquor can be used for cooking liquid.

Preparation of cooking liquid

Classification

Classify according to whether the factory has chemical recycling, using recycled liquid and commercial chemical configurations.
Commercial chemical configuration: refers to using purchased sodium hydroxide, sodium sulfide, etc. to configure cooking liquid in factories without alkali recycling.
The use of alkali-recycled white liquor to configure cooking liquid is based on Russian cooking requirements, white liquor concentration, sulfidation, etc., and an appropriate amount of chemical raw materials such as sodium sulfate and sodium carbonate are added to the alkali-recycled white liquor to meet the cooking needs.

Cooking calculation

When performing process calculations, different chemicals must be converted to a certain chemical, generally represented by Na₂O and NaOH.

1. Recovery from pulping wood is commonly expressed as the percentage, by oven-dry weight, of pulp obtained from the original wood weight. A recovery value of 45% means that for every 100 oven-dry pounds of wood processed, 45 oven-dry pounds of pulp is produced. ↩︎
2. Kappa number, also known as Kappa value, indicates the content of lignin in pulp. 2. It is one of the indicators reflecting the degree of delignification in pulp. ↩︎
3. “Black Liquor” is a by-product of pulp from mills that make products from trees, such as paper. It is currently used to recover cooking chemicals and produce high-pressure steam used in the pulp and paper-making process. It is composed of different ingredients from these processes such as lignin, hemicellulose, sodium hydroxide (NaOH) and sodium sulfide (Na₂S). The lignin compound in black liquor can be used to make biofuel but it can be expensive to produce and so fuel made from it is not very common. ↩︎
4. Green Liquor is an important by-product recovered from the kraft pulping of resinous woods in paper mills. It is the dissolved smelt of sodium carbonate, sodium sulfide, and other compounds from the recovery boiler in the kraft process. The liquor’s green color derives from the presence of colloidal iron sulfide. ↩︎
5. White liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is used in the first stage of the Kraft process in which lignin and hemicellulose are separated from cellulose fiber for the production of pulp.^[1] The white liquor breaks the bonds between lignin and cellulose. It is called white liquor due to its white opaque colour. ↩︎