Kraft paper is a roll thick brown paper that usually used for wrapping sandwich. The word kraft was stand by a German word for strength. This type of paper has higher tensile strength with a rough surface and has been manufactured since 1906. Commonly used as a grocery bag, cement packaging, food packaging, base for sandpaper, and wrapping paper. Kraft paper is produced in the kraft process.
Kraft pulping process is the most popular use of paper pulp process. It depends on Sulfate (NA2SO4) to create alkalinity in cooking, which is why it is also known as sulfate process. This process is patented by Dahl in 1884. Kraft or sulfate process replaced soda process because of its simplicity, rapid process, applicability to all wood species, and more valuable productions.
There are three main stages of pulp production :
Chemicals used in kraft paper manufacturing
Learn about the chemicals used in kraft paper manufacturing:
1. Sulfur – S
Sulfur is a bright yellow colored non-metallic chemical. Found widely in nature and is non-toxic. This element is widely used since ancient times. Sulfur can be used as fertilizer, pesticide, food preservation, pharmaceutical and furniture manufacturing. The chemical is used in wood pulping with oxygen and then converted into sulfur dioxide (SO2) and absorbed into water and become sulfurous acid (H2SO3).S + O2 → SO2
S2- + H2O → SH- + OH-
SH- + H2O → S2H + OH-
2. Sodium Sulfate – Na2SO4 (Soda)
With formula Na2SO4, This soda has a solid shape and mainly used for detergents and kraft process of paper pulping. Even if Na2SO4 is non-toxic, but it still can cause eye and respiratory organ irritation, thus still need to handle with care. Also read: Sodium Sulfate Functions
Together with NaOH and water, These chemicals create an opaque liquid called white liquor that can separate lignin and hemicellulose from cellulose fiber for pulp production. Later it will be reduced to sodium sulfide (Na2S) by the organic carbonNa2SO4 + 2 C → Na2S + 2 CO2
3. Sodium sulfide
Used as hydrolysis and solubilization of lignin and create Thiol (mercaptans). Thiol is an organosulfur compound which is a sulfur analog of alcohol. This chemical smell like garlic or a rotten egg.R-R’ + Na2S → Thiol (mercaptans)
4. Sodium Hydroxide – NaOH (Lye)
An inorganic compound shaped as colorless solid crystalline and highly soluble in water. This compound has alkaline character and may cause severe chemical burns. Sodium Hydroxide is often used as a base in industry and in textile, paper, drinking water, detergents, and soap manufacturing.
In kraft process, This chemical is used along with sodium sulfate as white liquor in the early stage known as impregnation and cooking where wood chips are steamed and cooked. The process will dissolve lignin and hemicellulose.R-R’|+ NaOH → R”COONa + ROH
NaOH + Na2S + wood → Na-org + S-org + NaHS
5. Calcium carbonate – CaCO3
Chemicals used in kraft paper manufacturing, Calcium carbonate naturally found as the main component of pearls, shells, snail, and egg. It is used a filler to make paper bulkier and brighter. It also regenerated to CaO and carbon dioxide in causticizing stage by burning it in the lime kiln.CaCO3 → CaO + CO2
6. Calcium oxide – CaO (Quicklime)
This alkaline compound is crystalline solid shaped and is an important commodity chemical. It made from the decomposition of limestone or seashells. Quicklime has a strong reaction with water and can cause severe irritation if inhaled or contacted.
In kraft pulping, calcium oxide is used to regenerate sodium hydroxide from sodium carbonate. It is then reacted with water to produce Calcium hydroxide.CaO + H2 O→ Ca(OH)2
7. Calcium hydroxide – Ca(OH)2 (Slacked lime)
Converted from Calcium oxide, Ca(OH)2 solutions can cause chemical burns. This chemical commonly used as water and sewage treatment. It is used with Sodium Carbonate (green liquor) in causticizing stage in kraft process as intermediates in the reaction in the production of sodium hydroxide. Also read: Calcium Hydroxide Uses in Food
8. Sodium Carbonate – NA2CO3 (Soda Ash)
This hygroscopic chemical is shaped as a white and odorless powder. Soda ash is great for grease, oil and wine stains remover, these properties make it used as a water softener in laundering. It is also used in glass, paper, and rayon manufacturing. Beside manufacturing industry, sodium carbonate is used as a food additive as raising agent and stabilizer.
This chemical used in early stage of kraft processing along with sodium hydroxide and sodium sulfate and in causticizing stage along with calcium hydroxide.Na2CO3 (aq) + Ca(OH)2 (s) → 2NaOH (aq) + CaCO3 (s)
Also read: Sodium Carbonate Uses in the Home
9. Hydrogen Peroxide – H2O2
Commonly used as an oxidizer, bleaching agent and antiseptic. This chemical compound has clear pale blue liquid form. In high concentration, uses of Hydrogen peroxide is an aggressive oxidizer and will corrode many materials. This is why the chemical concentration above 40% is regulated. In pulping, It is used as bleaching agent replacing chlorine.
10. Calcium Hypochlorite – Ca(ClO)2
Known as bleach powder and general oxidizing agent. Calcium Hypochlorite used as bleaching agent in kraft process that react almost only with carbonyl groups
Here are another chemicals used in kraft process in the small amount :
Chemicals used in kraft paper manufacturing:
- Hydrogen dioxide (H2O); water used in many processes of kraft process as alkaline dissolver
- Oxygen (O2); used with sulfur to create sulfur dioxide (SO2)
- Polyethylene (PE); sometimes used as kraft paper coating
- Chlorine (Cl); used as bleaching agent but produces dioxins and other desirable products
- Chlorine dioxide (ClO2); used as bleaching agent that reacts with free phenolic groups
- Ozone; used as bleaching agent that reacts with any phenolic groups
Kraft pulp process has faced environmental challenges as it produces TRS (total reduced sulfur gases, mainly H2S), SO2, and NO. Besides those main problems, there are also other gas emissions found such as HCl, NH3, CO, methanol and other hazardous compounds.
TRS is the prime suspect in kraft odor and subjected to control measures. However, the modern process develops “high-solids firing” which significantly increases steam generation and improve combustion stability, resulting in lower TRS and SO2 emissions.
There is also a future consideration of adding sodium borate (borax) into the liquor system. It will form tri-sodium borate (Na3Bo3) which will dissociate into sodium hydroxide (NaOH) and NaBO2. This will be a possible Na2CO3 causticizer in the recovery boiler.