Key Chemical of Paper industry-Caustic Soda

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The Sulfate Pulp process which goes by the trade name “Kraft Process” is followed widely by Paper and Pulp industry. Some of the key raw materials of Paper industry are described below as we go through the process.

In the first step of Kraft pulping process, the use of chemicals such as Sodium hydroxide, along with heat and pressure are used to dissolve the lignin in wood which frees cellulose fibers. Digester is used to remove sugar, lignin, and anything that is not needed in the final product.

To derive white paper, the pulp has to be placed in a bleach tower where it gets bleached. Pulp mills convert sodium chlorate into chlorine dioxide – an environmentally friendly bleaching agent for Pulp industry. The process of using chlorine dioxide to bleach pulp is referred to as “ECF” bleaching -meaning “Elemental Chlorine Free”. Also, Calcium carbonate is used in paper mill as a filler material in the alkaline papermaking process.

Sodium carbonate is used to make sodium bisulphite (NaHSO3) for the “sulfite” method of separating lignin from cellulose. Once the bleaching process is done, the pulp is placed on the beater, which modifies the pulp fiber. The product is then further refined.

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 Why is Sodium Hydroxide so important to Paper Industry?

Sodium hydroxide commonly known as caustic soda is produced industrially via Chlor-alkali process. This involves the electrolysis of aqueous Sodium Chloride solution, resulting in the formation of Sodium Hydroxide along with Hydrogen and Chlorine gases as a by-product. Another way of producing Sodium Hydroxide in the industry is through the metathesis reaction which involves the reaction of Sodium Carbonate with Calcium Hydroxide. This reaction will produce Sodium Hydroxide along with Calcium Carbonate as a by-product.

Caustic Soda is available in the market as High Purity Grade Caustic Soda & Standard Grade. High Purity Grade Caustic Soda also called as membrane, mercury, and rayon or low-salt grade has a similar concentration compared to the Standard Grade Caustic Soda. The main difference is that they contain lesser impurities such as iron or other salts. This grade is typically more expensive as compared to the lower Standard Grade. Standard Grade Caustic Soda is also known as diaphragm grade, which is named after the chlor-alkali production process using a diaphragm technology.

With China & USA being the largest exporter of caustic soda, Vietnam & UK stand at being the highest importers.

 How is Sodium hydroxide applied in this industry?

  • Sodium Hydroxide is widely used in chemical pulping of wood during the Kraft pulping process whereby it is present in the white liquor. Being a strong base, Sodium Hydroxide helps in delignification of woodchips to separate unwanted lignin from cellulose fibres.
  • Sodium hydroxide in paper industry is also used for paper recycling. It causes the fibers to swell when they are placed in a solution with the sodium hydroxide. This swelling helps to separate the ink that is attached to the fibers allowing the fibers to be reused.

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  • The final application of sodium hydroxide in paper industry is pH control. In these applications, the goal is to optimize the process in which chemicals are being used. It saves money and makes the process more environmentally friendly.The best example to explain is when sodium hypochlorite is used as a bleaching chemical; sodium hydroxide is used to help keep the pH close to 8.0 as much as possible, because sodium hypochlorite is most effective at that pH level.

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