Technical principles of oxidation bleaching of hydrogen peroxide (H2O2)

vacuum drum washer

According to different properties and action principals of pulp bleaching agents, the bleaching can be divided into two types: one is oxidation bleaching; the other is reduction bleaching. Oxidation bleaching agent can further remove the residue of lignin and other impurities as well as colored substances, which can not only improve the whiteness and purity of the pulp, but also can keep the whiteness being durable. For high yield pulps, for example, groundwood pulp (GP) or semi-chemical pulp, if we use reduction bleaching agent, we only can make chromogenic substances de-colored without resulting in the loss of fiber components. It can keep the original characteristics of the pulp, but the whiteness cannot be durable and easy to get back to yellow. This will not be good for making the paper for a long-term retention. Next, we will introduce the technical principles of hydrogen peroxide (H2O2) bleaching.

I. Hydrogen peroxide (H2O2) bleaching of pulp

1. Introduction of hydrogen peroxide (H2O2)

Hydrogen peroxide (H2O2) is a strong oxidizing and bleaching agent. Pure hydrogen peroxide (H2O2) is a colorless liquid. It is commercially available with various concentrations. We generally use 30% of aqueous solution of hydrogen peroxide (H2O2). For the anhydrous hydrogen peroxide (H2O2), its relative density is 1.46, the melting point -2℃ and the boiling point 158℃. It can be mixed with water and ethanol in any proportion with poor stability. Pure hydrogen peroxide (H2O2) will have a slow decomposition, but in case of containing metal impurities, it will accelerate the decomposition into water and oxygen.

2. The application and characteristics of hydrogen peroxide (H2O2) bleaching

The effect and stability of hydrogen peroxide (H2O2) bleaching is pretty good, which will not make the fabric yellowing and have the non-toxic and odorless characteristics that will not cause the pollution to the environment. Because it is suitable for various fabrics bleaching, it has the promising prospect of application. It has many advantages for us to use hydrogen peroxide (H2O2) bleaching agent for the pulp:

  • Strong adaptability for high yield pulp, chemical pulp, waste paper pulp and other pulps;
  • Good whiteness stability of the pulp after hydrogen peroxide (H2O2) bleaching, just light yellowing;
  • There is no toxic chloride in bleaching wastewater, reducing corrosion.
  • Chemicals and heat can be recycled, completely solving the problems of wastewater pollution.

Therefore, hydrogen peroxide (H2O2) bleaching has considered as main bleaching agent in the new process of development of chlorine-free bleaching.

II. Strengthening method of hydrogen peroxide (H2O2) bleaching process of existing pulps

bleaching-process-of-existing-pulps

1. Stabilizer used for hydrogen peroxide (H2O2) bleaching

In the hydrogen peroxide (H2O2) bleaching process, the stabilizer is added to be used for catalysis control of metal ions, such as sodium hydroxide and α-hydroxy acrylic polymer, alkali metal hydroxide and phosphate oligomer, Magnesium Oxide or soluble magnesium salt, citrate, magnesium acetate, phosphoric acid derivatives or substituted phosphoric acid, 2-picolinic acid, etc. At present, sodium silicate and magnesium sulfate are commonly used.

2. Chelator used for hydrogen peroxide (H2O2) bleaching

In the hydrogen peroxide (H2O2) bleaching process, the chelator is added to be used for ligation reaction of heavy metal ions and hardness ions Ca2+ and Mg2+ and forming the barrier to prevent the formation of free radicals resulting from the contact catalyzing of per-hydroxyl ions. The amino acid type of chelator is ethylenediaminetetraacetic acid (EDTA) is a commonly used. Polyhydroxy carboxylic acid type of chelators include citrate, tartaric acid, gluconic acid, etc. in the strong alkali bath, they show chelating value on metal ions. Recently a new type of chelating agent DTMPA has been developed.

3. Activator used for hydrogen peroxide (H2O2) bleaching

Bleaching activator is actually an acylating agent in combination of some acyl ionic groups, such as, TAED, SNOBS, TAGU, PAG, etc. Through hydrolysis reaction, they play their roles through making hydroxyl ions acylation and producing super acid and acyl anions. Peracid is the more active bleaching agent than hydrogen peroxide, which can do an effective bleaching at the range of 40-60℃.
The research has found that compared with pure H2O2 for the bleaching, H2O2 activated by TAED bleaching has the advantages of shorter time and higher whiteness. Through the transformation of sulfuric acid to hydrogen peroxide, the experiments have proved that the peroxidation single sulfate we got is stronger. It shows that the peroxidation sulfuric acid will be with low consumption in the process of delignification. It can improve the strength of high yield of the mechanical pulp or chemical mechanical pulp. Even for the production of chemical mechanical pulp, we can also use it to recover the strength of the unbleached softwood sulfuric pulp. In addition, it can also help the delignification on the agricultural residues to enhance the enzymatic processing ability.

4. Acid strengthening treatment of hydrogen peroxide (H2O2) before bleaching

It is a very effective method of removal of metal ions to make the acid strengthening treatment in hydrogen peroxide before the bleaching. We use hydrogen ions in sulfuric acid, hydrochloric acid and other inorganic acids to replace the metal ions out from the pulp, and then remove them through the dehydration washing. In the process of acid treatment, we can remove heavy metal ions of iron, copper and manganese. In the meanwhile, we also remove the metal ions of magnesium, calcium and other alkali. In order to stabilize the hydrogen peroxide and protect the carbohydrates, we usually add magnesium salt in the bleaching process.

5. The hydrogen peroxide (H2O2) bleaching treated by the strengthening of pressure and high temperature

Hydrogen peroxide (H2O2) is one of the most important bleaching agents. Its delignification or bleaching is usually limited to below 80℃, because most people is generally believed that at high temperature it will lead to become easy decomposition. This may arouse the decrease of chemical efficiency and pulp strength, resulting in economic losses.
Hydrogen peroxide (H2O2) is one of the most important bleaching agents. Its delignification or bleaching is usually limited to below 80℃, because most people is generally believed that at high temperature it will lead to become easy decomposition. This may arouse the decrease of chemical efficiency and pulp strength, resulting in economic losses.



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