In recent years, the impact of organic chlorides on the environment caused by the bleaching of release paper has promoted the development of environmentally friendly bleaching technology.
Hydrogen peroxide is the key chemical in TCF and ECF.
Most of H2O2 bleaching is used for lignin retention bleaching of mechanical pulp or semi chemical mechanical pulp, in which H2O2 is used to remove chromogenic groups from lignin structure.
The main reaction in the 2O2 bleaching system is the formation of hydrogen peroxide anion after adding alkali to H2O2. The bleaching effect of pulp depends largely on hydrogen peroxide anion. In addition, other reactions caused by the decomposition of H2O2 also affect the bleaching effect.
Due to the existence of transition metal ions such as manganese, iron and copper in bleaching chemicals, process water and unbleached pulp, it is very important to control the decomposition of H2O2 caused by transition metal ions. In mechanical pulping, the amount of transition metal ions should be reduced in order to obtain the highest concentration of hydrogen peroxide and remove chromogenic groups. In chemical pulp, partial decomposition of H2O2 is necessary for delignification.
The H2O2 consumed in the reaction of removing chromogenic group is only a part of total H2O2 consumed, and most of H2O2 is decomposed by transition metal ions. The traditional bleaching process uses mild alkaline conditions to control the decomposition of H2O2, stabilizes hydrogen peroxide by adding magnesium salt and silicate, or removes transition metal ions by pre pickling or adding chelating agent. Due to the different mechanism of H2O2 Decomposition caused by different transition metal ions, the concentration of transition metal ions must be considered when using H2O2 bleaching.
The decomposition mechanism of H2O2 solution containing pulp is different from that of H2O2 solution without pulp. The way in which transition metal ions react with H2O2 depends largely on the chemical environment in which it can be changed in many different ways by stabilizers. Under certain conditions, the stabilizer can slow down or prevent the reaction of transition metal ions with H2O2. However, they can also promote this reaction and accelerate the decomposition of H2O2.
Magnesium salt as stabilizer of release paper
In H2O2 solution, a small amount of magnesium salt can almost completely prevent the decomposition reaction. Many studies have reported the stability or instability mechanism of magnesium ions in H2O2 Decomposition Reaction Induced by transition metal ions. It has been found that magnesium ions can inhibit the catalysis of iron and copper ions, but can activate the catalysis of manganese ions. However, when H2O2 solution contains pulp, magnesium ions can even inactivate manganese ions.
Magnesium ions stabilize the basic H2O2 solution by capturing peroxy anion radicals, and then interrupt the decomposition reaction of free radical chain. MgO 2, formed by the combination of peroxy radical and Mg (II), can stabilize the decomposition reaction by interfering with free radicals.
Because iron ions can produce free radicals, the above theory is suitable for the presence of iron ions; the direct interaction between magnesium ions and metal ions is the main reason for magnesium ions to stabilize H2O2. The best way is to add magnesium ions to the pulp before alkali and H2O2 are added, which can reduce the formation of magnesium hydroxide without stabilizer effect. The active stabilizer is an organic metal complex of magnesium. The complex can be formed between magnesium ions and organic by-products of bleaching reaction. The complex of magnesium ions with pulp or chelate DTPA can reduce the decomposition rate of H2O2 more effectively than magnesium hydroxide.
Silicate as stabilizer of release paper
Sodium silicate has a beneficial effect on H2O2 bleaching. However, its specific action and mechanism are still uncertain. The main function of silicate is to inactivate transition metal ions, thus reducing their catalytic activity for H2O2 Decomposition. Manganese ions can be inactivated by the formation of manganese silicate.
At pH & lt; 11, sodium silicate has a good effect as a stabilizer in the presence of iron and manganese ions. However, when copper ion exists, it plays a catalytic role in decomposition. Silicate can also be used as buffer in H2O2 bleaching. In the presence of magnesium ions, silicate exists in the form of magnesium and iron (manganese) silicate as stabilizer.
Generally speaking, the concept of solid-state precipitation can be explained from solid state to stable state. Precipitation occurs when silicate comes into contact with calcium and magnesium ions. Silicate is more likely to react with magnesium ions. When the proportion of magnesium ion is greater than that of calcium ion, a small amount of precipitation will be produced. This is because the precipitation of magnesium ions is not as viscous as that of calcium ions.
However, the formation of silicate precipitates during bleaching is still a problem. On the other hand, the use of Mg (OH) 2 in the bleaching process can improve the effect of silicate as a stabilizer.
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