Brief introduction of preparation system of papermaking pulping oxidation white liquor

In the production of bleached sulphate chemical pulp, oxygen delignification is generally used to reduce the lignin content of the pulp prior to bleaching, and the Kappa number can be reduced by 40% to 60%. This greatly reduces the amount of bleaching chemicals and reduces the discharge of waste water. Pollution load.

The amount of oxygen delignification sodium hydroxide is generally about 2% to 3%. The reaction waste liquid is washed and mixed with the cooking black liquor and sent to the alkali recovery workshop for recycling, and used as a white liquor supply slurry plant for recycling.

Oxygen delignification uses sodium hydroxide, if you use the purchased base, on the one hand the cost is high, on the other hand, this part of the alkali into the alkali recovery system, will also cause the system of alkali excess (alkali self-sufficiency rate exceeds 100%), Therefore, modern pulp mills generally use oxidation white liquor in place of purchased base, which not only reduces the cost of alkali by 50%, but also better controls the balance of the system base. Therefore, in the production of bleached sulphate chemical pulp, the preparation device equipped with the oxidation white liquor is economically reasonable.

China Zhongguang International Engineering Co., Ltd., based on the absorption and jokes of foreign technologies, combined with the actual operation of several domestic companies, has developed an oxidation white liquid preparation device with independent intellectual property rights. This device has high efficiency, low energy consumption, no waste water discharge, low investment, cost is only 50% of the purchased base, and the economic benefits are good. It is expected that the investment can be fully recovered within two years. Users can choose air oxidation method or pure oxygen oxidation method to prepare according to the actual situation. Modular 3D design, flexible configuration. The whole device adopts an automatic control system, and it can also use the existing control system of the user's bleaching section to expand its capacity and save investment.

1. Oxidized white liquor preparation principle

The white liquor reacts with oxygen in the reactor to convert the sodium sulfide in the white liquor into sodium hydroxide and sodium thiosulfate. The reaction is as follows: 2Na2S + 2O2 + H2O → Na2S2O3 + 2NaOH

Side Reaction 1: Na2S2O3 + O2 + 2NaOH → 2Na2SO3 + H2O

Side Effects 2:2Na2SO3 + O2 → 2Na2SO4

After the oxidation reaction, the concentration of sodium sulfide in the white liquor after oxidation is ≤1.5 g/L.

2. Oxidized white liquor preparation method

2.1 Air Preparation

The air reactor is a circular tank, which is divided into two parts and consists of a small tank set inside a large tank to increase the reaction rate so that the concentration of sodium sulfide in the white liquor after oxidation is ≤ 1.5 g/L. After the white liquor enters the reactor, it enters the inner tank first, then passes through the outer tank, enters the buffer tank after two steps of reaction, and then is pumped to the oxygen delignification section for use.

Oxidized air is fed into the bottom of the reactor by a high-pressure blower and blown into the white liquid layer through the small holes of the air distribution tube to form bubbles which rise slowly. In this process, oxygen in the air reacts with sodium sulfide in white liquor. The generated exhaust gas rose to the top of the reactor and was discharged after removing the droplets. The reaction temperature was 85 degrees Celsius.

The main equipment used in the air preparation process includes white liquor reactors, white liquor buffer tanks, white liquor pumps and blowers, one for each.

2.2 Oxygen Preparation

The preparation of oxygenated white liquor by the oxygen process releases heat during the reaction and generates steam, which raises the pressure and temperature in the reactor. The top of the reactor is equipped with a safety valve protection device, and a release valve is provided to exclude nitrogen and the like. The temperature inside the reactor was controlled by pumping out the white liquor inside the reactor and cooling the circulation. The operating temperature is usually controlled at 130-150 degrees Celsius and the pressure is below 0.6 MPa.

The upper part of the reactor is a packed tower. White liquor enters the top of the reactor. It evenly sprays on the top of the packed bed and flows downwards. Oxygen flows from bottom to top and the two meet and react in the gas-liquid phase.

In addition, the reactor is provided with a set of jet circulation device. The white liquor is withdrawn from the lower part of the reactor's pumping tank with a circulating pump and circulated back to the upper part through the ejector. At the same time, the oxygen at the top of the reactor is sucked with the ejector to make the top pressure of the reactor. Lowering and increasing the flow of oxygen through the packed towers promotes the reaction of white liquor, fillers, and oxygen, which can increase the reaction efficiency and reduce the concentration of sodium sulfide in the white liquor. The main equipments used in the oxygen process include: white liquor reactor, white liquor cooler, white liquor pressurizing pump, white liquor circulating pump, jet mixer, separator, each one.

3 Conclusion
The preparation equipment of oxygen white liquor developed by China Light International Engineering Co., Ltd. with its own property rights only can be prepared by pure oxygen oxidation or air oxidation. It has high production efficiency, low energy consumption and good economic benefits.