1. technical name

Large-scale chlorine dioxide preparation system and key technologies for elemental chlorine-free bleaching of pulp.

2. scope of application

Clean production in the pulp and paper industry.

3. technical content

3.1 technical principle

(1) Comprehensive method chlorine dioxide preparation technology NaCl solution undergoes electrolytic reaction in a NaClO3 electrolytic cell to generate concentrated NaClO3 solution and hydrogen. The hydrogen is cooled and removed from water and then sent to the HCl synthesis unit to combust with weak chlorine gas generated by the ClO2 generation unit to prepare 32% hydrochloric acid. Concentrated NaClO3 solution and 32% hydrochloric acid are pumped to a chlorine dioxide generator to react to generate ClO2 gas. After condensation, the gas is transferred to the absorption/stripping tower and is absorbed by ice water to prepare a 9 g/L\~11 g/L ClO2 aqueous solution. The weak chlorine gas extracted from the gas is sent to the HCl synthesis unit for use. The generator reaction residue (containing sodium chloride) is returned to the NaClO3 electrolysis unit for recycling. (2) The reduction method raw materials concentrated H2SO4, NaClO3, and reducing agent solution enter the generator to react to generate ClO2. After condensing, ClO2 gas enters the absorption tower, and is absorbed by ice water to produce 9 g/L\~11 g/L ClO2 aqueous solution. The by-product of the reaction is washed and concentrated, the filtrate is returned to the generator for cyclic reaction, and the acid thenite crystals undergo a double decomposition reaction and are converted into neutral thenite. The acid solution after washing and concentration of neutral thenardite is returned to the generator to recover sulfuric acid. The neutral thenardite crystals are dissolved or dried and used in the alkali recovery workshop or sold out.

3.2 main innovations

3.2.1 Comprehensive method

(1) Sodium chlorate solution conditioning control is used between the chlorine dioxide generation and the sodium chlorate electrolysis section to eliminate mutual interference between sections caused by production fluctuations. (2) The chlorine dioxide generation efficiency increased from 90.0% to 93.5%, and the purity increased from 90.0% to 98.0%. (3) Using a gas-stripping circulation electrolysis system, optimized electrolytic cells and direct current-carrying technology, the electrolysis efficiency is as high as 95%, and the oxygen content is less than 2.6%.

3.2.2 Reduction method

(1) The conversion rate of sodium chlorate is increased by 2% compared with traditional processes, and the chlorine content is reduced by 50% to 60%. (2) The by-product acidic thenardite is converted into neutral thenardite for recycling. The amount of acid is reduced by 17% compared with the traditional process, and no solid waste and waste liquid are discharged.

4. Pollution control or environmental restoration effects

The chlorine dioxide prepared by the two processes has low chlorine content and is used in pulp and paper enterprises. The AOX content of the treated wastewater can meet the requirements of the "Discharge Standard for Water Pollutants in the Pulp and Paper Industry"(GB3544 - 2008), which is lower than that of traditional processes. The AOX of bleaching wastewater is reduced by more than 33%. The by-product acidic thenardite is converted into neutral thenardite for recycling, reducing system acid consumption and avoiding the discharge of solid waste and waste liquid.

5. Technology demonstration

It has been promoted and applied in more than 40 companies at home and abroad, and the application companies have produced more than 7 million tons of bleached pulp. The element-free chlorine-bleaching technical transformation project of Guangxi Guitang (Group) Co., Ltd. has built an 8 t/d reduction method chlorine dioxide preparation system, which is applied to the 270 t/d bleached pulp production line. APP Group Hainan Jinhai Pulp and Paper Co., Ltd.'s comprehensive chlorine dioxide preparation project has built a 35 t/d comprehensive chlorine dioxide preparation system, which is used in the company's expanded bleached pulp production line. The company's wastewater discharge AOX meets the requirements of the "Discharge Standards for Water Pollutants in the Pulp and Paper Industry"(GB3544 - 2008).

6. investment estimation

Guangxi Guitang (Group) Co., Ltd.(8 t/d) elemental chlorine-free bleaching technical transformation project has a total investment of 18.85 million yuan, including equipment investment of 16.44 million yuan. 7. Investment payback period The payback period of the (8 t/d) elemental chlorine-free bleaching technical transformation project of Guangxi Guitang (Group) Co., Ltd. is about 3 years.

8. Prospects for transformation and promotion of technological achievements

There are more than 3000 pulp and paper companies in my country, and new pulp and paper companies in some developed countries such as India and Indonesia also need to support chlorine dioxide preparation systems. Large-scale chlorine dioxide preparation systems can completely replace imported equipment and technology and have good prospects for promotion and application.