Abstract : Papermaking wastewater pollution is very serious, and its comprehensive treatment has always been a research hotspot in the papermaking industry and environmental protection industry at home and abroad. In this paper, the technical methods for the treatment of papermaking wastewater, especially papermaking black liquor, at home and abroad, such as black liquor alkali recovery method, acid precipitation method, flocculation method, membrane separation method, biological method, oxidation method, etc., are carried out in detail in recent decades. The review stated that the biological method is the ultimate solution to the water pollution of China's paper industry. At the same time, the application of supercritical water oxidation technology to the treatment of papermaking black liquor was proposed for the first time, which has potential feasibility and significant significance.

Key words: papermaking black liquor, comprehensive treatment, supercritical water oxidation

Research Progresses in Comprehensive Treatment of Black Liquor
SU Weifeng CHAI Liyuan WANG Yunyan
(College of metallurgical Science and Engineering, Central South University, Changsha, 410083 China)

ABSTRACT: The pollution caused by papermaking wastewater is very serious. The comprehensive treatment of that has been the focus point for papermaking industry and the environment field. In this paper, technology of papermaking wastewater especially black liquor treatment in recent decades, such as methods of单词suda recovery,acid recovery, flocculation, membrane separation,biology method and oxidation etc were reviewed. Biology method is regarded as the ultimate outlet for the settlement of the pollution of papermaking industry in our country. simultaneously, the application of supercritical water oxidation technology on The treatment of black liquor is pointed out for the first time, which has the potential feasibility and very important significance.
Key words: Black Liquor,Comprehensive Treatment,Supercritical Water Oxidation

The pollution problem in the paper industry is very serious and has received widespread attention. In the world, wastewater from papermaking industry is an important source of pollution. For example, Japan and the United States classified wastewater from papermaking industry as one of the six pollutions and one of the five major pollutions [1]. In China, wastewater from papermaking industry has become a key factor in the survival and development of papermaking and related industries. According to the statistics of the 1996 Environmental Protection Communique,[2] the discharge of pulp, paper, and paper products at or above the county level is 2.125 billion tons, accounting for 11% of the country's total industrial emissions, second only to the annual output of the chemical industry and the steel industry. Ranked third. Among them, 300 million tons of discharge standards were met, which accounted for only 14% of total papermaking emissions. The amount of chemical oxygen consumed in the discharged waste water was 3.07 million tons, accounting for about 45% of the total national emissions. During the "10th Five-Year Plan" period, China will require the paper industry to basically control the pollution of the wastewater. Therefore, the comprehensive treatment of black liquor from papermaking has been extensively studied.

The pollution of papermaking black liquor accounts for about 90% of the total pollutants [3]. Lignin and its degradants are the most important components in black liquor and can account for about 50% of total COD. Lignin is a three-dimensional network polymer with aromatic structure. It has two characteristics: it is not easy to degrade under natural conditions and it reduces the transparency of water. Different sources of black liquor have COD concentrations between 15.0 and 160 g/L [4]. If black liquor cannot be treated and discharged directly, it will not only seriously pollute the environment, but also cause a lot of waste of resources. Therefore, the key to solving the pollution emissions from the paper industry is the thorough and effective treatment of black liquor. The waste water pollution control situation in the paper industry is still very serious. The situation in which a paper mill pollutes a river has not yet been fundamentally changed. In particular, a large number of small and medium-sized paper companies that use grass fiber as their raw materials are not in urgent need of suitable pulping liquids to solve waste pollution discharges. problem.

The purpose of this paper is to review the research work done at home and abroad in the treatment of papermaking black liquor. At the same time, according to the characteristics of pollution in our country's papermaking enterprises, we propose new ideas and provide references for solving China's paper industry pollution.

1 The status quo of remediation technology for papermaking black liquor

As a large number of free alkalis, sulfides, and organics are present in black liquor from papermaking, the papermaking industry at home and abroad has been focusing on the recycling and comprehensive utilization of chemical and fiber raw materials in wastewater for many years. More extensive.

1.1 Alkali recovery method

At present, the alkali recovery rate of large-scale paper mills has reached 90%, and the alkali recovery rate of non-wood pulp mills (reeds, sucrose slag, etc.) has reached about 70%, and that of rice and wheat straw pulp mills is about 60%. The use of alkali recovery can greatly reduce the high load pollution of black liquor, BOD can be reduced by 80% to 85%, in addition to the recovery of heat energy, chemicals, etc., reducing costs and increasing economic benefits. The alkali recovery method is divided into combustion causticization, electrodialysis and black liquor gasification.

1.1.1 Burning causticization

The complete process of alkali recovery by combustion causticization is divided into four processes: extraction, evaporation, combustion, and caustic-lime recovery. The basic principle is to burn the black liquor in the combustion furnace to burn off the organic matter in order to recover the alkali and heat energy.

At present, various countries adopt the combustion method to recover alkali from black liquor. Most paper mills in China that use wood pulp as raw materials adopt this method. The silica content in the pulp black liquor is only 0.22%, so the alkali recovery proceeds more smoothly. However, more than 90% of China's paper mills use rice straw as raw material, and the black silicon content is about 3%. Silica reacts with alkali to generate sodium silicate, which is prone to fouling during the combustion process, which affects the process of alkali recovery. Smooth progress. At present, the recovery rate of alkali in some developed countries of the paper industry is 95 to 98%, and the energy efficiency is also high. The recovery rate of alkali in China is generally only 85 to 90%. The high content of raw silicon is one of the reasons that the alkali recovery rate is unsatisfactory. Synthetic silica removal technology of straw black liquor caustic soda is a new breakthrough in silicon removal technology [5], with the exception of the silicon rate of 70%, the alkali recovery rate of 10%, and the evaporation efficiency of 19%. The silicon removal technology has been successfully conducted in the production test and has a good promotion prospect.

1.1.2 Electrodialysis

The electrodialysis method [6] generally adopts a circulation type process. The black liquor is circulated through the anode chamber and the dilute alkali liquor is circulated through the cathode chamber. Under the action of a DC electric field, Na+ enters the cathode chamber through the positive membrane, and is combined with the OH generated by electrolysis to form NaOH to recover the alkali; the black electrolyte in the anode chamber is continuously acidified by electrolysis, and to a certain extent, most wood Element will precipitate out. The use of a single-membrane electrodialyzer to recover the alkali in the black liquor is technically feasible, with a simple process, convenient operation, low equipment investment, and easy automation. Chen Changchun et al [7] used single anode membrane to recover alkali from papermaking black liquor. The results showed that the electricity consumption of the 1t alkali recovered by this method can be stabilized at about 3000 kW.h, which is higher than that of caustic soda produced by the chlor-alkali plant and black liquor combustion. The consumption is low. When the recovery end of the black liquor pH is 7, the Na+ recovery is 50%, and the anode black liquor contains Na+ 5000-7000 mg·L-1. In the process, people are trying to improve the electrodialysis alkali recovery rate by improving the electrode and diaphragm. (to be continued)