The pulp and paper industry is a major water user, and it is also a major wastewater discharge. This paper analyzes the water-saving parts of the paper industry and summarizes the basic water-saving technologies and future development trends of water saving in the future.
The paper industry is a major water consumer. In the early days of the paper industry, the amount and quality of water used in papermaking were rarely considered. With the rapid development of the paper industry, the demand for and dependence on freshwater resources has increased dramatically. There is less and less fresh water available worldwide. According to statistics, the global land rainfall is about 100 000 mYd, of which about 25% of the water available for humans, and 50% and 60% of this part has been used for agricultural irrigation. Carrying out water-saving activities in the paper industry will not only reduce the amount of fresh water, but also reduce wastewater discharge, restore the damaged environment, and enable humans to eliminate the water crisis and environmental pollution nowadays. Embark on the road of sustainable development.
Water saving in the pulping process
Pulp and paper making refers to the process of using chemical methods, mechanical methods, or a combination of chemical and mechanical methods to dissociate plant fiber raw materials into natural pulp or bleached pulp, and then manufacture the pulp into paper products.
The process includes stripping, washing, slicing, screening of the bark of the logs, and dust removal and impurity removal of the grass materials. There are two kinds of preparation: wet method and dry method. At present, most paper pulp mills use wet peeling. It is also very important to consider dry peeling. The white water from the papermaking system can be used as a wood-working or wet-skinning water, which greatly reduces the generation and discharge of sewage.
There are three main types of pulping processes: chemical pulping, mechanical pulping and chemi-mechanical pulping. The water consumption of the three pulping processes is in turn mechanical pulping<chemi-mechanical pulping<chemical pulping. Therefore, reforming the existing pulping process will result in great benefits of water saving.
Washing and screening section:
The running, rushing, dripping and leaking of the pulp washing process are often serious. If the management of water quality and quantity is strengthened, it can be basically undrained; a large amount of wastewater will be generated during the screening stage, and the closed loop of the screening system can be considered to increase the efficiency, reduce the amount of fresh water.
Bleaching is the separation of residual lignin from the unbleached pulp. Therefore, the lignin-removing pulping process should be used to minimize the lignin content entering the bleaching pulp to reduce the water consumption during the bleaching stage. At the same time, the multi-stage bleaching process can also be used, recycle the alkaline waste liquid to achieve the purpose of water saving.
Water-saving technology development trend
At present, the water consumption of the paper industry is large, and reasons can be divided into the following aspects:
- The small scale of the enterprise, weak funds, outdated technique and equipment, and small production capacity;
- More than 60% of the chemical pulping material structure is grass raw materials, and the water consumption is large and medium-sized wastewater treatment is very difficult. And those large paper industries that use the commercial pulp and waste paper as raw materials, wastewater is easy to handle, and generally can meet the standard discharge;
- Process and equipment problems, backward pulp and paper traditional equipment and processes such as batch cooking, open washing, low concentration operation, and backward black liquor extraction process, no multi-stage bleaching process and low water-saving equipment relying on imports and low localization rate, which seriously restricts the development of water saving.
- The management work is weak (this is related to the mechanism is not smooth), the concept of water saving has not been deeply rooted in the hearts of the people, and the basic situation of water and water conservation is difficult to understand.
In this regard, we should continue to research and develop water-saving technologies and process equipment in the paper industry and also take corresponding countermeasures and measures.
High yield pulp and waste paper pulp
In the pulping industry in the past 20 years, high-yield pulp technology has developed rapidly due to its high yield and low pollution. For example, with the advantages of low energy consumption, high intensity, and low pollution, biochemical methods are becoming one of the mainstream developments. Chemical mechanical pulp is the main development direction of cleaning pulp production in the world. The raw material utilization rate of the method is as high as 80% (alkali pulping is only 40%). Compared with the traditional alkali pulping, the organic load of the pulping waste liquid is reduced by 80%, which is 10% lower than that of the alkaline recovery system.
Waste paper pulp is a secondary fiber that people attach great importance to now. It integrates water saving, environmental protection, and pollution reduction. It can also alleviate the contradiction between the rapid development of the paper industry and the shortage of timber resources. It is a direction worthy of further research.
High-efficiency black liquor extraction
The double-strand net extruder and the double-screw extruder can improve the cleaning degree of the coarse pulp, the extraction rate of the black liquor, and the black liquor concentration. At the same time, the washing system reduces the loss of pollutants, reduce the entrainment of the pulp into the bleaching system, in the alkali recovery system, improve the thermal efficiency of the evaporation system, save steam, reduce energy consumption
Water saving equipment
At present, the development of water-saving equipment is imminent. Such as the manufacture of high-yield refiners; double-stranded web extruder, double-screw extruder and twin-roll press in pulp extrusion dewatering and black liquor extrusion pre-extraction; pressure de-segmentation and pressure in closed screening systems Screening; medium-concentration pump and medium-concentration mixer in concentrated technology in the pulping system.