Environmental ink treatment in paper recycling

With the rapid development of information technology, the traditional carrier of information, paper, is growing at an alarming rate. Paper requires a lot of wood as the main raw material, and this is running counter to the aim of environmental protection. Therefore, countries have increased the recycling of waste paper, and both the utilization and the utilization have been increasing. In 1990, the global waste paper utilization was 84.8 million tons, and the utilization rate was 35.6%. In 1995, it increased to 115 million tons and 41.4% respectively. It is expected that the waste paper utilization and utilization rate in 2005 will reach 188 million tons and 48.1 respectively. %. China is in a period of rapid economic development. The demand for paper is also growing at a level that exceeds the world average growth rate. However, China’s forest resources are extremely scarce, and the per capita forest share is only 10% of the world level. Every year, a large amount of foreign exchange is required to exchange imported pulp and paper products to meet domestic demand. However, the waste paper consumption rate in China lags far behind that of some developed countries. For example, in 1997, the waste paper consumption rate in France (the ratio of recycled paper waste to total waste paper) was 48.9%. 59%, 71% in the Netherlands, 65.2% in Switzerland, 71.5% in the UK, and only 37.8% in China. Therefore, how to recycle waste paper efficiently and with high quality has become a hot topic for researchers in China and related professional researchers.

The key issue in the recycling process of waste paper is how to completely remove the ink and impurities from the paper fibers. The deinking is the difficulty in this key issue. Nowadays, there are many types of printing inks, such as lithographic offset printing inks, flexographic printing inks, screen printing inks, gravure inks, water-based inks, and UV inks (UV-curing inks). The degree of deinking of each printing ink is not limited. To the same extent, UV inks and water-based inks are the most difficult to deink. This article discusses the problems encountered in the deinking process of UV inks and water-based inks in waste paper processing, and how the current countries around the world have solved these problems.

Waste paper recycling process

In general, the deinking of waste paper can be roughly divided into five steps: crushing pulping, filtration, centrifugal cleaning, flotation to impurities, and rinsing, which are briefly described as follows:

1. Crushing pulping: In this step, first remove the relatively large domestic wastes in waste paper, such as plastic sheets and chopped trash, then pour the pretreated waste paper into a mixing kettle, then add water and caustic alkali. Enzymes are added to the kettle. After a period of soaking and mechanical agitation and comminution, the waste paper is made into a ready-to-use paper fiber slurry. Ink and other impurities can be separated from the paper fiber and get a certain size distribution.

2. Filtration: The resulting paper fiber slurry is passed through a rotating screen so that large particles of ink and impurities are retained on the screen. The size of the ink to be sieved can be controlled by a series of screen meshes. Generally, the ink particles with a particle size of more than 360 μm can be removed by this step.

3. Centrifugal cleaning: According to the difference in density, centrifugal force is used to separate the paper fibers from the non-paper fiber impurities. The key to the complete separation of the two substances lies in the difference in density between them. The greater the difference, the more complete the separation. Therefore, only 100 μm to 360 μm of ink impurities can be removed in this step.

4. Flotation method to remove impurities: The flotation agent (ie, surfactant) is added to the pulp first. This not only stabilizes the foam and facilitates flotation, but also enhances the hydrophobicity of impurities, making it easier to break away from the bondage of water. . At this time, air is introduced into the pulp to form air bubbles, and the impurities adhere to the air bubbles due to the action of the flotation agent, and float to the liquid surface with the air bubbles, and finally the impurities are removed. The key to this process lies in the weight of the impurities: it is necessary to have enough weight to penetrate the air-liquid interface of the air bubbles and adhere to the air bubbles, and to make the air bubbles have sufficient buoyancy to bring the impurities to the liquid surface. Therefore, most of the impurities are removed during this process. The impurity particle size should be controlled within 20 μm to 150 μm. This is the most important step in the entire recycling process. The quality level of recycled paper depends on the degree of impurities such as ink removed in this step.

Reprinted from: China Packaging News