It mainly includes waste paper, plastic, glass, metal, cloth and so on. Waste paper mainly includes: newspapers, magazines, books, all kinds of wrapping paper, office paper, cartons and so on. However, paper towels and toilet paper are too water-soluble to be recycled; Plastics mainly include all kinds of plastic bags, plastic packaging materials, disposable plastic lunch boxes and tableware, toothbrushes, cups, mineral water bottles and so on. Glass mainly includes all kinds of glass bottles, broken glass pieces, mirrors, light bulbs and so on. Metals mainly include cans and metal cans; Cloth mainly includes discarded clothes, towels, schoolbags and cloth shoes.
Non-recyclable garbage refers to garbage other than recyclable garbage. Common garbage is easily decomposed under natural conditions, such as skins, leaves, leftovers, flowers, branches and leaves. There is also harmful, polluted and unrecoverable garbage.
Abstract: The extensive use of polyethylene aluminum-plastic composite packaging materials has produced a large number of non-degradable wastes. Moreover, there are still many imperfections in the recycling of such materials at present, which brings another great pressure to environmental protection. In this paper, industrial pure acetic acid or formic acid is used as a delamination stripping agent to completely separate the recyclable substances in polyethylene (PE) aluminum-plastic composites. Compared with the current recycling technology of this kind of materials, the biggest advantage of this technology is that it can completely separate, recycle and reuse the waste polyethylene-aluminum-plastic composite packaging materials.
Key words: polyethylene aluminum-plastic composite packaging material; Acetic acid; Separation; Stripping agent; Recycling; reborn
1 Introduction
Polymer materials are widely used in daily life, all walks of life and high-tech fields because of their light weight, easy processing, beautiful and practical characteristics. However, while bringing people great material civilization, the generation of garbage also brings people serious problems. The recycling of waste plastics is of great social and economic significance for saving energy, reducing the amount of waste, reducing the harm of waste plastics to the environment and curbing the rise of oil prices. The main component of plastic is synthetic resin, which is a high polymer. In addition, in order to improve the properties of plastics, various auxiliary materials, such as fillers, plasticizers, lubricants, stabilizers, colorants, etc. , must be added to the polymer to become a plastic with good performance.
Polyethylene aluminum-plastic composite packaging material has the advantages of light blocking, constant temperature, non-toxicity and safety, low cost and so on, and is widely used in food, medicine, chemical industry, daily necessities and other packaging fields. For example, Tetra Pak's tetra pak series products and some bottle mouth sealing materials of milk-containing beverages belong to polyethylene aluminum-plastic composite packaging materials.
Due to the wide application and extensive use of polyethylene aluminum-plastic composite packaging materials, a large number of non-degradable wastes are produced, which will bring great pressure to environmental protection if not handled properly. In addition, the raw materials for producing this packaging material are: high-quality industrial aluminum, natural wood pulp paper and polyethylene plastic, which are all materials with high recycling value. Especially for industrial aluminum, the price is high and the supply is small. If recycled, the economic benefits will be considerable. Therefore, the recycling of polyethylene aluminum-plastic composite packaging materials has very important social significance and economic value.
2 Significance of recycling waste polyethylene
As we all know, polyethylene is one of the four general thermoplastic materials, accounting for 65%. With the continuous expansion of the polyethylene consumption market and the diversification of consumption varieties, the amount of polyethylene waste is also increasing. At present, our society can recycle about 1Mt of garbage every year, of which about 0.2Mt is actually used and 0.8Mt is not recycled. Most of the garbage is landfilled, which not only causes secondary pollution, but also causes extreme waste of materials. As early as June 65438+July 0975, the former Ministry of Light Industry hosted the "National plastic recycling Utilization Experience Exchange Meeting" in Shanghai, pointing out that the utilization of plastic recycling is not an expedient measure, but a long-term work. With the continuous progress of science and technology and the increasing demand for energy, how to recycle waste faster and better is related to the prosperity and progress of the country, and solving the recycling of polyethylene waste is one of the important contents of all waste recycling.
3 experimental principle and discussion
3. Separation principle of1polyethylene aluminum-plastic composite packaging material
Polyethylene aluminum-plastic composite packaging material is made by the interaction of aluminum foil, polyethylene plastic film and paper. The specific process is to melt the bonding surface of polyethylene (PE) plastic layer by high-frequency hot pressing, and then bond it with alumina and paper formed on the surface of aluminum foil. Because the oxide properties of aluminum are very stable, it is difficult to completely separate the three materials that are bonded together. Moreover, polyethylene plastic is resistant to acid and alkali corrosion, and ordinary organic solvents cannot dissolve or swell, which brings great difficulties to the recycling of this material. At present, there are two recycling processes for this material: one is to dissolve aluminum in the material by using the principle that both aluminum and alumina can be dissolved in acid and alkali, so as to recycle polyethylene plastic film and paper, and then make aluminum-containing waste liquid into polyaluminum chloride or aluminum sulfate; Secondly, Tetra Pak uses hydraulic method to separate the three materials into two parts: pulp and aluminum plastic, and then use them separately.
However, the above two methods have many shortcomings, such as low economic benefit and incomplete recovery. Therefore, according to the experimental results, this paper will recommend a new treatment method to achieve the best recovery effect.
Its working principle is: the aluminum foil, polyethylene plastic and paper layers in packaging materials are completely separated by using the characteristics that organic weak acid molecules such as acetic acid and formic acid can penetrate through polyethylene plastic layers and paper layers, and then the aluminum oxide bonded to each layer is dissolved, but the elemental aluminum is not dissolved.
3.2 Test process
(1) main reagents and raw materials
Acetic acid (industrial pure, 98. 5%, beijing chemical works), formic acid (industrial pure, 85%, Beijing Hongxing Chemical Factory), lime water (prepared into supersaturated solution), oxalic acid (CR, Beijing Yili Fine Chemical Co., Ltd.), tetra pak pillow for Mengniu pure milk, tetra pak brick for Mengniu pure milk, and Wahaha lactic acid beverage bottle mouth sealing material.
(2) weak acid layered separation method
Put the sample into a layered container filled with weak acid, and the weak acid molecules can be separated through the polyethylene plastic layer and the paper layer. Weak acid makes the alumina layer at the joint of each layer of the sample gradually dissolve, thus completely separating it. Drain the completely separated composite material, then soak it in saturated lime water and wash it until the pH value is between 6 and 8, and then rinse it with clear water. After washing, the stripped composite material was dried, and then put into a centrifugal separator for centrifugal separation to obtain clean PE plastic, aluminum foil and paper pulp.
3.3 Experimental results
Industrial pure acetic acid (98. 5%) and industrial pure formic acid (85%) have the fastest delamination speed, and no heating lasts for 4 hours and 4 hours respectively. A completely stratified sample can be obtained within 5 hours. When heated to 50℃, about 1 hour can be completely separated. Industrial pure acetic acid and formic acid can be recycled, but with the increase of use times, the concentration of acetic acid and formic acid gradually decreases and the stripping time will be prolonged. When the weak acid concentration is less than 15%, the stripping time will obviously increase, which takes about 12 hours.
4 Result analysis
During the experiment, oxalic acid with large dissociation constant (pKa) was also tried as a separating agent, but even when the solution was saturated and heated to boiling, the layers could not be separated successfully. This is because oxalic acid molecules are not easy to penetrate the paper layer and plastic layer and dissolve the alumina between the layers. The "weak acid separation method" using acetic acid or formic acid successfully realized the complete recycling of this substance.
Experiments show that the process is simple and feasible, and the separated materials have high purity and good quality, which can be used as high-quality raw materials in various related fields for reproduction. This method is superior to the hydraulic separation of aluminum-plastic composite packaging materials promoted by Tetra Pak Company at present, because hydraulic separation can only separate paper from aluminum-plastic, but can not successfully separate aluminum foil and plastic in aluminum-plastic. Although aluminum-plastic products can be made into colorful music boards and other products, it is difficult to recycle aluminum and plastic after these products are aged and discarded, resulting in secondary pollution. However, the technology proposed in this study separates aluminum foil, plastic and paper respectively, which realizes complete recycling and effectively avoids secondary pollution. In addition, because industrial pure acetic acid and formic acid can be recycled (new acid needs to be replaced when the concentration drops to 15%), the recovery cost is low and the economic benefit is considerable; The remaining weak acid for separation after separation is neutralized with weak alkaline lime water to eliminate environmental pollution. Therefore, this separation method can achieve good economic and social benefits at the same time.
5 Suggestions on the recycling of waste polyethylene products in the future
With the fluctuation of world oil price, the output and price of world polyethylene also fluctuate. China relies on imports for more than half of its polyethylene demand. Therefore, while improving China's polyethylene production capacity and technology, we must keep up with the development of polyethylene recycling technology. In addition to the above direct utilization and modified utilization, other comprehensive utilization ways of polyethylene should be added, such as thermal decomposition (oiling, vaporization and carbonization) and chemical decomposition (hydrolysis, alcoholysis and supercritical water chemical recovery). Although it will take some time to make a breakthrough in the application of recycling waste polyethylene, some universities and research institutes are conducting in-depth theoretical and applied research. For waste products that are really difficult to recycle, incineration energy can be recycled. In addition, because the recycling of films, especially agricultural films, has important practical significance, we should vigorously develop and apply new technologies to develop various degradable plastics, such as controllable biodegradable plastic films, photodegradable plastic films, controllable light and biodegradable plastic films. Thus, the production technology and recycling technology of polyethylene plastics in China will have three major effects in economy, environment and society, and promote the continuous improvement of China's scientific and technological level and comprehensive national strength.