introduction

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Microwave refers to electromagnetic waves with a frequency of 300MHz-300GHz. It is an abbreviation for a limited frequency band in radio waves, that is, electromagnetic waves with a wavelength between 1m (excluding 1m) and 1mm, which are decimeter waves, centimeter waves, and millimeter waves. And the general name of the submillimeter wave. The microwave frequency is higher than the general radio wave frequency and is also commonly referred to as "ultra-high frequency electromagnetic wave". Microwave as an electromagnetic wave also has wave-particle duality. The basic properties of microwaves usually appear as three characteristics of penetration, reflection and absorption. For glass, plastic and porcelain, the microwave is almost traversed without being absorbed. For water and food, it absorbs microwaves and heats itself. For metal things, microwaves are reflected. In recent years, the further development of high-efficiency heating characteristics of microwaves has led to the application of its applications from traditional communication fields to catalytic chemistry, material processing, pollution control and other fields. Among them, in the field of pollution control, especially in the treatment of solid waste such as industrial sludge, medical waste, used tires, electronic waste and construction waste, great progress has been made.

1 Principle of microwave heating

1.1 Mechanism of microwave heating

In the process of microwave heating, there are two kinds of mechanisms for converting microwave energy into heat energy, namely the dipole rotation mechanism and the ion conduction mechanism.

The dipole rotation mechanism is caused by microwave radiation causing the molecules inside the object to rub against each other to generate thermal energy. Natural media consists of molecules (or dipoles) with a positive charge at one end and a negative charge at the other end. In the natural state, the dipoles in the medium are disorderly moved and arranged. When the medium is in the electric field, the interior is re-arranged and becomes a polarized molecule with a certain orientation and regular arrangement. When the direction of the electric field alternates at a certain frequency, the polarization orientation of the dipole in the medium also changes at the same frequency. During the transition, thermal energy is generated due to friction and collision between the molecules. The faster the electric field changes, the faster the dipole rotates, and the stronger the thermal effect, and the electromagnetic frequency of the microwave band is as high as 108 orders of magnitude. Therefore, under microwave radiation, the heat generated by the dipole rotation is considerable, thus The system reaches very high temperatures in a short period of time. The heating efficiency of dipole rotation depends on the relaxation time, temperature and viscosity of the medium.

The ion conduction mechanism means that the dissociable ions generate a conductive movement in an electric field, and a thermal effect is generated due to the hindrance of the medium to ions. The heating efficiency produced by ion conduction depends on the size, concentration, charge amount and conductivity of the ions.

1.2 Characteristics of microwave heating

Conventional heating is carried out by conduction and convection. The container is first heated, and the container conducts heat to the surface of the object, and then heat is transferred from the surface to the inside of the object to obtain a heat balance condition, so heating takes a long time. However, the heating environment is generally not strictly insulated and sealed, and when heated for a long time, it is possible to dissipate a large amount of heat to the environment. Microwave heating is usually carried out in a fully enclosed state. Microwave power penetrates into the interior of the object at the speed of light, and is converted into heat energy in time, avoiding heat loss during long heating, and can "heat" the inside and outside of the object. Therefore, Compared with the traditional heating method, microwave heating has the characteristics of high efficiency, high speed and low energy consumption.

2 Application of microwave technology in solid waste treatment

2.1 Treatment of industrial sludge

Industrial sludge is an emulsion of oil and water containing solid debris. The annual production of oily sludge is as high as several billion tons. The conventional treatment process is: heating demulsification - centrifugal separation - landfill. Since the demulsification additive is often used during the heat demulsification, the residue generated is difficult to handle, and the amount of residue required for landfill disposal after deoiling is large, and the cost of landfill is high. The United States has developed a microwave deoiling technology for oil-bearing sludge in steel plants by mixing oil and metal sludge with additives, then receiving microwave irradiation for 10 min in a flow system, and finally separating the solid matter by centrifugation (mainly It is Fe, FeOx), oil and water. The separated solid can be reused as a steelmaking raw material, and the oil can be sold as a fuel. Studies have shown that the processing speed of the microwave demulsification deoiling system is 30 times faster than that of the conventional deoiling system, and the volume of the treatment system can be saved by 90%, which greatly reduces the solid waste residue required for landfill disposal and reduces the cost of landfill. The treatment of the sludge of the steel plant by microwave deoiling is reduced by 10 times compared with the conventional treatment method.

In addition, the sludge produced by the sewage treatment plant is used as a raw material, and the activated carbon can be prepared by microwave irradiation. There is also a mixed sludge of the primary treatment tank and the secondary sedimentation tank of the sewage treatment plant, which is activated by a phosphoric acid solution, and then irradiated with 400 W of microwave for 260 s to successfully produce sludge activated carbon. The iodine value of the activated carbon is 517.4 mg/ g. The treated sludge activated carbon was used to treat TNT wastewater, and the treatment efficiency was slightly higher than that of powdered commercial activated carbon.

2.2 Sterilization of medical waste

The mechanism of microwave sterilization is that microwave energy is converted into heat energy in the microorganisms, and the temperature of the organism itself is raised, so that the protein in the body is denatured and solidified to death. It is characterized by low operating temperature, short time and no secondary pollution. Under normal circumstances, common microorganisms such as mold and yeast are irradiated by microwave for 1 min, and can be killed by heating to 70-80 °C for sterilization purposes. Spores of penicillin can be killed by microwave irradiation for 2 min at 65-66 °C. . Microwave sterilization is a process based on steam that uses microwaves to generate moist heat and steam for sterilization. When sterilizing and disinfecting solid waste by microwave, the waste is first pulverized, then sent to a microwave heating furnace and heated by steam, and the volume of the waste is also compressed while being sterilized. Studies have shown that after the medical waste is soaked and pulverized, the waste is disinfected by microwave, the toxin is quickly eliminated, and the waste volume can be reduced by 80%. Many countries around the world produce large amounts of medical waste every year, causing serious environmental pollution. In Italy alone, there are 250,000 tons of medical waste generated each year. If microwave technology is used for sterilization, more than 60% of medical waste can be disposed of in landfill. Compared with the traditional incineration method, the medical waste treated by this method does not generate secondary pollutants such as toxic dioxins, and has short processing time, good effect and low energy consumption.

2.3 Recycling of used tires

The tire is a mixed material containing various substances such as rubber, carbon black, steel, and sulfur. The treatment of used tires is usually carried out by thermal decomposition. Compared with incineration, thermal decomposition under anoxic conditions can reduce NOx and SOx emissions and avoid soot production. However, conventional thermal decomposition often cannot be completely decomposed due to insufficient heating temperature. If microwave heating is used, the temperature will rise steadily and quickly reach 2000 °C. Moreover, the use of microwave heating to treat used tires can also achieve the reuse of rubber materials and energy recovery. Some factories in the United Kingdom use microwave heating technology to soften the waste tire rubber to break the CC and CS bonds in the rubber molecular structure, thereby recovering 36% of C (including high-quality carbonized products such as activated carbon and petroleum hydrocarbons). ), residual methane, hydrogen, etc. can also be used for heating the system. The heat treatment process must be carried out under strictly closed conditions to avoid the production of dioxins, soot and fly ash.

2.4 Processing of electronic waste

In recent years, millions of electronic components and printed circuit boards have been discarded from computers, automobiles, telephones, televisions, and other products. The conventional treatment of these electronic wastes is landfill, which results in the leaching of harmful metals and pollution of groundwater.

To this end, Professor DEClark of the University of Florida researched and developed the technology for recovering precious metals from microwave-destroyed printed circuit boards. The technology is to put the crushed waste circuit board into a fused silica crucible and heat it in a microwave oven lined with refractory material for 30 to 60 minutes. The organic substances such as benzene and styrene are volatilized and are carried by the carrier gas ( Compressed air) takes out the first microwave oven and enters the second microwave oven to be decomposed. The remaining material is coked below 1000 ° C, then the microwave oven power is increased, and the remainder (mostly glass and metal) is melted at a high temperature of 1400 ° C to form a vitrified material. After cooling this material, gold, silver and other metals are separated in the form of beads and can be recycled as a raw material for metal smelting. The remaining vitrified products can be recycled as building materials.

Microwave technology can reduce the volume of waste by 50%. No need to use any additives during the treatment process, it will not cause secondary pollution. The final vitrification product can firmly contain harmful components in it. Leakage, recycling of precious metals, and low processing costs.

2.5 Recycling of construction waste

Construction waste is generated during the construction process or during the repair and demolition of old buildings. It mainly contains wastes such as mortar, concrete, masonry, soil pile heads, metal, and wastes and packaging materials. It is estimated that [18], there are 40 million tons of construction waste generated by construction in China every year. Therefore, the comprehensive utilization of construction waste is an effective way to save resources and protect the ecology.

According to reports, microwave technology is an effective way to recycle and reuse construction waste.

US CYCLEAN company uses microwave technology to 100% recycle construction waste, regenerate old asphalt road fabric, the quality of recycled asphalt road fabric is the same as that of fresh asphalt road fabric, and the cost is reduced by 1/3, while saving garbage Clearance and handling costs. The use of microwave technology to recover construction waste not only solves the problems of secondary pollution, such as composting, incineration, landfill, etc., which is easy to cause secondary pollution, large investment, and large area, and can also recycle waste.

3 Conclusion

The development of microwaves is also manifested in the expansion of the scope of application. The most important applications of microwaves are radar and communications. Radar is not only used for national defense, but also for navigation, meteorological measurements, geodesy, industrial inspection and traffic management. Communication applications are mainly modern satellite communications and conventional relay communications. Radio telescopes, microwave accelerators, etc. are of great significance for the study of physics and astronomy. Millimeter-wave microwave technology provides an effective method for measuring plasma measurements of thermonuclear reactions. Microwave remote sensing has become an important means of studying celestial bodies, meteorology and geodesy, resource exploration and so on. The research on microwave production in industrial production, agricultural science, and the research and development of microwave in biology and medicine have been paid more and more attention (see microwave application, microwave energy application, microwave medical application, etc.). The use of microwave technology to treat solid waste is characterized by rapid and efficient operation, simple operation, low energy consumption, low cost, high resource recovery and utilization, and thus has broad application prospects. However, domestic research on the application of microwave technology in the treatment of solid waste started late, and most of the current work is limited to the level of laboratory research, and industrial applications are less. With the improvement of people's environmental awareness and the further improvement of microwave technology, as well as the development of microwave equipment for pollution control, microwave technology will be widely used in the treatment of solid waste and even the entire pollution control field.

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