Application of Microwave Technology in Mineral Processing

With the development of science and technology, mining development has made a qualitative leap. Mining enterprises are constantly seeking new equipment and new processes in order to achieve lower production costs, efficient operation of equipment, simplification of production processes, and comprehensive recycling of associated minerals, so that enterprises can obtain greater economic benefits.

Due to its unique selective heating, uniform radiation, high thermal efficiency and Low Energy Consumption, microwaves are attracting more and more people's attention. The new application fields are expanding and its application in mineral processing is becoming more and more possible. .

First, the microwave heating principle and its characteristics

The microwave is an electromagnetic wave having a frequency of approximately 300 MHz to 30 GHz and a wavelength of 100 cm to 1 mm. It is located between the infrared radiation (light wave) of the electromagnetic spectrum and the radio waves.

Microwave is a very special electromagnetic band. Although it is between radio waves and infrared radiation, its principle of generation, transmission and application is different. Only 915MHz and 2450MHz are used as civilians in the microwave.

(1) Principle of microwave heating

Microwave heating is different from conventional heating methods. Conventional heating is performed by external heat sources by heat radiation to conduct heat from the surface and inside, while microwave heating is the body heating caused by dielectric loss in the electromagnetic field. Microwave heating converts microwave electromagnetic energy into thermal energy. This process is closely related to the polarization of molecules inside the material.

According to the types of microscopic particles participating in polarization, the polarization of dielectric molecules can be roughly divided into electron polarization, atomic polarization, dipole steering polarization (orientation polarization), and interfacial polarization. The heating in the microwave field is mainly realized by dipole turning polarization and interfacial polarization. As a high-frequency electromagnetic wave, microwaves act on substances under the microwave field. The molecules in the substance are ionized and polarized under the action of an electric field to form polarized molecules. The polarized molecules have positive and negative dipoles, which are in the electric field. Produce an orientation. The polarized molecules in the material undergo high-frequency oscillations as the microwave electromagnetic field alternates, and the molecular motion generates heat, which is the principle of microwave heating.

(2) Characteristics of microwave heating

1. The immediacy of microwave heating. Heating the mineral material with microwaves is very rapid. As long as there is microwave radiation, the material is immediately heated; otherwise, the material does not get microwave energy and immediately stops heating. Microwave allows the material to get or lose heat in an instant, showing no inertness to the heating of the material. Under the action of an external alternating electromagnetic field, the polar molecules in the material are polarized and alternately oriented with the polarity of the applied alternating electromagnetic field. Polar molecules are frictionally lost due to frequent steering (about 108 times per second), which converts electromagnetic energy into heat. Therefore, the process by which microwave energy is converted into heat energy within the material has an immediate feature.

2. The integrity of microwave heating. Microwave is a kind of electromagnetic wave with strong penetrating power. It can penetrate into the inside of the object, radiate microwave electromagnetic field to the inside of the material to be heated, and promote the vigorous movement of its polarized molecules, so that the molecules collide with each other and friction to generate heat. Therefore, the heating process is carried out simultaneously in the whole object, and the temperature rise is rapid, the temperature is uniform, and the temperature gradient is small, which is a kind of “body heat source” heating, which greatly shortens the time of heat conduction in the conventional heating. In addition to the extremely large objects, it is generally possible to uniformly heat the surface together. This is in line with the requirements of industrial continuous production and automation control.

3. Selectivity of microwave heating. Not all materials can be heated in a microwave. Different materials have different responses to microwaves due to their different dielectric properties. According to the different reactions to microwaves, the materials are classified into microwave reflection type, microwave transparent type, microwave absorption type and partial microwave absorption type. Thus, the selective heating of the various components or parts of the mixture can be selectively effected by the selectivity of microwave heating.

4. The efficiency of microwave heating energy utilization. Microwave heating, dielectric material capable of absorbing microwave, and converted into heat, microwave heating device housing and the metallic material is a microwave reflective material which can not absorb microwaves reflected (or minimally absorb microwaves), so that the microwave The heat loss of the heating device itself is only a very small fraction of the total energy consumption. In addition, the microwave is an internal "body heat source", which does not require a high temperature medium to transfer heat. Therefore, most of the microwave energy is absorbed by the medium material and converted into heat required for temperature rise, forming a high efficiency characteristic of microwave energy utilization. Compared with the conventional electric heating method, it can generally save 30% to 50%.

5. Microwave heating is safe, hygienic and pollution-free. Conventional heating generally uses fossil fuels as an energy source, and the carbon dioxide produced by combustion is the main component of the "greenhouse effect". The energy used for microwave heating is electrical energy, and there is no pollution to the environment. When the organism is irradiated with microwaves, in addition to generating a microwave thermal effect, the biological activity of the organism can be suppressed or excited, that is, a non-thermal effect or a biological effect of the microwave is generated. Under the same temperature conditions, the mortality rate of bacteria by microwave is much higher than that of conventional heating.

It can be seen from the principle of microwave heating and microwave characteristics that microwave heating is a kind of heating method that goes deep into the material and is heated from the inside to the outside. It has the advantages that the traditional heating method can't match: selective heating of materials, fast heating rate and high heating efficiency; Promote endothermic reaction and exothermic reaction, have catalytic effect on chemical reaction; when microwave heating replaces traditional heating, smelting and other high-temperature chemical reactions can be carried out at very low temperature, that is, microwave heating has the effect of lowering the chemical reaction temperature; microwave It can make high-speed vibration of atoms and molecules, thus creating more favorable thermodynamic conditions for chemical reactions; microwaves can easily heat polar liquids (water, acid-base solution, etc.), thus increasing the leaching rate and reducing the energy consumption of the process. Easy to automate.

Therefore, by using the characteristics of microwave heating, the minerals can be processed to obtain better results.

Second, the application of microwave technology in mineral processing

Due to its special properties, microwaves are becoming more and more widely used. With an emphasis on field dressing microwave, microwave applications in mineral processing is also gradually show the superiority and practicality.

(1) Crushed mine

Sam Kingman of the University of Nottingham in the United Kingdom used seven years to study the use of microwaves to heat ore. He found that heating the ore with microwaves can smash the ore like a power crush, but it can save half the energy. Various ores react differently to microwaves, so different microwave parameters can be selected for different types of ore. In the ore, due to the different endothermic heat of various components, cracks in the ore are caused, which will help break the ore into particles of various compositions. For different minerals, choosing the right microwave frequency, strength and heating time are also critical factors.

(2) Grinding

In the mineral processing process, the minerals are generally ground to separate the valuable minerals from the gangue monomers, so that the subsequent process can be successfully completed. In this process, energy consumption is high, usually accounting for 50% to 70% of total energy consumption, but energy efficiency is often low, especially for dense ore.

According to the selective heating characteristics of the microwave, the ore can be pretreated before grinding to reduce the energy consumption of the grinding process.

Various minerals have different dielectric constants and different responses to microwaves. As described above, the medium can be classified into a microwave reflection type, a microwave transparent type, a microwave absorption type, and a partial microwave absorption type depending on the different reactions of the materials to the microwave. Therefore, the rate of temperature rise of various minerals constituting the ore in the microwave field is different. Under the same conditions, different minerals in the ore are heated by microwaves to different temperatures.

Most of the gangue minerals in the ore are quartz and calcite . From the data in the table, they have a very low heating rate, while the useful minerals have a higher heating rate, so they form between the useful minerals and the gangue minerals. Significant local temperature differences that cause thermal stress between them. When this thermal stress is large enough, cracks are formed at the interface between the minerals. The generation of cracks can effectively promote the monomer dissociation of useful minerals and increase its surface area, which is very important for reducing the grinding cost and improving the recovery rate of mineral processing.

Triticum aestivum L. grinding or the like to the kinetics and selectivity coefficient as a function of rupture, to prove the selective microwave heating of the ground magnet promotes ore, and quartz is not affected, so as to achieve that the object of finely ground magnetite selectivity. Microwave pretreatment for 5 min can increase the grain size yield of magnetite-0.3mm by more than 20%. Yuetie Bing et al [4] studied the effect of microwave grinding aid pyrite, kyanite ore, copper, molybdenum, tantalum and niobium ore, Copper, Lead and Zinc metal ore, the effect is obvious. Researchers at the Bandong Institute of Technology in Indonesia studied the effects of microwave pretreatment on the energy consumption of the Nishiganis gold ore. The researchers found that the gold ore's work index decreased by 20% to 35% after microwave energy was applied to the ore for different periods of time (5 to 300 s) before grinding. The University of Nottingham, the University of Birmingham and the University of Stirling Bosch have all studied the benefits of microwave energy in mining applications. Studies at the University of Birmingham have pointed out that microwave radiation is an effective way to improve ore grinding because it increases the cracks between the particles rather than the cracks between the particles.

(3) Flotation

University of Birmingham to study the effects of microwave titanium ore flotation. Studies have shown that microwave radiation pretreatment is effective in changing the surface characteristics of ilmenite and improving the flotation of minerals. When the microwave frequency is below 2.45 GHz and the power is 2.6 kW, the temperature of the loose ilmenite reaches 180 ° C after 10 s and reaches 720 ° C after 1 min, while quartz and feldspar have weak microwave heating characteristics, and after 1 min they are The temperature can only reach 53 ° C and 65 ° C, respectively.

After microwave irradiation, the recovery of ilmenite increased by nearly 20%. The specific surface of the ilmenite sample changes significantly with the increase of the microwave irradiation time, and a new phase can be observed. When ilmenite (titanium ferrite FeTiO ₃ ) is exposed to the air, oxidation occurs and Fe ²⁺ is oxidized to Fe ³⁺ . This oxidation is slow at room temperature, but its oxidation is enhanced as the temperature increases.

Microwave selective heating accelerates the oxidation of iron on the surface of ilmenite. Oxidation improves the adsorption performance of the flotation reagent and improves the flotation effect. Microwave treatment for 10 s increased the recovery of ilmenite by 10%. As the irradiation time is extended, the recovery rate is eventually increased from about 64% to 87%. Changes in the chemical nature of the ore surface can affect the chemicals required for mineral recovery through flotation. To some extent, microwave radiation reduces the dose of sodium oleate. For untreated ilmenite, to obtain a 60% flotation recovery, the concentration of sodium oleate is required to be 2 × 10 ^-4 mol / L, while the concentration of sodium oleate required for microwave treated ilmenite is only At 7 × 10 ^-5 mol / L, the dose was reduced by about 65%.

It can be seen that the effect of microwave is more obvious.

(4) Magnetic separation

Microwave technology can be used to convert non-magnetic minerals into magnetic minerals, which can be selected by magnetic separation. Coal pyrite pest belongs, using the flotation method reselection and are more difficult to handle. Microwave irradiation is used to convert pyrite (FeS ₂ ) into pyrrhotite (Fe _1-x S), which can be sorted out from coal by magnetic separation.

(5) Leaching

Most metal minerals exist in the form of sulfides in nature. The treatment methods for extracting metals are mainly fire roasting + leaching process. However, the fire method inevitably produces sulfur dioxide, causing environmental pollution. The use of microwave heating leaching can solve this problem well.

Ding Weian studied the leaching of ferric chloride from copper sulfide concentrate. Under the same temperature and the same FeCl ₃ concentration, the copper sulfide concentrate was leached by microwave heating and conventional heating, and the microwave method was not stirred. Microwave irradiation for 40 ~ 5Qmin, the leaching rate of copper can reach 98.8% ~ 99.05%, the slag contains less than 0.5% copper, and the traditional heating leaching takes more than 3 h to achieve the above indicators.

Peng Jinhui and Liu Chunpeng studied the microwave leaching of sphalerite. The results showed that under the same leaching conditions, the microwave leaching time was short and the leaching rate was high. After 60 minutes of leaching, the leaching rate of microwave leaching was 90%, while the traditional way The leaching rate is only 5l%.

(6) Restoration

The reduction of metal oxides into metals at high temperatures is the most important smelting process in pyrometallurgy and has a wide range of applications. Iron, tin , zinc, lead, manganese , chromium , etc. are produced by this method.

The identical hematite powder or magnetite powder, coke powder and lime powder are mixed in a certain proportion and reduced under the conditions of traditional heating and microwave heating respectively. The results show that under microwave heating, only 10 min reaction is required. At the end, the traditional method reaction has not completed 50% for 50 minutes. It can be seen that the reaction rates of the two differ greatly.

Hua Yixin et al studied the microwave reduction and traditional reduction of pyrolusite. The results show that the rate of microwave reduction is much higher than the traditional reduction rate.

(7) Ore pretreatment

Some gold ore is difficult to choose because it contains arsenic and carbon. After the gold ore or gold concentrate is pretreated by microwave roasting, the gold recovery rate can reach more than 90%, while the untreated material can only be about 30%. The use of microwave pretreatment of pyrite-coated gold concentrates increased the cyanidation leaching rate from 74% to over 90%.

(8) Other applications

In addition to the above applications, microwave technology is also used in mineral roasting, activated carbon regeneration, mineral powder drying, preparation of nano-products and sample testing.

Third, the conclusion

The application of microwave in ore dressing involves many aspects, and its application in industrial production has also been reported, but large-scale application has not yet been carried out. The main reason is that microwave research on the mechanism of action of minerals is insufficient, production equipment has certain defects, and peripheral equipment There are certain problems with the connection.

The application of microwaves to the beneficiation process, that is, the use of microwaves for the chemical, polarization and magnetic effects of minerals to promote the rapid and sufficient reaction of the minerals in the minerals, to achieve better results than the traditional heating method, will be the beneficiation process The development has opened up a new path. In particular, the characteristic heating of microwaves will help to reduce production costs.

At present, the research on microwave application in mineral processing is carried out at home and abroad. It is believed that in the near future, high-efficiency, high-quality, low-cost and pollution-free microwave beneficiation technology will be widely promoted.

Brief Introduction & Working Principle