Discussion on the new process route of powder drying granulation microwave heating (1)

Based on the discussion of the technical characteristics of powder drying, powder granulation and particle drying, a new process route of powder pre drying granulation microwave heating drying was established. That is to say, the material is dried at a constant speed stage, so that most of the moisture content of the material is removed, and the material with moisture content less than 20% (wet basis) is formed. The material with low moisture content is granulated by pressure method, which is not only easy to granulation and high granulation rate, but also has uniform particle size and high strength. The granular materials with low moisture content were dried by microwave heating. The characteristics of microwave heating make the moisture content in the particles quickly eliminated, so as to obtain qualified products. This paper discusses the scientificity, technicality, reliability and economy of the new process.

Key words: powder drying; granulation; microwave drying

1. Overview

With the development of industry, there are more and more kinds of powder materials in chemical industry, food industry, medicine industry, electronics industry, plastics industry, biochemistry industry and so on. For example, the particle size of ultrafine calcium carbonate is 5 μ m, titanium dioxide is 3 μ m, food yellow and tartar yellow are 10 μ m, dolomite is 15 μ m, yellow iron oxide is 5 μ m, aluminum silicate is 20 μ m, starch silica is 45 μ m, etc. Modern rapid development of ultra-fine powder - nano powder. The powder with very fine particle size has small bulk density and light weight. It is easy to fly in the process of operation, which not only causes material loss but also pollutes the environment. Among them, the release of toxic fine powder will cause serious harm to the environment. Because of the small bulk density of fine powder, it is inconvenient to transport. The fine chemical fertilizer is easy to be lost in the field, which has a negative side, so there is a need to aggregate the fine powder into a larger entity - granulation. The granulation methods include rolling method and pressure method. Rolling granulation is to add the loose wet material (fine powder and appropriate amount of wetting liquid) into the granulating device, stir and turn, and form the core of the pellet initially. Then, the core grows up in two ways: agglomeration and cladding. The agglomerated particles are spherical and irregular, and the surface is rough. The surface of cladding granulation is smooth and spherical, and the cross-section is "onion skin" structure, which can control the operation conditions, make one of the ways to become the dominant granulation method, and form smooth and regular spherical particles with high strength. Mixing and granulation, spray fluidized granulation and other methods are loose wet materials or paste or molten materials, solutions, slurry materials. Pressure granulation is to compact the fine powder material with low moisture content into granules under pressure or main shear force in pelletizer such as tablet press, roller press, screw extruder and so on. The roller press can realize the granulation under strong pressure. When the pressure range is 2.5MPa ~ 560mpa, the powder can be compacted very tightly, which makes the molecular force between the powders play a leading role, and makes the particles obtain greater tensile, compressive and abrasion strength. In the past 40 years, the granulation experiments of thousands of fine powder dry materials have been carried out, and the successful granulation data have been obtained. However, it is difficult to granulate the fine powder materials with low moisture content (such as less than 0.2%), fine particle size (D97 < 45um), small bulk density (< 200kgm-3), high porosity, low internal friction and good fluidity. It not only consumes a lot of energy, but also has low granulation rate and low yield per unit. If the appropriate amount of wetting agent is added to the materials with the above characteristics, the granulation conditions will be greatly improved. For example, high granulation rate, large single machine output and no dust flying in the environment can be achieved without strong granulation pressure. However, there will be a certain amount of moisture in the particles. There is no free moisture on the surface of the particles, and the internal moisture transfer becomes the controlling factor. At this time, the external variable conditions can not change the drying rate, that is, the drying of particles takes a long time and consumes a lot of energy. If microwave heating is used to dry the granular materials with low moisture content, the required dry degree can be quickly obtained. Based on this, the following granulation drying route is put forward, namely, the powder is dried to a moisture content of less than 20%) - double roller pressure granulation and particle microwave heating drying. This granulation drying route is scientific and reasonable with high economic benefits.

2. Drying in the early stage of powder granulation - the final moisture content should be controlled at 10% ~ 20%

Most of the powders are dry powders with average particle size less than 100 μ m, which are obtained by drying the solution of solid materials, filter cake and paste raw materials. For example, fertilizer, dyestuff and its auxiliaries, food and its auxiliaries, polymer resin and other powders are dried from the initial moisture content (30% ~ 80%) to the final moisture content of 0.02% ~ 9%. The average particle size is in the range of 5 μ m ~ 50 μ m, which requires high evaporation intensity of the dryer. Most of the time is spent on 20% moisture after drying. For example, silica powder is dried from paste containing 80% water to 6% water. The particle size is 45 μ m and the bulk density is 240 kgm-3. The silica powder is dried in an enhanced boiling dryer with a diameter of 150 mm, inlet air temperature of 300 OC and an hourly output of 5 kg. The black carbon black powder was dried from 92% of paste water to 2% of water content, and then dried in an enhanced boiling dryer. The drying diameter was 150 mm and the inlet air temperature was 300 OC. Only 1.3 kg dry powder could be obtained per hour. According to the drying mechanism and experimental drying curve analysis, in the constant speed drying stage, the heat of hot air is transferred to the material surface, which makes the surface free water evaporate rapidly, the skin moisture decreases, and the material begins to heat up, and forms a temperature gradient inside the material. The heat is transferred from the outside to the inside, and the moisture content migrates from the inside of the material to the surface Internal pressure caused by shrinkage of material volume during drying. Therefore, when the critical moisture content appears until the material is dried to a very low final moisture content, the internal moisture migration becomes the control factor. Some external variables such as the amount of hot air, temperature can not strengthen the moisture transfer rate, only by applying vibration, pulse, ultrasonic and other means to promote the internal moisture diffusion. Therefore, when the moisture content of general materials is lower than 20%, it is difficult to dry, which is called the slow drying stage. In this stage, the drying time is long and the energy consumption is more. In order to reduce the load of the general dryer, on the other hand, in order to maintain the specific moisture content requirements of materials, it has been convenient for processing, molding or granulation. At present, in order to reduce the loss of raw materials and the pollution of dust to the environment, it is required to granulate the powder materials with the particle size in the range of 20 mesh to 8 mm. When the moisture content of powder is 10% ~ 20%, it is beneficial to pressure granulation. The materials with high moisture content are heated and dried by convection, conduction and radiation, and the constant speed drying stage (i.e. the final moisture content is between 10% ~ 20%) is completed, and the loose wet materials are granulated. This kind of material is easy to form granules, and the particles are uniform, and there is no dust flying, that is, the material loss is small, and the environment is ^.
3. Powder granulation by pressure method

In the 1970s, the research on powder granulation began. Thousands of customers sent thousands of dry powder materials for granulation experiments. At that time, the granulation experiment was carried out on these materials without any binder, which was called forced dry granulation, and gratifying results were achieved. Most of the dry powder can produce various sizes of particles with qualified strength, but the granulation rate is not very high. Some dry powders are granulated under high pressure (350Mpa ~ 560mpa), and the granulation rate is less than 70%, and the return material is more than 30%. Therefore, the single granulation output is affected. Moreover, in the granulation project, the dust is flying during screening and the environment is polluted. If about 10% liquid binder (such as water, organic solvent, etc.) is added into the dry powder, the continuous rolling granulation process of the double roller press is smooth, not only the granulation rate is high, but also the particle size is uniform and the strength is high, and there is no dust flying in the granulation process.

In addition, in wet rolling granulation, the moisture content of loose wet material is required to be between 10% and 20%, up to 30%, and the particle size distribution of powder has strict requirements, such as large particle size of 30 ~ 50 mesh, at least 25% of fine powder particle size less than 200 mesh. In the feed of iron ore granulation, fine powder less than 325 mesh should account for 40% ~ 80%. However, when the moisture content is about 10%, there is no requirement for continuous granulation with roller press. When the pressure range is 2.5 ~ 140MPa, granulation can be achieved. The energy consumption is greatly reduced and the energy consumption is (2 ~ 4) kW / HT. If the binder, lubricant, plasticizer, wetting agent, bactericide, etc. are not added before granulation, but the wet materials which are not completely dried (the moisture content is between 10% and 20%) are directly used for continuous granulation by rolling, which is not only scientific and reasonable, but also has obvious economic benefits.

4. Final drying of granular materials by microwave heating

Microwave heating is no stranger to people, home microwave oven, urban residents are very skilled in the application. Especially for the reheating of cooked food, we all realize that the microwave heating speed is fast. For example, when heating steamed bread, bread and steamed bun, the above-mentioned food will be soaked in water first, and then heated in the microwave oven. In this way, not only the heating time is short, but also the bun and steamed bread are soft and not hard. Why is wet steamed bread hotter and faster than dry steamed bread in microwave oven? Why is it soft and hard? To answer these questions, we should understand the relationship between microwave and materials and the characteristics of microwave heating.
The so-called microwave is an electromagnetic wave with a frequency range of 3 × 108hz ~ 3 × 1011hz and a wavelength of 1m ~ 1mm. There are the following relationships between matter and electromagnetic wave:

(1) Conductor: this kind of material reflects electromagnetic wave, such as the metal shell of microwave oven, to store microwave energy, so that the microwave does not leak.

(2) Insulator: this kind of material does not reflect or absorb microwave. It is transparent to microwave. For example, microwave bowl and box are made of insulator such as glass, ceramics and PTFE resin.

(3) Dielectric: this kind of material absorbs microwave energy to varying degrees and converts it into heat energy. The dielectric parameter of water is large, that is, it is easy to absorb microwave energy into heat energy.

(4) Ferrite: this kind of material also absorbs, reflects and penetrates the electromagnetic wave, and interacts with the magnetic component of the electromagnetic wave to generate heat.

General powder materials are dielectric materials, and the moisture content is mostly water or organic solvent, such as ethanol. The dielectric parameters of wet fraction are far greater than those of solid. For example, water is about 80, while dry sand is only 2.55. It can be simply said that the ability of water to absorb microwave energy is more than 30 times higher than that of dry sand. It can be considered that most of the microwave energy is consumed in the moisture to be removed.

Microwave has wave particle duality. According to quantum theory, the energy of electromagnetic radiation is not continuous, but composed of "energy particles". Each quantum has energy proportional to its frequency.

E=hf

Where h = 6.626 × 10-34j. S Planck constant. This energy can be converted into heat energy in a dielectric. There are many mechanisms of energy conversion, such as ion conduction, dipole rotation, interfacial magnetization, hysteresis, piezoelectricity, electrostriction, nuclear magnetic resonance, etc. Ion conduction and dipole rotation are the main reasons for dielectric heating.

Ion conduction: charged particles (such as sodium chloride solution containing Na +, CL-1, (H3O) + oh-1 ions) will not be accelerated under the action of an external electric field, and will move along the direction opposite to its polarity. On the macro level, these ions will collide with other particles around them and transfer kinetic energy to these particles to intensify their motion. If it is in the high-frequency alternating electric field, the particles in the material will repeatedly change their direction, which will aggravate the collision, generate heat dissipation and energy conversion. The power generated per unit volume is:
Where: PV: power generated per unit volume, w / m3; E: electromagnetic field intensity vector, V / M; σ: conductivity, S / M;

Dipole rotation: under the action of external electric field, the dielectric material will produce displacement polarization (apolar molecular dielectric material) and steering polarization (polar molecule dielectric material). If the dielectric material is repeatedly polarized in the alternating external electric field, the dipole will continuously "orientation" and "Relaxation". In this way, due to the original thermal motion of molecules and the interaction between adjacent molecules, the molecules will change with each other The regular movement of the external electric field is disturbed and hindered, resulting in "friction effect". As a result, part of the energy is converted into kinetic energy of molecular thermal movement, which is expressed in the form of heat, which makes the material temperature rise.

The power of unit volume change is as follows:

Water is a polar molecule, and its relative permittivity is much higher than that of other dielectrics. The loss factor of water is similar to that of other dielectrics, so the tan δ value of water is large, and other liquids (such as ethanol and some organic solvents) also have strong dielectric properties. Therefore, the wet materials with water and solvent are suitable for microwave drying. In the microwave field, this kind of wet material can "heat locally, and the inside and outside are heated together". The temperature of moisture in the wet material rises rapidly and vaporizes. However, the solid powder only consumes a small amount of microwave energy.

It is scientific and economical to continuously granulate the loose and wet materials with moisture content of 10% ~ 20% by roller pressing method. However, what heating method is used to dry the moisture content in the particles? According to the second part of this paper, it is known that at the end of the constant speed drying stage, the material will enter into the stage of falling speed drying. Heating with convection, conduction and radiation can not strengthen the drying. Only microwave can effectively vaporize the internal moisture. Because the material in the microwave field "internal and external heat", the moisture inside the material quickly reaches the boiling point, and high-intensity evaporation occurs. The texture of the material hinders the water flow, so the pressure gradient is formed inside the material. Because the surface heat of "internal and external heat" is easy to dissipate, the positive temperature gradient and humidity gradient are formed inside the material. The three states can promote the moisture to be liquid, vapor or molecular The flow moves towards the material surface. It can greatly improve the drying speed of the slow drying stage, so that the materials which are difficult to dry can be quickly and evenly dried.

Microwave heating has many advantages in drying granular and low moisture content materials, such as fast drying speed, uniform drying, energy saving, moisture content control. Improve the quality (food, medicine will not produce secondary pollution, and can be sterilized). How about the development of microwave drying in China?