Liuchuan Yi Xi Tianpeng He Shian

(Nanjing Institute of Industrial Machinery granulation University Nanjing 210009)

Abstract: This paper discussed the powder dry, granulated powders, granules were dried and other technical characteristics of view, trying to scientifically establish a new pre-dried powder - granulation - Microwave Drying process route. The material starts drying phase constant speed, so that most of the moisture removal material, the material becomes less than 20% moisture content (wet basis). The low moisture content of the material pressure granulation, this time not only easy granulation, granulation rate, and particle size uniformity, high strength. Low moisture content of the particulate material, and dried by microwave heating. Microwave heating characteristics of the particles inside moisture quickly ruled out, thereby obtaining moisture eligibility requirements. Article comprehensively discussed the new process route scientific, technical, reliability and economy.

Keywords: powder dry; granulating; microwave drying

1. Overview

With the needs of industrial development in the chemical, food, medicine, electronics, bio-plastics and chemical industry applications more and more types of powder materials, and the size is getting smaller. Such as calcium carbonate ultrafine particle size is 5μm, titanium dioxide 3μm, Food Yellow, Tartrazine 10μm, dolomite 15μm, yellow iron oxide 5μm, aluminum 20μm, 45μm starch white carbon black and so on. The rapid development of modern ultra-fine powder - nano powder. Very fine particle size of the powder bulk density is small, light weight, easy to fly during operation, not only caused material damage and pollution of the environment, which stimulate toxic powder escaping to the environment can cause serious harm. Because the bulk density of the fine powder is small, inconvenient transportation. Fine chemical fertilizers applied to fields prone to erosion, thus creating a negative side, then there will be a fine powder to aggregate into larger entities - granulation needs. Granulation method rolling method and pressure method. Scroll granulation is wet loose materials (powder and the amount of the wetting liquid) was added to the granulation unit, flip stirring, initially formed the core of the pellet. Subsequently, in order to reunite the core and cladding are two ways to grow, agglomerated spherical particles of irregular, rough surface. Cladding granulation smooth spherical surface, the cross section of a layer packet layer "onion skin" structure, you can control the operating conditions, so that one way to become the dominant granulated to form spherical particles of high strength smooth rules. Mixing granulation, fluidized spray granulation methods are wet bulk materials or paste or molten material, solutions, slurry material. Granulation pressure is low moisture content of fine material under pressure in tableting machines, rolling machines, roller presses, screw extruders or pelletizer mainly by shearing force is compacted into tablets. Wherein the roller press can achieve extreme pressure granulation. Pressure range 2.5MPa ~ 560MPa, pressed powder can be very dense, so that inter-molecular forces play a leading role in the powder, the particles larger tensile, compressive, anti-abrasion strength. For almost four decades to thousands of dry powder materials for extreme pressure granulation experiment, data were obtained granulated success. But for those moisture content is very low (eg less than 0.2%), very fine grain size (d97 <45um), bulk density of small (<200kgm-3), porosity, small internal friction, and good mobility powder material pressure granulation is more difficult. Not only energy consumption, and granulation rate, low single output. If adding the corresponding material in an appropriate amount of wetting agent in the above-mentioned properties, the granulation conditions greatly improved, such as granulated without strong pressure can achieve granulation rate, stand-alone production, no dust and other environmental purposes. However, the particles will contain a certain amount of moisture. No free water surface of the particles, the internal migration of moisture to become controlling factors can not be changed at this time to strengthen the external variable conditions drying rate, namely dried particles take a long time, spent a lot of energy. If the microwave heating drying low moisture content of the particulate material, can quickly get the desired dry granular product uniformity. According to the powder to make the following preliminary constant drying stage (forthcoming powder dry to a moisture content of less than 20%) - Pressure on the roll granulation - dry granulated particles of microwave drying line. Such granulation drying line scientific and rational, high economic efficiency.

2. The dried powder granulation pre - final moisture content control in 10% to 20%

Most of the powder is a solid material from a solution, the average particle diameter of the filter cake, and dried to give a paste material powder is smaller than 100μm or less. Such as fertilizers, dyes and additives, and food additives, the polymer resin powder, by which the beginning of a moisture content (30% to 80%) and dried to a final moisture content of 0.02% to 9% of the dry powder, an average particle diameter in the range of 5μm ~ 50μm, the dryer requires a high evaporation intensity. Most of the time is spent drying the last 20% moisture. As silica powder by 80% aqueous paste, dried to a moisture content of 6% of powder having a particle size of 45 m, a bulk density of 240kgm-3, and dried in a desiccator promoting boiling dryer having a diameter of 150mm , intake air temperature of 300oC, the hourly output of 5kg. Black carbon black from the dried paste to a moisture content of 92% aqueous 2% powder, strengthen boiling drier, drying diameter of 150mm, inlet air temperature of 300oC, only an hour to get 1.3kg of dry powder. According to the drying mechanism and experimental drying curve analysis, the constant drying stage, hot air heat to the surface of the material, so that the surface free rapid evaporation of water, reduce epidermal water, the material began to heat up, and the formation of a temperature gradient in the interior, the heat from the outside spread inside, moisture migration to the surface from the interior of the material, moisture migration powered mainly by diffusion, capillary flow and internal pressure due to the shrinkage of the drying process the volume of material generated. Therefore, the critical moisture content of dried material to appear when a low final moisture content to internal moisture migration become controlling factors. Some external variable such as the amount of hot air, the temperature can not enhance its moisture migration rate only applied vibration, pulse, ultrasound and other means to promote their internal diffusion of moisture. So in general the material moisture content of less than 20%, drying is difficult, called falling rate drying stage. At this stage a long drying time, energy consumption and more. To reduce the general load on the dryer, on the other hand to retain the moisture content of the material-specific requirements, has been ease of processing, molding or granulation. World countries to reduce raw material losses and dust pollution on the environment, requires granulated powder materials, particle size in the range of 20 mesh to 8mm. When the powder material moisture content of 10% to 20% of the most favorable pressure granulation. Humidity content of materials with convection, conduction, radiation heating and drying of materials, constant drying phase is completed (ie final moisture content of between 10% to 20%) is terminated, the loose material was subjected to wet granulation. Such material is easy to granulation and uniform particles, no dust, the small material loss, environmental pollution.

3. Pressure granulation powder

Began in the 1970s research granulated powders, thousands of customers sent thousands of dry granulation of materials experiments, when these materials without any binder granulation experiment, called extreme pressure dry granulation and achieved gratifying results. Qualified majority Dry strength can be prepared a variety of different sized particles, but grain rate is not very high, and some dry powder under strong pressure (350MPa ~ 560MPa) granulated, the granulation rate of less than 70%, the material in the back more than 30%, so the single granulation production affected, but also in the granulation project, when screening dust, environmental contamination. If you add in the dry in about 10% of the liquid binder (such as water, organic solvents, etc.), with a continuous roll-to-roll press granulation process is very smooth, not only granulation rate, and uniform particle size, high strength, granulation process no dust.

Further when wet tumbling granulation, a loose moisture content of the wet material in 10% to 20%, up to 30%, the particle size distribution of the powder, there are strict requirements, such as a maximum particle size of 30 to 50 mesh, at least 25% of the fine particle size of less than 200 mesh. Granulated iron ore feedstock, less than 325 mesh powder should account for 40% to 80%. And a moisture content of about 10% fines with a roll press continuous granulation no requirements in this regard. Pressures in the range 2.5 ~ 140MPa, can be granulated, greatly reduce energy consumption, energy consumption is (2 ~ 4) kw / ht. If you do not pre-granulated additional binders, lubricants, plasticizers, wetting agents, fungicides, etc., but not directly dry wet material (wet amount between 10% to 20%) with a continuous roll granulated not only scientific and reasonable and has obvious economic benefits.

4. Microwave heating of granular materials for final drying

It has been no stranger to the microwave heating, domestic microwave oven, urban residents use very skilled. Especially for reheating cooked food, we all appreciate the speed of microwave heating, such as heating bread, bread, buns to, said food would be the first dip in the water, and then placed in the microwave, so that not only the heating time is short, and buns , soft bread will not harden. Why wet bread in a microwave oven heat than dry bread and fast? Why not harden soft? To answer these questions, we should understand the relationship between the material and the microwave and microwave heating characteristics.

The so-called microwave frequency range is 3 × 108Hz ~ 3 × 1011Hz between wavelength electromagnetic waves between 1m ~ 1mm. Between matter and electromagnetic waves have the following relationship:

(1) Conductor: such substances reflect electromagnetic waves, such as the metal housing of the microwave oven, microwave energy storage, the microwave does not leak.
(2) Insulator: these substances does not reflect nor absorb microwaves, the microwave is transparent, such as microwave bowls, boxes, all with an insulator such as glass, ceramic, resin or the like made of Teflon.
(3) dielectric: different levels of these substances absorb microwave energy is converted into heat, which the water dielectric parameters of the largest, most easily absorbed microwave energy that is converted to heat.
(4) Ferrite: such substances are absorbed, reflected, penetration of electromagnetic waves, electromagnetic waves with a magnetic field component generating action generates heat.

Powder materials are generally dielectric body, its moisture mostly water or an organic solvent, such as ethanol and the like, moisture dielectric parameters is much larger than the solid dielectric parameters, such as water and dry sand in about only 80 2.55 simply put, the water absorption of microwave energy capacity 30 times higher than dry sand. Microwave heating and drying can be considered, most of the microwave energy consumed in the desire to remove the moisture in.

Microwave wave-particle duality, according to quantum theory, the energy of electromagnetic radiation is not continuous, but rather one of "energy quanta" of, each having its frequency is proportional to the quantum energy.

E = hf (1)
Where h = 6.626 × 10-34J.S Planck constant. This energy can be converted to the dielectric body heat. Energy conversion mechanism has many kinds, such as ion conduction, dipole rotation, interface magnetic hysteresis, piezoelectric, electrostrictive and NMR. Wherein the ion conduction and dipole rotation is the leading cause of dielectric heating.

Ion conductivity: charged particles (e.g., sodium chloride solution contains Na +, Cl-1, (H3O) +, OH-1 four kinds of ions) external electric field is not accelerated, along with its polarity opposite directions movement, at the macro performance for conducting current, these ions collide with other particles in the course of the campaign will be its surroundings, while the kinetic energy of these particles pass to motion increases. If you are in a high-frequency alternating electric field, the material of the particles occurs becomes repetitive motion, resulting in increased collisions, resulting in consumption of heat energy conversion occurs, the power per unit volume is generated:
hg (2)

Where: PV: volume per unit of power generated, W / m3; E: the electromagnetic field intensity vector, V / m; σ: a conductivity, S / m;

Dipole rotation: Can cause displacement polarization (polar molecules dielectric) and steering polarization (polar molecules dielectric) under the action of an external electric field of the dielectric, if the alternating electric field, the dielectric was repeatedly polarized dipole constantly occur "orientation" and "relaxation", so, due to the interaction of the original thermal motion of molecules and the adjacent molecules, the molecule motion with the rules of the external electric field is disturbed and obstacles, generating "friction effect", the result part of the energy is converted to kinetic energy of the thermal motion of molecules, manifested in the form of heat, so that the material temperature.
The power unit of volume change:
dg
Water is a polar molecule, its relative dielectric constant is much higher than the relative permittivity of the dielectric other, with other water loss factor of dielectric loss factor similar, so the maximum value of tanδ of water, other liquids (and some other organic solvents such as ethanol) also showed a strong dielectric properties. Therefore, aqueous and solvent wet materials are suitable for microwave heating. Such wet material in the microwave field can be "in situ heat, with the heat inside and outside" the wet material in the rapid increase in water temperature, vaporized. The solid powder only consume a small amount of microwave energy.

Previously described by the roll method to a moisture content of 10% to 20% of the bulk materials for continuous wet granulation, both scientific and economic, but wet granules contained in sub-heating means what it dried it? According to the second article discusses the known, constant speed when the end of drying stage, proceeds falling rate drying stage, convection, conduction, radiation heating, drying can not strengthen, the microwave can only be effective to the internal water vaporized. Because the material in the microwave field "inside and outside with the heat," the water within the material quickly reaches boiling point, the occurrence of high strength evaporated material texture impede the flow of water, so the material inside a pressure gradient, due to "inside and outside with hot" surface heat easily lost, internal material to form a positive temperature gradient and moisture gradient, three states can promote moisture in the form of a liquid, vapor or molecular flow moves to the material surface. Greatly increased drying rate falling rate drying stage, so that it is difficult to quickly dried material to obtain a homogeneous dry.

Microwave heating of granular, low moisture content of dried material has many advantages, such as fast-drying, uniform drying, energy, moisture content and precise control. Improve the quality (food, medicine does not occur secondary pollution, and can be sterilized). Then microwave drying in the country's development situation?