1. Overview
With the needs of industrial development, more and more types of powder materials with finer particle sizes are being applied in industrial sectors such as chemical, food, pharmaceutical, electronic, plastic, and biochemical industries. The particle size of ultrafine calcium carbonate is 5 μ m, titanium dioxide is 3 μ m, food yellow and tartrazine are 10 μ m, dolomite is 15 μ m, yellow iron oxide is 5 μ m, aluminum silicate is 20 μ m, starch white carbon black is 45 μ m, and so on. Modern rapid development of ultra-fine powder - nano powder. Fine grained powders have a low packing density and light weight, and are prone to flying during operation, which not only causes material loss but also pollutes the environment. Among them, there are irritating and toxic fine powders escaping, which can cause serious harm to the environment. Due to the low bulk density of fine powder, transportation is inconvenient. Fine chemical fertilizers applied to fields are prone to loss, which creates an adverse effect. Therefore, there is a need to aggregate small powders into larger entities - granules. The granulation methods include rolling method and pressure method. Rolling granulation is the process of adding loose wet materials (fine powder and an appropriate amount of wetting solution) into a granulation device, stirring and turning them to initially form a core of granules. Subsequently, the core grows in two ways: agglomeration and cladding. The agglomerated particles have irregular spherical shapes and rough surfaces. The surface of the granulation layer is smooth and spherical, and the cross-section is a layer by layer "onion skin" structure. The operating conditions can be controlled to make one method dominant in granulation, forming smooth, regular, and high-strength spherical particles. Mixing granulation, spray fluidization granulation and other methods are loose wet materials or paste or molten materials, solutions, and slurry materials. Pressure granulation is the process of compacting fine powder materials with low moisture content into granules in granulators such as tablet presses, rolling presses, rolling presses, and screw extruders under pressure or mainly shear forces. The roller press can achieve strong pressure granulation. The pressure range is 2.5MPa~560MPa, which can compact the powder very tightly, allowing molecular forces between the powders to play a dominant role, and enabling the particles to obtain greater tensile, compressive, and abrasion strength. In the past forty years, thousands of fine powder dry materials have been subjected to high-pressure granulation experiments, and successful granulation data have been obtained. However, it is difficult to pressure pelletize fine powder materials with low moisture content (such as less than 0.2%), fine particle size (d97<45um), low bulk density (<200kgm-3), high porosity, low internal friction, and good flowability. Not only does it consume a lot of energy, but it also has a low granulation rate and low single machine output. If an appropriate amount of wetting agent is added to the materials with the above characteristics, the granulation conditions will be greatly improved. Without the need for strong granulation pressure, high granulation rate, high single machine output, and no dust flying in the environment can be achieved. However, the particles will contain a certain amount of moisture. There is no free moisture on the surface of the particles, and the migration of internal moisture becomes the controlling factor. At this time, external variable conditions cannot change the strengthening drying rate, which means that particle drying requires a long time and consumes a lot of energy. If microwave heating is used to dry low moisture content granular materials, the desired uniformly dried granular products can be quickly obtained. Based on this, the following granulation drying route is proposed: constant speed drying stage in the early stage of powder (i.e. drying the powder to a moisture content below 20%) - roller pressure granulation - particle microwave heating drying. This granulation drying route is scientifically reasonable and economically efficient.
2. Pre drying of powder granulation - final moisture content controlled at 10%~20%
Most powders are dry powders with an average particle size of less than 100 μ m obtained by drying solid material solutions, filter cakes, and paste like raw materials. Powder materials such as fertilizers, dyes and their additives, food and its additives, polymer resins, etc., are dried from an initial moisture content (30%~80%) to a final moisture content of 0.02%~9%, with an average particle size ranging from 5 μ m to 50 μ m, requiring high evaporation intensity from the dryer. Most of the time is spent on 20% moisture after drying. Like white carbon black powder, it is dried from a paste containing 80% water to a powder containing 6% water, with a particle size of 45 μ m and a bulk density of 240kgm-3. It is dried in an enhanced boiling dryer with a diameter of 150mm, an inlet temperature of 300oC, and an hourly output of 5kg. Black charcoal black is dried from a paste containing 92% water to a powder containing 2% water. It is then dried in an enhanced boiling dryer with a diameter of 150mm and an inlet temperature of 300oC. Only 1.3kg of dry powder can be obtained per hour. According to the drying mechanism and experimental drying curve analysis, it can be concluded that during the constant speed drying stage, the heat of the hot air is transferred to the surface of the material, causing rapid evaporation of surface free moisture and a decrease in skin moisture. The material begins to heat up and forms a temperature gradient inside. Heat is transferred from the outside to the inside, and moisture migrates from the inside of the material to the surface. The main driving force for moisture migration is diffusion, capillary flow, and internal pressure generated by the volume shrinkage of the material during the drying process. So when the critical moisture content occurs and the material dries to a very low final moisture content, internal moisture migration becomes the controlling factor. Some external variables, such as the amount of hot air, cannot enhance the rate of moisture migration due to temperature. Only by applying vibration, pulses, ultrasound, and other means can the diffusion of moisture inside be promoted. So when the moisture content of general materials is below 20%, drying is difficult, which is called the slow drying stage. At this stage, the drying time is long and energy consumption is high. In order to reduce the load of general dryers and maintain specific moisture content requirements for materials, it has been made easier to process, shape or pelletize. At present, in order to reduce the loss of raw materials and the pollution of dust to the environment, countries around the world require the granulation of powder raw materials with particle sizes ranging from 20 mesh to 8mm. When the moisture content of the powder material is between 10% and 20%, it is beneficial for pressure granulation. Heating and drying materials with high moisture content using convection, conduction, and radiation to complete the constant speed drying stage (i.e., the final moisture content is between 10% and 20%) is terminated, and the resulting loose wet material is granulated. This type of material is easy to form into particles, and the particles are uniform without dust flying, resulting in minimal material loss and environmental impact.
3. Powder pressure granulation method
Research on powder granulation began in the 1970s, with thousands of customers sending thousands of dry powder materials for granulation experiments. At that time, granulation experiments were conducted on these materials without adding any binders, known as high-pressure dry granulation, which achieved gratifying results. The vast majority of dry powders can produce various sizes of particles with qualified strength, but the granulation rate is not very high. Some dry powders are pelletized under strong pressure (350MPa~560MPa), but their granulation rate is less than 70%, and the return rate is over 30%. Therefore, the single machine granulation output is affected, and in the granulation project, dust flies during screening, causing environmental pollution. If about 10% liquid binder (such as water, organic solvents, etc.) is added to the dry powder, the continuous rolling granulation process using a roller press machine will be smooth, with high granulation rate, uniform particle size, high strength, and no dust flying during the granulation process.
In addition, during wet rolling granulation, it is required that the moisture content of the loose wet material be between 10% and 20%, up to 30%. There are strict requirements for the particle size distribution of the powder, such as a particle size limit of 30-50 mesh, and at least 25% of the fine powder particles must be smaller than 200 mesh. In the feed for iron ore granulation, fine powder with a mesh size less than 325 should account for 40% to 80%. However, there is no requirement for continuous granulation of fine powder with a moisture content of about 10% using a roller press. The pressure range is 2.5~140MPa, which can form particles and greatly reduce energy consumption, with an energy consumption of (2~4) kw/ht. If no additional binders, lubricants, plasticizers, wetting agents, disinfectants, etc. are added before granulation, and wet materials (with a moisture content between 10% and 20%) are directly used for continuous granulation by roller pressing, it is not only scientifically reasonable but also has significant economic benefits.
4. Microwave heating for final drying of granular materials
Microwave heating is no longer unfamiliar to people, and household microwave ovens are very proficiently used by urban residents. Especially for the reheating of cooked food, we all realize that the microwave heating speed is fast. For example, when heating Mantou, bread and steamed buns, the above food will be soaked in water first, and then heated in the microwave oven, so that not only the heating time is short, but also the steamed buns and Mantou will be soft and will not become hard. Why is wet Mantou hotter and faster than dry Mantou in the microwave oven? Why is it soft but not hard? To answer these questions, one should understand the relationship between microwaves and materials, as well as the characteristics of microwave heating.
The so-called microwave refers to electromagnetic waves with a frequency range of 3 × 108Hz~3 × 1011Hz and a wavelength range of 1m~1mm. There is a mutual relationship between matter and electromagnetic waves as follows:
(1) Conductor: This type of substance reflects electromagnetic waves, such as the metal shell of a microwave oven, stores microwave energy, and prevents microwave leakage.
(2) Insulator: This type of substance does not reflect or absorb microwaves and is transparent to them, such as microwave bowls and boxes, which are made of insulators such as glass, ceramics, polytetrafluoroethylene resin, etc.
(3) Dielectric: This type of substance absorbs microwave energy to varying degrees and converts it into thermal energy. Among them, water has a high dielectric parameter, which makes it easy to absorb microwave energy and convert it into thermal energy.
(4) Ferrite: This type of substance also absorbs, reflects, and penetrates electromagnetic waves, interacts with the magnetic field component of the electromagnetic waves, and generates heat.
Generally, powder materials are dielectric materials, and their moisture content is mostly water or organic solvents, such as ethanol. The dielectric parameters of the moisture content are much higher than those of the solid. For example, water has a dielectric parameter of around 80, while dry sand has a dielectric parameter of only 2.55. It can be simply said that water has a microwave energy absorption ability that is more than 30 times higher than dry sand. It can be considered that when microwave heating is used for drying, most of the microwave energy is consumed in the moisture to be removed.
Microwaves have wave particle duality. According to quantum theory, the energy of electromagnetic radiation is not continuous, but composed of individual "energy quanta", each quantum having energy proportional to its frequency.
In equation E=hf (1), h=6.626 × 10-34J S Planck constant. This energy can be converted into thermal energy in dielectric materials. There are many mechanisms for energy conversion, such as ion conduction, dipole rotation, interface magnetization, hysteresis, piezoelectricity, electrostriction, nuclear magnetic resonance, etc. Ionic conduction and dipole rotation are the dominant reasons for dielectric heating.
Ionic conduction: Charged particles (such as Na+, Cl-1, (H3O)+, and OH-1 ions in sodium chloride solution) will not be accelerated under the action of an external electric field and move in the opposite direction of their polarity. Macroscopically, they exhibit conductive current. These ions will collide with other particles around them during their movement and transfer kinetic energy to these particles, intensifying their motion. If it is in a high-frequency alternating electric field, the particles in the material will undergo repeated directional changes, resulting in intensified collisions, heat dissipation, and energy conversion. The power generated per unit volume is: Hg (2), where: PV： The power generated per unit volume, W/m3； E： For the electromagnetic field intensity vector, V/m； σ： For conductivity, S/m； Dipole rotation: Under the action of an external electric field, dielectric materials will undergo displacement polarization (non-polar molecular dielectric materials) and turning polarization (polar molecular dielectric materials). If the dielectric material is repeatedly polarized in an alternating external electric field, the dipole will continuously undergo "orientation" and "relaxation". Therefore, due to the original thermal motion of the molecule and the interaction between adjacent molecules, the regular motion of the molecule with the external electric field is disturbed and hindered, resulting in a "friction effect". As a result, some of the energy is converted into kinetic energy of molecular thermal motion, which is manifested in the form of heat, causing the temperature of the material to rise.
The power per unit volume change is: DG water is a polar molecule, and its relative dielectric constant 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 exhibit strong dielectric properties. Both wet materials containing water and solvents are suitable for microwave drying. This type of wet material can generate heat on-site in a microwave field, with both internal and external heating. The temperature of the moisture in the wet material rapidly increases and vaporizes. And solid powder only consumes a small amount of microwave energy.
The rolling method described earlier for continuous granulation of loose wet materials with a moisture content of 10%~20% is both scientific and economical. However, what heating method is used to dry the moisture content in the particles? According to the second part of this article, it is known that when the constant speed drying stage of the material ends, it enters the slow drying stage. Heating through convection, conduction, and radiation cannot enhance drying, and only microwave energy can effectively vaporize internal moisture. Due to the "internal and external heat" of the material in the microwave field, the moisture inside the material quickly reaches the boiling point and undergoes high-intensity evaporation. The material's texture hinders the flow of moisture, resulting in a pressure gradient inside the material. As the surface heat of the "internal and external heat" is easily dissipated, a positive temperature gradient and humidity gradient are formed inside the material. All three states can promote moisture to move towards the surface of the material in the form of liquid, vapor, or molecular flow. Greatly improve the drying speed during the deceleration drying stage, enabling materials that are difficult to dry to be quickly and uniformly dried.
Microwave heating has many advantages for drying granular and low moisture content materials, such as fast drying speed, uniform drying, energy saving, and moisture content control. Improve quality (food and medicine will not have secondary contamination and can be sterilized). So what is the development status of microwave drying in China?
