Abstract: Starting from discussing the technical characteristics of powder drying, powder granulation, and particle drying, this article attempts to scientifically establish a new process route for powder pre drying, granulation, and microwave heating drying. Start drying the material at a constant speed stage to remove most of the moisture and become a material with a moisture content below 20% (wet basis). Materials with low moisture content are granulated using pressure method, which not only facilitates granulation with high granulation rate, but also has uniform particle size and high strength. Low moisture content granular materials are dried by microwave heating. The characteristics of microwave heating enable the rapid elimination of moisture content inside the particles, thereby obtaining products that meet the moisture content requirements. The article comprehensively discusses the scientificity, technicality, reliability, and economy of the new process route.
Keywords: Powder drying; Granulation; microwave drying
1. Overview
With the needs of industrial development, more and more types of powder materials are being used in industrial sectors such as chemical, food, medicine, electronics, plastics, and biochemistry, with increasingly fine particle sizes. 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 white carbon black is 45 μ m, and so on. Modern rapid development of ultra-fine powder - nano powder. Fine grained powder has a low packing density and light weight, making it prone to flying during operation. This not only causes material loss but also pollutes the environment, with toxic and irritating fine powder escaping, which can cause serious harm to the environment. Due to the low packing density of fine powder, it is inconvenient to transport. Fine chemical fertilizers applied in the field are prone to loss, which creates a negative side. Therefore, there is a need to aggregate small powders into larger entities - granulation. There are rolling method and pressure method for granulation. Rolling granulation is the process of adding loose wet materials (fine powder and an appropriate amount of wetting solution) into the granulation device, stirring and flipping to initially form a agglomerate core. Subsequently, the core grows in two ways: agglomeration and cladding, and the agglomerated particles are spherical and irregular, with a rough surface. The surface of the coating granulation is smooth and spherical, with a cross section consisting of layers of onion skin. The operating conditions can be controlled to make one of the methods 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 particles under pressure or mainly shear force in pelletizers such as tablet presses, rolling presses, roller presses, and screw extruders. The roller press can achieve strong pressure granulation. The pressure range is 2.5MPa~560MPa, which can compact the powder and make the molecular forces between the powders play a dominant role, allowing the particles to achieve greater tensile, compressive, and wear resistance. In the past forty years, over a thousand types of fine powder dry materials have been subjected to strong pressure granulation experiments, and successful granulation data have been obtained. However, it is difficult to perform pressure granulation on 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 fluidity. 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. If strong granulation pressure is not required, high granulation rate, large 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 water on the surface of the particles, and the migration of internal moisture becomes a controlling factor. At this time, external variable conditions cannot change the enhanced drying rate, that is, particle drying requires a long time and consumes a lot of energy. If microwave heating is used to dry granular materials with low moisture content, the desired uniformly dry particle products can be quickly obtained. Based on this, the following granulation and drying route is proposed for the constant speed drying stage of the powder in the early stage (i.e. drying the powder to a moisture content below 20%) - granulation using the roller pressure method - and particle microwave heating drying. This granulation drying route is scientifically reasonable and has high economic benefits.
2. Drying in the early stage of powder granulation - controlling the final moisture content between 10% and 20%
Most powders are dry powders with an average particle size of less than 100 μ m obtained by drying solutions, filter cakes, and paste like raw materials of solid materials. Powders such as fertilizers, dyes and their additives, food and their additives, polymer resins, etc. are all dried from an initial moisture content (30%~80%) to a final moisture content of 0.02%~9%. Their average particle size is within the range of 5 μ m~50 μ m, requiring high evaporation intensity in the dryer. Most of the time is spent with 20% moisture after drying. For example, white carbon black powder is a paste with 80% moisture content, dried to a moisture content of 6%, with a particle size of 45 μ m and a packing 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 carbon black is dried from a paste containing 92% water to a powder containing 2% water. It is dried in an enhanced boiling dryer with a drying 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 water and a decrease in skin moisture. The material begins to heat up and forms a temperature gradient inside. The heat is transferred from the outside to the inside, and moisture migrates from the inside of the material to the surface. The driving force for moisture migration mainly relies on diffusion, capillary flow, and internal pressure generated by material volume shrinkage 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 a controlling factor. Some external variables, such as the amount of hot air, cannot enhance the moisture migration rate due to temperature. Only by applying vibration, pulses, ultrasound and other means can the diffusion of internal moisture be promoted. So when the moisture content of general materials is below 20%, drying is difficult, which is called the rate reduction drying stage. At this stage, the drying time is long and energy consumption is high. To reduce the load of general dryers, and on the other hand, to maintain specific moisture content requirements for materials, it is convenient for processing, molding, or granulation. At present, countries around the world require the granulation of powdered raw materials with a particle size range of 20 to 8mm in order to reduce the loss of raw materials and the pollution of dust on the environment. When the moisture content of the powder material is between 10% and 20%, it is beneficial for pressure granulation. Heat and dry materials with high moisture content using convection, conduction, and radiation, and terminate the constant rate drying stage (i.e., the final moisture content is between 10% and 20%) to obtain loose wet materials for granulation. This type of material is easy to form particles, and the particles are uniform without dust flying, which means that the material loss is small and the environment is clean.
3. Granulation using powder pressure method
Starting from the 1970s, research on powder granulation began. Thousands of customers sent thousands of dry powder materials for granulation experiments. At that time, these materials were subjected to granulation experiments without any binders, known as high-pressure dry granulation, which achieved gratifying results. The vast majority of dry powder can produce particles of various sizes with qualified strength, but the granulation rate is not very high. Some dry powders are granulated under strong pressure (350MPa~560MPa), with a granulation rate of less than 70% and a return rate of over 30%. Therefore, the single machine granulation yield is affected, and in granulation engineering, 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 pair of roller presses is smooth. Not only is the granulation rate high, but the particle size is uniform and the strength is high, and there is no dust flying during the granulation process.
In addition, during wet rolling granulation, loose wet materials are required to have a moisture content of 10% to 20%, up to 30%, and the particle size distribution of the powder has strict requirements. For example, if the upper limit of particle size is 30-50 mesh, at least 25% of the fine powder particles must have a particle size less than 200 mesh. In the feeding of iron ore granulation, fine powder with a mesh size of less than 325 should account for 40% to 80%. 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/h. If binders, lubricants, plasticizers, wetting agents, fungicides, etc. are not added before granulation, and wet materials (with a moisture content between 10% and 20%) are directly used for rolling continuous granulation, 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 is an electromagnetic wave with a frequency range of 3 × 108Hz~3 × 1011Hz and a wavelength of 1m~1mm. There are the following interrelationships between matter and electromagnetic waves:
(1) Conductor: This type of material reflects electromagnetic waves, such as the metal shell of a microwave oven, which stores microwave energy and prevents microwave leakage.
(2) Insulator: This type of material does not reflect or absorb microwaves, and is transparent to microwaves. For example, microwave bowls and boxes are made of insulators such as glass, ceramics, polytetrafluoroethylene resin, etc.
(3) Dielectric material: This type of material absorbs microwave energy to varying degrees and converts it into thermal energy. Among them, water has a large dielectric parameter, which makes it easy to absorb microwave energy and convert it into thermal energy.
(4) Ferrite: This type of material also absorbs, reflects, and penetrates electromagnetic waves, reacting with the magnetic field components of electromagnetic waves to generate heat.
Generally, powder materials are dielectric materials, and their wet content is mostly water or organic solvents, such as ethanol. The dielectric parameters of wet content are far greater than those of solids, such as water at around 80 and dry sand at only 2.55. It can be simply said that water has more than 30 times the ability to absorb microwave energy than dry sand. It can be considered that during microwave heating and drying, most of the microwave energy is consumed in the moisture content to be removed.
Microwave has wave particle duality. According to quantum theory, the energy of electromagnetic radiation is not continuous, but composed of individual "energy particles", each quantum having energy proportional to its frequency.
E=hf (1)
In the formula, 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. Among them, ion conduction and dipole rotation are the dominant reasons for dielectric heating.
Ionic conduction: charged particles (such as sodium chloride solution containing Na+ Cl-1 、 (H3O)+ OH-1 (four types of ions) will not be accelerated under the action of an external electric field, and will move in the opposite direction of its polarity. On a macroscopic level, it manifests as a conductive current. These ions will collide with other particles around them during their movement, while transferring kinetic energy to these particles, intensifying their motion. If it is in a high-frequency alternating electric field, particles in the material will undergo repeated directional movements, leading to intensified collisions, resulting in heat dissipation and energy conversion. The power generated per unit volume is:
Hg (2)
In the formula: PV: Power generated per unit volume, W/m3; E: is the electromagnetic field intensity vector, V/m; σ: is the conductivity, S/m；
Dipole rotation: Under the action of an external electric field, the dielectric material undergoes displacement polarization (unpolarized molecular dielectric) and turning polarization (polarized molecular dielectric). If the dielectric material is repeatedly polarized in an alternating external electric field, the dipole continuously undergoes "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, a part of the energy is converted into the kinetic energy of the molecular thermal motion, which is manifested in the form of heat, causing the material temperature 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 dielectric materials. The loss factor of water is similar to that of other dielectric materials, so its tan δ value is high. Other liquids (such as ethanol and some organic solvents) also exhibit strong dielectric properties. Therefore, both wet materials containing water and solvents are suitable for microwave heating and drying. This type of wet material can generate heat on site and heat both inside and outside in a microwave field. The temperature of moisture in wet materials rapidly increases and vaporizes. And solid powder only consumes a small amount of microwave energy.
The continuous granulation of loose wet materials with a moisture content of 10% to 20% using the rolling method described earlier 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 stage of material drying ends, it enters the deceleration drying stage. Heating with convection, conduction, and radiation cannot enhance drying, and only microwaves can effectively vaporize internal moisture. Because the material is heated both inside and outside in the microwave field, the moisture inside the material quickly reaches its boiling point and undergoes high-intensity evaporation. The texture of the material hinders the flow of moisture, resulting in a pressure gradient inside the material. Due to the easy dissipation of surface heat, a positive temperature gradient and humidity gradient are formed inside the material. All three states can promote the movement of moisture to the surface of the material in the form of liquid, vapor, or molecular flow. Greatly improve the drying speed of the deceleration drying stage, so that difficult to dry materials can be quickly and uniformly dried.
Microwave heating has many advantages in drying granular and low moisture content materials, such as fast drying speed, uniform drying, energy conservation, and moisture content control. Improve quality (food and drugs will not undergo secondary pollution and can be sterilized). So what is the development of microwave drying in China?
