Talking about Powder Granulation and Coating (Part 2)
In (1), we mentioned the difference and connection between powder granulation and coating. When the substrate powder particles are too small, it is too difficult to wrap a coating film on the outer layer of a single powder. At the beginning, it must be a granulation process, and after forming particles of a certain size, the subsequent step is coating. Can you explain the powder (wet granulation) process in more detail
When talking about granulation technology, we often hear a word: slurry addition. Slurry, also known as slurry, can be a solvent or a combination of a solvent and some adhesive material.
How to add slurry depends on the equipment used and the characteristics of the materials. When using high shear granulation in the early days, a liquid inlet was left on the lid of the granulator and poured directly in. Later on, I felt that this method of pouring in all at once was a bit rough, and the particles obtained were uneven. I needed to be gentle, so I used atomization (dispersion) to add liquid inside, which significantly improved the effect.
Did you know?
Under the centrifugal force of the stirring blade, the material exhibits a state of more edges and less middle;
If a single fluid nozzle can be used, try not to use the form of two fluid compressed gas atomization as much as possible;
It is preferred to use a "one" shaped nozzle;
The atomization angle of the nozzle should be calculated based on the diameter, shape, and material loading height of the pot. The atomization should be as dispersed as possible and not sprayed onto the pot wall;
According to the rotation direction of the stirring blade, the nozzle should be placed downstream of the cutting blade, not too close. The ideal position is directly opposite the cutting blade, which not only ensures that the liquid droplets do not spray onto the cutting blade, but also leaves enough travel for the material to mix with the liquid droplets;
If using a "one" shaped nozzle, it is better to be perpendicular to the radius of the pot body;
If using a two fluid atomizing nozzle, the atomization angle should not be too large and should not be too close to the edge.
We always enjoy exploring.
Since atomization (dispersion) can achieve good results, would it be better to make the atomized droplets smaller? Fine atomization has limited improvement on the granulation effect of common varieties, but it is helpful for varieties with special requirements for disintegration and dissolution. Fine atomization usually uses two fluid or three fluid nozzles. Due to the short liquid addition time, a large amount of compressed gas entering the gas seal of high shear granulator related components is challenging.
A friend noticed why the word 'dispersion' was added after the previous atomization, which is different from fine atomization. Dispersion is achieved by the pressure exerted on the liquid through a peristaltic pump or the pressure inside a pressure tank, causing it to disperse into several hundred micrometers or larger when passing through a nozzle. During high shear granulation, liquid droplets are sprayed into the interior of the pot, and the powder adheres to the surface of the liquid. Then, under the action of the stirring blade and cutting blade, the droplet nuclei gradually disappear and are distributed on the surface of the powder. The powder becomes the core, and smaller droplets wet the surface of the powder. Through the bridging of the liquid, the powder is turned into large particles and soft materials. After shear granulation, it enters fluidized drying to obtain the desired granular product.
When the slurry is a solvent
Yellow: Material powder
Blue: Adhesive powder
Grey: solvent droplets
Finished particles: Blue adhesive connects and fills between yellow particles.
The slurry is a pre configured adhesive
Yellow: Material powder
Light Blue: Adhesive Droplets
Finished particles: The blue binder forms a film on the surface of the yellow particles and fills them.
For one-step granulation in fluidized beds, the slurry addition method is fine atomization, and the spray guns used are all two fluid or three fluid spray guns, which atomize by compressed gas. The particle size is in the tens of micrometers, and there is no shear force to disperse the droplets. No matter how much larger the droplet is than the powder, the droplet acts as a wetting surface and liquid bridge.
Slurry as solvent
Yellow: Material powder
Blue: Adhesive powder
Grey: solvent droplets
Finished particles: Blue adhesive connects and fills between yellow particles.
The slurry is a pre configured adhesive
Yellow: Material powder
Light Blue: Adhesive Droplets
Finished particles: The blue binder forms a film on the surface of the yellow particles and fills them.
During high shear granulation, it is necessary to remove excess moisture through fluidized bed boiling drying. In order to achieve the desired particle size distribution, it is often necessary to dry the entire pellet afterwards. Fluidized bed one-step granulation is relatively uniform and does not require dry granulation. The obtained particles can be directly packaged into granules or sent downstream for total mixing, capsule filling, or tablet pressing.
There are many means of powder granulation. High shear granulator and fluidized bed are typical wet granulation. In addition, swing granulator and spray drying are also available.
Unlike wet granulation, there is no substrate in the drying tower during spray drying granulation. For some special products, production equipment needs to fully consider material characteristics. For example, some products are heat sensitive and can only adopt lower inlet and outlet air temperatures; Some products are prone to oxidation and can only be produced in an inert environment; Some materials use organic solvents or hazardous solvents instead of water, which not only prevent exhaust emissions into the air, but also consider environmental protection, explosion, safety, and other risks; Some materials have a high specific gravity and are prone to precipitation in suspensions.
Today we will focus on the latter one.
Materials with a relatively high specific gravity are typically new materials, such as metal powders used in the new energy industry, anode materials, solid electrolytes, or functional ceramic materials. In the suspensions made of these materials, because of the large proportion, the spray drying tower is challenged.
In spray drying, atomization is the first step, and the upstream of atomization is liquid preparation and material liquid transportation. Leaving aside explosion prevention, the key to these two aspects is how to prevent the precipitation of high density particles. Although some excipients may be added to the solvent to increase viscosity, precipitation cannot be ignored. Our approach is:
From liquid preparation to liquid inlet, a stirring blade needs to be installed on the liquid preparation tank;
When transporting materials and liquids, regardless of the flow rate, try to use pipelines with finer inner diameters. If you want to increase the flow rate, you can increase the speed/pressure of the inlet pump to achieve this;
The path of the liquid conveying pipe should be as short as possible, and if necessary, an oscillator can be installed on the conveying pipe;
The above methods can effectively reduce the sedimentation of the liquid during the preparation and transportation process. For nozzles, if a two fluid nozzle is used, a cleaning needle can be configured if necessary to automatically clear nozzle blockages; For pressure nozzles, high pressure can effectively prevent nozzle clogging. The nozzle can be selected according to the characteristics of the material. Ceramic slurry has high solid content and poor flowability, and pressure nozzles are relatively suitable.
The direction of the spray and the direction of the hot air have the design of downstream and countercurrent. For materials with large specific gravity, the countercurrent form is used when necessary, that is, the nozzle is installed at the bottom of the spray drying tower, and the hot air enters the tower from above. This design can significantly increase the droplet travel (first upward and then downward), improve drying efficiency, and avoid the rapid drop of droplets to the lower end due to the gravity and wind speed during downstream design, where the droplets/powder enter the cyclone separator before being completely dried.
If the solvent is an organic solvent or the product is sensitive to oxygen, a nitrogen environment is necessary. The source of nitrogen can be nitrogen cylinders, industrial nitrogen, or nitrogen generators. Considering the cost, environment, and safety of using nitrogen, a closed-loop design is appropriate.
The particle size of spray granulation products depends on the size of droplets and solid content. It can be imagined that the larger the droplet, the less solvent it contains, and the larger the particles formed after drying. Therefore, to achieve the target particle, we must first start from these two points. However, these two points are related to equipment capabilities and product characteristics. Imagine: it would be pointless to spend a lot of effort adjusting the droplets to make them bigger, but not to dry them. Larger droplets often require higher evaporation capacity, which is related to tower height/volume, air volume, and inlet temperature and humidity.
There is a hierarchy in the way of hearing, and a specialization in one's profession. The author's contact with things is complex and limited. I hope friends will forgive me and take the essence from it roughly. Please correct the shortcomings or mistakes in the text.