What is the spray granulation process of ceramic powder?

In the industrial production of ceramics, whether it is the classic hot isostatic pressing sintering or the new sintering technology, the mainstream ceramic molding process is based on dry or wet powder processing

Various types of dry-pressed ceramic parts

Why granulate?

For ultrafine ceramic powders, the biggest problem is that ultrafine powders have a strong tendency to agglomerate.

To give an example: there are two key problems in the preparation process of silicon nitride ceramics, which need to be solved by the granulation of raw material powder

Method of granulation

Commonly used methods of granulation are: general granulation, pressure granulation, spray granulation and freeze granulation

1Spray granulation method

The spray granulation method is to mix the blank and plasticizer to form a slurry with a certain solid content, and then spray the slurry into the granulation tower with a sprayer for atomization, and then a spherical shape with better fluidity can be obtained.

Spray granulation flow chart

The biggest advantage of spray granulation is: spray granulation in production can avoid re-agglomeration or sedimentation of each component in the slurry, so that the slurry remains uniform; at the same time, the slurry is uniformly atomized, which can obtain uniform particle size distribution and fluidity.

2The process of spray granulation

The spray granulation process can be divided into four steps: the atomization of the powder slurry, the mixing of the atomized slurry with the heat medium, the evaporation of the liquid and the collection of the dried particles.

(1) Atomization

Atomization is to turn the slurry into a large number of droplets, which increases the specific surface area; in general, the size of the final granulated powder particles is related to the size of the droplets obtained by atomization, so the final granulation can be achieved by controlling the size of the droplets.

A.

B.
Pressure nozzle type means that by increasing the spray pressure, the slurry is sprayed into the chamber at a high speed for drying, and finally atomization is realized, and the size of the formed droplet is positively related to the slurry viscosity and the length of the nozzle, which is directly related to the atomization.
pressure is negatively correlated
.
However, the disadvantage of this process is that the high speed of the slurry spraying causes the nozzle to be worn, and further affects the size and size distribution of the droplets, thereby affecting the performance of the spray granulated powder
.

C.
Centrifugal atomization is to use the high-speed rotating turntable located in the middle of the powder drying chamber to accelerate the slurry, and make the slurry spread on the turntable under centrifugal action to form a film, and the slurry continuously moves to the edge of the turntable Movement; due to the different shearing force of the slurry at different positions and friction with the gas, the slurry is dispersed into fine mist droplets, which are then mixed with the drying gas
.
The size and size distribution of the droplets can be controlled by adjusting the size, rotation speed, slurry viscosity and feeding speed of the turntable
.

(2) Mixing of droplets and gas

The movement mode of the gas in the drying chamber can significantly affect the movement time, drying degree, and drying time of the droplets, and also has an effect on the adhesion degree of the obtained powder to the drying chamber wall; It will lead to local overheating in the drying chamber, which will eventually reduce the performance of the granulated powder
.
The relative motion of drying medium gas and slurry droplets in the drying chamber is different, and can be divided into three types: co-current, counter-current and mixed-current
.

(3) Drying of mist droplets

The drying of the droplet is the evaporation process of the liquid phase in the droplet, as shown in the figure below, which can be divided into three stages
.
A~B is the beginning of drying, at this time, the evaporation rate of the mist liquid phase increases rapidly and reaches the maximum value
.
Then B~C is called the constant drying stage.
At this time, the liquid phase moves from the inside of the droplet to the outer surface, so that the surface of the droplet is saturated with liquid phase, and the droplet maintains a lower temperature; the time required for this stage is different from that of the droplet.
The viscosity, liquid phase content and humidity of the gas are temperature dependent
.
When reaching the point C, the liquid phase content at the critical time is reached.
Since the liquid phase content in the droplet decreases at this time, the evaporation rate of the droplet decreases, which increases the temperature of the droplet and achieves deceleration drying from C to
E.
When point D is reached, the drying process gradually penetrates deep into the inside of the droplet, and the drying rate keeps decreasing, and the outer part forms a crust due to drying
.
In the drying process, the inlet and outlet temperatures have a great influence on drying
.
If the inlet temperature decreases, the droplets will not be completely dried, and the granulation powder will agglomerate; if the inlet temperature increases, the surface of the droplets will dry too quickly, and the surface will quickly form a hard shell, which will hinder the normal shrinkage of the droplets.
Hollow irregular granulated powder
.
The outlet temperature has an influence on the moisture content of the powder, which has a great influence on the performance of the powder molding
.

Evaporation process of water in mist droplets

3  Influencing factors of spray granulation

In the spray granulation process, there are many factors that can affect the morphology and performance of the final granulated powder.
The more important factors are the solid content of the slurry, the content of the binder, and the inlet and outlet temperatures
.

(1) Slurry solid phase content

The solid content of the granulation slurry can significantly affect the morphology of the granulated powder and the fluidity of the granulated powder
.
The granulation slurry with high solid content can obtain regular spherical particles, while the slurry with low solid content will lead to the increase of hollow particles in the granulated powder and the poor fluidity of the granulated powder
.
Since the solid phase content will affect the viscosity of the slurry, that is, the viscosity increases with the increase of the solid phase content
.
As mentioned above, when the solid content is low, the fluidity of the powder will be poor, and when the solid content is high, the nozzle of the granulator will be easily blocked, which will adversely affect the performance of the granulated powder
.
Generally speaking, the bulk density of granulated powder will increase with the increase of the solid content of the slurry
.

(2) Slurry binder content

The effect of binder content on granulation process is similar to that of solid phase content, and both are closely related to slurry viscosity
.
An increase in the binder content in the slurry will increase the viscosity
.
When the amount of binder added in the slurry is low, the proportion of fine particles in the granulated powder becomes larger, and the granulated powder particles have lower strength.
When the content is too low, the spray granulation cannot form a regular shape, and there is no granulation.
meaning
.
If the binder content is too high, on the one hand, it will cause the viscosity of the slurry to be too high, which is similar to the problem of high solid phase content; on the other hand, if the binder content is too high, impurities will remain in the sintering process of the green body.
phase, which adversely affects sintering
.

(3) Inlet and outlet temperature

The inlet and outlet temperature of the slurry has an important influence on the granulation powder and the granulation efficiency during drying
.
If the slurry feeding temperature is high, the effect of the binder will be weakened, and regular particles cannot be formed
.
If the temperature at the outlet is high, the drying rate of the droplets will be accelerated, which will make the granulated powder particles finer and increase the bulk density to a certain extent, but the spray will be blocked because the powder is too fine
.
If the temperature of the feed inlet is too low, the drying of the droplets will be too slow, the particles will be broken, and the fluidity of the powder will be poor
.

In addition, other factors, such as feeding speed, type and content of dispersant, and spray pressure, will also affect the performance of granulated powder
.

References:

1.
A letter to welcome the spring
.
Exploration on the preparation process of large-size and high-precision ceramic guide rails (Department of Materials Science and Engineering, Tsinghua University)

2.
Wang Tengfei, Zheng Yu, Zhuang Xinjiang,
etc.
Research progress of silicon nitride ceramic powder granulation (Beijing Sinoma Intraocular Lens Research Institute Co.
, Ltd.
)

3.
Yang Zhanhou, Xu Ziqin, Wang Juncheng,
etc.
Preparation and Process Research of Spherical Alumina Powder by Spray Granulation (Xianyang Nonmetallic Mineral Research and Design Institute Co.
, Ltd.
)