Analysis of Technical Requirements for Diatomite Algae Type

Characteristics and classification of Diatomite algal types

The morphological characteristics of the main algal types

The performance of diatomite largely depends on the type of diatoms it is composed of. Based on microscopic observation and taxonomic research, Lycophyllum procellatum, Lycophyllum procellatum, Lycophyllum procellatum, and Lycophyllum procellatum are considered the most suitable algal types as carriers for vanadium catalysts. These algal types possess unique morphological characteristics and structural advantages.

Melosira:

The cells are cylindrical, with a diameter of approximately 12μm, a length of 15μm, and a cell wall thickness of 0.5μm
The shell surfaces are interlocked to form a chain-like group, featuring micropores arranged parallel to the long axis
There are approximately 12 micropores per micrometer, and each micropore contains several micropores, with a pore diameter of about 0.1 μm (1000 Å).
The wall thickness of the cylinder is 0.8-2μm, and there are approximately 24-30 rows of micro-holes on the shell wall
The pore structure of linear algae is regarded as the best among all algal types

Coscinodiscus:

The cell is disc-shaped and divided into an upper shell and a lower shell. The upper shell is embedded in the shell ring of the lower shell
The diameter of the shell varies greatly, ranging from 70 to 250μm
The shell surface has concentric radial micropores, shaped like a honeycomb, with a central pore diameter of 5μm and an edge pore diameter of 2μm
There are multiple micropores within the micrometer range, with a pore spacing of approximately 1μm. On top of each group of pores, there is often a layer of opal film that is only about 1μm thick
Its diameter is larger than that of linear algae, but its pore size is smaller. The micropores are dense and evenly distributed

Cyclotella:

Single cells or 2-3 cells connected, the cells are disc-shaped
The pattern features on the shell surface: The outer area is ribbed, while the central area is either plain or dotted
It mainly lives in freshwater and is one of the common algal types in diatomite

Fragilaria:

The cells are spindle-shaped, approximately 20-30μm in length and 5-8μm in width
There are micro-pores perpendicular to the long axis on the shell surface, which are few and sparse
The shell surfaces are often connected to form a band-like group

Synedra:

The cells are long lanceolate, with a pointed tip at the end, of equal thickness, and slightly thinner at both ends
The micro-holes on the shell wall are arranged longitudinally

Pore size distribution requirements

Influencing factors of pore size distribution

The pore size distribution of diatomite is one of the key parameters affecting its application performance in the sulfuric acid industry. Diatomite of different algal types has different pore size characteristics, which directly affect its performance in applications such as filtration and catalysis.
According to research data, the pore size distribution of diatomite has the following characteristics:

Overall pore size range: The pore size range of diatomite is very wide, ranging from the nanometer level to the micrometer level, mainly concentrated between 0.1 and 10μm. Among them, the small hole diameter is 20-50nm, the large hole diameter is 100-300nm, and the porosity is as high as 90%.

Algae type differences:

Linear algae: Pore size is approximately 0.1μm, belonging to the micropore range
Rhododendrons: The central pore size is 5μm, and the marginal pore size is 2μm, belonging to the mesoporous to macroporous range
Overall diatomite: Pore size range 0.1-3.0μm

Origin differences:

There are significant differences in the main pore diameters of diatomite from different origins
In Changbai and Linjiang of Jilin Province and Tengchong of Yunnan Province, the main aperture ranges from 100 to 800nm
Linqu, Shandong: Main aperture 50-500nm
Dunhua, Jilin: Main aperture 50-100nm
Zhejiang Shengxian and Yunnan Xundian: The main aperture ranges from 50 to 800nm

Aperture requirements for different application scenarios

Different application scenarios have different requirements for the pore size distribution of diatomite:

Catalyst carrier application:

The pore size distribution should be between 0.1 and 1.5μm, with a relatively wide pore size distribution range

Function: The moderate pore size is conducive to the diffusion and mass transfer of reactants and products, while providing sufficient specific surface area for the loading of active components
The pore size distribution of domestic diatomite is relatively narrow (mainly 0.5nm-0.5μm), while that of imported diatomite is relatively wide (mainly 0.1-1.5μm). This is an area that needs improvement when domestic diatomite is applied as a catalyst carrier

Filtration application:

The appropriate pore size should be selected based on the filtration accuracy requirements. Generally, it is required to be able to retain particles larger than 0.5μm

Function: A larger pore size is conducive to increasing the filtration speed, while a smaller pore size is beneficial to improving the filtration accuracy

Application: In the production of sulfuric acid, it is mainly used to remove fine particles of 0.5μm, so diatomite with a pore size distribution of 0.5-5μm is required

Adsorption application:

It is required to have a rich microporous (<2nm) and mesoporous (2-50nm) structure
Function: Micropores are mainly used for adsorbing small molecule substances, while mesopores are used for the adsorption and mass transfer of large molecule substances
Studies have shown that diatomite, mainly composed of micropores, has a better adsorption effect on small molecule substances (such as dye molecules) than mesoporous or macroporous materials

The regulation technology of aperture distribution

To meet the demands of different application scenarios, it is necessary to regulate the pore size distribution of diatomite. The main regulatory technologies include:

Acid washing modification: By treating with strong acids such as sulfuric acid and hydrochloric acid, impurities in diatomite can be removed and the pore size can be expanded at the same time. Studies have shown that acid treatment can reduce or eliminate other oxides in diatomite relative to SiO₂, thereby increasing its surface area and adsorption capacity.

Heat treatment modification: The pore structure of diatomite can be altered through high-temperature calcination (typically 500-1000℃). Moderate calcination can remove organic matter and increase porosity. However, excessively high temperatures may cause the collapse of the pore structure.

Mechanical treatment: Through mechanical methods such as grinding and sieving, the particle size distribution of diatomite can be adjusted, which indirectly affects its pore size distribution.
Chemical modification: By treating with chemical reagents such as surfactants and polymers, the pore size distribution of diatomite can be regulated. For instance, diatomite materials with a specific pore size distribution can be prepared by using the template method.

Specific surface area requirement

Measurement and characterization of specific surface area

Specific surface area is an important indicator for evaluating the performance of diatomite, directly affecting its effects in applications such as catalysis, adsorption, and filtration. The specific surface area of diatomite is usually determined by the BET (Brunauer-Emmett-Teller) method. According to literature reports, the specific surface area of diatomite ranges widely, from 10 to 200 m²/g. Some specially treated diatomites can even have a specific surface area of over 200 m²/g.
There are significant differences in the specific surface area of diatomite from different origins:

As shown in the table above, the diatomite from Linqu, Shandong Province, has the highest specific surface area (63.8 m²/g), which is related to its unique geological formation and algal composition. In contrast, the diatomite from parts of Jilin and Yunnan Provinces has a lower specific surface area (19.7-32.5 m²/g), but the diatomite from these regions typically has a higher SiO₂ content and better chemical stability.

The Impact of Specific Surface Area on Application Performance

Specific surface area has a significant impact on the application performance of diatomaceous earth in the sulfuric acid industry:

Catalyst Support Applications:

A high specific surface area is beneficial for the dispersion and loading of active components, improving catalyst activity.

Generally, the specific surface area of diatomaceous earth used as a catalyst support is required to be ≥20 m²/g.

Studies have shown that diatomaceous earth with a larger specific surface area may provide more reactive sites, but may also reduce thermal stability.

Adsorption Applications:

Specific surface area is a key factor affecting adsorption capacity; diatomaceous earth with a specific surface area greater than 50 m²/g has higher adsorption potential.
The ideal specific surface area of diatomaceous earth is generally required to be between 50-500 m²/g.
Diatomaceous earth typically has a specific surface area between 100-500 m²/g, giving it excellent application potential in adsorption, filtration, and catalyst support.

Filtration Applications:

A larger specific surface area can provide more adsorption sites, improving filtration efficiency.

However, an excessively high specific surface area may lead to increased filtration resistance, affecting filtration speed.

Specific Surface Area Optimization Strategies

To obtain optimal application performance, the specific surface area of diatomaceous earth needs to be optimized according to the specific application scenario:

Raw Material Selection: Prioritize diatomaceous earth raw materials with a suitable specific surface area. For catalyst supports, diatomaceous earth with a specific surface area of 20-60 m²/g is generally selected; for adsorption applications, diatomaceous earth with a specific surface area of  50-200 m²/g is selected.

Modification Treatment:

Acid Washing: Can remove impurities and increase specific surface area. Studies have shown that the specific surface area of 
Diatomaceous earth treated with sulfuric acid can increase by 25%.

Heat Treatment: Moderate calcination (600-800℃) can remove organic matter and increase specific surface area; however, excessively high temperatures may damage the pore structure.

Surface Modification: Chemical treatment can increase surface active sites and improve specific surface area.

Combined Modification: Combining multiple modification methods, such as "acid washing + heat treatment + surface modification," can achieve better results. For example, the "chemical treatment-physical centrifugation-cavitation synergistic technology" developed by the Hefei Institutes of Physical Science, Chinese Academy of Sciences, increased the proportion of mesopores (1-100nm) by 40% and the specific surface area by 25%.

Silicon Content and Other Chemical Composition Requirements

Importance of Silicon Content

Silicon content (SiO₂content) is the primary indicator for evaluating the quality of diatomaceous earth. The chemical composition of diatomaceous earth is mainly SiO₂, containing small amounts of Al₂O₃, Fe₂O₃, CaO, MgO, K₂O, Na₂O, P₂O₅, and organic matter. The SiO₂ content is typically above 60%, and in high-quality diatomaceous earth mines it can reach around 90%.

Different applications have different requirements for SiO₂content:

Catalyst support application:

Basic requirement: SiO₂ > 65%

Premium requirement: SiO₂ > 90%

Specific requirements: For diatomaceous earth used as a catalyst support, the SiO₂content should be ≥86%, Fe₂O₃content ≤1.5%, and Al₂O₃content ≤3.0%.

Filter aid application:

General requirement: SiO₂ content ≥88% (after acid washing and drying)

Domestic standard: GB/T 24265-2022 "Industrial Diatomaceous Earth" requires a silica (SiO₂) content of ≥85% for premium products.

Other applications:

Insulation materials: The SiO₂ content requirement is relatively low, generally 60-80%.

Filler application: Depending on specific requirements, the SiO₂content is between 70-90%.

Impact of Impurities

Impurities in diatomaceous earth significantly affect its application performance. The main impurities and their effects are as follows:

Fe₂O₃ (Iron Oxide):

Content Requirements: Generally, Fe₂O₃ < 1%-4%

High-quality diatomaceous earth requires Fe₂O₃ content between 1%-1.5%

Impact: Excessive Fe₂O₃ content affects the whiteness of diatomaceous earth and may react with active components at high temperatures, affecting catalyst performance.

Al₂O₃ (Alumina):

Content Requirements: Generally, Al₂O₃between 3%-6%

Impact: Appropriate amounts of Al₂O₃ can provide acidic sites, which are beneficial for certain catalytic reactions; however, excessively high Al₂O₃ content will reduce SiO₂ content, affecting chemical stability.

Other Metal Oxides:

CaO, MgO: Generally, content < 1%; excessive content will affect acid resistance.

K₂O, Na₂O: Generally < 1% 1% Too high a concentration will increase water solubility and affect application performance.

Loss on ignition:

Requirement: Loss on ignition < 10%

Impact: Loss on ignition mainly reflects the content of organic matter and water of crystallization. Too high a concentration will reduce the density and strength of diatomaceous earth.

Regional differences in chemical composition

Significant regional differences exist in the chemical composition of diatomaceous earth from major producing areas in China.

As shown in the table above, diatomaceous earth from Changbai and Linjiang in Jilin Province, and Tengchong in Yunnan Province is of the high-silicon, low-iron type, with SiO₂ content approaching 90% and Fe₂O₃ content around 1%, making it an ideal raw material for producing high-purity products. Diatomaceous earth from Linqi in Shandong Province, Xundian in Yunnan Province, and Shengxian in Zhejiang Province is of the low-silicon, high-iron, and aluminum type. Although its SiO₂ content is lower, its Al₂O₃ content is higher, which may offer unique advantages in certain catalytic applications requiring acidic sites.

Control and Optimization of Chemical Composition

To meet the needs of different application scenarios, the chemical composition of diatomaceous earth needs to be controlled and optimized:

Raw Material Selection: Select diatomaceous earth raw materials from suitable origins based on application requirements. For example, diatomaceous earth from Changbai in Jilin Province and Tengchong in Yunnan Province should be selected for producing high-purity catalyst supports; diatomaceous earth from Linqi in Shandong Province and Xundian in Yunnan Province can be selected for producing catalysts with special properties.

Purification Technologies:

Acid Laying: This method uses strong acids such as sulfuric acid and hydrochloric acid to remove impurities like Fe₂O₃ and Al₂O₃. Studies show that approximately 68% of domestic diatomaceous earth producers have adopted hydrochloric acid or sulfuric acid leaching processes. Some high-end products even combine this with hydrofluoric acid for deep impurity removal, increasing the silica content from 70%-85% in the raw ore to over 95%.

Magnetic Separation: This method uses a magnetic field to remove magnetic impurities, primarily Fe₂O₃.

Flotation: This method utilizes the differences in surface properties of different minerals for separation.

Composition Blending: Diatomaceous earth from different origins can be blended to obtain the optimal chemical composition based on application requirements. For example, mixing high-silicon, low-iron Jilin diatomaceous earth with high-alumina Shandong diatomaceous earth in a certain proportion can increase acidic sites while maintaining a high silica content.

Quality Control: A rigorous quality testing system is established, with regular testing of the content of major components such as SiO₂, Al₂O₃, and Fe₂O₃. Testing methods include X-ray fluorescence spectrometry (XRF) and chemical analysis.

Summary

The core technical requirements for diatomaceous earth encompass four aspects: Algal type, with five types including linear algae being the most suitable as vanadium catalyst carriers, and linear algae exhibiting the optimal pore structure; pore size distribution is influenced by algal type and origin, with varying requirements for different applications, which can be controlled through acid washing and heat treatment; specific surface area varies significantly by region, affecting catalytic performance and requiring optimization based on the application scenario; silicon content is the primary quality indicator, with varying requirements for different applications; impurities affect performance and can be controlled through raw material selection and purification; significant differences in chemical composition across regions necessitate targeted selection.