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Enhance Powder Flowability with ZLSIL™ & ZLCSIL™ Specialty Silica Solutions
Created on 05.17
Technical Manual for ZLSIL™ Specialty Silica and ZLCSIL™ Fumed Silica as Flow Aids and Anti-Caking Agents
Powder raw materials are widely and increasingly used in food, pharmaceutical, chemical, building materials, and many other industries. Excellent flowability is a core prerequisite for accurate powder handling, smooth silo discharge, and precise dosing. However, many powders are inherently highly cohesive and difficult to process. Meanwhile, affected by environmental humidity, temperature, pressure, and other climatic or operating conditions, most powders are extremely prone to caking during storage or transportation, which further increases the difficulty of application.
To address these industry pain points, ZLSIL™ specialty silica and ZLCSIL™ fumed silica series developed by Zhongqi (Shandong) Silicon Materials Co., Ltd. serve as high-efficiency flow aids and anti-caking agents. They target and solve the problems of poor flowability and easy caking in various powder systems, providing full-process stable guarantee for the processing, storage and transportation of powder products.
To accurately characterize the flow behavior of powders, a variety of standardized test methods are commonly used in the industry, with the core methods as follows:
This is a classic method for characterizing powder flowability. Powder is evenly dropped through a sieve onto the top of a metal cylinder, and the powder naturally forms a cone. When particles fall on the cone, they will adhere or roll according to the cone angle and particle viscosity. As particles continue to accumulate, the cone becomes steeper until gravity exceeds the inter-particle cohesion, finally forming a cone with a fixed height and slope. By measuring the height of the cone or the angle of the slope, the "angle of repose" of the powder can be obtained. The stronger the particle viscosity, the higher the angle of repose value, and the worse the powder flowability. Conversely, a lower angle of repose represents better powder flowability.
This is a rapid test method for powder flowability, which can be completed using a series of glass funnels with different outlet diameters. There are two core test modes: one is to determine whether the powder can flow out from the funnel of the corresponding caliber without interruption, so as to judge the flowability grade; the other is to measure the total time for the powder to pass through the funnel with a specific outlet diameter. The shorter the time, the better the powder flowability.
This is a more sensitive and easy-to-operate test method. During the test, the powder is poured into a nest of sieves (with the largest aperture sieve on the top), and the nest of sieves is oscillated for a fixed period of time. The powder will settle to the lower sieves according to its own flow characteristics. If the powder has extremely strong cohesion, most of the material will remain on the top sieve. The better the powder flowability, the larger the amount of material that passes through the sieve and settles, and the higher the proportion of material that finally falls into the sieve pan. After oscillation, the mass of residues on each layer of sieve is weighed, the mass of each layer is multiplied by the coefficient of the corresponding sieve and added up, and the final result is the quantitative measurement value of powder flowability.
This method has a more sophisticated test procedure, and the test data can be used to accurately calculate the design size of the silo to ensure smooth powder discharge. According to Jenike's classic theory, the ratio ffc of consolidation stress to bulk material strength is defined as powder flowability. The annular shear cell proposed by Schultze is also widely used in the industry as an optimized solution of the classic shear cell to further improve test accuracy.
According to the research of scholars such as Zimmermann, the tensile strength test is the core method to quantify the cohesion of powder in a low-density state. During the test, a probe with a thin layer of Vaseline film contacts the flat surface of the powder and is lifted at a constant speed. A high-sensitivity tensile strength tester records the force required to separate the upper layer of powder from the bottom layer. The smaller the force value, the lower the powder cohesion and the better the flowability.3 Mechanism of Action of Flow Aids
All powder particles adhere to each other through van der Waals forces. For ultrafine particles, the effect of van der Waals forces is much greater than the gravity that promotes particle separation and powder flow, which is the core reason for the generally poor flowability of fine powders.
Flow aids themselves are powder materials with extremely fine particle size, which can be evenly coated on the surface of host powder particles to increase the surface roughness of the particles, and the increase of surface roughness can significantly reduce the attractive force between two powder particles. ZLSIL™ specialty silica and ZLCSIL™ fumed silica can perfectly fit and cover the surface of host powder particles, effectively isolate the interaction force between particles, and greatly reduce the inter-particle attraction, which is the core advantage of them as high-efficiency flow aids and anti-caking agents.
In this manual, "caking" specifically refers to the phenomenon that the flowability of powder continues to decline over time due to long-term storage, and even forms an overall consolidated block in extreme cases. The corresponding "anti-caking agent" refers to a flow aid product that can continuously maintain good powder flowability during long-term storage.
To achieve the best flow improvement effect at a low addition level, the flow aid must be finely dispersed on the surface of the host powder. ZLSIL™ specialty silica and ZLCSIL™ fumed silica can easily achieve this fine dispersion through a simple mixing process, and are compatible with mixing equipment including plow shear mixers, paddle mixers or ribbon blenders.
Different models of ZLSIL™ and ZLCSIL™ products have differentiated dispersion characteristics. Professor Zimmermann's team at the University of Würzburg has conducted in-depth research on this effect. In the experiment, corn starch was used as the model matrix, and different models of ZLSIL™ and ZLCSIL™ products, as well as the industry-standard tricalcium phosphate (TCP) flow aid, were blended with corn starch for different mixing times using a Turbula® blender. Finally, the flowability of the system was evaluated through tensile strength test.
The experimental results showed that after short-time mixing, all tested flow aids could significantly improve the flowability of corn starch, among which hydrophobic products such as ZLSIL™ D 17 had a particularly outstanding improvement effect. However, when the mixing time was too long, the flowability of the system decreased again. The core reason is that the flow aid was over-dispersed on the surface of corn starch, resulting in reduced particle surface roughness and loss of flow modification effect.
The core reason for the poor flowability of moist powders is that the liquid film (the liquid can be water, aqueous solution or oil) existing on the surface of powder particles binds the particles together. Flow aids can improve the flowability by absorbing the liquid film on the particle surface.
To achieve efficient liquid absorption, flow aids must have high porosity and can absorb liquid into the pore structure through capillary action, which is one of the core reasons why ZLSIL™ specialty silica has become an excellent flow aid for moist powder systems with its high porosity characteristics. However, mixing conditions will directly affect the performance of the aid: if the silica agglomerates are over-depolymerized to submicron level, the pore structure will be destroyed and the absorption capacity will be greatly reduced. In other words, the fine dispersion that can improve the efficiency of the aid in dry powder systems will instead reduce the efficiency of the aid in moist powder systems.
Therefore, for moist powder systems, silica products with high porosity and stronger mechanical stability are more suitable. Zhongqi (Shandong) Silicon Materials Co., Ltd. has developed a variety of differentiated ZLSIL™ specialty silica and ZLCSIL™ fumed silica products for this scenario, which can accurately match the flow modification needs of different base powders.
A controlled experiment using moist sodium chloride mixture as the host powder showed that the moist salt system itself had extremely poor flowability, with a flow grade of 7 measured by the funnel method. After adding 0.6% of ZLSIL™ 22 S or ZLSIL™ 50 S, the flowability of the system could be increased to flow grade 2 within 1 minute of mixing. However, with the extension of mixing time, the silica was over-dispersed, the porosity was reduced, and the flowability of the system deteriorated again. Depending on the mixing intensity, the over-mixing effect of ZLSIL™ 22 S could occur within a few minutes, while ZLSIL™ 50 S could maintain high efficiency for a long time without performance failure due to extended mixing time. At the same time, the experiment confirmed that reducing the shear force and decreasing the mixing speed and intensity could significantly extend the effective action time of ZLSIL™ 22 S.
The liquids that cause moist powder stickiness can be divided into water-based and oil-based categories. For water-based liquid systems, in addition to improving flowability by absorbing liquid, hydrophobic silica can also be used for efficient modification, which has been proven to be a special high-efficiency flow aid for hygroscopic substances. Hydrophobic silica does not absorb the water film, but suspends on the water film to maintain a stable distance between particles, and can achieve flowability improvement at a lower addition level, with its efficiency not limited by its own absorption capacity.
It should be noted that hydrophobic silica is also sensitive to over-mixing. If excessive shear force is applied, the product may be wetted by water and lose its modification effect. The stronger the hydrophobicity of silica, the lower the sensitivity to over-mixing. The controlled experiment showed that for the moist sodium chloride system, after adding 0.4% of hydrophobic ZLCSIL™ D 10 and ZLCSIL™ D 17, the flowability of the salt could be improved immediately. However, after long-time mixing, the system flowability gradually deteriorated due to the wetting of silica caused by excessive shear energy. Among them, the over-mixing effect of ZLCSIL™ D 17 could occur within 3 minutes, while ZLCSIL™ D 10 had stronger anti-over-mixing ability, and could be mixed for up to 12 minutes at the given shear rate without any decrease in flow modification effect.
The depolymerization behavior of flow aids during mixing with host powder produces completely different effects in dry and moist powder systems:
1. In dry hard powder systems, the depolymerization of flow aids helps to more comprehensively cover the surface of host powder particles, thereby improving the modification efficiency of silica as a flow aid.
2. In moist powder systems, excessive depolymerization of flow aids will destroy the pore structure of silica, reduce the porosity, and then weaken the modification efficiency of silica as a flow aid.
Based on this, differentiated mixing processes and suitable silica models need to be selected for different types of powder systems.
Powders of soft materials such as fats, waxes, and emulsifiers are extremely difficult to handle during processing and transportation. This type of powder is particularly prone to severe caking during long-term storage or long-distance transport. The caking problem is further exacerbated when the product is exposed to fluctuating temperatures, as is common in ocean freight. Therefore, efficient anti-caking agents are essential for the long-distance transportation and long-term storage of these powders.
Soft or thermoplastic powders will undergo particle deformation and then adhere to each other when the temperature rises or under pressure. Silica can be evenly coated on the surface of soft powder particles to form an isolation layer and prevent particles from adhering to each other. However, compared with hard powder systems, to achieve the ideal anti-caking effect, especially long-term anti-caking effect, a higher addition amount of silica is required: usually the addition amount of silica in soft powder systems can be up to 5%, while hard powder systems usually only need an addition amount of less than 1% to meet the demand.
The core reason is that part of the anti-caking agent may penetrate into the surface of soft powder particles during storage, resulting in reduced surface isolation efficiency. Only by adding a sufficient amount of anti-caking aid can we ensure that enough aid remains on the surface of soft powder particles continuously to maintain the long-term anti-caking and flow modification effects.
The mixing process plays a decisive role in the final performance of flow aids, and the mixing equipment and process parameters need to be accurately matched according to the characteristics of different powder systems:
3. For dry hard powders, the mixing intensity must be high enough to fully depolymerize the silica agglomerates, achieve uniform coverage of the aid on the surface of the host powder, and full mixing can obtain better flow modification effect.
4. For moist powder systems, excessive mixing intensity will partially destroy the pore structure of silica, thereby reducing its anti-caking and flow modification performance, so a mild mixing process is required.
5. For soft powder systems, the mixing intensity needs to be adjusted according to the softening characteristics and particle strength of the powder, to avoid damage to the particle structure of the soft powder by excessive shear force, while ensuring uniform dispersion of silica.
The applicable scenarios of different mixing equipment are as follows:
• Tumble blender: can provide extremely mild mixing effect, especially suitable for the processing of ultra-soft powders.
• Conical mixer (such as Nauta mixer): also has mild mixing characteristics, with minimal damage to powder particles, but requires a longer mixing time.
1. Paddle mixer: the mixing process is mild, and at the same time can achieve excellent mixing uniformity on a macro scale. It is an ideal choice for soft powders and hygroscopic powders, and can fully retain the pore structure of silica.
2. Plow shear mixer: higher mixing energy, but still mild enough, will not press the flow aid into the surface of soft powders, suitable for many different types of powders, with shorter required mixing time, which can be flexibly adjusted according to actual needs: the mixing time can be shortened for hygroscopic powders, and appropriately extended for dry hard powders.
3. Ribbon blender: higher mixing intensity and large shear force, especially suitable for processing hard dry powders.
For the flow modification of spray-dried products, there is a special process optimization scheme: separating silica from the raw material slurry and directly adding it into the spray dryer can make the silica evenly disperse on the surface of new particles under the action of hot air flow, and completely avoid the damage to spray-dried powder caused by mechanical stress, which is an efficient scheme for flow modification of such products.
Core Dimension | Dry Hard Powders | Moist Hard Powders | Soft Powders |
Suitable Silica Type | Easy-to-disperse silica | High mechanical stability, high absorbency silica | Easy-to-disperse silica |
Recommended Addition Level | Low addition level, usually ＜1% | Addition level flexibly adjusted according to the liquid content of the system | High addition level, up to 5% especially for systems with long-term anti-caking requirements |
Mixing Process Requirements | High-intensity full mixing | Low-shear mild stirring | Moderate mixing: ensure full dispersion of silica without damaging the soft powder particle structure |
In actual industrial applications, powder products often have multiple characteristics at the same time. For example, fruit and vegetable powders may contain both hard and dry non-hygroscopic components such as starch, and hygroscopic components such as sugars; milk powder may contain both hygroscopic lactose and fat-based soft components. The flow behavior of these composite systems will integrate the characteristics of the single systems in the above table in a complex way, so targeted solutions need to be designed.
Zhongqi (Shandong) Silicon Materials Co., Ltd. can provide exclusive product selection and process optimization services according to the customer's specific material characteristics and application scenarios, and provide full technical support and application guidance for ZLSIL™, ZLCSIL™, ZQSIL™ series products for customers throughout the process.
Zhongqi (Shandong) Silicon Materials Co., Ltd. Specialized in R&D, production and sales of high-end food & pharmaceutical excipient grade silica. Core brands: ZLSIL™, ZLXIDE™, ZLCSIL™. Products meet global food-grade standards, providing compliant high-performance formulation solutions for global food & pharma enterprises.
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Contact: Tel +86 53188737397 | Email Levin@silicaplant.com
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All information herein and published content is for reference only, with no warranty of accuracy or completeness. No warranty is given for product use results. We accept no liability for improper product use or information reliance. We reserve the right to adjust product prices & specifications without notice. For infringement issues, contact us for removal.
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