Silkem has been actively developing and manufacturing zeolite products for over 25 years, and the company’s beginnings and growth are directly linked to them. Intensive development activities mean that new product types are continuously added to the range, expanding its applications.
Composition of Zeolites
A completely silicate structure composed only of SiO4 tetrahedra with a chemical formula of SiO2 is quartz, whose structure is neutrally charged. The incorporation of aluminum atoms into the silicate framework results in a negative charge, requiring extra-framework cations from elements in groups I and II of the periodic table, such as sodium, potassium, magnesium and calcium, or cations of organic origin to neutralize the charge of the crystal lattice.
The composition of a zeolite can be described with the formula 𝐌2/𝑛𝐎 ∙ 𝐀𝐥2𝐎3 ∙ 𝑦𝐒𝐢𝐎2 ∙ 𝑤𝐇2𝐎, where y represents values between 2 and ∞, n is the M cation valence, and w represents the water molecules in the zeolite’s pores. Structurally, zeolites could be described as complex crystalline inorganic polymers with an infinite three-dimensional framework composed of AlO4 and SiO4 tetrahedra linked by shared oxygen atoms. The amount of aluminum in the zeolite framework can vary over a wide range of SiO2/Al2O3 molar ratios, from a fully silicate structure to a zeolite with SiO2/Al2O3=2 molar ratio, which represents the lower limit according to the Loewenstein rule, which states that Al-O-Al (aluminum-oxygen-aluminum) motifs are avoided in zeolite structures.
The composition of the zeolite framework depends on the synthesis conditions or post-synthesis modifications. The hydrothermal stability and hydrophobicity of zeolites increase with the SiO2/Al2O3 molar ratio. The zeolite crystal structure contains intracrystalline channels or interconnected cavities occupied by cations or water molecules. The cations are mobile and can be exchanged with other cations. Water molecules can be reversibly removed from the zeolite framework, usually by applying heat, leaving behind a preserved porous structure in which micropores and cavities account for up to 50% of the volume.
The crystalline nature of the zeolite framework ensures the pores are uniformly sized throughout the crystal. This enables zeolites to separate molecules according to their size, which is why they are also known as molecular sieves. The zeolite framework can have a one-, two- or three-dimensional pore topology. In practice, zeolites with two- or three-dimensional pore systems are preferred due to the superior accessibility and interconnectedness.
Usage of Zeolites
In terms of indirect impact, many aromatic and olefinic hydrocarbons are produced using zeolite catalysts, which are then purified using various zeolite adsorbents. The compounds thus produced are then used as a source of raw materials in the production of textiles, furniture, food, construction materials, plastics, motor fuels, etc. Zeolites have a direct effect on our daily lives through their use in detergents, moisture and odor removers, in the production and purification of industrial gases, etc.
Although many new families of porous materials have been discovered, mainly based on aluminophosphates, mesoporous silicates and, more recently, metal–organic frameworks (MOFs), zeolites – materials first discovered in the mid-20th century – remain at the top in terms of effectiveness, affordability and use in industrial catalytic, adsorption and separation processes.
The outstanding properties of these microporous aluminosilicate materials, their simple preparation, their stability and the ecological and economic acceptability of their production form the basis of their usefulness in many industrial applications. New post-synthesis modification processes enable the production of zeolite materials with even better properties, which, after several decades, has refocused the attention of porous materials science on the basic and most industrially important zeolites such as zeolites A, X, Y, mordenite, beta and ZSM-5.
- High purity: the Al₂O₃ content typically exceeds 99%.
- High hardness: comparable to corundum (9 on the Mohs hardness scale), which makes it ideal for abrasive applications.
- Thermal stability: withstands extremely high temperatures, making it ideal for refractory materials.
- Chemical inertness: resistant to acids and bases, ensuring a long lifetime in harsh environments.
- Controlled particle size: available in different sizes for specific applications.
- Controlled specific surface area: can be adjusted to improve reactivity or inertness.
Detergents
The zeolites in laundry detergents facilitate the exchange of sodium cations for calcium and magnesium cations in the water. This softens the water and ensures the other components perform optimally.
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PVC stabilizers
An important application of zeolite 4A is in the production of thermal stabilizers for PVC manufacturing, where the zeolites act as HCl neutralizers and adsorbents.
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Adsorbents and molecular sieves
Powdered zeolites are used to make granular or pellet-form adsorbents, for zeolite-coated pharmaceutical and food packaging, and in polymer adsorbent production.
Agriculture
In agriculture, zeolite is used to enhance the flowability of fertilizers and ensure a slow, uniform release of active substances into the soil. They prevent nutrients from washing out of the soil.
Construction
Another important area of application of zeolites is construction, where they are mainly used in the production of various types of concrete.
Water treatment
As ion exchangers, zeolites are used to remove organic compounds, heavy metals and radioactive substances from water or contaminated soil.
Brake pad production
Zeolite is used as a friction regulator in the manufacture of brake pads. It acts as an adsorbent of moisture from the surface of the brake pad, reducing the noise produced by the friction between the brake pad and the brake disc.
Other applications
Zeolites are also used:
- as fillers in the manufacture of paints, coatings and paper;
- for removing unpleasant household odors (e.g. as additives in pet litter); and
- as catalysts in acid-catalyzed reactions (cracking, isomerization, alkylation).
| Type of Product | ZP-4A | ZP-4A-TSR | ZP-4A-LD | ZP-4A-HD | ZP-4AM | ||
|---|---|---|---|---|---|---|---|
| Na2O | % | 17 – 19 | |||||
| Al2O3 | % | 28 – 30 | |||||
| SiO2 | % | 31 – 34 | |||||
| H2O | % | 18 – 22 | |||||
| Na2SO4 | % | 0 | 1.5 – 2.5 | ||||
| Anhydrous solids content (1h/800˚C) | % | 78 – 82 | 79 – 85 | ||||
| Reflectance (R 460) | % | min. 94 | |||||
| Bulk density | g/l | 280 – 380 | – | – | 450 – 550 | ||
| Tapped density | g/l | – | – | max. 500 | min. 630 | – | |
| pH (5% Slurry) | 11 – 12 | 10 – 12 | |||||
| Calcium ion exchange capacity | mg CaO / g anhydrous | min. 160 | |||||
| Static water adsorption * | % | – | – | – | min. 25 | – | |
| Water adsorption capacity (WAC)** | % | – | – | min. 24 | – | – | |
| Tenzide absorption | g/100g | – | – | – | – | 45 – 55 | |
| Particle size distribution: | |||||||
| > 10 microns | % | max. 10 | – | ||||
| < 1 micron | % | max. 10 | – | ||||
| Average particle size d50 | µm | 3 – 5 | – | ||||
| Wet sieve residue (on 45 µm) | % | 0.2 | 0.0 | 0.2 | – | ||
| Characteristics | Fine white powder with no visible impurities | ||||||
*(25˚C, RH=55%, 24h)-climate chamber
**(23˚C +/-2˚C, RH=55%, 24h)-desiccator