Silica

Abstract

Silica (SiO 2 ) is a commonly occurring compound in nature. Various crystalline and noncrystalline silica minerals of inorganic and biogenic origin are known. Depending on temperature, pressure, composition of the precursor phases, etc., several different crystalline modifications are formed. Quartz is the most abundant and well‐known polymorph. Silica occurs as meter‐sized quartz crystals or as amorphous masses made up of submicroscopic crystallites having different microstructures or growth fabrics. In addition, synthetic polymorphs have been produced for which no natural counterparts exist. In industry, quartz sand and silica rocks are widely used as raw materials for production of glass, ceramics, and silicon. Single crystals of quartz are grown artificially for application as resonator crystals in clocks, communication technology, and computer electronics. Both natural and synthetic silica powders are used as fillers to improve the mechanical properties of plastics. The article contains sections titled: 1. Silica Modifications and Products 1.1. Occurrence and Geochemistry 1.2. Si‐O Bond and Crystal Structures 1.3. Crystalline Silica Phases 1.3.1. Crystalline Silica Minerals 1.3.2. Crystalline Nonmineral Silica Phases 1.4. Noncrystalline Silica Minerals 1.5. Colored Silica Minerals 1.6. Silica Rocks 1.7. Crystalline Silica Products 1.7.1. Cultured Quartz Single Crystals 1.7.2. Polycrystalline Silica Products 1.8. Noncrystalline Silica Products 2. Quartz Raw Materials 2.1. Physical Forms and Occurrence 2.2. Processing 2.3. Uses 3. Diatomites 3.1. Introduction 3.2. Formation, Composition, and Quality Criteria 3.3. Occurrence and Mining 3.4. Processing 3.5. Analysis 3.6. Storage and Transport 3.7. Environmental and Health Protection 3.8. Uses 3.9. Recycling 4. Colloidal Silica 4.1. Introduction 4.2. Structure of Colloidal Silica Particles 4.3. Physical and Chemical Properties 4.4. Stability 4.5. Production 4.6. Analysis and Characterization 4.7. Uses 4.8. Storage, Handling, and Transportation 4.9. Economic Aspects 5. Silica Gel 5.1. Introduction 5.2. Structure, Properties, and Characterization 5.3. Production 5.4. Uses 5.5. Economic Aspects 5.6. Legal Aspects 6. Pyrogenic Silica 6.1. Flame Hydrolysis 6.1.1. Production Process 6.1.2. Morphology 6.1.3. Solid‐State Properties 6.1.4. Surface Chemistry 6.1.5. Surface Modification with Silicon Compounds 6.1.6. Characterization 6.1.7. Uses 6.1.8. Industrial Hygiene and Safety 6.2. Electric‐Arc Process 6.3. Plasma Process 7. Precipitated Silicas 7.1. Introduction 7.2. Production 7.3. Properties 7.3.1. Physicochemical Properties 7.3.2. Surface Chemistry and Surface Modification 7.3.3. Chemical Composition and Analysis 7.4. Uses 7.5. Industrial Hygiene and Safety 8. Porosils 8.1. Introduction 8.2. Physical and Chemical Properties 8.2.1. Zeosils 8.2.2. Clathrasils 8.3. Manufacture of Porosils 8.3.1. Synthesis of Porosils 8.3.2. Dealumination of Aluminosilicate Zeolites 8.3.3. Formation of Melanophlogite 9. Toxicology 9.1. Experiences with Humans 9.2. Animal Experiments

Possibilities of rice husk ash to be used as reinforcing filler in polymer sector -a review

“…This process consists in the synthesis of silica in a vapor phase at high temperatures [45]. The silica is prepared from SiCl4 and is known as fumed silica while silica obtained from quartz sand is known as arc silica.…”

“…Once the silica is separated, the nucleation takes place followed by polymerization of silicate tetrahedron in a random amorphous manner until forms small spheroids called micelles. The solution with micelles is known as colloidal silica or silica sol (suspension of SiO2 particles size between 100 and 150 nm) [45]. The liquid where silica is dispersed can be water or organic solvent which are known as hydrosol or organosols respectively [44].…”

“…Similarly, colloidal silica may also be produced by: i) peptization of silica gel, ii) hydrolysis of a silica compound and iii) dispersion of a pyrogenic silica as shown in Figure 1 [45].…”

“…This step is known as gelation and produce the hydrogel. After the gel formation, the solid gel is washed and dried at specific conditions to obtain the desired physical properties such as porosity, pore size and surface area [45]. In general terms, there are three types of silica gel: hydrogel, xerogels and aerogels.…”

“…The production process consists of the following steps: precipitation, filtration, drying, grinding, and, in some cases, compacting and granulation [45].…”