Silicon dioxide is a kind ofinorganic compound, chemical formula is SiOtwoSilicon atoms and oxygen atoms form crystalline silicon dioxide by long-term orderly arrangement, and amorphous silicon dioxide is formed by short-term orderly or long-term disordered arrangement.[1]
In the silicon dioxide crystal, the silicon atom is located in the center of the tetrahedronOxygen atomOn the four top corners of the regular tetrahedron, many such tetrahedrons are connected by the oxygen atoms at the top corners. Each oxygen atom is shared by two tetrahedrons, that is, each oxygen atom is combined with two silicon atoms.[1]
The simplest formula of silicon dioxide is SiOtwo, but it does not represent a simple molecule (it only represents the ratio of the number of atoms of silicon and oxygen in the silicon dioxide crystal).[1]
Pure natural silicon dioxide crystal is a hard, brittle, insoluble, colorless and transparent solid, which is commonly used in the manufacture of optical instruments.[1]
Chinese name
silicon dioxide
Foreign name
Silicon dioxide
chemical formula
SiOtwo[11]
molecular weight
sixty point zero eight four
CAS login number
14808-60-7[11]
Melting point
1723 ℃[11]
Boiling point
2230 ℃[11]
Water solubility
Insoluble
Density
2.2 g/cm³[11]
Appearance
Hard, brittle, insoluble, colorless and transparent solid
Security description
S24/25
Hazard symbol
R37
Hazard description
Xn
Toxicity and hazard
Non toxic, but silicosis is easily caused by long-term inhalation
Silica existing in nature, such asquartz、Quartz sandAre collectively referred to as silica.[2]
Pure quartz is colorless crystal, large and transparent prismaticquartz crystalbe calledcrystal, purple with trace impuritiesamethyst, light yellow, golden yellow and brownSmoky crystal。[2]
Chalcedony、agateandJasperAll are colored quartz crystals containing impurities.Sand is fine quartz mixed with impurities.Opal and diatomite are amorphous silica.[2]
The melting point of crystalline silicon dioxide is 1723 ℃, the boiling point is 2230 ℃, and it is insoluble in water.Except for fluorine gas and hydrofluoric acid, silicon dioxide does not react with halogen, hydrogen halide and inorganic acid, but can be dissolved in hot concentrated alkali, molten strong alkali or sodium carbonate.[2]
In addition, at high temperature, silica can be reduced by coke, magnesium, etc.Strong alkali solution and SiO at normal temperaturetwoIt will react slowly to generate silicate, so the glass bottle storing strong alkali solution cannot use ground glass stopper (the glass contains SiOtwo）, otherwise viscousSodium silicateNatwoSiOthreeMake the cork and bottle mouth stick together.Due to SiOtwoIt can react with hydrofluoric acid, so it cannot be usedGlass containerIn full bloomhydrofluoric acid。[2]
Physical and chemical properties
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physical property
Physical properties of silica
Crystalline silicon dioxide
density
2.2 g/cmthree
melting point
1723℃
boiling point
2230℃
Refractive index
one point six
Change when heated
Melt with strong alkali when heating, formingsilicate
solubility
Insoluble in water, can react with HF to generate gaseous SiFfour
chemical property
The chemical properties of silica are relatively stable.Does not react with water.It has high fire resistance, high temperature resistance, small thermal expansion coefficient, high insulation, corrosion resistance, piezoelectric effect, resonance effect and its unique optical characteristics.[11]
Silicon dioxide is inactive in nature and does not react with halogens, hydrogen halides, sulfuric acid, nitric acid and perchloric acid (except hot concentrated phosphoric acid) except fluorine and hydrogen fluoride.[2]
The common concentrated phosphoric acid (or pyrophosphoric acid) can corrode silicon dioxide and generate heteropoly acid at high temperature. The molten borate or boric anhydride can also corrode silicon dioxide at high temperature. In view of this nature, borate can be used as a flux in ceramic firing. In addition, hydrogen fluoride can also dissolve silicon dioxide and generate easily solublewaterOfFluosilicic acid。[2]
Silicon and carbon have similar properties, but their oxide properties are very different.COtwoIt is a molecular crystal, while SiOtwoIt is an atomic crystal.SiOtwoYesSilica tetrahedronIt is a three-dimensional network structure formed by the basic structure. In the crystal structure, four silicon atomsvalence electronAnd 4 oxygen atoms to form 4covalent bond，The Si atom is at the center of the tetrahedron,The O atom is at the apex of the tetrahedron.
Each silicon atom is connected to four oxygen atoms, and each oxygen atom is connected to two silicon atoms.The smallest ring in the crystal is composed of 12 atoms (6 silicon atoms and 6 oxygen atoms), each silicon is shared by 12 rings, and the number ratio of silicon to oxygen atoms in the crystal is 1:2.[2]
Natural classification
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In sedimentary rocks, silica minerals mainly occur in clastic rocks and siliceous rocks.It can be used asterrigenous mineral , can also be submittedAuthigenic mineralappear.The genesis of these two silica minerals is very different, and they must be distinguished.[5]
terrigenous mineral
Terrestrial minerals are detrital quartz grains.Some detrital quartz grains contain inclusions, which can be divided into gas-liquid inclusions and mineral inclusions.Mineral inclusions can also be divided into granular, acicular and flaky minerals according to the crystal form of minerals.The type of inclusions is related to the source of quartz.
Generally speaking, most of the gas-liquid inclusions (milky quartz) come from quartz veins;There are few gas-liquid inclusions in quartz in schist and other high-grade metamorphic rocks;Quartz in volcanic rocks is usually as clear as water, with few inclusions;However, those with rutile acicular inclusions are mostly from granite.However, due to the complex occurrence of inclusions in the parent rock, very small particles and difficult identification, their application is quite limited.[5]
According to the extinction characteristics of quartz, it can be divided into two types: non wavy extinction quartz and wavy extinction quartz.Wavelike extinction is a simple optical performance of strain crystals. Most rocks will suffer some deformation either during crystallization or after the formation of crystals.[5]
According to statistics, in plutonic intrusive rocks and metamorphic rocks, there are few quartz without wavy extinction, generally less than 10%;On the contrary, in volcanic rocks, wavy extinction quartz is rarely seen, generally less than 10%.[5]
The content of wavy extinction quartz increases with the age, so the extinction characteristics of quartz are not reliable indicators of the parent rock type. Some people even think that it is invalid to use wavy extinction to determine the nature of the parent rock.[5]
According to the internal structure of quartz, it can also be divided into single crystal quartz and polycrystalline quartz (also called polycrystalline quartz).Single crystal quartz refers to particles composed of a single crystal, while polycrystalline quartz refers to the aggregate of two or more crystals, including particles from igneous, metamorphic and sedimentary rocks.Generally speaking, the grains of polycrystalline quartz are relatively large, and the polycrystalline form is relatively unstable.[5]
Authigenic mineral
There are three variants of authigenic silica minerals, namely opal, chalcedony and quartz.This kind of mineral can not only form rock alone, but also occur as cement.[5]
Opal precipitates SiO directly from aqueous solutiontwoThe colloid is dehydrated, so it is amorphous, and its water content is very high (up to 10%).In the thin section, opal is colorless and transparent, with negative high projections.Due to dispersion effect, the opal is yellow at the contact with gum or quartz.No cleavage, full extinction under orthogonal microscope.
Protein stone is easy to contactVolcanic glassThe fragments are mixed, but the glass fragments have a special shape, and their refractive index will not be lower than 1.490.Opal is also easy to mix with analcite, but analcite has cleavage.
The difference between fluorite and fluorite is that fluorite has crystal shape and cleavage.Opal is a quasi stable mineral, easy to recrystallize and transform into chalcedony.Therefore, it only exists in young strata.[5]
The chalcedony is cryptocrystalline SiOtwoThe mineral is crystallized from opal, so it is a transitional type between opal and quartz.In the thin section, chalcedony is colorless and transparent, with negative low protrusions.Under the orthogonal microscope, fibrous crystals can be seen in the form of petals or fan-shaped aggregates. When turning the animal platform, the chalcedony fibers reach the extinction position in turn, and the extinction shadow moves in a wavy manner. This extinction feature is called fan-shaped extinction.Sometimes chalcedony is a spherical aggregate with cross extinction.Level I gray interference color, mainly parallel extinction.[5]
Fokker once proposed to pay special attention to a type of authigenic chalcedony in carbonate rocks, which can be used to judge sedimentary and diagenetic environments.Fokker divided this type of chalcedony into negative ductility chalcedony (negative), positive ductility chalcedony (positive), aqua chalcedony and mottled chalcedony.The former is the product of sedimentation or metasomatism in freshwater environment, and generally chalcedony belongs to this category.The last three types of chalcedony may be SiO in evaporation environmenttwoDirectly precipitated, or SiOtwoFormed by replacement of evaporated minerals.[5]
The authigenic quartz is mostly recrystallized from the opal through the chalcedony stage, or it is directly and slowly precipitated from the solution.The main feature of authigenic quartz is that there is no abrasion trace in its shape, and it is often inlaid with each other, sometimes it is self shaped crystal, and sometimes its shape is suitable for its space.[5]
The authigenic quartz is produced at the same time as the secondary quartz. The secondary quartz is bright and clean, and has the same composition and optical orientation as the surrounding quartz particles, but they are often separated by a ring of iron or clay film.[5]
Manual preparation
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Amorphous silica
The preparation of amorphous silicon dioxide includes five steps, namely, gel preparation, granulation process, sintering process, cleaning process and drying process.[3]
Preparation methods of amorphous silicon dioxide
working procedure
method
Preparation of silica gel
Hydrolyze silicon tetrachloride to generate silica gel, or hydrolyze organic silicon compounds such as tetramethoxysilane to generate silica gel, or use gaseous silica to generate silica gel.[3]
Granulation process
The silica gel is dried to form a drying powder. After crushing the drying powder, the silica powder is graded to obtain the desired average particle size.[3]
Sintering process
The silica powder obtained in the granulation process is sintered at a temperature of 800 ℃~1450 ℃. Using the spheroidizing process of thermal plasma, the silica powder obtained in the sintering process is put into the plasma torch that introduces argon at a predetermined flow rate and generates plasma at a predetermined high-frequency output power,Heat and melt at a temperature from 2000 ℃ to the boiling point of silica.[3]
Cleaning process
The micro powder attached to the surface of the spheroidized silica powder after the spheroidizing process is removed.[3]
Drying process
Dry the silica powder after the above cleaning process.[3]
Crystalline silicon dioxide
Mix the raw material (silicon source) containing silicon dioxide, water, structure directing agent, alkali or acid in a certain proportion, and put them into the pressure resistancereaction kettleInner sealing, then heating to 100 – 220 ℃, constant temperature for 5 hours to 10 days.[4]
After the reaction is completed, the reaction kettle is cooled rapidly, the reaction products are washed with water or dilute acid to pH 8 – 11, and dried to obtain the raw powder. The raw powder or products formed by adding binder are calcined and activated in muffle furnace or tubular furnace.[4]
When silicon dioxide is crystallizedcrystalAfter gelatinization and dehydrationagate, aqueouscolloidAfter solidification, it becomesOpalWhen the silicon dioxide grain is less than several micronsChalcedony, flintSecondary quartzite。[7]
Mineral resources with very stable physical and chemical properties, crystalTrigonal systemThe oxide mineral, namely low-temperature quartz (α - quartz), isQuartz group mineralsThe most widely distributed one inMineral species。[7]
The generalized quartz also includes high-temperature quartz (β - quartz).Quartz block, also called silica, is mainly the raw material for producing quartz sand (also called silica sand), as well as quartz refractory and raw material for firing ferrosilicon.[7]
In addition, silica can also be used as a lubricant and an excellent flow promoter, mainly as a lubricant, anti sticking agent and flow aid.[8]
It is especially suitable for granulation of oil and extract drugs, and the prepared particles have good fluidity and compressibility.It can also be used as a flow aid in direct tablet pressing.[8]
As a disintegrating agent, it can greatly improve the fluidity of particles, increase the bulk density, increase the hardness of the prepared tablets, shorten the disintegration time, and improve the dissolution rate of drugs.[8]
Granules can be used as internal desiccant in manufacturing to enhance the stability of drugs.It can also be used as filter aid, clarifying agent, defoamer, suspension aid and thickening agent for liquid preparations.[8]
Edible standard
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FAO/WHO (2001) stipulates the maximum use amount for milk powderCocoa powder, sweetened cocoa powder, edible sodium fatCocoa butter, dosage 10mg/kg;1 g/kg cream fat;Sucrose powder, glucose powder, soup powder and soup block for coating are 15g/kg;The FDA stipulates that the maximum amount of this product as an anti caking agent is 2%.
China GB 2760 – 2011《National Food Safety Standard for Use of Food Additives》It is specified to use it in egg powder, milk powder, cocoa products, dehydrated protein products, sugar powder, fat planting powder, solid drinks, and formula food for pregnant and puerperal women, with a maximum use of 15g/kg;Spices, solidCompound seasoningThe maximum dosage is 20g/kg;The maximum consumption of bean products processing is 0.025g/kg.[9]
In addition to the above, there areTricalcium phosphateAs an anti caking agent, it is commonly used in milk powder and cream powder, with the maximum dosage of 10.0g/kg;Wheat flour, the maximum dosage is 0.03g/kg;Compound seasoning, the maximum dosage is 20.0g/kg;Solid beverage, the maximum dosage is 8.0g/kg;Fried snacks, the maximum dosage is 2.0g/kg.[9]
Health hazards
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Silica has important uses in daily life, production and scientific research, but sometimes it can also cause harm to human body.[10]
The dust of silica is very fine,Specific surface areaUp to 100mtwo/Above g can be suspended in the air. If people inhale dust containing silica for a long time, they will suffer fromSilicosis(Because silicon was formerly called silicon, silicosis was formerly calledsilicosis）。[10]
SilicosisIs a kind ofOccupational DiseasesIts occurrence and severity depend on the content of dust in the air, the content of silica in the dust, and the contact time with people.People who work in places with high content of silica dust for a long time, such as mining, sand turning, sand blasting, ceramics manufacturing, refractory manufacturing and other places, are prone to this disease.[10]
Therefore, in these dusty workplaces, strict labor protection measures should be taken, and a variety of technologies and equipment should be used to controldustTo ensure the health of the staff.[10]
