Amorphous silica application. Silicon: application, chemical and physical properties. Advantages and disadvantages

Amorphous (non-crystalline) silicon dioxide with a high specific surface area is almost never found in nature in its pure form. It can only be obtained through technology. The high-purity synthetic silica (amorphous silicon dioxide) produced by us under the trademark KOVELOS is a very light micronized (particle size, depending on the brand, from 6 to 40 microns) tasteless and odorless white powder with a nanoporous structure of particles, with pronounced sorption properties. Its specific surface area is 350-400 sq.m. per 1 gram. Oil absorption - 300-340 g / 100 g.

Among solids, amorphous silicon dioxide has the lowest thermal conductivity (0.02 W / (m. K)), sound propagation speed (100 m / s) and dielectric constant. Amorphous silica is heated (at temperatures above 1000 degrees C) into a crystalline form.

Synthetic silica (amorphous silicon dioxide) is indispensable in many sectors of the modern world economy due to the fact that

it is neutral and chemically resistant to almost all mineral and organic substances that exist on our planet. That is, it is harmless to living organisms, non-toxic, fire and explosion-proof in the external environment.

has a high specific surface area, due to the fact that the particle of amorphous silicon dioxide contains a huge number of nanosized pores. This complex nanopore structure (highly developed surface) of the particle determines the excellent sorption properties of synthetic silica. It can selectively absorb or bind gases, vapors and solutes from the environment. Interestingly, during the synthesis of amorphous silicon dioxide, it is possible to set the surface parameters in advance (modify the surface), and thereby obtain a product with selective sorption.

Thus, chemical neutrality and a huge specific surface area (highly developed surface) of amorphous (non-crystalline) silicon dioxide can impart new characteristics to various compositions, materials, products without changing their chemical properties. In particular, high-purity fine synthetic silica with a developed surface can:

thicken (increase viscosity) fluid formulations up to a free-flowing state (depending on the required degree of thickening, from 1.5% to 33% synthetic silica is introduced into the composition). This property is used in the production of paints, varnishes, adhesives, sealants, pastes, ointments, lubricants, etc.;

increase flowability crushed and/or powdered solids (spices, chips, crackers, bread, milk powder, dry mixes, animal feed, washing powders, toners, medicines, etc.) and protect them from clumping, thereby increasing their shelf life .

improve strength characteristics and wear resistance materials (plastics, resins, rubbers, rubbers, concrete, asphalt, etc.)

improve thermodynamiccharacteristics(heat resistance, thermal conductivity) of materials;

improve tribological characteristics(increases resistance to abrasion);

be used as an additive in oils and lubricants for any units and mechanisms where there are metal friction pairs. In this case, from amorphous silicon dioxide during the operation of mechanisms on the surface of rubbing pairs about silicate films are formed, which restore the geometric dimensions of units and mechanisms to their original state, which reduces the degree of wear by several times.

be a carrier of active substances in pharmaceutical and cosmetic products;

be used as a gentle abrasive in perfumery and cosmetics (skin peeling, sorption of dirt on the skin), in the production of silicon semiconductor wafers, etc. (as a polishing suspension);

for growinglarge crystals, which cannot be grown in water. In this case, a silica gel medium is used for growth. The structure of the silica gel prevents convection and allows the diffusion process of the components to proceed evenly;

for the preparation of synthetic clay materials. Thus, kaolin in the presence of amorphous silicon dioxide is formed under hydrothermal conditions at 200–300°C.

bind and excrete animal and human various toxins, salts of heavy metals, radionuclides;

be used as a raw material for the production of special quartz glasses with a transparency of more than 99.5% for optical radiation with a wavelength of 248 nm and more than 98% for optical radiation with a wavelength of 193 nm, for the production of optical fibers;

be used as a raw material for the manufacture of high-purity silicates used to cover television and lighting tubes;

in the production of microcircuits and other electronic components be used as an insulator, applied by spraying or as a special. films;

serve as source material to obtain high-purity silicon used in the production of solar cells and in the synthesis of organosilicon compounds;

used as a heat insulator and sound absorber in rocket and jet engines. It is a good heat insulator for various kinds of conducting systems with heating temperatures up to 1000 °C;

used in fire extinguishing powders for extinguishing fires of classes A (smoldering materials), B (flammable liquids), C (flammable gases), as well as electrical installations under voltage up to 1000 V.

Also, the use of amorphous silicon dioxide speeds up the production process (by simplifying the technological cycles, reducing the production cycle time) and requires less energy. For example, to thicken liquid formulations with synthetic silica, room temperature is sufficient.

The scope of application of high-purity amorphous silicon dioxide in the world economy is expanding every year, its role in the development of modern industries and in the creation of new materials is growing.

Silicon dioxide (silica, Silicon dioxide, silica) is a substance consisting of colorless crystals with high strength, hardness and refractoriness. Silicon dioxide is resistant to acids and does not interact with water. With an increase in the reaction temperature, the substance interacts with alkalis, dissolves in hydrofluoric acid, and is an excellent dielectric.

In nature, silicon dioxide is quite widespread: crystalline silicon oxide is represented by such minerals as jasper, agate (fine-crystalline compounds of silicon dioxide), rock crystal (large crystals of the substance), quartz (free silicon dioxide), chalcedony, amethyst, morion, topaz (colored crystals silicon dioxide).

Under normal conditions (at natural ambient temperature and pressure), there are three crystalline modifications of silicon dioxide - tridymite, quartz and cristobalite. When the temperature rises, silicon dioxide first turns into coesite, and then into stishovite (a mineral discovered in 1962 in a meteorite crater). According to research, it is stishovite - a derivative of silicon dioxide - that lines a significant part of the Earth's mantle.

The chemical formula of the substance is SiO 2

Obtaining silicon dioxide

Silicon dioxide is industrially produced in quartz factories that produce pure quartz concentrate, which is then used in the chemical and electronic industries, in the production of optics, fillers for rubber and paintwork, jewelry, etc. Natural silicon dioxide, otherwise called silica, is widely used in construction (concrete, sand, sound and heat insulating materials).

Obtaining silicon dioxide in a synthetic way is carried out by means of the action of acids on sodium silicate, in some cases - on other soluble silicates, or by the method of coagulation of colloidal silica under the influence of ions. In addition, silicon dioxide is obtained by oxidizing silicon with oxygen at a temperature of about 500 degrees Celsius.

Application of silicon dioxide

Silicon-containing materials are widely used both in the field of high technologies and in everyday life. Silicon dioxide is used in the production of glass, ceramics, concrete products, abrasive materials, as well as in radio engineering, ultrasonic devices, lighters, etc. In combination with a number of ingredients, silicon dioxide is used in the manufacture of fiber optic cables.

Non-porous amorphous silicon dioxide is also used in the food industry as an additive, registered under the number E551, which prevents clumping and caking of the main product. Food silicon dioxide is used in the pharmaceutical industry as an enterosorbent drug, in the production of toothpastes. The substance is found in chips, crackers, corn sticks, instant coffee, etc.

The harm of silicon dioxide

It is officially confirmed that the silicon dioxide substance passes through the gastrointestinal tract unchanged, after which it is completely excreted from the body. According to a 15-year study by French experts, drinking water with a high content of dietary silicon dioxide reduces the risk of developing Alzheimer's disease by 10%.

Thus, information about the dangers of silicon dioxide, which is a chemically inert substance, is false: the E551 food supplement, taken orally, is completely safe for health.

The invention relates to the technology of chemical processing of mineral raw materials, in particular to methods for producing highly dispersed silicon dioxide - an analogue of white soot, used as a mineral filler in industries using highly dispersed fillers. The method includes the stages of grinding the initial silicon-containing raw material, which is used as a natural rock - diatomite with a high, up to 70-75%, content of bound amorphous silica, preparing a charge at a ratio of W:T equal to 4-6:1, processing the latter to obtain a solution liquid glass, separating the precipitate formed, precipitating silicon dioxide from the resulting liquid phase with mineral acid in stages under the control of the pH of the medium, isolating the formed target product by filtration followed by repeated washing with water and drying, while diatomite is crushed to obtain a fraction with a grinding fineness of not more than 0, 01 mm and preliminarily subjected to firing at a temperature of 600-900°C for 1-1.5 hours, and the processing of the charge is carried out in the mode of a cavitating medium created by an electric pulse or hydrodynamic method. The technical result of the invention is to simplify the process by creating a non-autoclave, energy-efficient process and obtaining a product with high reactivity and a wide range of industrial properties. 6 w.p. f-ly, 3 tab., 4 ill., 13 pr.

Drawings to the RF patent 2474535

Amorphous silica is a multi-purpose material and is used in various industries. It is most widely used for the production of a special type of silicone rubber, as an adsorbent, or as an integral part of building dry mixes and in the paint and varnish industry, moreover, it is a constant component for many products and products of the perfume industry. For some types of tire rubber used for the production of high-quality tires, amorphous silica with rather stringent technical characteristics can be used as a filler.

Silica is a general term for compounds having the chemical formula SiO 2 . Amorphous silica (highly dispersed silica - VDS) is a highly dispersed chemical compound, which is a loose white powder containing at least 95-99.8% silicon dioxide. Its features are a high specific surface, loose packing of primary particles and aggregates, which leads to a large pore volume and, accordingly, high moisture and oil absorption, good thickening and structure-forming properties.

The entire VDC is conditionally divided into two classes - pyrogenic and precipitated.

Pyrogenic VDC is obtained by burning silicon tetrachloride in a stream of oxygen and hydrogen, resulting in highly dispersed amorphous silicon dioxide and hydrogen chloride in a gaseous state. This production requires high energy consumption and serious measures for explosion safety. Unlike pyrogenic, precipitated VDC is obtained, for example, from natural mineral raw materials - rocks such as perlite, obsidian, diatomite, nepheline, tripoli, from silicate raw materials, quartz sand [patent RU No. 2085488, cl. C01B 33/18, publ. 27.07.97] and from "semi-finished products" - high-silicon ferrosilicon waste [patent RU No. 2036836, class. C01B 33/12, publ. 06/09/95], the production of boron or borosilicate materials [patent RU No. 2170211, class. C01B 33/12, 07/10/2001], from the waste of apatite production [ed. certificate SU No. 856981 30.01.93] and ferroalloy production [RF patent No. 2237015, class. C01B 33/18, publ. 27.09.2004], from flue dust from the process of gas cleaning of electrothermal silicon production at aluminum industry enterprises [patent RU No. 2031838, 27.03.95] and others.

A known method is the isolation of silicon dioxide from a glassy volcanic rock, which is used as perlite, obsidian, pumice with a silica content of 69-75% [Patent RU No. 1791383, C01B 33/12, 30.01.93 g].

The method includes grinding silica-containing raw materials to obtain a fraction of the order of 0.1 mm, treatment with an alkali solution at a concentration of Na 2 O - 100-200 g/l and a ratio W:T=2-4 for 1-5 hours, followed by removing the precipitate from the liquid phases. The latter is subjected to magnetic treatment at an electromagnetic field strength of 500-1100 kA/m and a liquid phase velocity of 2-4 m/s, the solution thus treated is heated to boiling, calcium oxide and aluminum nitrate are added and boiled. The mass is filtered, and the resulting liquid glass is treated with mineral acid. The precipitated silica is filtered off, washed and dried.

The time of the entire process - 8-10 h, the yield of the target product - (by weight of the feedstock) - 30-60%, the content of SiO 2 in the final product up to 98%.

The disadvantage of this method is the complexity of the process, regulated by the feedstock used, the low degree of extraction of silicon dioxide from mineral raw materials and the insufficiently high specific surface area of the resulting product.

A known method for producing amorphous silicon dioxide, including grinding silicon-containing raw materials, processing the latter with an alkaline reagent at 150-170°C, separating the precipitate formed and precipitating silicon dioxide from the obtained liquid phase with mineral acid, separating the resulting silicon dioxide by filtration, followed by washing and drying [Patent No. 2261840, class. C01B 33/12, C01B 33/18, publ. 2005.10.10].

As the initial silica-containing raw material, natural rock - marshalite is used, grinding is carried out in a centrifugal apparatus with a speed of at least 10,000 rpm and a centrifugal factor of at least 20 g. to obtain a fraction with a grinding fineness of 10-15 microns, the latter is subjected to alkaline treatment at a pressure of 4.5-5.5 atm. 8-10% sodium hydroxide solution, taken in the ratio W:T=4.5-5.5:1, the precipitation of silicon dioxide is carried out with 45-50% nitric acid at the ratio W:R=3-3.5 :1 by dosed loading of nitric acid for 0.5-1 h until a neutral pH value is obtained, first, the target product is washed with 10-12% nitric acid, and then with at least five times the amount of hot water and dried.

The disadvantage of this method is the limited raw material base for the production of silicon dioxide.

In addition, the known method does not allow obtaining a final product with predetermined properties, for example, with a certain specific surface area of silicon dioxide, and the statement that it is possible to obtain a product with a wide range of specific surface area has not been confirmed experimentally.

The closest technical solution is a method for producing amorphous silicon dioxide, in which natural rocks with a high content of bound amorphous silica up to 70-75% are used as the initial silicon-containing raw material, for example, perlite, obsidian, pumice, vitroclastic tuff, diatomite, kieselguhr, volcanic ash with a minimum content of the crystalline phase, not more than 10-15%, etc. [US Pat. RF No. 2261840, C01B 33/12, 33/18, 06/18/2004].

The method includes the stages of grinding silicon-containing raw materials, carried out in a vibrating mill with ceramic or agate balls to obtain a fraction with a grinding fineness of not more than 10 μm, processing the latter with an alkaline agent in an autoclave at elevated pressure and temperature (180-200°C and 6.5 atm) to obtain a solution of liquid glass, and to obtain amorphous dioxide with a given specific surface area, treatment with an alkaline agent is carried out with its concentration selected according to the nomogram, separation of the precipitate formed, precipitation of silicon dioxide from the obtained liquid phase with mineral acid, while precipitation of silicon dioxide from the obtained liquid phase carried out with mineral acid by initially loading it in an amount that ensures a pH of 12 of the mixture, holding the mixture with constant stirring for 10-15 minutes, and then loading the acid in an amount that provides a pH of 10 of the mixture, repeated exposure of the mixture with constant stirring for 10- 15 min and final download to acid to obtain a pH of 7 mixture, while before each introduction of acid into the liquid phase additionally introduce water in the amount of 8-10%.

The resulting target product is separated by filtration, followed by repeated washing with hot water and drying.

To determine the concentration of an alkaline agent, it is proposed to use a nomogram consisting of two ordinate axes, one of which indicates the specific surface area of amorphous dioxide, and the other is the value of the alkali concentration, the common abscissa axis, which indicates the modulus of the obtained liquid glass, and two experimentally constructed curves, the first of which displays the dependence of the specific surface on the conditional glass modulus, and the second displays the dependence of the glass modulus on the alkali concentration used.

Treatment with an alkaline agent is carried out at a ratio W:T equal to 6-6.5:1 for 1-2.5 hours at a temperature of 170-200°C, a pressure of 6.5-7 atm.

This method makes it possible to obtain a product with predetermined physical and technical characteristics, however, in general, the method is rather complicated to implement, it requires very precise execution of the technological schedule, the creation of high temperature and pressure.

The objective of the invention is to simplify the process by creating a non-autoclave, energy-efficient method for producing amorphous silicon dioxide

The objective of the invention is also to obtain a product with a high reactivity and a wide range of industrial properties.

The tasks are solved by the fact that in the method for obtaining amorphous silicon dioxide, including the stages of grinding silicon-containing raw materials, to obtain a fraction with a grinding fineness of not more than 0.01 mm, preparing a charge at a ratio of W:T equal to 4-6:1, processing the latter with obtaining a solution of liquid glass, separating the precipitate formed, precipitating silicon dioxide from the obtained liquid phase with mineral acid step by step under the control of the pH of the medium, isolating the formed target product by filtration, followed by repeated washing with water and drying, the crushed silicon-containing raw material is first subjected to firing at a temperature of 600-900 ° C, and the processing of the mixture is carried out in the mode of a cavitating medium created by an electric pulse or hydrodynamic method.

It is preferable to use natural rocks with a high content of bound amorphous silica, up to 70-75%, mainly diatomite, as the initial silicon-containing raw material.

It is advisable to roast the crushed silicon-containing raw materials for 1-1.5 hours.

It is preferable to process the charge at a temperature of 80-90°C in an electric pulse reactor at a voltage and power of 5-10 kV and 1.2-1.5 kW, respectively, and a pulse repetition rate of 2-7 Hz or in a cavitation disperser for 2.5 -3.5 h at 1500-3000 rpm and 80-90°C.

It is advisable to process in an electric pulse reactor for 1.0-2.0 h with periodic mixing of the charge every 0.5 h, and use nitric, sulfuric or hydrochloric acids as mineral acid, mainly 40-50% nitric acid, and washing the target product is additionally carried out with a weak 2-5% solution of nitric acid.

Figure 1 shows the IR spectra of silica deposited from water glass, synthesized by autoclave.

Figure 2 - IR spectra of silica deposited from liquid glass, synthesized by electropulse method.

Figure 3 - electron microscopic image of silica synthesized by electropulse method.

a) electron microscopic image of silica synthesized by electropulse method,

b) electron microscopic image of silica obtained in an autoclave.

The spark discharge used in this process has not only a huge destructive power, but also affects the nature of the course of the resulting chemical reactions and their final results.

Let us briefly list the factors that act during an electric pulse discharge:

(a) High and ultra-high impulsive hydraulic pressures arise, with which shock waves are associated, moving at sonic and supersonic speeds of the order of hundreds of meters per second.

(b) In addition, powerful pulsed cavitation processes begin to act, capable of covering relatively large volumes of liquid. The formation of cavitation cavities occurs as follows. In places where ultrasonic waves penetrate, compression or tension periodically occurs. Ultrasonic waves in places of rarefaction cause the fluid to break with the formation of a microscopic cavity. Vapors and gases dissolved in it penetrate into this cavity from the surrounding liquid. The resulting cavities quickly collapse under the influence of subsequent compression. This phenomenon is called cavitation. The life span of a cavitation bubble is almost commensurate with the period of a sound vibration. In the range of high ultrasonic frequencies, it is millionths of a second. It is assumed that large electrical stresses and high temperatures occur in the cavitation cavity. Under these conditions, the molecules and atoms of gases present in the cavitation cavity undergo ionization and dissociation processes. In the cavitation cavity, for example, H 2 O and OH molecules dissociate. Due to the fact that energy-rich substances (ionized and activated molecules and free radicals) arise in the cavitation cavity, a number of phenomena were discovered that indicate the occurrence of fundamentally new reactions. Until now, this part of the electropulse effect has not been fully studied.

(d) An electric shock can cause delamination of a solid at the molecular level, for example, associated with the details of the structure of the crystal lattice of the mineral, including polymerization, breaking of sorption and chemical bonds, and changing other details of synthesis.

These processes contribute to a finer disintegration of the initial substance during the transition of diatomite to liquid glass and give additional energy impulses to liquid glass. The study of electrochemical processes associated with the synthesis of silica suggests that the electropulse effect affects the chemical properties of the SiO 4 structural groups that are part of the liquid glass synthesized under its action, perhaps this manifests itself in the strengthening of silonol bonds. At the same time, a special "crystal-chemical memory" is preserved, which affects the structure of the precipitated silica. The electrical impulse effect creates additional Si-O-Si bonds in liquid glass, which then appear in silica. In addition, silica synthesized from highly reactive liquid glass has a higher negative charge on the particle surface.

The process of synthesizing silica from diatomite includes the following steps:

1 - grinding diatomite to a fraction of 0-0.01 mm,

2 - firing of ground diatomite in an electric furnace,

3 - batch preparation,

4 - processing of the charge in the EI reactor,

5 - cooling and filtering the suspension;

6 - precipitation of silica,

7 - suspension filtration: silica + Na sulfate,

8 - washing of silica,

For the implementation of electrochemical processes associated with the synthesis of silica, when choosing a rock, it is necessary to keep in mind two features: the reactivity of the rock and its chemical composition. Reactivity refers to the ability of a rock to react with alkaline solutions.

The second feature of the rock, which is suitable for obtaining liquid glass, is that the content of SiO 2 silica in the rock is at least 70-80%. Diatomite also satisfies these two conditions.

Macroscopically, diatomite is represented by a weakly cemented rock of light gray color with an indistinctly expressed layered texture.

Table 1 shows the chemical composition of natural samples of diatomite from the Akhmetovsky deposit (wt.%), where 1, 2 - natural diatomite from the Akhmetovsky deposit, a - in its natural form, b - in terms of dry matter.

Table 1
oxides	1	2
a	b	a	b
SiO2	78,16	85,8	79,58	87,80
TiO2	0,52	0,58	0,37	0,4
Al2O3	5,6	6,16	5,6	6,1
Fe2O3	3,07	3,38	3,11	3,43
CaO	0,42	0,47	0,27	0,29
MgO	0,80	0,89	0,79	0,87
Na2O	0,00	0,00	0,00	0,00
K2O	1,61	1,78	1,16	1,28
SO 3	0,84	0,93	0,12	0,13
P.p.p.	8,9	-	9,44	-
Sum	100	100	100	100

The unburned silica rock contains a fairly significant amount of OHn groups in the form of hydroxyl, molecular water and organic matter in the form of CHn groups. The organic matter, reacting with an alkaline solution during the experiment, gives the liquid glass a black color and makes it difficult to purify the synthesized SiO 2 , significantly increasing the washing process. At a temperature of 600° and above, the organic matter burns out and thereby ensures the high purity of the synthesized amorphous silica. In this case, the target product of a light cream shade with a minimum content of organic impurities is obtained.

In the event that it becomes necessary to reduce iron oxides in diatomite, it may be recommended to roast the diatomite powder at a temperature of about 900°C. At the same time, iron oxides present in diatomite transform into the mineral form of hematite and are easily removed by electromagnetic separation without contaminating the target product.

Thus, the above data clearly show the need for pre-firing at 600-900°C.

The method is carried out as follows.

1. Raw diatomite in a lumpy state is dried at a temperature of 100-105°C, then crushed in a mill to a fraction of 0.2 mm.

2. The roasting of ground diatomite is carried out in an electric furnace in special metal carriages with low sides, at a temperature of at least 600 ° C for 1 hour.

4. Preparation of the mixture.

The mixture can be prepared directly in the reactor. Depending on the parameters of the liquid glass (mainly module) required by the recipe, the initial components are introduced into the reactor in the following quantities:

- water	- 4000 cm 3.
- diatomite	- 1000 g,
- solid NaOH	- 500 g.
Ratio	W:T=8:3

The sequence of mixing the components is as follows: water solid NaOH diatomaceous earth. Next, the mixture is stirred in any possible way at room temperature for 10-15 minutes. The finished suspension is sealed in an electric pulse apparatus for the synthesis of liquid glass.

4. Processing of the charge in the EI reactor.

To create electro-hydraulic shocks, a power source is needed in the form of a capacitor, which is a store of electrical energy. The voltage on the capacitor rises to a value at which a spontaneous breakdown of the air forming gap occurs, and all the energy stored in the capacitor instantly enters the working gap in the liquid, where it is released in the form of a short electric pulse of high power. Further, the process at a given capacitance and voltage is repeated with a frequency depending on the power of the pulse source. The liquid, having received acceleration from the discharge channel expanding at high speed, moves from it in all directions, forming in the place where the discharge was, a cavity of considerable volume, called cavitation, and causing active (main) hydraulic shock. Then the cavity also closes at high speed, creating a second cavitation hydraulic shock. At this point, the single cycle of the electrohydraulic effect ends, and it can be repeated an unlimited number of times, according to the given discharge repetition rate.

As a source of electromagnetic pulses, a ZEVS-25 installation was used, which is a capacitive electrical energy storage device with stored energy in one pulse up to 600 J. The voltage on storage capacitors with a total electrical capacitance of 8 μF can be adjusted from 5 to 12 kV.

The pulse repetition rate is 2-7 Hz. During operation of the unit, the power consumed from the 220 V network is no more than 1.5 kW.

The temperature in the reactor does not exceed 80-90°C.

The time for the synthesis of liquid glass is 1-2.0 hours. To reduce the synthesis time and reduce the amount of unreacted initial diatomite, the process is carried out with periodic (every 0.5 h) after the start of the process, a stop to stir the suspension for 15-20 minutes.

1st mode.

Voltage - 10 kV,

Distance between electrodes - 10 mm.

Pulse frequency 7 Hz.

Power - 1.5 kW.

Synthesis time - 1 hour.

2nd mode.

Voltage - 5 kV.

The distance between the electrodes is 5 mm.

Pulse frequency 4 Hz.

Power, - 1.2 kW.

Synthesis time - 1.5 hours.

As a result of experiments in any of the above modes, a suspension is produced: liquid glass + unreacted diatomite in the form of solid particles.

In the first mode, all processes occurring in the reactor are intensified, which makes it possible to reduce the time of the experiment and reduce the amount of unreacted diatomite.

Cooling for 15–20 min is necessary for the safe opening of the reactor.

Separation of liquid glass from the solid phase is carried out using a vacuum pump and a ceramic filter or by slow decantation of the liquid after exposure for 10-12 hours.

As a result of filtration or decantation, a precipitate is obtained, which is a mixture of quartz, zeolite of the analcime type and iron hydroxides, and a relatively homogeneous liquid glass of a yellowish-cream color.

6. Silica precipitation.

Sulfuric acid is slowly added to a container with liquid glass in two possible ways: precipitation: fast and slow. As a result, silica precipitates out of the liquid glass. For fast precipitation, 35% sulfuric acid is used, for slow precipitation, 14% sulfuric acid is used. Not only the state of aggregation of silica depends on the mode of deposition, but also such properties as the specific surface area and the presence of extraneous mineral impurities. The criterion for the completeness of the precipitation regime is the pH value.

During the two-stage neutralization, the precipitation of silica was carried out in the following sequence.

1 stage. A significant amount of highly dilute sulfuric acid is added to a solution of water glass. The amount of acid is regulated by the pH of the solution and depends on the volume of liquid glass. The pH of the solution should be around 8-9. This is followed by exposure for 20-30 minutes and then the rest of the sulfuric acid is slowly added with constant stirring and frequent measurement of pH. The process stops at pH=7-7.5.

With multi-stage neutralization, 14% sulfuric acid is gradually added to the resulting volume of liquid glass, equal to 3-3.5 liters.

Stage 1: add 200 cm 3 of dilute sulfuric acid, followed by an exposure of 20 minutes.

Stage 2: 50 cm 3 of sulfuric acid is added, holding for 20 minutes, precipitation of rare silica precipitates is observed. The neutralization process is completed at a pH of 7.

The general principle of liquid glass neutralization and silica precipitation is as follows. Before the massive precipitation of silica, it is necessary to create conditions for uniform and rapid precipitation. Therefore, a certain staging of the process is necessary. Conventionally, two main stages of precipitation can be distinguished. In the first stage, the alkalinity of the water glass solution decreases from 12-13 to approximately 9-10 pH. Thus, we are approaching the equilibrium precipitate - solution.

In this stage, massive formation of silica nuclei occurs. In order for nucleation to occur most completely, an exposure of approximately 3-0-40 minutes is required. The second step is followed by the addition of acid and massive precipitation.

Several variants of pulp neutralization have been developed: at room temperature and at a temperature of 60-80°C. If the glass modulus is high, above 2.3, then silica deposition is recommended to be carried out at room temperature, with a relatively low modulus (<2,3). Кремнезем более интенсивно осаждается при 60-80°C.

7. Filtration is carried out under reduced pressure (the vacuum is 0.01 atm). Filtration is carried out in 2 stages.

The first stage is filtration through a ceramic filter. After separating the filtrate from relatively large particles, the filtered water glass is filtered through a rag-and-paper filter.

8. Washing.

Washing is carried out with distilled water in 3 stages.

9. Drying is carried out at 600°C for 1 hour.

silica aging.

It has been experimentally established that the properties of silica can change if, before drying, the gelatinous precipitate of silica is kept for some time (1-2 days) under stationary conditions.

The conducted studies also showed that the following modes of carrying out the process of synthesis of liquid glass are very stable for obtaining liquid glass:

To obtain silica with a specific surface area of 150-200 m 2 /g

Diatomaceous earth 1000 g

Water 4000 cm 3

Voltage V=5 kV

Time 1.5 h.

Mode: 0.5 h (stopping, stirring) 0.5 h (stopping, stirring) 0.5 h (completion of the experiment, opening the reactor) draining the pulp into another container

Pulse frequency 5 Hz

Power - 1.5 W.

Synthesis time - 1.5 hours.

Precipitation in 2 stages. With increasing time, the yield of liquid glass of the required module increased significantly.

With an increase in the concentration of alkali in liquid glass, an increase in the specific surface of the precipitated silica occurs. The minimum specific surface area of precipitated silica was obtained at an alkali concentration of 6%.

The highest specific surface area - 700 m 2 /g was obtained with the content of NaOH 600-700 g per 1000 g of diatomite, other parameters are the same.

Raw diatomite of the Akhmatovsky deposit (composition SiO 2 - 78.16, TiO 2 - 0.52, Al 2 O 3 - 5.6, Fe 2 O 3 - 3.07, CaO - 0.42, MgO - 0.80, Na 2 O - 0.00, K 2 O - 1.61, SO 3 - 0.84, P.p.p. - 8.9) in a lumpy state is dried at a temperature of 100 ° C, crushed in a mill to a fraction - 0.2 mm, ground diatomite is fired in an electric furnace at a temperature of 600°C for 1 hour.

Water - 4000 cm 3,

Diatomaceous earth - 1000 g,

Solid NaOH - 500 g,

Ratio W:T=8:3.

Stir at room temperature for 10 min. The finished suspension is sealed in an electric pulse apparatus for the synthesis of liquid glass.

As a source of electromagnetic pulses, a ZEVS-25 installation is used with stored energy in one pulse up to 600 J. The voltage on storage capacitors with a total electrical capacitance is 8 μF, the distance between the electrodes is 5 mm, the voltage is 5 kV.

The pulse repetition rate is 4 Hz. When the unit is operating, the power consumed from the 220 V network is no more than 1.2 kW.

The temperature in the reactor is 85°C.

The time for the synthesis of liquid glass is 1.5 hours, with a periodic (every 0.5 h) after the start of the process, a stop to stir the suspension for 15 minutes.

The separation of liquid glass from the solid phase is carried out by slow decantation of the liquid after holding for 10 hours.

In a container with liquid glass 3 l at a temperature of 75°C slowly gradually add 14% sulfuric acid. First, 200 cm 3 of dilute sulfuric acid are added, followed by an exposure of 20 minutes, then 50 cm 3 of sulfuric acid are added, an exposure of 20 minutes. The neutralization process is completed at a pH of 7.

Filtration is carried out under reduced pressure (vacuum is 0.01 atm). Filtration is carried out first through a ceramic filter, and then through a rag-and-paper filter. Next, wash with distilled water in 3 stages.

The gelatinous precipitate of silica is kept for a day under stationary conditions and dried at 600°C for 1 hour.

As a result, silica of the following properties was obtained:

snow-white powder, bulk density 250 kg/m 3 , SiO 2 content - 99.93%; the content of impurities (Al, Fe) does not exceed 0.07%. Specific surface, according to BET - 670 m 2 /g, the particles are spherical, the size is 8-10 nm in diameter, 40% of the pores have a diameter<2 нм, остальные 60% >2 nm.

Table 2 presents the results of 11 experiments using an electric pulse installation (examples 2-12). The first 2 of them were short-term 5 and 10 min at 5 and 10 kV and aimed to prove that this method could be used in principle for the synthesis of SiO 2 . As a result, sodium trisilicate was obtained with a very low density and correspondingly low rheological properties. When neutralized with sulfuric acid, rare flakes and needle-shaped silica crystals fell out of the solution.

Were obtained IR spectra of silica deposited from water glass, synthesized by electropulse method (figure 2), from which it is seen that the bond Si-O-Si ghb 1161-1211, characterizing the chemical activity of silica is very clear.

For comparison, IR spectra were obtained of silica deposited from water glass synthesized by autoclave (figure 1). The Si-O-Si bond is most clearly manifested at 1084 cm -1 , the second bond at 1161 cm -1 is just emerging.

In addition, electron microscopic studies were carried out. Figure 3 shows the image of silica synthesized by electropulse method. Spherical particles are homogeneous. The average size is 6-8 nm. Micropores are visible, contributing to an increase in the specific surface area. This pore size is most favorable for catalytic reactions. The content of SiO 2 over 99.2%.

In addition, figure 4 shows: a) - electron microscopic image of silica synthesized by electropulse method, b) electron microscopic image of silica obtained in an autoclave. On figa) shows that the resulting spherical particles do not exceed 6-8 nm in diameter, when as in figa) shows that the particle size is not constant about 100-200 nm.

The cavitation effect in a liquid can occur not only under the action of a spark discharge, but also with a local decrease in pressure caused by the passage of a liquid from a region of high energy potential to a region with low energy potential. During the formation of liquid glass, cavitation is used to homogenize the suspension and transfer suspended particles into a colloidal liquid state. The mixture is processed in a cavitation disperser operating in a closed cycle.

From the working chamber of the dispersant, the alkali solution is fed into the block with spinnerets. Dies are narrow cylindrical holes in a metal blank that rotates around an axis. From the block with dies, the liquid enters the chamber, in which, due to the size and ejector shape, a significantly lower pressure can be provided than in the high pressure chamber. Between the working chamber and the block with dies there is a damper that rotates autonomously. It controls the diameter of the inlet. This feature of the mechanism allows you to adjust the process of work in various environments.

When filling the working chamber, the pump is turned on, while the damper closes momentarily. As soon as it opens, the solution rushes into the low pressure chamber. The speed of passage of liquid through the dies is very high, while maintaining a high pressure.

Ground diatomite with a certain amount of water is preliminarily placed in the working chamber. Due to the pressure difference in the high-pressure chamber and in the working chamber, cavitation cavities arise, causing the material to be crushed. In this case, as in the electric pulse, gases enter the cavities. The cavities close and produce work in a fraction of a second due to the hydraulic wave. The operation is repeated as many times as required by the regulations.

For the experiment, lumpy diatomite of the Akhmatovsky deposit was taken (composition SiO 2 - 79.58, TiO 2 - 0.37, Al 2 O 3 - 5.6, Fe 2 O 3 - 3.11, CaO - 0.27, MgO - 0.79, Na 2 O - 0.00, K 2 O - 1.16, SO 3 - 0.12, P.p.p. - 9.44).

In a lumpy state, diatomite is dried at a temperature of 200°C and crushed in a mill to a fraction<0,2 мм.

The mixture is prepared in the reactor in the following quantities:

Water - 180 l,

Diatomaceous earth - 50 kg,

Solid NaOH - 20 kg,

Ratio W:T=3:1.

The mixture is pre-mixed for 15 min at room temperature and placed in a rotary cavitation disperser (possible loading volume 300 l, power 100 kW, power supply 3 phase 380 V).

The mixed mixture is processed by cyclic pumping in a closed circuit in cavitation mode at a temperature of 90°C for 3 hours 40 minutes with a rotor rotating at 3000 rpm.

Get 100 liters of liquid glass of bright red color.

Silica precipitation is carried out as follows.

The system is cooled for 1 h to a temperature of 35°C.

Liquid glass is separated from the solid phase in portions (5 l in 1 portion) using a vacuum pump and a ceramic filter.

Silica is also precipitated from liquid glass in portions obtained after filtration. A total of 20 servings are processed.

1.5 kg of silica was obtained from each serving. This staging was caused solely by the difficulties that arise when washing large amounts of silica.

The following neutralization scheme was adopted. In the first stage, 3000 cm 3 of dilute sulfuric acid are added, followed by an exposure of 20 minutes, then 1000 cm 3 of sulfuric acid are added, an exposure of 20 minutes. The neutralization process is completed at a pH of 7.

Filtration is carried out under reduced pressure (the vacuum is 0.01 atm).

The gelatinous precipitate of silica is kept for a day under stationary conditions and dried at 500°C for an hour.

As a result, 30 kg of silica with the following properties were obtained:

bulk density of silica 250 kg/m 3 , SiO 2 content - 97.92%; the content of impurities (Al and mainly Fe) does not exceed 2.17%, the specific surface according to BET is 512 m 2 /g, the particles of silica proper are spherical.

Silica, like glass, is colored red with a brownish tinge.

A high-resolution electron microscope study showed that the main pigment responsible for the red color of the synthesis products is an admixture of a newly formed ferruginous mineral of the goethite type. Goethite penetrating silica crystals has the form of needles up to 2-3 nm long and up to tenths of nm thick. The content of needles is 15-20% of the total amount of silica obtained.

Table 3 presents examples 14-16 of the implementation of the method using a rotary cavitation disperser.

Thus, the developed electropulse and hydrodynamic methods for producing liquid glass make it possible to exclude the expensive process of obtaining a “silica block” from which liquid glass is produced.

The developed method is much cheaper and more progressive, and the use of high-silicon rock - diatomite allows you to expand the range of raw materials used, the reserves of which are practically unlimited in the region.

The use of electropulse and hydrodynamic methods for the production of liquid glass are energetically more favorable processes compared to the autoclave one due to time savings, and make it possible to impart to the final product, in particular, increased reactivity, which manifests itself in specific industries, and a high specific surface area (see Fig. 3-4).

The choice of varieties is dictated by the primary need of production: for the tire industry, for the paint and varnish industry, the production of various building composites and high-purity products used in pharmacology and medicine.

According to the proposed method, it is possible to obtain 3 grades of silica, differing in the size of the specific surface and the content of SiO 2:

The first type of product is intended for use in cosmetics and perfumery, in particular in the production of silicone pastes.

Grade At-2 is most effective in the production of a special type of varnishes and paints.

Grade At-3 can be successfully used in the production of silicone rubber, adhesives and sealants, silicone elastomers and in the processing of rubbers for various purposes.

The developed method allows the final properties of silica to be laid at the stage of obtaining liquid glass and subsequent modes of deposition and washing, and are determined by the specific requirements of production.

CLAIM

1. A method for producing amorphous silicon dioxide, including the stages of grinding the initial silicon-containing raw material, which is used as a natural rock with a high content of bound amorphous silica, up to 70-75%, preparing a charge at a ratio W:T equal to 4-6:1 , processing the latter to obtain a solution of liquid glass, separating the precipitate formed, precipitating silicon dioxide from the resulting liquid phase with mineral acid in stages under the control of the pH of the medium, isolating the resulting target product by filtration followed by repeated washing with water and drying, characterized in that as the specified mountain rocks use diatomite, which is crushed to obtain a fraction with a grinding fineness of not more than 0.01 mm, the crushed diatomite is preliminarily fired at a temperature of 600-900 ° C, and the charge is processed in the mode of a cavitating medium created by an electropulse or hydrodynamic method.

2. The method according to claim 1, characterized in that the firing of diatomaceous earth is carried out for 1-1.5 hours.

3. The method according to claim 1, characterized in that the processing of the mixture is carried out in an electric pulse reactor at 80-90 ° C, at a voltage and power of 5-10 kV and 1.2-1.5 kW, respectively, and a pulse repetition rate - 2-7 Hz.

4. The method according to any of claims 1 and 3, characterized in that the processing in an electric pulse reactor is carried out for 1.0-2.0 hours with periodic mixing of the mixture every 0.5 hours.

5. The method according to claim 1, characterized in that the mixture is processed in a cavitation disperser for 2.5-3.5 hours at 1500-3000 rpm and a temperature of 80-90°C.

6. The method according to claim 1, characterized in that nitric, sulfuric or hydrochloric acids, preferably 40-50% nitric acid, are used as the mineral acid.

7. The method according to claim 1, characterized in that the target product is additionally washed with a weak 2-5% solution of nitric acid.

The article describes a food additive (anti-caking agent and anti-caking agent) amorphous silicon dioxide (E551), its use, effects on the body, harm and benefits, composition, consumer reviews

Functions performed

anti-caking agent and anti-caking agent

Legality of use

Ukraine

Russia

What is food additive E551 - amorphous silicon dioxide?

Silicon dioxide is an inorganic compound with little activity under normal conditions. At room temperature, it does not dissolve in water, does not interact with it and with other substances. This oxide is acidic and under certain conditions can form salts of silicic acid, which are called silicates.

Silicon dioxide is widely distributed in nature, it is part of many rocks and minerals. In everyday life, it is known to everyone as ordinary (quartz) sand. There are several types of crystalline modifications of this substance.

The amorphous form of silicon dioxide is used in pharmaceuticals as an auxiliary and basic substance. Amorphous silicon dioxide is a food additive E551, which is used in the food industry to prevent caking and clumping of dry powder products.

In industry, silicon dioxide is used in the production of building materials, ceramic products, abrasives, fiber optic cables. For technical purposes, a product from natural sources is used. In the food and pharmaceutical industries, silicon dioxide synthesized by oxidation of silicon at a very high temperature is used as an additive E551.

Amorphous silicon dioxide, E551 - effect on the body, harm or benefit?

Additive E551 is one of the safest compounds for health. This substance is absolutely insoluble in the esophagus and is excreted from the body unchanged. In addition to a positive effect on food quality, the E551 supplement can have a cleansing effect on the intestines. It is no coincidence that silicon dioxide is used in practical medicine as an enterosorbent. This substance is present in many toothpastes and contributes to the mechanical and microbiological cleaning of the oral cavity.

Given the insolubility of silicon dioxide, people who have problems with the excretory system should not abuse food products with the addition of E551. When large amounts of this substance enter the body, its accumulation in the ducts of the urinary system cannot be completely excluded, especially in cases where they are deformed or spasmodic.

Food additive silica amorphous - food application

Additive E551 prevents caking of dry food products, the formation of lumps in them. It is used for packing spices and other mixtures. The addition of amorphous silica is especially relevant when dry food products are wrapped in foil. The maximum concentration of E551 in one kilogram of food mixtures should not exceed 30 grams. Silicon dioxide is approved for use as a food additive in all countries.

Amorphous silica can be classified into three types:

1. Quartz glass made by melting quartz (as well as high-temperature hydrolysis of silicon tetrachloride or its oxidation in a low-temperature plasma).

2. Silica M - amorphous silica obtained by irradiation with fast neutrons of amorphous or crystalline varieties of silica. In this case, the density of the initial amorphous silica increases, while that of the crystalline silica decreases. Silica M is thermally unstable and transforms into quartz at 930C for 16 hours. Its density is 2260 kg / m 3 (for quartz glass - 2200).

3. Miroamorphous silica, including sols, gels, powders and porous glasses, which consist mainly of primary particles with a size of less than one micrometer or with a specific surface area of more than 3 m 2 /g.

Microamorphous silica synthesized under laboratory conditions can be divided into three classes:

I Microscopic varieties obtained by special processes in the form of leaves, ribbons and fibers.

II Conventional amorphous forms consisting of elementary spherical SiO 2 particles smaller than 100 nm in size, the surface of which is formed either from anhydrous SiO 2 or from SiOH groups. Such particles can be separate or connected in a three-dimensional network: a) discrete or isolated (particles, as is the case in sols; b) three-dimensional aggregates connected in chains with a siloxane bond at the points of contact, as in gels; c) bulk three-dimensional particle aggregates, as observed in aerogels, silica of trogenic origin, and some dispersed silica powders (see Fig. 1.13).

III Hydrated amorphous silica in which all or almost all of the silicon atoms are held by one or more hydroxyl groups.

Rice. 1.13. Elementary particles of common forms of colloidal silica. The figure is presented flat, but in fact the aggregation of particles is three-dimensional: a - sol, b - gel, c - silica powder

Microamorphous silica of layered, ribbon and fibrous microforms is obtained:

1. The formation of particles at the gas-liquid interface as a result of the hydrolysis of SiF 4 in the gaseous state at 100 or the hydrolysis of SiCl 4 vapors at 100C. The flakes are thin films of silica gel formed on the contact surface of extremely reactive SiF 4 vapors with water droplets. The "fluffy" character of the powder prepared from SiF 4 is manifested in its very low apparent density value of 25 kg/m 3 and also in the "fluidity" of the powder, similar to that of water. Irregular silica gel flakes, about 1 µm in diameter and 1/10 µm thick, contain 92.86% SiO 2 and 7.14% H 2 O.

2. Formation of silica sols by freezing. When a solution of colloidal silica or polysilicic acid is frozen, the growing ice crystals will displace the silica until the latter accumulates between the ice crystals as a concentrated sol. Such silica then polymerizes and forms a dense gel. The subsequent melting of ice produces silica in the form of irregularly shaped flakes formed between the smooth surfaces of ice crystals. Vacuum dried silica powder contains approximately 10% H 2 O.

The most common silica in amorphous form is silica gel and quartz glass. Silica gel is obtained by heating silica gels to temperatures not exceeding 1000C. Ready technical silica gel is solid translucent granules of white or yellowish color. Widely used as a moisture absorbent.

The silica melt is easily supercooled to form quartz glass. Quartz glass used in engineering is a one-component silicate glass. It is obtained by melting natural or artificial varieties of silica of high purity.

With an increase in pressure, modification transformations were also established for non-crystalline silica - quartz glass. When glass is compressed, the Si-O-Si bonds in it are bent. With an increase in pressure to 3100-3300 MPa, a transition is observed, accompanied by a sharp change in density (transformation of the second kind). Glass produced at this pressure is called suprapiezoglass(abbreviated S-P-glass).

With an increase in pressure above 9000 MPa, the density of glassy silica again begins to increase and at 20000 MPa becomes equal to 2.61. 10 3 kg/m 3 , which is close to the density of quartz, but the material remains amorphous. Such glass does not elastically return to its original volume when the pressure is removed, and thin discs of superdense (condensed) quartz glass can be preserved. This compacted quartz glass is called condensed.

Characteristics of polymorphic modifications of SiO 2 are given in table 1.1.