RU2134657C1 - Method of producing thermally expanded graphite - Google Patents

Abstract

FIELD: carbon materials. SUBSTANCE: invention is designed for mechanical engineering, chemical, nuclear, and aerospace industries and can also be used when manufacturing graphite foil, heat-resistant insertions and sealing means, sorbents, and composites. Oxidized graphite and air are mixed at volume ratio 1:(750-1400). Double-phase stream is fed into thermal shock zone (950-1400 C) of vertical tubular heater. Feed rate of stream is determined by annular flow condition. Minimum heater residence time for particles is 0.1 to 0.4 s. Expanded graphite is cooled when withdrawn over inclined or horizontal pipeline at the expense of destruction of annular regime and formation of liquid deficient regime of stirring- providing flowing stream. Graphite expansion degree is 25 to 150 and loose density 1.7 to 1.9 kg/cu.m. EFFECT: optimized process conditions. 3 cl, 1 dwg, 2 ex

Description

 The invention relates to the technology of carbon-graphite materials, in particular to the production of thermally expanded graphite (TEG), used to produce graphite foil, heat-resistant gaskets and seals, and also as a sorbent in the treatment of water and soil contaminated with oil or oil products.

Known methods for producing TWG are based on the influence of thermal shock (900 - 1400 ^o C) on the powder of oxidized graphite (OG). One of the possible schemes for organizing the process involves dosing oxidized graphite and simultaneously supplying nitrogen, as a result of which a two-phase flow of “gas-particles of graphite” is formed. Then, the two-phase flow is directed to the thermal shock zone created at the inner walls of the hollow tubular electric heater (patent SU 1657473, 1991).

 However, neither temperature nor the residence time of oxidized graphite in the thermal shock zone is optimized. This leads either to insufficient expansion of the particles of oxidized graphite upon local or general underheating, or to partial oxidation upon overheating. In both cases, this affects the characteristics of the TWG, especially those that are important in its further use to obtain the sorbent.

A known method of producing TWG, comprising mixing oxidized graphite with a heated carrier gas and feeding the mixture from above into a vertical pipe heated to 600 - 1000 ^o C. Thermally expanded graphite additionally foams when leaving the pipe into a cylindrical chamber, and then enters the receiver-drive , where it is separated from the carrier gas and flue gases (ed. certificate. SU 1480304, 1994). The method allows to obtain TWG with a bulk density of 2-10 kg / m ³ , however, the heat treatment lasts from 7 to 20 minutes, which significantly limits the performance of the process.

 These disadvantages are eliminated if a vertical tube heater is used for thermal expansion of oxidized graphite, in which microwave heating, ohmic heating, direct fuel combustion, or any other heating principle can be used.

 According to the invention, an upward two-phase flow is formed in the heater pipe, in which the entire dispersed phase is located on the periphery of the stream along the heated walls of the pipe, and only the gas phase (air, nitrogen or inert gas) moves in the central part of the pipe. Such an annular flow regime is known for gas-liquid flows in pipes (Mamaev V.A., Odisharia G.E., Semenov N.I., Tochigin A.A. Hydrodynamics of gas-liquid mixtures in pipes.- M .: Nedra, 1969). For mixtures of gases with bulk solids in pipes, an annular flow regime can also be arranged. It depends on the type of gas and bulk material, gas flow rate and volumetric content of the discrete phase, as well as on the inner diameter of the pipe, the state of its surface and heating temperature. However, for specific plants operating with a certain bulk material and gas at a stable heating temperature, the annular flow regime is achieved either by adjusting the gas flow rate or by changing the dosage rate of oxidized graphite powder. The process can be optimized for both of these characteristics, but most often only the gas flow rate is regulated, since the powder flow rate is usually set to the maximum allowable.

 In an upward two-phase flow, the discrete phase, under the action of gravitational forces, tends to reverse motion at a speed called the reverse speed. But, if the velocity of the upward flow is high enough, the particles can be kept in equilibrium. This flow rate is called the particle velocity. With a further increase in the gas flow velocity, the particles will move along the pipe wall in the direction of gas movement.

Since particles of oxidized graphite can vary in size, they will be located at different heights along the surface of a vertical tubular heater and in a short time of the order of 0.1 - 0.4 s will be heated to a temperature of 900 ^o C and above, at which rapid expansion of oxidized graphite occurs. In this case, the particles expand by 25-150 times and due to a sharp increase in their windage, they are carried out by the gas flow outside the thermal shock zone.

 The determination of the optimal process characteristics in which an annular flow regime of a two-phase flow occurs can be carried out during testing of apparatuses for producing thermally expanded graphite with varying air flow rate and / or dosing rate of oxidized graphite powder. If quartz glass tubes are used in the heater, the process can be observed directly through the transparent walls of the tubes. More accurate data on the achievement of the annular flow regime can be obtained using scanning equipment, such as tomographs.

 Achieving the optimal flow regime with a minimum residence time of particles in the thermal shock zone can be more easily determined indirectly by the quality of the obtained TEG. Thus, the achievement of an optimal annular flow regime can be evidenced by a uniformly matte surface observed on the reflected beam of a powerful (for example, krypton) light source, on which there are no brilliant inclusions of unexpanded exhaust particles. In the presence of brilliant impregnations, the gas flow rate is changed, and then, if the inclusions in the resulting product do not disappear, the dosing rate of the initial exhaust powder is reduced.

 The results obtained are drawn up in the form of tables or nomograms and can be included in the accompanying documents of the device. In the future, when the apparatus is operated, depending on changes in the particle size distribution of individual batches of oxidized graphite powder, only a slight adjustment of the gas flow rate or the powder dosing rate will be periodically required.

 With the optimal process parameters found, the TWG should have a stable bulk density, which is continuously or periodically measured at the outlet of the installation by an automatic densitometer. When the bulk density changes (for example, as a result of a drop in temperature in the heat-shock zone or a change in the quality of the raw materials), the densitometer generates a signal that serves as a command to further control the gas flow rate and / or the dosing rate of the exhaust gas powder. The entire adjustment process can be fully automated using a computer.

A method for producing thermally expanded graphite includes dosing oxidized graphite powder, forming a two-phase annular stream of powder particles in a gas stream, supplying this stream to the thermal shock zone of a vertical tubular heater with a temperature of 950 - 1400 ^° C, followed by removal of expanded graphite from the heater while cooling it. In this case, a two-phase flow is fed into the tubular heater from below with a speed of not less than the speed of the particles of oxidized graphite, and the expanded graphite is cooled while it is removed from the heater through an inclined or horizontal pipe due to the destruction of the annular flow mode and the formation of a dispersed flow regime that provides mixing phases.

 The volumetric ratio of powder and gas during dosing and the formation of a two-phase flow is mainly maintained in the range from 1: 750 to 1: 1400. The minimum residence time of particles in a tubular heater is from 0.1 to 0.4 s.

 When moving inside the heater, the main gas stream (usually air) does not come into contact with the hot walls. Having low thermal conductivity and significantly exceeding the dispersed phase in volume, it practically does not have time to heat up while it is in the heater. Due to this, the process of expansion of oxidized graphite occurs at relatively low energy costs, as it avoids the consumption of heat for useless heating of air.

 Moreover, the proposed method allows you to cool the resulting TEG with the same unheated gas (air) directly in the pipeline connecting the heater to the receiving hopper of the finished product. To this end, the TEG particles moving along the periphery of the stream are mixed with gas moving in the central part of the pipe. This is achieved by changing the flow direction from ascending to inclined, horizontal or descending, due to which the annular structure of the two-phase flow is destroyed and the hot particles cool, mixing with a colder gas.

 The drawing shows a diagram of a process for producing thermally expanded graphite according to the invention.

 Position 1 in the diagram indicates the dispenser of oxidized graphite powder; position 2 - gas inlet to form a two-phase flow; 3 - pipeline supplying a two-phase flow into a vertical tubular heater 4; 5 - horizontal pipe-cooler expanded graphite (pipe-cooler can also be installed with a slight slope up or down); 6 - output of the finished product.

 Information confirming the possibility of implementing the method.

 Example 1

In the experimental setup for the implementation of the claimed method, a heater was used with a pipe height of 80 cm and an inner diameter of 2.2 cm. At an air flow rate of 50 l / min, the flow rate of oxidized graphite powder with a density of 400 kg / m ^{3 was} maintained at 0.045 l / min. The temperature of the inner walls of the tubular heater was 980 ^° C. The average residence time of particles in the heat-shock zone was 0.36 s at a two-phase flow velocity at the outlet of their heater - 220 cm / s. When the length of the horizontal cooler pipe equal to 0.7 m, the temperature of expanded graphite due to mixing of the two-phase flow decreased at the outlet to 78 ^o C. The finished product was a homogeneous gray flocculent powder with a bulk density of 1.75 kg / m ³ .

 Example 2

Under the conditions of example 1, but when passing a two-phase flow with a speed three times greater (that is, with an average time of particles in the heat-shock zone of 0.11 - 0.12 s), a homogeneous expanded graphite powder with a bulk density of 1 was obtained. 9 kg / m ³ .

 Industrial applicability.

 The invention can be used to obtain thermally expanded graphite in various fields of technology.

 Materials based on TEG are used in mechanical engineering, chemical, nuclear, aerospace, heat engineering and other industries as sorbents, fillers, protective coatings, gaskets and seals that work at elevated temperatures and are resistant to chemicals, heat and flame retardant composites, etc. .d.

Claims (3)

1. A method of producing thermally expanded graphite, comprising dosing an oxidized graphite powder, forming a two-phase stream of powder particles in a gas, supplying this stream to a thermal shock zone of a vertical tubular heater, then removing expanded graphite from the heater and cooling it, characterized in that the two-phase stream is supplied a tubular heater below a rate to provide an annular flow regime the two-phase stream at a temperature in the thermal shock zone 950 - 1400 ^o C, and cooling the expanded th graphite performed during its removal from the heater along an inclined or horizontal line by breaking the flow of the annular flow mode and flow mode of formation of the particulate providing a phase mixing.  2. The method according to claim 1, characterized in that the volumetric ratio of powder and gas during dosing and the formation of a two-phase flow is maintained within the range of 1: 750 - 1: 1400.  3. The method according to claims 1 to 2, characterized in that the minimum residence time of particles in a tubular heater is 0.1 - 0.4 s.