The method of obtaining carbon products

 

The invention relates to the field of production of carbon-graphite materials and, in particular, can be used in the manufacture of nuclear fuel and powder metallurgy. The inventive method of obtaining carbon products is that preparing a mixture of a graphite powder, a binder and MICROTEL. The resulting mixture is subjected to a preliminary pressing, then carry out a heat treatment, during which periodically reduce the temperature of products in the areas of phase transitions 10-15C, and the temperature interval between the depressions temperature chosen in the range of 20-30C. advantages of the invention lie in the decline of marriage in the products by preventing crack formation. 1 table, 2 Il.

The invention relates to the field of production of carbon-graphite materials and can be used in the technology of nuclear fuel and powder metallurgy.

Currently, carbon-graphite materials have found application in high temperature gas cooled reactors (HTGR), in which fuel elements (cartridges) made in the form of carbon-graphite matrix containing microtia is arbid silicon.

After pre-mixing MT, graphite powder and a binder (coal tar pitch, phenol resin, etc.) billet rods pressed and then subjected to heat treatment, in which the polymerization of the binder, the carbonization of the binder due to pyrolysis emitting solid coke residue and gaseous pyrolysis products (phenol, carbon monoxide, hydrogen, and others). The last stage of thermal treatment is heating up to 1800With the final removal of gaseous products.

In the process of obtaining fuel, the yield decreases, mainly due to the appearance of cracks in the products. Cracks caused by internal pressure of the gaseous products, which in the conditions of pyrolysis tar grows to a significant size. So, when the heat source blend containing 18 wt.% phenol-formaldehyde resin in the sample volume of 10 cm3and a mass of 10 g is allocated 1 normal liter of gaseous products. If we consider that the porosity of the billet after extrusion is 40%, then when the pyrolysis temperature of the workpiece develops a pressure of 300 ATM.

Prevention of crack formation provide dopolnitvah powder (application No. 6-36052 from 12.12.88,, Japan, ΜM G 21 3/62). The disadvantage of this method lies in the complexity of additional technological operations.

Another way to prevent cracking is to carry out heat treatment under pressure in the mold (application No. 19837989 from 21.08.98, Germany, MKI G 21 21/00). The disadvantage of this method lies in the complexity of instrumentation, which leads to the impossibility of application of the method for mass production of fuel elements.

The closest in technical essence to the problem at hand is a method of heat treatment of carbon products (rods and absorbing elements of HTGR) (patent No. 3435863 Germany, MKI G 21 3/22, 21/04, Appl. 29.04.84,, publ. 03.04.86,) consists in preparing a mixture of graphitized grains of coke and phenol-formaldehyde resin, mixing microtubule binding, preliminary compressing the mixture, napressovannye for the provision of a fuel rod casings of grains of coke followed by heat treatment up to 2000C. the Disadvantage of this method is similar to the lack of first presents unique and lies in the complexity of additional technological operations and cracks.

The present invention is the task of reducing defects in the products due predoc, pressed and thermoablative, when this heat treatment is conducted periodically reducing the temperature of products in the field of phase transitions 10-15C, and the temperature interval between the depressions temperature chosen in the range of 20-30C.

The proposed method differs from the known mode of heat treatment of carbon products.

The authors of the invention on the basis of the research found that the emission in the areas of phase transitions can be stopped by reducing the temperature of the product. In Fig.1 presents derivatograph phase changes in the workpiece cylindrical fuel rod with a diameter of 12.9 mm, height 53 mm (1 - curve of temperature rise; 2 - the mass of the sample; 3, 4 - differential temperature and sample mass, temperature, pending on the y axis refers to curve 1, all other curves are given without scale), the initial mixture consisted of 73 wt.% graphite, 9 wt.% MT-0.7-0.8 mm and 18 wt.% phenol-formaldehyde resin. The curves on the curve 2 in the temperature range of 70-130To meet stage polymerization resin in the temperature range 240-400C - stage pyrolysis (with sequential allocation of phenol, water, carbon monoxide, wadati changes in sample mass. At the same time eye-catching coke residue begins to impede diffusion of the products of pyrolysis of the sample. The meaning of the invention is to stutter passing regions of phase transitions for a large part of the gaseous products was prediffusion from the sample before the coke residue will close the pores of the sample.

In Fig.2 shows thermograms of carbonization processes billet rods on the prototype method (1) and proposed method (2). It is seen that the interval 240-400With corresponding phase transitions during carbonization, passed with a ninefold decrease in the temperature every 20C. we Can assume that if complete blockage of pores at 380-400From inside the sample remains 1/9 of the total number of released gases and vapors, which dramatically reduces the likelihood of cracking. Heat treatment is not brought up to 1800-2000(The prototype), because cracks occur at the stage of carbonization at 200-800C.

The amplitude reduction temperature is selected based on the sensitivity of thermocouples (5-10C). The time interval of the temperature reduction is dictated inertsionnosti temperature reduction (10-15C) occurs after 10-20 minutes the Temperature interval between the peaks of the temperature reduction (20-30C) choose from considerations reduce the gas pressure in 6-9 times in the range 240-400With nine peaks in the interval 20With six peaks in the interval 30C).

The proposed method of producing carbon products in addition to the main task of eliminating cracks has the following additional advantages compared with the known:

1. Has a wide scope for the products in the form of cylinders, spheres, prisms and products more complex geometry.

2. Has a universal character, since this mode of treatment does not depend on the type of graphite, resin type, size of the graphite particles and the MT, the ratio of components in the product.

The proposed method is as follows. The graphite powder with a particle size of 30-100 μm is mixed with a solution of phenol-formaldehyde resin in alcohol and MT diameter of 0.8 mm, the Composition of the mixture (excluding alcohol): graphite 73 wt.%, resin 18 wt.%, MT 9 wt.%. After preliminary pressing portions 10-11 g of a mixture in the matrix by bilateral compression samples are loaded into a quartz furnace for heat treatment, which is the province of the imps and the proposed method are presented in the table. In the experiments used two types of graphite: PGM-KS (Russia) and CL (France).

Examples of implementation are presented for two-phase composite material, rather than for graphite, because it is the composites have high hardness, low ductility, so the voltage in such materials relax by fracturing (Mechanics of composite materials. Ed.J. Sendecki. M., Mir, 1978, S. 32).

These tables show that when the modes presented in the claims, provided the yield (no cracks) 100%. When carrying out the heat treatment according to the method of the prior art (examples 1, 2) yield is 50-75%.

Claims

The method of obtaining carbon products, including mixing graphite powder, microtalon and a binder, pre-compressing the mixture and heat treatment, wherein the heat treatment is carried out periodically reducing the temperature of products in the areas of phase transitions 10-15C, and the temperature interval between the depressions temperature chosen in the range of 20-30C.

 

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