RU2559966C1 - Annealing of large-size blanks of fine-grain graphite by isostatic pressing - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: claimed invention relates to production of large-size articles from fine-grain graphite, over 800 mm in length and over 300 mm in diameter. Every said blank is arranged in annealing furnace chamber cartridge at loading thereto. It is arranged vertically in a special individual shell with wall depth making at least 10 mm to allow uniform temperature field over the blank entire surface. Aforesaid shell is made of metal (steel or aluminium) or ceramic material with high heat conductivity and capacity comparable with those of metals. Sizes of shell inside sizes allow a uniform gap between blank side surface and shell wall of at least 25 mm. Said gap is filled with heat insulating disperse hydrocarbon material or the other one that features heat conductivity factor smaller than that of the other filling used in loading of annealing furnace. Annealing is executed on schedule for at least 320 hours to maximum temperature of 1100-1500°C.

EFFECT: higher yield owing to decreased amount of rejects at annealing stage in multichamber furnace of industrial furnaces.

1 tbl, 1 ex

Description

The present invention relates to the production of graphitized carbon structural materials, and in particular to the operation of firing large-sized workpieces of fine-grained isostatic pressing graphite with a length l> 800 mm and a diameter> 300 mm in industrial multi-chamber shaft furnaces "Ridhammer".

Currently, such billets are fired in Ridhammer furnaces according to a technology that provides for the placement of large-sized pieces of fine-grained isostatic pressing graphite in the oven cassettes in a vertical position. At a distance of 90-100 mm from the walls of the cassette of the kiln and from adjacent workpieces and 100-150 mm along the end surfaces from other workpieces laid lower or higher in this cassette. The preforms are poured with a heat-shielding charge, which is used as graphite coke breeze fraction of 0.5-4 mm or graphite pecocoke breeze of the same fraction. With a width of the cartridge of the chamber of the kiln of firing of 650-700 mm and a length of 1500-1700 mm, one-sided side heating of the workpiece is obtained with the heat coming from the nearest wall of the cartridge.

The rate of temperature rise under the roof of the chamber of the kiln is low, because the firing cycle is 350-400 hours when the furnace charge is heated from 0 ° C to 1000-1100 ° C. But due to the low thermal conductivity of the shipping materials and the workpiece itself, when it is heated unilaterally, the temperature field in the workpiece body is very uneven. Moreover, a high temperature non-uniformity is observed not only for each section of the workpiece, but also for its height. Measurements show that in the channels of the walls of the kiln, the temperature of the heating gases drops by about 50 ° C over a length of 1000 mm along the height of the furnace chamber.

It is known that the process of destruction of the pitch of the binder in the pressed billet, that is, its coking, most intensively with the formation of a solid takes place in a fairly narrow temperature range of 550-600 ° C. For a billet with a length of 800 mm, this means that in different parts along the length various processes can take place that comprise the firing operation: heating the billet, melting the pitch, its destruction with the formation of a solid coke residue, and shrinkage of the billet material. Especially at the stage of shrinkage of the material in the body of the workpiece, internal stresses arise, which are further exacerbated due to the heterogeneity and unevenness of the flow of these processes in various parts of the body of the workpiece. This is especially dangerous for large-sized preforms of fine-grained graphite hydrostatic pressing due to their higher and more uniform density over the body of the preform. During the firing process of such preforms, internal stresses lead to the appearance of cracks. The yield of fine-grained graphite hydrostatic pressing blanks with dimensions: 338 mm in diameter and 810 mm in length when fired in an annular multi-chamber firing kiln under the above conditions is usually 50-55%. Workpieces are rejected due to cracks.

A known method of firing carbon blanks in a multi-chamber annular furnace with shaft chambers equipped with vertical muffle channels, comprising supplying chilled air to chambers with annealed billets and heating the billets in the chamber with a gas-air mixture using burners in the first of the heated chambers and discharging the exhaust gases into the burs. [one]. With this method, the loading of blanks into the chamber space is carried out without the use of containers.

The closest is the container method of roasting large-sized blanks of graphitized electrodes, the process is conducted according to a schedule of at least 320 hours for electrometallurgical furnaces [2]. Sizes of the workpieces: diameter 500-600 mm, length 2000-2500 mm. Containers with lids were made of sheet steel up to 5 mm thick. Their dimensions ensured the placement of an electrode surrounded by thermal insulation, graphitized by coke breeze of a fraction of 0.5-4 mm. From the side surface of the electrode billet, the insulation layer was 70-80 mm, and from the end, 150-200 mm. Containers with blanks were placed in the oven cassette without external heat-insulating transfer. The temperature distribution over the height of the container corresponded to the temperature distribution over the walls of the cassette. Low specific gravity of the container, i.e. the mass per unit surface area of the workpiece does not allow heat to be accumulated and redistributed over the container to equalize the temperature both along its height and around the circumference in each section.

The containerized method of firing large-sized billets does not eliminate the main disadvantage of traditional firing in industrial multi-chamber furnaces, i.e. does not lead to uniform distribution of temperature over the entire side surface of the workpiece. However, in the electrode blanks having a medium- and coarse-grained structure, the level of thermal stresses arising is lower than in the fine-grained large-sized hydrostatic pressing blanks and does not lead to cracking and marriage. The container firing method simplifies loading and unloading operations of the kiln during the firing of large electrodes. That is why it is more preferable in this case.

The basis of the present invention is the task of increasing the yield of large-sized preforms of fine-grained graphite of isostatic pressing by reducing the marriage of cracks at the stage of firing in multi-chamber industrial furnaces. This is achieved by equalizing the temperature along the entire side surface of the preform during the firing process and thereby ensuring that the corresponding processes determining the firing process occur simultaneously in different parts of the preform, such as:

- melting the binder pitch (temperature 300-400 ° C);

- destruction of the binder pitch with the formation of a solid workpiece structure, starting at a temperature of more than 400 ° C and most intensively going in the temperature range of 550-600 ° C;

- subsequent shrinkage of the workpiece material, ending at a temperature of 1100-1150 ° C.

The technical result of the task is achieved by the fact that each large-sized workpiece of fine-grained isostatic pressing graphite is placed in the furnace chamber cassette when loaded into the kiln, installing it while standing, to increase the heat transfer area along the side surface, in a special individual shell with a wall thickness of at least 10 mm, it is this wall thickness that provides sufficient mass for the shell to accumulate heat and redistribute it over the entire area, in order to obtain a uniform temperature fields over the entire surface of the workpiece. The shell is made of metal or a highly heat-conducting ceramic material having high thermal conductivity and heat capacity, comparable with the same properties of metals. The dimensions of the inner cavity of the shell should provide a uniform gap between the side surface of the workpiece and the wall of the shell of at least 25 mm. The resulting cavity between the preform and the shell is filled with a heat-insulating dispersed carbon or other material with a thermal conductivity coefficient lower than that of the rest of the overburden used when loading the kiln. This provides a thermal barrier between the shell and the workpiece, which does not allow the rapid removal of heat from the shell from the sides on which it receives heat from the heating walls of the firing chamber, thereby enhancing the effect of uniform heat distribution throughout the shell and thereby the uniformity of the temperature field along the side surface of the workpiece.

Thus, the shell is an accumulator of heat coming from the heating walls of the furnace chamber cartridge. At close values of the specific heat, for example, carbon dispersed powder and steel or aluminum, from which the shell can be made. The metal has a significantly larger mass, so the shell can accumulate heat. At the same time, the presence of a barrier heat-insulating inner layer between the shell and the side surface of the workpiece does not allow to remove the same amount of heat through this layer that the shell receives from the nearest heating wall of the furnace cassette. Therefore, the "excess" heat obtained by part of the shell due to the high thermal conductivity of the material of the shell is distributed to other parts of the shell, thereby equalizing the temperature across the entire inner surface of the shell and thereby on the outer surface of the workpiece.

Concrete example

The proposed firing method was tested when firing preforms of fine-grained isostatic pressing graphite with dimensions of ⌀338 × 850 mm in an industrial twenty-chamber annular Ridhamer firing furnace with a 360-hour firing schedule. The shells were made of steel 3 (st3) and had dimensions ⌀420 × ⌀400 × 850 mm. Shells with blanks placed in them were installed at a distance of 50-60 mm from the walls of the cassette. The distance between adjacent shells in the cartridge was 20-25 mm. As the main heat-insulating overburden, graphite pitch-coke fines were used with a fraction of 0.3–4 mm. The inner cavity of the shell, i.e. the gap between the side surface of the billet and the inner surface of the shell was filled with non-graphitized pecococcal fines of a fraction (0-0.3) mm. The end layers of thermal insulation were 200 mm thick.

When firing such billets without the use of shells, the total end-to-end output after graphitization was no more than 30%, and only for firing operations, the reject was 35-40%. The main type of marriage is transverse cracks in the part that is average in height of the workpiece, which indicates high longitudinal stresses in the body of the workpiece.

In the case of using steel shells when firing such billets and using Pecococcal fines, a fraction (0-0.3) mm for the inner insulation layer, the through yield of billets after firing and graphitization was 90-95%, which ensured a high profitability of the firing operation. The increase in the yield of finished billets is achieved precisely through the use of the described method of firing. The use in the firing operation of heat-accumulating massive steel shells in combination with a barrier heat-insulating layer that reduces heat outflow normal to the inner surface of the shell, contributes to a uniform temperature distribution on this surface and, thereby, a decrease in the longitudinal temperature gradient over the workpiece, i.e. reducing the level of longitudinal stresses in the body of the workpiece.

Table 1 shows the thermal conductivity coefficients of graphitized pitch-coke breeze (PCM) fraction (0.3-4) mm and non-graphite PCM fraction (0-0.3) mm used for the firing of the proposed method of fine-grain graphite isostatic pressing ⌀338 × 850 mm loaded into the furnace in steel shells, as well as for lamp black, as promising for a barrier thermal insulation layer.

Table 1 Thermal insulation material λ W / m ° C at T ° C twenty 550 Graphite RMB fr. (0.3-4) mm 0.25 1.09 Annealed RMB (1000 ° C) fr. (0-0.3) mm 0,03 0.25 Lampblack
Bulk weight 0.17 g / cm ³ 0.012 0.09

Information sources

1. USSR author's certificate No. 1736923 of 05/30/1992.

2. Pirogov A.V., Seleznev A.N., Pirogov V.I. and others. "Firing of impregnated electrode blanks in metal containers without overfilling", Non-ferrous metals, 2006, N 6, S.50-54

Claims (1)

The method of roasting large-sized preforms of fine-grained graphite of isostatic pressing in industrial multi-chamber furnaces, comprising placing the preforms in the furnace cassette, standing, surrounded by the appropriate sizes of layers of heat-insulating dispersed powder in the form of graphite coke or pitch coke fines, and conducting the process according to a schedule of at least 320 hours, characterized in that each blank is installed (placed) in a separate special shell with a wall thickness of at least 10 mm and with dimensions that ensure equal The black gap between the side surface of the workpiece and the shell wall is at least 25 mm, made of metal or a high-temperature and high-heat-conducting ceramic material, and the formed internal cavity is filled with dispersed heat-insulating material with lower thermal conductivity than that of the material surrounding the shell.