CS246353B1 - Protective layer of carbon materials, especially graphite electrodes and method of its production - Google Patents

Abstract

The protective layer of carbon materials, especially graphite electrodes, plasma-deposited consists of 65 to 99 wt. % of metallic aluminum, 1 to 20 wt. % of a mixture of metallic silicon and silicon dioxide and up to 15 wt. % of oxygen-containing aluminum compounds and has a specific resistance of 0.07 . 10 to 0.3 . 10"6 ohm.m at 20°C and 0.12 . 10-6 to 0.7 . 1C-6 ohm.m at 400°C. It is produced by feeding aluminum with a granulometry of 0.09 to 0.180 mm and silicon with a granulometry of 0.07 to 0.165 mm in an amount of 85 to 95' wt. % aluminum and 1 to 15 wt. % silicon into the flame of a plasma torch, preferably a torch with water stabilization.

Description

The invention relates to a protective layer of carbon materials, in particular graphite electrodes, against lateral opal during melting in electric arc furnaces, as well as to a method for producing this protective layer.

Metal, ceramic and metal-ceramic coatings are known. Their composition varies according to the desired layer properties. The metal protective layers find particular application in the protection of the substrate against corrosion or in cases where it is necessary to create an electrically conductive surface of the substrate. Ceramic protective layers are used in particular against the effects of high temperatures, against abrasion, etc. Metal and ceramic protective layers then have the properties of both metal and ceramic layers.

However, in some cases the properties of said protective layers are not sufficient. This is the case, for example, where the protective layer is intended to protect the substrate against corrosion at high temperatures, while at the same time providing electrical power to the substrate, such as, for example, graphite electrodes used in arc furnaces. It is known that at temperatures above 600 ° C there is already a noticeable opal to the graphite electrodes. In the literature (Metallurgy 1/1980, page 12) the value of the opal is 0.7 kg / m ² hours at 600 ° C, 5.5 kg / m ² hours at 1000 ° C and 10 kg / m ² hours at 1600 ° C.

It is known to reduce opal. No. 1,271,007] to use a protective layer comprising aluminum, silicon carbide and other refractory materials. From him. No. 1 671 065 discloses a protective layer consisting of a base layer consisting of a silicon substrate and a coating consisting predominantly of aluminum, which are deposited by flame deposition; a base layer and a cover layer thereon. No. 1 671 065 modified a fiber interlayer.

V ow. No. 827460 discloses a protective layer made by depositing a composition of TIBg and water glass on the electrode surface followed by plasma treatment at 30006000 ° K at an anode voltage of 9 & 10 kV and an anode current of 3.8 & 5 A and a plasma flame length of 80 c 800 mm for 3 < 10 min.

Z čs. No. 217,720, a protective layer is known which is based on an oxide ceramic and a metal filler such as copper or nickel.

The description of British Patent No. 1,419,302 and Bulgarian Patent Specification No. 11029 describes a process for producing a protective layer on carbon products, in particular electrodes, which is carried out by first depositing an aluminum layer on the product by electrometallization, then applying it at normal temperature. sprayed / eg. spray gun / paste made of aluminum, silicon carbide, tannic oxide and boric acid, arc fired, re-metallized, pasted again and fired, then metallized with aluminum again, graphite paste applied and fired and polished.

The prior art layers have a lower adhesion to graphite, especially at temperatures above 800 ° C with cyclic heating and cooling. Very often cracks are formed and peeling occurs, often during electrode storage. In addition, some layers, as well as processes for their production, are complicated and laborious, and thus economically inadequate. Some protective layers also change the resistivity during storage.

The object of the invention is a protective layer of carbonaceous materials, in particular a graphite electrode deposited by plasma, which consists of 65 to 9% by weight. 1 to 20 wt. % of a mixture of metallic silicon and silica and up to 15% by weight of aluminum compounds. The layer according to the invention is electrically conductive and preferably has a resistivity of 0.07.times.10@-4 ohm.m at 20 DEG C. to 0.3.times.10@-6 ^.

- 3,246,353 and 0.12.10 & to 0.7.10 ohm.m at 400 ° C. The layer thickness is preferably 0.3 to 1.5 mm. The specific gravity of the protective layer is 1900 to 2300 kg / m @ 2. The specific resistance of the protective layer decreases by 10 to 15% after the first heating to 400 ° C and cooling to 20 ° C.

The protective layer according to the invention is produced by supplying to a flame of a plasma torch, preferably a water-stabilized torch, aluminum with a granulated Lometri granulated 0.09 to 0.180 mra and silicon with a granulated granulated lometry 0.07 to 0.165 mm in an amount of 85 to 99 X wt. % aluminum and 1 to 15 wt. silicon. These metals may be introduced into the flame of the plasma torch alone or in a mixture.

The choice of granulometry of the feed materials is important.

The plasma spraying of too large particles produces a porous layer with a higher resistivity. As the feed particle size decreases, the resistivity decreases to a certain value. From this value, the resistivity of the plasma feed increases with decreasing feed particle size again. The increase in resistivity of small particle plasma injections is due to the increasing proportion of oxygenated compounds formed on the superheated particles of the feed material.

To increase the efficiency of the plasma deposition, aluminum and silica can be fed into the flame by one or more, preferably two or three, inlets distributed uniformly around the flame. The feed to the flame can be carried out by compressed air or other pressurized gaseous medium. Advantageously, for example, nitrogen, carbon dioxide, hydrogen, argon, propane-butane, acetylene and the like can be used to reduce the oxidation of the superheated coating material particles. The gases may be used alone or as mixtures thereof. The plasma deposition rate is preferably 0.3 to 0.8 m.s. and the total amount of feed to the plasma torch is preferably 12 to 60 kg / h. The application of the material can be repeated several times, preferably two to four times, depending on the desired thickness of the protective layer.

Plasma-coated silicon with aluminum improves the adhesion of the layer produced at high temperatures, causes chemical bonding between the layer and the carbon material, and at higher temperatures improves the resistivity of the protective layer. The optimum amount of silicon deposit to obtain said properties is in the range

246 353 to 10 wt.

For the production of the protective layer according to the invention, technical silicon, e.g. with a content of 9699% Si and technical aluminum of conventional quality, can be used successfully.

The protective layer according to the invention has high resistance to high temperatures and good adhesion to the carbon material even at temperatures above 800 ° C with cyclic heating and cooling. They do not form cracks and do not dull the layer either during prolonged storage of electrodes coated with the present invention or when applied in arc furnaces. The process according to the invention is simple and efficient.

The layer is well conductive both cold and at higher temperatures. Its resistivity does not change during storage.

The protective layer according to the invention can be formed in this way on all carbon materials, both planar and rotational from the smallest diameters (e.g. 3 mm /), such as graphite cover plates, closures, crucibles, electrodes for arc furnaces of different diameters for single use, continuous operation, electrode baking etc.

In the following, the invention is illustrated in more detail by means of exemplary embodiments.

Example 1

On the roughened electrode with a diameter of 350 mm and a length of 1800 mm, a protective layer of 0.45 mm thickness with a resistivity of 0.136 was applied with a 160 kw plasma torch. 10 “ ⁶ ohm.m at 20 ° C and specific gravity 2120 kg / m \

A 92% by weight mixture was used. % of aluminum (0.09 - 0.118 mm / 1/3 + / 0.118 to 0.175, mm / 2/3 and 8 wt. silicon (0.071 0.112 mm) administered at 13 kg / h to the burner flame. Injection was performed with three burner runs of 4 minutes from a distance of 220 °. 250 mm at 35 electrode revolutions per minute (ie, at a spray rate of 0.62 m / s). The electrode was mounted on an electric arc furnace with a capacity of 40 t for melting medium-alloy and carbon steel. The graphite electrode savings were 15-20X.

- 5 Example 2

246 3S3

On a roughened graphite electrode of 350 mm length and 1800 mm length, a 160 kw plasma torch was applied with a 0.5 mm thick protective coating with a resistivity of 0.115 · 10 “ ⁶ ohra.ra at 20 ° C and a specific gravity of 2180 kg / m. ³ so that a mixture of 94% by weight of aluminum / 0.09 <gst was supplied to the flame of the plasma torch

0.180 mm / + 6 X wt. Silicon (0.071-0.1112 mm) administered from two feed sites placed opposite each other at 13 kg / h, i.e. 26 kg / h in total. Injection was carried out by two taxing the plasma torch after

3.5 min from a distance of 230 * 250 mm, at a spray rate of 0.45 m / s. The electrode was mounted on an electric arc furnace with a capacity of 40 t for melting medium-alloy and carbon steel. The graphite electrode savings were 18 X,

Example 3

On a roughened graphite electrode with a diameter of 100 mm and a length of 1200 mm, a water-stabilized plasma torch of 160 kU was applied with a 0.7 mm thick protective layer with a resistivity of 0.20.10 ~ ⁶ ohms at 20 ° C and a density of 2070 kg / m ³ . An aluminum grit having a granulometry of 0.118 mm < 0.175 mm, fed from one feed point in an amount of 13.6 kg / h (85% by weight), and a silica particle with a granulometry of 0.112 to 0.165 mm fed from the second feed point to the flame 4 kg / h (15% w / w). The administration sites were placed opposite each other. Spraying was performed by two runs of the plasma torch from a distance of 240 mm at a spray rate of 0.71 m / s. The electrode was used to burn the tap resistance of a silicon arc furnace. There is no oxidation corrosion at higher temperatures and weakening of the electrode cross-section at the critical point. The loss of electrodes by fracture was significantly reduced. The graphite electrode saving was around 35 X.

Example 4

On a roughened graphite electrode with a diameter of 100 mm and a length of 1200 mm, a water-stabilized plasma torch of 160 kU was applied with a 0.5 mm thick protective layer with a resistivity of 0.17.10 ~ 6 ohra.ra at 20 ° C and a specific gravity of 2080 kg / m ³ . A mixture of 90% by weight was used. of aluminum with granulometry1

- 6 246 353

0.0 ^ 0.180 mm and 10 X wt. 0.071 ^ 0.165 mm silicon stranded into the torch flame with three inlets of 15, 16 and 18 kg / h symmetrically positioned around the flame 200 mm. The electrode was used to burn the tap hole of an electric arc furnace for melting silicon. The burn-out electrodes did not cause lateral opal of the electrode and considerably reduced the loss of electrodes by fracture. The graphite electrode savings were 33%.

Claims (7)

CLAIMS 1. A protective layer of carbonaceous materials, in particular graphite electrodes, which is plasma deposited, characterized in that it consists of 65 to wt. of aluminum aluminum / 1 to 20 2 wt. % of a mixture of metallic silicon and silica and up to 15 wt. of aluminum oxide compounds. 2. The protective layer according to item 1 _? characterized in that it has a resistivity of 0 / 07.10 ⁶ to 0.3.10 ^-6 ohm.m at 20 ° C and 0.12.10 ^-6 to 0 / 7.10 " ⁶ ohm.m at 400 ° C. 3. The protective layer of claim 1 having a density of 1900 to 2300 kg / m &lt; 2 &gt; and a thickness of 0/3 to 1/5 mm. 4. A method according to any one of claims 1 to 4, characterized in that aluminum with a granulometry of 0/09 to 0/180 mm and silicon of a granulometry of 0/07 to 0 are fed to the flame of a plasma torch _from a preferably water stabilized torch. / 165 mm in an amount of 85 to 99 wt. of aluminum and 1 to 15 2 wt. silicon. 5. The method of claim 4 wherein the plasma deposition rate is 0.3 to 0 _{out of} 8 ms and the total amount of material fed to the plasma torch is 12 to 60 kg.h. 6. Method according to claims 4 and 5, characterized in that the TLM _of aluminum and silicon is fed into the flame of plasma torch alone or in admixture with one or more, preferably two or three / inlets uniformly distributed around the flame. 7. A method according to any one of claims 4 to 6, wherein the aluminum and silicon are supplied to the flame of the plasma torch with a pressure gas medium such as air / nitrogen _from carbon dioxide / hydrogen, argon, propane-butane or acetylene either alone or in combination.