|Title of Invention||
A METHOD AND A DEVICE FOR APPLYING A PROTECTIVE CARBON COATING ON METALLIC SURFACES
|Abstract||This invention is about the process and equipment used for developing a protective carbon 'coating by partial burning of hydrocarbon gases on the internal surfaces of metallic trays that carry metallic powders (like iron powder) for thermal treatment (reduction, annealing etc.) in pusher and walking beam furnaces which prevents powder from sticking to trays. The inflammable hydrocarbon gas passes through thin slits provided on 'height-adjustable' metallic burners, screwed on to equidistant branched metallic pipes connected to a metallic main pipe having a regulator. The partial burning of regulated hydrocarbon gas and oxygen flow create laminar flame ensuring uniform deposition of protective carbon layer on internal surface of trays placed one at a time on the array of burners. After coating the tray is removed. This carbon layer gasifies during thermal treatment and does not contaminate the powder while facilitating its smooth unloading from the trays.|
This invention relates to the method of and the device for applying protective, non-sticky coatings to 'surfaces ' of metallic trays that carry different powders for their high temperature processing to render the powders non-sticking to those 'surfaces'. These metallic trays carry the powders through high temperature furnaces such that the powders get processed to be suitable for further operations.
More particularly, this invention relates to a method of applying non-sticky protective coatings to the internal surfaces of metallic trays into which iron or iron alloy powders are loaded during their reduction and/or annealing in hydrogen atmosphere furnaces at high temperatures (600 to 1200(C). The method of the present invention can also be used in other fields where the application of uniform carbon black protective coating on plain unmoving surface is required. The invention also relates to a device for carrying out the above said process.
Typically, 'industrial scale annealing' of iron powders and atomized steel powders at the said high temperatures is carried out in three types of furnaces - high temperature conveyor-belt type furnaces, pusher type furnaces and walking beam type furnaces. In the conveyer furnaces, the carrier that holds the powder for its transportation through the furnace is the conveyer belt itself; in the pusher furnaces and walking beam type furnaces, the carriers that hold the powder for its transportation through the furnaces are metallic trays. The powder, loaded in the transporting metallic trays, during the process of annealing, gets sintered into a hard cake, and, in the absence of a protective, non-sticky layer on the internal surface of the metallic trays, gets stuck to the internal surfaces of the trays. This creates problems of removing processed powder (in the form of hard cake) from the tray after the completion of the process. To eliminate the sticking effect of the processed powder-cake to the transporting metallic trays, a protective layer is usually applied on the latter.
The most effective protective, non-sticky coating employed is carbon black. During the process of annealing, this layer of carbon black prevents sticking of the powder-cakes to the surface of the metallic trays and facilitates easy separation of the cake from the metallic trays. Further, the carbon black gasifies during the process and does not contaminate the product.
Background of invention:
Carbon black can be deposited by different methods. The most widely used method is to obtain the carbon black layer by combustion of different types of hydrocarbon gases. Usually the hydrocarbon
gases are fed through a slit in a burner system at low velocities and burnt to obtain free flowing smoky flame. By placing the carrier tray surface (the surface that holds the powder) over this flame, it is possible to obtain a non-sticky layer of carbon black deposited on the surface.
To make such layer effective, it must uniformly cover the entire surface being coated. Attaining this uniformity using a series of burners has some inherent difficulties. If the tray is placed in a fixed position over one or several arbitrarily located slits, the coating tends to be a non-uniform - in the form of discreet strips following the pattern in which the burners are located.
To obtain the uniform coating, complicated devices are available wherein a lateral movement for the surface being coated is created in the direction perpendicular to the burner slits.
Some of the practically used methods for applying the protective coating involves partial burning of different hydrocarbons and are described below.
In a conveyer furnace, while undertaking annealing or reduction of powders, more particularly iron or iron alloy powder, a protective non-sticky coating is applied on the belt of the conveyer furnace, before loading iron or iron alloy powder on it. In order to obtain uniform coating across its width, the hydrocarbon gases are fed in the form of laminar jets through one or several slits (in the burners) located perpendicular to the edges of the belt. During this, a part of the feeding gas is burnt, thus, creating a condition for the decomposition of the remaining gas with the emission of carbon black, which gets deposited on the surface of the belt. A partly uniform coating of carbon black on the surface of the belt can be achieved by providing smooth movement to the belt. The density of the coating, on the other hand, depends on the velocity of the moving belt and the number of the burners located at various distances form each other
The disadvantages of such a coating method are that the burners are located at arbitrary distances from one another. Thus, the laminar jets issuing from the slits provided on the various burners do not overlap and mix. This results in the production of non-uniform, separated bands of carbon black coating, each band corresponding to each burner. Consequently, between different bands there are uncoated or relatively less densely coated layers, which adversely affect the uniformity and thus the efficacy of the coating.
Another method for carbon black coating exists as a part of the pusher-type annealing furnace ELINO. Acetylene is the hydrocarbon used for this purpose. The device through which this process is carried out is equipped with measuring and gas locking accessories, with fixed gas collector with holes distributed along the length for gas feeding, and a movable frame for supporting the trays, with a mechanism for displacing the said frame. The coating procedure consists of locating a gas distributor (with holes through which the gas issues out) under a frame supporting the trays such that, the holes through which acetylene is fed are located under the trays. Acetylene is burnt and the smoky flame that issues deposits the carbon black on the surface being coated. The reciprocal movement of the frame supporting the trays facilitates more uniform coating of carbon black on the surface.
The main disadvantage inherent in this method is with respect to complexity of the design of the device which demands movable parts (reciprocating movement to the frame supporting the trays, in the absence of which the coating would be non-uniform will be in the from of discreet bands), the mechanisms for their displacement and hence the operational complexity and low operational reliability of the device.
There is another method of obtaining non-sticky coatings on surfaces, other than that of depositing carbon black. In this method carbon-based coating is applied which also renders the surface non-sticky to the powder being processed at high temperatures in hydrogen-based atmospheres.
This is achieved though a device which is a part of Furnace LOI and involves, first, preparation of a graphite and liquid organic binder based emulsion. This emulsion is then sprayed uniformly on to the surfaces of the trays. After this, the trays are filled with iron powder and are fed into furnace for annealing.
However, coating by this method has disadvantages, especially in the form of complicated infrastructure required for the preparation of emulsions, complicated spraying equipment and equipment required for drying.
The main objective of the present invention is therefore to provide an improved method for applying protective coatings on the surfaces located in direct contact with iron or iron alloy powder during annealing.
Another objective of the present invention is to provide an improved method of applying protective coatings on the surfaces coming in direct contact with iron or iron alloy powder during annealing conducted in high temperature furnaces containing hydrogen atmospheres.
ill another objective of the present invention is to provide an improved method of applying protective carbon black coatings on the trays, which give a uniform coating in the internal surfaces of fixed trays.
A further objective of the present invention is to provide an improved device with increased operational reliability to carry out the method of coating mentioned above.
The present invention provides an improved method for applying a protective carbon black coating in such a way so as to obtain a uniform coating (not discreet bands) on the internal surface of metallic trays, without incorporating any moving parts, i.e, keeping the tray fixed and also to create a device with simplified design with increased operational reliability.
Many factors influence the actualization of the above objectives:
1. Overlapping fluxes of hydrocarbon gas flames to obtain uniform coating
2. Adjustment of the distance between the surface to be coated and the hydrocarbon flames such that the required overlapping occurs
3. Adjustment of the distance between the sources of the flames (burners) such that no discreet bands of carbon black coating forms on the surface being coated
4. Ensuring optimum supply of air to the hydrocarbon gas such that partial burning of the gas occurs and highly active carbon black layer is formed.
In order to actualize the above requirements, in an embodiment of the present invention the trays holding the powders whose surfaces have to be coated are kept at a distance in the range of 0.5 to 0.66, preferably 0.55 to 0.6 times of the potential unhindered flame height (unhindered flame height refers to the height the hydrocarbon flame would issue forth if it is not hindered by the surface to be coated kept at the height specified above).
According to another feature of the present invention there is provided an improved device for carrying out the method for applying protective carbon black coating on the surfaces of carriers of powders for
their high temperature processing to render the surfaces non-sticky as shown in figure 1 of the drawing accompanying this specification which comprises the following:
The partial burning of hydrocarbon gas is achieved through burners with slits (7) with width 'b'. These burners are threaded onto columns (6), which are fixed equidistantly and placed perpendicular to a set of pipes (4), which supply hydrocarbon gases to the burners. The slits (7) are aligned with the axes of pipes (4). These pipes (4) branch off from a common hydrocarbon supply pipe (5) into which the flow of hydrocarbon gas is controlled through valve (1). The air required for the-partial burning of the hydrocarbon gases is fed through inlet duct (2) and the amount of air is controlled by slide gates (3) provided above the branch pipes carrying hydrocarbon gases. The whole device is enclosed in a frame to which supports (8) are fixed for holding the trays whose internal surfaces need to be coated.
The distance between the branch pipes in reference to the axes of the slits is maintained at 75 - 85 times b. The slide gates for air supply are located above the branch pipes (4) at a height of 15 - 20 times b. The opening of the slide gates for air is adjusted 8-15 times b depending on the type of hydrocarbon gas being used. The side walls (frame) of the device, after rising upto the height of the slits in the burner, rise further up to a tune of 25 - 40 times b. The ranges of all the dimensional relationships specified in reference to the width of the said slit are provided to cater to different hydrocarbon gases that can be used; the heavier the hydrocarbon gas, more the respective said dimensions.
By controlling the slide gates, the air supply is restricted to suit the number of carbon atoms available in the type of hydrocarbon gas used such that optimum partial burning is achieved and highly active carbon black emission is achieved.
The number of carbon atoms may range from 3 to 10. The preferred gas used may be selected from C3H8, C4H10, C5H12, C6H14, C7H16, C8H18, C9H20. C10H22, Liquid Petroleum Gas, Natural gas and their combinations.
In actual operation, the hydrocarbon gas is tapped from its source, through to a pressure regulator at 0,1 to 0.5 kg/cm2 gauge pressure and through a flow meter (0.05 to 0.3 m3/h), to be led into the main gas carrying pipe from which the gas flows into the perpendicular branch pipes and then into individual burners with slits located equidistant from one another, while the air required to carry out the partial
burning of the hydrocarbon gas is taken into the system though the openings provided in air ducts so that when the mixture is externally lit, smoky flames issue out from each of the burners and with the surface to be coated placed on a frame in a stationary mode on the top of the burners such that the surface hinders the flow of the flames to spread the flames on the surface providing as large area of coverage as possible resulting in a uniform coating all over the area being covered.
The metallic trays are fixed in the supports provided on the device such that their inner surfaces are exposed to the flames. The height of the surface being coated from the slit is fixed at 0.5 to 0.66 of the free flame height. The width b of the slit is preferably is 0.8 to 1.0 mm such that the hydrocarbon gas is partially burned. However, in the periphery of the flame, the gases are consumed completely and the partial burned gases are available within the flame.
For obtaining uniform coating on an area of about 750 cm2, 2.5 - 4.0 kg/hour of hydrocarbon gas (particularly Liquid Petroleum Gas - LPG) is required, 10 % of which is burned in free flame while regulating the airflow.
The apparatus as shown in figure 1 was used to obtain protective non-sticky coating to the inside surfaces of Stainless Steel (SS 310) trays for annealing of iron powders. The hydrocarbon gas used for obtaining the coating was Liquid Petroleum Gas (LPG).
LPG was tapped at 0,4kg/cm2 gauge pressure, its flow, controlled through valve (1), being maintained at 0.15 m3/h and was allowed to flow out through burner slits (7) of 0.8 mm width. A total number of 12 burners arranged in a 4 x 3 matrix threaded to column (6) were used to cover the tray area 750 cm2. The air required for the partial burning LPG was obtained through the slide gates (3), which were opened by 9 mm. After lighting the LPG, the trays were placed on the supports (8) such that their internal surfaces was at a height of 85 mm from the slits of the burners. The trays were removed after 45 seconds exposure to partially burning smoky flame to obtain a uniform carbon black coating of 40 microns thickness. The amount of LPG consumed for the operation was 3.6 kg/h out of which 3.24 kg/hr was consumed for partial burning, while 0.36 kg/h of the gas burnt off completely.
Advantages of the invention
Some of the advantages of the present invention are mentioned below:
1. Obtaining uniform carbon black coating across the entire cross section of the metallic surfaces which are used in high temperature furnaces (600 - 1200C)
2. The said carbon black coating is of uniform thickness over the entire surface of the metallic trays that are coated.
3. The said coating does not contaminate the iron or iron alloyed powders in the metallic trays during their high temperature processing in a furnace.
4. The metallic surface during the coating process is kept stationary and hence no complicated moving parts are present in the apparatus.
5. The said coating facilitates easy removal of the processed powder cakes from the trays after their high temperature processing in the furnace and thus prevents distortion of the metallic trays and their subsequent repair.
6. Another advantage of the non-sticky protective nature of the carbon black coating is that the total quantity of the powder fed in the trays is completely retrieved after processing in the furnace.
1. The method for applying protective carbon black coating on the internal surfaces of metallic trays that carry iron or iron alloy powders into hydrogen atmosphere high temperature (600 to 1200(C) furnaces for processing to render the surfaces non-sticking, the said method comprising of feeding of a hydrocarbon gas in a distributed jet form through a plurality of burners in a laminarly distributed form along with formation of mutually overlapping flows resulting in smoky flames which spread along the inner static surfaces of the powder trays which are to be coated and placed above the burners, passing air in quantities required for the partial burning of the hydrocarbon gas to produce flames at the peripheral layers so to as to deposit a unifonn carbon black coating of maximum 100 microns thickness from the smoke of the flame on the internal surfaces of the metallic trays.
2. The method as claimed in claim 1, wherein non-sticky carbon black protective layer is developed which does not contaminate the iron and iron alloy powders being processed.
3. The method as claimed in claim 1 and 2, wherein a uniformly distributed carbon black coating of maximum 100 microns thickness is obtained.
4. A method as claimed in claims 1 to 3, wherein the powder holding trays whose surfaces have to be coated are kept at a distance in the range 0.50 to 0.66 times of the free flame height.
5. The method as claimed in claims 1 to 4 wherein any hydrocarbon gas having carbon atoms ranging from 3 to 10 is used,
6. The method as claimed in claims 1 to 5, wherein the hydrocarbon gas used is selected from CsHa, C4H10. C5H12, C6H14, C7H16, C8H18, C9H20, C10H22. Liquid Petroleum Gas, Natural gas.
7. The method as claimed in claims 1 to 6 wherein 90% of the hydrocarbon gas fed undergo partial burning to produce smoky flame required for deposition of carbon black on the internal surfaces of powder carrying metallic trays.
8. The device to carry out the method as claimed in claims 1 to 7 for applying carbon black coating on the surfaces of powder carrying trays for their high temperature (600 to 1200(C) processing to render the surfaces non-sticky as shown in the figure 1 of the drawing accompanying this specification, the said device comprising of burners with slits (7) with width 'b', the said burners being threaded onto columns (6), which are fixed equidistantly and placed perpendicular to a set of pipes (4), which supply hydrocarbon gases to the burners, while the slits (7) are aligned with the axes of pipes (4) branching off from a common hydrocarbon supply pipe (5) into which the flow of hydrocarbon gas is controlled through valve (1) and the air required for the partial burning of the hydrocarbon gases is fed through inlet duct (2) and the amount of air is controlled by slide gates (3) provided above the branch pipes carrying hydrocarbon gases, the whole device being enclosed in a frame to which supports (8) are fixed for holding the powder carrying metallic trays whose internal surfaces need to be coated.
9. The device as claimed in claim 7, where-in, on the periphery of the flame, the gas burns completely such that partially burnt gas is accessed from within the flame and maximum spread of the partially burnt flame on the surface being coated is obtained with thickness of slit 'b' = 0.8mm.
10. The device as claimed in claims 7, wherein the distance between the slits on any two adjacent burners on both the axes of the burner matrix is 75 to 85 times 'b'.
11. The device as claimed in claim 7, wherein the height difference between the hydrocarbon gas distribution pipes and slide gates for air passage is 15 to 20 times 'b'.
12. The device as claimed in claim 7, wherein the distance between the burner slit and the surface being coated is 0.5 to 0.66 times the height of the unhindered flame.
13. The device as claimed in claim 7, wherein, the opening of the slide gates for air is adjusted to 8 -15 times 'b'.
14. The device as claimed in claim 7 wherein the side walls of the device, after rising upto the height of the slits in the burner, rise further up to a tune of 25 - 40 times 'b'.
15. The method for applying protective carbon black coating on the surfaces of powder carrying metallic trays for their high temperature processing to render the surfaces non-sticking as herein before described with reference to the accompanying drawing.
16. The device for carrying out the method for applying protective carbon black coatings on the surfaces of powder carrying metallic trays for their high temperature processing to render the surfaces non sticking as herein before described with reference to the accompanying drawing.
|Indian Patent Application Number||719/MAS/1999|
|PG Journal Number||02/2008|
|Date of Filing||08-Jul-1999|
|Name of Patentee||M/S. INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW MATERIALS (ARCI)|
|Applicant Address||GOVT, OF INDIA, OPP: BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD - 500 005,|
|PCT International Classification Number||B32B 009/00|
|PCT International Application Number||N/A|
|PCT International Filing date|