WO2023012580A1

WO2023012580A1 - A process for electric degassing of molten aluminum

Info

Publication number: WO2023012580A1
Application number: PCT/IB2022/056874
Authority: WO
Inventors: Mohan Agarwal
Original assignee: Cmr Green Technologies Limited
Priority date: 2021-08-04
Filing date: 2022-07-26
Publication date: 2023-02-09

Abstract

A process for electric degassing of molten aluminum, wherein the said process comprises the steps of: (a) Melting the alloy and transferring the liquid melt to holding furnace; (b) Maintaining the temperature of liquid metal between 750°C to 780°C in holding furnace; (c) Pouring the liquid melt into a ladle with a filter; (d) Stirring the top layer of the molten melt; (e) Placing the electrodes inside the ladle such that the electrodes touch liquid melt; (f) Connecting the Electrodes with DC power source and turning the power supply on; (g) Disconnecting the power source and removing the Electrodes from the ladle.

Description

A PROCESS FOR ELECTRIC DEGASSING OF MOLTEN ALUMINUM FIELD OF THE INVENTION The present invention relates to a method of electric de-gassing of molten metal, particularly a kind of degassing method for molten aluminum. BACKGROUND OF THE INVENTION In a foundry, a high-temperature molten metal melted in a melting furnace or the like is carried to a pouring facility and the product is cast by pouring the molten metal into the mold in the pouring facility. Porosity is one of the major defects in aluminum alloy castings because it can be detrimental to the mechanical properties and the pressure tightness of a casting. Porosity occurs in castings because gas precipitates from solution during solidification or because the liquid metal cannot feed through the inter-dendritic regions to compensate for the volume shrinkage associated with solidification. Hydrogen is the only gas that is appreciably soluble in molten aluminum. Thus, the removal of the dissolved hydrogen from the molten aluminum alloy is critical for the production of high-quality castings. Several methods are currently in use to degas aluminum. These methods include the use of nitrogen or argon, or a mixture of either of these with chlorine, as a purge gas. Other techniques include tablet degassing by use of hexachloroethane tablets, vacuum degassing, electric degassing and ultrasonic degassing. Conventionally, Rotary Degassing is one of the important method usually performed in most of the industries and foundries to remove hydrogen and other constituent. In Rotary degassing usually inert gas (Argon or Nitrogen gas) is purged via rotating Shaft or Rotor. As the energy of the Rotating Shaft provides causes large number of bubbles, smaller the bubble better the hydrogen removal. As it increases surface area to volume ratio. So larger surface area provides effective diffusion of hydrogen to bubbles and moves to top. In certain cases fluxes can be added along to increase the temperature or to improve the efficiency of hydrogen removal. The rotary degassing is highly dependent on some of the important factors such as rotational speed, gas flow rate and design of the rotor. The wrong combination of these parameters also effect quality of the melt. However, the Rotary Degassing system suffer from several disadvantages, some of them being: as several moving/rotating parts in the degassing system are involved, the system is prone to break downs; capital investment /operating cost involved in setting up and running the Rotary Degassing system is high; use of inert gases like nitrogen, argon and chlorine is required; dross formation etc. The melt surface is disturbed during rotary degassing leading to dross formation. In addition, the Rotary Degassing method has several energy and environmental related disadvantages, some of them beings: wastage of energy due to more dross formation; low productivity due to higher processing time; environmental disadvantages from elimination of inert gases from the process. Therefore, there is a need in the art for a system and method for economical, reliable and environmentally friendly process of degassing of molten aluminum. Electric Degassing is relatively new technique of degassing to remove hydrogen from the liquid melt. It works on principle of electrochemical reaction to remove hydrogen from the melt. It consists of Electrodes (one is anode and another is cathode). Electrodes were connected with DC power source to supply current. Melt are treated with current for removal of hydrogen. Indian Patent Number 246682 describes a process of degassing of molten Aluminum which comprises (i) placing graphite crucible in a furnace and preheating at temperature in the range between 750 - 800°C, (ii) charging aluminum pieces into the preheated crucible and allowing the to melt fully, at temperature in the range between 750 - 800°C, (iii) dipping electrodes in the above aluminium melt in such a way that one electrode immersed fully in the molten bath and other electrode just touches the upper part of the melt, (iv) passing current through the molten aluminium bath in the range of 3 - 5 A for a period ranging between 5-15 min and maintaining temperature in the range between 750-800°C, (v) casting the molten aluminium in the conventional method. Indian Patent Number 226266 described a process for the casting of aluminum (Al) and aluminum-silicon (Al-Si) alloys. In the process of the invention, the AC or DC current agitates the liquid metals while passing through it. This leads to fracture of growing dendrites, refine grain size, homogeneity of molten pool of metal, reduce segregation, reduce dissolved gas etc. and thereby improving the mechanical properties of casting. However, the above referred processes do not disclose a reliable, scalable method, particularly a method which provides excellent hydrogen removal and a resultant melt which has quality equivalent to the quality obtained by Rotary Degassing. As the above referred prior arts do not provide a solution which provide the desired quality product and is fit to be used on an industrial scale, there remains an unmet need in the art for a new process of electric degassing of molten aluminum. Further, there remains a need in the art for a method for reproducible, secure, easy to use, economical, reliable and environment friendly process for electric degassing of molten aluminum. The inventors of the present invention have now with much experimentation been able to address the problem in the prior art, by devising an electric degassing process which is scalable, provides desired results on an industrial scale and maintains the inherent benefits i.e. economic and environmental. OBJECT OF THE INVENTION The main object of the present invention is to provide a process of Electric Degassing of molten metal, with high reproducibility, cost-efficiency, cleanliness and hydrogen control. Another object of the invention is to provide a process of Electric Degassing of molten aluminum which provides consistent high levels of quality and hydrogen control on an industrial scale, quality results whereof are comparable to those obtained by Rotary Degassing techniques. In addition, the object of the present invention is to provide a process of Electric Degassing of molten aluminum which is cost effective, environment friendly, has higher productivity due to less dross generation. SUMMARY OF THE INVENTION The present invention relates to a method of Electric Degassing of molten aluminum comprising the steps of: (a) Melting the alloy and transferring the liquid melt to holding furnace; (b) Maintaining the temperature of liquid metal between 750°C to 780°C in holding furnace; (c) Pouring the liquid melt into a ladle with a filter; (d) Stirring the top layer of the molten melt; (e) Placing the electrodes inside the ladle such that the electrodes touch liquid melt; (f) Maintaining the temperature of the molten metal between 750°C to 780°C; (g) Connecting the Electrodes with DC power source and turning the power supply on; (h) Extracting and casting the molten aluminum. The present invention will become fully understood from the detailed description given below. However, the detailed description of the specific embodiments are only illustrations of the desired embodiments of the present invention, and are given so only for explanation. Various possible changes and modifications will be apparent to those of ordinary skill in the art on the basis of the detailed description. The applicant has no intention to dedicate to the public any disclosed embodiment. Among the disclosed changes and modifications, those which may not literally fall within the scope of the present claims constitute, therefore, a part of the present invention in the sense of the doctrine of equivalents. The use of the articles “a”, “an” and “the” and similar referents in the specification and claims are to be construed to cover both the singular and the plural form of the noun, unless otherwise indicated herein or clearly contradicted by the context. The use of any and all examples, or exemplary language (e.g. “such as”) provided herein is intended merely to better illuminate the invention, and so does not limit the scope of the invention, unless otherwise stated. BRIEF DESCRIPTION OF THE DRAWINGS A complete understanding of the system and method of the present invention may be obtained by reference to the following drawings: As shown in Figure 1, is the representation of electric degassing system comprising, Electrode Holding Stand (1); Power Supply (2); DC Power supply (connected in series) (3); Supporting stand 1 (4); Ladle (6); Supporting stand 2 (7); Spout (8); Electrode – Cathode (9); and Electrode – Anode (10). Figure 2 is a view of the ladle transport step involving pouring of the Liquid Melt in Ladle according to one embodiment of the present invention; Figure 3 represents the step of checking temperature of the Liquid Melt according to one embodiment of the present invention; Figure 4 represents the step of Electric Gassing in progress according to one embodiment of the present invention; Figure 5 represents rotary degassing of Molten Aluminum; Figure 6 is a representative Densitometer employed in the present invention; Figure 7 is a representative view of the DC Power Supply employed in the present invention; Figure 8 is a representative RPT equipment used in the present invention; DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described hereinafter with reference to the accompanying drawings in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiment set forth herein. Rather, the embodiment is provided so that this disclosure will be thorough, and will fully convey the scope of the invention to those skilled in the art. Many aspects of the invention can be better understood with references made to the drawings below. The components in the drawings are not necessarily drawn to scale. Instead, emphasis is placed upon clearly illustrating the components of the present invention. Moreover, like reference numerals designate corresponding parts through the several views in the drawings. Before explaining at least one embodiment of the invention, it is to be understood that the embodiments of the invention are not limited in their application to the details of construction and to the arrangement of the components set forth in the following description or illustrated in the drawings. The embodiments of the invention are capable of being practiced and carried out in various ways. In addition, the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. The inventors of the present invention have devised a process for Electric Degassing of molten aluminum which comprises: (i) The required alloy is melted and the liquid melt is transferred to holding furnace; (ii) Temperature of liquid metal is maintained between 750°C to 780°C in the holding furnace; (iii) The liquid melt is poured into a ladle with the fiber glass filter cloth of 400 mesh size; (iv) The top layer of the molten metal is stirred at 150 RPM for 1 minute using machine stirring; (v) The electrodes are placed inside the ladle with support such that one Electrode is inserted from the top and another in spout (sideways), such that the electrodes touch liquid melt; (vi) Electrodes are connected with DC power source, power supply is turned on and DC power supply set at 2 Volts and current of 90 Amps for 15 minutes; (vii) Electrodes are disconnected and removed from the ladle. The Inventors of the present invention through rigorous experimentations set upon the task of finding the present Electric Degassing process, including steps and parameters required to obtain the product with optimum density index. Once the Inventors discovered the process steps and parameters, they then set upon the task of comparing the density index results obtained by using the Electric Degassing process of the present invention via-a-vis those obtained by using the conventional methods of Rotary Degassing. Based on the results of the comparisons, the Inventors did further research and experimentations and finally devised the claimed process which is dependable, reproducible, cost-effective and provides equivalent levels of hydrogen control in comparison to Rotary Degassing processes without the harmful environmental effects of the conventional processes. Density Index The method of estimating the degree of gassing is done by using the Density Index values. Due to this reason, to examine the effect of test pressure on the amount of the porosity, the density of each sample was calculated according to Archimedes Principle stated below. (by weighing in air and in water):

The densitometer gives the density in digital form, when the sample is placed in air and water. The densitometer automatically noted the weight of the sample in air and water, as it is programmed to give the density values. Then the corresponding Density indexes values to be calculated based on

To devise the process steps and parameters which provides optimum results in terms of quality of the final product, the inventors started their research by conducting a series of experiments. The experiments were conducted by gradually changing certain variables for example:- • Temperature of metal • Time taken • Current used for degassing • Polarity of the Electrodes The results of the experiments are as provided below:

Distinguished Chart w.r.t to different parameters on Average Density Based on the above referred experiments, the Inventors of the present invention concluded that the optimum parameters for Electric Degassing are as follows: • Temperature range of 740°C to 780°C • Positive Polarity • Current range of 90 Amp • For 15 minutes Once the broad optimum parameters of the electric degassing process were finalized, the Inventors of the present invention embarked upon the task of comparing the results obtained by using the above referred Electric Degassing process vis-à-vis Rotary Degassing process. The following broad methodology was adopted by the inventors while undertaking the task:

Following process steps were adopted while comparative experimentations: (i) The required alloy is melted and the liquid melt (temperature around 680°C -700°C) is transferred to holding furnace of 15 MT capacity; (ii) Temperature of liquid metal maintained between 750°C and 780°Cin holding furnaces; (iii) After reaching the required temperature for operation, the liquid melt is poured into ladle with the fiber glass filter cloth of 400 mesh size. As it is poured around ≃800 kg for 1 MT capacity ladle i.e. around 80% of the capacity. (iv) Temperature of the liquid metal has noted down, as it must be between 750°C to 780°C; (v) Initial samples are taken before degassing, as it is poured in 1mm vacuum sample die, one sample cooled in air and another is vacuum cooled in Reduced Pressure Test equipment (RPT); (vi) Then the electrodes are placed inside the ladle with support .One Electrode is inserted from the Top and another in spout (sideways) while ensuring that the electrodes touches with melt. (vii) Electrodes are connected with DC power source that is connected in series. (viii) Power supply is turned on and initial time is noted. (ix) The current is controlled by modifying the DC power source. DC power supply set at 2 Volts and current of 90 Amps. (x) After 15 mins, the power is turned off and final temperature is to be noted down. (xi) Two samples are taken out after degassing, one sample cooled in air and another is vacuum cooled in RPT. It is maintained at 28 inch Hg or 700 mm Hg for 3 mins; (xii) Simultaneously from the same heat Rotary Degassing is also to be conducted for comparison of results (both air cooled and vacuum cooled after degassing); (xiii) Then density is measured with Densitometer and density index is calculated Experiments with Density Index Calculations: To determine the Quality of melt with respective to Density Index both air cooled and vacuum cooled sample of Electric Degassing as well as Rotary Degassing from the same heat is to be experimented at the same time. It is done to compare the Density index between the two Degassing Processes. The results are stated below.

As better Density Index values were obtained while keeping the time to 15 minutes in Electric Degassing process, the time was kept at 15 mins for further experimentations.

From the experimental results, average densities has to be calculated to determine average Density Index values as it is listed below :

Average Density Index values of Electric and Rotary Degassing Results of the Experimentations: From the average values, it was found that in electric degassing, the Density Index is (3.53 %). However, literature is replete with the fact that for good Metallurgical Quality Alloy, the Density Index should be less than 3%. In Rotary Degassing, the Density Index is less than 1% which evidently suggested that the Quality was better in comparison to Electric Degassing. Accordingly, it was found that the Electric Degassing performed using the steps and parameters described above were yielding results. It was found that the said process was now reproducible, scalable on an industrial set up and was showing cost efficiency along with environmental benefits. However, the melt was still not achieving the required quality as shown by the higher density index values i.e. above 3.53%. Therefore, the inventors of the present invention embarked upon the task of further experimentations and analysis to improve the quality of the resultant melt. It was during these further experimentations, that the inventors found that by incorporating another process step and parameters, the resultant melt from Electric Degassing process surprisingly started showing the desired density index values. It was found that once the liquid melt is poured into a ladle with the fiber glass filter cloth of 400 mesh size, stirring of the top layer at 150 RPM for 1 minute, surprisingly and unexpectedly improved the density index value of the resultant melt. It was therefore found that by inclusion of these two process step and parameters i.e. filtering the melt before pouring in the ladle and stirring the top layer at 150 RPM for 1 minute improved the quality of the melt such that the desired density index value of less than 3 were achieved. The above said process step and parameters therefore unexpectedly improved the quality of the resultant product. The above said surprising results are further described in the below mentioned experiments: Manual Stirring: Experiments were initially conducted with manual stirring using dross removal tool as follows:

From the Electric Degassing experiments it was found that inclusion of manual stirring started showing better Density Index values (<3%). However, as the process had to be scaled up for commercial utilization, it was imperative that manual stirring be replaced by machine stirring. Machine Stirring: The inventors therefore conducted a series of experiments using different parameters i.e. different RPM, different time duration of for stirring, in an effort to achieve the results achieved using manual stirring. The series of experiments conducted with stirring time of 1 Minute are as follows:

Stirring Time: In order to arrive at the most favorable stirring time i.e.1 Minute, stirring operations were conducted using different stirring time intervals i.e. from 1 Minute, 3 Minute and 5 Minute at 150 RPM as follows.

It was concluded from the result above that the optimal parameter for stirring before electric degassing is for a time period of 1 Minute. From the above referred experiments, Average Density and Density index values were calculated to determine the better optimal parameters for Electric Degassing with stirring as follows:

Pursuant to the above referred experiments, it was observed that the Density Index values were improved in Electric Degassing process after inclusion of the stirring step. Moreover, it was observed that the said partial stirring also eliminated bi-film and improved grain refinement during solidification. Based on the above referred experiments, it was concluded that the optimum density index values were achieved while keeping the stirring parameters to 150 RPM for 1 minute followed by Electric Degassing for 15 minutes. Cost Analysis: It was observed that Electric Degassing process described in the present invention leads to substantial cost benefits as under:

It is therefore concluded that Electric Degassing is much better in terms of Cost optimization compared with Rotary Degassing. Thus, the present invention therefore provides a process for Electric Degassing of molten aluminum, such that the final product has optimum Metallurgical Quality while achieving the cost and environmental benefits.

Claims

CLAIMS 1. A process for electric degassing of molten aluminum, wherein the said process comprises the steps of: (a) Melting the alloy and transferring the liquid melt to a holding furnace; (b) Maintaining the temperature of liquid melt in the holding furnace between 750°C to 780°C; (c) Pouring the liquid melt from holding furnace into a ladle with a filter; (d) Stirring the top layer of the molten melt; (e) Placing the electrodes inside the ladle such that the electrodes touch the liquid melt; (f) Connecting the Electrodes with DC power source and turning the power supply on; (g) Disconnecting the power source and removing the Electrodes from the ladle.

2. The process as claimed in claim 1 wherein the filter is fiber glass filter cloth of 400 mesh size.

3. The process as claimed in claim 1 wherein the liquid melt is poured to 80% of the capacity of the ladle.

4. The process as claimed in claim 1 wherein the stirring is conducted at RPM range between 380 RPM and 150 RPM for 1 minute using machine stirring.

5. The process as claimed in claim 1 wherein the stirring is conducted at RPM 150 for 1 minute using machine stirring.

6. The process as claimed in claim 1 wherein the Electrodes are placed in an manner such that one electrode is placed on the top and another in placed sideways through the spout.

7. The process as claimed in claim 1, wherein the DC power source is connected in series.

8. The process as claimed in claim 1 wherein the DC power supply is set at 2 volts and current of 90 Amperes.

9. The process as claimed in claim 1 wherein the DC power supply is provided for a period of 15 minutes.

10. The process as claimed in claim 1, wherein the temperature of the molten melt is maintained at 750°C.