Title of Invention

"AN IMPROVED PROCESS OF DEGASSING OF MOLTEN ALUMINIUM"

Abstract An improved process of degassing of molten Aluminium which comprises (i) placing graphite crucible in a furnace and preheating at temperature in the range between 750 - 800°C, (ii) charging aluminium pieces into the preheated crucible and allowing the aluminium 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.
Full Text The present invention relates to an improved process of degassing of molten aluminum. This invention will be useful for the Industries, which are engaged in production of aluminium and aluminium base alloys.
BACKGROUND OF THE INVENTION
Aluminium and aluminium base alloys absorb or dissolve harmful quantities of hydrogen gas. Sources of hydrogen are (a) Water vapor in atmosphere; (b) Water vapour from burner fuel; (c) Damp refractories and crucible lining; (d) Damp fluxes; (e) Oily or dirty scrap charges; (f) Dirty or damp foundry tools etc. At the melting point an abrupt increase in solubility occurs. If the solubility limit is reached at pouring temperature, subsequent cooling and solidification result in gas evolution, gas or pinholes and microscopic gas porosity. Reference may be made to K. Strauss, Applied Science in Casting of Metals, Pergamon Press, Oxford, UK, 1970, wherein molten aluminium contains ~0.65 ppm of hydrogen (at 660°C). But the solid aluminum at the same temperature contains ~0.18 ppm of hydrogen. Therefore, a considerable amount of hydrogen is liberated through the molten aluminum during solidification. A fraction of the liberated hydrogen is removed to the atmosphere. The remaining part is entrapped in the frozen/solidified metal. This results in the formation of pores.
Different groups recommend several techniques adopted to remove hydrogen from the molten metal. Amongst mem most popular is the treatment with hexachloroethane. This compound liberates chlorine gas when comes in contact with molten aluminium and this gas flushes off hydrogen gas from the molten metal. Thus, the hydrogen content in molten metal reduces below the solubility limit of solid metal. The major disadvantage of this process is the liberated chlorine gas from hexachloroethane, which is injurious to health. This is banned by Europian Community since April 1,1999. Ref: "Foseco Non-Ferrous Foundrymen's Handbook", 11th Edition, 1999, Butterwoth-Heinman Publication,
Another process for removal of hydrogen is purging of molten metal with nitrogen or argon gas or combination of nitrogen and chlorine gases. This leads to (a) an increase in treatment cost and (b) a considerable amount of loss of heat during the process. The gas should be of high purity, otherwise, the impurities like moisture and oxygen present in the gases like nitrogen or argon will create problem due to hydrogen pick up and oxide formation respectively. Reference may be made to A. Prodhan, M. Carpenter and J. Campbell, CIATF Technical Forum, GIFA 99, Dusseldrof, Germany, June 9 - 15, 1999, pp. 106-115, wherein the problem with oxides of Al is thoroughly explained. It acts as a crack and the fatigue properties are deteriorated significantly.
A new variety of degasser developed by Foseco; reference may be made to J. R. Brown, "Foseco Non-Ferrous Foundrymen's Handbook", 11* Edition, 1999, Butterwoth-Heinman Publication, is known as Degasser - 900. Where they claim that it is available in tablet form and it liberates nitrogen gas when plunged in molten aluminium or aluminim base alloys. This liberated gas purges the dissolved hydrogen. But on a recent enquiry to M/s. Foseco, India, Pune, revealed that the material is not in use.
U. S. Patent No. 4,265,432 (dt 05.05.1981) describes a patent by Dimitri E. Lajovic and Hans Lassner on "Degassing of molten metals" filed on June 19,1979. It described an apparatus for the continuous degassing of molten aluminium. It consisted of an inlet chamber adapted to receive untreated metal, an outlet chamber through which treated metal may be removed, a channel existing beneath and interconnecting the chambers to provide a metal flow path there between, the chambers are separated at least in a part by an electrically conducting wall for defining with the molten metal in the channel an electrically conducting loop sufficient to maintain metal within the chambers and channel in a molten form and means are provided for introducing a purging gas beneath the surface of the molten metal.
U. S. Patent No. 4,032,124 (dt. 28.06.77) describes a patent by John C. Yarwood, James E. Dore and Robert K. Preuss on "Apparatus and method for in-line degassing and filtration of molten metal" filed on Feb. 2,1976. It describes an improvement in degassing and filtration in molten aluminium using an apparatus which employs a pair of sequentially placed, removable filter-type plates and at least one fluxing gas inlet positioned there between. Fluxing gas is provided to the melt through the inlet and flows upward through the first of said plates into counter-current in contact with the melt The plate serves to break up the fluxing gas into a fine dispersion to assure its intimate and extensive contact with the melt. Dissolved gases and non-metallic inclusions are thereby abstracted and removed from the melt.
U. S. Patent No. 4,052,198 (dt 04/10/1977) describes a patent by John C. Yarwood, James £. Dore and Robert K. Preuss on "Methods for in-line degassing and filtration of molten metals" filed on Dec 6, 1976. An improvement in the degassing and filtration of molten aluminium, using an apparatus that employs a pair of sequentially placed, removable filter type plates and atleast one fluxing gas inlet positioned there between. Fluxing gas is provided to the melt through the inlet and flows upward through the first of said plates into counter current contact with the melt Said plate serves to break up the fluxing gas into a fine dispersion to assure its intimate and extensive contact with the melt. Dissolved gases and non-metallic inclusions are thereby abstracted and removed from the melt.
The drawbacks of the above processes are the chlorine gas from hexachloroethane is injurious to human and therefore is banned by European Council. Degassing by nitrogen or argon bubbling is not that efficient and economic. Secondly, it cools the molten metal, which needs to be compensated by electrical heating or other heat source.
The main object of the present invention is to provide an improved process for degassing of molten aluminium which obviates the drawbacks as detailed above.
The another object of the invention is to provide an environment-friendly and economic process for degassing of molten aluminium.
Accordingly, the present invention provides an improved process of degassing of molten aluminium which comprises: (i) placing graphite crucible in a furnace and preheating at temperature in the range between
750-800°C, (ii) charging aluminium pieces into the preheated crucible and allowing the aluminium 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,
In an embodiment of the present invention the position of the electrodes may be maintained in such a way that the current flows through the entire mass of the molten metal.
The present concept of degassing treatment is based on principle of "Sievert's Law", where, degassing is done by increasing the partial pressure of a gaseous phase. This is done either by purging with neutral gas (Like nitrogen or argon) or plunging hexacloroethane-based tablets (which evolves chlorine gas in contact with molten metal). The chlorine gas in not environment-friendly and nitrogen/argon gas treatment is quite costly and that takes away a considerable amount of heat from the system. Which requires to be compensated by electrical energy or any other fuel.
In a gas fired or electric furnace a clay bonded graphite crucible was placed and preheated to 750 - 800°C. Pieces of aluminum or aluminium alloys are charged to the preheated crucible (~ 5 Kg.) and allowed to melt completely. Sample was collected by sucking the molten metal into a quartz tube and reported as Time 0. The first experiment was conducted to find out the effect of degassing treatment with nitrogen blowing and is presented in Example -1. Samples were collected at the intervals of 5 min and reported as 5 min, 10 min and 15 min. The experimental data is reported in Table -1. Other experiments (Example - II and Example - III) were conducted to find out the effectiveness of degassing treatment by current (AC / DC) treatment Similar procedures were followed to get molten aluminium or aluminum alloy melt at metal 750 - 800°C. Then coated graphite electrodes were inserted into the melt and positioned as shown in the figure 1. The current in the range of 3 - 5A (AC / DC) was passed through the molten aluminum for 5 -15 min. Samples were collected at the intervals of 5 min and reported as 5 min, 10 min and 15 min. The results are presented in Table II & Table HI.
The comparison of results shows mat to reduce hydrogen content of molten metal from ~ 4.6 ppm to ~ 2.2 ppm required about 45 lit. of nitrogen gas. Whereas, ~ 0.025 kWh of electricity can reduce hydrogen content from ~ 7ppm to ~ 2.6ppm.
Gas purging to molten metal takes away a part of sensible heat from the metal, which was compensated by external heat source. In case of current treatment, there was no such heat loss.
The novelty of the present invention is that AC or DC current agitates the liquid metal while passing through it Its agitation liberates the dissolved gases from molten pool of metal and thereby removes hydrogen from the molten metal.
The following examples are given by the way of illustration and therefore should not be
construed to limit the scope of the present invention.
EXAMPLE-I
Laboratory scale tests were conducted in Clay-bonded Graphite Crucible where ingot-pieces of Aluminum (5kg.) were melted in a gas-fired furnace. After melting, samples were taken from the liquid aluminium by sucking the melt in a quartz tube. Then melt was treated with Nitrogen (IOLAR -1) at 4 - 5 lit / min for 10 - 15-min. Samples were collected at an interval of 5 min and the results of hydrogen analysis is presented in Table -1.
TableI: Degassing of molten aluminium by nitrogen gas purging (Table Removed)
EXAMPLE-II
Laboratory scale tests were conducted in Clay-bonded Graphite Crucible where laboratory generated Aluminum-scrap (5kg.) was melted in a gas-fired furnace. After melting, samples were taken from the liquid aluminium by sucking the melt in a quartz tube. Then the melt was treated with 50 Hz AC supply for a maximum period of 15-min. Samples were collected at intervals of 5 min and the results are shown in Table - II. The treated liquid-metal then solidified in the same graphite crucible.
TableII: Degassing of molten aluminium by Current (A. C.) treatment (Table Removed)
EXAMPLE -ffl Laboratory scale tests were conducted in Clay-bonded Graphite Crucible aluminium was
(the frozen metal as mentioned above) melted in the same gas-fired furnace. After melting,
samples were taken from the liquid aluminium by sucking the melt in a quartz tube. Then melt
was treated with DC supply for a maximum period of 10 min. Samples were collected at an interval of Smin and the results are shown in Table - III.
Table III: Degassing by Current (D. C.) treatment of Aluminium
(Table Removed) The above-mentioned examples I - III show that the current treatment considerably reduces hydrogen content of molten metal in comparison to treatment with Nitrogen gas blowing. The main advantages of the present invention are:
1. This process does not use any degassing element like hexachloroethane, which pollutes the atmosphere by liberation of chlorine gas during the treatment.
2. This process does not use any other gaseous media like Nitrogen, Argon or mixture of Argon & Chlorine to remove gas from molten Al or Al-alloy. Therefore, the benefits are (a) cost reduction (as gas is not used for degassing) and (b) avoid loss of sensible heat (which needs to be compensated by other source of heat).







We claim:
1. An improved process of degassing of molten Aluminium which comprises
(i) placing graphite crucible in a furnace and preheating at temperature in the range between
750 - 800°C,
(ii) charging aluminium pieces into the preheated crucible and allowing the aluminium 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.
2. An improved process as claimed in claim 1 wherein the position of the electrodes is
maintained so that current flows through the entire mass .
3. An improved process of degassing of molten Alumimiun substantially as herein described with reference to the examples.

Documents:

2312-DEL-2004-Abstract-(11-02-2011).pdf

2312-del-2004-abstract.pdf

2312-DEL-2004-Claims-(11-02-2011).pdf

2312-del-2004-claims.pdf

2312-DEL-2004-Correspondence-Others-(11-02-2011).pdf

2312-del-2004-correspondence-others.pdf

2312-del-2004-correspondence-po.pdf

2312-del-2004-description (complete).pdf

2312-del-2004-drawings.pdf

2312-DEL-2004-Form-1-(11-02-2011).pdf

2312-del-2004-form-1.pdf

2312-del-2004-form-18.pdf

2312-del-2004-form-2.pdf

2312-DEL-2004-Form-3-(11-02-2011).pdf

2312-del-2004-form-3.pdf

2312-del-2004-form-5.pdf


Patent Number 246682
Indian Patent Application Number 2312/DEL/2004
PG Journal Number 11/2011
Publication Date 18-Mar-2011
Grant Date 10-Mar-2011
Date of Filing 19-Nov-2004
Name of Patentee COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 ANJAN PRODHAN NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR, JHARKHAND, INDIA.
PCT International Classification Number C22B 9/03
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA