GB2220424A

GB2220424A - Degassing and cleaning system for molten metals

Info

Publication number: GB2220424A
Application number: GB8816004A
Authority: GB
Inventors: Christopher John English
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-07-05
Filing date: 1988-07-05
Publication date: 1990-01-10
Also published as: GB8816004D0

Abstract

Contaminated molten metal is introduced into series of interconnected columns such that molten metal enters the system at (1) is directed so that it forms a downward moving column (2) into which gas is injected through a porous plug (3) so that bubbles rise through the column (2) countercurrent to the movement of the molten metal fed in at (1), partially degassed and cleaned metal is then directed into a second column (4) and raised by a gas lift driven by bubbles from a second porous plug located at its base and discharged into the top of a third column (2) and the process repeated until the contaminates are reduced to the desired level. Particulate material in suspension is raised by floatation to a level above the feed and discharge levels and forms a removable layer of dross (9). <IMAGE>

Description

DEGASSING AND CLEANING SYSTEM FOR MOLTEN METALS.

In the casting of metals, particularly Aluminium and it's alloys, it is necessary to remove dissolved gasses and harmful particulate matter from the melt.

It is common practice to bubble inert gasses such as Argon and Nitrogen, sometimes mixed with small quantities of active gasses such as Chlorine, Dichlorodifuoromethane, Sulphur Hexafluoride etc. to scavenge or react with dissolved gasses in the melt, and remove particulates by floatation as the bubbles pass through the molten metal.

The effectiveness of these processes depends on the size of the bubbles, their residence time in the melt, the relative partial pressures of the subject gas in the bubbles and in the melt, and the reactivity and concentration of the active gas in the bubbles and the dissolved gas in the melt.

As small bubbles have a greater surface area, are more numerous per unit volume and rise more slowly through the melt, the degassing, reaction and floatation actions become more efficient in terms of time and gas consumed, as the bubble size diminishes.

As the scavenging action progresses the partial pressure of the subject gas rises in the bubbles and lowers in the melt thus slowing the action down. As this is a progressive relationship it is desirable to present fresh uncontaminated gas bubbles to the partially degassed metal periodically to maintain the maximum degassing and reaction rates.

The residence time of gas bubbles in the molten metal is increased and their effective de-gassing action is increased if the metal is caused to flow downwardly along a path whilst bubbles of a gaseous fluid rise through the metal in the path.

To accomplish these conditions a substantially vertical flow path is defined where molten metal is introduced at a high level and discharged at a lower level and means are provided to disperse bubbles of a gaseous fluid under pressure at or near the lower level of the flow path. At the same time the direction of the metal flow is arranged to oppose the direction of the gas bubble flow thus reducing the effective rising velocity of the gas bubbles and increasing their residence time and consequently their reaction time in the melt.

The metal flow can also be arranged so that it is concurrent with the gas flow should this be required for structural, architectural or other technical reasons recognising that the degassing action though still present will be less effective than in parts of the system where where the gas and metal flows are countercurrent.

The effectiveness of the system is increased by directing the partially cleaned and de-gassed metal into subsequent paths into which fresh clean gas is injected and repeating the process as many times as is necessary to get the contaminate and gas concentrations to an acceptable level.

This invention relates to a method and apparatus for gas sing and removing particulates from molten metals, particularly aluminium and it's alloys, by injecting a swarm of small bubbles at or close to the base of a moving column of molten metal, directing the partially treated metal into a second moving column into which a fresh swarm of small bubbles is injected and repeating the process until the gas and particulate content of the metal being treated is brought to the desired level.

In order to prevent oxides and other contaminating material which float from being drawn back into the bulk of the molten metal as it enters, passes through and exits from the cleaning apparatus it is desirable to arrange the entry, transfer and exit points so that the metal flow takes place substantially away from the surface of the molten metal.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a diagrammatic representation illustrating the functional features of the system Molten metal contaminated with dissolved gas and non-metallic and inter-metallic particles enters the system at (1) and flows in a downward moving column (2) countercurrent to a swarm of fine bubbles released from a porous diffuser (3) located at or near the base of the column. The downward movement of the molten metal in the column (z) is brought about by gravity and/or by the hydraulic lift action of a second swarm of bubbles released from a porous diffuser at or near the base of column (4) connected to the base of column (2).In this way the passage of the molten metal along the return column (4) exposes it to a fresh swarm of bubbles which, in addition -to acting as a hydraulic lift, will contribute to the degassing and floatation actions of the apparatus.

For the sake of clarity a single gas injection point (5) is shown leading into a manifold feeding all columns but it is recognised that individual feeds will be used necessary to ajust the metal aand gas flows.

By suitably sizing the columns (2) and (4) adjusting the metal and gas flows the downward metal velocity and the upward bubble velocity in column (2) can be brought into near balance so that the residence time of the gas bubbles in column (2) is significantly increased over that obtainable in a static column and the effective use of the scavenging gas is thereby improved.

The partially de-gassed and cleaned metal is then directed, by gravity and/or a gas lift, into the top but below the surface of a second downwardly moving column, operating in the same way as the first, and by exposing the molten metal to a fresh uncontaminated swarm of bubbles the maximum partial pressure difference between the dissolved gas in the melt and the scavenging gas is re-established.

The process is then repeated as often as is required to bring the gas and particulate content to an acceptable level.

De-gassed and cleaned metal exits at (6), heat to maintain the required operating temperature is provided by electric heating elements (7) though it is recognised that that other locations and heating media such as oil, gas or electric induction could be used.

The containing vessel is preferably made out of a heat conducting material which is resistant to molten aluminium such as graphite, carbon or silicon carbide. Insulation is provided by a suitable insulating refractory material.

A removable roof (8) is provided to contain the heat and allow access to enable dross (9) to be skimmed off. The dross is held at a level above the metal discharge point (6) by virtue of the gas-lift action promoted by the scavenging bubbles in the columns (2) and (4) and is thus isolated from the cleaned metal.

Claims

CLAIMS.

1. A method of reducing contamination of a molten metal wherein the metal is caused to flow downwardly along a path whilst bubbles of a less dense fluid rise through the metal in the path.

2. A method of reducing contamination of a molten metal wherein the metal is caused to flow along a path having distinct first and second portions, bubbles of a less dense fluid are caused to rise through the metal in the first portion of the path and further bubbles are caused to rise through the metal in the second portion of the path.

3, A method according to claims (1) and (2) wherein there is an accumulation of molten metal above the position where the metal is introduced into said path.

4, A method according to claims (1), (2) and (3) wherein said path includes first and second distinct, substantially vertical portions in succession, said less dense fluid is introduced into the metal in said first portion and a further supply of fluid is introduced into the metal in said second portion.

5. Apparatus for use in a method according to claims (1) and (2) and (3) comprising means defining a path for molten metal, the path having distinct first and second portions, first dispensing means for dispensing bubbles of a less dense fluid in the first portion of the path and second dispensing means for dispensing bubbles of a less dense fluid in the second portion of the path, the arrangement being such that, in use of the apparatus, molten metal flows through said portions sequentially.

6, Apparatus for use in a method according to claims (1) to (5) comprising means defining a first, substantially vertical portion of said path, an inlet for admitting molten metal to said first portion at a high level, an outlet for molten metal from said first portion at a lower level, dispersing means for dispersing bubbles of a less dense fluid in said first portion at or near to the lower level and means for supplying the less dense fluid under pressure to the dispersing means.

7, Apparatus according to claims (5) and (6) wherein the inlet lies at a level below the upper end of the said first portion of the path.

8. Apparatus according to claims (5) to (7) wherein the vertical separation between the inlet and the outlet exceeds each horizontal dimension of the path at levels between the inlet and the outlet.

9. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.

9. Apparatus according to any one of claims (5) to (7) wherein the dispersing means is fixed with respect to the means defining said first portion of the path.

10. Apparatus according to any of claims (5) to (9) wherein the means defining said first portion of the path further defines a second substantially vertical portion of the path and an intermediate portion of the path providing for flow of molten metal from the outlet of the first portion to an inlet of the second portion, the second portion having an outlet for molten metal at a level below that of the inlet of the second portion and further dispersion means being associated with the second portion of the path.