Process for extracting zinc present in liquid cast iron, means f

Details Process for extracting zinc present in liquid cast iron, means f Process for extracting zinc present in liquid cast iron, means f

1993-09-17

1995-02-07

Rosenberg, Peter D.

Specialized metallurgical processes, compositions for use therei

Processes

Producing or treating free metal

75658, 75663, C21C 104, C22B 904

Patent

active

053872754

DESCRIPTION:

BRIEF SUMMARY
The invention pertains to a process for extracting zinc present in liquid cast iron.
The invention also pertains to the means for implementing the above-mentioned process and the products thus obtained.
More specifically, but not exclusively, the invention pertains to a process applicable to industrial settings and especially to a foundry.
Precisely, but not restrictively, the invention pertains to the elimination of zinc in the cast iron produced by melting salvaged ferrous materials.
The generalization of the process of galvanizing steel to protect it against corrosion has led to the increasingly frequent presence of zinc in salvaged scrap iron.
Said zinc, which is present in the cast iron produced from the above-mentioned type of scrap iron, involves problems, including the problem of inhibiting the action of magnesium used to ensure the formation of spheroidal graphite in said cast iron ("Effets du zinc introduit dans les charges en fonderie de fonte" [Effects of zinc added to batches in iron casting operations], S. Parent-Simonin, Fondeur d'Aujourd'hui No. 62, 1987).
It is known that the amount of zinc which can be present in solution in liquid cast iron cannot physically exceed a given level, which is a function of the temperature, pressure, and composition of the cast iron.
It is accepted that the maximum zinc content of conventional cast iron (liquid iron saturated with carbon) under an air pressure equal to a normal atmospheric pressure of 1 bar is 2.2% at 1,200.degree. C.; 1.6% at 1,300.degree. C.; 1.2% at 1,400.degree. C.; and 0.9% at 1,500.degree. C.
The contents found in practice are not as high as the maximum contents listed above.
Such contents are generally approximately 0.2% and only in rare cases exceed 0.4%, at least in 1990.
Zinc was still found to create problems, even when present in cast iron at a level of 0.1%, or 1,000 ppm.
The techniques now proposed to eliminate zinc consist, before melting, of either treating the scrap iron at a temperature higher than 910.degree. C. in order to reach the boiling point of the zinc with a view to vaporizing it, or applying chemical treatment to scrap materials and dust using a reduction agent, then collecting the zinc.
The technique according to the invention aims to eliminate the zinc not from the scrap iron, but from the liquid cast iron, and the corresponding treatment can be applied either while the liquid cast iron is soaked in an induction furnace, or in a ladle located between the outlet of the furnace and the casting station.
The temperature of the liquid cast iron is conventionally between 1,100.degree. C. and 1,600.degree. C.
Of course, these are extreme values, because in practice we avoid dropping below 1,300.degree. C. and heating above 1,500.degree. C.
An average temperature of 1,400.degree. C. is an effective compromise between the different limits.
The fraction of zinc dissolved in said liquid cast iron in excess of the maximum content progressively evaporates naturally, and with increasing speed the higher its initial concentration.
Therefore, it is the considerable portion of the zinc which naturally remains in solution in the liquid cast iron, thus referred to as "residual zinc," which the invention aims to eliminate completely, or at least to lower its concentration in the bath.
In order to extract zinc from cast iron, it is known (CA-A-1,251,328):
- In a first stage, with the cast iron being held in a container, to confine the volume of gaseous fluid located above the cast iron bath in a sealed manner;
- In a second stage, to place the confined atmosphere under a partial vacuum, the level of which can be varied in a controlled manner.
Said process has its advantages, but causes the cast iron to spatter, which is incompatible with its use in an industrial installation for casting parts in a plant.
Indeed, such cast iron spattering is highly damaging.
To that end the object of the invention is a process according to which, prior to the casting station, in order to trap the zinc:
- In a first stage, with the cast iron being held

REFERENCES:
patent: 2721813 (1955-10-01), Holmberg
Patent Abstracts of Japan, vol. 10, No. 350 (C-387) (2406) (Nov. 26, 1986) as it relates to JP-A-61-149415 (Sumitomo) (Jul. 8, 1986).
Patent Abstracts of Japan, vol. 8, No. 3 (C-203) (1440) (Jan. 7, 1984) as it relates to JP-A-58-171,519 (Kubota) (Oct. 8, 1983).

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