Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
1999-12-08
2001-09-25
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S655000, C148S553000
Reexamination Certificate
active
06293993
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the domain of recycling brass foundry waste. More particularly, the invention relates to a pyrometallurgical recycling process for zinc-rich dross and dust that also contain copper, and involving a compaction step.
DESCRIPTION OF RELATED
Pyrometallurgical processes for processing waste containing zinc, largely in oxide form, are already known. These processes are designed mainly for applications such as recycling iron-rich steelworks fines, or chlorine-rich household waste. In all cases, there is a reduction step in which zinc is released in vapor form. This reduction takes place either entirely in the solid phase, the temperature of the material to be treated being increased in a reducing atmosphere in a sintering furnace or a fluidized bed furnace, or with a pre-reduction step in the solid phase followed by a fusion step, the material previously being agglomerated in the form of pellets with a reducing agent and a flux and then put back into a blast furnace. In all cases, the zinc is recovered by vaporization and condensation either in the form of pure zinc or in the form of compounds (oxides, chlorides, etc.). These processes are capable of separating the various metallic elements (iron, lead, cadmium, etc.) present in the waste.
Furthermore, calamine (zinc ore containing blende (ZnS), zinc carbonate (SnCO
3
) and zinc oxide (ZnO) have been used to make brass ever since ancient times. This was done using the cementation process called the “calamine process”. This process was first described by Pseudo-Aristotle in 300 BC in “Mirabiles Auscultationes”, and later by Pliny and Discoride in the first century AD.
In this process, finely ground calamine is mixed with ground charcoal. An organic binder (urine, rice water, resin, tallow, salt, etc.) is added and the preparation is usually agglomerated in pellets. In ancient times, this mix was then made to react with pure divided copper for very long periods, between 8 and 24 hours. This resulted in a brass that typically contained less than 30% of zinc. This process has no longer been used since the beginning of the last century, when brass was made from metallic copper and zinc. Recent texts by authors such as Rolandi and Scacciati (1956), and Werner (1970), have described attempts to simulate the process described above at temperatures below 1050° C., to avoid zinc losses by vaporization. These tests in the solid state form the state of the art.
There are two different sources of brass foundry waste. The first is formation of dross on the surface of the liquid bath in furnaces containing up to 60% by weight of Zn and more than 10% of Cu. This dross is currently ground on production sites only and then separated by size grading. Large copper-rich fractions are recycled on site. However, small size grading fractions called “fines” with a low copper content are sent and treated in specialized companies. The second source is the recovery of flue-dust in foundry intake system filters, composed essentially of zinc oxide with up to 80% by weight of Zn, and also containing between 2 and 20% of Cu. This flue-dust is also treated on specialized sites.
However, this waste that cannot be recovered by the foundries that generate it, is not sufficiently rich in copper to be attractive in copper recovery systems, and it contains too much copper for conventional systems for recycling waste containing zinc. Therefore, it is becoming more and more difficult to find industries interested in their recovery. Furthermore, most treatment sites use hydrometallurgical processes that generate effluents that are highly noxious for the environment.
Finally, these fines and flue-dust require more and more particular precautions for transport (powder and polluting materials), to satisfy increasingly severe regulations. This is even more of a nuisance since treatment plants are frequently built in a different country and consequently different types of legislations are applicable, requiring a variety of authorization requests.
Therefore, the applicant has searched for and developed a process to:
economically and ecologically recycle fines and flue-dust generated by brass foundries;
preferably, be able to perform this recycling on the site on which the waste is produced, in order to eliminate risks due to long distance transport;
reuse the waste in brass production.
SUMMARY OF THE INVENTION
The purpose of the invention is a process for recycling waste (dross, flue-dust, etc.) generated in brass foundries in order to recover metal contained in this waste (Cu, Zn) on the site on which this waste is generated, and their reinjection into the brass production process.
In the process according to the invention, recycling of zinc-rich solid waste (typically dross and flue-dust) mainly in the form of zinc oxide, and containing Cu oxides and impurities normally present in brass and generated in brass foundries:
a) the said waste is mixed with a finely divided solid reducing agent and a binder;
b) the said mix is compacted in agglomerates with open pores;
c) the said agglomerates are added into a treatment reactor containing a bath of liquid copper or brass metal, where they are reduced;
d) after the said waste has been reduced, a new brass is formed and the waste generated in the treatment reactor is separated and recovered;
e) the said new liquid brass may then be transferred into a holding and casting furnace, where the proportion of Zn, or any other alloying element (Pb, Sn, Ni, etc.) in the composition of the said brass may be adjusted if necessary to obtain the required product, and waste generated in this furnace is separated and recovered;
f) the brass, possibly after its composition has been adjusted, is then cast by a process capable of producing plates, billets, wires, parts and products with the required dimensions;
g) the said waste recovered in steps d and e is recycled according to steps a to f.
The critical steps of the process are steps a to c, particularly the formation of agglomerates with open pores comprising the said waste, a reducing agent and a binder.
Tests carried out by the applicant have shown the need for this combination of essential means.
In particular, the applicant carried out recycling tests of the said waste directly with a reducing agent, in a liquid metal bath, without forming agglomerates. In this case, the test efficiency, and therefore the recovery rate of Zn and Cu containing the said waste, was very low, unlike tests according to the invention.
It is assumed that the high efficiency obtained with the process according to the invention is due to several factors:
the agglomerates formed are composed of an intimate mix of reagents (Cu and Zn oxides to be treated, reducing agent and accelerator binder agent), unlike a simple mix of powders that would be added to the said liquid metal;
these agglomerates maintain their integrity as agglomerates due to the binder in the said liquid bath, at least long enough for the said reducing agent to reduce Cu and Zn oxides;
contact between the zinc vapor formed and the liquid copper leading to the formation of a brass that can contain up to at least 35% zinc, is optimized due to the open pores of the agglomerates that enable infiltration of liquid copper within the agglomerates and a maximum exchange surface area;
finally, the agglomerates formed may easily be kept in contact with the said liquid metal, unlike a simple mix of powders that tends to rise and float on the surface of the said liquid metal.
It is important to note that the process for treating the zinc ore (calamine) since ancient times was known for being very slow, since the treatment duration could be as long as 24 h.
By comparison, the recycling process according to the invention is fairly fast.
Furthermore, it should be noted that the process according to the invention can result in brasses that contain up to at least 35% of zinc by weight, whereas the calamine process apparently cannot produce brass with more than 30% of zinc by weight.
According to the applicant's studies,
Revet Florence
Welter Jean-Marie
Dennison, Scheiner Schultz & Wakeman
Trefimetaux
Wyszomierski George
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