US3145097A - Shock-chilling condensers for a zinc blast-furnace - Google Patents

Description

United States Patent 3,145,097 SHQCK-CHTLLTNG CONDENSER?) iitfiit. A ZHNC BLAST-FURNACE Leslie .lach Derham, Avonmouth, England, assignor to Metallurgical Processes Limited, Nassau, Bahamas, a corporation of the Bahamas, and The National Smelting Company Limited, London, England, a British company, doing business as Metailurgical Development Company, Nassau, Bahamas No Drawing. Filed .Tune 26, 1961, Ser. No. 110,308 Claims priority, application Great Britain July 4, 1960 2 Ciaims. (Cl. 75-63) This invention relates to shock-chilling condensers for a zinc blast-furnace.

In the production of zinc in a blast-furnace, the gases produced are conducted to a condenser Where they are brought into contact with a circulating stream of molten lead to dissolve the zinc vapour. Typically, the lead enters the condenser at 450 C. and containing 2.0% zinc in solution, and leaves at 550580 C., containing 2.25% zinc in solution. This lead is then cooled to 450 C. by passing through Water-cooled launders or other suitable cooling devices to a settler Where the excess zinc separates out, leaving 2.0% zinc in solution in the lead that is recirculated to the condenser.

Zinc concentrates often contain arsenic. Some of the arsenic is volatilized and dissolved by the circulated lead in the condenser. When this lead is cooled outside the condenser, arsenic separates with the Zinc. Typically, after cooling to 450 C. the lead contains 0.0004% As, together with 2% zinc, these concentrations of arsenic and zinc apparently corresponding to the solubility product of zinc arsenide (Zn As in lead at 450 C.

Referred to the zinc content of the gases leaving the furnace, the arsenic content of these gases may typically be 0.2% to 0.8%, depending mainly on the character of the recirculated materials and the arsenic content of the sinter. Assuming the percentage of arsenic to be 0.8% by weight of the zinc, in passing through the condenser, if the zinc taken up by the lead is 0.25 of the weight of lead circulated, the arsenic taken up is 0.002% of the weight of lead. With the lead entering the condenser containing 2.0% zinc and 0.0004% arsenic, the hot lead leaving the condenser will then contain 2.25% zinc and 0.0024% arsenic.

The presence of this arsenic is disadvantageous, firstly, because means have to be found for removing it from the zinc to make saleable metal. Another disadvantage is that, as the lead is being cooled, some zinc arsenide solidifies on the walls of the launders or other cooling devices, forming an adherent coating that has to be removed periodically. A further disadvantage is that, as We have discovered, the presence of arsenic greatly increases the rate at which molten zinc oxidizes and thereby increases the loss of zinc metal during its flow through the cooling launders and the settler, and during casting. Even when a flux, such as zinc ammonium chloride, is used to cover the molten zinc-containing lead and the separated zinc as it flows into the settler and in the settler, a considerable amount of oxidation occurs.

An important object of the present invention is to avoid this oxidation and the consequent increase of dross formation. In the absence of aluminium a loose oxide skin is formed as the lead, with its supernatant layer of zinc, flows through the launders. The presence of even a small amount of aluminium produces a very thin protective oxide film under which the separated zinc can flow without further oxidation.

The present invention consists in the addition of aluminium to the circulated lead in amount sutficient to form the compound AlAs with the arsenic and consehldiifih? Patented Aug. 118, 1964i quent almost complete removal of both the arsenic and the added aluminium as a precipitate of composition AlAs.

In practice it is dilficult to make aluminum additions exactly equivalent to the arsenic present, and the exact procedure to be followed depends upon circumstances.

The aluminium also forms the compound AlPe with the iron that is present. In practice therefore, suflicient aluminium to form AlFe and AlAs with the iron and the arsenic is needed; the amount of aluminium needed for removing the small amount of iron is small compared With that required for removing the arsenic. This removal of iron is advantageous, since for most purposes zinc with low iron content is desirable.

Under the temperature conditions prevailing, aluminium acts as a deoxidant and inhibits oxygen pick-up by the lead-zinc mixture.

The aluminium arsenide is precipitated in the molten lead within the condenser. It forms part of the dross formed in the condenser and, if the condenser is fitted with a dross extractor of the type in which an inclined screw-conveyor takes up dross from the surface of the molten lead and allows entrained molten lead to run back into the condenser, this dross is removed partly by the extractor and partly from the pump sump outside the condenser.

Aluminium is only very silghtly soluble in lead but is highly soluble in liquid zinc. Therefore, most of any excess aluminium separates with the zinc when the lead is cooled outside the condenser, only a fraction of the excess aluminium would be retained in the lead returned to the condenser.

For certain uses of zinc, as in some types of galvanizing, a small aluminium content is required in the zinc. When the zinc is destined for such uses, aluminium is added to the lead (from which zinc has been separated in the settler) before it enters the condenser in amount sufficient to precipitate the arsenic and to provide the amount of aluminium required in the Zinc.

When aluminium is not required in the zinc advantage may be taken of the fact that aluminium is almost completely removed from zinc by reaction with molten zinc chloride. As zinc chloride is conveniently used as a fiux cover for preventing oxidation, excess aluminium can be added to the lead entering the condenser to ensure the complete removal of arsenic and the excess aluminium can then be removed by reaction with a flux covering of zinc chloride over the molten zinc in the settler. If, on leaving the cooling launders, the lead, together with the separated molten zinc, is allowed to fall through a layer of the zinc-chloride flux cover, removal of aluminium is further facilitated.

Alternatively, a slight deficiency of aluminium can be added, so that all the aluminium is precipitated but some of the arsenic remains unprecipitated during its flow through the cooling launders and the settler, and during casting. For instance, if the arsenic taken up each time the lead passes through the condenser is 0.002% of the weight of lead, 0.0007% of aluminium is added to the lead before it enters the condenser, this being sufficient to remove 0.0007 /27=0.00194% of arsenic. This leaves 0.00006% of arsenic not removed, and this must finally appear in the 0.25% of separated zinc, which will therefore contain 0.024% arsenic; this small amount of arsenic can be removed from the zinc by treatment with sodium metal.

The aluminium added to the lead is most conveniently added as an aluminium-zinc alloy of reasonably low melting point (below 500 C.) and containing not more than about 20% aluminium. Such an alloy can be prepared from clean scrap aluminium and zinc. Alternatively scrap aluminium-zinc die-casting alloy containing about 4% aluminium, can be used. For instance, an aluminium-silicon alloy of 10W silicon content can be used; the silicon then helps in the iron removal. A mixture of scrap aluminium and aluminium-silicon alloy with scrap zinc-aluminium die-casting alloy can be used.

Various modifications may be made within the scope of the invention. For instance, instead of adding aluminium to the lead before the lead enters the condenser, the aluminium may be added to the pump sump. This procedure has similar advantage (i.e., inhibition of oxidation at the launder surfaces, and no growth of zinc arsenide on the launder sides) as the procedure outlined above.

I claim:

1. In the blast furnace smelting of zinc, in which zinc vapour coming from the furnace is subjected to shockchilling With circulating molten lead to condense the zinc vapour, metal impurities, arsenic and iron, present in the zinc vapour also being condensed with the zinc, the improvement in combination therewith which comprises adding an excess of aluminium to the circulating molten lead in the condensing system to cause the formation of compounds between the aluminium and said metal impurities, and to reduce the rate at which the molten zinc oxidizes, precipitating said compounds in the molten lead, allowing the precipitated compounds to gather at the surface of the circulating molten lead, removing the precipitated compounds from the surface of the molten lead to inhibit the formation of accretions of the compounds in the condensing system, and permitting said aluminium to form a protective oxide film over the molten lead-zinc mixture.

2. Process according to claim 1, in which the excess aluminium is subsequently removed from the molten lead-zinc mixture with molten zinc chloride.

References Cited in the file of this patent UNITED STATES PATENTS 1,957,819 Cowan July 19, 1930 1,954,463 Thompson et al Apr. 10, 1934 1,957,837 Kern May 8, 1934 1,976,225 Hiers Oct. 9, 1934 2,119,197 Betterton et al May 31, 1938 2,411,940 Shmopshire Dec. 3, 1946 2,781,257 Wilkins Feb. 12, 1957 2,886,309 Derham May 12, 1959 OTHER REFERENCES Mathewson: Zinc, the Metal, its Alloys, and Compounds, Reinhold Publishing Co., 1959, pages 286-297.

Claims (1)

1. IN THE BLASST FURNACE SMELTING OF ZINC, IN WHICH ZINC VAPOUR COMING FROM THE FURNACE IS SUBJECTED TO SHOOCKCHILLING WITH CIRCULAING MOLTEN LEAD TO CONDENSE THE ZINC VAPOUR, METAL IMPURITIES, ARSENIC AND IRON, PRESENT IN THE ZINC VAPOUR ALSO BEING CONDENSED WITH THE ZINC, THE IMPROVEMENT IN COMBINATION THEREWITH WHICH COMPRISE ADDING AN EXCESS OF ALUMINUM TO THE CIRCULATING MOLTEN LEAD IN THE CONDENSING SYSTEM TO CAUSE THE FORMATION OF COMPOUNDS BETWEEN THE ALUMINUM AND SAID METAL IMPURITIES, AND TO REDUCE THE RATE AT WHICH THE MOLTEN ZINC OXIDES, PRECIPITATING SAID COMPOUNDS IN THE MOLTEN LEAD, ALLOWING THE PRECIPITATED COMPOUNDS TO GATHER AT THE SURFACE OF THE CIRCULATING MOLTEN LEAD, REMOVING THE PRECIPITATED COMPOUNDS FROM THE SURFACE OF THE MOLTEN LEAD TO INHIBIT THE FORMATION OF ACCRETIONS OF THE COMPOUNDS IN THE CONDENSING SYSTEM, AND PERMITTING SAID ALUMINUM TO FORM A PROTECTIVE OXIDE FILM OVER THE MOLTEN LEAD-ZINC MIXTURE.