Dross is a material, which forms on the surface of molten non-ferrous metal, such as aluminum or zinc, during melting, metal holding and handling operations when the molten metal is in contact with a reactive atmosphere. Dross normally consists of metal oxides entraining a considerable quantity of molten free (unreacted) metal, and for economic reasons it is desirable to extract the free metal before discarding the residue. Zinc Dross is obtained from galvanizing plants, which contains > 80% of Zinc metal. It is contaminated with Zinc oxide, Iron and Chlorine. During galvanizing of steel about 25% of the zinc metal is degraded to dross or ash. Cold zinc dross is crushed in a ball mill followed by sieving to separate the coarse metal portion from the fine oxide residue.
The study explains to determination of ZnO content as well as the other impurities present in Zn dross which may interfere the production of Zn metal powder. This study is done to finding the usability of the Zn dross in the metal industry.
Experimental – Sample of zinc dross collected was crushed to smaller size fraction. A representative sample was made and analyzed for zinc, iron, Aluminum, Copper, Zinc Oxide contents. A cut piece of zinc dross sample was subjected to TGA and FTIR studies .The chemical analysis of zinc dross is given in Table 1.
Except Zn and ZnO, other elements are analyzed by ICP-OES.
Table 1: Chemical Analysis of Zinc Dross
FTIR study of the Zn Dross:
FTIR study was done to find out ZnO content in the Zn dross. Pure Zinc Oxide was used as Standard in ratio made of 1% ZnO + 99% KBr to find out the absorption maxima for ZnO in IR region. The maxima was at 1629 per cm. Sample was also studied by FTIR. Initially it was scanned throughout the region it was found at near 1600 per cm. Then that region was zoomed and compared with 1% pure ZnO spectra. From comparison of the two heights ZnO was found to be 2.34%
1% Std ZnO +99% KBr
FTIR Scan of 50% Zn dross
Zoom image of the absorbance
FTIR study also revealed that there were two other peaks (absorption). Those two peaks may be due to ZnCO3 and Zn(OH)2. At present due to non-availability of pure ZnCO3 and Zn(OH)2 the percentage of Carbonate and Hydroxide forms could not be estimated.
Thermal study of zn dross
Thermal study of this sample was done if there would be any change during pyrometallergical treatment. It was found that the sample is not changing too much in its composition during heating in Nitrogen atmosphere up to 950 degree C.
This Study revealed the average composition of Zinc Dross about its constituents both cation as well as anion to find out its suitability of Zn Powder and Zn ingots by metallurgical process. The study was also made to find out the pyrometallergical stability of a Zn dross in nitrogen atmosphere.
Contributed by: Ms. Alakta Saha under guidance of Prof. Barun Gupta