True dross is an alloy of 6% iron and 94% zinc. Dross is a mass of solid impurities which floats on a molten metal or it may be dispersed in the metal such as in wrought iron. It generally forms on the surface of low melting metals such as tin, lead, zinc or aluminium. For high melting metals such as steel the oxidised impurities melt and float making them easy to separate. Dross normally consists of metal oxides along with a considerable quantity of free (unreacted) metal, and for economic reasons, it is desirable to extract the free metal before discarding the residue.
The experiment involves the estimation of total zinc, zinc oxide, zinc chloride, aluminium and iron out of which analysis of total zinc, zinc oxide and zinc chloride have been carried out by wet chemical approach whereas aluminium and iron content were determined by ICP-OES.
To estimate total zinc, the sample was dissolved in aqua regia followed by sulphuric acid fuming. After the completion of fuming (30-40 minutes) on a hot plate, the fumed mass was cooled, water was added to the cooled mass and boiled briefly. The solution was filtered and after R2O3 separation with NH4Cl and NH4OH, the filtrate was taken in a 500 ml volumetric flask. From the filtrate, total zinc was estimated on 0.1 gm basis by complexometric titration with standard EDTA solution using xylenol orange indicator at pH 5.6.
ZnO and ZnCl2
For the determination of zinc oxide and zinc chloride, the sample was treated with hot 10% ammonium acetate solution, since zinc oxide and zinc chloride present in dross are soluble in hot ammonium acetate solution. After boiling for 10-15 minutes the solution was filtered and from the filtrate ZnO was estimated by complexometric titration with std. EDTA solution using xylenol orange as indicator at pH 5.6
From the same filtrate, chloride concentration was evaluated by std. AgNO3 titration using potassium chromate as indicator and the concentration of ZnCl2 was determined from the equivalent of chloride present in the sample.
Aluminium & Iron
From another treatment, Al & Fe have been evaluated by ICP-OES analysis. In this method, the sample was treated with aqua regia and after complete dissolution of the sample, the solution was taken in a 250-ml volumetric flask and from this solution the concentration of Al & Fe was determined at the wave-length of 396.153 & 238.204 respectively.
Total Zinc = 98.21%
ZnO = 1.62%
ZnCl2 = 0.23%
Al = 0.37%
Fe = 1.17%
Determination of zinc salts present (especially oxide and chloride) in zinc dross is of enormous importance as it forms bubbles of oxide and chloride over the metal surface on which galvanization is done. As a consequence, the estimation of these impurities prior to galvanization is important and after determining these impurities, the quality of the metal is determined. Here we present an easy and convenient approach to estimate the constituents present in zinc dross.
Contributed by: Kajal Ray & Priya Das under the guidance of Prof. Barun Gupta