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Alternative fluxes for lead bullion refining
Asante, Samuel
Asante, Samuel
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2022
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The Harris process for lead softening uses sodium hydroxide and sodium nitrate to remove arsenic, tin, and antimony from molten lead by oxidation. It is known to be better than oxygen softening, however the process has a high cost in terms of reagent used. Sodium hydroxide is corrosive and expensive. In this research, calcium carbonate, calcium hydroxide, and magnesium hydroxide were used as alternatives for sodium hydroxide in the softening process. Based on the initial results, the research focused on using calcium carbonate for subsequent softening process. In this research, the activities of the impurities: arsenic, tin and antimony were calculated and compared with that obtained with sodium hydroxide as the melt. The softening experiments were conducted on lead bullion by design of experiment (DOE) method by varying temperature, time, and reagent quantity. The drosses obtained were analyzed chemically and with x-ray diffraction (XRD) to identify the phases formed.
In using stoichiometric amount of calcium carbonate and sodium nitrate (i.e., 0.019g CaCO3 :1g crude lead), 99.99% of tin and 95% of arsenic were removed. However, the removal of tin and arsenic were 65% and 35% respectively when reagents quantities were reduced by 84% of the stoichiometric amounts. The optimized parameters obtained from the DOE analysis were temperature: 500oC; reaction time: 15 minutes; and mass ratio of calcium carbonate to crude lead of 0.0064 : 1. Under these conditions, CaCO3-NaNO3 process produced 98.7 wt. % Pb whereas NaOH-NaNO3 process produced 99.04 wt. % Pb. The average residual calcium in the softened lead bullion was 0.0127 wt.%. The CaCO3 generated solid dross with little or no entrained lead. It was discovered from thermodynamic data that both CaCO3 and NaOH cannot remove arsenic from molten lead due to high positive Gibbs free energy values. The removal of arsenic is achieved by atmospheric oxidation prior to the addition of NaOH or CaCO3.
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