Abstract
Previous studies on the dissolution of zinc oxide have concentrated on the effects of
temperature, reagent concentration, particle size and agitation rate, among other factors.
However, there is dearth data on the effect of a combination of product concentration and pH
on leaching rates. This work examines the effects of the concentration of pH and zinc ions on
the dissolution kinetics of zinc oxide. The results showed that the rate of zinc dissolution was
greatest at lower pHs. This is because at lower pH, the concentration of acid (H+ ions) is also
expected to be high (pH = log10[H+]), thus increasing the reaction kinetics in accordance
with the kinetic molecular theory. The rate of reaction was found to increase with an increase
in the concentration of zinc ions (Zn2+), which is in contrast to established theories. This
observation can be explained by the fact that since heat energy produced (via an exothermic
reaction) was continuously being dissipated through a constant temperature water bath it
favoured zinc oxide dissolution. Zinc oxide ore at 62.5% purity also comprised of other metal
oxides (e.g., iron, copper, manganese, etc) which were simultaneously leached. Since the
solution was recycled, the concentration of iron, for example, could have also increased.
Since zinc is more reactive than iron, two reaction mechanisms could have possibly been
occurring at the same time, i.e., the dissolution of zinc oxide ore by sulfuric acid and the
displacement of iron in solution by zinc in zinc oxide forming iron hydroxide.