Porewater quality of uranium tailings during laboratory aging and its relation to the solid phase

Abstract

After treatment to dissolve and to extract uranium, tailings from Ranger Uranium Mine had a low pH of about 1.8. However, this rose rapidly during initial stages of aging in the laboratory, due to the neutralizing effect of chloritic minerals present in the ore. Within two weeks at 22°C, the pH had reached 3.1, and it continued to rise with time and increasing temperature. The maximum value observed was 5.4 after six weeks at 70°C. Associated with (and acting against) the rise of pH was precipitation of iron, aluminium and silicon oxyhydroxides and hydroxy-sulfates from pore solution into the solid phase. These compounds acted as scavengers to reduce the concentrations of potentially toxic elements copper, zinc, cobalt, nickel, cadmium and lead in porewaters to levels which eventually became comparable to those in limed tailings. Concentrations of radionuclides were also reduced by sorption/coprecipitation reactions with these scavengers, but levels in solution were continually replenished by slow dissolution of a residual uranium mineral, probably brannerite. As a result, radioactivity in porewaters after prolonged aging was appreciably higher than that in limed tailings. Salt concentrations were high, being composed essentially of magnesium, ammonium and manganous sulfates. Ionic strengths were nominally of the order of 1.5 M, but in reality were close to that of seawater (0.7 M) if complex ion formation was taken into account. Concentrations of the major ions magnesium, ammonium, manganous and sulfate, as well as those of calcium and sodium, did not change appreciably during aging at the various temperatures. In general, porewater compositions of limed tailings were similar to those of aged acidic tailings, but the solid phase of limed tailings contained much more gypsum. Also, there appeared to be differences in the behaviour of iron and aluminium hydroxy compounds in the two systems, probably because the hydroxy compounds were precipitated very rapidly during liming and recrystallized into less hydroxylated forms with time. Porewaters from tailings neutralized with magnesium oxide rather than lime had higher total salt concentrations, because, unlike calcium, magnesium salts did not precipitate unless porewaters were further concentrated by evaporation. Manganese precipitated from porewaters at about pH 8.5, and this precipitate also helped to retain heavy metals and radioelements within the solid phase.