The microbiological extraction of zinc from a high-grade zinc sulfide concentrate has been investigated, using a pure strain of Thwbacillus ferrooziduns. Conditions such as temperature, pH, pulp density, nutrient concentration, and specific surface of solids have been studied in terms of their effects on zinc extraction rate and in some instances on final zinc concentration in solution. Where appropriate, optimum conditions for leaching have been specified.

SUMMARYOptimum growth and maximum aflatoxin production by Aspergillus flavus were obtained on a basal medium containing 0.8 mg of zinc per liter. The effect of several other trace metals on the toxin production was evaluated. Aflatoxin production was inhibited by the presence of barium ions in the medium. Cadmium ions, in contrast, stimulated aflatoxin production.

The effects of carbon dioxide‐enriched air on the rate of zinc extraction during the microbiological leaching of a, high‐grade zinc sulfide concentrate by Thiobacillns ferrooxidans have been studied. Under normal air‐aeration conditions, the leach rate is limited initially by the availability of solid substrate surface area per unit volume of leach liquor, then by availability of carbon dioxide. If carbon dioxide‐enriched air is supplied, along with excess substrate, the zinc extraction rate increases as the carbon dioxide content increases until some other, as yet unknown, factor becomes limiting.