Bioleaching of lateritic nickel ore by ultrasound
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I.
INTRODUCTION
LATERITIC nickel ore constitutes about 80 pct of the presently known sources of nickel metal. Because of the small quantity of nickel present in the laterites, its economic production by conventional methods has been difficult. One of India's largest lateritic nickel ore deposits, which contains about 64 million tonnes, is at Sukinda Valley (Orissa, India). Bioleaching techniques, using organic-acid-producing heterotrophic microorganisms, have been reported to cause nickel solubilization, v-7] Among the heterotrophic microorganisms, the filamentous fungus Aspergillus niger has been most extensively studied for its organic acid production.tSm Native organisms were found to be more suitable, due to their greater tolerance and adaptability to metals present in the ore environment, t4'71 Sukla et al.tlo] have also reported the dissolution of Cu, Ni, and Co from copper converter slag using A. niger culture filtrate. The metal-leaching ability of heterotrophic fungi depends on their growth and the production of acidic metabolic byproducts, which are excreted into the culture medium. A variety of acidic metabolites, such as citric, oxalic, tartaric, malic, ketoglutaric, succinic, fumaric, and gluconic acids, are secreted by the genus Aspergillus.Vq Such metabolites induce metal dissolution by forming salts and chelates. Application of ultrasound plays an important role in extractive metallurgy and has been reviewed by PolyukhinY 21 Sonochemical extraction techniques, when used in combination with conventional methods, yielded faster and selective extraction of metals, t13,m Recently, Swamy et al. u51 have reported an enhanced growth rate of A. niger, which in turn was capable of producing more acid, by the application of ultrasound in controlled doses. In the present article, we report the results of further investigations into the effect of ultrasonic pretreatment of lateritic ore for bioleaching of nickel using a native strain of A. niger.
II.
EXPERIMENTAL PROCEDURE
A. Materials and Methods Lateritic nickel ore was collected from major deposits from Sukinda. For leaching experiments, the ore was ground and sieved. The - 1 5 0 BSS fraction (105-/xm size) was used for all leaching experiments. Elemental analysis of this fraction was done using an atomic absorption spectrophotometer, after digestion with dilute hydrochloric acid (I: 1) according to Vogel.t161 The Chemical composition of the ore is presented in Table I. The analyses given in Table I are acid soluble components, except for SiOz, which was estimated using hydrogen fluoride treatment.
B. Microorganisms A strain of A. niger isolated earlier from a freshly collected lateritic ore sampleI6j was used for the leaching experiments. The organism was cultured in 2 pct potato-dextrose broth until sporulation. For the preparation of the culture, medium 200 g potato tubers were washed, peeled, cut finely, put into cold distilled water, and boiled for 30 minutes. The liquid extract was filtered on cotton wool with a cheese cloth. Then 20 g of dextros
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