Kinetic and mechanistic studies on the biological and chemical leaching of nickel from sulphide ores.
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Date
1974
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Abstract
The aim of this investigation was to extend the knowledge of the
bacterial leaching of copper and zinc sulphides into the area of
nickel sulphide leaching.
By far the major portion of both theoretical and practical
expertise which is available in the field of bacterial leaching
is based on the treatment of copper and zinc sulphides. As yet
there is little information available on the bacterial leaching
of nickel sulphides to meet the growing interest in this field
both in South Africa and elsewhere.
To a large degree, it was necessary to start from basic principles
in this novel extension of bacterial leaching technology so that
the work covers a fairly wide field in general rather than one
particular aspect in detail.
A strain of 'nickel adapted' Thiobacillus ferrooxidans was
isolated from the tailings dam of a disused nickel mine. The
growth characteristics of this strain were studied in some detail
on sulphur using both batch and continuous techniques. This was
done as it was considered that growth on sulphur would provide
useful information which could be correlated with the mineral
leaching results.
The mineral pentlandite (NiFeS2) was chosen as the one with which
to work because of its economic importance. This was prepared in
a highly purified form from a concentrate of the Rhodesian
Shangani deposit.
Bacterial leaching tests in both batch and continuous operation
were then carried out in order to define the effects of various
physico-chemical parameters on the leaching of nickel from this
mineral. As a preliminary to these tests, a detailed chemical
kinetic study in the absence of bacteria of the leaching of
nickel was carried out using similar physico-chemical conditions.
The results of the bacterial and chemical leaching tests were
then compared and used to postulate a mechanism and model for
the process.
It was found that the rate of leaching of nickel from pentlandite
in acid ferric sulphate solutions was directly proportional to
the concentration of ferric ions and speed of agitation of the
stirrer and to the square root of the oxygen concentration. The
form of the rate expression was interpreted in terms of a mixed
diffusive and chemical rate controlling mechanism.
Bacterial growth rates on flowers of sulphur were found to be
controlled by the rate of dissolution of oxygen from the gas
bubbles into the bulk solution. When this latter condition was
made non-rate limiting, it was found that growth rates were still
dependent on the rate of agitation, implying mass transport
control by another mechanism.
The batch bacterial leaching results showed a linear pattern of
nickel leaching and bacterial growth, with a marked dependence on
oxygen concentration and rate of agitation. A mechanism in
accordance with the batch data was postulated, which proposed
that the rate of bacterial leaching was proportional to the
concentration of bacteria attached at the mineral surface and
to the square of the oxygen concentration. The rates of
bacterial leaching were computed by taking the difference between
the overall measured leach rate and the chemical leach rate based
on the chemical kinetic data.
The leach rates in continuous bacterial leaching were higher than
those predicted from the batch data. This effect was interpreted
in terms of higher specific growth rates being achieved in
continuous operation.
An economic assessment was made of the process based on the
optimum leach rates obtained in continuous leaching and found to
show some promise.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1974.
Keywords
Leaching., Sulphide minerals., Nickel--Metallurgy., Theses--Chemical engineering.