Optimization of the flotation of copper smelter slags from Namibia Custom Smelters' slag mill plant
Date
2016-03-17
Authors
Sipunga, Elias
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Abstract
Namibia Custom Smelters treats complex copper concentrates. At the time of the research, three
furnaces were in operation namely, the Top Submerged Lance Furnace, the reverberatory furnace
and the converters. All these three furnaces produce fayalite slag as a byproduct. The slags
contain copper with the content ranging between 0.8 to 5% copper. The slags cannot be
discarded due to the high content of copper and hence they are sent to a milling and flotation
plant (Slag Mill Plant) for liberation and recovery of copper. This work focuses on optimizing
the recovery of copper minerals in copper smelter slags by means of froth flotation. Most of the
copper minerals present in the Namibia Custom Smelters’ slags are free sulphide minerals,
however a certain proportion is copper minerals that cannot be floated easily because they could
either be in oxide form or locked in or occluded in the gangue matrix. This residual copper
content continues to present a tough challenge in the copper mineral recovery process. It was
initially postulated that these ‘refractory’ copper minerals were oxide minerals and an acid
solubility test was performed after fine griding in an attempt to validate this hypothesis. It is
known that oxides tend to dissolve in dilute sulphuric acid more readily while sulphides are
refractory to acid leaching. The acid liquor obtained from the acid tests were found to contain
dissolved copper which re-enforced the school of thought that the copper minerals were oxide
minerals, however doubt still remained because literature has shown that copper sulfide also
dissolves slightly before passivation sets in.
To further investigate the nature of the refractory copper minerals in the slag, mineralogical
analysis on the slag samples were performed using X-Ray Diffraction (XRD) and Scanning
Electron Microscopy (SEM) techniques.
Elias Sipunga 2015 iii
Both these techniques did not reveal the presence of copper oxide minerals, most scans showed
that copper sulphide minerals are locked and occluded in fayalite gangue. The slags were milled
to 75 percent less than 45 microns in order to liberate the copper minerals sufficiently for the
flotation tests which were performed on slag samples from three different sources (TSL furnace,
copper converters, reverberatory furnace) to recover the copper minerals. A range of commercial
flotation reagents including xanthates, dithiophosphates, mercaptobenzothiazole,
thionocarbamates, fatty acids, sulphides and sulphates were tested in the flotation tests. The
highest recovery obtained in the first mill-float stage was about 75.8%. Flotation tailings were
milled and floated again in a Mill-Float-Mill-Float (MF2) configuration. After re-grinding and
floating again, the highest recovery (cumulative) obtained was 92.2% which represented a
significant improvement in the copper recovery. The best flotation performance was obtained
with the reagent FC 4146 which contains thionocarbcarbamate and methyl isobutyl carbinol as a
froth enhancer.
Description
A research report submitted to the Faculty of Engineering and the Built
Environment, University of the Witwatersrand, in partial fulfillment of the requirements
for the degree of Master of Science in Engineering
Johannesburg, South Africa
August, 2015