| Summary: | Sulfate-based leaching processes for chalcopyrite (CuFeSa) are attractive because of
their inherent simplicity. Unfortunately, high copper extractions are not attainable in a
reasonable residence time unless the leaching temperature exceeds 200°C (oxygen pressure
leaching) or chalcopyrite is "activated" by a pretreatment method prior to leaching. In the
present work, the oxygen pressure leaching behaviour of chalcopyrite in the temperature range
110-220°C was studied to shed new light on the reasons for the slow leaching.
At temperatures where molten elemental sulfur is stable (120-180°C), the reaction was
found to be prematurely stifled by liquid sulfur formed during leaching. The liquid sulfur wetted
and agglomerated the unreacted mineral particles, effectively stopping the leach at 40-70%
copper extraction. The feasibility of using surfactants to disperse the sulfur and enhance the
copper extraction in the temperature range 125-155°C was investigated. Most of the surfactants
tested decomposed too rapidly to be of benefit. The best results were obtained with
orthophenylenediamine (OPD) when it was added continuously at a low temperature (125°C)
and a high dosage (50 kg/t). Under these conditions the surfactant was successful in dispersing
liquid sulfur and it effected a modest increase in copper extraction over that obtained at 110°C,
but only after prolonged retention times (6 hours). It was found that chalcopyrite leached slowly
even if molten sulfur was prevented from wetting the mineral surfaces. This led to the
conclusion that the reaction rate is ultimately controlled by a passivating mechanism unrelated to
elemental sulfur formation.
Oxidized chalcopyrite mineral surfaces were examined by Auger electron spectroscopy
and X-ray photoelectron spectroscopy to identify the presence of any passivating layers that may
form during leaching. The results indicated that an iron-deficient, copper-rich sulfide layer
forms on the surface of chalcopyrite as a result of solid state changes that occur in the mineral
during leaching. This layer is thought to be a copper polysulfide, CuS[sub]n, where n > 2. The layer
is quite thin (-10-100 nm) and hence is not detectable by conventional electron microscopy. The
copper polysulfide is believed to passivate the chalcopyrite.
The addition of silver ions at a temperature below the sulfur melting point (110°C) was
found to catalyze the leach, resulting in the copper extraction increasing from 53% to 96% after 3 hours leaching. Silver alone was ineffective at 125°C and 155°C because the leach was stifled
by liquid sulfur. However, when silver was used in conjunction with OPD at 125°C the copper
extraction was again enhanced significantly, to 91%. The OPD prevented the sulfur from
wetting the mineral surfaces, allowing the catalytic effect of the silver ions to be maintained.
A simple mathematical model was formulated to explain the leaching and passivation of
chalcopyrite at low temperature (110°C). The reaction kinetics can be explained in terms of a
mixed diffusion/chemical reaction model where the reaction rate is initially dependent on the
rate at which copper and iron diffuse through the thickening passive layer. The passive layer
also leaches, but at a slower rate than the chalcopyrite. Eventually, the passive layer reaches a
steady state thickness and the kinetics are then controlled by the rate at which the passivating
layer decomposes. The linear rate constant k\ for the decomposition of the passive layer was
calculated to be 5.0x10⁻³ um/min. The low value of k\ explains why inordinately long residence
times are required to obtain a high copper extraction from chalcopyrite at 110°C.
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