Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group ib metal
Reexamination Certificate
1999-11-24
2001-08-21
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group ib metal
C423S028000, C423S041000
Reexamination Certificate
active
06277341
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the leaching of chalcopyrite in order to recover the copper values therein and more particularly to a process in which ferric ions which are preferably, but not necessarily, bacterially generated, are employed to effect oxidation of the sulphide material in the chalcopyrite.
BACKGROUND TO THE INVENTION
The leaching of sulphide materials using ferric sulphate to oxidise the sulphide mineral usually leads to rapid oxidation and thus dissolution of the required values. The surface potential of the mineral in such a case is usually in the range of 550 to 660 mV and the rate of leaching is henerally expected to increase with increasing potential. This is the case for such sulphide minerals as pyrite (FeS
2
); arseno-pyrite (FeAsS); chalcocite (Cu
2
S); and sphalerite (ZnS).
However, in the case of chalcopyrite, leaching using ferric sulphate takes an inordinate length of time, presumably as a consequence of passivation of the exposed surfaces of the sulphide material. The exact nature of the mechanism whereby passivation occurs is not entirely clear and is presumed to be due to the formation of a passivating film on the chalcopyrite crystal surface which thus forms a diffusion barrier.
One attempt to overcome this difficulty has been the proposed use of activators or catalysts in the leaching system. One successful catalyst has been shown to be silver (see for example U.S. Pat. No. 3,856,913). However, as far as Applicant is aware, no commercial process has been developed to operate utilising such a catalyst presumably because of the high cost of silver and difficulties experienced in recovering it for recycle purposes. Other proposals have been the use of HgS; As
2
S
3
; SnS; and CoS. None of these have proved to be commercially successful as far as the Applicant can determine.
Ahonen and Tuovinen, Hydrometallurgy, Vol. 24, No. 2, March 1990, 219-236 discloses the catalytic effects of silver in the microbiological leaching of finely ground chalcopyrite-containing ore materials in shake flasks. U.S. Pat. No. 4,571,387 relates to leaching copper from copper sulphide containing ore using sulphide oxidizing bacteria and a catalytic amount of silver.
It is, accordingly, the object of this invention to provide an effective process for leaching chalcopyrite without the use of any costly or deleterious activators or catalysts.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a process for leaching chalcopyrite in which ferric sulphate is used to oxidise the sulphide material, the process being characterised in that the process conditions are actively controlled to cause the surface potenfial of the chalcopyrite to be maintained within the range of 350-450 mV measured against a standard Calomel reference electrode for at least a significant time period during which leaching is carried out.
Further features of the invention provide for the process conditions to be actively controlled by controlling at least the ferric ion (Fe
3+
) to ferrous ion (Fe
2+
) ratio with such control being carried out by controlling the oxygen supply to the ferrous ion oxidation process, and for the oxidation of the ferrous ions to ferric ions to be carried out using a bacterial oxidation process, preferably conducted in a separate reaction vessel in which case the residence time of circulating leach solution in the bacterial oxidation vessel may be actively controlled.
Additional features of the invention provide for the chalcopyrite to be fine milled prior to leaching in order to increase the available surface area thereof and promote crystallographic dislocations; for the oxidation of elemental sulphur products to be prevented during the leaching process; and for leaching to be carried out at elevated temperature.
Still further features of the invention provide for the leach solution to be circulated from the leach process to a copper removal step such as a solvent extraction or an electro-winning process in which dissolved copper is removed to some extent prior to circulating the leach solution to a bacterial ferric ion regeneration step preparatory to the leach solution being returned to the leach process in a continuous circuit.
In the implementation of this invention, it is important that the surface potential of the chalcopyrite be maintained in a narrow band within that specified. In this regard it is to be noted that the equilibrium potential measured using a platinum electrode in circuit with a Calomel electrode the slurry redox potential may be found to represent a mixed potential consequent on the contact between the “corroding” sulphide particles and the platinum electrode. Factors such as the presence of more electro-chemically active minerals such as chalcocite with the chalcopyrite, or the contact of less electro-chemically active minerals such as pyrite with the chalcopyrite (galvanic effects), will accentuate the difference between the true surface potential and the slurry potential measured using a platinum electrode. Ideally, the surface potential of the chalcopyrite should thus be measured directly using a chalcopyrite crystal electrode or composite electrode made from chalcopyrite powder or concentrate. Moreover, due to the slow kinetics associated with chalcopyrite leaching the difference between the slurry redox potential and the chalcopyrite surface potential will in all probability be small (less than 20 mV).
Impurities present in the chaloopyrite are also thought to play a catalytic role in promoting chalcopyrite instability and consequent solubility. This effect, plus the effectiveness of fine milling on promoting instability, is sample specific and is dependant on the origin of the sample. This will result therefore from differing geophysical conditions under which the chalcopyrite was formed and the different levels of impurities embodied therein.
The most cost effective process to be used on any particular ore or concentrate will be determined empirically with the four basic variables being:
(i) the surface potential of the chalcopyrite;
(ii) the temperature at which the leach is effected;
(ii) the pH at which leaching is effected; and
(iv) the fineness to which the ore or concentrate is ground.
In regard to the above it has been found that the surface potential range or “window” varies for different chalcopyrites, and apparently depends on mineralogical characteristics related to crystal structure, crystal impurities and associated minerals and impurities present in the ore being treated. Similarly, the most cost effective temperature, pH, and fineness of ground will vary.
The application of the process of this invention can be expanded to include not only processes treating chalcopyrite-containing concentrates by stirred tank leaching methods, but also processes for treating chalcopyrite-containing ores by simple heap percolation leaching methods.
Control of the surface potential or more generally, in a leach process, the slurry redox potential, can be achieved in any convenient manner as will be apparent to those skilled in the art such as, for example, by controlling the supply of O
2
to the oxidation process and/or by varying the residence time in a separate ferric ion generation reaction.
In order that the invention may be more fully understood, various tests which have been carried out will now be described with reference to the accompanying drawings.
REFERENCES:
patent: 2829964 (1958-04-01), Zimmerley et al.
patent: 3305353 (1967-02-01), Duncan et al.
patent: 3856913 (1974-12-01), McElroy et al.
patent: 3962402 (1976-06-01), Touro
patent: 4115221 (1978-09-01), Wadsworth et al.
patent: 4256553 (1981-03-01), Baczek et al.
patent: 4571387 (1986-02-01), Bruynesteyn et al.
patent: 1467-94 (1998-03-01), None
patent: 1674-96 (1999-02-01), None
patent: 1 528 061 (1978-10-01), None
Mateos, et al., “The passivation of chalcopyrite subjected to ferric sulfae leaching and its reactivation with metal sulfides,” Hydrometallurgy, 19, 1987, no month, pp. 159-167.*
Ahonen, et al., “Redox potential con
Myburgh Peter J.
Pinches Anthony
van der Merwe Charmaine
Bos Steven
Christensen O'Connor Johnson & Kindness PLLC
Mintek
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