Candidate of technical
sciences, senior researcher, associate professor Oryngojin Ye.S., magistrate
student Oryngojina S.Ye.
Karaganda
State Technical University (Karaganda)
ON THE ISSUE OF GOLD
EXTRACTION FORM PERSISTENT SULFIDE ORES
The
Republic of Kazakhstan is one of the most important sources of mineral resource
of Central Asia.
In
Kazakhstan there are significant reserves of non-ferrous, rare, radioactive
metals and gold. On the territory of Kazakhstan there are more than 2 thousand
field and gold appearances, including nearly 200 surveyed, tens of developed
and operative ones. The regions of developed gold mining are
Stepnyak-Bestobinskiy, Maikainskiy, Rudnoaltaiskiy, Kalbinskiy, Mugodjarskiy,
Yujno-Djungarskiy, Shu-Iliiskiy, Kokshetauskiy regions.
Gold fields of Kazakhstan have been formed in various geotectonic conditions with the formation of wide diversity of gold-containing formations (gold-cobalt-copper, gold-molybdenum-copper, gold-tellurium-bismuth, gold-silver, gold-arsenic, kolchadan) and wide associativity of gold with sulfides: pyrite, arsenopyrite, sulfide copper etc. 21 gold fields with the total reserves of nearly 1,2 ths.t at the average content of 4,76 g/t are under the development [1].
The
processing of the mined ores is connected with the application of complex
technological processes of concentration and metallurgy characterized with high
material costs, metal waste due to the cycles of technological limit,
environmental pollution with highly toxic agents and production waste. To
destroy sulfides and to release gold, highly oxidizing conditions are needed,
in particular oxidizing roasting with subsequent cyanidation of pyrite cinders.
Simple ore cyanidation does not result in high values of gold extraction. The
oxidizing roasting requires the application of highly expensive equipment and
complex gas purifying facilities to trap gold and arsenic from gas phase.
Toughening
of environmental safety requirements in the production of gold of arsenous ores
and the absence of effective, environmentally clean technology of their
processing hinders the capacity upbuilding at one of the largest field of
Kazakhstan – Bakyrchikskoye.
To
extract gold from pyrite and arsenopyrite ores with the cyanides of alkali
metals, preliminary preparation of raw material is needed. The technology of
preparation of such ores for cyanidation based on the oxidation with nitric
acid results in the escape of large amount of sulfur which significantly
reduces the gold extraction into the solution in the course of cyanidation.
The
technology of pressure oxidation of sulfide ores is related to high capital and
operational costs. Biometallurgy is overly sensitive to temperature, light conditions
and bacteria dieting. In the conditions of industrial production the required
conditions are difficult to provide. Therefore in recent years the world gold
mining industry has been actively searching for new technological solutions for
sulfide ore processing.
At
present time there are still no quite manufacturable, low cost, environmentally
clean and effective methods of processing of sulfide gold-containing products.
Destruction of sulfides and release of gold are possible only at significant
pressure oxidation or at oxidizing roasting with the subsequent
hydrometallurgical rerolling of pyrite cinders. For the oxidizing roasting,
expensive kilns and large amount of gas purifying facilities are needed. In the
most productive kilns with boiling bed, the flue dust escape, containing gold
in gas phase, amounts to nearly 30-35%. Besides, the gas purification from the
arsenic containing in it is very complicated.
Simple
cyanidation without sulfides oxidation will not result in high values of gold
extraction. Cyanide technology requires the application of highly toxic agent,
results in large accumulation of cyanide-containing water, sharp increase of
costs for nature protection activities and at that does not always ensure the
completeness of gold extraction from sulfides. The replacement of toxic agent
(sodium cyanide) with other, non-toxic one, and the expansion of the pH range
of environment would ensure safer labor conditions, increase of intensity and
completeness of gold extraction and environmental cleanness of production
technology of this metal. Therefore to develop the gold mining works in the
Republic of Kazakhstan, the vital remains the issue of the search for
relatively inexpensive, environmentally clean and special methods of sulfide
ore processing, which are effective with regard to gold extraction.
The technologies for processing of
persistent sulfide ores applied in Kazakhstan are characterized for high
material and labor costs, are multi-operational and are not environmentally
safe.
The review of material and resource base of gold mining industry of
Kazakhstan revealed that the main typomorphic elements of the majority of gold
fields of Kazakhstan are the ones associating with gold Ag, As, Sb, Pb, Zn, Cu,
Bi, Te, Mo, Sn, Co, Hg and other elements. Association of gold with sulfides,
availability of tellurides, gold chalcophile property, connection with arsenic, antimony, lead, bismuth and other
elements hinder gold extraction. Dense films of argentine, arsenopyrite,
galenite, ferric oxide etc., which are difficult to destroy, commonly found on
the minerals hinder gold extraction. At this the method of gold extraction is
connected with the form of its situation and concentration in ores. All
existing methods of production of noble metals from persistent ores are based
on sulfide decomposition. According to the decreasing sensitivity to
self-oxidation sulfide can be placed in the following order: FeAsS>FeS2>CuFeS2>ZnS>PbS>Cu2S.
Sulfide
oxidation takes the highest speed at the beginning, the sulfides of old
geological formations are more stable than that of later ones and that sulfide
mixtures are oxidized quicker than individual minerals. Arsenopyrite and pyrite
are chemically stable compounds and resist direct leaching with acid and alcali
solutions; their decomposition becomes possible only in the presence of
oxidizer able to replace their potentials toward positive value. As it was
described before, to oxidize the sulfide minerals the oxidizing roasting
(pyrometallurgy) with subsequent hydrometallurgical rerolling of pyrite cinders
is applied. The roasting requires expensive kelns and large amount of gas
purifying facilities. Cyanide technology requires the application of highly
toxic agent, harsh conditions with regard to ðÍ(Î-Ç) and Eh(≥+1000 ìÂ), leads
to large-scale accumulation of cyanide-containing ores, sharp increase of costs
for nature protection activities and at that does not ensure the needed
completeness of gold extraction.
The
increase of cyanidation efficiency is possible both by means of improvement of
the cyanidation process itself and by means of preliminary extraction of large
particle of gold from ores during the grinding circuit. The most successful
method of processing of gravity concentrates is their leaching with
concentrated solutions of cyanide with the use of pure oxygen as an oxidizer.
At the
cyanide leaching of preliminary oxidized sulfide ores, the process of gold
solution is expressed with the equation:
4Àu + 8NaCN + 2H2O + O2 =
4Na[Au(CN)2] + 4NaOH.
As a
result of side reactions, the part of active groups of sorbent is occupied with
anions of admixtures that reduces the sorbent capacity with regard to noble
metals. From the solution of complex cyanide Na[Au(CN)2] the gold is
effectively extracted with the use of resins, for instance Amberlit IRA – 400.
For
selective gold extraction in this case sorption should be carried out in the
presence of admixture of organic solvents with mineral acids (for instance
acetone and nitric acid).
Semi-functional
anionites are also effective for gold sorption, since they selectively sorb
gold-cyanide complex from cyanic solutions and pulps of complex composition.
Electrolytic sorption and electric desorption can be used quite effectively. In
the course of work with gold-containing ore pulps, the increase of efficiency
of ore pulp cyanidation can be expected in case of implementation of
countercurrent leaching; this would ensure the increase of extraction and
content of metals in liquid phase.
Environmental
safety of gold production can be improved by replacing tixic reagent-cyanide
with other, non-toxic reagents and by expanding the pH range of solutions.
To some extent this direction
can result in the increase of gold extraction.
The
search for cyanides replacement should be done in the presence of complexing
agents with respect to gold, the constants of which are stable and standard
oxidizing and reduction potentials of the system are compatible with the
analogous values for cyanide-ions.
1. Halogenide-ions in the same way
as ligands and conjugated compounds as oxidizers. So well
known are: family of bromic reagents, chlorination with gaseous chlorine in
hydrochlorid-acid environment, preliminary treatment with dry chlorine, sodium
hypochlorite in the presence of ligand non-halogenide,
treatment with chlorine-containing solution with oxidizer non-halogenide (CuCI2, FeCI3), treatment with
pulp hypochlorite in the presence of
excessive sodium chloride, acidation of chlorine-containing
solution with hydrochloric and nitric acids, fluorination.
2.
Thiourea in the presence of various oxidizers (air, oxygen Ñè (II) - ion, Fe(III) - ion.
3. Thiosulfate.
4.
Thiocyanate with oxidizers.
The
increase of efficiency of application of sorption without filter technology of
gold extraction from pulp-free ores should be done in the following directions:
improvement of quality of ores and resins, improvement of countercurrent
corption, introduction of sludge and associated arresters, application of
electroacoustic pulp treatment etc.
The
study of the issue of gold extraction efficiency increase from sulfide ores
revealed that basically the assigned task can be solved with cyanideless method
with the use of the cheapest and the most available reagent – sodium salt
(NaCl).
Electrochemical
technology of gold leaching from sulfide ores can become the alternative for
the existing cyanic technology.
Literature:
1.
Bolotova L.S., Romenenko A.G., Zaiceva V.N., Suvorova Ye.N. The latest in the
technology of gold extraction “resin in pulp”. // News of Kazakhstan science.
Scientific and technical collection. Integrated use of mineral resources of
Kazakhstan. Almaty, 1997.