Candidate of
technical sciences, senior researcher, associate professor
Oryngojin Ye.S.
ELECTRODE POTENTIALS OF SULFIDE
MINERALS AND MINEARLS BEARING GOLD
Sulfides
are salts of hydrosulphuric acid and they are formed in nature in the course of
binding of metals with sulfur. Many sulfides are resulted at the impact of
hydrogen sulfide on the metal salts solved in water.
Gold is
close associated with sulfide minerals. Various sulfides are to different
extent resistant to oxidizing processes. Thus, Veinig and Carpenter [1] put
sulfides in the following line in descending sensitivity to spontaneous
oxidation: FeAsS > FeS2 > CuFeS2 > ZnS > PbS
> Cu2S, tetrahedrite. They showed that oxidation takes the
highest speed at the beginning, that sulfides of old geological formations are
more stable than that of later ones and that sulfide mixtures are oxidized
quicker than individual minerals. It is found out that in pH interval from 3 to
12, sulfides retain stability, and pure pyrite is the most difficult to
oxidize [2].
The most easily decomposed
sulfides are: pyrrhotite, sphalerite and chalcocite; and the most poorly
decomposed are: pyrite, argentite, enargite, halenit; the rest of sulfides are
intermediate.
Polymineral sulfide ores (gold-containing) represent
complex electrode, consisting of individual mineral electrodes with relatively
positive and negative values of electrode potential. The potential of minerals
included in the content of polymineral ore significantly differs from the
stationary potential of minerals represented individually. At that, the
potential of the system becomes dependent upon the correlation of surface areas
of all composing minerals, their specific resistance, the action of
potential-determining and poling factors and depolarization phenomena.
Material content of gold-containing ores is one of the
main factors affecting the gold extraction factors. For instance, the
gold-containing sulfide ores of Vasilkov field (JSC “Kazakhaltyn”, the Republic
of Kazakhstan) are represented mainly by silicate minerals: albite (Na[AlSi3O8]),
microcline (Ê[AlSi3O8]), quartz (SiO2), kaolinite (Àl(OH)8[Si4O10]),
biotite (Ê(ÌgFe)3(OH,F)2[AlSi3O10]),
hornblende (Na3 Fe3 Fe2 Si2 O23 (OH)). Out of metallic
mineral there are Èç ðóäíûõ ìèíåðàëîâ ïðèñóòñòâóþò arsenopyrite (FeAsS), pyrite (FeS2), chalcopyrite (Ñu FeS2),
haematite (Fe2O3), bismuthine (Bi2S3),
native bismuth (Bi), which are evenly distributed in quartz veins. Their gold
content is 1,5-4 g/t.
The ores of the field Bestobe (JSC “Kazakhaltyn”, the
Republic of Kazakhstan) belong to gold-quartz and gold-sulfide-quartz
formation. The main metallic minerals are pyrite, arsenopyrite, antimonite; the secondary ones are halenite, scheelite,
sphalerite. The prevalent part of gold and silver is represented in the form of
native alloy electrum.
Crystal optics and
MRS are analyses that confirm the availability of gold and silver in arsenopyrite ore at the field Bestobe in
the form of native alloy electrum, containing up to 91% of gold and 10 % of
silver. Fineness of electrum according to the gold mass is 885 parts per
thousand, the rest 115 parts fall mainly at silver. Other chalcophile elements are copper, iron, tellurium;
selenium is present in hundredth fractions of a percent.
There
is small amount of silver and gold in copper- ferriferous alloy, but this form
is rarely found in the ore taken for electrochemical gold extraction and
therefore it cannot be viewed as the source of noble metals for electrochemical
processes.
Native
copper found in the ore is the bearer of gold and silver.
The
main part of noble metals in the ore is found in the native alloy of gold and
silver – electrum, the gold fineness of which is 885 parts per a thousand and
115 parts – for silver. Extractions of electrum are sized from the first
microns to 160-180 microns. The wide range of alloy dispersion can cause
complications in the course of the analytical control of solutions with regard
to the content of gold and silver as well as to affect the extent of their
extraction in the course of ore processing with the method of electrochemical
extraction with the use of sodium salt. In the process of electrochemical gold
leaching from the ore pulp, 20-30 mg/l of gold may be transferred to the
solution. Gold extraction (80-87 %) in the course of electrochemical process
will ensure the reduction of deficit and expensive solvents consumption by
several times – cyanide or thiourea, used in traditional processing methods of
such ore.
Availability
of gold in arsenopyrite ore at the
field Bestobe is determined with the presence of electrum alloy with other
sulfide minerals.
After
the study of the minerals composing the ore, it is possible to state that for
the most part the pyrite does not contain gold; chalcopyrite, arsenopyrite
and pyrrhotite contain gold but in small quantity. The same can be said of the
silver content in these minerals. The major gold content is in electrum (up to
91%). This fact should be considered at its electrochemical extraction. Binnite
- antimonous copper sulfur salt - contains up to 0,8 % of silver and it can be
referred to the silver collector.
As we
can see from the above-mentioned example, the gold-containing ores of any field
differ with variety of mineral forms and nature of gold mineralization. The
availability of fine ingrained mineralization of gold in ore-forming sulfide
ores significantly increases the obstinacy of gold extraction. As a rule, chalcophile property of gold is rarely
evident in sulfide ores. Chemical affinity with sulfur results in gold
concentration in crystal lattice of sulfides. The examples of domination is
arsenic and antimony. Fine ingrained gold is effectively released at the
oxidizing roasting of minerals. Fine impregnation with pronounced chalcophile property, with the
availability of sulfur minerals, antimony, arsenic, lead, tellurides etc. to
great extent increases the obstinacy of gold-containing raw material to
solvents. To oxidize the sulfide minerals and to extract gold it is required to
have quite strong oxidizing conditions with high oxidizing potential of the
environment.
Literature:
1.
Korostyshevskii M.B. Metallurgy of gold and silver //Achievements of science
and equipment. Metallurgy of non-ferrous metals: Tr.VINITI, 1987. v.17.
2.
Bolotova L.S., Romenenko A.G., Zaiceva V.N., Suvorova E.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.