ISSN 0012�5008, Doklady Chemistry, 2011, Vol. 436, Part 1, pp. 15–18. © Pleiades Publishing, Ltd., 2011.Original Russian Text © M.A. Medkov, A.I. Khanchub, V.P. Molchanov, D.G. Epov, G.F. Krysenko, L.P. Plyusnina, 2011, published in Doklady Akademii Nauk, 2011, Vol. 436,No. 2, pp. 210–213.

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Complex noble�metal deposits in carbon�bearingrocks have recently been discovered, which are nowconsidered as a new promising source of gold, plati�num, and other useful elements [1]. The main diffi�culty in developing methods for analysis and extrac�tion of noble metals from this type of the raw materialis the presence of carbon matter. Carbonaceous rocksare difficult to analyze because they contain a widevariety of noble�metal compounds: from organome�tallic compounds to graphitized substances, in whichmetal atoms are located between graphite layers andbound directly to carbon atoms [2–4]. The occurrenceof noble metals as cluster carbon–metal compoundscomplicates the analysis and extraction of metals. Theproblem of analysis of noble metals in carbonaceousrocks has been actively discussed in the literature [3,5]. Gold�containing graphite ores, in which metalsform the strongest bonds with carbon matter, are mostdifficult to analyze. Because of the specificity of its lay�ered hexagonal structure, graphite has a unique prop�erty to form gold�in�graphite interstitial compounds.To rupture these bonds, long�term thermal oxidationis necessary. A consequence of it is metal loss duringacid leaching.

For the prompt development of graphite depositsdiscovered in the Far East and the Primorskii andKhabarovsk krais [6, 7], it is necessary to perform inte�grated studies aimed at creating an environmentallyclean low�waste technology for processing graphite�bearing rocks for maximum possible extraction of use�ful components, first of all, graphite, gold, and plati�num metals.

In this context, the purpose of this work was todevelop energy�efficient and environmentally safemethods for integrated processing of graphite�con�taining ores by the example of a certain deposit. Forour study, we chose the Turgenevskoe deposit of theTamgino�Turgenevskaya group, Primorskii krai, Rus�sia. This deposit is one of the Russia’s largest graphitedeposits and simultaneously holds significant reservesof gold and platinum metals [7]. In this work, we forthe first time investigated the possibility of gold extrac�tion from graphite ore and also specific features of thegold distribution in preliminary ore dressing and atvarious stages of chemical treatment of graphite�bear�ing rocks.

Noble�Metal Analysis Method

The main difficulty in reliable determination ofgold and platinum metals in graphite�bearing rocksis that carbon matter in such rocks is graphite,which complicates their decomposition. Therefore,for determining gold content, we proposed amethod of instrumental neutron activation analysisusing a californium neutron source, the sensitivityof which is determined only by nuclear�physicsproperties of elements.

In the experiment, we detected that the reliabilityof the analysis is dependent on the sample weight andthe gold concentration in it. The determination of theoptimal sample weight (300 g) eliminates the questionof sample representativeness. It was proved that onlyhigh (more than 200 g/t) concentrations of fine goldhave a noticeable effect on the analysis error. Thiseffect is related to self�shielding: a part of the neutronflux is absorbed by surface layers, because of whichinner layers are less activated. In analyzing sampleswith dispersed�gold concentration below 200 g/t, theself�shielding is insignificant.

However, there are other factors that may influ�ence the gold determination error. The ores

CHEMICALTECHNOLOGY

The Development of a Method for Extracting Noble Metalsfrom High�Carbon Raw Material

M. A. Medkova, Academician A. I. Khanchukb, V. P. Molchanovb, D. G. Epova, G. F. Krysenkoa, and L. P. Plyusninab

Received July 20, 2010

DOI: 10.1134/S0012500811010046

a Institute of Chemistry, Far East Division, Russian Academy of Sciences, pr. Stoletiya Vladivostoka 159, Vladivostok, 690022 Russia

b Far East Geological Institute, Far East Division, Russian Academy of Sciences, pr. Stoletiya Vladivostoka 159, Vladivostok, 690022 Russia

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MEDKOV et al.

studied contain much carbon, which is an efficientneutron moderator because of the large scatteringcross�section. Therefore, theoretically, carbon inrock can affect the neutron activation analysisresults. To test this assumption, we determined goldfrom samples weighing up to 300 g with varying car�bon concentration by neutron activation analysis.The experiment detected no influence of the carbonconcentration on the neutron activation analysisresults. These observations were corroborated byexperimental data on changes in neutron fluxes forsilicate and graphite samples at a total weight ofloaded rock of ~3 kg. No effect of carbon on theneutron fluxes in this case was detected. Thus, neu�tron activation analysis is a reliable method forinvestigation of the gold distribution in high�carbonrocks.

Extraction of Useful Components

The object of investigation was chosen to be a largesample of graphitized granite gneisses from the Tur�genevskoe deposit. Neutron activation analysisdetected low gold contents (at the instrument sensitiv�ity limit), which required then preliminary flotationdressing of samples.

The frother was pine oil, the main components ofwhich are terpene alcohols, and the collector was along�chain amine. Much (~90%) of the graphite and apart of the quartz under the flotation condition wereconcentrated in the froth product. According to neu�tron activation analysis, the froth product containedno gold and the gold concentration in the flotationtailings was 2 g/t. The table presents the results ofX�ray fluorescence analysis of the graphite producedby flotation. Thus, by flotation, one can obtain GOST(State Standard) 8295�73 graphite (graphite for pro�

ducing lubricants, grade P electrically conductive rub�ber coatings, etc.).

The investigation showed that the flotation tailingswas dominated by microscopic (70–100 µm) platelikeand spheroidal gold particles, which are, in manycases, associated with silicates. The fact that graphite�bearing rocks of the Turgenevskoe deposit contain notonly gold but also other native metals (aluminum,iron, copper, etc.) is indicative of low sulfur activity atthe first stage of mineral formation and the related lowdevelopment of sulfidization processes within the Tur�genevskoe deposit [7].

To extract and additionally concentrate gold, theflotation tailings were fluorinated by adding ammo�nium hydrodifluoride and the gold distributionbetween the phases at various treatment stages wasstudied. Such hydrodifluoride treatment is an uncon�ventional approach to solving the problem of goldextraction because elemental fluorine and other fluor�inating agents, e.g., BF3 and KBF4, are so active reac�tants that they react with gold to form anionic fluorocomplexes. However, using ammonium hydrodifluo�ride NH4HF2, one can find a suitable variant of treat�ing raw material where only gold�holding minerals aredestroyed and gold is concentrated from gold�con�taining ore [8].

The workflow of ore treatment by ammoniumhydrodifluoride is based on physicochemical proper�ties of ammonium fluorometallates [9, 10], whichform in the initial ore treatment and then are separatedbecause of different volatilities or solubilities. Goldand other noble metals do not react with ammoniumhydrodifluoride. Moreover, for ammonium hydrodif�luoride, there are simple regeneration schemes, inwhich the theoretical regeneration yield is 100%.

According to X�ray powder diffraction, the flota�tion tailings mostly consisted of two minerals, quartzSiO2 and anorthite CaAl2Si2O8 . Natural aluminosili�cates containing neither hydroxyl nor crystallizationwater are generally high�melting, thermally stable,and water�insoluble. They are always decomposed byhydrofluoric acid, and the result of their reactionswith ammonium hydrodifluoride depends on theirstructure. However, layered, band, chain, and islandsilicates and hydromicas, which have complex chem�ical composition, react with NH4HF2 even duringmixing.

Thus, the treatment with ammonium hydrodifluo�ride for fluorinating the major components of the flo�tation tailings produced from graphite�bearing rock ofthe Turgenevskoe deposit can be represented by thereactions

Composition of dried froth product

Element Concentra�tion, wt % Element Concentra�

tion, wt %

C 97.17 Cr 37.05 × 10–6

Na 0.026 Mn 0.002

Mg 1 × 10–6 Fe 0.101

Al 0.116 Rb 20.6 × 10–6

S 0.081 Sr 0.007

K 0.224 Ti 0.009

Ca 0.362 Si 1.894

DOKLADY CHEMISTRY Vol. 436 Part 1 2011

THE DEVELOPMENT OF A METHOD FOR EXTRACTING NOBLE METALS 17

SiO2 + 3NH4HF2 (NH4)2SiF6 + 2H2O + NH3↑,

CaAl2Si2O8 + 13NH4HF2 = 2(NH4)3AlF6 + 2(NH4)2SiF6 + CaF2 + 3NH3↑ + 8H2O.

Ammonium hexafluorosilicate (NH4)2SiF6 is ahigh�volatile compound, passes on heating into thegas phase at 300–350°C, and condenses on cooling at200°C. Ammonium hexafluoroaluminate (NH4)3AlF6,which forms in the fluorination of the silicate compo�nent of ore, is water�soluble. On heating, it undergoesthermal decomposition to release ammonia andhydrogen fluoride into the gas phase. Entering a con�denser, the two substances combine to give NH4F.High temperatures of evaporation of fluorides of alu�minum, iron, and calcium create conditions for sharpseparation of volatile (silicon) and nonvolatile (alumi�num, calcium, admixture of iron, etc.) fluorides,which form a sludge that concentrate gold and othernoble metals.

Ammonium hexafluoroaluminate (NH4)3AlF6 isan intermediate product in aluminum fluoride pro�duction, and ammonium hexafluorosilicate(NH4)2SiF6 is widely used in production of high�purity amorphous silica. This will favor the maximumpossible extraction of useful components from graph�ite�bearing ore of the Turgenevskoe deposit and thecreation of a low�waste technology for processinggraphite�bearing gold�containing ore by ammoniumhydrodifluoride.

The flotation tailings were mixed with NH4HF2 inthe 1 : 2.3 weight ratio and were heated to 450°C. Thisprocess involved the fluorination of the main compo�nents of ore to form fluoroammonium salts of variouselements, thermal decomposition of fluoroammo�nium salts of aluminum and iron, and transition ofammonium fluoride and ammonium hexafluorosili�cate into the gas phase. As a result of this treatment,gold was concentrated in a nonvolatile residue, theweight of which constituted about 50% of the initialsample weight.

Thus, the “dry” initial treatment of graphite�bearing gold�containing ore by ammonium hydrodi�fluoride doubles the concentration of the extractedmetal. A higher gold concentration can be reached bycombining the dry initial treatment and the hydro�metallurgical processing of the product. Such anapproach allows one to convert aluminum and ironfluorides into soluble ammonium fluorometallates tobe removed by washing. In this case, all gold aftersample dissolution is concentrated in the insolubleresidue.

According to X�ray powder diffraction, the nonvol�atile residue obtained at 450 C contained slightly solu�ble aluminum and iron fluorides NH4AlF4, AlF3, and

FeF3. To convert them into a soluble form, they wererefluorinated by ammonium hydrodifluoride at200°C, after which the forming fluoroammonium saltsof aluminum and iron were leached with water. As aresult of this treatment, gold was concentrated in aninsoluble residue, a yellow viscous product. Its weightwas ~12% of the initial sample weight. The gold con�centration in the insoluble residue reached 21 g/t.Treatment of the produced insoluble residue byhydrofluoric acid gave a gray granular product, inwhich individual native gold particles 200–450 µm insize were seen.

Thus, for the first time, we have demonstrated thepossibility to extract useful components from graphiteore and determined specific features of the gold distri�bution in the preliminary ore dressing and variousstages of chemical treatment of graphite�bearingrocks. Using hydrodifluoride treatment in the initialtreatment of gold�containing graphite�bearing oresallows one not only to concentrate gold but also toextract accompanying useful components. This willfavor the maximum possible extraction of useful com�ponents and the creation of a resource�saving technol�ogy for processing this type of solid mineral resource.The developed methods for gold extraction open theway to create a resource�saving technology for utiliz�ing high�carbon technogenic raw material.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (project nos. 09–05–12060and 09–05–98545) and the Presidium of the Far EastDivision of the Russian Academy of Sciences (grantnos. 09�III�A�08�446 and 15�IN�09).

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