Tuesday, 25 November 2008

Test Results and Development of a Technology of Gold-copper ore Cyanidation

Olga D. Khmelnitskaya, Vasily M. Lodeischikov, Zinaida A. Marinyuk,

Grigory I. Voiloshnikov

IRGIREDMET, Russia

Email: gold@irgiredmet.ru, labl5@irgiredmet.ru



ABSTRACT


The paper presents the results of research on the development of gold recovery technology from oxidized gold-copper ore of one of deposits in Russian Federation. Gold and copper grades in the ore are 9.5 g/t and 0.4%, respectively. Various options of flowsheets, such as direct cyanidation, thiourea leaching, flotation and gravity concentration followed by subsequent cyanidation of concentration products, were tested. It was established that gravity-cyanidation technology with filtration leaching tails and the use of solutions in the recirculation is the optimal option for ore treatment.



The proposed flowsheet provides gold recovery for 91.0% at NaCN consumption 2.5 kg/t according to the results of pilot testing.



Ammonia-cyanide gold leaching was also examined. In comparison with cyanidation, gold leaching solution in system NaCN-NH4OH-(NH4)2CO3 allows NaCN consumption to be reduced three times from 4.5 kg to 1.5 kg per 1 ton of the ore and provides the same gold recovery. Based on the laboratory and pilot plant tests results of gravity-cyanidation technology of gold dissolution, process design criteria were developed.



Keywords: Pyrite cinders, Gold, Silver, Copper, Cementation, Recovery Zinc, Water washing, Extraction, Cyanidation,









INTRODUCTION


Gold-copper ores are considered to be the ores, where gold represents the main value and copper is by-product. Such ores are processed at gold-mining plants using cyanidation process completed if necessary by gravity and flotation operations (Mitrofanov, 1970; Chamberlin, 1996). If the ores contain refractory gold associated with iron sulfides (pyrite, arsenopyrite), the processes of oxidative roasting, autoclave (POX) or biochemical oxidation (BIOX) are used to liberate gold (Chamberlin, 1996). A number of copper minerals including chalcopyrite CuFeS2 and chrysocolla CuSiO3.nH2O actively react with alkaline cyanides binding ions CN into copper complexes. The result of these reactions is increased cyanide consumption as well as problems connected with tails detoxification, for which very strict Russian environmental regulations (comparable to CN- and CNS-). Thus, copper in gold ores and ore concentrates subjected to cyanidation plays a role of accompanying valuable and detrimental impurity complicating gold recovery. One of the ways of solving the problem of copper in gold ores is to remove it from the ore prior to cyanidation by flotation methods or selective leaching by H2SO4 solutions. However, there is a category of so-called mixed, i.e. semi-oxidised, copper-gold ores. The outlined processes of gold and copper flotation and hydrometallurgical selection cannot be used for these ores due to technological and economic considerations. The ore under investigation provides an example.




THE CHARACTERISTIC OF THE OBJECT


Gold-copper ore of one of deposits in Russian Federation was tested. Table 1 shows

chemical ore composition. Table 2 shows mineral ore composition.

Table 1 Chemical compositon of the initial ore





Table 2 Mineral composition of the initial ore



It was found that silicium dioxide, magnesium, calcium, iron and copper oxides are the main ore components. Copper content in the ore is 0.4%, mainly, in oxidized form. Copper carbonates, such as malachite, azurite, present mainly of copper minerals. Mineral ore composition is represented by host minerals by 93%, of which the main ones are pyroxene (34%), serpentine (25%), carbonates (11%) and chlorites (12%). Clay mineral, montmorillonite, presents in the ore in the amount of 2.3%. Of ore minerals, magnetite constitutes the main part in the amount of 6.5%. Sulfides are not more than 0.3%. They present as chalcopyrite, pyrite, bornite ant etc. Iron and manganese oxides (0.4%) are observed in small amount. Degree of oxidation of the given sample is 52% that features it as the ore of mixed type.



Gold in the ore presents as native and is associated with magnetite and sulfides. Size of gold particles varies from 0.005 mm to 0.15 mm. Gold chemical composition is heterogeneous. Its fineness is within 778—904 parts per thousand.




THE RESULTS OF LABORATORY TESTS


Various options of flowsheets, such as direct ore cyanidation, thiourea leaching, gravity and flotation beneficiation followed by subsequent cyanidation of concentration products were tested.



It has been found experimentally that direct ore cyanidation of optimal fineness of grind (85% 90% passing minus 0.074 mm) in the following regime (concentration NaCN 1 .01 .3 gIL, duration of leaching 32 h, pH = 10.511.0) provides gold recovery at a level of 92%93%. However, cyanide consumption is more that 4 kg per 1 ton of the ore and copper concentration in leach solution reached 1.0-1.3 gIL that makes this process technically and economically rather difficult.



Copper removal from the ore by flotation prior to cyanidation was not effective due to bad floatability of oxidized copper minerals (malachite, azurite). Preliminary copper leaching by sulphuric acid was also unacceptable due to high acid consumption (180 kg per ton of ore) as a result of the reaction with carbonate minerals. For the same reasons, other copper and gold (non-cyanide) lixiviants, in particular thiourea, acting under acidic conditions, were not used.



Gravity concentration was tested as 59.1% of free gold was determined by diagnostic leaching of ore sample.

According to the results of the experiments up to 40% of the gold is recovered into a rich gravity concentrate with its subsequent smelting to metal Dore.

Gravity tailings combined with the middlings of gravity concentrate recleaning were treated by cyanidation. Gold grade was 5.1 g/t, copper was 0.4%, fineness of grind was 90% 95% passing minus 0.074 mm in the mentioned product. Conditions of ore cyanidation, such as concentration 1.OgNaCN/L, ratio L:S 1.5:1, pH = 10.5-41.0, were taken as a basis when conducting tests. Leach time was varied from 8 to 24 hours. According to obtained results it is necessary to conduct leach process within 24 hours. Gravity gold recovery was 91.8% - 93.4% at NaCN consumption of 4.2 kg/tin the mentioned conditions. Copper concentration in the leach solution was 1.0 g/L. Gravity-cyanidation flowsheet was recommended for pilot plant testing.




PILOT PLANT TESTING


Technological flowsheet of ore processing included two-stage grinding, gravity generation of smeltable grade gravity concentrate, gravity tailings and middlings of gravity concentrate recleaning, gold desorption from loaded carbon, gold electrowinning and adsorption tailings detoxification.



Ore processing by gravity flowsheet allowed to generate smeltable high grade gravity concentrate with gold grade 5D—400 kg/t at the recovery of 15% 20%. Combined gravity tailings and product of concentrate recleaning, containing 5.1 gAu/t were treated by CIL process.



Cyanidation process tested is shown in Fig. 1 (Option 1).



The conditions of gravity tailings cyanidation in CIP regime (fineness of grind 90% passing minus 0.074 mm, L:S = 1.5:1, NaCN 1.0 g/L, CaO 0.2 g/L, duration of preliminary cyanidation 14h, adsorption leaching lOh), desorption and electrowinning were worked during pilot plant testing. Cathode precipitates and smeltable high grade gravity concentrate smelting to metal Dore was conducted by standard methods. Adsorption tailings detoxification was treated by sodium hypochlorite.





Fig. 1 Pilot plant testing. Option 1





According to the results of the tests, for gold was 4.1 mg/g, for copper was 2.1 mg/g. Gold and copper concentration in a liquid phase of adsorption tailings corresponded to 0.2 mg/L and 1200 mg/L. Taking into account losses of dissolved gold, recovery of gold was 87.9% at residual grade in cakes 0.42 g/t and NaCN consumption 4.0—4.5 kg/t, 21 kgCa(OC1)2/t (Table 3).



To reduce losses of dissolved gold and NaCN consumption, the option with adsorption cyanidation, adsorption tailings filtration and the use of solutions in recirculation (Fig.2, option 2) was tested. Table 3 presents the results.



It was shown that when the solutions were used in recirculation, copper concentration stabilized at a level of 2.5-2.7 g/L. Loading of loaded carbon for gold was 4.2 mg/g, for copper was 4.6 mg/g and average gold grade in adsorption tailings was 0.42 g/t. NaCN consumption reduced up to 2.5 kg/t and calcium hypochlorite up to 12 kg/t. Recovery of gold was 91.0%.


Table 3 Main technological indices of ore treatment by two options







Fig. 2 Pilot plant testing. Option 2



Considering high copper content in CIL time on loaded carbon (independent on technology option), process of preliminary copper desorption by solutions with content 10 gNaCN/L at the temperature of 20°C was tested. When treating carbon by ten bed volumes of eluates, average copper concentration in eluates was 100-140 mg/L, residual copper onto carbon was 1.0 mg/g. The obtained eluates can be directed to preliminary cyanidation cycle.



Gold desorption was conducted in a following regime: 40 gNaCN/L, temperature 170°C, duration lh. Residual gold onto eluted carbon was 0.060.08 mg/g.



Carbon regeneration included acid treatment by hydrochloric acid at the consumption of 40 kg/t and thermic reactivation of the temperature of 650°C.



Gold electrowinning from eluates containing 150-250 mg/L of gold provided recovery into cathode precipitate at a level of 97% 98%.



The given option was selected to project a gold-copper ore plant of one of the deposits in Russian Federation.

AMMONIA CYANIDATION


Ammonia cyanidation which allows NaCN consumption to be reduced is known to be of interest for gold-copper ores (Deng and Ma, 1996). The given option was tested in laboratory scale. Head ore gold grade was 9.1 g/t. Gold leaching process was conducted in NaCN-NH4OH-(NH4)2CO3 system.



To determine gold recovery indices, influence of ammonia (0.6-6 g/L) and sodium carbonate (10-40 g/L) concentrations, L:S ratio =(1-1.5:1) and NaCN (1-1.5 kg/t) were studied. Table 4 shows the results.

Table 4 The results of ammonia cyanide leaching optimisation tests


It was found that when 0.6 gNH4OH/L and 10 g(NH4)2CO3/L are added to leaching cycle, gold recovery is 93.8%, the same as for conventional ore cyanidation (without ammonia addition). However, NaCN consumption reduces by 4.2 times (from 4.2 kg up to 1 kg per 1 ton of ore).



Test results demonstrate the perspective of the given process, but it is necessary to carry out additional research on ammonia cyanidation tailings detoxification.


CONCLUSIONS


Based on the results of the conducted research on gold-copper ore treatment, the technology involving gravity with smeltable high grade concentrate, adsorption cyanidation of gravity tailings and the product of middlings, subsequent activated carbon eluation and regeneration, electrolytic Au precipitation with cathode gold containing precipitates and smeltable high grade concentrate smelting for metal Dore, adsorption cyanidation tailings filtration with filtrates direction to the main hydrometallurgical cycle was recommended.



The results of the research and pilot plant testing have been the basis of developed process design criteria for plant designing.



Ammonia-cyanide leaching was examined as a prospective option. The given option is of great interest and can be recommended for subsequent and technical and economical evaluation.



Gold recovery from such complex materials which are semi-oxidised Au-Cu ores can be evaluated. It was shown that process flowsheet option tested is efficient.



Tests results were used for economical evaluation of ore process flowsheet options and can be used for the same gold-copper ores.






REFERENCES

Chamberline, PD, 1996. Process selection for Gold/Copper Ores.

Proceedings of Randol Gold & Silver Forum’96 Squaw Creek, California USA 2 1-24April 1996, 303-306.

Deng, T and Ma, Y, 1996. Improvement of Gold Recovery From Gold Copper Ores by ammoniacal cyanidation. Proceedings of Randol Gold & Silver Forum’96 Squaw Creek, California USA 21-24April 1996, 307-309.

Lodeischikov, VV, 1999. Technology of gold and silver recovery from refractory ores, Irgiredmet: Irkutsk, 1-2: 342, 452.

Mitrofanov, S 1, 1970, Combined methods of oxidized and semi-oxidized copper ores (theory and practice), 286p (Nedra:Moscow).

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