APPLICATION OF HIGH-POWER ELECTROMAGNETIC PULSES TO

DESINTEGRATION OF GOLD-CONTAINING MINERAL COMPLEXES ∗

V.A. Chanturiya, I.J. Buninξ, A.T. Kovalev Research Institute of Comprehensive Exploitation of Mineral Resources, Russian Academy of Sciences,

4 Kryukovsky Tupik, Moscow, 111020, Russia

∗ Work supported in part by the President of the Russian Federation under contract number НШ−472.2003.5. ξ email: bunin_i@mail.ru

Abstract The application of High-Power Electromagnetic Pulses (HPEMP) irradiation in dressing of resistant gold-containing ores appears attractive as this technique provides for a significant increase in precious metal recovery (30−80% for gold and 20−50% for silver), therewith helping reduce both energy consumption and the cost of products. This study deals with plausible mechanisms of disintegration of mineral particles under the action of nanosecond HPEMP with high electric field strength E∼107 V/m. Experimental data are presented to confirm the formation of breakdown channels and selective disintegration of mineral complexes as a result of pulse irradiation, which makes for efficient access of lixiviant solutions to precious metal grains and enhanced precious metal recovery into lixivia during leaching. We studied the influence of HPEMP on the technological properties of particles of refractory gold- and silver-containing ores and beneficiation products from Russian deposits. Preliminary processing of gravity concentrate of one deposit ore with a series of HPEMP resulted in significant increase of gold and silver extraction into lixivia during the cyanidation stage, with gold recovery increased by ∼31% (from 51.2% in a blank test to 82.3% after irradiation) and silver recovery increased by 47% (from 21.8% to 68.8%). Gold recovery from stale gold-containing dressing tailings of the two integrated mining-and-dressing works increased after pulses-irradiation from 8−12% to 80−90%.

I. INTRODUCTION In Russia, like elsewhere in the world, development of primary gold deposits is considered a first-priority line of development for gold mining industry. Most of the gold-containing ores characteristic of Russian gold deposits are resistant ores with gold content varying between 3 and 5 ppm, usually showing quite low gold and silver recovery by cyanidation. Processing resistance of gold-containing mineral complexes is related to the presence of gold

particles of submicrometric size (<1.0 µm), mostly associated with pyrite and arsenopyrite. The problem of proper utilization of resistant ores and enhancement of precious metal recovery presently takes on ever increasing significance. In the mineral processing process, from 70% to 90% of electrical power is consumed for the crushing and grinding of the ore. The power consumption for the crushing process amounts to 20−40 kWh/ton, and even more. Moreover, in some cases, the increasing of grinding fineness does not result in the increasing of the degree of mineral granular disclosure. Therefore, one of the basic tasks in this domain is the reduction of the power consumption and the elevation of the degree of mineral granules (transgranular) disclosure. This paper reviews current research in high-power energetic technologies for processing of gold-containing resistant ores and beneficiation products, a branch of experimental engineering physics which critically depends on national priority research projects for its dynamic development. The aim of the review is basically to show progress in the study of nanosecond processes involved in the disintegration and breaking-up of mineral complexes with disseminated fine noble metals. Results of experimental studies of the mechanisms of non-thermal action of HPEMP with nanosecond leading edge and pulse duration and high electric field strength on complex natural mineral media are presented. Experimental data are presented to confirm the formation of breakdown channels and selective disintegration of mineral complexes as a result of pulse irradiation, which makes for efficient access of lixiviant solutions to precious metal grains and enhanced precious metal recovery into lixivia during leaching.

II. NON-TRADITIONAL METHODS OF TREATMENT OF RESISTANT GOLD-

CONTAINING ORES Let’s review some kinds of known technique of natural media modifications, which are closely related to the method proposed in this paper. The electrochemical

methods should be mentioned first. The essence of these methods consists in the increase of defects concentration and arising of a great number of microcracks under the polarization processes of sulfide and oxide minerals with semiconductor properties [1]. The defects concentration raise and microcracks emergence are caused under these conditions by the electrochemical reactions which take place on the mineral grain boundaries. In practice, the electrochemical action is performed in the process of grinding by the application of direct current of 3−6 A/m2 density and of 6−12 V voltage inside the ball mill. The electric power consumption in this case amounts to 0.2−0.4 kWh/ton and the degree of disclosure increases by 20−25%. Considerably better results were obtained under the ores exposure to the accelerated electron beam with energy of 1−2 MeV and current density of 1−5 µA/cm2 before grinding [2]. The physical background of the effect is the electric charge of the natural media of weak conductivity. This causes the emergence of microcracks, which lead to the softening of mineral components. The 20−80% increase of grinding efficiency, as well as the 15−20% raise of technological characteristics is observed under these conditions for all types of ores. One of the noteworthy attempts to solve the problem of disintegration of resistant ores and beneficiation products was the irradiation of the ore by the microwave generator [3]. The microwave generator provided a continuous radiation of 0.9-2.5 GHz frequency. The roast of the medium up to 360° C increased the yield of gold in several experiments but no convincing results were obtained. In UHF treatment, heterogeneous (non-uniform) absorption of microwave energy by different components of the mineral complex results in embrittlement of the mineral matrix and destruction of its skeleton along the intergrowth boundaries, which "unseals" the valuable components, making them easier to extract. In addition, intense physicochemical processes occur on the surfaces of the sulphide samples exposed to UHF treatment: pyrite oxidizes to hematite and elemental sulphur, and arsenopyrite oxidizes to magnetite, arsenic sulphide and (minor) SO2, which helps increase gold recovery up to 95% [4]. However, excessive UHP heating results in unwanted effects, such as fusion and sintering of the material and closure of as-formed cracks. In addition, this procedure is energy-intensive, with energy consumption of at least 3−5 kWh per ton required to provide for plant capacity of 5−10 tons per day. Magnetic pulse treatment of gold-containing ores is meant to reduce energy expenditure for milling and increase gold recovery [5]. This technique is realized by passing the ore (or pulp) through a dielectric pipeline segment enclosed in a system of electromagnetic coils which, constantly generates electromagnetic field pulses with repetition frequency up to 50 Hz. It is worthwhile to implement this technique in ore processing just before milling and to include it in the cyanidatlon procedure,

which proves to yield a 1−1.5% gain in gold recovery in all. A group of researchers affiliated in the Electrophysical Institute of the Uralian Branch of Russian Academy of Sciences (Yekaterinburgh) designed a plant for electrohydraulic treatment of resistant materials by nanosecond pulses with a positive polarity, a magnitude of up to 250 kV, and a repetition rate of up to 300 Hz [6]. This device does perform the mechanism of nanosecond breakdown of water (the electrohydraulic method proposed by L.A.Yutkin) with suspended microparticles, yet having significant limitations on efficiency, capacity and energy consumption, and some other technological restrictions. In essence, electrohydraulic treatment is realized through exposing the test material immersed in liquid, to shock waves generated by electrical breakdown of the liquid, with an aim to destruct the resistant particles. The essential disadvantages of this method are the necessity of performing the process in a liquid medium with solid-to-liquid ratio S:L=1:1, which decreases plant capacity and increases energy consumption, and non-controllable changes in ionic composition of the aqueous phase of the pulp. In particular, experiments with samples of stale tailings from the Uchala concentration plant revealed a sizable increase in concentration of Cu, Zn and Fe ions in the aqueous phase of the pulp after electrohydraulic pulse treatment, which may disturb further processing and have negative environmental sequels. All the above discussed high-energy treatment methods have the following disadvantages in common: high energy consumption, overheating of the material subject to processing, and certain intensification of sulphide leaching with uncontrollable passage of metal ions into the liquid pulp phase. In this paper we present a treatment method developed by IPKON RAS and IRE RAN researchers, which appears to be free of the above listed disadvantages. This non-traditional, highly efficient and environmentally safe method of breaking up mineral complexes with disseminated fine gold is based en non-thermal action of nanosecond High-Power Electromagnetic Pulses on resistant gold-containing ores and beneficiation products [7,8].

III. THE EFFECT OF HPEMP ON BREAKING-UP OF GOLD-

CONTAINING MINERAL COMPLEXES We have studied three plausible mechanisms of disintegration of mineral particles under the action of nanosecond HPEMP with high electric field strength Ep

∼107 V/m [9]. The first mechanism consists in loosening of the mineral structure due to electrical breakdown effects, which only occurs in cases where small, highly conductive inclusions are hosted in dielectric media. The second mechanism is related to development of

thermomechanical stresses at the boundary (interface) between the dielectric (or semiconductor) and conductive mineral components, being only realized in cases where these components are comparable in size. The third mechanism, assuming essentially non-thermal action of HPEMP on mineral complexes, is related to electromagnetic energy absorption by thin metallic films or layers much thinner than the characteristic skin layer (skin effect). Figure 1a presents an image of a fragment of spallation surface of a pyrite specimen after irradiation with a series of nanosecond pulses. Although the action of the pulses on the specimen surface was initially uniform, electric breakdown developed quite unevenly, predominantly close to rough edges of the specimen and along the intergrowth boundaries (Figure 1b). These experimental data are presented to confirm the formation of breakdown channels and selective disintegration of mineral complexes as a result of pulse irradiation, which makes for efficient access of lixiviant solutions to precious metal grains and enhanced precious metal recovery into lixivia during leaching. For practical realization the specialists affiliated in IPKON RAS designed a plant with capacity of 50−100 kg (of ore subject to processing) per hour using a conveyer mode of conveying ore into the zone of electromagnetic pulse treatment. The plant includes the following units: voltage converter, master pulse generator, capacitive energy accumulator, transportation system and electrode unit. The efficiency of disintegration of mineral complexes and breaking-up (″unsealing″) of precious metal particles is controlled by the development of a streamer discharge in the gap between the electrodes through proper selection of the magnitude, duration and shape of pulses. The required "dose" of electromagnetic pulse effect for the specified mass of the mineral material to be processed is attained by varying the speed of conveyer belt movement and repetition frequency of pulses from the pulse shaper. The flow of the material subject to processing is conveyed (with equalized thickness and limited width) into the unit of high-energy treatment with nanosecond high-voltage pulses with the following characteristics: voltage amplitude 20−50 kV, pulse front duration 1−5 ns, pulse repetition frequency 50−1000 Hz, with total plant power consumption not greater than 3 kW. The employment of HPEMP in dressing resistant gold-containing ores and beneficiation products appears attractive as it provides for maximum breaking-up efficiency for the mineral complexes being processed and a significant gain in valuable components recovery (30−80% for gold and 20−50% for silver), therewith helping reduce both energy consumption and the cost of products. Experiments on HPEMP-induced effects were performed with various materials, including samples of resistant ores, beneficiation products (gravitational and flotation concentrates) and stale tailings from

concentration plants. A feature in common to all the materials selected for study was the presence of finely dispersed gold and silver (hundredths and thousandths of µm), much of this gold being related to sulphide minerals, predominantly pyrite and arsenopyrite.

a)

b) Figure 1. SEM image of the microstructure of destructive zones of pyrite after HPEMP irradiation: a) partial breakdown of surface in the vicinity of metallic inclusion, and b) opening of the intergrowth boundaries.

The experimental procedure included pre-treatment of mineral particles with a series of HPEMP, followed by cyanidation to extract precious metals. The experiments involved both dry samples and samples wetted with water in amount not greater than enough to fill the pores in mineral particles, i.e., to attain the solid-to-liquid ratio S:L=(5−10):1. The number of pulses in a series and the irradiation parameters (pulse shape and duration) varied depending on particular experimental conditions. The appropriate value of electric field strength magnitude of the electromagnetic field (varying from 5 to 50 MV/m) exceeding the electrical strength of the material was attained through adjusting the gap between the electrodes and their insulation. Data on gain in gold recovery by

cyanidation from gold-containing ores, concentrates and other processing products from different deposits after HPEMP treatment are given in Table 1. Table 1. Effect of HPEMP irradiation on gold extraction by cyanidation from resistant gold-containing ores and

beneficiation products. Deposit; gold content, ppm

Size class, µm

Gain in gold recovery

(from → to), % Initial ore

Kyuchus; 24.2 −1000

12,11 (66.67 → 78.78)

Nevskoye; 1.3−1.8 −500 4,4

(91.2 → 95.6) Olimpiadinskoye; 2.4 −100 8,33

(60.0 → 68.33) Сoncentrates gravitational

−50 6,4

(77 → 3.4) Nezhdaninskoye; 80

−500 31.08 (51.22 → 82.3)

flotation

−20 5.7 (82 → 87.7) Kumtor

(Kyrgyzstan); 45 −140 7,9

(63.1 → 71) Tailing from concentration plants

Aleksandrinskoye; 2.34 −74 31.2

(52.56 → 83.76) Gai; 2 −315

80 (11 → 91)

Uchala; 2.1 −74

30 (12,86 → 42,86)

Urup; 1.02 −315 71.1

(8.5 → 79.6) Uzelga; 2.24 −74 36,61

(6,25 → 42,86) A series of process experiments confirmed the theoretical assumption that maximum breaking-up efficiency after НРEМР treatment would be expected from gold-containing sulphides not finer grained than 200−100 µm, and that the effect of formation of breakdown channels and selective desintegration is enhanced predominantly for wet samples. In particular, for a gravitational concentrate of ore from the Nezhdaninskoye deposit exposed to HPEMP rather high gain in precious metal recovery was obtained with minimum energy expenditure of just 2 kWh per ton of concentrate being processed, while energy consumption in a process involving mechanical grinding of the −500 µm ore to −50 µm were about 20−25 kWh per ton of ore.

IV. SUMMARY The treatment of gold-containing raw material by High-Power Electromagnetic Pulses allows one to achieve the maximum completeness of the intergranular breakdown of the mineral components with minimum expenditures of the electric energy (the efficiency coefficient of transformation of the industrial frequency energy into the pulse energy amounts to more than 90%). This fact predetermines the creation of a fundamentally new, highly-efficient, energy-saving technology of the ore treatment. This will exclude the necessity to make investments into the power-consuming and ecologically hazardous process of oxidative roasting, or into the expensive autoclave technology of concentrate breakdown. Consequently, this will make it possible to reduce the distance from raw material to final commodity.

V. REFERENCES [1] V.A. Chanturiya and V.A. Vigdergauz, "Electrochemistry of Sulphides. Theory and Practice of Flotation," Moscow: Nauka, (1993). [2] V.A. Chanturiya and V.A. Vigdergauz, "Scientific basis and prospects of commercial application of accelerated electron energy in mineral benefication processes," Mining Journal (Gorny Zhurnal), no 7, pp. 53-57, Jul. 1995. [3] S.W. Kingman, "Recent developments in microwave-assisted comminution," Int. J. Miner. Process., vol. 74, pp. 71-83, Jan. 2004. [4] A.V. Khvan, et. al., "Feasibility of using UHF field effects for ore preparation in gold production," Mining Bulletin of Uzbekistan (Gorny vestnik Uzbekistana), vol. 2, no 9, pp. 56-60, Sep. 2002. [5] S.A. Goncharov, et. al., "Employment of electromagnetic treatment of gold-containing ores in grinding and cyanidation processes," Information and Analytical Mining Bulletin, no 7, pp. 5-7, Jul. 2004. [6] Yu.A. Kotov, et. al., "All-round treatment of pyrite waste products from mining-and-dressing works with nanosecond pulses," Repts. Rus. Acad. Sci. (Doklady RAN), vol 372, no 5, pp. 654-656, May, 2000. [7] V.A. Chanturiya, et. al., "The opening of the refractory gold−containing ores under high-power electromagnetic pulses," Repts. Rus. Acad. Sci. (Doklady RAN), vol 366, no 5, pp. 680-683, May, 1999. [8] I.J. Bunin, et al., "Experimental studies of non-thermal action of high-power electromagnetic pulses on resistant gold-containing mineral products," Proc. Rus. Acad. Sci. (Izvestiya RAN). Ser. Phys., vol. 65, no 12, pp. 1788-1792, Dec. 2001. [9] V.A. Chanturiya, I.J. Bunin, and A.T.Kovalev, "Mechanisms of disintegration of mineral media exposed to high-power electromagnetic pulses," Proc. Rus. Acad. Sci. (Izvestiya RAN). Ser. Phys., vol 68, no 5, pp. 630-632, May, 2004.