The development of technologies for processing of strategic raw materials is necessary to maintain the world economy at the current level in connection with the depletion of mineral resource base. Worsening the quality of gold- and silver-containing raw materials, involvement of lean and refractory ores in processing owes the actualization of scientific research aimed at increasing the recovery of valuable components into concentrates [1]. The analysis of existing studies has shown that some of the topical areas in the field of mineral beneficiation are increasing the efficiency of disintegration of mineral raw materials [2, 3], application of machine vision technology at various stages of beneficiation, synthesis of new flotation reagents [4–6], and the development of new reagent regimes [7–9].

The refractoriness of gold-bearing ores can be caused both by the presence of substances sorption-active in relation to dissolved gold and inclusions of fine noble metals in mineral-carriers, mainly in such minerals as pyrite, arsenopyrite, galena, etc. Ores in which both these features are present are referred to ores of double refractoriness. “Invisible” gold is referred to submicroscopic gold particles that are 1–100 nm in size and are not detected using optical or electron microscopy [10]. The presence of “invisible” gold and silver in ores complicates the selection of flow charts for their processing and owes the necessity for the development of new process solutions and improvement of existing ones.

The carbonaceous material contained in the double-refractory ores contaminates the concentrates produced and results in significant losses of valuable components at the stage of metallurgical processing. The actual task of the existing research is to involve in processing carbonaceous products, going to tailings at most of the processing plants, in order to reduce losses of valuable components such as gold and silver [11–13].

In addition to native metal, silver in ores can be represented by dispersed inclusions in the host minerals, either as metallic or chemically bound silver. Chemically bound silver refers to various silver sulfides, for example, Ag2S. The presence of fine silver associated with sulfide minerals, as well as in the case of gold, complicates its recovery at the cyanidation stage [14]. The recovery requires destruction of the matrix of the host minerals in order to provide the contact of reagents with the noble metal to dissolve it.

To date, there is a large amount of researches aimed at finding process solutions for the processing of refractory raw materials [15–17]. For double refractory ores, pretreatment to reduce the refractoriness of gold-bearing ores is required. The most common pretreatment methods are roasting, chlorination, pressure oxidation, bioleaching, and Albion technology. However, in addition to the traditional methods, research into the feasibility of using energy-based methods of action has become widespread [18, 19]. Ultra-high frequency (UHF) treatment is one of the promising methods whose advantages include fast and selective heating [20–22]. The selectivity of heating minerals is due to the differences in their heating rate, which in turn is connected with specific heat capacity, specific thermal conductivity, and relative dielectric constant. Recent researches with the use of microwave treatment are aimed at increasing the efficiency of disintegration of mineral raw materials [23-25], studying the feasibility of using microwaves for sorting ores [26], reducing the content of harmful impurities in ores [27], studying the effect on surface properties and flotability of minerals [28], involvement of cyanide [29] and sulfide [30] tailings in the treatment. The melting temperature of “invisible” noble metals is much lower than the melting temperature of visible particles that predetermines the feasibility of their coarsening in the process of energy actions. For example, for Au, the melting point of a 1.6 nm diameter cluster is 257.85 °C [33], while that of a 1.9 nm diameter cluster is 318.85 °C [34]. For Ag, the melting point of a 4 nm diameter cluster is 449.85 °C [5], while that of a 5 nm diameter cluster is 509.85 °C [34]. Carbonaceous flotation products, as a rule, contain insignificant amounts of ore minerals, which requires sufficiently long time for microwave treatment in order to coarsen the particles. In this regard, the addition of magnetite in the process of treatment allows the creation of local heating centers. Table 1 systematizes the data of dependence of the achieved maximum heating temperature on the treatment time for sulfide mineral-concentrators of gold and products of their destruction after roasting.

Analysis of historical studies confirms the fact that magnetite is more active in relation to ultra-high frequency electromagnetic radiation and, in comparison with other minerals, reaches significantly higher heating temperatures. The low melting temperature of silver nanoclusters confirms the prospect of investigating the feasibility of their coarsening, since it is lower than the magnetite treatment heating temperature.

Thus, the purpose of this study was to reveal the mechanism of coarsening fine silver in the course of microwave treatment with the addition of magnetite using model samples and to justify the required content of magnetite to confirm the feasibility of silver coarsening in the course of microwave treatment of carbonaceous flotation concentrate samples to reduce losses of valuable components in tailings. 

Table 1

Summary data on dependence of maximum microwave heating temperature on treatment time

The following carbon-containing materials were chosen as the research subjects:

The source of silver in the model sample was silver leaf, containing 99.9% Ag. Preparation of the model samples included: grinding of activated carbon, preparation of leaching solution, transition of silver leaf to liquid phase for two days (leaching), its subsequent contacting with activated carbon for three days. The leaching reagent is a mixture of iodine complex, amino acids, sodium chloride, urea, ammonium chloride, and sodium carbonate. After contacting the noble metal with activated carbon, filtration and drying of the obtained cake was carried out for subsequent microwave treatment.

The initial carbonaceous gold-bearing ore is characterized by the presence of fine inclusions of gold and insignificant amount of silver in the mineral-concentrators, such as pyrite and arsenopyrite, as well as organic carbonaceous matter sorption-active in relation to dissolved gold. The main valuable component is gold, the grade of which in the initial ore is 5.99 ± 0.29 g/t. Silver is a by-product, the grade of which is 0.29 ± 0.1 g/t.

Separation of carbonaceous flotation concentrate from refractory gold-bearing ore was carried out using pneumomechanical flotation machine Flotation Bench Test Machine of Laarmann company with cell volume of 1.5 liters. The initial ore was subjected to grinding to particle size of 60% passing 71 μm, after which carbonaceous flotation was carried out with the addition of oxal frothing agent, the consumption of which was 85 g/t. The results of studying the obtained flotation products are presented in Table 2.

The obtained carbonaceous flotation concentrate after drying was subjected to microwave treatment, while the tailings of carbonaceous flotation went for additional grinding and subsequent sulfide flotation.

Table 2

Results of research of refractory gold-bearing ore flotation products

Microwave treatment of carbon-containing materials without and with the addition of different amount of magnetite (3, 5, 10, 15% by weight of a sample) was carried out using a Sineo UWave-2000 microwave oven, the range of possible setting of power in which was 100–1000 W. The coarseness of the added magnetite was −50 μm.

In order to confirm coarsening fine silver in model samples and carbonaceous flotation concentrate after microwave treatment with the addition of magnetite to create active centers of local heating, the study of the samples before and after the treatment was carried out using a Vega 3 LMH scanning electron microscope, combined with a system of X-ray energy dispersive microanalysis Oxford Instruments INCA Energy 250/X-max 20. The fine silver coarsening study was carried out with the determination of sizes of a minimum of 200 particles in each of the samples obtained. The carbonization of the samples was carried out using a Q150R E sputtering apparatus manufactured by Quorum Technologies Ltd.

Measuring the achieved temperature of microwave heating of samples was carried out using a FinePower DIN21H laser pyrometer. The measuring temperature range was −50 to 1,100 °C with an accuracy of ±2%.

In order to confirm the feasibility of coarsening fine silver in the course of microwave treatment, an initial model sample with adsorbed silver before treatment was studied for comparison. Figure 1 shows the results of the sample study before treatment using scanning electron microscopy. Table 3 shows the findings of the elemental composition study for the spectrum shown in Fig. 1.

Based on the results obtained, the absence of visible silver was revealed, which was confirmed by the results of the elemental composition study, thus confirming its “invisibility”.

Fig. 1. Results of investigation of the initial model sample before microwave treatment using scanning electron microscopy

Table 3

The results of the study of elemental composition for the spectra presented in Fig. 1

In order to confirm the necessity of adding magnetite to silver-containing model samples at microwave treatment for coarsening fine silver, the studies were carried out on model samples without magnetite addition. Figure 2 shows the results of scanning electron microscopy examination of the treated samples without magnetite addition. The results of the study of the elemental composition for the spectra in the studied samples are shown in Table 4.

Fig. 2. Results of scanning electron microscopy study of model samples with coarsened silver after microwave treatment without magnetite addition

Table 4

The results of the study of elemental composition for the spectra presented in Fig. 2

The analysis of the obtained data allowed confirming the feasibility of coarsening fine silver particles using microwave treatment without addition of magnetite up to the average size equal to 5–10 microns. Despite the fact that coarsening is achieved at the treatment without the addition of magnetite, the coarseness achieved in these studies does not allow extracting coarsened particles by traditional beneficiation methods. This fact predetermines the need to study the effect of the addition of different magnetite amounts to the studied model samples on coarsening fine silver particles.

The studies aimed at establishing the necessary magnetite content to achieve maximum coarsening fine silver in the course of microwave treatment were carried out using silver-containing model samples. The magnetite contents were chosen at 3, 5, 10, and 15%. To confirm the generation of active centers of local heating in the course of microwave treatment of the model samples with the addition of magnetite, heating temperatures were measured over the whole area of the treated samples using a laser pyrometer. Fig. 3 shows the temperature mapping using an example of microwave treatment of a silver-containing model sample with 10% magnetite with sampling points for scanning electron microscopy study. The microwave heating temperature was measured immediately after the treatment. The results of the elemental composition study for the spectra shown in Fig. 3 are presented in Table 5.

Fig. 3. Temperature mapping of silver-containing model sample at microwave treatment with addition of 10% magnetite with the results of sample examination at selected sampling points using scanning electron microscopy (gray – magnetite; black – model sample)

Table 5

The results of the elemental composition study for the spectra presented in Fig. 3

The interpretation of the obtained data allows confirming arising active centers of local heating in the course of microwave treatment when magnetite was added to the model samples. In Fig. 3, the temperature peaks at the magnetite addition locations, which reach about 600 °C and higher, are clearly visible. At the same time, the heating temperature of the model sample itself is 540–560 °C. The presence of magnetite is confirmed by the results of the elemental composition study.

The treated silver-containing model samples with magnetite addition were studied using a scanning electron microscope. Figs. 4 and 5 show the results of the study of these samples with magnetite content of 3, 5, 10, and 15%, respectively. The results of the study of elemental composition for the spectra shown in Figs. 4 and 5 are shown in Table 6.

Fig. 4. Results of scanning electron microscopy study of model samples with coarsened silver after microwave treatment with addition of 3 (left) and 5 (right) % magnetite

Fig. 5. Results of scanning electron microscopy study of model samples with coarsened silver after microwave treatment with addition of 10 (left) and 15 (right) % magnetite

Table 6

The results of the elemental composition study for the spectra presented in Figs. 4 и 5

The interpretation of the obtained data allows to confirm coarsening fine silver at addition of magnetite as a result of microwave treatment to the particle sizes exceeding the particle size formed in the samples without magnetite, as well as the increase of silver content in the coarsened particles. However, it is worth noting that at a magnetite content of 10%, if compared to the coarsened gold presented earlier in [36], spherical-shaped silver particles with an average size of 20–40 μm were found in the silver-containing model samples. This is because the melting point of silver is much lower than that of gold.

The obtained data on average and maximum sizes of coarsened noble metal particles (aggregates) depending on the magnetite content are presented in Table 7. Besides, Table 7 presents the degree of coarsening measured as the ratio of the average size of the coarsened silver particles to the estimated coarseness of the “invisible” silver.

Table 7

Results of the study of the effect of magnetite content on coarsening fine silver in model samples with and without microwave treatment

Based on the obtained data analysis, it can be concluded that the addition of magnetite, which forms active centers of local heating, contributes to coarsening fine silver particles in the course of microwave treatment, since the treatment of magnetite-free model samples allowed to coarsen the noble metal particles to smaller sizes if compared with the samples with magnetite. The obtained results allow justifying the necessary magnetite content of 10% to achieve maximum coarsening fine silver at microwave exposure to a coarseness of 20–40 μm. The formation of spherical silver particles can be explained by the lower melting point of silver compared to gold.

The feasibility of silver coarsening with the addition of the previously substantiated magnetite amount in the course of microwave treatment was investigated on carbonaceous flotation concentrates. In [37], the feasibility of coarsening fine Au particles using 10% magnetite at microwave treatment of carbonaceous concentrate was confirmed. Fig. 6 presents the results of a scanning electron microscope study of the treated carbonaceous concentrate, confirming coarsening silver in the course of microwave treatment when added with magnetite in the amount of 10% of the sample weight. The elemental composition of the coarsened particles is presented in Table 8.

Fig. 6. Results of the study of treated carbonaceous concentrates with addition of 10% magnetite using scanning electron microscopy

Table 8

The results of the elemental composition study for the spectra presented in Fig. 6

The interpretation of the obtained results allows to confirm coarsening silver particles in the course of microwave treatment of carbonaceous flotation concentrate added with 10% magnetite. The size of coarsened particles, which are characterized by spherical shape, after treatment is 20–50 microns. It is worth noting that, besides silver, characteristic peaks of gold were found in the coarsened aggregates according to the results of the study of the elemental composition. This indicates that silver was coarsened together with gold.

Microwave treatment contributes to increasing the temperature of the model sample, the main part of which is represented by activated carbon. When it is ignited in an oxidizing environment, the following phenomena occur:

Fig. 7 shows a schematic representation of the mechanism of fine silver coarsening in the course of microwave treatment.

Fig. 7. Schematic representation of fine silver coarsening in the course of microwave treatment

Fine silver coarsening is confirmed by the results of scanning electron microscopy examination of the treated silver-containing model samples. In the figures presented in this paper, the coarsened particles are observed both free (not tied with activated carbon and magnetite) and at the initial stage of coarsening directly on the activated carbon surface itself.

The results of the microwave treatment of carbonaceous flotation concentrate with the addition of 10% magnetite also confirmed the feasibility of fine silver coarsening. The achieved coarseness and the content (grade) of gold and silver in the coarsened spherical particles makes it possible to further extract them and, as a consequence, to reduce losses of valuable components with tailings. Centrifugal concentration can be considered as a potential method for the extraction of the coarsened particles [38].

The findings of the studies on microwave treatment of silver-containing model samples allowed substantiating the necessity of adding magnetite for fine silver coarsening. Based on the temperature mapping, the formation, in the course of the treatment, of active centers of local heating in the locations of magnetite addition was confirmed. The fine silver coarsening to spherical silver particles, whose average size was 20–40 μm, was achieved at a microwave oven power of 1.0 kW and a treatment time of 3 min with 10% magnetite added. It was revealed that at similar microwave oven power and treatment time of 5 min, with 10% magnetite added, the greatest sizes of coarsened particles containing gold and silver equal to 20–50 μm were achieved. The microwave treatment findings showed that the silver coarsening occurs forming a gravity-extractable form of silver that allowed further investigation of its recovery using gravity concentration methods.