Introduction

The use of recycled water in the concentration circuits of complex sulfide ores is an industrial practice that has recently been implemented for more efficient water usage [1,2,3]. However, process water used in mineral processing may contain various contaminants that can adversely affect the flotation of sulfide mineral species. Some of the most common contaminants include calcium, magnesium and sulfate. These contaminants originate from the use of lime (CaO, Ca(OH)2) as a pH regulator, the presence of magnesium species in the ore (e.g., dolomite, brucite), and from the natural oxidation of sulfur from sulfides due to atmospheric oxygen absorbed in the flotation pulps, respectively [4].

Previous studies have shown that the presence of calcium and magnesium ions in process water used for sulfide ores flotation, negatively impacts the recovery of galena and sphalerite [5, 6]. Furthermore, literature reports suggest that precipitating these ions with sodium carbonate (Na2CO3) has a beneficial effect on the flotation of the aforementioned sulfides [1, 2, 7,8,9]. However, up to date, there is no fundamental explanation for such behavior. In this context, the present study aims to characterize the solid produced by adding Na2CO3, using tools and techniques such as SEM, XRD, and TGA. The final aim is to achieve a better understanding of the behavior of these ions in the flotation process.

Experimental methodology

The experimental methodology used in this study consisted in preparing synthetic solutions with a composition similar to those encountered in the complex sulfide flotation water, that is, about 0.016 mol/L of CaSO4(aq) [5, 10] and 0.02 mol/L of MgSO4(aq). Deionized water, gypsum (CaSO4.2H2O), MgSO4·7H2O and Na2CO3, all analytical grade sourced from Sigma Aldrich, were used to prepare the solutions. The tested concentrations of Na2CO3 (0.008, 0.016 and 0.024 mol/L) were selected based on the results of a thermodynamic analysis of the reaction system. The experiments were conducted at room temperature (25 °C); sodium carbonate was introduced into the synthetic solution and maintained under magnetic stirring at 400 rpm throughout the test. The Na2CO3 treated solution was subjected to filtration using a vacuum pump and filter paper to retain 2.5 μm size particles as a precipitate (solid). This solid was characterized using XRD, TGA and SEM–EDS analyses.

Characterization of precipitated solids

In order to characterize the precipitated solids, a combination of analytical techniques was used. The solids obtained by the precipitation of calcium and magnesium were filtered, dried at room temperature and analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A Bruker D8 Advance diffractometer and a Phillips XL30 ESEM microscope were used to achieve these analyses. Additionally, to investigate the interaction between natural galena and calcite, a specimen was prepared using a pure galena sample embedded in a metallographic resin. This specimen was polished employing silicon carbide abrasive sheets, spanning grit sizes from No. 80 to 2400.To analyze the interaction between natural galena and calcite, 1 g/L of CaCO3 was added, and then the specimen was analyzed by SEM–EDS.

Furthermore, the solids due to the magnesium precipitation, already identified through XRD analysis as amorphous, were subjected to thermogravimetric analysis (TGA) using the CaOx New Balance 271,020 method and an SDT Q600 V20.9 Build 20 equipment. This enabled identifying the products of their thermal decomposition. The temperature range tested was 30–900 °C, with a heating rate of 10 °C/min and using a dynamic nitrogen atmosphere with a flow rate of 100 mL/min. Platinum crucibles and an average 10.4 mg sample were used.

Results and discussion

Characterization by XRD of the obtained solids

The resulting solids were characterized by X-ray diffraction. The corresponding diffractograms obtained at the different experimental conditions are presented in Fig. 1. In the case of treatment of calcium bearing solutions (see Fig. 1a), the diffractograms were examined and the peaks corresponding to calcite and vaterite (a polymorph of calcite) were identified using the diffraction patterns 00-005-0586 and 01-072-0506, respectively. No other phase was observed, which indicates that the obtained precipitates are practically pure CaCO3. The diffractograms show that the intensity of the peaks of the samples changes slightly, and therefore, it may be inferred that the concentration of sodium carbonate only slightly affects the crystallization of the particles [11].

Fig. 1
figure 1

X-ray diffraction spectra of the solids obtained from the precipitation of a Ca in solution by the addition of sodium carbonate, b MgSO4 in solution by the addition of sodium carbonate

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In the case of magnesium bearing solution, the solid obtained from the precipitation of a solution with a concentration of 0.02 mol/L magnesium sulfate and 0.08 mol/L Na2CO3 addition, was also characterized by XRD. The results of this analysis are presented in Fig. 1b, which shows that the precipitates obtained present wide and poorly defined diffraction peaks, which identifies an amorphous solid. In samples (a) and (b), reaction times of 15 and 360 min were tested, respectively; however, a crystalline structure was not achieved despite the longer reaction time allowed. It is concluded that with the concentration of Mg tested (0.02 mol/L), the precipitating solid is amorphous. The solid may correspond to an amorphous magnesium carbonate, as suggested by some authors [12, 13]. The results corroborate the difficulty of obtaining magnesite (MgCO3) at room temperature and reaction times of 15 and 360 min [14].

Thermogravimetric analysis (TGA) of magnesium precipitates

A conventional thermogravimetric analysis (TGA) was conducted on the solids obtained in the magnesium precipitation tests and both the percentage of mass loss and its derivative was plotted against temperature; this was done to identify the products of thermal decomposition of the solids and to determine the temperature at which they occur.

Figure 2 presents the TGA of sample (a) showing the suggested stages of its thermal decomposition. Three well-defined events are observed in the TGA curve. The first stage of decomposition (A), is observed in the temperature range of 30–240 °C, with a mass loss of 21.78%; in this temperature range the mass loss is assigned to the loss of hydration water [15]. The second stage (B), occurring between 240 to 421 °C, with a mass loss of 16.32%, is assigned to the thermal decomposition of the MgCO3·Mg(OH)2 species, that is, a basic magnesium carbonate, resulting in the formation of H2O, CO2 and MgO, according to Chen et al. [16]. The third stage, occurring between 421 and 550 °C, with a mass loss of 22.10%, is suggested to be due to the decomposition of the remaining magnesium carbonate. After this stage, a residual mass of 39.80% was observed, which is attributed to the remaining MgO present in the original solid. This last suggested event is consistent with literature reports of thermal decomposition of MgCO3 starting at around 547 °C and resulting in a mass loss of 50%. With regard to the above behavior, Choi et al. [17] reported that at 495.3 °C, MgCO3 shows a mass loss in the order of 50%, and suggest that the decomposition of MgCO3 begins around 547 °C.

Fig. 2
figure 2

TGA curve of the thermal decomposition of sample (a), the solid obtained from the precipitation of magnesium with the addition of sodium carbonate, [Mg] = 0.02 mol/L, t = 15 min

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Based on the thermogram of Fig. 2, a mass balance was carried out and the results of the analysis are reported in Table 1.

Table 1 Results of the TGA of sample (a)
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The obtained results suggest considering that the solid is a hydrated basic magnesium carbonate, with the following formula:

$${\text{Mg}}_{3} \left( {{\text{CO}}_{3} } \right)_{3} \cdot {\text{Mg}}\left( {{\text{OH}}} \right)_{2} \cdot 4.5{\text{ H}}_{2} {\text{O}}$$
(1)

It is worth noting that previous literature has defined hydromagnesite with the alternative formula Mg3(CO3)3 Mg(OH)2·3H2O [18], which is similar to the suggested stoichiometry. Considering that the nucleation rate of magnesite is slow compared to that of calcite [14], the results obtained under the conditions established in this study suggest that in the precipitation of the basic magnesium carbonate, the nucleation mechanism predominates, that is to say, in the oversaturated solution of sodium carbonate, disordered precipitation predominates over ordered crystallization. The results of the thermogravimetric analysis suggest that the amorphous solid may correspond to a hydrated basic magnesium carbonate, precursor of hydromagnesite (Mg5(CO3)4(OH)2·4H2O).

Characterization by SEM–EDS of the obtained solids

The characterization of solids obtained in the tests of precipitation of calcium with sodium carbonate was complemented with the use of SEM–EDS. Figure 3a shows the micrograph obtained, which reveals the presence of calcite and vaterite (as identified by XRD) The semi-quantitative analysis conducted using EDS detected the presence of Ca, C and O, with an atomic percentage composition of 17.13% Ca, 20.15% C and 62.07% O. This corroborates the presence of calcium carbonate. Additionally, two distinct morphologies were observed, one in the form of spheres and rhombohedral the other. This supports the existence of both calcite (rhombohedral) and vaterite (dihexagonal) species in the solid.

Fig. 3
figure 3

a SEM and EDS calcium precipitated by addition of sodium carbonate. b SEM micrograph of magnesium precipitated by the addition of sodium carbonate

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The superficial interaction between natural galena and calcite was also observed by SEM–EDS. An interaction within the system was identified: CaCO3 particles appeared to adhere to the surface of the galena sample. This observation is particularly significant in the context of the study, as the primary objective is to characterize the solids obtained from the precipitation process to describe the mechanism of interaction between these solids and the sulfide minerals.

Figure 3b shows the micrograph of the amorphous solids resulting from the precipitation of magnesium in solution (0.02 mol/L) with the addition of 0.08 mol/L of sodium carbonate and a reaction time of 30 min. The micrograph was obtained using secondary electrons, which facilitated the observation of the solid’s morphology. It is observed that the particles are submicronic and tend to agglomerate, despite not exhibiting a defined crystalline structure. Previous studies have reported the synthesis of hydromagnesite with a crystalline structure, but these were conducted with higher concentrations and temperature (e.g., 70 °C). This suggests the possibility of obtaining hydromagnesite with a defined crystalline structure [19]. However, given the ambient conditions and concentrations of the current study, the solid obtained is an amorphous species.

Conclusion

The addition of Na2CO3 to a saturated solution of calcium sulfate (0.016 mol/L CaSO4) promotes the precipitation of calcium as calcite and vaterite (a polymorph of calcite). Adding Na2CO3 to magnesium sulfate solutions with concentrations of 0.02 mol/L of Mg results in the precipitation of an amorphous solid. The results corroborate the difficulty of obtaining magnesite (MgCO3) at room temperature and reaction times of 15 and 360 min. The results of the thermal analysis suggest that adding 0.08 mol of sodium carbonate to a solution of 0.02 mol/L of magnesium, results in the precipitation of an amorphous solid, which appears to be a hydrated basic magnesium carbonate, precursor of hydromagnesite (Mg5(CO3)4(OH)2·4H2O).