Recycling of tailings from the Barruecopardo tungsten deposit for the production of glass

Tailings from tungsten mining activities in the vicinity of Barruecopardo (Salamanca) represent high environmental pollution. In this paper we present a study of the use of these wastes as raw materials for the manufacture of glass. This procedure aims to contribute to environmental remediation of mining areas through vitrification, a process which offers an alternative for stabilization of hazardous wastes. In addition, the marketing of the obtained product would provide an additional income to the mining areas. The chemical composition of the tailings to be used as raw materials was determined by X-ray fluorescence and their mineralogy by X-ray diffraction. Wastes are of granitic composition enriched in potentially toxic elements. For this study, a representative sample of mining wastes of sandy grain size was used to make the glass. On the basis of its composition, glass was formulated by adding 29.28 mass% of CaCO3 and 14.03 mass% of Na2CO3 and a green glass was produced. Crystallisation temperatures, obtained by DTA, were 875 and 1022 °C and the melting temperature was 1175 °C. The transition temperature of glass was of 644 °C. The temperatures for the fixed viscosity points, and the working temperatures were obtained. A thermal treatment induced devitrification to produce a glass–ceramic made of nepheline and wollastonite. Leaching tests of the obtained glass confirm its capacity to retain potentially toxic elements.


Introduction
Mining wastes constitute a serious environmental risk. To achieve the purity or concentration required to obtain the marketable product, it is necessary to apply processes that generate high volumes of tailings. The impact of mining wastes in the environment is even more concerning because, besides occupying large areas, it can lead to the emission of potentially toxic elements, as often the destabilization of contained sulphides produces acid mine drainage.
Intense mining to obtain tungsten in deposits associated with granitic rocks has long existed at the western part of the Iberian Peninsula. These activities have caused significant pollution in soils and water, especially with high levels of emission of As, as is the case with the old tungsten mines in Barruecopardo, Salamanca [1][2][3]. The strategic importance of tungsten [4] makes this activity of interest and currently many mines of this commodity are in process of reopening. This activity implies that economic and environmental aspects related to tailings management must be taken into account.
Vitrification is an alternative of environmental remediation through the transformation of wastes to glass in order to stabilize their pollutant components because their constituents are bound in a glassy stable matrix [5,6]. Nevertheless, it is an expensive process that can usually only be applied if the obtained glass is reused in high-value applications [7].
The chemical composition of mining wastes originated from granitic rocks is similar to the most commonly used raw materials in glass making process, thus being suitable to make glass and ceramic materials [12][13][14][15][16]. The high environmental risk produced by tailings result of the tungsten mining [17] has led to several investigations to reduce this problem using these wastes to obtain glass-ceramic products [18], ceramic materials [19] or polymer-based composite materials [20].
The present work aims to determine the potential of tailings from W-rich granitic rocks from Barruecopardo to be used as raw materials in glass and glass-ceramic production. The tailings and the obtained glass have been characterised.

Samples
The Barruecopardo mine, located in Salamanca, is a W greisen-type ore deposit that consists mainly of granitic rocks with scheelite, wolframite, pyrite, abundant arsenopyrite and minor amounts of chalcopyrite, molybdenite and cassiterite. The activity at the Barruecopardo ore deposit developed from 1918 to 1985 and left millions of tons of mineral waste. Around 5 Mm 3 of waste material, occupying a surface of 28·10 4 m 2 , are stocked in the Barruecopardo tailings. A systematic sampling was undertaken in order to obtain 11 samples which are representative of the different areas of the tailing. The wastes were oven-dried at 90 ºC during 24 h; afterwards they were quartered and pulverized to a diameter under 45 µm to obtain homogeneous samples.

Methods
The chemical compositions of raw tailing were determined by X-ray Fluorescence (XRF) using a Sequential X-ray Spectrophotometer (Philips PW 2400). Major elements were analysed in fused pearls (1/20 dilution in lithium tetraborate), two pearls for each sample. Trace elements were determined on pressed powder pellets. The spectrometer is calibrated by a set of more than 60 international standards.
The mineralogy of the tailing samples and the obtained glass-ceramic was determined by X-ray powder diffraction (XRD). The spectra were measured from powdered samples in a Bragg-Brentano PANAnalyticalX'Pert Diffractometer system (graphite monochromator, automatic gap, Kα-radiation of Cu at λ = 1.54061 Å, powered at 45 kV-40 mA, scanning range 4-100 º with a 0.017 º 2θ step scan and a 50s measuring time). Identification and semiquantitative evaluation of phases was made on PANanalyticalX'Pert HighScore software.
The mineral phases of the glass ceramic were observed by scanning electron microscopy (SEM) in order to determine the crystal morphologies and textural indicators of growth. The equipment was an environmental electron microscope ESEM Quanta 200 FEI, XTE 325/D8395 with an energy-dispersive X-Ray spectrometer (EDX) After the evaluation of chemical composition results and considering the homogeneity of the tailing, the composition of a representative sample was used to formulate a glass with the following composition: 57% BP-7 waste, 29 % of CaCO3 (PANREAC, cod.121212) and 14 % of Na2CO3 (PANREAC, cod.131648). The addition of CaO and Na2O had the function of lowering both the melting temperature of the raw materials and the viscosity of the melt.
These additives ensure that the original glass can be produced at temperatures under 1500 ºC. The batch was introduced in an alumina-mullite crucible and placed inside a globular alumina furnace equipped with molybdenum disilicide Super Kanthal® and a Eurotherm® 902 programmer. Heating began up to 450 ºC at 1 ºC min -1 , followed by a second step of 2 ºCmin -1 up to 1450 ºC, with a 2h long isotherm. The melts were cast into a metallic mould preheated to 350 ºC, and annealed near the glass transition temperature (Tg) at 450 ºC during 30 min, followed by free cooling inside the kiln.
Colour glass was defined by measuring the spectral diffuse reflectance according to the CIE, 1931 XYZ colour space (CIE, Comission Internationale de l'Eclairage), which measures colour spaces and calculates the chromatic parameters. These parameters were measured with a Minolta CM-503i spectrophotometer over the visible range (400nm to 700nm wavelength range). The spectrophotometer was fitted with a barium sulphate coated integrating sphere, and a standard illuminant C was used as a light source. A colorimeter is designed to evaluate the colour of a material according to international standards [21]. The tristimulus method is equivalent to the human eye system. It always has the same illuminant and measurements are performed under the same instrumental light source and illumination.
Tg and dilatometric softening temperature (Ts) have been measured and calculated using a Linseis horitzontal dilatometer L76/1550. A powdered sample (diameter under 45 µm) was introduced in a SP5856/3605/10 sample carrier, then placed in a horizontal furnace and heated up to 1000 ºC at a 10 ºCmin -1 . Viscosity at Tg in these conditions has a constant value, independent of composition, of 10 12.3 Pa·s [22].
Experimental viscosity-temperature curves (η-T) have been drawn using the fixed points defined by [23] for hotstage microscopy (HSM) according to rule [24] and Tg (obtained by dilatometry). 3 mm-high test cylinders were conformed using samples powdered under 45 µm and bound using a 1/20 solution of Elvacite® in acetone, conformed in an uniaxial press. Test cylinders were heated at a 5 ºCmin -1 rate from room temperature to 1500 ºC in air atmosphere.
All this process is recorded in pictures with ProgRes Capture Pro software. Picture analysis was performed with Hot-Stage software, developed by the Departament de Llenguatges i Sistemes Informàtics, ETSEIB, UPC [25]. The fixed viscosity points are plotted in a graph and then fit to Vögel-Fulcher-Tammann (VFT, Eq. 1).
Where η is the viscosity and parameters A, B and T0 are determined from iterations of VFT equation.
The chemical composition of the obtained glass was used to calculate the theoretical viscosity-temperature curves using the model defined by Fluegel [26].
Leaching tests in acidic solution according to EPA SW 846 [27] standard have been used to establish the heavy metal inertization efficiency of the glass. The obtained solution has been analysed by inductively coupled plasma mass spectrometry and optical emission source (ICP-MS and ICP-OES).
Thermal evolution of the original glass was obtained by Differential Thermal Analysis and Thermogravimetry (DTA-TG) using a Netzsch equipment (STA 409C model). Analyses were carried in a dry air atmosphere with a flux of at 80mLmin -1 constant flow ratio, using an alumina crucible at a temperature range from 25 to 1300 ºC with a linear temperature gradient set to 10 ºCmin -1 .

Results and discussion
The tailings from Barruecopardo are homogeneous both from the chemical and the mineralogical point of view. Table 1 shows the chemical composition of tailing samples.

Glass
The major components of glass calculated from the composition of the raw materials are SiO2, Al2O3, CaO and Na2O.
The glass network-forming oxides are SiO2, 59.0 mass% and Al2O3 (10.7 mass%), with relatively low contents of SiO2 and high contents of Al2O3, when comparing to a soda-lime silicate glass (  A mathematical model for industrial glasses [26] predicts an evolution of viscosity with temperature which is not in good agreement with the experimental results ( Figure 2). Although at the glass transition (10 12.3 Pa·s) the theoretical curve only differs from the experimental value by a few degrees, the deviation grows quickly with increasing temperature. In the low viscosity range (10 2 -10 4 Pa·s) measured viscosity is more than 300 ºC higher than predicted viscosity. This model is designed for industrial glasses that have more restricted compositions, such as lower Al2O3 and which do not bear trace amounts of potentially toxic elements (the case of BP-7 and most glasses made of wastes) as the combination of these elements influences viscosity in still not completely understood mechanisms.
The obtained significant workability values are the lower annealing point, 10 13 and melting range, 10 1-2 Pa·s > 1556 °C. Table 3 summarizes the temperatures of the upper and lower annealing points,   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65   5 forming interval, interval conditioning and melting range together with their corresponding viscosities [30]. The viscous behaviour of the glass-forming melt at a certain temperature allows casting the melt inside a mould along a fairly wide temperature range, by various processes, such as straining, blowing, stretching, rolling or pressing. Each conformation method will require appropriate thermal conditions in the work area in order to stabilize and maintain its viscosity enough time to ensure casting. The producer must reach a compromise between the viscosity required for each moulding method and the cadence and performance in manufacturing automated systems. In the present case, the range of workability of the glass is between 1190 and 1556 ºC.
The temperature increase in the 10 6 -10 3 Pa·s viscosity range for BP-7 is 326 ºC (Figure 2). Temperature variations under 400 ºC in this interval correspond to the rheological behaviour of so-called short glasses [30], which are suitable for automatic manufacturing. When this difference is larger than 400 ºC, they are called long glasses, and are suitable for manual shaping.
The analysis of leachates shows that the cations have been introduced in the glass structure because the concentrations of potentially toxic elements in the liquid are either very low (Zn, Cr, Ni, Pb, Ba) or directly below the detection limit (As, Se, Cd, W, Ag). These results comply with the requirements stated in EPA SW 846 [27] as the obtained glass has a good chemical durability (with a negligible leaching).
DTA obtained from annealed original glass powders is shown in Figure 3. The formation crystalline phases is represented by the occurrence of a single exothermal event. In this study, a small exothermic peak at 875 ºC and another, more prominent, at 1022 ºC, correspond to crystallization temperatures (Tc). The last peak is an endothermic event at 1200 ºC attributed to the melting of the system. The glass has been thermally treated in the furnace on the basis of these values in order to make a glass-ceramic and determine which mineral phases are formed during the heating process.
A glass-ceramic was produced by crystallization of the original glass. The first phase crystallized at 875 ºC and the second at 1022 °C. After crystallization, melting occurs at 1175 ºC, which is the temperature of the eutectic of NaAlSiO4-CaSiO3 system.
XRD patterns corresponding to glass treated over Tc show the mineral association produced in the devitrification process, constituted by 47% nepheline (Na,K)AlSiO4 and 53% wollastonite (CaSiO3) as presented in Figure 4. The bump of the diffractogram between 20 and 40 2θ/º evidences the existence of a remaining amorphous phase.

Glass-ceramic
The newly formed phases in thermally treated original glass are shown in Figure 5. The two different morphologies formed, which can be correlated with XRD results, are equidimensional crystals of nepheline and needle-like wollastonite crystals. The amorphous phase deduced in the diffractogram has also been identified by SEM in the central part of thermally treated glass, corresponding to residual glass matrix. In the region observed in Figure 5A, nepheline and wollastonite crystals are immersed in the glassy matrix. In the fully crystalline region, nepheline and wollastonite are intergrown ( Figure 5B, C). The typical fibrous texture of wollastonite is shown in Figure 5D.
In order to further prevent waste generation and accumulation and to limit consumption of certain raw materials, some waste products from other industries could be used as fluxes in the manufacture of this glass. CaO may be obtained from marble dust generated in the quarries of this material during cutting operations. In the case of sodium, the perfect option would be to use a waste product of sodium carbonate process. 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65   6 The Barruecopardo tailing has homogeneous chemical and mineralogical composition. The SiO2 and Al2O3 contents make it suitable to produce glass after the addition of CaO and Na2O as modifying oxides. Therefore, vitrification could be an efficient solution for the valorization and inertization of these wastes, which are enriched in toxic elements such as As, Ba, Cd, Cr, Pb and Zn (concentration over EPA limits for solid wastes).

Conclusions
The glass transition temperature is 644 ºC; the glass production should reach temperatures of at least 1556 ºC (lower limit of the melting range). As these temperatures are too high for an industrial process, the composition of the original glass should be changed to increase the content of modifiers, moving its composition closer to the most basic rocks, and therefore lowering its melting temperatures.
The original glass has a green colour and it could be used with marketable purposes, thus giving an economic value to the residues and at the same time minimizing the environmental problems. Leaching tests of the obtained glasses confirm their capacity to retain potentially toxic elements. The main crystalline phases obtained from the devitrification process of the glass are wollastonite and nepheline.