Thermodynamics of Tungsten Ores Decomposition Process Options

Abstract

The thermodynamics of tungsten ore decomposition in mineral acid and alkaline solutions were studied. The published thermodynamic data of tungsten minerals were collected and assessed. The Gibbs energies of CaWO₄ (−1538.43 kJ/mol), FeWO₄ (−1053.91 kJ/mol), MnWO₄ (−1206.08 kJ/mol), H₂WO₄ (−1003.92 kJ/mol), and Na₂WO₄ (−1455.58 kJ/mol, aq) at 25 °C were adopted in the calculation using HSC software. The results show that CaWO₄ is decomposed more readily in Na₂CO ₃ solution than in NaOH, while FeWO₄ and MnWO₄ are more reactive in NaOH solutions. From a thermodynamic point of view, tungsten ore decomposes easily in acid solutions despite most $\Delta G_{T}^{o}$ increasing slightly with temperature . Oxidizing Fe²⁺ to Fe³⁺ in acidic solutions facilitates decomposition of FeWO₄, and the reaction of CaWO₄ in H₂SO₄ solution occurs more easily than in other mineral acids, due to the formation of sparsely soluble CaSO₄. The results fit well with the experimental data and industrial experience previously reported in the literature.

Introduction

Tungsten , a strategic and rare metal, has a wide range of industrial applications, such as hard materials, high temperature technology, lighting, X-ray technology, machine and engine construction, chemical industry, laser technology, armaments and aerospace industry. The key properties are low vapor pressure and thermal expansion , high density and melting point, good thermal and electrical conductivity, and high modulus of elasticity [1, 2]. Due to the lack of viable substitutes, the European Union has included tungsten in the list of critical raw materials [3]. Similarly, the British Geological Survey ranked it as number two in a supply list containing forty one critical elements [4]. The main tungsten minerals in nature are scheelite (CaWO₄) and wolframite ((Fe,Mn)WO₄), with approximately two-thirds of the world tungsten reserves consisting of scheelite [1, 2]. China has more than 60% of tungsten reserves in the world, and accounts for about 80% of the world tungsten production [5]. Ammonium paratungstate (APT ), as the main intermediate in industrial tungsten production, is usually obtained from tungsten concentrates by a caustic/soda or hydrochloric acid treatment [6].

In modern industrial production technologies, caustic digestion is employed to treat wolframite producing soluble sodium tungstate and insoluble iron or manganese hydroxide at either atmospheric pressure (75–145 °C), in concentrated NaOH solutions (≥10 mol/L) [7], or under high pressures (135–255 °C), in a dilute NaOH solution (1.25–3.75 mol/L) [8]. When tungsten concentrate contains scheelite , Na₂CO ₃ is added in NaOH solution to extract tungsten in an autoclave operating under elevated pressures (180–310 °C) [9]. Subsequently, solvent extraction or ion exchange is used to convert Na₂WO₄ to (NH₄)₂WO₄, which requires excess acid to treat the Na₂WO₄ solution and a large amount of water to dilute the leach solution [10]. Obviously, the caustic/soda leaching method needs a large excess of reagents, and discharges huge volumes of high-salinity wastewater [11]. Furthermore, Leal–Ayala et al. [12] pointed out that 4% of tungsten would be lost in the APT production.

Acid processing, as a classical method, can avoid the use of high temperatures and a large excess of leaching reagents, and treating or even discharging large amounts of sodium salts solutions. Hydrochloric acid , nitric acid, or sulfuric acid is used to decompose tungsten ores and produce insoluble tungstic acid. However, a solid or colloidal H₂WO₄ layer was observed covering the surfaces of the unreacted tungsten particles, thus hindering the reaction rate and yield. Therefore, some strategies are usually adopted to increase the feasibility, such as reducing particle size [13], using a large excess of concentrated HCl [14], utilizing heated ball mill reactors [15], and employing organic acids to dissolve solid tungstic acid [16].

Additionally, many researchers have focused on discomposing tungsten concentrates to produce soluble tungsten compounds. Martins leached synthetic scheelite in hydrochloric or nitric acid at pH 1.5–3.0 in a temperature range of about 25–100 °C to obtain soluble metatungstates [17, 18]. Potashnikov et al. [19] and Kalpakli et al. [20] treated scheelite by oxalic acid to form soluble hydrogen aqua oxalate tungstate (H₂[WO₃(C₂O₄)H₂O]). Hydrochloric, nitric or sulfuric acid in the presence of phosphorus was found to decompose scheelite and form a water–soluble phosphotungstic acid chelate compound (12-tungstophosphoric heteropoly acid, H₃PW₁₂O₄₀) [21–24]. In previous work, we systematically studied the conversion of scheelite and mixed scheelite -wolframite concentrate in sulfuric acid , and found that the tungsten concentrates can be completely converted in H₂SO₄ solutions by adding an oxidizing agent and controlling the H₂WO₄ formation in moderate acid concentrations [25, 26].

Even though tungsten extraction technology has been developed for more than a hundred years since R. Oxiland in 1874 [1], there is limited thermodynamic information on the tungsten leaching chemistry. Osseo-Asare [27] applied the Pourbaix diagram concept to tungsten hydrometallurgy , but stated a lack of sufficient thermodynamic data. Martins [28] determined the equilibrium constants for wolframite and scheelite leaching systems, and also pointed out some inherent limitations due to the estimation of selected thermodynamic data. Zhao and Li [29] used a pseudo-ternary-system phase diagram to explain scheelite digestion in high concentration NaOH and Na₂CO ₃ solutions.

Therefore, this paper focuses on collecting and assessing the thermodynamic data of tungsten minerals previously published. Gibbs free energy changes versus temperature were calculated with HSC software for tungsten minerals decomposing in mineral acid and alkaline solutions, using the adopted thermodynamic data. The calculations were finally compared with the experimental and industrial results previously reported in the literature. This work is beneficial to better understanding of various technologies in industrial tungsten processing.

Thermodynamics

Thermodynamic Equation

For a chemical reaction, the Gibbs standard free energy as a function of temperature can be calculated by following equations:

$\Delta G_{T}^{o} = \Delta H_{T}^{o} - T \times\Delta S_{T}^{o}$

(1)

$\Delta H_{T}^{o} = \Delta H_{298}^{o} + \int_{298}^{T} {\Delta C_{p} dT}$

(2)

$\Delta S_{T}^{o} = \Delta S_{298}^{o} + \int_{298}^{T} {\frac{{\Delta C_{p} }}{T}dT}$

(3)

$\Delta G_{T}^{o} = \Delta G_{298}^{o} - \left( {T - 298} \right)\Delta S_{298}^{o} + \int_{298}^{T} {\Delta C_{P} dT - T} \int_{298}^{T} {\frac{{\Delta C_{P} }}{T}dT}$

(4)

where, $\Delta G_{T}^{o}$ is the standard Gibbs energy of reaction at temperature T, (kJ mol⁻¹); $\Delta H_{T}^{o}$ the standard enthalpy of reaction at temperature T, (kJ mol⁻¹); $\Delta S_{T}^{o}$ the standard entropy of reaction at temperature T, (J mol⁻¹); $\Delta C_{P}$ the molar heat capacity of reaction at constant pressure at temperature T, (J mol⁻¹ K⁻¹); T is the temperature (K).

In order to calculate more accurate $\Delta G_{T}^{o}$ , the estimation of the heat capacity term $\Delta C_{P}$ plays a critical role. Barner and Kust [30] has assessed the various approaches for estimating $\Delta C_{P}$ . The molar heat capacity $C_{P}$ of each substance relating to temperature T was used in the calculation, as shown in empirical Eq. (5).

$C_{P} = {\text{A}} + {\text{BT}}\cdot10^{ - 3} + CT^{ - 2} \cdot10^{5} + DT^{2} \cdot10^{ - 6}$

(5)

The general equation describing the temperature dependence of equilibrium constant can be derived from Eq. (6). The determination of equilibrium constants is dependent on the availability of the relevant thermodynamic data, and this knowledge is essential for understanding the leaching processes, especially when taking the substance concentration into account.

${ \ln }\,{\text{K}} = \frac{{ -\Delta G_{T}^{o} }}{RT}$

(6)

Thermodynamic Data

Standard–state thermochemical properties are prerequisites for the modeling of chemical reactions. Various thermodynamic properties of tungsten compounds have been reported. However, there is no assessment of the thermodynamic data available. So the primary target is to select the reliable data for tungsten compounds.

A number of values of $\Delta H_{f}^{o}$ have been reported for scheelite (CaWO₄) at 298 K. The proposed value changes in the range of −1623.00 to −1683.70 kJ mol⁻¹, as reviewed by Wood and Samon [31], and the adopted value is −1645.1 kJ mol⁻¹. This value is widely accepted by Lange’s Handbook of Chemistry [32], HSC software [33], and last by Martins [28]. King and Weller [34] determined the heat capacity of scheelite at low temperatures (up to 298.15 K), and from this they derived a third law entropy, $S_{298}^{o}$ = 126.4 J K⁻¹ mole⁻¹, which has been adopted in all major compilations of thermodynamic data on scheelite . Combined with the heat capacity of scheelite (298–1000 K) from Barin and Knacke [35], the parameters for temperature dependence of $C_{P}$ are obtained. The values of $\Delta H_{298}^{o}$ and $S_{298}^{o}$ result in $\Delta G_{298}^{o}$ = −1538.43 kJ mole⁻¹. The thermodynamic data adopted for scheelite are shown in Table 1.

Table 1

Adopted thermodynamic data for tungsten compounds

Compound	$\Delta H_{298}^{o}$	$\Delta G_{298}^{o}$	$S_{298}^{o}$	$C_{P} = {\text{A}} + {\text{BT}}\cdot10^{ - 3} + CT^{ - 2} \cdot10^{5} + DT^{2} \cdot10^{ - 6}$
Compound	$\Delta H_{298}^{o}$	$\Delta G_{298}^{o}$	$S_{298}^{o}$	A	B	C	D
CaWO₄	−1645.10	−1538.43	126.4	134.56	20.67	−24.4	0
FeWO₄	−1154.80	−1053.91	131.8	132.42	29.75	−23.93	0
MnWO₄	−1035.83	−1206.08	127.4	120.68	36.51	−3.69	0
H₂WO₄	−1131.77	−1003.92	112.6	13.25	350.6	10.1	−186
Na₂WO₄(aq)	−1554.02	−1455.58	160.3	105.41	118	0	0

$\Delta H_{298}^{o}$ and $\Delta G_{298}^{o}$ , kJ mol⁻¹; $S_{298}^{o}$ and $C_{P}$ , J mol⁻¹ K⁻¹

Similarly to scheelite , the values of $\Delta H_{f}^{o}$ reported for ferberite (FeWO₄) at 298 K have been also reviewed by Wood and Samon [31]. As with scheelite , the standard entropy of ferberite, $S_{298}^{o}$ = 131.8 J K⁻¹ mole⁻¹, was calculated from the low-temperature calorimetric measurements of heat capacity by Weller [36]. This value was adopted in this research. Combined with the heat capacity of ferberite (5–500 K), measured by Lyon and Westrum [37] using adiabatic calorimetry, the parameters for temperature dependence of $C_{P}$ were fitted by HSC software. However, there are big differences in the values of $\Delta H_{298}^{o}$ and $\Delta G_{298}^{o}$ for ferberite. The more negative values are in the range −1184 to −1192 kJ mol⁻¹ and −1085 to −1089 kJ mol⁻¹, while the opposed higher values range from −1154 to −1158 kJ mol⁻¹ and −1054 to −1057 kJ mol⁻¹ [31]. We favor the higher values because thermodynamic calculation result show an accordant trend with the experimental results [26], while the more negative values seem more suited to pyrometallurgical calculations. So we accept, respectively, −1154.8 and −1053.91 kJ mol⁻¹ for $\Delta H_{298}^{o}$ and $\Delta G_{298}^{o}$ of ferberite in this paper, which was also accepted in the recent research of Martins [28] and Lange’s Handbook of Chemistry [32].

There are limited thermodynamic data available for huebnerite (MnWO₄). In the HSC Database, the $\Delta H_{298}^{o}$ , $\Delta G_{298}^{o}$ , and $S_{298}^{o}$ for huebnerite are −1305.83 kJ mol⁻¹, −1053.08 kJ mol⁻¹, and 140.58 J K⁻¹ mole⁻¹, respectively. These data, together with the parameters for $C_{P}$ , originate from Barin and Knacke [35], and they are very similar to the values adopted by Horner [38].

The thermodynamic value for H₂WO₄, $\Delta G_{298}^{o}$ , is in the range of −1003.9 to −1006.9 kJ mol⁻¹ except for the estimate by Osseo-Asare [27], which has been compared by Martins [28]. The value in the HSC Database is −1003.92 kJ mol⁻¹, similar to Lange’s Handbook of Chemistry [32]. This was adopted in the calculations together with the other data.

The $\Delta G_{298}^{o}$ of Na₂WO₄ in aqueous solution was reviewed by Martins [28], ending up with a value of about −1455 kJ mol⁻¹. This value is also available in the HSC Database. As for CaWO₄ and FeWO₄, the entropy, $S_{298}^{o}$ = ~160.3 J K⁻¹ mole⁻¹, was calculated from low-temperature calorimetric measurements of heat capacity by King and Weller [34]. Combined with the heat capacity in Barin and Knacke [35], the parameters for temperature dependence of $C_{P}$ were fitted at temperatures of 275–800 K (see Table 1).

The adopted thermodynamic data for tungsten compounds are presented in Table 1. The other data for substances used in the calculation were taken from the HSC Database. They are close to the values adopted by Martins [28], except the $C_{P}$ parameters.

Thermodynamic Results

Tungsten Ores Decomposing in NaOH and Na₂CO ₃ Solutions

The decompositions of tungsten ores (CaWO₄, FeWO₄, and MnWO₄) in NaOH and Na₂CO ₃ solutions are expressed with Eqs. (7)–(12). The standard Gibbs energy changes versus temperature for these reactions are depicted in Figs. 1 and 2 for sodium hydroxide and carbonate solutions, respectively.

../images/468727_1_En_206_Chapter/468727_1_En_206_Fig1_HTML.gif — Fig. 1
Standard Gibbs energy changes versus temperature of tungsten ores decomposing in NaOH solutions

../images/468727_1_En_206_Chapter/468727_1_En_206_Fig2_HTML.gif — Fig. 2
Standard Gibbs energy changes versus temperature of tungsten ores decomposing in Na₂CO ₃ solutions

In NaOH solutions

${\text{CaWO}}_{4} \left( {\text{s}} \right) + 2{\text{NaOH}}\left( {\text{aq}} \right) \rightleftharpoons {\text{Ca}}\left( {\text{OH}} \right)_{2} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{aq}} \right)$

(7)

${\text{FeWO}}_{4} \left( {\text{s}} \right) + 2{\text{NaOH}}\left( {\text{aq}} \right) \rightleftharpoons {\text{Fe}}\left( {\text{OH}} \right)_{2} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{aq}} \right)$

(8)

${\text{MnWO}}_{4} \left( {\text{s}} \right) + 2{\text{NaOH}}\left( {\text{aq}} \right) \rightleftharpoons {\text{Mn}}\left( {\text{OH}} \right)_{2} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{aq}} \right)$

(9)

In Na ₂ CO ₃ solutions

${\text{CaWO}}_{4} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{CO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{CaCO}}_{3} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{aq}} \right)$

(10)

${\text{FeWO}}_{4} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{CO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{FeCO}}_{3} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{aq}} \right)$

(11)

${\text{MnWO}}_{4} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{CO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{MnCO}}_{3} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{aq}} \right)$

(12)

The results in Fig. 1 indicate that raising temperature favors the decomposition of tungsten ores in NaOH solution thermodynamically, especially for CaWO₄. The standard Gibbs energy change of CaWO₄ becomes negative at the temperature of above about 220 °C, while for FeWO₄ and MnWO₄ it decreases slightly with increasing temperature but is negative over the whole temperature range. The results indicate that the decomposition of CaWO₄ in NaOH solutions should be done at elevated temperatures.

When these tungsten compounds are decomposed in Na₂CO ₃ solutions, as shown in Fig. 2, the trend is similar to that in NaOH solutions. However, the temperature , at which the standard Gibbs energy change of CaWO₄ decomposition in Na₂CO ₃ solutions becomes negative, significantly decreases to ~75 °C, 80 °C, and 90 °C when producing calcite, aragonite, and vaterite, respectively. The standard Gibbs energy changes of FeWO₄ and MnWO₄ decomposition in Na₂CO ₃ solutions are more positive than in NaOH solutions at the same temperature .

It should be noted that the equilibrium constants of CaWO₄ decomposition in NaOH and Na₂CO ₃ solutions are positive at temperatures above 50 °C and 25 °C, respectively, as reported by reference [28]. These results differ from the above calculation according to Eq. (6). Wan et al. [39] synthesized scheelite by adding Ca(OH)₂ into Na₂WO₄ solution in a temperature range of 20–110 °C. However, Zhao et al. [40] studied the kinetics of sodium hydroxide leaching of scheelite at 60–100 °C. The results seem to be in disagreement. Considering that Zhao et al. [40] used NaOH concentrations in the range of 8–16 mol/L, these conditions cannot be assumed to be ideal as adopted in thermodynamic calculation . Kinetic theory can be used to explain those results. Chen et al. [41] extracted tungsten mechanochemically using caustic processing of scheelite according to reaction (13), and found a reverse reaction when washing the products in water. So NaOH concentration plays a key role in CaWO₄ decomposition. This is evidence that our thermodynamic calculation is reliable for CaWO₄ decomposing in NaOH solutions.

${\text{CaWO}}_{4} \left( {\text{s}} \right) + 2{\text{NaOH}}\left( {\text{s}} \right) \rightleftharpoons {\text{Ca}}\left( {\text{OH}} \right)_{2} \left( {\text{s}} \right) + {\text{Na}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)\left( {\Delta G_{298}^{o} = {-}30\,{\text{kJ}} \cdot \text{mol}^{{{-}1}} } \right)$

(13)

Furthermore, the stability of the products seems to influence the thermodynamic reaction sequence based on Figs. 1 and 2, especially for scheelite which has a very small equilibrium constant. The solubility product K_sp values at 298 K for Ca(OH)₂, CaCO₃, Fe(OH)₂, FeCO₃, Mn(OH)₂, and MnCO₃ are 10^−4.38, 10^−7.91 (calcite), 10^−8.22 (aragonite), and 10^−8.47 (vaterite), 10^−16.31, 10^−10.5, 10^−12.72 and 10^−10.63, respectively [32, 42], which is in accordance with thermodynamic calculation sequence. Chen et al. [41] used a 0.1 M Na₂CO ₃ solution washing the compositions of Na₂WO₄ and Ca(OH)₂ by forming more stable CaCO₃ and added Al(OH)₃ to the co –grinding of CaWO₄ and NaOH for producing more stable katoite [Ca₃Al₂(OH)₁₂] and in order to avoid the reverse reaction. All these observations verify the above analysis.

Tungsten Ores Decomposing in Mineral Acid

The decompositions of tungsten ore (CaWO₄, FeWO₄, and MnWO₄) in HCl, HNO₃, and H₂SO₄ solutions occur according to reactions (14)–(23). The oxidization of Fe²⁺ to Fe³⁺ is also taken into account below, due to the strong oxidation power of HNO₃ solutions.

In HCl solutions

${\text{CaWO}}_{4} \left( {\text{s}} \right) + 2{\text{HCl}}\left( {\text{aq}} \right) \rightleftharpoons {\text{CaCl}}_{2} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(14)

${\text{FeWO}}_{4} \left( {\text{s}} \right) + 2{\text{HCl}}\left( {\text{aq}} \right) \rightleftharpoons {\text{FeCl}}_{2} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(15)

${\text{MnWO}}_{4} \left( {\text{s}} \right) + 2{\text{HCl}}\left( {\text{aq}} \right) \rightleftharpoons {\text{MnCl}}_{2} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(16)

In HNO ₃ solutions

${\text{CaWO}}_{4} \left( {\text{s}} \right) + 2{\text{HNO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{Ca}}\left( {{\text{NO}}_{3} } \right)_{2} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(17)

${\text{FeWO}}_{4} \left( {\text{s}} \right) + 2{\text{HNO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{Fe}}\left( {{\text{NO}}_{3} } \right)_{2} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(18)

${\text{MnWO}}_{4} \left( {\text{s}} \right) + 2{\text{HNO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{Mn}}\left( {{\text{NO}}_{3} } \right)_{2} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(19)

${\text{FeWO}}_{4} \left( {\text{s}} \right) + 4{\text{HNO}}_{3} \left( {\text{aq}} \right) \rightleftharpoons {\text{Fe}}\left( {{\text{NO}}_{3} } \right)_{3} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right) + {\text{NO}}_{2} \left( {\text{g}} \right) + {\text{H}}_{2} {\text{O}}\left( {\text{aq}} \right)$

(20)

In H ₂ SO ₄ solutions

${\text{CaWO}}_{4} \left( {\text{s}} \right) + {\text{H}}_{2} {\text{SO}}_{4} \left( {\text{aq}} \right) \rightleftharpoons {\text{CaSO}}_{4} \left( {\text{s}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(21)

${\text{FeWO}}_{4} \left( {\text{s}} \right) + {\text{H}}_{2} {\text{SO}}_{4} \left( {\text{aq}} \right) \rightleftharpoons {\text{FeSO}}_{4} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(22)

${\text{MnWO}}_{4} \left( {\text{s}} \right) + {\text{H}}_{2} {\text{SO}}_{4} \left( {\text{aq}} \right) \rightleftharpoons {\text{MnSO}}_{4} \left( {\text{aq}} \right) + {\text{H}}_{2} {\text{WO}}_{4} \left( {\text{s}} \right)$

(23)

As seen in Fig. 3, raising temperature does not favor the tungsten ore decomposition in HCl solutions thermodynamically. The standard Gibbs energy changes of CaWO₄, MnWO₄, and FeWO₄ decomposition increase slightly, but keep a relatively negative value with increasing temperature . In addition, the preferential conversion order or driving force of reaction is FeWO₄ > MnWO₄ > CaWO₄ at <100 °C, from a thermodynamic point of view.

../images/468727_1_En_206_Chapter/468727_1_En_206_Fig3_HTML.gif — Fig. 3
Standard Gibbs energy changes versus temperature for tungsten ore decomposition in HCl solutions

The relationships of the standard Gibbs energy changes of CaWO₄, MnWO₄, and FeWO₄ decomposition in HNO₃ solutions versus temperature are depicted in Fig. 4, showing similar trends as in HCl solutions. Considering the oxidation of ferrous to ferric by HNO₃, the standard Gibbs energy change of FeWO₄ decreases significantly in HNO₃ solution. This implies that the equilibrium end product in the solution has an obvious influence on the decomposition.

../images/468727_1_En_206_Chapter/468727_1_En_206_Fig4_HTML.gif — Fig. 4
Standard Gibbs energy changes versus temperature of tungsten ore decomposition in HNO₃ solutions

Figure 5 presents the standard Gibbs energy changes versus temperature for the decomposition of CaWO₄, MnWO₄, and FeWO₄ in H₂SO₄ solutions. The trends for FeWO₄ and MnWO₄ are similar to those in HCl and HNO₃ solutions. The standard Gibbs energy change of CaWO₄ notably decreases in H₂SO₄ solutions. The effect may be increased by the lower concentration of Ca²⁺ compared to Fe²⁺ and Mn²⁺ in the leaching solutions, due to the formation of sparingly soluble CaSO₄ (K_sp = 10^−4.38). The solubility of CaSO₄ decreases with increasing temperature [43, 44], which further explains this trend reversal.

../images/468727_1_En_206_Chapter/468727_1_En_206_Fig5_HTML.gif — Fig. 5
Standard Gibbs energy changes versus temperature of tungsten ore decomposition in H₂SO₄ solutions

Our previous research [26] showed that increasing Fe²⁺ in the solution hinders the FeWO₄ decomposition, while oxidization of Fe²⁺ to Fe³⁺ or/and renewing the acid solution by a two–stage decomposition favors FeWO₄ reaction. This explains the impact of oxidization by HNO₃ in Fig. 4 and the difference between scheelite and wolframite decomposition in sulfuric acid in Fig. 5.

The current thermodynamic results give an explanation of wolframite treatment by caustic soda when scheelite is digested by soda in modern industrial APT practices. When tungsten ore is decomposed in acid solutions, a solid H₂WO₄ product layer will form on the surfaces of unreacted tungsten mineral particles. It can be dissolved incorporating actions, such as controlling the acid concentration and adding an oxidizing agent [25, 26, 45]. Due to the absence of thermodynamic data, unfortunately, this paper does not carry out a detailed thermodynamic analysis on tungsten ore decomposition to H₂[WO₃(C₂O₄)H₂O] and H₃PW₁₂O₄₀.

Conclusion

Thermodynamics is fundamental in metallurgical research. It plays an active role in high-temperature metallurgy, but seems to have been left out of hydrometallurgy in recent years. This paper studied the thermodynamics of tungsten ore decomposition, combining the available experimental and industrial experience. The previously published thermodynamic data of tungsten minerals were assessed to obtain a more reliable thermodynamic picture. The results explain why, in modern industrial APT practice, wolframite is usually treated with caustic soda while scheelite is treated with soda ash. Tungsten ores decompose easily in acid solutions from a thermodynamic point of view. The concentration of metal ion influences the decomposition of tungsten ores, which can be explained by kinetics, such as oxidizing to high valence ion, forming insoluble product, and multi-step decomposition. For that purpose, a dilute solution approach is inadequate in strong e.g. caustic or salt solutions, and proper solution models are needed to describe the solution properties accurately [46, 47]. This work is beneficial for further understanding the tungsten minerals decomposition process thermodynamically.

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