Introduction

Due to the global rise in population and economic growth, the need for the continuously increasing demand for many strategic and industrial metals that were previously not utilized in large quantities requires urgent attention, specifically, for such versatile raw materials including high-grade scheelite of industrial values whose supplies are important to the human, defense, medicine, high-tech, and emerging innovations. This among others has prompted global attention by governments, industries, academics, and research agencies if the supplies of those versatile raw materials would satisfy the industrial demands or be tedious to obtain in the nearest future [1, 2].

In recent years, the continuous decline of high-grade tungsten ore deposits especially the low-grade tungsten ores such as wolframite, scheelite, and stolzite ores with an appreciable amount of tin has attracted the modern researchers [3, 4]. Tungsten, discovered by a Swedish chemist, Karl Wilhelm Scheele in 1781 [5], also referred to as wolfram, along with other transition metals such as chromium and molybdenum has five isotopes (W-180, W-182, W-183, W-184, and W-186) with the relative abundance of 30.64% W-184, 28.43% W-186, and 26.49% W-182, respectively [6].

However, tungsten as a strategic and industrial metal due to its excellent properties such as corrosion resistance, hardness, high modulus of elasticity, highest melting point, armaments, and tensile strength as well as vapor pressure makes its wide uses possible as a catalyst, cemented carbide, alloys and steels, lighting technology, electronics, X-ray tubes, and photo/electro-chromic usages [7,8,9,10]. According to the United States Geological Survey (USGS) statistical data in 2018, the world tungsten reserves including scheelite, wolframite, and its varieties are estimated to be around 3.3 million tons [11]. In Nigeria, for example, the wolframite ore deposits contain admixtures of scheelite and stolzite, which could be easily treated and beneficiated to obtain high-grade tungsten compounds of industrial utilities [7]. Nowadays, scheelite is the most common tungsten compounds with varied colors ranging from pale white to brown-orange, though dominated with blue coloration under UV, and often changes with increasing molybdenum content [7, 12].

For obtaining pure and high-grade scheelite products for industrial and manufacturing purposes, leaching techniques involving the ore in alkaline media such as sodium carbonate or sodium hydroxide by autoclaving [13,14,15,16] or acidic media such as hydrochloric acid [17, 18] or sulfuric acid [19, 20] to give tungstic acid (H2WO4) are mostly employed. Depending on the source of tungsten ores (wolframite, scheelite, or stolzite), various extraction routes are often used for purifying the leach liquor that arose from the leaching operations. For instance, the traditional processes of scheelite in acidic media include three underlisted ‘win–win’ routes, Eqs. (1–3) [6]:

  1. (i)

    Dissolution and leaching by hydrochloric acid or sulfuric acid to separate calcium as summarized in Eqs. (1) and (2):

    $$ {\text{CaWO}}_{{4\left( {\text{s}} \right)}} + 2{\text{HCl}}_{{\left( {{\text{aq}}} \right)}} \to {\text{H}}_{2} {\text{WO}}_{{4\left( {\text{s}} \right)}} + {\text{CaCl}}_{{2\left( {{\text{aq}}} \right)}} $$
    (1)
    $$ {\text{CaWO}}_{{4\left( {\text{s}} \right)}} + {\text{H}}_{2} {\text{SO}}_{{4\left( {{\text{aq}}} \right)}} \to {\text{H}}_{2} {\text{WO}}_{{4\left( {\text{s}} \right)}} + {\text{CaSO}}_{{4\left( {{\text{aq}}} \right)}} $$
    (2)
  2. (ii)

    Extraction and purification of tungsten compound by ammonia solution as shown by the relation (3):

    $$ {\text{H}}_{2} {\text{WO}}_{{4\left( {\text{s}} \right)}} + 2{\text{NH}}_{4} {\text{OH}}_{{\left( {{\text{aq}}} \right)}} \to ({\text{NH}}_{4} )_{2} {\text{WO}}_{{4\left( {{\text{aq}}} \right)}} + 2{\text{H}}_{2} {\text{O}}_{{\left( {{\text{aq}}} \right)}} ,\;{\text{and}} $$
    (3)
  3. (iii)

    Evaporation and crystallization

The reaction mechanism of scheelite in alkaline media such as sodium hydroxide is represented by the following relation (4):

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

However, the scheelite ores treated with an alkaline solution appeared to be uneconomical due to the large volume of leachant and waste resources and often give rise to environmental pollution as a result of the release of toxic gases coupled with high intensive energy requirement during operation [21]. Moreover, the liquid waste emanated by these routes requires sophisticated equipment for processing the waste to meet the World Health Organization (WHO) Wastewater Discharge Standard [22].

Hence, to overcome the aforementioned setbacks, hydrochloric acid, owing to its cheapness, simple equipment design operation, and eco-friendly considerations, the treatment of indigenous scheelite mineral was examined in this study.

Materials and Method

Materials

The scheelite mineral used for this study was obtained from the Owa-Kajola axis, Ifelodun Local Government Area of Kwara State, Nigeria. The raw sample was crushed, ground, and sieved into three particle sizes: < 75 µm, 75 µm, and 90 µm, respectively. The smallest particle size (< 75 µm) assumed to have a larger surface area unless otherwise stated was used throughout the experiment. The hydrochloric acid (HCl) leachant used was of analytical grade and doubly distilled water was used in the preparation of all aqueous solutions.

Leaching Procedure

The leaching tests were performed in a 600 mL Pyrex leaching reactor equipped with a magnetic stirrer and a thermostat to control the reaction temperature. The hydrochloric acid solution at predetermined concentrations (0.1–2.5 mol/L) was added into the beaker and then heated to a preset temperature at different leaching times (5–120 min). This was followed by adding 10 g/L of the < 75 µm scheelite smallest particle size to the reactor and the contents were thoroughly stirred. After a specific duration, the resultant solution was filtered, water-washed, and oven-dried at 40 °C for 24 h. The fraction of ore reacted in the leachant was calculated from the difference between the mass of the reacted and unreacted at various contact times after respective drying [7, 23, 24]. For a better understanding of the nature of the unreacted species, the residual product at optimal leaching conditions (2.5 mol/L HCl, 70 °C, < 75 µm, 120 min) was accordingly analyzed by X-ray diffraction (XRD) using PHILIP PW 1800 X-ray diffraction with Cu Kα1 (0.154 nm) radiation graduated at 40 kV and 55 mA, Scanning electron microscopy (SEM) with NanoSEM 230, and Energy dispersive spectroscopy (EDS) with an oxford X-max EDS detector using INCA software, respectively. Also, the dissolution kinetics mechanism for the scheelite ore dissolution was determined by appropriate shrinking core models (SCM) [7, 23, 25].

The elemental composition of the major components of a Nigerian sourced scheelite ore examined by Skyray EDXRF 3600B Energy Dispersive X-ray fluorescence is summarized in Table 1.

Table 1 Chemical composition of raw scheelite by EDXRF
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The mineralogical purity by XRD analysis affirmed that the raw ore was made up of admixtures scheelite (Ca4.00W4.00O16.00: 96-900-9627) and quartz (Si6.00O6.00: 96-900-5019) as the major attributed peaks as shown in Fig. 1.

Fig. 1
figure 1

X-ray diffraction of raw scheelite mineral

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However, the surface morphological of the scheelite ore under investigation by FEI Nova NanoSEM 230 equipment (Fig. 2) affirmed that tungsten-bearing scheelite is majorly associated with quartz impurities coupled with a polishing surface of the metallic phase.

Fig. 2
figure 2

SEM images of raw scheelite ore associated with quartz impurities (sch scheelite, qtz quartz)

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Results and Discussion

Leaching Investigation

Effect of Hydrochloric Acid Concentration

The influence of hydrochloric acid concentration varying from 0.1 to 2.5 mol/L at a reaction temperature of 50 °C and a particle fraction of < 75 µm on the dissolution efficiency of scheelite ore was examined. The results in Fig. 3 showed an increase in scheelite dissolution reaction with increasing hydrochloric acid concentration at various leaching times (Fig. 3).

Fig. 3
figure 3

Effect of hydrochloric acid concentration on the dissolution rate of the scheelite ore. Experimental conditions: [HCl] = 0.1–2.5 mol/L; reaction temperature = 50 °C; particle size =  < 75 µm

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For example, at 2 h of contact time, the dissolution reaction yielded 38.4% using 2.5 mol/L HCl solution, whereas the dissolution rate was 16.1% with 0.1 mol/L HCl solutions. This affirms that the concentration of hydrochloric acid has a considerable effect on the dissolution of scheelite ore which can be attributed to the presence of hydrogen [H+] ion from hydrochloric acid solution [7].

Effect of Reaction Temperature

The influence of reaction temperature on the leaching reaction was investigated at 27–70 °C for the < 75 µm particle size by 2.5 mol/L HCl solution as depicted in Fig. 4.

Fig. 4
figure 4

Effect of reaction temperature on the dissolution rate of the scheelite ore. Experimental conditions: [HCl] = 2.5 mol/L; reaction temperature = 27–70 °C; particle size =  < 75 µm

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The result in Fig. 4 shows that the reaction temperature had a spontaneous effect on the scheelite ore dissolution rate. At 70 °C, 88.5% of scheelite ore reacted within 120 min. However, at 27 °C, the dissolution efficiency decreased to 27.1% for 120 min and thereby confirmed that the reaction temperature significantly influences the possible complexation ability of hydrochloric acid [26]. Thus, the higher the reaction temperature, the faster the dissolution reaction.

Effect of Particle Size

The result on the influence of the particle fractions on the dissolution efficiency was investigated using the three particle fractions: < 75 µm, 75 µm, and 90 µm by 2.5 mol/L HCl solution and 70 °C reaction temperature as summarized in Fig. 5.

Fig. 5
figure 5

Effect of particle size on the dissolution rate of the scheelite ore. Experimental conditions: [HCl] = 2.5 mol/L; reaction temperature = 70 °C; particle size =  < 75–90 µm

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From Fig. 5, it is evident that the smaller ore particle diameter gave a faster dissolution rate. For instance, almost 89% of the scheelite reacted using the < 75 µm smallest particle diameter within 120 min, while the largest ore particle diameter gave 59.3% dissolution efficiency at optimal conditions. Thus, the finer the particle of the scheelite ore, the higher the dissolution rate, as the finer particle exhibited possible high surface properties.

Discussion

Dissolution Kinetic Model

For a better understanding of the reaction mechanism exhibited by using scheelite ore with hydrochloric acid leachant in this study, it is mandatory to investigate its dissolution kinetic models using appropriate shrinking core models. However, two models vis-à-vis chemical reaction control and internal diffusion control equation models according to Eqs. (5) and (6) were employed [27, 28]:

$$ 1 - \left( {1 - \alpha } \right)^{1/3} = kt\quad {\text{chemical control}} $$
(5)
$$ 1 - \frac{2}{3}\alpha - \left( {1 - \alpha } \right)^{2/3} = kt\quad {\text{diffusion control}}, $$
(6)

where k is the specific rate constant (min−1); t is the leaching time (min.); and α is the amount of fraction of the scheelite ore reacted at different leachant times, t (min). Consequently, the experimental data obtained from Figs. 3 and 4 were appropriately subjected to Eqs. (5) and (6) to determine the reaction order and activation energy for kinetic assessment through relevant Arrhenius plots. During the preliminary trials with the two appropriate models, it was found that only shrinking core model Eq. (6) fitted perfectly the dissolution data with the average correlation of R2 = 0.978 > R2 = 0.585 using Eq. (5) model. Hence, the diffusion equation model (6) was used in the treatment of dissolution results in this study. For example, the dissolution data in Fig. 3 was treated with Eq. (6) to obtain the graph shown in Fig. 6.

Fig. 6
figure 6

A graph of \({1-\frac{2}{3}\alpha -(1-\alpha )}^{2/3}\) versus contact time at different acid concentrations. Conditions: Same as in Fig. 3

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The experimental rate constant, k, was estimated from Fig. 7 and the plot of ln k versus ln [HCl] gave the reaction order of 0.96, assumed to be the first-order relation.

Fig. 7
figure 7

A graph of ln k versus ln [HCl]. Conditions: Same as in Fig. 3

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However, the experimental data obtained from Fig. 6 were also substituted into Eq. (6) to obtain the plot affirmed in Fig. 8.

Fig. 8
figure 8

A plot of \({1-\frac{2}{3}\alpha -(1-\alpha )}^{2/3}\) versus contact time at different reaction temperatures. Conditions: Same as in Fig. 4

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Also, the temperature dependence of the specific rate constant can be estimated from the Arrhenius expression:

$$ k = A^{{e - E_{{\text{a}}} /RT}} , $$
(7)

where A is the frequency factor; k is the specific rate constant; Ea is the activation energy (J/mol); R is the universal gas constant (8.314 J/mol/K); and T is the absolute temperature (K).

Hence, Eq. (7) was linearized to obtain Eq. (8):

$$ \ln k = \ln A - \frac{{E_{a} }}{RT}. $$
(8)

The activation energy, Ea, for the leaching reaction estimated from the slope of Fig. 9 gave 22.94 kJ/mol (< 40 kJ/mol), supporting the diffusion-controlled reaction mechanism as the rate-determining step for the dissolution reaction, consistent with the following stoichiometry [29,30,31]:

$$ {\text{CaWO}}_{{4\left( {\text{s}} \right)}} + 2{\text{HCl}}_{{\left( {{\text{aq}}} \right)}} n{\text{H}}_{2} {\text{O}} \to {\text{CaCl}}_{2} .n{\text{H}}_{2} {\text{O}}_{{\left( {\text{s}} \right)}} + {\text{H}}_{2} {\text{WO}}_{4} , $$
(9)
Fig. 9
figure 9

A plot of ln k versus 1/T(K−1). Conditions: Same as in Fig. 8

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and found to be kinetically more feasible with less activation energy as compared to other tungsten-bearing minerals from other sources (Table 2). The differences in the activation energies could be due to variation in geographical and environmental factors of the sourced materials.

Table 2 Activation energy feasibilities of tungsten-bearing minerals from different sources
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Residual Product Analysis

The EDS analysis adopted to examine the elemental composition of the residual product at optimal leaching conditions (2.5 mol/L HCl, 70 °C, < 75 µm, 120 min) is depicted in Fig. 10.

Fig. 10
figure 10

EDS spectrum of the residual product at optimal conditions containing 77.59 wt% C, 20.89 wt% O, 0.59 wt% Si, 0.39 wt% K, and 0.54 wt% Ca

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Also, the morphological structure of the residue by SEM at optimal conditions is as shown in Fig. 11.

Fig. 11
figure 11

SEM images of residual product at optimal conditions (a = 2 µm, b = 100 µm; sch scheelite, qtz quartz, chl calcium chloride)

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From Fig. 11, it is clear that the W-dominated region is bright due to the possible high atomic number and the bombardment of the leaching residue starts at the outer surface and then proceeds to the inner layer progressively [34]. However, the undissolved product (18.5%) analyzed by XRD was affirmed to majorly dominated by scheelite (CaWO4: 96-900-7271) with few traces of calcium chloride (CaCl2: 96-900-6902), and could, respectively, serve as valuable materials for some defined indigenous steel and coating industries.

Conclusion

Scheelite mineral has tremendously received the attention of global scientists due to being the major primary source of tungsten. Therefore, the development of effective and efficient routes for extraction and purification of tungsten from scheelite ores is pertinent for producing tungsten compounds of industrial value. Thus, in this study, the effects of leachant concentration, reaction temperature, and particle size on the optimization of scheelite ore dissolution were examined. The results affirmed that leachant concentration and reaction temperature strongly affect the ore dissolution. For instance, 88.5% of the ore reacted at optimal leaching conditions (2.5 mol/L HCl, 70 °C, < 75 µm) within 120 min. The experimental data obtained were subjected to the shrinking core model (SCM), where the diffusion-controlled reaction is the rate-determining step. The apparent activation energy was estimated to be 22.94 kJ/mol and supported the proposed reaction mechanism. Hence, the leaching kinetics via the established low activation energy prepared the indigenous scheelite ore used in this study for defined industrial applications.