1. Introduction

Barium, an alkaline earth metal of group IIA in the periodic table, is found mostly in nature in the forms of barite (BaSO₄) [1] or witherite (BaCO₃) [2], often appearing as large tabular crystals, or divergent plates [3]. Barite solubility is low and has been the subject of many research interests in both geological and chemical processes since the late 18^th century [4]. This has affected the industrial practice of Barium ion extraction for barium compounds production ever since, which has taken place customarily by reducing BaSO₄ in reaction with coal in a process known as ‘black ash’ (in a rotary kiln at 800–1200°C) to form water-soluble BaS (as per reaction (1)), reacting further with suitable acids to produce useful barium salts [5, 6, 7]. Dissolution of barite, on the other hand, could offer a cheaper, cleaner, and more sustainable process with better yield, and in this work, we have tried to study and optimize it, with a view to employing this knowledge in designing a more environmentally sustainable process. But first, important aspects of the common black ash process development are reviewed here to highlight the practical aspects involved.

BaSO_4(s) + 2C_(s) → BaS_(s) + 2CO_(g)                      ΔH°rx  = 208.388 kJ/m                           (1)

In Eq.1, the reduction reaction efficiency depends on the reactant’s ratio [8] and the carbon type used, offering various activation energy [7] and volatile matters [9]. Considering the intense ever-increasing environmental concerns, new improvements are being made to the black ash method in developed countries. However, a considerable proportion of barium salts produced globally originates in developing countries where weak monitoring and less motivation for progress, have made ‘black ash’ the prevalent production method. In practice, reactions 2 and 3 occur [7,10], indicating high energy consumption and polluting carbon dioxide emissions [11].

BaSO_4(s) + 4CO_(g) → BaS_(s) + 4CO_2(g)                                                                                   (2)

C_(s) + CO_2(g) → 2CO_(g)                                                                                                              (3)

The application of other agents such as natural gas, methane, and carbon monoxide has also been suggested to reduce barite in a fluidized bed [12]. However, this has offered few significant improvements in energy and pollution [13]. Even though the barite reduction with hydrogen (see Reaction 4) proposed by some workers [14] is said to circumvent the carbon dioxide emission fully, the method is not cost-effective in terms of energy.

BaSO_4(s) + 4H_2(g) → BaS_(s) + 4H₂O_(g)                                                                                   (4)

In another study [11], sulfur was used as a barite-reducing agent according to reactions 5 and 6 to produce barium sulfide and sulfuric acid. This scheme seems to be capable of addressing the energy savings and emission discount, as the SO₂ gas is turned into a useful common industrial acid and hence, the approach could be considered as a useful contender for replacing the black ash method.

BaSO_4(s) + S_2(g) → BaS_(s) + 2SO_2(g)                                                                                        (5)

S_(s) + O_2(g) → SO_2(g)                                                                                                                   (6)

With around US$ 1.6bn market size, North America remains the largest global market in 2022 for barite [15]. India accounts for 35% of the global barite mine production followed by China at 25%, while Morocco, Kazakhstan, Mexico, USA, Turkey, Iran, and Russia, together account for around 40% in 2022 [16]. The global barite reserves in 2021 were estimated to be around 390 million tons of which Iran with more than a quarter of this, holds the greatest reserve [17]. More than 80% of barite mined globally is used as drilling mud in oil and gas wells, while the rest is used for barium compounds production such as BaSO₄, BaS, BaCO₃, BaCl₂, Ba(NO₃)₂, BaTiO₃, Ba(CH₃COO)₂, BaO, BaF₂, Ba(ClO₃)₂, etc. in a variety of industrial and medical applications including pigment, filler, glass and lenses, advanced electronics and, X-ray examination[18].

Most research on barite solubility published in the literature is related to oil and gas industries where insolubility of barite presents substantial challenges. Many workers have focused on evaluating barite dissolution in different Aminopolycarboxylic acids as chelating agents including ethylene diamine tetra acetic acid (EDTA), diethylene triamine penta acetic acid (DTPA), and hydroxyethyl ethylene diamine tri acetic (HEDTA) acid, etc., where parameters such as pH, temperature, concentration, catalyst, chelating agents base, presence of oxalate ion, agitation effect, and dissolution time [19,20,21] are investigated. In a recent study, ‘macropa’, a macrocyclic descaling agent was proposed to remove barite, claiming to offer better solubility performance of around 11.4 g/l at pH of 8, as compared to 5 and 8 g/l, for EDTA and DTPA, respectively [22]. However, in general, there are reservations about relying upon such data primarily due to a lack of chemical understanding of the subject. Others have developed thermodynamic solubility models based on the Pitzer ion interaction theory to predict and control barite scale formation in different apparatuses [23-26] whereby barite solubility is estimated over wide ranges of temperature, pressure, and ionic compositions in geochemical and industrial processes in the presence of mixed electrolytes.

Furthermore, barite solubility has been investigated in sulfuric acid by several workers. Trenner and Taylor in 1931 studied the solubility relations for the BaSO₄-H₂SO₄-H₂O system at 25°C and the concentration range of 83-100 wt% acid. The solubility in absolute acid with a density of 1.853 g/cm³ was reported as 15.89 wt%. The system, however, demonstrated two solid phases at 25°C BaSO₄ and probably BaSO₄.H₂SO₄, with a transition at an acid concentration of about 85wt%. Using an accurate, and convenient conductance method, a distinct compound from the solid solution and adsorption was formed [27]. Interestingly the solubility decreased sharply to 0.05 wt% in a solution containing 83% concentrated acid [28]. Considering modern theories of strong electrolytes, this incredible decrease is somewhat irregular whilst realizing that the dielectric constant of absolute sulphuric acid is of the same order as water [27]. In another research, various combinations of acids and peroxide (H₂SO₄, HNO₃, HF, H₂O₂) were employed in an attempt to determine the solubility of barite. However, these endeavors proved unsuccessful in yielding the desired information about barite’s solubility. This outcome arose primarily from the omission of a crucial step involving the dissolution of barite with the subsequent formation of barite as a precipitate. The only exception to this was the dissolution method employing sulfuric acid, which effectively and completely dissolved the barite sample while producing barite as a precipitate [29]. In their investigation of receiving barium sulfate from barite, Yermukhanova and Khatsrinov proposed the use of Oleum as a solvent for barite but did not offer additional details or information on this approach [2].

In this paper, the solubility of barite is investigated using oleum and sulfuric acid experimentally to measure barite’s maximum solubility. To produce barium salts, the initial step involves finding the optimal conditions for dissolving barite in a solvent capable of reacting salt metathesis. While previous research has shown a significant reduction in the solubility of barite in sulfuric acid as its concentration decreases, this study delved into various other factors impacts such as solution temperature, barite particle size, pressure, stirring speed (rpm), reaction time, and acid concentration on the barite solubility process. Then the solution temperature, barite particle size, and acid concentration were selected as operating parameters and the process was optimized using experimental design software. Moreover, in pursuit of achieving the highest possible barite solubility, both oleum and sulfuric acid were investigated as potential solvents for barite. It is worth noting that the nature of these two solvents bears a striking resemblance, albeit with a key distinction: oleum contains an elevated concentration of dissolved SO₃ gas, enhancing its acidity and differentiating it from standard sulfuric acid.