Industrial radioactive barite scale: suppression of radium uptake by introduction of competing ions
Introduction
The formation and deposition of mineral scale is a common problem related to oil and gas production, mineral extraction and some chemical processing procedures (Dunn and Yen, 1999). This is a world-wide problem that causes considerable production losses and involves expensive cleaning procedures once formation has occurred (Dunn et al., 1999). Barite is one of the most common scale-forming minerals due to its low solubility (pKsp=9.96 at 20 °C (Blount, 1977)) in aqueous media and the relative abundance of barium and sulphates in these solutions. Furthermore, barite can form a solid solution (SS) by substitution of Ba2+ with a wide spectrum of other alkali and alkali-earth cations. Substitution is controlled by the degree of similarity in charge, ionic radius and electronegativity between substituting ions and Ba2+ (Table 1; Alpers et al., 2000). Because of their isomorphic structure (both orthorhombic) and the similar ionic radius and charge of the dominant cations, solid solutions between barium sulphate and strontium sulphate are common. Metal partitioning can also take place during barite crystallization with the incorporation of metals (e.g. Pb) and radioactive elements (e.g. Ra) that may lead to serious problems of industrial contamination in soil and groundwater (Zachara et al., 1991).
The main radioactive element recorded in barite scale samples, as a substituted, cation is radium. Its presence has been detected previously and levels of radioactivity recorded (Smith, 1987; Wilson and Scott, 1992; Fisher, 1998; Worden et al., 2000). 226Ra (t1/2=1600 years) plus its daughters 210Pb (t1/2=22.2 years) and 210Po (t1/2=138 days), and the shorter lived 228Ra (t1/2=5.76 years) are the dominant isotopes (Zielinski et al., 2001). Barite scale and barite-bearing sludge derived from oil abstraction processes can contain tens to thousands of becquerels per gram (Bq/g) of radium, compared to the typical soil values analysed by the American Petroleum Institute of 1.85–18.5 mBq/g (Zielinski et al., 2001). This radioactively contaminated scale material is often denoted Naturally Occurring Radioactive Material (NORM). However materials designated as NORM include not only radioactive barite scales and radioactive barium sulphate sludge but also industrial contaminated equipment such as metal pipes, centrifuges, filters etc. Thus, the formation of mineral scale with significant levels of radioactivity raises serious concerns in terms of worker safety and land contamination.
Section snippets
Aims and objectives
The work presented in this paper represents a preliminary stage of a project to investigate the formation processes and element substitution in radioactive barite scale. The main aim of the project is to prevent the formation of scale. The following are specific objectives:
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To simulate the growth of radioactive barite scale in a static system as a function of temperature, pressure and fluid composition, and to observe and quantify its mineralogy and mineral chemistry changes using advanced
Experimental design
A static double diffusion system allowing slow crystallization from aqueous solutions in a porous medium (Prieto et al., 1997) has been used to prepare radioactive barite crystals. The system consists of two mother solution reagents, separated by a column of silica hydrogel (Fig. 1). This allows the growth of barite crystals with appreciable dimensions (300 μm (Putnis et al., 1995)) due to the high degree of supersaturation and to restricted mass transport through the hydrogel column. The gel
Characterization of barite crystals
X-ray pictures of the hydrogel columns were taken in order to clearly identify the distribution of the crystals within the column. The X-ray picture was taken with a Hewlett Packard Cabinet X-ray System (Faxitron Series 43855A). The columns were then sliced into sections, which were analysed with the residual mother solutions by γ-spectrometry in order to build up an activity profile along the column. Gamma detection was performed on a B5030 Canberra Broad Energy Germanium Detector (3800 mm2
Reservoir solutions
In the first experiment two reservoirs (A and B, Fig. 1) contain, respectively, BaCl2 (0.5 M), spiked with a 226Ra solution, and Na2SO4 (0.5 M). In this case the diffusion of the ions through the hydrogel column leads to the formation of pure barite crystals (column 7, Table 2), which incorporate the available radium in the lattice from the point of first nucleation (typically after 40 days) throughout the crystal growth process (1 month). In columns 3, 4 and 6 SrCl2 at different concentrations
Conclusions
Production of radioactive barite mineral scale is a common issue, which affects the oil, mining and chemical industries. During the industrial process, the reaction environment might become locally supersaturated with respect to BaSO4. This results in the precipitation of BaSO4, which could incorporate radioactive isotopes such as 226Ra naturally present in the reacting slurry. In this work radioactive barite and barite/celestite solid solutions have been synthesized through a double diffusion
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