Recovery of gallium and vanadium from gasification fly ash

Abstract

The Puertollano Integrated Coal Gasification Combined Cycle (IGCC) Power Plant (Spain) fly ash is characterized by a relatively high content of Ga and V, which occurs mainly as Ga₂O₃ and as Ga³⁺ and V³⁺substituting for Al³⁺ in the Al–Si fly ash glass matrix. Investigations focused on evaluating the potential recovery of Ga and V from these fly ashes. Several NaOH based extraction tests were performed on the IGCC fly ash, at different temperatures, NaOH/fly ash (NaOH/FA) ratios, NaOH concentrations and extraction times. The optimal Ga extraction conditions was determined as 25 °C, NaOH 0.7–1 M, NaOH/FA ratio of 5 L/kg and 6 h, attaining Ga extraction yields of 60–86%, equivalent to 197–275 mg of Ga/kg of fly ash. Re-circulation of leachates increased initial Ga concentrations (25–38 mg/L) to 188–215 mg/L, while reducing both content of impurities and NaOH consumption. Carbonation of concentrated Ga leachate demonstrated that 99% of the bulk Ga content in the leachate precipitates at pH 7.4. At pH 10.5 significant proportions of impurities, mainly Al (91%), co-precipitate while >98% of the bulk Ga remains in solution. A second carbonation of the remaining solution (at pH 7.5) recovers the 98.8% of the bulk Ga. Re-dissolution (at pH 0) of the precipitate increases Ga purity from 7 to 30%, this being a suitable Ga end product for further purification by electrolysis. This method produces higher recovery efficiency than currently applied for Ga on an industrial scale. In contrast, low V extraction yields (<64%) were obtained even when using extreme alkaline extraction conditions, which given the current marked price of this element, limits considerably the feasibility of V recovery from IGCC fly ash.

Introduction

Gallium and certain gallium compounds, such as GaAs and GaN, are suitable for the manufacture of high technological optical devices [1], [2], such as advanced semiconductors, DVD's, laser diodes and other electronic devices [1], [2]. These advanced applications, have created strong interest in the recovery of Ga from a variety of sources. Given the geochemical affinity between Al and Ga, the latter occurs mainly concentrated in bauxites and hosted by diaspore, various aluminosilicates (such as clays), apatite, nepheline and frequently alunite [3]. Ga also has a chalcophile affinity, and thus, may occur as gallite (CuGaS₂), and is frequently substituted for Zn and Cu in sulphides, mainly in sphalerite (ZnS), germanite (Cu₂₆Fe₄Ge₄S₃₂), and chalcopyrite(CuFeS₂) [1], [2], [3], [4]. Given this widely disseminated occurrence of Ga in the earth's crust [2], ore deposits rarely contain >0.1% of Ga, which is usually commercially recovered from bauxites and sphalerite as a by-product of the processing of Al and Zn, respectively [1], [2], [4]. To a lesser extent Ga is also recovered from industrial Cu processing and technological scrap [4]. The recovery of Ga from bauxite ores is based on the Bayer process, in which Al is extracted by hot alkaline digestion [6], Ga being concentrated in the Bayer liquors up to 0.19 g/L. The recovery methods of Ga from these liquors are based on Al–Ga precipitation by CO₂ and subsequent NaOH re-dissolution [6], on selective Ga extraction using liquid–liquid solvent extraction and ion exchange methods [1], [5], [7], [8] and on employing Hg amalgam with subsequent addition of NaOH [1]. The Ga recovery from acidic solutions produced during Zn processing also involve liquid–liquid solvent extraction [11], [12], while other Ga recovery methods from liquors include the use of insoluble amphoteric adsorbents [9] or membranes [10]. However, the difficulties associated with isolation Ga from Al always necessitate an electrolysis procedure to obtain high purity Ga end products [1].

The main producers of primary Ga are Australia, China, Germany, Japan and Russia while France is the main manufacturer of refined Ga [1], with the final price of this element ranging from 225–450 €/kg (Market Prices, 2003) depending upon purity. Despite the large estimated resources of Ga in bauxites (1 billion kg) and Zn deposits, only a small percentage (40%) is economically recoverable [1], [2], so there is considerable interest in evaluating Ga recovery from other significant sources, such as phosphate ores and coal [1], [2], [3]. Although the latter typically contains up to 0.10%wt Ga, this element, along with other valuable elements, is concentrated in coal fly ash during coal combustion [13], [14], [15] and gasification [16], [17]. Such concentration of metal content in fly ash and the potential recovery of these valuable elements, has led to considerable interest in metal extraction methods from coal combustion fly ash [18], [19], [20]. Previous studies have proposed fusion [18] and alkaline methods [19] for the recovery of Ga and V from coal combustion fly ash, respectively. Despite this proven suitability of coal fly ash for the extraction of potential metal ore, the most common commercial applications still involve its use in cement and other building materials.

Studies on speciation of valuable elements in the Integrated Coal Gasification Combined Cycle (IGCC) fly ash from Puertollano power plant [16], [17], [21], have yielded relatively simple and cheap extraction methods. The gasification fly ash is characterised by a relative enrichment in several valuable elements, such as Ge, Ni, Ga, and V, and by the occurrence of fine crystalline sulphides (rather than oxides) for many metals [16], [17]. Within the content of these studies, Ge have been shown to occur as water soluble species, generating the possibility of water based extraction methods for this element [21] and allowing for starting the up-scale development of low-cost and environmentally acceptable Ge extraction techniques.

Since Ga occurs in relatively high contents (up to 320 mg/kg) in the Puertollano IGCC fly ash the research has focused on development of feasible extractive process for this element. Much of the Ga occurs as oxide in four-fold coordination, mainly as free oxide but also as Ga oxide substituting for aluminium in aluminium-silicate framework structures. A minor proportion of Ga occurs in sulphides, substituting for Zn in the sphalerite/wurtzite structure [16]. Based on this speciation a NaOH-based extractive process for Ga from the Puertollano IGCC fly ash would appear to be the most appropriate.

Along with the studies on Ga, potential extraction of vanadium from this fly ash was also investigated. Vanadium is commonly recovered from titaniferous magnetite ores and as V₂O₅ from by-products of crude oil, being mainly used to produce high-strength and low-alloy steels [22]. Since 50:50% coal/petcoke blend is gasified in Puertollano IGCC plant, fly ash is predictably enriched in V (up to >6000 mg/kg). Although the main proportion of V (>90%) is in the third valence state into the Al–Si fly ash matrix [17], it may be oxidised to V⁴⁺ and V⁵⁺ during NaOH leaching, then extracted as highly soluble vanadates in alkaline media [22], [23].

This paper reports on the evaluation of the potential recovery of Ga and V from IGCC Puertollano fly ash using several NaOH-based extraction tests under different temperatures, NaOH concentrations, NaOH-solution/fly ash ratios (NaOH/FA) and extraction times (t). Pre-concentration and recovery methods were also investigated.