Extraction of Nb, Ta, Zr and Hf from tin slags originating from cassiterite beneficiation

Abstract

The extraction of refractory metals such as Nb, Ta, Zr, and Hf from secondary resources like tin slags typically requires aggressive fluoride-based reagents, leading to environmental and operational challenges. This study proposes a fluoride-free pyro-hydrometallurgical route involving thermal treatment with sulfuric acid followed by aqueous/oxidative leaching. Thermodynamic simulations are integrated to optimize the formation of soluble sulfate complexes and enhance metal recovery. The process aims to generate a pregnant leach solution (PLS) rich in target metals, suitable for downstream solvent extraction. The approach reduces volatilization losses and energy consumption compared to conventional high-temperature processes.

1. Introduction

Extraction of refractory metals from tin slags requires multiple sequential pyrometallurgical and hydrometallurgical processes. A common practice in the commercial extraction of these metals involves the use of hydrofluoric acid and disaggregates with sulfuric acid mixtures through hydrometallurgical routes, leading to strong complexation interactions with hard donors (fluoride and sulfate ions) and generating Pregnant Leach Solution (PLS) with high acidity levels (pH < 1) [14–19]. However, operational practices in the presence of fluoride ions remain a challenge in conventional metallurgy due to the impacts generated during and after the process [9].

An economical fluoride-free method for the reprocessing of these metals from secondary resources involves the use of sulfate-based routes at temperatures above 300°C [9]. Sulfuric acid is the most preferred reagent in the metallurgical industry due to its effectiveness as a solvent and its hygroscopic activity toward refractory materials, being capable of overcoming the energy barrier of Nb5+/Ta5+/Zr+4/Hf+4–O2− molecules [20–21]. The action mechanism of the oxoanions is based on the acid attack of the M(metal)–O2− ligands (<13012 kJ·mol−1), which are more chemically susceptible compared to the Si–O bonds (>13012 kJ·mol−1) of complex matrices [22]. This attack allows the metallic cations to be released and bound together with oxygen from the environment (air or moisture) or from the matrix, forming soluble salts on the outer surface rim of the material and soluble sulfates in an acidic medium [23], as shown in Fig. 1.

Its application in leaching processes, as well as in sequential thermal treatment–leaching processes, has demonstrated PLS with acceptable extraction levels [15,24–30]. After leaching, the solubilized species are commercially recovered and separated downstream by solvent extraction [31–32]. The challenge of extractive metallurgy within non-ferrous metal value chains involves avoiding the use of fluoride ions and the generation of large amounts of residual material after processing. Thermal treatment followed by aqueous/acid leaching could offer a viable alternative for the extraction of refractory metals through pyro-hydrometallurgical routes from secondary resources. To date, the literature has discussed thermal processing approaches using concentrated H2SO4, which may limit their large-scale applicability for this type of secondary residue. However, a comprehensive and systematic approach to modeling and analyzing the behavior of chemical species under optimal conditions is still lacking. The integration of thermodynamic simulations into these processes enables an understanding of sulfate complex formation and offers advantages such as reduced volatilization losses, lower energy consumption compared to the higher temperatures reported in the literature, and the optimization of valuable metal recovery processes.

In this context, the present study focuses on a prospective process aimed at maximizing metal extraction from tin slags. The process involves, as a first step, thermal treatment with sulfuric acid, previously simulated thermodynamically, followed by aqueous/oxidative leaching as promoters of the extraction of the target metals, with the goal of generating a PLS enriched with the desired metals, suitable for purification in subsequent stages.