Arsenic inertization through alunite-type phasesApplication to copper pyrometallurgy

Resumen

Nowadays, arsenic is an important problem in water pollution. Non-ferrous metallurgical industries generate arsenic residues because the ores contain this mineral. The high technology improvement is increasing the demand of some metals such as copper. This increasing demand and the scarce of copper ores with low arsenic content is generating a problem with arsenic wastes in lots of countries, but especially in Asia and in South and Central America. In these countries, groundwater is polluted by arsenic and generates health problems to the inhabitants. With the aim to precipitate and control the arsenic generated by these industries, different methods has been studied: Calcium arsenate, ferrihydrite and scorodite. The former is just a precipitation method with a very low arsenate stability. On the other hand, ferrihydrite and scorodite pass the TCLP from different countries, however, the long-term stability is not satisfactory, and many studies indicate that these phases easily decompose in long-term storage producing arsenic release. Moreover, these are iron-phases, that’s mean that not only the arsenic release can also be easily produced in reductive conditions. With the aim to found a phase with a good stability in long term, this thesis proposes the arsenic inertization through alunite-type phases, concretely with aluminum in the structure. Aluminum was chosen instead of iron because its good properties in reductive environments, which make it very difficult to be reduced. These phases has the characteristic to be able to substituted part of the sulfate present in the structure by arsenate. This substitution is the one studied in natroalunites, alunites and hydroniumalunites, as well as in analogous members. The incorporation of arsenate is possible in natroalunites and alunites members with the same partition: (AsO4/(SO4+AsO4))S ? 0.5(AsO4/(SO4+AsO4))L. With the arsenate incorporation in the natroalunite/alunite structure, the c cell parameter increased with a slope of ~0.58 Å. However, a higher initial c cell parameter does not have any effect in the arsenate incorporation in the structure. Other alunite-type phases such as hydronium-alunite, barium-alunite and leadalunite were also investigated with different results. Hydronium-alunite was formed under the same conditions of alunite and natroalunite. However, with the addition of arsenic in the medium this phase formation decreased, and arsenic was practically not incorporated in the structure. Barium-alunite and lead-alunite syntheses were performed from barium and lead sulfates respectively. However, the insolubility of its sulfates made not possible the syntheses of these alunites, instead, hydronium alunites were precipitated with barium or lead sulfate, and depending on the arsenic concentration with mansfieldite. Arsenical natroalunites were also performed with real wastes from copper pyrometallurgical plants. These arsenical natroalunites give the same results as the synthetic, with the same incorporation of arsenate and the same c cell parameter increase. Short, medium and long-term stability tests were performed in all synthesized natroalunites, in some arsenical alunites and in scorodites (synthetic and natural). Arsenical natroalunites and arsenical alunites gave good results in short-term tests (0.01-0.03 mg As/L). In long-term tests (up to 6 months) were done in natroalunites. These tests also gave a great stability of arsenical natroalunite, which were stabilized at 0.1 mg As/L. Medium-term tests (up to 5 weeks) were performed in arsenical alunites and scorodites (natural and synthetic). Arsenical alunites were stabilized at 0.3 mg As/L, similar values were obtained for natural scorodite (0.4 mg As/L). Whereas synthetic scorodite presented higher values (1.3 mg As/L) than arsenical natroalunites and arsenical alunites, indicating less stability in the same conditions.