Arsenic inertization through alunite-type phases: Application to copper pyrometallurgy

Abstract

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.

Avui en dia l’arsènic és un problema de contaminació de les aigües important. Una de les fonts de generació de residus amb arsènic són les indústries metal•lúrgiques no ferroses. En la metal•lúrgia del coure, la important demanda d’aquest metall està obligant a obtenir el coure de menes que cada cop contenen més arsènic. Així doncs, cada cop es generen més residus amb arsènic que han de ser abocats a llocs controlats. En alguns països, aquests residus estan provocant problemes de contaminació a les aigües freàtiques. Per tal d’evitar aquests problemes, s’han fet nombrosos estudis per a la precipitació i estabilització de l’arsènic. Malgrat tot, totes les fases estudiades contenen ferro, que és pot reduir fàcilment en sòls reductors. A més a més, molts estudis indiquen que totes aquestes fases, a llarg termini s’acaben descomponent permetent la dissolució de l’arsènic que contenen.

Amb la intenció de trobar una solució a aquest problema, en aquesta tesi es proposa la inertització d’arsènic a través de fases del supergrup de l’alunita, concretament les que porten alumini i no ferro en l’estructura. Aquestes fases presenten la característica de poder substituir part del sulfat de l’estructura per arseniat. Aquesta substitució és la que s’ha estudiat en natroalunites, alunites i hidroni alunites, així com en d’altres membres anàlegs. La incorporació d’arseniat ha estat possible en natroalunites i alunites, seguint la mateixa partició: (AsO4/(SO4+AsO4))S ≅ 0.5(AsO4/(SO4+AsO4))L. Amb la incorporació d’arseniat el paràmetre de cel•la c s’incrementava amb una pendent de ~0.58 Å. No obstant, un paràmetre de cel•la c inicial de l’alunita més gran, no té efecte en la incorporació d’arseniat a l’estructura. Finalment, l’estabilitat a curt, a mig i a llarg termini de les natroalunites i les alunites ha resultat sé molt bona. A curt termini aquesta era de 0.01-0.03 mg As/L a pH’s entre 5 i 8. A llarg termini (6 mesos) la natroalunita es va estabilitzar a 0.1 mg/L. En tests a mig termini (5 setmanes) l’alunita es va estabilitzar a 0.3 mg As/L. Aquests valors de dissolució són molt més bons que en d’altres fases utilitzades, com per exemple les escorodites, les quals presenten un valors a curt termini un ordre de magnitud més elevats. A mig termini es va veure que l’escorodita natural presenta un valor similar al de l’alunita (0.4 mg/L), mentre que l’escorodita sintètica presenta un valor de 1.3 mg/L, molt més elevat que els obtinguts per alunites i per natroalunites.