Abstract Coal mine waste rocks produced during mining and beneficiation can affect the environment due to land degradation and acid mine drainage generation. However, proper characterization and treatment of mineral processing wastes creates options for safe disposal, enhancing the environmental performance. The aim of this study was to use static and kinetic tests to assess properties of coal waste produced at the Moatize Mine in Mozambique, one of the largest "world-class" coal deposits. The study included sampling and characterization of coarse and fine wastes generated during coal beneficiation. Both materials were analyzed in terms of granular properties, static procedures - immediate composition, sulfur forms, mineralogical composition, elemental composition, acid-base balance - and kinetic behavior in humidity cell tests. The conjugation of these techniques indicates that these wastes have a low water acidification potential and low geochemical mobility of toxic elements. Therefore, it was concluded that fine and coarse wastes could be used in land shaping procedures.

Abstract The main coal seams explored in the coal region of Santa Catarina are Barro Branco (BB) and Bonito (BO). Owing to the association with the mineral matter, the tailings generated in the beneficiation are arranged in disposal areas, and subject to the generation of acid mine drainage (AMD). The objective of this study was to evaluate the use of pyrite present in the coal rejects of the BB and BO seams and the environmental gains with desulfurization. For this purpose, densimetric separability test, ash, sulfur, and AMD generation analyses were performed. In addition, the amount of pyritic concentrates and sulfuric acid were estimated considering the current level of production. Three densimetric fractions were obtained: less than 2.2 (energetic material), between 2.2 and 2.7 (low sulfur material), and greater than 2.7 g cm-3 (pyrite concentrate). The results revealed that the two seams could be beneficiated by gravimetric processes, obtaining pyrite concentrates with approximately 60% pyrite. By converting pyrite to sulfuric acid would represent an increase of 500,000 tons per year in the Brazilian production of this material and, in environmental terms, a reduction of up to 90% of the acidity potential to be disposed in the environment in the case of the BB seam and 65% for the BO seam. It was also observed that the “desulfurized” fraction of the BO had higher levels of pyrite and a higher potential for acidity generation than the BB seam, implying greater risks of environmental contamination and higher acid water treatment costs.

RESUMO: Rejeitos de carvão contêm vários minerais, entre os quais a pirita, que se oxida e gera a drenagem ácida de minas (DAM), um efluente ácido com alto poder poluidor. Atualmente, o setor de mineração de carvão brasileiro controla a DAM por meio do tratamento pelo método de neutralização/precipitação de metais, que consome substanciais quantidades de reagentes e gera um grande volume de lodo. O objetivo deste trabalho foi estudar a minimização da geração da DAM por outra abordagem, o método preventivo de isolamento e exclusão de sulfetos. O processo consiste na remoção da pirita da massa de resíduos, reduzindo a geração de acidez e minimizando a carga de metais nas estações de tratamento. A metodologia do trabalho consistiu na separação gravimétrica por meio denso de uma amostra típica de rejeito de carvão, obtendo-se três frações densimétricas: inferior a 2,2, entre 2,2 e 2,7 e superior a 2,7. Cada fração foi submetida às análises imediata e elementar. Também foram realizados ensaios estáticos e cinéticos em células úmidas para avaliação da geração de acidez no rejeito bruto e na fração com densidade intermediária. Os resultados demonstraram que, por intermédio do beneficiamento gravimétrico dos rejeitos, é possível reduzir em 30% o volume de material descartado. Esse material apresenta um potencial de geração de acidez líquido de 80 a 90% inferior ao do rejeito de carvão bruto. Além disso, estima-se que os custos com reagentes no tratamento de águas ácidas produzidas da mineração de carvão possam ser reduzidos em aproximadamente 85%.

ABSTRACT: Coal wastes contain many minerals, including the pyrite, which oxidize and generates the acid mine drainage (AMD), a highly polluting effluent. Currently, the Brazilian coal mining operations emphasise AMD control using waste water treatment systems which consume substantial amounts of reagents and generate large amounts of sludge. The aim of this work was to study the acid mine drainage control by another approach, the sulphide exclusion preventive method. The method comprises in removing the pyrite form coal tailings, reducing the acid generation and minimizing the metals load in wastewater treatment plants. Accordingly, a typical coal waste sample was submitted to a density separation process to divide the sample in the following relative density ranges: below 2.2, from 2.2 to 2.7 and above 2.7. All density fractions were submitted to ultimate and proximate analysis. Static and kinetic tests were carried with the raw waste and the pyrite-free fraction. The results showed that, by gravity processing of coal tailings, it is possible to decrease 30% the volume of wastes. The remaining material presents an acid generation potential of about 80-90% lower than the raw waste. Furthermore, it is expected a reduction in costs of reagents of approximately 85% in acid mine treatment plants.