Authors: Hamman, JPE; Belseran, REP; Kriska, B; Ramadhan, T; Sugiyanto, AY; Banda, H

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Hamman, JPE, Belseran, REP, Kriska, B, Ramadhan, T, Sugiyanto, AY & Banda, H 2023, 'An acidic water corrosive environment and ground support strategies at the Grasberg Block Cave mine, Indonesia', in J Wesseloo (ed.), Ground Support 2023: Proceedings of the 10th International Conference on Ground Support in Mining, Australian Centre for Geomechanics, Perth, pp. 427-438,

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The Grasberg Block Cave (GBC) mine is below the Grasberg open pit at PT Freeport Indonesia (PTFI). The production and infrastructure levels are predominantly in the Grasberg Igneous Complex (GIC) and less so in the surrounding sedimentary rocks of the Waripi and Faumai formations. Development of the production levels in the GBC Production Block 2 South started in 2016, and undercutting commenced in 2018. A prominent mineralised zone – the heavy sulphide zone (HSZ) with 10–75% pyrite and pyrrhotite – forms the outer boundary of the GIC along the country rock. High levels of oxidation and subsequent corrosion associated with the HSZ were identified within the first two years of development. Corrosion rates indicated an aggressive corrosive environment with pH levels of less than 4 (identified as a potential hazard) that could lead to support deterioration and excavation instability in the production and mine infrastructure levels. A corrosion assessment based on the presence of sulphides, water volumes and water chemistry was conducted to determine the corrosivity of the groundwater. Corrosion maps were developed in the initial stages of development and followed up by actual corrosion mapping to determine the accuracy and severity of the expected corrosion zones. During this study, it became clear the parameters that played an important role in determining the corrosivity of the acidic groundwater in the GBC differ greatly from the salinedominated corrosive environments identified in Australia and other areas of the mining industry. Numerous groundwater datasets, laboratory analysis and measured ground support corrosion rates formed the basis of this study to determine the relationship between the pH, sulphate ions, electric conductivity and corrosion rates. Factors that increase and delay the corrosion rates were observed and documented. Various corrosive resistant ground support types were evaluated to determine their effectiveness in acidic environments. This paper briefly describes the derivation of an acid-dominated aqueous corrosion design table that could be used in the industry to determine potential corrosion rates. Corrosion of ground support, as well as the management of corrosion, are also discussed.

Keywords: corrosion, acidic environments, sulphides, corrosion rates, pH levels, ground support strategies

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