Índices de estimativa de custos de fechamento de pilhas de estéril e barragens de rejeitos

Abstract

Mining generates large volumes of waste that are disposed in structures known as waste rock piles and tailings dams. Mining projects have a well defined life cycle on which the last stage is the closure. In order to build a proper financial planning for a mining project, it is necessary to estimate the closure costs at the very beginning of mine life cycle. The main issue on estimating closure costs in advance is the dynamic nature of mining and uncertainties regarding the final configuration of mining structures at the time of closure. The objective of the present study is to propose closure costs indexes for waste rock piles and tailings dams based on characteristics of those structures. The volume of waste and the area of the storage facility are the quantitative characteristic most easily obtained on the beginning of mine life cycle and are directly related to closure costs. The study was developed based on closure costs of different mines. Linear regression plots based on closure cost vs. quantitative characteristic were prepared for each type of structure (waste rock pile and tailing dam). Trend lines were adjusted to the plots according to the linear equation y = ax + b, where y is the closure cost and x is the structure characteristic. The trend line coefficient a represents the closure cost estimate index that correlates the structure characteristic (volume or area) with the closure cost. Box plots were prepared to identify distortions on the linear regression results considering the data used as well as to provide a variation margin around the cost estimate index. In general the cost estimate indexes based on the area of the structure presented better results than indexes based on the volumes of residues. The best cost estimate index obtained for waste rock piles was R$ 5,68/m² if surface area, considering the interval between R$ 5,50/m² and R$ 6,98/m². The best cost estimate index obtained for inert tailings dams was R$ 12,72/m² of reservoir area, considering the interval between R$ 8,40/m² and R$ 15,05/m². The best cost estimate index obtained for sulfide tailings dams was R$ 91,77/m² of reservoir area, considering the interval between R$ 73,57/m² and R$ 96,34/m². As expected, the geochemistry of waste rock and tailings significantly affected the closure costs. Sulfide material bearing structures presented closure cost indexes 5 to 9 times higher than inert material bearing structures.