Influence of water level in mine sequencing applied to strategic mining planning

Marcélio Prado Fontes,Rodrigo De Lemos Peroni

doi:10.1590/0370-44672019730082

Abstract

Abstract Developing mine sequencing involves a number of factors and a large amount of information, and consequently the profitability of the project will strongly depend on the production schedule. A mining project may be conditioned to non-optimal sequencing, which may affect the economic results of the project and also lead to an inadequate utilization of the mineral resources. The conventional method of mining sequencing is divided into three main steps: first, the delineation of the final pit; second, subdivision of the final pit in operational pushbacks (mining advances) and third, sequencing the blocks in each of these pushbacks, taking into consideration mine, processing plant and market capacities. However, there are some aspects that are not usually incorporated in production scheduling, including ore mining below groundwater level. The objective of this study is to demonstrate the relevance and impact on the results of the Net Present Value (NPV) from groundwater level as a constraint related to the need for its drawdown, also considering grades and Stripping Ratio (SR) variability during mining sequence for a phosphate mine. The results show a difference of U$ 140 million for the evaluation considering and not considering groundwater level, without considering other restrictions.

Highlights

In accordance with Hartman and Mutmansky, (2002), open pit mining is a method of operating a surface mine which is simple in concept but complex in its cost and efficiency requirements
When groundwater level was considered as a constraint in sequencing, it was necessary to establish a new final pit shell since the water level surface was overlying the original final pit bottom in some places
This "new" pit bottom considered the results of the numerical model simulating the groundwater level drawdown which was based on the number and the flow rates estimated for each drawdown well

Summary

Introduction

In accordance with Hartman and Mutmansky, (2002), open pit mining is a method of operating a surface mine which is simple in concept but complex in its cost and efficiency requirements. Samis (2001), Dessureault et al, (2007), Dehghani and Ataee-pour, (2012) emphasize that mining projects demand constant risk assessment This is because the value of the project is influenced by many uncertainties such as economy (commodity price, operating costs, production sequence, discount rate), geology (contents, density, hydrogeology, hardness) and underlying physical issues (environmental concerns, leasing limits). The distribution of variables representing the quality of minerals, such as grain size, grade, groundwater level and amount of waste to be removed may hinder or even make the economic exploitation of a mineral deposit impossible In this respect, the conventional method for open-pit planning starts with modeling the orebody based on the borehole data and geological information. Evatt et al, (2012) highlight the uncertainties of the mineral reserve estimate and their effects on the duration of the enterprise

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Conclusion