Representative samples of ore containing precious metals is a difficult task. The lower the grade and the higher the nugget and/or cluster effect, the more complex and difficult extracting samples that are both accurate and precise. Reconciliation practices can be used as an effective tool to evaluate sampling accuracy throughout grade control processes. However, a proper reconciliation system must be based on reliable data and, therefore, optimisation of sampling techniques is a must for development of a reliable reconciliation system. This paper is a result of an extensive reconciliation study carried out at a copper and gold mine in Brazil, where a significant reconciliation problem took place while using manual sampling for grade control and short-term modeling. After analysing several sampling equipment/sample selection techniques, the authors suggested the use of a reverse circulation drilling rig with an automatic sampling system for grade control sampling. The samples generated by this automatic system were compared with the manual samples collected from the piles generated by the previous percussion rotary air blast drilling rig. Also, three pairs of twin holes were drilled in order to validate the new reverse circulation approach. Results allowed estimation of the bias related to the increment weighting error (IWE) generated by manual sampling, and show that the reverse circulation rig eliminates significant sampling biases, thus improving the general sample representativeness by increasing both sample accuracy and precision.

The mining sector is looking for new technologies to minimise its environmental impacts and to maximise its reserves. The need to increase the life of mines has led to investigations considering the options to maximise Brazilian bauxite mine reserves. This work evaluates the use of a flip screen in the loader bucket to recover ore from the overburden. The study investigates both the financial and technical feasibilities of this option. This device results in a decrease in the amount of waste and an increase in the life of mine. An average mass recovery of 48% is achieved. The concentration of the bauxite present in the overburden is possible using only screening and the oversized (>12.7 mm) product contains 40% of the available alumina, with less than 4% reactive silica and 0.5% organic carbon, which is acceptable for the Bayer process.

ResumoNo contexto da indústria mineral, reconciliação pode ser definida como a prática de comparar a massa e o teor médio de minério previstos pelos modelos geológicos com a massa e teor gerados na usina de beneficiamento. Essa prática tem se mostrado cada vez mais importante, visto que, quando corretamente executada, aumenta a confiabilidade no planejamento de curto prazo e otimiza as operações de lavra e beneficiamento do minério. No entanto, a utilidade da reconciliação depende da qualidade e confiabilidade dos dados de entrada. Uma boa reconciliação pode ser ilusória. Em muitos casos, erros cometidos, em determinado ponto do processo, são compensados por erros cometidos em outros pontos, resultando em uma reconciliação excelente. Entretanto, esse fato mascara os erros do sistema, que, mais cedo ou mais tarde, podem se revelar. Frequentemente, os erros de amostragem podem levar a uma análise errônea do sistema de reconciliação, gerando consequências graves à operação, principalmente quando a lavra alcança regiões mais pobres ou mais heterogêneas do depósito. Como uma boa estimativa só é possível com práticas corretas de amostragem, a confiabilidade dos resultados de reconciliação depende da representatividade das amostras que os geraram. Esse trabalho analisa as práticas de amostragem, em uma mina de cobre e de ouro para fins de reconciliação. Os resultados mostram que a reconciliação aparentemente ótima entre mina e usina, é ilusória, consequência da compensação de diversos erros, na etapa de coleta de amostra, para o planejamento de curto-prazo. Palavras

No abstract

In the mining industry, reconciliation can be defined as the practice of comparing the tonnage and average grade of ore predicted by the geological models with the tonnage and grade generated by the processing or metallurgical plant. This practice is of increasing importance, since, if correctly executed, it allows the reliability of short-term planning to be improved and the mining and processing operations to be optimised. However, the usefulness of reconciliation relies strongly on the quality of the input data, which is generated by many different sampling methods across the industry. In fact, successful reconciliation can be illusory-errors generated at one point of the process can be offset by errors generated at other points, resulting in apparently excellent reconciliation. Such a situation will in fact also hide compensating biases in the system that will, unavoidably, surface some other day. When this happens, sampling errors are masked and may lead to an erroneous appreciation of the reconciliation system as a whole, which results in serious consequences for the mine operation, especially when reaching poorer or more heterogeneous areas of the deposit. Since valid estimation is only possible with TOS-correct sampling practices, the reliability of reconciliation results depends critically on the representativeness of the samples that generated them. This contribution a summary of an analysis of the manual sampling practices carried out at a copper and gold mine in Goias, and proposes a more reliable sampling method for reconciliation purposes. Results show that the apparently excellent reconciliation between the mine and the plant was in fact illusory; here a consequence of accidental compensation of many errors due to sampling practices for short-term planning.