Boliden’s mineral processing plant in Garpenberg wanted to investigate their zinc-regrinding circuit. The re-grinding circuit had performed subpar when running as the total recovery of zinc was lowered and problem with the dewatering of the final concentrates was also noted. Therefore, the zinc re-grinding circuit is currently not being used. Furthermore, it was noted during investigations about silicate depressant that the zinc circuit had problems with coarse sized sphalerite particles locked with silica. Which could perhaps be liberated with the re-grinding circuit. The purpose was therefore to investigate how a re-grinding step could be used to liberate locked sphalerite particles from gangue. The practical part of the thesis involved lab-scale SMD-mill, re-grinding of the scavenger concentrate and cleaner tailings which are the two streams being fed to the SMD-mill in the plant. The re-grind was done in three fractions: bulk, +90µm and +125 µm fraction with subsequent lab-scale flotation afterwards to identify how the re-grinding effected the flotation results. Also, a small QEMSCAN analysis was performed for the +90 µm fraction to identify how liberation was improved by re-grinding. The flotation trials were performed with two references and two different intensities for the re-grinds.  The results showed an overall increase in grade of zinc in the concentrate with increased grinding for all the trials. SiO2 and MgO content (typical elements for silicate gangue) was also reduced in the zinc concentrate, showing that re-grinding helped liberate locked sphalerite particles. The QEMSCAN results showed that the liberation of sphalerite particles in the +90 µm fractions increased with re-grinding. For the bulk fraction scavenger concentrate the zinc recovery increased slightly, for the +90 µm fraction scavenger concentrate, a slight decrease in zinc recovery could be seen after re-grinding. The decrease in recovery was larger for the cleaner tailings (bulk and +90 µm) and was decreased further with increased grinding. However, for the +125 µm fraction tests, zinc recovery was increased for both scavenger concentrate and cleaner tailing. The references showed an overall high recovery but a zinc grade in the concentrate close to the feed grade of zinc and contained a high grade of SiO2. Indicating that without re-grinding the amount of free sphalerite is low. Overall, the re-grind and flotation tests pointed towards that value can be created for the plant by re-grinding the scavenger concentrate and cleaner tailing.

A importância dos circuitos de moagem nos setores de mineração, pelotização e cimenteiro - seja por determinar o desempenho de processos subsequentes, seja por sua significativa parcela nos custos operacionais - encorajou o desenvolvimento de extensivos estudos na área. Dois campos continuam, contudo, pouco explorados: a moagem a seco e a moagem fina de concentrados (ou remoagem). Por outro lado, os circuitos de cominuição têm ganhado gradativamente mais importância ao longo dos anos para o mercado de minério de ferro, devido ao aprofundamento das cavas em operação e, por consequência, ao tratamento de minérios mais compactos e com menores teores de ferro. Por si só, essas novas características geraram a necessidade de moer produtos cada vez mais finos e de etapas adicionais de concentração e pelotização. Nesse ínterim, a moagem fina ganhou extrema importância. Este trabalho se concentrou no desenvolvimento de um método de ensaio de laboratório para a previsão do consumo específico de energia na etapa de remoagem a seco, adequado à aplicação em minérios de ferro itabiríticos. Foram investigadas diversas variáveis que podem interferir nos resultados de laboratório e definidas as melhores condições para os ensaios. Em seguida, os resultados dos ensaios foram confrontados com dados de uma operação de remoagem a seco industrial. O método proposto apresentou boa aproximação com os resultados industriais em relação à geração de finos (porcentagem passante em 45 ?m). Em relação à geração de área superficial - parâmetro importante para a pelotização -, os desvios foram maiores devido à sua correlação com a composição mineralógica para as misturas investigadas. Contudo, a relação entre a geração de área superficial com as diferentes mineralogias para o minério de ferro, especialmente o de Carajás, demanda maiores investigações. A relação linear proposta por Rittinger entre a geração de superfície e o consumo específico de energia na moagem também pode ser comprovada para a moagem a seco. / Studies on grinding area are vast due to its importance for mining, pelletizing and cement industry: grinding operations define both the throughput of following processes and operational costs. However, there are two grinding processes that lack deeper understanding: dry and fine grinding (re-grinding). On the other hand, as rich iron ore deposits get depleted and the necessity of treating more complex ores increases, the necessity of re-grinding concentrates prior further steps of concentration grows. Therefore, fine grinding of iron ores is progressively gaining importance. This study focused on the development of a bench scale test to predict the specific energy consumption of dry re-grinding circuits for iron ore. The variables that can affect grinding tests results were evaluated and the best conditions were defined. In sequence, the results of a series of lab tests were compared with data from an industrial circuit. The proposed test presented good correlation with industrial results regarding the generation of fines (percentage passing 0.045 mm). Regarding new surface generation - parameter of great importance for ore pelletizing - the differences between lab and industrial results were bigger, due the correlation of this parameter with the mineralogical composition of the samples investigated. However, the correlation between surface generation and mineralogical composition for iron ores, especially ores form Carajás complex, demands deeper investigations. Moreover, the linear relationship between energy consumption and surface area generation, as proposed by Rittinger was also observed for dry grinding.