Thesis (MScIng)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: Due to economic and environmental constraints mining operations are placed under increasing pressure to effectively manage and operate tailings disposal operations. Restrictions imposed on water usage and tailings operations footprint have led to higher density and wider particle size distribution slurries conveyed to tailings areas. One means of efficiently disposing the tailings is co-disposal. In this method a concentrated fine vehicle slurry is used to convey a coarser fraction. This produces a higher density of tailings, with a number of advantages both upstream and downstream of the tailings process. Limited research has been conducted on the effect of coarse particles on the non-Newtonian rheological properties of these slurries. This lack of information complicates the design and reliable operation of these systems. This project aims at gaining a clearer understanding as to the mechanisms involved in the addition of coarse particles to a fine clay slurry vehicle; and to provide a means of estimating the measured slurry rheological properties. A number of experiments were designed to test the slurry (both Kaolin only, and Kaolin-coarse particle mixtures) rheological properties using a Couette viscometer (for the dynamic flow properties of yield stress and plastic viscosity) and a vane instrument (for the static yield stress measurements). The slurries were prepared in varying Kaolin clay solids concentrations with reverse osmosis water. Glass beads and two types of industrial sand were used as the coarse fractions. All of the coarse particles had a similar size but varied significantly in shape. Slurry pH and temperature readings were monitored throughout the tests. Tests were done initially on clay only slurries. The rheological properties of these slurries were repeatable, and no noticeable variations of properties with time were observed. The yield stress (both static and dynamic) and plastic viscosity data were well correlated with established relationships. Coarse particles were added to the clay only slurries, and then removed. The remaining clay only slurry exhibited the same rheological properties as the initial clay only slurry. The presence of coarse particles increased all the measured rheological properties (i.e. dynamic yield stress, Bingham viscosity, and static vane yield stress) in a fashion resembling the effect of adding clay to a clay only slurry. In addition, the change in measured rheological property by addition of coarse particle was independent of the clay fraction in the clay slurry. Furthermore, with both the clay only slurries and clay and coarse sand slurries, a constant linear relationship existed between the static and dynamic yield stress. Several correlations from the literature were found to provide reasonable prediction of the rheological property variations observed. These empirical and semi-empirical models however did little to explain the mechanisms involved in coarse particle addition. A new correlation has been proposed, Residual Clay Concentration, which predicts the change in rheological property based on an additional clay concentration, which in turn is a linear function of the coarse particle concentration. The accuracy of this model further strengthens the belief that the coarse particle acts in a similar fashion to a floc. By means of a case study example the importance of selecting an appropriate model for design was illustrated. The Residual Clay Concentration method provided the most conservative results. This combined with its theoretical basis strengthens the models recommendation for use in design. / AFRIKAANSE OPSOMMING: As gevolg van ekonomiese en omgewings beperkinge word mynwese nywerhede onder toenemende druk geplaas om doeltreffende afvalstroom bestuur en operasie toe te pas. Beperkinge geplaas op water gebruik en afvalstroom area-groote ly tot hoër digthede en wyer partikel-grooteverspreidings van flodders vervoer na afval areas. Een manier om van die afval doeltreffend ontslae te raak en te berg is deur medeberging. In die metode word ‘n gekonsentreerde fyn flodder gebruik as draer van ‘n growwer partikel-fraksie. Dit ly tot ‘n hoër digtheid flodder, met verskeie voordele in beide die op – en afstroom prosesse. Beperkte navorsing is gedoen op die effek van growwe partikels op die nie-Newoniese rheolgiese eienskappe van hierdie flodders. Hierdie tekort aan informasie maak die effektiewe, betroubare bedryf en operasie van die sisteme meer ingewikkeld. Hierdie projek is daarheen gemik om ‘n beter begrip te ontwikkel met betrekking tot die meganismes betrokke in die byvoeging van growwe partikels aan ‘n fyn klei-agtige flodder draer; en om ‘n manier te voorsien wat die rheologiese eienskappe kan beraam. Verskeie eksperimente was ontwerp om die flodders (beide slegs Kaolien, en Kaoliengrowwe partikel mengsels) se rheologiese eienskappe te toets deur die gebruik van ‘n Couette-viskometer. Die Couette viskometer was gebruik om die dinamiese eienskappe (van grens-spanning, en plastiese viskositet) te meet. ‘n Vaan apparaat is gebruik om die eienskap van statiese grens-spanning te meet. Die flodders was voorberei in verskeie Kaolien konsentrasies met tru-osmosis water. Glas krale en twee tipes industriële sand is gebruik as die growwe fraksies. Al die growwe partikels het soortgelyke groottes gehad, maar het grootliks verskil in vorm. Die flodder pH en temperatuur lesings is deurentyd nagegaan. Toetse was aanvanklik gedoen op die klei-alleenlike flodders. Die gemete reologiese eienskappe van die flodders was herhaalbaar, en geen opmerkbare veranderinge van die eienskappe met betrekking tot tyd is gemeet nie. Die grens-spanning (beide statiese en dinamiese) en plastiese viskositeit is goed gekorrelleer met gevestigde verhoudinge. Growwe partikels is aan die klei-alleenlike flodders bygevoeg, en daarnae verwyder. Die oorblywende klei-alleenlike flodder het dieselfde gemete rheologiese eienskappe getoon as die oorspronklike klei-allenlike flodder. Die teenwoordigheid van growwe partikels het na ‘n toename van al die gamete rheologiese eienskappe gelei wat fisies baie soortgelyk is aan die byvoeging van klei tot ‘n klei-alleenlike flodder. Verder, met beide die klei-alleenlike en klei-growwe partikel flodders het ‘n konstante liniëre funksie tussen die statiese en dinamiese grens-spannings bestaan. Verskeie verhoudings uit die literatuur het goeie korrelasie bewerkstellig met die waargenome rheologie veranderinge. Hierdie empiriese en semi-empiriese modelle doen egter min om die megansimes betrokke in die toevoeging van growwe partikels te verduidelik. ‘n Nuwe korrelasie is voorgestel, naamlik die Residu Klei Konsentrasie. Hierdie model voorspel die verandering in reologiese eienskappe gebaseer op ‘n addisionele klei konsentrasie, wat ‘n liniëre funkise is van die growwe partikel konsentrasie. Die goeie korrelasie gesien met die model versterk die idée dat die growwe partikel in ‘n soortgelyke manier as ‘n flok gedra in die teenwoordigheid van ander flokke. Deur middel van ‘n tipiese industriële voorbeeld is die belangrikheid in die keuse van die regte korrelasie geillustreer. Die Residu Klei Konsentrasie metode het die mees konservatiewe resultate gelewer. Hierdie feit gekombineerd met die model se soliede teoreitiese beginsels versterk dit as voorgestelde korrelasie vir ontwerp.

Research was conducted at Cyprus Pima Mine, an open pit copper mine located southwest of Tucson, Arizona, in 1980 and 1981 to study the establishment of annual and perennial grasses on the slopes of copper mine tailing ponds. In 1980, the north and south slope exposures were hydroseeded with barley (Hordeum vulgare L.). Plots were capped with topsoil prior to planting or left uncapped, and mulched with wheat straw after planting or left unmulched. Desert soil was classified as Palos Verdes-Sonoita Complex and Detrital-Sonoita Complex. Barley provided a quick, temporary cover and served as a mulch for later plantings. The number of seedlings established was the same for both exposures. Barley grew taller on the north slope possibly due to cooler temperatures. Barley growth was more vigorous on mulched areas. In 1981, the same north and south slopes were hydroseeded with Lehmann lovegrass (Eragrostis lehmanniana Nees.), buffel grass (Pennisetum cilare (L.) Link), blue panicgrass (Panicum antidotale Retz.) and bermudagrass (Cynodon dactylon (L.) Pers.). Prior to planting perennials, barley residue was incorporated into the slopes using a spike-tooth chain drag and a sheepfoot roller, or was left unincorporated. Incorporation of barley residue aided in its breakdown and helped prepare a good seedbed. The spike-tooth chain drag produced better growth than the sheepfoot roller, possibly due to excessive soil compaction with the sheepfoot roller. Capping tailing slopes with desert soil produced the most significant improvements in barley and perennial grass growth. Soil analyses of pure tailing and capped tailing revealed both were low in nitrogen and phosphorus. All plots were amended with these nutrients. Organic matter, although low for both samples, was higher in the capped tailing sample. Organic matter improves soil structure and provides nutrients and its increased presence may have been the main reason for improved growth on capped slopes.

Os rejeitos de mineração provenientes do beneficiamento do minério, quando dispostos de maneira inadequada, podem causar a contaminação do solo e das águas superficiais e subterrâneas. Dependendo do tipo de minério tratado, tais rejeitos podem conter elevadas concentrações de metais potencialmente tóxicos e passar a atuar como fontes persistentes de liberação de tais metais através dos ecossistemas. Assim sendo, a caracterização desses rejeitos é extremamente importante. Neste contexto, foram realizadas caracterizações geológicas e geotécnicas nos rejeitos de mineração provenientes da região do Vale do Ribeira (município de Adrianópolis-PR), visando avaliar o grau de mobilidade e toxicidade dos metais contidos na porção superficial deste depósito. Para tanto, foram realizadas caracterizações mineralógicas (DRX e MEV/EDS), granulométricas, determinação dos parâmetros físico-químicos (pH, Eh e CE), além de ensaios que verificaram a mobilidade, especiação química e toxicidade das amostras: ensaios de solubilização, lixiviação, ensaio de variação de pH, extração sequencial e ecotoxicidade aquática envolvendo o organismo teste Daphnia similis. Além dos rejeitos foram realizadas análises mineralógicas e determinação dos parâmetros físico-químicos em amostras de estéril. A partir destas caracterizações, foi possível verificar que a composição mineralógica predominantemente no rejeito e no estéril é o carbonato, reflexo da litologia local. Tal composição refletiu também nos elevados valores de pH, que variaram de 7,6 a 8,0. Esses rejeitos apresentaram granulometria fina (predominância de areia muito fina, fina e silte), além de altas concentrações de metais, principalmente Pb (concentração média de 5.236,67 mg.kg-1 ) e Zn (7.726,53 mg.kg-1 ). Dos metais analisados, o Pb excedeu o valor preconizado na norma ABNT NBR 10.004:2004, tanto nos extratos lixiviados como nos solubilizados, para todas as amostras analisadas, o que permitiu classificar tais resíduos como Classe I - Perigoso. No ensaio de variação de pH, de forma geral, o Zn apresentou maior liberação na condição inicial mais ácida (pH 3), com média de liberação de 11,54 mg.L-1 após sessenta minutos de ensaio; embora nas outras condições também tenha ocorrido liberação considerável. A partir da extração sequencial, foi possível verificar que todos os metais (com exceção do Cu) estavam associados à fase de óxidos/hidróxidos, porém as fases consideradas de maior mobilidade (trocável e carbonática) estavam presentes em concentrações significativas para todos os metais analisados. As cinco amostras analisadas mostraram-se heterogêneas sendo que a RP5 e a RP7, apresentaram maior mobilidade dos metais Pb e Zn, além de terem apresentado maior toxicidade nos ensaios de ecotoxicidade. Para o ensaio de ecotoxicidade aquática, tais amostras apresentaram imobilidade dos neonatos até mesmo nas diluições menos concentradas (diluições contendo somente 6,2 e 3,1% de extrato solubilizado). Assim, concluiu-se que o rejeito pode ser considerado perigoso do ponto de vista ambiental, já que as altas concentrações de metais potencialmente tóxicos presentes estão passíveis de liberação e migração no ecossistema, podendo interagir com a biota aquática e terrestre. / Tailings from the ore processing when improperly disposed, can cause contamination of soil, surface water and groundwater. Depending on the treated ore, the mining waste may contain high concentrations of toxic metals and start acting as persistent metals sources to surrounding ecosystems. Therefore, the characterization of this material is extremely important. In this context, geological and geotechnical characterization were performed on tailings from Ribeira Valley (Adrianópolis-PR), in order to evaluate the mobility and toxicity of metals present in the superficial portion of the deposit. For the current paper the following tests were performed: mineralogical characterization (XRD and SEM/EDS), particle size distribution, determination of physical and chemical properties (pH, Eh and EC), solubilization and leaching tests, pH variation, sequential extraction and aquatic ecotoxicity were carried out. Besides the tailings analyzes, mineralogical characterization and pH measurements were also performed in samples from Perau (waste rock). The results showed mineralogical compositions predominantly carbonate in both tailing and Perau samples, as a reflection of the local lithology. This composition also reflected in the higher pH values, which ranged from 7.6 to 8.0. The tailing had fine particle size (predominance of silt, very fine and fine sand), and high concentrations of metals, especially Pb (average concentration of 5236.67 mg.kg-1) and Zn (7726.53 mg.kg-1). The metal Pb was above of the recommended value in ABNT NBR 10.004:2004 in leaching tests, solubilization tests and for all samples. This fact allowed the classification of the waste as Class I-Dangerous. In testing of pH variation, the Zn showed higher release in initial acidic condition (pH 3), releasing average 11.54 mg.L-1 after sixty minutes of test; although in others pH conditions have also been considerable release. In sequential extraction, the results found that all metals (except Cu) were associated with phase oxides/hydroxides, but the phases considered more mobile (exchangeable and carbonate) were also present in significant concentrations for all metals. The five samples were considered heterogeneous and two of them (RP5 and RP7) had greater mobility of Pb and Zn, besides a higher toxicity in ecotoxicity tests. In aquatic ecotoxicity test, the samples showed immobility of neonates organisms even in less concentrated dilutions (dilutions containing only 6.2 and 3.1% solubilized extract). In conclusion, the waste may be considered problematic for environment, because the high concentrations of toxic metals present in tailing can be released from it and migrate to the ecosystems, causing exposure to terrestrial and aquatic organisms.

Abstract Millions of tons of mine tailings are generated worldwide annually. Since many valuable metals such as Ag, Cu, Pb, Zn, Au and Ni are usually incorporated into sulphidic minerals, a large proportion of the tailings generated contain high amounts of sulphates and heavy metals. Some of these tailings are used as paste backfill material at mining sites, but large amounts are still being deposited into the tailings dams under water coverage. Sulphidic minerals are stable underground but after mining of the ore and several processing steps these minerals can be oxidized when they come into contact with water and air. This oxidation generates acid and thus reduces the pH of the surrounding environment. Furthermore, the heavy metals present in the mine tailings can be leached into the environment. This phenomenon, called Acid Mine Drainage (AMD), is one of the most critical environmental issues related to the management of sulphidic-rich tailings. Since AMD generation can still occur hundreds of years after closure of the mine, the mine tailings need stable, sustainable and economically viable management methods in order to prevent AMD production in the long term. The aim of this PhD thesis was to study various solidification/stabilization (S/S) methods for the immobilization of sulphidic mine tailings. The main focus was to develop a suitable chemical environment for achieving effective heavy metal (mainly arsenic) and sulphate immobilization while simultaneously ensuring good mechanical properties. Three treatment methods were tested: alkali activation, stabilization using hydrated lime (Ca(OH)2) and blast furnace slag (GBFS), and calcium sulphoaluminate-belite (CSAB) cement stabilization. The mine tailings used in this study contained large amounts of sulphates and heavy metals such as Cr, Cu, Ni, Mn, Zn, V and As. The leaching of arsenic and sulphates from powdered tailings exceeded the legal limits for regular and inert waste. All treatment methods were found to generate a hardened matrix that was suitable for use as a backfilling or construction material, but the calcium-based binding system was the most suitable for effective immobilization of all the heavy metals (including arsenic) and the sulphates. Precipitation in the form of calcium sulphates/calcium arsenate and the formation of ettringite are the main stabilization methods employed in calcium-based stabilization/solidification (S/S) systems. Some evidence of physical encapsulation occurring simultaneously with chemical stabilization was noted. These results can be exploited further to develop more sustainable mine tailing management systems for use in the future. The tailings could be stored in a dry landfill area instead of in tailing dams, and in this way a long-term decrease in AMD generation could be achieved, together with a high potential for recycling. / Tiivistelmä Monet arvometallit kuten kulta, kupari ja nikkeli ovat sitoutuneena sulfidipitoisiin mineraaleihin. Louhittaessa ja rikastettaessa näitä sulfidimineraaleja syntyy miljoonia tonneja sulfidipitoisia rikastushiekkoja vuosittain. Rikastushiekat voivat sisältää myös runsaasti erilaisia raskasmetalleja. Osa rikastushiekoista hyödynnetään kaivostäytössä, mutta suurin osa rikastushiekoista läjitetään edelleen ympäristöön rikastushiekka-altaisiin veden alle. Kun sulfidipitoinen malmi kaivetaan ja käsitellään, sulfidiset mineraalit hapettuvat ollessaan kosketuksissa veden ja hapen kanssa. Hapettuessaan ne muodostavat rikkihappoa, laskien ympäristön pH:ta jolloin useimmat raskasmetallit liukenevat ympäristöön. Muodostuvia happamia kaivosvesiä voi syntyä vielä pitkään kaivoksen sulkemisen jälkeen ja ovat näin ollen yksi suurimmista kaivosteollisuuteen liittyvistä ympäristöongelmista. Lisäksi suuret rikastushiekka-altaat voivat aiheuttaa vaaraa myös ihmisille, mikäli altaan rakenteet pettävät. Rikastushiekkojen kestäviä ja ympäristöystävällisiä varastointimenetelmiä täytyy kehittää, jotta näitä ongelmia voidaan tulevaisuudessa ehkäistä. Tässä työssä tutkittiin menetelmiä, joilla kultakaivoksella syntyvät sulfidipitoiset vaaralliseksi jätteeksi luokitellut rikastushiekat saataisiin stabiloitua tehokkaasti. Työssä keskityttiin kolmeen erilaiseen menetelmään: alkali-aktivointiin, stabilointiin kalsiumhydroksidin ja masuunikuonan avulla ja stabilointiin CSAB sementin avulla. Valmistettujen materiaalien mekaanisia ja kemiallisia ominaisuuksia arvioitiin. Tavoitteena oli ymmärtää, miten eri menetelmät soveltuvat raskasmetallien (erityisesti arseenin) ja sulfaattien sitoutumiseen ja mikä on eri komponenttien rooli reaktioissa. Alkali-aktivoimalla rikastushiekkaa sopivan sidosaineen kanssa saavutettiin hyvät mekaaniset ominaisuudet ja useimmat haitta-aineet sitoutuivat materiaaliin. Ongelmia aiheuttivat edelleen sulfaatit ja arseeni. Kalsiumpohjaiset menetelmät sitoivat raskasmetallit (myös arseenin) ja sulfaatit tehokkaimmin. Sulfaatit ja arseeni saostuivat muodostaen niukkaliukoisia komponentteja kalsiumin kanssa. Samanaikaisesti rakenteeseen muodostui ettringiittiä, jolla on tutkitusti hyvä kyky sitoa erilaisia raskasmetalleja rakenteeseensa. Raskasmetallit myös kapseloituivat rakenteen sisään. Työn tuloksia voidaan hyödyntää, kehitettäessä rikastushiekkojen turvallista varastointia. Kun materiaalille saavutetaan riittävän hyvä lujuus ja kemiallinen stabiilius, rikastushiekat voitaisiin läjittää tulevaisuudessa kuivalle maalle altaan sijaan. Näin vältyttäisiin rikastushiekka-altaiden rakentamiselta ja voitaisiin vähentää happamien kaivosvesien muodostumista pitkällä ajanjaksolla. Saavutettujen tulosten perusteella rikastushiekkoja voidaan mahdollisesti tulevaisuudessa hyödyntää myös erilaisissa betonin tapaisissa rakennusmateriaaleissa.

Thickened tailings production and disposal continue to grow in importance in the mining industry. In particular, the transport of oil sands tailings is of interest in this study. These tailings must be in a homogeneous state (non-segregating) during pipeline flow and subsequent discharge. Tailings are often transported in an open channel or flume. Slurries containing both clay and coarse sand particles typically exhibit non-Newtonian rheological behaviour. The prediction of the flow behaviour of these slurries is complicated by the limited research activity in this area. As a result, the underlying mechanisms of solids transport in these slurries are not well understood. To address this deficiency, experimental studies were conducted with kaolin clay slurries containing coarse sand in an open circular channel.<p> A numerical model has been developed to predict the behaviour of coarse solid particles in laminar, open channel, non-Newtonian flows. The model involves the simultaneous solution of the Navier-Stokes equations and a scalar concentration equation describing the behaviour of coarse particles within the flow. The model uses the theory of shear-induced particle diffusion (Phillips et al., 1992) to provide a number of relationships to describe the diffusive flux of coarse particles within laminar flows. A sedimentation flux has been developed and incorporated into the Phillips et al. (1992) model to account for gravitational flux of particles within the flow. Previous researchers (Gillies et al., 1999) have shown that this is a significant mechanism of particle migration.<p> The momentum and concentration partial differential equations have been solved numerically by applying the finite volume method. The differential equations are non-linear, stiff and tightly coupled which requires a novel means of analysis. Specific no-flux, no-slip and no-shear boundary conditions have been applied to the channel walls and free surface to produce simulated velocity and concentration distributions. The results show that the model is capable of predicting coarse particle settling in laminar, non-Newtonian, open channel flows. The results of the numerical simulations have been compared to the experimental results obtained in this study, as well as the experimental results of previous studies in the literature.

Thickened tailings production and disposal continue to grow in importance in the mining industry. In particular, the transport of oil sands tailings is of interest in this study. These tailings must be in a homogeneous state (non-segregating) during pipeline flow and subsequent discharge. Tailings are often transported in an open channel or flume. Slurries containing both clay and coarse sand particles typically exhibit non-Newtonian rheological behaviour. The prediction of the flow behaviour of these slurries is complicated by the limited research activity in this area. As a result, the underlying mechanisms of solids transport in these slurries are not well understood. To address this deficiency, experimental studies were conducted with kaolin clay slurries containing coarse sand in an open circular channel.<p> A numerical model has been developed to predict the behaviour of coarse solid particles in laminar, open channel, non-Newtonian flows. The model involves the simultaneous solution of the Navier-Stokes equations and a scalar concentration equation describing the behaviour of coarse particles within the flow. The model uses the theory of shear-induced particle diffusion (Phillips et al., 1992) to provide a number of relationships to describe the diffusive flux of coarse particles within laminar flows. A sedimentation flux has been developed and incorporated into the Phillips et al. (1992) model to account for gravitational flux of particles within the flow. Previous researchers (Gillies et al., 1999) have shown that this is a significant mechanism of particle migration.<p> The momentum and concentration partial differential equations have been solved numerically by applying the finite volume method. The differential equations are non-linear, stiff and tightly coupled which requires a novel means of analysis. Specific no-flux, no-slip and no-shear boundary conditions have been applied to the channel walls and free surface to produce simulated velocity and concentration distributions. The results show that the model is capable of predicting coarse particle settling in laminar, non-Newtonian, open channel flows. The results of the numerical simulations have been compared to the experimental results obtained in this study, as well as the experimental results of previous studies in the literature.

The use of geosynthetic clay liners (GCLs) as (i) covers for arsenic-rich gold mine tailings and landfills, (ii) subsurface barrier for migration of hydrocarbons in the Arctic, and (iii) basal liner for sewage treatment lagoons were examined. After 4 years in field and laboratory experiments, it was found that best cover configuration above gold mine tailings might include a layer of GCL product with polymer-enhanced bentonite and a geofilm-coated carrier geotextile serving above the tailings under ≥ 0.7 m overburden. However, acceptable performance could be achieved with using a standard GCL with untreated bentonite provided that there is a minimum of 0.7 m of cover soil above the GCL. When GCL samples were exhumed from experimental landfill test cover with complete replacement of sodium in the bentonite with divalent cations in the adjacent soil, it was observed that the (i) hydraulic head across the GCLs, (ii) size of the needle-punched bundles, and (iii) structure of the bentonite can all significantly affect the value of the inferred in-situ hydraulic conductivity measured at the laboratory. The higher the hydraulic head and the larger the size of the needle-punched bundles, the higher the likelihood of internal erosion/structural change of bentonite at bundles that will cause a preferential flow for liquids to occur. A key practical implication was that GCLs can perform effectively as a single hydraulic barrier in covers provided that the water head above the GCL kept low. The hydraulic performance of a GCL in the Arctic was most affected by the location within the soil profile relative to the typical groundwater level with the highest increase in the hydraulic conductivity (by 1-4 orders of magnitude) for GCL below the water table. However, because the head required for jet fuel to pass through the GCL was higher than that present under field conditions, there was no evidence of jet fuel leakage through the barrier system. The leakage through GCLs below concrete lined sewage treatment lagoons was within acceptable limits, in large part, due to the low interface transmissivity between GCLs and the overlying poured concrete. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2014-02-28 08:53:29.171

Once a source of environmental concern, mine tailings could now contribute to the fight against climate change. Greg Dipple and team are discovering how mines can potentially ofset their own emissions.

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O problema da disposição de resíduos de mineração e a ruptura de barragens de rejeitos preocupa engenheiros e órgãos ambientais nacionais e internacionais. Estes materiais possuem características geotécnicas distintas dos geomateriais encontrados em depósitos naturais, têm granulometria predominantemente siltosa e condutividade hidráulica intermediária. Como o ensaio de palheta constitui-se em uma técnica rotineira na estimativa da resistência ao cisalhamento não-drenada de argilas, procura-se através da presente pesquisa ampliar este conhecimento a solos siltosos. Dentre os fatores que influenciam as medições da resistência não-drenada, a velocidade de rotação pode ser considerada o mais importante, particularmente quando há ocorrência de drenagem parcial durante o cisalhamento. A ocorrência de drenagem parcial durante ensaios de campo em resíduos siltosos gera incertezas na interpretação e na estimativa de parâmetros de projeto. Assim, o objetivo deste trabalho consiste em avaliar a influência da velocidade de rotação da palheta na resistência não-drenada de um material siltoso, de granulometria semelhante aos resíduos de ouro, através de ensaios de laboratório utilizando uma mini-palheta. Para tal, procedeu-se a automatização e adaptação de um equipamento de palheta de laboratório, inicialmente manual, para aplicação de diferentes velocidades de rotação. Os ensaios de mini-palheta foram realizados a velocidades de rotação entre 0,5 e 54 rpm, que correspondem a velocidades adimensionais (V) entre 2 e 276. Os resultados dos ensaios de mini-palheta mostraram, de modo geral, um aumento na resistência com a diminuição da velocidade de rotação da palheta, devido à ocorrência do adensamento a baixas velocidades de rotação. O tempo de ruptura diminuiu e o deslocamento até a ruptura aumentou com o aumento da velocidade de rotação. Observou-se uma grande dispersão nos resultados dos ensaios, provavelmente devido à relação entre o tamanho da palheta e o tamanho dos grãos do solo. Com relação às condições de drenagem impostas ao solo durante o cisalhamento, observou-se que as condições não-drenadas foram satisfeitas para valores de V superiores a 10+2, enquanto as condições drenadas ocorrem para V inferior a 1. Assim, quando 1<V<10+2 há ocorrência de drenagem parcial durante o cisalhamento. A interpretação através da normalização de resultados, correlacionando a velocidade adimensional V ao grau de drenagem U, possibilita a identificação da ocorrência de drenagem parcial, evitando a superestimativa das propriedades geotécnicas dos solos em ensaios de campo. / The problem of tailings disposal and dams failure concerns engineers and national and international environmental agencies. These materials have geotechnical characteristics that differ from geomaterials found in natural deposits, are predominantly silty and have intermediate hydraulic conductivity. As the vane test is a usual technique to estimate the shear strength of clays, the present study extend this knowledge to silty soils. Among the factors that influence the undrained strength measurements, the rotation rate can be considered the most important, particularly when partial drainage during shear occurs. The occurrence of partial drainage during field tests on silty tailings generate uncertainties in the interpretation and estimation of design parameters. The purpose of this study is to evaluate the influence of mini-vane test rotation rate on the undrained strength of a silty soil, with a particle size similar to the gold tailings, through laboratory mini-vane tests. Thus, the automation and customization of a mini-vane test equipment initially manual was developed for the application of different rotation rates. Mini-vane tests were carried out at rotation rates between 0.5 and 54 rpm, corresponding to dimensionless velocities (V) between 2 and 276. The mini-vane tests results showed, in general, an increase in resistance with decreasing the rotation rate due to the occurrence of consolidation of soil at low shearing rates.The time to failure decreased and the displacement to failure increased with increasing rotation rate. There was a wide dispersion of test results, probably due to blade size to soil grain size ratio. Regarding the drainage conditions imposed to the soil during shear, the tests have shown that the undrained conditions are satisfied for V values greater than 10+2, while the drained conditions occurs for V values lower than 1. When 1<V<10+2 partial drainage occurs during shear. The interpretation using normalized results correlating the dimensionless velocity V to the drainage degree U allows the identification of partial drainage occurrence, avoiding the overestimation of soil geotechnical properties in field tests.