Academic literature on the topic '628.161.2'

Magnetorheological Finishing, MRF can achieve deterministic results on different surface shapes such as flat surfaces, spherical surfaces and aspheric surfaces. MRF can also overcome many drawbacks of the conventional polishing process. The schedule uncertainties driven by edge roll and edge control are virtually eliminated with the MRF process. This paper presents a polishing wheel that combines rotation motion with revolution motion while renewing the MR fluid. Stable removal characteristic can be obtained by using the composite-rotation wheel. This paper also presents some recent results of the deterministic finishing typified by the tool and its MR fluid circulation system. An example of finishing a square optical workpiece with spherical surface will be reviewed.

A voltammetric procedure for the determination of dissolved and colloidal iron in mine-waters has been developed. Whilst mine-waters are of course enriched in iron. we are remarkably ignorant of the physical state and chemical speciation of the iron. This is a problem since the physical and chemical state of iron is central to understanding a range of processes relevant to mine-water geochemistry and remediation. Examples include hydrolysis of dissolved Fe (III) to release protons. the adsorption of trace metals onto iron colloids and the bioavailability of iron within wetlands designed to remediate acidic waters. In this work, we have developed differential pulse voltammetry (DPV) as a rapid and robust method of determining the concentration of truly dissolved and colloidal iron in 0.45 J.lm filtered waters from a series of mine-water discharges and remediation sites in NE, England. Mine-water samples were collected from CoSTaR sites: these are abandoned mine sites in the UK, designated by the UK Coal Authority for remediation research and routine monitoring of water quality. The sites comprise of six full-scale bioreactors receiving a wide range of mine-waters with pH ranging from 3 to 5 and concentrations of < 0.45 Jlm iron between 30 and 800 mg L-1 across the sites. Monthly samples were collected over the period March 2006 to April 2007. The samples were analysed directly using differential pulse voltammetry (DPV) at gold electrode. The results show that our analysis provides data for total dissolved iron of comparable analytical quality to the established mine-water analysis techniques based on inductively couple plasma spectroscopy (ICP-OES). The good agreement between the iron concentrations measured in acidified samples electrochemically and by ICPOES validates the accuracy of DPV as an analytical method for iron. Colloidal and particulate iron was also determined since DPV measures only dissolved iron. particulate (>0.45 Jlm) and/or colloidal «0.45 Jlm) iron can then be estimated as the difference between the voltammetric responses of natural samples and samples in which the solid phase iron has been dissolved by the addition of He!. v The percentage dissolved iron ranged from 60-90% (in most cases) in unfiltered samples, while the percentage of colloidal iron varied widely across the sites; from 25-45% in unfiltered samples and 50-75% and 38-85% for dissolved and colloidal iron in the 0.45 Jim filtered samples. The ratio of Fe (II) to Fe (III) in the dissolved fraction was detennined usmg ultramicroelectrodes (UME) method. Iron ratio varied widely for the three sites studied. However, in general, the ratio is 1: 1 for the surface influent waters. 1:3 for the sub-surface waters (underground water-Shilbottle site) and 3: 1 for most of the effluent samples. Results suggest that in general, the influent waters are more oxidised and the effluent more reduced. Finally, characterisation of solid phase iron was done usmg a wide range of spectroscopic techniques. Atomic Force Microscopy (AFM) shows that iron colloids range from nm to Jim for lower pH mine waters; at higher pH, particles mainly aggregates on the Jim to mm scale. FT-IR, XRD, TEM and EDX show that the most common colloidal phase is poorly crystalline Fe oxyhydroxides, however certain unusual crystalline phases, e.g., Schwertmannite were found.

Current design practice for aerobic wetlands treating net-alkaline mine water in UK applications of passive treatment is based on zero-order kinetics for pollutant removal; the commonly used area-adjusted removal formula. Lagoons are designed to allow 48 hours of estimated retention time. However, there is significant variation in performance between systems. Neither of these approaches takes account of the hydraulic factors that may influence treatment performance. Therefore, this study aimed to improve understanding of both hydraulic and geochemical factors that govern contaminant behaviour, such that future design of treatment systems is able to optimise treatment efficiency and make performance more predictable, and improve performance over the long-term. Assessment of the hydraulic behaviour (flow pattern) of the treatment systems was accomplished by means of tracer tests. The tracer tests and simultaneous sampling of mine water were undertaken at eight UK Coal Authority mine water treatment systems (lagoons and wetlands) within Northern England (main study areas) and part of southern Scotland. Analyses of mine water samples were also undertaken in the laboratory alongside the field tests for assessment of geochemical processes controlling iron removal in the lagoons and wetlands studied. Analyses of the tracer test results were performed using a residence time distribution (RTD) analysis to account for the different shapes of tracer breakthrough curves observed. There appear to be multiple influences that possibly affect the RTDs in lagoons and wetlands e.g. vegetation and seasonal variation (growing or non-growing season), system age, flow and geometry (length-to-width ratio and depth). The RTD analysis shows that lagoons generally have a more dispersed flow pattern, associated with a more pronounced short-circuiting effects and a long tail compared to wetlands. A modelling approach using a tanks- in-series (TIS) model was adopted to precisely analyse and characterise the RTDs, in an effort to account for the different flow patterns across the treatment systems. Generally, lagoon RTDs are characterised by a greater flow dispersion compared to wetlands (i.e. higher dispersion number, D and lower number of TIS, n). Consequently, the hydraulic efficiency, e for lagoons is much lower than wetlands (mean of 0.20 for lagoons compared to 0.66 for wetlands). This is attributed primarily to a much lower volumetric efficiency, ev in lagoons, meaning that a greater proportion of the total volume of the lagoon system is not being involved in the flow of water through them, with implications for design to optimise performance. In contrast, in wetlands a greater volumetric efficiency is evident, and there is therefore a longer relative mean residence time for retention and attenuation of iron. On the evidence of field data, in lagoon systems the iron removal processes are primarily controlled by ferrous iron oxidation, whilst in wetlands the removal is controlled by iron settlement. The time- and concentration-dependence of iron removal (oxidation and / or settlement rate) has also been investigated in the laboratory alongside the field data. The rates are faster in lagoons compared to wetlands due to higher concentration of iron available for the processes. General trends showed that efficient treatment performance for iron removal corresponds with greater system hydraulic efficiency in wetlands compared to lagoon systems. The greater hydraulic efficiency in wetlands was mainly attributed to a grea ter volumetric efficiency in the wetland systems. In contrast, shorter relative mean residence time was found in lagoons, thus a lower retention time for iron attenuation and lower removal efficiency as a consequence. For lagoon systems, performance can be optimised by ensuring greater volumetric efficiency (hence residence time), which can be achieved with a large length-to-width ratio system (up to a ratio of 4.7), but also a greater depth (up to 3.0 m), though only if systems are regularly maintained (dredged). For wetlands, the use of the area-adjusted removal rate formula appears to work well for the design of aerobic wetlands, despite the observed concentration-dependence of iron removal processes. However, use of first-order removal formula (TIS basis) would be a more appropriate approach to the design of mine water treatment systems since it takes account of the flow pattern effect on pollutant removal processes, in addition to the first-order kinetics (concentration-dependence) for iron removal. Regular sludge removal (yearly) is recommended in lagoons to provide longer residence time because lagoon depth and volume tends to rapidly decrease over time due to build up of ochre and debris (7- 49% depth reduction per year). Thinning of reeds is recommended whenever apparent channelisation would otherwise dominate the flow pattern, and therefore limit the capacity for adsorption and settlement of precipitated iron hydroxide.

This study discusses the effects of past mining activities on sediment and water quality in streams and rivers in the Tamar catchment. High trace element concentrations, both in water and sediments, were observed in streams and rivers draining areas associated with abandoned mine sites. Maximum concentrations were observed in the Gunnislake/Calstock mining district, where intense metalliferous mining took place during the 19th century. Mine waste from abandoned mine sites in this area contained up to 6.3% arsenopyrite, 5.8% pyrite, 0.3% chalcopyrite and 24% scorodite. As a result, high concentrations of trace elements of up to 180000 mg kg‾¹ As, 6500 mg kg‾¹ Cu were determined in these wastes. Sequential extraction of the mine waste revealed that in most cases, the oxidisable fraction accounted for large proportions of mobile species, followed by the reducible fraction. The exchangeable fraction was relatively low, except for Cu in samples from fine grained waste heaps, in which significant amounts of secondary minerals, such as Fe oxides/oxyhydroxides and Fe-As-O minerals were observed, suggesting trace elements had the tendency to be retained and recycled within the fine grained waste heaps. The Fe oxides/oxyhydroxides can contain up to 12% As and Fe-As-O minerals can contain up to 25% As and 6% Cu, indicating that the As and Cu associated with Fe oxide phases represent their reducible fraction. The coarse grained waste heaps, with higher permeability and low cohesion characteristics, had a higher potential to produce acid leachate and were more susceptible to erosion than the fine grained waste heaps. Contaminants from abandoned mine sites entered aquatic systems within the catchment, as shown by the high concentrations of trace elements (up to 25000 mg kg‾¹ As, 28000 mg kg‾¹ ' Cu, 32000 mg kg‾¹ Mn, 9200 mg kg‾¹ Pb and 2700 mg kg‾¹ Zn) observed in sediments in water channels draining these mine sites. Some streams and adits draining abandoned mine sites carried acidic waters with pH values frequently below pH 4. Dissolved concentrations up to 560 μg L‾¹ As, 7600 μg L‾¹ Cu, 3800 μg L‾¹ Fe, 5700 μg L‾¹ Mn, 170 pg L‾¹ Pb and 2500 μg L‾¹ Zn, and particulate concentrations up to 1600 μg UI As, 7900 μg L‾¹ Fe, 290 pg U' Ni, 11 pg U' Pb and 91 μg L-¹ Zn were observed in channels draining abandoned mine sites. In total, the annual flux of trace elements from 25 studied streams and adits input ca. 13,000 kg Fe, 4300 kg Mn, 4200 kg Cu, 3600 kg Zn, 1400 kg As, 400 kg Ni, 350 kg Co, 43 kg Pb, and 6.6 kg Cd into the Tamar estuary. Seven important point sources of metals to the River Tamar were identified. The mass balance calculation revealed that over 50% of trace elements were not accounted for by the studied point sources, suggesting an importance of diffuse sources. The inputs of solid and dissolved contaminants from the intensive mining district affect the water and sediment quality of the Tamar estuary, an important ecosystem in southwest England. This work has provided important information on the relative importance of point and diffuse sources, ‾which is essential in the formulation of effective catchment management strategies