Limestone Drain Design Parameters for Acid Rock Drainage Mitigation

Abstract.  Limestone drains are an integral component of some of the most efficacious passive systems for the treatment of acid rock drainage (ARD). A critical design parameter for a limestone drain is the mass of limestone that will be required for effective treatment. This in turn depends on the flow rate, limestone dissolution rate, and associated hydraulic detention time necessary to achieve a certain effluent alkalinity for a given design life. Rates of alkalinity generation and limestone dissolution, and the quality of the limestone in terms of weight fraction of CaCO₃ and percentage CaCO₃ available must be known to determine the required mass of limestone. These parameters were experimentally determined for a natural and synthetic suite of ARD waters. The experimental results show that the empirical change in the alkalinity over time cannot simply be modeled as a first-order process. During the initial stage, the concentration increase is extremely fast, giving a linear and steep rise in the alkalinity. Alkalinity concentrations peaked at elapsed times of 90 to 180 minutes and then declined to a nearly constant value. The decline in alkalinity corresponds with a decline in concentrations of dissolved iron and aluminum, implying that the consumption of alkalinity by the hydrolysis of these metals is faster than the rate of alkalinity production by the dissolution of limestone, so that the reaction appears zero order in the early stage and pseudo-first order later. Because the changes in concentration were complex, considerable uncertainty exists in the rate constants for estimating alkalinity concentration. This causes problems in design equations based solely on a limestone dissolution rate that is estimated from alkalinity generation rates. A more applicable design procedure, combining the kinetics of alkalinity production and consumption with the hydraulics and chemical equilibrium of the system, is illustrated. Research was conducted while the first and third authors were with Ecology and Environment, Inc, Dallas, TX, USA;     

Note: Compact high voltage pulse transformer made using a capacitor bank assembled in the shape of primary

The experimental results of an air-core pulse transformer are presented, which is very compact (<10 Kg in weight) and is primed by a capacitor bank that is fabricated in such a way that the capacitor bank with its switch takes the shape of single-turn rectangular shaped primary of the transformer. A high voltage capacitor assembly (pulse-forming-line capacitor, PFL) of 5.1 nF is connected with the secondary of transformer. The transformer output voltage is 160 kV in its second peak appearing in less than 2 μS from the beginning of the capacitor discharge. The primary capacitor bank can be charged up to a maximum of 18 kV, with the voltage delivery of 360 kV in similar capacitive loads.     

XYLANASE EFFECTS ON PULP DELIGNIFICATION

Xylanase effects on softwood pulp delignification were investigated experimentally and using mathematical models. The effect of xylanase molecular size on pulp delignification was investigated. As xylanase molecular weight decreased from 67 kDaltons to 20 kDaltons, lignin removal from pulp increased from 30 wt% to 48 wt%, respectively. The rate of xylanase-aided pulp delignification was studied using a batch system. The rate-data was fitted to a mathematical model of the batch system that enabled estimation of process parameters including xylanase and lignin effective diffusivities, lignin mass transport coefficient, and effective particle diameter for mass transport. Parameter values thus obtained were used to simulate the semi-batch delignification process, which predicted 84.6% of lignin available to UGA xylanase (MW=39,000 Daltons) would be removed in 2.5 h.     

Performance Analysis of Ferroelectric GAA MOSFET with Metal Grain Work Function Variability

Silicon - This work represents a unique GAA MOSFET with metal work-function variations (WFVs) and ferroelectric material as dielectric. A random distribution of metal grain (TiN) with grain size...     

Online monitoring of laser remelting of plasma sprayed coatings to study the effect of cooling rate on residual stress and mechanical properties

This investigation deals with laser remelting of plasma sprayed alumina and chromia coatings. The time-temperature history of the laser remelted zone was recorded using an infrared pyrometer during the remelting operation. Cooling rates, under varying scanning speed, were determined from the time temperature curve. Surface morphology, microstructure, and phases of the laser treated and as-sprayed coatings were characterized using scanning electron microscopy, optical microscopy, X-ray diffraction, respectively. X-ray diffraction was also employed to measure the surface residual stress of the coatings. Inherent features of plasma sprayed coatings like porosity and inter-lamellar boundary were obliterated upon laser remelting. A columnar grain growth perpendicular to the laser scanning direction was observed. The range of roughness of the as-sprayed coatings reduced from 6 to 8?µm to 1–2?µm in the remelted layers. For both coatings, more than 90% reduction in porosity was found upon laser remelting. Surface residual stress of the as-sprayed alumina and chromia coatings was found to be tensile and compressive, respectively. Within the limits of the testing condition the tensile residual stress of the remelted layers increased by up to around 500% in the alumina coatings. In the chromia coating a decrease of compressive stress by up to around 80% was recorded. In the remelted layer the tensile nature of the stress showed a tendency to increase with an increase in the cooling rate. However, the state of stress of the as-sprayed layer, i.e., tensile or compressive, was retained in the remelted layer. The residual stress was found to decrease in the remelted layer with an increase in the degree of overlap of the remelted tracks.