Alison E. Lewis

A mathematical model of a high sulphate wastewater anaerobic treatment system

As an aid to the design and operation of anaerobic digesters treating high sulphate waste waters, a mathematical model describing this treatment process has been developed. Apart from sulphate reduction, the model includes those reactions which occur either prior to sulphate reduction or in competition with it. These include, hydrolysis of solid substrates, acidogenesis. beta oxidation of long chain fatty acids, acetogenesis and methanogenesis. By incorporating terms for these reactions the model is able to simulate sulphate reduction using a wide range of carbon sources. Acid/base equilibrium chemistry is included in order to predict the pH and unionized component concentrations, needed for calculating inhibition. An activity based model is used, with the activity coefficients calculated using Debye-Hückle theory. The mass transfer rates of hydrogen, methane, carbon dioxide and hydrogen sulphide from the liquid to the vapour phase are also included. A number of different reactor types may be simulated, including a dynamic batch. steady state CSTR and dynamic CSTR. By separating the hydraulic and solids residence times, high rate reactors such as UASB and packed bed reactors may also be simulated. The model has been used to successfully predict the dynamic and steady state behaviour of a number of different reactor types, utilizing both simple and complex carbon sources.

Effect of solution chemistry on particle characteristics during metal sulfide precipitation

Metal sulfide precipitation forms an important component of acid mine drainage remediation systems based on bacterial sulfate reduction. The precipitation reaction is thermodynamically favorable, but a number of technical issues remain. In this study the effect of metal to sulfide molar ratio and operating pH on the nature and settling characteristics of copper and zinc sulfide precipitates was studied in a CSTR. A large number of small copper sulfide particles, with highly negatively charged surfaces and poor settling characteristics, were formed in the presence of a stoichiometric excess of sulfide at pH 6. The size and the settling characteristics of the particles were significantly improved, while the number of particles and magnitude of their zeta potential decreased when experiments were conducted at pH values <6. By comparison, for zinc sulfide, a small change in the number and size of the particles was observed for all metal to sulfide molar ratios and tested operating pH values. Precipitates generated at pH 6 had the most negative zeta potential, while operating at pH values <6 reduced the magnitude of the negative surface charge and improved the settling and dewatering characteristics of the precipitate. The data indicated that the amount of reactive sulfide species (HS(-) and S(2-) ions) available in solution during the precipitation process was important in determining the nature and surface characteristics of the particles produced and this was mainly dependent on pH.

Metal sulphides from wastewater: assessing the impact of supersaturation control strategies.

Metal sulphide precipitation forms an important component of acid mine drainage remediation systems based on bacterial sulphate reduction. However, the precipitation reaction is inherently driven by very high levels of supersaturation with the generation of small particles with poor solid-liquid separation characteristics. In this study, the effect of strategies used to manage supersaturation was investigated during copper and zinc sulphide precipitation reactions. Initial batch studies showed the origin of sulphide (biological or chemical) had no significant effect on the efficiency of zinc sulphide precipitation. For copper, low metal removal efficiency was obtained at metal to sulphide molar ratios below 1.6 in the synthetic sulphide system. This was improved in the biogenic sulphide system, due to the presence of residual volatile fatty acids, but the presence or absence of particulate organic matter had no effect on recovery. Subsequent studies, conducted using synthetic sulphide solutions in a seeded fluidised bed reactor with multiple reagent feed points (2FP and 6FP) and different recirculation flow rates (300 and 120 mL min(-1)) showed efficient zinc sulphide precipitation, but limited (<10%) deposition on the seeds. Increasing the number of sulphide feed points (2-6) reduced precipitate loss as fines by approximately 10%. Zinc sulphide fines could be effectively recovered from suspension by settling under quiescent conditions. In the copper system, metal recovery was low (ca 40%) due to the formation of very small copper sulphide particles (mean particle size of ca 0.01 μm). Increasing the number of reagent feed points did not affect supersaturation to the extent of altering particle characteristics. The copper sulphide fines could not be recovered by settling, remaining in a stable colloidal suspension due to their highly charged surfaces (zeta potential -50 mV). The change in recirculation flow rate had a limited effect (ca 5% improvement) on process efficiency. The results show that the extremely high supersaturation prevalent during metal sulphide precipitation is difficult to control using conventional approaches and suggest that the seeded fluidised bed reactor is not suitable for this application.

Prioritising objectives for waste reprocessing: a case study in secondary lead refining

Secondary lead refining produces a sulphidic slag that also contains varying quantities of lead. Initially, the objectives of this project were to treat the slag in order to recover the valuable lead as well as to render the slag environmentally benign. However, in keeping with the principles of clean technology and, specifically, the approach of clean production, the project was redefined with the following priorities: waste characterisation; waste minimisation through process improvement; waste modification; identification of slag treatment methods. Characterisation of the waste facilitated an overall process understanding and aided in identifying process deficiencies. Process improvement was aimed at reducing both the quantity of slag produced as well as the lead loss to the slag. Waste characterisation combined with local hazardous waste regulations enabled desirable waste modifications to be identified. These waste adaptations were implemented through process modification. Lastly, treatment methods for the slag were identified.

Classical and Nonclassical Theories of Crystal Growth

In this chapter, we discuss classical and nonclassical concepts of crystal growth that coexist in the literature as explanations for the formation of both mono- and polycrystalline particles, often of the same substances. Crystalline particles with intraparticle nanosized subunits, nanoparticulate surface features, and complex morphologies have led to the development of new nonclassical theories of crystal growth based on the aggregation of nanocrystals in solution. At the same time, similar morphologies are explained by monomer incorporation at conditions of stress incorporation, which results in nucleation at the growth front and accompanying branching at the nanoscale. The two mechanisms are differently affected by important process variables like supersaturation, temperature, or additives and are analyzed with respect to their capability of predicting crystal growth rates. A quantitative description of the formation kinetics of the solid phases is essential for the design and operation of industrial precipitation and crystallization processes and for the understanding of fundamental principles in material design and biomineralization processes. In this chapter, we emphasize the importance of supersaturation in order to account for the extensive nanoparticle formation required to build micron-sized particles by nano-aggregative growth, as well as the accompanying change in the population density.

Treatment of textile wastewaters using Eutectic Freeze Crystallization

A water treatment process needs to recover both water and other useful products if the process is to be viewed as being financially and environmentally sustainable. Eutectic Freeze Crystallization (EFC) is one such sustainable water treatment process that is able to produce both pure ice (water) and pure salt(s) by operating at a specific temperature. The use of EFC for the treatment of water is particularly useful in the textile industry because ice crystallization excludes all impurities from the recovered water, including dyes. Also, EFC can produce various salts with the intention of reusing these salts in the process. This study investigated the feasibility of EFC as a treatment method for textile industry wastewaters. The results showed that EFC can be used to convert 95% of the wastewater stream to pure ice (98% purity) and sodium sulfate.

Three phase mixing studies for nickel precipitation

Abstract Hydrodynamics, temperature, pH and various other physico-chemical factors influence the morphology of nickel produced via hydrogen reduction. The focus of the current work is the effect on hydrodynamics of changing the impeller and reactor configurations in a 75 l stirred vessel with draft tube and baffles. The aim was to determine which configuration resulted in maximum particle suspension and local gas hold-up while using the minimum impeller speed and power consumption. A response surface methodology of experimental design was employed. This ensured that the number of candidate variables to be tested was reduced to a minimum and that interactive effects between variables were taken into account. The impeller configurations tested were a single Rushton turbine, a single axial flow impeller, and a double impeller system consisting of a combination of the two. The reactor configurations, tested at different gas flow rates, were varied to test the effects with and without baffles. It was found that optimum mixing could be achieved using a baffled vessel with an upper axial flow impeller and a lower Rushton turbine, and by keeping a minimum impeller clearance from the vessel bottom. This is in agreement with [Mineral Processing, 1–2 August 2002].

Improving platinum precipitation processes

Abstract The precipitation of platinum is often enhanced by the addition of wood flour; a very fine, nonvaluable material as seed. The wood flour acts as surface area for precipitate deposition that ensures the formation of reasonable sized particles and prevents undesirable mirroring and scalingon the reactor walls. However, the downstream removal of wood flour is time consuming and expensive. This work investigated the potential of platinum precipitation without wood flour. The ideal product would be grey, dense, have large particle size, large surface area, little evidence of platinum flakes (these being undesirable to the customer) and have no mirroring or scale formation on the reactor wall (causing lost platinum). The method and results are presented in a way that does not disclose priority knowledge.

Improved calcium sulfate recovery from a reverse osmosis retentate using eutectic freeze crystallization

A novel low temperature crystallization process called eutectic freeze crystallization (EFC) can produce both salt(s) and ice from a reverse osmosis (RO) stream by operating at the eutectic temperature of a solution. The EFC reject stream, which is de-supersaturated with respect to the scaling component, can subsequently be recycled back to the RO process for increased water recovery. This paper looks at the feasibility of using EFC to remove calcium sulfate from an RO retentate stream and compares the results to recovery rates at 0 and 20 °C. The results showed that there was a greater yield of calcium sulfate obtained at 0 °C as compared with 20 °C. Operation under eutectic conditions, with only a 20% ice recovery, resulted in an even greater yield of calcium sulfate (48%) when compared with yields obtained at operating temperatures of 0 and 20 °C (15% at 0 °C and 13% at 20 °C). The theoretical calcium recoveries were found to be 75 and 70% at 0 and 20 °C respectively which was higher than the experimentally determined values. The EFC process has the added advantage of producing water along with a salt.

The Dewatering Behaviour of Transformed Ferri-Oxyhydroxide Precipitates Formed Under Moderate Temperature and Varying Fe(III) Concentrations

Mining operations generate Acid Mine Drainage (AMD) that poses a significant threat to the natural environment. AMD treatment using Ca(OH)2 leads to the precipitation of a sludge dominated by a mixture of ferri-oxyhydroxides. This sludge has poor dewatering tendencies and is deposited in landfills with potential for metal remobilization. This study investigated the dewatering behaviour of a precipitate formed during elevated temperature treatment of a primary, pre-settled sludge from the neutralisation of aqueous acidic Fe2(SO4)3 and Ca(OH)2 solutions in an MSMPR reactor. The resulting treated precipitates were analysed for micro-properties, with results showing that an increase in the secondary reactor temperature from 25 °C up to 50 °C led to an increase in mean particle size, a decrease in the number of particles and improved dewatering behaviour at a ferric concentration of 300 mg/L. This was ascribed to the attainment of a circum-neutral surface charge that favoured agglomeration and a change in micro and nano-structure that allowed for better water passage.