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- Maronga, Savini
(author)
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On the optimization of the fluidized bed particulate coating process
- 1998
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Doctoral thesis (other academic/artistic)abstract
- Different aspects that influence the fluidked bedparticulate coating process have been investigated. An easy touse procedure for creating temperature and humidity profilesfor the gas inside the bed was developed. The procedureinvolves measuring a limited number of points inside the bedand using these points generate more data points used for thecreation of the profiles. The profiles revealed that differentparts of the bed have a different involvement in the coatingprocess. Apart from showing the hydrodynamic conditions insidethe bed, the profiles can be used to optimize the size of thebed, to map the net migration of particles and to set thespraying rate limits.The three-domain representation of the bed was used to modelthe coating process. The model showed that large beds and theexistence of stagnant region within the bed widen the toatingdistribution. Increasing the rate of transfer to the sprayingdomain or increasing the overall mixing of the bed Will havethe opposite effect of narrowing the coating distribution. Theunequal transfer between domains can be used to manipulate thetoating distribution. The combination of bed ske, differentrates of transfer between domains, size of spraying domain andthe rate of spraying can be used to obtain particles that arecoated to a particular distribution.The mechanism and kinetics of growth in a top sprayingprocess were determined by coating two poly-distributed seedparticles with a cellulose under different operatingconditions. The resulting particle distribution reveals thatnot all particles in the bed are equally coated. For the topspraying process, smaller particles Capture more toating thanlarger particles. A narrowing of the seed distribution wasfound to increase the chance of toating the large particles.The results also confirmed that particles are coated only aftervisiting the spraying region, which is small compared to therest of the bed. A growth model developed using theexperimental results introduces a segregation factor whichrepresent the probability of different particle sizes beingcoated. For the top spraying coating of lactose particle with acellulose, the segregation factor was found to be anexponential decaying function of the particle weight.Keywords:granulation, coating, surface layering,segregation, coating uniformity, temperature protile, humidityprofile, growth kinetics, particle coating.
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- Puura, Erik
(author)
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Weathering of mining waste rock containing alum shale and limestone
- 1998
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Doctoral thesis (other academic/artistic)abstract
- Geochemical processes occurring in the waste rock dumps ofan old phosphate mine, located in Maardu, northem Estonia, havebeen studied and modelled. The waste rock contains pyritic,metalliferous and organic-rich shale, as well as limestone.Production of acidity during pyrite oxidation is a potentialmechanism for heavy metal release. Under certain conditions,such as close proximity to steep slopes, active oxidation ofpyritc and organic matter leads to the spontaneous combustionof the waste rock.The study combines the results of field observations andsample analyses with physico-chemical modelling of thepredominating processes. The main approach has been to specifythe solid phase transformations and use reaction frontmovements for indicating and quantifying the development ofthese processes. The model of low temperature shale pyriteoxidation calculates two-stage diffusional transport of oxygen:(1) in the gaseous phase, through large dump pores, and (2)dissolved in the water, through shale pores. The concept oflotal hydrochemical equilibrium is used to describe theinteraction between acidic water and calcite. From themodelling results for different scenarios it appests, that inthe long-tenn, the areas where the shale was dumped selectivelybelow the rest of tbe overburden, are most likely to generateacid leachate.The other important phenomena are the dissolution of illitewith the formation of smectite and K-jaros&and thedissolution of magnesium-containing limestone followed bygypsum formation and a build-up of a high Mg/Ca ratio in theleachate. Inclusion of these factors into the model explainsthe leachate formation as a sequence of chemical reactions. Themain uncertainty remaining is the decrease in availablecarbonate buffering capacity due to the encapsulation of thelimestone lumps by secondary precipitates. The phenomena ofspontaneous combustion removes pyritic acidity from the dump ina gaseous phase. A variety of different altered shale types andnew minerals appear in the heated areas.The basic message forenvironmentally aware shale handling in association with theestablishment of new phosphate open pits in Estonia is toundertake storage under anoxic conditions.Keywords:Estonia, modelling, waste rock, alum shale,heavy metals, pyrite, oxidation, leachate, diffusion,buffering, pH, PHREEQC, transport, equilibrium, convection,spontaneous combustion, jarosite, ferrit oxyhydroxides, gypsum,illite, smectite.
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- Yan, Jinying
(author)
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Major leaching processes of combustion residues : characterisation, modelling and experimental investigation
- 1998
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Doctoral thesis (other academic/artistic)abstract
- Characterising leaching behaviour provides ample evidence toidentify the majorleaching processes of combustion residues.Neutralisation and chemical weatheringgovern the leachingreactions and control the release of major, minor and traceelementsfrom combustion residues, and are thus considered to bethe major leaching processes.Several geochemical models are used to describe theneutralisation and chemicalweathering processes based onleaching kinetics and the features of leaching systems.Areaction path model is used to evaluate the neutralisingprocesses in a batch system. Theneutralising processes in aflow-through system are assessed using an equilibriumreactivetransport model which accounts for most neutralising reactionsunder the experimental conditions. A kineticreactive transportmodel taking full kineticconsideration for heterogeneousreactions is used to simulate long-term chemicalweathering.According to experimental investigations and geochemicalsimulations, the leachingkinetics of buffering materials arekey issues for the understanding of the neutralisingprocesses.The acid neutralising capacity (ANC) at different pH levelsdepends mainly onthe mineralogy of the combustion residues. Thetime-dependent and pH-dependent neutralising behaviours aredetermined by the matrix phases of a solid waste. Incombustionresidues, the dissolution of glass phases is expected to playan important rolein a long-term neutralising process. Theneutralising process in a flow system issignificantly differentfrom that in a batch system. In general, the informationobtainedfrom batch experiments cannot directly to be used in aflow system. The neutralising ability of a combustion residuemay be strongly affected by solute transport and carbonationreactions in a natural leaching environment.The chemical weathering mainly involves the matrix ofcombustion residues consistingmostly of glass phases. Thedissolution kinetics of waste glass and other possibleprocesses involved in the chemical weathering have beeninvestigated and incorporatedinto a kinetic reactive transportmodel. Most important processes in the chemical weathering canbe simulated simultaneously using this model. The results showthat thereis a complicated relationship between the factorscontrolling the long-term chemicalweathering. The dissolutionof the waste matrix is strongly affected by itsdissolutionkinetics and weathering environment. Theenvironmental impact of the glass dissolutioncannot beneglected. Although the glass dissolution provides considerablebufferingcapacity in long-term weathering, the carbonate isusually a dominant buffering mineralin actual weatheringprocesses. The transformation of carbonate should be consideredasan important process in the chemical weathering. Theformation of secondary minerals,clay-like minerals (e.g.illite) and amorphous silica, may considerably alter themineralogy of the waste, and thus change the leachingbehaviours of the combustion residue duringlong-term chemicalweathering.Keywords:Leaching; neutralisation; chemical weathering;solid waste; combustionresidues; long term; geochemicalmodelling; reaction path model; reactive transportmodel
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