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- Emborg, Mats
(författare)
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Thermal stresses in concrete structures at early ages
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the problem of formulating constitutive models for the analysis of early-age thermal stresses in concrete due to hydration. The main features of the work are constitutive modelling of early-age concrete behaviour experimental studies to check the models and to determine material parameters application of the models to some design and construction situations. The proposed model for thermal stress analysis in young con- crete is based on a rate-type viscoelastic law which is supplemented with a strain-softening model for high stress level nonlinearity in tension and by an adjustment for low- stress nonlinearity in compression. In order to demonstrate the potential of the model it has been applied to some typical cases in practical engineering. Thus, the risks of thermal cracking are computed for A - a section poured between inflexible supports B - a differential temperature field across a wall section C - a wall cast on a concrete slab D - a box tunnel element E - a concrete cover layer applied on a bridge pier. From the computations it is concluded that the magnitude of thermal stresses do not only depend on temperature variations but to a critical extent also on the mechanical behaviour of the hardening concrete as well as on the nature and degree of the restraint. The thermal stresses are in most cases not proportional to the temperature field within the structures as defined by maximum temperature rise, maximum temperature differences etc. It is evident from this study that, in some early-age applications, an analysis solely based on the temperature field may not even reveal whether a part of the concrete structure is in tension or compression.
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| 4. |
- Lager, Thomas
(författare)
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The technology of processing antimony-bearing ores
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Several aspects of the process technology for treatment of antimony-bearing ores have been studied. The feasibility of separating contaminant antimony minerals as early as the mine planning stage is discussed. The possibility of separating contaminant antimony minerals depends on the mineralogy of the ores and the flotation properties of the antimony minerals. The flotation properties of stibnite jamesonite, gudmundite, boulangerite and bournonite have been studied in laboratory experiments. Information has been obtained which is fundamental to assessment of the possibilities for separating each of these antimony minerals in normal process environments for beneficiation of complex ores. Flotation of stibnite probably occurs as a result of formation of antimony xanthate on the mineral surface at low pH.
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- Sinkala, Thomson
(författare)
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Hole deviations in percussion drilling and control measures : theoretical and field studies
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, we study the problem of hole deviations in percussive drilling and present a solution to minimize the deviations. Field tests are performed to study the interactive behaviour of drilling parameters. Standard theories in beam bending are used to study the effect of thrust, drill string weight and drill string inclination on drill string deflection, an important source of hole deviations in percussive drilling. The effect of drill string torsion on penetration rate is also studied. A simple model is developed to estimate the minimum thrust required for bit-rock contact before succeeding stress wave. The drill string behaviour predicted by the models is validated with the results obtained from field tests. The effect of the bit-rock contact on percussive drilling, also an important source of hole deviation, is also studied. A new bit-rock contact model is developed to estimate the minimum torque required to maintain constant bit rotation. Results obtained from this model agree very satisfactorily with those obtained from further field tests. The understanding from results of our theoretical and field studies of drill string deflection and bit-rock contact is then used to formulate a solution to the problem of hole deviations in percussive drilling. In the solution, torque is used as a key parameter for automatic control of the drilling process. The solution is tested in the field and its performance is considered to be satisfactory.
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- Wang, Xianghuai
(författare)
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The chemistry of flotation activation and depression of iron-containing sulphide minerals
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thermodynamic calculations, electrochemical techniques, flotation and adsorption tests were used to study the physico-chemical principles involved in the flotation, activation and depression of the common ironcontaining sulphide minerals. Thermodynamic calculations on iron-xanthate-sulphur-water systems indicate that insoluble ferric hydroxo- xanthates can be obtained at a very low xanthate concentration in the neutral to weakly alkaline pH range, in which no liquid dixanthogen is formed. Dixanthogen is not the sole species for the flotation of iron-sulphides. Ferric (hydroxo-) xanthates play an important role in the flotation of the minerals. Good flotation of pyrite and other iron-containing sulphides occurs only when a monolayer of xanthate ion and/or dixanthogen is co-adsorbed on the ferric (di)hydroxo xanthate sites. Cyclic voltammetric investigations show that the initial oxidation of pyrite occurs simultaneously with the oxidation of hydroxyl ions. This facilitates the formation of iron hydroxide on the pyrite surface. The surface of pyrite oxidized at lower overpotentials is a sulphur-rich, surface but is covered with hydrophilic ferric hydroxide. The oxidation of pyrite at high overpotentials involves mainly sulphur and sulphide and is nearly independent of the previous history of the pyrite electrode surface. The initial oxidation of arsenopyrite surface produces ferric hydroxide and a realgar-like compound on the surface. At higher potentials, the oxidation of arsenopyrite results in elemental sulphur, arsenate and ferric hydroxide. Arsenate is adsorbed on the surface. The oxidation of arsenopyrite commences at lower potentials than that of pyrite. The separation of arsenopyrite from pyrite can therefore be achieved by selective oxidation of arsenopyrite. Flotation studies demonstrated that freshly ground arsenopyrite and pyrite are non-floatable in the absence of xanthate as a collector or strong iron-complexing agent: EDTA (hydrophilic). Good flotation was observed for both minerals with EDTA or ethyl xanthate. The floatability depends strongly on the pH value and concentration. With EDTA the minerals show good floatabilities in the pH range 6-10 for a concentrations <10-3M. Flotation is completely depressed for concentrations ≥10-2M. The principles for rendering the minerals hydrophobic by EDTA and xanthates are also discussed in this thesis. Studies of the activation of sphalerite, arsenopyrite, pyrite and pyrrhotite by Cu(II) ions by using adsorption tests and cyclic voltammetry showed that the activation reactions between the sulphides and copper(II) depend on the solution pH. The activation process in acidic media is essentially an electrochemical one. Sulphide activation by Cu(II) results in a sulphur-rich copper sulphide surface. In alkaline media, activation proceeds by a surface precipitation. The quantity of copper(II) adsorbed by the sulphides depends on the solution pH, Cu(II) concentration and oxygen content. In general, oxygen decreases the adsorption rate and the quantity of Cu(II) on the sulphide surface. The effect of oxygen on the activation of sulphide by Cu(II) decreases with an increase in pH value. Results from thermodynamic calculations for the cyanide-metalxanthate-sulphur systems and the literature show that the mechanisms of sulphide depression and deactivation by cyanide are extremely complex. Sulphide minerals are rendered hydrophilic and depressed by any of the following processes: the use of cyanide through adsorption of cyanometal complexes, dissolving metalxanthates and metal-sulphides, reducing the pulp potential to prevent chemical adsorption of xanthate ion and the formation and adsorption of dixanthogen, removal of heavy metal ions (activators) by forming cyano-metal complexes, and removal of elemental sulphur from the mineral surfaces.
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