Fundamental study of taphole clay behaviour with respect to the binder system and evaluation of alternative non-toxic binders

Abstract

The aim of this study was to determine the critical properties that a binder for use in taphole clay should have, identify alternative non-toxic binders that could potentially replace the currently used toxic high-temperature coal tar (CTht), and evaluate taphole clays in which these alternative non-toxic binders were used. The work presented in the first part of this thesis investigates the behaviour of the CTht. Analytical techniques were employed to describe the molecular composition and toxicity (Fourier-transform infrared spectroscopy, gas chromatography-mass spectroscopy; targeted and untargeted), thermal behaviour (thermogravimetric analysis, differential scanning calorimetry) and rheological properties of this binder. A set of alternative binders from various sources, including coal sources (a coal tar pitch-merisol oil blend, a low-PAH synthetic coal tar pitch), petroleum sources (a crude waxy oil, a distilled crude waxy oil, a mesophase-forming pitch), wood/plant sources (a beechwood tar, a pinewood tar, a vegetable tar), and a glycerine-resole resin mixture, were then considered and evaluated as non-toxic alternatives to CTht. Using a ranking system based on the results from the analytical techniques, the most suitable non-toxic binders to CTht were identified. The results indicated that the binders had molecular structures that were either cyclic aromatic hydrocarbons (aromatic benzene), chain hydrocarbons (aliphatic), or a combination of both. The toxicity (BE-values) ranged from 0.03-1.67 with the coal-based binders having the highest toxicity values. The thermal analysis indicated an average mass loss of the binders ranging from 0.28-1.79 g/°C with the glycerine-based binders having the highest average mass loss. Rheology results indicated that the vegetable tar and pinewood tar had limited thermal stability to be used as binders and was disqualified as potential alternative binders. The characterization and ranking process indicated that beechwood tar and a glycerine-resole resin mixture were the most suitable replacements for CTht in taphole clay, with BE-values (toxicity) of zero. Pinewood and vegetable tars were disqualified as potential binder replacements due to their limited thermal stability. In the second part of this thesis, the evaluation of the top two selected non-toxic alternative binders in a taphole clay formulation is discussed. Both non-standardized tests (workability and extrusion pressure ageing, hardenability, strength development) and standardized tests (volatile organic compounds, cold crushing strength, apparent porosity, carbon yield) were used to describe the behaviour of the taphole clay and the changes that occur due to the binder substitutions. Mixing procedures during the pilot-scale manufacturing of the taphole clay highlighted that beechwood tar was not a suitable binder for use in taphole clay. The crude waxy oil and the glycerine-resole resin mixture were selected to be evaluated in the taphole clay. The glycerine and phenolic resin binder caused the lowest decay in plasticity (7%) and smallest increase in extrusion pressure (17.5%) during aging of the THC. Thermal ageing results indicated that glycerine and phenolic resin binder had the lowest hardenability whilst the crude waxy oil had a higher hardenability. The strength development results indicated insufficient strength development for the crude waxy oil-containing clay which was manifested in the fact that the sample could not keep its shape after firing. The glycerine and phenolic resin binder clay had comparative compressive strength values (2.4 MPa) compared to the reference clay sample (3.1 MPa). The clay containing glycerine and phenolic resin binder had a lower volatile organic compound release. The taphole clay evaluations confirmed that the glycerine-resole mixture was the most suitable replacement for CTht in taphole clay due to similar process parameters (workability and extrusion pressure ageing, hardenability, strength development) as well as standardized test results showing similar performance to the clay with the CTht binder.