To overcome the difficulty of incorporation of hydrophobic carbon materials into refractory castables, TiC and SiC coatings with much better water-wettability were prepared on carbon particles from metallic powders (Ti or Si) by using a novel low temperature molten salt synthesis technique. The preparation conditions were optimized by varying processing parameters including synthesis temperature, holding time, salt assembly and metal/carbon molar ratios.
Homogeneous TiC coatings were prepared on carbon black (CB) particles by firing them with Ti powders in KCl or KCl-LiCl at 750-850 C for 4 hours. Alternatively, TiC coatings could be prepared at a lower cost by firing the mixture of TiO2 and Ti (in molar ratio of 1/3) with CB in KCl at 950 C for 4 hours. High quality SiC coatings were prepared on CB spheres after firing them with Si powders in a binary NaCl-NaF salt for 6 hours at as low as 1100 C. NaF was proven to be essential in the molten salt synthesis of SiC and its optimal amount was 2.5-5 wt% in the binary salt. In addition, graded SiC/SiO2 composite coatings were prepared by controlled oxidation of SiC-coated CB in air at 450 C for 90 minutes to further improve their water-wettability.
Carbide-coated CB spheres were identified as having a core-shell structure by scanning/transmission electron microscopy (SEM/TEM) and the thicknesses of TiC and SiC shells (Ti/C or Si/C =1/8 in molar ratio) were estimated as ~10 nm and ~12 nm, respectively. Nevertheless, the coating thickness and corresponding particle density could be readily tailored by controlling the metal/carbon molar ratio in the initial batch mixture to meet practical requirements in real castable systems. The coated CB particles retained similar morphologies and sizes to as-received CB, indicating the formation of carbide coatings in molten salt at test temperatures was governed by a template growth mechanism: dissolution of Ti or Si in molten salt and subsequently fast delivery of dissolved Ti or Si species to the surface of carbon particles, forming carbide coatings on the template. The growth of carbide coatings was dependent on the inward diffusion of Ti or Si and outward diffusion of carbon through an initially formed carbide coating layer.
The water-wettability, dispersion behaviour and flowability of CB after carbide coating were evaluated by sedimentation comparison, zeta potential measurement and rheology testing. Owning to the formation of hydrophilic Ti-OH and Si-OH groups on the surface of carbide-coated CB particles, they were able to be immediately wetted by water and well dispersed in aqueous solutions. Moreover, improved dispersivity and flowability of CB after carbide coating were verified by the increased zeta potential values (e.g. at pH=10, ~46.1 mV for TiC-coated CB, ~54.7 mV for SiC-coated CB and ~65.9 mV for SiC/SiO2-coated CB but only ~22.6 mV for uncoated CB) and lowered apparent viscosity (e.g. the apparent viscosity of suspensions containing 25 wt% coated CB was over one order of magnitude lower than that containing as-received CB) of coated CB containing suspensions. In addition, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) verified that the improvement in oxidation resistance of carbon after carbide coating was limited, however, the annealing treatment at 1200-1500 C could promote the growth of carbide nanocrystals and make the carbide coating denser, thus effectively improving carbon’s oxidation resistance. Both weight-loss curve (TGA) and exothermic peaks of carbon oxidation (DSC) were right shifted to higher temperatures. It was also found that annealing atmosphere and temperature were influential on the oxidation resistance of coated CB particles.
To investigate the effect of carbide coating on water demand for preparation of carbon-containing castables, both uncoated and carbide-coated carbon particles (carbon black and graphite fakes) were incorporated into model low cement Al2O3-C castables. The water addition was found to decrease dramatically, from 8.5-9.7 wt% required for uncoated carbon containing castables to 6.5-7.0 wt% for carbide-coated carbon containing castables when both of them reached the similar flow values. The evident decrease in water addition led to a considerable drop in apparent porosity and increase in bulk density. As a result, castables containing carbide-coated carbon particles after coking at 1500 C showed over 6 times higher compression strength and 3-5 times higher bending strength than uncoated carbon containing castables. Furthermore, oxidation resistance of carbon-containing castables was improved as well. Uncoated CB containing castable was severely oxidised at 1000 oC for 3 hours and showed the decarbonized depth of 10.48 mm, whereas TiC-coated and SiC-coated CB containing castables showed respectively 6.82 mm and 6.35 mm decarbonized depths under the same oxidation conditions.
