Bitumen surface charging

The adsorption of anionic surfactant molecules directly affects the electrophoretic mobilities of dispersed bitumen droplets, gas bubbles, and fine solid particles. These electrophoretic mobilities are directly linked to the Zeta potentials at the surfaces and therefore to the surface electric charges on the drops, particles or bubbles. Reference [100] shows how to convert the mobilities into Zeta potentials or surface charges. Although, as will be seen later, the shapes of the processibility curves can vary considerably, the various surface charges are always quite negative... 

After bitumen-solid separation, bitumen-air attachment has to occur. The process conditions that most favour bitumen-solids separation, that is a high degree of electrostatic repulsion due to charged surfactant molecules at the interfaces, also tend to oppose gas-bitumen attachment since the gas bubbles also acquire a surface charge of the same sign [JJ2] (see Figure 14). In comparison, mineral flotation involves gas-solid attachment without filming and such electrostatic repulsion is not as... 

Figure 18. Correlation between process aid additions for maximum primary bitumen recovery and process aid additions required to attain maximum bitumen/aqueous surface charge (Zeta potential). The data are for continuous pilot plant operation (Hj and laboratory batch extractions (%). (From Schramm and Smith [111].)...

Not all of the emulsifier in an emulsion is associated with the bitumen droplets. The amount of free emulsifier decreases over time as more emulsifier is adsorbed on to the bitumen droplets during storage, a process related to the migration of polar materials in the bitumen droplet to the interface. The rate of setting is related to the concentration of free emulsifier which can adsorb on to the aggregate surface and reduce the charge. 

Electrophoresis of the bitumen droplets to the aggregate surface depends both on the charge on the asphalt droplets and on the aggregate surface, as well as the size of the bitumen droplets - small particles mean faster-setting emulsions. The rise in pH resulting from the contact of emulsion with the aggregate surface or filler in the mixture, can result in a deprotonation of the cationic emulsifiers and consequent destabilization of the emulsion. 

The suspension of bitumen particles is achieved by charging their surface with the same electric charge after the addition of the emulsifier, or emulsifying agent, during the emulsification process. 

Natural Surfactants and Interfacial Properties. The action of the natural process surfactants has been studied in some detail [100-104]. The impact arises due to their adsorption at surfaces and interfaces, by which they alter surface electric charges and interfacial tensions. Figure 5 shows an example of the steps involved in determining the surface, interfacial, and other properties of dispersed bitumen drops, solid particles, and gas bubbles in aqueous solution. The samples analysed would be based on batch extraction tests involving different oil sand types and different process conditions. 

The existence of optimum concentrations for separation- and flotation-aid surfactants is qnite common in mineral processing operations. In oil sand processing, reasons for reductions in process efficiency at higher than the optimum concentrations may be due to the formation of different adsorption layer orientations and/or multiple adsorption layers at interfaces. Either or both of these could explain the observed reductions in surface electric charge (Zeta potential) at the bitumen/aqueous and gas/ aqueous interfaces (see Figure 14). 

It has been shown [lOJ] that the simple dilution model can be used to fairly accurately predict how to increase slurry water addition levels as well as, for example, to counteract overdosing with NaOH in the slurry. It has also been shown [JOJ] that the correlations between maximum primary bitumen recovery and surface electric charges (Zeta potentials) are also preserved when varying the slurry water addition levels. 

---