Though it’s more common to refine asphalt, many plants from all around the globe also extract asphalt or bitumen directly from deposits, such as from the Dead Sea. The whole process can be chopped up into three parts: extraction, recovery, and solvents.
There are three primary asphalt extraction methods: centrifuge, reflux, and vacuum. All three have varying levels of effectiveness when it comes to dissolving the binder and maintaining its elasticity. For example, the centrifuge method is a relatively safer extraction method that’s also sufficiently effective for most use cases of the asphalt binder throughout the world.
The key differences among the three methods of asphalt extraction revolve around the use of heat, solvent types, and the technique of treating the mixture. Centrifuge and reflux methods are used more than the vacuum method because of their simplicity and ease of use.
There’s another method, essentially an improvement on these two, called the US Strategic Highway Research Program or SHRP method.
More complete extraction is possible if more than one extraction techniques are leveraged on the same batch of bitumen. The consistency of results is still quite unmeasured and user safety remains to be a prime concern among all types of asphalt extraction methods. Scientists have been developing and testing safer extraction methods that are partly or fully automated, but no such method has seen the light of day, or at least widespread usage, so far.
The Abson recovery method was introduced in 1933 and the rotary evaporation was introduced more recently in the 70s. Both differ significantly.
The rotary evaporator, for example, is well-known for its convenience. Compared to the Abson method, it requires less binder modification and is much easier to do in general.
Both methods need a hot temperature. The Abson method is relatively inexpensive and consequently, is used much more than rotary evaporation. This method, however, has a couple of pretty glaring disadvantages: binder aging and the leaving behind of residual solvent. The rotary evaporation recovery method has remarkable improvement over the Abson method on both fronts, and also requires less labor, but becomes significantly more expensive to carry out.
Two of the most common solvents used in asphalt extraction today are chlorinated solvents and npropyl bromide. Both are theoretically reusable solvents, but they have their own sets of concerns regarding the safety of handling them. Also, their effectiveness is dubious for a large part. Plants rarely use bio-sourced solvents because they need higher quantities to have the same impact, but are much better for the environment.
Historically, humanity has used a large number of solvents for asphalt extraction, such as Carbon Disulfide (phased out due to high volatility and flammability) and Benzene (known to gel nicely with the Abson recovery method in the 1930s).
Apart from chlorinated solvents and n-propyl bromide, toluene is also commonly used for solving asphalt. Bio-solvents are also seeing ever-increasing usage whereas another solvent, Xylene, is used widely in Vermont DOT (though it has severe health concerns).
Let’s go over all the major solvents we have ever used or can use for asphalt extraction:
1. Carbon Disulfide: Low-cost, but phased out due to its high volatility and flammability.
2. Benzene: Another low-cost solvent, but has pretty much the same disadvantages as Carbon Disulfide.
3. Trichloroethane, Trichloroethylene, and Dichloromethane: All three are used widely because they evaporate quickly and are sufficiently reusable, but are extremely harmful to the environment as well as the users. They also have some negative effects on the asphalt binder.
4. Toluene: Relatively safer, and great for PMAs, but in general classified as a fire hazard.
5. Xylene: Xylene barely has anything going for it, because the significant health risks that it brings with itself far outweigh any measurable advantage. It also has a pretty high evaporation point.
6. n-Propyl Bromide: Quick to evaporate, reusable, and good with rap, but is somewhat hazardous for the environment and the users, is corrosive, and also comes with a few health concerns.
7. Bio-sourced solvents like d-Limonene: The best thing about bio-solvents is that they are a green alternative. They don’t harm the environment at all. But these are very expensive, have a high boiling point, and are required in much larger quantities to solve a batch of asphalt than any other solvent on this list.