This is a fairly simple procedure. A bitumen tank with a propeller stirrer with enough agitation action to create a vortex is recommended. The best choice is a "lightning" mixer or some other type of electrically powered mixer. An explosion proof motor is preferred if large dust concentrations are likely to occur.
natural bitumen should be added slowly at the vortex. Provisions should be made to recirculate the hot bitumen through recirculation piping. The most important item is that the minimum temperature should be about 170 to 175° C. Anything significantly less than this will extend mixing time. For typical (5-10%) substitution concentrations, 2-4 hours of mixing after addition is completed should be sufficient. For master batch concentrations (over 10% natural bitumen) recirculation overnight is preferred.
If the bitumen cannot be heated higher than 170°C then you may consider using Selects other Grade (60/70 mesh) or Selects Grade (200mesh) rather than HMA Modifier Grade (175°C softening point) for modification. However, each one of these Selects grades will require slightly more natural bitumen to be added, relative to HMA Modifier grade, to achieve the same level of bitumen modification. The dry natural bitumen should be poured into the hot bitumen slowly. If it is added too fast then it may agglomerate, or "ball up" at the surface. If this happens then some manual stirring to disperse the agglomerations may be required.
If a horizontal, cylindrical tank is used, then natural bitumen should be added at an opening at the top (about 0.5-1.0 meters in diameter). Again, it should be poured in slowly and stirred with a propeller mixer or a manual paddle so it does not "ball up" or agglomerate. Recirculation piping will be necessary to insure some agitation effect and proper dissolving.
Recirculation is very important to achieve proper dissolution. If mixing is done in a horizontal tank then it is essential that the natural bitumen-modified bitumen be recirculate from the front of the tank to the back, or vice versa. This should be accomplished, even if some re-plumbing of the tank is necessary. natural bitumen does not dissolve instantly. Mixing a tank containing 10-15 MT bitumen and 5% natural bitumen addition will take about 2-4 hours to add in, and an additional 2-4 hours mixing time afterwards. Naturally, higher natural bitumen dosage levels will require longer mixing times.
Unfortunately, during mixing there is no test or checklist to determine whether the natural bitumen is blending well in the bitumen. However, if it is not, then large balls or chunks will be visible in the bitumen if it is in an open tank.
Afterwards, the best method to check whether natural bitumen was fully mixed into the bitumen is by comparing the original and final penetrations of the bitumen.