Mixing Gilsonite into Bitumen
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.
Gilsonite 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 175Â° 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% Gilsonite) recirculation overnight is preferred.
If the bitumen cannot be heated higher than 170Â°C then you may consider using Selects other Grade (60/70 mesh) or Selects Grade (200mesh) rather than HMA Modifier Grade (175Â°C softening point) for modification. However, each one of these Selects grades will require slightly more Gilsonite to be added, relative to HMA Modifier grade, to achieve the same level of bitumen modification. The dry Gilsonite 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 Gilsonite 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 Gilsonite-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. Gilsonite does not dissolve instantly. Mixing a tank containing 10-15 MT bitumen and 5% Gilsonite addition will take about 2-4 hours to add in, and an additional 2-4 hours mixing time afterwards. Naturally, higher Gilsonite dosage levels will require longer mixing times.
Unfortunately, during mixing there is no test or checklist to determine whether the Gilsonite 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 Gilsonite was fully mixed into the bitumen is by comparing the original and final penetrations of the bitumen.
Batch Plant (Pug Mill) Mixing
First, Gilsonite should be added during the dry cycle of the mixing procedure, onto the hot aggregate rocks, before the bitumen is added. We recommend extending the total cycle time by about 15 seconds to insure proper dissolution.
PUB (Pars Universal Bitumen) Gilsonite Company has found that 5 additional seconds of dry mixing and 10 additional seconds of wet mixing maximized the Marshall Stability of the paving mix.
The Gilsonite can be stored in an additional silo at the pug mill and sprayed into the mixer. In Iran, there is frequently a dry mineral (ground limestone) silo and this material is sprayed onto the mix. Gilsonite would be handled in this same manner. A screw feeder or vane feeder that measures out the amount of Gilsonite per batch can be calibrated to measure the dosage level of Gilsonite per batch. Afterwards, the only residue left behind in the silo will be Gilsonite powder that can be easily cleaned out.
It may be much easier to pre-package Gilsonite into small, polyethylene bags with a measured amount of Gilsonite and toss them onto the hot aggregate in a batch plant. The sidewall thickness of the bag should be about 2 mils (0.005 cm). The aggregate temperature should be around 180Â°C. It is the aggregate temperature that is melting the bags and the Gilsonite, not the heat from the bitumen. Therefore a temperature of 150-165Â°C entering the pug mill is acceptable, as long as the aggregate is sufficiently heated.
In either case, spraying Gilsonite onto aggregate or tossing bags into the pug mill, we conservatively recommend increasing the mixing time an extra 15 seconds. This will insure the Gilsonite is melted properly and dissolving into the bitumen. Finally, it is possible to just scoop or shovel a precise number of kilos of Gilsonite per batch onto the hot aggregate, no re-packaging — just hand labor.
Continuous Plant (Drum) Mixing
Gilsonite may be introduced into a continuous mixing plant via a screw auger. It should be added at a point inside the drum where the bitumen is added. The screw auger should be controlled by a drive motor that is calibrated to the plant’s production rate. The auger should enter the drum at the opposite end from the flame. Care should be taken to insure that Gilsonite is not caught up in the air stream and delivered to the bag house. It should be added right under the bitumen output so that a part of the Gilsonite is taken down by the bitumen to mix with the aggregate.
PUB(Pars Universal Bitumen) Gilsonite Company does not recommend any special paving procedures just because Gilsonite resin has been added to the paving mix. After modification with Gilsonite, the final bitumen will have a significantly lowered penetration, a significantly increased viscosity and a moderately increased softening point. Gilsonite modification creates a highly stable, easily workable paving mix.
We recommend normal paving procedures and normal lay-down temperatures. If the increase in viscosity resulting from Gilsonite’s modification presents any flow ability problems, we recommend the contractor operates at the higher end of his normal operating lay-down temperature range. During cold weather, 5 to 10Â°C, Gilsonite-modified mixes may tend to set-up slightly faster than standard mixes. In this case, the initial roller may follow the paver a little close and the finish roller may not have to stay back as far.