Page 130 - Synthetic Fuels Handbook

116 CHAPTER FOUR

earlier systems used. In the current mining systems, trucks capable of hauling up to 420 tons of
material are loaded by electric- and hydraulic-power shovels with bucket capacities up to
3
60 yd . The trucks transport the oil sands to ore preparation facilities where the ore is crushed
and prepared for transport to the extraction plant (where bitumen is separated from the sand).
In their early operations, Suncor and Syncrude used long conveyor systems for ore
transportation. These systems have been replaced by hydrotransport with the first com-
mercial applications of this technology occurring in the early 1990s. For hydrotransport,
the oil sands ore is mixed with heated water (and chemicals in some cases) at the ore prepa-
ration plant to create oil sands slurry that is pumped via pipeline to the extraction plant.
Hydrotransport preconditions the ore for extraction of crude bitumen and improves energy
efficiency and environmental performance compared to conveyor systems.

4.5 BITUMEN RECOVERY

In terms of bitumen separation and recovery, the hot water process is, to date, the only suc-
cessful commercial process to be applied to bitumen recovery from mined tar sand in North
America. Many process options have been tested with varying degrees of success, and one
of these options may even supersede the hot water process.

4.5.1 The Hot Water Process
The hot water process utilises the linear variation of bitumen density and the nonlinear
variation of water density with temperature so that the bitumen that is heavier than water
at room temperature becomes lighter than water at 80°C (176°F). Surface active materi-
als in the tar sand also contribute to the process. The essentials of the hot water process
involve a conditioning and separation. Other ancillary steps are also used but will not
be covered here.
In the conditioning step, also referred to as mixing or pulping, tar sand feed is heated
and mixed with water to form a pulp of 60 to 85 percent by weight of solids at 80 to
90°C (176–194°F). First the lumps of tar sand as-mined are reduced in size by ablation,
that is, successive layers of lump are warmed and sloughed off revealing cooler layers.
The conditioned pulp is screened through a double-layer vibrating screen. Water is then
added to the screened material (to achieve more beneficial pumping conditions) and the
pulp enters the separation cell through a central feed well and distributor. The bulk of the sand
settles in the cell and is removed from the bottom as tailings, but the majority of the bitu-
men floats to the surface and is removed as froth. A middling stream (mostly of water
with suspended fines and some bitumen) is withdrawn from approximately midway up
the side of the cell wall.
Froth from the hot water process may be mixed with a hydrocarbon diluent, for exam-
ple, coker naphtha, and centrifuged. The Suncor process employs a two-stage centrifuging
operation and each stage consists of multiple centrifuges of conventional design installed
in parallel. The bitumen product contains 1 to 2 percent by weight of mineral (dry bitumen
basis) and 5 to 15 percent by weight of water (wet diluted basis). Syncrude also utilises a
centrifuge system with naphtha diluent.
As noted, tailings are a byproduct of the oil sands extraction process. For each ton of
3
3
tar sand in-place has a volume of about 16 ft , which will generate about 22 ft of tailings
giving a volume gain on the order of 40 percent. If the mine produces about 200,000 tons of
tar sand per day, the volume expansion represents a considerable solids disposal problem.
Tailings from the process consist of about 49 to 50 percent by weight of sand, 1 percent by