Water Management in Oil Sands Mining Facilities
Oil sands mining facilities are fairly water intensive. The relatively dry oil sands deposit requires large volumes of water to produce an oil sands slurry that can be pumped and gravity separated throughout the process plant. The oil sands to water ratio is typically about 1-to-1 (or 50% water in the average slurry stream). A typical oil sands mining facility requires 9 barrels of water per barrel of bitumen produced. However, most of this water is recycled internally, with fresh water requirements closer to 3 barrels per barrel of bitumen.
SOURCES OF WATER SUPPLY
A majority of the slurry water used is recycled from within the process plant (about 80%). The balance comes from the Athabasca River. The volume of river water consumed is carefully metered. Each operator is allocated a quota (or water license) for their production. If the water level in the Athabasca River is too low, oil sands operators may be cut-off completely and not allowed to draw river water for the duration of the low-flow period. This commonly occurs in winter months.
Although the exact configuration of each facility can vary, a typical water distribution circuit for oil sands mining facilities is generally as follows:
SOURCES OF WATER:
- fresh water from the Athabasca River
- connate water contained within the oil sands
- precipitation, such as rain or snow
- water trapped within the fine tailings
- trace amounts lost with bitumen product
- pond evaporation, especially in summer.
The importance of a reliable water supply to the plant cannot be understated. Although it is obvious large volumes of process water are required to slurry the oil sands, there are smaller streams of water that are equally critical. For example, demineralized boiler feed water is needed to generate steam and power. Clean cooling water is critical to the operation of Froth Treatment and the upgrader. Small volumes of gland water are pumped throughout the main plant and tailings corridors, which keep the slurry pumps in service. Each of these water streams are all crucial to maintaining safe operation of the facility. All facilities contain significant volumes of water inventory in order to maintain production (or at least initiate a safe shutdown) in the event of a supply disruption.
WATER PONDS EXPLAINED
RAW WATER POND
The Raw Water Pond serves as a storage facility for Athabasca River water. During the winter months, the Athabasca River experiences below-average water levels due to ice build-up. Water levels can also be low during an exceptionally dry summer. Oil sands operators can sometimes be banned from withdrawing river water during these periods of low-flow. The Raw Water Pond can provide critical fresh water make-up, normally a few days worth. River water quality is exceptional good and is commonly used as boiler feed water in Utilities or even drinking water.
RECYCLED WATER POND
The Recycled Water Pond also serves as a critical water storage facility. This pond sources about 80% of its water from the Tailings Pond and the balance from the Raw Water Pond (or Athabasca River Water). If water cannot be reclaimed from the Tailings pond (often due to equipment outages), the Recycled Water Pond provides critical process water to the processing plant for at least a few days. Since Recycled Water is mostly comprised of water reclaimed from the Tailings Pond, this water can contain silt and a high concentration of ions. It is generally not the best quality water.
TAILINGS PONDS: A CRITICAL COMPONENT IN WATER MANAGEMENT
Tailings ponds (or tailings storage facilities) play a critical role in managing water flows within an oil sands operation. Tailings produced in the Extraction plant normally contains about 50% water. Once the tailings are deposited into the large storage pond, the solids settle to the bottom, allowing the clarified water to be recycled back (or reclaimed) into the process. This reclaimed water provides over 80% of the facility's water requirements.
COMPOSITION OF TAILINGS POND WATER
Water reclaimed from the tailings pond (generally referred to as reclaimed water) is about +99% water with up to 1% fine solids and can contain trace volumes of hydrocarbons. The solids are generally composed of very fine clays that do not settle easily in the tailings pond. Tailings pond water is recycled for use as process water throughout the bitumen extraction plant.
Depending on the age of the tailings pond and salt content of the mined oil sands, this water can also contain a significant concentration of calcium, magnesium or chloride ions, which can be detrimental to the process and accelerate corrosion rates.
RIVER or RAW WATER: Fresh water supply from the Athabasca River.
PROCESS-AFFECTED WATER: Water that has been in contact with the oil sands or the process in general. Process-affected Water is 100% contained on-site and recycled back into the process. Process-affected water is never released back to the river or the environment.
RECLAIMED WATER: Tailings streams produced by oil sands mining operations contain about 50% water (the balance mostly sand and clays). The tailings storage pond act as settling basin for these solids. Once the solids settle, the clarified water is then reclaimed (or recycled) back to the process plant. Reclaimed water is considered process-affected and is never allowed to be released back to the environment.
PROCESS WATER: Process water is mixture of recycled water (reclaimed from the tailings pond) and fresh water (from the Athabasca River). Process water is typically 80-90% recycled water. This water is mainly used to slurry the oil sands.