Abstract

The large quantities of sludge and extraction tailings water contained in pondsat the closure of oil sands leases have been identified as significantenvironmental concerns,

Tests have been conducted to assess changes in tailings pond water quality overtime. Biological, toxicological, chemical, and physical parameters have beenconsidered. Results indicate that the quality of top water in an abandoned pondmay be anticipated to Improve with time to an environmentally acceptablecondition. The rate of improvement can be manipulated utilizing known watertreatment technologies.

The lake concept is an application of the information available on addressingthe sludge handling concerns in which the sludge is disposed of under a layerof reclaimed top water.

A scheme for using the lake concept has been Incorporated Into a leaseabandonment plan for the Suncor oil sands project. The plan, which incorporatesexisting knowledge on sludge properties and dynamics, demonstrates a practical, environmentally sound approach for a facility which is in currentoperation.

Introduction

The orderly development of oil sands resources in north-eastern Alberta isbecoming increasingly important as conventional crude oil reserves decline.Currently, the two Fort McMurray commercial ventures use the Clark Hot WaterExtraction Process to remove oil from oil sands. Tailings ponds are constructedto contain recycled waters and wastes from the extraction and upgradingprocesses. Included in the wastes is a stable sludge formed from clays whichoriginate from the ore body in the mine.

Mining of the current Suncor lease will be completed by about 2003. Uponabandonment of the lease, one pond, which will contain all the sludge from theyears of operation, will be left as a lake. The decision to pursue the lake or "wet pond" option was reached after many alternatives were analyzedextensively.

The key factors considered in this analysis were geotechnical stability, technical feasibility and environmental acceptability. A wealth of knowledgeon sludge fundamentals and sludge water reclamation provides the foundationupon Which the current reclamation and abandonment plans have been built. Thispaper overviews Suncor's knowledge of sludge fundamentals and environmentalimplications as well as the conceptual plan for sludge handling at leaseabandonment.

Sludge Origin

Clays are associated with the oil sands ore body. Sometimes the clays are Insuch large bands that they can be excluded from the mining operation. At othertimes they are so interspersed in the ore that they are carried with the oilsands to the extraction plant.

Clay lumps are screened out of the feed destined for bitumen separation. Thisoversized clay is transported from the extraction plant for deposit into awaste dump.

The remaining clays are carried into the process but nearly all are rejected tothe tailings stream at some point. The clays together with debitumenized sand, recycled water, and unrecovered hydrocarbons are discharged to large tailingsponds. At the tailings pond most of the clays become closely associated withthe deposited sands In impoundment dykes. However about 32% of the claysbecome part of the sludge at the bottom of the tailings ponds.

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