ABSTRACT
Corrosion rates in model oil sands coarse tailing slurries were investigated using a linear polarization resistance probe. The study was conducted in a pilot-scale horizontal slurry flow loop. Slurry consisted of either unimodal (sand) or bimodal (sand and rocks) solid particles distribution mixed with municipal water. The sand had median size, d50 ranges from 0.240 to 0.776 mm (9.4 to 30 mils) and spread (d90 / d10) ratios in the range of 1.9 to 3.1. The rocks size ranged from 6 to 12 mm (0.25 to 0.5 inches). The study was done in a 193.7 mm (7.625 inches) carbon steel pipe flow loop with mixture velocity and solids concentration of 4.1-6.0 m/s and 19.3-29.1 v/v%, respectively. The oxygen was injected to achieve dissolved oxygen concentration of 1.0 ppm to 11.7 ppm. In general, the presence of the solids affected the corrosion rates measured using the linear polarization resistance probe. The slurry effect on corrosion was not linear but depends highly on slurry flow regime and dissolved oxygen concentration.
BACKGROUND
Slurry pipeline systems are used for the extraction of bitumen from mined ore in the oil sands industry in Alberta, Canada. Most of these extraction processes are open to atmosphere resulting in significant air ingress and entrainment within the slurry pipelines used to transport mine ore and tailings. In addition, for short hydrotransport slurry pipelines, the slurry is conditioned by air to create bubbles coated with a bitumen film called “air-sacks”. These air-sacks are formed by the coalescence of air bubbles and bitumen droplets that both move in the dense slurry, Sanders et al.1 The presence of entrained air in the slurry at high pipeline pressures (up to 4 MPa or 600 psi) results in high dissolved oxygen concentration and associated corrosion rates. Considering the additional synergistic of erosive and corrosive contributions from the presence of solids mixture (mostly silica) and water containing up to 750 ppm concentration of chlorides, the handling and processing of oil sands slurries result in significant pipe wall material losses. Therefore, mitigation of observed corrosion, erosion or abrasion and their synergistic effects is required. One obvious mitigation strategy is to employ wear and/or corrosion resistance materials. However, the cost of this strategy can be prohibitive, especially in the pipeline sections where corrosion contribution is not significant. Another strategy is to utilize a tool to predict the pipe wall material loss and aid in development of a process-based maintenance schedule. Through an informed design decision, both capital expenditures (CAPEX) and operation and maintenance (OPEX) costs can be reduced. Therefore, understanding the synergistic effects of corrosion and erosion could provide some basis for developing a mechanistic material or wall loss predictive tool.
