Alum

Key Takeaways

• Aluminum sulfate coagulates suspended oil and solids in produced water by releasing Al³⁺ ions that hydrolyze to form Al(OH)₃ floc, which adsorbs oil droplets and particles through charge neutralization and sweep flocculation, with the treated water then passing through dissolved air flotation or plate pack separators to meet injection water quality specifications for sandstone waterflood reservoirs: When aluminum sulfate dissolves in produced water at pH 6 to 8, the Al³⁺ ions rapidly form monomeric and polymeric aluminum hydroxide colloids carrying a positive charge that neutralizes the negative surface charge of oil droplets and suspended clay particles (zeta potential shifted from -30 to -50 mV toward zero). The neutralized particles aggregate and are swept into the growing Al(OH)₃ floc gel during slow mixing (30 to 60 rpm, 15 to 30 minutes). Typical alum doses for Cardium and Viking waterflood produced water in central Alberta range from 50 to 200 mg/L, reducing oil-in-water content from 50 to 500 mg/L to below 10 mg/L and suspended solids from 15 to 150 mg/L to below 5 mg/L, meeting the injection water quality standard for 10 to 200 mD sandstone reservoirs. High-salinity Mannville heavy oil produced water (TDS 15,000 to 50,000 mg/L) requires higher doses of 150 to 300 mg/L because dissolved salts compress the electrical double layer and reduce coagulation efficiency at equivalent doses used for lower-salinity Cardium and Viking streams.
• Optimal alum dose for each produced water stream is determined by jar testing at the field laboratory, evaluating multiple doses and pH values to identify the treatment conditions achieving the lowest residual oil-in-water and turbidity at minimum chemical cost, with results translated into continuous-flow operating setpoints at the waterflood battery injection facility: Jar testing for WCSB produced water treatment evaluates alum doses from 25 to 400 mg/L in 500 mL volumes, measuring residual turbidity (NTU), oil-in-water (mg/L by infrared spectroscopy), and dissolved iron after 30 minutes of settling, with pH adjusted using NaOH or H₂SO₄ to maintain the 6.5 to 7.5 optimal window for Al(OH)₃ floc formation. The pH sensitivity of alum is critical: below pH 5.5, Al³⁺ remains soluble and no floc forms; above pH 8.0, the amphoteric Al(OH)₄⁻ aluminate ion forms (no floc), so produced water with high bicarbonate alkalinity (greater than 800 mg CaCO₃/L, common in Belly River and Milk River aquifer water) requires acid addition ahead of the alum injection point. Jar tests for Mannville heavy oil produced water in the Lloydminster area, where oil-in-water concentrations of 200 to 600 mg/L are common, typically determine optimal doses of 150 to 300 mg/L alum plus 3 to 8 mg/L cationic polyacrylamide as a flocculation aid, achieving OIW below 15 mg/L before the water enters the disposal or injection pump header.
• In oil sands operations at Fort McMurray, aluminum sulfate coagulates fine clay particles in process-affected tailings pond water to reduce turbidity before recycling to primary bitumen extraction, supporting AER Directive 085 progressive MFT volume reduction requirements and reducing freshwater intake obligations from the Athabasca River Water Act licence: Mature fine tailings ponds contain kaolinite and illite clay at 0.1 to 20 micron diameter suspended at 200 to 400 g/L, remaining colloidal for years because electrostatic repulsion between negatively charged clay surfaces prevents aggregation. Alum coagulation at 300 to 1,200 mg/L, combined with polyacrylamide flocculation at 20 to 80 mg/L, reduces suspended solids in the turbid decant water layer (5,000 to 15,000 mg/L) to below 500 mg/L for recycle to primary extraction, reducing freshwater intake from the Athabasca River by 20 to 35% per unit of bitumen produced. The AER's Directive 085 compliance reporting requires operators (Syncrude, Suncor, CNRL Horizon, Imperial Oil Kearl) to demonstrate annual reductions in MFT inventory, driving investment in large-scale alum-based coagulation-flocculation systems with capital costs of CAD 15 to 50 million per installation at each mine site, treating 50,000 to 200,000 m³/day of process-affected water.
• Aluminum sulfate accelerates Type G and Class A cement hydration in permafrost cementing at 2 to 10°C by providing aluminate ions that seed calcium aluminate hydrate crystal growth, reducing thickening time from 6 to 12 hours to 2 to 4 hours at near-freezing temperatures, allowing surface and intermediate casing strings to be cemented on schedule without excessive waiting-on-cement delays: In permafrost zones of northern Alberta (Colville Hills, Liard Basin) and British Columbia, static bottomhole temperatures at 100 to 400 m depth are 2 to 8°C, where Type G cement hydration kinetics are so slow that thickening times exceed waiting-on-cement windows before the next casing string can be run. Aluminum sulfate at 0.5 to 2.0% by weight of cement (BWOC) accelerates the tricalcium aluminate (C₃A) and tricalcium silicate (C₃S) reactions by providing additional aluminate reaction sites, reducing thickening time at 4°C from 8 to 12 hours to 2 to 4 hours and compressive strength development from 24 to 48 hours to 10 to 18 hours. Alum accelerator is combined with Class A or Class C cement (higher C₃A content than Class G) and sometimes with calcium chloride at 2% BWOC for maximum acceleration synergy at bottomhole temperatures below 4°C, as specified in cementing programs submitted to the AER or BC Oil and Gas Commission for exploration wells in northern gas fields.
• The principal environmental constraint on alum use in oilfield water treatment is residual dissolved aluminum in treated effluent, which is acutely toxic to freshwater fish above 0.1 mg/L and must meet the Canadian Water Quality Guideline of 0.005 mg/L total Al in any surface discharge, a requirement that drives WCSB operators to use alum in closed-loop produced water reinjection systems rather than discharge-to-surface treatment trains wherever possible: Dissolved Al³⁺ above 0.1 to 0.5 mg/L is acutely toxic to rainbow trout and other salmonids by precipitating Al(OH)₃ on gill surfaces at pH 5.5 to 7.0, causing asphyxiation within 24 to 96 hours, with chronic toxicity at concentrations as low as 0.05 mg/L. Alberta Environment and Parks' Canadian Water Quality Guideline for the Protection of Aquatic Life (CWQG-PAL) specifies 0.005 mg/L total aluminum (5 micrograms per litre) at pH greater than 7, applying to any produced water, process water, or drilling waste discharged to surface watercourses. Because most WCSB waterflood produced water is reinjected rather than discharged, the main alum-related environmental compliance issue arises at oil sands sites with surface discharge permits for treated process water, where operators must demonstrate through continuous monitoring that aluminum concentrations in seepage or overflow reaching the Athabasca River remain below the CWQG-PAL, achieved by maintaining discharge water pH above 7.5 (to keep Al as insoluble Al(OH)₃) and monitoring at licensed tributary entry points under the Alberta Water Act.