Grantee Research Project Results

Final Report: Carbonation of Mine Water to Increase Limestone Dissolution and Alkalinity Generation

EPA Contract Number: 68HERC21C0049
Title: Carbonation of Mine Water to Increase Limestone Dissolution and Alkalinity Generation
Investigators: Hedin, Robert
Small Business: Hedin Environmental
EPA Contact: Richards, April
Phase: II
Project Period: April 1, 2021 through March 31, 2023 (Extended to March 31, 2024)
RFA: Small Business Innovation Research (SBIR) - Phase II (2021) Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Mining and Mine Waste Management

Description:

The neutralization of acidity is a common requirement in environmental and industrial environments. Acidity arises from dissolved hydrogen ion, H+, and also from the presence of metals that release H+ during their precipitation. Acid drainage is a widespread mining problem caused when pyritic minerals are disturbed and subsequently oxidized to sulfuric acid and dissolved contaminant metals. This drainage is referred to as acid mine drainage or AMD. Coal mining in the eastern and midwestern US commonly generates AMD containing high concentrations of iron (Fe), aluminum (Al), and manganese (Mn). Hundreds of AMD discharges are currently treated by responsible parties in the eastern US. However, thousands of untreated discharges from legacy coal mines pollute tens of thousands of miles of US waterways. Metal mines also produce acidic discharges with metal contaminants. AMD from metal mines occurs throughout the US but is particularly problematic in the western states where flows from thousands of abandoned mines pollute waterways and water supplies.

The treatment of AMD requires neutralization of the acidity. Neutralization reagents generate alkalinity which increases pH and promotes the removal of metal contaminants through oxidation and hydrolysis reactions. The primary neutralization reagents used for active AMD treatment operations are sodium hydroxide (NaOH) and hydrated lime (Ca(OH)2). Sodium hydroxide is a caustic solution that is hazardous, creates copious sludge, and is costly. Its simplicity – generally a tank and a pond are sufficient -- makes it attractive for small and remote AMD flows. Hydrated lime is a calcium product that is manufactured through the calcination of limestone. The dry powder product is delivered to the treatment plant, where it is mixed with water to produce a caustic solution that is used to neutralize acidity. In a large treatment operation, hydrated lime is the least expensive neutralization option. However, the management and mixing of the dry lime requires electrical equipment and is labor-intensive. Lime slurry is a pre-mixed liquid lime product that is increasingly favored because of its simplicity of use, but it is much more expensive than dry hydrated lime.

Limestone (CaCO3) is a natural source of alkalinity that is mined throughout the US and is routinely processed to a product suitable for acid neutralization. Limestone can be added directly to AMD to neutralize acidity (reaction 1) and generate alkalinity through reaction with carbonic acid (H2CO3) as shown in reaction 3.

 

CaCO3 + H+ -> Ca2+ + H2O + CO2 acid neutralization (1)

CO2 + H2O -> H2CO3 carbonic acid formation (2)

CaCO3 + H2CO3 -> Ca2+ + 2HCO3- alkalinity generation (3)

 

Carbonic acid is present naturally in the mine water and forms through reactions with CO2 produced by acid neutralization (reaction 2). Limestone dissolution generates carbonate alkalinity which is not caustic and does not present safety and environmental concerns. Limestone is much less expensive than sodium hydroxide and hydrated lime. Cost data contained from the US Office of Surface Mining, Reclamation, and Enforcement (OSMRE) AMDTreat software program indicates that when considered on a common neutralization basis (one ton of CaCO3 neutralization), limestone is one-sixth the cost of hydrated lime and one twentieth the cost of sodium hydroxide.

Despite the cost advantage, limestone’s use in AMD treatment is currently limited to passive systems. This is primarily due to solubility and kinetic factors. Calcite solubility equilibria limit alkalinity generation by limestone to approximately 300 mg/L CaCO3, considerably less than sodium hydroxide and hydrated lime. The dissolution of limestone can be slow. The default retention time for a limestone reactor in AMDTreat is 12 hours. Slow kinetics makes limestone treatment impractical at sites with large flow rates and/or limited space.

Both of these limitations can be lessened through the carbonation of mine water in a limestone treatment system. This concept was the basis of Hedin Environmental’s original EPA SBIR Phase I project. Commercially available CO2 was added to mine water containing high concentrations of Fe and Mn which then flowed through a bed of limestone aggregate. In the absence of CO2, the limestone bed produced 260 mg/L alkalinity with 8 hours of contact time. With CO2 addition, 260 mg/L alkalinity was obtained in 4 hours and concentrations as high as 500 mg/L were achieved.

Summary/Accomplishments (Outputs/Outcomes):

The EPA SBIR Phase II project expanded on these findings and developed three important aspects of the CO2/limestone technology. The technology was applied to a full-scale passive limestone bed that generated insufficient alkalinity due to calcite solubility limits. Carbonation of the influent to the existing limestone bed increased its alkalinity generation by 100 mg/L CaCO3, which resulted in an effluent that was fully neutralized. Thus, an underperforming passive limestone system was made fully functional simply through carbonation of the mine water flowing into it. This result has implications for under-performing limestone beds and for AMD-impaired watersheds that would benefit from increased alkalinity generation by existing passive limestone treatment systems.

The technology’s use was expanded to include a wider range of AMD chemistry and shorter retention times. A CO2/limestone system was installed at a site where low pH mine water contaminated with Al is treated with limestone. Carbonation increased alkalinity generation by 80 mg/L at a retention time of 100 minutes; approximately one-eighth the retention time generally recommended for limestone treatment. This result extends the applicability of the technology to low pH AMD that is common at both coal and metal mining regions. The shortened reaction time makes feasible the installation of substantially smaller limestone-based treatment systems. It also provides a method to temporarily “turn up” the performance of a passive limestone treatment. This would be a valuable feature at sites with highly variable flow rates – as occurs seasonally in the western US during spring freshet events.

Lastly, the project expanded utilization of the technology to an active treatment environment. To this point, the carbonation experiments were conducted with pressurized CO2 injected with a nozzle into a passive limestone treatment system (no pumps or electrical needs). A carbonator used in industrial-scale beverage bottling operations was obtained. The device used a membrane carbonator to transfer CO2 to liquid in a reactor pressurized with an electric pump. Mine water was carbonated with the device and the output was injected into a limestone bed at a retention time of 100 minutes. The CO2/limestone unit produced up to 300 mg/L extra alkalinity. The relationship between CO2 injection (mg/L) and alkalinity generation (mg/L CaCO3) was highly significant, indicating very precise control of alkalinity generation. Alkalinity generation by the membrane carbonator system was twice as efficient (mg/L alkalinity per mg/L of CO2) as the passive nozzle injection method. The electrical requirements of the membrane carbonator were minor – well within the capacities of a conventional AMD treatment plant.

Conclusions:

The cost of producing an alkaline reagent with the active CO2/limestone technology was compared to the costs of sodium hydroxide, lime slurry, and hydrated lime (Table 1). Sodium hydroxide and hydrated lime costs were developed using AMDTreat. Lime slurry costs were developed from realized expenses at an operating facility. The CO2/limestone costs were developed assuming $570,000 in capital expenditures (CO2 storage tank, membrane carbonators, and reaction tanks) which were amortized over 10 years at 8%. The annual reagent costs were 1,460 ton/year limestone ($50/ton) and 1,750 ton/year CO2 (variable price). The primary driver of the CO2/limestone technology was the price of CO2. The modeling indicates that the CO2/limestone technology is less costly than sodium hydroxide and lime slurry. The CO2/limestone technology becomes cost competitive with hydrated lime if CO2 cost decreases to $80 through either price reductions or eligibility of the technology for federal CO2 credits.

The CO2/limestone technology has a smaller carbon footprint than hydrated lime and utilizes CO2 recovered from industrial processes. These benefits will make the CO2/limestone technology competitive with alternative reagents without steep decreases in the price of CO2.

Table 1. Costs for alkaline reagents used for in mine drainage treatment systems

Alkaline reagent	Assumptions			$/ton CaCO3
Sodium hydroxidea	AMDTreatb: 20% NaOH, $0.70/gal, $72K equipment			$573
Lime Slurry	Realized cost at operating treatment system			$626
Hydrated Limea	AMDTreat: $200/ton, 80% utilization, $100K equipment			$237
		CO2 credit $/ton	CO2 cost $/ton
CO2/limestonea	$50/ton limestone, $570K equipment	$0	$160	$349
CO2/limestonea	$50/ton limestone, $570K equipment	$35	$160	$300
CO2/limestonea	$50/ton limestone, $570K equipment	$0	$80	$237
CO2/limestonea	$50/ton limestone, $570K equipment	$35	$80	$188

^a treatment assumptions: 2,000 gpm of mine water with 300 mg/L acidity
^b AMDTreat is mine water treatment cost model developed by US OSMRE

 

SBIR Phase I:

Carbonation of Mine Water to Increase Limestone Dissolution and Alkalinity Generation  | Final Report