Ettringite Formation as a means of Treating Sulfate in Mine Water

OLI Simulation Conference 2016

Krystal Perez, Mary McCloud and Christina Joiner

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Agenda

• Overview of Ettringite Precipitation Process for Sulfate Removal from Mine Water

• Role of OLI Studio: Stream Analyzer

• Two Example Projects (Confidential Clients)

– Typical acid mine drainage water

– High-sodium mine water

U.S. EPA (https://www.epa.gov/polluted-runoff-nonpoint-source-pollution/abandoned-mine-drainage)

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Overview of Ettringite Precipitation Process for Sulfate Removal from Mine Water

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Ettringite

• Ettringite is a complex hydrated calcium aluminum sulfate compound with this overall formula in nature:

Ca6Al2(SO4)3(OH)12·26H2O

• Theoretically, a precipitation process forming ettringite can drop dissolved sulfate levels to 50 mg/L or less under the right conditions

• In the laboratory ettringite is precipitated at elevated pH, approximately 11.6, in the presence of calcium and aluminum

• The general stoichiometry is:

Calcium/aluminum/sulfur—6/2/3

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Three-stage Process for Sulfate Reduction

In the case of waters with lower sulfate content, where pretreatment for gypsum (CaSO4*2H2O) precipitation is not economically justified, Stage 1 treatment can be eliminated and the water can go directly to Stage 2

GYPSUM

SOLIDS

LIME

DISCHARGE

STAGE 1,

PRETREATMENT/

GYPSUM

PRECIPITATION

STAGE 2,

ETTRINGITE

PRECIPITATION

LIME

ALUMINUM REAGENT

ETTRINGITE

SOLIDS

STAGE 3, WATER

RECONDITIONING

CARBON

DIOXIDE

RAW

WATER

CALCIUM CARBONATE

AND ALUMINUM HYDROXIDE

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Role of OLI Software on Project

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OLI Studio: Stream Analyzer Modeling

• Ettringite is a species in the MSE Thermodynamic Framework

• Stream Analyzer used to develop the best “recipe” for optimizing ettringite formation

• CH2M ran preliminary models to better understand the underlying chemistry. Initial models showed promising results.

• Provision for modeling added to bench-scale study work to help identify target operating conditions and estimate performance

• Additional Stream Analyzer modeling was run alongside testing to compare to the results of the bench-scale work

• Error in early modeling run, and corresponding results, led to interesting insights into ettringite precipitation chemistry, which we hope to follow-up on

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Example Stream Analyzer Plot – Ettringite Precipitation

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Case Study 1Ettringite Bench Study for Sulfate Removal

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Project Background & Ettringite Bench-Scale Study

• Project: Surface mine in Canada expected to receive new discharge limits for sulfate, selenium, nitrate, cadmium, calcium

• Objective: Evaluate the feasibility of ettringite chemical precipitation as an alternative to evaporation/crystallization for removal of sulfate from nanofilter (NF) concentrate

• Testing:

– Bench-Scale testing/analysis by CH2M’s Applied Sciences Laboratory (ASL)

– Testing was conducted on various composites of three raw water sources (~ 500 to 800 mg/L SO4) and on one NF concentrate (~ 2,500 mg/L SO4), with target removal to 250 mg/L SO4

– Bench work designed to simulate 3-stage process (gypsum pretreatment, ettringite precipitation, recarbonation of high pH samples) on raw waters and NF concentrate

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Ettringite Bench Testing Results – Raw Water

Sulfate Reduction

– Raw water was successfully treated to ~ 250 mg/L SO4

– At higher calcium aluminate cement dosages, SO4 was reduced to <10 mg/L

Impact of Aluminum Dose on Reaction Rate

– Higher aluminum doses found to speed the reaction rate and yield lower final SO4 conc

Seeded Precipitations

– Ettringite seed found to speed reaction rate

Solids produced after aluminum addition settled in less than 20 mins

with no polymer addition

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Ettringite Bench Testing Results – Raw Water

710

50

100

150

200

250

300

350

400

450

500

550

600

0 20 40 60 80 100 120 140 160 180

Sulf

ate

Co

nce

ntr

atio

n (

mg

/L)

Elapsed Time (minutes)

Sulfate Results for Variable Cement Dosages

1.2 g cement/g SO4 (“half-dose”)

2.3 g cement/g SO4 (“single-dose”)

4.6 g cement/g SO4 (“double-dose”)

6.9 g cement/g SO4 (“triple-dose”)

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Ettringite Solids from Treated NF Concentrate

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Case Study 2Ettringite Bench Study for Sulfate Removal from High-sodium Mine Water

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Project Background & Ettringite Bench-Scale Study

• Project: Two mines in the eastern U.S. The mines have or are expected to receive new NPDES limits for sulfate and chloride. This case study had higher concentrations in raw water, including sulfate and sodium.

• Objective: CH2M conducted a conceptual engineering evaluation to address compliance for the mines, which included a robust technology confirmation, with both bench- and pilot-scale testing

• CH2M Lab Testing:

– Higher SO4 concentration in raw water (~4,000 and 5,000 mg/L SO4) and in NF concentrate (5,300 mg/L SO4). Target removal again to 250 mg/L SO4

– Bench work again was designed to simulate stages of the three-stage process for sulfate reduction

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Ettringite Bench Testing Results

• Sodium Impact:

– Several literature references state that the presence of sodium (as well as potassium and other monovalent cations) in the water increases the solubility of sulfate ion in ettringite precipitation process

– The impact of sodium ion concentration was evaluated using simple synthetic high sodium brines (~ 7,000 mg/L Na) and two additives for mitigation of sodium impacts

– Stream Analyzer modeling and bench-testing both confirmed that the presence of sodium does affect ettringite precipitation and that addition of a “pretreatment agent” (confidential) may be needed in cases of high sodium concentrations

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Sodium Impacts

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

0 20 40 60 80 100 120 140 160 180 200 220 240 260Sulf

ate

Co

nce

ntr

atio

n (

mg

/L)

Elapsed Time (minutes)

Without Pre-Treatment Agent OLI Results* without Pre-Treatment Agent

With Pre-Treatment Agent

* Note: OLI model used slightly lower aluminum dosage than jar tests

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Ettringite Bench Testing Results

Sulfate Reduction

– Final SO4 concentration highly dependent on the pretreatment agent dose

– With excess aluminum and pretreatment agent, the SO4 conc was low in both raw water and NF concentrate

Reaction Time

– Reaction time to reach low SO4 conc (<250 mg/L) is long at room temp and gets longer when run at cold temps (about 4.5°C). Shortened reaction time by increasing above room temp

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Conclusions

• Ettringite precipitation is a technically feasible treatment option for achieving low sulfate concentrations as determined using OLI Studio: Stream Analyzer modeling and laboratory bench-scale testing

• Results of Stream Analyzer modeling were generally consistent with results of the bench work

• Higher chemical dosages led to shorter reaction times and lower final sulfate concentrations

• Evaluating potential water chemistry impacts (monovalent cations, e.g. sodium) was critical

• Treatment via ettringite precipitation may not be economically feasible based on high chemical dosages and long residence times required. Economics would be dependent on the initial wastewater stream composition and target SO4 effluent concentration.

• CH2M has found ettringite precipitation to be cost competitive with other treatment options in at least 1 prior project

Thank You.