1、1Operational and Financial Studies of Hydrogen Peroxide versus Hydrated Lime and Hydrogen Peroxide versus Sodium Hydroxide at Two Pennsylvania Mine Drainage Treatment SitesBrent Means1, Richard Beam and Don Charlton AbstractThe Pennsylvania Department of Environmental Protection (PA DEP) oversees a
2、number of mine drainage treatment trust funds. Recent financial market conditions have reduced revenue generation needed to pay for treatment. Cost-reduction evaluations were performed at two sites: the Mon-View Mathies and LTV Banning facilities. At Mon-View, 20% sodium hydroxide solution (w/w) was
3、 used for treatment and at Banning hydrated lime (Ca(OH)2) was used. The evaluations consisted of characterizing chemical consumption and costs, pilot testing alternative treatment strategies and conducting a cost and treatment performance comparative analysis. The evaluation of the original Mon-Vie
4、w sodium hydroxide system showed hydroxylation and ferrous iron (Fe(II) removal were the dominant alkali-consuming reactions and calcite formation was minor. The actual NaOH dose was less than half the theoretical dose required to neutralize all alkali-consuming reactions. The treatment process util
5、ized influent alkalinity contained in the mine drainage to aid in neutralization and saves $108 per day in NaOH costs. While this treatment scheme effectively utilized influent alkalinity, avoidable hydroxylation reactions doubled alkali consumption and increased costs. The evaluation of the origina
6、l LTV Banning hydrated lime system showed the daily Ca(OH)2 cost was $569, of which $190 was spent on hydroxylation reactions, $39 on Fe(II) removal and $340 on calcite formation. Unlike Mon-View, this treatment system did not utilize influent alkalinity and dosed at the theoretical rate required to
7、 neutralize alkali-consuming reactions. A small portion of alkali added actually contributed to Fe(II) removal, which was the sole parameter targeted for treatment. The evaluations showed significant chemical costs as a result of alkali consumption due to hydroxylation and calcite-formation reaction
8、s. Year-long hydrogen peroxide (H2O2) field trials were conducted at each site to eliminate costs due to hydroxylation and calcite formation. At Mon-View, a 35% H2O2 dosing rate of 14 gallons per day achieved effluent concentrations of total iron similar to NaOH treatment. The H2O2 reduced the daily
9、 chemical cost by 34% and produced a yearly cost savings of $25,500. At Banning, a treatment strategy of 50% H2O2 and flocculent aids were used; however, the H2O2 treatment produced a difficult-to-settle iron floc that discharged elevated suspended solids. The treatment scheme was modified to a comb
10、ination of 50% H2O2 and hydrated lime, with the lime serving as a settling agent. Dosing rates of 25 gallons per day and 1.2 tons per day produced acceptable effluent and a yearly savings of $120,000. Cost savings are expected at other net alkaline mine drainage sites treating for iron. 1Brent Means
11、 is a hydrologist with the Office of Surface Mining (OSM), Harrisburg, PA, bmeansosmre.gov (corresponding author). Richard Beam is a geologist with the PA DEP and Don Charlton is President of AMD Industries. 21.0 IntroductionEnsuring perpetual treatment of coal mine drainage (CMD), post bankruptcy,
12、is a challenging task for state programs. PA DEP inherited treatment responsibilities after the bankruptcy of LTV Steel Corporation (LTV) in 2001 and Mon-View Mining in 2005. LTV operated nine treatment facilities, with flow rates ranging from 925 to 4,500 gallons per minute (gpm), and Mon-View Mini
13、ng operated a single facility. During bankruptcy proceedings, PA DEP used historical treatment cost data to calculate the perpetual treatment liability for each site. In both cases, the assigned assets were less than the amount requested by PA DEP resulting in partial funding of the perpetual liabil
14、ity. Liquid assets were placed in a growth income trust, whose revenue is utilized to pay for the ongoing operation and maintenance of mine drainage treatment facilities. The trust is financially managed by a third party, the Clean Streams Foundation, and PA DEP conducts oversight and approves all t
15、rust expenditures. To further stress the already underfunded trusts, the onslaught of The Great Recession limited revenue generation and contributed to dramatic fluctuations in commodity pricing. Pricing for sodium hydroxide, the treatment chemical used at Mon-View, tripled over two years. PA DEP, w
16、ith assistance from OSM, responded to the situation by performing cost-reduction evaluations at two sites, the Mon-View treatment site and the LTV Banning treatment facility. The Banning site was selected from the nine LTV sites based on its relatively high annual costs and because a second plant, t
17、he Euclid facility, which is essentially the same design as Banning, is operated in conjunction with Banning in order to control the mine pool. Consequently, any cost reduction strategies realized at Banning could be applied to Euclid as well. The major costs at Mon-View consisted of labor, treatmen
18、t chemical and sludge disposal. The major costs at Banning were pumping, labor and treatment chemical. Since annual chemical costs were a large percentage of the overall costs, the cost-reduction evaluation focused on evaluating whether the current chemical selection and usage were optimized and the
19、 most cost-effective option available. A five step methodology was used in the cost-reduction evaluation: 1) Measure the current chemical dosing rates, 2) Quantify treatment process chemistry to identify the chemical fate of alkali treatment reagents, 3) Develop alternative treatment strategies, 4)
20、Pilot test alternative treatment strategies, and 5) Perform a cost and performance evaluation between treatment strategies. This paper presents the results of the cost-reduction evaluation. First, the history and treatment configuration for each site is presented. Second, the methodologies used to p
21、erform the cost-reduction evaluation are presented. Lastly, the results of cost-reduction evaluation are presented.2.0 Site Description2.10 Mon-View Mathies The Mon-View Mine is located near the Town of Monongahela, PA. The 12,835 acre underground mine complex, operated since 1944, mined the Pittsbu
22、rgh Coal Seam using both conventional and longwall mining techniques until its abrupt closure in 2001. A mine pool quickly developed and iron-laden water started to gravity drain from an unreclaimed portal. The discharge flow ranges from 300 to over 2,000 gpm and quickly responds to precipitation ev
23、ents because of the subsidence features caused by the longwall mining techniques and historic room-and-pillar mining that occurred under shallow cover. The mine is currently 60% flooded (Ziemkiewicz et.al, 2004).3The Mon-View facility treats mine water that can be classified as “net alkaline” (Cravo
24、tta and Kirby, 2004). Table 1 shows the influent pH is 6.8 and contains 385 mg/L of alkalinity (as CaCO3). Total iron is the only parameter targeted for treatment. Both Table 1 and visual inspections show the water is partially oxidized and contains suspended iron hydroxide as it emanates from the m
25、ine portal. The total and dissolved iron concentrations are 46 and 34 mg/L, respectively. As the gravity discharge emanates from the mine portal, a 1,200 foot pipe conveys the water to ponds and a wetland. A 20% NaOH solution (w/w) is added directly into the conveyance pipe for pH adjustment. The tu
26、rbulence and retention time within the pipe acts as a reaction tank that mixes the NaOH with the mine drainage to increase pH and promote Fe(II) oxidation. During the treatment evaluation, the retention time between NaOH addition and the pipe outlet was measured at 3.5 minutes. The conveyance pipe d
27、ischarges the water to two oxidation/settling ponds and a wetland, configured in series. The wetland discharges the final effluent to the receiving stream, Mingo Creek, a trout-stocked fishery. 2.20 LTV Banning This underground mine complex is located near West Newton, PA, and was operated as early
28、as 1889 by the Pittsburgh Coal Company. The mine was then operated by Republic Steel Corporation and finally by LTV until closure in 1982. Mine water treatment began while the mine was still in operation in the mid-1960s as a result of legislative requirements enacted in 1966 by the Commonwealth of
29、PA which required all active underground mine operators to obtain discharge authorizations and treat all water pumped or otherwise discharged from their operations. The Pittsburgh Coal Seam was mined at this site by room-and-pillar methods. The Banning Mine complex and associated mine pool encompass
30、es a 28,000 acre area and is 43% flooded (Ziemkiewicz et.al, 2004). Two treatment plants, Banning and Euclid, pump the mine complex at a combined pumping rate of 6,500 gpm to prevent an artesian discharge into the Youghiogheny River Basin. The Banning mine pool must be maintained at an elevation bel
31、ow 775 mean sea level in order to prevent a breakout of the mine pool that would occur immediately adjacent to the Youghiogheny River in the Town of West Newton, located approximately one mile downstream. Euclid pumps from the deepest part of the mine and Banning pumps from a 150 foot shallower sect
32、ion located two miles from Euclid. The Banning facility treats net alkaline mine water containing an influent pH of 6.8 and alkalinity of 394 mg/L as CaCO3 (Table 1). Total iron is the only constituent targeted for treatment and Table 1 shows its concentration ranges from 10 to 18 mg/L. The Banning
33、facility pumps at 2,310 gpm and uses Ca(OH)2 slurry that is made on site using bulk delivery hydrated lime and treated effluent as slurry makeup water. The raw water is pumped to a rectangular reaction tank where Ca(OH)2 slurry is added and pneumatically mixed for twenty minutes. After the reaction
34、tank, the water flows to a flocculation tank and is then discharged to a circular clarifier having a retention time of approximately forty minutes. The precipitated sludge is continuously siphoned from the clarifier bottom and returned to the mine by injection boreholes. 4Site Sample Date Alkali Rea
35、gent Flow Field pH Field Alkalinity Ca - D Ca - T Fe - D Fe - T Mg - D Mg - T Mn - D Mn-T Na - D Na - T Sulfate Cl- TDS 105 CMon-View Untreated 8/10/2010 20% NaOH (w/w) 396 6.86 385 96.5 100 34.8 46.3 37.5 39.7 1.34 1.41 448 468 948 86.5 1858Banning Untreated 7/28/2011 Ca(OH)2 2310 6.89 394 114 112
36、18 18.0 37.8 37.4 0.42 0.42 434 432 888.1 119.9 1918* Flow = gpm, all concentrations in mg/L, Alkalinity =mg/L as CaCO3, D = Dissolved, T = TotalTable 1: Untreated water quality characteristics at Mon-View and Banning. 53.0 MethodologyThis section describes the methods used to measure flow and alkal
37、i dosing rates at both sites. In addition, this section describes the methods used to compute the chemical consumption due to alkali-consuming reactions encountered during the treatment process. The method used to validate the computed consumption is also presented. 3.10 Flow and chemical dosing rat
38、es The gravity flow rate at Mon-View was measured using a Marsh McBirney Model 2000 ultrasonic flow meter. The pumping rate at Banning was measured using an unobtrusive Greyline PT400 Portaflow ultrasonic flow meter secured to the pumping pipeline away from turbulent zones. Alkali dosing rates were
39、quantified using two methods. The first method entailed using the measured flow rate and collecting the dispensed chemical for a specified time period to determine the dosing rate. The authors recognized that dosing variability exists over short time frames. Therefore, measured dosing rates were val
40、idated by collecting a series of water samples directly before and after chemical addition to measure the mass increase in calcium Ca and sodium Na. The increases were expressed in terms of the treatment chemicals and adjusted for reagent purity to compute dosing. Results from both methods agreed (
41、10% difference) with the chemical purchasing records for each site. 3.20 Computed chemical consumption due to Fe(II), hydroxylation and calcium carbonate (CaCO3) formation The alkali requirement to achieve a desired target treatment pH is a function of the total hydroxyl-consuming reactions that occ
42、ur when pH is adjusted. Identifying the reactions responsible for hydroxyl consumption is important for predicting alkali requirements and for developing treatment strategies to reduce avoidable consumption. Since both of these waters are circumneutral pH net alkaline, the common hydroxyl-consuming
43、reactions encountered during treatment include Fe(II) removal, hydroxylation of aqueous species and calcite (CaCO3) formation. 3.21 Consumption due to Fe(II) removal Fe(II) is commonly removed from CMD by adding alkali chemical to a targeted treatment pH of between 7.5 and 8.5. Within this pH range,
44、 Fe(II) is removed by two different mechanisms occurring simultaneously. The removal mechanism, ferrous hydroxide formation (Fe(OH)2) (Equation 1), is the dominant mechanism at the upper end of the pH treatment range. The other mechanism, Fe(II) oxidation (Equation 2), is the dominant reaction at th
45、e lower end of the pH treatment range. Figure 1 shows that dissolved Fe(II) concentrations in excess of 10 mg/L will persist at a treatment pH of 8.5, if Fe(OH)2 formation is the sole control of iron solubility. Field experience and treatment performance data show dissolved iron is routinely below 0
46、.5 mg/L at a treatment pH of 8.5. A combination of both Fe(II) removal mechanisms occurring simultaneously explains the discrepancy between theory and field observations. In addition to Fe(OH)2 formation at pH 8.5, rapid Fe(II) oxidation at air saturation will reduce the dissolved Fe(II) concentrati
47、on by half in less than ten seconds using rate constants reported by Dempsey et al (2001). Figure 1 shows that Fe(II) oxidation will further reduce the residual dissolved Fe(II) concentration, due to Fe(OH)2 solubility, to less than 0.5 mg/L. Fe2+ + 2OH- = Fe(OH)2 (1) Fe2+ + .5H2O + .25O2 + 2OH- = F
48、e(OH)3 (2) 6NaOH and Ca(OH)2 consumption due to Fe(II) removal was determined by collecting total and dissolved water samples at the influent and effluent of the reaction tank. Sample results were used to quantify Fe(II) removal and Equations 3 and 4 were used to compute the NaOH and Ca(OH)2 consump
49、tion due to Fe(II) removal. NaOH consumption (ml/L) = (CFe initial CFe reactor effluent) * 0.006 (3) Ca(OH)2 consumption (mg/L) = (CFe initial CFe reactor effluent) * 1.33 / % purityCa(OH)2 (4) Figure 1: Solubility of Fe(OH)2 and Fe(OH)3, considering OH-, CO2(aq) and SO42- aqueous complexing.3.22 Consumption due to hydroxylation Hydroxylation is defined herein as the reaction of hydroxyl ion (OH-) with aqueous species to form water and other aqueous species. For example, as Ca(OH)2 dissociate
