Treatment methods of acid mine drainage and a case study on selective recovery of metals

By,V.GOUTHAM13MI60R09

MINING ENGINEERING

Overview of AMD, its generation and effects

Introduction• Acid mine drainage refers to the outflow of acidic water from (usually

abandoned) metal mines or coal mines.• a serious long-term environmental problem associated with mining• Its causes, prediction and treatment have become the focus of a number

of research initiatives, the mining industry, universities and research establishments, environmental groups.

• highly acidic discharges where the ore is a sulfide mineral or is associated with pyrite EX: Zn, Cu, Ni.

• ore of copper is chalcopyrite. Thus copper mines are often major culprits of acid mine drainage.

Chemistry of AMDGeneral equations for this process are:• 2FeS2 + 7O2+ 2H2O → 2Fe2+ + 4SO4

2- + 4H+

• 4Fe2+ + O2 + 4H+ 4Fe3+ + 2H2O• 4Fe3+ + 12H2O 4Fe(OH)3 + 12H+ • FeS2 + 14Fe3+ + 8H2O → 15Fe2++ 2SO4

2- + 16H+ The net effect of these reactions is to release H+, which lowers the pH, produces

sulphate ions.

Effect scenarios in some places• About half of the coal mine discharges in Pennsylvania have pH under 5.• In US >12,000 miles of river and 180,000 acres of lakes/reservoirs are adversely

affected. • US companies now spend >$1million/day to treat CMD prior to discharging;• In the coal belt around the south Wales valleys in the UK highly acidic nickel-rich

discharges from coal stocking sites have proved to be particularly troublesome.

Effects of Acid mine drainage• causes disrupted growth and reproduction of aquatic plants and animals. • source of freshwater metal contamination, land contamination and sparse

vegetation. • may leach into surrounding watercourses, enters waterways and disrupts

ecosystems by lowering the pH and coating stream bottoms with iron hydroxide, aluminium, black-manganese.

• Precipitation of Fe(OH)3 resulting in Increased turbidity and decreased photosynthesis;

• direct toxicity (benthic algae, invertebrates, and fish); Harm to fisheries. • In low pH conditions, the metals are dissolved and leave no physical indication

of their presence. However, acid mine drainage eventually leaves its mark.CONTROL• primary prevention of the acid-generating process; • secondary control, deployment of acid drainage migration, prevention

measures;• tertiary control, or the collection and treatment of effluent.

Treatment of AMDImportance• Varied objectives• Recovery and reuse of mine water within the mining operations• protection of human and ecological health• aims to remove the pollutants contained in mine drainage to prevent or mitigate

environmental impactsSelection of Treatment Method• treatment approach will be influenced by a number of considerations.• A clear statement and understanding of the objectives• treatment must be evaluated and implemented within the context of the

integrated mine water system.• based on mine water flow, water quality, cost, and water use(s) and receptors.• chemical properties of mine drainage relate to acidity/alkalinity, sulfate content,

salinity, metal content, etc.• Handling and dis posal of treatment plant waste and residues such as sludges

and brines and their chemical characteristics

This presentation gives a review on wetlands and mainly focuses on simple

treatment methods which are economic and integrated methods helping in individual metal recovery and treatment of acid mine drainage.

PASSIVE TREATMENT SYSTEMSWetlands

• compliance using passive treatment systems at low cost, producing pollution free treated water, and fostering a community responsibility for acid mine water treatment.

• AMD sources may remain active for decades or even centuries after mine closure. So, we need infinite amount of reactants in active systems which problem is solved by wetland technology.

• absorb and bind heavy metals and make them slowly concentrated in the sedimentary deposits to become part of the geological cycle

• The wetland’s ability to treat AMD depends on: water flow distribution, residence time, seasonal, climate.

Advantages:• Viable treatment system for many mining regions• Passive, low-cost remediation• removes iron and other metals from the water column, sulfate reduction• alleviation of extreme acidic conditions• Acidity, iron, manganese and aluminum concentrations are in AMD are

declined• Removed on average 33% of acidity and 20-40% of metal inputs• Enables vegetation establishment• Precipitates the metals.• Adaptability to acid drainage and elevated metals • Low capital costs of natural wetland systems • Low operational costs for constructed wetland• Provide wildlife habitat and flood control

Heavy metal removal in Wetlands• natural and artificially constructed wetlands offer an efficient treatment technology. • biological filters• These systems rely on several basic processes to purify contaminated wastewater:

uptake of nutrients by plants; bacterial degradation and oxidation of contaminants; sedimentation and adsorption of particles and dissolved substances in the waste on to the substrate.

Mechanism of Wetlands• the three main compartments of a wetland, i.e. (1) Soil and substrate, (2) Hydrology,

(3) Vegetation.• Hydric soils long enough during the growing season to develop anaerobic conditions

in the upper portion of the soil for hydrophytic vegetation.• plants create reduced carbon compounds that serve as food for a variety of

organisms.• Plants remove trace metals from the water through biological uptake and surface

adsorption• Decomposition of organic matter is by reduction and accumulation on the sediment

surface. The resulting organic sediment surface is responsible for scavenging heavy metals from influent AMD.

Processes in wetlandsThe primary removal processes in these systems include sedimentation, adsorption,

ion exchange, complexation, which are finite; removal will cease unless new removal sites are generated.

Physical removal processes• settling and sedimentationChemical removal process• Sorption, adsorption, Oxidation and hydrolysis of metals, Precipitation and co-

precipitation, Metal carbonates, Metal sulphidesBiological removal processes• Micro-organisms

Performance:• Beining and Otte (1996, 1997) reported about a natural wetland of a lead-zinc

mine of life 120 yrs. At surface mine sites with continual contaminant production or at systems constructed to treat drainage from underground mines or coal refuse disposal areas. High metal removal rates of close to 100% have been reported in both natural and artificially constructed wetlands.

TREATMENT USING SULPHATE REDUCING BACTERIA

• Sulphate reducing bacteria oxidize organic compounds under anaerobic conditions and reduce SO4

2- to sulphides. As the organic compounds are oxidized, alkalinity in the form of HCO3- is produced.

• 2CH2O + SO42- + H+ → H 2S + 2 HCO3+

• where CH2O represents the organic compound• The sulphides produced in the reaction (H2S, HS-, S2-) react with metals to form insoluble

metal sulphide precipitates, where M2+ represents metals such as Cd2+, Cu2+, Fe2+, Ni2+, or Zn2+ (Kaksonen & Puhakka, 2007). Precipitation occurs in order of increasing solubility product (Ksp), which is the product of ion concentrations.

• These reactions result in an increase in pH and alkalinity, and a decrease in SO42- and metals

concentrations, consistent with the objectives for treatment of the site AMD. Longevity of a system involving SRB can be limited by the depletion of the organic sources and clogging due to precipitation of the metal sulphides (Kaksonen & Puhakka, 2007).

• organic substrates: including plant materials, and agricultural, industrial and municipal wastes. Examples of the organic substrates include alfalfa, rye grass, hay, straw, rice stalks, peat, spent mushroom compost, municipal and leaf compost, molasses, sewage sludge, manure, paper products, plant hydrolyzate, cellulose, sawdust, and wood (Kaksonen & Puhakka, 2007).

• Excellent removal of metals including Cd, Cu, Ni, and Zn has been demonstrated in a number of laboratory and field scale studies completed using low-cost carbon sources to support SRB (Clyde et al., 2010; Costa, Martins, Jesus, & Duarte, 2008; Waybrant et al., 1998; Zagury et al., 2006;)

Selective recovery of Cu, Zn, Fe from acid mine drainage by intergrating sulfidization with neutralization

• The most commonly used process for acid mine drainage (AMD) treatment today is lime neutralization.

• accompanied by the treatment of produced metal hydroxide precipitate which have costly disposal requirements.

• There is a decrease in the capacity of landfill disposal site.• There is also an increase in the price of base metals such as copper and

zinc in recent years. • For the subsequent smelting process, the major issue is how to separate

the copper, zinc and iron from AMD as selectively as possible• it is expected that not only to treat but also to recover these base

metals from AMD.• by modifying the present lime neutralization treatment process with

sodium hydrosulfide (NaHS) sulfidization.

Process of study:• At first, lime neutralization was applied to the AMD to find the

precipitation behaviours of Cu, Zn, and Fe.• Next, NaHS sulfidization as well as the integration with lime neutralization

were conducted to separately precipitate Cu, Zn and Fe from the AMD.• Finally, Modified treatment approaches for selectively recovering the

metals from the AMD were proposed. Sulfidizing agent used: NaHS• more adequate than H2S• preferred over H2S for safety consideration• easier to handle in practical applications than the biogenic sulphide.

Case study:• An AMD sample generated from an abandoned copper mine of Ashio

Mines located in east Japan was utilized in this study.• The Cu mine in our case study, discharges about 12 m3/min of AMD, which

is gathered and treated in the downstream purification plant.• AMD contains copper, zinc , and iron. Other heavy metals such as lead,

cadmium, and arsenic in the AMD are always below the Japanese effluent standards.

Materials used in the process:• AMD sample taken: pH 2.8, Cu 4.05mg/L, Zn 9.86mg/L, Fe 6.58 mg/L• filtration before experiment• Lime (calcium hydroxide)• NaHS (70%sodium hydrosulfide)• 30 wt.% hydrogen peroxide (H2O2) solution.• All reagents were analytical grade.

Experiment results and discussion is

carried out.Precipitation of Cu, Zn and Fe with Lime

neutralization• Fe: pH 4 to pH 5. • Cu: pH 5–6 to pH 8. • Zn: pH 7, to pH 9.• both Cu and Zn in the AMD precipitated

between pH 6–8, which suggested that these two metals cannot be separated from each other in the present lime neutralization treatment process.

• Some factors can affect the metal precipitation process so that the observed pH when precipitation occurs spans a range.

2. Precipitation of Cu, Zn, Fe by NaHS sulfidization• According to the results in 1, the sulfidization is to be conducted below pH 5• At pH 2.8, Cu: 5 mg/L NaHS to 20 mg/L• Fe and Zn remained unchanged.• >20 mg/L NaHS at pH 2.8 can selectively precipitate Cu over Fe Theoritical dosage of sulfidizing agent:• The solubility equilibrium condition and solubility product (Ksp) of a metal

sulfide(MS) are: MS = M2+ + S2- Ksp = [ M2+ ][ S2- ]

• The theoretical solubility products of CuS, ZnS and FeS are 4.53*10-37,

1.67*10-25 and 1.18*10-19 respectively.• The solubility equilibrium conditions of each metal sulphide are drawn

using log scales.• Cu begins to precipitate when S2- concentration is >2.99*10-29mol/L,

whereas Zn when >1.1*10-21mol/L S2- is introduced, and Fe at 1*10-15mol/L S-15 is introduced. Therefore, it is theoretically possible to selectively precipitate Cu over Zn and Fe from the AMD by sulfidization.

• The introduced NaHS (sulfidizing agent) exists as H2S, HS-, and S2-. The theoretical dosages of NaHS needed are 5.26×10-19 mol/L at pH 2.8, and 3.32×10-21 mol/L at pH 5.

• The experimental dosages of NaHS for precipitating Cu are 2.5×10-4 mol/L (20mg/L) at pH 2.8 and 1.25×10-4 mol/L (10 mg/L)at pH 5,which are much higher than the theoretical values.

• Because of generated H2S and escapes into the atmosphere from the solution and is also unfavourable.

• Although NaHS sulfidization may be conducted at a higher pH so as to avoid H2S formation and reduce the NaHS dosage needed, however, Cu and Zn might co-precipitate during pH adjustment and the selective sulfidization may become difficult.

Precipitation of Zn, Fe by lime

neutralization after Cu precipitation by sulfidization

• Zn: pH 7 to pH9. • Fe:pH 6–7, pH 9. • Fe co-precipitated with Zn at pH 7–9, • introducing H2O2 (30 wt.%

hydrogen peroxide solution). By adding more than 0.02 vol.% H2O2, all Fe precipitated at pH 5 whereas Zn did not.

Proposed approach:

• For the purpose of selectively recovering Cu and Zn over Fe from AMD, the

feasibility of integrating NaHS sulfidization with conventional lime neutralization treatment was investigated.

• Cu, Zn, and Fe in the AMD were removed and separated into individual precipitates and the concentrations of Cu, Zn, and Fe were able to meet the Japanese effluent standards.

• The present study was a collabora tive research project between the University of Tokyo, Japan and the Furukawa Co., Ltd., Japan & supported in part by the 21st Global COE program, ‘‘Mechani cal Systems Innovatio n’’, MEXT,Japan.

A novel low pH sulfidogenic bioreactor using activated sludge as carbon source to treat AMD and selective recovery of metal

sulfides:

Case study• In this work, a pilot scale sulfidogenic bioreactor was used to treat acid

mine drainage from Zijinshang copper mine using less expensive & easily available carbon source and economic treatment process

• Zijinshan copper mine is the largest secondary copper sulfide mine in China (Ruan et al., 2011), large amount of AMD was produced.

• The AMD produced by mining activity at Zijinshan plant was treated by membrane technology with a capacity of 3300–3600 m3/days (Ruan et al., 2011). Due to relative high cost of membrane technology, other alternative AMD treatment process is needed.

• Bacterial sulfate reduction has been applied.• The major goal of the study aims at evaluating the feasibility of the system,

with real AMD from Zijinshan Copper Mine, activated sludge as carbon source and no pH control, and to provide data for the technical and economical evaluation for a larger scale.

Process:• In this process, S2- produced in the Up-flow Anaerobic Sludge Bed (UASB) reactor were

recycled in the two precipitation tanks for copper and iron precipitation, activated sludge from local wastewater treatment plant was used as the carbon source.

• The reactor were steady operated in acid condition (with no pH control) for 4 month,• AMD with a Cu concentration of 100–120 mg/L, Fe 170–200 mg/L, sulphate of 2000–2500

mg/L and pH 2.34–2.56, were feeding into the reactor under a feed rate of 1 m3/days and HRT of 3 days

Materials used in the process:• The feeding AMD of this study was from Zijinshan Copper Mine. The chemical characteristic

of the influent were as follows: pH 2.34–2.56; Cu2+ 100–120 mg/L; Fe 170–200 mg/L; SO4 2-

2000–2500 mg/L.• Two stainless precipitation tanks with a working volume of 1.5 m3 • The bioreactor used in this research was a stainless UASB reactor with a working volume of 3

m3. • AMD was fed into copper precipitation tank by pump, and from there to iron precipitation

tank by gravity. A pump fed AMD from iron precipitation tank to the UASB reactor, and the water was re-circulated back to the iron precipitation tank by gravity. The H2S gas produced from the UASB reactor was directly fed into the copper precipitation tank. Excess activated sludge from Shanghang wastewater treatment plant with a water content of 70% was used as carbon source and directly feeding into the UASB reactor. The feeding rate was 25 kg/day.

Sulfate was determined by ion chromatography. Copper and total iron concentrations were determined by atomic absorption spectrophotometer.

Results obtained in process1. Cultivation of sulfate reducing bacteria

(day 0–68)• AMD was neutralized to pH 7 by NaOH

and precipitated to removal copper and iron.

• water only contains SO42- with a concentration range from 1000 to 2450 mg/L.

• Sulfate removal was over 80% in the first 10 days, then decreased to around 40% and maintain steady.

2. Sulfate removal during the start-up stage

(69–210 days)In the period I, the AMD was diluted by equal

volume of water before feeding into the system. In period II, the volume of water add into AMD was stepwise decreased, and in period III, the AMD was directly feeding into the system without dilution.

• In period I, the sulfate removal is maintained around 40%

• In the beginning of period II, the sulfate removal rate is decreased

• decrease of influent sulfate concentration at period II

• At the end of the whole stage, sulfate removal was only 38%

• carbon source fed & Relative high Cu, Fe in the influent low pH

Copper removal during the start-

up stage (69–210 days):• The high copper removal rate

up to 80% in period I • At the initial stage of period

II, the copper removal decreased significantly from 80% to 10%

• At the end of period II, copper removal increased to around 60%

• At period III, copper removal was maintained around 60%

Iron removal during the start-up stage (69–210 days):• Due to the pH decrease, the influent iron

concentration was significantly increased from 10 to 160 mg/L in the period I

• While effluent iron concentration was maintain at low level (0.1– 5.8 mg/L)

• In the initial stage of period II, when the feeding iron concentration were around 80–120 mg/L, the effluent iron concentration increased to 29 mg/L

• in the following stage of period II, when the feeding iron concentration increased to 104 mg/L, the effluent iron concentration decreased from 29 mg/L to 13 mg/L

• iron removal of 92.85% and was maintained.

discussionIn terms of toxicity and H2S effects:• In all literature on SRB bioreactor: the Biosulfide and the Thiopaq processes

(Johnson and Hallberg, 2005). • In the Biosulfide- like system, AMD is directly fed into a precipitation tank; H2S

produced from the biological reactor was fed into precipitation tank for the metal recovery. High concentration of water soluble H2S will poison microbial community in the bioreactor and thus decrease the sulfate reduction rate.

• In the Thiopaq-like processes, AMD and carbon source were directly feeding into bioreactor, and in the reactor both sulfate reduction and metal precipitation occurs. High concentration of metal ion will have toxic effect on the microbial community, and the recovery of metal will be a problem.

• In our novel process, by using water recycle, the produced water soluble H2S entered precipitation tank and precipitated with metal to form metal sulfides, thus the H2S will maintain lower than water soluble concentration. Meanwhile, the metal was removed from the AMD at precipitation tanks, thus when the AMD entering bioreactor (UASB), only low concentration of metal ion within the AMD. Such process will decrease the toxicity of both metal ion and H2S to the microbial community and increase the sulfate reduction rate

In terms of sulfate reduction and carbon source• It is documented that SRB were sensitive to acidity (Hard et al., 1997), and

most of the reported papers were operated at neutral pH .• While other researchers reported that SRB could remove 38.3% of sulfate

at pH of 3.25 and 14.4% at pH of 3.0 by using lactate as carbon source • In our novel process, though feeding pH lower to 2.5 and effluent pH lower

to 3.8, still 38% of sulfate removal was reached, which is comparable with Elliott’s results. Consider the carbon source used in our process were activated sludge, the sulfate reduction is promising.

• Attempts are made at isolating pure culture of acid-tolerant SRB failed. Four 16S rRNA gene clone libraries have be constructed to analyze the microbial community structure inside the UASB reactor, results showed the existence of SRB including Desulfomicrobium, Desulfoviobrio psychrotolerans, Desulfovibrio aminophilus and some other uncultured sulfate-reducing bacteria (unpublished data). Works have been carrying on isolating the acid-tolerant SRB from this reactor.

In terms of High iron removal• Our work showed relative high iron removal by the system.• Adsorption might be one reason that previous research noted that Fe2+

could be bound to the solid phase that contains a residual negative charge on its surface (Cohen, 2006; Sheoran et al., 2010).

• At lower pH conditions, substitutes of SO42- for OH- occurs, resulting in the formation of ‘‘oxyhydroxysulphate’’ minerals such as schwartmanite (Sheoran et al., 2010).

• Another thing worthy to note is that Na+ and K+ and other monovalent cation from the broken cell by the sludge digestion in the bioreactor could form ferrite and jarosite and other iron complex with iron ion. Hydrolysis of iron (III) during the pH increase and produced water soluble CO2 also could attribute to the high iron removal.