Generation and Neutralization of Acid in Mining Environments

1
Generation and Neutralization of Acid in Mining
Environments

• A) Definition of hydrolysis
• B) Acid generation
• 1 Sulfide oxidation
• 1.1 By oxygen
• 1.2 By Fe3
• 1.3. Kinetics of sulfide oxidation depends on
• a) mineral reactivity
• b) oxidant
• c) pH conditions
• d) bacteria activity
• 2. Precipitation of hydroxides /hydrosulfates
• C) Acid consuming minerals
• 1. Carbonates
• 2. Hydroxides
• 3. Aluminosilicates
• 4. Dissolution kinetics of these gangue minerals
• D) Determination of neutralization potential
• 1. Mineralogical calculations

2
A) Definition of hydrolysis

• All geochemical reactions which change pH are
  hydrolytical reactions
• Hydrolysis can be defined as a reaction with
  water constituents (H or OH-) resulting in the
  formation of solids or stable complexes in
  solution.
• Example of complex formation
• CaCO3(calcite) H2O Ca2 HCO3- OH-
• Example of mineral precipitation
• 2Fe3 3H2O  2Fe(OH)3 (s) 6H
• Note all equations must balance in terms of
  elements and charge

3
B) Acid generation

• The amount of acid generated is a complex
  function of
• (i) the type of sulfide minerals present
• (ii) their resistance to weathering,
• (iii) whether the sulfides contain iron,
• (iv) whether oxidized or reduced metal species
  are produced by the oxidation,
• (v) whether elements such as arsenic are major
  constituents of the sulfides,
• (vi) whether oxygen or aqueous ferric iron is
  the oxidant, and
• (vii) whether hydrous metal oxides or other
  minerals precipitate as a result of the oxidation
  process.
• In the mining environment, there are two
  important classes of acid producing reactions,
  oxidation of sulfides and precipitation secondary
  minerals.

4
Sulfide oxidation 1.1 Reactions with oxygen.

• Sulfide minerals that are exposed by erosion or
  mining are unstable in the presence of
  atmospheric oxygen or oxygenated ground waters.
• Sulfides with metal/sulfur ratios ltI,
• Iron sulfide (pyrite, FeS 2 marcasite, FeS2
  pyrrhotite, Fel-xS),
• Arsenopyrite (FeAsS) and sulfosalts such as
  enargite (Cu3AsS4)
• generate acid when they react with oxygen and
  water.
• FeS2 (pyrite) 3.5O2 H2O Fe2 SO42- 2H
  
• Fe0.9S (pyrrhotite) 1.95O2 0.1H2O 0.9Fe2
  SO42- 0.2H
• FeAsS (arsenopyrite) 3.25O2 1.5H2O Fe2
  HAsO42- SO42- 2H
• Sulfides with metal/sulfur ratios I,
• sphalerite (ZnS), galena (PbS), and chalcopyrite
  (CuFeS2)
• do not produce acid when oxygen is the oxidant
• MeS 2O2 Me 2 SO42- MeZn ,Pb, Cu2
• For example
• FeCuS2 (chalcopyrite) 4O2 Fe2 Cu2
  2SO42-

5
1.2 Oxidation by Fe 3

• Ferric ion forms by Fe2 0.5O2 H Fe3
  0.5H2O
• Aqueous ferric iron is a very aggressive oxidant
• When it reacts with sulfides, it generates
  significantly greater quantities of acid than
  oxygen driven oxidation.
• Pyrite
• FeS2 14Fe3 8H2O 15Fe2 2SO42- 16H
• Chalcopyrite
• CuFeS2 16Fe3 8H2O Cu2 17Fe2 2SO42-
  16H
• Sphalerite, galena, covellite Me Zn, Pb, Cu2
• MeS 8Fe3 4H2O Me2 8Fe2 SO42- 8H
  
• In general, sulfide-rich mineral assemblages with
  high percentages of iron sulfide or sulfides with
  iron as a constituent (such as chalcopyrite or
  iron-rich sphalerite) will generate significantly
  more acidic water than sphalerite- and
  galena-rich assemblages without iron sulfide.

6
1.3 Kinetics of sulfide oxidation a) mineral
reactivity

• The relative reactivities of sulfides in tailings
  differ depending on the type of experiment or
  environment. However, a general sequence from
  readily attacked to increasingly resistant is
• Pyrrhotite(po ) gt galena-sphalerite(sph) gt
  pyrite(py)-arsenopyrite(asp) gt
  chalcopyrite(cp)

Ruttan, Leaf Rapids Corroded sphalerite in
oxidized tailings. The rim around the sphalerite
grain does not contain Zn. Pyrite is much less
altered.
sph
py
Chalcopyrite (cp) disseminated within a quartz
grain and armoured against alteration.
cp
py
Snow Lake Residue Pile
7
b) oxidant

• Sulfide oxidation by ferric iron occurs more
  rapidly than by oxygen alone.
• Ferric iron plays a crucial role in determining
  whether acid will be generated during weathering.
• c) pH conditions
• If reaction occurs under acidic conditions
  (pHlt3.5), then a significant quantity of the Fe3
  can remain in solution to react with sulphides.
• Fe2 0.5O2 H Fe3 0.5H2O
• When the pH is greater than 3.5, Fe3 is removed
  from solution by the precipitation of
  ferrihydrite or ferric hydroxide
• 2Fe3 3H2O  2Fe(OH)3(s) 6H
• d) bacteria activity
• Bacteria play catalytic role in ferric ion
  formation increasing ferrous ion oxidation rate
  by 105 over abiotic rate.
• Laboratory microbial oxidation rates and field
  sulfide oxidation rates are the same, therefore
  acid generation, is controlled by
  mineral-microbial interaction.

8
e) Macro and Microstructural Features.

• Macrostructure.
• Ore with a fine grained structure reacts faster
  than coarse grained ore.
• Single sulfide crystals can only be attacked on
  the surface.
• For aggregate grains, oxidants can diffuse along
  crystal boundaries and may oxidize inside the
  grain. These sulfides have larger reactive
  surfaces.
• A single crystal grain may many fractures cracks
  after active mining and milling allowing passage
  of oxidants.
• Natural acid drainage (minimum of pH 2) is
  not as acidic as mine drainage (minimum pH -3
  at Iron Mountain CA), because explosions inside
  mine open pathways for oxygen diffusion.
• Microstructure
• Crystal defects and isomorphic mixtures may
  affect the rate of sulfide dissolution.
• Structural defects (point or linear) reduce
  atomic binding energy and atoms could be pulled
  out from crystal structure. The more defects the
  faster the rate of oxidation.
• Isomorphic mixtures can also reduce energy of the
  crystal lattice, because binding energy of
  impurity atoms may be lower than that of matrix.

9
2. Precipitation of hydroxides /hydrosulfates
and carbonates

• Precipitation of hydrous oxides during the
  sulfide oxidation process lead to the formation
  of acid.
• 2Fe3 3H2O  2Fe(OH)3(s) 6H
• Some non-sulfide minerals such as siderite
  (FeCO3) and alunite (KAI3(SO4)2(OH)6) can
  generate acid during weathering if hydrous iron
  or aluminum oxide precipitates.
• K 3Al 3 SO42- 6H2O KAl3(SO4)2(OH)6
  (alunite) 6H
• Fe2 HCO3- FeCO3 (siderite) H
• Twice as much acid generates from same quantity
  of pyrite when iron hydroxide precipitates.
  
• FeS2 (pyrite) 3.5O2 H2O Fe2 SO42-
  2H
• FeS2 (pyrite) 3.75O2 3.5H2O SO42- 4H  
  Fe(OH)3(s)

10
C) Acid consuming minerals

• Acid neutralization is the most significant
  process controlling pollutants being immobilized
  into a solid phase
• In most mineral deposits, acid-generating sulfide
  minerals are either intergrown with or occur in
  close proximity to a variety of carbonate and
  aluminosilicate minerals that can react with and
  consume the acid generated during sulfide
  oxidation.
• However, like the sulfides, the ease and rapidity
  with which these minerals can react with acid
  varies substantially.
• 1. Carbonates
• Alkaline earth carbonates such as calcite
  (CaCO3), dolomite (Ca, Mg)(CO3)2 and magnesite
  (MgCO3) react with acid according to
• MCO3 (S) H M 2 HCO3-
• This reaction goes as rapidly as acid generates
  from oxidized sulfides, therefore carbonates are
  most significant group of acid consuming
  minerals.
• Carbonates buffer the pH of solution from 5 to 8
  depending on the ratio of Ca Fe Mg.
• A variety of metal carbonates, such as those of
  zinc (smithsonite), and copper (malachite and
  azurite) occur in the oxidized zones of sulfide
  mineral deposits in dry climates. These minerals
  are also effective acid consumers.

11
Development of pH buffering zones during early,
intermediate and late stages of sulphide oxidation
12
2. Hydroxides, hydrosulfates and oxides

• Hydrous iron or manganese oxides form as a result
  of the dissolution of their respective carbonates
  (siderite and rhodochrosite).
• If the pH decreases during waste-rock weathering
  hydroxides and hydro sulfates are dissolved
  consuming protons.
• Iron and aluminum hydroxides buffer solutions at
  pH of 4 - 4.5 pH
• 2Me(OH)3(s) 6H 2Me3 3H2O MeAl3, Fe3
  
• Jarosite and alunite may control pH at about 3.
  
•  KFe3(SO4)2(OH)6 (jarosite) 6H K 3Fe3
  SO42- 6H2O
• Iron-, manganese-, and aluminum- oxides, such as
  hematite, magnetite and pyrolusite can
  theoretically react with acid in an arid climate.
• e.g.
• Fe2O3 (hematite) 6H 2 Fe3 3 H2O

13
3. Alumosilicates

• Aluminosilicate, calc-silicate, and some
  metal-silicate minerals are common components of
  many mineral deposits or their host rocks.
• Acid reacts with aluminosilicate minerals dirong
  weatherting processes. These acid-consuming
  reactions can release constituents into solution
• Mg2(SiO4)3 (forsterite) 4H 2Mg2
  H4SiO4(aq)
• Other constituents can be transformed into more
  stable minerals.
• e.g. potassium feldspar forms aqueous potassium
  and silica, and solid hydrous aluminum oxide,
• KAlSi3O8 H K 3H4SiO4(aq) Al(OH)3(S)
• Anorthite forms kaolinite,
• CaAl2Si3O8 2H H2O Ca2 Al2Si2O5(OH)4
  (kaolinite)
• Silicates could buffer solutions at pH 7, but
  there are not naturally observed equilibria for
  the dissolution of sulfides and silicates,
  because silicates dissolve very slowly compared
  with carbonates.

14
4. Dissolution kinetics of gangue minerals

• Carbonates are dissolved as fast as acid
  generates from oxidized sulfides.
• The weathering of silicates is orders of
  magnitude slower than for carbonates and has been
  shown in laboratory experiments to follow the
  trend of Bowens reaction series
• olivine gt augite gt hornblende gt biotite
• gt K-feldspar gt muscovite gt quartz,
• A comparable series applies to the plagioclase
  feldspars, with calcic varieties being the most
  susceptible to destruction
• CaAl2Si2O8 (anorthite) gt NaAlSi3O8 (albite)

15
D) Determination of neutralization potential

• Determination of neutralizing potential (NP) is a
  technique to forecast the quantity of acid that
  could be neutralized by the rock-forming minerals
  present.
• NP is measured as kg CaCO3 -equivalent/tonne
• 1. Mineralogical calculations
• Mineralogical calculations are based on
  whole-rock analysis of tailings, study of
  petrography and empirical data on the resistance
  of minerals, assuming stoichiometry and kinetics
  of the reaction
• If you know whole rock composition and rock
  forming minerals present, you may calculate
  mineralogical (normative) composition of the rock
  
• Then you can calculate NP contribution of each
  acid consuming mineral
• e.g. anorthite
• NP contribution Mol wt. calcite x 17.1 x
  0.40 27.2 kg CaCO3 equivalent/tonne
• Mol wt. anorthite 100
• NP of tailings is the sum of NP contributions
  from all minerals.
• Advantages not expensive (lt 50)
• Disadvantages takes time (weeks), and is based
  on many assumptions.

16
2. Chemical tests

• This approach is based on chemical experiments,
  assuming that the sample is a black box.
• No assumptions of stoichiometry or rates of
  reaction before experiment.
• a) Static tests
• Sample is boiled in HCl acid or H2O2
• Proton concentration (pH) changes before and
  after experiment are measured.
• Under such strong chemical conditions, all
  reactions taking many years in natural conditions
  go fast,
• Sulfides oxidize and carbonates dissolve
  completely.
• Advantages Cheap 35-135, fast (from hours
  to days) ,
• Disadvantages overestimates NP

17
Comparison of NP from static (Sobek) test and
calculation
18
b) Kinetic tests

• Leaching test are long term tests. Their
  objectives are
• to confirm, or reduce the uncertainty in, static
  tests,
• identify dominant reactions, acid generation
  rates and temporal variety in water quality.
• Conditions of experiment are more close to
  natural conditions.
• Advantages Best NP prediction results
• Disadvantages Expensive (lt5200), takes months
  or sometimes years to complete