Iron Oxidation
Acid mine drainage usually contains significant iron concentrations that result from the oxidation of pyritic minerals present in coal seams. The effect of the process of oxidation on pyrites produces ferrous sulphate and sulphuric acid. These salts readily dissolve in water forming the AMD.
Iron is initially present in the ferrous (Fe2+) form. Ferrous iron can be oxidized to ferric iron which is much more soluble and hence can be precipitated as a hydroxide to effluent quality levels below the allowable pH of 6.0. The minimum solubility of ferric iron occurs at a pH of 8.0 (Figure 4.5), while ferrous iron does not reach minimum solubility until the pH approaches 11.0.
FIGURE 4.5 Solubility of ferric and ferrous iron at various pH.7
At the maximum allowable discharge pH of 9.0, ferrous iron is soluble to about 4 mg/L, which exceeds the discharge limitations for new sources. Therefore, in most AMD treatment systems, it is imperative to oxidize any ferrous iron to the ferric form and then to remove it at lower system pH. Methods used for this oxidation include:
- • Mechanical aeration
- • Chemical oxidation
- • Biological systems
Aeration Systems
Ferrous iron, when exposed to oxygen, oxidates to ferric iron. The rate of oxidation depends upon the ferrous iron concentration, the dissolved oxygen concentration, and the pH of the solution. At pH values higher than 6.0, the reaction occurs according to the following rate equation:
The reaction is a first-order reaction with respect to the ferrous iron and the dissolved oxygen concentration. The oxidation rate decreases as the concentration of ferrous iron or oxygen decreases. The reaction rate is second order with respect to the hydroxyl ion (OH-) concentration for pH values >6.0. The reaction rate increases 100 times for each one-unit rise in pH above 6.0. The rate of ferrous iron oxidation is extremely slow at a pH of <3.0, slow in the pH range of 3.0-6.0, moderate to fast in the pH range of 6.0-8.0, and rapid above this point. At a pH level of 5.0, ferrous iron will precipitate as ferric hydroxide sufficiently to meet the effluent limits at a pH of 5.0. At this pH value, however, the oxidation rate for ferrous iron is slow. Until the pH is 8.0 or greater, the oxidation rate does not increase until the pH value reaches 8.0 or higher. When iron occurs mostly in the ferrous form, aeration processes are most efficiently operated within a pH range of 8.0-9.0, when oxidation takes place in a matter of minutes. At this stage, the controlling parameter for the design of the aeration unit becomes a function of the oxygen transfer efficiency and not the chemical reaction of oxygen and iron. The aerator should be designed to provide dissolved oxygen saturation in the aeration basin with maximum oxygen transfer.
The capacity of the aeration system is determined by the amount of iron to be oxidated. If the oxygen requirements cannot be met, the oxidation will be incomplete. The oxidation rate increases as the concentration of oxygen dissolved in water increases to its saturation point. Aeration capacity in excess of the saturation point is not beneficial.
The rate of oxygen transfer into water depends on the initial oxygen deficiency of water. It is easier to dissolve oxygen by aeration if the initial oxygen concentration is lower.
The oxidation rate of ferrous iron depends on the dissolved oxygen concentration, with the maximum rate occurring at saturation. For optimization of the aeration process, these two mechanisms must be compromised. At a pH >8.0 the oxidation rate is sufficient so that oxygen concentration near saturation level is not necessary. If aeration is performed at a pH level <8.0, then a fairly high level of dissolved oxygen should be maintained.
The chemical equations for the oxidation of ferrous iron to ferric iron and hydrolysis of ferric iron are
These chemical equations indicate that 1 kg of oxygen will oxidate 7 kg of ferrous iron under ideal conditions. In these reactions, 1 mol of acidity (as H2S04) is formed for each mole of ferrous iron oxidated. Sufficient alkalinity must be added to neutralize the extra acid formed and to maintain optimum pH conditions.
The aeration system chosen must meet the oxygen demand for ferrous iron oxidation. The theoretical oxygen demand for any mine water can be calculated from the following equation:
where
02=theoretical 02 demand (kg 02/hr)
QW = AMD flow rate (L/sec)
Fe=Initial concentration of Fe2+ (mg/L)
Atmospheric air contains about 21% of oxygen by volume. Only a fraction of the oxygen in the air that comes in contact with water, called oxygen transfer efficiency, is actually absorbed by water. This fraction differs for each aeration system and operating conditions. The total air needed to supply the theoretical oxygen quantity demanded can be calculated by the following equation:
where
Qa=total air demanded (m-7min)
02=theoretical oxygen demanded (kg/hr of oxygen)
E=oxygen transfer efficiency (as %)
Oxygen transfer efficiencies (E) range from 3% to 25% depending upon the type and size of the aerator and the depth of submergence.
The aeration system must also be capable of keeping the ferric hydroxide solids and unreacted reagent in suspension. If the mixing is insufficient these solids will settle at the bottom of the basin.
Settled solids will reduce the aeration volume and aeration time, causing incomplete ferrous iron oxidation. Thus, the aerator must be designed to meet both oxygen and mixing requirements.
Aeration processes that dissolve atmospheric oxygen in mine drainage water can be classified into four types:
- • Mechanical surface aeration
- • Submerged turbine aeration
- • Cascade aeration
- • Diffused air aeration
The aeration basin should be efficiently designed for complete oxidation of ferrous iron without requiring excessive aeration time. Important parameters are basin plan, depth, and inlet and outlet structures. Aeration basins are excavated in the ground and levelled with riprap or asphalt. They also can be constructed as concrete or steel structures.
Mathematical models have been developed to predict aeration times for the oxidation of ferrous iron at varying pH ranges. The nature of mine drainage is variable, and the effects of the other dissolved ions on the reaction are not well known. Therefore, laboratory tests are the most reliable way to optimize the aeration system design. The tests should be conducted on a sample containing the maximum expended ferrous iron concentration and at the lowest anticipated operating pH level in the AMD.
The detention time needed for ferrous iron oxidation must be multiplied by a safety factor for the design of the operating aeration basin. The capacity of the aeration basin is determined from the following formula:
where
V=volume (m3)
Q = flow (m Vsec)
Dt=detention time (sec)
/= safety factor
The aeration basin must be designed to efficiently accommodate the aerator, and the entire volume of the basin should be well mixed and aerated.
Besides aeration, chemicals have been used for iron oxidation. Ozone and hydrogen peroxide have been used.
Theoretically, 1.0 kg of ozone will oxidize 2.3 kg of ferrous iron. The same amount of acid is released during ozone oxidation as during aeration. At 86% ozone utilization, 1.0 kg of ozone will oxidize 2.0 kg of ferrous iron. The advantages of ozone treatment over lime aeration include: [1]
FIGURE 4.6 Hydrogen peroxide feeding system.7
The chemical reaction of ferrous iron with hydrogen peroxide is given by the following equation:
According to the above equation, 0.45 kg of H202 will oxidize 1.5 kg of ferrous iron. A hydrogen peroxide feeding system is shown in Figure 4.6.
- [1] The oxidation reaction is efficient and quick. • Close process control needed in the lime treatment process is not required. • Neutralization to pH 6.0 is all that is needed. • The sludge produced by the limestone-ozone reaction is denser than lime sludge, reducingsludge handling requirements. Hydrogen peroxide can be used when specific conditions exist: • Alkaline mine drainage (pH higher than 6.0) with 10 W oxygen requirements for ironoxidation values • Need for a supplemental oxidizing source when the system is overloaded with iron andexpansion is impossible
