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Controlling Iron in Plant Water Systems

Soluble iron in quantities of 0.3 ppm (as Fe) and above is considered to be objectionable in industrial water uses.

Iron can either be removed or controlled in order that it will not create a problem.

Removal of Iron

  1. The most common method of removing iron is aeration, followed sometimes by settling and then filtration. The most common type of aerator is the coke tray aerator.

    Only about 1 ppm of O2 is required to remove 7 ppm of ferrous iron. Assuming the iron to be in the bicarbonate form, the reaction is:

    4Fe(HCO3)2 + O2 + 2H2 O -- 4Fe(OH)3 + 8CO

    The speed of reaction is dependent on several factors. High pH tends to speed the reaction; the optimum pH values are 7.0 to 9.0. Other factors that tend to increase the rate of precipitation are alkalinity, high hardness and high total dissolved solids.

    As would be expected, water of low alkalinity, hardness and total dissolved solids tends to precipitate iron at a slower rate. Large amounts of free CO2 also tend to slow the reaction—even at equal pH values.

    Following aeration, the water may be directed to a settling tank for a short retention time and then filtered. For iron quantities under 5 ppm, the settling tank may be omitted and the water passed directly to filtering equipment.

    Sand filter of either pressure or gravity type are used to remove the water insoluble ferric hydroxide precipitate from the water. In some plants, reduction of the ferric form to the ferrous state occurs in the filter bed. It appears that bacterial growth, which develops in the filter, is responsible for the passage of iron through the filter. One remedy is to assure that the media always is in an aerobic state. This can be accomplished by the continuous addition of chlorine.
  2. The cold lime treatment process also is commonly used to remove iron from water. At times, an aerator is mounted directly above the lime treater. In addition to removing iron, the aerator reduced the CO2 level. If the iron or CO2 level is not high, the aerator is omitted. The agitation and pH in the lime treater usually provides sufficient oxygen and alkalinity to convert the iron from the ferrous to the ferric form.
  3. If the iron-bearing water is not allowed to come in contact with oxygen, sodium zeolite softeners may be employed to exchange sodium for iron. If the water is not clear, a filter should be used ahead of the zeolite resins.
  4. For low quantities of iron (1 ppm), the manganese-zeolite method may be used. This may be either batch or continuous.

    Manganese-zeolite is represented by the formula Z · MnO2. In the batch process oxidation of iron changes the resin to Z · Mn2O3. As the oxidizing capacity of the manganese-zeolite is exhausted, leakage of iron occurs. The resin must then be oxidized again with potassium permanganate. In addition to regeneration, the unit is backwashed whenever the backpressure loss reaches 7 or 8 psi.

    A refinement of the batch process is the continuous addition of potassium permanganate before the manganese-zeolite unit. The permanganate oxidizes the ferrous iron and the zeolite resin acts as the oxidizing agent. If an overdose of permanganate is fed, it serves to regenerate the manganese-zeolite resin. Care should be taken to avoid excessive potassium permanganate, which would be passed into system waters.

Control of Iron

  1. Instead of removing iron, it may be prevented from precipitating by the addition of polyphosphates. It is necessary that polyphosphate be added before the iron is subject to oxidation.

    The addition of oxygen or chlorine after polyphosphate addition will convert the iron to the ferric form, but it will remain dispersed.

    The required amount of polyphosphate is 2-4 ppm per 1 part of iron. The limit on the amount of iron to be controlled by this method is 3-4 ppm.
  2. Recently developed organic phosphorous compounds have been used quite extensively to sequester iron; and in most cases are more effective than the polyphosphates, particularly where the iron concentration exceeds 1.0 ppm. The usual dosage is also 2 to 4 ppm per part of the iron. The organic phosphorous compounds should also be added before the iron is allowed to be oxidized. Plants using water containing up to 15 ppm iron have operated without problems using these newer products.
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