Ironbased Compounds

Iron compounds have pH coagulation ranges and floe characteristics similar to aluminum sulfate. The cost of iron compounds may often be less than the cost of alum. However, the iron compounds are generally corrosive and often present difficulties in dissolving, and their use may result in high soluble iron concentrations in process effluents. Among the most commonly used iron compounds used in wastewater treatment applications are ferric chloride, ferrous

Liquid ferric chloride is a corrosive, dark brown oily-appearing solution having a weight as shipped and stored of 11.2 to 12.4 lb/gal (35 percent to 45 percent FeClj). The ferric chloride content of these solutions, as FeCl3, is 3.95 to 5.58 lb/gal. Shipping concentrations vary from summer to winter due to the relatively high crystallization temperature of the more concentrated solutions. The pH of a 1

The molecular weight of ferric chloride is 162.22. Viscosities of ferric chloride solutions at various temperatures are can be found in reference 13. Ferric chloride solutions are corrosive to many common materials and cause stains which are difficult to remove. Areas which are subject to staining should be

Normal precautions should be employed when cleaning ferric chloride handling equipment. Workers should wear rubber gloves, rubber apron, and goggles or a face shield. If ferric chloride comes in contact with the eyes or skin, flush with copious quantities of running water and call a physician. If ferric chloride is

Ferric chloride solution can be stored as shipped. Storage tanks should have a free vent or vacuum relief valve. Tanks may be constructed of FRP, rubber-lined steel, or plastic-lined steel. Resin-impregnated carbon or graphite are also suitable

It may be necessary in most instances to house liquid ferric chloride tanks in heated areas or provide tank heaters or insulation to prevent crystallization. Ferric chloride can be stored for long periods of time without deterioration. The total storage capacity should be 1.5 times the largest anticipated shipment, and should provide at least a ten-day to two-week supply of the chemical at the design average dosage. It may not be desirable to dilute the ferric chloride solution from its shipping concentration to a weaker feed solution because of possible hydrolysis. Ferric chloride solutions may be transferred from underground storage to day tanks with impervious graphite or rubber-lined self-priming centrifugal pumps having Teflon rotary and stationary seals. Because of the tendency for liquid ferric chloride to stain or deposit, glass-tube rotameters; should not be used for metering this solution. Rotodip feeders and diaphragm metering pumps are often used for ferric chloride, and should be constructed of materials such as rubber-lined steel and plastics.

Materials for piping and transporting ferric chloride should be rubber or Saran-lined steel, hard rubber, FRP, or plastics. Valving should consist of rubber or resin-lined diaphragm valves. Saran-lined valves with Teflon diaphragms, rubber-sleeved pinch-type valves, or plastic ball valves. Gasket material for large openings such as manholes in storage tanks should be soft rubber; all other gaskets should be graphite-impregnated blue asbestos, Teflon, or vinyl. System pacing and control requirements are similar to those discussed previously for liquid alum. Ferrous chloride, FeCl2, as a liquid is available in the form of waste pickle liquor from steel processing. The liquor weighs between 9.9 and 10.4 lb/gal and contains 20 percent to 25 percent FeCl2 or about 10 percent available Fe2+ . A 22 percent solution of FeCl2 will crystallize at a temperature of - 4 °F. The molecular weight of FeCl2 is 126.76. Free acid in waste pickle liquor can vary from 1 percent to 10 percent and usually averages about 1.5 percent to 2.0 percent. Ferrous chloride is slightly less corrosive than ferric chloride.

Waste pickle liquor is available in 4,000 gal truckload lots and a variety of carload lots. In most instances the availability of waste pickle liquor will depend on the proximity to steel processing plants.

Since ferrous chloride or waste pickle liquor may not be available on a continuous basis, storage and feeding equipment should be suitable for handling ferric chloride. Therefore, the ferric chloride section should be referred to for storage and handling details.

Ferric sulfate is marketed as dry, partially-hydrated granules with the formula Fe2(S04)3-X H20,where X is approximately 7. Typical properties of a commercial product are given in Table 3.

Table 3. Typical Properties of Ferric Sulfate

Property

Value

Units

Molecular Weight

526

Bulk Density

56-60

lb/cu ft

Water Soluble Iron Expressed as Fe

21.5

percent

Water Soluble Fe +3

19.5

percent

Water Soluble Fe +2

2.0

percent

Insolubles Total

4.0

percent

Property

Value

Units

Free Acid

2.5

percent

Moisture @ 105° C

2.0

percent

Reactions involving ferric sulfate are...

Fe2(S04)3 2Fe(OH)3

Ferric sulfate is shipped in car and truck load lots of 50 lb and 100 lb moisture-proof paper bags and 200 lb and 400 lb fiber drums.

General precautions should be observed when handling ferric sulfate, such as wearing goggles and dust masks, and areas of the body that come in contact with the dust or vapor should be washed promptly.

Aeration of ferric sulfate should be held to a minimum because of the hygroscopic nature of the material, particularly in damp atmospheres. Mixing of ferric sulfate and quicklime in conveying and dust vent systems should be avoided as caking and excessive heating can result. The presence of ferric sulfate and lime in combination has been known to destroy cloth bags in pneumatic unloading devices. Because ferric sulfate in the presence of moisture will stain, precautions similar to those discussed for ferric chloride should be observed. Ferric sulfate is usually stored in the dry state either in the shipping bags or in bulk in concrete or steel bins. Bulk storage bins should be as fight as possible to avoid moisture absorption, but dust collector vents are permissible and desirable. Hoppers on bulk storage bins should have a minimum slope of 36°; however, a greater angle is preferred.

Bins may be located inside or outside and the material transferred by bucket elevator, screw, or air conveyors. Ferric sulfate stored in bins usually absorbs some moisture and forms a thin protective crust which retards further absorption until the crust is broken.

Feed solutions are usually made up at a water to chemical ratio of 2:1 to 8:1 (on a weight basis) with the usual ratio being 4:1 with a 20-minute detention time. Care must be taken not to dilute ferric sulfate solutions to less than 1 percent to prevent hydrolysis and deposition of ferric hydroxide. Ferric sulfate is actively corrosive in solution, and dissolving and transporting equipment should be fabricated of type 316 stainless steel, rubber, plastics, ceramics, or lead,

Dry feeding requirements are similar to those for dry alum except that belt-type feeders are rarely used because of their open type of construction. Closed construction, as found in the volumetric and loss-in-weight-type feeders, generally exposes a minimum of operating components to the vapor, and thereby minimizes maintenance. A water jet vapor remover should be provided at the dissolver to

Ferrous sulfate or copperas is a by-product of pickling steel and is produced as granules, crystals, powder, and lumps. The most common commercial form of ferrous sulfate is FeS04-7H20,with a molecular weight of 278, and containing 55 percent to 58 percent FeS04 and 20 percent to 21 percent Fe. The product has a bulk density of 62 to 66 lb/cu ft. When dissolved, ferrous sulfate is acidic. The composition of ferrous sulfate may be quite variable and should be established by consulting the nearest manufacturers. Ferrous sulfate is also available in a wet state in bulk form from some plants. This form is likely to be difficult to handle and the manufacturer should be consulted for specific information and instructions.

Dry ferrous sulfate cakes at storage temperatures above 68° F, is efflorescent in dry air, and oxidizes and hydrates further in moist air. General precautions similar to those for ferric sulfate, with respect to dust and handling acidic solutions, should be observed when working with ferrous sulfate. Mixing quicklime and ferrous sulfate produces high temperatures and the possibility of fire. The optimum chemical-to-water ratio for continuous dissolving is 0.5 lb/ gal. of 6 percent with a detention time of 5 minutes in the dissolver. Mechanical agitation should be provided in the dissolver to assure complete solution. Lead, rubber, iron, plastics, and type 304 stainless steel can be used as construction materials for handling solutions of ferrous sulfate.

Ferric sulfate and ferric chloride react with the alkalinity of wastewater or with the added alkaline materials such as lime or soda ash. The reactions may be written to show precipitation of ferric hydroxide, although in practice, as with alum, the reactions are more complicated than this.

Ferrous hydroxide is rather soluble and oxidation to the more insoluble ferric hydroxide is necessary if high iron residuals in effluents are to be avoided. Flocculation with ferrous iron is improved by addition of lime or caustic soda at a rate of 1 to 2 mg/mg Fe to serve as a floe-conditioning agent. Polymers are also generally required to produce a clear effluent.

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