COLLECTED FACTS ON WATER TREATMENT - Part Two

Symposium on Boilers and Ancillary Equipment - The Certificated Engineer July 1965.

By H. T. TUCKER, Ph. D. (Visitor) 

WATER SOFTENING

Impurities in the water are effectively removed by chemical reaction with combinations of lime and soda.

On the addition of lime and soda to hard water containing bicarbonates, it is possible to convert the soluble calcium and magnesium bicarbonates into the insoluble calcium carbonate and insoluble magnesium hydroxide.

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O

Mg(HCO3)2 + 2Ca(OH)2 → Mg(OH)2 + 2CaCO3 + 2H2O

To precipitate the non-carbonate salts of calcium and magnesium, some soda ash is also added.

CaSO4 + Na2CO3 → Na2SO4 + CaCO3

MgSO4 + Ca(OH2) + Na2CO3 → Mg(OH)2 + Na2SO4 + CaCO3

After the addition of the lime and soda, the precipitates are permitted to settle or are removed by filtration. The process is a continuous one and the retention time is approximately 60 to 90 minutes.

The removal of hardness by precipitation

Calcium and magnesium are present in water as either carbonate or non-carbonate hardness. Bicarbonates of calcium and magnesium are the carbonate hardness. Boiling the water will partially destroy this by driving off carbon dioxide and precipitating carbonates. Free carbon dioxide and water (carbonic acid) react with calcium hydroxide to form calcium carbonate. Calcium carbonate then reacts with more carbonic acid to produce soluble calcium bicarbonate and if sufficient carbon dioxide is present all carbonates will be converted to soluble bicarbonates. If there were just sufficient carbon dioxide to change all hydroxide to carbonate form and only part of the carbonates to bicarbonate, then the water would contain both carbonates and bicarbonates but no free carbon dioxide. All the carbon dioxide in the carbonate radical would be firmly bound with half of that in the bicarbonate. If this water is then boiled to remove carbonate hardness, then the half-bound carbon dioxide from the bicarbonate will be removed and the bicarbonates will be converted to carbonates.

Non-carbonate hardness is present as sulphate, chloride and nitrate of calcium and magnesium. These neutral salts are not precipitated by heat. The calcium sulphate forms a hard scale when water is evaporated and concentrated.

Precipitation takes place in simple cases of boiling or evaporation because the solubility limit of the compound is exceeded. When several substances, however, are dissolved in the same solution it becomes a more complex problem. As inorganic chemicals dissolve in water they dissociate or break up into ions. Sodium phosphate separates into sodium and phosphate and calcium sulphate into calcium and sulphate ions.

Treatment chemicals

Water treatment chemicals are generally used to lower the solubility product of scale-forming impurities and so precipitate them as a sludge. Lime or caustic soda produce hydroxides for the precipitation of magnesium hydroxide. Soda ash provides the carbonate needed to precipitate calcium. Although raw water may contain enough magnesium to reduce silica, it is usual to add magnesium oxide in its pure form or as a component of dolomite lime for this purpose.

Water which is treated by this cold process is never completely softened because the reactions do not have sufficient time to proceed to completion and both calcium carbonate and magnesium hydroxide are slightly soluble in water. The hardness can, however, be reduced to about 25 ppm and the silica brought down to about 3 ppm. Post precipitation of calcium carbonate sludge is one problem to be overcome. One possible solution to this problem is to re-carbonate the water by passing carbon dioxide gas through the softened water. This will neutralize any excess lime and convert residual calcium carbonate to soluble bicarbonate. The addition of organic and phosphate surface-active agents also retards post precipitation.

The chemical dosage of lime and soda ash required can be calculated as follows from an analysis of the raw water sample.

Let H = Total hardness expressed as ppm CaCO3

k = Total alkalinity expressed as ppm CaCO3

g = Magnesium hardness expressed as ppm CaCO3

Taking the equivalent weight for calcium carbonate as 50

Lime (calcium hydroxide) of 90 per cent = 41.1

Soda ash (sodium carbonate) = 53

Let us assume that the raw water has a total hardness of 150 ppm and total alkalinity of 125 ppm and magnesium hardness of 50 ppm all expressed as CaCO3.

Then: Amount of lime used = [(k+g)/50]41.4 = [(125+50)/50]41.4 = 143.8 ppm is equivalent to 1 438 lb per million gallons.

Amount of soda ash used = [(H-k)/50]53 = [(150-125)/50]53 = 26.5 ppm is equivalent to 265 lb per million gallons.

Hot process softeners

These are used in the treatment of boiler feed water and operate at 212°F or higher. The heat source is usually live exhaust steam.

Coldwater on entry is heated by spraying into an upper steam space. The inlet flow activates a proportioning device to control the amount of lime and soda ash added. The chemical reaction as previously described in the cold process takes place almost instantly. Sedimentation proceeds at a rapid pace because of the elevated temperature.

Sludge is collected and removed and the clarified water is passed through an anthracite filter. This process is capable of treating large flows of water in relatively small units, while the chemical consumption is lower than that of the cold process. The treated water is comparatively stable compared to that obtained by the cold process.

Zeolite procedure

The zeolite system of water treatment is based on the property of certain substances named zeolites which cause the substitution of ions in the zeolite for Ca and Mg ions in the raw water. Natural zeolite minerals are complex hydrous sodium aluminium silicates, the general composition of which is Na2O.A12O3 .4SiO2 2H2O. The symbol is usually abbreviated to Z or Na2Z the subscript 2 indicating that there are two replaceable sodium ions.

The zeolite mineral is used in the form of a filter d through which the raw water is passed. Water to be treated in a zeolite softener must be neutral and at a temperature not exceeding 110°F. In this treatment, the bulk of the silica is removed by first adding dolomitic lime or magnesium oxide to the settling tank. The reaction between the zeolite and the scale-forming salt may be represented by the following equations:

CaSO4 + Na2Z = Na2SO4 + CaZ

CaC12 + Na2Z = 2NaC1 + CaZ

The CaZ is insoluble and remains in the zeolite bed.

After all the sodium zeolite becomes changed to calcium zeolite the bed is exhausted and no further softening takes place. The filter bed may then be regenerated by passing a strong brine solution through it. 

2 Na CI + CaZ = Na2Z + CaC12

The calcium chloride is soluble in the water and is washed out to waste. The filter bed is once again washed to remove the brine after which the unit is ready for re-use.

It can be seen that in this reaction two sodium atoms are substituted for one calcium atom or one magnesium atom, and there will be an increase in the total dissolved solids in the treated water.

Natural zeolites have a capacity of 3 000 to 5 000 grams per cubic foot of the bed. Artificial zeolites are now manufactured which have the same chemical composition but with more than twice the capacity of the natural zeolites. The term zeolite has been extended to cover other substances that have been developed such as synthetic resins.

Ion exchange

Impurities which dissolve in water dissociate to form positively and negatively charged particles known as Ions. These impurities or compounds are termed electrolytes, the positive ions are known as cations because they migrate to the cathode which is the negative electrode in the electrolytic cell.

Negative particles are known as anions because they are attracted to the anode. Both cations and anions exist throughout the solution and act almost independently of each other. Magnesium sulphate, for example, will dissociate in solution to form positive magnesium ions and negative sulphate ions. All-natural waters contain a variety of electrolytes in varying concentrations. Ion exchange is the process by which one ion is exchanged for another, this ion being held temporarily in chemical combination until it is again exchanged by a stronger ion in the regenerating solution. Ion exchange material such as greensand has been in use for some time. More recently developed are those commonly known as resins which are strongly basic anions. The styrene cation resins are probably the most widely used' at present. Ion exchange equipment is similar to a pressure filter where the raw water is introduced, brought into contact with the exchange material when it undergoes the exchange process. Units require facilities for washing, backwashing and regeneration. In the exchange process the raw water exchanges calcium and magnesium for sodium cations. This exchange process continues until there are no more sodium cations available. At this stage, the plant must be stopped and the exchange material regenerated to restore the sodium cations. Before regeneration, it is advisable to backwash the exchange material by passing water through the exchanger bed in a reverse direction, at the rate of 5 to 7 gallons per minute per square foot of resin, for approximately 10 minutes to remove any dirt, etc., which may have accumulated on top of the material. The backwash cycle will also help to regrade the resin and will prevent packing and channelling. Regeneration is done by passing a saturated brine solution downwards through the exchanger bed. This salt solution reverses the exchange reaction when the sodium from the salt displaces the accumulation of calcium and magnesium on the exchange material. After regeneration, the exchanger bed is flushed with rinse water of which approximately 25 gallons per cubic foot of exchanger material is required. The complete regeneration and rinse cycle takes 1 hour and can be done automatically by time control valves.

It is advantageous to operate two or more such units in paral1el if a continuous flow of treated water is required. Water leaving the sodium cation exchanger has little hardness and contains primarily sodium salts. The amount of total dissolved solids is not reduced by the sodium exchange process.

Exchange cycle:

Ca(HCO3)2 + NaE = CaE + 2Na HCO3

(Calcium bicarbonate + Sodium cation exchanger) = (Calcium cation exchanger + Sodium bicarbonate)

Regeneration cycle:

CaE + H2SO4 = H2E + CaSO4

(Calcium cation exchanger + Sulphuric acid) = (Hydrogen cation exchanger + Calcium sulphate)

The water leaving the hydrogen cation exchange unit requires additional treatment before it is suitable for use since the pH of the water is extremely low and it contains carbon dioxide. There are two possible ways of handling this additional treatment, one of which is to remove the carbon dioxide by aeration and then adjust the pH to 7 by the addition of caustic soda or some other suitable alkali material.

A much better method is to mix the water from the hydrogen cation exchanger with water from a sodium cation exchanger and then de-aerate this blended stream to remove the carbon dioxide. In this system, the acid in the hydrogen stream is neutralized by the alkalinity in the sodium stream and together they produce water containing alkalinity of between 5 and 15 ppm.

Mixed-bed exchanger

This is a single unit containing an intimate mixture of cation and anion exchange material. The cation resin is of the styrene base type.

The mixed bed system is equivalent to an almost infinite number of two-step systems in series. In a mixed bed demineralising system the anion resin bead immediately snatches the acid produced by the neighbouring cation bead and removes the acid as it forms. This allows the reaction to proceed almost to completion with the result that the water obtained from this system is near to pure water and the specific conductance should be in the range of 0.17 micromhos or less. The total dissolved solid will below. A typical design of a mixed bed system is shown in Fig. 5.

In a mixed bed system a slightly modified form of the regeneration technique is required. The resin is first backwashed to clean the beads and to classify the resins, the lighter anion resin rises to the top while the heavier cation resin settles to the bottom. A sharp line of demarcation is usually visible through a sight glass. The cation resin is regenerated with acid and is followed by a caustic solution for regenerating the anion resin. The beads are rinsed and the unit drained before the air mix, after which the unit is refilled and rinsed before it is returned to service. (See Fig. 6.)

BOILER WATERS

One of the main reasons for boiler water treatment is the elimination of the troubles caused by scale, corrosion, caustic embrittlement, etc. Under pressure and temperature in the boiler, each ppm of bicarbonate alkalinity liberates 0.79 ppm of carbon dioxide gag while each ppm of carbonate liberates 0.35 ppm of carbon dioxide. This carbon dioxide later dissolves to form carbonic acid and with dissolved oxygen forms a mixture which can severely corrode pipelines carrying condensate.

The greatest danger from the scale is in the radiant heat zone where even a thin coat of scale can retard the removal of the heat by the mixture of steam and water. As a result, the temperature of the metal tube increases until it becomes hot enough for the metal to stretch and rupture. Rupture of the tub can also be caused by accumulated corrosion products inside the tube itself. Once the scale has been allowed to form the only remedy is to remove it either mechanically or chemically. The correct treatment of boiler feed water can prevent hard scale forming. Feedwater which contains appreciable amounts of dissolved and suspended solids will result in scale formation. This concentrate of calcium and magnesium salts is sometimes found cemented into a hard mass by the silica present. The formation of both types of scale can be prevented by the correct pre-treatment of the feed water when the calcium, magnesium and silica are removed before the water is fed into the boiler. If this is not possible then internal treatment will have to be used. The normal blowdown of a boiler also helps to remove the concentrated sludge which has formed and is a necessary function of boiler operation. Metal corrosion is possible if the feed water is acid or if it contains excessive amounts of dissolved oxygen, carbon dioxide or other corrosive gases. These problems can be prevented by neutralizing any acid present, de-aerating to remove the gases, and adding a reducing agent to combine with the remaining traces of oxygen. The effect of carbon dioxide can be avoided by the addition of a neutralizing amine which will form a protective film on the surface of the metal.

The following table shows the limits of boiler feed water.

It is realized that several industrial plants will be found in which water with a higher content of impurities than those specified in this table are running satisfactorily. For any specific case, however, the tolerances for process water may be determined by laboratory or plant scale studies in which the economic factors must be considered.

Internal treatment

Internal treatment aims to treat the water in the boiler while evaporation is taking place. It is possible to treat the water chemically and so prevent scale formation, corrosion, contamination and metal embrittlement. Most boiler plants use some form of internal treatment in which calcium and magnesium salts are precipitated in some desirable form. This removes hardness but produces an equivalent amount of sludge which must be controlled by blowdown. Scale can be form d by the mechanical attachment of these suspended solids if not properly controlled.

The calcium hardness can be reduced by the addition of phosphates. Sodium Hexametaphosphate has a chelating effect, the calcium becomes non-ionic and is taken up in the anion radicle.

The other forms of phosphate used are monosodium (acid), disodium (nearly neutral) and trisodium (alkaline). The reactions of these phosphates are similar in that calcium precipitates as a flocculent phosphate water reaction zone is high enough. Thus hydroxyapatite (mixed phosphate and hydroxide) is to be preferred because it is not sticky like the tricalcium phosphate. The ideal method of control is one in which the water will not concentrate caustic on evaporation and will contain enough alkalinity to give corrosion protection. This can be done by keeping only bonded alkali-metal phosphates in the boiler water, for without caustic there cannot be a caustic attack. The dry alkaline attack must come from the relatively weak phosphate alkalinity. For control, a graph is used in which pH is plotted against phosphate concentration. Samples of the boiler water are taken and the pH determined; if this falls on the curve then it can be expected that all phosphate present is in the trisodium form. Should the pH value fall above the curve then the indication is that free caustic is present. If the pH value is below the curve, then acidic phosphate salts are present and sodium hydroxide should be added to increase the pH and bring it up to the curve when all phosphates will be in the tri odium form. (See Fig. 7.)

Caustic alkalinity also precipitates magnesium salts as the insoluble magnesium hydroxide.

In some low-pressure boilers, soda-ash is added to avoid calcium sulphate scale. The calcium carbonate which is less soluble than the calcium sulphate is then precipitated. This method is usually more economical with hard waters (60-80 ppm). The converse is also true in that the phosphate treatment handles the waters with a lower hardness figure at higher temperatures. (See Fig. 8.)