TABLE
Solubility of SiO2 at Various pH Levels

Magnesium Silicate Scale

Soluble silica reacts readily with cations to form various complex silicates.
If magnesium is present in high enough concentrations. magnesium silicate
scaling will occur at pH of 8.5. Deposition of magnesium silicate can be
prevented by keeping

1. (Mg ppm as CaCO3) x (SiO2 ppm) <35,000 ppm @ pH<7.5
2. (Mg ppm as CaCO3) + (SiO2 ppm) <17,000 ppm @ pH>7.5

or more generally by keeping silica levels <200 ppm in the recirculating
water. Magnesium silicates form by a two-step process: First, magnesium
hydroxide precipitates, then reacts with dissolved and colloidal silica to
form scale. Magnesium silicate scale occurs when the pH level is higher
than 7.8. Magnesium silicate does not occur if the pH of the circulating
water is lower than the pH of saturation of magnesium hydroxide. This pH
of saturation can be determined using the Table below, which is also helpful
in predicting the deposition of magnesium silicate.

TABLE
Prediction of Magnesium Silicate Scale Deposition

pHs = MG + T - C

Example: MgH of makeup = 20 ppm, Cycles of concentration = 10 and pH of
circulating water = 8.7
Calculation of pHs at temperatures of 130, 150, 170, and 190 deg F:

This water would deposit magnesium silicate in coolers whose temperature
exceeds 170 deg F.

Iron-Based Scales

Iron exists in two states, Fe+2 and Fe+3. The chemistry of iron compounds
is considerably more complex than alkaline earth carbonate and sulfate
scale-forming materials. When these two oxidation states (Fe+2 and Fe+3)
of iron join with the same anion, the result is usually the formation of
compounds with significant differences in solubilities. Iron deposits
typically are in the form of Fe2O3, FeO, FeS, iron silicate, etc. Often
manganese is also detected in these deposits.

Iron fouling occurs as a result of corrosion processes throughout the system.
Thus, it is very important to implement a good corrosion control program.
Iron fouling sometimes occurs in cooling waters as a result of carryover
from clarifiers, where iron salts may be used as coagulants or where raw
water (such as well water) may be high in iron. Iron levels of 2 ppm or
greater in the recirculating water can be controlled through the use of
iron dispersants or deposit control agents. Use of an iron dispersant is
strongly recommended to avoid major equipment failure problems. In cooling
waters, Fe2O3 (hematite) and FeO are the most common iron deposits.
Magnetite (Fe3O4) is rarely found in cooling systems. Magnetite needs high
temperatures and/or anaerobic conditions. Most magnetite found in cooling
systems arrives via airborne or waterborne solids. If iron levels exceed
4-5 mg/l, mechanical or other means to remove iron should be strongly
considered, such as oxidation, aeration, coagulation, etc.

Zinc Phosphate and Zinc Hydroxide

Zinc hydroxide scale is normally found when pH exceeds 7.6 or when over-feed
of a zinc product occurs. Zinc hydroxide is a grayish-white precipitate.
With increasing pH, precipitate amounts increase. As pH is lowered below
7.0, zinc hydroxide will resolubilize. Above 140 deg F (60 deg C) it has
an inverse temperature solubility.. The most common methods to prevent
scale in cooling water systems are:

using scale inhibitors.

adjusting pH.

softening makeup water.

Estimating the Scale-Forming Tendency of Water

Scale forming and/or corrosive tendencies of water can be predicted
qualitatively by using the Langelier Index, Ryznar Solubility Index, or
the Stiff and Davis Index.

The Langelier Index:

For any given temperature and water composition,
one can calculate the tendency of the water to form calcium carbonate scale
by using the Langelier Index.

LSI = pH - pHs

where

pHs = pCa + palk + C

where
pHs = saturated pH
pCa = calcium hardness factor (expressed as ppm CACO3,)
palk = M alkalinity factor (expressed as ppm CACO3,)
C = total solids (expressed as ppm at the temperature of the water)

The Langelier Saturation Index (LSI) is defined as the difference between
the actual pH and the calculated saturated pH (pHs),

LSI = pH (actual) - pH (calculated)

LSI = pH - pHs

When the actual pH is equal to the saturated pH, the Langelier Saturation
Index is zero (LSI = 0 when pH = pHs). A saturation equilibrium exists.
There is no scale formation and corrosive attack is minimized.

When the actual pH is greater than the saturated pH, the Langelier
Saturation Index is positive (LSI when pH>pHs). Supersaturation of CACO3,
exists with respect to alkalinity and total solids at that temperature.
There is a tendency to form scale on heat transfer surfaces-

When the actual pH is less than the saturated pH, the Langelier Saturation
Index is negative (LSI = - when pH<pHs.). Any scale previously formed will
be dissolved. Corrosion of unprotected or bare metal will occur.

The Langelier Index does not tell whether scale will actually form, nor
how rapidly it will form.

The prediction of water characteristics by LSI appears in the Table below:

TABLE
Prediction of Water Characteristics by the Langelier Index

Ryznar Stability Index (RSI)

It is possible for a low-hardness water and a high-hardness water to have the same LSI value.
Using operational data and experience with scale and corrosion in a large
number of systems, Ryznar refined the Langelier Index to allow for a
distinction between two waters having the same Langelier Index. The
Ryznar equation is:

Ryznar Stability Index (RSI) = 2pHs - pH

The predictive nature of the Ryznar Index is shown in the Table below:

TABLE
Prediction of Water Characteristics by the Ryznar Index

The Stiff and Davis Index (SDI):

The Langelier Index has
been further modified by Stiff and Davis. Their index takes into account
the influence of high levels of dissolved solids on the solubility of
calcium carbonate. The SDI Index was developed for use in the oil field,
where highly saline waters or brines are produced. Where fresh water is
being used as makeup water for a cooling system, there is no need for the
refinements of the Stiff and Davis Index. However, as zero blowdown systems
become more common, as more and more reuse of water becomes necessary, and
as recovered wastewater is used in larger quantities as a makeup source, it
may become necessary to use the Stiff and Davis Index. The index is defined
as:

SDI = pH - pCa - pAlk - K

K = a constant based on the total ionic strength and temperature.

Methods of Scale Inhibition

Lime or Lime/Soda Softening

For cooling water applications, only partial lime or lime/soda softening is
economical. Often, sidestream lime softening is used as a supplement or
replacement for softening during external pretreatment prior to cooling
water applications. Occasionally, hot process lime or lime/soda softening
is used for silica removal in cooling water applications. In boiler
applications, however, hot or cold lime-soda softening is commonly used
for external treatment. This reaction is as follows:

2HCO3- + Ca(OH)2 --> CaCO3(s) + CO3-2 + 2H20

(M alkalinity)

Mg+2 + Ca(OH)2 --> Mg(OH)2(s) + Ca+2

Ca+2 + CO3-2 --> CaCO3(s)

Acid Treatment

The traditional method of calcium carbonate scale control is to reduce the
water's alkalinity sufficiently with sulfuric acid to create non-scaling
conditions.

Ca+2 + 2HCO3- + 2H++SO4-2 --> Ca+2+SO4-2 + 2CO2(g) + 2H20

The calcium bicarbonate is converted to volatile carbon dioxide and calcium
sulfate. Calcium sulfate is more soluble than calcium carbonate. The
cycles of concentration are limited by the solubility of calcium sulfate.
If the makeup water is high in sulfates, hydrochloric acid may be used to
reduce the alkalinity-, but chloride ions may cause problems because they
can penetrate oxide films, setting up local anodic cells and causing
corrosion.

Scale Inhibitors and Dispersants

The answer to the question of how scale inhibitors work is fraught with
complexities. One of the more logical explanations is that sub-
stoichiometric amounts of certain types of chemical additives can have
very marked effects on the growth rate of crystals deposited in a scaling
environment. These threshold inhibitors function by adsorbing onto the
growing crystals and distorting the lattice, which disrupts the crystal
growth process. The most commonly used threshold scale inhibitors are
inorganic polyphosphates, organophosphorous compounds, synthetic organic
dispersants and natural organic dispersants. Threshold treatment is the
term that describes the effective use of a scale inhibitor at concentrations
below the level required to sequester, or donate electrons to the metal ions
forming a water-soluble metal ion complex. The dispersant products are also
used to prevent scale formation by modifying the crystal structure of
the deposit-forming substance. This crystal distortion prevents deposition,
and the highly irregular stressed crystals tend to slough off as crystal
growth occurs.

Conventionally, scale inhibitors are classified as inorganic scale inhibitors
(such as sodium hexametaphosphate, sodium polyphosphate, sodium pyrophosphate)
or organic phosphorus compounds (such as the phosphate esters and
phosphonates). The inorganic phosphates used to treat scale contain
repeating oxygen/phosphorus bonds (-O-P-0-PO-P-). These bonds are highly
unstable in aqueous solutions, and as a result, they hydrolyze, or react
with water, and end up as ineffective orthophosphate. The hydrolysis of
such products is also referred to as reversion. Organic phosphorus
products are also subject to hydrolysis. The main factors that effect the
rate of reversion are pH and temperature. It is also influenced by
complexing cations, concentrations, holding time index, ionic environment
in the solution, and other factors. Environmental restrictions imposed upon
the industry have made organic treatments almost a necessity. The following
discussion about organic scale inhibitors and dispersants will be useful in
understanding the types of products.

Phosphate Esters

A typical phosphate ester functional group may be represented as follows:

                           O
                           ||

R - C H 2 - O - P - O H

                            |
                         O H

Phosphate esters are less likely to hydrolyze than inorganic polyphosphates
and more likely to hydrolyze than phosphonates. They exhibit varying degrees
of threshold scale inhibitor properties. In fact, some phosphate esters are
ineffective as scale inhibitors.

Phosphonates

A typical phosphonate has the following structure:

                     O
                     ||

R - C H 2 - P - O -

                      |
                     O -

This structure with C-P-0 bonding is more stable to hydrolysis than the
phosphate ester.

A variety of amino methylene phosphonic acids and their salts are available
commercially as scale inhibitors. They are effective inhibitors for both
calcium sulfate and calcium carbonate scale at very low threshold
concentrations. Non-nitrogen-containing phosphonic acids are widely used
because they are more compatible with chlorine than the aminomethylene
phosphonic acids. Phosphonates are believed to inhibit scale formation by
being adsorbed on active crystal growth sites, where they decrease the
crystal growth rate and decrease the nucleation rate. A number of blends
or combination products that incorporate low molecular weight dispersant
polymers with phosphonates are available. Phosphonates containing nitrogen
are good for iron control because the close proximity of the nitrogens allows
for formation of strong five-membered ring metal complexes with iron. The
structure of a typical phosphonate is as follows:

HEDPA

              OH      OH     OH
                 |           |          |
HO    -    P    -    C    -    P    -    OH
                ||          |          ||
                O       CH3     O

AMP

     /C H 2 P O 3 H
N - C H 2 P O 3 H
       C H 2 P O 3 H

Synthetic Polymeric Dispersants

 A large number of synthetic polymers are used as scale inhibitors and as
dispersants. Most of the synthetic polymers have a molecular weight
below 50,000 and are polymers of acrylic acid, acrylamide, methacrylic acid,
maleic acid, maleic anhydride, styrene maleic blends and maleics
(methylvinyl ether), etc. The effectiveness of the polymer decreases as the
chain length increases. A length of about 10-15 repeating units is best.
At this molecular weight, adsorption is maximized without causing bridging,
and dispersion is most efficient. Synthetic polymers such as polyacrylates
are believed to inhibit scale deposition by adsorption on growing crystals.
This is followed by both crystal structure modification and by a charge
repulsion mechanism of negative charges, which leads to destabilization of
the crystallites. Polymers are also used for iron control.

Natural Organic Polymers - Lignosulfonates

Lignosulfonate derivatives come from pulsing operations where wood is
sulfonated at high temperatures to remove the lignin binder from the
cellulose by forming a water-soluble polymer. This polymer is a highly
complex natural derivative and its structure is not well defined.
Lignosulfonates are classified as anionic polyelectrolytes. Those used
as dispersants usually have a molecular weight between 1,000 and 10,000.

Lignosulfonates are usually poor scale inhibitors. However, these organic
polymers are reported to be good dispersants for calcium phosphate,
suspended matter and iron oxides. Lignosulfonates function as dispersants
by limiting attractive forces between particles, thereby reducing
sedimentation.

Inorganic Polyphosphates

Polyphosphates are commonly used in once-through and municipal applications
to minimize scaling and corrosion at dosages of 1 to 5 mg/l of polyphosphate
as PO4. Thus, polyphosphates function as threshold treatments because they
are effective at dosages much less than those stoichiometrically required to
react with calcium, magnesium or iron. The repeating (P-0-P) structure is
characteristic of inorganic polyphosphates.

Although the advantages of low cost and threshold activity are attractive
for the use of polyphosphates, they are ineffective as scale inhibitors at
high levels of calcium. High iron and manganese levels can restrict the
effectiveness of polyphosphates by forming stoichiometric iron-polyphosphate
complexes that hinder the performance of polyphosphates as inhibitors for
calcium and magnesium deposits. In addition, polyphosphates act as nutrients
for bacteria. As a result, higher biocide dosages may be required if
polyphosphates are used for scale or corrosion control.

Surfactants

Surfactants are used in cooling systems either to emulsify or disperse
hydrocarbons, or to penetrate biomasses. Their primary purpose is to
prevent hydrocarbon and microbial deposition on heat transfer surfaces.
Surfactants also aid in de-oiling surfaces and keeping suspended solids,
including biomass, dispersed. Surfactant types are generally classified
as anionic, cationic, nonionic and amphoteric.

Surfactants work in one of two ways. If they are nonionic or amphoteric,
they reduce surface tension or interfacial tension at liquid-solid,
liquid-air, liquid-liquid and solid-air interfaces. This changes the
wetting characteristics of solids, making them either water-wet or oil-wet.
If the surfactants are anionic or cationic, they work by charge enhancing
the solids, either oil or biomass, and forcing those solids to repel one
another. This keeps the solids dispersed in the system water for later
removal. Reduction of interfacial tension is also achieved through the
use of anionic and cationic surfactants.

Problems Caused by Scale or Fouling Deposits in
Cooling Systems

Reduced or uneven heat transfer

Unexpected equipment shutdown - loss of production

Shortened equipment life

Increased pumping costs

Equipment corrosion

Product loss due to ineffective cooling

Increased product cost due to above factors

Profit Loss