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T he solubility of amorphous silicais important to the operation ofwater-dominated production

processes. In areas such as Texas, NewMexico, Arizona, parts of California,southern Europe, the Pacific Rim andLatin America, the water used for indus-trial applications contains high silica con-centrations (50 parts per million [ppm]to 100 ppm, expressed as silicon dioxide[SiO2]). These concentrations result fromquartz (crystalline SiO2) dissolution fromrock formations into the groundwater.

The potential for silica-scale deposi-tion poses serious problems in waterwith a high dissolved silica content. Infact, silica-scale problems could betermed the Gordian Knot of water treat-ment — such problems are extremelydifficult to resolve. Personnel responsi-ble for power plants, evaporative coolingsystems, semiconductor manufacturingand geothermal systems must monitorwater silica levels very closely.

Silica precipitation/deposition fre-quently is encountered in evaporativecooling systems, where salt concentra-tions increase through partial evapora-tion of the cooling water. Silica solubilityin water generally is 150 ppm to 180ppm, depending on water chemistry andtemperature. This imposes severe limitson water users, leading either to opera-tion at very low cycles of concentrationand consuming enormous amounts ofwater, or to use of chemical water treat-ment techniques that prevent silica-scaleformation and deposition.

Silica and/or silicate deposits are par-ticularly difficult to remove once they

form. Harsh chemical cleaning (basedon hydrofluoric acid) or laboriousmechanical removal usually is required.In addition, the potential for silica scalewith high calcium and magnesium levelslimits the use of polysilicate as a steeland aluminum inhibitor, especially inlow-hardness cooling waters.

Silica speciation and depositionSilica-scale formation is a highly complexprocess. It is usually favored at a pH ofless than 8.5, whereas magnesium silicatescale forms at a pH of more than 8.5.

Data suggest silica solubility is largelyindependent of pH in the range of 6 to 8.Silica exhibits normal solubility charac-teristics, which increase proportionally totemperature. In contrast, magnesium sili-cate exhibits inverse solubility.

Silica formation is actually a poly-merization event. When silicate ionspolymerize, they form a plethora ofstructural motifs, including rings ofvarious sizes, cross-linked polymericchains of different molecular weights,oligomeric structures, etc. The resultingsilica scale is a complex and amorphous

Water Treatment’s‘Gordian Knot’

To avoid silica-scale problems in cooling towers, plant personnel turn to unconventional methods

C O R R O S I O N C O N T R O L

By Konstantinos D. Demadis, Ph.D.

A t the end of a prosperous reign, the King of Phrygia died, leaving no heir. The Phrygians,distraught and without leadership, decided to send some of their people on a long, dan-

gerous journey to the Oracle at Delphi to ask for a prophecy. The Oracle told the people their next king would arrive on a wagon drawn by oxen. When

they returned to Phrygia, the emissaries described what they had heard, and as they told thenews, Gordius and his wife rode into town on their oxen-drawn wagon. In keeping with theOracle’s declaration, the elders made Gordius king.

Gordius dedicated his wagon to the deity of the Oracle. To remind himself of his humblebeginnings, he tied the wagon to a post in front of his palace with an enormous knot.

Gordius ruled well. In time, his son Midas took the throne. Phrygia prospered. But whenMidas stepped down from the throne, he left no heir to rule in his stead. Once again, the peo-ple were without a leader.

Once again, they made the difficult trip to consult the Oracle. This time, the Oracle toldthem that he who unraveled the knot tied by Gordius — termed the “Gordian Knot” — wouldbe their next ruler.

Many years passed, and many men tried to untie the famous Gordian Knot. None suc-ceeded. Then one day, Alexander the Great traveled to Phrygia flanked by a large army. FirstAlexander attempted to untie the knot like everyone else. When it became apparent that con-ventional means would not work, he drew his sword and sliced the knot in half. The elders ofPhrygia were initially wary of his solution, but they crowned him king.

As the years passed and Alexander conquered Asia, it became evident that his uncon-ventional solution was the first of many wise decisions.

The legend of the Gordian Knot

product (colloidal silica) — a compli-cated mixture of the above components.

Operation in a high-pH regime is notnecessarily a solution for combating silicascale. Water system operators must takeinto account the presence of magnesium(Mg2+) and other scaling ions such as cal-cium (Ca2+). A pH adjustment to greaterthan 8.5 might result in massive precipi-tation of magnesium silicate if high levelsof Mg2+ are present or in calcium car-bonate (CaCO3)or calcium phosphate ifhigh levels of these ions are overlooked.

Silica precipitation also can beaggravated by the presence of metal ionssuch as iron (Fe2+/3+) or aluminum(Al3+) and their hydroxides. Corrodedsteel surfaces (e.g., on pipes or heatexchangers) are very prone to silicafouling. Iron oxides/hydroxides act asdeposition matrices for silica (eithersoluble or colloidal) deposits.

Silica-scale controlCurrent practices for combating silica-scale growth in industrial waters includeoperation at low cycles of concentra-tion, prevention of other-scale forma-tion, pretreatment and inhibitor ordispersant use.

Operation at low cycles of concentra-tion is a common practice, but one thatconsumes large amounts of water. In acooling tower operating at a pH of lessthan 7.5, silica generally should be main-tained below 200 ppm (as SiO2). For apH greater than 7.5, silica should bemaintained below 100 ppm (as SiO2).

Keep in mind that Mg2+ levels alsoshould be taken into account at a pHlevel greater than 7.5. In this case, the

product (ppm Mg as CaCO3) � (ppmSiO2 as SiO2) should be below 20,000ppm. The figure gives additional rules-of-thumb for silica-bearing process waters.

Prevention of other-scale formationindirectly interferes with the propensityof silica scale to co-precipitate with otherscales. The method is based on preven-tion of other scaling species such asCaCO3 or calcium phosphate and indi-rectly benefits the whole cooling toweroperation. CaCO3 precipitates provide acrystalline matrix in which silica can beentrapped and grow. In environments in

which CaCO3 or any other mineral pre-cipitate is prevented completely, highersilica levels generally are tolerated in theprocess water than in those environ-ments in which other scales are con-trolled ineffectively (Gill, 1998).

Pretreatment involves reactive or colloidal silica removal in precipitationsofteners through an interaction betweensilica and a metal hydroxide. Both ironhydroxide, Fe(OH)3, and aluminumhydroxide, Al(OH)3, have shown silica-removal capabilities, although magne-sium hydroxide, Mg(OH)2, is consideredmore effective.

In addition, silica can be removedthrough reverse osmosis (RO) and ionexchange techniques, as well as desilicizers.RO membranes are not immune to silicascale, which forms as a gelatinous mass onthe membrane surface. It then can dehy-drate, forming a cement-like deposit.

The use of inhibitors or dispersants tocontrol silica scale generally follows twoapproaches: inhibition and dispersion.Inhibition is defined as the preventionof silica oligomerization or polymeriza-tion. As a result, the silica remains solu-ble and, therefore, reactive.

Dispersion, on the other hand, isthe prevention of particle agglomera-tion to form larger-size particles andthe prevention of the adhesion of theseparticles onto surfaces.

A number of products are availablecommercially for silica-scale control inRO, geothermal and evaporative cool-ing water applications. Discussion ofthese products is not the intent of thisarticle. However, much informationabout commercial silica-scale treat-ment can be found on the Webthrough any of the popular searchengines. In addition, several propri-

etary technologies can be found inpatent literature.

Mechanism ofsilica-scale inhibitionAmorphous silica formation is governedby several equilibria. Silica depositionresults from silicic acid self-condensa-tion. This reaction is first-order and iscatalyzed by OH- in the pH range of 5 to10. Reports have shown that the reactionyielding a silicic acid dimer is kineticallyslow in contrast to the reactions giving atrimer, tetramer, pentamer, etc., which arevery fast. All these equilibria are very sen-

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C O R R O S I O N C O N T R O L

SiO2 < 200 ppmMg � SiO2 < 40,000 ppm

SiO2 < 100 ppmMg � SiO2 < 20,000 ppm

pH > 7.5pH < 7.5

Mg is expressed as ppm CaCO3, and SiO2 as ppm SiO2

General Treatment Guidelines for Silica-Scale Control

“Operation in a high pH regime is not necessarily

a solution for combating silica scale.”

sitive to pH and tend to be accelerated bymetal ions that form hydroxides, e.g.,Fe2+, Mg2+ or Al3+.

Polymerization of silicic acid isbelieved to occur through a mechanisminvolving a deprotonated siliconmonoxide (Si-O-) and the Si center ofsilicate, Si(OH)4. Inhibition of this stepshould be critical in the inhibition ofsilica-scale formation. Some reportsindicate that orthosilicates hydrolyzemore rapidly than other silicate speciessuch as disilicates, chain silicates, cross-linked oligomers and polymers, suggest-ing that bridging oxygens are muchmore resistant to attack than nonbridg-ing oxygens. Above a pH of 2, thismechanism involves polymerizationwith condensation, catalyzed by OH-.

Silica-scale formation involves con-densation between Si-OH groupsformed at the material surface and Si-OH of the dissolved silicate presentin water. Condensation between the Si-OH units formed at a glass surfaceand dissolved Si-OH can be the domi-nant mechanism (Hayakawa, et al.).

Silica polymerization is governedlargely by pH. Unfortunately, silica is afoulant not easily cured through pHadjustments. For example, CaCO3 scalevirtually can be eliminated if a coolingtower system is operated at a lower pH.With water containing a high concen-tration of silica, operation at a higherpH generates the problem of magne-sium silicate scale. Lowering the pH (byfeeding acid) does not eliminate theproblem; it just shifts it from magne-sium silicate to silica.

A low operational pH also increasesthe corrosion rates of metallic surfaces,ultimately leading to material failure.Silica solubility is very high at a pHgreater than10, but this pH regime isnot an operational option for coolingtower systems.

Dissolved silica precipitates out ofsolution principally in three ways:through surface deposition, through bulkprecipitation or in living organisms.

Surface deposition. This occurs as adeposit on a solid surface where the[Si(OH)4-x]x- condenses with any solidsurface possessing -OH groups. Ifthe surface contains M-OH moieties

(M = metal), this reaction is enhancedfurther. Such pronounced silica deposi-tion phenomena in the water treatmentindustry are evident on metallic surfacesthat have suffered severe corrosion, with asurface covered with metal oxides/hydroxides. Once the receptive surface iscovered with silica scale, additional sil-

ica is deposited on an already-formedsilica film.

Bulk precipitation. This occurs ascolloidal silica particles grow throughthe condensation reaction. The particlescollide with each other and agglomer-ate, forming larger particles.

In living organisms. This form of silicais called biogenic and appears in certainmicroorganisms such as diatoms thathave the ability to remove and deposit sil-ica from highly undersaturated solutionsinto precisely controlled structures ofintricate design. It should be mentionedthat sessile microorganisms in a biofilm-fouled heat exchanger can entrap col-loidal silica. The high affinity of solublesilica toward exocellular biopolymerssuch as polysaccharides has been recog-nized (Gill, 1998).

The precise mechanism of silica for-mation is not well understood. Anyinterference with the condensation reac-tion could lead to silica-scale growthinhibition. A relevant example is silicainhibition by orthoborate, which reactswith silicate ions to form borosilicates.These products are more soluble inwater than are silica/metal silicates.

Silica-scale inhibition monitoringAny chemical treatment program req-uires proof of its performance. For exam-ple, programs to control CaCO3 aremonitored by measuring Ca hardness inthe recirculating water and by comparing

the measured and theoretical amounts.Inhibitor levels also are monitored closelyto ensure the presence of sufficientinhibitor and dispersant polymer.

For silica, an easy field test is basedon the silicomolybdate method. Thismethod is based on a yellow complexthat forms between molybdate and sili-

cate under certain conditions and canbe measured spectrophotometrically.

Phosphate can interfere with themeasurement by forming a similarcomplex with molybdate. Its interfer-ence can be eliminated by the action ofoxalic or citric acid.

The silicomolybdate test measuressoluble (reactive) silica. It does notmeasure colloidal silica. It is worth not-ing that reactive silica does not refer toonly monomeric silica (silicate ion). Italso includes other oligomeric speciessuch as dimers, trimers, tetramers, etc.

For all practical purposes, the silico-molybdate test offers a good indication ofthe soluble silica in the system. However,occasional measurements of total silica— i.e., soluble and colloidal — are rec-ommended to ensure no silica is lost toprecipitation and deposition. Thesemeasurements require sample treatmentor elaborate instruments and techniques(e.g., inductively coupled plasma atomicemission or atomic absorption spec-troscopy) and are not practiced routinelyin the field. Samples usually are sent to anindependent laboratory for study.

ConclusionsSilica is an undesirable scale for severalreasons. It severely impedes heat transfer.It is tenacious and costly (and potentiallyhazardous) to remove. It is extremelyprone to co-precipitation with otherscales, particularly iron (hydr)oxides. It isoften the limiting factor for limiting high

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C O R R O S I O N C O N T R O L

“Dissolved silica precipitates out of

solution principally in three ways:

through surface deposition, through bulk

precipitation or in living organisms.”

cycles of concentration.The amorphous character of silica

deposits precludes the use of conven-tional crystal modification technolo-gies. Molecules such as phosphonatesthat are effective mineral-scale thresh-old inhibitors provide virtually no ben-efit for silica-scale inhibition. Theyprovide only an indirect benefit by

maintaining acooling towerfree of otherdeposits thatcan act as pre-cipitation nucleifor silica or cat-alyze silica pre-cipitation in thebulk.

Just like theGordian Knot— the difficult,intractable and

often insolvable problem — resolutionof the silica problem might require non-conventional means. Several factors,unique to the individual system, must betaken into consideration. These factorsinclude water chemistry (presence ofother scaling ions), nature of the silica(colloidal and/or reactive) in the make-up water, target cycles of concentration

32 | May 2003 www.chemicalprocessing.com

(need for water conservation), feasibilityof mechanical silica removal (filtrationand softening), capital costs (for chemi-cals and/or equipment) and many oth-ers. Careful selection of a general scaletreatment program, combined with a sil-ica-scale inhibitor and/or dispersant, is agood starting point for attaining a silicadeposit-free cooling water system. CP

A bibliography for this article is pro-vided on p. 34.

Demadis is an assistant professor ofchemistry at the University of Crete inGreece. He has extensive experience inchemical water treatment research anddevelopment and also in technical andfield support. Contact him +3 2810393651, or via e-mail at kostasdemadis@yahoo.com.

C O R R O S I O N C O N T R O L

“Just like the Gordian Knot —the difficult, intractable andoften insolvable problem —

resolution of the silica problem might require

nonconventional means.”

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