Recent Silicate Binder Developments

7/28/2019 Recent Silicate Binder Developments

http://slidepdf.com/reader/full/recent-silicate-binder-developments 1/10

Recent Developments inSoluble Silicate Based Binders and Coatings

Mike McDonald, Janice Hamilton, National Silicates, Toronto, ON, and JudyThompson, PQ Corporation, Valley Forge, PA

ABSTRACT

While soluble silicate binders have been used effectively for decades in variousend uses, recent developments and discoveries may improve and expand theuse of silicates. This paper reviews the traditional use and chemistry of sodiumsilicate binders and also describes how surfactants, viscosity modifiers,lubricants and non-traditional setting agents can provide new characteristics andflexibility in the use of silicate binders. The discussion also includes specialty

uses for potassium and lithium silicates. These developments should further broaden the range of materials that can be agglomerated using soluble silicatebased binders, thus providing more formulation options.

1. INTRODUCTION

Soluble silicates have been used as binders for many years and in manyapplications1,2. In fact, a 1949 reference paper from the PQ library lists 28 ores or fuels and 10 patents where sodium silicate was used as the binding agent. These

“classic” silicate-based binder formulations either utilize a high SiO2/Na2O sodiumsilicate by itself or exploit the setting reaction of sodium silicate with a commoncalcium salt or cementitious based material. Iron ore, silica, detergents androofing granules are a few examples of materials that have used silicate basedbinders and coatings for decades.

Recent trends have tended to favor the traditional advantages of silicate basedbinders. As inorganic binders, soluble silicates do not face the handling, safetyand environmental issues associated with NOX and VOC’s. Furthermore, thepricing of soluble silicates is not subject to the same market fluctuations aspetroleum based or sugar based binders.

Another trend that has favored sodium silicate is the agglomeration of waste or low value materials into value added products. Typically, these value addedproducts require a higher level of binder performance such as strength, durabilityand water resistance vs. agglomerated material that is to be recycled. Anexample of such application is the use of a modified sodium silicate to makelight-weight porous spheres out of recycled container glass

3



Recent developments and ideas summarized in this paper address someimprovements that can be made to better utilize soluble silicates includingsodium, potassium and lithium silicates.

2. SILICATE CHEMISTRY REVIEW

Soluble silicates are manufactured by fusing high purity quartz sand (SiO2) withsodium or potassium carbonate (Na2CO3 or K2CO3) in an open hearth furnace at1100 – 1200°C. The resulting glass is then dissolved using high pressure steamto form liquid silicate or “waterglass” which is clear and slightly viscous. The keyparameter that determines the properties of soluble silicate solutions is theweight ratio of SiO2/Na2O. For example, a “3.2” r atio sodium silicate has 3.2 kgof SiO2 for every 1kg of Na2O. The typical range of commercially available ratiosis 1.6 to 3.2.

At the molecular level, the fundamental building block of silicate species is thesilica tetrahedron consisting of the silicon atom at the center of an oxygen-

cornered pyramid. Each oxygen atom may be associated with a hydrogen atoman alkali metal (Na, K, Li), or it may be linked to another silica tetrahedron. Thesilica can link to form chains, cyclic and larger polymeric structures. (Figure 1)The species typically carry an overall negative charge having the monovalentalkali atoms in loose association.

Monomer Linear Trimer Cyclic Trimer

Figure 1. Monomer Silica Tetrahedron, Linear and Planar Cyclic SilicaSpecies

Soluble silicates then are inorganic, polymeric, alkaline materials. They are alsomoderately strong buffers and can be involved in four basic types of chemicalreactions, each of which can play a role in binder applications. The chemicalreactions are:

Gelation

Metal Ion Reactions / Precipitation

Hydration / Dehydration

Surface Charge Modification



3. SODIUM SILICATE AS A BINDER

Agglomerated materials require a binder in order to achieve acceptable strength.In general, binders can be divided into three groups: matrix, film and chemical.4 Sodium silicate is unique in that it can serve in all three of these capacities. For

example, as a matrix binder, sodium silicate would be used in conjunction withPortland cement or pozzolan blended cement binders. Further discussion on filmforming and chemical binding attributes follows.

3.1 SODIUM SILICATE AS A FILM FORMER

Film forming binders are like glues and function by the evaporation of water or other solvent. Commercially available sodium silicates contain 45-65% water byweight. Loss of a small portion of this water, even under ambient conditions, willresult in a strong, rigid, glassy film. Rate of drying will depend on ratio,concentration, viscosity, film thickness as well as temperature and relative

humidity. The silicate binder may be subject to dissolution depending on useconditions; however some moisture resistance can be obtained by simply dryingthe silicate more completely through the addition of heat.

3.1.1 Surface Tension

Sodium silicate has a naturally high surface tension that is close to that of water i.e. ~ 76 dynes. The penetration and adhesive quality of soluble silicates can beimproved with surfactant/wetting aids. With reduced surface tension, sodiumsilicate can penetrate through smaller openings and spread over a greater area.Table 1 provides a summary of the physical properties with the addition of ananionic surfactant. Wetting agents appear to strengthen the bond by increasingthe spread, although if the spread is too great, decrease in bond strength canresult.

Table 1. % Surfactant vs. Surface Tension, Wetting and Drop Shape

% Surfactant 0 0.05 0.1 0.2 0.4

Surface tension* 76.7 46.6 39.9 34.7 35.3

Seconds to wet(darken) fiber

>600 >420 <180 <30 <15

Drop shape Beads Beads,somespreading

Slightlyflat

Flat Flat

*Du Noüy Ring method



3.1.2 Viscosity and Texture

An increase in the viscosity of the silicate will decrease the wetting coefficient,the amount of water absorbed and rate of absorption. Aside from adding water,the viscosity of sodium silicate can be decreased by the addition of a small

amount of potassium hydroxide or by gently warming the silicate. (Figure 2)

0

50

100

150

200

250

v i s c o s i t y ( c p s )

21 30 38 43 50

temperature C

sodium silicate

sodium silicate + 2% KOH

Sodium silicate – 3.2 ratioKOH ~45% solids

Figure 2. Addition of 45% KOH reduces the viscosity of 3.2 ratio sodium

silicate. Decrease in viscosity as a function of temperature is also shown.

The presence of clay in sodium silicate does not influence the total water loss butdoes serve to give a tougher, more coherent film, free of bubbles and dryingcracks. Glycerin and other plasticizers such as sorbitol may also aid by retainingmoisture to produce a smooth film.

3.1.3 Lubrication

Good lubrication properties are important in briquetting because reducing theparticulate friction and improving the flow onto the rolls provide for better and

more efficient compaction. Almost all liquid binders will also act as lubricants;however some chemicals are better at performing this dual task. High quality,unadulterated silicate has a very low coefficient of friction (CoF). Regrettably, thislow CoF is lost with the addition of solids and/or water. Other industries usingsilicate (the oilfield industry in particular) have expressed a need for solublesilicate with a lower CoF. Consequently, research was conducted to try to lower the CoF by either adding a lubricant to the silicate or by chemically modifying thesilicate with respect to how it interacts with metal surfaces.



In developing a lubricant for alkali silicate, the basic criteria included: effective,compatible with silicate, environmentally friendly and relatively low cost. A widerange of chemical lubricants was investigated using a block and ring lubricitytester. These materials included: glycols, olefins, phosphates, surfactants,

glucosides, asphaltenes and esters. Of these, the most effective were ester based, and to a lesser extent, glycols and certain glucosides. (Table 2) Alsoinvestigated and found effective was the modification of sodium silicate by theaddition of tetralkylammonium compounds such as tetramethyl ammoniumhydroxide (TMAH). It is thought that TMAH reacts with sodium silicate to formtetramethylammonium silicate with improved lubricity coming via the methylgroups.

Table 2. Effect of Lubricants on Coefficient of Friction

CoF

Water 0.36Sodium silicate 0.05

Sodium silicate + water 0.40

+ 3% glycol 0.29

+2% ester 0.18

+ 1% TMAH 0.30

3.2 SODIUM SILICATE AS A CHEMICAL BINDER

Chemical binders function by reacting with the material being agglomerated or by

formulating with multiple components that will react with each other. Sodiumsilicate has a long history of being used as a chemical binder. The best exampleis the use of sodium silicate with a soluble source of calcium (i.e. the Midrexprocess). The reaction of calcium salts with silicate forms calcium silicatehydrate. Other traditional setting aids used with sodium silicate are shown inTable 3.

Table 3: Traditional Setting Agents

Category Example

Inorganic Salts CaCl2, Ca(OH)2, Mg(OH)2,NaH2BO3, Na2CO3

Mineral Acids H2SO4, HClOrganic Acids CO2, acetic acid, citric acid

Inorganic Oxides ZnO, CaO, MgO

A study was carried out to demonstrate the strength and durability of materialsbound with sodium silicate. Figure 3 shows the benefits of time in developingstrength properties at 3% and 10% of binder matrix.



Figure 3. Compressive Strength of Sodium Silicate Binder in LimestoneGravel Matrix

Recently, development efforts have focused on non-traditional setting agentssuch as glycolic acid, sodium acid pyrophosphate (SAPP), and calciumlignosulfonate. The goal was to achieve one or more of the following attributes:

Higher final strengthBetter control of set

Longer setEnvironmental benefits

Figure 4 provides a compressive strength comparison of the non-traditionalversus other setting agents with 30% concentration sodium silicate in a limestonegravel matrix at 25°C.

0

100

200

300

400

500

600700

800

900

3Days 3 Months

C o m p r e s s i v e S t r e n g t h

k P a

3% of Mix

10% of Mix



Figure 4. Compressive Strength Comparison of Setting Agents at 25°C.

Lignin products are used in many applications that are similar to silicate enduses

5. Traditionally, the two technologies have been viewed as competitive.

For example, lignins and silicates are used as dispersants and can sequester metals. However, recent work has shown that there may be somecomplementary applications, particularly for calcium lignosulfonate. A laboratorystudy was designed to investigate the interaction between calcium lignosulfonateand silicates over a range of concentrations. The interactive effects wereobserved with both sodium and potassium silicates. The results of this workshowed that the setting characteristics of these systems were not typical of the

rapid interaction seen with other calcium salts and allowed for much moreflexibility in the binding process. Table 5 shows the setting effects of mixingcalcium lignosulfonate with sodium silicate. Calcium lignosulfonate is available asa solid or liquid (~50% solids) and either form can be used with silicates.

Table 5: Setting Times of Sodium Silicate with Calcium Lignosulfonate

Sodium Silicate N(3.2 ratio) %w/w

Calcium Lignosulfonate%w/w

Setting Time

5 5 3hr

5 10 4-5 min

10 25 1-2 min

At present, sodium silicate remains the workhorse of silicates for binder applications primarily because it is the least expensive of the alkali silicates. For specialty applications, however, there may be justification to use potassium or lithium silicate as highlighted below in the following paragraphs.

0

20

40

60

80

100

120

140

160

180

C a C l 2

H C l

A c e t i c

C a L i

g

S A P P

G l y c o l i c

C a S

O 4

H 2 S

O 4 C

o m p r e s s i v e S

t r e n g t h k P a



4. POTASSIUM SILICATE

Potassium silicate is typically used when sodium is undesirable for a particular

end use. For example, potassium silicate is preferred for the clays and fluxingmaterials used in welding rod applications. It is also used to bind vanadiumpentoxide catalysts for sulfuric acid manufacture and in acid-resistant cements.More recently potassium silicate is being considered as a binder for fertilizer applications. In this case, the more soluble potassium silicate can strengthenprills or pellets but then dissolve in use to release the nutrients of the fertilizer while providing additional potassium and silicon for plants. Potassium is aprimary plant nutrient, and silicon has gained significant recognition as abeneficial plant nutrient that helps to physically strengthen plant tissue as well asimprove nutrient uptake and alleviate salt and drought stress.

5. LITHIUM SILICATE

Lithium silicate is a specialty silicate that is now being manufactured in the US.The properties of lithium silicate are intermediate between those of sodiumsilicate and organic ammonium silicates. The difference in properties betweenlithium and sodium silicate can be ascribed to the fact that hydrated lithium ion islarger than the hydrated sodium ion and therefore can stabilize more silica in thecolloidal state. Table 6 provides a comparison between high ratio lithium,potassium and sodium silicate.

Table 6: Property Comparison of Various High Ratio Alkali Silicates

Lithisil 25 Kasil 1 N

Wt % Alkali 2.5% Li2O 8.3% K2O 8.9% Na2OWt % SiO2 20.5 20.8 28.7Weight ratio 8.2 2.5 3.2Molar ratio 4.1 4.1 3.2pH 10.8 11.3 11.2Sp. Gr. 1.2 1.26 1.38Viscosity (cp) 20 40 180Cation radius of

hydration (nm)

634.0 27.6 23.2

Compared to sodium and potassium silicate binders, certain surfaces canachieve better adhesion with lithium silicate. As a binder or coating, lithiumsilicate can also impart high levels of strength and water resistance. While lithiumsilicate is considerably more expensive than sodium silicate, improvements inadhesion can also be seen when a small amount of lithium silicate is blendedwith sodium or potassium silicate7.



Lithium silicates are somewhat compatible with water-miscible organic solventsand slightly compatible with non-polar solvents (Table 7). This behavior isdifferent from that of sodium silicate solutions which are not compatible withorganic solvents to any extent at comparable SiO2 concentrations. Lithium

silicate would allow for better bonding of material covered with organic solvents.

Table 7. Compatibility of Lithium Silicate with Water-Miscible OrganicSolvents

Solvent Composition Final

Mixture Stability

% SiO2 % Solvent

5.06 Mole Ratio Lithium Silicate

Methanol 18.5 7.1 7 Days

17.9 10.0 <10 min.

13.7* 19.7 3 hoursEthanol 19.0 4.4 GelAcetone 17.9 10.2 40 min.Dioxane 19.9 ------ Gel

Tetrahydrofuran

19.2 3.5 >7 days

19.0 4.2 >7 days

7.52 Mole Ratio Lithium Silicate

Methanol 20.3 11.3 50 min.

19.2 15.7 15 min.

13.4* 18.0 45 hrsEthanol 19.6 8.8 <10 min

15.5* 5.7 >7 daysAcetone 18.8 12.3 3 days

Acetone 13.3* 19.0 45 min.Dioxane 19.9 7.4 GelTetrahydrofuran

20.0 6.7 3 days

* From stock lithium silicate solutions containing about 17% SiO2. Allothers from stock solutions containing about 20% SiO2

ACKNOWLEDGEMENT

The authors would like to thank PQ Corporation and National Silicates for permission to publish this paper.

BIBLIOGRAPHY

1. W.L. Schleyer, “Sodium Silicate As A Briquetting Binder”,Proceedings of the 9th Biennial Briquetting Conference, 1965



2. R.E. Wright, “Smelter Dust Agglomeration with Sodium Silicate”,Proceedings of the 15th Biennial Conference of IBA, 1977

3. United States Patent 4430107 4. “Selecting Binders and Lubricants for Agglomeration Processes” by

Karl R. Komarek (Chemical Engineering Magazine, December, 1967)

5. SRI Consulting, 2005 Chemical Economics Handbook 6. .“Synthetic Inorganic Silicates,” Encyclopedia of ChemicalTechnology, Vol. 18, 2nd ed., 1969 by John Wiley & Sons, Inc., p150.

7. US Patent 4,347,890; Allyn-Pyzik & Falcone, PQ Corp

Recent Silicate Binder Developments

Documents

Transcript of Recent Silicate Binder Developments