Volume 4  Number 8                            Dennis R. Dinger                                1 June 2006

Updates

"... for Ceramists" Series Books

The paperback version of Characterization Techniques for Ceramists is available on the Books and Downloads page at the web site!    Retail price is $29.95 plus shipping and handling. The book has 256 pages and it covers 34 different characterization techniques that are commonly used by ceramists.  Order your copy NOW!

The book sets on the web site have also been revised to include this new book.  A 3-book set of paperbacks, including one each of Particle Calculations for Ceramists, Rheology for Ceramists, and Characterization Techniques for Ceramists, is now available for $64.85 plus shipping and handling.  This is a $10 saving off the total retail price of the 3 paperback books.  A 3-book set of downloads is also available for $52.85.  This, too, represents a $10 saving off the total retail price of the 3 downloadable books.  

The E-Book version of Characterization Techniques for Ceramists is available for downloading at the Books and Downloads page of the website for $24.95.  The download is a 2.889 Mb self-extracting Zip® file for the Windows® environment which unzips to the 2.998 Mb book in PDF file format.  Those of you who order the downloadable book will want to know that the PDF book is formatted to print on 5.5" X 8.5" paper (i.e., 8.5" X 11" sheets cut in half.)

The other two books, Rheology for Ceramists and Particle Calculations for Ceramists, continue to be available for purchase as downloadable E-books and as paperback books at the Books and Downloads page of the web site.

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The Effects of Sodium and Silicate Ions on Whiteware Suspensions

 

Introduction

This subject refers primarily to whiteware ceramics, but it applies to any and all bodies which use clays and kaolins as ingredient materials.  We will break this subject down into three categories for discussion:  the sodium ion, the silicate ion, and the effect of sodium silicates on clay/kaolin suspensions.

The Sodium Ion

The sodium ion is considered to be a deflocculating cation.  It is a relatively large cation with a low charge (+1).  More highly charged ions usually have smaller ionic radii because the higher net positive charges pull the electrons in tighter -- which produces smaller ionic radii.  But since the sodium ion only lacks one electron, the weak positive (+1) charge does not draw remaining electrons into tight orbits.  Therefore, the +1 sodium ion is a large fluffy ion which disrupts gel structures.  Smaller, more highly charged cations tend to fit within forming gel structures and pull the structures tighter -- which strengthens them.  Examples of higher charged, smaller cations are Mg++ and Al+++.

The sodium ion is large and weakly positive.  Rather than reaching out and strengthening forming gel structures in its vicinity, the sodium ion disrupts forming gel structures.

Sodium ions can be added to slips and slurries as a variety of soluble salts -- sodium hydroxide, sodium chloride, sodium sulfate, and sodium silicate, for example.  Most sodium salts are soluble, so there are many possible sodium salts that can be selected to add sodium ions to suspensions.  Of the four examples mentioned above, one should consider:  (1) Sodium hydroxide, which has a major effect on pH, is a good choice to add sodium cations to a suspension;  (2) sodium chloride is an inexpensive sodium salt, but once chloride ions are added to slips and slurries, there is no way to easily remove them;  (3) sodium sulfate could be used, but one must then consider the consequences of adding the sulfate ions to suspensions;  and (4) sodium silicate is often used in clay/kaolin slips and slurries, not because of the sodium ions, but because it is the primary source of silicate ion additions to suspensions.

Two questions need to be answered when selecting sodium salts:  (1)  How many sodium ions are too many?  and (2) Which anion should accompany the sodium cations?

Since almost all sodium salts are soluble, it is difficult (impossible??) to remove excess sodium ions from suspension.  When too many sodium ions have been added to a suspension, conductivity within the interparticle fluid will reach high levels.  When interparticle fluid conductivity is extremely high, electrostatic attractive/repulsive forces between particles no longer function.  At this point, any further additions of flocculants or deflocculants will cause flocculation.  So beware:  high sodium ion concentrations and high interparticle conductivities will eliminate your ability to deflocculate your suspension.  At that point, the suspension is ruined.

Summary:  Some sodium ions help.  Too many hurt. 

The Silicate Ion

The anion most often used with sodium ions in clay/kaolin suspensions is the silicate ion.  Sodium silicate is generally used to add silicate ions to suspensions.  The sodium ions simply go along for the ride.  Why do we use sodium silicate?  The two main flocculating cations present in whiteware suspensions (Ca++ and Mg++) are insoluble in their silicate forms, although they are soluble as sulfates and chlorides.  Magnesium and calcium ions are frequently present in raw clay and kaolin materials, and they are added to suspensions as flocculating cations in their soluble forms. 

When a suspension is too flocculated, additions of sodium silicate will deflocculate it.  To reduce the concentrations of soluble Mg++ and Ca++ in suspensions, sodium silicate additions should be used.  Deflocculating sodium cations replace the flocculating Mg++ and Ca++ cations, while the silicate anions combine with and precipitate Mg++ and Ca++ as insoluble silicates. 

When a suspension is too deflocculated, magnesium and calcium sulfate additions will flocculate it.  Magnesium and calcium ions cause flocculation.  Some believe the sulfate ions are also necessary for proper suspension flocculation and control, so both cations and anions increase the flocculation of suspensions.

Here's the summary:

▪  If a suspension is too flocculated because too many calcium and magnesium ions are present in suspension, silicate ions (from sodium silicate) will combine with them and precipitate them as insoluble silicates.

▪  If a suspension is too deflocculated, calcium and magnesium ions can be added as a variety of soluble salts (sulfates, chlorides, etc.)

▪  If too many sulfate ions are present in a suspension, barium hydroxide additions can be used to precipitate the excess. 

▪  If too many sodium ions are present, there's nothing that can be done to remove them. 

▪  If too many chloride ions are present, there's nothing that can be done to remove them.

Each additive generally has a counter-additive which effects its removal.  Silicate ions are never usually in excess in suspensions because they precipitate a variety of cations as insoluble salts.  Excess magnesium and calcium ions can be removed as insoluble silicates.  Excess sulfate ions can be removed as insoluble barium sulfate.  Sodium and chloride ions, however, cannot be removed.  For this reason, no more sodium ions should ever be added than absolutely necessary -- and if you need sodium and/or calcium ions, they should not be added as chlorides.  Once chloride ions are present in solution, there's no way to remove them.

One of the main considerations for selecting additives to suspensions is to pay attention to how, if necessary, excess ions can be removed?  Some can be.  Some can't.

Effects of Sodium Silicate

The primary reason for using sodium silicate is add the silicate ions, not the sodium ions.  Sodium silicate happens to be soluble, so it is an excellent way to add silicate ions to suspensions.  Silicate ions are generally added to remove unwanted flocculating cations such as magnesium and calcium ions.  Sodium silicate deflocculates by removing flocculating cations.  If you need to deflocculate, but you don't need to remove excess flocculating cations, you can use one of the many available organic deflocculants instead of sodium silicate.  Organic deflocculants (which are also frequently sodium salts) deflocculate by increasing the effective electrostatic charges on the surfaces of particles. 

Sodium silicate, therefore, is an excellent deflocculant.  It adds the deflocculating cation sodium (Na+) as the silicate ions remove flocculating cations from solution. 

This is the main difference between sodium silicate and organic sodium deflocculants.  Both types of deflocculants add sodium ions to suspensions.  But sodium silicate removes unwanted flocculating cations, while organic deflocculants mask, hide, and/or counter the effects of the unwanted flocculating cations without removing them. 

Which Additive to Use?

When adding flocculants and deflocculants to clay/kaolin-containing suspensions, one should consider if it is possible (and how one can do it) to remove any excess ions.  Silicate ions can be removed by highly charged cations such as magnesium, calcium, and aluminum.  Highly charged cations are removed by silicate ions.  Highly charged cations can be added as sulfates, chlorides, nitrates, etc.  Sulfates can be removed by barium hydroxide additions.  Sodium, chloride, and nitrate ions, however, cannot be removed.

Consider the following experiment which new ceramic engineering grad students were asked to perform.    They were told to over-flocculate a sample of a clay suspension (a whiteware body slip) with calcium chloride (rock salt).  When they achieved an over-flocculated suspension, they were then told to over-deflocculate it with sodium silicate.  When its viscosity was once again too low, they added more calcium chloride and repeated the cycle -- back and forth and back and forth -- over-flocculated, over-deflocculated, over-flocculated, over-deflocculated, etc.  They were to measure viscosity and rheology after each addition.  They learned that this process quickly reached the point where sodium silicate no longer deflocculated.  It only caused more flocculation. 

This experiment showed that too much of a good thing is bad.  I'm guilty of doing this during slip preparation when I was teaching the sophomore ceramic lab.  I wanted to flocculate the suspension quickly, so I frequently added too much flocculant.  Then, to compensate, I added too much deflocculant, which required that I continue to add more and more of each to counter my mistakes.

Try this experiment yourselves to see what happens.

What does this experiment actually accomplish?  Sodium silicate additions remove Mg++ and Ca++ ions as insoluble silicates.  Net effect:  sodium ions are added to the suspension.  With calcium chloride additions, flocculating calcium ions, as well as chloride ions, are added.  More additions of sodium silicate remove more calcium ions.  The two ions that are not removed in this experiment, however, are the sodium and chloride ions.  With each addition of the flocculant of deflocculant, the sodium and chloride concentrations increased in the interparticle fluid. 

The short cut to the end of this experiment is to just add an excess of table salt to the suspension.  The net effect of table salt additions, or the end point of the actual experiment, is a highly conductive interparticle fluid -- which renders all electrostatic attractive and repulsive forces ineffective. 

Any suspension with a highly conductive interparticle fluid will be flocculated.  Such suspensions are generally useless as production suspensions.

Summary

When adding flocculating and deflocculating ions to suspensions, one should consider the method of flocculation and deflocculation accomplished by each additive.  One should also consider whether excess concentrations of ions can be reduced to adjust suspension rheological properties.

Silicate ions deflocculate by removing flocculating cations.  Once such suspensions equilibrate, unwanted calcium and magnesium cations (which precipitate as insoluble silicates) are no longer available to interparticle fluids.

Organic anionic deflocculants deflocculate by coating flocculating cations and particle surfaces with high density electrostatically negative charges.  They deflocculate by increasing electrostatic charge densities which cause all particles to repel one another.  Since these additives function by coating particles and cations (i.e., by not removing anything), shear might change the equilibrium nature of these suspensions by freeing the unwanted flocculating cations so they can once again function.  Once added, however, these anionic deflocculating ions cannot be removed.

Silicate ions can be removed from suspension by highly charged cations which combine with the silicates to form insoluble salts.

The reverse is true as well -- highly charged flocculating cations can be removed by soluble silicate additions.

Sulfate ions, which are frequently used to add calcium and magnesium ions (plaster and epsom salts) to suspensions, can be removed by adding soluble barium ions.  The result will be insoluble barium sulfate.

Sodium ions, which accompany many deflocculating ions, and chloride and nitrate ions, which accompany many flocculating ions, are not easily removed from suspension.  Once present, they remain dissolved in the interparticle fluids and their concentrations increase as more and more additives are added to tune the slips.

At the extremes, when too many soluble ions have been added to suspensions, the suspensions will be severely and irreversibly flocculated. 

 

 

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

 

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