Silicate Chemistry

Sodium silicates (Na2O • XSiO2) are metal oxides of silica. All soluble silicates can be differentiated by their ratio, defined as the weight proportion of silica to alkali (SiO2 / Na2O). Ratio determines the physical and chemical properties of the product. 

Four major chemical processes account for the usefulness of silicates in industry, construction and other fields. Each of the hundreds of applications for silicates takes advantage of one or more of these phenomena: 

  • Hydration and Dehydration

  • Metal Ion Reactions

  • Surface Charge Modification

  • Gelation and Precipitation Reactions

Hydration and Dehydration 

Hydration is the pick-up of moisture by a solid material from the air or from a water source (dissolving).  Dehydration is the loss of moisture from a material to the air or to another substrate.

Soluble silicates are unique in the way they dissolve from their solid forms (powders and glasses) and the way they dry from liquid solutions.  The more water a solid silicate contains, the faster it will dissolve. Also, the lower the ratio of silica to alkali in the solid silicate, the faster it will dissolve into solution. 

Dehydration of silicate liquids at equivalent solids depends greatly on ratio. The higher the ratio, the faster the drying rate, because high ratio products have low alkali levels.  Alkalis tend to hold moisture more readily.

The glassy nature of silicates imparts strong and rigid physical properties to dried films or coatings.  Silicates air-dry to a specific moisture level, according to ambient temperature and relative humidity.  Heating is necessary to take these films to complete dryness—a condition in which silicates become nearly insoluble. Reaction with other materials, such as aluminum or calcium compounds, will make the film or coating completely insoluble.

Metal Ion Reactions 

Using silicates to tie up metal ions is an inexpensive way to enhance the performance of many processes.  Soluble silica reacts with all multivalent cationic metal ions to form the corresponding insoluble metal silicate. These multivalent cationic metal ions include:

  • Calcium 
  • Manganese
  • Magnesium 
  • Cadmium
  • Iron
  • Lead
  • Zinc
  • Nickel
  • Chromium
    
Silicates will precipitate these metals out of solution and render them insoluble and non-reactive. The reactant byproduct normally displays long-term stability. The bond formed can be broken only by extremely aggressive chemical action.

Detergency, pulp and paper, waste treatment, and water treatment applications all benefit from the silicate reaction that eliminates or ties up undesirable metals.  Left in the process to react with other more expensive chemicals, these metals degrade the functionality of those chemicals or render the application less effective.

Surface Charge Modification 

All dissolved silicas have anionic charge.  In a suspension, silicate anions can adsorb to other materials dispersed in water and cause them to be negatively charged.  Suspended particulate solids of negative charge can repel each other for full dispersion in water.  This phenomenon is the basis for the dispersive and deflocculating effects of silicates.

These effects are important in applications such as detergency and clay manufacture.  In detergency, the silica prevents the redeposition of soils on clothing.  In clay processing, the silica helps keep the clay dispersed using substantially less water than would be needed in the absence of silicate. 

Precipitation and Gelation Reactions 

In essence, these reactions involve the destabilization of liquid silicate solutions.  When the pH value of a liquid silicate is modified to a point below 10.7, the silica is destabilized and the system polymerizes or gels.

As the alkali is neutralized (pH decreases), soluble silicates polymerize, molecular weight increases, and the silica becomes insoluble.  The reaction results in a precipitation of silica, a colloidal silica suspension, or a continuous gel, depending on the silica concentration.  pH neutralization is one method of polymerizing silica and is widely used in industry with feedstock silicate to manufacture colloidal silicas, silica gels, and precipitated silica. 
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