Silicate Solution Chemistry
INTRODUCTION 
Sodium and potassium silicates are composed of a mix of silicate anions whose composition and handling can substantially affect the performance of the products in which they are used. Knowledge of these anionic species distributions, the properties one can expect from different types of distributions, and the factors that affect distributions can be valuable in developing product and process designs.

PQ, as the leader in silicate technology, continues to develop characterization methods that contribute to the ability of our customers to (1) understand and manipulate silicate product properties, and (2) to improve their own products and processes in which silicates are used, in order to be more competitive in their marketplace.

WHAT IS SILICATE SPECIATION?
The fundamental building block of silicate solutions is the tetrahedral silicate anion with a silicon atom at the center of an oxygen-cornered, four sided pyramid (illustrated as the monomer in Figure 1a). Associated with each oxygen atom is, typically, a hydrogen, sodium or potassium atom. In some cases, the oxygen atom may be linked to other silicon atoms through tetrahedral coordination.


Figure 1 a. Silicate Anion Structures
Linear and planar cyclic silicate anions

      
  Monomer                  Linear Trimer            Cyclic Trimer

A shorthand method of representing these silicate products uses the ratio of SiO2 to Na2O as follows: xSiO2:M2O.

M is an alkali metal, either sodium (Na) or potassium (K).  X represents the weight ratio of silica to metal oxide. For example, N® brand liquid sodium silicate has a ratio of 3.22 SiO2 to Na2O.  PQ manufactures liquid sodium silicates with SiO2:Na2O ratios as low as 1.6, and anhydrous sodium metasilicate with a ratio of 1.0. Potassium silicates are produced with weight ratios that range from 1.8 to 2.5 SiO2:K2O.

The mix of silicate anions in a soluble silicate solution is much more complex than a simple ratio representation may suggest. The tetrahedral monomers can be linked through a shared oxygen (hence the SiO2 representation, instead of SiO4) in two- and three-dimensional structures. The electrical charges of the anions are balanced by the sodium or potassium cations. Figure 1b shows examples of the types of anions that can make up silicate solutions, including linear, planar cyclic and three-dimensional silicate anion structures, with dots representing silicon atoms. The oxygens linking the silicon atoms lie between the dots and are not shown.

Figure 1b. Silicate Anion Structures 
Linear, planar cyclic and three dimensional silicate anion structures, with dots representing silicon atoms. The oxygens linking the silicon atoms lie between the dots and are not shown.

     Linear 
     Trimer
     Cyclic 
     Trimer
     Cyclic 
     Tetramer
   Prismatic 
   Hexamer
    Cubic 
    Octamer

Silicon29 nuclear magnetic resonance (NMR) spectroscopy provides a basic shorthand method for characterizing silicate anion mixtures and helping to understand how they change under different conditions. Silicon NMR uses the relationship between each silicon atom and its neighbors.  The Qx nomenclature is used with "x" referring to the number of Si atoms connected via oxygen to the reference Si atom.  

  • Q0 — The silicon atom is not bonded to any other silicons. This characterizes the silicon in the monomeric silicate anion SiO4-4.
  • Q1 — The silicon atom is bonded to one other silicon atom. Examples are silicon atoms in a two-atom chain or at the ends of longer chains.
  • Q2 — The silicon atom is bonded to two other silicon atoms. Examples are silicon atoms in the middle of a linear anion or silicon atoms that form a planar cyclic structure.
  • Q3 — The silicon atom is bonded to three other silicon atoms. An example is silicon atoms at the corner of a three dimensional structure.
  • Q4 — The silicon atom is bonded to four other silicon atoms. Examples are silicon atoms in the interior of polymeric colloidal silica or silicon atoms in the interior of silica gel particles.
  • HOW DOES IT SILICATE SPECIATION VARY? 
    There are two major factors that influence the distribution of anions—the ratio of silica to alkali (pH), and the concentration of solids.

    Moving from high alkali, low ratio products to low alkali, more siliceous high ratio products represents a change in silicate species distribution:  from high monomer (Q0) content and few complex structures (Q4) to reduced monomer content and a greater number of complex structures.

    The change in distribution ranges through a rise and fall in content of intermediate chains and cyclic trimers, and larger rings (Q1, Q2, Q3).  At ratios greater than 2.0, colloidal structures begin to form in the solution. At very high ratios, the structures result in gelling of the solution.

    SILICATE CHEMISTRY IN APPLICATIONS 
    Just as silicate products of differing ratios differ in their anionic species distributions, chemical alterations of an individual silicate product can result in a chemical environment that changes in anionic species distribution. The following changes in chemical environment will shift anionic species distribution. The changes occur at varying rates, depending on a variety of conditions. 

    Alkali Change. (1) Adding alkali to a high-ratio silicate to reduce the ratio of SiO2 to Na2O, or (2) adding colloidal silicate or another silica source to a high-ratio silicate to increase ratio will result in a shift in anionic species distribution (respeciate) to the distribution of the new ratio, according to the new SiO2 or Na2O content. The equilibrium time for this shift depends on the magnitude of the change, the initial concentration of the silicate, the temperature, and agitation. A full strength silicate at room temperature with slow stirring will take more than 48 hours to respeciate. Dilute solutions (<1%) can respeciate in minutes.

    Dilution. Similarly to addition of alkali to silicate, addition of water causes respeciation governed by the same parameters of concentration, temperature and agitation.  Respeciation will be less pronounced than the change due to alkali addition.

    Other ingredients or contaminants. Transition metals such as iron, or trivalent elements such as aluminum or boron can be incorporated into the structure of silicate anions. There is a threshold level beyond which types of anions do not incorporate; the level varies with the element. An example of aluminum’s incorporating into the silicate anion is the aluminosilicate gel formed as a precursor in zeolite synthesis. These ions have limited impact on species distribution.

    Silicate anions provide cation exchange sites. Within the anion distribution, determined by ratio and solids concentration, alkali ions tend to be distributed to a greater degree on the smaller anions. Precipitates or flocculates may form when hardness ions such as calcium or magnesium exchange with alkali cations associated with three-dimensional anions found in higher ratio silicates. 

    MEASUREMENT 
    As the leader in silicate technology, the PQ Corporation employs modern chemical characterization methods, such as Si29 NMR and vibrational spectroscopy, to obtain detailed insight into the complexities of silicate solution chemistry. Armed with this knowledge, we are able to:

    1. Produce developmental products at silica to alkali ratios beyond the limits imposed by conventional furnace processes. 

    2. Improve the performance of products to enhance customer value. 

    3. Support our customers when they require assistance in the optimal use of liquid silicates in their processes.

    We are happy to discuss with our customers, present and prospective, how any of the characterization methods PQ has developed can be of use in improving their products or processes.

    CONCLUSION 
    Commercial silicate solutions are complex distributions of a range of silicate anions. For product quality and consistency, it is important for users to control these distributions. Unique among silicate manufacturers, PQ is advancing silicate technology. We are using a fundamental knowledge of silicate solution chemistry to assist our customers in using silicates successfully for enhanced product value. We encourage our customers as partners to explore silicate solution chemistry and control for future advances of their own technology.

    As part of PQ Corporation's commitment to Continuous Quality Improvement, we apply our discoveries in silicate solution chemistry to our existing product lines. In this way we make certain for our customers that PQ products will maintain their exceptional quality, consistency, and value.

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