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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
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| 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. 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 but 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 but are not shown.
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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, counting the number of other silicon atoms to which each silicon atom it is connected to through an oxygen atom.
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Q0 — The silicon atom is not bonded to any other silicons. This characterizes the silicon in the monomeric silicate anion SiO4-4.
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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.
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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.
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Q3 — The silicon atom is bonded to three other silicon atoms. An example is silicon atoms at the corner of a three dimensional structure.
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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.
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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 (left to right in Figure 2) moves through a change in
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 as solids 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 incorporate 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
above the limits imposed by conventional furnace processes.
2. Improve the
performance of its existing product line 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 high value users to control these
distributions. Unique among silicate manufacturers, PQ is advancing silicate
technology. We are using 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.
For more detailed information please refer to the following document:
Anion Distributions in Sodium Silicate Solutions. Characterization by 29Si NMR
and Infrared Spectroscopies, and Vapor Phase Osmometry
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