“How do I know when my slurry is going bad?” I have heard that question many times and I imagine standing over the slurry tank admonishing it by saying, “Bad Slurry! Bad Slurry!” But, let’s put humor aside, take a step back and understand the parameters that impact slurry stability. Most people understand that low pH is an indication of compromised slurry health. But, conductivity is frequently overlooked as another important indicator.
Conductivity is a numerical expression of the ability of an aqueous solution to carry an electric current. This depends on a number of factors including the presence of ions, total concentration and temperature.
In slurries using colloidal silica, the conductivity value reflects the ionic charge across the microscopic amorphous particles of silica dispersed in the aqueous solution. Let me try to explain it in slightly simpler terms.
An aqueous solution means that water is the solvent. As a reminder, water is made up of Hydrogen ions (H+) and Hydroxide ions (OH−). Colloidal silica particles are spheres of silicon dioxide (SiO2)with a negative surface charge (SiOH−). The water ions (H+ and OH−) form an electric double layer that results in a neutral charge surrounding the silicon dioxide molecules.
As the concentration of ions increase, i.e. conductivity increases, there is less space between the ions and the double layer begins to collapse and the silica particles are forced together forming the following permanent bond:
SiO2 – Si – O – Si – SiO2 + H2O
This reaction is what forms the gel network that holds the molds together when molten metal is poured into the shell. However, we want this reaction to take place during the shell building process, not in the slurry tank. If this reaction is occurring in the slurry tank, shell strength may be reduced.
Each colloidal silica has an optimal conductivity range so check with your supplier for specific information.
Now that you have a basic understanding of conductivity, next week we will discuss parameters that impact conductivity and how to address those issues.