Feature Article

Effects of Freezing Condition on Colloidal Silica

by Kevin lee, PhD, Gavin Dooley, PhD, Bob Brown, REMET UK LTD W

hat happens when the binder is kept in an environment of temperature < 0º?

When a colloidal silica binder

is subject to freezing, usually a few degrees below the freezing point of water, the water (in fact a solution of water and sodium salts) in the colloidal silica will start to freeze. The water molecules freeze from the coldest area (usually starting from the surface of liquid) inwards.

As the water molecules start to crystallise they push the colloid particles away from the solidification front (see Figure 1). This increases the colloid concentration in the unfrozen area and increases the possibility of colloidal sol aggregation the precursor of gelation.

Once agglomeration occurs it is

irreversible even when the temperature of the binder is increased back to room temperature (~20°C). However, if this issue is noticed early enough (before

aggregation occurs) then the

silica particle can be re-dispersed and aggregation can be avoided. Any aggregation occurred during the freezing process will reduce the concentration of freely dispersed colloidal silica and thus reduces the free binding surfaces.

At the same time as water freezes, it releases latent heat and changes the equilibrium of the system, as a result it is not easy to know the condition of a binder after 5,12, 24 hours under freezing conditions. This depends on other factors including what is the temperature of the binder prior to exposing it to subzero, wind chill effect, total mass of binder exposed to extreme

18 ❘ November 2019 ®

Figure 1: Advancing of ice crystals during solidification (1).

condition etc. We recommend that colloidal silica

should always be stored in temperatures above 2°C (35°F). Freezing temperatures cause irreversible aggregation of the silica.

Where possible drums and

bulk storage tanks should be stored in a heated location, where this is not possible, they should be stored in a location that will protect them from freezing conditions. Tanks that must be located outside should be provided with external heating and lagging. The temperature of the colloidal silica must be kept high enough so that the air in the tank stays above freezing; otherwise, icicles and frost may form above the liquid. While it is necessary to prevent freezing, it’s not desirable to store colloidal silica for lengthy periods at high temperatures. Continuous exposure to high temperatures will shorten the shelf life, avoid storing the product in

areas where the taemperature routinely goes above 43°C (110°F).

Common Signs that the Binder Has Been Exposed to Subzero Conditions After the silica sol has aggregated and precipitated, the process is not reversible. In occasion when the binder exposed

to extreme conditions, the

binder will not flow smoothly and has agglomerated particles in it even after returning to above 0°C. The appearance is like sand or ice sludge in water (see Figure 3).

Process Impacts of a Binder That Has Been Exposed to Subzero Conditions A binder that has begun to aggregate, will show very typical tests on QC / Goods Inward Testing. The pH will be lower, and if left longer to aggregate the silica content can be lower too. In gross cases there may even be a precipitate at

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