The literature values vary for the freezing point of 40 wt% FeCl3
solution. From the more reliable sources, I give the value as -14+
To understand the variation in freezing point with composition, look at the solubility table & H2
O phase diagram given by Solvay: http://www.solvaychlorinatedinorganics.com/docroot/chlo_inorg/static_files/attachments/pch_1610_0007_w_en_ww.pdf
O(ice) and FeCl3.
O form a eutectic, with eutectic point 33-34 wt% FeCl3
C. So, starting with pure ice, increasing FeCl3
lowers the freezing point from 0 o
C to a minimum of -55o
C at the eutectic, whereupon further increasing FeCl3
raises the freezing point. From data in Perry's, 7th Edn.
, the solubility limit is 42.7 wt% FeCl3
C. Interpolation gives a freezing point of -15o
C at 40 wt% FeCl3
. Pretty good agreement with the -13.5o
C given in the Solvay table.
There are 2 points worth considering:
1) The slope of the FeCl3.
O-rich liquidus at the 40% FeCl3
composition is about 5.5o
C per wt.%.
So rather than heating, you can lower the freezing point to -25o
C by increasing the water content by 2 wt%.
2) The liquid only partially freezes as long as the temperature doesn't drop all the way to -55o
C. The fraction that solidifies can be calculated from the lever rule for phase diagrams. Note that FeCl3.
O is 60 wt% FeCl3
Example: 40 % FeCl3
solution that begins freezing at -14o
C is cooled to -24o
C. The liquidus composition at this temperature is 37.5 wt%. The weight fraction of solid FeCl3.
O will be (40-37.5)/(60-37.5) = 0.111 or 11%.
It is unlikely that this slush would damage your system, although it may be difficult to pump. If the piping is buried in soil, it will be warmer, and thus less problematic.