The literature values vary for the freezing point of 40 wt% FeCl
3 solution. From the more reliable sources, I give the value as -14
+2
oC.
To understand the variation in freezing point with composition, look at the solubility table & H
2O(ice)-FeCl
3.6HO phase diagram given by Solvay:
H
2O(ice) and FeCl
3.6HO form a eutectic, with eutectic point 33-34 wt% FeCl
3, -55
oC. So, starting with pure ice, increasing FeCl
3 lowers the freezing point from 0
oC to a minimum of -55
oC at the eutectic, whereupon further increasing FeCl
3 raises the freezing point. From data in
Perry's, 7th Edn., the solubility limit is 42.7 wt% FeCl
3 at
oC. Interpolation gives a freezing point of -15
oC at 40 wt% FeCl
3. Pretty good agreement with the -13.5
oC given in the Solvay table.
There are 2 points worth considering:
1) The slope of the FeCl
3.6HO-rich liquidus at the 40% FeCl
3 composition is about 5.5
oC per wt.%.
So rather than heating, you can lower the freezing point to -25
oC 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 -55
oC. The fraction that solidifies can be calculated from the lever rule for phase diagrams. Note that FeCl
3.6HO is 60 wt% FeCl
3.
Example: 40 % FeCl
3 solution that begins freezing at -14
oC is cooled to -24
oC. The liquidus composition at this temperature is 37.5 wt%. The weight fraction of solid FeCl
3.6HO 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.