It was once used as a fire extinguishing agent, used in portable and wheeled fire extinguishers. Known as Super-K dry chemical, it was more effective than sodium bicarbonate-based dry chemicals and was compatible with protein foam. This agent fell out of favor with the introduction of potassium bicarbonate (Purple-K) dry chemical in the late 60s, which was much less corrosive and more effective. Rated for B and C fires.
KCl is also an optical crystal with a wide transmission range from 210 nm to 20 µm. It was formerly often used in the infrared spectrum range, and still is from time to time. While cheap, KCl crystal is hygroscopic. This limits its application to protected environments or short term uses such as prototyping. Exposed to free air KCl optics will "rot". Today, much tougher crystals like ZnSe have replaced it for IR spectral range applications.
Optical data of KCl
:
Transmission range: 210 nm to 20 µm
Transmittivity = 92% @ 450 nm and rises linearly to 94% @
16 µm
Refractive Index = 1.456 @ 10 µm
Reflection Loss = 6.8% @ 10 µm (two surfaces)
dN/dT (expansion coefficient)= −33.2×10−6/°C
dL/dT (refractive index gradient)= 40×10−6/°C
Coefficient of absorption: 0.001 cm−1
Thermal conductivity = 0.036 W/(cm·K):
Damage threshold (Newman & Novak): 4 GW/cm2 or 2
J/cm2 (0.5 or 1 ns pulse rate)
Damage threshold (Kovalev & Faizullov)= 4.2 J/cm2
(1.7 ns pulse rate)
It has also been used to create heat packs which employ exothermic chemical reactions, but these are no longer being created due to cheaper and more efficient methods such as the oxidation of metals ('Hot Hands', one time use products) or the crystallization of sodium acetate (multiple use products).
A very bulk use of Potassium chloride is in mines and Oil Drilling mud and chemicals.