"Thermal dissolutionThe thermal dissolution method, which is...

"Thermal dissolution
The thermal dissolution method, which is used at several sites,
is based on the temperaturedependant dissolution behaviour:
While halite dissolves in water equally well nearly regardless of
the temperature, the solubility of potassium chloride increases
with the temperature. For example, only 137 grams of potassium
chloride can be dissolved in one litre of brine at 25 °C, whereas
265 grams can be dissolved per litre at 95 °C.
To extract potash from the crude salt, a saline solution which
is saturated with halite and KCl is heated. After it has been
heated, the solution continues to be saturated with halite but
the solubility of sylvite increases with the rising temperature.
Crude salt is now added to the hot leaching brine, the potassium
minerals dissolve, the halite and kieserite remain undissolved
as solid material. The solid residue is separated by filtration or
centrifugation and subsequent clarification. Potassium chloride
is then extracted from the hot, clear solution with a purity of up
to 97% by cooling in a vacuum crystallisation system."

https://www.kpluss.com/.downloads/agriculture/brochures/en-int-fertiliser-strong-minerals-A4-1611.pdf

SOP behaves in the same way.