SOP is best fertiliser, premium-priced product and in short supply, page-5

SOP production
(From IC Potash web site)

Sulfate of potash (SOP) is not a naturally occurring mineral and is produced by chemical methods. Only a few of these processes exist, and the Ochoa process is projected to be one of the lowest cost methods worldwide.

Mannheim Process
The most common method of producing potassium sulfate is the Mannheim process, which is the reaction of potassium chloride with sulfuric acid at high temperatures.

Potassium Chloride and Sulfate Salts
Potassium chloride can be reacted with various sulfate salts to form a double salt that can be decomposed to yield potassium sulfate.

Naturally Occurring Brines
Some operations produce SOP from the salt mixtures harvested from natural brines. Three companies produce potassium sulfate in such a way on a large scale: GSL Minerals (Great Salt Lake, Utah), SQM (Salar de Atacama, northern Chile) and Luobupo Potash (Lop Nur, northwest China). This method requires brines with high sulfate levels such as those found within these salt lakes. The sulfate is typically present in the harvest salts in the form of the double salt kainite, which is converted to schoenite by leaching with sulfate brine. The leach process is hampered by high sodium chloride content in the harvest salts and the halite is first removed by flotation. After thickening, the schoenite is decomposed by simply adding hot water, whereupon the magnesium sulfate enters solution leaving SOP crystals.

This process is currently the lowest cost method to make SOP. As lakes with sufficient brine mineral levels are rare, this method only accounts for 15% to 20% of global supply.

Ochoa Process
IC Potash Corp.'s Ochoa process will convert polyhalite into SOP using unit operations common to the industrial minerals industry. Processing polyhalite to produce SOP involves the following steps: crushing and washing, calcination, leaching, crystallization, and granulation.