Potassium Sulfate

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Natural resources

The mineral form of potassium sulfate, arcanite, is relatively rare. Natural resources of potassium sulfate are minerals abundant in the Stassfurt salt. These are cocrystallizationsof potassium sulfate and sulfates of magnesium calcium and sodium.

Relevant minerals are:

    Kainite, MgSO4·KCl·H2O

    Schönite, K2SO4·MgSO4·6H2O

    Leonite, K2SO4·MgSO4·4H2O

    Langbeinite, K2Mg2(SO4)3

    Glaserite, K3Na(SO4)2

    Polyhalite, K2SO4·MgSO4·2CaSO4·2H2O

The potassium sulfate can be separated from some ofthese minerals, like kainite, because the corresponding salt is less soluble inwater.

Kieserite, MgSO4·H2O, can be combined with a solution of potassium chloride to produce potassium sulfate.

Production

Approximately 1.5 million tons were produced in1985,typically by the reaction of potassium chloride with sulfuric acid, analogous to the Leblanc process. Potassium sulfate is produced according to the followingreaction, which is conducted in so-called Mannheim furnaces:

2 KCl + H2SO4 → 2 HCl + K2SO4

The Hargreaves process uses sulfur dioxide, oxygen and water and potassium chloride as the starting materials to produce potassiumsulfate. Hydrochloric acid evaporates. SO2 is produced through theburning of sulfur.

Structure and properties

Two crystalline forms are known. Orthorhombic β-K2SO4 is the common form, but itconverts to α-K2SO4 above 583 °C. These structures are complex, although the sulfate adoptsthe typical tetrahedral geometry.


Structure of β-K2SO4.


Coordination sphere of one oftwo types of K+ site.


SO4 environment in β-K2SO4.

It does not form a hydrate, unlike sodium sulfate. The salt crystallize as double six-sided pyramids,classified as rhombic. They are transparent, very hard and have a bitter, saltytaste. The salt is soluble in water, but insoluble in solutions of potassium hydroxide (sp. gr. 1.35), or in absolute ethanol.

Uses

The dominant use of potassium sulfate is as a fertilizer. K2SO4 does not contain chloride, which can be harmful to some crops. Potassiumsulfate is preferred for these crops, which include tobacco and some fruits andvegetables. Crops that are less sensitive may still require potassium sulfatefor optimal growth if the soil accumulates chloride from irrigation water.

The crude salt is also used occasionally in themanufacture of glass. Potassium sulfate is also used as a flash reducer in artillery propellant charges. It reduces muzzle flash, flareback and blast overpressure.

It is sometimes used as an alternative blast mediasimilar to soda in soda blasting as it is harder and similarly water-soluble.

Reactions

Acidification


Potassiumhydrogen sulfate (also known as potassium bisulfate), KHSO4, is readily produced bymixing K2SO4 with an sulfuric acid. It forms rhombic pyramids, which melt at 197 °C (387°F). It dissolves in three parts of water at 0 °C (32 °F). The solution behavesmuch as if its two congeners, K2SO4 and H2SO4, were present side by side of each other uncombined; an excess of ethanolthe precipitates normal sulfate (with little bisulfate) with excess acidremaining.

The behavior of the fused dry salt is similar whenheated to several hundred degrees; it acts on silicates, titanates, etc., the same way as sulfuric acid that is heated beyond its natural boiling point does. Hence it isfrequently used in analytical chemistry as a disintegrating agent. Forinformation about other salts that contain sulfate, see sulfate.

Reduction

At high temperatures, it is reduced to potassiumsulfide by the action of carbon monoxide.