- Potassium nitrate
- Monopotassium phosphate (MKP)
- Various K/Mg mixes/crystals
- Chemical grade potash
The third most widely used potassium salt in agriculture is potassium nitrate. An ore (caliche) containing sodium nitrate, potassium nitrate, some chlorides, and sulphates is mined in Chile; Figure 1. The ore is leached, and part of the sodium nitrate is crystallised and removed. Potassium chloride is added to the brine, and the resulting sodium chloride is separated. Cooling to 5°C crystallises the potassium nitrate, which is then recovered with centrifuges. Residual brine is concentrated by solar evaporation to remove salt and sodium sulphate, more KCl is added, and KNO3 is again crystallised. KCl or K2SO4 may also be used in fractional crystallisation processes with NH4NO3 or Ca(NO3)2 to produce potassium nitrate.
Potassium nitrate is also made synthetically, Figure 2. Nitric acid reacts with potassium chloride according to the equation: KCl + HNO3 -> KNO3 + HCI. Several variations on this process (different temperatures, ion exchange, etc) have been applied. The limiting factor is the production of by-product HCl solution which must find a home in a downstream product (PVC, phosphoric acid) since otherwise authorities will not give a license to operate.
Major uses of potassium nitrate are in fertilizers, rocket propellants and fireworks. It is one of the major constituents of gunpowder (black powder). Potassium nitrate is used in fertilizers as a source of nitrogen and potassium – two of the macronutrients for plants; its fertilizer designation is 13-0-44. Most forms of potassium nitrate are considered an oxidizer.
Potassium nitrate is moderately soluble in water, but its solubility increases with temperature. The aqueous solution is almost neutral, exhibiting pH 6.2 at 14 C (57 F) for a 10% solution of commercial powder.
In the far past, a major natural source of potassium nitrate was the deposits crystallizing from cave walls and the accumulations of bat guano in caves.
Monopotassium phosphate (MKP)
MKP is a fully soluble P+K containing salt which is used for hydroponics (green house) applications with very high purity. It is either produced from KOH + H3PO4 feed grade or from KCl + H2SO4 + rock phosphate (ICL process). The final product is a fine powder.
K-struvite is a product that comes from the precipitation of P2O5 and K-rich waste steams (for example calf manure, but also industrial waste streams) after the addition of a soluble Mg-salt (for example, MgCl2). The product is not water-soluble but is plant available (P2O5 soluble in neutral ammonium citrate solution) and therefore an interesting Circular Economy product (as is ammonium struvite, NH₄MgPO₄·6H₂O). Depending on the waste stream composition, the product can vary in composition and might contain some contaminants from co-crystallised mother liquor.
Various K/Mg mixes/crystals
Various minerals are based on combinations of MgCl2, MgSO4, KCl, NaCl, K2SO2 etc. If deposits of these minerals are found in rather pure form, they can be exploited and brought to the market as inorganic fertilisers, creating an added value. Examples of such materials are polyhalite and langbeinite. These products can be in powder form or as coarse oversize in the dimensions of granular fertilisers (2-5 mm).
In 2021 polyhalite was approved for usage in organic farming by the Control Union organisation, which has confirmed that both the standard and granular grades comply with both European and US regulations and can also be accepted in other parts of the world. This is because polyhalite is available in its natural state and is mined, crushed, screened and bagged, with no additional chemical separation or other industrial processes.
Chemical grade potash
About 15% of potash production is used in industrial applications. The industrial potash (chemical grade) has a higher purity than fertiliser-grade potash. Most of the chemical-grade potash is used for the production of potassium hydroxide. Potassium hydroxide (KOH) and its derivative potassium carbonate are the next largest industrial potassium compounds. The KOH is made by the electrolysis of KCI in installations like caustic soda/chlorine production (now membrane type, in the past mercury type). The main use of caustic potash is in the manufacture of liquid soaps; textile operations; production of grease, catalysts, alkaline batteries, electropolishing, and rubber production.
Anonymous. 1991. “SOQUIMICH Increases its Potassium Nitrate Capacity,” Phosphorus and Potassium, 173:37-40.
Crozier, R. D. 1981. “Chilean Nitrate Mining,” Mining Magazine, September, pp. 160-173.
Hager, T. (2008). The Alchemy of Air. Harmony Books, New York, United States of America. ISBN 978-0-307-35178-4. OCLC 191318130.
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