Contents
Potassium sulphate is the second largest tonnage potassium compound, and it is also used primarily as a fertiliser. The sulphate or other non-chloride forms of potassium are preferred for certain crops that do not tolerate the chloride ion well, e.g., tobacco and some fruits and vegetables. Non-chloride potash sources are also needed in areas where chloride accumulation in the soil is a problem. This is important in arid areas where chloride salts from irrigation water accumulate or in areas of very intensive agriculture, e.g., in the Netherlands. Potassium sulphate may be preferred because of its sulphur content where soils are deficient in both potassium and sulphur.
Potassium sulphate production can be split in two main lines of processes:
Mannheim process
Historically potassium sulphate has been made primarily from KCl and sulphuric acid in this process (and a small amount from KCl and SO2) when the by-product HCl was the dominant product. However, over the years the HCl market has had more competition, and “natural” K2SO4, with lower capital and operating costs has begun to dominate its production in some countries with natural complex salts.
The reaction is two-stage:
a. Exothermic reaction
KCl + H2SO4 -> KHSO4 + HCl
b. Endothermic reaction
KHSO4 + KCl -> K2SO4 + HCl
The potassium chloride reacts during slow mixing in the heated Mannheim furnace with sulphuric acid, producing gaseous HCl and K2SO4. The furnace is heated by natural gas or fuel oil. The product K2SO4 is cooled in a cooling drum. Lump material from the cooler is crushed and finished or can be compacted and granulated as with KCl. The K2O unit-price of agricultural grade SOP is twice as high as for MOP.
The HCl gas is cooled in a graphite heat exchanger and absorbed in water in two stages to produce 33% hydrochloric acid as a by-product. The process gives an excellent quality of potassium sulphate that contains over 50% K2O and less than 1-2% chloride. Nowadays the by-product HCl application can be a limiting factor since authorities will not accept HCl effluent coming from the process anymore.
Recovery of potassium sulphate from natural complex salts
The chief natural complex salts that are the source of the potassium sulphate are:
• Kainite (KCl•MgSO4•3H2O)
• Langbeinite (K2SO4•2MgSO4)
The natural process involves the initial conversion with recycled K2SO4 end liquor of “mined” kainite (from the solar evaporation of Great Salt Lake brines, or deposits in Sicily and Carpathia), or langbeinite (from Carlsbad) to form an intermediate product, schoenite (or leonite if done hot). All processes are based on intracrystalline reactions of ion exchange.
The basic reactions leading to potassium sulphate from kainite by transformation of the kainite into schoenite and later water leaching are:
2KCl•MgSO4•3H2O -> K2SO4•MgSO4•6H2O + MgCl2
K2SO4•MgSO4•6H2O -> K2SO4 + MgSO4 +6H2O
The Italkali’s Pasquasia potassium sulphate process based on the latter alternative comprises four basic units, Figure 1:
• Preparation of the ore and flotation.
• Production of schoenite and its recovery.
• Leaching of the schoenite to potassium sulphate.
• Liquor treatment.
The kainite is repulped with recycled brine, screened, and directed to ball mills and hydro classifiers. Overflows go to a thickener and main filter and underflows to flotation and filtration. Raw material, after filtration, is combined with the solid fraction from the main filter and directed to the schoenite reactors and separating cyclones. After a two-step hydro separation, centrifugation, and filtration, schoenite is directed to the leaching reactors. After decomposition of the schoenite, product is directed to a thickener. The underflow solids are sent directly to final centrifuges and a dryer; the overflows are cooled and crystallised. After additional thickening, the product is centrifuged and dried. The product specification ensures that the K2O content is not lower than 50% and the chlorine content is less than 1%.
In the recovery unit, Italkali forms syngenite [K2Ca(SO4)2•H2O] to moderately increase the recovery of schoenite from the plant’s end liquor. The brine from the schoenite filter is reacted with recycling gypsum. The schoenite formed is combined with main flow to the leaching reactors.
Other processes involve adding sylvite to kainite, langbenite, kieserite, etc. The schoenite intermediate can be formed by reacting KCl with mined kieserite (MgSO4•H2O) or with epsomite. A similar intermediate, glaserite, is formed from the reaction of KCl with salt cake or Glauber salt, Na2(SO4)•10H20 (Searles Lake, Spain, Canada). Either double salt can be transformed into pure K2SO4 by a partial leach or in greater yield by reacting it with additional KCL Where solar or plant evaporation can be done economically, the yields can be further improved by evaporating the schoenite or glaserite end liquor and recycling the salts.
The reactions are as follows:
a. by mixing kainite and sylvite
KCl + KCl•MgSO4•3H2O -> K2SO4 + MgCl2 +3H2O
b. by mixing sylvite with kieserite and other magnesium salts to give kainite.
KCl + MgSO4•H2O + 2H2O -> KCl•MgSO4•3H2O
The production of potassium sulphate from langbeinite is possible with a large amount of muriate of potash by mixing langbeinite and sylvite:
4KCl + K2SO4•2MgSO4 -> 3K2SO4 + 2MgCl2
2KCl + 2(K2SO4•2MgSO4)-> 3(K2SO4•MgSO4) + MgCl2
The Bureau of Mines (U.S.A.) developed the process, Figure 2.
The langbeinite ore is separated from sylvite and halite by selective washing, froth flotation, or heavy media separation. The commercial langbeinite used in the process must be pulverised in ball mills, and fine powder is mixed with a solution of the muriate of potash. The muriate of potash is dissolved and clarified in a separate unit. The reaction in the presence of water yields potassium sulphate in a crystalline form and brine. Crystals are centrifuged or filtered, dried in a rotary dryer, sized, and finished. The finishing methods either produce coarse material or granulated product. The brine is evaporated, crystallised, and filtered. The mixed salts are added to the sulphate reactor. The liquor is discarded as a waste.
References
Searls, J. P. 1991. “Potash,” Mineral Industry Surveys, Bureau of Mines, U.S. Department of Interior, Washington, D.C, U.S.A.
Hancer, M. and Miller, J. D. 2000. “The flotation chemistry of potassium double salts: schoenite, kainite, and carnallite, Minerals Engineering, Vol. 13, issues 14 – 15, pages 1483–1493
Links to Related IFS Proceedings
297, (1999), Tailoring Potash to the Needs of the Fertiliser Industry, H. Rug, K. Kahle
857, (2021), Mineral Sizing Journey from Pit to Port and its Influence on Fertiliser Products, R McConnell
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