Potassic fertilizer

K fertilizers

 POTASSIC FERTILIZERS

         The Latin / German word for the element with symbol K is" Kalium". It isderived from the Arab word "el-kali" means ash (Potash in English, Potassic in French).

         The term potash generally refers to any salt used to provide fertilizerpotassium . In early days, potassium carbonate was produced from solutionsleached form wood ashes evaporated in iron pots, hence term potash meaning"pot ashes" .The crude potassium salts ( K-minerals with impurities ) were found in theprocesses of drying up of sea water in former ocean basins which occur largelyduring the permian period some 200 million years ago .The sea water saltscrystallized in the order of solubility i.e., (common salt ) NaCl, strata are overlain by K-minerals . In course of time they were covered by many geological strata and converted to a hard rock . Crude potassium salts are thus natural sea waterminerals .

         Potassium content of earth crust is 2.40 % ,whereas the content ofphoshorus is only 0.11% .Potassium reserves are large .The large deposits ofpotassium salts are in North America (Canada,USA) , USSR, West Germany,East Germany and France in that order .The important potash minerals of theworld occurring as deposits and their chemical composition are given

Principal potash minerals of the world

S.NO. Mineral                   Chemical formula                         Approx. Content of K (%)

1      Carnallite                 KCl MgCl2 6H2O                                        14.10

2      Kainite                          KCl Mg SO4 3H2O                                     15.7

3      Langbeinite             K2SO4 2 Mg SO4                                          18.8

4      Nitre                        KNO3                                                          38.6

5      Polyhalite                K2SO4 MgSO4 2CaSO42H2O                 12.9

6      Sylvite                     KCl                                                             52.4

7      Sylvinite                        KCl NaCl                                                    29.4

         All the fertilizers potassium viz., KCl and K2SO4 used in India is importedas on today. Recently Geological Survey of India has reported the occurrence of K-deposits in Leh (Jammu and Kashmir) and Bikanir (Rajasthan)which await commercial exploitation . Potassium schoenite a double salt of K2SO4 and MgSO4 obtain from salt bitterns is recently being produced on a pilot scale at Bhavanagar (Gujarat).

Manufacturing processes of potassic fertilizers

         Two types of potassium fertilizers are widely produced in India

I.Potassium chloride (Muriate of potash)

II. Potassium sulphate (Sulphate of potash )

I. POTASSIUM CHLORIDE (KCl) [MOP] :

         Potassium chloride is popularly known as muriate of potash [MOP] .The termmuriate is derived from muriatic acid ,a common name for hydrochloricacid(HCl)) .It is the most important K-fertilizer used directly or in conjunctionwith P and N fertilizers.

Raw materials:

1. Sylvinite (or) brine

Unlike phosphate rock , K mineral salts do not require heat or strong acidtreatment as they are water soluble. Potassium chloride is recovered, from sylvinite adopting either of the two processes viz.,

1. Crystallization process

2. Flotation process

1. Crystallization process:

Principle: Crystallization process employed in separating potassium chloridefrom sodium chloride (Sylvinite: KCl .NaCl) is largely dependent on their differentsolubilities in hot (100 0C) and cold (20 0C) water . The solubility of KClincreases rapidly with a rise in temperature whereas the solubility of NaClvaries very slightly .Cool brine (20 0C) saturated with both the slats is heated to (100 0C)and passes over the finely ground sylvinite ore, when KCl rich brine solution isobtained .It is cooled by vacuum evaporation which produces KCl crystals,which are centrifuged , washed ,dried and packed .The filtrate (brine rich NaCl)is recycled for treating fresh ore.

2. Flotation process:

         Separation of potassium chloride from its ore by mineral flotation is widelypracticed allover the world relative to the crystallization process.

Principle: Flotation is a separation process in which a solid treated with a selective agent is suspended in an aerated aqueous liquid. The treated solid adheres to the bubbles of air, and the froth is floated off the suspension. Non floating material is removed as pulp. The sylvinite ore is a mixture of interlocked crystals of potassium chloride and sodium chloride plus small quantities of clay and other impurities.

Raw material : Sylvinite ore

Sylvinite ore is ground to a particle size of 10 mesh

·         Pulped in a saturated NaCl-KCl brine and scrubbed (Centrifuge) to disperse clay and other impurities .

·         The resultant slurry is thus deslimed in spiral classifiers to remove finely divided clay slimes.

·         Deslilming or removal of clay is the most important step since these material consume large quantities of flotation reagents.

·         The deslimed slurry relatively free of clay is treated with binding agents such as starch or mannogalactan gums

·         The reagents used for sylvinite flotation is a mixture of primary aliphaticamine salts derived from beef tallow ,which selectively film the sylviniteparticle so that they will float

·         The conditioned slurry is carried to flotation cells where air is drawn into the slurry.

·         The air bubbles attach themselves to the reagent treated sylvinite particle in the pulp causing them to float to the surface as froth,which ismechanically skimmed off by paddles

·         The concentrate rich in KCl is further purified by sending it to cleaner cells to remove residual NaCl

·         The MOP is separated from brine by centrifuging and drying in rotary driers ,screened to a desirable particle size and finally sent for productstorage

Physical properties of MOP:

1) MOP in pure form is white crystalline salt. However, colour ranges from white to red based on the impurities present in K minerals and methods ofrefinement.

2) It has a solubility of 37 grams per 100 grams of water at 30 0C.

3) The crystalline MOP is not very hygroscopic,flows freely and does not cake

4) Has specific gravity is 1.98.

Chemical properties of MOP:

1) MOP contains 58% by weight of K2O and about 47 % chloride

2) MOP is neutral in reaction and does not produce acidity and alkalinity on soilapplication.

II. POTASSIUM SULPHATE(K2SO4) [SOP] :

The entire potassium sulphate fertilizer used in India is imported and it is more expensive than MOP.

Raw materials: 1) Langbeinite

The manufacturing process is of two types viz.,

1. Langbeinite process

2. Mannheim furnace process

1. Langbeinite process:

The ore Langbeinite is a double sulphate of potassium and magnesium (K2SO4 2Mg SO4).

In this process Langbeinite is ground and dissolved in water and concentrated solution of KCl (brine) is added, when the potassium sulphate precipitates and is separated by centrifuging. The wet material is dried,screened and sent to storage .The chemical reaction involved is

         K2SO4 2Mg SO4 +4KCl → 3 K2SO4 +2MgCl2

2. Mannheim furnace process:

         In this process, potassium chloride (KCl) is reacted with sulphuric acid in a special furnace provided with rotary plough to form potassium bisulphate (KHSO4) in exothermic reaction and to form potassium sulphate in endothermic reaction. HCl gas is evolved as a by product which is cooled and absorbed inwater.  Reactions that occur are

         KCl +H2SO4 → KHSO4 + HCl→ [Exothermic reaction]

         KCl + KHSO4→ K2SO4+ HCl → [Endothermic reaction]

Physical properties of SOP

         1. Potassium sulphate [SOP] is a white crystalline salt, less hygroscopic ascompared to MOP and free flowing.

         2. It has specific gravity of 2.66 and has a solubility of 13 g/100 grams of water at 30 0C.

Chemical properties of SOP:

1. SOP contains 48 % K2 O and 18 % S by weight.

2. Like MOP, it is also neutral salt providing neither acidity nor alkalinity on soil application.

3. The SOP is widely desirable .The chloride content of KCl effects the burningquality of tobacco .Never use KCl (MOP) on tobacco crop.Characteristics of K fertilizers 

     Potassium content of potassic fertilizers is usually expressed as potassium oxide (K2O) referred as potassium.  Potassium fertilizers are soluble in water, but non hygroscopic in nature.  Nutrients of these fertilizers are readily available to the plants.  The fertilizers have little (or) no effect on soil pH as they are neutral in reaction.  As a mobile cation , K is readily susceptible to leaching. Most of the Indian soil contains a sufficient amount of K. Potassic fertilizers should therefore be applied only to such soils are definitely known to be deficient to K (or) those which respond to K application such as sandy soils. Potassium is more widely distributed and less frequently deficient in soils than N and P. 

(a) Muriate of potash (Potassium chloride) 

             Potassium chloride is the most common K fertilizer having K in the chloride form (KCl). It contains 50-60% K2O, the whole of which is readily available to the plants. It is highly soluble in water and it ionizes (to dissociated into) K+ and Clin the soil. In the case of crops, like tobacco, potato, coffee, tea and grapes, application of large amount of KCl reduces their quality. In such cases, the K has to be applied as K2SO4. KCl is prepared from ore such as silvite, carnallite (KCl MgCl2, 6H2O), kainite (KCl MgSO4 3H2O) etc. by crystallization and flotation method. 

Uses 

       Potassium chloride is suitable for a wide range of soils and crops. But in case of crops like potato, tobacco, tea, coffee and grapes large dose of KCl are considered to lower the quality. So that for these crops, K has to be supplied as K2SO4. It can be applied at sowing or prior to sowing. Now a days application of this fertilizer as a topdressing is as good as N fertilizer. 

Production of MOP (KCl) 

        Most of the chloride ores contains KCl, NaCl, MgCl2 and MgSO4. Hence, muriate of potash is produced by refining these ores where sodium chloride, magnesium chloride and magnesium sulphate are removed. 

(i) Removal of NaCl 

    Sodium chloride is removed by the processes as discussed below: 

(a)Fractional crystallization 

  The ore such as sylvinite is mixed with hot brine to dissolve all its potassium chloride. Its sodium chloride component does not dissolve and remains as solid. Solid sodium chloride is separated. The hot solution of potassium chloride is then cooled to crystalize potassium chloride. After separation of the crystals the mother liquor is reused to dissolve potassium chloride from ore.

 (b) Froth floatation 

The ore such as sylvinite is crushed to separate potassium chloride and sodium chloride crystals and other salts. It is then mixed with brine and treated with aliphatic amine acetate to form potassium chloride crystals. Then air bubbles are formed. The filmed potassium chloride crystals is dried and screened to have different sizes of potassium chloride crystals. The sodium chloride particles accumulate at the bottom. They are collected and separated from brine. The brine is reused in the process.

 (ii) Removal of magnesium chloride 

The ore, such as carnallite is mixed with brine to separate its components. MgCl2 is then removed by bleeding with brine.

 (iii) Removal of magnesium sulphate

 Magnesium sulphate such as Kieserite and sodium chloride are separated from ore by Hardselz refining process. As magnesium sulphate dissolves very slowly and sodium chloride dissolves rapidly, sodium chloride is separated by dissolving it with water. Magnesium sulphate is left as solid.

 Potassium sulphate (K2SO4)

 It is also called as sulphate of potash. It contains about 50 per cent K2O and 18 per cent S. Because its chloride content is below 2.5 per cent, it is used for Clsensitive crops such as tobacco, potato, tea, grapes etc. It accounts for about 6 per cent of total agricultural K sales. K2SO4 can be used where Cl buildup becomes a problem. 

Production of K2SO4 

       The K2SO4 is produced by several processes as follows

 (i) Decantation process (or) Langbeinite process

 Langbeinite is treated with concentrated solution of KCl to yield precipitates of K2SO4. K2SO4.2MgSO4 + KCl 3K2SO4 + MgCl2 The precipitates of K2SO4 is separated from solution by decantation. As Langbeinite is used, the process is termed as Langbeinite process and as decantation is involved in the process, it is also termed as decantation process.

 (ii) Glaserite process

 In this process, Na2SO4 is reacted with KCl to yield K2SO4. Na2SO4 + 2KCl K2SO4 + 2NaCl This reaction occurs in 2 steps as follows: a)Here an intermediate compound, glaserite is formed 4Na2SO4 + 6KCl Na2SO4+ 3K2SO4 + 6NaCl b) Here KCl reacts with glaserite to yield K2SO4 2KCl + Na2SO4.K2SO4 2NaCl + 4K2SO4

 (iii) Manheim process

 Sulphuric acid reacts with KCl to yield K2SO4 H2SO4 + 2KCl K2SO4 + 2HCl (iv) Hargreaves process Sulphur is burnt to yield sulphur dioxide. S + O2 SO2 Sulphur dioxide is reacted with air (O2), water vapour (H2O), and hot KCl to yield K2SO4. SO2 + ½ O2 + H2O + 2KCl K2SO4 + 2HCl 

Properties of K2SO4 

              1. It’s colour is white

             2. It is readily soluble in water 

              3. It contains 41.6 to 44.1 per cent K (or 50-53 per cent K2O). It is usually 90-95 per cent K2SO4.

              4. It is a neutral salt and neutral fertilizer. 

Sulphate of potash – Magnesia (K2SO4.MgSO4) (or) Potassium schoenite

  It is also called as potassium magnesium sulphate. It contains about 22 per cent K2O, 11 per cent Mg and 22 per cent S. It cocurs in nature as the mineral, langbeinite, which is refined to the commercial fertilizer product. It is a good source of water soluble K and Mg. It is a very important fertilizer where Mg and S are deficient. 

Potassium nitrate 

It is also known as salt petre. It is a white crystalline solid. It contains 13% N, 46-58% K2O.

 Potassium metaphosphate (KPO3)

      It contains 39.8% K2O and 60% P2O5. It is soluble in water and available to the plants.

 Reaction of K fertilizers in soil 

Both chloride and sulphate of K are soluble in water and on application to the soil, they ionize into K+ , Cland SO4 2- ions. The released K+ from the fertilizer gets adsorbed on the soil colloids and also available to the plants through cation exchange reactions. Based on the chemistry of chloride in the soils, it is found that under acidic soil conditions, Clreplaces OHassociated with the free ion oxides and therefore, in such soils, MOP likely to give a greater response than K2SO4. Besides the Clare less strongly retained on soil colloids than SO4 2- ions. In alkaline soils, when MOP (KCl) is applied, the accumulation of Clcreates toxicity to plants. So in K deficient soils with alkaline reaction, it should be applied along with organic matter. The application of potassic fertilizers has little (or) no effect on soil reaction.