Potash industry

Potash industry

Previous to the invention of the Leblanc Soda Process, the most important alkali was potassium carbonate, - potash, which was nearly all derived from wood ashes. But with the development of the soda industry, the demand for potash was greatly diminished, and at the present time, soda has replaced it for all except a few special purposes.
The chief sources of potassium salts now are:-

Wood ashes.
Beet-sugar molasses and residues.
Wool scourings. (Suint.) .
Stassfurt salts.

Land plants take up considerable quantities of potassium compounds from the soil. When the plants are burned, about 10 per cent of the weight of the ashes is potassium carbonate,* which may be obtained by lixiviation. Potash from wood ashes is now chiefly made in Russia, Sweden, and America, the woods most employed being elm, maple, and birch. Sometimes the stumps and small branches only are burned, the trunks being used for timber. The ashes are moistened slightly, put into tanks having false bottoms on which straw is spread, and then lixiviated with warm water. The lye so obtained is evaporated (sometimes by the waste heat from the burning wood) in iron pots until it solidifies on cooling. The dirty brown mass is then calcined in a reverberatory furnace until all the organic matter is destroyed. The product is known as potash or crude pearlash. It is white or gray in color, and contains about By redissolving the crude potash in water, settling and concentrating the solution until the sulphates and chlorides separate as crystals, a concentrated and pure lye is obtained. When this is evaporated to dryness and the residue calcined, it yields a much purer product, known as "refined pearl ash," and containing from 95 to 97 per cent of K2COS' It is necessary that a low heat be employed in the calcination, since the charge fuses at a moderate temperature.
Often, some quicklime is put in the bottom of the tanks before the ashes are introduced. On leaching, the solution of potassium salts reacts with the lime, forming insoluble calcium salts, and yielding more or less potassium hydroxide in the lye. The resulting product is then a mixture of potash and caustic potash.
In the manufacture of beet sugar, a very impure molasses remains, containing among other things a large amount of soluble potassium salts. This molasses is now generally fermented, ill which process the sugary ,substances are converted into alcohol, which is distilled off, leaving the mineral salts in the liquid residue, called 'cinClsse or schlempe. If this is evaporated to dryness and the mass calcined, the organic potassium salts are decomposed, leaving in the cinder about 3,3 per cent potassium carbonate, and a .large amount of chloride and sulphate, together with sodium salts. If the vinClsse be evaporated to dryness and the residue destructively distilled in retorts, a distillate is obtained, containing organic compounds of which methyl alcohol CH30I-I, ammonia, and tri- /CHs methylamine, :x - Cll3 are valuable. The cinder in the retort contains potassium salts, which are obtained in solution by lixiviation, and a considerable quantity of potash is thus recovered. Very often, however, the ash is used as a fertilizer, thus returning the potash to the soil. ,y001 scourings furnish some potash in countries where much wool is washed. Sheep's wool as it comes from the animal contains from 30 to 75 per cent of its weight of impurities, consisting of dirt, sand, dung, etc.; ~vool grease or "yolk," a fat-like substance, made up of cholesterine and compounds of it with oleic, stearic, and palmitic acids; and "suint," which consists chiefly of potassium salts of oleic, stearic, and other organic acids, with small quantities of chlorides and sulphates and nitrogenous matter. The" suint" exudes from the animal ill the perspiration, and is deposited on the wool bye evaporation. It is soluble in cold water, and is thus removed in the scouring process. If these wash waters, containing wool grease and suint, are run into the drains or streams, pollution of the water results. The prevention of this nuisance, as well as the value of the potash, has necessitated attempts to dispose of the washings in some economical manner, and they are usually evaporated to dryness and calcined. If the calcination is done in closed retorts, a considerable quantity of ammonia is obtained. The cinder is lixiviated, and on evaporation, the solution yields, first, chlorides and sulphates of potassium and sodium, and finally a very pure potash, which averages a little less than -1pel' cent of the weight of the raw wool scoured.
For the recovery and treatment of wool grease.
This utilization of wool grease and suint is mainly practised in France, Belgium, and Germany, and in these countries this is done chiefly to prevent the pollution of the streams. Cheap fuel is very essential to a successful working of the process. On a small scale it cannot be carried on profitably, and the wash waters are often run onto the fields as fertilizer.
For potassium carbonate from potassium chloride, see p. 135. By far the most important source of potassium compounds at the present time is the great natural deposit of potassium salts found at Stassfurt and Leopoldshall, near :Magdeburg, Germany. This consists of immense beds of various salts, which have been deposited from sea water. They were discovered in attempting to reach the underlying rocle-salt, but because of the large proportion of potasshun and magnesium chlorides, the material was at first thrown aside as worthless, the name applied to it, -" abmuinsalze," - indicating the small value attached to it. But in 1861-4 methods were devised by which potassium chloride and sulphate could be obtained cheaply from the Stassfurt salts, and since these furnish a valuable source for nearly all other potassium salts, a rapid development of the industry followed.
Sea water contains about. 3.5 per cent of solids, consisting of: - 70..!Oper cent· 10.20 "
0.51 "
3.m "
l.08 "
Sodium chloride .
Magnesium chloride
Magnesium sulphate
Calcium sulphate
Potassium chloride .
Magnesium bromide
Calcium bicarbonate, etc .. :} 0.85 "

salts, together with various double salts, formed by mutual interreactions, crystallize in the order of their relative insolubility. 'fhe Stassflll't deposit was undoubtedly formed by the evaporation of sea water, under peculiar eonuitions. The mode of formation has been studied by many investigators, to whose memoirs the reader is referred for full explanations.* The deposit is nearly 3000 feet thick, and about 1Gdifferent salts have been identified in the various strata. The more important salts and their composition, are given below:- Gypsum .
I~ainite .
Kieserite .
Sylvine .
'fachydri te
Roracite .
Astrakanite .
CaS04 . 2 H20
K2S04, l[gS04, lIIgCl2· 0 H20
KCl, :lgCl2 • 0 H20
11gS04• H20
K2804, l!gS04, 2 CaS04 .2 H20
CaCI2, 211gCl2 • 12 H20
2 (:'IlgsRsOI") + :lgCl2
:lgS04, Na2S04' 4 H20
K2S04, llgS04' 0 H20
The beds are not sharply defined layers of separate salts, the deposit being generally regarded as containing four principal" regions." The rock-salt 01' anhydrite region is the lowest of these. This consists of thin layers of very pure rocle-salt, separated by narrow strata (one-fourth of an inch thick) of anhydrite. The anhydrite is separated from the salt mechanically, and the latter is then ground for use directly. This bed is nearly 2000 feet thick in places. The poly halite region, about 200 feet thick, is above the rocksalt region. It is composed of 91 per cent of rock-salt, and Gt per cent of polyhalite, with smaller quantities of other salts. The kieserite region, lying next above, is about 185 feet thick, and contains G5 pel' cent rocle-salt, 17 per cent of kieserite, 13 per cent carnallite, and 5 per cent of other salts. The carnallite region lies nearest the surface, and is about 140 feet thick. This is the most important and contains:- Carnallite
Kieserite .
55-00 per cent
20-25 per cent
10 per cent
4 per cent

In parts of this region, changes have taken place action of water, by which considemble deposits of sylvine have been formed. The composition of mw about as follows: -
through the kainite and carn:1llite is II Potassium chloride 16.2 per cent 15.7 per cent Magnesium chloride 21.3 " 21.8
Sodium chloride 18.7 " 21.5 "
Calcium chloride 0.2 " 0.3 "
Magnesium sulphate 9.7 " 18.0
Calcium sulphate 2.1 " 00.0 "
Water 28.8 " 26.2
Insoluble 00.0 " 2.0
The crude carnallite is often colored a deep red by the presence
of iron compounds.
The present commercial supply of potassium chloride, and incidentally of other potassium compounds, is obtained from carnallite. The crude material is treated with the hot mother-liquor from a previous lot, in an iron kettle having a stirring apparatus and a false bottom. This mother-liquor contains about 20 per cent JIgC12, which prevents the solution of the rock-salt and kieserite, but does not hinder the dissolving of the carnallite. The action of the magnesium chloride solution is continued until the hot liquor reaches a density of 1.32 sp. gr., when it is drawn off from the sludge and allowed to cool slowly. At this density, the greater part of the potassium chloride crystallizes on cooling, leaving the magnesium chloride and some potassium chloride still in solution. This liquor is then further concentrated, until it contains about 30 per cent magnesium chloride. On cooling, crystals having the composition KCl, lfgCl2 • 6 H20, - artificial carnallite, - separate, leaving only the excess of magnesium chloride in solution. The artificial carnallite is decomposed with water, and the potassium chloride crystallized out, leaving the magnesium chloride in solution; a part of this liquor, diluted with the wash water from the sludge, is used to extract the next portion of mw carnallite. The potassium chloride is washed with a small portion of very cold water, to remove the common salt.
The residue from the solution of the raw carnallite consists largely of kieserite mud C~IgS04 . H2O), which is insoluble in water; but on standing for some time in contact with water, it passes over into the soluble Epsom salts Ci[gS04 . 7 H20). At an intermediate stage of the hydration, the mud solidifies in a manner similar to plaster of Paris when mixed with water. ·When this solidification is about to take place, the mud is moulded into blocks, which become very hard, and in which form it is shipped. But after some time they take up moisture from the air, and fall to a powder of Epsom salt.
Glauber's salt is made at Stassfurt in the winter time as fol. lows: Solutions of common salt and magnesium sulphate (e.g. from kieserite) when kept below 0° C. will react together, thus:- :JIgS04 + 2 NaCI = ::IgCI2+ Na2S04, and at the low temperature, the sodium sulphate crystallizes to form
Na2S04• 10 H20.
Kainite (K2SO., :JIgS04, ?IgCI2• 6 H20) is extensively used in the
crude state as a fertilizer. Some of it, however, is treated for
potassium sulphate, by the method of H. Precht. ,Then heated
with water under pressures of four or five atmospheres, kainite
decomposes into a double potassium.magnesium sulphate, magnesium
chloride, and potassium chloride, thus:-
3 (K2S04, lIgS04, lIgCI2• 6 H20) =2 (K2S04, 2 lIgS04 • H20) + 21IgC12+ 2 KCI + 16 H20. The double potassium-magnesium sulphate separates in crystals, and is freed from chlorides by washing; during the washing, one molecule of the magnesium sulphate is also removed, and a salt of the composition, K2S04, lIgS04, remains. This is dried and calcined, and sold as double potassium-magnesium sulphate; or it may be decomposed directly by treating with a solution of potassium chloride of 1.142 sp. gr. :-
K2S04, lIgS04 + 2 KCI = ?IgCl2 + 2 K2S04•
The potassium sulphate is separated from the magnesium chloride by crystallization.
Potassium sulphate, made from kainite as above, or by the action of sulphuric acid on potassium chloride, is largely used as a fertilizer and for the manufacture of potassium carbonate. Potassium chloride, chiefly obtained from carnallite, is extensively used for preparing other potassium salts, especially the nitrate, sulphate, and carbonate. Potassium carbonate or potash is made from potassium chloride by the Leblanc process, in the same way as soda-ash from salt. But the ammonia process cannot be employed, because the acid carbonate not precipitate.
Potassium carbonate is sold in trade under the name of potash or pearl ash, and is used chiefly in the glass industry, for caustic potash and for chromates of potassium. A considerable quantity is bought by soap makers, and causticized, the solution being used for soft soaps.
Caustic Potash is made in the same way as caustic soda. The mother-liquors from the black-ash lixiviation are decomposed directly with slaked lime. Caustic potash is much more deliquescent than caustic soda, and is generally made where it is to be used. In soap making, it was formerly customary to saponify the fat with caustic potash, and then to add common salt. An interchange between the potasslllm and sodium took place, and a hard sodium soap resulted. But as soda is now cheaper, and yields a hard soap directly, potash soaps are only used for special purposes. Potassium nitrate. (See p. 120.)
Potassium bichromate (K2Cr;0;) is made by roasting chromite (a native oxide of chromium and iron) with potash, lixiviating the fused mass with water, and adding enough sulphuric acid to convert the neutral potassium chromate into bichromate. The reactions involved are as follows:-

Cr203 +3 0 = 2 Cr03 ;
Cr03 + K2C03 = K2CrO. + CO2;
2K2Cr04 +H2S04 = K2S04 + K2Cr20; +Hp.

The finely powdered chrome ore is mixed with lime and potash, and roasted at a bright red heat, with free access of air and frequent stirring. After several hours the chromic oxide is all oxidized to chromium trioxide (Gr03), which combines with the lime and potash to form neutral chromates of calcium and potassium. The mass is then treated with a hot solution of potassium sulphate, which forms potassium chromate from the calcium chromate. The solution of neutral 'potassium chromate, when saturated, is drawn off and settled. It is then decomposed in lead-lineu tanks, by the addition of sulphuric acid. Since potassium bichromate is very much less soluble in cold solution than the neutral chromate, about threefourths of the total amount of bichromate formed precipitates. The remaining liquor, containing potassium sulphate, is used to leach a new portion of cinder. The precipitated bichromate is recrystallized from water.
The addition of lime to the furnace charge is necessary to prevent the fusion of the mass, and to keep it porous, so that the oxidation of the chrome is more complete. Potassium bichromate is much used as a source of other chromium compounds; as an oxidizing agent in dyeing and making coal-tar dyes; as a mordant; as a bleaching agent for oils and fats; and for the preparation of leather in the chrome tannage processes.

Organic Chemistry for the industry

Inorganic Chemistry for the industry

  • Lixiviation
  • Levigation
  • Evaporation
  • Distillation
  • Sublimation
  • Filtration
  • Crystallization
  • Calcination
  • Refrigeration
  • Density
  • Fuels
  • Liquid fuels
  • Gaseous fuels
  • Water
  • Sulphur
  • Sulphur Derivatives
  • Sulphuric Acid
  • Sulphuric acid burners
  • Fuming Sulphuric acid
  • Salt
  • Hydrochloric Acid
  • Soda Industry
  • Caustic Soda
  • Treatment of tank
  • Ammonia Soda
  • Cryolite Soda process
  • Chlorine Industry
  • Electrolytic Chlorine
  • Hypochlorites
  • Chlorates
  • Nitric Acid
  • Nitrates
  • Ammonia
  • Potash Industry
  • Fertilizers
  • Lime, Cement
  • Cement
  • Glass
  • Ceramic Industries
  • Pigments
  • Bromine
  • Iodine
  • Phosphorus
  • Boric Acid
  • Arsenic Compounds
  • Peroxides
  • Oxygen
  • Sulphates
  • Alum

  • Lenntech BV

    Rotterdamseweg 402 M
    2629 HH Delft
    The Netherlands

    tel: +31 15 261 09 00

    fax: +31 15 261 62 89

    e-mail: info@lenntech.com

    Bookmark and Share