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The Pamphlet Collection of Sir Robert Stout: Volume 73

Stassfurt Potash Salts

Stassfurt Potash Salts.

About fifty years ago strong brine was brought to the surface when boring for rock-salt at Stassfurt, in Prussia. The bed of rock-salt itself—a large one—was duly reached; and in carrying out the necessary works-boring, shaft-sinking, tunnelling, &c.—for exploring and developing the rock-salt mine the discovery of potash and magnesia salts, lying in great quantity geologically above the rock-salt, was made. This was the beginning of the greatest saltworks and potash industry in modern times. Little importance was at first attached to this discovery: indeed, as it was rock-salt (chloride of sodium) they were seeking, anything else was looked on as an impurity. By-and-by, however, the value of the potash-salts was recognised, and during the last thirty years a very large industry has gathered around Stassfurt for the preparation of these for the manure market. The salt minerals found in the Stassfurt beds may be put down as (1) common salt or chloride of sodium (NaCl); (2) sylvin or chloride of potassium (KCl); (3) chloride of magnesium (MgCl26H2O); (4) sulphate of potassium (K2SO4); (5) sulpnate of magnesium (MgSO4, H2O); and lastly, but in small proportion, and well down towards the base of the beds among the common salt, sulphate of lime or gypsum, or, when crystallized, selenite (CaSO42H2O). It must not be supposed, however, that these six different kinds of salts are lying in the Stassfurt deposit, pure and simple, a layer of one lying above a layer of another, in regular order, without admixture among themselves. On the contrary, the layers are in most cases of complex composition, generally containing at least two, often three, and sometimes four or five, or even the whole six, of the salts named above. The contents of these Stassfurt beds point clearly to a marine origin. A body of sea-water of great depth became isolated; and as the water became less and less (gradually drying up by sun and wind), while the amount of salts remained the same, a time came when there was not enough water left to hold the salts in solution. Then, of course, the less soluble salts began to crystallize and fall to the bottom, and there they page 33 laid the foundation of the first bed. This crystallization and subsidence of salts went on till the water was all dried up, leaving in its place the solid mass of salts that make up the Stassfurt mineral deposit.

The order in which the salts contained in sea-water crystallize out on boiling it down is virtually the same as found in these deposits. The first crop of deposit is sulphate of lime and then common salt, but the salt has begun to drop out before all the sulphate of lime has been precipitated, so that the sulphate-of-lime sediment contains an admixture of salt, and the salt an admixture of sulphate of lime. Then, after a time, when the common salt is nearly gone, magnesium and potash salts make their appearance, mixed with more or less of the other salts named. Formed in this way, for each bed or layer there is a preponderating ingredient of, say, common salt, sulphate of magnesia, sulphate of potash or chloride of potash, as the case may be, and in the very centre of each bed such salt is in many eases nearly pure.

The thickness and order of these salt beds at Stassfurt are given as follows: First, down below at the greatest depth yet reached it is common salt nearly pure and of unknown depth, the bottom having not yet been reached. This bed contains occasional thin layers of sulphate of lime: and it is expected, reasoning from analogy, that this mineral (sulphate of lime or gypsum) will be the true bottom layer resting on the bed-rock. The next layer, and resting on the pure rock-salt (common salt), is a bed 100ft. to 200ft. thick of rock-salt containing here and there layers of a mixture of the sulphates of potash and magnesia. This potash mixture is put into the market as "polyhalite," and is one of the best potash manures sent out from Stassfurt. Above this there is a bed (90ft. to 100ft.) of "kieserite," sulphate of magnesia (allied to Epsom salts), mixed with "carnallite" (chlorides of magnesia and potash). And lastly, crowning all, and therefore the last deposited because the most soluble, "carnallite" itself, nearly pure. Interspersed in thin layers in this last bed there is also a pure chloride of potassium or "sylvin," KCl; so that the whole formation may be described as a vast deposit of the salts of soda, potash, magnesia, and lime, with chloride of sodium in immense quantities at the bottom, and chloride of potassium in much less quantity at the top, with mixtures of these and other sea salts between.

It is for their potash that these salts are of any value as manures. They are now treated by solution and various boiling-down and crystallizing processes on a very large scale at the Stassfurt works for the separation of these constituents, which are then sent all over the world under various names as "potash manures." Of these, perhaps the best known is (1) kainit. This is a mixture of sulphate of potash (K2SO4), sulphate of magnesia (MgSO4, 7H2O), chloride of magnesium (MgCl26H2O), and chloride of sodium (NaCl). The sulphate of potash ranges from 13 per cent, to 27 per cent., averaging in good samples about 24 per cent., or 24 units, and worth something like 5s. per unit per ton. (2) Polyhalite, already described, is much of the same character, but contains about twice as much sulphate of potash as kainit, and less or no chloride of magnesium. (3) Sylvin or muriate of potash (KCl) is another of these products, and contains, as sent into the market, from 60 to 98 per cent, or units of chloride of potassium. Though richer in real potash than an equal weight of the sulphate, the muriate is charged with making potatoes waxy by diminishing the proportion of starch in them, and also with (in some cases) injuring young and tender plants in the same way as a large dose of common salt does. For these reasons, though the muriate is the richer in potash, the sulphate (in kainit and polyhalite) is generally preferred.

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Carbonate of Potash from Merino Wool.

The washings of merino wool are a curious source of potash, and it is worthy of some notice in a country like New Zealand, where so much merino wool is grown. The unwashed wool contains from one-third to one-half (in some cases, analysed here, nearly two-thirds) of its weight of dried-up sweat or suint or "grease," most of which is dissolved and removed when the wool is washed: that is, 100lb. of raw merino wool contains usually from 40lb. to 70lb. of wool and from 30lb. to 60lb. of grease or suint. The suint itself contains about 50 per cent, of potash salts and 50 per cent, of organic matter; and when the suint is burnt to ashes the ash amounts to half the weight of the suint, and consists almost entirely of carbonate of potash (K2CO3) with a small proportion of the sulphate and chloride of the same metal. It is only in France, however, so far as the lecturer knew, that the wool washings have been utilised for this product. The process is something like this:—

1. The wool is washed in the smallest possible quantity of water, the water being used over and over again, till it contains as much dissolved suint as it can hold. This suint water is boiled down to dryness in large iron boilers, and the residue is then converted into carbonate of potash by roasting. In this industry the chief difficulty is in the washing in such a way as to get the water saturated with the suint. This process would be more easily illustrated than described. Imagine a series of square tanks [see diagram] (capacity, say, 200 gallons each) arranged side by side in a row, and numbered 1, 2, 3, 4, 5, 6, 7, 8; No. 8 being placed on a level 6in. higher than 7, No. 7 6in. higher than No. 6, No. 6 6in. higher than No. 5, and so on, each successive tank being 6in. higher than the one next it on the left. Imagine also the one end of an iron tube (1in. wide) inserted into No. 8 1in. above its bottom, the tube being bent so that the other end of it opens into the mouth of No. 7, so that when the water in No. 8 is within 4in. or so of the top it will (being at the same level in the tube) pass over by the tube into No. 7. No. 7 is connected in the same way with No. 6, No. 6 with No. 5, and so on down to No. 1. Imagine, next, a square wooden tub, H, with perforated bottom (bottom pierced with, say, forty or fifty ½in. holes), containing the wool to be washed, and capable of being raised or lowered into the vats successively by means of a rope and pulley running on a wire above, and in line with, the row of tanks.

diagram of an aquaduct

page 35

This wooden tub with the raw wool in it is lowered into tank No. 1 and there washed. From No. 1 it is raised into tank No. 2, leaving much of its suint in solution in tank No. 1. In No. 2 it is further washed, and so on successively in Nos. 3, 4, 5, 6, 7, and 8, in the last of which it leaves very little of its suint, and from which it is finally withdrawn as scoured or washed wool. Another wooden tub similarly charged with raw wool has been following up the first tub, and another and another, till the tank No. 1—having always got the first and richest contribution of suint from each tub—is saturated, when it is removed and No. 2 takes its place, the other numbers following on and replacing each other in their order, while a fresh pure-water tank becomes the new No. 8, which is always supplied by a tap with fresh water. In this way each tank becomes No. 1 and fully saturated in its turn, and the final dipping of the wool in No. 8 is in clean water; the wool is thoroughly washed, and the suint is all saved in the smallest possible quantity of water. Of course any soft potash soap used with the washing-water would also be saved and reduced to potash salts in this process.

The lecturer had no information as to whether the potash thus saved would, with the high price of labour in New Zealand, pay for its making. There is no doubt, however, that when large wool-washing operations are going on it might be profitable to give the land the benefit of the potash by an inexpensive system of irrigation on to the adjacent grass-paddocks. Of course, other wools as well as merino produce suint, though in much less quantity.

In estimating the quantity of potash removed from the soil by farming and grazing, this source of loss should not be quite overlooked, as all the potash in the suint comes through the grass from the soil.

Another and more prolific source of potash salts (chiefly carbonate) in Europe is the syrupy liquid left after the sugar has crystallized out, in the beet-sugar industry.

The sugar-beet is largely grown in Belgium, France, and many parts of Germany, and would probably thrive well in New Zealand.

Beets take a good deal of potash from the soil. Twenty tons per acre would be a good crop, and would, if burnt, yield about 470lb. of ash, of which about 200lb. would be pure potash. This (see table under "Potash Salts," above) is equal to the potash removed by the same weight of mangels, and exceeds the quantity removed by a good yield of any of the other crops named in that table. The potash contained in the syrupy residues is recovered in the following way: (1.) The syrup is diluted largely with water. (2.) It is then fermented by the addition of yeast. By this process the sugar is converted into alcohol and carbonic acid, while the potash salts remain unchanged. (3.) The liquid is then distilled, and the alcohol, being of economical value, is saved. (4.) The residual liquid is then boiled down to dryness and roasted in an iron pot, the calcined mass or ashes consisting mainly of crude carbonate of potash, but containing also considerable quantities of the sulphates and chlorides. About twenty-five years ago the annual output of potashes from this beet-root residue amounted to something like 10,000 tons. A good deal of potash is, of course, brought up to the surface-soil by old clovers and other long-rooted plants. It is advisable, however, seeing that all the cultivated crop plants remove it from the soil, that some potash salts—such as kainit, carnallite, polyhalite, or saltpetre, or, better still, wood ashes, if they can be procured cheaply enough—should be occasionally administered to the soil, especially with such crops as turnips, mangels, potatoes, beets, and clovers, which are all very partial to that fertiliser.