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DISCOVERY 



Modem Industries — VI 



Salts, Brines, and Alkalis 



By R. C. Skyring Walters, B.Sc, 

 Assoc. M. Inst. C.E. 



Salts — their Distribution and Composition 



The salts that are distributed over the globe give 

 rise to many industries, some great, some small, and 

 several large volumes would be required to describe 

 adequately the geological, metallurgical, chemical, 

 mechanical, and economic considerations upon which 

 such industries are based. Here, however, a few 

 typical industries are outlined to show as simply as 

 possible, firstly, the complexity of the problems con- 

 nected with the formation of the raw materials, and, 

 secondly, the complexity of the problems involved in 

 their conversion into finished products for the use and 

 convenience of man. 



Without going into statistics, we all know sodium 

 chloride, or ordinary common salt, is one of the most 

 widely distributed of the salts in nature, in which state 

 it is called rock-salt. It is also probably the salt most 

 used by man, not only directly for flavouring and 

 preserving food, but indirectly for the manufacture of 

 alkalis, washing-soda, and soap. Salt, Geikie states. 

 occurs in rocks of all ages, in the pre-Silurian rocks of 

 the Punjab, in the Silurian of America, in the Carboni- 

 ferous of Australia, in the Permian and Trias of Ger- 

 many and England, in the Tertiary of Poland and 

 France, and is being formed at the present day in the 

 salt-lakes of Utah, the Dead Sea, and in the desert of 

 Kirgis Steppe in Siberia. 



In England, salt is usually associated with the Red 

 Keuper Marl of the Trias series. These rocks mould 

 the scenery of a large portion oi central England. The 

 hard red sandstone beds generally form conspicuous 

 hills, and the marl and clay beds, the low-lying ground 

 around them, and the floors of valleys in between. 

 In Cheshire the Keuper Marls, which contain the salt- 

 beds, are some 3,000 ft. thick, and were all originally 

 deposited in water ; the sands and clays being the 

 settlement of the muds of the ancient seas washing 

 against ancient land cliffs, or of muds brought down by 

 rivers draining the ancient land-surfaces ; the rock-salt 

 (and other things) being the chemical precipitation of 

 certain substances originally in solution, which have 

 separated out owing to certain physical conditions 

 prevailing at the time. 



In these seas and sediments several materials existed, 

 in solution or as solids ; a few of the salts may be noted 

 thus; Salt (Sodium Chloride), CarnalHte (Potassium 

 and Magnesium Chloride), Calcium Chloride, Sodium 

 Sulphate (Glauber's Salt), Kainite (Potassium and 



Magnesium Sulphate with Potassium Chloride), Gypsum 

 (Calcium Sulphate), Celestine (Strontium Sulphate), 

 Natron or Washing-soda (Sodium Carbonate), 

 Potassium Carbonate, Magnesite (Magnesium Carbon- 

 ate), Chalk or Limestone (Calcium Carbonate). 



It would take too long, however, to explain, even 

 if it were possible, how these several substances, at one 

 time more or less in solution, came to be crystallised 

 out so as to form the solid deposits on the earth's crust 

 as we now see them. The conditions for crystallisation 

 of more than one substance in the same solution is very 

 complicated. If one substance only is in solution the 

 case is simple, as the water can only contain so much 

 of that one substance, and if excess of that substance 

 is added, it remains undissolved. But if two sub- 

 stances are dissolved in the same water — for instance, 

 salt and gypsum — and the water is evaporated, some 

 of the gypsum will crystallise before the salt. Thus 

 seams of gypsum are sometimes found below seams of 

 salt. If five or six different substances are in a solution, 

 it is found not only that some crystallise out before 

 others, but that some combine, forming double salts 

 which have each its own rate of crystallisation. Again, 

 some salts dissolve others ; thus while pure water 

 dissolves 254 parts per 100,000 of gypsum (Calcium 

 Sulphate), strong brine dissolves 719 parts per 100,000, 

 and, to complicate matters still further, certain of 

 them cannot exist together in solution at certain 

 temperatures. 



How is Rock-salt Formed ? 



An idea of the possible origin of rock-salt, however, 

 may be easily gleaned in the following way : Let 

 the reader imagine a lake or sea with one inlet and 

 one outlet for water. Now as salt is distributed in 

 nature in small quantities and is soluble in water, 

 water which flows into the lake carries salt with it ; 

 thus, water that has flowed over vegetation will bring 

 in about 2 parts per 100,000, spring-water about 4, 

 and even rain-water about -36. These waters, with a 

 minute quantity of salt dissolved in them, flow into the 

 lake, and carry with them silt and mud which, in 

 depositing, forms a bed of clay or marl. When this 

 process has taken place the inflowing water stops and 

 the water in the lake is evaporated, possibly by a tropical 

 sun, and as the salt dissolved in the water does not 

 evaporate, the proportion of salt in the water con- 

 tinually increases as the latter is being evaporated. 

 This is comparable to what goes on in a steam-boiler. 

 The solution becomes more salty than the sea (that 

 is, more than 2 to 3 per cent, of salt) until strong brine 

 containing 26 per cent, of salt is formed. Now let 

 us see what happens when the evaporating process is 

 continued upon the brine-solution. The brine is not 

 able to dissolve any more salt ; if any salt were put 



