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Permutit Process

Permutit Process, fraught with great possibilities, lies in the utilisation of certain hydrated silicates of aluminium and the alkali metals. Such substances are found in nature and are known to the mineralogist as zeolites - a generic term which covers a series of well defined and inter-related salts. They are secondary minerals, occurring in cavities and veins of basic igneous rocks, and when heated they swell up and appear to boil, whence the name zeolite, from Greek lew, to boil, and Aitfos, stone. A curious property of the zeolites is the readiness with which they exchange their bases without altering their aluminium and silicon contents. Thus, for example, a sodium zeolite reacts with a calcium salt yielding, according to the law of mass action, a certain amount of calcium zeolite. For example, in the case of calcium sulphate,

Sodium zeolite + CaSO4 ⇔ Calcium zeolite + Na2SO4.

If, therefore, a hard water containing a dissolved calcium or magnesium salt is passed through a layer of sodium zeolite, the calcium is abstracted and the corresponding sodium salt passes into solution. In this way the water is readily softened.

In 1906 Gans succeeded in producing zeolites synthetically and named his product permutit, from Latin permutare, to exchange. The sodium salt may be made by fusing together sodium carbonate, silica and alumina or kaolin. The product is treated with water and yields a crystalline substance of empirical composition represented by the formula NaAl(SiO3)2.2H2O, which thus closely approximates to the natural zeolite analcite, NaAl(SiO3)2.H2O. The commercial product is usually prepared by fusing together soda, clay, felspar, and kaolin. In the Permutit Process, the permutit is placed in a suitable container and hard water allowed to percolate through. The whole of the hardness is thereby removed. The sodium permutit is thus gradually converted into the calcium or magnesium salt. Thus:

Na2Pm + CaH2(CO3)2 = 2NaHCO3 + CaPm
Na2Pm + MgSO4 = Na2SO4 + MgPm,
and so on.

The regeneration of the permutit is effected by soaking with a 10 per cent, solution of sodium chloride, whereby sodium permutit is regenerated and soluble calcium and magnesium chlorides pass into solution and are washed away.

CaPm + 2NaCl = CaCl2 + Na2Pm
MgPm + 2NaCl = MgCl2 + Na2Pm.

When water is very hard it is sometimes partially softened by a preliminary treatment with lime and soda. In some cases the permutit plant fed with such treated waters has ceased to work satisfactorily after a number of years, the permutit becoming contaminated with chalk.

Sodium permutit is gradually attacked by dissolved carbon dioxide and more rapidly by mineral acids. Waters therefore containing acids in solution are filtered through limestone or marble before reaching the permutit.

In this connection it may be mentioned that a manganese permutit filter has been invented for the removal of ferrous salts, which become oxidised in some manner, possibly in part through catalysis, but also through reduction of the manganese permutit, which requires periodic revivifying by treatment with a solution of permanganate of lime.

The following analyses of water before and after softening by permutit are interesting. The data are expressed as parts per million (mg. per litre):

Before SofteningAfter Softening
Iron oxide and aluminatracetrace
CO2 - bound81121
CO2 - free or half bound11178
KOH, NaOH58234
Total hardness11.20.6
Permanent hardness0.9nil

The water after Permutit Process is beautifully softened, but is rendered more corrosive towards iron and steel.

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