The Cryolite Soda Process

The Cryolite Soda Process

Cryolite is a double fluoride of sodium and aluminum, found as a mineral in southern Greenland. As no other important deposit has been found, the supply is limited, and only two or three manufactories using this process are in operation, one of which is in this country. The reactions involved are as follows:-
 
1) AlF3 3 NaF + 3 CaC03 =NaAI02 +Na20 + 3 CaF2+ 3 CO2,
2) NaAl02 + Na20 = Na2Al03.
3) 2 Na3Al03 +3 H20 + 3 CO2 = 3 Na2C03 -+ 2 Al(OH)3

The ground cryolite is mixed with powdered limestone, and calcined at a red heat. Carbon dioxide escapes, and a mixture of calcium fluoride, sodium oxide, and sodium aluminate remains. On lixiviating this mixture with water, another sodium aluminate is formed and goes into solution, leaving the calcium fluoride as an insoluble residue. The solution of sodium aluminate is then decomposed according to the third reaction, by passing into it purified lime-kiln gases, or the furnace gases of the calcining operation. Hydrated alumina is precipitated, while sodium carbonate remains in solution. Sal-soda may be made by evaporating the solution, and was formerly the chief source of bicarbonate for culinary and medicinal purposes. If carried to complete dryness and calcined, a high grade of soda-ash is obtained. By causticizing, it yields a very excellent caustic.
The by-products aluminum hydroxide and calcium fluoride are used in the alum and glass industries respectively.

   Many other processes for the manufacture of soda from salt have been proposed, but none of them are now of any commercial importance. A small amount of soda is still made from kelp or varec, which is the ash of seaweeds.
    A new process for making soda has been proposed, * which is interesting and may be developed in the future, but has not as yet been placed on a practical basis. Salt-cake is made from salt by the Hargreaves process; then in the same cylinder and at the same temperature, it is treated with water gas. This reduces the saltcake to sodium sulphide, whi1e water, carbon monoxide, and hy(1rogen escape. These vapors are cooled, the water condensed, and the mixture of gases burned, the products of combustion, carbon dioxide and water, passing into the cylinders containing the sodium sulphide. Hydrogen sulphide and sodium carbonate are formed, and as the temperature is much above 100 C., no water can combine with the carbonate. The hydrogen sulphide is burned to sulphur dioxide, and the latter returned to the Hargreaves process. The reactions involyed are as follows:-

1) 2NaCl + SO2+ H2O + 0 = Na2SO4 + 2 HCl.
2) Na2SO4+ 5CO +5H2 = Na2S + 4 H20 + 5 CO +H2.
3) CO + H2 +O2 = CO2+ H2O
4) Na2S + CO2 + H2O = Na2CO3 +H2S.
5) H2S +3 0 = H2O + S02

This process seems to offer several advantages of which the following are the chief:-

1. Cheap materials.
2. Small outlay for labor, - the materials not been handled from the time the salt is charged into the cylinders until the soda-ash is raked out.
3. No waste products nor nuisance.
4. The temperature constantly decreases, being highest when the furnace is charged and lowest when the soda-ash is finished.
5. The process yields hydrochloric acid which can be utilized
for making chlorine.

   For the methods of producing caustic soda and chlorine by electrolysis of urine, see Chlorine.