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NATURALLY OCCURRING SOLUBLE SALTS 757

and is one of our cheapest and most abundant necessities of life.

Salt is

obtained (1) by mining rock salt, and (2) by the evaporation of brine

from the ocean or salt wells. Many large deposits of salt are widely

distributed in the United States. For example, the Michigan salt

beds, 32,000 square miles in area and from 1000 to 5000 feet thick,

contain enough salt to supply the world for 1,000,000 years.

In addition to its use in foods, salt is used as follows: (a) with ice

as a refrigerant, (b) to melt ice and snow on sidewalks and railroads,

(c) to kill poison ivy, (d) in making soap, (e) in tanning leather, (/) as a

flux in steel-making, (g) in refining of oil, gold, silver, and copper, and

(h)

as a raw material in the electrochemical industry.

The Refining of Common Salt Is a Typical Process.

The modern salt refinery utilizes methods and equipment which are

almost universally employed in the refining of naturally occurring

salts such as soda, borax, and potash or in the refining of such products

as sugar and hundreds of other crystalline substances which are soluble

in water.

The steps in the refining of common salt are as follows:

a) The brine is pumped into large tanks, where lime and alum or

other chemicals are added which will form gelatinous precipitates with

impurities in the water. These precipitates settle out along with

other suspended matter.

b) The brine is then evaporated in multiple-effect evaporators,

arranged in series so that the vapor from the first evaporator is used

to heat the second evaporator, etc., the boiling-point in each successive

evaporator being lowered by decreasing the pressure of the atmosphere

above the solutions in the successive evaporators in a stepwise fashion.

c) The salt which has crystallized in the last evaporator is separated

from the mother liquor by centrifuges, in which pure water is sprayed

to wash out the remaining mother liquor which contains soluble

impurities. In some refining processes, an Oliver filter replaces the

centrifuge. An Oliver filter is a metal drum perforated with small

holes. A reduced pressure is maintained inside the drum, which thus

draws the mother liquors from the salt.

d) The salt is then dried in rotary kilns.

Sometimes the salt is dissolved in water, recrystallized, filtered or

centrifuged one or more times before drying, when exceptional purity

is desired.

Soap Is Considered to Be a Necessity by Modern Civilization.

Soap has been used for hundreds of years. Formerly it was prepared

by boiling grease or fat with the liquid obtained by leaching wood

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