A natural history of salt

Ventura County Star

July 21, 2002

It is the only rock we eat. Its Latin name, salax, is the root of the English words "salacious" and "salary," placing it at the heart of two of humanity’s most basic and enduring preoccupations: sex and money. Indeed, it has been used both as a medium of exchange and as a guard against impotence, although its most common uses today are as an ingredient in plastic and as a method of ridding highways of winter snow and ice.

According to the salt industry, there are 14,000 commercial uses for salt, which is a source of sodium and chlorine – basic components of an array of materials, such as plastics, glass, synthetic rubber, cleansers, pesticides, paints, adhesives, fertilizers, explosives and metal coatings. Sodium and chloride are required for cells to function, and cannot be produced by the body, making salt an essential nutrient.

Cargill Inc. harvests about 700,000 tons of salt a year from its San Francisco Bay evaporation ponds, a small fraction of the nearly 29 million tons of dry salt produced annually in the United States. Produced primarily by three processes – solar evaporation, deep-shaft mining of solid deposits and solution mining – salt is a mineral crop worth $1.2 billion a year in the United States, the world’s leading producer.

In solution mining, water is injected through wells drilled into an underground salt bed or salt dome, usually between 500 and 5000 feet deep. Dissolution of the salt forms a void or cavern in the salt deposit. Salt brine is pumped from the cavern and transported by pipeline to an evaporating plant to make dry salt, or to a chemical processing plant for use in manufacturing.

Underground mining of solid salt beds – typically formed as deposits at the bottom of ancient seas – usually involves digging two 20-foot-wide shafts to the salt deposit. After the shafts are sunk, the salt is excavated by blasting out a series of "rooms." Rectangular pillars in a checkerboard pattern, typically 35 percent to 50 percent of the original salt, remain to support the mine roof. The blasted salt is loaded into large trucks and hauled to a crusher, after which it is hoisted to the surface.

Solar evaporation is the simplest and perhaps most ancient method of producing salt. The Ohlone tribe, indigenous inhabitants of the San Francisco Bay area, are believed to have gathered salt deposited naturally through evaporation along the shallow edges of the bay, and commercial production using essentially the same process as that employed today began in 1854.

It takes about five years for bay water (which is only 2.5 percent sodium chloride, vs. the ocean's 3.5 percent) to become crystalline salt at Cargill’s ponds and processing plants. The process begins when bay water is introduced into the intake ponds, and continues as the increasingly saline brine is pumped or allowed to flow by gravity from one pond to the next around the southern edge of the bay. At each step, more of the water evaporates under the influence of summer sunshine and steady coastal wind.

When the brine is as salty as it can become without the salt actually precipitating out of solution, it is pumped into the crystallizer beds at the company’s salt production plants. The beds are shallow ponds with clay bottoms that have been carefully compacted and leveled. Evaporation continues, and salt drops out of solution to form a deposit on the bottom of the bed.

The remaining brine – a dark red liquid packed with other minerals, such as magnesium, and referred to as "bittern" – is drained off to leave a bed of pure sodium chloride 5 to 8 inches deep. (Bittern is toxic to marine life and cannot be discharged into the bay; some has been used to produce highway de-icer but the rest is simply stored in ponds and poses a major cleanup chore and liability issue during the anticipated ecological restoration of Cargill’s property.)

Using machinery that resembles a cross between a snowplow and a grain combine, workers carefully scrape up the salt and transfer it to rail cars, which carry it to the plant for washing. Once the dust and other impurities have been removed, it is dried and readied for sale. Industrial salt is shipped in bulk, salt for food and pharmaceutical uses undergoes further refining before being packaged.

The salt harvest follows many of the same rhythms of any other crop harvest: It is conducted in the fall, generally from September to December, and can be interrupted by rain; crews work 24 hours a day, seven days a week to bring in the crystalline crop before bad weather arrives.

Sources: "Salt: A World History" by Mark Kurlansky (2002: Walker Publishing Co.); Salt Institute (www.saltinstitute.org/); Cargill Inc. (www.cargillsalt.com/sfbay/index.html).