Salt: Production, Uses, and Health Impact

Background

Salt, chemically known as sodium chloride (NaCl), crystallizes as transparent cubic crystals. While most people recognize salt as a kitchen staple, less than 5% of U.S. production is destined for that purpose. Approximately 70% is supplied to the chemical industry—primarily as a chlorine source—while the remainder supports diverse applications such as road de‑icing, water softening, food preservation, and soil stabilization for construction.

The earliest humans harvested salt from natural licks and from meat. Coastal communities also obtained it by chewing seaweed or evaporating seawater in shallow pools. As hunting and animal domestication progressed, meat and milk became the main domestic sources. Today, some groups—including the Inuit, Bedouin, and Maasai—continue to rely exclusively on natural salt.

With agricultural expansion and population growth, the need for reliable salt supplies spurred new extraction methods. The first large‑scale technique was solar evaporation of seawater, well suited to hot, arid regions. It has persisted in many coastal and desert locales. Soon after, quarrying of exposed rock salt and underground mining of salt deposits expanded production. In ancient China, wells tapped deep underground brine pools, some exceeding 1 km in depth.

In climates unsuitable for solar evaporation, salt water was boiled over fire or heated with burning wood. Roman-era shallow lead pans gave way to iron pans heated with coal in the Middle Ages. The 1860s saw the Michigan (grainer) process, where steam passed through submerged pipes to evaporate brine—a method still used for certain salts. By the late 1880s, closed‑pan, multiple‑effect vacuum evaporators—borrowed from the sugar industry—replaced open pans.

Today the United States leads global salt production, followed by China, Russia, Germany, the United Kingdom, India, and France.

Raw Materials

Salt is sourced from rock salt (halite) and brine. Rock salt forms from the evaporation of ancient oceans and is found in the U.S., Canada, Germany, Eastern Europe, and China. Salt domes—pressure‑driven thrusts of salt—are common along the Gulf Coast of Texas and Louisiana.

Brine—water saturated with salt—derives primarily from the ocean, but also from hypersaline lakes such as the Dead Sea and underground brine pools. Major brine deposits exist in Austria, France, Germany, India, the United States, and the United Kingdom. Brine can be naturally occurring or artificially produced by dissolving mined salt or by pumping water into salt wells.

Natural brines contain additional minerals—magnesium chloride, magnesium sulfate, calcium sulfate, potassium chloride, magnesium bromide, and calcium carbonate—that can be commercially valuable. Rock salt purity varies; some deposits contain rocky impurities like shale and quartz.

Table salt is typically iodized to prevent goiter. A small amount of potassium iodide is added, followed by anti‑caking agents—magnesium carbonate, calcium silicate, calcium phosphate, magnesium silicate, or calcium carbonate—to maintain free flow.

The Manufacturing Process

Processing Rock Salt

• 1. Exploration starts with drilling for water or oil; a diamond‑tipped, hollow drill extracts core samples to evaluate mining viability.
• 2. Selected sites receive shafts, and a large‑scale “chain‑saw” machine cuts a 15 cm high, 20 m wide, and 3 m deep undercut. Subsequent drilling with tungsten‑carbide bits and explosive charges—dynamite or ammonium nitrate—blasts the rock in a room‑and‑pillar pattern, leaving salt pillars that support the roof.
• 3. Blasted salt is hauled to an underground crushing area. A grizzly screen retains pieces smaller than 23 cm, while larger fragments are crushed in a rotating cylinder. The salt is then transported to surface crushers that reduce particle size to about 8 cm, after which picking removes metals (via magnets) and rocky material (via a Bradford breaker). A tertiary crusher further downsizes to ~2.5 cm. If higher purity is required, the salt is dissolved to form brine for re‑processing; otherwise, the product is screened, bagged, and shipped.

Processing Brine

• 4. Solar evaporation is ideal for hot, dry climates. Brine is pooled in shallow ponds, allowing insoluble impurities (sand, clay) and slightly soluble salts (calcium carbonate) to settle. Sequential pond transfers remove calcium sulfate and other moderately soluble salts, preserving the remaining brine for final evaporation.
• 5. Salt crystals are collected on temporary rail tracks and washed with supersaturated salt water, which removes trace impurities without dissolving the salt. A brief rinse with fresh water and a 2–3 month drainage period yields 99.4% pure salt suitable for industrial use. For food‑grade salt, a secondary wash, brief drainage, and hot‑air drying at ~185 °C produce 99.8% pure crystals.
• 6. Most commercial brine is processed in a multiple‑effect vacuum evaporator: chemically de‑mineralized brine enters the lowest cylinder, where steam‑heated tubes evaporate the liquid. The resulting steam propagates to the next cylinder, lowering pressure and temperature and enabling efficient evaporation. Salt slurries are filtered, dried, and screened, producing vacuum‑pan salt—small, cubic crystals.
• 7. The grainer method involves heating brine to ~90 °C in a long open pan. Flakes crystallize on the surface, sink, and are harvested. Lower temperatures yield larger flakes; higher temperatures produce finer crystals. The Alberger process combines partial vacuum evaporation with grainer crystallization, yielding a mix of flakes and cubes.
• 8. Finished salt is bagged or boxed. For iodized table salt, potassium iodide is added, followed by anti‑caking additives, before final packaging.

Quality Control

Specifications differ by application. Food‑grade salt must exceed 99.99% purity, while de‑icing salt tolerates up to 4% impurities, often giving a gray, pink, or brown hue. Solubility tests involve dissolving 20 g of salt in 200 mL water, expecting complete dissolution within 20 minutes.

Higher purity is essential because even trace amounts of calcium, magnesium, copper, or iron can affect food quality: calcium can toughen vegetables, copper/iron can degrade vitamin C and accelerate rancidity, and magnesium/calcium increase hygroscopicity, causing caking.

Health Aspects

Current guidelines recommend 6–11 g (0.2–0.4 oz) of salt per day for healthy adults—equivalent to 2,400–4,400 mg of sodium. Individuals with hypertension may benefit from a low‑sodium diet (<2,400 mg/day). While some advocate universal sodium reduction, evidence for health benefits in otherwise healthy adults remains inconclusive.