What is Magnesium Chloride Hexahydrate used for?

What Is Magnesium Chloride Hexahydrate?

Magnesium chloride hexahydrate (MgCl₂·6H₂O, CAS 7791-18-6) is a white to off-white crystalline salt with a molecular weight of 203.30 g/mol. It is the hydrated form of magnesium chloride — each magnesium chloride unit is associated with six water molecules, giving it excellent water solubility and a distinctive hygroscopic (moisture-absorbing) character.

MgCl₂·6H₂O is one of the most versatile inorganic salts in commerce. It occurs naturally in seawater (magnesium chloride accounts for roughly 3.7% of dissolved salts in the ocean), salt lake brines, and underground mineral deposits such as bischofite. Commercially, it is typically sold in flake, pellet, or granular form and serves as a key input across industries ranging from winter road maintenance to construction, water treatment, and food processing.

Unlike the anhydrous form (MgCl₂, CAS 7786-30-3), the hexahydrate is stable at room temperature, easier to handle, and more economical to transport and store — making it the predominant commercial form traded globally.

Chemical & Physical Properties

Understanding the properties of magnesium chloride hexahydrate is essential for proper application and handling across different industrial contexts.

Key Behavioral Characteristics

Hygroscopicity and Deliquescence. Magnesium chloride hexahydrate aggressively absorbs moisture from ambient air. Above approximately 33% relative humidity, it will absorb enough water to dissolve into a liquid brine — a property known as deliquescence. This behavior is both a practical advantage (enabling its use in dust control and de-icing) and a storage challenge (requiring sealed, moisture-proof containers).

Endothermic Dissolution. Unlike calcium chloride, which releases significant heat when dissolved in water, MgCl₂·6H₂O dissolution is slightly endothermic — meaning it absorbs a small amount of heat from its surroundings. This does not impair its de-icing performance, as the freezing-point depression is driven by ion concentration (colligative property), not heat release.

Freezing Point Depression. A 30% MgCl₂ brine has a freezing point of approximately -33°C (-27°F). In practical de-icing applications, MgCl₂ remains effective down to approximately -25°C (-13°F), making it a strong performer across most winter conditions except extreme arctic environments.

Thermal Decomposition. When heated, MgCl₂·6H₂O does not simply melt — it undergoes stepwise dehydration, losing water molecules of crystallization between approximately 117°C and 235°C. Above 300°C in the presence of moisture, it can hydrolyze to form magnesium oxide (MgO) and hydrogen chloride (HCl), which is a critical consideration in metallurgical and high-temperature industrial processes. 

How Is Magnesium Chloride Hexahydrate Produced?

Magnesium chloride hexahydrate is manufactured through several routes, with the choice of process driven by local raw material availability, energy costs, and target purity.

1. Brine Evaporation & Crystallization (Most Common)

The dominant production method involves extracting magnesium-rich brine from salt lakes (e.g., Great Salt Lake, USA; Qarhan Salt Lake, China; Dead Sea, Israel/Jordan) or underground deposits. The brine — a natural solution concentrated over geological timescales — is pumped into solar evaporation ponds where water evaporates progressively, precipitating less soluble salts (sodium chloride, potassium chloride) first. As the remaining brine becomes enriched in magnesium chloride, it is further concentrated and cooled under controlled conditions to crystallize MgCl₂·6H₂O.

Typical purity: 46–47% MgCl₂ content (industrial grade), with further purification yielding 98%+ purity.

2. Reaction of Magnesium Oxide/Hydroxide with Hydrochloric Acid

For higher-purity grades (food, analytical reagent), magnesium oxide (MgO) or magnesium hydroxide (Mg(OH)₂) is reacted with hydrochloric acid (HCl):

MgO + 2 HCl + 5 H₂O → MgCl₂·6H₂O

The resulting solution is filtered, concentrated, and crystallized. This route allows precise control over impurity profiles and is commonly used for FCC (Food Chemical Codex) and ACS (American Chemical Society) reagent grades.

3. By-Product Recovery

Magnesium chloride is also recovered as a by-product from several industrial processes:

Potash production:

● When potassium chloride is crystallized from complex brines, the remaining magnesium-rich liquor can be processed into MgCl₂·6H₂O.

Lithium extraction:

● Brine-based lithium carbonate production from salt lakes generates magnesium chloride as a co-product or waste stream, presenting an opportunity for integrated recovery.

Seawater bittern:

● After sodium chloride (common salt) is crystallized from seawater, the residual bittern is rich in magnesium chloride and can be further processed.

Industrial Applications

Magnesium chloride hexahydrate's unique combination of high solubility, hygroscopicity, and magnesium ion availability makes it indispensable across a broad spectrum of industries. Below is a comprehensive breakdown of its major commercial applications.

1.De-Icing & Anti-Icing

Magnesium chloride is one of the most widely used de-icing agents for roads, highways, airport runways, and pedestrian surfaces.

How It Works. When applied to ice or snow, MgCl₂ dissolves in the thin layer of surface moisture and dissociates into three ions — one Mg²⁺ and two Cl⁻ — per formula unit. This high ion count produces strong freezing-point depression, breaking the bond between ice and pavement. In anti-icing (pre-treatment) mode, MgCl₂ brine is sprayed ahead of a forecast storm to prevent ice from bonding to the surface in the first place.

Effective Temperature Range. MgCl₂ remains effective down to approximately -25°C (-13°F). Below this temperature, calcium chloride (effective to -32°C / -25°F) is preferred.

Advantages Over Rock Salt (NaCl):

● Works at significantly lower temperatures (NaCl stops around -9°C / 15°F)

● Less corrosive to steel and concrete — roughly 30–50% less corrosive than NaCl by weight

● Lower vegetation damage along treated roadways

● Leaves minimal white residue compared to rock salt

● Pre-treatment (anti-icing) with MgCl₂ brine can reduce total material usage by 30–50% compared to reactive de-icing

2.Dust Control & Road Stabilization

Dust suppression is one of the largest-volume industrial applications for magnesium chloride hexahydrate, particularly in mining, construction, and rural road management.

Mechanism. The same hygroscopic property that makes MgCl₂ effective for de-icing also makes it an excellent dust suppressant. When sprayed onto unpaved surfaces, MgCl₂ draws moisture from the air and holds it at the road surface, keeping fine particulate matter (PM10 and PM2.5) bound to the ground instead of becoming airborne. It also promotes compaction, increasing surface density and reducing the generation of new dust.

Key Benefits:

● A single MgCl₂ treatment can replace 3–4 water truck passes, dramatically reducing fuel, labor, and water consumption

● Less corrosive to vehicles and equipment compared to calcium chloride, an important consideration for mining fleet managers

● Biodegradable and low environmental persistence

● Effective on unpaved roads, construction sites, mine haul roads, solar farm access roads, and aggregate storage yards

3.Construction & Building Materials

Sorel Cement (Magnesium Oxychloride Cement). One of the most distinctive construction applications of magnesium chloride is in Sorel cement — an air-hardening cement formed by mixing MgO powder with concentrated MgCl₂ solution. The resulting magnesium oxychloride phases (3Mg(OH)₂·MgCl₂·8H₂O and 5Mg(OH)₂·MgCl₂·8H₂O) form a dense, high-strength binder.

Sorel cement offers several unique properties:

● Rapid setting and high early compressive strength (can exceed 70 MPa)

● Excellent adhesion to a wide range of fillers — wood flour, sawdust, mineral aggregates, cork, and glass fiber

● Fire resistance; the bound water and magnesium hydroxide matrix provide inherent flame retardancy

● Used in industrial flooring, fireproof boards, grinding wheels, and decorative architectural panels

Note on water resistance: Sorel cement has poor water resistance and is generally limited to interior or protected applications. Recent research into supplementary cementitious materials and phosphate modification is improving moisture durability for broader use.

Concrete Acceleration. Like calcium chloride, MgCl₂ can accelerate the hydration of Portland cement in cold-weather concreting. However, due to concerns about long-term chloride-induced reinforcement corrosion, its use in reinforced concrete is restricted by building codes in many jurisdictions. MgCl₂ is more commonly employed in plain (non-reinforced) concrete or Sorel cement formulations.

Refractory Binder. In the metallurgical industry, MgCl₂ serves as a binding agent for magnesia-based refractory bricks and monolithic linings used in furnaces, ladles, and kilns.

4.Water Treatment

Magnesium chloride hexahydrate plays multiple roles in industrial and municipal water treatment processes.

Coagulation & Flocculation. When added to alkaline wastewater, MgCl₂ reacts to form magnesium hydroxide (Mg(OH)₂) — a gelatinous precipitate with a large active surface area. This precipitate physically entrains suspended solids, emulsified oils, and heavy metals through sweep coagulation, enabling removal by sedimentation or dissolved air flotation. Mg(OH)₂ precipitation is particularly effective for:

● Heavy metal removal (lead, copper, zinc, nickel, cadmium)

● Color removal from textile dye wastewater

● Phosphate precipitation and nutrient management

pH Adjustment. MgCl₂ solutions are slightly acidic (pH 5–7 at 5% concentration), making them useful for neutralizing alkaline process streams without introducing sodium, which can be detrimental in certain industrial contexts.

Drinking Water Treatment. Food-grade MgCl₂ is used in some drinking water treatment plants as a coagulant aid and for controlled magnesium supplementation in desalinated or softened water — restoring mineral balance without adding sodium or calcium hardness.

Cooling Tower & Boiler Water. MgCl₂ can be used to adjust water chemistry parameters, though care must be taken with chloride levels in systems with stainless steel components due to chloride stress corrosion cracking risk.

5.Oil & Gas Industry

In petroleum extraction and well operations, magnesium chloride hexahydrate serves several critical functions:

Drilling Fluid Additive. MgCl₂ is added to water-based drilling muds as a shale stabilizer. The magnesium ions inhibit clay swelling by cation exchange — replacing hydrated sodium ions in smectite clays with less-hydrated magnesium ions, preserving wellbore stability through reactive shale formations.

Completion & Workover Fluids. Clear brine fluids based on MgCl₂ are used during well completion and workover operations. These solids-free fluids provide hydrostatic pressure control (densities up to ~1.30 g/cm³ at saturation) while minimizing formation damage — unlike solids-laden drilling muds that can plug reservoir pore throats.

Advantages in Oilfield Use:

● Compatible with most reservoir formations

● Lower environmental toxicity compared to zinc bromide or cesium formate brines

● Cost-effective for moderate-density applications

● Does not introduce sulfates (unlike seawater-based fluids), avoiding barium sulfate scale risks

6.Textile Manufacturing

In textile processing, magnesium chloride hexahydrate serves primarily as a:

Sizing Agent Component. MgCl₂ improves the viscosity stability and film-forming properties of starch-based and PVA (polyvinyl alcohol) size formulations, which protect warp yarns from abrasion during high-speed weaving.

Dyeing Auxiliary. In reactive dye systems, MgCl₂ functions as a mild electrolyte and pH buffer, promoting dye exhaustion and fixation on cellulosic fibers. It is particularly useful for:

● Vinyl sulfone reactive dyes, where controlled alkalinity is critical

● One-bath dyeing processes requiring precise electrolyte profiles

Flame Retardant Finishing. MgCl₂ can be incorporated into flame-retardant formulations applied to cotton, polyester-cotton blends, and other textiles — leveraging the same thermal decomposition chemistry that releases bound water and forms a protective magnesium oxide char layer.

Wastewater Decolorization. Textile effluent containing residual reactive and direct dyes can be treated with MgCl₂, which — at alkaline pH — precipitates Mg(OH)₂ that adsorbs and removes color bodies. This is a cost-effective primary treatment step before biological processing.

7.Fire Retardant Applications

The fire-retardant properties of magnesium chloride hexahydrate stem from its thermal decomposition behavior: when heated, it releases its six water molecules of crystallization (endothermic cooling effect), and at higher temperatures, the remaining MgCl₂ decomposes to form a magnesium oxide (MgO) char layer that insulates the underlying material.

Applications:

Wood products:

● Particleboard, fiberboard, plywood, and structural timber can be pressure-treated or surface-coated with MgCl₂-based fire retardants

Paper & packaging:

● Corrugated cardboard, insulation paper, and construction-grade paper products

Textiles:

● As discussed above, cotton and blended fabrics for upholstery, drapery, and protective clothing

Wildfire prevention:

● Long-term fire retardant formulations for vegetation management in fire-prone regions

MgCl₂-based fire retardants are valued for being halogen-free in their active mechanism (the chloride is already present in the compound; no organohalogen flame retardants are generated) and for producing low smoke density during combustion compared to brominated or phosphorus-based alternatives.

8.Metallurgy & Magnesium Production

Magnesium chloride hexahydrate is a critical raw material in primary magnesium metal production:

Electrolytic Magnesium Production. In the dominant commercial process for magnesium metal, anhydrous MgCl₂ is fed into electrolytic cells operating at ~700°C. Direct current splits the molten salt into magnesium metal (at the cathode) and chlorine gas (at the anode, which is recycled to produce more MgCl₂). Because the electrolytic process demands anhydrous MgCl₂, the hexahydrate must first be dehydrated — a technically challenging step due to the hydrolysis side reaction at high temperatures. Modern plants use partial dehydration followed by chlorination in the melt, or they feed MgCl₂·6H₂O directly into special cell designs that tolerate some oxide content.

Pidgeon Process (Thermal Reduction). In the silicothermic Pidgeon process, calcined dolomite (MgO·CaO) is reduced with ferrosilicon under vacuum at ~1200°C. MgCl₂ does not play a direct role here, but some integrated operations use MgCl₂-derived MgO as a feedstock.

Refractory Binder. As noted in the construction section, MgCl₂ serves as a bonding agent for basic (magnesia-rich) refractory bricks and castables — essential consumables in steelmaking, non-ferrous smelting, and cement kilns.

Flux in Aluminum Recycling. MgCl₂ is a component of salt flux formulations used in secondary aluminum melting, where it protects the molten metal from oxidation, strips oxide inclusions, and improves metal recovery from dross.

4.9 Food-Grade Applications

When produced to FCC (Food Chemical Codex) or food-grade purity standards, magnesium chloride hexahydrate has a long history of safe use in food processing.

Tofu Coagulation (Nigari). In traditional Japanese tofu making, MgCl₂ — known as nigari — is the preferred coagulant. When a dilute MgCl₂ solution is stirred into hot soy milk (70–80°C), the magnesium ions neutralize the negative surface charge on soy protein micelles, causing them to aggregate into a soft, silken curd. Nigari tofu is prized for its delicate, slightly sweet flavor and soft texture compared to tofu coagulated with calcium sulfate (gypsum). Typical dosage: 1–2 teaspoons of food-grade MgCl₂ per liter of soy milk.

Brewing & Water Chemistry. Brewers add MgCl₂ to adjust the mineral profile of brewing water. Magnesium (10–30 ppm) is an essential yeast nutrient and co-factor for key fermentation enzymes, while chloride enhances mouthfeel and malt sweetness. A higher chloride-to-sulfate ratio favors malt-forward beer styles (stouts, porters, British ales), while sulfate-forward water emphasizes hop bitterness.

Mineral Fortification. MgCl₂ is an FDA GRAS (Generally Recognized as Safe) ingredient under 21 CFR 184.1426 for direct addition to food. It is used as a bioavailable magnesium source in:

● Sports drinks and electrolyte beverages

● Bottled mineral waters

● Fortified soy and plant-based milk alternatives

Other Food Uses:

Firming agent:

● Maintains texture and crispness in thermally processed fruits and vegetables

Cheese making:

● Occasional use as a coagulant aid in some artisanal cheese varieties

Salt substitute blends:

● Contributes magnesium with reduced sodium content

10.Agriculture & Horticulture

Magnesium is the central atom in chlorophyll — without it, plants cannot photosynthesize. MgCl₂·6H₂O provides a highly soluble, readily available magnesium source for agricultural applications.

Foliar Fertilizer. Dissolved MgCl₂ can be applied as a foliar spray to rapidly correct magnesium deficiency, which typically manifests as interveinal chlorosis (yellowing between leaf veins) on older leaves. Foliar application bypasses soil fixation issues and delivers magnesium directly to plant tissues. Typical concentration: 1–2% MgCl₂ solution.

Soil Amendment. For magnesium-deficient soils, particularly in intensive cropping systems, MgCl₂ can be broadcast or fertigated as part of a balanced nutrient program. It is especially relevant for:

● Sandy soils with low cation exchange capacity

● High-rainfall regions where magnesium has been leached

● Soils with imbalanced Ca:Mg ratios due to excessive liming

Cotton Defoliant. MgCl₂ solutions are used as a mild defoliant in cotton production, promoting leaf drop before mechanical harvesting without the phytotoxicity associated with stronger chemical defoliants.

Hydroponic Nutrient Solutions. MgCl₂ is a standard component in hydroponic and controlled-environment agriculture nutrient formulations, often paired with magnesium sulfate (Epsom salt) to balance the chloride and sulfate anion profiles for specific crops.

Livestock Mineral Supplement. In animal nutrition, MgCl₂ serves as a bioavailable magnesium source in mineral premixes and feed formulations, particularly for ruminants where magnesium is critical for preventing grass tetany (hypomagnesemia).

Available Grades & Specifications

Magnesium chloride hexahydrate is commercially available in several grades, each designed for specific end-use requirements:

Frequently Asked Questions

What is the difference between magnesium chloride hexahydrate and anhydrous magnesium chloride?

Magnesium chloride hexahydrate (MgCl₂·6H₂O, MW 203.30) contains six water molecules of crystallization per MgCl₂ unit. The anhydrous form (MgCl₂, MW 95.21) contains no water. The hexahydrate is the stable, commercially dominant form at ambient conditions — it is easier to handle, less hygroscopic when sealed, and more economical to transport. Anhydrous MgCl₂ is primarily used in electrolytic magnesium metal production, where the presence of water would cause undesirable side reactions at the ~700°C operating temperature of electrolytic cells.

How does magnesium chloride compare to calcium chloride for de-icing?

Magnesium chloride is effective to approximately -25°C (-13°F), while calcium chloride works to about -32°C (-25°F). MgCl₂ is less corrosive to steel and concrete, safer for vegetation, and leaves less residue. CaCl₂ generates more heat on dissolution and performs better in extreme cold. In practice, many road authorities blend the two for optimal temperature coverage, or use MgCl₂ as the default with CaCl₂ reserved for the coldest conditions.

Is magnesium chloride hexahydrate corrosive?

Yes, like all chloride salts, MgCl₂ is corrosive to unprotected metals — but it is significantly less aggressive than sodium chloride (rock salt) and calcium chloride. The relative corrosion severity generally follows the order: NaCl (most corrosive) > CaCl₂ > MgCl₂ (least corrosive among the three). Stainless steel, plastics, and coated metals are compatible for long-term equipment contact.

Can magnesium chloride hexahydrate be used in food production?

Yes. Magnesium chloride hexahydrate is FDA GRAS (Generally Recognized as Safe) under 21 CFR 184.1426 when produced to FCC or food-grade specifications. Its most well-known food application is as nigari — the traditional Japanese coagulant for tofu. It is also used in brewing, sports drinks, and as a firming agent for canned vegetables. Only FCC or food-grade material should be used in food applications; industrial/technical grade contains uncontrolled impurities.

What causes magnesium chloride to turn into liquid during storage?

This is deliquescence — the property MgCl₂ shares with other highly hygroscopic salts. When the ambient relative humidity exceeds approximately 33%, MgCl₂·6H₂O absorbs enough water vapor from the air to dissolve into a concentrated brine. The solution is a puddle of magnesium chloride brine, not a product degradation, but it renders the material difficult to handle and measure. The only prevention is airtight, moisture-proof storage.

What is the magnesium content of MgCl₂·6H₂O?

Magnesium chloride hexahydrate contains approximately 11.96% magnesium by weight (as elemental Mg). This is a useful figure for calculating application rates in agriculture (magnesium fertilization), water treatment (dosing for Mg(OH)₂ precipitation), and food fortification — bearing in mind that the actual bioavailable or reactive fraction depends on the specific chemical environment.

Is magnesium chloride hexahydrate safe for dust control near water bodies?

While MgCl₂ is less environmentally persistent than many synthetic dust suppressants, chloride runoff into freshwater ecosystems should be managed. Best practices include:

● Applying only the recommended rate — do not over-apply

● Maintaining setback distances from streams, wetlands, and lakes

● Avoiding application immediately before heavy rainfall

● Using containment berms or drainage controls on sloping sites