In drip-irrigated greenhouse production, magnesium nitrate hexahydrate is used as a fully water-soluble source of nitrate nitrogen and magnesium where sulfate and calcium must be excluded from the injection event. The crystalline form, Mg(NO₃)₂·6H₂O, has a molecular mass of 256.41 g mol⁻¹ and delivers approximately 9.5 % Mg and 10.9 % N by mass in the solid product. Solubility in water at 20 °C is approximately 1250 g L⁻¹, which permits concentrated stock tanks in the 100–200 g L⁻¹ range without crystallization under ordinary greenhouse conditions. The agronomic rationale for selecting magnesium nitrate over magnesium sulfate is the absence of sulfate, allowing the nitrate salt to be blended with calcium nitrate in closed drip-tape systems without forming calcium sulfate deposits. Emitter scaling becomes a measurable risk when the combined nutrient solution electrical conductivity exceeds 3 mS cm⁻¹, particularly in low-flow tape with discharge below 1.0 L h⁻¹ per emitter. Because the hexahydrate is deliquescent above approximately 54 % relative humidity, bulk storage silos and injection skids require vapor sealing or humidity control to avoid caking and erratic stock concentration. Foliar sprays are typically diluted to 0.5–1.0 % w/w to limit osmotic damage, and spray-drying on the leaf surface under relative humidity below 40 % can raise local salt concentration above the injury threshold even when the tank concentration is within label limits. Fertigation programs using magnesium nitrate fall under EU 2019/1009 for fertilizing product compliance, and nitrate vulnerable zones require recorded application according to Council Directive 91/676/EEC.

Concentrated stock compatibility data for magnesium nitrate hexahydrate at 20 °C

Combination	Concentrate behaviour	Process control
Mg(NO3)2 + Ca(NO3)2	Clear solution; no precipitation	Maintain combined stock below 200 g L−1 total salt
Mg(NO3)2 + KNO3	Clear solution; no precipitation	Agitate before injection if batch is older than 24 h
Mg(NO3)2 + KH2PO4	White magnesium phosphate precipitate above pH 5.5	Acidify to pH 5.0 or use separate injection tanks
Mg(NO3)2 + NH4H2PO4	Slow flocculation at concentrate strength	Separate stock solutions; avoid simultaneous injection at the same manifold point
Mg(NO3)2 + K2SO4	No direct precipitation	Avoid if calcium nitrate is also present in the blended feed due to calcium sulfate precipitation risk

Compatibility with phosphate salts is the principal formulation limit in open-head injection tanks. Concentrated magnesium nitrate and monoammonium phosphate or diammonium phosphate can form magnesium phosphate flocs above pH 5.5, so acidification to pH 5.0 or separate injection ports are standard inline measures. Diaphragm metering pumps with EPDM or PTFE wetted parts are used on production-scale fertigation skids, and pump calibration is verified against a nitrate ion-selective electrode before each new stock batch is brought online.

What Limits Thermal Stability in Magnesium Nitrate–Potassium Nitrate Molten Blends?

The thermal storage pathway starts from the hydrated precursor rather than an anhydrous melt, and the hydration state dictates start-up procedures in concentrating solar power and industrial waste-heat battery pilot plants. Mg(NO₃)₂·6H₂O melts at approximately 89 °C in its own water of crystallization, and direct heating above 180 °C without staged dehydration produces acidic condensate and NO_x through partial hydrolysis. Molten salt formulations containing magnesium nitrate are prepared by pre-drying the hexahydrate at 120–150 °C until residual moisture is below 1 wt%, followed by controlled addition to a pre-melted potassium nitrate or lithium nitrate–potassium nitrate base under a nitrogen sweep. Published data for Mg²⁺-containing ternary nitrate melts indicate reduced liquidus temperatures relative to solar salt, but the upper service temperature is constrained by nitrate decomposition onset rather than by freezing point. Decomposition of nitrate to nitrite and oxide becomes measurable above 400–450 °C depending on gas atmosphere, with magnesium oxide precipitation raising viscosity and eventually forming a sludge layer in the receiver sump. Thermal cycling between 200 °C and 420 °C has shown that magnesium nitrate additions can shift the solidification point of binary solar salt downward, but the exact eutectic composition is influenced by water and carbonate residuals. Differential scanning calorimetry according to ASTM E794-06 is used to determine solidus and liquidus onset, while thermogravimetric analysis according to ASTM E1131 tracks early mass loss from nitrate decomposition. Corrosion coupons tested with ASTM G1-03 provide material qualification data for stainless steel wetted parts, but operational data at production scale for magnesium nitrate-rich melts is limited; rigorous chloride and sulfate impurity control below 100 ppm is necessary because these species accelerate stress-corrosion cracking and alter the melt solidus. Plant designers generally derate maximum film temperature to 380 °C when magnesium nitrate is present unless continuous blanketing and makeup salt purity are tightly managed.After solution combustion in a citrate–nitrate system, the calcination step determines whether magnesium nitrate-derived powder meets ceramic or electronic-grade specifications. The Pechini route uses magnesium nitrate hexahydrate as both the metal source and the oxidizer. A 1:1 molar ratio of Mg(NO₃)₂ to citric acid is dissolved in deionized water, adjusted to pH 7 with ammonium hydroxide, and evaporated at 80–90 °C until a glassy resin forms. Ignition above 250 °C propagates through the nitrate–citrate matrix and leaves a porous precursor with low residual carbon when the nitrate-to-fuel ratio is held in the fuel-lean regime. Subsequent calcination at 600–800 °C converts that precursor to periclase MgO with a crystallite size typically below 20 nm as estimated by X-ray line broadening. When the target phase is spinel, the nitrate-derived MgO is co-processed with aluminum nitrate, and solid-state spinel formation begins at 900–1000 °C instead of the 1300 °C required for coarser mixed oxides. The off-gas contains NO_x and ammonia when ammonium hydroxide is used, so production kilns are fitted with selective catalytic reduction or wet scrubbers to meet EU 2010/75 emission limits for nitrogen oxides. Batch-to-batch variability in tray hearth furnaces is closely tied to bed depth; beds thicker than 5 mm develop thermal gradients that produce bimodal particle populations and localized sintering. Finished powders are accepted only after particle size distribution is verified by laser diffraction according to ISO 13320:2020, and residual chloride is kept below 50 μg g⁻¹ to avoid ionic conduction losses in ceramic bodies.

Collection System Sulfide Control with Nitrate Respiration

Addition of magnesium nitrate to municipal wastewater collection systems targets the metabolic competition between sulfate-reducing bacteria and facultative nitrate-reducing bacteria. In a septic gravity main, dissolved sulfide is generated at the biofilm–water interface when oxidation–reduction potential drops below approximately −100 mV and sulfate is present. Nitrate fed as Mg(NO₃)₂·6H₂O raises the biofilm redox potential and shifts electron flow to nitrate respiration, reducing the formation of H₂S gas in downstream headworks. Dosing strategies are typically based on dissolved sulfide concentration measured by SM 4500-S²⁻ D or vapor-phase sulfide logging; reported control bands often target 0.7–1.2 g NO₃-N per gram of dissolved sulfide, although the stoichiometric ratio is modified by chemical oxygen demand, temperature, retention time, and sulfate loading. Unlike calcium nitrate, magnesium nitrate does not add calcium hardness to the return activated sludge stream, but it raises magnesium concentration in the plant water loop, which can shift the saturation index for struvite precipitation. At facilities that practice phosphorus recovery as struvite, magnesium nitrate addition must be re-evaluated against the plant’s actual pH and dissolved ammonia profile to avoid unintended precipitation in dewatering liquors. Chemical feed skids for this service use progressive cavity or diaphragm pumps with stainless steel or polymer wetted parts, and injection points are positioned upstream of the turbulent zone where dissolved oxygen is below 0.1 mg L⁻¹. Injecting into an aerobic reach consumes the nitrate through ordinary heterotrophic respiration and reduces sulfide-control efficiency. Online nitrate analyzers using UV absorption are calibrated against ISO 7890-1:1986, and grab samples collected at the dosing point are preserved with sulfuric acid at pH 2 or below until analysis.

Extractive Distillation with Nitrate Salt Break Agents

Concentration of weak nitric acid by extractive distillation can incorporate magnesium nitrate as a salt break agent that depresses water activity and shifts the vapor–liquid equilibrium of the HNO₃–H₂O system. In this configuration, dilute acid at approximately 55–65 wt% HNO₃ is fed to a distillation column while a concentrated Mg(NO₃)₂ solution is circulated through the upper stripping section. The nitrate salt increases the relative volatility of nitric acid to water and reduces the azeotropic limitation that caps ordinary fractional distillation at around 68 wt% HNO₃ at atmospheric pressure. Reboiler temperature, column pressure, and circulation salt concentration are linked: raising the Mg(NO₃)₂ concentration above a service-specific threshold can achieve overhead acid strength in the high-90 wt% range, but the solubility of the salt in the descending acid and the viscosity of the spent solution impose practical limits. Materials of construction for column internals, reboiler, and salt-circulation piping require special attention because hot nitric acid above 40 wt% is strongly oxidizing. Low-carbon stainless steel grades with sufficient chromium and nickel, or titanium, are separated in industry by operational temperature and acid purity, while gaskets and pump seals are selected from PTFE or fluoroelastomer materials. The atmospheric vent from the column is scrubbed to prevent discharge of nitrogen oxide mists. Published data for specific column configurations with magnesium nitrate is limited in comparison to sulfuric acid dehydrating routes; most process design data is retained as proprietary engineering package information rather than openly published, so a pilot-scale vapor–liquid equilibrium test under site-specific pressure and salt concentration is generally required before scale-up.

When Nitrate-Derived Spinel Is Spray-Dried for SOx Transfer in FCC

Fluid catalytic cracking regenerators require SOx transfer additives that combine an oxidizing function, a reversible sulfate storage function, and sufficient attrition resistance to survive circulating fluidized-bed service. Magnesium oxide in a stable aluminate spinel matrix is conventionally prepared from magnesium nitrate and aluminum nitrate via coprecipitation or spray drying, followed by calcination at 650–850 °C to form MgAl₂O₄ spinel. The nitrate route enables intimate mixing of Mg²⁺ and Al³⁺ at the atomic scale in the liquid phase, avoiding the slow diffusion kinetics of solid-state mixing between MgO and alumina. In a production spray dryer, the mixed nitrate solution is atomized through rotary or two-fluid nozzles into a hot gas stream at inlet temperatures of 250–350 °C; the resulting microspheres are then calcined in a rotary or fluid-bed kiln where NO_x off-gas is routed to a wet scrubber. The attrition resistance of the finished additive is tested according to ASTM D5757-11, because soft microspheres fracture in the regenerator and elutriate from the unit, increasing catalyst make-up rate. In regenerator service at 650–760 °C, the spinel additive first oxidizes SO₂ to SO₃ and then captures SO₃ as a metal sulfate; in the riser and reducing environment, sulfate is reduced and sulfur leaves as H₂S in cracked gas. The magnesium nitrate-derived spinel is preferred over free MgO on mechanical and chemical stability grounds because free MgO sulfates to MgSO₄ and can swell, whereas the spinel lattice accommodates sulfate formation with lower volumetric change. Production-scale FCC experience shows that additive activity is strongly dependent on accessible MgO surface area and on pore size distribution, which is measured by mercury intrusion porosimetry according to ISO 15901-1:2016. A narrow mesopore population above 20 nm improves SO₂ transport without sacrificing attrition resistance. The process is limited by the availability of low-iron magnesium nitrate; iron above 500 μg g⁻¹ in the nitrate feed can promote coke-making side reactions and reduce selectivity.For solid-base catalysis, the nitrate-derived MgO route offers control over basic site density and phase purity that is difficult to achieve with mineral magnesite. In a typical production sequence, magnesium nitrate hexahydrate is dissolved in demineralized water and precipitated with aqueous sodium hydroxide or ammonium carbonate at pH 10–11 and 60 °C. The resulting Mg(OH)₂ or hydroxycarbonate cake is washed until the supernatant conductivity falls below 100 μS cm⁻¹, dried at 110 °C, and calcined at 550–700 °C for 3–5 h. This yields periclase MgO with basic site densities measurable by CO₂ temperature-programmed desorption; the ratio of medium-strength basic sites to weak sites controls catalytic activity in vegetable oil transesterification. In a continuous packed-bed reactor containing extrudates of 2–4 mm diameter, soybean oil and methanol are fed at a molar ratio of 6:1 to 12:1, and reactor outlet conversion remains stable as long as free fatty acid in the feed is below 2 mg KOH g⁻¹ and water content is below 0.1 wt%. The product ester layer is separated and distilled, with final biodiesel quality assessed against ASTM D6751. Sodium nitrate residue from the precipitation step is the principal waste stream, requiring controlled discharge under EU 2010/75 where applicable. Catalyst deactivation is linked to organic adsorption on strong basic sites and to trace leaching of Mg into the glycerol phase, so feedstock pre-treatment and periodic catalyst regeneration determine the practical run length.