Nitric acid magnesium nitrate process

Nitric acid-water Maximum-hoiling azeotrope Sulfuric acid, magnesium nitrate for salt process Sulfuric acid process relies heavily on boundary curvature... 

Figure 8. Flowsheet of the concentration of nitric acid by the magnesium nitrate process...

Vinyl acetate-ethyl acetate Propane-propylene Ethanol-isopropanol Hydrochloric acid-water Nitric acid-water Close-boiling Close-boihng Close-boihng Maximum-boiling azeotrope Maximum-boiling azeotrope Phenol, aromatics Acrylonitrile Methyl benzoate Sulfuric acid, calcium chloride for salt process Sulfuric acid, magnesium nitrate for salt process Alternative to simple distillation Alternative to simple distillation, adsorption Alternative to simple distillation Sulfuric acid process rehes heavily on boundary curvature Sulfuric acid process rehes heavily on boundary curvature... 

In the sulfuric acid process, which po.ses considerable corrosion problems, medium concentrated nitric acid is first produced using conventional methods (e.g. in a M/M-type unit) as in the magnesium nitrate process. Concentrated sulfuric acid is fed in at the head of the concentrating tower. During the extractive distillation, diluted sulfuric acid accumulates in the sump and 99% nitric acid is driven off. The diluted sulfuric acid is then concentrated by vacuum distillation and recycled. 

In the magnesium nitrate process weak acid is distilled with 72% magnesium nitrate solution, whereupon highly concentrated nitric acid is driven off at the head of the dehydration tower. The sump product is then concentrated by vacuum distillation. 

The manufacture of concentrated nitric acid (98—99%) can be done by two methods, direct and indirect. The indirect method, mainly used in the United States, is performed industrially by two different systems, the sulfuric acid and magnesium nitrate processes. Both processes involve the dehydration of nitric acid with concentrated H2S04 or Mg(N03)2. The sulfuric acid process has acute corrosion problems. This process utilizes azeotropic rectification to produce concentrated nitric acid. 

Fig. 28.14. Magnesium nitrate process for nitric acid concentration. (Hydrocarbon Processing, 58, No. 11, 209, Nov. 1979, copyright 1979by the Gulf Publishing Company. By permission.)...

Magnesium nitrate is prepared by dissolving magnesium oxide, hydroxide, or carbonate in nitric acid, followed by evaporation and crystallization at room temperature. Impurities such as calcium, iron, and aluminum are precipitated by pretreatment of the solution with slight excess of magnesium oxide, followed by filtration. Most magnesium nitrate is manufactured and used on site in other processes. 

Thermo dynamic data for nitric acid are given ia Table 2. Properties for the ternary systems sulfuric acid—nitric acid—water (5,14) and magnesium nitrate—nitric acid—water (11,15—17) used ia processes for concentrating nitric acid are available. 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

The Extractive Distillation Process for Nitric Acid Concentration Using Magnesium Nitrate... 

For a continuous extractive distillation process to be possible there must be adequate enhancement of the nitric acid-water relative volatility, and a system equilibrium which permits virtually complete separation of nitric acid from magnesium nitrate, the latter taking up the water content of the weak acid feedstock. This requires addition to the weak nitric acid of solutions of magnesium nitrate usually containing 60 wt% or more of Mg(NC>3)2. Under these conditions a nitric acid-water relative volatility of greater than 2.0 is obtained at the low end of the liquid phase concentration at a nitric acid mole fraction below 0.05 (4, 7). 

Since in an extractive distillation process based on this ternary system the extractive agent is nonvolatile and remains in the liquid phase, and since because of the similarity of the molar latent heats of nitric acid and water there is substantially constant molar liquid overflow, the mole fraction of magnesium nitrate remains almost constant throughout the process. It is appropriate to represent the equilibrium situation as a pseudo-binary system for each magnesium nitrate concentration, and Figure 7 shows vapor-liquid equilibria on a nitric acid-water basis at a series of magnesium nitrate concentrations from zero to 0.25 mole fraction in the liquid phase. 

Thermal data for the ternary system have not been widely reported, but may be evaluated as required for process calculations from available data for the nitric acid-water and magnesium nitrate-water binary systems. 

In 1957 Hercules Inc. started the first unit that produced concentrated nitric acid for commercial sales using magnesium nitrate as the extractive agent. In this process (see Figure 2) the weak nitric acid product from an AOP is fed to the appropriate tray of a distillation column. A concentrated solution of magnesium nitrate and water is fed to the proper tray in sufficient quantity to enrich the vapors to a concentration greater than 68 wt % nitric acid. The overhead product from the column is concentrated (98-99.5 wt %) nitric acid. A portion of the concentrated nitric acid is returned as reflux to aid in rectification. The... 

---