Purification by precipitation

The removal of copper from the pregnant nickel solution in the Sherritt-Gordon process is an example of purification by precipitation of a fairly insoluble compound. First, in the copper boil step, ammonia is driven off by heating the solution, and some copper sulfide precipitates. The residual copper is removed by a dding hydrogen sulfide for the chemical precipitation of mote copper sulfide. 

The above keto-nitrile (15 grams) was methylated with a solution of diazomethane in ether. (The diazomethane solution was prepared using 20 grams of N-nitrosomethylurea.) The ether and excess diazomethane were evaporated on the steam bath and the oil dissolved in ethanol (50 ml). To this was added a solution of guanidine in ethanol (100 ml) (prepared from 8.1 grams of the hydrochloride). The solution was refluxed for 5 hours, the alcohol removed and the residue treated with 5N sodium hydroxide. The insoluble material was then filtered. After purification by precipitation from dilute acetic acid with sodium hydroxide and by recrystallization from ethanol the product formed clear colorless needles (8.0 grams), MP 218°-220°C as described in U.S. Patent 2,602,794. 

Under N2, clean Li metal (0.17 g, 25 mmol) was placed in a round-bottom flask with a solvent mixture of MeOH (3 mL) and pcntan-t-ol (17 mL). The mixture was heated under N, until the reaction with Li was complete. Then, naphthalene-2,3-dicarbonitrilc (2 g, 11 mmol) was added to the mixture which turned green-brown the mixture was refluxed for 3h. The brown powder, obtained after cooling and removal of the solvent under reduced pressure, was dissolved in anhyd acetone (20 mL) and then hexane (70 mL) was added. The green precipitate was separated from the brown solution by filtration. This purification by precipitation was repeated twice. The green precipitate was placed in a Soxhlet extractor and extracted for 3 h with acetone (200 mL) in order to separate the product from the insoluble metal-free species and LiOH. The acetone solution was evaporated down to a volume of 20 mL. The product precipitated after the addition of hexane (70 mL). This latter purification step was performed several times yield 1.29 g (64%). 

Note Purification by precipitation results in the elimination of low-molar-mass oligomers and cyclics and yields polyesters with narrower distributions than the theoretical one (/p = 2). 

The purification by precipitation in methanol of both samples allows to obtain a very good stability for 6 months. 

EtG was distilled under vacuum over phosphorus pentoxide (P2O5). A clear yellow liquid was obtained (100°C/50 mbar). A solution of EtsN (initiator) in dichloromethane (2 pL/mL) was added to the monomer (50% v/v) and the solution was stirred one hour at -20°C. A transparent viscous medium was obtained. An excess of phenyl isocyanate was then added to end-cap the polymer in the presence of DBTL as catalyst. The solution was stirred overnight at room temperature. Purification by precipitation into methanol was realized and the polymer was dried under vacuum at 50°C for 5 hours. 

In this country within the last few years an important improvement in process increases the yield of high purity crystals at the expense of those of low purity. Many salt wells contain measurable quantities of barium salts, which are decidedly poisonous and have caused much loss through the use for stock of low purity salt containing salts of barium. The improvement in process is purification by. precipitation of impurities. The brine as it comes from the wells receives a calculated quantity of salt cake (acid sodium sulphate), which, being a byproduct of nitric acid manufacture, is very cheap. The sulphuric-acid radical combines with the barium salts to form the insoluble barium sulphate, which is removed by a settling process and the practically barium-free brine concentrated as usual. Crystallization by concentration has been treated thus fully because it is a somewhat rare case. 

The study carried out on this process showed the possibility of decreasing significantly the production costs,and time of operation, increasing product quality and process safety by using ultrafiltration in place of purification by precipitation with organic solvents (e.g. methanol). 

Purification by precipitation, liquid extraction, or ion exchange chromatography and... 

A significant amount of fission products passes into waste water. Storage of great quantities of high activity waste water is a relatively complicated problem. At the present time this is mostly solved by storage in underground tanks, concentration and conversion to solid forms, and purification by precipitation and ion-exchanging procedures. 

As in the mercury process, the brine is dechlorinated and recirculated, which requires solid salt to resaturate the brine. The life of the expensive membrane depends on the purity of the brine. Therefore, after purification by precipitation-filtration, the brine is also purified with an ion exchanger. 

The following procedure Is, however, an excellent example of uranium purification by precipitation methods. 

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