Nitric acid leaching

Several different flow sheets have been developed around DEHPA, which, as discussed earlier, vary according to the precise composition of the feed material. A typical process using a nitric acid leach of a xenotime ore follows as an example. 

Recovery from Nitric Acid Leach Liquors... 

Selenium. Selenium has been determined colorimetrically by formatrion of a diaminobenzidine complex, 420 nm, after an oxidation and distillation (13). Method S-190 for selenium in air extracts the analyte from filters by 0.1 N nitric acid leaching. The nitric acid extracts are aspirated into an argon/hydrogen-air flame, and the absorbance is recorded for the 196 nm analytical line. The method discourages the use of a carbonaceous flame like air-acetyelene. Many analysts use air acetylene because it is convenient. Background correction is required for either flame type. 

Baroch et al. (Bo) used a nitric acid leach in their proposed process for... 

The magnesium reduction slag is recovered by nitric acid leaching and solvent extraction in a manner exactly analogous to the recovery of calcium fluoride slag. 

A nitric acid leaching system is being deveioped by Direct Nickei, but there is no commercial plant as yet. 

Uranium Extraction from Ore Leach Liquors. Liquid—Hquid extraction is used as an alternative or as a sequel to ion exchange in the selective removal of uranium [7440-61-1] from ore leach Hquors (7,265,271). These Hquors differ from reprocessing feeds in that they are relatively dilute in uranium and only slightly radioactive, and contain sulfuric acid rather than nitric acid. 

Opa.nte. There are two methods used at various plants in Russia for loparite concentrate processing (12). The chlorination technique is carried out using gaseous chlorine at 800°C in the presence of carbon. The volatile chlorides are then separated from the calcium—sodium—rare-earth fused chloride, and the resultant cake dissolved in water. Alternatively, sulfuric acid digestion may be carried out using 85% sulfuric acid at 150—200°C in the presence of ammonium sulfate. The ensuing product is leached with water, while the double sulfates of the rare earths remain in the residue. The titanium, tantalum, and niobium sulfates transfer into the solution. The residue is converted to rare-earth carbonate, and then dissolved into nitric acid. 

Glass offers good resistance to strong acid at high temperatures. However, it is subject to thermal shock and a gradual loss in integrity as materials such as iron and siUca are leached out into the acid. Nonmetallic materials such as PTFE, PVDC, PVDF, and furan can be used for nitric acid to a limited degree, but are mainly restricted to weak acid service at ambient to moderate temperatures. 

AH operating facilities shear the spent fuel elements into segments several centimeters long to expose the oxide pellets to nitric acid for dissolution. This operation is often referred to as chop-leach. The design and operation of the shear is of primary importance because (/) the shear can be the production botdeneck, and (2) the shear is the point at which tritium and fission gases are released. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

Waters Seawater (National Research Council Canada 1992) was collected in the North Atlantic Ocean at a depth of 10 m, 35 km southeast of Hahfax, Nova Scotia, Canada. The water was peristaltically pumped through cleaned polyethylene-hned ethyl vinyl acetate tubing and 0.45-pm acrylic copolymer filters. It was acidified to pH 1.6 with ultrapure nitric acid during its immediate transfer to 50-L acid-leached polypropylene carboys, previously conditioned with ultrapure water acidified to pH 1.6. The seawater was later homogenized in two linked 800-L polyethylene tanks in a clean room and immediately bottled in cleaned 2-L polyethylene bottles. Randomly selected bottles were used for analytical measurements. 

A mineral was being leached by heating with 7 M nitric acid in a PTFE-lined bomb heated by immersion in a silicone oil bath, and at 195°C a violent explosion occurred. This was attributed to prior leakage of oil into the pressure vessel, which had been immersed in the oil at 120°C, then allowed to cool, before being heated to the higher temperature. Appropriate precautions are recommended. 

Reynolds Metal A process for extracting aluminum from clay by leaching with nitric acid. An essential feature is the pelletizing of the clay by calcination with kaolin in order to pro-... 

In 1997 and 1998, the authors306 307 also examined acid leached Raney copper catalysts, whereby the alloy was leached with either nitric or perchloric acid of 5, 14, or 27.5 wt% strength. The acid solution was added dropwise over 15 min to an equal volume of deionized water containing the alloy particles. After leaching at 50 °C, the particles were removed and washed to a pH of 7. Air drying at 120 °C was then carried out for one hour. The dissolution rates of catalyst components were observed to be functions of the extraction time (Table 56). 

Lynch et al. [21] have described a method for the determination of organic carbon in silty lake sediments. In this method the air-dried and sifted (-250 mesh) sample is leached with 4M nitric acid-0.1M hydrochloric acid for 1.5h at 90-95°C, and the extinction of the cooled, clean solution is measured at 500nm. The extinction correlates well with weight loss (%) on heating the sample between 120 and 800°C. The precision is 26%. The same leach solution can be used for trace-metal determinations. 

Environmental water samples to be analyzed for metals are best stored in quartz or Teflon containers. However, because these containers are expensive, polypropylene containers are often used. Borosilicate glass may also be used, but soft glass should be avoided because it can leach traces of metals into the water. If silver is to be determined, the containers should be light absorbing (dark colored). Samples should be preserved by adding concentrated nitric acid so that the pH of the water is less than two. The iron in well water samples, for example, will precipitate as iron oxide upon exposure to air and would be lost to the analysis if not for this acidification. 

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