Extraction/leaching results

SEMI has recently released provisional specifica-tions ] for particulate, anionic, and metallic contamination in parts. Specifications for particle release are not included in this document and are yet to be determined. Metallic extractables are based on leaching in ultra-pure water. Section 3.7 of this specification states that " The relative leach-out performance of polymer components in actual use with other chemicals (e.g., acids and bases) cannot be directly derived by using the UPW data. It is incumbent upon the user to determine if a component is suitablefor use based on these requirements. " For all practical purposes, the utility of the new SEMI specification is limited to ultrapure water. Table 15.6 shows a comparison of extraction/ leaching results (ICP-MS) for wet-bench plastics in different acids and ultra-pure water. The amounts of metals leached out of plastics by different fluids vary significantly hydrofluoric acid extracts 10-500 times more metal than nitric acid. 

Some contradictory results are obtained in the comparison of results in the determination of arsenic after ultrasonic slurry sampling — the use of sampling is not most appropriate in this case as the sampling step is before the slurry formation — ultrasound-assisted extraction — leaching has been the correct word in this case — and microwave-assisted digestion [28], which will be treated in detail in Chapter 5. 

Chelators such as EDTA, nitrilotriacetic acid (NTA), 1,2-aminocyclohexane 7V,7V,7V ,N7-tetraacetic (DCyTA), and ethylene glycol-bis(2-aminoethyl)-(V,(V, 7V ,7V -tetraacetic acid (EGTA) have been studied extensively and are well summarized (Peters, 1999). Chelator concentration and reaction pH influence metal complexation and the success of removal from soils. Sun et al. (2001) observed that batch extraction methods result in 1 1 molar extraction ratios of EDTA/metal (Pb, Cd, Zn, Cu) and reveal which metal is more or less soluble in EDTA solutions. Column leaching studies, however, relate the elution patterns and recalcitrance of the metals to desorption and dissolution by EDTA. There is concern over the detrimental effects on soil quality from using chelators because of their biotoxicity, persistence in soil environment, and their removal of beneficial micro-and macronutrients, which leave the washed soil infertile for revegetation when it is backfilled. 

Extractables. Development data can often show that a test for extractables is redundant, but when leaching of extractables can result in interaction between a product and its container/label/closure system, an extractables test might be appropriate. 

In the separation of Ca(N03)2 from Sr(N03)2 by extraction (leaching) better results were obtained when acetone was used as the solvent Instead of a (I + l) mixture of ethanol and diethyl ether [30]. 

The assay of solid phase ferrous and ferric iron has a long tradition due to the early interest in soil chemistry. Publications on extraction / leaching conditions and results from varying soils and sediments are extensive and thus, within the scope of a textbook, only important principals and a description of the (subjectively) most important extractions can be given. Since a... 

Careful consideration should be given to each test article to determine an appropriate approach to the testing. For example, a medical device may be a thin, sealed titanium can containing electronics. Extracting this device with the electronics contained within would not be appropriate, because there is little to no likelihood that a patient will be exposed to the electronics, and it would add mass to the test article and therefore increase the amount of extraction vehicle the can is exposed to. The result is a dilution of extractants leaching from the can, which can potentially mask toxic responses. In the case of hemodialyzers, the hollow fibers and housing are often tested separately, because of the different amount of extraction vehicle required. 

In the early days of the lithium industry considerable attention was paid to the recovery of lithium from moderately high-lithium Clay. Lien (1985) noted that in laboratory tests some clays could have as high as an 80% lithium extraction with a simple sulfuric acid leach, but that most required a more complex process. In brief tests a roast at 750°C with two parts of clay and one part limestone, followed by a leach with an excess of 20% hydrochloric acid gave a 70% lithium yield. In a second series of tests five parts of clay, three parts of gypsum and three parts of limestone were roasted at 900°C. A water leach resulted in an 80% recovery of lithium as lithium sulfate. In the later process the raw materials were first groimd together to a -100 mesh size and then formed into 6.5 mm pellets before being roasted. The pellets reduced the dust loss and increased the particles contact with the flue gas. 

Historically, soda ash was produced by extracting the ashes of certain plants, such as Spanish barilla, and evaporating the resultant Hquor. The first large scale, commercial synthetic plant employed the LeBlanc (Nicolas LeBlanc (1742—1806)) process (5). In this process, salt (NaCl) reacts with sulfuric acid to produce sodium sulfate and hydrochloric acid. The sodium sulfate is then roasted with limestone and coal and the resulting sodium carbonate—calcium sulfide mixture (black ash) is leached with water to extract the sodium carbonate. The LeBlanc process was last used in 1916—1917 it was expensive and caused significant pollution. 

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

Chemical Precipitation. The product of the extraction processes, whether derived from acid or carbonate leach, is a purified uranium solution that may or may not have been upgraded by ion exchange or solvent extraction. The uranium ia such a solution is concentrated by precipitation and must be dried before shipment. Solutions resulting from carbonate leaching are usually precipitated directly from clarified leach Hquors with caustic soda without a concentration step, as shown ia equation 9. 

Extraction of Bertrandite. Bertrandite-containing tuff from the Spor Mountain deposits is wet milled to provide a thixotropic, pumpable slurry of below 840 p.m (—20 mesh) particles. This slurry is leached with sulfuric acid at temperatures near the boiling point. The resulting beryUium sulfate [13510-49-1] solution is separated from unreacted soflds by countercurrent decantation thickener operations. The solution contains 0.4—0.7 g/L Be, 4.7 g/L Al, 3—5 g/L Mg, and 1.5 g/L Fe, plus minor impurities including uranium [7440-61-1/, rare earths, zirconium [7440-67-7] titanium [7440-32-6] and zinc [7440-66-6]. Water conservation practices are essential in semiarid Utah, so the wash water introduced in the countercurrent decantation separation of beryUium solutions from soflds is utilized in the wet milling operation. 

Pressure-acid leaching was used to extract cobalt from Blackbird mine ores before its closing in 1974. The result was a very fine cobalt powder which was subjected to a seeding process to produce cobalt granules. Leaching methods are also used in the refinement of lateritic ores. 

A similar process has been devised by the U.S. Bureau of Mines (8) for extraction of nickel and cobalt from United States laterites. The reduction temperature is lowered to 525°C and the hoi ding time for the reaction is 15 minutes. An ammoniacal leach is also employed, but oxidation is controlled, resulting in high extraction of nickel and cobalt into solution. Mixers and settlers are added to separate and concentrate the metals in solution. Organic strippers are used to selectively remove the metals from the solution. The metals are then removed from the strippers. In the case of cobalt, spent cobalt electrolyte is used to separate the metal-containing solution and the stripper. MetaUic cobalt is then recovered by electrolysis from the solution. Using this method, 92.7 wt % nickel and 91.4 wt % cobalt have been economically extracted from domestic laterites containing 0.73 wt % nickel and 0.2 wt % cobalt (8). 

Uses. The extraction or cyanidation of precious metal ores was the first, and is stiU the largest, use for black cyanide (71). The leaching action of the cyanide results from the formation of soluble cyanide complexes. 

The optimal temperature range for the fluorination process was found to be about 230-290°C. The resulting cake was leached with water. The prepared solution was separated from the precipitate by regular filtration and the separated insoluble precipitate was identified as lithium fluoride, LiF. The solution contained up to 90 g/1 Ta205. Solution acidity was relatively low, with a typical pH = 3-4, and was suitable for the precipitation of potassium heptafluorotantalate, K2TaF7, tantalum hydroxide or further purification by liquid-liquid extraction after appropriate adjustment of the solution acidity [113]. 

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