Leaching test atmosphere

Leaks in medical sources and seeds are detected by a vacuum immersion leach test. Because the internal volume of the medical sources is so small (3.6 x 10 mL, in the case of the ALC-P4C seed), the conventional helium leak test is not a valid leak test procedure. About 45 minutes is required to pump down the system before helium measurement is begun. If the internal volume of the test specimen is small, trapped helium would escape before helium assay begins. Therefore, leaks in encapsulated medical sources are detected by measuring the alpha activity of a nitric acid penetrant solution in which the source had been immersed. After immersion, pressure above the liquid is decreased to 2.5 psia for 3 min before venting to atmosphere. This procedure is repeated twice, then the sources remain in acid a minimum of 16 h at 20°C. 

Comparative pressure leach and atmospheric leach tests and residue treatment tests were carried out in the Dynatec laboratory in Fort Saskatchewan in 1999 and 2000. The procedures and results arc summarized in the following sections. 

The pressure and atmospheric leach tests were performed in a 3.S L titanium autoclave equipped with baffles, dual axial impellers and a water cooled condenser. The operating conditions are presented in Table III. Samples were taken throughout and at the completion of the leach tests. 

The particle sizes of the zinc concentrates fed to the pressure leach tests were all in the range of 90% passing 32 to 38 pm. Greater than 98% zinc extraction was achieved with all three concentrate feeds within 60 minutes. The third concentrate was very reactive under pressure leach conditions, achieving 99.1% zinc extraction after only 30 minutes. The three concentrates were also subjected to atmospheric leaching. The atmospheric leach tests results are summarized in Table VI. 

The residues from the atmospheric and pressure leach tests were floated and the flotation solids were analyzed for zinc. Based on these assays, and the weights and assays of the feed to the leaching step, the overall deportment of zinc to the flotation tailings was calculated and is presented in Table VII. The overall deportment of zinc to tailings was in the range of 0.8 to 2% in the atmospheric leach tests, and in the range of 0.1 to 0.5% in the pressure leach tests. 

Final solutions from both pressure and atmospheric leach tests were submitted for chemical analysis, and the results are presented in Table VIA. The concentrations of cobalt, nickel and selenium in the final solutions were not significantly different. Arsenic, antimony and tellurium levels were about the same or lower in the solution generated in pressure leaching (the arsenic concentration was substantially lower in the pressure leach liquor for... 

The physical properties of lead and several of its compounds are listed in Table 3-2. Lead readily tarnishes in the atmosphere but is one of the most stable fabricated metals because of its corrosive resistance to air, water, and soil (Howe 1981). A waste that contains lead or lead compounds may (or may not) be characterized a hazardous waste following testing by the Toxicity Characteristic Leaching Procedure (TCLP) as prescribed by the Resource Conservation and Recovery Act (RCRA) regulations. 

Several studies investigating the environmental effects of controlled tyre combustion have been conducted. It is evident that atmospheric emissions can be greatly reduced if proper air pollution control systems are installed. Laboratory and field data provide evidence indicating that concentrations of some environmental pollutants, especially NOx, may decrease due to tyre combustion, whereas others increase compared to pure coal combustion. Zinc is an example of an element that increases in both solid combustion products and atmospheric emissions. The geochemical impact of higher Zn contents in fly and bottom ash on leaching processes in disposal sites remains to be tested. 

A Study of Leaching of the Infiltrated Concrete. The distilled water experiments provided the most information for examining the reactions involved in the leaching of sulfur-infiltrated concrete in aqueous media. Products selected from the specimens were analyzed, and the experiment was continued in a controlled environment. One specimen was partly immersed in distilled water under a nitrogen atmosphere. Fragments of the concrete were also sealed in a test tube. 

Zinc extractions of about 95% were achieved with each of the concentrates within 24 h under atmospheric conditions in the batch tests. However, regrinding was necessary to further reduce the particle size of Concentrates A and B to achieve 95% zinc extraction. The zinc extraction rates obtained from Concentrate A with varying particle size and different modes of leaching are compared in Figure 1. 

It should be noted that none of the concentrates tested by Dynatec yielded 95% zinc extraction in less than 24 hours in an atmospheric leach. Based on the results for the different concentrates presented in this paper, the extraction in the atmospheric leach is assumed to be 95%, compared with 99% extraction in the pressure leach. The overall zinc recoveries are assumed to be 1% lower in each case. The difference in zinc recovery between pressure and atmospheric leaching is 1,600 t/y, which must be supplied with additional zinc concentrate at a cost of 0.53 million per year. 

For the zinc concentrates tested by Dynalcc an expansion of an existing roast-leach-electrowin plant using pressure leaching offers economics that arc favourable in comparison to those of an atmospheric leach. 

In the late 1990 s a series of flowsheets were proposed and tested involving simultaneous atmospheric leaching, iron precipitation and acid regeneration. These were considered particularly suited to sequential leaching of limonite and saprolite ores. A number of laterite sulfuric acid leaching processes utilising this chemistry have been put into commercial operation. 

With respect to chlorides (Cl ), the most widely used procedure is the chloride candle, a gauze wick in a flask of reagent water. The candles tire exposed for a fixed period of time, usually 30 days, then removed. After leaching the exposed gauze into the flask water, the total chlorides are determined analytically, and reported as chloride deposition per unit of exposed area per unit of time (mg/m /day). This method is described in ASTM G 140, Test Method for Determining Atmospheric Chloride Deposition Rate by Wet Candle Method. The other method more recently introduced uses a dry fabric panel, which is also exposed for a predetermined length of time, usually 30 days. Then the chlorides are washed out of the fabric and analyzed. Both procedures are currently described in ISO Standard 9225 however, the dry plate method has been found to produce inconsistent results, and probably will be dropped from the ISO document at its next revision. 

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