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Organic Compounds of Sulphur, Selenium, and Tellurium

Reviews 0. Ref legal event code : ST. Decopperized anode slime containing gold, silver, selenium, and tellurium is leached with nitric acid and the resulting solution is subjected to the following steps: silver chloride precipitation; simultaneous denitration and chlorination; separation of selenium and tellurium by liquid-liquid extraction; stripping of tellurium; and reduction of selenium and tellurium by sulphur dioxide.

The residue from the nitric acid leaching is subjected to aqua regia leaching; separation of gold by liquid-liquid extraction; and reduction of gold from the gold-loaded extractant directly. In the last three decades, pyrometallurgical processes have been used to recover gold, silver, copper, and selenium from anode slime. The conventional process Figure 1 involves numerous, and often complex, hydro-and pyrometallurgical operations.

The first step is to remove copper. Copper present in the slimes occurs usually as metallic copper dislodged from the anodes, together with copper sulphide, selenides, and tellurides. The copper sulphate solutions obtained by these methods are usually contaminated with selenium and silver, which must be removed before the copper is recovered by electro- winning, cementation, or crystallisation as copper sulphate.

Selenium is present in the slimes in the elementary form as well as metallic selenides. It is generally recovered by smelting the decopperized anode slimes with soda or soda nitre and dissolving the resulting water-soluble sodium selenite from the crushed slag.

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Elementary selenium is precipitated by passing sulphur dioxide through the acidic leached solution. Both of the above recovered selenium products should be purified by re-dissolution, re-precipitation, and final distillation to obtain a marketable product. The decopperized anode slimes, containing varying amounts of.

This metal is processed electrolytically using the Moebius system to refine silver and separate gold and platinoids. The Moebius cell cathodes receive a deposit of silver crystals with "three nines" quality or better. The gold and platinoids, together with some silver, are collected as an anode sludge in a fabric bag. This sludge may be treated by boiling with sulphuric acid to remove the silver and impurities, and high purity gold may then be recovered from the residue by refining in Wholwill cells. The above-described conventional pyrometallurgical process to recover the precious metals is getting harder and harder to operate because energy costs have soared, strict pollution laws have been enacted, and high grade ore has been depleted.

In order to cope with soaring energy costs, to comply with strict pollution laws, and to face up to the depletion of high grade ore, a hydrometallurgical process to recover the precious metals has been developed and tested successfully by us. The process is economic, energy-saving, and practically pollution- free. The hydrometallurgical process consists of 1 a leaching step, where the precious metals are introduced to the aqueous environment, 2 a solvent extraction step, where the metals are separated and purified, and 3 a reduction step, where the metallic ions are reduced and precipitated from solution.

Owing to the low energy consumption: x10 6 Btu xJ of energy consumed per ton of gold recovered calculated for the new process against 59,x10 6 Btu 62,x10 9 J of energy consumed per ton of gold recovered from the conventional process H. Kellogg, 'sizing up the requirements for producing primary materials', Engineering and Mining Journal, April, , pp.

In the preferred process, all of the organic media and most of the mineral acids are recycled and reused. The hydrometallurgical procedures outlined above have the following advantages over the pyrometallurgical art. To more fully illustrate the process of the invention, the following Example is presented, it being understood, however, that the example is given by way of illustration and not by way of limitation.

The Example corresponds to the flow sheet of Figure 2. Decopperized anode slime leaching was carried out experimentally in a flask equipped with an agitator, a condenser, heating apparatus, and a temperature indicator and controller. The mixture was then filtered and the residue was sent to an aqua regia leaching step see below for the recovery of gold. The filtered solution was found to contain To the filtered solution, a minimum amount of hydrochloric acid was added and the resulting precipitated silver chloride was filtered, washed, and dried to give a purity of Impurities such as lead and tellurium are less than 0.

After recovering silver chloride, the filtered solution was sent to denitration and chlorination simultaneous using a liquid-liquid extraction technique. The experiment was carried out in a six- stage mixer-settler in practice more than six stages may be used. The filtered solution with a nitrate ion level of 5.

An organic extractant was prepared containing about 30 parts by volume of kerosene and 70 parts by volume of tri-n-butyl phosphate. This organic extractant was fed to the sixth stage of the mixer-settler at a rate of The system, as described above, was operated for about 16 hours at the beforementioned flow rates. Samples were collected for every other 10 minutes in the first hour, 20 minutes in the second hour, and 30 minutes in the rest of the operating period.

The aqueous raffinate containing most of the metal ions left the sixth stage at a nitrate and chloride ion level of 0.

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Besides carboxylic and amino groups, it contains a pair of sulfur atoms linked to a methylene group in a way similar to that of thioeters, but different from that of sulfur atoms present in other aminoacids. It was shown to have a striking relationship to a sulfur-containing aminoacid cysteine [ 1 ]. DjA is mostly found in djenkol beans and, as stated in an earlier publication [ 2 ], severe poisoning may result from its ingestion, probably because of the neurotoxic effects of carbon disulfide.

In the long run, djenkolate urolithiasis can also occur due to the precipitation of DjA in the kidneys. For a long time DjA has been provided for educational purposes only and was not intended for medical use. Recently, however, it has been mentioned as a component of skin care cosmetics and dermatological preparations acting as antioxidants [ 3 ]. It seems paradoxical, but no crystal structure of djenkolic acid has been determined by X-ray diffraction.

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The data on interatomic distances and angles have actually been refined only for its derivative, S,S' -metylenebis cysteine monohydrochloride [ 4 ]. Lattice parameters are available for a series of salts of djenkolic acid with transition metals [ 5 , 6 ]. There do not seem to have been any investigations related to the synthetic preparation of DjA analogs with S atoms replaced by other chalcogens, e.

In this context, the molecular mechanics technique may be appropriate in calculating the missing parameters for comparing the structural arrangements of all three aminoacids in detail. The comparisons of the isolated models with their substituted counterparts may prove useful for our general understanding of the structure of real compounds and our ability to assess their stability and chemical properties, as well as for developing ideas for pharmacological applications.

To the best of our knowledge, the X-ray structure determination for djenkolic acid has never been performed, while the other two substances were never synthesized. As a result, the reference compounds are limited to the substituted methionine and cysteine along with their derivatives. As mentioned elsewhere [ 7 , 8 ], we do not perform any systematic energy sampling for searching conformational energy space.

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Ten conformers for each aminoacid studied were built at random for structural comparisons. Djenkolic acid and its substitution derivatives do not contain the dichalcogen bonds Ch-Ch between cysteine residues.

Pure and Applied Chemistry, , Volume 11, No. , pp.

Thus the total number of conformers compared with cysteine and cystine should be smaller due to a lack of symmetry folding being impossible. The models obtained using the molecular mechanics techniques are shown in Figure 2 , Figure 3 and Figure 4 , where they are oriented in a comparable way.

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  8. As far as homolog conformers are concerned, one can see that the chains containing chalcogens are basically the same and their geometries can be compared in further detail using the set of interatomic distances and bond angles listed in Table 1 , Table 2 and Table 3. Model of djenkolic acid.

    Model of djenkolic acid containing selenium. Model of djenkolic acid containing tellurium. The typical C-C bond lengths 1. All distances C-N are exactly the same 1. The latest contribution to this topic is a report on the structure and dynamics of l -selenomethionine in the solid state [ 13 ] which successfully combines X-ray studies and solid state NMR spectroscopy. In DjA all C-S distances are in accordance with the average value 1.

    They are also in accordance with later published sources [ 8 , 10 , 11 ]. As for the same distances in DjSe, they are identical with those reported on the basis of X-ray studies 1. The X-ray data for DjTe are missing in literature, so we compared our value with data for the C-Te distance characteristic of tellurium-containing organic compounds, which is 2.

    When plotted vs the chalcogen anionic radii Figure 5 the bond lengths C-Ch showed a net linear dependence on this parameter. This seems to indicate that the cleavage of the central ditelluride unity would occur easier, and as a result pharmacological properties may be impaired.

    The angles C-Ch-C are smaller than the average value of They decrease as follows: Naturally, this contraction would lead to increased distances between chalcogen atoms, a fact which is actually confirmed by the corresponding calculated values: 2. It must also be borne in mind that the angles C 1 -C 2 -C 3 do not show any substantial scattering in the three compounds studied.

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    By consequence, this indicates that a rigid structural block must be located in the same plane. This condition limits the number of possible conformers because of hindered rotation about C 4 , so most of the rotamers must be generated in the peripheral areas. In our case of two cysteine molecules linked through a methylene group these distances are even shorter: 2.

    Organic Compounds Sulphur Selenium Tellurium

    As for the remaining angles of the main chain C 1 -C 2 -C 3 and C 5 -C 6 -C 7 , they do not appear to be sensitive to the chalcogen nature, being within a range of The isolated nonionized forms of djenkolic acid and its derivatives are intrinsically flexible systems, so they are expected to display the largest variety of torsion dihedral angles.

    With the exception of the angle comprising the amino group, these values are close to those published for d,l -selenomethionine and substituted methionines [ 8 , 10 ]. The energy barriers that separate different conformers are typically rather small for many conformations so that the thermal energy at room temperature enables the molecule to freely change from one conformation to another [ 17 ].

    The values of minimal potential energies are given at the bottom of Table 1 , Table 2 and Table 3.