The relation between the composition of the electrolyte and the various conditions of current-density, temperature and the like has been studied by F.
Oettel, using a 20% solution of potassium chloride, obtained the best yield of hypochlorite with a high current-density, but as soon as II% of bleaching chlorine (as hypochlorite) was present, the formation of chlorate commenced.
With high current-density, heating the solution tended to increase the proportion of chlorate to hypochlorite, but as the proportion of water decomposed is then higher, the amount of chlorine produced must be less and the total chlorine efficiency lower.
He also traced a connexion between alkalinity, temperature and current-density, and showed that these conditions should be mutually adjusted.
A high current-density being employed, the turn-over of gold is rapid - an essential factor of success when the costliness of the metal is taken into account.
For instance, in practical working it has been found that a furnace return of oï¿½ 504 lb per kilowatt hour is brought down to 0.406 lb per kilowatt hour when the material has been broken up, sorted and packed in air-tight drums. In the tapping process a fixed crucible is used, lined with carbon, the electrode is nearly as big as the crucible and a much higher current density is used.
Goldstein 7 was able to show that an increase in the current density is capable of destroying the well-known spectra of the alkali metals, replacing them by quite a new set of lines.
The chemistry of the process has been studied by Martin Kiliani (Bergand Hiittenmdnnische Zeitung, 1885, p. 2 49), who found that, using the (low) current-density of 1 .
Of the metals which dissolve, none (except bismuth, which is rarely present in any quantity) deposits at the anode so long as the solution retains its proper proportion of copper and acid, and the current-density is not too great.
The dependence of the mechanical qualities of the copper upon the current-density employed is well known.
A very weak current gives a pale and brittle deposit, but as the current-density is increased up to a certain point, the properties of the metal improve; beyond this point they deteriorate, the colour becoming darker and the deposit less coherent, until at last it is dark brown and spongy or pulverulent.
The necessity for adjusting the current-density to the composition and treatment of the electrolyte is thus apparent.
Such an irregular distribution of the bath, with strong copper sulphate solution from the anode at the bottom and acid solution from the cathode at the top, not only alters the conductivity in different strata and so causes irregular current-distribution, but may lead to the current-density in the upper layers being too great for the proportion of copper there present.
By introducing perforated shields of ebonite between the electrodes, so that the full current-density was only attained at the centres of the jets, these ill effects could be prevented.
One of the chief troubles met with was the formation of arborescent growths around the edges of the cathode, due to the greater current-density in this region; this, however, was also obviated by the use of screens.
The chief differences between the commercial systems of refining lie in the arrangement of the baths, in the disposition and manner of supporting the electrodes in each, in the method of circulating the solution, and in the current-density employed.
critical densityemperature above 90 K is readily obtained, however the critical current density is only of order of 10 5 A/cm 2.
At the active electrode there is a high current density due to the small area of the electrode.
It is only by paying great attention to the current density that good results are obtained, since metals other than that sought for may be deposited.
Kiliani found that the sponge was produced chiefly when a weak solution, or a low current-density, was used, and that hydrogen was usually evolved simultaneously; sound deposits resulted from the use of a current-density of 200 amperes, or more, per sq.
From these and other considerations it is obvious that (I) the electrolyte must be such as will freely dissolve the metal to be refined; (2) the electrolyte must be able to dissolve the major portion of the anode, otherwise the mass of insoluble matter on the outer layer will prevent access of electrolyte to the core, which will thus escape refining; (3) the electrolyte should, if possible, be incapable of dissolving metals more electro-negative than that to be refined; (4) the proportion of soluble electro-positive impurities must not be excessive, or these substances will accumulate too rapidly in the solution and necessitate its frequent purification; (5) the current density must be so adjusted to the strength of the solution and to other conditions that no relatively electro-positive metal is deposited, and that the cathode deposit is physically suitable for subsequent treatment; (6) the current density should be as high as is consistent with the production of a pure and sound deposit, without undue expense of voltage, so that the operation may be rapid and the "turnover" large; (7) the electrolyte should be as good a conductor of electricity as possible, and should not, ordinarily, be altered chemically by exposure to air; and (8) the use of porous partitions should be avoided, as they increase the resistance and usually require frequent renewal.
With a current-density of 130 to 140 amperes per sq.
The bath is used with a current-density of loo amperes per sq.
The electrolyte commonly contains about a lb of copper sulphate and a lb of strong sulphuric acid per gallon, and is worked with a current density of about ro amperes per sq.
For instance, in practical working it has been found that a furnace return of oÃ¯¿½ 504 lb per kilowatt hour is brought down to 0.406 lb per kilowatt hour when the material has been broken up, sorted and packed in air-tight drums. In the tapping process a fixed crucible is used, lined with carbon, the electrode is nearly as big as the crucible and a much higher current density is used.
The operation is conducted at a dull red heat (about 760Ã‚° C. or 1400Ã‚° F.), the current density being about 0.64 amperes per sq.
The current density is about 700 amperes per sq.
The current from a Wilde's dynamo was passed, apparently with a current density of 5 or 6 amperes per sq.
p. 51) has found that with neutral solutions a 5% solution of copper sulphate gave no good result, while with a 20% solution the best deposit was obtained with a current-density of 28 amperes per sq.
Whilst the jet was playing, a good deposit was formed with so high a current-density as 280 amperes per sq.
In copper-refining practice, the current-density commonly ranges from 7.5 to 12 or 15, and occasionally to 18, amperes per sq.
The relatively electro-negative character of silver ensures that with moderate current densities no metal (other than precious metals) will be deposited with it; hence, while the solution is pure a current-density of 30 amperes per sq.
of cathode may be used, but as copper accumulates in it, the current-density must be diminished to (say) 15 to 20 amperes per sq.
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