Current density, chloride electrolysis

Pure aluminum is used in the electrolysis protection process, which does not passivate in the presence of chloride and sulfate ions. In water very low in salt with a conductivity of x < 40 yUS cm" the polarization can increase greatly, so that the necessary protection current density can no longer be reached. Further limits to its application exist at pH values < 6.0 and >8.5 because there the solubility of Al(OH)3 becomes too high and its film-forming action is lost [19]. The aluminum anodes are designed for a life of 2 to 3 years. After that they must be renewed. The protection currents are indicated by means of an ammeter and/or a current-operated light diode. In addition to the normal monitoring by service personnel, a qualified firm should inspect the rectifier equipment annually. 

The electrolyte is made by in situ chlorination of vanadium to vanadium dichloride in a molten salt bath. Higher valent chlorides are difficult to retain in the bath and thus are not preferred. The molten bath, which is formed by sodium chloride or an equimolar mixture of potassium chloride-sodium chloride or of potassium chloride-lithium chloride or of sodium chloride-calcium chloride, is contained in a graphite crucible. The crucible also serves as an anode. Electrolysis is conducted at a temperature about 50 °C above the melting point of the salt bath, using an iron or a molybdenum cathode and a cathode current density of 25 to 75 A dnT2. The overall electrochemical deposition reaction involves the formation and the discharge of the divalent ionic species, V2+ ... 

Consolidative reduction has been employed when the object is so badly corroded that it becomes extremely fragile and all surface details are just a mass of corrosion products. In this case, one must apply a low-current density over a prolonged period. In some instances, it is necessary to hold together the loose crusts during the electrolysis, which are then ultimately reduced in situ into a more coherent but porous metallic network. In the case of a completely mineralized silver lyre from the Royal Graves at Ur, partially-rectified current was used to reduce silver chloride [298]. 

Melhylpentanoyl chloride (26.8 g) was dissolved in anhyd HF (I L) in a cell. The evolution of gas (presumed to be HC1) was observed. Then the electrolysis was carried out with an anodic current density of 3.5 A dm 2, a cell temperature of 5-6 C, and a cell voltage of 6.4-6.8 V. Helium (ca. 100 mL min ) was bubbled from the bottom of the cell in order to agitate the electrolyte during fluorination. The operation was continued until the voltage reached 10 V. The total load passed was 507.6 kC in a period of 250 min. 

On crystallisation, copper selenate separates, contaminated with about 1 per cent, of cupric chloride. The latter may be removed by extraction with acetone, in which it is readily soluble, whereas the selenate is only very slightly soluble after this operation the copper selenate is finally purified by recrystallisation from water. The copper may then be removed by electrolysis,1 using low current density, when selenic acid free from selenious acid and chlorine remains in the electrolyte. The solution may be concentrated until it contains about 82 per cent, of the acid by evaporating at 95° C. under reduced pressure. 

In the patent referred to above, the cell described is suitable for the electrolysis of alkali chloride and is of the bell type, but it is particularly suitable for electrolysing alkali nitrate. Pure nitric acid is formed at the anode inside the bell and is removed by distillation, which is effected by working under reduced pressure and by heating the bells with superheated steam. The nitrite which is formed at the cathode is drawn off continuously and separated outside the cell. The cell itself acts as cathode, and the anode is of such size as to almost dll the bell and thus reduce the working space of the eleotrolyte. High current density (16 amps, per dm.8), reduced pressure and high temperature, are favourable to the distillation of a large amount of concentrated nitric acid. 

When the operation started, the iron tank and the individual boxes were filled with saturated solution of sodium (or potassium) chloride while solid salt was put into the stoneware cylinders. The level of electrolyte in tho anode boxes was always somewhat higher than in the tank. During electrolysis the temperature was kept at 85 °C by steam heating steam entered the heating element of the tank through piping H. The voltage across the electrolyzer was 3.5 to 4.0 V. The total current load was about 2500 A which corresponded to a current density of some 2 A/sq. dm. 

Fig. 121. The interdependence of the anode potential and current density during the electrolysis of a normal solution of sodium hypochlorite and of a normal solution of sodium chloride on smooth platinum electrodes.

A clear picture of the course of electrolysis of sodium chloride solution with a concentration of 5.1 moles of NaCl per litre at 12 °C and a current density of 6.7 A/sq. em can be seen in the graphical representation in Fig. 125. 

It follows from the above explanation that electrolysis of alkali chlorides in an electrolyzer without a diaphragm must be interrupted before curve h which represents the concentration of hypochlorite oxygen changes into a horizontal line only under this condition is the process economical, as a prolonged electrolysis would result in a waste of current without any further increase in th<) hypochlorite content. Moreover, care should be taken to prevent the hypochlorite ions formed from being electrochemically oxidized, as this would result in lower current efficiency and lower hypochlorite concentration in the liquor produced. This process is influenced by a number of factors, e. g. brine concentration, hydrogen ion concentration, anode material, current density, temperature, and last but not least a suitable design of the electrolyzer. 

Perchlorate formation in drinking water electrolysis is a serious problem. In experiments using a semitechnical bipolar cell with BDD electrodes and drinking water (40 ppm chloride) even for the lowest current density applied (50 A m-2) perchlorate was measured at 30ppb using a residence time of approximately 1 s. This behaviour does not recommend BDD cells for drinking water treatment without additional measures. 

The method most generally applied to the isolation of lithium is based on the decomposition of the fused chloride by electrolysis, modifications in practical details having been introduced by various experimenters. Bunsen and Matthiessen1 passed the current from six Bunsen cells through the fused chloride contained in a porcelain crucible, with a carbon rod as anode and an iron wire as cathode. Troost employed a similar method. Guntz2 mixed lithium chloride with potassium chloride, but his product contained 1-3 per cent, of potassium. His current was 10 amperes at 20 volts, with a cathode of iron wire 3-4 mm. in diameter. Borchers3 added chlorides of other alkali-metals and alkaline-earth-metals and a small proportion of ammonium chloride, and employed a current density of 10 amperes per 100 sq. cm. Tucker 4 electrolyzed the chloride without the addition of other material. 

Numerous attempts have been made to prepare iron by the electrolysis of ferrous chloride, but with this salt an elevated temperature is essential for good results, namely, 60° to 70° C. The current density at the cathode should not exceed 0-4 ampere per sq. decimetre, and the quality of the deposit is improved by rotation of the cathode.2... 

The silver nitrate is purified by repeated crystallization from acidified solutions, followed by fusion. The purity of the salt is proved by the absence of the so-called volume effect, the weight of silver deposited by a given quantity of electricity being independent of the volume of liquid in the coulometer this means that no extraneous impurities are included in the deposit. The solution of silver nitrate employed for the actual measurements should contain between 10 and 20 g. of the salt in 100 cc. it should be neutral or slightly acid to methyl red indicator, after removal of the silver by neutral potassium chloride, both at the beginning and end of the electrolysis. The anode should be of pure silver with an area as large as the apparatus permits the current density at the anode should not exceed 0.2 amp. per sq. cm. To prevent the anode slime... 

If the soln. of sodium chloride be made alkaline by.the addition of alkali hydroxide the maximum concentration of the hypochlorite becomes less as the alkalinity of the soln. is increased and the chief products of the electrolysis are chlorate and free oxygen. The relative yields of hypochlorite and chlo- I rate during the electrolysis of soln. of 200 grms. of sodium chloride, and various amounts of the hydroxide per litre of soln., are indicated in Fig. 6, with an anode current density of O OI amp per sq. cm. It is assumed that the reaction representing the formation of chlorate in neutral, and in slightly and in strongly alkaline soln,... 

F. Winteler has studied the conditions favourable for the production of perchlorates, and found that if the concentration of the chloride exceeds 10 per cent., very little perchlorate will be formed in warm soln. The anode temp, should be kept low by artificial cooling, since the yield falls when the temp, is raised and it is preferable to have a high concentration of chlorate. In the electrolysis of alkali chloride soln., no perchlorate is formed until nearly all the chloride has been converted to chlorate. The yield is reduced in alkaline soln. probably owing to the smaller number of chlorate ions and the increasing number of hydroxyl ions liberated as the alkalinity is increased—hydroxyl ions are more easily discharged than chlorate ions. Increasing the current density counteracts the effect of increasing alkalinity. 

The INCO, Thompson plant in Manitoba, Canada, electrolyzes 240 kg sulfide anodes in a sulfate-chloride electrolyte. The approximate composition of the electrolyte is 60 g L x Ni2+, 95 g L 1 SC>42, 35 g L 1 Na+, 60 g L 1 Cl-, and 16 g L 1 H3BO4, and the temperature is 60 °C. Nickel, cobalt, and copper dissolve from the anode, while sulfur, selenium, and the noble metals form an insoluble sludge or slime, from which they can be recovered. The anode sludge contains 95% elemental sulfur, sulfide sulfur, nickel, copper, iron, selenium, and precious metals. Nickel is deposited on to pure nickel starting sheets. The anode cycle is 15 days and the cathode cycle is 5 to 10 days. Electrolysis is carried out at a current density of 240 A m-2 giving a cell voltage of 3 to 6 V [44, 46]. 

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