Zinc sulphate electrolyte

The galvanic stripping reactions involved in separating iron from a zinc sulphate electrolyte using solvent extraction can be assumed to comprise the following half cell reactions. The major anodic and cathodic steps are ... 

SEPARATION OF IRON FROM A ZINC SULPHATE ELECTROLYTE BY COMBINED LIQUID-LIQUID EXTRACTION AND ELECTRO-REDUCTIVE... 

Iron is an important impurity in many process solutions. The environmentally friendly removal of iron and its downstream valorisation has attracted the attention of many researchers in industry and universities. In the last decade, much effort has been dedicated to the removal of iron from zinc sulphate electrolytes in the hydrometallurgical processing of zinc feeds (1-7). Similar problems of iron control, however, occur in other acidic sulphate-based process solutions in hydrometallurgy, in electroplating [conditioning and control of iron (III) in iron-zinc co-deposition baths (8)] or in the treatment of spent zinc galvanizing pickle liquors (9). Numerous extractants or extractant mixtures for the selective removal of iron from acidic sulphate solutions have been proposed. 

X. Tang, Pu Yu, T. J. O Keefe and G. Houlachi, Characterization of Antimony-Gelatin Additives in Zinc Sulphate Electrolytes Using Impedance Analysis, Aqueous Electrotechnologies Progress in Theory and Practice. D. B. Dreisinger, Ed., The Minerals, Metals and Materials Society, Warrendale, PA, U.S.A., 1997, 115-125. 

When such a cell is in action the zinc enters the electrolyte as zinc sulphate, and the nitric acid is reduced. The reduction products, however, depend upon the concentration of the acid, the nature and condition of the electrode and other factors. They may be any of the oxides of nitrogen, nitrogen itself, or even ammonia. Under these conditions it is evidently not possible to consider the measured electromotive force of such a cell as a measure of the decrease of the Gibbs free energy of any particular reaction. 

A plot of some typical activity coefficient ratios as a function of the square root of the concentration is shown in Fig. 1. The emf data for hydrochloric acid are from the work of Harned and Ehlers10 on cells of the type represented by equation (21). The mean activity coefficient ratios were computed by arbitrarily assuming that the most dilute solution, 0.003205 normal (which is the most nearly ideal) has the activity coefficient of unity. It will be observed that as the concentration increases the mean activity coefficient, on this arbitrary basis, steadily decreases whereas, if the ions were perfect solutes the activity coefficient ratios would be unity throughout the concentration range. The shift of activity coefficient ratios in the range of concentration included in this plot is roughly typical of the behavior of uni-univalent electrolytes. Fig. 1 also shows the activity coefficient ratios of zinc sulphate from the work of Cowperthwaite and LaMer,11 based on emf measurements at 25°, of the cell... 

The principle behind all galvanic cells can be explained with reference to one of the simplest, the Daniell cell, which was invented in 1836. It consists of a zinc rod in a solution of zinc sulphate, and a copper rod in a solution of copper sulphate. To complete the circuit, a porous solid layer, which allows ions to pass between the sulphate electrolytes, and an external metallic conductor between the zinc and copper, are needed. In this case, electrons then pass around the external circuit and ions travel through the electrolyte solutions (Figure 9.2). 

Following zinc dust purification, the hot solution is cooled to about 2S°C in two stages of cooling. During cooling, gypsum is precipitated and is then separated fix>m the zinc sulphate solution in two clarifiers. The zinc sulphate solution is forwarded to the electrolytic plant. 

The process for treating the baghouse dusts consists of leaching the dusts with a sulphuric acid solution, generating a lead and silver residue as a by-product, which is returned to the lead smelter, and a zinc sulphate solution, that is purified to remove Pb, Fe, Cd and As. Zinc extraction is carried out by a solvent extraction process based on Lurgi technology. The zinc-rich solution obtained in the solvent extraction plant is sent to the electrolytic zinc plant for zinc recovery and an ammonium sulphate and chloride salt is obtained fix>m the raffinate for agricultural applications. The plant has a capacity of 5,000 totmes Zn/year. 

Voltaic cells can be regarded as made up of two half cells, each composed of an electrode in contact with an electrolyte. For instance, a zinc rod dipped in zinc sulphate solution is a ZnlZrf half cell. In such a system zinc atoms dissolve as zinc ions, leaving a negative charge on the electrode... 

Daniell cell A type of primary voltaic cell with a copper positive electrode and a negative electrode of a zinc amalgam. The zinc-amalgam electrode is placed in an electrolyte of dilute sulphuric acid or zinc sulphate solution in a porous pot, which stands in a solution of copper sulphate in which the copper electrode is immersed. While the reaction takes place ions move through the porous pot, but when it is not in use the cell should be dismantled to prevent the diffusion of one electrolyte into the other. The e.m.f. of the cell is 1.08 volts with sulphmic acid and 1.10 volts with zinc sulphate. It was invented in 1836 by the British chemist John Daniell (1790-1845). 

The incorporation of dimethylamine borane complex (BH3NH(CH3)2) and zinc sulphate into a nickel plating electrolyte results in co-deposition of boron (B) and zinc (Zn). Table 2 presents the chemical composition of Ni-B and Ni-B-Zn alloy coatings. It can be noticed that both boron (B) and Zinc (Zn) have been successfully co-deposited with nickel forming Ni-B and Ni-B-Zn alloy coatings on the surface of the steel substrate. The addition of zinc into Ni-B matrix resnlts in change in metallic luster and chemical composition. These findings are consistent with previous studies [26]. 

For zinc the electrolyte is sulphuric acid and zinc sulphate (0.5—1.0 M) and the usual additives are silicate or animal glue. The current density is 30—75 mA cm , requiring a cell voltage of about 3.3 V and an energy requirement of 3000—3500 kWhton- ... 

It is clear that the electrochemical capacity of this composite electrode depends on its water content. When using zinc as metal and HUP as electrolyte, the hydrated sodium phosphate NaaPO. I2H2O is very well adapted for the insertion of zinc cations, but the results are better if hydrated zinc sulphate is added to this mixture. The potential of this electrode is —0.75 V versus Hj. 

Figure 2.39. Electrolytic galvanizing of a steel specimen is shown schematically. At the positive zinc anode, Zn atoms are oxidized to Zn ions which are dissolved. At the negative cathode formed by the specimen, Zn++ ions are reduced to Zn atoms which are deposited on the surface of the steel. The process can be carried out in a solution of zinc sulphate ZnS04 and may be driven by an external generator at a voltage of 4-6 volts.

In principle, the electric potential difference over the boundary layer cannot be measured for an individual electrode Such measurement would mean that electrode and electrolyte have simultaneous access to metallic wires from a voltmeter, whereby a new, unknown electrode is introduced into the measurement circuit. However, for a pair of electrodes, the measurement can be made as described. Assume that the electrolytes described above are in mutual electric contact, for example, achieved by inserting a porous wall between the solution of zinc sulphate and the solution of copper sulphate. In this way, electric contact between the two electrolytes is achieved without mixing them. Designating the electric... 

When the electrolyte contains more than one cation, their simultaneous discharge becomes possible. This will occur if their deposition potentials (under the conditions used) are close to one another, and it can lead to alloy electrodeposition (q.v.). The most general case, however, is of course where hydrogen ion is the second cation, and the simultaneous evolution of hydrogen is a common accompaniment of metal deposition. The amount of current that will be dissipated in this way can be calculated if the current-potential curves for the two processes are known. It must be remembered that the curve for H2 evolution depends on the nature of the cathode surface as well as on the pH of the solution. Figure E.4 illustrates the deposition of zinc from a neutral solution of zinc sulphate. At the point at which the curves intersect the current efficiency for zinc deposition is 50%, but at high current densities it is much greater. 

Zinc is prepared electroanalytically from an acid solution of purified zinc sulphate, using aluminium sheets as cathodes and anodes of pure lead. The anodic reaction is the liberation of oxygen, and so the concentration of free sulphuric acid tends to increase continuously to counter this the electrolyte is circulated, and the more acid solutions are returned to the leaching process. 

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

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