Metal-ion liquors

The major sources of dilute, metal ion liquors are identified within the metals production/processing and chemical industries. Problems associated with traditional methods of metal ion removal are highlighted and the developing role of electrochemical techniques is discussed. Electrode and cell reactions are illustrated via typical examples from laboratory and industrial practice. The need to select an appropriate cell design and to control the reaction conditions is emphasised via consideration of the problems caused by secondary reactions. Important design criteria for electrochemical reactors are summarised. Available reactors are classified according to the nature of the product which may be metal flake or powder, a metal deposited onto a disposable substrate, a metal ion concentrate or an insoluble metal compound. The applications for electrochemical techniques in environmental treatment are illustrated by examples which show features of reactor construction and their typical performance. Current trends are summarised and recommendations are made for further work in critical areas. 

Aqueous liquors which contain low levels (less than 1 g dm ) of dissolved metal routinely arise during the industrial production, processing and surface finishing of metals. In addition, many sectors of the chemical industry also utilise metal containing cleaners, catalysts, and process reagents. A survey of these industrial sectors [1] is summarised in table 1 which indicates the diversity of such metal ion liquors with respect to ... 

Traditional treatment methods for these dilute metal-ion liquors usually involve combinations of the following ... 

Figure 1. The origin and treatment of dilute metal-ion liquors...

So far, this paper has considered the sources of metal-ion liquors and problems with traditional methods of treatment and the importance of a point of source strategy towards wastes has been emphasised. The possible role of electrochemical techniques has been outlined and illustrated by typical electrochemical reactions. In practice, the choice of reactor design and the reaction conditions are extremely important. 

An initial screening test was developed to measure the peracid release of a fully formulated chelant-free detergent powder to which equimolar amounts of chelants were added. Peracid release was measured at 40 °C over 30 min in a synthetic hard water with transition metal ions added at ppm levels representative of those found in wash liquors of a typical washing machine with added soil [36] Ca 76, Mg 15, Fe... 

Far less selectivity can be achieved in cyanide solution because many metals, both precious and base, form anionic complexes in even very dilute cyanide solution. This is illustrated by the equilibrium absorption isotherms for various metal ions in a leach liquor from a gold cyanidation plant on a strong-base resin (Figure 14).358... 

Another process that has benefited from procedures such as that outlined above is the removal of cobalt from leach liquors derived from the extraction of nickel-bearing concentrates or mattes.406 Traditionally, the removal of cobalt from such acidic sulfate solutions has been based on the affinity of the trivalent metal ions (such as those of iron and cobalt) for hydroxide, which... 

Most base-metal sulfides are very insoluble compounds and, in principle, processes based on the differences in their solubilities can be used in the selective precipitation of sulfides. For example, the strong affinity of copper ions for sulfide ion is used to good effect in the removal of traces of copper from leach liquors in the recovery of nickel,406 cobalt407 and manganese.408 For manganese, other impurity metal ions such as cobalt, nickel and zinc are also selectively precipitated by the use of sulfide ions. 

The coordination chemistry of both metal ions involved plays a vital role in governing the thermodynamics and the kinetics of such reactions. This will be illustrated by reference to the chemistry involved in the precipitation of gold from cyanide leach liquors by metallic zinc powder — the most widely applied method for the recovery of gold from solutions.415 The overall reaction is... 

Bipolar membrane synthesis also holds promise for regenerating spent pickling liquors in stainless steel manufacture. As shown in Fig. 43(b), waste acid laden with metal ions can be continuously neutralized, filtered to remove the precipitated metal oxide, and the clarified salt solution split into its acid and base components in a bipolar membrane unit [Type (IV) shown in Table IX], As much as 95% of the hydrofluoric and nitric acid used are returned to the pickling bath, thereby solving a waste... 

The rate-determining step in an ion-exchange process is the diffusion of the adsorbable ions into the resin matrix. Retention times of 2-10 min are used in the uranium industry to attain full equilibrium. The metal ion to be recovered must almost completely occupy the resin fimctional sites to attain a very high degree of selectivity. Resins should be useful for at least two years if clean clarified leach liquors containing no poisonous ions are used. A drastic reduction in the usefulness of the resins is observed in the presence of such ions. Everest et al. (Ell) studied in detail the deleterious effects of thiocyanates, polythionates and sulfur, cobalt... 

Conversely, the calibration for the cyanide determination poses no problem. Free cyanide is found, for example, in untreated liquors from electroplating processes. Since these samples also contain transition metal ions, part of the cyanide exists in complexed form, whereby the various metal cyano complexes differ significantly in their stability. 

Figure 4-3. A typical chelate structure (Courtesy of Dow Chemical Co.), water and in pectins of cotton. The positively charged metal ions, particularly Fe and Ca are readily available for reaction with any negatively charged anion such as OH or COj and insolubilise soap in the fibre which may disturb subsequent operation. This problem is much more acute when scouring is carried out in continuous process involving padding bath where liquor ratio is much lower than the batch process [5]. Thus, the functions of the chelating agents in the soap and detergent formulations are for the prevention of-...

If, on boiling with ammonia, virtually no colour migrates into the liquor, a mordanted or premetallized dye is probably present. This is confirmed by testing for the presence appropriate metallic ions. To do this the material must be ashed, and the method recommended by Clayton is to heat about 1 sq in., if possible, in a platinum or silica dish until charring has taken place. The dish is then cooled and the residue is covered with a hot saturated solution of sodium nitrate, the water is evaporated, and heating is continued until all the carbon has burned away. The ash is extracted with water or dissolved with the aid of a little hydrochloric acid, and the usual qualitative tests for metallic ions are applied. 

The selenites were generally prepared by mixing a 0.1 to 0.2 M solution of the metal ion and a 0.1 M solution of sodium selenite in stoichiometric proportions. In some cases the precipitate was aged in the mother liquor before separation and drying. The specimens were analysed but only the ratio metahselenium is reported. X-ray diffraction patterns were not registered. It is therefore not clearly established whether the preparations were crystalline or (aged) amorphous phases. 

Alkaline hydrogen peroxide in the presence of sodium silicate bleach mechanical pulps (SOW, TMP, CTMP). In order to further stabilize the bleaching liquor against metal ion induced decomposition, the pulp must be pretreated with a chelating agent, usually EDTA or DTPA. Typically, the bleaching is carried out at a temperature of 60-70°C during 60-120 min. Under these conditions, the yield loss is low,... 

Continuous and semicontinuous electrochemical reactors are normally employed for effluent metal ion remediation, where the anode reaction is usually oxygen evolution from water [compare with Equation (26.4)]. After the metal contaminant is captured on the cathode, the cathode can be discarded, the collected metal can be resold, or the deposited metal can be chemically or elecfro-chemically etched into a small volume of a suitable leaching liquor (e.g., water) so as to increase its concentration substantially. 

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