Massive zinc

In rechargeable nickel/zinc and sil-ver/zinc batteries this problem is partly compensated for by provision of a massive zinc reserve. The cells are cathode-limited and the amount of anode material exceeds the theoretically required mass by a factor between two and three. 

The electrochemical way can also be used to activate massive zinc being under plates or rod forms. First, the electrooxidation of the zinc anode by a catalytic amount of electricity allows one to strip the surface similarly to an acidic treatment. Second, the electrolytic deposit of a small amount of zinc on a massive zinc surface also leads to the activation of the metal according to a process which is not understood yet. 

Other electrochemical ways have been attempted to generate an activated zinc surface from massive zinc (rod or plate). Two types of activation have been developed. In the first one, a low current density is applied on the massive zinc, which is used as the anode. The electroscoring of the zinc surface makes it reactive towards organic halides that are easily reducible. These reactions are catalytic in electricity. This electroscoring, which is all the more efficient as the applied current density is low, is equivalent to an acidic treatment of the surface. 

The massive zinc (rod or plate) reacts spontaneously with activated bromides provided the preliminary electroreduction of a catalytic amount of zinc salt (ZnBr2 or ZnCl2) occurs. Reactions are carried out in nitrile solvents (CH3CN, PhCN,. ..) or their mixture with dichloromethane. An undivided cell fitted with a zinc anode and an indifferent cathode (gold, nickel, carbon, zinc,. ..) is used. As observed with benzylic bromides, the activation leads to an organozinc compound able to react with either the nitrile solvent or an electrophile reagent. The process is depicted in equation 12. 

The reactions of massive zinc with the organic halides are relatively fast. Moreover, the rate of the reaction can be increased with a low current intensity being applied during the process between the cathode and the anode. 

Anyway, this massive zinc activation process has been successfully used to achieve a number of reactions which usually require more delicate and uncertain zinc activation processes. 

The Daniell gravity cell was one of the earliest galvanic cells to find widespread practical application. It was used in the mid-1800s to power telegraphic communication systems. As shown in Figure 18F-1, the cathode was a piece of copper immersed in a saturated solution of copper sulfate. A much less dense solution of dilute zinc sulfate was layered on top of the copper sulfate, and a massive zinc electrode was located in this solution. The electrode reactions were... 

In contrast to zinc coatings, massive zinc parts in seawater also suffer from pitting corrosion. In zinc coatings, local corrosion is prevented by the circumstance that the outer layer of pure zinc has a less noble potential than the iron-zinc alloy layer, resulting in cathodic protection. 

In commercial alloys, 2inc is usually dissolved in the magnesium matrix and in the hard magnesium—aluminum phase when aluminum is present. Zinc additions to magnesium—aluminum alloys change the eutectic stmcture to a so-called divorced eutectic, characteri2ed by the presence of massive compound particles surrounded by a magnesium-rich sohd solution. 

Sulfide collectors ia geaeral show Htfle affinity for nonsulfide minerals, thus separation of one sulfide from another becomes the main issue. The nonsulfide collectors are in general less selective and this is accentuated by the large similarities in surface properties between the various nonsulfide minerals (42). Some examples of sulfide flotation are copper sulfides flotation from siUceous gangue sequential flotation of sulfides of copper, lead, and zinc from complex and massive sulfide ores and flotation recovery of extremely small (a few ppm) amounts of precious metals. Examples of nonsulfide flotation include separation of sylvite, KCl, from haUte, NaCl, which are two soluble minerals having similar properties selective flocculation—flotation separation of iron oxides from siUca separation of feldspar from siUca, siUcates, and oxides phosphate rock separation from siUca and carbonates and coal flotation. 

Other electropositive elements have been used (e.g. Li, Na, K, Be, Ca, Al, Fe), but the product is generally amorphous and contaminated with refractory impurities such as metal borides. Massive crystalline boron (96%) has been prepared by reacting BCI3 with zinc in a flow system at 900°C. 

Single oral doses >350 mg Zn/kg BW were fatal to rats, although doses of 320 mg/kg BW were tolerated (Table 9.9), suggesting a rapid breakdown in ability to regulate zinc in a relatively narrow critical threshold range. More research seems needed on zinc regulation of massive doses. 

There are two major groups of arsenical gold ores of economical value. These are the massive base metal sulphides with arsenical gold (i.e. the lead-zinc Olympias deposit, Greece) and arsenical gold ores without the presence of base metals. Massive, base metal... 

The unique aspect of electrochemistry lies in the ability to change the electrode potential and thus concentrate an applied perturbation right at the interface. Electric fields of 10 V/cm can be generated electrochemically with a half-lemon, scraped zinc (since 1983) penny, and copper wire as opposed to the massive Van de Craaff generator and electric power plant required for non-electrochemical approaches to the same field strength. If UHV models are to provide useful molecular-scale insight into electrochemistry, some means of controlling the effective electrode potential of the models must be developed. 

MetaUiferous sediments are composed of metal-rich oxides and sulfides. At the ridge crests, the sediments can form huge moimds that are mostly metallic sulfides. Because of their large size—thousands to millions of tonnes—and diversity in metal enrichments— including copper, zinc, silver and gold—these deposits are termed polymetallic massive... 

Metal contents of the sulfide samples collected from the Key Anacon deposits have been classified using the Cu and Zn ratios (Fig. 5). The majority of the massive sulfide samples plot in the Zn-Pb-Cu type (of. Large 1992 Fig. 5) however, the semi-massive to disseminated sulfides in the footwall are enriched in Cu, Co, and Bi and lower in Zn and Pb, typical of stringer zone mineralization. These plots in the Cu type field and one sample contains enough zinc to be placed in the Zn-Cu group (Fig. 5). 

The evolution of metallicity as a function of redshift, and hence as a function of time, provided that we have selected our cosmological model, teaches us mainly about the evolving rate of SNII explosions, for it is in these that zinc originates. It thereby informs us of the rate at which massive stars were forming. 

ZnO films for use as buffer layers in photovoltaic cells (see Chap. 9) have been chemically deposited from aqueous solutions of ZnS04 and ammonia [57]. The solution was heated to 65°C, and adherent, compact Zn(OH)2 + ZnO films were formed after one hour. Low-temperature annealing converted the hydroxide to oxide. The solution composition will be important in this deposition. On one hand, increased ammonia concentration will increase the pH and therefore the homogeneous Zn(OH)2 precipitation in solution. However, further increase in ammonia concentration will redissolve the hydroxide as the ammine complex. There will clearly be an optimum ammonia (and zinc) concentration where Zn(OH)2 does form, but slowly enough to prevent massive homogeneous precipitation. The use of ammonia in (hydr)oxide deposition derives, in part at least, from its gradual loss by evaporation if the system is not closed [58], Any open solution of an ammonia-complexed metal ion (which forms an insoluble hydroxide or hydrated oxide) should eventually precipitate the (hydr)oxide for this reason alone. 

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