Zinc sulfide particles

FIG. 1.13 Spherical and cubic model particles with crystalline or amorphous microstructure (a) spherical zinc sulfide particles (transmission electron microscopy, TEM, see Section 1.6a.2a) x-ray diffraction studies show that the microstructure of these particles is crystalline (b) cubic lead sulfide particles (scanning electron microscopy, SEM, see Section 1.6a.2a) (c) amorphous spherical particles of manganese (II) phosphate (TEM) and (d) crystalline cubic cadmium carbonate particles (SEM). (Reprinted with permission of Matijevic 1993.)... 

The principal source of zinc is zinc sulfide or zinc blende. This ore occurs mainly in Australia, Canada and the US. The zinc ore is first concentrated by a process called froth flotation. The crushed ore is fed into tanks of water containing a chemical frothing agent. Air is blown through so that the whole mixture froths up. The rock particles become soaked with water and sink to the bottom of the tank. However, the zinc sulfide particles, into which the water cannot soak, are carried to the top of the tank by the air bubbles and are skimmed off and dried. This ore now contains 55-75% of zinc sulfide. The zinc sulfide is then heated very strongly in a current of air in a furnace (Figure 10.16) to convert it to the oxide ... 

H., Synthesis of controlled spherical zinc sulfide particles by precipitation from homogeneous solutions, J. Am. Ceram. Soc., 81, 2699, 1998. 

Pugh, R.J. and Tjus. K., Electrokinetic studies on Cu(II) hydroxy coated zinc sulfide particles, J. Colloid Interf. Sci., 117, 231, 1987. 

Englemann, R. J. (1965). Rain scavenging of zinc sulfide particles. J. Atmos. Sci. 22, 719-729. 

Sequential addition of reactants into a W/O microemulsion consisting of AOT and SDS, heptane and deionized water was the method used by Smith etal [379]. The sources of Zn and Mn were aqueous solutions of Zn(C104)2 and Mn(N03)2 with the atomic ratio Zn VMn = 10, while the source of sulfur was an aqueous solution of (NH4)2S. All of the reactants were added to the microemulsion the cation sources added slowly and the sulfur source very rapidly under vigorous agitation. A small quantity of methyl methacrylate was also added to avoid precipitation. The zinc sulfide particles thus obtained had a mean diameter of 1.2 nm. 

Zinc ores are generally floated at the mine (18). In the case of simple zinc sulfide ores, flotation is carried out by treatment with copper sulfate to activate the sphalerite causing it to be wet by the organic collector (eg, xanthate). The now-hydrophobic zinc ore particles attach themselves to the rising bubbles. Oxidized ore particles present must be sulftdized with sodium sulfide to be floated (19). Flotation produces concentrates which are ca 50—60% zinc. In mixed ore, the lead and copper are usually floated after depressing the sphalerite with cyanide or zinc sulfate. The sphalerite is then activated and floated. 

For environmental and economic reasons, the eady practice of roasting zinc sulfide and discharging the sulfur dioxide to the atmosphere gave way to plants where the sulfur dioxide is converted to sulfuric acid. Desulfurization takes place while the ore particles are suspended in hot gases. Called flash-and fluid-bed roasters, these processes are described below. Some plants use combinations of roasters and sintering for desulfurization. 

The experiment conducted by Rutherford and his co-workers involved bombarding gold foil with alpha particles, which are doubly charged helium atoms. The apparatus used in their experiment is shown in Figure 14-9. The alpha particles are produced by the radioactive decay of radium, and a narrow beam of these particles emerges from a deep hole in a block of lead. The beam of particles is directed at a thin metal foil, approximately 10,000 atoms thick. The alpha particles are delected by the light they produce when they collide with scintilltaion screens, which are zinc sulfide-covered plates much like the front of the picture tube in a television set. The screen... 

Baral S, Fojtik A, Weller H, Henglein A (1986) Photochemistry and radiation chemistry of coUoidal semiconductors. 12. Intermediates of the oxidation of extremely small particles of cadmium sulfide, zinc sulfide, and tricadmium diphosphide and size quantization effects (a pulse radiolysis study). J Am Chem Soc 108 375-378... 

The luminescence of macrocrystalline cadmium and zinc sulfides has been studied very thoroughly The colloidal solutions of these compounds also fluoresce, the intensity and wavelengths of emission depending on how the colloids were prepared. We will divide the description of the fluorescence phenomena into two parts. In this section we will discuss the fluorescence of larger colloidal particles, i.e. of CdS particles which are yellow as the macrocrystalline material, and of ZnS particles whose absorption spectrum also resembles that of the macrocrystals. These colloids are obtained by precipitating CdS or ZnS in the presence of the silicon dioxide stabilizer mentioned in Sect. 3.2, or in the presence of 10 M sodium polyphosphate , or surfactants such as sodium dodecyl sulfate and cetyldimethylbenzyl-ammonium... 

It is found that the dissolution of zinc sulfides occurs more rapidly when they are in contact with copper sulfide or iron sulfide than when the sulfides of these types are absent. This enhancement is brought about by the formation of a galvanic cell. When two sulfide minerals are in contact, the condition for dissolution in acidic medium of one of the sulfides is that it should be anodic to the other sulfide in contact. This is illustrated schematically in Figure 5.3 (A). Thus, pyrite behaves cathodically towards several other sulfide minerals such as zinc sulfide, lead sulfide and copper sulfide. Consequently, pyrite enhances the dissolution of the other sulfide minerals while these minerals themselves understandably retard the dissolution of pyrite. This explains generally the different leaching behavior of an ore from different locations. The ore may have different mineralogical composition. A particle of sphalerite (ZnS) in contact with a pyrite particle in an aerated acid solution is the right system combination for the sphalerite to dissolve anodically. The situation is presented below ... 

Several reviews on ore processing by flotation are available.17-21 In addition to providing details of the chemistry of collectors they describe the use of activators and depressants. The former usually convert the surfaces of an ore particle which does not bind strongly to conventional collectors to one that does. The addition of Cu2+ ions to enhance the flotability of minerals such as sphalerite, a zinc sulfide, has been exploited for some time.4 Formation of a surface layer of CuS has been assumed to account for this, but the mechanisms and selectivities of such processes continue to be investigated.18,22,23... 

Some of the discharged sulfide particles settle onto the chimney s exterior, where they are buried by the outward growth of anhydrite. Sulfide precipitation within the chimneys, causes copper, zinc, and iron sulfides to deposit and partially replace the anhydrite. Chimneys can build to several meters in height and their orifices range in diameter from 1 to 30 cm. Both the smoke and the chimneys are composed of polymetallic sulfide minerals, chiefly pyrrhotite (FeS), pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite or wurtzite (ZnS). 

The roasting (or oxidation) of sulfide ores to yield the metal oxides. For example, in the preparation of zinc oxide the sulfide ore is mined, crushed, separated from the gangue by flotation, and then roasted in a reactor to form hard white zinc oxide particles according to the reaction... 

Unlike other sub-atomic particles, electrons are easily detached from the atom. The picture on your TV set is produced by firing electrons at the screen, which has been coated with zinc sulfide,... 

Submicrometer uniform crystalline spheres of silver-doped zinc sulfide (ZnS Ag) were prepared by aging 0.04 mol dm-3 Zn(N03)2 and 2.80 X 10-6 to l. 68 X 10-5 mol dm-3 AgN03 with 0.4 mol dm-3 TAA for up to 100 min at initial pH 1.52 and 73°C (15). The authors found that the final particle density decreased with increasing content of silver ions, whereas the total reaction rate was virtually unaffected by the significant difference in the total surface area of the particle. In fact, the final particle diameter increased from 0.3 to 1.1 pm with increase in the content of silver ions from 5.6 to 16.8 pmol dm-3, as shown in Figure 3.1.5. Figure 3.1.6 shows the time evolutions of [Zn2+] and [Ag+] in the solution phase. 

Chiu prepared monodisperse crystalline particles of metal sulfides, such as lead sulfide (PbS cubes 100 A) (I), cupric sulfide (CuS hexagonal bipyramids 200 A) (2), and zinc sulfide (ZnS multifaceted spheres 0.1-0.4 p,m) (3) by introducing hydrogen sulfide gas into dilute acidic solutions of the ethylenediamine tetraacetic acid (EDTA) complexes of the corresponding metal ions (10 4-10-1 mol dm-3) for several minutes at room temperature. 

Figure 7. Scattering of white pigments as a function of particle size (A = 550 nm) a) Rutile b) Anatase c) Zinc sulfide d) Zinc oxide e) White lead f) Barium sulfate...

Figure 21. Scanning electron micrograph of lithopone. The larger particles are barium sulfate (mean size 1.0 pm) and the smaller particles are zinc sulfide (mean size 0.3 pm).

Since the 1960 s, stamp paper has been treated with fluorescent or phosphorescent phosphors. Since the stamp is stuck to the top right-hand corner of the letter, its recognition by the machine serves to establish the position of the letter and enables machine control of the correct positioning for postmarking and reading. Doped zinc sulfides with average particle sizes of 5-6 pm are used as afterglow phosphors. 

Commercially, zinc sulfide is available in the standard untreated grade and in several grades having particles treated by surfactants (qv) to improve their dispersability in either aqueous or organic media. 

Both zinc sulfide and barium sulfate are insoluble in water. To improve the stability of lithopone, a small amount of a cobalt salt is added to the precipitated mixture. The mixture has to be filtered off, dried, and calcined. The calcination is carried out in rotary calciners at temperatures between 600 and 700°C. During calcination, the particle size of zinc sulfide grows from its original size (about 0.1 Jim) to the pigmentary optimal size of 0.4—0.6 Jim. 

Zinc and lead usually occur together in nature as sulfides. Earlier separation processes involved the fine grinding of the combined sulfides and then treating the particles with chemical reagents to cause one sulfide to be preferentially wetted and rhns the two sulfides separated by the froth flotation process. In a first stage, the lead sulfide is floated while the zinc sulfide sinks to the bottom of the tank. In the second stage, the process is reversed and the zinc sulfide is floated. Gangue and other nonmetals collect at the bottom of the tank. The separated sulfides are dewatered to a 6-7% moisture content and are referred to as the zinc concentrate and the lead concentrate. 

Depressants (or deactivators) are chemicals that ensure that undesired particles remain hydrophilic and therefore do not get floated. Conversely to the activation of zinc sulfide by copper ions above, zinc ions from zinc sulfate act as a depressant for zinc sulfide. Another example is the use of cyanide to complex with copper and prevent adsorption of collectors in the flotation of base-metal sulfides with xanthates. There are many other depressants but they tend to be quite specific to one of a few types of minerals. In some cases, such as some uses of cyanide as a depressant, the mechanism of depressant action remains unclear. 

Spanhel, L., H. Weller and A. Henglein (1987). Photochemistry of semiconductor colloids. 22. Electron ejection from illuminated cadmium sulfide into attached titanium and zinc oxide particles. Journal of the American Chemical Society, 109, 6632-6635. 

Also in the early 1900s Ernest Rutherford performed his famous Gold Foil Experiment. Rutherford set up an experiment in which a radioactive substance released alpha particles. These particles were aimed at a thin sheet of gold foil. A screen coated with zinc sulfide was set up around the gold foil to detect the alpha particles when they hit the screen. Rutherford s experiment... 

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