In leaching

Many substances used in modem processing industries occur in a mixture of components dispersed through a soHd material. To separate the desired solute constituent or to remove an unwanted component from the soHd phase, the soHd is contacted with a Hquid phase in the process called Hquid—soHd extraction, or simply leaching. In leaching, when an undesirable component is removed from a soHd with water, the process is called washing. 

Separating the soHd and Hquid phases in leaching operations, where a valuable material is contained in the Hquid phase, so its recovery is to be maximized. 

Nickel and cobalt are recovered by processes that employ both pressure leaching and precipitation steps. The raw materials for these processes can be sulfide concentrates, matte, arsenide concentrates, and precipitated sulfides. Typically, acidic conditions are used for leaching however, ammonia is also effective in leach solutions because of the tendency for soluble cobalt and nickel ammines to form under the leach conditions. 

Berty, Laboratoty reactors for catalytic studies , in Leach, ed.. Applied Industrial Catalysis, vol. 1, Academic, 1983, pp. 41-57. 

Vinyl chloride is made from ethylene and chlorine with Cu and K chlorides. The Stauffer process employs 3 multitubular reactors in series with 25 mm (0.082 ft) ID tubes (Naworski and Velez, in Leach, ed.. Applied Industrial Catalysis, vol. 1, Academic Press, 1983, p. 251). 

Gold recovery Operation of carbon in leach, carbon in pulp, and heap leach circuits Recovery of gold from tailings dissolved in sodium cyanide... 

Rainfall, besides wetting the metal surface, can be beneficial in leaching otherwise deleterious soluble species and this can result in marked decreases in corrosion rate . A recent survey of rainfall analyses for Europe has shown that, with the exception of the UK, the acidity and sulphate content of rainfall markedly increased in the period 1956 to 1966, pH values having fallen by 0 05 to 0-10 units per ann. The exception of the UK may be due to anti-pollution measures introduced in this period. However, even in the UK a pH of 4 is not uncommon for rainfall in industrial areas. The significance of electrolyte solution pH will be discussed in the context of corrosion mechanisms. The remaining cases of electrolyte formation are those in which it exists in equilibrium with air at a relative humidity below 100%. 

Similar results were reported in deforested Amazonian rainforests (66). Within three years following forest clearing and burning, nutrient concentrations of soil leachates had returned to levels typical of primary forests of the area. A combination of high rates of immobilization and storage by successional vegetation, coupled with a decline in easily decomposable substrates, was attributed to the reduction in leaching losses. 

Two situations are found in leaching. In the first, the solvent available is more than sufficient to solubilize all the solute, and, at equilibrium, all the solute is in solution. There are, then, two phases, the solid and the solution. The number of components is 3, and F = 3. The variables are temperature, pressure, and concentration of the solution. All are independently variable. In the second case, the solvent available is insufficient to solubilize all the solute, and the excess solute remains as a solid phase at equilibrium. Then the number of phases is 3, and F = 2. The variables are pressure, temperature and concentration of the saturated solution. If the pressure is fixed, the concentration depends on the temperature. This relationship is the ordinary solubility curve. 

In most unit operations it is of considerable importance that material is transferred from one phase to another across a boundary. The transfer of material from a solid phase to a liquid phase (as typically in leaching), or the transfer of material between one liquid phase to another liquid phase (as typically in molten metal and molten slag phases), extraction or between liquid and vapor phases (as typically in distillation) are well-known examples encountered in practice. 

Thermodynamic considerations are important in leaching in that they provide basic guidance in choosing the combination of reagents and their concentrations so as to obtain favorable free energy changes associated with any proposed reaction. Kinetic considera-... 

It has often been found that for deriving the optimum results in leaching, it becomes necessary to have the starting feed material pretreated. These pretreatments generally fall in to the following categories comminution, concentration, and pyrotreatments. 

The film diffusion stages (i) and (v) are similar to the external mass transfer stages in leaching and may be dealt similarly (see Section 2.1.3). Solution flow conditions for resin touching the fixed beds are likely to be in the laminar flow range. 

The seepage and leaking of sulfuric acid solutions used in leaching can also produce liquid wastes however, this potential is offset by the copper producer s interest in collecting as much of the copper-bearing leachate as possible. Older operations generally do not have protective liners under the piles and experience some loss of leachate. New leaching operations use impermeable membranes to confine leach solutions and channel them to collection ponds. 

Nolan BT, Dubus IG, Surdyk N, Fowler HJ, Burton A, Hollis JM, Reichenberger S, Jarvis NJ (2008) Identification of key climatic factors regulating the transport of pesticides in leaching and to tile drains. Pest Manage Sci. doi 10.1002/ps,1587... 

CIL [Carbon In Leach] A process for extracting gold from cyanide leach liquors using activated carbon. 

More research and demonstration activity should be devoted to water harvesting, which can be considerably useful not only in reducing irrigation requirements but also in the reduction of overland flow and consequently in the protection of soils from water erosion, as well as in leaching soils from salts accumulated with irrigation water. The solution of tied ridges, or diked furrows, to be obtained either by animal energy or when possible with the use of mechanical equipment, has been... 

In the 1990s a large variety of organometallic catalysts were physically entrapped in leach-proof silica gels showing enhanced activity and... 

Acid soak cleaners, for electroplating, 9 783 Acid soap, 22 728 Acid soluble dyes, 7 373t Acid solutions hydrazine, 13 587 in leaching chemistry, 16 152 Acid sulfite process, 23 540-541 Acid sulfite pulping, 21 22-23, 26 Acid treatment, in petroleum refining,... 

Alkaline solutions effect on wood, 26 352 in leaching chemistry, 16 152 sodium nitrite in, 22 853-854 Alkaline sulfite pulping, 21 22, 25 Alkalinity... 

Complex-forming solutions, in leaching chemistry, 16 152-153 Complex hydrides, 13 607, 613-621... 

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