Overall leaching

Taking all these factors into account, overall leaching losses from organic farms tend to be... 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

Mechanism Leaching may simply result from the solubility of a substance in a liquid, or it may be enabled by a chemical reaction. The rate of transport of solvent into the mass to be leached, or of the soluble fraction into the solvent, or of extract solution out of the insoluble material, or of some combination of these rates may influence overall leaching kinetics, as may an interfacial resistance or a chemical reaction rate. 

TOC gives an indication of the overall leaching of organic substances from a material. While there is no direct relationship between the organic matter (TOC gives no indication of its composition), and any particular effect on water quality and risk for the consumer, it is necessary to minimize aity contribution from materials on this... 

In general, the rate of transfer of the solvent from the bulk solution to the solid surface is quite rapid, and the rate of transfer of the solvent into the solid can be somewhat rapid or slow. These are not, in many cases, the rate-limiting steps in the overall leaching process. This solvent transfer usually occurs initially when the particles are first contacted with the solvent. The dissolving of the solute into the solvent inside the solid may be simply a physical dissolution process or an actual chemical reaction that frees the solute for dissolution. Our knowledge of the dissolution process is limited and the mechanism may be different in each solid (Kl). 

The rate of diffusion of the solute through the solid and solvent to the surface of the solid is often the controlling resistance in the overall leaching process and can depend on a number of different factors. If the solid is made up of an inert porous solid structure with the solute and solvent in the pores in the solid, the diffusion through the porous solid can be described by an effective diffusivity. The void fraction and tortuosity are needed. This is described in Section 6.5C for diffusion in porous solids. 

In general, the foUowing steps can occur in an overall Hquid—soHd extraction process solvent transfer from the bulk of the solution to the surface of the soHd penetration or diffusion of the solvent into the pores of the soHd dissolution of the solvent into the solute solute diffusion to the surface of the particle and solute transfer to the bulk of the solution. The various fundamental mechanisms and processes involved in these steps make it impracticable or impossible to describe leaching by any rigorous theory. 

Desorption is the reverse of the sorption process. If the pesticide is removed from solution that is in equdibrium with the sorbed pesticide, pesticide desorbs from the sod surface to reestabUsh the initial equdibrium. Desorption replenishes pesticide in the sod solution as it dissipates by degradation or transport processes. Sorption/desorption therefore is the process that controls the overall fate of a pesticide in the environment. It accomplishes this by controlling the amount of pesticide in solution at any one time that is avadable for plant uptake, degradation or decomposition, volatilization, and leaching. A number of reviews are avadable that describe in detad the sorption process (31—33) desorption, however, has been much less studied. 

The interaction of all these factors makes it difficult to predict an overall effect of conservation tillage on the potential leaching of a pesticide compared to that in a conventionally tilled field. However, it was found that a prolonged rain immediately after appHcation resulted in short-term levels of pesticide in groundwater to be greater under no-tiU than under conventional till plots, which suggested that preferential transport in no-tiU had occurred... 

The zinc sulfide in the concentrate is always converted to oxide by roasting. An exception is the direct leach process described below. The principal overall roasting reaction is strongly exothermic and provides excess heat which is recovered. 

The dispersion of amphiboles in concentrated HQ. solutions also leads to partial leaching, the rate of which depends on the metal cations present. With crocidoHte, only small amounts of magnesium and sodium are extracted in these conditions, whereas amosite Hberates substantial quantities of iron and magnesium. Overall, tremoHte appears to exhibit the highest resistance to acid leaching. 

Usually, however, it is not feasible to establi a stage or overall efficiency or a leaching rate index (e.g., overall coefficient) without testing small-scale models of likely apparatus. In fact, the results of such tests may have to be scaled up empirically, without explicit evaluation of rate or quasi-equilibrium indices. 

Repair and maintenance records were analyzed to determine failure rates and distribution of failure modes. Preliminary findings are reported which include the Weibull distribution characteristics. Failure mode distributions are approximate. Overall mean-time-between-failure is given for the kiln, leach tank, screwfeeder, tank pump, tank gearbox, and kiln gearbox. The study was confined to an analysis of unscheduled repairs and failures. 

After ten consecutive runs the overall turnover number reaches up to 3500 mol 1-octene converted per mol Rh-catalyst. In agreement with these recycling experiments, no Rh could be detected in the product layer by AAS or ICP, indicating leaching of less then 0.07 %. In all experiments, very good selectivities for the linear aldehyde were obtained, thus proving that the attachment of the guanidinium moiety onto the xanthene backbone had not influenced its known positive effect on... 

The process is selective for nickel and cobalt, with iron, the major component of the ore, remaining in the leach residue. Overall metal recovery is low - typically 75-80% recovery of nickel and only 40-50% recovery of cobalt. 

It can be seen, therefore, that ferrous iron and chalcopyrite oxidation are acid-consuming reactions, while pyrite oxidation and iron hydrolysis are acid-producing reactions. Thus, whether the overall reaction in a dump is acid producing or acid-consuming depends on the relative proportions of chalcopyrite and pyrite and on the pH conditions. In practice, sulfuric acid additions to the leach solution applied to the dump are usually required to overcome the acid consuming reactions of the gangue minerals and to keep the pH in a suitable range, typically 2 to 2.4, to optimize bacterial activity and minimize iron hydrolysis. 

The last reaction cited above as shown is very effectively catalyzed by bacterial action but is very slow chemically by recycling the spent ferrous liquors and regenerating ferric iron bacterially, the amount of iron which must be derived from pyrite oxidation is limited to that needed to make up losses from the system, principally in the uranium product stream. This is important if the slow step in the overall process is the oxidation of pyrite. The situation is different in the case of bacterial leaching of copper sulfides where all the sulfide must be attacked to obtain copper with a high efficiency. A fourth reaction which may occur is the hydrolysis of ferric sulfate in solution, thus regenerating more sulfuric acid the ferrous-ferric oxidation consumes acid. 

---